Kinematic Optimization of a Five Degrees-of-Freedom Spatial Parallel Mechanism With Large Orientational Workspace

This paper deals with the kinematic optimization of a five degrees-of-freedom (DoFs) spatial parallel mechanism with three kinematic chains. Inspired by the structure of the icosahedron, the base of the discussed mechanism has been designed into a compact and light-weight frame. Due to the potential advantages, this mechanism is used as a movable plug-in module in a multi-axis machine center to process large-scale parts with rotary contour surfaces. To derive its optimal parameters, kinematic optimization based on the motion/force transmissibility is carried out. The parameter design space (PDS) is generated first. Then, the performance evaluation index (i.e., local transmission index (LTI)) is derived sequentially. On this basis, the good transmission positioning workspace (GTPW) for a given orientation is defined by constraining the value of LTI with a certain metric. Thereafter, the atlases of the GTPW and the optimal region satisfying the workspace constraint are derived in the PDS. Within this region, a set of optimal parameters without dimension are selected. Consequently, the cuboid workspaces within GTPWs are identified in detail. By using the ratio between required workspace in application and the derived cuboid workspaces, optimal geometric parameters with dimension are derived. Workspace analysis results show that, for an arbitrary orientation between the vertical and horizontal directions, there is always a cuboid workspace within GTPW larger than required workspace. In addition, the orientational capability of the mechanism can reach more than 90 deg, and the flexible 2DoFs rotations can also be realized. The work in this paper is very helpful to the development of a mobile machining module.