Disturbance-observer-based second-order adaptive nonsingular terminal sliding mode control for exoskeletons

Abstract

Reliable assistance from a hip exoskeleton depends on its ability to maintain accurate torque regulation despite unknown loads, modeling errors, and environmental disturbances. To improve tracking robustness under these conditions, this study develops a control architecture that combines disturbance estimation with a second-order adaptive nonsingular terminal sliding-mode scheme. The observer reconstructs the lumped effects of external perturbations and parameter uncertainties, while the nonlinear sliding-mode component is used to reduce control oscillations and accelerate error convergence. The proposed method is assessed through trajectory-tracking simulations in which measured normal-gait profiles serve as the desired hip-joint motion. The simulation results demonstrate that, compared to a conventional first-order sliding mode controller, the proposed method effectively mitigates the chattering, accelerates system convergence, minimizes steady-state error, and achieves superior dynamic regulation performance.