Introduction:
Active hood 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 end (windshield side) of the hood lifts a few centimeters.
The pedestrian experiences a less severe impact, reducing the chance of injury with this technique. 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 (F10 and F11 models) for the active hood hinges. Below the image, the system is explained.
- right hood latch with actuator;
- right hood hinge with actuator;
- Bowden cable (for hood release);
- left hood hinge with actuator;
- left hood latch with actuator;
- optical fiber cable;
- pyrotechnic actuator on left hood hinge;
- sensor;
- connecting elements.
On the bumper beam of the body, there is a fiber optic cable (6). The sensor sends an optical (light) signal through the fiber optic cable to the loop in the other bumper corner. The light signal arrives back at the sensor. In a minor collision, the front bumper bends slightly. The fiber optic cable gets compressed between the front bumper and the bumper beam. Due to this pressure, the fiber optic cable breaks at the connecting elements (9). The transmitted light signal does not reach or is at least weakened by the sensor, enabling the connected control unit to detect a collision.
Instead of the fiber optic cable, deceleration sensors can also be used that measure both the deceleration and deformation of the bumper.
When the control unit detects a collision, it activates the pyrotechnic actuators of the hood hinges, and in this case, also of the hood latches. The hood lifts by approximately 50 mm and often moves around 20 to 30 mm backward.
The system operates at speeds between (approximately) 20 to 55 km/h, depending on the manufacturer. After the pyrotechnic initiators have fired, a fault message appears on the vehicle’s display. In most cases, the hood can be closed again by pushing it down and forward in one motion at the corners. The pyrotechnic initiators 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 initiators, no components need to be replaced.
Pyrotechnic hood hinge in detail:
The image shows two situations where the hood hinge is positioned.a0
The hinge in the left image is intact; the hood is closed (visible by the hood mount). The pyrotechnic cylinder with the yellow connector is clearly visible.
The right image shows the activated hinge. The pyrotechnic cylinder has pushed the hinge with the hood upward.
