AIR BAGS

INTRODUCTION

For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debated about their safety, especially relating to children. But over time, mush of the country adopted mandatory seat-belt laws. Statistics have shown that the use of seat belts has saved thousands of lives that might have been lost in collisions.
Air Bags have been under development for many years. The attraction of a soft pillow to land against in a crash must be very strong – the first patent on an inflatable crash-landing device for airplanes was filed during World War II. In the 1980’s the first commercial air bags appeared in automobiles.
Since 1988, all new cars have been required to have air bags on both driver and passenger sides (Light Trucks came under the rule in 1999). To date, Statistics show that air bags reduce the risk of dying in a direct frontal crash by 30 percent. Newer than steering Wheel mounted or Dashboard-mounted bags, but not so widely used, are seat-mounted and door mounted side air-bags. Some experts say that within the nest few years, our cars will go from having dual air bags top having six or even eight air bags. Having evoked some of the controversy that surrounded seat-belt use in its early years, air bags are the subject of serious government and industry research and tests.








THE BASICS OF AIR BAGS
Before looking at specifics, let’s review our knowledge of the laws of the motion. First, we know that moving objects have momentum (the product of the mass and velocity of an object. Unless an outside force acts on an object, the object will continue to move its present speed and direction. Cars consist of several objects, including the vehicle itself, Loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a collision.
Stopping an object’s momentum requires force acting over a period of time. When a car crashes, the force required to stop an object is very great because the car’s momentum has changed instantly while the passengers’ has not much time to work with. The goal of any supplemental restraint system is to help stop the passenger while doing as little damage to him or her as possible.
What an air bag wants to do is to slow the passengers’ speed to zero with little or no damage. The constraints that it has to work within are huge. The air bag has the space between the passenger and the steering wheel or dashboard and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion.







DEVELOPMENT OF AIR BAGS
The idea of using a rapidly inflating cushion to prevent crash injuries has a long history. The first patent on an inflatable crash-landing device for airplanes was filed during World War II.
Early efforts to adapt the air bag for use in cars bumped up against prohibitive prices and technical hurdles involving the storage and release of compressed gas.

· If there was enough room in a car for a gas canister.
· Whether the gas would remain contained at high pressure for the life of the car.
· How the bag could be made to expand quickly and reliably at a variety of operating temperatures and without emitting an ear-splitting bang.

They needed a way to set off a chemical reaction that would produce the nitrogen that would inflate the bag. Small solid-propellant inflators came to rescue in the 1970’s.
In the early days of auto air bags, experts cautioned that the new device was to be used in tandem with the seat belt. Seat belts were still completely necessary because airbags worked only in front-end collisions occurring at more than 6 Kmph. Only Seat belts could help in side swipes and crashes (Although side-mounted air bags are becoming more common now), rear end collisions and secondary impacts. Even as the technology advances, air bags still are only effective when used with a lap/Shoulder seat belt.


MAIN PARTS OF AIR BAG
There are three parts to an air bag that help to accomplish this feat:
1. Bag
2. Sensor
3. Inflation system
BAG
The bag itself is made of a thin, nylon fabric, which is folded into the steering wheel or dashboard or, more recently, the seat or door.The powdery substance released from their sir bag, by the way, is regular cornstarch or talcum powder, which is used by the air bag manufacturers to keep the bags pliable and lubricated while they’re in storage.

SENSOR
The sensor is the device that tells the bag to inflate. It works with the control module to discriminate between crash and non-crash events. These sensors measure the severity of the impact. Inflation happens when there is a collision force equal to running into a brick wall at 16 to 24 Km per hour. They are setup so that sudden negative acceleration will cause the contacts to close, telling the control module that a crash before airbag deployment.

INFLATION SYSTEM
The air bag’s inflation system reacts sodium azide(NaN3) with potassium nitrate (KNO3) to produce large volume of nitrogen gas. Hot Blasts of the nitrogen inflate the air bag from its storage site up to 322Kmph. A Second later, the gas quickly dissipates through a tiny holes in the bag, thus deflating the bag so you can move.

CONSTRUCTION OF AIR BAGS
Airbag are assemblies consisting of the airbag (made of Nylon), inflator modules and sensor housing, electrical connectors (Clock spring), airbag retainer and the cover. The driver’s side bag is mounted in the center of the steering wheel as shown in fig. 1.
SENSOR TYPES


Spring and Roller type


By function, there are 2 types- Impact sensors and safing sensors. The Forward sensors are located in various locations forward of the passenger compartment. Some are located inside the fenders, some are on the cowl, and some are attached to the core support in front of the radiator.
Rear Sensors are also known as safing sensors as their functions is to determine that a crash has occurred. Rear safing sensors are located in various locations in the passenger compartment depending on the manufacturer. Some are integrated with the control/Diagnostic Module.
The Rear safing sensor must close before the forward sensors to avoid airbag deployment in cases where the impact is not severe enough to cause deployment. When the vehicle is parked with ignition off deployment is very unlikely because there is no power to the circuits for deployment this means that someone can hit your car and sound the alarm but not deploy the airbags.

