Theoretical Maximum Velocity in a PCP

[lloyd-ss]

I am trying to improve on the sensors for that lab setup, and adding something just beyond the muzzle could be helpful. Seeing the acceleration plunge from thousands of g's to very negative would be educational, to say the least.
What about using some of these self contained photo detectors to add to the setup? They have some with gaps big enough to shoot thru. Maybe coupled with an Arduino?

[WhatUPSbox?]

Lloyd,
In the middle of George's Hacking the CP-2 thread, there is some discussion on this. Page 9 https://www.gatewaytoairguns.org/GTA/index.php?topic=137566.160

I ended up using a laser detector module and a pointer laser diode. Both are set up to run off of 5V so hook-up is trivial. The cost is minimal (~$3 for both).
Cindi suggested using the photointerruptor. I don't know if she actually implemented it. Looking at the parts, the response time varies.

There are some Arduino based chrony projects out there. The impression I get is that you need to use the interrupts to get timing that is fast enough. I am not an Arduino expert.

[lloyd-ss]

You guys got me thinking about this again, and how to instrument a barrel to read travel time at numerous locations along the inside of the barrel. I had been doing some fiddling with home built alternators for wind and water power. Ah ha! Magnets and coils!

This looks like it will work, and it is much simpler and MUCH more reliable than the previous methods I was using.  However, it does require attaching a small rare earth magnet, probably spherical or cylindrical, to the front of each pellet that is tested. The neodymium magnets are itty bitty, and cheap. Shouldn't be a big deal.
The best thing about this is that you don't have to drill any holes into the barrel, with the associated miserable deburring that is required. But you do still have to sacrifice a barrel.

The magnet, attached to the front of the pellet, will be fired thru the barrel. At each location that you want to read a pulse as the pellet/magnet passes by, a groove is machined in the O.D. of the barrel. Each groove creates 2 perpendicular surfaces in the steel barrel that will interrupt and amplify (kind of) the flux lines of the magnet as it passes by.  There is a coil of very fine wire (a coil salvaged from a relay) that slides over the barrel. The coil is positioned over top of one of the grooves that are machined in the barrel.  The 2 wires from the coil are attached to the input of a single channel of the o-scope. A voltage blip (sine wave, kind of) shows on the o-scope as the magnet passes thru the coil, or surprisingly, when it passes thru any of the other machined O.D. grooves.

I set up an experimental barrel, coil, and magnet. as shown in this picture. The magnet is the tiny thing taped to the front of the 3/16" wooden dowel.
The barrel has 3 grooves machined in it, all located near the muzzle end of the barrel. The coil is positioned over the center groove.





For the experiment, the magnet on the end of the dowel was pushed into the barrel about 12". To simulate the magnet being fired out, I just pulled the dowel out of the barrel as fast as I could.




Here is the scope trace. You can see 4 distinct events in the trace. The first three are the magnet passing from left to right thru the 3 O.D. grooves. The 4th event, on the left, is where the magnet left the barrel.
One very nice feature of this arrangement is the precise timing on the events on the trace. You can see the vertical line at each event, where it drops straight down from maximum plus voltage to minimum negative voltage. That is where the magnet is passing thru the center of the groove. You can see where the magnet comes out of the end of the barrel, that the trace is distinctly different on the negative (bottom) side.



I am not sure if a separate coil will be needed at each groove or not. More experimentation is needed, but this looks very promising.

[rsterne]

Cool idea, Lloyd.... *looking at my watch*…. and BTW, Happy New Year (almost)…. *LOL*….

How sad is it that we are on the Forum right now?.... *eyeroll*….

Bob

[George Schmermund]

Lloyd - This is an interesting experiment, but Faraday's law and Lenz's law will be working against you if you're planning to use this for anything quantitative relating to ballistics. It would make an interesting braking system, though.

