How do air bags activate?

How do airbags activate?
Airbag deployment has saved thousands of lives and allowed people to survive a crash that otherwise might have resulted in serious injury or death.
An airbag is a stretchable fabric that can be tightly packed into various locations in a vehicle and can be deployed in milliseconds by filling the bag with a gas to help cushion the driver and passengers.
The most important part of the airbag is the crash sensor. A motorist wants the airbag to deploy in a crash, but not when he bumps into a car ahead while texting during a traffic jam!
The crash sensor responds to different inputs, the most important being a sudden stop, Other sensors measure wheel speed, seat occupant status, and brake pressure. Some sensors can activate seat belt locks and automatic door locks, in addition to airbag deployment.
There are two basic airbag sensors; electrical and mechanical. One common sensor is termed a “ball and tube”, in which a ball is held by a small magnet. When a collision occurs, the ball is dislodged from the magnet, rolls forward in the tube, and hits a switch that activates the airbag.
Another popular and modern airbag sensor is the MEMS accelerometer, a small integrated circuit with internal micro mechanical elements. The mechanical element moves with a rapid stop, causing a change in capacitance which is detected by the electronics on the chip. The chip activates the airbag. Most autos have some sensors inside the car, and some on the outside.
Once a sensor detects an actual crash, the next step is bag inflation. And it has to be fast, so fast that the driver’s head doesn’t smash into the steering wheel. The bag must be inflated with nitrogen gas within 55 milliseconds. A millisecond is one-thousandths of a second.
The decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds of the start of the crash. Airbags are fully inflated within 75 milliseconds. The bag has to deploy at a speed of about 200 mph. If the deployment is too slow, the passengers risk injury from the airbag moving toward the passenger at the same time the passenger is moving toward the airbag.
Some manufacturers use an igniter pin that is driven into a sodium azide packet that produces the gas used to inflate the airbag. Then the bag has to deflate on its own once deployed. The gas escapes out tiny vent holes.
The automobile people say the airbag can hurt a person if they are out of position. That’s why they preach that seatbelts must be worn if airbags are to be effective. Airbags are placed in the steering wheel for the driver and dashboard for the front passenger. That dashboard airbag on the passenger side is larger and more expensive than the driver’s airbag, simply because it is bigger. Side doors hide airbags. Modern cars can wrap a person in a cocoon of airbags.
Deployed airbags have been known to kill kids in the front seat. Most states have rules for kids in the front seat. Those laws are based on age and weight.
Airbags have been used by spacecraft landing on Mars, the F-111 fighter/bomber, and the Army’s Blackhawk and Kiowa Warrior helicopter.
Mecedes-Benz was the first production car to install airbags. They started in 1980. A poignant milestone occurred in April, 1990. Two cars, both Chrysler LeBarons, both equipped with airbags, collided head-on at Culpeper, Virginia. The estimated combined speed was 70 mph. One car strayed over the centerline initiating the crash. One driver had a cut on his elbow and a bruised knee. The other driver had a bloody nose and minor bruises. Both walked away. The press headlined the accident as “dueling airbags”.

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