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Robotics_Rookie
02-19-2009, 08:56 PM
I bought the flexiforce sensor kit and I am just trying to figure out how I am going to use this thing. Eventually I want to be able to use it in some sort of controller where I would make changes to an output based on the Vout from the sensor system (simple 2-D lookup table) but at this point I'd even accept just seeing the voltage change show up on a voltmeter while I push on the sensor. Any help would be greatly appreciated.

Robotics_Rookie
02-19-2009, 09:23 PM
Well I seriously "rigged" this thing and I think I got it to at least pretend to work. I put a 9v battery to the edges of the op amp and then 3 AAA batteries as the 5v supply. Then hooked up a volt meter to the variable signal and it goes from about 4.71 volts down to about 3 volts (when my finger starts to hurt) when I push on the sensor.

Robotics_Rookie
02-19-2009, 09:40 PM
Guess it's supposed to start at zero. Still can't quite figure this thing out.

robologist
02-19-2009, 10:20 PM
The Flexiforce sensor is a variable resistor that changes charecteristics based on force. If you are wanting a voltage to measure from it, it should be in a resistive divider circuit. From their data sheet, it appears if using a 100 lb sensor, ranges from 100K to maybe 8k in having 0 to 100 lb applied (Which sensor rating is being used?). So take a 20k resistor and tie this to +5 volts, connecting the other end of the resistor to an outside line of the sensor, then the other outside line to ground. Measure the voltage at the connection between the resistor and the sensor. WIth no force applied, the reading should be near 4.2 volts. If you are able to apply 100 pounds to the sensor, the reading shoud be around 1.5 volts.

Using their example circuit (http://www.tekscan.com/flexiforce/samplecircuit.html), a much greater range that is also linearized can probably be achieved. The Flexiforce manual (http://www.tekscan.com/pdfs/FlexiforceUserManual.pdf) can help a lot.

Robotics_Rookie
02-20-2009, 09:08 AM
The Flexiforce sensor is a variable resistor that changes charecteristics based on force. If you are wanting a voltage to measure from it, it should be in a resistive divider circuit. From their data sheet, it appears if using a 100 lb sensor, ranges from 100K to maybe 8k in having 0 to 100 lb applied (Which sensor rating is being used?). So take a 20k resistor and tie this to +5 volts, connecting the other end of the resistor to an outside line of the sensor, then the other outside line to ground. Measure the voltage at the connection between the resistor and the sensor. WIth no force applied, the reading should be near 4.2 volts. If you are able to apply 100 pounds to the sensor, the reading shoud be around 1.5 volts.

Using their example circuit (http://www.tekscan.com/flexiforce/samplecircuit.html), a much greater range that is also linearized can probably be achieved. The Flexiforce manual (http://www.tekscan.com/pdfs/FlexiforceUserManual.pdf) can help a lot.
I watched the video and decided to just hook a 9v battery to the power/ground and then read the voltage between the signal and ground and it worked. I played with the potentiometer until my full force pushing down on the sensor was over 4v. The first time I pushed on it my full force was only in the 2v range so I think the adjustment worked. Eventually this will be part of a bigger system but I just wanted to play with it a little bit so I didn't have to troubleshoot all 20 sensors at once.

Thanks for the help. I'll probably have many more dumb questions before long!