Compared to electromechanical drives, hydraulic drives can be a good actuation alternative for exoskeleton devices due to their advantages in terms of high force density and capabilities of energy recuperation, motion damping and locking. However, to promote a wider adoption of hydraulically driven exoskeleton devices, key challenges related to the compact lightweight design and energy efficient operation and control are needed to be addressed.

To tackle these challenges, the authors have recently developed a novel design of digital hydraulically driven knee exoskeleton. In this work, the authors address the control challenges during the operation of the knee exoskeleton over multiple gait cycles. Proper control methods for different phases of the gait cycle are adopted. Furthermore, these methods are augmented with strategies to accomplish knee motion repeatability across gait cycles. These strategies involve steps to ensure that at the beginning of every gait cycle: (a) the knee angle returns to the same position, and (b) the pressure in the hydraulic chambers return to the same levels.

These control strategies are investigated via a numerical model of the knee exoskeleton. The simulation results indicate that the device is able to repetitively track the motion of the knee over multiple gait cycles while delivering the required torque.

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