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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 collisions the airbag deployment caused serious injuries to a number of short stature drivers and several children that were seated in the front passenger seat. 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. Those measures addressed the immediate issues, but also reduced the overall crash protection, afforded by airbags. 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 stature, closely seated drivers and small children. Those same airbags had, however, been remarkable successful in preventing the death of occupants in high speed accidents, saving some 1200 lives per year. The less powerful airbags, installed under the "depowered rule", could provide protection to unbelted occupants only up to circa 25mph. That takes a whole story off the building. With a national information campaign and "click-it-or-ticket" enforcement campaign the government tried to increase the seat belt use rate, to prevent the measure from costing more lives than it saved. While simulation analysis was well established for occupant protection evaluation, in 1996, the tools to analyze injury risk to occupants in close proximity to the airbag were in their early development stage. It would take another 4 years, before those were sufficiently developed to provide design directions. 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 inches (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 would 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.

If the bag is folded incorrectly, it may lock-up under the chin of a closely seated occupant

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 head now has to pull the body rearwards which increases the neck loads.

Without tethers an airbag will balloon into a spherical shape.

While the depowing of airbags and passenger airbag switch off feature delt with the pressing injury risks, it did not eliminate all inflation induced injury risk, and put some classes of occupants at risk, that had previously been protected. Then there were classes of occupants that were not well protected by either system.

The industry went to work to gain back, what was lost. 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 that would allow a short driver to keep more distance from the steering wheel.

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, which is less time than it takes to blink your eyes. 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).
No two crashes are alike. The vehicle has a totally different response whether it drives into a tree or into the back of a truck. Often multiple sensors and sophisticated software algorithms are needed to make the distinction.

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. No sensor technology can make a child safer than the simple action of properly restraining them in the back seat. Henk Helleman has been on the forefront of the development of the next generation airbag systems. This culminated in his co-authorship of the TOPS™ strategy for airbag deployment. (fellow authors are Dr. Russel Brantman and Dr. Said Nakhla). TOPS™ is an acronym for Tailorable Occupant Protection System. It lays out in a clear progression how the new sensors and technology are used to address the safety of specific groups of occupants and what groups are left lacking full protection without these technologies being implemented. TOPS™ was unique in the industry in that it allowed a phased introduction of third generation airbag systems with increased levels of sophistication. This was important as it allowed cost effective development and a rapid return to full protection airbag systems. Cars with advanced airbags systems are typically equiped with these components:

• A crash sensor that can determine multiple levels of crash severity.
• A gas generator with a low and high output level, known as a dual stage inflator.
• A control module that can vary the timing between airbag stages.
• A seat belt usage sensor.
• A seat position sensor.
• A passenger seat weight sensor.

More manufacturers offer vehicles with adjustable pedals and telescoping steering column adjustment. These can be used by shorter drivers to keep a distance of 10" minimum from the airbag module.

Seat belts have become smarter too. Pretensioners, activated by the crash sensors, take the slack out of the belt, making them more effective. Digressive load-limiters then limit and lower the belt force, as more belt material spools off the reel. That makes them work better in combination with the airbag. Bi-level load-limiter, can set the load levels higher or lower, depending on the size of the occupant. It takes its input from the amount of belt used by the occupant or from the seat position sensor, to gauge whether the occupant is small or large. A heavier occupant needs more restraining force than a smaller occupant under the same crash conditions (according to the old F = m x a) and fortunately usually also can sustain higher forces, before injuries occur.

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:

Belt Use	Crash Severity1	Seat Position2	->	Airbag Deployment
yes	medium	forward	->	none
yes	medium	rearward	->	none
yes	high	forward	->	none
yes	high	rearward	->	low level deployment
no	medium	forward	->	low level deployment
no	medium	rearward	->	low level deployment
no	high	forward	->	low level deployment
no	high	rearward	->	high level deployment

The airbag deployment strategy for the passenger side might look like this:

Belt Use	Crash Severity	Occupant Weight3	->	Airbag Deployment
yes	medium	low	->	none
yes	medium	high	->	none
yes	high	low	->	none
yes	high	high	->	low level deployment
no	medium	low	->	none
no	medium	high	->	low level deployment
no	high	low	->	low level deployment
no	high	high	->	high level deployment

These systems deal with the two most pressing problems:

• Avoid deployment of the passenger airbag if a child seat is placed there.
• Reduce the airbag deployment force for closely seated drivers.

Sensor accuracy (gray areas) and the realization that everything man makes can fail, make that the real deployment strategy is somewhat more complicated than the one described above.
Sensors that can dynamically (i.e. during the crash) gauge the distance of the occupant to the airbag module, have not been widely implemented. These "proximity sensors" were foreseen by TOPSTM to affect the airbag deployment in case the occupants are thrown out of position prior to impact.

Fortunately the simulation analysis techniques caught up too, so that the interaction of the occupant with the deploying airbag can be studied. This helps the engineers balance the airbag strength between providing sufficient protection and minimizing injury risk.

Driver Airbag Single Stage Deployment Simulation	Driver Airbag Dual Stage Deployment Simulation

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