It's been 20 years since airbag systems started appearing in automobiles. Just like other automotive systems, airbags have undergone major changes since their introduction and the newer designs protect us better than ever.
The first airbags were mounted only in the steering wheel, for driver protection from the impact against the steering column. These were simple systems compared to the current designs, but still had to be robustly designed to work the first time, every time, even after years of service. They used mechanical sensors to determine impact loads: some of them have a gold-plated steel ball held in a tube by a magnet. An impact with enough force would jar the ball loose from the magnet and it would roll ahead, touching two gold-plated contacts to complete an electrical circuit that ignited the airbag. Gold is used so the electrical contacts won't corrode over time. Many of these early systems ignited sodium azide, a solid rocket propellant, to inflate the airbag in a fraction of a second. About the mid nineties, we started seeing airbags on the front passenger's side. These bags are larger, because typically they are further away from the vehicle occupants. Sodium azide was used to inflate some of these systems but others used a compressed gas cylinder. An explosive charge punctures the end of the cylinder allowing the gas into the airbag. Heat from the charge helps the gases expand so the airbag deploys quicker. Side impact and side curtain airbags also started appearing in the late 1990s. Each has a distinct purpose. The side impact bags are often mounted in the side of the front seats, although there are some side airbags located behind the door panels or in the doorjamb post (B pillar). These side impact airbags help protect passengers from a side impact, and because there is very little time from the point of impact until the passenger compartment is affected, the side impact airbags have to deploy much faster than frontal airbags. Side curtain airbags are located above the doors under the vehicle headliner. They were initially designed to protect passengers if the vehicle was involved in a roll over, but the newer systems may also deploy in a side impact if the collision is severe enough. Side curtain airbags are big, covering the complete side glass. Some systems cover only the front and second row seats, while others will cover three rows of seats if the vehicle is so equipped. Side curtain airbags also deploy for much longer than other airbags. Within a second, other airbags will be collapsing, but side curtain airbags may stay inflated for up to 6 seconds. They are calibrated this way because a roll over event can take several seconds to occur, until the vehicle stops. The next big step in airbag development came with dual-stage frontal airbags. These systems have two separate charges and igniters. Depending on the severity of the impact, one or both stages may deploy. The sensors used on these systems are much more sophisticated, using G-sensors and load cells to send information on the severity of the impact to a computer. The computer also monitors things like seat belt latches and front seat position to determine how to deploy the airbags. By 2007, vehicles in Canada required an occupant sensor for the front passenger seat. These sensors, and there are several different types, from ultrasonic to a capacitance mat, tell the airbag computer if there is a passenger in the seat. Some systems will signal if a passenger is in the correct position. If there is no passenger, the passenger side airbags should not deploy in a collision, saving on repair costs. There are still more advances in airbag technology to come. Just this past week, Honda announced a new driver's side airbag with continuously staged inflation. This airbag has a spiral-shaped seam structure and gas release control valve to control airbag deployment. It can be deployed faster but also reduces occupant impact compared to a conventional airbag by having a longer inflation time. Remember that when we talk about long inflation times, it is still measured in milliseconds. Without a high-speed camera recording the deployment, you would never see it. This new airbag design also contains all the inflation gases until it is time to deflate, unlike present systems that start exhausting gases even as the airbag inflates. This allows the new Honda system to remain inflated longer. The new Honda airbag will be introduced in November in the Japanese market, on the Honda Life minicar, but I suspect we will see it here in short order too. Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC). [Non-text portions of this message have been removed]
