Wheels are great for traversing flat, paved surfaces, but when the terrain is covered with loose sand and large rocks, it's time to grow feet. Scientists built a six-legged robot after studying the movements of lizards, and the breakthrough may mean a fast walking rover we send to Mars that'll leave its predecessors in the dust.
"Having appendages like legs or limbs can be useful and beneficial" when the ground gets less even and solid, Daniel Goldman, a physicist at the Georgia Institute of Technology, told NBC News.
Researchers use physics theories and computer simulations to design airplanes that fly with the grace of birds and ships that ply waters with the ease of dolphins. Until recently, however, they knew little about the interaction of forces between scampering feet and sandy particles. How lizards could move across sand with amazing speed remained a mystery.
"That's in large part because, for one, it is hard to see what goes on underneath the sand," Goldman said.
He and graduate students Chen Li and Tingnan Zhang used X-rays and other observational techniques to study lizards running on sandy surfaces and came up with a theory to describe the physics of how the animals move.
The team tested the theory with first with computer simulations, then by building six-legged robots that could scamper across a sandbox, one filled with grains ranging from poppy seeds to glass beads.
The theory proved accurate within 5 percent to 10 percent over a wide range of material conditions, Goldman said. All that's needed for it to work is one measurement of how hard it is push vertically into the sandy material.
His team found a six-legged robot with "C"-shaped feet is the optimal design for running across their sandbox. The robot is five-inches long and weighs about one-third of a pound, perhaps ideal for search-and-rescue missions or scouting out unknown environments such as the surface of Mars, they noted.
"What that now means is not only can we understand potentially why certain leg shapes of lizards work well, we can tell our engineering colleagues if you want to make a robot that runs on loose material … here is the beginning of the tool that will allow you to," Goldman said of the work, which was published Thursday in the journal Science.
Going forward, the team aims to improve the model to account for the flow of sandy, granular material on hilly terrain such as dunes. Understanding interactions in wet sand, too, is also needed.
"It is also very challenging and exhausting to walk up a sand dune; it is two steps forward while you're sliding one step back," said Melany Hunt, a mechanical engineer at the California Institute of Technology. "Hence, a better understanding of the interaction between loose terrain and vehicle would be useful."
Hunt, who studies granular material flows, wrote an accompanying perspective article in Science, saying that the new line of research "may improve the performance of roving and walking robots."
Her own fieldwork on desert sand dunes illustrates the difficulty of designing a vehicle to move across rough and uneven terrain, she noted in an email to NBC News. Hers often gets stuck in the sand.
Eventually, the new tools should allow NASA scientists to launch giant robotic lizards on missions to Mars, to scamper over hilly, sandy terrain and perhaps — if theories of ancient Martian lakes are true — dip a toe in a little wet sand.
John Roach is a contributing writer for NBC News. To learn more about him, check out his website.