Research on Trajectory Tracking Control of Lower Limb Exoskeleton Based on Fractional Order Sliding Mode and Disturbance Observer

Abstract The walking stability of the lower limb exoskeleton is affected by nonlinear dynamics, perturbations and noise, which easily triggers the trajectory deviation and jitter vibration of the traditional control method. For this reason, this study proposes an improved sliding mode control method based on fractional‐order nonlinear disturbance observer (FONDOB) and extreme learning machine (ELM). First, a FONDOB is constructed to enhance the dynamic tracking ability of the mismatch disturbance by introducing fractional‐order differential operators, and a Lyapunov stability criterion is established to ensure the exponential convergence of the observation error. On this basis, a new sliding mode controller with fractional‐order sliding mode surface is designed, which is combined with the ELM algorithm to estimate the system uncertainty in order to reduce the system jitter. The numerical simulation method of the fractional‐order system is also designed in Simulink environment and simulation experiments are carried out. The experimental results show that the proposed fractional‐order controller has better control performance than the traditional integer‐order sliding mode control.