Professor Daniel Goldman and researcher Nicole Mazouchova watch FlipperBot move through a bed filled with poppy seeds (CREDIT: Gary Meek)

Turtle robot reveals principles of moving on sand

A robot mimicking the way turtle hatchlings move over sand has shed light on the principles of locomotion on granular surfaces.

For sea turtle hatchlings struggling to reach the ocean, research has shown success may depend on having flexible wrists that allow them to move without disturbing too much sand.

Research published in the journal Bioinspiration & Biomimetics today describes how a similar wrist also helps a robot known as "FlipperBot", designed by researchers at the School of Physics at the Georgia Institute of Technology, move through a test bed.

Daniel Goldman, associate professor at the university’s School of Physics, has conducted previous research in the field, which he dubs “terradynamics" – the science of legged animals and vehicles moving on granular and other complex surfaces.

He hopes this latest research could help robot designers better understand locomotion on complex surfaces and lead biologists to a clearer picture of how sea turtles and other animals like mudskippers use their flippers. The research could also help explain how animals evolved limbs – including flippers – for walking on land.

"We are looking at different ways that robots can move about on sand," says Goldman. "We wanted to make a systematic study of what makes flippers useful or effective. We've learned that the flow of the materials plays a large role in the strategy that can be used by either animals or robots."

Both the baby turtles and FlipperBot run into trouble under the same conditions – traversing granular media disturbed by previous steps – and information from the robot research helped scientists understand why some of the hatchlings they studied experienced trouble..

The research began in 2010 with a six-week study of hatchling loggerhead sea turtles emerging at night from nests on Jekyll Island, one of Georgia's coastal islands, with Nicole Mazouchova, then a graduate student in the Georgia Tech School of Biology, studying the baby turtles using a trackway filled with beach sand housed in a truck parked near the beach.

She recorded kinematic and biomechanical data as the turtles moved in darkness toward an LED light that simulated the moon and when Mazouchova and Goldman studied data from the 25 hatchlings, and were surprised to learn that they managed to maintain their speed regardless of the surface on which they were running.

"On soft sand, the animals move their limbs in such a way that they don't create a yielding of the material on which they're walking," said Goldman. "That means the material doesn't flow around the limbs and they don't slip. The surprising thing to us was that the turtles had comparable performance when they were running on hard ground or soft sand."

The key to maintaining performance seemed to be the ability of the hatchlings to control their wrists, allowing them to change how they used their flippers under different sand conditions.

"On hard ground, their wrists locked in place, and they pivoted about a fixed arm," Goldman explained. "On soft sand, they put their flippers into the sand and the wrist would bend as they moved forward. We decided to investigate this using a robot model."

That led to development of FlipperBot, with assistance from Paul Umbanhowar, a research associate professor at Northwestern University.

The 19cm robot has two flippers driven by servo-motors and, like the turtles, the robot has flexible wrists that allow variations in its movement as it moves through a track bed filled with poppy seeds that simulate sand.

Mazouchova, now a Ph.D. student at Temple University, studied many variations of gait and wrist position and found the free-moving mechanical wrist also provided an advantage to the robot.

"In the robot, the free wrist does provide some advantage," said Goldman. "For the most part, the wrist confers advantage for moving forward without slipping. The wrist flexibility minimizes material yielding, which disturbs less ground. The flexible wrist also allows both the robot and turtles to maintain a high angle of attack for their bodies, which reduces performance-impeding drag from belly friction."

While the results may not directly improve robot designs, the researchers hope they should contribute to a better understanding of the principles governing movement using flippers, which would be useful to the designers of robots that must swim through water and walk on land.

"A multi-modal robot might need to use paddles for swimming in water, but it might also need to walk in an effective way on the beach," Goldman said. "This work can provide fundamental information on what makes flippers good or bad. This information could give robot designers clues to appendage designs and control techniques for robots moving in these environments."

The work was supported by the National Science Foundation, the US Army Research Laboratory's Micro Autonomous Systems and Technology (MAST) Program, the US Army Research Office, and the Burroughs Wellcome Fund.

A video of FlipperBot is available here.

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