外文翻译-计算机辅助设计

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计算机辅助设计一个好的机械设计产品必须保证产品的部分或全部功能在产品寿命中能够正确有效地实现。在设计中产品功能的研究包括：重力、强度、热、温度特性、运动学和动力学。评价设计的好坏与否可以通过比较产品的实际性能与设计要求的差距来衡量。设计除了功能达到设计要求之外，其它方面的实际同样也十分重要。比如说经济性方面的要求。这就是说最终的产品要近可能地接近设计要求。如果要求支撑设计能承受1000N/min的能力，然而设计成能承受2000N/min，则没有必要也不经济。因此机械设计必须处理好产品的功能与其经济性之间的矛盾。产品的性能受零部件的结构、材料属性和人机环境的影响；产品的经济性则和材料的成本以及市场因素有关。在机械设计过程中70%的制造成本已经决定了。这就是说设计者把产品交给制造之前，绝大部分主要的成本已经决定。比如：材料尺寸、公差、表卖弄粗糙度及过程成本的其它参数。因此，在加工过程中，我们在制造过程中再努力也只能减少%30的花费。这个比例说明设计过程起着置关重要的作用。我们应该清楚计算机能够帮上很大的忙。计算机辅助设计对不同的人意味着不同的意思。对一些人来说，它意味着计算机辅助绘图和草绘，对另外一些人来说，他则意味着完全自动化设计，设计者只要规定设计的功能，则计算机能设计出满足要求的产品，甚至经过优化的设计。以上则是计算机辅助设计。他的目标就是通过人类的智慧实现后面叙述的一种情形。计算机辅助设计仍停留在计算机辅助绘图阶段。大多数简单的CAD只能进行自动草绘功能。更高级的系统呢功能进行分析引导工程师得到必须平衡的关系。因为不同的卖主提供具有不同特点的计算机辅助设计系统，这些系统之间具有各自不同的数据标准。他们之间的交流和数据转化变得非常有意义。IGES在不同系统之间数据格式转换中扮演了中间者的角色。不同软件厂商只要提供与IGES格式相互转换的模块系统，各种计算机辅助设计软件能导入和导出IGES格式的文件，同时这种格式的文件在3D图形转换种也有根广泛的应用。目前一种基于IGES格式的实体建模技术的数据格式(PDES)也得到很大的发展。这种格式的文件占用系统空间小、速度快、可靠性高。由于国外也有各种不同的标准存在，预计不久这些标准将导致一个国际标准规格，叫产品模型数据(STEP)的交换的标准。硬件和软件硬件在计算机辅助设计系统之间存在很大的差别。服务器早就用于计算机辅助设计系统。处理速度迅速增加的微型和小型计算机也可以进行的计算机辅助设计系统的运行。网络一般被使用以便设计可以被中心储存，数据可以被分给设计者。很多输入装置使用的非常广泛。普通I/O设备包括：键盘，鼠标，跟踪球，数子化仪，控制杆，辅助键盘和光笔数字化仪。典型的输出装置通常是屏幕，绘图仪，打印机、移动硬盘和软盘。有多种计算机辅助设计软件提供给工作站和微型计算机。数据包在某种程度上以绘图信息形式的交换文件变化。这些文件可以被输入到其他软件中帮助可编程的机床。计算机辅助设计系统的要素计算机辅助设计系统的设计过程由四级组成，如下描述。几何学模型化。在几何学模型化过程中，将一个物体或者它的任何部分进行分析后用数字描述。首先设计者给出命令建立或者修改能够完成二维或三维的线条、表面、实体、尺寸和正文构造几何学模型。几何学模型可以用三种不同的方式呈现。如下图（线框内；图10.ld），全部边缘都象固体的线一样可见。这幅图像可能是含糊的，特别是复杂的形状。不过，各种各样的颜色一般用于物体的不同的部分，因此使物体更容易想象。在表面模型里，全部可见的表面被用模型显示，并且在实体模型里，全部表面被显示，但是内部以数据的形式描述（数字10.1e，f）设计分析和最优化。设计几何学特征有可能由分析压力、疲劳、挠度、振动组成。这阶段温度分布或者容忍。各种各样复杂软件包能计算出质量的准确并且快速的变化。设计评论和评估。设计过程忠一个重要的设计阶段是检查在各种各样的组成部分之间是否存在任何干涉，是否有自由度，例如将要作为运动构件的连杆。计算机软件能利用动画模拟运动的构件及其它运动状态。在设计过程的评论和评估阶段，零件已经具有制造需要的准确尺寸和公差。文档编制和草绘。在那些先前阶段完成之后，那些设计被自动化草绘仪复制作为文档编制和参考用，细节和图纸也被冲印。计算机辅助设计系统能绘制零件的剖视图、局部视图，并且能提取零件的各种各样的视图。以前，计算机辅助设计系统里的大部分设计过程是在被连接了一台计算机主机的工作站上进行，但是趋势已经迅速发生了强有力的改变，高性能和非常经济的32位台式Unix工作站及性能更好的64位的处理器也能运行这些计算机辅助设计软件。设计过程在深入研究计算机辅助设计之前，理解计算机辅助设计过程是非常重要的。计算机技术和它基于的理论一样非常有价值。在通常一个设计的短的讨论之后，计算机怎样能帮助每个设计阶段，我们将探索。希格利认为一个反复的设计过程由6阶段组成的：1需要的认知2具体问题的确定3问题综合4分析、优化