structure of cardboard process

Field

The invention relates to a system and method for measuring a moving layered sheet structure of a cardboard process.

Background

A simple cardboard sheet comprises a corrugated sheet and a non- corrugated sheet which are adhered to each other using glue at the ridges of the corrugated sheet, the ridges being pressed against the non-corrugated sheet. Sometimes it happens that the gluing fails, which may be caused by the gluing process or the corrugating process. The failures are difficult to detect and defected cardboard may be delivered to a customer. There are ultrasonic and optical methods which are used to detect the failures but their ability is limited and they do not particularly suitable for automatic control of the production.

Hence, there is a need for improvement.

Brief description

The present invention seeks to provide an improvement in the production of cardboard.

The invention is defined by the independent claims. Embodiments are defined in the dependent claims.

List of drawings

Example embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which

Figure 1 illustrates an example of a system for measuring a moving layered sheet structure of a cardboard manufacturing process;

Figure 2 illustrates an example where the system comprises a first light source; Figure 3 illustrates an example where the system comprises a second image capturing device;

Figure 4 illustrates an example where a data processing unit matches timings of captures of images of the first section and the second section;

Figure 5 illustrates an example of a first light source and a second light source illuminate the layered sheet structure with optical pulses;

Figure 6A illustrates an example of an optical component that receives optical radiation from the first edge surface and the opposite sheet surfaces of the layered sheet structure;

Figure 6B illustrates an example of the optical component with two or more objective lenses;

Figure 7 illustrates an example of the optical component with two or more mirrors;

Figure 8 illustrates an example of the second image capturing device with an optical component that receives optical radiation from the second edge surface and opposite sheet surfaces;

Figure 9 illustrates an example of a corrugation system with a corrugator and an adhering apparatus;

Figure 10 illustrates an example of failures in adhesion between the corrugated sheet and the non-corrugated sheet;

Figure 11 illustrates an example of a removal apparatus at a known position;

Figure 12 illustrates an example of a block diagram of the data processing unit and the controller of the cardboard manufacturing process; and

Figure 13 illustrates an example of a flow chart of a measuring method.

Description of embodiments

The following embodiments are only examples. Although the specification may refer to "an" embodiment in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may contain also features/structures that have not been specifically mentioned. All combinations of the embodiments are considered possible if their combination does not lead to structural or logical contradiction.

It should be noted that while Figures illustrate various embodiments, they are simplified diagrams that only show some structures and/or functional entities. The connections shown in the Figures may refer to logical or physical connections. It is apparent to a person skilled in the art that the described apparatus may also comprise other functions and structures than those described in Figures and text. It should be appreciated that details of some functions, structures, and the signalling used for measurement and/or controlling are irrelevant to the actual invention. Therefore, they need not be discussed in more detail here.

Figure 1 illustrates an example of a system for measuring a moving layered sheet structure 100 of a cardboard manufacturing process. The system comprises a data processing unit 102, and a first image capturing device 104. The data processing unit 102 may be included in a controller 344 of the cardboard manufacturing process or it may transfer its processed data to a separate controller 344 of the cardboard manufacturing process. The first image capturing device 104 has is farther from a longitudinal centerline 106 of the layered sheet structure 100 than an edge 108 of the layered sheet structure 100 in a direction orthogonal to both the centerline 106 and a thickness of the layered sheet structure 100. Then a distance D1 between the first image capturing device 104 and the centerline 106 is longer than a distance between the edge 108 and the centerline 106. The layered sheet structure 100 comprises a corrugated sheet 114 and a non-corrugated sheet 116 which are adhered together with adhesive at ridges 118 of the corrugated sheet 114. The adhesive may include a suitable glue or the like. The corrugated sheet 114 is made of paper or board. Correspondingly, the non-corrugated sheet 116 is made of paper or board. The corrugated sheet 114 has a wavy form with furrows and ridges which extend in the transverse direction with respect to the machine direction and which are one after another in the machine direction.

Sometimes there is something wrong in the adhering process and/or in a corrugation process for example, and the corrugated sheet 114 and the non- corrugated sheet 116 are poorly or not at all adhered to each other.

