11. EEE2221_Automatic Controllers , systems and distributed control systems..pdf

A control system is a set of mechanical or electronic devices that regulates other devices or systems by way of control loops. Typically, control systems are computerized.

Control systems are a central part of production and distribution in many industries. Automation technology plays a big role in these systems. The types of control loops that regulate these processes include industrial control systems, such as supervisory control and data acquisition,

1. 1
EEE 2221
Instrumentation and Electrical Control
Nayeema Hasan
Lecturer
Dept. of EEE, KUET.

2. Automatic Controllers
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Anautomaticcontrollercomparestheactualvalueoftheplant
outputwiththereferenceinput(desiredvalue),determinesthe
deviation,andproducesacontrolsignalthatwillreducethe
deviationtozeroortoasmallvalue.
Themannerinwhichtheautomaticcontrollerproducesthecontrol
signaliscalledthecontrolaction.
Thecontrollerdetectstheactuatingerrorsignal,whichisusuallyat
averylowpowerlevel,andamplifiesittoasufficientlyhighlevel.
Theoutputofanautomaticcontrollerisfedtoanactuator,suchas
anelectricmotor,ahydraulicmotor,ora-pneumaticmotoror
valve.(Theactuatorisapowerdevicethatproducestheinputtothe
plantaccordingtothecontrolsignalsothattheoutputsignalwill
approachthereferenceinputsignal.)

3. Automatic Controllers
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Figure: Block diagram of an industrial control system, which consists of an
automatic controller, an actuator, a plant, and a sensor , (measuring element).

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Classification of Industrial Controllers
Industrialcontrollersmaybeclassifiedaccordingtotheircontrolactionsas:
1.Two-positionoron-offcontrollers
2.Proportionalcontrollers
3.Integralcontrollers
4.Proportional-plus-integralcontrollers
5.Proportional-plus-derivativecontrollers
6.Proportional-plus-integral-plus-derivativecontrollers
Mostindustrialcontrollersuseelectricityorpressurizedfluidsuchasoil
orairaspowersources.Controllersmayalsobeclassifiedaccordingtothe
kindofpoweremployedintheoperation,suchaspneumaticcontrollers,
hydrauliccontrollers,orelectroniccontrollers.
Whatkindofcontrollertousemustbedecidedbasedonthenatureofthe
plantandtheoperatingconditions,includingsuchconsiderationsas
safety,cost,availability,reliability,accuracy,weight,andsize.

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Two-position or on-off controllers
Inatwo-positioncontrolsystem,theactuatingelementhasonly
twofixedpositions,whichare,inmanycases,simplyonandoff.
Two-positionoron-offcontrolisrelativelysimpleandinexpensive
and,forthisreason,isverywidelyusedinbothindustrialand
domesticcontrolsystems.
Lettheoutputsignalfromthecontrollerbeu(t)andtheactuating
errorsignalbee(t).Intwo-positioncontrol,thesignalu(t)remains
ateitheramaximumorminimumvalue,dependingonwhether
theactuatingerrorsignalispositiveornegative,sothat
 u(t)=U
1,fore(t)>0
u(t)=U
2,fore(t)<0
whereU
1andU
2areconstants.
TheminimumvalueU
2isusuallyeitherzeroor-U
l.

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Two-position or on-off controllers
Figure (a) Block diagram of an on-off controller
Figure (b) block diagram of an on-off controller with differential gap

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Two-position or on-off controllers
DifferentialGap:Therangethroughwhichtheactuating
errorsignalmustmovebeforetheswitchingoccursiscalled
thedifferentialgap.
Suchadifferentialgapcausesthecontrolleroutputu(t)to
maintainitspresentvalueuntiltheactuatingerrorsignalhas
movedslightlybeyondthezerovalue.
Insomecases,thedifferentialgapisaresultofunintentional
frictionandlostmotion;however,quiteoftenitis
intentionallyprovidedinordertopreventtoofrequent
operationoftheon-offmechanism.

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Proportional Controllers

9. 9
Integral Controllers

10. 10
Proportional-plus-integral Controllers

11. 11
Proportional-plus-derivative Controllers

12. 12
Proportional-plus-integral-plus-derivative
Controllers

13. 13
Proportional-plus-integral-plus-derivative
Controllers
Figure: Block diagram of a proportional-plus-integral-plus-derivative controller

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Fuzzy Controllers
Afuzzycontrolsystem:isacontrolsystembasedonfuzzylogic-a
mathematicalsystemthatanalyzesanaloginputvaluesinterms
oflogicalvariablesthattakeoncontinuousvaluesbetween0and
1,incontrasttoclassicalordigitallogic,whichoperateson
discretevaluesofeither1or0(trueorfalse,respectively)
Thewordfuzzyreferstothingswhicharenotclearorarevague.
Anyevent,process,orfunctionthatischangingcontinuously
cannotalwaysbedefinedaseithertrueorfalse,whichmeans
thatweneedtodefinesuchactivitiesinaFuzzymanner.
FuzzyLogicwasintroducedin1965byLoftiA.Zadehinhis
researchpaper“FuzzySets”.Heisconsideredasthefatherof
FuzzyLogic.

