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Smart head restraint system
M. 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 of
Crashworthiness, 12: 4, 429 — 435
To link to this Article: DOI: 10.1080/13588260701483367
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Smart head restraint system
doi:10.1080/13588260701483367
M Acar, S J Clark and R Crouch
School 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 automotive
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applications to reduce whiplash injuries to occupants. This head restraint mechanism is combined with
a 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 into
an 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 the
desired position. A system demonstrator is built and successfully tested.
Key words: Head restraint, whiplash, smart/intelligent head restraint, occupant protection, rear impact.
specifically head restraint height and horizontal distance
‘setback’ of the head restraint from the occupant’s head can
have a significant influence on the likelihood and severity of
a whiplash injury in rear impact collisions. Recent research
by Thatcham indicated that 72% of front seat occupants
failed to adjust their head restraints correctly or had head
restraints that were incapable of offering any protection [3].
The Insurance Institute for Highway Safety in the USA
published similar results [4]. Thatcham’s research suggests
that the car occupants do not take adjusting head restraints
seriously and therefore there is a need for a solution to
alleviate the problem of whiplash injuries caused by rearend automotive crashes. Whiplash can be prevented with
a correctly positioned good head restraint system. To be
effective, a head restraint must be as close to the back of
the head as possible (touching is best) and the top of the
restraint should be as high as the top of the head. An active
head restraint mechanism that would adjust itself to the
correct position would potentially be a solution towards
solving the problem. The head restraint would always be
in the correct position to reduce the chance of receiving
whiplash injuries in the event of a rear impact.
This paper reports the design and development of a
concept for a smart head restraint system that adjusts itself
automatically to the correct position in order to reduce the
amount of whiplash injuries suffered by millions of people
all over the world.
INTRODUCTION
Whiplash is an injury of the cervical spine soft tissue,
caused by the sudden acceleration of the head relative to the
torso, which occurs during a rear-end collision, mostly occurring at low impact velocities, typically less than 20 km/h
[1, 2]. Injury statistics indicate that almost every fourth injury to car occupants is related to rear-end crashes, with
three quarters of these injuries being in the neck. Although
officially classed as a minor injury, whiplash can lead to
long, painful and debilitating symptoms for many years following a crash. Whiplash injuries typically cost the British
insurers over £1 billion annually and account for over 80%
of the total cost of personal injury claims [3]. It has been estimated that the direct cost of whiplash injuries in the USA
is approximately $4.5 billion annually. This cost shows an
increasing trend and reflects the insurance premiums paid
by the motorists. There are further unaccounted medical
costs and the cost of lost working days to the economy
because of whiplash injuries. Moreover, the reduction in
the quality of life of the whiplash sufferers is difficult to
quantify.
The primary method of preventing whiplash-induced
injury in motor vehicles is the use of a head restraint.
A well designed and correctly aligned head restraint will
vastly reduce the risk of injury to the head, spine and neck
during rear-end collisions [4]. Head restraint geometry,
Corresponding Author:
M Acar
School of Mechanical and Manufacturing Engineering
Loughborough University, Leicestershire
LE11 3TU, UK
Tel: +44 1509 227533; Fax: +44 1509 227648
E-mail: [email protected]
C Taylor & Francis Group
Copyright CURRENT WHIPLASH REDUCTION SOLUTIONS
Industrial ‘anti-whiplash’ products
Most head restraints are either of a fixed type or manually adjustable in the vertical direction for height setting. Some also have a tilt mechanism to allow limited
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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 suggested
and implemented. Some notable ones are mentioned here.
The self-aligning head restraint (SAHR) system has
been in production since 1997. During a rear-end impact,
the head restraint moves upward and forward via a mechanical linkage, supporting the head before the relative motion
of the head and torso becomes significant. The system is
activated by the force of the occupant’s body pressing the
seat backrest during a rear impact. When the load of the
occupant decreases, the mechanism returns the head restraint to its original position. No replacement of the seat
is required after an accident since it resets itself. Saab’s
SAHR was found to be effective in reducing whiplash injuries [5]. Similar designs were also adopted by a number
of other companies.
