Adaptive nonsingular fast terminal sliding mode control with disturbance observation and input saturation compensation for a lower-limb exoskeleton

Most existing controllers for lower-limb exoskeleton robots perform poorly when faced with model uncertainties, input saturation in actuators, and external disturbances, which significantly degrade system stability and performance. In this paper, we propose a novel disturbance observer-based adaptive nonsingular fast terminal sliding mode control (NFTSMC) strategy to address these issues. The method introduces an input saturation compensation mechanism to mitigate actuator constraints, ensuring efficient system operation. A disturbance observer is then employed to estimate lumped uncertainties and external disturbances, while an adaptive parameter tuning mechanism adjusts the observer’s error-bound parameters in real-time, reducing the need for prior knowledge of disturbance bounds and minimising chattering effects. By integrating the disturbance observer with the NFTSMC controller, we guarantee finite-time tracking performance. Comparative experiments on a lower-limb exoskeleton platform demonstrate that the proposed controller achieves improved hip and knee tracking accuracy and smoother control responses under various walking speeds.