Airbag System

In Automobile Safety System the airbag system function is to absorb the impact of the human body during collision.

The Airbag System has become a major technological advancement in the automotive history. The effectiveness of the Airbag in an automobile collision has made this technology a standard in all modern vehicles produced today. It works in tandem with the Seatbelt System to form the Safety-Restraint System (SRS), which minimizes the risk of fatal injury during an automobile accident.

Along with seatbelts, the airbag is a crucial part of the safety restraint system. The safety restraint system reduces the risk of injury through the absorption of energy exerted through the human body during impact. Upon the moment of impact, the seatbelt retracts any slack to absorb some of the inertia of the human body moving forward. The strength of this force requires the gradual release of the seatbelt as the body moves forward, in which the airbag is deployed to absorb the remaining amount of energy and safely bring the human body to a complete stop without serious injury.

The History of Airbags:

In 1951, the first air bag safety system was developed by Walter Linderer, a German, and John Hedrik, an American. This device initially operated using compressed air that proved to be ineffective due to its lack of speed in deployment.

In the 1971 Ford began experimenting with airbags while in 1973 Chevrolet began offering them for government vehicles. Also in 1973, General Motors offered the first automobile intended for the public with airbags, the Oldsmobile Tornado.

By 1975 General Motors had expanded their airbag offerings to other automobiles. They did not become commonplace until the late 1980's and were mandated by the government to be in all vehicles in 1998.

The first modern gas-inflated airbag was introduced in 1994. While airbags previously focused on reducing the risk of injury during front-end collisions to the driver and the passenger, today they are being used in door panels, side curtains, and even rear columns to prevent serious injury in many different types of accidents.

Airbags – How do they work?

Airbags deploy when a sensor detects a collision of a certain threshold, typically equivalent to a 14 mile per hour barrier collision. When a collision exceeds this threshold, a signal is sent to an inflator by the sensor. The inflator contains an igniter that ignites sodium azide, a solid propellant. This creates a large volume of nitrogen gas and inflates the airbag in one twenty-fifth of a second. The airbag is coated in powder to reduce friction and immediately begins to deflate to decrease the force at which the occupant comes into contact with it.

The first airbag systems were installed in top-of-the-range vehicles in the mid-19s. The end of the 19s saw the introduction of the front passenger airbag, and this was gradually followed by other variants such as the head airbag and side airbag. Nowadays, airbag systems are provided as standard in vehicles.

AIRBAG COMPONENTS : Functions

Airbag control unit

The control unit is the heart of the airbag system and is installed centrally in the vehicle. It can generally be found in the dashboard area, on the center tunnel.

It is responsible for the following:

Detecting accidents.

Detecting the signals sent by the sensors in a timely manner.

Triggering the necessary firing circuits in a timely manner.

Providing the power supply to the firing circuits using the capacitor, independently of the vehicle battery.

Running self-diagnostics on the entire system.

Storing faults in the fault memory.

Activating the airbag indicator lamp if the system fails.

Connecting to other control units via the CAN bus.

Alongside the crash severity, the control unit also takes into account information about the direction of the accident (the application of force), for example 0° or 30°, and the type of accident in order to determine the deployment strategy. In addition, it considers whether or not the occupants are wearing their seat belts.

Crash sensors

Depending on the airbag system and the number of airbags installed, the crash or acceleration sensors are installed either directly in the control unit or as satellites in the vehicle front end or at the side of the vehicle.

Safing safety sensor

The safing sensor is responsible for preventing the airbags being deployed unintentionally. It is series-connected with the front sensors. The safing sensor is integrated into the airbag control unit. It comprises a reed contact in a resin-filled tube and a ring-shaped magnet. The open reed contact is located in a resin-filled tube over which the ring-shaped magnet is placed. The magnet is held by a spring at the end of the housing. If force is applied, the magnet slides over the resin-filled tube against the spring force and closes the reed contact. This closes the contact for firing the airbags. Front sensors are always provided in duplicate. These sensors usually operate according to the spring-mass system. With this system, the sensor houses a weighted roller which is filled with standardised weights. A bronze spring hinge is wound around the weighted roller and its ends are attached to the weighted roller and the sensor housing. As a result, the weighted roller can only move if the force is applied from a certain direction. If force is applied, the weighted roller rolls against the bronze spring force and closes the circuit to the control unit using a contact. The sensor also houses a high-impedance resistor in order to run self-diagnostics.

