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The development of automatic data handling in instrumentation and control
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The Development of Automatic Data Handling In Instrumentation and Control C. A. LAWS Introduction The modern graphic panel requires no introduction. The interesting point about it is that it appears to represent the culminating phase in present instrument practice. It is therefore a very suitable starting point for a discussion of new approaches to the subject. The need for an overhaul of instrumentation and control methods has arisen as a result of the emphasis being placed on such questions as plant efficiency and product quality. Technical developments have made improvements in these possible. Automatic control, which is often the key to such improvements, is based on automatic data processing, and the type of information required for the supervision of future plant is likely to change as a result. The purpose of this paper is to trace current trends in industrial instrumentation and data handling and to show their relation to the advancing science of control engineering.
15 Ib ./in.2 by existing primary transducers, or, for temperature measurement, from thermocouples and resistance thermometers. The data are sampled sequentially, each value being converted to digital form in the appropriate scale before being passed to an electric printer. The flow diagram of a typical logging system is shown in the right-hand section of Figure I. Under Set-point controls
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Automatic Logging The general realization that early methods of manual data handling have reached their limit has given rise to the development of automatic logging systems. Usually, the data logger obtains its input data from a series of transducers, frequently fed with pneumatic signals at 3 to
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Figure 1. Simple data-reduction system
typical conditions the values of perhaps 200 variables would be measured and logged once each hour, each complete operation taking about 31' minutes. The conversion of the input signal to digital form can be carried out in many different ways, depending on the application, and at speeds varying from a few seconds to a few microseconds. The most usual method for process applications is to use an industrial type self-balancing bridge to obtain a shaft position proportional to the input potential, the shaft carrying a commutator type digitizer. The coded on/off contact pattern which this provides is converted to the required units by a series of interconnected relays, the contacts of which supply current to the appropriate solenoids of an electric typewriter. Where shorter conversion times or higher sampling rates are required, use can be made of pulse techniques involving voltageto-time conversion, and such systems are capable of operating in periods as short as one millisecond. For even higher speed operation comparison techniques involving the summation of currents in binary grade resistors are frequently used. Data Reduction The operator is primarily concerned with values which have deviated beyond prescribed limits, and deviation detectors have
397 26
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The collected information is almost invariably available in the form of manually compiled log-sheets and criticism can often be levelled at this method of compilation in respect of the labour involved, the questionable accuracy of recording, and the difficulty of extracting significant data. The fact that the compiled data are seldom used in any significant way to modify hour-to-hour running conditions was evident and pointed to the big potential for measurement-and-control equipment capable of carrying out a continuous function of optimization.
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Data Recording At a recent meeting of the S.I.T.l - J in London, a number of engineers concerned with the planning and use of instrument data were asked to describe the way in which they are put to use. The main points can be summarized as follows: (2) To provide a record of raw materials and fuel consumed and of the products produced (3) To enable the efficiency of plant units to be derived and the need for overall adjustment or replacement observed (4) To enable trends to be observed and maintenance and trimming requirements predicted (5) To enable the best average process conditions to be assessed (6) To provide a basis for cost accounting of various types.
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therefore been introduced which enable the logger to discriminate between normal and abnormal data. The deviation detector is shown in Figure I, the complete system forming what is now known as a data reduction unit. In operation, the deviation channel is fed with the same transducer inputs as the logger, but in this case they are compared with potentials from a panel of set-point potentiometers defining high and low limits. The presence of a deviation is detected in a comparator unit and recorded by the appropriate channel of a simple twostate memory unit. This leads to the operation of an alarm indicating the measurement point involved, and to the operation of the logger which measures the value and records it, usually in red type. It will be apparent that the alarm and point identification function has a wide field of application quite apart from the logger, and limit scanners of this type are now widely used. It cannot be over-emphasized that a data-reduction system is not a replacement for all existing types of instrumentation. The fact is that systems of this type represent the beginning of a relatively new approach to instrumentation, and only when they are regarded in this light do the full implications become evident. The maximum potential of such devices is unlikely to be realized if comparatively slow manual methods are used for evaluation, and so long as this fact remains unrecognized, unsatisfactory comparisons will be drawn.
running total of all inputs. Facilities are also provided for pre-selecting the weight of cargo assigned to a given customer, the unloading operation being interrupted automatically as the required total is reached. A statement of total goods delivered, to which a computed cost could be readily added, is thus prepared whilst the actual operation is being performed, and because this information is compiled without delay it is possible to use it for on/off control. The final phase in streamlining this operation and one which is due to be added, involves the transmission of the digital data over telephone lines to the city office so that the invoice is prepared at this point. Figure 3 illustrates a plant used for freezing packeted foods, the central processing unit being the frost er. This consists Temperature
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Applications of Automatic Logging Techniques Many applications of data handling techniques exist outside the chemical industry and since these provide interesting evidence of the increasing role of simple automatic data handling in process management, it will be of interest to review one or two typical examples. The first, shown in Figure 2,
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Figure 2. Data handling applied to dockside cranes
demonstrates how a complete process has been effectively streamlined by the introduction of logging techniques combined with those of electric weighing. The' process' involves a dockside crane installation. The cranes are each fitted with a load cell providing an electric potential proportional to the grab load, and a digitizer for converting it to digital form. The digital information from each crane is relayed to a central office where it is recorded on an electric printer, together with a
essentially of an insulated cabinet containing some 20 refrigerated plates which can be separated by a hydraulic ram to take trays of packeted foods. After loading, the froster doors are closed and the ram operated to ensure positive contact between the packets and the refrigeration plates. A wide range of foods is processed in cartons of various sizes, and each particular product has a specific processing time which must be adhered to if frosting is to be effective. At the end of the allotted period, the products are withdrawn and despatched to the cold store. The data system was required as an aid to the manual operation of the plant, the industry being very much a seasonal one and much of the labour therefore casual. The objectives were: (a) to enable a final product of consistent quality to be obtained ; (b) to streamline the cost accounting system; (c) to improve the plant utilization factor. The factors most affecting the final product quality were: (1) variations in plate temperature; (2) variations in process timing (manual); (3) interruption of pressure on the ram; (4) opening of doors during processing. A system has therefore been designed in which the supervision of each of these functions is carried out automatically. Supervision of plate temperatures is carried out by temperature scanner units allocated to each group of four frosters. The plates, totalling some 200, are each equipped with a specially mounted thermistor element and the scanner units arranged to produce an alarm condition, and to indicate plate and froster numbers, whenever a plate temperature rises above a predetermined limit. The froster and plate numbers are visually displayed in a central office allocated to the quality-control
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THE DEVELOPMENT OF AUTOMATIC DATA HANDLING IN INSTRUMENTATION AND CONTROL
