Force passivity in fixturing and grasping

While the classical notion of force closure is defined for actively controlled and coordinated robotic fingers, passive contacts play an equally important role in workpiece fixturing and often in robotic manipulation. This paper presents a description of passive forces arising at the normal and frictional contacts by passive physical means. The passive contacts generate reactive forces only as a response to an external force and/or any active force. Within the framework of rigid body contact, a contact system with passive forces is generally undeterminate. We present a general approach based on an application of the minimum norm principle. The model reveals some intricate properties of the passive contact forces, including internal forces at the passive and/or active contacts. Some practical implications of the passive nature in fixture design are discussed.