Low Slip Bipedal Locomotion on Granular Slopes via Cleated Foot Interaction

Bipedal locomotion on steep granular slopes can be challenging due to slipping and tilting, typically resulting from media yielding and downhill flow [Marvi et al, Science, 2014]. To gain insight into the relevant granular physics and develop robots that can traverse sandy slopes, we performed systematic experiments on a planar biped robot that self-deforms via hip, knee, and ankle actuation. Due to limited understanding of feedback control mechanisms on flowable terrain, we first develop open-loop gait trajectories to generate stable locomotion: trajectories on flat media are hand-tuned to slopes (consisting of loosely packed poppy seeds) by shifting the robot's center of mass accordingly, preventing the robot from tilting. We achieve stable locomotion on uphill/downhill granular slopes of ±10 degrees, slipping at each step at most 10% of its step length. To extend locomotion capability to steeper slopes, we replace the flat feet with cleated feet consisting of thin blades emanating vertically from the foot. The blades insert fully into the medium with low intrusion force, allowing the material to remain near or below yield stress. Cleated feet allow robot locomotion with moderate slip (%50) on granular slopes close to the angle of maximum slope stability (± 27 deg).