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No Driving For Spirit On Sol 38, Minor Glitch
DavidVicari
post Feb 11 2004, 09:12 PM
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I heard on the news that Spirit was unable to drive today because of a pointing error with the High Gain Antenna. Turns out it was too cold in the morning because it was in the shadow of the Pancam mast.

They use hard stops to calibrate the HGA position and they know they hit the hard stops when the current reaches a certain limit. The limit was set too low for the extreme cold. Very minor problem and they got the HGA to work perfectly later in the day, but the day was wasted troubleshooting.
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Rupertg
post Feb 11 2004, 11:05 PM
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A bit worrying that this happened just after the first autonomous drive - it's not a problem with the AE-35 unit, is it?

Rupert
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djellison
post Feb 11 2004, 11:08 PM
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No - it seems they understand it very well, it's just not something they expected to happen.

Doug
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Rupertg
post Feb 11 2004, 11:17 PM
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Yes, I read the mission report - something like the motors were in shadow so the heaters didn't get them warm enough to give them a reasonable start current, so the computer thought they were stalled at the end stop. Entirely reasonable, although it's a shame to lose a day to what looks like a tolerance calculation mishap!

My reference was to a previous mission where an autonomous system apparently misdiagnosed an aerial pointing subsystem fault, losing contact with Earth and requiring some fairly advanced faultfinding to rectify. Stanley Kubrick did rather a good documentary about it.

Rupert
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DavidVicari
post Feb 11 2004, 11:27 PM
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My guess is the thermal people just made a mistake and forgot to consider the shadow. I'm sure the thermal guys will learn from this and will be more careful in the future.

There are dozens of heaters on the rover for the motors. There isn't enough power to run them all, so they only run heaters before they are about to use a device. I bet they did calculations on how early they had to turn the heater on to get the HGA warm enough to move. However, they likely just used previous data when the sun was on it. So the heaters didn't have enough time to get it warm. From previous press conferences, I know they consider shadows in their models of the solar power system.

I wonder if they would consider turning the vehicle in place at the end of every day so that the PMA shadow doesn't land on the HGA. The drive to the crater is probably going to take about ten days and is going to be in the same orientation every day.
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