Idler Wheels
Trusty's two main drive wheels determine whether he moves forward, backward, or turns. But without at least one more wheel, Trusty can't balance and will tip forward or backward. Trusty's third
wheel is a good example of an idler wheel. An idler wheel provides support for the robot but doesn't constrain its motion. It does this by rotating freely on a vertical axis. The front wheels on
shopping carts are idler wheels, as are the casters under furniture. Ideally, an idler wheel can easily swivel in response to changes in the robot's direction. You've already seen Trusty's idler wheel
in Figure 2-8.
Make your idler wheels sturdy! One of Trusty's early designs could follow a line just fine. But after about a minute of back-and-forth turning, the idler wheel broke off the bottom, and Trusty was
left lying on his back, twitching.
Using Two Light Sensors
You can buy more light sensors for about $20US each (see the ''Online Resources" section at the end of this chapter). By adding one more light sensor to Trusty, we can simplify his program and
make his movement along the black line smoother.
Adding the Sensor
To add the second light sensor to Trusty, just move over the one that's already there. You can leave the original one attached to input 2 and just add the second one to input 3. Figure 3-6 shows
how to do this.
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Figure 3-6.
Adding a second light sensor to Trusty
Programming
With two side-by-side light sensors, Trusty's algorithm is pretty simple. If both sensors are on the line, we just drive forward. If only one of the sensors is on the line, we simply turn back to the
line. If both sensors go off the line, we don't do anything; the robot must already be turning and should return to the line soon. This logic can be represented by a simple map from the sensor values
to the motor settings, as shown in Table 3-1.
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