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Airbag Deployment Systems

Basic Description

Airbags are passive safety devices that are mandatory on all vehicles sold in the United States. Airbags are a critical part of the Supplemental Restraint System (SRS) in most vehicles. The objective of the airbag, which is deployed when the vehicle suddenly decelerates (as in a collision), is to prevent the vehicle occupants from hitting any rigid surfaces and cushion the forces on their heads and upper or lower bodies. Airbags are typically made of nylon fabric and are hidden behind panels at various locations in the vehicle, including the steering wheel.

airbag deployment illustration

Depending on the crash severity, the rate at which the airbags are deployed is decided by the airbag control unit. In the event of a crash, the crash sensor (an accelerometer) sends a signal to the airbag control unit. This control unit triggers the inflation device, which generates nitrogen gas by igniting a mixture of sodium azide (NaN3) and potassium nitrate (KNO3). The time between crash detection and complete deployment of the airbag is approximately 0.05 seconds. The airbag speed is about 200 mph, which itself can be harmful in certain cases. This has given rise to adaptive airbag systems that employ multiple inflators to produce either low-level or high-level deployments. These systems can adjust the airbag pressure depending on factors such as seat position, size of passenger, crash severity and seat belt use.

Most systems use a weight sensor in the front passenger seat to determine if the seat is unoccupied. If it is, the passenger airbag will not deploy. The weight sensor can also discriminate between children and adults who may be occupying the seat. The U.S. Federal Motor Vehicle Safety Standard 208 requires airbag deployment systems to detect whether a child is seated in the front passenger seat. Typically, airbag deployment will be suppressed if a sensor identifies a low-weight condition. Additionally some systems can detect child's safety seats that are equipped with special sensors as defined by the technical specification ISO/TS 22239.

In 2012, Volvo became the first automotive manufacturer to introduce a Pedestrian Airbag System in its V40 model. It uses a pedestrian contact sensing system. When impact with a pedestrian is sensed, the hood opens from the back and an airbag is inflated over the windshield-wiper area.

In 2013, GM announced a new airbag for side impact crashes. It inflates near the center console and provides padding between the front passengers or support for the solo driver. GM has also modified the front airbag to include a vent which opens when the passenger compresses the bag. This provides a similar effect to the more expensive dual stage airbags without the increased cost. Due to the vent being closed during initial deployment, it can inflate with lower pressure. This will reduce some inflation-related injuries with smaller drivers who sit closer to the steering wheel and benefit drivers who sit further back due to reduced premature deflation.

Sensors
Accelerometers, wheel speed sensors, brake pressure sensors, seat occupancy sensor
Actuators
Airbag inflation device, passenger airbag ON/OFF indicator
Data Communications
Typically CAN
Manufacturers
Autoliv, Bosch, Delphi, Denso, Calsonic Kansei, Continental, Mitsubishi Electric, Takata, TRW
For More Information
[1] Car Airbags - Explained, YouTube, Apr. 2008.
[2] Airbag Deployment in Slow Motion, YouTube, Oct. 1, 2008.
[3] The History of Airbags, Mary Bellis, About.com.
[4] History of Airbags, Motorvista.
[5] Airbag, Wikipedia.
[6] How Airbags Work, Autoevolution.com, May 5, 2009.
[7] Ford's Inflatable Safety Belts, YouTube, Nov. 5, 2009.
[8] Gas Laws Save Lives: The Chemistry Behind Airbags, Rachel Casiday and Regina Frey, Washington University, Oct. 2000.
[9] 2011 Dodge Durango Side Impact Test, YouTube, Aug. 3, 2011.
[10] Volvo V40 Pedestrian Airbag, YouTube, July 30, 2012.
[11] The Future of Car Airbags, R. Montoya, Edmunds.com, May 21, 2013.