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In 1950's a very little attention was paid to the whiplash problems; as the whole focus was to develop automobile seats to accommodate occupants in one single structure. No Head restraints were in use at this time and seatback height was normally 600 mm. According to Vianno (2002), the basic automobile front seat was in the form of a bench, with about 363 Kg in strength equally distributed at each side structure. A moment arm of about 400 mm was assumed based on the available occupant loading data. Anderson (1961) at General Motors Laboratory carried out a comprehensive evaluation of these seats in case of rear impacts focussing on whiplash problems and found out that the 145 kg-m level provided the right balance between strength and neck loading. With the use of anthropometric data he also succeeded to show that an increase of 65mm -70 mm in a seatback height would provide head neck support for a larger proportion of occupants.
In 1968, several crash tests with seats of varying strength were carried out by Severy et al, 1968. .Their work has been considered as a milestone in setting the standard for head restraints. In these crash tests they considered 50% male driver seated in a bucket seat of considerably high strength compared to the previous versions of seats, with setback height of approximately 720 mm and 95% male, right front occupant in similar kind of seat with approximately 75mm offset of head to the seatback. They observed that rigid driver's seat inclined to 41 degrees in crash and center of gravity (CG) moved rearward over the top of the seatback.Driver's neck extension was 79 degrees and whiplash motion caused its hyper-extension and contraction with the head acceleration of 42g at the juncture. Thus it was concluded that the rigid seats with fixed head restraint cannot fully support the head during the crash and yielding seatbacks were found more fruitful to better support the head and neck alignment as the occupant is accelerated .(Blaisdell et al.,1993;Prasad et al.,1997,Viano et al.,2007,2008).
Thus in 1970's bucket seats made their appearance providing individual adjustment for the front seats. This ultimately gave rise to the seat recliners which allowed altering the seatback angle. With the progression of time, head restraints were added to the seatbacks as discussed earlier.
During 1970 -1980, certain fixed constraints were established for the seat designs restricting the maximum range for seat recliner angle to 10 degrees at the target load and seatback frame was restricted to return to within 5 degrees of its initial position after unloading. In 1980's, the highest load supported by seat was well above that of all the previous versions of automobile seats. But all the designs either failed to sustain these criteria during this period (e.g. the head restraints adjusted at highest position allowed whiplash) or it was the discrepancy in the occupant's performance which resisted the overall development in the seat design. This fact was first reported by O'Neil et al., (1972) and remained unaltered till next 20 years.
In 1982, Kahane evaluated the effectiveness of the safety head restraints complying with the regulations, set up by the US Federal Motor Vehicle Safety Standard (FMVSS 202) in attempt to reduce the risk associated with the whiplash injuries. This study was based on the research carried out by US national crash severity study, the National accident sampling system, and fatal accident reporting system. The conclusion of the study considering seated height of 50% US male in erect position as 700 mm, the head restraint above this height would give the benefits of injury prevention.
In early 1990's, in depth study was undertaken by GM focussing on seat characteristics to improve occupant safety in rear impacts (Viano,2002,2003a-i),which led to minimum seatback strength of 1700Nm in 1995 using quasistaic seat test(QST).This involved 50% male dummy loaded rearward into seatback through lumbar joint. The dummy was free to move up-down and sideways during rear impact. This test was similar to that of sled and vehicle testing in terms of loading the seatback. This test provided data on seat stiffness(K), ultimate load(F),frame strength(j),bending moment(M) and energy transfer(E) and showed that as seats increased in strength over the time, seat stiffness also increased, which was responsible for exerting much higher magnitude of load on occupant's torso than anticipated and resulting into greater neck displacements before head restraint contact(Viano 2003h).This was supposed to be one of the main reasons for the failure to reduce whiplash phenomenon from 1970s to 1990s.
With the rise of 21st century, many facts regarding the whiplash phenomenon and corresponding car seat designs are brought into limelight. The criterion for evaluating the severity of whiplash phenomenon was proposed by Viano and Davidsson in 2002 observing neck displacements in BioRid P3 and Hybrid III dummy loaded with load cells at the upper and lower portion of neck.
