Chronograph for bullets

[wannabegeek]

Hi all,

I recently thought of an idea for a bullet chronograph and Power distribution estimator.

There is a group of peole out there who target shoot and hand load, who need to know the velocity of the bullets.

The chronographs I've seen used don't allow for in-situ data between material like wood boards and ballistics gel.

I propose:
A copper trace that is one wire, snaked across a film covering a lot of surface area. This is what the bullet breaks to initiate a detection.
The trace together with a resistor in series, forms a resistor in a Wheat Stone bridge.

When the trace is intack, there's 0 V at the bridge output.
When the trace is broken, there's a fraction of Vcc at the output.

The outputs go to an OP-AMP with gain of 10.
There's undoubtably instability in the bridge, hopefully on the order of 0.1V or lower, after balancing it with a linear pot and using 2% resistors.

An output of 0.1 gets up to 1 V with gain of 10 but with a broken trace, the bridge output is say 10 V, then 10 * 10 gets me up to the rail.
I still get garbage but I filter it later in MATLAB and with some analog filters.

Two or more such configurations are set up serially in the path of the bullet.
Each OP-AMP alternates from non-inverting to inverting configuration.
Each of these networks also has an anti aliasing filter.


The ouputs are connected in parallel to old computer sound card sampling at 48 kHz. This gives a sample window of ~21 us. For a bullet travelling at 1145 fps or 350 m/s, on the cm scale, that's a period of 28.6 us per cm.

I think the quantizing error sucks though.

I then use MATLAB to further filter the data and pick out the peaks in the signal. SInce I know the sample rate, I know the time between pulses and I know the distance.

Of course a better system would use a National Instruments data aquisition card. Then I get into more trouble with software and the like however.

Does anyone see a major flaw or unrealistic design expectation here?
I need to work out the sample related error for sure. I think the electronics are simple enough.
thanks !
wbg

 

[CDRIVE]

I didn't want you to think that you're being ignored. I'm a hand loader and competitive shooter, so this interests me. I'm going to sleep on this but off hand I think there's got to be a much better way of doing this.

 

[poor mystic]

I wonder whether you have considered optics. Bright led's and phototransistors are cheap and fast, and require very little interfacing electronics.

 

[CocaCola]

IMO using a physical barrier only adds errors... How are you going to repeatably and consistently establish the distance between two barriers to establish a base for your measurements? What kind of deflection is the barrier going to have before the 'break' point? Although small a physical barrier will effect the bullet...

BTW watch this video... Pay attention at 0:50 - 0:59

 

[shrtrnd]

I'm not sure what you're exactly trying to do here, I think we could use a definition of
power distribution.
*(Average Velocity in feet per second)*
Average Velocity X Average Velocity X Bullet Weight (in grains) divided by 450,240 =
Energy (in foot pounds)

Is this what you're asking?

 

[CDRIVE]

BTW watch this video... Pay attention at 0:50 - 0:59

This is great! I simply love seeing how great minds solved complex physics long before electronics and computers.

Chris

 

[poor mystic]

I agree Chris, and isn't it instructive to notice that where difficult problems can be reduced to an exercise in first principles they become easier to solve! It's so easy to look for high technology solutions to everything but high tech is often the less effective approach
Mark.

 

[sparky_joe]

I invented a low-tech chronograph using IR LED's and a 1 MHz oscillator with 7490 counters to measure elapsed time between triggers. I've used it to do various science experiments such as measuring the speed of a BB. It works great, it just requires the projectile to pass within the narrow beam width of two LED's. If you want to see pictures and schematic, send an e_mail to [email protected] (that's a Q, not a G!)

 

[wannabegeek]

Hi all,
So many responses...!

I'll definitely think about a photo technique. I really want to be able to hit a fairly large target area so people don't need a bench to shoot.

I like the idea of using a sampler, since it's easy to know the elapsed time between
detections. Instead of a set of comparators or amps, perhaps I should use a 555 timer in astable mode to generate pulses.

As for the "Power Distribution".

