Page 351 - Lego Mindstorms

Robot 10 • RIS Turtle

Choosing a Pen

Now that you are done building the Turtle, you need to choose a pen to use
with it. Felt tip pens or markers make the best pictures, because the robot
doesn’t press a pencil or ballpoint pen against the paper hard enough. I used a
Staedler felt tip pen designed for overhead projectors. Sharpie markers also
work well, but they are shorter than the Staedler, so they can be held only at
their end. If you want to use the Sharpie marker, you need to remove the gray
2x2 plate and the yellow 2x4 plate from the penholder. The unfortunate thing
about markers is they tend to bleed a lot, making large dots where the robot
turns. You could prevent this by lifting the pen before turning, but then the
lines may not connect as well. Once you have chosen a pen, you need to make
sure it fits properly in the penholder. When you put the pen in, turn the Turtle
around and check where the pen tip comes through. To make nice drawings,
you want the tip to be as close to the center of turning as possible. Check this
by seeing if the tip is in line with the two drive axles. You can adjust the posi-
tion of the pen in the holder by either adding or removing plates. If you want
to be overly precise, you could wrap layers of tape around the pen until it lines
up exactly in the center. When you put the pen in, the easiest way to set the
correct height is by turning the secondary motor by hand until the penholder is
in the lowest position. Then you can gently push the pen down until it touches
the paper.

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