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193

2.11 Hierarchical Control

J. W. BERNARD (1970) T. J. WILLIAMS (1985, 1995) B. G. LIPTÁK (2005)

In modern, computer-controlled control systems (Figure 2.11a),several layers of control can be superimposed onto the basiccontrols, which are physically connected to the process. Suchhierarchical control uses several levels of computer systems inan extended master–slave relationship to carry out not only theprocess control but also the supervisory, management and busi-ness functions in controlling industrial plants. This sectiondescribes the historical evolution of hierarchical control andsome typical forms of such control systems, including the dis-tribution of the potential duties among the several levels ofcomputers involved.

HIERARCHICAL LEVELS

There are basically four levels in the hierarchy:

1. All direct connections to the process are uniformlyreferred to as Level 1. These basic controls include all

sensors, final control elements, continuous PID con-trols, sequential interlocks, alarms, and monitoringdevices. In some systems, where several levels of basiccontrols exist, they are referred to as Levels 1A and 1B.

2. Supervisory control is universally called Level 2. Thecontrol functions of this level are targeted to increasedproduction, improved environmental protection, energyconservation, optimization, predictive maintenance,and plant-wide safety. The means used to achieve thesegoals include envelope control, model-based controls,fuzzy logic, statistical process control, and other typesof expert systems, which are all described in thissection.

3. In some plants the area controls or inter-area coordi-nation is called Level 3, while in other plants it isincluded in Level 2. From this example it can be seenthat not even the language has yet been standardized;this whole field is still in a process of evolution.

FIG. 2.11aModern control systems are well suited for hierarchical control because Level 1 controls, which consist of devices with direct connectionsto the process, can be cascaded to supervisory and management levels of control. (Courtesy of Control Associates, Inc.)

Internet/Intranet

Controllers

I/O Communication Bus(Several options)

High-SpeedEthernet

Workstation

© 2006 by Béla Lipták

194 Control Theory

4. In some plants the management functions are referredto as Level 3, while in others the scheduling and man-agement information functions are called Level 4.Often this level is split into Levels 4A and 4B, where4A covers contact with the plant (scheduling, etc.) and4B covers contact with upper management. Manage-ment functions include marketing, financing, humanresources, production scheduling, and inventory tasks.Some generalized software packages already exist forenterprise resource planning (ERP) and materialrequirement planning (MRP).

HISTORY

Prior to the introduction of the use of computers for industrialcontrol applications, the standard industrial control systemconsisted of a large number of single-loop analog controllers,either pneumatic or electronic, as diagrammed in Figure 2.11b.Note in the figure that Level 1A refers to those instrumentsthat have no signal readouts that are readily available to theoperator (e.g., some field-mounted controllers, ratio instru-ments, etc.). Level 1B devices have transmitters and provideindication to the operator by a pointer or on a strip chart, etc.These latter devices include the standard PID (proportional,integral, and derivative control mode) controllers.

Computers were first used in industrial plants not ascontrollers but as monitors or data loggers (see Figure 2.11c).However, these installations were quickly followed by thefirst supervisory controllers (early 1960s), which used thecomputer to change the set points of the analog controllers(Figure 2.11d).

It was the desire of most control engineers who wereworking with early computer control systems to completelybypass the analog controllers of Figure 2.11d and have thecomputer directly control the valves. This resulted in directdigital control (DDC), as illustrated in Figure 2.11e and

Figure 2.11f. Figure 2.11f includes specialized dedicated dig-ital controllers to illustrate the relationship to Figure 2.11b.Although common today, these devices were not available inthe earliest DDC systems. In these early systems, all com-putational work was carried out in the digital computer ofLevel 2 (Figure 2.11d).

Although direct digital control always had the potentialfor assuming an unlimited variety of complex automatic con-trol functions, most early DDC systems were implementedas digital approximations of conventional three-mode analogcontrollers. Thus the computer system of Figure 2.11f isconsidered most often as a direct substitution for the analogcontrol system of Figure 2.11b.

