Continental Concept Vehicle Shortens Reaction and Pressure Build-up Time
With Braking Distance Reduced,
Reaction Distance Is Now Shorter, Too
Frankfurt, September 2001
At the IAA 2001, Continental AG is presenting a technological innovation based on a compact-size car that clearly reduces the stopping distance using sensor technology for monitoring the vehicle environment and the driver’s reaction. This device boosts the company’s
efforts to increase traffic safety in general. As a first step, the company project group “Reduced Stopping Distance” concentrated on the optimization of the pure chassis system reactions. This led to a concept vehicle based on a compact car fitted with concept tires,
electro-hydraulic brake (EHB) and controlled air-suspension chassis achieving a stopping
distance of 30 meters and less at 100 km/h in December 2000. Series vehicles in this
class achieved approx. 38.5 meters.
Minimizing the reaction distance
Continental combined together it´s corporate know-how and optimized the driver’s/vehicle’s reaction time in the second phase of the “Reduced Stopping Distance” project. That not only means an improvement of driving safety but also a dramatic increase in
traffic safety overall. Tests carried out by TÜV Rhineland/Brandenburg show that 50% of
all rear-end collisions and accidents at crossroads as well as 30% of all car-to-car frontend impacts could have been avoided if this reaction time for the driver/vehicle system had
been reduced by only 0.5 seconds. These accidents are responsible for 40% of all road
traffic deaths.
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The reaction time for the driver/vehicle system consists of three phases:
1. The reaction time (human factor) is the time the driver requires to become aware of a
critical situation using his senses, to process this awareness mentally and to then move
his foot from the accelerator to the brake pedal.
2. The pressure build-up time (human/machine factor and their interaction) is the time period starting from when the driver first touches the brake pedal and increases pressure
on the pedal and in which the brake pressure builds up to the final value that the driver
gives to the brakes.
3. The braking time (machine factor) is the time span in which the wheel brakes eliminate
the remaining kinetic energy of the vehicle up to a full standstill at full braking effect.
Corresponding distances are allocated to these times in which the vehicle continues to
move forward. If a hesitant car driver of normal experience brakes in an emergency, it is
not unusual for these distances to add up to a stopping distance of 80 meters. If one assumes a reaction time of 1.0 second, the vehicle will cover 27.8 meters at a speed of 100
km/h before the driver makes his first contact with the pedal. Creating an optimization here
will shorten the stopping distance considerably as the car continues to drive on during the
reaction time at almost brakeless starting speed. Only the drag torque from the engine and
drive train has a slight delaying effect. There is still a great deal of potential for shortening
stopping distance during pressure build-up time. This was used in greater depth in the
second phase of the project.
Human action moves into the center of the development
Human action and human ergonomics are increasingly becoming part of the brake system
design in the reduction of reaction and pressure build-up distance. It is the developers' objective to support the driver in coping with critical situations by integrating technology actively into his actions. In brief, the car should help the person at the wheel to carry out an
emergency stop better and faster. Development is chiefly aimed at average drivers of
normal experience, as these constitute the main body of road users all over the world.
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At the same time the highly concentrated “driving pro” must of course not be at a disadvantage. The development work has therefore not left out active driving pleasure.
The concept vehicle presented at the IAA 2001 has a system network of selected components that show a promising way of shortening all three phases of the stopping distance
(reaction distance, pressure build-up distance and braking distance) for the normal driver.
The technological innovation
The vehicle runs on the concept tires with SWT that were already used for the 30-meter
car project at the end of last year. The tire's rubber compound had been optimized with a
view to a later series manufacture. Further components are an electro-hydraulic brake with
brake assist function and a controlled air-suspension chassis. These chassis subsystems
that communicate with one another, give priority to shortening the stage of pressure buildup distance dominated by the braking system as well as the car’s braking distance.
The concept vehicle for the year 2001 also has a sensory mechanism for monitoring the
vehicle’s environment and the driver’s reaction, shortening the reaction time and the pressure build-up time phase determined by human action.
Front-mounted radar monitors the vehicle environment
The sensory mechanism consists of a 77GHz front-mounted radar made by A.D.C. (ContiTemic), which is also a central component of the headway distance control system ACC
(Adaptive Cruise Control). This radar monitors traffic on the lane in use and enables the
ACC to adjust the set desired speed to the actual traffic situation taking the specified safety distance into consideration. The front-mounted radar in the concept vehicle also fulfils
the task of measuring the distance to the car in the front. The technological innovation also
monitors driver behavior with the aid of sensors.
