Proprioceptive sensing to aid with locomotion adaptation in mud*

Moving on natural muddy terrains such as river beds, forest floors, and nearshore is extremely challenging, as subtle changes in mud composition and water content can lead to significant differences in mechanical behaviors. To enable terrestrial robots to effectively determine mud properties and flexibly adapt their locomotion strategies accordingly, in this study we explore proprioceptive terrain-sensing methods for robots to estimate substrate mechanical properties through joint torques during locomotion. We performed highly-controlled shear and penetration experiments with systematically-varied mud properties and found that mud resistance forces depended sensitively on both water content and clay-to-sand ratio. Interestingly, when scaled by a "distance from jamming'' variable, all shear force measurements from the high clay content regime collapsed to a non-dimensional master curve. This suggested that the distance from jamming could be used to determine the mechanical properties for a wide range of natural cohesive terrain materials. We demonstrated that based on the distance from jamming estimated from its limb joints, a flipper-based terrestrial robot could adapt its locomotion strategies and robustly move through muddy substrates with widely-varied properties.