The Central Computer

The early digital control systems suffered from many draw-backs. Among these were:

1. The computers were all drum machines, i.e., the mem-ory was located on a magnetic drum. As a result, theywere very slow. A typical addition time for suchmachines was 4 milliseconds.

2. The memories were very small, usually 4000 to 8000words of 12 to 16 bits each.

3. All programming had to be done in machine language.

FIG. 2.11bThe basic old analog control system operated at Level 1 only.

Process

(Level 1B)

(Level 1A)

Operator’sdisplay

Analogcontrollers

Specializedcontrollers

Plant inputs to control systemControl signals to plant

Operator communication signals

FIG. 2.11cThe early computers served only for data collection and monitoring.

FIG. 2.11dLevel 2 supervisory computer used to adjust the set-points at Level 1.

(Level 1)Data and

alarm outputdevices

Data collectionand monitoring

computer

Process

(Level 2)

(Level B)

(Level 1A)

Supervisor’sconsole

Supervisorycontrol level

computer

Operator’sdisplay

Analogcontrollers

Specializedcontrollers

Process


2.11 Hierarchical Control 195

4. Neither vendors nor users had any experience in com-puter applications. Thus, it was very difficult to size theproject properly within computer capabilities. Mostprojects had to be reduced in size to fit the availablemachines.

5. Many of the early computer systems were very unre-liable, particularly if they used temperature-sensitivegermanium rather than silicon circuitry, as many did.In addition, these computers depended on unreliablemechanical devices, such as air conditioners, for theirsuccessful operation.

In response to these problems, the computer manufactur-ers came out with a second-generation computer, which wasmuch larger and had a magnetic core memory, wired-in arith-metic functions, etc., which made it much faster. But the highcost of core memories and the additional electronic circuitrymade the system much more expensive. To help justify thecost, vendors advocated the incorporation of all types of com-puter functions, including both supervisory and DDC, into asingle computer or mainframe in the central control room(Figures 2.11g and 2.11h.)

Although these computers had much greater speed andmemory size compared to earlier systems, their use led tostill further problems:

1. Most were marketed before their designs were thor-oughly proven or their programming aids (compilers,higher-level languages, etc.) were fully developed. Thusthere were many frustrating delays in getting theminstalled and running.

2. The vast amount of wiring and communication systemrequired to bring the plant signals to the centralizedcomputer location and return the control signals to thefield was very expensive, and unless it was very care-fully designed and installed, it was prone to electricalnoise problems.

3. Because all of the control functions were located in onecomputer, the possibility that the computer might failresulted in demands for a complete analog backup sys-tem. The resulting system, illustrated in Figure 2.11i,was a combination of the systems shown in Figures2.11d and 2.11g, and it greatly increased the cost.

4. To compensate for these high costs, users and vendorsalike attempted to squeeze as many tasks as possibleinto the computer system, thus drastically complicat-ing the programming and aggravating the difficultiesdescribed in item 1 above.

As a result of these difficulties, management in many com-panies reacted disapprovingly to computer control, and therewas a hiatus in computer system installations until the 1970s.

FIG. 2.11eBlock diagram of a direct control system.

FIG. 2.11fDigital substitution for the analog system in Figure 2.11b.

Process

D/P celltransmitter

MeasurementCommon equipment

Multi-plexercontrol

Outputcontrol

Inputmulti-plexer

Outputmulti-plexerAnalog to

digitalconverter

Digitalcomputer

Consolecontrol

Console Console Console

Valveoutput

(Level 1B) Operator’sconsole

Specializeddedicated

digital controllers

Direct digitalcontrol level

(Level 1A)

Process

FIG. 2.11gTwo-level hierarchical control with supervisory control (Level 2)and direct digital control (Level 1).

(Level 2)

Operator’sconsole



Centralizedcomputer system

Direct digitalcontrol level(Level 1)

Process


196 Control Theory

Distributed Control

Because of the problems experienced with the centralizedcomputer systems of the late 1960s, most of the new computerprojects of the early 1970s were relatively small, specializedprojects that took advantage of the capabilities of the newlyarrived minicomputer. Many such projects flourished. Theygenerally followed the lines of Figures 2.11d or 2.11i anddiffered from earlier undertakings mainly in their smaller size.

