机电一体化专业英语Unit ppt课件.ppt

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1、IntroductiontoProgrammableLogicControllerPartITechnicalandPracticalReadingPassageAProgrammableLogicController(PLC)Thissectiononprogrammablelogiccontrollersillustratesjustasmallsampleoftheircapabilities.Ascomputers,PLCscanperformtimingfunctions(fortheeq

2、uivalentoftime-delayrelays),drumsequencing,andotheradvancedfunctionswithfargreateraccuracyandreliabilitythanwhatispossibletouseelectromechanicallogicdevices.MostPLCshavethecapacityforfarmorethansixinputsandsixoutputs.Thefollowingphotographshowsseverali

3、nputandoutputmodulesofasingleAllen-BradleyPLC(Fig.1).Witheachmodulehavingsixteen"points"ofeitherinputoroutput,thisPLChastheabilitytomonitorandcontroldozensofdevices.Fitintoacontrolcabinet,aPLCtakesuplittleroom,especiallyconsideringtheequivalentspacetha

4、twouldbeneededbyelectromechanicalrelaystoperformthesamefunctions(Fig.2).OneadvantageofPLCsthatsimplycannotbeduplicatedbyelectromechanicalrelaysisremotemonitoringandcontrolviadigitalcomputernetworks.BecauseaPLCisnothingmorethanaspecial-purposedigitalcom

5、puter,ithastheabilitytocommunicatewithothercomputersrathereasily.Thefollowingphotograph(Fig.3)showsapersonalcomputerdisplayingagraphicimageofarealliquid-levelprocess(apumpingstationforamunicipalwastewatertreatmentsystem)controlledbyaPLC.Theactualpumpin

6、gstationislocatedmilesawayfromthepersonalcomputerdisplay.ThecharacteristicstructureofprogrammablelogiccontrollersisillustratedinFig.4.Provisionismadetoacceptinformationfromvariousinputswitchingdevicessuchaspush-buttons,limitswitches,andrelaycontacts.Al

7、soprovidedareterminalstowhichoutputdevicessuchassolenoids,relaycoils,andindicatorlightscanbeconnected.Therearenodirect-wiredconnectionsbetweentheinputsandoutputs.Instead,theswitchedconditionsoftheinputsareconvertedtologic-levelsignalsthatareinputstoadi

8、gitalcomputerinthecontrolunit.Aprogramstoredinthecomputerspecifieswhichoutputsshouldbeenergizedonthebasisofthepresentandpaststatesoftheinputsignals.Logic-leveloutputsfromthecontrolunitareconvertedtothevoltagelevelsrequiredtoenergizeorde