Himmetoglu et al., 2008 focussed on exploring the whiplash injury mechanism. They also suggested different features to be included in the car seat to reduce neck internal motion giving rise to whiplash injuries in a rear crash phenomenon. For this study, 50th percentile biofidellic human male Hybrid III dummy and IIWPG standard crash pulse (medium 16g and severe 35g) was used as an input. Using the same driving posture as that in JARI (Japan Automobile Research Institute) sled tests (Davidson et al, 1999), several anti-whiplash car seat design concepts were considered. The special feature of these car seats was, they consist of specially designed anti whiplash devices (AWD), which includes active head restraint, car seat damper. The main idea behind the design was that car seat should move rearwards and down at the same time to that of rear impact to minimize the distance between head and the head restraint to reduce risk of whiplash injury. It was argued as a conclusion that the proposed seat designs successfully limit the neck internal motion and hence reduce the risk associated with the 'S' shape deformation, based on the knowledge of the whiplash injury mechanism also developed by Himmetoglu et al,2008. This project to design a car seat damper is based on the work done by Himmetoglu et al, 2008 which will be discussed later in detail. Some remarkable work was carried out by M.J Van der Horst, 2002 in the area of human neck response in frontal lateral and rear end impact in order to determine the mechanism of whiplash injury. LopÑz et al, 2005 carried out a research to study the viability of current neck injury criterion proposed by Viano and Davidsson,2002 used for whiplash analysis. Some manufacturers had begun to introduce designs at the start of 21st century that go beyond simple geometric improvements. Toyota has devised a seat that it calls the "Whiplash Injury Lessening" (WIL) seating system (Sekizuka, 1998). It is designed to allow an occupant's upper back to sink farther into the seatback, thus reducing the differential movement of the head and torso in a sudden acceleration. It also has a slightly higher seatback and head restraint than earlier seating systems (Figure 1). The WIL system was introduced in some versions of the 1999 Lexus RX300 and then made standard for the 2000 Toyota Avalon, Celica, and Echo.
Figure 1 (a) 1999 Toyota Avalon vs. (b) 2000 Toyota Avalon with Whiplash Injury Lessening (WIL) system.
The Saab Active Head Restraint (SAHR) is a mechanical system that moves the head restraint upward and forward in response to sudden pressure from an occupant against the seatback (Wiklund & Larsson, 1998) (Figure 1). It was first introduced in Europe in the 1998 Saab 9-5, and in the United States in the 1999 Saab 9-3 and 9-5. SAHR is meant to overcome the problem of poorly adjusted head restraints by automatically moving into an optimal position when needed. Laboratory testing has shown that SAHR, even when adjusted to its lowest position, controls the head and neck movement of an average-size male as well as a fixed head restraint with good geometry (Linder et al., 2001; O'Neill, 2000; Zuby et al., 1999).
Figure 2 SAAB Active Head Restraint
General Motors has incorporated an active head restraint similar to SAHR in its "catcher's mitt" seating system, beginning with the 2000 Buick LeSabre, Pontiac Bonneville, and Oldsmobile Aurora. Nissan also has adopted a similar active head restraint, beginning with the 2000 Infiniti Q45 (Viano and Olsen, 2001).
Volvo, whose head restraints were receiving good ratings for geometry even in 1995, introduced the Whiplash Injury Prevention System (WHIPS) for the 1999 Volvo and 2000 Volvo S70 (Lundell et al., 1998). The head restraint is unchanged but a hinge at the base of the seatback yields and partially rotates under sufficient loading, thus reducing the forward acceleration of the occupant's torso (Figure 3).
The three new approaches to reducing whiplash injury risk-WIL WHIPS, and active head restraints-have been justified both theoretically and in the laboratory (Farmer et al, 2003).
Figure 3 Volvo WHIPS seat mechanism
However, with the exception of the Viano and Olsen (2001) study, they remain unproven in real-world crashes. Improved head restraint geometry has been linked to lower neck injury rates (Chapline et al., 2000; Farmer et al., 1999), but the comparisons were made among different vehicle models. A better comparison would be between vehicles of the same model, yet with differing head restraints.
Golinski and Gentle, 2010 proposed an innovative design of anti whiplash device. They used a collapsible spring principle as sometimes used in the safety devices on elevators. According to them, this device could be mounted in the existing car seats parallel to the main axis of the car.
This proposed design consists of two shallow angled conical tubes mounted on the double ended conical mandrel. One tube would be connected to the car seat and the other one would be connected to the floor below the car. When floor is loaded due to rear impact, the conical tubes will be pushed onto the cone and deform elastically following the elastic behaviour of a linear spring. But the friction between the metal surfaces will resist the device springing back and hence will be locked at the position. The collapsible springs are to be used on the both sides of the car seat. In order to achieve self locking of the spring, very high unloading stiffness is assigned to the spring of the order of 1E+8 N/mm, which successfully prevented spring from bouncing back after the impact. The maximum loading force of 3500 N was applied to the elements. Spring length was based on the safety of the rear passenger. It was observed that rearward movement of about 1o cm would not cause harm to the rear occupant, but the length of the deformable spring was set to 5 cm to allow strict safety measures.
