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1、 中国地质大学长城学院本科毕业论文文献综述学 院工程技术学院学生姓名陈瑀专 业电气工程及其自动化学 号0431301442017 年 4 月 20 日第12页 共13页外文资料PLCtechniquediscussionandfuturedevelopmentWrite by T. J. byersFrom: Electronic Test Equipment-principles and Applications. Princeton University. America.Alongwiththedevelopmentoftheages,thetechniquethatisnowadays
2、isalsograduallyperfect,thecompetitionplaysmorestrong;theoperationthatlistdependstheartificialhasalreadycantsatisfiedwiththecurrentmanufacturingindustryforeground,alsocantguaranteetherequestofthehigherquantityandhighnewtheimageofthetechniquebusinessenterprise.Thepeopleseeinproducepractice,automatebro
3、ughtthetremendousconvenienceandtheproductquantitiesforpeopleupofassurance,alsoeasedthepersonnelslaborstrength,reducetheestablishmentonthepersonnel.Thetargetcontrolofthehardrealizationinmanycomplicatedproductionlines,wholeandexcellentturn,thebestdecisionetc.,well-trainedoperationwork,technicalpersonn
4、elorexpert,governorbutcanjudgeandoperateeasily,canacquirethesatisfiedresult.Theresearchtargetoftheartificialintelligencemakesuseofthecalculatorexactlytocarryout,imitatetheseintelligencesbehavior,moderatingtheworkthroughpersonsbrainandcalculators,withthemodethatpersonsmachinecombine,forresolvethevery
5、complicatedproblemtolookforthebestpathWe come in sight of the control that links after the electric appliances in various situation, that is already the that time generation past, now of after use in the mold a perhaps simple equipments of grass-roots control that the electric appliances can do for
6、the low level only; And the PLC emergence also became the epoch-making topic, adding the vivid software control through a very and stable hardware, making the automation head for the new high tide. The PLC biggest characteristics lie in: The electrical engineering teacher already no longer electric
7、hardware up too many calculations of cost, as long as order the importation that the button switch or the importation of the sensors order to link the PLC up can solve problem, pass to output to order the conjunction contact machine or control the start equipments of the big power after the electric
8、 appliances, but the exportation equipments direct conjunction of the small power can.PLCinternalcontainmenthavetheCPUoftheCPU,andtaketohaveanI/Oforexpandofexteriortoconnectapeoplesaddressandsavingmachinethreebigpiecestoconstitute,CPUcoreisfromanormanyistiredtoaddthemachinetoconstitute,mathematicsth
9、attheyhavethelogicoperationability,andcanreadtheproceduresavethecontentsofthemachinetodrivethehomologoussavingmachineandI/Ostoconnectafterpassthecalculation;TheI/Oaddinnerpartistiredtheinputandoutputsystemofthemachineandexteriorlink,anddeposittherelateddataintotheproceduresavingmachineordatasavingma
10、chine;ThesavingmachinecandepositthedatathattheI/Oinputinthesavingmachine,andinworkadjustingto become tired to add the machine and I/ Os to connect, saving machine separately saving machine RAM of the procedure saving machine ROM and dates, the ROM can do deposit of the data permanence in the saving
11、machine, but RAM only for the CPU computes the temporary calculation usage of hour of buffer space.ThePLCanti-interferenceisveryandexcellent,ourrootneednotconcernitsservicelifeandtheworksituationbad,theseallproblemshavealreadynolongerbecomethetopicthatwefail,butstaytoourisaconcerntocometointernalres
12、ourcesofmakeuseofthePLCtostrengthenthecontrolabilityoftheequipmentsforus,makeourequipmentsmoregentle.PLClanguageisnotweimagineofeditcollectedmaterialsthelanguageorlanguageofCstocarryonweavingthedistance,butthetrapezoiddiagramthattheadoptionisoriginalaftertheelectricappliancestocontrol,maketheelectri
13、calengineeringteacherwhileweavingtowritetheprocedureveryeasycomprehendedthePLClanguage,andalotofnon-electricityprofessionalalsoveryquicklyknowandgodeepintotothePLC.TounderstandthePLCscommunicationsversatility,letsfirstdefinethetermsusedindescribingthevarioussystems.ASCII:ThisstandsforAmericanStandar
14、dCodeforInformationInterchange.AsshowninFig.1,whentheletterAistransmitted,forinstance,itsautomaticallycodedas65bythesendingequipment.Thereceivingequipmenttranslatesthe65backtotheletterA.Thus,differentdevicescancommunicatewitheachotheraslongasbothuseASCIIcode.ASCIImodule:ThisintelligentPLCmoduleisuse
