机电一体化英文论文

Usingmechatronicsinearlydesign

ThomasR.Kurfess*,JohnG.Witzel

TheGeorgeW.Woodru󰀌SchoolofMechanicalEngineering,GeorgiaInstituteofTechnology,

Atlanta,GA30332-0405,USA

Abstract

Thispaperpresentstheresultsofintegratingmechatronicsintoalargesecondyeardesigncourse.Issuesrelatedtocourseobjectives,implementation,costsandresultsfromacurricu-lumperspectivearepresented.Thesystemsdevelopedforthiscoursearetheresultofthreeyearsofsystemdesignandmodi®cation.Theresultsofthispaperdemonstratethatforre-alisticcostsande󰀤orts,mechatronicscanbeintegratedintoasophomoreleveldesignclassthatservicesapproximately300studentsperyear.Furthermore,resultsfromtheseniorlevelcapstonedesigncourseindicatethattheuseofmechatronicspermeatesthroughoutthestu-dents'career.Ó2002ElsevierScienceLtd.Allrightsreserved.

1.Introduction

Mechatronicsisabuzzwordthatcanbeseeninawidevarietyofcurriculathroughouttheworld.Itisthecombinationofmechanicalandelectronicsystemdesignandimplementation.Therealityisthatmostengineersworkintheworldofmechatronics,astherearerelativelyfewsystemsthatarepurelymechanicalorelectronic.Thedi󰀲cultyfacingourstudentsisthatelectromechanicalsystemdesignandimplementationisnottaughtuntiladvancedcoursesintheundergraduatecurriculum.Alltooften,thesecoursesareelectivecoursesandarenottakenbyallofthestudents.Toaddressthiscurricularshortcoming,thedesigncoursetakenbysecondyearstudentsintheWoodru󰀤SchoolofMechanicalEngineeringatGeorgiaTechhasbeenenhancedtoincorporateahands-onmechatronicsexperience.

Tofullypreparestudentsforindustrialenvironments,theymustbecapableofaddressingavarietyofbasicissuesinelectromechanicalsystemdesignandim-

*Correspondingauthor.Tel.:+1-404-4-0301;fax:+1-404-4-9342.E-mailaddress:thomas.kurfess@me.gatech.edu(T.R.Kurfess).

0957-4158/02/$-seefrontmatterÓ2002ElsevierScienceLtd.Allrightsreserved.PII:S0957-4158(01)000-2

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plementation.Thecourseprovidesaplatformtoaccomplishthistask.Targetedatmechanicalengineeringstudents,itprovidesthemwiththenecessaryelectronicscontrolbackgroundtoaddressavarietyofsituationsintherealworld.

Sincethestudentsdesignandbuildtheirelectromechanicaldevices,theyareaf-fordedtheopportunitytoactuallyreceivesomehands-onexperienceinmachiningandelectronicassembly.Asthisisthe®rstdesigncoursetakenbythestudents,itprovidesthemwithsomemotivationfortheirothercoursesduringtheremainderoftheiryearsatGeorgiaTech.Italsoprovidesastrongplatformfortheirmoread-vancedesignandlaboratorycoursesyieldingbetter-preparedengineers.

2.Theelectricalinterfacesystem

Thecontrolleror``systembrains''employed,isabasicstamp2SX(BS2SX)(seeFig.1).Itisbasedonaperipheralinterfacecontroller(PIC)chipthatistypicallyfoundonappliancessuchasmicrowaveovensandwashingmachines.TheBS2SXiscapableofexecutingapproximately10,000instructionspersecondandisdesignedtobeasimpletoolforcontrollingelectricalsystems.Theelectricalinterfacesystem(EIS)wasdesignedintheSchoolofMechanicalEngineeringatGeorgiaTechasthefoundationbywhichthestudentscanmakeuseoftheBS2SX.TheMarkIIIEIS,showninFig.2,isthethirdgenerationoftheunitandistheresultofthreeyearsof®eldtestingandredesigningthesystem.Itconsistsofasingleprintedcircuit(PC)boardthatwasdesignedandpopulatedatGeorgiaTech.TheBS2SXisdirectlyintegratedintothePCboard.Thedesignfortheboardiscompletelyelectronic(usingastandardCADsystem)andisreadilyreplicatedviaavarietyofPCboardmanufacturingtechniques.Adescriptionofthelessonslearnedwithrespecttohardwareispresentedlaterinthispaper.

