毕业设计英文翻译Programmablelogic.docx

毕业设计英文翻译Programmablelogic.docx

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毕业设计英文翻译Programmablelogic

Programmablelogiccontroller

CynthiaCooper

FromWikipedia,thefreeencyclopedia

Aprogrammablelogiccontrollerorsimplyprogrammablecontrollerisadigitalcomputerusedforautomationofindustrialprocesses,suchascontrolofmachineryonfactoryassemblylines.Unlikegeneral-purposecomputers,thePLCisdesignedformultipleinputsandoutputarrangements,extendedtemperatureranges,immunitytoelectricalnoise,andresistancetovibrationandimpact.Programstocontrolmachineoperationaretypicallystoredinbattery-backedornon-volatilememory.APLCisanexampleofarealtimesystemsinceoutputresultsmustbeproducedinresponsetoinputconditionswithinaboundedtime,otherwiseunintendedoperationwillresult.

Features

ControlpanelwithPLC（greyelementsinthecenter）.Theunitconsistsofseparateelements,fromlefttoright;powersupply,controller,relayunitsforinputandoutput.

ThemaindifferencefromothercomputersisthatPLCsarearmoredforseverecondition（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.

PLCswereinventedasreplacementsforautomatedsystemsthatwouldusehundredsorthousandsofrelays,camtimers,anddrumsequencers.Often,asinglePLCcanbeprogrammedtoreplacethousandsofrelays.Programmablecontrollerswereinitiallyadoptedbytheautomotivemanufacturingindustry,wheresoftwarerevisionreplacedthere-wiringofhard-wiredcontrolpanelswhenproductionmodelschanged.

ManyoftheearliestPLCsexpressedalldecisionmakinglogicinsimpleladderlogicwhichappearedsimilartoelectricalschematicdiagrams.Theelectricianswerequiteabletotraceoutcircuitproblemswithschematicdiagramsusingladderlogic.Thisprogramnotationwaschosentoreducetrainingdemandsfortheexistingtechnicians.OtherearlyPLCsusedaformofinstructionlistprogramming,basedonastack-basedlogicsolver.

ThefunctionalityofthePLChasevolvedovertheyearstoincludesequentialrelaycontrol,motioncontrol,processcontrol,distributedcontrolsystemsandnetworking.Thedatahandling,storage,processingpowerandcommunicationcapabilitiesofsomemodernPLCsareapproximatelyequivalenttodesktopcomputers.PLC-likeprogrammingcombinedwithremoteI/Ohardware,allowageneral-purposedesktopcomputertooverlapsomePLCsincertainapplications.

UndertheIEC61131-3standard,PLCscanbeprogrammedusingstandards-basedprogramminglanguages.AgraphicalprogrammingnotationcalledSequentialFunctionChartsisavailableoncertainprogrammablecontrollers.

PLCcomparedwithothercontrolsystems

PLCsarewell-adaptedtoarangeofautomationtasks.Thesearetypicallyindustrialprocessesinmanufacturingwherethecostofdevelopingandmaintainingtheautomationsystemishighrelativetothetotalcostoftheautomation.PLCscontaininputandoutputdevicescompatiblewithindustrialpilotdevicesandcontrols.PLCapplicationsaretypicallyhighlycustomizedsystemssothecostofapackagedPLCislowcomparedtothecostofaspecificcustom-builtcontrollerdesign.Ontheotherhand,inthecaseofmass-producedgoods,customizedcontrolsystemsareeconomicduetothelowercostofthecomponents,whichcanbeoptimallychoseninsteadofa"generic"solution。

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.

PLCsmayincludelogicforsingle-variablefeedbackanalogcontrolloop,a"proportional,integral,derivative"or"PIDcontroller."APIDloopcouldbeusedtocontrolthetemperatureofamanufacturingprocess,forexample.HistoricallyPLCswereusuallyconfiguredwithonlyafewanalogcontrolloops;whereprocessesrequiredhundredsorthousandsofloops,adistributedcontrolsystem（DCS）wouldinsteadbeused.However,asPLCshavebecomemorepowerful,theboundarybetweenDCSandPLCapplicationshasbecomelessclear-cut.

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,ananalog4-20mAor0-10 Vinputwouldbeconvertedintoanintegervalueof0-32767.

Example

Asanexample,sayafacilityneedstostorewaterinatank.Thewaterisdrawnfromthetankbyanothersystem,asneeded,andourexamplesystemmustmanagethewaterlevelinthetank.

Usingonlydigitalsignals,thePLChastwodigitalinputsfromfloatswitches（tankemptyandtankfull）.ThePLCusesadigitaloutputtoopenandclosetheinletvalveintothetank.

Whenthewaterleveldropsenoughsothatthetankemptyfloatswitchisoff（down）,thePLCwillopenthevalvetoletmorewaterin.Oncethewaterlevelraisesenoughsothatthetankfullswitchison（up）,thePLCwillshuttheinlettostopthewaterfromoverflowing.

Ananalogsystemmightuseawaterpressuresensororaloadcell,andanadjustable（throttling）drippingoutofthetank,thevalveadjuststoslowlydripwaterbackintothetank.

Inthissystem,toavoid'flutter'adjustmentsthatcanwearoutthevalve,manyPLCsincorporate"hysteresis"whichessentiallycreatesa"deadband"ofactivity.Atechnicianadjuststhisdeadbandsothevalvemovesonlyforasignificantchangeinrate.Thiswillinturnminimizethemotionofthevalve,andreduceitswear.

Arealsystemmightcombinebothapproaches,usingfloatswitchesandsimplevalvestopreventspills,andaratesensorandratevalvetooptimizerefillratesandpreventwaterhammer.Backupandmaintenancemethodscanmakearealsystemverycomplicated.

Systemscale

AsmallPLCwillhaveafixednumberofconnectionsbuiltinforinputsandoutputs.Typically,expansionsareavailableifthebasemodeldoesnothaveenoughI/O.

PLCsusedinlargerI/Osystemsmayhavepeer-to-peer（P2P）communicationbetweenprocessors.Thisallowsseparatepartsofacomplexprocesstohaveindividualcontrolwhileallowingthesubsystemstoco-ordinateoverthecommunicationlink.ThesecommunicationlinksarealsooftenusedforHMI（Human-MachineInterface）devicessuchaskeypadsorPC-typeworkstations.Someoftoday'sPLCscancommunicateoverawiderangeofmediaincludingRS-485,Coaxial,andevenEthernetforI/Ocontrolatnetworkspeedsupto100Mbit/s.

Programming

EarlyPLCs,uptothemid-1980s,wereprogrammedusingproprietaryprogrammingpanelsorspecial-purposeprogrammingterminals.theyweredesignedtoreplacerelaylogicsystems.ThesePLCswereprogrammedin"ladderlogic",whichstronglyresemblesaschematicdiagramofrelaylogic.ModernPLCscanbeprogrammedinavarietyofways,fromladderlogictomoretraditionalprogramminglanguagessuchasBASICandC.AnothermethodisStateLogic,aVeryHighLevelProgrammingLanguagedesignedtoprogramPLCsbasedonStateTransitionDiagrams.

Recently,theInternationalstandardIEC61131-3hasbecomepopular.IEC61131-3currentlydefinesfiveprogramminglanguagesforprogrammablecontrolsystems:

FBD（Functionblockdiagram）,LD（Ladderdiagram）,ST（Structuredtext,similartothePascalprogramminglanguage）,IL（Instructionlist,similartoassemblylanguage）andSFC（Sequentialfunctionchart）.Thesetechniquesemphasizelogicalorganizationofoperations.

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