WO2001053603A1

WO2001053603A1 - Method and device in connection with the production of paper or paperboard, and paper or paperboard product

Info

Publication number: WO2001053603A1
Application number: PCT/SE2001/000039
Authority: WO
Inventors: Ulf Andersson
Original assignee: Stora Kopparbergs Bergslags AB
Current assignee: Stora Enso Oyj
Priority date: 2000-01-18
Filing date: 2001-01-11
Publication date: 2001-07-26
Anticipated expiration: 2002-07-18
Also published as: SE515640C2; SE0000135D0; SE0000135L; AU2001228948A1

Abstract

Method for the characterisation of a fibre suspension stream (6), which fibre suspension stream flows from a head box (2) in a paper or paperboard machine and out onto a wire (4), the true direction and velocity of the stream (6) being determined at at least one measuring point in the cross direction of the paper or paperboard machine. The invention also relates to a system for implementing the method, and the product obtained.

Description

METHOD AND DEVICE IN CONNECTION WITH THE PRODUCTION OF PAPER OR PAPERBOARD, AND PAPER OR PAPERBOARD PRODUCT

TECHNICAL FIELD

The present invention relates to a method for, and a system intended for, the characterisation of a fibre suspension stream, which fibre suspension stream flows from a head box in a paper or paperboard machine and out onto a wire. The invention also relates to a paper or paperboard product which is obtained by utilising the method and/or the system according to the invention.

STATE OF THE ART

In the production of paper or paperboard, a stock consisting of a fibre suspension is formed on a wire in a paper or paperboard machine. In this connection, the fibre suspension is distributed, in the form of a fibre suspension stream which is elongate in the cross direction (CD) of the machine, from a head box and out onto the wire. The properties of the elongate fibre suspension stream in terms of the stream direction and the stream velocity often vary to a certain extent over the width of the stream in the CD, which in turn results in a fibre orientation and quality properties in the finished paper or in the finished paperboard which vary in the CD. In this connection, it is also the case that the head box can be designed optimally only for a specific nominal flow. All deviations from this nominal flow lead to increased variations in the CD as far as the direction and velocity of the stream are concerned.

It is also generally the case that a defect in fibre orientation, fibre distribution or grammage cannot be rectified later in the production process. On the contrary, it can be worsened or accentuated during the subsequent treatments in the production process.

In the production of paper or paperboard, it is also the case that the wire can run at the same, higher or lower velocity than the velocity of the fibre suspension stream, the wire then contributing to orienting the fibres in the working direction of the paper or paperboard machine, the machine direction (MD). This is desirable on condition that the fibre suspension stream is straight in the MD, but can create problems if the fibre suspension stream is to any extent at an angle. This is because if the stream leaves the head box at an angle, the lowest fibres in the stream, that is to say the fibres which meet the wire first, will be deflected more from the MD than the fibres above this lowest layer will. The result is a gradient in the thickness direction of the fibre suspension stream, where the lowest fibres lie at an angle in relation to the MD, owing to the stream angle and the velocity difference, and the uppermost lie aligned in the stream direction. In the finished product, after shrinkage associated with drying, this is apparent in the paper or the paperboard having a tendency to "warp", curl or bend at the corners/edges.

Another problem is that defective and uneven fibre distribution and fibre orientation means that the paper or the paperboard often has to be given unnecessarily high grammage in order for its quality and strength properties to be guaranteed. This of course increases the costs of the paper or the paperboard.

A further problem is that the finished paper or paperboard often has defects at the edges, which is due to the fact that, with modern technology, it is difficult to measure and control the conditions at the short ends of the head box. The fibre suspension stream is affected here by friction against the short end sides of the head box, which results in reduced velocity and/or deflection of the stream. A pressure sensor is often mounted on the inside of one or both of the short end sides of the head box. The purpose of the positioning of the pressure sensor is to provide input data for the speed of the mixing pump, for adjusting the stream velocity, but this is consequently rendered more difficult by virtue of the fact that the pressure sensor is affected by the flow variations in the head box, especially at its short ends. Defective edges on the finished product are removed as what is known as broke, or render degrading necessary, which can entail a significant financial loss for the paper or paperboard mill.

It is also a great disadvantage for the paper or paperboard mill not to be able to check how even and straight a fibre suspension stream the head box can provide. It is then not possible to verify the guarantee commitments made by the manufacturer of the head box.

It is therefore highly desirable to be able to measure and control, in the CD, the velocity and direction of the stream, so that the distribution of fibres and the fibre orientation is as uniform as possible. Traditionally, input data for control has been produced by laboratory analysis of the finished product, after which the operating parameters of the head box were adjusted in a manner which it is accepted from experience would give optimum fibre distribution and fibre orientation in the finished product. The result of the adjustment was then analysed in the finished product for any further readjustments. The finished product can only be laboratory-analysed, however, when what is known as a tambour roll is finished and has been taken out of the paper or paperboard machine, which means that the lead time between an adjustment of the operating parameters of the head box and analysis of the result may be at least one hour and often a number of hours. In the production of a laminated multi-ply product, it is also difficult or impossible to laboratory-analyse the individual plies after lamination.

