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International Journal of CrashworthinessPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t778188386

Smart head restraint systemM. Acara; S. J. Clarka; R. Croucha

a School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, UK

To cite this Article Acar, M. , Clark, S. J. and Crouch, R.(2007) 'Smart head restraint system', International Journal ofCrashworthiness, 12: 4, 429 — 435To link to this Article: DOI: 10.1080/13588260701483367URL: http://dx.doi.org/10.1080/13588260701483367

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Smart head restraint systemdoi:10.1080/13588260701483367

M Acar, S J Clark and R CrouchSchool of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, LE11 3TU, UK

Abstract: This paper reports the design and development of a novel head restraint system for automotiveapplications to reduce whiplash injuries to occupants. This head restraint mechanism is combined witha pair of ultrasonic sensors, actuators and control algorithm to form an active head restraint system,which detects the position of an occupant’s head when seated in the car and moves the head restraint intoan optimum position to reduce the chances of receiving whiplash injuries in the event of a rear impact.The system monitors significant changes in the position of the head and moves the head restraint to thedesired position. A system demonstrator is built and successfully tested.

Key words: Head restraint, whiplash, smart/intelligent head restraint, occupant protection, rear impact.

INTRODUCTION

Whiplash is an injury of the cervical spine soft tissue,caused by the sudden acceleration of the head relative to thetorso, which occurs during a rear-end collision, mostly oc-curring at low impact velocities, typically less than 20 km/h[1, 2]. Injury statistics indicate that almost every fourth in-jury to car occupants is related to rear-end crashes, withthree quarters of these injuries being in the neck. Althoughofficially classed as a minor injury, whiplash can lead tolong, painful and debilitating symptoms for many years fol-lowing a crash. Whiplash injuries typically cost the Britishinsurers over £1 billion annually and account for over 80%of the total cost of personal injury claims [3]. It has been es-timated that the direct cost of whiplash injuries in the USAis approximately $4.5 billion annually. This cost shows anincreasing trend and reflects the insurance premiums paidby the motorists. There are further unaccounted medicalcosts and the cost of lost working days to the economybecause of whiplash injuries. Moreover, the reduction inthe quality of life of the whiplash sufferers is difficult toquantify.

The primary method of preventing whiplash-inducedinjury in motor vehicles is the use of a head restraint.A well designed and correctly aligned head restraint willvastly reduce the risk of injury to the head, spine and neckduring rear-end collisions [4]. Head restraint geometry,

Corresponding Author:M AcarSchool of Mechanical and Manufacturing EngineeringLoughborough University, LeicestershireLE11 3TU, UKTel: +44 1509 227533; Fax: +44 1509 227648E-mail: [email protected]

specifically head restraint height and horizontal distance‘setback’ of the head restraint from the occupant’s head canhave a significant influence on the likelihood and severity ofa whiplash injury in rear impact collisions. Recent researchby Thatcham indicated that 72% of front seat occupantsfailed to adjust their head restraints correctly or had headrestraints that were incapable of offering any protection [3].The Insurance Institute for Highway Safety in the USApublished similar results [4]. Thatcham’s research suggeststhat the car occupants do not take adjusting head restraintsseriously and therefore there is a need for a solution toalleviate the problem of whiplash injuries caused by rear-end automotive crashes. Whiplash can be prevented witha correctly positioned good head restraint system. To beeffective, a head restraint must be as close to the back ofthe head as possible (touching is best) and the top of therestraint should be as high as the top of the head. An activehead restraint mechanism that would adjust itself to thecorrect position would potentially be a solution towardssolving the problem. The head restraint would always bein the correct position to reduce the chance of receivingwhiplash injuries in the event of a rear impact.

This paper reports the design and development of aconcept for a smart head restraint system that adjusts itselfautomatically to the correct position in order to reduce theamount of whiplash injuries suffered by millions of peopleall over the world.

CURRENT WHIPLASH REDUCTION SOLUTIONS

Industrial ‘anti-whiplash’ products

Most head restraints are either of a fixed type or man-ually adjustable in the vertical direction for height set-ting. Some also have a tilt mechanism to allow limited

Copyright C© Taylor & Francis Group 429 TCRS 2007 Vol. 12 No. 4 pp. 429–435

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M Acar, S J Clark and R Crouch

setback adjustments. In recent years, a number of novel‘anti-whiplash’ head restraint designs have been suggestedand implemented. Some notable ones are mentioned here.

