GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: George Schmermund on July 30, 2020, 02:13:11 AM
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I was sorting through a composting pile of airgun paraphernalia the other day and found a couple of scopes that I had bought for my one and only break barrel rifle. Neither of these scopes proved to be up to the task of being useful for break barrel duty despite the manufacture's claims. The power plant in this rifle is a gas spring type. I think the gun is capable of generally good shooting performance, it just beats up the scopes very quickly. That's when I switched to the CO2 guns outlined in my previous "Hacking" threads.
After having found these old scopes I felt myself being mystically drawn toward another one of my "spiraling into the sunset" bench testing boondoggles.
Relocating the Benjamin rifle, which had been safely sequestered out of sight in a remote corner of the attic, an investigation into the destructive power of spring airguns was afoot. My delusional hopes were that it might provide a possible means of taming spring air rifles to the point where the mortality rate of garden variety scopes would be lower. To that end I began looking for any actual metrics that the internet could provide as a starting point.
There were plenty of sites and videos available out there that described or demonstrated the effects of spring airgun recoil, but nothing much that truly addressed the actual testing and measurement numbers and how they were collected. I'll leave this previous comment as an open invitation for Stan to apply the powers of his search juju.
In the mean time I've come up with the first approximation of an experimental arrangement for generating some starting point numbers. My main interest is to quantify the actual forces, their directions, and the time intervals of each shot cycle. The basic setup includes a 1" steel bar that sits in the scope mounts on the rifle. The bar's forward facing end has a B&K force transducer mounted on it and is pressed against a rigid structure . The rear end of the bar has a B&K accelerometer attached. The total mass of the bar and transducers is close to the same mass as one of the previously mentioned scopes. A Combro cb-625 MK4 Chronoscope is mounted on the muzzle end of the barrel for measuring the exiting pellet's metrics.
The instrumented rifle is supported at its balance point by using a rest. A foam block is used at the butt plate of the stock to simulate a loose "artillery grip" hold. The rifle is basically very stable at this point and will be easy to remove and then return to a reproducible position for each shot's measurement.
The photos show the basic physical arrangement with everything mounted on the milling machine's table. Next is to connect the transducers to the electronics and document the experiments.
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Definitely looking forward to this one. :D
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I'm interested too in seeing your test results. Thanks for sharing.
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Nice to hear from you George, of course you picked a rifle that I happen to have so there is some chance I'll get sucked into this thread. I think that is the sturdiest shooting bench I've seen on GTA.
If you need some axial compliance in the forward support, there is always the advanced artillery carriage in this thread.
https://www.gatewaytoairguns.org/GTA/index.php?topic=175397.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=175397.0)
Looking forward to your testing.
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Stan - Your printing project is very interesting. The progressively uniform angle upward on your target is similar to the one I got last week when this new testing fever overtook me.
The testing started with the rifle upside down horizontally with just the barrel clamped solidly in the workbench vise. This allowed me to not only cock the gun without removing it from the vise, but also to sight down the bore from the breach end with the laser aiming device while the gun was open.
The photo shows that the target was turned with the top at the 9 o'clock position for these shots. The rifle requires ~45 lbs to cock so it seemed likely that the POI was moving because of the cocking force. After even more severe vise tightening the pattern still remained. I was nonplussed at this outcome until I came to my senses and realized that if the gun was loose in the vise jaws, the POI pattern would be vertical. I then re-tightened the vise's swivel clamp with a small cheater bar and proceeded to shoot the lower set of shots. The target distance was only 15 feet. It amazed me that the recoil could still cause the vise to rotate that much even though trying to move the rifle by hand seemed to produce no effect. The rotation was in the direction of the arrows. This would indicate that the recoil in the forward direction is much greater than the initial backward recoil when the piston is launched.
I'm going to propose the your rail is falling pray to the same effects. This assumed supporting information from your experiments is cause enough to add some level of ambition to my approach to finding the actual impulse magnitude of the recoiling forces. The goal is to see if the shot cycle can be modified enough to eliminate the need for an artillery grip (or rail). If nothing else I'll have learned more than I know now by the time something else distracts me!
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I like your scope surrogate mount for the sensors, I'll have to make something similar. It will be interesting to see the comparison between the accel and the force transducer. I'm not sure how the contact interface will track the reversing recoil cycle. I think the damage mechanism to the scopes is the acceleration term.
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The plan at this point is to just get some preliminary numbers. The instruments as currently arranged will provide simultaneous impulse force in N sec. and g force. I've attached the force transducer solidly to the front end of the bar and allow just the tip to be in contact with the block. This will produce good numbers in the forward motion. The reverse motion will be captured by the accel, but not exclusively. The force transducer is designed with a preload that allows it to measure force in both directions. The rounded tip attached to the transducer has a finite mass to it and will therefore measure some relative fraction of the reverse impulse along with timing information.
By using the testing assembly this way it will be easy to remove the rifle from the mill's table for reloading. If all goes well I'll will then remove the tip and fix the force transducer solidly to both the block and bar for more "advanced"(?) measurements down the road. The current arrangement is just proof of concept.
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Did some actual shooting today and collected some numbers. This is the first go-round with the measurements so the scope images will need some interpretation. This will be sort of a ruff sketch of instrument settings and the results they indicate.
First off the B&K 8200 output was connected to a B&K 2635 preamp whose output was then connected to ch 1 of the scope. The signal from the 2635 was set to 10mV/N. The scope's ch1 was set to 500mV/div. vertically.
Next the accel was connected to the input of a Kistler 504 preamp whose output was set to 200g/volt and then connected to ch2 on the scope. Ch2 was also used to trigger the measurement and was set to 1V/div. vertically.
The scope's timebase was set to 1ms/div. horizontally.
The photos show an assortment of cursor measurements and signal overlays. The gist of what's shown include the spring and piston release and recoil. About 7ms later comes the recoil from the piston hitting the front of the cylinder. There are many interesting features that show up in these screen shots and I'm sure Stan will have his way with the info.
It will be instructive to include a muzzle tripwire that can be connected to ch3 and see where the exit time of the pellet ends up. Surly it must be between the two recoil peaks!
I'm open to any and all interpretations of these experiments.
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George, thank you for posting these. They go well with my morning coffee (actually a good measure of how long it takes the caffeine to get to my brain).
A couple of questions (as usual): For the force transducer, is compression positive, and for the accel, which direction is positive? Also is there any gain/attenuation set in the oscilloscope?
I was trying to think what the shot cycle should look like before you posted. There is a technical derivation here https://www.airrifle.co.za/library/Ballistics_Internal_Ballistics_of_Spring_Piston_Airguns_Tavella.pdf (https://www.airrifle.co.za/library/Ballistics_Internal_Ballistics_of_Spring_Piston_Airguns_Tavella.pdf) . If you skip the derivation (way more coffee needed) and look at the firing cycle section starting on page 26, it gives some guide to what the accelerometer should see. The first few millisec after trigger release, it should be a spring (or air spring) mass system with the recoil having a ramp profile. The gas spring will have a different profile depending on the charge pressure, etc.
I think your idea of adding a trip wire or equivalent to the muzzle to anchor the timeline is great. If you get a chance, a shot that is relatively axially unconstrained with only accel data and a soft or sling support in the front would be interesting.
Yeah you gave me incentive to go dust off the oscilloscope.
Thanks
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Thanks for the questions. The polarity of the force transducer is positive for compression. The accel is positive when pushing up from the mounting base through the body. The scope basically acts as a graphing voltmeter without gain or attenuation. I can invert the signals for convenience when doing overlays, but that doesn't effect their values.
I had down loaded a copy of your link a couple of weeks ago. I slipped into a coma while trying to read it. I prefer to just do the experiments and then figure out what they mean after the fact. All roads lead to Rome in the end.
The thrust (?) of these test is to see what a scope is in for when mounted onto a springer rifle. There are several ways to evaluate the forces involved, so the experiments will expand along the way. The pellet exit time is an easy test to throw in, but doesn't have much to do with the scope's torment. I'm very curious about it none the less.
The image here indicates the time to peak for the spring and piston launch. The previous post has a time to peak image of the piston's impact at the end of travel.
It's nice to know that you might get lored back to the test bench again.
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Once you get a data set for this rammer, it would be interesting to see you do it again with a similar rifle set up with a traditional spring. It is known that the spring and gas spring/rammer are similar, but different. Seeing the differences mapped out might be quite instructive. I'd expect to see more peaks, albeit small ones, between the two you're already showing and after the second one as the coils of the spring tend to rebound before coming to rest. Thank you for sharing this experiment with us.
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That's very cool data. Thank you.
I've been creating a computer simulation based on some common springer dimensions and measured spring (coil and gas) forces. My computer sim shows some similar timing results with about 7.5 ms between piston launch and full compression (piston stop and/or bounce). My sim data indicates the piston "stop" impulse is 3-5 times greater than the piston "launch" impulse. A reasonable indication of why many scopes don't fare well on magnum springers.
The sim also indicates that the compression tube pressure is not high enough to start moving the pellet until about 5 mm before piston stop. That's only about 0.5 milliseconds before piston stop. Pellet acceleration time in the barrel is about 3 ms (16" barrel to 900 fps) so I believe you will find that the pellet does NOT exit the barrel between recoils. It will likely exit about 2.5 ms AFTER the second recoil.
If my sim is "ballpark accurate" then your pellet exit measurement should confirm this timing. As a scientist and engineer I am open to any repeatable data that may validate or invalidate my sim results. Contrary data will actually help me refine the simulation to improve the results. Looking forward to your pellet exit time data.
IMO all the rifle motions and vibrations followed by a late pellet exit explain why it can be so difficult to attain PCP-like accuracy from a powerful springer. The discipline of the shooter to learn the best hold for a particular rifle and maintain a consistent "follow-through" is the key to shooting springers with any accuracy.
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Michael, If you don't mind sharing the gas ram compressed and extended force data, it would help bound the piston launch event. I think rather than an impulse, that should look like the start of a ramp down. Depending on the support conditions, and the mass/inertial of the rifle, the magnitude of the acceleration could be estimated (more easily than the end of stroke) and provide a rough check on the measured results. You probably have something different that the Titan but it is likely to be similar.
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OK George, you pulled me into this. I started adding some ornaments to the Titan.
I took the laser/detector that I used in the CP-2 thread and packaged it to fit onto the end of the Titan Muzzle break to provide pellet exit (first image). It runs off of 5v and provides a sharp drop from 5V when the beam is obscured (channel 1 in image 2).
I had some real estate available on the muzzle module, so I attached and old 2 axis accelerometer board I had (images 3&4). It is a low g device and not fast enough for any amplitude measurement but it may detect the key events. I want to use it for some measurements outside of the shot cycle.
With all that attached, I took a test shot (image 5). The accel (channel 3 is axial) does provide a cycle start and perhaps piston impact as timed to pellet exit. There is a little over an inch between the actual muzzle (recessed in the muzzle break) and the laser beam, but that can be calculated. These plots aren't useful yet but got the process going
I plan to add some of my other poor-man sensors and clean up the wiring, and maybe learn something.
Looking forward to your high fidelity data.
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Looks like you're back in business, Stan. The exit gating is really clean. I'm looking forward to your progress.
Here are a few images of the recoil signals with the rifle just balanced on the pivot rest without the 8200's tip against the block or the butt plate's foam brace. Everything else is the same as the previous post. The timing and magnitudes can be evaluated with the screen divisions.
Included is a photo of the tripwire accessory machined out of Delrin. The wire is the same species of wire wrap used in the previous Hacking threads. The wire sits about .1" in front of the muzzle. It shouldn't take long to get ch 3 up and running.
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I switched to the microphone based sensors that I used in the CP-2 thread and mounted one axially on a scope surrogate and one laterally on the side of the tube (image one). I did a quick axial tap of the surrogate to see what frequencies are in the system. Looks like there are some in the 1+ Khz range (image 2).
Image 3 shows an overview of a typical shot cycle measurement. The Titan is shooting .22 Basics, 11.9 gr, in the past they measured at 740 ft/s. The pellet exit is the trigger for each trace (marked with the T flag). The laser gate is one inch past the muzzle so at that speed, there is about an extra 0.11 msec in the timeline.
Next few images are for a particular shot with some zoom in for different parts of the shot cycle. They are probably best viewed after an adult beverage or three to fully get on board with the unsubstantiated theorizing I'll be adding. ::)
Image 4 shows the overall timeline of shot 5. The blue (channel 2) is the axial sensor mounted on the surrogate and the purple (channel 3) is mounted on the tube. The shot duration is typical and in this shot takes 13.3 millisec from the first disturbance to pellet exit (T marker). These are not calibrated accelerometers, so the absolute amplitude does not mean anything, though within a trace there may be some qualitative information. The overall shape of channel 2 (axial sensor) is encouraging. It starts in one direction and then reverses, to a potentially higher value before it appears to impact about 11 msec later.
Image 5 shows the first section of the plot. The second cursor line is placed about where it is passing through zero, about 7.1 msec after start. If this is not an instrumentation artifact, it should be where the recoil starts to reverse.
Image 6 shows the possible piston impact (or stop) point. The channel 2 curve is flat (saturated) but the channel 3 (tube noise) shows an event at 2.4 msec before pellet exit. After that, there is some ringing, I'll have to see if I can determine if that is the barrel. There is nothing I can see that would show when the pellet motion started.
Fun stuff, all this info and $5 will get you a cup of coffee at the fancy place.
I'm looking forward to George's more rigorous data, which will probably blow all these theories into the weeds.
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The tripwire accessory works really well. The break is much cleaner than the one used to get the exit timing on the CP-2 experiments. Using the ohmmeter as the voltage source simplifies things.
The photos are self explanatory and clearly demonstrate that the pellet exits the barrel early in the shot cycle. The pellets are .22 CPHP.
This is another instance of spreadsheets vs measurements. No judgement, of course, just an observation.
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The photos are self explanatory and clearly demonstrate that the pellet exits the barrel early in the shot cycle. The pellets are .22 CPHP.
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This is fascinating. Thank you for applying your talent, knowledge, and equipment to springer investigations!
The timing doesn't seem right, though. If the pellet is out of the barrel so soon, wouldn't it mean that any piston movement past that moment does nothing for power? In other words, wouldn't a 30mm stroke make the same power as a 100mm stroke?
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The shot cycle is most likely longer than the scope's time window. My original plan was to just get some numbers to work with and apply them to impulse remediation for optical sights. Keep in mind that the accel scale is set to 200g's/volt just to keep all of the signal on the screen. That would mean the first division of the acceleration ramp is 200g's. There can be a lot of the early shot cycle action going on well before the scope's trigger requirement is met. How much time is spent in early spring/piston movement and compression? I don't know. That wasn't even a consideration when I designed the experiment. It did seem intuitive though, (from the CP-2 testing) that the pellet should leave the muzzle between the peaks.
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George, do you think the piston release is before the first peak on your plot?
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Stan - Is this a trick question?
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No
I read your earlier comment on the scope trace not containing the full shot cycle and there being piston movement prior to the trace. I don't think the piston release is a 200g event.
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Thanks, Stan. I think I understand the question now. I hope this answer will make things clearer. My comment was about the DSO's trigger set-point and not about the piston actually being physically released.
I'll expand a little further for those readers not overly familiar with a DSO (Digital Storage Oscilloscope). Using the images that I post, if you look above the data box at the top left of DSO's screen you'll see the carrot marker that indicates the time when the signal's magnitude crosses the DSO's trigger set point voltage requirement. (This carrot will be there with or without the data box.) The trigger's zero voltage is referenced to the screen's center horizontal line of dots. Everything to the left of the carrot is pre-trigger information. The DSO's trigger voltage set-point, polarity, and channel# is shown in a box at the lower right hand corner of the screen. For the last batch of DSO images with a data box the trigger point value is ~ 200g's.
There could be lots of action to the left of the screen that is pre-trigger info and isn't displayed. The confusion is probably in my not indicating which trigger I was referring to (rifle or DSO). I'll try to reference which trigger I mean in the future.
