Spaceflight Insider

New driving algorithm helps protect Curiosity rover’s wheels

Jim Sharkey
July 4th, 2017
A "scarecrow" rover at NASA's JPL drives over a sensor while testing a new driving algorithm. Engineers created the algorithm to reduce wheel wear on the Mars Curiosity rover. Photo Credit: NASA/JPL-Caltech

A “scarecrow” rover at NASA’s JPL drives over a sensor while testing a new driving algorithm. Engineers created the algorithm to reduce wheel wear on the Mars Curiosity rover. Photo Credit: NASA / JPL-Caltech

The six wheels of NASA’s Curiosity Mars rover have experienced considerable wear and tear since the one-ton rover landed on Mars on August 6, 2012. However, a new algorithm is helping the rover drive more carefully over rocks on the Martian surface to reduce wheel wear.

The new software, called traction control, adjusts the speed of the rover’s wheels depending on the rocks it’s rolling over. The software was uploaded to Curiosity in March, following 18 months of testing at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

Curiosity's wheels have received considerable damage since the rover touched down on the surface of Mars in 2012. Photo Credit: NASA / JPL

Curiosity’s wheels have received considerable damage since the rover touched down on the surface of Mars in 2012. Photo Credit: NASA / JPL

The software was approved for use by mission managers on June 8, after extensive testing at JPL and multiple tests on Mars. The traction control software is currently on by default but can be turned off as needed, such as during scheduled wheel imaging when the Curiosity team assesses wheel wear.

Even before the wheels began to show visible signs of wear, engineers at JPL had started studying ways to reduce the damaging effects of the Martian surface.  All of the rover’s wheels turn at the same speed on level ground, but as a wheel goes over uneven terrain, the incline causes the wheels behind or in front of it to begin slipping.

Such changes in traction are particularly troublesome when going over pointed rocks that are embedded in the Martian surface. When this occurs, the wheels in front pull the trailing wheels into the rocks; the wheels behind push the leading wheels into rocks.

The treads on Curiosity’s  wheels, called grousers, are designed for climbing rocks. The spaces on the wheels in between the grousers are more at risk of cracks and punctures.

“If it’s a pointed rock, it’s more likely to penetrate the skin between the wheel grousers,” said Art Rankin of JPL, the test team lead for the traction control software. “The wheel wear has been [a] cause for concern, and although we estimate they have years of life still in them, we do want to reduce that wear whenever possible to extend the life of the wheels.”

The traction control software uses real-time data to adjust the speed of each wheel, thus reducing pressure from the rocks. The software measure changes to the rover’s suspension system to determine the contact points of each wheel. The algorithm then calculates the correct speed to avoid slippage and improve Curiosity’s traction.

To test the effectiveness of the software, the wheels were driven over a six-inch (15-centimeter) force torque sensor on level ground. According to Rankin, leading wheels experienced a 20 percent load reduction and middle wheels experience an 11 percent load reduction.

The traction control software will also help with the problem of wheelies. Sometimes a climbing wheel will keep rising, lifting off the surface of a rock until it is free-spinning. Wheelies increase the forces on the wheels that remain on the ground. When the traction control software detects a wheelie, it adjusts the speeds of the other wheels until the rising wheel is back into contact with the surface.

Video courtesy of NASA

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Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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