Steve Canfield and his marvellous mechanical joint

In previous posts I’ve mentioned that when I first got to NASA I worked in the Propulsion Research Center, which was a fun place to work because you got to think about and try just about anything you wanted to so long as you could get funding, and there was this sugar-daddy at NASA named John Cole who would fund all kinds of crazy stuff. I never got any funding from John but my patron was Les Johnson, who was kind of like NASA’s “point-man” on tether technology. After about two years in the PRC, Les told me it was about time to quit fooling around and become a serious-type manager like him, and to come and join him in the newly forming In-Space Propulsion project.

So in the fall of 2002 that’s what I did, and before long I was writing NRAs (National Research Announcements) to solicit universities and corporations to bid on technology work for tethers. We put the first NRA out for tethers and got responses and had a meeting where a committee picked the winners in March 2003. After that things started getting serious. We had real money for the first time to do momentum-exchange tether work, and there were still so many unanswered questions that needed to be solved.

Sometimes fate or luck or serendipity drops things in your lap. In the summer of 2002, I met one of the most clever and hard-working people I’ve ever had the good fortune to meet–Dr. Stephen Canfield of Tennessee Technological University. The next summer he was down at MSFC and I was in the middle of trying to figure out the answer to a very thorny problem: if you have a tether that’s spinning, how do you keep the solar panels pointed at the Sun? My friend Kyle Frame and I would sit in my cubicle for long stretches of time with pieces of paper pretending to be solar panels and pencils and sticks standing in for the tether, trying to figure out some way to do it that wasn’t totally foolish.

One day Steve Canfield stopped in and asked us what we were up to. We described the problem and he asked a simple question:

“Do you care what orientation your solar panel is in so long as it is pointed at the Sun?”

I said no, we didn’t care, and then he showed me something he’d been working on since he was a grad student. It looked like this:
Basic Canfield Joint
He called it a “Trio-Tristar Carpal Wrist Joint.” I thought that sounded like a real mouthful so I just called it “Canfield’s joint” and eventually everyone (except Canfield) began to call it a Canfield joint. It was kind of a crazy looking thing that you couldn’t figure out what to do with it unless you held it in your hands and started playing with it. Unfortunately, in a blog post I can’t reach out of your screen and hand you your own Canfield joint to play with, because if I could you’d figure out in a few seconds what I’m talking about, but the real magic of the Canfield joint is that you can point the joint anywhere in a hemisphere without winding up anything.

The joint has several parts. There’s the “base plate” which stays attached to whatever the joint is mounted to, like your spacecraft, and then there’s the “distal plate”, which points to whatever it is that you want to point at. There are six legs on the joint, in three units. The joint is called a “parallel structure” because there’s more than one load path for the loads to follow, and this is what gives it its potential strength. Where the legs mount to the plates is a simple revolute joint. I didn’t know what that meant so I asked Canfield and he said that it just meant that it was a simple, one-degree-of-freedom (one way to move) joint or hinge. Where the two legs come together you could have a spheric joint (a ball and socket with two degrees-of-freedom) or you could have three revolute joints in series. That’s what we usually do.

I asked Canfield what the joint was for. He said that he originally wanted to use it to replace the CV joints in cars, since if it had all revolute-joints then it wouldn’t need a boot. If I hadn’t had to replace the boot on the CV joint in my car when I was in college and dirt-poor, I wouldn’t have had any idea what he was talking about, but the loss of money was still burned in my mind, so I appreciated that application.

Well, to make a too-long story shorter, I learned how the Canfield joint worked and figured out how to solve my little problem on the tether. Tell me if you like the result:
Canfield Joint on MXER Tether
Medium View of Canfield Joint
Closeup of Canfield Joint

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MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999

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17 Responses to Steve Canfield and his marvellous mechanical joint

  1. That’s so neat.

    Hey Kirk, who is pushing for a MXER Tether Flagship Technology Demonstration? There was a guy at ISDC working on Solar Sails who said he was pushing for a FTD for them.

  2. Ah, it’s not my fight anymore. If I actually thought that NASA MSFC gave a rip about low-cost space access, I might consider it. But over the last five years watching Constellation/Ares mutate into ever more expensive and hideous forms, this idea has been driven out of my mind.

  3. David says:

    This is very neat indeed. I am really enjoying your posts. The quality of the topics are very high. Thank you very much.

  4. Kirk, to me that kinda sounds like “I’m gunna move to Canada because the US is going down the drain” .. you could have stuck around and made a difference. 🙂

  5. johnhare john hare says:

    Many battles have been lost by trying to hold indefensible ground, and many have been won by retreating to a position that can be held. Sir Liddel Hart has a lot to say on the subject of the indirect approach.

