Nonlinear disturbance observer based torque control for series elastic actuator

This paper presents a practical control approach for series elastic actuators(SEAs) to generate the desired torque. Specifically, the controller is applicable to both linear and nonlinear SEAs and it works well even in the presence of unknown payload parameters and external disturbances. Via the analysis and transformation of the SEA dynamics, a lumped disturbance signal is constructed and a form convenient for controller design is developed. Then, a nonlinear disturbance observer(NDOB) and a sliding-mode control scheme are introduced to synthesize the control law. The performance of the proposed controller is theoretically ensured by Lyapunov analysis. Taking a nonlinear SEA for instance, a series of simulations and hardware experiments are carried out. The results suggest the effectiveness and superior performance of the proposed method for SEA torque control by comparing it with the cascade-PID controller.