What exactly is a safety light curtain? | SICK

The light curtain can safely detect patterns, objects, and direction, maintaining the safety while still allowing known patterns to enter and ... Home SICKSensorBlog Whatexactlyisasafetylightcurtain? Home SICKSensorBlog Whatexactlyisasafetylightcurtain? Whatexactlyisasafetylightcurtain? Dec8,2020 Thesafetylightcurtainisreliedontoprotectthesafetyoffactorypersonnelallovertheworld.Yet,althoughthetechnologywasdevelopedalmost70yearsago,manyapplicationsmaystillnotbemakingfulluseofitscapabilities.Ihavetriedtore-appraisewhatlightcurtainscouldbecontributingtoyourmachinerysafety,factorycontrol,andproductivitywithabreakdownofthefunctionsandfeaturesthatarecurrentlyavailable.Thelightcurtainhasbecomesoubiquitous,thatitisalltooeasytoregarditasasimpleon/offdevice-afail-safe,fit-and-forgetsubstituteforprotectivefencing.However,anexpandingandversatilerangeoflightcurtainshasevolvedtoperformmultiplefunctionsandtheycancontributepositivelytoproductivityandintegratewithfactorylocalareanetworkstosupportbetterplant-wideautomation. HistoryofthelightcurtainIn1951,ErwinSickpresentedthefirstwoodenmodelofhislightcurtaintothe“GermanInventorandNewDevelopmentTradeFair”inMunich.Laterthesameyearapatentregistrationofthelightcurtainbasedontheautocollimatorprinciplewasregardedasthetechnicalbreakthroughthatbeganthedevelopmentoftechnologyweknowtoday–onethathassavedcountlesspeoplefrominjuryanddeathatthehandsofdangerousmachinery.Sincethattime,hardwareandsoftwaredevelopmenthavetransformedtheversatilityanduseofsafetylightcurtains.Thekeyattributesoflightcurtainsremainthesame,buttheirpotentialmaystillbeunder-exploited. LightcurtainspecificationsTherereallyisnosuchthingasastraightlike-for-likeofftheshelf‘menu’ofsafetylightcurtainssinceallmanufacturersoffertheirownrangesincludingdifferentsizes,shapes,andresolutionsetc.However,thestartingpointofanysafetyapplicationisalwaysthedangeryouwanttoguardagainstandthespacethatitisin.Basedonthis,therearedifferentattributestoconsiderregardingthephysicalpropertiesofthelightcurtain.Currently,thedocumentENISO13855istheharmonizedstandardforpositioningSafetylightbeamsystems,anditcanassistindeterminingwhichofthesephysicalattributestogoforandinthissectionIhavetriedtoexplainthemainspecificationstoconsider.However,therearealsoadditionalpropertiessuchasIPrating,currentconsumption,visibleindicationsetc.thatarenotcoveredhere.ScanningrangeTherangeisthemaximumdistancebetweensenderandreceiver.Incaseofactive/passiveunitsthedistancebetweentheactivepartandthepassivepartiscalledscanningrange,too.Productsareavailablewitheitherafixedrange,aselectablerange(e.g.low/high)ortheyhaveanauto-adjustingrange(atstart-up,thesensitivityisadjustedbasedonthedistancebetweensenderandreceiver).ChoosingthecorrectrangeensuresthattheSafetylightbeamsystemsisbestfitfortheapplication,forexample,ifahighpower,longrange,deviceisusedoverashortdistance,reflectionsoffnearbysurfacesmaymaketheinstallationunsafe.Ontheotherhand,alongrangedevicemayberequiredforprotectinglargeareaswhichrequirehighbeam.ProtectiveFieldHeightTheheightofaSafetylightbeamsystemmustbechosenbasedontheaccesstothehazard.Forexampleifalightcurtainisprotectingagainst"pointprotection"thenthelightcurtainmustcoverthewholeaperture.However,iftheSafetyproductisforaccessprotectionthendifferentheightscanbeusedbasedonthenumberofbeamsrequiredandtheheightfromthefloorpluspossiblereachover. Figure1:Pointandaccessprotection