Dynamically Stable Legged Locomotion

Abstract : This report documents recent progress in exploring active balance for dynamic legged systems. Balance in 3D can be achieved with a very simple control system. The control system has three separate parts, one that controls forward running velocity, one that controls body attitude, and one that controls hopping height. Experiments with a physical 3D machine that hops on just one leg show that it can hop in place, travel at a specified rate, follow simple paths, and maintain balance when disturbed. Top recorded running speed was 2.2 m/sec (4.8 mph). The 3D control algorithms are direct generalizations of those used earlier in 2D, with surprisingly little additional complication. Computer simulations of a simple multi-legged system suggest that many of the concepts that are useful in understanding locomotion with one leg can be used to understand locomotion with several legs. A planar model with two legs trots and bounds with the same three part control decomposition used for the one-legged systems. We have designed a four-legged running machine in order to experiment with balance in systems with more than one leg. The machine is arranged like a large dog, with narrow hips, and a long body. We have begun to study gait in terms of coupled oscillations. We have found that changes in the ratio of leg stiffness to hip stiffness change the pattern of rocking and swaying motions. For legged systems to be maneuverable, they must be able to traverse arbitrary paths in the horizontal plane.