JP2025051073A - Method and device for measuring flatness of plate material - Google Patents

Abstract

To provide an inexpensive flatness measurement method and a flatness measurement device for a plate that are capable of suppressing halation without using a plurality of LEDs.SOLUTION: A flatness measurement device includes: a projector 13 which has a single light source 13a and which projects a linear pattern 12 comprising a plurality of parallel lines 14 drawn on a slide onto a surface of a steel plate 11; and a camera 15 for imaging the linear pattern projected onto the steel plate 11. In measuring the flatness of the steel plate 11 by analyzing an image of the linear pattern 12 imaged by the camera 15, when light radiated from the light source 13a is normally reflected by the steel plate 11 and reaches the camera 15, the flatness is measured by using a slide where shading is provided at a place where the light passes.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method and device for measuring the flatness of a plate material, which measures the flatness from an image captured by projecting a linear pattern onto the plate material.

Plate materials used in various industrial products require a certain degree of flatness in order to ensure the quality of the industrial products.
For example, in the manufacture of hot-rolled steel sheets, which are one type of plate material, a slab is heated in a heating furnace, then passed through a roughing mill, a finishing mill, and a cooling device, and finally wound into a coil by a down coiler. The flatness of the hot-rolled steel sheet is mainly determined by the manufacturing conditions of the finishing mill and the cooling device. For this reason, flatness gauges are installed at the exit side of the finishing mill and the exit side of the cooling device, and the finishing mill or the cooling device is feedback-controlled based on the measured flatness to ensure the flatness.
In order to do this, it is important to measure the flatness, and therefore many inventions have been made regarding the measurement of flatness.

For example, Patent Document 1 discloses an invention for a flatness measurement method in which a linear pattern consisting of multiple parallel lines is projected onto the surface of a plate material using a projector, and this linear pattern is photographed by multiple cameras and subjected to image analysis to measure the flatness of the plate material.
However, when projecting a linear pattern using a projector as in this invention, if the specularly reflected light from the plate material is directly reflected by the camera, so-called halation occurs, making it difficult to capture the linear pattern sufficiently and making image analysis difficult.

Patent Document 2 discloses an invention for a flatness measurement method that uses an LED light source equipped with multiple LEDs instead of a projector (metal halide lamp), projects a light and dark pattern onto the surface of the plate material using the light emitted from the LED light source, and reduces the output of the LED only in the areas that are specularly reflected, suppressing halation and measuring the flatness of the plate material.

JP 2008-58036 A International Publication No. WO 2011/145168

When performing flatness measurement as described above, halation can be suppressed by using a plurality of LED light sources and adjusting the output of the LEDs.
However, LEDs are expensive, and preparing a plurality of LEDs creates a cost problem.
In addition, in order to reduce the amount of light in the areas where halation occurs (specularly reflected areas), it is necessary to fine-tune the light amount of each LED, which makes it difficult to control the entire LED light source.

LEDs are expensive, and using multiple LEDs in a flatness measuring device increases the overall device cost, which is not preferable. In addition, a control device is required to control the amount of light emitted by the LEDs, and this control device increases the overall device cost.
Therefore, the present inventors have devised a method for measuring the flatness of a plate material inexpensively without using a plurality of LEDs.

A method for measuring flatness without using multiple LEDs is disclosed in the above-mentioned Patent Document 1. In this invention, a slide is formed by forming a linear pattern on a quartz glass substrate by vapor deposition of Cr, and a metal halide lamp is used as a single light source to project the linear pattern onto a steel plate (see paragraph 0107 of Patent Document 1).
When a single light source is used, the amount of light is greatest at the center and smallest in the peripheral areas away from the center. Increasing the output of the light source also increases the amount of light, causing halation where the light is reflected specularly from the steel plate. Lowering the output of the light source can suppress halation, but it also reduces the amount of light in the peripheral areas, making it difficult for the camera to capture the reflected light, and making it impossible to measure the flatness of the plate material.

