air bag

An automobile airbag, like this one in a crashed SEAT Ibiza car, inflates and deflates within a fraction of a second (about 0.8 seconds).

An airbag, A.K.A. a Supplementary Restraint System (SRS), an Air Cushion Restraint System (ACRS), or the Supplemental Inflatable Restraint (SIR) is a flexible membrane or envelope. Air bags are most commonly used for cushioning, in particular for rapid inflation in the case of an automobile collision. The number of lives saved by air bags is hard to pin down. One study, cited below, puts the number at just under 400 per year (6000 total), and another study indicates that air bags reduce fatalities by 8% when seatbelts are worn.

History

1975 Buick Electra Limited with ACRS

Since the start of 1994, Ford made airbags standard across their entire range of cars sold in Europe (except for the Maverick which was outsourced from Nissan).

Invention

The airbag was invented by Patrick W. Hetrick of Newport, PA, U.S. in 1951 and he patented the device the following year. Hetrick came up with the idea to help protect his own family using expertise from his naval engineering days. Throughout the years, the saving of many lives have been attributed to Hetrick's invention and highlighted in television shows such as Nova on PBS, or as winners of awards such as the Golden Gear award. There have been devices similar to airbags for airplanes as early as the 1940s, with the first patents filed for those devices in 1958. Early air bag system origins traces back to air filled bladders. These systems were large and bulky, and primarily consisted of compressed or heated air, compressed nitrogen gas (N₂), freon, carbon dioxide (CO₂), or a mixture of water and potassium (KH₂O).^[1]

The American inventor Allen K. Breed then developed a key component for automotive use - the ball-in-tube sensor for crash detection. He marketed this innovation first in 1967 to Chrysler.{fact}

First automotive applications: "replacing" the seatbelt

The device was briefly available in the United States in the mid-1970s.

During this era, Americans were infrequent users of seat belts and a means of offering seat belt-like levels of occupant protection to unbelted occupants in a head-on collision was felt to be a valuable innovation.

Ford built an experimental fleet of cars with airbags in 1971. General Motors followed with a fleet of 1,000 experimental vehicles in 1973, and these Chevrolet cars equipped with dual airbags were sold to the public through GM dealers two years later. GM called this the Air Cushion Restraint System.^[2] In the seventies GM cars had two-stage deployment similar to newer air bags.^[3]

The design is conceptually simple—accelerometers trigger the ignition of a gas generator propellant to inflate a nylon fabric bag very rapidly, which reduces the deceleration experienced by the passenger as they come to a stop in the crash. The bag has small vent holes to allow the propellant gas to be (relatively) slowly expelled from the bag as the occupant pushes against it.

Before these Chevrolets were sold, airbags were made available to the public in November 1973 when General Motors began offering dual airbags as an extra-cost option on several 1974 model full size cars made by the Buick, Cadillac and Oldsmobile divisions. This system was known as the Air Cushion Restraint System.

The market did not appear to reward this innovation. Airbags were first implemented by GM and Ford in low-volume production (in approximately 12,000 automobiles in the 1973-76 era), then abandoned by GM and Ford. The passenger side airbag on 1970s cars was located in the lower part of the dashpad and it also acted as a knee restraint.^[4] The lower part of the dash on the driver side was also different on cars with air bags as it was padded.^[5]

The 1970s fleet of 10,000 airbag-equipped GM experienced seven fatalities. One is now suspected to have been caused by the airbag. The crash severity was only moderate and at the time a heart attack was suspected. The victim was cremated without autopsy.

The standard shoulder belts were removed on these cars, as they were designed to replace seat belts.

Front air bags are not designed to deploy in side impact, rear impact or rollover crashes. Since air bags 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 air bag's benefits and they help restrain occupants during the initial and any following collisions.

Rebirth: supplemental restraint

Car designers have moved on from the initial view of the airbag as a seat belt replacement. Automobile airbags are now designed and sold as Supplemental Restraint Systems (SRS).

