TheairbagisanadditionalrestraintsystemwhichwasconceivedinEurope;itisusedinthecountrieswherethesafetybeltsarecompulsory,andisdesignedtobeusedincombinatioltwiththesafetybelt.Inthecaseofanaccident,withinacertainimpactspeedrange,itactstolimittheextentandtheentityofinjuriestotheoccupantsofthevehicle.TheUSAisanexceptionbecause,eventhoughtheuseofthesafetybeltiscompulsoryinalmostallstates,rulesstatethattheoccupantshaveonlytobeprotectedbyanairbag.Inthecaseofanaccident,orifanactionwithcertainforceisexercisedonthevehicle,oneormoresensors,whichareappropriatelylocatedonvehicle,sendanelectricsignaltoanelectroniccontrolunit.ThisunitestablishesthetypeandmagnitudeoftheimpactanddefineswhichARS(additionalrestraintsystem)hastobeputinaction,sendinganelectricsignalwhichprimesthepropellantactivatingtheadditionalrestraintsystem.Theairbaghasthefunctionofinteractingwiththeoccupantsanddissipatethekineticenergytogetherwiththesafetybelts.Theuseofonlyasafetybelt,evenwhenitiswelldesignedandmounted,enablestheoccupantstoberestrainedbut,ifcertaindecelerationlevelsareexceeded,cancausecrushingofthethorax,givingrisetosecondaryinjuries.Forexample,duringafrontalimpact,thedriverwearingonlyasafetybeltreceivesaveryhighloadonthethorax,withavaluenearto10kN;instead,withanairbag,thisloadisreducedbyabout30%.Moreovertheairbagdistributesitspressureonalargerbodysurfaceareaand,attheend,enablesanimportantreductiontotheaccelerationlevelsexperiencedbytheheadwhicharedangerousforthebrain.Anairbagsystemautomaticallyinflateslargenylonbagsimmediatelyafterthestartofamajorcollision.Theairbagsystemisdesignedtosupplementtheprotectionprovidedbyseatbelts.Themajorpartsofanairbagsysteminclude:(1)Airbagsensor一inertiasensorsthatsignalthecontrolmoduleintheeventofacollision.Fortheairbag,sensorshavetobeelectronicduetotheirpositiononthevehicleandtheinformationtobeanalyzed.Thesensorscanbedividedinto:·Sensorsforimpact(frontandside);·Sensorsfortheoccupantsofvehicle(presenceofoccupants,weightofoccupantsandthepositionoftheoccupantsontheseats).Thesesensorsenabletheairbagsystemtobemanagedwhileavoiding,forexample,activatingthepassengerairbagifthereisnopassengeronthatseator,ifthereisadualstageairbag,allowpilotfillingasafunctionoftheweightandpositionofthepassengerontheseat.(2)Airbagmodule一containstheinflatormechanismandthenylonairbagthatexpandstoprotectthedriverand/orpassengersduringthecollision.Anairbagmoduleconsistsoffourbasicparts:gasinflator,bag,housingandcover.Thegasinflatoristhebasiccomponentand,forthisreason,usuallythenameoftheinflatordefinesthenameofairbagmodules.Themissionoftheinflatoristoproducethegasmixtureinquantityandwithinaspecifiedtime;thisgasfillsandinflatesthebag.Thegasinflatoriscomposedbyanigniter,comprisingalittlemetalhousing,whichcontainsasmallquantityofexplosivecharge(usuallyblackpowder),andanelectrictrigger;thetriggerispilotedbythecontrolunitandmakesalittleexplosionthat,inturn,triggersthesolidpropellant.Explosiongeneratesanontoxicgasmixture(nitrogenandparticulates).Usuallythebagismadeofapolyamidefibrefabricwithaspecificweave.Itwon'ttearorignitewhenfilledwiththegasmixtureduringthefillingphase.Theshapeandvolumeofthebagsdependonthespacebetweentheoccupantandtheinteriorofthecar,andonthetypeofprotectionthatthemodulehastoensure(driverfrontimpact,ratherthanheadprotectionforsideimpact).Thefoldingofthebagisveryimportantintermsofminimizingriskofinjuryduringdeployment;forexample,thesteeringwheelairbagisstarfolded.Tooptimizeperformanceduringtheopeningphase,tearseamsorstrips(tethers)canbeusedtooptimizethepositionofthebagincomparisontotheoccupant.(3)Airbagcontroller一computerthatoperatesthesystemanddetectsfaults.(4)Dashwarninglamp一dashbulbthatgrowswithsystemproblemandgoesoutassystemarms.Anelectricairbagsystemdeploysaninflatablenylonbagtohelpprotectthedriverduringacollision.Itusesimpactsensorstodetectaseverecollision.Thesensorsfeedtheirsignalstotheairbagcontroller.Whenatleasttwoimpactsensorsareenergized,thecontrolleractivatestheairbagmodule.Theairbaginflatesinabout1/20thofasecond,wellbeforethedriver'sbodyfliesforwardfromthecollision.Thetoughnylonbagcaneasilyabsorbtheforwardinertiaofahumanbody.Thishelpsprotectboththedriverandthepassengers.Alllate-modelvehiclesareequippedwithdualairbags:adriver-sideairbagandapassenger-sideairbag.Thedriver-sideairbaglocatesinthesteeringwheel.Thepassenger-sideairbagdeploysfromtherightsideofdash.Thedriver-andpassenger-sideairbagswillonlydeployduringfrontalimpacts.Steeringwheelanddashmountedairbagsmaynotdeployinsideimpact,rearimpactorrolloversituation.Acollisionmustoccurwithinabout30degreeofthevehicle'scenterlinefortheseairbagstoinflate,showninFig.10.2.Side-impactairbagscanbelocatedinthedoorpanelsorontheoutsideedgeofeachfrontseat.Thesesmallairbagsdeploywhenthevehicleishitfromtheside.Th(:vgenerallydonotdeployduringafrontalimpact.Side-impactairbagsarebecomingmorecommonandareusedbyseveralautomanufactures.Whenavehicleishitfromtheside,injuryusuallyresultswhentheoccupant'sshoulderandheadflythroughthesidewindowglass.Aside-impactairbagsystemsensesthesidethrustoftheimpactanddeploysasmallairbagtocushionthepersonasheorsheispropelledsideways.Fig.10.2showsadistributedsensingsystem,wheremultiplesensorsarecombinedtodetectacollision.Anelectroniccontrolunit(ECU)oftenmonitorsthetriggeringofmultiplesensorsandcontrolstheactivationoftheairbags.