WO2013043103A1

WO2013043103A1 - Method for measurement of topography on the surface of a material web

Info

Publication number: WO2013043103A1
Application number: PCT/SE2012/000145
Authority: WO
Inventors: Thim Jan Peter LUNDGREN; Mattias O'NILS; Anatolly MANUILSKIY; Anzar Mohammad ALAM
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-09-25
Filing date: 2012-09-24
Publication date: 2013-03-28
Anticipated expiration: 2014-03-25
Also published as: SE1100700A1; SE536900C2

Abstract

Procedure used for, in essential real time, measuring the topography of the surface of a material web (3) which is in motion, by illuminating the surface with a Cross machine Directional, or essentially Cross machine Directional line of optically radiated light that is emitted from at least one optical radiating light source (4) where the reflected light from the illuminated surface (2) of the web (3) is gathered (registred) by at least one image sensor or sensors (5), whereafter the, by the image sensor or sensors, gathered information is compiled and processed by at least one processing device (6) into topographical information regarding the surface of the web, and whereafter the topographical information is stored on a storage device and/or is transmitted to at least one other unit. The uniqueness of the invention described in this patent application is that at least one image sensor (5) gathers information that is transformed into a very wide spatial wavelength spectrum, and that the measurement is done in the Cross machine Direction, or essential Cross machine Direction, of the web (3), and that the measurement occurs when to movement velocity of the web is at least 10 meters per minute.

Description

METHOD FOR MEASUREMENT OF TOPOGRAPHY ON THE SURFACE OF A MATERIAL WEB

Technical field

The invention regards a method and a device for measuring topography on the surface of a material web. More specifically, the invention regards a method and a device for measuring topography on the surface of a material web in motion, in accordance with the patent demands.

Technical background

Measurements of topography on the surface of material webs are an important part in securing the quality of a number of different products. For example, topography can be measured in the manufacturing of paper and paper related products. The measurement of the topography is essential to determine that the surface of the paper, the printability and the quality of the final product corresponds to the demands of the customer.

The topography of a paper web has significance for the printability and the perceived print quality of a paper. The way a print is perceived on a paper is highly dependand on the spatial wavelengths of the surface of the paper. It is therefore customary to use spectral analysis of the spatial wavelengths as a measure of how well the surface responds to print. These are also simplified memasures such as surface roughness that describe the shorter spatial wavelengths in a combined value.

General methods for measuring topography on a surface can be devided into contact based methods and optical methods. The contact based methods of types Perthometer and AFM are not suitable for measurements of a surface in movement because of frictional wear and vibrations. There are many optical methods for gathering information of a topographical surface, which can be categorized into methods that are able to measure topography in a single point, methods that are able to measure topography for a line and methods that are able to measure topography for a surface area. Methods that are used for measuring topography in a single point are auto focus based methods, white light interferometer based methods and triangulation in a single point. The problem with point based methods is that some are too slow (auto focus based methods and white light interferometer) to enable use on moving objects. Furthermore, in order to enable surface characterization in a right angle to the surface motion direction, it is required that the measuring point moved very quickly over the moving surface. Methods that characterize a surface area can be methods based on light spread and/or shadow based methods. These methods are only able to characterize very short topographical wavelength spectra with low dynamic resolution. Standard methods for characterize topography with a line are based on triangulation, and have limitations in the dynamic range related to the accuracy of the measurements.

One challenge with measuring topography on a material web in an industrial process is that the material webs move with a high velicity in the length direction (machine direction) of the web. There are many examples of webs of paper i paper manufacturing machines that have a movement velocity of over 400 meters per minute. At these velocities, there are great challenges in gathering data for measurements of the webs topography.

