Method for measuring inclination angle of belt wire rod

Abstract

To provide a method for measuring an inclination angle of a belt wire rod included in a tire which prevents measurement errors without requiring much labor.SOLUTION: A method for measuring an inclination angle of a belt wire rod for determining an inclination angle of a belt wire rod included in a belt of a tire to a tire circumferential direction when being viewed from outside in a tire radial direction includes the steps of: acquiring a belt image that is an image in which a plurality of belt wire rods are projected when being viewed from the outside in the tire radial direction; subjecting the belt image to image processing including binarization, and thereby acquiring a binarized image that is an image where the belt wire rods and other part are divided by white and black; and determining inclination angles of each of the belt wire rods to the tire circumferential direction, on the basis of the binarized image.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a method for measuring the inclination angle of belt wires.

Background Art

[0002] Pneumatic tires (hereinafter referred to as "tires") are provided with a plurality of belts. Each belt is formed by covering a large number of belt wires arranged in parallel with rubber. The belt wires are inclined with respect to the tire circumferential direction.

[0003] By the way, since the inclination angle of the belt wires with respect to the tire circumferential direction affects the characteristics of the tire, it has been conventionally investigated. A conventionally common investigation method was a method in which an operator prepared a cut sample obtained by dividing the tire in the circumferential direction, peeled off the rubber on the tread side of the cut sample to expose the belt wires, and measured the inclination angle of the belt wires with a protractor.

[0004] Also, as in Patent Document 1, a method for measuring the inclination angle of belt wires in a belt alone rather than a tire by electrical means has been proposed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the conventionally common investigation method had problems such as being laborious and prone to measurement errors. Also, with the method of Patent Document 1, the inclination angle of the belt wires inside the tire could not be measured.

[0007] Therefore, an object of the present invention is to provide a method for measuring the inclination angle of a belt wire included in a tire, which is relatively easy and less likely to cause measurement errors.

Means for Solving the Problems

[0008] The present invention includes the embodiments shown below.

[0009] [1] In a method for measuring the inclination angle of a belt wire included in a tire belt, with respect to the tire circumferential direction as viewed from the outside in the tire radial direction, in the method for measuring the inclination angle of the belt wire, a step of obtaining a belt image which is an image having a predetermined length in the tire axial direction and the tire circumferential direction respectively, and in which a plurality of belt wires as viewed from the outside in the tire radial direction are imaged; a step of obtaining a binarized image which is an image in which the belt wire and other parts are separated into white and black by performing image processing including binarization on the belt image; and a step of obtaining the inclination angle of each of the belt wires with respect to the tire circumferential direction based on the binarized image. The method for measuring the inclination angle of the belt wire includes these steps.

[0010] [2] The method for measuring the inclination angle of the belt wire according to [1], wherein a CT image as the belt image is obtained based on data obtained by photographing an inflated tire with a CT apparatus.

[0011] [3] The method for measuring the inclination angle of the belt wire according to [1] or [2], wherein the length in the tire circumferential direction of the belt image and the binarized image is 58 pixels or more.

[0012] [4] The method for measuring the inclination angle of the belt wire according to any one of [1] to [3], wherein brightness correction is performed on the belt image so that the average brightness at each location in the tire axial direction is the same, and the binarization is performed after the brightness correction.

[0013] [5] In the binary image, where the belt wire material is white and other parts are black, convert the part where the number of consecutive white pixels in the tire circumferential direction is less than the number of pixels of one belt wire material in the tire circumferential direction from white pixels to black pixels. The method for measuring the inclination angle of the belt wire material according to any one of [1] to [4].

[0014] [6] In the binary image, where the belt wire material is white and other parts are black, convert the part where the number of aggregated white pixels is less than the number of pixels representing one belt wire material from white pixels to black pixels. The method for measuring the inclination angle of the belt wire material according to any one of [1] to [5].

[0015] [7] In the binary image, obtain the end points that are the center points in the tire axial direction at both ends of the belt wire material in the tire circumferential direction, and use the inclination angle of the straight line connecting the end points at both ends in the tire circumferential direction with respect to the tire circumferential direction as the inclination angle of the belt wire material. The method for measuring the inclination angle of the belt wire material according to any one of [1] to [6].

[0016] [8] Use the average value of the inclination angles of a plurality of the belt wire materials at the same position in the tire axial direction as the inclination angle of the belt wire material at that position in the tire axial direction. The method for measuring the inclination angle of the belt wire material according to any one of [1] to [7].

Advantages of the Invention

[0017] According to the present embodiment, the inclination angle of the belt wire material can be measured with relatively little effort, and measurement errors are also less likely to occur.

Brief Description of the Drawings

[0018]

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Mode for Carrying Out the Invention

[0019] The embodiments will be described with reference to the drawings. Note that the embodiments described below are merely examples, and those appropriately modified without departing from the gist of the present invention are included in the scope of the present invention.

[0020] This embodiment is executed by a CT (Computed Tomography) apparatus that acquires data on the internal structure of a pneumatic tire (hereinafter referred to as "tire") using X-rays, and a calculation apparatus that processes the data acquired by the CT apparatus and calculates the inclination angles of the belts and belt wires included in the tire.

[0021] The CT apparatus is an apparatus that acquires three-dimensional data on the internal structure of a tire using X-rays and generates a tire internal image, which is an image representing the inside of the tire, based on the three-dimensional data. In the tire internal image, portions where X-rays are more easily absorbed appear brighter, and portions where X-rays are more easily transmitted appear darker. Therefore, metal parts such as belt wires (also called cords), bead cores, and rims appear bright, and rubber parts (hereinafter referred to as "rubber parts") appear dark. According to the CT apparatus, information on the internal structure of the tire can be acquired based on the tire internal image with light and dark without cutting the tire.

[0022] The calculation apparatus is realized by a computer including a processing apparatus, a storage apparatus, an input apparatus, and a display apparatus. As the storage apparatus, a RAM (Random Access Memory), a ROM (Read Only Memory), an HDD (Hard Disk Drive), etc. are provided. Programs for executing this embodiment and tire internal images acquired by the CT apparatus are stored in the storage apparatus. The processing apparatus is composed of a CPU (Central Processing Unit) etc. The processing apparatus reads out and executes the programs stored in the ROM etc. on the RAM to execute the method of this embodiment. The input apparatus is, for example, a mouse and a keyboard, and receives inputs from a user of the calculation apparatus (hereinafter referred to as "user"). The display apparatus is, for example, a display, and displays an input screen for inputting by the input apparatus, a tire internal image, various data, calculation results, etc.