In order to shorten the lead time to some extent, on-line measurement of the fibre orientation in the formed and pressed sheet before the product leaves the paper or paperboard machine has been introduced in some cases. This is performed using a meter with a reflecting laser light, which measures the fibre orientation in the surface of the product. The measurement provides a basis for calculating and adjusting the operating parameters of the head box. A disadvantage of such on-line measurement of the fibre orientation is that it is only in the surface of the product that the fibre orientation can be measured. In particular in the production of a multi-ply product, it is clear that the values for the surface do not provide adequate input data for adjusting the head boxes which distribute fibre suspension for the plies lying below the surface ply. These head boxes can be arranged in one and the same paper or paperboard machine, the different plies being put together in the wet end of the machine.

According to a standard method, velocity control for a fibre suspension stream has been performed by calculating, from a desired velocity, the necessary pressure in the head box, which pressure is generated by a mixing pump. In this connection, the accuracy of the calculation depends on the accuracy of the calculation formula used, which formula provides a mean value over the width of the machine. The pressure is controlled via a regulator which controls the speed of the mixing pump, which does not afford the possibility of controlling the fibre suspension stream in the CD. According to a modified version of the velocity control, use has been made of a contact-free stream velocity meter in order to measure the velocity of the fibre suspension stream in the machine direction (MD), which offers the possibility of correcting the control for inaccuracies in the calculation formula, so that the desired stream velocity can be achieved. The value measured by the velocity meter can also be used in order, via the speed of the mixing pump, to control directly the pressure for the desired stream velocity.

A major problem associated with known types of stream velocity meter is that they can measure only in a preset MD, it being possible, by traversing of a measuring head of the stream velocity meter, to obtain a velocity profile in the CD. In the event of variations in the direction of the fibre suspension stream along the CD, however, only a velocity profile which at every measuring point shows the component, that is to say the projection, of the true velocity at the measuring point, in the MD, is obtained. In this connection, it is also the case that the velocity component is calculated on the basis of a distance between two sub-measuring points, which distance, when the measuring direction differs from the true direction of the stream, does not correspond entirely to the correct distance for calculating the velocity component. This means that the calculated velocity component contains a small error when the measurement is performed in a measuring direction (that is to say the MD) which does not correspond to the true direction. (This applies generally, that is to say for the method and the device according to the invention also, although this does not have any significance in the method and the device according to the invention because, as will emerge below, use is made according to the present invention of measurement values which are recorded in a measuring direction which corresponds to the true direction of the stream.)

There is today no known way of, at every measuring point along the CD, measuring the true velocity vectors, that is to say the true velocity in its true direction. As there is no possibility of measuring these true velocity vectors, there is no possibility either of using the true direction of the fibre suspension stream to influence the adjustment of the operating parameters of the head box. Nevertheless, the direction of the fibre suspension stream at every point is of decisive significance for the characteristics of the product in terms of fibre distribution and fibre orientation. Furthermore, the velocity values collected provide inaccurate assumptions for the adjustment of the operating parameters of the head box because the velocity measured is not the true velocity but only a calculated component in the MD, including calculation errors, of the true velocity.

DISCLOSURE OF THE INVENTION According to the invention, the above mentioned problems are reduced or eliminated by using a method in which a fibre suspension stream, which fibre suspension stream flows from a head box in a paper or paperboard machine and out onto a wire, is characterised by virtue of the fact that the true direction and velocity of the stream are determined at at least one measuring point along the cross direction of the paper or paperboard machine. The characterisation is preferably performed at a number of measuring points in the cross direction (CD) of the paper or paperboard machine, so that a vector field for the true direction and velocity of the stream can be determined as a profile in the CD. By virtue of the fact that the true direction and true velocity of the fibre suspension stream, that is to say its velocity vector, can be determined at one or more measuring points along the CD, a unique possibility is afforded of direct and feed-back control of the direction and velocity of the fibre suspension stream along the CD. The fibre suspension stream is therefore already characterised when forming of the product begins, and is characterised by more, and more accurate, measurement values than previously possible, as a result of which control/adjustment of the operating parameters of the head box can be optimised so as to obtain a product with uniform grammage, fibre distribution and fibre orientation over the cross section.

According to one aspect of the invention, this is brought about by virtue of the fact that a number of components of the direction and velocity of the stream, in a measurement plane which is parallel to a plane of the wire, are determined for a measuring point in the cross direction of the paper or paperboard machine, after which the direction and velocity of that of these components which has the greatest velocity, or the direction and velocity of that of the components which has the greatest correlation factor, are used as the direction value and velocity value for the true direction and velocity of the stream, or are used for calculation of the true direction and velocity of the stream, at the measuring point, after which the characterisation preferably continues in the same manner at a following measuring point in the cross direction of the paper or paperboard machine, for the formation of said profile.

