Air bags have been touted as the greatest automotive safety devices since seat belts. But as more drivers gain experience with these life-saving systems and the number of air bags installed in vehicles rises, questions crop up.

Companies that develop air bag system components are starting to address the concerns, which include the safety of children in rear-facing child seats and how to prevent deploying an air bag into an unoccupied passenger seat.

At first people whose lives were saved by air bags complained about face bums, abrasions and breathing propellant. Now worries include paying air-bag replacement costs that can range from $1,500 to $2,500, as well as child safety.

Recent accounts of children being injured by air bags smacking into child safety seats led the National Highway Traffic Safety Admin. (NHTSA) to issue a warnings to parents about putting infants in rear-facing child seats in the front seat of cars with passenger-side air bags.

The government has been concerned about air bag safety for some time. In September 1993, NHTSA required air-bag-equipped vehicles to have warnings on the sun visor and information in owners manuals. Five months later the agency asked for similar warning on rear-facing child seats. In vehicles in which infant restraints can fit only in the front seat, such as pickups and sports cars, the government is allowing manufacturers to install a manual cut-off switch to deactivate the passenger air bag as a temporary measure to address this problem until a better solution can be devised.

Well, that better solution appears to have arrived, and more are around the corner. Siemens Automotive says it will produce the first integrated child seat presence and orientation detection (COD) and passenger presence detection (PPD) system for 1997 production vehicles.

The new integrated safety system, which likely will be installed on the full line of BNM and Mercedes-Benz models midway through the 1997 model year, was developed in cooperation with International Electronics and Engineering of Luxembourg. Siemens says the new system has met with equal enthusiasm by North American and other European carmakers.

The COD function of the Siemens system senses if a child carrier seat is present as well as determining the child seat orientation. If the seat is rear-facing, the system automatically deactivates the passenger air bag. The PPD function of the Siemens system detects the absence of a passenger in the seat and prevents the air bag from deploying.

A force-sensing resistor foil in the passenger seat detects the presence of a passenger. Similarly, the child seat detection antennae, which also are built into the seat, sense the presence of two resonators incorporated into the base of the child seat during the manufacturing process.

The position of the resonators, with reference to the location of the antennae in the passenger seat of the vehicle, determines whether the child seat is rear- or forward-facing. An in-seat processor sends signals to the air bag system controller, which makes the final decision on deploying the passenger air bag, based on OEM specifications.

"It's not the most advanced technology, but it's the best technology available on the market," says Andreas Hirl, product development manager at Siemens. "And it's completely reliable. It's the best solution to the problem right now."

Mr. Hirl anticipates the Siemens system will add less than $50 to the cost of a car seat. He says that the majority of European child-seat manufacturers have committed to use the Siemens resonators, which will add an estimated 10% to the cost of the seats "because the resonators have to be integrated into the seat."

Siemens has been working on this and similar systems for many years but, Mr. Hirl says. "Suddenly the market picked up after a few accidents." Siemens may be the first to market with a child/passenger sensing system to optimize air bag deployments, but it is not the only supplier working on the problem.

Two years ago, Robert Bosch Corp. announced that it was developing a detection system using wires in the seat to detect the electrical properties of the seat when a person (who is comprised mostly of water) is sitting in the seat. Bosch reportedly had a deal with an OEM, but the system never went into production. Bosch now says it's evaluating the market to see which sensor device being developed outside the company will work best with its ECU.

Delco Electronics Corp. has three types of occupant sensing: passenger presence, child-seat presence and occupant position. Like the Siemens system, the Delco version of passenger detection consists of a force-sensitive resistor pad and an electronic module and is installed in the seat by the seat manufacturer. The resistor interprets total weight and orientation of the weight.

Delco's child seat sensing system also requires an electronic "tag" that the in-car system uses to determine if a child seat is in position or not. Delco expects it to add $10 to $20 to the cost of a child seat. To design its system, Delco gathered data from adults ranging in weight from 88 to 270 lbs. (40 to 123 kg), infants from 16 to 37 lbs. (7 to 17 kg), children from 40 to 105 lbs. (18 to 48 kg), more than 35 infant seats, and a variety of crash test dummies to determine more than 2,100 point-pressure patterns. Designers took into account occupant position, seat belt usage, seat angle, eight different vehicles, four seat types, and leather and cloth seat covers, Says Delco.

As more information is gathered by sensors and interpreted by electronics, air bags will one day be able to inflate to best protect a vehicle's particular occupant(s). "This could lead to staged deployment," says Siemens' Mr. Hirl. "(The industry) doesn't have staged deployment yet, but that's one of the hottest things under development. It's probably two or three years away."

TRW Inc. is working on an occupant and child-seat sensing system of its own that it hopes to incorporate into a smart restraint system. As in the Siemens set up, the TRW system uses a magnetic coil installed in the child seat that interacts with an antenna in the car seat.

TRW's advanced occupant-sensing system would use pressure sensors and ultrasound to monitor occupants' weight, distance from the air bag, whether or not their seat belts are buckled and crash severity, sending the data to the ECU. The ECU would then optimize the protection offered by the air bag and seat belts by managing the air bag inflation, tensioning the seat belt accordingly and preventing deployment if no one is in the seat or if a child seat is there and out of position.

In TRW's system, an array of sensors and logic devices monitor and update occupant information as often as every 10 milliseconds to maintain a rolling, real-time definition of occupant size and position.

Temic, an AEG Daimler-Benz company, Takata Inc. and Morton International also have developed occupant sensors.

Siemens' Mr. Hirl says his company is developing advanced passenger sensing systems as well, but adds that he doesn't expect systems based on optics, ultrasound, infrared or radar to be in use until around 2004.

Current Delco development projects include, ultrasonic and optical sensing for measuring the distance between an occupant and the air bag module. As in the TRW system, these measurements would allow optimal air bag fill rates depending on weight and sitting position of occupants and crash severity. And like Temic, Delco also is looking at infrared beams in the instrument panel and in the headrest or dome light to sense occupant position.

Since they were first introduced, seat belts have become high-tech. So, it seems, will the air bag system, thanks to advances in sensor and other electronic technology. It looks like the greatest safety device since the seat belt may become even greater.