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I started using the pellet exit as a sharp DSO trigger, and then use the buffer to capture the entire cycle.
I'm still trying to understand the second peak in your trace, the one after the pellet exit. I wonder if the Titan can be rocking on the single support point and the butt knocking against the table. For my shots, I've been holding the Titan with just a soft, one finger support under the stock and by the trigger.
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The second peak is the piston slamming against the front of the cylinder. The force signal's time to peak indicates a very high impulse value.
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I was thinking that the piston impact signal would be opposite in sign from the piston release event.
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You're right that the assumed polarity of the second impact signal would be in the oposite direction. The ch#1 box at the lower left corner of the screen indicates that the signal is inverted. That would allow the traces to be overlaid more clearly. I can't remember if I inverted it because of the preamp's signal conditioning. Either way that leaves something interesting to unravel. The tip mass on the 8200 could also be adding another twist to things. This is all early stuff to be sorted out.
On another note, I've noticed that the shots with no cushion or stop on the butt plate always ends up with the rifle pushed backwards. It's hard to tell if it goes in both directions or only goes back. This motion may be too fast to normally see.
My curiosity got the best of me so the decision was made to design something for that measurement. Borrowing your Pro Construction techniques for assembling the instrument I was able to confirm that the direction of motion was only to the rear. The displacement was .3". This was one of my usual seat of the pants tests, but reliable to a certain level. More grist for the mill.
The photo shows the completed apparatus.
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Nice!, though the Home Depot paint stir sticks are my usual go to. I wonder if the motion would stay one sided if you had a sling support instead of the rigid notch (or one of those state of the art 3D printed linear bearing artillery carriages).
So for your scope traces, the accelerometer is positive for gun acceleration to the right (rear). The force transducer is (due to inversion) also positive for gun acceleration to the right (which due to the attached mass is creating tension in the transducer). Do I have that right?
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Those are really good suggestions. I await your measurement results!
Meanwhile I'll be working on the transducer enigma.
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OK, Since I made the suggestion, here is a shot (pun?) at it.
I set up the Titan on a roller in the front and my linear bearing in the back. I adjusted the front roller height until the underside of the portion of the stock that was resting on it was level so that there would not be a gravity drift. In this position you can move the gun with one finger and no return motion. I marked the starting points and put the paint stick(Lowes)/sharpie indicator in place. The results are traced on the white electrical tape (I added the start/end verticals). The carriage moves .266" to the right total travel and somewhere in the cycle recovers about .078" of that. I think there is some slip at the forward roller, I'll try some cloth tape to get the friction up.
Overall, it is a promising configuration. Since the travel is relatively short, I'm wondering if I could use a linear potentiometer to track the motion throughout the cycle.
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Nice work. I think we're moving in the right direction(?) for getting a much clearer view of the recoil dynamics.
While Stan and I are setting up our new experiments it may be instructive to see what one manufacturer is doing to try and get a springer rifle's recoil under control: https://www.youtube.com/watch?v=IRH0_3zAkpg (https://www.youtube.com/watch?v=IRH0_3zAkpg)
Here's another one of the same rifle in action. I consider this video to be much more informative. If you change the settings speed down to .25 times normal and advance the video forward to 5:20 you'll see some very interesting things. Watching a replay from 5:20 at least ten times with your eyes fixed on different parts of the gun and scope you might be left wondering what would be happening WITHOUT the shock absorber! : https://www.youtube.com/watch?v=PNgbF7EzAPU (https://www.youtube.com/watch?v=PNgbF7EzAPU)
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Most of the acquired data here seems to be consistent with the basic physics back-of-the-envelope calculations (see attached pdf).
The rifle should move rearward about a quarter of an inch from piston acceleration forward.
The acquired data showing pellet exit about 2.5ms after the beginning of a large impulse is consistent with those calculations, IF that impulse is the piston reaching the end of its travel. I think this is what is actually occurring in those scope traces. The "rifle trigger" (piston launch) event should be about 7-8 ms earlier off to the left (not visible) of the scope trace and up to a factor of 10 smaller in amplitude.
I suspect the second impulse after pellet exit on the scope traces may be a piston bounce event.
The simple back-of-the-envelope calculations ignore friction and air pressure forces so a piston bounce event is not considered.
However, in my computer simulation friction and air pressure forces ARE included and a "piston bounce" event can occur about 7-10 ms after the initial piston "slam-home event".
This thread has peaked my interest in actually measuring one of my rifles. Probably one of my Hatsan Edge rifles w/coil spring will be easiest to set up. If that goes well I can swap in a Crosman NP1 gas spring to see if there are any significant differences.
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Regarding the Umarex Stop-Shox system, I don't think this helps "scopes" at all. It DOES help isolate the action (and scope) from the stock and shooter so the shooter feels less rear recoil. The rear recoil on springers already feels trivial to me, so I'm not buying this marketing ploy.
The de-coupling of the action from the mass of the stock and shooter means rear recoil acceleration of the now lower mass action and scope alone will be HIGHER than it was without the Stop-Shox decoupling system. This means the scope will now feel a higher recoil rearward. Probably still too low to damage most scopes, BUT ... if the action is moving rearward faster with Stop-Shox when the piston slams home and everything STOPS, the forward recoil/impulse deceleration (g's) will actually be LARGER and potentially MORE DAMAGING to scope internals than it was without Stop-Shox. IMO it's the sudden STOP that ENDS the rear recoil that kills most springer scopes.
I think Umarex came out with this around 2016(?) and it is still in a few of their rifles. If Stop-Shox was as good as the marketing made it look, wouldn't it be in ALL their break barrels from 2016 until the end of time? Question to ponder... are there any "newly designed" break barrels from Umarex with Stop-Shox?
I'm a hard skeptic for marketing hype that seems too good to be true and I think Stop-Shox falls into this category. So you don't think my analysis is correct? No problem. FWIW I'm not trying hurt Umarex sales. I have several Umarex & Ruger branded break barrel air rifles. I like them ALL and they all shoot very well for me. None of them have or need the Stop-Shox gimmick.
Be skeptical of the marketing hype AND other peoples opinions. Do your own research and buy what makes YOU happy!
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The only system I know of for truely isolating/decoupling rifle motion from the scope is the Diana Bullseye Zero-Recoil mount.
https://www.youtube.com/watch?v=bBLiIeoxGTg (https://www.youtube.com/watch?v=bBLiIeoxGTg)
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Yeah, that 5:20 point is interesting. It would help to know what frame rate that portion was originally shot at.
I did a little more testing just to give physics a chance to screw up my conclusions.
I added some cloth tape to the front roller to get rid of the possible slip. That seemed to work but the sharpie trace changed to a single leg without overshoot. I took the tape off and recreated both the slip condition and the original sharpie trace with overshoot. Not quite sure what that implies other than that my support rollers are still influencing the results. I have a few more support configurations to try and figure out a way to monitor position within the ~14msec shot cycle.
Image 1 - cloth tape interface - little or no slip
Image 2 - bare interface - slip
Image 3 - tape results (left) and no tape result (right)
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Here's a rally good video. About 30 seconds in it shows a 1000 frames per second video of the pellet leaving the muzzle during forward recoil.
https://youtu.be/hIsfDfOd3vY
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Michael - It will be interesting to see your approach to setting up the instrumentation for making measurements. Comparing notes can always improve these threads.
Stan - I'm enjoying the results of your continuing refinements. It won't be long before you'll have to sharpen your Sharpie. I did check out the polarity issue you pointed out. It wasn't the preamp's fault. I had inverted it for the convenience of being able to overlay the 2 traces. You're right that the second peak is inverted. I've now swapped out the Kistler preamp for another B&K 2635 in order to reduce my confusion when setting the dials. I was trying to be impartial by putting the Kistler in with the mix for this batch of experiments, but it's just a different breed. Getting attached to vintage test gear must be like collecting vintage cars........ But I'll never really know.
Dan - I don't think there is any useful information in the video. I've seen this one before and was surprised at the lack of any real measurements. In one of his videos the view is from the other side of the test stand. It shows an enormous solenoid mounted to the same frame as the gun and the stroke on it would suggest a huge kinetic impulse would be generated when it was energized. I didn't see any transducers or means of capturing their outputs. Am I missing something?
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Dan - I don't think there is any useful information in the video. I've seen this one before and was surprised at the lack of any real measurements. In one of his videos the view is from the other side of the test stand. It shows an enormous solenoid mounted to the same frame as the gun and the stroke on it would suggest a huge kinetic impulse would be generated when it was energized. I didn't see any transducers or means of capturing their outputs. Am I missing something?
I think it is interesting how the pellet exit and recoil movement shown in the video relate to your graphs. It seems to illustrate mikeyb's idea that the impulses on your graph might be the piston slamming home, then bouncing and/or hitting home again.
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Michael, thank for posting the output of your spreadsheet (envelope). In thinking about it, I guess my recoil stroke measurements were basically measuring the motion the rifle needs to make to keep the rifle/piston center of mass in the same place when the piston moves over its stroke (neglecting the small pellet momentum).
I guess I need to dust off and reconnect my physics neurons
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I watched a couple of Dirt E. Harry's videos at 1000 fps. That is way too slow to make any solid conclusions. A minimum 10,000 frames per second should begin to provide some reliable data.
In one video D.E. Harry states that the pellet has left the rifle long before any recoil(s) have occurred. Sorry, but that is not correct when discussing spring piston air rifles. The pressure in the compression tube does not get high enough to begin moving moving a pellet until the piston has traveled at least 92% of its full sweep length. Basically the "magic" happens in that last 1/3" of a 4" piston stroke. In terms of piston travel time (8ms), the pellet doesn't start to move until approximately 0.5 ms before the piston slams home or about 7.5 ms after piston launch.
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Many years ago I was able to work with a professional using a 10,000 frame per second Redlake HyCam (https://en.wikipedia.org/wiki/High-speed_photography#Rotary_prism). An amazing film (anyone remember film?) camera which required great skill to operate. That camera was still too slow for the event we were studying. A 100,000 frame per second video camera was brought in, cutting edge tech for that time, and it provided clear images which were consistent with proposed theory. I'd love to have access to a camera that fast for some springer analysis.
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The Diana ZR-Mount video (~time 1:13) at 10,000 frames per second is pretty good and validates some of the ballpark calculations. The rifle moves rearward about 1/4" at a leisurely pace (for that camera) which is expected. Then there is a sudden STOP of rearward motion with significant vibration. I'm confident that is the piston slamming home collision. Note the compression tube end cap and the safety lever doing an interesting "dance". Even the compression tube seems to want to jump out of the stock! We know that springers can shoot 10,000+ shots without completely falling apart. That seems an impressive feat after watching the slow-motion violence of a typical shot cycle.
If you watch at 1/4 speed you can see the frame timer clock-off about 8 ms of rearward motion which is consistent with the expected piston travel time.
Then it appears the rifle has reversed direction and is actually moving FORWARD a bit? Or is the scope now moving back slightly in its mount? Since there is no precise frame of reference in the video I can see, it's difficult to gauge how much forward rifle movement there really is.
Regardless, the sudden STOP is bad enough. ADDING a direction reversal will produce a MUCH larger forward acceleration to stress scope mounts and scramble scope internals. That supports the theory that it's not the first rearward recoil acceleration doing the scope damage.
As the saying goes, "It's not the fall that kills you, it's that sudden STOP (and bounce) at the end that really hurts!"
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I added some notes to the simplified calculation sheet in case someone is actually reading it and running through calculations of their own. I don't think I made any errors, but welcome input to correct any if found.
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Ordered some sensors this morning. Now I've really got the curiosity bug ;-)
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Thank you for the updated notes.
Which sensors did you order?
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I was already placing a driver order at Parts Express for a transmission line speaker box I want to build and added some small audio exciters to that order. They "might" work for my airgun experiment. Voice-coil motors can become good motion/vibration sensors under the right conditions and these are an inexpensive way to find out. Amplification and filtering using op-amps will be easy IF needed. I suspect that won't be necessary with these ...
https://www.parts-express.com/dayton-audio-daex9ct-4-coin-type-9mm-exciter-05w-4-ohm--295-212 (https://www.parts-express.com/dayton-audio-daex9ct-4-coin-type-9mm-exciter-05w-4-ohm--295-212)
I started ordering audio related items from Parts Express around 1988, but haven't had the "play" time to go back for almost 25 years. Glad to see they are still doing well.
My plan right now it to sense piston launch using a contact wire through a scope mount/stop hole. I can see the rear edge of the piston skirt through one hole when the rifle is cocked. As soon as the piston launches the wire will lose contact. I can make a simple circuit than should provide a nice clean 0-5v step when the piston begins its journey.
I used muzzle mounted wire-break (actually metal foil strips) sensors many years ago to measure supersonic rail-gun projectile velocity (2 spaced conductors) with good results. In that case the huge EM fields and blinding arc flash from the rail-gun caused many measurement problems. "Simple" techniques worked the best. The break-wire at the muzzle used in this thread is simple and reliable. My first choice for pellet exit timing.
An audio transducer glued to a 1" slug mounted in a scope ring will be my attempt to capture rifle acceleration data. I'm only interested in axial acceleration (reverse and forward recoil) for now. Other motion and vibrations are not on my list until I can see some kind of correlation between the theoretical and measured parameters.
To "gently" start the shot cycle I plan to use a small R/C servo to move the trigger blade. I DON'T want to use a large solenoid which may product unwanted vibrations and possible EM noise.
This may not work well, but I'll have some fun trying.
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A "thought experiment" on piston bounce...
I'm assuming the piston rebounds some distance off the high pressure column of air and it finally slams "home" sometime after the pellet has exited the muzzle. There should no more air "cushion" to slow the piston and I would expect the second piston impact to have a faster rise to peak and possibly more "ringing" from exciting rifle resonances. Kind of like a dry-fire, but with a good portion of the pistons kinetic energy already transferred to the pellet.
My contribution to unsubstantiated theorizing ;)
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Case closed?
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George, that is pretty nice.
Is your accel still at 200g/V?
From a purely a timeline stand point, It is interesting (to me) how the data compares to what I was getting.
Your overall span from start to pellet exit is about 12.4 msec, I was getting about 13.2 msec.
Your time to piston impact looks like 10.4 msec mine was about 10.9 msec
You've got a lot of great data there, it will take some time to work though it.
Great job
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The the accel preamp is now set to 10mv/g so that would be 100g/V. I haven't really spent any time piecing the whole thing together yet. My motivation for this project is basically to find out what the rifle scope sees as impulses.
My only claim so far is that the pellet exits between the 2 big recoils. If the second recoil can be attenuated significantly it won't help the pellet, but would sure help the scope. The rest of the data can be generally squabbled about and I'm certain there's plenty for all of us to learn. Thanks for what you've posted so far and keep us informed of your discoveries as they are unearthed.
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The the accel preamp is now set to 10mv/g so that would be 100g/V. I haven't really spent any time piecing the whole thing together yet. My motivation for this project is basically to find out what the rifle scope sees as impulses.
My only claim so far is that the pellet exits between the 2 big recoils. If the second recoil can be attenuated significantly it won't help the pellet, but would sure help the scope. The rest of the data can be generally squabbled about and I'm certain there's plenty for all of us to learn. Thanks for what you've posted so far and keep us informed of your discoveries as they are unearthed.
Your expanded scope traces verify what I thought was happening inside a typical springer. Thank you!
I was trying to find out what caliber your Benjamin Titan NP was by going back through the posts. Is it a 177 or 22 caliber?
Asking because I think piston bounce and that second large impulse is different for different calibers and pellet weights. I suspect you can "tune out" that second peak using a larger caliber bore and a properly chosen pellet weight. First large peak will probably still kill many budget scopes, but tuning out that "double-tap" might help.
Today I finally read that excellent analysis linked by Stan in post #9. All the data posted here seems consistent with results presented in that analysis. I only absorbed about half of it this read. To digest the other half I'll need to dig my college diff-eq textbooks out of storage, brew a fresh pot of coffee, and hope there are enough grey-cells left for the upload. Not today though... maybe another day ;)
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Thanks, at 100g/V the launch amplitude is starting to make sense.
