[philso]

Micheal

The part that Joe linked to is a good quality part and should work just fine for you. It would normally be found in high quality industrial or military grade equipment unlike the consumer grade parts found at Radio Shack.

You want to use something in the range of 2kOhm to 20kOhm. The industrial equipment I design (similar to the CRio) is frequently connected to 10kOhm potentiometers. In most robotics applications, you will want to use a "linear" potentiometer with a 1/4 diameter shaft.

The shafts are typically made from aluminum and are easy to cut if too long. Just clamp the end of the shaft in a bench vice and cut it with a hacksaw. Mount it by drilling a hole that fits the threaded part of the body. Mark the location of the anti-rotation device that sticks out from the body. Drill a second hole that is large just enough to accomodate the anti-rotation device. Make sure you use the washers and lock washers under the nut.

The coupling to your arm does not need to be very substantial since it should take very little force to turn the potentiometer shaft. We used a piece of surgical tubing forced over the shaft then pushed into the shaft (1/2 inch OD tube, 3/8 inch ID) of the dumper my son built.

Make sure that the potentiometer still has travel beyond the two ends of the 90 travel that you want. Lastly, do not use the potentiometer as the "hard stop" that prevents the arm from travelling beyond a certain point or you will break the potentiometer.