Structure Synthesis of 6-DOF 3-3 Decoupled Parallel Manipulators

In parallel manipulator design, usual 6-DOF PMs, like the Stewart platform, suffer from the disadvantages of difficult forward kinematics, coupled motion, and small workspace, so that the motion planing and control become very complicated and difficult to be implemented in real application. Most of the PMs with lower mobility are over-constrained when assembly errors are considered [1]. Therefore, these PMs cannot be assembled without joint clearence which imposes negative impacts on the accuracy of the end-effector. Kinematically 6-DOF decoupled PMs are the best candidates to cope with these drawbacks while keeping the advantages of general PMs. To reduce the complexity in the generic problem of type synthesis of 6-DOF decoupled PMs, the decoupled PMs are classified, and the concept of Group Decoupling (GD) is introduced. An idea of sharing sub-chain composed by only passive joints is disclosed. A systematic approach based on this idea is proposed for structure synthesis of 3-limb symmetrical 6-DOF PMs with 3-3 decoupled motion, i.e., 3-DOF translation and 3-DOF rotation are independently controlled, or 3-DOF planner motion and the other 3-DOF spatial motion are independently controlled. Two classes of 3-3 decoupled parallel manipulators are obtained, in which seven structures are novel.