Airbag system principle and structure analysis

Since the adoption of civilian vehicles in the 1980s, airbags have become a very important passive safety device for automobiles. The number of airbags has become one of the references for measuring the safety of vehicles. The structure and principle of airbags How? What conditions do the airbags need to be opened? What are the disadvantages of the airbags? What should you pay attention to during the use? Let's explain them one by one.

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Principle and structure of airbag

The airbag is part of the SRS, primarily to prevent injury from collisions between the occupant and the interior of the vehicle during a car collision. It is usually used as an auxiliary safety device for the seat belt. collective effect. The protection principle of the airbag is: When the car is subjected to a certain collision force, the airbag system will cause a chemical reaction similar to the explosion of a micro-explosive. The airbag hidden in the car will be inflated in an instant, in the body and inside of the occupant. The parts can be placed in time before the collision. When the human body touches the airbag, the airbag starts to vent through the air holes on the surface of the airbag, thereby laying a cushioning effect, reducing the impact of the body, and finally achieving the effect of reducing the occupant's injury.

Schematic diagram of the airbag system of the usual model

The commonly used car airbag system consists of a collision sensor, a control module (ECU), a gas generator, and an air bag. The following are the main components.

Airbag system sensor

The airbag sensor is also generally called a collision sensor. According to the application, the collision sensor is divided into a trigger collision sensor and a protection collision sensor. The triggering collision sensor, also called the collision intensity sensor, is used to detect the acceleration change during the collision, and transmits the collision signal to the airbag computer as the trigger signal of the airbag computer; the protection collision sensor is also called the safety collision sensor, and the triggering collision sensor In series, used to prevent airbags from accidentally exploding.

According to the structure, the collision sensor can be divided into an electromechanical collision sensor, an electronic collision sensor and a mechanical collision sensor. The protective collision sensor generally adopts an electronic structure, and the trigger collision sensor generally adopts an electromechanical combined structure or a mechanical structure. The electromechanical combined collision sensor uses mechanical motion (rolling or rotating) to control the action of the electrical contacts, and then the contacts are opened and closed to control the opening and closing of the airbag circuit. Commonly, there are ball and eccentric hammers. Collision sensor. The electronic collision sensor has no electrical contacts. Currently, there are two types of resistance strain type and piezoelectric effect type. A common type of mechanical collision sensor is a mercury switch type, which utilizes the conductivity of mercury to control the opening and closing of the airbag circuit.

Airbag collision sensor mounted on the front longitudinal beam of the engine compartment, mostly electromechanical

Control module (ECU)

For early cars, there are usually multiple trigger collision sensors. The installation position is generally in the front and middle of the body, such as the inside of the fenders on both sides of the body, under the headlight brackets, and on both sides of the engine radiator bracket. With the development of collision sensor manufacturing technology, some cars will trigger a collision sensor installed in the airbag system ECU. Protective impact sensors are typically assembled with the airbag system ECU, most of which are mounted under the center console in the cockpit. The ECU is the core component of the airbag system and is mostly installed under the center console in the cockpit.

Most airbag control modules (ECUs) are installed in the middle of the body near the handlebars.

Typical airbag system control circuit schematic

During the driving process, the sensor system continuously sends speed change (or acceleration) information to the control device, and the airbag control module (ECU) analyzes and judges the information if the measured acceleration, speed change or other index exceeds a predetermined value ( That is, a collision actually occurs, and the capsule control module issues an ignition command to the gas generator.

The gas generator

The gas generator is mainly used to generate a large amount of gas in a short period of time (about 30ms) to fill the airbag, the generated gas must be harmless to the human body, and the temperature cannot be too high, and the gas generator is required to be highly reliable. Sex and stability. At present, gas generators are mainly in the form of compressed gas, pyrotechnic and hybrid. The hybrid gas generator is a combination of a compressed gas type and a pyrotechnic type, and is also widely used as a gas generator.

Modules (back), including gas generators and airbags, are generally not allowed to be disassembled

The gas generator stores substances such as sodium nitride or ammonium nitrate. When the car is subjected to a violent impact during high-speed driving, these substances will rapidly decompose and generate a large amount of gas (non-toxic and odorless nitrogen accounts for more than 70%), which is filled with airbags. The newer airbags incorporate a device that can be graded or released to prevent the damage caused by the enormous pressure generated by a sudden burst. The grading point blasting device, that is, the gas generator is blasted in two stages, the first stage generates about 40% of the gas volume, far below the maximum pressure, which has a buffering effect on the movement of the human head, and the second stage blast generates residual gas. And reach the maximum pressure. The grading release pressure method is to open a pressure relief hole or a pressure adjustable hole on the airbag bag, and the pressure can be instantaneously released at the beginning of the pressure to avoid excessive damage to the passenger.

