[gpetilli]

To control the location of an arm, a potentiometer (aka pot) works better than an encoder since it does not lose "home" when powered down. To find home at power-up, you would need to add a limit switch (or use a magnetic encoder with an index). Guessing at what your system looks like, I recommend a single turn pot connected to an analog input of either the roboRio or a Talon SRX. The voltage from the pot will tell you the angle of the arm. Mechanically, it wants to be mounted directly at the pivot of the arm such that the arm turns the shaft and the body is mounted to the frame (doesn't turn). Many shafts have a 1/4" center hole, which you can press a 1/4 dowel into and then make a coupler from surgical tubing and zip-ties.

The string pot will have a multi-turn (10 turn ?) potentiometer inside; since an arm normally does rotate more than 360deg, most of the string pot will go unused, reducing accuracy - it can work if you have nothing else but just use the pot and lose the string and spool parts. Potentiometers are available from Digikey and Mouser.