[0002] The invention further relates to an apparatus for controlling the moisture profile of the paper web on a paper machine that comprises at least a headbox, a former, a press unit, a dryer unit and a reel.

[0003] As velocities increase, the quality of paper and runability both on the paper machine and in the further processing become more and more important variables. At the same time the number of online processes grow in paper machine concepts, both in new machines and in renovations of old paper machines. For instance, in online after-treatment, particularly in online multi-nip calendering of demanding grades extremely high requirements are set for cross machine direction, i. e. CD, and machine direction, i. e. MD, profiles of the paper. A moisture profile is one of these profiles. A poor mois- ture profile is most typically the major cause of variation in the physical proper- ties and further in the tension profile.

[0004] Currently, the moisture profile of a paper web is typically measured after the dryer section, and the moisture profile is adjusted e. g. by a steam box of the press section or later in a dryer section, at its end, or after the dryer section. The article"Optimize or compromise? The art of former section tuning", Odell M. 51St Appita Annual General Conference 1997 Proceedings, Volume 1, discloses one solution of this kind. However, this solution is not able to eliminate well enough defects in the properties of the finished paper result- ing from the combined effect of draw differences and moisture profile defects on the paper machine. The combined effect of the draws in the dryer section and the moisture profile defects, as well as drying shrinkage, pose the greatest problem. The combined effect is due to amounts of various tensile components stored in the paper, which produce defects in the stretch at break profile, ten- sion profile and tensile strength profile of the web such that the greater the draw the greater the defects in the finished paper.

[0005] The article"Kosteusprofiilin mittaus paperikoneen marassa paassa (Moisture profile measurement at the wet end of the paper machine)", Riikka Gerlander, Paperi ja Puu-Paper and Timber, vol. 82, issue 6/2000, sets forth moisture profile measurement of a paper web immediately after the press section. A measuring head traversing the paper web in the cross ma-

chine direction is employed in the measurement. The publication also refers to a possibility of controlling the operation of the steam box by means of the data collected on the moisture profile. However, said solution is not able to eliminate sufficiently well defects in the properties of the finished paper resulting from the combined effect of draw differences and moisture profile defects on the paper machine. In addition, a traversing measuring method is excessively slow for providing an effective and rapid control.

[0006] DE publication 29,923,284 discloses a paper machine, in which a paper web is conveyed from a former, through a press section, to the beginning of a dryer section such that the paper web is all the time supported by at least a roll or a band. A baby press comprising a suction roll and a press roll is arranged in connection with the end section of the former. A steam box is arranged between the former and the press section. A sensor, which measures a moisture profile of a paper web, is arranged at the beginning of the dryer section. The measurement can be used for controlling the steam box, the par- tial vacuum of the suction roll and the pressing force of the press roll. The pre- sented solution is not able to adjust the moisture profile of the paper web effec- tively and rapidly enough in order to eliminate sufficiently well defects in the properties of the finished paper resulting from the combined effect of differ- ences in paper machine draw and moisture profile defects.

[0007] FI publication 104,988 discloses a solution, in which a sur- face temperature of first drying cylinders of a dryer section and dry mat- ter/moisture content and temperature of a paper web are measured prior to first drying cylinders. The paper web moisture is controlled on the basis of these measurements. However, it would be useful to be able to control the pa- per web moisture profile more rapidly and effectively, so that defects in the fin- ished paper resulting from the combined effect of the differences in paper ma- chine draw and moisture profile defects could be eliminated efficiently enough.

[0008] Currently, one more typical manner is to overdry paper by means of the dryer section of the paper machine so as to equalize the mois- ture profile of the paper. Overdrying deteriorates the quality of the paper, and on the other hand, when the dryer section is used for overdrying the paper, the drying capacity of the dryer section is relatively limited.

