Development, Analysis, and Experimental Realization of a Direct-Drive Helical Motor

Safety issue is a great concern for rehabilitation robots that are expected to contribute to future aging society. Appropriate compliance is required for their joints. However, combination of servomotors and high-ratio gears, such as harmonic gears, makes the joint of robots nonbackdrivable. The nonbackdrivability causes lack of adaptability and safety. On the other hand, conventional direct-drive systems, including linear motors, are relatively big for such application. This paper presents the development and analysis of compact high-backdrivable direct-drive linear actuator. The motor consists of a helical structure stator and mover. The mover does not contact the stator and moves helically in the stator under a proper magnetic levitation control. Thus, the motor realizes direct-drive motion without mechanical gears. Decoupling control is proposed and integrated with disturbance observer to achieve robustness against model uncertainties and input disturbance. The main contribution of this paper is to experimentally realize the direct-drive feature of the helical motor.