Enhanced terminal sliding mode control for gait exoskeleton device: experimental investigation and validation

The design of pediatric lower-limb exoskeleton devices demands advanced control strategies that ensure safe, stable, and accurate real-time gait tracking despite model uncertainties and external disturbances. This study introduces an improved fast terminal sliding mode (IFTSM) control framework incorporating an adjustable exponential reaching law to enhance the reaching-phase dynamics of conventional terminal sliding mode schemes. The proposed formulation accelerates attraction to the sliding surface for large tracking errors while ensuring smoother finite-time convergence near equilibrium. Lyapunov-based analysis establishes finite-time stability and bounded tracking performance of the coupled subject-exoskeleton system. The controller is implemented on an existing pediatric lower-limb exoskeleton and evaluated under passive-assist operation with one healthy child (12 years, 40 kg, 132 cm) and one child with spastic cerebral palsy (12 years, 35 kg, 131 cm). Comparative experiments with multiple classical and advanced control schemes demonstrate superior real-time tracking behavior of the proposed IFTSM, with tracking error deviations relative to IFTSM on the order of 40-65% for conventional controllers and about 5-20% for advanced sliding-mode variants across the lower-limb joints, together with improved convergence characteristics and reduced cumulative control effort for the healthy child. Over a 25-day experimental protocol, consistent within-subject reductions in tracking error were observed for the CP-affected subject (37.54% hip, 49.47% knee, and 16.29% ankle), together with modest within-subject changes in joint kinematics and ∼10% increased alignment toward a healthy reference. These pilot-level results demonstrate the technical feasibility, robustness, and stable real-time performance of the proposed IFTSM framework for pediatric exoskeleton gait tracking, while broader statistical validation and functional outcome assessment remain topics for future investigation.