Machine Design

Motors bring Mars mission to life

Small, efficient motors help NASA search for signs of life on the Red planet.

Unlike previous Martian rovers, new rovers dubbed Spirit and Opportunity carry a variety of scientific instruments used for analyzing rocks.
Maxon's custom engineering and design work included providing a protective fiberglass bandage over motor windings for additional structural support.

This January two new NASA rovers -- Spirit and Opportunity -- will reach Mars and examine rocks for signs of water and ancient life. Many rover functions depend on one of 78 motors aboard that come from Maxon Precision Motors, Burlingame, Calif.

The rovers are bigger than previous Martian explorers such as the Sojourner which was about 2 ft long and weighed about 22 lb. The two new rovers are 4.9-ft high X 7.5-ft wide X 5.2-ft long, and each weighs 384 lb.

NASA engineers wanted better position feedback than they had with Sojourner. One of the most important features of the customized motors is the use of magnetorestrictive encoders. They're the same diameter as the motors with each multipole encoder adding less than 5 mm to the length of the motor. For Sojourner, NASA attempted to build its own encoders, but the devices required a lot of machining and design work. The encoders Maxon supplied are much smaller and weigh less than the encoders NASA built. They give each motor at least 32 counts/rev of position feedback.

The heavier rovers can carry more instruments for conducting experiments. The instruments include four cameras, three spectrometers, an imager, a rock abrasion tool, and magnetic arrays. The rovers can carry the instruments longer distances and maneuver over larger objects compared to their predecessors.

The new rovers used RE 20 motors to get more power and force in a much shorter package than RE 16s. And they have only a slightly bigger diameter, so they are somewhat easier to fit into the small spaces.

Maxon says it got the rover job because it can do custom work per NASA specifications. Changes included motors with special ball bearings and lubricants, special brush material, a fiberglass structural support, a special circuit board, and encoders.

The ironless core motors have high power density which is important for weight concerns. The RE-25 ironless core motors weigh 130 gm and the RE 20 motors weigh slightly less than 60 gm. A patented rhombic moving coil design provides long life, low electrical noise, fast acceleration, and high efficiency. The ironless rotor allows for zero cogging and simple accurate control.

The operating temperature range was also important. Surface temperatures on Mars average -64°F (-53°C), while temperatures can vary from -199°F (-128°C) during polar nights to 80°F (27°C) at the equator during midday at the closest point in orbit to the Sun. The Maxon motors have a temperature range from -80 to 204°C. The risk for damage is not the cold in itself, but the accumulative effects of temperature cycling that can cause fatigue and cracking failures.

A few of the motors reside in systems that support the mission. For example, 16 of them operate air bag retraction systems on the lander. After traversing more than 300 million miles and landing on air bags, the rovers unfold their solar-array panels and roam the planet surface. The solar-array panels unfold using five RE20 motors. Two more RE 20 motors unfold the rover from its flight configuration by deploying the front wheels. Another two RE 20s on the rocker-bogie mobility system assist the front-wheel deployment. All six of the rover wheels use the larger RE 25 motors. Four additional RE 25 motors operate the steering and a fifth handles driving. All remaining motors are the smaller RE 20 motors, which power the rover's science and geological tools.

Five small motors move a robotic arm called the instrument deployment device used for maneuvering a rock abrasion tool, an alpha particle spectrometer, a Mossbauer spectrometer, and a microscopic imager. The abrasion device uses an additional two motors for drilling into rocks and obtaining samples for analysis.

The thermal emission spectrometer sees infrared radiation, which helps scientists determine from afar the mineral composition of Martian surface features. An RE 20 motor also points the instrument upward to make the first-ever high-resolution temperature profiles through the Martian atmosphere's boundary layer.

The geological instruments are aided by a rock abrasion tool that uses a grinding wheel for exposing rock patches approximately two inches in diameter. The grinding wheel motor rotates at 10,000 rpm.

Three motors sit atop a 5-ft-tall camera mast to move panoramic and navigation cameras as well as a mini-thermal emission spectrometer. The panoramic camera images provide angular resolution more than three times higher than that of the Mars Pathfinder cameras. The extra detail will help scientists pick rocks and solid formations to analyze and find features carved by ancient waterways.

For more information on on the Mars Rover, visit
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