economy in equipment, is shown in Figure 4. The example is that of an oil-fired power station for which the value of B.Th.V. (fuel)/kWh is derived as a measure of overall efficiency. To obtain this a corrected flow signal is integrated to provide a potential representing the quantity of fuel consumed. A factor is introduced which takes account of calorific value so that the potential E becomes proportional to the heat value of the fuel. A similar potential is derived by integration for the power output, and the division of one by the other is carried out, as shown, by the digitizer servo. The availability of computed information of this simple type at the time of the event, rather than hours or days later, gives a new incentive to the operator as well as a basis for controlling his plant more effectively. A somewhat more advanced data system, developed for the British gas industry, is of particular interest in that specialpurpose computing techniques have again been used, this time on a fairly large scale. A comparatively simple equipment has resulted from the combined use of pneumatic, electrical , mechanical and pulse techniques, but even so it appears that for this size of undertaking the greater flexibility afforded by the use of a small general-purpose digital computer might well be advantageous. This plant is concerned with the production of gas from oil and the function of the data-handling equipment is first to log some 60 variables. Approximately 40 of these are flow rates which it is necessary to integrate continuously for hourly periods and, generally, summate before conversion to digital form. The integrator, the logical arrangement of which is shown in Figure 5, makes use of a bellows-operated arm riding on a
supervisor and, in addition, all information relevant to temperature faults is automatically typed on a log-sheet. The correct selection and control of process time in relation to the product is carried out in the data processing centre, information on product type being communicated to the centre by 'posting' a coded plate into a box located at each froster. Each plate bears a picture and description of one product and the operators each have a set appropriate to the products they are controlling. The plates operate a series of contacts providing a binary coded signal, the selection of switches being determined by cut-outs in the metal plates themselves. On reception of this information at the processing centre, a timing unit is automatically selected and set up with the appropriate timing instruction. When switches operated by the ram and doors have been closed, 'Process On' information is fed back to the annunciator display situated above each froster unit and the timing cycle begins . The product number derived from the coded plate is also displayed at the supervisory panel and typed on the log-sheet. An interruption of the ram pressure during a process, or the opening of a door, will give rise to an appropriate alarm and to a typed statement of the duration of the fault. The four factors affecting product quality are therefore supervised automatically and departures from the correct procedure, accidental or otherwise, are recorded. This installation provides an example of the use of a planned information handling system for coordinating the workings of an existing manually operated plant, no changes to the actual production equipment having been made. Introduction of Simple Computing Aids
It will be noticed that in the above examples of industrial installations, use is already being made of very simple computing-or compiling-techniques. These simple operations represent first steps in deriving information not present in the initial data; operations which were previously carried out manually. A digitizing servo can be used to carry out simple computations, and since the servo shaft carries the digitizer, the answer is directly available in digital form for printing or display. One way in which advantage has been taken of this, with consequent
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square law cam to effect a conversion from the input pressure P to a time interval T. The rotating cam lifts the rider arm for a period each revolution determined by its deflection, the arm serving to open and close a pair of contacts acting as a gate to a continuous train of pulses. The number of pulses passing through the gate during the period of integration is therefore determined by the pneumatic pressure applied to the bellows and, since this is proportional to the differential pressure across an orifice plate in the gas flow line, the pulse count is directly proportional to flow. The groups of pulses are passed to a series of simple stepping switches arranged as a decade counter, the contacts being maintained at suitably graded potentials to give an output proportional to the time integral of the flow. Summation of accumulated flows is readily attained in a resistance network common to the integrators involved . Where it is necessary to multiply one integrated quantity by another, for example volume of crude gas x calorific value, this is done by using the output of one integrator as the input reference potential for the other. Now in addition to the measurement, integration and logging
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C. A. LAWS
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400
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THE DEVELOPMENT OF
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of variables, the equipment also carries out a number of hourly calculations relating the volume of gas produced to the quantity of oil consumed. As an example of the operations involved, the method adopted for obtaining the average calorific value is shown diagrammatically in Figure 6. Squares A, B, C and D represent flowmeter transmitters in the four units of the plant, the outputs being integrated to give volumes A , B, C and D . Since both the volume of gas and its calorific value may differ in each unit, it is necessary to multiply each volume by its own average calorific value to give individual B .Th.V. These are then added to give total B.Th.U. At the same time the outputs from the four integrators are added to give total volume, and this is divided into the total B.Th.V. to give the average calorific value for all four units of the plant. This va lue is printed out as such and is also used in calculating the therms of gas produced for each gallon of oil. Thus a record of plant production is obtained which takes account of product quality. The automatic optimizing of plant performance requires the acc umulation of a great deal of data, and thi s installation illustrates one very sound and practical way in which such data may be acquired. The recorded effects of changes made to plant conditions become available in the shortest possible time, and provide a guide to short-term improvements as well as material for analysis. The control room , including the data logger and efficiency computer, is shown in Figure 7.
Electronic Instruments Nowadays, the complexity and size of plant , coupled with the market requirement for ever more rigid control of product quality, has resulted in a considerable increase in the use of instruments and in their attendant problems. At the same time, the idea of continuously integrated control as a means of optimizing process conditions has already profoundly affected thinking on the subject of instrumentation, as a part of data handling, and it is within this new framework that the immense potential of electronic methods has become apparent. ] n this section , one approach to electronic instrumentation is used to illustrate the great flexibility of such systems when dealing with the more complex measurements involving computation 4 . The sensitive element used is a differential transfornler in which, with the core centrally placed , current fed to the primary coil results in zero output from two counter-wound secondary coils. Displacement of the core from its central position then results in an unbalance signal in the secondary. The output of such a device can be made linear with displacement and it may be used as a pick-off with any measuring device producing a small proportional displacement . The basic measuring circuit, shown in Figure Sea), is a simple self- balancing servo in which a small servomotor is used to adjust a differential transfo rmer TA in the feedback loop . The movement of the shaft, which is proportional to the input signal , is conveyed either to an indicating pointer or to a chart pen and, since a common a.c. supply is used for the input and feedback transformers, the measurement is insensitive to supply varia tions. The signal input to the amplifier is EX, where X represents the core displacement of the transducer transformer T I, and that from the feedback loop is Efl, where fl represents the core displacement of TA ' Since at balance EX = Efl, it follows that fl = X. The equivalent servo using potentiometers instead of differential transformers is shown at Figure S(b). Servo indicators of this type can be used to carry out a very impressive range of process calculations, and some of these are illustrated. Square roots may be found in the manner shown at Figure S(e), both the shaft position and the electrical output TB representing ../ XI' The replacement of input E to trans401
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former TB by a second variable, EX2 , results in the solution of fl = (Xd X 2)! as shown at Figure Sed), one application being the measurement of flow ratio. Figure See) shows an arrangement for solving an equation of the form (A - B) /C. The variables Hs , Hw and HF chosen represent measurements on a boiler system in which Hs is the heat in the steam, Hw is the heat in the feedwater, and HF is the heat in the fuel. fl = (Hs - Hw) /Hp therefore represents boiler efficiency. This circuit differs from the previous ones only in that the variables Hw and H s are combined in a simple subtracting network consisting of two resistors. The solution is available as a shaft position and as an electrical output from TB. Figure SC!) shows the manner in which the servo indicator may be adapted to the requirements of a heat exchanger. The pressure signal EP, derived from a differential transformer is applied as the input to a second transformer measuring the differential pressure C!.P. The output from this, EP.6.P, is a pplied as one input to the servo amplifier. The second input is obtained from transformers TA and TB in cascade, and since TB is fed with a temperature signal ET, the shaft position fl is proportional to (P. 6.P/I I)!, which is the mass flow. This quantity is indicated on the dial or recorder chart. If a third transformer Tc is now fed with a signal representing the temperature drop across the heat exchanger, the output from this will be 6.T(P. c!'P/TI)t , or the heat output. The possible variations on this general theme are clearly numerous but sufficient has been said to indicate the enormous potential of electronic instruments. Jt is perfectly feasible with such a system to carry out continuous performance calculations-as well as some of those required for control- in the recording and indicating instruments used at the supervisory panel. An arrangement of this type suitable for a power plant is suggested in Figure 9.