Prior art

Several methods for measuring topography of material webs are known. For example, in the patent WO 2009/083655 A 1 is decribed a method that uses a formation of an uncertainty value of flashes of light from sources that emit a wide spectrum of light. This method is fundamentally different from the method of the invention described by this patent, in that the light is not formed as a line that indicate the actual topography in the Cross machine Direction (CD), but rather takes a mean value of the relative hight change over a surface. With the invention described by this patent, the formation of a wavelength spectrum is a result of the measurement, which is not possible to achieve in WO 2009/083655A1. Only one image sensor is used in WO

2009/083655A1, which indicates that longer wavelengths are not included in the measurement, which is also an important different compared to the invention described by this patent.

In the patent US 4202630 A a method is described for measuring topographical information on a surface. The method in US 4202630 A does involve a line of light, but rather a series of light spots ordered on a line. The large difference is here that with a series of light spots (in US 4202630 A) the relative distance between the spots is measured and the distances are then converted into topographical heights. With such a metod the size of the spots should be significantly smaller than the distance between the spots, which severely limits the resolution or at the very least sets unreasonable demands on the light source and grid used, in order to achieve the same spatial resolution as in the invention proposed in this patent application. Furthermore, the resolution of an image sensor (in a renewed version of US 4202630 A) must be considerably higher that the final spatial resolution in the measurement. The method in US 4202630 A is suitable for measurements where the spatial resolution is not critical. If the measurements should be done in the micrometer range, however, the method in US 4202630 A is in the best of cases ineffective. In the invention described by this patent application a continous line of light is projected onto the surface and the image sensor or sensors in combination with the processing device measures the relative divergencies in a right angle to the line direction. This means that the spatial resolution is limited by the pixel size in the image sensor and o the number of pixels over the width of the line, which makes the invetion described by this patent application far more technology efficient than the method in US 4202630 A.

In the patent JP 57049805 A a bethod is described where you in a point (in the cross section of the line of light source and the line sensor) measure the intensity function of the cross section of the line of light and then, by studying at what degree this function resembles a rectangular or a Gaussian distributed shape, makes a decision of the surface roughness. This measurement has nothing to do with topographical heights. It is furthermore not possible to extract spatial wavelength spectra from the method in JP 57049805 A, which is an important part of the invention described by this patent application. If the method in JP 57049805 A were to be expanded with a two dimensional sensor or even multiple sensors, one would still not be able to measure topography or be able to extract wavelength spectra. One could achieve mean values of the cross section profile over multiple strips and thereby get a more accurate measure, that would still be based on an empirically founded estimation of the shape of a function.

In the patent EP 1898207 Al multiple image sensors are used, which is the only similar detail to the invention described by this patent application. In the metod in EP 1898207 Al the sensors image the surface with no specific light source. A direct spectral analysis is made on tva or more columns in the image sensor. This to see if there are frequency peaks in the spectrum that would indicate a, in the mchine direction, irregularity. This is an effective method to detect drag marks on a surface, which is the limit of the functionality of the method.

In the patent US 4019066 A a relatively simple method is described to estimate (rather that measure) surface roughness on a surface. In US 4019066 A the surface is illuminated and the relative intensity differences of the entire imaged surface are used to form a mean estimation of the surface roughness. It is a quite unprecise method that does not image the topography and cannot be used to extract spatial wavelength spectra. It is, however, an interesting simple method for measuring surface roughness that in most respects has little in common with the invention described by this patent application.

In the patent text WOOO/68638 Al a method is described for measuring roughness and gloss, rather than topography. A surface is illuminated and a mean value of roughness and gloss is produced. The text in WOOO/68638 Al does not describe any measures of spatial wavelength analysis or Cross machine Direction topography. Furtermore, the txt in WOOO/68638 Al does not describe online measurements.

In the patent text SE 51 1985 C2 a method is described to extract topography from a surface by illuminating the surface with diffuse light. The illuminated area is very limited (5 x 5 mm), which results in the neglect of longer spatial wavelengths measures. The patent text in SE 511985 C2 is furthermore different from the invention described by this patent in that the device in SE 51 1985 C2 is not designed for online measurements.

