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JamesS.McDonaldDepartmentofEngineeringScience

TrinityUniversitySanAntonio,TX78212

Abstract

Thispaperdescribesaninterdisciplinarydesignprojectwhichwasdoneundertheauthor’ssupervisionbyagroupoffourseniorstudentsintheDepartmentofEngineeringScienceatTrinityUniversity.Theobjectiveoftheprojectwastodevelopatemperaturecontrolsystemforanair-filledchamber.Thesystemwastoallowentryofadesiredchambertemperatureinaprescribedrangeandtoexhibitovershootandsteady-statetemperatureerroroflessthan1degreeKelvinintheactualchambertemperaturestepresponse.Thedetailsofthedesigndevelopedbythisgroupofstudents,basedonaMotorolaMC68HC05familymicrocontroller,aredescribed.

Thepedagogicalvalueoftheproblemisalsodiscussedthroughadescriptionofsomeofthekeystepsinthedesignprocess.Itisshownthatthesolutionrequiresbroadknowledgedrawnfromseveralengineeringdisciplinesincludingelectrical,mechanical,andcontrolsystemsengineering.

1Introduction

Thedesignprojectwhichisthesubjectofthispaperoriginatedfromareal-worldapplication.

AprototypeofamicroscopeslidedryerhadbeendevelopedaroundanOmegaTMmodel

CN-390temperaturecontroller,andtheobjectivewastodevelopacustomtemperaturecontrolsystemtoreplacetheOmegasystem.Themotivationwasthatacustomcontrollertargetedspecificallyfortheapplicationshouldbeabletoachievethesamefunctionalityatamuchlowercost,astheOmegasystemisunnecessarilyversatileandequippedtohandleawidevarietyofapplications.

ThemechanicallayoutoftheslidedryerprototypeisshowninFigure1.Themainelementofthedryerisalarge,insulated,air-filledchamberinwhichmicroscopeslides,eachwithatissuesampleencasedinparaffin,canbesetoncaddies.Inorderthattheparaffinmaintaintheproperconsistency,thetemperatureintheslidechambermustbemaintainedatadesired（constant）temperature.Asecondchamber（theelectronicsenclosure）housesaresistiveheaterandthetemperaturecontroller,andafanmountedontheendofthedryerblowsairacrossthe

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heater,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

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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.Chambertemperatureissensedusinga

pre-calibratedthermistorandinputviaoneofthe6805’sanalog-to-digitalinputs.Finally,apulse-widthmodulation（PWM）outputonthe680