Let's see if you can help me... (Motors)

[Alan Anderson]

Given the motors you've chosen, I think the most appropriate fix would be a mechanical one. Provide a counterweight or spring to balance the weight of the arm, so you don't have to worry about gravity when moving it.

[Dave K.]

A counter balance would be best, and you still might want to couple that with a position sensor, such as a potentiometer, and a closed loop position control system. Given that your arm wants to sink to the ground, the control loop would cause apply enough energy to counteract that force and essentially keep the motor stalled to hold the arm in place. This may result in excessive heating of the motor, and subsequent failure.

If you could balance the arm AND provide a closed loop control of the arms position, then it would easily maintain position.