Figure 9. Power plalll scheme
Use of Digital Computers A stage has now been reached in the V.K. where the digital computer is about to take a significant place in both process data handling and control. The principal factors responsible
C. A. LAWS
for this are, first, the educational process which has been going on in the more progressive industries since about 1945, and which has resulted in a wider understanding of the potential of the 'Integrated system'. The second factor is the successful development of the small reliable transistorized computer. The flexibility inherent in a tape programmed machine gives it a great advantage over the fixed-programme special-purpose type discussed earlier, particularly where a policy of continued development leads to changing requirements. The justification for employing a digital computer in a data handling system usually hinges on such considerations as the volume of information being handled, the complexity and/or variability of the operations to be performed, the extent of the storage required, and the speed at which the handling must be effected. Additionally the situation needs must be reviewed in the light of the equipment which would otherwise be necessary -if an alternative is possible, effect on production, and overall outlay. An example is provided by an equipment designed to monitor 600 temperature points in a nuclear reactor. Both the temperature and the rate at which it changes are of great importance to the safety of the plant and it is therefore necessary to store at least two temperature measurements per point. In fact, six measurements are stored in digital form for each input point, a total of 3,600 values. As each new scan takes place the latest measurement is compared with the previous one, already stored, and the first difference is taken as a measure of rate-of-change. At the same time the earliest of the six measurements in store is deleted and the new value added. Both the temperature reading and the extracted rate-of-change are compared in the computer with set-point values, also stored digitally, and an alarm is sounded when either is exceeded. Print-out is normally hourly, but is brought into continuous operation under alarm conditions or when demanded manually. The small computer employed uses transistor-driven core logic and a ferrite core store with a capacity of 2,048 thirty-three bit words. It provides a compact solution to the problem and, having regard to the storage requirements, has obvious advantages over analogue methods. However, the data provided, whilst essential to the safe running of the reactor, are in no way associated with its control. In fact, although a great deal of thought is being given to the use of digital computers in industrial control very few examples so far exist. It will therefore be appropriate to conclude this paper with a brief description of one such project which, following a period of study, is being implemented in the U.K. for a large steel company. The purpose of the system (Figure 10) is to control the cutting of hot steel billets after their emergence from the finishing mill on to the cut-up table. The length of the billet, which may be as much as 400 ft., is measured by a self-calibrating pulsecounting technique as it begins its final run through the mill. By the time it has fully emerged the computer, which has been fed with length information, discard instructions, and the customer's specification, has calculated and issued setting instructions to the various cutting stations along the line. Due to limitations of space, it has not been possible to locate the various cutting stations along the length of a single straight roller table, and recourse has had to be made to a shunting technique by means of which sections of the billet are sent back along a second line as shown. The first cut is made by the parting saw, the maximum permissible length being limited to
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Figure 10. Computer control of a billet cut-up line
180 ft. However, the exact length has to be determined with reference to the calculated number of cuts of 30 ft. and under required at the first saw, which in turn depends on the number of cuts under 10 ft. required at the ganged shear. The situation is complicated by the fact that individual statistical allowances must be made for the cutting 'spread' at each saw, and by the fact that customer specifications can take any of a number of different forms. For instance, the customer can specify exact lengths; alternatively he may specify minimum and maximum lengths and be willing to take any billet size within these limits; yet again he may specify minimum and maximum lengths, but require that cuts taken between these limits shall be at increments of, say, 2!- ft. It will therefore be clear that the calculation and distribution of cuts between the various saw positions, so as to minimize wastage, can be quite a complex problem, and it is not surprising that present methods relying on manual estimation yield a higher percentage wastage than is ideally necessary. The information from the computer is fed to digital displays at the saw and billet-stop positions and the machines are brought to the positions indicated under manual control. When the system has been fully proved in practice, the manual link will disappear, and it will then be fully automatic. In addition to issuing cutting instructions, the computer will operate a printer setting out information relevant to the requirements of accounting and stock control. At an estimated overall cost of £100,000 per annum per 1 per cent of waste, the ability to reduce this by even 1 per cent is clearly attractive, and the indications are that a greater saving than this is to be expected. References 1 NONHEBEL, G. The use of process data on steam raiSIng by industry. Symposium on the Use ol Data Recorded all Industrial Plant. S.I.T. Meeting. March 1959 2 KEATING, J. M., MILLIEN, A. and TOWNEND, D. S. Informationcommunication and usage in an oil refinery. ibid. 3 WEBSTER, F. G. The handling of data from process operations in a chemical plant. ibid. 4 NEEDHAM, M. V. Electronic Systems for Industrial Measurement and Control. Control. July, Aug., Se pt. 1948
Summary Data handling is the transforming agent in the concept of process automation, and the intelligent handling of information can bring about a surprising degree of integration, even in the simplest processes. 402
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Data reduction is seldom an end in itself and the use of simple computing techniques can usually extend the immediate value of collected data considerably. These and other computations can
THE DEVELOPMENT OF AUTOMATIC DATA HANDLING IN INSTRUMENTATION AND CONTROL
often be carried out in electronic panel instruments of suitable design. An advanced logger-computer is operating in an oil-gasification plant and its use is expected to result in higher operating efficiency. The small transistorized digital computer sets new standards in reliability and is a serious competitor, both technically and financially, to the special-purpose type often designed for a particular
application. A digital computer of this type is being used to supervise a nuclear reactor and to give warning on the basis of computed trends. Another computer of this type is being used to optimize the cutting of steel billets from a rolling mill so as to minimize waste. Practical examples are given of all the systems and techniques discussed.
