基于单片机的低成本教学数字温度控制器英文.docx
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基于单片机的低成本教学数字温度控制器英文.docx
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基于单片机的低成本教学数字温度控制器英文
Teachingdigitalcontrolusingalow-costmicrocontroller-basedtemperaturecontrolkit
Abstract
Thedesignofalow-costdigitaltemperaturecontrolkitisdescribed.Thesystemenablesthestudentstoimplementvariouscontrolstrategiesusingamicrocontroller.Thekitisintendedtobehelpfulinacontrollaboratoryasacomplementtothedigitalcontrolsystemtheorytaughttoundergraduatestudents.
Keywords
Microcontroller-basedcontrol;teachingdigitalcontrol;temperaturecontrol
Withtheavailabilityoflow-costcomputersandmicrocontrollers,digitalcontrolhasgainedpopularityandmostcurrentcontrolsystemsarebasedondigitaltechniques.Thesameistrueforsimulation.Digitalsimulationtechniqueshavereplacedanaloguesimulators.Thesesimulatorsareintheformofinteractivecomputerpackagesusedintheindustry,inresearchlaboratories,collegesanduniversities.EarlysimulationpackagessuchasACSL1weredesignedforlargemainframecomputersandonlylargeorganizationscouldaffordtopurchaseandusesuchpackages.
Currently,softwarepackagessuchasTUTSIM,220-sim,3programCC,4VisSim,5Extend,6andMATLAB7areavailablefordesktopPCsandtheyarewithinthebudgetsofmostuniversitiesandcolleges.Somepackages(e.g.MATLABandprogramCC)arealsoofferedintheformofalow-coststudenteditionandstudentscanpurchaseandusethesepackagesontheirownPCs,awayfromtheuniversitylaboratories.Simulationisaninvaluabletoolinteachingthetheoryofcontrolsystems.Forexample,thestudentscanplottheaccurateroot-locusofacomplexsysteminamatterofafewminutesratherthanspendingseveralhours.Similarly,thetimeandfrequencyresponsesofasystemcanveryeasilybeplottedwiththeaidofasimulatorpackage.Althoughthesimulatorsareveryusefultoolstheyarenotthesameasrealworldsolutions.Therearealsocasesinwhichcomputermodelsmaybeinappropriateorthesystemistoocomplextodescribebymathematicalequationsinacomputer.Itistheauthors’experiencethatstudentslearnanengineeringtopicbestwhentheyseethephysicalresultsoftheexperimentstheyperform.Simulationcanstillbeusedatthefirstleveloftheanalysisbutthisshouldbesupportedwithrealphysicallaboratoryexperiments.Thus,simulationshouldbeacomplementarytoolratherthantheonlytoolwhenteachinganengineeringtopic.Oneoftheproblemswithcommerciallyavailablephysicallaboratoryexperimentsisthattheexperimentalkitsareusuallyveryexpensive,especiallywhenanumberofsimilarkitsarepurchasedforteachingpurposes.Suchkitsmayalsorequirefrequentcalibrationandmaintenanceservicesasaresultofcomponentfailuresandageing.Laboratorykitsalsodobecomeobsoletequiterapidlyasnewproductsaredeveloped.
Thispaperdescribesalow-costtemperaturecontrolkitwhichisdesignedandusedintheengineeringteachinglaboratoriesoftheNearEastUniversity.ThekitisbasedonthepopularPIC16F877modelmicrocontroller,manufacturedbyMicrochipInc.8Theoverallcostofthekitislessthan$200,whichiswellbelowthecostofsimilarcommerciallyavailableeducationaltemperaturecontrolkits.Thedesign,modellinganddigitalcontrolofthekitaredescribedindetail.
Temperaturecontrolkit
Educationaltemperaturecontrolkitsarenotnew.Manycompaniesmanufacturinglaboratorykitsalsooffersomekindofgeneralprocesscontrolortemperaturecontrolkits.TCL-1byKuruganti9isatemperaturecontrollooptrainerwhichisintendedtoshowhowthetemperatureinaheatexchangercanbecontrolled.TCT-1,alsobyKuruganti,isanon/off-basedtemperaturecontrolteachingkit.PROCONbyFeedbackInstruments10isaprocesscontrolsystem,whichincludesrigsforlevel,flowtemperatureandpHcontrol.Here,thetemperaturecontrolsystemuseswaterastheprocessfluidandthekitprovidesPIDcontrolwithauto-tunefacilities.G34/EVisaPID-basededucationaltemperaturecontrolunitmanufacturedbyelectronica.TheunitcanbeinterfacedtoaPCandconsistsofaPIDcontroller,poweramplifier,andsignalconditionerfortemperaturesensors.NearEastUniversityoffersundergraduateandgraduatelevelengineeringcoursesandcontrolengineeringisoneofthesetopics,whichistaughtforonesemester.TherearenopracticalexperimentsandstudentshavebeenusingtheMATLABpackagetodesign,simulateandtesttheircontroltheory.Itwasfeltnecessarytoprovidesomepracticalexperimentstothestudentsasawayofsupportingthetheoreticalconceptstaughtintheclassroom.Themainreasontodesignacontrolkitratherthantobuyacommerciallyavailableonewasthecost.
