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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

@MorningR Apologies, just getting back to the forums after a trip to our manufacturer and the holidays. These look great! We’ve limited the motion of the arms in SW to prevent them from hitting the face of the robot, but using your magnetic arms could allow us to increase to the full range or even 360, which opens a lot of new uses.

From an engineering perspective, I like the light saber arm. that magnet seems strong enough to keep the arm from wobbling. Have you driven the robot around with the arms attached? I wonder what magnet strength is required to counter the vibrations of driving?

From a novelty perspective I like the hitchhiking thumb and wings. they lend well to building personality into Misty.

Are you planning to share the CAD and assembly instructions?


@miles Welcome back!

I had someone help me with the CAD. I am Tinkercad, and he is Solidworks, if the level of capability matches the software you use :wink: . The shoulder nub is important to get right, so I had a professional ensure the interface to Misty was correct and provide options for the magnetic attachments. The nub turned out many times better than I could have done on my own. Here is the Thingiverse link:

The magnetic shoulder nub provides a good base to play with. I made the different arms in Tinkercad just playing around - these are not pretty as I was trying to get them done quickly. (I will share specific ones if someone requests them, however, I think anyone could build better ones.)

Engineering details:

  • For friction, an o-ring was added. The o-rings (from a plumbing kit assortment at hardware store) are a #14 O-Ring OD=15/16" ID=3/4" Wall = 3/32" and a #13 (OD=7/8" ID = 11/16" Wall = 3/32"). The #14 just fits and the #13 stretches a little. I find the #13 works better and is what I presently use.

  • For the magnets, I purchased a variety from K&J Magnetics, and settled on using the RA24 in the shoulder nub (K&J Magnetics: RA24) Then, depending on the arm, can either use another RA24 magnet, another magnet, or any magnetic metals. The light-saber arm uses a RA24 magnet, and the arms in the obstacle and dancing video also use the RA24. This is as strong as I am going for now, and will increase friction if needed for future arms. The magnets are hot glued in place.

  • The nub and arms are printed in PLA at 0.2mm (Normal) resolution.

There is always something to test and improve. I have driven around with arms, and they work for props. If you look at my obstacle video and dance video that is presently where the arms are at. They will not pick up anything substantial, but move when they should. If you look at the last take in the obstacle video, the arm did not fall off dragging a football!

Note: The “wing” should look familiar - it is your design, the one you shared to hold a card. Nice job and thanks for sharing.

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@MorningR thanks for posting the CAD, the addition of the O-ring for friction is a great idea. Also, I’m glad you found the card-holder arm useful!

@aubreyshick These might work really well for holding props. See the videos Aaron posted. Do you have the capability to print some to see if they work for you?

@miles I don’t, but @johnchoi does. John can you see the CAD? This might work for holding books and cards for teaching accessories. Thanks!!