Sommaire La manipulation de donnees est l'agent de transformation dans le concept d'automatisation des processus, et la manipulation intelligente de I'information peut apporter un degre surprenant d'integration, me me dans les processus les plus simples. La concentration de donnees est rarement une fin en soi et I'emploi des techniques simples de calcul pe ut habituellement augmenter considerablement la valeur immediate des donnees collectees. Ces calculs peuvent etre sou vent effectues par des instruments de tableau de conception convenable. Dans une installation de gazeification de combustible liquide on utilise un consignateur-calculateur dont on espere une amelioration du rendement de l'installation. La petite
calculatrice numenque transistorisee est une concurrente serieuse, aussi bien techniquement que financierement des calculatrices speciales souvent proposees pour une application particuliere. Une calculatrice numerique de ce type est en usage pour surveiller un reacteur nucleaire, et pour donner I'alarme sur la base des tendances calculees. Une autre calculatrice de ce type est utilisee pour optimiser la coupe des billettes d'acier sortant d'un laminoir, de fa.;on it reduire la perte au minimum. On donne des exemples pratiques de to us les systemes et techniques discutes.
Zusammenfassung Unter Datenverarbeitung versteht man die Mel3wertverarbeitung bei der Automatisierung in der Verfahrenstechnik. Eine gunstige Informationsverarbeitung bietet einen uberraschend hohen Grad von Vollkommenheit auch selbst bei einfachsten Verarbeitungsverfahren. Eine Datenumsetzung kommt fUr sich allein selten vor, da die zusatzliche Benutzung einfacher Rechner gewohnlich den unmittelbaren Wert der gesammelten Daten betrachtlich erhohen kann. Diese und andere Rechenoperationen konnen oft von elektronischen Registriergeraten geeigneter Konstruktion ausgefUhrt werden. Eine weiterentwickelte registrierende Recheneinrichtung arbeitet in einer Olgasgewinnungsanlage, und ihre Benutzung lal3t einen hoheren
Wirkungsgrad erwarten. Der kleine dazu verwendete Transistorrechner zeichnet sich durch hohe Zuverlassigkeit aus und ist fUr Spezialgerate, die oft fUr Sonderzwecke entwickelt wurden, sowohl in technischer als auch in finanzieller Hinsicht ein ernster Konkurrent. Ein Digitalrechner dieser Art wird zur Oberwachung eines Kernreaktors verwendet und gibt auf Grund berechneter Tendenzen Warnsignale ab. Ein weiterer Rechner der gleichen Art wird verwendet, um das Beschneiden von Stahlbarren in einem Walzwerk zu verbessern und damit die Verluste zu verringern. Fur alle Systeme sind praktische Beispiele angegeben und die AusfUhrungsformen werden eriautert.
DISCUSSION
D. 1.
AGEYKIN
are conflicting: at the input to the apparatus for observing deviations the switch must act continuously, it must have a higher speed and a much greater life, in terms of the number of
(U.S.S.R.)
In your report mention is made of the possibility of changing the operating programme of the scanning control device, depending on the deviation of this or that variable. Is this possibility realized in practice anywhere? Could you describe in more detail the pulse-counting technique for measuring the length of a milled bar? V. L.
EpSHTEIN
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How is the mathematical problem of defining the optimum cutting programme set up? How is this problem solved? I should be grateful to hear of other known or intended work on the automatic control of cutting billets. J. M.
SHENBROT
FiUl/re A "
switchings. But it is possible for a known error to be introduced. At the input to the digital recording apparatus the switching is much less frequent, in accordance with the necessity of recording
(U.S.s.R.)
In his paper Mr. Laws has given an excellent survey of the contemporary state of the technology of data handling in industry. He has given a number of examples which show graphically the advantage of using circular systems of central control, but unfortunately, he has paid insufficient attention to the internal structure of such a control system. On Figure 1 of his paper he introduced a widely used block diagram, which can be represented simply in the form shown in Figure A. Here, one input switch P operates to switch the n control points I-n to the apparatus for observing the deviations (00), as well as to the apparatus for digital recording (TsR). The operation of a reliable, fast and accurate input switch P is attended by technical difficulties. Among these is the fact that in the majority of applications the requirements on the switch for input to the apparatus for observing the deviations, and to the apparatus for digital recording,
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information, but a greater accuracy in the transmission of the measured quantity is necessary during switching. In connection with these considerations it is convenient to use a different block diagram (Figure B). The continuously
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acting input switch PI is constructed from transistors, and the switch P 2 from sealed electromagnetic relays. The switch P 2 permits the automatic selection of any point of measurement in any order corresponding to the instruction, either from the apparatus for observing the deviations, or from an electrical clock, or according to the demand of the operator. C. A. LAWS in reply. Automatic programme changing has been used in the U .K. in cases where certain measurements are of great importance under 'Alarm' conditions. On the operation of the 'Alarm', the programme is changed to give priority to these inputs, they are sampled much more frequently than the others. With the computer system (609) almost any automatic programme change can be provided for. The generator consists of a number of bar magnets mounted round the periphery of a drum mounted on the mill shaft. A pick-off coil produces a pulse each time one of the magnets passes its pole-piece. The magnets are mounted at intervals of 3 in.
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The computer programme is, basically, a loop in which control is continually returned to the starting point. The procedure is to take the longest billet which the customer order allows and then try to obtain a better fit with shorter lengths that are still within the specification . The problem is complicated by the fact that the 'as rolled' billet must be cut into a number of sections before it can be transferred to the second section of the line, and by the error at the saws and shears. It was not clear to me whether Mr. U. M. Mesropoinsk is using his transistor circuit for scanning high-level inputs or for driving the relay unit. In the reactor scanner described in the paper the scanning is carried out by mercury relays, which are driven by the transistor logic in the computer. It would certainly be possible to use mixed scanning techniques, the input level to the transistor switch being limited to a minimum of 50 ,tV.
