Method of controlling operation of a winder for a fiber web

1. A method of controlling operation of a winder for a fiber web while forming at least one fiber web roll, comprising the steps of:

guiding the fiber web onto the web roll via a nip formed between a first support drum and the web roll;

applying a controllable torque to the first support drum with a first drive assembly;

applying a winding force to the web roll by a second drive assembly;

wherein the winding force is controlled by executing at least the following steps:

(a) setting an initial value for an indicative coefficient (μ n );

(b) determining a set value for the winding force using a function F s =f (μ n , N)

wherein:

F s =set value for winding force [N/m],

μ n =the indicative coefficient,

N=nip force [N/m] at a nip over which the winding force is transmitted,

(c) at least the second drive assembly is controlled by using the set value for the winding force;

(d) determining a first indicative speed of at least one of the first drive assembly and the first support drum, and determining a second indicative speed of the second drive assembly;

(e) comparing the second indicative speed with the first indicative speed, and determining a difference value;

(f) when the difference value, is different than a predetermined set difference, the indicative coefficient value is corrected; and

(g) the steps (b) to (f) are repeated so long as the winder is in operation.

2. The method of claim 1 wherein the first indicative speed is measured using a first predetermined time interval, and the second indicative speed is measured using a second predetermined interval.

3. The method of claim 1 wherein step (f) comprises a further control rule according to which, in case the difference value is smaller than the predetermined set difference, the indicative coefficient value is increased.

4. The method of claim 1 wherein step (f) comprises a further control rule according to which, in case the difference value is greater than the predetermined set difference, the indicative coefficient value is decreased.

5. The method of claim 1 wherein the web roll is supported by a second drum support member, comprising a drum or a belt assembly, and the drum support member is driven by the second drive assembly applying a controllable torque to the drum support member.

6. The method of claim 1 wherein step (c) the second drive assembly is controlled by using the set value for the winding force, such that a maximum torque which the second drive assembly applies to the drum support member is calculated from the set value for the winding force.

7. The method of claim 1 wherein the indicative coefficient is a function of at least one of the following variables: the indicative speed of the first drive assembly, the indicative speed of the second drive assembly ( 22 ), a thickness of a separate surface layer of a drum, a thickness of a belt in a belt assembly, nip force and roll diameter.

8. The method of claim 7 wherein the value of μ n is updated based on detected change of at least one of the following: the indicative speed of the first drive assembly, the indicative speed of the second drive assembly, a thickness of a separate surface layer of a drum, a thickness of a belt in a belt assembly and a nip force.

9. The method of claim 1 wherein the set value for the winding force is calculated using the function F s =μ n ·N.

10. The method of claim 1 wherein during winder operation the speed of the winder is accelerated or decelerated.