The longitudinal centerline 106 is parallel to the machine direction of the corrugation process 900. Thus, a distance between the first image capturing device 104 and an edge 108 may be measured along a shortest route of the surface of the layered sheet structure 100. A direction parallel to the thickness of the layered sheet structure 100 is perpendicular to both the machine direction and the transverse direction, the transverse direction and the machine direction being orthogonal with respect to each other.

The first image capturing device 104 may comprise a camera which may be a ID camera or a 2D camera that has a semiconductor cell for converting an optical image into an electrical form. The ID camera may be a line camera which has detector elements arranged one dimensionally and which captures line images one after another while the layered sheet structure 100 is moving in the machine direction. A 2D image may be made by stitching the line images together in a capturing order. The 2D camera has detecting elements arranged two dimensionally and a 2D image is formed each time an image is captured. The first image capturing device 104 may comprise a CCD camera (Charge-Coupled Device camera) or a CMOS camera (Complementary Metal-Oxide-Semiconductor camera), for example. The first image capturing device 104 may be sensitive to ultraviolet light, visible light and/or infrared light for detection.

The first image capturing device 104 is directed towards a first edge surface 110 of the layered sheet structure 100 such that said first edge surface 110 is within a field-of-view 120 of the first image capturing device 104. The light of the first light source 122 illuminates a first edge area 124 of the first edge surface 110. The first edge area 124 may be within the field-of-view 120 of the first image capturing device 104 in order to capture an image. An image of the first edge area 124 may also be made by combining a plurality of ID images or 2D images covering a smaller area of the layered sheet structure 100.

The edge surface 110 has typically a larger measure in the machine direction than in a direction that is used to measure a thickness of the layered sheet structure 100, because the layered sheet structure 100 is continuous in the machine direction but typically only a few millimeters or a few centimeters thick. A normal of the layered sheet structure 100, the normal possibly being an averaged normal, is also parallel to the direction of the thickness.

The first image capturing device 104 comprises or is connected with the data processing unit 102. The image capturing device 102 captures repeatedly the images that represent the first edge surface 110, and the images are transferred to the data processing unit 102.

The ridges 118 are meant to be pressed against the non-corrugated sheet 116. In order to find out, if the combination of the corrugated sheet 114 and the non-corrugated sheet 116 have been successfully attached together, the data processing unit 102 detects a non-zero distance at one or more ridges 118 between the corrugated sheet 114 and the non-corrugated sheet 116. Alternatively or additionally, the data processing unit 102 detects a non-zero distance at the ridges 118 between a piece of the adhesive and either of the corrugated sheet 114 and the non-corrugated sheet 116. Still alternatively or additionally, the data processing unit 102 detects a non-zero distance at the ridges 118 between a piece of the adhesive on the corrugated sheet 114 and a piece of the adhesive on the non- corrugated sheet 116. The data processing unit 102 detects the non-zero distance in the direction of thickness of the layered sheet structure 100 in all these examples. The detection of the non-zero distance is based on an image processing of the images, which include the first edge surface 110. A non-zero distance can easily be detected at the first edge area 124.

In an embodiment, the data processing unit 102 may perform the detection by comparing an image of the first edge 124 with an average image of the first edge 124. If a difference between an image and the average image is within an accepted variance, the data processing unit 102 may determine that no detection of a non-zero distance is made. If, on the other hand, a difference between an image and the average image is not within an accepted variance, the data processing unit 102 may determine that a detection of a non-zero distance is made. The average may be measured from a plurality of acceptable layered sheet structures 100. The comparison may be made using correlation, for example. An accepted layered sheet structure 100 has a rather constant appearance in the image, i.e. a variance in the images of an accepted layered structure 100 is small. Thus, any larger change in the image with respect to the average indicates a defect.

In an embodiment, the data processing unit 102 may measure a power spectral density of the corrugation from an image, or the like for example, and compare such a measured power spectral density of the corrugation with an average power spectral density of the corrugation. If a difference between a power spectral density and the average power spectral density is within an accepted variance, the data processing unit 102 may determine that no detection of a non zero distance is made. If, on the other hand, a difference between a power spectral density and the average power spectral density is not within an accepted variance, the data processing unit 102 may determine that a detection of a non-zero distance is made. The comparison may be made using correlation, for example. An accepted layered sheet structure 100 has a rather constant power spectral density of the corrugation in the image, i.e. a variance in the power spectral density of the corrugation of an accepted layered structure 100 is small. Thus, any larger change in the power spectral density of the corrugation with respect to the average indicates a defect.

