IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-18, NO. 2, MAY 1971

A Batch-Chemical-Process Controller

ROBERT YOUNG

Abstract-The essential characteristics of a minicomputer sys-tem designed to control a batch-process chemical plant are presented.The installed hardware and software concepts are presented. Thesystem incorporates direct digital control (DDC) of analog processvariables with its real-time software.

INTRODUCTIONF OR most process control applications it is more

economical to design the control system around adigital computer than hardware logic components

and analog set-point controllers. Prices of minicom-puters start at $3000 to $4000 without core memory.For all but the simplest systems the cost of the com-puter will be less than the cost of the hardware compo-nents it replaces. In addition, the overall effort requiredto design computer software, even using assembly-language programming, is less than that required todesign equivalent hardware. Computer programs canbe much more easily modified than can hardware. So-phisticated control algorithms which can result in con-siderable savings of operating cost but are difficult toimplement with hardware can usually be programmedinto a computer with little difficulty.With the aim of demonstrating the preceding advan-

tages, this paper describes the application of a mini-computer to a batch-chemical-process control system.The process control system described performs a varietyof functions typical of computer-based systems. Thesefunctions include "contact closure" input and output,analog input and output, direct digital control (DDC)of analog process variables, timing and sequencing ofprocess events, and logging of process variables andevents as printed output. A description of the computerand associated hardware is given first, followed by adiscussion of some of the programming techniques whichwere used for the system.

COMPUTER HARDWARE

Fig. 1 shows a block diagram of the computer systemwhich is built around a Digital Equipment CorporationPDP-8L computer and Peripheral Equipment Corpora-tion 7820-9 magnetic tape unit. Additional equipmentconsists of A/D and D/A converters, contact closureinputs and outputs, a time-of-day clock used for eventlogging, a 60-Hz interval timer used as the time base forthe entire system, a stall alarm, and a model 33ASR

Manuscript received August 26, 1970. This paper was presented atWESCON, Los Angeles, Calif., August 25-28, 1970.

The author is with Emery Industries, Inc., Cincinnati, Ohio.

CHRONO-LOG PERIPHERALDIGITAL EQUIPMENTCLOCK MAG - TAPE

r LIMIT DIGITAL SOLENOIDSWITCHES (21) EQUIPMENT VALVES (47)I ~~~~CORP

I LEVEL PDP8L ~ALARM 9aL|DEVECTORSL(4)> PDP8 L iANNUNCIATORS |£DETECTORS (4)

C- ALARM CPU INDICATOR 0SWITCHES(24) LAMPS (23)

4096 iOPERATOR 12 BIT WORD MOTOR

a CONTROLS(29) CONTROLS(5)

SPARES (18) KP8/L SPARES (18)POWER FAIL

RAYTHEON TIME RAYTHEON 0>ocen COMPUTER INTE-RRUPT COMPUTER ODo S 24 CHANNEL 7CHANNEL X< z ANALOG ANALOG H<- INPUT MADC 10 OUTPUT MDA 2

MOTOR CONT

TELETYPEASR - 33

Fig. 1. System block diagram.

Teletype used for logging. Operator messages are pre-sented through an annunciator panel. The computermemory which holds the control program and the pa-rameters associated with each product which is to bemanufactured consists of 4096 twelve-bit words of mag-netic core memory with a 1.6-,us cycle time.The computer input-output facilities consist of 12

input data logic lines and 12 output data lines. Data canbe selected and placed on the output lines or acceptedfrom the input lines at appropriate times under controlof the program. In addition, six address lines are pro-vided to be used by the external logic in routing inputor output data to or from external equipment (contactclosure sensors, D/A converters, etc.). Control lineswhich can be pulsed or tested by the program are pro-vided for synchronization of the external logic with thecontrol program.The contact closure outputs consist mainly of solid-

state devices; triacs for ac output and transistorswitches for dc output. A few relays driven by transis-tors are used where it is desirable to have continuity inthe event of a logic power supply failure. The contactclosure outputs are arranged in groups of 12 correspond-ing to the 12 data output lines of the computer. Eachcontact closure output has a storage flip-flop which re-ceives and holds the data from one of the output lines.Groups of contact closure outputs are selected to receive

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YOUNG' A BATCH-CHEMICAL-PROCESS CONTROLLER

data by codes provided on the six address lines. Thecontact closure outputs include signals to the annunci-ator panel, motor start stop signals, and signals tosolenoid-operated pilot valves which provide air to air-operated process valves.

