Opportunity Sol 925 (Spirit Sol 946)

Well, the tests on the IDD have gone well, and yesterday they even did real work with it. Maybe it was just a transient failure, after all.

However, we're still nervous about it, mostly because we're in for pain if it fails while we're trying to stow. During the stow and unstow process, the IDD maneuvers itself through a number of tight spaces, and getting it back out can be a real bitch. And stowing contains several small joint-1 moves, so if we didn't just have a transient failure -- if we have a real, ongoing problem -- then it's possible we'll fault out when that happens.

Fortunately, we can do something a little more active than just cross our fingers. We can walk the IDD partway through the stow process manually, at least as much as is relatively easy to sequence -- and to undo, if we get stuck. That will either build our confidence that the real thing will succeed, or tell us we shouldn't bother trying the real thing.

Happily, Eric Baumgartner left behind excellent documentation, including the exact IDD joint angles we need in order to do this test. So we look those up, put the sequence together -- and get shot down. Well, not permanently, but the SOWG chair's call is that since our doing this would put thisol's MB observation at risk, we should punt. And I can't really disagree, since we have a chance to do it again tomorrow. So we put our nearly complete sequence on the shelf, point the next day's RPs to it, and move on with the rest of the day's IDD sequencing. At least it won't be work wasted.

[Next post: sol 954 (Opportunity sol 931), September 9.]


Opportunity Sol 921 (Spirit Sol 942)

Grim news from the Land of Opportunity today. The drive's off, because yestersol's IDD work on the trench failed. On the very first IDD move of the sol, we got a joint-1 stall at 80 ohms, which we haven't seen before. And the most likely interpretation of that is that the IDD has continued to degrade, and we're about to see a whole lot more failures. We might have lost the shoulder joint altogether. Jake Matijevic looks unhappy, which is never a good sign. "I'm concerned whether we'll ever be able to reliably stow again," he mutters to me sotto voce.

The only ray of hope is that the 80 ohms value isn't necessarily a magic number. It's a number we picked because it was supposed to work 90% of the time, or something like that. And if this is the first time it's failed in lots and lots of tries, maybe we just had some kind of bad luck, a purely transient failure due to ambient temperature or something. It has been particularly cold lately.

And Kirk Fleming makes a good point. "The first move of the day can be kind of sludgy," he notes as we're sitting in the fishbowl reviewing the data. "The stall doesn't seem to be resistance-related, or we wouldn't have hit the current limit. So either something else is wrong with the same actuator, or we just had a transient glitch."

Well, that is good news, and real reason for hope at last. Anyhow, we'll know more tomorrow than we do today. We're planning a test of the IDD in which we do a series of small shoulder-az motions, at increasing resistance values, until we get a success. That, of course, is hoping we do get a success, which is by no means assured at this point.

After a lot of discussion, we end up with a fairly simple scheme. We're going to repeat a test at three different times of day, so that we can test at different temperatures. Each test moves the IDD to the left and right with a rotor resistance of 80 ohms, then 90, then 100, the max we can command. If we succeed in moving it both directions at a certain resistance setting, we don't try the next-higher setting, since that's potentially dangerous.

And we'll take lots of pictures as we do it. Lots of pretty pictures. Which, we hope, will -- like the other data -- show nothing anomalous. That would leave us, perhaps, with a mystery. But given the alternatives at this point, it's a mystery I think I could live with.

[Next post: sol 946 (Opportunity sol 925), August 31.]


Opportunity Sol 919 (Spirit Sol 938)

I take a personal satisfaction in today's activity: trenching with a stuck wheel.

Sure, I'd rather be high-tailing it toward Victoria. In my absence, the team got us to Beagle and beyond. Now nothing stands in our way -- well, nothing except the science team, who feel there's some science work we really need to do first.


The good part is that we're already prepared to do what they want to do. A few weeks before we reached the annulus, I got to thinking about that big old unknown sand sheet, and what we'd want to do once we got there. Trenching was one thing I knew they'd want to do, and I knew we hadn't done it since Opportunity's RF wheel lost the ability to steer, which makes our usual trenching sequence unworkable. So I got Paolo, Jeff Biesiadecki, and (bless his heart) Rob Sullivan to go down to the testbed and work out how to trench all over again.

Squyres said -- very, very politely -- that this was a waste of time. But I had a feeling it would prove otherwise, and it looks like I was right and he was wrong.

Well, it had to happen eventually.

So that makes the sol easy, at least: the trenching routine's already fully worked out, and we just have to send it to the spacecraft.

But you knew it wasn't going to be that simple, right? We're actually finished with the CAM when I realize what's wrong. The new trenching sequence draws a long, shallow scrape with the RF wheel, then turns slightly and makes a second scrape overlapping the first, thus broadening the trench. Now, the turn between the two scrapes is performed in an unusual way. Rather than turning through an explicitly commanded angle, which causes the rover to turn until the IMU -- roughly, her internal compass -- says it's time to stop, the mechanism used in the sequence simply runs the wheels a certain number of revolutions.

When I change this to an explicit turn for simulation purposes, the error is clear: we're turning way, way too far.

I call Paolo and confirm the problem. They worked this sequence out in the testbed, and the number of wheel revolutions to use in the turn was calculated empirically. In the testbed -- in the particular soil material in use there, in Earth gravity, and so on -- this does the right thing. On Mars, it might do the right thing, or it might turn the rover way too far.


The change to the sequence is simple enough; we just have to replace the turn with an explicit one. I'm glad I caught this, but, boy, do I feel dumb for not catching it sooner. My first day back from England, and we're redelivering post-CAM.

Just like the old days, really.

[Next post: sol 942 (Opportunity sol 921), August 27.]