Many people do not understand the operation and inherent
dangers of airbags. While airbags are one of the best safety features
introduced in vehicles since the seatbelt, they do contain certain dangers due
to their designing and operation. This
is especially true if the vehicle’s occupants are taking certain care as
recommended.
In design,
the airbag system is fairly simple. An Airbag Control Unit (ACU) is a
specifically designed Electronic Control Unit (ECU) or computer which receives
signals from a number of sensors in the vehicle including impact sensors, side
door pressure sensors, wheel speed sensors, brake pressure sensors,
accelerometers, gyroscopes, and seat occupancy sensors. Upon receipt of the
required signals (information) from the sensors, the ACU will send out an
electronic signal to the trigger in the required airbag or airbags and other
related items such as seat belt pretensioners.
The trigger then sets off a chemical reaction, which creates a gas that rapidly
inflates the airbag.
To explain
this in a little more detail, after the ACU receives an electronic signal and
sends out a trigger signal, it reaches a pyrotechnic device called an initiator
or an electric match. An electric match is generally an electrical conductor
wrapped in a combustible material, which will burn with just one to three amps
of charge in less than two milliseconds. This ignites a gas generator, which in
the case of an airbag is a mixture of a number of chemicals. In the most commonly
used mixture, this sets off a series of three separate chemical reactions to
create nitrogen gas. The time lapse from impact to receipt of the signal is
normally 15 to 30 milliseconds, with a further 20 to 40 milliseconds to full
inflation.
The one
chemical used in most airbags is sodium azide, which is highly reactive and
gives off toxic byproducts, which are neutralized in the third step of the
process. This is the reason many people complain of burning of their skin or in
their throat after an airbag deployment.
Sometimes the byproducts do not get 100% neutralized, resulting in some
irritation, but there are no known permanent effects. There are replacement chemicals, which have
been tested, and many manufacturers are using in the latest generation airbags.
Though, many of the older airbags will be around for a long time. There is also
testing on another chemical ongoing, which holds even more promise as a
suitable replacement. The reason that many of the original chemicals were
chosen is that the chemicals cannot be hygroscopic, in other words, they cannot
absorb water, which would affect the operation of the airbag. The need to
totally isolate the effects of water from the system is the reason for many of
the connectors having a thin gold plating to prevent corrosion.
After the
third stage of the reaction is completed, the bags have fully inflated. There
are small bleed holes in the bag to allow a controlled deflation as the body
comes forward to contact the bag. This cushions the body and head and the controlled
deflation allows a slowed forward movement rather than a sudden stop. Another
advantage of the newer chemicals is that the operation temperature of the
reaction is lower, so there is less chance of getting any burns from the
airbag.
Unlike
crash tests into barriers, real world crashes occur at all sorts of angles, so
a crash sensor must be designed to recognize this and to send the proper
signals. The presently used sensors are called a MEMS accelerometer. It
contains a small integrated circuit with microprocessors and a micro-mechanical
element that moves quickly upon deceleration. This changes the built-in
resistance of the unit, which triggers the electronics to send the signal to
the ACU.
At present,
the federal requirement is that an airbag must deploy at a 14 miles per hour barrier
collision or the equivalent deceleration. Another requirement is in the event
of a fire in the vehicle, when a threshold temperature of 300 to 400 degrees F
is reached, the airbags will automatically deploy. This is called auto ignition
and is to prevent a total explosion of the entire airbag module.
The newest
airbag technologies are being designed to adjust to factors such as the
severity of the crash: the size, seating position and posture of the occupants,
seatbelt usage, and how close the person is seated to the airbag. They use
multi-stage inflators with specific sensors to adjust the force of the
deployment.
The dust
released during deployment has long been a source of complaint from people who
experienced an airbag deployment. It is usually cornstarch, French chalk, or
talcum powder, which is used to lubricate the bag while it deploys. It is not harmful.
It may cause some minor irritation of the throat and
eyes. However, that usually occurs only
if the person remains in the vehicle for a period of time after the crash, such
as during an entrapment.
Some
automakers such as Mercedes Benz call for the replacement of any un-deployed
airbags after a period of years in order to guarantee their effectiveness in
the event of a crash in an older airbag equipped vehicle. Most manufacturers
who do recommend replacement consider a lifespan to be about fourteen years. On
the other hand, Volvo, a very safety-oriented company has said, “airbags do not
require replacement during the lifetime of the vehicle except after deployment.”
Next week we’ll look at side airbags, seat belt
pretensioners and other airbag systems. Some
information for this article was obtained from Wikipedia.org.
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