Rovers, no matter what world they're designed to explore, tend to be designed like little cars, equipped with wheels that spin on fixed axles. But that can leave the vehicles vulnerable to getting stuck, as Spirit infamously did on Mars.
That's why engineers are exploring new ways for rovers to move on the moon, Mars or other planetary bodies. One team doing just that is Daniel Goldman's lab at Georgia Tech in Atlanta, which presented recent experiments on robot movement at the March meeting of the American Physical Society in Boston. The researchers are looking for ways to prevent future rovers from meeting the same fate as Opportunity's twin, Spirit, which lost contact with Earth when it became stuck in a sand trap.
"You can see that the wheel kept trying to rotate and paddle and sweep, and no matter what it did, it wasn't able to free itself," Siddharth Shrivastava, an undergraduate student at Georgia Tech, said of Spirit's downfall during a news conference held at the meeting.
Similar problems may pop up on the moon as well, particularly near the poles, which is where scientists have spotted water ice. That's an appealing target for space travelers looking to harvest drinkable water or rocket fuel on the moon, but dust and rock in the moon's surface at the poles appears to be less consolidated than it is near where the Apollo missions landed. That could mean rovers looking for water could get stuck, just as Spirit did.
So, the team tested new gaits for a proposed lunar-resource prospector, including one in which the rover can pick up each leg and swing it forward, like a human step. "When it encounters a risky substrate, it can initiate this walking pattern, in addition to the wheeled motion to get across it, to kind of paddle across it," Shrivastava said. The scientists ran experiments using a mock-up of the rover in a test bed they could tilt, allowing them to analyze all of the factors involved.
"In the most trivial case, the rover will roll, it will get stuck, and after it's stuck, it will add this legged motion in addition to the rotational motion of its wheels," Shrivastava said. "This gait works pretty well, and it's independent of how deep the rover is." That said, wheeled rovers aren't going anywhere: The team has found that the combination of spinning wheels and a walking motion is more effective than just walking.
At higher inclines, rovers can become unstable and topple over. The team has found that in these cases, too, a combination of walking and spinning can help get a rover moving. That pair of motions causes mounds to form between the rover's front and rear wheels, and those mounds give the front of the rover more traction on the slope, letting it pull itself forward.
That's not without a cost, however: The rover is actively changing the environment it's exploring. "Essentially, you're intentionally terraforming in order to locomote," Shrivastava said, "which is quite contrary to what Spirit and previous rovers have done, where they're intentionally trying not to model their terrain."
Despite their exotic aspirations, the test rovers practice their moves in tanks of a mundane substitute for lunar or Martian regolith: buckets' worth of poppy seeds. Without bagels involved, poppy seeds are the right size and have an irregular structure that mimics extraterrestrial regolith, the team said. And it's safer than other options, which can get into the air and damage rovers.
"These poppy seeds will crush well," Andras Karsai, a doctoral student in the lab, said during the news conference. "Regular sand made of quartz, if that gets stuck between the fine gears of your delicate robot, that can scratch it up and cause damage. However, poppy seeds are much softer … they'll just crush down to a little fine powder usually."
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