Development of a damping seat slide to reduce whiplash injury

Transcription

1 949 Development of a damping seat slide to reduce whiplash injury K-U Schmitt1,2*, M Muser2, M Heggendorn1, P Niederer1 and FWalz2 1Institute of Biomedical Engineering, University and Swiss Federal Institute of Technology ( ETH), Zurich, Switzerland 2Working Group on Accident Mechanics, University and Swiss Federal Institute of Technology ( ETH), Zurich, Switzerland Abstract: To prevent whiplash injury, a new seat slide was developed. In the case of a rear-end impact, the device allows the vehicle seat to slide backwards while damping this motion. Deformable steel pro les serve as damping elements. To activate the device, a trigger system is used which detects an acceleration threshold. Sled test experiments were performed to analyse the behaviour of the new system. The tests were conducted to mimic rear-end collisions with a delta-v of 16 km/h. A BioRID dummy was used as a human surrogate. The results indicate the bene cial in uence of the damping seat slide on the occupant kinematics. In particular, the so-called S-shape deformation of the neck assessed by the neck injury criterion (NIC ) is reduced. Comparing a standard car seat with and without the damping seat slide, it was shown that the NIC is reduced by approximately 40 per cent. Hence it was shown that besides the head restraint and the recliner that are used in other whiplash protection systems, the seat slide has the potential to prevent whiplash injury. Keywords: whiplash injury, seat, seat slide, neck, biomechanics 1 INTRODUCTION Neck injuries resulting from low-speed rear-end collisions have attracted increasing attention in recent years with statistics from several countries indicating an increase in the occurrence during the last decades [ 1 3]. Although those injuries, often also called whiplash associated disorders ( WAD), are usually not life threatening, their high incidence rate together with possible long-term consequences make them very costly for society. Consequently, the reduction of such neck injuries is a major subject in tra c safety. To date, the underlying injury mechanism has not yet been established. Several hypotheses have been suggested by various researchers, but are not conclusive. Nonetheless, it seems to be generally agreed upon the fact that such injury is related to the sudden movement of the head torso complex (e.g. see reference [4]). The presence of the so-called S-shape, i.e. the characteristic deformation of the neck, is well established today and was observed in various experiments [5 7]. The MS was received on 22 July 2002 and was accepted after revision for publication on 20 June * Corresponding author: Institute of Biomedical Engineering, University and Swiss Federal Institute of Technology (ETH), Gloriastrasse 35, 8092 Zurich, Switzerland. Developments in seat design aim at providing seats that next to other speci cations o er good whiplash protection. How can demand be met if the cause of injury is unknown? In a rather holistic approach to prevent whiplash injury general design guidelines for seat development can be established (e.g. see reference [ 8]). Such guidelines attempt to address all existing hypotheses concerning the injury mechanism. Thus, by trying to cover all possible scenarios, a reduction of the occupant loading should be ensured. The occupant acceleration must be reduced. It is assumed that without acceleration, in particular without relative acceleration between the head and torso, no whiplash injury will be sustained. Bearing in mind that the neck injury criterion (NIC ) [ 9] is widely used to assess the risk of soft tissue neck injuries, it is especially this relative acceleration that has to be reduced in order to obtain good NIC values. Taking into account hypotheses that claim the relative movement between adjacent vertebrae to be causal for whiplash injury, such motion must be avoided. Hence, the curvature of the spine must be kept unchanged during the impact. Additionally, the rebound phase has to be considered [10]. To minimize the interaction with the seat belt, rebound must be reduced. The potential of a seat to prevent whiplash injury is

