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Pedestrian Protection System: Description and Operation





Pedestrian Protection System

COMPONENT LOCATION









INTRODUCTION

WARNING: All pyrotechnic devices are dangerous. Before performing any procedures on any pyrotechnic device, read all information contained within the Standard Workshop Practices section of this product.
For additional information, refer to Supplemental Restraint System (SRS) Health and Safety Precautions Supplemental Restraint System (SRS) Health and Safety Precautions
The pedestrian protection system control module receives an impact signal from PCS (pedestrian contact sensor) and two accelerometers mounted on the front bumper beam. The control module interprets the signals to determine whether impact is with a pedestrian or other object, such as a traffic cone. When the control module detects a valid impact signal and the vehicle is in the 15 to 45 km/h (9 to 28 mph) speed range, it fires the two pyrotechnic hood latch release units and two pyrotechnic actuators which opens the hood approximately 130 mm (5.1 in.).
The pedestrian protection system is factory fit only to non Federal vehicles.

CONTROL MODULE





The control module is secured with two screws to the underside of the LH (left-hand) front fender reinforcement, behind the wheelarch liner. The control module is hardwired to the other system components and is also connected to the high speed CAN (controller area network) bus. The control module receives ambient air temperature, ignition state and vehicle speed on the high speed CAN (controller area network) bus.
The control module uses signals from the PCS and the accelerometers to determine if the impact on the bumper is a pedestrian or some other object. The system will always deploy where the control module determines a pedestrian impact. The system minimizes deployment for other impacts that are similar to pedestrian impact, such as traffic cones.

PEDESTRIAN CONTACT SENSOR









The PCS is located in a slot behind the bumper foam on the front bumper beam and is secured with a single fixing.
At the end of the sensor next to the fixing there is a PCB (printed circuit board) and fibre optic transceiver, which is connected to a fibre optic cable configured in a loop. The PCS uses this fibre optic loop to detect an impact by measuring the amount of light returned to the fibre optic transceiver. A reduction in light levels occurs when the PCS is deformed by an impact. The PCS monitors the level of light returned from the loop, if a reduction of a predetermined magnitude is measured a trigger signal will be sent to the control module.

HOOD LATCH









The hood latches are located at either side of the trailing edge of the hood. Each latch is released by a small pyrotechnic pin, which is fired in response to a signal from the control module. The pyrotechnic pin engages with the latch pawl which is holding the latch claw closed. Once the pawl is moved by the pyrotechnic pin the latch claw is released and the return spring moves the latch claw to the open position allowing the hood to move open under the pressure of the hood actuator.

Hood Latch Deployment









HOOD ACTUATOR





The hood actuators are pyrotechnic air bags. The hood actuators are located just forward of the hood latches on either side of the hood. The actuators comprise a pyrotechnic device to raise the hood, a secondary hood latch mechanism and a tether sleeve. Once fired a locking device mounted on top of the actuator couples with the secondary hood latch before the air bag inflates to raise the hood. This ensures that the hood can only deploy to a pre-determined height (approximately 130 mm (5.1 in.)). As the hood actuator is fired two prongs located in the secondary latch housing are moved out of their retaining clamp. The prongs are spring loaded to grip the secondary hood latch striker securing it to the actuator tether cone.

Hood Actuator and Secondary Latch









ACCELEROMETER





Two accelerometers are mounted on the rear of the bumper beam. The accelerometers measure the amount of inertia when an impact occurs and send a signal to the control module. This signal is used by the control module to calculate the firing of the pedestrian protection system. The accelerometers are modified crash sensors. The sensors have a lower detection range than front/side crash sensors to enable them to detect lower level impacts and therefore assist in the object discrimination process.

CONTROL DIAGRAM

NOTE:
A = Hardwired; D = High speed CAN (controller area network) bus









PRINCIPLES OF OPERATION
The system is designed to help reduce head and lower leg injuries when a pedestrian has been hit by a vehicle. This is achieved in two main ways:
- Bumper design
- Pyrotechnic deployment of the hood.
The front bumper of the vehicle has been designed with a profile which encourages the pedestrians lower leg to lift upwards rolling around the front of the bumper surface. The bumper is constructed of energy absorbing foam and plastics and is designed to be softer around the top part of the bumper than the middle. This helps reduce injuries to the pedestrians lower leg.
As the pedestrians leg rolls around the bumper it deforms the bumper and the sensor beneath. If the predetermined parameters are met the pedestrian protection system control module fires the pyrotechnic hood latch pins which opens the hood latch claw. This is followed by the pyrotechnic actuator firing to force the hood upwards. As the hood moves upwards the secondary hood latch automatically engages with the secondary hood latch striker plate locking the hood and the body together via the tether cone attached to the hood actuator. This ensures that the hood only rises a set amount. The raising of the hood creates a more cushioned effect between the vehicle and the pedestrian. The hood actuators will deflate within 3 seconds of deployment to improve visibility. The secondary hood latch remains connected to the hood when the hood actuators have deflated and can be released by squeezing the two latch forks together, either by hand or with a pair of pliers.
The hazard warning lights will be activated and can only be switched off by pressing the engine start/stop button to turn the engine OFF and ON again (without pressing the brake pedal) to revert to the ignition convenience mode.
A warning message CHECK PEDESTRIAN SYSTEM will appear on the message center. The vehicle must not be driven once the bonnet has been deployed.

Hood Actuator-Deployed