SYSTEMS Proportional plus reset control is a combination of
the proportional and integral control modes.
EO 1.4DESCRIBE the characteristics of the following types of automatic
control systems:
d. Proportional plus reset control system
Proportional Plus Reset
This type control is actually a
combination of two previously discussed control modes, proportional and
integral.
Combining the two modes results
in gaining the advantages and compensating for the disadvantages of the two
individual modes.
The main advantage of the
proportional control mode is that an immediate proportional output is produced
as soon as an error signal exists at the controller as shown in Figure 22. The
proportional controller is considered a fast-acting device. This immediate
output change enables the proportional controller to reposition the final
control element within a relatively short period of time in response to the
error.
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Figure 22 Response of Proportional Plus Reset
Control
The main disadvantage of the
proportional control mode is that a residual offset error exists between the
measured variable and the setpoint for all but one set of system conditions.
The main advantage of the
integral control mode is that the controller output continues to reposition the
final control element until the error is reduced to zero. This results in the
elimination of the residual offset error allowed by the proportional mode.
The main disadvantage of the
integral mode is that the controller output does not immediately direct the
final control element to a new position in response to an error signal. The
controller output changes at a defined rate of change, and time is needed for
the final control element to be repositioned.
The combination of the two
control modes is called the proportional plus reset (PI) control mode. It
combines the immediate output characteristics of a proportional control mode
with the zero residual offset characteristics of the integral mode.
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