Autoliv developed the Whiplash Protection System
(WHIPS) for Volvo that has proven to be effective in rearend impacts. Initially, a pair of four-bar linkages on both
sides of the seat that hinge the backrest to the seat pan allow
the seatback to move backwards, thus allowing the torso to
move together with the head, reducing relative displacement. Then a plastically deformable element allows tilting
of the seatback rearwards to allow further energy absorption, which needs to be replaced after a rear-end collision
[6, 7].
The Toyota Whiplash Injury Lessening (WIL) system
aims to reduce the relative motion between occupant’s head
and torso [8]. This is achieved by seatback and cushion
design that yields in a controlled manner during a rear
impact to reduce the whiplash effect. The WIL seat was
included as a standard component for 2000 Toyota Avalon,
Celica and Echo [9].
Mercedes Benz have recently introduced an active head
restraint, using an on-board crash sensor that activates a
linkage in the head restraint which pushes it forward and
upward to close the gap between the occupant’s head and
head restraint [3].
Further ‘anti-whiplash’ concepts
In 1999, Autoliv launched the Self Inflating Head Restraint
(SIHR), an anti-whiplash system specially developed for
rear-seat occupants. During rear impact, air is pressed from
a large bag in the backrest to a small bag in the head restraint, 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 a
similar mechanical principle to that developed by Autoliv
was reported [13]. The difference was that the airbag is
inflated around the neck supporting it, instead of extending
outwards by the pressure the head exerts on the headrest
itself.
An inflatable head restraint, with an integrated airbag
into the foam, was also reported [14]. It would enlarge
sufficiently in all directions to protect occupants of all sizes
independently from the pre-adjusted position.
A whiplash protection device in the form of a collapsing
spring consisting of two shallow angled cone tubes on a
double-ended conical mandrel was suggested [15]. One of
the tubes is attached to the seat and the other to the floor.
During rear impact the conical tubes are pushed onto the
mandrel and deform elastically allowing a translational motion of the seat and locking the tubes preventing springing
back.
A damping seat slide system was developed which reduces the acceleration of the occupant, thus reducing the
relative motion between the head and torso, which in
turn reduces the whiplash effect [16]. The seat slide was
mounted between the seat base and the car floor, allowing
a damped translational motion of the seat backwards. The
damping mechanism is triggered by a sensor.
SENSOR APPLICATIONS IN OCCUPANT DETECTION
The primary application of sensor systems in passenger
cars is generally for occupant detection in conjunction with
airbag deployment. Stereovision systems provide information about the occupant presence and location within
the vehicle to improve the control of the airbag firing
[17]. Autoliv also have an occupant detection and classification system ready for production which uses an arrangement of five ultrasonic sensors in combination with a
weight sensor integrated into the seat, a seat position sensor and a sensor in the buckle to determine the size and
sitting position of the seat occupant [18]. Various other
sensing technologies have been tested for similar applications but are not expected to reach production within
the 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 head
restraint applications. Massara [22] reports experiments
using ultrasonic sensors mounted on top of a head restraint. He successfully used this arrangement to detect
Whiplash protection for the aftermarket
The Autoliv Anti-Whiplash System (AWS) for the aftermarket for front-seat occupants consists of a metal sheet
with tear grooves that is installed between the seat rails
and the seat. When the vehicle is hit from behind, the tear
grooves absorb the impact by gradually ripping apart. The
seat then pivots rearwards in a controlled manner instead
of violently pushing the occupant’s shoulders and lower
neck forward [10].
WipGARD, also developed by Autoliv, is a yielding
device that could be fitted to the existing seat rail, which
allows the seat to move back horizontally and rotate to the
rear. WipGARD reduces the severity of the S-curve and
decreases the rebound speed at which the body will be
thrown forward [11]. It is also designed to give protection
to smaller, lighter occupants such as women.
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Smart head restraint system
Figure 1 The horizontal motion (setback) mechanism: (a) top view showing details of the design, (b) front view with setback
wheel in the low position, and (c) with setback wheel in the high position.
The horizontal and vertical movements of the head restraint are controlled by two separate DC motors placed in
the backrest of the seat.
the presence and seated height of occupants with a range
of hairstyles and sitting positions. He concluded that ultrasonic technology provides a cost-effective, high performance solution and that a sensor-based active head restraint
was feasible. He did not however build a full prototype; neither did he use the ultrasonic sensors to detect the backset.
