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SEATCOMPONENTTOPREVENTWHIPLASHINJURYK.-U.Schmitt1,2,M.Muser2,M.Heggendorn1,P.Niederer1,andF.Walz21InstituteofBiomedicalEngineering,UniversityandSwissFederalInstituteofTechnology(ETH)Zurich,Switzerland2WorkingGrouponAccidentMechanics,UniversityandSwissFederalInstituteofTechnology(ETH)Zurich,SwitzerlandPaperNo.224

ABSTRACT

A new seat slide was developed toprevent whiplashinjury. The behaviour of the seat during low-speedrear-endimpactsisimproved.Allfunctionsaswellasthebehaviour of the seat in other impact conditionsremainunchanged.Thesystemconsistsofadampingmechanismwhichistriggeredby a sensor. A certain vehicle accelerationfollowed

by

an

impact

force

extended

by

the

occupanton the seat back is needed to activate the system.

Unintendedactivationisprevented.Oncethesystemisactivated, theseat isallowed tomovebackwards inapurely translational manner while the motion isdamped.Adistanceofapprox.40mmcaneffectivelyreducetheoccupantneckloading.Sled test experiments wereperformed to analyse thebehaviourofthenewsystem.Thetestswereconductedtomimicrear-endcollisionswithadelta-vof16km/h.ABioRIDdummywasusedasahumansurrogate.Theresultsindicatethebeneficialinfluenceofthedampingseatslideontheoccupantkinematics.Inparticulartheso-calledS-shapedeformationoftheneckassessedbythe neck injury criterion NICmax is reduced.Comparing a standard car seat with and without thedamping seat slide, it was shown that theNICmax isreducedbyapprox.40%.Henceitwasshownthatbesidestheheadrestraintandtherecliner thatareusedinotherwhiplashprotectionsystems,alsotheseatslidehasthepotentialtopreventwhiplashinjury.INTRODUCTION

As of today several seat systems arepresented thatintend toprevent whiplash injury. Simplifying, suchsystems aim at reducing the relative motionbetweenheadandtorsoassuchmotionisoftensuspectedtoberelated to whiplash injury [e.g. Ferrari 1999].Generally,thesystemscanbedividedintotwogroups:active and passive systems. Active systems arecharacterised by a mechanical mechanism thatinfluencesthekinematicsoftheoccupantsittingontheseatby allowing or enforcing additional interaction,

oftenamotionof the seat, incaseofan impact. TheSAHR system [Wiklund and Larsson 1998], forinstance,representsanactivesystem.Itconsistsofanactivehead restraint thatautomatically movesupandclosertotheoccupants'headinrear-endimpacts.Thusthe distance between head and head restraint isreduced.Volvopresented theWHIPSseat[Lundelletal.1998]whichisequippedwithareclinerthatallowscontrolledbackwardsmovementofthebackrestduringrear-endimpact.Themotionisperformedintwosteps:a translationalrearwardsmovementof thebackrest isfollowedbyarotationalmotionrecliningthebackrest.An other system, called WipGARD [Zellmer et al.2001], also enables the backrest to perform atranslation followedby a rotation. Both the WHIPSand the WipGARD require a critical load to activatethesystem.A typicalexample forapassivesystem, i.e.asystemwithout mechanical mechanism, is a add-on headrestraintpadding to reduce the head to head restraintdistance. Such additional padding is available fromvariousmanufacturers[e.g.ContiTech2000].Inthisstudyanewactivesystem,adampingseatslidetoprevent whiplash injury ispresented. A functionalmodel of the seat slide wasbuilt and mounted to astandard car seat. Itspreventivepotential was shownbyperformingsled testexperiments.ThesystemwaspatentedattheEuropeanPatentOffice(No.EP05405537.8).

MATERIALANDMETHODSA new seat component was developed based on astandardseatslideasusedinarecentcarseat.Inorderto ensure that the basic functions of the seat slidepersisted,modificationswerenotallowedtoinfluencethe translational adjustability and the stiffness of thestructure.Changestothefixingoftheslidetothecarwereavoidedsuch that theaccording regulations likeECE R-17 are still fulfilled. Thus the seatcharacteristics for other impact conditions than low-speed rear-end are meant tobe sustained. Also theseatingposture of an occupant had tobe maintainedwiththenewdevice.Furthermore,itwasdefinedthat

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repairspossibly needed after the new seat slide wasactivatedhadtobeinexpensiveandeasytocarryout.Principle

