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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.

Hydraulic actuation of exoskeletons has gained interest among researchers due the potentials of high power density and energy recuperation allowing the reduction of mass and space used by the device (when compared to the traditional electrically actuated exoskeletons). However, developing a light and cost-effective design of such exoskeleton remains a key challenge. In this work, a novel design of digitally driven knee exoskeleton is presented. The design uses simple hydraulic cylinders instead of multi-chamber cylinders (which are typically used in digital actuations and are expensive). The design also includes a unique mechanism that is able to satisfy the peak torque requirements during a typical gait cycle with a smaller hydraulic force. This ensures small size of hydraulic components and a moderate power demand from the energy source. To study this exoskeleton design, a numerical model of the exoskeleton and the lower limb is developed in this work. The simulation results show that the design is able to track the motion of the knee in a typical gait cycle as well as satisfy the necessary torque requirements.