三坐标测量机CAD尺寸检测新算法概要Word格式文档下载.docx

三坐标测量机CAD尺寸检测新算法概要Word格式文档下载.docx

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2000Springer-VerlagLondon

Limited

ANewAlgorithmforCAD-DirectedCMMDimensionalInspection

Y.-J.LinandP.Murugappan*

DepartmentofMechanicalEngineering,TheUniversityofAkron,Ohio,USA

Therecentdevelopmentsincomputerintegratedmanufacturing（CIMsystemshavemadethetraditionaldimensionalinspec-tionsbottlenecksintheproductionline.Toovercomethesebottlenecks,computerintegrateddimensionalinspectionwasproposedwiththecoordinatemeasuringmachine（CMMbeingthekeydevice.Inthisinvestigation,aframeworkforintegrat-ingtheCMMintothecomputer-aideddesign/computer-aidedmanufacturing（CAD/CAMenvironmentisdevelopedtoauto-matetheprocessofdesign,manufacturingandinspection.Analgorithmtogenerateanoptimumcollision-freeCMMprobepathisproposed.Thisalgorithmusestheraytracingtechniquetolocatethecollisionofthepossiblepathswiththeworkpiecetobeinspected,betweentheinitialprobepointandthetargetpoint.Ifthereisacollision,thealgorithmwalksthroughthetopologicalstructureofthepartandselectsthemidpointoftheedgesharedbythefacewithwhichthepathcollidesandtheadjacentfacenearesttothetargetpoint,asthenextprobepoint.Thisprocedureisfolloweduntilthetargetpointisreached.ThefirsthalfoftheproposedalgorithmisimplementedusingMechanicalDesktopastheCADsystemandAutoCADRuntimeExtension（ARXastheapplicationprogramminginterface,runningonaWindowsNT4.0plat-form.Theeffectivenessoftheproposedalgorithmisverifiedbytheresultsoftheimplementationwhichdemonstrateoptimumcollision-freedimensionalinspectionpathgenerationforthreeprismaticparts.

Keywords:

Computerintegratedmanufacturing（CIM;

Co-ordinatemeasuringmachine（CMM;

Dimensionalinspection1.Introduction

Acoordinatemeasuringmachine（CMMisthekeydeviceusedincomputer-integrateddimensionalinspection[1].TheessentialsystemcomponentsofaCMMare,themechanical*Presentaddress:

TheStructuralDynamicsResearchCorporation,Cincinnati,Ohio,USA.

Correspondenceandoffprintrequeststo:

DrY.-J.Lin,DepartmentofMechanicalEngineering,TheUniversityofAkron,Akron,OH44325–3903,USA.E-mail:

YLȰuakron.eduarrangementwiththethreeaxesandthedisplacementtrans-ducers,aprobeheadwithatouchtriggeroranalogueprobetoprobetheworkpiecesinallspatialdirections,acontrolunitandacomputerwithperipheralequipmentandsoftwaretocalculateanddisplaytheresults[2].

AlthoughCMMhasaconsiderablelevelofautomationcomparedwithtraditionalmethods,CMMsstillhavetobemanuallyoperatedtocarryoutthefirstpartinspection,inthecaseofon-lineinspectionplanning.TheCADsoftwarethengeneratestheDMIS（dimensionalmeasurementinterfacespecificationprogram.DMISprovidesaneutralinterchangeformatbetweenCADsystemsandthedimensionalmeasuringequipment.Theprogramisthenreplayedforinspectionoftheremainingparts,inabatchproductionenvironment.Inthecaseofoff-lineinspectionplanning,thewholeprocedureissimulatedbytheCADsoftware.Inordertoachieveahigherlevelofautomation,CAD-directedinspectionplanningisessential.

InCMMinspectionpathplanning,optimisationoftheinspectionpathplaysamajorroleinincreasingthethroughputoftheCMM.ThepathplanningforaCMMissimilartothatofaCartesianrobotpathplanning,thegoalbeingtogenerateanoptimumcollision-freepath.Intheinspectionprocess,theCMMprobehastomoveintheworkspacetotouchdifferentareasofthepartformeasuringpurposes.Sincetheprobeworksintheproximityofthepart,thegeometryofthepartandfixturescreateobstaclesthatinterferewiththemovementoftheprobe.Hence,thegenerationofanefficientandoptimumcollisionfreeCMMinspectionpathformsamajorproblemintheinspectionplanning.

AnewalgorithmisproposedtogenerateanefficientCMMinspectionpathbyaccessingtheCADdatabaseforthepartgeometryandtraversingthroughsolidrepresentationsintheCADsystemforcollisiondetectionandpathgeneration.Theobjectsconsideredaresimpleprismaticsolids.Tolaythegroundforthepresentationofthisnewalgorithm,pastrelatedworkonpathplanningisbrieflyreviewedandtheoverallframeworkandthegoaloftheproposedalgorithmaredescribedinSection2.TheproposedalgorithmanditsverificationarediscussedinSection3.TheresultsareshowninSection4.FinallytheconclusionsandrecommendationsforfuturestudyaregiveninSection6.

