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ProgrammableLogicController(PLC),Automation,MicrocontrollerApplications,IndustrialElectronics,DigitalControl,On-OffControl,DigitalElectronics
1.INTRODUCTION
1.1PROBLEMDESCRIPTION
Weallknowthatindustrialautomationisthebackboneofanationforitsprosperitysince,itreducestheproductioncostmanyfoldsandalsoincreasesproductivity.ForthispurposeWLC(i.e.WiredLogicController)wasintroduced.Itwasthefirststepintheindustrialautomation.LateronmicrocontrollerswereintroducedwhichhadtheadvantageofgreaterflexibilityovertheWLC.
Eventually,averysophisticatedversionofacontrollerwasintroducedwithaverylargeI/OhandlingcapabilityandextremeflexibilitycalledtheProgrammableLogicController,commonlycalledthePLC.Siemensintroducedmanyseriesofcontrollerswithdifferentcapabilities.OneofthefirstserieswastheSIMATIC®
5Controllers.ThePLCavailabletousistheS5-100U-100.LateronSiemensintroducedtheSIMATIC®
7seriesofcontrollersandnow,wealsohavetheS7-314IFMPLC.
ThePLCsarenotalientotheindustriesofPakistan.Theyhavebeenaroundforquitealongperiodandarebeingemployedextensively.Eventhoughthedemandofthisproductisconsiderablehere,therehasbeennoefforttodevelopitindigenously.Sincetherehadbeennodevelopmentinthisfieldinourcountryandbeingarequisiteofthenation,soasanexerciseofLogicDesign,DigitalElectronics,InterfacingandSoftwareEngineering,weundertookthistaskofmakingaPLCbetterthantheS5controllersandcomparabletosomeextenttotheS7controllers.ThePLCthatwehavedesignedcanalsobeupgradedtosupport
RemoteDataAcquisition.
1.2BLOCKDIAGRAMANDBRIEFDESCRIPTION
ThecompletegeneralblockdiagramofthePLCisdepictedinFig.1-1.JustliketheSIMATIC®
5systems,ourPLCcanalsobebroadlyclassifiedintothreedistinctunits:
a.CPU
b.Busunit,and
c.Modules(I/Oandfunction)
Fig.1-1BlockdiagramofthePLC
Eachunitcanbediscussedasfollows:
1.2.1CPU
TheCPU,asthenamesuggests,isthebrainofthePLCsystem.Withoutitsbrain,thePLCsimplycannotfunction.
Inoursystem,theCPUcomprisesoftwoATMEL®
89C52microcontrollers.Asineverymultiprocessorsystem,oneisthemasterandtheotheristheslave.Thefunctionsperformedbythemasterorthecontrollingunit(Fig.1-1)canbesummarizedasfollows:
∙MonitorandcontroltheoperatingmodesofthePLCusingtheoperationcontrols.
∙ConfigurethePLCmodules.
∙Checkprogramintegritybymonitoringscancycles.
Theslavemicrocontroller(executingunit,Fig.1-1)functionstoruntheuserprogram.Itisresponsiblefor:
∙Takingtheappropriateactionsduringstartup.
∙Issuingthe
signalatthestartofeveryscancycleandthesubsequent
addressingandtransactionofdatawiththemodules.
∙Handlingthecountersandtimers.
TheCPUsectionalsohastheresponsibilityofcommunicatingtheprogrammingdevicei.e.thecomputerthroughaserialport.Thecommunicationchannelworksaccordingtothesemaphorealgorithm(commonlyknownastokenalgorithm).Itistobenotedherethatthecommunicationlinkcurrentlydoesnotfunction,asmentionedearlier,becauseitwasapartoftheplannedGUI,whichhasnotbeenimplementedduetotheconstraintoftime.
1.2.2BUSUNIT
Thebusunitfunctionssimilartothemotherboardinacomputer.Thebusunitthatwehaveconstructedholdsupto4modulesandtheCPU;
wenameditthemainbusunit.Moreover,thebusunitalsocarriestheextensionconnectorbywhichotherbusunitscanbecascadedtoincreasetheI/OhandlingcapabilityofthePLC.ThebusunitsthatwillbecascadedwillnothaveaslotforconnectingtheCPU;
onlymodulescanbeconnected.
Themainfunctionofthebusunitistobufferallthesignalsi.e.thedatabuses,addressbuses,andthecontrolsignals,toeliminatetheloadingoftheCPU.Thisfunctionisalsoperformedbythemainbusunitbutthisalsoperformsanextrafunctionduringtheconfigurationstage.Thebusunitcontroller,seeFig.1-1,selectseachslotoneatatimesothatitcanbeconfiguredforthemoduleconnectedinitandalsosendsasignaltothecontrollingunitwhenalltheslotshavebeenselectedandconfiguredonce.
