1958 IRE TRANSACTIONS ON INSTRUMENTATION 95

Practical Information Theory Aspects0f High-Speed Data Handling

BRUCE K. SMITHt

IT is superfluous, perhaps, to note that an industry the formulation of its control decisions implies com-will not spend large sums of moniey on any project puter-like abilities and, in common with computers, anwithout some consideration of the economic or adequate definition of the task before its performance.

charitable worth of that project. Inasmuch as there is But, definition can be a truly formidable proposition.little likelihood that the manufacturers of electronic The petroleum process industry, for example, spendsdata systems will legally qualify for charity, we must several million dollars a year in the gathering and corre-assume that their products have at least an apparent lation of information for optimizing control decisions.economic value to explain the monies which have been They are concerned not only with the quality and pro-spent for them by industry. duction cost of each of the many derivatives from the

In a few instances, the purchase rationalization is crude oil input, but also with matching the productionsimply that the sale of the company's product might be ratios of those products to the changing demands ofenhanced by advertised association with a mysterious their customers. Identical concerns may be found in al-electronic inspector. The output from more than one most every industry.production line has been noticeably increased due to the The urgency of many military development programspresence of an electronic gadget along it, whether or not has forced the derivation of complex control equationsthe gadget made any direct contribution to production by the methods of trial, error, and income tax. Beingquality or economy. We may expect the sale of such neither the recipient of charity nor taxes, an industrynebulous black boxes to continue until the capabilities desiring to apply comparably advanced technology toof human inspectors return to fashion, or until more production control must first discover an economicaluseful electronics makes their presence superfluous. method for obtaining comprehensive control informa-

Fortunately for the consumer and for the future of tion.electronic system manufacturers, the major efforts in A second and equally imposing barrier which elec-system design have been directed toward the more tronic system manufacturers must circumvent is trust.tangible goals of obtaining better and cheaper produc- No responsible head of a petroleum industry, for exam-tion. Paradoxically, most of those industries which have ple, is apt to entrust the operation of his refineries to a

achieved any measure of success in the application of computer-like control system when the computer in hissuch systems are remaining discreetly quiet about their accounting department has exhibited an average error-existence until public acceptance of automation has be- free operating time of about one day. In the latter in-come more universal. stance, the presence of either built-in or programmed

In actuality, true automation can hardly be said to be checking will stop the computer, and the error is thenwith us. While numerous examples have received news- corrected by a rerun of that part of the program or by a

paper attention, the plain and inescapable truth is that call to the local serviceman. But real time industrialthe majority of our major industries has excellent rea- control must be full-time control to meet most economicsons for believing that electronic closed-loop control has acceptance standards. In the petroleum industry, againyet to cross the threshold of economic advantage. They by example, control loss can easily mean product lossdo not, by any means, question that a world ringed by representing an order of magnitude more dollars thansatellites does not possess the technical know-how to the cost of the control system. Push-button warfare isconstruct such control systems. In fact, virtually all of more concerned with the relative reliability and effec-the basic techniques required for optimum automatic tiveness of rival push buttons. Any major advance incontrol in industry have been demonstrated in the potential push-button ability may be worth the gamblehardware of -today's push-button war and defense con- of the push button not working when it is needed.triv ances..Irrespecti ve of electronic control loop closure, a largeAn automatic control system which takes into ac- amount of data must be gathered for control decisions.

count all of the many interacting process variables in Demonstrably greater accuracy and lower instrumenta-tion cost are obtainable through the use of multiplexers,

* Manuscript received by the PGI, December 4, 1957. Pre- amplifiers, normalizers, and analog-to-digital converterssented at the Third IRE Instrumentation Conferenlce, Atlanta, Ga.; similar to those which would be employed at the front

Not Epsro, Bos3ton, Mass. end ofS a computing control system. The purchase of

96 IRE TRANSACTIONS ON INSTRUMENTATION March

electronic data acquisition means, instead of conven- appropriate high and low limit values and types out thetional control room metering, has frequently been justi- measurement only when the voltage is found to be offfled on the basis of cost alone. However, in many in- limit. Generally, the alarm type-out is supplemented bystances the improved accuracy is recognized as valuable a complete type-out of all values at periodic intervals,to both immediat-e and long-range control objectives, for the principal purpose of assuring all concerned thatMoreover, experience with electronic equipment from the system is still alive and attentive to assigned duties.firms which are one and all vocal in their claims for But those of us who have been deprived of our auto-reliability is an important basis for evaluating the prob- motive gauges by supposedly equivalent alarm lightable reliability of a complete con-trol system. bulbs will appreciate that fixed limits convey only aThe fact that repeat orders for electronic data acqui- fraction of the information which we could obtain from

