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92 CHAPTER 8 Reactor control
8.3.2 On-off controller
A common and simple controller is an on-off controller. The control action takes on
two values. Examples of the on-off controller include applications in space heating,
air-conditioning systems, and a household oven. Fig. 8.3 shows the actions of an on-
off controller.
The control action is on when the error is positive and is off when the error is
negative. To avoid frequent actuation of the switch near zero error, a dead-band
may be provided. In the dead-band region, the controller action does not change. This
modification is shown in Fig. 8.4.
With a dead-band controller, the control action does not change for a small, spec-
ified band of error on either side of the zero error. An example is a room thermostat
controller with a dead band of 2°C. This modification could avoid serious prob-
lems in the actuators, such as, frequent starting and stopping of electric motors or
frequent actuation and turning off of a heater.
f(t)
A
0 Error, e(t)
FIG. 8.3
A typical on-off controller without a dead-band.
f(t)
0 Error, e(t)
Dead Band
FIG. 8.4
A typical on-off controller with a dead-band.
