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Extricating Spirit, Digging out from Troy
Keatah
post Dec 9 2009, 07:16 PM
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Perhaps this is the correct place to post this or not.. (moderators??)

Why didn't the mer design team use brushless motors? Most certainly the tech has been developed for the past 50 years and would prove much more reliable imho. Was it a cost issue? Weight/size? Power output/efficiency?

With 2 dead wheels on the right side I don't see how we can have a mobile rover, let alone escape from the trap.
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djellison
post Dec 9 2009, 08:06 PM
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Remember - they were only specified for 90 sols of driving, approx 600-1000 metres.

They have both done many many times that. So it's only fair to say that the motors have far outperformed their requirements.

Brushless are a newer technology, a more complex technology, and are heavier

http://starsys.spacedev.com/starsysproduct...rticle.asp?id=1

http://trs-new.jpl.nasa.gov/dspace/handle/2014/38930

For what was required - brushed was the proven choice.
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marsophile
post Dec 9 2009, 08:12 PM
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QUOTE (Keatah @ Dec 9 2009, 12:16 PM) *
With 2 dead wheels on the right side I don't see how we can have a mobile rover, let alone escape from the trap.


I believe one of the "plan Bs" (or maybe "plan Z" for this one) is to try energizing the right front wheel again in the hope that it might somehow have started working again. They have said they are reluctant to try this because of the possibility that it is shorted, in which case energizing it might cause damage to other driving circuits. However, if the RR wheel is truly gone, there might be little to lose in trying that at least.
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Tesheiner
post Dec 9 2009, 08:34 PM
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QUOTE (marsophile @ Dec 9 2009, 09:12 PM) *
However, if the RR wheel is truly gone, there might be little to lose in trying that at least.

The problem is "what would be damaged if they try to energize the RF wheel?"
Would it be limited to the RF wheel control circuitry or would it extend to other areas like the IDD? If it's the former I would concur with you but I fear it is the latter and in this case we would lose almost all science capability and not only the driving.
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djellison
post Dec 9 2009, 08:38 PM
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They can't move both IDD and wheels at the same time - I think they may be tied on the same power distro. board.
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marsophile
post Dec 10 2009, 04:44 PM
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Why is it that energizing the RR wheel in the current diagnostics does not carry the same risk of propagating a short as would energizing the RF wheel? What was the basis for suspecting a short in the RF wheel and why does that not apply to the RR wheel?
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centsworth_II
post Dec 10 2009, 05:15 PM
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QUOTE (marsophile @ Dec 10 2009, 11:44 AM) *
....What was the basis for suspecting a short in the RF wheel...
Elevated current. I don't know if any elevated current has been seen in the RR.

Edit: Actually, from this report, it looks like the RR may have the opposite of a short:
"The resistance tests indicate anomalously high resistance in the motor winding..."
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James Sorenson
post Dec 10 2009, 06:37 PM
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Since the RR wheel is submerged completely, have they thought of possible soil particles that may have found its way into the gearbox itself? Does the resistance test's show that signature?
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marsophile
post Dec 10 2009, 08:24 PM
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QUOTE (centsworth_II @ Dec 10 2009, 10:15 AM) *
from this report, ... anomalously high resistance in the motor winding..."[/i]


The report says: "a curious transition from anomalously low resistance to high resistance was observed very briefly..." Doesn't that suggest an initial short followed by a broken circuit?
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centsworth_II
post Dec 10 2009, 10:47 PM
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QUOTE (marsophile @ Dec 10 2009, 03:24 PM) *
...an initial short followed by a broken circuit?
If so, there may be no further risk of short circuit. But I don't know what all the data suggests.
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briv1016
post Dec 10 2009, 10:52 PM
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New update on the Free Spirit site.

http://www.jpl.nasa.gov/freespirit/index.cfm


Two interesting quotes:

"The plan ahead, still being developed, will likely include more rotor resistance tests, an attempt to apply higher voltage to the right-rear wheel to see if any movement will occur, and a check of the right-front wheel to confirm its status and to see if it may offer insight into the right-rear wheel's condition."

"Because of the current rover tilt, the environmental conditions and dust accumulation on the solar arrays, Spirit is at risk of inadequate power for surviving through the next southern Mars winter, which reaches solstice on May 13, 2009."
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Mixer
post Dec 10 2009, 11:18 PM
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QUOTE (briv1016 @ Dec 11 2009, 09:52 AM) *
"Because of the current rover tilt, the environmental conditions and dust accumulation on the solar arrays, Spirit is at risk of inadequate power for surviving through the next southern Mars winter, which reaches solstice on May 13, 2009."


One presumes they mean 2010 wink.gif
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fredk
post Dec 10 2009, 11:30 PM
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The check of the RF wheel definitely sounds interesting. I wonder if that means an attempt to drive it.

We'd heard about the danger of the coming winter, but not this bit:
QUOTE
Even if extrication is not possible, some limited rover motion may be able to improve rover tilt and increase the chance of winter survival.
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BrianL
post Dec 11 2009, 04:28 AM
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Given the current position, how would that be possible other than to intentionally dig the front in even deeper to get more northerly tilt? Perhaps the statement might better read, once we determine that extrication is impossible, some limited rover motion....

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Stu
post Dec 12 2009, 11:48 AM
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One of the new hazcam images suggests NASA has called in extra help as it attempts to free Spirit...

Attached Image


smile.gif


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