Theoretical Maximum Velocity in a PCP

[lloyd-ss]

Travis, You're up bright and early this morning, but I think that's standard for you, LOL.

Thanks for those thoughts, I had been pondering what might happen when i actually fire the magnet thru barrel. That is going to happen within the next few days. Right now, the barrel has a .437 O.D., and the grooves where the coils are wound directly on the barrel are .327 dia. That leaves a wall thickness of .038"..... about as thin as I am willing to go. The only magnet I have is 6mm dia x 3mm long. So that is a total gap of .045" to the coil, per side. I doubt I can get the magnet  any closer to the coil than that.  Your thought of the voltage spike degrading to an indistinct "bump" when the magnet passes by at high speed sounds realistic. We'll see when I fire the magnet. But if the spike does disappear, the backup plan is to try a longer magnet. Instead of 6mm dia x 3mm long, maybe 6x6 or 6x10. Too bad I don't have a handy "Magnet Store" down the street, LOL.
Lloyd

[oldpro]

I can see it now Lloyd casting magnet/epoxy mix Bullets!!!  That just might work! LOL  The older I get the less I sleep so I just get up and start working. I’m tinkering with a program to tune my Subaru ecm right now. Nothing I own is  unmodified 😂

[lloyd-ss]

...............................................................
I’m tinkering with a program to tune my Subaru ecm right now. Nothing I own is  unmodified 😂

Do you ever look at it from your wife's perspective? "What the heck is that crazy @#x%* doing now. I am just going to turn around and pretend like I never saw that."  
My wife has a tag line she uses on facebook called "Married to an engineer." She uses it for "Guess what this is," and good natured laughs. Yes, she puts up with a lot. But not everyone has a silent exhaust fan under the kitchen sink so that the whole kitchen doesn't smell like a trashcan full of mashed garlic and old shrimp." She didn't even know she needed it, but she loves it. Right?

[WhatUPSbox?]

Could you use your test barrel and existing probe with shop air (blowgun) to explore the response at higher (moderate) velocities?

[lloyd-ss]

Yes, I will creep up on the velocity and hopefully not loose my only magnet in the process.  The catch can for the pellet/magnet is filled with plastic grocery bags. They exhibit tremendous elongation when hit with a pellet, spreading the deceleration over a distance of 6 or more inches.
I am anxious to see how it goes.

[lloyd-ss]

Keeping with the cautionary note of creeping up on the task, I managed to successfully record some 400psi, 350 fps shots. They all went well, but a few tweaks will be done before the velocity is bumped up.

I won't be able to give full documentation until later tonight, but here are pics of the test setup.

This is the overall setup.
The .25 cal Disco-based gun is strapped into place. It shoots thru the birdhouse muffler. The metal rod above the gun barrel is an alignment bar for the hole thru the birdhouse.  If the gun's barrel is parallel to that rod, the pellet should pass thru without hitting anything. Beyond the birdhouse is the chronograph with light bar above it, and then the bullet trap.  For this exercise a 6x6x6 open-fronted cardboard box is filled with grocery bags, and placed in front of the normal trap, to catch the fired pellet.  To the right side is the trusty laptop with the PC based o-scope program ready to go. It has 2 channels. One channel records the valve stem movement, and the other channel records the pellet/magnet movement past the sensor coils.



Here is the pick-up for the valve stem movement. As Rube Goldbergish as it looks, it is about the 20th attempt at a reliable (ha ha) valve stem movement pickup. But, it does work! Sticking out of the window in the side of the air tube is a 1/4" steel rod that is threaded ontothe end of a special valve stem. It travels in unison with the valve stem as a single entity. Sticking out of the top of that valve stem bar is a short length of .050 hex key. The hex key pivots where the little set screw is visible, and the end of the hex key swings an arc across a portion of carbon track from a trim pot. The white bead on the end of the hex key is a teflon ring that helps the wiper maintain a proper pressure ont eh carbon throughout its swing of travel. There is about a 3 to 1 travel increase ratio of the wiper across the carbon track. A 1-1/2 volt battery and a resistors plus the carbon track, make a voltage divider circuit to provide a voltage to the o-scope that is proportional to the travel of the valve stem. There is a torsion spring wound around the hex key pivot point to remove any backlash from the setup. The voltage seems to track well with the movement, although I am sure there is some lag and overshoot at the extremes of travel. But again, it does work.