I don't know what a scope that isn't designed for reverse recoil can take. There are limits to what you can do to attenuate the first peak. It is the nature of gas compression that it all happens at the end of stroke. Your data (as well as mine) shows deceleration in the last 1.5-2 msec before the peak. If you try to increase the dead volume to reduce the peak pressure (perhaps by deep seating the pellet), I think you will get piston impact. It kind of looks like that happened in the second shot.
The second peak looks like piston impact. It would be interesting to compare a springer to the gas ram since they have different preload levels. If the piston had a soft face, you could probable reduce the second peak.
Putting compliance into the scope mount (like the Diana ZR mount) is probably the most direct fix. Of course that becomes a precision mechanism so figuring out how much stroke is needed is useful.
Enough Sunday morning theorizing
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Stan - Here's one with no interference from the 8200. The vertical is still 100g/V and the polarity is correct even with the input inverted. Ch#3 is true zero. I moved the Ch#1 trace up so that the full signal could be displayed. It just fit. The rest of the info is on the screen.
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Thanks George. A lot of information in that trace to chew on. Was anything different in that shot that would have reduced the amplitude of the second peak or is it just shot-shot variation? What weight pellets are you shooting?
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Stan - I like your idea about using a potentiometer to measure the rifle's displacement. I'm assuming that you'd use a linear as opposed to a rotary one. You could try to incorporate some form of damping to null the pendulum effect of your suspension system. A DSO trace would provide much better metrics than pen and ink, but it would lose that classical look. I'm going to look around and see if I have an old slide type one to try.
The pellet weight is 14.3 gr. Any difference in the scope data is just shot variance. It varies dramatically for about 3 shot after a (dumb) dry fire.
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Did you capture data in the dry fire? That should show what a metal first impact looks like.
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The experiment has moved up to using 2 accelerometers on the test bar and setting the force transducer aside for the time being. The rifle scope wouldn't be up against a block in normal use anyway, but the stock's butt plate against a block may be interesting to look at later. The accels are twin B&Ks except that one has the spigot on top and the other has it on the side. It's a handy arrangement. See photo.
With the 2 accels measuring the same shot cycle I can set one preamp to read acceleration and the other to read velocity or displacement. That will allow a mix of any two values simultaneously.
I didn't save the DSO screen on the dry fire, but I can assure you that it wasn't pretty. It's a good thing that these accels are shock rated at 20,000g's!
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Nice,
Since you commented on my needing to sharpen the Sharpie in my displacement measurement, I took a shot at it. I realized I had a second laser/receiver combo so I tried setting up an optical encoder. I used the roller as the mount for the indicator and placed the encoder pattern at about 4X gain from the roller. Tonight I just rolled it by hand. I'll see if it works for a shot tomorrow. The hope is that I can get the speed variation. Unfortunately, changes in direction will be tough to establish. I may still need the potentiomenter.
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That's an interesting approach for sure. My guess is that the roller inertia is going to be problematic. Especially if it has to reverse directions. If you have any of that blue silicone rubber sheet material that's used for getting tight twist lids off of jars it could be helpful. It's very nonskid, but not tacky.
Keep the creative juices flowing!
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The 2 accels were tested today to see how well they would match on the same impulse signal. The photo shows the 2 outputs on top of each other when the muzzle of the rifle was tapped with one of my modal hammers. I'll have to confess that I'm impressed with how well these vintage instruments perform.
An interesting (to me) feature of the 2635 preamps is that they will convert the acceleration to velocity or displacement. The preamp can also apply a 1 volt sinewave at 159.2 Hz on the input when needed. This frequency corresponds to 1000 radians/sec. It's a handy calibration signal that separates the overall gain of the V, and D at the output by 20 dB and 40 dB respectively, compared to the acceleration value.
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Here's a shot that may be interesting to any timing sleuths following these experiments. The photo is of a 14.0gr RWS wadcutter's first ~.010" travel at the breech. The test was made by using a piece of wire wrap strategically placed at the tip of a marshmallow roasting stick and then fed down the barrel. The signal comes from a 9V battery with a 10K ohm resister in series with one of the leads. It's a good clean make and break timing marker for when the pellet starts to move and when the stick and pellet leave the muzzle. This pretty much follows the testing done in the CP-2 thread.
Can't learn much from the exit time, but the ramp signal's timing should be the same as most of the previous ones. My interest is in other parts of the cycle. There is some enticing stuff in the impulse though.
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George,
Thanks for doing that. I was trying to figure out how to (safely) launch a harpoon in my small test range. It was easier when I was testing the 2240.
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Make sure that you have at least the sticks length between your trap setup and the muzzle. Don't ask me how I know about this.
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I made some updates to my instrumentation. I added a 3-axis accel mounted to a larger scope surrogate (image 1). The accel has 200g capability so hopefully it will capture some interesting data. The roller support and encoder sensor are still in place (left side of image 2) to look at overall airgun recoil motion.
Image 3 shows an overall trace. The trace gets triggered by the light gate at the muzzle (channel 1). The microphone is still mounted to the front of the surrogate (channel 2). The X axis of the new accel is aligned with the surrogate, positive towards the muzzle (channel 3). Channel 4 is the encoder on the support roller.
The trace starts at the typical 13.7 msec before pellet exit at the light gate. Image 4 shows the peak for the piston impact and what appears to be the second impact 4.5 msec later.
Need to do some more testing.
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I did a few tap tests of the mounted scope surrogate. Not a detailed survey but just to see what is there in the timescales where the shot cycle operates.
Channel 1 is the ADXL377 Y direction (lateral)
Channel 2 is the end mounted mic as before (X direction)
Channel 3 is the ADXL377 X direction (boresight)
Channel 4 is the ADXL377 Z direction (vertical)
Image 1 is a tap on the end of the surrogate in the X direction towards the muzzle. Looks like there is a 1.4 Khz mode. Just something to be aware of in looking at the other data.
Image 2 is a tap on the Titan tube between the two scope mounts, roughly in the Y-Z direction. Looks like the mode drops down a bit to 1.2 Khz.
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You're collecting some decipherable signals now. It's interesting what just a single tap produces. You're out-festooning my rig! Now I'm going to have to add a pressure transducer to the mix just to keep up. I found a few Kistler miniature quartz piezo types in a draw. They're about the same size as the Endevco ones that were used in the previous Hacking threads. The one I'll probably use is rated at 5000 psi and has excellent specs.
I want to look more closely at the piston ramp and pellet break-loose timing. A pressure measurement at that point would definitely help my mental construct about what's happening.
The first major peak to peak force is ~ 1000 g's on some of these shots. Maybe I'll try some mitigation there first. I'll have to disembowel the rifle to do the machining needed to get into the transferport anyway.
By the way, What camera did you use to take image 1 with a couple of posts back? It made me want to take my camera and throw it into a ditch!
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Funny, I'm thinking I need more oscilloscope channels. Though cocking an instrumented break barrel is a pain, we had it good testing the CO2 guns.
Actually, all the photos are with my phone camera. With those small sensors, the depth of field for close up work is so good that I usually don't bother with the DSLR.
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Here's the first attempt to use an accel to measure displacement. The Ch #2 preamp output is 1v/mm. The Ch #1 preamp is still 10mV/g. The top of the screen is the zoom window and the brackets therein show what will be displayed on the main screen. The timebase changes with the zoom so have a care when counting horizontal divisions. There may be some interesting stuff here to get started with. These are all from the same shot.
You're on your own with these images.
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It is hard to see the relationship between those two curves. When you mentioned the preamp capability, I tried to find what starts the integration period for the preamp to get to velocity and distance, but could not find it in documentation. My scope can do at least a single integral. I need to read on how that works, or maybe just export the captured waveform and do it in the computer.
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The preamp does all of the heavy lifting. The DSO just records the signal. I was surprised myself when I saw the first measurements. Here's some info about the preamp and how it does the magic: https://www.bksv.com/-/media/literature/Product-Data/bp0099.ashx (https://www.bksv.com/-/media/literature/Product-Data/bp0099.ashx)
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Yeah, I read that documentation. I'm still not sure of what those integrators are doing, including the low frequency filters they use. But that may be just me.
Maybe try them with a cleaner waveform like a tap test.
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The integrators act like low pass filters, but each one also contains a constant of integration. This constant is mainly drift and can go to infinity over time. Using a low frequency cutoff filter and short measurement times will reduce the double integration constants to negligible levels.
The impulses that we've been looking at are mostly in 10 and 20ms wide windows. There is a much lower frequency component that is suggested in some of the DSO shots, but you can't really see them until the time window is opened to 500ms wide. The gain difference between preamps also goes from 10 mV/unit to 1000mv/unit. The DSO's trigger is still on ch#1 so the initial impulse is captured as acceleration, but the displacement signal on that channel is buried much deeper than an 8 bit DSO can see.
I hope this helps to make sense out of what we're looking at with these measurements.
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Thank you, that helps with understanding the preamp. I was still expecting to see some features on the accel curve that correspond to the change in slope of the displacement curve. Maybe it is low amplitude, long duration.
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I didn't have full confidence in the friction coupling between the stock and my roller/encoder. I used some thin rubber sheet but even with that I wasn't confident that there wasn't slip occurring. So I went on Thingiverse and found a rack/gear model and scaled it to fit my roller diameter (image 1). I used some strong double sided tape to attach it to both the stock and the roller. I took a couple of shots and the motion during the cycle is very repeatable. The final rest point is influenced by the cog interval.
Here are some results from the second shot:
Image 2 - overview of the cycle. The trigger (time=0) for the trace is the light trap at muzzle detecting the pellet exit. There is a 0.1 msec delay there because the trap is one inch in front.
Image 3 - shows the timing of the cycle start (-14 msec) and the piston impact (-1.9 msec). these numbers have been very consistent for all of the Titan shots
Image 4 - shows the first encoder cycle (slot to slot motion). I measured all of the increments to generate the recoil motion in image 5. Each encoder increment represents 0.63 mm of rifle motion as mounted. The total motion (8.16 mm) matched the length of the sharpie mark to my ability to measure.
Image 5 - Shows a plot of the recoil motion and the velocity with some of the key cycle events noted. It is just my assumption that the second peak in the accel trace is the piston bouncing.
More data to wrap some theorizing around ::)
CORRECTION: There was a typo in the excel plotting. I replaced the recoil chart (sorry)
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Stan - I was just posting this when I saw you new one. I'll put this one up and then got back and look at yours.
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If you look carefully at the images again you'll see what your looking for. First keep in mind that there is a difference of 100:1 in the signal levels after one of them has been integrated twice. Now look at the slope of the displacement curve at the time when the first (ramped) impulse signal drops through it on the screen. The slope of the curve has momentary changed.
The second interesting event is in the next image when the rifle moved forward during the shot and the front accel just sightly touched the face of the block. This was unintended, but fortuitous in the overall outcome. I put one of the cursors there to mark the event. If you look at the third image you can see a the other cursor at the top of a very smooth curve. Looking back at the first cursor you'll see that the curve is flattened out there due to the block having stopped the accel (and gun) from proceeding further forward, therefore limiting the displacement.
If the DSO had more bits to improve the resolution the images would be much more dramatic in presentation.
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Stan- I like your electromechanical approach to getting the recoil data. Printing a gear set is a clever idea.
I've been staring at your plot. I think it might help if I go lay down for a while.
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I hope you are looking at the corrected plot, not the original.
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After staring at this data for a while, some observations (speculations) come to mind.
1) Fairly confident in the major events (start, piston impact, pellet exit) in the timeline. These are distinct and don't depend on absolute amplitude calibration. The timing is in general similar to George's data. In my trace, my calling the second peak piston bounce is speculation. The timing of the accel zero crossing prior to piston impact is similar to George's trace and the overall shape of the pre-impact trace is similar as well.
2) The displacement data from the encoder is interesting (i.e. I don't have it figured out). The encoder mask was made with a CNC encoder (though as a trial it was made from a piece of CD-ROM disk). I calibrated the spacing to rifle motion with an indicator at 5 cycle intervals and it was uniform, though cycle to cycle variations could look like ripples on the velocity plot. The encoder can't tell direction reversals, but since the overall span matched the length of my sharpie trace and there was no indication of reversal I'm going with single direction motion.
3) Given all that, the velocities derived from the encoder motion are lower than you would expect from rigid body acceleration either as measured in the front part of the cycle or from the spreadsheet (F=ma) estimate that Michael provided for a 180lb force gas ram.
All good head scratching stuff.
By the way if someone has a kid/grandkid that needs a politically incorrect STEM project for school, well, step right up, this will keep them busy
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A general idea about what might be going on with this recoil testing is starting to materialize. The highest forces that could damage a rifle's scope are most likely in the first impulse set. I'm going to focus(?) on these for the first attempt to rein them in. This choice is driven mostly by the fact that the power plant may be readily accessible by removing it's rear end cap. I think. There might be some butchering needed to the stock in that area, but if easy and repeated access can be gained by just removing the end cap and not having to dismantle the whole gun every time, I'm willing to make some sacrifices. That's why these are called "Hacking" threads.
I'm going back to putting the piece of foam behind the butt plate to act as my proxy shoulder. I want to record what forces the scope is really exposed to during the shot cycle. The pivot yoke now has a piece of sheet Teflon between it and the stock. This should reduce any of the drag.
The photos show the reconstructed testing arrangement.
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I have not taken it apart but I thought there is some significant preload on the gas ram. Looking at the parts drawing, it may be tricky to get access. https://support.crosman.com/hc/en-us/article_attachments/201592640/BW8M22NP_EVP___PL.pdf (https://support.crosman.com/hc/en-us/article_attachments/201592640/BW8M22NP_EVP___PL.pdf)
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That will be an interesting test. I think the acceleration spikes we've been measuring are due to the internal pressure spike decelerating the piston and are transmitted to the scope through the ring mounts. It will be interesting what an external force measurement on the rifle through the scope looks like.
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I was looking at that parts list a couple of weeks ago and thought about just buying some parts to build most of the rifle sans stock. Then I decided to just do exploratory surgery on what I've got since I'm not really much interested in using it for anything but bench experiments. The thing is like handling some sort of elephant gun compared to the CO2 carbines I've gotten used to.
Here's a link to a YouTube teardown of basically the same powerplant. It looks pretty straight forward which is what got me to thinking about expedients. https://www.bing.com/videos/search?q=rebuilding+the+benjamin+air+rifle&docid=608033129985672106&mid=8E672C9716D77B0925DF8E672C9716D77B0925DF&view=detail&FORM=VIRE (https://www.bing.com/videos/search?q=rebuilding+the+benjamin+air+rifle&docid=608033129985672106&mid=8E672C9716D77B0925DF8E672C9716D77B0925DF&view=detail&FORM=VIRE)
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Reassured that the 2 accels and preamps have been previously confirmed to produce ostensibly identical signals, we can trust that the signals can be massaged without altering the original information. This will allow a look at the bigger picture which would include the shot follow-through.
Here are 2 images with longer time windows. Ch#2 is still 1mm/V. The differences between the peak distances I'll attribute to the foam at this point.
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I thought you were going to try to make the piston/ram removable in the stock. That looked hard. Yes bench top it is straightforward.
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At some point I'll have to do a total disassembly to be able to machine the transfer port for a pressure transducer. In the end (pun?) I just want to be able to remove and replace the things behind the rear of the gas spring while the rest of the gun is fully assembled. I'll probably have to do some machining and woodwork at that point.
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Just to keep things moving along here I decided to machine a mount for the force transducer to fit the butt plate. This will allow it to measure the recoil force at the shoulder and see how well it tracks the displacement signal. I would expect some phase shifting during the shot cycle.
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Here's A pair of images for Stan.
I tapped the front of the muzzle with a hammer. There was a thin piece of wood shim between the hammer and muzzle to control the impulse spectrum. Ch #2 is still using the displacement setting on the 2635 and Ch #1 is using the acceleration setting.
