Safety issue is 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 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.

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In this paper, modeling and control of spiral motor is proposed. The voltage equation and motion equation of the spiral motor is proposed. Based on this modeling, control system for the spiral motor is proposed. The proposed controller consists of three parts; the first part is PI current controller with back EMF compensation specialized for spiral motor, the second part is disturbance observer based PD controller for linear and gap motion interacting each other, and the third part is zero-power controller for equilibrium fluctuation of gap displacement. It is confirmed that the proposed controller achieves independent linear position and gap control simultaneously.

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