WO2008034954A2

WO2008034954A2 - A method and a device for the measurement of properties of a moving web

Info

Publication number: WO2008034954A2
Application number: PCT/FI2007/050503
Authority: WO
Inventors: Tuulikki Manninen
Original assignee: Metso Paper, Inc.
Priority date: 2006-09-22
Filing date: 2007-09-21
Publication date: 2008-03-27
Also published as: FI20065586A0; FI20065586L; WO2008034954A3

Abstract

A method and a device for measuring at least one property of a moving web, wherein the web (W) is illuminated with a light source (1) and detected with a camera (2) substantially at the illuminated area. A diode laser light source is used for illuminating the web (W).

Description

A method and a device for the measurement of properties of a moving web

Field of the invention

The invention relates to a method for the measurement of properties of a moving web according to the preamble of the appended claim 1. The invention also relates to a device for implementing the aforementioned method in accordance with the preamble of the appended claim 12.

Background of the invention

In the manufacture of paper, cardboard and cellulose webs, the manufacturing process is continuously monitored by means of various measurements. These are used to secure that the process operates in the desired way and that the quality of the web meets the target values. Measurements are taken from the process devices, from the pulp used for manufacturing the web, as well as from the finished web.

It is known to monitor the above-mentioned manufacturing processes by measurement methods based on on-line camera imaging. In these methods, the measurement target is illuminated with light and detected with a camera. The camera produces image material that is analyzed with a data processor. The method of illumination and the illumination wavelength as well as the type of the camera used as the detector are selected according to the property to be measured. The illumination may be continuous or pulsed illumination. The camera may be, for example, a line camera, a matrix camera, a CCD camera, a CMOS camera, or a video camera.

The most common measurement methods employing a camera as the detector are measurements related to web formation, fiber orientation and roughness, as well as measurements and detection of defects of the web. A camera is also used for measuring pulp properties. Video cameras are normally used for monitoring the running of the web and defects occurring therein, such as holes and tears. Patent publication DE 19930154 discloses a device for measuring the formation and the formation index of a moving paper web, wherein the paper web is illuminated from one side of the web with light emitted by a LED array. The illumination is pulsed. At the corresponding location on the opposite side of the web, at least one matrix camera is arranged to form images of the web and to lead the image information into a data processor for processing.

Document US 5,113,454 discloses another system for measuring the formation of a moving paper web, wherein the illumination from one side of the web is implemented as continuous diffuse illumination. At the corresponding location on the other side of the web, a CCD camera is provided to produce image information to be led into a data processor.

The above-mentioned methods and devices for measuring the formation involve the problem of not being capable of measuring the formation of thick paper and cardboard webs. This is due to their poor illumination. Normal LED technology or continuous diffuse illumination is not sufficiently effective to penetrate the web and for the image information recorded by the camera to be sufficiently clear for determining the formation. This problem is particularly difficult in the measurement of the formation of thick webs, such as cardboard and cellulose webs with great web thicknesses, the grammage ranging from 600 to 1200 g/m². At present, the formation of cellulose webs is thus not measured on-line at all.

The formation can also be measured in a laboratory, where samples taken from the web are measured by methods utilizing e.g. a camera.

The laboratory measurements are accurate, but the illumination used in them is not sufficient for the measurement of thick web samples.

Furthermore, the paper samples taken from the web only represent very small areas of the whole manufactured web, not the formation of the whole web. Defects occurring in the moving fiber web, such as holes, tears and markings caused by different parts of the web manufacturing machine, such as wires and felts, have also been monitored and measured with devices comprising a camera as the detector. Such methods are presented, for example, in WO publications 99/10833 and 2004/063664. A problem in these methods is that in the absence of effective illumination, the quality of the image material recorded by the camera is poor. For this reason, it is difficult to analyze the defects from the image material.

