Decision Elements

Perhaps the most important or significant contribution to instru-ments and controls is the recent introduction of decision elements, which, taking advantage of the theoretical advances in symbolic logic, permit a control instrument to choose alternate courses of action. Fig-ure 5.14 shows one version of these building blocks which are basically switches without moving parts. The two inputs are, for instance, of the nature of "yes-no" intelligence, or in the binary code, 0 or 1. A

INDUSTRIAL INSTRUMENTATION AND CONTROL 107 third input is a clock voltage which triggers the answer "yes" or "no,"

depending on the inputs. These devices, working at high frequencies (megacycles), can be used to play games, or, more important, to es-tablish a strategy of procedures for acting under pre-eses-tablished emergency patterns (the old safety interlock), or to choose optimum procedures (automatic telephone scanners for open connections be-tween cities), or even to choose different modes of control to satisfy predetermined safety or stability specifications.

Fig. 5.14 Magnetic decision elements (Courtesy Minnesota Electronics Corp.) With such a formidable array of new tools at his disposal-the list is by no means complete-the instrument designer of today is better equipped than ever before to tackle the problems of industry. In passing, the absence of one still unavailable basic device should be mentioned in the hope that this may stimulate thinking and trigger inventive ideas. There is no instrument as yet that can identify patterns, faces, signatures, or the identity of landscapes in winter and summer. We shall see later on that the lack of this basic building block is one of the major handicaps in the further develop-ment of full automation and in partiCUlar of automatic inspection.

(For example, signatures must still be identified by a human checker in banks.)

5.5 THE SIGNIFICANCE OF MEASUREMENTS

In deciding on the measurement of variables in the industry to con-trol a process, some basic questions must be asked. One, for instance, is what does this variable mean?

The most frequently measured and controlled variables like level, pressure, and temperature are actually energy balance indicators which give some information on the difference of input and output energy flow rates and the. prevailing energy level. Thus, constant pressure in a pipeline means that as much gas is supplied per time unit as is withdrawn. The same applies to level for fluids and liquids, and temperature as an index of thermal equilibrium. Of similar nature is the pH level as a chemical ion balance indicator.

But coming back to pressure, we find it used, as an example, in open-hearth furnaces as an indicator of the balance between the incoming combustibles, combustion products, air infiltration, and the exhaust flow rate. However, what does this pressure mean, and where should it be taken? The inside pressure profile of a furnace is by no means constant, and the result of the effect of many variables, including gas temperatures, velocity distributions, location of scrap charges, furnace geometry, open or closed doors, etc. Why is a pressure signal taken at the center of the furnace roof acceptable in spite of this multitude of effects?

The answer is one of experience: It "works"-which means that experience in production has shown a correlation between the end-product quality based on such measurements and customer acceptance that is great enough to be useful. /

5.5.1. Correlation between Measured Variable and Desired Property In general, such practical and seemingly arbitrary choice of indices for somewhat remote measuring targets is very widely used by ex-perienced operators. Its soundness should not be underestimated by the academic purist who will, for instance, shudder at the idea of measuring air flow in a boiler as a pressure. drop in the combustion chamber-a method violating the most fundamental laws of any be-ginner's textbook on flow measurements.

To systematize this choice a group of German engineers under the leadership of Dr. Ing. K. Daeves have given this correlation problem much thought; they have developed rather simple techniques to make such a correlation evaluation a general tool for industry. Unfortu-nately, no one has taken the time so far to translate Dr. Daeves' most

INDUSTRIAL INSTRUMENTATION AND CONTROL 109

useful and stimulating book, Praktische GrosszahlJorschung (see reference 8).

(a)

t

b

Propertyb (b)

t

b

(c)

t

b

Influencing factor a

---;-Probability of

_-.--+-+-<.-.-~~

^__

-t73ence

Influencing factor d ~

Fig. 5.15 Evaluation of correlation of various variables and the desired property of an end-product (After Daeves)

Although the conventional engineering approach hopes for the per-fect-law relationship between two variables a and b (Fig. 5.15a) that is expressed by a narrow band curve (preferably a line), there are many cases where the observed data cover a broad area (Fig. 5.15b). To

110 AUTOMATION IN BUSINESS AND INDUSTRY

handle, these data, Daeves connects the outer rim of the data area by a continuous border line and then establishes the center of gravities of sections parallel and perpendicular to the abscissas. Connecting their respective centers of gravities, he obtains two curves with an in-tersection and an inin-tersection angle (a).

If a = 0, we obtain (Fig. 5.15a) the classical law (100 per cent . correlation), or if a

=

90 degrees in Fig. 5.15c, zero correlation.

Fig. 5.16 An x/y recorder (Courtesy Librascope Inc.)

The important, and at present mostly overlooked implication (known to the old-timer exper1r-but often overlooked by the young engineer) is that adjusting not only b but also c and other influencing factors for the optimum of the desired property b, is likely to give better results, often with less-complicated procedures, than concentrating on the usually preferred 100 per cent correlation factors.

For the purpose of simplifying or mechanizing this type of study,

"x/y recorders" become almost a necessity. Figure 5.16 shows a late model of such an x/y recorder which can handle digital as well as analog input data directly from transducers, plotting continuously or in batches, the correlation diagrams of b =

f

(a, c, or d) of Fig. 5.15, or data fed into it from tape records or from punched cards. This

INDUSTRIAL INSTRUMENTATION AND CONTROL 111 approach of systematically exploring the influence of variables on the desired property of the end products is a very powerful tool which will gain more importance in the future. It will become standard practice in process analysis as soon as all instruments speak the same output tongue, or common language.

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