108Y.-J.LinandP.Murugappan

2.ReviewofRelatedWork

CMMinspectionpathplanninghasreceivedwideattentioninrecentyears.TherehasbeenresearchusingCADdatabasesforCMMinspectionpathgeneration.Acomputer-integratedenvironmentforautomateddimensionalinspectionwaspro-posed[1]whichplanstheinspectionpathbysearchingforfunctionalfeaturesfromtheCADdatabaseusingthespecifi-cationmodule,andgeneratesthepathusingtheplanningmodule.Thepathgeneratedischeckedforcollisionsusingtheverificationmoduleandisexecutedusingtheexecutionmodule.Themeasureddataisthencomparedwiththespecifieddimen-sionsandtolerancesusingtheanalysismodule.

Thespecificationmoduleacceptsvariousinspectionattributesfromuserstogeneratetheinspectionspecificationwhichisusedbytheinspectionplanningmoduletosearchforthefeaturesanddividethemintosinglefeaturesandrelatedfea-tures.Basedonthegeometryofeachindividualfeatureanditsrelationshipwiththeinspectionspecification,thenumberofinspectionpoints,pointlocations,probingvectors,backoffdistance,theprobingsequenceisdetermined.Thesecompo-nentsarecheckedforinterferencewithpartsurfacesandacollision-freeinspectionpathisgenerated.InthisapproachmucheffortisrelatedtosearchingtheCADdatabaseusingthespecificationgiven,ratherthanusingthepartgeometryorthesolidrepresentationofthefeaturesandgeneratingacolli-sion-freepath.

Graphicrepresentationmethodscanbeclassifiedintosweeprepresentationmethods,configurationspacemethodsandspacesubdivisionmethods.Sweeprepresentationdefinesanewobjectbysweepinganobjectalongatrajectorythroughspace.Thentheoverlaporintersectionbetweenthesweptvolumesandtheobstaclesisdetermined,andanewpathisproposed.Sincesimplesurfacesareusedtomodeltheobjectaccurately,itisdifficulttodeterminewhethertwosuchmodelsoverlap.More-over,theproposedpathprovidesonlylocalinformationaboutthepotentialcollision,forexample,theshapeoftheintersec-tionsofthevolumes,ortheidentityoftheobstaclegivingrisetothecollision.Thisinformationsuggestsonlylocalpathchanges,butnotaradicallydifferentpathifthatisrequired[3].Thisresultsinanexpensivesearchofthespaceofpossiblepaths.

Inaconfigurationspacemethod[4–6]theobjectsconsideredarepolyhedrons.Configurationspacecharacterisesthepositionandorientationofanobject.Thisisformedbyshrinkingtheobjecttoapointwhileexpandingtheobstaclestoaccountfortheobjectdimensions.Theremainingfreespaceislefttotranslatethepointobjectbetweenthestartingandfinalpos-itions.Thismethodusestheverticesofallthetransformedobstaclepolyhedronstoformeverypossiblevisiblepath.Avertexgraphisthenusedtosearchfortheshortestvertexpath.Thecomputationaltimeincreasesexponentiallyasthenumberofobstaclesincreases[7].InaCMM,thepartisfixturedtoitsbase,sothisleadstoalongsearchtimeasthefixturesbecomeobstacles.

Thespacesubdivisionmethodcanbedividedintotwoapproaches,namely,spatialenumeration[8]andoctreedatabase[9,10]methods.Manyalgorithmshavebeendevelopedbasedonanoctreedatastructure.Octreeisarecursivemethodofrepresenting3Dobjectsinahierarchicaleight-arraytreestruc-ture.Themovingobjectandtheobstaclesaremodelledbysweepingvolumesinoctreemethods,whichmakescollisiondetectiondifficult.Acommonprobleminthesemethodsisthezigzagnatureofthepathwhichinvolvesaccelerationanddecelerationoftheprobe.This,inturn,leadstolowpathefficiencyduetovibrationinducedmeasurementerrorsandlongertravellingtime.

Luetal.[11]proposedanalgorithmwhichusesamodified3Doctreeraytracingtechniquetosearchforthecollidingobstaclesinanoctreedatabaseonaselectedpath.Italsousestheglobalinformationabouttheobstacleverticestoreducethezigzagnatureofthepathgeneratedbytheoctreebasedmethods.Thisapproachrepresentsthepartusinganoctreedatastructure,whichisnotwidelyusedinCADsolidmodel-ling,sinceitapproximatesthegeometryofthepartwithcells.ThecommonlyusedsolidrepresentationsareB-reps（boundaryrepresentationsandCSG（constructivesolidgeometry.More-over,sincetheCADmodelcontainscompleteinformationaboutthepartdimensions,tolerances,geometry,orientation,etc.,thisinformationcanbeuseddirectlyforinspectionpathplanning.Therefore,itavoidstheprocessofrepresentingtheworkspaceandobjectineitherspecialfunctionsoragraphicdatabase,whichistimeconsuming.

ThisresearchdevelopsaframeworkforCADdirectedinspectionplanningandimplementsthebasicstructureoftheframework[12,13].

3.ProposedFramework

Themainobjectivesoftheproposedframeworkare:

1.TointegratetheCMMintotheCAD/CAMenvironment,sothatthepartdesignedandmanufacturedcanalsobeinspectedinasinglecycle.

2.Toincorporatefeaturebasedinspectionplanningatthedesignstage.

3.Togenerateanefficientandoptimumcollision-freeCMMinspectionpath.

4.TogenerateaDMIS（dimensionalm