Advanced Airbag Restraint Systems
In 1996 reports were getting stronger that airbags were not the magic
protective restraint system some had hoped. In several moderate speed
(i.e. 12 to 20 mph) collisions the airbag deployment caused the death of
a number of short stature drivers and several of children that were
seated in the front passenger seat.
The first generation of airbags was designed to catch unbelted adult
male driver and passenger in a 30mph frontal crash (straight on and at a 30 degree angle).
If a speed of 30 mph doesn't sound too serious, realize that it is the equivalent of falling
from a three story building onto the pavement. There aren't too many people that have fallen
from a three story building and lived to tell about it.
The airbag power needed to cushion this blow proved to be too much for short statue, closely seated drivers and small children.
The government responded by first changing the requirements such that
the industry could de-power the airbags.
Then, bit by bit, new technologies were introduced: Dual threshold crash sensors;
Dual stage inflators; Occupant classification sensors; Seat position sensors;
Belt usage sensors; As well as pedal positioning and steering column adjustments.
All that eventually lead to the "Advanced Airbag Rule" in which the government
mandated new certification testing with different speeds, belted and unbelted dummies,
and with dummies positioned right on top of the airbags during deployment.
The advanced airbags were born.
While simulation analysis was well established for occupant protection analysis, back in 1996, the tools to analyze injury risk to occupants in close proximity to the airbag were in their early development stage. Testing was therefore the only viable solution to investigate the issue. While some research had shown that for a person, who was right on top of the airbag module, only the strength or aggressivity of the inflator determined the injury risk, this was less clear for a person sitting a mere 6" (150 mm) away from the airbag module. Laboratory testing with the 5th percentile female dummy (height 5'0") showed that module design and airbag fold could also play a role.
It was found that airbag modules that have a single piece cover
increase the chance that the airbag gets caught under the chin of a
closely seated occupant. Once caught there, the force of deployment will
snap the head back with great force, increasing the risk of neck injury.
Single piece covers are often desired by vehicle stylists. Optimizing
the airbag fold such that the airbag deploys free from interaction with
the occupant is an engineering challenge.
It is more difficult than with
module designs that have a cover that splits through the center which
therefore have the engineer's preference.
An other challenge engineers face is the ongoing cost cutting to
remain competitive. At less than $2 a set, airbag tethers are
frequently targeted by the bean counters. Testing showed, however, that
they are a worthwhile expense to improve airbag safety. Tethers are
strips of fabric, hidden inside the airbag, that span from the back to
the front of the cushion. They reduce the throw of the airbag during
deployment and determine the final thickness of the cushion. Without
such thickness control the airbag will expand into a spherical shape.
The extra thickness increases the load on the head of a closely seated,
short stature occupant.
The Magic ReturnsStep by step the airbag systems were improved. As an intermediate step the National Highway Traffic Safety Administration sanctioned the de-powering of airbags by changing the certification requirements. The resulting airbags were some 30% softer than the first generation. Furthermore, the NHTSA allowed the passenger airbag to be manually turned off in case a rearward facing infant seat had to be placed in the front passenger seat. You will find either of these modifications in most vehicles between model years 1998 and 2002. While depowered airbags reduce the chance of airbag induced injury to the short stature drivers and young front seat passengers, it also reduces the amount of protection to larger occupants and occupants in more severe crashes. This affects the area airbags have been proven to be very successful. The government and industry set to work to regain what was lost.
One of the first technologies to be introduced were dual-output inflators. Those have the ability to deploy the airbag with either a part or a full charge of gas. They were first combined with dual-threshold crash sensors that would signal if the crash was of moderate or high severity. The full gas charge would only be released if the crash sensor signalled a high severity crash.
That may sound easier than it is. The crash of a vehicle driving at 30mph into a rigid barrier
only lasts about 100 milliseconds, or 0.1 seconds, or about the time it takes to blink your eyes.
Out of this time the crash sensor only has about 15 to 20 milliseconds to determine that a crash is
happening and whether or not it is a severe one. By that time the vehicle may have only slowed down
2 mph (from 30mph to 28mph).
Next on the scene were weight sensors that would determine the presence of a passenger and determine whether it was an adult or a small child. It was without any doubt that an infant in a rearward facing child seat had no benefit whatsoever from a deploying airbag. But what about a six year old child, involved in a high speed crash? Well, a 12-month old in an infant seat, covered with a blanked weighs about as much as a 3-year old child. So if we turn the airbag off for the infant, then the weight sensor will also cause the airbag to be turned off for the 3-year old child. A 3-year old, sitting in a child seat can weigh as much as a 6-year old child sitting on the seat by itself. So, the weight sensor might decide that the airbag will be switched-off for them too. Only sophisticated, pattern recognition software could tell the difference, but not in 100% of the cases.
Henk Helleman has been on the forefront of the development of the next generation airbag systems. This culminated in his co-authorship of the BREED Technologies' TOPSTM strategy for airbag deployment. (fellow authors are Dr. Russel Brantman and Dr. Said Nakhla)
TOPS is an acronym for Tailorable Occupant Protection System. It combines several of the new and improved sensors with the dual-output gas generators to tailor the airbag for the (crash) situation at hand. TOPS was unique in the industry in that it allowed a phased introduction of third generation airbag systems with increasing levels of sophistication. This is important to allow cost effective development and a rapid return to full protection airbag systems.
Cars of model years 2003 and newer may have airbag systems that typically comprise of
Furthermore there is an increasing chance that your car of choice will
be equipped with seat belt pretensioners. These will take the slack out
of the belt in the early stages of a crash, making the belts more
effective (only if you wear them, of course). In addition to this the
seat belts might incorporate a load limiter. This device will cap the
seat belt load at a certain maximum, which will reduce the force of the
belt on the chest and the shoulder. This reduces the chance of injuries
caused by the seat belt, such as cuts and broken ribs (yes, belts can
cause injuries too...)
With all this in place the restraint system can be tailored to the
needs of the occupant. The airbag deployment strategy on the driver side
might look like this:
The airbag deployment strategy for the front passenger side might look like this:
These systems will deal with the two most pressing problems:
Sensor accuracy (gray area's) and the realization that everything
man makes can ultimately fail, make that the real deployment
strategy is somewhat more complicated then the one described above.
Fortunately at long last the simulation analysis techniques have caught up too, so that the interaction of the occupant with the deploying airbag can be studied. This helps the automakers balance the airbag strength between providing sufficient protection and minimizing injury risk.
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