Design and Evaluation of a Load Control System for Biomechanical Energy Harvesters and Energy-Removing Exoskeletons

Controlling the timing and magnitude of electricity production is a critical factor in reducing the metabolic cost of walking with an energy-removing exoskeleton. This article outlines a novel power electronic control system designed to apply a mechanical loading profile onto the user that extracts kinetic energy. This energy extraction assists the user's muscles, thereby providing metabolic assistance while simultaneously producing electrical power. This open-loop control system estimates the state of both the exoskeleton and the user's lower limbs and uses this estimation to identify and apply a desired knee flexion moment during the terminal swing phase. The control system was evaluated using human treadmill walking experiments and benchtop testing, which determined that the system could identify the user's stride period, ground contact timing, and the device's moment arm with high accuracy and precision. Furthermore, the system could apply the desired cable force within +2.6 and −2.3 N and the muscle-centric knee moment profile within +0.05 and −0.04 N·m. Through proper load control, a user would benefit from walking with an energy-removing exoskeleton, regardless of the need for portable power.