[meteorite-list] Shoulder Motor Balks on Opportunity Rover's Robotic Arm

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 24 Apr 2008 11:16:33 -0700 (PDT)
Message-ID: <200804241816.LAA03262_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2008-068

Shoulder Motor Balks on Opportunity Rover's Robotic Arm
Jet Propulsion Laboratory
April 23, 2008

A small motor in the robotic arm of NASA's Mars Exploration Rover
Opportunity that began stalling occasionally more than two years ago has
become more troublesome recently.

Rover engineers at NASA's Jet Propulsion Laboratory, Pasadena, Calif.,
are diagnosing why the motor, one of five in the robotic arm, stalled on
April 14 after much less motion that day than in the case of several
earlier stalls. They are also examining whether the motor can be used
and assessing the impact on Opportunity's work if the motor were no
longer usable.

The motor controls sideways motion at the shoulder joint of the rover
robotic arm. Other motors provide up-and-down motion at the shoulder and
maneuverability at the elbow and wrist. A turret at the end of the arm
has four tools that the arm places in contact with rocks and soils to
study their composition and texture.

"Even under the worst-case scenario for this motor, Opportunity still
has the capability to do some contact science with the arm," said JPL's
John Callas, project manager for the twin rovers Opportunity and Spirit.
"The vehicle has quite a bit of versatility to continue the
high-priority investigations in Victoria Crater and back out on the
Meridiani plains after exiting the crater."

The performance of the motor in the past week is consistent with
increased resistance in the electrical circuit, such as from degrading
of wire in the winding, rather than a mechanical jam. Additional tests
are planned for checking whether the apparent resistance is localized or
intermittent.

Opportunity and Spirit landed on Mars in January 2004 to begin missions
originally planned for three months. They have continued operating for
more than four years, though each with some signs of aging.

Opportunity's balky shoulder motor began stalling occasionally in
November 2005. The motor could still be operated by applying increased
voltage. Engineers assessed it has an increased likelihood of becoming
unusable, however, so the team changed its standard procedures for
stowing and unstowing the arm.

Until then, on days when the arm would not be used, the team kept it
stowed, resting on a hook under the front of the rover deck. Motion of
the stall-prone shoulder motor is necessary to unstow the arm, so if the
motor were to become unusable with the arm in the stowed position, the
arm could not be deployed again. With diminished confidence in the balky
motor, the team began unstowing the arm at the end of each day's drive
rather than leaving it stowed overnight. This keeps the arm available
for use even if the motor then stops working.

This spring, Opportunity is crossing an inner slope of Victoria Crater
to reach the base of a cliff portion of the crater rim, a promontory
called "Cape Verde." On April 14, Opportunity was backing out of a sandy
patch encountered on the path toward Cape Verde from the area where the
rover descended into the crater. As usual, the commands included
unstowing the arm at the end of the day's short drive. The shoulder
motor barely got the arm unstowed before stalling.

"We'll hold off backing out of the sand until after we've completed the
diagnostic tests on the motor," Callas said. "The rover is stable and
safe in its current situation, and not under any urgency. So we will
take the time to act cautiously."

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Exploration Rover project for the NASA Science Mission
Directorate, Washington. Additional information about Spirit and
Opportunity is available online at http://www.nasa.gov/rovers.

------------------------------------------------------------------------

Guy Webster (818) 354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
Received on Thu 24 Apr 2008 02:16:33 PM PDT