Design and transmission analysis of an asymmetrical spherical parallel manipulator

•An asymmetrical wrist with unlimited-torsion motion and high positioning accuracy is developed.•The transmission quality of the manipulator is evaluated by means of some virtual coefficient transmission indices.•Two optimal sets of design parameters for the asymmetrical wrist.•A comparative study with two symmetrical counterparts shows the advantages of the proposed asymmetrical wrist.

This paper presents an asymmetrical spherical parallel manipulator and its transmissibility analysis. This manipulator contains a center shaft to both generate a decoupled unlimited-torsion motion and support the mobile platform for high positioning accuracy. This work addresses the transmission analysis and optimal design of the proposed manipulator based on its kinematic analysis. The input and output transmission indices of the manipulator are defined for its optimum design based on the virtual coefficient between the transmission wrenches and twist screws. The sets of optimal parameters are identified and the distribution of the transmission index is visualized. Moreover, a comparative study regarding to the performances with the symmetrical spherical parallel manipulators is conducted and the comparison shows the advantages of the proposed manipulator with respect to its spherical parallel manipulator counterparts.