基于单片机的温度控制系统(英语)

TemperatureControlUsingaMicrocontroller:

AnInterdisciplinaryUndergraduateEngineeringDesignProject

JamesS.McDonald

DepartmentofEngineeringScience

TrinityUniversity

SanAntonio,TX78212

Abstract

Thispaperdescribesaninterdisciplinarydesignprojectwhichwasdoneundertheauthor’ssupervisionbyagroupoffourseniorstudentsintheDepartmentofEngineeringScienceatTrinityUniversity.Theobjectiveoftheprojectwastodevelopatemperaturecontrolsystemforanair-filledchamber.Thesystemwastoallowentryofadesiredchambertemperatureinaprescribedrangeandtoexhibitovershootandsteady-statetemperatureerroroflessthan1degreeKelvinintheactualchambertemperaturestepresponse.Thedetailsofthedesigndevelopedbythisgroupofstudents,basedonaMotorolaMC68HC05familymicrocontroller,aredescribed.Thepedagogicalvalueoftheproblemisalsodiscussedthroughadescriptionofsomeofthekeystepsinthedesignprocess.Itisshownthatthesolutionrequiresbroadknowledgedrawnfromseveralengineeringdisciplinesincludingelectrical,mechanical,andcontrolsystemsengineering.

Introduction

Thedesignprojectwhichisthesubjectofthispaperoriginatedfromareal-worldapplication.AprototypeofamicroscopeslidedryerhadbeendevelopedaroundanOmegaTMmodelCN-390temperaturecontroller,andtheobjectivewastodevelopacustomtemperaturecontrolsystemtoreplacetheOmegasystem.Themotivationwasthatacustomcontrollertargetedspecificallyfortheapplicationshouldbeabletoachievethesamefunctionalityatamuchlowercost,astheOmegasystemisunnecessarilyversatileandequippedtohandleawidevarietyofapplications.

ThemechanicallayoutoftheslidedryerprototypeisshowninFigure1.Themainelementofthedryerisalarge,insulated,air-filledchamberinwhichmicroscopeslides,eachwithatissuesampleencasedinparaffin,canbesetoncaddies.Inorderthattheparaffinmaintaintheproperconsistency,thetemperatureintheslidechambermustbemaintainedatadesired(constant)temperature.Asecondchamber(theelectronicsenclosure)housesaresistiveheaterandthetemperaturecontroller,andafanmountedontheendofthedryerblowsairacrosstheheater,carryingheatintotheslidechamber.Thisdesignprojectwascarriedoutduringacademicyear1996-97byfourstudentsundertheauthor’ssupervisionasaSeniorDesignprojectinthe

DepartmentofEngineeringScienceatTrinityUniversity.Thepurposeofthispaperis

10.25"TopView

Electronicsenclosure( Tempe工苜tuiuizontfuLlut

FigureLSlided/yerwechantcallayoui

todescribetheproblemandthestudents’solutioninsomedetail,andtodiscusssome

ofthepedagogicalopportunitiesofferedbyaninterdisciplinarydesignprojectofthistype.Thestudents’ownreportwaspresentedatthe1997NationalConferenceonUndergraduateResearch[1].Section2givesamoredetailedstatementoftheproblem,includingperformancespecifications,andSection3describesthestudents’design.Section4makesupthebulkofthepaper,anddiscussesinsomedetailseveralaspectsofthedesignprocesswhichofferuniquepedagogicalopportunities.Finally,Section5offerssomeconclusions.

ProblemStatement

ThebasicideaoftheprojectistoreplacetherelevantpartsofthefunctionalityofanOmegaCN-390temperaturecontrollerusingacustom-designedsystem.Theapplicationdictatesthattemperaturesettingsareusuallykeptconstantforlongperiodsoftime,butit’snonethelessimportantthatstepchangesbetrackedina“reasonable”manner.Thusthemainrequirementsboildownto

allowingachambertemperatureset-pointtobeentered,

displayingbothset-pointandactualtemperatures,and

•trackingstepchangesinset-pointtemperaturewithacceptablerisetime,steady-stateerror,andovershoot.

Table1.TemperatureconWollerspecifications

Set-pointtemperatureentiy

Range

Precision

60-99°C

1nC

Set-pointtemperaturedisplay

RangePrecision

60-99°C

1℃

ChambeiLtemperaturedisplay

RangePrecisionAccuracy

60-99°C

1℃±1℃

Chambertemperaturestepresponse

Range(steadystate)Accuracy(steadystate)MaximuinovershootSettlingtime(to±LC)

60-99°C士1℃

1℃

120s

AlthoughnotexplicitlyapartofthespecificationsinTable1,itwasclearthatthecustomerdesireddigitaldisplaysofset-pointandactualtemperatures,andthatset-pointtemperatureentryshouldbedigitalaswell(asopposedto,say,throughapotentiometersetting).

