Modeling and Experimental Evaluation of a Variable Hydraulic Transmission

A low-loss hydraulic transmission offers an efficient and lightweight means to route power from a central source to distal actuators, with example applications in legged and wearable robots. For example, one could develop a completely passive hydraulic transmission for an exoskeleton that routes power from a healthy limb to a limb weakened by neurological injury to enable self-assist or rehabilitation. In this article, we introduce and model a variable hydraulic transmission that features rolling diaphragm cylinders and a discretely variable transmission ratio. The variable transmission utilizes digital hydraulics, wherein valves switch redundantly mounted cylinders in and out of the hydraulic circuit. The article discusses the impact of configuration on the available transmission ratios and passivity. Our modeling efforts focus on the parasitic effects that arise with power losses between the driving and driven joints, compliance and energy storage in the transmission, and drift during switching. We carefully validate the model, comparing analytically predicted fine and gross behaviors to experimental observations. Experimental results demonstrate the feasibility and utility of the proposed device and the associated model which serves as an important tool for the design and control of digital hydraulic transmission systems.