In the patent text SE 528526 C2 a method is described that is intended as a complement to surface roughness measurements, where larger defects on a paper surface are detected with the aid of illuminated light on the surface at a glancing angle. This gives a numerical measure on how even the surface is. The method as it is described in SE 528526 C2 does not measure the actual topography in the Cross machine Direction of the paper. In difference to the invention described by this patent the method in SE 528526 C2 is only intended for laboratory studies rather than online measurements.

In the patent texts FI 914794 A and FI 91676 C methods are described where a laser point is swept ovr the surface, in stead of projecting a line of light, as in the invention described by this patent. The described metods also only use one image sensor. The combination of these features means that longer spatial wavelengths are lost. A correct topographical representation in the Cross machine Direction is not achieved, but rather a mean value of the surface roughness.

In regard to the problems with current techniques and methods, there is an obvious need for substantially improved equipment for measuring the topography of a material web in real time. Brief description of the concept of the invention

The main pupose of the invention described by this patent is to create a sunstantially improved metod for measuring topography of a material web. Another purpose with the invention described by this patent is to measure the topography of a material web i movement. Yet another purpose of the invention described by this patent is to create a method for measuring the topography of a paper web in the Cross machine Direction. It is furthermore a purpose of the invention described by this patent to measure in a wide spatial wavelength spectrum with a high dynamic range.

Brief description of the drawings

In the following detailed description of the invention described by this patent, references to the following figures will be made. Each figure is here described in short. Note that the figures are schematic and that detailes therefore may be left out. The examples of applications described by the figures are not limiting the width of protection for this patent application.

Figure 1 shows, from a side view, schematically the principle for a device in accordance with this patent application.

Figure 2 shows, in a view from above and the side, schematically the principle for a device in accordance with this patent application. Here, the mechanical suspentions of the system parts have been left out.

Figure 3 shows, in a view from above and the side, an alternative form of execution of the invention described by this patent. Here, the mechanical suspentions of the system parts have been left out.

Detailed description of the invention

In reference to the figures a device 1 is schematically shows with the the purpose of, in real time, measuring the topography of the surface 2 of a material web 3, being in a state of motion in the Machine Direction of the material web.

The material web 3 is signified by a web moving at a velocity of a t least 10 meters per minute relative the device 1. In the preferential form of execution of the invention described by this patent, the material web 3 consists of a web in a machine that manufactures paper, paperboard, card board or similar products where the material web 3 has a movement velocity over 100 meters per minute. Preferentially the material web 3 has a movement velocity over 400 meters per minute. In anternative forms of execution it is thinkable that the material web 3 consists of another type of web, which has a movement velocity over 10 meters per minute and

preferentially over 100 meters per minute.

Preferetially the topography of the surface 2 is measured in the Cross machine Direction of the web 3 or in the essential Cross machine Direction. In anternative forms of execution it is thinkable that the measurement of the topography of the surface 2 of the web 3 is conducted in a different direction than the Cross machine Direction or the essential Cross machine Direction.

The device 1 comprises at least one electromagnetically radiating source, preferentially an optical radiating source 4, at least one image sensor 5 and at least one image processing device 6. The image processing device 6 process the gathered information from the image sensor or image sensors, 5. The image processing device 6 is assumed to have appropriate equipment for storage and processing of the gathered information from the image sensor or image sensors, 5.

Preferentially the processing device also has a function for transferring the gathered and/or processed information to another type of unit or similar.

In Figure 1 the electromagnetically radiating source (or sources) 4 and the image sensors 5 are attached to a frame, structure, or similar. The technical effect of the frame, or similar, is that this positions the radiating source, or sources, and the image sensors relative to each other and in relation to the web 2. The design of the frame, or similar, can vary greatly within the boundaries of the invention described by this patent. The optical radiating source 4 can be comprised of one or more optical radiating sources.

Preferentially the optical radiating source 4 emits optical radiation within the wavelength range 100 nm to 1 mm.