Brief description of the invention

It is thus an aim of the present invention to provide a method for measuring the properties of a moving web that avoids the above- mentioned problems and can be used for accurate and reliable on-line measurement of properties of the web.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 12.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that a moving fiber web is measured with a measuring device, in which the illumination of the web is implemented with a diode laser light source. The diode laser is capable of emitting very exact short light pulses with a high pulse power. As a result, the illumination has good spatial resolution and it can be used for the measurement of thick material layers. Furthermore, the resolution of cameras that are commercially available today is sufficiently good for stopping the movement of the target and for capturing images at a high frequency.

The diode laser light source may consist of one or more diode lasers. When a light source composed of several diode lasers is used, it is possible to combine their light power and to direct the light to the target in a desired way, for example by means of an optical fiber. Such sophisticated diode laser light sources are capable of producing even 100-fold brightnesses compared with currently available advanced LED light sources. This is due to the fact that the pulse power of the diode laser light sources may range from about 500 W to 1 kW. The length of the light pulse can also be adjusted, and it may vary from 1.5 to 10 μs. The wavelength of the light emitted by the diode laser light source can also be adjusted; depending on the application, it may be either 680 nm or 808 nm. The effective light source is crucially important for visualizing a moving target.

The camera applied in the device for detection may be any camera suitable for measurement purposes. It is possible to use, for example, a camera equipped with an external shutter release that is capable of capturing still or multiple exposure images. Such cameras are inexpensive and they still have a good resolution to secure sharp image quality, for analyzing desired process features or properties of the web.

An imaging device applying a pulsed diode laser light source and a still camera, is capable of imaging a web running at a speed as high as 150 to 2000 m/min. It is possible to capture, for example, 200 images per a measurement point. Thanks to the high-power illumination provided by the diode laser light source, the image material obtained in detecting has a significantly better quality than material obtained with devices of prior art.

The light pulse emitted by the diode laser light source is so intense that the device can also be used for measuring a web on a supporting means, such as a wire or felt. The light source is placed on one side of the supporting means to illuminate the web through the supporting means, and the camera is arranged to detect, that is to form images of the web from its other side. For example, the formation of the moving fiber web can be measured so that the running web is supported by the supporting means. Thanks to the intensity of the light pulse, it is possible to measure the properties of not only thin paper grades but also the thickest cellulose webs. It has been shown by experiments that the device can be used for measuring, for example, the formation of webs with a grammage of about 150 to 1100 g/m². The supporting member is visible in the captured images of the web. In the measurement of certain properties, such as formation, it does not disturb the measurement of the property itself. If necessary, the effect of the wire on the measurement can be eliminated by image processing methods.

It is an advantage of the invention that quantitative image material can be produced at a rate corresponding to the operation of a high-speed camera at a MHz rate, but by using components that are significantly less expensive. Thanks to this, the device can be used for on-line monitoring of the manufacturing process of paper, cardboard and cellulose webs. Analyzes on the operation of the process and its parts can be made quickly from the accurate image material obtained from the monitoring. Furthermore, the device can be arranged to measure the properties of a moving paper, cardboard or cellulose web continuously on-line. Thanks to the accuracy of the image material obtained from the device, the results are accurate and reliable. When defects occurring in the web are monitored by the device, even the smallest defects are visualized quickly and the web manufacturing process can be readjusted to reduce them.

The device also makes it possible to measure certain properties, such as formation, of thick fiber webs, which cannot be measured with the measurement devices of prior art at all.

A number of advantages arise with the possibility of using the device for the measurement of a fiber web supported by a supporting means. The web can be measured closer to the web forming part than before. The structure of the web no longer needs to be strong enough to withstand a so-called free transfer, where the web runs without the supporting means and where the measurements are conventionally made. Furthermore, there is no longer a need to arrange free transfer for measurements at all. This supports the current trend of aiming at continuous supporting of the web in the web manufacturing machines.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a schematical principle view of the device according to the invention, seen from the side,

Fig. 2 shows a schematical side view of a device according to the invention,

Fig. 3 shows a schematical side view of a second device according to the invention, and

Fig. 4 shows a supporting surface from above.

In Figs. 1 to 4, the same numerals refer to corresponding parts and they will not be explained separately later on, unless required for the illustration of the subject matter.