Inflator Assembly
This is a diagram of a typical inflator assembly behind the steering wheel.


When the control Module activates the airbag assembly, an electric current is sent to the detonator, which ignites the sodium azide pellets. When it burns, it releases nitrogen gas very quickly and in large quantities. This is what inflates the airbag.

Sodium Azide
Sodium Azide is Rocket fuel. Sodium azide is a solid propellant with a very high gas generation ratio. It is very stable in this application.
When Sodium azide burns, its major product is nitrogen gas, which makes up around 78% of the Earth’s atmosphere. One of the other by-products is sodium hydroxide. This is commonly known as Lye, which is a caustic compound. The quantities produced are very small and present a very small risk of burns. The white powder residue seen after inflation is common cornstarch, used as lubricant for expansion of the airbag. Testing is underway with inflators that release argon gas.


WORKING OF AIR BAGS
Air Bags are designed to inflate in frontal or frontal-angle impacts in which the car strikes an immovable object at more than about 16 Kilometers per hour or another car at twice that speed. After a collision, sensors sense an electric current to an igniter system or, in some cases, to the computerized control unit. This unit evaluates the situation and then sends an electrical impulse to the igniter system. The electric current heats a filament (wire), which then ignites a capsule. The Ignited capsule supplies the heat to ignite gas-generating pellets. In most systems, the pellets are made of sodium azide and produce nitrogen gas when they burn. In other systems, pressurized argon gas is used instead. The gas then expands quickly and inflates the airbag, which then breaks through a plastic cover in the steering wheel or, the dashboard on the passenger side. The whole process takes about 0.1 second from the exact moment the crash is detected. The air bag starts to deflate immediately, venting the harmless gas through holes in the back of the bag of the through the fabric itself.

MODERN TYPES OF AIR BAGS
Curtain Airbags
Curtain airbags are airbags that inflate in front of vehicle windows to provide passengers better head and neck protection. The curtain airbags are part of new rollover protection system. Most equipped cars will have vertically inflating curtain airbags in the headliner trim just above the windows, while some will have them inflate horizontally from the side pillar between the doors. In some cases, curtain airbags will deploy in a fraction of a second.

Sensors in the side pillar will measure the force of side-impact Collisions. If the sensors measure a strong impact, then the curtain airbags will deploy in a fraction of a second. The curtain airbag will drop straight down over the windows to keep the occupants’ heads from striking the window or the intruding nose of another vehicle.


The system will use sensors that measure tipping rather than impact. These sensors send information to a central module, which will determine if the vehicle is beginning to roll over. If the vehicle is tipping into a roll, the sensors will deploy the curtain airbag. Stored in the headliner above the doors, the cells of the inflatable curtains are inflated in less than 25 thousands of a second in a triggering accident. To avoid stitches from sewing the bag, its cells are woven on the loom directly from the yarn using Autoliv’s one piece-weaving technology. Therefore the bag can remain inflated for several seconds, which is imperative in roll-over accidents. Cool gas will keep it inflated for up to six seconds.
Laboratory tests have shown that the so-called Head Injury Criterion (HIC) can be reduced by approximately 80%. The Inflatable Curtain was developed in cooperation with Mercedes and Volvo.
Car manufacturers selling car with these as standard equipment are Alfa Romeo, Audi, BMW and Volvo.

2. Head-Thorax Bag
The Head-Thorax Side-Impact Airbag has an extension from the regular Thorax Bag that protects the head. It was introduced in 1998 and developed in cooperation with Ford and Renault.

3.Inflatable Tubular Structure (ITS)
The Inflatable Tubular Structure(ITS), the world’s first airbag for head protection, was introduced on BMW cars. It consists of a unique nylon tube, installed in the head-liner above the frontal doors that inflates to a diameter of about 15 centimeters(5 inches).



FUTURE OF AIR BAGS

Smart Systems
The smart airbag of the future is not just the airbag, but also a redesign of the components in the current airbag system. Features include:
Weight Sensors
This is a new sensor for the passenger seat to classify the weight and to determine what type of occupant is in the seat, i.e. adult or child.

Infrared Occupant detection
This system will use infrared beams (just like in your TV remote control) to detect the distance the passenger is from the airbag and adapt the force of deployment accordingly.

Capacitive Reflective Occupant Sensing
These sensors will be located in the seat backs and in the dash to identify the distance you and or your passengers are from the dashboard. These sensors will be able to discriminate between a human occupant and inanimate objects like your groceries. This alone will save thousands of dollars in the cases where the driver is the only occupant in the front seat.

