Manufacturers are using a variety of sensors including ultrasonic, infrared, and piezoelectric to prevent air-bag deployment that could cause injury.
First installed in 1986 in some luxury cars, air bags are now standard equipment in virtually every automobile sold in the United States. Essentially a nylon pouch folded up like a parachute, the bag is intended to supplement the seat belt by cushioning the passenger during a crash.
Air bags, however, are also highly controversial because they have killed 67 people and injured more with severe trauma to the neck and head. Most victims have been either children or small women riding in the passenger seat. Critics contend that air bags are unnecessary and seat belts are enough to maximize passenger safety. They also claim that the deployment threshold set by some air-bag manufacturers is too low, and air bags inflate too quickly.
Sensors such as the piezoelectric sheet under the seat deploy an air bag more softly or deactivate it if the passenger is in a potentially dangerous position
Proponents of the device are quick to point out that air bags are credited with saving 1,900 lives so far. The apparent recent increase in the number of deaths, they say, is due more to the higher number of vehicles equipped with air bags than to the bags becoming less safe. Most importantly, almost every person killed by an air bag was not wearing a seat belt.
Automakers have known about the potential dangers ever since air bags were first developed. For this reason, vehicles have warning labels to instruct passengers on how to avoid potential dangers. No rear-facing child seat is supposed to be put in the passenger seat. Children under the age of 12 should also ride in the back seat. Above all, passengers should always wear their seat belts. Still, some say that the vague warnings, which often are placed under dashboards, do not provide enough information and do not properly convey the dangers of noncompliance.
Manufacturers Take Interim Steps
Electronics in the air-bag system continuously monitor the output of an accelerometer installed in the vehicle's chassis. The air-bag manufacturer sets a deployment threshold that is intended to reflect the deceleration of a potentially harmful crash. If the accelerometer reading exceeds this threshold, the system sends a signal to an igniter embedded within a block of sodium azide. The block explodes, sending a flood of nitrogen gas into the air bag and inflating it at more than 200 miles per hour. The total time from collision to air-bag inflation is less than 1 second.
Although virtually all of the bag's safety problems would be eliminated if passengers wore seat belts, auto manufacturers are redesigning air-bag systems around the fact that that not all passengers do. Most safety improvements revolve around sensor-based systems that detect the size and position of the passenger seat's occupant and prevent the air bag from deploying if there is the potential for harm. However, these systems are still in various stages of development and will not be incorporated into production vehicles for several years.
Data from the piezoelectric sheet are sent to a processor, where fuzzy-logic-based algorithms determine whether to deactivate the air bag
Meanwhile, several measures are being taken to address safety concerns. For example, the Ford Motor Co. in Dearborn, Mich., is among the U.S. automakers working on making its bags deploy more softly, which will increase safety for children and small women. This action follows a ruling by the National Highway Traffic Safety Administration (NHTSA) that allows automakers to reduce air-bag power by 20 to 35 percent.
When air bags were first designed, the inflation speed was optimized to provide the highest level of protection for an average-size adult not wearing a safety belt in a 30-mile-per-hour crash. Therefore, although depowered bags provide more protection for smaller passengers, they are less effective for average and large adults. Vehicles manufactured in the 1998 model year will be the first to have the air bags depowered along NHTSA guidelines.
The agency has also passed a ruling allowing new cars and trucks to incorporate an on/off switch so passengers can control whether the air bag will activate. Initially, the plan was to permit the switch only in vehicles without a back seat.
In existing vehicles, NHTSA regulations will enable dealers to retrofit cars on a case-by-case basis. Even before this ruling, some owners have taken it upon themselves to disconnect the air bag. Indeed, this has blossomed into a cottage industry in some areas of the United States, because not all car owners have the technical know-how to disable the air bag themselves. However, a deactivated air bag will not pass vehicle inspection in many states.
Some automakers already have systems in place that make air bags safer. For example, Mercedes Benz in Montvale, N.J., has a dual-threshold system that considers not only how severe the crash is but also whether the passenger is wearing a seat belt. If the crash happens below the lower threshold, the air bag will not deploy. Between the two thresholds, the bag deploys if the passenger isn't wearing a seat belt, but it won't deploy if the passenger is belted. (A simple electrical circuit tells the system that the seat belt is buckled or unbuckled.) The bag deploys regardless of seat-belt status if the crash occurs above the upper threshold.
Mercedes has also introduced a feature in some vehicles called BabySmart, which is intended to protect babies from air bags. The cornerstone of the system is a resonating device built into the child seat. The device—similar to anti-shoplifting tags that some stores place on expensive merchandise—is completely passive and requires no energy source. A low-power signal from the car causes the resonator to send a reply signal. If the system receives this signal, it knows not to activate the air bag. A light on the center console confirms that the air bag is deactivated. Mercedes first installed BabySmart in certain vehicles that lack a back seat. The company will begin putting the feature in vehicles with a back seat, even though it still recommends that children ride in the back.
