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Unread 03-12-2012, 21:55
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Alan Anderson Alan Anderson is offline
Software Architect
FRC #0045 (TechnoKats)
Team Role: Mentor
 
Join Date: Feb 2004
Rookie Year: 2004
Location: Kokomo, Indiana
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Re: Balancing an Arm

The TechnoKats "Overdrive" robot had a perfectly counterbalanced windmilling trackball manipulator mechanism. It used a pneumatic cylinder which could be selected to two different pressures in order to balance it both empty and when holding a trackball. The design feature that made it perfectly balanced in any orientation was that the cylinder was connected to a sprocket that turned with the "arm", so that it applied maximum upward force when the arm was horizontal and zero force when it was vertical. The arm would stay exactly where it was placed with no motor power applied at all.

The main benefit I saw of the perfect counterbalance was in the control software for setting the arm position. Gravity simply was not an issue, and a single set of PID constants worked for the entire circle of arm travel. As a programmer, I consider the fact that we didn't burn out Fisher-Price motors by stalling them against the weight of the arm to be of secondary importance.