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Rolta Academy: Engineering Design Services

CHAPTER 8

DISTRIBUTED COTRO! S"STE#S8$% De&inition o& a DCS

As implied by its name a distributed control system is one whose functions aredistributed rather than centralized. A DCS consists of a number of microprocessor-based modules that work together to control and monitor a plantsoperation. The modules are distributed geographically. This reduces field wiringand installation costs. It also reduces risk by distributing the control functionthroughout a number of small modules rather than concentrating it in one largemodule.

A DCS is a computer network. It differs from a office or personal computer network a DCS does real time computer processing as opposed to batchprocessing done by data processing computers. The difference between the realtime and data processing computers is the way they e!ecute their programs.Data processing computers typically do a single program operation at a time. Theprogram will start with some fi!ed data" perform a comple! set of calculationsand pro#ide a set of results. $nce the program has done its %ob it stops until it isinstructed to run again with new data. An e!ample would be the monthlyprocessing of in#oices by a utility company.

As in batch processing the real time computer also e!ecutes its program by usingfi!ed data" performing calculations and pro#iding a set of results. The difference

howe#er is that it runs the same program repeatedly with updated data" sometimes se#eral times a second. A simple e!ample of a real time operation is thecomputerized cruise control on a car. &or e!ample assume the speed set point is'((mph)*(kph+. The real time computer continuously scans the cars actualspeed .if the speed is lower say ,(mph) kph+" the computer will increase the espeed by calculating and increasing the amount of fuel to the engine. Similarly if the measured speed is higher say ''(mph)* kph+ the computer will decrease

the fuel intake. It continuously makes tiny incremental ad%ustments se#eral timesa second by scanning the actual speed comparing it to the set speed andrecalculating the fuel reuirements. A DCS does e!actly the same repetiti#e

scanning and recalculating for hundreds or e#en thousands of de#icesthroughout a plant.

8$' BASIC DCS (UCTIOSThe DCS like /0C is connected to primary control elements such as temperatureand pressure transmitters" flow meters" gas analyzers p1 and conducti#itysensors" weigh scales" contact switches #al#es and motors and so on. &romthese field de#ices it recei#es electrical signals for e!ample 2-3(mA" '-4 DC" 32

Instr)mentation Engineering 1

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Rolta Academy: Engineering Design Services4 AC" and '3( 4 AC. The DCS con#erts these signal )digitizes them+. $ncecon#erted they can be used by the computer to '. Control loops3. 5!ecute special program logic

6. 7onitor inputs2. Alarm the plant operations. trend" log and report data and*. /erform many other functions.

8$* DCS ARCHITECTURE

8$*$' Over all str)ct)re

The structure of a DCS is often referred to as its architecture. In terms of functional modules DCS from the #arious #endors ha#e a lot in common. These

section therefore e!amine these functional modules from the point of #iew of ageneric system that is representati#e for all manufacturers. &igure 8.' illustratesthe architecture of a generic system in terms of functional modules. The key wordis functional .the modules do not necessarily represent physical componentssome manufactures may combine two or more functions in one physicalcomponent.In addition to the process instruments )such as temperature transmitters" flowmeters" p1 sensors #al#es and so forth+ that are common to any process controlapproach there are si! generic functional modules'. Input 9output or I9$ modules scan and digitizes process instrumentinput9output data. Some may perform elementary simple logic.

3. The local I9$ bus links I9$ modules to controller modules.6. Controller modules reads and update field data and perform controlcalculations and logic to make process changes.2. :ser interfaces include operator interfaces and engineering workstations.. The data high way is a plant wide communication network*. Communication modules pro#ide a link between the data high way andother modules typically controller modules and user interfaces.5ach DCS #endor has a proprietary approach and it is possible for e!ample for the function of control and I9$ to be combined in the same physical component.;e#ertheless it is still possible e#en preferable for a DCS to be described bymeans of generic functional modules.

8$*$* In+)t,O)t+)t #od)les

I9$ modules pro#ide the main interface between the DCS and the process beingcontrolled. They con#ert the information pro#ided by the process instruments intodigital form. They also pro#ide signal filtering" contact debouching and in someinstances they can also do alarming" signal characterizing" and low le#el logic.&our basic types of signals connect to I9$ modules


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Rolta Academy: Engineering Design Services'. Analog inputs also called analog ins or AIs3. Analog outputs also called analog outs or A$s6. Digital inputs also called digital ins or DIs2. Digital outputs also called digital outs or Dos.

Analog inputs are gradually #arying signals )as opposed to two position+ typicallyconnected to sources such as 2-3(mA and '- # dc transmitters" thermocouplesand <TDs. Analog outputs are gradually #arying signals usually 2-3( mA typicallyconnected to de#ices such as #al#es dampers and #ariable speed motors.Digital inputs are typically connected to two position de#ices such as limitswitches relays and pulse contacts. Digital outputs are contact openings andclosings that operate controlled de#ices )such as #al#es dampers and motors+ ina two-position manner.

