Design and development of high-performance torque-controlled joints

Dynamic decoupling, motion and force control of manipulators rely on the ability of the actuation system to provide accurate joint torques. However, this ability is considerably restricted by the nonlinearities and friction inherent in the actuator-transmission systems of most industrial robots. This paper is concerned with the development of high-performance torque controlled joints and focuses on two basic issues: sensor design and torque control. The first part of the paper describes a conceptually new type of torque sensor that uses contactless inductive transducers. The new sensor provides a substantial increase in accuracy over conventional strain gauge sensors, achieves higher mechanical robustness, and presents lower sensitivity to electrical noise. The second part of the paper presents an analysis of the effect that the manipulator´s transmission and structural properties have on the joint torque controller design. Two manipulators with very different mechanical characteristics are used in this analysis: the PUMA 560 manipulator and Artisan, an eleven-degree-of-freedom manipulator currently under development at Stanford. The experimental results obtained with a prototype link of Artisan are presented and compared to those previously obtained with the PUMA