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How exactly does an airbag work?

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05/02/2019
 

Driver assistance and safety systems help prevent accidents. At VisionZeroWorld we explain how ABS, ESC and other safety-related technologies work. This time it’s the turn of the airbag.


It’s probably fair to say that Walter Linderer has acted as guardian angel for countless drivers. Although not much is known about this visionary German engineer, what we do know is that in 1951 he filed a patent that was to have a global impact on the auto industry – and has since saved any number of lives. It was a patent for an airbag. Linderer called his invention a “Device for the protection of vehicle occupants against collision injuries," and described it as “An inflatable container … mounted in a folded state in front of the seat of the occupant to be protected, which inflates automatically or by voluntary triggering in the event of danger”. Two years later, the German Patent Office granted the patent, publishing it on November 12, 1953. So now it was official: Walter Linderer had invented the first vehicle airbag.


But Linderer was way ahead of his time. The technology he would have needed to implement this early form of airbag had yet to be invented. Linderer’s patent proposed using compressed air to inflate a folded bag in the event of an accident. However, the necessary pressure had to be built up in milliseconds, and this would not have been possible using compressed air. Nor were sufficiently tear-proof materials available for the airbag itself. So for the time being, Linderer had invented the airbag in theory only. But even though its practical realization was still a pipedream, the idea itself still had the power to inspire others, and the quest to perfect this new safety system continued, not only in Germany but also in the USA. Because as the car finally became a mass consumer product in the sixties, the number of accidents began to increase as well.


The first airbags were fitted on the Mercedes S-Class in the early 1980s. Photo: Daimler

 

In 1981, thirty years after Linderer filed his patent, the Mercedes-Benz S-Class became the first ever German automobile to be fitted with an airbag, initially as an option at extra cost, then later as standard. Today this safety system, which is fitted in all new cars, comes in many different shapes and sizes. There are steering-wheel-mounted front airbags, seat-mounted side airbags, headliner-mounted head airbags and knee bags mounted low on the instrument panel. So how does the airbag work? And what technology is used in these systems that wasn’t yet available in 1951?

 

The quest for a tear-resistant material for the airbag, which had defeated Linderer, eventually resolved itself with the decision to use the artificial fabric nylon. This material is highly stress-resistant and what’s more, it is not prone to wear, however many years it spends folded up behind its panel in the airbag module. And so it was that in 1981 a Supplemental Restraint System (SRS) – comprising a driver airbag and belt-tensioner – made its debut as optional equipment for the Mercedes-Benz S-Class. Priced at DM 1525.50, this airbag was the culmination of an intensive research and development program at the company that had begun in the mid-sixties. When the system’s sensors detected a collision, a pyrotechnic charge was ignited, generating nitrogen gas to inflate the airbag, which was made from a special fabric. This all took place in just fractions of a second. The inflated airbag then cushioned the impact between the occupant and the instrument panel. While the driver’s airbag was mounted in the steering wheel, the earliest versions of the passenger airbag were fitted in the space formerly occupied by the glove compartment. On later vehicles it was then possible to reinstate the glove compartment, because by this time smaller, more sophisticated airbag modules were available.

Continental develops and produces control units for airbags and other vehicle safety systems. One example is the Safety Domain Control Unit (SDCU) for improved pedestrian and occupant protection. Photo: Continental

 

But how does the system know whether – and when – to trigger the airbag? This all-important decision is the responsibility of the Airbag Control Unit (ACU) and the various crash sensors, some of which are fitted in the ACU and some in the vehicle itself. Further important information is supplied by acceleration sensors that measure the negative acceleration in the event of a sudden drastic reduction in speed. To prevent false triggering of the airbag, at least two such sensors are always fitted. The airbag is only deployed if all of the sensors detect a given level of deceleration. Essentially, the Airbag Control Unit acts as the brain of the airbag system, evaluating incoming data from the sensors and deciding whether airbags need to be deployed, and if so which ones. Continental is among a number of companies that currently develop control units and sensors of this type.

 

Continental’s development work in the field of restraint systems also extends to systems of the future. When autonomous vehicles take to the road, this will have an impact not only on driving styles but also on occupant sitting positions. Occupants will no longer be limited to sitting in an upright position but will have greater freedom of movement. At the Consumer Electronics Show Asia 2017 in Shanghai, Continental presented a Safety Domain Control Unit (SDCU) which is set to play a pivotal role in improving pedestrian and occupant protection in the future. This control unit fuses data not only from the usual airbag-related sensors but also from vehicle dynamics sensors and from all the various radar and camera sensors installed around the vehicle. This makes it possible to detect an imminent collision much earlier, while also providing significantly improved detection of accident severity. All these new possibilities are being opened up courtesy of the increasing connectivity of today’s sensor- and radar-equipped vehicles. In the context of greater connectivity, more effective occupant protection and maximized safety, the airbag forms part of an increasingly integrated safety system that is helping to bring Vision Zero ever closer.

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