[JCharlton]

We haven't tried it yet, but our solution to the same issue is to use an accelerometer. Should read the absolute angle of the arm with respect to the floor. Some filtering for hits may be required.

Check your torques and all, my suspicion (as a mechanical guy) is that you'll need a larger gear reduction to move the arm with enough torque, control and to avoid stalling your motors at high amperage for too long. A very good PID might alleviate it, but lots of reduction will help.