The airbag is an additional restraint system which was conceived in Europe; it is usedin the countries where the safety belts are compulsory, and is designed to be used incombinatiolt with the safety belt. In the case of an accident，within a certain impact speedrange, it acts to limit the extent and the entity of injuries to the occupants of the vehicle.The USA is an exception because, even though the use of the safety belt is compulsory inalmost all states，rules state that the occupants have only to be protected by an airbag.
In the case of an accident，or if an action with certain force is exercised on the vehicle,one or more sensors, which are appropriately located on vehicle, send an electric signal toan electronic control unit. This unit establishes the type and magnitude of the impact anddefines which ARS（additional restraint system) has to be put in action, sending anelectric signal which primes the propellant activating the additional restraint system.
The airbag has the function of interacting with the occupants and dissipate the kineticenergy together with the safety belts. The use of only a safety belt, even when it is welldesigned and mounted, enables the occupants to be restrained but, if certain decelerationlevels are exceeded, can cause crushing of the thorax, giving rise to secondary injuries.For example, during a frontal impact，the driver wearing only a safety belt receives a veryhigh load on the thorax, with a value near to 10 kN；instead, with an airbag，this load isreduced by about 30％．Moreover the airbag distributes its pressure on a larger bodysurface area and, at the end, enables an important reduction to the acceleration levelsexperienced by the head which are dangerous for the brain.
An airbag system automatically inflates large nylon bags immediately after the start ofa major collision. The airbag system is designed to supplement the protection provided byseat belts. The major parts of an airbag system include：
(1) Airbag sensor一inertia sensors that signal the control module in the event of acollision.
For the airbag, sensors have to be electronic due to their position on the vehicle andthe information to be analyzed. The sensors can be divided into：
·Sensors for impact (front and side)；
·Sensors for the occupants of vehicle (presence of occupants, weight of occupantsand the position of the occupants on the seats)．
These sensors enable the airbag system to be managed while avoiding, for example,activating the passenger airbag if there is no passenger on that seat or, if there is a dualstage airbag，allow pilot filling as a function of the weight and position of the passenger onthe seat.
(2) Airbag module一contains the inflator mechanism and the nylon airbag thatexpands to protect the driver and/or passengers during the collision. An airbag moduleconsists of four basic parts：gas inflator, bag, housing and cover.
The gas inflator is the basic component and, for this reason, usually the name of theinflator defines the name of airbag modules. The mission of the inflator is to produce thegas mixture in quantity and within a specified time; this gas fills and inflates the bag. Thegas inflator is composed by an igniter, comprising a little metal housing，which contains asmall quantity of explosive charge（usually black powder)，and an electric trigger; thetrigger is piloted by the control unit and makes a little explosion that，in turn, triggers thesolid propellant. Explosion generates a non toxic gas mixture (nitrogen and particulates).
Usually the bag is made of a polyamide fibre fabric with a specific weave. It won't tearor ignite when filled with the gas mixture during the filling phase. The shape and volumeof the bags depend on the space between the occupant and the interior of the car, and onthe type of protection that the module has to ensure (driver front impact, rather than headprotection for side impact）．The folding of the bag is very important in terms ofminimizing risk of injury during deployment；for example, the steering wheel airbag is starfolded. To optimize performance during the opening phase, tear seams or strips (tethers)can be used to optimize the position of the bag in comparison to the occupant.
(3) Airbag controller一computer that operates the system and detects faults.
(4) Dash warning lamp一dash bulb that grows with system problem and goes out assystem arms.
An electric airbag system deploys an inflatable nylon bag to help protect the driverduring a collision. It uses impact sensors to detect a severe collision. The sensors feedtheir signals to the airbag controller. When at least two impact sensors are energized, thecontroller activates the airbag module.
The airbag inflates in about 1/20th of a second, well before the driver's body fliesforward from the collision. The tough nylon bag can easily absorb the forward inertia of ahuman body. This helps protect both the driver and the passengers.
All late-model vehicles are equipped with dual airbags：a driver-side airbag and apassenger-side airbag. The driver-side airbag locates in the steering wheel. The passenger-side airbag deploys from the right side of dash. The driver- and passenger-side airbags willonly deploy during frontal impacts. Steering wheel and dash mounted airbags may notdeploy in side impact，rear impact or rollover situation. A collision must occur withinabout 30 degree of the vehicle's centerline for these airbags to inflate，shown in Fig. 10. 2.
Side-impact airbags can be located in the door panels or on the outside edge of eachfront seat. These small airbags deploy when the vehicle is hit from the side. Th(:vgenerally do not deploy during a frontal impact. Side-impact airbags are becoming morecommon and are used by several auto manufactures. When a vehicle is hit from the side,injury usually results when the occupant's shoulder and head fly through the side windowglass. A side-impact airbag system senses the side thrust of the impact and deploys a smallairbag to cushion the person as he or she is propelled sideways.
Fig. 10. 2 shows a distributed sensing system, where multiple sensors are combinedto detect a collision. An electronic control unit（ECU) often monitors the triggering ofmultiple sensors and controls the activation of the airbags.