[procrj]

Related Thread : Link

Supplementary Restraint Systems (SRS) in modern day cars are quiet complex and Airbags and Seat Belts with pretensioners are key components that help keep one safe during accidents. I have broken down this thread into the following components in order to aid easy reading

• History
• Key components of Airbag Systems
• How does it work
• Deconstructing the Airbag Module
• Types of Sensors
• Modern day Airbags system
• Some Questions
• What you need to know

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History
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John W. Hetrick of Newport, Pennsylvania, who came up with the idea after an accident in which he swerved his car off the road into a ditch to avoid hitting a rock, almost throwing his daughter through the windshield. Hetrick filed his patent for a Safety cushion assembly for automotive vehicles on August 5, 1952 (it was granted as US Patent #2,649,311 on August 18, 1953). Although a German inventor named Walter Linderer filed an airbag patent several months before Hetrick, it was granted after Hetrick's, and it seems likely that the two men came up with the same idea independently.

Source: http://www.explainthatstuff.com/airbags.html

Artwork: John Hetrick's original airbag design from 1953, which I've colored to make it easier to follow. There are three separate drawings here, showing the main mechanism (occupying most of the picture), a driver's perspective view of the steering wheel (bottom right), and a view of the inflated bag from the side (top right). The bag is triggered by a heavy weight (blue) restrained by a spring (yellow) inside the red cylinder on the right. After an impact, the weight pushes the spring to the right, opening a valve inside a pipe (turquoise) that allows compressed air to flow out from a cylinder (green) and inflate the airbag cushion. Artwork courtesy of US Patent and Trademark Office. Read a full description in Safety cushion assembly for automotive vehicles (via Google Patents)

Source: http://www.explainthatstuff.com/airbags.html

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Key components of Airbag Systems
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To keep it simple, a modern day automotive airbag system contains the following key components:

• Airbag Sensors - are electro mechanical or MEMS (Micro electro mechanical systems) based chips that are used to provide various kinds of inputs to the airbag control unit
• Airbag Control Unit(ACU) - is like the ECU in the car. It analyzes various inputs from sensors and makes the decision to deploy an airbag
• Airbags - are typically nylon bags that are inflated with an inert gas (typically nitrogen or argon) to protect the occupants from impact

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How does an Airbag System work
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Here is the simple explanation of how the system is designed to work


Source:http://www.explainthatstuff.com/airbags.html

• 1. When a car hits something, it starts to decelerate (lose speed) very rapidly
  2. Airbag sensor detects the change of speed
  3. If the deceleration is great enough, the sensor sends a signal to the ACU
  4. The ACU passes an electric current through a heating element
  5. The heating element ignites a chemical explosive. Older airbags used sodium azide as their explosive; newer ones use different chemicals
  6. As the explosive burns, it generates a massive amount of harmless gas that quickly fills up the nylon airbag
  7. As the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance such as talcum powder to help it unwrap smoothly
  8. The driver (moving forward because of the impact) pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should have completely deflated

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Deconstructing the Airbag Module
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Source: https://www.autoliv.com/ProductsAndI...s/default.aspx

The Airbag module typically consists of 4 main components:

• 1. An inflator that produces sufficient gas to inflate the folded bag. The inflator basically contains the chemicals and the heating element that are required to generate the inert gas that fills up the airbag
  2. A textile cushion made of thin, woven nylon or polyester fabric, which is the actual airbag
  3. A housing for storing the folded bag and the inflator. The housing can be made of steel, plastic or even textile.
  4. A cover that opens as the cushion inflates.

https://www.youtube.com/watch?v=I_hkGN8TiJY&t=1s

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Types of Sensors
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Here is where things can get a little complex. Based on my understanding, sensors can be broadly classified into 3 types:

• Crush zone sensors - these are sensors that are placed within crush/crumple zones to help measure impact using deceleration/inertial changes
• Impact sensors - these are placed just inside the crush/crumple zone to provide the ACU with additional information
• In cabin sensors - these mostly sit within the ACU and help provide passenger side inputs to the ACU & help it make the deploy decision

