Washington, February 10: The Burroughs Wellcome Fund and the U. S. Army Research Laboratory are funding a research into why present-day advanced mobile robots have difficulty traversing granular surfaces-like sand, dirt, rubble or slippery piles of leaves-despite being able to explore complex terrains across the globe, and even on Mars.

The researchers attached with the study have thus far suggested that robots attempting to move across sandy terrain should move their legs more slowly, especially if the sand is loosely packed.

"We have discovered that when a robot rotates its legs too fast or the sand is packed loosely enough, the robot transitions from a rapid walking motion to a much slower swimming motion," said Daniel Goldman, an assistant professor in the School of Physics at the Georgia Institute of Technology.

A research article in the journal Proceedings of the National Academy of Sciences says that the physics of movement on granular media has been largely unexplored systematically, which is why Goldman and his colleagues decided to probe the performance of a small six-legged device called SandBot.

The report revealed that the robot was designed by Haldun Komsuoglu and Daniel Koditschek at the University of Pennsylvania.

"This is new territory because researchers have not examined the interaction between an animal''s foot and sand like they have a whale or duck''s flipper and water. Sand is a uniquely challenging terrain because it can shift quite easily from solid to fluid to solid and requires different locomotion strategies," said Goldman.

For their experiments, the researchers built a trackway for SandBot to run along, which consisted of an eight-foot-long poppy seed-filled container with tiny holes in the bottom, through which air could be blown.

The air pulses elevated the granules and caused them to settle into a loosely packed solid state, allowing the researchers to closely control the density of the material.

"We used poppy seeds as the granular material because they were large enough not to get into the SandBot motors but light enough to be manipulated with our air blowers. We have done experiments with small glass beads, which more closely approximate desert sand, and found no qualitative change in the results," said Goldman.

While typical volume fractions for granular media in the desert range from 55 to 64 percent, the researchers packed the poppy seeds to a volume fraction of 63 percent. They placed SandBot onto the surface and set its c-shaped legs to rotate five times per second.

The little robot, which could bounce quickly across hard ground, became completely stuck in the granular material after just a few steps.

The researchers found that the problem was the rotational motion of the robot''s limbs. They observed that the SandBot moves its limbs in an alternating tripod gait and during a rotation, each limb moves fast while it is in the air and slow while it is in the ground.

The team further noted that the robot could walk across the sand quickly at a speed of one body length per second, if the rotation frequency was fixed and three parameters were adjusted: the durations of the slow and fast phases and the angle at which the limb changed from slow to fast.

"A systematic study of the motion then revealed that changes in volume fraction of less than one percent resulted in either rapid motion or slower swimming. We saw similar sensitivity when we changed the limb rotation frequency," added Goldman.

Goldman believes that this study''s experiments and model describing the basic behaviour of motion on granular media will help biologists understand how animals appear to move effortlessly across a diversity of complex substrates.

He also plans to use the information to help roboticists design devices with the appropriate feet and limb motion to move well in complex terrain, including sand. (ANI)