A MINICOMPUTER CONTROL SYSTEMFOR A BATCH CHEMICAL PRODUCTION FACILITY

Dr. Robert YoungEmery Industries, Inc.Cincinnati, Ohio

ABSTRACT

A batch process control system built arounda Digital Equipment Corporation PDP8L minicomputeris described both in hardware and software. Thissystem allows two levels of operator interactionwith the normally automatic production of any ofa wide family of chemical products. The widevariety of functions performed by this systemrange from simple alarm-point monitoring and pro-cess logging to on-line closed-loop control.

I NTRODUCT ION

For most process control appl ications it ismore economical to design the control system arounda digital computer than hardware logic componentsand analog set-point controllers. Prices of mini-computers start at $3,000 to $4,000 without corememory. For all but the simplest systems the costof the computer will be less than the cost of thehardware components it replaces. In addition, theover-all effort required to design computer soft-ware, even using assembly-language programming, isless than that required to design equivalent hard-ware. Computer programs can be much more easilymodified than can hardware. Sophisticated controlalgorithms which can result in considerable savingsof operating cost but are difficult to implementwith hardware can usually be programmed into acomputer with little difficulty.

With the aim of demonstrating the above ad-vantages, this paper describes the application ofa minicomputer to a batch chemical process controlsystem. The process control system described per-forms a variety of functions typical of computer-based systems. These functions include "contactclosure" input and output, analog input and output,direct digital control (DDC) of analog processvariables, timing and sequencing of process eventsand logging of process variables and events asprinted output. A description of the computer andassociated hardware is given first, followed by adiscussion of some of the programming techniqueswhich were used for the system.

COMPUTER HARDWARE

Fig. 1 shows a block diagram of the computersystem which is built around a Digital EquipmentCorporation PDP-8L computer and Peripheral Equip-ment Corporation 7820-9 magnetic tape unit. Add-itional equipment consists of analog-to-digitaland digital-to-analog converters, contact closureinputs and outputs, a time-of-day clock used forevent logging, a 60 Hz interval timer used as thetime base for the entire system, a stall alarm anda model 33ASR Teletype used for logging. Operatormessages are presented through an annunciator panel.The computer memory which holds the control program

Dr. Dean E. SvobodaJackson AssociatesColumbus, Ohio

and the parameters associated with each productwhich is to be manufactured consists of 4096twelve-bit words of magnetic core memory with a1 .6 m i crosecond cycl e t ime.

The computer input/output facilities consistof twelve input data logic lines and twelve outputdata lines. Data can be selected and placed onthe output lines or accepted from the input linesat appropriate times under control of the program.In addition, six address lines are provided to beused by the external logic in routing input oroutput data to or from external equipment (contactclosure sensors, D/A converters, etc.). Controllines which can be pulsed or tested by the programare provided for synchronization of the externallogic with the control program.

The "contact closure" outputs consist mainlyof solid state devices; triacs for AC output andtransistor switches for DC output. A few relaysdriven by transistors are used where it is desir-able to have continuity in the event of a logicpower supply failure. The contact closure out-puts are arranged in groups of twelve correspond-ing to the twelve data output lines of thecomputer. Each contact closure output has astorage flip-flop which receives and holds thedata from one of the output lines. Groups ofcontact closure outputs are selected to receivedata by codes provided on the six address lines.The contact closure outputs include signals to theannunciator panel, motor start/stop signals andsignals to solenoid-operated pilot valves whichprovide air to air-operated process valves.

Contact closure inputs are arranged and add-ressed in groups of twelve in a manner similar tothat used for the outputs. DC input circuitsconsist of RC filters which filter out contactbounce effects followed by Schmitt triggers toconvert to logic levels. AC inputs include iso-lation transformers and diodes to convert to DC.Contact closure inputs include signals from theannunciator panel, operator push-button signals,valve position limit switches, level detectors,etc.

The digital-to-analog converters accepts thecomputer output data as binary numbers and producea corresponding analog voltage. A separate D/Sconverter is used for each analog output. Outputrange is -10 to +10 volts. Each D/A converter hasa separate six-bit address. Analog output voltagesare used to operate panel meters to display processvariables and to drive electrical-to-pneumatic(E/P) converters which provide air to proportional(modulating) process valves.

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Analog voltage inputs are multiplexed to asingle analog-to-digital (A/D) converter wherethey are converted to corresponding binary numbersto be placed on the input data lines of the com-puter. Process variables such as temperature,pressure and flow are entered into the computerfrom electrical transmitters via the A/D converter.Manual set-points supplied by the operator throughpotentiometers are also entered via the A/D con-verter.

The 60 Hz interval timer provides a timeinterrupt signal to the computer every 1/60 of asecond. This signal is used as a basic timer forsequencing the process. It also provides a timereference for integral and derivitive control inthe DDC loops.

The stall alarm consists of two 10 millisec-ond timers which can be reset by the program. Thetimers must be reset so that at least one is run-ning at any given time. Otherwise, an alarm sig-nal is produced. If a program error or hardwaremalfunction alters the normal sequence of theprogram the timers will not be reset often enoughand a stall alarm will be produced.