Based on their construction, Sensors can also be classified into:

• 1. Electromechanical - are inertia based sensors which use the sudden deceleration during impact to send signals to the ECU. These are basically of 2 types
  
  a.Mass type sensor
  

  During a collision with another moving vehicle, the sensing mass is forced forward into the gold-plated contacts as a result of change in the state of motion. Following movement of this metal ball into the contacts, this metal mass makes contact with electrical terminals at either side of the metal ball which alerts the central unit to a collision

  Source: https://www.azosensors.com/article.aspx?ArticleID=40
  
  
  Source: https://www.azosensors.com/article.aspx?ArticleID=40
  
  
  b.Roller type sensor
  

  The roller-type sensor involves a weight connected to a coil spring component. Like the mass-type sensor, during impact with an oncoming vehicle, the metal weight is forced forward which alters the tension on the coil spring to manipulate the electrical circuit that closes off the sensor contact.

  
  Source: https://www.azosensors.com/article.aspx?ArticleID=40
  
  Do watch this video for a better understanding of the same and how it actually looks when a sensor is broken down.
  
  

  https://www.youtube.com/watch?v=mWSlwhYyOhI

  
  
  
• 2. Micro electro mechanical systems (MEMS) - Think of these as small chips which contain integrated circuits as well as mechanical membranes that help sense inertial change. Thanks to MEMS a lot of components can be include in a small chip that is either an integral part of the ACU or sits separately and feeds data to the ACU.
  
  Here is a small clip from bosch that explain what an MEMS Accelerometer works
  

  https://www.youtube.com/watch?v=9eSnxebfuxg

  
  
  The two most common applications of MEMS from an airbags control point of view
  
  a.Accelerometers - used to measure the rate of acceleration/deceleration. They are not only used to measure horizontal accelerations but can also be used to measure lateral acceleration. Today its also possible to include 3 accelerometers within a chip to measure change over all 3 axes
  
  b.Gyroscopes - are used to detect roll over and help the ACU take a decision on deployment

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Modern day SRS system
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Here is the simplest representation of a modern day SRS system


Source:https://www.slideshare.net/abhisheks...-in-automobile

• SAT - Satellite sensors, which are basically various types of sensors that are placed in various sections of the car depending on the number of airbags
• ECU - ACU
• ROS - Roller over sensors
• WS - Weight sensors
• BS - Buckle switches
  
  1.Satellite sensors can be electromechanical or MEMS based sensors which provide signals to the ACU
  
  2.The ACU then combines these signals along with signals from its in built sensors and based on certain thresholds, decides to deploy airbag.
  
  3.Rollover sensors, Weight sensors provide inputs to the ACU to decide what airbags needs to be deployed. For example if the passenger seat is not occupied, then the passenger weight sensor will not send a signal to the ACU and hence the passenger airbag will not deploy, thereby reducing replacement cost
  
  4.Buckle switches get input from the ACU and the pretensioners kick in to restrain the passenger
  
  
  Event Data Recorders (EDR) are also included in a lot of advanced SRS systems.
  

  Popularly called a "black box," an event data recorder is a device that records certain information from a vehicle immediately before and/or during most serious crashes. Police, crash investigators and others can download the data from the EDR's memory to help them better understand what happened to the vehicle and how the safety systems performed, and in some cases, help establish culpability. Most EDRs are built into a vehicle's airbag control module and record information about the deployment of airbags and belt tensioners, as well as pre-crash data from the engine control module, such as engine speed, throttle position and vehicle speed.

  Source: http://www.iihs.org/iihs/topics/t/ai...data-recorders
  

Here is a slow motion video of what happens when the seatbelt pretensioner works.

https://www.youtube.com/watch?v=6phdJWRi_vw

[procrj]

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Some Questions
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• Why didn’t the airbag deploy when I hit a pole?
  