The relative accelerationbetween head and T1, andconsequentlyalso theNICmaxvalue, is tobereducedto prevent whiplash injury. Therefore a device wasdeveloped whichallowsa translational motionof theseatrelativetothecarwhiledampingthismotion.Thisleadstoadelayofthebuildingupofthetorsoloading.Thus the main effect is the synchronisation of theloadingoftheheadandtheuppertorso.Thesystemismountedbetweentheseatbaseandthecar floor (Figure 1) and consists of a dampingmechanismwhichistriggeredbyasensor.Thissensordetects the vehicle acceleration and, when a certainlimitisexceeded,thedeformableelement isreleased.Additionally, thedampingelement requiresa trippingenergythresholdtobedeformed.Thus,asameasuretoprevent unintended activation, the system needs acertainaccelerationfollowedbyanimpactforceoftheoccupantagainsttheseat.Oncethesystemisactivated,the seat is allowed to movebackwards in apurely

translational manner while the motion is damped(Figure2).FunctionalmodelTo show the feasibility of the principle describedabove, a functional model of the seat slide wasdevelopedandincorporatedintoastandardcarseat.Aspring-mass system is used to detect the accelerationnecessary to activate the system. If the accelerationthreshold is reached, the system is released, i.e. atranslationalmotionoftheseatrelativetothecarfloorisallowed.Inthiscasetheseatslidesalongadefinedpathasaglidingpartisguidedalongaslide.The activation mechanism releases both dampingcomponents simultaneously. The damping isperformed by deformation of steel profiles on eachside of the system, i.e. the mechanism is symmetric(Figure3). Thegeometryandstrengthof theprofileswerechosen on thebasisofanalytical design studiesandtensiletestingexperiments.Ithastobenotedthatduetothematerialoftheprofileused,acertainelasticforce limit mustbe exceeded to start the damping.Preferably the profiles show a small elastic range,

Figure1. Principleoftheseatslide:Theseat(1)ofavehicle ismountedtoaseatslide(2)whichitselfismountedtothecarbody(3)(left).Incaseofarear-endimpact,atriggersystemreleasesthenewseatslidewhichthenenablestheseattomovebackwards(relativelytothecar)whiledampingthismovement(right).

Figure2. Dampingofthetranslationalbackwardsmovementoftheseatbaserelativetotheseat;initialstate(left)anddeformedstate(right).

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followedby a largeplateau region until eventually ahardening effect is observed. The distance of thebackwards movement results from these deformationcharacteristics.SledtestsTo analyse whether the new seat slide offers thepossibility to prevent soft tissue neck injuries, sledtests were performed. A recent car seat from aEuropean manufacturer was used and tested in itsoriginalconfigurationandwiththenewseatslide.Thesled tests were carried out in collaboration withAutoliv (Germany) GmbH and all tests wereperformed in the same manner according to the testprocedure for the evaluation of the injury risk to thecervicalspineinalowspeedrear-endimpactproposedfor the ISO/TC22 N 2071 and ISO/TC22/SC10,respectively[Museretal.1999,Museretal.2002].Theseatsweremountedonthesledandadjustedsuchthat the angle of the seat ramp and the recliner read

121and252,respectively.AnH-pointmachineaccordingtoSAEJ826wasusedfortheadjustments.As ananthropomorphic testdevice (ATD), a BioRIDwas used. The dummy was positioned in the seataccording to theprocedures described in ECE R 94.The ATD was instrumented with accelerometerslocated at thehead, lower neck, chestandpelvis.Onthe upper neck (C1 level) forces in x and z directionwere recorded and thebendingmoment around theyaxiswasmeasured.Figure4showsthetestset-up.Theseatingposture was similar to the one in theoriginalseat.Targetsfixedtotheseatbaseandtotheseatslidewereusedtoanalysethetranslationalmotion.The crashpulse used in the tests was a trapezoidalshapedpulsewithanaveragesleddecelerationof61g and with rise and fall times of 10 - 20 ms. Theresultingchangeofvelocity(v)ofthesledwas161km/h.Asfor theevaluation,theneck injurycriteriaNICmax[Bostrom et al. 1996] andNkm [Schmitt et al. 2002]werecalculatedusingthefollowingequations:

(1)

(2)wherearelandvreldenotefortherelativeaccelerationand velocity of the highest (occipital condyles) andlowest (C7/T1) point of the cervical spine,respectively. NICmax is the maximum value of theNIC(t)curveduringtheretractionphase.CurrentlythecriticallimitfortheNICmaxis15m2/s2[Bostrometal.1996].Fx(t) and My(t) are the shear force and the flexion/extensionbending moment, respectively. Both valuesareobtainedfromtheloadcellpositionedattheupperneck. Fint and Mint represent critical intercept values

NI C t( ) 0.2 are l t( ) vre l t( )( )2

+=

Nkm

t( )F

xt( )

Fin t

-------------M

yt( )

Min t

--------------+=

Figure 3. DeformableU-shaped steel profile asused in the functionalmodel;undeformedprofile(left)anddeformedprofile(right).

