Active Hood Hinges


  • Preface
  • Required sensors and actuators
  • Pyrotechnic hood hinge in detail

Active hood hinges serve to protect a pedestrian as much as possible against injury in the event of a collision.

The electronics in the car ensure that the hood at the rear (window side) moves up a few centimeters.
The pedestrian experiences a less hard blow and, thanks to this technique, has less chance of injury. Sometimes this system is extended with a pedestrian airbag, which inflates near the windshield.

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.

  1. right bonnet lock with actuator;
  2. right hood hinge with actuator;
  3. bowden cable (for the hood release);
  4. left hood hinge with actuator;
  5. left bonnet lock with actuator;
  6. optical fiber cable;
  7. pyrotechnic actuator on left hood hinge;
  8. sensor;
  9. connecting elements.

There is a fiber optic cable (6) on the bumper beam of the body. The sensor sends an optical (light) signal through the fiber optic cable to the loop in the other bumper corner. The light signal returns to the sensor. In a minor collision, the front bumper bends slightly. The fiber optic cable is then clamped between the front bumper and the bumper beam. This pressure causes the fiber optic cable to break at the connecting elements (9). The transmitted light signal does not arrive at the sensor, or at least weakens it, as a result of which the connected control unit recognizes a collision.

Instead of the fiber optic cable, deceleration sensors can also be used that measure both the deceleration and the deformation of the bumper.

The moment the control unit recognizes that there is a collision, it controls the pyrotechnic actuators of the bonnet hinges, and in this case also the bonnet locks. The bonnet rises about 50 mm and often moves back about 20 to 30 mm.

The system operates at speeds between (approximately) 20 to 55 km/h, depending on the manufacturer. After the pyrotechnic igniters have gone off, a fault message appears in the vehicle display. In most cases, the hood can be closed again by pulling it downwards and forwards in the corners in one movement. The pyrotechnic igniters must be replaced; they only work once. After replacement, the error message can be deleted.

If the hinges are equipped with mechanical actuators instead of pyrotechnic igniters, no components need to be replaced.

Pyrotechnic hood hinge in detail:
The illustration shows two situations where the hood hinge can go. 

The hinge in the left image is intact; the bonnet is closed (indicated by the bonnet mounting). The pyrotechnic cylinder with the yellow plug is clearly visible.

The right image shows the activated hinge. The pyrotechnic cylinder has pushed up the hinge with the hood.

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