8$*$- !OCA! I,O BUSThe local I9$ bus pro#ides a bridge between the I9$ and controller modules and

by definition is restricted in terms of geographical area and data loading. Ittypically operates at a lower speed than the plant wide data highway althoughcommunication rates can range from ,*(( to 3(((( to ' million bits per second.

I9$ buses connect #arying numbers of I9$ and controller modules. The manner inwhich they pro#ide communications can also #ary from polling or scanning of theI9$ by the controller modules to serial communication between I9$ and controller modules. They can also be arranged for serial or parallel communications or combination of both.=hile I9$ buses are seldom a bottleneck or a limitation they become a criticalcomponent if they fail. The loss of a single I9$ bus can affect the control of many

end de#ices.

8$*$. COTRO!!ER #ODU!ES

Controller modules are the true brain of DCS their primary function is to usecontinuously updated information from I9$ modules and then perform thecomple! logic and analog loop calculations needed to produce the controller output signals that keep process #ariables at the desired #alues. It is at thecontroller modules that many DCS functions such as the following are performed'. I9$ signal characterization3. Signal filtering

6. Alarming I9$ modules2. <anging and engineering units. Control logic*. Control interlocks>. Seuencing8. ?atch control,. /assing on of trend information'(. /assing on of report information


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Controller modules are microcomputers and as such ha#e similar limitationsalthough the #arious numbers associated with the #arious types of controller modules can ha#e a mesmerizing effect not all of these numbers are important in

ones e#aluation of controller module performance. The key ones are

'. A#ailable memory for configuration 3. A#ailable idle time )based on a gi#en scan rate+

6. I9$ loading or criticality

2. ;umber of a#ailable software addresses for input9output blocks.

. ;umber of a#ailable software addresses for control blocks

In sizing and selecting of a DCS it is #itally important to ensure that there isenough processing power not only to ser#e the acti#e I9$ and control functionsbut also to pro#ide spare capacity for future I9$ e!pansion additional logic ande!tra thing such as totalizes. This is an important consideration because addingthis processing power after the fact doubly penalizes the owner. &irst their addedcost of the e!tra modules and other associated euipments such ascommunication modules" power supplies and cabinets. This added cost is oftendetermined on a non-competiti#e basis and is therefore higher than it would ha#ebeen if purchased as part of the initial contract.

The second penalty is inferior performance due to e!tra loading put on theoriginal and the new controllers modules the communication modules and thedata high way. This e!tra loading is the result of controller modules doing linkcommunication instead of simple control. 0ink communications are those thatpass high #olumes of information between control processors. Suchcommunication consume large amount of memory and scan time in theassociated controller and communication modules and load the data highway. Asimple ay to a#oid this potentially reduced performance is to specify suitable#alues of I9$ loading memory usage and idle time for controller modules. &or e!ample for a gi#en scan cycle )'92"'93 or 's on a#erage+ one can specify theamount of spare memory and idle time to be a#ailable in the controller module

after e!ecution of the I9$ and the control functions. Spare memory and idle timeshould normally range from 3(@ to *(@ depending on the application. 0imitingthe number of I9$ and control function e!ecuted in a controller module is a goodidea for three reasons'.it ensures the a#ailability of the microprocessor power needed to carry out thespecified functions and thereby simplifies configuration engineering.3.it allows for easier more fle!ible future e!pansions and reduces the risk of linkcommunications


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Rolta Academy: Engineering Design Services6.it reduces the criticality of any gi#en controller module by limiting the number of I9$s and loops controlled" thus limiting the damage caused by failure of themodule.

8$*$/ CO##UICATO #ODU!ES Communication modules are also microcomputers but they differ from controller modules in function. <ather than e!ecute control strategies communicationmodules manage the flow of information between the data highway and controller modules" user interfaces and gateways to host computers and /0Cs. Althoughthere is always a physical limit to the amount of data that communicationmodules can handle" they are not often a bottleneck.If problem do occur the communications rate and memory capacity should bechecked. /erformance impro#es if one either decreases the number of communication modules or decreases the number of de#ices ser#ed by singlemodules. Again there should be room for e!pansion. Communication modules

are critical to proper operation of DCS without them the operator may be blind tothe process.

8$*$0 REA! TI#E DATA HI1H 2A"<eal time data high way come in many #ariations. Topologies can be linear" loopor star and they may or may not include traffic controllers since a data highway isa microprocessor based module it should be #iewed as considerably more thanone or two cables strung out across the plant.The following are the principal issues to be addressed in the e#aluation of a DCSdata highway'.synchronized #ersus non synchronized

3.deterministic #ersus non deterministic6.token passing #ersus report by e!ception2.4ariation in protocol types..peer to peer #ersus collision detection based communications*.speed of data transmission>.ma!imum transmission distance