The Development of Automatic Data Handling In Instrumentation and Control C. A. LAWS Introduction The modern graphic panel requires no introduction. The interesting point about it is that it appears to represent the culminating phase in present instrument practice. It is therefore a very suitable starting point for a discussion of new approaches to the subject. The need for an overhaul of instrumentation and control methods has arisen as a result of the emphasis being placed on such questions as plant efficiency and product quality. Technical developments have made improvements in these possible. Automatic control, which is often the key to such improvements, is based on automatic data processing, and the type of information required for the supervision of future plant is likely to change as a result. The purpose of this paper is to trace current trends in industrial instrumentation and data handling and to show their relation to the advancing science of control engineering.
15 Ib ./in.2 by existing primary transducers, or, for temperature measurement, from thermocouples and resistance thermometers. The data are sampled sequentially, each value being converted to digital form in the appropriate scale before being passed to an electric printer. The flow diagram of a typical logging system is shown in the right-hand section of Figure I. Under Set-point controls
I "I
Ref. potential generator
Automatic Logging The general realization that early methods of manual data handling have reached their limit has given rise to the development of automatic logging systems. Usually, the data logger obtains its input data from a series of transducers, frequently fed with pneumatic signals at 3 to
1498
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Figure 1. Simple data-reduction system
typical conditions the values of perhaps 200 variables would be measured and logged once each hour, each complete operation taking about 31' minutes. The conversion of the input signal to digital form can be carried out in many different ways, depending on the application, and at speeds varying from a few seconds to a few microseconds. The most usual method for process applications is to use an industrial type self-balancing bridge to obtain a shaft position proportional to the input potential, the shaft carrying a commutator type digitizer. The coded on/off contact pattern which this provides is converted to the required units by a series of interconnected relays, the contacts of which supply current to the appropriate solenoids of an electric typewriter. Where shorter conversion times or higher sampling rates are required, use can be made of pulse techniques involving voltageto-time conversion, and such systems are capable of operating in periods as short as one millisecond. For even higher speed operation comparison techniques involving the summation of currents in binary grade resistors are frequently used. Data Reduction The operator is primarily concerned with values which have deviated beyond prescribed limits, and deviation detectors have
397 26
Plant variables
11- '\)
Deviation store
(I) To provide a record of plant operation and performance
The collected information is almost invariably available in the form of manually compiled log-sheets and criticism can often be levelled at this method of compilation in respect of the labour involved, the questionable accuracy of recording, and the difficulty of extracting significant data. The fact that the compiled data are seldom used in any significant way to modify hour-to-hour running conditions was evident and pointed to the big potential for measurement-and-control equipment capable of carrying out a continuous function of optimization.
:" '
~--"-~
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Data Recording At a recent meeting of the S.I.T.l - J in London, a number of engineers concerned with the planning and use of instrument data were asked to describe the way in which they are put to use. The main points can be summarized as follows: (2) To provide a record of raw materials and fuel consumed and of the products produced (3) To enable the efficiency of plant units to be derived and the need for overall adjustment or replacement observed (4) To enable trends to be observed and maintenance and trimming requirements predicted (5) To enable the best average process conditions to be assessed (6) To provide a basis for cost accounting of various types.
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A. LAWS
therefore been introduced which enable the logger to discriminate between normal and abnormal data. The deviation detector is shown in Figure I, the complete system forming what is now known as a data reduction unit. In operation, the deviation channel is fed with the same transducer inputs as the logger, but in this case they are compared with potentials from a panel of set-point potentiometers defining high and low limits. The presence of a deviation is detected in a comparator unit and recorded by the appropriate channel of a simple twostate memory unit. This leads to the operation of an alarm indicating the measurement point involved, and to the operation of the logger which measures the value and records it, usually in red type. It will be apparent that the alarm and point identification function has a wide field of application quite apart from the logger, and limit scanners of this type are now widely used. It cannot be over-emphasized that a data-reduction system is not a replacement for all existing types of instrumentation. The fact is that systems of this type represent the beginning of a relatively new approach to instrumentation, and only when they are regarded in this light do the full implications become evident. The maximum potential of such devices is unlikely to be realized if comparatively slow manual methods are used for evaluation, and so long as this fact remains unrecognized, unsatisfactory comparisons will be drawn.
running total of all inputs. Facilities are also provided for pre-selecting the weight of cargo assigned to a given customer, the unloading operation being interrupted automatically as the required total is reached. A statement of total goods delivered, to which a computed cost could be readily added, is thus prepared whilst the actual operation is being performed, and because this information is compiled without delay it is possible to use it for on/off control. The final phase in streamlining this operation and one which is due to be added, involves the transmission of the digital data over telephone lines to the city office so that the invoice is prepared at this point. Figure 3 illustrates a plant used for freezing packeted foods, the central processing unit being the frost er. This consists Temperature
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Applications of Automatic Logging Techniques Many applications of data handling techniques exist outside the chemical industry and since these provide interesting evidence of the increasing role of simple automatic data handling in process management, it will be of interest to review one or two typical examples. The first, shown in Figure 2,
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Figure 2. Data handling applied to dockside cranes
demonstrates how a complete process has been effectively streamlined by the introduction of logging techniques combined with those of electric weighing. The' process' involves a dockside crane installation. The cranes are each fitted with a load cell providing an electric potential proportional to the grab load, and a digitizer for converting it to digital form. The digital information from each crane is relayed to a central office where it is recorded on an electric printer, together with a
