Design and Validation of a Lightweight Adaptive and Compliant Locking Mechanism for an Ankle Prosthesis

Over the last decade, active lower-limb prostheses demonstrated their ability to restore a physiological gait for transfemoral amputees by supplying the required positive energy balance during daily life locomotion activities. However, the added-value of such devices is significantly impacted by their limited energetic autonomy and excessive weight preventing their full appropriation by the patients. There is thus a strong incentive to reduce both the overall power consumption and weight of active prostheses. To address these issues, we developed a novel parallel spring mechanism, tailored to the dynamical behavior of an ankle prosthesis. The first contribution is the development of a lightweight and adaptive locking system, comprising an energy efficient ratchet and pawl mechanism with electromagnetical actuation. As second contribution, the required compliance is directly materialized within the structure of the prosthesis with no additional parts, taking advantage of fused filament fabrication (FDM) technology with carbon fibers reinforcement. Our system provides an elastic torque during flat ground walking, corresponding to 41% of the peak torque produced by an healthy ankle (50 Nm), at a negligible energetic cost (0.5 J/stride). By design, the novel parallel spring mechanism is lightweight (140 g), can engage at any plantarflexion position with a locking discretization of 0.3°, and is self-unlocking.