The rolling hills of Mars or the Moon are a long way from the nearest tow truck. That’s why the next generation of exploration rovers will need to be good at climbing hills covered with loose material and avoiding entrapment on soft granular surfaces.
Built with wheeled appendages that can be lifted and wheels able to wiggle, a new robot called the “Mini Rover” uses complex locomotion techniques robust enough to help it climb hills covered with such granular material — and avoid the risk of getting ignominiously stuck on some remote planet or moon.
Using a complex move called “rear rotator pedaling,” the robot can climb a slope by using its unique design to combine paddling, walking, and wheel spinning motions. The rover’s behaviors were modeled using a branch of physics known as terradynamics.
When loose materials flow, that can create problems for robots moving across it. By avalanching materials from the front wheels, it creates a localized fluid hill for the back wheels that is not as steep as the real slope. The rover is always self-generating and self-organizing a good hill for itself.
A robot built by NASA’s Johnson Space Center (JSC) pioneered the ability to spin its wheels, sweep the surface with those wheels, and lift each of its wheeled appendages where necessary, creating a broad range of potential motions. Using 3D printers, researchers collaborated with JSC to re-create those capabilities in a scaled-down vehicle with four wheeled appendages driven by 12 different motors.
The rover was developed with a modular mechatronic architecture, commercially available components, and a minimal number of parts, enabling the robot to be used as a robust laboratory tool without concern about damaging the rover, service downtime, or hitting performance limitations. The rover’s broad range of movements gave the research team an opportunity to test many variations that were studied using granular drag force measurements and modified Resistive Force Theory. Testing began with the gaits explored by the NASA RP15 robot and were able to experiment with locomotion schemes that could not have been tested on a full-size rover.
The researchers also tested their experimental gaits on slopes designed to simulate planetary and lunar hills using a fluidized bed system known as SCATTER (Systematic Creation of Arbitrary Terrain and Testing of Exploratory Robots) that could be tilted to evaluate the role of controlling the granular substrate.
By creating a small robot with capabilities similar to the RP15 rover, researchers could test the principles of locomoting with various gaits in a controlled laboratory environment. In the tests, researchers primarily varied the gait, locomotion medium, and slope the robot had to climb and iterated over many gait strategies and terrain conditions to examine the phenomena that emerged.
One gait allowed the rover to climb a steep slope with the front wheels stirring up the granular material — poppy seeds for the lab testing — and pushing them back toward the rear wheels. The rear wheels wiggled from side to side, lifting and spinning to create a motion that resembles paddling in water. The material pushed to the back wheels effectively changed the slope the rear wheels had to climb, allowing the rover to make steady progress up a hill that might have stopped a simple wheeled robot.
The experiments provided a variation on earlier robophysics work that involved moving with legs or flippers, which emphasized disturbing the granular surfaces as little as possible to avoid getting the robot stuck.
In the researchers’ previous studies of pure legged robots modeled on animals, they determined the secret was to not make a mess, which causes the robot to paddle and dig into the granular material. For fast locomotion, the material has to be kept as solid as possible by tweaking the parameters of motion. But simple motions had proved problematic for Mars rovers, which got stuck in granular materials.
The combination of lifting, wheeling, and paddling, if used properly, provides the ability to maintain some forward progress, even if it is slow. Testing showed principles that could lead to improved robustness in planetary exploration — and even in challenging sur faces on Earth. The researchers hope next to scale up the unusual gaits to larger robots and to explore the idea of studying robots and their localized environments together.