GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: George Schmermund on January 06, 2018, 06:09:15 PM
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The CP-2 from Mrodair arrived a couple of days ago and I've had a chance to give it a cursory inspection. So far I've just done a mechanical inspection and switched the 2 barrels in and out and pushed around the rest of the moving parts. I haven't loaded and used the magazines yet, but the single shot pellet tray registers well.
The pistol grip is somewhat awkward for my hand, but the shoulder stock arrangement for converting it to a carbine makes a more comfortable grip. The length of pull with the stock installed is quite short for me. The overall ergonomics seem to be designed for the average Chinese customer. None of these traits are really a negative to me because my plan is to customize the gun to what I want anyway. This is a project gun and the basic foundation appears to be sound.
Most of the action parts have a gritty feel to them, but that's all part of tuning things up. The left-side bolt is desirable in my case. The 'thing?' that screws onto the end of the barrel is very effective. I took it apart and there is an assortment of unusual compartments inside. I think they are for storing extra pellets when you're traveling.
Today's project is to clean the barrels and do some testing from the vise. I'm going to remove the open sights and replace then with a red dot or telescopic sight. I'm curious to see how the CP-2 accuracy compares to the Vigilante carbine build.
There are several tests and experiments that I've been planing to do with this new gun. This should be good year for the test bench!
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In for following.... 8)
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subscribed ;D
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Can't wait to get more information. Keep us updated
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Subscribed... 8)
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This should really be interesting.
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I had a chance to spend some time with the CP-2 this afternoon. I started with cleaning the longer barrel. After having spritzed the bore with some Rem Oil, a cleaning patch was pushed through using a marshmallow roasting stick . What I witnessed coming out on the first pass was, well.... unspeakable! I was going to call my wife in to confirm what I was seeing, but thought it would be wiser to just go and get another beer, knowing I'd need a bracer to do the second pass. After the 10th patch I started to see some white corners of the patch coming out and continued for several more passes. They never really came out clean, but I decided to let the pellets finish the job.
Anyhow, It was getting late in the afternoon and I wanted to see what the starting point in accuracy testing was going be. Some preliminary shots were done in the shop using the vise and the results were nominal at 15 feet. I decided to take the gun out to the back patio and test it at 30 feet. It was probably not the best choice for doing the test seeing how we're in the dead of winter here in San Diego and the temperature by late afternoon had plummeted to a brutal 64º F. I ventured to give it a try anyway and went back inside for my mittens. I couldn't find them so I moved everything back to the shop. At that point my wife insisted that I have a couple snifters of brandy to return some color to my visage. That sort of wrapped up the testing for today. I'll give it another go tomorrow.
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lol, this week there is an aftermarket trigger being released that will have the trigger at 1lb a few ounces.. made by a nasa scientist not to be mentioned here.
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After taking a few shots back in the shop with the short barrel in the vise there was nothing special about the results. The group of five shots was in the 1/2" range, but the target was only 15' away. This is a .22 cal version of the CP-2. I removed the barrel to clean it and got the same grossness as was in the longer barrel. The photo is of the muzzle. Obviously there are some burrs that snagged the patch material. The machining, as can be seen, is certainly not any better than the sample of the Crosman Vigilante barrel 'out of the box'.
I'm disappointed in the image quality. I guess it's time to go back to focus stacking. I just wanted to represent the view of an 'out of the box' CP-2 barrel. This is where I'll start the muzzle treatments.
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The short CP-2 barrel was going to start with just the muzzle fix and then be tested before doing the breech. After taking a look down the breech end with the borescope it became clear that just doing the muzzle was not going to be easy to evaluate alone. The machining at the breech is as horrific as the muzzle. Maybe even worse. As long as the barrel was out loose I decided to push a few pellets through by hand to see how what I saw with the scope would effect the smoothness of pellet loading in general.
I was thinking about setting up a load cell experiment to get some numbers to evaluate the before and after effects of redoing the machining. After viewing the breech and pushing some pellets by hand I decided that it wasn't worth the trouble at this point. The situation was too deplorable not to just address the problems and move on to do the shooting tests.
I recommend that anyone interested in experiencing what I'm talking about (if you own a CP-1 or CP-2 airgun) to remove the barrel and try pushing a pellet of your choice down the barrel from the breech. There are a series of resistance steps that will be met as you push. With the Barrel free from having to be loaded by the bolt probe the pellet should just drop into the barrel. This avoids the shearing effect that is usually felt as the pellet is forced into the breech during normal loading. This shearing can be seen on the sides of pellets when they are carefully and safely captured and examined after the pellet is normally fired. I attribute this shearing of the pellet on loading as the result of an insufficient entry slope.
Anyhow, the first big resistance when hand pushing will be just getting the pellet's skirt into the breech. The second resistance will be felt when the pellet drags past all of the burrs left in the grooves and lands of the rifling by the rough edges of the transfer port. The third big resistance step will be when the pellet finally gets pushed into the actual barrel's bore diameter.
I certainly wouldn't expect there to be no resistance in this process of loading a pellet, but I'm confident that the process can be made a lot smoother with some corrective machining operations.
My goal now is to rework the short barrel using the grinding and polishing techniques with the lathe and Dremel tool setup that was used to greatly improve the performance of the Vigilante barrels. The choke will be a little tricky due to the muzzle threads, but this can be dealt with when we get that far.
The CP-2 is now everything that I was hoping for in a project airgun. I think that just about everything important to it's performance can be improved with a little attention to details.
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sounds interesting.. the barrel will be quite the challenge reaching in there far enough and straight enough to properly deburr.. looking forward to pics ;D
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I sure do like the looks of the old school rifling that's used in all the SPA guns. It's hard to beat something that's been time tested in the powder burners for many decades. I never was a fan of the micro groove.
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I was going to just jump right into the breech machining and report my impression of improvements in loading pellets. I then decided that just doing that was sorta cheating and not doing any of the measurements that I got the gun for in the first place. Actually, I got pretty excited about offering up numbers instead of opinions because it's such an easy measurement to make.
The photos are of the lathe method of doing the pellet pushing that was also used in the Vigilante hack. I'll get the measurements done tomorrow and report back.
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I did a couple of quick tests pushing RWS wadcutters. The screen shot shows the 3 resistance points that the pellet encounters when being loaded. The full screen pressure value is 12 lbs. I'll try to get in some more measurements later if I get the time.
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that's a lot of resistance for just the rifling ( 10 lbs or so
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I did a little more push testing and there may be some good information here. The last post was what the RWS wadcutters looked like. In that post the photo showed the 3 peaks that were readily detected just by hand pushing. The scope showed more detail about the three major peaks. Each large peak was proceeded by a smaller peak. The smaller peaks are the head meeting the resistance followed by the large skirt peak. The trailing resistance after the last peak is the whole pellet engaged with the rifling. The rifling is obviously rough.
The photos here show what different pellets look like in the same setup. The first photo is a Crosman wadcutter. The 2nd one is a Crosman super dome. The 3rd one is a Crosman hollow point. There are only 2 distinct peaks with these pellets. There are virtually no small peaks in front of the larger peaks. This indicates that the heads are not engaging.
This may explain some things about why many guns are fussy about what they can shoot well. Without a choke at the muzzle some pellet heads just can't engage well with the rifling when they leave the muzzle. This is all speculation at this point, but I think it it's worth considering. I offer it up for your own speculation.
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Here's another photo of the 8" barrel. Still not so good for detail, but I wanted to get something for the 'before' regrinding and polishing treatment on the lathe. The microscope detail is much better without the camera. Notice that the machining burr is pushed up into the groves. This barrel will undoubtedly be much more accurate with a new crown.
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George you can sure tell that they've cut the corners they can by that closeup of the barrel. I like the brass screw method with some lapping compound, it really cleans up those rough cuts like that.
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There was a question about how to reach far enough in to grind and polish the deeper areas of the breach. This is the method I use for most barrel treatments. The tool is a Temo 4mm green rubber burr. The final polishing is done using traditional methods.
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Please disregard this post.
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Here are the results of the first breech treatments. The pellets used for this test are the same RWS wadcutters from the previous tests. The loading pressures are lower at each resistance point and the transfer port drag is down to virtually nothing of consequence. The last high pressure point where the pellet if pushed into the bore is greatly reduced. The actual rifling drag at this point is higher because the other pressure points haven't crushed the skirt as before. This would indicated that the gas sealing of the skirt to the rifling is now better. The 2 photos are for comparison.
I'm open to all other speculation that is offered up as to what is being shown in the comparative scope traces.
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I think you nailed it. Have you done a JB Borepaste treatment on the barrel? That might smooth it up a bit more if you haven't.
Also, have you pushed one through the muzzle yet? Just wondering about the presence or lack thereof of choking. My guess is that there is none on these cheap little blasters.
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When I first got my CP-1, the burrs/restrictions at the breach end swagged some of the pellets enough that the pellets fell out of the barrel. I opened it up some manually but probably have more work to do.
This seems like it would be a cheap process step fix at the factory.
Good data George
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Subscribed... 8)
Thanks, George!
While I am not in the least surprised at the looks of the factory machining, nor the crud-filled barrels, I am surprised at how darn accurate a couple of mine are in particular, without even cleaning the barrels. My .177 CP-1 is the worst shooter, by far, but my .177 CP-2 blows it away for accuracy..
After shooting a tin or two, my .22 CP-2 carbine smoothed out on it's own, and now loads like silk, and shoots cloverleafs at 20 yards rested on a windowsill..
Crazy how much potential these lil' shooters have, IMO.
Thanks again, George, for taking the time to investigate and document the details of your efforts to make 'em as good as they can be, very cool!
:-) chickie
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I've decided to tackle barrel harmonics for the next adventure into the unknown. The CP2 is a prime candidate with its carbine barrel and easy access for testing. This is a classic single tine tuning fork experiment. The bench in the machine shop area was chosen for this preliminary testing because I wanted to play with some of the vintage Bruel & kjaer instruments I've collected. They need more room to arrange for the testing and the shop has more free space for this approach.
After I've had my way with this arrangement I'm going to switch the testing back to the electronics lab area and use micro accelerometers to do a modal analysis with the HP digital signal analyzer (DSA). This will be interesting to me because the results from the analog testing can be compared to the digital instrument numbers.
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I should probably explain the test setup for doing the first steps in the barrel harmonic measurements. The lower photo in the above post is the arrangement for stimulating the harmonics. The device at the left with the cable coming from it is the stimulus driver. This is a variable reluctance transducer (VRT). A similar device at the right sitting under the threaded part of the muzzle is also a VRT, but it's used as the pickup. Either one can be used interchangeably as a driver or a pickup.
The stimulus VRT can be driven by a oscillator through a broad range of frequencies and cause the barrel to magnetically experience the driving force of those frequencies just the same as a loudspeaker can be driven. The response VRT senses the vibrations and generates a signal of varying amplitude at those output frequencies. The muzzle VRT stays at the muzzle position while the stimulus VRT can be moved back and forth along the bottom of the barrel. This allows the stimulus VRT to move between the nodes and antinodes of the barrel. The virtue of this method of stimulus/response is that there is no contact of either VRT with the barrel, therefore adding no mass to the device under test (DUT).
In the current configuration the induced vibrations are in the vertical direction. In the final experiments I'll use microaccelerometers at 90º to each other to generate Lissajous patterns for a first approximation of complex muzzle movement. The final testing will be done with fired pellets. I apologize if I'm preaching to the choir.
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Paul asked about bore polishing. I have not done anything to the bore yet. I'm interested in treating each element of the barrel separately and making measurements as each is done. I can report that the bore, as shipped, is as gritty as the Crosman Vigilante and 1077 barrels. Also, there is no indication that a choke exists at the muzzle. This means that producing a choke could be helpful to further increase accuracy.
My method of choice for choking the muzzle right now is to knurl it in. I'll have to remove the threads to do a proper knurl and then reinstall them. This will be an interesting process. I don't want to lose the effectiveness of the device that screws onto the threads.
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Here's a thought for you - ream the barrel under the threads, then knurl the area immediately behind them (towards the breech) to produce your choke. You'd have a slightly shorter bore, but wouldn't have to mess with the threads. Oh, and you'd want to clean up the reaming after knurling in order to produce a good crown back in the recess.
I can see it in my mind, don't know if I managed to transmit the idea accurately...
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I love the knurling a choke idea.. and given a rigid setup , should work flawlessly .. just time , pressure, and a touch of heat.
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Here's an interesting measurement anomaly that I recorded today. After determining that the major mode of vibration in the undamped barrel is about 33Hz vertically when driven by the VRT, I thought it was time to see what shooting a pellet down the barrel would look like. The photos show how the measurement was setup using the trap.
I was surprised to see that the 33Hz signal was in the envelope of a low beat frequency. This was unusual because I had done frequency sweeps with the oscillator looking for any significant output signals. I had also used a small damped hammer to physically induce greater force into the barrel than the VRT could produce. I saw no beat frequency signals using these methods.
I have to admit that I was nonplussed by the shape of this new waveform and couldn't figure out what had enough mass in the airgun to cause this low beat frequency. It was like someone (probably my wife) had come in when I had gone for another beer and surreptitiously installed a wha-wha pedal into my experiment. After a couple of more beers I figured it out. The muzzle blast from firing the pellet was strong enough to cause the heavy and not solidly mounted VRT arm to also get resonated. This caused the VRT to move up and down at arm's natural frequency and modulate the amplitude of the barrel signal.
This was actually a good outcome overall because it demonstrated that the barrel frequency at 70ºF is really close to 33HZ even with large interfering signals.
There's more to this story, but I thought it might be best to break it down into smaller segments as things proceed. I hope this stuff isn't too over the top. It's all R&D to me.
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Here's a photo for those who may be interested in what the barrel/muzzle look like when the front sight is removed. I was going to just tap the sight off, but it was on there really tight. It would take something like a slide hammer to remove it without scaring up the barrel. There were no indications that any adhesives were used by the factory. Cutting it off with the lathe was the most expedient way for me because I'm not interested in using the gun with open sights. Besides, I needed the muzzle area to be clear for knurling the choke.
Paul's suggestion about a method for doing the choke looks viable from what can be seen with the sight removed. I'm going to do some target shooting at 30 ft with this barrel before doing any knurling. The crown has already been attended to.
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I'm becoming excessively impressed with this .22 cal CP2 and it's not just the beer talking. After shooting it today in it's pistol form with just the basic barrel treatments, I was compelled to ordered another one in .177 cal.
I haven't even opened up the gun yet to examine it's private parts, but I know that there are things inside to massage that will provide even better performance.
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I'm becoming excessively impressed with this .22 cal CP2 and it's not just the beer talking. After shooting it today in it's pistol form with just the basic barrel treatments, I was compelled to ordered another one in .177 cal.
I haven't even opened up the gun yet to examine it's private parts, but I know that there are things inside to massage that will provide even better performance.
awesome to hear that.. I think you are gonna find there are ample power mods ..I had the stock valve throwing 25 cal 19 grains at 646 fpe with extensive work before I over did it ,, that was on the plinkster model.. not sure but the plinkster comes with a 20 inch barrel, I think these are a bit shorter.
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Great info throughout, George. About those harmonics- I've experienced stuff like this working with long ultra-rigid hydraulics. A dynamic-yet-slower-moving pressure wave (air behind a pellet) moving , just not moving as fast the resonance wave moving through the steel of the barrel itself. Never went anywhere with the hydraulic stuff since it was just "dumb old squid in a hydraulic shop playing with some fancy gear"... all we cared was that it worked without leaking. Still, food for thought.
Otherwise, all your R&D just proves my next airgun will be the CP2. Prolly gonna order in the next month or so, and likely in .22 as a mate for my 2400KT.
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In an effort to cover all of the bases I decided to remeasure the previous barrel measurements. The suggestion that a pressure wave behind the pellet may be modulating the natural frequency of the barrel called for another experiment. This time just the barrel was mounted in the vise without being attached to the gun. The barrel resonance was ~33Hz again. The VRT arm was then struck lightly with a damped hammer and resonated.
As can be seen the modulation envelope (~5Hz) is the same as before. This was without the agency of a fired pellet as a source of added energy. This is convincing evidence of the envelope's source to me.
I do appreciate the opportunity to retest any information that may need clarification. Don't be shy about questioning anything I post that may seem unreliable.
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George,
Based on some of your Vigilante results, the pellet transit time will be much less than the 30ms, which makes the timing of the first leg of the response interesting. Do you think mounting your lightweight accel near the valve would detect the hammer strike as a timing trigger?
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I would be honored to sit down with you and have a beer, discussing airgun physics. That being said, I'm drunk now and still can only grasp a little of what you post from your results. You sir, are an air gunner of the highest caliber, pun intended. ;D
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Stan - I've decided to put my thoughts about your question back into the oven for a little more baking. I need to do some more testing and get a better picture of what's going on.
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Since the barrel harmonics can be attended to without the need for the action parts of the gun, I finally decided to disembowel the the CP2 and spread its innards out on the bench. As Rob mentioned there's really nothing in there compared to the Vigilante. I'm not disappointed, but I'll admit to a sense of accomplishment when I could disassemble and rebuild the Vigilantes quickly without having to look at a drawing or video. By comparison Helen Keller would have no trouble working on a CP2.
There are several places to start tampering with the working parts and easy ways to make improvements. I chose the hammer for starters. It's the stuff that you can't see from the outside that seem to be the foible of all of these low cost airguns. The designs and most materials all seem to be basically good, but the finish work that you pay for in a truly fine gun are absent on many of the components. If your willing to apply your own olecranon lubrication these guns can be transformed into real gems.
With the main tube stripped down the view inside is somewhat grim. There is basically no lubrication and the deburring of the machined tube holes looks as though it might have been done with a tire iron. The scoring on the hammer itself is a good indication of how rough it is in there. I decided to address the deburring in the lathe with a slotted Delrin dowel and a a piece of abrasive cloth. They fit somewhat snugly into the tube. It didn't take long to get the hammer and tube smooth and shiny. There are still plenty plenty of scratches left in the wall, but they are below the surface. I left them there with the thought that they would act as lube reservoirs like hand scraped ways do on a good mill or lathe. One of the tests I did to mark progress on the tube/hammer treatment was to use a protractor to measure the angle that the tube had to be tilted for the hammer to slide smoothly down the tube. A very slight push is need to overcome the stiction. With the parts unlubed any angle below 20º is pretty good. Beware of the threads at the exit end.
The bolt got a good polishing in the lathe and the shoulder at the probe seal was given a slight radius to try and save the breech o-ring from getting sliced up.
The trigger and seer look good. Someone did spend some time on them to make the contacting surfaces very smooth, but not polished. The parts look like they've been tumbled. There is a tapped hole in the trigger that doesn't have a set screw in it. It would be for adjusting the trigger/seer setting, but I guess that one of the factory's lawyers said to leave the screw out. I'll take the tapped hole as a wink and nod from the designer.
I didn't do any mods to the action parts yet because I wanted to see what the gun would be like with just deburring and some moly lube. I'm thinking about how to get an accelerometer in a good place for doing timing experiments.
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George,
What is the cantilevered length of the CP-2 long barrel that you are testing? Also is it .475" OD like on the CP-1?
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The carbine barrel, when mounted, is 17". The diameter is .473" (nominal).
I'm thinking about some trick things that can be done to improve the hammer operation. Hammer bounce in the Vigilante was a none issue as far as I could tell, but these CP2 hammers are probably pretty bad in the bounce department. Dry firing the gun without a powerlet gives a suggestion of a problem and firing a pellet with a full powerlet can mask the bounce ringing.
Here's a test that I thought was instructional. If you loosen the barrel set screws and rotate the barrel 180º and then re-tighten the screws you can fire the gun without moving any gas into the barrel. There's basically no noise from the muzzle or any movement except the hammer hitting the valve stem and rebounding several times. The muted sound and feel of the vibration give a good impression of what must be happening when the gun is fired normally. The accelerometer will tell all when I get there, but this is a simple test that any CP(1-2) owner can do.
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After cleaning and deburring, the moving parts were greased and then reassembled. There is a small elastomeric ball That sits under a set screw and presses against the bolt. The pressure is adjusted with the screw and definitely effects the smoothness of the cocking action. I had to take the gun apart a couple of times to make the adjustment. No big deal.
The trigger action is simplicity itself. I reassembled it without polishing anything and the feel was in need of improvement. The empty trigger set screw hole was left empty because I didn't have a screw that would fit. There were some discussions about the screw in other threads that referenced the CP1 trigger and a set screw provided by Mrodair as an after sale item. Considering what was said about the process of adjustment I wasn't keen on following that procedure.
The decision was made to improve upon the factory approach with something that might be better. Looking around the shop I found a sheet of .030" thick Teflon. I cut a small strip of the material and shaped it as shown in the photos. The shape made the Teflon easy to install and hold in place. The .030" thickness took up most of the first stage motion and really smoothed out the trigger/sear engagement. The overall effect was very pleasing, but there was still a bit of grittiness. Poking around some it seems that the culprit was the spring that loads the sear. There actually was a small amount of factory grease on the ends of the spring. Even so, when the spring is compressed it scraps along the edges of the holes.
The spring is stiffer than I thought was necessary. Modifying it or replacing it with another spring could lighten the trigger pull, but not necessarily reduce the roughness during compression. My solution will be to fashion a leaf spring. The spacing between the top surface of the sear and the trigger frame looks perfect to accommodate the leaf. An assortment of different springs can be fashioned easily and tested without having to do more than remove the plastic grip. I'll report back when I've done some testing.
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theres actually an aftermarket supersear that brings the trigger to mere ounces. I just received mine , yet to install. I'm sure the trigger can be vastly impoved with the method you have outlined also. looking forward to before and after impressions
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Thank you for the clear photo of the new 2 stage trigger blade. I had not seen that. If you chose to put the second set screw in, I would be interested in how easy it is to dial in the 1st/2nd stages.
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I'm starting to see that the CP2 is a different gun than the CP1 in some ways. As far as the second screw in the trigger, I don't see any reason to install it compared to the sheet of Teflon. The .030" thickness actually adds .060" to the sear displacement because the sheet wraps around the on both sides of the trigger to also displaces it from the stop pin. I doubt that anything is going to make the sear/trigger interface any silkier that the Teflon. The 2 stages are very smooth and distinct for what I'm after at the moment. The sear spring needs to be attended to as described in a previous post.
Does the CP1 have the bolt drag adjustment? I haven't read about that yet in other threads. It really improves the feel of cocking and loading. It's important to thoroughly debur the bolt tunnel first to get the best effects of smooth cocking.
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The .177 version of the CP2 arrived from Mrodair. The moving parts seem a little better out of the box compared to the .22 gun when it arrived, but will still need attention. Most of the shooting that I've done with the .22 was with it clamped in a vise. After each lathe treatment it went back into the vise to compare the results. When I finally did some shooting with it myself it was with the carbine setup.
After spending some time with both guns last night while sitting in a comfortable chair I got some new impressions. On the positive side I can report(?) that the thing on the end of the .177 barrel is even more effective than the .22 version, though they seem to be the same devices. I guess that result could be expected.
On the down side I have to confess that the grip and trigger guard are small and awkward for my hand. With the narrow grip area the weight distribution feels unbalanced when trying to aim in a freehand hold. This would be a problem if I wasn't planning to use them both in the carbine setup. The balance and overall comfort is remarkably different and better for me in that mode. I'll have to figure out how to improve these issues.
Another thing that I should mention is that right out of the box the piercing pin would not puncture the powerlet. I turned the locking sleeve in until it was out of threads and still nothing. The powerlet was then removed and examined. There was only a dimple in the end of the powerlet seal. The remedy was to put a washer between the end of the powerlet and the inside of the locking sleeve. This was inconvenient, but it worked. I'll have to turn a couple of threads off of the end of the main tube, but I going to put the tube in the lathe for deburring anyhow. It's just another factory oversight. It's early and much can be done to make things better. I'm still impressed with what wonderful project guns these have turned out to be.
If I had my choice I'd have to admit that I wouldn't buy a CP1 over a CP2. The CP2 package is too good of a deal. Also, the CP1 is an unfinished design that finally blossomed into the CP2 and there's no turning back for my money.
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I'm glad that just the teflon set the two stages at good points. The second screw would be just for additional tuning. I'm wondering if the longer lever arm approach could be applied to the CP1 with a custom trigger blade.
No bolt drag adjustment on cp1
Yes, CP2 looks like a good evolutionary step and a great deal for the money
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I may be behind the curve on this, but I just looked at the Mrodair site and saw that they now show an exploded view of the CP2. The CP1 can now be compared with the CP2 for design differences and parts count. Hopefully there will be replacement parts available for both airguns soon. I'd like to get an extra 1 or 2 of those things that screw onto the end of the CP2 barrel (I don't mean the thread protector).
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To distract myself from other projects that have been started I decided to do some experiments with one of the accelerometers. The easiest place to attach the device is onto the bolt holding in the valve. This gets me very close to where the hammer will be striking the valve stem. The instrument will be fixed in the direction that will be most sensitive to forward and back motion parallel to the barrel's axis.
Hammer and valve stem interaction as well as transit time markers will be recorded. The transit time will be measured at the muzzle with the same force transducer that was used in the timing experiments that were done on the Vigilante.
The photos show the grip being milled out to accommodate the accelerometer and the basic arrangement for mounting. The wires can be easily routed along the inside of the grip along the action and will exit out the back.
I've returned to my usual inability to march a straight course while doing experiments.
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While I was setting up the accelerometer arrangement I got to looking at the main tube section and and decided to try a fit test with one of the heater experiments from the Vigilante days. It turns out that the heater is a perfect fit for the CP2 and a second pair of heaters will also fit. The extended grip in the front will cover the heaters and offer a good insulator so that the heat will conduct through the tube into the powerlet and the valve with little loss. This will be an interesting new project for these airguns.
The photo shows one of the powerlets and a pair of heater strips from the earlier Vigilante project.
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The sear spring project reemerged this morning. I was thinking about the roll that the spring played in the overall trigger's pressure response. I needed a ground zero place to start measurements. The obvious starting point would be to measure the trigger pressure without the spring involved. The results were enlightening, so I thought it was best to share what was found.
To start with I can report that the trigger is no trigger at all when the spring is removed and the gun is cocked in the normal upright orientation. The sear can't engage the hammer sans spring. The next thing was to remove the grip and install the gun upside down in the vise. Now the springless sear can be engaged when the bolt is pulled back and the hammer allows the sear to drop in. It's like setting a spring mouse trap.
The force gage was then setup to push on the trigger from a low friction surface as shown in the photos. A piece of paper was placed between the trigger and sear surfaces and could act as a feeler gage to detect when the the surfaces met. What surprised me was that the first stage trigger pull was reinstated without the agency of the missing sear spring. This was interesting. On further investigation it was revealed that the first stage is basically generated by the resistance of the surface of the sear sliding along the face of hammer's catch with a total force of .68 lbs. When the sear reaches the wall of the second stage the force increased to 1.58 lbs when the gun fires. This would indicate that the sear spring is only needed to return the trigger and sear to their original positions. Any trigger force above 1.58 lbs (in this case) is just extra baggage from the sear spring. It's the hammer spring and hammer/sear surface friction that runs the show.
My conclusion is that the real trigger 1st and 2nd stage force requirements are determined, for the most part, by friction of all the moving parts' surfaces. Now that I've got my baseline numbers I can do surface polishing (including the pivots) and see what happens. If These experimental results are true, then changing the hammer spring will also effect the trigger force.
As an aside I'll add that a good vise soft jaw solution for holding barrels is to cut a piece of PVC pipe and slot it as shown.
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The .177 CP2 was dismantled today and I was able to inspect the trigger parts for comparison to the .22 version. There was actually some lube on some of the parts. This seems To be as hit and miss as lube in the Vigilantes. There was a set screw (missing in the .22) installed in the extra trigger hole that can be used to adjust the first stage travel. The first stage force needed to get to the second stage wall was 1.22 lbs, the second stage break was in excess of 3 lbs. These CP2 triggers are very tunable with little work required to make big improvements. Replacing the sear return spring and polishing the contact surface are all that are needed.
A trigger shoe could also be fashioned to reduce the perceived trigger pressure. I'm giving that some thought because I've been spoiled by the generous blade width on the Vigilantes.
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The hammer was easy to improve upon by reshaping the slope of the sear ramp. The slope was cut to 30º and extended to within .040 of the edge of the sear latch. The ramp and latch face were then polished with the Dremel using a Scotch-Brite wheel.
I chose a small spring from a collection in the shop to replace the factory sear spring. As the photo shows, the new spring is ~ .130" shorter and ~ .004" larger in diameter. The larger dia. allows the new spring to be pushed snugly into the sear hole to keep it in place. This spring also requires less compression to do it's job. Being shorter than the original spring means that when the gun is in its normal orientation the trigger falls away from the sear if the gun is not cocked. This has the interesting effect of leaving the trigger's first stage loose with no spring preload when the gun isn't cocked. I going to consider this outcome to be a feature rather than a flaw because now I don't have to pull the cocking arm back to check it the gun is cocked and loaded. This will reduce the likelihood of jamming from double cocking when using the magazines. When the gun is cocked the first stage of the trigger reasserts itself do to the hammer and sear being engaged.
The trigger force is now .72 lbs for the first stage and 1.52 lbs for the second stage. The new ramp on the hammer is impressive in it smoothness. The first stage pull is slicker than..... The second stage wall is distinct and the break is very clean and positive. The Sta-Lube 3161 grease that I'm using is also very effective.
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The accelerometer testing got underway today. Both the .177 as shipped and the .22 with the hammer mod and the main tube deburring were tested next to each other. The photos show the 2 pistols with the accelerometers each mounted in the same place and the scope traces of the hammer strikes. Neither gun had a powerlet installed. It's just the hammer impacting the valve stem. The top 2 photos are of the .177 results with the traces separated and then overlaid. The bottom 2 photos are of the .22 results, also separated and then overlaid. All instrument settings were identical for these tests.
The red trace on each screen is the unfiltered signal coming from the accelerometer and the yellow trace is the same signal after it's passed through a 1 octave filter centered at 10KHz to eliminate the high frequency noise. The red trace shows a lot of signal that is not directly related to the hammer/valve interaction. Both signals on each screen are triggered simultaneously.
This is the tip of the iceberg for these tests. I'm also planning to put a mini-pressure transducer through the side of the valve so that the pressure/time curves can be synchronized with this hammer/valve timing. With a force transducer at the muzzle we'll probably see some interesting stuff.
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Things have gotten increasingly interesting with the impact testing. Using only the improved .22 gun, the measurements can proceed on a single platform. The photos shows what the accelerometer signal looks like when a powerlet is installed and the gun is pressurized. As with the Vigilante, the CP2 is much more under control as far as having a bunch of rattling of the moving parts. The impact signals are quite distinct now in the 10KHz filtered channel (#2). Channel #1 is still noisy with the high frequency vibration of the main tube longitudinal waves. Note that the vertical signal gain has been doubled to get the signal heights to about the same level as the unpressurized tests done previously. There is significant timing information that is starting to gather. This should be tantalizing stuff for the hammer bounce sleuths out there.
The next test will be to plug the barrel hole in the breech and install a pressure gage in one of the unplugged barrel clamping screw holes. This will correlate the hammer impact timing with the pressure/time signals. This will confirm if there is any hammer bounce to deal with.
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very cool , looking forward to the hammer /pressure test.
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Looking at the accelerometer signals in the previous post with the gun pressurized it is apparent that there is a low frequency signal hiding in the middle of things. After changing the octave filter from a 10KHz center frequency to one centered at 1KHz the hidden signal popped right out. As can be seen in the menu at the right of the screen the frequency is about 1.1KHz and appears to be a low Q undamped resonance. The scope ranges were changed to get better signal contrast.
After closer examination using my nifty little tapping hammer it turns out that this resonance signal is the bolt and hammer spring acting as a classic spring/mass system.
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George,
Interesting data, as always.
Can you describe the filter you applied? Since you are looking for data in the 1 to 10 Khz range how does the filter affect that?
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Stan - Thanks for your interest in the instruments and methodology I'm using for these measurements. These tests are done using analog gear because I'm so enamored with the exquisiteness of Bruel & kjaer equipment of that era. The HP Digital Signal Analyzer (DSA) will be used in the next iteration of tests. The DSA can run circles around the instruments used here, but I still enjoy playing with some of my older toys.
There's no compact way to properly explain how these measurements work without shortcutting the information and causing probable confusion. Here's a link to a PDF that covers it all very well. The measuring amplifier I'm using is the 2636.
[PDF]Third-octave and Octave Band Pass Filter — Type 1617 meteosat.pessac.free.fr/IMA/ressources/tp_avio/TP.../FILTRE_1617_bp0163.pdf
I hope this link works. It comes up on the top of the page on my Google search.
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Yes, thanks,
That filter spec helped quite a bit.
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Another aspect of the CP2 being a project airgun is the truly horrible grips they have. I wasn't very concerned about this issue because I envisioned the guns to be used exclusively in the carbine mode, which is a much more comfortable arrangement.
Now that I started to play with the .177 gun I've discovered that a target can actually be hit using the gun handheld even with just open sights. This was a revelation to me. Now I want to practice getting better at pistol target shooting which I had previously shunned.
Searching eBay I found a CP1 grip, which until now I considered to be silly looking, and bought it. From searches about custom grips it seems that these CP1 grips are supposed to look like just a chunk of 2x4 lumber so that they can be carved to custom fit the shooter's hand. This is a door to a new level of modding for me. A pistol that really fits my hand!
While daydreaming about how to do the shaping when the grip arrives, my mind was left to spin out of control again. It only took a few beers to come up with the idea of using my wife's slab pottery methods to produce the most outlandish designs that I've ogled on the internet. It would be fairly simple, with some guidance from my wife, to reify and kiln fire several designs at the same time. I could have a smorgasbord of grips. And then there's the glazing!
I wouldn't mind having one of these to put up on the mantle next to the anniversary clock. https://www.formgriffe.de/en/shpSR.php?A=86&p1=400&p2=255 (https://www.formgriffe.de/en/shpSR.php?A=86&p1=400&p2=255)
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Not sure if the CP1 grip is interchangeable with the CP2. The CP1 has an extra attachment screw at the endcap (item 44 in the parts diagram you linked). Some mod will be needed.
I have not tried it but I've read that one way to custom grip is to squeeze Bondo
Yes, the .177 is great fun as a target pistol, especially when detuned a bit to get the shot count up.
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This is the direction I'm going to head in for the first grip mod when the eBay score arrives.
http://anotherairgunblog.blogspot.com/2017/05/mrod-air-cp1-m-modifications.html (http://anotherairgunblog.blogspot.com/2017/05/mrod-air-cp1-m-modifications.html)
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In an effort to reduce pressure variations due to powerlet self-cooling I decided to start the heating project again. As can be seen in the photos 4 heaters can be installed under the main tube. This will allow for more power to be pumped into the gun than was doable with the same approach on the Vigilante which could only accommodate 2 heaters. There is also the bonus of more insulation with the front part of the grip pressing the heaters snugly against the tube.
This is just a fit test. The wires can latter be routed back down the inside of the grip to a jack that will be installed in the grip's base. This will be a good situation for the carbine setup. The power pack and temperature controller can be installed in the hollow of the stock.
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Now that I started to play with the .177 gun I've discovered that a target can actually be hit using the gun handheld even with just open sights. This was a revelation to me. Now I want to practice getting better at pistol target shooting which I had previously shunned.
Yes, indeed. "Long range" (for an air pistol) shooting can be a real hoot. Once I managed a 1 inch three shot group at fifty meters with my Baikal IZH 46M. Long range shooting is an art and a science. Elmer Keith is perhaps the best known proponent of long range pistol shooting, albeit with powder burning firearms. One of my favorite pass times is to shoot along some of our locals when they're playing with CO2 powered scoped carbines. My aim is to break the clay pigeons they're aiming at before they can line up their scope.
All that should probably be summed up as "You'd be amazed at what one can accomplish with a hand held open sighted air pistol." By all means, DO experiment with it - and stretch the distances as you gain dexterity at closer ranges.
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The CP1 grip just arrived. I really like it already. Some quick mill work to fit the gun and some carving to fit my hand and I'll be all set to do some target shooting!
Here are a few photos of how things look now.
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Some woodworking was done using the mill today. I liked the CP1 trigger guard because it was metal, but the CP2 plastic guard needed to be used because the bolt holes were moved on the CP2 tube.
The end plug holding in the spring and hammer in the main tube on the CP2 was modified to fit the new grip by machining off the locking stub. The CP1 instead uses a bolt that goes through the grip into the plug. This is fortuitous because an oversized bolt can be installed through the grip and into the plug. The new larger bolt can be drilled out and threaded to allow the instillation of an adjustment screw. This screw can be used to set the hammer spring just like on the Crosman 2240.
The new grip arrangement is working out better than anticipated.
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You're making some progress there George !!! I really liked the first grips on the CP1's they had the nice stippling on them.
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excellent , doesn't seem like crazy conversion at all.. Love the shop towel soft jaws, been there done that
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The new grip is coming along nicely. The main tube plug, after removing the CP2 nub, was drilled and tapped to fit a 10-32 cap screw and the assembly went together as shown. The grip was already factory drilled to allow the tube to be secured into the grip with a screw when it was originally designed as the CP1. This is very convenient. Now the spring loaded cap screw can be adjusted to change the compression on the hammer spring.
If this new arrangement is viable I'll make a new plug out of metal and remove the plastic one. The CP2 is a luxury hacking project compared to the Vigilante. I'm now consumed by the idea of making a custom grip from the CP1's wooden one.
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Looks great. A power adjuster would be a very useful upgrade.
I'm going by memory but doesn't a bolt pass through the cap to attach the back of the breech to the tube? did you modify that attachment?
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Some extensive CP1 Grip mods in this series if you hadn't already seen it.
http://anotherairgunblog.blogspot.com/2017/05/mrod-air-cp1-m-modifications.html (http://anotherairgunblog.blogspot.com/2017/05/mrod-air-cp1-m-modifications.html)
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The screw that was originally passing through the CP2 main tube end plug was removed. The CP1 grip has a hole drilled through the back of the grip to allow another screw to secure the tube. The new screw arrangement will allow the hammer spring to be adjusted while still attaching the tube/breech and grip together. This is not a permanent setup, but is serviceable for the time being. I'm working on an alternative way to attach the breech to the main tube and grip while still having the spring adjuster.
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Being inherently lazy I decided to use the filing machine to do the shaping.
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yep, off to Google I went....never seen a filing machine....interesting.
