毕业设计方案外文原文(4万字符)

编号：毕业设计外文翻译（原文）院（系）：电子工程与自动化学院专业：测控技术与仪器学生姓名：李XX学号：XXX指引教师单位：电子工程与自动化学院指引教师：XXX职称：XXX年5月20日InformationControlAutomaticControlAutomaticcontrolhasplayedavitalroleintheadvanceofengineeringandscience.Inadditiontoitsextremeimportanceinspace-vehiclesystems，missile-guidancesystems，roboticsystems，andthelike，automaticcontrolhasbecomeanimportantandintegralpartofmodernmanufacturingandindustrialprocesses.Forexample，automaticcontrolisessentialinthenumericalcontrolofmachinetoolsinthemanufacturingindustries，inthedesignofautopilotsystemsintheaerospaceindustries，andinthedesignofcarsandtrucksintheautomobileindustries.Itisalsoessentialinsuchindustrialoperationsascontrollingpressure，temperature，humidity，viscosity，andflowintheprocessindustries.Sinceadvancesinthetheoryandpracticeofautomaticcontrolprovidethemeansforattainingoptimalperformanceofdynamicsystems，improvingproductivity，relievingthedrudgeryofmanyroutinerepetitivemanualoperations，andmore，mostengineersandscientistsmustnowhaveagoodunderstandingofthisfield.Controlengineeringisbasedonthefoundationsoffeedbacktheoryandlinearsystemanalysis，anditintegratestheconceptsofnetworktheoryandcommunicationtheory.Thereforecontrolengineeringisnotlimitedtoanyengineeringdisciplinebutisequallyapplicabletoaeronautical，chemical，mechanical，environmental，civil，andelectricalengineering.Forexample，acontrolsystemoftenincludeselectrical，mechanical，andchemicalcomponents.Furthermore，astheunderstandingofthedynamicsofbusiness，social，andpoliticalsystemsincreases，theabilitytocontrolthesesystemswillalsoincrease.Acontrolsystemisaninterconnectionofcomponentsformingasystemconfigurationthatwillprovideadesiredsystemresponse.Thebasisforanalysisofasystemisthefoundationprovidedbylinearsystemtheory，whichassumesacause-effectrelationshipforthecomponentsofasystem.Thereforeacomponentorprocesstobecontrolledcanberepresentedbyablock，asshowninFigure8.1.Theinput-outputrelationshiprepresentsthecause-and-effectrelationshipoftheprocess，whichinturnrepresentsaprocessingoftheinputsignaltoprovideanoutputsignalvariable，oftenwithapoweramplification.Ingeneral，controlsystemscanbecategorizedasbeingeitheropen-looporclosed–loop.Thedistinguishingfeaturebetweenthesetwotypesofcontrolsystemsistheuseoffeedbackcomparisonforclosed-loopoperation.1．Open-loopControlSystemAnopen-loopcontrolsystemutilizesacontrollerorcontrolactuatortoobtainthedesiredresponse，asshowninFigure8.2.Anopen-loopsystemisasystemwithoutfeedback，simplestformofcontrollingdevices.Figure8.3illustratesasimpletank-levelcontrolsystem.WewishtoholdthetanklevelhwithinreasonableacceptablelimitseventhoughtheoutletflowthroughvalueV1isvaried.ThiscanbeachievedbyirregularmanualadjustmentoftheinletflowratebyvalveV2.Thesystemisnotaprecisionsystem,asitdoesnothavethecapabilityofaccuratelymeasuringtheoutputflowratethroughvalveV1,theinputflowratethroughvalveV2，orthetanklevel.Figure8.4showsthesimplerelationshipthatexistsinthissystembetweentheinput(thedesiredtanklevel)andtheoutput(theactualtanklevel).Thiscontrolsystemdoesnothaveanyfeedbackcomparison,andthetermopenloopisusedtodescribethisabsence.Figure8.3Tank-levelcontrolsystemOutput(actualtanklevel)OuputInputFigure8.1ProcesstobecontrolledProcessFigure8.3Tank-levelcontrolsystemOutput(actualtanklevel)OuputInputFigure8.1ProcesstobecontrolledProcessOutputActuatingdeviceProcessDisireouputresponseFigure8.2Anopen-loopsystemhControlsystem(valueandoperator)Input(desiredtanklevel)Figure8.4Tank-levelcontrolsystemblackdiagram2．