Ergonomic considerations for anthropomorphic wrist exoskeletons: A simulation study on the effects of joint misalignment

Abstract

This work focuses on anthropomorphic exoskeletons for the human wrist. We consider a 2 dof model for the human wrist with non intersecting joints and a similar model for the exoskeleton. We assume a viscoelastic attachment between the human hand and the handle of the exoskeleton which on one side allows the different kinematics of the exoskeleton to follow the human wrist and, on the other side, induces reaction forces at all joints, in particular causing discomfort. We quantify discomfort as the amount of potential energy stored in the deformation of the viscoelastic attachment. For a specific exoskeleton implementation, based on kinematic simulations, we report the kinematic mismatch (i.e. differences between the human joints and the corresponding exoskeleton joints) as well as the reaction forces arising when the human joints assume postures throughout their physiological range of motion. Considering a typical distribution of joint offset for humans (derived from literature) and the asymmetry in the discomfort function (derived from our simulations) we address the “one-size-fits-all” problem and propose an optimal joint offset for the exoskeleton, based on the minimization of the aggregate loss function.