PLC

Introduction

ProgrammableLogicController(PLC),adigitaloperationcontrollerwithmicroprocessorforautomaticcontrol,canloadcontrolinstructionsatanytimeIntothememoryforstorageandexecution.TheprogrammablecontrolleriscomposedoffunctionalunitssuchasCPU,instructionanddatamemory,input/outputinterface,powersupply,anddigital-to-analogconversion.Theearlyprogrammablelogiccontrollershadonlylogiccontrolfunctions,sotheywerenamedprogrammablelogiccontrollers.Later,withcontinuousdevelopment,thesecomputermoduleswithsimplefunctionsatthebeginninghaveincludedlogiccontrol,timingcontrol,analogcontrol,Thenameofvariousfunctionssuchasmulti-computercommunicationhasalsobeenchangedtoProgrammableController.However,duetotheconflictbetweenitsabbreviationPCandtheabbreviationofPersonalComputer(PersonalComputer),aswellashabitualreasons,peoplestilloftenuseit.Theterm"programminglogiccontroller"stillusestheacronymPLC.

Theprogrammablelogiccontrollerusedintheindustrynowisequivalenttoorclosetothehostofacompactcomputer,anditsadvantagesinscalabilityandreliabilitymakeitwidelyusedinvariouscurrenttypesIndustrialcontrolfield.NomatterinthecomputerdirectcontrolsystemorthecentralizeddistributedcontrolsystemDCS,orthefieldbuscontrolsystemFCS,therearealwaysalargenumberofvariousPLCcontrollersused.TherearemanyPLCmanufacturers,suchasSiemens,Schneider,Mitsubishi,Delta,etc.,almostallmanufacturersinvolvedinthefieldofindustrialautomationwillprovidetheirPLCproducts.

Developmenthistory

Origin

ThedevelopmentofproductiontechnologyrequirementsoftheAmericanautomobileindustrypromotedtheemergenceofPLC.Inthe1960s,GeneralMotorsCorporationoftheUnitedStatesWhentheproductionlinewasadjusted,itwasfoundthattherelayandcontactorcontrolsystemweredifficulttomodify,largeinsize,noisy,inconvenienttomaintain,andpoorinreliability,sothefamous"tenuniversal"biddingindicatorswereproposed.

In1969,theAmericanDigitalEquipmentCorporationdevelopedthefirstprogrammablecontroller(PDP-14).AftertrialingitontheproductionlineofGeneralMotors,theeffectwasremarkable;in1971,Japandevelopedthefirstprogrammablecontroller(PDP-14).Taiwanprogrammablecontroller(DCS-8);in1973,Germanydevelopedthefirstprogrammablecontroller;in1974,mycountrybegantodevelopprogrammablecontrollers:in1977,mycountrypromotedPLCinthefieldofindustrialapplications.

Theoriginalpurposeistoreplacemechanicalswitchingdevices(relaymodules).However,since1968,thefunctionofPLCgraduallyreplacedtherelaycontrolboard,modernPLChasmorefunctions.Itsuseextendsfromsingleprocesscontroltothecontrolandmonitoringoftheentiremanufacturingsystem.

Development

Themicroprocessorappearedintheearly1970s.Peoplesoonintroduceditintoaprogrammablelogiccontroller,whichaddedfunctionssuchascalculation,datatransmissionandprocessing,andcompletedanindustrialcontroldevicewithrealcomputercharacteristics.Theprogrammablelogiccontrolleratthistimeistheproductofthecombinationofmicrocomputertechnologyandtheconventionalcontrolconceptofrelays.Afterthedevelopmentofpersonalcomputers,inordertofacilitateandreflectthefunctionalcharacteristicsofprogrammablecontrollers,programmablelogiccontrollerswerenamedProgrammableLogicController(PLC).

Inthemid-to-late1970s,programmablelogiccontrollersenteredthestageofpracticaldevelopment.Computertechnologyhasbeenfullyintroducedintoprogrammablecontrollers,makingtheirfunctionsleapforward.Highercomputingspeed,ultra-smallsize,morereliableindustrialanti-jammingdesign,analogcomputing,PIDfunctionandextremelyhighcostperformancehaveestablisheditspositioninmodernindustry.

Intheearly1980s,programmablelogiccontrollershavebeenwidelyusedinadvancedindustrialcountries.Thenumberofcountriesproducingprogrammablecontrollersintheworldisincreasing,andtheoutputisrisingdaybyday.Thisindicatesthattheprogrammablecontrollerhasenteredamaturestage.

Fromthe1980stothemid-1990s,programmablelogiccontrollersdevelopedthefastestperiod,withanannualgrowthrateof30-40%.Duringthisperiod,PLC’sabilitytohandleanalog,digitalcomputing,human-machineinterfaceandnetworkcapabilitieshasbeengreatlyimproved.ProgrammablelogiccontrollershavegraduallyenteredtheprocessThedominantDCSsystem.

