Feasibility Study of a Four-Link Active Ankle Assist Mechanism with Motor-Based Control Implementation for Human Motion Stability

Abstract

This paper presents a feasibility study of an active ankle assist mechanism based on a four-link model representing single-leg stance dynamics. A dynamic model with two actuated ankle joints is derived, and an invariant-manifold-based nonlinear stabilization controller is designed. For practical motor implementation, three control schemes are formulated and compared: Ideal Torque control, Current-Based Torque control, and Angle-Based control with realistic motor current and position loops. Simulation results demonstrate successful stabilization under external disturbances for all implementations. The motor torque scheme closely reproduces the ideal torque benchmark and maintains stability over a wide feedback gain range. The angle-based scheme achieves comparable transient disturbance suppression but exhibits different robustness margins under feedback gain variations due to structural differences in gravity compensation and input realization. Frequency-domain analysis further reveals similar low-frequency disturbance attenuation and distinct higher-frequency gain characteristics across the implementations. These results confirm the feasibility of motor-based implementation and clarify the structural trade-offs between torque and angle realizations toward practical wearable assistive systems.