Thanks
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Well, the filing machine has it's place in the shop, but I'll stick with using it for metal. I should have used the oscillating spindle sander right off, but thought that it might be too aggressive for pistol grip 101. It turned out to be perfect for the job. I've only roughed out the shape so far and the results are already amazing. The more I read about how these types of grips works the more surprised I am that I haven't seen more people using them. It's another whole new world for me.
The photos show where the project is at this point. I'll do some shooting with it like this and then start the fine tuning. Up until now I thought that accuracy had everything to do with barrels and pellets. Custom grips never even entered my mind.
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I got to do some shooting today using the .177 CP2 and the roughed out grip. The pellets were the Crosman pointed and the RWS basic wadcutters. I've done no machining or deburring yet on the barrel so that I could get a baseline for performance. The pointed pellets had a strong tendency to jam when loading with the single shot tray. Once you get them started and they get stuck you have to really apply a lot of effort to get them in there. I've tried to push them out with a rod from the muzzle end, but the pellets get hung up on the o-ring. You just have to keep pushing with the bolt. I've read where other folks with SPA guns have broken the bolt's side arm when facing this same situation.
I struggled with getting 50 shots through and then switched to the RWS wadcutters. Assuming that they would be even more troublesome I was surprised by how well they loaded. I has to remind myself of the vow I took to never use cheap pellets again. I was getting better at this freehand, open sight shooting as the afternoon progressed. Much of the improvement is probably attributable to the 4 beers I had to steady my aim with.
I'm going to hack away at the grip some more now that I've located new places that need attention. My shoulder didn't appreciate having to hold the gun out at arms length for so long today, but I'm going to restore it with an internal alcohol rub this evening. Sometimes the disappointments of getting old can have their own perks!
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Now that I've started to do some target shooting with the .177 CP2 it's time to consider what makes a good open sighting system. I used the sights as they were set when shipped and was surprised how well they worked at 15'. I'll stick to this distance for the moment so as not to overly humiliate myself as I get started down this path. I was shooting outside and the lighting was bright sunlight. In the evening I pointed it around the room to practice just holding the gun up at arm's length and noticed how the front sight pretty much disappeared when sighting on darker things under lower light.
Now, a reasonable person would wisely just paint the front blade white and be done with it. That hardly seems to be an upgrade to me, though. Using the inspirational effects of a pint of strong ale it wasn't difficult to envision an interesting alternative to the paint.
It was easy to mentally deconstruct a red dot sight by stripping it down to just the micro LED and battery. I found some white LEDs on eBay that are ~.060"x.030" with leads attached. That's pretty small and will be easy to mount onto the base of the rear sight. The small lead wires can be routed down to the grip and attach to a battery in a small compartment. One of the trick parts of this scheme is to attach a small piece of 3M retroreflective tape, also available on eBay, to the front sight blade. The retroreflective optical property of this material is such that it has the ability to return a large part of the incident light back to the rear sight as a very narrow cone shaped beam of light. This ability would replace any of the optics that would be needed to project the LED onto the front glass in a red dot system.
The required light intensity in this new sighting system would be very low which would really stretch out the battery life. The added weight would only be a few grams.
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hahaha, electronic open sights , cant wait
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After cleaning and deburring, the moving parts were greased and then reassembled. There is a small elastomeric ball That sits under a set screw and presses against the bolt. The pressure is adjusted with the screw and definitely effects the smoothness of the cocking action. I had to take the gun apart a couple of times to make the adjustment. No big deal.
If the opportunity arises, I would appreciate seeing the location of the bolt adjustment ball/screw. Wondering if it might be worth doing to my CP1.
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Here's a shot of the set screw that holds in the rubber ball. It's a 4x3 mm screw. I've had no luck getting any email response from Mrodair about spare parts, so I guess we're all on our own.
Luckily the set screws are easy to find at the hobby store. The rubber ball can be substituted with a small slice from an o-ring that will fit into the screw hole. If you get the bolt section apart it's worth while to polish the bolt and all of it's contact areas and then apply some grease.
Another thought is that the set screw doesn't need to go in through the bottom. It could go in from the side and then be adjusted after the gun is reassembled.
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Today I opened up all the ports in my CP2, I've got the long barrel on mine right now along with the carbine stock. It was giving me about 9.5fpe and opening up the ports it brought it up to 11.5 - 12pfe. with 16 grain Air Arms pellets.
(https://farm5.staticflickr.com/4672/40272162051_c031ed052c_b.jpg)
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Here's a shot of the set screw that holds in the rubber ball. It's a 4x3 mm screw. I've had no luck getting any email response from Mrodair about spare parts, so I guess we're all on our own.
Luckily the set screws are easy to find at the hobby store. The rubber ball can be substituted with a small slice from an o-ring that will fit into the screw hole. If you get the bolt section apart it's worth while to polish the bolt and all of it's contact areas and then apply some grease.
Another thought is that the set screw doesn't need to go in through the bottom. It could go in from the side and then be adjusted after the gun is reassembled.
Thanks for the photos and details on the bolt adjustment.
FWIW, you can order parts from SPA. No barrels or grips but most other parts should be okay:
Julie at SPA airguns for parts
email- sales@china-airrifle.com
• Request China post if available for best shipping rate
• ~ 2 weeks delivery wait
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Yesterday I started to play with the .177 pistol using the 8" barrel and decided to record a before and after muzzle and breech treatment test. The lathe and Dremel tool were used as described in previous posts. I was more aggressive with the Dremel on the breech this time and the improvement in smoothness of loading dramatically improved. I'm starting to think that a good crown job will only get you halfway there without doing the breech, too.
The photos show the before and after performance using the same RWS .177 Basic Line 7.0 gr. pellets from the same tin. Shooting was done using the bench vise and the targets were at 15 ft. Each group is 10 shots. I'm pleased with the outcome even thought one pellet in the "after" group tried to express its individuality by attempting to become a flyer.
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Looking good!
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I've been thinking about the recent post that showed the 'before and after' results of machining the CP2 8" barrel. While staring at the 'after' group and trying to decide how large it actually was I put a pellet from the tin into the main group's hole. It was quite amazing to me that the skirt of an unfired pellet could support the whole pellet if I picked the target up.
This inadvertent test leads me to believe that these barrels (post machining) are forsooth worthy of competition shooting on some level. The other interesting thing from this group is that the rogue pellet was shot #9. Shot #10 fell back into the same hole where the first 8 pellets went. Nothing in the setup and testing changed except for the pellets. I'll leave it as food for thought as to why the #9 pellet deviated from its brethren. Hints: Weight? Head diameter? Ovality? Unbalanced axial precession? Damage? ........
The photo shows an unfired pellet placed in the group's center with the target raised.
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The other interesting thing from this group is that the rogue pellet was shot #9. Shot #10 fell back into the same hole where the first 8 pellets went. Nothing in the setup and testing changed except for the pellets. I'll leave it as food for thought as to why the #9 pellet deviated from its brethren. Hints: Weight? Head diameter? Ovality? Unbalanced axial precession? Damage? ........
The photo shows an unfired pellet placed in the group's center with the target raised.
Most likely a problem with the head, although an uneven skirt will also throw them out. It's interesting what can be done with carefully sorted pellets, although how one would improve on that grouping is a question worth contemplating. The main idea behind sorting is to eliminate the odd pellet that falls outside the main group by ensuring as much as possible that each pellet has as close as possible the same dimensions as all the other pellets. It'd be interesting to see how well it groups at a standard 10 meter competition distance.
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Something else to consider in the overall scheme of things is that I haven't choked the muzzle yet. Choking may be all that's needed to ameliorate the recalcitrant behavior of pellet #9. Its deviation from the herd is quite small.
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Since I had the .177 dismantlement to machine the hammer's sear ramp and do some polishing as was done on the .22 gun I decided to deal with the main tube end plug mod, too. This is the plug that holds in the hammer spring and it's guide. The nib on the plug that is used to secure the gun into the grip stock on the CP2 had to be removed in order to use a CP1 wooden grip. I had been using only the screw under the pellet loading tray to keep the bolt housing secured onto the tube while the feasibility of the grip conversion was evaluated.
Now that I know the conversion is doable and highly desirable it was time to get the rear of the bolt housing securely fastened to the main tube. As the photo shows, the plug was drilled through the axis to allow a screw to be added that fastened the new wood grip to the main tube's plug. The plug's side hole that provided for the original screw was enlarged to 1/4" to allow the head of a shorter screw to pass through the plug and sit in a counter sunk hole in the opposite side of the original hole. This allowed a new, shorter, screw to pass by the center hole in the plug, but still have enough material left to clamp the main tube tightly to the bolt housing.
The axial screw can now pass through the new wood grip and into the drilled and threaded plug. For now I'll just leave this screw recessed in the wood and keep it tight for clamping purposes. It will be easy to convert this screw to adjust the hammer spring tension in the future.
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The more time I spend tweaking these CP2 guns the more I like them. The paradox is that the more things that are improved, the more other things become obvious. Right now I'm back to the trigger. Using the pistol version is now focused in that direction. Not the trigger action, I love the way that's coming along, but the trigger blade (?) itself. I've removed the trigger guard and that helps to a large extent, but the actual trigger blade is like pressing against a piece of barbed wire. Well, maybe there's some hyperbole there, but then you should talk to my finger! It's now time to start thinking again about modifying one of the Vigilante triggers into a shoe. I'm wondering if I'll remember this as a project tomorrow. I'm sure it will all come back to me the next time I shoot the guns the way they are.
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Here's something else to play with - or not - once you get your other mods going. An air stripper. Attached is a picture of one done by Maverick airguns, it's for the ubiquitous Crosman barrels, but no reason why one couldn't be made up by someone for different barrels. What it does is allows for turbulent air (or CO2) to be diverted from the skirt of the pellet as it is released from the barrel.
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The more time I spend tweaking these CP2 guns the more I like them. The paradox is that the more things that are improved, the more other things become obvious. Right now I'm back to the trigger. Using the pistol version is now focused in that direction. Not the trigger action, I love the way that's coming along, but the trigger blade (?) itself. I've removed the trigger guard and that helps to a large extent, but the actual trigger blade is like pressing against a piece of barbed wire. Well, maybe there's some hyperbole there, but then you should talk to my finger! It's now time to start thinking again about modifying one of the Vigilante triggers into a shoe. I'm wondering if I'll remember this as a project tomorrow. I'm sure it will all come back to me the next time I shoot the guns the way they are.
Good thread!
From a CP1 thread (http://www.network54.com/Forum/79537/thread/1492210182/last-1492272034/Expand+Thread):
I took the sharp edge off the CP1 blade and polished it. It’s comfortable to shoot now but that might not be enough for your trigger. The pull is set to 15 ounces, so being thin isn’t as much of a problem than if the pull were heavier.
(http://www.crankshaftcoalition.com/wiki/images/b/b5/CP1M_TRGGR_PLSHED_w-text.jpg)
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For those CP2 owners that find the trigger and guard arrangement to be as untenable as I do here's a partial fix. I was going to use a more destructive procedure, but since the trigger guard is plastic It seemed reasonable to exploit it thermoplastic properties.
The guard was uninstalled from the gun and then clamped in a vise along with some spacers so that the jaws could get a good grip. The guard loop was then carefully heated with a hot air gun and softened to the point where a 7/8" socket could be pressed through. The socket was left in the loop until the parts were cool and then removed.
The resulting extra room is now a luxury. The trigger blade still not worthy, but that's for this afternoon's project. I've cut down one of the Vigilante triggers and now have to figure out how to attach it so that it can be made adjustable.
The photo shows where things are at this point.
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George,
Since you have machining resources, have you seen this page? http://anotherairgunblog.blogspot.com/2015/09/new-trigger-guard-for-cp1-m-pistol.html (http://anotherairgunblog.blogspot.com/2015/09/new-trigger-guard-for-cp1-m-pistol.html)
I do like your quick heatgun mod
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The trigger mod turned out to be much easier than I thought. The Vigilante 'shoe' was considerably softer metal than the CP2 trigger, which was very hard. After taking the picture of the trigger in the milling vise I realized that I'd need a carbide end mill to do the job.
The shoe can now be adjusted to the proper position for my first approximation of the feel I'm after and will be temporarily affixed to the CP2 blade. When everything feels right I'll epoxy it in more permanently.
I'm pretty sure that if I'd started off with a 2240 I wouldn't be getting the same satisfaction from modding it. My friend Dave is into the Crosman pistols and has a fair assortment of them. He's a machinist so he bought them to make custom parts. After showing him what the CP2 is capable of he's now on the verge of becoming a convert!
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looks excellent George.. Having a 2240 and the cp1 , I have to say the airmax guns are 10 times the quality for about 20 dollars more ( cp1 is 79.99 shipped
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Trigger feel to your finger is as important as the grip feel is to your hand. This is only news to me, I'm sure, but I'm coming along in the target shooting arena. The new Vigilante shoe conversion for the CP2 is working nicely. It's a rough lash-up that uses hot glue to put the 2 pieces together. The hot glue allows me to mate the floating shoe to the CP2 blade and still have time to squeeze the trigger to set the 2 together. I'm finding that the shoe is now offset to the blade in a way that combines the grip position to where my finger wants to be at the same time. The broad surface of the shoe significantly reduces the apparent pressure needed to break the second stage. This is all in the testing stage.
The usual trigger position in most of the entry level match pistols I've been seeing is vertical and at a right angle to the axis of the barrel. The more expensive pistols have an exotic looking trigger arrangement that I assume is designed to pivot to the angle of the shooter's finger. I'm guessing at all of this.
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Here's a look at one type of target trigger. One can move it forward or backward. The trigger "shoe" can be moved up or down AND it can be twisted so as to meet one's finger at a wide variety of angles. Put one on my IZH 46M and am tickled pink with it. It's much better than the original trigger, and even better than merely covering the trigger with a shoe. Of course, the trigger target shoe is MUCH better than a thin blade, it makes the trigger feel lighter and tends to improve one's trigger control. Of course, a trigger like the one posted here is even more better. I've no idea what it'd take to adapt one or something similar to your CP-1, but it's something to think about. ;D
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I decided to get another start on some measurements. This test is to get the hammer strike to trigger the scope on channel 1 using the accelerometer and time the pellet's exit with the force transducer on channel 2. Ch1 is red, ch2 is yellow.
The photo shows that the transit time is just over 2 mS using the .22 cal 8" barrel. This is just a setup test. Next is to use the 18" barrel and overlay it's transit time onto the barrel harmonics testing.
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Now that there is an accelerometer for the o'scope triggering signal I got sidetracked back to doing the pressure measurements. As the photo shows there is a perfect place to install the transducer right into the valve cavity itself. The screw that secures the front of the trigger guard and clamps the grip onto the main tube is semi-compatible with the 10-32 thread of the pressure transducer. It's a blind hole from the factory that goes into the valve body and can be drilled further as a through hole into the valve and then tapped for a true fit with the transducer.
I did the same thing with the Vigilante valve, but it was not nearly as simple as this. It's as though the DP2 was designed to be hacked to make these measurements.
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The pressure measurements got stalled because I wanted to do the same transit time tests on the .177 cal. unit while the instruments were still set up from the .22 tests. I'm glad I did. The photo shows the results from the .177 gun with its 8" barrel and powerplant. These pellets were Crosman 7.4 gr Destroyers. The temperature was 70º F.
This is definitely a different time sequence from the previous test. This would be expected. The highlights of interest are the differences in how the hammer strike effects the valve timing. The picture of events is much clearer than the .22 screen shots.
My interpretation of the time events, through the optics of a few beers, is that the first peak is when the hammer hits the valve stem and triggers the o'scope. The hammer then loose some energy and starts to slow down as the valve begins to open. The next peak is when the hammer slams into the end of the valve body and deposits the rest of its momentum into the gun using the valve body to transfer it. The valve spring and compressed CO2 then push back against the hammer and cause a spring / mass rebound to start. The hammer again hits the valve stem and begins another cycle. This is a simplified explanation and hopefully leads to a clearer idea of what the measurements are telling us. Keep the beer vision in mind, too.
OK, I know all of this is preaching to the choir, but what I'm finding interesting is how fast the first rebound occurs. The valve appears to have opened and close twice in less than 900 µS which means that the pellet is still in the barrel while that is happening. If I calibrate the force transducer It will determine the true exit velocity with more certainty than a typical chronometer. For now I'm just interested in collecting some numbers.
The pressure tests become a much more compelling measurement now because they can demonstrate that what we see with just timing information can be correlated with a time/pressure measurement.
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George,
Looking forward to the pressure timelines.
The hammer/hammer spring should be a fairly low frequency system. Not sure it can respond fast enough.
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Stan - You're probably right about the hammer/ hammer spring. I'm going to switch my guess to the valve stem/ valve spring. They're much less massive. I'll do some tapping with the small hammer and run the signals through the filter and measuring amplifier and see what I can find.
The time/pressure measurement scheme has changed from putting the pressure sensor through the side of the main tube and valve housing. The sensor is now going into the center hole of the breech lock down screws that hold the barrel in place. This change will allow the pressure information to come from the transfer port instead of inside of the valve. I'll drill through the set screw hole and the barrel opposite the transfer port hole and tap it to match the sensor. The pressure info will now come from right behind the pellet. A 10-32 set screw can then replace the sensor when the testing is done.
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Some progress was made today toward getting the pressure sensor into the breech and barrel. Next is to make an adapter to screw into the newly threaded hole to act as a height stop to keep the sensor out of the bore. The sensor will now look directly down into the transfer port and see the same pressure at the same time as the pellet. This would have been much harder to accomplish on the Vigilante. I'm hoping that the powerplant is clean enough not to foul or destroy the sensor.
The photo shows the set up in the mill with the .22 cal breech and barrel. I'll do the same procedure on the .177 pistol.
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I got to play with the pressure transducer installation today and It's about ready to start making some measurements. The placement over the transfer port was a good decision in the overall scheme of things. These miniature transducers from Endevco are really nice. They're right up there with my favorite B&K instruments.
I'm working on a calibration system for the accelerometers so that they can be used for measuring muzzle displacement when the testing gets back to barrel harmonics. Terry (dv8eod) sent me some interesting info back when the Vigilante barrel projects were alive. I've been integrating his information into the mix. It will be a good place to start the new measurements.
The photos shows how the pressure transducer is installed.
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It's easy to get distracted when instruments are sitting around the work area and asking to be put back in harness. I've spent the last couple of days setting up the barrel harmonics experiments. The measurements should be ready to go in another day or so. I'll be back to the pressure/time testing when the instruments for those tests are set up.
Here's the current calibration setup for doing the muzzle displacement measurements. I love these old analog instruments. It's really enjoyable to to turn the knobs and flip the switches on B&K stuff. It makes it very easy to see where the signals are coming from and going to when I'm trying to learn something new. I can break-in anywhere along the line if I think something anomalous is happening. My brain seems to work in the same block diagrams.
The instrumentation photos include:
HP 3310B Function Generator
B&K 2706 Power Amplifier
B&K 4810 Mini Shaker
B&K 4384 Calibration Accelerator
B&K 2511 Vibration Meter
B&K 1617 Band Pass Filter
B&K 2636 Measuring Amplifier
Owon Scope
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For clarity concerning the accelerometer calibration I want to add that the 4384 device is being used with the 4810 shaker to produce a traceable displacement signal. The photo shows how the Endevco 2250 low mass (0.4 grams) accelerometer is being calibrated. The 18" .22 cal barrel, including the thread protector, weighs 304 grams.
I'm confident that the added accelerometer mass won't interfere with the barrel harmonics measurement, but I'll still do a stimulus/response measurement with the non-contact VRT devices to make sure that the resonant frequency doesn't shift.
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That's a pretty nice collection, looking forward to the data
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I was sorting through some boxes in the attic and found one the Schlieren optics sets along with assorted other stuff. These mirrors are 10" f/1.6 type. This makes them very fast as mirrors go. Their short focal length will be an asset for setting them up on a reasonable size optical table.
I wanted to see what the arrangement would look like so I set them up on the dining room table. My wife came in and asked what the equipment was for. I informed her that it was for doing some ballistics experiments. She then informed me that if I thought I was going to do any airgun testing in HER dining room I'd be looking for more than just a new hobby! Something about her grandmother's china. Wives and physics are not always the best mix in this household.
Anyhow, now I have a feel for the footprint of this experiment and can plan accordingly. I'll have to make some mounting hardware and arrange some precision translation instruments to get things going, but the project is becoming more compelling as time goes on. I'm already a year behind on this project.
The photos shows the main hardware needed to get started. This includes the mirrors and a high intensity light source with an adjustable aperture and flexible fiber optic cable. When the system is all adjusted the Micro Flash system will replace the conventional light source.
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Funny how wives are not always as enthusiastic as their husbands about experiments in the dining room, kitchen, etc.
;D
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Dabbling in air pistol timing .... without George's toolbox
I wanted to try some of the timing measurements with amateur sensors. I set up a 2240 just because it was convenient...and happened to have a powerlet in it. I ended up using a pair of small microphones and an amplifier from a Velleman kit (MK136 <$10). I fed this to a scope, but a soundcard app may work as well.
Initially, I compared the impulse (yes, I used George's mousetrap clapper) response of the amplified microphones to raw response output from a couple of small (headset) speakers and there didn't appear to be significant lag through the amplifier so I went with the convenience of the microphones.
The first picture shows the 2240 with one microphone stuck to the barrel band and the other loosely hanging to decouple it from the barrel. The second shows the trace for a shot without a pellet. The channel 1 (yellow) is the suspended microphone its lag is consistent with speed of sound at 74 micro-sec/inch. The second spike in the yellow trace is the echo from the table top 8" away. The channel 2 (blue trace) is the mounted mic and has both acoustic and mechanical responses. The speed of sound lag helps identify the acoustic ones.
The third picture just shows the clapper board test result. The interesting part is the very front where the amplified microphone and the small speaker raw responses (channels 3&4) happen within about 20 micro-sec of each other ( the yellow trace is the suspended mic and the lag is generally consistent with speed of sound).
None of this replaces results from quality lab equipment, but it does give me a little better understanding of the results George is providing us...and gives me motivation to hunt down an accel or 2.
Now back to the real data in this thread.
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Stan - It's nice to see someone else taking the plunge into the mysteries of ballistic tests and measurements. Starting with things that are readily at hand and re-purposing them to do your bidding warms my heart. We need more experimentalists contributing to these "Engineering - Research & Development" threads.
I envy your 4 channel scope. That opens extra doors when making simultaneous measurements in a triggered time window. I've been on the fence about choosing one. How did you make the decision on the one you chose? There are a few questions I have about some of the data you've presented, but I'll consider them to be irreverent at this point in the measurements.
Adding accelerometers to the mix is certainly a leap forward in these tests. A cheap and expendable intermediate device can be fashioned from an old crystal/ceramic phono pickup. The old mono ones were usually designed to detect displacement in only the vertical direction. Their output is generally high so they don't need a preamp to produce line level signals. There are lots of them on eBay. Even lots of lots. I'd stay clear of the oldest ones. Something in the era of 45 RPM stuff should be useful. Try to get the ones that still have the cantilever visible. The condition of the actual needle itself doesn't matter.
With some creative fixturing you can arrange a moveable contact point that is very sensitive to vibration, but virtually immune to the acoustical output of the DUT. The mass loading is negligible. These are basically "throw away" sensors so they can even be used as targets. Raw piezoelectric discs are also available. These would also make good targets. A simple design could make the discs survivable for many tests.
Keep up the good work and report your findings!
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George, the 4 channel DSO is a new addition, my old analog scope wasn't useful for this. I blame Bargain Gate here because of the $50 off ebay coupon a couple of weeks back.
It is the Rigol 1054z. https://www.rigolna.com/products/digital-oscilloscopes/1000z/ (https://www.rigolna.com/products/digital-oscilloscopes/1000z/) They also have a promo through the end of the month that adds a bunch of software functionality. I got it through one of their distributors but they also have their store...I was tempted by their open box items.
I'm not a EE so I relied on a decent (enthusiastic) online review
https://www.youtube.com/watch?v=ETCOhzU1O5A (https://www.youtube.com/watch?v=ETCOhzU1O5A).
Basically I was looking for 4 channels and the ability to store data to USB...The rest of the features I'll have to learn as I go along. I've also been poking my nose into the arduino ecosystem and its wealth of dirt cheap sensor modules so that helped justify the purchase....but mostly it was an (another) impulse buy because of bargain gate here.....
The scope is very pleasant and intuitive to use, even for a non-EE
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Hi George,
I'm enjoying your thread, of course it has my brain hamster running amok. :o
The pressure sensor looks like an Endevco 8510B series strain gage based transducer.
Which Endevco transducer are you using?
What is the cost range for the ones you have?
Have you been able to take any pressure measurements on your CP-2 project ?
Why I'm interested:
I'm trying to determine the cost of putting together a pressure measurement setup for a PCP air gun suitable for measuring the air pressure rise and fall time behind the pellet after the hammer strike hits the discharge valve. Given the pressure levels and valve timings, I think that an 8511A series transducer will be needed. It is unclear what I will need to capture the measurements. Dataq has some low cost USB data loggers that can cycle up to 160KHz, but will need an instrument amp to boost the signal level.
-Cindi
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Thanks for the scope info. Your choice looks like a good one. Lots of capability there for the price. I'm still vacillating about what to get. In the mean time I've fallen prey to a B&K 2515 Vibration Analyzer I found on eBay. I'm setting it up now for an enhancement of the barrel harmonics displacement measurement tests. There's a point at which I'll come to my senses and switch everything over to the HP DSA and be done with it, but like they say " The journey is the reward!". I'm in no rush to finish this project.
As far as what to use as a pressure transducer, there are quite a few choices out there. Right now I'm using an Endevco 8510B - 500. It has a conservative rating of 500 PSI, but it specs out to 3X pressure. I won't be using it at more than 2X with CO2. The 8511 series would be good for higher pressures but their form factor is considerably larger than the 8510 series. The higher pressure transducers I've collected are Kistler 601 series 1500 PSI and 5000 PSI. I did some preliminary P/T measurements in the "Hacking the Crosman Vigilante" thread. The CP2 in coming up next.
A signal conditioner is a good device to have so that you can adjust the signals from the transducer to calibrate them. Check out the transducers' websites and study the brochures and tech notes to find really good information.
I've got a couple of the Dataq acquisition devices and they have their places in some experiments, but I find their software to be somewhat clunky and not all that friendly. The software I'm using came with the units so I can't really complain about the price. A data logger is another animal in many respects so be sure of what and how you want to use them.
A simple digital scope would be a necessary instrument for these measurements in my opinion. If you're already familiar with scopes you're well on your way to making useful measurements.
All of the above hardware, and lots more, is available on eBay for very low prices if you are a good shopper.
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Thank you George,
I download the tech info and contacted Meggitt Sensing Systems with some detail questions. I now believe that an Endevco 8510B - 2000 is a good candidate for what I want to do, as you noted, they test them to 3x the rated measurement range, they don't guaranty linearity beyond 2K psi (for that model), but they stated the data is still sensible. The response time looks good for a PCP pressure curve, unfortunately the list price made my jaw drop a bit, equivalent to a very nice PCP air rifle purchase. I've set up some ebay searches to try and mitigate the cost.
Its been multiple decades since I've used a scope and don't have digital scope currently (although they have come down in price significantly), I'm comfortable hacking software and communication protocols (which Dataq publishes) didn't look terrible to work with. This darn PCP rabbit hole is consuming lots of dollars, I guess I'll need to fire up the CNC machines and see if I can make some bucks to afford new test equipment and sensors. ::)
Also, I'm part way through the "Hacking the Crosman Vigilante" thread, my complements to you, you are having the kind of fun I'm interested in. I'm also great at self induced exploration side trips.
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Before I made the impulse buy on the Rigol DSO, I was looking at the DS212 handheld scope. 2 channel, 1 mhz bandwidth and very portable. all for around $100.
Here is a review https://www.youtube.com/watch?v=lxxS6rvrk3o (https://www.youtube.com/watch?v=lxxS6rvrk3o)
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Thanks for the post, Stan. I enjoy your sense of humor!
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A few posts back I mentioned that the CP2 sights were pretty much useless in low light conditions. I suggested that a micro lighting system incorporating an LED and a piece of retroreflective tape might solve the problem. Rob thought that the idea was amusing. His comment was "hahaha, electronic open sights , cant wait". After his comment sank in I got a good laugh out of it myself. What was I thinking?
Well here it is. Electronic Open Sights! I went out to the shop this evening after a few beers and was going to play with the newly acquired B&K 2515 and do some calibration. I inadvertently started sorting through some of the composting piles of project material that had finally arrived from China some weeks ago. In the middle of the heap were the vanishing small LEDs that were ordered. Then I found the strip of retroreflective tape under some pellet tins and the intended course of the evening got derailed.
Being Saturday night by this time I relied on a pint of locally brewed strong ale to see me through this unexpected detour. The photos show what the basic scheme is. I didn't spend a lot of time doing the camera work because the focusing seemed to become erratic. Strangely, the camera's performance always gets much better by the next morning.
The battery was from a collection of worn out ones that were in red dot scopes that I forgot to turn off. The battery didn't last long, but got through this session as a proof of concept test. I think I'll pursue this protect.
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I did a little more tinkering with the approach of using miniature microphones for timing dynamic events. I wanted to attenuate the acoustic response and leave the shock response through contact. I made a thick aluminum cap that fit snugly on the front of one of the mics (I used a cap because for now I wanted it removable, otherwise bonding on a thick disk would serve the same function). The other mic is bare. In this case they are both suspended in air about 3" from impact. The first image shows the response to the clapper impact. The blue trace is the bare mic and responds to the acoustic signal. The capped mic (yellow) is quiet...so far so good.
I had some copper tape so I upgraded the clapper with a digital trigger (image 2). This seems to work well. It has bounce as expected but triggers on the initial impact.
So I mounted the capped mic to the clapper board and left the other mic suspended (image 3) and ran a trace (image 4). The clapper (purple) triggers the sequence, about 80 micro-sec later the mounted, capped mic (yellow) responds through contact, and at around 268 micro-sec from trigger the suspended mic (light blue) responds. The suspended mic is about 3.5" from impact so at 74 micro-sec/inch (speed of sound) there should be a 259 micro-sec delay. Pretty close. This seems to indicate that the basic microphone and amplifier does not generate a large delay. The delay for the mounted mic is harder to predict since it goes through some bonded joints.
I'm encouraged enough to invest the $5 it will take to get a fistful of 6mm mics and modify some of them with the cap/plate. The 6mm size should be more convenient for sticking onto airguns. Also get some amplifier boards ($1-5) to get more channels.
Yes, having fun in the instrumentation rabbit hole
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George, the sights look great. Now I don't have to order those glow in the dark jelly fish tentacles to glue onto the front sight. I guess you have several options for the rear blade.
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There are tritium vials available that are suitable for gun sights. I got a few to replace the night sights of my G22- cost for the trit vials and UV cure adhesive was about $15, new night sights are well over $100.
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Looks like you may be close to the point of no return on making AG measurements, Stan. The microphone approach is a good starter exercise. they're cheap enough to be expendable. When you start upgrading your transducers you'll be glad the you laid the ground work with the cheap stuff.
I noticed that some of your signals are clipped on the peaks. This can really overload the amplifiers and their recovery time in some measurements. It may be worthwhile to check each amp in the string including the front end of the scope. Phase shift is also worth looking at. A tunable filter is also good to have when you're trying to control noise levels.
Here's a link to a nifty little hammer: https://www.harborfreight.com/watchmakers-hammer-with-6-heads-99895.html (https://www.harborfreight.com/watchmakers-hammer-with-6-heads-99895.html) . You can use this for many test setups. The interchangeable heads offer lots of creative opportunities. One would be to attach a microphone to one end and use the copper tape trick that you applied to the snapper on the impact end. With the hammer set up as described you can trigger the scope with the tape and look at rise and delay timing and phase shift of the mike (I'll stick with the old abbreviation). With the interchangeable heads you can make a very useful 'instrumented hammer' (worth Googling). You can't beat the price.
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Thanks George, I'm intrigued with how far one can push very low cost instrumentation if you set up the conditions appropriately. Yes, I'm sure I'll get sucked into the better transducer cycle.
I saw the clipping on the traces, I postponed tweaking that since I was looking for the initial response. The scope has some filter capabilities built in that I also need to explore.
Thanks for reminding me of the little HF hammer. I was planning to make a poor man's smart hammer from some delrin rod I have but the HF approach is more flexible...I don't need much of an excuse to go there.
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I was day dreaming between beers this afternoon and had another one of my induced epiphanies. I was quite impressed with these micro LEDs and thought about how easy it would be to EDM some 0.038" - 0.040" holes through a barrel at intervials. A series of light gates could then be made by putting an LED on each side of the bore to get really good acceleration numbers to overlay onto the P/T curves. Don't calculate what you can measure. The photos show an LED.
Of course the measurements would be best served if I had a slick 4 channel scope like Stan has. I feel as though I'm being inexorably 'channeled' toward the edge of another hardware precipice.
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Wouldn't those 16 channel cheap logic analyzers do the job just fine.
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After giving more thought to using the LED's for pellet acceleration measurements I realized that they can be made using analog methods. I've collected a substantial number of nuclear instrumentation modules (NIM) over the years and the bins that they operate with. It will be easy to use an Ortec 533 Dual Sum and Invert Amplifier and then run the summed LED output to a Canberra 2025 AFT Research Amplifier. The conditioned signal can then be routed to an Ortec 551 Timing Single Channel Analyzer (TSCA). The TSCA output pulses would then go to one channel on the scope. The LED timing pulses could then fall into the same time window as the P/T curve. This arrangement would allow me to put off the new scope decision for a while longer.
This is actually an easy setup to put together. The NIM system is standardized across the industry so the you can mix and match different manufactures. The instruments are like Legos.
I'll do some simple scope tests to make sure that the LEDs will work in pairs and talk to each other. This may be the answer to my transit time/acceleration questions. The data may also be of interest to some of the spread sheet folks out there.....
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Further testing on the LED timing experiment was an enjoyable distraction to confirm that the devices will talk to each other. The polarity of the listener needs to be reversed in this experiment. The talker was driven directly from the sine wave output of the 3310B @ 25 Hz. I didn't measure the voltage, but the 50 ohm output had no trouble lighting things up.
The photos show the scope trace of the listener LED and the lashed up "optical cavity" used for the test. The Al foil was folded and used to shield most of the wiring. This experiment indicates that the barrel light gates should be easy to implement.
I'll probably use one of the Vigilante barrels to set up the first gating system. The CP2 barrels are at a premium right now, so I hesitate to start making holes in them until this measurement is proven to be headed in the right direction.
I'll have to start pedaling backwards now to get back to the barrel harmonics experiments.
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OK, a little bit more tinkering with instrumentation. I set up the 2240 again and added a probe to try to detect hammer release (image 1). The probe is an insulated rod (barbecue skewer) with an electrode (brass plated pin) in the tip (image 2). The probe contact is a little finicky to set (should have left in a bit of compliance) but seems to trigger cleanly and repeatably. I mounted the capped microphone onto the barrel band and suspended the bare mic near (1.5 inches) the muzzle.
Image 3 shows a timeline trace for the 2240 without a pellet. Channel 1 (yellow) is the capped mic, channel 2 (blue) is the bare mic, and channel 3 (purple) is the probe. Looks like the overall timeline from hammer release to muzzle blast is around 13 milliseconds with most of that in hammer travel. I'll have to weigh the hammer and measure the spring to see if a prediction comes anywhere close.
Image 4 is more detail around the time of valve impact. Not sure how to read the activity on channel 1. I'm trying to think of an explicit way of detecting hammer impact. Maybe drill a hole in the side and put in one of George's fireflies.
Fun stuff, though George's nuclear instrumentation modules sound way cooler than a barbecue skewer.
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You're really moving along with the testing, Stan. That new scope sure collects a nice ensemble of inscrutable data. Don't forget the phono pickups. The stylus cantilever is very small and can easily be extended into an orifice.
I like the blood on the business end of the skewer. Nice touch.
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I was getting back to the Pressure/Time measurements on the .22 pistol today. When the pellet loading was tested after reassembly, the force needed to insert a pellet with the probe was excessive. I haven't played with this gun (.22) for a while since most of my time has been spent with the .177. The same problem with hard loading with the .177 was cured by doing the Dremel treatment with the green grind/polish burr.
I had previously done a light Dremel treatment to the .22, but not as extensively as with the .177. I dissembled the .22 again and inspected it. The breech entry had the sharp factory edge broken, but the corner was still pretty much square to the bore. Being late in the afternoon I was able to focus my attention more closely through the usual agent. Now it became apparent that the large force needed to seat the pellet into breech was due to the skirt pressing into the o'ring. This caused the o'ring to then compress onto it's front sealing surface with no place to relieve the extruding pressures.
The barrel was placed back into the lathe for another Dremel treatment with the larger burr. This time the the corner that was previously just broken was now ground and polished to form a chamfer. The chamfer allows the o'ring to extrude into it and dramatically reduce to pressure need to load a pellet. The o'ring is now less traumatized by the loading exertion and recovers itself to still make a good seal against the probe.
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The barrel was placed back into the lathe for another Dremel treatment with the larger burr. This time the the corner that was previously just broken was now ground and polished to form a chamfer. The chamfer allows the o'ring to extrude into it and dramatically reduce to pressure need to load a pellet. The o'ring is now less traumatized by the loading exertion and recovers itself to still make a good seal against the probe.
Would it be possible to see a pic of that? I lightly rubbed my entry for a touch smoother entry the idea it may allow the oring to jam was a background worry as well.
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Here's a shot of the chamfered breech. This is the .177 barrel. The treatment is about the same as the .22 breech got. I have no dimensions because nothing was measured. Just guess work until it looked about right. This process can't be easily reversed, so you're on your own.
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Here's a screen shot from of the first Pressure/Time tests. The red trace is without a pellet. The aqua trace is with a pellet. There's some interesting stuff in here already. Pressure scale is ~100 PSI/vertical division. the time scale is 2.5 mS/horizontal division. Nothing is calibrated yet (except the scope's time base), but these numbers are close to real. The traces were offset a little for clarity.
Next will be to add the force transducer to the muzzle to get the pellet's transit time in the barrel. This is starting to get interesting. I'll probably move the measurements into the electronics area since the footprint of the instruments is comparatively small and I prefer to use the Lecroy scope for these experiments.
