High-performance series elastic stepper motors for interaction force control

A series elastic actuator (SEA) consists of an actuator in series with a compliant element. By measuring and controlling the displacement of the compliant element, an SEA provides better forward and inverse force control accuracy than traditional stiff actuators. SEAs are suitable for robots that need interact properly with human or environment. Most SEAs use brushless DC motors as the actuators. The merit of using stepper motors as the actuators has not been explored. Stepper motors have a higher torque-to-weight ratio than other DC motors. With proper rotor position feedback, stepper motors can achieve smooth and accurate dynamic response similar to brushed or brushless DC motors. This paper develops a dynamic model of a series elastic stepper motor and presents its prototype. Forward and inverse force tracking responses of the series elastic stepper motor will be demonstrated to show its force control performance. We expect that the results presented here can provide an alternative for the actuator selection of series elastic actuators when higher torque-to-weight ratio is required.