Automotive Airbag Sensors

By Kal Kaur

Introduction

The Mercedes S-Class car is to be launched in North America in 2013. What stands out about this new model is its airbag control technology. Mercedes have now introduced a beltbag airbag for backseat passengers. The airbag works as an inflatable seatbelt that inflates upon frontal impact from another vehicle. In this new model, the purpose of this 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 to 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 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 generate 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 change in the state of motion. Following 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 to a collision (i.e., the electrical contact completes the circuit).

Roller-Type Sensor

The roller-type sensor involves 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 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 collision to determine release of an airbag. In the event of one impact sensor and safety sensor being 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 and give off nitrogen gas, and it is this gas that starts to fill 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 dynamic-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 diversifies 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 has been application 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 a car to help avoid collision and improve safety measures.