Study on the Influence of Rotation Axis Misalignment of the Exoskeleton Knee Joint on Human Knee Joint Torque

Misalignment between the rotation axis of a lower-limb exoskeleton knee joint and the human knee joint in the sagittal plane introduces additional human-exoskeleton interaction forces, affecting gait and assistance efficiency. This study proposes an equivalent stiffness-damping scheme that simultaneously accounts for the serial characteristics of both the exoskeleton's strapping and human soft tissues, and establishes a human-exoskeleton coupling model based on OpenSim that allows for precise adjustment of the misalignment magnitude along the sagittal coordinate system. Simulation results indicate that the effects of misalignment exhibit significant directional dependence: at a misalignment of 0.05 m, the peak increase in extension torque in the positive y-axis direction reached 38.9%, while the peak increase in flexion torque in the negative x-axis direction reached 211.2%. The trends in human-exoskeleton interaction torque and electromyographic signals measured experimentally were consistent with the simulation results. Considering that, at a misalignment of 0.02 m, the maximum difference in torque in the negative x-axis direction was 54.65% and the assist efficiency in the positive x-axis direction dropped to -5.13%, it is recommended that the misalignment of the exoskeleton knee joint be controlled within 0.02 m. This provides quantitative evidence for the structural design and wear calibration of the exoskeleton.