Figure1:Pointandaccessprotection ResolutionTheresolutionofalightcurtaincanalsoaffectthemountingpositionwithrespecttothehazard.Theterms“fingerprotection”and"handprotection"areoftenusedwhichrefertoresolutionsof14mmand30mmrespectively.However,lightcurtainsandmultiplebeamsystemscanalsohavedifferentresolutionsorbeamseparations(e.g.20mm,40mm,200mmetc.).Thehighertheresolution,thenthebetterthedetectioncapabilityisforasafetylightcurtainandthereforethedevicecanbemountedclosertoahazard(i.e.a14mmlightcurtaincanbemountedcloserthana30mmlightcurtain).ENISO13855containslotsofdifferentcalculationsformountingdistancesandresolutioncanhavealargeimpactbutthebasicequationis,figure2: Figure2:Calculationofminimumdistancetohazardouspoint Figure2:Calculationofminimumdistancetohazardouspoint Thestandardalsocoversadditionalthingssuchasdirectionofapproach,possiblereachoveretc.Forlargerdistancesandaccessprotectionabeamseparationof400mmisnormallysufficient,butthenadditionaldevicesmayberequiredforpresencedetectionifthemachinecanberesetwhilstapersonisbehindthelightcurtainormultiplelightbeamsafetydevice.AthoroughriskassessmentaccordingtotheharmonizedstandardENISO12100shouldbeperformedtotakethesepointsintoconsideration.Higherresolutionlightcurtainsareusuallymoreexpensivethanlowerresolutions,butswitchingtoalowerresolutionmeansthatthedistancebetweenthesafetylightcurtainandthehazardneedstobeincreasedwhichmaybeimpracticalornotpossibleonthefactoryfloor.TypeWhenchoosingasafetydeviceforanapplication,theharmonizedstandardENISO13849orIEC62061canbeusedtodesignthesafetyrelatedpartsofthecontrolsystem(SRP/CS).Followingthedefinitionofasafetyfunction(e.g.initiatingastopusingasafetylightcurtain)itisimportanttodeterminewhatperformancelevel(PL)orsafetyintegritylevel(SIL)thattheassociatedSRP/CSshouldachievebasedontherisk.Iwillnotgointodetailaboutfunctionalsafetyherebutforelectrosensitiveprotectiveequipment,theSafetylightbeamsystemsaccordingtoIEC61496isdirectlyrelatedtowhatPL/SILthatthesafetyfunctioncanachieve.Thefollowingfigurereflectstherelationship,figure3. Figure3:RelationshipbetweenESPETypeandPL/SIL Figure3:RelationshipbetweenESPETypeandPL/SIL ThereforeifPLe/SIL3isrequired,forexampleforprotectingpowerpresses,atype4devicewouldberequired.Active/passivesystemsTosaveonwiringandtokeepcostsdownforaccessprotectionapplications,activepassivesystemscanbeused.Anactive/passivesystemconsistsoftwodevices.Onedevicecontainsbothemittingelementsandreceivingelementsandtheotherdevicecontainse.g.mirrorsthatareusedtodeflectthebeamsbacktothereceivers,figure4.Mirrorsreducethestrengthofalightbeamandthereforeactive/passivesystemswillnormallyhaveamuchshorterdistancethansender/receiversystems. Figure4:Active/Passivesystem Figure4:Active/Passivesystem  ESPEFunctionsofSafetylightbeamsystemsThesimplestfunctionofalightcurtainiswhenlightbeamsareobstructedandtheoutputsturnoff,whichsignalsastoptotheassociatedhazardousmovement(s).Ifnoincidenthasoccurredandtheareaissafe,aresetcanbeperformed.Thisstop/startstyleofsafeoperationcanbedisruptivetofactoryflowandproductivityandthereforethelightcurtainhasevolvedoverthelasthalfcenturytoperformmultiplefunctionssuchasinternalreset,externaldevicemonitoring(EDM),beamcoding,muting,blanking,patternrecognitionandbeamcoding.RestartinterlockThetwomaininterlockingmodesthatcanbesetinareautomated–Outputsafetyswitchingdevice(OSSD’s)gohighwhenthebeamsarefree.Manual–OSSD’sgohighwhenthebeamsarefreeandthenaresetbuttonispressed.Itisimportantthatanautomatedrestartcanonlybeusedinspecificcasewhenitisnotpossibletostandbehindthelightcurtainwithoutbeingdetected(e.g.smallpresswitharmonlyaccess),figure5.Furtherrequirementsmayalsoapply. Figure5:Smallpresswithpointprotection