Therefore, the present inventors conceived of a method for suppressing the occurrence of halation by conversely devising a method for the slide side on which the linear pattern is formed, if the flatness is to be measured using a single light source.
Specifically, halation is suppressed by adjusting the color tone of the background of the physical slide on which the linear pattern to be projected is drawn. Alternatively, halation may be suppressed by adjusting the color tone of the background of a soft slide created on a PC instead of a physical slide.
If halation can be suppressed in this way, the linear pattern can be clearly captured by the camera, and the flatness can be easily measured by image processing.

The present invention was made based on the above findings, and aims to provide a method and device for measuring the flatness of plate materials at low cost, capable of suppressing halation without using multiple LEDs.

In order to achieve the above object, the present invention provides a method for measuring the flatness of a plate material, comprising: a projector having a single light source and projecting a linear pattern consisting of a plurality of parallel lines drawn on a slide onto a surface of the plate material;
a camera for capturing an image of the linear pattern projected onto the plate material;
A method for measuring flatness of a plate material, comprising: analyzing an image of the linear pattern captured by the camera to measure the flatness of the plate material;
The flatness is measured using a slide with shading at the points through which light passes when the light emitted from the light source is specularly reflected by the plate material and reaches the camera.

Here, the "shaded slide" may be a physical slide on which the linear pattern to be projected is drawn with its background color adjusted, or it may be a soft slide created on a PC (personal computer) rather than a physical slide with its background color adjusted.

In addition, in the above-mentioned configuration of the present invention, flatness measurements may be performed on a strip-shaped plate material flowing from the upstream of the line to the downstream of the line.

The present invention also provides an apparatus for measuring flatness of a plate material, comprising: a projector for projecting a linear pattern consisting of a plurality of parallel lines onto a surface of the plate material;
a camera for capturing an image of the linear pattern projected onto the plate material;
The method for measuring flatness of a plate material is characterized in that the flatness of the plate material is measured by the method for measuring flatness of a plate material.

The present invention makes it possible to suppress halation without using multiple LEDs, and to provide a method and device for measuring the flatness of plate materials at low cost.

1 is a diagram showing a schematic configuration of a flatness measuring device according to an embodiment of the present invention; FIG. 2 is a diagram showing the arrangement of a projector and a camera in the flatness measuring apparatus according to the embodiment of the present invention. 1 is a diagram showing the relationship between the height h of a plate wave (a corrugated steel plate), its wavelength L, and the length S along the plate wave. 1 is a schematic diagram for explaining the measurement principle of a pattern projection type flatness meter; 1A and 1B are diagrams showing projected stripe patterns and video images taken before and after halation countermeasures are taken. This is a graph showing the results of verification of the accuracy of measurements made according to the present invention, in which the vertical axis shows the steepness measured by the present invention and the horizontal axis shows the steepness measured by hand, in order to compare the steepness at the same position on the coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a method and an apparatus for measuring flatness of a plate material according to the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a schematic configuration of a plate material flatness measuring device 10 according to the present embodiment.
In this embodiment, a steel plate 11 is used as the plate material to be subjected to flatness measurement.
The flatness measuring device 10 includes a projector 13 and a camera 15. The projector 13 has a single light source 13a and projects a linear pattern 12 consisting of a plurality of parallel lines 14 onto the surface of the steel sheet 11, and for example, a PJHD5452 type projector manufactured by RICOH is used. The linear pattern 12 projected onto the surface of the steel sheet 11 is depicted by a plurality of parallel lines on a slide (not shown).
The camera 15 photographs the linear pattern 12 projected onto the steel plate 11, and is, for example, a gigabit camera of the CM-040GE type manufactured by JAI Corporation.

