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An airbags, in its most elemental forms is an automotive safety restraint system designed to inflate a cushioning bag extremely quickly, then rapidly deflate it in a controlled manner, during a collision. They’re considered a passive restraint system, because unlike seatbelts, they require no interaction by the occupant for their operation.
SYSTEM DESIGN
An airbag system is fundamentally composed of one or more inflation mechanisms located primarily within the steering wheel for the driver and the upper dashboard for the front passenger. These inflation mechanisms are controlled by a centralized system that continuously monitors for impact events using as little one to dozens of sensors, depending on the system’s sophistication. Once this system detects an impact, one or several inflation mechanisms are pyrotechnically triggered by an electrical signal, causing a gas generating propellant to be ignited, rapidly inflating a bag that is folded within each inflation mechanism. While simple in concept, the difference between an airbag’s deployment protecting an occupant, and causing traumatic or even deadly injuries, comes down to the precise millisecond timing of its operation.
ANATOMY OF AN COLLISION
This incredibly narrow window to act within the first ⅓ of the entire collision duration is due to the airbags needing to deploy before the occupants contact any portion of the vehicle interior as it crushes, and before the limits of the seat belt’s stretch are reached. The airbag’s inflation must also be timed so that it is fully inflated before the occupant engages with it, to minimize trauma from the inflation process itself.
COMPRESSED AIR
Both systems were based on a store of compressed airbags that would inflate the airbag using mechanical trigger valves. By the 1960s, practical airbag systems for vehicles were being explored by the major manufacturers and from this decade of research it was determined that compressed air systems were far too slow reacting to be effective. These flaws made the mechanical compressed air airbag system completely unsuitable for commercial adoption.
A BREAKTHROUGH
Allen K Breed would make a breakthrough that finally made airbags commercially viable, with the development of the ball-in-tube electromechanical crash detection sensor. When a collision occurs, the ball is separated from the magnet, moving forward to electrical contacts and closing the trigger circuit. Breed also pioneered the use of a gas-generator as a method for rapidly inflating an airbag. Breed devised an inflation mechanism that used just 30-150 grams of the solid-fuel, sodium azide as a gas generating agent for airbags. The sodium azide would then exothermically decompose rapidly to sodium and nitrogen, fully inflating the airbag with the resultant gas, within just 60-80 milliseconds.
AIRBAG HISTORY
Any car sold in the United States must now be certified to meet the Federal Motor Vehicle Safety Standards or FMVSS, a comprehensive set of regulations on vehicle design, construction, and performance. The NHTSA began to prepare for a second wave of mandates during the 1970’s, specifically targeting a push for new safety technologies, with the airbag being a prime technology for regulatory compliance. The first mass-produced vehicle to have an airbag system was introduced on a government-purchased in 1973. Called the The Air Cushion Restraint System or ACRS, General Motors employed impact sensors mounted in the vehicle’s front bumper in order to deploy the airbags embedded in the steering wheel, for the driver, and in the dashboard for the passenger.
By 1984, the NHTSA would reach a compromise with the industry, and with this agreeing to the introduction of a passive restraint system mandate for all new vehicles produced in the US, beginning on April 1, 1989. Manufacturers had 2 options, either an automatic seat belt system or the airbag.
The 1980s saw the shift of the industry’s view of airbag as a primary safety system to one designated as a supplemental restraint system or SRS, or the less common designation of supplemental inflatable restraints or SIR.
THE NEXT WAVE OF AIRBAG TECHNOLOGY
This proliferation led to the development of a new generation of airbag systems during the 1990s that overcame the flaws of earlier systems through the use of recent breakthroughs in the semiconductor industry.
ALGORITHMIC CRASH DETECTION
The electronic control unit that formed the backbone of airbag systems, called the airbag control unit or ACU, would now become an embedded computer, relying on a fusion of MEMS sensor data and other vehicle inputs, to employ algorithms that could now manage a larger spectrum of collision types and inflation response profiles.
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