Study on Case-Based Fixture Design(基于案例學(xué)習(xí)的夾具設(shè)計(jì))

StudyonCase-BasedFixtureDesign

SHIQi,YANGHai-chcng,UYuan,

SlateKeyLaboratoryofCAD/CAM,NorthwesternPolytechnical

University,Xi'an710072,P.R.China

Abstract:Fixtureisanimportantmanufacturingactivity.Afixturedesignsystembasedoncase-basedreasoning(CBR)isproposedinthispaper.AnewmethodofcaserepresentationonthebasisoffixtureJunctionispresented,wherethecaserepresentationisconstitutedofworkpieceknowledge,processingfeatureknowledge,andfixturefeatureknowledge.Runningtheprototypesystemshowsthattheknowledgerepresentationmethod,usingcases,isabetterwaytotransformandexplainthedesignknowledge.

Keywords:case-basedreasoning,fixturedesign,knowledgerepresentation,caseretrieval

1.Introduction

Fixturesareworkholdingdevicesandareusedinmostofthemanufacturingoperationssuchasmachining,assembly,inspection,etc.Aspartofmanufacturingplanning,fixturedesigncanbeamajorcontributortothemanufacturing,lead-time,andthecostofaproduct.Owingtothegreatvarietyofpartsandmanufacturingoperations,theprocessesoffixturedesignbecomemorecomplexanduncontrolled.Moreover,thedesignofafixturereliesheavilyonthedesignersexpertiseandexperienceaswellasthedatafrommanufacturing,processing,technical,andparts.Acomputer-aidedfixturedesign(CAFD)techniquehasbeendevelopedaspartofCAD/CAMintegration.Nowadays,CAFDplaysanimportantroleinflexiblemanufacturingsystems(FMS)andcomputer-integratedmanufacturingsystems(CIMS).Thefirstthingthatmustbeconsideredbyemployingacomputerinfixturedesignistoestablishasuitabledesignplanningmodel.Thepresentationmethodofknowledgeusedindesignandthedesigner'sexperienceareessentialtoestablishasuccessfulfixturedesignsystem.

Comparedtoclassicalrule-basedreasoningandmodel-basedreasoning,case-basedreasoning(CBR)attemptstosolvenewproblemsbyusingpreviouscasesfromacasebaseandadaptingthesolutionofasimilarproblemaccordingtotheparametersofthenewsituation.Themajoradvantageofthisapproachoverrule-basedreasoningormodel-basedreasoningisthattheknowledgeusedindesignistoocomplextobereducedtorulesormodels.Theapproachholdskeybenefitswithrespecttosolvingmechanicaldesignproblems.TherearesystemsthathavebeendocumentedasdesignsystemsintheCBRliterature.OnesystemcalledDEJAVUusescase-basedreasoningtoassistmechanicaldesign.AnothersystemcalledCADSYNisahybridcase-baseddesignmodelwhichisproposedtointegratespecificdomainknowledge.Inthispaper,casesareusedtoelaboratethedomainknowledgeanddesignexperience.Amethodofcaserepresentationoffixturedesignbasedonfixturefunctionisproposed.Section2describestheconceptofcase-basedreasoning.Section3providesthecaserepresentationoffixturedesignandthestructureofthecasebase.Section4presentsthedetailsofthearchitectureofthecase-basedfixturedesignsystem.Theconclusionsfollow.

2.Case-basedreasoning

Case-basedreasoningreliesonstoringsolutionsaswellasproblemsandadaptingthesesolutionstosolvethenewsimilarproblems.Designisanill-structuredproblem.Designexperienceandheuristicsplayimportantrolesinthedesignprocess.However,mostexperienceisdifficulttoarticulateascompiledandformulated,soinarule-basedsystem,knowledgeacquisitionwouldbeanobstacle.ACBRsystem,incontrast,solvesproblemsdirectlybyusingexperiencegainedfromsolvingsimilarproblemsinthepastratherthanbycompilingandgeneralizingit.Acaserecordsnotonlytheresultbutalsohowthisresultwasobtainedsuchasinwhatsituation,accordingtowhatconditions,etc.Someengineerspioneeredcase-basedreasoningasanalternativetothetraditionalrule-basedandmodel-basedreasoningtechniques.

AtypicalCBRprocessincludesthestepsasshowninFigure1.First,theproblemshouldbeidentifiedanddescribedandthisdescriptioninformationisthenusedtoretrieveoneormoresimilarcasesfromacasebase.Caseretrievalmaybebasedonsomealgorithmstoimprovetheaccuracy.Theresultofcaseretrievalwillbeshowntothedesigner.Thechosencaseisadaptedwhenneededtomeettherequirementsofthenewproblem.Finally,thesolutionsaswellasthenewproblemarestoredinthecasebaseasanewcase.

