ProgrammablelogiccontrollerAprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,amusementrides,orlightingfixtures.PLCsareusedinmanyindustriesandmachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.ThePLCwasinventedinresponsetotheneedsoftheAmericanautomotivemanufacturingindustry.Programmablelogiccontrollerswereinitiallyadoptedbytheautomotiveindustrywheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.BeforethePLC,control,sequencing,andsafetyinterlocklogicformanufacturingautomobileswasaccomplishedusinghundredsorthousandsofrelays,camtimers,anddrumsequencersanddedicatedclosed-loopcontrollers.Theprocessforupdatingsuchfacilitiesfortheyearlymodelchange-overwasverytimeconsumingandexpensive,aselectriciansneededtoindividuallyrewireeachandeveryrelay.In1968GMHydramatic(theautomatictransmissiondivisionofGeneralMotors)issuedarequestforproposalforanelectronicreplacementforhard-wiredrelaysystems.ThewinningproposalcamefromBedfordAssociatesofBedford,Massachusetts.ThefirstPLC,designatedthe084becauseitwasBedfordAssociates'eighty-fourthproject,wastheresult.BedfordAssociatesstartedanewcompanydedicatedtodeveloping,manufacturing,selling,andservicingthisnewproduct:Modicon,whichstoodforMOdularDIgitalCONtroller.OneofthepeoplewhoworkedonthatprojectwasDickMorley,whoisconsideredtobethe"father"ofthePLC.TheModiconbrandwassoldin1977toGouldElectronics,andlateracquiredbyGermanCompanyAEGandthenbyFrenchSchneiderElectric,thecurrentowner.Oneoftheveryfirst084modelsbuiltisnowondisplayatModicon'sheadquartersinNorthAndover,Massachusetts.ItwaspresentedtoModiconbyGM,whentheunitwasretiredafternearlytwentyyearsofuninterruptedservice.Modiconusedthe84monikerattheendofitsproductrangeuntilthe984madeitsappearance.TheautomotiveindustryisstilloneofthelargestusersofPLCs.EarlyPLCsweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedin"ladderlogic",whichstronglyresemblesaschematicdiagramofrelaylogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,averyhigh-levelprogramminglanguagedesignedtoprogramPLCsbasedonstatetransitiondiagrams.ManyearlyPLCsdidnothaveaccompanyingprogrammingterminalsthatwerecapableofgraphicalrepresentationofthelogic,andsothelogicwasinsteadrepresentedasaseriesoflogicexpressionsinsomeversionofBooleanformat,similartoBooleanalgebra.Asprogrammingterminalsevolved,itbecamemorecommonforladderlogictobeused,fortheaforementionedreasons.NewerformatssuchasStateLogicandFunctionBlock(whichissimilartothewaylogicisdepictedwhenusingdigitalintegratedlogiccircuits)exist,buttheyarestillnotaspopularasladderlogic.AprimaryreasonforthisisthatPLCssolvethelogicinapredictableandrepeatingsequence,andladderlogicallowstheprogrammer(thepersonwritingthelogic)toseeanyissueswiththetimingofthelogicsequencemoreeasilythanwouldbepossibleinotherformats.EarlyPLCs,uptothemid-1980s,wereprogrammedusingproprietaryprogrammingpanelsorspecial-purposeprogrammingterminals,whichoftenhaddedicatedfunctionkeysrepresentingthevariouslogicalelementsofPLCprograms.Programswerestoredoncassettetapecartridges.Facilitiesforprintinganddocumentationwereveryminimalduetolackofmemorycapacity.TheveryoldestPLCsusednon-volatilemagneticcorememory.Morerecently,PLCsareprogrammedusingapplicationsoftwareonpersonalcomputers.ThecomputerisconnectedtothePLCthroughEthernet,RS-232,RS-485orRS-422cabling.Theprogrammingsoftwareallowsentryandeditingoftheladder-stylelogic.GenerallythesoftwareprovidesfunctionsfordebuggingandtroubleshootingthePLCsoftware,forexample,byhighlightingportionsofthelogictoshowcurrentstatusduringoperationorviasimulation.ThesoftwarewilluploadanddownloadthePLCprogram,forbackupandrestorationpurposes.Insomemodelsofprogrammablecontroller,theprogramistransferredfromapersonalcomputertothePLCthoughaprogrammingboardwhichwritestheprogramintoaremovablechipsuchasanEEPROMorEPROM.ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.Regardingthepracticalityofthesedesktopcomputerbasedlogiccontrollers,itisimportanttonotethattheyhavenotbeengenerallyacceptedinheavyindustrybecausethedesktopcomputersrunonlessstableoperatingsystemsthandoPLCs,andbecausethedesktopcomputerhardwareistypicallynotdesignedtothesamelevelsoftolerancetotemperature,humidity,vibration,andlongevityastheprocessorsusedinPLCs.Inadditiontothehardwarelimitationsofdesktopbasedlogic,operatingsystemssuchasWindowsdonotlendthemselvestodeterministiclogicexecution,withtheresultthatthelogicmaynotalwaysrespondtochangesinlogicstateorinputstatuswiththeextremeconsistencyintimingasisexpectedfromPLCs.Still,suchdesktoplogicapplicationsfinduseinlesscriticalsituations,suchaslaboratoryautomationanduseinsmallfacilitieswheretheapplicationislessdemandingandcritical,becausetheyaregenerallymuchlessexpensivethanPLCs.Inmorerecentyears,smallproductscalledPLRs(programmablelogicrelays),andalsobysimilarnames,havebecomemorecommonandaccepted.TheseareverymuchlikePLCs,andareusedinlightindustrywhereonlyafewpointsofI/O(i.e.afewsignalscominginfromtherealworldandafewgoingout)areinvolved,andlowcostisdesired.Thesesmalldevicesaretypicallymadeinacommonphysicalsizeandshapebyseveralmanufacturers,andbrandedbythemakersoflargerPLCstofillouttheirlowendproductrange.PopularnamesincludePICOController,NANOPLC,andothernamesimplyingverysmallcontrollers.Mostofthesehavebetween8and12digitalinputs,4and8digitaloutputs,andupto2analoginputs.Sizeisusuallyabout4"wide,3"high,and3"deep.MostsuchdevicesincludeatinypostagestampsizedLCDscreenforviewingsimplifiedladderlogic(onlyaverysmallportionoftheprogrambeingvisibleatagiventime)andstatusofI/Opoints,andtypicallythesescreensareaccompaniedbya4-wayrockerpush-buttonplusfourmoreseparatepush-buttons,similartothekeybuttonsonaVCRremotecontrol,andusedtonavigateandeditthelogic.MosthaveasmallplugforconnectingviaRS-232orRS-485toapersonalcomputersothatprogrammerscanusesimpleWindowsapplicationsforprogramminginsteadofbeingforcedtousethetinyLCDandpush-buttonsetforthispurpose.UnlikeregularPLCsthatareusuallymodularandgreatlyexpandable,thePLRsareusuallynotmodularorexpandable,buttheirpricecanbetwoordersofmagnitudelessthanaPLCandtheystillofferrobustdesignanddeterministicexecutionofthelogic.4.PLCTopicsThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditions(suchasdust,moisture,heat,cold)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsystems.Someusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelays,solenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.4.2SystemscaleAsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodelhasinsufficientI/O.ModularPLCshaveachassis(alsocalledarack)intowhichareplacedmoduleswithdifferentfunctions.TheprocessorandselectionofI/Omodulesiscustomisedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkisusedsothatrackscanbedistributedawayfromtheprocessor,reducingthewiringcostsforlargeplants.4.3UserinterfacePLCsmayneedtointeractwithpeopleforthepurposeofconfiguration,alarmreportingoreverydaycontrol.Asimplesystemmayusebuttonsandlightstointeractwiththeuser.Textdisplaysareavailableaswellasgraphicaltouchscreens.Morecomplexsystemsuseaprogrammingandmonitoringsoftwareinstalledonacomputer,withthePLCconnectedviaacommunicationinterface.PLCshavebuiltincommunicationsports,usually9-pinRS-232,butoptionallyEIA-485orEthernet.Modbus,BACnetorDF1isusuallyincludedasoneofthecommunicationsprotocols.OtheroptionsincludevariousfieldbusessuchasDeviceNetorProfibus.OthercommunicationsprotocolsthatmaybeusedarelistedintheListofautomationprotocols.MostmodernPLCscancommunicateoveranetworktosomeothersystem,suchasacomputerrunningaSCADA(SupervisoryControlAndDataAcquisition)systemorwebbrowser.PLCsusedinlargerI/Osystemsmayhavepeer-to-peer(P2P)communicationbetweenprocessors.Thisallowsseparatepartsofacomplexprocesstohaveindividualcontrolwhileallowingthesubsystemstoco-ordinateoverthecommunicationlink.ThesecommunicationlinksarealsooftenusedforHMIdevicessuchaskeypadsorPC-typeworkstations.PLCprogramsaretypicallywritteninaspecialapplicationonapersonalcomputer,thendownloadedbyadirect-connectioncableoroveranetworktothePLC.TheprogramisstoredinthePLCeitherinbattery-backed-upRAMorsomeothernon-volatileflashmemory.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.UndertheIEC61131-3standard,PLCscanbeprogrammedusingstandards-basedprogramminglanguages.AgraphicalprogrammingnotationcalledSequentialFunctionChartsisavailableoncertainprogrammablecontrollers.InitiallymostPLCsutilizedLadderLogicDiagramProgramming,amodelwhichemulatedelectromechanicalcontrolpaneldevices(suchasthecontactandcoilsofrelays)whichPLCsreplaced.Thismodelremainscommontoday.IEC61131-3currentlydefinesfiveprogramminglanguagesforprogrammablecontrolsystems:FBD(Functionblockdiagram),LD(Ladderdiagram),ST(Structuredtext,similartothePascalprogramminglanguage),IL(Instructionlist,similartoassemblylanguage)andSFC(Sequentialfunctionchart).Thesetechniquesemphasizelogicalorganizationofoperations.WhilethefundamentalconceptsofPLCprogrammingarecommontoallmanufacturers,differencesinI/Oaddressing,memoryorganizationandinstructionsetsmeanthatPLCprogramsareneverperfectlyinterchangeablebetweendifferentmakers.Evenwithinthesameproductlineofasinglemanufacturer,differentmodelsmaynotbedirectlycompatible.5.PLCcomparedwithothercontrolsystemsPLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation,andwherechangestothesystemwouldbeexpectedduringitsoperationallife.