However, at this time, two other developments were underway that would forever change digital computer–based con-trol and indeed all process control. The first of these was therapid development of the integrated circuit and the resulting

production of the all-important microprocessor or microcom-puter. The second development was the courageous effortbegun by the Honeywell Company in 1969 to design analternative to the unwieldy and unreliable centralized com-puter control system. This alternative was the distributedcomputer system.

The idea behind the distributed computer system was tohave a set of small, widely distributed computer “boxes”containing one or more microprocessors. Each of the boxescontrolled one or a very few loops. All of them were con-nected by a single high-speed data link that permitted com-munication between each of the microprocessor-based“boxes” and with a centralized operator station or console.

This became the TDC 2000 system, the principles ofwhich were widely followed by other process control systemvendors. These systems solved reliability problems in twoways. First, these units controlled only a few process loops;thus any single failure involved only those few loops. Second,a digital backup capability was developed, that is, backupcomputer systems were included to take over the duties ofany failed components.

Figure 2.11j illustrates this concept. Comparing this fig-ure with Figure 2.11h shows what was accomplished withthis new concept. Almost universally, the distributed com-puter systems offered the following features and capabilities,which greatly fostered their acceptance over electronic ana-log or centralized computer-based control systems:

1. A modular system development capability that is easyto use, particularly with the configuration aids avail-able from the vendor.

2. A color-CRT–based, largely preprogrammed operatorinterface system that is easy to adapt to the individualplant situation.

FIG. 2.11hThe use of centralized computer in a supervisory mode over analog controls located on an analog panel board and on a backup board(circa 1965–1975).

Central control room

Computer

FIG. 2.11iComplete secondary digital control: supervisory plus direct digitalcontrol with associated analog control backup.

(Level 2) Supervisor’sconsole


Centralizedcomputer system


Analog back-updevices

Process

Operator’sdisplays(Level 1)



3. A preprogrammed menu type of instruction systemfor the microcomputers of the controller box. This per-mits the final programming (configuration) of the totalsystem to be done by pushing a few buttons on thekeyboard.

4. A very wide selection of control algorithms or com-putational schemes within the preprogrammed menu,which permits easy selection and testing of alternatecontrol schemes for a process.

5. Data highway based data transmission and commu-nications capabilities between separate units of thesystem. Data highways provide very-wide-band com-munications and the possibility of redundancy for extrasafety.

6. Relatively easy communications with mainframe com-puter systems for supervisory control or other higher-level process control or hierarchy control functions.However, these new control systems themselves aregenerally restricted to supplying the needs of the 1Aand 1B level or dynamic control. Supervisory controlis externally supplied.

7. Extensive diagnostic schemes and devices for easy andrapid maintenance through replacement of entire cir-cuit boards.

8. Redundancy and other fail-safe techniques to help pro-mote high system reliability. These features are oftenstandard, but may be optional.

What was achieved with these systems is illustrated byFigures 2.11h and 2.11j, as noted above. These particularsketches are adapted from Honeywell drawings, but they arejust as applicable to almost any other vendor’s process controlsystem.

HIERARCHICAL CONTROL

The development of the distributed digital control systemgreatly simplified the connection of the computer to the pro-cess. (Compare Figure 2.11k to Figure 2.11d.) Also, sinceredundancy or other backup devices could be incorporateddirectly into the digital system, the need for analog safetybackup systems was minimized.