[0023] Also, the tire in this embodiment is a tire having two belts. The two belts are a first belt that is wide and located on the inner side in the tire radial direction, and a second belt that is narrow and located on the outer side in the tire radial direction.

[0024] Each belt is arranged such that a number of belt wires are parallel, and these belt wires are coated with rubber and formed into a single sheet. The interval between the belt wires is the same for the first belt and the second belt. Also, each belt wire is inclined with respect to the tire circumferential direction. Further, each belt wire is formed by twisting together a plurality of thinner metal wires. Such a belt wire is also referred to as a cord.

[0025] In the present embodiment, first, the user takes a picture of the tire with a CT device and acquires three-dimensional data of the internal structure of the tire. Here, the tire is photographed in an inflated state, that is, in a state where it is mounted on a rim and internal pressure is applied. Next, the user specifies a portion to be imaged in the three-dimensional data of the tire and acquires an internal image of the tire at that portion.

[0026] In the present embodiment, as the internal image of the tire, a belt image and a tire axial direction cross-sectional image are acquired. The belt image is an image obtained by projecting one of the plurality of belts included in the tire from the outside in the tire radial direction. The actual belt is in the shape of a curved surface that bends particularly greatly on both sides in the tire axial direction, but the belt image is a planar image. The belt image is a rectangular image that includes both ends of the belt in the tire axial direction and is short in the tire circumferential direction. Also, the tire axial direction cross-sectional image is an image when a plane passing through a perpendicular to the tire outer peripheral surface and the tire rotation axis is taken as the cross-section.

[0027] Coordinates are set for the belt image and the tire axial direction cross-sectional image with an arbitrary reference point being set to 0 (for example, the center position in the axial direction of the tire is set to 0 in the left-right direction coordinates of the image). Since the relationship (calibration value) between one pixel of the image and the length in the actual tire is known in advance, the coordinates can be expressed in mm. When the calibration values are different for the belt image and the tire axial direction cross-sectional image, the coordinates (mm) in each of the belt image and the tire axial direction cross-sectional image are calculated using their respective calibration values.

[0028] FIG. 1 is a belt image showing the first belt, FIG. 2 is a belt image showing the second belt, and FIG. 3 is an image obtained by cutting out the portion of the tire axial direction cross-sectional image where the belt is shown. As shown in FIGS. 1 and 2, in each belt image, a number of belt wires are shown as bright lines extending obliquely with respect to the tire circumferential direction. Further, as shown in FIG. 3, in the tire axial direction cross-sectional image, the cross-sections of the respective belt wires are shown as bright points, the rubber portions such as the tread rubber and the sidewall rubber are shown relatively dark, and the air portion is almost black.

[0029] Next, the user reads the tire internal image into the calculation device. The calculation device performs the processing shown in FIG. 4 on the read tire internal image. Specifically, based on the belt image, the calculation device calculates the inclination angle of the belt wire when viewed from the outside in the tire radial direction (this inclination angle is referred to as the "temporary inclination angle") (S1), calculates the belt gradient with respect to the tire axial direction based on the tire axial direction cross-sectional image (S2), and calculates the three-dimensional inclination angle with respect to the tire circumferential direction based on the temporary inclination angle and the belt gradient (S3).

[0030] First, the calculation of the inclination angle of the belt wire (temporary inclination angle) when viewed from the outside in the tire radial direction (S1 in FIG. 4) will be described. Since the same processing is performed on the two belt images, the first belt will be described here.

[0031] As can be seen from FIG. 5, which is an enlarged view of a part of the belt image, the belt wires shown in the belt image do not have a constant thickness and brightness. There are some thicker parts (indicated by reference numeral 1 in FIG. 5) and some thinner parts (indicated by reference numeral 2 in FIG. 5), or the brightness is slightly darker in some thinner parts. This is because there are changes in the twist of the belt wires formed by twisting thin wires, resulting in parts where the thin wires are dense and sparse. Due to such changes in thickness and brightness in each belt wire, the belt image appears to have wavy unevenness. Also, although the belt wires shown in the belt image appear to be straight, they are actually curved.

[0032] First, the belt image read into the calculation device is converted into a grayscale image (S1-1 in FIG. 6). By this conversion, the belt image becomes an image with 256 gradations from 0 to 255, where black is 0 and white is 255. In the following description, the numerical value representing the gradation is referred to as luminance. The larger the luminance, the closer it is to white, and the smaller the luminance, the closer it is to black. Note that when the belt image is substantially a 256-gradation grayscale at the time of being read into the calculation device, the conversion to grayscale may be omitted.

[0033] Next, the portion where the belt wire is not shown is cut out from the belt image, and only the portion where the belt wire is shown is cut out (S1-2 in FIG. 6). In the following description, when simply referring to the belt image, it indicates the image after the portion where the belt wire is shown is cut out. The 1st belt is shown from one end to the other end in the tire axis direction in the belt image. The left-right direction of the belt image corresponds to the tire axis direction, and the up-down direction of the belt image corresponds to the tire circumferential direction. The belt image is shorter in the up-down direction than in the left-right direction.

[0034] The length of the belt image in the left-right direction corresponds to the length of the 1st belt in the tire axis direction when viewed from the outside in the tire diameter direction. Also, the length of the belt image in the up-down direction is preferably 58 pixels or more. The reason why 58 pixels or more is preferable will be described later.

[0035] As shown in FIG. 7, there is unevenness in luminance in the belt image. Specifically, unevenness occurs such that it is dark at the center in the left-right direction of the image and bright on both sides in the left-right direction of the image.

[0036] Therefore, luminance correction of the belt image is performed so that the average luminance at each location in the left-right direction of the belt image becomes the same (S1-3 in FIG. 6). Specifically, first, the average value (average luminance) of the luminance of all the pixels arranged in the up-down direction at each position in the left-right direction of the image is calculated. The average luminance is distributed such that it is small at the center in the left-right direction of the image and large on both sides in the left-right direction of the image, as indicated by reference numeral 3 in FIG. 8.

