WO1999066125A1

WO1999066125A1 - Method for regulation of a roll adjustable in zones

Info

Publication number: WO1999066125A1
Application number: PCT/FI1999/000515
Authority: WO
Inventors: Pekka Kivioja; Juha Ehrola
Original assignee: Valmet Oy
Current assignee: Valmet Technologies Oy
Priority date: 1998-06-15
Filing date: 1999-06-14
Publication date: 1999-12-23
Anticipated expiration: 2000-12-15
Also published as: AU4784399A; FI981385A7; FI111744B; FI981385A0; DE19983348T1

Abstract

The invention concerns a method for regulation of a roll adjustable in zones, which roll (10) adjustable in zones forms a nip (N) with a backup roll (20). A paper or board web (W) is passed through the nip (N) in view of controlling the cross-direction profile of the web (W), and the nip (N) is loaded adjustably by means of hydraulic glide shoes (13) which form the zones in the roll (10) adjustable in zones. In the method, the cross-direction profile of the web (W) is measured constantly after the nip (N), and a differential profile between the measured cross-direction profile and the target cross-direction profile is computed, which differential profile is used as a regulation parameter in a closed regulation circuit so as to regulate the nip profile to the desired form. In the method in accordance with the invention, a regulation model is used in which substantially all the forces applied to the roll mantle (12) are formed as models each of them separately so that these forces are also treated each of them separately in the regulation process.

Description

Method for regulation of a roll adjustable in zones

The invention concerns a method for regulation of a roll adjustable in zones, which roll adjustable in zones forms a nip with a backup roll, through which nip a paper or board web is passed in view of controlling the cross-direction profile of the web and which nip is loaded adjustably by means of hydraulic glide shoes which form the zones in the roll adjustable in zones, in which connection, in the method, the cross- direction profile of the web is measured constantly after the nip, and a differential profile between the measured cross-direction profile and the target cross-direction profile is computed, which differential profile is used as a regulation parameter in a closed regulation circuit so as to regulate the nip profile to the desired form.

In paper machines and in paper finishing devices, a number of rolls are used which form a dewatering press nip, an equalizing nip, a size-press nip, a calendering nip, or equivalent with a backup roll. In such sites of operation and applications of operation, it is important that the distribution of nip pressure, i.e. the nip profile in the axial direction of the rolls, remains invariable or that this profile can be adjusted in the desired way, for example, in view of controlling the cross-direction moisture profile or thickness profile or any other, corresponding property profile of the web. For regulation of the nip profile, various adjustable-crown and variable-crown rolls are used, in particular rolls adjustable in zones, by whose means the nip profile can be controlled in a suitable way. Currently, as such variable-crown rolls or rolls adjustable in zones, most commonly such rolls are used as comprise a stationary roll axle and a roll mantle arranged revolving around said axle as well as hydraulically loaded glide shoe arrangements fitted between the roll mantle and the axle and acting upon the inner face of the mantle, by means of which glide shoes the axial profile of the mantle at the nip can be aligned or regulated in the desired way. In a roll adjustable in zones, two or more adjacent glide shoes always form one zone, which is controlled by means of a zone-specific hydraulic valve. The number of the glide shoes included in different zones can differ from zone to zone in a way required by a purposeful control of the compression force between the zone roll and its backup roll.

The constructions of rolls adjustable in zones have been developed so that, in principle, by their means, it would be possible to provide a very precise regulation of the profile if the system of control of the roll were capable of controlling the roll in the desired way. On a level of principle, regulation of the nip profile by controlling a roll adjustable in zones is quite simple, for, firstly, the starting values or initial values in compliance with which the nip profile must be regulated are determined by the paper grade that is aimed at. The zones in the roll are then regulated and controlled in a purposeful way in compliance with these target values. After the nip, it is then measured from the paper web how successful the regulation has been, i.e. how well the values aimed at have been achieved by means of regulation of the roll. When measured values differ from the target values, the values of regulation of the zones in the roll are changed so that, after this correction, the measured values correspond to the target values with an optimal and adequate accuracy. Thus, the principle is quite simple. However, ever higher and higher requirements are constantly imposed on the quality of paper, and this is why it is also necessary to be able to improve the accuracy of regulation of a roll adjustable in zones and in particular of the system of control of the roll.