The steel plate 11 is elongated in the left-right direction of FIG. 1 (hereinafter referred to as the "longitudinal direction"). A projector 13 is installed above the steel plate 11, which projects a linear pattern 12 onto the surface of the steel plate 11. The linear pattern 12 is composed of a plurality of parallel lines 14. The linear pattern 12 is projected onto the surface of the steel plate 11 such that the longitudinal direction of the lines 14 is perpendicular to the longitudinal direction of the steel plate 11 (hereinafter referred to as the "width direction"). A camera 15 facing the linear pattern 12 is installed above the steel plate 11.

In addition, a plurality of cameras 15 may be arranged. For example, when two cameras 15, 15 are used, the cameras 15, 15 are used which have different sensitivities and/or observation angles for photographing the linear pattern 12 projected on the steel plate 11. When the sensitivities of the cameras 15, 15 are different, the luminance of the images is different. Therefore, by using the images of the cameras 15, 15 in different ways, a clear linear pattern 12 can be obtained. Furthermore, when the sensitivities and observation angles (photographing angles with respect to the longitudinal direction of the steel plate 11) of the cameras 15, 15 are different, images with different luminances can be obtained not only from the sensitivity but also from the position where the reflection is received. Therefore, even if the images of the cameras 15, 15 have parts with saturated luminance or parts with weak luminance, a clear luminance distribution can be obtained by using the images of the cameras 15, 15 in different ways.
In the above, the imaging angle is changed with respect to the longitudinal direction of the steel plate 11, but the angle in the width direction may be changed. Also, the sensitivity of the cameras 15, 15 may be made the same by changing only the imaging position depending on the situation.

2 is a diagram showing the arrangement of the projector 13 and the camera 15 in the flatness measuring device 10. The projection direction of the projector 13 is obliquely downward in the opposite direction to the rolling direction of the steel sheet 11, and the projection angle is β with respect to the vertical direction. On the other hand, the shooting direction of the camera 15 is obliquely downward in the rolling direction of the steel sheet 11, and the shooting angle is α with respect to the vertical direction. When multiple cameras 15 are arranged, the multiple cameras 15 are arranged at the same shooting angle α.
The camera 15 is connected to a PC (personal computer) 16 for analysis, and the projector 13 is connected to a PC (personal computer) 17 for projection.

Furthermore, if the angle between the central axis of the projector 13 and the central axis of the camera 15 is the crossing angle γ, in this embodiment, the crossing angle γ is set to 40°, and the stripe pitch (the distance between parallel and adjacent lines 14, 14) of the linear pattern 12 on the slide is set to 10 mm.
In this embodiment, the crossing angle γ is set to 40°, but the crossing angle γ is preferably 35° to 45°. If the crossing angle γ is smaller than 35°, it becomes difficult to capture the unevenness of the plate shape with the camera 15, and the flatness cannot be analyzed. If the crossing angle γ is larger than 45°, the field of view becomes narrow and the measurement accuracy decreases. In addition, the stripe pitch is preferably 5 to 15 mm. If the stripe pitch is too narrow (less than 5 mm), the resolution of the projector 13 and the camera 15 becomes too large compared to the resolution required for measuring the steepness, resulting in excessive performance and also in a decrease in measurement accuracy. If the stripe pitch is too wide (more than 15 mm), the measurement accuracy of the steepness decreases.

In such a flatness measuring device 10, a linear pattern 12 (see FIG. 1 ) is projected onto the surface of the steel sheet 11 by drawing the linear pattern 12 to be projected onto the surface of the steel sheet 11 on a slide (soft slide) formed on a PC 17. This linear pattern 12 is photographed by a camera 15, and the flatness is calculated by a PC 16 from the image.
Further, the flatness of the strip-shaped plate material (steel material 11) flowing from the upstream side of the line to the downstream side of the line is measured by the flatness measuring device 10.

