PSN-L Email List Message

Subject: Re: VBB response, contd.
From: barry lotz gbl@.......
Date: Thu, 27 Jan 2000 20:07:13 -0800

    Well a few years ago 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. To give you the end of the story first,
I got confused as to what I was measuring and gave up.
  The sensor: I'll try to describe the sensor with the dimensions from memory.
There is a plastics shop near me that sells acrylic plate and tubing. The sensor
exterior was a ~2.5" diameter tube about 5" long. At the ends I made caps of the
next size larger tube that telescoped over the main tube about 0.5". To form the
end I glued a ~3" square plate at the end of the cap tube. I now had a "sealed"
tube. For the sensor(s) I bought two radio shack piezo disks. Each disk was the
same diameter as the main tube. I put the disk at the end of the main tube them
put the cap over that. I did leave a small spacer inside the cap so the disk
wouldn't touch the cap plate. Ok, for the mass I used a ~1" diameter steel round
stock. The idea was to position the round stock inside the main tube and have 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. I won't go to much into the disk mechanical attachment to
the mass at this time. 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. Therefore was the sensor I described above
outputing the differential of acceleration since I was measuring velocity rather
than displacement? I'm not sure. I also wasn't sure about how to apply damping to
the sensor short of maybe filling the interior with oil or some such material. I
would like any feedback as to what I was measuring. Could piezoelectric mat'l  be
used for hinge material and their output measured? I still think there maybe some
possibilities for them in strong motino sensors.

ChrisAtUpw@....... wrote:

> In a message dated 26/01/00 20:52:58 GMT Standard Time, sean@...........
> writes:
> --snip---
>     Has there been any use of piezo electric accelerometers, either directly
> or with the output integrated to give velocity? (I am not talking about
> silicon chip devices.) What period would you recommend, please? I can extend
> the response out to ~15 sec without problems.
>     Regards Chris


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Larry Cochrane <cochrane@..............>