Anti whiplash Device (Golenski and Gentle, 2010)
The basis of a current project:
This project is primarily based on the work carried out by S.Himmetoglu, 2008 in his Ph.D. thesis titled "Car seat design and human body modelling for rear impact whiplash mitigation". The force-displacement profile obtained for translational crash energy absorbing AWD by testing and validating the actual whiplash conditions by S.Himmetoglu largely forms the basis of the project.
The thesis provides an insight of the subject including preliminary research, testing and validation of actual whiplash conditions using JARI (Japanese automobile research Institute) (refer figure 1) sled tests and biofidelic 50th % male multi-body human model, whiplash injury mechanism, affecting factors and the evaluation of injuries. The theoretical information has been supported with the experimental analysis carrying out a series of multi-body simulations to study the effects of car seat design to mitigate whiplash injuries.
Figure 1. JARI sled test setup (Lundell et al., 1998)
The brief review of work done by Himmetoglu et al. forming the basis of this project is provided as below.
A number of energy absorbing car seat concepts with anti- whiplash devices (AWD's) have been proposed by Himmetoglu et al., 2008 and the motions induced by these AWD's on human body model, subjected to rear impact are studied. These motions enable seatback and seat pan to function independent of each other. They are controlled by passive response units such as springs and dampers. These passive devices become active only when a particular level of force and/or torque has been exceeded during the rear impact. These passive devices are designed to absorb optimum crash energy at various severities. The characteristics of these passive devices are determined using a wide range of crash pulse (âˆ†V between 4.5 and 35Kph) with different severities and shape of crash pulse (Linder et al., 2001, 2003; see also http://www.folksam.se). The figure follow shows the six different car seat design concepts designed and developed, where few main components represent the total mass of car seat collectively (Verver, 2004).
Figure 2. Anti-whiplash car seat design concepts (HR: head restraint, SB: Seatback, SP: seat-pan, OF: outer seatback frame, P: translational AWD, R &R*: rotational AWD) developed by S.Himmetoglu, 2007.
During simulation, the modelled seats were used in combination with a virtual 'test dummy' called BioRID dummy, as illustrated in Figure 2. This 'test dummy' was modelled in such a way that the important data pertaining to whiplash injury could be calculated with the help of the optical sensors and recorded such as relative intervertebral angles and accelerations of the head and neck. This data was subsequently used to calculate NIC (neck injury criterion) and S-Shape values and help evaluate each seat concept.
Figure 3. Dynamic test initial configuration of BioRID test dummy
(Himmetoglu et al, 2008)
Each seat concept was subjected to a range of acceleration pulses from the rear, to simulate a rear impact collision. The pulses ranged from âˆ†V = 4. Insurers Whiplash Prevention Group (IIWPG 5 kph (low severity impact) to âˆ†V = 35 kph (severe impact) and included the International 'Standard Pulse' of âˆ†V = 16 kph.
Figure 4. IIWPG standard pulse
This pulse, shown in Figure 15, is used to evaluate a seat against a range of criteria developed by the IIWPG which has the aim of setting minimum standards for car seat responses in rear impacts. IIWPG 2006 specifies these evaluation criteria as head restraint contact time, maximum T1 forward acceleration, upper neck (rearward) shear and tension forces for the dynamic rating of seats and head restraints.
Himmetoglu evaluated each of the seats to the IIWPG criteria and also used NICmax and maxSSI values to compare each of the concepts with each other. These results are shown in Figure 5a & 5b.
Max NIC (m2/s2)
Figure 5a. Maximum NIC Values- IIWPG standard pulse
Max. S shape index (deg.)
Figure 5b. Maximum S shape deformation (max SSI) - IIWPG standard pulse
As shown in figure 2, the abbreviation RG indicates basic rigid seat, used to simulate the seat used in JARI volunteer sled test.RO (recliner only) concept represents a rotational spring damper which allows the seatback to rotate with respect to fixed seat-pan. SPO (seat Pan only) has a horizontal translational spring damper which enables the whole seat to translate rearwards in order to facilitate the early contact between the head and the head restraint.
The project is primarily focussed on SPO concept design and thus performance of damper as stated in this case of rear impact is of special interest. It is important to specify at this stage that there is no rotational motion between seatback and seat pan. The seat design concept WMS represent the features of both SPO and RO respectively. DWMS concept is the extension of the WMS concept with an additional translational AWD inclined at 300 with horizontal providing g both rearward and backward motions simultaneously. The activation points for these rotational and translational AWD's are at âˆ†V Kph) > 4.5 and âˆ†V> 10 Kph respectively.
