Improving the Dynamic Force Control of Series Elastic Actuation Using Motors of High Torque-to-Inertia Ratios

A series elastic actuator (SEA) includes an elastic spring in series with an actuator. SEAs provide more accurate force and impedance control than conventional rigid actuators. They are ideal for robots and machines that need to interact safely with the environment. The majority of existing SEAs uses brushless or brushed DC motors as the actuators. The advantages of using step motors as the actuators of SEAs have not received enough attention. Step motors have much higher torque-to-weight ratio and torque-to-inertia ratio than other DC motors. Hence they can provide better stability and high-speed accuracy of force control while maintaining lightweight. When the rotor position feedback is used, step motors can achieve accurate dynamic position response smoothly. This paper develops the dynamic model of a linear series elastic step motor and presents its prototype. Force and impedance control responses will be provided to show the advantages due to the high torque-to-inertia ratio of step motors. It is expected that the results presented here can offer a better actuator selection of SEAs when high-performance dynamic force control is required.