精品中英文外文翻译--PLC和微处理器-定.doc

精品中英文外文翻译--PLC和微处理器-定.doc

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IntroductionsofPLCandMCU

APLCisadevicethatwasinventedtoreplacethenecessarysequentialrelaycircuitsformachinecontrol.ThePLCworksbylookingatitsinputsanddependingupontheirstate,turningon/offitsoutputs.Theuserentersaprogram,usuallyviasoftwareorprogrammerthatgivesthedesiredresults.

PLCareusedinmany“realworld”applications.Ifthereisindustrypresent,chancesaregoodthatthereisaPLCpresent.Ifyouareinvolvedinmachining,packaging,materialhandling,automatedassemblyorcountlessotherindustries,youareprobablyalreadyusingthem.Ifyouarenot,youarewastingmoneyandtime.AlmostanyapplicationthatneedssometypeofelectricalcontrolhasneedforPLC.

Forexample,let’sassumethatwhenaswitchturnsonwewanttoturnasolenoidonfor5secondsandthenturnitoffregardlessofhowlongtheswitchisonfor.Wecandothiswithasimpleexternaltimer.Whatiftheprocessalsoneededtocounthowmanytimestheswitchindividuallyturnedon?

Weneedalotofexternalcounters.

Asyoucansee,thebiggertheprocessthemoreofaneedwehaveforaPLC.WecansimplyprogramthePLCtocountitsinputsandturnthesolenoidsonforthespecifiedtime.

Wewilltakealookatwhatisconsideredtobethe“top20”PLCinstructions.Itcanbesafelyestimatedthatwithafirmunderstandingofthereinstructionsonecansolvemorethan80%oftheapplicationsinexistence.

That‘sright,morethan80%!

Ofcoursewe’lllearnmorethanjusttheseinstructionstohelpyousolvealmostALLyourpotentialPLCapplications.

ThePLCmainlyconsistsofaCPU,memoryareas,andappropriatecircuitstoreceiveinput/outputdata,asshowninFig.19.1WecanactuallyconsiderthePLCtobeaboxfullofhundredsorthousandsofseparaterelays,counters,timeranddatestoragelocations.Dothesecounters,timers,etc.reallyexist?

No,theydon’t“physically”existbutrathertheyaresimulatedandcanbeconsideredsoftwarecounters,timers,etc.Theseinternalrelaysaresimulatedthroughbitlocationsinregisters.

Whatdoeseachpartdo?

INPUTRELAYS-（contacts）Theseareconnectedtotheoutsideworld.Theyphysicallyexistandreceivesignalsfromswitches,sensors,etc...Typicallytheyarenotrelaysbutrathertheyaretransistors.

INTERNALUTILITYRELAYS-（contacts）Thesedonotreceivesignalsfromtheoutsideworldnordotheyphysicallyexist.TheyaresimulatedrelaysandarewhatenablesaPLCtoeliminateexternalrelays.Therearealsosomespecialrelaysthatarededicatedtoperformingonlyonetask.Somearealwaysonwhilesomearealwaysoff.Someareononlyonceduringpower-onandaretypicallyuserforinitializingdatawhatwasstored.

COUNTERSTheseagaindonotphysicallyexist.Theyaresimulatedcountersandtheycanbeprogrammedtocountpulses.Typicallythesecounterscancountup,downorbothupanddown.Sincetherearesimulated,theyarelimitedintheircountingspeed.Somemanufacturersalsoincludehigh-speedcountersthatarehardwarebased.Wecanthinkoftheseasphysicallyexisting.Mosttimersthesecounterscancountup,downorupanddown.

TIMERSThesealsodonotphysicallyexist.Theycomeinmanyvarietiesandincrements.Themostcommontypeisanon-delaytype.Otherincludeoff-delayandbothretentiveandnon-retentivetypes.Incrementsvaryfrom1msthrough1s.

OUTPUTRELAYS-（coil）Theseareconnectedtotheoutsideworld.Theyphysicallyexistandsendon/offsignalstosolenoids,lights,etc…Theycanbetransistors,relays,ortriacsdependinguponthemodelchosen.

DATASTORAGE-Typicallythereareregistersassignedtosimplystoredata.Thereareusuallyusedastemporarystorageformathordatamanipulation.Theycanalsotypicallybeuserpower-uptheywillstillhavethesamecontentsasbeforepowerwarremoved.Veryconvenientandnecessary!

APLCworksbycontinuallyscanningaprogram.Wecanthinkofthisscancycleasconsistingof3importantsteps,asshowninFig.19.2Therearetypicallymorethan3butwecanfocusontheimportantpartsandnotworryabouttheothers.Typicallytheothersarecheckingthesystemandupdatingthecurrentandtimervalues.

Step1-CHECKINPUTSTATUS-FirstthePLCtakesalookateachinputtodetermineifitisonoroff.Inotherwords,isthesensorconnectedtothefirstinputon?

Howaboutthesecondinput?

Howaboutthethird…Itrecordsthisdataintoitsmemorytobeusedduringthenextstep.

Step2-EXECUTEPROGRAM-NextthePLCexecutesyourprogramoneinstructionatatime.Maybeyourprogramsaidthatifthefirstinputwasonthenitshouldturnonthefirstoutput.Sinceisalreadyknowswhichinputsareon/offfromthepreviousstep,itwillbeabletodecidewhetherthefirstoutputshouldbeturnedonbasedonthestateofthefirstinput.Itwillstoretheexecutionresultsforuselaterduringt