Making Better Medical Devices with Multisensor Better Medical Devices with Multisensor Metrology ...…

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    MakingBetterMedicalDeviceswithMultisensorMetrologybyNateJ.Rose,ChiefApplicationsEngineer,OpticalGagingProducts(OGP)

    Multisensormetrologyisbecomingapreferredqualitycontroltechnologyformanufacturersto

    develop,maintain,andimprovethequalityofmedicaldevices.

    Articlesaboutmultisensormeasurementmachinesoftenexplainthetypesofsensorsavailableand

    whattheydo.Inallcases,measuringsensorsacquiredatapointsfromapartundertest.Thosedata

    pointsareusedinthesystemsoftwaretodeterminegeographicrelationshipsthatarecomparedtothe

    partdesigndrawing.Somesensorsaremoreefficientthanothersatacquiringdatapointsfromcertain

    areason,across,orwithinapart.Thisarticledescribessensorsinthecontextofmedicaldevices,butit

    goesastepfurtherbydescribingwhatcanbedonewiththecloudsofdatapointsacquiredbyanyorall

    sensorsonamultisensormeasurementmachine.Sincethegoalofanymeasurementiscomparisonto

    thedesigndrawings,a3DfitofallthedatapointscomparedtotheCADdesigndatacanactuallymake

    decisionmakingbasedonlargesetsofdatapointseasierthaninterpretingprintoutsofindividual

    measuredvalues.Althoughitmayseemcounterintuitive,usingmultiplesensorstogetlotsofdatacan

    actuallymakedecisionsaboutpartsandprocesseseasier.

    SpecializedpartsdemandspecializedmeasurementMedicaldevicesandtheircomponentpartsareusuallyextremelyspecializedinformandfunction.

    Forexample,theymaybeverysmall,likethepartsthatformmiddleearprosthesesforENT

    applications.Regardlessofsize,medicaldevicepartsarealsoalmostalwaysfabricatedtoextremely

    tighttolerances.Measurementsystemsthatcharacterizethesemedicaldevicepartsmustbecapableof

    highprecision,oftentothesubmicronlevel.

    Orthopedicimplants,suchasprosthetichipballjoints,ortibialandfemoralkneeandankleimplants,

    areachallengetomeasureaccurately.Thesurfacesthatdefinetheirshapearehigherordercurvesmade

    upofNonUniformRationalBSplines(NURBS)orpointcoefficients.Theformstheyembodyare

    oftenvariableandorganicallycurved,becausethepartsmustfitwithmatingprostheticpartsandeven

    matewithpartsinsidethehumanbody.Theircomplex3Dcurvesmakeitdifficulttomeasureall

    surfacesfromasingledirection,makingitsimplyimpossibleforcertaintypesofsensorstomeasure

    them.

    Videobasedmeasurementsystemsarewellsuitedtomeasureprismaticparts,whichbydefinition

    containintersectingplanes.Whereplanesintersect,thereareedges,andedgesareeasytomeasurewith

    video.Orthopedicimplants,however,areoftenmadeupofregularcontinuouscurves(hipcomponents)

    orcomplexcontouringsurfaces(kneecomponents)shapesthatmimictheorganiccontoursofnatural

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    bodyparts.Thesetypesofsurfaceshavefewornoplanesorintersectingedges.Sincevideosensors

    excelatmeasuringedges,videomeasurementofthesepartswouldbelimitedtowidthsofouteredges

    illuminatedfrombehind.Videocanmeasuresurfacepoints,butusingmultiplefocuspointstogather

    enoughdatatosupportevenalinearsectionofacontouredsurfacewouldbelaboriousandimpractical.

    Atouchtriggerprobewouldhaveasimilarlimitationbecauseeachsinglepointrequiresapproach,

    trigger,andbackoff possiblebutimpracticalinamanufacturingenvironment.

    AcaseinpointmeasuringareplacementkneeAgoodwaytoverifythattheorganicallycurvedprofileof a

    replacementkneematchesitsdesigniswithalaser.Lasersensorsin

    multisensorsystemsworkbyprojectinglighttowardsasurface,

    collectingthereflectedand/orscatteredlightonadedicatedsensor,and

    automaticallycalculatingthedistanceofameasuredpointbetweenthe

    laserandthepartin3Dspace.Lasermeasurementcanbe

    accomplishedforasinglepoint,oralternatelyaseriesofdatapoints

    canbegatheredandcalculatedasthepartistranslatedbeneaththelaser

    orthelaserismovedoverthepart.Thepointspacingandsampling

    ratecanbeuserspecified.Metrologysoftwarecontinuouslycalculates

    thedistancebetweenthelaserandthepartsurfaceasthelaserbeam

    movesacrossthepart,keepingthelasersensorwithinitscapturerange

    throughclosedlooppositioningofthemultisensorsystemsZaxisstagecontrol.Bykeepingthelaser

    sensorwithinitscapturerange,precisepointpositionscanbecollectedquickly.Laserfocusisfaster

    andmoreaccuratethanvideoautofocus,andsinceitisnoncontact,itavoidspotentialdamagetothe

    partsurfaceandcontaminationofsterileparts.

