Abstract: A system having an accelerometer and a micro-controller responsive to acceleration signals for deploying an air bag as well as activating a pretensioner upon frontal impact includes an algorithm for activating the pretensioner upon rear impact. A frontal crash algorithm is executed when the acceleration exceeds a negative threshold and a rear impact algorithm is enabled when the acceleration exceeds a positive threshold. Rear impact severity is tested by integrating positive acceleration samples and issuing an activation command when the integrated value attains a severity threshold value. Noise immunity results from the positive threshold and from resetting the integrated value to zero whenever a negative acceleration sample intervenes in the severity test.

Abstract: A two-step deployment control method for a supplemental inflatable restraint (SIR) in a vehicle. The deployment decision includes a "time to wait" step in which the control determines "when to deploy", and a "severity" step in which the control determines "whether to deploy". The "time to wait" step is based on an estimation of occupant displacement due to the crash, and the "severity" step is based on an estimation of the crash severity. The control method is modified to initiate earlier deployment when the acceleration data is indicative of a localized impact which does not involve the full front structure of the vehicle, such as an angle or pole impact.