Right front and left rear wheels not operating as
commanded
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.