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I've still got the instruments from the earlier CP2 measurements in the machine shop area, so a few more experiments were in order before things go back to the electronics room. The force transducer, used back in the Vigilante era, was again employed this evening to see what the transit time would look like with the simple Owon scope. This is starting to be a compelling set of measurements even with these quickie tests. The pellet transit time is ~ 3 mS with some gas left over. Here's where tuning would be helpful.
Things are starting to gel.
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The pressure in the power plant is starting to drop and the pellet's exit time is starting to increase. There are some interesting pressure bumps that occur after the pellet exits that can be examined in more detail with this simple setup. I'll put another pressure transducer at the muzzle to get much more detail from the bumps.
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With the gun setup in the vise it was easy to swap the .177 modified piston cover over to the .22 main tube. The screw can now be used to adjust the hammer spring tension and direct measurements of the effects. I did a couple of quick tests and the results were very interesting. I'll do some more testing this evening and report the findings.
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Some testing was done today to see what tightening the power adjustment spring would do. The photos show the preliminary results. The screw was just touching the spring at 1/2". This would be the factory setting. The second photo is a screen shot of the 1/2" results. The third photo is a screen shot with the 1/2", 1/4", and ~ 3/4" settings (where the spring bottomed out) overlay of the 3 tests.
The pressure is ~500 psi for each shot. The transit times changes from 2.88 mS @ 1/2"(no compression) to 2.76" mS @ 1/4" to 2.72 ms @ ~3/4" (full compression). This is just a first look at things with the limited Owon scope. I'll switch the experiments over to the Lecroy scope and have it do the integral calculus for me while I drink a few beers. The numbers that it generates will be very enlightening to me for sure and others, maybe.
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Now that the P/T curves and transit time markers can be measured I'd like to add an accelerometer closer to the valve stem and another pressure transducer at the muzzle. I could just use the ext. trigger as a third channel using the accelerometer output, but that won't let me see the hammer/valve stem impact waveform.
I'm designing this more extensive experiment in order to force myself to get a 4 channel scope. After viewing Stan's post about the EEVblog video review of the Rigol DS1054Z I checked out the same reviewer's assessment of the GW Instek GDS-1054B 4 channel scope. I found myself enamored with most of what the reviewer said he didn't like about it.
I like a larger front panel. There's more room between knobs and switches. These are assets when you're getting old and feeble. Beer vision is a bonus when you're creating experiments in your head, but in the real world of making measurements a little extra room on the front panel is very helpful when you're fumbling with adjustments. I want a knob for each channel to reduce the amount of thinking required to set things up. Also, I'm not inclined to make the choice of an instrument based upon what color it is. Maybe a camo front panel would be more universally accepted nowadays.
At this entry level of scope hardware it mostly boils down to choosing a Ford or a Chevy. My personal choice is the GDS-1054B.
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George,
Not sure I understood the 3/4" setting in your earlier post. I thought you started at 1/2" and moved in.
I'm designing this more extensive experiment in order to force myself to get a 4 channel scope.
I understand that process well
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Here's a shot of the chamfered breech. This is the .177 barrel. The treatment is about the same as the .22 breech got. I have no dimensions because nothing was measured. Just guess work until it looked about right. This process can't be easily reversed, so you're on your own.
this looks incredible George.
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Thanks for catching that, Stan. I meant to say that the screw was turned all the way in toward the zero point, which would have been the full 1/2" compression. It didn't quit make it because the spring bottomed out first. I hope that makes more sense.
Rob, the chamfering process was simple to achieve. I used the Dremel in the tool post on the lathe. The Dremel holder is available on eBay in several forms. I use the TEMO green rubber burrs from amazon. I like them because they're flexible and basically eliminate any chatter that might occur from the Dremel's bearings. They get used up pretty fast, so you need to keep an eye on them when your grinding/polishing (using your safety squint, of course).
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Now that I've pulled the trigger(?) on the 4 channel scope purchase I have time to get back to barrel harmonics. I've decided to use the thread protector on the muzzle as a fixture to mount a pair of micro accelerometers at 90º to each other. This will allow the simultaneous measurement of multiple degrees of freedom in the muzzle displacement. These measurements could resolve a lot of speculation in this arena.
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I'm waiting for the new o'scope to arrive so I thought that it would be a good time to get reunited with the barrel harmonics project. The thread protectors that comes with all of the assorted barrels is a good place to mount the micro accelerometers. 2 flats were milled into the device and the transducers were glued onto the flats at 90º to each other. An o'ring was installed between the barrel shoulder at the end of the threads. This way the thread protector can stay under some compression and still allow adjustment of the right angle of the transducers to be rotated around the muzzle.
The overall weight of the transducers and thread protector comes to less than 6 grams. This added mass should not interfere with the resonance measurements.
The photos show the basic arrangement when things were fit tested on the 8" .22 pistol barrel.
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very nice,assuming the thread protectors are a solid fit on the barrel threads ( some are and some are not , I don't have this model
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Rob - The threads on the barrel and the thread protector are machined well enough to work for this preliminary set of tests. I use a small amount thick vacuum grease on the threads to fill any dead spaces between mating surfaces. The time constant for the stiff grease to extrude itself out of place is longer than the impulse signals that are going to be measured.
The o'ring allows for some useful compression to keep things snug during the measurement. It also allows the thread protector to be rotated around the muzzle while still keeping the transducers orthogonal to each other's axial translation. There are more considerations to how this test was designed, but I think things are off to a good start for now.
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There was some discussion a while back about what might be appropriate transducers for testing airgun pressure/time curves. My choices for doing measurements at the breech end of the barrel has been the Endevco 8510B sensors and the Kistler 601B/C sensors. They're both small devices and add little dead space to the measurement. The 8510B devices are my current choice as the best of equals.
The other sensor mentioned was an Endevco 8511A. It's a somewhat larger device with what would have, by default, more of a dead space issue if used in the breech of a typical airgun. As an extended thought about this type of measurement I consider the the 8511A to be an good choice where the volumes at the measurement point would make the 8511A's dead space to be negligible such as at the muzzle end of a barrel.
Other things to consider are how the transducer is mounted and how the signals are conditioned for display and analysis. More on this subject if anyone is interested, though it's not a requirement to follow this thread.
The photo shows the 8510B and the 8511A for comparison.
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So while we are waiting for George to get his new quadraphonic rig, I tried the broken wire approach for detecting the pellet leaving the muzzle. The first picture shows the setup. I still used the hammer probe but added a couple of resistors in a voltage divider to place the broken wire on the same channel as the probe. The two microphones are still attached as they were before, one suspended from the foam and the other hard mounted to the barrel band.
The second picture shows the broken wire detail. I used some 0.1mm magnet wire I had. The two alligator clips are insulated from their support. The wire is less than 1/8" in front of the opening and the actual muzzle is recessed about 0.17" from the opening.
The third image is the trace of the 2240 shooting a destroyer (14.3gn) pellet. The purple line (ch-3) shows the hammer release (the scope trigger) and then 15.2 milliseconds later the wire being broken by the pellet. Need to think about what the two microphones are doing in the roughly 2.4 millisecond timeframe before the pellet exits the muzzle.
I also ran a trace (image 4) without a pellet to make sure the wire didn't brake from the muzzle blast. I had originally tried Al foil, edge on but that didn't survive. The time delay between the two microphones is a little shorter without the pellet.
Fun stuff. I need to calculate the flight time of the 2240 hammer. I don't have the 2240 hammer weight in my notes, if someone has that handy, otherwise I'll take it out and weigh it. Still thinking about how to explicitly detect hammer strike and pellet transition time. For now, detecting pellet exit helps a bunch.
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Stan - I'm glad to see that you're still at it. Looks like it's you and me against the tide of spread sheets! I took a look at a 2240 cross section and it doesn't seem to be much different than the DP2 hammer/valve stem arrangement.
If you're not adverse to drilling a hole through the side of the gun at the hammer/valve contact point you could set the trigger point there. What I'm thinking is that a small Al foil switch could be slipped into the hole and placed against the face of the valve. The 2 slivers of foil can be isolated fro each other by a thin film of grease. When the hammer strikes the foil the grease will move and allow signal contact. The surface offset by the foil will be hammered flat and should have no deleterious effect on the valve/hammer interface alignment.
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I'm not adverse to drilling holes in the tube but there are a couple of other things I want to try.
In the meantime USPS delivered my huge shipment of microphones....amazing what you can get for under $6 to your door...now to make some caps for a few of them.
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One of the things that I try to get right is the viability of the experiments and data that is provided in this (these) thread(s). Sometime I get caught up in the exercises of calibrating the instruments that are used here. Today's question is about how accurate are the pressure measurements that are being made. I'm a first principles type of guy and generally fall back on the known values of specific physical quantities. I tend to use the known values of quantities that have been arrived at by many tests and observations over long periods of time.
Here's a solution to getting accurate, though not absolute, values for pressure up into the range of 1,000 PSI. The measurement relies upon the characteristics of gasses that can be forced into a state where the they exist in both the liquid and gaseous phases at the same time. My gas of choice for these measurement is CO2 since that's what I'm measuring at the moment. Without getting too far into this I'll just show a picture of the calibration device that I'm assembling to test the pressure transducers used in these CP2 experiments.
There are a couple of more fittings that I'll have to make to complete the calibrator, put the photo shows where the starting point is.
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George, that looks great. Interested in the parts list when you are done. Having a compact general purpose high pressure source may be useful.
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Here's the main component for the pressure calibrator. It's a quick change CO2 adapter from eBay. There are plenty to choose from.
The other important part is a valve so that the gas can be turned on and off. I've chosen the small Hoke valve because it was in my collection and is rated at 3000 PSI. The rest of the parts are just couplings to make everything go together. Keep in mind the pressure requirements for everything.
If you're going to use it as a calibrator you'll need a way to adjust and control the temperature. To work properly as a calibrator the temperature must be kept below the critical point.
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Thanks, looks like the version with the paintball threads mates with the available fill adapter/valve/gauge that gets you to 1/8 NPT
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as an aside, for those wanting to make something similar, most of the metal ball valves we see are rated between 600 and 1000 psi ( common home depot metal ball valves ) and when I searched ebay , I saw many rated even higher..
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I would advise a safety margin of at least 3 times the highest pressure you plan to try and contain when building something like this. Here's where one of our resident spread sheeter's comments would be useful for all of us.
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I would advise a safety margin of at least 3 times the highest pressure you plan to try and contain when building something like this. Here's where one of our resident spread sheeter's comments would be useful for all of us.
very good point , safety margins still Apply..
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Thanks for pointing out the valve adapter, Stan. I had one of the intermediate adapter couplings already, but didn't think of the valve. This will save a lot of fooling around to make the calibrator. The 1/8" port on the valve is ideal for my application.
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The barrel harmonics testing is coming along nicely. I decided that it would be good to confirm if and where any nodal points might be. A small super magnet was glued to the bottom of another 2250a accelerometer that can now be placed anywhere along the barrel and around the circumference as the photo shows. The added mass produces a negligible effect on the natural frequency of the barrel. That new 4 channel o'scope will sure be useful for these and other upcoming measurements! Looking forward to it's arrival.
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I've been out in the shop this afternoon amusing myself with the calibration of some of the very cool (to me) vibration instruments I've acquired over the last couple of months. Now there's even more hardware on the way. After a few beers it occurred to me that I wouldn't be enjoying this time doing these experiments if there weren't folks that seem to enjoy this themselves even if it's just vicariously. There are few who actually post, but the view counter indicates that this thread isn't dead in the water at all.
Therefore, I've listened to the beer telling me that I should express my gratitude to all of you that continue to follow this wayward journey into the ostensibly bottomless chasm of airgun ballistics. Some, like Stan, have fallen victim to the insidious contagion of experimentalism and others suggest that the fever may already be on their brows.
As I've mentioned in the Vigilante thread, I'm not a good conversationalist in a forum venue. I don't do social networking, so GTA and eBay are pretty much the internet to me right now. The winds do blow me astray from time to time, but like the Prodigal Son, I always come back home.
I've had many kind and generous comments about these posts over the last year or so. One of the most endearing was from Terry (Dv8eod). I wish that I had the good sense to respond to these fine gestures in a timely fashion. I was raised better. In my defense, though it's no excuse, I'll say that it's the effect of becoming an old man who now communicates many of my ideas sitting if front of a monitor using a key board to try and express myself to others. It's become so impersonal that it's easy to forget that there are real people on the other side.
I thank you all and apologize for my lack of civility, but I probably won't change much.
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Can't speak for the rest of the lurkers, but my ability to contribute to your posts in a constructive way tends to be limited - but am definitely along for the ride, looking to learn from the experiments others do.
They say there are three kinds of learning.
1. Learn from reading.
2. Learn from watching others.
3. Take a leak on the electric fence yourself.
I tend to the first two methods... ;D
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George,
I'm enjoying your multi-stop journey delving into airgun characteristics and the bottomless toolbox of instrumentation that you've brought to the task. So far, I think the glass lathe is my favorite.
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The new o'scope arrived today and it's a beauty! They've changed the front panel layout and color. I like it even more. The new firmware version is stated to be V1.19. I was mainly interested in getting 4 channels to make simultaneous measurements and a good XY mode for Lissajous patterns. Using just test signals to take the instrument on a shakedown cruise proved to be quite rewarding. I'll post some photos as some as I'm sure of what I'm seeing.
These next measurements on real airgun stuff are going to be very interesting and easily worth the investment in new hardware.
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awesome, pics when ya can
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Having 2 accelerometers on the end of a barrel has given me some head scratching moments recently. Being able to measure the barrel vibrating in both the horizontal and vertical directions simultaneously has produced a lot of unexpected information. Since the accelerometers are very sensitive in only one axis and have negligible response to anything transverse to that axis, I was surprised to see a large signal coming from both transducers when the gun was fired. Using the tapping hammer I was able to excite either the vertical or horizontal modes individually. Tapping the barrel at a 45º angle between the sensors again produced a pair of ~ equal amplitude signals. The surprise was the consistency of the signals. One of them was at 104 Hz and the other was at 167 Hz on the same piece of 8" barrel mounted in the same hole in the breech!
I double checked the mounting screws and they were tight. Using the third transducer, which was mounted on the magnet, I could move it around and confirm that the only node was at the breech end of the barrel where it was clamped. Having become nonplussed, I fell back to the safe haven of a few beers.
After regaining my composure, I ventured back onto the battle field. It turns out that the end of the breech where the barrel mounts has 2 modes of flexibility. The vertical mode makes the breech act as a flexure point where the pellets are loaded. The horizontal mode has more stiffness because the whole breech is secured better and is also tightly straddling the main tube. This accounts for the two well defined frequencies.
The photo shows where the problem lies.
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George,
For the ~8 inch barrel, the first mode, cantilevered should be about 230 hz
http://www.amesweb.info/Vibration/Cantilever-Beam-Natural-Frequency-Calculator.aspx (http://www.amesweb.info/Vibration/Cantilever-Beam-Natural-Frequency-Calculator.aspx)
http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowCircle.aspx (http://www.amesweb.info/SectionalPropertiesTabs/SectionalPropertiesHollowCircle.aspx)
I know, I know....spreadsheets....but sometimes they are useful
So a couple of additional areas of compliance might be: 1) the two orings at the barrel/breech interface may not be compressed by the two set screws so the barrel is not cantilevered and 2) I don't remember how the block is attached to the tube. If it is effectively cantilevered itself through the section under the loading shelf, from an attach point closer to the bolt, then that will drop the frequency as well. Both of these would be different in the two axes.
To check out your instrumentation, a simply supported configuration may be cleaner, though you would need to move your accel, and the frequency will be above 600 hz
http://www.amesweb.info/Vibration/Simply-Supported-Beam-Natural-Frequency-Calculator.aspx (http://www.amesweb.info/Vibration/Simply-Supported-Beam-Natural-Frequency-Calculator.aspx)
Or you could clamp the barrel directly
....Still waiting for that first screenshot with 4 traces on it
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I went back and tested the barrel again. The measurements show high Q peaks at 104 Hz and 164 Hz. The measurement bandwidth is 4 Hz to 1 KHZ. The vertical display is 60 dB. An interesting dip on the right side of the 104Hz peak is a partial phase cancellation from the 164 Hz peak. The very low Q humps around 800 Hz are at least 20 dB down and of no consequence in these tests. The CRT screen is not as easy as an LED screen to get a good photo from without some effort with camera settings, but this should do for a quick test.
The 2515 analyzer comes up with pretty much the same numbers as the o'scope. Someone should now check their spread sheets for sanity. Thanks for the nudge, Stan.
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I can go back to the Complex Modulus Apparatus (CMA) to test just the barrel as was done on the Vigilante 10" barrel in the other thread. That will give complete isolation for the natural frequency test. That exercise was enjoyable in itself.
The thing to keep in context here is that I'm trying to do real world measurements on the gun. The tests presented so far are those of the barrel as it is used when firing the gun. The rest would be academic, but still interesting.
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Someone should now check their spread sheets for sanity.
Yes, that is a great point. The web is full of calculators and databases of all sorts. In this example I found those after just a quick google search. In reality, I have no idea who owns that site and how thorough they are in proofing their tools. This is the general risk of using other peoples spreadsheets, calculators, material data etc. In this case I checked their cantilever results against other handbook derived results and they were consistent. I also like that they show the equations used and reference the handbook source. For this application the frequency results are not, in general a safety issue and the material properties for stiffness are pretty consistent for steel. On the same site they have other calculators for stresses in pressure vessels as an example, as well as strength properties for materials. These need to be approached with caution. Equation errors, misunderstanding idealizations of the handbook approach, scope of what the factors of safety cover, and uncertainty in process driven material properties can all lead to an unsafe design decision.
The thing to keep in context here is that I'm trying to do real world measurements on the gun. The tests presented so far are those of the barrel as it is used when firing the gun. The rest would be academic, but still interesting.
Agreed, I always try to have a handbook or back of the envelope evaluation for any configuration I'm looking at. It tends to provide a good bounding value for either instrumentation or detail analysis results. In this particular case the calculator provides 1-D results for a uniform tube, fixed to an infinitely stiff mount. In predicting your test configuration, only the first mode prediction is useful, you will have modes in other directions that will show up after the first mode. You also have minor lumped mass at the tip and the mount is not infinitely rigid. All of these will drive the frequency down. So given all that, if your results showed a first mode above the ~230hz, I would be scratching my head. The fact that they are significantly lower, starts the search for the extra compliance. This can be useful for the actual barrel study. For example if the compliance is in the barrel setscrew/oring mount, then it may respond to torque values, additional shimming, or material choice for the orings. If the compliance is due to cantilever/softness in the mounting block then additional shimming or clamping may influence the results.
None of this probably matters for the accuracy with the 8" barrel in pistol form but may be more significant for the 18" barrel.
Finally, I find comparing analytical predictions with tests result to be lots of fun (at least if the test results didn't include one's safety)
Carry on...great stuff
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The rest of the parts for the pressure calibrator arrived today. Everything went together without a hitch or leak and I'm very pleased with the outcome so far. The only downside is the painful reliance on... you guessed it... online calculators and spreadsheets! To arrive at the actual pressures above CO2 in the liquid/gas phases is like throwing darts at a board. Well, I'm OK with that at this point because the differences between various sources isn't enough to be a problem with the measurements right now. The real solution is a dead weight tester. The one that I have now doesn't go high enough to for these pressures so I'll just keep an eye on eBay.
These small Endevco transducers are really nice and the older Endevco 4428A conditioner/readouts that I've been collecting are very good and fairly plentiful on eBay. Their analog output makes them compatible with the way I do most of my measurements. There are some perks to being from an older technology era.
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looking great 8)
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I really like that co2 adapter system...can't think right now what I would use it for but it is on the list
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Continued to poke at how far one can take hobby level instrumentation. I dug out a 2 axis accel dev board I've had forever. It uses a now obsolete ADXL202 mems type sensor. Similar boards currently use the ADXL335. They are cheap, light, don't require conditioning electronics and provide analog voltage output. I first set up a 2240 barrel in a suspended free-free (first picture) to see if the output was useful and played nice with the spreadsheet prediction. It did pretty well, the predict was in the range of 1.53 to 1.55 Khz and the results came in at 1.47 Khz. Also note the George recommended tap hammer was acquired and used...worked great. The board mounting to the barrel worked well but may need to be changed for any shot response tests.
I also looked at the cantilever configuration, primarily to see what drives the results away from prediction. The second picture shows the same barrel clamped into a v-block, in what appeared to be a rigid mount. With 6" cantilevered, the predict for a bare barrel is around 380 hz, with the 4gm sensor at the end, it should drop to around 350 hz. The tests showed 313 hz in the horizontal and 265 hz in the vertical. I'm guessing some of that compliance is in the line contacts in the v-block, and some is in the bench top. I was surprised at the end mass effect, as well as the what I thought was a stiff mount. But mostly it was to shake-out the board so to speak.
I also tried the miniature microphones for these tests. They did well in the free-free test with similar results. The cantilever test was much more difficult with ambiguity on the directionality of the response.
I think the microphones can work well at timing sharp events, their low cost and tiny size make them worthwhile.
For frequency, the hobby accelerometer boards can provide useful data.
For anything requiring higher fidelity or amplitude accuracy, the instrument grade accels that George is using are the way to go.
Fun stuff
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Stan - Nice experiment. It's pleasing to see someone else getting dual simultaneous modes out of these simple tests. It must all lead somewhere.
I did a simple test this morning to evaluate the difference between the the way we might be using our hammers. I converted mine into an "instrumented" hammer for this test. The demonstration is to show how the hammer's impact tip material will effect the force/time impulse. I used a rubber cap like the one's use to protect things like threads on fittings and such. This one has a nub on the end that makes it ideal for getting good taps.
The red trace is the impulse waveform with just the bare steel hammer tip and the aqua one is a tap with the hammer's tip covered with the rubber cap. As the photo shows, the impulses have very different time constants. The rubber cap extends the impulse time which allows much better low frequency excitation. By changing the tip material you can make simple mechanical filters for this type of testing. You might want to give this a try.
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Interesting, I'll have to try out some different tips. I just tried to get a clean tap without a double tap.
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OK, another little packet made it over the pacific ocean. The thought was to replace the broken wire timing approach with some form of a light gate. I found a laser receiver module (first image) that met my criteria of cheap and easy to use. I added a laser source ( a dollar store type pointer in module form) to set up a light gate across the muzzle (second image). The response time of the receiver is pretty good (third image), less than 50 nano-sec.
I tried a few locations but ended up about .25" in front of the muzzle. The fourth image shows the response of the Destroyer pellet fired from the 2240. One interesting note is the Destroyer pellet is .228" inches long, the little point in the center makes it .257". So if you take the 46.4 micro-sec transit time it works out to 410 and 462 ft/sec respectively. I'm not suggesting this is chrony quality data but it is nice to be in the ballpark.
Right now it looks like a good low cost addition to the toolkit. I'll have to run it with the broken wire to confirm it is doing what I think it is.
A couple of side notes. I had it just grazing the muzzle and had some additional transients on the trace. Turns out with the 2240 held in a lightweight panavise clone, I was catching transients of the barrel recoil. Interesting, and may be useful later to time recoil to pellet exit. Also CO2 generated clouds showed up once or twice (I have the power adjuster turned in a bit). This could be more interesting info if timed to pellet exit.
Next is to fold this in with the rest of the timing instrumentation and then read the tea leaves
More fun stuff.
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OK, Stan, now that you've thrown down the cheapskate gauntlet I have no choice but to follow suite. These piezoelectric transducers came in the mail today all the way from China, too. That's how far I'm willing to go to meet your challenge! These are the 10 PAK of 35mm disks for $1.78, free shipping. There is a 20 PAK of 12 mm disks on the way that cost $1.49 , free shipping.
To keep the playing field level I used the flying wire and Scotch tape attachment method. Also included was the odd piece of cat hair into the mix as a flourish. Kitchen scissors were used in an indiscriminate way to make a smaller sensor. These disks can sure "take a licking and keep on ticking" (plug for Timex).
The signal from the sensor was connected to the o'scope with a 10X probe with no conditioning. That shows a pretty good output for a small tortured device. The sensor and tape weigh in at less than 1 gram. This could easily be cut in half again, obviously. The o'scope shows the same fundamental frequency (in one axis) as the other accelerometers.
I should have chosen a higher contrast background for the before and after disk photo, but I think that it still works. I'll blame shift the oversight to the beer.
PS - I just saw your new post as I was posting this one. Nice work. I am surprised at how clean the break points are in your o'scope images.
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I don't want all this talk of low cost to imply that I skimped on the optical chopper used to evaluate the detector response......
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George and Stan: Both of you "enablers" are making my piggy bank nervous! Every time you have interesting data, you are showing off your wonderful multi channel DSOs. A DSO was not in my budget this year.
I did appreciate the helpful bargain pointer, however I'm not going to buy a tiny & cheap DSO either, my eyes are not happy with tiny displays, and I get frustrated with fiddly little controls.
Back to sensors:
I did find an Endevco 8511a-5k at a low cost and purchased it. Still looking for a 8510B-2000 at an affordable cost (list price is around $1,100).
At a cost between the broken wire and the Laser Pointer+Laser Detector combination are Slot Type Optical Photointerrupters with a large gap. The sensor slot is much smaller than a .177 pellet. A pair of these could be the basis for an end of barrel chronograph.
Example 1:
Digikey P/N 160-1936-ND $1.11 15MM gap (Horizontal) and 1OMM clearance (Vertical), Photo transistor output
(https://media.digikey.com/Photos/Lite%20On%20Photos/LTH-301-32_sml.jpg)
Digikey P/N 160-1936-ND $1.11 15MM gap (Horizontal) and 1OMM clearance (Vertical), Photo transistor output
Example 2:
Digikey P/N 365-1768-ND, 9.5MM gap (Horizontal) and 1O.8MM clearance (Vertical)
TTL Output (buffered totom-pole) [p/n OPB910W55Z $5.35]
Open Collector output version available [p/n OPB901W55Z $5.33]
(https://media.digikey.com/Photos/TT%20Electronics-Optek%20Photos/OPB9-W_sml.jpg)
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Cindi,
Yes, I'm looking at the photo interrupters as well. Started with the laser detector just because it is more general purpose....and it uses a laser!
If you can wait for the transpacific they are less than a cup of coffee (I paid $3.28 for two and free shipping). The lasers that run straight off of 5 V are $1.30.
I have not found a low cost, fast pressure transducer in this pressure range.
As far as the DSO is concerned, I blame George. My old analog scope will be looking for a new home. I looked at the PC based scopes but decided I wanted a standalone.
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OK, Stan, you're not going to out cheap me on this one. Washer - found it in a box. Sensor - 18 cents. Post-it notes - poached from my wife's desk.
This device is going to be a muzzle exit detector. It's ostensibly immune to acoustical excitation and the trigger signal for the DSO can be set to ignore the pressure pulse. The pre-trigger signal will still catch the pressure pulse for inspection (maybe).
The sticky part of the pads can be trimmed off of the note part and be used as the expendable part of the transducer. I tried a Scotch tape pull test on the sensor's coating and nothing came off with the tape. The sticky part of the note pad is much less aggressive and should allow many tests if the sensor itself isn't hit by a bullet. A pair of these would make a cheapie chronoscope.
I'll test this out this afternoon and report back.
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Well if you can work without a laser.........
I like your design. It will be interesting how the post-it responds. When I was setting up the broken wire I tried Al foil on edge. That blew off without a pellet. Maybe if you ran a case with a piece of thin wire taped across (or a cat whisker).
Tuning the position of the washer relative to the muzzle may create a useful pressure wave sensor that can be combined with a second pellet detector, and some accels to give a fuller picture of the timing at the muzzle.
A while back you brought up Schlieren photography of the muzzle. I found this video. There is an interesting puff that takes place long before the pellet gets there.
https://www.youtube.com/watch?v=HZD-IgMvt54 (https://www.youtube.com/watch?v=HZD-IgMvt54)
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Stan - That's the best pellet gun video I've ever seen. That initial shock pulse in the video is the one that I was getting when the Vigilante's muzzle blast was done with the microphones. That was back when we were discussing N waves. The string of pressure pulses that were recorded back then without a pellet being fired are the same thing, but with the air plug out in front of the CO2 plug. The strings that were shown back then, including the heavy duster gas injection was also captured in the string of pulses, though nothing like this.
The microphone's o'scope images are like a Hilbert transform slice through this video. It would be very interesting to know the details of the video's timing sequence. The tail-end image may be hammer bounce. I'm actually astounded by the density of information you've just presented.
After watching your video ten times I feel totally defeated and deflated in all of this test and measurement stuff. Beer won't do it tonight. Where's my gin bottle?
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Yeah, I could make that my screensaver.
If you watch it on youtube you can bump up the resolution/size a little bit and play it at .25 speed. A little bit more detail pops out. They don't provide much background other than it was shot at a frame rate of 15K. With a guess on pellet size, might be able to derive a speed.
I would not give up hope. I think your piezo-donuts might work out well as pressure wave sensors. If you add the chamber pressure, an explicit pellet detection, and a microphone. Substitute in a couple of accel readings after a few shots and you got a pretty thorough data set.
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OK, found one more. 2700 frames per sec...not sure why it says no pellet. looks like one at the 1 sec mark
https://www.youtube.com/watch?v=_yC_-CWvMDI (https://www.youtube.com/watch?v=_yC_-CWvMDI)
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Cindi,
I found I had a generic photo interrupter. It is too small to send a pellet through but I triggered it with a dremel driven chopper. The generic one had a relatively slow response time (~150 micro-sec). I think the ones you are looking at from Digikey...with a spec sheet...are the way to go. It will be interesting to see if they respond to muzzle blast without a pellet.
I'm with you on the tiny screen & fiddly controls...to take that to an absurd level there is a 4 channel version of that tiny DSO
https://www.amazon.com/SainSmart-Handheld-Pocket-Sized-Oscilloscope-Bandwidth/dp/B0788HLXKP (https://www.amazon.com/SainSmart-Handheld-Pocket-Sized-Oscilloscope-Bandwidth/dp/B0788HLXKP)
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Last evening was spent consuming therapeutic doses of tincture of juniper berries and other botanical restoratives. Today I was able to climb back to the helm and continue this voyage into the unknown. I thought it best to start in slowly and get my sea legs again by getting back to hammer/valve timing.
The previous effort on this task was to use an accelerometer attached to the breech above where the action was happening. This choice was expedient, but the transducer picked up a lot more activity then was needed. In an attempt to get a cleaner signal and better timing info, the choice of sensor and placement was moved.
After the success with making the "donut" sensor for muzzle measurements I decided to change the scaling and make one that would fit around the valve stem and get struck directly by the hammer. I'll need to arrange for some type of shock absorber to spare the disk from impact destruction. I'll mill a small grove into the face of the valve for an electrode lead and bring the wire out through a hole in the side if the main tube. This approach should also give good information about hammer bounce.
The photo shows where things are at this point.
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Stan - In your free-free test you used rubber bands as a compliant overhead suspension to support the barrel. I've seen several YouTube videos about doing modal testing in a free-free arrangement where they use a soft egg crate foam supporting the DUT. Since you already have the earlier data, what do you think about rerunning the test with the egg crate foam method as a comparison?
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yes, I have some soft foam, I'll give it a try.
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George, Here is the 2240 barrel on foam. It worked well, though you can only tap it in one direction, not an issue here with enough signal in both channels.
The predicts were 1.54 khz for the first mode and 4.24 khz for the second. The measured results were 1.57 khz and you can see the second mode out around 4.2 Khz.
Note: I really need to learn how to use the built in fft math function (as well as the others). I am using the trace fft which is near real time but has limited number of points. You can also operate on a stored waveform with much higher number of points. So I'm not sure what the uncertainty is in the numbers above. Of course there is a small uncertainty in the modulus number used for the predict as well. Overall I think it is a decent match and the free-free numbers don't apply directly to a real world configuration, but provide a sanity check.
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Stan - Thanks for doing the followup test using the foam. It confirms that the method is a viable shortcut for doing some of these experiments.
I've been playing with my new DSO and grinding through the almost 200 pages of manual. Entry level instrumentation is a lot different than it used to be. I'm slowly getting it to do my bidding, but getting data into a postable form for this thread has been a challenge. I think I've got it working, but I can't seem to get a screen shot without having the menus captured too.
Being a computer dummy doesn't help, but I like this scope too much to be overly disappointed. I'll just keep after it and see where it ends up. I'm getting ready to do some multi-channel tests in the shallow end of the pool with it. I should have something that isn't just a camera screenshot sometime this week if I stay busy.
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Yeah, how to turn off a display item was on page 147 in mine. The soft menus don't turn off. There is a convenient print button that grabs a screenshot to USB but any cropping is done in the PC.
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OK, down another level in this rabbit hole.
I was thinking of ways to monitor what the hammer is doing. The hope is to tie it in with some of the other measurements and start to fill in the timeline. George is looking into the piezo-bumper on the valve face approach, so I thought I would see if the skewer probe concept can be stretched any further. I changed the probe from being a continuity type signal to breaking an optical beam. I added a flag to the exposed end of the probe. The width of the middle position (Step 2) approximates the static stroke of the hammer. The other two are a little bit on each side of that length. The laser/detector combo is used as the detector. The first image shows the parts involved overlayed on the 2240 (thank you to whoever made that great section view). When the hammer is cocked, the flag moves to the left and the laser dot is just at the right edge of the flag. The probe tip inside is trapped between the hammer and the spring so it follows the hammer. When fired the flag is pulled to the right and the laser is blocked until it reaches the left side of the flag. In the Step-2 position this roughly corresponds to contact with the valve stem. Any time spent with the laser exposed (signal at 5V) the valve is compressed (approximately).
Image two shows the test configuration. The 2240 is held in a cross vise that allows me to start each run with the laser beam just on the verge of being broken. The laser/detector are on a scissor-jack platform that allow me to move it up and down to use the different steps on the flag. The laser beam and detector have a finite width that effectively shortens the flag width. I estimate it at 0.05" but I still need to measure it. (it amazes me that these <$1 laser modules can be focused). These initial shots were dry fired without CO2. I want to add the microphones and see if the hammer motion is consistent with anything in the mic traces. Without CO2 pressure I'm guessing there is a lot more bouncing than in real shots. For anyone looking at the results, only the initial hammer flight would relate to a CO2 powered shot. Again, this is only to try out the test method the actual dry-fire data is not relevant real world performance.
Image 3 shows the trace with the middle (step 2) position being used. I listed my guesses of what is happening and the time line for each. Note from the earlier tests with the skewer probe, the pellet is gone by 15 ms or so. Again once the trace starts, each time it reads 5V the laser is unblocked on the left side of the flag.
In image 4 I overlayed two separate shots (using step 2) they are surprisingly repeatable.
In image 5 I used the full width of the flag. As expected the first leg is longer since it now includes some of the valve compression. The number of bounces is less since the smaller bounces stay obscured.
In image 6 I used the shortest (step 1) part of the flag. Again, the number of bounces shown is less because the smaller bounces don't compress the hammer spring enough for the beam to be obscured. Note that the first shot didn't achieve the expected shorter first leg (the repeat did though) I think this is Murphy taking his crack at the repeatability claim.
It will be interesting to see if this probe approach helps clarify what happens and when between hammer release and pellet exit. Still trying to figure out how to fully check out this measurement and whether it is useful. Comments are always welcome.......wife still shakes her head when she walks by.
Fun stuff
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Stan - Your experiment looks interesting. Except for a small error in step 9 of your guess list, you seem to be on the right track with this method. The ~ 18% trigger offset might be do to the compressed time scale view or else something is not resetting to the same place in the hardware.
It's nice to see a different approach to these measurements. I look forward to your progress.
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George, yes I think step 9 is the hammer coming to rest on the valve stem....I have no idea if the valve opens. It is also ambiguity in the exact size of the blocking flag. I expect the small bounces to go away once the system is under pressure
Note the plots labeled as overlays are just graphical overlays meant to qualitatively compare the repeatabilty. They were overlayed in a graphics program. (yes I need to learn how to store and retrieve waveforms). The 18% difference in the last (step 1) results were not the trigger points but the measured duration of the first hammer motions.
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Thanks for clearing up the overlay anomaly. My comment on the step 9 point was really about a simple arithmetic error. It now seems petty to even mention it, but that's what peer review is supposed to be about. Numbers are different than things likes spelling errors and grammatical mistakes, unless the latter causes confusion. I'll attempt to defocus my perusal of postings and pay attention to more egregious things.
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Thanks for catching that. I agree number typos should be addressed
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Some of my most enjoyable experiences in Research and Engineering was the interaction with my peers even if it just about attention to minor details, it made us all more mindful of the complexities & subtleties of our work, peer review helped my team achieve the best results.
The two of you are courteous and show each other respect, I have been enjoying this thread (it's the first one I look at in the morning) and hope to have some experiments of my own going in the future.
Best of the Day to both of you
-Cindi
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Another incremental step, I added the microphone in contact with the 2240 tube in the valve area (image 1). Contact is maintained with double sided tape. The rubber band just keeps it in compression and avoids having it knocked off. I also made a new flag for the probe that more closely spans the two trip points for the two static positions of the hammer.
Image 2 shows the trace for a no CO2 dry fire. The middle purple 5v line is the contact with the valve. Looks like both the probe and the mic compare well on the timing of that event. Image 3 just zooms in. the difference between probe trace step and the mic event is about 80 micro-sec. That's probably mostly in the probe trigger position uncertainty. System is too sloppy to improve on that without doing some extra work. It will be interesting to see if the mic results are as distinct once there is CO2 in the system.
Yes Cindi, I've always enjoyed peer reviews and robust technical discussions. Though they seem to be a bit of a fading art.
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George,
Do you think the valve in the pressure calibrator you assembled is a good enough long term seal that it could be a useful source of compressed gas for blowing out dust, etc. when away from a compressor?
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I'm not sure about the long term sealing on the calibrator. The best test for long term would be to weigh it. My needs are for intermittent calibration, so small leaks over time are not a consideration. I'm willing to spend 50 cents on a powerlet for it's occasional use. I don't think you'll do better that a 2 or 3 pack of canned duster gas at Fry's or some other outlet.
I don't use my shop compressor for general dusting because it always seems to make a bigger mess than what I started with. Then I have to breath the stuff. I use the can duster exclusively around the machine tools. high pressure compressed air blows swarf into places it should never go. I use chip brushes to clean up the bigger piles.
Considering how seldom I actually cleanup any of my messes I can get a lot of mileage from a can of duster gas. Hope this helps.
