Analysis of the Principle of Automobile Collision Sensor

The collision sensor is a control signal input device in the airbag system. Its function is to detect the intensity signal of the car collision when the car collides, and input the signal into the airbag computer. The airbag computer determines whether to detonate the inflation element to inflate the airbag according to the signal of the collision sensor. Most collision sensors use an inertial mechanical switch structure, which is equivalent to a control switch. Its working state depends on the magnitude of the acceleration of the car during the collision.

The function of the collision sensor is to detect the inertial force under extreme deceleration when the car collides, and input the detection signal to the electronic control device of the airbag system.

Automobile collision sensor principle

Collision sensors are widely used, especially in automobile safety systems, and now some robots are also equipped with collision sensors. Here we first understand the principles of automobile collision sensors.

Generally, collision sensors are installed in three main places on the car: left front, right front and center.

Automobiles are generally equipped with multiple trigger collision sensors, which are generally installed in the front and middle of the car body, such as the inner side of the fenders on both sides of the car body, under the headlight bracket, and on both sides of the engine radiator bracket. With the development of collision sensor manufacturing technology, some cars install trigger collision sensors in the airbag computer. Protection collision sensors are generally assembled with airbag computers, and most are installed under the central console in the cockpit.

Airbags need to be used in conjunction with seat belts to work, and the first condition for the normal operation of airbags is that the car collision sensor must work normally and give a signal to the central controller within a very short time after the collision occurs. The central controller issues a command to eject the airbag.

The central controller is the control center of the airbag system. Its function is to receive signals from collision sensors and other sensors, determine whether to ignite and inflate the airbag, and perform self-diagnosis of system faults.

The central controller consists of an ignition controller, a drive circuit, a storage circuit, a diagnostic circuit, etc. The key in the ignition controller is the ignition control algorithm, which determines the airbag's detonation time and the rough road anti-interference characteristics. This is the core technology of airbag control. Due to the different actual vehicle structures and collision characteristics, many ignition control algorithms have been developed to adapt to different situations, and they are still being developed.

The methods currently used include velocity variable method, acceleration slope method, specific power method, etc. For a head-on collision, the collision sensor and control system generally need to determine whether ignition is required within 20ms, and the airbag must be fully filled with gas 30ms after ignition.

When the car speed is above 30km/h and is hit head-on (the collision angle is within 30° with the car's central axis) or sideways, the collision sensor installed on the front or side of the car uses the inertial force generated during the collision to detect the collision time, car deceleration, and collision intensity. It is then compared with the parameters set before leaving the factory to see if the set value is reached. If it exceeds the set value, an alarm signal will be given.

Airbag working principle diagram SRSECU compares the collision signal sent by the collision sensor with the collision trigger data stored in the ECU. If it is determined that the collision intensity reaches or exceeds the specified value, the command connects the working circuit of the airbag detonator, and the detonator explodes and burns rapidly, and ignites the gas generator in the gas generator. The combustion of the gas generator is very intense, and a large amount of gas is released in an instant. After filtering and cooling, it is filled into the folded airbag, so that the airbag breaks through the cushion in a very short time and expands rapidly into a flat sphere. When the driver or passenger's head, chest or body rushes forward or to the side due to the recoil force during the collision, the bulging airbag forms an elastic buffer air cushion between the driver or passenger's front or side body hardware, and absorbs and disperses the impact energy of the driver and passenger by using the damping effect of the gas itself or the damping effect of the exhaust throttling of the exhaust hole on the back of the airbag. After the airbag bulges, part of the gas is discharged from the small hole on the back of the airbag and becomes deflated. The soft airbag surface can effectively protect the human head, chest and other parts of the body from impact damage or reduce the degree of damage.