In regulation of a multi-zone roll, when conventional models of regulation have been used, it has been a considerable factor limiting the accuracy of regulation that the system of regulation has not been sufficiently well, or actually at all, capable of taking into account that there are cross-effects in different zones of a roll adjustable in zones, or that the effects of different zones are not equal, among other things, owing to different numbers of shoes in the zones and owing to locations of the zones. Moreover, in this way, a control of the physical limitations of the actuators to be monitored has become difficult. Earlier, for improving the accuracy of regulation, besides the regulation in zones, regulation by means of temperature has also been employed so that the roll has been heated from outside and locally in the axial direction of the roll. Such use of differences in temperature for regulation of the profile and especially the effect of this regulation was reasonably local, and this regulation by means of temperature did not have significant cross-effects. Thus, with this regulation by means of temperature, it was locally possible to correct and to adjust the effects of regulation in zones. However, even this mode of regulation did not provide an adequate accuracy of regulation, and in view of simplification of the regulation, it was desirable to get rid of such temperature regulation.

Earlier, attempts were made to make the models of regulation as simple as possible, however, so that an adequate accuracy could be achieved by their means. Thus, in earlier models of regulation, beams were used in order to illustrate the conduct of rolls that are in nip contact. In a beam model arrangement, consideration is given to the length dimension of the piece and to certain properties of rigidity, whose function is to represent the cross-section of the piece. Such a model of regulation also operates reasonably well when it is not necessary to employ extensive profiling and when the nips are not operated in curved form. This is why, in such a model of regulation, it has been necessary to adopt certain assumptions and simplifications, which are not correct in reality. These include, for example, the effects of bearing forces on the nip load, the effects of backup zones on the nip load, and computing of the stresses of the roll mantle. Among simplifications connected with earlier models and routines of computing, it can be stated, among other things, that in the prior art it was assumed that the direction of a force has no significance for the magnitude and the form of the response. Likewise, earlier, in respect of stresses exclusively the portion arising from bending was taken into account. Out of these reasons, in some cases, the regulation had operated entirely illogically, and it was not possible to take advantage of the whole potential of the mantle in the profiling. Further, in prior-art computing routines, for example the pressures present in the same sections, the stresses effective in the same direction, etc. were given certain limit values, between which said value had to remain. In the prior art, in optimizing of regulation, linear limiters were used in the pressures of the hydraulic system. The object of the present invention is to provide a significant improvement over the prior-art methods for regulation of the nip profile by means of a roll adjustable in zones. In view of achieving this objective, the method in accordance with the invention is mainly characterized in that, in the method, a regulation model is used in which substantially all the forces applied to the roll mantle are formed as models each of them separately so that these forces are also treated each of them separately in the regulation process.

In the formation of models, elements are used as an aid by whose means it is possible to take into account local deformations, for example shell elements and/or solid elements.

As compared with the prior-art methods of regulation, the present invention provides significant advantages, and of these advantages it is possible to state, among other things, that, by means of the method of regulation in accordance with the present invention, in the regulation of rolls adjustable in zones, a considerably better accuracy is achieved, as compared with earlier solutions, because the model of regulation on which the method of regulation in accordance with the invention is based corresponds to the factual conduct of the constructions better. Besides improved accuracy, it is a further advantage that, when the method of regulation in accordance with the invention is used, it is possible to carry out extensive profiling reliably, and, further, the nips can be operated in curved form. Such curved nips are used commonly, for example, in soft calenders and in OptiLoad™ calenders. An OptiLoad™ calender is described, for example, in the US Patent 5,438,920. The further advantages and characteristic features of the invention will come out from the following more detailed description of the invention.

In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing.

Figure 1 is a schematic illustration of a pair of rolls which comprises a roll adjustable in zones and a backup roll and which pair of rolls forms a nip. Figure 2 is a schematic illustration of a closed circuit of regulation for use in regulation of the nip profile.