Attheendofthe20thcentury,thecharacteristicsofthedevelopmentofprogrammablelogiccontrollersweremoreadaptedtotheneedsofmodernindustry.Duringthisperiod,mainframesandultra-minicomputersweredeveloped,variousspecialfunctionunitswereborn,andvariousman-machineinterfaceunitsandcommunicationunitswereproduced,makingiteasiertomatchindustrialcontrolequipmentusingprogrammablelogiccontrollers.

Basicstructure

Programmablelogiccontrollerisessentiallyacomputerdedicatedtoindustrialcontrol.Itshardwarestructureisbasicallythesameasthatofamicrocomputer.ThebasiccompositionisshowninFigure1.Thebasicstructureisdescribedindetailasfollows:

Powersupply

ThepowersupplyisusedtoconvertACpowerintoDCpowerrequiredbythePLC.Atpresent,mostPLCsuseswitched-modestabilizedpowersuppliesforpowersupply.

CentralProcessingUnit

Thecentralprocessingunit(CPU)isthecontrolcenterofthePLCandthecorecomponentofthePLC.ItsperformancedeterminestheperformanceofthePLC.

Thecentralprocessingunitiscomposedofacontroller,arithmeticunit,andregisters.Thesecircuitsareallconcentratedonachip,andareconnectedtotheinput/outputinterfacecircuitofthememorythroughanaddressbusandacontrolbus.Theroleofthecentralprocessingunitistoprocessandrunuserprograms,performlogicalandmathematicaloperations,andcontroltheentiresystemtocoordinate.

Memory

Memoryisasemiconductorcircuitwithmemoryfunction.Itsfunctionistostoresystemprograms,userprograms,logicvariablesandotherinformation.Amongthem,thesystemprogramistheprogramthatcontrolsthePLCtorealizevariousfunctions.ItiswrittenbythePLCmanufacturerandsolidifiedintheread-onlymemory(ROM),whichcannotbeaccessedbytheuser.

Inputunit

TheinputunitistheinputinterfacethatconnectsthePLCtothecontrolleddevice.ItisthebridgeforthesignaltoenterthePLC.Itsfunctionistoreceivethemastercomponentandthedetectioncomponent.Signal.TheinputtypesareDCinput,ACinput,ACandDCinput.

Outputunit

TheoutputunitisalsotheconnectionpartbetweenthePLCandthecontrolleddevice.ItsfunctionistotransmittheoutputsignalofthePLCtothecontrolleddevice,thatis,sendthecentralprocessorTheweakcurrentsignalisconvertedintoalevelsignaltodrivetheactuatorofthecontrolleddevice.Theoutputtypesincluderelayoutput,transistoroutput,andthyristoroutput.

Inadditiontotheabove-mentionedparts,PLCalsohasavarietyofexternaldevicesaccordingtodifferentmodels.Itsfunctionistohelpprogramming,realizemonitoringandnetworkcommunication.Commonlyusedexternaldevicesareprogrammers,printers,cassettetaperecorders,computersandsoon.

Workingprinciple

Whenaprogrammablelogiccontrollerisputintooperation,itsworkingprocessisgenerallydividedintothreestages,namelyinputsampling,userprogramexecutionandoutputrefresh.Completingtheabovethreestagesiscalledascancycle.Duringthewholeoperationperiod,theCPUoftheprogrammablelogiccontrollerrepeatedlyexecutestheabovethreestagesatacertainscanningspeed.

Inputsampling

Programmablelogiccontroller(2photos)

Intheinputsamplingstage,theprogrammablelogiccontrollerisscannedsequentiallyReadinallinputstatusanddata,andstoretheminthecorrespondingunitintheI/Oimagearea.Aftertheinputsamplingisover,itgoestotheuserprogramexecutionandoutputrefreshstage.Inthesetwostages,eveniftheinputstatusanddatachange,thestatusanddataofthecorrespondingunitintheI/Omapareawillnotchange.Therefore,iftheinputisapulsesignal,thewidthofthepulsesignalmustbegreaterthanonescanperiodtoensurethattheinputcanbereadunderanycircumstances.

Userprogramexecution

Intheuserprogramexecutionstage,theprogrammablelogiccontrolleralwaysscanstheuserprogram(ladderdiagram)sequentiallyfromtoptobottom.Whenscanningeachladderdiagram,thecontrolcircuitformedbythecontactsontheleftsideoftheladderdiagramisalwaysscannedfirst,andthelogicoperationisperformedonthecontrolcircuitformedbythecontactsintheorderofleftandright,upanddown.,Andthenaccordingtotheresultofthelogicoperation,refreshthestateofthecorrespondingbitofthelogiccoilinthesystemRAMstoragearea;orrefreshthestateofthecorrespondingbitoftheoutputcoilintheI/Oimagearea;ordeterminewhethertoexecutetheladderdiagramThespecifiedspecialfunctioninstructions.