TheEISispoweredbytwo,6Vleadacidbatterieslocatedbeneaththeboard.Studentsaresuppliedwithatricklechargertokeepthebatteriescharged.TheEISisprogrammedinaWindowsenvironmentusingfreesoftwaresuppliedbythe

Fig.1.Thebasicstamp.

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Fig.2.TheMarkIIIEIS.

produceroftheBS2SX.TheEISisconnectedtotheprogrammingcomputereitherviaaserialportorviaanRFlinkthatisintegratedintothesystem.TheRFlinkenablesreal-timedebuggingwithouttheworryofacableconnection.2.1.Actuatorsandsensors

Presently,theactuatorsemployedbythestudentsincludetwosteppermotors,twoDCmotors,twosolenoidsandtwoshapememoryalloy(SMA)actuators.Thesystembatteriesstoreenoughenergytopermitseveralhoursoftestingandopera-tion.Thus,thestudentsareinstructedtochargetheirbatteriesovernight,everynight.Theleadacidbatteriesdonothaveanymemory;thus,dailychargingdoesnotreducethelifespanofthebattery.

Tosupportthesteppermotors,twosteppermotorcontrolsarebuiltintotheEIS.Infact,thesteppermotorcontrolsaredesignedonanotherPICchip.InsupportoftheDCmotorsaswellastheSMA'sandsolenoids,severalpoweramplifyingcircuitsareavailableonthesystem.AscanbeseeninFig.2thereareavarietyofsocketsontheEIS.Thesearespeci®callydesignedsuchthatthevariousactuatorscannotbeattachedtotheincorrectport,asituationthatmayresultindamage(aswaslearnedontheMarkIEIS).

Someexcellentlessonscanbetaughtwiththeseparticularactuators.First,thefunctionofasteppermotorisaclearlessonthatthestudentslearn.Theyalsolearnthedi󰀤erencebetweenasteppermotorandaDCservomotor.Furthermore,theylearnabitaboutpowerampli®cation.Thus,inthefuturetheywillnottrytodrivelargemotorsorsolenoidswithlowpowercircuitssuchascomputerparallelports.Finally,theylearnaboutpulsewidthmodulation(PWM)asthisishowtheDCmotors'velocityiscontrolled.Infact,theBS2SXactuallyhasaPWMcommandthatpulsewidthmodulatesaspeci®cpin'sTTLsignal.Thissignalisthenampli®edand

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senttothemotor.Again,noneoftheseideasisdi󰀲cultataconceptuallevel,andthatiswhatispresentedtothesecondyearstudents.

Thesensorsusedbythestudentsincludeavarietyofmicroswitches,aninfrared(IR)range®nder(10±100cm)andavariableresistance¯exsensor.The¯exsensorisa¯exiblestripwhoseresistancevarieswiththeamountanddirectionofthesensor's¯exing.

Thesesensorso󰀤ersomeinterestinginsightinmechatronicsandsystemsforthestudents.Firstofall,justtheconnectionofaswitchtothecontrollerdemonstratesthemostbasicmeansbywhichonecommunicatestoacomputer.TheIRrange®nderrequiresanA/Dconverter.AsimpleA/Dconverterchip,costingapproxi-mately$1.50,isincorporatedintotheEISandcommunicateswiththeBS2SXviaserialcommunication.TheA/Dchipdemonstrateshowonetakesananalogsignalfromtherealworldandconveysthatinformationtothedigitalworldofthecom-puter.Furthermore,inordertocommunicatewiththeA/Dchip,thestudentsneedtounderstandsomebasicserialcommunicationsincludinghandshaking.Alloftheseconceptsaretaughttothestudents,andtheyreadilycomprehendthem.