The self-aligning head restraint (SAHR) system hasbeen in production since 1997. During a rear-end impact,the head restraint moves upward and forward via a mechan-ical linkage, supporting the head before the relative motionof the head and torso becomes significant. The system isactivated by the force of the occupant’s body pressing theseat backrest during a rear impact. When the load of theoccupant decreases, the mechanism returns the head re-straint to its original position. No replacement of the seatis required after an accident since it resets itself. Saab’sSAHR was found to be effective in reducing whiplash in-juries [5]. Similar designs were also adopted by a numberof other companies.

Autoliv developed the Whiplash Protection System(WHIPS) for Volvo that has proven to be effective in rear-end impacts. Initially, a pair of four-bar linkages on bothsides of the seat that hinge the backrest to the seat pan allowthe seatback to move backwards, thus allowing the torso tomove together with the head, reducing relative displace-ment. Then a plastically deformable element allows tiltingof the seatback rearwards to allow further energy absorp-tion, which needs to be replaced after a rear-end collision[6, 7].

The Toyota Whiplash Injury Lessening (WIL) systemaims to reduce the relative motion between occupant’s headand torso [8]. This is achieved by seatback and cushiondesign that yields in a controlled manner during a rearimpact to reduce the whiplash effect. The WIL seat wasincluded as a standard component for 2000 Toyota Avalon,Celica and Echo [9].

Mercedes Benz have recently introduced an active headrestraint, using an on-board crash sensor that activates alinkage in the head restraint which pushes it forward andupward to close the gap between the occupant’s head andhead restraint [3].

Whiplash protection for the aftermarket

The Autoliv Anti-Whiplash System (AWS) for the after-market for front-seat occupants consists of a metal sheetwith tear grooves that is installed between the seat railsand the seat. When the vehicle is hit from behind, the teargrooves absorb the impact by gradually ripping apart. Theseat then pivots rearwards in a controlled manner insteadof violently pushing the occupant’s shoulders and lowerneck forward [10].

WipGARD, also developed by Autoliv, is a yieldingdevice that could be fitted to the existing seat rail, whichallows the seat to move back horizontally and rotate to therear. WipGARD reduces the severity of the S-curve anddecreases the rebound speed at which the body will bethrown forward [11]. It is also designed to give protectionto smaller, lighter occupants such as women.

Further ‘anti-whiplash’ concepts

In 1999, Autoliv launched the Self Inflating Head Restraint(SIHR), an anti-whiplash system specially developed forrear-seat occupants. During rear impact, air is pressed froma large bag in the backrest to a small bag in the head re-straint, which then moves forward to reduce, or eliminate,gaps between the occupant’s head and the head restraint[12].

An airbag-based inflatable system that operates on asimilar mechanical principle to that developed by Autolivwas reported [13]. The difference was that the airbag isinflated around the neck supporting it, instead of extendingoutwards by the pressure the head exerts on the headrestitself.

An inflatable head restraint, with an integrated airbaginto the foam, was also reported [14]. It would enlargesufficiently in all directions to protect occupants of all sizesindependently from the pre-adjusted position.

A whiplash protection device in the form of a collapsingspring consisting of two shallow angled cone tubes on adouble-ended conical mandrel was suggested [15]. One ofthe tubes is attached to the seat and the other to the floor.During rear impact the conical tubes are pushed onto themandrel and deform elastically allowing a translational mo-tion of the seat and locking the tubes preventing springingback.

A damping seat slide system was developed which re-duces the acceleration of the occupant, thus reducing therelative motion between the head and torso, which inturn reduces the whiplash effect [16]. The seat slide wasmounted between the seat base and the car floor, allowinga damped translational motion of the seat backwards. Thedamping mechanism is triggered by a sensor.

SENSOR APPLICATIONS IN OCCUPANT DETECTION

The primary application of sensor systems in passengercars is generally for occupant detection in conjunction withairbag deployment. Stereovision systems provide infor-mation about the occupant presence and location withinthe vehicle to improve the control of the airbag firing[17]. Autoliv also have an occupant detection and clas-sification system ready for production which uses an ar-rangement of five ultrasonic sensors in combination with aweight sensor integrated into the seat, a seat position sen-sor and a sensor in the buckle to determine the size andsitting position of the seat occupant [18]. Various othersensing technologies have been tested for similar appli-cations but are not expected to reach production withinthe next five years. These include: infrared displacement[19], thermal imaging [20] and omni-directional cameras[21].

