This is a sensor used to trigger an airbag. At impact the cylinder rolls out against the roller spring to close the electrical contact and set off the bag.

Airbags are passive restraint systems. Passive systems are those which do not require the user to do anything in order to be protected. In contrast, most seatbelts are active systems, the user must put them on. Because airbags, as passive restraints, are currently required to protect occupants who are not belted up, they are large and powerful.

Small adults and children are especially vulnerable to airbag injuries, although potentially anyone can be injured with some systems if they are close enough to the bag when it deploys. In general, protected occupants should be as far away as possible from strike surfaces in the vehicle's interior - including the airbag- regardless of the type of protection system in use. Belted occupants without airbags should also be as far from the dash and steering wheel as interior packaging and ergonometrics allow. It is not clear that manufacturers have provided adequate warning on this point. Air bags have killed scores of people so far while they are credited with saving over 1100 lives in the past ten years.

You want to run into the airbag, you don't want the airbag to run into you. This generally means sitting back about 12" from the airbag housing although the exact distance required can be hard to determine without knowing the size of the deployed bag and the bag's deployment characteristics.

Airbags deploy in about 0.06 seconds at speeds of over 200 mph. They must be this quick because the typical automobile collision lasts about 0.125 seconds and unprotected occupants will hit the the vehicle's interior before the primary impact is over. That is, the second collision occurs while the first collision is still going on. Deployment is effected by a gas generator which usually employs the combustion of sodium azide to produce the volume of gas required. Deployment is essentially the result of a small, contained explosion.

The sensor, or more likely, the sensor in conjunction with a computer, decides whether or not to deploy the bag based on the severity of the accident. The usual deployment algorithm involves in effect only the velocity change of the vehicle although more complicated algorithms are being incorporated into newer systems.

Airbags first appeared in the early 1970's in GM and Ford test fleets, widespread introduction did not take place until Chrysler introduced them into its American made vehicles in 1989 as a result of government and consumer group pressure. They were delayed because of cost and competitiveness objections raised by vehicle manufactures

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