The term correlation factor means that, for each measuring point, measurement data/a measurement signal is recorded at two sub-measuring points which lie a short distance, preferably 1-5 mm, and even more preferably 1.5-3.5 mm, from one another along a line which follows the measuring direction concerned at the time, that is to say the component direction. The measurement data/measurement signals for the two points are then compared with one another, and a correlation factor can then be determined, which correlation factor constitutes a measure of how close to the true direction of the fibre suspension stream the measuring direction concerned lies. A prerequisite for it being possible to measure a high correlation factor is, in addition to the measurement taking place in the true stream direction, that the fibre suspension stream has constant data in this measuring direction, which is an assumption which can be made with great certainty. The correlation factor is determined in this way for a desired number of measuring direction at the measuring point, and at the same time the velocity of the stream is measured in each measuring direction by recording the time it takes for the stream to move from the first sub-measuring point to the second sub-measuring point. The velocity which is measured in the different measuring directions is then a component of the true velocity. The correlation factors recorded for the different measuring directions at the measuring point are then used in order to decide which of the measured components lies closest to the true direction and velocity of the stream at the measuring point, the direction value and velocity value of that of the measured components which has the greatest correlation factor then being used as the direction value and velocity value for the true direction and velocity of the stream at the measuring point. In this connection, the number of measuring directions is selected with a sufficiently small incremental angle between them, preferably 0.001-1°, even more preferably 0.01-0.5°, and most preferably 0.05-0.5°, in order that the component selected as the true value can be indicated in terms of direction and velocity with the desired accuracy.

Alternatively, the component which lies closest to the true value is selected on the basis of the velocity value of the components. The direction and velocity values for the component which has the greatest velocity are then selected as the true direction and velocity. It is then preferable to use a laser Doppler meter in order to avoid problems with errors in the velocity calculation as mentioned above.

If the requirement for accuracy is great, or if for any reason the incremental angle cannot be selected so as to be as small as required for the desired accuracy, the recorded correlation factors can be used in order to decide which two of the measured components he closest to the true direction and velocity of the stream at the measuring point. In this connection, use is made of the direction and velocity of those two of the measured components which have the greatest correlation factor for calculating the direction value and velocity value for the true direction and velocity of the stream at the measuring point.

Alternatively, use is made of direction and velocity values for the two components which have the greatest velocity value for calculating the direction value and velocity value for the true direction and velocity of the stream at the measuring point.

According to one aspect of the invention, the vector field profile in the cross direction of the paper or paperboard machine determined during characterisation is used in order to adjust the operating parameters of the head box for optimising the properties of the fibre suspension stream and thus of the paper or paperboard produced. The operating parameters of the head box which are adjusted are one or more of the parameters temperature compensation, pressure balancing, edge valve setting, diluting flow and discharge opening, or any other relevant parameter. To the extent that these parameters can be adjusted variably along the CD, the head box can therefore be set optimally for each measuring point, or at least for different measuring point areas. The characterisation of the vector field and the adjustment of the operating parameters of the head box are preferably performed on-line, using direct and feed-back regulation, which means that the characteristics of the fibre suspension stream can be regulated directly against desired reference values, that is to say normally to flow straight in the MD.

What is meant by the operating parameters of the head box and their adjustment is well known to the person skilled in the art and is therefore described only briefly below. Temperature compensation normally means that parts of the head box, principally its bottom, can be heated by means of, for example hot water in a surrounding casing, so that the head box is at the same temperature as the incoming fibre suspension. This is appropriate principally when the head box is started up after the system has been out of operation, and its purpose is to counteract thermal stresses in the different parts of the head box. It is then the flow and temperature of the heating medium, the hot water, which can be regulated.

Pressure balancing means regulating the pressure provided by the mixing pump, and/or regulating a circulation valve which maintains a counter-pressure in a circulation which distributes the fibre suspension over the width of the head box at the inlet to the head box. It is a great advantage in utilisation of the invention that the mixing pump can be controlled directly on basis of the true velocity of the stream.

Edge valve setting means adjustment/setting of an edge valve in each short end side of the head box, which edge valves are arranged so as to reduce or increase a flow of fibre suspension at the short ends of the head box, in order to counteract the effect of friction on the fibre suspension stream. By adding or drawing off fibre suspension via the edge valves, the stream can therefore be aligned on the MD at the edges.

Diluting flow means a flow of diluting liquid which is added to the fibre suspension stream, usually at a selected position along a tube bank which distributes the fibre suspension over the cross section of the head box. By adding diluting liquid to the fibre suspension flow, the local fibre concentration of the stream and thus the grammage of the product can be adjusted and the CD profile can be optimised. In connection with this, any influence on the local direction and velocity of the stream can be established by means of the method and the device according to the invention.

Discharge opening means the gap-like opening which the head box has in the CD, from which opening the fibre suspension stream flows out onto the wire. At least the upper edge of the opening can be regulated along the CD, by virtue of the fact that the "upper Up", that is to say the front wall and the rim, can be regulated upwards or downwards to increase or reduce the height of the opening. Along the upper lip/rim, there is also a number of uniformly distributed adjusting screws, by means of which the rim, which is to some extent flexible, can be raised or lowered locally. In this way, the discharge opening can be regulated over the CD.

