SAW sensors

robski · Sep 15, 2012

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 ahve 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

davenn · Sep 16, 2012

I have personally never seen SAW units being used for anything but filters and occassionally in oscillator circuits
Virtually every reference is to a SAW filter.
It is possible they would pick up vibrations if placed against something, I just dont know how you would interpret the signal.
Normally they would have a RF ( radio frequency) signal of some broad frequency going in one side of the SAW filter and a much narrower/cleaner RF signal coming out the other side. The frequencies of which are determined on what the SAW filter is designed for

Sensing vibrations on a object surface would be accomplished much more easily using piezo transducers

Dave

wingnut · Sep 16, 2012

Have a look at this website. It uses a system of bouncing light off the vibrating surface (similar to spy technology of having a laser beam bounce off a window to eavesdrop conversations). They use sunlight, but a laser beam could be used.

http://www.creative-science.org.uk/lightbeam.html

robski · Sep 16, 2012

Thanks for the replies. Although the website suggested is extremely interesting it probably isn't quite what i'm after. My intention is to detect the veibration from a surface. It is to detect whether a fluid is there or not so a different vibration will be given when there is a fluid against the wall a dn another when there is not.
Any advice on this would be greatly appreciated.
Thansk
Rob

wingnut · Sep 17, 2012

Rob

Maybe there is a far simpler solution.
A pipe filled with fluid weighs more, so resting on a switch would switch on.

A fluid flowing through a pipe could be as silent as an empty pipe.

But a stethoscope attached to the pipe, taken to a microphone - with the signal taken to a microcontroller or pc to analyse the signal??

Or a transparent pipe to see or detect the fluid with photosensors.

Or different fluids diffract differently, so shine a laser through a window and measure the diffracted angle - which will differ for air or liquid.

Or have a waterwheel (like a centrifugal pump) which is turned by the liguid flow.

All I am saying is that there may be a simpler way if one thinks out of the box.

robski · Sep 17, 2012

Hi Wingut,
Yeah 'im pretty sure there are many different, easy ways of carrying this out but the application is quite restricted. Perhaps it would be best if i explain the job in hand properly.
It is for monitoring the level of fuel in a diesel generators tank. The tanks are very large, as are the generators.
The generators are hired out so any cutting into the wiring, breaking into pipes, etc, is out of the question. The device has to be non-invasive.
I got the idea from putting your hand on a large tank and tapping the side. Where there is fluid acting against the wall inside, the the vibration is different compared to where there is no fluid inside.
I was trying to get a form of sensor, possibly piezo contact mic, etc, to get it to give a differnt reading on points on the side of the tank where there is fluid and where there is not. The reading i get from it would need to be the raw data so i can determine the difference between them. I'm assuming the difference between a value with fluid inside and one without will only be small so i need to able to decifer between the 2. Hopefully i can set this up within the program of the microcrontroller.

The rest of the project, (which i'n not diving into yet) involves then sending this information back to a hub to monitor the levels (1 for now but maybe more generators on the same system later on)
I'm relativley new to this type of electronics so still feeling my way. All the help given on here is greatly appreciated.
Many Thanks
Rob

wingnut · Sep 17, 2012

I took an empty beer can and attached a tone generator and crystal microphone to it. I then increased the frequency. At a certain frequency band (3620Hz as can be seen on the frequency counter), the amplitude increased by about double.

This suggests the system has hit its natural resonance frequency. See the attached photo of my Winscope oscilloscope and frequency counter.

A full beer can resonates at a different frequency, and it definitely conducts sound better so that it picks up many more frequencies on the meter.

Maybe this setup could be used in your case, that above the liquid level it would resonate at a different frequency to below the liquid level.

robski · Sep 17, 2012

Yes that's interesting. That definitely sounds like the sort of thing I'm after. If I use piezo sensor, do you know what kind of raw values I would get from it. Just to try and get an idea of what will be feeding into the microcrontroller.
Thanks very much.
Rob

wingnut · Sep 18, 2012

That I cannot say Rob. Piezo sensors I believe are only good for big changes, like being struck. They don't work well for subtle changes, so my guess would be that any other type of mic would be better.

SAW sensors

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