Novel three-DOF ankle mechanism for lower-limb exoskeleton: Kinematic analysis and design of passive-type ankle module

In this paper, a novel three-DOF ankle mechanism is introduced. A lower-limb exoskeleton is developed by our research group to strengthen wearer´s muscle power in military missions. Owing to the tough operational condition, each module of the exoskeleton should be carefully designed for safety and to reduce the sense of fatigue of the wearer. To this end, it is desirable for the ankle module to be lightweight and compact and to have a remote center of rotation which is closely located to the rotation center of the wearer´s ankle. In order to satisfy these design considerations, the proposed ankle mechanism consists of the parallel connection of a spherical five-bar linkage and an RSU (revolute-spherical-universal)- serial chain. The spherical five-bar linkage realizes two-DOF ankle rotations of inversion/eversion and internal/external rotation, and the RSU-serial chain controls the ankle dorsiflexion/plantar flexion. The forward and inverse kinematics are derived, and instantaneous kinematics as well as force relations are analyzed by deriving the screw-based Jacobian and the reciprocal Jacobian. Finally, based on the gait data obtained from a normal subject with 4km/h of walking speed and 45kg of backpack load, the initial version of it is designed as a fully passive-type mechanism with a torsional spring. It is shown from a simulation that 42% reduction of the wearer´s ankle peak torque is expected by using the proposed ankle exoskeleton.