[0037] Next, the target luminance is multiplied by the luminance of each pixel so that the value obtained by dividing the target luminance by the average luminance at the horizontal position of the image to which the pixel belongs. Here, the target luminance is the desired luminance as the average value of the luminances of all pixels in the belt image after luminance correction, and is, for example, about 170.

[0038] Expressed in a mathematical formula, the luminance B'ij after luminance correction of the pixel at the i-th position in the horizontal direction of the image and the j-th position in the vertical direction of the image is obtained by the following formula using the luminance Bij before luminance correction of the pixel, the target luminance BG, and the average luminance BAi at the horizontal position of the image to which the pixel belongs.

[0039]

Number

[0040] By this luminance correction, unevenness in luminance in the horizontal direction of the image is eliminated. The distribution of the average luminance after luminance correction is shown by reference numeral 4 in FIG. 8.

[0041] Next, binarization processing of the belt image after luminance correction is executed (S1-4 in FIG. 6). By the binarization processing, pixels with a luminance greater than the threshold value are converted into white pixels. Also, pixels with a luminance less than the threshold value are converted into black pixels. The threshold value for the binarization processing is determined as appropriate and is, for example, about 190. By the binarization processing, a binarized image in which the original belt wire having a large luminance is white and the others are black is obtained. The binarized image of the belt image is referred to as a binarized belt image. In the binarized belt image, each belt wire is a set of a plurality of white pixels.

[0042] As can be seen from FIG. 9, the binarized belt image contains a plurality of noises. As noises, there are white linear noises extending in the horizontal direction of the image (this noise is pointed by an arrow in the figure) and small white dot-like noises. Therefore, removal of these noises is executed from the binarized belt image (S1-5 in FIG. 6).

[0043] As can be seen from FIG. 10 which is an enlarged view of a part of the binarized belt image, in the part of the belt wire, a large number of white pixels are continuous in the vertical direction. On the other hand, in the part of the white linear noise extending in the left - right direction of the image (the part pointed by the arrow in FIG. 10), the number of consecutive white pixels in the vertical direction is less than that in the part of the belt wire. This can also be confirmed by counting the number of consecutive white pixels on the straight line L - L in FIG. 10. Therefore, a process is executed to convert the part where the number of white pixels continuous in the vertical direction of the image is less than the number of pixels in the vertical direction of the image of one belt wire from white pixels to black pixels.

[0044] Specifically, a predetermined number is determined such that the part where the number of white pixels continuous in the vertical direction of the image is more than the predetermined number becomes the part of the belt wire, and the part where the number of white pixels continuous in the vertical direction of the image is less than or equal to the predetermined number becomes the white linear noise. This predetermined number is set as the "linear noise removal reference number". The linear noise removal reference number is appropriately set according to the thickness of the belt wire and the linear noise, and is, for example, 3 pixels.

[0045] Then, the part where the number of white pixels continuous in the vertical direction of the image is less than or equal to the linear noise removal reference number is converted from white pixels to black pixels. By repeating this conversion process from one end to the other end in the left - right direction of the image, the white linear noise extending in the left - right direction of the entire binarized belt image is removed.

[0046] However, in the vicinity of the upper and lower sides of the binarized belt image, even for the belt wire, the number of consecutive white pixels is less than or equal to the linear noise removal reference number. Therefore, the range within the linear noise removal reference number in the vertical direction of the image on the upper side and the lower side of the binarized belt image is excluded from the range of this noise removal process.

[0047] Also, for small white dot-like noises, the number of aggregated white pixels is less compared to the part of the belt wire material. Therefore, a process is executed to convert the part where the number of aggregated white pixels is less than the number of pixels representing one belt wire material from white pixels to black pixels. Here, "aggregation" means being in contact with each other.

[0048] Specifically, a predetermined number is determined such that the part where the number of aggregated white pixels is more than the predetermined number becomes the part of the belt wire material, and the part where the number of aggregated white pixels is less than or equal to the predetermined number becomes small white dot-like noises. This predetermined number is set as the "dot-like noise removal reference number". The dot-like noise removal reference number is appropriately set according to the size of the belt wire material and dot-like noises, and is, for example, 10 pixels.

[0049] Then, the part where the number of aggregated white pixels is less than or equal to the dot-like noise removal reference number is converted from white pixels to black pixels. By executing this conversion process for the entire binarized belt image, small white dot-like noises are removed from the entire binarized belt image.

[0050] Furthermore, as one of the noise removal processes, dilation and erosion processing is executed. The execution order of the dilation process and the erosion process depends on the type of noise to be removed. For example, when trying to remove a chip occurring in the belt wire material, one erosion process is executed after one dilation process.

[0051] Here, three types of noise removal processes are executed, but the execution order is, for example, the removal of white linear noises extending in the left-right direction of the image, the removal of small white dot-like noises, and the dilation and erosion processing, in this order.

[0052] Next, labeling is performed to assign numbers to each belt wire in the binarized belt image after noise removal (S1-6 in FIG. 6). The labeling is performed by assigning different numbers to each continuous portion of white pixels in the binarized belt image. Such labeling can be performed using the bwlabel function, which is a function of MATLAB (registered trademark), a numerical analysis software by MathWorks. By means of labeling, each belt wire can be managed by a number.

[0053] Incidentally, since the belt wire has a thickness corresponding to a plurality of pixels, each belt wire has a thickness of a plurality of pixels in the left-right direction of the image at the upper and lower sides of the binarized belt image. Therefore, next, the center coordinates in the left-right direction of the image of each belt wire (that is, the portion of white pixels) at the upper and lower side positions of the binarized belt image are calculated (S1-7 in FIG. 6). This calculation can be performed using the regionprops function of MATLAB. The center points in the left-right direction of the image of the belt wire at the upper and lower side positions of the binarized belt image are used as the end points of the belt wire.

[0054] Next, the inclination angle of each belt wire in the binarized belt image with respect to the tire circumferential direction is calculated (S1-8 in FIG. 6). The inclination angle is calculated as the inclination angle of the straight line connecting the end points on both sides in the extension direction of the belt wire with respect to the up-down direction of the image (the up-down direction of the image coincides with the tire circumferential direction).

[0055] Specifically, first, the upper and lower two endpoints belonging to the same belt wire are identified. Here, the fact that the upper and lower two endpoints belong to the same belt wire is recognized by the fact that those endpoints belong to the belt wire with the same number in the above labeling. Next, from the coordinates of the two endpoints, the interval in the left - right direction of the image and the interval in the up - down direction of the image between the two endpoints are calculated, and the inclination angle is calculated using the inverse tangent function with those intervals. In this way, since the inclination angle of the straight line connecting the endpoints on both sides in the extension direction of the belt wire is calculated, an inclination angle that is not affected by the bending of the belt wire and the change in thickness between the upper side and the lower side of the binarized belt image is calculated. By this specific method, the inclination angles of all the belt wires in the binarized belt image are calculated.

[0056] The calculated inclination angle of each belt wire is treated as the inclination angle of each belt wire at the position of the lower side of the binarized belt image.

[0057] By the way, when the binarized belt image is short in the up - down direction of the image, even if the position of the endpoint of the belt wire changes by 1 pixel in the left - right direction of the image, the inclination angle of the belt wire with respect to the up - down direction (tire circumferential direction) of the image changes greatly. Therefore, the number of pixels in the up - down direction of the binarized belt image and the belt image on which it is based is determined so that even if the position of the endpoint changes by 1 pixel in the left - right direction of the image, the influence on the inclination angle of the belt wire with respect to the up - down direction of the image is less than 1°. The number of such pixels P is obtained as a value that satisfies the following formula.

[0058]

Equation

[0059] The minimum value of the number of pixels P that satisfies this formula is 58. Therefore, the length of the belt image in the up - down direction is preferably 58 pixels or more.

[0060] In this way, the inclination angles of the belt wire materials at respective positions in the left-right direction of the image (tire axis direction) are obtained. However, in order to improve the accuracy, at each position in the left-right direction of the image, the average of the inclination angles of a plurality of belt wire materials at the same position in the left-right direction of the image is calculated (S1-9 in FIG. 6). Then, the average value of the inclination angles of the plurality of belt wire materials at the same position in the left-right direction of the image is taken as the inclination angle of the belt wire material at that position in the left-right direction of the image.

[0061] For example, in FIG. 9, since there are 7 belt wire materials at the position of the vertically long frame F, the average of the inclination angles of these 7 belt wire materials is calculated. Then, that average value is taken as the inclination angle of the belt wire material at the position of the vertically long frame F.

[0062] In the regions on both sides in the left-right direction of the binarized belt image (the regions indicated by reference numeral 5 in FIG. 9), the belt wire material reaches only one of the upper side and the lower side of the binarized belt image. For such a belt wire material, the inclination angle may not be calculated, or it may be calculated. When it is calculated, the end point of the upper side or the lower side in the binarized belt image and the end point of the left side or the right side in the binarized belt image are specified, and it is calculated as the inclination angle with respect to the up-down direction of the image of the straight line connecting these two end points belonging to the same belt. Note that the end point of the left side or the right side in the binarized belt image is the center point in the up-down direction of the image of the belt wire material at the position of the left side or the right side of the binarized belt image.

[0063] When the inclination angle is not calculated for the belt wire materials that reach only one of the upper side and the lower side of the image on both sides in the left-right direction of the binarized belt image, the average value of the inclination angles of the belt wire materials at the same position in the left-right direction of the image is calculated based on a smaller number of belt wire materials than the center in the left-right direction on both sides in the left-right direction. Also, when the inclination angle is calculated for the belt wire materials that reach only one of the upper side and the lower side of the image, the average value of the inclination angles of the belt wire materials at the same position in the left-right direction of the image is calculated including those belt wire materials. Then, such an average value is taken as the inclination angle of the belt wire material at that position in the left-right direction of the image.

[0064] In this way, the inclination angle of the belt wire with respect to the tire circumferential direction (image vertical direction) at each position in the tire axial direction (image left - right direction) is calculated. The distribution of the calculated inclination angle with respect to the image left - right direction is, for example, as shown in FIG. 11.

[0065] For the second belt as well, the inclination angle of the belt wire when viewed from the outside in the tire radial direction is calculated in the same way as for the first belt.

[0066] In this way, the inclination angle of the belt wire with respect to the tire circumferential direction, which is calculated based on the belt image when the belt is viewed from the outside in the tire radial direction, is defined as the provisional inclination angle.

[0067] Next, the calculation of the belt gradient with respect to the tire axial direction based on the tire axial - direction cross - sectional image (S2 in FIG. 4) will be described.

[0068] First, the tire axial - direction cross - sectional image read into the calculation device is converted into a grayscale image (S2 - 1 in FIG. 12). By this conversion, the tire axial - direction cross - sectional image becomes an image with 256 gradations from 0 to 255, where black is 0 and white is 255. Note that if the tire axial - direction cross - sectional image is substantially a 256 - gradation grayscale image when read into the calculation device, the conversion to grayscale may be omitted.

[0069] Note that in the tire axial - direction cross - sectional image, the lower side is the inside in the tire radial direction and the upper side is the outside in the tire radial direction. Therefore, the second belt is shown above the first belt. The first belt extends on both sides in the tire axial direction more than the second belt. Also, the cords of the carcass ply are made of organic fibers and are not shown in the tire axial - direction cross - sectional image.

[0070] Next, from the tire axial - direction cross - sectional image, the part on the outer - end side from the center of the height from the inner - end to the outer - end in the tire radial direction is cut out. Thereby, unnecessary parts such as the bead core are cut off, and the part where the whole belt is shown is cut out (S2 - 2 in FIG. 12). The cut - out image is the image in FIG. 3.

[0071] In the following description, an image in which unnecessary parts such as bead cores are cut off and the part where the entire belt is shown is cut out is defined as a tire axial direction cross-sectional image. The tire axial direction cross-sectional image includes the first belt and the second belt from one end to the other end in the tire axial direction. The left-right direction of the tire axial direction cross-sectional image corresponds to the tire axial direction, and the up-down direction of the tire axial direction cross-sectional image corresponds to the tire diameter direction. The tire axial direction cross-sectional image is shorter in the up-down direction than in the left-right direction.

