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    Reliability Growth

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    ObservingandPredictingTrendsinReliability

    Today’scomplexproductsrequirethatcompaniesfocusonestablishingdetailedplansandproceduresfordevelopingandmanufacturingproductsthatmeetspecifiedreliability,maintainability,andotherperformancerequirements.Reliabilitygrowthreferstoawell-definedprocessforidentifyingandcorrectingreliabilityproblemsearlyinthedesignprocesssothatthereliabilityofaproductincreasesor”grows”astheproductgoesthroughsuccessivedevelopmentstages.Reliabilitygrowthprogramsshouldbeestablishedforallnewproductsandforallexistingproductsundergoingmajorredesignssothattheimprovementduetochangesindesignandmanufacturingprocessescanbeeasilytracked.
    Duringtheearlydesignstage,areliabilitygoalissetforaproduct.Becausefailuredatafromprototypetestingisnotyetavailable,theinitialreliabilitygoalisoftenbasedoneitherthefailuredataforsimilarproductsorthefailuredataforthesubcomponentsoftheproduct.Duringthedevelopmentstage,productprototypesgenerallyundergoextensivetestingsothatdeficienciesinrespecttodesign,engineering,andmanufacturingcanbeidentifiedandcorrected.
    Atypicalreliabilityimprovementtestconsistsofoperatingproductprototypesforseveralweeksinthesametypesofenvironmentsinwhichcustomerswilleventuallyoperatetheproduct.Ateamofprojectengineersandtechniciansanalyzeeveryfailurethatoccurs,determiningrootcausesforfailuresanddevelopingdesignandmanufacturingimprovementsthataretoeithereliminateorreducetherecurrencesofthesefailures.Asthetestingcontinues,theimprovementsdevelopedbytheteamareincorporatedintotheprototypesothatproductreliabilitycontinuestoimprovethroughoutthetestingperiod.

    AchievingReliabilityGrowth
    Thethreemostimportantstepsintheiterativeprocessforachievingreliabilitygrowtharedepictedinthefeedbackloopthatfollows:
    [attach]177[/attach]
    Mostoftheproblemsencounteredduringtestingarelikelytobecomponentfailuresandmanufacturingdeficienciesthatcouldnotbeforeseenintheearlydesignphase.Becausevariousperformancerequirementscanconflict,optimizingadesigntomeetonerequirementcancausethedesigntofailtomeetanotherrequirement.Thus,iterationsofdesignsareoftenneededtocorrectallofthecomponentselectionandmanufacturingdeficienciesthatarefoundduringprototypetesting.
    Inadditiontoimprovingreliability,earlyimplementationofareliabilitygrowthprogramminimizestheimpactonproductionschedulingandtotalproductcost,especiallysincethecostsassociatedwitheitherredesigningaproductlateinthedevelopmentcycleorretrofittingproductsalreadyinthefieldareextremelyhigh.Developingaproductthatmeetsreliabilityrequirementsalsoensuresthattheproductultimatelymeetsuserneedsandhasanacceptabletotallifecyclecost.Settinginterimreliabilitygoalsthataretobeachievedduringtestingensuresthatresourcesareallocatedefficiently.