    Do you know offhand if this joint is proprietary or open to general use?

  6. Bram Cohen says:

    Possible uses for this doohickey, both assuming the spinning part is on the inside rather than the outside as in the example you’ve made:

    A flexible extension for a drill, for cases where the drill itself is too heavy or large to be maneuvered.

    A flexible transmission for buses which are long enough that power to the back wheels needs to go around a variable bend.

  7. Mike Lorrey says:

    Thats a really sweet joint design. As I spent my dot-com bust as a mechanic, I can really appreciate this.

  8. John Bossard says:

    It should also be noted that Kirk was instrumental in putting together a program in which a small thruster could be attached to the Canfield joint, thus making a gimballing spacecraft thruster. The thruster’s feed-lines could be run up through the legs of the joints. Since all joints are hinged, this configuration results in no pressurization-rigidization loads on the joint, unlike what can happen when using flex-lines. This thruster on a Canfield joint was demonstrated with a small GO2/GH2 thruster in 2005. It was recently publicized in NASA Tech Briefs, Vol. 34, No. 5, pp. 55.
    I don’t know about the Canfield joint in general, but the gimballing spacecraft thruster is indeed quite available for general use. Contact Sammy Nabors, MSFC Commercialization Assistance Lead at Refer to MFS-32520-1.
    Lastly, I will totally vouch for Kirk’s comments about Dr. Steve Canfield. Steve was almost embarrassed about the joint being called “the Canfield joint”. But I think its a great name, totally appropriate, and way cooler sounding than the “Trio-Tristar Carpal Wrist Joint”. Thank you, Kirk, for your support and dissemination of this technology. Without you, it would not have reached the TRL that it currently resides at.

  9. Hey John, thanks. I’ll be talking about the gimballing thruster in an upcoming post.

  10. There’s (very brief) mention of “Electrodynamic Tether Propulsion. Artist’s Concept of ISS Reboost.” in the NASA Chief Technologist’s town hall meeting.

  11. Patrick says:

    That’s the coolest mechanical doodad I’ve ever seen. And “Canfield Joint” sounds like something out a Heinlein novel. Excellent!

    Is the

  12. Charles Grimm says:

    I forwarded this to my son, the mechanical engineer, who had this response:

    It looks cool, but it’s kind of an overcomplicated solution, in my opinion. If you replaced each of those legs with a prismatic joint, in the form of a linear actuator, then, not only do you have fewer things to worry about, you’ve answered question #2: How do we drive this thing?

    Anyway, the question itself seems flawed. Why not take your tethered craft and rotate it in a plane perpendicular to the sun’s radius? That is, parallel to a plane tangent to the sun’s surface. Then, one side of your craft is always facing the sun, and one side is always away.

    However, if for some reason that wouldn’t work, and you only need to drive the panel through a few specific orentations, it would be a good candidate for a Spherical Four-Bar, like the ones I did in grad school. The benefit being that it would only take a single motor to drive through the cycle, instead of 3.

    Sounds like something that needs a few napkins and a beer to work through.

  13. I’m a mechanical engineer myself, and Canfield’s the best mechanical engineer I’ve ever met. You might suggest that your son take a longer look at the design. Rotating the tether in the plane he describes isn’t feasible for a variety of reasons.

    We went through many napkins and a bunch of Diet Cokes to get to this point. But he’s invited to improve on it if he can.

  14. Charles Grimm says:

    Thanks, Kirk, I’ll give him a poke. I’d like to get him to work on space applications. His master’s thesis was on robotic joint movement with curved arms, if I can oversimplify. Interesting applications, fewer motors for the same movements, which is why I forwarded the Canfield joint to him.
    Since I haven’t met Stephen Canfield, the best ME I’ve ever met is my dad. He got a dollar each for his pile of patents, including some for a process called Xerography. (James J Grimm,
    I’d like my son to follow in his grandfather’s footsteps, but a recent grad recently married may not have space applications on the front burner :(.

  15. If he’s interested in robotics, he ought to think about Canfield as an advisor for grad school. Canfield does lots of robotics work, not so much aerospace. I’ve been the one coming up with aerospace applications for his ideas.

  16. Ed Minchau says:

    I really like this design. Look what happens when you connect several of them in series and drive them all at the same rate: ExtendaTruss. This could also make serpentine robots easier to use and more capable. Can you imagine what the nanotech version of this would enable? Yoikes.

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