[0072] By the way, in the tire axial direction cross-sectional image, the luminance is not constant in the left-right direction of the image, and there is unevenness such that a part in the left-right direction of the image is dark and other parts are bright. Therefore, luminance correction for eliminating such unevenness is performed (S2-3 in FIG. 12). However, in the case of the tire axial direction cross-sectional image, the average luminance calculated from all the pixels arranged in the up-down direction of the image is small at the location where the main groove is present (therefore, at the location where there are many pixels with low luminance), and the average luminance calculated from all the pixels arranged in the up-down direction of the image is large at the location where the main groove is not present. Therefore, the luminance correction performed in S1-3 for the belt image is not optimal as a method of luminance correction for the tire axial direction cross-sectional image.

[0073] Therefore, the following luminance correction is performed on the tire axial direction cross-sectional image. First, for pixels with a luminance of a predetermined value (for example, 50) or less, the luminance is converted to 0. This is a process for making the luminance of the air portion around the tire 0. By this process, the tire axial direction cross-sectional image becomes an image in which only the tire portion made of belt wire and rubber has a luminance greater than 0.

[0074] Next, a calculation range is set that has a length greater than the interval between the two belt wires (the length of the portion without belt wires between the two belt wires) in the left - right direction of the image, and has a range that includes at least from the outer surface to the inner surface of the tire (for example, the range from the upper side to the lower side of the tire axial - direction cross - sectional image) in the up - down direction of the image. Here, the reason why the length of the calculation range in the left - right direction of the image is greater than the interval between the two belt wires is to ensure that pixels of the belt wire with high luminance always fall within the calculation range.

[0075] Next, the calculation range moves in the left - right direction of the image, and the maximum luminance and the minimum luminance in each calculation range are detected. Here, the amount of movement of the calculation range in the left - right direction of the image once is the same as the length of the calculation range in the left - right direction of the image, so that all pixels are included in the calculation range once. Also, the maximum luminance is the luminance of the pixels of the belt wire, and the minimum luminance is the luminance of the pixels of the rubber part. Pixels with luminance of 0 are not treated as pixels of the minimum luminance.

[0076] For each calculation range, a luminance correction value C is obtained from the maximum luminance Bmax and the minimum luminance Bmin in that calculation range according to the following formula.

[0077]

Equation

[0078] Next, the luminance B'ij after luminance correction of the pixel at the i - th position in the left - right direction of the image and the j - th position in the up - down direction of the image is obtained from the luminance Bij before luminance correction of that pixel, the minimum luminance Bmin of the calculation range to which that pixel belongs, and the luminance correction value C of the calculation range to which that pixel belongs according to the following formula.

[0079]

Equation

[0080] As can be seen from this equation, the luminance of each pixel after luminance correction is calculated based on the difference between the maximum luminance and the minimum luminance in the calculation range to which the pixel belongs, as the ratio of the difference between the luminance of the pixel with respect to this reference and the minimum luminance of the calculation range to which the pixel belongs. Therefore, in the tire axial cross-sectional image after luminance correction based on this equation, not only is the unevenness of luminance in the left-right direction of the image eliminated, but also the contrast increases and the luminance of the belt wire becomes very large. It can also be seen from FIG. 13, which is a tire axial cross-sectional image after luminance correction, that the unevenness of luminance has been eliminated and the contrast has increased compared to before the luminance correction.

[0081] By the way, the difference between the maximum luminance and the minimum luminance becomes smaller in the left-right direction of the image, i.e., in a place where there is no belt wire, that is, in a place where there are no pixels with high luminance. Therefore, as can be seen from the equation [Equation 3], and as shown in FIG. 14, which is a distribution diagram of the luminance correction values in the left-right direction of the image, in a place where there is no belt wire (the place surrounded by the dashed circle in the figure), the luminance correction value becomes large.

[0082] As a result, as can be seen from FIG. 13, in a place where there is no belt wire in the left-right direction of the image (the place surrounded by the dashed ellipse in the figure), pixels with high luminance are generated even though it is not a belt wire. Such pixels with high luminance, even though they are not belt wires, are scattered on both the left and right sides of the tire axial cross-sectional image and are later removed as noise.

[0083] Next, binarization processing of the belt image after luminance correction is executed (S2-4 in FIG. 12). By the binarization processing, pixels with luminance greater than the threshold value are converted into white pixels. Also, pixels with luminance less than the threshold value are converted into black pixels. The threshold value for the binarization processing is determined as appropriate and is, for example, about 120. By the binarization processing, a binarized image is obtained in which the original belt wire with high luminance is white and the rest is black. The binarized image of the tire axial cross-sectional image is referred to as a binarized cross-sectional image. In the binarized cross-sectional image, each belt wire is a set of a plurality of white pixels.

[0084] Next, noise removal from the binarized cross-sectional image is performed (S2-5 in FIG. 12).

[0085] First, on both the left and right sides of the tire axial direction cross-sectional image before binarization, as described above, due to the formula [Equation 3], there are pixels with high luminance even though they are not belt wires. These pixels remain as white pixels in the binarized cross-sectional image. Therefore, from the binarized cross-sectional image, the belt portion and a slightly upper and lower portion thereof are cut out. Thereby, the left and right side portions with pixels having high luminance even though they are not belt wires are removed. Such cutting is performed manually by the user or by automatic detection.

[0086] In addition, the binarized cross-sectional image contains small white dot-like noises. The small white dot-like noises have a smaller number of aggregated white pixels compared to the belt wire portion. Therefore, a process of converting portions where the number of aggregated white pixels is less than the number of pixels representing one belt wire from white pixels to black pixels is performed. Here, aggregation means being in contact with each other.

[0087] Specifically, a predetermined number is determined such that a portion where the number of aggregated white pixels is more than the predetermined number becomes the belt wire portion, and a portion where the number of aggregated white pixels is less than or equal to the predetermined number becomes small white dot-like noise. The predetermined number is set as the "dot-like noise removal reference number". The dot-like noise removal reference number is appropriately set according to the size of the belt wire and dot-like noise, and is, for example, 3 pixels. Then, portions where the number of aggregated white pixels is less than or equal to the dot-like noise removal reference number are converted from white pixels to black pixels. By performing this conversion process on the entire binarized cross-sectional image, small white dot-like noises are removed from the entire binarized cross-sectional image.

