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Feb. 6th, 2022

Yo-yo robot part 4: control, slow-mo, and yo-ing.

Occasionally being pleasantly told...  Oh no odrv0 disappeared 


Ok so here comes the part we've all been waiting for — the controls and actual yo'ing of the yo-yo. This is a complicated task of course as we need to have the robot release the yo-yo, wait for it to get to the bottom of the string, and then pull the yo-yo up again so it re-winds. The first half of this post will likely be just understanding the physics of yo'ing so we can replicate the behavior in real life. 

As a side note, we actually did found a robot that yo-yo's a yo-yo, the video is here. Our approach to understanding yo-yo'ing will be to take a bunch of videos of ourselves yo-yo'ing and analyze the data that comes from it.


We have a few hypothesis we will list below.

  • Based on our current yo-yo'ing experience, a shorter yo-yo string means a smaller motion will allow us to retract the yo-yo.

  • A heavier yo-yo will require more force to retract but will be more stable in flight due to the higher inertia.

We will observe ourselves yo'ing the yo-yo to see if we can come up with a control strategy based on what we see. We will take slow-motion videos and edit them in iMovie to add markers and such to draw general conclusions about yo'ing. 

Here's a very basic video of me trying to yo a yo-yo. Already from this video we can see with the full-length string, the arm needs to travel almost as much vertical distance as around HALF the total length of the yo-yo string to properly yo the yo-yo. That and the hand yo'ing the yo-yo and the yo-yo itself all roughly stay on the same vertical line with very little arc'ing motion. 

Just to confirm our findings, we're going to try again, with a much smaller yo-yo string (half the size?) to see if the same trends still hold.

Now here's the second video with the shorter string. In this case the arm has to travel around three-quarters of the distance to get the yo-yo back up to the original point and past it. So I think it's safe to say the arm needs to travel at least half if not one whole yo-yo string length to yo the yo-yo.

It's also interesting to point out while the reflexes need to be slightly quicker for the shorter string, it's easier to yo the yo-yo when the string is shorter very likely because we don't need to make as large of a movement. Between the two trials the arm seemed to travel a less distance during the second, shorter yo-yo string.

Note Feb 6, 2022_edited.jpg

There's also an interesting "phase" relationship going on here — the hand starts moving first of course, supplying the initial energy to the whole system. Close to the bottom of the hand's trajectory the hand lets go of the yo-yo and the string unfurls allowing the yo-yo to glide down and continue along its path. The hand give the yo-yo a tug and the string starts to shorten pulling the yo-yo back up. All three curves meet at the "capture point" where the yo-yo enters the hand.

Note that the red and green curves will always add to give the blue curve due to geometric constraints. We can also see as we shorten the string, the blue curve approaches the green curve for a given hand trajectory. This limiting case makes sense since if the string is of length zero, the yo-yo position and hand would have to match.

The final note is these curves should not be "sine waves" or trig functions. They're more like logarithmic distribution curves where the peak appears rapidly and there's a longer tail response.

A few small updates of the general kind before we begin — the first 

- start by slow-mo yo'ing us and doing measurements
- see the other yo-yo robot online here
- winding the yo-yo so shorter string
- adding a release mechanism to hold the yo-yo on until we're ready to yo


Note Feb 6, 2022_edited.jpg
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