[lloyd-ss]

Lloyd - This is an interesting experiment, but Faraday's law and Lenz's law will be working against you if you're planning to use this for anything quantitative relating to ballistics. It would make an interesting braking system, though.

George, I agree that a braking effect will be induced in the magnet as it is propelled thru the barrel, but that force will be orders of magnitude less that the propelling force of the air pressure on the pellet. For a crude comparison of the braking force, pushing the wooden dowel with its attached magnet thru the barrel offers much less resistance than pushing just a plain pellet thru the barrel. I tried pushing the magnet both fast and slow and resistance was only slightly perceptible in both cases.

It will be easy to quantify the actual resistance that the magnet adds by by firing the pellet with magnet thru the barrel and measuring its MV, and comparing that to the MV of a plain lead pellet of the same weight and configuration.  I doubt that the difference in velocity will be consistently measurable, but the experiment will show the answer.

But back to point of the experiment, it is just another way to measure the acceleration of a pellet thru a barrel.

[PakProtector]

the braking force is a function of power taken away. With a large monitoring load( Mega-Ohm+ ), the system will just about only create voltage. This will cause a negligible drag. Now if you short the coils, there will be non-trivial braking...
cheers,
Douglas

[lloyd-ss]

The coil in the setup is 80 ohms, so lots of turns of small gage wire, but the coil does produce pretty spikes, from the groove it is directly over, and from the grooves that are nearby. That makes connecting to the o-scope very easy. But I am wondering if just hand winding a tiny coil in each groove along the barrel might work?  But how would they be connected? Since the idea is to just produce a clean indication at each groove, I could see the numerous coils interfering with each other.
The only magnet wire I have is 20 gage and I was able to get about a dozen wraps in one groove. By pulling the magnet thru the barrel with a stretched bungee, I could barely detect some sort of event on the scope from the 20 gage coil.  Also, I am not sure what passing the magnet thru at pellet velocities will do, but I think it will improve the indications.
Lloyd

[WhatUPSbox?]

Looking forward to the results.
The rare earth magnets are brittle and I don't think the resulting dust is all that great. A dedicated soft pellet trap full of old clothes should help keep them intact.

[lloyd-ss]

Good thoughts gentleman. Thank you.

[Motorhead]

Sort of an odd question on determining losses over the short distance a light test pellet travels.

Could you not start with say the rearmost sensor @ 2 feet from muzzle & take velocity shot.
Then move the Chrony backwards so the Front screen as at the 2 foot position & take a velocity shot.
* Both figures added & divided by 2 should give a accurate average if the CENTER between screens was a 1.5 feet from muzzle.

Then do the test again at say 6 to 10 feet and come up with DECELERATION figure over distance.  * As in FPS per inch of loss

Knowing that you should be able to now add back the speed into the 1.5 foot distance to muzzle and be close to exit velocity.

Or something like this

[lloyd-ss]

Scott,
Very good idea, but my guess is that the shot to shot velocity variation would be too great to get any sort of repeatability.
BUT,  if you had two chronys that read the same (or within a few fps of each other), then your idea would work great getting 2 measurements from each shot.
Lloyd

[lloyd-ss]

I am pretty sure this way of measuring internal velocities will work, so I am getting excited.

I wound coils in the 3 grooves I had machined in the .25 cal barrel. I'm still just pulling the magnet thru the barrel by hand. And believe me, the hand is NOT quicker than the eye, LOL.
Each coil is approx 90 turns of 32 gage magnet wire. The grooves are just randomly placed to see if I could get some readings.

Here's a pic of the 3 small coils on the barrel.



Here are pics of the o-scope pics with the coils connected in series, and with them connected in parallel. I am a little surprised that each groove shows up very distinctly, with no type of cross talk.  The voltage spikes are a bit taller with the coils connected in series, so that is how I will try it installed ona  gun. I am anxious to see what happens at airgun velocities, seeing that the calcs show that i pulled the magnet thru the barrel at a blazing 12.8 fps. Doh! 