Buffer airbag

The airbag is generally made of polyamide fabric with good crack resistance. It is a semi-hard polymer material that can withstand large pressures; after vulcanization treatment, it can reduce the impact force when the airbag is inflated. To seal the gas, the bladder is coated with a coating material. The size, shape and air leakage performance of the airbag are important factors in determining the protective effect of the airbag, and must be determined according to the actual conditions of different vehicles. The airbag is folded into a bag when it is at rest, and is placed between the upper part of the gas generator and the airbag cover. The surface of the airbag cover is molded with a light print so that the airbag can tear the cover when the air is inflated and reduce the resistance of the cover. . A vent hole is provided on the back or top of the airbag, and when the driver presses on the airbag, the airbag is pressurized and then exhausted from the venting hole.

In addition, the airbag system has a backup Power Supply, and the backup power supply circuit is composed of a power supply control circuit and a plurality of capacitors. When the collision of the car causes the battery and the generator to be disconnected from the airbag system, the backup power supply can maintain the power supply of the airbag system for a certain period of time (generally 6 seconds).

In order to ensure that the steering wheel has a sufficient rotation angle without damaging the connection harness of the airbag module, a spiral beam is used between the steering wheel and the steering column, that is, the wire harness is installed in the spiral spring (the spiral beam is also called a coil spring, Hairspring or spring spring).

Airbag initiation condition

In order to ensure that the airbag is opened at the appropriate time, the car manufacturer has specified the detonation conditions of the airbag, and only when these conditions are met, the airbag will explode. Although in some traffic accidents, the occupants in the car have broken their blood flow and even life-threatening, the vehicle is close to scrapping, but if the airbag explosion conditions are not met, the airbag will not open.

The airbag opening requires a suitable speed and collision angle. In theory, only when the front of the vehicle is about 60° to the left and right and hits a fixed object at a speed higher than 30 km/h, the airbag can be opened. The speed mentioned here is not the speed of the vehicle that we usually understand, but the speed at which the vehicle collides with the relatively rigid fixed obstacle in the laboratory. The speed of the car in the actual collision is equivalent to the speed of the test to open the airbag.

In the crash test, the rigid object is struck, and the actual collision speed is equivalent to the predetermined speed, and the airbag will open.

In the collision of the car, the main force is the passenger compartment skeleton and the body longitudinal beam. In order to cushion the impact force during the collision, the front part of the vehicle body is mostly designed with a collision buffer, and the rigidity of the vehicle body is also uneven. In some accidents, such as when a car has a drilling rear-end collision with a truck without a rear guard, or a rollover accident occurs after the car collides with the guardrail, or a side collision occurs, such an accident often does not have a direct impact on the front of the vehicle body. The main part is the collision between the upper part and the side of the vehicle body. The rigidity of the collision part of the vehicle body is very small. Although the cabin has undergone a great deformation, the occupant in the vehicle is injured or killed. However, due to the wrong collision position, the airbag cannot be opened sometimes.

Airbags make defects

As one of the effective measures to improve the safety of automobiles, airbags have received more and more attention. In some actual collision accidents, airbags have proved to have the effect of reducing occupant casualties, but some problems existed. The problems with airbags in use are:

The airbag may be turned on at very low speeds. When a car crashes at a very low speed, the occupant and the driver can wear a seat belt, and the airbag is not required to be deployed. If the airbag is deployed at this time, it will cause unnecessary waste, and may even cause collision damage due to the deployment of the airbag.

The activation of the airbag can cause injury to the occupant. When the airbag system is activated, the airbag cover will be opened and the air is inflated in an instant, which may cause an impact on the occupant; the generated hot gas will also burn the occupant and the driver. According to calculations, if the car is driven at a speed of 60km, the sudden impact will cause the vehicle to stop within 0.2 seconds, and the airbag will pop out at a speed of about 300km/h, and the impact force generated by this will be about 180kg. This is difficult for the head, neck and other parts of the human body that are more vulnerable.

When the passenger deviates from the seat or the seat is unoccupied or the child is riding, the activation of the airbag system not only fails to provide the protection it deserves, but may also cause certain damage to the occupant.

The co-pilot airbag that can be closed is still very practical.

Airbag use

Because the airbag system has certain dangers and needs to be used correctly to play the role of the SRS system, there are some things to be aware of when using the airbags in daily use. In the case of ensuring the correct driving posture, the driver should move the seat as far as possible so that there is enough space for the airbag to fully protect its protection after an accidental expansion, and the driver must fasten the seat belt. Children under the age of 12 should sit in the back of the car and fasten their seat belts. They must not put the children in the front row. The huge impact of the airbags may even be fatal to them!

For models with airbags, rear child safety seat fixtures should be more fully utilized


Although the current car manufacturers use airbags as an important selling point to promote, but the airbags are a double-edged sword that can be used as a double-edged sword. When you use it properly, it can play a good protective role. Or when it is used unreasonably, it will have side effects. Again, all safety equipment is auxiliary, and safe driving is the most important guarantee for safety.

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