A robot walks, sinks, and swims in granular media

Inspired by crabs, cockroaches, and other nimble creatures, engineers at the University of Pennsylvania have designed robotic vehicles to traverse complex terrain such as deserts and presumably the Martian landscape. Equipped with six spring-loaded and synchronously rotating C-shaped limbs, the robots outmaneuver current military and rescue vehicles over coarse but rigid terrain. On granular media, however, that agility comes at a cost: The latest robot model, the 30-cm-long SandBot, shown in the image and designed by Georgia Institute of Technology physicists in collaboration with the UPenn engineers, drops from a speed of 60 cm/s on a rigid surface to a crawl of 2 cm/s in a bed of poppy seeds. (See videos with this item at http://www.physicstoday.org .) Only when the researchers empirically tweak the limb-control parameters does the speed approach a respectable 30 cm/s. A team led by Daniel Goldman at Georgia Tech set out to determine how the robot’s speed is influenced by the angular frequency of its limbs and the granular medium’s packing fraction. The experiments revealed that the robot’s legs sink into the fluid-like medium, then slip, before walking forward. Below a critical packing fraction, and at high limb frequencies, a sharp transition from rotary walking to a slower swimming motion was observed. The researchers say that understanding the physics associated with crater formation and collapse in granular media will lead to advances in limb geometry and robot locomotion. (C. Li et al., Proc. Nat. Acad. Sci. USA 106 , 3029, 2009 http://dx.doi.org/10.1073/pnas.0809095106 .)