title: From the Cover: Sensitive dependence of the motion of a legged robot on granular media.
abstract: Legged locomotion on flowing ground (e.g., granular media) is unlike locomotion on hard ground because feet experience both solid- and fluid-like forces during surface penetration. Recent bioinspired legged robots display speed relative to body size on hard ground comparable with high-performing organisms like cockroaches but suffer significant performance loss on flowing materials like sand. In laboratory experiments, we study the performance (speed) of a small (2.3 kg) 6-legged robot, SandBot, as it runs on a bed of granular media (1-mm poppy seeds). For an alternating tripod gait on the granular bed, standard gait control parameters achieve speeds at best 2 orders of magnitude smaller than the 2 body lengths/s (approximately 60 cm/s) for motion on hard ground. However, empirical adjustment of these control parameters away from the hard ground settings restores good performance, yielding top speeds of 30 cm/s. Robot speed depends sensitively on the packing fraction phi and the limb frequency omega, and a dramatic transition from rotary walking to slow swimming occurs when phi becomes small enough and/or omega large enough. We propose a kinematic model of the rotary walking mode based on generic features of penetration and slip of a curved limb in granular media. The model captures the dependence of robot speed on limb frequency and the transition between walking and swimming modes but highlights the need for a deeper understanding of the physics of granular media.
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.
authors.forename: Chen
authors.initials: C
authors.lastname: Li
authors.forename: Paul B
authors.initials: PB
authors.lastname: Umbanhowar
authors.forename: Haldun
authors.initials: H
authors.lastname: Komsuoglu
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Koditschek
authors.forename: Daniel I
authors.initials: DI
authors.lastname: Goldman