Type synthesis of kinematically redundant 3T1R parallel manipulators

A kinematically redundant parallel manipulator (PM) is a PM whose degrees-of-freedom (DOF) are greater than the DOF of the moving platform. It has been revealed in the literature that a kinematically redundant PM has fewer Type II kinematic singular configurations (also called forward kinematic singular configurations, static singular configurations or parallel singular configurations) and/or constraint singular configurations than its non-redundant counterparts. However, kinematically redundant PMs have not been fully explored, and the type synthesis of kinematically redundant PMs is one of the open issues. This paper deals with the type synthesis of kinematically redundant 3T1R PMs (also called SCARA PMs or Schoenflies motion generators), in which the moving platform has four DOF with respect to the base. At first, the virtual-chain approach to the type synthesis of kinematically redundant parallel mechanisms is recalled. Using this approach, kinematically redundant 3T1R PMs are constructed using several compositional units with very few mathematical derivations. The type synthesis of 5-DOF 3T1R PMs composed of only revolute joints is then dealt with systematically. This work provides a solid foundation for further research on kinematically redundant 3T1R PMs.