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Airbag
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An airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly in an automobile collision, to prevent vehicle occupants from striking hard interior objects such as steering wheels.
use no action by the vehicle occupant is required to activate or use the airbag, it is considered a passive safety device.
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Encyclopedia
An airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly in an automobile collision, to prevent vehicle occupants from striking hard interior objects such as steering wheels.
Terminology
Because no action by the vehicle occupant is required to activate or use the airbag, it is considered a passive safety device. This is in contrast to seat belts, which are considered active safety devices because the vehicle occupant must act to enable them. Terminological confusion can arise from the fact that passive safety devices and systems — those requiring no input or action by the vehicle occupant — can themselves operate in an active manner; an airbag is one such device. Vehicle safety professionals are generally careful in their use of language to avoid this sort of confusion, though advertising principles sometimes prevent such syntactic caution in the consumer marketing of safety features.
Various manufacturers have over time used different terms for airbags. General Motors' first bags, in the 1970s, were marketed as the Air Cushion Restraint System. Common terms in North America include Supplemental Restraint System (SRS) and Supplemental Inflatable Restraint (SIR); these terms reflect the airbag system's nominal role as a supplement to active restraints, i.e., seat belts.
History
Invention
An American inventor, Dr. Allen S. Breed, invented and developed a key component for automotive use: the ball-in-tube inertial sensor for crash detection. Breed Corporation then marketed this innovation first in 1967 to Chrysler. A similar "Auto-Ceptor" crash-restraint, developed by Eaton, Yale & Towne Inc. for Ford was soon offered as an automatic safety system in the USA, while the Italian Eaton-Livia company offered a variant with localized air cushions.
As an alternative to seatbelts
Airbags for passenger cars were introduced in the United States in the mid-1970s, when seat belt usage rates in the country were quite low. Airbags were marketed as a convenient alternative to seat belts, while offering similar levels of protection to unbelted occupants in a head-on collision.
Ford built an experimental fleet of cars with airbags in 1971, followed by General Motors in 1973 on Chevrolet vehicles. In 1974, GM made dual-stage twin front airbags optional in full-size Buick, Cadillac and Oldsmobile models and called them the "Air Cushion Restraint System". The early fleet of experimental GM vehicles equipped with airbags experienced seven fatalities, one of which was later suspected to have been caused by the airbag.
The development of airbags coincided with an international interest in automobile safety legislation. Some safety experts advocated a performance-based occupant protection standard rather than a standard mandating a particular technical solution, which could rapidly become outdated and might not be a cost-effective approach. As countries successively mandated seat belt restraints, there was less emphasis placed on other designs for several decades.
As a supplemental restraint
Frontal airbag
The auto industry and research and regulatory communities have moved away from their initial view of the airbag as a seat belt replacement, and the bags are now nominally designated as Supplemental Restraint System (SRS).
In 1980, Mercedes-Benz introduced the airbag in Germany that it had patented in 1971, as an option on its high-end S-Class (W126). In the Mercedes system, the sensors would tighten the seat belts, and then deploy the airbag on impact. The airbag was thus no longer a means of avoiding seat belts, but as a way to obtain an extra margin of occupant safety or a supplemental restraint.
In 1987, the Porsche 944 turbo became the first car in the world to have driver and passenger airbags as standard equipment. The Porsche 944 and 944S had this as an available option. This year also saw the first airbag in a Japanese car, the Acura Legend.
Audi was relatively late to offer airbag systems on a broader scale; until the 1994 model year, for example, the 80/90, by far Audi's 'bread-and-butter' model, as well as the 100/200, did not have airbags in their standard versions. Instead, the German automaker until then relied solely on its proprietary procon-ten restraint system.
Airbags became common in the 1980s, with Chrysler and Ford introducing them in the mid-1980s; the former made them standard equipment across its entire line in 1990.