In 1980, Mercedes-Benz re-introduced the airbag in Germany that it had patented in 1971 as an option on its high end S-Class (W126), which also offered such other exotic options as hydropneumatic suspension. In the Mercedes system, the sensors would tighten the seat belts and then deploy the airbag on impact. The airbag was thus no longer marketed as a means of avoiding seat belts, but as a way to obtain an extra margin of occupant safety.

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 Honda 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 belt tensioner 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.

Airbags become mandatory

On July 11, 1984, the U.S. government required cars being produced after April 1, 1989 to have driver's side air bags or automatic seat belts (the automatic seat belt was a technology, now discarded, that "forced" motorists to wear seatbelts). Airbag introduction was stimulated by the U.S. DOT.^[6] However, airbags were not mandatory on trucks until 1995.^{[citation needed]}

In 1998, dual front airbags were mandated by the National Highway Traffic Safety Administration (NHTSA), and de-powered, or second-generation air bags were also mandated. This was due to the injuries caused by first-generation air bags that were designed to be powerful enough to restrain people who were not wearing seatbelts.

Airbags branch out

Side airbag.

As they gain experience, automakers have created new forms of airbags, like the side curtain airbag to protect occupants in side impact crashes.

The Swedish company Autoliv AB, today Autoliv, was granted a patent on side airbags, and torso side protection airbags were first offered as an option on the 1995 model year Volvo 850. The first head protection system airbags were included as standard equipment in 1997 on the 1998 BMW 7-series and E39 5-series.

In 2006, Honda introduced the first motorcycle airbag safety system on its Gold Wing motorcycle.

Benefits

Air bags supplement the safety belt by reducing the chance that the occupant's head and upper body will strike some part of the vehicle's interior. They also help reduce the risk of serious injury by distributing crash forces more evenly across the occupant's body.

"One recent study concluded that as many as 6,000 lives have been saved as a result of airbags."^[7] Given that airbags have roughly been around for 15 years, that comes out to perhaps only 400 lives per year, perhaps many of those the result of parents putting their children in the back of the car, due to airbag risk.

Costs

Airbags cost about $500 (USD) per vehicle from OEMs, who typically pay the supplier less than $100. If they are deployed in error or stolen, the registered vehicle owner is required to repurchase them. Since they are an integral part of the vehicle design, it is not usually possible to retrofit airbags to a vehicle that does not have them, though in the early 1990s Breed Technologies of Lakeland, Florida, offered a retro driver side system, the SRS-40 for seven of the top selling vehicles of the time.

The SRS-40 system used the same sensor as the early 1990s Jeep Cherokee. When the vehicle exceeded a preset negative acceleration threshold for a specific length of time, the sensor released two firing pins, initiating the deployment. This simple all-mechanical system eliminated the need for any electrical connections. Sensor calibration was changed from vehicle to vehicle to allow for the structural differences. The kits supplied by Breed included a new steering wheel.

Most manufacturers specify the replacement of undeployed airbags after a certain period to ensure their reliability in an accident.

Although self installation of used or surplus components may result in considerable cost savings, air bags should only be serviced by those who are properly qualified. Accidental deployment while servicing could result in severe injury and an improperly installed or defective airbag unit may not provide sufficient protection in an accident. For these reasons, laws limiting sale, shipping, handling and maintenance have been imposed in several countries. In Germany, where some of the strictest laws are in place, airbags count as harmful explosives and only car mechanics with additional special training are allowed to service airbag systems. Under German federal law, used but intact airbags are to be detonated under secure conditions and must not be passed on to third parties in any way, and private (i.e. layman) individuals are not allowed to handle airbag units under any circumstances. Legal purchase is restricted to buying a new replacement unit for immediate installation by the seller's qualified personnel.

Airbag injuries and fatalities

Airbags involve the extremely rapid deployment of a large cushion. While airbags can protect a person under the right circumstances, they can also injure or kill. To protect occupants not wearing seat belts, U.S. airbag designs trigger much more quickly than airbags designed in other countries. As seat belt use in the U.S. climbed in the late 1980s and early 1990s, auto manufactures were able to adjust their designs. Today all airbag control units recognize if a belt is used and set the trigger time accordingly.

