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
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 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.
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 airbag inflation process must be immediate and precise.
Here are a couple of units that display common inflation techniques.
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.
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.
Heated gas inflator (HGI)
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.
Similar to pyrotechnical inflators, the initiator in HGIs is also an electric match.
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.
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 airbags prevent impact between the driver and passenger or between the rear passengers.
Inflatable seatbelts increase the surface area against an occupant’s body to reduce injury.
Knee airbags specifically prevent knees from shattering on impact.
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