Design of a Bistable Inertial Clutch for a Passive-Elastic Ankle Exoskeleton

Abstract

Unpowered ankle exoskeletons are advantageous over powered ankle exoskeletons owing to their low weight. However, passive ankle exoskeletons without force transmission control can cause discomfort in certain situations. In this study, we designed a bistable inertial clutch that controls spring force transmission using phase-specific inertial forces generated by foot movement during walking. It features a ratchet-pawl mechanism, dual passive springs, and an inertial weight to switch between engaged and disengaged states. The design facilitated the timely storage and release of elastic energy at a certain walking speed and ensured that interruptions to the slow movement of exoskeleton wearers were minimal. We determined the optimal mass of an inertial weight of the clutch using an optimization method that synchronizes the clutch motion with changes in the walking phase. Eight subjects walked on a treadmill at five speeds (0.8-1.6 m/s), and foot angle, heel acceleration, and foot pressure were measured using a custom-made walking boot. Optimal inertial weights for each speed were obtained via a nonlinear optimization algorithm that minimized the least-squares error between foot pressure data and simulated clutch switching. Pilot testing showed that the clutch with the optimal weight for 1.2 m/s began switching between engagement and disengagement once the treadmill speed exceeded 1.2 m/s, as predicted. Thus, the clutch operated in sync with the stance and swing phases at sufficient speeds and remained stationary during low-acceleration leg movements.