Hello All,
Being working with Airbag systems for quite some time, i think i can throw some light on to this subject. The airbag system ideally consists of the following components:
1. Airbags (Driver, Passenger, Side, Curtain, Knee etc...)
2. ACU (Airbag control Unit, or Airbag ECU)
3. Seat Belt Pretensioners
4. Impact Sensors
5. Seat Position sensors
6. Weight Sensors
7. Buckle sensors
PS: This is a sort of theroetical explanation. So Sorry for writing long paragraphs.
Airbags can be single stage or dual stage. The low end vehicles are normally having the single stage. That means, what ever be the level of impact or whoever be the person sitting in the vehicle, it will deploy to its full. (Level of impact should be above the threshold of the impact, which varies from vehicle to vehicle). The dual stage airbag fires in two stages depending on the impact level as well as the condition of the passenger inside. The airbag module consists of the nylon bag, then an igniter and the chemicals which react to give the gas. Normally Sodium Acetate and Pottassium Nitrate (which is a class II explosive) are used to give nitrogen gas which fills the bag in case of a deployment. Presence of Pottassium nitrate makes the module a dangerous thing to handle unless trained. That is why it is always recommended not to repair or handle airbag modules by youreself.
Seat belt pretensioners are used to pull you back and hold you to the seat to a certain extend in case of an impact. It removes the slackness of the seat belt. And as per my knowledge, it is system requirement that all air bag equipped vehicles will have a pretensioner also. In case of an impact, the pretensioner is activated first.
ACU controls the whole airbag system. In low end models, the impact sensor will be located only inside the ACU. There need not be dedicated sensors in the front or side of the vehicle. The sensor inside is basically an accelerometer which detects the impact and if the impact passes a certain threshold value, it will send the deploy signal to the seat belt pretensioner followed by the airbag module.
Impact sensors are normally not provided on low end models with airbag as it increases cost. These are located on the front side of the vehicle and side of the vehicle if it has a curtain or side airbag. Again these are accelerometers which are impact rated.
Seat position sensors and weight sensors combined will give the system an idea about the person sitting on the seat. Position and weight sensors together will identify if a child is sitting on the seat or if an adult is sitting.
These sensors are calibrated for the standard average weights of persons around the world and it varies from market to market.
Buckle sensors are small switches inside the seat belt buckle which will sense the condition of the seat belt. As already mentioned in the discussion, previously the idea was that the airbag should not be deployed in case the seat belt is not worn as it can be disastrous. But now with the introduction of multi stage airbags, the airbags will fire even if the seat belt is not worn, but with a lesser deployment force.
Now coming to the topic, in most vehicles coming in India at the entry level are having only the ACU, Driver and passenger airbags (Single stage) and seat belt pretensioner. As such in India, there are no regulations which mandates the use of seat position, weight sensors, and bucle swithes. But these are mandatory in US market. In India some vehicle are providing the switch for passenger airbag which facilitates manually turning off that particular airbag. So if the vehicle doesnt have a switch and it doesnt have a buckle switch or seat position sensor or seat weight sensor, all the airbags will deploy in case of an impact.
Now coming to child seats, and seating of children in vehicle which doesnt have any such disabling options, it is always recommended to have the children sitting on the rear seats. This is because in case of an impact, if the airbag is deployed, the airbag will hit the child directly on the head and mostly due to the small height of the child, will press him down to the seat. The force with which the airbag opens is too high and if it doesnt hit you in a proper intended way, it will prove disastrous.
Finally all of you should have seen the words SRS written over the airbag modules in the vehicles. It means Supplimentary Restrain System (sometimes referred as Secondary Restrain System) and the primary restrain system is the seat belt. So the SRS will not help you if you have not using the PRS
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ideally.
Thanks and regards,
Harikrishnan.