Emerson 54e pH/ORP Proportional Mode Gain, Proportional Gain Plus Integral Reset

MODEL 54e pH/ORP	SECTION 6.0
	THEORY OF OPERATION

6.9PID CONTROL (continued)

Proportional Mode (Gain)

The simplest control is proportional. Proportional may also be referred to as sensitivity or gain. Although these terms may refer to a different version of proportional, the control function is still fundamentally the same - the error from set point is multiplied by this factor to pro- duce the output.

The Model 54e pH/ORP's proportional mode is referred to as proportional "band" which is config- urable from 0 to 299%. For good control of a specific process, the proportional band must be properly adjusted. The proportional band is the percent of the analog output span (the difference between the 4 (or 0) mA and 20 mA settings) through which the measured variable must move to change the output from mini- mum to maximum. The larger the proportional band, the less the controller reacts to changes in the measured variable. As the proportional band is made smaller, the reaction of the controller increases. At 0 proportional band, the proportional- only controller behaves like an on/off controller (an alarm set at 20 mA).

Most processes require that the measured variable be held at the set point. The proportional mode alone will not automatically do this. Proportional alone will only stabilize the measured variable at some offset to the actual control point. To control at an exact setpoint, proportional plus integral mode is used.

Proportional (Gain) Plus Integral (Reset)

For the automatic elimination of deviation, I (Integral mode), also referred to as Reset, is used. The propor- tional function is modified by the addition of automatic reset. With the reset mode, the controller continues to change its output until the deviation between measure- ment and set point is eliminated.

The action of the reset mode depends on the propor- tional band. The rate at which it changes the controller output is based on the proportional band size and the reset adjustment. The reset time is the time required for the reset mode to repeat the proportional action once. It is expressed as seconds per repeat, adjustable from 0-2999 seconds.

The reset mode repeats the proportional action as long as an offset from the set point exists. Reset action is cumulative. The longer the offset exists, the more the output signal is increased.

The controller configured with reset continues to change until there is no offset. If the offset persists, the reset action eventually drives the controller output to its 100% limit - a condition known as "reset windup". To prevent reset windup, a controller with reset mode should never be used to control a measured variable influenced by uncorrectable conditions. Once the con- troller is "wound up", the deviation must be eliminated or redirected before the controller can unwind and resume control of the measured variable. The integral time can be cleared and the "windup" condition quickly eliminated by manually overriding the Model 54e pH/ORP's analog output using the simulate tests feature (detailed in Section 5.4).

Control Loop Adjustment and Tuning

There are several methods for tuning PID loops includ- ing: Ziegler-Nichols frequency response, open loop step response, closed loop step response, and trial and error. Described in this section is a form of the open loop response method called the process reaction curve method. The reaction times and control charac- teristics of installed equipment and real processes are difficult to predict. The Process Reaction Curve Method of tuning works well because it is based on the response of the installed system. This procedure, out- lined in the following paragraphs, can be used as a starting point for the P and I settings. Experience has shown that PID controllers will do a fair job of control- ling most processes with many combinations of rea- sonable control mode settings.

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