Triggering unit

1. A triggering unit to trigger a safety device in a motor vehicle to protect an occupant of the vehicle in an impact situation, in which the vehicle impacts with an impacting object, the triggering unit being configured to actuate the safety device only if the total stiffness between the vehicle and the impacting object exceeds a predetermined stiffness value, the unit incorporating a calculating arrangement to calculate stiffness from the sensed acceleration of the vehicle and sensed relative velocity between the vehicle and impacting object, the stiffness being defined as the quotient between: the force between: the vehicle and the impacting object, and the decrease in distance between the centers of gravity of the vehicle and the impacting object after contact between the vehicle and the impacting object.

2. A unit according to claim 1 wherein the unit is configured so that the value of the total stiffness is updated during the course of the impact.

3. A unit according to claim 1 where in the unit is configured to trigger the safety device if the change in velocity of the vehicle also exceeds a specific predetermined value.

4. A unit according to claim 1 where there are two said predetermined stiffness values, the triggering unit being adapted to actuate the safety device in a first mode of operation if a first predetermined threshold stiffness value is exceeded and in another mode of operation if a second predetermined threshold stiffness value is exceeded.

5. A triggering unit to trigger a safety device in a motor vehicle to protect an occupant of the vehicle in an impact situation in which the vehicle impacts with an impacting object, the triggering unit being configured to actuate the safety device only if the total stiffness between the vehicle and the impacting object exceeds a predetermined stiffness value, the unit incorporating a calculating arrangement to calculate stiffness from two or more sensed parameters in the form of a sensor to sense relative velocity between the vehicle and the impacting object immediately prior to impact, and an accelerometer provided on the vehicle, the stiffness being calculated as a function of sensed acceleration and as an inverse function of relative velocity and elapsed time.

6. A unit according to claim 5 , wherein total stiffness is calculated as being related to (M 1 .a 1c )/(v r .t) where M 1 is the mass of the vehicle, a 1c , is an output from a central accelerometer on the vehicle, v r is the relative velocity between the vehicle and the impacting object prior to impact and the t is the time elapsed from the commencement of the impact.

7. A unit according to claim 5 wherein the unit is configured so that the value of stiffness is updated during the course of the impact.

8. A unit according to claim 5 wherein the predetermined stiffness value is substantially inversely proportional to the pth power of the relative speed, where p>1.

9. A unit according to claim 5 wherein the unit is configured to trigger the safety device if the change in velocity of the vehicle also exceeds a specific predetermined value.

10. A unit according to claim 5 where there are two said predetermined stiffness values, the triggering unit being adapted to actuate the safety device in a first mode of operation if a first predetermined threshold stiffness value is exceeded and in another mode of operation if a second predetermined threshold stiffness value is exceeded.

11. A triggering unit for triggering a safety device in a vehicle for protecting an occupant of the vehicle in an impact situation in which the vehicle impacts with an impacting object the unit incorporating an accelerometer to determine acceleration applied to the vehicle, and a sensor to sense the relative velocity between the vehicle and the impacting object immediately prior to impact, the triggering unit being configured to actuate the safety device only if the determined acceleration multiplied by relative speed, raised to the power q, where q>0, divided by the time from the start of impact is greater than a predetermined invariable value.