control system

7/29/2015 ControlsystemWikipedia,thefreeencyclopedia

https://en.wikipedia.org/wiki/Control_system 1/8

ControlsystemFromWikipedia,thefreeencyclopedia

Acontrolsystemisadevice,orsetofdevices,thatmanages,commands,directsorregulatesthebehaviorofotherdevicesorsystems.Industrialcontrolsystemsareusedinindustrialproductionforcontrollingequipmentormachines.

Therearetwocommonclassesofcontrolsystems,openloopcontrolsystemsandclosedloopcontrolsystems.Inopenloopcontrolsystemsoutputisgeneratedbasedoninputs.Inclosedloopcontrolsystemscurrentoutputistakenintoconsiderationandcorrectionsaremadebasedonfeedback.Aclosedloopsystemisalsocalledafeedbackcontrolsystem.Thehumanbodyisaclassicexampleoffeedbacksystems.

Contents

1Overview2Logiccontrol3Onoffcontrol4Linearcontrol

4.1Proportionalcontrol4.1.1Underdampedfurnaceexample4.1.2Overdampedfurnaceexample

4.2PIDcontrol4.2.1Derivativeaction4.2.2Integralaction

4.2.2.1RampUP%perminute4.3Othertechniques

5Fuzzylogic6Physicalimplementations7Seealso8References9Externallinks

Overview

Theterm"controlsystem"maybeappliedtotheessentiallymanualcontrolsthatallowanoperator,forexample,tocloseandopenahydraulicpress,perhapsincludinglogicsothatitcannotbemovedunlesssafetyguardsareinplace.

Anautomaticsequentialcontrolsystemmaytriggeraseriesofmechanicalactuatorsinthecorrectsequencetoperformatask.Forexamplevariouselectricandpneumatictransducersmayfoldandglueacardboardbox,fillitwithproductandthensealitinanautomaticpackagingmachine.Programmablelogiccontrollers



Abasicfeedbackloop

areusedinmanycasessuchasthis,butseveralalternativetechnologiesexist.

Inthecaseoflinearfeedbacksystems,acontrolloop,includingsensors,controlalgorithmsandactuators,isarrangedinsuchafashionastotrytoregulateavariableatasetpointorreferencevalue.Anexampleofthismayincreasethefuelsupplytoafurnacewhenameasuredtemperaturedrops.PIDcontrollersarecommonandeffectiveincasessuchasthis.Controlsystemsthatincludesomesensingoftheresultstheyaretryingtoachievearemakinguseoffeedbackandsocan,tosomeextent,adapttovaryingcircumstances.Openloopcontrolsystemsdonotmakeuseoffeedback,andrunonlyinprearrangedways.

Logiccontrol

Logiccontrolsystemsforindustrialandcommercialmachinerywerehistoricallyimplementedatmainsvoltageusinginterconnectedrelays,designedusingladderlogic.Today,mostsuchsystemsareconstructedwithprogrammablelogiccontrollers(PLCs)ormicrocontrollers.ThenotationofladderlogicisstillinuseasaprogrammingidiomforPLCs.[1]

Logiccontrollersmayrespondtoswitches,lightsensors,pressureswitches,etc.,andcancausethemachinerytostartandstopvariousoperations.Logicsystemsareusedtosequencemechanicaloperationsinmanyapplications.PLCsoftwarecanbewritteninmanydifferentwaysladderdiagrams,SFCsequentialfunctionchartsorinlanguagetermsknownasstatementlists.[2]

Examplesincludeelevators,washingmachinesandothersystemswithinterrelatedstopgooperations.

Logicsystemsarequiteeasytodesign,andcanhandleverycomplexoperations.SomeaspectsoflogicsystemdesignmakeuseofBooleanlogic.

