Decibel Sensor

I am looking for a sensor that can detect an audible signal around 95 decibels. I have an Allen Bradley MicroLogix PLC controlling a motor. When the the sensor does not detect a report I would like to send a signal to the PLC to stop the motor and timer. Does such a sensor exist? I have a hand held unit that uses sound to detect rate of fire and count shots, but I cannot find a sensor to send a signal to a PLC.

You're using a PLC to control a machine gun?

WOW. Talk about 21st Century warfare.

- Leigh

A $10 VU meter kit connected to a mic will provide a time-averaged display (on LEDs) of an audio event. You can adjust gain, frequency weighting, and peak/average timing. Event stops=LEDs go out=trigger PLC. Once proof-of-concept is established, re-design for production/environment.

Chip

One point to consider regarding VU meters...

They have a fast-attack slow-decay characteristic that might or might not impact their suitability for use in this particular application.

- Leigh

I thought about that, and it could be adjusted in the "final" version to be anything they want. But they probably don't want the motor to stop between shots, but rather between "volleys". Hence the integration is "a good thing". Maybe. Details are few and far between. OP?

Chip

I really have no experience with this, but I'd like to hazard a guess anyway. Take what I write with a grain of salt.

My guess would be find something to convert microphone signal to PLC-friendly analog signal (0-10V), very likely an amplifier and possibly a filter circuit.
Since you're just looking for peaks and not a specific frequency, you could look for all analog values above a certain level and count/average/etc. per your application.

Chip Chester said:

Details are few and far between. OP?
Chip

Click to expand...

Thanks for all the replies. The audible signal will be continuous at 15 cycles/second. If the sensor does not detect a signal for 1/2 a second it should shut the motor off that activates the trigger. A typical test is 100,000 cycles.

Kyle Smith said:

The audible signal will be continuous at 15 cycles/second.

Click to expand...

That statement is ambiguous.

Do you mean:
1) The signal is a sine wave with a frequency of 15 Hz, or
2) The signal is a sequence of impulses numbering 15 in a second.

If the latter, what's the duration of the impulse?

Also, in your original post you mention 95 dB. Relative to what? The decibel is a relative measurement. Without a stated reference it's meaningless.

- Leigh

The latter, I do not know the duration of the sound impulse. And thanks to wiki for anybody who could not understand what I meant, "The decibel is commonly used in acoustics to quantify sound levels relative to a 0 dB reference which has been defined as a sound pressure level of .0002 microbar.[11] The reference level is set at the typical threshold of perception of an average human"

Decibel - Wikipedia, the free encyclopedia

I didn't think it was relevant to my question, but if it will some how help you to answer it, there you go.

I have a few questions, too...

-1. What are the stakes if the test goes south? If the results are inaccurate? What's your budget?

0. Is this for an automatic weapon? Gas-driven or "new tech"?

1. Is this for a one-off lab or field test scenario, or does the methodology need to survive into production and be "combat-field rated"?

2. Is the noise the device makes the only way to test that it works? Or could optical or metal-detecting sensors at the "business end" do the job, too?

3. Are you testing that the "gun" works 100k times, or that the ammo works 100k times?

If "95 dB" is interpreted to mean the signal is 95 dBanything above ambient noise floor, then detecting it is easy. If the "turn motor off" signal is any 1/2 second interval that elapses without at least 8 "cycles", then a mic and a zener diode will generate a suitable nearly-square-wave trigger signal. But like many questions posed in a vacuum, the environmental factors, reliability requirements, portability, lifespan, etc. make all the difference.

And then, to reveal how little I know about automatic weapons -- doesn't the exhaust gas from the chamber power the reload cycle? And then, wouldn't it just stop on it's own if a round or two didn't fire -- regardless of whether the trigger was pulled? Or is there some sort of electric "helper" on the firing pin if there's a bad round? Not my area of expertise, to be sure.

Chip

PS... just saw your wiki ref... that's usually referred to as "dB SPL" for sound pressure level. Power and voltage are also measured using dB scales, once you convert the sound pressure into electricity. Above, I've assumed that whatever the sound is, it's significantly louder than the ambient sound, or ambient noise in the electrical signal. Or, 95 dB above whatever sound it makes when it doesn't work right.

Chip Chester said:

...doesn't the exhaust gas from the chamber power the reload cycle?

Click to expand...

It sounds as though the OP is working with a gattling gun type of weapon, being a multi-barrel device that's controlled entirely by external mechanisms, not relying on gas expansion or recoil for operation.