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you are right, duster gas is the way to go......I'm still trying to find a use for that adapter set
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I shall consider this to be my piece-de-resistance in the trigger timing challenge. The only thing cheaper and easier then this would be a thought experiment. The parts were free. The laser was added only to comfort Stan. (It doesn't really work anymore.)
The materials list is:
1 small piece of 5mil brass shim stock (could also be a piece of kitchen foil)
2 Pieces of electrical tape (could be Scotch tape, etc)
1 multimeter (could be free at Harbor Freight)
The photos show how it want together. The voltage from the ohms setting on the multimeter is connected to the gun frame and the brass strip. The scope probe is connected to the the meter probes. When the hammer pinches the brass between itself and the valve body the meter circuit closes and the voltage pulse is generated.
The DSO traces tell the whole story. The switch closes in 2 µS and any real hammer bounce is recorded as a 2nd (or N) full voltage pulse(s). The valve stays open for ~1.5 mS. This test was done without a powerlet installed.
I'm now getting very close to putting the new DSO out on the playing field.
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The only thing cheaper and easier then this would be a thought experiment.
Not true a successful thought experiment usually includes a pint in the consumables list.
Nice result. What keeps the shim off of the valve after it closes? Just the spring of the shim? Do you have the hammer spring backed out so it does not preload the hammer after a shot?
Yes the laser adds tech cred.
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Well, I consider the pints to be an overhead expense so they're amortized into the cost of life itself. Sort of like eating and breathing.
The shim is resilient and has a small displacement so it returns to it's rest position just above the face of the valve body. The small signals just after the switch closes are probably the settling time of the squish. The hammer spring is at the factory setting with no preload. I expect more interesting things to happen when the powerplant is pressurized.
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One of the problems that occurs when doing the barrel harmonics testing is that there is a high probability of getting double strikes caught up in the measurement as Stan has attested to. I decided to do another cheapskate approach to building a better instrumented hammer for doing the tapping. This was prompted by the arrival of the 20 PAK of 12 mm piezoelectric disc ($1.48, free shipping).
The decision was made to get the next size up hammer from the Harbor Freight watch & jewelry tool collection ($7). I actually like it better then the smaller metal handled one. The size is perfect for the new 12 mm discs. A little lathe time with the new hammer and a small piece of Delrin and I was ready to do some tap dancing on everything within reach of the cabling.
The scope traces shows the difference between tapping the wood benchtop (stored wave A) and the jaw of the vise (channel 2). The disc was wired in behind the brass hammer tip. This effort was a simple proof of concept project that turned out quite well. There are several refinements that will be added to improve the overall performance, but I'm really happy with this new toy for now.
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George, Wow, great job on that hammer! I looked and Piezotronics has one for $760... looks like you have enough left over for a pint or two on overhead.
When I was testing the 2240 barrel I gravitated towards the dome or chisel tips to get a single point contact without rocking.
Again, great job
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Stan - you're right about the shape of the tapping end of the hammer. I've got to do some machining on the shape and type of material for the faces. Right now I'm working on characterizing the hammer itself. The wooden handle may end up having some desirable traits compared to metal or other composition materials. This hammer project adds a whole new layer of interesting investigation to the barrel harmonics measurements.
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OK, it is only fair to show the dead ends and partial fails.
I added the two axis accel near the muzzle. I used a block (a keen eye will recognize it as the base from a clamp-on desk lamp) because I wanted to try to get the two axes normal to the barrel. The block also had some convenient cross holes for mounting a second laser/receiver set for detecting pellet exit. The first image shows the configuration. I used Al tape as shim to make it a snug fit on the barrel. You can also see the laser receiver mounted but not yet wired up (next step).
The second image shows the overall trace for a dry fire, no CO2. The blue (ch4) trace is the laser/receiver on the hammer probe it triggers on hammer release. The yellow (ch1) is the mic on the tube. The blue (ch2) and purple (ch3) traces are the accel X (horiz) and Y (vert) axes respectively. The hope was to get some clear event timing marks on the accel channels. The accels detect the start of hammer motion well, the valve impact is detectable but the mic trace is probably better. After that there is too much ringing to discern specific events.
The third image shows details of the initial motion. Looks like the vertical accel axis pick up some of the hammer release noise, then both channels respond to the hammer flight, and then respond to the valve impact.
The hope was to be able to use the accels to generate some event timing points but also see if it can detect recoil when hand held. Combined with the pellet muzzle exit, it would make for an interesting timeline. Looks like right now the mic is a little clearer. I may try the accels on the barrel band to see if that quiets it down. In the worst case I'll have to break out the manual and learn about the built in filters.
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I like your instrumented pistol, Stan. It looks like you're getting ready with a top level entry for a steampunk convention! The laser pellet exit detector will probably work well. The hammer start detector is clean, but really needs to be at the point where it strikes the valve stem. That would be an easy machining job, but not an easily reversed commitment.
The rest is to busy for anything to be very well defined. It's all a good start, though.
I've played some with my DSO's digital filters and they might have some interesting capabilities. It's worth the time to figure them out just to know how they work and what they will and won't do. I still prefer the analog world for frequency tuning at this point.
It's nice to know that there is someone else out there trying to make some measurements. So far it's been like being stranded on the moon and then finding out that there's actually another person up there trying to signal back home, too.
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It's sure easy to get distracted when you don't really know were you're going. This vibration measurement project has been consumingly interesting for the last couple of weeks. The hammer project is turning into a didactic tsunami of practical experiments using transient signal analysis.
The brass tapping end of the hammer has been reshaped to produce very reliable single strikes and the force transducers are being switched alternately between the PZT discs and quartz crystals. The quartz crystals are considerably more expensive, but are very stable with changes in temperature. The cheapie PZT discs are more temperature sensitive, but they're generally quite useful in these experiments.
By good fortune I had a fixture on the shelf that was designed to test mechanical features of certain silicon wafers, but it's also ideal for characterizing these impulse transducers. Preloading can be added at the top of the fixture which is very handy.
The scope image is of a hammer tap at the top of a 5 lb preload weight sitting atop the fixture. The blue trace is the hammer. Of interest in the hammer trace is the lack of any resonances that might interfere with the measurement. I use a small sheets of different types of rubbery stuff to simulate different tips for the hammer. Different tapes from electrical to packaging types are also very instructive. I'm still trying to figure out how to copy a scope image without the menus in the image. All in good time.
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Interesting, is the preload weight loose on the threads or is it bottomed out? For the trace, is that the same size transducer in the hammer as in the test stand?
Great work
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At this point the weights are just stacked on top of the flat end of the shaft. The separate weights are 5 lbs and 10 lbs plus the 2.3lb shaft and upper platen. This can give a preload of up to ~86.5 PSI on the discs which are all ~1/2" in diameter.
The disc in the fixture is the same PZT device as the one in the hammer. This is all ground zero for these tests, so I'm only after first approximations to get things going. With increased attention to details I anticipate some interesting and reproducible experiments. One of the first test will be to do a free-free measurement of the shaft and platen's natural frequencies using the soft foam method that Stan has already confirmed as being viable.
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I decided to get back on track today do some measurements on the pressure/time curves and the pellet transit times.
The image shows the simultaneous events of the hammer/valve signal, the pressure transducer output, and the force transducer at the muzzle.
The yellow trace (#1) is the hammer/valve contact signal. This signal is used to start the timing sequence.
The aqua trace (#2) is the output of the pressure transducer. This sensor is measuring the pressure just above the transfer port and just behind the pellet loaded into the breech.
The purple trace (#3) is the signal from the force transducer at the muzzle and is recording the moment the pellet leaves the muzzle.
The two purple cursor lines (1&2) mark the transit time of the pellet through the barrel.
The hammer signal is bouncing, but that seems to be the shim and not the actual valve. The pressure peaks out at ~500 psi in ~600 µS. The pellet exits after 2.66 mS. The temperature was 74º F.
Some adjustment on the hammer's signal shim can probably clean up the bouncing. I'd still like to put another pressure transducer at the muzzle to get the full pre-exit pressure profile since I've now got a 4th channel to collect more data on.
It may also be possible to put a micro hot wire anemometer into the transfer port and get the actual flow rate simultaneously with the pressure curve. Then, of course, a high speed micro thermistor could be added on each side of the transfer port and measure the CO2's cooling rate as it expands behind the pellet.
These extended measurements would require setting up the HP data acquisition and control system. Unfortunately the system runs with LabVIEW and I'd rather have a root canal then get involved with that system again.
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George, that looks great
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I got to thinking about the valve open and close time when the hammer's trigger signal is being used. Maybe the timing is different when viewed from the pressure curve numbers. Since the pressure sensor is located just above the transfer port it seemed to be a good place to do another test.
The barrel and 2 of the clamping screws were removed to reduce the flow impedance as much as possible. The bolt and probe were still in the breech. The test was done with a new powerlet installed.
The photo and scope image show the setup and response of the pressure sensor. I'll assume that the noise on top of the signal is turbulence from the probe and bolt being between the transfer port and the sensor. The time interval between the cursors is 2.40ms. The end of the turbulent portion may be when the valve is closing. The rest of the curve may be bleed down. The negative dip just past the 2nd cursor might be a pressure drop due to the measuring geometry and the refrigerant property of the CO2.
Actually, this test result is anybody's guess. What I'm interested in finding out is the release time when the pellet starts moving. This would give the real transit time for the pellet to get to the muzzle. I think that if the whole pressure/time curve's derivative is taken a much better picture will be seen.
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Here are a couple of more images of the valve pressure/time measurements. This is the same setup as the last post, but with the transfer port removed. The pressure has dropped as can be seen on the vertical scale. The time between the cursors is 3.16 ms, but they are set wider than the last ones.
The 2nd image shows what might be some valve or hammer bounce. Both images now show a pressure drop on each side of the curves. These results are all quite sketchy and will have to be reassembled when all of the results are in. You're all welcome to guess as to what any of these tests might mean.
The next test will be with the bolt and probe removed. I'll try that tonight if I get time.
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The bolt w/probe was removed from the breech and several shots were fired, again without the barrel and it's clamping set screws. It seems now that the valve dwell time is in the vicinity of about 2 ms. It may really be back down to ~ 1.5ms that was indicated by the original hammer/valve stem contact measurements. I'm not sure what the actual decay time from the peak pressure value is considering how open the test volume is now. Keep in mind that we're measuring the timing of a valve and not a switch.
The noise on top of the main pressure curve can't be from the bolt/probe turbulence. My current guess is that a breech resonance might be the cause. The next test will include an accelerometer on the breech and we'll see if the frequency matches the pressure curve noise.
Anything that can be gleaned from these images is up for grabs. I'll just keep on testing and we can all have a go at what might be happening.
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George, do you still have have the hammer/valve signal hooked up? Maybe if the shim material is tweaked to more explicitly show hammer/valve contact, it may help in understanding the pressure curve. As you have it set up does the hammer spring preload the hammer against the valve at rest?
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The shim is sitting just above the valve's face. The hammer is not touching the valve stem when the spring is not cocked so there is no preload. I'll be able to measure the hammer/stem offset later today. I'll also measure the actual valve displacement when it's fully open.
Looking back at the DSO image in post #213 there is some interesting info at the foot of the left hand side of the curves. Using the increased acuity of beer vision I've started to see an overlooked part of the record. It shows that the hammer/stem strike lags the pressure curve by about 100 µs. This would mean that the valve takes that long to fully open.
A new set of experiments can now be done to expand that small time window and rearrange the shim to be struck between the hammer and stem. Then I'll redo the shim position at the valve face and pay more attention to detail. Fortunately, the pressure transducer's rise time and sensitivity are up to the task.
I'm most interested at the moment in finding out the time of release of the pellet at the breech and what the pressure is at that moment. I thought it would be easy to see in the P/T curve, but if it's there it's not distinct. It should be quite simple to make this measurement in a much more direct way. I do enjoy having a menu of experiments to choose from.
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George, getting the valve open duration would be great data. Earlier I missed the the fact that you had the brass shim on the valve body, I thought it was on the stem. I don't know if you have some insulation on the hammer side of the shim so it is forced to ground through the valve stem rather than through the hammer. Ultimately, you could have two pieces of shim, one on the stem and one on the body to get both the start and fully open points. Unfortunately the shape of the hammer face makes that a little trickier.
I've been thinking about the pellet start measurement. Other than drilling portholes into the barrel for an optical measurement, an insulated bolt with a wired tip (similar to my original skewer hammer probe) may be easier.
Great work as always
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Things are getting more interesting with these new measurements. The photo shows the basic setup for finding the break free time on a pellet in the breech. I'm using the skewer method that Stan used on the other end of his pistol to get the hammer information. This test uses the same ohmmeter switch circuit that was used to get the valve timing info in a previous post. A piece of wire wrap is at the end of the skewer and it's pushed in until it makes contact with the end of the pellet and closes the circuit. It's then backed out until the circuit just opens. The pellet only has to travel a few thousandths to make contact. This arrangement is very reliable and surprisingly durable. I've used the same stick and wire for several shots and haven't had to replace anything yet. This test method obviously has it's hazards but the metal cylinder and trap keep things under control.
To synopsize the DSO image I'll just point out that the pellet starts moving 375 µs after the valve begins to open. The pressure behind the pellet at skewer contact is 284 psi. The timing for this experiment is based on the valve actually starting to open and not when the hammer strikes the valve stem or bottoms out. That information can be gathered as a separate experiment.
When we get further along with all of the test data there will be a set of Lego blocks that can be used to put the whole time sequence together to make one clear picture. I think these smaller parts are much easier to digest.
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I like your probe approach. I was looking at a very thin probe (only weighed 4 grains) that the pellet could push to initiate the transition time with a laser sensor at the muzzle. Your approach can time not only start of pellet motion but potentially the acceleration down the barrel by moving the contact point outwards in sequential shots.
Interesting, the previous (213) pressure curve achieved peak pressure in about 500 micro-sec, in this curve it is more than twice that
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Thanks for catching the discrepancy, Stan. I'll redo the tests and see what's going on with the measurement numbers. It must be the new DSO. I'm sure that it can't be the beer.
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The only thing that comes to mind as a possible reason for the difference in the 2 curves is that the stick and wire are causing the effect. If you look at the probable time of exit on the #213 image it would be about twice as long.
I did a few shots this morning using a 4 event average from the acquire mode. The averaging does some nice smoothing on the P/T curve as shown in the image. I'm confident that the system is working properly.
The incremented timing for acceleration measurements is a good idea except for the air plug in front of the pellet. I doubt that it technique would give real world timing info unless the stick and wire influence could be reduced to a very small number. I was thinking about using a straw in place of the stick in order to reduce the effect of the barrel air column as the distance between the trigger wire and the pellet increases.
Your 4 grain probe is interesting. It would have to be very stiff with a small cross section. I suppose that if you could place one end into a hollow point pellet to keep the probe centered and then shoot it straight up it might work. Can you give us more details about your plan?
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Interesting, I was guessing that the pressure system didn't know about the probe&wire until the pellet moved, and yet the 221 curve in 375 micro-sec gets to 2.84v at the time the pellet moves and the 224 curve gets to about the same voltage in about 250 micro-sec and the pressure rise is about the same on the 213 curve.... interesting.
For the incremental method, a thin gauge (.020?) solid wire may be sufficient and would block a minimal portion of the cross section. The straw approach should work as well. The one I tried was just a little too big for a .22 barrel....I'll order a few more manly umbrella drinks to look for a smaller straw.
For the lightweight probe, I planned to use some high tech garden stuff. We have some Mexican grass tree https://www.monrovia.com/plant-catalog/plants/815/mexican-grass-tree/ (https://www.monrovia.com/plant-catalog/plants/815/mexican-grass-tree/) specimens. When dry the tip sections (~.070") weigh about 4-4.5 grains for a 7" section for the 2240 barrel. I don't need them to survive, I was just going to use them to trigger the start of motion by unblocking the laser trap at the muzzle. After that it can come apart. The pellet itself would have its normal timing interval going through the trap. I figured a 10 gn wadcutter + 4.5 gn probe = ~ 14.3 gn CPHPs. All that said, I like your blocking probe better.
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I needed a diversion today that would allow me a play session with the new DSO. The decision was to drift back to some of the barrel harmonics experiments. Since the measurements were already done to find the natural frequencies of the mounted 8" barrel it was easiest to start there. 2 sine wave generators were set up to reproduce the 2 frequencies that the accelerometers recorded. The information about those experiments is somewhere in this thread.
What I want to show in this post is the combined output of the sensors. This is real world stuff and not spread sheet voodoo. When an impulse of sufficient force, whether by firing the gun or striking it with an instrumented hammer, is injected into a cantilevered barrel it will resonated at specific frequencies. As demonstrated in the previous experiments the CP2 breech mounting allows 2 distinct modes of vibration. I doubt very much that this is a condition unique to this gun.
Anyhow, these 2 harmonics can be accurately simulated by the 2 signal generators as mentioned above. When these signals are combined simultaneously and plotted on a Cartesian coordinate system a Lissajous pattern can be drawn to show where the barrel is pointing at any given moment in time. To interpret the graph assume that you're facing the end of the barrel of an unloaded gun that has just been struck be an instrumented hammer.
The images show a lot of good information about time and the positions of the muzzle activity. Across the top of the o'scope's screen is a time vs amplitude trace of the 2 sine waves as would be normally seen using any conventional o'scope. It's obvious that the 2 signals are out of phase because they're at 2 different frequencies. The lower square display is what the 2 signals look like when they're combined to show an amplitude vs phase angle plot. I've chosen to show only a 10 ms window of these signals for clarity because the plot gets much more crowded and confusing with longer time intervals. The cursor markers in the first image can be seen at each end of the 10 ms window of the Lissajous pattern.
The 2nd image shows the markers at the start and exit time of the averaged pellet shots from the previous post #224, i.e. 2.86 ms.
Please keep in mind that the magnitude of these Lissajous patterns are exaggerated in these examples, but the patterns are very real. Also keep in mind that the 2.86 ms window can be moved around to any position on the pattern.
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George,
If I understood correctly, what you plotted here is an example using a function generator source. In the real case, those will be replaced with your accel outputs. How does one get from the accel plot to where boresight is pointing?
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I guess that the previous post was a bit cryptic in it's presentation. I'll try to demystify it. First we can go back to post #166 and see that the two accelerometers have well defined high Q peaks at 104 Hz and 164 Hz. These are the real signals from the barrel at its resonances which are simultaneous, but 90º apart. Since we now know these frequencies from the gun, it's an easy matter to use a pair of signal generator to do a bench test and time window the measurements with them. Stan, this method is a juxtaposition of your comment "In the real case". The signal generators are now replacing the accelerometers for doing these measurements.
The starting point for both sensors is before the shot is fired which would mean that the end of the barrel is at rest and there is no signal from either sensor. In this bench test we're running both of the generator signals continuously, but they can represent the barrel at rest when both signals cross the zero voltage point at the same time. This is a fleeting moment, but neither signal has any magnitude there. The only time that the 2 signals (or the barrel) come to the zero or rest point again is about 32.6 ms later. This is long after the pellet has left the muzzle. The 1st image shows what the 2 sine wave look like in a conventional o'scope mode. The 2 cursors show the time of the zero crossing points.
The 2nd image shows the XY mode of the 2 waves as they cross the zero point where the barrel is at rest. The time vs amplitude trace at the top of the screen shows the crossover point of the 2 waves at zero magnitude. The XY plot shows both the #1 and #2 cursors on top of each other at the zero magnitude point for each wave. The scales on each side of the XY plot represent the plus and minus values of each of the waves. the yellow Lissajous pattern is the continuous time line for both waves.
The 3rd image shows the same traces at the top of the screen, but with one of cursors moved from the barrel at rest point to the 2.86 ms point. That's when the pellet leaves the muzzle and also where the barrel is pointed at that moment.
I'll reiterate that the magnitudes of barrel displacement are exaggerated, but the changes of position in time are real. I also find it interesting that the barrel will never cross the "at rest" point again during the shot cycle.
I hope this helps in the interpretation of the images.
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The instrumented hammer is becoming a preoccupation. One of the requirements for proper use of the device is to be able to calibrate it. I've been going round and round with all of the possibilities. Most of the standard methods that are easy seemed to fall short in one area or another. While reflecting upon the situation during a beer induced meditation I was able to recall that there was a resiliometer up in the attic. Hastily untangling myself from the Yogic Sleep pose I managed to limp up the ladder into my hardware cornucopia and retrieve the apparatus. It was just how I remembered it, though not exactly were I remembered it having been.
Anyhow, the plan is to use it in reverse of it's intended application and measure the impulse force instead of the rebound height. This procedure gives me a calibrated weight impact on the hammer and a DSO capture of the event. I tried a few drops and it really works well for this repurposed application.
The photo shows the setup.
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George, Thank you for the extra clarification on the harmonics plot. I guess my question was more on the relationship of the measurements to changes in boresight. My understanding is that boresight error at the time of pellet exit is the slope of the tip of the barrel relative to the breech and possibly any lateral velocity the muzzle may have at that time (I need to pull out the dreaded spreadsheet to see if the second one is negligible). Your configuration provides acceleration data. To get to lateral velocity and/or slope, some time history is needed. It is not clear to me what the time history should be given the hammer and pressure rise sequence. To get the slope some mode shape consideration comes into play.
All very interesting. I was wondering if an optical test of boresight could be done, perhaps using something like a quad-cell (expensive).
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I did a little more exploration with the 2240 setup. In the spirit of you can't have too many wires, I added a few (image 1). The setup has the laser gate on the hammer probe set to trigger on valve impact (rather than on hammer release) this is channel 4 (dark blue) in the traces. The microphone is still attached to the tube near the valve (channel 1 - yellow). I added a laser gate near (.25") the muzzle (channel 4 - purple). I also included the broken wire at the muzzle (channel 2 - light blue).
Image 2 shows the overall trace. The hammer probe and the mic are in pretty good agreement in detecting hammer/valve impact. With the valve pressurized, I did not see any of the bounce on the hammer probe.
Image 3 shows some of the timing details. The valve duration was typically just under 2ms, pellet exit at about 2.7ms. I did have some additional trips on the laser gate at the muzzle. The powerlet was new and there was probably more CO2 spray than before. The pellet preceded the additional trips as you would expect with CO2 spray. I'll have to go figure out what the large signal is in the mic at the tail end of the sequence, it is at about 350hz.
Next is to go try some of the options for detecting pellet start to complete the sequence.
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Very impressive measuring arrangement, Stan. I hope your wife lets you put it up on the mantel when your done testing. Your DSO images are becoming intrepretable with your new sensor array. The screens are much easier to decipher now that I'm looking at pretty much the same displayed info on my scope, it's just in different places on my screen.
The broken wire detector signal is certainly clean. How have you removed the hammer travel to get the valve info?
The microphone appears to hear 2 different impacts. Are they the stem and then valve's face? The larger slow signal may be the mass/spring system of the mike and rubber band.
A pressure transducer behind the pellet at the breech would a good addition a this point seeing how there's room left to festoon some more wires. Then adding a pellet release detector would seem the be all that's need to complete the kit.
For clarity, how are you using the terms slope, lateral, and boresight? I'm pretty sure I know, but I don't want to guess and confuse the discussion.
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The hammer probe has two options: trigger on release or on valve contact. For valve contact, I adjust the position of the 2240 with the cross vise so the left edge of the flag on the probe is just tripping the laser gate when in the post fired position, i.e. the hammer resting on the valve stem (the hammer spring has preload). If you look closely you will see the red laser dot just on the left edge of the Al flag. When the hammer is cocked, the flag moves to the left and the beam is obscured. When the hammer is released the beam stays obscured until it travels past the valve stem contact point. The signal then stays high until the valve closes and pushes the hammer away from the contact point. There is a limit to the accuracy given the probe & flag hardware used, but in a half dozen shots, the probe signal and the mic signal were within 0.1 ms of each other (std dev 0.055). With a little bit more care, that could probably be tightened up.
I don't think the large slower signal is the mic mount, it has sticky tape under it and is fairly firm. If it came loose it would be very slow. It may be how the 2240 is held in the vise. 350 hz is still pretty firm. I didn't want to clamp the gun by the tube like you use because I wanted to give the mic a chance at hearing the valve impact. Since it is after the pellet exit point, I set it aside for now.
The pressure transducer is a longer term project. I did find a low cost 1000 psi unit that might work but it hasn't started its swim across the pacific yet. The home grown (literally) pellet probe is in work.
I'm loosely using boresight for the direction defined by the last inch or so of the barrel when at rest. It has some fixed relationship back to whatever the sighting system is established during the zeroing process. I use boresight error for the change in that relationship at the instant the pellet is passing through that section on the way out the muzzle. This is an angular error so I used the term slope to capture the change from the at rest condition, of course it has two directions involved. I used lateral for the two directions at right angles to the barrel axis. These are the axes which your accels are aligned with. I'm starting with the assumption that angular errors are more important than translation, but that can be verified.
I noticed that your screen shots still have the save menu overlay. On my DSO there is a print or hardcopy button on the front panel. If there is no printer attached but there is a USB thumbdrive in place it dumps a screenshot to the drive without bringing up any menus (you can preselect the image format) maybe yours has something similar.
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For clarity I'd like to show this graph that I've copied from the internet. It's a generic plot of the relationship of acceleration, velocity, and displacement. If we were to drop a vertical line through the graph at any point along the time line the intersections would give the instantaneous values of A, V, and D and how they are related to each other. Therefore, taking the integral of acceleration once will give the velocity and taking it twice will give the displacement at any point on the acceleration timeline.
The trace of the two accelerometer signals plotted simultaneously on the same XT timeline shown at the top of the DSO image is also displayed in the XY plot as a Lissajous pattern. Where the cursors are shown on the XY pattern are in fact the exact position of the barrel at the assumed exit time of the pellet if the double integral of A is taken. This displacement position is in reference to the origin or rest position of the barrel as displayed by the upper trace.
The position of the pellet on exit has to be determined by the exit time in relation to the the start of the barrel movement. The clock really starts when the barrel begins to oscillate. The pellet exit time has to be referenced to the start of that clock. I'm a few pints into this post so please point out any errors or oversights that may have gotten incorporated into it.
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Stan, you're right! There is a hardcopy button on the front panel. I had to get a flashlight to read the label, but It works just like you said. I like this DSO even more now that I can copy screenshots without the menus in the way. Thanks for the tip!
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George, I noticed my DSO (with royalties to Sir Isaac I'm sure) will do an integral of the waveform, though I'm not sure it will do it twice, probably need to pull the data out for that. Yours may do that as well.
On the other hand I was wondering if you could also use a direct displacement measurement of the muzzle when you do the pellet shot runs. The VRT you were using in Jan is a displacement measurement isn't it? I may have a cheap hall sensor that I'll tinker with. I was even thinking about your phono cartridge suggestion earlier.
Glad you found the print button. Some of these labels are definitely meant for younger eyes than mine
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Doing a direct displacement measurement would be interesting and possibly useful, but you would have to contrive a way of measuring the instantaneous time at any point along a dynamic displacement curve. It will be hard to beat an accelerometer, or better yet a pair of them, for making this measurement. My DSO has the advanced math capabilities, but the vintage B&K 2515 may be a better instrument for these measurements because it's designed to work with accelerometers.
After playing with the piezoelectric discs the crystal phono pickups look far less attractive. Being able to cut and shape the discs provides almost unlimited possibilities for making simple vibration measurements. The prices on eBay are ridiculously cheap.
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The thought was the VRT could provide a real time displacement measurement and then a broken wire or any of the other pellet exit indicators could provide the time marker. With a little bit of history on each side of pellet exit, the velocity can also be obtained. It would be an interesting comparison to trying to integrate a complex and perhaps noisy accel trace of a pellet shot.
The phono cartridge thought was going after direct displacement and velocity measurement, not a force measurement like the piezos.
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The VRT is considered to be more of a detection device than a measurement device. it's used where detection of vibratory motion needs to be done without contact or adding mass to the DUT. It's not used where absolute amplitude is needed. Also, it's a velocity detector and would still need the signal to be integrated once to get displacement. That gives it 2 strikes right out of the gate.
Virtually all common accelerometers in use today use either quartz or PZT piezoelectric or piezoresistive crystals. The accelerometers use a seismic mass attached to the crystal and the force detectors use a preload on the crystal. The crystal phono pickups use neither and are generally torsional sensors. The discs allow experimentation with all 3 methods of generating a signal. They're too cheap not to play with and you can determine your own favorite method.
I'm glad that you're giving thought to all of this. Keep experimenting and posting your results!
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Thank you for the clarification on the VRT characteristics. I realize that instrumentation grade accels are the standard for this kind of measurement but I was poking at other methods to see if they may be useful, especially for directly measuring velocity or displacement.
I was looking at what the powder burner folks are doing on the barrel harmonics front. There are several pages on this site http://www.varmintal.com/aeste.htm (http://www.varmintal.com/aeste.htm) that combine FEM analysis with empirical point of impact results. Granted this is for high velocity rounds, but their heavier barrels keep the frequencies in the general range of interest. The FEM results also discuss the muzzle slope and vertical velocity contributions to POI spread. Not sure how accurate the pressure analysis is as a forcing function but the characteristics of the barrel FEM should align with handbook results. Interesting reading.
For the 2240 with 7" barrel and barrel band that I've been playing with, the affect of barrel harmonics on accuracy is primarily a thought exercise.
Fun stuff
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Thanks for posting the link. I've seen it before and it does have some interesting drama in it. The real foible it demonstrates is that the results are obtained when the analysis is confined to just 1 degree of freedom. Most (if not all) of what is presented represents only the vertical motion of the barrel. This makes great graphs and animations, but I highly doubt that they represent more than what can be seen when you only have half of the information. I see this as the trap of spread sheets. The pressures and temperatures achieved with PB guns is a whole other can of worms compared to airguns. I'll stick with my own measurements in that arena.
Keep in mind (pun?) that thought experiments are a close relative of spread sheets. They're good as trailheads, but they can't be reified until that first step onto the path is actually taken. Of course they're a lot easier to tote around compared to experimental hardware.
Your 2240 experiments are still valuable exercises even if you conclude that there's not much going on that needs correction in terms of barrel vibration. When you get your inevitable CP-2 with the 18" barrel you'll have all of the needed hardware ready to go!
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The testing is back to the pressure and time of release of the pellets. I'm finally getting comfortable with the settings and displays on the DSO. This will improve the quality of what is reported as the testing proceeds.
One of the things that can be reported on is the durability of bamboo skewers over thin wooden dowels. They're about the same diameter and fit nicely down the barrel for doing the pellet release experiments, but the wooden ones just about explode on the first shot. The bamboo ones hold up very well for up to a half dozen shots. Their mode of failure is typically splitting, but they stay mostly in one piece. This got me to thinking about the fibrous structural differences between the 2 materials. I was going to haul down on of the microtomes from the attic and do some cross and longitudinal sections that could be stained and then microscopically inspected to ascertain whether bamboo was a monocot or dicot. After a couple of more beers I decided that I might be able to save myself a week's work by just looking it up on the web. Bamboo is a monocot. This explains a lot and may be good news for Stan when he makes his grass tree (monocot) probes.
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OK, Since George requested monocot probes, here we go. (I'm glad George introduced the monocot label, with all the legalization going on in CA, I was afraid people would get the wrong idea from a "grass tree" probe)
Image 1 shows one of the early probes with the Al flag attached. The weight ranged from 3.6 to 4.6 gn. I also switched to Hobby wadcutters at about 12gn in the 2240. Some of the probes were straighter than others, all were dry to minimize weight.
Image 2 shows the probe in the barrel, wire across the muzzle (I subsequently cleaned up the broken-wire system), and the laser light trap set at the outside edge of the probe flag.
The hammer probe and the tube/valve mic are in place and used as before
Image 3 shows an overall trace. As before, yellow (1) is the mic on the tube, dark blue (4) is the hammer probe, light blue (2) is the broken-wire across the muzzle, and the pink (3) is the trap at the muzzle. The hammer probe triggers the trace ~when the hammer hits the valve.
Image 4 shows the timing of the valve. The mic and the hammer probe detect hammer impact, independently, within about .05 ms fairly consistently. The hammer probe shows the valve closing after 1.86 ms (this ranged from about 1.6 to about 1.9 ms)
Image 5 shows the pellet timing. The pellet/probe begin to move at .45 ms and exits the muzzle at 2.59 ms for a 2.14 ms transit time. There is a little bit of ambiguity on the exit time since the probe could break the wire as well but the wire and the light trap are within .13 ms of each other and are generally consistent with the pellet only shots
Some observations: Both the hammer probe and microphone appear to capture the hammer strike event pretty well. The pellet probe was consistent at .45 +/-.05 ms and should remain valid for other shots where the probe is not used. I think doing sequential shots with different length pellet probes could map the acceleration of the pellet. Best timing tool for the muzzle exit is the wire but it is probably best to establish exit time in separate shots, without the probe to avoid the chance of the probe breaking the wire (or tripping the light trap). My original question was whether you can get useful information on the 2240 event timing using just hobby detectors. These results are good enough to answer my curiosity, and other than the DSO hit, the wallet was unharmed in the process.
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Your measurements are looking really good. The laser/flag signals can't get much cleaner than what you're showing now. It might be of interest as an exercise to try some digital filtering on the mike channel. There seem to be at least 4 signals on it.
How well are your probes holding up as projectiles?
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So as not to get left behind on these measurements I did a quick test with a Crosman 14.3 gr hollow point. This current arrangement for doing pellet release time and pressure is working well. The yellow channel is the total T/P curve for the shot and the aqua channel is the probe/pellet contact time. The cursors show the release time to be 390 µS. The pressure at that time is 232 psi. The measurement panel gives the peak pressure to be 504 psi.
It will be interesting to do an average of a few shots with these CPHP pellets and then see how it compares to other pellet averages.
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At 4 gn each, even the mighty monocot material does not survive impact. The range is short enough that the pellet may help in the destruction. Definitely want to have a scatter shield around the path.
Yes, I definitely want to do a dive into the filtering and waveform math capabilities of the DSO. The slower features on the right side of the first trace image are around 400 hz. I see that with a vertical tap on the barrel. Not sure if one can ID any of the Khz stuff.
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George,
I really like that pressure curve and pellet release data. There are a lot of pellet sizing and bolt tip designs that this will shed some light on.
It is probably just a coincidence but the pellet release times being similar for the two guns is intriguing.
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Stan, I like your probe alliteration. Even if it's a 1 shot probe, from the image's I've seem of your source you should have access to plenty of spares.
Now that we're into Probe Wars 2 I'll offer up a new contender. Though not the bantamweight that your putting into the ring, I'm confident that my guy can become a worthy opponent after he gets a proper workout.
He's a relic of a bygone era when wire wrapping circuit boards together was King. Who ever thought that he could make a comeback?
To start with he stands 9"tall and weighs in at a flabby 5.6872 gr. I'm going to put him through a tough workout regime that will force him into a better shape.
First I'm going to put him on an improvised Inquisition rack (a vise and pair of pliers) and see how much he can take. He should come out taller then when he went in. Once he's cut back to 9" again he will have lost some weight and have been hardened by the exercise. This should keep him standing upright against an inevitable pellet haymaker.
One of his sterling virtues is that he's silver plated and has a very tough hide. I'd add that he's got a wiry build, but I don't want to give too much away before we get into the ring.
The photos show a 14.3 gr CPHP and my guy before conditioning.
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The smallest solid wire I had was .018" core and with the insulation it started to get chunky. I also wasn't sure that the leads would look like. I considered using the magnet wire on the monocot probe but it seemed like too much work per shot, so I stayed with the opto-mechanical approach.....and it had a laser
I do have scale envy though...very nice
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Now, this is a monocot probe. I pulled this bamboo skewer out of the trap after it had already been used several times for pellet timing shots. It speared either a CPHP or CP domed pellet that I had been shooting. I think I'll build my next house out of bamboo.
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Most of the accelerometers used in the type of vibration measurements we've been discussing come in 2 flavors if they are piezoelectric. Typically the crystal elements will be quartz or PZT. The two main construction methods are to either compress or shear stress the crystal by using an attached seismic mass. The mass is attached to the crystal and is free of the base and housing. The crystal can then produce a signal that is proportional to the acceleration of the seismic mass. This is a simplistic description, but useful for designing a small DIY device.
That being said, I decided to explore the possibilities of a start from scratch compression style device. What I've come up with is about as simple as it gets as far as raw materials go. So far things are not much different than Stan's microphone devices. The main ingredients include a pellet tin lid, some single and double backed tape, and 2 pieces of thin shim stock. Also included is a double duty fridge door mini magnet that was pilfered from the kitchen. The double duty part is by using the magnet as both the compressor and the seismic mass.
As can be seen in the photos the crystal is sandwiched between the magnet and the lid. Also seen is another disc like the one used in this experiment before a 1/4" piece was punched out of the center of the first one. The DSO image is the device's output from a tap on the lid from a monocot probe. This is a proof of concept device. The next one will dispense with the lid as a building platform.
This test opens the door to more and better DIY designs that are easy to accomplish and cost a total of about 50 cents each.
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Stan - Thanks for taking note that these experiments could have a valid use for anyone interested in the finer details of how some airguns work. It's still early in the game, but the results are getting better and easier to arrive at. Your involvement in your own experiments is further indication that this can be an interesting and meaningful endeavor. The clever approaches to instrumentation you've come up with may push someone else over the edge. This is after all an R&D area for posting.
I see the measurement results that coincide with 2 different guns and methods of generating data as confirming that if we both use the same caliber pellets and push them with similar power plants the release times and pressures should be similar. What happens by the time the pellet gets to the muzzle may have a larger variance.
It may be just you and me in the lab, but there seem to be a lot of people looking in through the windows!
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Yes, a lot of the fun has been in breaking down the airgun timeline into steps that can be measured individually by relatively simple instruments. Other than aiguns not being politically correct these days, these measurements would make a great STEM project for someone interested. The DSO is convenient, but with a little cleverness I think most of the timing measurements could be captured with a sound card, someone mentioned a logic analyzer, or a microprocessor board like Arduino or Raspberry Pi.
I think the pressure, pellet release timing, and final muzzle velocity is an interesting relationship. Bolt tip design, pellet placement, and pellet skirt sizing all play in this and there are plenty of ideas and explanations to consider. For example, I still don't understand why the pressure rise in your pellet probe cases is significantly slower than the ones without the probe.....calls for more of those thought experiments
Fun stuff as always
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The measurements that I've been doing up until now are pretty much seat of the pants stuff to test out the hardware. Now I need to test a series of pellets from the same tin and see if there is a set of predictable curves that will somewhat define the lot. If that's done for different pellets there might be some interesting characteristics that can ascribed to them when they're all shot from the same barrel under the same conditions. 1 point doesn't define a trend, so we'll need lots of points.
