Twin Tweaks ...
VEX GEAR SET.
VEX OMNI WHEELS.
up with a configuration that allowed us
to fit all of the LEDs.
The compass module is powered
by a nine-volt battery, and when it is
turned on the four LEDs will light up in
different patterns to signify different
directions. This makes the compass
module an entertaining project in and
of itself, because it can be used with
only the pieces included in the kit (with
the exception of the battery, which you
have to get yourself).
The manual goes on to specify
several techniques to save power, and
it even has good instructions for an
interface circuit. The problem with
this is that the circuit itself is fairly
complicated, and tinkerers not willing
to buy the interface kit from Images SI
will have a lot of parts to round up. The
manual also includes several programs
for the PIC microcontroller, but since
we wanted to use it on the Vex robots,
that didn’t really help us.
Instead of focusing on the interface
of the compass module, we decided to
move onto the Vex robots and simply
take the experience as a mere taste of
the challenge that Grand Challenge
teams face when outfitting their vehicles
with full sensory suites. These vehicles
need much more than just compasses,
because they also need to detect
obstacles and react to vastly complicated terrain. We shudder to think at the
interface circuits that they have to craft.
The Vex Robotics Design System is
a great tool to learn about and experiment with robotics, even with just the
basic starter set. This set, however,
does have some limitations. Most of
VEX AUTOMOTIVE IN PROGRESS ...
them can be overcome by good old-fashioned ingenuity, but the Vex folks
have also put out a variety of expansion kits to kick-start your imagination.
We got our hands on two such kits — a
gear set and an omni wheel set. With
such additions, it seemed only natural
to create a versatile Vex vehicle capable
of tackling tons of tough terrain.
The gear set most notably includes
some worm gears (for lots of torque) and
a rack set. The rack set is the vital component for a rack and pinion steering
system that was absent from the starter
kit. To complete the bill of materials
needed for automotive steering, the kit
also comes with beveled gears and a
pumpkin to make a differential. The omni
wheel sets come with two omni wheels
each. With the new parts, we were ready
to make some all-terrain Vex vehicles.
We did plan to make more than one
vehicle, for the very reason that omni
wheels and automotive steering don’t
exactly mix. If one was to put omni
wheels on the front wheels of an automotive vehicle, it would be kind-of like
driving the vehicle on ice. The wheels
would turn but the vehicle would keep
moving straight ahead because of the
rollers on the omni wheels. Since we
weren’t really interested in any ice
capades, we planned to use the omni
wheels on a vehicle for which they
would be advantageous — tank drive.
We have had some personal experience with a situation where omni
wheels could certainly have been very
helpful in the FIRST Robotics competition. In 2003, our robot MO was a robot
with tank style drive, but we had some
trouble steering it. MO had a long
wheelbase and high friction rubber
wheels, which created a lot of
scrubbing and made steering
difficult. Omni wheels could
have eliminated the scrubbing
on the back wheels and had
MO turning on a dime rather
than turning like a super-tanker.
Our omni wheel tank drive
robot was quite straightforward to build. We chose to go
with a long wheelbase for the
68 SERVO 09.2007