intimidation factor and also to showcase
the advantages of the omni wheels. For
the power transmission, we went with a
super simple, four wheel direct drive system. Normally, we would like to include
some sort of gear reduction for more
torque, but a direct drive is so much
simpler, and the omni wheels make the
lower torque design plenty feasible.
One of the things that makes Vex
robots such great rapid prototyping
tools is that they are so easy to build —
after putting together the drive train,
we just had to fasten the microcontroller, transmitter, and batteries to the
frame and we were ready to roll. We
determined that one last touch
was necessary to compete with the
coolness of the automotive drive
vehicle — some carbon fiber plates to
define the front end.
The automotive drive vehicle was a
much greater challenge. The rack and
pinion steering was difficult to recreate
with only Vex parts, but the new bits in
the gear set gave us a solid start. The
great thing about building an automotive bot was that it would utilize two of
the unique parts from the gear set: bits
for a differential and a rack.
We thought that the differential
would be a good place to start. The
pumpkin has room for three beveled
gears, but, if you want to utilize one
shaft so both wheels turn the same
direction, you can only employ two of
the three beveled gears. The pumpkin
is also designed to rotate, and we had
to constrain that degree of freedom for
our design. That object was easily
achieved by meshing the teeth around
the pumpkin with a gear that we fixed
to be stationary. The motor drove the
back wheels by connecting to one of
the beveled gears, and we had our
fixed diff drive train.
This design more closely resembles
a racing type of differential that is
referred to as a spool type rear end
since both wheels turn exactly the
same amount all the time. A real street
car differential has the ability —
through more gears — to allow each
rear wheel to turn at different speeds
when necessary. This can be achieved
with this kit by utilizing two axle shafts
and the three bevel gears while having
OMNIWHEEKTANK IN PROGRESS.
the whole pumpkin rotate. Since we
are all about high-performance, we
used the racing design.
Rack and Roller
The back wheels were the easy
part. To recreate rack and pinion
steering with Vex parts, we had to
build pivoting mounts for the wheels
that could be slightly rotated by the
linear motion of the rack via pivot
points and kingpins. We tackled it one
bite at a time, starting with mounts for
the wheels. Those went together fairly
easily, and then we were able to see
how the robot might sit given different
placements for the wheels. With the
biggest Vex wheels available, the robot
would sit nicely parallel to the ground
with the given height of our mounts.
We moved onto the rack when we
saw how things could come together.
Rack and pinion steering works by having a fixed pinion (a gear) drive the rack
back and forth. For our rack to move
back and forth, we had to put together
a slider, and branching off of the slider
were the pivot arms. The wheel mounts
VEX RACK.
Robot vs. Wild
OMNI WHEEK TANK DRIVE ROBOT.
were structurally attached to the frame
of the bot with bits called kingpins —
the shafts that the wheel mounts
would rotate around.
Getting everything to come
together was a challenging undertaking, but we were actually able to build
the automotive steering robot completely out of Vex parts with the minor
exception of some Nylock nuts. We
downloaded a quick Easy C program to
the robot to make sure that the
controls were as we wanted them, and
we were ready to test our creations.
The tank drive vehicle performed as
we expected. When we tried to turn
while scrubbing the normal wheels, its
performance was lackluster, but scrubbing the omni wheels gave us a tight
turning radius worthy of a high performance vehicle. The robot was fairly peppy
by virtue of its four wheel drive, and
even without gear reduction it hopped
over obstacles like feet with ease.
The automotive steering vehicle
drove well enough, but it did have
some trouble making tight turns while
moving forward. In reverse, it turned
like a dream, but in the forward direc-
VEX DIFFERENTIAL.
SERVO 09.2007 69