The downward motion is introduced to assist reducing the compressive forces induced due to spine straightening at very early stage of impact and a 300 rampage is allowed as smaller angles were not found to provide the necessary reduction in compressive forces, while higher angles were not found to be useful to limit neck internal motions (Himmetoglu et al., 2008) as well as they may give rise to large normal and frictional forces between the translational AWD and the supporting seat structure.
In RFWMS and DRFWMS, an inner seatback frame (SB) pivots about outer seat back frame at 'R*' as shown in figure above. When the pressure exerted by human torso on the inner seatback frame 'R' exceeds the breakaway torque at the rotational AWD at R*, the rotation at 'R* occurs in the opposite direction to that of outer seatback frame 'R' providing better occupant retention at high severity impacts by reducing the effective seatback angle. It also reduces the effective setback distance ensuring head to head restraint contact at the early stages of the impact. In both of these concepts, AWD's at R, R* and P are activated for âˆ†V (kph) > 4.5, 10.5 and 10.5 respectively. Thus it has been seen that sufficient care has been taken to ensure that activation of these passive AWD's is possible at a point higher than âˆ†V= 4.5kph to prevent activation during normal daily use.
Though a difference observed among the performance of last four anti-whiplash seat design concepts regarding their responses to the IIWPG standard pulse is very little, it is concluded that seats with inner seatback frame possess some advantages over the ones without the inner seatback frame. DRFWMS seat design concept performs better than all the other car seat design concepts as it does not let the head rise over the head restraint and also lowers the position of head relative to the vehicle floor. This offers good perception for the tall and unbelted occupants during rear impact. Therefore DRFWMS concept has been selected for further development especially for design of passive AWD's (In this case it is a translational damper related to point 'P' in design concepts) to reduce rearward displacement of seatback minimizing head to head restraint contact time.
Though DRWMFS seat design concept offers optimum solution to mitigate whiplash related injuries, in this project an attempt has been made to design a translational damper positioning parallel to the ground as discussed in car seat design concept RWMFS which is a combination of SPO and WMS with the addition of rotation of rotation of inner setback frame relative to outer one.
The damping profile obtained for translational AWD 'P' by S.Himmetoglu while is shown in figure 3.
Damping and stiffness profile for translational AWD in RFWMS
According to the results obtained through the simulation study, for seats designed to absorb energy (WMS, DWMS, RFWMS, and DRFWMS), maximum rearward displacement of the seat pan (âˆ†x SP) varies between 6 to 7 cm at severe crash pulse, and 4.6 to 5.4 cm for IIWPG crash pulse.
Development of whiplash testing:
Whiplash may occur in all impact directions but the injury has been most frequently observed in rear end impacts. The biomechanical guidelines available on the subject are not sufficient, due to the limited knowledge of the whiplash associated injury mechanism; though most of the researchers in the similar or the associated area believe that in case of rear impact collisions, improving design for the head restraint as well as vehicle seat improves safety against whiplash injuries.
Using the assumptions that reducing the load on the neck of the passenger lessens the probability of whiplash associated injuries, the first assessment method for vehicle seat and head restraint was established by 'International insurance whiplash protection group' popularly known as IIWHS (Research council for automobile repairs,2006) and the 'Swedish Road Administration' (SRA) (Folkswam,2005;Kraft et.al.,2005). Though both these organizations were established for the same purpose of improving car safety features; their viewpoints were different in terms of selecting criterion for seat performance. IIWHS concentrated more on the seat design validation using real world data while SRA put much emphasis to the plausible hypothetical cause of whiplash injuries.
Euro NCAP Test:
Euro NCAP (European New Car assessment Programme) testing procedure was established in 1997 with the purpose of providing consumers with a safety measures for most of the popular cars available in the European car market. The intention of this test from the beginning is to encourage car manufacturers to pay attention to the overall safety features of the car rather than concentrating on a particular part of the car (Hobs et.al, 1998).
The overall objective of Euro NCAP whiplash test is to reduce the real world whiplash associated injuries in EU-27 by promoting the best practise in seat designs among seat manufacturers and the customers.
Euro NCAP test procedure is an effective combination of IIWPG and SRA procedure with an element of amendment, due to the slight difference observed with the real world responses of above two mentioned test procedures with no substantial improvement in the knowledge of Whiplash injury mechanism (Kullergen et. al, 2007).
