re located in a compartment in the dashboard, usually identified with an embossed (raised) or debossed (indented) SRS or SIR logo (Arnold 48).
Standard air bag systems deploy only in the case of frontal impacts. Here, "frontal" impact means any impact where the angle of collision is within 30 degrees of the vehicle's centerline. Because SRS systems only provide protection in frontal impacts, protection in all other types of accidents must be provided by the seat belt and shoulder harness. In fact, because of the nature of air bag deployment, seat belts must protect front seat passengers against harm from any impacts that follow the initial frontal impact. Types of impact which might occur include flipping over or additional chain collisions from the rear.
While systems vary from one manufacturer to the next in terms of the location of components, the number of impact sensors, and the exact type of inflation mechanism, the typical supplemental restraint system begins deployment when an impact causes the vehicle to decelerate sharply. Initially, this closes the normally open contacts in the arming sensor, which, in turn, assures that voltage is present at the impact sensors. The arming sensor is always set at a lower threshold than the impact sensor, so that its contacts close first.
In a case where the vehicle's deceleration does not continue to increase, as when the bumper contacts a parking barrier at low speed, the arming sensor merely resets itself, with no further consequence. If the magnitude of the deceleration continues to grow until it trips the impact sensor as well as the arming sensor (indicating a real collision), then the air bag deployment sequence continues.
The closing of the impact sensor's contacts sends a signal to the electronic control module, which interprets the signal and responds by sending a trigger current to the deployment module.
The electrical current in most units heats a squib locat...