基于单片机的低成本教学数字温度控制器英文.docx

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