From the applicant's earlier FI Patent No. 80,522, a solution is known in which the pressure between a curved measurement rib and a web and, on its basis, the tension of the web are measured. The pressure between the curved rib and the web is passed to a pressure measurement detector, in which connection, by means of a pressure- difference detector operating as the measurement detector, the pressure between the measurement rib and the web is measured from a so-called air cushion.

From the applicant's earlier Patent Application FI-930366 an arrangement of a measurement device is known in which the cross-direction variation in the web tension can be changed to the desired level. The web is made to run over a so-called airborne rib, which consists of a curved face, in which connection the air pressure formed between the airborne rib and the web is measured, and, on the basis of said measurement values, positions signals are passed to an actuator, in which connection the cross-direction variation in the web tension can be changed to the desired level.

However, the measurement of tension is not used for regulation of the difference of draw in the web in any of the prior-art solutions described above.

Determination of the difference of draw has taken place mainly empirically. In such a case, for example, in connection with a free draw, visual observation has been used for determination of the difference of draw. Separation of the web in connec- tion with a free gap, for example, from the centre roll in a press section has been observed.

In the present patent application, it is suggested that the difference of draw be determined and regulated based on measurement of the tension of the web.

In an embodiment of the invention, the measurement of tension takes place by means of a so-called weighing roll. At the ends of the weighing roll, the loading of the roll is measured by means of detectors, and, when the load becomes higher as the web loads the roll with a certain tension, the web tension can be computed directly from the increase in loading measured by means of the detectors. In an embodiment of the invention, measurement of the web tension is based on measurement of the pressure in an air space between the web and a backup face.

Thus, in the commonest embodiment of the invention, the difference of draw is regulated based on the result of measurement of the web tension. The difference of draw is preferably regulated in a paper/board machine by regulating the motors of the drives at the inlet side and outlet side of a free gap. Advantageously, the measurement is carried out between the drives at the inlet side and outlet side of a so-called free gap, i.e. a gap not supported by a wire. For measurement of the web W tension, it is possible to use a weighing roll, in the way described above, in which connection the load applied by the web to the roll is observed by means of detector devices of the weighing roll, and on this basis the tension of the web at the roll is determined. In the way described above, measurement of the web tension can be based, for example, on measurement of a pressure which is measured between a backup face in the free gap, for example a roll, and the web. Thus, it has been realized to use measurement of pressure for fitting together of the web tension profile and the difference of draw. From the air space between the web W and the measurement face, the air pressure is measured, and by means of said pressure data the draw difference of the web is regulated so that the desired tension is obtained for the web. The measurement face can be a separate face of a measurement rib or the face of a separate roll. In such a case, the pressure is passed from between the roll face and the web into a pressure space placed in the interior of the roll and further to a measurement detector, which may be placed separately or in connection with said pressure space in the roll. The roll may be revolving or non-revolving. The

pressure can be measured from each hole separately (tension profile in the cross direction of the machine), or several holes can communicate with one measurement detector.

When a non-revolving roll is used, the construction of the measurement device may consist of a roll in which the measurement chamber is fitted in the interior of the roll and holes have been fitted to be opened into the roll interior over a certain sector across the entire width of the sector. In such a case, the measurement pressure can be obtained from a wider area, and a better average value can be obtained from the measurement. When a revolving roll is used, it is enough that perforations have been made through the roll mantle into the measurement chamber in one row in the longitudinal direction of the roll.

When the web has a speed V1 in the first nip and a speed V2 in the second nip, the relative difference in speed that is obtained is: <BR> <BR> <BR> <BR> <BR> V2 - V1 <BR> <BR> V2 - V1 V1 When the relative difference in speed E becomes higher, the tension produced in the web, i.e. the web tension T, also becomes higher. With a certain value of e the web is broken, and, on the other hand, when E is too low, i.e. the difference in speed is not sufficient, fluttering arises in the web.

The web tension T is formed by the sum T1 + T2, wherein the tension T1 is com- puted from the formula: T P= R

wherein P is the pressure applied to the web, and R is the curved face over which the web is passed, for example a roll used for guiding the web. T2 is composed of a mass and speed term, in which M is the basis weight of the web and V is the speed of the web. Thus, for the overall expression, the value T = P R + MV2 is obtained.

When the speed of the web and the pressure applied to the web are known, from the above formula it is possible to compute the overall tension of the web, and, based on said computed value, if necessary, it is further possible to adjust the difference in speed, i.e. it is possible to act upon the values V1 and V2 in the vicinity of the measurement point.

In a preferred embodiment of the invention, the tension of the web is estimated by passing the web over a curved measurement face, for example a roll, and by measuring the pressure P present between the web and the measurement face. Based on the measured pressure, the tension of the web is detected at the measurement point and preferably, in a paper/board machine, at a free and unsupported draw of the web, and based on the measurement data thus obtained, the difference of draw between the inlet side and the outlet side of the free gap is regulated, for example, by regulating the drives of rolls at the inlet side and at the outlet side.

