Design, control, and computational validation of a mechanically decoupled 3-DOF ankle rehabilitation device

This paper presents the design methodology, kinematic analysis, finite element structural validation, and closed-loop computational simulation of a novel 3-degree-of-freedom (3-DOF) ankle-foot rehabilitation exoskeleton. The proposed exoskeleton addresses the simultaneous rehabilitation of dorsiflexion/plantarflexion (DF/PF), abduction/adduction (AB/AD), and inversion/eversion (INV/EV) in a seated configuration suited to patients with weight-bearing restrictions. Three NEMA 23 stepper motors independently actuate each axis through custom two-stage spur gear transmissions, achieving positive torque margins of + 6%, + 35%, and + 25% and sub-degree angular resolution. Full kinematic decoupling between all three axes is established through two independent architectural strategies. A finite element analysis (FEA) conducted on all four critical Al 6061-T6 structural components confirms safety factors between 2.82 and 4.87 under worst-case loading. A closed-loop PID control framework with adaptive range-of-motion (ROM) regulation, simulated in MATLAB/SIMULINK, demonstrates tracking performance characterised by maximum overshoots of 0.02°-0.12°, settling times of 3.7-7.8 s, and steady-state errors below ± 0.04° across all nine simulation scenarios. The results confirm the mechanical feasibility, structural integrity, and control effectiveness of the proposed exoskeleton as a viable platform for multi-DOF seated ankle rehabilitation.