[GBC] Akiyuki's Catch & Spin Robots : way of working, mods and improvements

[Thierry-GearsManiac]

Goals for this topic :

1. Describing the inner workings of the module : highlighting the tricks used by Akiyuki for automating the robots' movements (without compromising his copyrights on the model and building instructions --sold at a very reasonable price, around 8EUR)
2. Showing my mods and improvements on this module, and gather other people's ones and comments too, of course

Like everybody, I was impressed when discovering this module for the first time. And although I partially guessed its working, I only discovered some mechanical tricks much later when watching the video again... and even a last one a few weeks ago when I built it ! That's why I want to share all of this.

Official Akiyuki's video, as a reference :

Now, (1) how it works (hidden for people who don't want to get spoiled, but prefer guessing)

Spoiler

The whole cycle of actions of a robot consists in 4 stages.

The 3 first ones are ensured by the mechanism inside the robot itself, featuring 3 degrees of freedom.

As we can see, a vertical driveshaft powers the robot's cranksaft responsible for the hands' motion. It also acts as a central pivot for the spinning of the robot.

The crankshaft is part of a 4-bar linkage moving the head and the hands (1st degree of freedom). But the lower bar isn't bound to the base, but part of a parallelogram linkage (2nd degree of freedom) which will raise the front of the robot if a ball ever get caught between its hands and blocks the cranking motion, overcoming the gravity which previously held the front of the robot down.

A roller at the bottom of the front of the robot initially rests in a well on the edge, preventing the robot's body from spinning freely (3rd degree of freedom), unless the front of the robot rises enough.

So :

1. Robot in its resting position, front down because of gravity, prevented from spinning. No ball between its hands = hands clapping.
2. A ball between its hands, then the cranking motion gets blocked and, since the body can't spin yet (roller down the well), the front will rise.
3. Now that the front is in its up position, the gravity and/or the reaching of the front's stop position forces the whole robot's body to spin (at the same speed of the vertical driveshaft)
4. But that's not all. As soon as the robot has traveled approximately 135° from its resting position, an extra mechanism comes in action, increasing the rotation speed of the robot's body higher than its vertical driveshaft's one. Therefore, from the robot's point of view, this driveshaft spins backwards, and the robot releases the ball ! (and the robot's body acceleration ends slightly below 360° of rotation)

This accelerator mechanism can be seen in action at 1mn26s in Akiyuki's official video and is located behind the robot. It works like a half circle gear which would be mounted on the robot's body and meet one high speed pinion, if built in non-LEGO technology. But with LEGO's technical limitations, it has been done by :

• placing the "half circle gear" system remotely = behind the robot's body, while driving it in a 1:1 gear ratio, hence the 2nd Z28 turntable (sometimes replaced by the recent Z28 gear, Id=46372, see)
• emulating the half circle gear with an eccentric Z24 gear which meets three Z16 fast-spinning gears one after another (I only noticed this subtle detail / clever trick a few days ago when building the module, although I already understood the partial gear trick roughly)

 

(2) My mods and improvements

Here is a bottom view of my build, where we can see the most :

1. (a very very simple mod) Bringing the mechanical power from outside :
   replacing the M motor with an axle which comes out at the right (sorry, hidden by the white beam supporting it)
2. (mod) Spinning the robots in opposite directions :
   replacing part of the central transmission unit (see Akiyuki's build manual p79-83) : central drive of both robot units made through a right angle gearing, see the 7L axle with Z12T gears at each end and a Z20T gear inside the fork. I also replaced the two Z16 input gears (below the 'L' beams) by Z20 ones.
   I forgot to say that the robots must be mirror-built too (otherwise one robot will have its crankshaft spinning in the wrong direction, and its front being pushed downwards instead of upwards when it grabs a ball).
   (for information, the original transmission has the Z20T gear parallel to the robot units' bases, driving them through a parallel gear train ==> odd number of gears from one robot unit to the other ==> same direction)
3. (mod & improvement) Robot's position auto-reset if it loses its ball :
   When the robot begins to spin with a ball in its hands (stage 3), it holds it too weakly because it is too light for the roller to roll down the tilted platform's slope and maintain the hands firmly enough. Although Akiyuki saw and tried to fix this problem by adding adjustable friction to the spinning of the robots' bodies, this makes the problem even worse when a robot drops a ball while in stage 3 (or sometimes when the robot exits its fast spinning phase = stage 4, without its roller falling into the well) because it stays in its position forever without human intervention.
   That's why I replaced the adjustable friction generators with a detent mechanism which forces the robot to return to its home position (or to hold the ball more firmly), see the Z12 pinions at the rear, the partially hidden Z24 gears (with rollers mounted on opposite round holes) and the blue angled 4/4 liftarms acting as pawls.
   
   My initial try was a simple crankshaft with a spring-loaded piston, but the shaft's torque around the home position was too weak, whereas it was too high around 90°(applying too much force on the robot's hands mechanism which has a high flexibility and backlash, so that it skips a turn, making the robot throw the ball away and returning home).
   The alternate solution is a cam with the correct shape... which doesn't exist or can't be built in an enough small size to fit in the remaining room. And the solution came by dividing the robot's body rotation by two, hence a bi-foil cam looking like a technic beam (here two rollers in order to decrease wear) !

For more clarity, I can make 3D mockups if needed.

Edited April 20, 2020 by Thierry-GearsManiac
correcting typos and completing the post, URL updates

[Ankoku]

Good work.

