How would you motorize the joints in an Iron Man suit? You need something fairly shallow, I would think, probably dual servos sitting on either side of an elbow or knee joint or either side of your hips, but how do you get motorized action there without dramatically adding to the thickness of the joint?

Bicycle-style chain drives wouldn't work, I would think, since the length of the chain would need to vary depending on what position you're in for at least a lot of joints.

How would you motorize the joints?

  • 1
    $\begingroup$ Where "fairly shallow" is a concern, consider a ring of a large number of tiny motors all the way around a ball and socket, rather than the (effectively) 3 big motors to replicate human shoulder-joint motion. $\endgroup$ Commented Jul 18, 2013 at 2:27
  • $\begingroup$ The ring of tiny motors is awesome. Also, not sure how I missed the existence of a robotics exchange. If a mod feels it would be more appropriate to punt this question to robotics, please feel free but I won't dupe it. $\endgroup$
    – Erik Reppen
    Commented Jul 18, 2013 at 2:36
  • $\begingroup$ Thanks :-) I dabble in some mechanical engineering problem solving / CAD in my spare time ;-) $\endgroup$ Commented Jul 18, 2013 at 2:37
  • $\begingroup$ Hydraulic actuators like the terminator arm. $\endgroup$
    – Nick Alexeev
    Commented Jul 18, 2013 at 2:48
  • 1
    $\begingroup$ Mods: as a R.SE mod, I'm happy for this to be passed over to Robotics $\endgroup$
    – Andrew
    Commented Jul 18, 2013 at 12:16

3 Answers 3


Addressing the shoulder joint, which is rather more complicated than elbows and knees... After this one, the other joints become far simpler to visualize or engineer.

Here is how the ball and socket shoulder joint could work:

Shoulder Joint

  • Freedom of movement: Approximately 60 degrees end to end, all around. Less than for humans, but it can be tweaked to around 80 degrees without difficulty just by making the half-torus narrower.
  • The static half-torus (shown in shiny steel here) is for strength, and would be joined to the torso exoskeleton by a rigid set of struts. Half torus = torus with an inner coaxial cylinder excised, tentative diameter of cylindrical hole 145 mm.
  • The inner blued-steel hollow sphere is where the arm goes through, just beyond the actual shoulder. Tentative diameter of sphere 138mm, diameter of inner cylindrical hole 100 mm.
  • Rings of little stepper motors on both front and hidden edges of half-torus, gripping inner hollow sphere.
  • Stepper motors have outer ring rotor, inner stator, and rubberized omniwheel "teeth" to move the inner sphere.
  • Inner sphere will of course be attached to a rigid exoskeleton matching the upper arm dimensions, up to the elbow joint.

This is how the motors act upon the inner sphere, close up:

Close-up of joint

This is how each omniwheel stepper motor will work:

Omniwheel Stepper

The omniwheel design is required for the teeth to allow frictionless motion axial to the rotation direction.

  • $\begingroup$ Is this Autodesk Inventor? $\endgroup$ Commented Jul 18, 2013 at 6:50
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    $\begingroup$ @trav1s Yes, with some image editing to merge wireframe and shaded outputs. $\endgroup$ Commented Jul 18, 2013 at 6:56
  • $\begingroup$ Anindo is building his own Iron Man suit. This is proof! $\endgroup$
    – JYelton
    Commented Jul 18, 2013 at 7:39
  • $\begingroup$ @JYelton I bet I'd do it cheaper and better engineered too. All I need is a 10 million dollar sponsor, and a couple of dozen minions. Any day now. $\endgroup$ Commented Jul 18, 2013 at 8:01
  • $\begingroup$ Okay, did some back and forth on it. I think joe actually has the elbow solution but this is just !@#$ing badass for a shoulder joint and I kind of went more general with the question than elbow. Although I do wonder if it doesn't represent a lot of little extra things that can go wrong. Syncing the motors would be a challenge. And what happens when one of them craps out or loses its place or something... which isn't to say I don't think this is really cool. $\endgroup$ Commented Jul 19, 2013 at 2:10

Erik, your wish is my command:

"High torque pancake stepper motor" comes to mind. Can you develop one, and a couple of extra's? :)

Seriously, you're looking at a combination of technologies and disciplines. Precision positioning of a "High torque pancake stepper motor" might be an acceptable direction, but it'll probably end up simply being the driver behind Nick Alexeev's (Terminator) hydraulics, after all, the back-hoe surely has its place in "motive amplification," if I may use the term sloppily. :)

  • $\begingroup$ I was thinking more a combo of Anindos ring of motors and the pancake but I'll give you the answer. $\endgroup$
    – Erik Reppen
    Commented Jul 18, 2013 at 5:08
  • $\begingroup$ Okay I gave you the answer pre-migration but dude.. he did an auto-desk. Upvoted again, and the minions will still deliver should they ever exist. $\endgroup$ Commented Jul 19, 2013 at 2:01

I'll chime in.

  1. You could use piezoelectric inchworm motors. The approach is outlined here: https://www.techbriefs.com/component/content/article/tb/techbriefs/mechanics-and-machinery/2273

  2. you could use a ball robot motor system, upside down with a spherical bearing rather than actuating with a toroidal motor. The ball robot is well explained here: https://www.youtube.com/watch?v=bI06lujiD7E Bearings are available here: https://www.acornbearings.co.uk/bearings/spherical-roller-bearings. an omniwheel appraoch is here: https://www.semanticscholar.org/paper/Concept-of-a-three-D.O.F-spherical-joint-gripper-Weyrich-Abdullah/10699c6f1c39a8700c903d407a110910bef53f3c

  3. You could make a rare earth magnet ball rotor with an electromagnetic. See these two links for a better idea. enter image description here and enter image description here

  4. or a delta robot through a SR joint.

  5. I'm guessing you are already familiar with stewart plaforms

Hope this helps. I can sketch stuff if this isn't clear, but I'm not fast enough at CAD to draw this all out. I didn't post pictures because it seemed terribly slow.


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