Problemdescription

Casebase

Case-basedreasoning

Problemspec.

Designsolutions

Retrievalmechanisms

DesignKnowledge

Adaptaionmechanisms

Designsolution

Figure1Acase-baseddesignsystem

3.Representationofdesigncasesandcasebaseorganization

3.1Thefeatureoffixturedesign

Inamanufacturingsystem,fixturedesigninvolvesthreeactivities:set-upplanning,fixtureplanning,andfixtureconfigurationdesignasshowninFigure2.Theobjectiveofset-upplanningistodeterminetheoperationsequence,thepositionandorientationoftheworkpieceineachset-upandthenumberoftheset-ups.Fixtureplanningdeterminesthelocatingandclampingpointsontheworkpiecesurfaces.Thetaskoffixtureconfigurationdesignistomakedecisionsofhowtolocateandhowtoclamp.Thefixturecomponentsarealsoselectedandplacedintopositiontolocateandholdtheworkpiece.

Fixturedesign

Fixtureconfiguration

Elementselection

Positiondetermination

Fixtureplanning

Locatingsurface(points)

Clampingsurface(points)

Setupplanning

Operationsequence

Workpieceorientation

Figure2Fixturedesignprocess

Themainpurposeofset-upplanningandfixtureplanningistoaccomplishfixtureconfiguration.Twokindsofinformationareneededduringset-upandfixtureplannings.Oneisworkpieceinformation;theotherisprocessinginformation.BothofthesetwokindsofinformationarederivedfromtheoriginalCADmodelandtheproductprocessplanningmodel.AcaseinaCBRfixturedesignsystemiscombiningworkpiecestructureinformation,processinginformationandfixtureconfigurationinformation.

3.2Abstractcasefunctionfeatures

Themostimportantissueincase-basedreasoningistheinformationdescriptionofdesigncases.Feature-basedtechnologyisusedtodepicttheparts'shape,functionandstructure.Traditionally,itisthelimitationsofthegeometricalmodeloftheCADsystemthatitcanonlydescribeapart'sgeometricalinformationratherthanthedesignexperience.Feature-basedtechnologycandescribetwokindsofknowledge;datarelationalknowledgesuchasshape,function,precision,material,andsoon,andtheknowledgeoftherelationshipamongdata,suchasdesigndomainknowledge,designer'sexperience,andformulas.Casesareaggregatesofasetoffeatures.

Twopartsofknowledgemustbekeptincasesstoredinacasebase:descriptionofdesignproblemanddesignplanning.Descriptionofdesignproblemconsistsofasetofcasefeaturesfromdesignproblems.Designplanningisfeaturesabstractedfromsolutions.Infixturedesign,thefixturemodelintegratesthreetypesofknowledge;fixturesolution,workpieceinformation,andprocessinginformation.Accordingtothemethodofproblemreductioninartificialintelligence,acomplexdesignproblemcanbeseparatedintoseverallesscomplexsub-problems,andthesub-problemscanbeseparatedintomoresimplesub-sub-problems,andthelike.Inthissystem,adesigncaseisorganizedinahierarchythatconsistsofthreekindsoffeatureinformation;fixturefeatures,workpiecefeatures,andprocessingfeatures,andinthesystemthefixturefeaturescanbedecomposedtofunctionfeatures,behaviorfeatures,andstructurefeatures.Fixturebehaviorfeaturesshowtherelationshipbetweenfixturefunctionsandstructure.Fixturefunctionfeaturecanbefurtherdividedintolocatingfeatures,clampingfeatures,supportingfeatures,andguidingfeatures.TheinformationonafixturecasemodelisshowninTable1.

Theimportanceofcasefeaturesisdefinedinthecontrolknowledgebaseofthesystem.Caserepresentationisdoneonlybysummarizingthedesignfeatures,wheretheimportanceandtheinterrelationofthecasesneednotbeconsidered.Therearetwoadvantages:(1)itsimplifiescasestructureandcasestorageinthedatabase,(2)anewreasoningstrategyindependentofcaserepresentationandcasebaseorganizationcanbeestablished.

Case

information

Features

Value(s)

Name

Type

Axis,dish,cover,cabinet,gear,andothers

Workpiece

Quantity

Mass,medium,small-lot

Material

Size

Lengthxwidthxheight

Partdrawing

Filenameandpath

Machinename

Lathe,milling,planer,grinder

Contentsofprocessing

Plane,outercylindricalsurface,innerholesurface,slot

Feedtype

Line,circle

Processingfeatures

ProcessingPrecision

Finishmachining,roughmachining

Typeofprincipalaxis

Horizontal,vertical

Typeofprincipallocatingsurface

Plane,outercylindricalsurface,innerholesurface,curvedsurface

Precisionofprincipallocatingsurface

Finishmachining,roughmachining,intherough

Partsscheduledrawing

Filenameandpath

Fixturename

Fixturetype

Lathefixture,millingfixture

Locatingmode

Plane,cylindricalsurface

Fixture

Locatingdevice

Two-pin,v-block,etc.

features

Clamprequirement

Verticalclamping,horizontalclamping

Typeofdampingcontact

Point-contact,surface-contact

Power

Manualclamp,etc.

