ATLAS: An Active Multi-Axis Ankle-Foot Prosthetic With Adaptive Tendon Stiffness For Locomotor Assistance and Stability

Overview

Ankle-foot prosthetics aim to recreate the biological motion of the ankle and restore natural gait to transtibial amputees. Achieving this requires motion in all three biological planes and a variable-stiffness actuator mimicking the adaptive behavior of the Achilles tendon. Prior research has not met this goal, as most devices augment only plantarflexion/dorsiflexion and rely on fixed-stiffness springs that cannot adjust energy return in a fixed travel range across gait speeds. This project presents ATLAS, a three-degree-of-freedom ankle-foot prosthesis with a variable-stiffness artificial Achilles tendon designed to replicate multi-plane ankle mechanics and adaptive energy storage. ATLAS uses a parallel J-hook linkage driven by brushless motors and 100:1 harmonic gearboxes to guide the heel in a natural arc about the tibia. This configuration produces high torque with low backlash while preserving multi-plane motion. A two-stage artificial Achilles tendon provides adaptive stiffness: an outer spring remains constantly engaged, while an inner spring engages under compression, with servo-actuated supplemental springs enabling spring rate shifts from 67 N/mm to 200 N/mm. Results reveal normalized gait at 0.2 and 0.6 m/s with peak plantarflexion torque of 140 Nm. Engaging one supplemental tendon servo increased peak plantarflexion torque by 5 Nm with only 1 mm additional compression. Additionally, all three degrees of freedom were achieved and verified during bench trials. These findings demonstrate that multi-plane actuation combined with adaptive stiffness can better replicate biological ankle mechanics and improve propulsion in transtibial prostheses. Future work will reduce system weight and refine adaptive control algorithms.