To overcome the tradeoff between torque density and response of the backdrivable actuators, actuation by electro-hydrostatic actuators (EHA) is effective. While their backdrivability and energy efficiency was shown in the previous studies, their closed-loop dynamic behavior was not discussed in detail. In this paper, we present the analysis and experimental evaluation of the force control performance of the electro-hydrostatic actuator for the humanoid robot 'Hydra'. We first present a simplified model of EHA and show that EHA can be simplified as a mass-spring-damper model if all values such as pump torque/velocity and fluid pressure/flow-rate are expressed in the equivalent value seen from the actuator. We also show the comparison between the model and experimentally acquired open-loop dynamic behavior. Then, the evaluation on the force measurement and control performance is shown. The static friction on the rod-seal was 0.46 of the maximum piston force, and with additional strain gauge information, the error can be reduced to 0.28 of the maximum force. We also show that our developed EHA has a pressure control bandwidth of 100 Hz in the fixed piston configuration, which is higher than other state-of-the-art series elastic actuators, in the last of paper, the joint level position and torque control performance of Hydra is examined.
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