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1、PLCControloftheApplication1MotivationProgrammableLogicControllers(PLC),acomputingdeviceinventedbyRichardE.Morleyin1968,havebeenwidelyusedinindustryincludingmanufacturingsystems,transportationsystems,chemicalprocessfacilities,andmanyothers.Atthattime,thePLCreplacedthehardwiredlogic
2、withsoft-wiredlogicorso-calledrelayladderlogic(RLL),aprogramminglanguagevisuallyresemblingthehardwiredlogic,andreducedtherebytheconfigurationtimefrom6monthsdownto6days.AlthoughPCbasedcontrolhasstartedtocomeintoplace,PLCbasedcontrolwillremainthetechniquetowhichthemajorityofindustri
3、alapplicationswilladhereduetoitshigherperformance,lowerprice,andsuperiorreliabilityinharshenvironments.Moreover,accordingtoastudyonthePLCmarketofFrostandSullivan[1995],anincreaseoftheannualsalesvolumeto15millionPLCsperyearwiththehardwarevalueofmorethan8billionUSdollarshasbeenpredi
4、cted,thoughthepricesofcomputinghardwareissteadilydropping.TheinventorofthePLC,RichardEMorley,fairlyconsidersthePLCmarketasa5-billionindustryatthepresenttime.Particularly,practicalproblemsinPLCprogrammingaretoeliminatesoftwarebugsandtoreducethemaintenancecostsofoldladderlogicprogra
5、ms.ThoughthehardwarecostsofPLCsaredroppingcontinuously,reducingthescantimeoftheladderlogicisstillanissueinindustrysothatlow-costPLCscanbeused.Ingeneral,theproductivityingeneratingPLCisfarbehindcomparedtootherdomains,forinstance,VLSIdesign,whereefficientcomputeraideddesigntoolsarei
6、npractice.ExistentsoftwareengineeringmethodologiesarenotnecessarilyapplicabletothePLCbasedsoftwaredesignbecausePLC-programmingrequiresasimultaneousconsiderationofhardwareandsoftware.Thesoftwaredesignbecomes,thereby,moreandmorethemajorcostdriver.Inmanyindustrialdesignprojects,moret
7、hanSO0/aofthemanpowerallocatedforthecontrolsystemdesignandinstallationisscheduledfortestinganddebuggingPLCprograms[Rockwell,1999].Inaddition,currentPLCbasedcontrolsystemsarenotproperlydesignedtosupportthegrowingdemandforflexibilityandreconfigureabilityofmanufacturingsystems.Afurth
8、erproblem,impellingtheneedforasys