Onoffcontrol

Athermostatisasimplenegativefeedbackcontroller:whenthetemperature(the"processvariable"orPV)goesbelowasetpoint(SP),theheaterisswitchedon.Anotherexamplecouldbeapressureswitchonanaircompressor.Whenthepressure(PV)dropsbelowthethreshold(SP),thepumpispowered.Refrigeratorsandvacuumpumpscontainsimilarmechanismsoperatinginreverse,butstillprovidingnegativefeedbacktocorrecterrors.

Simpleonofffeedbackcontrolsystemslikethesearecheapandeffective.Insomecases,likethesimplecompressorexample,theymayrepresentagooddesignchoice.

Inmostapplicationsofonofffeedbackcontrol,someconsiderationneedstobegiventoothercosts,suchaswearandtearofcontrolvalvesandperhapsotherstartupcostswhenpowerisreappliedeachtimethePVdrops.Therefore,practicalonoffcontrolsystemsaredesignedtoincludehysteresiswhichactsasa



deadband,aregionaroundthesetpointvalueinwhichnocontrolactionoccurs.Thewidthofdeadbandmaybeadjustableorprogrammable.

Linearcontrol

Linearcontrolsystemsuselinearnegativefeedbacktoproduceacontrolsignalmathematicallybasedonothervariables,withaviewtomaintainthecontrolledprocesswithinanacceptableoperatingrange.

Theoutputfromalinearcontrolsystemintothecontrolledprocessmaybeintheformofadirectlyvariablesignal,suchasavalvethatmaybe0or100%openoranywhereinbetween.Sometimesthisisnotfeasibleandso,aftercalculatingthecurrentrequiredcorrectivesignal,alinearcontrolsystemmayrepeatedlyswitchanactuator,suchasapump,motororheater,fullyonandthenfullyoffagain,regulatingthedutycycleusingpulsewidthmodulation.

Proportionalcontrol

Whencontrollingthetemperatureofanindustrialfurnace,itisusuallybettertocontroltheopeningofthefuelvalveinproportiontothecurrentneedsofthefurnace.Thishelpsavoidthermalshocksandappliesheatmoreeffectively.

Proportionalnegativefeedbacksystemsarebasedonthedifferencebetweentherequiredsetpoint(SP)andprocessvalue(PV).Thisdifferenceiscalledtheerror.Powerisappliedindirectproportiontothecurrentmeasurederror,inthecorrectsensesoastotendtoreducetheerrorandthereforeavoidpositivefeedback.Theamountofcorrectiveactionthatisappliedforagivenerrorissetbythegainorsensitivityofthecontrolsystem.

Atlowgains,onlyasmallcorrectiveactionisappliedwhenerrorsaredetected.Thesystemmaybesafeandstable,butmaybesluggishinresponsetochangingconditions.Errorswillremainuncorrectedforrelativelylongperiodsoftimeandthesystemisoverdamped.Iftheproportionalgainisincreased,suchsystemsbecomemoreresponsiveanderrorsaredealtwithmorequickly.Thereisanoptimalvalueforthegainsettingwhentheoverallsystemissaidtobecriticallydamped.IncreasesinloopgainbeyondthispointleadtooscillationsinthePVandsuchasystemisunderdamped.

Inrealsystems,therearepracticallimitstotherangeofthemanipulatedvariable(MV).Forexample,aheatercanbeofforfullyon,oravalvecanbeclosedorfullyopen.AdjustmentstothegainsimultaneouslyaltertherangeoferrorvaluesoverwhichtheMVisbetweentheselimits.Thewidthofthisrange,inunitsoftheerrorvariableandthereforeofthePV,iscalledtheproportionalband(PB).Whilethegainisusefulinmathematicaltreatments,theproportionalbandisoftenusedinpracticalsituations.Theybothrefertothesamething,butthePBhasaninverserelationshiptogainhighergainsresultinnarrowerPBs,andviceversa.