What I want, is a way to measure how much KE is being used to break through various barriers per unit time. So if I know the KE right before the bullet hits a piece of 1 cm wood, I can subtract the KE lost, using the work energy theorem. I don't want to count the energy of deformation since it doesn't go into breaking the barriers.

KE =0.5 * m * v^2 = integral( F dot distance);

A good micrometer is important here of course but that error is much smaller than other sources and will be neglected.

I'm about to leave on vacation so I may not be able to respond to this thread until next week. However I'll be thinking about it the whole time...

cheers,
wbg

 

[CocaCola]

This is great! I simply love seeing how great minds solved complex physics long before electronics and computers.

Chris

Yep, it's great to see how it was done at one time but the repeatable accuracy and overall accuracy without a lot of controls in place simply isn't practical vs the new digital solutions...

 

[wannabegeek]

Hi all,

I'm in the Florida keys on vacation but of course still thinking about my projects.

I wanted to get back to the idea of using an LED based system for bullet tracking.

There is an obvious advantage here as long as the bullet is only being tested for it's free flight velocity.

However, my original idea of a physical barrier is tempting to me, b/c I could track the trajectory as the bullet moves through materials, simulating various conditions.

As well I can track the amount of KE and force through each material. With an LED system, this would be impractical.


However, using a sound card as a crude sampler is really only going tobe good for prototyping the detection circuit.

I have only used a counter once and never a full counter/sampler circuit to measure events.

Does anyone mind chiming in on doing this for the first time ?
I image the sampler chip has a clock output pin that goes to the counter. And a counter trigger input pin for the record events ? I suppose I'll need a decoder too for counter to get decimal numbers on a read out.

In stead comparators/amps I'll need a pulse generator like a 555 in astable mode. I have never used one in this configuration before, but it sounds easy enough. Does anyone suggest a much better astable vibrator for pulses less than 10 us apart ?

cheers,
wbg

 

[CDRIVE]

Good grief man, you're in the Keys! You should be diving, fishing and drinking. Then more drinking, more diving and drinking. Hell, it's hot down here at this time of the year!

 

[john monks]

It would seem to me that if I wanted to measure the velocity of a bullet I would simply use a digital oscilloscope and a microphone. Then I could look for the sound signature of the bullet leaving the gun and hitting the target. The rest is just math.

 

[CDRIVE]

Sound is a very poor measurement medium for firearms. Many varmint calibers are nearing muzzle velocities of MACH 2. On top of that the environment on a typical rifle range can sound like a Hatfield/McCoy skirmish.

 

[wannabegeek]

It would seem to me that if I wanted to measure the velocity of a bullet I would simply use a digital oscilloscope and a microphone. Then I could look for the sound signature of the bullet leaving the gun and hitting the target. The rest is just math.

This is a good first principal's idea....actually I'm not just looking for the velocity of the bullet. I want the velocity, for 0.5*m*v^2, the KE, spent in each material the bullet passes through and to plot this as a function of time. This gives me the distribution of power expended in each material.

Now back to your idea.

In basic principal it's a good guess. But, in practice, even on a private shooting range, it's not practical at all. The 'sound' of a gun firing is a very complicated signal. The high velocity bullets being faster than sound aside, the harmonics should be insane to filter out. Recall that the more discontinuous the signal ( such as a heavy-side step function ), the more harmonics and the more filtering you need.

For a good velocity measurement in air, the best idea was proposed earlier in the thread. LED's and photo-transistors. An array of them would be best for repeated use without a bench to shoot from.

As for my idea, I'm thinking of the detector prototype to be made with 2% carbon resistors in a Wheat Stone bridge. Then move on to a current mirrors or look for a bridge chip.

@CDROM

thanks for the encouragement....I don't drink much anymore...it is hot....and my wife is sleeping as we wait to go snorkeling this afternoon...so I get a chance to think about electronics projects....

 

[wannabegeek]

I'm such a noob...I have been thinking about this more.
I don't need to 'sample' the detection pulses, just use a counter with two triggers. Send in a clock pulse from a 555 or bench function
generator and se the triggers.... duh.

