Predicting an Accident to Save Lives

For its future car models, Mercedes-Benz is developing an innovative safety concept that will reduce the risk of injury for car occupants even further. The system known as PRE-SAFE, which Mercedes-Benz announced this month, recognises a potential collision in advance and activates special protection systems even before the impact. These include new belt tensioners and automatically adjusting seats.

For the more distant future, other systems such as extending bumpers, ‘smart’ crash boxes in the front-end structure or movable interior components are conceivable, systems which are able to keep the occupants away from the deformation zones. If the collision is avoided at the last moment the PRE-SAFE systems reset themselves to their original status, thereby avoiding expensive repairs.

This future-oriented concept is based on the findings of Mercedes accident researchers, which show that, in approximately two-thirds of all traffic accidents, there is a relatively long time interval between recognition of an impending accident and the impact itself. Thus, in future, this can be used to activate the vehicle’s protection systems.

“Our present protection systems such as airbags, windowbags or belt tensioners must ensure safety in a matter of milliseconds, even though accident recognition can be measured in seconds. Making use of this interval opens up new dimensions in occupant protection,” explains Dr. Rodolf Schoeneburg, head of safety development at Mercedes-Benz.

His vision has the following scenario: Even before the driver suddenly applies the brakes, things begin to happen in his car… the seat belts tension and restrain his torso, preventing his body from moving forward during the braking manoeuvre and ensuring a safe seating position. At the same time, the seat cushions of the front passenger and rear seats are tilted to the rear, while the door panels move into the car’s interior and mould themselves around the hips of the occupants like protective shields.

Fractions of a second before, the sensors in his car have detected another vehicle that is on a collision course with his own. An accident is possible. As a precaution, the onboard computer therefore activates a number of protective systems to reduce the risk of injury to the occupants.

Innovation for the future
PREventive occupant SAFEty – PRE-SAFE for short – is what the safety specialists at Mercedes-Benz call this vision of an innovative safety system for the cars of the future. Already, engineers at the Mercedes Technology Centre have spent the last few months in intensive study of this trailblazing concept, which will make further significant advances in the vehicle safety sector possible in the future. They have already installed a number of conceivable PRE-SAFE systems in a test vehicle: automatically extending bumpers, movable interior door panels, sensor-controlled seats and other ideas for the preventive occupant protection of the future.

In the view of Mercedes engineers, the early recognition of an impending accident and the development of suitable, situation-related occupant protection represents the greatest safety engineering challenge of the next few years.

This innovative technology is based on systems that continuously monitor the driver, vehicle and road surface with the help of sensors and are automatically activated when danger is detected. Such systems are already installed as standard in Mercedes models today, and prove their worth millions of times every day by recognising critical situations at an early stage and preventing accidents: the anti-lock braking system (ABS), Brake Assist (BAS) and the Electronic Stability Program (ESP). These driving safety systems form the basis for the innovative PRE-SAFE concept.

In the field of occupant protection, too, Mercedes-Benz initiated the concept of preventive measures some time ago. As early as 1989, the Stuttgart company set a milestone by equipping the SL-Class Roadster with an automatic rollover bar that snaps into position in just 0.3 seconds if a rollover threatens. The CLK-Class Cabriolet is likewise equipped with such an anticipatory rollover protection system.

With high-performance adaptive airbags, side and windowbags, belt tensioners, belt force limiters and crash-optimised body structures, most of the potential in passive safety technology has now been exploited according to safety specialists. New concepts are required if further advances are to be made. This is why, at Mercedes-Benz, the term ‘safety’ no longer refers only to crash safety. “For us, PRE-SAFE means the logical continuation of our long-held safety philosophy,” says Dr. Schoeneburg. “In future, we will also be able to use the time interval between recognising a dangerous situation and the point at which the actual impact occurs, to prepare the car for the impending crash and therefore give the occupants the best possible protection. Our previous protection concept will therefore be supplemented with an additional PRE-SAFE phase.”

The phases are as follow:

Warning phase: Sensors on the axles and bodywork monitor the vehicle’s behaviour. When the dynamic limits are reached, a yellow warning symbol lights up in the cockpit.

Assistance phase: Standard dynamic systems such as ABS, Brake Assist and ESP automatically intervene in critical situations and help to make these controllable for the driver.

PRESAFE phase: The sensors detect the enhanced probability of an accident. Various safety systems are activated as a precaution to prepare for the possible accident and condition both the vehicle and its occupants. If the accident is avoided, the PRE-SAFE systems are automatically reset to their original status.

Light impact: In the case of impacts up to 15 km/h, the bumpers and crash boxes in the front-end module absorb the impact energy. The inertia-reel belt system is blocked. Sensors monitor the impact severity and prevent activation of the airbags if their additional protection is not required.

Minor accident: During a more serious impact, robust structural members in the front end, sides and rear end absorb energy and distribute the forces over a wide area. The belt tensioners go into action. Depending on impact severity, the front airbags inflate less than fully to ensure optimal occupant protection. Sidebags and windowbags provide protection during a lateral impact. The automatic front passenger and child safety seat recognition system prevents activation of the front passenger airbag and belt tensioner if they are not required. The fuel supply to the engine is interrupted.

