CHAPTER 25 ■ LAUNCHING THE LINE-FOLLOWER
There’s a problem, though. Check out the position of the right wheel at the time the right motor was
first disengaged (frame two). Now compare that to the wheel’s position before the right motor is reengaged
(frame seven). The wheel has continued to roll toward the line, probably about 6 cm (see Figure 25-12), even
though the right motor is turned off.
Figure 25-12. Frames two and seven superimposed to show wheel roll. If the wheel had stayed in place, the
“x” marks would be directly on top of each other. (Please excuse the quality of this image; it came from a video
camera. Even so, it does show that the wheel has moved forward instead of pivoting in place.)
To make tighter turns, the robot must pivot in place.Braking to Pivot
In order to pivot in place rather than allowing a wheel to roll forward, the robot needs to apply an electrical
brake. It’s easier than it sounds. In the present circuit design, when a motor is disengaged it is simply
disconnected from the positive power bus. If instead it were then connected to the negative power bus, the
motor would brake.
To achieve that, two more transistors would need to be added (one for each motor), and a push-pull
comparator (or a pull-up resistor and a 4069 inverter logic chip) would need to be substituted. These are just
suggestions; I’m not going to add them to the line-following robot in this book.
A major downside of braking the motors is that the overall progress of the robot will be slowed when
driving straight. This is because the opposite wheel no longer rolls forward. To fix that, a microcontroller
could be programmed to only apply the brakes when turning.
This is getting complicated! To truly improve the performance of Sandwich, it needs faster sensors,
motor circuitry that can brake, and a microcontroller for better control. Not surprisingly, the line-following
robot in Intermediate Robot Building has these features.