Ankle Exoskeleton with a Symmetric 3 DoF Structure for Plantarflexion Assistance

Miha Dezman, Charlotte Marquardt, Tamim Asfour

Abstract: Ankle exoskeletons can assist the ankle joint and reduce the metabolic cost of walking. However, many existing ankle exoskeletons constrain the natural 3 degrees of freedom (DoF) of the ankle to limit the exoskeleton’s weight and mechanical complexity, thereby compromising comfort and kinematic compatibility with the user.This paper presents a novel ankle exoskeleton frame design that allows for 3 DoF ankle motion using a symmetric parallel frame design principle resulting in a strong frame while weighing 1.8 kg. Furthermore, a cable routing method is proposed to actuate the plantarflexion of the ankle. The kinematic compatibility of the proposed exoskeleton frame is evaluated in straight- and curve-walking scenarios with four users. The study demonstrates that the exoskeleton frame adapts to the natural 3 DoF ankle motion and the range of motion (RoM) during walking. The actuation in plantarflexion is evaluated in a stationary torque experiment demonstrating the ability of the frame to transfer large torque loads of up to 57.4 Nm. This work contributes to the design and development of more flexible and adaptable ankle exoskeletons for walking assistance.