the airbag is opening

Figure 1: The airbag in the car.



Cars have an airbag that everyone doesn't want to use because it plays a role in car collisions, but the airbag is on standby all the time, with the effect of a knock sensor. This sensor is monitoring your car at any time. And when there is a collision, the signal will be transmitted to the airbag, so that the airbag can operate normally.

The knock sensor is a device in the supplement restraint system (SRS). It can detect the situation of the collision of the car, and send out a signal at the same time, and then input the signal into the SRS. The microcomputer in the airbag can determine whether it is necessary to detonate the inflator according to the input signal.

Effect of a Knock Sensor

The knock sensor is used to detect the inertia force under the great deceleration when there is a car crash and input the detection signal to the electronic control device. Most of the knock sensors adopt the structure of the inertial switch, which is similar to a control switch, and its working state depends on the acceleration of the vehicle during the accident.

As we know, the sensor is a detection device. And the main role of the knock sensor is to detect or judge the strength of the impact signal when the car collides, and to judge whether the airbag should be exploded according to the signal strength. The airbag is positioned between the driver and the steering wheel, creating a cushion that can protect the driver from a hard impact and injury.

The knock sensor located on the side of the car is mainly used to detect the acceleration signal at the installation position of the sensor on the body of the car, and send the signal to the airbag ECU.

The ECU recognizes acceleration signals, so as to determine whether it needs ignition. When a car is running, it is stimulated internally and externally by the engine, transmission, transmission shaft and road, which covers frequencies from low frequency to medium and high frequency. The side knock sensor mounting point may resonate due to internal and external excitation.

When resonance occurs, a large amplitude will appear at the installation point. At this time, the abnormal acceleration signal collected by the side knock sensor will be transmitted to the ECU of the airbag. When the ECU mistakenly reaches the collision deceleration threshold, it will lead to an accidental explosion, which will bring unnecessary losses to passengers' personal safety and property.

Therefore, if you want to avoid airbag explosion, you must find a way to control the resonance degree of the side knock sensor installation point, so that it meets the requirements of the sensor manufacturer.



benz knock sensor

Figure 2: The knock sensor.

Components of a Knock Sensor

Knock sensor switch

The working principle of the switch is very simple, completely relying on the internal mechanical structure to complete the circuit conduction and interruption. When the feeler arm is impacted, the downward force drives the reed inside the switch to toggle, thus changing the conduction state of the circuit.

The switch is cheap, generally only a few dollars each, and is simple to use with a wide range. There is no limit to the environmental conditions. But the knock sensor switch also has one obvious flaw that it can detect obstacles only after the collision, which makes it less practical in actual applications. In addition, after being used for a long time, switches are prone to mechanical fatigue and cannot continue to work properly.

Three-core cable.

Set screws.

Knock Sensor Type

Function

Knock intensity sensors are used to detect the degree of collision intensity.

The protection knock sensor is also known as a safety knock sensor or knock detection sensor, referred to as a protection sensor, and is generally installed in internal SRS ECU.



an srs control module used in the car

Figure 3: The airbag control module of the SRS.

Installation Site

The front knock sensor includes the left and right front knock sensors. It is installed inside the left front and right front fender board of the car, under the two sides of the headlight bracket, the left and right sides of the engine radiator bracket, under the left and right dashboard.

The central knock sensor is installed in the airbag control unit used to evaluate the sensor data and trigger the corresponding supplement restraint system.

Structure

* Electromechanical Knock Sensor

It is a sensing element that uses the mechanical movement (rolling or rotating) to control the electric shock movement of electrical appliances, and then controls the opening and closing of the igniter circuit of the airbag by the electric shock.

Rolling ball type knock sensor

Also known as a bias magnet type knock sensor, it is mainly composed of an iron ball, permanent magnet, guide cylinder, fixed contact, shell.

Working principle
When the sensor is in a static state, under the action of a permanent magnet, the ball in the guide cylinder is drawn to the magnet, and two contacts are separated from the ball. The sensor circuit is in the off state. When the car suffers a collision and deceleration reaches a set threshold, the inertia force generated by the ball will be greater than the electromagnetic attraction of the permanent magnet.

Under the action of inertia force, the ball will overcome the magnetic force and move along the guide cylinder to the two fixed contacts and connect them. When the sensor is used as a knock sensor, the knock signal is input to the SRS ECU after the fixed contact is switched on. When the sensor is used as a knock protection sensor, the ignition power is switched on.

Roller type knock sensor

When the sensor is in a static state, the roller is rolled to the end of the stop pin under the elastic force of the sheet spring, and the rolling contact is disconnected from the fixed contact. When the car suffers a collision and deceleration reaches the set threshold, the inertia force generated by the roller will be greater than the elastic force of the sheet spring.

Under the action of inertia force, the roller overcomes the spring force and rolls to the right, and the rolling contact contacts with the fixed contact. When the sensor is used as a knock sensor, the knock signal is input to the SRS ECU after the fixed contact is switched on. When the sensor is used as a knock protection sensor, the ignition power is switched on.

Eccentric hammer type knock sensor

It is also known as the eccentric rotor type knock sensor. At present, Toyota, Mazda vehicles SRS use this sensor.

Working principle
When the sensor is in a static state, under the action of the return spring, the eccentric hammer keeps in contact with the block, the rotor is always in a static state, and the rotating contact is disconnected from the fixed contact.

* Electronic Sensor

It uses the deformation of the strain resistance during the collision process, so that the resistance value changes. Or because of the piezoelectric crystal force, the output voltage changes to control the airbag circuit.

* Mercury Switch Sensor



mercury knock sensor 8m6003194

Figure 4: A mercury knock sensor.



It is made of mercury with good conductive characteristics. Besides, it consists of the mercury, shell, electrode and sealing screw.

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