Active ball joint mechanism with 3 DoF based on spherical gear meshings (2021)

[luma]

Here’s a video of this linkage in action: https://youtu.be/AHUv9Zda_48

[jcims]

Some incredible explanatory animations.

[ge96]

at 2:01 that differential pinion... trying to figure out how that would work... it looks like it's locked in place yet it spins.

edit: oh maybe it's some kind of worm gear/helical gear to translate 90 deg or maybe weird bevel gear

[Ord3rChaos]

I saw this last year, really cool demo. Stronger materials are needed to make this useful and that looks like a nightmare to manufacture at scale. Also mechanical advantage seems to be a major tradeoff.

There is a reason biology tugs on ropes and lets reaction surfaces slide around instead.

[hwillis]

> There is a reason biology tugs on ropes and lets reaction surfaces slide around instead.

And that reason is that it's very difficult to have a rolling joint or muscle because you can't get blood into it. It's largely fundamentally inaccessible to an organism that must grow itself. There are massive advantages that are unexplored by evolution.

Even further, gears are inherently asymmetric, meaning they are even harder to evolve. Nevertheless nature DOES use gears[1] when it can.

> You can shape the surfaces the way you want for the movement you need.

You can actually do the same with gears and screws: https://www.youtube.com/watch?v=100is1XpXcE

[1]: https://www.smithsonianmag.com/science-nature/this-insect-ha...

[miketery]

What is meant by "reaction surfaces slide"? Is this referring to joints?

[busyant]

> Also mechanical advantage seems to be a major tradeoff. There is a reason biology tugs on ropes and lets reaction surfaces slide around instead.

I'm not sure I understand. Do you think that there's a biological benefit to ropes and sliding (compared to this)? Or is it simply that ropes/sliding are easier to evolve? You probably know this, but (in case others are unaware) there are rotary "engines" in biology (e.g., bacterial flagellum and F0F1 ATPase), but, obviously, they are not quite like this spherical gear.

[rolph]

its actually like a hydraulic cushion , with a self renewable matrix between the bones. we are close to mimicking tissue renewal with some of the recent materials advances.

https://en.wikipedia.org/wiki/Shoulder_joint

https://en.wikipedia.org/wiki/Self-healing_material

[the_cat_kittles]

the ball gear might be a good candidate for metal 3d printing

[jacquesm]

There appears to be an improved version that they have in the works:

https://link.springer.com/chapter/10.1007/978-3-031-08140-8_...

[hinkley]

> the main disadvantage of Abe et al.’s design is that it is an over-actuated mechanism: it requires four instead of only three actuators. In this paper, we propose a variation on this mechanism which requires three actuators, thus simplifying its control and its potential cost.

That’s sort of the problem. Four motors and nine gears, versus three motors and six gears. I suspect their solution here may be as simple as having two rolling motors at angles, and only one motor for twisting.

[Animats]

That's very clever. I've seen videos of this before. It's a robotic actuator.

Notice that it's overconstrained. The ball has 3 degrees of freedom, but is driven by 4 motors, which must maintain some invariant relationship. It's kind of like a mechanical implementation of a quaternion.

[dtgriscom]

Good point. I wonder how they manage the inevitable mis-match of the motor positions? Seems like it would be easy to get the system into a high-tension state, with motors fighting each other.

[carabiner]

Previously: https://news.ycombinator.com/item?id=28556208

[henearkr]

Here we are! "spherical gear" (used in your link) is a waaayy better name than "abenics" (used in the link of the present HN submission)...

[mhb]

Among others: https://hn.algolia.com/?q=spherical+gear

[CamperBob2]

Very cool design (and analysis, and animation) -- this is one case where I'm glad the authors paid up to make the paper available to the public. (And equally annoyed that they had to.)