[0009] US patents 4,801,809 and 5,172,005 disclose solutions, in which paper web properties are measured at several points simultaneously utilizing CCD or other camera technology. DE publication 19,830,323 discloses

measuring the thickness of a paper web such that a measuring beam is di- rected to the paper web with optical fibers and the beam from the web is di- rected to a measuring sensor with optical fibers. A plurality of optical fibers are thus arranged in parallel extending substantially across the whole paper web in the cross machine direction. Various solutions for measuring paper web prop- erties have also been set forth in FI publication 73,319, DE publication 3,336,659, WO publication 98/40727 and US patent 5,019,710. However, none of the above-mentioned publications discloses a solution for controlling a mois- ture profile of a paper web.

[0010] The object of the present invention is to provide an improved solution for controlling the moisture profile of the paper web.

[0011] The method of the invention is characterized by measuring the moisture profile of the paper web prior to drying the paper web such that measuring is carried out by a measuring device which measures a cross direc- tion profile of the paper web substantially simultaneously such that the measur- ing device measures moisture in the infrared range, whereby the measuring device comprises optical fibers, by means of which radiation is transmitted to the paper web and from the paper web at a measuring point, and a plurality of parallel measuring heads, in which the ends of the optical fibers are arranged, and the measuring heads are moved back and forth, part of the web width, perpendicularly to the travel direction of the paper web (3), and by means of said measurement the moisture profile of the paper web is controlled before drying the paper web.

[0012] Further, the apparatus of the invention is characterized in that the apparatus comprises a measuring device, which is arranged in the paper machine before a dryer section or at the beginning of the dryer section to measure the moisture profile of the paper web by infrared measurement prior to drying the paper web, the measuring device being arranged to meas- ure the cross direction profile of the web substantially simultaneously, whereby the measuring device comprises optical fibers, which are arranged to apply radiation to the paper web and from the paper web at a measuring point, measuring heads, in which the ends of the optical fibers are arranged, and an oscillation means for making the measuring heads traverse the paper web part of the way back and forth in the cross machine direction, and that the appara- tus comprises a profiling device for controlling the moisture profile of the paper

web on the basis of the measured moisture profile prior to drying the paper web.

[0013] The basic idea of the invention is that the moisture profile of the paper web is measured prior to drying and on the basis of said measure- ment alone the moisture profile of the paper web is controlled prior to drying.

Further, it is essential that the measurement is performed with a measuring device which measures the cross direction profile of the web substantially si- multaneously. The basic idea of one preferred embodiment is that the moisture profile is also measured after the drying and on the basis of said measurement the moisture profile of the paper web is controlled during the drying.

[0014] The invention has an advantage that the moisture profile can be equalized when the dry matter content is low before the paper web moves to a dryer section, and consequently, before the web will have its final struc- ture, it is possible to eliminate defects resulting from the combined effect of a possible moisture profile defect and differences in draw in the drying section.

Thus, paper web defects in tensile stiffness module and stretch at break can be eliminated, and a defect in the tensile profile can be prevented. Further, edge defects in the web, edge elongation and web flutter can be reduced. All in all, it is possible to reduce defects produced during the drying of paper, and this results in a more uniform quality of paper, better reelability and better runability also in further processing. By regulating the wet draw it is also possi- ble to control paper properties, such as tensile stiffness module and stretch at break, in a desired direction. The moisture profile having been equalized al- ready before the dryer section, there will be no need to overdry the paper in the dryer section in order to equalize the moisture profile. Hence, the solution increases drying capacity, because average moisture in the paper can be higher than previously. In addition, by measuring the paper moisture substan- tially simultaneously across the paper web, it is possible to obtain an accurate CD resolution of the measurings, which is of assistance in the web drying. The simultaneous profile measurement also makes the control faster, as compared with the traversing measurement that is currently in common use.

[0015] In the present document, the term'paper'refers not only to paper, but also to paper board and tissue.

[0016] The invention will be described in greater detail in the at- tached drawings, wherein Figure 1 is a schematic side view of a paper machine;

Figure 2 is a schematic view of a measuring device for measuring paper web properties, seen in the travel direction of the paper web; Figure 3 is a schematic side view of a press section and beginning of a dryer section of the paper machine; Figure 4 is a schematic side view of the press section and the dryer section of the paper machine; Figure 5 is a schematic view of a spectrometer.