In Europe, airbags were almost entirely absent from family cars until the early 1990s, except for Saab, who made them standard on the 900 Turbo in 1989 and on all models in 1990. The first European Ford to feature an airbag was the facelifted Escort MK5b in 1992; within a year, the entire Ford range had at least one airbag as standard. By the mid 1990s, European market leaders such as Vauxhall/Opel, Rover, Peugeot, Renault and Fiat had included airbags as at least optional equipment across their model ranges. By the end of the decade, it was very rare to find a mass market car without an airbag, and some late 1990s products, such as the Volkswagen Golf Mk4 also featured side airbags. The Peugeot 306 was a classical example of how commonplace airbags became on mass market cars during the 1990s. On its launch in early 1993 most of the range did not even have driver airbags as an option. By 1999 however, side airbags were available on several variants.
During the 2000s side airbags were commonplace on even budget cars, such as the smaller-engined versions of the Ford Fiesta and Peugeot 206, and curtain airbags were also becoming regular features on mass market cars. The Toyota Avensis, launched in 1998, was the first mass market car to be sold in Europe with a total of nine airbags.
Variable force deployment front airbags were developed to help minimize injury from the airbag itself.
Shaped airbags
The Citroën C4 provides the first "shaped" driver airbag, made possible by this car's unusual fixed hub steering wheel.
Side airbag
There are essentially two types of side airbags commonly used today, the side torso airbag and the side curtain airbag.
Side-impact airbags or side torso airbags are a category of airbag usually located in the seat, and inflate between the seat occupant and the door. These airbags are designed to reduce the risk of injury to the pelvic and lower abdomen regions. Some vehicles are now being equipped with a different types of designs, to help reduce injury and ejection from the vehicle in rollover crashes.
The Swedish company Autoliv AB, was granted a patent on side airbags, and they were first offered as an option on the 1995 model year Volvo 850, and as standard equipment on all Volvo cars made after 1995.
Side tubular or curtain airbag
In late 1997 the 1998 model year BMW 7-series and E39 5-series were fitted with a tubular shaped head side airbags, the "Head Protection System (HPS)" as standard equipment. This is an industry's first in offering head protection in side impact collisions. This airbag also maintain inflation for up to seven seconds for rollover protection. However, this tubular shaped airbag design has been replaced by an inflatable 'curtain' airbag in non-BMW's for superior protection.
In May 1998 Toyota began offering a side curtain airbag on the Progrés.
In 1998 the Volvo S80 was first given curtain airbags to protect both front and rear passengers. They were then made standard equipment on all new Volvo cars from 1998. The Volvo Curtain Airbag design is now considered to be superior by most car manufacturers and is the one mostly used; in some cars it has been designed to stay inflated for rollover crashes.
Roll-sensing side curtain airbags found on vehicles more prone to rollovers such as SUV's and pickups will deploy when a rollover is detected instead of just when an actual collision takes place.
Curtain airbags have been said to reduce brain injury or fatalities by up to 45% in a side impact with an SUV. These airbags come in various forms (e.g., tubular, curtain, door-mounted) depending on the needs of the application. Many recent SUVs and MPVs have a long inflatable curtain airbag that protects all 3 rows of seats.
Knee airbag
In 2003, the The Toyota Avensis was the first vehicle in Europe to be sold with a driver’s knee airbag. This airbag is located at the bottom of the dashboard and is designed to reduce injuries in a frontal collision where driver and if offered the front passenger's knees make contact with the dash.
Rear curtain airbag
The Toyota iQ launched in 2008 featuring a rear curtain shield airbag to protect the rear occupants heads in the event of a rear impact.
On motorcycles
Various types of airbags were tested on motorcycles by the UK Transport Research Laboratory in the mid 1970s. In 2006 Honda introduced the first production motorcycle airbag safety system on its Gold Wing motorcycle. Honda claims that sensors in the front forks can detect a severe frontal collision and decide when to deploy the airbag, absorbing some of the forward energy of the rider and reducing the velocity at which the rider may be thrown from the motorcycle.
Airbag suits have also been developed for use by Motorcycle Grand Prix riders. They are connected to the motorcycle by a cable and deploy when the cable becomes detached from its mounting clip, inflating to protect the back.
