Control of a Direct-Drive Arm

Abstract

12 direct-drive arm is a new mechanical arm in which the shafts of articulated joints are directly coupled to the rotors of high performance torque motors. Since the arm does not contain any gears or transmission mechanisms between the motors and their loads. the drive systems have no backlash, small friction and high mechanical stiffness, all of which are desirable for fast, accurate and versatile robots. At the Robotics Institute of Carnegie-Mellon University, we have built a first prototype direct-drive arm (referred to as CMU DDArm hereafter). This paper presents the characteristic analysis and the design of the control system. First, we describe an outline of the developed CMU DDArm and compare its characteristics with conventional indirect-drive arms. Sccond, we discuss basic feedback control for single-link drive systems in the frequency domain. Third, we apply a feedforward compensation to the control of multi-degree-of-freedom motion in order to compensate for interactions among multiple links, and Coriolis, centrifugal and gravitational forces. Finally, the steady-state characteristics are discussed with respect to servo stiffness and positioning accuracy. The experiments show the excellent performance of the direct-drive arm in terms of speed and accuracy. Throughout the paper comparison with indirect-drive methods is made to contrast the advantage of the direct-drive method.