Bi-directional Cable-driven Ankle Exoskeleton Coupled with Series Elastic Actuator for Compliant Gait Assisting*

Abstract

This paper presents a lightweight bidirectional cable-driven ankle exoskeleton system (total mass: 2.6 kg) based on a series elastic actuation architecture (actuator module mass: 1.05 kg). The system utilizes a waist-mounted drive unit and Bowden cables to deliver bidirectional assistance to the ankle joint (nominal force: 460 N, peak force: 680 N). By integrating a dynamically coupled adaptive oscillator (AO), the system achieves robust gait synchronization across a range of walking speeds (0.6 – 1.8 m/s, phase estimation RMSE <2.48%, stride frequency estimation RMSE <0.1 Hz). This is complemented by a Gaussian Process (GP)-based torque planner and a cascaded torque control framework, ensuring seamless coordination with natural gait. Experimental characterization of the actuator demonstrates its high dynamic performance (torque bandwidth: 12.5 Hz) and low-impedance characteristics (peak passive backdrive torque: 0.97 N · m). Human trials involving five participants show that the system significantly expands the ankle joint range of motion (up to [-15.64 °, 20.67 ° ] at high speeds) while reducing peak muscle activation levels in the tibialis anterior (18.54%–30.21%) and gastrocnemius (19.34%– 25.45%). This design, combining lightweight construction with adaptive control strategies, provides a highly effective solution for daily mobility assistance and rehabilitation applications.