Robotic locomotion in complex terrains

Legged robots have a long way to go before they can attain the speed and control exhibited by living organisms, especially on granular surfaces. Granular materials exhibit a wide range of material properties and can behave as solids, gases, liquids, or glasses depending on the applied stress. Adapting to such a shape-shifting base exceeds the capability of most robots. Chen Li et al. conducted experiments with SandBot, a bioinspired robot with six Cshaped legs driven by rotating axles, and noted that the machine’s performance depends on the frequency of its leg rotation and the packing fraction of the granular material that provides traction. The authors observed that, as the leg frequency increases or the packing fraction decreases, the robot undergoes a transition from walking to less-efficient ‘‘swimming.’’ The transition occurs over a remarkably narrow window. A model developed by Li et al. captures most of the behavior and shows that the failure transition is typically due to the increasing penetration of the limbs into the granular stratum and the disturbances in the material from the flailing legs. A better understanding of granular physics could enable higher performance by the robot, according to the authors. — K.M.