Simulation Study of a Digital Hydraulic Drive for a Knee Joint Exoskeleton

Even though the majority of currently published exoskeletons prototypes employ electrical drives, great opportunities are seen for hydraulic drives. Their main advantages are the unrivalled force and power density, facilitating the low additional masses at the peripheral limb joints, and the simple realization of locking, damping, and recuperation functions. The latter function is feasible with some hydraulic control concepts, like primary or secondary motion control. In this paper a digital cylinder drive is studied for getting up from a crouch. This motion is a sound benchmark to test the ability of the drive for exoskeleton knee joint actuation. Digital cylinders can realize output torques only in steps and the transition between different steps can create jerky motions. In this study the motion quality and the losses are evaluated for a binary stepped digital cylinder with four different chambers.