LOW-TORQUE-RIPPLE SWITCHED RELUCTANCE MOTORS FOR DIRECT-DRIVE ROBOTICS

A direct-drive robot consists of a mechanical arm with electrical motors directly coupled to the joints. The benefit of improved mechanical design has already been conclusively demonstrated by the introduction of the parallel drive mechanisms. The need for improved electrical design is just as great since these robots require new types of motors that operate at low speed with high torque and that perform commutation with satisfactory smoothness. In this paper, we examine one contender for the direct-drive application, the switched reluctance motor. We first describe how the number of teeth on the rotor and stator can be selected to achieve an electromagnetic gearing, with the goal of eliminating the need for a mechanical gearbox. We also discuss how the tooth arcs of the rotor and stator can be adjusted so as to reduce the torque ripple of the motor during commutation. Our treatment of these subjects includes a detailed case study in which the performance of four related motors is compared using finite-element analysis with a commutation algorithm for torque control. A contribution of this paper is the discovery that, although a tradeoff exists between peak torque and torque ripple, the maximum smooth torque and minimum torque ripple can be achieved by the same direct-drive motor design.