Emerson Rosemount 56, Инструкция по эксплуатации

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Section 7: PID Control Instruction Manual
April 2017 LIQ-MAN-56
56 PID Control
Example of direct acting control: Lower the pH of a solution at 10 pH by adding acid to
control it at 8 pH with the Gain parameter assumed to be 1.0. The higher the measured pH,
the more acid is required to lower the pH toward the setpoint, but as the pH approaches the
setpoint less acid is required:
7.1.2.2 Reverse Acting Control Action
Reverse acting control action, decreases the control output as the measured variable
increases toward the setpoint. The LRV is usually set to the setpoint value, so that the
control output is 0% at the setpoint, and the URV is less than the setpoint value so that the
100% control output is at a lower measurement value. The Gain can then be adjusted to
produce the desired gain.
Example of reverse acting control: Add base to a solution at 8.0 pH, to control the pH to
10.0 pH with an assumed Gain parameter of 1.0. The lower the measured pH, the more
base is required to raise the pH toward the setpoint, but as the pH approaches the setpoint
less base is required:
Fig. 7-2 Example of Direct Acting Control
Fig. 7-3 Reverse Acting Control