from one crash to the next, making the forecasting of the impact's severity daunting, experts say. Head-on wrecks into poles particularly dumbfound current software.
The impact of these collisions is so concentrated that, at first, its energy goes more into folding the car's grille around the pole than into decelerating the vehicle, tricking air bag controllers into predicting a minor collision. When the pole finally meets the engine block, the sudden enormous deceleration flings the car occupants forward. By then, it may be too late because air bags have not fired or are in the process of filling, possibly adding to injuries from the crash.
Algorithms can be made to do much better, engineers say. Some air bag suppliers are building more complete libraries of possible crash profiles into controllers and speeding up the comparison between those profiles and accelerometer data. Others have learned to distill more accurate predictors of crash behavior from accelerometer signals and are writing software to make those calculations more nimble.
Beyond getting smarter about a crash in progress, some air bag researchers have proposed giving vehicles the ability to prejudge a crash before it begins. Such precrash sensors could include radar systems now being developed for collision avoidance. The warnings that such sensors could provide might boost safety by making it possible to inflate air bags sooner in a severe crash.
While better crash profiles will help, gathering information about the people inside the vehicle is also vital to boosting air bag safety, engineers say. Most often, it's small people who end up dangerously close to air bags.
Small, adult drivers may sit forward to reach controls, putting them up against the steering-wheel air bag. Children's faces may hover at the same height as the passenger-side air-bag compartment positioned at an adult's chest level. Child seats, especially rear-facing infant seats, often protrude into the space into which an air bag inflates-called the keep-out zone.
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When an air bag occasionally does smack someone, small people, with their more delicate frames, also tend to suffer worse injury than bigger folk.
Advanced air bag controllers need answers to many questions about a car's occupants in order to recognize who is most vulnerable to air bag-induced injuries and to solve physics equations for the forces and motion that vehicle restraints are meant to control.
Is there a front-seat passenger? A child seat? How much do the driver and passenger each weigh? Are they wearing seat belts? Where are parts of their bodies relative to the keep-out zone for each bag?
Restraint makers have yet to settle on the best ways to get those answers.
A group of engineers from TEMIC Telefunken Microelectronic GmbH, a German firm, have developed a sensing method using infrared light. They described it in February 1997 at the Society of Automotive Engineers' International Congress and Exposition in Detroit.
Mounted in front of a seat and above it, the device shines beams of infrared light onto the seat from top to bottom. By comparing transmitted and reflected beams, it constructs a profile of the seat or whoever is sitting in it.
Another company, unidentified in the JPL report, would electrify the seats to detect the presence and size of an occupant. Four electrodes would generate an oscillating electric field whose properties change when someone sits down. The system would sense the human body's ability to store electric charge, or its capacitance, which increases with body volume.
Other engineering teams have built instruments that bounce sound waves inside the vehicle compartment at frequencies above the human hearing range. Detectors pick up ultrasound echoes, which the circuitry can use to determine shapes and positions of people and objects in the vehicle.
According to the JPL report, restraint equipment suppliers are also investigating optical camera-based sensors, radar, and other approaches.
Weight sensors already developed can determine if someone is present and how heavy they are by measuring the pressure in a gas-filled bag in the seat or by detecting the change in current through force sensitive resistors. Although tests have shown that these sensors lack the accuracy needed, for instance, to reliably distinguish between a child and a slightly heavier small woman, the weight-sensing technology, which is inexpensive and sturdy
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has proved useful.
Mercedes-Benz customers can opt for a front passenger seat weight detector that shuts off the air bag if it senses less than 30 kilograms of force, indicating that the seat is unoccupied or that a child is present.
The strategies of advanced air bag controllers will rely so heavily on information indicating whether an occupant is belted or not that engineers have felt compelled to introduce a more reliable seat belt sensor.
Currently, a simple electric contact switch typically monitors whether the seat-belt tongue is in the buckle, but its contacts can become dirty Newly developed sensors register an altered magnetic field when the tongue is in place.
Along with more information, restraint designers also plan to give air bag controllers more ways to respond to a crash. Manufacturers have developed air bags with two gas sources rather than the typical single source. The change gives controllers four levels of response: no inflation, a relatively slow inflation, a faster rate, and finally, a simultaneous firing of both inflators for the quickest bag filling.
As an example of how the options might provide an advantage, suppose that a small woman is driving when she has an accident at low speed but still severe enough to cause injuries. The current options are for the air bag to do nothing, risking broken cheek bones or worse, or for it to fire, possibly causing neck injury or other damage.
With the two-inflator bag, the controller could choose its gentlest inflation, possibly avoiding injury either from crash or bag. This technology will be available in at least one 1999 model of European car this year, according to TRW Vehicle Safety Systems of Washington Mich. Air bag makers also are exploring adding even more inflation levels and developing fully adjustable inflators that could give a continuous range of rates.
The sophistication of seat belts is growing too, giving controllers more factors to consider and more options for responses.
Belt manufacturers have developed ways to draw belt webbing back into the reel as a crash begins, cinching the occupant more tightly against the seat-a safer way to ride out a crash. Engineers are also designing load limiters into seat belts to reduce tension when it reaches potentially harmful levels. Inflatable seat belts may be in the offing too. Their shoulder straps would puff up to hold and cushion occupants during a crash.
In the next few years, as these new technologies come into play, many people will be watching to see whether air bag-induced death and injury rates drop and crash survival improves.
Whatever the outcome, the dark side of air bags won't disappear entirely. As the JPL report notes, sometimes air bags will fire when they shouldn't and not fire when they should, no matter how reliably they are made. The nature of technology is that it sometimes fails.
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