Status Report

NASA Mars Rover Status Report 3 November 2004

By SpaceRef Editor
November 3, 2004
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SPIRIT UPDATE: The engineering team is keeping Spirit moving –
sol 285-291, November 03, 2004

Spirit employed its full instrument suite on Sols 285 through 291 to
study “Uchben,” an interesting rock encountered on the way into the
Colombia Hills. The engineering team continued to diagnose and study
work-arounds for a problem with the steering brake relay. An anomaly
related to electric-circuit grounding came to light during this period
and is also being studied by the engineering team. Neither problem has
hampered Spirit’s daily operations. Spirit is otherwise healthy and
ready to continue its trek further into the Columbia Hills.

The engineering team has been studying recovery options for steering
brakes that apparently failed to release on two previous sols. On Spirit
and Opportunity, dynamic braking is accomplished using a relay switch to
place a short across the motor windings of an actuator that is not being
used. If that actuator starts to move unexpectedly, the motor acts as a
generator and the short provides an electrical load that slows the motor
down. The same principle is used to generate electrical energy for
hybrid cars when the brakes are applied. Thanks to forethought on the
part of the rover design team, it is possible to disable the dynamic
braking function using ground commands. Those commands deliberately and
safely blow a fuse that is in line with the brake relay circuit. The
absence of the braking function for the steering actuators in question
(right front and left rear steering) will not affect the accuracy of our
drives or the rover’s safety when we are stopped. Until this problem is
fully resolved, we will continue to drive with the right front and left
rear steering actuators disabled, using tank-like steering.

Regarding the grounding anomaly, the engineering team regularly receives
telemetry that tells them the voltage difference between “rover chassis”
and “power bus return”. The rover chassis is the conductive structure of
the rover akin to an automobile chassis. The power bus return is a
collection of wires designed to carry current back to the rover power
source (battery or solar array). Ideally, all rover current flows in a
loop from the battery or solar array, returning by way of the power bus
return wires. No current is supposed to flow in the rover chassis
though, in reality, some leakage paths exist that allow current to
return by way of the rover chassis. When these currents flow across the
circuitry that separates the rover chassis and power bus return, they
create a small voltage that is measured and reported in telemetry. Until
sol 287, the reported voltage was typically in the range of 0.6 to 0.8
volts. On Sol 287, that voltage dropped to 0 volts. The 0 volt reading
could indicate that there is a problem with the measurement circuit, or
it could indicate that power bus return and rover chassis are now
shorted (making direct contact). The rover can operate when the chassis
and power bus return are shorted together or when they are separated
from each other by electrical circuitry. In the shorted case, however,
the rover is more susceptible to permanent damage if another short
occurs somewhere else. Engineers are looking at when the short
indication occurred for clues about its possible root cause.

Sol specifics:

On 285, Spirit continued systematic atmospheric observations on this and
all sols during this period using the miniature thermal emission
spectrometer and panoramic camera. The rock abrasion tool was employed
to drill a shallow hole at “Koolik,” a location on the rock Uchben.

On sol 286, Spirit took microscopic images of the Koolik rock abrasion
tool hole and placed the alpha particle X-ray spectrometer on Koolik for
an overnight observation. The alpha particle X-ray spectrometer works
best when cold.

During sols 287 through 289, Spirit placed the Mossbauer spectrometer on
the Koolik rock abrasion tool hole for several observations over the
Earth weekend. The Mossbauer spectrometer radiation source has weakened
significantly since landing, through normal decay, so longer integration
times are now required to get acceptable data.

On sol 290, Spirit performed tests to diagnose the root cause of the
indication that steering brakes had failed to release, but the tests
were inconclusive. Spirit then used the rock abrasion tool to brush
“Chiikbes,” another location on Uchben. Spirit placed the alpha particle
X-ray spectrometer on Chiikbes for an overnight observation.

On sol 291, which ended on Oct. 28, Spirit took microscopic images of
the Chiikbes brush site, and then placed the Mossbauer spectrometer on
Koolik to improve the data from that location.

SpaceRef staff editor.