Yo-Yo Launcher! - Living out my FRC High School Dreams Round 2

A close friend of my family’s who invented the yo-yo style called Soloham (2 offstring yoyos at a time on one string) had a request for me which I had a bit of fun making. He wants to try using 3 offstring yoyos at a time in his yoyo performances but has a problem; there’s not really a reasonable way to hold and throw 3 of these giant offstring yoyos. And so he asked if I could make a robot/device that could help him with this.

Taking inspiration from my high school days in FRC, I figured a magazine of yoyos and a flywheel shooter would be the easiest method to get it done right on the first try. I’m fairly busy with many robot projects and work these days (as might be apparent by my lack of blogging!) but I decided to fit this in.

To put things into perspective I started this project on a Friday night and finished it by Sunday night. My 2 Ender 3s printed continuously the entire time. The design/build process felt very reminiscent of my HackMIT/MakeMIT days which was nice.

I started by digging through my robot parts and found some Banebots wheels of the same diameter as my typical offstring yoyos and this strange brushless motor. It seems normal at first until you realize its an 850kv ~50mm outrunner! This high kv is rather uncommon (due to there being a finite number of ways to rewind for different kvs) but the way the company did this was by reducing the number of pole pairs. This is a 4 pole pair motor (and only half the perimeter is covered in magnet - yuck). Luckily this was the perfect way to use this motor. I wanted high speed (yoyos spin at around 6000-10000 rpm) and high power to make sure things worked, and the inefficiency from the magnet situation wouldn’t be a problem. I hadn’t heard of this company before but Austin told me this was the Hobbyking before there was Hobbyking.

My plan was to gear down the flywheel to the speed I wanted the yoyos to spin and decided on a 3:1 reduction which I did with some spare GT2 belts and printed pulleys. I turned the ends of this 1/2” hex shaft to fit bearings on the lathe. A live shaft was easiest for interfacing with the Banebots wheels and I figured frame structure would be fine without a dead shaft.

Much CAD later and I had a design ironed out. I had originally wanted to get a full 180 degree sweep of wheel-yoyo contact to maximize time I could put energy into the yoyo. I had also considered ordering the large flat side plates from SendCutSend, but I wanted to at least test the concept of flywheel yoyo shooter first. Soooooooo I chopped the hood of the shooter to ~120 degrees which was the max I could fit on my printer and started printing some test parts!

I promise I’m not usually the kind of person who would print EVERYTHING but I apologize since my mill was down (blog post on that later) and I wanted to knock out everything in a weekend. Also spoiler alert, this worked so well I ended up not SendCutSending anything and just ran with the first iteration of all parts I made lol. You can see I even had to reposition the binder clips to fit these parts.

This physical cross section of the shooter shows how it works remarkably well! Since the yoyo and the wheel are both not very compliant, I used some closed cell foam on the back of the hood (Thanks Jeremy for the tip!) and it worked fantastically.

I designed the magazine to be modular where you could print repeating elements. A small servo with an arm forces one yoyo at a time into the flywheel and also blocks the next yoyo from falling in.

There’s a physical button on the side for an assistant to launch the yoyos, as well as the XT30 at the top that leads to an optional long cable and foot pedal for one-man use. The electronics are all shoved in an orange plastic pocket (redbrick esc and arduino nano) and hopefully won’t be touched again. The green knob is for speed control.

There was a little bit of a hack to get this all done in the weekend. I only had a continuous rotation pot which wouldn’t be great for someone who was not me to use and so I mechanically constrained it with a groove and dowel pin.

Although I designed the launcher specifically for my favorite offstring at the moment, the SkyDancer, the compliant hood allows for basically every yoyo I own to be launched effectively. I took these to the park and had some fun.

And the finished product! I’ll post a video here soon but its able to launch some yoyos ~50ft in the air and they have enough spin to do Soloham tricks so I view this as a great success. I still need to coordinate if they want the launcher to be battery or power supply powered but that’s just a matter of swapping in a 24V power supply.

If I were to redo this, I would have had another wheel at the exit to help reduce linear speed while increasing spin, but I’d rather not have to redo this project for the time being. It was a LOT of printing and I have other projects to get back to!

I’m happy to report that every part was only printed once with the exception of one… The speed control knob haha. I had to print that one twice to get it to fit nicely lol.

Video coming soon!