The perfect constraint was round, rubbery, and came
with a hub for a 3/4” shaft. The 10” Colson wheels from
Robot MarketPlace that came with a hub would be the
perfect fit for our bot, and much more cost-effective than a
large scale version of the tracks that we put on our tiny tank.
Their diameter provided a perfect design constraint: They
would provide the maximum height for the robot because
we wanted the whole thing to be invertible. The amount of
recoil we saw during our high pressure test at 1,000 PSI — at
one third of the maximum pressure — had us thinking that a
full pressure shot had a real chance of flipping the whole
robot over. So, with about 3/4” of clearance on the top and
bottom, we had a major dimension of the robot. Our main
challenge would be fitting everything under and around the
cannon, and the first step in sorting out those placements
would be to figure out our frame.
Given the heaviness of the cannon (which clocks in at
about 120 lbs), we wanted to go with a lightweight frame
so as not to stray too far afield from the weight
restrictions on heavyweight robots. The extruded
aluminum frame of Protobot had served us so
well that we decided to keep it, and we stripped
off all the components that had made it an
excellent prototyping platform for over a decade.
The resulting skeleton was fearfully lightweight, so
one of our early goals with the structural design
of the base would be to add some much needed
One of our favorite ways to do that is with a
floor panel, and the Protobot frame rails had a U-shape perfect for attaching one. We didn’t want
to invest in a large panel of 7075 aluminum just
yet, so for mockup purposes we used a sheet of
particle board. It would still be in the same plane
as the final aluminum panel, and would help us
mock up some possible component placements.
We planned on attaching the floor to the bottom
lip of the frame rails, giving the frame kind of a
sunken living room look.
Our initial thought was to attach the bearing blocks to
the top of the frame rails, using the same wheel wells as we
did on Protobot. We cut up a 2x4 to make mock bearing
blocks, using lightweight aluminum conduit for mock axles.
To complete the mockup, we put some horizontal struts
across the top of the inner bearing blocks (more wood) so
that we could set the cannon on top.
Our wood-aided design (WAD perhaps) made some
things immediately apparent. The first is that we would not
be able to mount the cannon on top of the bearing blocks; it
would be too tall. We would need another way to mount the
cannon while keeping enough room inside of the bot for all
of our components and trigger mechanism.
We didn’t like the idea of attaching the cannon mounts
directly to the floor given the crazy amount of force they
might experience when the cannon fires. We happened to
have some nice 3” wide 1/4” thick planks of 2024 that
would make some ideal horizontal struts extending
between the tops of the frame rails. The beefy struts
would be the perfect attachment point for the cannon
Having the mockup in front of us helped us quickly
determine an effective placement for the horizontal
struts. Our first instinct was to place them between the
bearing blocks, but we saw that such a placement
would wreak havoc on the placement of other
components by criss-crossing the prime component-placing real estate on the floor. If we placed the
horizontal struts under the bearing blocks, however, we
would have plenty of room for our other components.
The most critical items were the motors. They were
certainly the biggest, and we were confident that the
smaller parts could be arranged around them. However,
our WAD mockup immediately revealed a major
56 SERVO 08.2017
SKETCHING OUT SOME BEARING BLOCKS.
TESTING OUT SOME LAYOUT IDEAS.