OUT

I1

L/L

(Continued)

78 RATIO, OVERRIDE, AND SELECTIVE CONTROL

TABLE 5-1.1 Continued

Selector: OUT = maximum of inputs I1, I2, I3

OUT = minimum of inputs I1, I2, I3

I1

I1

OUT

OUT I2

I2

HS LS

I3

I3

Dead time: OUT = input delayed by t0

OUT

I1

DT

TABLE 5-1.2 Programming Language

Input/output: AIN = analog in; AOUT = analog out

Format:

In variable = AIN (input channel #, low value of range, span of transmitter)

“In variable” will be returned in engineering units.

Out variable = AOUT (output channel #, out variable)

“Out variable” will be returned in percent.

Mathematical symbols: +, -, *, ^, /, <, >, =

Statements: GOTO; IF/THEN/ELSE

Controller:

Output = PID (variable, set point, low value of range of variable, span of variable)

“Output” will be returned in percent.

Every term in the PID argument must be in engineering units.

Comments: To insert a comment in any line, use a semicolon followed by the comment.

implemented. The necessity and meaning of the additional calculations are

explained by the following. Consider a tank, shown in Fig. 5-1.1, where temperature

transmitters with different ranges measure temperatures at three different locations

in the tank. The ¬gure shows the transmitter ranges and the steady-state values of

each temperature, which are at midvalue of each range. It is desired to compute the

average temperature in the tank. This computation is straightforward for the control

system that reads each signal and converts it to engineering units. The three values

are added together and divided by 3; the program in Fig. 5-1.2 does just that. The

¬rst three lines, T101, T102, and T103, read in the temperature, and the fourth state-

ment calculates the average temperature, TAVG.

For control systems that treat each signal as a percent of span, this simple com-

79

SIGNALS AND COMPUTING ALGORITHMS

50“150 C

TT

100 C 101

DCS

TT

50 C 102

25“75 C

TT

25 C 103

0“50 C

Figure 5-1.1 Tank with three temperature transmitters.

1 T101=AIN(1,50,100) ; reads in T101

2 T102=AIN(2,25,50) ; reads in T102

3 T103=AIN(3,0,50) ; reads in T103

4 TAVG=(T101+T102+T103)/3 ; calculates average

Figure 5-1.2 Program to read in temperatures, in engineering units, and calculate average

temperature.

1 T101=AIN(1) ; reads in T101

2 T102=AIN(2) ; reads in T102

3 T103=AIN(3) ; reads in T103

4 TAVG=(T101+T102+T103)/3 ; calculates average

Figure 5-1.3 Program to read in temperatures, in percent of span, and calculate average

temperature.

putation would result in an answer without much signi¬cance; Fig. 5-1.3 shows this

program. That is, because each signal is 50% of its range, the computation result

would also be 50%. However, 50% of what range? How do we translate this answer

into a temperature? Furthermore, notice that even though every input signal is 50%,

their measured temperatures are different because the ranges are different. Thus,

for the computation to “make sense,” the range of each input signal, and a chosen

range for the output variable, must be considered. The consideration of each range

will ensure compatibility between input and output signals, and it is called scaling.

Reference 1 presents the method to scale the computations.

5-1.4 Signi¬cance of Signals

During the presentation of the types of ¬eld signals in Chapters 1 and 4, and in the

discussion earlier in this section, it was mentioned that signals are used by the instru-

ments to convey information and that, therefore, every signal has physical signi¬-

cance; that is, every signal used in the control scheme has some meaning. Signals are

in percent, but percent of what (pressure, temperature, ¬‚ow, etc.)? The what is the

80 RATIO, OVERRIDE, AND SELECTIVE CONTROL

meaning of the signal. It is now important to stress this fact again as we embark on

the design of complex strategies to improve control performance.

As mentioned earlier in this chapter, the new strategies frequently require the

manipulation of signals in order to calculate controlled variables, set points, or

decide on control actions. To perform these calculations correctly, it is most impor-

tant to understand the signi¬cance of the signals.

Very often, the ¬rst step in the design of a control strategy is to give a signal,

sometimes referred to as the master signal, a physical signi¬cance. Then, based on

the given signi¬cance, the strategy is designed. Currently, this presentation may

seem somewhat abstract; however, as we continue with the study of different control

strategies, the presentation will become clear and realistic.

To help keep all the information in order and to understand the calculations, we

indicate next to each signal its signi¬cance and direction of information ¬‚ow. This

practice is not common in industry, but it helps in learning and understanding the

subject.

5-2 RATIO CONTROL

A commonly used process control technique is ratio control, which is the term used

to describe the strategy where one variable is manipulated to keep it as a ratio or