How airbags work
The design is conceptually simple; a central "Airbag control unit" (ACU) (a specific type of ECU) monitors a number of related sensors within the vehicle, including accelerometers, impact sensors, side (door) pressure sensors, wheel speed sensors, gyroscopes, brake pressure sensors, and seat occupancy sensors. When the requisite 'threshold' has been reached or exceeded, the airbag control unit will trigger the ignition of a gas generator propellant to rapidly inflate a nylon fabric bag.
As the vehicle occupant collides with and squeezes the bag, the gas escapes in a controlled manner through small vent holes. The airbag's volume and the size of the vents in the bag are tailored to each vehicle type, to spread out the deceleration of (and thus force experienced by) the occupant over time and over the occupant's body, compared to a seat belt alone.
The signals from the various sensors are fed into the Airbag control unit, which determines from them the angle of impact, the severity, or force of the crash, along with other variables. Depending on the result of these calculations, the ACU may also deploy various additional restraint devices, such as seat belt pre-tensioners, and/or airbags (including frontal bags for driver and front passenger, along with seat-mounted side bags, and "curtain" airbags which cover the side glass). Each restraint device is typically activated with one or more pyrotechnic devices, commonly called an initiator or electric match. The electric match, which consists of an electrical conductor wrapped in a combustible material, activates with a current pulse between 1 to 3 amperes in less than 2 milliseconds. When the conductor becomes hot enough, it ignites the combustible material, which initiates the gas generator. In a seat belt pre-tensioner, this hot gas is used to drive a piston that pulls the slack out of the seat belt. In an airbag, the initiator is used to ignite solid propellant inside the airbag inflator. The burning propellant generates inert gas which rapidly inflates the airbag in approximately 20 to 30 milliseconds. An airbag must inflate quickly in order to be fully inflated by the time the forward-traveling occupant reaches its outer surface. Typically, the decision to deploy an airbag in a frontal crash is made within 15 to 30 milliseconds after the onset of the crash, and both the driver and passenger airbags are fully inflated within approximately 60-80 milliseconds after the first moment of vehicle contact. If an airbag deploys too late or too slowly, the risk of occupant injury from contact with the inflating airbag may increase. Since more distance typically exists between the passenger and the instrument panel, the passenger airbag is larger and requires more gas to fill it.
Front airbags normally do not protect the occupants during side, rear, or rollover accidents. Since airbags deploy only once and deflate quickly after the initial impact, they will not be beneficial during a subsequent collision. Safety belts help reduce the risk of injury in many types of crashes. They help to properly position occupants to maximize the airbag's benefits and they help restrain occupants during the initial and any following collisions.
In vehicles equipped with a rollover sensing system, accelerometers and gyroscopes are used to sense the onset of a rollover event. If a rollover event is determined to be imminent, side-curtain airbags are deployed to help protect the occupant from contact with the side of the vehicle interior, and also to help prevent occupant ejection as the vehicle rolls over.
Triggering conditions
Airbags are designed to deploy in frontal and near-frontal collisions more severe than a threshold defined by the regulations governing vehicle construction in whatever particular market the vehicle is intended for. U.S. regulations require deployment in crashes at least equivalent in deceleration to a 23 km/h (14 mph) barrier collision, or similarly, striking a parked car of similar size across the full front of each vehicle at about twice the speed. International ECE regulations are performance-based, rather than technology-based, so airbag deployment threshold is a function of overall vehicle design.
Unlike crash tests into barriers, real-world crashes typically occur at angles, and the crash forces usually are not evenly distributed across the front of the vehicle. Consequently, the relative speed between a striking and struck vehicle required to deploy the airbag in a real-world crash can be much higher than an equivalent barrier crash. Because airbag sensors measure deceleration, vehicle speed and damage are not good indicators of whether an airbag should have deployed. Airbags can deploy due to the vehicle's undercarriage striking a low object protruding above the roadway due to the resulting deceleration.