Serious accident: In the event of even greater risk to the occupants a second, additional stage is activated in the front airbags. At the same time, the seatbelt force limiters are activated.

Recovery phase: The doors are automatically unlocked after the accident. The emergency call system TELEAID (an optional feature) alerts the emergency services and guides them to the scene of the accident. The hazard warning flasher system is switched on.

Precautionary protection as found in nature
Accident analyses carried out by engineers in Sindelfingen show that in two thirds of all collisions, enough time elapses before the impact to activate protective systems. In roughly 60% of the more than 1,000 reconstructed accidents, the vehicles involved were in a dynamic state which indicated an impending impact. Dr. Schoeneburg asked: “In critical situations like these, why should we wait until the crash actually happens?”

Mother Nature is a perfect model for the PRE-SAFE concept, for every living being follows an inborn safety strategy. Take the domestic cat as an example: if a cat is obliged to jump from a great height to escape a pursuing enemy, it instinctively stretches its legs to their fullest extent in order to absorb the landing impact with maximum “suspension travel”. People also have precautionary reactions: if we see a snowball flying towards us, we close our eyes as a reflex reaction. Both are optimal, situation-related precautions on the part of nature.

Sensors play vital role
The PRE-SAFE system with which future Mercedes models will be equipped relies on sensors that not only recognise a critical dynamic situation, but also provide data from which the computer is able to predict an accident with a high degree of probability. This enables the electronics to initiate suitable protective measures even before the crash. Mercedes-Benz takes a multi-stage approach with this sophisticated sensor system, which is divided into four sub-systems:

Dynamic sensor system: This evaluates the actual vehicle dynamics and detects critical deviations from the nominal dynamic values. For this purpose it measures, for example, the speed, braking torque, brake pedal speed, wheel slip, acceleration around the vertical axis, spring travel, steering speed and tyre pressure.

Pre-crash sensor system: This monitors the surroundings using, for example, ultrasonic, infrared or radar technology and image processing and registers the distance from a possible obstacle, the relative speed and the possible angle of impact. This provides information concerning the nature and severity of an accident.

Crash sensor system: This recognises an actual impact within a few milliseconds with the help of acceleration, pressure, intrusion and contact sensors that can even provide information about the final severity of the accident.

Interior sensor system: This establishes which seats in the vehicle are occupied, measures the weights of the passengers, monitors the seat belt buckles and will in the distant future also register the positions of the occupants using infra-red signals or image processing.

On the basis of all this sensor data, a powerful computer prepares the vehicle components and vehicle structure for an impending accident. This involves, for example, extending the bumpers and activating the crash boxes in the front-end structure so that depending on accident severity, they will generate suitable forces to absorb a large proportion of the impact energy before it can act on the vehicle occupants. In the event of an impending lateral collision with an off-road vehicle, van or truck, the computer uses the Active Body Control system to raise the vehicle body and improve compatibility. At the same time, the sliding roof is automatically closed to prevent the occupants from being thrown out in the event of a rollover.

Things also start to move in the interior if the electronics have calculated the high probability of an accident. The purpose of these measures is to ensure that the occupants are placed in the most protected position possible even before the impact.

Automatic seat adjustment lowers the seat cushion to the rear if required and adjusts the backrest to the best possible position so that the airbag and seat belt can function with optimum effect. The belt tensioners are already activated before the impact, preventing the occupants from moving too far forward during emergency braking or preventing excessive lateral movements of the torso if the car begins to skid.

Tests carried out by Mercedes engineers show that the PRE-SAFE belt tensioner is able to reduce forward body movement by up to 150 millimetres and lateral movement by up to 300 millimetres. This means that the occupants are in the best possible seating position when the impact occurs, enabling the airbags to carry out their protective function to the full.

A knee protector automatically extends from the instrument panel to reduce the forces acting on the legs during a frontal impact. It would also be conceivable to install interior door panels that move towards the occupants before the crash and keep them away from any body components that might penetrate into the interior during the accident. Extending panels on the insides of the B-pillars could carry out the same protective function.
Mercedes-Benz has already developed the sensor systems and actual protective components of the PRE-SAFE system to the stage that they can be subjected to practical vehicle trials. An important principle applies here: these innovative systems must not impair the driveability of the car. If the accident can be avoided at the very last moment, they must reset themselves to their original status.

Accident-free driving with “thinking” systems
Mercedes-Benz has set its safety objectives even higher for the long-term future. There are plans for a “thinking” car that is able to recognise its surroundings. This means not only recognising the road ahead or other vehicles, but also traffic lights, road signs and pedestrians.

In addition, the cars of the future will be able to interpret traffic scenarios and if necessary initiate accident prevention measures by themselves. This is how automobile researchers gradually intend to approach their long-term objective: almost completely accident-free motoring.