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A control strategy for stable passive running

We present a control strategy for a simplified model of a one legged running robot which features compliant elements in series with hip and leg actuators. Proper spring selection and initial conditions result in "passive dynamic" operation close to the desired motion, without any actuation. However, this motion is not stable. The proposed controller is based on online calculations of the desired passive dynamic motion and stabilises any fixed robot speed. It also tracks large changes in desired robot velocity and remains largely passive for a wide range of velocities, despite a fixed set of springs, masses and inertias. To this end the desired motion is expressed as a function of a normalized "locomotion time" parameter. Comparisons of simulated runs with direct hip actuation show dramatic energy savings of 95% at 3m/s. Such energy savings are critical for the power autonomy of electrically actuated legged robots.