In order for it to be possible to form a relevant vector field profile in the CD, measurements should be performed at at least two measuring points along the CD, preferably three points or at least three points and even more preferably five points or at least five points along the CD. How many measuring points are analysed depends on which type of analysis or which type of operating parameter adjustment is concerned. When edge valve setting or pressure balancing is to be carried out, for example, three measuring points may be sufficient, one reference measuring point in the centre of the CD being analysed, and one in the vicinity of each short end of the head box. If the intention is to adjust the discharge opening, on the other hand, it may be justified to analyse measurement data at up to twice as many measuring points as there are adjusting screws, that is to say to analyse measurement data with a measuring point density with interspaces of the order of 5-10 cm. If an analysis of what is known as streakiness is desired, measurements at 2-3 measuring points for each tube in the tube bank may be required, which can mean a measuring point density with interspaces of the order of 5-40 mm, or around 10-20 mm in the CD. It is also possible to provide a fixed meter which analyses the fibre suspension stream in the centre of the CD, plus a traversable meter which analyses a profile in the CD. Measurement data from the fixed meter can then be used in order to compensate the profile measurements for pulsations and regulating variations of the mixing pump etc.

According to a further aspect of the invention, two or more fibre suspension streams in a paper or paperboard machine can be characterised individually. These different fibre suspension streams are then intended for different plies in a multi-ply product, which is preferably put together in the wet end of the machine. This is a very great advantage of the invention because, in conventional terms, there is no possibility of, by means of conventional laboratory tests or on-line measurements on the surface of the product, controlling fibre distribution and fibre orientation in the individual plies of a multi-ply product.

Utilisation of the invention affords a unique possibility of characterisation of the fibre suspension stream with regard to its true direction and velocity at each given measuring point. The result can then be used for adjusting the fibre suspension stream to the desired appearance. In this way, a possibility is afforded of counteracting the formation of stresses in the paper or the paperboard on account of uneven and defective fibre orientation, of controlling grammage and fibre orientation in a pertinent manner, of reducing the grammage of the paper or of the paperboard while preserving the strength, of reducing the quantity of broke, of checking guarantee commitments given, of forgoing pressure sensors which are affected by the stream and can thus provide inaccurate regulating data etc.

DESCRIPTION OF THE FIGURES The invention will be described in greater detail below with reference to the figures, in which:

Fig. 1 shows diagrammatically different sections in a paper or paperboard machine, with a head box which is controlled by means of characterisation equipment according to the invention,

Fig. 2 shows diagrammatically a measuring head, according to a first embodiment, for the characterisation equipment in Fig. 1, Fig. 3 shows diagrammatically a measuring head, according to a second embodiment, for the characterisation equipment in Fig. 1, Fig. 4 shows in principle an example of a recorded correlation factor graph, and Fig. 5 shows in principle an example of a recorded vector graph.

Fig. 1 shows a paper or paperboard machine, seen from the side, in which the invention has been implemented. A fibre suspension, that is to say a stock, is pumped from a mixing pump 1 to a head box 2 in order then, via a discharge opening 3, to be distributed as a fibre suspension stream 6 onto a wire 4. The discharge opening consists in principle of an elongate gap, the extent of which is parallel to the cross direction (CD) of the machine. Characterisation equipment 5 according to the invention is mounted at the line where the fibre suspension stream 6 meets the wire 4, that is to say at a line where the fibre suspension stream 6 runs parallel to the wire 4 but has not yet had time to be affected by contact with the wire, or at a line where the fibre suspension stream has just arrived on the wire, only the bottom surface of the stream having begun to be influenced by contact with the wire. When the fibre suspension stream has arrived fully on the wire 4, a fibre web is formed, with certain characteristics in terms of fibre distribution and fibre orientation, which characteristics it is not possible to influence further in a positive manner in the continued production process. In the event of a lack of space, the characterisation equipment 5 according to the invention can be mounted in a position before the fibre suspension stream 6 encounters the wire 4, the characterisation equipment then being arranged, however, to measure at right angles to the fibre suspension stream.

The fibre web continues in the wet end 7 of the machine and on into the press section 8 and the dry end 9, in order then to be rolled up as what is known as a tambour roll 10. Laboratory samples can be taken from the tambour roll 10.

In Fig. 1, in accordance with known art, an on-line fibre orientation meter 11 has also been arranged, which measures the fibre orientation in the surface of the fibre web in the dry end 9.

The characterisation equipment 5 is arranged so as to record a number of components of the direction and velocity of the fibre suspension stream 6 in a measurement plane which is parallel to the wire 4, or at least parallel to the fibre suspension stream. In this connection, a measuring head 12 is arranged traversably, on a traversing beam, in the CD, so as to be capable of recording a profile over the CD. Recorded measurement data is analysed by means of a computer 13 which also, if appropriate, receives input data from a pressure sensor 14 at the discharge opening 3 of the head box. The computer 13 is programmed to analyse measurement data from the measuring head 12 and to determine from this the true direction and the true velocity of the fibre suspension stream 6 at each measuring point along the CD, for formation of a profile. This profile is then compared with a reference value profile, after which the operating parameters of the head box 2, including the mixing pump 1, are regulated against the reference value profile.