We use the test of the SRS produced by Bosch, Germany, on Audi cars to illustrate the working state of the airbag. When the car hits the obstacle in front at a speed of 50km/h, the protective action process of the airbag system can be divided into four steps as shown in Figure 9-5. 1) See Figure 9-5 (a). About 10ms after the collision, the SRS reaches the detonation limit. The detonator detonates to generate a large amount of heat energy, ignites the gas generator sodium azide tablets, and decomposes them under heat. At this time, the driver has not yet leaned forward due to the inertia of the collision. 2) See Figure 9-5 (b). About 40ms after the collision, the airbag is fully inflated and expanded to the maximum volume. The driver rushes forward due to the inertia of the collision. At this time, the seat belt tied to the driver is quickly tightened, absorbing part of the impact energy. 3) See Figure 9-5 (c). About 60ms after the collision, the driver's head and upper body quickly press against the inflated airbag, and the impact energy of the human body is absorbed and diffused by the elastic airbag. The exhaust holes on the back of the airbag exhaust gas outwards under the action of gas tension and human body pressure, and the exhaust throttling damping further absorbs the kinetic energy generated by the elastic collision between the human body and the airbag, effectively protecting the driver's life. 4) See Figure 9-5 (d). About 110ms after the collision, most of the gas has escaped from the airbag, and the airbag has deflated, preventing the driver from being suffocated by the inflated airbag. Under the action of the seat belt, the driver leans back on the seat back and the front of the car is restored. About 120ms after the collision, the kinetic energy hazard generated by the car collision is completely eliminated, and the vehicle speed is reduced to zero. The commonly used collision sensors in the automobile airbag system can generally be divided into six categories: rolling ball collision sensor, roller collision sensor, eccentric hammer collision sensor, mercury switch collision sensor, resistance strain gauge collision sensor, and piezoelectric effect collision sensor. The working principles of these six types of collision sensors will be introduced in detail below. 1. Working principle of collision sensor: rolling ball collision sensor. Rolling ball collision sensor is also called biased magnet collision sensor. Its structure is shown in Figure 1. It is mainly composed of iron rolling ball, permanent magnet, guide cylinder, fixed contact and shell.

The two electric contacts are connected to the sensor lead terminals respectively. The ball is used to sense the deceleration and can move or roll in the guide cylinder. The housing 5 is printed with an arrow mark. The direction is related to the sensor structure. Some regulations point to the front of the car (such as Toyota Lexus LS400 sedan), and some regulations point to the rear of the car. Therefore, when installing the sensor, the arrow direction must comply with the instructions for use. The working principle of the ball collision sensor is shown in Figure 2. When the sensor is in a stationary state, under the action of the permanent magnet, the ball in the guide cylinder is attracted to the magnet, the two contacts are separated from the ball, and the sensor circuit is in a disconnected state, as shown in Figure 2. When the car is hit and the deceleration reaches the set threshold, the inertial force generated by the ball will be greater than the electromagnetic attraction of the permanent magnet. Under the action of the inertial force, the ball will overcome the magnetic force and move along the guide cylinder to the two fixed contacts and connect the fixed contacts, as shown in Figure 2. When the sensor is used as a collision sensor signal, the fixed contacts are connected to input the collision signal into the SRS ECU; when the sensor is used as a collision protection sensor, the ignition power supply is turned on. 2. Working principle of collision sensor: roller type collision sensor The structure of roller type collision sensor is shown in Figure 3.

One end of the leaf spring 6 is fixed on the base 5 and connected to a lead terminal of the sensor, and the other end is wound on the roller 2. The rolling contact 3 is fixed on the leaf spring of the roller part and can rotate with the roller. The contact 4 is fixed on the base 5 and insulated from the leaf spring 6, and is connected to another lead terminal of the sensor. When the sensor is in a stationary state, the roller rolls to one end of the stop pin under the elastic force of the leaf spring, and the rolling contact and the fixed contact are in a disconnected state, as shown in Figure 3, and the sensor circuit is disconnected. When the car is hit and the deceleration reaches the set threshold, the inertial force generated by the roller will be greater than the elastic force of the leaf spring. Under the action of the inertial force, the roller will overcome the spring elastic force and roll to the right, and the rolling contact will contact the fixed contact, as shown in Figure 3. When the sensor is used as a collision signal sensor, the rolling contact contacts the fixed contact and the collision signal is input into the SRSECU; when the sensor is used as a collision protection sensor, the igniter power supply circuit is turned on. 3. Working principle of collision sensor: eccentric hammer collision sensor The eccentric hammer collision sensor is also called eccentric rotor collision sensor. Toyota and Mazda SRS adopt this sensor, and its structure is shown in Figure 4.