Figure5:Smallpresswithpointprotection Amanualresetcanbeperformedusingeitheranexternalsafetyrelay,safetyPLCorinternallyiftheSafetylightbeamsystemshasthecapability.BeforesafetyPLC/Relaystheresetwasperformedusingrelaylogic.EternalDeviceMonitoring(EDM)Thelevelofsafetyperformanceforasafetyfunctioncanbeincreasediffaultdetectionisimplemented.TheharmonizedstandardENISO13849,whichisusedforcalculatingperformancelevelofaSRP/CSdescribesthetermdiagnosticscoverage(DC)whichmeasuredthiscapability.Itisameasureofhowmanydangerousfailuresaredetected.Onemethod,listedinthesestandardsandusedextensivelyinindustry,hasadiagnosticscoverageof99%(candetect99%ofdangerousfailures)andiscalledExternalDeviceMonitoring(EDM)or“checkback.”Thismonitoringfunctionisameansinwhichasafetydeviceactivelymonitorsthestateofexternaldevicesthatitcontrolssuchasmachineprimarycontrolelements(MPCEs),contactors,orrelays.Thisisneededbecausetheexternaldevicesusuallyhavenodiagnosticsoftheirown.Ifanunsafestateisdetectedintheexternaldevicethenthesafetydevicecanlockout.Thisisperformedbymonitoringcomplimentarychannelsoftheexternaldeviceswhicharephysicallylinkedinternally.Iftheoutputsofasafetydevicegohigh,thereturningEDMsignalfromthefinalswitchingdevicesmustgolowifthesystemisinasafecondition.Otherwise,thereisafault.SeeFigure6forexamplesofimplementationinasafetyrelayanddirectlyinasafetylightcurtain. Figure6:TwoexamplesoftheimplementationofEDM Figure6:TwoexamplesoftheimplementationofEDM BlankingInsomeapplications,itmightbedesirabletopermitcertainobjectstoprotrudethroughthelightcurtainfieldwithoutsignalinganobstructionofthedetectionzone.Forexample,whenusingasupporttableorrackrequiredtofeedthemachinewithaheavywork-piece,ablankingfeaturecanbeappliedtomodifythedetectioncapabilityofthelightcurtainandnotinterrupttheworkflow.Theblankingcaneitherbefixedorfloatingtoallowavariablepositionfortheobject.SeeFigure7. Figure7:Exampleoffixedblanking Figure7:Exampleoffixedblanking EnablingMaterialThroughput-MutingInmanyapplicationsofSafetylightbeamsystems,thesafetydevicemustbeabletoallowobjectsthroughwhilststillreactingtopresenceofpeople.Thisisparticularlyimportantonconveyinglineswithgoodspassingfromoneareaofproductiontoanotherorinautomatedwarehousingoperationswherestaffareexcludedfromtheautomatedstackingandshelvingareas.Sensorscanbeusedtoidentifytheobject(suchasacarbody)andallowpassageintoahazardzoneduetotheshapeandsize,whilestilldetectingpresenceofaperson.Thisisautomaticandknownasmuting.Thetwomostpopularmethodsofmutingarecrossbeamandfoursensormuting,asshowninFigure8. Figure8:Crossbeamandfoursensormuting Figure8:Crossbeamandfoursensormuting