Underdampedfurnaceexample



AblockdiagramofaPIDcontroller

Inthefurnaceexample,supposethetemperatureisincreasingtowardsasetpointatwhich,say,50%oftheavailablepowerwillberequiredforsteadystate.Atlowtemperatures,100%ofavailablepowerisapplied.Whentheprocessvalue(PV)iswithin,say10oftheSPtheheatinputbeginstobereducedbytheproportionalcontroller(notethatthisimpliesa20proportionalband(PB)fromfulltonopowerinput,evenlyspreadaroundthesetpointvalue).Atthesetpointthecontrollerwillbeapplying50%powerasrequired,butstraystoredheatwithintheheatersubsystemandinthewallsofthefurnacewillkeepthemeasuredtemperaturerisingbeyondwhatisrequired.At10aboveSP,wereachthetopoftheproportionalband(PB)andnopowerisapplied,butthetemperaturemaycontinuetoriseevenfurtherbeforebeginningtofallback.EventuallyasthePVfallsbackintothePB,heatisappliedagain,butnowtheheaterandthefurnacewallsaretoocoolandthetemperaturefallstoolowbeforeitsfallisarrested,sothattheoscillationscontinue.

Overdampedfurnaceexample

Thetemperatureoscillationsthatanunderdampedfurnacecontrolsystemproducesareunacceptableformanyreasons,includingthewasteoffuelandtime(eachoscillationcyclemaytakemanyminutes),aswellasthelikelihoodofseriouslyoverheatingboththefurnaceanditscontents.

Supposethatthegainofthecontrolsystemisreduceddrasticallyanditisrestarted.Asthetemperatureapproaches,say30belowSP(60proportionalband(PB)),theheatinputbeginstobereduced,therateofheatingofthefurnacehastimetoslowand,astheheatisstillfurtherreduced,iteventuallyisbroughtuptosetpoint,justas50%powerinputisreachedandthefurnaceisoperatingasrequired.Therewassomewastedtimewhilethefurnacecrepttoitsfinaltemperatureusingonly52%then51%ofavailablepower,butatleastnoharmwasdone.Bycarefullyincreasingthegain(i.e.reducingthewidthofthePB)thisoverdampedandsluggishbehaviorcanbeimproveduntilthesystemiscriticallydampedforthisSPtemperature.Doingthisisknownas'tuning'thecontrolsystem.Awelltunedproportionalfurnacetemperaturecontrolsystemwillusuallybemoreeffectivethanonoffcontrol,butwillstillrespondmoreslowlythanthefurnacecouldunderskillfulmanualcontrol.

PIDcontrol

Apartfromsluggishperformancetoavoidoscillations,anotherproblemwithproportionalonlycontrolisthatpowerapplicationisalwaysindirectproportiontotheerror.Intheexampleaboveweassumedthatthesettemperaturecouldbemaintainedwith50%power.Whathappensifthefurnaceisrequiredinadifferentapplicationwhereahighersettemperaturewillrequire80%powertomaintainit?Ifthegainwasfinallysettoa50PB,then80%powerwillnotbeappliedunlessthefurnaceis15belowsetpoint,soforthisotherapplicationtheoperatorswillhavetorememberalwaystosetthesetpointtemperature15higherthanactuallyneeded.This15figureisnotcompletelyconstanteither:itwilldependonthesurroundingambienttemperature,aswellasotherfactorsthataffectheatlossfromorabsorptionwithinthefurnace.



EffectsofvaryingPIDparameters(Kp,Ki,Kd)onthestepresponseofasystem.

Toresolvethesetwoproblems,manyfeedbackcontrolschemesincludemathematicalextensionstoimproveperformance.Themostcommonextensionsleadtoproportionalintegralderivativecontrol,orPIDcontrol.

Derivativeaction

Thederivativepartisconcernedwiththerateofchangeoftheerrorwithtime:Ifthemeasuredvariableapproachesthesetpointrapidly,thentheactuatorisbackedoffearlytoallowittocoasttotherequiredlevelconverselyifthemeasuredvaluebeginstomoverapidlyawayfromthesetpoint,extraeffortisappliedinproportiontothatrapiditytotrytomaintainit.

Derivativeactionmakesacontrolsystembehavemuchmoreintelligently.Oncontrolsystemslikethetuningofthetemperatureofafurnace,orperhapsthemotioncontrolofaheavyitemlikeagunorcameraonamovingvehicle,thederivativeactionofawelltunedPIDcontrollercanallowittoreachandmaintainasetpointbetterthanmostskilledhumanoperatorscould.

Ifderivativeactionisoverapplied,itcanleadtooscillationstoo.AnexamplewouldbeaPVthatincreasedrapidlytowardsSP,thenhaltedearlyandseemedto"shyaway"fromthesetpointbeforerisingtowardsitagain.

Integralaction

Theintegraltermmagnifiestheeffectoflongtermsteadystateerrors,applyingeverincreasingeffortuntiltheyreducetozero.Intheexampleofthefurnaceaboveworkingatvarioustemperatures,iftheheatbeingapplieddoesnotbringthefurnaceuptosetpoint,forwhateverreason,integralactionincreasinglymovestheproportionalbandrelativetothesetpointuntilthePVerrorisreducedtozeroandthesetpointisachieved.

RampUP%perminute

Somecontrollersincludetheoptiontolimitthe"rampup%perminute".Thisoptioncanbeveryhelpfulinstabilizingsmallboilers(3MBTUH),especiallyduringthesummer,duringlightloads.Autilityboiler"unitmayberequiredtochangeloadatarateofasmuchas5%perminute(IEACoalOnline2,2007)".[3]

Othertechniques



ItispossibletofilterthePVorerrorsignal.Doingsocanreducetheresponseofthesystemtoundesirablefrequencies,tohelpreduceinstabilityoroscillations.Somefeedbacksystemswilloscillateatjustonefrequency.Byfilteringoutthatfrequency,more"stiff"feedbackcanbeapplied,makingthesystemmoreresponsivewithoutshakingitselfapart.

Feedbacksystemscanbecombined.Incascadecontrol,onecontrolloopappliescontrolalgorithmstoameasuredvariableagainstasetpoint,butthenprovidesavaryingsetpointtoanothercontrolloopratherthanaffectingprocessvariablesdirectly.Ifasystemhasseveraldifferentmeasuredvariablestobecontrolled,separatecontrolsystemswillbepresentforeachofthem.

ControlengineeringinmanyapplicationsproducescontrolsystemsthataremorecomplexthanPIDcontrol.Examplesofsuchfieldsincludeflybywireaircraftcontrolsystems,chemicalplants,andoilrefineries.Modelpredictivecontrolsystemsaredesignedusingspecializedcomputeraideddesignsoftwareandempiricalmathematicalmodelsofthesystemtobecontrolled.

Fuzzylogic

Fuzzylogicisanattempttoapplytheeasydesignoflogiccontrollerstothecontrolofcomplexcontinuouslyvaryingsystems.Basically,ameasurementinafuzzylogicsystemcanbepartlytrue,thatisifyesis1andnois0,afuzzymeasurementcanbebetween0and1.

Therulesofthesystemarewritteninnaturallanguageandtranslatedintofuzzylogic.Forexample,thedesignforafurnacewouldstartwith:"Ifthetemperatureistoohigh,reducethefueltothefurnace.Ifthetemperatureistoolow,increasethefueltothefurnace."

Measurementsfromtherealworld(suchasthetemperatureofafurnace)areconvertedtovaluesbetween0and1byseeingwheretheyfallonatriangle.Usually,thetipofthetriangleisthemaximumpossiblevaluewhichtranslatesto1.

Fuzzylogic,then,modifiesBooleanlogictobearithmetical.Usuallythe"not"operationis"output=1input,"the"and"operationis"output=input.1multipliedbyinput.2,"and"or"is"output=1((1input.1)multipliedby(1input.2))".ThisreducestoBooleanarithmeticifvaluesarerestrictedto0and1,insteadofallowedtorangeintheunitinterval[0,1].

