Introductio
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.
Progressushistory
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.
Progressus
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.
Centralis processui unitas
Thecentralprocessingunit(CPU)isthecontrolcenterofthePLCandthecorecomponentofthePLC.ItsperformancedeterminestheperformanceofthePLC.
Thecentralprocessingunitiscomposedofacontroller,arithmeticunit,andregisters.Thesecircuitsareallconcentratedonachip,andareconnectedtotheinput/outputinterfacecircuitofthememorythroughanaddressbusandacontrolbus.Theroleofthecentralprocessingunitistoprocessandrunuserprograms,performlogicalandmathematicaloperations,andcontroltheentiresystemtocoordinate.
Memoria
Memoriaisasemiconductorcircuitwithmemoryfunction.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.OrthestateanddataintheRAMstorageareaofthesystemmaychange,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.
Contentio theoutputation of the userprogramscanning execution.
⑤ Contenta the outputlatchis determinatur per datam, ut putatur imago regis perdurante ultima putri.
⑥ Praecurrente actu de statuo PLCis determinatur per contentum theo-putlatch.
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.One of the biggestadvantages of PLCis that fordifferentfield signa (ut DCorAC, mutans, digitaloranalog, voltageorcurrent, etc.), sunt correspondentia templata quae possunt esse sine industria machinarum (qualia globulis, mutando, digitaloranalogo, voltageorcurrent, etc.), sunt correspondentia contemplationum quae possunt esse sine industria machinarum (qualia globulis, mutando, digitaloranalogo, voltageorcurrent, etc.).
(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,functiontablediagramsandrelatedprogramminglanguageswillhelpshortentheprogrammingtime.Therefore,whenselectingandestimatingengineeringdesign,thecharacteristicsoftheprocessandcontrolrequirementsshouldbeanalyzedindetail,andthecontroltasksandscopedeterminationrequirementsshouldbeclarified.Thenaccordingtothecontrolrequirements,estimatethenumberofinputandoutputpoints,therequiredmemorycapacity,determinethefunctionoftheprogrammablelogiccontroller,thecharacteristicsoftheexternalequipment,etc.,andfinallyselecttheprogrammablelogiccontrolleranddesignwithahighercostperformanceThecorrespondingcontrolsystem.
Estimationofthenumberofpoints
WhenestimatingthenumberofI/Opoints,anappropriatemarginshouldbeconsidered.Usuallybasedonthenumberofinputandoutputpoints,anexpandablemarginof10%to20%isadded.,Astheestimateddataofthenumberofinputandoutputpoints.Whenactuallyordering,theinputandoutputpointsneedtoberoundedaccordingtotheproductcharacteristicsofthemanufacturer'sprogrammablelogiccontroller.
Memoriacapacity
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),threegraphicallanguagesandstatementlist(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-ad-digitalconverterisasystem that convertsanalogsignalsintodigitalsignals.Itisaprocessoffiltering, specimen-tenens, andencoding.
Theanalogsignalisband-spatiatus limitatum, exemplum et circumitus, signum inaladder-informatum, ac deinde per modum encoder, sohateach-linea inadder-signalintoabinarycode.
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.
Functionrequirements
Generaliter parvae functiones habent sicut logicoperationes, leo et numerans, quod potest forde- viscere quod solum imperium requirit.
Forsystemsthatarebasedonon-offcontrolandwithasmallamountofanalogcontrol,anenhancedlow-endsystemwithA/DandD/Aconversionunits,plusandminusarithmeticoperations,anddatatransmissionfunctionscanbeusedPLC.Formorecomplexcontrol,itisrequiredtorealizefunctionssuchasPIDcalculation,closed-loopcontrol,communicationnetworking,etc.,dependingonthesizeandcomplexityofthecontrol,selectmid-rangeorhigh-endPLC.However,mediumandhigh-endPLCsaremoreexpensive,andaregenerallyusedinlarge-scaleprocesscontrolanddistributedcontrolsystems.
Responspeed
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.