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How Vehicle Airbags Work

Vehicle collisions are a leading cause of injury or death due to vehicle occupants being subject to a significant amount of "G" forces. These forces are caused by the rapid deceleration of the vehicle and the body. Passengers in vehicles lose their velocity slower than the vehicle they are in, that is why unbelted occupants in a vehicle seem to be thrown forward. In reality they are just not slowing as fast as the vehicle.

Seat belts are designed to prevent injuries caused by this different change in velocity. They do this by preventing the person from continuing forward into the steering wheel or windshield. Airbags are a supplement to seat belts which is why they are called a supplemental restraint system or SRS. They do this by inflating rapidly at the onset of a collision and then deflating slowly as a person makes contact with it.

Airbags are controlled by a computer module which is part of the "black boxes" I explained the use of in a prior article (see How Automotive Black Boxes Work). Different manufacturers call their black boxes by different names, for this article we will use General Motors terminology of Sensing Diagnostic Module or SDM.

The primary function of the SDM is to measure the forces being applied to a vehicle during a collision and to determine if the vehicle's air bags should be deployed. The SDM also runs diagnostic tests on the air bag module and air bag related wiring. Finally a part of the SDM called the Event Data Recorder records various conditions of the vehicle such as vehicle speed, throttle position, engine speed, as well as other conditions just prior to airbag deployment. The event data recorder is digital however it records similar to a short loop of magnetic recording tape in that it only records a certain amount of information, based on a set time, and then writes over the old information. For instance some save five seconds of data recorded at one second intervals. A new second of data pushes the oldest second of data out of the memory. In the event of an airbag deployment the system saves the last five seconds of data and locks it into its memory. If a car is started and driven after this collision the pre-collision data is not lost. This allows investigators access to this data even if a car is operated after a collision.

Inside the SDM are accelerometer(s), a microcomputer, and the Event Data Recorder. The accelerometer is designed to measure a vehicles change in velocity or speed. It is lined up

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