The function of the airbag system is to inflate the airbag to protect the occupants before a secondary collision occurs during a crash, preventing the occupants from colliding with the vehicle's interior components and thus suffering severe injuries. The vehicle's airbag system works in conjunction with the seat belts. The Supplemental Restraint System (SRS), commonly known as the airbag, serves as an auxiliary device to the seat belt's occupant restraint system. It is a supplementary protection system that includes components such as the sensor assembly, inflator, folded airbag, igniter, solid-state nitrogen, and warning light. The airbag system consists of the airbag and inflator (gas generator) forming an integrated airbag module, a collision sensor system that detects impacts and sends deployment commands to the airbag module, and wiring harnesses that transmit signals from the sensors. According to vehicle test data, when a collision occurs, the deployment of the airbag can reduce the probability of head injuries by approximately 25% and facial injuries by about 80%. Sensors and microprocessors are used to assess the severity of the crash and transmit signals. The gas generator, upon receiving the signal, ignites the solid fuel to produce gas and 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 the airbag has high tensile strength, typically made of nylon, and the folded surface is coated with dry powder to prevent the airbag from sticking together and rupturing during deployment. To prevent gas leakage, the inner layer of the airbag is coated with sealing rubber. Additionally, the airbag is equipped with a safety valve that automatically releases excess gas if over-inflation or excessive pressure occurs, avoiding injury to passengers. A typical airbag system consists of two main components: the collision detection ignition device (or sensor) and the gas generator airbag (or airbag). When the sensor switch is activated, the control circuit begins operating and uses detection loops to determine if a collision has occurred. The airbag will only deploy if signals are received simultaneously from two sensors. Since the vehicle's alternator and battery are usually located in the front of the car, which is prone to damage, the airbag control system has its own backup power supply to ensure functionality. Once the conditions for airbag deployment are confirmed, the control circuit sends current to the igniter, which rapidly heats up and ignites the sodium azide propellant inside. The airbag system mainly consists of four parts: the collision sensor, airbag control unit (SRS computer), SRS indicator light, and airbag assembly. Collision Sensor: The collision sensor is the primary input device for control signals in the airbag system. Its role is to detect the intensity of a collision and send the signal to the airbag control unit, which then determines whether to trigger the inflator to deploy the airbag. Most airbag systems are equipped with 2-4 collision sensors, typically located on the left and right front fenders, the front bumper, and sometimes inside the cabin. Most collision sensors use an inertial mechanical switch structure. The collision sensor consists of a housing, eccentric rotor, eccentric weight, fixed contact, and rotating contact. A resistor R is also fixed outside the sensor to check for open or short circuits in the wiring between the airbag control unit and the front collision sensor during system self-tests. Under normal conditions, the eccentric rotor and weight are pressed against the stopper connected to the housing by the force of the spiral spring, keeping the rotating contact separate from the fixed contact (switch "OFF"). During a collision, the eccentric weight's inertia causes the rotor to overcome the spring force and rotate. If the collision intensity meets the threshold, the rotor's rotation closes the contacts, sending an "ON" signal to the airbag control unit. The airbag will only deploy upon receiving this "ON" signal. Some vehicles also have side airbags, which require additional side collision sensors. Airbag Control Unit (SRS Computer): The airbag control unit is the system's central control, receiving signals from collision sensors and other inputs to determine whether to deploy the airbag. It also performs self-diagnostics for system faults. The control unit continuously tests critical circuits (e.g., sensor circuits, backup power, ignition circuits, SRS indicator) and displays results via the SRS light and stored fault codes. The SRS indicator on the dashboard provides the driver with system status. Fault codes can be retrieved using specialized tools for inspection. Signal Processing Circuit: Comprising amplifiers and filters, this circuit shapes, amplifies, and filters sensor signals for the SRS computer to process. Power Supply: 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 up to 6 seconds if the main power is cut during a collision, allowing the airbag to deploy. Beyond 6 seconds, the backup power's capacity diminishes, potentially preventing deployment. Protection and Voltage Regulation: To safeguard against voltage spikes from inductive loads in the vehicle's electrical system, the SRS module includes protection circuits. A voltage regulation circuit ensures stable operation despite power fluctuations. SRS Indicator: This light on the dashboard indicates system status. If it lights up or flashes for about 6 seconds after ignition and then turns off, the system is functional. Persistent illumination or sudden activation while driving signals a fault, requiring diagnosis via fault codes. Note that fault codes may not be retrievable after an airbag deployment, necessitating SRS computer replacement. Airbag Assembly: Comprising the inflator and airbag, this non-serviceable unit is installed in the steering wheel or dashboard. The inflator contains a squib, igniter powder, and gas-generating agent. Upon collision impact, sensors trigger the SRS computer to activate the squib, igniting the powder to produce gas. The cooled gas inflates the nylon airbag, which bursts through the steering wheel to cushion occupants. A spiral cable (clockspring) connects the squib to the SRS computer, allowing steering wheel rotation. The airbag system is a passive safety feature that, when used with seat belts, provides effective crash protection by distributing impact forces evenly across the head and chest.
