Mars Rover Spirit Down a Wheel

riflemann writes "NASA is reporting that two years into its 90-day mission, Spirit has lost one wheel and is now running on five wheels, dragging the broken wheel. With this reduced mobiity, the rover still needs to make its way to a slope where it can catch enough sun over the Martian winter to keep it operating. 'Even though the rovers are well past their original design life, they still have plenty of capability to conduct outstanding science on Mars.', says project leader Dr. John Callas."

It's not "dragging" the sixth wheel

[parc]

It's more accurate to say that the wheel is free-spinning. It isn't contributing to drive power, but it's not drawing any current, either. It can still steer, so it's not off at some odd angle.

Additionally, there's only been a couple days worth of data -- noone really knows why the motor stalled.

Re:It's not "dragging" the sixth wheel

[cmacb]

Doesn't really LOOK like it's free spinning:

http://marsrovers.jpl.nasa.gov/gallery/all/2/f/781 /2F195698779EFFAQ15P1210L0M1.JPG [nasa.gov]

Re:At first read, I get dissapointed

[Anonymous Coward]

That was in a TNG episode with La Forge.

Re:Still a tossup - Rovers dead before Vista relea

[rspress]

The computer in each Mars Exploration Rover runs with a 32-bit Rad 6000 microprocessor, a radiation-hardened version of the PowerPC chip. This chip is used in IBM's heavy duty computers and is the same family of chip in the pre-intel Macs, Xbox 360, GameCube and the Cell version will run the PlayStation 3.

The chips are fairly cheap and have lightning fast floating point calculations via alti-vec. They are also very easy to program for.

NASA claimed they would only work for 90 days due to the high iron content of martian soil. In 90 days the solar cells were supposed to be covered with magnetized iron dust and the cells would not get enough sun to charge them. That never happened. Considering the cold, dusty, unsheilded environment they are in it is amazing they have lasted over two years.

Re:Even if it can't make it up the slope...

[deong]

It's not that simple. The rovers are full of fairly sophisticated sensor packages, most of which can't handle the extremely low temperatures on the Martian surface. They need the batteries to basically, well, run the heater.

The principle investigator for the missions has written a book, "Roving Mars", that really is worth the read.

Re:It's not "dragging" the sixth wheel

[Tablizer]

They stopped using the wheel about a year ago for a while because it was having problems as if the lubricant was wearing off, and it indeed did *not* free-spin when power was not sent to it. They simply dragged it around by running the rover backward. They found it easier to control the rover by dragging the bum wheel rather than by pushing it. They only used the wheel for close-up control when rocks were being targeted. Eventually it started working properly for a while, and now won't turn at all. It does not appear they have a "free spin" mode. Dragging is it.

I beleive they have a video about the last time the wheel was left dragging. They did some test-bed simulations of an Earthly rover copy. Page down to the "Driving Uphill Backwards" portion, about half-way down the page:

http://marsrovers.jpl.nasa.gov/gallery/video/spiri t01.html [nasa.gov]
     

Re:Tire tracks, not water

[Dr. Eggman]

Not quite. Martian weather is quite likly to blow away the tracks before anything found them.

You can get your Martian weather forecast here: http://astro.sci.uop.edu/~harlow/weather/mars.html [uop.edu]

Re:Even if it can't make it up the slope...

[ScottMaxwell]

It's not that simple. The rovers are full of fairly sophisticated sensor packages, most of which can't handle the extremely low temperatures on the Martian surface. They need the batteries to basically, well, run the heater.

Bingo. Indeed, it's even worse than that: if you can't run the heaters, all of the electronics undergo more extreme thermal cycling. This causes components to contract, flex, break, etc. Several critical components -- e.g., the CPU -- have no redundancy; if one of those goes, the whole rover goes.

This failure is the most dangerous thing to happen to Spirit since the flash anomaly on sol 18, when we effectively lost contact entirely for several days. Frustratingly, we're within sight of a safe haven -- only about a football field away -- but we might not be able to get there. Some people on the team think that if we have to drag a wheel, we can't climb the slopes we need to climb to make it to safety. I would just hate for Spirit to go this way; it would be like dying of thirst within sight of water, and she deserves better. (On the other hand, one thing I've learned is this: never bet against the rovers.)

The principal investigator for the missions has written a book, "Roving Mars", that really is worth the read.

Agreed! And since Steve's such a great guy, I'll linkify [barnesandnoble.com] that. :-)

Also looks like it's coming out in paperback [barnesandnoble.com] soon.