In the figures in the drawing, the roll adjustable in zones has been denoted generally with the reference numeral 10. The roll 10 comprises a stationary roll axle 11, on which a tubular roll mantle 12 has been fitted revolving. The roll mantle 12 is supported on the roll axle 11 by means of hydraulic glide shoes 13 acting upon the inner face of the roll mantle, by means of which glide shoes the nip N is loaded adjustably. In the figure, it is shown further that the roll 10 is also provided with backup shoes 13a, which act in a direction opposite to the glide shoes 13 in the nip plane. The glide shoes 13 have been fitted in a certain way in groups and divided into zones Zi ...Z₄ in the axial direction of the roll 10 so that each zone is regulated separately. Thus, in Fig. 1, four zones have been illustrated, but the number of the zones is by no means confined to this number. Similarly, it is shown in Fig. 1, that the backup shoes 13a have also been fitted in groups in the zones Za₁...Za₄, each of which zones is regulated separately. These backup zones Za! ...Za₄ are not needed in all cases, and, on the other hand, if backup zones are employed, their arrangement and number can differ from the arrangement and number of the zones proper. Further, it is shown in Fig. 1 that the roll 10 adjustable in zones is additionally provided with end bearings 14, by whose means the roll mantle 12 is supported revolvingly on the roll axle 11. In Fig. 1 , the end bearings 14 are illustrated fully schematically. As the end bearings 14, it is possible to use either conventional roller bearings or, in their place, the end bearings 14 can be accomplished as modern glide bearings. In order that the regulation of the profile in the nip N could be carried out reliably over the entire axial length of the nip, it is, however, important that the bearings 14 have been accomplished so that they permit radial movement of the roll mantle 12 also in the areas of said bearings.

In the figures in the drawing, the reference numeral 70 denotes a regulation and setting device, which communicates, through pressure ducts 15, with the glide shoes

13 in each zone Z_j ...Z₄ and, similarly with the backup shoes 13a in each backup zone Za_j ...Za . As is shown in the figures, the roll 10 adjustable in zones forms a nip N with the backup roll 20. The nip N can be, for example, a press nip, an equalizing nip, a size-press nip, a calendering nip, or equivalent. As is shown in Fig. 2, the paper or board web W has been passed through said nip N.

Fig. 2 illustrates a closed circuit of regulation, which is used in compliance with the method in accordance with the present invention for regulation of the nip profile. The linear load in the nip N is regulated by means of regulation of the set pressures passing to the hydraulic glide shoes 13 and backup shoes 13a in the roll. As is shown in Fig. 2, a paper or board web W or equivalent has been passed to run through the nip N, and by means of said nip, which is, for example, a calendering nip, attempts are made to produce a certain effect in the web W that runs through the nip N. In the case of a calender, attempts are made to keep the distribution of the linear load in the nip, i.e. the profile in the axial direction of the rolls, as uniform and invariable as possible. Thus, into the system, the data have been fed concerning the nip profile aimed at, i.e. the effect that is supposed to be produced by means of the nip N in the web W that runs through the nip.

On the run of the web W after the nip N, a measurement equipment 30 has been fitted, by whose means the cross-direction profile of the web W is measured con- stantly in order to find out how well the cross-direction profile aimed at is achieved by means of the set values that have been fed into the zones Z_j.. ^ and backup zones Za_j...Za₄ in the roll 10 adjustable in zones. The equipment 30 for measurement of the cross-direction profile can comprise, for example, a measurement beam passing across the web W, on which beam measurement detectors have been installed with a specified spacing in the cross-direction of the web so as to measure the cross-direction profile. The measurement equipment can also comprise a measurement head or equivalent traversing back and forth across the web W. Thus, as a result of the measurement of the cross-direction profile, information is obtained concerning the measured profile, i.e. the factual profile 40. This measured factual profile 40 is then passed as input data 41 into the computing unit 50 included in the system, into which unit the data concerning the target cross-direction profile have been fed in advance. The computing unit 50 compares the measured profile with the target profile and, on this basis, performs a calculation in order to form an error profile or a differential profile, i.e. in order to find out to what extent the measured profile differs from the target profile. Further, based on the differential profile thus determined, this computing unit 50 carries out a computation in order to regulate the linear load so that, in the nip N, the cross-direction profile aimed at could be achieved based on the computed regulation parameter.

