[Jon Stratis]

I would also look for a mechanical solution. Holding an arm in place with a motor means stalling the motor, and a lot of motors don't like that - they could burn out on you. That problem increases as the weight the motor has to hold (arm + game piece typically) increases. Decreasing that weight makes it much easier for the motor to hold- if the weight was low enough, I would be comfortable just stalling the motor. For example, in 2013 we had an arm that would move a set of hooks from one level of the pyramid to the next. The arm weighed very little, it wasn't lifting game pieces, and only had to hold its position briefly before the winch would take over and pull on the hooks. So, we let the motors handle it that year.

I would start by looking for a way to coubterbalance the arm - weights or springs can work wonders, and give you almost a neutral weight for the arm! After that, mechanical brakes are certainly a good option. I've seen teams use disc brakes from bikes (light weight, able to hold a large force, and easy to work with a pneumatic piston). Using something similar to the ratchet-modified gearbox a lot of teams used in 2015 is also an option, but is a little less flexible and needs a little more thought to make work properly.

Maybe one of my favorite arms from a few years ago (I can't remember the team it is as on) used a 500-series motor driving a long worm drive that turned the gear on the arm pivot. Simple, worked well with their design, got the weight of the motor and gearbox in the bottom of the robot, and prevented the arm from back driving.