The paper reports on a hydraulic robotic leg, a research platform suitable for exploring high-performance legged locomotion. We propose to use hydraulic linear actuators combined with lightweight links made out from carbon-fiber-reinforced plastic so that we can maximally enjoy their innate high load-to-weight ratio. The robot is designed so as to have a one-to-one mass ratio between the actuators and other parts. Based on the hydraulic servo actuator dynamics, the paper describes the details of velocity and force control of the robot joints, along to our passivity-based force control framework. Details on the hardware including the mechanisms, microcontrollers, and simulators are also described. Finally, the paper provides experimental results on zero-force tracking control, gravity compensation, task-space impedance control, and jumping.
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