基于单片机的温度控制外文文献及中文翻译.docx

基于单片机的温度控制外文文献及中文翻译.docx

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基于单片机的温度控制外文文献及中文翻译

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.

AprototypeofamicroscopeslidedryerhadbeendevelopedaroundanOmegaTMmodel-390temperaturecontroller,andtheobjectivewastodevelopacustomtemperaturecontrolsystemtoreplacetheOmegasystem.Themotivationwasthatacustomcontrollertargetedspecificallyfortheapplicationshouldbeabletoachievethesamefunctionalityatamuchlowercost,asthe

Omegasystemisunnecessarilyversatileandequippedtohandleawidevarietyofapplications.

ThemechanicallayoutoftheslidedryerprototypeisshowninFigure1.Themainelementofthedryerisalarge,insulated,air-filledchamberinwhichmicroscopeslides,eachwithatissuesampleencasedinparaffin,canbesetoncaddies.Inorderthattheparaffinmaintaintheproperconsistency,thetemperatureintheslidechambermustbemaintainedatadesired（constant）temperature.Asecondchamber（theelectronicsenclosure）housesaresistiveheaterandthetemperaturecontroller,andafanmountedontheendofthedryerblowsairacrosstheheater,carryingheatintotheslidechamber.Thisdesignprojectwascarriedoutduringacademicyear1996一97byfourstudentsundertheauthor5ssupervisionasaSeniorDesignprojectintheDepartmentofEngineeringScienceatTrinityUniversity.Thepurposeofthispaperistodescribetheproblemandthestudents5solutioninsomedetail,andtodiscusssomeofthepedagogicalopportunitiesofferedbyaninterdisciplinarydesignprojectofthistype.Thestudents'ownreportwaspresentedatthe1997NationalConferenceonUndergraduateResearch[1].Section2givesamoredetailedstatementoftheproblem,includingperformancespecifications,andSection3describesthestudents,design.Section4makesupthebulkofthepaper,anddiscussesinsomedetailseveralaspectsofthedesignprocesswhichofferuniquepedagogicalopportunities.Finally,Section5offerssomeconclusions.

Figure1.Slidediyerineclicmicallayoui

2ProblemStatement

ThebasicideaoftheprojectistoreplacetherelevantpartsofthefunctionalityofanOmega-390temperaturecontrollerusingacustom-designedsystem.Theapplicationdictatesthattemperaturesettingsareusuallykeptconstantforlongperiodsoftime,butifsnonethelessimportantthatstepchangesbetrackedina“reasonable”manner.Thusthemainrequirementsboildownto

•allowingachambertemperatureset-pointtobeentered,

•displayingbothset-pointandactualtemperatures,and

•trackingstepchangesinset-pointtemperaturewithacceptablerisetime,steady-stateerror,andovershoot.

Table1.Temperaturecontrollerspecifications

Set-pointtemperatureentry

RanaeJ.

60-99°C

Precision

1°C

Set-pointtemperaturedisplay

RanaeJ.

60-99°C

Precision

1°C

Chambertemperaturedisplay

RanaeJ.

60-99°C

Precision

1°C

Accuracy

±1°C

Chambertemperaturestepresponse

Range（steadystate）

60-99°C

Accuracy（steadystate）

±1°C

Maximumovershoot

1°C

Settlingtime（to±1°C）

120s

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

3SystemDesign

Therequirementsfordigitaltemperaturedisplaysandsetpointentryaloneareenoughtodictatethatamicrocontrollerbaseddesignislikelythemostappropriate・Figure2showsablockdiagramofthestudents5design.

Figure2.Temperaturecontrollerhardwcuvblockdiagram

Themicrocontroller,aMotorolaMC68HC705B16（6805forshort）,istheheartofthesystem.Itacceptsinputsfromasimplefour-keykeypadwhichallowspecificationoftheset-pointtemperature,anditdisplaysbothset-pointandmeasuredchambertemperaturesusingtwo-digitseven-segmentLEDdisplayscontrolledbyadisplaydriver.Alltheseinputsandoutputsareacmodatedbyparallelportsonthe6805.Chambertemperatureissensedusingapre-calibratedthermistorandinputviaoneofthe6805'sanalog-to-digitalinputs.Finally,apulse-widthmodulation（PWM）outputonthe6805isusedtodrivearelaywhichswitcheslinepowertotheresistiveheateroffandon.