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That's a great plan. The 232 psi on a .22 pellet should be a little under 9 pounds of force on the pellet, maybe a little less if the bolt tip is masking some of the pellet. I tried a crude version of the pellet push test using a postal scale and for the spare 2240 barrel, stock with no leade clean up the initial force was in the 3.5 to 4.5 lbs for the cphp and wadcutter respectively (note, I didn't check how far the stock bolt pushes the pellet into this skirt sizing so the actual firing force may be less). I would expect this value to increase when the skirt has outward pressure on it. I don't know if you did any pellet push test after you cleaned up the breech side of the barrel. It will be interesting what you get from different pellets, also if you pre-size a pellet using a spare barrel. This also ties into the pellet measurement exercise from a few months back...I thought that would be useful at some point.
The other question is what is the optimum pressure for pellet release and pellet drag in the barrel.
I'm probably 1-2 months from being able to do pressure measurements so....Go George!
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I've been reading some posts in other threads that are claiming that the pressures and times of pellet release in PCP guns is something very different than what Stan and I are measuring. I'm becoming fairly confident that the numbers we're arriving at while doing these experiments are real. This also would include notions about the real world of simple harmonic motion (SHM) as it relates to barrel harmonics.
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I'm editing this post at this point to just say that upon reflection it seems best if I just surrender to the fact that there are two camps reporting on how some airguns may actually work. Some posters are offering the results of instrumented test data and others are offering spreadsheet calculation data.
This is not to say that no calculations were done by the first group nor that there were no measurements made by the second group. The differences between the two camps can be viewed as a choice in the foundations on which they chose to build their understanding of how some airguns work .
For my part I differ with the idea that for a 1" long transfer port system, it takes about 5 uSec (0.000005 sec.) for the air to get to the pellet, and maybe only twice that for the pressure behind the pellet to reach nearly full valve (reservoir or plenum) pressure. Nor do I subscribe to the idea that we can consider the pressure rise at the pellet to be basically complete and instantaneous. I'm putting nothing in quotes here so that I don't set myself up for a claim that I'm taking anything out of context. I'd just like to say show me the measured P/T curves.
The same goes for single axis SHM reports. It requires at least 2 transducers to actually know what the barrel is doing during the shot cycle in my world. POA/POI information will always contain unknown and ostensibly uncontrollable variables.
Therefore, I'm going to take the 'separate but equal' view on these matters. If I embrace a "suspension of disbelief" approach to all of this, community harmony can prevail.
I'm here to learn and share.
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George,
If you don't mind, post a couple of links to the PCP threads. I would not be surprised if there was a broad range in times and pressures for pellet motion. When I first got my CP-1, the lack of a leade sized the pellet to where it would fall out the barrel if I pointed it down. I think the pellet fit is an additional tuning parameter.
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Stan - I've edited post #256 to try and avoid any conflicts if can. I don't own a PCP airgun yet, so maybe I'm just being naive about the differences of how they work. Take what you will from the edited post above.
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George, I don't know about others, but I don't see comparisons between analysis and test as conflict. In my past life these were always intertwined and mutually beneficial. Complex phenomena like the airgun cycle need to be broken down into steps that can be addressed sometimes with analysis and sometimes with tests. When the results differ, that's when the best learning takes place. Both approaches have an obligation to discuss and estimate the uncertainties in their approach.
After all, this is a hobby....here to learn
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Stan - I agree with you, this is a hobby. Let's move on.
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Here's a video of an impulse shock wave from an open ended shock tube. What I find interesting is that the tube is fired in a similar way that an airgun works. The pressure when fired is 750 psi. This is about the same pressure as the CO2 guns are working with. Note that there is an initial flat shock wave pulse that emerges from the muzzle. It's then followed by a series of diminishing vortex rings. There's more to be seen, but these are the early events that tell a lot about what happens between the breech and the Muzzle even when there's no pellet being fired. The main point being that the air sitting in the barrel doesn't whoosh or seep out of the muzzle when the valve discharge is initiated. The air at the muzzle end doesn't know it's being compressed right away. When it does come out it does it explosively as a series of shock waves.
These are the same events that were recorded with the microphone back when the muzzle measurements were made on the Vigilante with no pellet being shot. The mike was recording the initial pulse and the following vortex pressure pulses. If an axial slice was taken down through the shock tube pulse train in the video it would look similar to the o'scope display of the mike's pressure wave recording. These records can be seen in reply #138 in the Vigilante hacking thread. The use of different density gasses fired simultaneously w/o pellets in those experiments is a good demonstration of how these separate gas pressure plugs are formed and how they exit the barrel.
This information is probably an excellent way to evaluate the condition of the crown and how square it is to the end of the barrel. If your gun's muzzle treatment was as good as the shock tube's muzzle you should get the same images without a pellet being fired. We could then assume that any perturbation in the pulse train when shooting a pellet would be an anomaly introduce by the pellet itself. I've left plenty of room for second guessing here.
https://www.youtube.com/watch?v=61f0MrG6Zm8 (https://www.youtube.com/watch?v=61f0MrG6Zm8)
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Something you might want to know. Shock tube or Nonel, as it is commonly referred to is HMX explosive powdered finer than flour. It is blown through a hollow tube so that a film is deposited on the inside, leaving it still hollow down the center. Each particle is its own little explosion in a 360 degree sphere, restrained on the side where it is touching the tube wall. The touching particles continue the propagation of the explosive wave. Any air contained in the tube is compressed and the heat generated is also applied to the propagation.
While not quite as fast as PETN, it does zip along at a pretty good clip. You can see the light generated through the tube and follow its progress to the shot. When it's removed from the firing device, you can hear a hissing sound as the pressure inside bleeds out. Really neat stuff.
I just wanted to contribute Something to the discussion instead of being just a lurker... 😀
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I've decided to backtrack to some of the work that was done on the vigilante's 10" barrel mods. This was prompted by some information that was presented in one of those scholarly University papers that I read sometime back concerning airgun internal ballistics. It's been festering in my mind for a while and resurfaced today after a few beers. I can't remember the names of the author(s) or other such details, but I was left with a lingering impression about the data presented. There was a comment made about the difficulty of modeling the air in the barrel in front of the pellet as the pellet moved toward the muzzle.
The paper concluded that it was reasonable to assume that the mass of air and it's inertia were not significant in terms of how it might effect the projectiles flight down the barrel. It was therefore ratified that the air would move as a continuous slug and be pushed forward by the pellet as a single column out through muzzle.
Rather than make any direct claims to the contrary I'll just show a couple of DSO screen shots from experiments that were done about a year ago. The first image shows the timing from hammer's impact on the valve stem to the pellet impacting the force transducer at the muzzle to be ~ 3.2 ms.
In the 2nd screen shot image the top trace shows the time between the barrel's compressed air's shock wave in front of the pellet and the impulse as the pellet leaves the muzzle to be ~ 230 µs. The amplitudes of both shock waves is almost the same. As I recall the The SPLs were in the 150 dB range. This double report of the shock waves shows them both at about equal in energy. My conclusion from experimental evidence is that more may be going on then is assumed by some investigators. Again, I'll leave this open for second guessing.
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I thought that it might be interesting to look at the difference between the resonance information that is provided from an impulse induced signal and that produced by a burst of energy from a sine wave set to the natural frequency of the horizontal mode of a barrel vibration.
The sine wave test starts as a tone burst with zero amplitude and rises to full amplitude within 1/2 cycle the moment the generator is triggered. It continues at full amplitude for the rest of the test. As can be seen the barrel requires ~ 100 ms to come up to full displacement. The pellet is long gone by this time.
The impulse test can be initiated by either the instrumented hammer or actually firing the gun and illustrates that the maximum displacement of the barrel is within the first ~ 4 or 5 ms after the impulse. The pellet can still be somewhere in the barrel within this time window.
The point of this exercise is to show the difference between the gathering of reified data and that of potentially misleading data the is still in it's abstract form. this all relates to airgun barrel harmonics.
Virtually all vibration measurements used to be done using methods of continuous shaking of the test sample or structure at various frequencies and amplitudes until the natural frequencies of the DUT could be determined. This was expensive and time consuming in it's day. Transient events were very difficult to quantify back then.
Now most of the testing can be done easily and quickly using modal analysis and fast computers.
The photo shows the setup for doing the sine wave measurement. The barrel is being driven by the noncontact VRT in the B&K complex modulus apparatus. The barrel's vibration is being detected by an Endevco 2250 accelerometer in both tests. The DSO images should be self explanatory.
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George, good info, as always. Have you tried running a sine sweep to also identify the other modes?
It will be interesting to see the response to a shot compared to the pellet exit time. It is not clear to me when that response is initiated. Velocity response may also be of interest (good excuse to try out the math functions on that shiny new DSO).
Also, at some point it may be worth running a broken wire across the muzzle to quantify to offset between muzzle exit and your impact response. At 400 fps even an inch of effective offset is a fraction of a ms and may be of interest for the muzzle accel responses.
Thanks for the updates
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Here's a DSO image of the entire shot cycle showing the pressure curve on channel 1, the vertical accelerometer (XCL) at the muzzle on channel 2, and the impact sensor on channel 3. The time scale is 1 ms/division and each horizontal dot is 200 µs.
The measurement was triggered by ch 1 and the pressure rose to a peak of ~ 500 psi in ~ 300 µs. The pellet exited at ~ 2.9 ms after the clock started as shown by ch 3. This is all consistent with previous tests. A point of interest is the little ramp at the base of the ch 3 signal as it starts it's vertical assent. This small signal may be the time it takes for the pellet to flatten against the force transducer. The transducer is ~ 1/8" in front of the muzzle.
All of this same information has been presented before. The point of this test is to demonstrate that the ch 2 trace shows that the barrel has gone through a full negative displacement and is on the other side of it's excursion back up about 3/4's of the way while the pellet is still in the barrel. The positive half of the displacement cycle looks different because it includes the combined positive interference of the horizontal movement of the barrel. If we were to do an FFT on the signal from just this one XCL the result would be the 2 discrete frequencies of the barrel's harmonic signature.
And I've still got 1 channel left!.... I guess I need a laser.
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George, that is one beautiful chart. Kind of the Rosetta stone of the airgun cycle, everything in one place. Is the accel trace acceleration or has it been processed by your B&K box to be displacement? What is the sensitivity in g‘s/mv? Looks like at pellet exit the muzzle is near peak velocity.
Great job putting it all together. Channel 4 will be for the other accel?
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Stan - The XCL trace is from it's signal having been run through a Kistler 504E conditioning amplifier, then through a Kronhite 3500 band pass filter and then to the DSO. The data is for acceleration only. As far as I can tell the DSO's advanced math function will only take the integral once. This would only give velocity, so I'll stick with the B&K instruments for higher level operations. As you've pointed out it's easy to guess qualitatively what the velocity integral might be from the trace's information.
A more time consuming thing right now is to do the true calibration on the XCL and get real numbers for A,V,and D. The factory info says that A is 10 mV/g nominal, but considering the signal's path, I was only interest in getting each one of the transducers data displayed all in one place for these tests. Only the time and pressure information is accurate in the previously posted image.
All of the pressure measurements so far have a mild sort of roller coaster with bumps profile to them. It would be interesting to translate the ups and downs to the pellet's velocity at each point along the way and convert it into pellet drag information at any point as it moves down the barrel. The Endevco pressure transducer is exceptionally well suited to make these measurements. There's no shortage of things to experiment with as this project moves forward.
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George, I've been looking at the eye-candy chart in your post 266. Such great information. I went back and looked at some of the harmonics data you provided in the past. Back in P166 you measured the modes of the 8" barrel at 104 and 164hz. In P165, I took a quick pass at a prediction using rough dimensions from my CP-1 and came up with a first mode around 230hz. I didn't quite resolve that in my head but set that aside. In P215, you took the barrel off to do some of the pressure rise tests. Now with the barrel reinstalled, you did a tap in P264 that if I read the settings right came in at around 200hz for the first mode. Finally, in the shot chart in P266, the muzzle response is a 1/2 wave in about 2ms or about 250hz. Looks like even though at the time you said the mount screws were snug, reinstalling the barrel may have set it more firmly and perhaps the pressure sensor tightens up the barrel mount to get closer to a true cantilever condition.
I think the easiest way to get the displacement may be to store the trace data points and do the integration outside. I think this is just a test run for the longer barrel configuration. The muzzle velocity will be interesting to see if it reaches on the order of 1"/sec. For a 20 yd shot at 400 ft/sec, that would be about .15" error, it would be interesting to see how that compares to the velocity deviations people worry about. Any muzzle tilt error would combine with that.
For the accel calibration, does the little shaker you showed in an earlier post get you there? For the harmonics studies getting within 10% on amplitude is probably good enough.
Thanks for providing all this data to chew on
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Stan - The discrepancies in the resonant frequencies may well be caused by the varying stiffness of the breech and barrel mounting. Even if the barrel is really secured tightly into the breech block there can be variations in their compliance due to the way the breech/barrel combo is attached to the main tube. The screw beneath the pellet tray is really minuscule considering the job it's assigned to.
The bolt that holds down the breech block in the rear runs through the piston cover. This long bolt was much more substantial in it's original implementation, but I've modified it to allow a shorter bolt to work in it's stead. The new short bolt allows the clamping of the piston cover to pull the breech tightly against the main tube, but only uses the top half of the piston cover, which is plastic. This arrangement was done in order to allow another bolt to pass through the center of the piston cover for the possibility of setting the hammer spring tension. Thus, the breech block to main tube clamping force achievable is considerably lower than the force that can be developed if the longer bolt went through both sides of the main tube. See post #69 and #88. Also, some of the testing was done with and with out the pellet tray, which may change the breech block compliance at that point.
These are some possible conditions that may have effected the resonances values, but now need to be tested as new variables. Thanks for paying more attention than I do to these things. I'm still in my shotgun approach to doing all of this. Don't forget the beer factor.
As far as the using the time line as a source of frequency information it's important to remember that the first 1/2 cycle (and subsequent wave train) info contains a compound waveform of the real breech/barrel movement and gets more complicated as time goes on because there is no simultaneous phase, amplitude, and frequency separation. This is demonstrated very well in the Lissajous patterns.
The B&K mini shaker is the best method I have for doing the XCL calibration.
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George, I don't think the modifications you made to the rear cap significantly affect the first mode of the barrel. At the loads we are dealing with, as long as you have enough preload to compress the breech to tube interface, avoiding a point load, the stiffness of the bolt and to a great extent the plastic cap do not drive the stiffness of that joint. If you could get enough torque on it you could use a nylon bolt. The same is true for the small bolt up front, as long as there is enough preload, its small size does not drive it. I don't think the pellet tray is in the load path at all. If I understand what you did with the pressure sensor mount, I think what you did was augment the two small set screws that are radially compressing two o-rings with a large thread compressing an o-ring face seal against the barrel so you changed the preload and the geometry to something stiffer....Of course, this evaluation is only valid until the next test.
I understand the complex waveform concern, I was just noting (celebrating) the general agreement with your latest tap test (and of course, my analysis). One thing I've been wondering about is whether the force that initiates the natural frequency response is repeatable enough to always initiate the barrel motion in the same general direction. In that case, much of the error might be removed in zeroing the sights. I think as you add the other direction accel and run a several cases, that will become apparent.
I love that little B&K shaker. What is the bandwidth?
I've been thinking about how to characterize the transient response of a pressure sensor. I have a low cost, 1000psi unit coming across the pacific. Not much in the way of specs other than that it uses a ceramic sensor. It is water compatible so maybe a hydraulic shock configuration of some sort....another thought exercise.
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It was a good exercise to redo the barrel resonances. The vertical frequency was a reassuring 104 HZ and the horizontal 164 Hz. The screws were then loosened and the new H was 90 Hz and the V was 136 Hz. That's 12.2% and 13.5 % lower respectively. The Q dropped quite a bit too, as would be expected. The pellet tray, in or out, made no difference as Stan had predicted.
The mystery of the tapped resonance in the #264 being ~ 200 Hz was solved by taking a larger number of wave peaks from the DSO image and averaging them. I came up with 160 Hz. That's close enough for me. If I had done this first it would have been comforting to know that the measurements were good to start with, but I wouldn't have thought to do the loose screw tests. All's well that ends well.
As far as the XCL info on ch 2 in #266, I wouldn't read too much into it other than curiosity at this point. A lot more testing will have to be done with calibrated XCL's down the road. Here's a link to some info on the B&K 8210 mini shaker.
https://www.bksv.com/-/media/literature/Product-Data/bp0232.ashx (https://www.bksv.com/-/media/literature/Product-Data/bp0232.ashx)
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Awhile back I did a 4 shot average using CPHP pellets and recorded the pressure curve. The averaging smoothed out the trace, but still left the curve with some up and downs on the descending slope. What's interesting to me is that if those ups and downs weren't averaged out along with the noise, then they must be real and part of the pellet's flight down the barrel.
The pressure peak is 500 psi (5 divisions) and the full scale resolution of the screen is 256 steps in the full 8 divisions ( the image is cropped). The number of steps used to draw this curve is therefore only 5/8's of full resolution. These pressure changes would probably be variations in the barrel's diameter or roughness causing drag. The curves might be a good way to evaluate bore and rifling treatments such as moly coating, lapping, and chokes. Higher resolution data collection should show a much better picture of what's going on.
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After giving some thought to the pressure curve vs time experiment it came to me that there was an HP 7090A Measurement Plotting System up in the attic that hasn't been used for a long time. It was one of those instruments that became a hybrid between an digital plotter and a three channel data acquisitions system. It was born in the era of early PC's and laboratory measurements. One of it's virtues that I want to explore is the ability to do 12 bit data capture into a buffer and then plot the buffer out as a hardcopy. Sounds crude, I know, but it may be able to do an accurate plot of the actual pellet transit down the barrel with a resolution of over 4,000 vertical points instead of the 256 points of an 8 bit scope.
The data collection bandwidth is limited to about 3 KHz, so the rise time on the pressure will have to be slowed down. This can be done by reducing the throughput of the transfer port. The transit time on the pellet going down the barrel should be increased accordingly. The roller coaster shape of the pellet's barrel transit should look the same as the faster rise time data, just stretched out to allow the slower ADC to capture everything. This is another Saturday night beer infused thought experiment, but I'll try to reify it tomorrow.
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The HP 7090A system was put to the task today, but was found wanting. It was inspirational, though, because of the lack of noise on the curve. The short coming was the low 3.3 kHz bandwidth limitation of its ADC. I've been looking at some of the cheap Dataq instruments and may reinvest in a newer one of them for a followup of these time/pressure curves. I'm pretty sure that the release time can be detected with a faster and higher resolution ADC without the need for a probe down the barrel.
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The ability to see what the external hammer action was doing on the Vigilante hack was easy with an XCL attached directly to it. The CP2 in another matter because the hammer is buried inside the main tube. It wasn't hard to do the CP2 shim switch arrangement by milling a slot for hammer/valve access, but it still didn't give all of the sequence information nor do it reliably.
I'm going to try and install an XCL into the hammer and bring the leads out through another milled slot in the tube. this should give a blow by blow time sequence that will define the valve action with good timing information.
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George, with all that is going on with gas flow, I‘m not sure it will be easy to attribute pressure fluctuations to pellet motion. One possible check might be to compare the curves of a new pellet vs. one that has been pre-sized by pushing it down a barrel. The two should have enough starting force difference to see with the 8 bit system.
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I love those small accels. I was wondering if you could take one of your piezo disks and put it on the face of the hammer, covered with a steel disk. It may give a sharper valve closed indicator than the accel
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Stan - The mini 2250A XCL's are indeed nice little instruments. You got me thinking about what I'd be in for if I wanted to replace or add to any of the ones I've been using. I called Endevco to see what they cost. It turns out that they're over $1500 each! There's a 2% break for 11 of them. I think I'll start taking much better care of the ones I have.
On that note, your idea to use the < 1 cent piezo discs on the hammer is compelling. Originally the disc was going to be on the face of the valve as posted previously, but I chickened out because of the possibility of ceramic chips from the PZT crystal getting into the valve stem seal. With your suggestion of sandwiching the disc between the hammer face and a steel striker disc sounds much safer. Some sort of adhesive can bind everything together and hopefully contain the dust.
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I couldn't help myself, so a Dataq DI-1110 acquisition box is on it's way. It will replace the DI-155 box now collecting dust in the electronics room. I'm sure that it will be a useful addition to the ballistics experiments and many other projects. I'm not fond of their software, but it's usable if I buffer my exposure to it with a few beers.
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Looking forward to your thoughts on that unit. I have one of their old serial port units that I haven't done more than tinkered with. This looks more useful.
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The new Dataq acquisition system arrived. I had high hopes that it would work with the previous system's software, but it looks as though I'll need to install new drivers. That also means that the operating system will also have to be updated. There's no end to it all.
In the mean time I've been looking much more closely at the requirements of transient measurements using accelerometers. These measurements are considerably more difficult to evaluate than a continuous vibration signal would be. Having a lot of excellent equipment doesn't mean you'll get easily acquired good results. Many of these measurements have pitfalls that are so unintuitive and convoluted that it makes you want to just throw your hands up in the air and surrender. Fortunately, I have a plentiful source locally brewed strong ale to get me back on track.
What I'm finding to be perversely enjoyable right now is probing the limits of what I think I can understand about these measurements. For every hour spent doing experiments, I'm forced back into doing hours of studying the available literature just to evaluate the results of some new test. This usually yields just a glimmer of what I now need to know, but it does push me forward some to do more testing.
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The new Dataq acquisition system arrived. I had high hopes that it would work with the previous system's software, but it looks as though I'll need to install new drivers. That also means that the operating system will also have to be updated. There's no end to it all.
In the mean time I've been looking much more closely at the requirements of transient measurements using accelerometers. These measurements are considerably more difficult to evaluate than a continuous vibration signal would be. Having a lot of excellent equipment doesn't mean you'll get easily acquired good results. Many of these measurements have pitfalls that are so unintuitive and convoluted that it makes you want to just throw your hands up in the air and surrender. Fortunately, I have a plentiful source locally brewed strong ale to get me back on track.
What I'm finding to be perversely enjoyable right now is probing the limits of what I think I can understand about these measurements. For every hour spent doing experiments, I'm forced back into doing hours of studying the available literature just to evaluate the results of some new test. This usually yields just a glimmer of what I now need to know, but it does push me forward some to do more testing.
In other words, it's keeping you out of the pool halls and off the streets... ;D ;D ;D
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Since I'm trying to build some XCL's to test my understanding of how they work it seem practical to test how real ones respond to vibration. The setup shows how I'm measuring the displacement of a B&K 4384 transducer on the 4810 Mini-Shaker. The output of the XCL is sent to a B&K 2511 Vibration Meter with a Kronhite 3500 Band Pass Filter attached as an external pass through filter. The signal is also being monitored on the DSO.
By keeping the shaker's drive signal between 5 and 10 Hz the dial indicator seems to be able to track the XCL without breaking loose from the surface and the displacement indication is symmetrical and easy to read. This experiment in tracking the displacement is for my own edification to see if the electrical signal is the same as the mechanical test indicator.
There's a lot more to do to get a proper understanding of all this barrel harmonic testing, but my primary interest is to get accurate muzzle displacement numbers that are not distorted by phase angle lead or lag and the offsets of taking the double integral when looking at transient signals.
It might end up that a laser interferometer is the real answer to all of this, which should please Stan. In the mean time I'll keep playing with my collection of vintage instruments.
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George,
That mini-shaker runs open loop doesn't it? It does not have a control accel built in, right?
Thinking about the barrel measurements, I still think what will end up being useful will be the muzzle velocity and the muzzle angular displacement. The velocity should come out of integrating the two accel traces to the point of pellet exit. The angular displacement is harder. The low cost MEMS IMU type sensors they have for drones/quadcopters, I don't think they have the bandwidth. Maybe a pair of accels per axis near the muzzle. Or a laser/mirror/quad-cell detector setup, though I think one would need a larger quad-cell, the lower cost ones are pretty small.
As you say, a good thought exercise.
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The shaker has no built-in accelerometer, but if I mount the calibrated B&K 4384 onto the shaker's table the signal can be used to calibrate the signal conditioners and display instruments. The servo control loop is me. I can adjust the amplitude and frequency with excellent accuracy and repeatability.
The 2511 instrument is the current proving ground for the first and second integral test. Remarkably, it takes the integrals with what appears to be text book accuracy. This is reassuring, but the signals are all continuous sine waves at this point. The next step is to measure some transient signals. The 2515 analyzer will be able to sort out the 2 muzzle XCL signals as deconvolved FFT data and hopefully give the same accuracy with its integrators as the 2511 instrument.
The next consideration of the impulse signals is timing and phase shift of the signals after they pass through the preamplifiers and Kronhite bandpass filter. I know that the filter adds a phase shift to the signal which means a time shift is also there. This is an analog filter. It will be interesting to compare it with the DSO's digital filters.
Earlier, in post #228, the combined signals of the 2 muzzle XCL's were shown as a Lissajous pattern. The information contained in this XY pattern is remarkable to me because it demonstrates what the barrel is doing during the whole shot cycle in graphic form when looked at from the face-on view of the muzzle. All of the instantaneous information about the time and angular displacement of the muzzle can be tracked by the cursors and is displayed in the table at the right of the screen. The YT display at the top of the screen shows both signal in conventional O'scope form. This signal contains all of the A,V, and D information and now seem to be easily sorted out with the current bench instruments, but the information is from continuous sine waves. The graph in post #234 may be worth looking at again as a review.
What I'm after now is the accurate measurement of transient signals that only last a few milliseconds. The 2515 will do impulse analysis in both time and frequency domains simultaneously and may be the answer to how to make these measurements. As an aid to doing the impulse testing I'm going to put a vintage General Radio 1396-B Tone Burst Generator into the mix and see what happens.
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George,
What do the accel signals look like if you don't filter them?
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The barrel harmonics testing was back in play today. Stan wanted to see what the unfiltered signals look like compared to the filtered ones so here they are. The Kronhite 3500 Band Pass Filter was set to pass 20 to 500 Hz. Ch 1 (yellow) was unfiltered and ch 2 (blue) is the output of the filter. There is a phase lag in the filtered signal, but that can be compensated for in the final timing calculations. The filter's gain is unity.
An interesting part of the measurements was brought about by using the other power plant to do the testing with. The previous testing was done with the modified tube block which was altered to allow a hammer spring adjustment. It now lies dismembered on the dissecting table. That arrangement had allowed the breech clamping screw to tighten only the top side of the tube to the breech. Today's replacement main tube, which was left unaltered, is clamped through both sides of the tube and the spring block. The new tighter setup now has a resonance of 175 Hz horizontal and 148 Hz vertical. This was not anticipated. Barrel tuners take note!
Anyway, only the vertical XCL was used in these tests. Some preliminary numbers indicate that the barrel experiences a maximum of ~ 30 g's at 148 Hz and the displacement p-p is ~ .027" and the peak velocity is ~ 12.5"/sec. That's a higher displacement than I expected with this short of a barrel. Hopefully someone else out there is also making these measurements and can check my numbers. As an aside I'll mention that the higher frequency vibrations are of no consequence to these barrel harmonic measurements. They look enormous, but they're all well below 0.0001" in displacement effect.
Included with the DSO screen shots is a generic graph that was copied from the internet and shows how precipitously the displacement drops with frequency in regards to A and V.
The first DSO image is of a hammer tap used to force resonance. the second and third images show the actual shot induced vibrations.
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George,
Great, data rich, post as usual.
For the tap test, filtered/unfiltered results, is that two different taps or did you T the accel signal before it went to the Kronhite filter? What made you choose 500 Hz? I'm wondering if the tap test, using your instrumented hammer, could be used to evaluate the effect of the filter on time lag, velocity, and displacement for the first 3 msec of the response
As you mentioned, the results for the shot trace came up with fairly high velocity and displacement, how were these calculated? Also, is the muzzle exit still at ~3msec or did the hammer preload change affect that?
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Stan - I'm still staring at the images myself. I think that there may be more in there than what I assumed. The 3 images are all captured by triggering on ch 1. Both signals are, therefor, simultaneous with each other. Any time differences between the signals are from the filter's delay. The 500 Hz cutoff frequency was just for convenience since I only wanted a window to look at the 148 Hz vertical resonance.
What interests me most now is the action in the first 5 ms after triggering the DSO. I didn't use the exit detector on any of these tests, but there is no chance that the pellet hasn't left the barrel by this time. Most intriguing is that the the first 2 cycles of both the hammer tap and the pellet shots are at a higher frequency than the natural decay frequency of the barrel. This would mean that those first 2 cycles would be what really determines the position of the muzzle when the pellet exits and not the calculated or measured first resonance mode usually represented as a continuous or exponentially decaying sine wave.
For the time being let's forget the G, D, and V calculations. I've had a few beers, so I'll now confess that the numbers were arrived at through the spread sheet method..... because it's so easy. Dam_ those things! I've been snookered again!
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George,
Just to clarify in my mind, the Ch1 and Ch2 traces are the two horizontal and vertical accels as you have them mounted in post 264 or is it just the vertical accel and on the DSO you captured the input (Ch1) and output signal of the Kronhite?
I understand about the G,D, and V calculation method....I thought I detected a disturbance in the force
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The XCL's are still mounted as in #264, but only the signal from the vertical XCL is used for the above tests. It's signal is connected to the input of a Kistler 504E Dual Mode preamplifier. The output of the preamp is then teed into a direct connection to channel 1 of the DSO and also to the input of the Kronhite filter. The output of the filter is then connected to channel 2 of the DSO. The DSO is triggered by the channel 1 signal. This arrangement keeps any effect that the 504E may have to now be common to both signals.
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It might be useful to further expand on the timing offset of the 2 channels of data from the barrel's vertical XCL. As can be seen in the DSO's image the channel 2 signal (blue) coming from the Kronhite filter is delayed by 500 µs compared to the unfiltered (yellow) signal. This amounts to a significant part of the time that a pellet is actually in the 8" barrel and must be accounted for in the harmonics measurements.
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How does the filter frequency affect the delay?
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There is a varying phase shift within the pass band of the filter so any signal being measured must be corrected by comparing it to an unfiltered signal. This is pretty straight forward because the signal generator can be used as a timing reference. The generator and filter have a flat frequency response between the cutoffs.
I played with the DSO's digital filter using the same frequency cutoffs as the Kronhite and it was pretty messy. There didn't appear to be any phase shift, but the response wasn't flat and there was considerable distortion at some frequencies. Maybe I just need to spend more time with it.
Stan, this would be a good exercise to try on your DSO for a comparison.
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Yep, I need to explore the math functions
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While going through some DSO images from earlier posts I got distracted by one that showed the P/T curve of the 4 shots that were averaged. The image showed what the pressure changes in the pellets transit down the barrel looks like if most of the noise was removed from the pressure data. To make the measurement more interesting (to me) the image was rotated to make the down slope side of the graph horizontal.
All of the original image's data is the same, i.e. the time window from the valve starting to open to the pellets leaving the muzzle was ~ 2.8 ms; The pressure max was ~ 500psi; time to peak pressure was ~ 500µs; the barrel length is ~ 8"; the pellets release times and pressures were ~ in the middle of the increasing side of the pressure curve.
The actual values and uncertainties of these measurements are unimportant to the point of this observation. What we do know for sure is that the pressure transducer's rise time is more than 100 times faster than the pressure rise it's measuring. We also know that it's non-linearity, non-repeatability, and pressure hysteresis are below what the DSO's ADC can resolve.
If we now look at the rotated pressure curve It's clear that the pressure is rising and falling alternately as the pellets progress towards the muzzle. If we look back at the data that was collected in the pellet pushing tests done using a load cell and the Vigilante barrel (Hacking the Crosman Vigilante thread), the same type of behavior was observed. Lot's of room for speculation here!
What I'm getting out of this at the moment is that Stan and I need to get back to the barrel probe tests. I got distracted, as usual, after starting my low mass wire probe design and it fell by the wayside.
Yesterday I went back to making a few probes and was surprised at how well they worked without the supports that we were using in the earlier tests. The original probes that were placed a few mils in front of the pellet's face were not intrusive when testing the moment that the pellet was released and they worked very well. The new probes now have to be moved incrementally down the barrel and still not interfere with the measurement. The trick now is to allow unimpeded air movement in front of the pellet as it moves towards the probe. Anything past the point of contact is of no consequence, as long as you have a proper scatter shield. Stan and I both agree on that point!
Putting another pressure transducer at the muzzle will probably be an interesting addition to these experiments.
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Time to dust off the test rig and my brain and collect some data.
George, you did some clean up of the breech end of the barrel you are currently using for the pressure tests didn't you? Did you do a push test on that particular barrel after the clean up? I'm still wondering how much the pellet sizing in the leade affects the pellet release pressure and time.
I may be able to check the time (but not pressure)....time to head outside to my probe supply.
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Stan - I've been playing with the wire probes and am having good success with 16 mil copper that has an overall thickness of 27 mils including the insulation. I put a 12" length in a vise and pull on it slowly with a pair of pliers until it breaks. This makes the wire very straight and it work hardens the copper. The wire breaks at the pliers end and the insulation shrinks back about an inch from the end where the pliers were pulling it. This avoids having to strip back the wire for an electrical connection. The other end is cut to length for the test and doesn't need to be stripped. If you can see the copper at the cut end then the face of the pellet will find it on initial impact if you use wadcutter pellets. This will avoid inadvertent triggering of the DSO from premature contact with the barrel due to having stripped the wire back. No moncot support is required unless you're just doing the release time measurements.
I realize that there's no LASER involved in this approach, but it's always good to have a backup plan.
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Back in early April (#127) I was testing the possibility of using micro LED pairs to make light gates for detecting a pellet's transit down a barrel. The LEDs worked well and had sufficient sensitivity to see a pellet passing through the gate. The down side was that I'd have to drill holes in the barrel at intervals down it's length. This would also entail deburring the holes so that they didn't interfere with the pellets smooth (?) transition. This method would probably bring more uncertainty baggage to the measurement than would be practical to sort out.
After Stan and I were getting some good numbers when timing the first movement of the pellet it was obvious that we were onto a much better way to get timing information with probes down the muzzle than drilling holes in the barrel. The problem now was that for any detection of pellet movement greater than a few mils, the probe itself would grossly interfere with the measurement.
Now that I've put the barrel harmonics onto the back burner while the XCL measurements are being sorted out, I decided to go back to measuring barrel transit times. A couple of days ago I posted some info on making stiff low mass wire probes that didn't need any extra support. Since the barrel diameter is ostensibly 0.22" ID and the wire is 0.027 OD the ratio is in favor of much better numbers. After several tests to confirm that this new method was viable I made some probes of assorted lengths for a new round of testing.
The method of making the probes was discussed in the previous post. The inadvertent plus to this method is that the wire stretches and maintains its new length, but the insulation shrinks back to its original length when the wire finally breaks at the pulling end. This leaves a perfectly stripped and exposed end of the wire. If you've ever tried to strip the insulation from very small gage wire without damaging the wire itself you'll know what a time and headache saver this is. Also, these probes only take seconds to make and are as expendable as the pellets that impact them.
The photos show a few of the probes and how they are mounted onto the end of the barrel. The muzzle end of the wire is formed over a simple fixture to keep the length fixed and the o-ring keeps the probe in place until it is impacted. What happens after the impact is inconsequential and no longer part of the measurement.
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George,
Those probes look great, I'll have to add them to the arsenal so to speak.
I've started to dip my toes into the DSO's built in filter functions. This is not something I have a lot of background in so my controls manipulation is more Etch-A-Sketch than a technical review.
I dusted off my trusty Signal-O-Matic, checked for the NIST traceable cal sticker...nope still not there...and punched up a 1 Khz triangle wave.
I turned on the DSO's internal math and set it to low pass filter (2.5 Khz). I did not see any lag between the two traces. Tried a few other source and cut-off frequencies with similar results. I need to apply it to some of the transient responses to see how it works....and read up a bit on the buttons that should be pushed
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Stan - I had overlooked you're inquiry a couple of days ago in post #298 about whether the .22 barrel had gotten the same breech treatment as the .177. Yes it did. Post #20 shows the "before and after" results of the push tests that were done back in January.
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The wire probes are coming along nicely. I did a few tests this morning using CPHP pellets. My preference is to use wadcutters because of the flat face they have, but ran out of them last night.
The images show the results of 4", 3", and a 2" probes. The last image is a look at the rise time of the contact signal on impact. As can be seen using a 50 µs /div window the rise time won't even come close to interfering with the time of impact data.
The pressure for each shot stayed at ~ 500 psi. The vertical scale is 100 psi/div. The pressure rise part of the curve shows some interesting bumps that appear to be very quick and small pressure decreases with quick recoveries. I predict that these may be the actual pellet movements. Image 2 shows no distinct pressure reverse steps in the rise part of the curve and the pressure peak is lower than the 1 and 3 curves. Each shot had plenty of time to recover from the previous shot. The temperature was constant in the testing area.
The lack of steps and lower peak pressure of shot 2 may be related to CO2 blow-by before the pellet is fully seated in the rifling. This is the stuff I'm interested in looking at when the Dataq 1110 is up and running.
Lots to amuse one's self with here. I didn't use any cursors on any of these shots because that info, if anyone is interested in the numbers, can be gathered from the grids. These are only early tests of this probe method and I didn't want to clutter up the screen and disrupt how clean and unambiguous this method can really be.
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George,
Yep, not much ambiguity on the timing marks, those curves look great. Is the reason the probe signals roll off because you are powering it with a meter?
I wonder if the bumps on the pressure rise are some LCO2 spittle flashing
I am amazed how relatively repeatable the pressure curve is. That leveling off at ~2.3ms is consistent and occurs before the pellet exits
Great stuff
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Stan - There's lot's of stuff to speculate about in these tests. The contact signal is now using just a 9V battery as a power source. The decay side is after the fact and only of curiosity value, though it may be interesting. The wire is being pushed down the barrel and may or may not be in intimate electrical contact with the pellet. The only thing I'm interested in at the moment is the timing at the point of contact. As an aside, I've fired the pistol without a pellet loaded and then inspected some wires. They stayed straight and unaffected by the blast. I consider this to be a big step forward in getting good numbers from these probes.
The pressure transducer and associated amplifier are much faster than the powerplant so what you see in the P/T curves is real.
