3 ms·

Edit: Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I'm not sure others have read deeply into the article. Wh

Edit: Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I'm not sure others have read deeply into the article. While there's always some subtle ways friction can sneak its way, the authors account for the obvious. The article has a lot of definitions, which makes the crux difficult to pinpoint. They use the word "shape" many times to describe the configuration of the four motors. I think this adds confusion. Let me try to summarize as I understand it without using the word "shape." The authors are measuring the net position of the arm holding the two motor tracks. The arm can only rotate clockwise or counter-clockwise in the horizontal plane, so it suffices to measure the arm position using an angle - phi, with initial position being phi = 0. The two motor tracks are oriented perpendicular to one another. A key point is that the motors do not travel up, down, left, or right. That's Cartesian thinking. Rather, they travel clockwise or counter-clockwise along their respective tracks. One track is vertical, the other is horizontal. With this picture, the authors claim that the net horizontal position angle (phi) of the arm changes depending on how the motors move along their tracks. None of this is particularly impressive (e.g. the motion of the motors may cause the arm to "wobble" one way due to issues with synchronization, weight differences, and numerous other causes). The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed. Think about that for a moment...why should the path that the vertically-oriented motors take affect the horizontal rotation of the arm? In Newtonian physics, acceleration/force in one direction cannot affect acceleration/force in a perpendicular direction. At any given moment, the vertically-oriented motors are perpendicular to the arm's rotation direction, so their motion should not affect its horizontal rotation in space. In particular, both the curved-vertical and linear-vertical paths are perpendicular to the horizontal at all times. Newton would predict that changing the path of the vertical motors from linear to curved would not affect the horizontal rotation...but it does.

waterproof 4y ago
Thanks for the interpretation, very compelling if that's what they're showing. Sci-hub doesn't seem to have this paper so I haven't been able to read the whole thing. But from what I can see the data doesn't seem to agree with what you're saying - > The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed In the video, it looks like the same kind of net movement is observed with the cylindrical configuration as with the spherical configuration. With the cylindrical configuration the average position migrates by about 0.2rad and stays there, compared with 0.3rad for the spherical configuration. So the spherical configuration seems "better"(?) but it's hardly a convincing difference given how much slop there seems to be - there's even significant net rotation in the "plateau regime" where it's apparently supposed to stop migrating. And why is there a "plateau regime" anyway? If they can pull off a delta-V with no momentum transfer, I'd expect the robot to keep migrating around in a circle. But it stops. The abstract says: > While this simple geometric effect predominates over short time, eventually the dissipative (frictional) and conservative forces, ubiquitous in real systems, couple to it to generate an emergent dynamics in which the swimming motion produces a force that is counter-balanced against residual gravitational forces. I think this translates to "it just stops working after a bit". What's your take?
- aabajian 4y ago
  Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I think their figures answer your questions...both theoretically and empirically. Figure 3 shows the difference between curved/linear vertical motion.
tbrownaw 4y ago
> Think about that for a moment...why should the path that the vertically-oriented motors take affect the horizontal rotation of the arm? They effectively weigh less when they're far apart. If you look from above (like their video), the curved track means that projected down to the plane it's rotating in they're closer to the axis.

waterproof 4y ago

Thanks for the interpretation, very compelling if that's what they're showing. Sci-hub doesn't seem to have this paper so I haven't been able to read the whole thing. But from what I can see the data doesn't seem to agree with what you're saying - > The interesting part is this: If the vertically-oriented motors are made to move up-down on linear straight paths instead of curved paths, the net movement of the arm is not observed In the video, it looks like the same kind of net movement is observed with the cylindrical configuration as with the spherical configuration. With the cylindrical configuration the average position migrates by about 0.2rad and stays there, compared with 0.3rad for the spherical configuration. So the spherical configuration seems "better"(?) but it's hardly a convincing difference given how much slop there seems to be - there's even significant net rotation in the "plateau regime" where it's apparently supposed to stop migrating. And why is there a "plateau regime" anyway? If they can pull off a delta-V with no momentum transfer, I'd expect the robot to keep migrating around in a circle. But it stops. The abstract says: > While this simple geometric effect predominates over short time, eventually the dissipative (frictional) and conservative forces, ubiquitous in real systems, couple to it to generate an emergent dynamics in which the swimming motion produces a force that is counter-balanced against residual gravitational forces. I think this translates to "it just stops working after a bit". What's your take?

aabajian 4y ago
Here's the pre-print: https://arxiv.org/pdf/2112.09740.pdf https://arxiv.org/pdf/2112.09740.pdf I think their figures answer your questions...both theoretically and empirically. Figure 3 shows the difference between curved/linear vertical motion.

tbrownaw 4y ago

> Think about that for a moment...why should the path that the vertically-oriented motors take affect the horizontal rotation of the arm? They effectively weigh less when they're far apart. If you look from above (like their video), the curved track means that projected down to the plane it's rotating in they're closer to the axis.