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols;littleelectricaldesignisrequired,andthedesignproblemcentersonexpressingthedesiredsequenceofoperations.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofa"generic"solution,andwherethenon-recurringengineeringchargesarespreadoverthousandsormillionsofunits.Forhighvolumeorverysimplefixedautomationtasks,differenttechniquesareused.Forexample,aconsumerdishwasherwouldbecontrolledbyanelectromechanicalcamtimercostingonlyafewdollarsinproductionquantities.Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersupplies,input/outputhardwareandnecessarytestingandcertification)canbespreadovermanysales,andwheretheend-userwouldnotneedtoalterthecontrol.Automotiveapplicationsareanexample;millionsofunitsarebuilteachyear,andveryfewend-usersaltertheprogrammingofthesecontrollers.However,somespecialtyvehiclessuchastransitbusseseconomicallyusePLCsinsteadofcustom-designedcontrols,becausethevolumesarelowandthedevelopmentcostwouldbeuneconomic.Verycomplexprocesscontrol,suchasusedinthechemicalindustry,mayrequirealgorithmsandperformancebeyondthecapabilityofevenhigh-performancePLCs.Veryhigh-speedorprecisioncontrolsmayalsorequirecustomizedsolutions;forexample,aircraftflightcontrols.Programmablecontrollersarewidelyusedinmotioncontrol,positioningcontrolandtorquecontrol.SomemanufacturersproducemotioncontrolunitstobeintegratedwithPLCsothatG-code(involvingaCNCmachine)canbeusedtoinstructmachinemovements.PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,a"proportional,integral,derivative"or"PIDcontroller".APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCS)wouldinsteadbeused.AsPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessdistinct.PLCshavesimilarfunctionalityasRemoteTerminalUnits.AnRTU,however,usuallydoesnotsupportcontrolalgorithmsorcontrolloops.Ashardwarerapidlybecomesmorepowerfulandcheaper,RTUs,PLCsandDCSsareincreasinglybeginningtooverlapinresponsibilities,andmanyvendorssellRTUswithPLC-likefeaturesandviceversa.TheindustryhasstandardizedontheIEC61131-3functionalblocklanguageforcreatingprogramstorunonRTUsandPLCs,althoughnearlyallvendorsalsoofferproprietaryalternativesandassociateddevelopmentenvironments.6.DigitalandanalogsignalsDigitalordiscretesignalsbehaveasbinaryswitches,yieldingsimplyanOnorOffsignal(1or0,TrueorFalse,respectively).Pushbuttons,limitswitches,andphotoelectricsensorsareexamplesofdevicesprovidingadiscretesignal.Discretesignalsaresentusingeithervoltageorcurrent,whereaspecificrangeisdesignatedasOnandanotherasOff.Forexample,aPLCmightuse