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Misty Quick-Change Prop Arm

Hey @Miles, per your request to see the arm, this is for you. Feel free to comment and critique.

First, any good engineering problem starts with some requirements. Here are some top-level ones for my Misty arm:

-The arm should be quickly interchanageable with no tools. Think snap off, snap on.
-The arm should be able to reach close to the ground, or far enough in front to Misty to interact with other objects. Think buttons and switches, and maybe these will have to be specially designed for Misty.
-If the arm interacts with an external object, the event should not damage the object, the arm, or Misty. No robots or humans should be harmed by using the arm. It is OK for the arm to fall off in this case.
-The arm should not move on its own, and remain in the position it was originally snapped into placed.

-There is no lifting weight requirement (for now), just props
-There is no look requirement (for now)
-There is no “Misty should still fit in case for travel requirement” (for now)

The video shows a first iteration. This design uses an embedded magnet in a “shoulder nub.” The shoulder nub is installed onto Misty. The “upper arm” also has a magnet, and holds a concave cup to the shoulder nub. Two early iterations of “shoulder nub” are shown sitting next to Misty, one turned over so you can see inside.

The magnets in the arm and shoulder are from a local hobby store.

Next steps are to investigate friction and magnetic forces to see how much lifting ability is possible. And of course design some quick connect arms.

I will try to include links to the models in the near future. If you want the STL files sooner, let me know the best way to get them to you.


Love this - such a cool system.

That’s great! In all of our discussions about arms I recall talking about using magnets. Having the ability to swap appendages without tools will be really useful.

A couple thoughts to consider:

  • Misty’s arms only rotate through ~120 degrees, so it might be useful to have a registration dimple or tab to ensure the arm is not installed at the wrong angle.
  • Magnets are not super strong under shear forces so it may be difficult to use them to carry the torque from lifting something with the arm. I’ve seen some clever systems using tabs and grooves to carry the shear loads, while using the magnets to hold the arm onto the socket. If you made them different sizes that could also accomplish the bullet above.
  • Last thought: the arm motors in Misty II were not designed to provide serious torque so her lifting capability is pretty low. I like fact that the arms can pop off, both for safety reasons, and to protect the arm motors.

At any rate, this is a really cool idea, and I look forward to the next update!

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Very nice. I’ve always wanted to do a quick swap system but using something like a spring-lock push button to lock/unlock the arm on the shoulder.

Hey @Miles - here is a magnetic prop arm update. I had a chance to take a video of some attachments. Still working on magnet selection and friction.

One item I did not expect is that I care less about the absolute position of the prop arm. I program in a delta arm position change. If the arm is in the wrong position during a skill I just move it (for now). The arms are easy to switch into different poses, and you can either move them when they are still on Misty, or pull them off adjust and snap back on. The arms have hit Misty’s visor face, but no issues as the arms just move. It does not look graceful :upside_down_face:, but no risk of damage.

The lightsaber arm is the one that has the most powerful magnet - you can tell when I pull the arm off, Misty moves. The other arms all have different sizes of magnets.

Let me know which one is your favorite!


I like the wings