From the single driver-side airbag that was installed on all of the U.S. vehicles a decade ago, the number has increased to as many as a dozen today. Side airbags may seem like an obvious next step after front airbags, but a significant amount of engineering effort occurred behind the scenes. In 1994, TRW engineers did not have to start from scratch, but they did have to synthesize information from different sources and generate and analyze a considerable amount of data to take airbag protection to the next level. Evaluating field data of fatal accidents, the researchers sought out the next area where they could make a life-saving contribution. They relied on biomechanics data from researchers in other organizations, such as those at Wayne State University (Detroit, MI). With this data as a starting point, other steps in their design process provide insight into how engineers design under uncertainty.

Side Collisions Get Scrutiny

Side collisions were quickly identified as the next area that could benefit from added protection. The decision was easy, but solving the problems was not so simple. "In a frontal impact the car kind of crushes and the occupant compartment pretty much stays intact," says Charlie Steffens, director, TRW global systems technology. "In a side impact the occupant compartment is crushed, so the whole accident is quite a bit different."

Deploying the airbag in a frontal collision allows the occupant to ride into the expanding bag rather than hitting the steering wheel or dashboard. "The timeframe available to do this protection is greatly reduced from the side versus the frontal," adds Doug Campbell, vice president of engineering for TRW automotive's occupant safety systems.

With the side impact, the bag must absorb the impact of an intruding object. The crash must be detected within 5 to 8 ms, and within another 5 to 10 ms the occupant is already starting to contact the door. This means that protection must be implemented in about 1/3 of the time of the frontal impact making the side airbag system much more difficult. The differences don't end here.

Testing was one of the first process challenges the engineers tackled. Frontal testing with a high-speed sled was well established. "We took an existing sled and we turned the sled sideways," says Steffens. However, they did not rely on the accepted high-g ram procedure for frontal testing, where the vehicle is accelerated towards a barrier. Instead, using an earlier approach called the bungee sled, engineers simulated a field accident by propelling the object towards the vehicle, causing an intrusion into the passenger compartment.

Engineers also modified the frontal crash dummy to provide data for injuries in different locations and nicknamed it SID for side impact dummy. With the sled and SID, TRW could generate its own data and start working toward the side impact solution.

Simulate Before Building

To optimize the protection that would work the best for a variety of situations, engineers supplemented vehicle test data with simulation modeling of all crash events. "We will run hundreds or thousands of simulation runs before making the first part to come up with the very best scenario and then move forward from there," says Steffens. TRW employed standard simulation tools recognized by the industry including MADYMO, a software tool used to predict occupant kinematics and calculate injury criteria. They also developed some of their own in-house proprietary adaptations. The combination of these tools reduced the number of hardware iterations and the associated cost and timing delays.

Hardware Challenges and Changes

One of the most significant hardware challenges was the inflator. To make the inflator meet the reduced timing requirements, engineers had to trade off the time required for the inflator, versus bag inflation time, versus the time required for sensing and the decision.

"The squibs are essentially the same," says Jeff Aird, director for inflatable restraint systems in North America. "What is different in the side airbags is the gas delivery rate of the inflator, or gas generator." From the time the inflator is fired or deployed with the squib, the gas needs to flow out of the inflator into the airbag at a faster rate. Increasing the burn rate of the propellant, orifice size of the gas outlet and other parameters in the inflator allowed the rate to meet the design target of 13 ms.

When it was recognized it accounted for a larger percentage of the fatalities on the road, rollover protection was a logical extension. TRW started with side impact protection for head injuries with a curtain that came down to cover the upper area of the occupant. However, this protection was not aimed at protecting passengers during rollover.

Rollover sensing was a big challenge since it required a sensor that has the ability to discriminate for this event," explains Steffens. In addition, rollover is a very slow event compared to a side impact taking several seconds-not milliseconds. Since SUVs ride so high there may not be as much of a need for head curtains during side impacts; but when rollover is taken into account, the curtains are required.

Changes to the side airbag curtain to ensure rollover capability include a cold gas inflator that has the capability to allow bags to be deployed for up to 6 to 8 seconds, says Campbell. TRW also developed bag-sealing technology because the front airbags were initially designed to be porous to absorb energy.

Path to Future Development

To establish an airbag system architecture for future growth, TRW developed an airbag bus structure called Safe-by-Wire (SBW). This architecture greatly simplifies the complexity that side airbags and rollover and other hardware components add to the system.

With SBW, engineers can add safety components-such as side-impact air bags, kneebags, tubular restraints, pretensioners, and occupant sensors-without redesigning the entire system. This provides flexibility as well as saving time and resources. During power-up each node in the system is analyzed and an air bag or pretensioner can be added or removed, depending on the presence and size of the passenger, without completely reconfiguring the network. Configuration during power-up allows components from several different suppliers to be combined for the first time at vehicle assembly, and defective components can be replaced in service without special programming equipment. In operation, the master node polls each sensor for information, calculates whether an air bag needs to be deployed, and then issues a command to fire the appropriate squib.

The components in side protection are similar to those used for front airbags with a few significant exceptions in the sensing area. The accelerometer specification itself changes when sensing is closer to the crash event as it is in the side impact case. For frontal crash sensing, the centrally mounted accelerometers used today are in the 50-g range. In contrast, the side impact accelerometers or even front crush zone mounted sensors are in the 250-g range.

Safe-by-Wire allows the addition of RAS with minimal overhead increase on the microcontroller, and running a relatively simple algorithm doesn't require significant extra memory-typically only about 20 percent. Computing can be done at the RAS or in the central MCU. The 8-bit MCU that was sufficient to handle the algorithms in the initial front airbag systems has been increased to a 16-bit MCU for today's systems and will be increasing to 32-bit to handle the increasing system complexity.

The specifications for side airbags are still evolving and there is proposed rule-making to regulate rollover. No one knows exactly what the next federal requirements will be, but then engineers at TRW have plenty of experience working under uncertainty.

Contact Contributing Writer Randy Frank at [email protected].