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

Area of Science:

• Robotics
• Control Systems Engineering
• Biomedical Engineering

Background:

• Pediatric lower-limb exoskeletons require advanced control for safe and accurate gait tracking.
• Model uncertainties and external disturbances pose significant challenges in exoskeleton control.
• Existing control strategies may not fully address the dynamic complexities of pediatric gait.

Purpose of the Study:

• To develop and evaluate an improved fast terminal sliding mode (IFTSM) control framework for pediatric lower-limb exoskeletons.
• To enhance real-time gait tracking performance, stability, and robustness against uncertainties.
• To investigate the controller's effectiveness in children with and without spastic cerebral palsy.

Main Methods:

• An adjustable exponential reaching law was incorporated into a fast terminal sliding mode (FTSM) controller, creating the IFTSM framework.
• Lyapunov-based analysis was used to establish finite-time stability and bounded tracking performance.
• The IFTSM controller was implemented on a pediatric exoskeleton and tested with one healthy child and one child with spastic cerebral palsy.

Main Results:

• The IFTSM controller demonstrated superior real-time gait tracking compared to conventional and advanced sliding-mode controllers, with significantly reduced tracking errors.
• The controller achieved smoother finite-time convergence and reduced cumulative control effort in the healthy child.
• The child with cerebral palsy showed consistent within-subject reductions in tracking error (hip, knee, ankle) and improved alignment toward a healthy reference.

Conclusions:

• The proposed IFTSM control framework is technically feasible, robust, and provides stable real-time performance for pediatric exoskeleton gait tracking.
• The IFTSM controller offers significant advantages in reducing tracking errors and improving system dynamics.
• Further validation with larger cohorts and functional outcome assessments are warranted for broader clinical application.