The target 51 of linear load or the regulation parameter is, however, not transmitted directly to the regulation and setting device 70 that regulates the set pressures in the zones Z₁...Z and in the backup zones Za_j ...Za , but this target of linear load, i.e. regulation parameter computed in compliance with the differential profile, or a corresponding control signal is fed into an optimizing part 60, which is included in the closed regulation circuit and which contains an optimizing routine. The optimizing part contains advance information on the profile responses of the actuators and on the physical limitations of the actuators and of the whole roll 10, and so also on factors of non-linearity related to the roll. Further, the optimizing part contains information on the cross-effects of different actuators, i.e. on the way in which a change in the settings of one actuator acts upon the settings of some other actuator. In the novel and inventive system, non-linear limitations are included, which take into account, among other things, in what direction, for example, a pressure acts (towards the nip or away from the nip). Likewise, stresses are taken into account, above all stresses in the roll mantle and the directions of the stresses. If the directions of effect are not taken into account but the operation takes place exclusively with the aid of the limit values of the parameters, a regulation carried out on this basis can be fully erroneous, in particular if the need of regulation is large. The novel model of regulation, which is based on creating a model of a roll adjustable in zones by means of shell elements, corresponds to the factual conduct of the roll mantle better. In this regulation model, all forces applied to the roll mantle 12 have been formed as separate models in order that they could also be treated separately. In order that the regulation of the multi-zone roll should be as accurate as possible, certain factors of non-linearity must be taken into account when the distribution of linear load is computed. Such forces operating in a non-linear way are, among other things, bearing forces whose direction is changed. A change in the direction of bearing forces comes out quite well, for example, in supercalenders, in which running takes place with a what is called zero load or very close to this load. Computing of the stress level of the roll mantle at each time of optimizing also requires a non-linear computing model. When a roll adjustable in zones is being regulated, forces that act in the same section but at the opposite side of the roll mantle 12 in relation to the nip N have different linear-load responses, and a linear computing model cannot deal with this. Likewise, if it is desirable to control the stresses in the roll mantle (combined stress) during running, the optimizing must be capable of controlling a non-linear stress equation during the optimizing.

The novel optimizing routine, which is employed with the novel computing model, is, besides linear limitations, also capable of dealing with non-linear limitations. One such force that operates in a non-linear way is, for example, a bearing force, as was already stated above, whose response is changed in accordance with whether the direction of the force is towards the nip or away from the nip. In optimizing, such a force is illustrated so that just one of the forces can be different from the minimum, for example, in the following way:

(Pi ^" Plmin) (P2 ^" P2min) = °

wherein:

P_j = pressure at the nip side

P_lmin ⁼ minimum pressure at the nip side p₂ = pressure at the side opposite to the nip P₂min ^{= m}iⁿi ^um pressure at the side opposite to the nip. As regards limitations in stresses of the mantle, a non-linear property has also been employed. For the roll mantle, a maximum value of combined stress has been determined, which value must not be surpassed by the result of optimizing. During optimizing, the limitation is computed at each time of iteration by means of the formula of combined stress:

ø_j + σ₂

wherein:

σ_j = axial stress σ₂ = radial stress.

Thus, into the optimizing part 60, in addition to the information on the target 51 of linear load computed in accordance with the differential profile, the necessary data 52 are fed, among other things, concerning the bearing forces, the pressures in the backup zones Za_j ...Za₄ and their effects on the linear load, etc. Based on these data, the optimizing part 60 computes new set values at the same time for all actuators in order to minimize the overall error of cross-direction profile and the differential profile as well as determines the control necessary for the differential profile at each particular time. Thus, in this computing, the optimizing part 60 takes into account the cross-effects of all actuators, among other things of the zones Z_j ...Z₄ in the roll 10, the effects of the backup zones Za^ . -Za^ the effects of bearing forces, etc. and the physical limitations of the actuators, besides the linear limitations also the nonlinear limitations. Thus, the optimizing part attempts to take maximal advantage of the whole of the profiling potential of the roll 10 adjustable in zones.