Sensor-based systems have not yet been used in headrestraint applications. Massara [22] reports experimentsusing ultrasonic sensors mounted on top of a head re-straint. He successfully used this arrangement to detect

430TCRS 2007 Vol. 12 No. 4 Copyright C© Taylor & Francis Group


Smart head restraint system

Figure 1 The horizontal motion (setback) mechanism: (a) top view showing details of the design, (b) front view with setbackwheel in the low position, and (c) with setback wheel in the high position.

the presence and seated height of occupants with a rangeof hairstyles and sitting positions. He concluded that ul-trasonic technology provides a cost-effective, high perfor-mance solution and that a sensor-based active head restraintwas feasible. He did not however build a full prototype; nei-ther did he use the ultrasonic sensors to detect the backset.

SMART HEAD RESTRAINT DESIGN

The mechanical design concept

None of the so-called active head restraints currently onthe market are ‘active’ in the sense that they only move intoposition once an impact has occurred. The truly active headrestraint mechanism proposed in this paper will move thehead restraint into an optimum position and continuouslymonitor the changes in the position of the head and makeadjustments, and be always in a state of readiness for animpact to occur. The major benefit of this system is the factthat it will take over the responsibility of adjusting the headrestraint from the occupant and the restraint will always bein the optimum position before the impact takes place.

The system consists of mechanical horizontal and verti-cal adjustment mechanisms with actuators, sensors to de-tect and monitor the position of the occupant’s head, andcontrol software that continuously adjusts the position ofthe head restraint.

The horizontal and vertical movements of the head re-straint are controlled by two separate DC motors placed inthe backrest of the seat.

Setback mechanism (the horizontal motion)

Figure 1(a) shows the details of the design for the setbackmechanism which enables the horizontal motion of theheadrest. In normal operation the tension in the motorcable keeps the setback wheel in the down position (Figure1(b)). When the setback wheel is in the down position thesecond cable is taut and the sprung loaded locking pinsare not engaged with the setback shafts. This means themechanism can move back and forth freely in the horizontalplane.

In the event of a rear impact the occupant’s head hitsthe head restraint. As soon as the head restraint beginsto move rearwards the motor cable becomes slack. Thesetback springs are then compressed and absorb the kineticenergy from the head. Once the cable is slack, the sprungloaded locking pins are able to move outwards pulling thesetback wheel upwards (Figure 1(c)) and act like a ratchetmechanism preventing the setback springs forcing the headrestraint forward. The system is reset when the tension inthe cable provided by the motor becomes great enough topull the setback wheel downwards again, disengaging thelocking pins allowing free movement once more.

431Copyright C© Taylor & Francis Group TCRS 2007 Vol. 12 No. 4



Figure 2 The proof of the concept mechanical system designassembly.

Figure 3 Sensor performance after calibration.

The main advantage of this design is that the springsare able to absorb the energy from a rear impact in a con-trolled manner, without releasing that energy back ontothe occupant’s head.

Height adjustment (the vertical motion mechanism)

The vertical system is designed such that the restraint can-not accidentally be pushed in by passengers sitting at therear of the car by pulling it down. The horizontal systemalso has a spring mechanism that absorbs energy during theimpact. Hence, the vertical mechanism has been designedby using a 40:1 ratio worm and wheel. The vertical mecha-nism will be mounted in the seat back as low as possible tokeep the centre of gravity of the whole mechanism as low aspossible. The system resets itself and is therefore reusableunless permanent damage to the system occurs during asevere impact.

Figure 2 shows the complete assembly of the CADmodel for the proof of concept mechanical system.

Sensor system development

Several sensor types have been investigated and infraredand ultrasonic sensors have been identified as having suf-ficient potential for detection of the occupant’s head posi-tion. By testing a sample of each sensor it would be possibleto discover which is more appropriate for a smart head re-straint system.

Daventech SRF04 ultrasonic range finder and SharpGP2D12 infrared sensor were chosen for further inves-tigation. Calibration tests showed that both sensors werereasonably accurate in detecting distance as shown in Fig-ure 3. Readings were taken from the sensors at 5 cm in-tervals from 10 cm to 60 cm. The tests were conducted innormal daylight with no known other specific sources ofinterference existing.

The sensors ability to operate in a variety of conditionswas tested in order to simulate the vehicle cabin environ-ment. The first stage of testing was to see how the sensorsrespond to different hair types. Four volunteers, each witha different hair type, namely black, blonde and red hairand a bald head were tested. Readings were taken from thesensors at the same test conditions used during calibration.Ultrasonic sensors were least affected as shown in Figure 4by different hair types.

Common sources of interference such as bright light,darkness, loud music and a mobile phone were also tested.It is clear from the test results that the effect of the inter-ference sources is far less than the effect of the hair types

Figure 4 Sensor performance during volunteer testing.




Figure 5 Sensor performance during interference source testing.

Figure 6 Dual sensor arrangement.

