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DIY Chronograph, am I doing it right?

A place to ask general spud cannon related questions.
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Unread postAuthor: potatoflinger » Fri Jun 20, 2008 7:27 am

Yeah, I just built it with CDS cells, and it didn't work. (I didn't use a battery) I think I will make a trip to radioshack and buy phototransistors instead of the CDS cells, and I will add a battery to the circuit.
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Unread postAuthor: Antonio » Fri Jun 20, 2008 8:58 am

Ragnarok wrote: I have tried the "ants method"my air rifle, and the results seem decent enough, although I am slightly concerned about drag losses incurred between the muzzle and target.


Hey this all seems really interesting. It seems like a easier and more compact way of testing the veloctiy of our guns. If you are wondering about the drag issue. I did a test before, where I shot with a spring bb gun(constant velocity) at different ranges. Like this I could calculate the drag coefficient according to the graph and some facts like the frontal area of the bb etc. If you are interested just reply. I did this test some time ago, but I found it pretty interesting. I tried to find a formula or excel sheet for ppl to input and find the calculated muzzle velocity according to the technique above. About 5 shots at different ranges would be enough then.
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Unread postAuthor: Antonio » Fri Jun 20, 2008 9:13 am

Sorry for double post.

I found that the drag coefficient of my bb (0.12g, 6mm) balls was 0.25. The function that I got was y=0.1036x^2-5.773x+b where y is your velocity at while it has flow distance x, b is the muzzle velocity. Get 5 data points and fit these to this curve inorder to get b. This only applies for 0.12g 6mm plastic bbs (with no seam). Btw the units are metric(meters and m/s). If I have time I will make a more general formula, that would be pretty cool:)
The picture below shows a curve with a muzzle velocity of 160m/s:
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Unread postAuthor: Ragnarok » Fri Jun 20, 2008 11:21 am

jackssmirkingrevenge wrote:Since manufacturers have gotten all paranoid about UK legislation however, I doubt your AA's doing much more than 10 ft/lbs.

Well, I don't know.
The sound rec method did actually suggest an ME of a little over 11 ft-lbs, (I've put quite a lot of pellets through it, so it's quite well broken in) so I'm wary.

Just to be completely sure, rather than souping up my DIY models, I might either get a Combro, or I might go the whole hog and get something bigger that'll work on my spuddies too.
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Unread postAuthor: dongfang » Sun Jun 22, 2008 12:01 pm

Hi,

No, CDS won't work at all. They react very very slowly.

Regards
Soren
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Unread postAuthor: jimmy101 » Sun Jun 22, 2008 6:48 pm

dongfang wrote:Hi,

No, CDS won't work at all. They react very very slowly.

Regards
Soren

Have you actually tried it Soren?

I just took a RadioShack CdS (Cadmium sulfide) photocell (Part no 276-1657, dark resistance ~4K Ohms, direct sun ~60 Ohms) wired in series with a 1.5V AA battery and a 10K resistor. The voltage across the CdS cell was input into a PC soundcard's Line Input. To create a signal, I placed a 40mm 12V brushless CPU fan (5 blades) powered by a 9V battery above the CdS cell and pointed the whole thing at bright sky. (Basically, the spinning fan blades simulate a high velocity projectile moving past the CdS cell.)

Using Audacity I had the sound card record at 48KHz, 16-bit, obtaining a 465Hz distorted sine wave. Blocking the light with my hand removed the signal indicating it was from the light being chopped by the fan and not electrical noise from the fan or other sources.

The signal was about 20mV peak-to-peak (this is determined mostly buy the ratio of the CdS resistance to the 10K series resistor I used) and the time between peaks was 2.2mS. Since this is a five bladed fan the speed of the fan is (1/5)/2.2mS = 90 RPS = 5400 RPM which is a reasonable speed for a small DC fan.

So, it looks like a CdS cell will indeed work in a shooting chronometer. A 1000 FPS round passing between two gates one foot apart would take 1mS which the CdS cells shouldn't have any problems with.

I believe the CdS cells work even though they have very slow responses because you don't really need much response. Indeed, all you need is a detectable signal. Even though CdS cells typically have rise times in the 20~30mS range, and fall times in the 30~50mS range, you don't need the full response from the cells. The response times are typically for something like "10-90% response" of the cell. Since we don't care what the magnitude of the signal is, as long as it is detectable, we only need a small fraction of the total response. In a situation where signal magnitude isn’t critical the effective response is much faster than it is in a situation where you care about the magnitude of the signal.

