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-   -   Frontal Crash - Airbags didn't deploy. Why? (https://www.team-bhp.com/forum/road-safety/157842-frontal-crash-airbags-didnt-deploy-why-2.html)

Quite off topic, but I'm surprised to see the kind of wiring that the airbag sensor has. Airbag wires are enclosed in yellow sheath and they shouldn't be tampered with. Here I see two wires hanging freely. Throughout my car, any wire related to the airbag is in this way and its clearly mentioned not to tamper any such wire.

Correct me if I'm wrong. An airbag impact sensor is NOT something that works only if it gets an impact on it. It's a combination of accelerometers and impact sensor. If the sensor measures the deceleration value above the threshold set by the manufacturer it deploys the airbags no matter whether the impact sensors took a direct hit or not thereby preventing the driver from hitting the steering wheel/dash. The airbag ICU just not takes the information from the frontal impact sensors present but all the accelerometers present in the vehicle along with other gyroscopes and pressure sensors if any. So the ultimately it's the combination of vehicle speed and the deceleration it experience triggers the airbag sensor apart from the impact sensed.

IMHO it's not proper for a service center to ask the customer to "collide properly". If the impact is severe enough to cause injury for a driver it has to deploy the airbag to avert it.

The question of airbags deploying has already been taken up in another thread. The answer remains the same here too:

1. Airbag deployment conditions are manufacturer specific and the algorithms that activate them are based on various parameters such as vehicle stiffness to weight ratio etc.

2. They also usually need two consecutive pulses of ~1G for smaller vehicles and ~2G for larger vehicles. It is not necessary for the sensors to have a physical impact to be activated - the sensors would just need to sense an impact of ~1G to ~2G along with the minimum speed.

3. Usually the minimum speed for activating airbags is around 12 kmph (8mph)

4. In the example quoted above, if the impact on the car was while it was moving at a minimum speed of ~12 kmph and there were 2 pulses of around ~1G the airbags should have deployed unless Honda has calibrated different parameters into the Brio Airbag algorithms which is quite unlikely.

Not true in all cases (unfortunately).

Most common (low end) cars sold in India do not have accelerometers and gyroscopes and deceleration sensors etc. The sensors are usually of the electro-mechanical crush-dependent type, only working when physically broken / affected. Accelerometer-based sensors are obviously more expensive, and also more easily triggered in minor fender benders. And of course, a single accelerometer type sensor installed inside the cabin or engine compartment is good enough to sense the rapid deceleration caused by frontal impact, so why do we need two of those things on the two front corners, just behind the bumper frame? :)

(I can now foresee indignant members baying for manufacturers' blood, as to how they have been cheated by airbag set-ups which are less effective than what buyers thought they were.)

Disclaimer: I have not physically opened up and checked & researched types of airbag sensors used in different cars in this country, so while reading some of my generalized statements, please do not draw any conclusions that the same may be applicable to your own car. Check and confirm with your manufacturer.

As I mentioned in the earlier post, most Airbag units work with not one, but a combination of parameters. To name a few: Crush (in inches), change in velocity (delta V), jerk (change in acceleration), and acceleration, etc. If you look at most manufacturers and their patents, its a combination of these parameters that are used to make the "fire; no fire" decision.

For example, when the car goes through a deep pothole, there is no crush, little delta V, but huge jerk, the airbags would not deploy.

Also, as one can imagine, Crush depends on the weight (w) and the stiffness (k) of the vehicle, which in turn depends on its build and the physical parameters of the materials used. Thus each car would deploy the Airbags at different speeds; but should deploy between 8 MPH and 14 MPH.

As far as I know, MEMS based accelerometers are most commonly used. An "Analog Device ADXL78" accelerometer costs much less than $ 6 for the volumes we are talking. However, its best to get this info from someone in the forum who procures these products.

I am doubtful of this. Firstly, airbag components are imported for a small car like the WagonR itself. Hence, what is available elsewhere, ie same part numbers as international variants is available here. It isn't India or segment specific. Two sensors or more is needed to detect offset collisions. Suppose the car hits a barricade on the passengers side, the net deceleration that side will obviously be greater. Hence two or more sensors can be placed.

I am not a mechanical or automotive engineer but I do study about MEMS. A deceleration based sensor, as you say, cant be placed in the passenger compartment and made to trigger in the event of a collision. Then as you say, it will trigger in small accidents. The sensor, irrespective of its type is still mounted on a front structural member like the apron, or radiator cross member(not seen any).

