FI116403B - Procedure for regulating a property of a paper web - Google Patents

Description

116 4 ϋ '6

Method for adjusting the property of a paper web to be calendered - Förfar-ande för att reglera en egenskap hos en pappersbana

BACKGROUND OF THE INVENTION The invention relates to calendering a paper web with a calender having a profile adjustable device for adjusting paper properties such as thickness in the transverse direction of the web.

In calendering, the paper is driven through a nip formed by rotating rollers. In the nip, the paper is compressed at its thickest points. This results in a smoother surface for the paper-10, which also increases its gloss and smoothness.

Nowadays calendering also uses so-called calendars. profile adjustable multi-zone rolls to compensate for roll deflection across the web. Such a roller comprises loading shoes which are pressed radially in the axial direction of the roller into the roll shell, the pressure of each of which can be individually adjusted. This compensates for the profile change caused by roll deflection, whereby the nip gap is equalized over the entire width of the web.

GENERAL DESCRIPTION OF THE INVENTION * I · * t:. An adjustment method according to claim 1 has now been found. Certain preferred embodiments of the invention are set forth in the other claims.

• I

In accordance with the invention, a precise overall control of a variable property such as thickness in a transverse (CD) calender of a web is achieved in all production situations. The model-based adjustment method of the invention is suitable for both machine, soft and multi-roll calenders. The multi-roll calendar may be one where the rolls •. '. load separately (Valmet OptiLoad).

Hl · • t »25 The quality of the paper affected by the adjustment may be, for example, thickness, gloss: 'I *; it's smoothness. Of course, several sizes can be adjusted simultaneously.

In particular, the profiling device may be a calender roll which is compensated for deflection. It has si-1, one or more profiling members, in particular a radial piston for correcting deflection of the roll; However, the profiling device may also be, for example, a member affecting temperature 30, such as a heater. The heater may be, for example, an induction heater, possibly in combination with a thermal roller, or a heat blower.

2, 116405

Drawings

The accompanying drawings relate to a detailed description of the invention. In them: - Figure 1 shows a paper compression curve and a CD directional thickness profile by zone, - Figure 2 shows an on-line identification of the thickness change model parameters, - Figure 3 shows a block diagram of the CD controller.

Detailed Description of the Invention

According to the invention, the calenderer uses a profiling device for adjusting the desired paper quality across the direction. The calender may be one or more to 10 pins.

The profiling device may be a profile adjustable roll. It has at least one zone having a profile member inside the roll, by means of which radial adjustable force can be exerted externally on the roll shell. There are generally several such profiling members at intervals of, for example, about 10-20 cm. The profile members can be individually adjusted. Profiloin-15 treads are generally loading shoes pressed onto the surface of a roll shell. Usually they are hydraulic.

According to the invention, there is controlled a variable variable in calendering which is aimed at a certain value and which can be influenced by a profiling device.

* «* 'The size can be especially the thickness, gloss, smoothness or moisture of the paper. A model is defined in advance which illustrates the effect of the force of the profiling member on the quantity being monitored. After calendering, the size of the * *. * · Strip adjustable from the web is measured, and if it differs from the set, the model calculates the change in force required by the adjusting member to correct the difference. By adjusting each profile member individually, the quantity is also made as constant as possible across the web. The quantity is also measured before the calender, in which case the model counts to,, the change in the calender. Preferably, the measurand is the thickness of the paper.

* ... · A model is a mathematical model with parameters. In a normal production situation with no changes in machine speed and other things in the process. · ·. changes are slow in dynamics, feedback control can be used.

30 A mathematical model is preferably one that seeks to take into account all: ''; the effects of significant variables. The unknown parameters 3 1 1 6 4 u 6 included in the model are determined from experimental results, for example, by a recursive algorithm such as the Kalman filter or the least squares method. The parameters can be updated under production conditions. This can be done on a species or site basis. The update can be done on-line algorithm or based on laboratory measurements. Parameters 5 can be stored in paper type tables. Paper type-specific data can also be used to determine the roll constant and response pattern of a roll adjustable roll profile.

In a multi-nip calender, a mathematical model can be used to determine the change in each nip in the paper. By coupling the model equations of the various nipples, a concatenated model of the overall sequence is obtained. Thus, the large change output value calculated in the previous nip is then used as input data in the next nip. In Kalan blades with multiple adjustable rolls, the model can thus be used for chaining to describe the overall process of calculating nipple-specific adjustable roll line load profile setpoints.

15 The model may have parameters that depend only on the properties of the paper to be calendered, as well as parameters that depend on the calender. For example, the model coefficients can be determined experimentally for each paper grade, after which the model equation can be applied to any calender. Calender-dependent coefficients are determined separately for each calender.

The model may be formulated to have the desired shape, for example linear, with respect to unknown parameters. In this case, different off-line or on-line. ···. Identification methods can be applied directly to determine unknown parameters of the model. For example, the least squares method * or the Kalma filter applies.

• ,,.: 25 One model illustrating the relative compressibility of paper in a calender nip is the so-called.

Crotogino's equation (see, e.g., Crotogino et al., TAPPI Procee'dings, 1988 Finishing: '* *: and Converting Conference, 49-57) ε = (Βι-Bf) / B; = A + μΒί.

