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1992-01-25
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91-04/Force.Sensing.Resistors
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.
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 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.
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