Contact closure inputs are arranged and addressed ingroups of 12 in a manner similar to that used for theoutputs. Dc input circuits consist of RC filters whichfilter out contact bounce effects followed by Schmitttriggers to convert to logic levels. Ac inputs includeisolation transformers and diodes to convert to dc. Con-tact closure inputs include signals from the annunciatorpanel, operator push-botton signals, valve position limitswitches, level detectors, etc.The D/A converters accepts the computer output

data as binary numbers and produces a correspondinganalog voltage. A separate D/A converter is used foreach analog output. Output range is -10 to +10 V.Each D/A converter has a separate 6-bit address. Ana-log output voltages are used to operate panel meters todisplay process variables and to drive electrical-to-pneumatic (E/P) converters which provide air to pro-portional (modulating) process valves.

Analog voltage inputs are multiplexed to a singleA/D converter where they are converted to correspond-ing binary numbers to be placed on the input data linesof the computer. Process variables such as temperature,pressure, and flow are entered into the computer fromelectrical transmitters via the A/D converter. Manualset points supplied by the operator through poten-tiometers are also entered via the A/D converter.The 60-Hz interval timer provides a time-interrupt

signal to the computer every 1/60 s. This signal is usedas a basic timer for sequencing the process. It also pro-vides a time reference for integral and derivative con-trol in the DDC loops.The Teletype and time-of-day clock are used in a

conventional manner for event logging. The time inhours, minutes, and seconds can be read from the clockand printed on the Teletype followed by a message. Forexample,

08:16:57 THERMINOL FROM ESTERIFIERLOW FLOW

PROGRAMMING TECHNIQUES

It is the computer programming, or "software," whichdetermines the actual operation of a computer-con-trolled process. Thus it constitutes a major part of thedesign and development effort of such a system. Someof the general tasks for our particular system can bementioned here. They seem typical of a batch-processcontrol computer application.

First, the software determines the sequence of valveoperations, depending, of course, on the particularchemical which is to be manufactured. Also peculiar toa specific chemical is the timing and process status func-tion which decides when the next step should be taken.

Our computer senses six variable and 14 logical (yes orno) "endpoints," of which any combination can controlthe duration of a step and/or branching to one of severalpossible next steps. The computer software must alsocheck the status of 50 other temperatures, manual valvepositions, etc., which indicate whether the system isoperating normally. The software is such that criticalstatus errors may halt operation of the process. An an-nunciation function of the software puts out printedmessages on the Teletype console concerning the statusindicators and program flow. A self-checking functionwhich detects and annunciates computer malfunctionsis also normally called for. A final function to be morefully discussed is the DDC software, which replacesconventional analog control loops.

It is perhaps impressive that the minicomputer hassufficient capacity for all of these functions. The key tofitting them all in was the careful organization of thesoftware into subroutines. Besides the normal advan-tages of easier trouble shooting and simplified programchanges, an important feature of subroutines for ouruse is that a subroutine can be called many times duringa program sequence, preventing duplication of groupsof statements; a program of routine operations withunique sequence and duration of the operations greatlyshortens its length. A general principle of our softwareorganization is that every function which is routine isa subprogram, and only those functions unique to theparticular chemical product remain in the main pro-gram.An operation which serves to illustrate the application

of this principle is changing the combination of the 47on-off values of the process system. This change canhappen as many as 30 times during the batch process.This takes two main program statements, a valve-change subroutine call followed by an encoded combi-nation. The valve-change subroutine decodes the com-bination, selects the appropriate valves, and forms avalve-change message which gets printed by the annun-ciation subprogram. Finally, the proper valves getactuated by the update subprogram which does allinput-output functions.Another familiar programming tool used here is time-