I'll wait until the DI- 1110 system is in harness before commenting on more than my current guess about the upward pressure spikes. In the mean time I'm interested in putting another pressure transducer at the muzzle and measuring the P/T curves before and after the pellet leaves the barrel.
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Stan - I've been thinking about the gyrations that we've been going through to do the timing on the hammer launch, valve stem impact, valve opening time, pellet release, etc. It certainly has been a good circus. Now I'm thinking about what's really important in the overall scheme of things.
The shot cycle could be considered as starting at several point. I'm going for compacting everything down to my own most convenient assessment and say that the real cycle, as far as the pellet and I are concerned, begins when the valve starts to open. From then on the action really gets going! Any part of tuning an airgun that is applied to things like hammer or valve mass, spring coefficients, lubes,or anything else that may effect how and when the valve stem is impacted can be measured. This response to the impact can be determined by looking at the P/T curve. The P/T curve also communicates to the pellet what to do and when to do it. The pellet's response can be followed all the way until it leaves the muzzle. Any barrel treatments should also be evident in the curve.
This notion of all encompassing information in one graph may seem like a stretch, but I think that with sufficient resolution in the curve measurements it can be done. I'm interested in any arguments against this proposal.
As can be seen in the DSO images, even with just 8 bit resolution there is still a lot of information there. The main thing is that the pressure transducer signal is clean and fast compared to the other heroic methods Stan an I have been working on.
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George,
I agree that any of the hammer information is only really important in how it impacts the time initial valve contact and then the stroke and dwell time of the valve.
The pressure curve you are able to generate is where the action is, and it is in general unique in air gun investigations ( I think I found one paper where they instrumented and airsoft gun in a similar way). The curve is also all encompassing and benefits from additional event markers to help dissect what is going on. The hammer/valve contact and the start of pellet motion are two good examples.
The zoomed in pressure plots you posted have some characteristics that with a sensitivity of 100 psi/v represent about 5 psi deviations from a likely curve fit. With the pellet motion starting around 400 micro-sec, these sections of the plot are well before pellet motion. It is hard to further identify their cause. The flow through the valve and transfer port is anything but smooth. There may also be some liquid CO2 droplets that change phase. Once the pellet starts moving down the barrel, the pressure fluctuations may have an additional contributor in the varying pellet friction and anything that may happen in the air in front of it. How to dissect these combinations gets to be an intriguing challenge. The multi-sampling you did earlier is one step in the process.
I also think the relationship of the pressure curve to the time of pellet start is an interesting area. There is a large collection of bolt probe designs that try to affect the gas flow and the positioning of the pellet relative to the transfer port, that this data would shed light on. Similarly the affect of the pellet fit in the leade and pellet sizing would greatly benefit from direct pressure data as opposed to the more common chrony data.
I'm looking at ways I can contribute data with my testing but you have a truly unique and important test capability.
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OK, as a side note, it seems fair to also report the fails and dead ends.
A while back, I thought I found a pressure sensor (image 1) that could be put to use in this testing. It had some positive features:
<$20, 0-1000 psi measurement range, simple 5V power, 250 psi/v out, relatively compact with 1/8 npt threads, and "ceramic pressure chip sensor inside". Before I tried to make some kind of a test breech for the 2240, I tried to see what the response time of the sensor is. I thought if I could create a hydraulic shock, I might be able to see a fast enough response.
Image 2 shows the sensor with a 1/8-1/4 bushing, I made a plug that fit inside the bushing, filled with water and tapped on the plug with the George approved tap hammer.
After the first couple of taps I thought we had a winner but tapping it with the brass hammer, I was just putting some electrical noise into the system (image 3). With some insulation on the hammer, I did get a real response but unfortunately it comes in 2 ms chunks (image 4). Too slow
The sensor is nice and I'm sure I can use it for some other slower measurements but for this testing, it is going back on the shelf.
Unless I find something else, George is our pressure guy.
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Stan - Your image 3 with a 0.5 µs/div capture would have gotten me all excited, too! < 20 bucks, wow! Sorry it didn't work out. I'll readily admit that the testing of any new transducer of any type is always worth some level of excitement to me.
I'm glad that you've become another active experimentalist in this airgun ballistics arena. It appears to be uncharted territory for the advanced hobbyist.
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This morning a couple of shots were taken without using a pellet. I wanted to see what the P/T curve would look like with just the valve opening and closing. The first cursor is at the start of the opening and the second one is at an inflection point where I'm guessing that the valve has closed. This knee may indicate where the plenum is no longer being feed by the powerplant. the time is 2.4 ms. There is another knee that is more like a step at ~ 1.5 ms. My guess is that the valve just turned around and started to close at this point.
Of course there are other things that can be read into this image, but more resolution will be needed to tease out better guesses. I tried to get the DSO's math function to plot the derivative of the curve to accentuate the points of inflection, but without success. I'll have to spend more time figuring out why it wasn't successful.
In the mean time anyone can read anything they want into these digital tea leaves. Keep in mind that this curve was made without a pellet in the mix and therefor probably altered the timing some, but the numbers are real for the situation. My guesses are likely to change with more detailed information.
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George, early this month you were thinking about instrumenting the hammer. We talked about piezo and striker plate instead of the accels, another option might be a springier contact shim that's insulated on the hammer side and releases from the valve stem when the hammer retreats. Either of those may be able to identify the valve close event.
I'm not sure if these DSO's do any noise reduction before they take a derivative. If not, the derivative may be busy.
I really like these pressure curves, there is always new data in them
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The shim contactor did work well in the trial runs, though it had some noise issues. I got distracted by the piezo/hammer detector idea and stripped down the powerplant to do some of the hammer and main tube mods that would be needed to implement it. The parts are now in a tray of formaldehyde on the dissecting bench.
Since the hole for the shim has already been machined into the main tube I should go back and spend a little more time in reassembling the shim project more carefully in the configuration that you're describing. My problem with that is how embarrassingly simple that method is. As a final insult it will probably work really well.
I'll try to swallow my pride and put the shim experiment back together this weekend. As a cataplasm I'll now refer to the method as being elegant instead of simple.
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I did some tests today using the shim impact method and decided that it doesn't have a timing consistency that can compete with the pressure measurements. I'll try again with the piezo detector/striker plate method to see what it can do. I now think that maybe I'm on a fool's errand with valve timing measurements because all of the information about the valve workings up until the pellet moves is best seen in the P/T curves.
I well understand the siren call of cheapskate measurements because I try my hand at it often. At some point, though, it has to come down to what are we really trying to do? Stan and I have offered up several ways to make affordable airgun ballistics measurements, but it hasn't drawn anyone into the hobby on that level yet. What's the incentive? At some point I'll just return to doing the experiments with the best instrumentation I have available and continue to report my findings.
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George, Sorry to hear the shim-flexure didn't give clean data. I had hoped that with the shim-flrxure, heavily biased towards the hammer, would mark the valve closed event. I don't think of it as competing with the pressure curve, the valve-closed point would be a great addition to the Pt curve.
As far as drawing anybody into testing, maybe not, but Lloyd is doing some interesting testing in the Bob & Lloyd gate. At some point the data streams will cross.
I'll dabble in the low cost end, If you got better instrumentation for some of this, apply it, getting the data is the key
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Since the previous post #202 shows that the valve timing can be done with a shim switch it will be worth returning to that testing at some point. What the switch shows is a clean signal of when the valve face is contacted. This is the time that the valve is fully open. The total time for the valve to both open and close is yet to be determined. The method is easy to implement and will work if it is broken into 2 separate shim switches. The problems that I ran into yesterday had to do with using too thin of a shim stock.
I've had to remind myself that the valve timing is of little interest to me at this point except maybe the interest in making the measurement. I'm still an adherent to the idea that all of the important timing information is contained in the P/T curve including valve timing.
This all needs to be sorted out and the ballistics and DSO are now sending me back to the calculus books to get rebaptized. This is a good thing.
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OK George, Since I only had a weak lager with dinner instead of the memory enhancer, how is the cosine function related to the first derivative of the P-T curve?
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Well, it looks as though the strong ale memory enhancement only got me back as far as the barrel harmonics testing. Since the Diff function on the DSO didn't seem to be doing it's Job I fell back on making the sin(x)'s derivative, cos(x), to be the stand in. If we were doing accelerometer graphs it would have looked pretty good.
This morning I came partially to my senses and realized that I wasn't getting the slope information I wanted. I went back to the DSO Diff function and tried again. The problem I was having, aside from ignorance, was that the DSO was working overtime on the limited low resolution 8 bit data. I lowered the number of data points for it to collect and it really works well now. There's hope yet. I still think that the real shot cycle truth is buried in the derivative curve.
The image is of the DSO's calibration square wave and the slope information the diff function is capable of retrieving. It appears that I'm the weak link for the time being.
Thanks for continuing to keep me awake along the path, Stan!
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To be honest, I had a quick try at the DSO's calculus functions last week when I was trying the filter and was not successful. I think I need to follow your lead on how much I ask the DSO to process. The derivative will be interesting in trying to manage the noise without smoothing out the features of interest.
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Hello George,
About a week or two ago, Stan said your thread was something I needed to look at.... and he was right! ;D I had totally missed it as I've been off line for a while. Great work George, and a wealth of information, and of course, a wealth of new questions, too. I have only gotten through about half of the thread... it does take some time to digest, and honestly, some of it is out of my knowledge comfort zone, so I have to read up on what the heck you are saying, LOL. But, a day without learning something new is a day wasted. ;)
Your explanations are great, and I can certainly appreciate your stream of consciousness side trips where you get off into the weeds, but eventually make it back to the task at hand, whatever that really is. ::)
Your arsenal of electronic toys is impressive, but what is more impressive, is that you actually KNOW what to do with them, or dream up new ways to put them to practical use on this project.
I'll be catching up and following along to see what yummy tid bits you are throwing out to us. Kudos on your passion for the effort, and your willingness to share.
Lloyd-ss
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Back in April (post #180) I was playing with piezoelectric sensors and came up with a donut type that could be placed at the muzzle. I wanted to use it as a simple DSO trigger signal generator for when the pellet left the muzzle. Due to the number of distractions that I constantly surround myself with, the detector got sidetracked.
This morning I was shuffling things around on one of the test benches and the device rematerialized. Now that I'm consumed with pressure measurements, the original test as a pellet detector had now transmogrified into a slick and simple way of detecting and measuring pressure pulses at the muzzle.
It was a simple task to put it in front of the barrel and take a few shots through it. I was surprised at how well it worked with no changes to the original geometry. The duct seal putty is there to damp out the resonances of the disc, washer, and vise. The detector is in a vise for these first test runs, but it may get an smaller adjustable stand of its own. A pair of these would make a good muzzle chronometer. The need to move a typical chronometer away from muzzle because potential gas disturbances, particularity with CO2 guns, is negated because the detector ignores the pellet and just measures the impulse disturbances.
This sensor is fast and sensitive and I can see many hours of further distraction using it. Of great pleasure right now is to see how well presented the initial barrel air plug blast is captured. This plug is the same one that is shown in Stan's video (#181) as the very early shock wave. The shock wave has some energy number associated with and may be useful for the spreadsheeters out there looking for hidden drains on efficiency.
The last pulse coming out of Stan's video must be hammer or valve bounce and is also detectable on the P/T curve and the piezo signal in one of the DSO images. I'm still very impressed with Stan's excellent video post!
Rather than describe each DSO image and photo I'll just give a general idea. The photos should be self explanatory, though the DSO images are probably only a gourmet mystery meal for Stan. Others are also welcome to come in and help yourselves. The images are a sort of medley of several shots. Some are with and without pellets as the powerplant pressure drops. Some are with a new powerlet installed, again with and without a pellet.
I'll clean up the experiments with this sensor and present more coherent info sooner or later. Right now I'm going out back to shoot off some contraband firecrackers and shout out some July 4th hurrahs!
Lloyd - Welcome aboard!
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George, that is providing some nice data, I'm glad you rediscovered it. Were you ..errr.. running out of gas on the first 4 DSO's? The transit times are stretched and the pressure curve is depressed.
The last two traces are intriguing. Do you think you could fit your instrumented pellet catcher behind the sensor to get a timing mark for the pellet (or maybe a broken wire across the muzzle)?
Note my only contribution to the 181 video was a bit of google time.
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Stan - The last 2 images are with the sensor moved right up to the muzzle. That's why the air plug impulse is so attenuated. The timing should be very close to what the force transducer has measured.
The photos with the sensor at the muzzle weren't very informative about its build and arrangment so I didn't include them.
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I was wondering if the pellet was leading the activity around 2.7 msec or if there was a significant chunk of high speed air leading it. Kind of gets to the airmass discussion.
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If you go back and watch the video at .25 speed I think that most all of the information may be there or at least enough to remove a lot of the guess work.
It still leaves a lot to be deciphered, but that's why were doing this, I hope.
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yes, even at that speed I am always surprised how quiet it is between the smoke ring and just before the pellet.
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I did a few shots using the piezo sensor and a wire at the muzzle. The sensor was given an extra dose of putty to reduce the sensitivity to resonating. The wire was a truncated version of the wire probes with only a 90º bend at the business end that crossed the muzzle. This makes the wire easy to form and needs to be electrified at only one end. This also allows the disc and wire to be placed in close proximity to each other.
There was not a lot of time spent on the positioning of the 2 detectors, but this looks good for now. The DSO was triggered on ch 1 (pressure), ch 2 is the wire, and ch 3 is the disc.
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That is nice data. Interesting contrast to the no-pellet trace (DS0008). Looks like not much happens at the muzzle before the pellet gets there.
Doing a little wild speculation and hypothesizing.....
In DS0008 the piezo rings at about 0.6 ms. For an 8 inch barrel, that's right around 1100 ft/s ... Initial shock from the valve opening?
In DS0019 the little blip on the piezo curve is 1 ms out. Back in DS0003 (post 245) it took ~ .4 ms for the pellet to move so that gets back to .6 ms. So out here on the limb it looks like when the pellet breaks stiction it generates a mini sonic shock that the piezo picks up....
......unless of course it isn't
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I was curious about what the actual rise time capabilities of the pressure sensor might be in relation to the valve starting to open. The DSO was zoomed in to get a sub-microsecond view of the first start of a pressure rise. There's more that can be done to refine this measurement approach, but I find the info useful for now. This is just a quick snapshot.
The time scale is obvious from the grid. The pressure scale is the usual 100 psi/V.
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As long as the zooming exercise is ongoing I thought I'd look at the P/T curve more closely along the peak. It turns out that what I had previously suggested was probably turbulence while the valve was open is actually periodic.
The DSO images are of 2 different shots without a pellet. The first one is without any signal filtering and the second one is with a low pass 40 KHz digital filter from the DSO. I'm starting to get a feel for the capabilities of the filtering app that the DSO provides. With the filter it's clear that the ~ 20.8 KHz signal is being modulated by a lower frequency signal.
The 20.8 KHz signal is also on the peaks of P/T curves when a pellet is shot. My idea on this outcome is that there is a whistle being formed by some fixed part geometry somewhere in the power plant. Maybe it's the transfer port. Maybe it's the valve assembly. The signal's amplitude actually gets larger as the power plant pressure decreases, but the frequency stays the same. I'm liking this scope more every time I use it.
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George, it is great to see you are back at it. All I've done during our recent heat wave is make a list of the cases I want to run. Note to those of you in the south or southeast, replace the word heat wave with pleasant summer morning.
I love the data you get from that pressure sensor
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I've replaced the image in post #328 with the proper image that's shown here. I consider this test to be a milestone measurement regarding when the action really starts.
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Stan has mentioned the use of a quadrature cell and laser to do some barrel harmonics testing. I'm of the opinion the the information is obtainable with the use of a pair of very low mass accelerometers. Being a test and measurement type of guy, I'm always interested in trying new measurement techniques. There are few things in life, at my age, that are as satisfying as a pint of locally brewed IPA, and the correlation of 2 different testing methods that produce complementary experimental results.
To that end I decided to invest in a Keuffel &Esser 71-2627 quadrature sensor with target. It was another eBay sleeper ($20). The sensor is simplicity itself and works essentially the same as a group of solar cells. It's activation requires only a collimated light source and a mirror. The output signals can be routed to the previously discussed NIM instruments that were going to be used with the LED barrel timing experiment. The processed signal can then be routed to the DSO and be displayed in the XY mode as a Lissajous pattern.
The measurement can be made with a conventional light source and some optics. As a gesture to Stan's enthusiasm for the quad detector technique I'll fire up a vintage He-Ne laser for the test.
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$20 ??!! Wow, that is a great find.
I think the optical data complements the accel results, providing slope data that the accels don't measure.
The old school HeNe will be great to see.
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Stan - Could you expand on your slope comment?
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George,
I was envisioning a small mirror rigidly attached to the muzzle and at right angle to the boresight. With the laser source and detector on or near the breech (your clamping looks pretty rigid), the laser dot on the detector should only (ideally) respond to changes in tilt of the mirror and be insensitive to lateral motion. If you can synchronize the detector output to the pellet exiting the muzzle, the change in the tilt of the mirror should represent the change (compared to pre-shot) in the slope of boresight at the muzzle at the time of pellet exit. This pointing angle change would be one of the contributors to errors at the target (the other possibly being the muzzle lateral velocity from the accel data).
Some evaluation of the sensor for an optimum spot size will be needed. You could also add a second rigid mirror at the breech to set up multi bounce to increase the motion on the detector. Of course with longer path lengths you are more susceptible to air convection from temperature gradients caused by the presence of say a large cold beer.
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The distraction of too many projects has taken it's toll on airgun testing of late. One of the things that has brought me back to contemplating some new testing has been the results of earlier barrel harmonics measurements. The results contained in 2 DSO images that were originally shown in post #288 back in June have haunted me ever since.
In post #290 I commented:
"What interests me most now is the action in the first 5 ms after triggering the DSO. I didn't use the exit detector on any of these tests, but there is no chance that the pellet hasn't left the barrel by this time. Most intriguing is that the the first 2 cycles of both the hammer tap and the pellet shots are at a higher frequency than the natural decay frequency of the barrel. This would mean that those first 2 cycles would be what really determines the position of the muzzle when the pellet exits and not the calculated or measured first resonance mode usually represented as a continuous or exponentially decaying sine wave."
In both the hammer tap and the powerplant excitation a higher frequency oscillation can be seen to occur within the first ~ 3ms. The pellet can still be in the barrel during this time window and therefore never actually sees the real barrel harmonic motion. Since I didn't have a solid explanation for this initial barrel activity, other than seeing it was there, the data was generally ignored and things moved on.
Inadvertently, I've recently run across a wealth of published information about "forced oscillations and resonance". Of particular interest is a set of lecture notes that can be searched at:
Notes on Diffy Qs
Differential Equations for Engineers
byJiˇríLebl
In section 2.6, figure 2.7, I found the answer to my measurement dilemma. It's now clear (to me, at least) that the pellet experiences mostly the forced oscillations of the transient or impulse and not the barrel's natural frequency!
Spreadsheet rebuttals are always welcomed.
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George, I understand the interference of projects. Thank you for rescuing my brain from the - setting pavers and painting wood - cage it has been trapped in lately.
A while back I ran some numbers (spreadsheet...sorry) for just the CP-1 barrel fixed at one end. There was a mode at ~1,400hz. With the extra compliance in the breech, it would come down some and should be what you are seeing in the unfiltered tap test data.
Characterization of the forcing function over the ~3 msec the pellet is in the barrel is an interesting question. An ideal impulse is a single event and rings all the barrel harmonics. In the firing cycle there are several events in that 3 msec period: hammer strike, pellet release, pellet accelerating down the barrel, etc. How these interact with the barrel modes is not an easy assessment. The accel is sensitive and picks up a lot of the noise sources in the cycle. The really high frequency stuff won't generate a lot of displacement and probably won't create much lateral velocity. If your test rig is operational, getting a trace with the pressure curve and the accel data, and maybe your pellet exit trip wire would help shed light on whether any of the cycle events line up with the accel response. Eventually may have to introduce the DSO's low-pass filter to get rid of the very high frequency noise.
Unfortunately, I can't try this with the 2240.
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Now that I know what I'm looking for the measurements can be redone using the DSO filtering capabilities.
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The barrel harmonics project is getting increasingly interesting now that I've gotten better at using this new DSO properly. I've shifted my enthusiasm for analog signal filtering to becoming much more enamored with this scope's digital filtering capabilities. Being able to switch the DSO's filter in and out even after the signal has already been captured is really valuable for these vibration measurements.
I'm starting to get a better idea about the nature of the how the initial force is being applied in these tests. The definitions of these forces are becoming clearer in my mind and how to most properly use them. The reading that I've been doing lately has narrowed down my impression of what a transient is compared to an impulse. My current understanding is that a transient event is something (a force) that disturbs a system in a way to momentarily alter it's steady state condition. On the other hand an impulse is a single event that would look like a step function where the energy is deposited as a single nonrecurring event. A steady state condition in these barrel tests could be one where the barrel is at rest or one where it's running at one of its natural frequencies which would be the case of simple harmonic motion.
Stan has suggested that a transient event could be initiated by the sum of several forces that occur during the brief firing cycle. He also pointed out that an impulse could be initiated by a single hammer tap on the barrel. Both situations are capable of producing forced oscillations of the barrel that are unrelated to the barrel's own natural frequencies. These forced oscillations only last a few milliseconds before the system restores itself a to steady state condition, but that time interval can be close to the time when the pellet is still in the barrel.
Today's testing has captured some very interesting information that may reshape certain notions about what is really going on with barrel harmonics. I'll leave things here for now because I need to sort out all of the DSO images and also want to here any feedback about my understanding of proper nomenclature regarding what and how these things can be measured and explained.
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Here are 3 DSO images that illustrate the effects of forced oscillations on an 8", .22 cal. barrel. As can be seen the barrels natural harmonic (~ 800 Hz) takes over fairly quickly, but the first ~3ms of vibration are overwhelmingly erratic. This time window would be when the pellet is probably still in the barrel. These forced oscillations can probably be damped out, but I don't see any possibility of them being "tuned" out.
Please enlighten me if I'm wrong.
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George,
These are great curves.
I am surprised the barrel first mode in the ~150-200hz range didn't pop out in the tap test, perhaps the fft would bring it out.
It is interesting to follow the timeline. Looks like a small initial disturbance at hammer strike. Maybe a bit more at pellet release (~250 psi), and the bulk of the build-up once the pellet builds up speed down the barrel. If you get a chance to get a shot without a pellet for comparison.
I noticed the pressure peak went up a 100psi or so...a little warmer in Carlsbad than it was in May.
Great work
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George,
What is the g/v for the accel trace as you have it configured?
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Stan - The CO2 powerplant is making a good vapor pressure thermometer these days. Other projects are diverting my attention and I'm not putting much time into these measurements, but I can see some time becoming available to get back into it again. I've done some spurious measurements to keep it interesting when the spirits (?) move me, so I've rearrange some things on the bench.
The last couple of posts were about measurements made with the vise mounted .22 cal. pistol. The DSO's digital filtering is looking to be very good. There is no phase shift and things can be adjusted on the captured raw data. This is very handy. To get a better handle on the measurements I've now put the .177 barrel and breech block (sans powerplant assembly) into the vise to simplify the amount of measurements that are taken for each test. I'll just use the hammer for these tests.
The .177 arrangement doesn't have the pressure transducer installed, so we're just looking at the accelerometer in the vertical axis. You were wondering about the signal for the first harmonic. It's actually there, but is not well represented because of the short time window. These next tests with just the breech block and barrel will show much more of what's going on when the time windows are expanded out to longer snap shots.
The XLR has a nominal sensitivity of 10 mV/g. The data now is probably good within about +/- 5% if the Kistler signal conditioner is functioning properly. All of this instrumentation needs to be calibrated at some point.
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Here's a better DSO image of the two .177 modes when the breech is tapped lightly and shown in a time window that's 10X longer.
Things on the test bench have been dormant for a while due to other more compelling projects. I'll be setting up a new test bench that will give me more room to do these experiments. My wife has been bugging me about a new kitchen. I'm going to surprise her with a new lab setup instead. I thinked she'll be thrilled!
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In the course of setting up a new ballistics test bench I've decided to reduce the possibility of extraneous vibrations to a lower level then the table and vise offered out in the machine area. The approach is to use a high enough mass in the clamping arrangement that any vibrations from the vise will be well outside the frequencies of interest while making measurements.
The new configuration consists of a 4" vise bolted to a rectangular section of 1/2" thick steel tubing. A block of lead sitting on a piece of polymer damping material is placed inside the steel frame. The arrangement, as it sits now, weighs a little over 108 lbs. Once it's in position on the new bench I'll be able to use a hammer and an accelerometer to evaluate where to place more damping material if necessary.
The CP2 rifle barrels on the .177 and .22 versions haven't been looked at very closely yet and I'm interested in measuring the amplitude of forced oscillations while the pellets are still traveling inside these longer barrels.
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The new Ballistics bench was open for business today so I though I'd try making some measurements. The quest at the moment is to try and offer up some tests that have inadvertently shown up and accumulated using the previous version of a test bench. The decision was made to redo some of the P/T measurements.
The .22 cal. pistol was reassembled to the factory configuration in order to make realistic measurements, though the pistol was clamped into the new vise arrangement and not handheld. The vise was employed to keep the test setup reproducible.
Today's effort started with cleaning the pressure transducer and using the nifty calibration rig described in a previous post (#169) to make sure that it's working properly. I'll have to say that the Crosman Powerlets sure dispense some grungy gas.
The DSO images again demonstrate that the initial forced oscillations from the powerplant will swamp the barrel resonance for at least 3.5 ms in these tests. This further confirms that any form of barrel harmonics tuning would be ineffective when trying to control the chaos unleashed in this time window.
The same test was done using the "thing" (I'll refer to it as an LDC) that is included in the CP-2 kit. The added mass of this device can be readily seen in the resonance shifting to a lower frequency.
The next step will be to attach the trip wire to the muzzle so that the actual pellet exit time can be overlaid onto the P/T curve.
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These DSO images were acquired with the same setup as the previous post with the addition of channel 3 being the trip wire at the muzzle. The exit point of the pellet's head is the first vertical purple line. Today's testing was all done with CPHP pellets.
The filtering is as described in the previous post.
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Now that the system seems to be working properly I decided to make some pellet release measurements. The DSO images are made using a CP Domed pellet. A wire probe, as described in post #300, was used as the pellet release trigger. A domed pellet isn't the best choice for these tests, but I've run out of wadcutters.
Anyhow, the cursors mark the pressure and time info which turns out to be 204 PSI at release and 205 µs after the valve starts to open. There is still the pressure bump that tends to stand out on the up side of the pressure slope that needs to be explained. Stan suggested that it might be some CO2 liquid or ice getting past the valve, but my guess is that it has something to do with the pellet. There's always something more to investigate.
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George, thank you for posting these. I'm a few weeks from getting back to doing some testing so it is nice to see new results to ponder. Top notch data as always.
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THE MYTH of BARREL HARMONIC TUNING
I was going to make this a new thread, but thought that it might be safer to hide it in this thread. A fair amount of resonance and harmonic testing has been presented in this thread using various means of stimulating sustained resonances for evaluation. Having been seduced by a significant number of papers, reports(?) and You Tube videos, I ,too, fell into the trap (pun?) of misguided consensus.
I was going to get into a protracted effort to explain my myth claim, but decided that the information heading to this conclusion has already been presented in previous posts and it would be easier to just refer the reader to those posts and save all of us a lot of time.
First, I'll start by referring you to posts #226 and #228. They describe the methods and results of my naive (in retrospect) attempts to demonstrate what real barrel harmonic signatures look like when using the 2 accelerometer method. The methods and numbers, actually, are real and reproducible.
Next come posts #340 and 347. These demonstrate what is really happening at the muzzle of the barrel of an airgun while the pellet is still in the barrel. These measurements record the forced oscillations that occur during the shot cycle and eventually drive the barrel into its natural frequencies (harmonics). The point is that the pellet has typical already left the barrel before the stimulated natural frequencies have a chance to take control of the barrel's vibration.
This post goes back to using Lissajous patterns to illustrate what the muzzle is doing, when viewed end on, during the 2.8 ms interval when the pellet is still in the barrel. The pattern shown here is unfiltered because the DSO is unable to do the filtering while in the triggered XY mode. Therefore, the displacements can't be usefully evaluated, but the activity is real. The plan now is to set up the analog filters again and redo this test to get the actual displacement numbers.
I look forward to and encourage any and all information (not opinions) that would point things in another direction, pun intended.
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The above post (#350) was meant to be a first attempt at bring together information about the measurement and applicability of airgun barrel harmonics. The earlier posts that were referenced included the measurements that were conducted using sinewaves to replicate the 2 main barrel resonant frequencies as determined by a pair of accelerometers attached to the muzzle of a CP2 barrel. One of the DSO images referred to in the above post is re-posted below. As can be seen in the sinewave measurement table included in the first DSO image, the time window for the 2 markers is 2.86 ms.
The accompanying DSO image (same as #350) is of the real raw data from the same two accelerometers when viewed during the shot cycle when seen through a 2.80 ms time window. This data is admittedly unfiltered and therefore includes higher frequencies that are real, but of little consequence to muzzle displacement, but the filtered data still contains large amounts of chaotic forced oscillations that are unrelated to the barrel's actual natural frequencies.
The point here is that the sinewave model being used exclusively to describe muzzle displacement during the shot cycle, as far as I've seen, can only represent much longer time windows than the projectile will ever be exposed to. I see this as another predicament of over dependence on spreadsheet calculations.
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Here's some new info for those that I haven't caused to doze off yet. These 2 DSO images are of the usual P/T curve on channel 3 and one of the accelerometers on channel 1. Usually a measurement tries to stay well clear of the resonance frequency of any accelerometer because of the quickly rising nonlinearity of the amplitude response as you approach resonance. In this case it becomes a useful feature. The spec sheet claims a high Q resonance of 80 KHz for this model. The DSO measurement says 88KHz, so we're very close to a peak. The gain here is very high, though to measure it would be useless data.
The utility of this unorthodox signal that permeates the measurement is that the high gain allows a view of what otherwise might go unseen.
The first DSO image is of the unfiltered signals. The cursors show that the first signal starts to rise at ~104 µs before the first pressure step starts. We've already measured this first step to be sub-microsecond on previous P/t curves, therefore this first event must be the steel on steel of the hammer striking the valve stem. After that the valve is starting to open and the pressure is rising. The next big event is the hammer striking the face of the valve body as the pressure continues to rise. What we're seeing is only approximately the first 1 ms of the action, but it's a good start so far.
The second image is with a 2 KHz filter applied to both channels to smooth out the picture. The signals are rounded off, of course, but it might give a better idea of what's going on.
There's lot's more to see when the traces can be time zoomed over a longer data collection period. I'm playing with that now and should have some more revelations soon.
As usual I'm open to other interpretation of what's being presented here.
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Some of the measurements that have been taken when the testing was being done back in the machine area differ from the ones that are being done on the new test bench that's now set up in the electronics area. Winter's coming on. The vise arrangement used now is an improvement over the direct bench mounted one used previously. The most apparent differences seem to be the result of having reassembled the pistol version of the .22 cal gun and clamping it in the new vise using the plastic forearm. This allows something that may be closer to real world data as apposed to clamping the gun by just the powerplant tube. The vibration spectrum is sure different.
One set of measurements that I'm interested in following up on is related to hammer timing and hammer bounce. The best way to get some answers is to attach an accelerometer right onto the hammer and then see how it correlates with the P/T curve and the accelerometers at the muzzle. These tests will allow the use of all 4 channels on the DSO. I'm hoping that the hammer sensor will survive the tests.
The images show the hammer having a small shelf area milled into it for mounting the sensor and how the sensor will be mounted on same. The hammer appeared to be hardened to some extent, but I didn't bother doing a Rockwell test and opted to just use a carbide end mill.
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Here's how things look when reassembled for a fit test. I've also milled a slot next to the valve body for any future interest in putting a pressure transducer in the valve body itself.
The next task is to clean, lube, and reassemble the whole powerplant. The accelerometer can then be mounted onto the new hammer shelf. After that I'll mill out a section of the forearm to accommodate the accelerometer and it's wiring.
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Wow George, great testbed. How are you going to attach the accel?
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The recommended method of attachment for these small devices is to use superglue. We'll have to wait and see how that holds up.
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Can you get the sensor off with superglue? With a thin layer of silicone, you may be able to get a razor under it pry/cut it off
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The sensor actually comes with a wrench type tool for removing it from the DUT. Sometimes I just use a pair of needle nose pliers in tight spaces. The wire leads are much more fragile than the sensor itself. I try to beef up the wire terminations with shrink tubing if the device is going to get a lot of handling.
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Here's what today's project looks like in full dress. I thought I should get a picture if it before doing any testing just in case any of the parts decide to leave the test bench on their own to seek work elsewhere.
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The hammer sensor is holding up fine so far, but the real test will be with the powerplant charged up. I've included a few DSO images to show what's happening in these first runs sans CO2.
The images include some shots with the DSO's digital low pass filter set to 5 KHz. It can be turned on and off using the same data set. There's an extra hammer shot to illustrate the variance that can occur from shot to shot that makes it impossible to predict the muzzle movement during these forced oscillation periods.
I'll try to get some tests done with everything under power later this afternoon. I'm very interested in seeing what the P/T curve looks like with this ensemble.
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George, I was trying to understand your hammer sensor data. Have you taken any datasets with a longer timeframe? I was looking to see if the low speed motions of the hammer were present. When I was monitoring the hammer motion on the 2240 there was about 10ms of hammer flight time. The CP-2 should have something similar. During that time, starting with sear release, there should be a low frequency acceleration that starts high and may or may not hit zero before the valve is hit. Then a deceleration until the hammer bounces off of the valve. Without CO2, there may be several bounces. Noise from bouncing on the tube wall would be on top of that.
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Stan - I'm sure that the smaller signals would be detectable if the gain on the DSO was set higher. The noise floor on the 2250A is less than .002 g RMS. The shock limit is 2000 g peak. The stated calibration range of the device is +/- 500 g. The scale I setting the DSO to is 800 g full scale. Some of these hammer impacts are approaching full scale. Without the filter in place the signal has high frequency content that swamps the sensor's amplifier and causes massive clipping at 15 Volts!
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Here's a cornucopia of images with the powerplant in operation. These were all done with the 5 kHz filter on duty. I'm surprised that there was no indication of hammer bounce extending past the P/T peak.
Channel 1 is the hammer sensor.
Channel 2 is the muzzle sensor (only one of the sensors is being used for these tests).
Channel 3 is the pressure sensor.
Channel 4 is off duty.
The key to deciphering most of this info is at the bottom of the grid frame. The channel voltage/grid segment values are listed along with the time/grid segment. The channel being used as the trigger is in the lower far right. Stan was asking about the hammer sensor's sensitivity. It's output is 10 mV/g. Actually, all of the 2250A accelerometers I'm using have the same nominal output rating.
These measurements are all up for interpretation at this point so have your way with them.
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George, These are great. Definitely lots to chew on. Have you made a shot without a pellet? It would be interesting to see how the muzzle responds without the pellet excitation.
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Stan - Here are a couple of shots w/o a pellet.
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George has collected and posted a wealth of data on the shot cycle and vibration response in the CP-2. To help me try to understand what is going on, I took his graphics (from posts 363 and 365) and overlayed them onto a couple of charts. Since these are his results, I checked with him for his OK before posting. The charts are just an overlay of several runs indexed to roughly the same starting point on the hammer accelerometer curves. These are just graphical overlays, no numerical processing was done. I post them because they helped me understand a bit what was going on. Note, sorry for the artifacts left over from setting the transparency.
The first image overlays the three pellet shots (1-DS0004, 3-DS0010, 4-DS0011) I also added the dashed blue line to approximate the period when the pellet is moving in the barrel (from George's previous data). To me the repeatability of the traces, including the hammer impact event was something I didn't expect.
In the second image I added an overlay of the shot without the pellet (2-DS0003). For this I switched the colors of the hammer and muzzle traces to provide some contrast.
In reading the resulting tea leaves, It looks like the initial muzzle response is very similar whether there is a pellet there or not, and then after about 1 ms the pellet causes additional response at the muzzle. I also expected the pellet shots to have higher amplitude than the 2-3X seen. In fact the no-CO2 shot (5-DS0011) have similar muzzle response amplitude to the pellet shots.
No clue if my interpretations tie to reality but it is fun trying to figure out what's going on.
Thanks George
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Stan - That's some interesting work that you've done. Thank you for the effort. From the look of the overlays there seems to be something forming as a possibly repeatable pattern in the shots that you chose. The illusion is due to the 2 dimensional data that I used to test the hammer sensor's signal and just one of the muzzle signals at the same time.
The DSO images shown in this post are using both of the muzzle's 2250's being recorded simultaneously and with the 5 kHz filter in. As can be seen, they present a pretty wild ride after the first 1.5 ms. These shots were taken without the hammer signal included because the sensor got hit one too many times while working the bolt handle. That caused the lead wires to break. I'll confess to not having thought out the experiment well enough to realize that the access slot was milled on the wrong side of the powerplant tube. That can be corrected.
Another interesting part of these images is that the 2 muzzle sensors are producing signals that are 180º out of phase on some of the major peaks. This is probably do to the muzzle rotating and the fact that they're frequencies are different as shown in the Lissajous patterns. For curiosity I've inverted channel 4 in the 2nd image. Also,as a check, I tapped the top of each sensor to assure that the direction of displacement and output signals were both in phase.
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Stan has pointed out something very interesting with his overlays. The overlays showed what might be a pattern in the first part of the shot cycle time window when the forced oscillations are running the show. My point of difference was based upon the fact that only 1/2 of the information was available in the images that I posted. Subsequently, a DSO image of both muzzle sensors recording simultaneously was posted to illustrate the real complexity of these vibrations.
My assumption up until this point was that the situation was approaching chaotic randomness until the barrel's natural frequencies took over. This might have been true if we consider that most systems which are very sensitive to the initial conditions will lean towards chaos. After some thought about the overlays it was clear that the actual initial conditions were very close to reproducible because the action to produce the forced oscillations is pretty much the same force applied repeatedly to the valve's stem and face every time.
This new notion got me going onto another experimental set up. Since the DSO wouldn't do any filtering when using the XY mode of acquisition, I decided to employ a Krohn-Hite 3202 Dual Channel Tunable Variable Filter. This instrument allows both signals from the sensors to be filtered identically before they're passed on to the DSO. I also decided to lower the low-pass upper limit to 2 kHz. This cleans up the images and uses a more realistic frequency range where the displacement is concerned. Using the dual filter also eliminates any phase shift between the signals.