For the system to work, the car owner naturally must have a BabySmart-compatible child seat. So far, one manufacturer—Britax Co. in Marysville, Mich.—has incorporated the feature into its seat. The seat is available directly through Mercedes, which has made the technology available to any interested automakers and child-seat manufacturers. If several automakers support the system, more child-seat makers will incorporate the resonators into their seats, and the seats will be sold through traditional retail outlets rather than car dealerships.
Some critics say that a simple weight sensor installed under the seat could serve the same purpose as BabySmart. Mercedes contends, however, that its system is more reliable because a weight sensor can be fooled. For example, if a well-meaning parent tightens the child seat down very firmly with the seat belt, the seat's apparent weight might be much higher than it actually is and the system could deploy the air bag.
Nevertheless, Mercedes does have a weight sensor in its vehicles, but primarily for economic reasons. If the sensor does not detect a weight of more than 26 pounds, it will not fire an air bag. The main benefit of this feature is to prevent the air bag from deploying when no one is in the seat, saving more than $2,000 it would cost to replace and reset a deployed air bag.
New air-bag systems are being developed that mitigate many existing safety problems. In several of these so-called smart systems, the primary modification is the addition of one or more types of occupant sensors. Some manufacturers are also improving the air bag. For example, Automotive Systems Laboratory in Farmington Hills, Mich., a subsidiary of Japan's Takata Corp., is developing an air bag with two separate gas-generating chambers instead of one. Each can be triggered separately and has about half the explosive power of the original single chamber. The benefit of a dual-stage air bag is that it allows more operating options: Mild collisions will deploy only one chamber; more-severe collisions will deploy both. This system also takes into account whether the seat belt is buckled, thus giving it four different sensitivity thresholds.
Even with a change to the air bag, the heart of all new systems is some type of occupant sensor. All such sensors determine if someone is in the passenger seat, and some identify smaller passengers who might not be able to withstand air-bag deployment. Tests have shown that an occupant at least 8 inches away from the air bag is much less likely to be injured than if he or she is right up close, so some sensors also read the passenger's position. If conditions for deployment aren't optimal, the systems will either prevent the air bag from being released or—if possible—deploy the air bag at a reduced speed.
The high dielectric constant of the human body allows the capacitive sensor to gauge the passenger's position to within 0.1 inch but ignore clothing and cargo
The simplest approach is to install a weight sensor at a single point under the seat. This method has several limitations, however, which is why Mercedes does not consider its own weight sensor to be a safety device. For example, the sensor can tell that something is on the seat but gives no indication as to whether the weight is a person or cargo. Furthermore, a weight sensor cannot determine an occupant's position. These drawbacks mean no manufacturer of air-bag systems is using a weight sensor as the primary method of occupant sensing. If used at all, a weight sensor will serve as a backup to a system employing a different technology.
Infrared Ranging
Automotive Systems Laboratory has developed an infrared ranging system. Installed near the air-bag module, the system sends out an invisible infrared beam toward the passenger. A receiver perceives this beam as a spot on the occupant. The receiver, offset horizontally from the beam, uses triangulation to determine the passenger's distance from the air bag. The system operates in real time, so it can adjust if the passenger moves around. Other manufacturers are developing similar systems that use higher wavelengths, making them more similar to radar.
With initial work complete, the system is now in the hands of several automakers for evaluation. Tests are focusing on determining if the system recognizes baby seats and can distinguish passengers from cargo such as grocery bags. Any shortcomings will be addressed by tweaking the algorithm on the range sensor.
The Robert Bosch Corp. in Broadview, Ill., has developed a system that uses infrared and ultrasonic sensing technologies for redundancy and reliability. A unit that houses both types of sensors will be installed on the headliner near the rear-view mirror. The company says that this unit can also eventually house other types of sensors to provide functions such as climate control, headrest adjustments, and mirror dimming.
The ultrasonic component of the occupant detection system works by emitting 50-kilohertz sound waves from three ultrasound transducers that are installed above and behind the passenger. The sensor then picks up the resulting echoes. A microprocessor analyzes the data to extract the important information and calculate the passenger's position.
Advanced Safety Concepts Inc. in Santa Fe, N.M., has invented a capacitive occupant sensing technology, the Proximity Array Sensing System (PASS). The basis for this technology is the human body's high water and salt content, which gives a dielectric constant of approximately 80—one of the highest among commonly found materials. By contrast, the dielectric constant of air is 1.
Although the capacitive sensor that the PASS uses can be installed in the steering wheel or dashboard, the company says that the ideal place for it is directly over the passenger's head. The device creates a low-level (100-volt-per-meter) hemispherical electrical field. The presence of a human changes the field capacitance, and the sensor detects this change. Since the dielectric constant of a human versus the air is so different, it's very clear whether there is an occupant in the car.
