Nonlinear disturbance observer based flexible-boundary prescribed performance control for a lower limb exoskeleton

Abstract

In this article, a nonlinear disturbance observer based flexible-boundary prescribed performance controller (NDOPPC) is proposed to achieve outstanding transient and steady-state tracking performance for a lower limb exoskeleton. To estimate and compensate the lumped disturbance consisting of unmodelled dynamics and interaction torques applied by the wearer, a nonlinear disturbance observer (NDO) is utilised. The prescribed performance of the trajectory tracking of the exoskeleton is guaranteed by constraining the tracking error between the preset upper and lower boundaries. The boundaries of the proposed NDOPPC can be asymmetric functions in various forms, which increases the flexibility of the prescribed performance. The proposed NDOPPC is designed based on backstepping and the stability is analysed with Lyapunov theorem. Comparative simulations have been conducted with MATLAB/Simulink. Compared to computed torque controller (CTC) and nonlinear disturbance observer based computed torque controller (NDOCTC), the obtained results of comparative simulations demonstrate the effectiveness and superiority of NDOPPC. Finally, different types of boundaries are considered to demonstrate the flexibility of the proposed NDOPPC.