Figure3showsamoredetailedschematicoftheelectronicsandtheirinterfacingtothe6805.Thekeypad,aStorm3K041103,hasfourkeyswhichareinterfacedtopinsPA0{PA3ofPortA,configuredasinputs.Onekeyfunctionsasamodeswitch.Twomodesaresupported:

setmodeandrunmode.Insetmodetwooftheotherkeysareusedtospecifytheset-pointtemperature:

oneincrementsitandonedecrements.Thefourthkeyisunusedatpresent.TheLEDdisplaysare

drivenbyaHarrisSemiconductorICM7212displaydriverinterfacedtopinsPB0IPB6ofPortB,configuredasoutputs.Thetemperature-sensingthermistordrives,throughavoltagedivider,pin

ANO（oneofeightanaloginputs）.Finally,pinPLMA（oneoftwoPWMoutputs）drivestheheaterrelay.

Figure3.SchematicofinicrocontroUerboard

Softwareonthe6805implementsthetemperaturecontrolalgorithm,maintainsthetemperaturedisplays,andalterstheset-pointinresponsetokeypadinputs.Becauseitisnotpleteatthiswriting,softwarewillnotbediscussedindetailinthispaper.Thecontrolalgorithminparticularhasnotbeendetermined,butitislikelytobeasimpleproportionalcontrollerandcertainlynotmoreplexthanaPID.SomecontroldesignissueswillbediscussedinSection4,

however.

4TheDesignProcess

Althoughessentiallytheprojectisjusttobuildathermostat,itpresentsmanynicepedagogicalopportunities.Theknowledgeandexperiencebaseofaseniorengineeringundergraduatearejustenoughtobringhimorhertothebrinkofasolutiontovariousaspectsoftheproblem.Yet,ineachcase,realworldconsiderationsplicatethesituationsignificantly.

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

4.1MathematicalModel

Lumped-elementthermalsystemsaredescribedinalmostanyintroductorylinearcontrolsystemstext,andjustthissortofmodelisapplicabletotheslidedryerproblem.Figure4showsasecond-orderlumped-elementthermalmodeloftheslidedryer.ThestatevariablesarethetemperaturesTaoftheairintheboxandTboftheboxitself.Theinputstothesystemarethepoweroutputq（t）oftheheaterandtheambienttemperatureT¥.maandmbarethemassesoftheairandthebox,respectively,andCaandCbtheirspecificheats.p1andjj2areheattransfer

coefficientsfromtheairtotheboxandfromtheboxtotheexternalworld,respectively.

Figure4.Luinped-elemeiuthe/刀model

Ifsnothardtoshowthatthe（linearized）stateequationscorrespondingtoFigure4are

〃皿方-

〃坊-

“1（爲r—Tb）

（1）

“1（爲一Tb）_'Tb—TJ

（2）

TakingLaplacetransformsof

（1）and

（2）andsolvingforTa（s）,whichistheoutputofinterest,givesthefollowingopen-loopmodelofthethermalsystem:

加）•

K（xzs+1）1T

whereKisaconstantandD（s）isasecond-orderpolynomial.K,tz,andthecoefficientsofD（s）arefunctionsofthevariousparametersappearingin

（1）and

（2）.Ofcoursethevariousparametersin

（1）and

（2）arepletelyunknown,butifsnothardtoshowthat,regardlessoftheirvalues,D（s）hastworealzeros.Thereforethemaintransferfunctionofinterest（whichistheonefromQ（s）,sinceweJIIassumeconstantambienttemperature）canbewritten

_爲⑸_Kg+1）

力W（^15+1）（^254-1）

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

Illi

"p2-

-1/g-1/ji

Figure5.PoJe-zerodicigrcimofGaq（s）

Obtainingapletethermalmode