essentially of an insulated cabinet containing some 20 refrigerated plates which can be separated by a hydraulic ram to take trays of packeted foods. After loading, the froster doors are closed and the ram operated to ensure positive contact between the packets and the refrigeration plates. A wide range of foods is processed in cartons of various sizes, and each particular product has a specific processing time which must be adhered to if frosting is to be effective. At the end of the allotted period, the products are withdrawn and despatched to the cold store. The data system was required as an aid to the manual operation of the plant, the industry being very much a seasonal one and much of the labour therefore casual. The objectives were: (a) to enable a final product of consistent quality to be obtained ; (b) to streamline the cost accounting system; (c) to improve the plant utilization factor. The factors most affecting the final product quality were: (1) variations in plate temperature; (2) variations in process timing (manual); (3) interruption of pressure on the ram; (4) opening of doors during processing. A system has therefore been designed in which the supervision of each of these functions is carried out automatically. Supervision of plate temperatures is carried out by temperature scanner units allocated to each group of four frosters. The plates, totalling some 200, are each equipped with a specially mounted thermistor element and the scanner units arranged to produce an alarm condition, and to indicate plate and froster numbers, whenever a plate temperature rises above a predetermined limit. The froster and plate numbers are visually displayed in a central office allocated to the quality-control
398
1499
THE DEVELOPMENT OF AUTOMATIC DATA HANDLING IN INSTRUMENTATION AND CONTROL
economy in equipment, is shown in Figure 4. The example is that of an oil-fired power station for which the value of B.Th.V. (fuel)/kWh is derived as a measure of overall efficiency. To obtain this a corrected flow signal is integrated to provide a potential representing the quantity of fuel consumed. A factor is introduced which takes account of calorific value so that the potential E becomes proportional to the heat value of the fuel. A similar potential is derived by integration for the power output, and the division of one by the other is carried out, as shown, by the digitizer servo. The availability of computed information of this simple type at the time of the event, rather than hours or days later, gives a new incentive to the operator as well as a basis for controlling his plant more effectively. A somewhat more advanced data system, developed for the British gas industry, is of particular interest in that specialpurpose computing techniques have again been used, this time on a fairly large scale. A comparatively simple equipment has resulted from the combined use of pneumatic, electrical , mechanical and pulse techniques, but even so it appears that for this size of undertaking the greater flexibility afforded by the use of a small general-purpose digital computer might well be advantageous. This plant is concerned with the production of gas from oil and the function of the data-handling equipment is first to log some 60 variables. Approximately 40 of these are flow rates which it is necessary to integrate continuously for hourly periods and, generally, summate before conversion to digital form. The integrator, the logical arrangement of which is shown in Figure 5, makes use of a bellows-operated arm riding on a
supervisor and, in addition, all information relevant to temperature faults is automatically typed on a log-sheet. The correct selection and control of process time in relation to the product is carried out in the data processing centre, information on product type being communicated to the centre by 'posting' a coded plate into a box located at each froster. Each plate bears a picture and description of one product and the operators each have a set appropriate to the products they are controlling. The plates operate a series of contacts providing a binary coded signal, the selection of switches being determined by cut-outs in the metal plates themselves. On reception of this information at the processing centre, a timing unit is automatically selected and set up with the appropriate timing instruction. When switches operated by the ram and doors have been closed, 'Process On' information is fed back to the annunciator display situated above each froster unit and the timing cycle begins . The product number derived from the coded plate is also displayed at the supervisory panel and typed on the log-sheet. An interruption of the ram pressure during a process, or the opening of a door, will give rise to an appropriate alarm and to a typed statement of the duration of the fault. The four factors affecting product quality are therefore supervised automatically and departures from the correct procedure, accidental or otherwise, are recorded. This installation provides an example of the use of a planned information handling system for coordinating the workings of an existing manually operated plant, no changes to the actual production equipment having been made. Introduction of Simple Computing Aids
It will be noticed that in the above examples of industrial installations, use is already being made of very simple computing-or compiling-techniques. These simple operations represent first steps in deriving information not present in the initial data; operations which were previously carried out manually. A digitizing servo can be used to carry out simple computations, and since the servo shaft carries the digitizer, the answer is directly available in digital form for printing or display. One way in which advantage has been taken of this, with consequent
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square law cam to effect a conversion from the input pressure P to a time interval T. The rotating cam lifts the rider arm for a period each revolution determined by its deflection, the arm serving to open and close a pair of contacts acting as a gate to a continuous train of pulses. The number of pulses passing through the gate during the period of integration is therefore determined by the pneumatic pressure applied to the bellows and, since this is proportional to the differential pressure across an orifice plate in the gas flow line, the pulse count is directly proportional to flow. The groups of pulses are passed to a series of simple stepping switches arranged as a decade counter, the contacts being maintained at suitably graded potentials to give an output proportional to the time integral of the flow. Summation of accumulated flows is readily attained in a resistance network common to the integrators involved . Where it is necessary to multiply one integrated quantity by another, for example volume of crude gas x calorific value, this is done by using the output of one integrator as the input reference potential for the other. Now in addition to the measurement, integration and logging
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399
1500
C. A. LAWS
Total vol. Total B.Th.U Total vol.
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400
1501
THE DEVELOPMENT OF
AUTO~!ATI C
DATA HANDLING IN INSTRUMENTATION AND CONTROL
of variables, the equipment also carries out a number of hourly calculations relating the volume of gas produced to the quantity of oil consumed. As an example of the operations involved, the method adopted for obtaining the average calorific value is shown diagrammatically in Figure 6. Squares A, B, C and D represent flowmeter transmitters in the four units of the plant, the outputs being integrated to give volumes A , B, C and D . Since both the volume of gas and its calorific value may differ in each unit, it is necessary to multiply each volume by its own average calorific value to give individual B .Th.V. These are then added to give total B.Th.U. At the same time the outputs from the four integrators are added to give total volume, and this is divided into the total B.Th.V. to give the average calorific value for all four units of the plant. This va lue is printed out as such and is also used in calculating the therms of gas produced for each gallon of oil. Thus a record of plant production is obtained which takes account of product quality. The automatic optimizing of plant performance requires the acc umulation of a great deal of data, and thi s installation illustrates one very sound and practical way in which such data may be acquired. The recorded effects of changes made to plant conditions become available in the shortest possible time, and provide a guide to short-term improvements as well as material for analysis. The control room , including the data logger and efficiency computer, is shown in Figure 7.