Figure4. Principleof the testset-up for thesledtestsincludingthemeasurementtargetsontheseatandtheBioRIDdummy.

T11

T2

T3

T4

T12

TT1

B1

H

RR

B2

P

B

A

D

C

45.636.00

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used for normalisation. The threshold valuecurrentlyproposed is 1.0 for all possible Nkm load cases[Schmittetal.2002].RESULTS

Table 1 summarises the according results for thereference seat and two seatsboth equipped with theseat slide but using different deformation profiles.Theseprofileswereofsimilargeometry(asshowninFigure 3)but with different thickness (8 mm and 5mm). The deformation characteristics of theprofilesalong with the design of the seat slide resulted in abackwards motion of approx. 40 mm for alldeformableslides.Figure5illustratesthedeformationcharacteristics.

TheNICmax values were reduced from 13.4 of thereference seat to 8.1 for a modified seat. The Nkmvalues were all - even for the reference seat - wellbelow the proposed threshold and only slightdifferences between the seats were observed. Thereboundvelocitieswerealsoverysimilarforalltests,not indicating any significant changes causedby theseatslide.ResultsforthepelvisaccelerationareshowninFigure6. The influence of the damping device is clearlyindicated by the plateau value which limits theaccelerationforacertaintimeandthereforecausestheincreaseof thepelvis acceleration tobedelayed.Thechangeofthepelvisaccelerationconsequentlyaffectstheaccelerationofthefirstthoracicvertebra(T1).ThisisalsoreflectedintheNICmaxvalues(Figure6).Using the seat slide with the 5 mmprofile the T1acceleration is reduced in the retraction phase (i.e.whenNICmaxoccurs)anditsmaximumisshiftedsuchthat it meets the maximum of the head acceleration.The T1 and head accelerationpeak values, however,wereslightlyincreased.Tocheckfortheeffectofthisincrease,theheadinjurycriterionHICwascalculated.TheHICvaluesobtainedare75forthereferenceseat,122forthe8mmprofile,and105forthe5mmprofile,respectively.

Concerningthetiming,alaterheadcontactresultingina latermaximumof theheadaccelerationisobserveddue to the additional translational motion. TheNkmvalues were also reached later in time than for thereferenceseatexceptfortheNeawherethetimingwasinconsistent. TheNICmaxoccurred atabout the sametimeforallseats.

Table1.Resultsfromthesledtestexperiments:timeofheadcontact,NICmax(top),andNkmvalues(bottom).

seat headcontact NICmaxt[ms] NICmax[m2/s2] t[ms]

referenceseat 62 13.4 63profile1(8 mm) 92 10.7 63profile2(5 mm) 80 8.1 66

seat Nkm

Nep [ms] Nfp [ms] Nea [ms] Nfa [ms]

referenceseat 0.24 101 0.23 97 0.02 169 0.07 64profile1(8 mm) 0.26 155 0.45 126 0.05 81 0.05 96profile2(5 mm) 0.22 140 0.34 113 0.00 -- 0.06 81

Figure5. Deformationofasteelprofile.-45

-40

-35

-30

-25

-20

-15

-10

-5

0

0 0.05 0.1 0.15

time(s)

deformation

(mm)

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Additionally,profiles of different geometry and fromdifferent materials were tested. Those results wereeithercomparableordidnotimprovethebehaviourofthe seat in terms of neck injury criteria and arethereforenotdiscussedhere.DISCUSSION

In order to develop a countermeasure for whiplashinjury,anewseatslidewaspresented.The seatslideallows a translationalbackwards motion of the seatduringalow-speedrear-endimpactwhiledampingthismovement. A prototype of the seat slide was builtusing deformable steelprofiles to damp the motion.Theprototypewasmountedtoastandardcarseatandsled tests wereperformed to analyse thepotential ofinjurypreventionofsuchadevice.ItwasshownthattheseatslidereducedtheNICmaxtoapproximately60% of the value of the reference seat without such aslide.Thisindicatesthebenefitofcontrolleddampingto reduce neck loading, especially toprevent the so-calledS-shapewhichisassessedbytheNICmax.Thehigherheadaccelerationcanbepartlybeattributedtothe increased timeintervaluntil theheadcontactsthehead restraint.Inaddition, the reclinerproperties thatgovern the relative velocitybetween head and headrestraint are possibly accounting for a higher headacceleration.Whentheheadimpactstheheadrestraint(and thus is accelerated), thebackrest andwith it theheadrestrainthaveaslightlyhighervelocityrelativetothe head. The reason for such higher velocity canprobablybe found in the longerperiod for which thebackrest is accelerated before head impact. TheaccordingHICvaluesreflectthatbehaviour,although