So, pick up some CdS cells from the shack, chop up a piece of PVC pipe the same size as your barrel, glue two cells to the PVC, wire in series with a AA battery and perhaps a 1K resistor. Take the voltage drop across both cells (or just one, or the resistor) and input to the soundcard's line input.
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Unread postAuthor: rp181 » Sun Jun 22, 2008 7:14 pm

I didn't want to start a new topic, so if you want me to start another, just tell me =)

On the topic of chronographs, i was looking for a way to messure the speed of a railgun projectile, and beacuse of the muzzle flash, i decided optical wouldn't work. I also didn't want to use sound because 1)it would slow the projectile 2) i have to look at and decide where the peak start's, so i could easily mistake it, also it would be hard if there was backround noise. I was thinking along the lines of having 2 rings, each crisscrosed with wire, when the projectile passes through, connection would be broken, thus giving me a signal, same for the second one. The two could be hooked up to an O-scope, giving me clear signals.

Oh ya, and since the projectile will be well past supersonic (hopefully past mach 2 with a well designed injector), response time is crucial.
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Unread postAuthor: sandman » Mon Jun 23, 2008 4:07 pm

i would coils but i dont know how to hook them up i just know they work for metallic projectiles (soren used a tater with metal and cronyed it with coils)
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Unread postAuthor: rp181 » Tue Jun 24, 2008 7:37 pm

Well, i made a chrono with 2 phototransistors and 2 IR LED's, the phototransistors casing is black, infrared transparent, so ambient light doesnt matter much. Its consists of two of each, spaced 1ft, in 1in PVC. It seems preety good, my crappy 2 liter ball valve canon at ~40PSI with no barrel clocked in at 166 FPS. I would sugenst using

http://zeitnitz.de/Christian/Scope/Scope_en.html for debogging, very usefull.

To look at the waves and see time, i use

http://www.nch.com.au/wavepad/masters.html

You can cut away excess audio, zoom in, and select range, it says the span at bottom right corner. This program can only do 1000FPS (min time length is .001), unless you have a longer spacing, otherwise, find another.

I probably shouldn't say this, but Wavepad still works after evaluation is over if you set the system clock back, you probably shouldn't do that, it throws abunch of prgrams off.
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Unread postAuthor: TurboSuper » Tue Jun 24, 2008 9:11 pm

I was always wondering- why not use a very simple switch to measure velocity?

Just put two very thin metal wires next to each other, when the projectile passes through them, you get a signal!

Even better: You can use the projectile itself to close the gap, if it's made of metal (or something equally conductive).

Now, of course, you will need a signal conditioning circuit, and it will have to be right after the barrel, but I'm still curious as to whether it would work.
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Unread postAuthor: jimmy101 » Wed Jun 25, 2008 1:05 pm

TurboSuper wrote:I was always wondering- why not use a very simple switch to measure velocity?

Just put two very thin metal wires next to each other, when the projectile passes through them, you get a signal!

Even better: You can use the projectile itself to close the gap, if it's made of metal (or something equally conductive).

Now, of course, you will need a signal conditioning circuit, and it will have to be right after the barrel, but I'm still curious as to whether it would work.

1. Will only work with conductive ammo.
2. The signal will be very noisy so you need to clean it up a bit (google "switch bounce" )
3. You actually have to hit the wires, both at the same time.
4. Then you have to hit both wires on a second switch (it would be pretty difficult to measure the velocity from the open time of a single switch).

Break (trip) wires would be simpler to setup and simpler to get to work.

Phototransistors are very simple switches. Once you get past the perception that they are complicated devices, they are electricaly identical to a mechanical switch.
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Unread postAuthor: jimmy101 » Wed Jun 25, 2008 1:20 pm

RP181:

Good job, sounds like you've got it figured out. (you probably don't even need the IR LEDs, give it a try without them.)

You might download a copy of Audacity (freeware) to do the signal analysis. Audacity works at the full sample rate of the recording. On my soundcard, that's 48KHz (21 microseconds/sample, ~50x better than the 0.001 second resolution).

Lets see, 1000 FPS round through gates 1 foot apart at 1mSec resolution ... sample error is huge, really can't tell the difference between velocities between 500 FPS (2mSec) and 2000 FPS (0.5mSec).

You will definitely need Audacity (or something similar) for the analysis step. Same setup with 20uSec resolution (48KHz sample rate) gives ~50 data points between the two peaks. The sample error is ~2% (+/-20FPS at 1000 FPS).

Post a picture of your detector setup and a typical screen trace if you can.
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Unread postAuthor: TurboSuper » Wed Jun 25, 2008 2:09 pm

jimmy101 wrote:
TurboSuper wrote:I was always wondering- why not use a very simple switch to measure velocity?

Just put two very thin metal wires next to each other, when the projectile passes through them, you get a signal!

Even better: You can use the projectile itself to close the gap, if it's made of metal (or something equally conductive).

Now, of course, you will need a signal conditioning circuit, and it will have to be right after the barrel, but I'm still curious as to whether it would work.

1. Will only work with conductive ammo.
2. The signal will be very noisy so you need to clean it up a bit (google "switch bounce" )
3. You actually have to hit the wires, both at the same time.
4. Then you have to hit both wires on a second switch (it would be pretty difficult to measure the velocity from the open time of a single switch).