In one image the digital filter on Ch# 1 is set to be low pass and the cutoff is at 50 HZ. As can be seen the displacement information is there using just the filtered acceleration signal and it's 180º out of phase with the displacement.
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I set out to get a feel for the repeatability of the shot cycle and measurement. I used the setup as described before but added an extra roller under the stock (image 1). Previously the timing gear was supporting most of the weight of the Titan. It was OK but the lateral motion by hand was not smooth due to the weight on the printed gears. With the extra roller, it carries the weight and the timing roller is just closing the gap in the gears.
I took three shots. Image 2 shows an overlay of the first and third shot cycle as measured with the accel aligned with boresight. I overlayed them in a graphics program and aligned the trigger point (pellet at muzzle).
The third image is a plot of the encoder distance (count) measurements for all three shots.
I'm good with the repeatability. I was going to try a few shots with deep seated pellets. Does anyone know the length or volume of the transfer port as baseline?
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Since the gas spring and piston are acting as a damped oscillator I don't know why your displacement measurements aren't indicating a reciprocating signal somewhere in the stroke. We're both getting about the same total displacement numbers. What am I missing?
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My optical encoder does not provide direction information. I am fairly confident that the motion up until the piston impact is single direction. The impact brings it to a near stop but the post impact motion could be in either direction. The vertical axis in my plot should be distance traveled. When I had the gear supporting the weight of the rifle, there was more friction and the sharpie traces looked one directional. With the weight supported by the extra roller, there is little friction and some of the traces look like they reverse. I am also pinching the trigger with my fingers. This contact with the rifle is overwhelmed by the 180 lb gas ram force so I don't think it affects the pre-impact displacement curve, but could affect the settling point of the rifle some milliseconds after the cycle.
I am still thinking about how to implement a potentiometer. I would still like to use the gear approach to get some mechanical gain. I made the clamp on the roller with the thought of being able to switch out the encoder plate with one that has a gear section on it. I'm not sure how fast you can move a pot wiper and maintain contact
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How about blocking the middle slot on the encoder. Then you could tell each time it passed.
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Maybe, the big change in recoil velocity after impact make detecting a different spacing a little harder.
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This image shows a response to a modal hammer tap on the muzzle. A couple of details in it separate out well. The DSO trigger is on Ch #1 which is the signal from the forward looking accel. Ch #2 the rear facing accel and is set to read displacement. Ch #3 is the force transducer (8200) and it's poking into the butt plate.
Ch #1 clearly shows how the displacement signal is modulating the acceleration signal. Ch #2 confirms the timing of the modulation. There's probably a Doppler effect in there somewhere.
Ch #3 shows the compliance recovery time of the butt plate cushion. The cushion looks like it's damping out the displacement motion.
Now that the testing looks good with the hammer It will be interesting to see how it compares to an actual shot.
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Looking forward to your results.
I think I'll try using a limit switch to detect zero crossing for my encoder as a temporary mod until I try the potentiometer or equivalent.
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If you have a piece of fairly stiff foam about an inch or so thick it could be placed between the butt plate and something heavy to block it with. The muzzle can then be tapped sufficiently hard to assure a rebound. It works a lot faster than having to load and fire the gun. The black open cell foam that I used for my proxy shoulder tests worked really well.
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I changed the proxy shoulder to something possibly more realistic. The new foam and metal plate up against the butt pad now resemble more of a shoulder with a bone in it. This may actually have an effect on the recoil. we'll see. At any rate a butt pad against a shoulder is part of any real world shooting condition.
Hammer tapping the muzzle only gives the rifle a blow in one direction as can be seen in the previous post. There's a small signal in the opposite direction, but not much. The more time I spend staring at these DSO images the more pieces of a larger mosaic seem to be coming together. The hammer taps really do run the action in a form of slow motion. It also provides very fast feedback on any changes made to the experiments. The taps can be repeated as fast as the DSO can reset it's trigger.
At some point the modal hammer's signal could be run to ch#4 and a math function could then be used to standardize the other three channels. As always thought experiments easily outpace hardware experiments!
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Interesting, though I'm not sure the muzzle taps are exercising the action. Even at end of stroke, the gas ram has 140 lb preload (Michael's spreadsheet), so the internal parts are not moving. I think at the low frequencies the taps are exercising a rigid body suspension mode. For the accel, I think the high frequency content is the barrel harmonics and the tube/scope mounts. I still think the shot data is the way to go.
I kind of liked your previous configuration with the rigidly mounted force transducer pushing on the relatively rigid rubber. This should have given you the ram preload for the first part of the piston motion and then a sharp fall off towards impact.
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I should point out that my modal hammer enthusiasm wasn't intended to suggest that the hammer would or could replace the actual shots. The hammer just gives a good impulse to test the entire measurement system without having to cock, load, and then reposition the rifle and wiring where you want it. It's quite annoying to take a shot and realize a DSO or preamp setting needed to be reset. The tapping can be done (with care) while the rifle is ready to go and before you take the real shot.
If you go back to page 2 of this thread and look at the slo-mo or even real time YouTube video @5:20 (now disabled) you can see how much energy the shooter's shoulder is absorbing during the shot. This leads me to think that real world shooting of a springer (or any rifle) includes the shooters shoulder in the recoil absorption system.
My simplified objective at this point is to evaluate the magnitude of the optical scope's impulse loading. I fell that it isn't fully defined without the shooter's shoulder being taken into account. The 8200's measurement is more curiosity than anything else. The real stuff (for me) is happening where the 2 accels are mounted.
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Agreed, having an impulse for checking the instrumentation settings is a great idea. I've take a number of shots with the DSO not set right or a power supply not turned on.
So tonight I added a geared potentiometer to monitor the carriage block. This was to take the ambiguity out of the encoder readings by identifying any reversals. The rack and gear (just a scaled down version of the other set) are shown in image1. It is a 10K pot, that I'm using in the 2-3 Kohm region. There is a 1K resistor in series to provide a voltage divider.
Image 2 shows the overall trace out to 100 msec. The accel data (Ch-3) and timing is a repeat of the traces from the other day. The potentiometer data (Ch-2) does not clarify the position data. Based on both markings and measurement of the travel needed to get to the starting voltage, the difference between the start and end positions on the carriage is about 8.5 mm. The encoder (Ch-4) total count does not match that and the final position of the encoder is not 8.5 mm from the start. There appears to be some relative motion between the encoder and the carriage.
Well, more work to be done.
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Can you move the rifle back and forth some distance by hand to confirm that the pot is reversing the signal?
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Yes, I marked the carriage at the start and recorded the voltage. After the shot, I can roll back the carriage until I get the same starting voltage and I'm back to the marked starting position.
I think I need to add some constraints on how the rifle rests on the carriage. I've been depending on the low friction of the carriage bearings and the arch shape of the stock to maintain contact, but I think more needs to be done.
I should also start integrating the accel data as a comparison. The DSO only does single integration internally so I probably need to capture the dataset and do it externally.
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My guess was right. The restraint on the carriage block was not good enough (even strapped down) and there was about 3.5mm slip (image). I will add some mechanical constraints.
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You got me curious about the drag on my system. I decided to do a static friction test to see what the break-free force would be. It's pretty consistent at ~ 2 1/4 lbs in both directions. I'll go back at some point and see what the 8200 comes up with for actual shot forces in both directions.
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I'm making some progress with the potentiometer as a motion instrument approach. I added some blocks to control axial slips (image 1).
I also took some time to pick a portion of the potentiometer range to work in, calibrated the voltage to axial motion, and scaled the DSO accordingly.
I put it all together and took a shot. Image 2 shows the overview of the DSO trace. I took the timing data for the encoder and the voltage data for the potentiometer and converted them into motion (image 3). The plot also show a simple local (trailing data) slope calculation for the velocity, just to give a feel. The data later in the cycle is probably influenced by slow things like my hand but I'm starting to gain confidence in the shot cycle timing itself.
Some more testing to be done for sure, but it looks like there is some potential in that potentiometer.
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Very puny! It looks like some bidirectional action is starting to show up.
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Stan - Have you measured the cocking force on your Titan?
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I have not. I don't have the dimensions to calculate the leverage and right now I have wires down the barrel so clamping on a scale mount will take a little thinking. For now I've been working from Michael's estimate for a gas ram of 180 lbs max. Though my accelerations were showing less.
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I was thinking of just the effort you have to put into cocking it normally. I checked mine by putting the (gun's) butt on the bathroom scale and doing a normal cocking using the barrel. There's a cosine correction that's necessary to adjust the force normal to the scale, but a close guess is good for first approximations. Then subtract the weight of the rifle.
It helps if you have someone as a dial watcher if the scale doesn't hold the reading for a few seconds after applying the pressure. It can be a bit awkward at first so I'd recommend a few practice maneuvers before putting enough pressure on the barrel to actually hear the two clicks.
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Just curious what that information might be used for.
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It's just another baseline. Mine cocks at ~ 45 lbs. If yours was different it might account for any differences in some of our measurements. It would also be useful if you see a change in pellet velocity outside of the normal spread when using pellets from the same tin. Is it the spring or the seals? Why guess when you can measure?!
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To keep things here from getting too stale I decided try something different. This trio of images is a series with 1, 2, and 3 of the previous wadcutters incrementally stacked in the breech. there are some things going on that may run contrary to predictions so everything will have to be sorted out when I do this again tomorrow. I think a lot of it will be guess work without a pressure transducer in the transfer port. Seal leaks may be one of the issues.
One item of interest is that the wire is taking longer to break. 3 pellets at once are a slow lot when they reach the muzzle.
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That is an interesting dataset. A lot to chew on.
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I tried to measure the cocking force. The scale approach didn't work for me because our scale is old and does not do well at low forces (I don't like how it does at high forces either ::) ). I tried an electronic fish scale attached about an inch in from the muzzle. Not real happy with the approach but I got just about 30 lbs. I found a spec sheet that listed 31 lbs. https://www.pyramydair.com/product/benjamin-titan-gp-nitro-piston-air-rifle?m=2603 (https://www.pyramydair.com/product/benjamin-titan-gp-nitro-piston-air-rifle?m=2603)
I did not get close to the 45 lb number.
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Hi guys. I am not capable to understand your thinking, but I would love some practical answers that we, that like to shoot scoped springers, could use.
I know this is off subject regarding this thread, but I would like a kind of dialogue with your knowledge and ability.
Without rigor, I will try just to pass the idea.
The problems with a scope happen due the ‘efforts’ directly over it. So, I would try to connect the sensors directly to the scope (or to a “replica”, as a simple 1” tube filled with some weight). So, I will call the object being connected as “scope”.
Considering holes for stop pins becoming oval, I would elect the longitudinal axis of the rifle as the main goal to study. I will call this axis as – X.
I think we have 2 kind of dampa mounts easily available in the market.
For 1” we have the DM60 from Sportsmatch that works with rubber O-rings inside, placed horizontally; so, some part of the recoils along X are dampened.
We also have the ZR mount from Diana. Due the first and weaker recoil, when you release the piston, the little spring of the mount is compressed (to some extent). Then, the piston reaches the end of its course and you have the hardest recoil. The idea of the ZR is that when this second recoil happens, the little spring is still compressed, and the scope would be without a rigid point of support (relative to X), not suffering this specific great impact.
So, I would proceed as follows:
When I think I have a trustable setup to measure the efforts upon the “scope”, and relative to X, I would use a regular mount (not a ‘dampa’) and repeat some times. The idea is to know how consistent are the results. Let’s say we may achieve a range of results, with its maximum and minimum (no matter the causes). I would call it as ‘consistency interval A’.
Then, I would change the mount holding the “scope” to each of the dampa ones. Repeat the tests to each one and compare with the regular mount.
I think the practical/simple answers could be – compared with a regular mount, by how much would be dampened the impact of the second/hardest recoil if I use the DM or the ZR?
Again, it was just to pass the idea..
Thank you for your attention.
Marcos
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I think if you look at the early pages in this thread you will see the scope simulators we are each using.
I've looked on the web for any testing of the ZR or DM60. I found one blog that tried to instrument a ZR with an accelerometer but the configuration they chose made the data challenging. I have not seen any data for the DM60, not even on the manufacturer's sight.
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Marcos - Mikeyb (Michael) posted favorably about the Diana Bullseye Zero-Recoil mount. Maybe he'd be interested in doing the experiments that you've suggested. He mentioned that he might put together a test rig to do some of the things Stan and I have been amusing ourselves with.
My interest (this week) is in modifying the power plant part of a springer airgun to reduce recoiling. First I'll have to figure out how they work. This will probably take awhile at the present rate, but it's my way of seeking out the silver lining to a pandemic lockdown.
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Stan - your cocking force numbers are interesting. Maybe it just felt like 45 lbs when I did it. The old man effect probably. Now I'll have to contrive a dead weight test.
In the mean time here are a couple of images that may be helpful to use as a mental aid for visualizing what the DSO's acceleration trace means. The photo is obviously what the proxy scope looks like. I've rotated the DSO trace to show the direction the bar and accels are moving. I may be preaching to the choir, but the rotated trace keeps me oriented.
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I like the rotated plot. Which way is time flowing? The first large sharp peak should be to the left if I understand it correctly.
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Marcos,
I don't have either of the two damped mounts to include in the testing. I think the more challenging test for those mounts is how well do they re-establish zero. Given the general hold sensitivity of springers, I don't think that is an easy measurement to make to the accuracy of an expensive scope.
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How the heck do you guys turn all that off so you can go to sleep at night?
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Actually, it is like counting sheep. It distracts the brain cells from the rest of the $&%! going on in the world.
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Thanks, and good luck with your challenges.
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Bill - What is this "sleep" you speak of?
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Yeah, that would be me... no way am I on the level of you guys but Betty Lou says that when I get something on my mind it's one track until it's finished.
Wait.... was she complimenting or what?
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Stan - Time is the purple line. It increases going upward. It's traveling along the zero value point of acceleration. The piston moving forward is the acceleration ramp on the right. I'll agree with you (for the moment) about the sharp peak.
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Stan - Thanks for going to the trouble of testing your cocking force. I had become complacent about the digital bathroom scale I've been trusting to give fair weight numbers. It works very well with comparison body weight measurements against other scales. I've just never tested it for accuracy using lower proof-weights. It turns out that mine doesn't do well with light weights either. If I put a known 30 lb steel plate on it and then let the display time itself out It will come back on with the plate tared out and the display reads 0.0 lbs. If I then cock the rifle pressing down against the plate the scale now reads very close to the numbers you've posted.
This would be another good use for the information. I'm now a wiser man for your effort!
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I've been thinking that if the chunky end of the barrel were put in a v-block on a load cell of sorts, and the stock was used to cock the gun (upside down) then the force would be at right angles to the barrel for most of the motion and give the most consistent measurement. On the other hand it may be easier to just remove the gas ram and measure its compression force directly.
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I'm going to stick with the cocking force numbers we have now. There are other windmills to tilt at. At some point all of the parts will be spread out on the bench for individual evaluation.
I did a couple of tests to see what the pellet stacking experiments did to the power plant seals, if anything. It was a relief to see the peaks returning back to sort of normal. The muzzle wire break looks OK too. The cleanest breaks are with using the ohmmeter instead of the battery through the resistor. The battery method looks better for the contact signal when timing the pellet release at the breech.
Just to make sure that I could add some extra confusion to the mix I removed the rear facing accel and replaced it with a steel bumper. The bumper can be tapped repeatedly with the hammer and not ruin the bar's mating surface for future use . The net result was that I can confirm the forward facing accel's signal polarity by tapping the bumper.
The DSO image here is confirmation that the front mounted accel produces a positive going signal when the force is applied up through its mounting base and towards its body. Switching out 2 accels and a force transducer on the opposite ends of the same bar can produce very confusing information for a left handed old man.
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This is another trio of stacked shots. This time I switched from the wadcutters to the CPHP 14.3gr pellets. I'm starting to see some patterns in these tests that may be useful down the road.
Ch#2 is being used to mark zero acceleration and the cursors for better contrast.
