CHAPTER 6 ■ ROBOT LINE-FOLLOWING
Sandwich has two wheels. If the robot had four wheels, it would be very difficult for it to pivot. Mainly
the robot would go straight. On the occasions that it did turn, it would make a horrible vibrating noise as one
pair of wheels was dragged horizontally rather than in the forward direction they want to roll.
There are other wheel arrangements possible, such as a single caster or ball roller on the front or rear,
making a three-wheel robot. There are even omni-directional wheels available, containing multiple sets of
perpendicular rollers. Although not a critical factor for small, lightweight robots, wheel formation becomes
more significant with heavier or faster robots.
Mounting the two wheels toward the rear causes the front lip of the robot to drag on the ground. This
isn’t a big problem, although the plastic does scuff up a bit. Generally the plastic slides quite well in any
direction. This makes for reasonably smooth forward movement and agile turning.
Sometimes the front of Sandwich skips and hops due to uneven or gritty terrain. With fresh batteries
and tall wheels, Sandwich occasionally stands up on its rear and pops a wheelie! Even though that’s a blast to
watch, it causes the robot to momentarily lose sight of the line, sometimes causing it to head off course.
Fitting and Changing Wheels
LEGO makes fantastic wheels. A wide variety of shapes, styles, and sizes are available from LEGO kits in
retail stores, from online auctions, or from the LEGO Shop At Home web site. The prices are reasonable and
no one produces higher-quality parts.
Most wheels consist of a tire (the outer rubbery part) and a rim with hub (the inner hard part). Almost
all LEGO hubs are compatible with LEGO cross axles (see Figure 6-16).
Figure 6-16. LEGO wheel (left) and coupler on motor (right)
If there was a piece that could connect a motor to a standard LEGO cross axle, then your robot would
have access to almost the entire line of LEGO wheels. It turns out that you can make such as part, called a
coupler (see right side of Figure 6-16).
With a coupler, you can quickly slide different wheels on and off the robot, without needing tools.
This is great for experimenting. It can also save you from a disaster if you suddenly discover the wheels you
originally planned to use are too short or too wide.