sharing. For this application, simultaneous operation offunctions such as output printing, system error detect-ing, end-point detection, and DDC loop control wasdeemed necessary, and a time-interrupt system for time-sharing was devised. The executive control programdivides each second into 15 equal time slots, each ofwhich contains a part of the programming. At the endof each 1/60-s interval, the contents of the accumulatorand the address of the next statement to be executed inthat particular time slot are saved by the executivebefore going on to the next slot. At the start of the corre-sponding time slot during the next 4 s, the accumulatoris restored by the executive, and the program proceedsas if the interruption had not taken place. An executive

65

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND COINiTROL INSTRUMENTATION, mAAY 1971

"fork control" subroutine permits programming in oneslot to alter the flow of that in another slot. Parametervalues in one time slot can be read or modified fromanother slot also.One time slot previously alluded to and which de-

serves further explanation contains the DDC loops.There are nine process variables which are controlled byfive proportional (modulating) valves for our system.The DDC loops are the digital-computer equivalent ofanalog control loops for controlling these valves. Eachloop has set-point inputs from the main program andactual process variable inputs from the update program.The control algorithm closely resembles that of a normalanalog loop, except that summation replaces analogintegration and differencing replaces analog differentia-tion. A tune-up control panel supplied with the systempermits rapid optimization of gain constants, etc., forthese loops. Comparing the computer speed and thetime constants of our application, the DDC control isindistinguishable from analog control, but it is mucheasier to tune and modify.

Finally, in case of computer malfunctions, safeguardprocedures have been included in our software design.Provision for manual takeover of any valve, or manualtakeover of the DDC loop set points has been provided,and these options are compatible so that automaticcontrol can be re-established smoothly.As a safety as well as convenience feature, all of the

preceding software is stored on magnetic tape, which isread into the computer using a simple loader program.Normally, all subprograms are refreshed along with theidle system which runs between batches, and only thedesired main program is read in at the beginning of eachchemical process. Provision for updating the magnetictape has also been provided as part of the furnishedsoftware.

OFF-LINE CHECK OUT

In order to adequately check the controller prior toinstallation, it was mounted in a standard 19-in testrack and connected. Fig. 2 shows the setup.An essential part of the preinstallation test program

was adequate plant simulation. Shown to the right ofthe controller itself on the lower right side of Fig. 2 isthe plant simulator. A closer view of the simulator isprovided by Fig. 3. This simulator includes 96 switchclosure inputs and 20 analog inputs. Instruments areprovided for six analog outputs (five are used in thesystem). The 96 lamps indicate the digital outputs.By appropriate manipulation of the inputs, plant

conditions are simulated. The action taken by the con-trol is then shown by the outputs as well as by theTeletype.

Fig. 2. Computer control system.

Fig. 3. Plant simulator.

SUMMARY

The minicomputer batch-process control systemdescribed illustrates an exceptionally wide range ofminicomputer applications, all integrated into a flexiblesophisticated production tool. Every phase of real-timecomputer usage is represented, from simple alarm-pointmonitoring functions to unattended direct digital con-trol with self-checking features.The system has been designed so that the operator

has the capability to interact with the control systemto alter set points if he deems it necessary, or he cancompletely dominate the control system to handle un-foreseen process upsets manually if the need should everarise.Some of the real-time software concepts, which permit

many subroutines to be activated effectively simul-taneously, have added heavily to the flexibility of thiscontrol system. This organization permits a new mainprogram to be written for an entirely new product witha minimum of effort, inasmuch as the main programsconsist primarily of a sequence of calls to the variousutility subroutines, along with their required end pointsand set points.This minicomputer control system is considered to

be one of the most flexible and most completely auto-mated control systems in its industry, and yet its totalsystem cost is less than the cost of basic computer in thewell-known brands of larger process control computersystems.

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