Airbags serve as a Supplemental Restraint System (SRS), which is secondary to the seatbelt system.
Sensing and evaluating a crash
What happens in a crashWhen a crash happens,
sensors 1send data to the
ACU 2(Airbag Control Unit), which decides to deploy the airbags. Once the decision is made, the ACU instructs the
inflator 3to initiate. Inflators either burn fuel to create gas, or heat expandable gas, which fills the
The deployment process usually happens in under 0.03 seconds. That's about ten times faster than the blink of an average human eye!
Sensing and evaluating a crash
Crash sensors located throughout the vehicle collect information and send it to the ACU. Most modern crash sensors are microelectromechanical systems (MEMS), which are tiny devices that detect mechanical stress and convert it into an electrical signal.
The pressure sensors use a force collector, such as a spring or diaphragm, to determine the amount of pressure being applied and convert the pressure into an electrical signal.
2 Front crash sensorsand
3 Side crash sensorsare similar in design and function, and detect external forces.The
4 Seat occupancy sensoruses a silicone bladder on top of a pressure sensor to sense the weight of the passenger. If a passenger is detected, the sensor sends a signal to the ACU, and the corresponding airbag is deployed.
1 Wheel speed sensors
Wheel speed sensors use a toothed wheel attached to the vehicle’s wheel and a magnet surrounded by a coil to determine wheel speed. As the teeth pass by the magnet, the magnetic field fluxes and creates a current.
The ACU consists of a processor core, memory, inputs, and outputs. It uses a logic gate programmed with algorithms to "decide" whether to deploy the airbags. If the resulting velocity and acceleration of a crash surpasses a set threshold, the ACU sends a signal to the inflation unit of the airbags. The ACU also performs regular diagnostics and, in most modern cars, contains a "black box" or Event Data Recorder (EDR) that saves crash information.
The accelerometer detects changes in acceleration by measuring the displacement of a small mass, and then converting the movement into an electrical signal. Frontal crashes cause rapid deceleration, while side crashes cause rapid acceleration.
A vibrating gyroscope senses angular velocity (change in rotational angle over time) by measuring the directional change of vibration (as the vehicle’s orientation rotates) in the mechanical structures and converting the movement into an electrical signal.
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The inflation unit inflates the airbags, which then slightly deflate to cushion the occupants of the vehicle. Pyrotechnical and heated gas inflators are the main types that are used. On average, the entire inflation process takes only 10-15 milliseconds (0.01 – 0.015 seconds).
The initiator is an electric match, which is a bridgewire (an electrical conductor) wrapped in combustible material. Electrical currents heat up the bridgewire, which ignites the combustible material.
The initiator heats a solid propellant that rapidly decomposes and produces nitrogen gas. The most common propellant chemicals used are phase-stabilized ammonium nitrate and nitroguanidine.
Nitroguanidine, for example, could break down something like this: CH4N4O2 (nitroguanidine) 2N2 (nitrogen gas) + CO2 (carbon dioxide) + 2H2 (hydrogen gas). The gas that fills the bag (nitrogen), nitrogen, and the byproducts exit through metal mesh filters (which cool and remove particles from the gas) and into the airbag.
Heated gas inflator (HGI)
Similar to pyrotechnical inflators, the initiator in HGIs is also an electric match.
4 Compressed gas cylinder
The compressed gas consists of hydrogen, oxygen, and nitrogen.
5 First burst disk
The initiator ruptures the first burst disk and heats the compressed gas.
6 Second burst disk
The internal pressure increases enough to rupture the second burst disk.
The heated gas exits the cylinder through the diffuser and into the airbag.
1 Woven nylon fabric
The airbag assembly usually contains talcum powder or cornstarch as a lubricant so the airbag doesn’t get stuck on deployment.
Airbags are folded and stored in a compartment inside the vehicle, which has designated seams that tear open during deployment.
Airbags have vents in the back that allow them to slightly deflate and cushion the vehicle’s occupant as they make contact with it.
Types of airbags
The location and purpose of airbags determine their shapes, sizes, firmness, and how quickly they inflate.
1 Front airbags
Front airbags inflate at a rate determined by seat occupancy and crash severity.
2 Side airbags
Side torso airbags have a lower, firmer chamber to support the pelvis and an upper, softer chamber to cushion the ribcage.
Side curtain airbags stay inflated for several seconds to protect occupants if the vehicle rolls over.
3 Knee airbags
Knee airbags specifically prevent knees from shattering on impact.
4 Center airbags
Center airbags prevent impact between the driver and passenger or between the rear passengers.
5 Seatbelt airbags
Inflatable seatbelts increase the surface area against an occupant’s body to reduce injury.
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