If you're interested in counterbalancing your arm, this year we figured out a way of using surgical tubing to perfectly balance our collector arms throughout their range of motion, which could possibly be upscaled to deal with heavier arms.
I'm hoping that the student who worked on that system will chime in here (I can only tell you that it worked like a charm and integrated into our design quietly, inside a frame tube where nobody ever noticed it without us pointing it out). The value of counterbalancing the arm was that it allowed us to use a much smaller pneumatic cylinder than we otherwise would have, reducing air usage while giving us very quick actuation in both directions. Similar benefits would have been in store if we'd used a motor to actuate the arms.
The links show a mechanism (and components) for counterbalancing a lift gate that works on the same principles. The first link also has an entertaining video that shows it in action:
http://www.northerntool.com/shop/too...BTOWbnB85IBZSh
http://www.etrailer.com/Accessories-...Fc1afgod9HgA3A