Abstract: The accuracy and rigidity of a parallel manipulator decrease much because of its joint clearance, so the redundant control was presented to improve the accuracy. Firstly, the reason that the effect of the joint clearance on the accuracy of the parallel manipulator is the same as the length error of the leg was given, and then it can be derived that the error workspace is the intersection of several parts of the sphere. The errors between two mechanisms in the whole workspace were compared after the solution to get the error workspace volume is given. The conclusion that redundant control can improve accuracy was drawn at last.

Key words: parallel manipulator, redundant control, joint clearance, error, workspace