    Inmostinstances,theusermaynotbeabletofixturethekneereplacementtoassureadirectlineof

    sightbetweenallitscriticalsurfacesandthelasersensor.Inthosecases,mountingthereplacementina

    rotaryindexermayprovideasolution,aswellasanopportunitytospeedmeasurementthroughputby

    eliminatingmultiplepositionalfixtureswhilecuttingdownonmanualparthandling.Typicallyadatum

    isestablishedfromsurfacesonthebackofthekneewithatouchprobe.Then,therotaryindexerrotates

    thekneereplacementtopresentitsdesiredsurfacetothelasersensorformeasurement.Sincethe

    oppositesideofthemeasuredsurfacesdefinesthedatum,itisimperativethatthemetrologysystembe

    equippedwithfully3Dcapablemetrologysoftwarethatrotatesthecoordinatesystemwhentheindexer

    rotates.Inthisway,everydatapointcapturedbythelasercanbetrackedin3Dspacebythemetrology

    software,regardlessoftherotaryindexerposition.

    Aprosthetickneemountedtoarotaryindexerbeingscannedbyalasersensoronamultisensormeasurementmachine.

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    Adifferentmethodofmeasuringthecomplexcontoursofaprosthetickneeimplantiswitha

    multisensorsystemequippedwithaRenishawSP25scanningtouchprobe.Likethelaser,theuser

    specifiesstartandendpointsforthescanonthekneereplacement,butinthiscase,theprobetip

    maintainsconstantcontactwiththeimplantsurfaceasthesystemmovesitalongthepartsurface

    acquiringdatapointsasitgoes.Unliketouchtriggerprobesthatmustapproachthesurface,trigger,then

    backoff,theSP25scanningprobemaintainsconstantcontact.Aswithalaser,thedatapointdensity

    andscanrateareuserdefinable.ThemultisensorsystemmustbeconfiguredfortheSP25,andalso

    mustbeequippedwith3DcapablemetrologysoftwaretotrackthedatapointsinXYZspace.

    Thereareotherwaystomeasureakneereplacementthatisfixturedinarotaryindexer.As

    mentionedabove,alinearlaserorcontactprobescancanbeperformedacrosstherotaryfixturedknees

    topsurface.Sincesuchalinescanrepresentsasectionacrossthe3Dpart,thatsectionmaybe

    measurableasanedgeusingavideosensor.Byrotatingtheknee90,thatsectionbecomesadistinct

    edgewhenthepartisilluminatedfrombehind.Thistechniquerequiresagoodmetrologylenssystem

    thathasalongworkingdistanceandlimitedinfluencefromthekneessteepsurfaces.Sincethe

    sectionislargerthananopticalfieldofview,functionssuchasEdgeTracearegoodforthis

    applicationwherethesystemautomaticallyfollowstheedgeovermultiplefields,acquiringpointsat

    eachposition.

    Whenmountedinarotaryindexer,thekneesentiresurfacecanalso

    bemeasuredbyrotatingitincrementally,afewdegreesatatime,and

    performingmultiplelinearscans(oredgetraces).Withtightpoint

    samplingdensity,thecollectionofallthepointsfromthesemultiple

    scanswillyieldpointcloudsofdata.Thesepointcloudscanbeimported

    into3Dcapablefittingsoftware,which,byknowingthecenterof

    rotation,canshowhowallthepartdatacoincidewiththeCADmodelof

    thepart.Somefittingsoftwareevenhasthecapacitytoperforma

    GD&Tanalysisofthepointclouddatasatisfyingsimultaneousrequirements,andshowinggraphically

    anydeviationsfromthedesignfile.Notonlycanthisinformationbeusedforacceptancetestingofeach

    part,themanufacturingengineercanusetheinformationtomakechangestomanufacturingprocessesto

    enhanceaccuracy and/orefficiencyforsubsequentparts.