As illustrated in Figure 2.11k, there are three levels ofcontrol devices. Each has its distinct duties, as they forma hierarchical computer system in which upper-level com-puters depend on lower-level devices for process data, andthe lower-level systems depend upon the higher ones fordirection that serves more sophisticated control functionssuch as overall plant optimization. This configuration makesit possible to design a computer control system — one thatcombines the company’s production scheduling and man-agement information functions with the process controlfunctions to form a total plant hierarchical control system,as illustrated in Figure 2.11l. This figure outlines most ofthe probable functions of such a hierarchical computer con-trol system, but the magnitude of the tasks to be accom-plished in the upper levels of the system is better indicatedby the expanded version shown in Figure 2.11m. Such asystem represents the ultimate dream of many process con-trol system engineers. While what has already been achievedin the field of computer control fits easily within the frame-work shown in this diagram, this framework is only one ofseveral possible structures for such a system and is not nec-essarily the optimal one for the task. Nevertheless, it is anexcellent vehicle for our purpose here since it allows us totreat all the possible benefits of the computer system withone example.

FIG. 2.11jMicroprocessor-based distributed direct digital computer control system (circa 1975).

Controller

Controller

Centralcontrol room

Data highway

Controller

Controller


198 Control Theory

It should be noted that the several levels shown inFigure 2.11m are operational levels and do not necessarilyrepresent separate and distinct computer or hardware compo-nents. Although in large systems each level is likely to behandled by a separate computer, in small systems two or moreoperational levels might be combined into a single computer.

The dedicated digital controllers at Level 1A require no humanintervention since their functional tasks are completely fixedby systems design and are not altered online by the operator.All other levels have human interfaces as indicated in thefigure. It should also be noted that each element of the hierar-chy can exist as an individual element. Indeed, all of the earlierforms of industrial digital computer systems (Figures 2.11c,2.11d, 2.11f, and 2.11i) still exist and will no doubt continueto be applied where their particular capabilities appear to bestfit the application at hand.

The paragraphs below explain the basis for the organizationof the six-level hierarchical system depicted in Figure 2.11m.

Overall Tasks of Digital Control Systems

Automatic control of any modern industrial plant, whether itis by a computer-based system or by conventional means,involves an extensive system for the automatic monitoring ofa large number of variables, each having its own dynamics.The generation of control output signals requires the develop-ment of a large number of functions, some of which might bequite complex. These control corrections must be transmittedto a large number of widely scattered final control elementsof various types. Because of the nature of the manufacturingprocesses involved, these control corrections often require theexpenditure of very large amounts of material and energy.

FIG. 2.11kComplete secondary digital control: supervisory plus direct digitalcontrol.

(Level 2)

Operator’sconsole

Specialized dedicateddigital controllers



Direct digitalcontrol level(Level 1B)

(Level 1A)

Process

FIG. 2.11lHierarchical organization for a complete process computer control system.

(Level 2)

Operator’sconsole



Control signals

Plant inputs(Analog and digital)

Communicationswith other

direct controlmachines


supervisorycontrol

machines

Plant inputs(Analog and digital)


Digital Data Link

Digital Data Link

Digital Data Link


Managementdata

presentation

Productionscheduling andmanagement

information level

(Level 1B)

(Level 1A)

Process



Also, both operators and managers must be kept aware of thecurrent status of the plant and of each of its processes.

In addition, an industrial plant is faced with the continualproblem of adjusting its production schedule to match itscustomers’ needs, as expressed by the order stream beingcontinually received, while maintaining high plant produc-tivity and the lowest practical production costs. Productionscheduling today is usually handled through a manual, com-puter-aided production control system in conjunction with anin-process and finished goods inventory judged adequate byplant personnel.

Another role of digital computer control systems in indus-trial plants is as a “control systems enforcer.” In this mode, themain task of the lower-level computers is continually to ensurethat the control system equipment is actually carrying out thejob that it was designed to do in keeping the units of the plantproduction system operating at some optimal level — that is,to be sure that the controllers have not been set on manual,that the optimal setpoints are being maintained, etc.

Often the tasks carried out by these control systems areones that a skilled and attentive operator could readily do.But the automatic control systems offer a much greaterdegree of attentiveness over much longer periods of time.

