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I'd like to measure the velocity of a projectile, but I'm not sure how I'd do it.
I've been researching for the past few hours, and pretty much all of the results seem to be using an LED and either a photoresistor or a phototransistor but it is also mentioned several times that these are too slow.
Would a photodiode work instead? There are a few results with them, and I haven't seen any one report any speed issues.

Would this be possible? I'd like to measure speeds of 200~500 FPS, and plan on using an Arduino Nano v3.0. I don't actually physically have one yet, as I'd like to know if what I want to do is possible before committing any money to it.


I have no experience with an Arduino, and little experience with hardware in general, but I'm fairly familiar with C and C++, and this seems like a reasonably simple first project.


Thanks!
« Last Edit: February 12, 2012, 12:21:43 am by Cress » Logged

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Much depends upon your requirements for precision and repeatability, and how far apart your photosensors can be. In practice, however, achieving 500fps with a photodiode circuit is entirely doable, using careful component choices (we've done it smiley-wink )

There's no reason why you couldn't do it with an Arduino Nano, though the clock speed and photodiode spacing trade off to determine resolution. The faster the clock speed of your processor, the closer together you can space your photodiodes to achieve the same resolution.

A lot also depends upon whether or not you can constrain the projectile path (inside a tube, e.g.), or whether you need to sense over a relatively large area (e.g., a firearm speed trap).

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I plan on mounting two pairs of photodiodes and LEDs six inches apart inside a PVC pipe.

So I'd need:
1x Arduino Nano
1x LCD display
2x IR LED
2x Photodiode

Do I need anything else? It seems a bit too simple, haha.
If anybody could point me towards a specific model of any of the parts that'd work, that'd be really cool, too!


And again, I have very limited experience with electronics, so any suggestions or comments are appreciated!  smiley
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500 fps, 6" apart, that's 500 microseconds, should be doable.
Connect the sensors to an interrupt, note the times, quick calculation, all done?
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Hrm. One of the things I'm worried about as of now is how accurately I need to shoot in order to get a reading.

If I'm using and LED and a photodiode, would I have to be directly in between them to get a reading?
Because I'd also like to get an rate of fire reading, which won't really work if only every fifth shot is detected.
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I plan on mounting two pairs of photodiodes and LEDs six inches apart inside a PVC pipe.
Quote
One of the things I'm worried about as of now is how accurately I need to shoot in order to get a reading.

Well you have to shoot through the pipe for a start.

But as mentioned before:-
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A lot also depends upon whether or not you can constrain the projectile path (inside a tube, e.g.), or whether you need to sense over a relatively large area (e.g., a firearm speed trap).

Quote
would I have to be directly in between them to get a reading?
Well how else would a signal be produced on the sensor?
« Last Edit: February 12, 2012, 03:10:56 pm by Grumpy_Mike » Logged

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why don't you describe the full setup. not just the speed measuring part?
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would I have to be directly in between them to get a reading?
Well how else would a signal be produced on the sensor?
I was thinking more of how large of a tolerance I'd have for the sensor to detect it; would it work if the BB( which is only 6mm, by the way ) is blocking only half of the sensor? Or would it have to be exactly in the center?

And I'm not completely sure what you mean by the full setup, it should just be the speed measuring part hooked up to the Arduino and an LCD screen for displaying the results.


I apologize for my ignorance.
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I meant things like knowing the size of the object and the diameter of the pipe. That way we can visualize the problem better.
Ok so it's a 6mm BB pellet, how wide is the tube?
One possibility is to use mirrors to bounce the IR signal around so it covers more of the diameter of the tube
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I was thinking more of how large of a tolerance I'd have for the sensor to detect it;
It depends on if you are reading the analogue output of the sensor or the digital output.
With digital there is a threshold but it is fast with analogue it is proportional but slower. A lot depends on your actual setup which is why we are asking.
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I have done some competition shooting and a chronograph is a very common thing to see on the range. I have sometimes wondered how they detect the bullet passing through the detectors. I have never seen something as small as a 6 inch pipe. Usually they use some kind of detector thats a foot or so square.

500 feet per second is going to be a bit on the low side. That will be subsonic rounds, like a BB pellet or a .45ACP. If you want to do +P 9mm or .357SIG you'll want to go faster, and if you want to do long guns faster still. You might want to think about going to at least 3,000fps or faster (like a 5.56 round shot from an M4). Just separate the detectors a bit more.  

The software is easy. I would think reliable and precise spacing of the detectors would be the challenge.

Oh, for those who dont know, typically the bullet is as small as 5.588mm (.22LR) or as big as .45 inch (11.43mm).

  
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Also, the best case (or worst case, depending upon which end your on)  cyclic firing rate of, say, an M-16, is 950 round per minute. In terms of a microprocessor, this is an eternity. Plus, to measure rate of fire, you only need one detector.

Regarding the detectors, I would think a bullet passing through the "screen" is not going to cut off all the light. So there is probably a lot of noise. You may have to experiment and see what kind of signal you see as the round passes by the photocell. Assuming you're even using a photocell.
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This sounds like it calls for a grid of reflected laser beams so that a projectile passing through anywhere will break the beam reliably.

I don't think you've said what sort of resolution and accuracy you want on the speed measurement. This will affect the spacing/timing constraints which in turn will affect the rate-of-fire constraints.
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I think you would be better off using a metal detector type circuit. This can be wound on a frame about a foot square and will detect metal anywhere inside the frame so there is no need to break any beam.
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Hrm.
Well, I plan on using it mainly for airsoft, so any metal detector type unit won't affect plastic BBs.

And accuracy-wise, I'd like it to be within, say, 20 FPS-ish? I don't need extreme accuracy, just a basic idea of how fast the gun is shooting.


And I am considering just using multiple photodiodes mounted in a circle around the pipe; they are pretty cheap and it shouldn't be too hard.
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