Wheel Trouble, ...down to 5 good wheels? |
Wheel Trouble, ...down to 5 good wheels? |
Mar 14 2006, 12:29 AM
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#1
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Junior Member Group: Members Posts: 90 Joined: 13-January 05 Member No.: 143 |
I hate to be the bearer of bad news, but I just heard that there is a serious problem with Spirit's right front drive actuator after yestersol's drive. (This is the actuator that had high currents in the past and led to the backward-driving strategy.) It will take awhile to evaluate the situation, but early speculation is grim.
In a worst-case scenario, they can still drag the bum wheel along, but don't expect another summit view. Best-case scenario would be a rock or debris jammed in the wheel. This is a heads-up, sorry I can't really give more info at this time. I still thought you all would like to know. |
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Guest_BruceMoomaw_* |
Mar 14 2006, 04:13 AM
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#2
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Guests |
Do you mean that the wheel won't turn even if its motor drive is disengaged, or just that the motor has failed?
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Mar 14 2006, 05:15 AM
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#3
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Senior Member Group: Members Posts: 1229 Joined: 24-December 05 From: The blue one in between the yellow and red ones. Member No.: 618 |
Do you mean that the wheel won't turn even if its motor drive is disengaged, or just that the motor has failed? Do we have six clutches on this thing? Can we disengage the motor and free-wheel? -------------------- My Grandpa goes to Mars every day and all I get are these lousy T-shirts!
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Mar 14 2006, 04:37 PM
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#4
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Junior Member Group: Members Posts: 90 Joined: 13-January 05 Member No.: 143 |
Do we have six clutches on this thing? Can we disengage the motor and free-wheel? The short answer: I'm 99% sure the answer is "no". The long answer: There are two things that resist turning of the wheel when the motor isn't energized: dynamic braking and static detents. Dynamic braking occurs from back-emf in the motor electrical circuit, and provides a resistive torque when the wheel is turned but no force when the wheel is stopped. Dynamic braking can be disengaged in the electrical system. The static detents, on the other hand, are magnets that cause the motor to preferentially stay at certain turn positions. These are necessary to keep the wheels from rolling after the rover comes to a stop, and cannot be disengaged. Because of the high gear ratio of the drive gearbox, it takes a hefty torque to the wheel from the outside to overcome the detent. Bottom line: assuming the motor stall can't be corrected, they will probably have to drive backwards dragging a stuck RF wheel. This is something they have done before in the West Spur area when they were afraid the RF motor was nearing end of life. Though at that time, I think they periodically turned the RF wheel a bit to drive over the buildup of dirt in front of the wheel. |
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Mar 15 2006, 12:49 AM
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#5
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Senior Member Group: Members Posts: 1636 Joined: 9-May 05 From: Lima, Peru Member No.: 385 |
The short answer: I'm 99% sure the answer is "no". The long answer: There are two things that resist turning of the wheel when the motor isn't energized: dynamic braking and static detents. Dynamic braking occurs from back-emf in the motor electrical circuit, and provides a resistive torque when the wheel is turned but no force when the wheel is stopped. Dynamic braking can be disengaged in the electrical system. The static detents, on the other hand, are magnets that cause the motor to preferentially stay at certain turn positions. These are necessary to keep the wheels from rolling after the rover comes to a stop, and cannot be disengaged. Because of the high gear ratio of the drive gearbox, it takes a hefty torque to the wheel from the outside to overcome the detent. Bottom line: assuming the motor stall can't be corrected, they will probably have to drive backwards dragging a stuck RF wheel. This is something they have done before in the West Spur area when they were afraid the RF motor was nearing end of life. Though at that time, I think they periodically turned the RF wheel a bit to drive over the buildup of dirt in front of the wheel. Would you post the rover wheel in graphical description? I still have no a good idea about its design. By now, I tought that there are four motors for motion: 2 in front and 2 in back and the motor axis is connected to the wheel. The others motor are for turning control which are connected at the top of every four motors: 2 front and 2 back. Isn't that correct? P.D. If that looks so, I think that design is the most elegant but not the most robust. Hope this would provide a good lesson for the next Mars rover : MSL (I am afraid that its wheel design is very similar to MER according to the very generic picture.) Rodolfo |
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