英文翻譯--計(jì)算機(jī)與設(shè)計(jì).doc

ComputerandDesignThecomputerhashadamajorimpactonthewayeverydaytasksassociatedwithdesignareaccomplished.Itcanbeusedinmanywaystodomanythings.However,alldesigntasksaccomplishedusingacomputerfallintooneoffourbroadcategories:*designmodeling*designanalysis*designdocumentation1.DesignModelingInCAD/CAMdesignmodeling,ageometricmodelofaproductisdevelopedthatdescribesthepartmathematically.Thismathematicaldescriptionisconvertedtographicformanddisplayedonacathoderaytube(CRT).Thegeometricmodelalsoallowsthegraphicimagetobeeasilyeditedandmanipulatedoncedisplayed.TheprocessbeginswhenadesignercreatesagraphicimageusingaspecialICGsoftwarepackage.Theimageiscreatedbyenteringcommandsthatcausethecomputertoconstructtheimageoutofpoints,lines,circles,andcurves.Tocreateagraphicimage,thecomputermusttranslatethegeometriccharactersintoacorrespondingmathematicalmodel.Whatappearstothedesignerasagraphicimageisstoredinthecomputerasaseriesofmathematicalcoordinates.Asthedesignerissuescommandstoeditormanipulateagraphicimage,thecomputerrevisesthemathematicalmodel.Thecomputermustfirstchangethegeometricmodelbeforeitcanchangethegraphicimage,anditmustcalculatethemathematicalcoordinatesofageometriccharacterbeforeitcandisplaythatcharacter.Graphicimagesaredisplayedinoneofthreeformats:two-dimensionalor2-Dtwo-and-a-half-dimensionalor21/2-Dthree-dimensionalor3-DThesethreeformatsareillustratedinFig.2.3,2.4and2.5.A2-Dgraphicimageshowsanorthographic(flat)representationandusuallyshowstwoormoreviews(e.g.,top,front,andrightside).A21/2-Dgraphicimageisanobliquerepresentation.A3-Dgraphicimagemaybeawireframemodeloratrue3-Dsolidmodel(Fig2.6).Communicationcanbeenhancedfurtherwhengraphicimagesaredisplayedincolor.ManycapabilitiesofCAD/CAM,includingfinite-elementanalysis,wireframe3-D,andsolidsmodeling,aremoreeasilyunderstoodwhenthegraphicimagesareFig1displayedincolor.Specificapplicationssuchaspipingdrawingsarealsoeasiertoreadwhendisplayedinanappropriatevarietyofcolors.Whengraphicimagesaredisplayedinonlyonecolor,itissometimesdifficulttodeterminewhenlinescrossoverorunder,whatisnearandwhatisfaraway,whatistheoriginalimageandwhatisthedistortedimage(asinthecaseoffinite-elementanalysis),andwhichlayeragivencomponentison.Theuseofcolorallowsforamoredefinitivedistinctionbetweenoverandunder,nearandfar,andoriginalanddistorted.Italsohelpsclarifyobjectsconsistingofnumeroussurfacesorseveralcomponentparts.2.DesignAnalysisThecomputerhassimplifiedthedesignanalysisstageofthedesignprocesssignificantly.Onceaproposeddesignhasbeendeveloped,itisnecessarytoanalyzehowitwillstanduptotheconditionstowhichitwillbesubjected.Suchanalysismethodsasheattransferandstress-straincalculationsaretime-consumingandcomplex.WithCAD/CAM,specialcomputerprogramswrittenspecificallyforanalysispurposesareavailable.Fig2Onesuchprogramthathassimplifiedtheanalysisofmanufacturedproductsiscalledfinite-elementanalysis.Finite-elementanalysisinvolvesbreakinganobjectupintomanysmallrectangularortriangularelements(Fig.2.7),thenanalyzingeachindividualelementbycomputer.Thisapproachgivesathoroughanalysisthatmightnotbefeasiblewithouttheaidofacomputer.Italsoofferstheadvantageofspecificallypinpointingthelocationsofproblemssothatdesigncorrectionscanbemoreeasilymade.Withfinite-elementanalysis,thecomputermusthaveapowerfulprocessingcapability.Thecomputeranalyzesthewholeobjectbyanalyzingeachindividualinterconnectedfiniteelement.Byanalyzingtheresponseofeachfiniteelementoftheobjecttothestress,strain,heat,orotherforceactingonit,thecomputercanpredictthereactionofthewholeobject.ModernCAD/CAMsystemswithfinite-elementanalysiscapabilitymaketheprocesssimpletoachieve.Usersdefinetheareathatistobedivided.Thecomputerthenautomaticallydividestheareaintotheinterconnectednetworkoffiniteelements.Fig3Aparticularlyvaluablecharacteristicoffinite-elementanalysisisitsabilitytovisuallydisplaytheresultsoftheanalysis.Forexample,ifapartistobeanalyzedtodeterminehowitwillbehavewhensubjectedtoaspecifiedamountofstress,thecomputerisablesuperimposetheimageofthestressedpartovertheunstressedpart.Inthisway,theresultantdistortioncanbeeasilyseen.Suchvisualevidenceoftheresultscanmakeiteasierforthedesignertopinpointthenecessarydesignchanges.3.DesignReviewAnotherstepinthedesignprocessthathasbeensimplifiedbythecomputerisdesignreview.Thisinvolvescheckingtheaccuracyofallaspectsofthedesign.ThereareseveralICGcapabilitiesthatmakedesignreviewinCAD/CAMeasierthanwithmanualdesign.ThefirstisthesemiautomaticdimensioningcapabilityofmanyCAD/CAMsoftwarepackages.Toproduceagraphicimage,anICGsystemmustfirstcreateamathematicalmodel.Thedatainthismodelcanbeusedbythecomputerforcalculatingdimensionsautomatically.MostgoodCAD/CAMsoftwarepackagescandisplaythesedimensionsaccordingtostandarddimensioningandtolerancingpractices.Thismeansfewerdimensioningerro