[QVTom]

Great stuff Lloyd!

Tom

[MJP]

One more coil over the valve stem and / or hammer and you'll have complete monitoring over the shot cycle.

Marko

[PakProtector]

Not sure what your scope has in terms of input voltage tolerance, but since voltage is a function of speed, do make sure that your first shot does not burn out input circuitry... The old tube Tek's I use for preliminary investigations don't have this issue...they also don't have Record the trace capability either...LOL
cheers,
Douglas

[lloyd-ss]

Tom, Thanks for commenting. It is fun working with the electronics and actually making progress.

Marko, I was definitely thinking about how to use the other channel in the scope. The coil winding is easier than I thought it was going to be, and hopefully, there is a combination of coil arrangement and magnet that will yield both velocity and displacement for a valve stem with a single trace. Maybe?

Douglas, The scope is a PC based Velleman PCSU200 2 channel rig. About $120 and I am just scratching the surface with it. I am a novice at this, but managing to fumble along.  Your comment about the voltage from the coils when the magnet is actually fired is well taken. The magnet velocity could be 100 times as fast as the demo, and the coil output voltage is supposed to be proportional to the speed of the magnet. The scope also has a high impedance probe with a 10x setting.  Maybe I should take the first shots with that probe and the voltage set to the max setting on the scope.  Would that be prudent? Right now, the coils only have 90 turns, maybe fewer will be needed. I don't know.

Lloyd

[Sfttailrdr46]

Lloyd fewer turns might be prudent otherwise there is definitely a strong possibility of overloading the scope input. Even if the input is has over limit protection the result would be useless data

[lloyd-ss]

Don, I read your post just as I was getting ready to post this one. You've made a good suggestion. Fewer turns in narrower grooves sounds like a safer way to go.
Lloyd
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This is from last night's experimentation:

I was wondering if the coils could be made as pre-wound bobbins that just slipped onto the barrel, eliminating the need to machine grooves into the O.D. of the barrel.
Here are 2 coils that I made and tried.

The one on the low carbon steel bobbin was a complete fail. It has about 100 turns of 32 gage wire, but thinking about it now, the steel bobbin seems to have shielded the coil from the magnetic lines from the passing magnet. There was basically no indication from this magnet on the scope. The coil checked out ok: good continuity and no shorts, but no output. Oh well.

The other coil is about 300 turns on a nylon bobbin. This coil definitely showed a trace on the scope, but it needs improvement. This coil is twice as wide as the coils in the grooves in barrel. This extra width makes a trace with longer duration rise and fall on the scope. And despite having 3 times as many turns as the coils in the grooves, the voltage level was much lower. Overall, the coil worked, but it's trace was not as precise as the narrower coils in the grooves.

 Since I've basically already ruined this .25 cal barrel, I will continue working with it and machining additional grooves into it to get more coil locations. But first I have to machine the breech end and add the barrel port, and see what kind of output I get from the 3 coils that are already there.

[oldpro]

I as you know am a tech geek lol and I have probably 3 working scopes and a few more for parts and have dabbled a bit in building pic up sensors for ignition systems so I have a few pointers that I can share. The closer the magnet is to the pick up or in your case coil spool the better a signal you will see now I have a feeling that at speed you will most likely not see any spike at all or just a bump in the signal ( I hope I’m wrong). When building the pick ups for a high reving Seadoo engine (10k rpm) if the gap got more than .030 the signal was lost at speed so I wonder how the barrel thickness will play into this at speed. I would in this case thin the barrel as much as possible where the pick ups go on or even better thin the barrel and put down a thin insulation barrier and wrap directly to the barrel and epoxy over it you don’t want any wire movement it will show up on the scope. Do this every 6 inches and run the pick ups in series with insulation tape between them to help prevent any bleed over. Even going to this extreme I’m not sure at Speed you will see a good strong signal