Page 331 - Robot Builder's Bonanza

300 BUILD ROBOTS WITH WHEELS AND TRACKS

Pneumatic wheels: Traditional foam and rubber tires are merely fitted over their hubs. In
a pneumatic wheel, the tire is filled with air, which gives the wheel more bounce, but
with added rigidity. Wheels for wheelbarrows and some wheelchairs are pneumatic.
Airless tires: Similar in concept to the pneumatic wheel, airless tires are hollow and filled
not with air but with a rubber or foam compound. They are common on wheelchairs
and heavy- duty materials- handling carts. They’re great for larger bots that have to carry
a lot of weight.

Wheel Diameter and Width
There are no standards among wheel sizes. They vary by their diameter, as well as their tread
width (the tread is the plastic, foam, or rubber material that contacts the ground).

• The larger the diameter of the wheel, the faster the robot will travel for each revolution of
the motor shaft. You can quickly calculate linear speed if you know the speed, in revolu-
tions per minute or second, of the motor. Simply multiply the diameter of the wheel by pi,
or 3.14, then multiply that result by the speed of the motor. See the section “Using Wheel
Diameter to Calculate the Speed of Robot Travel” for more details.
• The larger the diameter of the wheel, the lower the torque from the motor. Wheels follow
the laws of levers, fulcrums, and gears. As the diameter of the wheel increases, the amount
of torque delivered by the wheel decreases.
• Wider wheels provide a greater contact area for the wheel, and therefore traction (from
friction) is increased.
• The wider the wheels, the more the robot will tend to stay on course (called tracking). With
narrow wheels, the robot may have a tendency to favor one side or the other when there
is even the slightest misalignment of the wheels. Conversely, if the wheels are too wide, the
friction created by the excess wheel area contacting the ground may hinder the robot’s
ability to make smooth turns.

When selecting the wheel diameter and width, match the wheel to the job. A robot with
modest- size wheels of fairly narrow proportions (say, 1/4″ wide for a wheel of 2.5″ to 3″ in
diameter) will be more agile than if it were equipped with much wider wheels.

Wheel Placement and Turning Circle
Where the wheels are located on the robot base affects the turning circle of the robot. When-
ever possible, locate the wheels within the body of the base, rather than outside it. This
decreases the effective size of the robot and allows it to turn in a tighter circle. Figure 26- 3
shows wheels mounted both within the area of the base and outside it.

UNDERSTANDING WHEEL TRACTION

As in a car, wheels on your robot are meant to grip the driv ing surface. This provides traction
and allows it to move forward. Yet, oddly enough, with robots both too little and too much
grip can be a bad thing.
Picking up from Chapter 20, “Moving Your Robot,” let’s look at how a differentially steered
robot is designed. It has two motors and wheels mounted on opposite sides. Traction going
straight ahead is simple: when both motors are activated in the same direction, the robot
moves forward or backward in a straight line.
Wheel traction becomes an issue in turns. There are two ways to turn a differentially

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