Hacking the CP-2

[George Schmermund]

I noticed some recent activity in this thread. There was a request to expand upon what the barrel harmonics testing might really mean. Going back over the information that Stan and I posted on the subject it still looks to me as though the idea of barrel harmonics during the shot cycle have little to no effect on a pellet's point of impact. The real driver of the muzzle's position at the time the pellet is released still seems to be controlled by forced oscillations.

Stan posted some interesting overlays, but I'd want to see more data that would compliment what he has shown. My thinking at this point is that barrel harmonics controlling POI is still a myth.

I'm open to any and all experimental evidence that might change my mind. I'm not interested in opinions.

[WhatUPSbox?]

George,
Nice to hear from you and see that you are still interested in this stuff. It might be useful to clarify your distinction between forced oscillation, harmonics, and rigid body motion of the muzzle so that everyone is on the same page.

As an aside, I was always curious if your pressure sensor survived the high temperature peak in the Titan shot cycle test last year.

[George Schmermund]

Stan - Here's how I'm looking at the information we've accumulated so far:

A forced oscillation is something that pushes or pulls on a body in a way that causes motion that is not synchronous with the body's natural frequency. If this forced oscillation (an impulse(s) in our case) contains any frequency that includes any of the resonant frequencies of the driven body (the barrel in our case) then the body will continue to oscillate at resonance until any form of damping within the system reduces the amplitude(s) to zero. These resonances are the barrel's harmonics.

The forced oscillations are transient in nature and decay quickly, but not before the pellet leaves the barrel in our case.

A rigid body, by definition, is an idealization of a body that does not deform or change shape. A barrel driven by forced oscillations and harmonics doesn't act as a rigid body.

As far as the survival of the pressure transducer goes I'll have to check the calibration on it. As I recall it's still functional.

[WhatUPSbox?]

I mention the rigid body motion because the spring/piston system is offset from the CG as well as from the support conditions so a rifle rigid body motion (rotation) is present along with any barrel deformation and can be difficult to separate using muzzle located instrumentation.

[George Schmermund]

It's not clear to me how a rigid body rotation could effect the barrel harmonics. Rotation as you've described it may be a component of the shot cycle that could effect the POI though. The magnitude of the rotation could be evaluated as a separate set of experiments if you think its contribution to POI is significant.   

[WhatUPSbox?]

My point was that rigid body motion is present in the accelerometer measurement at the muzzle.

[George Schmermund]

I've been doing some thought experiments that might provide an indication that rigid body (RB) rotational forces could be an influencing factor in POI. The measurements using accelerometers at the muzzle of the CP-2 were all done while the gun was clamped in a vise. Any RB rotation contribution would have to include the simultaneous rotation of the vise and bench that the vise was bolted to. I find this contribution to be implausible when considering the measurements at hand.

These experiments don't account for Stan's argument about RB motion when shooting freehand. I haven't made any muzzle measurements when the gun wasn't in the vise that I can recall. I'm not going to chase my tail trying to prove a negative, but if anyone can come up with a persuasive experiment to support the argument I'll listen. The ground rules would be a demonstrable nontrivial rotation within  0.004 seconds or less from the time the piston is released. I look forward to any suggestions.







 

[WhatUPSbox?]

yes, no need to instrument the vise.

Though if you did any testing in 2020, you might check this chart of earth rotation anomalies to see if any of the dates align.

https://www.timeanddate.com/time/earth-faster-rotation.html

[RScott]

I am starting a journey to get as good a score in 25 meter benchrest that I can.  My plan is to test lots of ideas related to pellets, gun harmonics/vibration, benchrest equipment, FPS, trigger technique, etc.  I'll probably start a new thread after I get underway, but along with shot precision measurements, I'd like to see what impact some of the modifications have on the gun's vibration, along the lines of all your tests of the CP-2.  The biggest decisions to make on measurement equipment are:
1) Use a Microtrack recorder I got for lock time (aka shot development time) measurements or get a Rigol DS1054Z Digital Oscilloscope like you guys (George and Stan) have used.  The Microtrack is an audio recorder but it seems to measure vibrations when the microphones (actually repurposed earbuds) are taped to the barrel and receiver.  Advantage is cost.  Disadvantage is 2-channel limit and need to download files to my PC and use of Audacity to analyze the data.  The Rigol is expensive and may take a lot of effort to learn how to use.
2) Use the microphones like I used for the lock time work or a Endevco Triaxial PE accelerometer Model 23 that I bought on eBay last year for a bit over $100.  I've hooked the Model 23 to the Microtrack but don't seem to get great x,y separation in the readings.  I can't even figure out which is x vs y vs z yet.

