外文翻譯-計算機輔助設計

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計算機輔助設計一個好的機械設計產品必須保證產品的部分或全部功能在產品壽命中能夠正確有效地實現(xiàn)。在設計中產品功能的研究包括：重力、強度、熱、溫度特性、運動學和動力學。評價設計的好壞與否可以通過比較產品的實際性能與設計要求的差距來衡量。設計除了功能達到設計要求之外，其它方面的實際同樣也十分重要。比如說經(jīng)濟性方面的要求。這就是說最終的產品要近可能地接近設計要求。如果要求支撐設計能承受1000N/min的能力，然而設計成能承受2000N/min，則沒有必要也不經(jīng)濟。因此機械設計必須處理好產品的功能與其經(jīng)濟性之間的矛盾。產品的性能受零部件的結構、材料屬性和人機環(huán)境的影響；產品的經(jīng)濟性則和材料的成本以及市場因素有關。在機械設計過程中70%的制造成本已經(jīng)決定了。這就是說設計者把產品交給制造之前，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。這個比例說明設計過程起著置關重要的作用。我們應該清楚計算機能夠幫上很大的忙。計算機輔助設計對不同的人意味著不同的意思。對一些人來說，它意味著計算機輔助繪圖和草繪，對另外一些人來說，他則意味著完全自動化設計，設計者只要規(guī)定設計的功能，則計算機能設計出滿足要求的產品，甚至經(jīng)過優(yōu)化的設計。以上則是計算機輔助設計。他的目標就是通過人類的智慧實現(xiàn)后面敘述的一種情形。計算機輔助設計仍停留在計算機輔助繪圖階段。大多數(shù)簡單的CAD只能進行自動草繪功能。更高級的系統(tǒng)呢功能進行分析引導工程師得到必須平衡的關系。因為不同的賣主提供具有不同特點的計算機輔助設計系統(tǒng)，這些系統(tǒng)之間具有各自不同的數(shù)據(jù)標準。他們之間的交流和數(shù)據(jù)轉化變得非常有意義。IGES在不同系統(tǒng)之間數(shù)據(jù)格式轉換中扮演了中間者的角色。不同軟件廠商只要提供與IGES格式相互轉換的模塊系統(tǒng)，各種計算機輔助設計軟件能導入和導出IGES格式的文件，同時這種格式的文件在3D圖形轉換種也有根廣泛的應用。目前一種基于IGES格式的實體建模技術的數(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準存在，預計不久這些標準將導致一個國際標準規(guī)格，叫產品模型數(shù)據(jù)(STEP)的交換的標準。硬件和軟件硬件在計算機輔助設計系統(tǒng)之間存在很大的差別。服務器早就用于計算機輔助設計系統(tǒng)。處理速度迅速增加的微型和小型計算機也可以進行的計算機輔助設計系統(tǒng)的運行。網(wǎng)絡一般被使用以便設計可以被中心儲存，數(shù)據(jù)可以被分給設計者。很多輸入裝置使用的非常廣泛。普通I/O設備包括：鍵盤，鼠標，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、移動硬盤和軟盤。有多種計算機輔助設計軟件提供給工作站和微型計算機。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床。計算機輔助設計系統(tǒng)的要素計算機輔助設計系統(tǒng)的設計過程由四級組成，如下描述。幾何學模型化。在幾何學模型化過程中，將一個物體或者它的任何部分進行分析后用數(shù)字描述。首先設計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、實體、尺寸和正文構造幾何學模型。幾何學模型可以用三種不同的方式呈現(xiàn)。如下圖（線框內；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，并且在實體模型里，全部表面被顯示，但是內部以數(shù)據(jù)的形式描述（數(shù)字10.1e，f）設計分析和最優(yōu)化。設計幾何學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。各種各樣復雜軟件包能計算出質量的準確并且快速的變化。設計評論和評估。設計過程忠一個重要的設計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，例如將要作為運動構件的連桿。計算機軟件能利用動畫模擬運動的構件及其它運動狀態(tài)。在設計過程的評論和評估階段，零件已經(jīng)具有制造需要的準確尺寸和公差。文檔編制和草繪。在那些先前階段完成之后，那些設計被自動化草繪儀復制作為文檔編制和參考用，細節(jié)和圖紙也被沖印。計算機輔助設計系統(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，計算機輔助設計系統(tǒng)里的大部分設計過程是在被連接了一臺計算機主機的工作站上進行，但是趨勢已經(jīng)迅速發(fā)生了強有力的改變，高性能和非常經(jīng)濟的32位臺式Unix工作站及性能更好的64位的處理器也能運行這些計算機輔助設計軟件。設計過程在深入研究計算機輔助設計之前，理解計算機輔助設計過程是非常重要的。計算機技術和它基于的理論一樣非常有價值。在通常一個設計的短的討論之后，計算機怎樣能幫助每個設計階段，我們將探索。希格利認為一個反復的設計過程由6階段組成的：1需要的認知2具體問題的確定3問題綜合4分析、優(yōu)化