A zero distance refers to nongaseous material coupling at a ridge 118 between the corrugated sheet 114 and the non-corrugated sheet 116. The corrugated sheet 114 and the non-corrugated sheet 116 may physically touch each other. The sections of the corrugated and non-corrugated sheets 114, 116 that touch each other at the one or more ridges 118 then act as the nongaseous material that couples the corrugated sheet 114 and the non-corrugated sheet 116 together in a continuous manner. The corrugated sheet 114 may have adhesive at the one or more ridges 118, and the adhesive and the non-corrugated sheet 116 may physically touch each other. The adhesive then acts as the nongaseous material that couples the corrugated sheet 114 and the non-corrugated sheet 116 together in a continuous manner. The non-corrugated sheet 116 may have adhesive, and the adhesive and the one or more ridges 118 of the corrugated sheet 114 may physically touch each other. The adhesive then acts as the nongaseous material that couples the corrugated sheet 114 and the non-corrugated sheet 116 together in a continuous manner. The nongaseous material includes at least one of the following: a solid material, which may be hardened adhesive, and viscous material such as adhesive that has not been hardened or that does not require hardening.

The non-zero distance means that there is only environmental gas between at least the following: the corrugated sheet 114 and the non-corrugated sheet 116, a piece of the adhesive and either of the corrugated sheet 114 and the non-corrugated sheet 116, and a piece of the adhesive on the corrugated sheet 114 and a piece of the adhesive on the non-corrugated sheet 116. The environmental gas may include air. Instead of an environmental gas it is possible that only vacuum is between at least the following: the corrugated sheet 114 and the non-corrugated sheet 116.

The corrugated sheet 114 and the non-corrugated sheet 116 may be physically separated from each other at the one or more ridges 118. Then there is no continuous nongaseous connection between the corrugated sheet 114 and the non-corrugated sheet 116 at any such ridge 118. The corrugated sheet 114 may have adhesive at the one or more ridges 118, and the adhesive and the non- corrugated sheet 116 may be physically separated from each other. Then there is no continuous nongaseous connection between the corrugated sheet 114 and the non-corrugated sheet 116 at any such ridge 118.

The non-corrugated sheet 116 may have adhesive, and the adhesive and the one or more ridges 118 of the corrugated sheet 114 may be physically separated from each other. Then there is no continuous nongaseous connection between the corrugated sheet 114 and the non-corrugated sheet 116 at any such ridge 118. In an embodiment an example of which is illustrated in Figure 2, the system comprises a first light source 122 that may illuminate the layered sheet structure 100 within the field-of-view 120 of the first image capturing device 104 such that an optical axis 250 of the first light source 122 is at an angle a larger than about 45° with respect to the direction of the thickness. A wide angle of the illumination illuminates the first edge area 124 efficiently and makes it easy to reveal a non-zero distance associated with the corrugated sheet 114 and the non- corrugated sheet 116 adhered together.

In an embodiment, the first light source 122 may illuminate the layered sheet structure 100 with pulsated optical radiation. In this embodiment, the first light source 122 may thus output optical pulses repeatedly one after another in order to illuminate the layered sheet structure 100 with each of the optical pulses. The first light source 122 may output the optical pulses regularly or irregularly. The use of pulsated light saves energy.

In an embodiment, each of the light pulses of the first light source 122 may exposure the first edge area 124 of the first edge surface 110 within the field- of-view 120 of the first image capturing device 104 in order to capture an image with at least one of the optical pulses. In an embodiment, a single optical pulse may exposure of the first edge area 124 for capturing an image of the first edge area 124. In an embodiment, a plurality of the light pulses of the first light source 122 may be used to exposure the first edge area 124 of the first edge surface 110 in order to form an image of the first edge area 124 by a combination of images, each of the combined images covering only a section of the first edge area 124. In this manner, the first image capturing device 104 does not necessarily need a shutter for the exposures.

In an embodiment an example of which is illustrated in Figures 1 and 3, the system comprises a second image capturing device 200, which is farther from the longitudinal centerline 106 of the layered sheet structure 100 than an edge 108 of the layered sheet structure 100 in a direction orthogonal to both the centerline 106 and the thickness of the layered sheet structure 100. Then a distance D2 between the second image capturing device 200 and the centerline 106 is longer than a distance between the edge 108 and the centerline 106. Within the explained possibilities, the first image capturing device 104 and the second image capturing device 200 may be similar or they may differ from each other.

