1ApplicationanddevelopmentO fO O O casebasedreasoninginfixturedesignAbstract:Basedonthecasebaseddesigning (CBD) m ethodology, thefixturesim ilarity isintworespects: thefunctionandthestructureinform ation.Then, thecom puter aidedfixturedesignsystem is created on case basedreasoning(CBR),inwhichtheattributesof them ain featuresof workpieceandstructureof fixtureascaseindexcodeare designedfor the retrieve of thesim ilar cases, and the structure andhierarchical relationof caselibraryareset up for store. Meanwhile, thealgorithm based on the knowledgeguidedinthe retrieve of the sim ilarcases, thestrategyof caseadapt at ionandcasestorageinwhichthecaseidentifcat ionnum ber isusedtodistinguishfrom sim ilar cases arepresented. Theapplication of the system in som eprojectsim proves thedesignefficiencyandgetsagoodresult.K eywords:K K K case basedreasoning ;fixture design; com puteraideddesign(CAD)Fixturesaredevicesthat serveasthepurposeofholdingtheworkpiecesecurely and accurately, andm aintaining a consistent relationshipwith respect to the tools whilem achining. Because the fixturestructuredependsonthefeatureof theproduct andthestatus of theprocessplanningintheenterprise, itsdesignisthe bottleneck during m anufacturing,which restrains to im prove theefficiency and leadtim e. And fixture2designisacom plicated process, basedon experience that needscom prehensive qualitative knowledgeabout a num ber of design issuesincluding workpiece configuration,m anufacturing processesinvolved, andm achining environm ent. Thisisalsoaverytim e consum ing workwhenusingtraditional CAD tools (such asUnigraphics, CATIAor Pro/E), whicharegoodat perform ing detaileddesigntasks, but provide fewbenefits fortakingadvantageofthepreviousdesignexperience andresources, which arepreciselythekeyfactorsinim provingtheefficiency. Them ethodology ofcasebased reasoning (CBR) adapts thesolutionof apreviouslysolvedcasetobuildasolutionforanewproblem withthefollowingfoursteps: retrieve, reuse,revise, andretain1. This is am oreuseful m ethod thantheuseofanexpertsystem to sim ulate hum an thoughtbecauseproposingasim ilar caseandapplyingafewm odifications seem s tobeself explanatoryandm ore intuitiveto hum ans .So various case baseddesign support tools have beendeveloped for num erous areas2-4,suchasininjectionm olding anddesign,architectural design, die casting diedesign, process planning, andalsoinfixturedesign. Sunusedsixdigitalstocom pose theindexcodethat includedworkpiece shape, m achine portion,bushing, the1st locatingdevice, the2ndlocatingdeviceandclam ping device5.But thesystem cannot beusedforotherfixturetypes except for drill fixtures,andcannot solvetheproblem ofstorageofthesam e indexcodethat needstoberetained, whichis veryim portant inCBR6.1ConstructionofaCaseIndexand3CaseLibrary1.1CaseindexThe case index should becom posed of all features of theworkpiece, which are distinguishedfrom different fixtures. Usingall ofthem would m ake the operation inconvenient. Becausetheform s of theparts arediverse, andthetechnologyrequirem ents of m anufacture in theenterprise also develop continuously,lotsof featuresusedasthecaseindexwill m ake thesearchrateslow, andthem ain feature unim portant, for thereasonthat therelativeweight whichisallottedtoeveryfeaturem ust dim inish.Andontheother hand, it is hardtoinclude all the features in the caseindex.Therefore, considering thepracticalityandthe dem and of rapiddesign, thecaseindexincludesboththem ajor featureof theworkpieceandthestructureoffixture. Thecaseindexcodeis m ade upof 16digits: 13digitsforcase features and 3 digits for caseidentificationnum ber.The first 13digits represent 13features. Eachdigit iscorrespondingtoanattributeof thefeature, whichm aybeoneof“*”,“？”,“1”,“2”,“A”,“B”,“Z”, etc. Inwhich,“*”m eans anyone,“?”uncertain,“0”nothing.The system rules: fixture type,workpieceshape, locatingm odel cannotbe“*”or“?”