Design and control of a prototype platform manipulator for workholding and workhandling applications

This paper presents the strategy of combined workholding and workhandling to reduce machine setup time using a three degrees-of-freedom (DOF) in-parallel actuated manipulator. The manipulator is characterized by its short-arm rigidity, high force-to-weight ratio and relatively simple inverse kinematics. It has two orientation freedoms to adapt to a complex surface and a translation freedom for clamping actuation. Unlike workhandling which requires the robotic arm to have the ability to follow a path in three dimensions at specified accuracy and speed, workholding requires the part and the manipulator to be over-constrained. The reactions result in time-varying, path-dependent and load-dependent Coulomb friction and stiction, which have significant influences in joint motion. To investigate the effects of the nonlinearities on the joint motion control, a prototype manipulator has been built. A digital tracking control algorithm under the influences of the nonlinearities was experimentally evaluated.