Stabilizing passive dynamic walk by constraining leg angles by foot shape

This paper proposes a foot shape design to enhance stability of passive dynamic walk, by constraining fall down phenomenon in both sagittal and lateral planes. We focus on the over swinging situation for a passive dynamic biped walker, where exceeding sideway and forward swing of a leg causes fall down phenomenon. From geometrical analysis, we found that stability under a wide range of slope inclinations will be achieved by limiting swing legs spatially in a certain angle. We also found that the limiting leg´s angles are achieved by a certain type of a 3D sole shape, which provides stability in a compact size. In lateral plane, limiting swing from side to side is effective to avoid falling down in this plane. This is realized by preparing limiter shape in lateral direction, such as two separate sub-foots placed apart in parallel. In sagittal plane, stability of walk is kept by constraining fall down phenomenon toward moving direction, by a sharp edge at the head of a foot. A prototype of a foot that realizes these two constraining effects in a 3D sole shape design is proposed. We show that the constraining effect by the proposed foot shape is effective for a walk comparing to an arctic foot shape.