Closed-loopControlSystemIncontrasttoanopen-loopcontrolsystem，aclosed-loopcontrolsystemutilizesanadditionalmeasureoftheactualoutputtocomparetheactualoutputwiththedesiredoutputresponse.Themeasureoftheoutputiscalledthefeedbacksignal.Asimpleclosed-loopfeedbackcontrolsystemisshowninFigure8.5.Afeedbackcontrolsystemisacontrolsystemthattendstomaintainaprescribedrelationshipofonesystemvariabletoanotherbycomparingfunctionsofthesevariablesandusingthedifferenceasameansofcontrol.OutputComOutputComparisonControllerProcessDesireoutputresponseMeasurementFigure8.5 Close-loopcontrolsystemClosed-loopcontrolsystemsderivetheirvaluableaccuratereproductionoftheinputfromfeedbackcomparison.Anerrordetectorderivesasignalproportionaltothedifferencesbetweentheinputandoutput．Theclosed-loopcontrolsystemdrivestheoutputuntilitequalstheinputandtheerroriszero．Anydifferencesbetweentheactualanddesiredoutputwillbeautomaticallycorrectedinaclosed-loopcontrolsystem．Throughproperdesignthesystemcanbemaderelativelyindependentofsecondaryinputsandchangesincomponentcharacteristics．Figure8.6illustratesanautomatictank-levelcontrolversionofthesystemshowninFigure8.3.ItcanmaintainthedesiredtanklevelhwithinquiteaccuratetoleranceseventhroughtheoutputflowratethroughvalueV1isvaried.Ifthetanklevelisnotcorrect,anerrorvoltageisdeveloped．ThisisamplifiedandappliedtoamotordrivethatadjustsvalueV2inordertorestorethedesiredtanklevelbyadjustingtheinletflowrate．AblockdiagramanalogoustothissystemisshowninFigure8.7.Becausefeedbackcomparisonispresent,thetermclosed-loopisusedtodescribethesystem’soperation.Input(desiredtanklevel)hInput(desiredtanklevel)h+v-vPoweramplifierMotordriveFloatErrorFigure8.6 Automatictank-levelcontrolsystemPoweramplifierTankFloatMotordriveFigure8.7 Blackdiagramofautomatictank-levelcontrolsystemOutput(actualtanklevel)Error+-Duetotheincreasingcomplexityofthesystemundercontrolandtheinterestinachievingoptimumperformance，theimportanceofcontrolsystemengineeringhasgrowninthepastdecade.Furthermore，asthesystembecomemorecomplex，theinterrelationshipofmanycontrolledvariablesmustbeconsideredinthecontrolscheme.AblockdiagramdepictingamultivariablecontrolsystemisshowninFigure8.8.Theintroductionoffeedbackenablesustocontroladesiredoutputandcanimproveaccuracy，butitrequiresattentiontotheissueofstabilityofresponse.ControllerControllerProcessMeasurementDesiredoutputresponseOutputvariablesFigure8.8 MulutivariablecontrolsystemMicrocontrollerAmicrocontroller(sometimesabbreviatedµC，uCorMCU)isasmallcomputeronasingleintegratedcircuitcontainingaprocessorcore，memory，andprogrammableinput/outputperipherals.ProgrammemoryintheformofNORflashisalsooftenincludedonchip，aswellasatypicallysmallamountofRAM.Microcontrollersaredesignedforembeddedapplications，incontrasttothemicroprocessorsusedinpersonalcomputersorothergeneralpurposeapplications.Microcontrollersareusedinautomaticallycontrolledproductsanddevices，suchasautomobileenginecontrolsystems，implantablemedicaldevices，remotecontrols，officemachines，appliances，powertools，toysandotherembeddedsystems.Byreducingthesizeandcostcomparedtoadesignthatusesaseparatemicroprocessor，memory，andinput/outputdevices，microcontrollersmakeiteconomicaltodigitallycontrolevenmoredevicesandprocesses.Mixedsignalmicrocontrollersarecommon，integratinganalogcomponentsneededtocontrolnon-digitalelectronicsystems.Somemicrocontrollersmayusefour-bitwordsandoperateatclockratefrequenciesaslowas4kHz，forlowpowerconsumption(milliwattsormicrowatts).Theywillgenerallyhavetheabilitytoretainfunctionalitywhilewaitingforaneventsuchasabuttonpressorotherinterrupt；powerconsumptionwhilesleeping(CPUclockandmostperipheralsoff)maybejustnanowatts，makingmanyofthemwellsuitedforlonglastingbatteryapplications.Othermicrocontrollersmayserveperformance-criticalroles，wheretheymayneedtoactmorelikeadigitalsignalprocessor(DSP)，withhigherclockspeedsandpowerconsumption.EmbeddeddesignAmicrocontrollercanbeconsideredaself-containedsystemwithaprocessor，memoryandperipheralsandcanbeusedasanembeddedsystem.Themajorityofmicrocontrollersinusetodayareembeddedinothermachinery，suchasautomobiles，telephones，appliances，andperipheralsforcomputersystems.Thesearecalledembeddedsystems.Whilesomeembeddedsystemsareverysophisticated，manyhaveminimalrequirementsformemoryandprogramlength，withnooperatingsystem，andlowsoftwarecomplexity.Typicalinputandoutputdevicesincludeswitches，relays，solenoids，LEDs，smallorcustomLCDdisplays，radiofrequencydevices，andsensorsfordatasuchastemperature，humidity，lightleveletc.Embeddedsystemsusuallyhavenokeyboard，screen，disks，printers，orotherrecognizableI/Odevicesofapersonalcomputer，andmaylackhumaninteractiondevicesofanykind.1．InterruptsMicrocontrollersmustproviderealtime(predictable，thoughnotnecessarilyfast)responsetoeventsintheembeddedsystemtheyarecontrolling.Whencertaineventsoccur，aninterruptsystemcansignaltheprocessortosuspendprocessingthecurrentinstructionsequenceandtobeginaninterruptserviceroutine(ISR，or“interrupthandler”).TheISRwillperformanyprocessingrequiredbasedonthesourceoftheinterruptbeforereturningtotheoriginalinstructionsequence.Possibleinterruptsourcesaredevicedependent，andoftenincludeeventssuchasaninternaltimeroverflow，completingananalogtodigitalconversion，alogiclevelchangeonaninputsuchasfromabuttonbeingpressed，anddatareceivedonacommunicationlink.Wherepowerconsumptionisimportantasinbatteryoperateddevices，interruptsmayalsowakeamicrocontrollerfromalowpowersleepstatewheretheprocessorishalteduntilrequiredtodosomethingbyaperipheralevent.2．ProgramsMicrocontrollerprogramsmustfitintheavailableon-chipprogrammemory，sinceitwouldbecostlytoprovideasystemwithexternal，expandable，memory.Compilersandassemblersareusedtoconverthigh-levellanguageandassemblerlanguagecodesintoacompactmachinecodeforstorageinthemicrocontroller’smemory.Dependingonthedevice，theprogrammemorymaybepermanent，read-onlymemorythatcanonlybeprogrammedatthefactory，orprogrammemorymaybefield-alterableflashorerasableread-onlymemory.HigherintegrationIncontrasttogeneral-purposeCPUs，micro-controllersmaynotimplementanexternaladdressordatabusastheyintegrateRAMandnon-volatilememoryonthesamechipastheCPU.Usingfewerpins，thechipcanbeplacedinamuchsmaller，cheaperpackage.Integratingthememoryandotherperipheralsonasinglechipandtestingthemasaunitincreasesthecostofthatchip，butoftenresultsindecreasednetcostoftheembeddedsystemasawhole.EvenifthecostofaCPUthathasintegratedperipheralsisslightlymorethanthecostofaCPUandexternalperipherals，havingfewerchipstypicallyallowsasmallerandcheapercircuitboard，andreducesthelaborrequiredtoassembleandtestthecircuitboard.Thisintegrationdrasticallyreducesthenumberofchipsandtheamountofwiringandcircuitboardspacethatwouldbeneededtoproduceequivalentsystemsusingseparatechips.Furthermore，onlowpincountdevicesinparticular，eachpinmayinterfacetoseveralinternalperipherals，withthepinfunctionselectedbysoftware.Thisallowsaparttobeusedinawidervarietyofapplicationsthanifpinshaddedicatedfunctions.Micro-controllershaveprovedtobehighlypopularinembeddedsystemssincetheirintroductioninthe1970s.SomemicrocontrollersuseaHarvardarchitecture：separatememorybusesforinstructionsanddata，allowingaccessestotakeplaceconcurrently.WhereaHarvardarchitectureisused，instructionwordsfortheprocessormaybeadifferentbitsizethanthelengthofinternalmemoryandregisters；forexample：12-bitinstructionsusedwith8-bitdataregisters.Thedecisionofwhichperipheraltointegrateisoftendifficult.Themicrocontrollervendorsoftentradeoperatingfrequenciesandsystemdesignflexibilityagainsttime-to-marketrequirementsfromtheircustomersandoveralllowersystemcost.Manufacturershavetobalancetheneedtominimizethechipsizeagainstadditionalfunctionality.Microcontrollerarchitecturesvarywidely.Somedesignsincludegeneral-purposemicroprocessorcores，withoneormoreROM，RAM，orI/Ofunctionsintegratedontothepackage.Otherdesignsarepurposebuiltforcontrolapplications.Amicro-controllerinstructionsetusuallyhasmanyinstructionsintendedforbit-wiseoperationstomakecontrolprogramsmorecompact.Forexample，ageneralpurposeprocessormightrequireseveralinstructionstotestabitinaregisterandbranchifthebitisset，whereamicro-controllercouldhaveasingleinstructiontoprovidethatcommonly-requiredfunction.Microcontrollerstypicallydonothaveamathcoprocessor，sofloatingpointarithmeticisperformedbysoftware.ProgrammingenvironmentsMicrocontrollerswereoriginallyprogrammedonlyinassemblylanguage，butvarioushigh-levelprogramminglanguagesarenowalsoincommonusetotargetmicrocontrollers.Theselanguagesareeitherdesignedspeciallyforthepurpose，orversionsofgeneralpurposelanguagessuchastheCprogramminglanguage.Compilersforgeneralpurposelanguageswilltypicallyhavesomerestrictionsaswellasenhancementstobettersupporttheuniquecharacteristicsofmicrocontrollers.Somemicrocontrollershaveenvironmentstoaiddevelopingcertaintypesofapplications.Microcontrollervendorsoftenmaketoolsfreelyavailabletomakeiteasiertoadopttheirhardware.Manymicrocontrollersaresoquirkythattheyeffectivelyrequiretheirownnon-standarddialectsofC，suchasSDCCforthe8051，whichpreventusingstandardtools(suchascodelibrariesorstaticanalysistools)evenforcodeunrelatedtohardwarefeatures.Interpretersareoftenusedtohidesuchlowlevelquirks.Interpreterfirmwareisalsoavailableforsomemicrocontrollers.Forexample，BASIContheearlymicrocontrollersIntel8052；BASICandFORTHontheZilogZ8aswellassomemoderndevices.Typicallytheseinterpreterssupportinteractiveprogramming.Simulatorsareavailableforsomemicrocontrollers，suchasinMicrochip’sMPLABenvironmentandtheRevolutionEducationPICAXErange.Theseallowadevelopertoanalyzewhatthebehaviorofthemicrocontrollerandtheirprogramshouldbeiftheywereusingtheactualpart.Asimulatorwillshowtheinternalprocessorstateandalsothatoftheoutputs，aswellasallowinginputsignalstobegenerated.Whileontheonehandmostsimulatorswillbelimitedfrombeingunabletosimulatemuchotherhardwareinasystem，theycanexerciseconditionsthatmayotherwisebehardtoreproduceaswellinthephysicalimplementation，andcanbethequickestwaytodebugandanalyzeproblems.Recentmicrocontrollersareoftenintegratedwithon-chipdebugcircuitrythatwhenaccessedbyanin-circuitemulatorviaJTAG，allowdebuggingofthefirmwarewithadebugger.TypesofmicrocontrollersThereareseveraldozenmicrocontrollerarchitecturesandvendorsincluding：ParallaxPropellerIntel8051SiliconLaboratoriesPipelined8051MicrocontrollersARMprocessors(frommanyvendors)usingARM7orCortex-M3coresaregenerallymicrocontrollersSTMicroelectronicsSTM8(8-bit)，ST10(16-bit)andSTM32(32-bit)AtmelAVR(8-bit)，AVR32(32-bit)，andAT91SAM(32-bit)HitachiH8，HitachiSuperH(32-bit)MIPS(32-bitPIC32)NECV850(32-bit)PowerPCISEPSoC(ProgrammableSystem-on-Chip)Rabbit(8-bit)TexasInstrumentsMicrocontrollers：TIMSP43016-bitMicrocontrollers，C(32-bit)，andStellaris(32-bit)ToshibaTLCS-870(8-bit/16-bit)andmanyothers，someofwhichareusedinverynarrowrangeofapplicationsoraremorelikeapplicationsprocessorsthanmicrocontrollers.Themicrocontrollermarketisextremelyfragmented，withnumerousvendors，technologies，andmarkets.Notethatmanyvendorssell(orhavesold)multiplearchitectures.Examplesofmicrocontrollers1．Intel8051The8051architectureprovidesmanyfunctions(CPU，RAM，ROM，I/O，interruptlogic，timer，etc.)inasinglepackage.Anewandparticularlyusefulfeatureofthe8051corewastheinclusionofaBooleanprocessingenginewhichallowsbit-levelbooleanlogicoperationstobecarriedoutdirectlyandefficientlyoninternalregistersandRAM.Thiscriticalfeaturehelpedcementthe8051’spopularityinindustrialcontrolapplicationsbecauseitreducedcodesizebyasmuchas30%.Anothervaluedfeatureisfourseparateregistersets，whichgreatlyreducesinterruptlatencycomparedtothemorecommonmethodofstoringinterruptcontextonastack.2．ARMARMisa32-bitreducedinstructionsetcomputer(RISC)instructionsetarchitecture(ISA)developedbyARMHoldings.ItwasknownastheAdvancedRISCMachine，andbeforethatastheAcornRISCMachine.TheARMarchitectureisthemostwidelyused32-bitISAintermsofnumbersproduced.TherelativesimplicityofARMprocessorsmakesthemsuitableforlowpowerapplications.Asaresult，theyhavebecomedominantinthemobileandembeddedelectronicsmarket，asrelativelylow-cost，smallmicroprocessorsandmicrocontrollers.ARMprocessorsaredevelopedbyARMandbyARMlicensees.ProminentARMprocessorfamiliesdevelopedbyARMHoldingsincludetheARM7，ARM9，ARM11andCortex.3．AtmelAVRTheAVRisamodifiedHarvardarchitecture8-bitRISCsinglechipmicrocontrollerwhichwasdevelopedbyAtmelin1996.TheAVRwasoneofthefirstmicrocontrollerfamiliestouseon-chipflashmemoryforprogramstorage，asopposedtoone-timeprogrammableROM，EPROM，orEEPROMusedbyothermicrocontrollersatthetime.TheAVRisamodifiedHarvardarchitecturemachinewhereprogramanddataisstoredinseparatephysicalmemorysystemsthatappearindifferentaddressspaces，buthavingtheabilitytoreaddataitemsfromprogrammemoryusingspecialinstructions.NatureofMechanicalDesignTheultimateobjectiveofmechanicaldesignistoproduceausefulproductthatsatisfiestheneedsofacustomerandthatissafe，efficient，reliable，economical，andpracticaltomanufacture.Thinkbroadlywhenansweringthequestion，“whoisthecustomerfortheproductorsystemIamabouttodesign?”Considerthefollowingscenarios:Youaredesigningacanopenerforthehomemarker.Theultimatecustomeristhepersonwhowillpurchasethecanopeneranduseitinthekitchenofahome.Othercustomersmayincludethedesignerofthepackagingfortheopener，themanufacturingstaffwhomustproducetheopenereconomically，andservicepersonnelwhorepairtheunit.Youaredesigningapieceofproductionmachineryforamanufacturingoperation.Thecustomersincludethemanufacturingengineerwhoisresponsiblefortheproductionoperation，theoperatorofthemachine，thestaffwhoinstallthemachine，andthemaintenancepersonnelwhomustservicethemachinetokeepitingoodrunningorder.Youaredesigningapoweredsystemtoopenalargedooronapassengeraircraft.Thecustomsincludethepersonwhomustoperatethedoorinnormalserviceorinemergencies，thepeoplewhomustpassthroughthedoorduringuse，thepersonnelwhomanufacturetheopener，theinstallers，theaircraftstructuredesignerswhomustaccommodatetheloadsproducedbytheopenerduringflightandduringoperation，theservicetechnicianswhomaintainthesystem，andtheinteriordesignerswhomustshieldtheopenerduringusewhileallowingaccessforinstallationandmaintenance.Itisessentialthatyouknowthedesiresandexpectationsofallcustomersbeforebeginningproductdesign.Marketingprofessionalsareoftenemployedtomanagethedefinitionofcustomerexpectations，butdesignerswilllikelyworkwiththemasapartofaproductdevelopmentteam.Manymethodsareusedtodeterminewhatthecustomerwants.Onepopularmethod，calledqualityfunctiondeploymentorQFD，seekstoidentifyallofthefeaturesandperformancefactorsthatcustomersdesireandtoassesstherelativeimportanceofthesefactors.TheresultoftheQFDprocessisadetailedsetoffunctionsanddesignrequirementsfortheproduct.Itisalsoimportanttoconsiderhowthedesignprocessfitswithallfunctionsthatmusthappentodeliverasatisfactoryproducttothecustomerandtoservicetheproductthroughoutitslifecycle.Infact，itisimportanttoconsiderhowtheproductwillbedisposedofafterithasserveditsusefullife.ThetotalofallsuchfunctionsthataffecttheproductissometimescalledtheproductrealizationprocessorPRP.SkillsneededinmechanicaldesignProductengineersandmechanicaldesignersuseawiderangeofskillsandknowledgeintheirdailywork，includingthefollowing:（1）Sketching，technicaldrawingandcomputer-aideddesign.（2）Propertiesofmaterials，materialsprocessing，andmanufacturingprocesses.（3）Applicationsofchemistrysuchascorrosionprotection，plating，andpainting.（4）Statics，dynamics，strengthofmaterials，kinematics，andmechanisms.（5）Oralcommunication，listening，technicalwriting，andteamworkskills.（6）Fluidmechanics，thermodynamics，andheattransfer.（7）Fluidpower，thefundamentalsofelectricalphenomena，andindustrialcontrols.（8）Experimentaldesignandperformancetestingofmaterialsandmechanicalsystems.（9）Creativity，problemsolving，andprojectmanagement.（10）Stressanalysis.（11）Specializedknowledgeofthebehaviorofmachineelementssuchasgears，beltdrives，chaindrives，shafts，bearings，keys，splines，couplings，seals，springs，connections(bolted，riveted，welded，adhesive)，electricmotors，linearmotiondevices，clutches，andbrakes.Itisexpectedthatyouwillhaveacquiredahighlevelofcompetenceinmulti-disciplinaryknowledgeandskillsthataredevelopedcontinuouslythroughoutyouracademicstudyandthroughexperience.Functions，designrequirements，andevaluationcriteriaInordertoemphasizetheimportanceofcarefullyidentifyingtheneedsandexpectationsofthecustomerpriortobeginningthedesignofamechanicaldevice，youcanformulatethesebyproducingclear，completestatementsoffunctions，designrequirements，andevaluationcriteria:Functionstellwhatthedevicemustdo，usinggeneral，nonquantitativestatementsthatemployactionphrasessuchastosupportaload，toliftacrate，totransmitpower