In the present patent application, the invention is not supposed to be confined exclusively to such regulation of a difference of draw and, thereby, to such regula- tion of the web tension in which the measurement device is placed exclusively in connection with a roll. As was described above, the web may also be made to run over a curved rib face or equivalent, which rib face has a certain curve radius and from which rib face flow openings are opened through the rib across the width of the web, said openings passing into a pressure monitoring space placed at the opposite side of the rib.

In the solution in accordance with the invention, the web tension is regulated, on the basis of the pressure measured at a free draw (unsupported draw) of the web

between a measurement face and the web, by regulating the difference in speed between the drive motors placed at both sides of the free gap.

In accordance with the invention, the difference of draw is regulated preferably at the drives between the third press and the fourth press, between which presses the web has a free draw (an unsupported web draw), and favourably said difference of draw can also be regulated between the fourth press and the first drive motor of the dryer section, between which drives there is also a free draw of the web.

Thus, based on the difference in pressure between the web and a measurement face, the web tension is measured, and based on said data on tension, the difference of draw is regulated preferably between a drive that precedes a free draw of the web and a drive following after said free draw by regulating the speeds of rotation of the motors of said drives.

The method and the equipment in accordance with the invention are characterized in what is stated in the patent claims.

The invention will be described in the following with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying draw- ings, the invention being, yet, not supposed to be confined to said embodiments alone.

Figure 1 A illustrates a principle of measurement, wherein the web is made to run along the measurement face, preferably the face of a measurement rib, and in the solution of equipment the pressure is measured from the air space placed between the web and the face of a measurement rib.

Figure 1B illustrates a second solution of the mode of measurement, wherein the pressure is measured from between the web and the face of a roll while the roll comprises perforations through which the pressure is passed to the detectors.

Figure 1C shows such a roll for use in measurement as comprises perforations provided in the roll face on a certain sector, in which connection the pressure can be measured from the area of a certain sector.

Figure 1D illustrates a so-called weighing roll.

Figure 2 illustrates the principle of carrying out the regulation of the difference of draw in accordance with the invention.

Figure 3A illustrates an application of the principle of regulation as shown in Fig.

2, in accordance with the invention, in the press section of a paper machine.

Figure 3B shows a second embodiment of the regulation in accordance with the invention, wherein the difference of draw is regulated between the dryer section and the last press, i.e. a so-called separate press, in the press section.

As is shown in Fig. 1A, the web W is transferred over the measurement face 100.

In the embodiment shown in the figure, the measurement face 100 is the outer face of a prior-art measurement rib 110. From the measurement face 100, a duct 120 passes to the detector 170, in which connection the pressure P is measured from the air space E between the web W and the measurement face 100. Thus, based on said pressure data, the tension of the paper web W is obtained indirectly. Based on said tension data, the web tension is regulated further to the desired level by regulating the difference of draw between the drives provided at both sides, i.e. at the inlet side and at the outlet side, of a so-called free draw of the web W.

Fig. 1B illustrates a second embodiment of the mode of measurement in accordance with the invention, wherein the web is made to run along the face 150 of the roll 140, and the pressure is measured from the air space E between the web W and the face 150. As is shown in the figure, the roll 140 includes perforations 160a1 ,160a2... on its face, through which perforations the pressure is passed to the detector 170 in the measurement chamber C. The roll can be revolving or non-

revolving. The holes 160a1, 160a2... are opened into the measurement chamber C in the interior of the roll, in which chamber the detector/detectors that 170 measures/measure the pressure p is/are placed.

In the solution in accordance with the invention, each perforation extending across the width of the roll or a certain group of holes across the distance of the length concerned can comprise a separate detector, in which case it is also possible to determine the tension profile of the web across the width of the web.

Fig. 1C shows an embodiment corresponding to Fig. 1B, but in the embodiment of Fig. 1C the measurement data are passed within a certain sector S to the measure- ment detector, in which connection the pressure data obtained correspond to a certain average pressure value over a wider sector-shaped measurement area. The roll 140 shown in the figure can also be non-revolving. The holes 160a1,160a2... are opened over a wider sector area into the measurement chamber C in the interior of the roll 140, in which chamber the detector 170 for measurement of pressure is placed.

Fig. 1D shows a so-called weighing roll 200, by whose means it is possible to measure the tension of the web W. Loading detectors 200al and 200a2 have been fitted at the ends of the roll 200. By means of the detectors 200al and 200a2, the increase in load produced by the web W is measured, and thereby it is further possible to compute/determine the tension of the web W at the roll 200.

Fig. 2 illustrates the main principle of the method in accordance with the invention.

Between the press nips N1 and N2 of the presses PI and P11, there is a free draw F, in which connection the pressure p is measured from the space E between the measurement face 100 of the rib 110 and the web, and by means of said measured pressure the difference of draw V1 - V2 between the driven presses PI and PII is regulated. V1 -V2 is the difference between the speeds of the paper/board web W as measured at the presses PI and P11. V1 is the speed of the web W at the press PII, and V2 is the speed of the web W at the press PI. Thus, as is shown in the figure,

the speed(s) of rotation of the drive motor M10 of the rolls T1 and T2 that form the nip N1 and/or of the drive motor M20 of the rolls T3 and T4 that form the nip N2 is/are regulated. It is the main principle that the difference in speed V2 - V1 of the web W between the nips N1 and N2 is adjusted by regulating a drive motor. In order that a tension could be formed in the web W over the free gap F, the speed V1 of the web W in the nip N20 and in the press PII is higher than the speed V2 of the web W in the nip N10 and in the press P1.

