Sony designed its robotic dog Aibo to bark and roll over. The Jet Propulsion Laboratory designed its robotic rover not to roll over. That's harder than it sounds.
The challenge, according to Issa A.D. Nesnas, principal investigator for JPL on this NASA project, was to make Axel both symmetrical and simple. "We designed it to be symmetrical because it's working on rocky, steep slopes. If it falls off a ledge, or flips upside down, it can still operate." That's where its cylindrical design and paddlewheel feet come in.
But it also uses as minimalist a design as possible. "If you make it simple and small and lightweight enough, you can use multiple rovers when you go to high-risk areas. Because we're targeting high-risk areas where a lot of things could go wrong, having multiple rovers manages the risk for the overall mission." This means that none of the components of any one Axel require backup, which keeps the weight down.
The current Axel prototype has an onboard computer, power system, and wireless communications. It also has a unique arm, or link, that can be used as a tether for climbing, Nesnas says. JPL is testing it in what it calls its "Mars yard," an expansive area approximately 60 yards by 45 yards built to simulate the surface of Mars, including steep promontories. (You can view the Mars yard here, pictures of Axel here, and a video of Axel here.)
There are multiple challenges in designing Axel, Nesnas noted, not the least of which was getting all of its instrumentation to fit into a cylindrical shape. "Inside the cylinder is an electromechanical system, a lot of sensors, and cameras. But we also had to figure out how to the arm move all the way around the cylinder, how to keep it clean in a dusty environment, and how to keep it at the right temperature so that it wouldn't overheat."
At the same time, in order to make Axel easy to repair in a hostile environment, certain components had to be easily accessible. "We had a few instances where we had to repackage the components to make it easier to reach and repair them. We designed it with a lot of things in mind, but we learned a lot when we did a second iterative design process," Nesnas adds. Finally, from a mechatronics standpoint, the system integration - getting all the components to talk to each other - was a challenge.
Nesnas co-leads the project with Joel Burdick, a mechanical and bioengineering professor at Caltech, who supervises a handful of Caltech graduate and undergraduate students working on the rover system. The JPL-Caltech team demonstrated Axel at the Smithsonian Institution in Washington, D.C., celebrating NASA's 50th anniversary.
Nesnas stresses that Axel is still a research prototype, because the components are not yet flight-qualified, and there is no time frame for launching Axel into space. The rover may be used for search-and-rescue operations in the future, but while there are no partnerships in place for such development, Nesnas isn't averse to discussing them. "We're always interested in talking if something from the space program has terrestrial applications."