Morphology-Changeable Soft Pads Facilitate Locomotion in Wet Conditions

Stable robot walking requires effective gait planning and control. The mechanical properties of contact between feet and ground are important factors in this process. In this paper, we propose and test a design for a robot foot that houses soft pads, which adapt to facilitate locomotion on wet and dry floors. The morphology of each soft pad distributed in the sole of the foot is changed from a thin elastic sheet to a spherical dome that protrudes from the sole with the use of compressed air to adapt the foot for walking. We constructed an analytical model to estimate how soft pad dome shape is effected by air pressure and associated properties regarding friction and forces when in contact with a dry or wet floor. The contribution of wet adhesion to enhance shear force among the floor and the soft pads was elaborated theoretically. We report results of practical experiments with a sole (35 mm×15 mm) housing 14 such soft pads (radius 1.5 mm) comparing walking in dry and wet conditions. Finally, we present a setup of a walking robot leg with the proposed foot and soft pad on flat ground. The results demonstrate that soft pad domes improved walking stability under wet conditions on flat ground whereas they made no noticeable difference under dry conditions. Changeable morphology of the soft pads (stiffness, radius of the domes), which is controlled by adjusting air pressure, contributed to adaptive walking. The results obtained here may inform attempts at morphological design of soft pads to facilitate locomotion of legged robots under various terrain conditions.