Detailed description of the invention

Figure 1 shows a principle view of a device for measuring a web. In a machine for manufacturing a fiber web, such as a paper, cardboard or cellulose web manufacturing machine, which machine is not shown in the figure for clarity, the web W to be manufactured moves forward in the direction of an arrow A. On one side of the web W, that is on the side of its second surface S2, a light source 1 is arranged to illuminate the web W from the side of its second surface S2. In the corresponding location on the other side of the web W, a detector is provided, that is, a camera 2 arranged to form images of the web from the side of its first surface S1 substantially in the location exposed to illumination. The camera and the light source are thus on opposite sides of the web W. The image material produced by the camera 2 is led via a line 3 to data processing equipment 4.

The data processing equipment 4 comprises software and algorithms, by means of which the image material can be processed. The data processing equipment also comprises means for receiving and storing the image material. The camera 2 can also be controlled by the data processing equipment 4, for which purpose the data processing equipment comprises means for transmitting control commands to the camera. The light source 1 can also be controlled by the data processing equipment 4. The data processing equipment 4 and the light source 1 are connected to each other via a line 8.

The data processing equipment 4 is connected to a user interface 5 comprising a display device 6 and one or more input devices 7. The display device 6 may be a display based on a cathode tube, a flat panel display, or an image projected onto a substrate. The input device

7 may be a conventional keyboard, a mouse, or another data input device known in the field. By means of the display device 6, the operator may visually observe, that is, look at the image material produced by the camera, and by means of the input device 7, it is possible to control the camera 2 and the light source 1.

The light source 1 is a diode laser light source emitting light pulses with a high pulse power. The illumination emitted by the diode laser can penetrate the web W and makes it possible to see, for example, structural elements forming the web, such as fibers, in the image material produced by the camera 2. This measurement configuration is thus suitable for measuring, for example, the formation of the web or the fiber orientation. The diode laser source consists of at least one diode laser. When several diode lasers are used, their illumination is focused in such a way that their light power is combined. As a result, only one, very intense light beam is directed to the web. The area illuminated by the light beam may vary from a very small spot of light to a large illuminated area.

Figure 1 also shows another measurement configuration, in which the light source V is placed on the same side of the web W as the camera 2. The light source 1 ', indicated with broken lines in the figure, is a diode laser light source. The light source V illuminates the web W from the side of its first surface S1 at a given angle, and the camera 2 is arranged to detect, that is to form images, from the same side as the illumination, substantially the location of the web which is exposed to the illumination. The image material produced by the camera 2 is led via a line 3 to data processing equipment 4. The light source V can also be controlled by means of the data processing equipment 4. The data processing equipment 4 and the light source V are connected to each other via a line 9. This measurement configuration is suitable for the monitoring of a moving web, for example for detecting defects in the web, such as holes or marking caused by machine parts in contact with the web.

Figure 2 shows an embodiment for measuring web properties. The web W moves in the direction of the arrow A in the web manufacturing machine. On one side of the web, in the vicinity of the second surface

S2 of the web, an illumination platform 10 is arranged and provided with a window 11. The window 1 1 is placed in vicinity to the second surface S2 of the web W. The window 11 is embedded in the illumination platform 10 on the web side so that the surfaces of the window 11 and the illumination platform 10 that face the web are substantially on the same level and form a uniform flat surface that does not prevent the travel of the web. The illumination platform 10 may be in contact with the web W to be measured, or there may be a small air gap between the illumination platform 10 and the web W. Figure 4 shows the illumination platform 10 seen from above. The window 11 is fixed substantially in the center of the illumination platform 10 so that it is surrounded on all sides by the edges of the illumination platform. The suitable shape and size of the window 11 are selected according to the measurement target. In the example of the figure, the window is square. The sides of the square may have a length ranging from about 60 to 120 mm and being preferably about 70 mm. The window 11 may also have another shape, for example, annular, rectangular or polygonal. During the illumination, the whole window is illuminated, illuminating the web over an area having the size of the window. The illumination platform 10 is equipped with means for distributing light evenly over the window 1 1. The means may comprise mirrors, grates, optical fibers or other elements distributing light evenly over the window 11.

