4/21/2011 Sprint #2 Update

I’m tired, but I wanted to give a quick update on the day. Sorry I don’t have any eyecandy to show off, since most of the CAD work is being done by other people who aren’t collocated with me. We still got a ton done though, so here’s some quick details.

  • AJ got a basic kinematics analysis done on our gripper concept. We are trying to use an unactuated gripper device that can conform to even a low mass target with a low enough conforming force that the target doesn’t accelerate away before you can lock it down. We also want to be able to handle misalignments and relative velocity. While some of the “velocity matching” will be provided by steering and extending/retracting the boom, there’s still going to be some errors in real life. So AJ made some models to verify that even with decent offsets, the gripper would still lock onto the ball.
  • We had a meeting after that to discuss the “joint locking” mechanism options. One of the keys with this gripper (what makes it tricky) is that you want it to go limp as a noodle right as it touches the target, so it’ll conform well, but you want it to lock back up as soon as it has conformed to the right shape, otherwise it might apply peel loads to the pads, and Electroadhesion, like most adhesives doesn’t do as well in peel. We found some really tiny electromagnetic brakes (about 9/16″ diameter by 7/8″ long) that we can use as the hinge axes, and we came up with enough ideas on how to integrate them to the gripper that AJ is off to the races on making CAD models based on that and the kinematics.
  • We also did some work on brainstorming ideas for the EA pad mounting and configuration. We’ll hopefully be doing more of that tomorrow.
  • I also helped Mike and Forrest with motor controller selection and motor sizing. The motor would drive the friction roller which feeds the boom out. We used the torque specs that Mike got last night (and better specs he got today) to figure out a good motor/geartrain option. Ironically, we found a McMaster Carr gearmotor option that was pretty much perfect for the job–the rated speed was within 1rpm of our target, and the rated torque at that speed was within 5% of our torque target. I’m sure its stall torque is several times higher than that (I’ll look it up once I get the part in) so we should have plenty of torque margin. And it’s on the shelf…Have I mentioned that McMaster Carr is one of those other rare companies that actually wants to sell us stuff? It’s a bit heavier than I would’ve liked, but it’ll get the job done. We can go a lot lighter in the future if we want to.
  • Mike and Steve spent a lot of time today getting more data on the boom deployer, and working on refining the concept. We’re hoping to have a conceptual design review tomorrow, so we can start getting detailed parts drawings made soon. We pretty much need the parts assembled by the end of next week, so we don’t have a huge amount of time.
  • A few days ago we ordered a lightweight 10″ stainless steel sphere to use as one of the capture targets. It’s the same size as the “Orbiting Sample Canister Simulator” that Aurora used for their MSR capture tests previously. We’ll also get a hamster ball in the same size to serve as an additional simulator. When I saw these globe balls, I was reminded of the Palantirs from Lord of the Rings. If I had spare time it would be fun to do an Eye of Sauron motif on one of them. But most likely we’ll just spraypaint the hamster ball some solid color like Aurora did for theirs. The stainless one should be in tomorrow. It weighs about 3lb. The hamster ball will likely be the acid test for our gripper, because it is so light that any “conforming force” is going to be pushing against very little inertia.
  • The one last challenging unknown was how we were going to get power into the boom. In our last device, the EA pads were powered by a small 1/2 AA battery in a plastic box zip-tied to the end of the boom. We’re going to have a lot more electronics and instrumentation out at the gripper this time, so we’ll likely be putting a small microcontroller and circuit board out inside the end of the boom to run all that and do all the data logging. But that requires us to pass at least power, and probably signal out into the boom. You can get thin-sheet flexible “cables” (like ribbon cables but even thinner), so that part isn’t hard. The challenge is getting the power and signals from the rotating boom coil to the stationary box. This would be a lot easier if the axis of the boom coil stayed stationary, but from Mike and Steve’s observation, that isn’t necessarily a given. We should have a second Rolatube boom showing up soon that has a neutral bias (ie when you start uncoiling it, it doesn’t want to extend or retract), so that may behave differently. But for at least right now we had to assume that the axis of rotation of the coiled part of the boom is going to shift around as the boom is wound up or deployed… But we think we have a good solution for this too. We’re going to need to do some modeling and experiments, but we think we’ve got a good lock on a solution for this.

So, most of today was figuring out solutions to the key problems for this design, but the good news is that we found at least a one promising solution for most of the big technically risky areas going into the day. Hopefully tomorrow I’ll have some more pictures/youtubes to show off.

6 Responses
  1. Alan Long

    Hi Jon,
    Following your progress with baited breath. My gut tells me this technology will someday be everywhere in space. An obvious quick and dirty solution to the power and signal problem would be to duct tape the wires to the edge of the flattened boom. It looks like the near end is rolling off a reel. If so make the connections through the axle. Check with your supplier. These things were made to be battlefield radio masts. Yours may already be able to carry power and or some sort of signal. I am sure they already know how to run power and data connections. According to their web site, once you “get a grip” (so to speak) on your needs they can build it directly into the booms.

    Good Luck,
    Alan Long

  2. Two different ideas on data & power cables.

    1. Quick and dirty (it might work with modification and some kind of support): use the coiled kind that partially extends and retracts by itself. There might only be power cables of this kind but I think it should work to wind ordinary data cable around the “springs” of such cable and let it be carried with it. Perhaps ordinary coiled “twirly” phone cable (the kind attached to the receiver on really old phones) is good enough as long as one can find a way to properly wind it back in so it doesn’t obstruct the Rolatube.

    2. A “reversing spool”, during the last hours I’ve built a really crap model that halfway works, taken blurry pictures, and tried to write down how the idea works (and model fails). Like the above idea it enables both endpoints to remain fixed/non-rotating. Thinking about sending you a mail with all of it, if nothing else it might help brainstorming. I’ve given a more “official” mail address with this comment and will send it from that address if I convince myself during the next hour or two that I won’t be wasting your time ^_^

  3. Joseph

    Hi Jon,

    It is good to see you busy. I certainly appreciate the open window you have provided into your team’s thoughts, design, and implementation processes while you pursue the goal of a sticky boom.

    I couldn’t resist chiming in my two cents worth of armchair engineering (yea, I have done a little of that, the engineering I mean.). It sounds like you have assembled a crack team of very talented individuals and I mean no slight to any of you…I still can’t resist.

    Wireless. I dunno your requirements for power and data rates nor do I claim to know the kinds and number of sensors you use. Perhaps this has already been mentioned, however, the ZigBee modules are small, low power and, if I recall correctly, have at least 6 ADC lines at 10-bit res and a few data I/O line. The ZigBee modules are down to 1mW radiated power output with higher powers available. You could then, based on your requirements, have the microcontroller at the base unit to record the data stream…or hang a dongle off of your laptop and directly record your data.

    If it is required to have a microcontroller on the end of the boom for more data lines, higher res ADC, and/or more ADC lines, stream the data via bluetooth directly to a laptop.

    Benefits low mass, low cost, removes the requirement for data logging at boom tip, provides ‘real time’ data/control, and solutions are widely available and well supported. Visit the SparkFun site….I am sure they would be happy to work with a local start-up space company.

    Anyway, my apologies for the interjection, thought I would share at the risk of…well…you know making myself look like an over sized donkey.

    Love the work you are doing and have followed your blog from way before your first little one!


  4. Bob Steinke

    How about a telephone cord for the power and data? I mean the old style non-cordless coiled up cord that goes from the base to the handset. Just hang it out there parallel to the boom It still might have a tendency to tangle when it’s really slack, but for a first cut option maybe it would work well enough.