PSN-L Email List Message

Subject: Re: VBB response cont., Piezo Acc.
From: "steve hammond" shammon1@.............
Date: Sat, 29 Jan 2000 11:59:42 -0800

I'm working from memory here so I can't be too specific.  In 1991 Bob Ogborn
in San Jose who later moved to Texas, but who may still be lurking around
the list, built a number of piezoelectric sensors and was thinking about
building an array with them. I seem to remember that he tried the RS device
but they would break because they were brittle. Bob told me this. #1 they
are only good for very local events because they are velocity devices and
"need to have the ground moving around you to induce an output". At the
time, Bob, Dick, Bruce and I all lived within close proximity and there were
a number of event Bob never recorded. I've often thought that the device
might be more suited for strong motion designs.

There are a number of thing I can tell you about the characteristics of
Bob's design.
One of the first things you notice is that if you held one in your hand and
shook it while holding the bare wires, you got a shock. Bob is an excellent
electronics tech and would grin at you every time you did it.  He used 1/2
of a whisper fan out of an original Mac that was a piezoelectric device that
had 2  flippers sticking out of one end that were 3x1/2x1/16-inch that
sandwiched and isolated  a piezoelectric disk at the other. When power was
applied, the free ends of the flippers would flutter freely in the air from
the current applied to the piezoelectric disk and move the air around them.
Bob, simply broke off one flipper and epoxied a 1-oz. fishing weight to the
free-end of the flipper.  Walla -ka-zam, the remaining 1/2-fan was now
transformed into the pendulum and mass creating a velocity transducer.

Next he took 4-inches of common PVC 1-inch pipe and capped one end of it off
with a pipe cap which he had drilled a 1/4-hole into it for filling the
device with damping oil. In the second pipe cap he drilled another hole, put
the single wires through it and filled the cap 1/2-full with enough cement
to seal-up the hole and hold the piezoelectric block in the center so the
mass weight would extend into the PVC pipe without contacting the walls of
the pipe. Once both caps were mounted, he filled the device with 10W oil and
capped the fill-hole off with a rubber plug.

Bob, tried an number of weights of oil for damping, but he also spent time
trying different mass weights and damping flags. By adding a copper flag at
the end of the pendulum, he could drill different hole patterns in it and
achieve different degrees of damping. If you took a 4x4 wood block and
placed it on the ground a few feet away and stood on it with both feet and
hit the end once with a slug hammer (without falling off...), you could
calibrate each of the devices output magnitude to different combinations of
oil and drill patterns.

 Bob really knew electronics and I won't even go there. I'm sure there are a
lot of people within the forum that can talk about the circuit for this kind
of device. I know this, if you play with it, be sure to use a pair of
clamping zineor diodes to protect the electronics from being charbroiled by
the output because the ones that Bob built  could really put out high
current and draw a good spark when you smacked them.  Another characteristic
is that they were EXTREMELY directional.

Steve Hammond  PSN Aptos, California

-----Original Message-----
From: ChrisAtUpw@....... 
To: psn-l@.............. 
Date: Saturday, January 29, 2000 4:34 AM
Subject: Re: VBB response cont., Piezo Acc.

>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
>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
>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 &
>~ 0.7 damping. Peizoelectric material seems to output velocity. I also
>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
>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
>very high impedance voltmeter, the charge just sits there, otherwise it
>away. If you then unload the disk, you get a charge of the opposite
>The first resonant frequency of a large size unloaded disk is usually ~500
>>>Could piezoelectric material  be used for hinge material and their output
> C> small piezo laminated bars are sold just for this job, but their
>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
>sec. It is not too difficult to increase this by another order of
>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
>leaks in water mains, by pushing the spike into the ground and listening
>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
>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
>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,
>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.
>Public Seismic Network Mailing List (PSN-L)
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Larry Cochrane <cochrane@..............>