In addition to tons of FIRST stuff, I also go to college (surprise!). For RPI’s Intro to Engineering Design class, we start the first month of the semester by working on a “mini project”, where teams compete in an engineering challenge. My small team (3 people) competed in a marshmallow shooting competition.

For the marshmallow shooting competition, students were given 5 attempts to shoot one marshmallow a distance between 5 and 20 feet to be determined on competition day. In addition, teams had to display competency by firing a marshmallow over 20 feet. The primary design challenge essentially was consistently shooting a marshmallow a varying distance.

We followed a standard engineering design process fairly “to the T” for the project as we were being taught and graded on it. Stared off by figuring out our priorities and identifying the problem. Got all the fun technicalities out of the way (no, we couldn’t shoot more than one marshmallow per trial for a “shotgun approach”; the marshmallow had to remain intact, the firing mechanism could not leave the starting area except for random strings or whatever, etc etc). Then we went to our design specifications, pulling most from the manual and grading rubric and some “for fun”, though unfortunately we failed at making our project able to put on a laser light show. I quickly figured out that it’s pretty similar to what I’ve done in FIRST every year, which got me thinking of a concept.

Eventually it came time to prototype. Most teams basically drew on their past experiences and picked whichever design seemed simplest to them, but my team had a few different ambitious designs and the fallacious idea that we had a lot of time to complete the project. The first idea was similar to a lot of designs: Stored energy (in the form of a mousetrap) hitting the marshmallow and using the angle of launch to adjust the landing position. The second idea was one I drew on from FIRST: a flywheel shooter that used 1-2 high velocity wheels to throw a marshmallow super far. We decided thanks to my Vex kit that made building my prototype super fast and easy that we had time to build prototypes of both designs and go from there.

In about a day I whipped this little thing up together:

Vex Flywheel Shooter

Geared 1:25, nothing could go wrong!

It spun the wheels at about 2ooo RPM, which translated into about a 6-8 foot shot for a marshmallow. It didn’t fully work, but it was relatively consistent (all shots landed within 6 inches of each other) and the team was very attracted to the idea of varying PWM signals for pre set distances and ranges. Our plan to get the necessary range out of the thing was to set the machine’s angle to 45 degrees, raise it about a foot off the ground just within the size requirement, and to use the Vex High Strength Motors to get the machine spinning about 60% faster. With the success of the prototype above we chose this direction to go.

Unfortunately, the High Strength Vex Motors were on backorder, so we ultimately decided to see if it was at all possible to make do with the current Vex motors and just gear them even faster. Now, in retrospect overgearing a vex motor 35:1 sounds really, really stupid, but at the time… Nah, it was still stupid then too.

At least the structure was coming along nicely.

Anyway, the gearup was so severe that one motor was spinning significantly slower than the other. We tried changing out shafts, gears, and the motor itself until we came to the conclusion that it was acting that way because the weight of the gears was pressing down on the motor shaft on one side, and the other side did not have this problem. With any reasonable loading at all this difference is absolutely negligible, but not with a 35:1 overgear with Vex’s “slop-py” holes. Ultimately we quickly ended up going with a DC motor and AC-DC voltage regulator to power said motor. Since DC motors don’t really couple with Vex out of the box, we had to get creative…

Try and tell me this isn't cool.

Using some tube clamps, foam tape, plastic tubing, and some set screws, we made a custom mount and flex coupling for the motor that mated the pulley at the end of it to Vex shaft. With this major change, we were able to get the wheel spinning and shooting 20 feet. Of course, like all good engineering projects, it was completed the day before, at about 1:30 AM in my room.

A scene I knew far, far too well by the end of the night.

We ended up using only one shooter wheel and a backspin plate for more repeatability, and ultimately did alright in competition. We lost, but we also were the first people to ever attempt this project electrically. This didn’t make much sense to me, as our entire group was straight up mechanical. I guess approaching the project through gear ratios and varying motor speeds “seemed like a good idea at the time”. Ultimately we were beaten by a stupid simple project: A slingshot with two rubber bands and a peg board to adjust wind up distance. D’oh.

If you want, I can include my technical paper on this whole project if you’re interested in the design details and everything, but I don’t think it’s a particularly good paper so I don’t want to just post it in public if I don’t have to…

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