Why?
Consider a car that suffers from frontal impact at a moderate speed. This car has two sensors mounted on the apron. The deceleration faced by you is very, very lesser compared to the deceleration experienced by an apron that's getting crushed into a crumple of metal. Imagine, if that force can mangle metal, if the body, or the passenger compartment had been through the same, I doubt the human body would have withstood it. Roughly saying, Deceleration is a method of resisting force. Hence, when the airbag sensor is attached to an apron, the deceleration it can measure is a lot more than the same sensor being placed in a passenger compartment. If we analyse a crash in milliseconds' resolution, the engine compartment takes in the blow, crushes the apron, this force triggers the airbags quite quickly before the passenger compartment can realise the crash.

Suppose the deceleration was measured elsewhere, or inside the cabin, either the airbags would have deployed late(if sensitivity accurate) or would have deployed unnecessarily(sensitivity high) or not at all deployed(less sensitivity). Hence, the sensors aren't placed in the front to get 'hit' by the crash. They need a mounting point that can let them measure deceleration when they require and exactly how they require. The apron of my car, for example is a crumple zone. The sensor is mounted right ahead of the crumple zone.


Even in the case of this brio, the impact hasnt reached the sensor because the forces have been absorbed by the bumper and bonnet, leaving lesser force to the point where the airbag sensors are. The sensor neednt get crushed. The part holding it should be displaced at a high rate thats it.

Yes airbag sensors are MEMS devices and as the day progresses and the technology will become cheaper, so will the cost of such components. As for now, the problem in India is these components are largely imported. Else they can manufacture safety equipments for lesser prices.

I don't understand why they consider speed of the vehicle as an entity. Let's assume someone has parked their vehicle properly and some idiot coming in the opposite direction rams the car at high speed, shouldn't the airbag deploy in this case? makes no sense to me.

I had raised the same doubt on the other thread and this is what Rehaan had to say:

Link

As far as I have understood Honda's literature, airbags can deploy at a lesser threshold of impact if the seatbelts are not latched/ locked to provide additional safety to the unbelted passengers. In case the occupants are belted, the airbags deploy at a higher impact threshold; only to supplement the seat belts. In this case, the driver was belted, hence the airbags would have deployed at a higher impact threshold. Also, it is possible for only one airbag to deploy.

Regarding the impact sensors, if I remember correctly, there is a magnet/ steel ball setting, which works as a sensor. When it experiences a heavy impact, the steel balls rolls away from the magnet (due to impact), eventually completing a circuit which in turn triggers the airbags. I have no idea if vehicle speed is taken into consideration by the ECU to trigger the airbags or not.

Regards,
Saket

AFAIK, the magnetic or spring-loaded ball sensors are obsolete for some time now, though your description of their working is correct. All manufacturers have moved to MEMS based capacitive or piezoelectric sensors.

This is interesting. The airbag sensors in my Mitsubishi are located dead centre. They are right behind the front impact protection beam and a good 1.5 foot off the ground. The black balloon shaped thingy (above the horns) houses the sensors for both sides and this car is not equipped with 'passenger side airbag off' option.

This placement is true for all markets where the Cedia is sold which essentially translates to 'all over the World'.

My question is whether the placement of sensors in corners for South/South East specific models is due to cost cutting achieved by enabling only the airbag in the impact zone to inflate thereby negating the need for a "passenger air bag OFF' circuit as well as reducing the cost of air bag replacement OR is it because of traffic conditions in these markets OR some other reason.

This implies that airbags should not deploy if there is a collision when a moving object hits a stationary vehicle.

But that shouldn't be the case, right?

It essentially makes passengers sitting ducks if there is a head-on/rear collision with a vehicle at high speeds.

While I see why they might want to have a min. speed requirement, it doesn't cover all possibly scenarios where airbags could help.

You're right. If the minimum speed aspect is to be considered, airbags wouldn't inflate if the car is stationary. Hence, as I explained previously, there is a minimum deceleration to cause them to inflate and not minimum velocity.

Consider a car is stationary. Once its hit by an opposite vehicle, the crash sensors will experience deceleration. If you recall physics, deceleration is not only slowing down, but is also to do with negative acceleration. Hence, the crash sensor from rest will be accelerated in the opposite direction with respect to the forward driving direction. Hence this will indeed trigger the airbags.

That was my first thought.

I'm curious, would the airbag mechanism rely on the speed reading from the airbag sensors or the car electronics?