The electronic brake controller now analyzes all information (that from the front-mounted
radar, as well as the pedal sensors) and with the aid of special analysis algorithms
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recognizes hazardous situations that arise from the driving speed of both vehicles, the distance between them and the speed with which the distance between the vehicles changes.
The system introduces measures in various degrees for reducing the stopping distance
dependent on the profile of this hazardous situation.
Assistance in an emergency situation
If the vehicle in front slows down appreciably, the ACC-front-mounted radar recognizes
this faster than the driver with average experience. Relative acceleration and distances are
difficult to estimate even for the most attentive person. Under certain circumstances inattentive drivers are not aware of approaching danger. The system now prompts the person
to ensure a safe distance by using optical or acoustic warning signals. Should he not react,
further system prompts are possible such as soft braking to bring the driver’s attention
back to what is happening in the traffic.
Monitoring driver reaction
If the driver has now recognized the hazardous situation himself – possibly thanks to assistance from the car – and decides to do something about it, the analysis of his foot
movements gives the vehicle electronics clear information on the approaching emergency
stop. To apply the brakes fully the driver must remove his foot very rapidly from the accelerator, place it on the brake pedal as quickly as possible, and push the brake pedal down
fast and forcefully.
The sensory mechanism in the concept vehicle recognizes each of these phases. The
sensor on the e-gas device signals the fast return of the accelerator, the electronic system
recognizes that the foot is moved quickly from the accelerator to the brake pedal from the
short time span between leaving the accelerator and first contact with the brake pedal.
Brake assist sensors, and of course the EHB sensor, measure the high brake pedal
speed.
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The way in which the electronic co-driver supports the driver in slowing down the car depends on the deceleration requirement that results from the distance to the vehicle in front
as well as from the sequence of events in the time the driver takes to react. It ranges from
slight application of the brake linings to the brake discs to minimize system reaction times,
soft braking when the accelerator is released with an adjustment of the system thresholds
to the activation of the brake assistant, to the triggering of the brake assistant when the
driver’s foot steps rapidly on the brake pedal.
Long-term objective: collision mitigation
Should it be required by legislation, it will be possible in the future to produce an emergency stop introduced by the car itself, if it is clear that the driver himself cannot avoid the impending accident. The following scenario to support the driver is conceivable in the case of
an impending accident: The system continuously monitors the area surrounding the vehicle and what is happening in front of the car, such as the speed and distance of a vehicle
traveling in front using cameras or sensors. If the situation in the surrounding area clearly
indicates a hazardous situation, an escalating warning strategy starts depending on the
probability of an impending accident. Warnings of the hazardous situation can be optical,
acoustic or haptic, as already described.
Automatic emergency stop
If the driver does not react to the flashing control lamp, the warning sound or the car’s own
slight braking and if an accident cannot be avoided, the system will automatically introduce
an emergency stop to minimize the seriousness of the collision and consequently the seriousness of the accident. Hitting the correct point in time for this emergency stop is a task
which requires the greatest care and extensive testing. The latest time lies a few fractions
of a second after the moment in which a professional test driver would have been able to
just prevent the collision by steering and/or braking. It is useful for the driver with average
experience that the automatic emergency stop be designed with less stringent thresholds.
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The environmental information required for active collision mitigation can also help to condition further active or passive safety systems in the car in the pre-crash phase. If there's
danger of impact with an obstacle, the ESP can, for example, be sensitized for evasive action, and intelligent restraint systems can be prepared for the best passenger protection.
To use such a system in series production, the foresighted environment sensory mechanism needs to be developed further to meet with the requirements of the new function. The
fusion of different physical sensor types is necessary to produce a complete picture of the
vehicle environment in the vehicle computer. Work is already being carried out on relevant
technologies such as radar, laser and video cameras at Continental's development departments.
Electro-hydraulic brake (EHB)
Brake-by-wire systems as remote controllable braking systems with high system dynamics
are of central importance for implementing the vision of an intelligent vehicle that avoids
accidents or drastically reduces the consequences of an accident through the electronic
co-driver. The electro-hydraulic brake (EHB) fulfils these requirements very well. EHB can
be controlled without a great deal of effort and contributes to optimum deceleration with
short reaction and pressure build-up times. With pedal characteristics that, to a great extent, can be freely designed, EHB offers advantages in ergonomics. These advantages
take effect when the driver and not the car initiates an emergency stop.
