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 Wheat stone 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.

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.

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.

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.

Workshop Hint 

Wheatstone Bridge circuits are used to measure resistance, inductance and capacitance. They consist of 4 resistors arranged in a diamond orientation. Excitation voltage is applied between the top and bottom of the diamond, output voltage is measured across the sides. One or more of the legs contain a resistive transducer, a strain gauge. Based on the change in resistance of the strain gauge the legs go from "balanced" to "unbalanced".