Processcontrolisaveryimportantfieldofautomaticcontrolengineeringandasafirstinitiativeitwasdecidedtodevelopadigitaltemperaturecontrolexperimentbaseduponamicrocontroller.Oneoftheaimsduringtheimplementationofthislaboratorykitwastouselowcost,butindustrialequipmentinordertoensurethenecessaryrobustnessforitsuse.Itishopedtodevelopmorecontrol-basedexperimentsinthenearfuturewiththeparticipationofmembersofthefacultyandstudents.TheblockdiagramofthedigitaltemperaturecontrolkitisshowninFig.1.Theworkingprincipleofthisexperimentconsistsofheatingthewaterinasmallcontainerusingalow-voltageelectricheatingelementandasimpleMOSFET-basedpowercontrollercircuit.AtemperaturesensorisimmersedintothewaterwhoseoutputsignalissenttoaPIC-type8microcontroller.ThissignaliscomparedwithareferencetemperaturesignalandaPIDcontrolleralgorithmisimplementedbythemicrocontrollertoachievetherequiredtemperaturecontrolaction
Fig.1Schematicofthetemperaturecontrolkit
Thisprocessisusedtoteachthefollowingimportantconceptstothestudents:
•Modellingandidentificationofarealphysicalprocess
•UsingtheZiegler-Nichols11,12tuningmethod
•Usingmicrocontrollersinprocessautomation
•DevelopingandexperimentingwithdigitalPID13,14,15controllers.
Figure2showsapictureoftheprototypeexperimentkit.Thekitisrathersimple,
consistingofonlylowcostmaterials.Aroundplasticcontainerisusedtostorethe
water.Theheaterelementandthesensorareimmersedinthiscontainer.Thetemperatureissensedusingalowcostsemiconductorsensor,whichisprotectedinsideaglasstube.Theheatingelementisthetypewhichisusedincampingandotheroutdooractivitiesinordertowarmupliquidinacup,forexampleformakingcoffee.Theheateroperateswith12V,draws10Aofcurrentandprovidesapower
of120W.Alaboratorypowersupplytoprovidesuchhighpowerisusuallyrather
Fig.2Thetemperaturecontrolkit
expensivesoastandard350WPCpowersupplyisusedinstead,costingnomore
than$50.Usingalowvoltageinanexperimentalkithastheadvantagethatthe
systemissafeasthereisnoriskofelectricshock.Figure3showstheelectricalcircuitdiagramofthekit.Thecircuitisrathersimple,consistingofonlyafewparts.LM35DZ16istheanalogsemiconductortemperature
sensor,PIC16F877isthemicrocontroller,andIRL100417isapowerMOSFET
switch,usedtodrivetheheaterelement.
Thetemperaturesensor
Thetemperaturesensorusedintheexperimentisa3-pinsemiconductorsensorwith
anoutputvoltagedirectlyproportionaltothetemperature.Theoutputofthesensor
isconnectedtooneoftheA/Dconverterinputsofthemicrocontroller.Therewastheoptionofusingadigitaloutputsensor,buttheyareusuallymoreexpensiveanditwasalsofeltnecessarytouseananalogsensorandteachthestudentsthepracticalapplicationsofA/Dconverters.
Themicrocontroller
Inordertolowercosts,weneededamicrocontrollerwithabuilt-inA/Dconverter.
Processcontrolalgorithmsrequiretheuseoffloating-pointarithmeticandasaresult,
Fig.3Circuitdiagramofthekit.
amicrocontrollerwithalargedatamemorywasalsorequired.Anotherrequirement
tolowerthecostwasabuilt-inpulsewidthmodulated(PWM)output,whichwasusedtodrivetheheatercircuitrylinearly.Thedevelopmentoffloating-pointarithmeticroutinesisverycomplexusingtheassemblylanguageanditwasdecidedtoprogramthemicrocontrollerinahigh-levellanguagewhichalsosupportedthefloating-pointarithmetic.
ThePIC16F877microcontrollersatisfiedallofourrequirements.Thisisoneofthemostpopularmicrocontrollersusedinindustryanditoffersthefollowingfeatures:
•8K*14flashprogrammemory
•368bytesRAMmemory
•256bytesEEPROMmemory
•8*10bitA/Dconverters
•Pulsewidthmodulated(PWM)output
•High-levellanguagesupport
TheFEDC18compilerwasusedforprogramdevelopment.ThisisaWindows-based
low-costcompilerforthePICfamilyofmicrocontrollers.Thecompileroffersa
largenumberofstandardClibraryfunctions,includingsupportforfloating-point
arithmetic.
Theheaterdriver
AnIRL1004powerMOSFETswitchisusedtodrivetheheaterelement.This
MOSFETcandissipateupto200Wwhenmountedonasuitableheatsink.The
heatingelementisconnectedtothedrainpinoftheMOSFETandthegateinputis
controlledfromthemicrocontroller(seeFig.3).Largeindustrialtemperaturecontrolsystemsarebasedona.c.powercontroltechniquesusingthyristorsandtriacsandappropriatetheoryisgiventothestudentsonthistopic.
Modelling
Thesystemcanbeapproximatedtoafirst-ordersystemwithatimelag.Asimplifiedmathematicalmodeloftheoverallsystemcanbederivedasdescribedhere.
Mathematicalmodelofthetank
Theheat-balanceequationforthetankcanbewrittenas:
Heatinputtothesystem=heatincreaseinthesystem+heatlosses
Ifwel
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