Themeansbywhichthe¯exsensorresistanceiscomputedisquiteingeniousandworthdiscussing.TheresistancetobemeasuredisconnectedtoacapacitortoformasimpleRCcircuit.Oneendofthecircuitisconnectedtoground,theothertoapinontheBS2SXthatissettooutputmode.(Note,thepincaneitherbe0or5V,itisastandarddigitalTTLpinsignal.)Thepinisactivated,chargingtheRCcircuit.AfterthecapacitorhasbeenchargedtheBS2SXpinischangedtoinputmodeandthecapacitorispermittedtodischarge.Simultaneously,aninternalclockintheBS2SXisstarted.WhenthevoltageofthecircuitdropsbelowthethresholdvoltagefortheBS2SX(1.5V)theinternalclockisstopped.The3.5Vdropinconjunctionwiththevoltagedroptimeandthecapacitor'svaluepermitthecalculationoftheresistance.Inreality,thecalculationoftheresistanceisfairlyinaccurate.However,thisap-proachprovidesexcellentfeedbackwithrespecttoapotentiometer'schangeinre-sistance.Thisapproachisanexcellentexampleforthestudents,andasallofthestudentshavehadE&Mphysics,theyunderstandthebasicprinciplesemployed.2.2.Costs

Allofthecomponentsusedforthesystemsareo󰀤theshelfitemstokeepthecostminimal.Asanexampleofcosts,themicrocontrollerwithamanualisapproxi-mately$60,A/Dchipsusedforsensorinterfacingcostapproximately$1.50,standardmotorsthatcanbeusedbythissystemscostapproximately$7.Therealityisthatthesystemsarerelativelyinexpensive($350pergroup);however,itmustberememberedthatthereisalargenumberofgroupsinthecourse(approximately50groupspersemester).

Whilethecostoftheindividualsystemsisquitereasonable,otherbaselineex-pensesmustbeconsidered.Thereareasetofmajorstart-upcostsinvolvedinde-velopingandfabricatingthemechatronicsystems.Thebulkofthesecostsareduetothesmallmachinetoolsusedbythestudents($500permachinetool)andtheprintedboardprototypingsystemfortheelectronicsfabrication.Othersigni®cantstart-up

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Fig.3.Standardlathe.

Fig.4.Standardmill.

costsincludethebenchesrequiredtohousethelathesandmills(seeFigs.3and4),andbasictooling(mills,boringbars,drills,taps,etc.).Theothermajorcostcom-ponentispersonneltime,includingtheteachingandsupportpersonnel.ForthelargeclasssizeatGeorgiaTech,theequivalentofonefulltimetechnicianand8TA'sareemployedbythecourse.Thisisasubstantialcommitmentinfunds.3.Thecompetition/project

ThestudentsusetheEISinaprojectthatisusuallyawell-speci®edand®xedcompetitionassignedtoteamsconsistingofthreetofourstudents.Thecompetitionrequiresthestudentstodesignelectromechanicalsystemsthatperformaspeci®ctask.Thecourseisdesignedtobefunandtopermitawidespectrumoflearningexperiences.However,astudentteam'sprojectgradeisdependentonhowwelltheirsystemperformsinthecompetition.Thecourseisdesignedsuchthatevenastudentwhosesystemsperformpoorlyinthecompetitiondoesnotirreversiblyharmtheirgrade.Duringtheprojectthestudentsaregivenasetofrawmaterialsthattheyusetofabricateasystem.Nominally,theyusethesmallmachinetools(millsandlathes,aswellasbandsawsanddrillpresses)inthedesignstudioforthistask.Tohighlight

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thedi󰀤erencesbetweendesignsusingpurelymechanicalcomponentsandonesthatemploymechatronics,aprojectwasdesignedthatcouldbeexecutedbyeitherapurelymechanicalsystemsoroneaugmentedwithmechatronicssystems.Forthisprojectastudiosectionof20students(®veteamsoffourstudents)employedmechatronic(EIS)systems.SeveralotherprojectswhereallofthestudentsemployanEIShavebeenrun.However,itisworthwhiletoreviewthisparticularprojectsinceitdemonstratesthefactthatgoodmechanicaldesignandathoroughunder-standingoftheprojectcanmitigateanyadvantagesthatthemechatronicsystemsmighthave.Theprojectisdescribedintheremainderofthissection.