The airbag sensor is a MEMS accelerometer, which is a small integrated circuit with integrated micro mechanical elements. The microscopic mechanical element moves in response to rapid deceleration, and this motion causes a change in capacitance, which is detected by the electronics on the chip that then sends a signal to fire the airbag. The most common MEMS accelerometer in use is the ADXL-50 by Analog Devices, but there are other MEMS manufacturers as well.
Initial attempts using mercury switches did not work well. Before MEMS, the primary system used to deploy airbags was called a "rolamite". A rolamite is a mechanical device, consisting of a roller suspended within a tensioned band. As a result of the particular geometry and material properties used, the roller is free to translate with little friction or hysteresis. This device was developed at Sandia National Laboratories. The rolamite, and similar macro-mechanical devices were used in airbags until the mid-1990s when they were universally replaced with MEMS.
Nearly all airbags are designed to automatically deploy in the event of a vehicle fire when temperatures reach 150-200 °C (300-400 °F). This safety feature, often termed auto-ignition, helps to ensure that such temperatures do not cause an explosion of the entire airbag module.
Today, airbag triggering algorithms are becoming much more complex. They try to reduce unnecessary deployments (for example, at low speed, no shocks should trigger the airbag, to help reduce damage to the car interior in conditions where the seat belt would be an adequate safety device), and to adapt the deployment speed to the crash conditions. The algorithms are considered valuable intellectual property. Experimental algorithms may take into account such factors as the weight of the occupant, the seat location, seatbelt use, and even attempt to determine if a baby seat is present.
Inflation
When the frontal airbags are to deploy, a signal is sent to the inflator unit within the airbag control unit. An igniter starts a rapid chemical reaction generating primarily nitrogen gas (N2) to fill the airbag making it deploy through the module cover. Some airbag technologies use compressed nitrogen or argon gas with a pyrotechnic operated valve ("hybrid gas generator"), while other technologies use various energetic propellants. Propellants containing sodium azide (NaN3) were common in early inflator designs. However, propellants containing the highly toxic sodium azide were widely phased out during the 1990s in pursuit of more efficient, less expensive and less toxic alternatives.
The azide-containing pyrotechnic gas generators contain a substantial amount of the propellant. The driver-side airbag would contain a canister containing about 50 grams of sodium azide. The passenger side container holds about 200 grams of sodium azide. The incomplete combustion of the charge due to rapid cooling leads to production of carbon monoxide (CO) and nitrogen(II) oxide as reaction by-products.
The alternative propellants may incorporate, for example, a combination of nitroguanidine, phase-stabilized ammonium nitrate (NH4NO3) or other nonmetallic oxidizer, and a nitrogen-rich fuel different than azide (eg. tetrazoles, triazoles, and their salts). The burn rate modifiers in the mixture may be an alkaline metal nitrate (NO3-) or nitrite (NO2-), dicyanamide or its salts, sodium borohydride (NaBH4), etc. The coolants and slag formers may be eg. clay, silica, alumina, glass, etc. Other alternatives are eg. nitrocellulose based propellants (which have high gas yield but bad storage stability, and their oxygen balance requires secondary oxidation of the reaction products to avoid buildup of carbon monoxide), or high-oxygen nitrogen-free organic compounds with inorganic oxidizers (e.g., di or tricarboxylic acids with chlorates (ClO3-) or perchlorates (HClO4) and eventually metallic oxides; the nitrogen-free formulation avoids formation of toxic nitrogen oxides).
From the onset of the crash, the entire deployment and inflation process is about 0.05 seconds — faster than the blink of an eye (about 0.2 seconds). Because vehicles change speed so quickly in a crash, airbags must inflate rapidly to reduce the risk of the occupant hitting the vehicle's interior.
Variable-force deployment
Advanced airbag technologies are being developed to tailor airbag deployment to the severity of the crash, the size and posture of the vehicle occupant, belt usage, and how close that person is to the actual airbag. Many of these systems use multi-stage inflators that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the airbag control unit know if someone is occupying a seat adjacent to an airbag, the mass/weight of the person, whether a seat belt or child restraint is being used, and whether the person is forward in the seat and close to the airbag. Based on this information and crash severity information, the airbag is deployed at either a high force level, a less forceful level, or not at all.
