Engineers are getting closer to building a legged robot that can run across rocks and up stairs, but they admit that they’re still a long way from replicating the abilities of creatures like lizards, big cats or even mules -- let alone people. A new research finding out today describes the physics of artificial legs on shifting sands and could be another step closer, so to speak, in this quest for a powerful, real-life running machine.

Running robots are already being eyed for obvious tasks like conducting search-and-rescue missions during emergencies or hauling gear for soldiers in the jungle or woods. But they could eventually come in handy around the house as well, according to Dan Koditschek, professor of electrical engineering at the University of Pennsylvania, who developed the R-Hex legged robot more than a decade ago.

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“We want our robots to go indoors and outdoors,” Koditschek said.

Building robots the size of people that can actually help make the bed, prepare a meal or dig a ditch means figuring out a way to get them to walk on all kinds of different surfaces.

Koditschek said the roboticists and engineers are doing a pretty good job at getting machines to run on level, hard surfaces, but not so much when things get bumpy or soft.

“Where we begin to not do so well, is where the environment becomes flowable or if suddenly the ground changes,” he said.

Today in the journal Science, a group of researchers at Georgia Tech have come up with some mathematical formulas that describe these “flowable surfaces” comprised of things like poppy seeds, glass beads or pebbles. The group led by biophysicist Daniel Goldman came up with predictive models that might help other engineers build better mobile robots.

“You need to have some model on how the (robot) legs interact with the ground,” Goldman said. “They react in more complex ways than tanks and tires.”

Goldman and a former doctoral student, Chen Li -- now at the University of California, Berkeley -- developed a piece of software that allows other researchers to compute these forces that the robot legs exert on different kinds of shifting ground. See a video here of it working in a Lizard Robot.

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Then the team built a small six-legged robot with plastic legs made on a 3-D printer to test their predictions. The mini-bot was able to scurry across the sand just like a lizard, Goldman said.

“It was our first attempt and we’re happy it works,” he said.

The lizard bot will join several other bio-inspired walking, legged robots that have been developed in research labs in the past few years. They including MIT’s Cheetah; the Stickybot; and the Big Dog by Boston Dynamics.

Koditschek said the goal of researchers like himself are to capture the abilities of fast moving, highly energetic animals, rather than slow-moving rovers. Still, there are plenty of engineering and software related obstacles to overcome, he said, including how to distribute enough power to the various arms and legs, and how use integrate information from sensors to algorithms that tell the device where to take its next step.