Method of controlling operation of a winder for a fiber web

Abstract

This invention relates to a method of controlling the operation of a winder in which method while forming at least one fiber web roll the fiber web is brought on the web roll via a nip formed by a first support drum and the web roll, which first support drum is driven by a first drive assembly (20) applying controllable torque to the drum and the winding force is applied to the web roll by a second drive assembly (22). In the method the second drive assembly (22) is controlled based on the indicative speed difference of the second drive assembly (22) and setting a friction coefficient for determination of maximum winding force.

Claims

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 (.sub.n); (b) determining a set value for the winding force using a function F.sub.s=f (.sub.n, N) wherein: F.sub.s=set value for winding force [N/m], .sub.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 .sub.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.sub.s=.sub.n.Math.N.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which;

(2) FIG. 1 illustrates a winding section according to an embodiment of the invention, and

(3) FIG. 2 illustrates a chart of an operation of an exemplary embodiment according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIG. 1 depicts schematically a winding section in a slitter winder for a fiber web. The figure shows a so called two drum winder 10, where parallel web rolls 12 to be wound from partial webs w are formed in support of a front drum 16 and a rear drum 18, as well as by a press roll 28. In the winding section there is a slitting section 11 to which a full width web w is guided and in which the web w is slit into at least two partial webs w while the web is running under control of a number of guide rolls. The parallel partial webs w are guided to the rear drum 18 and are brought on the set of rolls 12 via a nip 18 formed between the rear drum 18 and the set of web rolls 12. Both the rear drum 18 and the front drum 16 are provided with a dedicated drive 20, 22 in order to controllably rotate the drums. Here the drive alone or a combination of the drum and its drive are called a drive assembly. The winding section 10 is provided with a control computer 100 which is arranged to control the operation of the drives, such as electric motors 20, 22. Under normal conditions the control of the motors is based on torque control whereas the rotational speed of the motors is maintained as close as possible to a set value by controlling the torque applied by the motor.

(5) Each one of the drums 18, 16 is provided with a speed sensing device 24, 26, which comprises suitable sensors. The speed sensor is used to determine the speed, acceleration or deceleration of the drum. As an example, such sensors may use a magnet and a Hall-effect sensor, or a toothed wheel in connection with the drum and an electromagnetic coil to generate a signal. The signal is made available to the controller computer 100 for use in controlling the operation of the winder section 10.

(6) When the winder is running the rear drum 18 is operated such that the speed of the web w is controlled by the rear drum 18 and the tension of the web is controlled by an unwinder (not shown) from which the web w is fed to the winder section 10. The front drum 16 is configured to provide torque to web rolls 12, which torque is controlled based on special rules which are configured to the computer controller 100. By means of the torque applied by the front drum 16 it is possible to have an effect on the formation and quality of the web rolls 12.

(7) In the FIG. 1 there is shown an embodiment of the invention where the front drum 16 has been replaced by a belt assembly which supports the rolls 12 over a wider area. The belt assembly, which is depicted by a dashed line here, is provided with guide rolls and is driven by a motor. Therefore the invention is similarly applicable to such a winder also. The belt assembly may also be referred to as a drive assembly. When a speed is discussed in this context it means the web speed or a surface speed of a drum or a belt unless otherwise specifically mentioned.

(8) Now, the traction control unit 102 for the front drum 16 is provided or configured into the control computer 100. The traction control unit 102 comprises instructions to control the drive 22 of the front drum 16 in order to prevent loss of traction of the driven front drum 16 against the web roll 12. The traction control unit 102 controls the torque i.e. winding force applied by the front drum 16 to the web roll 12 by means of specific executable instructions. This way, depending for example on the properties of the web and surface speed of the drum 16, a maximum torque set value is determined such that loss to traction can be practically avoided. The winding force applied to the web roll is proportional to the torque applied to the front roll 16 and therefore, the other one can be determined if the other one is known, since the radius of the front drum 16 is also known.

(9) The controller computer, and specifically the traction control unit 102 thereof according to an embodiment of the invention comprises instructions to

(10) (a) setting an initial value for an indicative coefficient (F.sub.s),

(11) (b) setting the winding force using a function
F.sub.s=f(.sub.n,N) where: F.sub.s=set value for winding force [N/m], .sub.n=the indicative coefficient, N=nip force [N/m] at a nip over which the winding force is transmitted, stored in the controller computer,
(c) controlling at least the second drive 22 assembly by using the set value for the winding force,
(d) determining an indicative speed of the first drive assembly 20 and/or the first support drum using a first predetermined time interval, and determining an indicative speed of the second drive assembly 22 using a second predetermined interval,
(f) comparing the indicative speed of the second drive assembly 22 with the indicative speed of the first drive assembly 20, and
(g) in case the difference between the indicative speed of the second drive assembly and the indicative speed of the first drive assembly is greater than a predetermined set difference, correcting the indicative coefficient value,
(h) repeating steps (b) to (g).

(12) The method is executed by practicing at least the following steps. The method can be realized by operating the controllable computer 100 by executing the instructions stored in the control computer 100.

(13) Firstly an initial value is set for an indicative coefficient (step a). The indicative coefficient represents a friction coefficient between the front drum 16 and the web roll 12 (or between the belt assembly and the web roll, if that is the case). The indicative coefficient is at its simplest form a friction coefficient, which may in some practical application provide adequate operation. The initial value is set based on empirical data relating to the practical parameters of the case, like surface properties of the fiber web. It is set substantially close to a best estimate of the correct value and during the execution of the method the value is corrected on-line based on the detected traction performance. The indicative coefficient may include correction factors which take into account, for example, the speed of the web which effects on tendency of the air entering between the web layers.