SystemDesign

Therequirementsfordigitaltemperaturedisplaysandsetpointentryaloneareenoughtodictatethatamicrocontrollerbaseddesignislikelythemostappropriate.Figure2showsablockdiagramofthestudents’design.

Themicrocontroller,aMotorolaMC68HC705B16(6805forshort),istheheartofthesystem.Itacceptsinputsfromasimplefour-keykeypadwhichallowspecificationoftheset-pointtemperature,anditdisplaysbothset-pointandmeasuredchamber

temperaturesusingtwo-digitseven-segmentLEDdisplayscontrolledbyadisplaydriver.Alltheseinputsandoutputsareaccommodatedbyparallelportsonthe6805.Chambertemperatureissensedusingapre-calibratedthermistorandinputviaoneofthe6805’sanalog-to-digitalinputs.Finally,apulse-widthmodulation(PWM)outputonthe6805isusedtodrivearelaywhichswitcheslinepowertotheresistiveheateroffandon.

Figure3showsamoredetailedschematicoftheelectronicsandtheirinterfacingtothe6805.Thekeypad,aStorm3K041103,hasfourkeyswhichareinterfacedtopinsPA0{PA3ofPortA,configuredasinputs.Onekeyfunctionsasamodeswitch.Twomodesaresupported:setmodeandrunmode.Insetmodetwooftheotherkeysareusedtospecifytheset-pointtemperature:oneincrementsitandonedecrements.Thefourthkeyisunusedatpresent.TheLEDdisplaysaredrivenbyaHarrisSemiconductorICM7212displaydriverinterfacedtopinsPB0{PB6ofPortB,configuredasoutputs.Thetemperature-sensingthermistordrives,throughavoltagedivider,pinAN0(oneofeightanaloginputs).Finally,pinPLMA(oneoftwoPWMoutputs)drivestheheaterrelay.

Figure3.Schematicof basrd

Softwareonthe6805implementsthetemperaturecontrolalgorithm,maintains

WV

Heater

12TABcDAE

thetemperaturedisplays,andalterstheset-pointinresponsetokeypadinputs.

Becauseitisnotcompleteatthiswriting,softwarewillnotbediscussedindetailinthispaper.Thecontrolalgorithminparticularhasnotbeendetermined,butitislikelytobeasimpleproportionalcontrollerandcertainlynotmorecomplexthanaPID.SomecontroldesignissueswillbediscussedinSection4,however.

TheDesignProcess

Althoughessentiallytheprojectisjusttobuildathermostat,itpresentsmanynicepedagogicalopportunities.Theknowledgeandexperiencebaseofaseniorengineeringundergraduatearejustenoughtobringhimorhertothebrinkofasolutiontovariousaspectsoftheproblem.Yet,ineachcase,realworldconsiderationscomplicatethesituationsignificantly.

Fortunatelythesecomplicationsarenotinsurmountable,andtheresultisaverybeneficialdesignexperience.Theremainderofthissectionlooksatafewaspectsoftheproblemwhichpresentthetypeoflearningopportunityjustdescribed.Section4.1discussessomeofthefeaturesofasimplifiedmathematicalmodelofthethermalpropertiesofthesystemandhowitcanbeeasilyvalidatedexperimentally.Section4.2describeshowrealisticcontrolalgorithmdesignscanbearrivedatusingintroductoryconceptsincontroldesign.Section4.3pointsoutsomeimportantdeficienciesofsuchasimplifiedmodeling/controldesignprocessandhowtheycanbeovercomethroughsimulation.Finally,Section4.4givesanoverviewofsomeofthemicrocontroller-relateddesignissueswhichariseandlearningopportunitiesoffered.

MathematicalModel

Lumped-elementthermalsystemsaredescribedinalmostanyintroductorylinearcontrolsystemstext,andjustthissortofmodelisapplicabletotheslidedryerproblem.Figure4showsasecond-orderlumped-elementthermalmodeloftheslidedryer.ThestatevariablesarethetemperaturesTaoftheairintheboxandTboftheboxitself.Theinputstothesystemarethepoweroutputq(t)oftheheaterandtheambienttemperatureT¥.maandmbarethemassesoftheairandthebox,respectively,andCaandCbtheirspecificheats.四1and四2areheattransfercoefficientsfromtheairtotheboxandfromtheboxtotheexternalworld,respectively.

