Page 37 - Instrumentation and Measurement

3.2.1 Open- and Closed-Loop Control

Consider two alternative ways of heating a room to some required temperature. In the first instance,
there is an electric fire which has a selection switch which allows a 1 kW or a 2 kW heating element
to be selected. The decision might be made that to obtain the required temperature it is only necessary
to switch on the 1 kW element. The room will heat up and reach a temperature which is determined
by the fact the 1 kW element is switched on. The temperature of the room is thus controlled by an
initial decision and no further adjustments are made. This is an example of open-loop control. Figure
3.4 illustrates this. If there are changes in the conditions, perhaps someone opening a window, no
adjustments are made to the heat output from the fire to compensate for the change. There is no
information fed back to the fire to adjust it and maintain a constant temperature.

FIGURE 3.4 Open-loop control.

Now consider the electric fire heating system with a difference. To obtain the required temperature,
a person stands in the room with a thermometer and switches the 1 kW and 2 kW elements on or off,
according to the difference between the actual room temperature and the required temperature in
order to maintain the temperature of the room at the required temperature. There is a constant
comparison of the actual and required temperatures. In this situation there is feedback, information
being fed back from the output to modify the input to the system. Thus, if a window is opened and
there is a sudden cold blast of air, the feedback signal changes because the room temperature changes
and so is fed back to modify the input to the system. This type of system is called closed loop. The
input to the heating process depends on the deviation of the actual temperature fed back from the
output of the system from the required temperature initially set, the difference between them being
determined by a comparison element. In this example, the person with the thermometer is the
comparison element. Figure 3.5 illustrates this type of system.

FIGURE 3.5 The electric fire closed-loop system.

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