Keep Safe: A Novel Static Balance Control Strategy for Lower Limb Exoskeletons

While powered lower limb exoskeletons assist paraplegic patients in static standing positions, uncontrollable physical disturbances may lead to imbalances in the human-machine system and cause injury. To address this issue, we propose an innovative static balance control strategy for the lower limb exoskeleton. Three balance strategies that include an ankle strategy, a hip strategy, and a stepping strategy were analyzed and validated by using a balance disturbance experiment conducted on unimpaired individuals. Among them, only the stepping strategy could be adopted to restore balance by the human-machine system due to limitations of the system's joints. A regression model was trained on experimental data of the falling point of the center of mass (COM) of unimpaired individuals following a physical disturbance. This model was then used to predict the falling point of the COM of the exoskeleton of a human-machine system. These predictions suggest that the falling point of crutches and footholds are necessary to maintain stability. Considering both stability and arm pressure, a weighting factor was employed to identify an optimal foothold of the human-machine system that ensured minimum pressure and maximum stability. Finally, a simulation showed that the application of the balance strategy enabled the exoskeleton human-machine system to recover balance over the tested controlled disturbance range.