Impact Sensor: Description and Operation
Impact Sensor
The MRS Control Unit contains 2 internal collision detection sensors to determine the severity of the crash.
Main Sensor
The main sensor is a deceleration detection device which is contained in the MRS control unit. The sensor consists of a spring and weight system which is attached to strain gauges in a Wheatstone bridge circuit. If a change in strain gauge resistance is greater than a preset value, it corresponds to a crash condition of sufficient severity to warrant MRS component deployment. In this case, the processor provides a signal to initiate airbag module and/or seat belt pre-tensioner deployment. Deployment will only be carried out if a confirmation signal that a crash condition is occurring is received by the MRS control unit. Crash condition confirmation is achieved by the simultaneous actuation of the safing sensor.
Side Impact Sensors
For side impacts, the side impact crash sensors provide additional inputs to the control unit for determining and confirming a crash condition in conjunction with the control unit's internal accelerometer. The side impact crash sensors must be titled in the correct orientation, and there must be no gap between the sensor and the body mounting position. Ensure the mounting screws are tightened to the correct torque. Be careful when refitting trim molding not to damage sensors or sensor harness. The side impact sensors are located behind the rear lower quarter trim moldings, and mounted to the body floor via their own mounting brackets, in alignment with the rear seat cushion front mountings. Each sensor is attached to their body mounting bracket by two Torx bolts.
Each side impact sensor consists of an electronic accelerometer, microprocessor and serial link circuit. The sensor uses the accelerometer to determine the severity of a side impact, and if the impact is great enough to warrant multiple restraint operation, the sensor communicates the need for deployment to the MRS control unit via a single line serial link.
Front
For front angled impacts, the control unit acts in co-operation with the side impact crash sensors, to determine which airbags and seat belt pre-tensioners need to be deployed.
Rear
For rear impacts, the control unit uses a rear impact trigger threshold to determine that a severe rear collision has occurred and deploys all seat belt pre-tensioners, to restrain vehicle occupants.
Safing Sensor
This sensor is also contained within the MRS control unit and is included in the control unit internal circuitry to prevent unintentional detonation of MRS components. The safing sensor is connected in series with the main sensor and operates at comparatively lower rates of deceleration. When the safing sensor closes in conjunction with the main sensor exceeding its trigger value, electronic switches are activated, allowing electrical current to be supplied to the driver and passenger airbags and the relevant seat belt pre-tensioners. The operation of the side (thorax) airbag modules and the Advanced Head Protection System (AHPS2) airbags are controlled by electronic switching in response to the threshold value for the side impact sensors being exceeded.
Inertia Sensor
The inertia sensor is responsible for certain functions previously handled by the MRS control module.
Passive Safety functions handled by Inertia Sensor:
- Fuel pump shut-off.
- Door unlock signal to BC1.
- Window down Signal to BC1.
- Hazard warning lights on signal to BC1.
The inertia switch has a threshold of 14G's. When the threshold is exceeded the switch goes open. This open causes the shutoff of power to the fuel pump relay.
The BC1 sees the open and activates the door lock motors to the unlock position, supplies power to the windows for down operation for 750ms. and activates the hazard warning lights.
All of these functions were previously controlled directly by the MRS control unit.