The results were enlightening and I want to thank Stan for his sizable contribution to my understanding of what these measurements can reveal.
I'll try to get some images posted tomorrow. The new Lissajous patterns contain much more information now that they're filtered.
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Here are some XY plots of the pellet's potential position after the shot cycle begins. The first image is from the naive days of using pure sine waves to test barrel harmonics. This image was posted already some time ago. The markers are positioned selectively at a crossover point that would represent the face of the muzzle at rest. The second marker is placed at 2.86 ms from the first marker to illustrate where the pellet might be at that time if the barrel was truly in resonance at both it's vertical and horizontal frequencies.
The next 3 images are the real patterns of the barrel's vibrations during the full shot cycle at the muzzle. Each is marked with overlapping points at 100 µs intervals on either side of the time when most pellets leave the barrel. The lead or lag timing of the marker in these images could easily be real by this amount of time for several quantifiable reasons.
The time position of both cursors can be seen in the YT trace at the top of the screen. The Krohn-Hite filter was in play for these last images.
The next interesting experiments would be to delay the trigger time in order to capture just the window on either side of when the pellet leaves the muzzle. This could be done by using the already reliable muzzle contact wire.
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There was a thread somewhere along the way where someone was talking about testing the hardness of the lead that they were using to cast pellets/bullets with. I got to thinking that this would be a good thing to distract myself with while I work on the design and construction of an accelerometer calibrator.
Being able to test the hardness of the alloys being used by different manufacturers would be interesting along with the alloy's composition. This alloy notion was brought up back in the days of the Vigilante hacking thread. ( https://www.gatewaytoairguns.org/GTA/index.php?topic=118339.60 (https://www.gatewaytoairguns.org/GTA/index.php?topic=118339.60) ) It was post #70 and the experiment showed that a piece of solder was easy to analyze with some simple stuff found around the shop. Pellet alloys would be just as easy to analyze, but I got distracted back then, as usual, and never got around to reassembling the experiment.
Anyhow, now that I've wrestled this sliding microtome out of the attic I might as well put it to good use. I'll have to say, I was a younger man when I put this beast up there. Somehow it acquired significantly more mass during it's stay.
The first testing setup to confirm that it still works was done on a .22 wadcutter that was just clamped unsupported in the mounting vise. I wanted to make sure that the blade was up to the task. Well, it didn't even know that the pellet was there. The slicing thickness can be set from 1 to 40 microns per slice and automatically advances the sample up by the assigned thickness on each stroke. It's really an amazing instrument to operate. The blades are as sharp as you can get steel and feels like you can almost get cut just staring at them! This instrument uses a 10" blade which is quite large as bench microtomes go.
The real cross sectioning will be done with the pellets mounted in a hard wax and then sectioned properly. These sections can then be etched to see if the cooling rate or other factors involved in the manufacturing process have any significant effects. This will allow me to use some of my other metallurgical toys. Idle hands are the devil's tools!
The photos are for those who may still be following my "Mr. Toad's Wild Ride" down into the rabbit hole. Disregard any cat hair that she somehow manages to sneak into my photos.
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Wow George, I think I'd keep that in a display cabinet. Looks great...waiting to see how you'll put it to use.
Won't ask what tests you were running on Fluffy that needed a 1 micron shave off the top.
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I've decided that the sliding microtome was not the best match for slicing up pellets. The next choice was to go with a rotary microtome. It's scaled much closer to the job. Fortunately the collection of these instruments has grown over the years and fulfilled an assortment of applications. There are some things that you might want to see the inside of without having to use x-ray imaging. Of coarse the x-ray imaging is nondestructive in use, but for pellets, physically slicing them open affords the best view of the interior and they can be sacrificed.
My interest at the moment is to see what the cross sections look like. There are lots of pellet photos out there, but very few of them are inside views with much detail. A good cross section will allow the sample to be chemically etched to give high contrast to the grain structure for comparison to other samples. If nothing else they should be interesting to look at. It doesn't take much to get me chasing my tail with another experiment!
The .22 cal. wadcutter in the image is only tacked onto the sample holder for a sense of scale compared to the sliding microtome. To do a proper sectioning will require that the pellet be embedded in the appropriate mounting medium.
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The accelerometer used in the hammer experiment was short lived because of my lack of foresight about which side of the main tube to mill out the access slot on. Since I'll have to take the gun apart to move the slot onto the other side of the bolt, The idea of replacing the accelerometer was worth considering. After rummaging through a few drawers I found an Endevco model 22 device.
I thought the 2250A was small. This on is claimed to be the world's SMALLEST! It weighs in at 0.14 gm! There won't be any mass loading issues with this flyweight. The next thing that needs doing is to see if it still works. The 2250A has a built-in preamplifier that requires a current source signal conditioner to get the signal out to the measurement system. The model 22 needs a charge coupled preamplifer to get the signal out. Fortunately I'm well stocked in both requirement.
The specs on the 22 are excellent. The amplitude linearity is 1% out to 4,000 g. The frequency response is +/- 1dB from 3 to 12,000 Hz. I'm looking forward to getting the gun back on the test bench.
The image is for comparison of the 2 sensors.
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I got a chance to check out the new micro accelerometer and it works to spec. This pushed me to start the machining for the hammer to be instrumented on the other side of the bolt. The photo shows the new slot and hammer in place. I'll still have to machine a new shelf for the sensor on this side of the hammer.
The rifle barrel will probably be the next project for new vibration testing now that the pistol arrangement has be overly measured at this point. The rifle barrel will need to be machined to accommodate the pressure sensor.
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The new accelerometer's ledge was milled into the hammer and fit tested today. Progress is slow, but little steps will get this project back onto the test bench. Tomorrow I may be able to mount the accelerometer and also machine the pressure sensor port into the .22 cal rifle barrel.
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The .22 cal rifle barrel was drilled and tapped today so that the pressure sensor could be mounted. The photos show the barrel in the lathe getting the front sight turned down after the blade was bandsawed off. The breach was then deburred after the drilling and tapping operations and polished using the Dremel tool.
Next is to reassemble the gun and make some measurements with the new smaller accelerometer installed on the hammer and with the longer barrel.
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I finally got around to finishing the machine work on the carbine version of the CP2. This included the milling on the pistol grip to accommodate the new hammer and accelerometer position on the side opposite the bolt lever.
The gun was reassembled without using the barrel band and tested with some RWS wadcutters at the 15" target in the machine shop area. These guns are really impressive for the price.
Installation of the new micro accelerometer for the hammer was abandoned because of the precarious mounting necessary to install it safely for such hazardous duty. The previous 2250A device is better suited for this job and is safer now that it's terminal wiring won't get clobbered by the bolt action.
The next tests will include vibration testing with and w/o the barrel band along with timing info for the full shot cycle. Another pressure sensor at the muzzle is in the works and should provide some interesting numbers along with the exit timing.
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I shot 10 pellets with the new mounting and everything stayed in place. Next is to attach the signal wires and make some measurements with this longer barrel. I'm trying to weave this project into something less protracted, but Ive got a lot of irons in the fire at the moment.
I've become preoccupied with designing and building new versions of the earlier accelerometer and instrumented hammer boondoggle. That and a bunch of other things.
The ballistics experiments are still fascinating, though.
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Well, It's the end of another year. I decided to use today as a reminder that I'm somewhere in the middle of a barrel test on the CP2 carbine. I just need to attach the new hammer/accelerometer arrangement up to the preamp and get a few DSO screen shots posted. It's amazing how time flies by when there are other distractions.
Speaking of which, the instrumented hammer and accelerometer project has made considerable headway lately. The parts and methods of assembly are closing in on something resembling standardization. This includes a small shaker for calibration and small structure excitation table. There may be a market for these devices as an entry level front-end for a modal analysis systems. If not at least I'll have it all for my own experiments.
Part of the above system includes a force transducer that gets built into the impulse hammer. I've posted several variants of these sensors in the "hacking" threads and how they can be used to make an assortment of measurements. Now that I've arrived at a somewhat standardized force transducing device it would be interesting to use one as a "ballistic pendulum" sans the pendulum part. The output signal from these sensors is very reproducible and could be useful for measuring a pellet's energy at the muzzle and at a target some distance away. There could be some very interesting info relating to energy loss vs distance if it's done right. There would be some challenges in the design and construction, but that's what a New Year's project list is all about!
This is my second full year of experiments in the airgun world and I want to say "Thank You" to all of the people who have been keeping track of these experiments for all this time. I know that the replies have been few and far between this year, but the thread doesn't seem to be withering on the vine, so I'll keep posting as long as folks stay interested in reading about my perpetual follies!
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George,
I'm looking forward to the 2019 explorations. You have some unique test capabilities in the airgun world.
Thanks for all your hard work
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Here are a couple of New Year's Eve photos as a last hurrah for 2018! The first image shows the gap that is between the face of the hammer and the tip of the valve stem. I'm not sure why the gap is there, but I've seen similar gaps in previous re-assemblies after a hammer treatment for whatever reason.
The second image is a DSO record of a CPHP .22 shot cycle. The yellow trace is the accelerometer that is attached to the hammer. The scope is triggered on this channel at the 50% point on the signal as indicated by the arrow at the top. I've offset the vertical so that the full signal trace can be seen.
The blue channel is the pressure sensor's signal. It's channel is triggered by the yellow channel. Any timing info will have to be divined from any numbers you can find on the screen.
The point of this exercise, besides the last minute effort to squeeze it in this year, is that the hammer-to-valve stem gap can be clearly seen in the yellow trace as valve rebounds. It drives the hammer backwards and produces a sharp negative going signal that exists for a finite amount of time. The hammer then recovers quickly to continue its previous course.
The yellow channel gets a lot of action of all sorts during the shot cycle so I've used the DSO's digital low pass filter set at 5 kHz to make things easier to see.
This post is for Stan's amusement, but others are welcome to have a go at it.
HAPPY NEW YEAR!
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the hammer-to-valve stem gap can be clearly seen in the yellow trace as valve rebounds.
George, Thanks for posting the data, a great start to the year.
I'm not quite in seeing the gap segment on the trace . If it is free flight, it should be averaging zero acceleration for a while. Maybe between 1.5 and 2 msec?
I checked my CP-1 and it has a very slight preload on the hammer spring when uncocked.
Any idea why this hammer accel trace is so different from the 10/7/18 data (post 363)?
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I'm sure it has do with how the hammer spring is installed. There's a plug that's normally installed into one end of the spring (when I remember to look for during re-assembly) that probably has most to do with the different hammer/valve activity. This would also account for the gap I notice from time to time after a mod. It's always nice to mix up the variables to keep things interesting. The hammer mass has also been reduced after milling the new shelf for the accelerometer.
I can also do some blame-shifting on having given up beer and gin drinking. I'm now a bourbon devotee´. Good ole Jerry Brown has legalized micro-distilleries in California. Micro-breweries are now becoming passe´. I have a couple of friends who have gotten their distilling licenses and have their stills up and running. I find the distilling process much more intriguing than brewing beer.
Feeling somehow obligated to support their courageous endeavors, I've offered to experiment with an accelerated aging process. Under law bourbon requires aging in a new charred oak barrel for longer than they want to wait. There are lots of other rules and regulations that apply to making and labeling different whiskies, but I'll leave that to them. I'm just working on making a 'tunable' proxy barrel for them. Lots of sampling needs to be done so I had my liver tested recently. The results came back and they indicate that I'm fit for active duty!
This is a great way to bring in the New Year, helping friends and all, but the airgun Sirens are still calling. More than the usual allowances will probably have to be made when trying to decipher some of my postings this year.
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It's been awhile since I've spent much time on the airgun ballistics project, but it hasn't been completely ignored. Having been seduced by the whiskey aging project along with the modal hammer/accelerometer experiments and testing sessions it's finally time to return to the ballistics bench.
The new focus has been on developing an instrument for measuring the real energy of a pellet at the target. Most of what I've seen are numbers generated at or near the muzzle of the gun using MV metrics to calculate the fpe of a projectile. The target energy seems to be based on spreadsheet calculations or ignored completely.
I've decided to raise the bar on how to arrive at the actual target energy numbers by using a force transducer that is a modified version of the modal hammer transducer. The starting mass of this device is 3 lbs, 4.5 oz. The dimensions are 2.5" dia. and 2.75" long. This should be a good match for what I'll be shooting at it.
I'll begin with a cable connection to the scope for preliminary measurements and then, hopefully, move on to a Bluetooth or Wi-Fi arrangement along with an app to do the calculations and display the data on a smartphone.
The photos show the parts and an assembled prototype instrument that should be ready for testing within the next week.
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Welcome back, George! Was wondering just the other day about you. Good to see you're still cogitating on various and sundry projects. :D
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Yep, welcome back George....Now to go dust off my thinking cap and get ready for your posts.
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George, i was hoping you would know how to make a transducer to measure the pressure in the transfer port?
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A DIY pressure transducer isn't difficult to build. A miniature high pressure one with any accuracy, linearity, high speed response, reliability, easy to mount, etc., would be nearly impossible to make at the hobbyist level in my estimation. Your best bet would be to put Endevco and Kistler into your eBay search list and wait for a sleeper that fits your needs. Keep in mind that you'll also need the right cable along with a display of some sort that will allow you to connect a scope to the output.
I've had good luck with these transducer manufactures on eBay and there are others that make similar products. An important requirement with any of these devices when used for measuring transfer port characteristics is that they must be very fast and introduce as close to zero dead volume as possible.
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thanks for the reply. i am a complete novice and needs an "idiot's guide to build a pressure transducer" book.
looking at the suggested brands i would need a 3 to 5 ksi range. i have no idea how fast i would need. even miniature models looks pretty big to be fitted on a small power wheel. since you said it needs to be close. i can always relocate to the front of the barrel and measure the muzzle end pressure instead. but it looks like i may need a complete kit then just a transducer. and i have no idea what else i may be. wires, read out monitor and power source?
can you help me with a mini version of how to build one?
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Stay with what reliable manufactures can already supply. Go back over what I've already done. What you'll need is detailed over the last couple of years in the CP2 and Vigilante threads. You'll just have to choose a transducer in the pressure range you want to work with.
It will take time and some money to get to where you want to go. Any DIY shortcuts will probably be dangerous!
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Check out George's description on page 8 of this thread for the sensor he is using. Unfortunately they are industrial quality sensors with a price tag and also the need for supporting electronics which also have a price tag. You'll see in the thread that I tried a low cost sensor. It was low cost and easy to hook up to a readout but unfortunately not fast enough. You'll see in George's scope traces for pressure that you are looking for fractions of a milisec.
You could possibly build a DIY sensor that captures the timing of a pressure pulse. But getting a pressure amplitude would be more difficult.
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At 3-5,000 psi I'm not going to advise anything in the DIY sensor realm. At either end of the barrel.
If it's a manufactured device with that pressure range capability, then that's different.
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The preliminary testing of the new prototype 'Kill Zone' fpe transducer came out really well. The next step was to do some more machining to allow the sensor to be preloaded from the back of the of the device. This allows for easy calibration to standardize the product. There's still quit a bit of testing to do, but it's off to a good start.
The device is machined from mostly steel bar stock with the odd piece of stainless steel for the compression preload adjustment. The steel parts needed to be protected from rusting so I opted for trying my hand at making a magnetite conversion coating. Converting the iron back to a glass hard mineral at it's surface was just the challenge I was looking for.
Most everything I read about the process warned against attempting it on the DIY level at home because of the high probability of explosive boiling at some point in the procedure. Having molten lye in combination with the main constituent of gun power suddenly getting out of control while you're working with it was another one of those Sirens I find irresistible.
Ignoring my own advice about dangerous DIY projects (only a post or two ago) I proceeded forward. By good fortune I had everything needed for this doomsday project close at hand. My wife suggested that I seal my fate somewhere else than at the kitchen stove so I set up shop in the back yard. This would allow me room to run at least some distance from a disaster even though blinded and maimed. There are times when I think that her prescience transcends intuition.
Anyhow, All's well that ends well. At least that's what they told me at the ER.... just kidding. I cautiously engaged the process one beer at a time and everything worked out fine.
The photos are of the new assembly with the magnetite finish.
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George, That is a work of art....now I need to search info on magnetite conversion.
Just curious, how are you going to calculate the energy at the target?
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https://www.epi.com/black-oxide/steel/ (https://www.epi.com/black-oxide/steel/)
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The process of calibration for the Kill Zone Quantifier device is pretty straight forward. I'm partial to using at least two different methods to arrive at traceable calibration numbers. Therefor I'll be using the usual muzzle Velocity X Mass method using my Combro chronograph and comparing it with the Mass X Acceleration method.
I demonstrated a simple calibration of the Combro in a post somewhere in the CP-2 / Vigilante threads and found it to reliable. The F=MA measurement is more involved, but it's what I use to calibrate the modal hammers with.
The photos show the F=MA apparatus with its calibrated mass and a calibrated Bruel & Kjaer 4384 accelerometer along with one of the prototype hammers. This hammer has the extended mass on the back. The output signals from the hammer and accelerometer are routed to a pair of B&K or Kistler charge amplifiers. The outputs of the charge amps then goes to an HP 35665A DSA and a digital scope for evaluation.
I can swap out the original mass (as shown) with the KZQ mass along with it's internal force transducer. I'll then attach a calibrated accelerometer onto the back of the KZQ. The simultaneous impact signals can then be evaluated.
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The KZQ project made some progress today. I moved the CP2 carbine from the bench setup out to the back patio range so that I could put more distance between the muzzle and the transducer. It was getting dark so I only had time for a quick test at 30 ft. I'll have more time to spend with the measurements tomorrow.
The image shows a basic test with a .22 CPHP. The timing window is set at 150 µS with the signal connected strait into the scope without any conditioning amplifier. The scope's digital filter is set to low pass at 14 kHz. This is all preliminary at this point so there are no comparison values yet. The next test run will be with the Combro installed.
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Looking forward to the secret decoder ring. The 14 Khz lowpass filter, 14 Khz is around 70 µs per cycle, is that fast enough?
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Stan - The filter is to block the ~ 25 kHz transducer resonance and the ~ 200 kHz internal pedestal compression wave resonance. The signal of interest is the initial impulse which is the negative waveform in the time window. The shape of the waveform is determined by the material attached to the front platen of the KZQ. This material controls the rate of change in velocity of the pellet as the pellet's velocity goes to zero.
The active material is ballistic putty (alias duct seal). This material is attached to a disc of the same ballistic tile material that I use in my traps for bench top airgun testing. These two components are then attached to the front of the KZQ like an Oreo with one of the cookies removed, the remaining cookie being attached to the KZQ platen.
The duct seal acts as a very high viscosity isotropic Newtonian fluid when struck by the pellet as far as I've been able to measure so far. This keep the shape of the impulse waveform fairly uniform in the time window at the different impact energies being used in these early tests. The time window and thickness of the putty can be modified as needed. The tile acts as a backstop for any over penetration and protects the KZQ.
The photo shows the effects of impact on the CPHP pellets. The left one hasn't been shot. The middle one is with the duct seal. The third one is straight onto the platen. The shots were taken at a distance of 1 ft. from the muzzle. As can be seen the putty captures the pellet with little energy lost to deformation. The flattened one still retained enough energy to rebound ~ 8" to 10" and was still quite warm when it was recovered. Remarkably, to me, the flattened pellet's impact didn't phase the magnetite coating!
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The KZQ transducer in v1.? has now been upgraded to v2.0. Testing so far with the v1.? has been very instructive in regards to resonances. The main goal from the outset was to be able to measure the energy of a pellet at the target accurately and without ambiguity. To that end v2.0 now has a single resonance of ~ 52 kHz which is easily seperated from the impulse energy measurement using a low pass filter. The signal of interest is now contained within a time window as an impulse that will be relatively easy to quantify. There is still work to do in fine tuning the instrument, but the new version is a big step from where it started.
The photos show the assembled transducer. The stand alone weight is 3.5 lbs. It's the same size as v1 and uses the same front energy absorber (not shown) as described in an earlier KZQ post.
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The next step in setting up the KZQ for further testing was to mount the device in a box that could provide a stiff mounting arrangement and add extra mass to it as a system. The box is actually a version of the quiet trap that I posted as a construction project some time back. The box is 13.5" square and 3.5" deep. This package is less deep and lighter than the original design and weighs in at 20 lbs. The original trap design was posted here: https://www.gatewaytoairguns.org/GTA/index.php?topic=127358.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=127358.0)
The photos show the KZQ mounted in the trap with and w/o the front impact absorber. I'll try to post some measurements in the next couple of days.
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It might be instructive at this point to show the difference between the impulse signal from the KZQ going straight into the scope and first running the signal through a charge amplifier. Keep in mind that the signal generated by the KZQ's piezoelectric crystal is actually a charge of only a few Picocoulombs (pC). Therefore any resistance in the signal path will begin to bleed this charge off the crystal while the measurement is still being made.
A charge amplifier has an extremely high input impedance compared to the scope's input. It also has a low impedance output signal that can easily drive the scopes input impedance load without loosing signal information.
As can be seen in the screen shot channel 2, being the charge amp's output, shows a good representation of a real world half sine pulse. The trace below it is a stored output of the KZQ crystal when it's loaded down by the scope's input without the benefit of the charge amp.
The cursor arrows indicate the time window where the signal can be integrated to quantify the pellets energy after striking the impact absorber. Obviously the lower trace is not useful for this purpose. Note that neither of the traces show any signs of extraneous resonances or other interfering signals coming from the v2 KZQ!
It should also be mentioned that the stimulus for the test signal is being generated by one of my nifty modal hammers with the compliant tip installed to simulate the front impact absorber.
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The testing and improvements on the KZQ continues and has now progressed to the v2.5 level of development. Each incremental step in the measurements allows a commensurate reduction in machining complexity. This will all lead to a more reliable and reproducible device.
In the meantime I thought that it might be useful and possibly interesting to review the reason for attempting to design the KZQ in the first place. I understand that there are different levels of consciousness involved in the process of hunting. My personal goal within this activity is to humanely dispatch rodents that become problematic on our property. I don't hunt them as such, but if they interfere with the quality of my life, on our property, I'm going to aggressively interfere with theirs. I have no other quarry at the moment. Errant rabbits are captured in 'catch and release' traps and then set free in the local canyons for the coyotes.... just supporting the food chain.
That being said, I'll report that coyotes are starting to come up out of the canyons around our neighborhood and are becoming aggressive towards pets according to the neighbors. I haven't seen evidence of them on our property yet, but if I do I'll address their trespassing the same way as I deal with the rats.
Should this situation come to pass I'll need something more substantial than a CO2 powered gun to stand my ground with. This will most likely push me over the edge into the world of PCP armaments. The ability of the KZQ will be even more relevant in this case.
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George, a word here if I may... don't try to dispatch coyotes with a pcp. Coyotes are wily, anxious, and don't generally allow you a good, close shot. Get a 30-06 if you can legally use it...
Wyo
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Thank you for the advice. I'll have to check with the city to see what level of protection I'm allowed to use in this situation.
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George,
I am picking back up on this very educational thread and absorbing what I can. Your unique approaches, throughout this thread, are appreciated and admired. I believe you are testing and documenting characteristics that have not been done before.
Regarding your latest creative challenge, the KZQ force/impulse transducer is quite intriguing. Are you satisfied with the calibration of the unit? You seem to really like solid, empirical data, and are not satisfied with "relative" measurements.
I truly believe his kind of brain challenging work keeps us young and vital. You are going to be around forever. ;)
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Hi George, I was reading about your thoughts on pressure transducers, their cost, size, responsiveness, etc. I built a few pressure transducers a while back but did not test the responsiveness of them. Your idea of a piston arrangement that you tap with a hammer is great and I think I will give that a try.
But if you are interested in trying to make one, and I see that you don't shy away from that, I started with Merit SMD series sensor dies. They came already mounted and compensated. Approx .28" square and .08" thick. Four wires must be soldered to them and then exited out through a pressure-proof seal. Magnet wire and epoxy worked well once the process was figured out. One of the sensors has 4 connector pins instead of wires. It was connected to an AD623 instrument amp and an A-D convertor. with a reference and excitation voltage. Here are pics (not that great) of 2 designs. One of the designs screws in like an SAE o-ring plug. The other has a 10-32 screw with a .050 dia hole thru it to bring in the air pressure. Making them was probably more trouble than they were worth, but they actually did work well for the intended purpose.
In both designs, free dead space was held to a minimum.
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Lloyd,
I found a spec sheet on their site for one of the other configurations, using the same technology. https://meritsensor.com/assets/documents/pdf/BP-series.pdf (https://meritsensor.com/assets/documents/pdf/BP-series.pdf) It mentioned a frequency response of 1200 hz. That may not be fast enough for a breech sensor. Have you tried just tapping the sensor housing to see if there is a response rise time?
Found an interesting, slightly vintage, discussion on the topic. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700010011.pdf (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700010011.pdf) Their dynamic calibrator is interesting, I think it would work well with George's accelerometer calibrator.
Thanks for the idea
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It's nice to see that Lloyd and Stan are keeping track of things here in this wilderness of airgun experiments! Stan's search juju is right up there with the best I've seen.
Lloyd's pressure sensor build is interesting. Characterizing its frequency response and other specs would be useful and important depending on it intended application.
The KZQ calibration is moving along at my usual distracted pace, but progress is being made. The current effort is to determine the uniformity and reproducibility of its impact surface. I've been playing with various methods of pellet impact simulation using the B&K 2515 Vibration Analyzer and the 4 ch DSO to evaluate these impact signals simultaneously. All of my B&K instruments are vintage, but they work well and still calibrate to the original specs. In general, accelerometer, force, and pressure transducers have very relaxed specs compared to audio (lunacy) requirements. I take comfort in knowing this when I use these hallowed instruments to make measurements with.
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George,
Are you going to integrate the force response by using the math within the DSO? I have not tried that yet with mine.
Lloyd,
I also found an interesting paper on optical pressure sensors. The paper is about biomed applications at relatively low pressures, but the approach should be applicable to higher pressures and would yield tiny sensors
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4541926/ (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4541926/) Section 3
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Stan - The testing that I'm doing at the moment uses the serendipitous confluence of the clean half sine wave output of the charge amplifier impulse response and the DSO's collective measurement screen when the measurement button is pressed. No need to get tangled up in the math functions section of the scope. The DSO's measurement panel has an area calculator that can use just the positive half of a single sine wave (it doesn't know that's what I'm giving it) and calculates the area (integral). The signal from the charge amp is also going to the 2515 which then measures the frequency spectrum of the same half sine wave. The numbers are remarkably close for being this early in the game!
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It might be of interest and some help to those following the KZQ project to see the general scheme that is being used for these measurements. The images are from the Endevco website and show the basics of modal impact testing signals. One image demonstrates the typical output of an instrumented hammer when measured in the time domain as would be seen on the DSO. The other image is what the same signal looks like when it's measured in the frequency domain on the vibration analyzer.
The overlaid plots in each image are results from the different tips that can be interchanged on the hammer. As can be seen the tips can determine a different range of outputs. The KZQ is in essence an instrumented hammer operated in a reverse mode where the pellet is generating the force to be measured. I'm probably preaching to the choir, but it may be helpful to some folks out there.
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A few posts back Lloyd asked if I was satisfied with the calibration of the KZQ. I have to say that the question is really jumping the gun (pun) at this point. There is still some amount of thought that has to be given as to how to best represent the POI (point of impact) metric and what method(s) give the most reliable numbers.
Generally, all of the 'chronys' that are popular with airgunners today report the projectile's velocity in ft/sec and energy in ft-lbs. The measurements are usually done at or near the muzzle and therefore can be comparatively interesting, but not overly useful at any distance beyond the muzzle.
The point of designing and building the KZQ is to enable shooters to accurately measure the amount of muzzle energy a projectile still has at the POI. To accomplish this in a meaningful way the projectile's mass and velocity at the POI must be converted to units of momentum (a directed force) and timed as the velocity drops to zero. This time to zero measurement then becomes an impulse. The impulse can then be integrated (the area under the half sine wave curve). This new unit is reported in lb-sec.
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A few posts back Lloyd asked if I was satisfied with the calibration of the KZQ. I have to say that the question is really jumping the gun (pun) at this point. There is still some amount of thought that has to be given as to how to best represent the POI (point of impact) metric and what method(s) give the most reliable numbers.
Generally, all of the 'chronys' that are popular with airgunners today report the projectile's velocity in ft/sec and energy in ft-lbs. The measurements are usually done at or near the muzzle and therefore can be comparatively interesting, but not overly useful at any distance beyond the muzzle.
The point of designing and building the KZQ is to enable shooters to accurately measure the amount of muzzle energy a projectile still has at the POI. To accomplish this in a meaningful way the projectile's mass and velocity at the POI must be converted to units of momentum (a directed force) and timed as the velocity drops to zero. This time to zero measurement then becomes an impulse. The impulse can then be integrated (the area under the half sine wave curve). This new unit is reported in lb-sec.
George, That is an interesting approach, and a bit confusing to me, because the concepts of kinetic energy and momentum seem similar but are actually different. So, you are thinking that you will know the velocity of the projectile as it hits the sensor and can therefore calculate the force that it has, and then see if the force-time integral that is measured by your system is the same as what the projectile arrives with? I hope it sinks in eventually, LOL.
Lloyd
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Lloyd - It may be useful as a thought experiment to consider the KZQ as an electronic ballistic pendulum. The force transducer and DSO combine to measure and store the momentum (which is always conserved in any collision) as being the pellet's impulse signature.
The charge generated in the piezoelectric crystal converts the pellet's kinetic energy into potential energy minus any of the usual losses do to capturing the pellet.
There are, of coarse, some things that need to be worked out to standardize the measurements and hardware design to make this a reliable transducer, but it all seems to be coming together using the approach I've chosen at this stage of the game.
I'm open to any and all comments and testable critiques.
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George,
I think your second sentence may be causing a little confusion. The way I understand your planned measurement. The piezo sensor will provide you (after calibration) a signal that is proportional to the force on its surface. The time integral of that trace will be the impulse or the change in momentum of the pellet. Since you are capturing the pellet and not bouncing it off, it is also the momentum (mv) prior to impact. From that you can calculate the velocity and then the energy. There is nothing in the measurement approach that measures the energy of the pellet directly.
The charge generated in the piezoelectric crystal converts the pellet's kinetic energy into potential energy minus any of the usual losses do to capturing the pellet.
I'm looking forward to some of the data.
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Stan - Thanks for pointing out the over simplification of the measurement process in my previous post. You are correct that the sensor only provides a signal that is proportional to the force and is not a direct measurement of the pellet's energy. I think the electronic pendulum analogy should still be valid though.
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Yeah, now that I know about them, I'm tempted to build a ballistic pendulum. I like the separate conservation conditions.
http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html (http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html)
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Stan - I hope that you reify your ballistic pendulum temptation. Would you pursue a mechanical or electronic version? Either way it would be more than interesting to compare notes with you on any experimental results.
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Interesting, if the math in the calculator I linked is correct, (I have not checked it) for a 14.3gr (.93g) pellet at 150 m/s, into a 100g target, you need to measure about 1.3 mm on a 97 mm rise to resolve 1 m/s. I think the classic instrumentation is a mechanical pointer. There should be a way to measure that electronically or optically.
Interesting
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Stan - Your idea of building a hybrid ballistic pendulum using an electro-mechanical arrangement sounds good. Of coarse you'll need to incorporate a LASER somewhere in the pendulum's mass so that the displacement can be projected onto a wall for magnification to improve the resolution.
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Well, maybe one of these AND a laser. https://digital.library.unt.edu/ark:/67531/metadc12516/m1/46/ (https://digital.library.unt.edu/ark:/67531/metadc12516/m1/46/)
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WOW!! With a saddle on that thing you could make some real money at the County Fair!
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Testing the KZQ requires a lot of tapping on the face of the transducer. The taps have to be square to the surface and repeatable in order that the output of the transducer can be characterized for uniformity.
I had been doing the testing using one of my modal hammers so that I could compare the input force to the KZQ's output signals. An easier method of getting good repeatable taps in a straight line across the face of the instrument was to devise a linear displacement sliding hammer arrangement that could be used as a drop test load. I've repurposed a gate latch as a quick proof of concept device for the tests.
The simple lash-up consists of the reshaped gate latch with an adjustable extension arm. The end of the arm has a calibrated B&K 8200 force transducer attached to it. The tapping surface of the 8200 can be fitted with any of the hammer faces from the modal hammers. This will offer the same input taps as the hammer would, but with much improved control and repeatability. The extension arm can also be loaded with variable weights.
The photos show the general set up and the new arrangement should be finished and ready for a shakedown (tapdown?) cruse later today.
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I have been away for a few days but see that there have been no idle hands or minds. I was also wondering about using a drop hammer, or swing hammer to aid in calibration. The pendulum steel balls hung on V-shaped strings that have the fully elastic collisions are certainly fun to play with but your idea is a fully inelastic collision with 100% of the momentum being transferred to the sensor. I have been involved with some Mil-S-901D light and medium weight pendulum hammer shock tests. The energy was always calculated beforehand so that the lift of the hammer could be set correctly, but the equipment being tested was always instrumented with plenty of accelerators so that the shape of the impulse could be used to verify that the equipment was indeed hit hard enough, with the correct pulse duration, etc. Your technique is similar, with a closely estimated (?) impulse that is then compared to the impulse that is measured and subsequently calculated. The shape of the impulse is usually a mystery without proper instrumentation.
It looks like you are now trying to use a pendulum hammer to hit your sensor. I can't tell the orientation of the swing, however. Will you use some duct seal or maybe some of the sticky mouse trap glue to capture the hammer on impact and thus not loose any energy to rebound?
It also seems that by adjusting the lift angle of the pendulum hammer and also tilting the inclination of the impact face,such that even though the hammer might be lifted a full 90 degrees, it might only swing 45 degrees before impact, that you might be able to come up with a method that almost eliminates any rebound. That way the calculated hammer hit could be reasonably accurate. Used in conjunction with testing by shooting pellets of known energy you could end up with all sorts of data that either doesn't correlate at all, or that puts a smile on your face.
Killing the hammer rebound is key, I think. What about a home made mini dead blow hammer?
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Lloyd - The KZQ is designed to be primarily an impact force quantifier. Knowing the impulse's integral, time to peak, and magnitude should provide a useful metric for POI evaluation. It's actually pretty much the opposite of the Newton's Cradle demonstrator as you've pointed out. The balls do eventually stop though, so "fully'" is something of a stretch. No matter, they're still interesting to watch.
The latest impactor (tapper) design will work along the lines of the modal hammer calibrator shown in post #396 in this thread. The KZQ will be held within the target box with it's impact face vertical. The impactor will swing down the same as a pendulum in instruments like an Izod and Charpy impact tester, but not as destructively. (Disregard the 'drop test' misnomer in the previous post.) The B&K 8200 will record the impact signal data and the KZQ will record the response signal data. The rebounding impactor is quite easy to catch after a strike when using any of the modal tips.
At this point in the measurements I'm only interested in the relative sensitivity of the KZQ at all points on the front surface within the area bounded by the machined flexure ring. The impactor can be adjusted to hit any area of this surface repeatably and it's stimulus signal can be compared to the response. This will allow me to map out the usable area for getting reliable numbers.
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Using the ballpark numbers mentioned before: .93 g (14.3 gr) pellet at 150 m/s (500 ft/s), the momentum (or impulse) magnitude is about 0.14 kg-m/s. It will be interesting to see if your tapper is of similar magnitude.
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The actual calibration of the whole KZQ system is going to take some time. The basic design concept was (and still is) to have a reliable and accurate instrument that can use WIFI as the data link to a smartphone app.
The central part of the concept was to design and build a force transducer that had a large enough target (KZ) surface area to represent the most common pests and small game that airgunners might want to assure humane kills with. This won't be an inexpensive system by the time it becomes a marketable product. It also won't be something that the average airgunner would be interested in investing in for only occasional use. Therefore it's design also incorporates probably the best quiet trap available on the market today. As a guiet trap it can be used for both indoor and outdoor target shooting.
The trap design has been linked in a recent post to another thread on this forum. That post describes details of the trap's construction. The original design was test marketed on eBay with good results, though only a few were offered up for sale.
The trap designed to hold the KZQ is an even more robust version of the original while maintaining about the same dimensions.
To change the instrument's use will require only a simple conversion; the removal of the KZQ transducer and replacing it with a ballistic plug insert along with a new target supporting front foam piece. The replacement foam is made of the same military grade closed cell material as the original.
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The new testing arm for the KZQ went on it's first bench trial today and worked out really well. I've added Delrin sleeves to the arm's pivot points to act as bearing surfaces and take up any slop in the hinge. The arm can now be slid from the center impact area to either side of the active area with good repeatability.
The photos show the current test bench arrangement. One DSO image show the conformal symmetry of the B&K 8200 stimulus and the KZQ response signal. This is an excellent start for a right out of the gate measurement. The signals from each transducer have been fed into a pair of B&K 2635 charge amplifiers and the gain is adjusted to give equal amplitude from each transducer.
The other DSO image is what the scope measures on an input signal when requested.
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George, that is nice data!
Do you have some preload on the KZQ, is that why there is little phase lag? Is channel 1 the stimulus since it is leading slightly or is that just different response time of the sensors? Do the charge amps provide any filtering?
I like that data summary. Does the area calculation represent the whole data set or can you window that down with cursors?
Great work.
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Stan - Both charge amps have adjustable filters at the high and low ends. They're both set at 2 Hz at the bottom and 10kHz at the top for these tests. The 2 transducers are greatly different is size and mass. Their crystal detectors are different too. The B&K is quartz and the KZQ is PZT. At this point I haven't looked closely at anything except basic shapes and amplitudes of the pulses. They're too similar right now to pay any close attention to.
Channel 1 is the KZQ. There are some extraneous wiggles on the stimulus channel (#2) caused by the arm resonance, but it's down at the noise level and not being fed into the KZQ measurement.
The measurement summery display is everything at once (no cursors) that can be squeezed out of the chosen channel. The cursors would allow more selective measurements along with the separate math functions. I'm still amazed that these DSOs are referred to as "entry level" scopes.
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The KZQ project has given me a new way to look at the POI units that seem to be popular in airgunning circles. Muzzle energy numbers are used rather causally probably because they appear so often in articles, ads, and conversations about pellet guns. Pellet muzzle velocity has gotten the same reputation as a quality or status metric for advertising purposes with many products. I'll assume that most of the airgun owners following this thread are well aware of this trend by now. If not it's no barrier to continuing down the path here.
