Comfort-Centered Design of a Lightweight and Backdrivable Knee Exoskeleton

TL;DR

This paper introduces a comfort-centered design approach for a lightweight, backdrivable knee exoskeleton, focusing on reducing discomfort through innovative structure, joint mechanisms, and low impedance transmission.

Contribution

It proposes novel design principles and mechanisms that improve comfort and backdrivability in knee exoskeletons, validated through simulations and experiments.

Findings

74% reduction in joint misalignment at maximum knee flexion

1.03 Nm RMS resistive torque in unpowered mode

0.31 Nm RMS torque tracking error in experiments

Abstract

This paper presents design principles for comfort-centered wearable robots and their application in a lightweight and backdrivable knee exoskeleton. The mitigation of discomfort is treated as mechanical design and control issues and three solutions are proposed in this paper: 1) a new wearable structure optimizes the strap attachment configuration and suit layout to ameliorate excessive shear forces of conventional wearable structure design; 2) rolling knee joint and double-hinge mechanisms reduce the misalignment in the sagittal and frontal plane, without increasing the mechanical complexity and inertia, respectively; 3) a low impedance mechanical transmission reduces the reflected inertia and damping of the actuator to human, thus the exoskeleton is highly-backdrivable. Kinematic simulations demonstrate that misalignment between the robot joint and knee joint can be reduced by 74% at maximum knee flexion. In experiments, the exoskeleton in the unpowered mode exhibits 1.03 Nm root mean square (RMS) low resistive torque. The torque control experiments demonstrate 0.31 Nm RMS torque tracking error in three human subjects.