Kinematic control of parallel robots in the presence of unstable singularities

Parallel mechanisms frequently contain an unstable type of singularity that has no counterpart in serial mechanisms. When the mechanism is at or near this type of singularity, it loses the ability to counteract external forces in certain directions. The paper considers an interesting alternative to redundant actuation through the application of additional kinematic constraint via passive joint braking in the close neighborhood of unstable poses. The brake has the advantage of not requiring additional actuator/sensor pairs or mechanism architecture redesign, and of being implemented only when necessary to stabilize the system. The kinematic analysis of braked mechanisms is supported with results using a six degree-of-freedom parallel machining mechanism.