(14) The next phase (step b) comprises determination of a set value for the winding force. The winding force is determined as a function which is using as its variables at least the indicative coefficient .sub.n, and nip force N at a nip over which the winding force is transmitted. As its simplest form the set value for the winding force is determined as F=.sub.n.Math.N. To be more specific, in case of the winder is a carrier drum winder, the winding force refers to the front drum (or a belt assembly). In case of a centerwind winder the winding force refers to the torque applied to the winding shaft of the web roll. The set value is utilized in the control method as a maximum value for the winding force.

(15) In the next step (step c) at least the second drive assembly 22 is controlled by using the set value for the winding force. In practice the winding force represents the torque set value assigned to a drive controller such as a frequency converter.

(16) In the following step (step d), an indicative speed of the first drive assembly 20 and/or the first support drum is determined using a first predetermined time interval, and an indicative speed of the second drive assembly 22 is determined using a second predetermined interval. Advantageously the first predetermined interval is equal to the second predetermined interval, such that a pair of indicative speeds is determined substantially simultaneously.

(17) Next (step e) the indicative speed of the second drive assembly 22 is compared with the indicative speed of the first drive assembly 20. In this step a possible loss of traction is revealed by any difference between the indicative speeds. Advantageously the actual difference between the indicative speed of the second drive assembly and the indicative speed of the first drive assembly is compared with a predetermined set difference value. The set difference may differ based on the case. The factors which effect the allowable difference in the indicative speeds comprises at least one of the following: fiber web grade, surface properties of the fiber web, resilient drum cover properties, nip loading and roll diameter.

(18) The indicative speed of the second drive assembly 22 and the indicative speed of the first drive assembly 20 may be the actual surface speed of the drums (or belt assembly). However, advantageously the indicative speed is based on the actual speed value which is corrected by certain factors.

(19) Such factors may be for example related to the surface of the drums, and particular to the surface of the belt assembly. When there is a resilient layer involved in the nip the control will be more accurate when for example the compression of such resilient layer is taken into account. This is because the compression effects the effective radius of the torque applied.

(20) In connection with an embodiment where the belt assembly is used as the front drum, the resilient belt brings more challenge to the application of the method. The belt wears out during use and it is compressed during the winding sequence when it supports the web rolls. These phenomena are taken into account when determining the indicative speed. Thus, the indicative speed follows a calibration curve which takes into account the characteristics of the belt and the change of thickness of the belt.

(21) Next, in case the actual difference between the indicative speed of the second drive assembly and the indicative speed of the first drive assembly deviates from the predetermined set difference value, i.e., the difference between the indicative speed of the second drive assembly and the indicative speed of the first drive assembly is greater than a predetermined set difference, the indicative coefficient value is corrected, (step f). Now, depending on if the difference between the indicative speed of the second drive assembly and the indicative speed of the first drive assembly is positive or negative, the indicative coefficient value is decreased or increased.

(22) More precisely, if the indicative speed of the first drive assembly is greater than the indicative speed of the second drive assembly the indicative coefficient value is decreased. This leads to a situation where also the set value for the maximum winding force is decreased. Based on this, also the drive controller decreases the torque set value.

(23) In the FIG. 2 there is shown an exemplary chart where the operation of the invention can be seen during an increase of speed. The horizontal axis represents time and the vertical axis represents magnitude of each variables in the chart, which variables are shown as lines with different patterns. The variables shown are: a speed difference 310 between the front drum 16 and the rear drum 18. The speed of the rear drum 18 can be considered to be substantially equal to the surface speed of web rolls 12; a nip force 320 at the nip over which the winding force is transmitted, i.e., between the front drum 16 and the web roll 12; a set value for the winding force 340 used in the traction control of the rear drum and in the traction control unit 102; actual winding force 360; speed of the web 380; and indicative coefficient 300 used in the traction control and the traction control unit 102. There is also shown a general reference value 400 for the winding force.

(24) The chart shows an exemplary situation where the set of rolls 12 are accelerated from standstill to a desired running speed which can be seen from the curve 380. The figure relates particularly to an effect of the air to the winding while the speed increases. At the beginning of the acceleration, the indicative coefficient 300 has a considerably high value. The initial value for the indicative coefficient can be set considerably close to the correct value and executing the method set the value substantially quickly to the appropriate level due to its on-line adjustment. The curve 310 shows that shortly after starting the acceleration, the speed difference between the front and the rear support drum increases steeply. When the speed difference increases above a predetermined set difference value, or a range 312, the value of the indicative coefficient 300 is decreased accordingly. This is clearly shown in the curve 300. This results in maintaining the speed difference between the front and the rear support drums at an acceptable or desired level. The actual acceptable speed difference is set suitably to assure that the fiber web roll quality does not suffer but still the acceleration stage is as short as possible.

(25) In the FIG. 2 there is shown a time period TC during which the method i.e. the traction control is active. During the time the traction control is active the actual winding force 360 is lower than the general reference value 400 indicating the effect of the invention. The value of the indicative coefficient 300 is decreased until the speed difference 310 is within the range 312 and the set value for the winding force 340 overrules the general reference value 400. After the traction control is deactivated the indicative coefficient is substantially constant. Since the indicative coefficient is corrected substantially continuously it is possible to use maximum winding forces without the risk of loss of traction and without the risk of unduly slowing down the speed.

(26) While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.