The e#aluation of the security and reliability of a data highway is notstraightforward because many factors are in#ol#ed. 7ost importantly speed isnte#erything. $ther key factors are module highway access message buffering andprioritizing and efficiency. for e!ample highways based on collision detection and

report by e!ception can lose >(@ to 8(@ of their rated capacity when messageloading increases due to alarm burst and process upset conditions. :nfortunatelyit is under such conditions that it is most important for the data high way toperform efficiently.Benerally one should e#aluate a data high way design based on the worst-casescenario. Consideration must be gi#en to'.the number of tags )I9$s and control loops+ that are connected to the high way3.how much trending and reporting information is being transferred


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Rolta Academy: Engineering Design Servicesalarm. $ther o#er#iew displays are obtained by page forward and page backprocess.Broup displays may contain indi#idual control loops with tag name bar graph of measured #ariable output #alue set point process #alue in engineering units set

point source )remote local or tracking+ output mode )auto" manual" tracking+ andalso alarm information. a page forward and page back techniue can be used toselect other groups.Trend displays indicate the rate of change of key #ariables in a process and areimportant indicators of plant status. This information can help the operator identify or anticipate process upset conditions. The user can select real time or historical mode )from one second to one month" typically+ and scale in bothpercent and engineering units for all #ariables and for the time base. Trends areoften recorded on a hard copy printer. The principal features to look in for trenddisplays are'.sample freuencies a#ailable )one per sec" one per fi#e sec" one per minute+

3. ;umber of #ariables that can be trended6.duration of trending periods a#ailable )one hour" one shift" one day" one week"one month" one year+2.amount of ram and amount of hard disk space a#ailable for trend files.how sampling and duration are related and how they affect each other.

Trending and reporting are methods of archi#ing information and reuire amethod of backing up hard disk files. It is important for the operator tounderstand how this back up is accomplished and how the data can be retrie#edand re#iewed later on.

Alarm displays pro#ide a list of alarms with their tags types descriptions prioritiesand acknowledgement status. Alarms are listed chronologically according to thetime of detection of the alarm. The size of an alarm list is defined by the user andusually contains appro!imately 3(( alarms. As the number of alarms e!ceedsthis figure the oldest alarms drop off. Dedicated keys on the keyboard permit theoperator to uickly identify acti#e alarm conditions to scroll up and down thealarm listings and to acknowledge alarms.The graphic display is a schematic of the process being controlled. The display isdynamic gi#ing the operator real time data concerning the condition of theprocess. The display is user definable and is constructed with a #ariety of geometric shapes te!ts and process control symbols. Braphic display indicates

loop tag names measured #alues output #alues output modes and engineeringunits. The operator can control directly from the graphic representation by meansof animated symbols and color changes of things such as tank le#els andprocess temperatures.

8$. Engineering 3or4stations

The engineering workstation is used principally to


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Rolta Academy: Engineering Design Services'. Configure the data base and console3. :pdate and decompile the data base and6. Implement application software

5ngineering workstations are usually physically separated from the location of the operator interface" which permits engineers to work independently before andduring and after installation of the DCS.5ngineering workstations are often put together with the same hardware as theoperator interface. They include C<Ts electronics keyboards hard disks floppydisks and9or tape dri#es. The key difference is the software. Thus an engineeringworkstation could be used as an operator interface if it had the proper operator interface software.The engineering workstation is typically used offline howe#er with the rightsoftware it can be used online as a diagnostic tool. In general howe#er theengineering and the operator functions are separated e#en by different physical

locations of the workstations. This is to minimize any potential interference in theday to day running of the plant

8$/ BASIC DCS SO(T2ARE #ODU!ES

8$/$' Programming conce+ts

All computer systems need software )programming+ to e!ecute their assignedtasks. The DCS which is highly computer based must be programmed withprocess information control algorithms and the operator interface instructionsthat are necessary for proper operation.

The system supplier can program some control function while the users controlsystem engineer must configure other functions. Computer based controlsystems come with standard fill in the blank software for data acuisitions"process control alarming and operator displays.Software can be classified as e!ecuti#e" system support and application e#enthough descriptions from the manufacturers .do not always see to fit neatly intothese categories.

8$/$* E5ec)tive So&t3are5!ecuti#e software is the operating systems of the computer .It consist of programs that o#ersee or super#ise the actual operation of the system while it isrunning. It performs such function as'.scheduling and starting the e!ecution of system application programs3.allocating main memory and loading programs into main memory from bulkmemory such as cassettes" drums" and floppy disks and6.super#ising I9$ operations.


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Rolta Academy: Engineering Design Services3.specifying the loops to appear on the operational displays6.specifying the graphics to be associated with the operational displays and2.specifying the users that are allowed access to each console.

Configuration techniues can #ary widely from manufacturer to manufacturer.they can range from synta! laden line by line programming to small interacti#elogic blocks or large multifunction blocks. A block for one DCS #endor does notmean the same thing as a block for another. In the same way a display pagemeans different things to different suppliers. Some are restructured while othersare customized graphics.


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(ig 8$' Bloc4 diagram o& DCS


Operator Engineering

Communic

ation

modules

Communic

ation

modules

Plant wide data igwa!

Communicati

on module

Controller

modules

"ocal #$O %us

#$O modules

Process instruments

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