[0017] Figure 1 shows schematically a paper machine. The paper machine comprises a headbox 1, from which stock is fed onto a former 2, where the stock is formed into a fiber web 3. The fiber web 3 is conveyed to a press unit 4 and further to a dryer unit 5. From the dryer unit 5 the web is con- veyed to a reel 6. The paper machine may also comprise other parts, such as size presses and a calender, which are not illustrated in Figure 1 for the sake of clarity. Furthermore, the function of the paper machine is known per se to the person skilled in the art, and will therefore not be further explained in this connection.

[0018] The paper machine further comprises a first measuring de- vice 7, which is arranged to measure a cross direction profile of moisture in a moving paper web substantially simultaneously prior to paper web drying. The first measuring device 7 is arranged in the vicinity of the press unit 4, for in- stance at the beginning of the dryer unit 5 or immediately after the press, as indicated by broken lines in Figure 1. The first measuring device 7 can also be placed in a location before the press section, between the former and the press section or immediately at the beginning of the press section before a first press roll nip, which locations are not shown in Figure 1 for the sake of clarity.

A second measuring device 8 is arranged after the dryer unit 5 for measuring the moisture profile after the drying. In connection with the press unit 4, there is further a steam box 9, by which the moisture profile of the paper web can be controlled.

[0019] Figure 2 shows schematically the structure and operating principle of the first measuring device 7. The first measuring device 7 com- prises a radiation source 10, which emits radiation most preferably in the infra- red range. The wavelength of the radiation can be e. g. within 1 to 2.5 pm, but when necessary, the wavelength can also be outside said range. The radiation source 10 can be a halogen lamp or a black body radiator or any other radia- tion source suitable for the purpose.

[0020] After the radiation source 10, there is arranged a chopper 11, which interrupts radiation emitted by the radiation source 10 in a manner known per se. The radiation is directed to the paper web with first optical fibers 12. The radiation reflected from the paper web 3 is forwarded with second op- tical fibers 13 to a spectrometer 14. The ends of both the first optical fibers 12 and the second optical fibers 13 are arranged in measuring heads 16. Neces- sary optical measuring accessories, such as lens or mirror arrangements or the like, can additionally be arranged in the measuring heads 16.

[0021] From the spectrometer 14 the measurement data is con- veyed to a data processing or control unit 15. The spectrometer 14 is an imag- ing spectrometer, i. e. it images the spectrum of a measuring point. In this con- nection, the spectrometer refers to a measuring device, which comprises at least a spectrograph 27, preferably an imaging spectrograph, and a matrix de- tector 28. The spectrograph splits the radiation conveyed by the fiber into dif- ferent wavelengths for the matrix detector 28. The imaging spectrograph 27 can be e. g. a PGP-type spectrograph or a grating spectrograph or any other spectrograph suitable for the purpose. Because measurement data are con- veyed with second optical fibers 13 from a plurality of measuring points simul- taneously, the apparatus determines the infrared spectrum reflected from the paper web 3 at a plurality of measuring points simultaneously. The apparatus thus provides a matrix, which describes the location and spectrum data of each measuring point. Most preferably the spectrum is measured in the near- infrared range, for instance within the range of 1.0 to 1.7 lim or 1.0 to 2.4 pm.

Infrared light is thus split into a spectrum with the spectrograph 27, and the split light is measured with the matrix detector 28. Figure 5 shows schemati- cally the spectrograph 27 and the matrix detector 28. The spectrograph 27 re- ceives data from each optical fiber at a particular point on a location axis. The data at each point is dispersed onto the matrix detector 28 at a particular point on the location axis such that the light is spread onto a spectrum axis. In Fig- ure 5, indications B1 and 2 illustrate the longest and shortest wavelength of the measuring zone.