It turns out I have an old Tektronix bench counter. But my function generator died
I just ordered a new one...was prolly for the best anyways....can't wait to get a new scope, Rigol 2 channel 50 MHz digital scope $355.

The Rigol has pretty good reviews on Amazon.

Hopefullly my ancient counter doesn't die too...that's more $$$.

I'm also excited to build a Wheat-Stone bridge from four current mirrors in a monolithic chip. I haven't seen any off the shelf bridges for my purposes yet.

 

[wannabegeek]

How to use/make counter/decoder for 1MHz events

Hi all...

I have a project where I want to measure the time between two or three events in micro seconds out to four sig figs.

Let's start with two events for now.

I have an old Tekronix DC503 50 MHz counter and have been playing around with it for prototyping. This bench model has a width measurement which is handy, so I don't have to build a Bi-stable oscillator or anything. I just use two comparators with opposite input voltages to get an 'up' and 'down' edge.

The counter has a built in time circuit in this mode and will measure the width (time) of the square pulse sent in.

Now lets say I want to make something I can take away from the bench.

Do most counters have two or more input triggers ? How do these triggers work ?

And do they typically have an input for a clock signal ?

And how about the decoder ?

Is that part pretty straight forward ?

TIA !

wbg

 

[CDRIVE]

I thought this was mentioned earlier. The counter in mS starts when the projectile breaks the first beam and stopped when it breaks the second beam, If the second beam is placed 1 foot behind the first one you can convert it to a direct Ft/sec reading.

 

[Harald Kapp]

The instrument you use may be called counter, but from your description ist is much more than a simple counter:
- counter
- frequency meter
- timer

Of course, all these are built on the principle of counting events, so the name "counter" for this type of instrument is o.k.

1) Let's start with the counter:
This is a digital counter which counts events. What an event is, is defined by the input and trigger circuit. For example you could set up the counter to count pulses by triggering/advancing the counter with each rising edge of a pulse. You could also use the falling edge of a pulse. Or you could (maybe not with your instrument, but in theory) use a more complex trigger condition like e.g. count pulses on input 1 but only if input 2 is HIGH.
This is not the function you need.

2) Use a counter for metering frequency.
This is basically a use of an extended trigger function as I described before: An internal signal (gating signal), probably not accesible on the outside of the meter, is active for a defined period of time. Only during the time this signal is active will the counter count pulses from the external input. If for example the gating signal is active for 1 second, then the number of pulses counted during that interval is exactly the frequency of the external signal (frequency = pulses/second). In practice the gating signal is active for a much shorter time (millisconds) and the number of pulses counted is scaled up to get the frequency.
This is not the function you need.

3) Timer.
This operates like a frequency meter but with the roles of the gating signal and the coun signal reversed. An internal signal creates a fixed timebase (known number of pulses/unit of time, e.g. 1 MHz). The external signal is used to gate the internal signal to the counter. The counter will count the internal signal pulses only as long as the external signal signal is active. Thus the count at the end of the active time of the external signal is a direct measure of the time during which the external signal is active.
That's the function you need.

So you need:
- a stable frequency reference
- a counter
- a gating circuit
- an input conditioning / trigger circuit

There used to be single chip solutions like ICM722 (http://www.hit.bme.hu/~papay/edu/Acrobat/icm7226.pdf), but I don't know whether they are still available. Today, you could use an inexpensive mircocontroller (PIC, AVR or similar) for the same task. Counting, gating and displaying you'd do in SW.

As for the input conditioning: that depends on the type and quality of your input signal:
If it is already at a digital level, maybe no conditioning at all will be required.
If it is noisy, a filter to remove noise may be required.
If it has slow edges, a Schmitt-trigger signal conditioner is recommended to generate clear signal edges.
If it is not at digital level suitabel to your counter, amplification or attenuation may be required.

Now that you should know what to look for: Google lists a lot of circuit examples.

Harald

 

[wannabegeek]

Thanks for the break down, that helps confirm what I am guessing at.

My detection circuit has been designed for a clean digital trigger to be sent already. I guessed that much up front.

I think I will try to use the single chip solution like the one you linked to. That's likely a good entry point.

TIA for your effort in responding,
wbg