The second image capturing device 200 may be on the opposite side of the layered sheet structure 100 with respect to the first image capturing device 104 in the transverse direction. The second image capturing device 200 may capture images of the layered sheet structure 100. The second image capturing device 200 may comprises or may be connected with the data processing unit 102.

The second image capturing device 200 may transfer images that are repeatedly captured by the second image capturing device 200 to the data processing unit 102.

In an embodiment an example of which is illustrated in Figure 3, the second image capturing device 200 may be directed towards a second edge surface 112 opposite to the first edge surface 110, such that said second edge surface 112 is within a field-of-view 202 of the second image capturing device 200.

In an embodiment an example of which is illustrated in Figure 3, the system may comprise a second light source 204, which may be on the opposite side of the layered sheet structure 100 with respect to the first light source 122 in the transverse direction. The second light source 204 may illuminate the layered sheet structure 100 such that an optical axis 254 of the second light source 204 is at an angle b larger than about 45° with respect to the direction of the thickness. In an embodiment, the second light source 204 may be on the same side of the layered sheet structure 100 as the first light source 122 in the transverse direction. In this manner, the edges surface 124, 256 are illuminated effectively.

In an embodiment, the second light source 204 may illuminate the layered sheet structure 100 with a pulsated optical radiation. In this embodiment, the second light source 204 may be on the same side of the layered sheet structure 100 as the first light source 122 or on the opposite side of the layered sheet structure 100 in the transverse direction. The pulsated illumination saves energy.

In an embodiment, each of the pulses from the second light source 204 may perform an exposure of a second edge area 256 of the second edge surface 112 within the field-of-view 202 of the second image capturing device 200 in order to capture an image. In an embodiment, a plurality of the light pulses of the second light source 204 may be used to exposure the second edge area 256 of the first edge surface 112 in order to form an image of the second edge area 256 by a combination of images, each of the combined images covering only a section of the second edge area 256. In this manner, the second image capturing device 200 does not necessarily need a shutter for the exposures.

In an embodiment, the first light source 122 may illuminate a first planar section 258 of the layered sheet structure 100. In an embodiment, the first planar section 258 may be within the field-of-view 120 of the first image capturing device 104. Alternatively, the first planar section 258 may be included in an image of the first edge area 124 by a combination of images, each of the combined images covering only a section of the first edge area 124. The images that are combined may represent successive sections of the layered sheet structure 100 as the layered sheet structure 100 is moving. The successive sections may be partly overlapping or they may touch each other without overlapping.

The second light source 200 may illuminate a second planar section 260 of the layered sheet structure 100. In an embodiment, the second planar section 260 may be within the field-of-view 202 of the second image capturing device 200. Alternatively, the second planar section 260 may be included in an image of the second edge area 256 by a combination of images, each of the combined images covering only a section of the second edge area 256. The images that are combined may represent successive sections of the layered sheet structure 100 as the layered sheet structure 100 is moving. The successive sections may be partly overlapping or they may touch each other without overlapping.

The first section 258 and the second section 260 may be overlapping or they may be non-overlapping. In an embodiment, the first section 258 and the second section 260 together may cover continuously a whole width of the layered sheet structure 100 in the transverse direction. In an embodiment, the first section 258 may cover X percent of the width of the layered sheet structure 100 and the second section 260 may cover 100 - X percent of the width of the layered sheet structure 100, for example. In this example, the first section 258 and the second section 260 may touch each other. A length in the machine direction depends on the field-of-views 120, 202 of the first and second image capturing devices 104, 200. This kind of imaging enables the data processing unit 102 to detect indirectly any non-zero distance, because the non-zero distances cause stripes (see Figure 10) .

In an embodiment examples of which are illustrated in Figure 4, the data processing unit 102 may match timings of captures of images of the first section 258 and the second section 260 such that a combination of the images of the first section 258 and the second section 260 represent areas of opposite sides of the layered sheet structure 100 that a line 400 parallel to the transverse direction passes through both of them. In this manner, sections 258, 260 facing each other may be imaged simultaneously. In an embodiment, the first sections 258 one after another in the machine direction touch or partially overlap each other. In an embodiment, the second sections 260 one after another in the machine direction touch or partially overlap each other. In an embodiment, the first section 258 and the second section 260 touch or partially overlap each other.

