From: campbell@gnu.ai.mit.edu (nobody:*:-2:60001:I. N. Cognito:/:)
Subject: Kynar (pyroelectric and piezoelectric plastic)
Date: 7 May 91 02:03:55 GMT
Organization: The Internet



        I just got an interesting little piece of plastic today. It's a
piece of PVDF (acid resistant) plastic sold under the brand name of Kynar.
The interesting feature of this little piece of plastic is that it is both
pyroelectric and piezoelectric.

        Pyroelectric means that if any heat or infrared light strikes it,
the plastic starts generating a voltage from one side to the other. It can
detect a man from about 50 feet away and is used in burglar alarms for that
purpose.

        Piezoelectric is where it really shines. If it undergoes stress,
vibration, or just about any movement, it also has a voltage. This is
reversible so that if you apply a voltage, it wiggles. It can be used as
a touch switch (the sample I was sent was about .5" X 1", enough to generate
about .5 volts when I tapped with a fairly resistive multimeter attached). You
can just tap the table and it picks up the vibrations. In fact, we had a hard
time zeroing it out (stand still and don't move or it will pick you up from
changes in heat or vibration).

        This stuff can be used as fans. You can use the little (~$1) pieces
for switches. Because it senses vibration, you got to have a way to have a
true 'continuous' switch. Their literature suggests that you cement two pieces
on either side of a solid metal strip and apply electricity to one so that it
vibrates while the other is hooked to sensors. When anything touches the metal,
it changes the resonant frequency and the sensor can detect this.

        As far as total voltage goes, I expected to have to measure in terms
of microvolts and to have to amplify it, but the range on this stuff is the
same as a lot of decent quality analog-to-digital convertors.

        In my own project, I want to construct a touch screen. I looked at the
wire and surface acoustic wave methods and decided that they vastly
overcomplicated/raised the expense of the project. With this stuff, I can use
a sheet of glass and mount sensors around the edges. By timing how long it
takes for a vibration to reach a particular sensor, I can easily triangulate
the exact position where the wave occurred. If I also watch how large the
vibration peaks are, I might be able to even have a crude estimate of the
amount of force applied, so position and pressure make a difference.