外文翻譯-計(jì)算機(jī)輔助設(shè)計(jì)

ComputerAidedDesignGoodengineeringdesignisnecessarytoguaranteetatapartormechanismfunctionscorrectlyandlastsforareasonablylongtime.Functionalconsiderationsduringdesigninvolve,amongotherthings,weight,strength,thermalproperties,kinematics,anddynamics.Theperformanceofadesigncanbeevaluatedbycomparingitsperformancemeasurementswiththerequiredspecifications.Asimportantassatisfactoryperformanceis,thereareotherareaspartshouldbedesignedeconomically.Thisimpliesthatthefinishedpartshouldbedesignedasclosetothespecificationsaspossible.Ifthefunctionofasupportingmemberofastructurerequiresthemembertowithstand10,000psiofcompressivestress,thendesigningthemembertowithstand30,000psiisunnecessaryandwillprobablybemoreexpensivethantherequireddesign.Engineeringdesign,thereforeshouldaddressfunctionalityandeconomics,Functionalityisdeterminedbyapart’sgeometry,materialproperties,andenvironment.Theeconomicfactorsincludematerials,processingcosts,andmarketingdetails.Asmuchas70%oftheproductioncostsofamanufacturedpartaredeterminedduringtheengineeringdesignprocess.Thismeansthatbythetimetheparthasleftthedesigner’shandsthelargemajorityofitsproductioncostshavebeenestablished.Theyaredefinedimplicitlybythematerials,dimensions,tolerances,surfacefinishes,andotherparameterswhichdetermineprocessingcosts.Therefore,only30%oftheparfscostissubjecttomoney-savingeffortsduringthemanufacturingplanningstage.This70/30ratioemphasizestheimportantofthedesigncanbeamajorhelpinassuringproperfunctionandreasonableproductioncosts.Computer-aideddesign(CAD)isatermwhichmeansmanythingstomanypeople.Tosome,itmeanscomputer-aideddraftingordrawing.Toothers,itmeanscompute-aidedanalysis.Andtostillothers,itsuggeststotallyautomateddesignwheretheengineerneedspecifyonlythefunctionofapartandthecomputerarrivesatasatisfactoryorevenoptimaldesign.CADisalloftheabove,aimingmostlyforthefastdescriptionthroughtechniquesinartificialintelligence.CAD,however,stillexistsseparatelyineachofthestagesmentioned.ProbablythemostcommonandsimplestCADsystemsarelimitedtoautomateddraftingcapabilities.Moreadvancedsystemscanperformanalysesandevenhelpguidetheengineertotheequationsnecessarytodeterminetheeffectivenessofthedesign.BecauseoftheavailabilityofawidevarietyofCADsystemswithdifferentcharacteristicssuppliedbydifferentvendors,propercommunicationandexchangeofdatabetweenthesesystemshavebecomeasignificantproblem.TheneedforasingleneutralformatforbettercompatibilityispresentlyfilledlargelybytheInitialGraphicsExchangeSpecificaton(IGES).Vendorsneedonlyprovidetranslatorsfortheirownsystemstopreprocessthedataintotheneutralformatintotheirsystem.IGESisusedfortranslationintwodirections(intooroutofasystem)andisalsousedwidelyfortranslationof3-Dlineandsurfacedata.Amorerecentdevelopmentisasolid-model-basedstandardcalledProductDataExchangeSpecification(PDES),whichisbasedonIGES.AlthoughIGESisadequatefromostrequirement,PDEShaslessmemorysize,requireslesstimeforexecution,andislesserrorprone.Becauseoftheexistenceofvariousstandardsinothercountriesaswell,itisexpectedthatinthenearfuturethesestandardswillculminateinaninternationalstandard,calledtheStandardfortheExchangeofProductModelData(STEP).HardwareandSoftwareHardwarevariesagreatdealbetweenCADsystems.MainframeshavelongbeenusedforCADsystems.TherapidincreasesinprocessingspeedshavemademicroandminicomputersviableCADstations.Networksgenerallyareusedsothatdesignscanbestoredcentrallyanddatacanbesharedamongdesigners.Manyinputdevicesareused.CommonI/Odevicesincludekeyboard,mouse,trackball,digitizingpad,joystick,keypad,andlightpens.Outputdevicesaretypicallyscreen,plotter,printer,andharddrivesanddisksforstorage.ThereisawidevarietyofCADsoftwareavailableformainframesengineeringstationsandmicrocomputers.Thepackagesvarytosomedegreeininformationintheformofdrawingexchangefiles.Thesefilescanbeimportedintoothersoftwaretoassistinprogrammingthemachinetool.ElementsofCADSystemsThedesignprocessinaCADsystemconsistsoffourstages,asdescribedbelow.Geometricmodeling.Ingeometricmodeling,aphysicalobjectoranyofitspartsisdescribedmathematicallyoranalytically.Thedesignerfirstconstructsageometricmodelbygivingcommandsthatcreateormodifylines,surfaces,solids,dimensions,andtextthattogetherareandaccurateandcompletetwo-orthree-dimensionalrepresentationoftheobject.Themodelscanbepresentedinthreedifferentways.Inthelinerepresentation(wire-frame;Figure10.ld),alledgesarevisibleassolidlines.Thisimagecanbeambiguous,particularlyforcomplexshapes.However,variouscolorsaregenerallyusedfordifferentpartoftheobject,thusmakingtheobjecteasiervisualize.I'igure10.J Jype函moJu蛔I,防'A]>Inthesurfacemodel,allvisiblesurfaceareshowninthemodel,andinthesolidmodel,allsurfacesareshown,butthedatadescribetheinteriorvolume(Figure10.1e,f)Designanalysisandoptimization.Afterthedesign’sgeometricfeatureshavema：consistofanalyzingstresses,strains,deflection,vibrations.Thisphasetemperaturedistribution,ortolerance.Varioussophisticatedsoftwarepackagescapableofcomputingthesequantitiesaccuratelyandrapidlyareavailable.Designreviewandevaluation.Animportantdesignstageisreviewandevaluationtocheckforanyinterferencebetweenvariouscomponentsinordertoavoiddifficultiesduringassemblyoruseofthepart,andwhethermovingmembers,suchaslinkages,aregoingtooperateasintended.Softwareisnowavailablewithanimationcapabilitiestoidentifypotentialwithmovingmembersandotherdynamicsituations.Duringthedesignreviewandevaluationstage,thepartispreciselydimensionedandtolerance,asrequiredformanufacturingit.Documentationanddrafting.Aftertheprecedingstageshavebeencompleted,thedesignisreproducedbyautomateddraftingmachinesfordocumentationandreference.Detailandworkingdrawingsarealsodevelopedandprinted.TheCADsystemiscapableofdevelopingdraftingsectionalviewsofthepart,scalingthedrawings,andperformingtransformationtopresentvariousviewofthepart.InCADsystems,muchofthedesignprocesswasformerlycarriedoutonworkstationconnectedtoamainframecomputer,butthetrendhaschangedrapidlytopowerful,high-performance,andmuchlessexpensivestand-alonedesktop32-bitUnixworkstation;64-bitprocessorswithevenbetterperformancearealsobecomingavailable.TheDesignProcessBeforedelvingintoCAD,itisimportanttounderstandthedesignprocessitself.Computertechniquesarenomorevaluablethanthetheoryonwhichtheyarebased.Afterashortdiscussionofdesigningeneral,wewillexplorehowthecomputercanhelpwitheachofthedesignstages.Shigleydefinesthedesignprocessasaniterativeprocedureconsistingofsixphase:RecognitionofneedDefinitionofproblemSynthesisAnalysisandoptimizationEvaluationPresentationThefirststep,recognitionofneed,arisesfromanidentifiedproblem.Suppose,forexample,thatanew,moreefficientjetengineisrequiredbecauseofafuelshortage.Step2,definitionoftheproblemismorespecific.Definingthespecificsofthejetengineinvolvesknowingthethrust,horsepower,allowableweight,etc.Someofthesespecificationsareconstraints;I.e.,theyarerequirementsthatmustbemetforthedesigntobefeasible.Thejetengineweightlimitmaybeoneconstraintontheavailableliftoftheplanewings.Amaximumweight,then,wouldbeaconstraint.Constraintsareusuallyexpressedasamaximumorminimumvalueorrageofvalues.Otherspecification,however,aretermedcriteria,i.e.,measuresofthegoodnessorqualityofthedesign.Criteriaareusedtomeasureadesignandcomparedesignsagainstoneanother,asoccursduringtheoptimizationstage.Onecriterionofthejetenginemaybegallonsoffuelburnedperhourorpermileoftravel.Criteriahavenomaximumorminimumlimitsonvalues.Asatisfactorydesignisoneinwhichalltheconstraintsaremet.Thisbestdesign,Iadditiontomeetingtheconstraints,willhavethebestcombinationofcriteriavalues.Selectionofconstraintsandcriteriaisanintegralpartofstep2,definitionoftheproblem.Followingdefinitionoftheproblem,thenextthreesteps(synthesis,analysis/optimization,andevaluation)areiterative.Synthesisandanalysisareperformedinconjunctionwithoneanother.Severaltypesofproblemscanariseduringthedesignprocess.Synthesisrequiresdeterminingtheoverallshapeandsystemcomponentspecifications.Uauallythistakestheformofengineeringdrawingsincludinggeometry,tolerances,materialtypes,andcomponentparnumbers.Analysismayrangefromasimpleestheticevaluationtothesolutionofasetofcomplexdifferentialequationswhichdescribetheperformanceofthepartorassembly.Theresultsofanalysescanbeexpressedintabularorgraphicalform.Itisthejobofthedesignertoevaluatetheanalysisdataandtoperformanyredesign.Evaluationsalsoatypeofanalysis,butusuallyrequiresanexpensiveprototypeandracksofinstrumentation.Thedesignprocesscanbeverycostlyandinvolvemanypeople.Designcostmaybelowed,however,byreducingthenumberofpeopleandtheprototypingprocessitself.AnalysisTheanalysisisthecornerstoneofCAD.Theresultsgeneratedbyanalysismodulesareusedbytheengineertodeterminethefeasibilityofadesignortooptimizeorredesigntheobject