外文翻译原文-单片机温度控制：一个跨学科的本科生工程设计项目

TemperatureControlUsingaMicrocontroller:AnInterdisciplinaryUndergraduateEngineeringDesignProjectJamesS.McDonaldDepartmentofEngineeringScienceTrinityUniversitySanAntonio,TX78212Abstract.ThispaperdescribesaninterdisciplinarydesignprojectwhichwasdoneundertheauthorssupervisionbyagroupoffourseniorstudentsintheDepartmentofEngineer-ingScienceatTrinityUniversity.Theobjectiveoftheprojectwastodevelopatemperaturecontrolsystemforanair-filledchamber.Thesystemwastoallowentryofadesiredcham-bertemperatureinaprescribedrangeandtoexhibitover-shootandsteady-statetemperatureerroroflessthan1de-greeKelvinintheactualchambertemperaturestepresponse.Thedetailsofthedesigndevelopedbythisgroupofstudents,basedonaMotorolaMC68HC05familymicrocontroller,aredescribed.Thepedagogicalvalueoftheproblemisalsodis-cussedthroughadescriptionofsomeofthekeystepsinthedesignprocess.Itisshownthatthesolutionrequiresbroadknowledgedrawnfromseveralengineeringdisciplinesinclud-ingelectrical,mechanical,andcontrolsystemsengineering.1IntroductionThedesignprojectwhichisthesubjectofthispaperorigi-natedfromareal-worldapplication.Aprototypeofamicro-scopeslidedryerhadbeendevelopedaroundanOmegaTMmodelCN-390temperaturecontroller,andtheobjectivewastodevelopacustomtemperaturecontrolsystemtoreplacetheOmegasystem.Themotivationwasthatacustomcon-trollertargetedspecificallyfortheapplicationshouldbeabletoachievethesamefunctionalityatamuchlowercost,astheOmegasystemisunnecessarilyversatileandequippedtohan-dleawidevarietyofapplications.ThemechanicallayoutoftheslidedryerprototypeisshowninFigure1.Themainelementofthedryerisalarge,insulated,air-filledchamberinwhichmicroscopeslides,eachwithatissuesampleencasedinparaffin,canbesetoncaddies.Inorderthattheparaffinmaintaintheproperconsistency,thetemperatureintheslidechambermustbemaintainedatade-sired(constant)temperature.Asecondchamber(theelectron-icsenclosure)housesaresistiveheaterandthetemperaturecontroller,andafanmountedontheendofthedryerblowsairacrosstheheater,carryingheatintotheslidechamber.Thisdesignprojectwascarriedoutduringacademicyear199697byfourstudentsundertheauthorssupervisionasaSeniorDesignprojectintheDepartmentofEngineeringSci-enceatTrinityUniversity.ThepurposeofthispaperistoSlideChamberSlideChamber161.756.2510.254.5HeaterFanElectronicsenclosureTopView8888TemperaturecontrollerFrontView4.5Figure1.Slidedryermechanicallayoutdescribetheproblemandthestudentssolutioninsomede-tail,andtodiscusssomeofthepedagogicalopportunitiesof-feredbyaninterdisciplinarydesignprojectofthistype.Thestudentsownreportwaspresentedatthe1997NationalCon-ferenceonUndergraduateResearch1.Section2givesamoredetailedstatementoftheproblem,includingperformancespecifications,andSection3describesthestudentsdesign.Section4makesupthebulkofthepaper,anddiscussesinsomedetailseveralaspectsofthedesignpro-cesswhichofferuniquepedagogicalopportunities.Finally,Section5offerssomeconclusions.2ProblemStatementThebasicideaoftheprojectistoreplacetherelevantpartsofthefunctionalityofanOmegaCN-390temperaturecontrollerusingacustom-designedsystem.Theapplicationdictatesthattemperaturesettingsareusuallykeptconstantforlongperi-odsoftime,butitsnonethelessimportantthatstepchangesbetrackedina“reasonable”manner.Thusthemainrequire-mentsboildownto#0Fallowingachambertemperatureset-pointtobeentered,#0Fdisplayingbothset-pointandactualtemperatures,and#0Ftrackingstepchangesinset-pointtemperaturewithac-ceptablerisetime,steady-stateerror,andovershoot.Table1givesamoreprecisestatementofspecifications.Table1.TemperaturecontrollerspecificationsSet-pointtemperatureentryRange6099#0ECPrecision1#0ECSet-pointtemperaturedisplayRange6099#0ECPrecision1#0ECChambertemperaturedisplayRange6099#0ECPrecision1#0ECAccuracy#061#0ECChambertemperaturestepresponseRange(steadystate)6099#0ECAccuracy(steadystate)#061#0ECMaximumovershoot1#0ECSettlingtime(to#061#0EC)120sAlthoughnotexplicitlyapartofthespecificationsinTa-ble1,itwasclearthatthecustomerdesireddigitaldisplaysofset-pointandactualtemperatures,andthatset-pointtem-peratureentryshouldbedigitalaswell(asopposedto,say,throughapotentiometersetting).3SystemDesignTherequirementsfordigitaltemperaturedisplaysandset-pointentryaloneareenoughtodictatethatamicrocontroller-baseddesignislikelythemostappropriate.Figure2showsablockdiagramofthestudentsdesign.RelayHeaterTemperatureSensorSlideChamberMicrocontrollerKeypadDisplays&DriversElectronicsEnclosureFigure2.TemperaturecontrollerhardwareblockdiagramThemicrocontroller,aMotorolaMC68HC705B16(6805forshort),istheheartofthesystem.Itacceptsinputsfromasimplefour-keykeypadwhichallowspecificationoftheset-pointtemperature,anditdisplaysbothset-pointandmea-suredchambertemperaturesusingtwo-digitseven-segmentLEDdisplayscontrolledbyadisplaydriver.Allthesein-putsandoutputsareaccommodatedbyparallelportsonthe6805.Chambertemperatureissensedusingapre-calibratedthermistorandinputviaoneofthe6805sanalog-to-digitalinputs.Finally,apulse-widthmodulation(PWM)outputonthe6805isusedtodrivearelaywhichswitcheslinepowertotheresistiveheateroffandon.Figure3showsamoredetailedschematicoftheelec-tronicsandtheirinterfacingtothe6805.Thekeypad,aStorm3K041103,hasfourkeyswhichareinterfacedtopinsPA0#7BPA3ofPortA,configuredasinputs.Onekeyfunctionsasamodeswitch.Twomodesaresupported:setmodeandrunmode.Insetmodetwooftheotherkeysareusedtospecifytheset-pointtemperature:oneincrementsitandonedecre-ments.Thefourthkeyisunusedatpresent.TheLEDdisplaysaredrivenbyaHarrisSemiconductorICM7212displaydriverinterfacedtopinsPB0#7BPB6ofPortB,configuredasoutputs.Thetemperature-sensingthermistordrives,throughavoltagedivider,pinAN0(oneofeightanaloginputs).Finally,pinPLMA(oneoftwoPWMoutputs)drivestheheaterrelay.7-segLEDdisplay2x2-digitABCDA1A2E1ICM7212RelaylineHeaterVddKeypadVdd28ThermistorMC68HC705B16PA0PA1PA2PA3PB1PB2PB3PB4PB5PB6PLMAAN0PB0Figure3.SchematicofmicrocontrollerboardSoftwareonthe6805implementsthetemperaturecon-trolalgorithm,maintainsthetemperaturedisplays,andalterstheset-pointinresponsetokeypadinputs.Becauseitisnotcompleteatthiswriting,softwarewillnotbediscussedinde-tailinthispaper.Thecontrolalgorithminparticularhasnotbeendetermined,butitislikelytobeasimpleproportionalcontrollerandcertainlynotmorecomplexthanaPID.SomecontroldesignissueswillbediscussedinSection4,however.4TheDesignProcessAlthoughessentiallytheprojectisjusttobuildathermostat,itpresentsmanynicepedagogicalopportunities.Theknowl-edgeandexperiencebaseofaseniorengineeringundergrad-uatearejustenoughtobringhimorhertothebrinkofasolu-tiontovariousaspectsoftheproblem.Yet,ineachcase,real-worldconsiderationscomplicatethesituationsignificantly.Fortunatelythesecomplicationsarenotinsurmountable,andtheresultisaverybeneficialdesignexperience.Theremainderofthissectionlooksatafewaspectsoftheproblemwhichpresentthetypeoflearningopportunityjustdescribed.Section4.1discussessomeofthefeaturesofasimplifiedmathematicalmodelofthethermalpropertiesofthesystemandhowitcanbeeasilyvalidatedexperimentally.Section4.2describeshowrealisticcontrolalgorithmdesignscanbearrivedatusingintroductoryconceptsincontrolde-sign.Section4.3pointsoutsomeimportantdeficienciesofsuchasimplifiedmodeling/controldesignprocessandhowtheycanbeovercomethroughsimulation.Finally,Section4.4givesanoverviewofsomeofthemicrocontroller-relatedde-signissueswhichariseandlearningopportunitiesoffered.4.1MathematicalModelLumped-elementthermalsystemsaredescribedinalmostanyintroductorylinearcontrolsystemstext,andjustthissortofmodelisapplicabletotheslidedryerproblem.Figure4showsasecond-orderlumped-elementthermalmodeloftheslidedryer.Thestatevariablesarethetemper-aturesTaoftheairintheboxandTboftheboxitself.Theinputstothesystemarethepoweroutputq#28t#29oftheheaterandtheambienttemperatureT.maandmbarethemassesoftheairandthebox,respectively,andCaandCbtheirspecificheats.1and2areheattransfercoefficientsfromtheairtotheboxandfromtheboxtotheexternalworld,respectively.Ta;ma;CaT1#28Ta,Tb#292#28Tb,T#29q#28t#29Tb;mb;CbFigure4.Lumped-elementthermalmodelItsnothardtoshowthatthe(linearized)stateequationscorrespondingtoFigure4aremaCaTa=1#28Ta,Tb#29(1)mbCbTb=1#28Ta,Tb#29,#28Tb,T#29(2)TakingLaplacetransformsof(1)and(2)andsolvingforTa#28s#29,whichistheoutputofinterest,givesthefollowingopen-loopmodelofthethermalsystem:Ta#28s#29=K#28z+1#29#28#29Q#28#29+1#28s#29T;whereKisaconstantand#28s#29isasecond-orderpolynomial.K,z,andthecoefficientsof#28s#29arefunctionsofthevariousparametersappearingin(1)and(2).Ofcoursethevariousparametersin(1)and(2)arecom-pletelyunknown,butitsnothardtoshowthat,regardlessoftheirvalues,#28s#29hastworealzeros.Thereforethemaintransferfunctionofinterest(whichistheonefromQ#28s#29,sincewellassumeconstantambienttemperature)canbewrittenGaq#28s#29=Ta#28#29Q#28#29=K#28z+1#29#28p+#29#28p2s+1#29(3)Moreover,itsnottoohardtoshowthat1=p1#3C1=z#3C1=p2,i.e.,thatthezeroliesbetweenthetwopoles.Bothoftheseareexcellentexercisesforthestudent,andtheresultistheopen-looppole-zerodiagramofFigure5.,1=p1ImRe,1=p2,1=zFigure5.Pole-zerodiagramofGaq#28s#29Obtainingacompletethermalmodel,then,isreducedtoidentifyingtheconstantKandthethreeunknowntimecon-stantsin(3).Fourunknownparametersisquiteafew,butsimpleexperimentsshowthat1=p1#1C1=z;1=p2sothatz;p2#190aregoodapproximations.Thustheopen-loopsys-temisessentiallyfirst-orderandcanthereforebewrittenGaq#28s#29=+(4)(wherethesubscriptp1hasbeendropped).Simpleopen-loopstepresponseexperimentsshowthat,forawiderangeofinitialtemperaturesandheatinputs,K#190:14#0E=Wand#19295s.14.2ControlSystemDesignUsingthefirst-ordermodelof(4)fortheopen-looptransferfunctionGaq#28s#29andassumingforthemomentthatlinearcon-troloftheheaterpoweroutputq#28t#29ispossible,theblockdia-gramofFigure6representstheclosed-loopsystem.Td#28s#29is1Ofcoursethesystemisnotactuallylinear,sotheapparentparametervaluesvarywithinitialconditionsandinputmagnitude.Theeffectonclosed-loopperformanceisnottooserious,butitgivesthestudentagoodideaofwhatnonlinearitymeansandhowfeedbacktendstomitigateitseffects.thedesired,orset-point,temperature,C#28s#29isthecompensatortransferfunction,andQ#28s#29istheheateroutputinwatts.Td#28s#29C#28s#29Ks+1Ta#28s#29Q#28s#29Figure6.Simplifiedblockdiagramoftheclosed-loopsystemGiventhissimplesituation,introductorylinearcontroldesigntoolssuchastherootlocusmethodcanbeusedtoarriveataC#28s#29whichmeetsthestepresponserequirementsonrisetime,steady-stateerror,andovershootspecifiedinTa-ble1.Theupshot,ofcourse,isthataproportionalcontrollerwithsufficientgaincanmeetallspecifications.Overshootisimpossible,andincreasinggainsdecreasesbothsteady-stateerrorandrisetime.Unfortunately,sufficientgaintomeetthespecificationsmayrequirelargerheatoutputsthantheheateriscapableofproducing.Thiswasindeedthecaseforthissystem,andtheresultisthattherisetimespecificationcannotbemet.Itisquiterevealingtothestudenthowusefulsuchanover-simplifiedmodel,carefullyarrivedat,canbeindeterminingoverallperformancelimitations.4.3SimulationModelGrossperformanceanditslimitationscanbedeterminedus-ingthesimplifiedmodelofFigure6,butthereareanumberofotheraspectsoftheclosed-loopsystemwhoseeffectsonperformancearenotsosimplymodeled.Chiefamongtheseare#0Fquantizationerrorinanalog-to-digitalconversionofthemeasuredtemperatureand#0FtheuseofPWMtocontroltheheater.Bothofthesearenonlinearandtime-varyingeffects,andtheonlypracticalwaytostudythemisthroughsimulation(orexperiment,ofcourse).Figure7showsaSimulinkTMblockdiagramoftheclosed-loopsystemwhichincorporatestheseeffects.A/Dconverterquantizationandsaturationaremodeledusingstan-dardSimulinkquantizerandsaturationblocks.ModelingPWMismorecomplicatedandrequiresacustomS-functiontorepresentit.A/DConverter0.392ScaleFactor+Sum80SetpointTemperaturePIDCompensator0.05Thermistor0.137295.2s+1G_aq(s)pwm.mPWMSFunctionSaturation/HeaterFigure7.Simulinkblockdiagramofclosed-loopsystemThissimulationmodelhasprovenparticularlyusefulingaugingtheeffectsofvaryingthebasicPWMparametersandhenceselectingthemappropriately.(I.e.,thelongerthepe-riod,thelargerthetemperatureerrorPWMintroduces.Ontheotherhand,alongperiodisdesirabletoavoidexcessiverelay“chatter,”amongotherthings.)PWMisoftendifficultforstudentstograsp,andthesimulationmodelallowsanex-plorationofitsoperationandeffectswhichisquiterevealing.4.4TheMicrocontrollerSimpleclosed-loopcontrol,keypadreading,anddisplaycon-trolaresomeoftheclassicapplicationsofmicrocontrollers,andthisprojectincorporatesallthree.Itisthereforeanexcel-lentall-aroundexerciseinmicrocontrollerapplications.Inaddition,becausetheprojectistoproduceanactualpackagedprototype,itwontdotouseasimpleevaluationboardwiththeI/Opinsjumperedtothetargetsystem.Instead,itsnecessarytodevelopacompleteembeddedapplication.Thisentailsthechoiceofanappropriatepartfromthebroadrangeofferedinatypicalmicrocontrollerfamilyandlearningtouseafairlysophisticateddevelopmentenvironment.Fi-nally,acustomprinted-circuitboardforthemicrocontrollerandperipheralsmustbedesignedandfabricated.MicrocontrollerSelection.Inviewofexistinglocalexper-tise,theMotorolalineofmicrocontrollerswaschosenforthisproject.Still,thisdoesnotnarrowthechoicedownmuch.Afairlydisciplinedstudyofsystemrequirementsisneces-sarytospecifywhichmicrocontroller,outofscoresofvari-ants,isrequiredforthejob.Thisisdifficultforstudents,astheygenerallylacktheexperienceandintuitionneededaswellastheperseverancetowadethroughmanufacturersselectionguides.Partoftheproblemisinchoosingmethodsforinterfac-ingthevariousperipherals(e.g.,whatkindofdisplaydrivershouldbeused?).AstudyofrelevantMotorolaapplicationnotes2,3,4provedveryhelpfulinunderstandingwhatbasicapproachesareavailable,andwhatmicrocontroller/peripheralcombinationsshouldbeconsidered.TheMC68HC705B16wasfinallychosenonthebasisofitsavailableA/DinputsandPWMoutputsaswellas24digitalI/Olines.Inretrospectthisisprobablyoverkill,asonlyoneA/Dchannel,onePWMchannel,and11I/Opinsareactuallyrequired(seeFigure3).Thedecisionwasmadetoerronthesafesidebecauseacompletedevelopmentsystemspecifictothechosenpartwasnecessary,andtheprojectbudgetdidnotpermitasecondsuchsystemtobepurchasedshouldthefirstproveinadequate.MicrocontrollerApplicationDevelopment.Breadboard-ingoftheperipheralhardware,developmentofmicrocon-trollersoftware,andfinaldebuggingandtestingofacustomprinted-circuitboardforthemicrocontrollerandperipheralsallrequireadevelopmentenvironmentofsomekind.Thechoiceofadevelopmentenvironment,likethatofthemicrocontrolleritself,canbebewilderingandrequiressomefacultyexpertise.Motorolamakesthreegradesofdevelop-mentenvironmentrangingfromsimpleevaluationboards(ataround$100)tofull-blownreal-timein-circuitemulators(atmorelike$7500).Themiddleoptionwaschosenforthisproject:theMMEVS,whichconsistsof#0Faplatformboard(whichsupportsall6805-familyparts),#0Fanemulatormodule(specifictoB-seriesparts),and#0Facableandtargetheadadapter(package-specific).Overall,thesystemcostsabout$900andprovides,withsomelimitations,in-circuitemulationcapability.ItalsocomeswiththesimplebutsufficientsoftwaredevelopmentenvironmentRAPID5.Studentsfindlearningtousethistypeofsystemchal-lenging,buttheexperiencetheygaininreal-worldmicro-controllerapplicationdevelopmentgreatlyexceedsthetypicalfirst-courseexperienceusingsimpleevaluationboards.Printed-CircuitBoard.Thelayoutofasimple(thoughdefinitelynottrivial)printed-circuitboardisanotherpracti-callearningopportunitypresentedbythisproject.Thefi-nalboardlayout,withpackageoutlines,isshown(at50%ofactualsize)inFigure8.Therelativesimplicityofthecir-cuitmakesmanualplacementandroutingpracticalinfact,itlikelygivesbetterresultsthanautomaticinanapplicationlikethisandthestudentisthereforeexposedtofundamen-talissuesofprinted-circuitlayoutandbasicdesignrules.Thelayoutsoftwareusedwastheverynicepackagepcb,2andtheboardwasfabricatedin-housewiththeaidofourstaffelec-tronicstechnician.Figure8.Printed-circuitlayoutformicrocontrollerboard5ConclusionTheaimofthispaperhasbeentodescribeaninterdisciplinary,undergraduateengineeringdesignproject:amicrocontroller-2pcbisfreelydistributablesoftwareforUNIX/X11.ItiswrittenbyThomasNau,AssistantDirectorofComputingatUniversitatUlm,Germany.HecanbecontactedatURLmailto:Thomas.Naurz.uni-ulm.de,thesoftwareisavailableatftp:#2F#2Fftp.uni-ulm.de#2Fpub#2Fpcb,andanemaillistcanbesubscribedtoatmailto:pcbmajordomo.uni-ulm.de.basedtemperaturecontrolsystemwithdigitalset-pointen-tryandset-point/actualtemperaturedisplay.Aparticularde-signofsuchasystemhasbeendescribed,andanumber