In a message dated 29/01/00 01:03:53 GMT Standard Time, gbl@....... writes:

>> I was very interested in those piezoelectric disks. Radio Shack has them 
for one. I thought they would make a good sensor without the use of magnets 
or excitation electronics. 
 >> The sensor exterior was a ~2.5" diameter tube about 5" long.  For the 
sensor(s) I bought two radio shack piezo disks. I put the disk at each end of 
the main tube. For the mass I used a ~1" diameter steel round stock inside 
the main tube and had it attached to the disks at both ends. When the mass 
moved relative to the tube there would be an output from both disks. I wired 
them so the flex output would sum rather than cancel. It did give an output 
when put it on my work bench and applied a tilt to the bench top.
 >>  The problem: As I understand it, for short period sensors 
(accelerometers), the displacement output is a directly proportional to the 
ground acceleration for periods below the natural period of the sensor &  at 
~ 0.7 damping. Peizoelectric material seems to output velocity. I also wasn't 
sure about how to apply damping. I would like any feedback as to what I was 
measuring. To give you the end of the story, I got confused as to what I was 
measuring and gave up. 

C> As you put a load on the center of a piezo disk the capacitative element 
of ~15 nF charges up. The more the load the bigger the voltage. If you have a 
very high impedance voltmeter, the charge just sits there, otherwise it leaks 
away. If you then unload the disk, you get a charge of the opposite polarity. 
The first resonant frequency of a large size unloaded disk is usually ~500 Hz.

>>Could piezoelectric material  be used for hinge material and their output 
measured? 
 C> small piezo laminated bars are sold just for this job, but their capacity 
is low at 750 pF, which makes long period response sensing more difficult. 
100 M Ohm in // with 750 pF has a decay time of 0.075 sec. You can exchange 
capacity for sensitivity if you place additional capacity in parallel with 
the piezo element. There are also well established feedback techniques for 
increasing the input impedance by up to x100. 
 
The larger disk elements with 15 NF capacity look much more promising at 1.5 
sec. It is not too difficult to increase this by another order of magnitude. 
They also seem to be more robust than the small bar elements.

C> I have been using a modified PZT 'sounder' disk mounted on a spike to find 
leaks in water mains, by pushing the spike into the ground and listening for 
the water hiss and the clink of pebbles. It seems to be an effective 
technique. You use an audio amplifier and if you turn up the gain, you can 
easily hear someone walking by. I could also hear the noise of traffic on the 
road 100 yards away quite loudly. 
The response extends to over 1 KHz if you don't filter it and it can be 
extended down to a few tens of seconds without much difficulty. I don't know 
is how this device would compare in sensitivity to a geophone, but it seems 
to have a respectably high sensitivity and it is a lot cheaper. As it is, the 
disks give a pure acceleration output. This could be integrated to give a 
vertical velocity output. The disk temperatures need to be quite closely 
regulated to eliminate drifts. 

    Chris.

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