1.2.3MODULES
ThemodulesservetoprovidetheinterfaceofthePLCwiththeplanttobecontrolled.Themodules,whichwehaddesignedweredigitalinput,digitaloutput,analoginputandanalogoutputmodules(verifiedusingOrCAD®
PSpice).Butweonlymanagedtomanufacturethedigitalinputandoutputmodules,eachcapableofhandling16devices.Thestandardsofthemodulesaresimilartothatofcommerciallyavailablemodules.
Themodulesareequippedwithanaddressdecodercircuit,whichiscommoninallthemodules,asthecircuithasbeendesignedtosupportautoconfiguration.
1.3POSSIBLESOLUTIONS
Nowthattheoverviewoftheprojectispresentbeforeus,wewilltakealookintothepossiblesolutionsthatcametoourmindwhenwecommencedbrainstorming.Theideascanbelistedasfollowsalongwiththeirsupportinganddisagreementcomments.
∙Useofasingleprocessor/microcontrollerwasthefirstideathatcametomind
becauseitwasmostsimpleandeasytoimplement.Theproblemwithsuchanideawasthatwewerestrivingforagoodscantimeduetowhichweneededafastprocessor(whichwasunavailable)orhadtogoformultipleprocessors.
∙Theuseofmicroprocessorinsteadofamicrocontrollerwasalsoamongtheoptionsbecausewehavebeentaughttheassemblylanguageofthisprocessorandwewerewellawareofit.ThedrawbackofsuchaselectionwasthatinaPLC,bitoperationsareplentywhilethemicroprocessorinstructionisnotverypowerfulforsuchoperations.
∙Usinga16-bitmicrocontrollerforeaseofworkwasagreatideabutitsmaterializationwasnotpossibleasitwasnotavailablehere.
∙Afteralltheseoptions,wewereleftonlywiththeoptionofusingtwomicrocontrollersintandemtoachieveourdesiredtask.
1.4REASONSFORSELECTION
Ofalltheabovesolutionsmentionedabove,wedecidedtoimplementthemultiprocessorCPUwiththebusunitschemeforconnectingthemodules.Theadvantages,whichalsoformthereasonsfortheselectionofthisdesign,areenumeratedbelow.
∙Thissolutionwasthefirstseeminglyfeasiblesolutionthatcametoourminds.Duringthecourseofwork,manymorecamebutthenitwastoolate.
∙Thedesignwasbasedonequipmentandtechnologyeasilyavailableinourcity.
∙Theuseofmicrocontrollerinsteadofamicroprocessorisjustifiedbythefactthatthemicroprocessorisincapableofbitoperations,whicharenumerousinaPLC.
∙Afastmicrocontrollerwasunavailablehencewedecidedtodistributetheworkloadandruntaskssimultaneouslybyusingtwocontrollerssothatthespeedisnotcompromised.
∙Theautoconfigurationfeatureallowsthedesignofthemodulestobequitegenericastheaddressdecodingsectionisidenticalhenceensuringeaseofmanufacture.
∙TheuseofthebusunitallowscosteffectivenessasspecializedcircuitsineachmodulearenotrequiredasfoundintheSIMATIC®
7controllers.
2.ANALYSISANDSIMULATION
Sinceoursystemisadigitalsystembasedonmicrocontrollers,itwasnotpossibletoobtainitsmathematicalmodel.Instead,wehavesimulatedourcircuitdesignwithallthereallifelimitationsinOrCAD®
9.1PSpice.Thecomputerhardwaresimulationresultsandtheiranalysisarediscussedbelow.
2.1COMPUTERSIMULATIONS-HARDWARE
Thesimulationswerecarriedindifferentstages;
theautoconfigurationprocess,inwhichthecontrolwordisreadandthentheappropriateaddress,isassignedtothemodule.Oncethiswastested,thecompletesequenceofreadingdatafromadigitalinputmodulewastested.Similarly,thesequenceofwritingtoadigitaloutputmodulewasalsotested.
Beforediscussingtheprocesses,onethingthatmustbestressedisthatthesignalsusedfortestingtheperformanceofthecircuitswereconstructedusingactualtimingdelaysthatwouldbeintroducedbythemicrocontrollers.Allthepropagationdelays,pulsewidths,etc.havebeencarefullyimplementedsothatoursimulationsareasneartopracticalresultsaspossible.
2.1.1THEAUTOCONFIGURATIONP