Monitoring web speed of material web

1. A method for monitoring a web speed of a material web moved by rollers, comprising:

driving a drive roller by a motor;

measuring an instantaneous rotational speed of the motor;

determining an instantaneous web speed from the rotational speed of the motor as measured and a diameter of the drive roller;

comparing the instantaneous web speed with a speed target value and a speed threshold value of the web speed;

switching off the motor when the instantaneous web speed is greater than the speed threshold value for longer than a tolerance period; and

switching off the motor when a deviation of the instantaneous web speed from the speed target value is greater than a tolerance deviation.

2. The method as claimed in claim 1 , wherein the drive roller is driven by the motor via gears.

3. The method as claimed in claim 2 , wherein a first tolerance period is predefined and the motor is switched off if the difference between the instantaneous web speed determined and the speed target value remains constant for longer than the first tolerance period.

4. The method as claimed in claim 3 , wherein a rotational speed target value for the rotational speed of the motor is determined from the speed target value and the diameter of the drive roller, and the rotational speed of the motor is adjusted to the rotational speed target value.

5. The method as claimed in claim 4 , wherein the diameter of the drive roller varies over time, as the material web is wound or unwound, further comprising periodically measuring the diameter of the drive roller and determining the instantaneous web speed as a function of the instantaneous diameter measured.

6. The method as claimed in claim 5 ,

wherein, during each revolution of the drive roller, diameters of the drive roller are measured at different points, for each revolution a largest and a smallest measured diameter of the measured diameters is determined, a tolerance difference for the diameters, a number of revolution N 1 and a tolerance number N 2 are predefined, and the motor is switched off if the difference between the largest and the smallest diameter measured is greater than the tolerance difference for more than N 2 of N 1 consecutive revolutions of the drive roller.

7. The method as claimed in claim 6 , wherein the motor is switched off if exactly the same rotational speeds are measured for a predefined number of consecutive motor revolutions.

8. The method as claimed in claim 7 , further comprising monitoring the web speed of the material web in at least one of a paper machine, a reel slitter and a coater.

9. A device for monitoring a web speed of a material web moved by a drive roller driven by a motor, comprising:

a speed measuring unit measuring a rotational speed of the motor; and

a control unit evaluating signals acquired by the speed measuring unit to determine an instantaneous web speed based on the rotational speed of the motor and a diameter of the drive roller, and switching off the motor when the instantaneous web speed is greater than a speed threshold value for longer than a tolerance period and when a deviation of the instantaneous web speed from a speed target value is greater than a tolerance deviation.

10. The device as claimed in claim 9 , further comprising at least one diameter measuring unit measuring the diameter of the drive roller because the drive roller has a diameter which varies over time as the material web is wound or unwound.

11. The device as claimed in claim 10 , wherein the at least one diameter measuring unit comprises a distance meter measuring a distance of the distance meter from an external surface of the drive roller.