Ergonomic advantages
The pedal forces required for strong braking are considerably smaller since the mechanical-hydraulic connection between the brake pedal and the wheel brake is separated. This
means that EHB merely requires approx. 200 Newton of pedal force to initiate an emergency stop whilst 1000 Newton is not unusual for today’s systems.
Women of a slight build are not the only drivers who can barely apply such a force and
lose decisive meters in an emergency.
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A further advantage of EHB is that the brake pedal can be mounted almost at the same
level as the accelerator. This means the driver can move his right foot fasterfrom one pedal to the other in dangerous situations.
EHB's high system dynamics
Even the higher system dynamics of EHB provide decisive benefits. A normally-trained
driver brings a servo assisted brake-system force regulator with vacuum-brake booster –
as is usual in today’s passenger cars – into the ABS-control range after about 900 milliseconds of pressure build-up time. If a brake assist supports the driver as he initiates full
brake application, the pressure build-up time is reduced by approx. 200 to 250 milliseconds and the stopping distance of 80 meters at 100 km/h is reduced by approx. 9.5 meters. EHB with brake assistant reacts again more dynamically and reduces the pressure
build-up time by a further 100 milliseconds.
Optimized ABS control
The EHB sensor concept allows precise control of the wheel brake pressure in an ABS
case. Fine adjustment possibilities can be implemented in an optimized way, especially for
tires with higher grip potential.
Stopping distance becomes 16 meters shorter
Compared with a conventional braking system featuring brake assist, these system advantages of the EHB with brake assist together with environment sensory mechanism together lead to the reduction in the reaction time for the driver/vehicle system of 500 milliseconds mentioned at the beginning. According to TÜV Rhineland/Berlin-Brandenburg,
this would reduce the number of road traffic deaths by 40%. The 500 milliseconds gained
correspond to a stopping distance reduced by 16 meters at a starting speed of 100 km/h.
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This means the car with EHB and additional sensory mechanism stops after 64 meters,
the reference car only after 80 meters. The next figure is even more impressive: where the
concept car has already decelerated to a stop, the reference car crashes into the obstacle
at a residual speed of 60 km/h.
Test set-up close to automotive reality
Continental checked and proved the effects of this technology network for driver assistance with the aid of a prototype vehicle and a simple test set-up simulating a typical scene
from everyday motoring life.
The vehicle follows the car in front at approximately 80 km/h. In the test this is a real rear
vehicle section that is guided by a mule car. The safety distance is approx. 20 meters,
which is much less than recommended in the rule of thumb “half the speedometer display”.
One glance at actual traffic conditions, however, show that this is completely realistic as
fines are only a threat when the safety distance measured is less than half of the specified
value, that is one quarter of the speedometer value (at 80 km/h this is 20 meters).
The vehicle rear, the dummy, guided by the mule car on a parallel track, slows down a bit
in the demonstration – a normal procedure, which a driver does not pay too much attention
to in everyday driving. But then suddenly the car in front, simulated by the dummy, hits the
brakes with full force.
Support for the driver with average skills
The driver of the prototype vehicle is informed of the changes in speed and distance,
which are only minor at the beginning while the front-mounted radar registers the reducing
safety distance at the initial stage. This attracts his attention. The braking system builds up
light pressure and applies the brake linings to the discs while he still has his foot on the
accelerator. The vehicle in front now brakes suddenly with full deceleration power, and the
driver in the following vehicle recognizes how serious the situation is. The braking system
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increases the braking pressure already as the driver is taking his foot off the accelerator.
When the sensory mechanism signals an unusually short changeover time from the accelerator to the brake pedal with subsequent fast braking, the brake assist is in alarm standby mode and can introduce full braking fractions of a second faster, which might be decisive in preventing the accident.
A mass-produced vehicle with a conventional braking system and without a sensory
mechanism can also stop in good time if the driver drives with full concentration and reacts
as fast as a trained professional. This proves, however, to be the exception because rear
impacts often couldn't be avoided in test drives with drivers of normal experience.
The corporate technology built into the concept vehicle cannot, however, prevent a collision in every case. It assists the driver, but doesn't tell him what to do. Vehicle occupants
would, however, suffer less serious consequences of an accident as the impact speed
would be considerably less than that of a vehicle with conventional equipment.
