Cliff-hangers: The all-terrain Mars rover

Cliffbot doing off-road tests
Cliffbot doing off-road tests (Image credit:JPL/NASA)

In a first-of-its-kind test, a JPL team of robotics experts recently began off-road exploration into how an ideal Mars rover might handle the bumps and cliffs expected on Mars. Maneuvering on sandy, natural terrain presents many new and high-risk engineering challenges.

According to Dr. Paul Schenker, of the Jet Propulsion Laboratory’s Robotics Group, the motivation in rethinking ways of roving the Red Planet follows from ambitious goals: many attractive science targets likely call for ruggedness beyond the reach of current rover designs.

"The objective of this work at large," says Dr. Schenker, "is simply to provide new rover capabilities–the underlying sensing and controls and overarching system concepts/architectures–by which surface mobility can reach into increasing high risk access areas with high value science." One such future candidate will likely be of keen astrobiological interest, mainly the sandy soils that may have once harbored ancient oceans.

Bolder with Boulders

One of the most intriguing prototypes demonstrated recently turns next generation rovers into climbing machines. For some tests, the terrain’s steepness tilts beyond a 50-degree slope. Where previous rovers slide or slip, the novel designs maneuver the steep ascents. With tires smaller than the rocks they are expected to drive over, the rover wheels can also act as climbers and overcome obstacles bigger than their wheel diameter. Recent JPL rover development can roll over rocks up to one and a half times its wheel diameter. Simpler four wheeled designs (simple angle arm rockers) can handle about 1.0 times wheel diameter. In analogy to a standard Earth-bound pickup truck, with 3-foot wheel diameters, the vehicle must drive over and not around a hip-high boulder.

Teamwork Meets the Grade

If one such all-terrain rover meets the challenge, why not employ many of them cooperating so that a kind of small mountain ascent team makes the grade? According to Dr. Schenker, "Two rover system prototypes and underlying sensing and control technologies have been developed by our group. The first of these systems is "All Terrain Explorer (ATE)" and the second is the "Cliff-bot" team. The first is a single rover; the second is a team of three cooperating robots".

The cliff-hanger arm in action showing a safety line tether
The cliff-hanger arm in action showing a safety line tether.
Image credit: NASA/JPL

Look-ahead to Adjust Rover Stance

"The ATE," says Dr. Schenker, "is based in the concept of a rover that is ‘behaviorally’ adaptive to its changing terrain: the rover looks several meters forward with its stereo visual sensors, estimates the shape of the terrain, analyzes the effect terrain slope/tilt will have on gravity derived forces/traction, and alters stance and center of gravity to provide better balance. …We have successfully extended the stable range of rover operations from 15 to 50 degree slopes with these techniques, operating in outdoor, natural and sandy terrain."

Cliffbot Expert Assistance

An often-used analogy for thinking about getting around in tight places is how two people carrying a ladder must share knowledge and look-ahead as they follow an obstacle course or maze. The general problem is difficult if the ladder-carriers don’t coordinate their paths. The ladder becomes a kind of constraint on what bends can be traversed, and the teamwork relies on communication. In the case of the JPL "Cliff-bot" design, three robots advise each other using networked signals. This climbing team of rovers does more than just share knowledge about the terrain it can also connect physically with a tether or lifeline.

Asking for a Lifeline

For the Cliff-bots, "there is a tethered robot that actively descends / ascends a cliff of up to 75 degrees, actively assisted by two others that reel out-in the tethers at the cliff edge above. There is control / communications network amongst all three robots which allows them to constantly be aware of mutual directions and forces being experienced during the cliff-bot descent, as well as anomalies such as tether interference and hang-up. It is a system very much like two expert assistants working with a skilled climber below, each having a clear model in mind of what is to be done…There is no single control or manager of the climb; rather, it is a shared expertise and goal." In practice, the cliff-bot idea embraces anchors and a cooperative climbing method.

What’s Next

Given that the surface area of Mars equals the landmass of Earth (minus the terrestrial oceans), the need for exploration and mobility remains an important feature of most Mars’ design concepts. The exciting and image-rich galleries now being returned from the Mars Odyssey increase the depth of terrain knowledge and also provide guideposts for likely landing targets for taking the off-road tests to the Red Planet itself.

Dr. Terry Huntsberger of JPL is the current task manager for the ATE Project. Team members are available here