You should post a video of it in action!

[Thierry-GearsManiac]

Thanks.

For the video, why not. But since it is impossible to show the mechanism (under/inside the module) in action and its effect on the robots, I'll first explain it with the help of a mockup, then I'll show the robots.

I'm not very skilled in videos (and not at all in video editing) but I'll try to do my best.

Coming soon...

[doug72]

When i built my Catch & Spin, the two robots were out of phase but decided not to adjust them to be in phase.
 Have had no problem with balls dropping.

Only problem i have had was a 5L axle with stop slipping out of engagement

https://www.eurobricks.com/forum/index.php?/forums/topic/117305-gbc-the-akiyuki-project/&page=91&tab=comments#comment-3177193

I used a 8L axle with stop as easy to fit, should be enough space to fit a tray.

 

 

 

 

 

 

 

Edited February 4, 2020 by Doug72
added correct link

[Thierry-GearsManiac]

@Ankoku :

Here is the video (not YouTube ==> not embeddable) :

https://diode.zone/videos/watch/289b6802-ac4b-4639-8bd5-ca63adec6999

(Disclaimer : I'm stammering in English and I'm not used to make videos)

This is a demo of my detent mechanism using a mockup which is built "open" for visibility purposes, whereas the real implementation is covered by a frame for supporting axles on both ends.

 

However, if Akiyuki's original central drivetrain is used (pages 79-82 on the instructions), there will be less room, so that the pawls will have to be built differently. I challenge you to publish a solution !...

[Ankoku]

Hey, I can't even speak a second language! You did a great job.

It is really nice to see it in action, especially with it running slowly via hand-crank.

You should create a Stud.IO file of just the bit you changed and a few pieces in white to show where it connects to.

[Thierry-GearsManiac]

Modeling it in Stud.IO this way, I had the idea in mind.

But this 1st version of the detent mechanism takes so much room that only my modified drivetrain can fit into the remaining space under the module (and, in turn, requires to mirror-rebuild one robot) : many cascading changes.

I'll do it, because people may be interested by these bundled changes too (i.e. including the symmetrical spinning of the robots).

What will be better in the future is a still more compact design which will be compatible with Akiyuki's drivetrain (his combined Z20+Z24 intermediate gears take the place of the pawls' axes).

 

[Ankoku]

I think people can figure out how to mirror a robot themselves, so I won't bother modelling that bit.

Edited February 15, 2020 by Ankoku

[Thierry-GearsManiac]

@Ankoku :

Here is the model :

http://www.mgtx.fr/LEGO/GBC/CatchAndSpinRobots_mods/CSRMods_DrvTrainNewAndDetent.io

The module's parts where the mod connects to are shown in translucent green (for a better constrast) and are reduced to a minimum, in order to comply with Akiyuki's copyright.

Prior to building it, one musts do the following :

• removing the two adjustable brakes (see submodels p.58 and 60)
• removing the large end of the main drivetrain (i.e. the spur gear side), only keeping the fork with the right angle gear pair which connects to the M motor
• replacing the outer Z16 gear by a Z20 gear on the input shaft of each robot unit (one can see this in my CAD model)

Then, the choice of which robot to rebuild in mirror and the position of the Z20T gear are tightly dependant, according to the desired rotation direction for the robots (inwards or outwards). In all cases, this must result in the robots' crankshafts spinning clockwise when observed from the right side of the module = the left side of the robots (so that they lift the ball they pick up, because the parallelogram linkage in their body moves clockwise).

For inwards spinning, the mirror-built robot is the right one and the Z20T gear is positioned on the left side (like in my CAD file). For outwards spinning, do the opposite.

At last, rubber bands must be added between each bushing-terminated pin and the towball-terminated pin.

@Doug72 :

I've understood. Mechanical vibrations make press-fit connections (here gear to axle) slip over time, and it looks like the grey (odd-sized) axles are even more slippery than black ones. An extra bushing and a black axle prevented the slipping.

Edited November 30, 2021 by Thierry-GearsManiac
correcting an unclear explanation ; updating URL ; Z20T

[Ankoku]

Great work. Did a quick render, so people can see what your file looks like!

You have done a good job at showing where it connects. Thank you!

Today I learned that transparent green doesn't ender well :D

I should have flipped it over to show your work, rather than lots of green.

[tismabrick]

@Thierry-GearsManiac, the links to the pictures and the model are not working anymore.

Edited April 19, 2020 by tismabrick

[Thierry-GearsManiac]

On 4/18/2020 at 7:26 PM, tismabrick said:

@Thierry-GearsManiac, the links to the pictures and the model are not working anymore.

URLs corrected (the two ones I forgot to update in my posts)...

[OldRed]

Hi, Do you have any instructions just for the opposite spinning mods by itself. The write up on the mod is not clear. I do have the Akiyuki instructions.

Thanks in advance

[Thierry-GearsManiac]

Unfortunately, I have never made build instructions for this associated feature. I only have the Stud.io 3D model for it, see the download link above.

(ans I even can't work again on it or export a more visible picture because a LINUX update broke the working of Stud.io under Wine and I failed to make it work again)

The only extra thing you need to do is to rebuild one of the robots mirrored, then to adjust the position of the single-bevel input or output gear in my new drivetrain.

Perhaps I can help you on specific points first : a difficulty to find which sub-elements to remove ? Or to see how to install the mod ?