In an embodiment an example of which is illustrated in Figure 5, the first light source 122 and the second light source 200 may illuminate the layered sheet structure 100 with pulses 500, 502 of the optical radiation in an alternating manner. The vertical axis denotes optical power P in an arbitrary scale and the horizontal axis denotes time T in an arbitrary scale. A pulse of repeated pulses 500 of light from the first light source may exposure the first field-of-view 120 covering the first edge area 124 of the first edge surface 110. A pulse P2 of the repeated pulses 502 of light from the second light source 200 may exposure the second field- of-view 202 covering a second edge area 256 of the second edge surface 112 at a moment between two temporally directly adjacent pulses PI, P3 of the repeated pulses 500 of light from the first light source 122. That is, the first light source 122 and the second light source 200 may illuminate by turns. The optical powers of the different light sources 122, 200 may differ or they may be the same (in Figure 5 they differ, for example). In an embodiment, first light source 122 may illuminate the layered sheet structure 100 in the first field-of-view 120 with a first band of optical radiation. The second light source 200 may illuminate the layered sheet structure 200 in the second field-of-view 202 with a second optical band of optical radiation. In an embodiment, the first optical band and the second optical band being non overlapping. Then the first optical band and the second optical band have no common optical wavelength. In an embodiment, the first optical band and the second optical band may be partially overlapping, and then a part of the optical wavelengths are common.

In an embodiment an example of which is illustrated in Figure 6A, the first image capturing device 104 may comprise an optical component 600 that receives optical radiation from the first edge surface 110 and the opposite sheet surfaces 300, 302 of the layered sheet structure 100. Such gathering of optical radiation to a common optical system 602 allows the first image capturing device 104 to capture images of the first edge surface 110 and the opposite sheet surfaces

300, 302 of the layered sheet structure 100. The common optical system may comprise one or more lenses and a semiconductor cell that converts an optical image formed thereon into an electric form.

In an embodiment, the optical component 600 may comprise an objective lens a diameter of which is larger than a thickness of the layered sheet structure 100, and the first edge surface 110 is fully within an entrance pupil of the first image capturing device 104 in the direction of thickness of the layered sheet structure 100. In an embodiment, the first edge surface 110 may be fully within extreme ends of the optical component 600 in the direction of thickness of the layered sheet structure 100. In this manner, an image contains the first edge area 124, the first planar area 258 on a sheet surface 300 and a corresponding planar area on the opposite sheet surface 302, which can be used to reveal non-zero distances on both of the sheet surfaces 300, 302.

In an embodiment an example of which is illustrated in Figure 6B, the optical component 600 may comprise two or more objective lenses 620, 622 that refract light from the two opposite sheet surfaces 300, 302 to the common optical system 602 for allowing the image capture. The lenses 620, 622 may have a space therebetween, the space facing the first edge surface 110 of the layered sheet structure 100. In an embodiment, at least one of the lenses 620, 622 may participate in an image formation of the first edge surface 110 onto the semiconductor cell. In an embodiment, both of the lenses 620, 622 may participate in the image formation of the first edge surface 110 onto the semiconductor cell of the common optical system 602.

In an embodiment an example of which is illustrated in Figure 7, the optical component 600 may comprise two or more mirrors 730, 732 that reflect light from the two opposite sheet surfaces 300, 302 to the common optical system 602 for allowing the image capture. In an embodiment, at least one of the mirrors 730, 732 may participate in an image formation of the first edge surface 110 onto the semiconductor cell. In an embodiment, both of the mirrors 730, 732 may participate in the image formation of the first edge surface 110 onto the semiconductor cell of the common optical system 602.

In an embodiment an example of which is illustrated in Figure 8, the second image capturing device 200 may comprise an optical component 800 that receives optical radiation from the second edge surface 112 and opposite sheet surfaces 300, 302 of the layered sheet structure 100. Such gathering of optical radiation to a common optical system 802 allows the second image capturing device 200 to capture images ofthe second edge surface 112 and the opposite sheet surfaces 300, 302 of the layered sheet structure 100. That is, the second image capturing device 200 may operate in a corresponding manner to that of the first image capturing device 104 which is why this embodiment is not explained in more detail here but instead the reader is advised to see Figures 6A, 6B and 7 and their explanation.

