I mentioned below that I made a giant high torque servo (160 in-lb) but I didn’t explain the feedback loop at all. You need to know your position, so I hose-clamped a bracket onto my motor and drilled a hole for a standard 5k linear pot. put + and – on the ends, and the wiper gives you a voltage that corresponds to position. When I finish tuning the pid, I’ll post the arduino (bullduino) code to make this work. Note there are 2 servos present on the device, which makes it a little tricky, as you’ll have to switch A/D converters and give it a little dwell time between reads. Here’s a picture of my linkage between the motor and the pot. Note the masking tape component. If this thing flails out of control during development, I don’t want anything damaged besides the sacrificial tape.