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I really like those three traces. Patterns yes, explanations, well that may take some head scratching.
Good work.
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I'd like to clarify the DSO images I've been posting. The rotated image in post #109 is just the trace of the acceleration as are most of the other images from the DSO. For super clarity (and more confusion) I refer you to the bottom image in post #79, which shows that the displacement is 180º out of phase with the acceleration. In other words the bar is moving in the opposite of most of the traces up until now. This phase change is due to the double integration of the acceleration.
The rotated DSO image in #109 will be more useful if this relationship between A and D is kept in mind.
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Here are images taken with a CPHP pushed 1" into the breech before the shot. The high frequency content is already filtered by the 2635's 10KHz roll-off filter so there must be lots of action above there. This indicates that the piston hit the front of the cylinder pretty hard before the rebound. More grist for the mill.
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I was about to do something similar, though I was going to come in through the shallow end of the pool. The transfer port is only about 1/8 inch diameter so roughly 1/4 inch of barrel has the same volume. I was/am going to try this at 1/4 inch increments.
That said, I am surprised that the second impact is so energetic. I was speculating that the piston would bounce more in the first impact when it was bouncing off of the high pressure air cushion that just a metal impact... So much for that.
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The shape of the initial piston acceleration ramp, peak, and pellet exit time are getting interesting. I'm going to add some marshmallow stick time markers again that will show on the ramp where the pellet starts to move. That will leave just a small and easy time window to play in. I've already been tampering with things as can be seen in these images.
At this point there seems to be little cost to the pellet's exit time (CPHP). The 2635 is still set to 100g/V.
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I took a shot (pun) at a couple of the off nominal runs. The first is a pellet recessed by 6 mm into the breech which about doubles the transfer port volume. The first image is a graphic overlay of the 6 mm recessed trace on top of a nominal run. The overlays were lined up to start the cycle at the same time. Interestingly (to me) the timing of both the impact and muzzle exit were unaffected but the second bounce was delayed. Also both impact peaks increased.
The second image is an overlay of a double pellet shot over the same nominal run. The shot cycle before impact didn't change much but the muzzle exit time increase by 1 msec (about 50%). The second peak is delayed and both peaks increased in amplitude.
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Here's an expanded time window of the acceleration ramp and pellet release as seen by the marshmallow stick. The timing is about the same as the earlier post, but this time the pellet and stick got stuck about 3/4 of the way up the barrel. Make what you please from the DSO image.
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This is an interesting(?) paper that can be used as an overview of the impact measurements Stan and I are using to try gaining some insight into the mysteries of springer airguns. The link gets you to a PDF download.
https://peer.asee.org/newton-s-law-and-accelerometer-integration-applied-to-impact-analysis
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Thanks George, I was actually planning a drop tube configuration like that but with a known stiffness spring to get some additional evaluations of the accel's response.
I thought I would summarize some of my August test results. I made a table of some of the measured timing points in the shot cycle for 14 nominal shots using 11.9gr Hobby wadcutters and also the two off nominal shots (recessed pellet and double pellet). The results in the table below show a fairly repeatable shot cycle and measurements. Surprisingly (to me) neither of the off nominal conditions affected the timing of the first impact.
If you add the pellet start data that George so bravely collected using his marshmallow stick harpoon technique, which has the pellet motion starting about .5 msec before the first impact , you start getting a picture of the cycle. My pellets would likely have a different start point because they fit into the breech more/less tightly, but it should be in the ball park. I'm going to try the pellet departure measurement but being less brave, I'm leaning towards using a foam plug.
I don't know if anyone on the forum has a transient model of a gas ram airgun. It would be interesting to compare these timing points with the analysis predictions
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Coffee break..
If/when you guys care to be famous in the airgun world, remember the case of the 2 'dampening' mounts..
Sorry for the interruption.
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This is another trio of stacked shots. This time I switched from the wadcutters to the CPHP 14.3gr pellets. I'm starting to see some patterns in these tests that may be useful down the road.
Ch#2 is being used to mark zero acceleration and the cursors for better contrast.
Sorry a bit late in response. I drafted this a couple days ago and finally had time to post it.
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So this stacked image sequence is shooting (1x 14.3 gr), (2x 14.3 gr) and (3x 14.3 gr) pellets?
A few comments...
1) I'm confident that first positive peak is the piston coming to a stop and rebounding off the high pressure air column.
2) The scope trigger point at approximately 500us before that first peak is pretty darn close to where/when I thought the pellet would start to accelerate down the barrel. My simulation confirms this value and I believe you have verified by measurement that the pellet does start moving about 500us before that first peak.
3) Based on the assumption that the pellet starts its journey at ~500us before peak pressure, measured (from your scope traces) pellet acceleration time in the barrel is approximately
t1 = 3.02 ms (M=14.3 grain)
t2 = 3.98 ms (M=28.6 grain)
t3 = 5.42 ms (M=42.9 grain)
4) Simple theory (calculation) indicates approximate pellet acceleration time in the barrel should be
t1 = 3.01 ms (M=14.3 grain, ~ 25 fpe)
t2 = 4.25 ms (M=28.6 grain, ~ 25 fpe)
t3 = 5.31 ms (M=42.9 grain, ~ 24 fpe)
Without chronograph data I'm estimating pellet fpe (velocity).
That's pretty close and IMO indicates consistent sensor data. I trust the sensor data for better accuracy since the simple theory is only an approximation that ignores friction and how the forces vary over time.
What I find interesting is the piston bounce/rebound time AND final impact force/energy are increasing for each larger pellet mass. The longer time suggests the piston is rebounding farther for the heavier pellet masses. That is expected. The larger final impact suggests less energy went into the pellet and more energy was dissipated in shaking the rifle. The measured impulse when the piston slams home for the last time on an empty barrel is larger than expected. I had thought the first piston stop/reversal was the primary scope killer, but it looks like that second impact can be worse than the first impact.
I've always wondered why some folks claimed that shooting heavy pellets shortened the life of coil steel springs. This data suggests that the heavier pellets cause harsher piston bounce. That rebound shock, probably approaching intentional dieseling (or detonation) with added oil, will tend to over-stress and reduce coil spring life.
I once repaired an abused B19 clone which had its main coil spring shattered into 5 pieces. The previous owner had intentionally added "fuel" to the rifle. Obviously a shattered coil spring isn't an issue with gas/air spring powered rifles.
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The 14.3gr CPHP is averaging about 700 fps at the muzzle. That works out to be ~ 15 -16 fpe.
Here's another quick and interesting(?) read. It's another one of Endevco's excellent papers relating to accelerometers and their uses. The last page has a good reference table.
https://endevco.com/contentStore/mktgContent/endevco/dlm_uploads/2019/02/TP321.pdf
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I agree with George. My Titan is not putting out 25 FPE. The spec page I listed earlier says 21 FPE but past chrony data has mine down around the 15 range.
In the simple theory model you use, what is the pressure-time profile that drives the pellet.
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I'm glad that you're taking notes along the way, Stan. I'm still at the panoramic view stage. I think it's about time to add a pressure transducer at the transfer port. That will force me to dissect the whole power plant and do some machining on the innards.
Since you're already contemplating a drop test apparatus you might enjoy this paper if you haven't already perused it.
DSTO-TR-2584 (pdf) authored by Andrew Krelle
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Nice report. Since it is Australian there must be a -1 in front of the g term somewhere. I have not found it yet.
The springer analysis models show a significant temperature spike along with the pressure. The very short duration probably won't affect your pressure transducer.
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The plan is to use a Kistler 601B1 miniature pressure sensor. It's rated at 260ºC continuous duty so it should be OK. I'm assuming that it has enough thermal mass to handle a couple of ms transient. Last week I put a piece of card stock behind a pellet and closed the breech to see what the piston ramp looked like with a little extra resistance. You could smell burning paper after the shot so your right about it getting pretty hot in there.
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Sorry, I was thinking the powerplant was similar to Trail XL, or about the same power as my Hatsan Mod125.
Was also using 16" barrel length with acceleration constant over the entire barrel length (too long and too simple).
Titan is similar to Vantage and you are correct as mine is shooting in the 15-16 fpe range and has a 15" barrel.
Recalculating the simplified equations using 15.5 fpe and a 15" barrel length...
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3) Based on the assumption that the pellet starts its journey at ~500us before peak pressure, measured (from your scope traces) pellet acceleration time in the barrel is approximately
t1 = 3.02 ms (M=14.3 grain)
t2 = 3.98 ms (M=28.6 grain)
t3 = 5.42 ms (M=42.9 grain)
4) Simple theory (calculation) indicates pellet acceleration time in the 15" barrel should be about
t1 = 3.58 ms (M=14.3 grain, 15.5 fpe, 699 fps)
t2 = 5.06 ms (M=28.6 grain, 15.5 fpe, 494 fps)
t3 = 6.20 ms (M=42.9 grain, 15.5 fpe, 403 fps)
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Not as close, but as stated I trust your measurerment data over the simple calculations. The calcs are just a reality check to verify the measurements make sense.
Possible source of mismatch between calculated and measured?
The pressure pulse time is pretty short and actual acceleration time in the barrel will also be short. These pellet(s) may be coasting or decelerating by the time they exit the muzzle. This has been discussed many times regarding springers. There should be an optimal barrel length for each combination of power-plant, bore diameter, and pellet weight. In some cases, like some of the larger caliber Hatsan springers (Mod130s), the barrel length is only about 10.5 inches. I'm betting their R&D folks found that a 10.5" barrel provided the best performance for those particular rifles.
I can assume a shorter acceleration length (trying 10.5") in the simple calculations, but keep the travel distance time to the full barrel length (Vantage = 15") the same. This will provide a new "pellet exits the muzzle time" that should align better with the actual measurements taken here.
Making these changes provides
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4) Simple theory (calculation) indicates pellet acceleration travel time in the barrel should be about
t1 = 3.04 ms (M=14.3 grain, 15.5 fpe, 699 fps muzzle, 15" barrel = 10.5" acceleration + 4.5" no drag coast)
t2 = 4.30 ms (M=28.6 grain, 15.5 fpe, 494 fps muzzle, 15" barrel = 10.5" acceleration + 4.5" no drag coast)
t3 = 5.27 ms (M=42.9 grain, 15.5 fpe, 403 fps muzzle, 15" barrel = 10.5" acceleration + 4.5" no drag coast)
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...considering the actual forces are not constant and friction/drag can be significant factors, that is pretty close IMO.
My only goal is to attempt to understand what is happening inside a springer by comparing theorized performance with actual measurements.
You guys are providing some excellent real world measurements on these air rifle. Thank you and please continue ;)
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Thank you Michael. It is nice to have analysis running in parallel with testing, even if it is not the full up model. Is your simple theory label a kinematics calculation with a prescribed acceleration profile or do you define a pressure profile and calculate an acceleration profile?
I also keep going back to the Tavella paper linked earlier for some sanity checks. While his is a springer and .177, it is a 15 FPE gun with a 15" barrel. A lot of his results are for for .67 gram pellets, which if you scale up (areal) to .22 are 15.4 grain. A little heavier than what George is using but in the ballpark. I probably should switch to at least CPHP's at 14.3 gr for my testing to get closer.
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Since various barrel lengths have been stated in some of these posts I'll declare that my Titan rifle has a barrel length of 18.5" from the face of the breech to the muzzle exit.
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Yeah, mine too. I don't know why Pyramyd has it spec. at 15"
OK George, the discussion on impact testing accels sent me down yet another side tunnel in this rabbit hole. I made the test block you suggested and covered it with accel modules (image 1). The three modules are all analog output, MEMS based with different g capabilities and with different capacitor set bandwidths. The intent was to see how they all respond to impulse inputs. The right hand face in the image has my microphone based sensor. It does not have amplitude fidelity but is fast response and provides an independent way of detecting/measuring event timing.
Right now I'm using a wrecking ball/pendulum (image 2) to provide the impulse. This is of course a sidetrack from airgun testing but gives me a chance learn how swapping around the capacitor filter affects the traces used in the tests.
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I don't think testing your measuring instruments is a sidetrack. It's more like an essential part of what this thread is all about.
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Stan - I was thinking about your mention of using foam in some fashion to time the pellet's release. I got to thinking that the wire could be fed down the barrel to the breech and then connected to a small plug that could be pulled back any distance into the barrel before the pellet was loaded. Very fine wire could be used if the plug was pulled back instead of pushed in. Is this what you had in mind?
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Yes, I was originally thinking of using those felt cleaning pellets with a radial slice and a fine wire with a solder ball at the end. I thought of the foam because I didn't feel like ordering the felt pellets. My only concern with the foam is whether it will gunk up the inside of the barrel (maybe drink some wine and cut some plugs from the corks). A push rod with a recess for the solder ball or a tube should be able to set the foam plug position precisely enough. A dowel with a metal tip can provide a continuity confirmation of the position.
One issue could be the trapped air between the pellet and the plug pushing the plug.
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One option might be to trim a flat stick to size and attach the wire to it. I seem to have an endless supply of these Popsicle sticks. A few seconds on the belt sander gets them to size. Maybe a 1/2 inch length with a small notch for the solder bead, pushed in from the breech side.
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I've decided against using the rear entry position (?) for something more prosaic. This will be my next attempt at getting good pellet release timing numbers. The coil is made with wire wrap that was slowly stretched by holding one end in a vise and pulling until the wire broke. This makes the wire very straight and hardens it nicely. The stretching also makes the wire noticeably thinner. It was then wound around a small Phillips screwdriver shaft. The coil fits with just enough clearance to make it easy to insert into the muzzle end until it touches the face of the wadcutter. An ohmmeter is used to adjust the contact clearance.
I haven't used this method to make a measurement yet, but it should work as a low friction sensor with negligible air resistance. This claim can be tested using the muzzle trip wire with and without the sensor in place.
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That looks like a nice solution. You still have the option of setting it at different positions in the barrel if you want pellet acceleration data.
I'm still distracted with evaluating the accelerometer modules. I changed my wrecking ball test rig so that the accel block swings down and impacts a spring loaded plunger. The thought is by adjusting the preload on the plunger, I can control the force the block sees and therefore the acceleration over the stroke defined by the block velocity. The first image shows setup. The calibrated popsicle stick sets the release height for the block. The results are promising, especially for the higher preload cases with the shorter stroke. An example is shown in the second image. Channel 1 is the accel and channel 3 is the mic. I use the mic to confirm any unique features in the accel trace. The intent is to better understand the response to shock type input in the millisec range.
More work to be done for sure but it looks promising.
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That's a good start on a pendulum impulse generator. I would expect more of a half sine wave signal from the accel though. You may be saturating the amp. Are you using a metal to metal contact surface on the block?
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The preload is intended to make the spring force almost constant. For that particular preload my estimate is that the force only varies about 10%. Right now it is sized and preloaded for the 16g accel that is mounted. When you reduce the preload and allow the plunger stroke to define the force, you get a shaped profile (see the overlay image).
I did an initial test with metal on metal contact but there appeared to be some ringing (second image). For now, I added one layer of electrical tape on the impact face. I want to add a laser gate to measure the speed before and after impact and see how the elastic collision assumption is holding up. I think I need to switch to a smooth rod with a threaded end or a shoulder bolt in the through hole region to get rid of some friction. A spring guide would help as well.
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Looks like I've got some work to do on the new pellet release timing wire. It gets the time right, but scrapes its way down the barrel. Maybe a straw or the like to guide and isolate the tip.
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Stan - The accel that you're using is really bandwidth limited for these measurements. The metal to metal impacts generate way too much high frequency information. The tape is a good start, but it would be better to try something like a sticky backed rubber bumper for kitchen cabinet doors type of thing. That will reduce the impulse bandwidth and extend impact time. You should end up with a textbook shaped half sine waveform and then work up from there.
The DSO's filters would also be good to experiment with at this point. I was amazed at how well they work after I figured out how to set them.
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The timing wire is tricky. I'm not sure you can get any more information out of it other than first contact. I think your (and mine) double pellet shots showed that any mass in front of the pellet changes the shot profile, including the first impact peak.