  Pole crashes are typically staged by automobile manufacturers during their airbag development programs, but they are limited in scope. They typically involve large poles that are one foot or more in diameter and are usually run at high speeds. It has been found, however, that thin poles at low speeds are much more difficult to enable proper sensing for airbag deployment than thick poles at high speeds.
  
  They are also much more common in the real world. Non-crush zone sensors have a particularly difficult time in sensing pole crashes especially those involving thin poles at low velocities, since the crash pulse is very soft until it is too late to initiate airbag deployment. Conventional crush zone sensors, such as the ball-in-tube sensors, function properly as long as the sensor is located in-line with the impact point of the pole. When this is not the case, and especially when the impact speed is low, these sensors can fail.
  
  A particular case, for example, involved a vehicle that has three ball-in-tube sensors mounted in the crush zone, one center-mounted and one on each side approximately in line with the rails. This vehicle impacted a pole at approximately 15 miles per hour at a point midway between the front center and side sensors. An examination of the vehicle showed that there was no crush at either of the sensor locations. In this case, the sensors triggered the airbag late based on the non-crush zone crash pulse as described in U.S. Pat. No. 4,900,880. Before the airbag deployed, the occupant had already impacted with the steering wheel and although conscious after the accident, later died from internal injuries

  Source: https://www.google.com/patents/US7744122
  
  Hence it's not necessary that the airbags deploy even during a frontal impact. If the impact does not occur inline or close to the sensor, the sensor might not trigger a signal to the ACU.
  
  
• Why didn’t the airbag deploy when I hit a stationary car?
  

  Passive safety electronics (ACU) measure deceleration, since vehicle speed is not an adequate indicator of whether an airbag should be deployed or not. Airbags and seatbelt pretensioners in frontal and near-frontal collisions are typically designed to deploy when the crash force is comparable to hitting a solid barrier at approximately 12 to 20 km/h (about 8 to 12 mph).This is, roughly speaking, equivalent to striking a parked car of similar size at about 43 km/h (about 27 mph), assuming 100% of the fronts of both vehicles are engaged in the crash.
  
  Unlike most 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. In addition, the parked car absorbs some of the energy of the crash and is pushed by the striking vehicle. Consequently, the relative speed between a striking and struck vehicle required to deploy the airbag in a real-world crash can be much higher than an equivalent barrier crash.

  Source https://www.autoliv.com/ProductsAndI...ectronics.aspx
  
  
  Given the volume of sensors and amount of intelligence that is fed into the ACU, the answer to this questions can be a very simple - airbag sensor did not trigger as there wasnt sufficient change in inertia to a more complex ACU related calculation.
  
  
• Can the airbag sensor fail?
  Most Indian cars (as far as my knowledge goes) use the ball in tube type/mass type sensor, which because of its placement, can easily fail for the following reasons: corrosion & small front end collisions that dont deploy airbags
  
  
• Will airbags work even if I don't wear seatbelts?
  Well the jury is out on this but based on info out there on quora etc. it seems like modern cars assume that if the car is moving, there is someone driving it and hence during a crash airbags should deploy (if the sensor & ACU triggers the same).
  
  
• Why is it important to restrain a passenger using additional force?
  

  Airbags have traditionally been designed based on the assumption that 30 milliseconds of deployment time is available before the occupant, as represented by an unbelted dummy corresponding to the average male, has moved five inches.
  
  An occupant can be seriously injured or even killed by the deployment of the airbag if he or she is too close to the airbag when it deploys and in fact many people, particularly children and small adults, have now been killed in this manner. It is known that this is particularly serious when the occupant is leaning against the airbag when it deploys which corresponds to about 12 inches of motion for the average male occupant, and it is also known that he will be uninjured by the deploying airbag when he has moved less than 5 inches when the airbag is completely deployed.

  Source:https://www.google.com/patents/US7744122
  
  
  Slow motion video of an airbag opening. Do check where the glass of water lands!
  

  https://www.youtube.com/watch?v=KRcajZHc6Yk

  
  
  
• What is a dual stage airbag?
  The common airbag module contains just 1 inflator assembly which once used, cannot be reused. A dual stage airbag contains 2 inflator assembly which can be used to control the rate of inflation of the airbag based on various parameters like weight sensor inputs, distance from the wheel etc.
  