Thatis,duringtheexecutionoftheuserprogram,onlythestateanddataoftheinputpointintheI/Oimageareawillnotchange,whileotheroutputpointsandsoftwaredevicesareintheI/Oimagearea.OrthestateanddataintheRAMstorageareaof​​thesystemmaychange,andtheladderdiagramslistedabovewillhaveaneffectontheladderdiagramslistedbelowthatusethesecoilsordata;onthecontrary,theladderdiagramslistedabovewillbeaffected.Intheladderdiagrambelow,thestateordataofthelogiccoiltoberefreshedcanonlytakeeffectontheprogramaboveituntilthenextscancycle.

Intheprocessofprogramexecution,ifyouuseimmediateI/Oinstructions,youcandirectlyaccessI/Opoints.EveniftheI/Oinstructionisused,thevalueoftheinputprocessimageregisterwillnotbeupdated.TheprogramdirectlyfetchesthevaluefromtheI/Omodule,andtheoutputprocessimageregisterwillbeupdatedimmediately,whichissomewhatdifferentfromtheimmediateinput.

Outputrefresh

Whenthescanninguserprogramends,theprogrammablelogiccontrollerenterstheoutputrefreshphase.Duringthisperiod,theCPUrefreshesalloutputlatchcircuitsaccordingtothecorrespondingstateanddataintheI/Oimagearea,andthendrivesthecorrespondingperipheralsthroughtheoutputcircuit.Atthistime,itistherealoutputoftheprogrammablelogiccontroller.

Summary

Accordingtothedescriptionoftheaboveprocess,thecharacteristicsofthePLCworkingprocesscanbesummarizedasfollows:

①ThePLCadoptstheworkingmodeofcentralizedsamplingandcentralizedoutput.Thiswayreducestheinfluenceofexternalinterference.

②TheworkingprocessofthePLCisacyclicscanningprocess,andthelengthofthecyclicscanningtimedependsonfactorssuchastheinstructionexecutionspeedandthelengthoftheuserprogram.

③Theinfluenceofoutputoninputhasahysteresis.ThePLCadoptsacentralizedsamplingandcentralizedoutputworkingmode.Whenthesamplingphaseisover,theinputstatechangewillnotbereceiveduntilthenextsamplingperiod.Therefore,thelengthofthislagtimemainlydependsonthelengthofthecycleperiod.Inaddition,thefactorsthataffectthelagtimeincludetheinputfiltertime,thelagtimeoftheoutputcircuit,andsoon.

④Thecontentoftheoutputimageregisterdependsontheresultoftheuserprogramscanningexecution.

⑤Thecontentoftheoutputlatchisdeterminedbythedataintheoutputimageregisterduringthelastoutputrefresh.

⑥ThecurrentactualoutputstateofthePLCisdeterminedbythecontentoftheoutputlatch.

Features

(1)Highreliability.SincePLCsmostlyusesingle-chipmicrocomputers,theyarehighlyintegrated,coupledwithcorrespondingprotectioncircuitsandself-diagnosticfunctions,whichimprovethereliabilityofthesystem.

(2)Easytoprogram.PLCprogrammingmostlyusesrelaycontrolladderdiagramsandcommandstatements,andtheirnumberismuchlessthanmicrocomputerinstructions.Exceptformediumandhigh-endPLCs,thereareonlyabout16smallPLCsingeneral.Becausetheladderdiagramisvisualandsimple,itiseasytomasterandeasytouse,anditcanbeprogrammedevenwithoutcomputerexpertise.

(3)Flexibleconfiguration.AsthePLCadoptsabuildingblockstructure,theusercanflexiblychangethefunctionandscaleofthecontrolsystembysimplycombiningit.Therefore,itcanbeappliedtoanycontrolsystem.

(4)Completeinput/outputfunctionmodules.OneofthebiggestadvantagesofPLCisthatfordifferentfieldsignals(suchasDCorAC,switching,digitaloranalog,voltageorcurrent,etc.),therearecorrespondingtemplatesthatcanbeusedwithindustrialfielddevices(suchasbuttons,switches,etc.),Sensorcurrenttransmitter,motorstarterorcontrolvalve,etc.)aredirectlyconnected,andconnectedtotheCPUmainboardthroughthebus.

(5)Easytoinstall.Comparedwithcomputersystems,PLCinstallationneitherrequiresadedicatedcomputerroomnorstrictshieldingmeasures.Whenusing,onlyneedtocorrectlyconnectthedetectiondevicewiththeactuatorandtheI/OinterfaceterminalofthePLC,anditcanworknormally.

(6)Fastrunningspeed.BecausePLCcontrolisexecutedbyprogramcontrol,regardlessofitsreliabilityoroperatingspeed,itisincomparabletorelaylogiccontrol.

Inrecentyears,theuseofmicroprocessors,especiallywiththelarge-scaleadoptionofsingle-chipmicrocomputers,hasgreatlyenhancedthecapabilitiesofPLCandmadethedifferencebetweenPLCandmicrocomputercontrolsystemssmallerandsmaller,especiallyThisisespeciallytrueforhigh-endPLCs.

