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1TemperatureControlUsingaMicrocontroller:AnInterdisciplinaryUndergraduateEngineeringDesignProjectJamesS.McDonaldDepartmentofEngineeringScienceTrinityUniversitySanAntonio,TX78212AbstractThispaperdescribesaninterdisciplinarydesignprojectwhichwasdoneundertheauthor’ssupervisionbyagroupoffourseniorstudentsintheDepartmentofEngineeringScienceatTrinityUniversity.Theobjectiveoftheprojectwastodevelopatemperaturecontrolsystemforanair-filledchamber.Thesystemwastoallowentryofadesiredchambertemperatureinaprescribedrangeandtoexhibitovershootandsteady-statetemperatureerroroflessthan1degreeKelvinintheactualchambertemperaturestepresponse.Thedetailsofthedesigndevelopedbythisgroupofstudents,basedonaMotorolaMC68HC05familymicrocontroller,aredescribed.Thepedagogicalvalueoftheproblemisalsodiscussedthroughadescriptionofsomeofthekeystepsinthedesignprocess.Itisshownthatthesolutionrequiresbroadknowledgedrawnfromseveralengineeringdisciplinesincludingelectrical,mechanical,andcontrolsystemsengineering.1IntroductionThedesignprojectwhichisthesubjectofthispaperoriginatedfromareal-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.Thepurposeofthispaperis2todescribetheproblemandthestudents’solutioninsomedetail,andtodiscusssomeofthepedagogicalopportunitiesofferedbyaninterdisciplinarydesignprojectofthistype.Thestudents’ownreportwaspresentedatthe1997NationalConferenceonUndergraduateResearch[1].Section2givesamoredetailedstatementoftheproblem,includingperformancespecifications,andSection3describesthestudents’design.Section4makesupthebulkofthepaper,anddiscussesinsomedetailseveralaspectsofthedesignprocesswhichofferuniquepedagogicalopportunities.Finally,Section5offerssomeconclusions.2ProblemStatementThebasicideaoftheprojectistoreplacetherelevantpartsofthefunctionalityofanOmegaCN-390temperaturecontrollerusingacustom-designedsystem.Theapplicationdictatesthattemperaturesettingsareusuallykeptconstantforlongperiodsoftime,butit’snonethelessimportantthatstepchangesbetrackedina“reasonable”manner.Thusthemainrequirementsboildownto·allowingachambertemperatureset-pointtobeentered,·displayingbothset-pointandactualtemperatures,and·trackingstepchangesinset-pointtemperaturewithacceptablerisetime,steady-stateerror,andovershoot.3AlthoughnotexplicitlyapartofthespecificationsinTable1,itwasclearthatthecustomerdesireddigitaldisplaysofset-pointandactualtemperatures,andthatset-pointtemperatureentryshouldbedigitalaswell(asopposedto,say,throughapotentiometersetting).3SystemDesignTherequirementsfordigitaltemperaturedisplaysandsetpointentryaloneareenoughtodictatethatamicrocontrollerbaseddesignislikelythemostappropriate.Figure2showsablockdiagramofthestudents’design.4Themicrocontroller,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.5Softwareonthe6805implementsthetemperaturecontrolalgorithm,maintainsthetemperaturedisplays,andaltersthes