Setback mechanism (the horizontal motion)
Figure 1(a) shows the details of the design for the setback
mechanism which enables the horizontal motion of the
headrest. In normal operation the tension in the motor
cable keeps the setback wheel in the down position (Figure
1(b)). When the setback wheel is in the down position the
second cable is taut and the sprung loaded locking pins
are not engaged with the setback shafts. This means the
mechanism can move back and forth freely in the horizontal
plane.
In the event of a rear impact the occupant’s head hits
the head restraint. As soon as the head restraint begins
to move rearwards the motor cable becomes slack. The
setback springs are then compressed and absorb the kinetic
energy from the head. Once the cable is slack, the sprung
loaded locking pins are able to move outwards pulling the
setback wheel upwards (Figure 1(c)) and act like a ratchet
mechanism preventing the setback springs forcing the head
restraint forward. The system is reset when the tension in
the cable provided by the motor becomes great enough to
pull the setback wheel downwards again, disengaging the
locking pins allowing free movement once more.
SMART HEAD RESTRAINT DESIGN
The mechanical design concept
None of the so-called active head restraints currently on
the market are ‘active’ in the sense that they only move into
position once an impact has occurred. The truly active head
restraint mechanism proposed in this paper will move the
head restraint into an optimum position and continuously
monitor the changes in the position of the head and make
adjustments, and be always in a state of readiness for an
impact to occur. The major benefit of this system is the fact
that it will take over the responsibility of adjusting the head
restraint from the occupant and the restraint will always be
in the optimum position before the impact takes place.
The system consists of mechanical horizontal and vertical adjustment mechanisms with actuators, sensors to detect and monitor the position of the occupant’s head, and
control software that continuously adjusts the position of
the head restraint.
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M Acar, S J Clark and R Crouch
Height adjustment (the vertical motion mechanism)
The vertical system is designed such that the restraint cannot accidentally be pushed in by passengers sitting at the
rear of the car by pulling it down. The horizontal system
also has a spring mechanism that absorbs energy during the
impact. Hence, the vertical mechanism has been designed
by using a 40:1 ratio worm and wheel. The vertical mechanism will be mounted in the seat back as low as possible to
keep the centre of gravity of the whole mechanism as low as
possible. The system resets itself and is therefore reusable
unless permanent damage to the system occurs during a
severe impact.
Figure 2 shows the complete assembly of the CAD
model for the proof of concept mechanical system.
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Sensor system development
Several sensor types have been investigated and infrared
and ultrasonic sensors have been identified as having sufficient potential for detection of the occupant’s head position. By testing a sample of each sensor it would be possible
to discover which is more appropriate for a smart head restraint system.
Daventech SRF04 ultrasonic range finder and Sharp
GP2D12 infrared sensor were chosen for further investigation. Calibration tests showed that both sensors were
reasonably accurate in detecting distance as shown in Figure 3. Readings were taken from the sensors at 5 cm intervals from 10 cm to 60 cm. The tests were conducted in
normal daylight with no known other specific sources of
interference existing.
The sensors ability to operate in a variety of conditions
was tested in order to simulate the vehicle cabin environment. The first stage of testing was to see how the sensors
respond to different hair types. Four volunteers, each with
a different hair type, namely black, blonde and red hair
and a bald head were tested. Readings were taken from the
sensors at the same test conditions used during calibration.
Ultrasonic sensors were least affected as shown in Figure 4
by 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 interference sources is far less than the effect of the hair types
Figure 2 The proof of the concept mechanical system design
assembly.
Figure 3 Sensor performance after calibration.
The main advantage of this design is that the springs
are able to absorb the energy from a rear impact in a controlled manner, without releasing that energy back onto
the occupant’s head.
Figure 4 Sensor performance during volunteer testing.
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Figure 5 Sensor performance during interference source testing.
Figure 7 Head restraint system demonstrator and the
volunteers; sensors are attached to the head restraint.
Figure 6 Dual sensor arrangement.
(Figure 5). The ultrasonic sensor performed very well on
all the interference tests. The ultrasonic sensor, therefore,
has been selected for the prototype development.
The compact size of the sensor allows a variety of attachment locations within the vehicle to be considered. The
sensor must not be at any great risk from accidental damage
and it must not pose any threat of injury to any vehicle occupant during normal operation or a crash situation. The
location of the sensor must also allow it to determine the
position of the occupant’s head over a wide range to ensure
moderate amounts of seat occupant movement can be detected. Various alternatives include roof lining and A and
B pillars of the car. However the head restraint is identified as the most suitable position for the sensors, since the
position of the head relative to the head restraint can be
determined directly. In addition there is sufficient space to
conceal the sensor and there will be no distraction to the
seat occupant as the sensor will be out of their line of sight.