All of these requirements must be factored into the designand operation of the control system that will operate the plant,including the requirements for maximum productivity andminimum energy use. As the overall requirements becomemore complex, more sophisticated and capable control sys-tems are necessary. Thus we enter the realm of the digitalcomputer-based control system.

System Capabilities To obtain the above described controlresponses, an overall system is needed that offers the follow-ing capabilities:

1. Tight control of each operating unit of the plant toensure that it is using raw materials and energy atmaximum efficiency and/or is operating at the most

FIG. 2.11mHierarchical computer system structure for a large manufacturing complex.

(Level 2)

(Level 3)

Operator’sconsole


Communications withother control level

systems


supervisory levels


areas

Salesorders


Intra-areacoordination

Supervisor’sconsoles

Operational andproductionsupervision

Productionscheduling

and operationalmanagement

Managementdata

presentation

Managementinformation

level


(Level 1B)

(Level 4A)

(Level 4B)

(Level 1A)

Process


200 Control Theory

efficient production level, based upon the productionlevel set by the scheduling and supervisory functionsof the control system. This control reacts directly toany emergencies that occur in its own unit. (Thesecontrol functions are executed by Levels 1A and 1Bin Figure 2.11m)

2. A supervisory and coordinating system that deter-mines and sets the local production level of all unitsworking together between inventory locations andoptimizes their operation. This system ensures that nounit exceeds the general area level and thus ensuresthat no unit uses unnecessary amounts of energy orraw materials. This system responds to emergenciesor upsets in any of the units under its control to shutdown or systematically reduce the output in these andrelated units. (These control functions are executed byLevels 2 and 3 of Figure 2.11m.)

3. An overall production control system capable of car-rying out the scheduling function for the plant basedon customer orders or management decisions so as toproduce the required products at the optimum combi-nation of time, energy, and raw materials, suitablyexpressed as cost functions. (This is the specific taskof Level 4A of the hierarchy.)

4. A method of assuring the overall reliability and avail-ability of the total control system through fault detec-tion, fault tolerance, redundancy, and other applicabletechniques built into the system’s specification andoperation. (This task is performed at all levels of thecontrol system.)

Because of their ever-widening scope of authority, con-trol tasks 1, 2, and 3 above can effectively become the distinctand separate levels of a hierarchical control structure. Also,in view of the amount of information that must be passedback and forth among these three control tasks, it appearsthat some sort of distributed computational capability, orga-nized in a hierarchical fashion, could be a logical structurefor the required control system.

In Figure 2.11m, we can see that the right-hand elementsof Levels 2 to 4A are all handled by computers of increasingcapability as one goes up the hierarchy. Level 4B alsocontains a computer, but it is used mainly for communica-tions and database management tasks. These elements arebest handled by data processing or scientific computerssince the noncontrol tasks at these levels far outnumberthe process control tasks. These noncontrol computationaltasks will thus determine the design and cost of these com-puters. As long as they are satisfactory for the purposes ofprocess control, these established computer models shouldbe used so that the economy gained from large-scale pro-duction can be capitalized on. This leaves Levels 1 and 2as candidates for the application of digital process controlhardware — the distributed, microprocessor-based systemsdiscussed earlier.

Detailed Task Listings

In the context of a large industrial plant, the tasks carriedout at each level of the hierarchy are as described in Tables2.11n to 2.11t. Note that in each table the tasks are subdi-vided into ones that are related to production scheduling,control enforcement coordination/reporting, and reliabilityassurance. This was described in items 1 through 4 previ-ously. Note also that the duties listed in Table 2.11o forLevels 1 and 2 begin with Item II, Control Enforcement,since the lower-level machines do not do any productionscheduling. Likewise, the upper-level machines do no con-trol enforcement since they have no direct connection to theprocess actuators.

Finally, Level 4B does neither since its main task ismanagement and staff function communications, with a pro-duction data file maintained by Level 4A, Production Sched-uling. These tables outline the tasks that must be carried outin any industrial plant, particularly at the upper levels of thehierarchy. Details of how the operations are actually carriedout will vary drastically, particularly at the lowest levels,because of the nature of the actual processes being con-trolled, but this does not change the basic definition of thesetasks.

