Method for evaluating tire grounding characteristics

Abstract

To evaluate the grounding characteristics of a tire by acquiring an accurate and wide image of a part where a tire is grounded.SOLUTION: A tire 1 is grounded on a dummy road surface plate 12 having a dummy road surface 12A which is as uneven as an actual road surface. A grounded part 5 to the dummy road surface 12A of the tire 1 is divided into plural regions when an electromagnetic wave having passed through the dummy road surface plate 12 is detected and an optical imaging is performed, and a plurality of images are thus acquired. After that, the images are synthesized and a synthesis image of the grounded part 5 is acquired. The synthesis image is binarized and a binary image is acquired, and the grounding characteristics are evaluated on the basis of the binary image.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the ground contact characteristics of a tire. [Background technology]

[0002] There is a demand for accurate evaluation of the ground contact characteristics of a tire on an uneven road surface simulating an actual road surface, and various proposals have been made. For example, Patent Document 1 discloses a method for measuring the ground contact condition of a tire on a wet road surface, in which a fluorescent liquid is placed on a transparent plate having an uneven surface equivalent to an actual road surface, a tire is placed on the plate, and excitation light is irradiated onto the fluorescent liquid to measure the luminance distribution of the fluorescence emitted from the fluorescent liquid. Patent Document 2 discloses a method for measuring the ground contact condition of a tire with high accuracy from the correlation between the contact area and the friction coefficient by using the measurement method.

[0003] Patent Document 3 discloses a method for evaluating the ground contact characteristics of a tire by irradiating a tire that is placed on a flat plate having projections and recesses with terahertz waves and detecting the terahertz waves reflected from the tire.

[0004] Patent Document 4 discloses a method for measuring the contact shape of a tire tread by reading the shadow of the tire contact area from below a road surface body made of a translucent material using an image reading means. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-084428 A [Patent Document 2] JP 2019-074335 A [Patent Document 3] JP 2018-119928 A [Patent Document 4] JP 2004-009880 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the above-mentioned tire ground contact characteristic evaluation method, when photographing a wide area of ​​the tire's contact area with the pseudo road surface all at once, it may not be possible to obtain an image of the contact area with sufficient accuracy due to, for example, constraints on the camera accuracy, constraints on the distance to the contact area with a simple device, constraints on the size of the pseudo road surface plate, constraints on the irradiation intensity of the electromagnetic waves and the intensity of the electromagnetic waves from the contact area, etc.

[0007] In view of the above, an embodiment of the present invention aims to provide a tire ground contact characteristic evaluation method that can acquire images of a tire ground contact portion with high precision and over a wide area, thereby evaluating the ground contact characteristics of a tire. [Means for solving the problem]

[0008] The present invention includes the embodiments set forth below. [1] A step of grounding a tire or a rubber test piece on a simulated road surface of a simulated road plate having an unevenness equivalent to that of an actual road surface; a step of obtaining a plurality of images by optically photographing the contact portion of the tire or rubber test piece with the simulated road surface by using an imaging device that detects electromagnetic waves transmitted through the simulated road surface plate and optically photographs the contact portion to obtain a plurality of images; combining the images to obtain a combined image of the ground contact portion; binarizing the composite image to obtain a binarized image; and evaluating ground contact characteristics based on the binarized image; A tire ground contact characteristic evaluation method comprising: [2] A tire ground contact characteristic evaluation method according to [1], further comprising synthesizing the multiple images using a feature calculation algorithm. [3] The tire ground contact characteristic evaluation method according to [1] or [2], wherein the composite image is binarized using a local adaptive threshold algorithm. [4] A tire ground contact characteristic evaluation method according to any one of [1] to [3], comprising the steps of: placing a fluorescent liquid between the pseudo road surface and the tire or rubber test piece, grounding the tire or rubber test piece on the pseudo road surface, irradiating the fluorescent liquid with excitation light from the side of the pseudo road surface plate opposite the pseudo road surface, detecting the fluorescence emitted from the fluorescent liquid and transmitted through the pseudo road surface plate, and performing the optical imaging. Effect of the Invention