Adaptive airbag systems may utilize multi-stage airbags to adjust the pressure within the airbag. The greater the pressure within the airbag, the more force the airbag will exert on the occupants as they come in contact with it. These adjustments allow the system to deploy the airbag with a moderate force for most collisions; reserving the maximum force airbag only for severest of collisions. Additional sensors to determine the location, weight or relative size of the occupants may also be used. Information regarding the occupants and the severity of the crash are used by the airbag control unit, to determine whether airbags should be suppressed or deployed, and if so, at various output levels.
Post-deployment
Once an airbag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it's sometimes called, the cushion) and cools. Deployment is frequently accompanied by the release of dust-like particles, and gases in the vehicle's interior (called effluent). Most of this dust consists of cornstarch, french chalk, or talcum powder, which are used to lubricate the airbag during deployment. Newer designs produce effluent primarily consisting of harmless talcum powder/cornstarch and nitrogen gas (about 80% of the air we breathe is nitrogen). In older designs using an azide-based propellant (usually NaN3), varying amounts of sodium hydroxide nearly always are initially present. In small amounts this chemical can cause minor irritation to the eyes and/or open wounds; however, with exposure to air, it quickly turns into sodium bicarbonate (baking soda). However, this transformation is not 100% complete, and invariably leaves residual amounts of hydroxide ion from NaOH. Depending on the type of airbag system, potassium chloride (a table salt substitute) may also be present.
For most people, the only effect the dust may produce is some minor irritation of the throat and eyes. Generally, minor irritations only occur when the occupant remains in the vehicle for many minutes with the windows closed and no ventilation. However, some people with asthma may develop an asthmatic attack from inhaling the dust.
Regulatory specifications
United States
On 11 July 1984, the U.S. government amended Federal Motor Vehicle Safety Standard 208 (FMVSS 208) to require cars produced after 1 April 1989 to be equipped with a passive restraint for the driver. An airbag or an automatic seat belt would meet the requirements of the standard. Airbag introduction was stimulated by the U.S. National Highway Traffic Safety Administration. Airbags were not mandatory on light trucks until 1995.
In 1998, FMVSS 208 was amended to require dual front airbags, and de-powered, or second-generation airbags were also mandated. This was due to the injuries caused by first-generation airbags, though FMVSS 208 continues to require that bags be engineered and calibrated to be able to "save" the life of an unbelted 50th-percentile size and weight "male" crash test dummy.
Outside the U.S.A.
Most countries outside North America adhere to internationalized European ECE vehicle and equipment regulations rather than the U.S. Federal Motor Vehicle Safety Standards. ECE airbags are generally smaller and inflate less forcefully than U.S. airbags, because the ECE specifications are based around belted crash test dummies. In the United Kingdom, and most other developed countries there is no direct legal requirement for new cars to feature airbags. Instead, the Euro NCAP vehicle safety rating encourages manufacturers to take a comprehensive approach to occupant safety; a good rating can only be achieved by combining airbags with other safety features. Thus almost all new cars now come with at least two airbags as standard.
Maintenance
Inadvertent airbag deployment while the vehicle is being serviced can result in severe injury, and an improperly installed or defective airbag unit may not operate or perform as intended. Some countries impose restrictions on the sale, transport, handling, and service of airbags and system components. In Germany, where some of the strictest laws are in effect, airbags are regulated as harmful explosives; only mechanics with special training are allowed to service airbag systems. Under German Federal Law, used but intact airbags are to be detonated under secure conditions, must not be passed on to third parties in any way, and no untrained person is permitted to handle airbags. Purchase is restricted to buying a new replacement unit for immediate installation by the seller's qualified personnel.
Some automakers call for the replacement of undeployed airbags after a certain period of time to ensure their reliability in an accident.
Safety performance
A U.S. study concluded that as many as 6,000 lives may have been saved as a result of airbags.
Injuries and fatalities
Airbags can injure or kill vehicle occupants. To provide crash protection for occupants not wearing seat belts, U.S. airbag designs trigger much more forcefully than airbags designed to the international ECE standards used in most other countries. Recent airbag controllers can recognize if a belt is used, and alter the bag deployment parameters accordingly.
