The airbag system (SRS) consists of airbags, belt tensioners, crash sensors and the SRS control unit. To determine when the airbag is activated, the SRS system has sensors to measure the deceleration of the vehicle. If the deceleration is high enough, the sensor sends a signal to the SRS control unit and together with other parameters, the SRS control unit sends an activation signal to the relevant airbag. Over the years, the number of crash sensors has grown considerably. In this newsletter, we want to demonstrate and elaborate on the way crash sensors work and the associated differences between them.
In the 90s, mechanical sensors were combined with the driver's airbag unit. This sensor is activated by a metal pin that is knocked against the ignition mass with the aid of a pre-tensioned spring.
PLEASE NOTE! The mechanical sensor in the airbag unit cannot be deactivated! That is why the CRS highlights this type of airbag with an extra red border.
Mechanical satellite sensors:
In the first SRS systems with satellite sensors at the front, mechanical sensors that worked with a spring/mass system (see illustration) were mainly used. These sensors react during a deceleration of 3-5G. PLEASE NOTE! These sensors are deactivated after disconnecting the battery!
Mechanical / pyrotechnical side airbag sensor:
Volvo Cars introduced the SIPS (Side Impact Protection System) in model year 1995. The SIPS airbag system (see illustration) consists of:
1. Mechanical/pyrotechnical sensor unit
2. Trigger circuit
3. Side airbag cushion
The SIPS airbag is a separate system for each front seat. The two SIPS airbags are therefore not connected. In the event of a side impact, the SIPS airbag is only activated on the side of the collision. The sensor unit is a mechanical / pyrotechnic impact sensor that is activated during a collision by the force of the door panel on the pressure plate (see illustration).
FOCUS POINT FOR THE FIRST RESPONDER: Due to the risk of activation during first-responder deployment, we have circled the sensor in red (see CRS screenshot). For emergency services, Volvo has even added a sensor protective cover on the inside of the seat panel.
The Safing sensor is used as a safety sensor in the SRS control unit. The Safing sensor in the SRS control unit is responsible for preventing the unintended activation of airbags. During a strong deceleration, the annular magnet overcomes the spring force, the magnet slides over the tube and the reed contact is closed.
In the Volkswagen Golf Cabrio, the location of the satellite sensor and the SRS control unit with Safing sensor can clearly be seen
Around the year 2000, more and more micro-mechanical sensors were used instead of mechanical sensors. Depending on the number of airbags, several micro-mechanical sensors are used at the front and/or side of the vehicle. The micromechanical sensor is built as a capacitor (see illustration).
Source: Audi How micromechanical sensors work and an example of a sensor
Side airbags were introduced because the crumple zone is much smaller in the event of a side impact than in a frontal collision. The detection of a side-impact must occur as quickly as possible to ensure that the side and curtain airbags are activated before the passenger or driver touches the side of the vehicle. For this, vehicle manufacturers have developed a pressure sensor that is located in the doors (see illustration).
Contemporary safety systems often have an airbag and sensor layout as shown in the images below. The system has 6 or more airbags and 6 crash sensors, of which 2 satellite sensors at the front, 2 pressure sensors in the front door and 2 satellite sensors in the C-pillar. Of course, models also differ from each other and the number of sensors only seems to increase. The SRS system of the new Porsche Cayenne and Cayenne Coupé, for example, includes 14 crash sensors, 6 at the front and 4 on each side of the vehicle for even faster accident detection. PLEASE NOTE! It is important to avoid these sensors during first-responder deployment. Thanks to the CRS you know where these sensors are located and you can act accordingly.
Example of a contemporary vehicle with 6 or more airbags and 6 crash sensors (source: Audi)
New Porsche Cayenne with 14 crash sensors! (source: Porsche)