However,therearemanydifferentmethodsofmutingincludingtheuseofphotocells,groundinductiveloops,theuseofsafetylaserscanners,partialmuting(partialblanking)etc.ThetechnicalspecificationIEC/TS62046canassistintheapplicationofprotectivedevicestodetectpersons.Itisimportanttounderstandthoughthatmutingisnotthesameasbypass-mutingisautomatedwhilstbypassisamodeselectedbyanoperatorwhilstotherprotectionmeasures(e.g.enablingdevice)i.e.manual.Anothertermrelatedtomutingis"override"whichisamanualtriggeringofthemutingafteranerrorinthemutingconditions.Thisisneededtomakeasystemclearforexamplewhenapalletisstuckinasafetylightcurtainwhichhasstoppedtheconveyorandtheconveyortemporarilyneedspowertoremovethepallet.EnablingMaterialThroughput-PatternRecognitionTherearesomesafetylightcurtainsthatcanperformcomplexalgorithmsonindividualbeams.Thismeansthatnotonlycanitdetectifthebeamsareobstructedornot,butalsowhichbeamsandinwhichsequence.Thelightcurtaincansafelydetectpatterns,objects,anddirection,maintainingthesafetywhilestillallowingknownpatternstoenterandleavethedetectionzoneofthelightcurtain.Furthermore,itdoesnotrequireanyadditionalsensorsorcontrol.Thispowerfulfunctioncandramaticallyincreaseavailabilityandincreasesafetywhileatthesametimeasallowingmaterialtopassthrough.TheC4000Fusionisanexampleofsuchadevice.SeeFigure9. Figure9:Objectpatternrecognition Figure9:Objectpatternrecognition BeamcodingBecauseandmultiplebeamsystemsworkoversimilaropticalfrequencyintheInfraRedspectrum,ifseveralsafetylightcurtainsareusedincloseproximityinterferencecanoccurbetweenthesystems.Thiscouldcauseadangeroussituation.Toovercomethissomesafetylightcurtainsofferbeamcodingsothatareceivercandistinguishthebeamsfromaspecificsenderandnotbeaffectedbyanotherdeviceincloseproximity.Whenbeamcodingisactivatedthough,thismayaffecttheresponsetimeand/orrangeofthesafetylightcurtainandshouldbetakenintoaccountasthismayaffectthemountingdistance.  Summary:WhataresafetylightcurtainsandmultiplelightbeamsafetydevicesToday’sSafetydeviceinstallationsaremuchmoreattunedtoproductionneeds,capableofawiderangeofmanualandautomatedactionsthatensureaSafetylightbeamsystemsassistssafetyandproductivityratherthansacrificingefficiency.However,itisimportanttoknowwhatfeaturesandfunctionsareavailableonasafetylightcurtainandalsoiftheapplicationrequiresthemornot?Safetylightbeamsystems.TheapplicationofaSafetyproductisnotalwaysasstraightforwardasstickingitinfrontofanaccesspointandfurtherthoughtmayberequired.  Safetymultibeamsensors Safetymultibeamsensorsareelectro-sensitiveprotectivedeviceswithtwoormorelightbeams.Whenatleastoneofthelightbeamsisinterrupted,aswitch-offsignalisdeliveredtothe... Safetylightcurtains Safetylightcurtainsprovidereliableandcost-effectiveaccessprotectionforhazardouspointsandhazardousareas.Dependingonthevariant,differentmachinefunctionsareintegratedorcan... VIDEO:#40SecondsOfSafety:Alignyoursafetylightcurtainin5simplesteps VIDEO:#40SecondsOfSafety:Alignyoursafetylightcurtainin5simplesteps  Readmore Playingitsafe:SwissdesignengineerschooseSICKsafetysolutions  more Safeandrugged:ExpertsatMAN,HELLERandSICKperfectthedeTecsafetylightcurtain more Quickdiagnosticsviaapp-deTec4safetylightcurtains more SICKSensorBlog SICKSensorBlog Allarticles MartinKidman ProductSpecialistMachinerySafetySICK(UK)Ltd.MartinKidmangainedhisPh.D.attheUniversityofLiverpoolin2010andhasbeeninvolvedinIndustrialAutomationsince2006workingforvariousmanufacturersofsensors.HehasbeenatSICKUKLtd.since2013asaproductspecialistformachinerysafetyprovidingservices,supportandconsultancyforindustrialsafetyapplications.HeisalsoacertifiedFunctionalSafetyEngineer(TUVRheinland,#13017/16). 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