Here, flatness is required for plate materials such as steel plates from the viewpoint of quality. Flatness is also required for stable production. Therefore, it has been a long-standing issue to properly manage flatness in the production process of plate materials. In general, the elongation percentage ε and the steepness λ are used as indicators of flatness.
As shown in FIG. 3, if the height of the plate wave is h, its wavelength is L, and the length along the plate wave is S, the elongation rate ε and the steepness λ are defined by the following equations (1) and (2), respectively.
ε=(SL)/L (1)
λ = h / L (2)

By approximating the shape of the plate wave to a sine wave, the relationship between the elongation rate ε and the steepness λ is expressed by the following equation (3).
λ=(2/π)(|ε|) ^1/2 (3)
The steepness is the ratio of the height of the plate wave to the wavelength, and has the advantages that it is easy to intuitively recognize the degree of shape defect, and that the measurement value can be transmitted to the rolling mill controller with fewer digits than the elongation difference.

Measurement principle of the pattern projection type flatness meter In shape measurement using the pattern projection method, a projector (light source) is used to project a periodic pattern (generally a stripe pattern) in the rolling direction onto the steel plate surface at an angle β from diagonally above the rolling direction, and the periodic pattern on the steel plate surface is then photographed with a camera installed at a different angle α in the rolling direction.
The distribution of inclination of the steel plate surface in the rolling direction is calculated from the change in pattern pitch obtained by analyzing the brightness distribution of the periodic pattern in the captured image, and the shape of the steel plate in the rolling direction is obtained. Figure 4 shows the principle of measuring the surface angle distribution using the pattern projection method. As shown in Figure 4(a), the light source (projector) projects obliquely downward in the opposite direction to the rolling direction of the steel plate, and the projection angle is β with respect to the vertical direction. On the other hand, the camera shoots obliquely downward in the rolling direction of the steel plate, and the shooting angle is α with respect to the vertical direction.

With the rolling direction as the x-axis, the pattern pitch distribution observed at the shape evaluation points in the width direction when there is no tilt distribution in the steel plate as in Figure 4(b) is Pr(x), and the pattern pitch distribution observed when tilt occurs in the steel plate due to a change in shape is Ps(x). When the steel plate shape is not flat and a tilt distribution occurs, the pattern pitch is observed to be wider in the part where the steel plate surface is tilted towards the camera as in Figure 4(c), and the pattern pitch is observed to be narrower in the part where the steel plate surface is tilted towards the projector as in Figure 4(d).

At this time, if the inclination of the steel plate surface is θ(x), the geometric relationship of formula (6) is established between the pattern pitch Ps(x) when the inclination occurs and the pattern pitch Pr(x) when there is no inclination. If a flat plate is placed in advance and the reference pattern pitch distribution Pr(x) of the projection range is measured, the angular distribution θ(x) in the rolling direction can be measured by measuring the pattern pitch distribution Ps(x) after the shape change, and the surface shape y(x) can be calculated by integrating in the rolling direction using formula (7). A flatness meter using this measurement method can take images with a short exposure time that does not blur the pattern, even when the steel plate is passed through while changing its shape at such a high speed that plate waves are present, and can measure the elongation distribution in the width direction, i.e., flatness, by analyzing the pattern pitch distribution at multiple width direction evaluation points.

Calculation of flatness and measurement range length In the calculation of flatness, the surface length Sm and the straight line length Lm along the rolling direction at each evaluation point in the width direction are calculated, the elongation εm is calculated by formula (1), and the steepness λm is converted by formula (3). However, if the measurement range length R in the rolling direction is short compared to the wavelength L of the plate wave, it is considered that it will affect the measurement accuracy of the steepness. If the rolling direction is the x-axis and the shape of the plate wave is assumed to be a sine wave, the plate wave shape y(x) of the height h of the plate wave and the wavelength L can be expressed by formula (8), and the gradient dy/dx of the steel plate surface can be expressed by formula (9). In addition, the difference between the maximum inclination angle and the minimum inclination angle of the plate wave can be expressed as formula (10), and is proportional to the steepness.

