to move, or what angles
you want it to turn (there
are many other parameters that you can control).
You no longer have to
worry about controlling
and monitoring each
individual wheel to do all
of this. The robot’s motion
is now controlled by a
small set of RS-232 serial
commands or I2C commands, and best of all, the
BASIC Stamp no longer
needs to update the servo
positions every 20 ms. This
frees up the BASIC Stamp
to focus on monitoring the line sensors and solving the maze.
When the robot reaches an intersection, it can query the
WheelCommander to see how far it has moved and can
record this value for later analysis.
Figures 2 through 13 show some assembly steps in
upgrading the Parallax SumoBot with WheelWatcher
encoders and the WheelWatcher Commander for motion
control. Figure 2 shows the original Parallax SumoBot (before
modifications) with the opponent infrared sensors removed.
Figure 3 shows the jumbled mess of wires after the BASIC
Stamp embedded in the SumoBoard has been removed.
Figure 4 shows the servo removed from the SumoBot
next to all of the parts that come with a Wheel Watcher kit.
All of the nylon spacers and washers shown here are not
used in the assembly process. Depending on the geometry of
the servo motor, different combinations of washers and
spacers are used to ensure that the Wheel Watcher board is
properly spaced on the servo.
The manual that comes with the WheelWatcher lists
several spacer and washer combinations to use with several
different types of servos. The Parallax servo shown here was
not included in the list in the manual, but it uses the same set
of spacers as the Futaba S3001; so use the short spacers and
Figure 3. SumoBoard removed
from the SumoBot.
Figure 4. Servo, wheel, and Wheel Watcher components.
the thick washers in the assembly process.
Figure 5 shows the WheelWatcher mounted on the
servo. The clear plastic disk on the servo’s output spline is the
alignment tool that comes with each Wheel Watcher kit. This
tool ensures that the encoders are mounted with the proper
orientation with respect to the servo shaft. Figure 6
shows the servo and Wheel Watcher mounted back on the
The existing Sumo ring edge sensor cannot remain
mounted in its original position using the existing 1-1/4 inch
long aluminum spacer. This will interfere with the connector
on the Wheel Watcher. It will have to be removed. Since line
sensors on line following robots are located more towards
the centerline of the robot, I turned the sensors around and
used the two existing holes at the base of the robot frame as
new mounting locations.
To conserve parts, I used two of the long spacers from
the Wheel Watcher kit and the small nylon spacers that were
on the original sensor post with a new 4-40 x 5/8” long
screw to remount the sensors. Figure 7 illustrates these
components and Figure 8 shows the bottom of the robot
with the new sensor orientation. Note that the distance
between the sensors is now about one inch.
Figure 5. Wheel Watcher mounted on the servo.
Figure 6. Wheel Watcher and servo mounted
back on the Parallax SumoBot.
SERVO 03.2008 21