15、dforconnectingPLCstootherdevicesalsocapableofcommunicatingusingASCIIcodeasavehicle.Bustopology:Thisisalinearlocalareanetwork(LAN)arrangement,asshowninFig.2A,inwhichindividualnodesaretappedintoamaincommunicationscableatasinglepointandbroadcastmessages.Thesemessagestravelinbothdirectionsonthebusfromth
16、epointofconnectionuntiltheyaredissipatedbyterminatorsateachendofthebus.CPU:Thisstandsforcentralprocessingunit,whichactuallyisthatpartofacomputer,PLC,orotherintelligentdevicewherearithmeticandlogicaloperationsareperformedandinstructionsaredecodedandexecuted.Daisychain:Thisisadescriptionoftheconnectio
17、nofindividualdevicesinaPLCnetwork,where,asshowninFig.3,eachdeviceisconnectedtothenextandcommunicationssignalspassfromoneunittothenextinasequentialfashion.Distributedcontrol:Thisisanautomationconceptinwhichportionsofanautomatedsystemarecontrolledbyseparatecontrollers,whicharelocatedincloseproximityto
18、theirareaofdirectcontrol(controlisdecentralizedandspreadout over the system).Hostcomputer:Thisisacomputerthatsusedtotransferdatato,orreceivedatafrom,aPLCinaPLC/computernetwork.Intelligentdevice:ThistermdescribesanydeviceequippedwithitsownCPU.I/O:Thisstandsforinputsandoutputs,whicharemodulesthathandl
19、edatatothePLC(inputs)orsignalsfromthePLC(outputs)toanexternaldevice.Kbps:Thisstandsforthousandbitspersecond,whichisarateofmeasureforelectronicdatatransfer.Mbps:Thisstandsformillionbitspersecond.Node:Thistermisappliedtoanyoneofthepositionsorstationsinanetwork.Eachnodeincorporatesadevicethatcancommuni
20、catewithallotherdevicesonthenetwork.Protocol:Thedefinitionofhowdataisarrangedandcodedfortransmissiononanetwork.Ringtopology.ThisisaLANarrangement,asshowninFig.2C,inwhicheachnodeisconnectedtotwoothernodes,resultinginacontinuous,closed,circularpathorloopformessagestocirculate,usuallyinonedirection.Som
21、eringtopologieshaveaspecialloopbackfeaturethatallowsthemtocontinuefunctioningevenifthemaincableissevered.RS232.ThisisanIEEEstandardforserialcommunicationsthatdescribesspecificwiringconnections,voltagelevels,andotheroperatingparametersforelectronicdatacommunications.TherealsoareseveralotherRSstandard
22、sdefined.Serial:Thisisanelectronicdatatransferschemeinwhichinformationistransmittedonebitatatime.Serialport:Thisthecommunicationsaccesspointonadevicethatissetupforserialcommunications.Startopology.ThisisaLANarrangementinwhich,asshowninFig.2B,nodesareconnectedtooneanotherthroughacentralhub,whichcanbe
23、activeorpassive.Anactivehubperformsnetworkdutiessuchasmessageroutingandmaintenance.Apassivecentralhubsimplypassesthemessagealongtoallthenodesconnectedtoit.Topology:ThisrelatestoaspecificarrangementofnodesinaLANinrelationtooneanother.Transparent:Thistermdescribesautomaticeventsorprocessesbuiltintoasy
24、stemthatrequirenospecialprogrammingorpromptingfromanoperator.Nowthatwerefamiliarwiththeseterms,letsseehowtheyareusedindescribingtheavailablePLCnetworkoptions.PLCnetworkoptionsPLCnetworksprovideyouwithavarietyofnetworkingoptionstomeetspecificcontrolandcommunicationsrequirements.Typicaloptionsincluder
25、emoteI/O,peer-to-peer,andhostcomputercommunications,aswellasLANs.Thesenetworkscanprovidereliableandcost-effectivecommunicationsbetweenasfewastwoorasmanyasseveralhundredPLCs,computers,andotherintelligentdevices.ManyPLCvendorsofferproprietarynetworkingsystemsthatareuniqueandwillnotcommunicatewithanoth
26、ermakeofPLC.Thisisbecauseofthedifferentcommunicationsprotocols,commandsequences,error-checkingschemes,andcommunicationsmediausedbyeachmanufacturer.However,itispossibletomakedifferentPLCstalktooneanother;whatsrequiredisanASCIIinterfacefortheconnection(s),alongwithconsiderableworkwithsoftware.RemoteI/
27、0systemsAremoteI/Oconfiguration,asshowninFig.4A,hastheactualinputsandoutputsatsomedistancefromthecontrollerandCPU.Thistypeofsystem,whichcanbedescribedasamaster-and-slaveconfiguration,allowsmanydistantdigitalandanalogpointstobecontrolledbyasinglePLC.Typically,remoteI/OsareconnectedtotheCPUviatwistedp
28、airorfiberopticcables.RemoteI/Oconfigurationscanbeextremelycost-effectivecontrolsolutionswhereonlyafewI/Opointsareneededinwidelyseparatedareas.Inthissituation,itsnotalwaysnecessary,orpracticalforthatmatter,tohaveacontrollerateachsite.NorisitpracticaltoindividuallyhardwireeachI/Opointoverlongdistance
