Page 81 - Understanding Automotive Electronics

2735 | CH 2 Page 68 Tuesday, March 10, 1998 10:55 AM

2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

and lower limit, and the variations can be graphed as a function of time, as
shown in Figure 2.28.
The amplitude of the temperature variations, called the differential, can be
decreased by reducing the difference between the maximum and minimum
temperature limits. As the limits get closer together, the temperature cycles
more rapidly (frequency increases) to hold the actual temperature much closer
to the desired constant temperature. Thus, the limit-cycle controller controls
the system to maintain an average value close to the command input. This type
of controller has gained popularity due to its simplicity, low cost, and ease of
application. Fuel control, one of the most important automotive electronic
control systems is, at least partially, a limit-cycle control system (see Chapter 5).

Figure 2.28
Oven Temperature
Graph

FPO

68 UNDERSTANDING AUTOMOTIVE ELECTRONICS