Zhang Dynamics based Tracking Control of Knee Exoskeleton with Timedependent Inertial and Viscous Parameters

Knee exoskeleton plays an important role in robot-assisted rehabilitation for impaired pilots to restore their motor functionality of lower extremity through producing external movement compensation. Tracking control of knee exoskeleton often encounters time-dependent (time-varying) issues reflected in its dynamic behaviors. In many applications, inertial and viscous parameters of knee exoskeletons are measured to be time-dependent due to unexpected mechanical vibrations and contact interactions, which increases difficultly of accurate control of knee exoskeleton to follow desired joint angle trajectories. This paper proposes a novel control strategy for controlling knee exoskeleton with time-dependent (time-varying) inertial and viscous coefficients. Such controller is designed based on Zhang dynamics (ZD) method and utilizes twice Zhang function (ZF) so as to make the tracking error of joint angle exponentially converge to zero. Illustrative simulation examples and experimental validation are presented to show efficiency of this type of controller based on ZD method. Comparisons with gradient dynamic (GD) approach are also presented to demonstrate superiority of ZD-type control strategy for tracking joint angle of knee exoskeleton.