Questions for you guys:
-Any thoughts on how useful the vibration data would be for my efforts?
-I've never used an oscilloscope before?  Will there be a big learning curve?  (I'm a long-ago retired chemical engineer)
-Does the Rigol give fairly immediate results, e.g., take a shot/meaurement, review it after a few seconds on the screen to decide whether to keep it or re-shoot with new data sampling?
-Is the Model 23 going to work well in this application or should I have bought something else to take barrel / shroud movement measurements?

Attached are pictures of the equipment I have and a summary of the lock time analysis.

Regards,
RScott
P.S.  I'll be using a .177 Daystate Pulsar shooting at 12-20 FPE at USARB 25m targets outdoors, and a modified Sinclair front rest.  I can achieve 730 scores in 75-shot benchrest with it as is, but I want to get 750 scores and as many "X"s as possible.

[WhatUPSbox?]

RScott, Nice to see someone diving into the measurement pool. If you have not seen it, this thread provides more information. https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg156145889#msg156145889

The basic question becomes what specific data will be useful to you. From the work I've done, microphones are useful for timing events. It gets more challenging if you want to measure amplitude of accelerations and the resulting motions. You'll see I used a simple light gate to detect when the pellet exits the muzzle. You can also use a microphone to detect an event in the shot cycle but it is not quite as direct.

I think you also need a charge amplifier for the model 23

[RScott]

Thanks for the link.  I had seen it a while ago but was glad I read it again.  I might look into filming a laser pointer near the target during the shot.  Will be easy to calculate when the pellet leaves the barrel based on when the target is hit.  I think I can also easily use a mic at the receiver to estimate when the pellet starts to move (having looked at a lot of Microtrack diagrams; and an electronic PCP's audio track is much easier to interpret than your springer) which then enables me to calculate when the pellet leaves the muzzle.  (The sound pattern at the muzzle is very messy for determining the time of pellet exit)

I infer from your charts that you have been able to reliably quantify muzzle movements using your DSO and accelerometers. 
You mentioned I might need a charge amplifier to use the Model 23. 
- I read about George's recommendation of a Bruel & kjaer 2635 Charge Amplifier.  That would put a further dent in my budget for this effort.
- Why would I need a charge amplifier?  I thought it was used to do additional integrations of the velocity data?  Wouldn't the Model 23 connect directly to the Rigol DS1054Z and provide acceleration and/or velocity data that I could put into Excel to calculate x,y coordinates?

And if I go down the path of DSOs, charge amplifiers, and accelerometers, would you be willing to help me troubleshoot the set-up issues I would likely run into?

[WhatUPSbox?]

I infer from your charts that you have been able to reliably quantify muzzle movements using your DSO and accelerometers. 

Well, that is a bit of an overstatement. The charts were a first look into what data may be attainable and whether it is useful. It is not at this point an established process with a reliable/tested result. I am not completely sure what the complete dataset for high precision shooting might be (muzzle linear accel, velocity, displacement, angular equivalents, etc.). I am going through a similar thought process on what it would take to monitor a pistol shot.

I have not used the model 23 accel but the spec sheet looks like it does not provide a voltage output by itself, so a charge amp may be needed (per channel).

In this thread as well the Titan thread, George and I took different approaches to taking measurements. George has a wonderful collection of laboratory equipment and is experienced in their use. The caveat on lab instruments is that you tend to need the matching equipment, even cables, and these can get expensive. I tried to look at what could be done with hobby level instruments.

The reason I mentioned the laser light trap is that it is a very cheap (<$5) way to get the pellet exit timing (the broken wire works as well but is tedious). Unless you have very high frame rate I don't think video will get you there. The entire shot cycle is shorter than one 60hz frame.

Depending on what you want to do, there are cheaper DSO options than the Rigol.

Maybe start a new thread and list what specific data you think will be useful, and I'll give you input as best I can.

[RScott]

Thanks.  Will do.  Though I might approach it by trying to use what I have and getting reactions and improvement suggestions from you, George, and others