Other items that typically might be found in a car seat, like boxes and bags, have dielectric constants from about 2 to 4, which are higher than air but still far lower than that of a person. Therefore, the system will never mistake cargo for a passenger. For the same reason, hats and newspapers, for example, will not skew the location of the passenger. Similarly, the mounting hardware used to hold the sensor in place and the fabric that covers the headliner of the car are also invisible to the sensor, so it can be installed above the fabric where it can't be seen.
A single capacitive sensor can determine the presence of a passenger as well as the radial distance of the passenger from the sensor. Two to four sensors installed at different locations can provide more complete information on the passenger's position in all three axes to within 0.1 inch. The sensor can take readings up to 2 feet away. The system can acquire data continuously at thousands of times per second if necessary.
According to Advanced Safety Concepts, the sensor will cost less than $4 when manufactured in the quantities needed for installation in automobiles. "An added benefit is that the sensor's accuracy allows it to detect subtle nods of the head that occur when a driver is becoming drowsy," said president and chief executive officer Philip Kithil. "If the system senses that the driver is falling asleep, an alarm could sound to wake the driver. The same sensor could do both functions, which makes it even more cost-effective." The system could also be used to adjust headrest height and mirror angles automatically.
One major U.S. manufacturer of air-bag systems has licensed the PASS, but it will still be several years before the system shows up in actual air bags.
Elsewhere, the Photonics Center at Boston University has developed a technique that uses a piezoelectric polymer, which is a plastic sheet coated with a conductive metal and about as thick as a transparency used for an overhead projector. The piezoelectric sheet reacts to weight placed on it by producing a proportional voltage output.
If a sheet of this material were embedded directly under the seat, it would react when a passenger sat down. Since the information is provided over an area and not at a single point, the sensor provides information on the occupant's position. Calculating the passenger's center of gravity also helps track position. Sensor output is sent to a logic-based controller that uses fuzzy logic to decide whether to deactivate the air bag or deploy it at high or low speed.
According to the laboratory, one of the strongest selling points of the piezoelectric system is its low cost. "We've had automotive suppliers suggest that the manufactured cost in production quantities would be about $1.75," said James Hubbard, senior systems engineer who began working on smart materials while at the Massachusetts Institute of Technology in Cambridge. "The system also overcomes some of the limitations of other technologies. For example, a passenger drinking a cup of hot coffee could throw off the readings of an infrared system. Also, holding up a newspaper could skew the readings of a radar-based system."
The piezoelectric system could gain greater functionality by developing more-advanced algorithms, such as one that compares the passenger's weight with a threshold weight to determine if the passenger is a child or an adult. The laboratory is also looking at an algorithm that uses subtle shifts in weight and position to distinguish between a passenger and cargo. The laboratory has built a prototype of the system, which automakers have already taken a look at on-site. Further discussion with interested parties is in various stages, ranging from patent review to field testing in real seats.
The Robotic Vision Laboratory of Sandia National Laboratories in Albuquerque, N.M., is taking what is perhaps the most unconventional approach: detecting the presence of a passenger with a video camera. This prototype camera was attached on the roof where the sun visor mounts to the ceiling of the car, but it could be mounted anywhere with a clear view of the seat. The camera's images would be analyzed using image-processing algorithms to determine whether the seat is occupied by nothing, an adult, or a rear-facing child seat.
The most commonly used cameras in video equipment capture images with a charge coupled device. However, cameras based on a complementary metal oxide semiconductor (CMOS) are quickly becoming a less expensive alternative. Originally, the system used a single CMOS camera, but Sandia then upgraded to a two-camera version, which captured range images instead of a simple black-and-white image. Range images are less sensitive to changes in illumination and other unimportant phenomena, such as shadows and variations in the color of the occupant's clothing. A stereo image from two cameras can also be used to measure the distance between the occupant and the dashboard.
The cameras themselves capture an analog image. A subcomputer then digitizes the image and sends it to another computer for more advanced image processing. "During the initial test, a large workstation processed the image," said John Krumm, a senior member of the laboratory's technical staff. "However, in an automobile, the processor needed could fit into a box about the size of a cigarette box."
In tests, the system made the correct classification 95 percent of the time. Unfortunately, the cost of the system is higher than other alternatives. "The major advantage of this technique," Krumm said, "is that this sensor lets you do things that would take multiple other sensors to do. A video system could replace a whole suite of other sensors and be the sole source of information, which would make it more cost-effective."
The initial research for this system has now been completed. Sandia could work with an air-bag manufacturer to make it into a production-ready system or do more research to exploit further capabilities of the system, such as distance detection and estimation of the occupant's weight. The direction that the project takes will depend on the needs and interests of the research partner that becomes involved.
