POSITION/TORQUE CONTROL OF A SPACE ROBOTICS ARM

Torque sensors are used in space robot joints to increase robot capabilities and operational flexibility. Cascaded to a load position loop, a torque loop increases the performance of a position servo facing large variations of load inertia, typical of space robot arms. The issue specifically addressed in this paper is to what extent non linear elements, like sensor resolutions and friction, affect performance of the cascaded position/torque control strategy for a joint of a space arm. The detrimental effects of the comparatively low resolution of the load position sensor are discussed. A solution to this problem is proposed based on the adaptation of a Kinematic Kalman Filter. Finally it is pointed out that the adoption of a PI torque controller, in addition to a PID position controller, easily induces limit cycles of the hunting type, thus supporting the adoption of a proportional torque controller.