On the design of gravity-compensated six-degree-of-freedom parallel mechanisms

The design of gravity-compensated six-degree-of-freedom parallel mechanisms - or manipulators with revolute actuators is studied in this paper. Two methods are studied for the static balancing of these mechanisms, namely, using counterweights and using springs. The first method leads to mechanisms with a stationary global center of mass while the second approach leads to mechanisms whose total potential energy (including the elastic potential energy stored in the springs as well as the gravitational potential energy) is constant. In both cases, the resulting mechanisms are fully compensated for gravity, i.e., the actuators do not contribute to supporting the weight of the moving links in any of the configurations of the mechanisms. The position vector of the global center of mass and the total potential energy of the manipulator are first expressed as functions of the position and orientation of the platform. Then, conditions for static balancing are derived from the resulting expressions. Finally, examples are given in order to illustrate the design methodologies.