Shape-Shifting Rovers

An innovative rover robot designed to explore planets and moons is undergoing final assembly this week in a lab at NASA’s Goddard Space Flight Center. The robot may also be useful in hazardous environments on Earth, its creators say.

Instead of driving, walking, or rolling around like other vehicles designed to traverse distant, rugged landscapes, the new rover changes its shape and topples along, veering a bit from side to side as it moves ahead. “We call it the drunken-sailor walk,” says Pamela Clark, one of the designers of the project at Goddard and a professor at Catholic University of America.

The minimalist device consists of an adjustable frame joined together at key points called nodes. The thin struts connect to the round nodes to form a tetrahedral shape, with another “payload node” at the center to hold the computer systems and sensors. The robot moves by extending or contracting its struts to change its configuration and shift its center of gravity until it tumbles over, then begins the process again. Depending on the terrain, its overall shape can change from tetrahedral to cubic to nearly spherical or flattened out. Ultimately, it should be able to negotiate its way across deep crevasses and climb steep cliffs by shifting its shape as needed.

Tumbling by changing the center of gravity may seem like an awkward and ungainly way of getting around, but Clark says it’s efficient and useful for dealing with obstacles, slippery surfaces, and steep slopes.

Last month, a small, single-tetrahedron version of the device had great success climbing the steep, rugged sides of Meteor Crater, in Arizona, in a test run. After months of tests in controlled lab environments, the device performed well despite extremely windy conditions. “We felt like we were about to be blown off the crater,” Clark said. “It was a good test.”

A larger, more advanced device made up of 12 tetrahedrons is now going through its final assembly in the lab, and it will be tested over the next several months. Clark says dividing up the structure into more tetrahedrons allows for much finer control over the shape and more efficient movement, with only minimal changes in the strut lengths.

Much of the work has been on the control algorithms. Clark says it’s tough to think intuitively about a robot that moves without wheels. “When there are 26 struts, there are little games that you can play to think of clusters of nodes in making it walk,” she said. Ultimately, the individual struts would be made interchangeable so they could be easily replaced in the field in the event of damage.

Now the main focus will be on developing a variety of “gaits” that the device can use to negotiate different kinds of surfaces, terrains, and slopes. This involves figuring out how far each strut should extend and in what order. Clark has just worked out a control sequence for what she calls an amoeboid gait, which makes the device look as though it’s slithering across a surface. “We set out to make the most efficient, low-to-the-ground gait we could,” Clark said. “An amoeba moves by trying to extend itself horizontally, with not very much fighting of gravity, which turns out to be very important in this.”

After a few months of tests in the lab, the team plans to take the new prototype to Sedan Crater, in Nevada. This crater, formed by an underground nuclear-test blast, has steep slopes of up to 40 degrees composed of loose sand. The steepest inclines in last month’s Meteor Crater tests were about 23 degrees and composed of rocky ground. Unlike wheeled rovers, the tetrahedral structure should be able to handle the steeper incline without slowing down.

The team has designed versions for both lunar and Mars exploration in hopes that they will be able to crawl into narrow cracks and around, over, and under obstacles that a wheeled rover can’t negotiate. More-advanced versions could even “chimney” up a space between vertical cliff faces. (This technique was developed by rock climbers to enable them to ascend a narrow vertical space: they lean against one wall with their legs against the other and gradually inch upward.) And since there’s inherently no risk of the device falling over and it is unlikely to get stuck, the vehicles could take more chances in exploring the difficult nooks and crannies that might be of great interest geologically and biologically.

Working in conjunction with conventional wheeled rovers, the two kinds of vehicles could form an effective team: the wheeled vehicles would be able to cross long flat distances and serve as a base, while the “tets” could scurry around as scouts, looking for the most interesting places and retrieving samples.

The devices could also find applications on Earth. Wheeled robots are now sometimes used for hazardous tasks such as finding land mines and exploring dangerous chemical-pollution sites and volcanic calderas. But the tetrahedra could do so in places the wheeled versions could never get to.

David Wettergreen, a robotics engineer at Carnegie Mellon University, says the Goddard design could be a useful adjunct to other rover designs. “You often want to specialize your mode of mobility for particular applications,” he said, and it is therefore useful to have different mechanisms available. But he adds that the computational challenges could be daunting: even working out different gaits for robots with four legs is a difficult task. “This could have huge potential, but it will also be a big challenge to figure out how to use that potential.”