Figure4.Lmiped-elemeiilthermalmodel

It’snothardtoshowthatthe(linearized)stateequationscorrespondingtoFigure

4are

(〃-㈤

-m~乙j

TakingLaplacetransformsof(1)and(2)andsolvingforTa(s),whichistheoutputofinterest,givesthefollowingopen-loopmodelofthethermalsystem:

whereKisaconstantandD(s)isasecond-orderpolynomial.K,tz,andthecoefficientsofD(s)arefunctionsofthevariousparametersappearingin(1)and(2).Ofcoursethevariousparametersin(1)and(2)arecompletelyunknown,butit’snothardtoshowthat,regardlessoftheirvalues,D(s)hastworealzeros.Thereforethemaintransferfunctionofinterest(whichistheonefromQ(s),sincewe’llassumeconstantambienttemperature)canbewritten

Moreover,it’snottoohardtoshowthat1=tp1<1=tz<1=tp2,i.e.,thatthezeroliesbetweenthetwopoles.Bothoftheseareexcellentexercisesforthestudent,andtheresultistheopenlooppole-zerodiagramofFigure5.

FigurexPole-zerodiagramofGaq(s)

Obtainingacompletethermalmodel,then,isreducedtoidentifyingtheconstantKandthethreeunknowntimeconstantsin(3).Fourunknownparametersisquiteafew,butsimpleexperimentsshowthat1=tp1_1=tz;1=tp2sothattz;tp2_0aregoodapproximations.Thustheopen-loopsystemisessentiallyfirst-orderandcanthereforebewritten

G/s)=含 ⑷

(wherethesubscriptp1hasbeendropped).

Simpleopen-loopstepresponseexperimentsshowthat,forawiderangeofinitialtemperaturesandheatinputs,K_0:14_=Wandt_295s.1

ControlSystemDesign

Usingthefirst-ordermodelof(4)fortheopen-looptransferfunctionGaq(s)andassumingforthemomentthatlinearcontroloftheheaterpoweroutputq(t)ispossible,theblockdiagramofFigure6representstheclosed-loopsystem.Td(s)isthedesired,orset-point,temperature,C(s)isthecompensatortransferfunction,andQ(s)isthe

heateroutputinwatts.

Figure6.Simplifiedblockdiagrctntoftheclosed-loopsystem

Giventhissimplesituation,introductorylinearcontroldesigntoolssuchastherootlocusmethodcanbeusedtoarriveataC(s)whichmeetsthestepresponserequirementsonrisetime,steady-stateerror,andovershootspecifiedinTable1.Theupshot,ofcourse,isthataproportionalcontrollerwithsufficientgaincanmeetallspecifications.Overshootisimpossible,andincreasinggainsdecreasesbothsteady-stateerrorandrisetime.

Unfortunately,sufficientgaintomeetthespecificationsmayrequirelargerheatoutputsthantheheateriscapableofproducing.Thiswasindeedthecaseforthissystem,andtheresultisthattherisetimespecificationcannotbemet.Itisquiterevealingtothestudenthowusefulsuchanoversimplifiedmodel,carefullyarrivedat,canbeindeterminingoverallperformancelimitations.

SimulationModel

GrossperformanceanditslimitationscanbedeterminedusingthesimplifiedmodelofFigure6,butthereareanumberofotheraspectsoftheclosed-loopsystemwhoseeffectsonperformancearenotsosimplymodeled.Chiefamongtheseare

quantizationerrorinanalog-to-digitalconversionofthemeasuredtemperatureand

theuseofPWMtocontroltheheater.

Bothofthesearenonlinearandtime-varyingeffects,andtheonlypracticalwaytostudythemisthroughsimulation(orexperiment,ofcourse).

Figure7showsaSimulinkTMblockdiagramoftheclosed-loopsystemwhichincorporatestheseeffects.A/DconverterquantizationandsaturationaremodeledusingstandardSimulinkquantizerandsaturationblocks.ModelingPWMismorecomplicatedandrequiresacustomS-functiontorepresentit.

Figure7,SiumUnkblockdiagramofclosed-loopsystem

ThissimulationmodelhasprovenparticularlyusefulingaugingtheeffectsofvaryingthebasicPWMparametersandhenceselectingthemappropriately.(I.e.,thelongertheperiod,thelargerthetemperatureerrorPWMintroduces.Ontheotherhand,alongperiodisdesirabletoavoidexcessiverelay“chatter,”amongotherthings.)PWMisoftendifficultforstudentstograsp,andthesimulationmodelallowsanexplorationofitsoperationandeffectswhichisquiterevealing.

