Opportunity Sol 706 (Spirit Sol 727)

Opportunity's start time is sliding later, thanks to the Earth-Mars day-length difference, but it's not late enough yet that I can actually come in to work any earlier -- between 9:30 and 11:30, there's no parking to be had.

As a result, I'm in a couple of hours before we actually start, and this gives me time to check out Ashitey's meeting with Erickson, Callas, Eric Baumgartner, Joe Melko, and so on, about the current state of the IDD investigation. The first question is whether we should finish the mini-Martian-tai-chi (our cute name for the recalibration activity we were doing) before stowing. Nobody wants to bother; they just want to get the damn arm stowed so we can drive. So much for that.

Next question: is it OK to increase the rotor resistance for the duration of the stow? This resistance value is the thing that lets us work around the broken motor winding, and which had been preventing stalls until this weekend. Eric's take on this, which mirrors email he wrote over the weekend, is that this will be fine. They'd worked out the present value partly empirically, and everyone's willing to believe that we simply needed a little more margin in that number.

What else do we need to do to get ready to drive? We won't be driving today -- we're just going to stow the IDD in its new stow configuration -- but it's what everyone's gearing up for. First, we have to make sure we're not driving over anything too big. Ashitey negotiates a slight relaxation of the definition of "too big," but we're still left with this rule: the front and middle wheels can't go over anything bigger than 3cm; the rear wheels, 4cm. This is about what I'd already planned for, so no problem there.

Next, we have to set up the new stow parameters. The FSW's Fault Protection code has a feature where you can tell it what joint angles represent a stowed IDD and have it check constantly whether we're in or out of bounds; if we're out of bounds, it will stop the drive. We never used this on Mars, because when the IDD is stowed the normal way, it hangs securely onto a hook. But we'll be setting it up today.

Finally, Ashitey suggests reducing the wheels' max speed by 66%, on the theory that that should make us come off of an abrupt drop (if we screw up and have one of those) a little more gently. To my surprise, this is rejected: the rovers already move so slowly that there's little to be gained by this.

Oh, and we have to clear the errors left over from the most recent stall fault. And Jim Erickson asks me to add a rule to our flight rule checker that warns if you send the "stow IDD" command on Opportunity, since what that command does is no longer what we want to do.

That's all for the meeting. As was the case Friday, the sequencing itself is simple. Today it's even simpler, since we're basically just repeating Friday's sequence (after clearing the errors and increasing the rotor resistance). But, once again, it's worrying about the upcoming drive that consumes the greater part of the day.

First, Ashitey relays a concern from the science side about closing the APXS doors. You close the APXS doors by rotating the turret to one of the extremes, and in the new stow configuration, we're commanding it pretty close to that value. In addition, the wrist and turret have more "play" than the other joints -- they'll jiggle around while we're driving -- and we might therefore end up crossing over the line we're starting so close to. This is worrisome because the doors are so hard to get open once they're closed. That wasn't always the case, but we started having trouble with it on the surface, so we decided just to leave the doors open permanently. What happens if we inadvertently close the doors and can't get them open again? We effectively lose that instrument, and with it the ability to sense the chemical composition of rocks and soil.

But we're dealing with a failing mechanism here, and we need to make some hard choices. And that's the risk we decide to take.

We also spend a considerable time working out the suspension limits. We frequently set these limits, especially on Opportunity (come to think of it, I don't know if we've ever set them on Spirit), to help us decide whether we're crawling over something bigger than expected. So in theory, we should be able to just work out how much a 3cm or 4cm rock would articulate the suspension, poke in those limits, and proceed with a bit of extra insurance.

The problem is that, as with so many things we do, this is beyond the limits of what the hardware is rated for. The potentiometers that sense the suspension aren't accurate to more than 2 degrees or so -- they're not intended for this level of precision -- and a 3cm rock changes the rocker differential by a little less than that. Ashitey wants to be fairly generous with the limits, but I don't. "We've already sold everyone on the concept that this might take us a couple of sols here," I point out. "We might as well be conservative." We add a little margin, but it's still possible we'll stop for no good reason. So be it. I'm getting paranoid.

By the end of the day, we've got a pretty decent drive sequence. It's not terribly complex; it's another one of those sequences with a high discussion-to-command ratio. But that's okay. We're doing something new and weird, and we can screw things up that we could never fix again. So the more talk, the better.

But first let's just hope we can stow the damn arm, so that we can find out whether this even works.


Buck said...

Very cool insight into the every-day problem solving that you folks go through.

Emily said...

Just found out about this blog (still catching up to this post, actually). It's very amusing ind interesting. Seeing as I am also in the research phase of a story based on Spirit's life, it is also an excellent resource. Keep up the good work at JPL and this blog.

Scott Maxwell said...

Buck and Emily: thanks to you both!

Emily, I'll look forward to reading your story when it's ready!