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FIGURE 4.7 Proportional controller for the control of temperature.
4.4.1 Proportional Band
Note that it is customary to express the output of a controller as a percentage of the full range of
output that it is capable of passing on to the correction element. Thus, with a valve as a correction
element, as in the float operated control of level in Figure 4.8, we might require it to be completely
closed when the output from the controller is 0% and fully open when it is 100% (Figure 4.9).
Because the controller output is proportional to the error, these percentages correspond to a zero
value for the error and the maximum possible error value. When the error is 50% of its maximum
value then the controller output will be 50% of its full range.
FIGURE 4.8 Water level control system.
FIGURE 4.9 Percentages.
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