Break (trip) wires would be simpler to setup and simpler to get to work.

Phototransistors are very simple switches. Once you get past the perception that they are complicated devices, they are electricaly identical to a mechanical switch.


Um, thanks? I think you pretty just reiterated a good chunk of what I said in different words. I know phototransistors are simple devices but as was mentioned before, if they are too close to a muzzle flash, they can get thrown out of whack. The idea is that these "wire sensors" would be inside the barrel, so hitting them both at once would be a snap. Trip wires are a good idea, but they'd need to be replaced every shot...
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Unread postAuthor: rp181 » Wed Jun 25, 2008 2:44 pm

it doesnt work without the IR LEDS, the signal is all distorted. The IR transistor is potted in black epoxy, thats infrared transparent, and theres hardly any IR from ambient light.

The only problem im seeing right now is that the vapor cloud (or even my breath) gives minor spikes, but it is easy to seperate from the large peaks of the projectile. But as projectile speed increases, peaks become small, and the only way i can tell is because after the peak (projectile entering) theres a negative peak (projectile leaving).

Couldn't you use the time between the first sensors start of the peak and end of the downward peak to confirm speed? speed and that time should be able to give you projectile length.


EDIT: i downloaded audacity, my computer sound card is 44100hz, so pleanty fast. when recording, is it supposed to lead in chunks?
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Unread postAuthor: jimmy101 » Wed Jun 25, 2008 6:04 pm

rp181 wrote:it doesnt work without the IR LEDS, the signal is all distorted. The IR transistor is potted in black epoxy, thats infrared transparent, and theres hardly any IR from ambient light.

The only problem im seeing right now is that the vapor cloud (or even my breath) gives minor spikes, but it is easy to seperate from the large peaks of the projectile. But as projectile speed increases, peaks become small, and the only way i can tell is because after the peak (projectile entering) theres a negative peak (projectile leaving).

Couldn't you use the time between the first sensors start of the peak and end of the downward peak to confirm speed? speed and that time should be able to give you projectile length.

EDIT: i downloaded audacity, my computer sound card is 44100hz, so pleanty fast. when recording, is it supposed to lead in chunks?

Are you firing the gun indoors? Is there a cover (intact piece of pipe)? Outdoors, and without any cover there should be more than enough IR from the sky. If not, oh well, probably should have used phototransistors (PTs) sensitive to visible light.

The vapor cloud is going to be a problem at the velocities you are talking about but it can be minimized. One easy solution would be to use both channels of the soundcard's line input, one for each of the detectors. A battery and say a 1K ohm resistor in series with the PTs. Take one stereo channel across one detector and the other stereo channel across the other. If you want I'll draw up a diagram.

You could also move the detectors farther from the muzzle. But then it'll get trickier to get the projectile to actually pass through the detectors.

The peak shape is dependent on exactly how the input of the soundcard is wired. Typically, the input(s) are "DC uncoupled" which means the input passes through a capacitor. If the peak width is narrow enough you get a peak or a square wave. If the peak width is wide enough the top of the peak starts to drift downwards. When the PT is uncovered the voltage swings past zero and then moves back to zero. The faster the projectile the more you'll get one peak, a very slow projectile, or a very long one, can give two peaks from a single detector, on peak positive and one peak negative. Figure the soundcard is designed for audio frequencies so any peak that looks like a frequency of less than about 50Hz is going to start to split into two peaks. Anything from 50Hz to 20,000Hz should be fine. Your 44KHz sound card should work all the way up to ~44KHz though the peak amplitude will no longer be linear with the input signal at frequencies above ~20,000Hz. That's fine since all you want is a detectable signal and a reliable time base.

If your peak sizes are getting too small you may have tweak the setup a bit. If you are using the MIC input then switching to the Line Input and add an AA battery and a resistor to boost the signal strength without introducing any more noise.

Do you have a Voltmeter? The problem with the soundcard is that it won't record DC signals so you can't just wave your hand in front of it and record an interpretable signal. Your hand moves too slow and the soundcard things the signal is DC and removes it. The solution is to connect a voltmeter across the detector, in parallel with the line to the soundcard. Now you can cover the detector, or hold the shell in place over the detector, and see exactly how much the voltage changes. Figure the sound card is probably designed for a 1 volt peak-to-peak range. So, tweak the system to get as close to a 1V signal change as you can.

You might be able to use the peak width or the positive to negative time to calculate the velocity if you know the ammo length. The problem is that you'll have very few data points in that very short time interval. The sampling error is basically +/- 1/2 data point. So, if there are only 10 data points in the peak you can only define the time as being between 9.5 data points and 10.5 data points, an error of 5%.


When Audacity is recording the actuall recording takes precedence over the display, so ya, it'll look like it is falling behind and/or windowing the output. The recorded signal is fine.

Try recording the signal from a TV remote control ...
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