Editorial Feature

Automotive Airbag Sensors

The Mercedes S-Class car was launched in North America in 2013. What stood out about this model was its airbag control technology. Mercedes introduced a beltbag airbag for backseat passengers. This airbag works as an inflatable seatbelt that inflates upon frontal impact from another vehicle. In this model, the purpose of the inflatable seatbelt is to reduce the strain that is normally placed on the ribcage during a collision.

Activation of automotive airbags is triggered by crash sensors (also known as impact sensors) that work to detect frontal impact and trigger a control unit that deploys the airbag to cushion the passenger.

Mass-Type Sensor

An impact sensor is normally fitted to the front of the vehicle as this is where a collision is likely to occur. The sensor is positioned inside the engine and a similar safety sensor is located inside the passenger zone of the vehicle. This safety sensor is required to measure the intensity of the collision to determine whether the impact is over a certain threshold to justify the release of an airbag.

Both types of sensors (termed inertia sensors) work on the principle of detecting a decrease in acceleration of a moving vehicle and generating an electrical impulse. Figure 1 is a schematic diagram of an inertial sensor.

Figure 1. Structural components to an Inertia sensor. Source: Duffy, J.E. (2001). I-Car Professional Automotive Collision Repair. New York: Delmar, a division of Thomas Learning.

During a collision with another moving vehicle, the sensing mass is forced forward into the gold-plated contacts as a result of the change in the state of motion. Following the movement of this metal ball into the contacts, this metal mass makes contact with electrical terminals at either side of the metal ball which alerts the central unit of a collision (i.e. the electrical contact completes the circuit).

Roller-Type Sensor

The roller-type sensor consists of a weight connected to a coil spring component. Like the mass-type sensor, during impact with an oncoming vehicle, the metal weight is forced forward which alters the tension on the coil spring to manipulate the electrical circuit that closes off the sensor contact. It is important to note that the impact and safety sensors must activate and close off at the same time to allow for the deployment of the airbag (figure 2).


Figure 2. Functional principle to a typical roller type airbag sensor. Source: Erjavec, J. (2010). Automotive Technology: A Systems Approach. New York: Delmar, Cengage Learning.

Airbag Control Unit

Upon signal of a collision, the controller interprets the electrical input and measures the level of a collision to determine the release of an airbag. In the event of one impact sensor and safety, sensor is closed, an electrical current is transmitted to an airbag module which contains the airbag and inflator assembly. Activation of the airbag results in an ignition that produces an electrical transmission between a pair of metal pins.

The electrical arc created between both pins activates a propellant (made up of sodium azide) that starts to burn as the compound decomposes to produce nitrogen gas which fills the airbag. The Volvo V40 model takes airbag technology to a new level by deploying a pedestrian airbag upon impact on the bumper to this car. The following video demonstrates the world’s first pedestrian airbag technology.


Similar airbag control units currently on the market include a model introduced by TRW. This integrated control module detects vehicle impact by using an occupant dynamics-based algorithm, which meets all North American and European regulations with a rollover sensor adapting a functional system similar to the type discussed in this article. The idea of an integrated airbag control unit has many advantages:

  • Increased sensitivity of the moving vehicle by placing the integrated control module in the vehicle’s center of gravity
  • Integrated crash sensors diversify the diagnostics on a collision
  • Increased precision of the integrated sensor technology to allow for better safety
  • Cost-effective if all crash sensor systems are integrated into one module

Traditional airbag systems have adopted mechanical airbag sensors, such as the mass and roller type sensors. Since the introduction of this technology, there have been applications of more sophisticated sensing systems such as the electronic crash sensors and sensor microsystems. Radar, infrared and image processing are currently implemented to work with a vehicle control unit to anticipate space and timing in relation to a possible collision. As the world continues to develop a more eco-friendly approach to living, it will be interesting to see how advanced environmental sensors contribute to the design and engineering of cars to help avoid collision and improve safety measures.

Sources and Further Reading

  • Valldorf, J., Gessner, W. (2005). Advanced Microsystems for Automotive Applications. The Netherlands: Springer Science and Business Media.
  • Duffy, J.E., Scharff, R. (2004). Auto Body Repair Technology. New York> Delmar Learning, a division of Thomas Learning, Inc.
  • TRW. Cognitive Safety Systems.
  • Krueger, S., Gessner, W. (2002). Advanced Microsystems for Automotive Applications: Yearbook 2002. New York: Springer Science and Business Media.
  • Erjavec, J. (2010). Automotive Technology: A Systems Approach. New York: Delmar, Cengage Learning.
  • Duffy, J.E. (2001). I-Car Professional Automotive Collision Repair. New York: Delmar, a division of Thomas Learning.

This article was updated on the 3rd October, 2019.



    which type of airbag sensor is used in the i10 cars


    which are the main sensors (speed sensor , airbag sensor , airbag diploid  sensor ,seat belt latching sensor  )and its specification used in car ?

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