24VDCI/O,withvaluesabove22VDCrepresentingOn,valuesbelow2VDCrepresentingOff,andintermediatevaluesundefined.Initially,PLCshadonlydiscreteI/O.Analogsignalsarelikevolumecontrols,witharangeofvaluesbetweenzeroandfull-scale.Thesearetypicallyinterpretedasintegervalues(counts)bythePLC,withvariousrangesofaccuracydependingonthedeviceandthenumberofbitsavailabletostorethedata.AsPLCstypicallyuse16-bitsignedbinaryprocessors,theintegervaluesarelimitedbetween-32,768and+32,767.Pressure,temperature,flow,andweightareoftenrepresentedbyanalogsignals.Analogsignalscanusevoltageorcurrentwithamagnitudeproportionaltothevalueoftheprocesssignal.Forexample,ananalog0-10Vinputor4-20mAwouldbeconvertedintoanintegervalueof0-32767.可編程邏輯控制器可編程邏輯控制器（PLC）或可編程序控制器是用于機電過程自動化的數字計算機，例如控制機械廠生產線、游樂設施或照明裝置?？删幊炭刂破髟谠S多工業和機器中使用。與通用的計算機不同的是，PLC是專為多個輸入和輸出管理，擴展溫度范圍、不受電磁噪音影響、抗震動和沖擊所設計。控制器的操作程序通常存儲在電池供電或非易失性的內存中。PLC是實時的系統，因為系統產生的輸出結果必須在有限的時間內回饋到輸入，否則會導致錯誤操作。1.歷史PLC發明是針對于美國汽車制造行業的需要?？删幊踢壿嬁刂破髯畛跬ㄟ^了在軟件版本更換硬連線的控制板生產模式更改時的汽車工業。在PLC之前，控制、程序化和安全聯鎖邏輯制造汽車是使用上百或上千的繼電器、凸輪計時器、鼓定序儀和專用的閉環控制器來完成的。在每年更新模型等設施轉變過程是非常耗時并且成本高昂的，這是因為電工需要單獨地再接電線給每個中轉。在1968年GMHydramatic（自動輸電分局）發布通用汽車公司的提議，電子替代布線中繼系統。獲獎的提案來自貝得福得，馬薩諸塞的貝得福得同事。第一個PLC選定084，因為它是貝得福得同事的第八十四個項目。貝得福得同事建立了一家新的公司致力開發、生產、銷售，和服務這一新產品：Modicon，代表模塊化數字控制器。迪克·莫利，被認為是PLC之父，他是從事該項目的人之一。1977年古爾德電子公司當前所有者收購法國施耐德電氣公司同德國公司AEG并售予該品牌為Modicon。084模型之一首次被設在北部安多弗的Modicon總部馬薩諸塞州。這是專門為通用汽車服務的，并且經過了近二十多年的不間斷服務。直至984出現，Modicon使用的84名字才在其產品范圍中結束。汽車工業仍是PLC的最大用戶之一。早期的可編程控制器是設計來取代繼電器邏輯系統。這些可編程控制器的“階梯邏輯”是與繼電器邏輯示意圖非常類似的。選擇此程序表示法的目的是為了減少對現有技術人員的培訓需求。其他早期的可編程控制器使用指令列表編程，基于一個堆棧編程邏輯求解器進行求解?，F代可編程控制器在各種各樣的方式可以被編程，從梯形邏輯語言到更加傳統的編程語言例如BASIC和C語言。另一個方法是狀態邏輯，被設計的一種非常高級編程語言根據狀態轉換圖的可編程控制器編程。很多早期可編程控制器沒有可編程終端的邏輯圖形表示法，邏輯反而是被描繪成一系列在一些版本的布爾格式的邏輯表達式，類似于布爾代數。隨著編程碼發展，由于上述原因它變成更常見的梯形邏輯語言。更新的格式如國家邏輯和功能塊（這是類似的邏輯描述使用數字邏輯集成電路時的方式）的存在，但它們仍沒有梯形邏輯語言流行。一個主要原因是可編程控制器解決問題用一個可預測和重復的序列的邏輯，并且梯形邏輯語言可以用其他格式讓程序員（寫邏輯）的人看到邏輯的時間，所有問題更加容易地程序化。編程早期的PLC，到80年代中期，都是用專有的編程版或專用編程終端，往往有專門的功能鍵，代表各種PLC程序邏輯元件。程序存儲在盒式磁帶盒上。由于缺少的內存容量很少用于打印設備。最古老的可編程控制器使用的是非易失性磁核心內存。最期PLC在個人計算機上使用應用軟件編程。計算機連接到PLC通過以太網RS-232,RS-485或RS-422纜線連接。編程軟件允許輸入梯式邏輯編程。通常，軟件提供了用于調試和故障排除的功能，例如在操作過程中或通過仿真的邏輯部分PLC軟件突出顯示當前狀態。該軟件將上傳和下載PLC程序以便備份和恢復。在某些型號的PLC中雖然程序寫入一個可移動的芯片，如EEPROM或EPROM，但該方案還是得從個人電腦傳輸到PLC編程版。PLC的功能經過多年的發展，包括連續的繼電器控制，運動控制，過程控制，分布式控制系統和網絡。一些現代PLC的數據處理，存儲，處理能力和通信能力相當于臺式電腦。PLC編程結合遠程I/O硬件，一臺通用臺式計算機允許在某些應用中重疊使用某一可編程控制器。在重工業中PLC被認為沒有這些桌面計算機為主的邏輯控制器的實際性強，因為PLC在臺式計算機系統中運行不是很穩定，并且，因為臺式計算機硬件沒有被設計成耐溫度、濕氣、振動和耐用作為可編程控制器的處理器。除桌面基于邏輯的硬件局限之外，例如Windows操作系統不適合自己的確定性邏輯的執行，結果是PLC邏輯不可能總是對規定邏輯變化的輸入狀態與極端性預計的時間一致。盡管如此，這樣桌面邏輯被應用在較不重要情況，像實驗室自動化和小型設施中使用該應用程序的要求不高，因為他們的價格一般都遠遠低于昂貴的PLC。在最近數年，小產品稱為PLR（可編程邏輯繼電器），并且因為名字相似，變得更常見并被接受。這些很像PLC已經應用于輕工業，它只有少部分的輸入/輸出（例如一些真實的輸入輸出信號）參與，低成本，很理想。這些小設備尺寸和形狀比較普通地幾位制造商制作，并且由更大的PLC制作商來填滿他們低端產品規格。俗名包括PICO控制器、納米PLC和其他的小控制器。多數這些控制器有在8到12數字輸入、4到8數字輸出，多達2個模擬輸入。尺寸通常是4英寸寬、3英寸高、3英寸深。大多數這樣的設備有一個小郵票大小的液晶屏幕來觀看簡化梯子邏輯的輸入/輸出點（只有一小部分程序被可見于給定的時間）和狀況，并且這些屏幕由一個電磁四通搖臂按鈕操縱加上四個不同的用于瀏覽和編輯的邏輯電鈕，類似于錄像機遙控按鈕?？刂破鞔蠖鄶涤幸粋€小插座為通過