Electronic Control Unit

Passive Safety Electronics
The electronic control unit (ECU) is the brain of the car’s passive safety system. It decides not only if, but also exactly when, the seatbelt pretensioners should be triggered and each airbag system should be deployed. The ECU contains crash sensors and a microprocessor, as well as back-up power in the event the connection to the car's main battery is cut off in the crash. The ECU is located in the middle of the vehicle where it is well protected during a crash.
Passive safety electronics measure deceleration, since vehicle speed is not an adequate indicator of whether an airbag should be deployed or not. Airbags and seatbelt pretensioners in frontal and near-frontal collisions are typically designed to deploy when the crash force is comparable to hitting a solid barrier at approximately 12 to 20 kmh (about 8 to 12 mph). This is, roughly speaking, equivalent to striking a parked car of similar size at about 43 kmh (about 27 mph), assuming 100% of the fronts of both vehicles are engaged in the crash.