The precise control of manipulators with high joint-friction using base force/torque sensing

Joint friction is a major problem in accurate robot position control, particularly during low-speed, small-amplitude tasks. This paper proposes a simple, practical, and effective method to compensate for joint friction, using a six-axis force/torque sensor mounted under the manipulator. From these measurements, joint torques are estimated and used in a torque controller, which virtually eliminates friction and gravity effects, providing high-precision motion control even for small motions at low speed. The method does nor require complex analytical friction models. The method also does not require expensive and unreliable internal joint-torque sensors. Experimental results demonstrate the effectiveness and practicality of the method for an electrical PUMA and hydraulic Schilling Titan II manipulator. (C) 2000 Elsevier Science Ltd. All rights reserved.