surface acoustic wave sensors

[robski]

Hi all,

Can anyone offer me some advice on surface acoustic wave technology. I need something to sense the vibrations of any surface. It will be mounted to something against its surface and pick up the vibrations. Can anyone help with the surface acoustic wave types. I have looked at a lot of information on them but it's difficult to find something exactly related to what i need. I have found some that are used for remote controls but i dont think this will work for my application.
Does anyone have an example of a circuit using an acoustic wave sensor??

Many Thanks for any help.
Rob

 

[alec_t]

Have you checked out 'SAW filter' technology?

 

[Mr RB]

I think you are looking at the wrong thing with S.A.W. searching.

Try a google for "contact microphone" or "pressure zone" "surface pressure zone" microphones. They are using in some sound recording and vibration recording applications.

 

[davenn]

I think you are looking at the wrong thing with S.A.W. searching.

Try a google for "contact microphone" or "pressure zone" "surface pressure zone" microphones. They are using in some sound recording and vibration recording applications.

agreed .... as I responded to the Op on another forum SAW elements are usually used for filtering RF signals

I suggested piezo transducers which are also used for glass break detectors in alarm systems

Dave

 

[robski]

Thanks for the responses. Clearly i'm headed in the wrong direction.
My intention is to mount a sensor to the side of a tank and detect whether there is fluid inside by the ammount of vibration sensed. The exact figures aren't important just as long as i can detect a reasonable difference between a signal with no fluid against the wall and one with fluid there.
I shall broaden my search with the suggested topics.
Thanks very much.
Rob

 

[robski]

The information you gave was very helpful. I've just been looking at info on piezo sensors and it appears they could be what i'm after. I was wondering if you could help with the set up of them. I take it that if i was to buy a simple piezo transducer i would need to drive it with a frequency as it is not created within it??
The value i get back will be fed into a microcrontroller so i will need the raw data.
Thanks again
Rob

 

[Boncuk]

Hi robski,

I used to make acoustic sensors to detect dry bearings on industry fans (diameter 125cm up to 285cm.)

Bearings make a lot of noise when running dry.

The setup can be made easily:

Use an electret microphone which you glue to the magnet of a defective loudspeaker. Build an AF amplifier directly above the mic with a gain factor of 1,000 (adjustable) and measure the sound volume either with an AC-multimeter or a scope. Remove the fabric on the front end of the microphone since it's glued there to suppress unwanted noise around the mic.

Disregard any clipping of the amplifier if it incorporates AGC (automatic gain control). The LM386 should do that easily.

You'll be able to measure object rpm, loudness (keep the volume control stable after testing.

Use a 3.5 mm mono-plug for the output into ear phones. Using a 3.5mm stereo-plug you might use the third connector for the positive power supply.

You might want to reduce cost using the diaphragm of a pieco-speaker, but you must make sure to glue only at the outer side onto the magnet.

If you drill three threaded sack-holes into the magnet you just need three standoffs to mount the cover of a spray can. That way you keep the interior free of dust/dirt and have a strong handle. The PCB must have three holes to slide over the bolts which must have two nuts on each standoff.

If you want to study similar projects have the German term "Koerperschallmikrofon translated and use that for further search.

Boncuk

 

[Mr RB]

Hi Boncuk, what role does the magnet play? Is that just a device to magnetically hold the electret mic in contact with the metal motor that you are listening to?

 

[Mikebits]

What you want is a geo phone. https://www.geophone.com/content.asp?Pageid=16

You can often find them as surplus or Ebay.

 

[dr pepper]

Boncuk: my mate at college did something like that only with an fft on the end, the idea was to detect bearings faults and indentify the actual fault.
You can get them readily now, I've used one the odd time.
I've wondered whether building something like you said and wiring it up to an spectrum analyser off an old stereo would do the trick.

 

[Boncuk]

Hi Boncuk, what role does the magnet play? Is that just a device to magnetically hold the electret mic in contact with the metal motor that you are listening to?

Yes, Mr RB,

the mic has to be held in a good and stable position when you start measuring. A fan of 2,85m diameter is generally used for wind channel applications (e.g. car body design for least cw-value for streamlining the body)

I don't want to stand in a 300mph wind when measuring. Therefor the mic has to be mounted with sufficient mechanical strength.

Magnet power won't interfere with the electret sensed output of the amplifier.

Boncuk

 

[Mr RB]

Thanks for the info!

 

[alec_t]

I take it that if i was to buy a simple piezo transducer i would need to drive it with a frequency as it is not created within it?? The value i get back will be fed into a microcrontroller

A simple tone burst generating circuit should do for transmitting, plus a (possibly tuned) pre-amp circuit (gated off during the transmission) for amplifying the resultant reverberation signal.