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Haldun Komsuoglu





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Sensitive Dependence of the Motion of a Legged Robot on Granular Media

Proceedings of National Academy of Science, February 2009

Li, C.*Umbanhowar, P. B. *Komsuoglu, H. †
Koditschek, D. E. † Goldman, D. I. *
*: Georgia Institute of Technology
†: University of Pennsylvaina

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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 bio-inspired legged robots display speed relative to body size on hard ground comparable to high performing organisms like cockroaches but suffer significant performance loss on materials like sand. In laboratory experiments we study the performance (speed) of a small ( 2 kg) six-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 two orders of magnitude smaller than the 2 bodylength/s (≈ 60 cm/s) for motion on hard ground. Empirical adjustment of these control parameters away from the best hard ground settings can restore the 30 cm/s performance. Robot speed depends sensitively on the packing fraction φ and the limb frequency ω, and a dramatic transition from walking to slow swimming locomotor modes occurs when φ becomes small enough and/or ω large enough. For hard surfaces, the forces developed during leg-ground contact that accelerate the robot are poorly understood. The corresponding forces on flowing ground remain unexplored. We propose a kinematic model of the 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.tal understanding of the collective physics can be achieved through interplay of experiment and theory. Unlike more heterogeneous real-world environments, granular media can be precisely controlled using laboratory scale devices [15, 16] to create states of varying material strength that mimic different deformable flowing materials produced during locomotion on complex terrains. Here we systematically explore the performance of a small legged device on granular media prepared in different packing states with volume fraction ranges typical of desert sand [17]. We find that locomotion is remarkably sensitive to substrate preparation and gait characteristics, which points to both the need for a more sophisticated understanding of the physics of motion within granular media and the possibility of better robotic control paradigms for locomotion on complex terrains.
BibTeX entry
  author = {Li, C. and Umbanhowar, P. B. and Komsuoglu, H. and Koditschek, D. E. and Goldman, D. I.},
  title = {Sensitive dependence of the motion of a legged robot on granular media},
  journal = {Proceedings of National Academy of Science},
  year = {2009},
  month = {February}
Related Sites
GaTech SandBot Project - SandBot project page maintained by the GaTech team.
SandBot - Information on robot platform, media appearances, more videos and pictures.

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