Now that side airbags are used to protect chest and head of the occupants, reliable detection of side impacts is of great importance. Compared to frontal collisions of the same impact speed, side collisions take only the third of time. Thus detection of a side impact at 50 km/h has to be done in less than 5 ms after the first contact. This challenging task cannot be achieved with a single crash sensor in the middle of the car, as used for the ignition of frontal airbags, but requires specialized sensors for the side impact detection.
At present two different principles of sensors are ready for production: devices that measure deformations and devices that use accelerations. Volkswagen utilizes acceleration-based sensors because they are less sensitive in regard to the impact points. However, placing this type of sensors is an extremely critical task. The pros and cons of some typical mounting locations like doors, sills or cross members are discussed in this paper. Also discussed are methods for optimizing the car in order to shorten the sensing time. They include the analysis of the transmissibility of shocks by experimental methods and the numerical calculation by FEM.
The airbag controlling computer processes the accelerations measured by the sensor and decides whether the airbag should be employed. The evaluation algorithm has to be designed for a specific car body. Not only the behavior of the car body in case of violent side collisions has to be taken into account, but unintended firing in case of light traffic accidents or abnormal driving conditions also has to be prevented.
By employing acceleration based side impact sensors a secure ignition of side airbags is possible. However, an adjustment by modifying merely the software of the controller does not lead to acceptable results. A good performance of the side impact protection system requires an adjustment of all components involved, i.e. the car body, the hardware of the sensor / controller and the software algorithm.