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by
Pete Miles
Q. I would like to know your opinion on performing
some simple modifications to my SumoBot from
Parallax so that it can be used to solve line mazes. It
is my understanding that I would need some encoders on the
wheels so that it can keep track of how far it has moved so
that it can repeat its previous paths. I am not an electronics
expert, so I am hoping that you can point me to something
that is more plug-and-play than build from scratch. Any help
would be greatly appreciated.
— Lynn Brown
LINE MAZE
Line Maze is typically a contest where a line is placed down
the center of a maze puzzle. There is no wall in this type of a
maze and the robot must use the line to solve the maze. Many
contests allow the robot to run through the maze several times,
and the fastest run time is used for the final score. Remembering
the maze path helps to greatly reduce the amount of time
required to solve the maze on subsequent attempts. One
source for a complete set of rules for this type of a contest can
be found at the Robothon website ( www.robothon.org).
Figure 1. The WheelCommander closed loop
differential servo drive motor controller.
Figure 2. Original Parallax SumoBot
before modifications.
A. This sounds like a fun project! I think I have the ideal
set of plug-and-play products for you. Yes, encoders
can be a big help in solving a line maze, but they are
not required. Many people do this without encoders.
As for simple off-the-shelf, plug-and-play encoders that
will work great with the SumoBot, take a look at the
WheelWatcher encoders from Nubotics ( www.nubot
ics.com). These encoders are designed to mount directly
to regular servo motors (that have been modified for
continuous rotation) like those used with the SumoBot.
These encoders will keep track of how many degrees each
wheel rotates and the direction of each rotation. These
encoders also come with a self-adhesive encoder disk that is
mounted to the inside of the wheel. With the SumoBot’s 2. 6
inch diameter wheels, these encoders — by themselves — can
keep track of positional accuracy of at least 0.25 inches of
travel (π 2. 6 inches/32 counts per revolution = 0.25 inches
per count). Technically, all the BASIC Stamp has to do is keep
track of the encoder counts for each wheel as the robot
moves through the maze.
But, having the BASIC Stamp keep track of all of
the encoder counting, direction
control, turning, updating the
servo motion commands every 20
ms, and monitoring the line
sensors while trying to solve the
maze is going to be a bit of a
programming challenge. That’s not
to say it can’t be done since many
people have been very successful
in doing this, but there is another
plug-and-play device that will
greatly simplify all this.
The WheelCommander from
Nubotics (Figure 1) is a closed loop
differential servo motor controller.
With this controller, all you have to
do is tell the robot how far you
want it to go, how fast you want it
20 SERVO 03.2008
