Airbag structure principle explained: teach you how to repair

    The airbag can cushion the vehicle in the event of a collision, thereby reducing the impact of the impact on the passengers in the vehicle. Many people equate airbags with SRS, which is inaccurate. In fact, airbags are just one type of SRS. SRS is the abbreviation of English Personal Restraint System. The Chinese meaning is auxiliary protection system. Common auxiliary protection systems include airbags and safety belts. Many cars are marked with SRS or AIR BAG on the steering wheel and above the glove box on the right side of the dashboard, which means that airbags are installed here.       Components of the airbag system   The airbag system mainly includes a collision sensor, an airbag computer, a system indicator, an airbag component and a connection line. The airbag component mainly includes an airbag, a gas generator, and an igniter.

  (1) Collision sensor　 For vehicles produced by various automobile manufacturers, the installation positions of the collision sensors are different, and the names of the collision sensors are not uniform. For example, some collision sensors are also called acceleration sensors according to the working principle.   ① According to different purposes, collision sensors are divided into trigger collision sensors and protective collision sensors. Triggered collision sensor is also called collision intensity sensor, which is used to detect the deceleration or inertia at the time of collision, and transmit the collision signal to the airbag computer as the trigger signal of the airbag computer. Crash sensors are connected in series to prevent accidental explosion of the airbag.   ② According to different structures, collision sensors are divided into electromechanical collision sensors, electronic collision sensors and mechanical collision sensors. The protective collision sensor generally adopts an electronic structure, and the triggering collision sensor generally adopts an electromechanical combined structure or a mechanical structure. The electromechanical combined impact sensor uses mechanical movement (rolling or rotating) to control the electrical contact action, and then opens and closes the contact to control the on and off of the airbag circuit. Common ball type and eccentric hammer type Impact sensor. Electronic impact sensors do not have electrical contacts. There are currently two types of resistance strain type and piezoelectric effect type. Mechanical collision sensors commonly have a mercury switch type, which uses the characteristics of mercury to control the on and off of the airbag circuit.   ③ For early cars, multiple trigger collision sensors are generally provided, and the installation positions are generally in the front and middle of the vehicle body, such as the inside of the fenders on both sides of the vehicle body, under the headlight bracket, and the sides of the engine radiator bracket . With the development of collision sensor manufacturing technology, some cars will trigger the collision sensor installed in the airbag computer. The crash protection sensors are usually assembled with the airbag computer (Figure 1), and most are installed under the center console in the cockpit.   Figure 1 Airbag computer with impact sensor

  (2) Airbag computer　   It is the core component of the airbag system and is mostly installed under the center console in the cockpit (Figure 2). After the airbag explodes, the collision data and fault codes are stored in the airbag computer. These fault codes cannot be cleared by ordinary instruments. In order to ensure the reliability of airbag work, many auto manufacturers recommend that the airbag computer be used once. However, the price of airbag computers is very high, so many instruments with data restoration functions of airbag computers have been developed. By reading and repairing collision data, the airbag computers can be reused. It should be noted that there are cases where the repaired computer is sold as a new accessory in the accessory market, and you should pay attention when purchasing accessories.   Figure 2 Passat B5 airbag computer The airbag system has two power sources, namely the car power source (battery and generator) and a backup power source. The backup power circuit is composed of a power control circuit and a number of capacitors. When the battery and generator are disconnected from the airbag system due to a car collision, the backup power supply can maintain the power supply of the airbag system for a certain period of time (typically 6 s). When repairing the airbag system, pay attention to the role of the backup power supply. After disconnecting the battery power, you still need to wait for a period of time to discharge the backup power supply. For the specific waiting time, refer to the relevant maintenance manual.  

(3) Airbag indicator   Installed on the dashboard to indicate whether the airbag system is functioning normally. Under normal circumstances, after turning on the ignition switch, the airbag indicator should light up for a few seconds and then go out. If the airbag indicator light is off, always on, or suddenly lit during driving, it means the airbag system is faulty and should be repaired in time.  

(4) Airbag   According to different installation positions, airbags can be divided into front airbags and side airbags (air curtains), or driver airbags, front passenger airbags, and rear passenger airbags. In order to improve passenger safety, some cars are also equipped with airbags on the side of the seat and the inside of the door.   ① The airbag is made of nylon, and the inner layer is coated with polychloroprene to seal the gas. When the airbag is at rest, it is folded into a bag and placed between the upper part of the gas generator and the airbag decoration cover. The surface of the airbag decoration cover is stamped with a light imprint so that the airbag tears when the airbag is inflated and reduces the resistance to punch out the decoration cover. . There is an exhaust hole on the back or top of the airbag. When the driver presses on the airbag, the airbag will be exhausted from the exhaust hole when the airbag is under pressure.   ② The driver's airbag is installed in the center of the steering wheel (Figure 3). The size of the airbag varies depending on the manufacturing company. For example, in many places in Europe and Japan, the driver's seat belt is mandatory. The driver's airbag is small, typically 40 L after inflation. ③ The front passenger airbag (Figure 4) is installed directly in front of the front passenger seat, between the glove box on the dashboard and the dashboard. Because the passenger airbag is relatively far from the passenger, the volume of the passenger airbag is larger than that of the driver airbag. In order to avoid unnecessary explosion of the front passenger airbag (for example, no passenger on the seat), the front passenger airbag can be closed by a switch or an instrument (Figure 5).  

