Manipulating rigid payloads with multiple robots using compliant grippers

This paper describes an approach to nonmodel-based decentralized controls of multirobot systems utilizing structural flexibility in gripper design to avoid large unwanted internal forces acting on multirobot systems. It is proven in theory that a simple proportional and derivative (PD) position feedback plus gravity compensation controller can regulate the desired position/orientation of a payload manipulated by multiple robots with compliant grippers and simultaneously damp vibrations of compliant grippers. By adding a force feedforward control to the PD scheme, a hybrid position/force control scheme is further developed to control internal forces between robots and the payload in the particular directions, in the event that the compliance of grippers is low or negligible in these directions. Experiments conducted with two CRS A460 industrial robots manipulating a beam, using a rigid and a compliant gripper, confirm these theoretical predictions.