Introduction:
Active bonnet hinges are designed to protect pedestrians as much as possible from injury in the event of a collision.
The electronics in the car ensure that the rear (windscreen side) of the bonnet moves up a few centimetres.
The pedestrian experiences a less severe impact and, thanks to this technology, has a lower risk of injury. Sometimes this system is supplemented with a pedestrian airbag, which inflates at the level of the windscreen.
Required sensors and actuators:
The image below shows the sensors and actuators with the legend used on a BMW 5 Series (models F10 and F11) for the active bonnet hinges. The system is explained below the image.
- right bonnet catch with actuator;
- right bonnet hinge with actuator;
- Bowden cable (for bonnet release);
- left bonnet hinge with actuator;
- left bonnet catch with actuator;
- optical fibre optic cable;
- pyrotechnic actuator on left bonnet hinge;
- sensor;
- connecting elements.
On the bumper bar of the body there is a fibre optic cable (6). The sensor sends an optical (light) signal through the fibre optic cable to the loop in the opposite bumper corner. The light signal then returns to the sensor. In a minor collision, the front bumper bends slightly. The fibre optic cable is then clamped between the front bumper and the bumper bar. Due to this pressure, the fibre optic cable breaks at the connecting elements (9). The transmitted light signal does not, or at least only in a weakened form, reach the sensor, causing the connected control unit to recognise a collision.
Instead of the fibre optic cable, deceleration sensors can also be used that measure both the deceleration and the deformation of the bumper.
At the moment the control unit recognises that there is a collision, it activates the pyrotechnic actuators of the bonnet hinges, and in this case also of the bonnet catches. The bonnet rises by approximately 50 mm and often moves around 20 to 30 mm backwards.
The system is activated at speeds between (approximately) 20 and 55 km/h, depending on the manufacturer. After the pyrotechnic igniters have fired, a fault message appears in the vehicle display. In most cases, the bonnet can be closed again by moving it downwards and forwards in one movement at the corners. The pyrotechnic igniters must be replaced; they only work once. After replacement, the fault message can be cleared.
If the hinges are equipped with mechanical actuators instead of pyrotechnic igniters, no components need to be replaced.
Pyrotechnic bonnet hinge in detail:
The image shows two situations in which the bonnet hinge can be found.
The hinge in the left image is intact; the bonnet is closed (visible from the bonnet mounting). The pyrotechnic cylinder with the yellow connector can be clearly seen.
The right image shows the activated hinge. The pyrotechnic cylinder has pushed the hinge and bonnet upwards.
