可编程序逻辑控制器外文文献翻译大学论文Word文档下载推荐.docx

可编程序逻辑控制器外文文献翻译大学论文Word文档下载推荐.docx

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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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