[0022] Each measuring head 16 and optical fiber 13 constitute an individual measuring channel and it is possible to arrange as many parallel measuring channels as necessary. If channel resolution is e. g. 5 mm in the cross machine direction, and a 10-metre wide paper web 3 is to be covered substantially throughout, 2,000 measuring points, i. e. measuring channels, will

be needed. However, the measuring heads 16 can be made to move back and forth part of the web 3 in CD by an oscillation means 17. By moving the meas- uring heads 16, for instance, for about 10 cm back and forth, the above- mentioned 10-meter wide paper web can be measured substantially through- out by utilizing 100 measuring channels. Such a short movement does not cause considerable mechanical stress to the fibers 12 and 13. Hence, it is possible to reduce the number of measuring channels considerably, and nevertheless, the measuring can be carried out in less than one second for the entire width of the paper web 1, as compared with the current situation, where traversing from one edge of the paper web to the other edge typically takes tens of seconds. By arranging the distance of the reciprocating movement to be slightly larger than the width of the channel, i. e. in the above case to be slightly larger than 10 cm, the adjacent measuring points also measure the same transversal position of the paper web 3, i. e. the measuring channels overlap. This measurement of the common point can be utilized, for instance, in making the measuring channels uniform, or even in transferring configura- tion.

[0023] The measuring apparatus also comprises a moving refer- ence unit 29, which can be moved across the path of the measuring beams of all measuring channels. In that case, the same reference/calibration plate is used for calibrating all measuring channels. In the reflection measurement shown in Figure 5, the calibration can be performed during the paper making process, i. e. the reference unit 29 is arranged to move between the measuring heads 16 and the paper web 3.

[0024] The measuring device 7 can be arranged in a relatively small space, because it will suffice that only the optical fibers 12 and 13 and the measuring heads 16 are arranged in a measuring bar required by the measur- ing device. The measuring arrangement of the measuring head 16 can also be implemented by mirror optics, whereby the arrangement will be particularly small in size. Other parts of the apparatus, such as the light source, the chop- per, the spectrometer and the control unit, can be placed separately in a suit- able location in the vicinity of the paper machine, with the proviso that the fi- bers retain their connection with said devices. The measurement can be per- formed either as reflection measurement as shown in Figure 2 or the second optical fibers 13 can be arranged on the reverse side of the paper web 3, whereby the second optical fibers 13 convey the measuring beam, which has

penetrated the paper web 3, to the spectrometer 14. The measurement can also be performed either when the web is against a wire, felt or roll, or in open draw. Instead of the imaging spectrometer 14, it is possible to employ e. g. an infrared line camera and to separate the wavelengths of the measuring points with interference filters placed in a rotating filter disc, for instance. With the measuring device 7 it is possible to measure, for instance, moisture, dry matter content, ash content or some other property of the paper web 3.

[0025] Figure 3 shows how the measuring device 7 is placed at the beginning of a dryer unit 5. Figure 3 shows some of the drying rolls, i. e. steam- heated cylinders 18 and vacuum cylinders 19, of the drying unit 5. The dryer unit 5 typically comprises several tens of steam-heated cylinders 18 and vac- uum cylinders 19. Figure 3 further shows a drying fabric 20. In principle, the measuring device 7 can be placed at any point in the dryer unit 5. However, if the paper web moisture is to be measured prior to drying, the measuring heads 16 of the measuring device 7 are most preferably arranged in connection with the first vacuum cylinder 19, as shown in Figure 3. Figure 3 also shows an al- ternative location, indicated by a broken line, for the measuring device 7 after the second vacuum cylinder 19. Even though the moisture measurement is not arranged immediately after the press 4, one or three drying rolls do not bring about any essential change in the moisture of the paper web 3, and therefore it can be defined that in both cases the paper web is measured for its moisture before it is dried. Inside the vacuum cylinders 19 there is vacuum, which makes the drying fabric 20 and the paper web 3 thereon adhere to the vacuum cylinder 19. The measurement is thus performed on top of the wire 20 and the roll 19, i. e. the paper web 3 is supported, and there is no need to arrange open draw for the measurement. The distance between the measuring heads 16 and the paper web 3 can be in the order of 10 cm.

