Specifying and achieving passive compliance based on manipulator structure

We explore the possibility of achieving passive compliance through the structure of the manipulator itself. The emphasis is on passive compliance because a minimum of passive compliance to prevent jamming will always be required even when active stiffness control is employed. Particular attention is given to the large class of robots with nonbackdrivable actuators, where the actuator must be commanded to move, and in which actuator forces or torques are not easily interpreted as end-effector forces and torques. We present a novel framework for specifying the desired end-effector compliance for several tasks in terms of stiffness matrices. We explore whether the desired stiffness matrix of a manipulator can be achieved by using the natural or designed stiffness of the manipulator limbs themselves. Several techniques for adjusting the manipulator stiffness matrix are proposed. Achieving this variable passive compliance allows the attainment of high stiffnesses for fast and accurate movements and low stiffness values for force control. Furthermore, achieving nondiagonal stiffness properties wherein there are force and motion coupling in different directions is shown to be useful to prevent jamming and contact induced vibrations