Systemintegration

Inthemanufacturingindustry,therearealargenumberofswitch-basedopen-loopsequentialcontrol,whichperformssequentialactionsaccordingtologicalconditionsandoperatesintimesequence;inaddition,thereareThecontrolofthechainprotectionactioniscarriedoutaccordingtothelogicalrelationshipregardlessofsequenceandtiming;andalargenumberofstatequantitiessuchasswitchquantity,pulsequantity,timing,counter,analogquantityover-limitalarm,etc.—discretedataacquisitionandmonitoring.Duetothesecontrolandmonitoringrequirements,PLChasdevelopedintoaproductthatreplacesrelaycircuitsandperformssequentialcontrol.PLCmanufacturershavegraduallyincreasedvariouscommunicationinterfacesontheoriginalCPUtemplate,andthefieldbustechnologyandEthernettechnologyhavealsodevelopedsimultaneously,whichhasmadetheapplicationscopeofPLCmoreandmoreextensive.PLChastheadvantagesofstability,reliability,lowprice,completefunctions,flexibleandconvenientapplication,andconvenientoperationandmaintenance.Thisisthefundamentalreasonwhyitcanoccupythemarketforalongtime.

ThehardwareofthePLCcontrolleritselfadoptsabuildingblockstructure.Therearemotherboards,digitalI/Otemplates,analogI/Otemplates,aswellasspecialpositioningtemplates,barcoderecognitiontemplatesandothermodules.UserscanfollowNeedtoadoptthemethodofexpandingonthemotherboardorusingthebustechnologytoequiptheremoteI/OslavestationtogetthedesiredI/Oquantity.

PLCiscapableofoutputtinganalogvoltageanddigitalpulsewhilerealizingvariousnumbersofI/Ocontrol,sothatitcancontrolallkindsofservomotors,steppingmotorsthatcanreceivethesesignals,Variablefrequencymotors,etc.,coupledwithtouchscreenman-machineinterfacesupport,PLCcanmeetyourneedsatanylevelinprocesscontrol.

Selectionrules

Whendesigningaprogrammablelogiccontrollersystem,thecontrolplanshouldfirstbedetermined,andthenextstepistoselecttheprogrammablelogiccontrollerengineeringdesign.Thecharacteristicsoftheprocessflowandapplicationrequirementsarethemainbasisforthedesignandselection.Programmablelogiccontrollersandrelatedequipmentshouldbeintegratedandstandard,accordingtotheprincipleofeasyintegrationwithindustrialcontrolsystemsandeasyexpansionoftheirfunctionsForawell-established,matureandreliablesystem,thesystemhardware,softwareconfigurationandfunctionsoftheprogrammablelogiccontrollershouldbecompatiblewiththescaleandcontrolrequirementsofthedevice.Familiarwithprogrammablecontroller,functiontablediagramsandrelatedprogramminglanguages​​willhelpshortentheprogrammingtime.Therefore,whenselectingandestimatingengineeringdesign,thecharacteristicsoftheprocessandcontrolrequirementsshouldbeanalyzedindetail,andthecontroltasksandscopedeterminationrequirementsshouldbeclarified.Thenaccordingtothecontrolrequirements,estimatethenumberofinputandoutputpoints,therequiredmemorycapacity,determinethefunctionoftheprogrammablelogiccontroller,thecharacteristicsoftheexternalequipment,etc.,andfinallyselecttheprogrammablelogiccontrolleranddesignwithahighercostperformanceThecorrespondingcontrolsystem.

Estimationofthenumberofpoints

WhenestimatingthenumberofI/Opoints,anappropriatemarginshouldbeconsidered.Usuallybasedonthenumberofinputandoutputpoints,anexpandablemarginof10%to20%isadded.,Astheestimateddataofthenumberofinputandoutputpoints.Whenactuallyordering,theinputandoutputpointsneedtoberoundedaccordingtotheproductcharacteristicsofthemanufacturer'sprogrammablelogiccontroller.

Memorycapacity

Thememorycapacityisthesizeofthehardwarestorageunitthattheprogrammablecontrolleritselfcanprovide,andtheprogramcapacityisthesizeofthestorageunitusedbytheuserapplicationinthememory,sotheprogramThecapacityislessthanthememorycapacity.Inthedesignstage,sincetheuserapplicationprogramhasnotbeencompiledyet,theprogramcapacityisunknowninthedesignstageandneedstobeknownaftertheprogramisdebugged.Inordertohaveacertainestimateoftheprogramcapacitywhendesigningandselecting,theestimateofthememorycapacityisusuallyusedinstead.

Thereisnofixedformulaforestimatingthememorycapacity.Manyliteraturesgivedifferentformulas,whicharegenerally10-15timesthenumberofdigitalI/Opoints,plusanalogI/O.100timesthenumberofpoints,thisnumberisthetotalnumberofwordsinthememory(16bitsisaword),and25%ofthisnumberisusedtoconsiderthemargin.