2 950 K-U SCHMITT, M MUSER, M HEGGENDORN, P NIEDERER AND F WALZ assessed by performing experiments measuring acceleration, force and moment and determining various neck injury criteria thereof, such as the NIC [9], the N km [11] or the neck displacement criterion (NDC ) [ 12]. To ensure a broad basis for analysis and assessment, none of the measures mentioned can serve alone, but all should be reduced. Due to the uncertainty with respect to the injury mechanism, an increase of any response related to the biomechanical guidelines even if accompanied by a clear reduction of another criterion is to be avoided. As of today, several seat systems are presented to prevent whiplash injury. The Saab active head restraint (SAHR) system [13], for instance, consists of an active head restraint that automatically moves up and closer to the occupant s head in rear-end impacts. Thus the distance between the head and the head restraint is reduced. Volvo presented the WHIPS seat [8] which is equipped with a recliner that allows controlled backwards movement of the backrest during rear-end impact. The motion is performed in two steps: a translational rearwards movement of the backrest is followed by a rotational motion reclining the backrest. Another system, called WipGARD [14], also enables the backrest to perform a translation followed by a rotation. Both the WHIPS and the WipGARD require a critical load to activate the system. In addition to these systems, a damping seat slide to prevent whiplash injury is presented in this study (patent pending). A prototype of the seat slide was built and mounted to a standard car seat. Its e ectiveness was shown by performing sled test experiments. 2 MATERIALS AND METHODS In an attempt to ful l the design criteria mentioned above reduction of occupant acceleration, reduction of relative movement between adjacent spine segments and reduction of rebound a new seat slide was developed. A standard seat slide as used in a recent car seat formed the basis for this development. In order to ensure that the basic functions of the seat slide persisted, modi cations were not allowed to in uence the translational adjustability and the sti ness of the structure. Changes to the xing of the slide to the car were avoided such that the according regulations like ECE R-17 were still ful lled. Thus the seat characteristics for impact conditions other than a low-speed rear-end impact are meant to be sustained. In addition, the seating posture of an occupant had to be maintained with the new device. Furthermore, it was de ned that repairs possibly needed after the new seat slide was activated had to be inexpensive and easy to carry out. 2.1 Principle The relative acceleration between the head and the rst thoracic vertebra ( T1), and consequently also the NIC max value, is to be reduced. The new seat slide is mounted between the seat base and the car oor and allows a translational motion of the seat base relative to the car while damping this motion (Figs 1 and 2). The damping mechanism is triggered by a sensor. This sensor detects the vehicle acceleration and, when a certain limit is exceeded, the deformable element is released. Additionally, the damping element requires a tripping energy threshold to be deformed. Thus, as a measure to prevent unintended activation, the system needs a certain acceleration followed by an impact force of the occupant against the seat. Once the system is activated, the seat is allowed to move backwards in a pure translational manner while the motion is damped. This leads to a delay of the building-up of the torso loading and thus synchronizes the loading of the head and the upper torso. 2.2 Prototype design To show the feasibility of the principle described above, one possible design of the damping seat component was developed. Here, the damping component consists of a deformable element on each side of the slide; i.e. the mechanism is symmetric. A single spring mass system Fig. 1 Principle of the seat slide: the seat (1) of a vehicle is mounted to a seat slide (2) which itself is mounted to the car body (3) ( left). In the case of a rear-end impact, a trigger system releases the new seat slide which then enables the seat to move backwards (relative to the car) while damping this movement (right)

3 DEVELOPMENT OF A DAMPING SEAT SLIDE TO REDUCE WHIPLASH INJURY 951 Fig. 2 Principle of the seat slide showing the deformation element in its initial position (top) and during impact (bottom) located in the middle under the seat base is used to detect the acceleration necessary to activate the system. If the acceleration threshold is reached, the system is released; i.e. a translational motion of the seat relative to the car oor is allowed. In this case the seat slides along a de ned path as a gliding part is guided along a slide. The activation mechanism releases both damping components simultaneously. The damping is performed by deformation of steel pro les (Fig. 3). The geometry and strength of the pro les were chosen on the basis of analytical design studies and tensile testing experiments. It has to be noted that due to the material of the pro le used, a certain elastic force limit must be exceeded to start the damping. Preferably the pro les show a small elastic range, followed by a large plateau region until eventually a hardening e ect is observed. The distance of the backwards movement results from these deformation characteristics. 2.3 Sled tests To analyse whether the new seat slide o ers the possibility of preventing soft tissue neck injuries, sled tests were performed. A recent car seat from a European manufacturer was used and tested in its original con guration and with the new seat slide. The sled tests were carried out in collaboration with Autoliv (Germany) GmbH and all tests were performed in the same manner according to the test procedure for the evaluation of the injury risk to the cervical spine in a low-speed rear-end impact proposed for ISO/TC22 N 2071 and ISO/TC22/SC10, respectively [ 15, 16]. The seats were mounted on the sled and adjusted such that the angle of the seat ramp and the recliner read 12 ±1 and 25 ±2 respectively. An H-point machine according to SAE J 826 was used for the adjustments. As an anthropomorphic test device (ATD), a BioRID dummy was used. The dummy was positioned in the seat according to the procedures described in ECE R 94. The ATD was instrumented with accelerometers located at the head, lower neck, chest and pelvis. On the upper neck (C1 level ) forces in the x and z directions were recorded and the bending moment around the y axis was measured. Figure 4 shows the test set-up. The seating posture was similar to the one in the original seat. Targets xed to the seat base and to the seat slide were used to analyse the translational motion. The crash pulse used in the tests was a trapezoidalshaped pulse with an average sled deceleration of 6 ±1g and with rise and fall times of ms (Fig. 5). The resulting change of velocity ( v) of the sled was 16±1km/h. For the evaluation, the neck injury criteria NIC [9] and N [11] were calculated using the max km following equations: NIC(t)=ka (t)+[v (t)]2, k=0.2 m rel rel N (t)= F x (t) + M y (t) km F M int int where a and v denote the relative acceleration and rel rel velocity of the highest (occipital condyles) and lowest (C7/T1) point of the cervical spine respectively. NIC max is the maximum value of the NIC(t) curve during the retraction phase. Currently the critical limit for the NIC is 15 m2/s2 [9]. max F (t) andm (t) are the shear force and the exion/ x y extension bending moment respectively. Both values are obtained from the load cell positioned at the upper neck. F and M represent critical intercept values used for int int Fig. 3 Deformable U-shaped steel pro le as used in the prototype: undeformed pro le ( left) and deformed pro le (right)

