I've worked on most bits of cars for many years, but have never tampered with airbags, or even investigated how they are actually triggered (I know an explosive device is used in the bag, but what provides the signal to detonate it?) Can someone please explain exactly what sort of sensor is used, where it's usually situated, what it looks like etc. Just curiosity you understand!
Let's face it, these bags must be pretty clever to be able to know what constitutes 'an accident', yet not go off in response to a minor bump or vibration from a poor road surface etc.
My C5 has about 6 bags as I recall - some of them 'curtain bags' for the sides. Does this mean there are sensors that respond to collision from the side as well?
Bearing in mind that you seldom hear of bags going off at the wrong time, the system must be amazingly reliable!
Graeme
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Simply put the system uses an inertia sensor or sensors plus some pretty sophisticated software to determine direction and force of the impact to detonate the bag(s). The systems (naturally) have become more complex and now take into account occupancy of each seat together with the weight and height above the seat base using ultrasonic scanning of the interior. Then the system can decide which bags to detonate, partially or fully, in conjunction with or without seatbelt pretensioners, active head restraints, collapsible steering column...........
Andrew
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Simplicate and add lightness!!
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Thanks Andrew!
I suspected there must be some sort of 'inertia' sensor, but didn't really think about the software that would be involved. Presumeably these things would pretty much be little computers in their own right.
Graeme
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There are several types of sensors available. Older designs used the 'Rolamite' sensor - rather like a rolled-up sardine tin lid with a heavy roller at the centre. When the car hits something the impact causes the thing to unroll and it hits a gold-plated contact which closes the sensing circuit. These are mounted in a little can and filled with an inert gas to reduce the possibility of corrosion.
More modern sensors use microengineered cantilevers which have a polysilicon resistor deposited on them (e.g. SensoNor sensors). Under impact the cantilever bends and the resistance of the PS resistor increases - this is detected by the ECU which can then determine the deceleration. These devices have the added bonus of being easy to self-test. When you switch on, a current is passed through the polysilicon resistor which heats up and expands, causing the cantilever beam to bend. The voltage/current ratio is then measured and any deviation from the stored program value indicates a faulty sensing resistor or beam.
In addition, most systems have a 'safing' sensor located centrally on the bodyshell. This measures the whole vehicle deceleration and hence provides a plausibility check on the front crash sensors - this is to ensure that the systems doesn't deploy in the event of a localised impact (eg a hammer blow).
Software in the ECU looks at inputs from other sensors too - such as seat occupancy and vehicle speed.
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Graeme also ask about the location of the sensors. So here's an example:
The new XJ has 9: two underbonnet, one each side (roughly just behind the headlights); one on the centre tunnel just forward of the J-gate; one each just above sill level on the inside panels of the A, B & C pillars.
The car also has occupancy sensing for the passenger seat. It uses four ultrasonic sensors in the pillar trim and headlining. It is looking for out-of-position passengers; if the passenger is o-o-p the bag will not go off.
The car also has weight sensing in the passenger seat frame. The information from this and the o-o-p sensors tells it the type of passenger - in particular it's looking for rear facing child seats!!!
Suffice to say that the software that reads all this and controls all 8 airbags is fairly substantial and cost a lot of money (and crashed cars!!) to develop.
Oh! don't forget 5 pretensioner seatbelts too.
I don't know whether the car will get more sensors when knee bolster bags are added.
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Thanks everyone - a fascinating insight!
My worry would be that cars that are (say) 10 years old might have bags that are either capable of deploying at the wrong moment or are not capable of deploying at all. The skill (and therefore cost) needed to check maybe 6 airbags in an old car would be considerable, so I guess that most of them would pass into a twilight zone where everyone 'hopes for the best'. It wouldn't surprise me if airbags became part of the MOT.
My car is very safe - it's got a big windbag on the steering wheel........... :-)
Graeme
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There is no need to be concerned about 10 year old airbags. Tests have been caried out which prove that 10-12 yr old bags are as good as new. The software will not degrade either unless exposed to some kind of EMC - but in that event your SRS light would come on.
Obviously there might be some concern if the structure of the car were seriously compromised (eg significant corrosion), but by this time the car would probably be in the scrap yard anyway.
www.honestjohn.co.uk/forum/post/index.htm?v=i&t=70...4
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