Emulating Biology For Robots With Rolling Contact Joints

Joints are an essential part in robotics, especially those that try to emulate the motion of (human) animals. Unlike the average automaton, animals are not outfitted with bearings and similar types of joints, but rather rely sometimes on ball joints and a lot on rolling contact joints (RCJs). These RCJs have the advantage of being part of the skeletal structure, making them ideal for compact and small joints. This is the conclusion that [Breaking Taps] came to as well while designing the legs for a bird-like automaton.

These RCJs do not just have the surfaces which contact each other while rotating, but also provide the constraints for how far a particular joint is allowed to move, both in the forward and backward directions as well as sideways. In the case of the biological version these contact surfaces are also coated with a constantly renewing surface to prevent direct bone-on-bone contact. The use of RCJs is rather common in robotics, with the humanoid DRACO 3 platform as detailed in a 2023 research article by [Seung Hyeon Bang] and colleagues in Frontiers in Robotics and AI.

The other aspect of RCJs is that they have to be restrained with a compliant mechanism. In the video [Breaking Taps] uses fishing line for this, but many more options are available. The ‘best option’ also depends on the usage and forces which the specific joint will be subjected to. For further reading on the kinematics in robotics and kin, we covered the book Exact Constraint: Machine Design Using Kinematic Principles by [Douglass L. Blanding] a while ago.

9 thoughts on “Emulating Biology For Robots With Rolling Contact Joints

  1. Dyneema fishing line? I did not know. Must get some. It is stronger than Kevlar/Aramid and can take higher temperatures. If I recall, also strong enough for a sky hook on the Moon and space elevator on Mars.

  2. Interesting video, the author was correct in that I’ve never heard of RCJs before. I do question whether they are that much better than a sleeved pined joint, no ball bearing, which can also be made small.

  3. RCJ’s are an innovative solution for robotics, especially for temperature and pressure sensors positioning on the distal parts (read: gripper) of a robot. And seeing the grooves for the cabling allows for placement of electrical wiring (26 AWG and smaller) within the joints. Of note: no lubrication required,

    1. I think eventually you would need some lubricant, or some even lower friction surface (teflon tape) to manage wear between parts still, as tighter the string, the more force between parts as this slowly sheer. Its a very neat concept and I agree. Allowing for sensor placement is a game changer to aid in actuation accuracy. I’m reading on this now and just blown away since like the video author mentioned hobby robotics typically sticks servos at the joint, but this causes a trade off between weight/power usage, and so I think you can use these joints + a smaller servo with a preloaded tension mechanism to actuate/stabilize a system with less active energy.

      I’d refrain from the groves for cabling more so out of fear of mechanical failure or dislocation would also destroy the wire itself, causing other problems.

  4. Contact Ball joints for robots could be vastly improved over nature. New non stick indestructible fry pan surfaces could be used for coatings far superior and more durable than teflon.
    360 degree infinitely rotatable designs could allow movement that nature cannot reproduce. Use of Thixomolded magnesium could produce limbs 30% lighter and stronger than Aluminum and allow for hollow metallic bones similar to birds for even greater weight reduction.
    Or the hollow metal bones could provide storage for batteries eliminating bulky back packs etc.
    Having batteries spread through the robot structure could reduce required wiring just needing an ether net loop to run functions.
    It may also be possible to offload compute capacity to a remote AI. Just using the robot as a wifi terminal.

  5. Talking of joints and robots, I noticed that the new Atlas001 from Bostom Dynamics can rotate its joints 360 degree, meaning it has slip rings, and that is new I think since they went electric with this new robot.
    Now since each joint has to transfer power and signals to the next joint in the arms/legs you now know that damage to that shoulder/hip would disable the rest of the arm/leg.

    Might be good to know too in case they end up setting them on you you know… ;)

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