Controlfunctionselection

Thisselectionincludestheselectionoffeaturessuchascalculationfunction,controlfunction,communicationfunction,programmingfunction,diagnosisfunctionandprocessingspeed.

1.Operationfunction

Theoperationfunctionofsimpleprogrammablelogiccontrollerincludeslogicoperation,timingandcountingfunction;theoperationfunctionofordinaryprogrammablelogiccontrolleralsoincludesdatashift,Comparisonandothercalculationfunctions;morecomplexcalculationfunctionsincludealgebraiccalculations,datatransmission,etc.;therearealsoanalogPIDcalculationsandotheradvancedcalculationfunctionsinlargeprogrammablelogiccontrollers.Withtheadventofopensystems,programmablelogiccontrollershavealreadyhadcommunicationfunctions.Someproductshavecommunicationwiththelowercomputer,someproductshavecommunicationwiththesameoruppercomputer,andsomeproductsalsohavecommunicationwiththefactoryorenterprisenetwork.Thefunctionofdatacommunication.Whendesigningandselectingmodels,weshouldproceedfromtherequirementsofactualapplications,andreasonablyselecttherequiredcomputingfunctions.Mostapplicationsrequireonlylogicoperationsandtimingcountingfunctions.Someapplicationsrequiredatatransmissionandcomparison.Whenusedforanalogdetectionandcontrol,algebraicoperations,numericalconversionandPIDoperationsareused.Todisplaydata,operationssuchasdecodingandencodingarerequired.

2.Controlfunctions

ControlfunctionsincludePIDcontrolcalculations,feedforwardcompensationcontrolcalculations,ratiocontrolcalculations,etc.,whichshouldbedeterminedaccordingtocontrolrequirements.Programmablelogiccontrollersaremainlyusedforsequentiallogiccontrol.Therefore,inmostoccasions,single-loopormulti-loopcontrollersareoftenusedtosolveanalogcontrol,andsometimesdedicatedintelligentinputandoutputunitsareusedtocompletetherequiredcontrolfunctionsandimprovetheProgramtheprocessingspeedofthelogiccontrollerandsavememorycapacity.Forexample,PIDcontrolunit,high-speedcounter,analogunitwithspeedcompensation,ASCcodeconversionunit,etc.areused.

3.Communicationfunction

Largeandmedium-sizedprogrammablelogiccontrollersystemsshouldsupportmultiplefieldbusesandstandardcommunicationprotocols(suchasTCP/IP),andshouldbeabletocommunicatewiththefactorywhenneededInternet(TCP/IP)connection.ThecommunicationprotocolshouldconformtotheISO/IEEEcommunicationstandard,anditshouldbeanopencommunicationnetwork.

Thecommunicationinterfaceoftheprogrammablelogiccontrollersystemshouldincludeserialandparallelcommunicationinterfaces,RIOcommunicationports,commonlyusedDCSinterfaces,etc.;largeandmedium-sizedprogrammablelogiccontrollercommunicationbus(includinginterfaceequipmentandcables)Itshallbeconfiguredwith1:1redundancy,thecommunicationbusshallcomplywithinternationalstandards,andthecommunicationdistanceshallmeettheactualrequirementsofthedevice.

Inthecommunicationnetworkoftheprogrammablelogiccontrollersystem,theupper-levelnetworkcommunicationrateshouldbegreaterthan1Mbps,andthecommunicationloadshouldnotbegreaterthan60%.Themainformsofthecommunicationnetworkoftheprogrammablelogiccontrollersystemareasfollows:

1)PCisthemasterstation,andmultipleprogrammablelogiccontrollersofthesamemodelareslavestationstoformasimpleprogrammablelogiccontrolControllernetwork;

2)Oneprogrammablelogiccontrolleristhemasterstation,andotherprogrammablelogiccontrollersofthesamemodelareslavestations,formingamaster-slaveprogrammablelogiccontrollernetwork;

3)TheprogrammablelogiccontrollernetworkisconnectedtoalargeDCSasasubnetoftheDCSthroughaspecificnetworkinterface;

4)Dedicatedprogrammablelogiccontrollernetwork(dedicatedprogrammablelogicofvariousmanufacturersControllercommunicationnetwork).

InordertoreducetheCPUcommunicationtask,accordingtotheactualneedsofnetworkcomposition,communicationprocessorswithdifferentcommunicationfunctions(suchaspoint-to-point,fieldbus,)shouldbeselected.

4.Programmingfunction

Offlineprogrammingmode:TheprogrammablelogiccontrollerandtheprogrammershareaCPU.Whentheprogrammerisintheprogrammingmode,theCPUonlyprovidesservicesfortheprogrammer.Fielddevicesarecontrolled.Aftertheprogrammingiscompleted,theprogrammerswitchestotheoperatingmode,andtheCPUcontrolsthefielddevicesandcannotperformprogramming.Offlineprogrammingcanreducesystemcosts,butitisinconvenienttouseanddebug.Onlineprogrammingmode:CPUandprogrammerhavetheirownCPU.ThehostCPUisresponsibleforon-sitecontrolandexchangesdatawiththeprogrammerduringascancycle.Theprogrammersendstheprogramordatacompiledonlinetothehost.Thenextscancycle,Thehostrunsaccordingtothenewlyreceivedprogram.Thismethodhasahighcost,butitisconvenientforsystemdebuggingandoperation,andisoftenusedinlargeandmedium-sizedprogrammablelogiccontrollers.