(Figure 5). The ultrasonic sensor performed very well onall the interference tests. The ultrasonic sensor, therefore,has been selected for the prototype development.

The compact size of the sensor allows a variety of at-tachment locations within the vehicle to be considered. Thesensor must not be at any great risk from accidental damageand it must not pose any threat of injury to any vehicle oc-cupant during normal operation or a crash situation. Thelocation of the sensor must also allow it to determine theposition of the occupant’s head over a wide range to ensuremoderate amounts of seat occupant movement can be de-tected. Various alternatives include roof lining and A andB pillars of the car. However the head restraint is identi-fied as the most suitable position for the sensors, since theposition of the head relative to the head restraint can bedetermined directly. In addition there is sufficient space toconceal the sensor and there will be no distraction to theseat occupant as the sensor will be out of their line of sight.One further major benefit of this location is that a singlesensor can detect both the distance and backset position ofthe head by moving the head restraint up and down to scanthe area in which the seat occupant’s head would be.

A dual sensor system as shown in Figure 6 has the bene-fit of increased area over which the head is detectable. Thiswould allow for a larger amount of OOP movement by theseat occupant before their head moved out of detectablerange. Precisely how much extra area is gained with a dualsenor arrangement is hard to quantify without mapping outthe detectable zone, but from inspection the dual sensorarrangement is clearly superior. There is also some reliabil-

Figure 7 Head restraint system demonstrator and thevolunteers; sensors are attached to the head restraint.

ity benefit: if one sensor should fail the other will providesome degree of operational ability until the system can befixed. Due to these factors a dual sensor arrangement willbe developed for the demonstrator model.

Integration and testing of the systems

Control software has been written and implemented toenable continuous measurements of the position of theoccupant’s head and adjustment of the head restraint toa position for reduced whiplash injury. The control soft-ware measures the horizontal and vertical distance betweenthe head restraint and the occupant’s head, and then com-pares this with the predetermined values. If the measureddistances are outside the acceptable range with respect tothe head restraint, then the control algorithm instructs thehead restraint to move to the correct position. The positionof the head is continuously monitored and the mechanismsare operated when an adjustment of the head restraint isnecessary. The sampling frequency of the measurement ofthe position of the head and the adjustment of the headrestraint can be chosen to avoid micro adjustments.

For testing purposes the sensors were simply attachedto the head restraint foam approximately 14 cm apart fac-ing outwards towards the head (Figure 7). In a produc-tion ready system the sensors would require a significantamount of concealment to ensure they are not susceptibleto accidental damage. First the mechanical system, sensorsand control circuitry were tested separately to ensure thateach system was operating correctly.




To conduct the testing on the smart head restraint sys-tem as a whole, the sensors and control circuitry were fullyintegrated. Then the accuracy, consistency and reliabil-ity of the system has been tested repeatedly to ensure itcould locate a seat occupant’s head and move the head re-straint into a position that would provide protection againstwhiplash injuries.

The conclusion to be drawn from the testing is that thesmart head restraint system concept determines the posi-tion of an occupant’s head and adjusts the head restraint tothe optimum position with reasonable accuracy. The inte-grated sensors, actuators, control software and mechanicalsystem worked well together.

DISCUSSION AND CONCLUSIONS

A full-scale, proof of concept working demonstrator of asmart head restraint system that detects and monitors theoccupant’s head and adjusts to the correct position au-tomatically was built and tested successfully. The smarthead restraint demonstrator proves that the concept of asensor-based system was entirely feasible offering an ef-fective solution to a problem that is not currently beingaddressed to a satisfactory degree in the automotive indus-try. This system greatly increases the likelihood of the headrestraint being correctly adjusted to a position at the pointof impact for maximum whiplash protection with potentialsafety benefits and associated economic savings.

Whilst the accuracy and consistency of the prototypehas been tested to a satisfactory standard, it would be ben-eficial to build a system to industry standards and conductindustrial trials. We demonstrated that the system has agreat potential to significantly reduce whiplash injuries inrear-end impacts and we anticipate that, not in a too distantfuture, vehicles will be fitted with such a system.

ACKNOWLEDGEMENTS

The authors wish to express their thanks to the organizersof the Enhanced Safety of Vehicles (ESV) InternationalCollegiate Student Safety Technology Design Competi-tion, June 2005, Washington DC, for the financial assis-tance that they received when the project was selected asone of the two representatives from the European regionto compete as one of the six world finalists at the compe-tition. Authors also gratefully received the award of ‘FirstPlace Paper’ of the 2005 Student Safety Engineering De-sign Contest of the Safety Engineering and Risk AnalysisDivision (SERAD) of the American Society of MechanicalEngineers (ASME).

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