This is the front end of the barrel with the 3 coils in place in grooves on the barrel.  More grooves and coils will be added after the bugs from these are worked out. The 3 coils are wired in series and connected to the second channel on the o-scope.  They work well, but the inter-connecting wiring of the coils needs to be done in a neater manner to reduce unwanted indications on the scope. But, it works, too. The patient is ready for the experiment to begin.



REAL results  tonight. 
Lloyd

[rsterne]

Hey, Rube.... nice setup !!!!

Can't wait to see the results.... *grin*….

Bob

[Sfttailrdr46]

What Bob just said. You can't help but smile when you see a setup like that gets the job done 

[oldpro]

Ok I’m excited!!! Popcorn at the ready

[MJP]

Looks a bit like my ecu bench setup at times. Wires soldered to the ecu board, going to open programmer with changed components hanging on ribbon cables.

But if it gets the job done it's enough. 

Good job Lloyd!

Marko

[lloyd-ss]

Thanks guys. When you can do it with stuff from your junk.... uh, project material collections. so much the better.

[rsterne]

Ohhhhhhhhhh man.... I just KNEW your junk was bigger than my junk.... 

Bob

[lloyd-ss]

Bob, and I just spent today putting more flooring in the attic to store MORE junk.
----------------------------------------

First internal velocity data.
There were some good recordings of 3 different shots, so far, but I have only annotated the 3rd one.
The platform was a modified (do ya think, LOL) Discovery, .25 caliber, with 128cc of reservoir. All shots were low pressure and low speed.
The pellet was a Beeman wadcutter pellet with a neodymium magnet attached with 2 faced tape to the flat front end. Total weight of the pellet and magnet was 32.6 gn.
This shot was fired at 386 psi, and the pressure after the shot was 354 psi.
Velocity across the chrony was 309 fps. Distance from muzzle to the first eye of the chrony was 30 inches.

After you get oriented with the graphs from the oscilloscope, they are pretty easy to follow.  There are 2 channels, red, and blue.  They read as voltage in the vertical axis and time in the horizontal axis.  The time scale for both channels is the same, and the total time across the ENTIRE graph is 11.5 milliseconds, or 1/2 millisecond per horizontal division line.  The vertical voltage scales are different for each channel, and are set to appropriate scales for the inputs.
For the blue channel, which is travel of the valve stem, the scale is 1 volt per large vertical division line. For the red channel, the output of the coils that are excited by the magnet, the scale is 30 milli volts per large vertical division line.

The blue trace is for the valve stem starts moving at time = 0. You can see it rise to max height at about 1.4 milli sec and then return down to its seat at about 3.4 millisec.  The overshoot, or the poppet reseeding into the seat  is probably just a function of over travel of the mecahnical indicator attached to the valve stem. Maximum stem height is about .12 inches, but I need to investigate that further.
However, I do think that the valve open time of approx 3.4 msec is reasonably accurate.

For the red trace, the travel of the pellet/magnet combo, the first real indication is where the magnet passes thru the first coil, at 17.7" down the barrel. That occurs at 6.2 msec after the valve stem starts to move. At 6.7 msec the pellet is at 20.15", at 7.0 sec the pellet is at 21.3", and at 7.5 msec, the pellet passes out the muzzle at 23.5".

 (Note- this picture is from 2 screen captures of the same shot that were combined to make a single, complete timeline of the shot. You have to scroll thru the o-scope screen to see the entire shot, so knitting the 2 together was necessary.)
Click on the picture to make it larger.


The extra humps and bumps in the red trace are due to a number of things that need to be cleaned up. The hose clamp at the front end makes the extra blip at the muzzle. The way the extra lengths of coil wires are soldered together and taped to the barrel also cause funny bumps near the coil indications. But a little bit of house keeping will fix all of that.

There is some definite overtravel in the valve stem indicator, and it might actually be running out of travel.... I am not sure, but it will be looked at. For the time being, it is functional.