When the invention is implemented in older systems, which do not afford the possibility of computer-controlled regulation, the regulation of the operating parameters of the head box can of course be performed manually.

Fig. 2 shows a measuring head 12 for the characterisation system in greater detail. The measuring head 12 is arranged so as, for each measuring point along the CD, to record measurement data at a first sub-measuring point 14, which constitutes a reference point, and a second sub-measuring point 15. The measuring head 12 and these sub-measuring points 14, 15 are oriented in such a manner that a measuring beam, preferably an optical measuring beam, and even more preferably a laser measuring beam, if appropriate a laser Doppler measuring beam, with a diameter of 20-150 μm, preferably 40-120 μm, through each measuring point 14, 15 will meet the fibre suspension stream 6 at right angles to the same, and preferably be oriented at right angles to the wire 4. The distance between the two sub-measuring points, and thus between the two measuring beams, is known, as mentioned above, and is 2.5 mm in the preferred embodiment shown. The figure is, however, not to scale. The two measuring beams, or in other words the two sub- measuring points 14, 15, are arranged so as to record individually measurement data for the fibre suspension stream 6, preferably in the form of analogue and/or digital images of the surface of the fibre suspension stream. The principle of this is known and has been in general use previously, examples of known measuring heads of this type being Dantec Sensorline 7510, 7520 and 7530. Briefly, the principle is based on the top surface of the fibre suspension stream being illuminated by the two laser beams, the reflection then forming measurement data in the form of an analogue signal which is then converted into a digital signal.

In the characterisation system, the measurement data/signal/image recorded at the first sub-measuring point 14 is compared/cross-correlated with the measurement data/signal/image recorded at the second sub-measuring point 15. Each image is recorded with an image area which corresponds to the diameter of the measuring beam, in the preferred embodiment 80 μm, the characterisation system being arranged so as to determine a correlation factor, or an image correlation factor, when comparison of the images is carried out. When the image from the first sub-measuring point 14 corresponds to the image from the second sub-measuring point 15, a maximum correlation factor is obtained, which means that the measuring direction corresponds to the true direction of the fibre suspension stream. If the correlation factor is lower than the maximum which may be expected, this indicates that only a part of the two images corresponds, which means that the direction between the two sub-measuring points 14, 15 does not correspond to the true direction of the fibre suspension stream. According to previously known art, lack of a correlation factor has not influenced the recorded velocity, which means that the recorded velocity was actually a projection of the true velocity.

The correlation factor can, in a manner known per se, be determined as correlation height or as correlation width. In this connection, a great correlation height or a low correlation width corresponds to a high correlation factor. Theoretically, a correlation height of 100% would correspond to complete correlation, that is to say indicate that the measuring direction concerned is the same as the true direction of the fibre suspension stream. In practice, however, it is been found that the correlation height, when the measuring direction corresponds to the true direction of the fibre suspension stream, has values around 45-50%, which is due to optical phenomena and deviations in measuring accuracy etc. As far as the correlation width is concerned, a correlation width of 3.2% would theoretically correspond to complete correlation. A more detailed definition of the terms correlation height and correlation width is not given here, but the terms are well known within the area of measurement technology concerned.

According to the invention, the measuring head 12 has now been arranged so as to be rotatable about the first 14 of the two sub-measuring points, or in other words about the optical axis constituted by a centre line of the measuring beam through the first sub- measuring point 14. In this connection, the measuring head is, according to the preferred embodiment, mounted in a first, externally annular mounting device 16, which first mounting device 16 is arranged rotatably in a second, internally annular mounting device 17. The second mounting device 17 is mounted in a mounting 18 which is arranged so as to move along the traversing beam.

The measuring head is rotatable, preferably by angle servo means, by angular increments with the desired interval, which increments have been indicated in Fig. 2 by the designations -3, -2, -1, (0), +1, +2, +3. It will be understood that there can be further angular increments on both sides of the zero point. The zero point/zero direction coincides with the MD and is calibrated by measuring the running direction of the wire, which by definition constitutes the MD. By virtue of the rotatability, measurement data can therefore be recorded in a number of different measuring directions for each measuring point along the CD. The result is then, for each measuring point along the CD, a correlation factor graph which is compiled by means of the computer 13. An example of such a correlation factor graph is shown in Fig. 4, the broken curve being a calibration curve relative to the running direction of the wire in the MD, and the solid curve being a measurement result for the fibre suspension stream in the various measuring directions. In the solid correlation graph shown, it can be seen that the top of the graph lies in the direction -1 and that the correlation factor is lower in all the other measuring directions. The greatest correlation factor, that is to say the top of the graph, corresponds to the true direction of the fibre suspension stream, for which reason the velocity of the fibre suspension stream recorded in this direction is noted as its true velocity in the true direction. The more accurate the measurement data desired, the smaller the incremental angles chosen for the measurements.

