Occupant restraint system

The term restraint system describes the entirety of passive safety devices in a vehicle that are suitable for securing vehicle occupants in their seat. Passive safety devices serve to reduce the consequences of an accident for everyone involved. Restraint systems are used in land, air and sea vehicles.

Task of the restraint system

Restraint systems have two goals. On the one hand, by fixing them on the seat, vehicle occupants should be protected from the effects of excessive accelerations and, in extreme cases, from being thrown out of the vehicle. Perhaps the decisive point, however, is the protection of the occupants from collisions with components of the vehicle structure such as the steering wheel , dashboard , door frame, etc. significantly reduced. The risks that some restraint system devices present (e.g. crushing from the seat belt , deaths from airbags in OOP situations - out-of-position, the occupant is in an unexpected position / posture, e.g. the passenger is tied up the shoes) weigh slightly against it.

Difference between passive and active devices

While some devices of the restraint system perform their function permanently (e.g. shape of the seat with pronounced lateral support), other devices are only activated when necessary (e.g. airbags). Devices of the first type are also called “passive”, those of the second type are also called “active” (however, active restraint systems also belong to the area of passive safety.) The difference becomes clear when looking at the different shapes of seat belts. The simplest seat belt (two-point lap belt, as it is still used in air and sea vehicles today) is a belt that is firmly attached to the structure on both sides of the seat. This belt holds the occupant in the seat with the same (maximum) force regardless of the traffic situation . In the automotive sector, the three-point belt with automatic retractor developed by Volvo caught on early on . A mechanism inside the automatic retractor prevents the belt from unrolling in the event of a backward force, such as occurs during a frontal crash. This seat belt is already one of the active devices, as the function depends on the external situation. Younger generation seat belts also have belt tensioners , small motors that roll up the belt with a predetermined force when necessary and thus actively pull the occupant into the seat. The seat belt tensioners are activated by a control unit that evaluates and interprets the signals from the crash sensors.

Construction of a restraint system with active devices

A restraint system consists of the various restraint devices and the control electronics, the core of which is its own control unit. Since this control unit became necessary with the use of airbags, it will i. General Called an airbag control unit, although this control unit controls all active, electronically controlled devices. The airbag control unit is located i. General in the vehicle tunnel under the center console between the front seats. There are various sensors in the airbag control unit , the signals of which are interpreted. These are essentially acceleration sensors that measure the deceleration of the vehicle in different spatial directions. If the deceleration in one direction exceeds a dynamic threshold, this behavior is interpreted as an accident and the corresponding restraint devices are activated. Modern, multi-stage restraint devices (especially smart airbags) are activated according to the severity of the accident. The evaluation algorithm must at least differentiate between front crash, left side crash, right side crash and rear impact so that only suitable restraint devices are activated. In addition, the rollover sensor should also be mentioned, but it did not find its way into the vehicle until the 1990s. Satellite sensors are often assigned to the airbag control unit . These are sensors located in the crash area with which the crash can be detected earlier than with the sensors in the control unit. In addition to acceleration sensors, pressure sensors are used as satellite sensors, which record the sudden reduction in the size of the inside door volume in the event of a side impact. The restraint devices are only triggered if the accident is detected by two independent signal paths (e.g. airbag sensors and mechanical trigger switch ), as incorrect triggering can have devastating consequences. On the one hand, many of the restraint devices are irreversible, and on the other hand, restraint devices (especially airbags) activated during normal driving can provoke an accident.

Overview of different designs of modern vehicle restraint systems

Multipoint seat belts with belt tensioners on all seats
Airbags (driver and front passenger airbags, side airbags, head airbags, knee bags, window bags, etc.)
Seat functions (active headrest , anti- submarining, etc.)
Wheelchair restraint systems in vehicles for the transport of the disabled

Restraint systems and pre-crash

The timing of an accident is divided into four phases.

Phase 1: normal driving
Phase 2: pre-crash (an accident is foreseeable)
Phase 3: In-Crash (the accident happens, the vehicle comes into contact with the obstacle)
Phase 4: Post-crash (the accident happened, everyone involved is calm)

Basically, the functions of the restraint system belong to phase 3. The risk of injury should be minimized during the accident. With the increasing spread of environmental sensors (e.g. radar ) and their use for passive safety in motor vehicles, the restraint system is increasingly effective as a means in phase 2. For example, the active seat elements are already activated in the event of danger. The occupant is brought into the correct sitting position at an early stage so that, on the one hand, the activated devices can work optimally in the event of a crash, and, on the other hand, reversible, slow devices can be used that would be too sluggish for pure use in phase 3 (e.g. automatic seat adjustment).

Special restraint systems for children

Because of their smaller size and weight, children need special additions to the restraint system, see child seat .