Once this set up is functioning at real airgun velocities, more coils will be added, including, probably, a couple just past the muzzle.
This seems to be working. Fingers crossed! 
Lloyd

[rsterne]

Very neat stuff, my friend.... watching with continued fascination....

Bob

[lloyd-ss]

I was studying the data from the shot in the graph in post # 392 and ran into a few things that surfaced some questions.
There are 3 magnetic coils near the muzzle end of the barrel. The o-scope indications of these 3 coils, plus the indication of the muzzle of the barrel, provide 3 segments along the barrel where average velocities can be calculated.  The first attempt at calculating velocities yielded questionable results.
The results were, starting with the one closest to the breech, 374fps, 342 fps, and 364 fps. And finally, velocity across the chronograph was 309 fps, although interestingly enough velocities across the chrony for the previous 2 shots (at very slightly higher pressures) were 366fps and 376 fps.

So, I remeasured, more accurately, the distances along the barrel, and also enlarged the o-scope trace so that calipers could be used to measure the time more accurately (fractions of the 1/2 msec time divisions).

The final results that I got were, again, starting closest to the breech, 366fps, 361fps, and 369fps. Still a bit odd with the slower velocity in the middle of the 3 segments. So, there are some possible explanations for the anomalies. One is  that the velocities are correct.  Another is that I need to remeasure everything for accuracy again. Another is that the magnetic center of the coils might not be where I think they are. Another is George's comment about the magnetic resistance of the magnet passing thru the barrel. Another is the affect of the muzzle crown on the magnetic end of the barrel.

The bottom line is that the setup is working, but we are down to splitting hairs on the velocity measurements, and that requires more accurate work on my part to reduce the tolerances wherever that can be done. The work continues.

[lloyd-ss]

Here are a few pictures of the method I used to retrieve the pellet and magnet after each shot. I've show this before, so please forgive my repetition.

This is a picture of pellet hole in the "catcher". The catcher is a 6" square box, with no cardboard on the front end ,just 5 sides to the box, not six. Plastic grocery bags are layered loosely in the box to stuff it slightly tight, but still soft, and then a single bag is tightly tied around the entire box to hold everything in place. The plastic grocery bags have tremendous elongation and slow the pellet very gently.

Here is a pic of the catcher with the pellet hole in it.



In this pic, the outer bag has been removed and you can see the funnel shaped hole into the bags. The nice thing is that the pellet stretches the first bags deep down into the bottom of the hole just like you were trying to push your foot out thru the toe of a sock.


Here is everything peeled back and you can see the pellet with the magnet attached with 2-faced tape. Also note that there is a single layer of shiny packaging tape wrapped around the magnet to stop it from making direct contact with the bore of the barrel. BTW, the magnet is a plain cylinder even though it doesn't look like it in the picture.


I've used this grocery bag method to catch everything up to 50 caliber. One thing to keep in mind is that the bags need to be re-fluffed after each shot because the stretched portions loose their elasticity and don't stretch for the next shot.

[WhatUPSbox?]

You might be able to corroborate the muzzle signal with a broken wire (or probe wire) approach using your magnet wire. You would lose the valve info for those shots but that would be OK.

One thought that would take more work would be to add the coils a small precise distance from the probe positions on your .22 test barrel.

Looks like for the shorter distances you need to pick the trigger point to about 5 micro-sec. When you expand the timescale on the scope, is that doable?

This is all great stuff, thanks for doing this.

[lloyd-ss]

Stan, I like those ideas and will give them a try, in particular, about verifying exactly when the magnet leaves the muzzle.
The trigger point can be adjusted on the scope, but the scope is constantly storing data until the trigger is triggered, LOL. So even if the trigger voltage has to be set a little high to avoid false triggering, the before-trigger data will be displayed on the graph. Is that what you were asking about? Sorry, but I don't know much of the proper terminology for o-scopes.

[WhatUPSbox?]

Lloyd, Sorry bad wording on my part. My question was whether the trace produced by the pellet exciting the coils has a sharp enough peak to allow resolving down to about 5 micro-seconds

[lloyd-ss]

Stan, I am not sure but I am about to find out.