[0088] Next, labeling is performed to assign numbers to each belt wire in the binarized cross-sectional image after noise removal (S2-6 in FIG. 12). The labeling is performed by assigning different numbers to each continuous part of white pixels in the binarized cross-sectional image. Such labeling can be performed using the bwlabel function in MATLAB. By labeling, each belt wire can be managed by a number.

[0089] Next, the separation of the first belt and the second belt is performed (S2-7 in FIG. 12). For this purpose, first, the centroids of all belt wires appearing in the binarized cross-sectional image are calculated. The regionprops function in MATLAB can be used to calculate the centroid.

[0090] Also, a predetermined range in the left-right direction of the image is set as the search range. This search range is approximately the same as the interval in the left-right direction of the image between two adjacent belt wires in one belt. As a specific method for setting this search range, first, the coordinate data of the centroids of all belt wires appearing in the binarized cross-sectional image are arranged in the order from the left to the right of the image. Next, the difference between the coordinates of two adjacent centroids in the left-right direction of the image (the difference in the left-right direction coordinates of the image) is calculated. Then, the largest of the calculated differences is set as the search range. The search range set by this method is usually the interval between two adjacent belt wires in the part of the first belt that does not overlap vertically with the second belt on both sides in the left-right direction of the image.

[0091] Next, belt wires are sequentially selected from one side to the other in the left-right direction in the binarized cross-sectional image, and an association process is executed each time a belt wire is selected. In the association process, the presence or absence of the centroid of other belt wires within the search range including the centroid of the selected belt wire (itself) is searched. When there are no other belt wires, itself is associated with the first belt. On the other hand, when there are centroids of multiple belt wires including itself within the search range, the belt wire with the centroid at the lowest side (i.e., the inner side in the tire diameter direction) is associated with the first belt.

[0092] Such linking processing is executed for all belt wires from one belt wire in the left - right direction of the image to the other belt wire. Then, all belt wires not linked to Belt 1 are linked to Belt 2. The belt wires linked to Belt 1 are treated as constituting Belt 1, and the belt wires linked to Belt 2 are treated as constituting Belt 2.

[0093] Next, the belt gradient at the position of each belt wire is calculated as an angle (S2 - 8 in FIG. 12). The method of calculating the belt gradient will be described taking Belt 1 as an example.

[0094] First, as shown in FIG. 15, from the coordinates of the centroids of two adjacent belt wires, the length l in the left - right direction (tire axis direction) of the image and the length h in the up - down direction (tire circumferential direction) of the image of these two centroids are calculated. The lengths l and h are calculated, for example, in terms of the number of pixels.

[0095] Here, the length h in the up - down direction of the image is a value with positive and negative signs. Specifically, taking one of the belt wires in the previously determined left - right direction as a reference among two adjacent belt wires, if the other belt wire in the left - right direction is in one of the up - down directions, the length h is positive, and if the other belt wire in the left - right direction is in the other up - down direction, the length h is negative.

[0096] Next, by the following formula, the belt gradient β (°) at the position of the reference belt wire (strictly speaking, the centroid position of the belt wire) is calculated as an angle.

[0097]

Equation

[0098] By this formula, the belt gradient at the positions of all belt wires belonging to Belt 1 is calculated.

[0099] However, due to the influence of the resolution of the binarized cross-sectional image, the calculated centroid position of the belt wire may not be accurate. For this reason, among others, the change in the image left-right direction position (tire axis direction position) of the calculated belt gradient may not be smooth. In that case, smoothing processing is executed (S2-9 in FIG. 12). The smoothing processing is a process of setting the average value of the belt gradients at a plurality of positions arranged in the image left-right direction as the belt gradient at any one of the plurality of positions.

[0100] FIG. 16 shows the distribution of the belt gradients before and after smoothing superimposed. The zigzag line depicts the distribution of the belt gradient before smoothing, and the smooth line depicts the distribution of the belt gradient after smoothing.

[0101] The belt gradients at each position in the image left-right direction (tire axis direction) of the second belt are also calculated in the same way as for the first belt.

[0102] Next, the calculation of the three-dimensional inclination angle of the belt wire with respect to the tire circumferential direction based on the provisional inclination angle calculated based on the belt image and the belt gradient calculated based on the tire axis direction cross-sectional image (S3 in FIG. 4) will be described.

[0103] The belt image and the tire axis direction cross-sectional image are obtained in such a way that the tire circumferential direction position at which the provisional inclination angle of the belt wire is calculated (i.e., the position of the lower side of the belt image) and the tire circumferential direction position at which the belt gradient is calculated (i.e., the position of the tire axis direction cross-sectional image) coincide, and the tire axis direction coordinates (tire axis direction position) of each belt wire at the tire circumferential direction position at which the provisional inclination angle is calculated (i.e., the position of the lower side of the belt image) and the tire axis direction coordinates (tire axis direction position) of each belt wire at the tire circumferential direction position at which the belt gradient is calculated (i.e., the position of the tire axis direction cross-sectional image) coincide. However, there may be cases where these do not coincide.

[0104] Therefore, a process is executed in which the tire axial coordinates of each belt wire appearing in the belt image are aligned with the tire axial coordinates of each belt wire appearing in the tire axial cross-sectional image, and the belt gradient at the aligned tire axial coordinates is calculated. In this paragraph and the next paragraph, the coordinates are strictly the coordinates of the center of gravity of the belt wire. Also, the same scale of tire axial coordinates with a common reference point (for example, the axial center position of the tire) set to 0 are set in the belt image and the tire axial cross-sectional image.

[0105] For this process, the tire axial coordinates of two adjacent belt wires in the tire axial cross-sectional image and the data of the belt gradients at these two coordinates are used. Based on this data, the belt gradient at any tire axial coordinate between two adjacent belt wires appearing in the tire axial cross-sectional image can be calculated by interpolation (for example, linear interpolation). Therefore, the belt gradient can be calculated at the tire axial coordinates of the belt wires appearing in the belt image.

