Design, Modeling, and Control of a Knee Exoskeleton for Underwater Load Assistance

This study presents the design and control of a knee exoskeleton de- veloped to assist humans while carrying loads through shallow or waist-level water. Conventional exoskeletons are designed for terrestrial applications and are inefficient in underwater conditions due to the effects of buoyancy, drag, and corrosion. To address this gap, an underwater knee exoskeleton is designed to augment strength while supporting a 51 kg load. The lightweight aluminium frame features adjustable lengths to accommodate users of varying anthropo- metric dimensions. To emulate the torque experienced at the knee joint dur- ing load-bearing, an existing knee joint kinematic dataset (angular motion and ground reaction force data) under aquatic conditions, corresponding to a 51 kg individual, is utilized in this work. A mathematical model was developed to es- timate the torque requirements during the normal gait cycle, taking into account buoyant force and hydrodynamic drag. Thereafter, closed-loop position control was implemented using a PID controller in MATLAB Simscape Multibody, and its performance was analyzed for different gain combinations. Finally, numeri- cal simulation results indicate that optimal PID gains achieved a mean absolute error of 4.05°, a root mean square error of 4.65°, and an integral absolute error of 4.07°. Moreover, a peak torque of 34.76 Nm is found to be necessary for the knee exoskeleton design to support additional loads of up to 51 kg in waist- level water. The results show that the proposed design and control strategy are effective for load augmentation in underwater conditions.