Many applications of manipulators to date have been based on position control, but when a robot manipulator makes contact with the environment, the control of force and position is required. One approach, impedance control, is to control the manipulator such that the relationship between position and force is well defined. For example, such that the robot behaves like a mass-spring-damper system whose parameters can be specified arbitrarily. In the recent years, many of the impedance control implementations were using force feedback from an end-effector force sensor and the impedance control was performed by forming the error in Cartesian coordinates. The implementation proposed here requires instead co-located torque sensors and the feedback control signals are based solely on joint variables, torque and displacement. Co-located torque sensors lead to a larger control bandwidth since the structural dynamics of the arm is seen as a perturbation, but is not part of the plant to be controlled. Simple SISO controllers, designed to modulate individual joint impedances, can achieve diagonal (decoupled) impedance matrices in Cartesian coordinates, provided that a set of SISO compensators, called her "decouplers", are set up to cancel the non-linear coupling among the joints. This strategy results in a simple computational architecture which does not require complex coordinates transformations to be performed at servo rate. The method can be used with non-redundant and redundant manipulators and experimental results are discussed using a seven DOF manipulator available at the Institut de recherche d'Hydro-Quebec.

Hayward, Vincent (Supervisor)