[0026] Figure 4 shows the principle of moisture control concept. The system comprises a control unit 21, whereto CD moisture data on the paper web 3, prior to drying, is transmitted from the first measuring device 7. On the basis of said measurement the moisture profile of the paper web 3 is controlled before drying the web. Said moisture profile control can be carried out, for in- stance, by means of a steam box 9 arranged in connection with the press unit 4. For controlling the moisture profile, it is also possible to use an infrared heater 22 or a microwave dryer 23, for instance. Furthermore, it is possible to control the press section 4 itself. If the moisture profile measurement is carried

out before the press section 4 or at the beginning of the press section 4, the moisture profile control is performed using feedforward control, for instance with a steam box 9 or other profile actuators of the press section 4.

[0027] The moisture profile control takes place such that the CD moisture profile of the paper web 3 having been obtained, the profile control device directs an operation to an excessively wet section of the paper web 3, for instance, which operation affects exactly said excessively wet section of the paper web 3. For instance, for the steam box 9 said control operation consists of increasing the amount of steam in said zone, whereby evaporation in- creases and the moisture profile can be rectified. The nip pressure of the press can also be controlled zone by zone, if the press comprises a roll enabling zone control. Also other above-mentioned profile control devices and, in addi- tion, e. g. an impinged blowing unit in the dryer section can be controlled zone by zone.

[0028] Because the measurement is carried out with the first meas- uring device 7, which measures the cross direction profile of the whole paper web substantially simultaneously, and because the moisture profile of the pa- per web prior to drying is controlled by means of said measurement, it is pos- sible to eliminate the defects in the finished paper resulting from the combined effect of drawing differences and moisture profile defects on the paper ma- chine. Most preferably the moisture profile control takes place the dry matter content of the web being 40 to 70 %. Because the measurement takes place rapidly and the profiling devices correcting the moisture profile of the paper web are most preferably arranged in the vicinity of the press unit 4, the mois- ture profile can be corrected without that a sudden disturbance in the moisture profile passes through the dryer unit 5 to the second measuring device 8. It is also possible to correct the moisture profile by means of the impinged blowing unit 24, in which case the impinged blowing unit 24 is most preferably arranged at the beginning of the dryer unit 5, when the paper web 3 is not yet completely dried. All in all, the control of the profiling device, i. e. the device correcting the moisture profile of the paper web 3, is not affected by control distorting factors, such as the dryer unit 5, and an equalized or smooth moisture profile can be achieved before the dryer unit 5. This also reduces, or even eliminates, a need to overdry the paper.

[0029] Most preferably the moisture profile is still corrected at a second stage by measuring the moisture profile of the paper web 3 with a sec-

ond measuring device 8 after the dryer section 5. On the basis of the moisture measurement of the paper web 3 performed after the dryer section 5, the mois- ture profile of the paper web is controlled in the dryer unit 5. Adjustable vari- ables of the dryer section, such as steam pressure of steam-heated cylinders of the dryer section or steam temperature or operation of vacuum cylinders, can then be adjusted, or the moisture profile control can employ a wetting box 25 or a steam box 26 arranged in the dryer unit 5. When the moisture profile of the paper web is also measured after the dryer unit 5, and the apparatus in the dryer section 5 for wetting or drying the paper web 3 is controlled on the basis of said measurement, it is possible to eliminate moisture profile defects and their effects appearing in the dryer section 5.

[0030] The drawings and the relating specification is only intended to illustrate the inventive idea. The details of the invention may vary within the scope of the claims. So, if desired, the moisture profile measurement of the paper web can be carried out with a measuring device other than the one de- scribed herein. It is substantial that the whole cross direction profile of the web can be measured substantially simultaneously. The invention is particularly well suited for the control of the CD moisture profile of the paper web.