Facts about Airbag Crash Sensors

The airbag system uses two electrical components to help make the decision on whether to deploy. These would be the crash sensors and the module. The electronic control module receives event data then makes the final decision on deployment. Unintended inflation is just as dangerous as when the bag does not pop out in a hard collision.

Drivers don’t want the system to activate when they slam into a pothole, but they also want it to deploy without hesitation in a crash situation. Learn about the different types of crash sensors and the fail-safe built into modern airbag systems.

Typical Crash Sensor Operation

The number of the crash sensors used in inflatable restraint systems varies between vehicle manufacturer and the year of the car. One thing many systems share is it takes activation of more than one sensor to deploy. Airbag crash sensors are mounted strategically throughout the automobile and include positions inside the passenger compartment. Typically the airbag won’t go off unless an outside and inside impact switch is closed.

In this type of setup the inside impact sensor will arm the system and then the external crash sensor will activate the igniter, setting off a chemical reaction creating gas to inflate the supplemental inflatable restraints. Requiring two separate switches to be closed before anything will happen helps reduce the chances of accidental deployment from defective crash sensors, pot holes or speed bumps.

Different Types of Crash Sensors

A common type of impact or crash sensor that’s used often is the roller switch. This type of sensor has a roller located on a ramp with the roller itself held in place by a light spring. If the vehicle is involved in a hard collision the roller will overcome spring pressure due to forward momentum and move up the ramp forcing the contacts together, completing the circuit and deploying the airbag.

Sudden Motion Sensors

Another popular crash sensor uses a corrosion resistant metal ball held in place by a magnet. When the vehicle stops moving suddenly, forward motion will break the ball loose from the magnet and it rolls into place completing a circuit between two contacts. In this situation the ball actually closes the circuit and allows current to flow through triggering an impact event.

Seat Cushion Sensors

Starting with the 2006 model year, automobiles must have a system that allows for airbags to be suppressed when infants or small children are in the front passenger seat. A seat cushion sensor determines the weight of the passenger in conjunction with data received by the seat belt tension sensor. When no one is sitting in a front passenger seat the bag will not deploy on the passenger side. This not only protects infants and small children that might sit there, but also saves the cost of replacing an airbag that was not needed during a crash event.

Electronic Control Module

The supplemental inflatable restraint control module not only monitors the readiness of the system, but performs self tests on individual components. It can set codes for failed parts and decides when all criteria have been met for deployment. In addition to making the decision of when to deploy it also controls the location of airbags inflated. On newer vehicles these modules can also contain backup power that is capable of supplying enough power to deploy the airbags even if the battery has become disconnected during the crash event.

With the importance of deployment being as critical as the bag not exploding in the face of the driver when it’s not needed, manufacturers have found it necessary to build in a fail-safe. One of the most common is to have two sensors closed before an airbag will deploy. This in addition to regular diagnostics run by the electronic control module will notify the driver by illuminating the airbag light if the system is not ready to deploy in an accident. Another fact about airbags is the effectiveness of this safety feature depends on the seat belts holding people in the right position when deployed.