Treads are a special kind of differential drive, in which two wheels are each driven by a motor. The wheels are mounted on either side of the robot, like the treads. Figure 2-7 shows a top view of

such a robot.

Independent drive wheels behave a lot like treads. If you run them both forward at the same speed, the robot moves forward. Run the drive wheels in opposite directions, and the robot will spin in

place. This design also exhibits zero turning radius, but without the slipping of the treads.

Zero turning radius is nice in robots that measure how far each wheel travels. (You could do this using rotation sensors.) Once you know how far each wheel on a differential drive robot has

traveled, you can calculate with a fair degree of accuracy the location of the robot relative to its starting point.

The idler wheels don't drive or steer the robot. They exist to keep the robot from tipping over. They can turn freely to point in whatever direction the rest of the robot is moving. Figure 2-8 shows a

typical idler wheel.

Figure 2-7.

Independent drive wheels

Figure 2-8.

An idler wheel

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