Eleven years ago this week (or very nearly), the Spirit rover was noodling around in Gusev Crater on Mars when it drove over a thin hard crust of soil and broke through into a layer of soft sand underneath. The rover was already a little bit hobbled (understandable, since Spirit was something like 2,000 days into what was originally planned as a 90-day mission), and after months of trying, it became clear that Spirit wasn’t likely to move again. Unable to reach a position where its solar panels could be tilted toward the sun, Spirit froze to death during the Martian winter.
This animation shows NASA's Spirit rover trying to drive through soft sand. Stuck in this location and unable to reach a position where its solar panels could be tilted toward the sun, Spirit lost power and its hardware froze during the Martian winter. Image Credit: NASA/JPL-Caltech
Larger rovers like Curiosity don’t have to worry about solar power, but getting stuck in soft surfaces is still a concern, since the options for getting a rover unstuck are limited—all you’ve really got to work with is the rover’s own mobility system.
In a paper published today in Science Robotics, researchers from Georgia Tech’s CRAB Lab led by Professor Daniel Goldman describe how they’ve worked with a NASA rover design to enable new mobility behaviors with actuated wheels that can avoid getting stuck. How do the wheels do that? By wiggling.
That full-size RP15 rover that you briefly saw in the video was a test bed for the Resource Prospector rover, which was going to do some exploring of potentially icy parts of Earth’s moon in 2022. Resource Prospector was canceled in 2018, and then some of what it was going to do was resurrected as part of the VIPER lunar rover. But VIPER’s wheel system is quite different. The RP15 prototype uses a four-wheel design, but the wheels are on a sort of actuated suspension system that allows them to move up and down and forward and backward as well as rotating around the vertical axis. All of these degrees of freedom mean that the wheels can do things that the Mars rovers, with their rocker bogie suspensions, just can’t.
No matter how many degrees of freedom your robot has, they’re not going to be all that useful unless you can get them to work in a coordinated manner toward a specific goal (I’m looking at you, snake robots). Georgia Tech’s Mini Rover is a scaled-down version of RP15 that uses the same wheel kinematics, allowing for testing of those degrees of freedom at smaller scale. The idea is finding the best way of moving a rover forward in granular terrain—poppy seeds in the scaled-down version. Effective gaits, as is turns out, are fairly complicated:
We first investigated an open-loop gait derived from tests of RP15’s crawling capabilities at JSC. Previous studies showed that various open-loop strategies for granular slope climbing were sufficient if the locomotor’s dynamics allowed it to repeatedly intrude into undisturbed media. We implemented this gait on the Mini Rover by cyclically sweeping rearward with three appendages while one appendage lifted to disengage from the medium and also spinning all four wheels at a constant rate of 2.1 rad/s. This gait is classified as a quadrupedal rotary sequence (RS) gait with regard to its foot placement, which cycles around the rover’s locomotion appendages.
While trying to drive with just the wheels spinning, the Mini Rover would just dig itself deeper into the poppy seeds, but the “quadrupedal rotary sequence (RS) gait” was able to consistently move the Mini Rover along, even if it only had three out of four wheel/leg appendages able to independently move.
The technique also works on most slopes, though very steep slopes (steep enough that the material the robot is trying to drive through will begin to avalanche downward if disturbed) require a modified gait:
To climb these steeper slopes in the laboratory, we developed a specialized hill-climbing gait that we refer to as “rear rotator pedaling”. Like the RS gait, the Mini Rover lifts and resets [alternate rear wheels]. Each wheel spins at 2.1 rad/s during the RRP gait, including the front wheels. These motions generate a periodic yaw oscillation of the rover. After spinning wheels for 30 s to embed the Mini Rover in the media, once the RRP gait initiates, then the rover initially slides backward until a sufficient mound develops behind its rear wheels, which prevents further rearward sliding and acts as a buffer the rear wheels can push on to move forward.
The RRP gait’s performance arises from a stable granular conveyance action that develops along the Mini Rover’s long axis. As the front wheels spin and shear the steep granular slope at the rover’s front, the material avalanches downhill and is transported between the rover’s front and rear wheels to an “intermediate mound” (IM) closer to the rear wheels’ reach. The yaw oscillation of the rover body pushes media in the IM rearward where the RRP gait’s sweeping reaches and pushes it into a rear mound to apply a net propulsive force. On steep slopes of bed > 20∘ , over time, the gait will dynamically reconfigure an initially insurmountable slope through avalanching and granular conveyance to create a terrain profile it can climb. This dynamic reconfiguration allows the Mini Rover to “swim” uphill in the agitated frictional fluid created by its wheel spin.
The researchers are well aware that the likelihood of NASA sending a rover somewhere with a surface that consists of poppy seeds is not very high, and that “planetary regolith is often very cohesive and polydisperse.” Yeah, that. Future research will mix up (literally and figuratively) the kinds of terrain that the Mini Rover will be tested in, and continue gait validation on NASA’s fuller-size RP15 rover.
Unfortunately, VIPER won’t benefit from the kinds of gaits being developed here, but other rovers, like the European Space Agency and Roscosmos’s Rosalind Franklin rover (which is scheduled to head to Mars in 2022) does have additional degrees of freedom in its mobility system. And longer term, the Georgia Tech researchers are hoping that what they learn will inform future rover designs, allowing them to explore more interesting places without stressing their creators out quite so much.