[0106] By performing such calculations for at least all the belt wires where the tire axial coordinates of the belt wires do not match between the belt image and the tire axial cross-sectional image, the belt gradient is obtained for the tire axial coordinates of all the belt wires appearing in the belt image. This process can be performed using the resample function in MATLAB. By this process, both the provisional inclination angle and the belt gradient are obtained for the tire axial coordinates of all the belt wires appearing in the belt image.

[0107] Next, at each tire axial position, the three-dimensional inclination angle of the belt wire with respect to the tire circumferential direction is calculated from the provisional inclination angle and the belt gradient. The calculation method will be described using FIGS. 17 to 20.

[0108] FIG. 17 is a perspective view of the belt 10, FIG. 18 is a view of the same belt 10 seen from the outer side in the tire diameter direction, FIG. 19 is a view of the same belt 10 on the tire axial cross-section, and FIG. 20 is a view for explaining the length and angle for obtaining the inclination angle. Only a part of the belt 10 is drawn in these figures, but actually the belt 10 extends around the tire in the circumferential direction. As can be seen from these figures, the belt wire 20 included in the belt 10 is inclined not only with respect to the tire circumferential direction when seen from the outer side in the tire diameter direction, but also along the belt gradient, so it is three-dimensionally inclined with respect to the tire circumferential direction.

[0109] First, looking at FIG. 18, between the inclination angle α of the belt wire 20 with respect to the tire circumferential direction (this inclination angle α is a provisional inclination angle), the length a in the tire circumferential direction of the belt wire 20 (the length between P and Q), and the length b1 in the tire axial direction of the belt wire 20 (the length between Q and R), when each is seen from the outer side in the tire diameter direction, the following equation holds. Note that the lengths a and b1 are the lengths within a certain range in the tire circumferential direction and within the range shown in FIGS. 17 to 20.

[0110]

Equation

[0111] Therefore, the following equation holds.

[0112]

Equation

[0113] Next, looking at FIG. 19, between the inclination angle β of the belt wire 20 with respect to the tire axial direction (this inclination angle β is the belt gradient), the length b1 in the tire axial direction of the belt wire 20, and the length b2 in the direction of the inclination angle β of the belt wire 20 (the length between Q and S), when each is seen on the tire axial cross-section, the following equation holds. Note that the length b2 is the length within a certain range in the tire circumferential direction and within the range shown in FIGS. 17 to 20.

[0114]

Equation

[0115] Next, referring to FIG. 20, it can be seen that the following equation holds for the three-dimensional inclination angle θ of the belt material 20 with respect to the tire axial direction.

[0116]

Equation

[0117] Therefore, the three-dimensional inclination angle θ of the belt material 20 with respect to the tire axial direction is obtained by the following equation.

[0118]

Equation

[0119] For each belt material, the three-dimensional inclination angle with respect to the tire circumferential direction is obtained by this equation. Also, for each of the first belt and the second belt, the three-dimensional inclination angles of the respective belt materials with respect to the tire circumferential direction are obtained.

[0120] The obtained three-dimensional inclination angles are displayed on the display device as a graph as shown in FIG. 21. Although only the inclination angle of the first belt is shown as a representative in FIG. 21, the inclination angle of the second belt is actually also displayed.

[0121] In FIG. 21, the solid line is the three-dimensional inclination angle of the belt material with respect to the tire circumferential direction at each tire axial direction coordinate. Also, the dashed line is the provisional inclination angle of the belt material at each tire axial direction coordinate. The center in the left-right direction of FIG. 21 coincides with the center in the tire axial direction, and both sides in the left-right direction of FIG. 21 correspond to both sides in the tire axial direction.

[0122] From the comparison between the solid line and the dashed line in Fig. 21, it can be seen that there is a difference between the three-dimensional inclination angle of the belt wire and the provisional inclination angle which is the inclination angle of the belt wire when viewed from the outer side in the tire radial direction on both sides in the tire axial direction. This is the influence of the large belt gradient on both sides in the tire axial direction.

[0123] When the calculation device captures the belt image and the tire axial direction cross-sectional image, it automatically executes up to the display of the graph as shown in Fig. 21 on the display device. However, between the capture of the belt image etc. and the display of the graph, the user may participate in the form of checking the results up to the intermediate stage of the process and instructing the continuation of the process, or inputting necessary information with the input device.

[0124] The three-dimensional inclination angle of the belt wire obtained in this way is used for various purposes. For example, in a tire completed through molding, it is possible to check whether the belt wire is arranged as designed, or to check the influence on the belt wire due to the formation of grooves in the tread during molding.

[0125] Next, the effects of this embodiment will be described. In this embodiment, a belt image which is an image showing a plurality of belt wires when viewed from the outer side in the tire radial direction is acquired, and by performing image processing including binarization on the belt image, a binarized image (binarized belt image) in which the belt wire and other parts are separated into white and black is acquired, and the inclination angle of each belt wire with respect to the tire circumferential direction is obtained based on the binarized image. Therefore, compared with the conventional method of peeling the rubber on the tread side of the cut sample of the tire to expose the belt wire and measuring the inclination angle of the belt wire with a protractor, the inclination angle of the belt wire can be measured without much effort and measurement errors are also less likely to occur.

[0126] In addition, since the belt image is acquired by a CT device that can acquire internal structure data non-destructively, there is no need to perform an act of peeling rubber from a cut sample, and it does not take much effort to acquire the belt image.

[0127] In addition, since a CT device is used, a belt image in an inflated tire can also be acquired. When the tire is inflated, it bulges outward in the tire radial direction, so the inclination angle of the belt wire in the tire changes with respect to that before inflation. The inclination angle of the belt wire in such an inflated tire could not be obtained conventionally, but can be obtained according to the present embodiment.

[0128] In addition, since the circumferential length of the belt image and the binarized image obtained by binarizing it is 58 pixels or more, as described above, the inclination angle of the belt wire with respect to the tire circumferential direction can be accurately obtained.

[0129] In addition, before binarizing the belt image, luminance correction is performed so that the average luminance at each location in the left - right direction (tire axis direction) of the belt image becomes the same. Therefore, even if there is unevenness in the belt image such that a part is dark and another part is bright in the left - right direction of the image, the unevenness is eliminated and an appropriate binarized image is obtained.