29、sbacktotheCPU.Forexample,remoteI/Osystemscanbeusedinacquiringdatafromremoteplantorfacilitylocations.Informationsuchascycletimes,counts,durationorevents,etc.thencanbesentbacktothePLCformaintenanceandmanagementreporting.InaremoteI/Oconfiguration,themastercontrollerpollstheslavedI/OforitscurrentI/Ostat
30、us.TheremoteI/Osystemresponds,andthemasterPLCthensignalstheremoteI/OtochangethestateofoutputsasdictatedbythecontrolprograminthePLCsmemory.Thisentirecycleoccurshundredsoftimespersecond.Peer-to-peernetworksPeer-to-peernetworks,asshowninFig.4B,enhancereliabilitybydecentralizingthecontrolfunctionswithou
31、tsacrificingcoordinatedcontrol.Inthistypeofnetwork,numerousPLCsareconnectedtooneanotherinadaisy-chainfashion,andacommonmemorytableisduplicatedinthememoryofeach.Inthisway,whenanyPLCwritesdatatothismemoryarea,theinformationisautomaticallytransferredtoallotherPLCsinthenetwork.Theythencanusethisinformat
32、ionintheirownoperatingprograms.necessarysoftwareinterface.Theeasiestwaytoprovidesuchaninterfacetofixed-formatintelligentdevicesistouseanASCII/BASICmoduleonthePLC.ThismoduleisessentiallyasmallcomputerthatplugsintothebusofthePLC.EquippedwithRS232portsandprogrammedinBASIC,themoduleeasilycanhandleASCIIc
33、ommunicationswithperipheraldevices,dataacquisitionfunctions,programmingsequences,numbercrunching,reportanddisplaygeneration,andotherrequirements.Access,protocol,andmodulationfunctionsofLANsByusingstandardinterfacesandprotocols,LANsallowamixofdevices(PLCs,PCs,mainframecomputers,operatorinterfacetermi
34、nals,etc.)frommanydifferentvendorstocommunicatewithothersonthenetwork.Access:ALANsaccessmethodpreventstheoccurrenceofmorethanonemessageonthenetworkatatime.Therearetwocommonaccessmethods.Collisiondetectioniswherethenodeslistentothenetworkandtransmitonlyiftherearenoothermessagesonthenetwork.Iftwonodes
35、transmitsimultaneously,thecollisionisdetectedandbothnodesretransmituntiltheirmessagesgetthroughproperly.Tokenpassingallowseachnodetotransmitonlyifitsinpossessionofaspecialelectronicmessagecalledatoken.Thetokenispassedfromnodetonode,allowingeachanopportunitytotransmitwithoutinterference.Tokensusually
36、haveatimelimittopreventasinglenodefromtyingupthetokenforalongperiodoftime.Protocol:NetworkprotocolsdefinethewaymessagesarearrangedandcodedfortransmissionontheLAN.Thefollowingaretwocommontypes.Proprietaryprotocolsareuniquemessagearrangementsandcodingdevelopedbyaspecificvendorforusewiththatvendorsprod
37、uctonly.OpenprotocolsarebasedonindustrystandardssuchasTCP/IPorISO/OSImodelsandareopenlypublished.Modulation:Networkmodulationreferstothewaymessagesareencodedfortransmissionoveracable.Thetwomostcommontypesarebroadbandandbaseband.NetworktransmissioninterfacesThevastmajorityofPLCcommunicationsisdonevia
38、RS232Candtwistedpaircables.MostPLCshaveanRS232portandarecapableofhandlingcommunicationswithhostcomputers,printers,terminals,andotherdevices.Maximumtransmissionspeedis19.2Kbps.Thedistanceanddatatransmissionratesarestandardsforthevariousinterfaces.Theiractualperformanceisafunctionofthedrivingdevicesan
39、dvariessignificantlybetweenmanufacturers.Assuch,youshouldconsultthemanufacturersspecificationsforactualdistanceanddatatransmissionratecapabilities.TheonlyreallimitationonRS232Cisthe50-ftrecommendeddistancebetweendevices.WhileRS232Cinstallationsoftencanachievecablingdistancesgreaterthanthis,theunbala
40、nceddesignoftheinterfaceresultsinagreatersusceptibilitytosurroundingelectricalnoiseandreduceddataintegrity.Thisisparticularlytruewhereelectromagneticinterference(EMI)andradio-frequencyinterference(RFI)areknowntoexist.Whenlongertransmissiondistancesareneeded,RS422isabetterchoice.UnliketheRS232Cinterf
41、ace,RS422isbalanced.Eachofitsprimarysignalsconsistsoftwowiresthatarealwaysatoppositelogiclevels,withrespecttosignalground.Asaresult,theinterfacecanachievelongertransmissiondistance(4000ft)andhigherdatatransmissionrates(upto90Kbps).Inshorterruns(lessthan50ft),datatransfercanreach10Mbps.Fiberopticcomm
42、unicationsaregaininggreateracceptanceandarebeingusedinmoreandmoreinstallations.Fiberopticcableisvirtuallyimpervioustoharshenvironmentalconditionsandelectricalnoise.Also,theselinkscanspanextremelylongdistancesandtransmitdataatveryhighspeeds.Forexample,insomeLANsystems,theselinkscantransmitatrelativelyhighspeedsandspanlongdistancesbefore