This paper presents preliminary bipedal running experiments with our Robotic Hexapod, RHex. The robot and the bipedal gait are under-actuated, using only one actuated degree of freedom per compliant leg. We doubled up the hind legs by attaching a duplicate set of hind legs at 180 degrees, forming S shaped hind legs. This reduces the actuator speed requirements during noncontact, while preserving the bipedal dynamics and control challenges. Stable running at an average 1.08 m/s with a success rate of 59% (100% without steering failures) over ten runs is obtained with only leg angle and body pitch angle feedback. The average specific resistance (cost of transport) of 1.2 is lower than previously reported numbers for pronk and bound gaits at similar speeds. clock- driven tripod gaits. These biologically motivated ideas, combined with sound scientific and engineering principles, have endowed RHex with a large repertoire of gaits, including walking over highly broken and irregular terrain 9, pronking 6, stair climbing 7,8, swimming 11, flipping 10 and quadrupedal bounding 2.
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