One further major benefit of this location is that a single
sensor can detect both the distance and backset position of
the head by moving the head restraint up and down to scan
the area in which the seat occupant’s head would be.
A dual sensor system as shown in Figure 6 has the benefit of increased area over which the head is detectable. This
would allow for a larger amount of OOP movement by the
seat occupant before their head moved out of detectable
range. Precisely how much extra area is gained with a dual
senor arrangement is hard to quantify without mapping out
the detectable zone, but from inspection the dual sensor
arrangement is clearly superior. There is also some reliabilC Taylor & Francis Group
Copyright ity benefit: if one sensor should fail the other will provide
some degree of operational ability until the system can be
fixed. Due to these factors a dual sensor arrangement will
be developed for the demonstrator model.
Integration and testing of the systems
Control software has been written and implemented to
enable continuous measurements of the position of the
occupant’s head and adjustment of the head restraint to
a position for reduced whiplash injury. The control software measures the horizontal and vertical distance between
the head restraint and the occupant’s head, and then compares this with the predetermined values. If the measured
distances are outside the acceptable range with respect to
the head restraint, then the control algorithm instructs the
head restraint to move to the correct position. The position
of the head is continuously monitored and the mechanisms
are operated when an adjustment of the head restraint is
necessary. The sampling frequency of the measurement of
the position of the head and the adjustment of the head
restraint can be chosen to avoid micro adjustments.
For testing purposes the sensors were simply attached
to the head restraint foam approximately 14 cm apart facing outwards towards the head (Figure 7). In a production ready system the sensors would require a significant
amount of concealment to ensure they are not susceptible
to accidental damage. First the mechanical system, sensors
and control circuitry were tested separately to ensure that
each system was operating correctly.
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M Acar, S J Clark and R Crouch
To conduct the testing on the smart head restraint system as a whole, the sensors and control circuitry were fully
integrated. Then the accuracy, consistency and reliability of the system has been tested repeatedly to ensure it
could locate a seat occupant’s head and move the head restraint into a position that would provide protection against
whiplash injuries.
The conclusion to be drawn from the testing is that the
smart head restraint system concept determines the position of an occupant’s head and adjusts the head restraint to
the optimum position with reasonable accuracy. The integrated sensors, actuators, control software and mechanical
system worked well together.
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4. Insurance Institute Highway Safety, ‘Head restraint can
protect our necks’, 2001, Insurance Institute Status Report. 36
(9), 8pp.
5. K Wiklund and H Larsson, ‘Saab active head restraint
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Effekanalys av nackskadeforskningen vid Chalmers, Vinnova
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DISCUSSION AND CONCLUSIONS
A full-scale, proof of concept working demonstrator of a
smart head restraint system that detects and monitors the
occupant’s head and adjusts to the correct position automatically was built and tested successfully. The smart
head restraint demonstrator proves that the concept of a
sensor-based system was entirely feasible offering an effective solution to a problem that is not currently being
addressed to a satisfactory degree in the automotive industry. This system greatly increases the likelihood of the head
restraint being correctly adjusted to a position at the point
of impact for maximum whiplash protection with potential
safety benefits and associated economic savings.
Whilst the accuracy and consistency of the prototype
has been tested to a satisfactory standard, it would be beneficial to build a system to industry standards and conduct
industrial trials. We demonstrated that the system has a
great potential to significantly reduce whiplash injuries in
rear-end impacts and we anticipate that, not in a too distant
future, vehicles will be fitted with such a system.
ACKNOWLEDGEMENTS
The authors wish to express their thanks to the organizers
of the Enhanced Safety of Vehicles (ESV) International
Collegiate Student Safety Technology Design Competition, June 2005, Washington DC, for the financial assistance that they received when the project was selected as
one of the two representatives from the European region
to compete as one of the six world finalists at the competition. Authors also gratefully received the award of ‘First
Place Paper’ of the 2005 Student Safety Engineering Design Contest of the Safety Engineering and Risk Analysis
Division (SERAD) of the American Society of Mechanical
Engineers (ASME).
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