I'd like to propose a modified interpretation of the POI units that may have more information about what actually happens when a projectile intercepts a target. To encourage more thought experiments in the realm of the impact/impulse arena I think that the link below is a good place to start. It may be helpful for laying the ground work for further consideration about reshuffling the deck.
http://www.lasc.us/TaylorHowHard.htm (http://www.lasc.us/TaylorHowHard.htm)
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I think this thread https://www.gatewaytoairguns.org/GTA/index.php?topic=147820.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=147820.0) chewed on the topic for a while. I think getting the pellet velocity at impact is the important variable. Whether it is used for a momentum or energy calculation is up to the user.
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George, your work continues to be quite impressive, even though I struggle to understand the more serious instrumentation theory and operation. But thank you for the thorough explanations that you do give. I am the kid in the back of the class struggling to make a passing grade; Stan is the kid on the front row who already "understands" the material, LOL.
I am a "pester", not a "hunter", so my field experience on lethality is rather limited. But I have always thought that the V "squared" factor of FPE gives an unfair advantage in the perception of how effective a projectile is. There are so many other factors that are involved that assigning a FPE number as THE most important, or the most important "relative" factor, almost seems to trivialize the complex process of what the pellet or bullet is being required to do.
I am sure there is a reason that the use of energy, rather than momentum, has some history behind it. Maybe a marketing ploy when major increases in velocity were being made by the industry? It is a little bit like the use of amp hours for car batteries instead of a more useful number like watt hours. It is very similar to your "area under the curve" method instead of an easier and more showy "peak value."
I will stay in the back of the classroom and absorb what I can. Thanks again!
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You guys are going to make me re-activate some long unused brain cells.
I think the problem with the observations in the link that George posted is that if you are going to do the ballistic pendulum test, you can't skip the momentum conservation step in the evaluation (sir Isaac would frown upon that). In an inelastic collision, the kinetic energy of the target after collision is very small, basically the ratio of the pellet mass to the combined mass. So the measurement needs to be carefully done. If your target mass is too large, the motion is hard to measure.
In terms of damage to the target, most of those deformations are inelastic (hopefully) and are based on energy needed to cause the deformation. Now if you are trying to blast that ground squirrel off of a cliff then momentum is the driver.
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Lloyd - You're welcome to sit anywhere in the 'room' you feel comfortable.
Stan - The link was to a PB project, but I thought it might stimulate some thinking here too. Seems to have worked.
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Yeah, physics is kind of like string to a cat...can't resist. I just hope I don't pull an atrophied brain muscle
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Getting back to the discussions from a few posts ago I'd like to add for clarity that my focus on the KZQ measurements is about reporting impulse numbers. I agree that velocity is a key factor in calculations of energy (ft-lb, a scaler quantity) and momentum (slug-ft/sec, a vector quantity). The unit of choice for my purposes will be force (a vector quantity having magnitude and direction) represented as an impulse (lb-sec, a vector quantity). After all, the KZQ is designed around the physics of a force transducer.
Not to belabor thought experiments here, but lets face it; they're cheap, easy to run, and akin to spreadsheets. Let's try this one:
We have a wooden plank and a large nail that's already started in the plank. We also have 2 hammers. One of them is a 16 oz. steel construction worker's hammer. The other one is a 16 oz. rubber car fender repairman's hammer. We want to drive the nail home into the plank. We give the nail a whack with each hammer swung at equal velocity (same mass times same velocity). What would we expect the results to be?
The graph image should be persuasive as to my choice for using impulse force as a defining quantifier for impact numbers at this point in the measurements.
Plot #1 is the rubber hammer. Plot#2 is the steel hammer. There's still lots to be understood about these measurements, but right now I'm just looking for the best way to piece it all together.
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George, I look at those two force-time graphs and I also see a mechanical punch press vs a hydraulic punch press.
Would you say that those graphs might also represent a slow moving heavy bullet and a fast moving light weight bullet?
Myth busters once did a segment about test methodology for head-on automobile collisions. The question was, given a head -on collision between 2 vehicles, both moving at the same speed, at what speed would a single vehicle need to hit a fixed concrete barrier to acquire equivalent damage. I think this also leads back to your original question about what metric quantifies real-world effectiveness of a bullet. Sorry if I am just muddying the water, but as you said, these mind experiments consume very little "real" budget.
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George,
For the scope trace in post 429, Do you have the lbs/V for channel 2, the stimulus trace?
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The posted graph image isn't meant to be analytical by any means. It's just to illustrate a concept in physics. The change of momentum produced by an impulse is numerically equal to the impulse. Or, the other way around, an impulse is equal to the change of momentum: Ft = m(V terminal - V original). V terminal will always be equal to zero when it's captured by the KZQ. V original will always be the velocity at the moment of impact. We end up with negative acceleration for V. It's F = ma from there out.
As far as plot 1 and plot 2 in the image go, we could switch the the hammer experiment for a KZ of either a vital organ shot or for a head shot. One is hide + flesh and the other is hide + bone (mostly). These forms of target can be replicated, to some level, by changing the KZQ's 'cookie'.
"Myth Busters" gets a "No comment". TV is what it is.
I have no 'real' numbers for the DSO traces. The charge amps were set to produce a 1 for the 8200 and the KZQ balanced at .685. That would be the gain ratio for these sensors in early tests. I'm only interested in relative measurements at this point so that the impact surface's sensitivity can be plotted for further tuning.
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As previously mentioned, an aspect of the KZQ front cookie is that it can be changed to represent differences in potential targets. Pellets of the same caliber and weight can have different geometries and materials at their tips. Comparative impulse profiles may be instructive for different applications. An impulse is generally considered to be a specific quantity measured as a time variant of momentum. It might also be viewable as an informative shape.
Much more testing will have to be done to make this device a reliable source for data, but some interesting things are beginning to show up in the DSO traces.
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The KZQ project has reached a point in the design and measurements stage that allows a certain level of predictability as to what the data window will look like. What happens within that window is what I'm sorting out now.
The measurements are such that the amplitudes and half sine frequencies are pretty well bunched into a very easily evaluated group of shapes that represent a pellet's impulse profile. This will allow the relevant data to be captured and stored on a much simpler acquisition system than my earlier ideas lead me to believe.
To that end I've chosen to continue the prototyping with just a very simple and inexpensive 1 channel DSO that is about the size an early smartphone. It can be connected to the output of the force transducer's charge amplifier and will be capable of storing many screen shots along with their data. This approach will eliminate the need for developing specific software and WIFI connections on an entry level system.
There was a time not too long ago when the idea of an average airgunner having a measurement system at the muzzle of their gun that would measure things like velocity, kinetic energy, and also be capable of doing a statistical analysis of exiting pellet groups would have been laughable. Now it's commonplace. Being able to measure and evaluate a projectile's impulse force at the POI is not that much of a stretch. It's just something new and different for the same airgunners.
The mini DSO that I'm starting with is really no more complicated to understand and operate than a stereo receiver's control panel or a modern car's dashboard controls. I've included a link to a YouTube video that gives a good introduction to this DSO. It's done by a guy that's sort of a 'Mr. Rogers' with and accent. https://www.youtube.com/watch?v=YzCkjb2ZlA8 (https://www.youtube.com/watch?v=YzCkjb2ZlA8)
I'd like to add that this instrument is not far removed from the mini DSO that Stan referred to back when we were both considering getting new o'scopes. Now I can see where these might fit into doing some level of ballistic testing!
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I've been giving some thought as to the best way to represent the impulse signals from the KZQ. Since the signal is, for the most part, a half sine wave and the impulse is numerically equal to the integral of the momentum there may be a better way to separate the two. My approach so far has been to inspect the maximum force signal as a time variant of the total momentum. I now think that there's an easier way to look at the data other than just the impulse's integral.
An alternative approach would be to consider the FWHM (full width half max) value. This method would only require measuring the impulse's maximum signal voltage and than measuring the time interval at 1/2 of that maximum value. There are many types of measurements that use the FWHM metric for device or circuit appraisal. Usually this metric is expressed as the "Q" value of what is being measured. This is a handy signifier since I've already got a "Q" in the new instrument's brand that describes the instrument's purpose as a Quantifier.
Any thoughts out there?
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George, I'm curious how repeatable the traces from actual pellet impacts are. The putty is a non-linear material and may have different thickness. There may also be small voids. While the area under the curve is dependent on the pellet, the shape of the curve is influenced by the putty and could vary. Do you have an example of a pellet strike?
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The ballistics test bench has been rearranged to make it easier to position the KZQ for testing and making measurements. I did some test shots with the muzzle to POI set at 14". The Combro chrony is installed on the carbine version of the CP-2. Having muzzle numbers for comparison with the KZQ data at different distances can finally be done.
These DSO images are just a couple of screen shots to make sure that everything is working. There are some numbers here that can be seen as samples of where the current testing is headed using the FWHM approach. I think this will be a good method for POI measurements if they compare favorably with the muzzle data. The 2 shots were done using CPHP 22 cal pellets aimed at different areas of the cookie. 1 image is of a shot aimed at the center of the cookie where the calibrated KZ is 3/4" in diameter. The second shot is closer to the flexure's edge on the face of the load transducer. The sensitivity drops near the edge because the cookie tile's base attaches to the very stiff body of the sensor.
The point here was just to capture the pellets without over penetration or rebound. The goal was to see if there were any noticeable distortions in the signal's shape or other perturbations that might give some merit to Stan's mention of putty nonlinearity. These were simple tests to confirm that things are ready for real testing and I'm satisfied that no problems have cropped up yet.
The 3rd generation of the load transducer is close to being finished and some important improvements have been made. The area of flat signal response of the KZ has been increased along with other improvements that should make the system fairly easy to use.
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The gen 2 KZQ force transducer has a central area that has the highest sensitivity to impacts. Now that I'm starting to standardize the testing and measurements using the gen 2 transducer until the gen 3 can be installed I thought it might be interesting to show the testings variables evolve. The confectioneries have changed from being an Oreo without the top cookie to a single cookie (ballistic tile) with an attached Milk Dud (putty) in the center. See photo.
The aim (pun) here is to try evaluating the putty in small amounts where the force transducer is most sensitive and reproducible. It's sort of a shotgun approach to getting initial values, but the testing will scale down to simple parameters quickly.
The photos are self explanatory to me, but may leave many readers nonplussed. I'm counting on Stan and Lloyd and anyone else out there to jump in and pry more information out of me. That would avoid a lot of possibly unnecessary details on my part.
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George,
Thank you for the photos. The test setup looks great, looks like the Mild Dud approach works well.
I'm still trying to understand the shape of the curves you are getting. I didn't expect them to be so symmetric. Earlier you said you had a 10 Khz filter on the amplifier. With most of the curve falling within .2 msec is 10 Khz high enough? Could it be smoothing the curve?
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Stan - Thanks for the input. The symmetry of the impulse's shape is do to the filtering I'm using to reduce signal noise and DC offset. I've chosen to filter the low end at 2 Hz and the high end at 3KHz. If you look back to post #412 in this thread you'll see how an impulse can generate continuous frequencies and amplitudes within selectable bandwidths. With the KZQ materials I've chosen for capturing the pellets along with the force transducer's design it's possible to carve out a half sine wave shape that will hopefully be compatible with using the DS211 DSO as an acquisition system.
Keep in mind that a square wave with frequencies broader than the described filter, when run through that filter, will have something (on one half of the wave) which will be similar to the shape of an impulse when it's viewed in the time domain. If you evaluate the same filtered square wave with a spectrum analyzer you'll see a series of odd harmonics that the signal is constructed from.
On the other hand, a true impulse signal is constructed from an infinite number of frequencies (in theory). The amplitudes within the filtered frequency band I've chosen should provide a uniform and reproducible signal representing the impact. This should hold as long as the force transducer is linear within that band and the impacted material stays consistent within the dynamic range of the system.
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I"ll have to think about that 3 Khz filter. Also, I thought the only assumption or feature of the impacted material was that it absorbs all the pellet momentum and there is no rebound. Adding a consistency assumption for the Milk Dud from shot to shot may be hard.
Perhaps it will all be clearer once you tie it together.
Thanks for the update.
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Here are a couple of photos to further explain the signal filtering along with the attempt to find the best size for a Milk Dud (MD) within selected energy ranges at the POI. The filter in use is set at 3KHz and is second order in effect. As can be seen it's not an axe like a digital brick wall filter would behave on this spectrum. The cursor is set at 2 KHz to indicate the approximate area where just a single peak would be found if the impulse were a typical sine wave. The impulse has generated ALL of the frequencies shown in the shaded area of the spectrum.
The other photo is of the MD being pressed to a defined thickness (.5") using a platen. The MD is first formed by rolling a predetermined amount of kneaded putty into a uniform ball between the palms of the hands. The size and thickness of the MD has an influence on the impulse signal.
The previous reference to consistency was regarding the geometry of the MD. So far there has been no sign of nonlinearity in the makeup of the putty itself. The kneading and palm rolling seems to be reliable at this point. The thickness will have to be adjusted to accommodate different energy ranges. This will be a trivial matter once these thickness vs energy ranges are determined.
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The gen 3 KZQ has gone through the preliminary tests with flying colors and is now ready to be installed into the trap. The clamping back plate just needs 2 holes for the bolts and a new round of testing can begin. The early tests on this new and improved design show a much higher sensitivity than the gen 2 and better linearity over a larger impact area.
When finished as a product the goal is to have an instrument that will give reliable POI information in the kill zone diameters for rats, squirrels, and rabbits. I suppose this could be referred to as the "Small Mammal Pest" model for humane kills or just a POI test instrument for those airgunners that want to raise the bar above spreadsheet guesses.
The photos show the gen 2 next to the gen 3 model.
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Lloyd mentioned something a few posts back about an analogy relating to punch presses and hydraulic presses. These presses use force differently. The punch press uses momentum which is a vector quantity and the hydraulic press uses pressure which is a scaler quantity. I didn't want to leave that concept just dangling out there.
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George
Thank you for the information you (and others) have provided.
I have read your first post, Hacking the Crosman Vigilante on another site and then here on GTA,
I have just finished reading your second post, Hacking the CP-2 , Great information, what little bit I understood,
I understood some of the information presented, even thou 99% was over my head.
In both of these post, 2 different sites, you have dangled a carrot, showed some photo's, in front of several of us,
and kept leading us down the rabbit hole path etc, are we going to get at least a nibble of the carrot?
I am sitting behind LLoyd and in the corner, as I know nothing.............. ;)
There is several of us that would like the info, and I for one am hoping you can provide it for this carrot:
The photo shows the Co2 cylinder heaters, Lithium-ion battery pack and the temperature controller, as they will be installed in the stock.
I'll make a mounting plate for these devices and do the simple wiring needed to power the heaters.
This has turned out to be much easier than what I thought it would be,
when the original heaters were installed into the pistol frame and powered using a bench power supply.
* 1-P1050159-001.JPG (92.91 kB, 1024x470
I am hoping that you planning to revisit this section? after all it is hunting season and too COLD to operate Co2 reliably,
at least please provide the proper names of the above equipment and the wiring diagram etc.
It would be greatly appreciated by us carrot nibblers, bring up the rear on this trail............
Thank you,
Don
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Don - That was a long time ago. I bought the parts on eBay and they've since been spirited away into who knows what other projects. The parts should still be available is my guess. I don't have a list or wiring diagrams. Never did.
If you and your friends are thinking about heating powerlets I advise great caution. The old adage "Don't step where you can't see the bottom" is applicable here.
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Thanks George
That is the thoughts I have, 2 different Co2 airguns to set up, A Crosman 160 and 150, 22 cals.
Both have massive heat sinks, with their tubes+barrels etc.
I am thinking about starting with the 85*F heater, and testing both, empty cylinders, and then an empty Co2 (fired) cylinder,
then a live cylinder in that order.
I have a very accurate temp meter with an 18" long probe so I can get temp reading right over the heater/Co2 cylinder/bbls etc.
What is the proper name for the temp controllers?
I for one will use extreme caution, I have had enough Kabooms in my life............Lol
Any and all info you can supply would be a great help.........
Thank you for your reply.
You can send a PM or email, whichever you feel is the best.................
Don
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Don, as said be careful. Having an accurate temperature meter can provide a false sense of security. The placement of the sensor relative to both the heater and what is being controlled, as well as the thermal response time of each, all go into the design of the control system.
There are self regulating heaters that could help but unfortunately the ceramic PTC type all control to higher temperatures. There is a PTC rubber heater that controls lower but I'm not sure they are available at the retail level.
Not sure if you are planning to control the individual powerlets like George suggested in the Vigilante application or the tubes. The tubes are probably easier but could lead to a bigger Kaboom.
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Hi Stan
I have some question as to the heating of the cylinders/tubes and the Co2 gas once released, at this time.
I would believe that, in this order, that the heating to xyz temps would be,
1. Co2 gas in the tube, easiest heating/fastest response time
2. Steel cylinder/tube and Co2 gas, would be slower to get to temp, but hold the temp longer etc
There is No way to heat just the Co2 cylinder in these type of airguns, 150 pistol and 160 rifles,
(like George did with the cylinders in the pistol grips) as the wires stop that process etc.
That leaves wrapping the heaters (depending on which size used) around the Co2 cylinder and,
the bbl right directly above where the Co2 cylinder sets.
I am not sure if heating the bbl has any advantage as compared to the Co2 cylinder with gas only etc.
I am also thinking of machining a Co2 cap that has a 1800 psi burst disc in the end,
I would like to find a 1000 psi burst disc, LOL.
Lots of item to be considered to avoid a Kaboom...........
Your thoughts or suggestions would be greatly appreciated and not held liable..........
Tia,
Don
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Don, excuse my ignorance of the classics. Do both the 160 and the 150 have a pressurized tube (like my QB78) and don't just press the powerlet against a seal like the 2240?
I think this would be easiest to implement on a bulk fill, then the 160, and the 150 being a little more challenging.
A custom cap would be my first choice. I don't think you need to heat the barrel. On the 160, I'm not sure you need to wrap wires around the tube (unless you are sneaking in a cozy hand warmer in the fore stock).
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Stan - Thanks for pointing out potential drawbacks to the heating of powerlets. Here's something more to add to the list. One of the most important, though often overlooked concepts in thermometry is this: 'A thermometer only reads it's own temperature'.
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Stan
Both the 150 & 160 have pressurized tubes, after the Co2 cylinder is punctured the gas fills the tube etc.
On the 150 there is a bare metal tube that the Co2 cyl fits into hanging out in the weather,
On the 160 it is the same, except the cylinder tube has a wooden stock (normal rifle configuration),
for some insulation from the weather etc.
The custom cap should be able to fit either air gun/pistol.
As far as the bulk fill, wouldn't that be a bigger problem, ie, a bigger Co2 tank to deal with in the cold weather,
not sure I am following along correctly with the "bulk fill" note?
George
Thank you for your input, and you'll not be held liable also.
The instrument I have reads temps via a multi meter gauge, with a "K" probe,
I have tested melted lead and have tested it on the wood stove for temps at various locations,
for setting the best location of the automatic off/on Piezo fan switch etc.
Thank you,
Don
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Don,
Here is the train of thought on the heating approach and the bulk fill. Note, this is just brainstorming, no engineering calculations or drawings.
I was assuming bulk fill where you use the tube volume, not the external paintball bottle (QB79) approach (for the paintball bottle w/burst disk, I think you just bond on some kapton heaters, mount a couple of TCs, slip on a koozie, and tune the PID controller).
For the tube only type bulk fill, I was thinking that you could make a custom cap that includes a heater/TC probe that is internal and immersed in the CO2. I thought it was easier since you have an unobstructed internal volume.
For the 160, the thought was that you could use the volume of the outer powerlet to have a more robust probe that also retained the inner powerlet. Fewer shots but hey if it is cold outside that may be OK.
For the 150, it may need to be just a longer, hollow cap that is heated and the cap has some internal porting to let the CO2 circulate somewhat.
The battery and controller would need to get packaged. For the 160 either under the barrel or in the stock like George suggested. For the 150, it is harder. perhaps a LiPo battery under a custom grip.
Like I said, just a train of thought....plenty of Kaboom potential that would need to get addressed.
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The new KZQ design is working better than anticipated. The sensitivity is ~ 10X higher than the previous generation. Some unexpected signals have shown up in the measurements that arise from the much lower resonance frequency of the transducer as a whole.
This much lower resonance has started me down another wayward path heading toward developing an interesting new way to build a short period seismometer. If the resonance could be reduced to ~ 1 HZ with a Q of .7 it may turn into another product. I have no idea of what the market might be, but I know that at least I want one!
The DSO images are from some tests I did today. The first one is of a pellet at ~ 465 fps that includes the half sine wave impulse along with the resonance sine wave. The new KZQ was attached to the back panel of the trap. The second one is of a double pellet shot with the KZQ detached from the back of trap. This is pellet stacking with one shot. The 2 pellet velocity was ~ 330 fps.
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Seismometer...an interesting thought. Somewhere I have a geophone, they measure velocity. I wonder if I put on a Milk Dud target plate, and suspend it properly, the velocity peak should be analogous to a ballistic pendulum.
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Your geophone might work if you can attach it to a large mass and very compliant spring. The qeophone would also have to be able to work in the horizontal position and move with a linear stroke. They're usually limited to only a couple of mm of displacement though. Damping is also an issue. If you used the modal hammer calibrator as a model for a short stroke linear displacement device it might work.
The project would certainly be educational and worth the effort in my opinion.
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Educational - yes, starting geophone 101
I set up a quick configuration to see how the geophone reacts to an impact, especially in a horizontal configuration. The geophone is clamped behind the target board and a supersized milk dud (modelling clay) catches the pellet. The assembly weighs 226 g (3488 gn) and is roughly horizontal. The pellet is a 7.5 gn wadcutter, fired hand held, maybe a foot from the target.
The results were relatively consistent, about +/- 5 %. It takes about 500 micro-sec to get to the voltage peak (7.44V). I assume this is the path through the milk dud. The two traces show the short and long time traces for two shots.
I have not tried to calibrate this with a chrony yet but I saw somewhere that 20V/m/s is in the ballpark for the geophones. Using that number, the 7.44V and the mass ratio of 7.5 gn to 3488, calculates out to about 570 ft/sec. Probably a little high but a starting point.
Some thoughts: I was generous in sizing the milk dud (did I mention hand held and point blank?). It is not clear how much of the milk dud is accelerated in that first 500 micro-seconds as opposed to getting stretched. Now that I know the geophone may be useful, Increasing the mass of the target plate (metal) and reducing the thickness of the milk dud should reduce this effect.
Doing some tap tests on the assembly and the geophone should be interesting, as well as running some tests in the vertical configuration.
Can't let George have all the fun.
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Stan - It's good to see that you're back in harness at your ballistics bench. I'm looking forward to seeing what you find using your geophone.
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I did a little more work with the geophone as a sensor. I made a target block out of some cast aluminum. For initial characterization, I used a pendulum mounted weight to repeatably strike the target. There is also a laser light gate to measure the speed of the weight just prior to impact.
What worked: The light gate and the calculated speed based on drop height were consistent with each other and repeatable....The law of gravity is safe (kudos Sir Isaac). With modelling clay at the impact, the geophone output (peak voltage) for the pendulum weight strikes was repeatable and consistent with what a reasonable sensitivity might be for the geophone. The pendulum weight had the same momentum as a 7.4gn pellet at 370 ft/s so it is a reasonable test.
What didn't work (yet): If you don't use the modelling clay at the strike point of the weight, or for actual pellet strikes, There is a higher frequency in the output that makes getting a good peak value unreliable. This frequency can be moved around some but is present if the target experiences a hard impact (the pellets cut through the clay) and a resonance somewhere in the system is excited. The options are a little constrained because as you soften the impact point you slow the response to where the geophone first mode starts affecting the peak voltage.
Not quite ready to give up on the geophone, there are still some mounting options to try as well as tuning the impact material. As a side interest, I may try using the light gate to measure the target velocity. This wasn't the original objective but diversions are not unheard of in this thread. Also looking forward to George's piezo results.
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Finally got some hopeful results with the geophone. I tried a number of mounting and testing configurations trying to get rid of the high frequency content that is present in pellet shot data that you see in post 466. I could get rid of the content in the relatively low velocity impacts of the pendulum weight (post 468), but the high velocity impacts of the pellet excited the higher frequency, no matter how I configured the clay buffer or the Geophone mount. I thought it was the assembly but then I tapped just the geophone itself and it does have an output mode in the 2 KHz range (note vertical and horizontal orientations behave similarly).
I switched to using thin LDPE bags as the breaking medium for the pellet (something Lloyd used in his high velocity tests). The target configuration is in the photo below. The PVC tube just provides containment for several plastic bags. The clay is there to primarily hold the PVC. The aluminum block provides the mass and the geophone (brass color) is clamped to the back. I also attached my microphone block to the back to provide a shock indicator. Everything is softly suspended.
The second image shows output that still has the high frequency content. This was from a shot that still had a bunch of clay inside the tube and less plastic bag so the pellet still hit the clay. The third image shows the final output without the high frequency content. I had taken out almost all of the clay and filled the tube with plastic bags and the pellet stops in the plastic and does not hit a hard surface. The hope was that the pellet deceleration would be slow enough so it does not excite the higher modes but is still well above the geophone first mode around 30 Hz. I could have used the O-scope low pass filter function but since I want to capture the voltage peak right after impact, I did not want to modify the waveform.
Next steps would be to do a number of shots to see if the results are consistent and set up the chrony to get a calibration of the sensitivity.
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Your experiments are looking interesting. Are you using a damping shunt resistor across the geophone's output? It would be instructive and useful to fully characterize the device without any additional mass and spring components.
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George, I did some simple tap tests of just the geophone softly suspended. I did include my microphone block (double sided tape mount) just to confirm any higher frequency features.
The geophone output is channel 1 one in each case. The first plot is vertical, the second is horizontal. It was meant to be a quick and dirty confirmation that there is high frequency content in the geophone itself.
It showed that there is higher frequency content to be avoided. I had also tried a saddle mount for the geophone and that had the content as well. This is why I went after softening the impact.
The geophone is marked 400 ohm and measures out at 387
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The suspension appears to be rubber bands which would add another spring into the system. Supporting the geophone on some very soft foam would be closer to a free-free condition.
The shunt resistor can be used to load the geophone's coils and reduce or eliminate overshoot and ringing. A damping factor (DF) of .7 is about ideal and can be found experimentally or from the manufactures specs.
The other detector adds mass to the geophone for the free-free measurement, but could still be helpful in the overall scheme of things at this point. In theory the housing and magnet of the geophone are anchored to the earth and follow the earth's movement while the spring suspended coil assembly, due to inertia, remains in a quasi-stationary position with the coils generating a velocity following signal. https://shop.raspberryshake.org/product/geophone-earth-sensor-cut-open/
I'm sure that you're well aware of all this stuff, but it might be helpful info to anyone trying to follow our continuing follies. It looks like you're making good progress. Please continue to post your experiments.
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Thanks George,
I used the rubber band suspension because it had a fairly specific suspension mode in the 1-2 Hz range and I was looking for KHz features. I'm a little cautious about loading the geophone circuit. Since I'm looking for peak amplitude somewhere around a msec in, I want to avoid filtering in that timescale.
I took the configuration shown in the last post and did some shots for repeatability. I used a P17 which puts out around 400 ft/s with a 7.5 gr wadcutter. The PVC snout is stuffed with 3 LDPE (newspaper) bags and each pellet is removed between shots.
For a string of 10 shots, I got a peak voltage average of 2.86 V and a standard deviation of 3.3% The first image below shows a typical clean trace. There were a few in the string that had some ripple on them.
I then took one of the bags out and got the second image which shows that the pellet tapped the block (I could see a small indent in the clay). If I'm reading the trace right, I think it shows the higher resonance being excited and affecting the waveform before the basic signal has reached its peak. That is what I'm trying to avoid. I think I'll use a longer PVC snout so I can get some margin in my baggie brake.
I think the next step is to set up the chrony and see if this all comes up with the right answers. I'll use one of the crosman pistols that has a power adjuster.
The geophone I'm using, I bought surplus long time ago (it has a Reagan era date stamp). I just ordered a couple more surplus units to see if they behave the same. I had forgotten how much fun they were to play with.
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The original KZQ device that morphed into a seismometer boondoggle as a substitute for the geophone front end on the Raspberry Shake instrument has taken another turn. The educational market won't have any money to spend on new lab course instruments anytime soon so I'm back to the POI airgun devise.
The KZQ in it's previous incarnation was being designed to measure the POI energy as an impulse number. This was going to introduce a new metric that would probably face considerable resistance in my targeted(?) market for the device. I've since come up with a compact dual diode array scheme that would bring the metrics back to a conventional set of numbers similar to what a generic chrony would provide. I'll report on progress as (if?) it progresses.
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Here are a couple of photos that show how the diode arrays are going to be assembled. They will be mounted in a pellet proof housing and finally mounted into a trap configuration about the same as the original KZQ system.
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Interesting, looking forward to the design
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Wow. I've just spent the better part of two days reading with great interest the exploits of George and his peer reviewer / collaborator Stan. I am blown away by their deep technical knowledge, intense curiosity, and ability to tee up a question and work it.
When the barrel harmonics work morphed in the KZQ quest, I became concerned that we will not see much more insight on the harmonics beyond Replies #366-369.
I learned a huge amount through that point - enough for me to conclude that the hammer-valve interactions and pellet movement dynamics are likely the biggest drivers of vibrations and barrel swings. And that damping vibrations might be a better strategy than timing the vertical barrel swing to improve pellet direction consistency. And enough to get me interested in putting sensors on the muzzles and receivers of my PCPs to see if I can correlate the x-y patterns and shot groupings, and then test ways to tame them. (I've already used audio techniques to determine their shot development times, and maybe can adapt that equipment for motion sensing)
So a big thanks for all your efforts even though a summary of findings and recommended approaches for future barrel harmonics hobbyists might never appear.
I hope George and Stan are well and enjoying their latest exploits.
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It was a bit of a shock to get the notice of a reply on this thread, hadn't seen it or thought of it in some time, life's been too busy. I hope you share your own journey and experiences with us, there is still a lot to learn.
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RScott, I'm glad you enjoyed the thread.
I took a brief look at muzzle motion on the last page of this thread https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.220 (https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.220)
I was looking at hold sensitivity but harmonics would be part of the motion.
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I noticed some recent activity in this thread. There was a request to expand upon what the barrel harmonics testing might really mean. Going back over the information that Stan and I posted on the subject it still looks to me as though the idea of barrel harmonics during the shot cycle have little to no effect on a pellet's point of impact. The real driver of the muzzle's position at the time the pellet is released still seems to be controlled by forced oscillations.
Stan posted some interesting overlays, but I'd want to see more data that would compliment what he has shown. My thinking at this point is that barrel harmonics controlling POI is still a myth.
I'm open to any and all experimental evidence that might change my mind. I'm not interested in opinions.
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George,
Nice to hear from you and see that you are still interested in this stuff. It might be useful to clarify your distinction between forced oscillation, harmonics, and rigid body motion of the muzzle so that everyone is on the same page.
As an aside, I was always curious if your pressure sensor survived the high temperature peak in the Titan shot cycle test last year.
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Stan - Here's how I'm looking at the information we've accumulated so far:
A forced oscillation is something that pushes or pulls on a body in a way that causes motion that is not synchronous with the body's natural frequency. If this forced oscillation (an impulse(s) in our case) contains any frequency that includes any of the resonant frequencies of the driven body (the barrel in our case) then the body will continue to oscillate at resonance until any form of damping within the system reduces the amplitude(s) to zero. These resonances are the barrel's harmonics.
The forced oscillations are transient in nature and decay quickly, but not before the pellet leaves the barrel in our case.
A rigid body, by definition, is an idealization of a body that does not deform or change shape. A barrel driven by forced oscillations and harmonics doesn't act as a rigid body.
As far as the survival of the pressure transducer goes I'll have to check the calibration on it. As I recall it's still functional.
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I mention the rigid body motion because the spring/piston system is offset from the CG as well as from the support conditions so a rifle rigid body motion (rotation) is present along with any barrel deformation and can be difficult to separate using muzzle located instrumentation.
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It's not clear to me how a rigid body rotation could effect the barrel harmonics. Rotation as you've described it may be a component of the shot cycle that could effect the POI though. The magnitude of the rotation could be evaluated as a separate set of experiments if you think its contribution to POI is significant.
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My point was that rigid body motion is present in the accelerometer measurement at the muzzle.
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I've been doing some thought experiments that might provide an indication that rigid body (RB) rotational forces could be an influencing factor in POI. The measurements using accelerometers at the muzzle of the CP-2 were all done while the gun was clamped in a vise. Any RB rotation contribution would have to include the simultaneous rotation of the vise and bench that the vise was bolted to. I find this contribution to be implausible when considering the measurements at hand.
These experiments don't account for Stan's argument about RB motion when shooting freehand. I haven't made any muzzle measurements when the gun wasn't in the vise that I can recall. I'm not going to chase my tail trying to prove a negative, but if anyone can come up with a persuasive experiment to support the argument I'll listen. The ground rules would be a demonstrable nontrivial rotation within 0.004 seconds or less from the time the piston is released. I look forward to any suggestions.
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yes, no need to instrument the vise.
Though if you did any testing in 2020, you might check this chart of earth rotation anomalies to see if any of the dates align.
https://www.timeanddate.com/time/earth-faster-rotation.html (https://www.timeanddate.com/time/earth-faster-rotation.html)
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I am starting a journey to get as good a score in 25 meter benchrest that I can. My plan is to test lots of ideas related to pellets, gun harmonics/vibration, benchrest equipment, FPS, trigger technique, etc. I'll probably start a new thread after I get underway, but along with shot precision measurements, I'd like to see what impact some of the modifications have on the gun's vibration, along the lines of all your tests of the CP-2. The biggest decisions to make on measurement equipment are:
1) Use a Microtrack recorder I got for lock time (aka shot development time) measurements or get a Rigol DS1054Z Digital Oscilloscope like you guys (George and Stan) have used. The Microtrack is an audio recorder but it seems to measure vibrations when the microphones (actually repurposed earbuds) are taped to the barrel and receiver. Advantage is cost. Disadvantage is 2-channel limit and need to download files to my PC and use of Audacity to analyze the data. The Rigol is expensive and may take a lot of effort to learn how to use.
2) Use the microphones like I used for the lock time work or a Endevco Triaxial PE accelerometer Model 23 that I bought on eBay last year for a bit over $100. I've hooked the Model 23 to the Microtrack but don't seem to get great x,y separation in the readings. I can't even figure out which is x vs y vs z yet.
Questions for you guys:
-Any thoughts on how useful the vibration data would be for my efforts?
-I've never used an oscilloscope before? Will there be a big learning curve? (I'm a long-ago retired chemical engineer)
-Does the Rigol give fairly immediate results, e.g., take a shot/meaurement, review it after a few seconds on the screen to decide whether to keep it or re-shoot with new data sampling?
-Is the Model 23 going to work well in this application or should I have bought something else to take barrel / shroud movement measurements?
Attached are pictures of the equipment I have and a summary of the lock time analysis.
Regards,
RScott
P.S. I'll be using a .177 Daystate Pulsar shooting at 12-20 FPE at USARB 25m targets outdoors, and a modified Sinclair front rest. I can achieve 730 scores in 75-shot benchrest with it as is, but I want to get 750 scores and as many "X"s as possible.
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RScott, Nice to see someone diving into the measurement pool. If you have not seen it, this thread provides more information. https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg156145889#msg156145889 (https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg156145889#msg156145889)
The basic question becomes what specific data will be useful to you. From the work I've done, microphones are useful for timing events. It gets more challenging if you want to measure amplitude of accelerations and the resulting motions. You'll see I used a simple light gate to detect when the pellet exits the muzzle. You can also use a microphone to detect an event in the shot cycle but it is not quite as direct.
I think you also need a charge amplifier for the model 23
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Thanks for the link. I had seen it a while ago but was glad I read it again. I might look into filming a laser pointer near the target during the shot. Will be easy to calculate when the pellet leaves the barrel based on when the target is hit. I think I can also easily use a mic at the receiver to estimate when the pellet starts to move (having looked at a lot of Microtrack diagrams; and an electronic PCP's audio track is much easier to interpret than your springer) which then enables me to calculate when the pellet leaves the muzzle. (The sound pattern at the muzzle is very messy for determining the time of pellet exit)
I infer from your charts that you have been able to reliably quantify muzzle movements using your DSO and accelerometers.
You mentioned I might need a charge amplifier to use the Model 23.
- I read about George's recommendation of a Bruel & kjaer 2635 Charge Amplifier. That would put a further dent in my budget for this effort.
- Why would I need a charge amplifier? I thought it was used to do additional integrations of the velocity data? Wouldn't the Model 23 connect directly to the Rigol DS1054Z and provide acceleration and/or velocity data that I could put into Excel to calculate x,y coordinates?
And if I go down the path of DSOs, charge amplifiers, and accelerometers, would you be willing to help me troubleshoot the set-up issues I would likely run into?
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I infer from your charts that you have been able to reliably quantify muzzle movements using your DSO and accelerometers.
Well, that is a bit of an overstatement. The charts were a first look into what data may be attainable and whether it is useful. It is not at this point an established process with a reliable/tested result. I am not completely sure what the complete dataset for high precision shooting might be (muzzle linear accel, velocity, displacement, angular equivalents, etc.). I am going through a similar thought process on what it would take to monitor a pistol shot.
I have not used the model 23 accel but the spec sheet looks like it does not provide a voltage output by itself, so a charge amp may be needed (per channel).
In this thread as well the Titan thread, George and I took different approaches to taking measurements. George has a wonderful collection of laboratory equipment and is experienced in their use. The caveat on lab instruments is that you tend to need the matching equipment, even cables, and these can get expensive. I tried to look at what could be done with hobby level instruments.
The reason I mentioned the laser light trap is that it is a very cheap (<$5) way to get the pellet exit timing (the broken wire works as well but is tedious). Unless you have very high frame rate I don't think video will get you there. The entire shot cycle is shorter than one 60hz frame.
Depending on what you want to do, there are cheaper DSO options than the Rigol.
Maybe start a new thread and list what specific data you think will be useful, and I'll give you input as best I can.
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Thanks. Will do. Though I might approach it by trying to use what I have and getting reactions and improvement suggestions from you, George, and others