Newer airbags trigger at a lesser speed; nonetheless, passengers must remain at least 25 centimeters (10 in) from the bag to avoid injury from the bag in a crash.

Injuries such as abrasion of the skin, hearing damage (from the sound during deployment), head injuries, eye damage for spectacle wearers and breaking the nose, fingers, hands or arms can occur as the airbag deploys.

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 airbags appeared on passenger cars in 2005. By that time, deaths related to airbags had declined, with no adults deaths and 2 child deaths attributed to airbags that year. Injuries remain fairly common in accidents with an airbag deployment.

Smoking a pipe should be avoided while driving. If the airbag inflates and hits the pipe, it is likely to be fatal, even if the crash is moderate.

The increasing use of airbags may actually make rescue work for Firefighters, EMS and Police Officers more dangerous. Every first responder should be properly trained on how to safely deactivate airbags or be aware of the potential hazards. Removing the car battery does not deactivate the airbags.

Triggering conditions

Airbag system.

Airbags are typically designed to deploy in frontal and near-frontal collisions, which are comparable to hitting a solid barrier at approximately 13-23 km/h (8-14 mph). Roughly speaking, a 23 km/h (14 mph) barrier collision is equivalent to striking a parked car of similar size across the full front of each vehicle at about 45 km/h (28 mph). This is because the parked car absorbs some of the energy of the crash, and is pushed by the striking vehicle. 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 air bag 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 air bag should have deployed. Occasionally, air bags can deploy due to the vehicle's undercarriage violently striking a low object protruding above the roadway surface. Despite the lack of visible front-end damage, high deceleration forces may occur in this type of crash, resulting in the deployment of the air bag.

The airbag sensor is a MEMS accelerometer, which is a small integrated circuit chip with integrated micromechanical 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, which 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.

There was some work initially in mercury switches but they did not work very 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 very little friction or hysteresis. This device was developed at Sandia National Laboratories. The rolamite and similar macro-mechanical devices were used in air bags until the mid-1990s when they were universally replaced with MEMS.

Nearly all air bags 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 air bag module.

Today, airbag triggering algorithms are becoming much more complex. They try to reduce useless 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 as very 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.

When there is a moderate to severe frontal crash that requires the frontal air bag to deploy, a signal is sent to the inflator unit within the air bag module. An igniter starts a rapid chemical reaction generating primarily nitrogen gas (N₂) to fill the air bag making it deploy through the module cover. Some air bag 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 very common in early inflator designs. However, propellants containing 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 may contain a canister which is 5,0 cm (2") in diameter, 3,8 cm (1.5") long, and contains about 50 grams of sodium azide. The passenger side container is 15 cm (6") long and contains 200 grams of sodium azide. ^[8] The incomplete combustion of the charge due to rapid cooling leads to production of carbon monoxide (CO) and nitrogen(II) oxide as reaction byproducts. ^[9]

The alternative propellants may incorporate eg. 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.^[10] Other alternatives are eg. nitrocellulose based bipropellants (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 (eg. 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 faster than the blink of an eye (about 0,2 seconds). Airbags deploy in about 0,05 seconds. Because a vehicle changes speed so fast in a crash, air bags must inflate rapidly if they are to help reduce the risk of the occupant hitting the vehicle's interior.

Once an air bag deploys, deflation begins immediately as the gas escapes through vent(s) in the fabric (or, as it's sometimes called, the cushion). 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 or talcum powder, which are used to lubricate the air bag 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 air bag 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. With the onset of symptoms, asthmatics should treat themselves as advised by their doctor, then immediately seek medical treatment.

Airbags in certain car models deploy twice, for two crashes; it first deploys and deflates, and then re-inflates upon a subsequent collision.

Airbags must inflate very rapidly to be effective, and therefore come out of the steering wheel hub or instrument panel with considerable force, generally at a speed of about 97,8 m/s (220 mph). Because of this initial force, contact with a deploying air bag may cause injury. These air bag contact injuries, when they occur, are typically very minor abrasions or burns. The sound of air bag deployment is very loud, in the range of 165-175 dB for 0,1 second. Hearing damage can result in some cases.