 (5) Gas generator   The function is to generate gas to inflate the airbag when the igniter detonates the igniter, so that the airbag explodes. The airbag generator is fixed to the airbag holder using special bolts, and can only be assembled using special tools. The gas generator should be replaced once every 10 years from the date of installation. ① In order to facilitate installation, the driver airbag gas generator is generally made in a circular shape. At present, most gas generators use thermal effects to generate nitrogen to fill the airbag. ② The gas generator of the front passenger airbag has a long cylinder shape, and its working principle is the same as that of the driver's gas generator.  

(6) Ignitor　   It is installed in the central position of the gas generator. When the collision sensor and the protective collision sensor are triggered to switch on the airbag circuit, the ignition agent is detonated and heat is generated to decompose the inflation agent.  

(7) Airbag system wiring harness connector and insurance mechanism　   In order to easily distinguish the airbag system wiring harness from other electrical system wiring harnesses, the airbag system wiring harnesses of most cars currently use yellow connectors (Figure 6), and dark blue or orange-red connectors are also used. The connector uses gold-plated terminals with good conductivity and durability, and is designed with a mechanism to prevent airbags from accidentally exploding to ensure the airbag system to work reliably.   Figure 6 Airbag wiring harness using yellow connecting wire ① The connector from the airbag computer to the igniter uses a short-circuit piece mechanism (copper spring leaf, also called short-circuit spring leaf) to prevent the airbag from accidentally exploding. When the connector is unplugged (the plug is unplugged or the plug is not fully combined with the socket), the short-circuit tab automatically shorts the 2 lead terminals near the plug or socket on the side of the airbag igniter to prevent the airbag from exploding due to static electricity or incorrect power. ② In order to ensure that the steering wheel has a sufficient rotation angle without damaging the connecting harness of the airbag module, a spiral harness is used between the steering wheel and the steering column, that is, the harness is installed in a spiral spring. In the repair manuals provided by different car manufacturers, the name of the spiral harness varies, for example, it is also called a coil spring, hairspring or spring. When the coil spring is installed between the steering wheel and the steering column, pay attention to the installation position and direction (Figure 7). When removing and installing the steering wheel, the steering column should be fixed in a "straight forward" position to avoid damaging the spiral wiring harness.

  Figure 7 Pay attention to the direction when installing the coil spring   Overhaul of airbag system   (1) Safety regulations Except for the originally designed wiring harness, it is strictly forbidden to connect other wiring harnesses to the airbag system wiring harness; the use of multimeters and other instruments capable of generating power to detect the igniter is prohibited; when storing the airbag, it should be placed with the airbag up and the connector down In case of accidental explosion, the risk of such placement is small; do not try to open the airbag or igniter of the airbag with tools, and prohibit heating it; remove the battery positive electrode before repairing and welding; check carefully before connecting the electrical wiring harness Whether the wiring harness is powered off.   (2) Disassembly precautions   The following operations should be performed before any disassembly work. ① First turn on the ignition switch and check whether the airbag indicator light on the dashboard is working properly. ② Turn off the ignition and remove the key. Disconnect the positive electrode of the battery and wait for more than 2 minutes. If the airbag indicator is not working properly, wait 10 minutes after disconnecting the positive electrode of the battery before proceeding. When removing the steering wheel, special tools should be used to lock the steering column in the "straight forward" position to ensure that the control device and spiral harness will not be damaged during installation. ③ Only the accessories with the same part number as the original car can be installed. The igniter has an expiration date. Observe the service life stated on the accessories. ④ After turning on the battery, when the ignition switch is turned on, the maintenance personnel should not put their body in the track of the airbag opening. ⑤ After installation, check whether the airbag indicator is operating normally. Airbag manufacturer Well-known companies that develop airbags include TRW, Breed, and Delphi in the United States, Autoliv in Europe, Bosch, and Siemens, and Japan. Denso, Toyota Gosei, etc. At present, most of the airbags installed in domestic cars are imported products. China ’s research on airbags started in the early 1990s. At present, there are more than a dozen companies producing airbag devices. But most of them are imported from foreign countries such as airbags and igniters for assembly and production. Some domestic airbag system manufacturers have tried to develop and master the control and matching technology of airbags, and have made certain progress. The localization of airbag systems is an inevitable trend.