Ankle springs instead of arc-shaped feet for passive dynamic walkers

Passive dynamic walking is interesting for humanoid robots because of the efficient, natural-looking, and naturally stable gait. However, most prototypes so far have been equipped with arc-shaped feet rigidly mounted to the shank, which has been deemed ´non-human´, and it prevents certain functions such as standing still. In this paper, we show that the rigid arc feet can be replaced by flat feet that are mounted on ankles with a torsional spring stiffness. The spring stiffness has a similar effect as the foot radius; it reduces the sensitivity to disturbances and thus improves the disturbance handling. We have already implemented the idea in various prototypes, but in this paper we focus on the disturbance behavior of a simple 2D straight-legged passive walking model.