[0009] According to this embodiment, it is possible to obtain highly accurate images of the tire contact portion over a wide area, and it is possible to evaluate the tire contact characteristics with high accuracy. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a front view of a measurement device used in a tire ground contact characteristic evaluation method according to an embodiment; [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of the measurement device. [Diagram 3] FIG. 1 is a diagram showing an example of a tire contact shape; [Figure 4] A diagram showing images G1 to G3 of a tire's contact area divided into multiple parts. [Diagram 5] 5A is a diagram showing a composite image obtained by combining images G1 to G3 of FIG. 4; FIG. 5B is a diagram showing a binarized image obtained by binarizing the composite image; [Figure 6] 1 is a flowchart of a tire ground contact characteristic evaluation method according to an embodiment; [Figure 7] Graph showing the relationship between tire contact area occupation rate and load [Figure 8] FIG. 1 shows a local region of interest in a binarized image. [Figure 9] A graph showing the relationship between the contact area occupancy rate and the load for a local region of interest. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, a tire ground contact characteristic evaluation method according to an embodiment will be described with reference to the drawings.

[0012] In the tire ground contact characteristic evaluation method according to the embodiment, a tire or rubber test piece is grounded on a pseudo road plate, electromagnetic waves transmitted through the pseudo road plate are detected and optically photographed, the contact portion with the pseudo road plate is divided into multiple parts and optically photographed, the multiple images obtained are synthesized, and the synthesized image is binarized to evaluate the ground contact characteristics.

[0013] Electromagnetic waves are a phenomenon in which periodic changes in electric and magnetic fields propagate through space as waves. There are no particular limitations on electromagnetic waves, and examples of such waves include X-rays, ultraviolet rays, visible light, infrared rays, and radio waves (including terahertz waves).

[0014] As an embodiment, a method for evaluating the ground contact characteristics of a tire on a wet road surface will be described below as an example, in which a fluorescent liquid is interposed between the tire and the simulated road surface, a tire is grounded on the simulated road surface, excitation light is irradiated onto the fluorescent liquid from the side opposite the simulated road surface of the simulated road surface plate, and the fluorescence emitted from the fluorescent liquid and transmitted through the simulated road surface plate is detected to perform optical imaging. Each substance having fluorescent characteristics has a specific absorption wavelength range and a fluorescence wavelength range that are unique to each substance. The fluorescence used for imaging is generally in the visible light range, but it may be electromagnetic waves invisible to the human eye, such as ultraviolet light or infrared light. Generally, ultraviolet light or visible light is used as the excitation light, but it is selected according to the absorption wavelength range of the substance of interest.

[0015] 1 is a diagram showing a measurement device 10 used in a tire ground contact characteristic evaluation method according to one embodiment. However, the measurement device used in the evaluation method is not limited to this. The measurement device 10 is a device that measures the ground contact state of a tire 1 that is grounded on a pseudo road surface 12A of a pseudo road surface plate 12 by detecting electromagnetic waves that have passed through the pseudo road surface plate 12.

[0016] 1, the measuring device 10 includes a pseudo road surface plate 12, a table 14 on which the pseudo road surface plate 12 is placed, a support shaft 16 that supports a tire 1, a pair of shaft supports 18, 18 that support the support shaft 16, and a base stand 20. The table 14 and the pair of shaft supports 18, 18 are provided on the base stand 20.

[0017] As shown in FIG. 2, the pseudo road surface plate 12 has a pseudo road surface 12A having projections and recesses corresponding to an actual road surface on one of the front and back surfaces, and in this example, the surface opposite the pseudo road surface 12A is flat. The pseudo road surface plate 12 is a flat plate having a rectangular shape in a plan view that is permeable to electromagnetic waves, and is preferably a transparent plate. There are no particular limitations on the method of producing the transparent pseudo road surface plate 12 having the pseudo road surface 12A, and the pseudo road surface plate 12 can be produced, for example, by molding asphalt corresponding to an actual road surface with silicone rubber, pouring a transparent resin into the mold, and curing it in a vacuum degassed state. An example of the transparent resin is a urethane-based resin.

[0018] The simulated road surface plate 12 is replaceably placed on a table 14, and a number of plates with various uneven shapes are prepared according to the road surface to be tested, and one of them can be selected and installed.

[0019] The table 14 includes a mounting stand 22 for horizontally mounting the pseudo road surface plate 12. The mounting stand 22 is provided on the top plate of the table 14. The pseudo road surface plate 12 is mounted on the upper surface of the mounting stand 22 with the pseudo road surface 12A facing upward.