Injuries such as abrasion of the skin, hearing damage from the extremely loud 165-175 dB deployment explosion, head injuries, eye damage, and broken nose, fingers, hands or arms can occur as the airbag deploys. Most vehicle airbags are inflated using hot gas generated by a chemical process. Using hot gas allows the required pressure to be obtained with a smaller mass of gas than would be the case using lower temperatures. However, the hot gas can pose a risk of thermal burns if it comes in contact with the skin during deployment and occupant interaction. Burns are most common to the arms, face and chest. These burns are often deep dermal or second-degree burns that take longer to heal and risk scarring.
In 1990, the first automotive fatality attributed to an airbag was reported, with deaths peaking in 1997 at 53 in the United States. TRW produced the first gas-inflated airbag in 1994, with sensors and low-inflation-force bags becoming common soon afterwards. Dual-depth (also known as dual-stage) airbags appeared on passenger cars in 1998. By 2005, deaths related to airbags had declined, with no adult deaths and two child deaths attributed to airbags that year. Injuries remain fairly common in accidents with an airbag deployment.
Serious injuries are less common, but severe or fatal injuries can occur to vehicle occupants very near an airbag or in direct contact when it deploys. Such injuries may be sustained by unconscious drivers slumped over the steering wheel, unrestrained or improperly restrained occupants who slide forward in the seat during pre-crash braking, and properly belted drivers sitting very close to the steering wheel.
The increasing use of airbags may actually make rescue work for firefighters, emergency medical service and police officers more dangerous, because of the risk of deployment while the emergency responder is assisting or extracting vehicle occupants.
Improvements in sensing and gas generator technology have allowed the development of third generation airbag systems that can adjust their deployment parameters to size, weight, position and restraint status of the occupant. These improvements have demonstrated a reduced injury risk factor for small adults and children who had an increased risk of injury with first generation airbag systems.
Air bag fatality statistics
From 1990 to 2008, the U.S. National Highway Traffic Safety Administration identified 175 fatalities as because of air bags. Most of these (104) have been children, while the rest are adults. About 3.3 million air bag deployments have occurred and the agency estimates more than 6,377 lives saved and countless injuries prevented.
A rear-facing infant restraint put in the front seat of a vehicle places an infant's head close to the airbag, which can cause severe head injuries, or death if the airbag deploys. Some modern cars include a switch to disable the front passenger airbag, (although not in Australia, where rear-facing child seats are prohibited in the front where an airbag is fitted), in case a child-supporting seat is used there.
In vehicles with side airbags, it is dangerous for occupants to lean against the windows, doors, and pillars, or to place objects between themselves and the side of the vehicle. Articles hung from a vehicle's clothes hanger hooks can be hazardous if the vehicle's side curtain airbags deploy.
Aerospace and military applications
The aerospace industry and the US Government have applied airbag technologies for many years. NASA, and US DoD have incorporated airbag systems in various aircraft and spacecraft applications as early as the 1960s.
Airbag landing systems
The first use of airbags for landing were Luna 9 and Luna 13, which landed on the Moon in 1966 and returned panoramic images. The Mars Pathfinder lander employed an innovative airbag landing system, supplemented with aerobraking, parachute, and solid rocket landing thrusters. This prototype successfully tested the concept, and the two Mars Exploration Rover Mission landers employed similar landing systems. The Beagle 2 Mars lander also tried to use airbags for landing, but the landing was unsuccessful for reasons which are not entirely known.
Occupant protection
The US Army has incorporated airbags in its UH-60A/L Black Hawk and OH-58D Kiowa Warrior helicopter fleets. The Cockpit Air Bag System (CABS) consists of forward and lateral airbags with an Electronic Crash Sensor Unit (ECSU). The CABS system was conceived and developed by the US Army Aviation Applied Technology Directorate, Fort Eustis, Va. It is the first conventional airbag system for occupant injury prevention designed and developed specifically for helicopter applications.
See also
External links
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