  

  Advanced airbag technologies are being developed to tailor airbag 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 actual airbag. Many of these systems use multi-stage inflators that deploy less forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let the airbag control unit know if someone is occupying a seat adjacent to an airbag, the mass/weight of the person, whether a seat belt or child restraint is being used, and whether the person is forward in the seat and close to the airbag. Based on this information and crash severity information, the airbag is deployed at either at a high force level, a less forceful level, or not at all.
  
  Adaptive airbag systems may utilize multi-stage airbags to adjust the pressure within the airbag. The greater the pressure within the airbag, the more force the airbag will exert on the occupants as they come in contact with it. These adjustments allow the system to deploy the airbag with a moderate force for most collisions; reserving the maximum force airbag only for the severest of collisions. Additional sensors to determine the location, weight or relative size of the occupants may also be used. Information regarding the occupants and the severity of the crash are used by the airbag control unit, to determine whether airbags should be suppressed or deployed, and if so, at various output levels

  Source:https://en.wikipedia.org/wiki/Airbag
  
  
• My car has an on/off switch for airbag. When do I use it?
  The National Highway Traffic Safety Administation has a very good guideline document which does provide guidance on when to turn off airbags.
  https://www.nhtsa.gov/sites/nhtsa.do...fobrochure.pdf
  
  
• What happens when someone crashes into the passenger side door? or rear doors?
  If you car has sensors placed on doors or the B pillar, they will record the impact and send signals to the ACU to deploy side/curtain airbags.
  

  Many Volkswagen models are fitted with ultra-fast pressure sensors in the front doors to detect a collision from the side. When the vehicle is hit from the side, the outer door panel is pushed inwards, creating excess pressure inside the door. If the change in air pressure exceeds a certain limit, the sensor will relay a corresponding signal to the airbag control unit. Acceleration sensors are furthermore fitted in the vicinity of the C-posts so that collisions from the side can also be detected in time.

  Source: http://en.volkswagen.com/en/innovati...hsensoren.html
  
  
• Are all airbags of the same size?
  No. Usually passenger airbags are bigger than driver airbags. Driver airbags are usually 50-60 liters and passenger airbags are 90-120 liters in volume. This difference in volume can be attributed to the steering wheel, which makes the driver airbags smaller.
  Recently there was discussion on Airbags size impacting the NCAP rating of the India Spec Duster where it was found that the India spec gets a smaller size airbag and hence had lower safety rating as compared to its Latin counterpart.
  
  
• Will airbags work when the car ignition is off?
  The ACU and other SRS components need your ignition to be turned on for them to be functioning. In case of a crash when you car was stationary and ignition was turned off, Airbags & SRS will not function. Hence if you are parked by side of the road and still sitting in your car, keep your ignition on
  
  
• Why shouldn’t we have front bull bars on a car?
  Based on the types of sensors and the way they work, Bull bars should not really have an effect on airbag deployment. If we assume that all Indian cars use the mass type sensor, which uses inertia/deceleration and not force of impact to trigger airbags, then bull bars should in no way impede the operation of the SRS system. Which is probably why some manufacturers do provide bull bars as accessories for cars with airbags.

_____________________________________

What you need to know
_____________________________________

• Its extremely important that you wear your seatbelts because if you don't and your airbags go off, you might be severely injured or might even die because you were too close to the airbags when it deployed
  
  
• Airbag deployment does not depend on just one sensor. The ACU is at the heart of the SRS system and a lot of modern day cars use multiple sensors to provide inputs to the ACU
  
  
• Modern day ACUs can infer acceleration/deceleration along all 3 axes
  
  
• ACUs do some massive computations within a few milli seconds to arrive at a decision. There are articles out there that talk about multi level neural networks (complex algorithms) that are used to evaluate inputs from various sensors and check if the airbag should be deployed
  
  
• Its only going to get more complicated as we add more airbags & all other kinds of safety features in a car
  
  
• The future seems to be integration of Electronic Stability Control (ESC), Anti-lock Brakes (ABS) and Automatic Traction Control (ATC) with the ACU to ensure better safety

Here is an interesting video from 1991

https://youtu.be/qdsCqizfSqU

[GTO]

Thread moved from the Assembly Line to the Technical Section. Thanks for sharing!

