Nonlinear Disturbance Observer-based Robust Motion Control for Multi-joint Series Elastic Actuator-driven Robots

Motion control of multi-joint Series Elastic Actuator (SEA)-driven robots still faces challenges including intrinsic oscillatory dynamics, high-order robotic dynamics, low-bandwidth inner loop, and dynamic nonlinearities. In this letter, a nonlinear disturbance observer (NDOB)-based robust controller with the singular perturbation theory is proposed to perform stable and precise motion control of multi-joint SEA-driven robots. First, a fast-time control term is designed according to the singular perturbation theory to stabilize the SEA-level dynamics. Then, for the robot-level dynamics, a NDOB is designed to estimate the effects of unmodeled dynamics and external disturbance. The NDOB is combined with a baseline computed torque controller (CTC) to construct a composite controller NDOB-CTC. In addition, bounded stability is achieved with Lyapunov-type analysis. Finally, the proposed controller was implemented on a 2 DOFs SEA-driven robot. Comparative experiments were conducted for validations.