Design and Experimental Characterisation of a Hydrostatic Transmission for Upper Limb Exoskeletons

This paper introduces a novel hydrostatic air-liquid torque transmission system for an upper limb exoskeleton. The proposed design is based on remote electrical actuation, with grounded motors, combined with high performance fluid power transmission employed to deliver the power to the joints of the exoskeleton. The fluid transmission is based on rolling membrane cylinders that guarantee leakage-free operation, no backlash, and virtually zero stick-friction. This solution makes it possible to obtain easy controllability, good efficiency, intrinsic backdrivable operation, and reduced mass/inertia of the links of the robot. Additionally, the proposed system can be potentially implemented at relatively low-costs thanks to the employment of standard components and an architecture based on a modular approach. A test bench of the fluid transmission system is developed and a campaign of experiments is conducted to characterize its static/dynamic response for different choice of design parameters. In addition, we present a preliminary complete integrated arrangement of an upper limb exoskeleton equipped with the proposed transmission system. Results confirm the feasibility of the proposed actuation approach for the envisaged application.