ThegoalofthisstudioprojectwastodesignadevicethatplacesaCDROMonatarget.Boththemechatronicsectionaswellasthepurelymechanicalsectionsareprovidedwiththesameproject.TheCDROMisplaced3ftinfrontofateam'sdeviceand2ftfromtheedgeofthetracktothedevice'srightasshowninFig.5.Thetargetislocated1ftinfrontoftheopponent'sdevice(4ftdirectlyinfrontoftheteam'sdevice).TheobjectiveistoplacetheCDusingonlyenergyfrom®vemousetrapsandgravity.Themechatronicsectionhasuseofseveralelectricalactu-atorsaswell.Theseactuatorsaredescribedinthefollowingsection.

Tomakethecompetitionmoreinteresting,anopposingteamwillattempttomovetheCDplaced1ftinfrontoftheirdeviceand1ftfromtheedgeofthetracktotheirrighttoatargetlocation.Pointsareawardedbasedontheaccuracyofthesystem;thus,theaccuracyofthedeviceisparamount,asisitsreliabilityandre-peatability.Eachdevicewillcompeteagainsttheotherdevicesinadoubleelimi-nationcompetition.Thewinnerofeachcompetitionisthesystemwho'sCDisclosesttothetargetposition.Thedevicemustconformtotherulesprovided.Partofthedevice'sperformancegradeisbasedonitsabilitytoplacetheCDROMontarget.ConcentriccirclesaredrawnaroundthetargetasshowninFig.6.ThepointvaluesofplacingtheCDwithineachcirclearealsoshowninthe®gure.

Forthisparticularcompetition,onesectionof20studentswasrunatGeorgiaTech.Thestudentswereprovidedwithalimitedsetofactuatorsandsensors,

Fig.5.Thecompetitiontrack.T.R.Kurfess,J.G.Witzel/Mechatronics12(2002)241±249247

Fig.6.Thetargetarea.includingDCmotors,solenoids,SMAlinearactuatorsandmicroswitches.Theirinitialramp-uptimeonthebasicstampwasapproximatelytwoweeks,afterwhichtheywerequitepro®cientwiththeunits.Theirdesignprojectsperformedtoallspeci®cations,andmettheexpectationsofthecourse.ThetwowinningmechatronicsdesignsareshowninFigs.7and8.

Theoverallcompetitionwasrunwithbothmechatronicssystemsaswellasme-chanicalsystems.Inthe®nalrounds,themechanicalandmechatronicsystemscompetedagainsteachother,inahead-to-headfashion.Theoverallwinnerofthecompetitionwasapurelymechanicalsystem.Thisresultwasduetotwomajor

Fig.7.Thewinningdesign.

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Fig.8.Thesecondplacedesign.

reasons.First,thecompetitionwastailoredspeci®callyformechanicalsystems.Second,thegroupsusingthemechatronicsystemstailoredtheirsystemstocompeteagainstothermechatronicsystems.Thus,theyweresubstantiallyslowerthanthepuremechanicalsystems.Intheend,thebestmechatronicdesignswerebasicallymechanicaldesignsaugmentedbymechatronics;ratherthanmechatronicsystemsaugmentedbyamechanicalsystem.Thisresultachievesoneofthecourseobjectives,topromotemechanicaldesignandaugmentwithelectronicsystems.Clearly,ifthemechanicaldesignisnotwellconceivedandexecuted,agoodmechatronicdesigncomponentcannoto󰀤setthisshortcoming.

