The Manipulation of Granular Media Flow Properties to Produce Stable, Uphill, Low Mass Bipedal Locomotion

Unlike multileg robots which are typically close to the ground and generally statically stable, bipedal robots must maintain balance in potentially shifting terrain. We performed a series of systematic experiments to enable a 7 degree-of-freedom planar biped walker (45cm tall) to robustly traverse granular inclines of 0 to 10 deg of 1 mm poppy seeds. Through gait optimization, center of mass (CoM) variation, and contact-based control, a robust open-loop (OL) system for low mass biped robotic locomotion on granular media (GM) was developed. This was achieved through manipulation of step length, L, and vertical CoM to minimize GM flow at the foot-media boundary. Because of nonlinearities involved in GM deformation, typical body and joint stabilization techniques used in biped robotics over hard ground are insufficient. Thus, we developed a control scheme encompassing static inertial changes through torso position, and contact area variation through dynamic expansion of the feet, to redistribute slip forces (characterized by foot intrusion or material flow). This allows for robust, steady gait over GM, and have found that this scheme enables the robot to remain upright for the duration of a trial (5 gait cycles) 90% of the time, compared to consistent OL failure within 1 gait cycle.