Updated sensors.
The updated sensors will have the capabilities of deploying the seatbelt pretensioners faster, so in a crash situation you will be in the best position to benefit from the airbag deployment.

Centralized electronic Control Unit
The new control units will be able to use all the input from the new sensor technology and through new software deploy what you need when you need it.
Prototype of a Smart Air bag

The Prototype smart air bag creates a weak electric field. Antennae located under fabric or in the seat cushion measure the field and instantly update the airbag controller about the size and position of the occupant.
When you are involved in a frontal collision somewhere in the neighborhood of at least 20 kmph, a number of things happen very quickly. The sudden deceleration of you vehicle causes 2 sensors to send an electrical signal; to the diagnostic module. The diagnostic module self tests to confirm that a crash event is taking place, and then it allows the signal to trigger the airbag deployment.


Airbag Deployment
Below is an illustration of a driver side airbag deployment. The first image is the airbag inflation, the second depicits your contact with the airbag, and the third is your coming to rest in the seat and the deflation of the airbag. This all takes place in about 30 milliseconds.
In fact the entire inflation/deflation cycle takes less than ½ second. The speed of the airbag inflating is around 320 kmph. The vehicle hasn’t come to a stop at this point, nor hopefully have you lost control of it.
1988 is the first model for depowered bags. Unfortunately these are not standard equipment on all cars.

Driver’s Side Airbag
The image below shows the parts of the airbag module in the steering wheel. From top to bottom we have the cover, the airbag assembly, the retainer attached to the steering wheel, the steering wheel and the clock spring.
The Clockspring allows the steering wheel to move while maintaining the electrical connection to the airbag module.

Passenger Airbag
The Passenger bag is mounted in the top of the dash on the passenger side of the vehicle.

Airbag Checks
When you get in your vehicle and turn the key, look at the dash to find your airbag or SRS light.
It should come on for 7-10 seconds and then go out. This tells you all is well with the airbag system.
· If the light does not come on, have it checked. After all, it could just be a burnt out Light bulb
· If the light does not go off after this period (usually 7-10 seconds) there is a problem
· If the Light comes on while you are driving, it also needs attention.

In addition to the front airbags, the car companies are putting airbags in the doors for side impacts that are not covered by the primary airbags.
They are putting them in the seats for the drivers and rear passengers as well. This increases the cost6 as well as the complexity of the systems.
Since 1990 the National Highway Traffic Safety Administration, which is responsible for the Vehicle safety rules and statistics relating to vehicles, found that of 19 drivers killed by airbags, only five of them were wearing seat belts and two of them were determined to be unconscious before the airbag deployed.


Depowered Airbags
Starting with the 1997 model year some automobiles will be available with depowered Airbag modules

Inflators
Inflators are undergoing redesign toward a hybrid design using argon gas and non-adized initiators.

DISARMING THE AIRBAG

Steps in disarming
· Disconnect the battery
· Wait 30 seconds. The air Bag is equipped with a capacitor and it’s important to make sure that it is completely discharges before continuing. Change the oil while waiting
· Remove the airbag fuse. The fuse Box is below the driver’s left knee; it folds down by squeezing on a couple of tabs near the top of the box. Fuse also checks the diagram on the fuse box lid before pulling the wrong one.
· Drop the sound insulator panel. This is a plastic panel underneath the steering column. It goes: steering column; interior colored panel; black colored panel. The black panel is the one to remove. There are two screws along the top and two finger-wing nuts along the back (one near the accelerator and one behind the emergency brake.)
· Fastened to the sound insulator panel is a yellow circuit. This is the airbag circuit. To disconnect the circuit, you’ll first have to remove a green connector Positive Assurance lock which is for safety catch. Both the green catch and the yellow connector are had to pull apart, but hey eventually will give.
· Thus the bag is disarmed. The only problems is, now the airbag light will be on all the time. The airbag is fastened to the steering wheel with four Torx screws (T-30), accessible from the behind the wheel. They won’t come all the way out they’re retained. Once you have them all loose, the airbag will try to fall out. The airbag is till connected by one or two wires. The yellow one is the airbag circuit again; it is unplugged the same way as before, except this time the safety catch is blue.
· Do not store it anyplace, which gets hotter than 130 degrees; do not heat it or apply electricity (even static).





CONCLUSION

The air bags are of greater importance in today’s vehicles since safety of human life is of prior importance. Since the count of automobiles is increasing tremendously on our roads, the probability of accidents is also more. So far a safe riding and for saving the precious life the safety bags must be implemented. Today it is the prevail age of the high class people who own high priced cars. Lets hope every automobile manufacturers implement the same since safety for life is inevitable.

















REFERENCES
¨Leonard Evans, “Airbag benefits,airbag costs”,journal of
Scince Servicing Society,2003
¨www.howstuff works.com
¨www.scienceservicinsociety.com