This test primarily focuses on the safety of the driver and the front passenger and has been extensively used before for frontal and sideways impact testing. Though Euro NCAP assessment procedure has been rarely used for rear crash testing, it has been considered as the probable cause of whiplash injuries and corresponding tests have been arranged as a part of future development strategy from 2000(Van Ratigen,2003).
The solution to reduce distance between head and head restraint in the early stages of rear impact was unanimously agreed. This includes supporting head in least possible time after an impact, so that head restraint can absorb the energy of impact and lower the differential movement between head and head restraint to reduce the risk of whiplash injuries. The overall performance of the seat system is governed by both geometric and dynamic characteristics; the test includes both static and dynamic assessments. The use of sled as a representative of cars during impact is promoted due to economic reasons. Though collision data collected in Euro NCAP protocol development programme indicates that majority of whiplash injuries occurred and sustained at âˆ†V of 16 Kph (10mph) in rear impact, insurance data indicates that injuries do occur at lower and higher speeds as well. Therefore in light of this Euro NCAP test consists of three sled tests simulating a variety of crash tests scenarios at varying âˆ†V values.
The test procedure involves three pulses of low, medium and high severities. The real world crash studies show that there exist a variety of crash pulses which forms the basis for the low severity crash pulse (European Car Assessment Programme, 2009). The medium severity pulse is a result of Insurance Industry Reo search featuring a number of car tests. High severity pulse is used to prevent long term injuries but these are not as frequently observed as the low and medium severity crash pulses. A BioRID III dummy is used and place in a standardised position restrained by a three point belt.
Thus JARI sled tests vary from these tests in the manner that in JARI sled tests only rigid seat was used for crash test simulation without any safety equipments such as seatbelt, but in Euro NCAP tests the BioRID dummy is restrained by seatbelt. Thus JARI sled test results as shown in published the research paper "Car seat design strategies to mitigate whiplash injuries" authored by Himmetoglu et al, 2007 considers extra measures for safety in the absence of seatbelt and head restraint.
PRODIUCT DSEIGN SPECIFICATION:
Damper System must allow controlled rearward displacement of whole passenger seat by absorbing crash energy in order to reduce the overall tensile and shear forces on the neck in the early phase of impact.
Seat Displacement must follow the force-displacement profile developed by S.Himmetoglu in his PhD thesis "Car seat design and human body modelling for rear impact whiplash mitigation", as closely as possible.
Maximum rearward displacement of seat pan (âˆ†x SP)with respect to fixed reference (essentially corner point of the seat) at severe crash pulse (âˆ†V=31KPh, a mean= 7.1g a max = 16 g) and IIWPG crash pulse (âˆ†V=16KPh, a mean= 5 g a max = 10 g) must be within 6 to 7 cm and 4.6 cm to 6.4 cm respectively.
The damping system must be activated when any of the following condition satisfies.
âˆ†V > 10.5KPh
Breakaway force (force exerted by occupant's torso on the seatback) â‰¥ 4KN
The passenger seat retraction after impact must be as quick as possible in order to assist the head restraint to minimize the head to head restraint contact time.
Damper unit must sustain maximum velocity of impact of 35Km/h.
Dimensions of the damper system must agree current average car seat dimensions. Ideally it should be placed under the seat to enable comfort of the rear passenger in case of family cars.
There should not be any positional limitations in case of sport CoupÑ as mentioned above.
The material selected for the damper application must be such that it should provide necessary damping profile for a passive translational AWD 'P' as shown in figure 3.
The weight of the material should be as small as possible, such that there should not be a significant increase in the gross weight of the car.
Ideally elastomeric materials should be considered for this application as they possess good damping properties and less weight per unit volume compared to that of metals as in case of metal actuators.
The damper system must work within the temperature range of -200C to 1000C.
The damper system must not produce any kind of hazardous gases, fumes or aerosols as a part of operation, which may cause dizziness to the occupants and or/ lower the performance of the car .
The market retail price of the damper unit must not exceed £ 20.
The damper unit must be build such that minimum or no modifications in the present design of car seat is necessary for most of the cars available in the market.
The damper unit must be easy to install and if possible should be modular to suit the requirements of different types of car seats.
The service life of the damper unit must be at least 10 years.
The damper unit must be aesthetic in overall appearance and suit the seat styling.
The damper system must provide optimum performance at all impact severities.
Weight of the damper unit must not exceed 5Kg.
Damper unit must be able to work with minimum monthly maintenance.
Ideally it must not involve any moving parts in order to facilitate the ease of maintenance.
The damper unit must not be operational within normal working conditions.