A deployed car airbag cannot be repaired; it must be replaced. This is because car airbags are single-use items. Once deployed in an accident, they become ineffective and require replacement with a new airbag. The primary gas in 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 ensuring full deployment. Nitrogen is non-toxic and odorless, and since it is a major component of air, it poses no harm to humans. The sound level during airbag deployment is approximately 130 decibels, which is within the tolerable range for humans. The powder released during deployment is a lubricant that prevents the folded airbag from sticking together and is also harmless to humans. These characteristics make nitrogen ideal as the primary component of airbags, ensuring passenger protection during accidents. The working principle of a car airbag is as follows: When a collision occurs while driving, the airbag sensor first receives the impact signal. If the impact reaches a specified intensity, the sensor activates and sends a signal to the electronic control unit (ECU). The ECU compares this signal with stored data. If the conditions for airbag deployment are met, the ECU sends an activation signal to the gas generator in the airbag assembly. Upon receiving the signal, the gas generator ignites the propellant, producing a large volume of gas. This gas is filtered and cooled before entering the airbag, causing it to rapidly inflate within milliseconds. The airbag forms an elastic cushion in front of the driver or passenger, then deflates and contracts to absorb impact energy, effectively protecting the head and chest from or reducing injury severity. A standard airbag must activate within 0.01 seconds after a collision, with the ignition device firing within 0.03 seconds, high-pressure gas entering the airbag within 0.05 seconds, outward expansion beginning within 0.08 seconds, and full inflation achieved within 0.11 seconds. The airbag system mainly consists of four components: collision sensors, the airbag control module (SRS computer), the SRS indicator light, and the airbag assembly. Collision Sensors: These are the primary input devices for the airbag system. They detect the intensity of a collision and send the signal to the SRS computer, which determines whether to activate the inflator. Most systems have 2-4 collision sensors, typically located on the left and right front fenders, the front bumper, or inside the cabin. Most sensors use an inertial mechanical switch structure. The collision sensor comprises a housing, eccentric rotor, eccentric weight, fixed contact, and rotating contact. An external resistor (R) checks for open or short circuits between the SRS computer and the sensor during system self-tests. Normally, the eccentric rotor and weight are held against a stopper by a spring, keeping the contacts open ("OFF"). During a collision, inertia causes the eccentric weight to rotate the rotor, overcoming the spring force. If the impact meets the threshold, the contacts close ("ON"), signaling the SRS computer to activate the inflator. Some vehicles also have side airbags with additional sensors for side impacts. SRS Computer: This is the control center of the airbag system. It processes signals from collision and other sensors, decides whether to deploy the airbag, and performs self-diagnostics. It continuously tests critical circuits (e.g., sensor, backup power, ignition, SRS indicator) and displays results via the indicator or fault codes stored in memory. The dashboard SRS light informs the driver of system status, while fault codes can be retrieved via specialized tools. Signal Processing Circuit: Comprising amplifiers and filters, it conditions sensor signals for the SRS computer. Power Sources: The system uses two power supplies—the vehicle battery/alternator and a backup power source (capacitors). The backup ensures operation for ~6 seconds if the main power is cut during a collision, allowing the SRS computer to function and the inflator to ignite. Beyond 6 seconds, backup power diminishes, risking deployment failure. Protection and Voltage Regulation: To safeguard against voltage spikes from inductive loads (e.g., coils, switches), the SRS module includes protection circuits. A voltage regulator maintains stable operation despite fluctuations in vehicle power. SRS Indicator: Located on the dashboard, it shows system status. A 6-second light or flash upon ignition indicates normal operation. Persistent or sudden illumination signals a fault, with codes stored for diagnosis. After a collision where the airbag deployed, fault codes aid in troubleshooting.