Electronic Instruments Nowadays, the complexity and size of plant , coupled with the market requirement for ever more rigid control of product quality, has resulted in a considerable increase in the use of instruments and in their attendant problems. At the same time, the idea of continuously integrated control as a means of optimizing process conditions has already profoundly affected thinking on the subject of instrumentation, as a part of data handling, and it is within this new framework that the immense potential of electronic methods has become apparent. ] n this section , one approach to electronic instrumentation is used to illustrate the great flexibility of such systems when dealing with the more complex measurements involving computation 4 . The sensitive element used is a differential transfornler in which, with the core centrally placed , current fed to the primary coil results in zero output from two counter-wound secondary coils. Displacement of the core from its central position then results in an unbalance signal in the secondary. The output of such a device can be made linear with displacement and it may be used as a pick-off with any measuring device producing a small proportional displacement . The basic measuring circuit, shown in Figure Sea), is a simple self- balancing servo in which a small servomotor is used to adjust a differential transfo rmer TA in the feedback loop . The movement of the shaft, which is proportional to the input signal , is conveyed either to an indicating pointer or to a chart pen and, since a common a.c. supply is used for the input and feedback transformers, the measurement is insensitive to supply varia tions. The signal input to the amplifier is EX, where X represents the core displacement of the transducer transformer T I, and that from the feedback loop is Efl, where fl represents the core displacement of TA ' Since at balance EX = Efl, it follows that fl = X. The equivalent servo using potentiometers instead of differential transformers is shown at Figure S(b). Servo indicators of this type can be used to carry out a very impressive range of process calculations, and some of these are illustrated. Square roots may be found in the manner shown at Figure S(e), both the shaft position and the electrical output TB representing ../ XI' The replacement of input E to trans401
1502
former TB by a second variable, EX2 , results in the solution of fl = (Xd X 2)! as shown at Figure Sed), one application being the measurement of flow ratio. Figure See) shows an arrangement for solving an equation of the form (A - B) /C. The variables Hs , Hw and HF chosen represent measurements on a boiler system in which Hs is the heat in the steam, Hw is the heat in the feedwater, and HF is the heat in the fuel. fl = (Hs - Hw) /Hp therefore represents boiler efficiency. This circuit differs from the previous ones only in that the variables Hw and H s are combined in a simple subtracting network consisting of two resistors. The solution is available as a shaft position and as an electrical output from TB. Figure SC!) shows the manner in which the servo indicator may be adapted to the requirements of a heat exchanger. The pressure signal EP, derived from a differential transformer is applied as the input to a second transformer measuring the differential pressure C!.P. The output from this, EP.6.P, is a pplied as one input to the servo amplifier. The second input is obtained from transformers TA and TB in cascade, and since TB is fed with a temperature signal ET, the shaft position fl is proportional to (P. 6.P/I I)!, which is the mass flow. This quantity is indicated on the dial or recorder chart. If a third transformer Tc is now fed with a signal representing the temperature drop across the heat exchanger, the output from this will be 6.T(P. c!'P/TI)t , or the heat output. The possible variations on this general theme are clearly numerous but sufficient has been said to indicate the enormous potential of electronic instruments. Jt is perfectly feasible with such a system to carry out continuous performance calculations-as well as some of those required for control- in the recording and indicating instruments used at the supervisory panel. An arrangement of this type suitable for a power plant is suggested in Figure 9.
Figure 9. Power plalll scheme
Use of Digital Computers A stage has now been reached in the V.K. where the digital computer is about to take a significant place in both process data handling and control. The principal factors responsible
C. A. LAWS
for this are, first, the educational process which has been going on in the more progressive industries since about 1945, and which has resulted in a wider understanding of the potential of the 'Integrated system'. The second factor is the successful development of the small reliable transistorized computer. The flexibility inherent in a tape programmed machine gives it a great advantage over the fixed-programme special-purpose type discussed earlier, particularly where a policy of continued development leads to changing requirements. The justification for employing a digital computer in a data handling system usually hinges on such considerations as the volume of information being handled, the complexity and/or variability of the operations to be performed, the extent of the storage required, and the speed at which the handling must be effected. Additionally the situation needs must be reviewed in the light of the equipment which would otherwise be necessary -if an alternative is possible, effect on production, and overall outlay. An example is provided by an equipment designed to monitor 600 temperature points in a nuclear reactor. Both the temperature and the rate at which it changes are of great importance to the safety of the plant and it is therefore necessary to store at least two temperature measurements per point. In fact, six measurements are stored in digital form for each input point, a total of 3,600 values. As each new scan takes place the latest measurement is compared with the previous one, already stored, and the first difference is taken as a measure of rate-of-change. At the same time the earliest of the six measurements in store is deleted and the new value added. Both the temperature reading and the extracted rate-of-change are compared in the computer with set-point values, also stored digitally, and an alarm is sounded when either is exceeded. Print-out is normally hourly, but is brought into continuous operation under alarm conditions or when demanded manually. The small computer employed uses transistor-driven core logic and a ferrite core store with a capacity of 2,048 thirty-three bit words. It provides a compact solution to the problem and, having regard to the storage requirements, has obvious advantages over analogue methods. However, the data provided, whilst essential to the safe running of the reactor, are in no way associated with its control. In fact, although a great deal of thought is being given to the use of digital computers in industrial control very few examples so far exist. It will therefore be appropriate to conclude this paper with a brief description of one such project which, following a period of study, is being implemented in the U.K. for a large steel company. The purpose of the system (Figure 10) is to control the cutting of hot steel billets after their emergence from the finishing mill on to the cut-up table. The length of the billet, which may be as much as 400 ft., is measured by a self-calibrating pulsecounting technique as it begins its final run through the mill. By the time it has fully emerged the computer, which has been fed with length information, discard instructions, and the customer's specification, has calculated and issued setting instructions to the various cutting stations along the line. Due to limitations of space, it has not been possible to locate the various cutting stations along the length of a single straight roller table, and recourse has had to be made to a shunting technique by means of which sections of the billet are sent back along a second line as shown. The first cut is made by the parting saw, the maximum permissible length being limited to
Cutting instructions Billet length
Discard Instructions Customer's
Figure 10. Computer control of a billet cut-up line
180 ft. However, the exact length has to be determined with reference to the calculated number of cuts of 30 ft. and under required at the first saw, which in turn depends on the number of cuts under 10 ft. required at the ganged shear. The situation is complicated by the fact that individual statistical allowances must be made for the cutting 'spread' at each saw, and by the fact that customer specifications can take any of a number of different forms. For instance, the customer can specify exact lengths; alternatively he may specify minimum and maximum lengths and be willing to take any billet size within these limits; yet again he may specify minimum and maximum lengths, but require that cuts taken between these limits shall be at increments of, say, 2!- ft. It will therefore be clear that the calculation and distribution of cuts between the various saw positions, so as to minimize wastage, can be quite a complex problem, and it is not surprising that present methods relying on manual estimation yield a higher percentage wastage than is ideally necessary. The information from the computer is fed to digital displays at the saw and billet-stop positions and the machines are brought to the positions indicated under manual control. When the system has been fully proved in practice, the manual link will disappear, and it will then be fully automatic. In addition to issuing cutting instructions, the computer will operate a printer setting out information relevant to the requirements of accounting and stock control. At an estimated overall cost of £100,000 per annum per 1 per cent of waste, the ability to reduce this by even 1 per cent is clearly attractive, and the indications are that a greater saving than this is to be expected. References 1 NONHEBEL, G. The use of process data on steam raiSIng by industry. Symposium on the Use ol Data Recorded all Industrial Plant. S.I.T. Meeting. March 1959 2 KEATING, J. M., MILLIEN, A. and TOWNEND, D. S. Informationcommunication and usage in an oil refinery. ibid. 3 WEBSTER, F. G. The handling of data from process operations in a chemical plant. ibid. 4 NEEDHAM, M. V. Electronic Systems for Industrial Measurement and Control. Control. July, Aug., Se pt. 1948
Summary Data handling is the transforming agent in the concept of process automation, and the intelligent handling of information can bring about a surprising degree of integration, even in the simplest processes. 402
1503
Data reduction is seldom an end in itself and the use of simple computing techniques can usually extend the immediate value of collected data considerably. These and other computations can
THE DEVELOPMENT OF AUTOMATIC DATA HANDLING IN INSTRUMENTATION AND CONTROL
often be carried out in electronic panel instruments of suitable design. An advanced logger-computer is operating in an oil-gasification plant and its use is expected to result in higher operating efficiency. The small transistorized digital computer sets new standards in reliability and is a serious competitor, both technically and financially, to the special-purpose type often designed for a particular
application. A digital computer of this type is being used to supervise a nuclear reactor and to give warning on the basis of computed trends. Another computer of this type is being used to optimize the cutting of steel billets from a rolling mill so as to minimize waste. Practical examples are given of all the systems and techniques discussed.