theyareallfarbelowanythreshold.Futureworkwillanalysethisbehaviourmoreclosely.The relative motion between head and neck wasreduced,becausethetimingoftheaccordingheadandT1 acceleration was improved such that bothaccelerations reach their maximum at the same time.Furthermore, the outcome for the Nkm, and thereboundvelocitiesaswellasthesittingpostureofthedummy were not significantly influencedby the seatslide.Nonetheless,theinfluenceoftheseatslideonthehead acceleration, for instance, demonstrates that aseatmustbeconsideredasaunit, i.e.abalancedseatdesign is - in the context of crashworthiness - onlyachievedby taking all seat components into account.Hencetheconstructionoftheseatslide,especiallythecharacteristicsofthedeformableelements,hasalsototakethosecomponentsintoaccount.Fortheseattestedherethisholdsparticularlytrueforthereclinerwhichseems to have a large influence on the overall seatbehaviour.

As for thepure translational motion of 40 mm, thisdistance seems acceptable for all two-seaters and forseatswherethebackrestdoesnot(dynamically)rotatemuch like the one analysed here. Further tests withother seats will investigate whether there ariseproblems for backseat passengers such that thetranslationshouldbelimited.Ingeneral, theprototypeprovedtoberepair friendly.Onlythedampingelementshadtobereplacedafteratest.Furthermore,asystemofthiskindseemssuitabletobefittedtodifferentseatsbyadapting theprofiles.As required the new system did not interfer with thetranslational adjustability of the seat, i.e. the seatremainedadjustableaftertheimpact.

Figure6. Pelvisacceleration for the reference seatand the seat equippedwith thenew slideanda5mmdeformationprofile.Additionally,atypicalheadaccelerationcurveisshown.ThereductionoftheNICmaxisindicatedschematicallybytheaccordingareas.

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Consequently,applyingtheprincipleofadampingseatslide shows that besides the recliner and the headrestraintthatareusedinexistingsystemslikeWHIPS,WipGARD,andSAHR,theseatslidehasthepotentialtopreventwhiplashinjury.CONCLUSIONS

The design of a new seat slide toprevent whiplashinjuryispresented.Adampingelementwasintroducedthatdeformsduring a translationalbackwardsmotionoftheseatincaseofalow-speedrear-endimpact.Theresults indicate that such a design has apotential toimprovetheseatbehaviour,particularlywithrespecttothe relative acceleration between head and neck. A40%reductionofthevaluefortheneckinjurycriterionNICmax was obtained. Although the design of theprototypeisnotyetsuitedfortheproductionoflargernumbers, the concept has proven to be suitable forsuch

apurpose.

Together

with

other

systems

presented,the new seat slide showed that for future car seats

different possibilities concerning all major seatcomponents are available to implement whiplashcountermeasures.

ACKNOWLEDGEMENTS

We are gratefully indebted to the Swiss Funds ofTrafficSafetyfortheirsupport.REFERENCES

Bostrom O, Svensson M, Aldman B, Hansson H,Haland Y, Lovsund P, Seeman T, Suneson A,SaljoA,OrtengrenT(1996):Anewneckinjurycriterioncandidatebasedoninjuryfindingsinthecervical spinal ganglia after experimental neckextensiontrauma;Proc.IRCOBIConf.;pp.123-136

Eichberger A, Steffan H, Geigl B, Svensson M,Bostrom O, Leinzinger P, Darok M (1998):Evaluationoftheapplicabilityoftheneckinjurycriterion(NIC)inrearendimpactsonthebasisofhuman subject tests; Proc. IRCOBI Conf.; pp.153-161

Ferrari

R

(1999):

The

whiplash

encyclopedia;

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MuserM,ZellmerH,WalzF,HellW,LangwiederK(1999) Testprocedure for the evaluation of theinjury risk to the cervical spine in a low speedrear end impact, Proposal for the ISO/ TC22N2071 / ISO/TC22/SC10 (collison testprocedures), http://www.biomed.ee.ethz.ch/~agu/

MuserM,WalzF,SchmittK-U(2002)Injurycriteriaapplied to seat comparison tests, Traffic InjuryPrevention,Vol.3(3),pp.224-232

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Zellmer H, Stamm M, Seidenschwang A, Brunner A(2001)Benefitfromaneckprotectionsystemforaftermarketfitting,Proc.IRCOBIConf.,pp.337-338