Otherwise, another approach is to use the measurement values in the direction -1, that is to say the direction which has the greatest correlation factor, together with the measurement values in another, adjacent direction which has the next greatest correlation factor, in order, taking the magnitude of these two greatest correlation factors into consideration, to calculate the true direction and the true velocity of the fibre suspension stream 6 at the measuring point. When the true velocity is calculated from measured components in directions which differ from the true direction, use should be made of laser Doppler in order to obtain exact velocity values.

Another variant for obtaining accurate results with regard to the true direction and velocity of the fibre suspension stream 6 at each measuring point can be to use vector analysis of the velocity vectors which are recorded in the various measuring directions at each measuring point. Fig. 5 shows a vector graph, in which each vector shows direction and velocity (the lengths of the vectors are proportional to the measured velocity in the respective direction). In the graph, vectors measured in different measuring directions from the first sub-measuring point 14 are shown as solid vectors. As all these vectors are components of the true vector along the respective direction, it follows that the true vector must lie between the two measured vectors which have the greatest length. The length of the true vector (broken) can then be calculated vectorially from these two longest measured vectors. A variant is also to select the measured vector which has the greatest velocity as the true vector. When the velocity values are used in order to select or calculate the true vector, it is preferred if measuring beams of the laser Doppler type are used.

It has also been found that the correlation factor graph recorded for each measuring point is symmetrical, or at least fundamentally symmetrical, about its top, that is to say about the true direction. This means that the method according to the invention can be performed more rapidly and easily by means of a measuring head of the type shown in Fig. 3. In the measuring head in Fig. 3, as in Fig. 2, the sub-measuring point 14 constitutes the reference measuring point, around which the measuring head is rotatable. In the measuring head according to Fig. 3, however, there are no fewer than three further sub-measuring points which are all located at the same distance from the first sub-measuring point 14, so that a second sub-measuring point 15a is arranged between a third 15b and a fourth 15c adjacent sub-measuring point. The three sub-measuring points 15a, 15b and 15c are preferably arranged exactly next to one another, but without overlapping one another, and with the same angle between the outer sub-measuring points 15b, 15c and the central sub-measuring point 15a on both sides of the central sub- measuring point 15a. Angle means the angle with its origin at the reference measuring point 14. The measuring head is rotated in the same manner as described previously, three different correlation factors and three corresponding velocity components being recorded at each angular increment, one for each sub-measuring point 15a, 15b and 15c respectively. By virtue of the fact that the correlation factor graph is symmetrical, the direction in which the central sub-measuring point 15a is oriented will then correspond to the true direction of the fibre suspension stream when the two outer sub-measuring points 15b, 15c record the same value of correlation factor, or fundamentally the same value of correlation factor, or the same value of velocity component, or fundamentally the same value of velocity component. In this way, the measuring head 12 only needs to be rotated, at one measuring point, until the same, or fundamentally the same, value of correlation factor or velocity is recorded for 15b and 15c, the direction value and velocity value for 15a then being used as the true values for the fibre suspension stream.

It is to be understood that the measuring head can also be supplemented with further sub-measuring points on each side of the outer sub-measuring points 15b and 15c shown in Fig. 3, in which case more than three correlation factors and velocity components can be recorded at each angular increment. This can be used to provide an additional guarantee that the top of the graph, that is to say the true direction, is identified, and also to reduce time expenditure.

When it is necessary for it to be possible to position the sub-measuring points at a small mutual angular distance, the sub-measuring points 15a, 15b, 15c etc. can be positioned at the same mutual angular distance relative to one another, but at different distances from the first sub-measuring point 14. The different known distances are then used for calculating the velocity in the respective direction. This makes it possible for the sub- measuring points to be located at a small mutual angular distance without the measuring beams overlapping one another.

It is also possible to exclude the central sub-measuring point 15 a, in which case the true direction and velocity of the fibre suspension stream can be calculated from the values for the sub-measuring points 15b and 15c when these record the same value of correlation factor, or fundamentally the same value of correlation factor, or the same value of velocity component, or fundamentally the same value of velocity component.

Two or more sub-measuring points 15a, 15b, 15c, over and above the first sub- measuring point 14, can also be used simply to reduce the time expenditure at each measuring point. The measuring head can then be rotated by greater angular increments, a number of directions being recorded at each angular increment, after which the true direction and the true velocity are selected in the manner described previously.

The method according to the invention, which method characterises the fibre suspension stream, does not normally have to be carried out continuously, although this is possible if so desired. Typically, it is used in the event of changed operating conditions, such as for example in the event of a change-over in production and/or a changed inflow of fibre suspension. It can also be implemented routinely, for example 1-3 times per day or once per tambour roll, or when defects are detected in the product, for example by on-line measurement of fibre orientation, on-line measurement of grammage, or laboratory tests.

The invention is not limited to the embodiments described but can be varied within the scope of the claims. It is to be understood therefore that, for example, the traversable measuring head can be replaced by a number of measuring heads arranged along the CD, although this would be a worse solution both practically and financially, at least if it is desirable for the measuring points to lie close together. In the analysis of which measuring direction corresponds to the true direction of the fibre suspension stream and which velocity arises in that direction, a combination of correlation height and/or correlation width and/or velocity can also be used.