sition systems have not met the system manufacturer's our old-fashioned dashboards. A jumpy ammeter or oilexpectations is only partly attributable to the reliabili- pressure needle conveys information, even when the ex-

ties of their equipments. At least one of the major proc- cursions are within prescribed boundaries. Rapidly ris-ess industries has abandoned electronic methods in ing temperature towards a danger limit is of greaterfavor of control rooms walled with meters because they interest than a slow variation in any direction. We are

have discovered that data are not synonomous with in- more apt to pay closer attention to a gauge which indi-formation and that their "old-fashioned" methods gave cates a condition close to danger, than to ones which are

them more useful control information per operator hour stable and safely within normal operation boundaries.than they were obtaining from the modern counter- Likewise, we learn to consider certain off-limit readingsparts. A sharp and necessary distinction exists between as normal under restricted circumstances as, for exam-

data gathering arnd information gathering facilities, pIe, unusually high ammeter and oil pressure gaugeUntil that distiniction is recognized and applied in sys- readings following a start on a cold winter morning.tem design, the system manufacturers will seldom be The frustrations of a gauge-acclimated operator of a

given the opportunity to close the control loops in indus- modern vehicle are of comparable origin to the feelingstry. of an operator of a typical electronic data monitoringFrom the beginninig of serious missile technology, it and alarm system. A system operator would also like to

was recognized that tremendous quantities of data know something about his process dynamics and thewould have to be derived from missile testing to dis- proximity of measurements to alarm limits. He is inter-cover the proper avenues for development. However, ested, of course, in a parameter passing an alarm preset,when elaborate telemetering systems were evolved to but a measurement which is rapidly advancing towardscollect such data and equally elaborate computers were that preset is obviously in alarm before it gets there andconstructed for the task of data reduction, it was dis- may be well beyond by the time he is informed by a

covered that the quantity of data far exceeded the diges- slow-speed, fixed-limit, scan monitor. Moreover, manytive capacities of the data translation and reduction process variables follow readily predictable courses ofequipments. This bottleneck was finally broken by the variation through alarm territory during warm-up or

addition of one or more types of "quick look" recorders change-over from one output product to another and,for picking the infor-mation worthy of computer atten- as such, are not truly in alarm unless they deviate fromtion from the redundant or fully predictable areas in the the normal pattern of variation.test recordings. A driver's love for gauges may not be a sufficient basis

This example is entirely analogous to the industrial for him to turn in his new car for an old one, but an in-data acquisition situation. An electronic data system dustrialist's concern for a properly controlled process iswhich types out measurements from several hundred often sufficient to turn him away from modern elec-transducers presents a tremendous barrier to real time tronic instrumentation. However appreciative he mayoperator analysis because of the great mass of redundant be of the accuracy of electronic data measurement, heand effectively informationless numbers surrounding has discovered that the information content of that dataone or two items of interest. For all their cost and ac- is largely obscured by either the region of silence be-curacy limitations, the rows of meters with red-lined tween fixed alarm limits or by the informationless dataoperating points or r-egions offer a much better "quick which engulfs items of interest during a demand or

lookz" facility than rows of numbers which must be read periodic loggying cycle. This is not the approval which

1958 Smith: Practical Information Theory Aspects of High-Speed Data Handling 97

While it necessarilyr includes elements unnecessary in namics, or the input circuit to the converter must in-more elementary data monitoring equipment, it may clude a precision sample and voltage storage device toalso do without some of the conventional hardware and obtain constancy artificially.thereby have an end cost which is entirely competitive. When a data monitoring system includes a memory toMost important of all, of course, is that the apparent store the measurements from the prior scan cycle, botheconomic justification for purchase will stand up under the converter programmer and the anialog storage cir-use if, but only if, the equipment shows reasonable re- cuits can be eliminated. The conversioni is reduced sim-liability. ply to reconstruction of the analog voltage correspond-

