A zebra-tailed lizard stands on a bed of tiny glass beads and shifts its weight. The beads slip underfoot, and the mottled beige creature stretches its spindly toes to get a better purchase. Suddenly it breaks into a run, blazing across the granular surface with stupendous agility, its toes stretching out flat as they hit the beads, its feet whipping back and forth in a blur. Each side of the lizard’s body stretches and then coils in turn as the reptile darts ahead at several meters per second.
Scooped up a year ago in California’s Mojave Desert and transplanted to a lab at Georgia Tech, the lizard holds our interest because of its truly peculiar feet. Those long, bony toes allow the reptile to navigate over sand, rocks, and the many other types of terrain it may face in the desert. In the lab, the bed of glass beads stands in for desert sand, and by blowing air through it or packing it down, we can make the ground looser or more solid. We then study how the lizard copes with the changes.
Our interest isn’t purely biological. We—Goldman at Georgia Tech, Koditschek and Komsuoglu at the University of Pennsylvania, in Philadelphia, and our other collaborators—are hoping that by studying the zebra-tailed lizard and a menagerie of other desert-dwelling creatures, we can create more agile versions of our six-legged robot, SandBot. When traversing solid ground, the robot runs at a steady clip of two body lengths per second. (For comparison, a trotting dog covers four body lengths per second.) But on its first outing across the glass beads, SandBot dug holes fruitlessly with its crescent-shaped feet and got stuck after just a few steps.
Sand, it turns out, is one of the most difficult terrains for a robot to conquer. Sand is slippery, for one thing, and it is also inherently unstable: Its properties can easily flip between solid and fluid behavior in the course of a single footstep. Physicists still don’t have a complete picture of the mechanics of sand, which is why we’ve turned our attention to the lizard and the clever strategies it has evolved to cope with sandy terrain. For example, we have noticed that the lizard’s long toes sink deep into the sand at each step. It appears that this allows it to push off from sand that’s deeper and more solid than the less stable surface layer. The effect, preliminary evidence suggests, is that the sinking enables the lizard to run as if on hard ground, allowing it to maintain speeds up to 75 percent of its pace on solid ground. Desert animals deal with sand with different levels of success, and their techniques provide valuable clues for refining SandBot.
Ultimately, we would like to build robots that can traverse any kind of terrain—bounding across hard ground like a gazelle, scaling tall trees and buildings like a squirrel, or maneuvering over slippery piles of leaves or mud like a snake. At least for short periods, a few robots already have managed to scale vertical walls, leaf-covered slopes, and even ice. Eventually, highly mobile robots could make a big difference in search-and-rescue missions and could explore all kinds of tricky terrain, not just on Earth but on the moon, Mars, and beyond.
First, though, our machines need to conquer sand. Had we been designing a wing for flying or a flipper for swimming, we would have been guided by the well-established rules for fluid flow, the Navier-Stokes equations. But for a complex material like sand, the equivalent models do not yet exist. So we had to start at the very beginning, by investigating the physical properties of granular materials. After about two years of study and experimentation, we in our small consortium of physicists, roboticists, and biologists think we have identified some basic rules describing movement across granular surfaces. Applying that knowledge to designing sandworthy robots, though, is not at all straightforward.
Consider how humans transport themselves over land. In places where massive investments have been made in roads and tracks, it’s relatively simple to move about by car or train. In fact, our vehicles require all of that engineered smoothness—without it, they can’t go far. But much of the Earth’s surface is largely inaccessible to vehicles, including robots. About 30 percent of the land area is desert, and one-fifth of that is covered by some kind of sand.
Sand isn’t the only issue. Disaster sites and battlefields—precisely the places where mobile robots are expected to be most useful—are full of unpredictable, impassable rubble. In 2001, for example, robots were sent in after the World Trade Center towers collapsed, but debris quickly clogged their tracks or caused the robots to flip over. Likewise, when a coal mine collapsed in Sago, W.Va., in 2006, a rescue robot made it about 700 meters past the mine’s entrance before getting stuck in mud. Even benign stuff like gravel and fallen leaves can stop a robot cold.