The primary gas inside an airbag is nitrogen, which exists in a solid form called sodium azide. During a severe collision, the solid sodium azide in the airbag rapidly decomposes to release nitrogen gas, inflating the airbag and allowing it to fully deploy. Nitrogen is non-toxic and odorless, and since it is the main component of air, it poses no harm to humans. The noise level during airbag deployment is approximately 130 decibels, which is within the tolerable range for humans. The powder released during deployment is a lubricating powder that prevents the folded airbag from sticking together and is also harmless to humans. Due to these properties, nitrogen is well-suited as the main component of airbags, ensuring passenger protection during vehicle accidents. An airbag system is a passive safety protection system (see vehicle safety performance) that, when used in conjunction with seat belts, provides effective collision protection for occupants. The airbag distributes impact forces evenly across the head and chest. The Supplemental Restraint System (SRS), commonly known as the airbag, serves as an auxiliary device to the seat belt's occupant restraint system. It includes components such as sensor assemblies, inflators, folded airbags, igniters, solid nitrogen, and warning lights. Vehicle test data indicate that deploying airbags during a collision can reduce the likelihood of head injuries by about 25% and facial injuries by approximately 80%. Sensors and microprocessors assess the severity of a crash, transmitting and sending signals. The gas generator then ignites solid fuel based on these signals, producing gas to inflate the airbag. The airbag is housed within the steering wheel hub, close to the cushion, with a capacity ranging from 50 to 90 liters. The fabric used for airbags has high tensile strength, typically made of nylon, and is coated with dry powder to prevent sticking during deployment. To prevent gas leakage, the inner layer of the airbag is coated with sealing rubber. Additionally, a safety valve is installed to release excess gas if pressure exceeds a certain threshold, avoiding injury to passengers. A typical airbag system consists of two main components: a collision detection ignition device (or sensor) and a gas-generating airbag (or air cushion). When the sensor switch is activated, the control circuit begins operation, using detection loops to determine if a collision has occurred. The airbag deploys only if signals are received simultaneously from two sensors. Since the vehicle's generator and battery are usually located in the front, the airbag control system includes a backup power supply to ensure functionality. Once deployment conditions are met, the control circuit sends current to the igniter, rapidly heating it to ignite the sodium azide propellant. The airbag system primarily consists of four parts: collision sensors, the airbag control unit (ACU), the SRS indicator light, and the airbag module. Collision sensors: These are the main input devices for control signals in the airbag system. They detect the intensity of a collision and transmit the signal to the ACU, which determines whether to trigger the inflator. Most systems include 2–4 collision sensors, typically installed on the front left and right fenders, the front bumper, and sometimes inside the cabin. Most collision sensors use an inertial mechanical switch structure. Collision sensors comprise a housing, eccentric rotor, eccentric weight, fixed contacts, and rotating contacts. A resistor (R) is also fixed outside the sensor to check for open or short circuits in the wiring between the ACU and the front collision sensor during system self-tests. Under normal conditions, the eccentric rotor and weight are held against a stopper by a spring, keeping the contacts open ("OFF"). During a collision, the weight's inertia causes the rotor to overcome the spring force, closing the contacts ("ON") if the impact exceeds a set threshold. The ACU triggers the inflator only upon receiving an "ON" signal. Some vehicles also include side airbags, which require additional side collision sensors. Airbag control unit (ACU): The ACU is the system's control center, receiving signals from collision and other sensors to determine whether to deploy the airbag. It also performs self-diagnostics. The ACU continuously tests critical circuits (e.g., sensor circuits, backup power, ignition circuits, and the SRS indicator) and displays results via the SRS light and fault codes stored in memory. The dashboard SRS light informs the driver of the system's status. Fault codes can be retrieved using specialized tools or through a serial interface for inspection. Signal processing circuits, consisting of amplifiers and filters, shape, amplify, and filter sensor signals for the ACU to process. The airbag system has two power sources: the vehicle's main power (battery and alternator) and a backup power (capacitors). The backup power ensures system operation for about 6 seconds if the main power is cut during a collision, allowing the ACU to function and the igniter to deploy the airbag. Beyond 6 seconds, the backup power's capacity diminishes, potentially preventing deployment. Protection circuits guard against voltage spikes from inductive loads or switches in the vehicle's electrical system, while voltage regulation circuits ensure stable operation despite power fluctuations. SRS indicator: This light, labeled "SRS" or "AIRBAG" on the dashboard, indicates system status. It should illuminate briefly (about 6 seconds) when the ignition is turned on, confirming normal operation. If it stays on, fails to light, or activates while driving, a fault exists, and diagnostics are needed. Fault codes stored in memory aid in troubleshooting, though codes may be unavailable after airbag deployment, necessitating ACU replacement. Airbag module: This non-serviceable unit, installed in the steering wheel or dashboard, combines an inflator (with a squib, igniter powder, and gas generant) and a nylon airbag lined with resin. During a collision, sensors trigger the ACU to activate the squib, igniting the gas generant. The resulting gas, cooled by a filter, inflates the airbag, which bursts through its cover to cushion occupants. A clock spring (spiral cable) connects the squib to the ACU, allowing steering wheel rotation. In summary, the airbag system is a passive safety measure that, alongside seat belts, distributes crash forces to protect occupants. The SRS includes sensors, an ACU, an indicator, and airbag modules to ensure effective deployment during collisions.
