Engineers on NASA's Mars Exploration Rover team are
investigating possible causes and remedies for a problem
affecting the steering on Spirit.
The relay for steering actuators on Spirit's right-front
and left-rear wheels did not operate as commanded on Oct. 1.
Each of the front and rear wheels on the rover has a steering
actuator, or motor, that adjusts the direction in which the
wheels are headed independently from the motor that makes the
wheels roll. When the actuators are not in use, electric
relays are closed and the motor acts as a brake to prevent
unintended changes in direction.
Engineers received results from Spirit today from a first
set of diagnostic tests on the relay. "We are interpreting
the data and planning additional tests," said Rick Welch,
rover mission manager at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "We hope to determine the best work-around
if the problem does persist."
Spirit and its twin, Opportunity, successfully completed
their three-month primary missions in April and five-month
mission extensions in September. They began second extensions
of their missions on Oct. 1. Spirit has driven more than 3.6
kilometers (2.2 miles), six times the distance set as a goal
for mission success. It is climbing into uplands called the
"Columbia Hills."
JPL's Jim Erickson, rover project manager, said, "If we do
not identify other remedies, the brakes could be released by
a command to blow the fuse controlling the relay, though that
would make those two brakes unavailable for the rest of the
mission." Without the steering-actuator brakes, small bumps
or dips that a wheel hits during a drive might twist the
wheel away from the intended drive direction.
"If we do need to disable the brakes, errors in drive direction
could increase. However, the errors might be minimized by
continuing to use the brakes on the left-front and right-rear
wheels, by driving in smaller segments, and by adding a
software patch to reset the direction periodically during a
drive," Erickson said. Engineers believe the steering-brake
issue is not related to excessive friction detected during
the summer in the drive motor for Spirit's right-front wheel,
because the steering actuator is a different motor.
Meanwhile, the team continues to use Spirit's robotic arm and
camera mast to study rocks and soils around the rover, without
moving the vehicle until the cause of the anomaly is understood
and corrective measures can be implemented.
JPL, a division of the California Institute of Technology in
Pasadena, manages the Mars Exploration Rover project for NASA's
Science Mission Directorate, Washington.
Additional information about the project is available from JPL at
http://marsrovers.jpl.nasa.gov/
and from Cornell University, Ithaca, N.Y., at
http://athena.cornell.edu .