Fivestandardizedprogramminglanguages:SequentialFunctionChart(SFC),LadderDiagram(LD),FunctionModuleDiagram(FBD),threegraphicallanguages​​andstatementlist(IL)andstructuredtext(ST)Textlanguage.Theselectedprogramminglanguageshouldcomplywithitsstandard(IEC6113123).Atthesametime,itshouldalsosupportmultiplelanguageprogrammingforms,suchasC,Basic,etc.,tomeetthecontrolrequirementsofspecialcontroloccasions.

5.Diagnosisfunction

Thediagnosisfunctionofprogrammablelogiccontrollerincludeshardwareandsoftwarediagnosis.Thehardwarediagnosisdeterminesthefaultlocationofthehardwarethroughthelogicaljudgmentofthehardware,andthesoftwarediagnosisisdividedintointernaldiagnosisandexternaldiagnosis.DiagnosingtheinternalperformanceandfunctionsofthePLCthroughsoftwareisinternaldiagnosis,anddiagnosingtheinformationexchangefunctionsoftheprogrammablelogiccontroller'sCPUandexternalinputandoutputcomponentsthroughsoftwareisexternaldiagnosis.

Thestrengthofthediagnosticfunctionoftheprogrammablelogiccontrollerdirectlyaffectstherequirementsforthetechnicalcapabilitiesoftheoperationandmaintenancepersonnel,andaffectstheaveragemaintenancetime.

6.Processingspeed

Programmablelogiccontrollersworkinscanningmode.Fromthepointofviewofreal-timerequirements,theprocessingspeedshouldbeasfastaspossible.Ifthesignaldurationislessthanthescanningtime,theprogrammablelogiccontrollerwillnotbeabletoscanthesignal,resultinginthelossofsignaldata.

Theprocessingspeedisrelatedtothelengthoftheuserprogram,CPUprocessingspeed,softwarequality,etc.Theprogrammablelogiccontrollercontactshavefastresponseandhighspeed,andtheexecutiontimeofeachbinaryinstructionisabout0.2-0.4Ls,soitcanmeettheneedsofapplicationswithhighcontrolrequirementsandcorrespondingrequirements.Thescanperiod(processorscanperiod)shouldsatisfy:thescantimeofasmallprogrammablelogiccontrollerisnotmorethan0.5ms/K;thescantimeofalargeandmedium-sizedprogrammablelogiccontrollerisnotmorethan0.2ms/K.

Controllertype

Programmablelogiccontrollersaredividedintointegraltypeandmodulartypeaccordingtothestructure.Accordingtotheapplicationenvironment,theyaredividedintotwotypes:on-siteinstallationandcontrolroominstallation;accordingtoCPUThewordlengthisdividedinto1bit,4bit,8bit,16bit,32bit,64bit,etc.Fromtheapplicationpointofview,itcanusuallybeselectedaccordingtothecontrolfunctionorthenumberofinputandoutputpoints.

TheintegralprogrammablelogiccontrollerhasafixednumberofI/Opoints,sousershavelessroomforchoice,whichisusedinsmallcontrolsystems;modularprogrammablelogiccontrollersprovideavarietyofI/OcardsOrinsertacard,sotheusercanselectandconfiguretheI/Opointsofthecontrolsystemmorereasonably,andthefunctionexpansionisconvenientandflexible.Itisgenerallyusedforlargeandmedium-sizedcontrolsystems.

Inputandoutputtype

Theswitchquantitymainlyreferstothebinaryinputandoutput,whichreferstotheauxiliarypointofadevice,suchastherelayofthetemperaturecontrollerofthetransformer.Auxiliarypoint(displacementaftertransformerover-temperature),auxiliarypointofvalvecamswitch(displacementaftervalveswitch),auxiliarypointofcontactor(displacementaftercontactoraction),thermalrelay(afterthermalrelayaction)Displacement),thesepointsaregenerallytransmittedtothePLCorcomprehensiveprotectiondevice.ThepowersupplyisgenerallyprovidedbythePLCorcomprehensiveprotectiondevice.Itdoesnothaveapowersourceitself,soitiscalledapassivecontact,alsocalledtheopeningofthePLCorcomprehensiveprotectiondevice.quantity.

1.Digitalquantities

Physicalquantitiesthatarediscreteintimeandquantityarecalleddigitalquantities.Thesignalthatrepresentsadigitalquantityiscalledadigitalsignal.Anelectroniccircuitthatworksunderadigitalsignaliscalledadigitalcircuit.

