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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-temcanbede?nedasthecontrollerthatconsistsofaCPUandmemoriestoeditandexecuteaPLCprogram.APLCsystemisasoftware-basedcontrolsystemthatenableseasyediting,execution,andmodi?cationofaPLCprograminsteadofusinghardwaresequencecont
5、rolthatcontrolsprocessesbywiringtogetherthehardwareelementssuchasrelay,timer,andcounter.Therefore,aPLCsystemhasthefollowingadvantagescomparedwithahardware-basedsequencecontrolsystem.?Flexibility:themodi?cationofcontrollogicispossiblebychangingaPLCpro-gram.?
6、Scalability:theextensionofasystemiseasilypossiblebyaddinglogicandthenchangingPLCprograms.?Economicef?ciency:thecostdecreasesthroughreductionofdesign,highrelia-bility,andconvenienceofmaintenance.?Miniaturization:thephysicalvolumerequiredforinstallationissmal
7、lcomparedwithhardwarecontrolsystems.2292307ProgrammableLogicControl?Reliability:theprobabilityonbreakingdownduetopoorcontactdecreasesbe-causethesystemconsistsofasemiconductorhavinganon-contactswitch.?Performance:advancedcontrolfunctionssuchasdatahandlingand
8、arithmeticcalculationareenabled.Thecomparisonbetweenhardware-basedsequencecontrolandPLCaresumma-rizedinTable7.1.Table7.1ComparisonbetweenhardwiredsequencecontrollerandPLCTypeSequencecontrolPLCLogicHard
