The Joint Torque Feedback Control of a Direct-drive Arm

Abstract

The direct-drive arm is a high performance manipulator arm having high-torque, low-speed motors directly coupled to the arm links. The elimination of reducers provides important features including small friction, no backlash and high mechanical stiffness, all of which are favorable for high precision force/torque control as well as for position control. However, the motors used for the direct-drive arm still have significant torque ripples and other nonlinearities. To improve the torque control accuracy, the feedback compensation of joint torques is discussed in this paper. First, the design of strain gage torque sensors built in a direct-drive motor is presented. The feedback control system of the joint torque is then designed. Through experiments, it is shown that the torque control accuracy is an order-of-magnitude improved. This allows us to vary the compliance of arm motion from an extremely low value to an extremely high range without causing any stability problems