Comparison of 3-DOF asymmetrical spherical parallel manipulators with respect to motion/force transmission and stiffness

This paper presents a comparative study of asymmetrical spherical parallel manipulators with respect to motion/force transmission and stiffness. The manipulators adopt a center shaft to both generate a decoupled unlimited-torsion motion and support the mobile platform for high positioning accuracy, where the difference lies in the rotationally/linearly actuated limbs in-parallel. The transmission indices are defined based on the virtual coefficient between the transmission wrench and twist screws for design analysis. Comparison of the manipulators is carried out based on a multi-objective optimization approach, which aims to maximize the workspace of high transmissibility and stiffness over a regular workspace. Performance isocontours are visualized to highlight the advantages/drawbacks among the manipulator counterparts.