I will say that I don't really get the problem being solved here, though. There are any number of ways to do this if you don't need to rotate the output arm around its own axis. If you do, simply adding another motor to the output arm seems like the most straightforward way to solve the problem, and with fewer compromises elsewhere. Can any robotics gurus comment on the relative advantages of the all-in-one approach using a single spherical gear?

[sacnoradhq]

PDF thesis about another design for a 3DOF active joint (2009 IEEE/RSJ) from different authors and different universities, coincidentally mostly from Japan: https://vigir.missouri.edu/~gdesouza/Research/Conference_CDs...

Also interesting OMNIA wheels:

https://www.omniawheel.com/omnia-wheels

[o-o-]

Honorable mention: Not a gear, however 4(!) DoF with a locking mechanism (the spherical bushing expands both inward and outward: https://www.youtube.com/watch?v=6qnYgE-qNHU

[jacquesm]

That's a beautiful write-up. At the end under 'media' there are two videos linked.

[xg15]

I buy everything up to the point where they present a second prototype with the motors opposite to each other, i.e. right in the gimbal lock position you'd usually be careful to avoid. (3:55 in the video)

How does that even work?

[philsnow]

I thought the same thing at around 1:17 in the video; if they stopped the pitch axis movement 90 degrees earlier (so that the poles were aligned), it wouldn't be able to do the roll axis movement because it'd be in a gimbal lock position, right?

At https://www.youtube.com/watch?v=AHUv9Zda_48#t=5m20s they show its behavior around the poles, but the motion it's doing right then is pitch, not roll.

[sacnoradhq]

OskarPuzzle 1D passive linkage in the recommendations, different than a U-joint that has much less 2D of freedom:

https://youtu.be/7Tv-aUC6lpM

[hinkley]

This solution seems to have a lot of “gimbal lock” positions so I’m not sure 3 degrees is really accurate. The gears are so large that you lose 120° of rotation along one axis. 2.6 DoF is probably more accurate.

[ajb]

Not sure - the controllers have 4 degrees of freedom, so maybe they managed to arrange only to lose at most one at any given time, so that they always have three?

[xpe]

What are some ways the robotics field / industry quantifies degrees of freedom?

[_a_a_a_]

Can't see teh video but I think I've seen this before - very neat but there seems to be a couple of conditions where a kind of lock up (gimbal lock?) could occur.

[leeoniya]

reminds me of the Triple Gear: https://www.youtube.com/watch?v=I9IBQVHFeQs

[henearkr]

Edited. Thought the name in the linked article ("ABENICS") was a reference to the former PM (yikes), but instead it is probably a reference to the author of the article himself.

Still weird to call a technology by one own's name... usually we let other people do it for us. Especially in Japan, where humbleness is valued. Thus I am still a bit suspicious.

I would still welcome an alternative name, more descriptive.

Edit: "spherical gear" is the name used elsewhere, and it fits nicely!

[jabbany]

Umm, you do realize that the first author's surname is also Abe? And that in general it's a common-ish surname...

[pimlottc]

It's a product they're looking to commercialize. Naming products after yourself is nearly as old as time.

[RobotToaster]

The fact that it's in capitals makes me think it's an acronym of some kind? Active Ball Effector?

Or since it's always written next to "active ball mechanism" it could be Active Ball mEchaNICS?

[formerly_proven]

Might be a pun on both one of the author's name and abenomics.

[HPsquared]

Perhaps someone else in the group coined the name?

[keepquestioning]

Can this be used in 3d printing?

[xpe]

To clarify: you are wondering if this has applications to make better 3D printers?

[samwillis]

This is far from trivial, there is significant control needed to keep the gears in sync and track the relative position of the main spherical gear.

[jacquesm]

Trivial??

[lizardactivist]

Just looking through the paper makes my head spin. The Japanese are undoubtedly the most skilled engineers in the world.

[keepquestioning]

Japanese and also German.

Old-school, driven by craft, not obsessed about clicks and profit. Just incredible utility to society and civilization.

Thank god for these countries.