To travel on rough terrain, a legged system must adjust the length of its steps so that the feet land on the available footholds. This paper explores the task of controlling step length in the context of a dynamic biped robot that actively balances itself as it runs. We explored three methods for con- trolling step length, each of which adjusted a different parame- ter of the running cycle. The adjusted parameters were forward running speed, running height, and duration of ground contact. All three control methods were successful in manipulating step length in laboratory experiments, but the method that adjusted forward speed provided the widest range of step lengths (0.1 to 1.1 m) with accurate control of step length (average absolute error of 0.07 m). The three methods for controlling step length manipulated the dynamics of the system so the feet could be placed on the available footholds without disturbing the system’s balance. An alternative approach was to ignore balance for a single step, placing the foot directly on the desired foothold, and recovering balance later. This approach generally resulted in very precise foot placements for a single target but could not be used to control many steps in a row. In laboratory demonstra- tions a biped running machine used these methods for adjusting step length to place its feet on targets, leap over obstacles, and run up and down a short flight of stairs.
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