Compact and Lightweight Hydraulic Actuation System for High Performance Millimeter Scale Robotic Applications: Modeling and Experiments

In this work, the design modeling and testing of a compact and lightweight hydraulic actuation system is presented. Compared to similar compact actuators found in the literature, our system considerably increases the work density while maintaining an equivalent force-to-volume ratio. An analytical model that is able to accurately predict the quasi-static behavior of the actuator has been developed and experimentally validated. Existing models in the literature are able to predict only one performance parameter at a time - either the force or the contraction - from the imposed pressure and the exteroceptive measurement of the other performance parameter. Due to the design configuration chosen for the actuator, our analytical model is able to simultaneously predict both the force and the contraction by using the knowledge of two proprioceptive parameters of the fluid circuit (imposed volume and measurement of the pressure). The latter is particularly interesting, as it enables a precise estimate of the muscle behavior, only through the known parameters located at the fluid-transfer system, and not directly in the muscle, which is of crucial importance to simplify instrumentation and compactness of the actuation system. Four in-house fabricated muscles, with diameters down to 1.5 mm, have been tested with internal pressures up to 1.7 MPa. The experimental results showed in all cases a very good agreement with the predicted performances, thus validating the analytical model developed.