[SunnyBoi]

Good info. Here's some vehicle specific info about the sensors and how they work from my Mitsubishi's manual.

[kirren07]

Thanks. Very valuable and useful information. What I would like to know is does it cost a lot to reset an airbag once it's deployed?

[motorguy]

@Procrj and @sunnboi - fabulous articles. Both of you have broken down the complexity of the SRS systems to something all of us can understand. Always wanted to know the intricacies of SRS systems.

On a humorous note, someone I know (and this is true) after seeing the words "Airbag" on the steering wheel and near the glove compartment, thought that there was actually a bag that could be used for a purpose (like a shopping bag container) and didn't actually know that it was a restraint system. She almost tried to take out the airbag by prying the cover with her hands to find out what this "air bag" was, before a friend stopped her and said you would have to pay a pricey amount to replace (as it can't be packed back into the steering wheel).

[vjkamal]

Originally Posted by procrj

[*]Will airbags work even if I don't wear seatbelts?
Well the jury is out on this but based on info out there on quora etc. it seems like modern cars assume that if the car is moving, there is someone driving it and hence during a crash airbags should deploy (if the sensor & ACU triggers the same).

Throwing more light on this subject, if you car doesn't have a belt pre-tensioning system or a seat belt reminder, that means your seatbelt is a full mechanical unit and has no interaction whatsoever with the car's ECU. So, the airbag will deploy regardless of you wearing a seat belt or not. In case, if it has a pre-tensioner or seatbelt reminder, then it will depend on OEM to OEM. Some manufactures choose to deploy the airbag while some, may not while the seatbelts aren't worn.

Originally Posted by procrj

[*]Why shouldn’t we have front bull bars on a car?
Based on the types of sensors and the way they work, Bull bars should not really have an effect on airbag deployment. If we assume that all Indian cars use the mass type sensor, which uses inertia/deceleration and not force of impact to trigger airbags, then bull bars should in no way impede the operation of the SRS system. Which is probably why some manufacturers do provide bull bars as accessories for cars with airbags.

Most manufacturers don't officially encourage installing a bull bar. The sudden deceleration (due to crash) experienced by the sensors would not be the same with/without a bull bar. The airbags will deploy in most cases, but the ACU may record a short peak of severe pulse due to the bull bar. While the ACU does this, it may not have an actual impact on the speed of occupant movement towards the steering wheel/ airbag. The ACU basically has an algorithm designated to use different TTF (time to fire) based upon the crash severity. In this case, we can expect the airbag to fire earlier than required which will make the airbag not 100% effective when compared to what could have been achieved had it been deployed at the correct time.

Again, everything happens in a matter of less than 80 ms. So the subject is very debatable with a lot of if's and why's.

Originally Posted by kirren07

Very valuable and useful information. What I would like to know is does it cost a lot to reset an airbag once it's deployed?

You can't actually 'reset' an airbag. You will need to replace the entire SRS. For sure, it will cost a fortune.

[vinit.merchant]

Thanks for the insights.
What this makes us know more is having an airbag equipped car is no longer a peace of mind. Manufacturers can still skip on some necessary sensors (and thereby save a few hundred rupees at the most). Skipping on an impact sensor, and in case of a crash, which as mentioned, in real life scenarios may not be head on, the airbag may not deploy, thereby making the entire purpose of having an airbag as useless.

[procrj]

Originally Posted by vjkamal

The sudden deceleration (due to crash) experienced by the sensors would not be the same with/without a bull bar.

