thinman@netcom.COM (Lance Norskog) (04/20/91)
Index Number: 15044 If you're building computer 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