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Yes, I reduced the output capacitors so the accel should have a bandwidth around 1.6 Khz. Full scale should be about 1.65 V when AC coupled and horizontal. It also has an internal resonance of 5.5 Khz. I am working with it because I want to do some non springer measurements down the road and it is a very convenient form factor/output. I want to add the speed measurement to see how well the elastic collision assumption is holding up. As I add cushioning at the impact point, that will fall off and my ability to predict the spring compression falls off. The other option may be to integrate the response curve, get a total impulse and compare it to the momentum at impact.
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So far we've been able to get good numbers for an assortment of parameters. The next thing that would help in the overall scheme of measurements could be a power plant pressure curve to tie things together better. This will require dismantling the rifle to get at the transfer port. It should be straight forward to drill, ream, and tap the holes for the transducer and its sleeving parts. I'll start on that next.
It would also be nice at this point to look for any resonances in the scopes to see if they match any peaks that the rifle is generating. It could be that just a set of forced oscillations from the total shot cycle are exciting and driving high level harmonics in the scope. This test could be done by mounting a scope onto a shake table and driving the shaker with white noise. Any peaks that come up out of the noise could then be investigated individually.
There are lots of things left to do that can be planned around the second wave of a pandemic quarantine this coming winter. As Ernest Rutherford said ..... “I am sorry for the poor fellows who haven't got labs to work in.”
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The Titan is really simple to take apart. Since the preload on the power plant needed to be relieved for further disassembly it seemed like a good time to measure how much pressure was really there. Testing in the lathe with a load cell made easy work of making the measurement. As can be seen in the photos the load amounts to ~ 140 lbs when the retaining pin is able to slide in and out smoothly. The next test will be to measure the air spring and its displacement once the spring is removed from the cylinder.
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Nice!
I like the Jenga tower load cell calibration. Hopefully no toes were harmed in the process.
https://www.guinnessworldrecords.com/world-records/500643-most-jenga-blocks-stacked-on-one-vertical-jenga-block (https://www.guinnessworldrecords.com/world-records/500643-most-jenga-blocks-stacked-on-one-vertical-jenga-block)
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I measured the compression on the gas spring today. The steps were in 1/2" increments out to 4". The settling time at each point was ~ 1 minute. Reversing out each distance step and pressure measurement indicated that there was no hysteresis of note in the actual pressure measurements. But there was a constant seal and shaft drag of ~ 25 lbs in these static measurements when the displacement was reversed by 0.010" at each measurement point. Maximum pressure at 4" was 213 lbs. There's lots left to explore.
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Great data.
Is the seal drag a stiction issue? When I was trying to get a cocking force number, there appeared to be stiction that needed be overcome.
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On closer inspection today I realized that there is .010" backlash in the bracket used to mount the digital readout on the tailstock. Since I (generally) only use the tailstock's feed for operations like drilling and reaming in the direction of the headstock, it never occurred to me to check for backlash in the other direction. Checking the previous measurements I can now attest to having found no pressure offset or apparent stiction in the air spring . This is another lesson for me to remember when using machine tools and other devices in unconventional ways.
I can also report that parts of the cocking linkage (on my Titan) have tight binding issues in part of their action. This may account for what Stan has experienced while cocking his Titan.
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I went back and printed out the 3 DSO images in post #121. There's a lot of information there if you study them for awhile. I think every thing I'm looking for at this point is in them, they just need to be inspected through another lens that can rotate the information to provide another view.
To that end I'll try recording the shot cycle from the back of the gas spring using a force transducer. By good fortune the factory parts that make up the rear of the power plant can be easily replaced with some simple machining. This will allow me to install a prototype transducer I'm working on directly into the power plant itself.
By the way, what ever happened to Mikeyb and the testing he was going to do with the exciter parts he ordered?
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I agree, I spent a lot of time looking at your post 121. Looking forward to a gas spring force measurement, great idea.
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By the way, what ever happened to Mikeyb and the testing he was going to do with the exciter parts he ordered?
Sorry, no experimental data on this end. Had some "play" time planned for this month. As usual plans fell apart. Busy work schedule and unexpected travel crushed my play plans for at least a couple months. I did find a little time to work on my computer simulation during travel, but all planned physical experiments are on hold and and even my weekend shooting sessions are severely curtailed :'(. Envious of those who can retire early. Please keep on measuring. At least I can read about your results while traveling.
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Michael,
Stay safe in your travels. Throw in some simulation results or other observations when you can.
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OK, I'm still reconnecting my Physics 101 brain cells with my pendulum evaluator for my accels. In the mean time a bag of parts showed up. After inhaling some solder fumes, I have an extra DSO channel (wasn't too bad, SMD's were already in, just through hole stuff). What I think will make it useful is that it will take an external trigger which my Rigol should provide. Once I figure out how to do that, I should have an extra parameter that I can record in an event. For less than $15 it is worth trying.
Picture below was the same event at the same settings. Reasonably close measurements.
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Just as a follow-up to the previous post, after some fiddling, I did get the external trigger to work both on the Rigol generated trigger and on my laser gate (the image below triggered on the blue (ch2) trace of the laser gate for both scopes) . I do need to fully understand the timing of the triggered waveform, but this gives either an extra channel or a very portable measurement. You can see in the photos that the amplitude measurements compare well to the Rigol (the spec claims 12 bit).
Progress
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That's an nice add-on for an extra channel. The 12 bit resolution is interesting, but I'll guess that you'd have to get the signal out your computer and use extra software to actually have access to the extra bits. The bandwidth seems OK for the mechanical realms we're looking at now. Sure can't beat the price!
I've been saving up for another DSO like the one I've got now. I agree with your idea that more channels may be needed to really see the full picture. The prices have been dropping on these DSO's so time is on may side. Unless I die first.
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but I'll guess that you'd have to get the signal out your computer and use extra software to actually have access to the extra bits.
So today the Amazon smile truck dropped off a couple of UART to USB converter boards. Add a standard terminal program and a spreadsheet, and I have a useful file.
With the converter board, I'm still under $20
I am embarrassed to to say I have not done the data capture/transfer with the Rigol DSO yet.
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While Stan's been making progress in the digital domain I'm still working in the analog realm developing a new approach to designing and building accelerometers and force transducers. Some of the assembly work can be operose even with the help of an assortment of Optivisors. Old eyes need all of the help they can get. Fortunately my beer hiatus is only temporary and will come back to the rescue when called upon!
The Titan power plant's custom force transducer design is moving along slowly due to the current heat wave, but some things are moving forward inside the house where it's cooler.
The point of this post is to show a device I found on eBay (~ $10) that may be useful for any other experimentalists out there that haven't discovered it yet. It's a simple device and readily modified for many positioning and/or assembly tasks. At a little over 4" long it can hold stuff up to about 1 7/16" dia.
The photos show the adjustable spring clamp holder while on assembly duty.
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I finally got around to building a new force transducer and installing it into the newly designed gas spring guide. There is a plain plate that fits between the back of the gas spring and the face of the transducer. Drawings of other Benjamin rifles indicate a cushion in place of the plain plate. It will be interesting to swap out other materials for the plain plate and see what the impulse results look like.
The photos show the original parts and the new arrangement for accommodating the transducer.
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Nice, what is the actual force transducer?
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The actual transducer is assembled inside a .45 cal spent casing. I punch out the primer and press in an insulated piezo crystal sandwiched between 2 machined anvils. The signal is on a wire that is fed through the primer hole.
I use the casing for these prototype designs because it saves me a lot of machining. The very high insulation (> 10^10 ohms), and signal output level can be tested before committing to the rest of the assembly process. One of the attributes of a good force transducer is it's rigidity. To achieve this with a shell casing it's pressed, after testing, into the steel housing of whatever project it was designed for. The brass is no longer a structural part of the device at that point.
The photo shows a typical test casing.
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First fit test.
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That looks great. I'm back to looking at my accelerometer response to step inputs.
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Second fit test.
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Since the power plant was already apart it seemed reasonable to go ahead and machine in a pressure transducer to complete the measurement ensemble. I chose the Endevco 8510B transducer for the first round because of its miniature size and excellent response characteristics including negligible sensitivity to temperature transients.
There's a small increase in the transfer port volume, but that shouldn't interfere with getting some good preliminary measurements. At this point I'll have used up all 4 DSO channels and we'll be able to see what a complete shot cycle looks like in a 10 ms time window. The only additional measurement would be the pellet release time, but that can be added as an overlay.
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I'm looking forward to the shot cycle data. I have not seen anything equivalent.
In the mean time, I've been shooting my pistols more and am interested in the recoil and how it relates to power settings. I made a test mount for the barrel of my 2240 to capture timing and acceleration.
The first image is the configuration. The glowing parts are the legs of the laser gates that form a speed trap. It provides trigger for the DSO and an attempt at muzzle velocity. On the side is a 3 axis accelerometer board (image 2). These form the 4 channels on the DSO. The 2240 has a power adjustor so I had a chance to compare higher and lower power shots.
Image 3 shows a comparison of the overall traces. I was surprised at the number of what appear to be bounces. I need to check that.
Image 4 shows a comparison of the bounce period. Sorry for the two different time bases, but the cursors capture the values.
Image 5 show the values of the speed trap trace on top (ch1). I have to compare them to a chrony to see how accurate they are. It was interesting that the higher power shots have some obscuration of the beam after the pellet leaves. Probably CO2 fog. I'll have to see at what power setting it shows up.
Even though it is convenient to have the instrumentation on one structure. The 3D printed part is probably not stiff enough for an accel mount and I'll make a dedicated block for the barrel.
To me, this is all interesting data, even though it doesn't quite provide the flinch detector I was thinking of.
This post is just to show a different tunnel in the rabbit hole that I wandered into. If I collect some more meaningful results, I'll start a new thread so it does not get confused with the Titan testing.
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The accelerometer traces have fairly good repeatability. The first image is an overlay of three lower power shots and the second is an overlay of two higher power shots.
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Just an update. I was worried that the laser speedtrap box was too flimsy and loose mounted to serve as an accel mount so I moved the accel module to a block clamped to the barrel (first image). I reran some of the shots. Image 2 shows a trace for the higher power shot. It looks very similar to the traces posted yesterday. Image 3 shows yesterday's and today's traces overlayed (4 total), pretty similar to my eyes.
Some thoughts:
I had hoped to create a configuration that could detect some aim shift due to recoil and determine which part was before pellet exit, and how it changed with hammer preload. At this point, that's a fail, though it does provide some insight into the shot cycle.
The speedtrap works...sort of. At higher powers, the second timing can be masked by the CO2 blast. At lower powers, this is not an issue. I can get around this with a second laser detector but I had hoped to stay on only one channel. Maybe some resistor network will work to keep it on one channel.
More work to be done....Now back to the regular programming on the Titan channel
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It's nice to see that you're still making measurements, Stan. You're welcome to post any of your work in this thread if you want. So far most of the posting has been about how to make measurements and then try to figure out what the results mean. It's all pretty much generic at this point.
I'm still staring at post #121 and have come to the temporary conclusion that the pellet is long gone before the second impulse happens. This would mean that the second impulse would have no effect on the POI, but could still have a substantial effect on the scope.
It's also clear that a heaver pellet (or stacked ones) can greatly reduce the bottom half of the first impulse. Since the direction of the recoil is 180º out of phase with the acceleration, the recoil would be moving the gun backwards in the conventional way most guns recoil. If this is true then the use of the "artillery hold" should be no more effective than with any other rifle. This is speculation at this point, but can be evaluated down the road.
The second impulse may end up just being a fact of life when it comes to break barrel rifles and some attention focused on the scope mounts, as suggested by Marcos in an earlier post, may prove fruitful.
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Now that an effort to improve scope mounting methods seems reasonable (to me) I've decided to do some testing. The photo shows a single degree of freedom optical rail arrangement for doing a transfer function measurement on one of my stray scopes.
The plan is to attach an accelerometer to the scope's body and then use an instrumented hammer to excite the scope's response modes. The frequency and amplitude characteristics can then be determined.
This simple lash-up should provide some quick information to use as guidelines for vibration isolation and damping experiments.
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I don't think creating compliance is an issue. It is interesting how the two commercial mounts chose to provide very different ranges of motion as seen in the two videos.
https://www.youtube.com/watch?v=AfSIeJI4y5Q (https://www.youtube.com/watch?v=AfSIeJI4y5Q)
https://www.youtube.com/watch?v=ZK2IsBMoi2A (https://www.youtube.com/watch?v=ZK2IsBMoi2A)
I have not found a parts diagram of the DM60 but post 14 in this thread describes it well https://www.airgunforum.co.uk/community/index.php?threads/sportsmatch-dampa-dm60-question.289657/ (https://www.airgunforum.co.uk/community/index.php?threads/sportsmatch-dampa-dm60-question.289657/).
I think the more difficult part is maintaining line of sight alignment. If you are mounting a scope with 1/4 MOA adjustments, you probably want the mount to be that or better, about a thou over 4". I'm not sure how you measure drift of that scale on a springer.
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Stan - I'm not sure how compliance (1/stiffness) plays a roll in these transfer function measurements. Could you elaborate some on it's applicability in the proposed experiments?
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The compliance observation and the reference to the two commercial designs was related to your mention of the scope mounts brought up by Marcos in the previous post, not your planned experiment.
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Thanks for the clarification. I agree that compliance itself isn't a major issue with the mounts shown in the videos. But both do show their foibles when the videos are slowed down to .25 speed. Repeated views of just the shot cycles demonstrate the amount of flexure in the scopes due to resonances. The double trigger pull in the Dampa one is interesting. Also, the piece of masking tape in the DM60 demo was less than quantitative.
A mount's return to mechanical "zero" after a shot cycle could be checked using fixed blocks on the barrel's scope rail and some feeler gauges. That would be pretty easy and there would be no moving parts to get out of adjustment between shots. The measurement could get much more advances than that, but not simpler.
Both mounts are attempts at addressing the recoil issue, but neither seem to do anything about the stored energy in resonances. A scope that won't hold zero is no better than one that's broken.
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I did some rainy day fiddling with the instrumented 2240 that I showed in posts 176 and 178. I drilled a hole in the power adjustor for a wire probe to reach the back of the hammer (first image). This acts as a simple switch when the hammer is released and the shot cycle starts. The laser trap still marks the pellet exit. Image two shows a trace with all of that information captured. Channel 4 (blue) and Channel 1 (yellow) are the hammer start and pellet exit. Ch 2 (green) and 3 (purple) are the vertical and horizontal accelerometers. From hammer start to pellet exit is 17.4 ms and the hammer flight takes up most of that. The trace also show what appears to be multiple hammer bounces.
This test was with a long soft hammer spring (third image) and the power adjustor moved out (stock dimensions shown in the picture) to provide about 350 ft/s for target shooting. The longer spring is 50 mm Vs. the stock 33.8 and for me the stiffness measured out at .84 N/mm Vs the stock 3.0 N/mm (sorry about the units) The plan is to get measurements using each spring with the power adjustor set to equivalent muzzle speed.
The hope is that the accel traces can be manipulated in Excel to get muzzle vertical speed and potentially motion. The fourth image shows the accel trace and the vertical velocity and displacement of the muzzle. The fifth image shows a comparison of two different shots. One shot's sample started earlier so it affects the integrals and generates the offset. These are just first examples, the process needs to get checked out.
The flight time for a 10m target shot is .09 to .1 sec so if the vertical velocity at the time the pellet leaves the muzzle is 6 cm/s, that is 6mm on target. Of course, these were loosely held shots into a table top trap, and what matters is the shot to shot variability (like any hold). But to me it is interesting enough to pursue to see if this is a useful tuning parameter for pistols. I'm already sure it is a primary suspect (scapegoat) for missed shots. ::)
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The distraction of having too many projects going on at the same time has taken its toll on the break barrel measurements. The force transducer has finally come to life and has produces some results that I've been staring at for several days now.