The optical radiating source 4 consits of, or include, LED, laser or for the purpose at hand suitable radiating source. In the preferential form of execution a NIR laser is used as an example, as the visual spectrum including ambient light can be filtered out. It is furthermore thinkable that different types of optical radiating sources are combined and used in the device 1.

The principle for measuring in accordance with the procedure of this patent application is that at least one surface in the Cross machine Direction, or the essential Cross machine Direction, is illuminated with at least one optically radiated line 7 emitted by at least one radiating source 4. The electromagnetically radiated light is projected at an incoming angle VI relative to the web surface. The angle VI consists of the angle between the web surface and the angle (essential angle) of the radiating light. The angle VI is preferentially in the range 20-90 degrees.

At the time of illumination of the surface, parts of the optical radiating light are reflected and parts are absorbed by the surface. The reflected optical radiating light in a particular direction to a particular point, line or surface area vary in relation to the topography of the illuminated web. This variation takes the form as lateral divergencies in the line of light relative a straight line, and is gathered via at least one sensor 5 or preferentially two or more sensors 5. The information from the sensor or sensors 5 is used in the invention in this patent application as incoming information to the processing device 6 in which the gathered information is processed into topographical information of the web surface.

The sensor or sensors 5 that gather information from the reflected light is/are preferentially mounted in an angle according to angle V2 in relation to the web. The angle V2 consists of the angle between the web surface and the gathering direction of the sensor or sensors. The angle V2 is preferentially in the range 20-90 degrees. Preferentially the design includes multiple sensors 5 that gather the information from the reflected light and transmits to and/or stores the gathered information from each sensor 5 in a, for the applicational purpose, suitable storage media.

Differences in the topographical height on the surface of the web yield different spatial divergencies of the line position in relation to a straight line. In the exemplifying form of execution of the method multiple sensors 5 have been placed in the Cross machine Direction, or the essential Cross machine Direction, of the web, so that each sensor gathers information of parts of the illuminated line formed in the Cross machine

Direction, or the essential Cross machine Direction. The gathered information from the sensors 5 is processed by the processing device and is combined so that the gathered information reflects the line of light in its full length. The angle between the image sensors 5 and the line of light 7 makes the line of light bend in accordance to the surface topography, see figure. These angles are optimized for a particular surface in a particular application, where a coarse surface need a larger angle between the web surface and the light source, as shadowing would otherwise distort the measurements, while a smooth surface need a smaller angle, as a smooth surface require a higher relative intensity in the topographical information. Thereafter the center line of the line of light is extracted.

This center line is then seen as an estimation of the surface topography along the line. Extracting the center of the line enables a reduction of data essential for real time processing of the information. The center line is then transformed to give information about the spatial wavelengths of the surface, where wavelength spectra is something that the paper industry use as a quality measure relating to the print quality. In order to describe the average topographical information of a paper, an average of several lines is also taken. In order to measure in high velocities the measurements of the lines are done with a particular time interval and the lines are then averaged for the surface average measure.

The uniqueness with the method is that is enables measurements of topographical information with a wide spatial wavelength spectrum, in the Cross machine Direction while the surface is moving with a high velocity. In the paper industry there have previously been trials conducted in measuring along the Machine Direction (MD), a term used by the industry. Experience have shown, however, that there are larger topographical variations in a multitude of wavelengths in the Cross machine Direction (CD), also a term used by the industry. This new method of measurement enables quality measures for print related surface topography in CD online in the paper manufacturing machine.

This method has a limitation in how short the wavelengths can be related to the image sensor resolution. Since the method uses the center of a line whose line width contains several pixels, the method cannot completely utilize the full resolution of the image sensor, but rather calculates a sub-pixel precision center based on several pixels. As a supplementary method to this methos, light 8 can be projected with a very small angle (-10-20 degrees) relative to the surface with a supplemental light source 9. In this case the shadows from the surface can be used in a measurement that can yield topographical information for very short wavelengths, and increase the intensity in the topographical values of short wavelengths. In this case, rows from the illuminated light, captured by the image sensors must be derivated before they are transformed in order to recreate the topography information distorted by the shadowing. This supplemental method is suitable for spatial wavelength <50um, and enables a further extension into very short wavelengths of the resulting total spatial wavelength spectrum.