Just an assumption on my part.

Your "dB SPL" notation is correct, and seemingly appropriate for this application.

A dB value by itself without a reference is like saying the temperature is "ten degrees hotter", without further explanation.

- Leigh

This is for a paintball marker. Currently I use a DC motor with a motor controller to turn a shaft that actuates a trigger. During the design stage, and less frequently during production, while being life cycle tested the marker will fail and not cycle. The test takes between 2 and 4 hours depending on the cycle rate, too long to stand and monitor the test. If the test is left unattended and is not cycling I currently have no way to know when the failure occurred. One option is a vane and switch mounted in the path of the exiting gas, as long as the marker is cycling the switch is made. The negative to that approach is, each product has different barrel lengths and placing the vane in the correct position is one more adjustment to make. This led me to the audible signal as a way of determining if the marker had cycled. The parts that reciprocate inside the marker are covered up and not accessible while shooting on most of our products.

What's a "marker"?

How about an optical curtain? Place it far enough down-range that it will accommodate any barrel length, so no adjustment is needed.

Very easy to implement and fool-proof.

- Leigh

A paint ball gun is called a marker, since the target is marked.

Using an optical curtain to detect a gas pulse does not sound all that well thought out.

Don't use slang

3t3d said:

A paint ball gun is called a marker, since the target is marked.
Using an optical curtain to detect a gas pulse does not sound all that well thought out.

Click to expand...

You still haven't told me what a "marker" is. I doubt that they're lobbing the gun downrange on every shot.

I interpreted the OP's question to mean that the device was firing projectiles of some sort. An optical curtain is certainly appropriate in that case.

This is one obvious problem with using job-specific slang as opposed to plain English.

- Leigh

To continue answering questions that do not help with my original question: Do you think any word that you do not know is slang? Marker is not "job specific", it is the industry accepted terminology for our products. This is to distinguish these types of products from guns or firearms.

I am not sure why your are nitpicking the minutia of every post Leigh, I have dealt with you in the past an not had any issues.

While testing we are not using any projectiles.

The you just need a sensitive pressure sensor. Freescale Semiconductor makes them. You just need one sensitive enough to register the pressure wave and feed it into the PLC.

Chris P

Muffler Bearing said:

I really have no experience with this, but I'd like to hazard a guess anyway. Take what I write with a grain of salt.

My guess would be find something to convert microphone signal to PLC-friendly analog signal (0-10V), very likely an amplifier and possibly a filter circuit.
Since you're just looking for peaks and not a specific frequency, you could look for all analog values above a certain level and count/average/etc. per your application.

Click to expand...

The best advice here so far.
Place a mic, run a FFT to find the frequencies of interest between good and bad, build a bandpass filter to isolate them, amplify as needed to trigger the PLC.
This is real basic control engineering and about as hard falling off a log. (no insult intended to the OP ).
Bob

Kyle Smith said:

While testing we are not using any projectiles.

Click to expand...

OK. Then what's coming out the end of the barrel?

Terms which are accepted standards in "the industry" are only meaningful to people familiar with "the industry". Since you didn't identify that in your original post, it's reasonable to ask.

- Leigh

Ok. So, to test the marker without the paintballs, you need a mechanical impulse "in", and to measure a pressure differential "out". Either "sound" or "wind", they're both moving air. Large-scale moving air (pressure) is better than sound, because it's less likely to be a false positive. It's possible the marker could "make a good sound" on a test firing, while in fact the sound is coming from a gaping hole that just blew out in the side of the thing, instead of the business end of the barrel. So pressure measurement at the correct point is somewhat better. With no projectile, the sensor could attach to the barrel, so placement with different barrel lengths is no problem. Others have posted appropriate devices which should to the trick.

But the big change I would make in your test setup is to use either a stepper motor and controller, or an actuator piston of some type. Both methods will allow you to have an official, unique "start" to test-firing number 99,234, whose result can then be measured for and logged. Milliseconds later, test-firing number 99,235 occurs, and those results can also be measured and logged. Data acquisition and processing should have no problem with logging events this fast. Then, all the guesswork of "when did it stop" is gone. You can also keep firing, say, 5 times after first failure, to see if the problem in the mechanism is intermittent or not. Cycle rates can also be evaluated, to see whether the mechanism is problem-free at 15 shots per second, continuous fire, but for some reason fails at 20 shots per second.
(Some of those kids have fast fingers...)

Chip