There were a couple of questions early on about when the shot cycle actually starts. At the time I only had the DSO data from an accelerator and force transducer that were attached to a proxy scope arrangement. The DSO's trigger was set to capture the force and vibration signals as an attached scope might experience them. Some rearrangements with the sensors was tried and the data collected in various assortments. None represented the full shot cycle from start to finish.
This new batch of tests utilizes the custom force transducer (FT for brevity) that is now installed inside the power plant and directly behind the cylinder body of the air spring. The DSO images show the signal from the FT on CH1 and the accel attached to the scope proxy on CH3. The vertical green line is CH2 which is the muzzle trip wire signal for the pellet exit timing. The real numbers are for the horizontal timing. The magnitude values are arbitrary. The trigger timing is the carrot at the top left side of the screen and set to CH1.
The 2 DSO images are of the same trace data with and w/o a 10 kHz low pass filter. They're separated some for clarity. The polarity of each signal is correct.
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Thanks for posting an update.
The force transducer provides an interesting trace. Just to understand it better, is the transducer a transient device where, even under the starting preload, it provides a zero output? In your trace for the FT, is compression positive? I was expecting the force transducer to follow the drop in the gas ram pressure with stroke, until the shock of the impact event.
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Stan - Thanks for the question. I've been trying to understand what's happening too. The FT settles to zero under static load. The preload when the rifle is cocked is ~ 200 lbs. The FT's output is positive when compressed and negative in tension. When the gas spring (GS) is fully extended it has a preload of ~ 140 lbs remaining. When the FT is installed into the receiver it is compressed slightly (?) more.
The accel's signal is positive when the force is applied up through its base. The DSO image shows the accel's response to the initial recoil as being positive as would be expected. Puzzling over several of these images (in my usual brain fog) I seem to have misrepresented the FT's polarity. It would make much more sense if the curve was inverted. The FT's signal should decrease, as you've suggested, when the GS is extending. The curve should then start increasing as the compression in the rifle's cylinder increases until the rifle's piston reaches the end of the cylinder. What happens after that is still unclear to me.
I'm going to do a few more tests today and reverse the FT polarity in the measurements and see what it looks like. I've got a dummy pressure transducer (PT) installed in the transfer port for the time being. I'm not ready to put a working PT into service yet because of the potential for dieseling to occur from the lube this soon after a rebuild. Actual pressure numbers during the shot cycle will be very useful.
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The polarity for both DSO channels should finally be right in this pair of images. I'm now using Fomblin PFPE oil for the rifle's piston/cylinder lubricant. It's an extravagance, but only takes a few drops and seems to be doing a good job so far. I didn't want to take a chance on any dieseling destroying the transfer port's pressure transducer.
The DSO's CH2 is now the accel and the scale is 100g's/V. CH3 is now the exit trip wire signal. Installing the FT into the rear end of the power plant seems to have tightened up a few thing's that might have been banging around in the earlier tests. The event timing seems to be about the same.
When the pressure measurements are added to the mix the full shot cycle dynamics record should add much more clarity to future interpretations. I'm counting on Stan to continue getting me back on course when necessary.
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Nice, the FT provides a clean trace. Is that the raw output of the piezo? It does not have some of the noise of the accel. I'm not sure I fully understand the initial drop but the roll offs prior to the two impacts make sense to me.
To be honest, I'm still trying to figure out the vertical recoil traces I posted earlier.
Great work George.
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Stan - your green trace appears to be a relatively hi Q ringing of a spring-mass system, obviously. You could expand the time base and measure the frequency in the largest envelope. Then put the gun on a foam pad and start exploring with a small tapping hammer. Something would probably give you a strong hint as to the source of the vibration. No need to actually fire the gun to get good signals, though dry firing it would be interesting.
How are you doing your 10M target timing?
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Both of my previous DSO images are with a 10 kHz filter in the line. Above that frequency it just gets unintelligibly noisy. The FT signal is cleaner because of it's rigid construction assembly using bolts instead of a pin. That end of the receiver is also bolted tightly to the stock which adds a lot of damping.
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Yes, I do need to do some tap tests to explore that ringing.
The 10M timing is just a rough estimate based on muzzle velocity. It is meant to see what the ballpark effect of vertical muzzle recoil might be.
I've been trying to figure out how to get an independent measurement of the muzzle recoil vertical velocity for the pistol to compare with the integral of the accel data. I tried the geophone but I'm not sure I like that data. I looked at the specs of the audio exciter that Michael posted, but it looks like it has a resonance at 1.2 kHz.
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It might be worthwhile to reconsider the humble phono pickup cartridge. a simple preamp w/o the RIAA compensation is probably a 1 chip solution. The stylus tip has very low mass and you could 3D print a holder for making measurements w/o having to mount a fixed transducer to the DUT. Another advantage is its extended frequency response. You could even get a complete tonearm assembly for cheap on eBay and modify it to due your bidding.
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If I understand the phono stylus/cartridge, I still need a suspended seismic mass to measure. I would like to get to where I can hold the pistol as normally fired on target and be able to record the pellet exit and muzzle vertical motion (a or V). I have a small portable 2 ch DSO that will let me do this away from the benchtop.
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I forgot that anyone shoots pellet guns anywhere besides clamped to a test bench.
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Here's a set of 4 DSO images of the same shot. I've misplaced a part for the pressure calibrator, so the actual value for Ch4 is guess work at this point. I'm sure that it's over 1,000 psi. Its double peak is a mystery at the moment. The pellet's exit time interval (~2ms) after the first force peak (Ch1) is what I use as a check value in the approximate overall shot-cycle timing. The DSO's timebase for the images is from 5ms/div down to 500µs/div.
This will all take some sorting out, but a guy needs something to do during a pandemic.
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I expect your pressure peak may be over 2000 psi once you have confirmed a calibration reference.
I've captured pellets from PCP rifles charged to 2000 psi and found solid rifling marks but minimal to no skirt expansion. Captured pellets from a similar FPE-at-muzzle break-barrel rifle had more defined rifling and severely "ballooned" pellet skirts. That suggests to me that the peak psi experienced by those break-barrel pellets is well over 2000 psi.
While it could be something else, I think the small double peak in the pressure trace can be explained by the pellet finally "popping" the lead and just starting its acceleration down the bore.
Thanks for keeping the information flowing. Wish I had the time to participate.
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George, great job getting the pressure sensor online.
The double peak is interesting. Going back to post 150, the pellet started moving at about 500 micro-sec before the accel peak. Also if I remember correctly from your CP measurements it took about 200-250 psi to get the pellet moving in that case. The double peak looks like it is in between events. I wonder if a piston seal leak would look like that. The other thing that is happening at that point is the temperature spike.
The pressure curve also shows very little pressure in the breach for the last milli-sec that the pellet is in the barrel. Some of this is because of the piston retracting but I wonder how this drop-off propagates down the barrel in this timeframe.
Thanks again.
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I've ordered another part for the pressure calibrator. It will probably be here before I manage to find the one that's missing. As it stands now the pressure curve is probably close to good up to ~ 1,000 psi. Each vertical division on Ch4 should be close to 200 psi. Stan's estimate of the pellet's release time and pressure is probably reasonable. Above 1,000 psi the transducer's electronics are most likely going to be nonlinear and may be the cause of the ragged double beaks. Stan's pointing out the possibility of thermal spiking also has merit.
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The part for the pressure calibrator arrived today. I checked the transducer and it appears to be functioning properly. I'll have to adjust the sensitivity setting to keep the output value below the 10V rating of the signal conditioner and readout. This should keep the transducer linear to within 1% up to 1500psi.
The temperature offset claims to be 0.02 psi/ºC up to 121ºC. I know it gets a lot hotter than that in this application, but the duration is only in the ms range. This leaves the potential for nonlinearity to be of little consequence at this point in the measurements as far as I'm concerned.
The photo of the calibration setup is from the archives of previous Hacking threads.
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The polarity for both DSO channels should finally be right in this pair of images. I'm now using Fomblin PFPE oil for the rifle's piston/cylinder lubricant. It's an extravagance, but only takes a few drops and seems to be doing a good job so far. I didn't want to take a chance on any dieseling destroying the transfer port's pressure transducer.
The DSO's CH2 is now the accel and the scale is 100g's/V. CH3 is now the exit trip wire signal. Installing the FT into the rear end of the power plant seems to have tightened up a few thing's that might have been banging around in the earlier tests. The event timing seems to be about the same.
When the pressure measurements are added to the mix the full shot cycle dynamics record should add much more clarity to future interpretations. I'm counting on Stan to continue getting me back on course when necessary.
George, can you tell me if I'm interpreting the DSO screen shots in reply #189 correctly...Is that plot showing peak accelerations close to 500 g's?
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David - Yes, you are correct. Keep in mind that the full peak-to-peak acceleration can be considerably greater.
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Thanks George! I've been playing with a low order model to look at the effect of isolation mounts for a rifle scope and was looking for "validation". Of course, my model has parameters for something in the neighborhood of a Slavia 630, which I'm going to guess is in a different power league than the Titan NP...
The model is greatly simplified, but at least I get peaks and curve shapes matching other simulations, so I'll call it close enough for some simple case studies. If I can't stand it, I might break down and implement a version of Tevalla's model, but for now it will do if we don't look too closely at the results ::).
For example, the spring rate I'm using with "average" rifle and piston masses yields peak accelerations of 200 g's. Among my simplifications is the air is essentially trapped in the chamber and piston bounces off it completely after crushing it to within about 2 mm of the end of the chamber with only some light damping losses. Good for a first bounce, but not to be believed after that ;)
It's all relative actually. If I increase the spring rate and don't like the result, I can add a wee bit more damping to the piston sliding and bring it back down to believable levels. More importantly, I just need to see that the piston moves about like it should, as it is the accelerations I am after. But the trends remain the same--isolation works, whether "free" slides like the Diana and Gamo mounts or plain old "rubber" bushings (never seen that done, but theoretically admissible for sure).
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Got back to doing a little testing. In addition to the Titan .22, I have a Quest 1000 in .177. I was curious how the recoil of the two compared. Both guns are original, without any tuning. Past Chrony testing has them in the 13-14 FPE range, with the Quest the higher of the two.
I used the ADXL377 accelerometer module mounted to aluminum round stock to be a surrogate for a scope (first image). I also had the laser light gate on the muzzle to track pellet exit.
The images below show overlays of 5 shots for each of the guns. They were all indexed to start at the same point. The final image is an overlay of one Quest and one Titan shot.
I was expecting, for equivalent FPE, the shot cycles to be more similar between the two guns.
Anyway, it was fun getting back into testing. I may get sucked into buying the more expensive ADXL1004 sensor......
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Good work Stan! You are making more progress than I have...things are busy here for a little while yet.
The eval board for that ADXL1004Z ain't cheap, is it? $75! (Looks like Analog Devices has some in stock; I'm not finding them anywhere else.)
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Got back to doing a little testing. In addition to the Titan .22, I have a Quest 1000 in .177. I was curious how the recoil of the two compared. Both guns are original, without any tuning. Past Chrony testing has them in the 13-14 FPE range, with the Quest the higher of the two.
I used the ADXL377 accelerometer module mounted to aluminum round stock to be a surrogate for a scope (first image). I also had the laser light gate on the muzzle to track pellet exit.
The images below show overlays of 5 shots for each of the guns. They were all indexed to start at the same point. The final image is an overlay of one Quest and one Titan shot.
I was expecting, for equivalent FPE, the shot cycles to be more similar between the two guns.
Anyway, it was fun getting back into testing. I may get sucked into buying the more expensive ADXL1004 sensor......
NICE!
Can you confirm the parameters below?
Titan, NP gas spring, 22 caliber, pellet mass 14.3 grain, rifle mass? (as tested here)
Quest1000, steel coil spring, 177 caliber, pellet mass 7.9 grain, rifle mass? (as tested here)
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For the Titon, yes CPHP's at 14.3gn. Test weight is 3.58 Kg. I have not taken it apart but the tube ID is ~ 25.1mm and the stroke looks to be ~ 97mm.
For the Quest, test weight is 3.19 Kg and I was using CPHP's at 7.9 gn
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Along with the accel measurements, I set up some microphones (first image) to get a noise trace. The silver bare mic is generic and goes through a kit amplifier and is captured by the DSO. The black mic is a better quality 6mm measurement mic that is sampled at 96Khz and fed into Audacity software on a PC. The kit mic is time synched to the DSO trace. I can then manually line up the Audacity trace to the DSO trace. The results are shown below (the Audacity trace is the blue lower one).
The microphones are 25 inches from the muzzle and about 40 inches from the breech. In the chart, I show the approximate speed of sound time delay. When you take that into account, a chunk of the sound energy happens before the pellet leaves the muzzle. I would be interesting to do some of this with an LDC.
More fun stuff.
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Tapping on the surrogate scope shows a mode in the 1.1 -1.4 Khz range. Probably shows up in some of the response after the initial impact.
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A few years ago I started reading about the TWANG that some springers have. Then, I suggested that the sound be recorded, before and after the tuning.
However, I didn't know what is needed for a decent recording.
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The recording traces I posted were intended as an example of what data can be collected but they were taken inside a relatively small room with the gun on a hard desk. Lots of bounces possible in that trace. To get cleaner signals, taking it outdoors on a grass field may be a better approach. I wonder how much of the perceived sound comes through your contact with the stock.
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"...contact with the stock."
A long time ago, when I was a teen, the first step with a girl in a party used to be ask her for slow dance and .. a very light and careful touch cheek to cheek ::)
Nowadays, shooting a magnum springer, I always remember that ;D
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"...contact with the stock."
A long time ago, when I was a teen, the first step with a girl in a party used to be ask her for slow dance and .. a very light and careful touch cheek to cheek ::)
Nowadays, shooting a magnum springer, I always remember that ;D
I think with a magnum springer it is more like a later slap in the face step. No? :-X
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David, did you pick the values for the R-C filter for the bare ADXL1004's you got?
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What a great thread!
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David, did you pick the values for the R-C filter for the bare ADXL1004's you got?
I haven't but it is a pretty easy calculation, if I'm not reading too much into the question. The data sheet gives some sample data on page 11 and you can try the numbers in an online calculator (e.g. half way down this page: https://electronicbase.net/low-pass-filter-calculator/ (https://electronicbase.net/low-pass-filter-calculator/)) to confirm.
I plan to scrounge for the components once I pick a frequency; I got plenty of old/used components to choose from with all the electronics labs we have going on at the university. Students leave stuff behind, etc. (If you need the a set for a filter, I can send something.)
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Thanks, I have R-C components. I was just curious what frequency you were going to set.
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I'll mount any filter parts on the terminal strip near the sensor and that will let me change them easily if needed. Then I'll see how it goes with a couple of trials (it's not like I only get one shot at good data...).
To tell the truth, I favor 2nd order or higher filters for their sharper roll-off, but then we're talking active filters and a lot more messing around to build things. In this case, for such a fast event just taking the data as fast as I can get it and keeping the low pass filter on the high side may cover my bases.
What I am looking forward to doing is integrating the acceleration data. It's not that I need to see velocity and displacement, but with such good control of the initial conditions it is a tempting numerical question. I am curious to see what I get.
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Since I had the instrumentation set up, I took some shots with a QB78 (stock, untuned). As you would expect, the accelerations are gentle by comparison (note the change in vertical scale for the accel trace). The values are in the range of the ADXL326 I was using on the 2240 and I'll have to redo it with that more sensitive sensor.
When you adjust for the speed of sound delay in the audio trace, looks like the bulk of the noise starts at pellet exit (as expected). Looks like the first event on the accel trace (channel1) sort of lines up with the star of the audio when you account for the longer distance to the breech area.
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Not sure who still visits this corner of the GTA universe, but here is a thought on testing.
Recently I got an accelerometer with a faster response/higher bandwidth. The thought was: Can one use a muzzle mounted accelerometer to measure/evaluate hold sensitivity?