, · ·, Μ on ns. nip intensity factor, and more specifically

: V: 30 μ = ao + aLlogL + aslogS + aRlogR + aeO + aMM

....: wherein B; is the initial volume (cm3 / g)

Bf is the final bulb (cm3 / g) 4 116 4 u 3 L is the nip load (kN / m) S is the machine speed (m / s) R is the equivalent roll radius (m) Θ is the temperature of the web in the nip (° C) 5 M is moisture content of the web (%).

For example, this equation can be used as a model in the control method of the invention.

Em. the model contains a set of parameters that depend exclusively on the properties of the paper being calendered. For example, model coefficients can be determined experimentally for each paper grade individually. For example, since the model equation does not include a term that takes into account the flexibility of a soft roll, the coefficients are determined for each calendar separately.

The most important control variables traditionally used to influence thickness change are the line load and the temperature of the die roll. The track speed and the radii of the rollers affect the 15 nip times, but generally the machine speed is maintained constant, except during controlled speed changes. By limiting, for example, the radius change of the rolls, the number of unknown parameters in the model equation can be controlled in a controlled manner if desired.

•. ·. Unknown parameters of the model can be determined using various off-line or on-| 4 20 line identification methods, such as a recursive least squares method or a Kalman filter.

: / .: Instead of a mathematical model, functions can be used for experimentally calendering: 'paper types that take into account the influence of process conditions; ''. TUS. These may be obtained, for example, from a quality measurement system or from laboratory test results. These can be represented, for example, by compression curves.

For a given paper grade experimentally, under certain process conditions (line load, roll surface temperature, web humidity, web temperature, machine speed), the compressibility curve can be determined. The curve can be determined by searching for an off-line method. * · ·. function measurements (e.g., least squares method) for measuring points, or using recursive on-line methods to determine noncompliance parameters of the compression curve. When using on-line methods, ':: thickness values corresponding to different line load points and curve shape are determined according to a separate program sequence, whereby the line load level is automatically changed.

5 1 1 i 4 υ i

The model or function obtained experimentally, as well as any recursive update of the model, can be calculated, for example, in an automation system or in a separate accounting unit directly connected to it.

An example of a compressibility curve in Fig. 1 to the right is the thickness of the paper as a function of the line load. When making a line load change Δι kN / m, the paper thickness Δι pm changes. Similarly, if a paper thickness change Δι pm is desired, a change in the line load at the corresponding line load level must be made Δι kN / m. Figure 1 to the left shows the thickness difference profile in the transverse direction in the various CD zones N. In each zone, the line load 10 is optimally altered by the CD adjuster as determined by the compression curve.

From the difference between the measured thickness profile and the target profile, the CD controller provides a control message that takes into account nip / profile specific compressibility. The patterns are used to calculate the control variables 15 for each zone of the profile-adjustable roll so that the target profile in the CD direction is achieved. Unknown model parameters or compression curves are specified for the paper type being run, or the corresponding data has been retrieved from the database. For each zone i of the profile adjustable roll, the corresponding difference profile Δ; pm Zone-specific line load change requirement Δ, kN / m corresponding to the need for thickness change.

116403 6

Figure 3 is a block diagram of the structure of the CD controller. In the feedback controller CD controller, the measured thickness profile and target profile are formed into a difference profile, which is derived to the controller / calculation block. The CD Controller - / - calculation block defines 5 profile control requests (kN / m) to be forwarded to the profile adjustable roll optimization calculation. The CD Controller / Calculator block uses the paper compression model to take into account the current state of the process. The information from the compression model is coupled to the zone specific A kN / m controls either by computation or by using the compression model to determine the optimum values for the controller tuning parameters (gain K, integration time Ti and derivation time Td) n (line load level). using a gain tabulation method or other method. The zone-specific tuning parameters (K, Ti, Td) of the controller are changed so that the difference profile is minimized in one control and the adjustment is significantly accelerated. This will generate a prediction of the controls that minimize the difference profile.

15 The compression model or compression curves are determined by a separate on-line or off-line method in the automation system or by a separate unit of account directly related to it. Model information is also used to optimize the profile-adjustable roll so that the base model constants of the roll model are coagulated with point-to-point and paper type-specific compressibility information. The values of the base constant 20 influence the shape of the response pattern either by highlighting or smoothing the peak. The coefficient of the constant constant can be made using either off-line or on-line methods.

♦ · «• *« · »! ) $ t «4 i»

* I

I «·» i t t t • 4 * »» · account *

Claims (4)

116403 7 A method for adjusting the property of a paper web to be calendered in a transverse web with a calender having at least one adjustable profiling device, characterized by predetermining a mathematical model having parameters to be determined from test results or by production on-line algorithm Calendering to a variable paper property, this property is set to a target value for calendering, measured before and after calendering at a position corresponding to the position of at least one profiling member in the web, and adjusting the profiling member to achieve the set value. A method according to claim 1 for adjusting the thickness, gloss or smoothness of a paper web. The method of claim 1 or 2, wherein the profiling device is a flexural compensating calender roll. The method of any one of claims 1 to 3, wherein the model is a mathematical model in which the parameters are stored in paper-type tables.