Servo - December 2015

54 SERVO 12.2015

ometimes the greatest gift a roboticist can give themselves is to revisit a project and find ways to optimize it. Getting an initial proof of concept working is always a marvelous achievement, but it often just scratches the surface of the project’s potential.

Last time, we built a compressed air robotic punkin chunker. A simplified isothermal model helped us determine some effective proportions for the chunker, and we crafted the Mark I from four inch diameter pressure-rated PVC. The resulting chunker was a pretty impressive sight — over 10 feet of tubing, painted a menacing flat black. Despite having over six feet of tank, the power of the chunker was limited by a literal bottleneck — the small diameter of the electric valve.

By revisiting our chunker, we could figure out a way to overcome this design limitation, improve the power of the cannon, and even devise a more seasonally appropriate test.

The Nightmare Before Christmas

Our moniker of “punkin chunker” was more or less aspirational — for our initial tests, we used foam balls because we weren’t sure how the chunker would work. In case the muzzle speed was approaching what we had calculated with our simplified model, we wanted to ensure that we wouldn’t be accidentally raiding our neighbor’s yard or house with a pumpkin like Jack Skellington stumbling into Christmas Town. Four inch diameter foam balls were a literal perfect fit, and in our initial tests they launched nicely (and pretty harmlessly). However, they didn’t fly too well because of the air resistance.

Our next set of tests was with a baseball, also about a four inch diameter. We wrapped a sheet of newspaper around it to give a snug fit in the barrel. The baseball sailed through the air as smoothly as a sleigh pulled by a fleet of reindeer, and we had no problem hitting (and sometimes seriously overshooting) a target that we placed 60 feet away.