The illumination platform 10 is connected to the light source 1 via a light guide 12. The light guide 12 is a single optical fiber or a fiber bundle consisting of several optical fibers. It is also possible to use other guides suitable for the transmission of light. The light source 1 is a diode laser light source. The light guide 12 transmits the light emitted by the light source 1 into the illumination platform 10, and the web W is illuminated through the window 11. By means of the light guide 12, it is possible to transmit the illumination into targets that are placed in locations that are difficult to reach in web manufacturing apparatuses. By using a light guide for the transmission of light, it is also possible to implement a uniform backlight for the measuring target and to illuminate the target from the imaging direction in different ways. A camera 2 is placed in the location corresponding to the illumination plane, on the side of the first surface S1 of the web W, and is arranged to detect substantially that area of the web which is exposed to the illumination.

Figure 3 shows a second embodiment for measuring web properties. In this embodiment, the web W is supported by a supporting means 13 moving substantially at the same rate and in the same direction as the web W. The running direction of the web W and the supporting means 13 is indicated with an arrow A in the figure. The supporting means 13 is a band-like endless belt loop, such as felt or wire, supported by guide rolls and running in the web manufacturing machine. It may also be a band-like loop of a supporting means installed in the machine entirely for the measurement. The second surface S2 of the web W is in contact with the first surface SS1 of the supporting means. On the other side of the supporting means 13, an illumination platform 10 is provided, which is in contact with the second surface SS2 of the supporting means 13.

As shown in the figure, the web W is illuminated through the supporting means 13. The camera 2 is placed on that side of the web W which is not supported by the supporting means, that is, on the side of the first surface S1 of the web. The camera 2 detects the web substantially in the location that is exposed to the illumination. As in the embodiment of Fig. 2, the web W is illuminated by light penetrating the window 1 1 arranged in the illumination platform 10. Also the camera and the light source are on opposite sides of the web W. The illumination platform 10 is connected to the light source 1 via a light guide 12.

The device according to this embodiment can be used to measure a fiber web running very fast. The web is supported all the time, wherein the risk of web breaks is very small. Because the illumination platform is in contact with the supporting means and not the web, possible markings caused to the web by the illumination platform are eliminated.

In the embodiments shown in Figs. 2 and 3, the image material produced by the camera 2 is led via the line 3 to the data processing equipment 4. The data processing equipment 4 is a device of the type described earlier, for receiving, processing and storing image material. It can also be used for controlling the camera 2 and the light source 1. The data processing equipment 4 and the light source 1 are connected to each other via a line 8.

In the embodiments of said figures 2 and 3, the data processing equipment 4 is, as described earlier, connected to a user interface 5 comprising a display device 6 and one or more input devices 7. Furthermore, the light source 1 shown in Figs. 2 and 3 is a diode laser light source. Preferably, it consists of more than one diode lasers so that the luminosity of the illumination emitted by the light source is sufficiently high. The light source 1 emits pulsed light.

The embodiments of Figs. 2 and 3 are particularly well suited for measuring the formation of the web. Thus, the camera used is a CCD or CMOS camera capturing still images of the web illuminated by a diode laser light source. The light power of the illuminating pulses emitted by the diode laser light source is intense, and the pulse power of the light pulses is high. In the measurement of the formation, this improves spatial resolution and makes it possible to measure thick webs. The light power of the illuminating pulses is so intense that they are capable of penetrating the supporting means shown in the embodiment of Fig. 3 and illuminating the fiber web on its other side so efficiently that the images of the web, captured by the camera, are sufficiently sharp images for determining the formation of the web. The performance of the device is sufficient for measuring the formation of thick webs, such as cellulose webs with a basis weight exceeding even 1000 g/m², through a supporting means.

Formation refers to the small-scale variation in the basis weight of the web. Variations in basis weight are due to the uneven distribution of the fiber pulp in the direction of the plane of the web, poor orientation of the fibers, and uneven distribution of possible fines in the web in the thickness direction of the web. In the measurement of the formation, variations in the grey tone levels of the image material produced by the camera are analyzed, and a formation index is determined from them.

