The first airbags were triggered by sensors in the bumper. In a crash, the bags deployed automatically and at full force.
Most vehicles have an airbag computer that governs airbag deployment. There are several types of smart airbag systems on 2004 model vehicles. The simplest systems add a sensor in the seat cushion that measures the weight of the occupant.
More complicated designs, such as the system in the Jaguar XK sports car, use ultrasonic sensors in the front pillars and roof. These sensors can detect the presence and position of the front seat passenger.
Other systems can detect whether a child safety seat is placed in the front passenger seat. If so, the system will deactivate the passenger-side airbag.
TRW, Delphi Automotive Systems and other suppliers are developing video cameras to detect passengers, but even those can be fooled. If the camera's vision is blocked, the system may not work.
Each system has weak points. Hard braking combined with a swerve just before a crash can cause passengers to be thrown out of position. That in turn could cause the smart airbag system to turn off the airbag or deploy it with the incorrect amount of force.
If a passenger leans forward, leans below the instrument panel, puts his or her feet on the instrument panel or doesn't sit upright with his or her feet on the floor, the smart airbag system also can be fooled.