Sommaire La manipulation de donnees est l'agent de transformation dans le concept d'automatisation des processus, et la manipulation intelligente de I'information peut apporter un degre surprenant d'integration, me me dans les processus les plus simples. La concentration de donnees est rarement une fin en soi et I'emploi des techniques simples de calcul pe ut habituellement augmenter considerablement la valeur immediate des donnees collectees. Ces calculs peuvent etre sou vent effectues par des instruments de tableau de conception convenable. Dans une installation de gazeification de combustible liquide on utilise un consignateur-calculateur dont on espere une amelioration du rendement de l'installation. La petite
calculatrice numenque transistorisee est une concurrente serieuse, aussi bien techniquement que financierement des calculatrices speciales souvent proposees pour une application particuliere. Une calculatrice numerique de ce type est en usage pour surveiller un reacteur nucleaire, et pour donner I'alarme sur la base des tendances calculees. Une autre calculatrice de ce type est utilisee pour optimiser la coupe des billettes d'acier sortant d'un laminoir, de fa.;on it reduire la perte au minimum. On donne des exemples pratiques de to us les systemes et techniques discutes.
Zusammenfassung Unter Datenverarbeitung versteht man die Mel3wertverarbeitung bei der Automatisierung in der Verfahrenstechnik. Eine gunstige Informationsverarbeitung bietet einen uberraschend hohen Grad von Vollkommenheit auch selbst bei einfachsten Verarbeitungsverfahren. Eine Datenumsetzung kommt fUr sich allein selten vor, da die zusatzliche Benutzung einfacher Rechner gewohnlich den unmittelbaren Wert der gesammelten Daten betrachtlich erhohen kann. Diese und andere Rechenoperationen konnen oft von elektronischen Registriergeraten geeigneter Konstruktion ausgefUhrt werden. Eine weiterentwickelte registrierende Recheneinrichtung arbeitet in einer Olgasgewinnungsanlage, und ihre Benutzung lal3t einen hoheren
Wirkungsgrad erwarten. Der kleine dazu verwendete Transistorrechner zeichnet sich durch hohe Zuverlassigkeit aus und ist fUr Spezialgerate, die oft fUr Sonderzwecke entwickelt wurden, sowohl in technischer als auch in finanzieller Hinsicht ein ernster Konkurrent. Ein Digitalrechner dieser Art wird zur Oberwachung eines Kernreaktors verwendet und gibt auf Grund berechneter Tendenzen Warnsignale ab. Ein weiterer Rechner der gleichen Art wird verwendet, um das Beschneiden von Stahlbarren in einem Walzwerk zu verbessern und damit die Verluste zu verringern. Fur alle Systeme sind praktische Beispiele angegeben und die AusfUhrungsformen werden eriautert.
DISCUSSION
D. 1.
AGEYKIN
are conflicting: at the input to the apparatus for observing deviations the switch must act continuously, it must have a higher speed and a much greater life, in terms of the number of
(U.S.S.R.)
In your report mention is made of the possibility of changing the operating programme of the scanning control device, depending on the deviation of this or that variable. Is this possibility realized in practice anywhere? Could you describe in more detail the pulse-counting technique for measuring the length of a milled bar? V. L.
EpSHTEIN
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(U.S.S.R.)
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How is the mathematical problem of defining the optimum cutting programme set up? How is this problem solved? I should be grateful to hear of other known or intended work on the automatic control of cutting billets. J. M.
SHENBROT
FiUl/re A "
switchings. But it is possible for a known error to be introduced. At the input to the digital recording apparatus the switching is much less frequent, in accordance with the necessity of recording
(U.S.s.R.)
In his paper Mr. Laws has given an excellent survey of the contemporary state of the technology of data handling in industry. He has given a number of examples which show graphically the advantage of using circular systems of central control, but unfortunately, he has paid insufficient attention to the internal structure of such a control system. On Figure 1 of his paper he introduced a widely used block diagram, which can be represented simply in the form shown in Figure A. Here, one input switch P operates to switch the n control points I-n to the apparatus for observing the deviations (00), as well as to the apparatus for digital recording (TsR). The operation of a reliable, fast and accurate input switch P is attended by technical difficulties. Among these is the fact that in the majority of applications the requirements on the switch for input to the apparatus for observing the deviations, and to the apparatus for digital recording,
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information, but a greater accuracy in the transmission of the measured quantity is necessary during switching. In connection with these considerations it is convenient to use a different block diagram (Figure B). The continuously
403
1504
C . A. LAWS
acting input switch PI is constructed from transistors, and the switch P 2 from sealed electromagnetic relays. The switch P 2 permits the automatic selection of any point of measurement in any order corresponding to the instruction, either from the apparatus for observing the deviations, or from an electrical clock, or according to the demand of the operator. C. A. LAWS in reply. Automatic programme changing has been used in the U .K. in cases where certain measurements are of great importance under 'Alarm' conditions. On the operation of the 'Alarm', the programme is changed to give priority to these inputs, they are sampled much more frequently than the others. With the computer system (609) almost any automatic programme change can be provided for. The generator consists of a number of bar magnets mounted round the periphery of a drum mounted on the mill shaft. A pick-off coil produces a pulse each time one of the magnets passes its pole-piece. The magnets are mounted at intervals of 3 in.
404
1505
The computer programme is, basically, a loop in which control is continually returned to the starting point. The procedure is to take the longest billet which the customer order allows and then try to obtain a better fit with shorter lengths that are still within the specification . The problem is complicated by the fact that the 'as rolled' billet must be cut into a number of sections before it can be transferred to the second section of the line, and by the error at the saws and shears. It was not clear to me whether Mr. U. M. Mesropoinsk is using his transistor circuit for scanning high-level inputs or for driving the relay unit. In the reactor scanner described in the paper the scanning is carried out by mercury relays, which are driven by the transistor logic in the computer. It would certainly be possible to use mixed scanning techniques, the input level to the transistor switch being limited to a minimum of 50 ,tV.