valve closed, we would need a mechanism that could
hold that force back until we wanted to fire, and then
release it quickly. This design, fortunately, required
less trial and error than the actuation mechanism.
In some of our initial trigger prototypes, we found
success with a simple pin and pulling mechanism. To
deal with the amount of force from Bruce Springsteel,
we knew we would need a little bit of mechanical
advantage. Being in the middle of our time crunch,
we ended up using what we had in Robot Central
— a racecar part, an old homework assignment, and
leftovers from a windshield installation.
The racecar part was the same kind of Maxon
motor that we used for Troublemaker’s drive, which
in its previous life operated a butterfly valve on the air
intake manifold of an Indy car engine. We knew the
motor was stout and lightweight, but that it could
use some more help — that’s where the homework
assignment came in.
As part of his studies as a mechanical engineering major
at UCSD, Evan had to machine some pulley blocks using a
mill and a lathe. We added mechanical advantage to the
Maxon with a double tackle pulley arrangement. Some nylon
rope (part of which had been previously used to install a
windshield) happened to be the perfect size for the pulleys,
so we ran that through, tied one end to a pulley on the
Maxon, and the other end to a hood pin that would lock the
lever arm. We installed an eye bolt on the end of the lever
arm, and a stop out of Teflon blocks and aluminum channel.
The trigger was done!
The working trigger was essential for testing the
actuation mechanism. Testing a giant nitrogen cannon is
dangerous, so the majority of our initial tests were done
unloaded (or with foam balls). For loaded tests, we fired
into hay bales, and slowly ramped up the pressure when we
knew the behavior of the projectile.
It was slow going. Every time we wanted to adjust the
spring steel, we had to remove the cannon. We ended up
changing the slot geometry, adding some more spring steel
layers, then taking some spring steel layers away. With six
layers of 0.050” spring steel, we sheared the 3/8” steel bolt
that held the spring to the lever arm while cocking the valve.
Thank goodness for safety glasses!
Ensuring that the spring steel had a nice smooth arc
was tough. There wasn’t a lot of clearance between the
bottom of the lever arm and one of the internal struts in the
bot, and the spring would tend to scrape on the frame. We
actually ended up shimming all of the cannon mounts to
raise the entire cannon a bit, and that gave a nice smooth
motion throughout the arc.
The very first time we successfully fired the cannon
remotely was a huge victory. When we fired it at
progressively higher pressures, we felt like we were getting
really close. We then weighed the robot in progress and
found we were 10 pounds overweight — with no body
We took dramatic action! We switched to a shorter
smaller diameter 2. 5” barrel and a lighter projectile (going
from 11 lbs to 5 lbs). We clad the robot in paper thin
titanium armor dust covers, painted it red, and literally
fastened all of the body panels to the robot for the very first
time on the first morning of filming.
Yes We Cannon
We’ve nearly hit our word limit and you are probably
thinking that we’ve yet to answer the most burning question
about Double Jeopardy — why the heck is there an old FRC
frame on a BattleBot? The short answer is time and weight.
When we really started building the cannon and drive
train in 2016 and 2017, there was not any season of
BattleBots on the horizon. So, we used an existing platform
to test out our cannon and drive train designs with Protobot.
Protobot has made numerous appearances in these
pages over the years. It was a prototyping platform we
originally built in 2003 with FIRST Team 1079 so that the
team would have something to experiment with in the
offseason. Protobot happened to be the right size for
carting around our cannon, so we used its bolted together
We never planned on using Protobot as the base for
a real competition-ready BattleBot — we figured we would
have the time to finalize our cannon design and then make a
welded steel frame that could fit everything.
However, with a six week turnaround — where we still
needed to figure out the cannon — a new frame just wasn’t
going to happen. So, yes — we brought a 15 year old FRC
frame into the BattleBox.
But don’t worry — we won’t be doing that next year.
WE FINALLY MADE
IT TO BATTLEBOTS!
SERVO 09/10.2018 73