Civil Engineering Reference
In-Depth Information
100
Gain increasing
Input
Figure 7.5
Input/output characteristics for a proportional controller with saturation
by limited controller output or travel of valve.
In general, a proportional controller has to have two variables which
can be set by the operator by means of front panel switches or knobs: the
set-point, which is the value of the controlled variable which the controller
is set to maintain, and proportional gain, which is in effect the slope of the
input/output curve. A third parameter named nominal control output is often
applied in the proportional controller, which can reduce the steady-state
offset as discussed later in this section.
Practical controllers can easily be provided with a temperature scale and
setting knob for the set-point (electronic and digital controllers may provide
a digital temperature display), but the proportional gain is often replaced by
an alternative, known as the
proportional band
. The proportional band is
illustrated in Figure 7.6. It may be defined as the change in the controlled
variable required to make the final control element move through its full
operating range or stroke.
Full Output Range of Controller
Proportional Band =
Proportional Gain
Normally, we consider the output of controller to be between 0 and 100 per
cent. Therefore, their relationship can be revised as:
100%
Proportional Gain =
Proportional Band
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