88 Supplementary restraint systems SRS

Airbags of the type shown in Fig. 8.18 and seat belt pre-tensioners, such as that shown in Fig. 8.19, are features of a basic supplementary restraint system. In the event of a frontal impact of some severity, the airbags and seat belt pre-tensioners are deployed. The airbags are inflated to protect those provided with them from impact with parts of the vehicle. The seat belt pre-tensioners are made to operate just before the airbags are inflated and they operate by pulling about 70 mm of seat belt onto the inertia reel of the belt. This serves to pull the seat occupant back onto the seat.

The deployment of these supplementary restraint devices is initiated by the action of the collision detection sensing system. A collision detection sensing

Fig. 8.18 Airbags for driver and front passenger

system normally uses signals from two sensors: a 'crash sensor' and a 'safing sensor'. The safing sensor is activated at a lower deceleration than the crash sensor (about 1.5 g less) and both sensors must have been activated in order to trigger the supplementary restraint system. The safing sensor is fitted to reduce the risk of a simple error bringing the airbag into operation. Both of these sensors may be fitted inside the electronic control unit which, in some cases, is known as a diagnostic and control unit (DCU) because it contains the essential self-diagnosis circuits in addition to the circuits that operate the SRS. Figure 8.20 shows the layout of a supplementary restraint system on a Rover Mini.

Airbags are made from a durable lightweight material, such as nylon, and in Europe they have a capacity of approximately 40 litres. The pyrotechnic device that provides the inert gas to inflate the airbag contains a combustion chamber filled with fuel pellets, an electronic igniter and a filter, as shown in Fig. 8.21. Combustion of the fuel pellets produces the supply of nitrogen that inflates the airbag. The plastic cover that retains the folded airbag in place at the center of the steering wheel is designed with built-in break lines. When the airbag is inflated, the plastic cover separates at the break lines and the two flaps open out to permit unhindered inflation of the airbag.

The seat belt pre-tensioners are activated by a similar pyrotechnic device. In this case the gas is released into the cylinder of the pre-tensioner, where it drives a piston along the cylinder. The piston is attached to a strong flexible cable which then rotates the inertia reel of the seat belt by a sufficient amount to 'reel in' the seat belt by approximately 70 mm.

The rotary coupler is a device which is fitted beneath the steering wheel to provide a reliable electrical connection between the rotating steering wheel and airbag, and the static parts of the steering column. The positioning of the rotary coupler is a critical element of the airbag system and it should not be tampered

Fig. 8.20 The elements of a supplementary restraint system (Rover Mini)
Fig. 8.21 A pyrotechnic device for inflating airbags

with. When working on supplementary restraint systems it is important that a technician is fully acquainted with the system and procedures for working on it.

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