Process Controls
PROPORTIONAL PLUS RATE CONTROL SYSTEMS
PROPORTIONAL PLUS RATE CONTROL SYSTEMS
Proportional plus rate control is a control mode in which a derivative section is
added to the proportional controller.
EO 1.4
DESCRIBE the characteristics of the following types of
automatic control systems:
e.
Proportional plus rate control
Proportional-Derivative
Proportional plus rate describes a control mode in which a derivative section is added to a
proportional controller. This derivative section responds to the rate of change of the error signal,
not the amplitude; this derivative action responds to the rate of change the instant it starts. This
causes the controller output to be initially larger in direct relation with the error signal rate of
change. The higher the error signal rate of change, the sooner the final control element is
positioned to the desired value. The added derivative action reduces initial overshoot of the
measured variable, and therefore aids in stabilizing the process sooner.
This control mode is called proportional plus rate (PD) control because the derivative section
responds to the rate of change of the error signal.
Definition of Derivative Control
A device that produces a derivative signal is called a differentiator. Figure 25 shows the input
versus output relationship of a differentiator.
The differentiator provides an output that is directly related to the rate of change of the input and
a constant that specifies the function of differentiation. The derivative constant is expressed in
units of seconds and defines the differential controller output.
The differentiator acts to transform a changing signal to a constant magnitude signal as shown
in Figure 26. As long as the input rate of change is constant, the magnitude of the output is
constant. A new input rate of change would give a new output magnitude.
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