    GD&Tanalysisofpointclouddatacouldpotentiallysupportacriticalconclusionthatwouldnotbe

    immediatelyobviousotherwise.Forexample,pointclouddatamayshowthattwoperpendicularlinear

    laserscansarewithinspec,butGD&Tanalysisofthebothofthosescanstakenasawhole,mayshow

    theentireparttobeoutoftolerance.

    AGD&TanalysisofapointcloudofdatafrommultiplelaserscansofaprosthetickneeshowingdeviationsfromtheCADmodel.

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    AcustomvideosolutionAttheotherendofthespectrumofmedicaldevices,seeminglysimpleplasticsyringebodiesrequire

    numerousdimensionalmeasurements.Sometimesacustomsolutionisthebestwaytomeasurebatches

    ofmedicaldevicepartslikethis.Asaminimum,thelengthandoutsidediameterofeachofthetubular

    syringesismeasured.Thiscouldeasilybeaccomplishedbyfixturingasyringehorizontallyonthestage

    ofamultisensorsystem,backlightingit,andusingavideosensortomeasuretheedgesthatdefineits

    outerdiameterandlengthtodeterminethedistancebetweenthoseedges.(SeeFig.1)Thesyringe

    bodiescouldbemeasuredoneatatimeasshownhere,butcouldbemoreefficientlymeasuredif

    mountedinamultipartfixture,bettersupportingproductionvolumesandloweringthecostpersyringe.

    Theabovesolutionworksfortheoutercylinder,butfortheseparts,diametersareimportanttoo.

    Videomeasurementoftheseperpendiculardiameterscannotbegatheredwhilethesyringebodyisina

    horizontalposition.Theusercouldremountthesyringesverticallyinaseparatefixture,butthatwould

    addparts,laborcosts,andtimetotheprocess.Itwouldalsobepossibletomounteachsyringe,oneata

    time,onarotaryindexer.Thatway,thelengthandoutsidediametercanbemeasuredwiththesyringe

    bodyinoneposition,thenrotated90soitsimportantdiameterscanbeimagedandmeasured.(SeeFig.

    2)Thiswouldautomatethemeasurementprocess,butagainitisnotefficient.Eachsyringebodymust

    beloadedandunloaded,andmeasuredoneatatime,andindexerrotationtakestime,too.

    Aspeciallydesignedmultipartfixturesolvesthismeasurementproblem.Withsyringesmounted

    horizontallyrelativetothevideooptics(inthesystemsXYplane),a45mirrorcouldallowdirect

    imagingandmeasurementofthediameterswiththepartinafixedposition.Byfixturinganumberof

    syringebodiessidebyside,thevideoopticscanperformallthemeasurementswithquickXYposition

    Fig.1:VideomeasurementoftheODandlengthofaplasticsyringebody.

    Fig.2:Videomeasurementofthediametersofaplasticsyringewhenrotated90.

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    movesandautofocus.Apartroutinecreatedforonesyringecaneasilybecopiedandreusedforall

    otherstospeedthroughput,andsincevideomeasurementisnoncontact,potentialpartdeformationof

    thesoftplasticsyringebodieswillnotbeaproblem.

    Thiscustomvideomeasurementtechniquewillonlyworkif

    thevideosensoropticshavealongenoughworkingdistanceto

    focusonthepartafterreflectingtheimagingpath.Asanoptical

    technique,thistypeoffixturecanworkwithaTTLlaser

    measurementsensor.Liketheoptics,theTTLlaserwould

    requirealongworkingdistance,aswellasminimalbeam

    triangulation.Somethroughthelenslasersystems,particularly

    theTeleStarTTLlasersfromOpticalGagingProducts(OGP,

    Rochester,NY),fulfillthoserequirements.

    ThenameofthegameMedicaldevicemanufacturersarerequiredtohavedocumented,controlledmanufacturingprocesses

    thatincludetheinspectionequipmentusedforqualitycontrolandmonitoring.Multisensor

    measurementsystemsarecapableofverifyingmanyoftheimportantdimensionsofmedicaldevices

    quickly,accurately,andwithminimalparthandling thisarticlehaspresentedonlytwoexamplesof

    themyriadofmedicaldevicescurrentlymeasuredbymultisensorsystems.Verifyingthatmanufactured

    partsmeetdesignspecificationsisthenameofthegame.Thefinaloutcomeaffectsthehealthof medical

    devicemanufacturersbalancesheetsandultimately,thehealthofthepatient.

    Amultipartfixturewithfoldmirrorformeasuringallthedimensionsofseveralplasticsyringebodiesinasinglesetup.