Robust Gait Tracking Control of a Pediatric Exoskeleton System: An Adaptive Non-Singular Fast Terminal Sliding Mode Approach

This study proposes an adaptive non-singular fast terminal sliding mode (ANSFTSM) control for an exoskeleton system in passive-assist mode. The exoskeleton serves the purpose of motion assistance and gait restoration for the pediatric group aged 8–12 years. The complexity of the dynamic model, characterized by uncertain dynamics and external perturbations, poses a challenge in developing a gait tracking control for a pediatric exoskeleton. The experimental setup and real-time control architecture are introduced, followed by the dynamic formulation of the subject-exoskeleton system. Thereafter, a non-singular fast terminal sliding surface (NSFTSM) is considered to design the proposed equivalent control scheme. The adaptive laws are introduced in the reaching control scheme. Lyapunov's theory is employed to verify the rapid convergence of the tracking error in a finite time. The proposed ANSFTSM control is implemented for a pediatric subject (12 years) coupled with the exoskeleton in passive-assist gait tracking. The robust control provides efficient gait tracking (RMSE
hip
: 2.06°, RMSE
knee
: 3.81°, RMSE
ankle
: 1.26°) with converging sliding surfaces (|s|
hip
< 0.03, |s|
knee
< 0.06, |s|
ankle
< 0.04) and negligible chattering in finite-time. This study showcases the potential to apply the proposed control to a broader range of pediatric subjects.