How Airbags Work

How Airbags Work

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Airbags serve as a Supplemental Restraint System (SRS), which is secondary to the seatbelt system.

When a crash happens, sensors send data to a central "brain" (called the "Airbag Control Unit"). The ACU evaluates all incoming information to decide if airbag deployment is necessary, and if needed, deploy specific airbags.

The process usually happens in under 0.03 seconds, and before a human body would move toward it because of the crash.

Sensing and evaluating a crash

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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.

Seat occupancy sensor

The seat occupancy sensor uses 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.

External sensors

External sensors have 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.

Airbag Control Unit (ACU)

The ACU consists of a processor core, memory, inputs, and outputs. It uses a logic gate programmed with algorithms to "decide" whether to deploy 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.

  • Accelerometer

    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.

  • Gyroscope

    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.

Airbag inflation

The airbag inflation process must be immediate and precise.
Here are a couple of units that display common inflation techniques.

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Pyrotechnical inflator

  • Propellant

    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.

  • Initiator

    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.

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Heated gas inflator (HGI)

  • Diffuser

    The heated gas exits the cylinder through the diffuser and into the airbag.

  • Second burst disk

    The internal pressure increases enough to rupture the second burst disk.

  • Compressed gas cylinder

    The compressed gas consists of hydrogen, oxygen, and nitrogen.

  • First burst disk

    The initiator ruptures the first burst disk and heats the compressed gas.

  • Initiator

    Similar to pyrotechnical inflators, the initiator in HGIs is also an electric match.

The airbag

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Woven nylon fabric

Powder

The airbag assembly usually contains talcum powder or cornstarch as a lubricant so the airbag doesn’t get stuck on deployment.

Storage

Airbags are folded and stored in a compartment inside the vehicle, which has designated seams that tear open during deployment.

Vents

Airbags have vents in the back that allow them to slightly deflate and cushion the vehicle’s occupant as they make contact with it.

Airbag types

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Side torso airbag

Side torso airbags have a lower, firmer chamber to support the pelvis and an upper, softer chamber to cushion the ribcage.

Side curtain airbag

Side curtain airbags stay inflated for several seconds to protect occupants if the vehicle rolls over.

Center airbag

Center airbags prevent impact between the driver and passenger or between the rear passengers.

Seatbelt airbags

Inflatable seatbelts increase the surface area against an occupant’s body to reduce injury.

Front airbags

Knee airbag

Knee airbags specifically prevent knees from shattering on impact.

References

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