On Campus: Steady and Sure

The creature crawls on the edge of a dormant volcano called Mauna Kea in Hawaii. As it rolls along, its chassis adjusts to the terrain, moving up and down to help the tires grip the black sandy surface. This beast resembles a bumper car on dirt-bike wheels, but it's no toy. Known as Scarab--because its hull is shaped like a beetle's body--this autonomous robot is a prototype for a lunar rover.

Onboard lasers scan the landscape and plot a map that looks like the simulated 3D topography inside a video game. The map will help Scarab find the best route down the steep hillside.

From a nearby base camp, associate research professor David Wettergreen and his team watch Scarab in action. He wants Scarab to descend the hill using a drainage ditch--the easiest way down the slope. But Wettergreen has to be patient as he observes the rover. It takes eight hours for Scarab to cover one kilometer.

It's critical that Scarab makes the right choice. NASA wants a lunar rover that can explore craters at the poles of the moon without human direction. If Scarab doesn't find the easiest path, Wettergreen and his team will have to rework the robot and its navigation software. Wettergreen and his team are studying how Scarab will climb slopes and drive through loose volcanic soil.

Scarab slowly turns and heads down the drainage ditch. Relief washes over Wettergreen and his colleagues. The rover made the right choice. As the robot performs test after test, Scarab capably avoids obstacles such as rocks. Instead of going around all of them, Scarab sometimes lifts up its body so the rocks slide underneath.

To support human exploration and missions to other planets, NASA intends to create a base on the moon. Before that can happen, the space agency needs vehicles that can explore parts of the moon that have never been seen by humans. Scientists think craters at the poles of the moon have frozen water in them and NASA wants rover that can drill and sample the dirt. That's why NASA scientists approached Wettergreen and Red Whittaker, Fredkin Professor of Robotics and director of the Field Robotics Center, who each have extensive experience creating rover prototypes.

Scarab is able to crawl in and out of craters with sides as steep as 29 degrees.  It can crawl over a surface until it arrives at a drill site. It then lowers its belly onto the soil to stabilize the rover as it drills into the surface and extracts a core sample of the material. On board Scarab is a drill developed by the Canadian Space Agency and an automated chemistry lab designed and built by NASA. After the drill extracts "regolith"--the loose combination of dirt that covers solid rock on the moon--it puts a sample in a crusher, which pulverizes it to a fine powder. Then it places the powder in an oven that heats it to 900-degrees Celsius, releasing all of the volatile materials as gasses and creating a sintered ash. From a chemical analysis of these gasses, Scarab's internal lab can determine how much of each chemical compound is in the ground. If there were water frozen below the surface of the moon, the rover's computer would be able to tell.

During testing in Hawaii, Scarab drilled five holes, each time taking anywhere from one to three hours. The processing of the rock took another eight to 20 hours.

Scarab's body was built by Fringe, one of the Carnegie Mellon student organizations that enter the annual Sweepstakes (or "buggy") races during spring carnival. It is made of carbon fiber and is shaped so that it won't snag obstacles and can support the weight of the robot when it lowers to the ground.

In October and November, Wettergreen and his team put Scarab through a battery of experiments on Mauna Kea, testing the robot's mobility and a new set of lunar wheels made by Michelin for exclusive use on the moon. When exposed to extremely cold temperatures--like those on Earth's moon--rubber tires will become brittle and crack. Scarab's special wheels were made of fiberglass and were designed to withstand low temperatures without compromising mobility.

It was the first time the wheels were tested outside of the lab and over long distances, Wettergreen says. However, as Scarab rolled over the ground again and again, the tread wore down and lost traction. To counteract this, Wettergreen and his team crafted "grousers"--new tread-like ridges of metal--to replace the worn-down tread on the wheels. These helped the wheels grip the soil, but they required more power to drive the rover. Without the grousers, the researchers discovered the lunar tires had 20 percent less traction but also used 50 percent less energy than rubber tires. Using less power is key, because it allows more of Scarab's energy to be devoted to other tasks.

"The object was to demonstrate the complete process of lunar prospecting," Wettergreen says. "We're developing models of vehicle performance that are calibrated well enough to predict how (such a rover) would work on the moon."
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Jason Togyer | 412-268-8721 | jt3y@cs.cmu.edu