Backlash-Compensated Active Disturbance Rejection Control of Nonlinear Multi-Input Series Elastic Actuators

Abstract

Series elastic actuators with passive compliance have been gaining increasing popularity in force-controlled robotic manipulators. One of the reasons is the actuator's ability to infer the applied torque by measuring the deflection of the elastic element as opposed to directly with dedicated torque sensors. Proper deflection control is pinnacle to achieve a desired output torque and, therefore, small deviances in positional measurements or a nonlinear deformation can have adverse effects on performance. In applications with larger torque requirements, the actuators typically use gear reductions which inherently result in mechanical backlash. This combined with the nonlinear behaviour of the elastic element and unmodelled dynamics, can severely compromise force fidelity.This paper proposes a backlash compensating active disturbance rejection controller (ADRC) for multi-input series elastic actuators. In addition to proper deflection control, a multiinput active disturbance rejection controller is derived and implemented experimentally to mitigate any unmodelled nonlinearities or perturbations to the plant model. The controller is experimentally validated on a hybrid motor-brake-clutch series elastic actuator and the controller performance is compared against traditional error-based controllers. It is shown that the backlash compensated ADRC outperforms classical PID and ADRC methods and is a viable solution to positional measurement error in elastic actuators.