This paper proposes a foot shape design to enhance the stability of passive dynamic walk by constraining fall down phenomenon in both sagittal and lateral planes. We focus on excessive side-to-side and forward leg swinging that causes a passive dynamic biped walker to fall over. Geometrical analysis showed that stability under a wide range of slope inclinations is achievable by limiting the swinging leg spatially to within a certain angle. Such a limit, or constraint, on swinging effectively prevents falling down on the lateral plane, while stable walking is maintained on the sagittal plane by constraining forward movement using a sharp edge at the head of a foot. We propose a foot prototype realizing these two constraints using a three-dimensional (3D) sole design and show that the proposed constraint is more effective for walking than an arctic foot shape. In verification experiments, the constraint stabilized the passive dynamic walker in a wide range of outdoor environments.

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