The first picture shows the instrumentation block clamped near the muzzle of the Titan I used for the other testing. The accelerometer is on the circuit board on the side, with the sensitive axis vertical. On top of the block is a piezo microphone (in the cylindrical mount) that I use for a qualitative sanity check on the accel. The black sleeve on the right side of the instrument block holds the laser/detector pair that detects when the pellet leaves the muzzle. The other instrument is an accel in the barrel direction mounted as before in the surrogate scope on top of the tube (second picture). The laser, two accels, and the mic. use up the 4 channels of the DSO.
I set up two different support conditions on the benchtop (picture 2), bag supported and, on a roller and linear bearing. I also tried offhand and my (amateur) artillery hold (open palm support and medium press into shoulder). For the two benchtop supports, I pinched the trigger to minimize my contact with the rifle. the thought being that my two fingers can't compete with the recoil forces of the Titan.
The third image shows all the data on the scope screen. It shows the start of the cycle and when the pellet leaves the muzzle. The axial accel provides the basic shot cycle. The vertical accel and mic provide the response at the muzzle. Interesting that the Titan now appears to have a rough spot in the piston travel that also shows up on the muzzle response. The trace in the middle of the chart labeled vertical velocity is an integral of the vertical accel done by the DSO. The repeatability of that trace I think is part of hold sensitivity.
In the fourth image, traces from two different artillery hold shots. The difference between the two velocity traces at the instant the pellet leaves the muzzle would be part of the hold sensitivity error.
The fifth image shows these velocity traces for the different support conditions and the sixth zooms in on the instant when the pellet is leaving the muzzle. Any difference between a pair of equivalent traces would project to be an error on target. At least from this limited trial, the artillery hold was much more repeatable. The actual numerical value can be used for calculating a POI spread.
The results look promising enough to do some more testing. It would be interesting to be able to have a number for the recoil portion of hold sensitivity.
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Thank you for sharing. I suppose the muzzle path due recoils is not only vertical. It would be interesting to complement your experiments with a laser device attached to the barrel and a slow motion camera filming the laser dot at the target.
That's my intuitive thinking to explain why a heavier pellet (slower..) has the POI changed to the side.
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I only have one of the accels at this point so I chose the vertical because there the recoil forces are not on centerline. There will be lateral accelerations at the muzzle as well due to vibration modes in the barrel and other sources so adding a second sensor is a logical step.
High speed optical measurements are challenging. High speed video would probably want 10,00 frames/sec to capture the instant the pellet is leaving the muzzle. You could use low speed video or still camera and pulse the laser briefly as the pellet leaves the muzzle.
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I was thinking that, once the muzzle path is recorded, the point corresponding to the moment the pellet leaves the muzzle would be determined by calculation.
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Regarding the learning curve for springers, I think it would be valuable to know about some basics.
Once squeezed the trigger, and due recoils, the muzzle usually has a path, for instance, in this 'form' (known from the video). Depending on the barrel length and the speed, a certain pellet would take a certain time to travel through the barrel. At the exact moment it's leaving, it will be being leaded by the muzzle to a certain direction.
(on the other hand, I suspect that, depending on the holding, and the resting, the path's 'form' may have relevant changes)
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In addition to the direction based on the tilt of the muzzle at pellet exit, the pellet also caries whatever sideways (vertical or lateral) velocity the muzzle has at the time the pellet exits. So if you are shooting at say 20m and the typical flight time is 0.1 sec, then, as an example, if the muzzle was moving at 5 cm/s up (or sideways) that would add 5mm of error on target in addition to any error due to pointing. That's what I'm trying to measure with this setup. To measure the pointing (tilt) I would need another accel near the rear of the rifle to calculate tilt. I have not seen any of the reasonable priced gyro modules (for RC drones, etc.) that output fast enough to measure the change in the pointing angle.
Without some expensive video equipment, I think you could record a streak made by a laser dot (flashed) on a still image and measure its length. That could give you some idea of the muzzle sideways motion.
When the pellet leaves the muzzle is not that hard to measure. Trigger release is harder.
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I suppose the muzzle path due recoils is not only vertical.
I took the accelerometer and moved it so that it measured the lateral direction (image 1). I then took 3 shots in the roller supported configuration. While it may not be realistic, it is relatively repeatable, and useful for checking things out. Unfortunately, at this time, I can't measure the vertical and the lateral with the same quality in one shot.
Image 2 shows a comparison of the muzzle motion (velocity) of the two sets of three shots.
Here is my speculation:
The vertical motion responds first to the piston being driven (first recoil) and then the piston being quickly stopped by the pressure spike (second recoil). The amplitudes are larger because the tube is offset from the center of gravity and from the support points. The motion in the lateral direction responds less to the first recoil and then more so to the sharper second recoil. Most of this motion is accounted for in the shooter's zeroing of the sights/scope using his/her preferred hold. The part that ends up in the error at the POI is the spread (at pellet exit) within each set of three shots in this case. Here the actual spread was similar for the two directions. If one does the calculation to get the muzzle position change, the results are similar, large difference in absolute value of the curves, but the spread is similar.
Taking a guess at why the the POI can move laterally for heavier pellets. When I did a double pellet shot earlier in this thread, the pellets spent an extra .001 sec in the barrel. So for that extreme weight change, the pellet exit point on the curve in image 2 would move to the right and the pellet lateral motion would have a new value, and a spread to go with it.
At this point this is speculation without enough testing to back it up, but that is how I interpret the trends.
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I am from another (distant) corner of the GTA universe. I would love to just point and shoot, with nothing in between in my mind. When I'm having a recurrent problem, the detective mode always needs a qualitative referential to start thinking about the suspects.
I understand your attention going to the quantitative. Thanks for your kindness by using "Brazilian" units..
Btw, I already had a misunderstanding when thinking only with the qualitative, until I found out the quantitative involved. Top quality pellet, relevant vertical strings.., and always worried about a supposed need to improve the seals (minimise ES). Then, I found out the 'vertical stringing' tool in ChairGun. It validated the rule of thumb .. while ES is up to 4% of the average fps, no worries (about vertical effects). For my needs, I was free to think about another suspects.
Besides the pellet weight thing, I am currently considering the possibility that, as a component of POI changes (same pellet), some lack of repeatability when holding and/or resting may cause relevant changes in the muzzle path, and the specific tilt when the pellet is exiting. To see some images would be fine.
But .. I am not a tech person. From my newbie times, I bought a Gamo laser device that I ended not using; it doesn't have a pulsing mode. I only have a Sony 'handycam'.
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I think with the Handycam you can measure you more than the rifle, which is actually useful.
If your handycam shoots at 60 frames per second, so about .017s per frame. The shot cycle for my Titan is about .014s so the rifle effects will look like a blur, probably split between two frames. On the other hand, everything you do prior to the pellet leaving the muzzle is slower so you may be able to get good information.
If you want to experiment, here is what I would try:
Set up a normal paper target to shoot at and sight your scope. Set up a second sheet of white paper off to the side where you can't see it in your scope (so it does not distract you). Somehow mount your Gamo or just a laser pointer so it hits the second sheet when your scope is on the bullseye. The mounting does not have to be very rigid since you will be measuring what happens before the recoil hits. Aim your handycam at the second sheet and measure the distance from the handycam to your muzzle. The speed of sound in air is about 343 m/s at 20C and 355 m/s at 40C so if you have the handycam 10m from the muzzle the time delay on the audio track is 10/343 = .029s or about two frames so you can get a better idea of when the pellet left the muzzle from the audio track. Looking at the video prior to that point will give you an idea of what your hold is doing up to the point when the recoil hits. You can compare that to how the pellets are hitting the bullseye. You probably want to use a multi-bull target (like benchrest) so you can keep track of each shot.
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When I say ‘repeatability’ of holding and resting, I was not referring to any motion from the shooter.. It was only about the position of the hand under the rifle, relaxed palm or some grip, materials where resting the hand, position of the elbows, pressure on the shoulder, ..
Your wider perspective is more realistic and thank you for the guidelines. Nevertheless, I don’t believe I would be able to have useful results to correlate the sound and the poor image within the very short timeline. So, I will remain curious about actually seeing the muzzle path, mainly after the second recoil.
Well, just figuring about the path reinforces to me two wise advices .. tame it = ‘artillery hold’, or similar. Let it be repeatable = ‘follow-trough’.
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I did a little more testing. This time I used, what I would call a casual artillery hold, off-hand sitting in a chair with the pellet box inches away. Since I'm starting the accelerometer clock at the point when the piston is released and tracking it for about 15 millisec, I think the typical slow pointing drift should not muddy the results too much. Since I only have one of the high bandwidth accelerometers, the vertical and the horizontal results are from separate shots (yeah, I probably should spend the $50 for another one). I made 3 shots with vertical data and 3 shots with horizontal data. I also included the two offhand vertical results from the pervious testing.
The first chart shows an overview of the muzzle vertical and horizontal velocity traces based on a simple integration of the accelerometer data. The integration is started with zero at -14 millisec (piston release). Each shot follows a similar trend, even though some of my pulling the trigger may still be included, the forces are probably small compared to the shot cycle.
The second chart just zooms in on the time between peak recoil (or surge) and pellet exit. Looking at the average vertical, for a 200m/s muzzle velocity, this corresponds to about 2.8 MOA with a spread of 2. For the horizontal it is about 1 MOA with a spread of 1.
The third chart does a second integration to get the muzzle vertical displacement. If the motion was a result of the rifle pivoting (as a rigid body) at my shoulder the average would be about 2.5 MOA and the spread larger at about 2.8 MOA. These MOA numbers would be larger if they were due to barrel flex (harmonics?) and smaller if they are just due to the riffle rising without tilt (yes, a second sensor would help here as well). These would be potentially additive to the vertical velocity generated MOA errors.
Is it real? Is it useful? (note: an adult beverage or two may help with this section)
Real? I admit, to make the data more robust, I need to proof and scrub the data processing in the spreadsheet for any errors.
Is it useful? It could be. I think you can use such results to evaluate tuning improvements, including potentially barrel harmonics tuning. It may also help finetune the hold technique. It may also help in picking the number of shots to use for zeroing a scope. You can also put printouts up on the wall of the man-cave to impress any visitors.
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Went back to thinking about the pellet motion in the shot cycle. I tried a couple of way of holding the probe wire inside the barrel. First was on the tip of a bamboo shaft (spear?). This worked OK but there is a chance that the shot cycle recoil/surge moves the probe tip and affects the measured launch time. I considered the coiled wire probe George used last year but I wasn't sure it would not jam even using wadcutters. I settled on a felt-like plug (image 1). This is similar to cleaning pellet plugs (which I didn't have on hand) but is cut square so there is some room for air to get by. Unfortunately I cut it from the pads you put under furniture feet and it is synthetic so I now have some plastic to clean out of the barrel. I think it worked well to capture the initial motion but I'll need something else to capture the motion later in the cycle as the gas motion builds up in front of the pellet.
The second image shows the results for the first inch or so of motion. It shows the shot cycle (Hobby wadcutter 11.9 gr) as captured by an axial accelerometer. The inset graph shows the position from multiple shots as a function of time. It also shows the volume behind the pellet (based on a .217 barrel diameter, but not including the skirt volume). As a comparison, my estimate for the transfer port volume is 154 mm^3.
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I took some more data to track the pellet motion during the surge spike which I'm treating as equivalent to the pressure spike in the compression chamber. I switched to a wooden holder for the probe wire (image 1). This is a tighter fit and has a narrower cross section so it is not moved by the air moving in front of the pellet at the higher velocities.
The position is mapped on the surge acceleration trace (image 2) and shows that the pellet is a little over 8 inches down the barrel when the surge is ending. The Titan has a 18.5 inch (470 mm) barrel.
The third image shows the same data plotted against time from the surge peak, I added a trendline and a simple estimate of the velocity from the slope of the trendline.
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Stan, I'm trying to figure out how you are using the wire. Is this inserted down the barrel or just at the muzzle? (It does go in the barrel, correct?)
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I thread the loose end in through the breech until the wood block hits the leade with the wire tip facing the transfer port. I then use a skewer to push the block/wire to the desired depth for that shot. For the very shallow ones, I check it with a depth gage but for the rest, a marking on the skewer is good enough. The wire is long enough to go out the muzzle and reach a connector on the barrel. The pellet closes the circuit on contact. The pellet, wood, and the wire go flying into a length of PVC pipe partly filled with plastic bags.
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I've been reading an excellent book that was published by Bruel & Kjaer back in 1980. It's titled Mechanical Vibration and Shock Measurements. This is the second edition and has many revisions not included in the first edition. Since my purpose for hacking the Titan was mainly to measure and evaluate what happens during the shot cycle I've found this tome to be rewarding, to say the least.
I decided after the last set of measurements (back when) that there was not going to be an easy path toward getting the second large impulse in the cycle under control. In the overall scheme of thing it seems that the gun has no big problems surviving the impulses. It's the scope that takes the real beating.
The Titan project was left there and I got distracted with several other experiments in different areas in the mean time. My mind did wander back occasionally to an assortment of unusual damping arrangements that might be investigated using mechanical impedance measurements. This book may instigate some experiments in that arena. It shouldn't be too difficult to put together an impedance head transducer to help design a new type of scope mount.
The photos show a copy of a page in this book that got me interested in making a few bench measurements. I set up some instruments that would allow me to use the DSO to simultaneously capture a single cycle (one period) of a 1 kHz sine wave along with its 2 integrals - velocity and displacement. I was pleased to see the DSO's resemblance to Fig E. 4. The 3 channel overlay gives some idea of the amplitude and phase relationship of the signals.
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Did you mean this pamphlet? https://www.bksv.com/media/doc/bn1330.pdf (https://www.bksv.com/media/doc/bn1330.pdf) (large pdf)
I'm impressed with your DSO's math function. I don't think I can do a double integral in mine, other than by post processing the waveform file.
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Yes, that's the publication. I bought it in book form from a bookstore online. It was about $8. Saved a lot of time, paper, and toner.
The measurement setup Includes several pieces of instrumentation. The DSO is only doing one of the integrations. The other one is done by a B&K charge amplifier which can do either or both integrals plus low and/or high pass filtering. The DSO's digital filters are used as bandpass limiters.
Mixing and matching between the 2 different instrument's integrators is interesting and instructive. They both do equally well at this task.
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Stan - One of the toys you might consider saving up for is a Bruel & kjaer 2635 Charge Amplifier. It would open up a whole new world of accelerometer measurement capabilities for you. They're real sleepers on eBay. I've acquired several of these amps over time and not had to pay more than $100 each for any of them. They're current catalog items from B&K and list for $7500 apiece. Go figure!?
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That's a good thought. I probably should think about having an industrial based comparison for some of my instrumentation. I've hesitated both because of the cost that comes with that eco system and because a good part of my interest in this testing was to see how far one can utilize the chip based sensors. One area where it is difficult to duplicate the industrial instrumentation is in fast response pressure measurements like you did with the Titan.
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Another advantage to having a 2635 conditioner is that it can be used with quartz pressure transducers. I have 3 Kistler 601B units (all eBay trophies at laughable prices). These devices look like just another charge generating transducer to the 2635 which means that the 601B's calibration sensitivity can be dialed into the 2635 and read out in pressure units. The Endevco instruments need a different readout arrangement. I've been using the Endevco 8510B devices because I've collected a fair amount of Endevco instruments and their pressure measurement system is pretty much plug and play.
There are several specification advantages in favor of the 601B when compared to the 8510B. The only downside to the Kistler transducer is that it requires extra machine shop operations to properly mount it. The machining isn't difficult, I'm just lazy about some things. When It become necessary to get better numbers the 601B will be pressed (?) into service. Up until now I've been more interested in piecing together the overall picture of the shot cycle.
As an aside I'd like to mention that there are considerable differences between "industrial" and "Laboratory" instruments and procedures. And these differences aren't philosophical interpretations... Just sayin'.
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As an aside I'd like to mention that there are considerable differences between "industrial" and "Laboratory" instruments and procedures. And these differences aren't philosophical interpretations... Just sayin'.
Fair enough. Some industries have much overlap between the two where this type of instrumentation would be used for both R&D as well as acceptance testing of deliverables, but I appreciate the difference.