4 952 K-U SCHMITT, M MUSER, M HEGGENDORN, P NIEDERER AND F WALZ Fig. 4 Principle of the test set-up for the sled tests including the measurement targets on the seat and the BioRID dummy Fig. 5 Typical crash pulse resulting in a v of 16 km/h normalization. Distinguishing positive shear, negative shear, exion and extension, the N km criterion identi es four di erent load cases: N fa, N ep, N fp and N ea. The rst index represents the bending moment (f, exion; e, extension) and the second indicates the direction of the shear force (a, anterior, i.e. in the positive x direction; p, posterior, i.e. in negative x direction). The sign convention according to SAE J211/2 was used. The threshold value currently proposed is 1.0 for all possible N km load cases [11]. 3 RESULTS During impact, the translational movement of the new seat slide was observed. Table 1 summarizes the results for the reference seat and two seats both equipped with the seat slide but using di erent deformation pro les. These pro les were of similar geometry (shown in Fig. 3) but with di erent thicknesses (8 and 5 mm). The deformation characteristics of the pro les along with the design of the seat slide resulted in a backwards motion of 40 mm for all deformable slides. It can be seen that the NIC max values were reduced from 13.4 for the reference seat to 8.1 for a modi ed seat. The N km values were all (even for the reference seat) well below the proposed threshold and only slight di erences between the seats were observed. The rebound velocities were also very similar for all tests, not indicating any signi cant changes caused by the seat slide. The pelvis acceleration is shown in Fig 6. The in uence of the new device is clearly indicated by the plateau value, which limits the acceleration

5 DEVELOPMENT OF A DAMPING SEAT SLIDE TO REDUCE WHIPLASH INJURY 953 Table 1 Results from the sled test experiments (a) Time of head contact and timing and value of NIC max NIC max Head contact Seat t (ms) NIC (m2/s2) t (ms) max Reference seat Pro le 1 (8 mm) Pro le 2 (5 mm) (b) N km values Seat N km N ep t (ms) N fp t (ms) N ea t (ms) N fa t (ms) Reference seat Pro le (8 mm) Pro le (5 mm) for a certain time and therefore causes the increase of the pelvis acceleration to be delayed. The change in the pelvis acceleration consequently a ects the acceleration of the T1 and the head. Figures 6 and 7 show these di erences. Using the seat slide with the 5 mm pro le, the T1 acceleration is reduced in the retraction phase (i.e. when NIC max occurs) and its maximum is shifted such that it meets the maximum of the head acceleration. The T1 and head acceleration peak values, however, were slightly increased. To check for the e ect of this increase, the head injury criterion (HIC ) [17] was calculated. The HIC values obtained are 75 for the reference seat, 122 for the 8 mm pro le and 105 for the 5 mm pro le respectively. Concerning the timing, a later head contact resulting in a later maximum of the head acceleration is observed due to the additional translational motion. The N km values were also reached later in time than for the reference seat except for the N ea case, where the timing was Fig. 7 Head and T1 acceleration of the reference seat (above) and the seat equipped with a 5 mm deformation pro- le (below) inconsistent. The NIC max occurred at about the same time for all seats. Additionally, pro les of di erent geometry and from di erent materials were tested. Those results were either comparable or did not improve the behaviour of the seat Fig. 6 Pelvis acceleration for the reference seat and the seat equipped with the new slide and a 5 mm deformation pro le

