Method for actuating a reversible belt pretensioner

1. A method for actuating a reversible belt pretensioner for pretensioning a belt webbing wound around a belt reel of a seat belt in a motor vehicle, the belt pretensioner having a drive which acts on the belt reel, and the seat belt having, as a belt extraction lock, a locking device which acts on the belt reel in an unwinding direction and is released only if the belt reel is rotated through a specific angle of rotation in a winding-on direction, comprising the steps of:

triggering the belt pretensioner by a hazardous situation;

satisfying a predefined vehicle operating state which indicates an end of the hazardous situation; and

actuating the drive of the belt pretensioner for opening the locking device in an opening mode wherein in a first winding-on phase the belt reel is rotated through the specific angle of rotation in the winding-on direction to an angular position and wherein in a subsequent, holding phase the drive of the belt pretensioner is actuated for a predefinable holding time (T H ) in such a way that the belt reel is held in the angular position.

2. The method according to claim 1 , wherein the holding time (T H ) is predefined in such a way that a vehicle-sensitive sensor of the belt extraction lock permits the opening of the locking device up to an end of the holding time (T H ).

3. The method according to claim 1 , wherein the holding time (T H ) is predefined as a function of at least one vehicle-movement dynamics parameter, wherein the parameter necessary for determining the holding time (T H ) is registered, and wherein the holding time (T H ) is determined by the registered parameter.

4. The method according to claim 1 , wherein the holding time (T H ) is a function of a vehicle longitudinal acceleration.

5. The method according to claim 1 , wherein the holding time (T H ) is a function of a vehicle transverse acceleration.

6. The method according to claim 1 , wherein the holding time (T H ) is a function of a coefficient of friction between a tire of the vehicle and a road surface.

7. The method according to claim 1 , wherein the holding time (T H ) is a function of a brake pressure.

8. The method according to claim 1 , wherein the holding time (T H ) is a function of a velocity of the vehicle.

9. The method according to claim 1 , wherein the holding time (T H ) is a function of a yaw rate of the vehicle.

10. The method according to claim 1 , wherein the holding time (T H ) is a function of a steering angle of the vehicle.

11. The method according to claim 1 , wherein the holding time (T H ) is a function of a signal of a vehicle-movement dynamics control system.

12. The method according to claim 1 , wherein the holding time (T H ) is a function of a signal of a brake booster system.

13. The method according to claim 1 , wherein the holding time (T H ) is a function of a velocity of the vehicle and of a signal of a brake booster system, the brake booster signal bringing about full braking and triggering of the reversible belt pretensioner in order to protect a vehicle occupant, and the holding time (T H ) being equal to a first time period (T 1 ) if, after a disappearance of the brake booster signal and/or after a termination of the full braking, the vehicle has a velocity which is less than or equal to a velocity threshold value, and the holding time (T H ) is equal to a second time period (T 2 ) if, after the disappearance of the brake booster signal, the vehicle has a velocity which is higher than the velocity threshold value.

14. The method according to claim 13 , wherein the first time period (T 1 ) is longer than the second time period (T 2 ).

15. The method according to claim 13 , wherein the velocity threshold value is equal to zero.