Agreed. Simple physics where F=M*A slightly higher mass and hence slightly lower deceleration.

but the ACU may record a short peak of severe pulse due to the bull bar.

Theoretically that pulse will be lower if you have a bull bar and hence the ACU might fire later that require thereby injuring the driver. Dont understand you reasoning behind early deployment.

Originally Posted by vinit.merchant

What this makes us know more is having an airbag equipped car is no longer a peace of mind.

Having a car today is no longer peace of mind jokes apart relevant analogy I can think of is Google maps. Most of the times its gets its traffic predictions right and you can rely on it but there will be those some times where you will be taken for a ride. Does that mean you stop using it? No. Ditto for Airbags & SRS.

For me its a trade-off between 100% likelihood of injury and a less likely chance of injury due to SRS. Also need to note that Airbags are only 1 part of SRS. Pretensioners today work quiet well and I have observed on many occasions that my seatbelt pretensioner held me in place during panic breaking or even when I suddenly move in front to get a little more comfortable.

[Sutripta]

How doesa one find out the weight of the person(kid/ adult) in the seat? Something a bit more detailed than 'with sensors'.

Regards
Sutripta

[ravib]

Originally Posted by procrj

[*]Will airbags work even if I don't wear seatbelts?
Well the jury is out on this but based on info out there on quora etc. it seems like modern cars assume that if the car is moving, there is someone driving it and hence during a crash airbags should deploy (if the sensor & ACU triggers the same).



What you need to know

[list][*]Its extremely important that you wear your seatbelts because if you don't and your airbags go off, you might be severely injured or might even die because you were too close to the airbags when it deployed

Thanks so much for sharing this useful info @procrj. Truly appreciated !
IIRC, I have read in other threads in TBHP that it is mandatory to wear seat belts in order for SRS Airbag to deploy (atleast in modern cars).

Aren't we getting mixed information ? Or Am i missing something.

I met with an accident last March in my i20 Asta. Because it was not a frontal impact, airbags did not get deployed, but we all walked out safely due to seat belts. However, when i dropped the car in Hyundai garage, i saw another i20 which had complete damage all the way till windshield due to head on collision. But the steering wheel was intact and Airbags didn't deploy because the driver was not wearing seat belts. This is my personal experience and confirmed by Hyundai service advisor.

-Ravi

[Jeroen]

Originally Posted by procrj

Agreed. Simple physics where F=M*A slightly higher mass and hence slightly lower deceleration.

.

True, but that would be a very small effect. I would think that the bull bar can change the way the car crush zone absorbs the crash and therefor impacts the deceleration.

Jeroen

[Jeroen]

Originally Posted by Sutripta

How doesa one find out the weight of the person(kid/ adult) in the seat? Something a bit more detailed than 'with sensors'.

Regards
Sutripta

There might be different ways about it, but essentially a weight system, e.g. Bladder, in the seat with some electronics.

http://auto.howstuffworks.com/car-dr...vices/ocs1.htm

[procrj]

Here is a more detailed explanation of what is called Occupant Detection Systems

While bladders have been in use for almost 20 years, today the systems are more advanced and make use of bolt type weight sensors, pressure sensitive mats and in some cases electrostatic capacitance sensors as well.

The entire system is called an Occupant Detection System and consists of a Weight Detection System and also an Occupant Position Detection System

Not sure how much of this is in use in Indian cars.

All 2012 and 2013 Impreza and 2013 BRZ models use a new type of Occupant Detection System called Electrostatic Capacitance Sensor ODS. This system does not use weight to determine whether to turn the Occupant Detection System ON and OFF.

The Electrostatic Capacitance represents a material's capability of storing an electrical charge. Have you ever been shocked when walking across a carpeted floor and touching a doorknob? This demonstrates your ability to store an electrical charge in your body.

When anyone is seated or when anything is placed on the front passenger seat, there is a change to that capacitance value. This change in capacitance value is what the Electrostatic Capacitance Sensor ODS uses to determine whether the front passenger seat airbag (SRS) will be ON or OFF.