Accident consequences become less serious even when the driver reacts
incorrectly
This can be calculated from sensored data that document all the relevant details of the test
procedures. The kinetic energy consumed in the reaction phase was used as a quality criteria to capture the use of the technology in figures. If the driver in a series production vehicle does not react at all, he will hit the obstacle with almost full power. If the driver of the
technological innovation does not react, the system can still reduce a part of the kinetic
energy and consequently the seriousness of a collision – without any action by the driver.
This is expressed in the phrase “Reduction of the Energy during the Reaction”. RER80 is
thus the standardized energy reduction in the stopping process at a starting speed of
80 km/h.
Concept tires with SWT
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The concept vehicle is equipped with a concept tire based on that of the “30-meter car”
project presented in winter 2000 and for the development of which information from bionics
was transferred to the world of technology. This bionic approach can be seen in the tire
tread that increases in size during braking, thus being able to transfer more braking power.
The contour and body of the tire were extensively redeveloped. Further development of
the principal used on the standard Continental Premium Contact tire provides for a highly
uniform distribution of pressure in the concept tire. This allows the transfer of greater longitudinal forces without early slipping when braking.
The tread compound has also been optimized to ensure transfer of higher braking forces.
Thanks to advanced silica compounds, it has been possible to minimize the number of
conflicting objectives without compromise during the last decade. Tires incorporating such
compounds have extremely high potential for transmitting forces on dry road surfaces, plus
excellent wet-weather qualities matching those of today's top-of-the-range production tires.
The tread was also designed with the goal of having high radial orientation of brake force
transmission. Its large share of positive tread elements provides large-area contact between the rubber and the road; the V-shaped tread grooves in the shoulder area, together
with the longitudinal grooves, contribute to the prevention of aquaplaning.
The tire's body performs other functions alongside its contribution towards increasing the
size of the contact area. Additional vibration reduction, for example, reduces the disruptive
interaction between the tire and the brake during pressure modulation by the ABS controller. At the same time, characteristics have been developed and implemented for the concept tire that are specifically tuned to the driving dynamic control system used. All these
measures together led, in the fall of 2000, to a tire that allows a braking deceleration of
1.31 g on a road with a high coefficient of friction. Mass-produced vehicles today average
around 1.0 g (g = gravitational acceleration of 9.81 m/s2).
SWT: Force measurement directly on the wheel
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Like the body, a further technical highlight of the concept tire is invisible: magnetized sidewalls turn the tire into a supplier of data providing the ABS computer with information
about the transmitted frictional forces. This data enables optimization of the ABS control
processes by keeping the brake pressure consistently in the area in which the tire can
transfer a high degree of braking force. The sidewalls, which are magnetized in 96 fields of
alternating polarity, together with the sensors mounted to the chassis are known as the
SWT System (Sidewall Torsion Sensor)
Air-sprung chassis with controllable dampers
Like the concept tire, the prototype's air-sprung chassis has no effect whatsoever on the
reduction of reaction and pressure build-up distances. This system doesn't exploit its advantages over the conventional steel-sprung chassis until the last phase of stopping distance, namely the braking distance.
It allows the electronically controlled air-sprung chassis to minimize, say, negative effects
of dynamic axle-load distribution on the front axle during braking. The rear wheels are relieved to the same degree as the front wheels are loaded. Because of this, they can transfer less braking force and make only a small contribution to driving stability. This is why the
chassis electronics lower the vehicle body at high speed by reducing the pressure level in
the air springs. As a consequence, the lower center of gravity provides a general improvement in driving dynamics as well as a reduction of stopping distance. The rear
wheels can transfer higher braking forces with improved cornering stability potential due to
the lower center gravity.
Damper adjustment decreases the stopping distance
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In addition to the air springs, the shock absorbers are also controlled electronically. The
mounted dampers are continuously adjustable. Their rebound and compression travel are
modulated for each wheel individually to reduce the stopping distance, so that wheel load
variations due to pitching motion of the chassis and uneven road surfaces are minimized.
This not only leads to an improved, continuous contact between the tire and the road, but
also to more effective ABS control.
Contact:
Lars Döhmann
Tel.: 0511/ 938-1370
Fax: 0511/ 938-12455
Lars.Doehmann@conti.de
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