The general duties of the different levels in the hierar-chical computer system are summarized in Figure 2.11t.

TABLE 2.11nDuties of Control Computer Systems

I. Production scheduling

II. Control enforcement

III. System coordination, reporting, and management information

IV. Reliability assurance

TABLE 2.11oDuties of the Control Levels (Levels 1A and 1B)


1. Maintain direct control of the plant units undertheir cognizance.

2. Detect and respond to any emergency condition in theseplant units

III. System coordination and reporting

3. Collect information on unit production raw material and energy use and transmit to higher levels.

4. Service the operator’s man/machine interface.

IV. Reliability assurance

5. Perform diagnostics on themselves.

6. Update and standby systems.



Lower-Level Computer Tasks

In the hierarchy shown in Figure 2.11m, all contact with thecontrolled process is maintained through the computers ofLevels 1 and 2. The distributed, microprocessor-based sys-tems, are all effectively stand-alone Level 1 and 2 systems.

According to Table 2.11o, the tasks of these systems areto maintain direct control of the process; to detect andrespond to timing signals, emergencies, and other events inthe process; to collect process data for the plant operators or

TABLE 2.11pDuties of the Supervisory Level (Level 2)


1. Respond to any emergency condition in its region of plantcognizance.

2. Locally optimize the operation of units under its control within limits of established production schedule; carry out all established process operational schemes or operating practices in connection with these processes.

III. Plant coordination and operational data reporting

3. Collect and maintain data queues of production, inventory, and raw material and energy usage for the units under its control.

4. Maintain communications with higher and lower levels.

5. Service the man/machine interfaces for the units involved.

IV. System reliability assurance

6. Perform diagnostics on itself and lower-level machines.

7. Update all standby systems.

TABLE 2.11qDuties of the Area Level (Level 3)


1. Establish the immediate production schedule for its ownarea, including transportation needs.

2. Locally optimize the costs for its individual production area as a basis for modifying the production schedule established by the production control computer system (Level 4A) (e.g., minimize energy usage or maximize production).


3. Make area production reports.

4. Use and maintain area practice files.

5. Collect and maintain area data queues for production, inventory, raw materials usage, and energy usage.

6. Maintain communications with higher and lower levels of the hierarchy.

7. Collect operations data and off-line analysis as required by engineering functions.

8. Service the man/machine interface for the area.

9. Carry out needed personnel functions (such as vacation schedule, work force schedules, and union line of progression).


10. Diagnostics of self and lower-level functions.

TABLE 2.11rDuties of the Production Scheduling and Operational ManagementLevel (Level A)


1. Establish basic production schedule.

2. Modify the production schedule for all units per order stream received, energy constraints, and power demand levels.

3. Determine the optimum inventory level of goods in process at each storage point. The criteria to be used will be the trade-off between customer service (e.g., short delivery time) versus the capital cost of the inventory itself, as well as the trade-offs in operating costs versus costs of carrying the inventory level. (This is an offline function.)

4. Modify production schedule as necessary whenever major production interruptions occur in downstream units, where such interruptions will affect prior or succeeding units.


5. Collect and maintain raw material use and availability inventory and provide data for purchasing for raw material order entry.

6. Collect and maintain overall energy use data for transferto accounting.

7. Collect and maintain overall goods in process and production inventory files.

8. Collect and maintain the quality control file.

9. Maintain interfaces with management interface level function and with area level systems.


10. Run self check and diagnostic routines on self and lower-level machines.

TABLE 2.11sRequired Tasks of the Intracompany Communications Control Sys-tem (Level 4B)


1. Maintain interfaces with plant and company management, sales personnel, accounting and purchasing departments, and the production scheduling level (Level 4A).

2. Supply production and status information as needed to plant and company management, sales personnel, and the accounting and purchasing departments. This information will be supplied in the form of regular production and status reports and in response to online inquiries.