.Fundamentaltoanalysisisdevelopedtoaccuratelypredicttheperformancebasedonthedesignvariablevalues.Typically,sophisticatedanalysisprogramsaretheperformancemodelsofthedesign.Theuserdoesnotneedtoformulateequationswhichdescribetheperformanceorfunctionality.Whattheusermustdoinsteadistomodeltheobjectgeometryinanappropriateformat.Usuallytieengineerwhoisdesigningacommonlyanalyzedpartorassemblycanpurchaseanalysissoftwaretoevaluatethedesign.Itisonlyinthecaseofanunusualdesignorperformancerequirement,whereoff-the-shelfsoftwareisnotavailable,thattheengineermustformulateauniquemodel.Inthiscase,theengineerwillusuallyderiveamathematicalmodelintheformofequationsusinghelawsofphysicsandwriteaprogramtoreadthedesigndescriptionandcalculateteperformanceparameters.Thesecommonlyavailableanalysismodulesreadthedesigndescriptionandgenerateperformanceresultintheformofnumbers,graphs,charts,etc.AnExampleofUsingCADCAD,normallyusedinengineeringdepartments,hasdrasticallychangedthesedepartments.Drawingusedtobemadeonpaperwithpencilorpenanddrawinginstruments.Theywerethencopied,andthecopiesweresenttothefloorforproduction.Theoriginalswerestoredinlargedrawers.Evenasmallenterprisecouldhavethousandsoflargeblueprintsonfile.Ifchangeswerenecessary,theengineerwouldgettheoriginaloutofthefiledrawer,maketheneedforallofthephysicalstorageofprints.Thecomputeralsoallowedforrapidandeasyprintmodifications.Theengineerordesignerfirstdrawsthepartonthescreenthispartdrawingistheactualpartgeometry.Thesizesandlocationareallcorrectsothattheinformationcanbeusedlatertocreateaprogramtomachinethepart.Theengineerordesignerfirstdrawsthepartonthescreen.Thispartdrawingistheactualpartgeometry.Thesizesandlocationsareallcorrectsothattheinformationcanbeusedlatertocreateaprogramtomachinethepart.Thedesignermustworkcloselywiththemanufacturingpeopletoestablishsomestandardsfordesign.CADallowsdifferentlayers(orlevels)tobecreated.Thisallowsthedesignertoputdifferentpotionsofthepartgeometryondifferentlayers.Forexample,imagineasimplepartthathasa1/2-inchslotmilledinitandfourholesdrilledthroughit.Thedesignerwouldputtheslotononelayerofthedrawingandtheholesonanotherlayer.Thiswillallowothersoftwaretotakethepartgeometryformeachlayerandassigndifferenttoolstoit.Thinkofdifferentlayersasdifferentmachiningoperationthatwouldhavedifferenttoolassignedtothem.Layersareliketransparency.Ifweputallofthetotaltransparenciesontopofeachother,weseethetotalpart.Butwecanstilltakeanyonetransparencyandisolatesomeofthepartgeometry.TheCADsystemwillprinttheblueprintsoitappearsthereisonlyonelayer,orindividuallayerscanbeprinted.Thedesigneralsodimensionsthepart.Thisisquiteautomatic.Thedesignerchooseswherehe/shewantsdimensionsplacedandthecomputerplacestheactualdimensions.Dimensionswouldbeonadifferentlayerthanpartgeometry.Thecomputerallowsdesignstobeviewedandtestedbeforetheactualpartisevenmanufactured.Thepartcanbetestedmakesureitfitswithanymatingpars.CADsystemcancheckforinterferenceofparts,whichcansavealotofwastedmachininganddevelopmenttime.CADsystemscanstress-testparttoseeiftheywillmeetthestrengthrequirementsoftheapplication.ThegraphicsabilitiesofCADsystemalsoallow3-Dviewingofpartfromanyangle.IntegratedCADTheemphasisofthechapterisintegratedCAD/CAM.Theintegrationaspectisaccomplishedwhenthedesignandanalysismodulesareunitedintoonepackage.Thedesigngeometryenteredbythedesignercanbeusedbytheanalysisprogramsandbythemanufacturing,inspection,inventory,andotheraspectsoftheproductliftcycle.Themanifestationofintegratingsoftwareisrecognizedwhentheusercansitdownatacomputerterminal,designapart,viewitonthescreen,analyzeitfordesignrequirements,andhittheproverbialmanufacturingbuttonthekeyboardtohavethe

partautomaticallymanufactured,inspected,anddelivered.domain.TheVariousstagesofintegrationdoexist,butnoneisascompleteastheexamplejustmentioned.Integrationimpliesautomation.AutomationimpliesthatthehardwareandsoftwarecancreateaCADdatabaseanduseittoanalyzeandmanufacturetheparttotallyautomatically,withlittleornohumanintervention.Thequestionastowhetherhumaninterventionisadvisableisstillopen.Certainlyhumanintelligenceisvaluableindevelopingadesigningeometryandfunctionality,recognizingcertainpatternsandshortcomingsofproducts,andformulatingmanufacturingplans.Ithasbeensuggestedthatartificialintelligence(AI)techniquescansubstituteformuchofthehumandesign/manufacturinginput.AIresearchhashadsuccessintheareasofpatternrecognition,planning,naturallanguageunderstanding,roboticcontrol,faultdiagnosis,andotherapplications.ThemostwellrecognizeareaofAIisthedevelopmentofexpertsystem.ExpertsystemssimulatehumanexpertisebystoringaknowledgebaseofdataaboutaspecificarchitectureofanexpertsystemisshowninFigure10.2.Theknowledgebasecontainsknowledgeaboutaspecificproblemdomainandfactsstatingtheproblem.domain.TheTheinferenceengineistheprogramcontrolwhichsearchestefactsabouttheproblemtosatisfytheknowledgebaseadabovetheproblem計(jì)算機(jī)輔助設(shè)計(jì)一個(gè)好的機(jī)械設(shè)計(jì)產(chǎn)品必須保證產(chǎn)品的部分或全部功能在產(chǎn)品壽命中能夠正確有效地實(shí)現(xiàn)。在設(shè)計(jì)中產(chǎn)品功能的研究包括：重力、強(qiáng)度、熱、溫度特性、運(yùn)動學(xué)和動力學(xué)。評價(jià)設(shè)計(jì)的好壞與否可以通過比較產(chǎn)品的實(shí)際性能與設(shè)計(jì)要求的差距來衡量。設(shè)計(jì)除了功能達(dá)到設(shè)計(jì)要求之外，其它方面的實(shí)際同樣也十分重要。比如說經(jīng)濟(jì)性方面的要求。這就是說最終的產(chǎn)品要近可能地接近設(shè)計(jì)要求。如果要求支撐設(shè)計(jì)能承受1000N/min的能力，然而設(shè)計(jì)成能承受2000N/min，則沒有必要也不經(jīng)濟(jì)。因此機(jī)械設(shè)計(jì)必須處理好產(chǎn)品的功能與其經(jīng)濟(jì)性之間的矛盾。產(chǎn)品的性能受零部件的結(jié)構(gòu)、材料屬性和人機(jī)環(huán)境的影響；產(chǎn)品的經(jīng)濟(jì)性則和材料的成本以及市場因素有關(guān)。在機(jī)械設(shè)計(jì)過程中70%的制造成本已經(jīng)決定了。這就是說設(shè)計(jì)者把產(chǎn)品交給制造之前，絕大部分主要的成本已經(jīng)決定。比如：材料尺寸、公差、表賣弄粗糙度及過程成本的其它參數(shù)。因此，在加工過程中，我們在制造過程中再努力也只能減少%30的花費(fèi)。這個(gè)比例說明設(shè)計(jì)過程起著置關(guān)重要的作用。我們應(yīng)該清楚計(jì)算機(jī)能夠幫上很大的忙。計(jì)算機(jī)輔助設(shè)計(jì)對不同的人意味著不同的意思。