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Fuzzy Controllers
Inthebasicfuzzysystemabove,thevaluesareindicatedbya
numberintherangefrom0to1.Here1.0representsabsolute
truthand0.0representsabsolutefalseness.Thenumberwhich
indicatesthevalueinfuzzysystemsiscalledthetruthvalue.

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Fuzzy Controllers
Figure:ThearchitectureofFuzzyLogicControl(FLC).

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Fuzzy Controllers
FollowingsarethemajorcomponentsoftheFLCasshownintheabove
figure:
Fuzzifier−Theroleoffuzzifieristoconvertthecrispinputvaluesinto
fuzzyvalues.
FuzzyKnowledgeBase−Itstorestheknowledgeaboutalltheinput-
outputfuzzyrelationships.Italsohasthemembershipfunctionwhich
definestheinputvariablestothefuzzyrulebaseandtheoutput
variablestotheplantundercontrol.
FuzzyRuleBase−Arulebaseisthesetofrulesforafuzzysystem.To
createarule,youmustspecifytheantecedents,orIFportions,and
consequents,orTHENportions,oftherule.
InferenceEngine−ItactsasacoreofanyFLC.Basicallyitsimulates
humandecisionsbyperformingapproximatereasoning.
Defuzzifier−Theroleofdefuzzifieristoconvertthefuzzyvaluesinto
crispvaluesgettingfromfuzzyinferenceengine.

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Distributed Control System (DCS)
Adistributedcontrolsystem(DCS)isadigitalautomated
industrialcontrolsystem(ICS)thatusesgeographically
distributedcontrolloopsthroughoutafactory,machineor
controlarea.
Controlloopsarecomposedofallthehardwareandsoftware
controlfunctionsneededformeasurementsandadjustmentsin
anindividualcontrolsystem.
ADCShasseverallocalcontrollersthroughoutthefactory.A
high-speedcommunicationnetworkconnectsthelocal
controllers.
Whileeachcontrollerworksautonomously,thereiscentral
supervisorycontrolrunbyanoperator.

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Distributed Control System (DCS)
Differencewithacentralizedcontrolsystem:
Unlikeacentralizedcontrolsystemthatoperatesallmachines,a
DCSenableseachpartofamachinetohaveitsowndedicated
controllerthatrunstheoperation.
Advantages:
Havingthecontrolsystemarchitecturedistributedaroundthe
factoryorplantprovidesaneffectivecontrol,efficiencyandprocess
qualitystrategy.
Additionally,ifapieceoftheDCSarchitecturefails,aplantcan
continuetooperate.Forthesereasons,DSCesarecommonlyfound
inindustrialfieldsandprocessingormanufacturingplants.
TheDCSconceptincreasesreliabilityandreducesinstallationcosts
bylocalizingcontrolfunctionsneartheprocessplant,withremote
monitoringandsupervision.

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Distributed Control System (DCS)
ThearchitecturaldesignofaDCSincludesthefollowing:
Anengineeringworkstation.Thisisasupervisorycontrollerfor
theDCSasawhole.Thestationtypicallyincludesconfiguration
toolsthatlettheusercompletefunctionssuchascreatingnew
loopsandinputandoutput(I/O)points,aswellasconfiguring
distributeddevices.
Anoperatingstation:Thisstationisusedforcontrol,operation
andmonitoring.
Aprocesscontrolunit:Thismicroprocessor-basedcontrolleris
designedforautomaticandcompoundloopcontrol.
Acommunicationsystem:Thissystembringsdatafromstation
tostationandisimportantindistributedcontrolsystems.
TypicalnetworkprotocolsusedincludeEthernet,andProfibus.
Smartdevices:Theseareanysmartdevicesorbustechnologies
thatareusedtoreplaceolderI/O.

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Distributed Control System (DCS)
Figure: Functional Levels of a Manufacturing Control Operation

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Distributed Control System (DCS)
Level0:containsthefielddevicessuchasflowand
temperaturesensors,andcontrolelements,suchascontrol
valves.
Level1:containstheindustrializedInput/output(I/O)
modules,andtheirassociateddistributedelectronic
processors.
Level2:containsthesupervisorycomputers,whichcollect
informationfromprocessornodesonthesystem,andprovide
theoperatorcontrolscreens.
Level3:istheproductioncontrollevel,whichdoesnot
directlycontroltheprocess,butisconcernedwithmonitoring
productionandmonitoringtargets
Level4:istheproductionschedulinglevel.

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Distributed Control System (DCS)
ApplicationsofDCS:
Distributedcontrolsystems(DCS)arededicatedsystemsusedin
manufacturingprocessesthatarecontinuousorbatch-oriented.
Chemicalplants,Petrochemical(oil)andrefineries,Pulpandpaper
mills,Boilercontrolsandpowerplantsystems,Nuclearpower
plants,Environmentalcontrolsystems,Watermanagementsystems,
Watertreatmentplants,Sewagetreatmentplants,Foodandfood
processing,Agrochemicalandfertilizer,Metalandmines,
Automobilemanufacturing,Pharmaceuticalmanufacturing,Sugar
refiningplants,Agricultureapplications.