As mentioned above, when a single light source is used, the amount of light is greatest at the center and smallest at the periphery away from the center. If the output of the light source is increased, the amount of light also increases, and halation occurs at the position where the light is specularly reflected from the steel sheet.
Therefore, in this embodiment, as a measure against halation, the PC 17 software changes the color tone of a circle on the slide to gray.

The projected stripe pattern and the video image before and after the halation countermeasure are shown in Fig. 5. As shown in Fig. 5, before the halation countermeasure, the projected stripe pattern is formed by a plurality (a large number) of black lines spaced apart in parallel on a white background, whereas after the halation countermeasure, the projected stripe pattern is formed by a plurality (a large number) of black lines spaced apart in parallel on a white background, and the color tone of the base (background) of the halation area where halation occurs is adjusted in advance to a gray base that is darker than the white base.
It can also be seen that before the halation countermeasure is taken, a halation area exists in the center of the video image captured by a camera (video camera), whereas after the halation countermeasure is taken, halation is suppressed in the video image.
As described above, in this embodiment, it is understood that the halation of the stripe pattern projected from the general-purpose projector (projector 13) is suppressed by software adjustment of the stripe pattern.

Furthermore, the accuracy of the measurement according to the present invention was verified by comparing the steepness measured according to the present invention with the steepness measured by hand.
Specifically, the steepness was measured while winding the steel sheet flowing through the continuous annealing line using a slide with an actual color change, and the steepness was obtained by unwinding the steel sheet (coil) downstream of the line and measuring it by hand. The hand measurement was performed by visually observing the appearance (flatness) of the coil while slowly unwinding it on the inspection line (RL, recoiling line), and measuring L and H (see Figure 3) of any defective flatness found by an operator on an inspection table using a straightedge.
The results of verifying the accuracy of the measurement according to the present invention are shown in Fig. 6. In Fig. 6, in order to compare the steepness at the same position of the coil, the vertical axis shows the steepness measured by the present invention, and the horizontal axis shows the steepness measured by hand.
It can be seen from FIG. 6 that the steepness measured after the halation countermeasure was taken was accurately measured.

As described above, according to the present embodiment, halation can be suppressed without using a plurality of LEDs, and a flatness measuring device 10 and a flatness measuring method can be provided at low cost.
If the flatness measuring device 10 of this embodiment is placed on a production line, the flatness of the plate material (steel plate 11) flowing on the line can be measured in real time. For example, if it is installed on the delivery side of a rolling mill and feedback control is applied, the operating conditions of the rolling mill can be changed immediately, and since major flatness defects can be detected immediately, the length of the flatness defect portion of the plate material (steel plate 11) can be significantly shortened.

10 Flatness measuring device 10
11 Steel plate (plate material)
12 Linear pattern 13 Projector 13a Light source 15 Camera 16, 17 PC

Claims (3)

a projector having a single light source and projecting a linear pattern consisting of a plurality of parallel lines drawn on a slide onto a surface of a plate;
a camera for capturing an image of the linear pattern projected onto the plate material;
A method for measuring flatness of a plate material, comprising: analyzing an image of the linear pattern captured by the camera to measure the flatness of the plate material;
A method for measuring the flatness of a plate material, characterized in that the flatness is measured using a slide with shading on the areas through which light passes when it is specularly reflected by the plate material and reaches the camera from the light source. The method for measuring the flatness of a plate material according to claim 1, characterized in that the flatness is measured for a strip of plate material flowing from the upstream of the line to the downstream of the line. A projector that projects a linear pattern consisting of a plurality of parallel lines onto a surface of a plate material;
a camera for capturing an image of the linear pattern projected onto the plate material;
3. An apparatus for measuring the flatness of a plate material, comprising: a measuring device for measuring the flatness of the plate material by the method for measuring the flatness of the plate material according to claim 1 or 2.

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