. Whenthesystem isdesigned, the attribute inform ation ofthe three item s does not have theseoptions, which m eans the certainattributem ust beselected.Thelast threedigits arethecaseidentificationnum ber, whichm eans the13digits of the casefeaturearethe4sam e, andthenum ber of thesethreedigitsisusedfordistinguishingthem .Thesystem alsorules:“000”isaprototype case, which is used forretrieval, andothercasesare“001”,“002”, whichareusedforreference cases to be searched bydesigners. If occasionallyoneof themneeds tobechangedas theprototypecase, first it m ust berequiredtoapplytochangetheoneto“000”, andtheform er is changedtoreferential caseautom atically.TheconstructionofthecaseindexcodeisshowninFig.1.1.2CaselibraryThecaselibraryconsistsoflotsofpredefinedcases. Caserepresentationisoneofthem ost im portant issuesincasebasedreasoning. Socom pounding withtheindexcode,.1.3Hierarchicalform ofCaseThe structure sim ilarity of thefixture is represented as the wholefixturesim ilarity, com ponents sim ilarityandcom ponent sim ilarity. Sothewholefixture case library, com ponents caselibrary, com ponent case library offixture are form ed correspondingly.Usually design inform ation of thewholefixtureiscom posed ofworkpieceinform ation andworkpiece procedureinform ation, whichrepresent thefixturesatisfying the specifically designingfunction dem and. The whole fixturecaseism ade upoffunctioncom ponents,whichare describedbythe functioncom ponents nam es andnum bers. Thecom ponents case represents them em bers. (function com ponent and5other structure com ponents，m aindrivenparam eter, thenum ber, andtheirconstrainrelations.) Thecom ponentcase(thelowest layer of thefixture) isthestructureoffunctioncom ponent andother com ponents. In the m odernfixture design there are lots ofparam etric standardpartsandcom m onnonstandardparts. Sothecom ponentcase library should record thespecificationparam eter andthewayinwhichitkeepsthem .2StrategyofCaseRetrievalIn the case based design offixtures,them ost im portant thingistheretrieval of the sim ilarity, whichcanhelptoobtainthem ost sim ilar case, andtocut downthe tim e of adaptation.Accordingtotherequirem ent offixturedesign, the strategyof case retrievalcom bines the way of the nearestneighborandknowledgeguided.Thatis,first searchondepth, thenonbreadth;theknowledgeguidedstrategym eans tosearchontheknowledgerulefrom roottotheobject, whichisfirstlysearchedbythefixturetype, thenbytheshapeoftheworkpiece, thirdlybythelocatingm ethod. Forexam ple, if thecaseindexcode includes the m illing fixture offixturetype, thesearchis just for allm illing fixtures, then for box ofworkpieceshape, thethirdfor 1plane+2pineoflocatingm ethod. Ifthereisnom atch of it, thenthesearchstops ondepth, andreturns totheupper layer,andretrieves all therelativecases onbreadth.Retrievalalgorithm s:1)According to the case indexinform ation of fixture case library,searchtherelevantcaselibrary;2)Matchthecaseindexcodewiththecodeofeachcaseofthecaselibrary,6andcalculatethevalueofthesim ilaritym easure;3)Sort the order of sim ilaritym easure, thebiggest value, whichisthem ost analogicalcase.Sim ilarity betweentwocases isbasedonthesim ilarity betweenthetwocases. features. The calculation ofsim ilarity m easure dependsonthetypeof thefeature. Thevalueof sim ilaritycanbecalculatedfor num erical values,for exam ple, com pareWorkpiece withtheweight of50kgand20kg. Thevaluecanalsobe calculatedbetween nonnum erical values, forexam ple, nowthefirst 13digits indexcode is all nonnum erical values. The sim ilaritym easure of afixtureis calculatedasfollows:whereSisthesim ilarity m easureofcurrent fixture, nisthenum ber oftheindexfeature, istheweight ofeachfeature, isthesim ilaritym easure oftheattribute ofthei2thfeaturewiththeattribute of relativefeature of the j-thcase inthe caselibrary. At the sam e tim e, thevaluecountsasfollows:. Where