Help with CIM motor locking system?

[CryptoStorm]

If you look at the video its clearly a cim connected to a standard gear box. a chain and sprockets deliver torque to a shaft going through the arm..

Its too hard to follow, however, if it were me and i was using this setup:

I'd attach a single turn pot to the end of the arm shaft and create a PID program to control the CIM output to match the analog value of the pot to a user defined value. pressing up increases the user defined value, pressing down decreases it.