6 954 K-U SCHMITT, M MUSER, M HEGGENDORN, P NIEDERER AND F WALZ in terms of neck injury criteria and are therefore not discussed here. 4 DISCUSSION In order to develop a countermeasure for whiplash injury, a new seat slide was presented in this study. The seat slide allows a translational backwards motion of the seat during a low-speed rear-end impact while damping this movement. A prototype of the seat slide was built using deformable steel pro les to damp the motion. The prototype was mounted to a standard car seat and sled tests were performed to analyse the potential of injury prevention of such a device. The results, in particular those for the NIC max were promising. It was shown that the seat slide reduced the NIC max to approximately 60 per cent of the value of the reference seat without such a slide. This indicates the bene t of controlled damping to reduce neck loading, especially to prevent the so-called S-shape which is assessed by the NIC max.the higher head acceleration can be partly attributed to the increased time interval until the head contacts the head restraint. In addition, the recliner properties that govern the relative velocity between the head and head restraint are possibly accounting for a higher head acceleration. When the head impacts the head restraint (and thus is accelerated), the backrest and with it the head restraint have a slightly higher velocity relative to the head. The reason for such a higher velocity can probably be found in the longer period for which the backrest is accelerated before head impact. The resulting HIC values re ect that behaviour, although they are all far below any threshold. Nonetheless, to comply with the design guidelines for whiplash protective devices, this behaviour has to be analysed more closely and changes to the seat design (e.g. head restraint thickness) might be introduced. The relative motion between the head and neck was reduced, because the timing of the according head and T1 acceleration was improved such that both accelerations reach their maximum at the same time. Furthermore, the outcome for the N km criterion and the rebound velocities as well as the sitting posture of the dummy were not signi cantly in uenced by the seat slide. Thus important requirements as described in the design guidelines are ful lled. Nonetheless, the in uence of the seat slide on the head acceleration and the N fp case, for instance, demonstrates that a seat must be considered as a unit; i.e. a balanced seat design is in the context of crashworthiness only achieved by taking all seat components into account. Hence the construction of the seat slide, especially the characteristics of the deformable element, has to be coordinated with those components. For the seat tested here this holds particularly true for the recliner, which seems to have a large in uence on the overall seat behaviour. For the pure translational motion of 40 mm, this dis- tance seems acceptable for all two-seaters and for seats where the backrest does not (dynamically) rotate, much like the one analysed here. Further tests with other seats will investigate whether problems arise for backseat passengers, such that the translation should be limited. In general, the prototype proved to be repair friendly. Only the damping elements had to be replaced after a test. Furthermore, a system of this kind seems suitable for tting to di erent seats by adapting the pro les. As required, the new system did not interfer with the translational adjustability of the seat; i.e. the seat remained adjustable even after the impact. However, the prototype presented is not at all an industrial design eligible for serial production. Due to limitations in manufacturing further development is necessary. A product based on the principle of the new seat slide might incorporate the system into a standard seat slide, making many parts of the prototype dispensable. Consequently, applying the principle of a damping seat slide shows that besides the recliner and the head restraint that are used in existing systems like WHIPS, WipGARD and SAHR, the seat slide has the potential to prevent whiplash injury. 5 CONCLUSION The design of a new seat slide to prevent whiplash injury is presented. A damping element was introduced that deforms during a translational backwards motion of the seat in the case of a low-speed rear-end impact. The results indicate that such a design has a potential to improve the seat behaviour, particularly with respect to the relative acceleration between the head and neck. A 40 per cent reduction of the value for the neck injury criterion NIC was obtained. Although the design of max the prototype is not yet suited for the production of larger numbers, the concept has proven to be suitable for such a purpose. Together with other systems presented, the new seat slide showed that for future car seats di erent possibilities concerning all major seat components are available to implement whiplash countermeasures. 6 ACKNOWLEDGEMENTS The study was supported by the Swiss Funds for Tra c Safety. Furthermore, the authors are indebted to Paul Lüthi for his help in manufacturing the prototype. REFERENCES 1 von Koch, M., Kullgren, A., Lie, A., Nygren, A. and Tingvall, C. Soft tissue injury of the cervical spine in rearend and frontal car collisions. In Proceedings of the

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