3 Nov 2004
(Source: Jet Propulsion Laboratory)
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 Columbia 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.
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.