Everything that is placed on the seat will affect a change in capacitance value. The system can determine whether to change the OFF status of an airbag to ON depending on what is on the passenger seat which in turn causes a change in capacitance value.

Some examples:

>A proper sized passenger will change the OFF status of an airbag to ON.

>Plastic, metals and other substances, even if at a heavy weight, will not change the OFF status of an airbag.

>A false electrical charge, for example, those caused by laptops that are processing data, cellphones that are on, or even an electric device connected to a car charger can cause the airbag status to switch ON.

>When water or anything with high moisture content is spilled on the front passenger seat cushion, the airbag is designed to remain OFF until the seat cushion is dry. This will also be the case if the passenger has moisture on their clothing, such as rain-soaked pants or rain gear. In each of these situations, the airbag status may remain OFF even if a passenger is seated.

Source: http://www.stanleysubaru.com/blog/20...-my-subaru.htm

How does a Weight Detection Sensor work?


Source: http://www.google.co.in/patents/US6161891

When an occupant sits on seat bottom 12, the seat occupant's weight is transferred from seat bottom 12 to seat pan 16, through sensor 40, to upper slide rail 25, then to lower slide rail 26, then to seat bracket 28 and then to floor 29. The entire weight of the seat occupant is transferred as a force through the four sensors 40. This weight causes strain in sensor 40 and is measured by sensor 40. An electrical output signal is generated and transmitted over a wire harness 50 to a conventional air bag controller (not shown).

Source: http://www.google.co.in/patents/US6161891

How does a Occupant Position Detection Systems?


Source: YouTube video below

You have pressure sensors in the seat back and side bolstering that will help determine the position of the passenger and send signals to and ECU located in the seat back, which then send inputs to the ACU about which airbags to turn on or off.

https://www.youtube.com/watch?v=akrvHDzKnPU

More info for those who are interested:

http://cpsboard.org/cps/wp-content/u...tic-On-Off.pdf

https://www.youtube.com/watch?v=2xITwigR2UA

Originally Posted by Sutripta

'with sensors'.

Originally Posted by ravib

Aren't we getting mixed information ? Or Am i missing something.

Ravi - yes its mixed info as there is no clear guidance which says that it will work or wont work without seatbelts. Hence to be on the safer side, always wear seatbelts

Originally Posted by Jeroen

True, but that would be a very small effect. I would think that the bull bar can change the way the car crush zone absorbs the crash and therefor impacts the deceleration.

Can you explain a little more? Not able to clearly understand the relationship between the crush zone and the change in deceleration experienced by the sensor.

[vjkamal]

Originally Posted by procrj

Can you explain a little more? Not able to clearly understand the relationship between the crush zone and the change in deceleration experienced by the sensor.

Let me try to explain in simple terms. A curve that represents the acceleration of the vehicle (measured at floor/pillar) with respect to the time is called pulse. The pulse of a typical monocoque bodied car with an offset deform-able barrier crash can be divided in to 3 phases:

1. The first phase is when the barrier hits first point of the vehicle. Both the parties start deforming. On the car side, it is mostly the soft parts (bumper/ radiator) that can absorb light impact.

2. The second phase is when the barrier enters in to the crumple zone present in between the engine and the bumper. This can be lateral members made of sheet metal or other part of BIW. This is a relatively harder phase.

3. The third phase is when the barrier has almost deformed (meaning it will not take impact in the same way as it did in the previous phases) and it reaches the engine. This is the most severe phase.

Now consider the case of a bull bar. Remember most of the bull bars are mounted on the underbody directly on some rigid area. The bull bar will be the first to contact the barrier where the pulse is expected to be softer. Even though the bull bar takes some load, the accelero-meter sensors may send to the ACU a sharp shock wave due to this. This may or may not trigger the restraint systems depending on case to case. If it does, the airbag may not be 100% effective in securing the occupant because early triggering means the bag will lose out some gas even before it is being loaded. The occupant head may bottom out. This is again, very subjective.