Claims

1. Method for the characterisation of a fibre suspension stream (6), which fibre suspension stream flows from a head box (2) in a paper or paperboard machine and out onto a wire (4), characterised in that the true direction and velocity of the stream (6) are determined at at least one measuring point along the cross direction of the paper or paperboard machine.

2. Method according to Claim 1, characterised in that a vector field for the true direction and velocity of the stream (6) is determined as a profile for a number of measuring points in the cross direction of the paper or paperboard machine, preferably at least three measuring points.

3. Method according to Claim 1 or 2, characterised in that a number of components of the direction and velocity of the stream (6), in a measurement plane which is preferably parallel to a plane of the wire (4), are determined at the measuring point, after which these components are analysed so that the component which corresponds to the true direction and velocity of the fibre suspension stream can be selected or calculated.

4. Method according to Claim 3, characterised in that the direction and velocity of that of these components which has the greatest velocity, or the direction and velocity of that of the components which has the greatest correlation factor, are used as the direction value and velocity value for the true direction and velocity of the stream (6), or are used for calculation of the true direction and velocity of the stream, at the measuring point.

5. Method according to Claim 3, characterised in that said components are determined by recording measurement data for the stream (6) at two sub-measuring points (14, 15; 15a) in the measurement plane, the measuring direction being varied by virtue of the fact that a second (15; 15 a) of said two sub-measuring points is rotatable about a first (14) of said two sub-measuring points, in the measurement plane, so that the velocity component for the stream (6), through the two sub-measuring points (14, 15; 15a), can be determined in selected measuring directions.

6. Method according to Claim 5, characterised in that said measurement data is recorded, preferably by means of a measuring beam at each sub-measuring point (14, 15; 15a), as a first digital and/or analogue signal at said first sub-measuring point (14) and a second digital and/or analogue signal at said second sub-measuring point (15; 15a), after which a correlation factor for the two signals is determined in the measuring direction concerned, which correlation factor constitutes a measure of correspondence between the two signals.

7. Method according to Claim 5, characterised in that measurement data for the stream (6) is also recorded at a third sub-measuring point (15b) in the measurement plane, which third sub-measuring point (15b) is arranged at the side of said second sub- measuring point (15 a), the measuring direction being varied by virtue of the fact that said second (15a) and third (15b) sub-measuring points are rotatable about the first (14) of said sub-measuring points, in the measurement plane, so that the velocity component for the stream (6), through the sub-measuring points (14, 15a; 14, 15b), can be determined in two different measuring directions at the same time.

8. Method according to Claim 5, characterised in that measurement data for the stream (6) is also recorded at a third (15b) and a fourth (15c) sub-measuring point in the measurement plane, which third (15b) and fourth (15c) sub-measuring points are arranged one on each side of said second sub-measuring point (15a), the measuring direction being varied by virtue of the fact that said second (15a), third (15b) and fourth (15c) sub-measuring points are rotatable about the first (14) of said sub-measuring points, in the measurement plane, so that the velocity component for the stream (6), through the sub-measuring points (14, 15a; 14, 15b; 14, 15c), can be determined in three different measuring directions at the same time.

9. Method according to Claims 4 and 8, characterised in that the direction value and velocity value for said second sub-measuring point (15a) are used as the direction value and velocity value for the true direction and velocity of the stream at the measuring point when the measuring direction is such that said third (15b) and fourth (15c) sub- measuring points record the same value of correlation factor, or fundamentally the same value of correlation factor, or the same value of velocity component, or fundamentally the same value of velocity component.

10. Method according to Claim 4, characterised in that said correlation factor is used in order to decide which of the measured components lies closest to the true direction and velocity of the stream (6) at the measuring point, said direction and velocity of that of the measured components which has the greatest correlation factor then being used as the direction value and velocity value for the true direction and velocity of the stream at the measuring point.

11. Method according to Claim 4, characterised in that said correlation factor is used in order to decide which two of the measured components lie closest to the true direction and velocity of the stream (6) at the measuring point, said direction and velocity of those two of the measured components which have the greatest correlation factor then being used for calculating the direction value and velocity value for the true direction and velocity of the stream at the measuring point.

12. Method according to Claim 3, characterised in that the direction and velocity of two of the measured components, preferably the two components which have the greatest velocity value, are used for calculating the direction value and velocity value for the true direction and velocity of the stream (6) at the measuring point.

13. Method according to any one of the preceding claims, characterised in that said characterisation is performed at at least one, preferably at least two, measuring points in the cross direction of the paper or paperboard machine, the characterisation being used in order to adjust the operating parameters of the head box (2, 1) for optimising the properties of the fibre suspension stream (6) and thus of the paper or paperboard produced.

14. Method according to Claim 13, characterised in that the operating parameters of the head box (2, 1) which are adjusted are one or more of the parameters temperature compensation, pressure balancing, edge valve setting, diluting flow and discharge opening.

15. Method according to Claim 14 or 15, characterised in that said characterisation and said adjustment are performed on-line, using direct and feed-back regulation (13).