4.Lessonslearned

Anumberoflessonswerelearnedduringthe®rstuseofthesesystems.The®rstandmostimportantlessonlearnedisthattheaveragestudenthaslittleifanyex-periencewithbothbasichardwareandlowlevelprogramming.Thisfactsurprisedthesta󰀤ofthedesigncourse.Thelargestproblem,byfar,wasthelackofhardwareexperience.Inparticular,anumberofEISunitswereshortedwhenthestudentsattemptedtodosomebasicwiringwhiletheunitwaspowered.Thisresultedinanumberofincidentsthatdamagedtheboardoritscomponents.Thestudentsquiteoftendidnotwiretheactuatorsandsensorscorrectly,eventhoughtheywerepro-videdwiththeappropriatewiringdiagramsandseverallecturesonwiring.Nomi-nally,thisdidnotdamagetheEIS;however,therewereinstanceswherethemotordriversweredamagedduetoincorrectwiring.Toaddressthisissue,theconnectionstotheEISwerestandardizedsuchthatactuatorsandsensorscanonlybeattachedasspeci®ed.Suchattachmentsreducethe¯exibilityofthesystem;however,theypro-videasubstantialreductionisdamagetotheEISandactuators.Furthermore,theMarkIIIEISunitsaredesignedwithcircuitbreakers.Therewasasigni®cantdebateastowhethercircuitbreakersorfusesshouldbeused.Fusescanbeby-passed(e.g.,

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aluminumfoilinthefuseholder)whilecircuitbreakerspermittheeasyignoranceofoverloadproblems.

Fromasoftwareperspective,itwasobservedthatwhilestudentshaveexperiencewithhigherlevelprogramminglanguagessuchasBASIC,CandC++,fewofthemhaveexperiencewithlowerlevelconceptssuchasbyteprogrammingorbitcontrolofports.Forexample,theconceptofsettingthe16I/OportsoftheBS2SXunitbysettingatwo-bytevariablebit-by-bit(16bits)toidentifywhetheraportisaninput(0)oranoutput(1),wasforeigntomoststudents.Terminologysuchasleastsigni®cantbit(LSB)andmostsigni®cantbit(MSB)wasgenerallyforeigntothestudents.Toremedythis,severalnewlecturesandstudioexercisesweredevelopedandimplemented.

Onemajorconcernofthecoursesta󰀤wasthepossibilityofanincreasedworkloadonthestudents.Thisconcerndidnotprovevalidwhentheworkloadofthespecialelectronicssectionofthecoursewascomparedtothatofthestandardsections.5.Conclusion

Asstatedpreviously,thesystemsbeingdevelopedatGeorgiaTechareproducedusingstandardtechniquesandtechnologythatarereadilytransferredtootheren-tities.Thus,otherinstitutionsinterestedinusingtheEISaswellastheactuatorsandsensorscanreadilyproducetheseunitsatareasonablecost.Withinoneyear,itisanticipatedthattheMarkIIIEISwillbetestedandfullydebuggedpermittingthedistributionofitsdesign.Onceaninstitutionhasthisdesign,itwillbepossibleforthemtooutsourcethefabricationofthemajorityoftheunit.IngeneralitisquiteeasyandfasttohavethePCboardsproducedandpopulatedatavarietyofcom-panies.The®nalpackagingoftheEISmayhavetobeaccomplishedinhouseattheacademicinstitution;however,thise󰀤ortshouldbeminimal.

WiththecompletionoftheMarkIIIEISdesign,itisanticipatedthatthestudentswilllearnmoreaboutmicrocontrollersandmechatronicsystems.Furthermore,theywillbemoreapttointegrateelectronicsintotheirmechanicaldesigns.Thishasbeendemonstratedbyanumberofseniorcapstonedesignprojectsthatmadeuseofthebasicstampintheirproject.Thecapabilitiesandqualityoftheseniordesignprojectshavebeensubstantiallyenhance.

Inconclusion,itispossibletointroducemechatronicstostudentsearlyintheiracademiccareers.Infact,ifintroducedproperly,mechatronicscanbeusedasastrongreinforcementoflessonspreviouslylearnedaswellasafoundationforfuturelessons.Finally,thestudentsenjoytheuseofthesesystems;thus,theyaremoremotivatedtolearnandunderstandthelessonstaughtintheclass.Acknowledgements

ThisworkwassupportedbytheGeorgeW.Woodru󰀤SchoolofMechanicalEngineering,theCollegeofEngineeringattheGeorgiaInstituteofTechnologyandtheGeorgiaInstituteofTechnology.