3. Supply order status information as needed to sales personnel.


4. Perform self check and diagnostic checks on itself.


202 Control Theory

for higher-level functions; and to ensure reliability by mon-itoring their own operation.

To do this, the system must monitor each importantplant or process variable on a regular basis. That is, it mustread the current value of the variable and, first, compare itwith a set of alarm limits to detect the presence of anyemergency situation and, second, compare it with the cur-rent operating set point to determine whether any correctionto the current control output is necessary. Close check isalso kept of the passage of time on the system real-timeclock since most systems are time coordinated. That is, themonitoring program is reinitiated at fixed time intervalsbased upon the required sampling interval for the processvariables.

As just indicated, plant variables are normally monitoredeither on a fixed time schedule, so that every variable is testedeach second or fraction of a second (common in micropro-cessor-based distributed systems) or on a variable scheduledepending upon the detected variable type. Table 2.11u liststhe frequencies at which the various process variables are

sensed. This second system has been popular for minicom-puter-based digital control systems that commonly do nothave the speed capability of the distributed systems. It is basedon the dynamics or speed of response of the process beingmonitored and controlled.

FIG. 2.11tSummary of the tasks of the hierarchical computer.

(Level 2)

(Level 3)

Operator’sconsole


Communications withother control level

systems


supervisory levels


areas

Salesorders


Intra-areacoordination

Supervisor’sconsoles

Operational andproductionsupervision

Productionscheduling

and operationalmanagement

Managementdata

presentation

Managementinformation

level


(Level 1B)

(Level 4A)

(Level 4B)

(Level 1A)

Process

Prod

uctio

n

Sche

dulin

g

And

Manag

emen

t

Infor

mation

Contro

l

Compu

tation

And

Contro

l

Enfor

cemen

t

TABLE 2.11uSampling Frequency for Process Variables

Variable Frequency

1. Flow Once each second

2. Level and pressure Once each 5 seconds

3. Temperature Once each 10 seconds

4. Queries from the operator’s console Once each second

Note: In some systems, process variables are sampled much morefrequently than this. These are minimum rate values establishedfrom plant experience.



The timing of the process variable sampling is normallybased upon a real-time clock included in the computer system.

If an emergency is detected, the computer’s program con-tains routines for correcting the emergency or calling the oper-ator’s attention to it. The routines are selected according to apriority interrupt scheme. Control correction computations arecarried out by means of a set of control algorithms or compu-tational routines also stored in the computer’s memory.

In addition to being used for emergency detection andcontrol corrections, the values of the process variables arestored in the computer’s memory in a process data base.These data are then available for readouts on the operators’consoles, for data logging, for historical records, for processefficiency calculations, and for readout to higher-level com-puters for optimization calculations, inventory monitoring,overall plant production status, historical file updates, andother necessary calculations. These calculations thus fulfillthe systems coordination and reporting functions listed inTable 2.11o.

The computer uses any spare computational time to runtest computations to check the validity of its own operationand of any companion computers in the system. These andother related tests fulfill the need for the reliability assurancefunctions listed in Table 2.11o.

Higher-Level Computer Tasks

As noted in the previous section, all contact with the con-trolled process is maintained through the input/output inter-faces of Levels 1A and 1B. These interfaces may be part ofa microprocessor-based distributed control system or of aminicomputer-based direct digital controller. Any other lay-out would require a reversion to the schemes of Figures2.11d, 2.11g, or 2.11i, all of which have distinct drawbackscompared to Figures 2.11k or 2.11m.

The upper-level machines are connected to the lower-level systems and to each other through the communicationssystem. Their major tasks, as outlined in Tables 2.11p to2.11t, are to carry out the extensive computations involvedin (1) the optimization of productivity and of raw materialand energy use in each process; (2) the development of thebest production schedule for the overall plant; and (3) theminimization of the plant’s inventory.

An equally important task is the processing of the plant’sproduction data as collected by the lower-level machines inorder to supply the proper information to plant supervisoryand management personnel and in order to maintain the plant’sproduction database for the company’s production, financial,and personnel reports.

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