Robust Autonomous Bipedal Robot Locomotion on Granular and Rigid Slopes

Bipedal robot locomotion on slopes is challenging, as the upright posture makes such devices prone to pitching failures. We are interested in developing a robophysical model that can give insight into how simply controlled bipedal systems can transition from flowable substrates like granular media to rigid ground without undergoing instabilities. We developed a 40 cm high, 1.6 kg planarized robophysical model and controlled limb trajectories to generate stable flat footed walking on rigid and granular inclined surfaces. When challenged with granular slopes, robot starts to slip significantly for slopes greater than 10 degrees. The addition of multiple thin vertical plates (3 cm "cleats") on each foot with appropriate spacing allowed for low penetration forces but resisted material downhill yielding; this allowed for climbing slopes up to ~30 degrees, near the angle of repose. On rigid substrates, the cleats caused instability due to the increased height of the robot's CoM. We therefore developed a motor driven cleat retraction system in the foot that could sense foot motor load and automatically withdraw (in rigid ground) or intrude (in granular media) the cleats. When augmented with a body pitch sensor, the robot could modify its posture to accommodate different slope angles. The interaction of the physics of collective intrusion and simple closed loop control allowed the robot to walk on homogeneous terrain as well as successfully transition between rigid and granular surfaces.