Page 80 - Understanding Automotive Electronics

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THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION 2

limits. Limit-cycle controllers often are used to control plants with nonlinear or
complicated transfer functions.
Limit-cycle control An example of a limit-cycle controller is the temperature-controlled oven
responds only when the depicted in Figure 2.27. The temperature inside the oven is controlled by the
error is outside a pair of length of time the heating coil is energized. The temperature of the oven is
limits. measured with a temperature probe, and the corresponding electrical signal is
fed back to the command to obtain an error signal. The control electronics
checks the error signal against the temperature control dial to determine if one
of the following two conditions exists:
1. Oven temperature is below minimum setting of command input.
2. Oven temperature is above maximum setting of command input.
The control electronics responds to error condition 1 by closing the relay
contacts to energize the heating element. This causes the temperature in the
oven to increase until the temperature rises above a maximum limit, producing
error condition 2. In this case, the control electronics opens the relay contacts
and the heat is turned off. The oven gradually cools until condition 1 again
occurs and the cycle repeats. The oven temperature varies between the upper

Figure 2.27
Limit-Cycle
Controller to Control
Oven Temperature

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 67