Forexample:whenusinganelectroniccircuittorecordthenumberofpartsoutputfromanautomaticproductionline,everytimeapartissent,asignalisgiventotheelectroniccircuittomakeitrecord1,andusuallynopartsaresenttotheelectroniccircuitThesignalis0,whereitiscounting.Itcanbeseenthatthesignalofthenumberofpartsisdiscontinuousinbothtimeandquantity,soitisadigitalsignal.Thesmallestquantityunitis1.

2.Analogquantity

Physicalquantitiesthatarecontinuousintimeorvaluearecalledanalogquantities.Thesignalthatrepresentsananalogquantityiscalledananalogsignal.Anelectroniccircuitthatworksunderananalogsignaliscalledananalogcircuit.

Forexample:thevoltagesignaloutputbythethermocoupleisananalogsignal,becausethemeasuredtemperatureisunlikelytohaveasuddenjumpunderanycircumstances,sothemeasuredvoltagesignaliseitherintimeorinThenumbersarecontinuous.Moreover,anyvalueofthisvoltagesignalinthecontinuouschangeprocesshasaspecificphysicalmeaning,thatis,representsacorrespondingtemperature.

Conversionprinciple

1.Adigital-to-analogconverterisasystemthatconvertsadigitalsignalintoananalogsignal,whichcangenerallyberealizedbylow-passfiltering.Thedigitalsignalisdecodedfirst,thatis,thedigitalcodeisconvertedintoacorrespondingleveltoformasteppedsignal,andthenlow-passfilteringisperformed.

Accordingtothetheoryofsignalsandsystems,adigitalsteppedsignalcanberegardedastheconvolutionofanidealimpulsesamplingsignalandarectangularimpulsesignal.Thenaccordingtotheconvolutiontheorem,thefrequencyspectrumofthedigitalsignalistheimpulsesamplingsignalTheproductofthefrequencyspectrumandtherectangularpulsefrequencyspectrum(thatis,theSafunction).Inthisway,thereciprocaloftheSafunctionisusedasthespectralcharacteristiccompensation,andthedigitalsignalcanberestoredtothesampledsignal.Accordingtothesamplingtheorem,thefrequencyspectrumofthesampledsignalisfilteredthroughideallow-passfilteringtoobtainthefrequencyspectrumoftheoriginalanalogsignal.

Ingeneralimplementation,itisnotdirectlybasedontheseprinciples,becausesharpsamplingsignalsaredifficulttoobtain.Therefore,thesetwofilters(Safunctionandideallow-pass)canbecombined(cascaded),andbecauseofthisThefilteringcharacteristicsofeachsystemarephysicallyunrealizable,soitcanonlybeapproximatedinarealsystem.

2.Theanalog-to-digitalconverterisasystemthatconvertsanalogsignalsintodigitalsignals.Itisaprocessoffiltering,sample-holding,andencoding.

Theanalogsignalisband-limitedfiltered,sample-and-holdcircuit,intoaladder-shapedsignal,andthenthroughtheencoder,sothateachlevelintheladder-shapedsignalintoabinarycode.

Choosemodel

TherearemanytypesofPLCproducts.DifferentPLCmodelscorrespondtotheirdifferentstructure,performance,capacity,instructionsystem,programmingmethod,price,etc.,andtheirapplicableoccasionsalsohavetheirownemphasis.Therefore,areasonableselectionofPLCisofgreatsignificanceforimprovingthetechnicalandeconomicindicatorsofthePLCcontrolsystem.

PLCmodel

ThechoiceofPLCshouldbecomprehensivelyconsideredfromtheaspectsofPLCmodel,capacity,I/Omodules,powersupplymodules,specialfunctionmodules,andcommunicationnetworkingcapabilities.ThebasicprincipleofPLCmodelselectionistostriveforthebestperformance-to-priceratiounderthepremiseofmeetingfunctionalrequirementsandensuringreliabilityandconvenientmaintenance.Theselectionshouldmainlyconsiderfactorssuchasreasonablestructuretype,choiceofinstallationmethod,correspondingfunctionalrequirements,responsespeedrequirements,systemreliabilityrequirements,andunifiedmodelsasmuchaspossible.

Structuraltype

PLCmainlyhastwostructuraltypes:integraltypeandmodulartype.

TheaveragepriceofeachI/OpointoftheintegratedPLCischeaperthanthatofthemodulartype,andthevolumeisrelativelysmall.Itisgenerallyusedinasmallcontrolsystemwitharelativelyfixedsystemprocess;whilethemodularPLCThefunctionexpansionisflexibleandconvenient.ThereisalargechoiceinthenumberofI/Opoints,theratioofthenumberofinputpointstothenumberofoutputpoints,thetypesofI/Omodules,etc.,andthemaintenanceisconvenient,generallyinmorecomplexcontrolsystems.

Installationmethod

TheinstallationmethodofPLCsystemisdividedintocentralizedtype,remoteI/Otype,anddistributedmultiplePLCnetworking.

