Response Electronics CL6011B Dokumentacja Pobierz pdf (Strona 30)

3Ć10 Theory of Operation: Point Processing

Original Ć June 1990CE4.2:CL6211

Note that like the P/PD with Bias PCA, the set point term of the PID

algorithm has been isolated so that rate action occurs only on changes

to the PV, and that the rate action has been filtered to prevent the

algorithm from over reacting to high frequency noise. Again, the

equations shown are for reverse acting loops, and the signs of the SP

and DN terms are inverted for direct acting loops.

A feature of the PID PCA that is not shown in the control equations is the

concept of one half proportional bands" that establish when the point's

IVP will come off of its limits. As their name implies, the one half

proportional band limits are positioned on either side of set point, with

each limit one half of a control proportional band away from set point.

Once the IVP of a DCP is output limited, at which point the integral term

of the equation stops winding up, it will not come off the limit until the

process variable crosses its one half proportional band point. By design,

the valve position will be 50% from the limit when the process variable

crosses its set point.

Figure NO TAG illustrates the one half proportional band concept. As an

example, one may consider the one half proportional bands of a DCP

with a gain of 10. The proportional band is 10%, with each one half

proportional band at 5% above and below the set point. If the set point

is 70%, the one half proportional bands will be at 65% and 75%.

Assuming the valve is closed and the IVP is at its low limit, as shown at

Point A, the valve will not start to open until the process variable

increases to 65% at Point B. When the process variable equals the set

point at Point C, the process variable will have moved 5% past the one

half proportional band limit at a gain of 10, which will cause the valve to

move 50%.

IVP

Setpoint

+1/2 P.B.

- 1/2 P.B.

50%

70%

65%

75%

Figure 3Ć2. One Half Proportional Band Concept

One half proportional bands are built into each of the PID based

algorithms, and are valuable on high gain DCPs where the valve output

is at its output limit during a portion of the control cycle.

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Response Electronics CL6011B Dokumentacja Strona 30