Re:Still a tossup - Rovers dead before Vista relea

[yoprst]

If memory serves me well (and it often doesn't), Rad 6000 is something like $600000

Re:Failed brushes?

[dougmc]

The rover uses maxon's motors which also are used in artificial hearts, surgical tools, and underwater robots

Maxon [slashdot.org] makes lots of motors, both brushed and brushless. If you look at their web site, you'll notice that most of their new motors are brushless.

As for the list of applications you gave, I'm guessing that new designs of these products use mostly brushless motors now, if only for the reason that they last longer and the brushes aren't ground down to dust over time.

those aren't your mother's DC motors 8D

My mother doesn't need a Rascal yet. In any event, I doubt these motors are that different from what use use down here on Earth. Sure, they're probably built better, with better materials and such, but the general design and layout is probably very similar.

Re:Failed brushes?

[dougmc]

Brushless motors are more complex

Actually, they're simpler, since there's no brushes. Generally the permanent magnets are on the shaft that rotates, and the electromagnets are on the part that doesn't, with three wires coming out (and possibly five more for a sensored model, but the sensorless models are more popular now.) Ignoring the older sensored models, the brushless motors are signifigantly simpler than brushed motors.

and require an array of active electronics inside them to produce the AC and modulating magnetic field they need to operate.

Actually, the electronics are generally outside the motor, in an ESC (electronic speed control), but I'll admit that it doesn't matter where they are. Brushed motors use an ESC as well to control the speed, so you've got some electronic parts either way. You're right that a brushless ESC is more complex than a brushed ESC, but the difference isn't really that signifigant.

Most brushless motors are lower torque than their brushed counterparts. (majority, I know there will be exceptions)

Torque is a function of motor design. It really has little to do with brushed or brushless, and you can certainly make high torque brushless motors if desired. If you need a motor with more torque but the same power (and less speed, since power = torque * speed) you either design an appropriate motor, or adjust your gear ratio so the amount of torque your motor does provide is appropriate for your use.

Brushed motors are more mechanical in nature and suffer from the usual mechanical issues, but they are less prone to failure than brushless.

And I disagree completely. Brushes wear out. (So do bearings and bushings, so it's a race to see which one wears out first, but in my experience, it's usually the brushes.) And for anything where you control the speed of the motor, you'll have an ESC (electronic bits) that can fail, but as a general rule of thumb, electronic bits are more reliable than mechanical bits.

Also, traveling through space and landing on a planet that may not have a protective magnetic field, active (transistor based) electronics must be carefully protected against emi that can disable or damage them.

Even the brushed motors will have ESCs on devices like the Mars Rovers (since the alternatives suck for a robot) and so either way you'll have active electronics to deal with. Also, the Mars rovers aren't operating in space -- the atmosphere (thin as it is) and magnetic field of Mars do provide considerable protection (compared to a satellite or something that is in space) against things like ionizing radiation.

This stuff isn't rocket science. Even things like scooters [rascalscooters.com], Segways [segway.com] and electric cars use similar technology.

The technology of brushless was available when the rovers were designed, and I can't imagine NASA not seriously considering them.

I'm guessing that the AC who posted in this thread was right on -- that NASA used brushed motors because they've used them in the past and they worked fine then, so they'll work fine now -- when you're spending billions of dollars on things that can't be repaired in the field, you tend to stick with what's tried and true rather than what's 15% more efficient but not quite so well tested. I suspect that future rovers will have brushless motors, however.

Re:Still a tossup - Rovers dead before Vista relea

[Moridineas]

Uhm, this gets informative? The Rad6000 chip runs at 20mhz according to wikipedia and 25mhz according to other sources. The chip is based on EARLY power cpu designs (think early/mid 1990s), and most definitely does NOT include any sort of altivec technology.

No radiation hardened space suitable chips are "cheap." Expect to spend tens, if not hundreds of thousands of dollars per CPU.

That's not to say it's not a great chip for what it does, but come on...

Re:At first read, I get dissapointed

[Dominic_Mazzoni]

That's partially true, but another important aspect of it is that if they had planned on a five-year mission up-front, the budget would have been several times larger - and in fact the project might never have been funded at all. So they decided on a mission length long enough to get some interesting science done, but short enough to look cheap.

After 90 days, they went and asked for additional money. What's NASA going to do, stop running the rovers because they're over budget? Of course not. Unfortunately now they're eating into money that would have gone to other Mars missions. But it's still far more sensible to spend a dollar on the rovers already on Mars than on a future rover that may or may not make it to Mars.