CentralizedtypedoesnotneedtosetupthedriveremoteI/Ohardware,thesystemrespondsquickly,andthecostislow;theremoteI/Otypeissuitableforlarge-scalesystems,thesystemdevicesaredistributedinawiderange,andtheremoteI/OcanbedispersedInstallednearthefielddevice,theconnectionisshort,butthedriveandremoteI/Opowersupplyneedtobeadded;thedistributednetworkofmultiplePLCsissuitablefortheoccasionswheremultipledevicesareindependentlycontrolledandconnectedtoeachother.SmallPLCscanbeused,butAcommunicationmodulemustbeattached.

Functionalrequirements

Generallysmall(low-end)PLCshavefunctionssuchaslogicoperations,timing,andcounting,whichcanbemetfordevicesthatonlyrequireswitchcontrol.

Forsystemsthatarebasedonon-offcontrolandwithasmallamountofanalogcontrol,anenhancedlow-endsystemwithA/DandD/Aconversionunits,plusandminusarithmeticoperations,anddatatransmissionfunctionscanbeusedPLC.Formorecomplexcontrol,itisrequiredtorealizefunctionssuchasPIDcalculation,closed-loopcontrol,communicationnetworking,etc.,dependingonthesizeandcomplexityofthecontrol,selectmid-rangeorhigh-endPLC.However,mediumandhigh-endPLCsaremoreexpensive,andaregenerallyusedinlarge-scaleprocesscontrolanddistributedcontrolsystems.

Responsespeed

PLCisageneral-purposecontrollerdesignedforindustrialautomation.TheresponsespeedofdifferentgradesofPLCcangenerallymeettheneedsofitsapplication.IfyouwanttousethePLCacrossarange,orwhencertainfunctionsorsignalshavespecialspeedrequirements,youshouldcarefullyconsidertheresponsespeedofthePLC.YoucanchooseaPLCwithhigh-speedI/Oprocessingfunctions,orchooseafastresponsemoduleandinterruptinputModulePLC,etc.

Reliability

ItcanmeetthereliabilityofgeneralsystemPLC.Forsystemswithhighreliabilityrequirements,considerationshouldbegiventowhethertoadoptaredundantsystemorahotstandbysystem.

Trytounifythemodelsasmuchaspossible

AcompanyshouldtrytounifythePLCmodels.Thefollowingthreeaspectsaremainlyconsidered:

1)Themodelisunified,anditsmodulescanbeusedassparesforeachother,whichisconvenientfortheprocurementandmanagementofspareparts.

2)Themodelisunified,anditsfunctionsandusagemethodsaresimilar,whichisconducivetothetrainingoftechnicalforceandtheimprovementoftechnicallevel.

3)Themodelisunified,theexternalequipmentisuniversal,theresourcescanbeshared,anditiseasytocommunicatewiththenetwork.Whenequippedwithahostcomputer,itiseasytoformamulti-leveldistributedcontrolsystem.

Applicationoverview

Openloopcontrol

TheopenloopcontroloftheswitchisthemostbasiccontrolfunctionofPLC.ThePLCcommandsystemhaspowerfullogicoperationcapabilities,anditiseasytoimplementvariouslogiccontrolmethodssuchastiming,counting,andsequence(stepping).MostPLCsareusedtoreplacetraditionalrelaycontactorcontrolsystems.

Analogclosed-loop

Foranalogclosed-loopcontrolsystems,inadditiontodigitalinputandoutput,theremustalsobeanaloginputandoutputpointsforsamplinginputandadjustmentTheoutputrealizescontinuousadjustmentandcontroloftemperature,flow,pressure,displacement,speedandotherparameters.ThecurrentPLCnotonlyhasthisfunctionforlargeandmedium-sizedmachines,butalsosomesmallmachineshavethisfunction.

Digitalcontrol

Whenthecontrolsystemhasarotaryencoderandpulseservodevice(suchasasteppermotor),PLCcanbeusedtorealizethefunctionofreceivingandoutputtinghigh-speedpulsestorealizedigitalControl,themoreadvancedPLChasalsospeciallydevelopedadigitalcontrolmodulethatcanrealizecurveinterpolation.Recently,anewtypeofmotionunitmodulehasbeenintroduced,anditcanalsoprovideaprogramminglanguagefordigitalcontroltechnology,makingiteasierforPLCtorealizedigitalcontrol.

Dataacquisitionandmonitoring

SincePLCismainlyusedforon-sitecontrol,itisaverynecessaryfunctiontocollecton-sitedata.Onthisbasis,connectthePLCtothehostcomputerortouchscreen,whichisThecurrentvalueofthesedatacanbeobserved,andstatisticalanalysiscanbeperformedintime.SomePLCshaveadatarecordingunit,andageneralpersonalcomputermemorycardcanbeinsertedintotheunittosavethecollecteddata.AnothercharacteristicofPLCisthattherearemanyself-checksignals.Usingthisfeature,thePLCcontrolsystemcanrealizeself-diagnosticmonitoring,reducesystemfailures,andimprovesystemreliability.