Development of a Direct-Drive Robot and Evaluation of Its Control Performance

Abstract

A direct-drive arm is a new mechanical arm in which the shafts of articulated joints are directly coupled to the rotors of motors with high torque. Since the arm does not contain any gears and transmission mechanisms between the motors and their loads, the drive systems have no backlash, small friction and high mechanical stiffness, all of them are desirable for fast, accurate and versatile robots. First, we describe the prototype robot we have developed, and the characteristics of the arm is compared with conventional indirect-drive arms. Second, we discuss a basic feedback control for single-link drive systems in frequency domain. Third, we apply a feedforword compensation to the control of m.d.o.f. motion in order to compensate interactions among multiple links, and Coriolis, centrifugal and gravity forces. Finally, the steady-state characteristics are discussed with respect to servo stiffness and positioning accuracy. The experiments show excellent performances of the direct-dirve arm in terms of speed and accuracy. Throughout the paper comparison with indirect-dirve methods is made to contrast the advantage of the direct-drive method.