wheelbase. The wheelbase of a robot dictates how well it
can turn. With a longer and narrower wheelbase, there is a
greater tendency for scrubbing. Scrubbing is where the
wheels simply drag across the ground instead of rotating.
(Like when you try to move a Hot Wheels’ car side to side
instead of forward and backward). Even with differential
drive, robots with long and narrow wheelbases will fall
victim to scrubbing.
This is actually something that happened to our first
FIRST robot — MO from Team 1079. MO had a long
wheelbase, and with rubber wheels on a carpet arena the
bot had a tough time turning. If MO had tracks, the long
wheelbase wouldn’t have been a problem.
Continuous tracks promised that our mini tank
wouldn’t have similar mobility problems. Now that we had
the base driving, we needed a cannon to put on top of it.
Don’t Tread on Me
A mini tank needs a mini cannon, and we knew exactly
the design we wanted to use. The small cannon is almost
an exact scale model of our larger chunkers — we even
used the same Schrader valve and the same gauge as we
used on the Mark I. Instead of 4” pipe, we went with 1-
1/2” pipe, and the 1” sprinkler valve looked to be a much
better fit. We laid out the valve on the top of the Agent
390 to see how big of a cannon we could put on top of
the base. The barrel was proportionally shorter than our
larger chunkers, but once it was all cemented together and
painted black, you could certainly see the family
resemblance. (The candy stripe must be a recessive trait.)
Before wiring in the cannon, we pressure tested it. At
first, we tried hooking the solenoid wires into a battery, but
we soon figured out that manually turning the purge valve
was a much easier way to test. The cannon fired with a
much higher pitched boom than our larger chunkers, and
sent a ping pong ball (which happened to be the perfect
size for the 1-1/2” pipe) skidding up the hill in the backyard
before languidly rolling back down.
Wiring up the cannon was simply a matter of hooking
the valve wires into the solenoid port on the Canipede
RCM. To mount the cannon, we originally had grand plans
for making custom brackets with all-thread bent into U
shapes, but zip ties were much easier and they didn’t scuff
the paint. Looping the zip ties through the hole pattern on
the top plate was easy enough — even when we had to be
careful to avoid all of the controller guts underneath.
As a final touch on the mini tank, we added some thin
aluminum body panels: one on the back so the cannon
didn’t look like it was hanging too far off of the end of the
robot, and a scoop on the front (even though the scoop
would limit the angle of approach the bot could make on
an incline). The top of the scoop provided some cover for
our master switch. With the cannon, tracks, and front
scoop, the bot looks a bit like a design you could use for
things beyond casual target practice.
However, casual target practice was the name of the
game for the time being. We used the same plywood
target that we used for the larger chunkers, even though it
was a bit oversized for the mini tank.
56 SERVO 04.2016
WIRING UP THE MASTER SWITCH.
READY FOR TARGET PRACTICE.