Development of a new suction gripper for gripping under-constrained workpiece with minimized contact

When gripping delicate workpieces such as a silicon wafer, contact should be minimized to protect the workpiece. Some existing suction grippers can grip a workpiece with only three contact points on its upper surface, which is minimal to fully constrain the workpiece. Further reducing the contact points will make the workpiece under-constrained and thus difficult to grip. This paper develops a new suction gripper that can grip an under-constrained workpiece with only two contact points at the edge of its upper surface. The uniqueness of the new gripper lies in that it uses feedback control to stabilize the unstable motion of the under-constrained workpiece. First, to overcome the negative-stiffness effect that makes the under-constrained gripping unstable, a zero-stiffness suction unit based on closed-loop pressure feedback is developed via optimal design. Next, a cooperative actuating mechanism based on four suction units is designed to actuate the workpiece in four different DOFs individually, so that the workpiece can be levitated stably with the contact forces being controlled. Finally, the dynamics of the gripping system is modeled, and an adaptive robust controller is designed based on the dynamics model. With the proposed controller, the gripper can handle workpieces with unknown inertial parameters and irregular upper surfaces. Experiments were conducted to verify the new suction gripper with the proposed controller.