Airbags

History For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debates about their safety, especially relating to children, but over time, much 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 1980s, the first commercial air bags appeared in automobiles.

To date, statistics show that air bags reduce the risk of dying in a direct frontal crash by about 30 percent

Types of Airbag There are two main categories of Airbag. Mechanically Triggered (no longer used??) Electrically triggered

THE SYSTEM

Effect of the restraint systems during minor accidents During minor accidents, the body absorbs most of the impact energy, using components such as high tech. bumpers and impact absorbers. The seat belts provide adequate protection; the belt tensioners restrain the car’s occupants in their seats. The belt force limiter reduces the risk of the belt causing injury. In this case, the airbags are not triggered.

The effect of the restraint systems during serious accidents During serious accidents, the car body absorbs as much of the impact energy as possible. The passenger compartment remains by and large intact and the airbags are triggered. In addition to the protection afforded by the seat belts, the restraint systems protect the front passengers from more serious injuries in the upper body and head areas. The car’s central locking system is opened.

1. Contact Reel (Rotary coupling/clock spring) 2. Air Bag Module Connector 3. Steering wheel securing bolt (use once only) 4. Steering Column 5. Harness Airbag Components

Airbag Control Unit Mounted directly to centre of vehicle floor Contains crash sensor, microprocessor, back up power supply and a memory unit.

Airbag Control Unit Airbag ECU’s usually house the following:-  Centre sensor. Usually solid-state deceleration sensor. Main sensor to initiate ignition of the squib. Must be fitted on the centre line of the vehicle mounted correctly

Airbag Control Unit Airbag ECU’s usually house the following:-  Safety sensor. Designed to prevent unintentional ignition of the airbag. Fitted in series with the power supply to the squib. Airbag will only inflate when the safety sensor is closed. Normally a mechanical switch (mercury switch or roller and ramp)

Airbag Control Unit Airbag ECU’s usually house the following:-  Ignition and Drive module The brain of the ECU. Contains a microprocessor, which will ignite the squib when the safety switch is closed and input data compares with programmed crash data stored in its ROM memory.

Airbag Control Unit Airbag ECU’s usually house the following:-  Diagnosis and memory module Works in conjunction with warning lamp. (Legal requirement on Electrically triggered system, Construction and use Regs.) When ignition is turned to on the warning lamp is illuminated for approx. 5secs, whilst a system self check is carried out. If no malfunction is detected the warning lamp is turned off and the system is made ready for action.

Airbag Control Unit Airbag ECU’s usually house the following:-  Diagnosis and memory module If a malfunction is detected the lamp remains on and the system will not operate. (Switched off) Whilst the vehicle is in motion, the system is continuously monitored and any malfunction will be stored in the fault memory as a fault code and the system will be deactivated until the fault code has been removed.

Airbag Control Unit Airbag ECU’s usually house the following:-  Diagnosis and memory module Fault Diagnostics The fault memory can be accessed via a code reader, which will normally indicate the exact fault detected (E.g. Drivers squib open circuit). Some systems may also have the facility to use a blink code system (no longer usual practise).

Airbag Control Unit Airbag ECU’s usually house the following:-  Diagnosis and memory module Safety Due to the dangers associated with airbag systems, only personnel who have been sufficiently trained should attempt repair to an airbag system. For safety most airbag systems have dedicated colour coded (yellow!) wiring looms.

Airbag Control Unit Airbag ECU’s usually house the following:-  Back-up power source Safety Due to the possibility of the battery/leads being damaged during frontal impact a back-up power source may be required. This will normally be a capacitor which can store enough charge to power the ignition of the squib. Disconnection from the power supply for ten minutes is advised before working on the system.

Airbag & Squib 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.

Airbag & Squib The air bag's inflation system reacts sodium azide (NaN 3 ) with potassium nitrate (KNO 3 ) to produce nitrogen gas. Hot blasts of the nitrogen inflate the air bag

The ‘Igniter’ or fuse is a weak point in an electrical circuit similar to a normal fuse, which is designed to get very hot very quickly and start the reaction.

Rotary Coupler

The rotary coupler or clock spring as it is sometimes referred to, is a spiral flat wiring loom. It allows connection between the ECU and Squib during full rotation of the steering wheel. It is therefore vital that it is fitted as per manufacturers instructions, as it will be manufactured either in the straight ahead, full left hand lock or full right hand lock position. If it is not fitted correctly it will fail almost immediately.

Operation Most Airbag systems are designed to work in conjunction with seat belt pre-tensioners. It is however imperative to wear a seatbelt when operating a vehicle with an Airbag, as failure to do so would result in the Airbag being ineffective.

Operation The timing of the detonation of seat-belts and air-bags are critical if they are to perform their required function. The following slide gives some example timings in milliseconds of a simple SRS system.

Operation 1.0 – 10 milliseconds: Impact identified and Pre-tensioner ignition started milliseconds: Airbag starts to inflate and Pre-tensioner moving milliseconds: Airbag is inflated and Pre-tensioning is complete milliseconds: Driver contacts airbag milliseconds: Airbag collapses and normal seat belt function occurs.

Following a Collision and Deployment Following airbag deployment the complete airbag system will usually require replacing. The airbag and squib/explosive will have been used and are obviously NOT reusable. The rotary coupler and wiring loom will in most likely hood be burnt by the explosive and good practise requires replacement.

Following a Collision and Deployment The ECU will have had any sensors contained within tripped and these may not be reset. The main memory module will have a collision stored in it’s permanent memory and this cannot be reset. This information can be used for criminal investigation.

Additional Information Most modern vehicles are equipped with more sophisticated systems than simply a drivers Airbag. They will include passenger Airbag, side airbags(incorporated in the seats) and may have up to twenty different airbags. These more complicated systems will of course be accompanied by more advanced impact sensors ( up to five may be situated around the vehicle)

Additional Information Some modern systems also incorporate an ‘Intelligent Inflation System’. These systems take into account seat position (distance from airbag) and severity of impact and can inflate softly/slowly or at full speed to ensure the occupant contacts a fully inflated airbag.

Additional Information Legislation With such a system there is of course bound to be detailed legislation on the installation and operation, of Airbag Systems. This is particularly so with warning lamps, On/Off switches for passenger Airbags.

Additional Information Useful websites (user name required)