TheMicrocontroller

Simpleclosed-loopcontrol,keypadreading,anddisplaycontrolaresomeoftheclassicapplicationsofmicrocontrollers,andthisprojectincorporatesallthree.Itisthereforeanexcellentall-aroundexerciseinmicrocontrollerapplications.Inaddition,becausetheprojectistoproduceanactualpackagedprototype,itwon’tdotouseasimpleevaluationboardwiththeI/Opinsjumperedtothetargetsystem.Instead,it’snecessarytodevelopacompleteembeddedapplication.Thisentailsthechoiceofanappropriatepartfromthebroadrangeofferedinatypicalmicrocontrollerfamilyandlearningtouseafairlysophisticateddevelopmentenvironment.Finally,acustomprinted-circuitboardforthemicrocontrollerandperipheralsmustbedesignedandfabricated.

MicrocontrollerSelection.Inviewofexistinglocalexpertise,theMotorolalineofmicrocontrollerswaschosenforthisproject.Still,thisdoesnotnarrowthechoicedownmuch.Afairlydisciplinedstudyofsystemrequirementsisnecessarytospecifywhichmicrocontroller,outofscoresofvariants,isrequiredforthejob.Thisisdifficultforstudents,astheygenerallylacktheexperienceandintuitionneededaswellastheperseverancetowadethroughmanufacturers’selectionguides.

Partoftheproblemisinchoosingmethodsforinterfacingthevariousperipherals(e.g.,whatkindofdisplaydrivershouldbeused?).AstudyofrelevantMotorolaapplicationnotes[2,3,4]provedveryhelpfulinunderstandingwhatbasicapproachesareavailable,andwhatmicrocontroller/peripheralcombinationsshouldbeconsidered.

TheMC68HC705B16wasfinallychosenonthebasisofitsavailableA/DinputsandPWMoutputsaswellas24digitalI/Olines.Inretrospectthisisprobablyoverkill,asonlyoneA/Dchannel,onePWMchannel,and11I/Opinsareactuallyrequired(seeFigure3).Thedecisionwasmadetoerronthesafesidebecauseacompletedevelopmentsystemspecifictothechosenpartwasnecessary,andtheprojectbudgetdidnotpermitasecondsuchsystemtobepurchasedshouldthefirstproveinadequate.

MicrocontrollerApplicationDevelopment.Breadboardingoftheperipheral

hardware,developmentofmicrocontrollersoftware,andfinaldebuggingandtestingofacustomprinted-circuitboardforthemicrocontrollerandperipheralsallrequireadevelopmentenvironmentofsomekind.Thechoiceofadevelopmentenvironment,likethatofthemicrocontrolleritself,canbebewilderingandrequiressomefacultyexpertise.Motorolamakesthreegradesofdevelopmentenvironmentrangingfromsimpleevaluationboards(ataround$100)tofull-blownreal-timein-circuitemulators(atmorelike$7500).Themiddleoptionwaschosenforthisproject:theMMEVS,whichconsistsof_aplatformboard(whichsupportsall6805-familyparts),_anemulatormodule(specifictoB-seriesparts),and_acableandtargetheadadapter(package-specific).Overall,thesystemcostsabout$900andprovides,withsomelimitations,in-circuitemulationcapability.ItalsocomeswiththesimplebutsufficientsoftwaredevelopmentenvironmentRAPID[5].

Studentsfindlearningtousethistypeofsystemchallenging,buttheexperiencetheygaininreal-worldmicrocontrollerapplicationdevelopmentgreatlyexceedsthetypicalfirst-courseexperienceusingsimpleevaluationboards.

Printed-CircuitBoard.Thelayoutofasimple(thoughdefinitelynottrivial)printed-circuitboardisanotherpracticallearningopportunitypresentedbythisproject.Thefinalboardlayout,withpackageoutlines,isshown(at50%ofactualsize)inFigure8.Therelativesimplicityofthecircuitmakesmanualplacementandroutingpractical—infact,itlikelygivesbetterresultsthanautomaticinanapplicationlikethis—andthestudentisthereforeexposedtofundamentalissuesofprinted-circuitlayoutandbasicdesignrules.Thelayoutsoftwareusedwastheverynicepackagepcb,2andtheboardwasfabricatedin-housewiththeaidofourstaffelectronicstechnician.

Figure&Printed-circidtlayoutformicrocontrollerboard

5Conclusion

Theaimofthispaperhasbeentodescribeaninterdisciplinary,undergraduateengineeringdesignproject:amicrocontroller-basedtemperaturecontrolsystemwithdigitalset-pointentryandset-point/actualtemperaturedisplay.Aparticulardesignofsuchasystemhasbeendescribed,andanumberofdesignissueswhicharise—fromavarietyofengineeringdisciplines—havebeendiscussed.Resolutionofthese