Page 306 - Lego Mindstorms

Robot 10 • RIS Turtle

The Logo Turtle used precise servo motors driving both wheels. This allowed
the speed of the wheels to be accurately controlled so the robot drove straight.
With LEGO, the speed of the motors cannot be controlled quite so accurately, so
a LEGO robot tends to drive in a wavy line. Our solution to this problem is
mechanical: Both wheels on the RIS Turtle are driven from the same motor, so
they always drive at the same speed. The way the robot turns is by shifting
gears on the right wheel to reverse its direction. When turning, the wheels both
turn at the same speed, but in opposite directions.
The second important feature a turtle robot must have is that it must be
able to measure distances when driving straight, and measure angles when
turning. The Logo Turtle most likely used an optical encoder (rotation sensor)
to accomplish this. Because you may not have a LEGO rotation sensor (the RIS
does not include one), we will improvise and use the light sensor pointed at a
rotating disk to measure distances and angles.
The third feature that a turtle robot must have is the ability to raise and
lower its pen. It is fairly straightforward to create a motorized penholder, but it
is important to remember that the RIS only includes two motors. One of the
motors must be used for driving the robot, and the second motor must be used
for shifting the gears to allow turning, so we need a third motor for the pen-
holder! Here is where some mechanical ingenuity comes in. You can use one
motor to do two different things with some special gearing involving the differ-
ential. Basically, the motor performs one function when driving forward and
performs the other function when reversing. We use this mechanism to allow
the second motor to control both the gear shifter and the penholder. All of this
should become apparent when you build the robot.
Once the robot is built, it certainly needs a program to control it. We wrote
a NQC (Not Quite C) program that allows users to program the RIS Turtle in a
way very similar to the Logo Turtle. The robot will understand commands such
as the following:

pendown();
forward(30);
right(90);
reverse(10);

You can even make the turtle robot “smart” by writing your own function such
as box(x), which could draw a box of any desired size:
void box(int x)
{
pendown();
repeat(4)
{
forward(x);
right(90);

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