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3Ć6 Theory of Operation: Point Processing

Original Ć June 1990CE4.2:CL6211

the IVP, they do provide the ability to establish a reference point for the

process (set point) and the ability to determine how the process variable

compares to that reference point for alarming purposes.

The modes of operation which are valid for the manual loader PCA,

manual (MAN) and Direct Digital Control (DDC), determine who is

allowed to change the IVP value. While in manual mode, the point

provides an operator with control of the IVP value. A host computer is

allowed to change the IVP value when the point is in DDC mode. The

operator is allowed to change the set point in either mode unless the set

point has been configured to track the process variable while in DDC

mode.

3.3.1.2 Bias & Gain

The Bias and Gain algorithm performs a linear transformation on the

process variable of the DCP to convert it to IVP. The algorithm adds the

bias value of the DCP to the process variable, and then multiplies the

result by the configured gain value. Both the bias value and the gain

value are tunable. Even though it has no effect on the IVP of the point, an

adjustable set point value is available as a reference value to be used for

alarms.

The two types of control action that are available for the Bias & Gain PCA

are Direct and Reverse. Direct action causes the IVP of the point to

increase if the PV increases. Reverse action causes the IVP to decrease

if the PV increases. The effects of bias and gain on the PV are shown in

Figure NO TAG for both direct and reverse control action.

It should be noted that the bias value and gain value are tuned

independently of the valve action. For example, two parallel valves in a

split range operation could both be direct acting even though one valve

might be a fail open" type and the other one a fail closed" type.

The following equations describe the Bias and Gain algorithm:

Direct Action:

Reverse Action:

IVPĂ +Ă GAINĂ @Ă(PV ) LOOP BIAS) ) TBIAS

IVPĂ +Ă 100 * GAINĂ @Ă(PV ) LOOP BIAS) ) TBIAS

Note that TBIAS is zero if transfer bias ramping is not enabled. Refer to

section NO TAG on page NO TAG.

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