基于单片机的温度控制外文文献及中文翻译.doc

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TemperatureControlUsingaMicrocontroller:

AnInterdisciplinaryUndergraduateEngineeringDesignProject

JamesS.McDonald

DepartmentofEngineeringScience

TrinityUniversity

SanAntonio,TX78212

Abstract：

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

1Introduction

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’ssupervisionasaSeniorDesignprojectintheDepartmentofEngineeringScienceatTrinityUniversity.Thepurposeofthispaperis

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

2ProblemStatement

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

·allowingachambertemperatureset-pointtobeentered,

·displayingbothset-pointandactualtemperatures,and

·trackingstepchangesinset-pointtemperaturewithacceptablerisetime,steady-stateerror,andovershoot.

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

3SystemDesign

Therequirementsfordigitaltemperaturedisplaysandsetpointentryaloneareenoughtodictatethatamicrocontrollerbaseddesignislikelythemostappropriate.Figure2showsablockdiagramofthestudents’design.

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

Softwareonthe6805implementsthetemperaturecontrolalgorithm,maintainsthetemperaturedisplays,andalterstheset-pointinresponsetokeypadinputs.Becauseitisnotcompleteatthiswriting,softwarewillnotbediscussedindetailinthispaper.Thecontrolalgorithminparticularhasnotbeendetermined,butitislikelytobeasimpleproportionalcontrollerandcertainlynotmorecomplexthanaPID.SomecontroldesignissueswillbediscussedinSection4,however.

4TheDesignProcess

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

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

4.1MathematicalModel

Lumped-elementthermalsystemsaredescribedinalmostanyintroductorylinearcontrolsystemstext,andjustthissortofmodelisapplicabletotheslidedryerproblem.Figure4showsasecond-orderlumped-elementthermalmodeloftheslidedryer.ThestatevariablesarethetemperaturesTaoftheairintheboxandTboftheboxitself.Theinputstothesystemarethepoweroutputq（t）oftheheaterandtheambienttemperatureT¥.maandmbarethemassesoftheairandthebox,respectively,andCaandCbtheirspecificheats.μ1andμ2areheattransfercoefficientsfromtheairtotheboxandfromtheboxtotheexternalworld,respectively.

It’snothardtoshowthatthe（linearized）stateequationscorrespondingtoFigure4are

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.

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

（wherethesubscriptp1hasbeendropped）.

Simpleopen-loopstepresponseexperimentsshowthat,forawiderangeofinitialtemperaturesandheatinputs,K_0:

14_=Wandt_295s.1

4.2ControlSystemDesign

Usingthefirst-ordermodelof（4）fortheopen-looptransferfunctionGaq（s）andassumingforthemomentthatlinearcontroloftheheaterpoweroutputq（t）ispossible,theblockdiagramofFigure6representstheclosed-loopsystem.Td（s）isthedesired,orset-point,tem