Clampdevice

Eccentricwheel,etc.

3.3Caserepresentation

Themethodofcase-basedknowledgerepresentationisusuallynotanewmethodbutthehigher-levelabstractofthecommonknowledgerepresentation,suchasfirstorderlogic,productionrule,frame,andsemanticnetwork.Theimplementationofcaserepresentationisbasedonthosecommonknowledgerepresentationmethods.Inthispaper,anobject-orientedknowledgerepresentationisusedtodescribefixturedesigncases.Essentially,theobject-orientedknowledgerepresentationiscombinedwithasetofcommonknowledgerepresentationmethodsintermsofobject-orientedtechnology.Thecaseknowledgecanbedividedintotwoparts:oneisspecifictupleoffeature-valuepair,andtheotherisdesignexperienceandrules.Therepresentationofthecaseissimilartotheconceptionofclassinobject-orientedtechnology.Thefeature-valuepairsaretheattributesinclass,whichcontainsthepreconditionandsolutioninformation.Designexperienceandrulesarethemethodsofclass.Thecontentsofacaseinobject-orientedtechnologyisillustratedasfollows.

Class<CASE_NAME)

{[(FEATURE_VALUE_PAIR_LIST>]/*listofcasefeaturevaluepair*/Structure

[(STATIC_STRUCTURESJMPL)]/*staticstructuresusedtodepictspecificdesignfunction

Methods

[(CASE_FUNCT101VS_LIST)]/*listofcasefunction*/|;where,

<CASE_NAME>::=<CASE_NAMEBYDEFAULT)|(CASE_NAMEBYUSER)<FEATURE_VALUE_PAIR_LIST)::={(FEATURE_TYPE)(FEATURE_NAME)

<FEATURE_VALUE>|f(STATIC_STRUCTURESJMPL>::=|STRUCTURE(STRUCTURE_NAME)

{STRUCTURE_VARIABLES_LIST)|*{CASE_FUNCTIONS_LIST)::={(FUNCTION_NAME)<FUNCT10N_C0NDITI0N)

(FUNCTION_CONCLUSION)}

Thelistofcasefunctionscontainsthemethodlistaboutthecasedesignexperienceanddesignrules.Anotherproblemthatshouldbeconsideredisthestructureofthecasebase.Acasebasecanbestructuredasalinedlistoraframewithahierarchicalstructure.Weselectthelatter,whichusesarelationaldatabasetostorecases.Theadvantageofrelationaldatabasemanagementconsistsofthreepoints-.(1)advancedabilitytodatamanagement;(2)highperformanceindataretrievalanddatamaintenance;(3)directoperationrelationshipbetweenrelationalcalculusandlogiccalculus.

4.Case-basedfixturedesignsystem

Basedonthecaserepresentationmodelmentionedabove,acase-basedfixturedesignsystemisbuilt,anditsarchitectureisshowninFigure3.Thesystemmainlyconsistsoffourcomponents,asystemcontrolmodule,acase-basedreasoner,fixturedesignmemory,andadesignevaluationmodule.Acase-basedreasoneristhecoreofthesystem,anditperformsthecaseretrieval,caseselection,andcasetransformation.Figure4isascreenfortheworkingprocess.Thesysteminputconsistsofthreekindsofinformation:workpiecegeneralinformation,suchasname,type,material,etc.workpieceprocessinginformation,suchasmachininginformation,precisionandmachinetype;fixturefeaturesinformation,suchasclamprequirement,contacttype,powertype,etc.Thesystemmodulescontainsthefollowing:

ControlModule;usedtomonitorandmanagedataflowandmessageflowamongothermodules;

CaseReasoningModule;canbeseparatedintosixsub-modules:weightproduction,judgmentoffunctionalsimilarity,judgmentofstructuresimilarity,caseretrievalmodule,fixturecaseadaptation,andcasestorage;

Casebase;usedforfixturecasestorage,establishcaseindex,andperformcasemaintenance;

Designevaluation:evaluatetheresultofreasoningandsubmitthesolutions.

5.Conclusions

TheCBRtechnologyisclosertohumanthought.Afixturedesignsystemusingthistechnologyisde-scribedinthi