，ortoholdtwostructuralmemberstogether.Designrequirementsaredetailed，usuallyquantitativestatementsofexpectedperformancelevels，environmentalconditionsinwhichthedevicemustoperate，limitationsonspaceorweight，oravailablematerialsandcomponentsthatmaybeused.Carefulpreparationoffunctionstatementsanddesignrequirementswillensurethatthedesigneffortisfocusedonthedesiredresults.Muchtimeandmoneycanbewastedondesignsthat，althoughtechnicallysound，donotmeetdesignrequirements.Designrequirementsshouldincludeeverythingthatisneeded，butatthesametimetheyshouldofferampleopportunityforinnovation.Evaluationcriteriaarestatementsofdesirablequalitativecharacteristicsofadesignthatassistthedesignerindecidingwhichalternativedesignisoptimum—thatis，thedesignthatmaximizesbenefitswhileminimizingdisadvantages.Evaluationcriteriashouldbedevelopedbyallmembersofaproductdevelopmentteamtoensurethattheinterestsofallconcernedpartiesareconsidered.Oftenweightsareassignedtothecriteriatoreflecttheirrelativeimportance.Safetymustalwaysbetheparamountcriterion.Differentdesignconceptsmayhavevaryinglevelsofinherentsafetyinadditiontomeetingstatedsafetyrequirementsasnotedinthedesignrequirementslist.Designersandengineersarelegallyliableifapersonisinjuredbecauseofadesignerror.Youmustconsideranyreasonablyforeseeableusesofthedeviceandensuresafetyofthoseoperatingitorthosewhomaybecloseby.Achievingahighoverallperformanceshouldalsobeahighpriority.Certaindesignconceptsmayhavedesirablefeaturesnotpresentonothers.Togethertheseelementscanbecalledthespecificationsforthedesign.MostdesignsprogressthroughacycleofactivitiesasoutlinedinFigure10.1.Youshouldtypicallyproposemorethanonepossiblealternativedesignconcept.Thisiswherecreativityisexercisedtoproducetrulynoveldesigns.Eachdesignconceptmustsatisfythefunctionsanddesignrequirements.Acriticalevaluationofthedesirablefeatures，advantages，anddisadvantagesofeachdesignconceptshouldbecompleted.Thenarationaldecisionanalysistechniqueshouldusetheevaluationcriteriatodecidewhichdesignconceptistheoptimumand，therefore，shouldbeproduced.ExampleoftheintegrationofmachineelementsintoamechanicaldesignMechanicaldesignistheprocessofdesigningand/orselectingmechanicalcomponentsandputtingthemtogethertoaccomplishadesiredfunction.Ofcourse，machineelementsmustbecompatible，mustfitwelltogether，andmustperformsafelyandefficiently.Thedesignermustconsidernotonlytheperformanceoftheelementbeingdesignedatagiventimebutalsotheelementswithwhichitmustinterface.IdentifycustomerrequirementsIdentifycustomerrequirementsDefinefunctionsofthedeviceStatedesignrequirementsDefineevaluationcriteriaProposeseveralalternativeDesignconceptsEvaluateeachproposealternativeRateeachalternativeagainsteachevaluationcriterionSelecttheoptimumdesignconceptCompletedetaileddesignoftheselectedconceptDefineSpecificationsCreateDesignConceptDecisionMakingDetailedDesignFigure10.1 StepsinthedesignprocessToillustratehowthedesignofmachineelementsmustbeintegratedwithalargermechanicaldesign，letusconsiderthedesignofaspeedreducerforthesmalltractor.Supposethat，toaccomplishthespeedreduction，youdecidetodesignadouble-reduction，spurgearspeedreducer.Youspecifyfourgears，threeshafts，sixbearing，andahousingtoholdtheindividualelementsinproperrelationtoeachother.Thedesignprocessasoutlinedhereimpliesthatthedesigncanprogressinsequence：fromthegearstotheshafts，tothebea