對一些人來說，它意味著計(jì)算機(jī)輔助繪圖和草繪，對另外一些人來說，他則意味著完全自動化設(shè)計(jì)，設(shè)計(jì)者只要規(guī)定設(shè)計(jì)的功能，則計(jì)算機(jī)能設(shè)計(jì)出滿足要求的產(chǎn)品，甚至經(jīng)過優(yōu)化的設(shè)計(jì)。以上則是計(jì)算機(jī)輔助設(shè)計(jì)。他的目標(biāo)就是通過人類的智慧實(shí)現(xiàn)后面敘述的一種情形。計(jì)算機(jī)輔助設(shè)計(jì)仍停留在計(jì)算機(jī)輔助繪圖階段。大多數(shù)簡單的CAD只能進(jìn)行自動草繪功能。更高級的系統(tǒng)呢功能進(jìn)行分析引導(dǎo)工程師得到必須平衡的關(guān)系。因?yàn)椴煌馁u主提供具有不同特點(diǎn)的計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)，這些系統(tǒng)之間具有各自不同的數(shù)據(jù)標(biāo)準(zhǔn)。他們之間的交流和數(shù)據(jù)轉(zhuǎn)化變得非常有意義。IGES在不同系統(tǒng)之間數(shù)據(jù)格式轉(zhuǎn)換中扮演了中間者的角色。不同軟件廠商只要提供與IGES格式相互轉(zhuǎn)換的模塊系統(tǒng)，各種計(jì)算機(jī)輔助設(shè)計(jì)軟件能導(dǎo)入和導(dǎo)出IGES格式的文件，同時(shí)這種格式的文件在3D圖形轉(zhuǎn)換種也有根廣泛的應(yīng)用。目前一種基于IGES格式的實(shí)體建模技術(shù)的數(shù)據(jù)格式(PDES)也得到很大的發(fā)展。這種格式的文件占用系統(tǒng)空間小、速度快、可靠性高。由于國外也有各種不同的標(biāo)準(zhǔn)存在，預(yù)計(jì)不久這些標(biāo)準(zhǔn)將導(dǎo)致一個(gè)國際標(biāo)準(zhǔn)規(guī)格，叫產(chǎn)品模型數(shù)據(jù)(STEP)的交換的標(biāo)準(zhǔn)。硬件和軟件硬件在計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)之間存在很大的差別。服務(wù)器早就用于計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)。處理速度迅速增加的微型和小型計(jì)算機(jī)也可以進(jìn)行的計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的運(yùn)行。網(wǎng)絡(luò)一般被使用以便設(shè)計(jì)可以被中心儲存，數(shù)據(jù)可以被分給設(shè)計(jì)者。很多輸入裝置使用的非常廣泛。普通I/O設(shè)備包括：鍵盤，鼠標(biāo)，跟蹤球，數(shù)子化儀，控制桿，輔助鍵盤和光筆數(shù)字化儀。典型的輸出裝置通常是屏幕，繪圖儀，打印機(jī)、移動硬盤和軟盤。有多種計(jì)算機(jī)輔助設(shè)計(jì)軟件提供給工作站和微型計(jì)算機(jī)。數(shù)據(jù)包在某種程度上以繪圖信息形式的交換文件變化。這些文件可以被輸入到其他軟件中幫助可編程的機(jī)床。計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的要素計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)的設(shè)計(jì)過程由四級組成，如下描述。幾何學(xué)模型化。在幾何學(xué)模型化過程中，將一個(gè)物體或者它的任何部分進(jìn)行分析后用數(shù)字描述。首先設(shè)計(jì)者給出命令建立或者修改能夠完成二維或三維的線條、表面、實(shí)體、尺寸和正文構(gòu)造幾何學(xué)模型。幾何學(xué)模型可以用三種不同的方式呈現(xiàn)。如下圖（線框內(nèi)；圖10.ld），全部邊緣都象固體的線一樣可見。這幅圖像可能是含糊的，特別是復(fù)雜的形狀。不過，各種各樣的顏色一般用于物體的不同的部分，因此使物體更容易想象。在表面模型里，全部可見的表面被用模型顯示，并且在實(shí)體模型里，全部表面被顯示，但是內(nèi)部以數(shù)據(jù)的形式描述（數(shù)字10.1e，f）設(shè)計(jì)分析和最優(yōu)化。設(shè)計(jì)幾何學(xué)特征有可能由分析壓力、疲勞、撓度、振動組成。這階段溫度分布或者容忍。各種各樣復(fù)雜軟件包能計(jì)算出質(zhì)量的準(zhǔn)確并且快速的變化。設(shè)計(jì)評論和評估。設(shè)計(jì)過程忠一個(gè)重要的設(shè)計(jì)階段是檢查在各種各樣的組成部分之間是否存在任何干涉，是否有自由度，例如將要作為運(yùn)動構(gòu)件的連桿。計(jì)算機(jī)軟件能利用動畫模擬運(yùn)動的構(gòu)件及其它運(yùn)動狀態(tài)。在設(shè)計(jì)過程的評論和評估階段，零件已經(jīng)具有制造需要的準(zhǔn)確尺寸和公差。文檔編制和草繪。在那些先前階段完成之后，那些設(shè)計(jì)被自動化草繪儀復(fù)制作為文檔編制和參考用，細(xì)節(jié)和圖紙也被沖印。計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)能繪制零件的剖視圖、局部視圖，并且能提取零件的各種各樣的視圖。以前，計(jì)算機(jī)輔助設(shè)計(jì)系統(tǒng)里的大部分設(shè)計(jì)過程是在被連接了一臺計(jì)算機(jī)主機(jī)的工作站上進(jìn)行，但是趨勢已經(jīng)迅速發(fā)生了強(qiáng)有力的改變，高性能和非常經(jīng)濟(jì)的32位臺式Unix工作站及性能更好的64位的處理器也能運(yùn)行這些計(jì)算機(jī)輔助設(shè)計(jì)軟件。設(shè)計(jì)過程在深入研究計(jì)算機(jī)輔助設(shè)計(jì)之前，理解計(jì)算機(jī)輔助設(shè)計(jì)過程是非常重要的。計(jì)算機(jī)技術(shù)和它基于的理論一樣非常有價(jià)值。在通常一個(gè)設(shè)計(jì)的短的討論之后，計(jì)算機(jī)怎樣能幫助每個(gè)設(shè)計(jì)階段，我們將探索。希格利認(rèn)為一個(gè)反復(fù)的設(shè)計(jì)過程由6階段組成的：1需要的認(rèn)知2具體問題的確定3問題綜合4分析、優(yōu)化