[0130] In the binarized image, the belt wire is white and other parts are black. For the binarized image, a process of converting a part where the number of consecutive white pixels in the up - down direction (tire circumferential direction) of the image is less than the number of pixels in the up - down direction of the image of one belt wire from white pixels to black pixels is executed. Therefore, even if linear noise extending in the left - right direction of the image is included in the binarized image, the noise can be removed.

[0131] In addition, for the binarized image, a process of converting a part where the number of aggregated white pixels is less than the number of pixels representing one belt wire from white pixels to black pixels is executed. Therefore, even if small noise is included in the binarized image, the noise can be removed.

[0132] In the binarized image, endpoints that are the center points in the tire axial direction at both ends in the tire circumferential direction of the belt wire are obtained, and the inclination angle of the straight line connecting the endpoints at both ends in the tire circumferential direction with respect to the tire circumferential direction is calculated as the inclination angle of the belt wire. Therefore, for example, even when there are wavy unevennesses in the belt image and there are variations in thickness when looking at each individual belt wire, the inclination angle can be calculated without being affected by this.

[0133] Also, since the average value of the inclination angles of a plurality of belt wires at the same position in the left - right direction of the image (tire axial direction) is taken as the inclination angle of the belt wire at that position in the left - right direction of the image, the accuracy of the calculated inclination angle is improved.

[0134] Various changes can be made to the above - described embodiments. Any one of the modification examples described below may be applied to the above - described embodiments, or any two or more of them may be combined and applied to the above - described embodiments. The combination can be freely made.

[0135] <Modification Example 1> The tire may be photographed by a CT device in a deflated state where it is not cut and no internal pressure is applied. Also, a cut sample in which the tire is cut from one end to the other end in the tire axial direction at two locations in its circumferential direction may be photographed by a CT device.

[0136] In any method, 3D data of the entire tire or the cut sample can be obtained by a CT device, and a belt image and a tire - axial - direction cross - sectional image can be obtained based on the 3D data.

[0137] <Modification Example 2> One or both of the belt image and the tire - axial - direction cross - sectional image may be obtained by an imaging device other than a CT device.

[0138] For example, the belt image may be an image obtained by peeling off the rubber on the tread side of the cut sample to expose the belt wire and then scanning the exposed surface where the belt wire is exposed with a scanner. Also, the tire axial cross-sectional image may be an image obtained by scanning the cut surface of the cut sample with a scanner and capturing it.

[0139] <Modified Example 3> As a method for obtaining both the provisional inclination angle and the belt gradient for each tire axial coordinate (tire axial position), for the tire axial coordinates of each belt wire appearing in the tire axial cross-sectional image, the tire axial coordinates of each belt wire appearing in the belt image are aligned, and a process of calculating the provisional inclination angle at the aligned tire axial coordinates may be executed.

[0140] For this process, the tire axial coordinates of two adjacent belt wires in the belt image and the data of the provisional inclination angles at these two coordinates are used. Based on this data, the provisional inclination angle at any tire axial coordinate between two adjacent belt wires appearing in the belt image can be calculated by interpolation (for example, linear interpolation). Therefore, the provisional inclination angle can be calculated at the tire axial coordinates of the belt wires appearing in the tire axial cross-sectional image.

[0141] By executing such calculations for at least all the belt wires where the tire axial coordinates of the belt wires do not match between the belt image and the tire axial cross-sectional image, the provisional inclination angle is obtained for the tire axial coordinates of all the belt wires appearing in the tire axial cross-sectional image. By this process, both the provisional inclination angle and the belt gradient are obtained for the tire axial coordinates of all the belt wires appearing in the tire axial cross-sectional image.

[0142] <Modified Example 4> The method of the above embodiment is applicable to tires with a number of belts other than two.

Explanation of Reference Numerals

[0143] 10… Belt, 20… Belt wire

Claims

1. In a method for measuring the inclination angle of a belt wire included in a tire belt, for obtaining the inclination angle of the belt wire with respect to the tire circumferential direction when viewed from the outside in the tire radial direction, a step of obtaining a belt image, which is an image having a predetermined length in each of the tire axial direction and the tire circumferential direction and in which a plurality of belt wires are shown when viewed from the outside in the tire radial direction; a step of obtaining a binarized image, which is an image in which the belt wire and other parts are separated into white and black, by performing image processing including binarization on the belt image; a step of obtaining the inclination angle of each of the belt wires with respect to the tire circumferential direction based on the binarized image; A method for measuring the inclination angle of a belt wire, comprising:

2. The method for measuring the inclination angle of a belt wire according to claim 1, wherein a CT image as the belt image is obtained based on data acquired by photographing a tire with a CT apparatus.

3. The method for measuring the inclination angle of a belt wire according to claim 1 or 2, wherein the length of the belt image and the binarized image in the tire circumferential direction is 58 pixels or more.

4. The method for measuring the inclination angle of a belt wire according to claim 1 or 2, wherein brightness correction is performed on the belt image so that the average brightness at each location in the tire axial direction becomes the same, and binarization is performed after the brightness correction.

5. In the binarized image, the belt wire is white and other parts are black, The method for measuring the inclination angle of a belt wire according to claim 1 or 2, wherein a portion where the number of consecutive white pixels in the tire circumferential direction is less than the number of pixels of one belt wire in the tire circumferential direction is converted from white pixels to black pixels.

6. In the binarized image, the belt wire is white and other parts are black, The method for measuring the inclination angle of a belt wire according to claim 1 or 2, wherein a portion where the number of aggregated white pixels is less than the number of pixels representing one belt wire is converted from white pixels to black pixels.

7. In the binarized image, end points that are the center points in the tire axial direction at both ends in the tire circumferential direction of the belt wire are obtained, and the inclination angle of a straight line connecting the end points at both ends in the tire circumferential direction with respect to the tire circumferential direction is taken as the inclination angle of the belt wire. The method for measuring the inclination angle of a belt wire according to claim 1 or 2.

8. The method for measuring the inclination angle of the belt wire according to claim 1 or 2, wherein an average value of the inclination angles of the plurality of belt wires at the same position in the tire axial direction is taken as the inclination angle of the belt wire at that position in the tire axial direction.

Images