    ReliabilityGrowthData
    Thebasicprincipleofanyreliabilitygrowthmodelistoapplythetestingresultsanddatapointstodetermineifthereliabilityoftheproductisgrowingsufficientlytomeetthereliabilityrequirementsfortheproduct.Thetypesofdatausedforpredictingreliabilitygrowthare:
    •ReliabilityData.Thereliabilityoftheproductisrecordedatdifferentpointsintime.Thereliabilityisaratioofthenumberofunitsstillfunctioningandthenumberofunitsthatenteredthestage.
    •Success/FailureData.Theitemistestedandcaneithersucceedorfail.Successdatacanconsistofacodeindicatingtheoutcome,suchasSforSuccessorFforFailure;or,itcanconsistofacodeindicatingthefailuremodeforeachfailure.Success/failuredatamightalsoindicatethenumberoffailuresthatoccurredwhenseveralunitsaretestedtogether.
    •FailureTimeData.Thetimetofailureforanitemistrackedusingeithercumulativeornon-cumulativeoperatingtimes.Failuretimedataisthemostcommonlyusedtypeofdatainreliabilitygrowth.Thisdatacanbeforonefailureorsystem,oritcanbeformultiplefailuresormultiplesystems,whichiscommonlycalledintervalorgroupeddata.
    Thefailuredatacollectedduringprototypetestingisusedtodeterminewhetherthereliabilitygoalfortheproductislikelytobemetorexceededbythetimetheproductisscheduledtobeputintofull-scaleproduction.Althoughmanymethodsexistformodelingthereliabilitygrowthprocess,theDuane/Crow-AMSAAmodelisconsideredthebestpractice.In1964,J.T.Duane,anengineerattheAerospaceElectronicsDepartmentofGeneralElectronicCompany,publishedapaperdemonstratinghowalearningcurveapproachcouldbeusedtomonitorthecontinuingreliabilityimprovementsintheearlystagesofdevelopingcomplexelectromechanicalandmechanicalsystems.Duaneexplainedhowthiswasbecausethelessonslearnedfromfailureswereusedtorefinedesigns.
    AccordingtoDuane,agraphofthecumulativeMTBFversusthecumulativeoperatingtimeplottedonlog-logpaperfellclosetoastraightline.Graphingthislearningcurveprovidesameansofmeasuringandpredictingreliabilityduringaperiodofproductchange.Dr.LarryCrowlateraddedpowerfulstatisticalcapabilitiestoDuane’spostulateforlearningcurvemodelingtocreatethemodelsowidelyrespectedandimplementedtoday.AnexampleofareliabilitygrowthchartgeneratedbytheDuane/Crow-AMSAAmodelfollows:
    [attach]178[/attach]
    Reliabilitygrowthisgenerallyquantifiedbygraphinganyofthefollowingthreemeasuresovertime:
    •Theincreaseinthemissionsuccessprobability(reliability).
    •TheincreaseinMTBF.
    •Thedecreaseinfailurerateasafunctionoftime.
    Reliabilitygrowthchartsdepicttrendsthatareusedtoforecastfailuresasafunctionofadditionaltesttimeorcalendartime,therebymakingplanningforredesignandtestresourceseasier.Inadditiontoallowingyoutodeterminewhetherreliabilityrequirementswillbeachieved,reliabilitygrowthchartscanhelpyoutodeterminethetimeneededtomeettheserequirementsandtheassociatedcosts.Extrapolatingagrowthcurvebeyondthecurrentlyavailabledatashowswhatreliabilityaprogramcanbeexpectedtoachieveprovidingthattheconditionsofthetestandtheengineeringefforttoimprovereliabilityaremaintainedattheirpresentlevels.
    Ifthereliabilitygrowthgraphindicatesthatthereliabilitygoalisnotgoingtobemetorexceeded,thentheproductdesignmustbeimproved.Thismightrequiretheuseofmorereliablecomponentsorredundancy.Additionalresourcesmightalsohavetobedevotedtodesigningandmanufacturingamorereliableproducttomeettherequireddeliverydate.

    OtherUsesforReliabilityGrowthCharts

    Inadditiontoanalyzingtheresultsfromtestingnewlydevelopedorredesignedproducts,theDuane/Crow-AMSAAmodelcanbeusedto:
    •Predictfuturefailures.
    •Trackfleetsofrepairablesystems.
    •Provideaccuratetrendingofsignificanteventsformanagement.
    •Forecastsafetyincidentsthatcanbecontrolled.

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    Lv.1
    很有参考学习的价值谢谢
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    goodinformation
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    不是可靠性增长测试??、、、、
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    Lv.1
    看了以后再评论
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    goodinformation
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