Airbag

The airbag is made from a highly durable polyamide fabric that resists aging. It has a low coefficient of friction to ensure it unfolds easily and makes gentle contact with the skin. The airbag is dusted with talcum powder to protect it and stop it sticking. When the airbag is deployed, this powder can be seen as a white cloud. There are retaining bands on the inside which maintain the shape of the airbag when it is being inflated. There are outflow openings on the rear which allow the gas to escape.

Volute spring

The volute spring establishes the connection between the rigid steering column and the moving steering wheel. It also ensures the connection between the airbag control unit and the gas generator when the steering wheel rotates. The conductor foil is wound so that it can follow the rotation for 2.5 revolutions in each direction.

Take particular care when removing and installing the volute spring. You must ensure that the steering is centred and the wheels are in the straight-ahead position. Do not twist the volute spring once it has been removed.

Seat occupancy detection

Seat occupancy detection is used to control airbag deployment more precisely and prevent airbags from being deployed unnecessarily. There are different ways to check whether or not the seat is occupied. Sensor mats comprising pressure sensors and an electronic evaluation unit are used. The sensor mats can only be integrated into the front passenger seat. The most state-of-the-art systems can, however, also be integrated into the driver's seat and rear seats. Both infrared and ultrasonic sensors can be used. They are installed in the area of the interior light/rear-view mirror and monitor not only whether or not the seat is occupied, but also how the front passenger is seated. In this way, the system detects if the occupant is "out of position", which would be problematic.

The information from the seat occupancy detection system affects the deployment of the airbags and the activation of the seat belt tensioners and active head rests. The airbag system detects if individual seats are not occupied and the corresponding protection systems are not activated in the event of an accident.

Airbag cables

The airbag connectors are bright yellow in order to more easily identify the airbag cables and connectors. Inside the connector there is a jumper which prevents the airbag from being deployed unintentionally if work is being carried out on the airbag system. This could occur as a result of static charging, for example. The jumper is a contact which connects the two contacts inside the connector when the plug connection is disconnected, in order to eliminate any potential.

Seat belt tensioner

The purpose of the seat belt tensioner is to eliminate slack in the seat belt in the event of an accident. This slack can arise as a result of generously sized, loose clothing or a "relaxed" seating position. The seat belt tensioner can be integrated into the belt buckle or the belt roller. If the seat belt tensioner has been installed in the belt buckle, it will comprise a retractor tube, cable, piston, gas generator, and firing pellet, for example. In the event of an accident, the gas generator is fired as is the case for an airbag. The gas spreads and moves the piston in the retractor tube. The cable connection between the piston and belt buckle pulls the belt buckle down and eliminates the slack in the belt. If the seat belt tensioner has been integrated into the belt roller, the slack is eliminated by means of a retractor mechanism.

In this case too, a generator is fired if the tensioner is triggered. This generator sets a series of balls in motion. The balls turn a reel connected to the belt roller. This rotation retracts the belt over a precisely defined distance. The balls then fall into a designated container to prevent any damage.

Another option is to use the "Wankel rotary engine" principle. With this method, when the tensioner is triggered the propellant drives a rotor, which eliminates the slack due to the rotation. In order to reduce the load exerted on the chest in the event of an accident, a belt force limiter is installed in the driver's and front passenger belts.

Belt force limiter

Belt force limiters are adaptive belt machines that use a gas generator, like in an airbag, to switch between a high and low level of force.

Thanks to the optimal coordination between the seat belt tensioner and airbag, the kinetic energy of the occupants is slowly dissipated over the course of the impact, which reduces the loads.

Battery switch-off

To avoid the danger of short circuits and resulting vehicle fires, the battery is disconnected from the vehicle electrical system in the event of an accident.

This is done using a cut-off relay or a gas generator. The battery disconnection signal is sent by the airbag control unit. The gas generator works in a similar way to the seat belt tensioner here. If the battery disconnection function is triggered, the connection between the battery and connecting cable is disconnected inside the terminal.