Decision ability, whether applied to the derivation of ing to the digital value in memory and a single compari-a control command from a complex interrelationship of son operation to determine whether the new value ismeasurements on several inlputs or to the determination more or less than the old one. An incremental adder,of what conditions should be brought to an operator's capable of either adding or subtracting one, is sufficientattention, necessarily implies certain of the character- for "updating" the measurement as it is returned toistics of a computer. The computiiig speed required for memory.optimum real time operation cani be deceptively high A system operating in this mannier makes use of thecompared to the inertia of the controlled process. While inertia of the process beinlg monitored to obtain thea high-speed, general purpose digital computer could be measurement accuracy of conventional analog-to-digitaladapted to these tasks, it is unilikely- that ty!pical appli- conversion. Since the conversion decisions occur onllycations can justify either the original price tag or the once per input per cycle and involve only one unit offrequent breakdowns common in complex computing change, a number of scan cycles must be gone throughmachinery. Fortunately, the apparent dilemma van- following the turn-on of the system before the measure-ishes if we remove the adjective "general purpose" from ments are meaningful, unless good approximations have"computer," and then examine the characteristics of the been left or placed in memory. However, once caught upinputs and required outputs of a black box which may with its inputs, a system of reasonable scanning speedor may not warrant the name of "computer." can easily follow changes occurring at the inputs. The

Use difficulties with conventional industrial data significant factor is that the system organization is de-logging and alarm systems have beeni shown to arise signed for the recognition of change and that the essen-primarily from the fact that most of the inputs remain tial qualifications of information can be readily derivedconstant or are slowly varying within their alarm from the change data.boundaries. Because almost everything being monitored Rate of change can be monitored by using a section ofhas inertia, it is customary to operate scanning multi- memory to keep track of updating activity over anyplexers at comparably leisurely paces, with a second or prescribed number of scan cycles, with a read-out com-so spent digitizing and logging the input from each mand issuing if the total deviation during that intervalchannel. This rate is still high for the digestive and de- exceeds a program prescribed limit. Another section ofcision abilities of an operator, yet may easily be too low memory can be used to store fixed or program variedto catch important changes at a giveni input when sev- operation points and to keep track of the amount oferal hundred inputs are surveyed during a complete deviation from optimum. The latter information can, ofscanning cycle. course, be used to command read-out in a manner com-The characteristics which define information are parable to that obtained from fixed alarm limits. It can

change, rate of change, direction of change relative to also be applied as a modifier on the rate of change logic,present operating point, and chainge from a predictable so that as a value progresses towards a boundary, anormal operating patterni. Silnce all of these imply input proportionately lower rate of change is permitted inactivity, an information filtering system should be opti- that direction.mized for the job of discerninig what its inputs are doing, These functions, and all others required for informa-rather than simply measuring their preselnt values. The tion recognition, canl be derived by incremental addi-two basic requisites of such a system are that it should tions and subtractions. One such system recently de-operate fast enough to monitor each of its inlputs as signed at Epsco for a specific monitoring applicationoften as required to interpret change properly, and that trades speed for hardware and has only one "updating"it should include a memory to store measurement his- circuit for both measurement and information recog-tory. nition. Comparable multiplexing of electronics through-A conventional analog-to-digital converter performs out the Epsco Information Monitor has salutary inlfiu-

an operation of measurement by trial and error match- ence in the areas of price and reliability, which are asing of a digitally controlled voltage against that applied important to over-all economic acceptability as the de-to the converter input. MNeasurement accuracy is deter- sign virtue of true information output.mined both by the intrinsic accuracy and resolution of Real-time control calculation can be accomplished bythe conlverter and by the conlstancy of the input during simple extensions of these incremental techniques. Thethe decision cycles. The conversion speed must be very nature of plant dynamics permits the substitution of anhigh compared to the frequencies of normal process dy- elementary updating operation for the series of ap-

98 IRE TRANSACTIONS ON INSTRUMENTATION March

proximations used in conventional analog-to-digital useful information depenlding on changes occurring inconversion. Similarly, the solution of complex equations the solutions. Because an incremental computer oper-involving the measurements as operands does not re- ates on change, the same circuits used for recognizingquire the blind start which characterizes a digital com- information in measurement data can be associatedputer solution, inasmuch as the solution during each with the computing logic and thereby provide an evenscan cycle is nearly identical to that on the previous one. higher refinement in information output.Computation by means of incremental techniques rep- Whether or not an information monitor includesresents surprisingly small logical involvement over a computational ability, it is the only form of industrialbasic information system and provides both the speed electronic data measurement system which can hope toand accuracy required for real-time industrial con- replace permanently conventional control room meter-trol. ing. Until such time as the economic advantage of elec-The inclusion of incremental computing logic in an tronics is firmly established in industry, the next step

information systenm can often be justified despite cus- of full automation will go unrealized. The proof of elec-tomer reticence towards closed-loop operation. Real- tronic capability lies first in the demonstration of realtime conitrol solutions may be regarded as simply a utility and, second, in the reliable conitiniuation ofhigh-order type of data, which nmay or may not contain utility.

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