A Backdrivable Axisymmetric Kinematically Redundant (6+3)-Degree-of-Freedom Hybrid Parallel Manipulator

A kinematically redundant (6+3)-degree-of-freedom (DOF) hybrid parallel robot with an axisymmetric workspace is proposed. By arranging the first revolute joint of each leg such that they have the same rotation axis, this robot can achieve an axisymmetric workspace, resulting in a large reachable workspace. In addition, type II singularities, which critically limit the orientational workspace, can be fully avoided by utilizing kinematic redundancy. A gripper mechanism is developed to increase the orientational workspace by exploiting the redundant DOFs. Moreover, the orientational workspace can be further increased by controlling one of the redundant DOFs to keep a certain constant angle. As a result, the proposed hybrid parallel robot achieves a high workspace-to-footprint ratio comparable to that of serial robots. A CAD model of the robot and computer animations are provided to demonstrate the large workspaces and the gripper mechanism. A significant advantage of the proposed robot over serial architectures is that the robot is backdrivable since it uses direct-drive or quasi-direct-drive actuators.