可编程序逻辑控制器外文文献翻译大学论文.docx
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可编程序逻辑控制器外文文献翻译大学论文
Programmable logic controller
From Wikipedia, the free encyclopedia
PLC & input/output arrangements
Aprogrammablelogiccontroller(PLC)orprogrammablecontrollerisadigitalcomputerusedforautomationofelectromechanicalprocesses,suchascontrolofmachineryonfactoryassemblylines,controlofamusementrides,orcontroloflightingfixtures.PLCsareusedinmanydifferentindustriesandmachinessuchaspackagingandsemiconductormachines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.
Features
ControlpanelwithPLC(greyelementsinthecenter).Theunitconsistsofseparateelements,fromlefttoright;powersupply,controller,relayunitsforin-andoutput.
ThemaindifferencefromothercomputersisthatPLCsarearmoredforsevereconditions(dust,moisture,heat,cold,etc)andhavethefacilityforextensiveinput/output(I/O)arrangements.TheseconnectthePLCtosensorsandactuators.PLCsreadlimitswitches,analogprocessvariables(suchastemperatureandpressure),andthepositionsofcomplexpositioningsystems.Someevenusemachinevision.Ontheactuatorside,PLCsoperateelectricmotors,pneumaticorhydrauliccylinders,magneticrelaysorsolenoids,oranalogoutputs.Theinput/outputarrangementsmaybebuiltintoasimplePLC,orthePLCmayhaveexternalI/OmodulesattachedtoacomputernetworkthatplugsintothePLC.
Systemscale
AsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodeldoesnothaveenoughI/O.
ModularPLCshaveachassis(alsocalledarack)intowhichareplacedmoduleswithdifferentfunctions.TheprocessorandselectionofI/Omodulesiscustomizedfortheparticularapplication.Severalrackscanbeadministeredbyasingleprocessor,andmayhavethousandsofinputsandoutputs.AspecialhighspeedserialI/Olinkisusedsothatrackscanbedistributedawayfromtheprocessor,reducingthewiringcostsforlargeplants.
Userinterface
Seealso:
Listofhuman-computerinteractiontopics
PLCsmayneedtointeractwithpeopleforthepurposeofconfiguration,alarmreportingoreverydaycontrol.
AHuman-MachineInterface(HMI)isemployedforthispurpose.HMIsarealsoreferredtoasMMIs(ManMachineInterface)andGUI(GraphicalUserInterface).
Asimplesystemmayusebuttonsandlightstointeractwiththeuser.Textdisplaysareavailableaswellasgraphicaltouchscreens.Morecomplexsystemsuseaprogrammingandmonitoringsoftwareinstalledonacomputer,withthePLCconnectedviaacommunicationinterface.
Communications
PLCshavebuiltincommunicationsportsusually9-PinRS232,andoptionallyforRS485andEthernet.ModbusorDF1isusuallyincludedasoneofthecommunicationsprotocols.Others'optionsincludevariousfieldbusessuchasdevicenetorprofibus.OthercommunicationsprotocolsthatmaybeusedarelistedintheListofautomationprotocols.
MostmodernPLCscancommunicateoveranetworktosomeothersystem,suchasacomputerrunningaSCADA(SupervisoryControlAndDataAcquisition)systemorwebbrowser.
PLCsusedinlargerI/Osystemsmayhavepeer-to-peer(P2P)communicationbetweenprocessors.Thisallowsseparatepartsofacomplexprocesstohaveindividualcontrolwhileallowingthesubsystemstoco-ordinateoverthecommunicationlink.ThesecommunicationlinksarealsooftenusedforHMI(Human-MachineInterface)devicessuchaskeypadsorPC-typeworkstations.Someoftoday'sPLCscancommunicateoverawiderangeofmediaincludingRS-485,Coaxial,andevenEthernetforI/Ocontrolatnetworkspeedsupto100Mbit/s.
PLCcomparedwithothercontrolsystems
PLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation,andwherechangestothesystemwouldbeexpectedduringitsoperationallife.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols;littleelectricaldesignisrequired,andthedesignproblemcentersonexpressingthedesiredsequenceofoperationsinladderlogic(orfunctionchart)notation.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofa"generic"solution,andwherethenon-recurringengineeringchargesarespreadoverthousandsormillionsofunits.
Forhighvolumeorverysimplefixedautomationtasks,differenttechniquesareused.Forexample,aconsumerdishwasherwouldbecontrolledbyanelectromechanicalcamtimercostingonlyafewdollarsinproductionquantities.
Amicrocontroller-baseddesignwouldbeappropriatewherehundredsorthousandsofunitswillbeproducedandsothedevelopmentcost(designofpowersuppliesandinput/outputhardware)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.[1]
PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,a"proportional,integral,derivative"or"PIDcontroller."APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem(DCS)wouldinsteadbeused.However,asPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessclear-cut.
PLCshavesimilarfunctionalityasRemoteTerminalUnits.AnRTU,however,usuallydoesnotsupportcontrolalgorithmsorcontrolloops.Ashardwarerapidlybecomesmorepowerfulandcheaper,RTUs,PLCsandDCSsareincreasinglybeginningtooverlapinresponsibilities,andmanyvendorssellRTUswithPLC-likefeaturesandviceversa.TheindustryhasstandardizedontheIEC61131-3functionalblocklanguageforcreatingprogramstorunonRTUsandPLCs,althoughnearlyallvendorsalsoofferproprietaryalternativesandassociateddevelopmentenvironments.Digitalandanalogsignals
Digitalordiscretesignalsbehaveasbinaryswitches,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,ananalog4-20mAor0-10Vinputwouldbeconvertedintoanintegervalueof0-32767.
Currentinputsarelesssensitivetoelectricalnoise(i.e.fromweldersorelectricmotorstarts)thanvoltageinputs.
Example
Asanexample,sayafacilityneedstostorewaterinatank.Thewaterisdrawnfromthetankbyanothersystem,asneeded,andourexamplesystemmustmanagethewaterlevelinthetank.
Usingonlydigitalsignals,thePLChastwodigitalinputsfromfloatswitches(LowLevelandHighLevel).Whenthewaterlevelisabovetheswitchitclosesacontactandpassesasignaltoaninput.ThePLCusesadigitaloutputtoopenandclosetheinletvalveintothetank.WhenthewaterleveldropsenoughsothattheLowLevelfloatswitchisoff(down),thePLCwillopenthevalvetoletmorewaterin.OncethewaterlevelraisesenoughsothattheHighLevelswitchison(up),thePLCwillshuttheinlettostopthewaterfromoverflowing.Thisrungisanexampleofsealinlogic.Theoutputissealedinuntilsomeconditionbreaksthecircuit.
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