In the detailed description of the invention in this patent application design details may be left out that are obvious to an expert in the field that the invention falls within. Such obvious design details include details in the magnitude required to achieve a fully functional device for the invention in this patent application.

Even if some preferential forms of execution for a final design have in this text been described in detail, variations and modifications may occur within the boundries of the invention and obvious for an expert in the field. All those variations are regarded to fall within the boundries of the following patent claims. It is, for example, thinkable that the measurement of topography is done in combination with at least one other quality of the web.

Advantages of the invention

There are a number of advantages with the invention described in this patent application. The most important advantage is that a substantially improved method is acquired for measuring the topography online in the Cross machine Direction of a paper web. With this new method the possibility arises to aquire a quality measure related to print quality in the Cross machine Direction in a paper manufacturing machine in real time.

Claims

1. Procedure for, in essential real time, measure the topography of the surface of a material web (3), that moves with a velocity of at least 10 meters per minute in the Machine Direction of the material web, by irradiating a cross directional, or essentially cross directional, surface of the web (3) with a line of optical light, emitted by at least one optical radiating source (4) where the reflected light from the web (3) is gathered (registred) by at least one image sensor (5), where the information gathered by the image sensor or image sensors (5) is compiled and processed by at least one processing device (6) into topographical information regarding the surface (2) of the web (3) characterized by the gathering of information is done by at least one image sensor (5) that gather information containing a wide spatial wavelength spectrum in regard to the lateral divergencies from a straight line in the Cross machine Direction, or essentially Cross machine Direction, of the material web (3), whose gathered information is transmitted to at least one processing device (6) with which the information gathered by the image sensor or sensors transforms the information into topographical information regarding the surface (2) of the web (3), whereafter the topographical information is stored on at least one storage media or and/or is transmitted to at least one other unit.

2. Procedure in accordance with patent claim 1 characterized by the sensor or sensors (5) measure in a wavelength range where the relation between the shortest and the longest spatial wavelength is at least 200 times.

3. Procedure in accordance with patent claim 1 characterized by the sensor or sensors (5) measure in a wavelength range where the relation between the shortest and the longest spatial wavelength is at least 1000 times.

4. Procedure in accordance with patent claim 1 characterized by that by combining the topographical information from several image sensors by correlation and koncatenation of the slightly overlapping imaged lines of light, create wavelength spectra containing longer wavelengths.

5. Procedure in accordance with at least one of the previous patent claims characterized by the lines being measured with a particular time difference, that the lines form a meanvalue and that the spatial wavelength spectra are formed from these mean values by utilizing transforms.

6. Procedure in accordance with patent claim 5 characterized by the center of the lines are calculated before the following processing starts, in order to reduce the quantity of data.

7. Procedure in accordance with at least one of the previous patent claims characterized by the material web in contained of paper, paperboard or cardboard.

8. Procedure in accordance with at least one of the previous patent claims characterized by at least one light source with low coherence is used to reduce the coherence noise in the imaged lines of light and thusly increase the measurement accuracy.

9. Procedure in accordance with at least one of the previous patent claims characterized by the measurements include at least one line of light running in the Cross machine

Direction and enabling the method to measure a surfave travelling at a velocity of over 400 meters per minute.

10. Procedure in accordance with at least one of the previous patent claims characterized by the procedure being supplemented with light emitted from at least one light source whose impact angle is in the range 10-20 degrees relative to the web surface, where the impacing light is used to more accurately calculate the intensities of the very short wavelengths (-10-100 μπι), where in this case the shadowing from the surface is used to increase the intensity of the topographical values, and that in this case the rows of the image sensors must be derivated before they are transformed in order to recreate the topographical information that have been distorted by the shadowing.