Twin Tweaks ...
VEX TANKALL DONE!
tion the wheels would scrub in protest
at the prospect of too tight a radius.
We wondered what the problem was,
and the answer lay in the geometry.
One of the reasons that our automotive Vex vehicle had a bit of trouble
turning was that we had difficulty
replicating the Ackermann geometry of
automotive steering with the Vex parts.
Ackermann steering geometry is a
specific arrangement of the steering
linkages in an automotive vehicle
that solves one of the big problems of
automotive steering — the fact that the
two front wheels need to trace different
radii throughout the course of a turn.
The solution presented in
Ackermann steering geometry simply
stipulates that you should be able to
draw two straight lines between the
steering pivot points and the steering
kingpins that intersect at the center of
the rear axle. Because of the limitations of using prefabricated Vex
parts, the steering geometry of our
Vex bot was a little less than ideal,
with the pivot points positioned
too far outboard on the robot.
Another problem stemming
from our less than ideal geometry
was the fact that the front
wheels of the robot were a
bit toed in. That means the
wheels pointed inwards instead
of straightforward. In real cars,
you actually want the wheels to
be a bit toed in, but it is a delicate
balance. Thankfully our pivot
arms had a bit of adjustment in them,
and we were able to sort out that
problem without a major redesign.
Apples and Rocks
In the end, our long wheelbase
tank drive vehicle greatly benefited
from the omni wheels and was a
breeze to drive despite its often
troublesome shape. This seems to beg
the question though — if omni wheels
are most useful for tank drives with
long wheelbases, then why not just
always design the drive train to have a
short wheelbase?
That would certainly fix the problem, but the truth is that a short wheelbase might not always be an option.
Maybe the robot needs to carry a long
payload, or maybe the robot needs
to navigate an area where a short
VEX OUT IN THEWILD!
wheelbase would not be effective. And
maybe the cool factor of omni wheels
is just too good to pass up.
Our automotive robot drove well
enough, especially gliding through
wide turns. The scrubbing during tight
turns is something that could be taken
care of with a bit of tweaking, but even
without any further redesigns the
automotive vehicle was much more
difficult to design with the Vex parts. It
was also a bit more difficult to drive,
even if only because tank style is the
more familiar configuration that we
have encountered when driving robots.
This seems to bring up another
question — if tank style drive trains are
generally easier to design, build, and
drive, then why were automobiles
designed in a more difficult fashion? It
might seem like a silly question to ask,
but if Henry Ford had made a tank
drive Model T, then that would most
likely be the norm.
So why did Henry Ford choose
what we now know as automotive
steering? A quick side note here —
early automotive steering systems were
a reticulating ball type. Rack and pinion
steering came along later, and is now
the norm.
There are several good answers to
this question. One looks back to the
origins of the personal automobile. The
predecessor of the automobile was the
carriage, which was drawn by horses.
Indeed, the first automobiles were
often referred to as horseless carriages.
Carriages, wagons and the like were all
essentially front wheel drive vehicles,
with the horses or oxen or whatever
livestock acting as the power source in
lieu of a mechanical engine. What we
know today as automotive steering is
simply the mechanical progression
from the horse drawn carriage.
Also, automotive steering vehicles
are advantageous in their power
transmission technique. Tank style drive
trains require at least two motors to
drive the vehicle — at least one for each
side. Our Vex tank required four motors,
and while this was simple because
we used direct drive, if we wanted to
implement a gear reduction to improve
torque, it would have significantly
increased the effort of construction sim-
70 SERVO 09.2007