Thelaststepisto"defuzzify"anoutput.Basically,thefuzzycalculationsmakeavaluebetweenzeroandone.Thatnumberisusedtoselectavalueonalinewhoseslopeandheightconvertsthefuzzyvaluetoarealworldoutputnumber.Thenumberthencontrolsrealmachinery.

Ifthetrianglesaredefinedcorrectlyandrulesarerighttheresultcanbeagoodcontrolsystem.

Whenarobustfuzzydesignisreducedintoasingle,quickcalculation,itbeginstoresembleaconventionalfeedbackloopsolutionanditmightappearthatthefuzzydesignwasunnecessary.However,thefuzzylogicparadigmmayprovidescalabilityforlargecontrolsystemswhereconventionalmethodsbecomeunwieldyorcostlytoderive.

Fuzzyelectronicsisanelectronictechnologythatusesfuzzylogicinsteadofthetwovaluelogicmorecommonlyusedindigitalelectronics.



Physicalimplementations

Sincemodernsmallmicroprocessorsaresocheap(oftenlessthan$1US),it'sverycommontoimplementcontrolsystems,includingfeedbackloops,withcomputers,ofteninanembeddedsystem.Thefeedbackcontrolsaresimulatedbyhavingthecomputermakeperiodicmeasurementsandthencalculatefromthisstreamofmeasurements(seedigitalsignalprocessing,sampleddatasystems).

Computersemulatelogicdevicesbymakingmeasurementsofswitchinputs,calculatingalogicfunctionfromthesemeasurementsandthensendingtheresultsouttoelectronicallycontrolledswitches.

Logicsystemsandfeedbackcontrollersareusuallyimplementedwithprogrammablelogiccontrollerswhicharedevicesavailablefromelectricalsupplyhouses.Theyincludealittlecomputerandasimplifiedsystemforprogramming.Mostoftentheyareprogrammedwithpersonalcomputers.

Logiccontrollershavealsobeenconstructedfromrelays,hydraulicandpneumaticdevicesaswellaselectronicsusingbothtransistorsandvacuumtubes(feedbackcontrollerscanalsobeconstructedinthismanner).

Seealso

ControltheoryPerceptualcontroltheoryDistributedcontrolsystemProgrammablelogiccontrollerPIDcontrollerHVACcontrolsystemDroopspeedcontrolControlengineeringSampleddatasystemsBuildingautomationVisSimEPICSSCADACoefficientdiagrammethodEducationandtrainingofelectricalandelectronicsengineersIndustrialcontrolsystemsProcesscontrolProcessoptimizationNetworkedcontrolsystemHierarchicalcontrolsystemMotioncontrolCyberneticsGoodRegulatorNumericalcontrolBehaviorTrees(ArtificialIntelligence,RoboticsandControl)



Wikibookshasabookonthetopicof:ControlSystems

References1. Kuphaldt,TonyR."Chapter6LADDERLOGIC"

(http://openbookproject.net//electricCircuits/Digital/DIGI_6.html).LessonsInElectricCircuitsVolumeIV.Retrieved22September2010.

2. Brady,Ian."Programmablelogiccontrollersbenefitsandapplications"(http://www.optimacs.com/wpcontent/uploads/2012/03/PLCreport.pdf)(PDF).PLCs.Retrieved5December2011.

3. http://www.seeei.org.il/prdFiles/2702_desc3.pdfABB:PowerGenerationEnergyEfficientDesignofAuxiliarySystemsinFossilFuelPowerPlants,Page262,Section:LoadFollowing

Externallinks

SemiautonomousFlightDirectionReferenceunmannedaircraft.org(http://www.unmannedaircraft.org/f/pat6460810.pdf)ControlSystemToolbox(http://www.mathworks.com/products/control/)fordesignandanalysisofcontrolsystems.ControlSystemsManufacturer(http://www.trminternational.com/)DesignandManufactureofcontrolsystems.Mathematicafunctionsfortheanalysis,design,andsimulationofcontrolsystems(http://reference.wolfram.com/mathematica/guide/ControlSystems.html)

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