The Teletype and time-of-day clock are usedin a conventional manner for event logging. Thetime in hours, minutes and seconds can be readfrom the clock and printed on the Teletype fol low-ed by a message. For example:

08:16:57 THERMINOL FROM ESTERIFIER LOW FLOW

PROGRAMMING TECHNIQUES

It is the computer programming, or "software",which determines the actual operation of a computer-controlled process. Thus, it constitutes a majorpart of the design and development effort of sucha system. Some of the general tasks for our par-ticular system can be mentioned here. They seemtypical of a batch-process control computer appli-cation.

First, the software determines the sequenceof valve operations, depending, of course, on theparticular chemical which is to be manufactured.Also peculiar to a specific chemical is the timingand process status function which decides when thenext step should be taken. Our computer senses 6variable and 14 logical (yes or no) "endpoints",of which any combination can control the durationof a step and/or branching to one of severalpossible next steps. The computer software mustalso check the status of 50 other temperatures,manual valve positions, etc. which indicate whetherthe system is operating normally. The software issuch that critical status errors may halt operationof the process. An annunciation function of thesoftware puts out printed messages on the Teletypeconsole concerning the status indicators andprogram flow. A self-checking function whichdetects and annunciates computer malfunctions isalso normally called for. A final function to bemore fully discussed is the Direct Digital Controlsoftware, which replaces conventional analog con-

trol loops.

It is perhaps impressive that the minicom-puter has sufficient capacity for all of thesefunctions. The key to fitting them all in wasthe careful organization of the software intosubroutines. Besides the normal advantages ofeasier trouble-shooting and simpl ified programchanges, an important feature of subroutines forour use is that a subroutine can be called manytimes during a program sequence, preventing dup-lication of groups of statements, and for a pro-gram of routine operations but unique sequenceand duration of the operations, greatly shortensits length. A general principle of our softwareorganization is that every function which isroutine is a subprogram, and only those functionsunique to the particular chemical product remainin the main program.

An operation which serves to illustrate theapplication of this principal is changing thecombination of the 47 on-off valves of the processsystem. This change can happen as many as 30 timesduring the batch process. This takes two mainprogram statements, a valve-change subroutine-callfollowed by an encoded combination. The valve-change subroutine decodes the combination, selectsthe appropriate valves, and forms a valve-changemessage which gets printed by the annunciationsubprogram. Finally, the proper valves get actu-ated by the update subprogram which does all input-output functions.

Another familiar programming tool used hereis time sharing. For this application, simultan-eous operation of functions such as output print-ing, system error detecting, endpoint detection,and DDC loop control was deemed necessary, and atime-interrupt system for time sharing was devised.The executive control program divides each secondinto 15 equal time-slots, each of which containsa part of the programming. At the end of each1/60 second interval, the contents of the accumu-lator and the address of the next statement to beexecuted in that particular time slot are savedby the executive before going on to the next slot.At the start of the corresponding time slot duringthe next 1/4 second, the accumulator is restoredby the executive and the program proceeds as ifthe interruption had not taken place. An exec-utive "Fork control" subroutine permits program-ming in one slot to alter the flow of that inanother slot. Parameter values in one time slotcan be read or modified from another slot also.

One time-slot previously alluded to and whichdeserves further explanation contains the DDCloops. There are 9 process variables which arecontrolled by 5 proportional (modulating) valvesfor our system. The DDC loops are the digital-computer equivalent of analog control loops forcontrolling these valves. Each loop has setpointinputs from the main program and actual processvariable inputs from the update program. Thecontrol algorithm closely resembles that of anormal analog loop, except that summation replacesanalog integration and differencing replaces ana-log differentiation. A tune-up control panelsupplied with the system permits rapid optimizationof gain constants, etc. for these loops. Comparing

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the computer speed and the time constants of ourapplication, the DDC control is indistinguishablefrom analog control, but it is much easier totune and modify.

Finally, in case of computer malfunctions,safeguard procedures have been included in oursoftware design. Provision for manual takeoverof any valve, or manual takeover of the DDC loopsetpoints has been provided, and these optionsare compatible so that automatic control can bere-established smoothly.

As a safety as well as convenience feature,all of the above software is stored on magnetictape, which is read into the computer using asimple loader program. Normally, all subprogramsremain in the computer core and only the mainprogram is read in at the beginning of each chem-ical process. Provision for updating the magnetictape has also been provided as part of the furn-ished software.

SUMMARY

The minicomputer batch-process control systemdescribed here illustrates an exceptionally widerange of minicomputer applications, all integratedinto a flexible, sophisticated production tool.

Every phase of real-time computer usage is repre-sented, from simple alarm-point monitoring func-tions to unattended direct digital control withself-checking features.

The system has been des'igned so that the op-erator has the capability to interact with thecontrol system to alter set points if he deems itnecessary, or he can completely dominate the con-trol system to handle unforeseen process upsetsmanually if the need should ever arise.

Some of the software concepts borrowed fromcomputer time-sharing technology, which permitmany subroutines to be activated effectively sim-ultaneously, have added heavily to the flexibilityof this control system. This organization permitsa new main program to be written for an entirelynew product with a minimum of effort, inasmuch asthe main programs consist primarily of a sequenceof calls to the various utility subroutines, alongwith their required end points and set points.

This minicomputer control system is consider-ed to be one of the most flexible and most com-

pletely automated control system in its industry,and yet its total system cost is less than thecost of basic computer in the well-known brandsof larger process control computer systems.

CHRONO-LOG PERIPHERALEQUIPMENT CORP

DIGITAL MAG - TAPE

CLOCK 7820-9

II ..

T

Fig. 1. Batch process control hardware configuration.

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