Bullet impact sensor help needed....

[Rapidrob]

Hi folks,new here. I have an unusual request for the experts.
I run a long range shooting club here in Albuquerque,NM. We have a need for a reactionary steel target that needs to sense a bullet impact. The normal kinetic energy that will be transferred to the steel plate will be in the neighborhood of 2,000 foot pounds traveling at 2,000 feet per second. I tried an acoustic sensor but were getting false target reactions due to the 180 dB sonic crack of the projectiles passing close to the target.
I'm thinking that a true impact sensor mounted to a thick steel block ( bullet resistant ) mounted to the rear of the steel target would sense the bullet impact. The target at the time of impact moves less than 3mm, however then energy transfer to the target is very high.
I want the sensor to close a relay to energize a motor drive that will withdraw the target from sight.
I'm not able to determine the resonate frequency of the steel plate upon impact,so I'm not sure if a simple glass brake sensor will work.
Thank you for any information.
Rob.

 

[btrueblood]

Well...do you want fancy and spendy, or quick & dirty?

Fancy would be accelerometers and a sensing circuit that would trigger the motor mechanism if the peak acceleration of the plate was above some level (you would need to test to find the appropriate level).

Quick and dirty would be to tear open an old pinball machine, and take out the "tilt" switch. A similar device could be cobbled together from a microswitch and a spring-loaded pendulum.

In the really fancy category: If you google "bullet passage sensor" you will be led to several patents for acoustic sensors that not only sense bullet passage, but can also be used to compute the apparent hit location of the bullet. I know the USMC uses a system similar to these for some of its training ranges.

 

[Bribyk]

Search for "impact transmitter" for reciprocating equipment. They detect looseness (parts beating themselves to death) not vibration and can be purchased off the shelf as two-wire 4-20mA transmitters. They are a piezoelectric accelerometer with integral signal conditioner. Metrix Instrument sells one such device and I'm sure there are a number of other models and manufacturer's out there.

 

[GregLocock]

Or wander by a scrap yard and pick up the knock sensor off an engine.

Not too sure what the interface is, but it is dead simple.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rapidrob]

All of the tips are interesting and helpful. KISS is the way to go. I was hoping for a low voltage ( 12 DC or less ) sensor that would function.
Thank you.

 

[Bribyk]

A knock sensor may do the trick. See if you can tune your plate's natural frequency to 6-7 kHz which is where an engine knock sensor should be tuned to be most effective.

 

[Rob45]

How about a mechanical filter (i.e., a sound barrier/absorber wrap) for your acoustic (microphone) sensor?
That way you won't need to have the sensor real near the target, in harm's way, and the frequency of the sonic boom ought to be singificantly different than the sound you get from impacting the plate.

Or you could simply require that the impacts be sufficiently centered that the plate is knocked down...
<grin>.

This is an interesting question; tell us what you end up doing.

 

[hydtools]

Maybe a car alarm impact switch/system:

http://auto.howstuffworks.com/car-alarm2.htm

Ted