The camera used in the device may, as such, contain software components for storing and processing the image material captured by the camera. The data processor may be a separate unit, or it may be integrated in the user interface. Thus, the user interface may be a mobile device, for example a portable computer. The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. Consequently, the invention is also suitable for determining the properties of fibers in stock.

Claims

1. A method for measuring at least one property of a moving web, in which method the web (W) is illuminated with a light source (1 ) and detected with a camera (2) substantially at the illuminated area, the camera (2) and light source (1 ) being placed on opposite sides of the web (W), characterized in that when illuminating the web (W), an illumination platform (10) connected to a diode laser light source is used.

2. The method according to claim 1 , characterized in that the web (W) is illuminated from the side of its second surface (S2), and the web (W) is detected from the side of its first surface (S 1 ).

3. The method according to claim 1 , characterized in that the illumination platform (10) is fitted in the vicinity of the web and provided with a window (11 ).

4. The method according to claim 1 , characterized in that the light emitted from the light source (1 ) is transmitted via a light guide (12) to the illumination platform (10).

5. The method according to claim 1 , characterized in that the web is illuminated by means of the illumination platform (10) which is in contact with the web (W).

6. The method according to claim 1 or 2, characterized in that the web (W) is supported by a supporting means (13) and that the web (W) is illuminated through the supporting means (13).

7. The method according to claim 6, characterized in that the web is illuminated by means of the illumination platform (10) which is in contact with the supporting means (13).

8. The method according to claim 1 , characterized in that the camera (2) captures image material which is transmitted into a data processing equipment (4), in which at least one property of the web (W) is determined from the image material.

9. The method according to claim 8, characterized in that the image material is observed visually, and the data processing equipment (4) is controlled by means of a user interface (5).

10. The method according to claim 1 , characterized in that at least one of the following properties is measured from the web (W): formation, fibre orientation, roughness, and defects in the web.

11. The method according to claim 1 , characterized in that the web (W) is one of the following: paper, cardboard or cellulose web.

12. A device for measuring at least one property of a moving web (W), which device comprises a light source (1 ) for illuminating the web (W) and a camera (2) for detecting the illuminated area of the web (W), the camera (2) and light source (1 ) being placed on opposite sides of the web (W), characterized in that the device comprises an illumination base (10) which is connected to a diode laser radiation source.

13. The device according to claim 12, characterized in that the light source (1 ) is arranged to illuminate the web (W) from the side of its second surface (S2), and the camera (2) is arranged to detect the web (W) from the side of its first surface (S 1 ).

14. The device according to claim 12, characterized in that the illumination platform (10) comprises a window (11 ), and that the illumination platform (10) is placed in the vicinity of the web (W).

15. The device according to claim 12, characterized in that the device comprises a light guide (12) for transmitting the light emitted from the light source (1 ) to the illumination platform (10).

16. The device according to claim 15, characterized in that the light guide (12) is a single optical fibre or a fibre bundle consisting of several optical fibres.

17. The device according to claim 12, characterized in that the illuminating platform (10) is arranged to contact the web (W).

18. The device according to claim 12 or 13, characterized in that the web (W) is arranged to be supported by a supporting means (13) and that the web (W) is arranged to be illuminated through the supporting means (13).

19. The device according to claim 18, characterized in that the illuminating platform (10) is arranged to be in contact with the supporting means (13).

20. The device according to claim 12, characterized in that the device comprises a data processing equipment (4), to which the image material captured by the camera (2) is arranged to be led and which data processing equipment (4) is arranged to determine at least one property of the web (W) from the image material.

21. The device according to claim 20, characterized in that the device comprises a user interface (5) for observing the image material visually and for controlling the data processing equipment (4).

22. The device according to claim 12, characterized in that the device is arranged to measure at least one of the following properties of the web (W): formation, fibre orientation, roughness, and defects in the web.

23. The device according to claim 12, characterized in that the web (W) is one of the following: paper, cardboard or cellulose web.