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The Wobble Ring Stepping Motor MK1

by Ndrew, published

The Wobble Ring Stepping Motor MK1 by Ndrew Nov 6, 2012

Description

Edit: This thing has been upgraded!
thingiverse.com/thing:52299


This is my humble start on an actuator that could be mostly printed on a 3d printer. Incomplete as it is, I thought I'd post it as it developed.

It is based on this:
act.sys.okayama-u.ac.jp/kouseigaku/research/okamoto_wobble_06/english.htm
Instead of air chambers, I plan on rolling my own solenoids, eventually.

It also uses hypo-epicycoidic gearing, based off this:
thingiverse.com/thing:28237

And it should move like this:
youtube.com/watch?v=UayBAHJQE_k
But instead the outer ring 'wobbles'.

The wobbleRing is 1 tooth larger than the drivenGear(51/50). This allows for 2.4 degrees between solenoid steps(360 / (50 * 3)). But that's with just one of the 3 solenoids pulling at a time.

Now why did I use cycloidic gearing? Well, I understand they wear a little slower for this application. Also I was really having a time getting the MCAD library to work. Please correct me if there is a better gear profile for this application.

I do also appreciate any constructive critism, as this is my first openscad project.

BEWARE: I have not printed this thing yet!
I really wish I could but I am still waiting for mine.

BEWARE: There be snakes in dem codes; look at the openscad code at your own risk!
I really do intend to add more paramaters too, just working out the basic form first.

Oh and for now there are no solenoids included. At this point I really don't know enough yet to design capable ones.

Recent Comments

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Yes that's about it. Every second quadruple of trapezoids could be made coplanar.

Wow I am having a hard time picturing that. I think I've got it though. Correct me if I am wrong then: each vertex of the 3 original rectangles is connected by an edge to the rectangle 1 size bigger. So that the smallest rectangle would form 4 trapezoids  when connected to the middle one, and the middle sized rectangle would form 4 again with the largest rectangle. The middle rectangle would not be filled. Is this about right?

Nope googling doesn't seem bring up anything that looks like my third option. What I meant is to make the inner walls from a (hollowed out) sequence of expandingfrustum-cylinder-reducingfrustum-cylinder-expandingfrustum-... and so on.

Still there are the chamber seperation walls blocking flexing a bit.

So now that I think of it in more detail the best solution would be to start with three (or more) concentric rectangles projected from the origin outward onto one chamber wall. Pull the wall with the outer two radially outward while holding the center part of the wall with the innermost rectangle in place. (translate into openscad-fu). Only the inner rectangle will be in contact with the "wobble ring". If the flexing force is consderably smaller than the pneumatic force: (pressure*areaOfInnerRectangle) the the motors torque can be precalculated more precisely.
I think it might be a good sign if the design is so sensitive that it would be destroyed if pressured without the "wobble ring" in place.

Then again If you make the motor very small chamber hight Increasement might be the only option.

Maybe print some pickle-jar-lid-style bellowchambers for testing-purpouses

One more thing: Maximize Toothcount to minimize necessary amount of flexing.

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Instructions

Really none yet :/
I don't pretend to know enough to instruct anyone else when I've not built anything myself.

If someone were to print just one or two pieces, I'd think that the "wobbleRing" and the "drivenGear" would be the most illuminating. Really curious how they mesh IRL...

Comments

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mechadense on Nov 22, 2012 said:

Nice too see my epihypo function used for gears :)

IMHO I'd go for the original pneumatic (or hydraulic) drive.
I think you are losing the benefits of high tourque density by using electromagnets instead of air pressure
which is the main selling point of this device. (ratcheting will kick in way below structural strength of the teeth)
Solenoids as pulling magnets tend to have a very nonlinear force with airgapthickness which
(unless special purpouse rod pulling magnets are used) further reduces force density.
Also you'll need transformer sheet cut which makes it hardly DIY.
Shapeway's "StainlessSteel" (which has reasonable magnetic properties) might be a viable alternative
but with 10$/ccm its awfully expensive.

Also before beginning constructions an estimative quick feasability assessment
(= rough estimation of minimum force to expect for a given highly approximated geometry)
is always a good idea. If you really wanna do your own coils read up on magnetic Circuits.

The slight flexibility of PLA/ABS would lend itself perfectly for the original closed air-chamber design pictured at the okamoto website.
Still you'll need valves. But I think an appropriate cycling-valve which encodes the sequence in hardware could be printed.
with a sequence like this:
(o=ambient; x=closed; *=pressured) (ABC...chambers)
(Ao B* Cx) (Ao B* C*) (Ao Bx C*) (LONG - Ao Bo C*)
(Ax Bo C*) (A* Bo C*) (A* Bo Cx) (LONG - A* Bo Co)
(A* Bx Co) (A* B* Co) (Ax B* Co) (LONG - Ao B* Co)
If you'll add a hydrodynamic speed limiter (turbolent wingwheel) for a valve rotor that encodes this sequence you'll get a constant speed motor from a constant pressure source. If you want stepping control you maybe could use small low force (comercial) pulling magnets to control the valve.
 
As for the construction: Its a bit blocky.
Try to minimize printing volume by looking where no/little mechanical forces occur and take away as much material as possible. Example:
"frameStabilizer" takes no force => thinner
"solenoidAnchor" tips take lots of force => thicker
Try to minimize screw count by using plastic shape to reduce DOFs. Example:
The top and bottom plate are only this thick for the screws to go in sidewards.
Suggestion: make the "framesides" wider & THINNER and let them be vertilally hold by the "frameStabilizers" (make two dents ito them)
so you can use one vertical instead of four ratial screws.

Ndrew on Nov 23, 2012 said:

 I appreciate all the input. I have still yet to print this (printer on its way) so I do appreciate the feedback. Probably just saved me a version or two.
As for the solenoids, I won't be using them like you suggested. I do intend to keep designing with the goal of replacing a stepper motor, and so will be using something similar to your chamber sequence.
As for chambers, I was thinking abs would be better. Is it more flexible? Perhaps I could make them out of silicone once I print off a universal paste extruder.

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