A Novel Series Elastic Actuator with Variable Transmission Ratio Using Reconfigurable Epicyclic Gearing for Hip Exoskeletons

Abstract

Series elastic actuators (SEAs) are extensively employed in exoskeletons due to high actuation transparency, reliable back-drivability, and accurate force control. However, existing SEAs designed for exoskeletons fail to deliver a wide range of torque-speed characteristics for activities including walking, running, ramp ascent, and ramp descent. In this study, a novel variable transmission SEA that incorporates reconfigurable epicyclic gearing and two clutches is presented to achieve three distinct speed-torque characteristics and enhance transparency. To ensure robust position tracking for the actuator in torque-speed characteristics switching, a disturbance observer is developed to estimate the lumped disturbance induced by external load and internal modeling uncertainties. Comparative experimental results demonstrate that, under unload and load conditions, the sinusoidal tracking error of the proposed controller is reduced by more than 30% compared to the controller without a disturbance observer, and the energy consumption of the actuator system is reduced by more than 40% compared to traditional SEAs.