ROVER WHEELS: Monitoring changes over time, NOTE: Read back through the thread to avoid repeating misconceptions Options

[djellison]

or last as long as Opportunity has

Why would wheel wear have an impact on rover longevity?

[RoverDriver]

Rover longevity no, but mission longevity probably yes.

Paolo

[Gerald]

I'm wondering, whether a wheel, severely damaged along the stiffening ring, needs to be the end of the wheel or the driving.
Wouldn't it be possible to intentionally split the inner two thirds completely from the outer third by patiently fatitiguing the remaining cleats (e.g. by driving back and forth over an appropriate rock a sufficiently large number of times); then driving with the outer third of the wheel through the removed inner two thirds, again using Martian rocks as tools?
The journey could be continued with the outer third of the wheel.
Certainly a (crazy) challenge, but did anyone try?

[elakdawalla]

In testing, they have found that the inner 2/3 of the wheel does, in fact, rapidly shred off of the rest of the wheel once all the chevron-shaped grousers have broken through. They are currently testing driving on the remaining 1/3 of the wheel, which does work. The problem is that to get to the 1/3 wheel state you have to go through a state where the inner 2/3 flaps around while you're driving, endangering the cable bundle that runs along the top of the wheel strut. If a sharp edge of shredded wheel slices into that cable, not only will the wheel possibly become unusable, but the short could affect the motor controller -- which runs wheels, HGA, and mast. Needless to say, they will not take any action that would knowingly risk failure of the motor controller.

[Gerald]

My hope has been, that the risk could be handled by staying at a well-known location, driving forth and back just several centimeters, plus steering, such that cables remain in safe distance from wheel shreds, until this intermediate state is overcome.

[fredk]

Damage to cables aside, it's not clear how you could drive the outer 1/3 through the inner 2/3. To be able to do that, you'd need to slice the inner 2/3 open, so it could be at least partially "unwrapped" along its circumference. For example, you could slice between two odometry grousers. But to completely slice through the inner 2/3, you'd need to slice through the inner rim. Most damage is away from the inner rim.

Maybe you could target the inner rim for damage. But then it's not clear if you'd be able to unwrap the inner 2/3 since once it's complely loose you'd be very limited in the forces you could apply to it.

[Gerald]

That's probably the more tricky ("crazy") part.
An idea would be to use a rock as a tool to slip the 2/3 over the 1/3 ("driving through"). For this to work, the 2/3 needs to be stretched (assuming it cannot be sliced).
The acute angle and the relatively low coefficient of friction between the two wheel parts should help to amplify the torque of the wheel actuator.
The first step would be to find out, whether this experiment is physically possible respecting the material parameters of the alloy and the torque of the actuators.
If it turns out to be physically possible, it's necessary to find out, whether the experiment can be executed in a reliable way.

Since the wheels have been constructed to dampen the landing, and the actuators are rather strong, I'd consider the idea as physically possible, but anything beyond is guesswork, without experiments. That's why I've asked, whether there exists already some experimental experience.

[serpens]

Somehow I think the extremely competent mission engineers, having access to a wealth of empirical trials data and theoretical considerations will have this matter well in hand and will have considered all options. However the capacity for mitigation is pretty limited and the main value will be in lessons learned and the reminder for future designs that no plan survives contact with the enemy (the enemy in this case being the Martian terrain). However perhaps we should also remind ourselves that the MSL fulfilled the last outstanding mission success criteria when it lasted one year on Mars. Possibly we have all been spoiled by the amazing longevity and transit distance achieved by Opportunity. However Opportunity's achievement should also be considered in light of the nail biting near failure of Spirit in the first few days which was rectified only because some very smart people provided for a low data rate communications back door. This enabled the problem to be identified and rectified in both Spirit and Opportunity. All in all the MSL wheels have done pretty well.

[algorithm]

All in all the MSL wheels have done pretty well.

IMHO, that's a pretty low expectation. Personally I wouldn't bolt on a nuclear generator just to enable a rover to last the lifetime of a wheel, especially if that wheel did 'pretty well' to last two years.

[serpens]

I would think the use of a nuclear generator would be a function of the requirement for a stable power level and invulnerability to dust storms rather than longevity. Regardless, if any mission critical part of the MSL gave up the ghost tomorrow the mission would have to be classified as a success having fulfilled prime mission requirements. Still there will be some pretty interesting risk analysis being undertaken in considering the alternatives of driving over hidden valley sand or the wheel damaging environment associated with bypassing the valley.

[Don1]

We're years away from wheel disintegration at this point, and it might never happen if the rover is lucky with the terrain. What worries me more is what the project might do to preserve the wheels. Wheel damage is a slow, degenerative disease, but getting stuck in soft sand could stop a mission overnight. Or the rover might get slowed down so much that all the instruments have packed up before we have finished on Mt Sharp.

People praise Opportunity, but they overlook that two of the instruments have gone and their loss means the rover can no longer detect clay minerals, which is a real problem when you are trying to explore clay rich regions.

The reason for the nuclear generator was to allow a wider choice of landing sites, in particular southerly sites like Eberswalde. It also simplifies operations, and makes the rover immune to planet wide dust storms.

[jmknapp]

Is current to the wheel motors monitored, or possibly some other means (slip signature?) of determining when one of the rover wheels is likely pinned against a rock? If so, could the rover do a little sidestep of some kind?

[Gerald]

Yes. Here an excerpt of Emily's blog post:

By implementing a "smart controller" on the wheel current and allowing wheel rotation rates to vary intelligently in response to sensed conditions, they might be able to mitigate the damage.

[jmknapp]

That'll teach me not to fall behind on Emily's blog.

[nprev]

Gentle mod note: Guys, MER solar panel cleaning has been exhaustively discussed for years on many, many threads. Let's keep posts relevant to the topic at hand; thanks!

EDIT: Per mcaplinger's suggestion, new topic created here for solar-powered Mars rover issues.