More serious injuries are rare; however, serious or even fatal injuries can occur when someone is very close to, or in direct contact with an air bag module when the air bag deploys. Such injuries may be sustained by unconscious drivers who are slumped over the steering wheel, unrestrained or improperly restrained occupants who slide forward in the seat during pre-crash braking, and even properly restrained drivers who sit very close to the steering wheel. Objects must never be attached to an air bag module or placed loose on or near an air bag module, since they can be propelled with great force by a deploying air bag, potentially causing serious injuries.

An unrestrained or improperly restrained occupant can be seriously injured or killed by a deploying air bag. The National Highway Traffic Safety Administration (NHTSA) recommends drivers sit with at least 25 cm (10") between the center of their breastbone and the center of the steering wheel. Children under the age of 12 years should always be properly restrained in a rear seat ^[11]. A rear-facing infant restraint must never be put in the front seat of a vehicle with a front passenger air bag. A rear-facing infant restraint places an infant's head close to the air bag module, which can cause severe head injuries or death if the air bag deploys. Modern cars include a switch to turn off the airbag system of the passenger seat, in case a child-supporting seat must be installed.

Advanced airbag design

Many advanced air bag technologies are being developed to tailor air bag 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 air bag module. Many of these systems will use multi-stage inflators that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the air bag diagnostic unit know if someone is occupying a seat in front of an air bag, whether the person is an adult or a child, whether a seat belt or child restraint is being used and whether the person is forward in the seat and close to the air bag module. Based on this information and crash severity information, the air bag is deployed at either a high force level, a less forceful level or not at all.

Many new vehicles are also equipped with side air bags. While there are several types of side air bags, all are designed to reduce the risk of injury in moderate to severe side impact crashes. These air bags are generally located in the outboard edge of the seat back, in the door or in the roof rail above the door.

The Citroën C4 provides the first "shaped" driver airbag - made possible by this car's innovative fixed hub steering wheel.

Seat and door-mounted air bags all provide upper body protection. Some also extend upwards to provide head protection. Two types of side air bags, known as inflatable tubular structures and inflatable curtains, are specifically designed to reduce the risk of head injury and/or help keep the head and upper body inside the vehicle. A few vehicles are now being equipped with a different type of inflatable curtain designed to help reduce injury and ejection from the vehicle in rollover crashes.

Airbag landing systems

NASA engineers test the Mars Pathfinder airbag landing system on simulated martian terrain.

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.

See also

• Automobile safety
• Safety standards

Other information

On some vehicles, airbags are labelled SRS (Supplemental Restraint System) or SIR (Supplemental Inflatable Restraint), reflecting the airbag system's claimed role as an adjunct to conventional restraints such as seatbelts.

References

1. ^ How Products Are Made: Air Bags
2. ^ http://img145.imageshack.us/img145/2507/acrs9ad.jpg
3. ^ http://img145.imageshack.us/img145/8004/acrs56dt.jpg
4. ^ http://img219.imageshack.us/img219/5651/acrs41ub.jpg
5. ^ http://img145.imageshack.us/img145/7784/acrs31zu.jpg
6. ^ http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/Esv/esv16/98S8P12.PDF In 1984, a U.S. Department of Transportation edict was issued to try to induce a large percentage of states to adopt their own mandatory buckle-up laws, or else passive restraints (airbags or automatic seatbelts) would be federally mandated...[]...Thus stimulated, airbags finally came into mass-production implementation by most auto manufacturers in the early- 1990s.
7. ^ http://www.productsliability.net/resources/legal_articles/generation_i_airbags
8. ^ http://www.sdearthtimes.com/et0800/et0800s9.html
9. ^ http://www.patentstorm.us/patents/5806888-description.html
10. ^ http://www.freepatentsonline.com/6306232.html
11. ^ Air Bags - National Highway Traffic Safety Administration (NHTSA).

External links

• Airbag Information and Airbag Deployment video in slow motion
• Howstuffworks : "How Air Bags Work"
• Chemistry behind airbags

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