Improving the kinematic performance of a planar 3-RRR parallel manipulator through actuation mode conversion

The small workspace and complicated singularities severely limit the application of parallel manipulators. The methodology of redundancy is commonly used to eliminate singularity and enhance the kinematic performance of parallel manipulators. However, redundancy results in apparent drawbacks including intensified internal force coupling and complicated dynamic control. A new potential method to improve the kinematics and eliminate singularities, named actuation mode conversion, is investigated in this study. First, a planar 3-RRR parallel manipulator that can realize 27 actuation modes is designed. Based on the matrix orthogonal degree, indices for evaluating the transmission performance are defined, and kinematics of the 3-RRR manipulator under different actuation modes is analyzed. Results indicate that the actuation modes significantly affect the kinematic performance of the parallel manipulator. A reasonable selection of the actuation modes can effectively improve the kinematic performance of the planar 3-RRR parallel manipulator and eliminate type II singularities. Finally, dimensional parameters of the variable-actuated 3-RRR manipulator are optimized via the atlas-based method considering actuation mode conversion, and the optimal conversion map of the actuation modes is obtained.