Figure 9 illustrates an example of a corrugation system 900 which includes a corrugator 902 and an adhering apparatus 904. The corrugator 902 corrugates the sheet 910 that is fed to the corrugator 902. The corrugator 902 may comprise corrugating rolls 950, 952. Then the adhering apparatus 904 adheres a non-corrugated sheet 116, which maybe called a liner, to the corrugated sheet 114, which in turn may be called a fluting. The combination of the non-corrugated sheet 116 and a corrugated sheet 114 is a single-wall corrugated sheet structure. Pieces or drops of an adhesive 912 is transferred to the ridges 118 of the corrugated sheet 114 and the non-corrugated sheet 116 is attached to the pieces of adhesive 912 thereby adhering the corrugated sheet 116 and the non-corrugated sheet 114 together. Sometimes the adhering fails and a non-zero distance D remains between at least one of the following: the corrugated sheet 114 and the non-corrugated sheet 116, a piece of the adhesive and either of the corrugated sheet 114 and the non-corrugated sheet 116 (examples of these are shown in Figure 9). Additionally or alternatively, a non-zero distance D may be between a piece of the adhesive on the corrugated sheet 114 and a piece of the adhesive on the non-corrugated sheet 116 (this situation is not shown in Figure 9 but is easily understandable on the basis of the other examples of the non-zero distance D). Such an adhering failure causes a defective section in the layered sheet structure 100.

Figure 10 illustrates an example of failures in adhesion between the corrugated sheet 114 and the non-corrugated sheet 116. The non-zero distances between at the ridges may be observed as stripes 1000 in the images of the non- corrugated sheet 116 and/or the corrugated sheet 114. The stripes 1000 of the non-zero distances, which are defected sections, can be considered bubbles or delamination. The non-zero distance and the stripes may be caused by an improper corrugation, for example. The non-zero distance and the stripes may be caused, for example, by an adhesive roll of the adhering apparatus 904 that fails to apply the adhesive to the ridges of the corrugated sheet 114. The failure in the application of the adhesive may be due to an improper suction operation of the corrugator roll(s) 950, 952, for example. However, whatever the reason the non-zero distance and the stripes can be detected and a delivery of an end product with adhesive failures to a customer can be prevented. Figure 10 illustrates an example of a double wall layered sheet structure 100 which is may be examined using the embodiments of Figures 6, 6B and 7, for example.

A first image capturing unit 104 may reside behind an adhering apparatus 904 in the machine direction. Namely, a layered sheet structure 100 may have one or more non-corrugated sheets 116 and one or more corrugated sheets 114 which may require one or more adhering apparatuses 904 in the production line (the double wall layered sheet structure of Figure 10 may require two adhering apparatuses; a triple wall may require three adhering apparatuses and so on). In a corresponding manner, the number of the first image capturing units 104 may be more than one. In an embodiment, a first image capturing unit 104 may reside behind and directly adjacent to the adhering apparatus 904. The layered sheet structure 100 may be examined using the first and/or second image capturing apparatus 104, 200 at any moment before delivering the layered sheet structure 100 to a customer or bending the layered sheet structure 100 in a shape which prevents its examination from the edge surface 110. Often any bent shape prevents the examination. The shape may include bending the layered sheet structure 100 at a 90° angle. Alternatively or additionally, the shape of the layered sheet structure 100 may form a surface, which encloses a volume, the surface being at least almost closed. The surface that encloses a volume may be (like) a box or an envelope, for example. Correspondingly, a second image capturing unit 200 may reside behind an adhering apparatus 904 in the machine direction. In an embodiment, the first and/or second image capturing unit 104, 200 may reside before hot plates and/or at the hot plates. Additionally or alternatively the first and/or second image capturing unit 104, 200 may reside behind but directly adjacent to the hot plates in the machine direction.

In an embodiment, the data processing unit 102 may determine a speed of the layered sheet structure 100 on the basis detected movement of the layered sheet structure 100 in one or more images. In a single image, a blur or inaccuracy of an object caused by the movement within the exposure may be used to determine the speed of the layered sheet structure 100. The length of the blur in the machine direction may be used to determine the speed on the basis of the duration of the exposure, for example. In an embodiment, two or more images captured one after another may be used to determine the speed of the layered sheet structure 100. The two or more images may be compared with each other, and a length of the movement of the layered sheet structure 100 may be determined on the basis of the time between the captures of the two or more images. To determine a length of a movement, correlation between the images may be used. On the basis of the speed of the layered structure 100 it is possible to determine a size of a defected section of the layered sheet structure 100. Additionally, a location of any point or section of the layered sheet structure 100 at any moment may be determined within the corrugation process.