16. Method according to any one of the preceding claims, characterised in that two or more fibre suspension streams in a paper or paperboard machine are characterised individually, which fibre suspension streams are intended for different plies in a multi-ply product.

17. System for a head box (2) in a paper or paperboard machine, which system comprises means for characterisation of a fibre suspension stream (6), which fibre suspension stream flows from said head box and out onto a wire (4), characterised in that said system is arranged so as to determine the true direction and velocity of the stream (6) at at least one measuring point in the cross direction of the paper or paperboard machine.

18. System according to Claim 17, characterised in that said system is arranged so as to determine a vector field for the true direction and velocity of the stream (6) as a profile in the cross direction of the paper or paperboard machine.

19. System according to Claim 17 or 18, said means comprising a measuring head (12) which is arranged so as, at a measuring point in the cross direction of the paper or paperboard machine, to record measurement data for two sub-measuring points (14, 15; 15 a), so that a velocity component for the stream (6), in a measurement plane which is preferably parallel to a plane of the wire (4), through these two sub-measuring points (14, 15; 15a) can be determined, characterised in that the measuring head (12) is arranged so as to be rotatable about a first (14) of said two sub-measuring points, so that velocity components for the stream (6) can be determined in a number of different measuring directions in the measurement plane, the system also comprising analysis and/or calculation equipment (13) capable of determining, from the velocity components measured in the various measuring directions, the true direction and velocity of the stream (6) at the measuring point.

20. System according to Claim 19, characterised in that said measuring head (12) is traversable in the cross direction of the paper or paperboard machine for measuring at a number of measuring points, so that said vector field profile in the cross direction of the paper or paperboard machine can be generated.

21. System according to Claim 19 or 20, characterised in that said measuring head (12) consists of a contact-free, preferably laser-based, surface velocity meter arranged so as to measure the surface velocity of the fibre suspension stream (6).

22. System according to any one of Claims 19-21, characterised in that the measuring head (12) is arranged so as, preferably by means of a measuring beam at each sub- measuring point (14, 15; 15a), to record said measurement data as a first digital and/or analogue signal at said first sub-measuring point (14) and a second digital and/or analogue signal at a second (15; 15a) of said two sub-measuring points, said means being arranged so as to determine a correlation factor for the two signals, which correlation factor constitutes a measure of correspondence between the two signals.

23. System according to any one of Claims 19-22, characterised in that said means is arranged so as to measure the time it takes for the fibre suspension stream (6) to move between the two sub-measuring points (14, 15; 15a), which time measurement is used together with a known, preferably constant, distance between the two sub-measuring points in order to determine said velocity component for the fibre suspension stream (6).

24. System according to Claim 22, characterised in that the measuring head is also arranged so as, preferably by means of a measuring beam at each sub-measuring point (15b, 15 c), to record measurement data as a third digital and/or analogue signal at a third sub-measuring point (15b), and if appropriate further digital and/or analogue signals at further sub-measuring points (15c).

25. System according to Claim 24, characterised in that all the sub-measuring points (15a, 15b, 15c), over and above said first sub-measuring point (14), are located at the same distance from said first sub-measuring point (14), and with the same mutual angle relative to the origin at said first sub-measuring point (14).

26. System according to Claim 24, characterised in that at least two sub-measuring points (15a, 15b, 15c), over and above said first sub-measuring point (14), are located at different distances from said first sub-measuring point (14), and in that all the sub- measuring points (15a, 15b, 15c), over and above said first sub-measuring point (14), are located with the same mutual angle relative to the origin at said first sub-measuring point (14).

27. System according to any one of Claims 17-26, characterised in that the measuring head (6) is rotatable, preferably by servo means, and is arranged so as to rotate by increments with the desired interval, preferably 0.001-1°, even more preferably 0.01- 0.5°, and most preferably 0.05-0.5°.

28. System according to any one of Claims 17-27, characterised in that said measuring head (12) is mounted in a first, externally annular mounting device (16), which first mounting device is arranged rotatably in a second, internally annular mounting device (17), which second mounting device is mounted in a mounting (18).

29. System according to any one of Claims 17-28, characterised in that the system is arranged so as to use the true direction and velocity for at least one, preferably at least two, measuring points in the cross direction of the paper or paperboard machine determined during characterisation in order to adjust the operating parameters of the head box (2, 1) for optimising the properties of the fibre suspension stream (6) and thus of the paper or paperboard produced.

30. System according to Claim 29, characterised in that the operating parameters of the head box (2, 1) which are adjusted are one or more of the parameters temperature compensation, pressure balancing, edge valve setting, diluting flow and discharge opening.

31. System according to Claim 29 or 30, characterised in that said characterisation and said adjustment are performed on-line, using a direct-acting and feed-back regulating system (13).

32. System according to any one of the preceding claims, characterised in that the system is arranged so as to characterise two or more fibre suspension streams in a paper or paperboard machine individually, which fibre suspension streams are intended for different plies in a multi-ply product.

33. Paper or paperboard product, characterised in that it is produced using a method and/or a system according to any one of the preceding claims.