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1、Chapter7ProgrammableLogicControlTheProgrammableLogicControl,PLC,istheautomationcomponentforcontrollingtheexecutionsequenceofmachinesorproductionprocesses.IntheCNCsystem,thePLChasthetaskofcontrollingthemechanicalbehaviorofthemachinewiththeexceptionofaxiscont
2、rol.Inthischapter,thegeneralarchitecture,function,andbehaviorofaPLCwillbeaddressedandtheroleandcharacteristicsofPLCsintheCNCsystemwillbedescribed.Also,IEC-1311-3,theinternationalstandardPLCprogramminglanguage,willbeintroducedandSoft-PLCwillbeintroduced.Fina
3、lly,withrespecttothedesignofaPLCsystem,thearchitectureandexecutionconditionswillbedescribedandanexecutorofaPLCprogramwillbeimplemented.7.1IntroductionIngeneral,aPLCsystemcontainsalogicaloperator,relay,counter,timer,andarith-meticcalculationfunctionstocontro
4、lvariousmachinesandprocesses.APLCsys-temcanbedefinedasthecontrollerthatconsistsofaCPUandmemoriestoeditandexecuteaPLCprogram.APLCsystemisasoftware-basedcontrolsystemthatenableseasyediting,execution,andmodificationofaPLCprograminsteadofusinghardwaresequencecont
5、rolthatcontrolsprocessesbywiringtogetherthehardwareelementssuchasrelay,timer,andcounter.Therefore,aPLCsystemhasthefollowingadvantagescomparedwithahardware-basedsequencecontrolsystem.•Flexibility:themodificationofcontrollogicispossiblebychangingaPLCpro-gram.•
6、Scalability:theextensionofasystemiseasilypossiblebyaddinglogicandthenchangingPLCprograms.•Economicefficiency:thecostdecreasesthroughreductionofdesign,highrelia-bility,andconvenienceofmaintenance.•Miniaturization:thephysicalvolumerequiredforinstallationissmal
7、lcomparedwithhardwarecontrolsystems.2292307ProgrammableLogicControl•Reliability:theprobabilityonbreakingdownduetopoorcontactdecreasesbe-causethesystemconsistsofasemiconductorhavinganon-contactswitch.•Performance:advancedcontrolfunctionssuchasdatahandlingand
8、arithmeticcalculationareenabled.Thecomparisonbetweenhardware-basedsequencecontrolandPLCaresumma-rizedinTable7.1.Table7.1ComparisonbetweenhardwiredsequencecontrollerandPLCTypeSequencecontrolPLCLogicHard