Method for actuating a reversible belt pretensioner

1. A method for actuating a reversible belt pretensioner for pretensioning a belt webbing, unwound from a belt reel, of a seat belt in a motor vehicle, wherein the belt pretensioner has a drive configured to act on the belt reel, and the seat belt has a belt extraction locking device which acts on the belt reel in the unwinding direction and whose locking can be released only if the belt reel is rotated about an angle of rotation in the winding-on direction,

comprising initiating, after a triggering of the belt pretensioner caused by a hazardous situation and sensing when a variable indicative of the belt force indicates a decrease in the belt force, an opening mode in which the belt pretensioner drive is actuated to open the locking device so that, at a beginning of the opening mode, the belt reel rotates through the angle of rotation in the winding-on direction.

2. The method as claimed in claim 1 , wherein the opening mode is initiated only if a predefined vehicle operating state is present which is indicative of an end of the hazardous situation.

3. The method as claimed in claim 2 , wherein in a braking operation, the vehicle speed reaching or falling short of a first speed threshold value is indicative of the end of the hazardous situation, and the traveling speed, as a variable indicative of the belt force, reaching or falling short of a second speed threshold value is indicative of the decrease in the belt force.

4. The method as claimed in claim 3 , wherein the first and second traveling speed threshold values are zero.

5. The method as claimed in claim 2 , wherein the indicative variable of the belt force is a belt-force expected variable which is determined by a belt-force model using vehicle-movement-dynamics parameters, and a decrease in the belt force is concluded if the belt-force expected variable decreases by at least one predefinable decrease threshold value.

6. The method as claimed in claim 5 , wherein at least the vehicle speed and at least one of a brake-system variable indicating a braking action and a signal of a brake booster system are utilized as model parameters for the belt-force model.

7. The method as claimed in claim 5 , wherein a signal of at least one of a vehicle-movement-dynamics control system, yaw rate and steering angle are utilized as model parameters.

8. The method as claimed in claim 1 , wherein a vehicle with a brake booster system in which a brake booster system signal brings about a triggering of a braking action with a predefined, strong braking effect, an omission of the brake assistance signal is indicative both of the end of the hazardous situation and also of the decrease in the belt force or the fact that the belt force falls short of a threshold value, and initiates the opening mode.

9. The method as claimed in claim 1 , wherein motor current during a predefinable actuation phase of the belt pretensioner drive is utilized as a variable indicative of the belt force.

10. The method as claimed in claim 9 , wherein the predefinable actuation phase is a motor-current test phase during which a predefinable single or multiple actuation of the belt pretensioner drive in order to determine the motor current is carried out so that the belt reel rotates only to an extent such that the locking device can not yet be released.

11. The method as claimed in claims 1 , wherein, in order to determine the variable indicative of the belt force, at least one force- or strain-measuring devices are operatively arranged in or on the belt webbing or another part of the belt system.

12. A method for actuating a reversible belt pretensioner for pretensioning a belt webbing, unwound from a belt reel, of a seat belt in a motor vehicle, wherein the belt pretensioner has a drive configured to act on the belt reel, and the seat belt has a belt extraction a locking device which acts on the belt reel in the unwinding direction and whose locking can be released only if the belt reel is rotated about an angle of rotation in the winding-on direction,

comprising initiating, after a triggering of the belt pretensioner caused by a hazardous situation and sensing when a variable indicative of the belt force indicates the fact that the belt force falls short of a predefinable threshold, initiating an opening mode in which the belt pretensioner drive is actuated to open the locking device so that, at a beginning of the opening mode, the belt reel rotates through the angle of rotation in the winding-on direction.

13. The method as claimed in claim 12 , wherein the opening mode is initiated only if a predefined vehicle operating state is present which is indicative of an end of the hazardous situation.

14. The method as claimed in claim 13 , wherein in a braking operation, the vehicle speed reaching or falls short of a first speed threshold value is indicative of the end of the hazardous situation, and the traveling speed, as a variable indicative of the belt force, reaching or falling short of a second speed threshold value is indicative of the decrease in the belt force.

15. The method as claimed in claim 14 , wherein the first and second traveling speed threshold values are zero.

16. The method as claimed in claim 13 , wherein the indicative variable of the belt force is a belt-force expected variable which is determined by a belt-force model using vehicle-movement-dynamics parameters, and a decrease in the belt force is concluded if the belt-force expected variable decreases by at least one predefinable decrease threshold value.

17. The method as claimed in claim 16 , wherein a signal of at least one of a vehicle-movement-dynamics control system yaw rate and steering angle are utilized as model parameters.

18. The method as claimed in claim 13 , wherein at least the vehicle speed and at least one of a brake-system variable indicating a braking action and a signal of a brake booster system are utilized as model parameters for the belt-force model.

19. The method as claimed in claim 12 , wherein a vehicle with a brake booster system in which a brake booster system signal brings about a triggering of a braking action with a predefined, strong braking effect, an omission of the brake assistance signal is indicative both of the end of the hazardous situation and also of the decrease in the belt force or the fact that the belt force falls short of a threshold value, and initiates the opening mode.

20. The method as claimed in claim 12 , wherein motor current during a predefinable actuation phase of the belt pretensioner drive is utilized as a variable indicative of the belt force.

21. The method as claimed in claim 20 , wherein the predefinable actuation phase is a motor-current test phase during which a predefinable single or multiple actuation of the belt pretensioner drive in order to determine the motor current is carried out so that the belt reel rotates only to an extent such that the locking device can not yet be released.

22. The method as claimed in claim 12 , wherein, in order to determine the variable indicative of the belt force, at least one force- or strain-measuring devices are operatively arranged in or on the belt webbing or another part of the belt system.