Performance Level (PL) of safety-related control systems

The PL (Performance Level) is the discrete level used to specify the ability of SRPs / CSs to perform a safety function under foreseeable conditions. The PL is ... Skiptocontent Home»Safetyofmachinery»PerformanceLevel(PL)ofsafety-relatedcontrolsystems SicurezzadelleMacchine WhatarethePerformanceLevelsofsafety-relatedcontrolsystemsofmachiney? Themachinerydirective2006/42/ECrequiresthatsafety-relatedcontrolsystemsofthemachinesaredesignedandconstructedinordertoavoidtheoccurrenceofdangeroussituationsevenintheeventofafailureinthehardwareorsoftwareofthemachinecontrolsystem. Inordertomeetthisrequirement,itispossibletousetheUNIENISO13849-1:2016standard. SRP/CSreferstothesafety-relatedpartofacontrolsystem,i.e.thatrespondstosafety-relatedinputsignalsandgeneratessafety-relatedoutputsignals. TheprocessforthedesignandimplementationoftheSRP/CSenvisagedbytheUNIENISO13849-1:2016standardincludesthefollowingactivities: identificationofthesafetyfunctionstobeperformedbytheSRP/CS(forexampleemergencystop,interlockingofmovableguards,manualreset,two-handcontrol,etc.)anddefinitionofthecharacteristicsofeachsafetyfunction; determinationofthePLr(requiredPerformanceLevel); designandimplementationoftheSRP/CS; estimateofthePL(PerformanceLevel)foreachsafetyfunctionimplementedthroughtheSRP/CS; foreachSRP/CS,comparisonbetweenPLrandPL;thePLmustbegreaterthanorequaltothePLr;ifthePLachievedislessthanthePLr,theSRP/CSmustbemodifiedinordertoincreaseitsreliabilityi.e.thePL. WhatisthePLr(requiredPerformanceLevel)? TherequiredperformancelevelPLristheperformanceleveltobereachedinordertoachievetherequiredriskreductionforeachsafetyfunction.ThePLrthereforeindicateshowreliabletheSRP/CSshouldbe. ThedeterminationofPLristheresultoftheriskassessmentandreferstotheextentoftheriskreductionforthesafety-relatedpartsofthecontrolsystem. ThegreatertheamountofriskreductionrequiredbytheSRP/CS,thehigherthePLrmustbe. PLPerformanceLevel:thelevels ThePL(PerformanceLevel)isthediscretelevelusedtospecifytheabilityofSRPs/CSstoperformasafetyfunctionunderforeseeableconditions.ThePLisexpressedthroughfivelevels(“a”,“b”,“c”,“d”,“e”)withincreasingreliability. Performancelevela Itisthelowestlevel,ithasnoequivalentintheSILestimatedinaccordancewiththeCEIEN62061:2005standard. Performancelevelb Controlcircuitscapableofachievingperformancelevelbcanbeimplementedusingsinglechannelarchitectureswithoutmonitoring. Performancelevelc Single-channelarchitectureswithoutmonitoringandwell-testedcomponentsandsafetyprinciplescanbeusedtocreatecontrolcircuitscapableofachievingperformancelevelc. Performanceleveld Toobtaincontrolcircuitsthatareabletoreachaperformanceleveld,singlechannelarchitectureswithoutmonitoringarenolongersufficient,buttheprinciplesofmonitoringandpossiblyofredundancymustbeadopted. Performancelevele Theperformanceleveleisthehighestachievableandcanonlybeachievedbyusingredundantandmonitoredcircuits. Performancelevel(PL) Averageprobabilityofadangerousfailureperhour(PFHD)[1/h] to 10-5≤PFHD<10-4 b 3x10-6≤PFHD<10-5 c 10-6≤PFHD<3x10-6 d 10-7≤PFHD<10-6 And 10-8≤PFHD<10-7 HowmanyPLsdoesamachinehave? Asmentionedintheintroduction,oneofthemostdifficultrequirementsoftheMachineryDirectivetosatisfyisthatrelatingtothereliabilityofsafety-relatedcontrolsystems;inessence,thisrequirementdemandsthatthemachinedoesnotbehavedangerouslyeveninthepresenceoffaults. Inpracticethistranslatesintoassessingthatthesafety-relatedcontrolsystemsaresufficientlyreliable,thereforethattheprobabilitythattheyfailcreatingdangeroussituationsisquitelow. Whatcanbeadangeroussituationgeneratedbyafailure? Forexample,itcouldhappenthatbypressinganemergencystopbutton,amotordoesnotstopbecausetheremotecontrolswitchthatcontrolsitdoesnotopenthepowercontacts. ThereliabilityofthecontrolcircuitsofamachinecanbeestimatedaccordingtothemethoddescribedintheUNIENISO13849-1standard,whichallowstodeterminethelevelofperformance(PL).ThePLofacircuitisexpressedbyaletterfrom"a"to"e",where"a"istheleastperformingleveland"e"isthemostreliable. However,itisnecessarytostartwiththeassessmentoftherisksofthemachine:onthebasisoftheresultsoftheriskassessment,themanufacturerofthemachinewilldefinewhichprotectionmeasuresheintendstoadopttoreducetheriskspresenttoanacceptablelevel. Theseprotectivemeasuresmayormaynotincludecontrolcircuits: forexample,ifadangerousareaisprotectedwithafixedguard,itwillnotinvolvecontrolcircuits; If,ontheotherhand,aninterlockedmovableguardisused,itwillbenecessarytoimplementacontrolcircuitthatstopstheprotecteddangerouselementswhentheguardisopenedandpreventsthemfrombeingrestartedbeforetheguardisclosed:thisisasafetyfunction. WillthenumberofPLstobeestimatedforamachinethencorrespondtothenumberofprotectivedevices? Forexample,forasimplemachinethathasanemergencystopbuttonandaninterlockedmovableguard,itwillbenecessarytoestimatetwoPLs? Thisisnotalwaysthecase. Inthepreviousexample,itwillbenecessarytoevaluatewhichandhowmanydangerouselementsmustbestoppedwhenthemovableguardisopened;infact,therecouldbeaspindledrivenbyanelectricmotor,apusherdrivenbyapneumaticcylinder,etc.Eachoftheseelementsmustbestoppedbyitsowncontrolcircuit,thereforeitwillbenecessarytoestimateaPLforeachofthesecircuits. Thesameinterlocksensorofamovableguardcouldthereforebelongtoseveraldifferentcontrolcircuits,eachofwhichstopsadangerouspartofthemachine. ThemachineryPL Onethingmustbeclear:thereisno"PLofamachine",butthePLmustbeestimatedforeachdifferentsafety-relatedcontrolsystem. OnlyoncetheriskassessmentprocesshasbeencompleteditwillbepossibletodeterminewhatthesecircuitsareandthereforehowmanyPLsshouldbeestimated. PerformanceLeveldinroboticcells Inthecaseofsafety-relatedpartsofcontrolsystemsofroboticcells,thisisreferredtoasPerformanceLeveld. Let'sseewhy. TheUNIENISO10218-2:2011standardappliestoindustrialroboticislands,whichdefinesthemas“oneormoreroboticsystemsincludingmachinery,equipment,protectedspaceandassociatedprotectionmeasures”. AccordingtothisharmonizedtypeCstandard,applyingwhichthemanufacturerensuresthepresumptionofconformitywiththemachinerydirectiveoftheroboticcell,thesafety-relatedpartsofthecontrolsystemmustbedesignedtobecompliant(UNIENISO10218-2:2011,§5.2.2): atPerformanceLeveldwithcategory3structureasdescribedintheUNIENISO13849-1:2016standard; toSIL2withhardwarefaulttoleranceof1withaperiodicverificationintervalofnotlessthan20yearsasdescribedinthestandardCEIEN62061:2005. Anyfailureofthesafety-relatedpartsofthecontrolsystemmustcauseacategory0or1stopinaccordancewiththestandardCEIEN60204-1:2018(UNIENISO10218-2:2011,§5.2.1). Interfaceswithcontrolcircuitshavingsafetyfunctionsofotherpartsoftheintegratedproductionsystem(IMS)mustbecompliantwithwhatisindicatedabove(UNIENISO10218-2:2011,§5.9.3). Theresultsofanoverallriskassessmentcarriedoutontheroboticsystemmaydeterminethatdifferentperformanceofthesafetyrelatedpartsofthecontrolsystemisjustifiedfortheapplication(UNIENISO10218-2:2011,§5.2.3). Inthesecasesthecriteriausedtodefinetherequiredperformancelevelsmustbespecificallyidentifiedandadequatelimitationsandcautionsmustbeindicatedintheinstructionsforuse. Therefore,thistypeCstandardnotonlyrequiresaPerformanceLevel(PerformanceLeveld)forthesafety-relatedpartsofthecontrolsystem,butalsorequiresthatthecorrespondingcircuitsbeimplementedinaccordancewiththerequirementsofcategory3. TheUNIENISO10218-2:2011standardindicatesthelevelsofreliabilitythatmustbeachievedbyvarioussafetyfunctionsofroboticcells(exceptinthecasewheretheriskassessmentjustifiesdifferentcriteria),including: theinterlockingsensorsofmovableguardsmustmeetthePerformanceLevelofcategory3oftheUNIENISO13849-1:2016standard(orSIL2withfaulttolerance1oftheCEIEN62061:2005standard); thelimitedspeedmonitoringsystemmustmeetthePerformanceLevelofcategory3oftheUNIENISO13849-1:2016standard(orSIL2withfaulttolerance1oftheCEIEN62061:2005standard); themethodsusedtolimitthemovementofrobotsmustmeetthePerformanceLevelofcategory3oftheUNIENISO13849-1:2016standard(orSIL2withfaulttolerance1oftheCEIEN62061:2005standard); theemergencystopfunctionmustmeetthePerformanceLevelofcategory3oftheUNIENISO13849-1:2016standard(orSIL2withfaulttolerance1oftheCEIEN62061:2005standard). WhattodotoincreasethePL(PerformanceLevel)? ToincreasethePLoftheSRP/CS,thefollowingmeasuresarepossiblewhichcanbeappliedindividuallyorincombination: reducingthelikelihoodofdangerouscomponentfailures; improvementofthestructureoftheSRP/CSormodificationofitscategory.   Ifyouareamachinerydesigner,andyouthinkyouneedqualifiedtechnicalsupportforthedeterminationofthePLrorfortheestimatationoftheachievedPLoradvicetoincreasethePL,youareintherightplace. Wecanprovideyouwithspecializedtrainingonthetopic,alsothroughdirectcoaching,ortechnicaladviceforsolvingproblems. Alltraining,orcoaching,canbefullyfinancedwithFondimpresaorotherfundingorganizations. If,ontheotherhand,youprefertodoityourself,youcanbuyourtechnicalmanualonPLpublishedbySole24OREandreadourarticleontheBLOG.   Haiundubbio,unproblemaounadomanda? PossiamofornirtilaconsulenzadicuihaibisognoancheDAREMOTO VEDIILCOSTODELLACONSULENZA VUOISCARICAREILCAPITOLOSULLAVALUTAZIONEDEIRISCHI DELNOSTROLIBROSULLADIRETTIVAMACCHINE? 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