On the basis of the size and a location of a defected section of the layered sheet structure 100 as a function of time in the cardboard manufacturing process it is possible to cut out the defected section from the layered sheet structure 100 in an embodiment. Namely, the cardboard manufacturing process may include, as an example illustrates in Figure 11, a removal apparatus 1100 at a known position. The removal apparatus 1100 may be controlled by the data processing unit 102 or the controller 344 to cut out defected section(s) 1102 when the data processing unit 102 or the controller 344 estimates that the defected section(s) 1102 is within the operational range of the removal apparatus 1100 on the basis the speed of the layered sheet structure 100 and detection by the first and/or second image capturing devices 104, 200. The layered sheet structure 100 may be cut into desired forms before or after the removal of the defected section(s) 1102. The removal apparatus 1100 may comprise transversal cutters, for example, in order to cut out the defected section (s) 1102.

In an embodiment, the data processing unit 102 or the controller 344 may control the corrugating system 900 of the corrugating process such that any detected defect can be avoided after the detection. That is, the operating parameters may be modified. For example, suction of the corrugator roll may be increased or decreased. Additionally or alternatively, a distribution of suction force along the roll length may be modified and/or a circumferential distribution of the suction force may be modified. In these manners, stripes of the corrugated sheet 114 may be eliminated or their number or strength may be decreased.

In an embodiment, the data processing unit 102 may control the adhering apparatus 904 to increase or decrease an amount of the adhesive that is applied to the ridges. Additionally or alternatively, the data processing unit 102 or the controller 344 may alarm the user and through a user interface 342, if there is a malfunction in the adhering apparatus 904 or in any other actuator related to the corrugation process (such as the corrugator roll, for example). The data processing unit 102 may identify the malfunctioning actuator such that the malfunctioning actuator can be repaired or replaced.

Figure 12 illustrates an example of a block diagram of the data processing unit 102 and the controller 344 of the cardboard manufacturing process. In an embodiment, the data processing unit 102 and the controller 344 may comprise at least one processor 1200 and at least one memory 1202. The image processing and the operation of the cardboard manufacturing system 900 is based on a sequence of program commands of a computer program run in the data processing unit 102 and the controller 344, respectively. The computer program may be stored in the at least one memory 1202.

Figure 13 is a flow chart of the measurement method. In step 1300, images of a first edge surface 110 of the layered sheet structure 100 are captured repeatedly using a first image capturing device 104 that is farther from a longitudinal centerline 106 of the layered sheet structure 100 than the first edge surface 110 in a direction orthogonal to both the centerline 106 and a thickness of the layered sheet structure 100, and that is directed towards the first edge surface 110 of the layered sheet structure 100, the layered sheet structure 100 comprising a corrugated sheet 114 and a non-corrugated sheet 116 adhered together with adhesive 912 at ridges 118 of the corrugated sheet 114. In step 1302, the images are transferred to a data processing unit 102. In step 1304, a non-zero distance at the ridges 118 is detected between at least one of the following based on an image processing of images: the corrugated sheet 114 and the non-corrugated sheet 116, a piece of the adhesive and either of the corrugated sheet 114 and the non- corrugated sheet 116, and a piece of the adhesive on the corrugated sheet 114 and a piece of the adhesive on the non-corrugated sheet 116.

The method shown in Figure 13 may be implemented as a logic circuit solution or computer program. The computer program may be placed on a computer program distribution means for the distribution thereof. The computer program distribution means is readable by a data processing device, and it encodes the computer program commands, carries out the measurements and optionally controls the processes on the basis of the measurements.

The computer program may be distributed using a distribution medium which may be any medium readable by the controller. The medium may be a program storage medium, a memory, a software distribution package, or a compressed software package. In some cases, the distribution may be performed using at least one of the following: a near field communication signal, a short distance signal, and a telecommunications signal.

It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the example embodiments described above but may vary within the scope of the claims.