Singularity analysis of a kinematically redundant (6+2)-DOF parallel mechanism for general configurations

Abstract

Parallel mechanisms suffer from type II singularities which reduce their useful orientational workspace. Adding kinematic redundancy in parallel mechanisms enhances their orientational workspace by providing singularity avoidance capabilities. However, an increasing number of kinematically redundant degrees of freedom (DOFs) requires additional actuators and makes the redundancy resolution more complex. Moreover, examples in the literature exist where a minimal number of kinematically redundant DOFs was used to produce a singularity-free orientational workspace for planar mechanisms. In this work, the architecture of a kinematically redundant (6+2)-DOF parallel mechanism akin to the well-known Gough–Stewart platform is studied, and its singularity locus is derived. The results show that, while some singularities still remain in the useful workspace of the mechanism, they can be accurately localized with simple closed-form analytical expressions for trajectory planning purposes. Furthermore, the redundancy resolution may find itself easy to handle, since the avoidable singularities and mechanical interference can be mapped into the 2-D space of the redundant parameters. Finally, the proposed architecture is considered as a compromise between obtaining a singularity-free workspace and handling easily the redundancy resolution for the trajectory planning.