thinman@netcom.com (Lance Norskog) (04/18/91)
If you're building VR input devices, you want to check this out:
Interlink Electronics sells a type of force sensor called a Force
Sensing Resistor (TM). They were originally developed to build
velocity-sensitive synthesizer keyboards.
FSR's transduce force into resistance. Without force, they read
out at 10 megohms. With a very strong finger-press, they
slide down to 2 kohms. The response curve is not linear, but
is logarithmic instead. This gives a response curve which is
"subjectively linear".
The basic one-dimensional pad model comes from 1/2 inch to 1-1/4 inch
squares and circles. They also make a 4" linear model with separate
connectors for force and position on the line, and a 4" square model
with three sets of connectors for X, Y, and force.
FSR's are peculiarly suited to computer-human
interaction. They look like band-aids with electrodes sticking out.
You could build a guitar fretboard with them and use finger pressure
to bend the pitch. I found out about them from a friend who is using
them in a sports simulation. I'm using them for something that may
be patentable, so I'm keeping quiet. Keytronics is using them in
a laptop computer they're building: you grab a stick on the keyboard
and push it. FSR's on all four sides measure the force and the system
software turns it into mouse-style input.
The company literature includes a big book of basic circuits for
translating current, voltage, resistance, et al, so for the
electronics-impaired (I flunked electronics) it should be pretty
simple to build something fun. Four FSR's can be hooked directly to the
IBM PC joystick port, since it's designed to handle a variable resistance
from 0 to 100kohm. If you want to sample more, you could multiplex them
with analog multiplexor chips controlled from a parallel port. But, the
joystick card is a very cheesy design and you should get a lab I/O
card instead. You also want a lab I/O card if you use the 4" square
pad for a (very small) force-sensitive tablet, because you need a
high input sample rate that you won't get off a joystick card.
I agree with their claim of subjective linear force response. Others
may not. I'm a classical pianist.
The address is:
Interlink Electronics
P.O. Box 40760
Santa Barbara, CA 93103
vox: 805-684-2100
fax: 805-684-8282
When you ask for a literature kit, they throw in a small FSR free.
The little pads range from $3-$5, and the linear and square pads
are $45.
Specs:
Size range: Max 20" x 30"
Min .2" x .2"
thickness: .008" to .050"
force: 30g to 10kg
pressure: .45 psi to 150 psi w 1cm2 actuator
part variance: +- 15%
resolution: 0.5% of force
switch size: zero travel
rise time: 1-2 milliseconds
lifetime: 10 million actuations
temp range: -30 C to 170 C
max current: 1 ma/cm2 of force
vibration: not sensitive
EMI: not sensitive
In short, they're very slick.
Lance Norskog