Fig. 3A illustrates the method in accordance with the invention as applied to a press section P. The web W is passed between the felts H1 and H2 on arrival in the press section P. The felt H1 is passed over the felt guide rolls 10a1,10a2...10an, and similarly the felt H2 is passed over the felt guide rolls llal,lla2...11an. The web is passed into the first press 12 in the press P, which press 12 comprises the press rolls 12a1,12a2. From the nip N1 the web is transferred further into the second press 120, into its nip N2 formed between the centre roll 13 and the press roll 12a1, and further, along the centre roll 13, into the third press 14 in the press section P, into the nip N3 formed between the press roll 14a1 and the centre roll 13. After this the web W is transferred from the nip N3 of the third press 14 through a free draw F1 onto the transfer roll 15. The transfer roll 15 comprises a measurement face 15', in which connection, in the solution of equipment, the pressure is measured from between the web W and the measurement face 15', and by making use of said pressure data, the tension of the web W is determined, and based on said determined web W tension, the difference of draw V1 - V2 in the web W and, thus, the tension of the web W, are regulated to the desired level. Advantageously, the regulation of the difference of draw V1 - V2 in the web is carried out between the drive at the inlet side and the drive at the outlet side of the so-called free draw, i.e. unsupported draw F1, of the web. The web W is passed further from the roll 15 into connection with the transfer felt 114 and along with said felt into the fourth press 16, into the nip N4 formed between the press rolls 16al and 16a2. The felt H4 has been passed over the felt guide rolls 17a1,17a2...17an. Similarly, the felt H3 has been passed over the felt guide rolls 170a1,170a2... In the regulation of the difference of draw V1 - V2, in the embodiment shown in Fig. 3A, V1 is the speed of the paper or board

web W at the outlet side of the free gap F1 at its first drive, in the embodiment of Fig. 3A at the driving nip N4 of the fourth press 16. V2 is the speed of the paper or board web at the inlet side of the free gap F1 at the first drive related to its inlet side, in the embodiment of Fig. 3A at the driving nip N3 of the third press 14.

From the fourth press 16, the web W is passed further through the free draw F2 onto the transfer roll 18 and further into the dryer section. The air pressure p is measured from between the face 15' of the transfer roll 15 and the web W, and, based on the measurement data, thus, the tension of the web W is determined indirectly, and on its basis the web W tension is regulated further by regulating the difference of draw.

The web is passed further from said second transfer roll 18 into the dryer section R into its first drying group RI onto the first drying cylinder K1 in said group along with the lower face of the wire 115 of the single-wire draw, for example, supported by a vacuum suction.

As is shown in Fig. 3A, the difference of draw is regulated by regulating the speed of the web W between the roll 16a1 of the fourth press 16 and the roll 14a1 of the third press 14. In accordance with the invention, the drive motor M2 of the fourth press 16, preferably the motor M2 that drives the roll 16a1, is regulated. Before this, information concerning the operation has been received from the drive motor M1 of the third press 14, preferably from the drive motor M1 of the roll 14a1, concerning its speed of rotation. The regulation can be carried out by regulating the motor M1 or M2 or both of said motors.

Fig. 3B illustrates a second embodiment of the invention, in which the drive motor M3 of the first drive in the dryer section R, i.e. of the drive of the drying cylinder K1, and the drive motor M2 of the press roll 16al of the fourth press 16 are regu- lated. For the purpose of regulation, the data concerning the speed of rotation of the drive motor M2 of the press roll of the fourth press 16 are passed into the central unit 1000, and, based on the data on the pressure in the air space E between the web

W and the measurement face 100 of the measurement rib 110, the tension of the web is determined. The data concerning this are transferred into the central unit 1000, and, within the range between the maximal and minimal values of the tension, an instruction of speed is regulated for the drive motor M3 of the drying cylinder K1 of the first drying group R1 in the dryer section R so as to operate the drying cylinder K1 at a certain speed of rotation. Regulation of the difference in speed, i.e. of the difference of draw, V1 - V2 can be carried out by regulating the speed of rotation of the motor M3 or M2 or by regulating both of the motors M2 and M3. In stead of a measurement rib 110 it is also possible to employ a roll 18, as is illus- trated in Figs. 3A, 1B and 1C, or the tension can be measured in said position by means of a weighing roll 200. When the difference of draw V1 - V2 is being regulated, in the embodiment of Fig. 3B, V1 is the speed of the paper or board web W at the outlet side of the free gap F2 at the drive first connected with the free gap, in the embodiment of the figure, at the drive of the drying cylinder K1. V2 is the speed of the paper or board web W at the inlet side of the free gap F2 at the drive most closely connected with the inlet side of the free gap F2, in the embodiment of Fig. 3B, at the nip N4 of the fourth press 16.