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Wiggly ‘mini rover’ robot climbs every otherworldly mountain

19 May 2020

Built with multifunctional appendages able to spin wheels that can also wiggle and lift, researchers modelled the “Mini Rover” on a novel NASA rover design.

(Image: Georgia Tech)

A new robot called the “Mini Rover” has complex locomotion techniques robust enough to help it climb hills covered with granular material.

It may also avoid the risk of getting stuck on some remote planet or moon.

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, the new Mini Rover could be the right robot for the job.

Using a complex move the researchers dubbed “rear rotator pedalling,” it can climb a slope by using its unique design to combine paddling, walking, and wheel-spinning motions. The rover’s behaviours were modelled using a branch of physics known as terradynamics.

“When loose materials flow, that can create problems for robots moving across it,” says Dan Goldman, a Professor in the School of Physics at the Georgia Institute of Technology.

“This rover has enough degrees of freedom that it can get out of jams pretty effectively. By avalanching materials from the front wheels, it creates a localised 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 Centre 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 in-house 3D printers, the researchers collaborated with the Johnson Space Centre to recreate those capabilities in a scaled-down vehicle with four-wheeled appendages that 12 different motors drive.

“The rover was developed with a modular mechatronic architecture, commercially available components, and a minimal number of parts,” says Siddharth Shrivastava, an undergraduate student in Georgia Tech’s George W. Woodruff School of Mechanical Engineering.

“This enabled our team to use our robot as a robust laboratory tool and focus our efforts on exploring creative and interesting experiments without worrying about damaging the rover, service downtime, or hitting performance limitations.”

The rover’s broad range of movements allowed the research team to test and study many variations using granular drag force measurements and modified Resistive Force Theory.

The researchers also tested their experimental gaits on slopes designed to simulate planetary and lunar hills using a fluidised 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, we could test the principles of locomoting with various gaits in a controlled laboratory environment,” Karsai says. 

“In our tests, we primarily varied the gait, the locomotion medium, and the slope the robot had to climb. We quickly iterated over many gait strategies and terrain conditions to examine the phenomena that emerged.”

In the paper, the authors describe a gait that 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 in Goldman’s group that involved moving with legs or flippers, which had emphasised disturbing the granular surfaces as little as possible to avoid getting the robot stuck.

“In our previous studies of pure legged robots, modelled on animals, we had kind of figured out that the secret was to not make a mess,” says Goldman. “If you end up making too much of a mess with most robots, you end up just paddling and digging into the granular material. If you want fast locomotion, we found that you should try to keep the material as solid as possible by tweaking the parameters of motion.”

However, simple motions had proved problematic for Mars rovers, which got stuck in granular materials. Goldman says the gait discovered by Shrivastava, Karsai, and Ozkan-Aydin might be able to help future rovers avoid that fate.

“This combination of lifting and wheeling and paddling, if used properly, provides the ability to maintain some forward progress even if it is slow,” Goldman says. “Through our laboratory experiments, we have shown principles that could lead to improved robustness in planetary exploration – and even in challenging surfaces on our own planet.”

The researchers hope next to scale up the unusual gaits to larger robots and to explore the idea of studying robots and their localised environments together.

“We’d like to think about the locomotor and its environment as a single entity,” Goldman says. “There are certainly some interesting granular and soft matter physics issues to explore.”

Though the Mini Rover was designed to study lunar and planetary exploration, the lessons learned could also be applicable to terrestrial locomotion – an area of interest to the Army Research Laboratory, one of the project’s sponsors.

“This basic research is revealing exciting new approaches for locomotion in complex terrain,” says Samuel Stanton, Program Manager at the Army Research Office, an element of the US Army Combat Capabilities Development Command’s Army Research Laboratory.

“This could lead to platforms capable of intelligently transitioning between wheeled and legged modes of movement to maintain high operational tempo,” he says.

Video courtesy of Georgia Tech.

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