isthevalueoftheindexattributeofthei-thfeature, and is7thevalueofattributeoftherelativei-thfeatureofthej-thcaseincaselibrary.Sotherearetwom ethods toselecttheanalogical fixture. Oneistoset thevalue. If the values of sim ilaritym easure ofcurrent caseswerelessthanagivenvalue, thosecaseswouldnot beselectedasanalogical cases. Whenthecaselibraryisinitiallyset up, andthereareonlyafewcases, thevaluecanbeset sm aller. If there are lots ofanalogical cases, thevalueshouldgetlarger. The other is just to set thenum ber of theanalogical cases (suchas10), whichis the largest value ofsim ilarity m easure from the sortedorder.3 Case adaptation and CaseStorage3.1CaseadaptationThe m odification of theanalogical case inthe fixture designincludesthefollowingthreecases:1)Thesubstitutionof com ponentsandthecom ponent;2) Adjusting the dim ension ofcom ponents andthecom ponent whiletheform rem ains;3)Theredesignofthem odel.If thecom ponents andcom ponentofthefixturearecom m on objects, theycanbeedited, substitutedanddeletedwithtools, whichhavebeendesigned.3.2CasestorageBeforesavinganewfixturecaseinthe case library, the designer m ustconsiderwhetherthesavingisvaluable.If the case does not increase theknowledge of the system , it is notnecessarytostoreit inthecaselibrary.Ifit isvaluable, thenthedesignerm ustanalyze it before saving it to seewhether the case is stored as aprototypecaseor asreferencecase. A8prototypecaseis arepresentationthatcandescribethem ain featuresofacasefam ily. Acasefam ily consistsof thosecaseswhoseindexcodeshavethesam efirst 13digits anddifferent last threedigitsinthecaselibrary. Thelast threedigits of aprototypecasearealways“000”.Areferencecasebelongstothesam e fam ily astheprototypecaseandis distinguishedbythe different lastthreedigits.From the concept that has beenexplained, thefollowingstrategies areadopted:1) If a newcase m atches anyexistingcasefam ily, it has the sam efirst 13digitsasanexistingprototypecase, sothecaseisnot savedbecauseitis representedwell bythe prototypecase. Or is just savedas areferencecase(thelast 3digitsarenot“000”,andnot thesam e withothers) inthecaselibrary.2) If a newcase m atches anyexistingcasefam ily andisthought tobe better at representing this casefam ily thanthepreviousprototypecase,thentheprototypecaseissubstitutedbythis newcase, and the previousprototypecaseissavedasareferencecase.3) If anewcasedoes not m atchanyexistingcasefam ily, anewcasefam ily will begeneratedautom aticallyandthecaseisstoredastheprototypecaseinthecaselibrary.4 Process of CBRin FixtureDesignAccordingtothecharacteristicsoffixturedesign, thebasicinform ation ofthefixturedesignsuchasthenam e offixture, part, product andthedesigner,etc. m ust beinput first. Thenthefixturefileisset upautom atically, inwhichall9com ponents of the fixture are puttogether. Then the m odel of theworkpiece is input or designed. Thedetailed inform ation about theworkpieceisinput, thecaseindexcodeisset up, andthentheCBRbeginstosearchtheanalogical cases, relyingonthe sim ilarity m easure, andthe m ostanalogical case is selected out. Ifneeded, thecaseis adaptedtosatisfythecurrent design, andrestoredintothecase library. The flowchart of theprocessisshowninFig.3.5IllustratingforFixtureDesignbyCBRThisisaworkpiece(seeFig.4). Itsm aterial is45#steel. Itsnam e isseat.Itsshapeisblock, andtheproduct batchsizeism iddle, etc. Afixtureisturningfixture that serves toturnthe hole,whichneedstobedesigned.Thevalueof feature, attribute,case index code and weight of theworkpieceisshowninTab.2.10Throughsearching, andcalculatingthesim ilarity, thecaseindexcodeofthem ost sim ilar case is19325513321402000, andthedetailedinform ation isshowninTab. 3.Thesim ilarity iscalculatedasfollows:Sothevalueofsim ilarity m easureof the fixture which needs to bedesignedwiththem ost analogical casein case library is 0.806, and thestructureof th