英文原文JournalofSoftwareEngineeringandApplications,2011,4,172-180doi:10.4236/jsea.2011.43019PublishedOnlineMarch2011(http:/www.SciRP.org/journal/jsea)DevelopmentofEquivalentVirtualInstrumentstoPLCFunctionsandNetworksMohammadA.K.Alia,TariqM.Younes,MohammadAbuZalataMechatroncisEngineeringDepartment,FacultyofEngineeringTechnology,Al-BalqaAppliedUniversity,Amman,Jordan.Email:,,ReceivedFebruary20th,2011；revisedMarch5th,2011；acceptedMarch10th,2011.ABSTRACTThisresearchisacontinuationtoourworkwhichwaspublishedin1.EightdifferenttimingVIsaredesignedandtested.TheseincludeON-Delay,OFF-Delay,SingleShot,RetriggerableMonostable,andAccumulativesoftware-basedtimers.Usinghardwareprogrammablecounter/timerchip(DAQ-STC-24bit)andPCIMIO-16E-1DAQboard,anothertwoprecisetimersaredesigned.Attheendofthepaper,forillustrationpurposes,anelectro-pneumaticdrivesystemwasdevelopedandcontrolledutilizingdesignedon-delaytimersVIfunctions.ResultsofexperimentshowcompletecoincidencebetweenthePLC-basedcontrolandVirtualPLC-basedprogramresults.Keywords:PLC,VirtualPLC,LabVIEW,ProgrammableTimers1.IntroductionInourwork“DesignofavirtualPLCusingLabVIEW”wehaveshownhowitispossibletocreateLabVIEWVIswhichrepresentPLCfunctionsandnetworks.WecomparedbetweenPC-basedandPLC-basedcontrolsystems,andcametothefactthatbothsystemsarecontinuouslydevelopinginthesamedirectioninordertoobtainbetterprogrammability,connectivityandcommunicationinterfacing.AtthetimebeingthePC-basedDCSsaresuitedforindustrialapplications.Theyarerobustandtheyeasilyworkinanopenarchitecturemode,whilePLCsareequippedwithspecificMMIsoftwareandpseudo-standardcommutationsoftwarealso.WehaveshownthatinordertoimprovetheprogrammabilityofPACs,wepracticallybroughtthePLCtothecomputerutilizingbythatnumerousadvantagesofcomputerssuchasmultitasking,unlimitedmemory,highspeedandthepossibilityofcreatingunlimitednumberofprogrammableobjectssuchascounters,timers,shiftregistersandothers.Becauseofthelimitedsizeofpreviouswork,wewerenotabletocoverotherimportantVIswhichmaybeusedalsoastheanalogofPLCfunctions.InthispaperweshalldevelopdifferenttypesofprogrammabletimersusingLabVIEWsoftware2andNIDAQboardhardwarealso.TheLabVIEWbasicfunctionsthatprovidetimingonmillisecondlevelarethe“wait”and“waitforNextmsMultiple”VIs.Botharebasedonthesameunderlyingmechanism.MostapplicationsworkcomfortablywithavailableLabVIEWmeasurementsthatresolvemilliseconds,andmanymoreoperatewithsecondresolution3-4.Afewapplicationsdemandsub-millisecondresolutionandresponsetime,whichisproblematicdueprimarilytooperatingsystemandnotaLabVIEWlimitation5.Iftheapplicationrequireshigheraccuracyorresolutionthanthebuilt-intimingfunctionscansupply,thenonewillhavetousesomeadditionalhardware,suchasNI-DAQboardsoranexternalclock6.NI-boardshavetwo24bitcounterchipsandseveralon-boardclocksthatcanbecountedtoproduceaccuratetiming(intervals).WiththeDAQcounter-timerVIs,onecanconfiguretheon-boardversatilehardwareforavarietyoftasksincludingtheaccurategenerationoftimedpulses,countingevents,andthemeasurementofperiodsandfrequencies.Thecounteroutputgeneratesapulsewhenapreprogrammedterminalcount(TC)isreached.Thepulsemaybeusedforsequencingpurposes.Similarhardware-basedtimingmaybeperformedusingwindowsAPIfunction“Queryperformancecounter”.Thisfunctionlooksatahighresolutionsystemhardwarecounterthatrunsatapproximately1.2MHzor0.8microsecondcount.Theactualresolution,onceweaccountforthedelayincallingthefunction,willbeconsiderablyless,butstillfarbetterthanonemillisecond.ConcerningReal-Timeoperatingsystems(RTOS),theyaredesignedtorunasingleprogramwithveryprecisetiming.Theycanallowtorunloopswithnearlythesamethingeachiteration(typicallywithinmicroseconds).TimingforhardRTOSscanbeperformedusingtheDAQcardsinternalclock,givingbetteraccuracythansoftwaretimingfunctions7.Atthetimebeing,somehardwareplatformsfeatureanon-boardFPGA,thatmaybeprogrammedusingLabVIEWFPGAmodule.NIComactRIOandsingle-boardRIOareexamples.ThedefaultclockrateofLabVIEWFPGAis40MHz.GeneralFPGAtimingVIs2maygenerateoneclockperiod.One-shotpulseormeasuretheperiod,pulsewidth,accumulateperiodoveraspecifiednumberofpulsesandcountpulsesoveraspecifiedperiodoftime.NeverthelessFPGAVIsdonotincludereadyOn-delaytimers,OFF-delaytimersandmomostableretriggerabletimerswhichfindextensiveapplicationsinPLCsequentialcontrolprograms.Buildingontheabove,thetargetofthisworkistoillustratethedesignofdifferenttypesoftimingVIsusingLabVIEWsoftwareinordertobeusedasprogrammingelementsinvirtualPLCprograms.2.ON-DelayTimer1)ON-DelayTimer-1Figure1showsthefrontpanelandtheblockdiagramcomponentsofasoftware-basedON-DelayTimer.Theloopiterationisindicatedinseconds.Becausetheloopiterationstartsfromzero,theincrementfunctionisaddedinordertostartitatone.Sincethewaiticonhas100msdelaybetweeneverytwoiterationsafactorof10ismultipliedbytimerpreassignedvalue,inordertomeasurethetimedelayinseconds.Aftertheapplicationofenablesignalittakessomedelaytimeintervalfortheequalfunctiontohaveatruestateattheoutput.Iftheinputsignalisdisabled,thetimeroutputinstantlychangestolowstate.2)ON-DelayTimer-2ThecomponentsoftheVIareshownintheblockdiagram,Figure2.Initiallytheinputsignalisnotenabledandthefalsecaseisactivated.Theoutputofselecticonwillbezero,whichislowerthanthetimerpresetvalue,andasaresultofthattheoutputofthetimerisOFF.Whentheinputsignalisenabledthetruecaseisexecutedandtheselecticonwilloutputthevaluethatcomesformtheoutputofthecasestructure.Theinitialvalueoftheiterationlocalvariableiszero,thenitwillbeincrementedafteradelaycausedbythewaiticon,andthencomparedbytimerpresetvalue.Whentheoutputofthecomparisonfunctionistrue,theoutputofthetimerbecomeshigh.Whentheenableinputsignalbecomeslow,theoutputofthetimerbecomeslowsimultaneously.InthisVI,thecheckingofthecasestructureiscontinuousatascanrateequaltoonemillisecond,whichisacceptedformanyapplications.（a）（b）(c)Figure1.On-delaytimer-1,(a)TheBlockDiagram；(b)TheFrontPanel；(c)Subicon3.OFF-DelayTime1)OFF-DelayTimer-1ThefrontpanelandblockdiagramareshowninFi