Design, Development, and Validation of a Self-Aligning Mechanism for High-Torque Powered Knee Exoskeletons

Powered knee exoskeletons aim to assist individuals with lower limb impairments by providing power and torque at the joint level. However, if the anatomical and exoskeleton joints are not perfectly aligned, the exoskeleton assistance may result in spurious forces and torques transferred to the user's limb. These spurious forces and torques can then generate undesired loads on the user's joint and shear stress on the user's skin, causing pain and ultimately undermining the exoskeleton assistance. To address this issue, we propose a novel powered knee exoskeleton with a self-aligning mechanism using a prismatic-revolute-revolute (PRR) configuration. The proposed self-aligning mechanism weighs 190 g (i.e., 5.3% of the total exoskeleton weight) and can transmit up to 120 Nm of torque at the knee joint (i.e., biological peak torque for a 50th percentile male climbing stairs). Our experiments show that during assisted sit-to-stand transfers the peaks of the spurious torques and forces are below 0.5 Nm and 5 N, respectively, even if misalignments are intentionally added between the user and the exoskeleton. This study supports the use of self-aligning mechanisms in powered exoskeletons.