Variations on Obstacle Detection

By using the NXT motor’s built-in rotation sensor, you can experiment with indirect obstacle detection. Program the robot to monitor the rotation count while in motion. If both motors are on forward, but the count doesn’t increase, the robot knows an obstacle has blocked it. As a positive side effect, the rotation count allows you to use the same platform for experimenting with navigation, applying some of the concepts about dead reckoning explained in Chapter 13.

You can also implement indirect obstacle detection using a “drag sensor.” The idea requires that your robot keep a mobile part in touch with the ground, and that the friction this part exerts against the floor surface when the robot moves activates a touch sensor. For example, you can use the friction of a rubber tire to oppose the force of a rubber band that keeps a touch sensor closed. When the robot moves, the friction of the tire on the floor overcomes the force of the rubber band and opens the touch sensor; as soon as the robot stops—or has been blocked by an obstacle—the friction disappears and the touch sensor closes.