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Passive safety systems now also intervene before the crash - a new approach to occupant protection.

Active safety includes systems that are activated before the accident (ABS, ESP), and passive safety includes those devices that come into play after the crash (airbag, seatbelt tensioners). Systems such as Pre-Safe, developed by specialist TRW and commercialised by Mercedes, remove the classic boundaries between active and passive safety.

Reversible belt tensioner

The core of Pre-Safe is a reversible seatbelt tensioner that works with an electric motor. In contrast to conventional seatbelt tensioners equipped with explosive charges, this one does not have to be replaced after a crash and, if necessary, is activated even before the accident occurs. This is done via constant control of ABS, brake assistant and stability programme ESP.

An impending collision is to be detected from a data mix of these three devices. If an accident is imminent, the belt is tightened with 250 Newtons in just 120 milliseconds and the driver is pressed into the seat. A clutch serves as power transmission, separating the electric motor from the mechanical belt retractor during normal operation.
If a crash actually occurs, an additional belt tensioner is triggered. If the collision could be prevented, the electric pretensioner is released again.

Future: object recognition

In practice, Pre-Safe intervenes relatively late; actually only when you lose control of the car. The fact that the windows and sunroof are automatically closed and the seats are moved into position is nice, but hardly significant from a safety point of view. By the time the roof has been leisurely closed, the accident has probably already happened.
In the future, such pre-crash systems are to be expanded with detection sensors that provide more precise data on the time, place and possibility of a collision. A 'visual belt' around the vehicle will provide total surveillance. The car of tomorrow will thus also be able to identify the opponents in an accident. If the object is a pedestrian, for example, spring force or pyrotechnics will be used to raise the bonnet and thus create deformation space.

Source: Automobil Revue No. 7

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