The new set values of the actuators are transmitted as input data 61, i.e. as regulation parameters, to the regulation and setting device 70 for regulation of the set pressures for the zones and backup zones. In the non-linear regulation model in accordance with the invention, the response of a regulation to a regulation parameter is very precisely known, for which reason, depending on the optimizing routine, i.e. on the quality of the model employed by the optimizing, the speed of regulation can be increased considerably in this way. In the method in accordance with the inven- tion, the required movement of the actuators placed at the regulation locations is determined so that the known profile response of each regulation location, i.e. the change in the cross-direction profile caused by a movement of the actuator, is compared with the differential profile prevailing at said regulation location and that corresponding regulation impulses are given to the actuators, while taking into account the cross-effects, physical limitations and, in particular, non-linear limitations of the actuators so as to correct the differential profile. Besides regulation taking place during running, by means of this method it is possible, among other things, to predict a need of profiling after a break and, thus, to shorten the time of recovery after the break. In the closed regulation circuit shown in Fig. 2, measure- ment of the profile and regulation carried out on the basis of the measurement are carried out during running constantly and without interruption.

Above, the invention has been described by way of example with reference to the figures in the accompanying drawing. The invention is, however, not confined to the exemplifying embodiments illustrated in the figures alone, but different embodiments of the invention can show variation within the scope of the inventive idea defined in the accompanying patent claims.

Claims

1. A method for regulation of a roll adjustable in zones, which roll (10) adjustable in zones forms a nip (N) with a backup roll (20), through which nip a paper or board web (W) is passed in view of controlling the cross-direction profile of the web (W) and which nip (N) is loaded adjustably by means of hydraulic glide shoes (13) which form the zones (Z₁...Z₄) in the roll (10) adjustable in zones, in which connection, in the method, the cross-direction profile of the web (W) is measured constantly after the nip (N), and a differential profile between the measured cross- direction profile and the target cross-direction profile is computed, which differential profile is used as a regulation parameter in a closed regulation circuit so as to regulate the nip profile to the desired form, characterized in that, in the method, a regulation model is used in which substantially all the forces applied to the roll mantle (12) are formed as models each of them separately so that these forces are also treated each of them separately in the regulation process.

2. A method as claimed in claim 1, characterized in that, in the formation of models, element are used as an aid by whose means it is possible to take into account local deformations.

3. A method as claimed in claim 2, characterized in that the elements are shell elements and/or solid elements.

4. A method as claimed in any of the preceding claims, characterized in that, in the regulation model, an optimizing routine is employed in which a regulation parameter which represents said differential profile is fed into an optimizing part (60) of the system, into which optimizing part information is fed constantly also concerning the profile responses, cross-effects, and linear and non-linear limitations of the actuators in the roll (10) adjustable in zones, in which connection the optimizing routine computes new set values for the roll adjustable in zones in order to minimize the overall error of the cross-direction profile and the differential profile while taking into account the limitations fed into the optimizing routine, in which connection the set values that have been computed are fed as regulation parameters to the regulation devices that regulate the nip profile so as to correct the cross- direction profile.

5. A method as claimed in any of the preceding claims, characterized in that, in optimizing, out of limitations of regulation, their non-linear properties are used.

6. A method as claimed in any of the preceding claims, characterized in that, during optimizing, each non-linear limitation is constantly computed again and is taken into account when set values are fed to the actuators of the roll (10) adjustable in zones.

7. A method as claimed in any of the preceding claims, characterized in that the computed set values are fed to the regulation and setting device (70) for the glide shoes (13) and for possible backup shoes (13a), if any, in the roll (10) adjustable in zones in view of regulation of the set pressures for the zones (Z₁...Z₄) and for the backup zones (Za_j ...Za ), respectively.

8. A method as claimed in any of the preceding claims, characterized in that the required movement of the actuators placed at the regulation locations is determined so that the known profile response of each regulation location and the profile response that is derived from the non-linear limitations that are being computed constantly during optimizing are compared with the differential profile prevailing at the regulation location concerned, and a corresponding regulation impulse is given to the actuator in view of correcting the differential profile.