[0020] The installation stand 22 is provided with an observation window 24 so that the contact portion 5 between the tire 1 and the simulated road surface 12A can be observed from below through the simulated road surface plate 12. The simulated road surface plate 12 is installed on the installation stand 22 so as to cover the observation window 24 from above.

[0021] Below the installation stand 22, which is the top board of the table 14, there are provided an irradiation device space 30 for installing the irradiation device 28 and an imaging device space 34 for installing the imaging device 32, which are divided into upper and lower spaces by a shelf board 36.

[0022] An irradiation device 28 for irradiating electromagnetic waves to the contact portion 5 between the tire 1 and the pseudo road surface 12A through the pseudo road surface plate 12 is provided in the irradiation device space 30 above the shelf board 36.

[0023] In this example, the irradiation device 28 includes a light source 38, a filter 40, and a dichroic mirror 42, as shown in Fig. 2. The light source 38 irradiates excitation light onto the fluorescent liquid 2 interposed between the tire 1 and the simulated road surface 12A. The filter 40 transmits and separates only light of a specific wavelength from the light irradiated from the light source 38. The dichroic mirror 42 reflects only the light of the specific wavelength separated by the filter 40. The irradiation device 28 is configured to be movable in the horizontal direction so that the irradiation position of the excitation light on the ground contact portion 5 can be changed.

[0024] The photographing device 32 is provided in the photographing device space 34 below the shelf board 36 for optically photographing by detecting electromagnetic waves transmitted through the artificial road surface plate 12. The photographing device 32 is installed on a photographing device installation stand 44 provided on the base stand 20.

[0025] The imaging device 32 detects electromagnetic waves that travel downward from the contact portion 5 between the tire 1 and the simulated road surface 12A through the simulated road surface plate 12. For example, the imaging device 32 may detect reflected waves that are formed when the electromagnetic waves irradiated from the irradiation device 28 are reflected by the contact portion 5, or may detect electromagnetic waves emitted from the contact portion 5 due to the electromagnetic waves irradiated from the irradiation device 28.

[0026] In this example, the photographing device 32 detects the fluorescence emitted from the fluorescent liquid 2 present in the ground contact portion 5 and transmitted through the pseudo road surface plate 12. The photographing device 32 is equipped with a filter 46 that transmits and separates only light of a specific wavelength from the fluorescence that has transmitted through the pseudo road surface plate 12, and a camera 48 that photographs the fluorescence that has transmitted through the filter 46 (see FIG. 2). A single-lens reflex camera, for example, may be used as the camera 48. The photographing device 32 is configured to be movable horizontally in accordance with the above-mentioned movement of the irradiation device 28.

[0027] The light source 38 can be appropriately selected to match the excitation spectrum of the fluorescent dye contained in the fluorescent liquid, and a light source having a peak wavelength near the peak wavelength of the excitation spectrum (e.g., a mercury lamp or an ultraviolet LED) is preferably used.

[0028] The dichroic mirror 42 and the filters 40, 46 can be appropriately selected and used according to the excitation spectrum and fluorescence spectrum of the fluorescent dye to be used. Examples of the filters 40, 46 include a wavelength-selective fluorescence filter that removes noise when detecting fluorescence, a high-pass filter (long-pass filter) that cuts off light on the shorter wavelength side than a specified wavelength and transmits light on the longer wavelength side, a low-pass filter (short-pass filter) that cuts off light on the longer wavelength side than a specified wavelength and transmits light on the shorter wavelength side, and a band-pass filter that transmits only light in a certain wavelength range and cuts off other light on the shorter and longer wavelength sides.

[0029] 1, the support shaft 16 is an axis member (shaft) that supports the tire 1 so as to be able to contact the pseudo road surface 12A above the pseudo road surface plate 12, and is provided above an installation stand 22. The tire 1 is mounted on the rim of a wheel 3, and the support shaft 16 is attached so as to pass through the center of the wheel 3. The wheel 3 is fixed to the support shaft 16 by using a wheel fixing disk 50.

[0030] The shaft supports 18, 18 are provided in a pair on the left and right so that the support shaft 16 supporting the tire 1 can be supported horizontally on both axial sides of the tire 1. The pair of shaft supports 18, 18 are columns provided upright on the base stand 20 and support the support shaft 16. A shaft chuck 51 is provided at the upper end of the shaft support 18, and the support shaft 16 is attached to the upper end of the shaft support 18 using the shaft chuck 51. The pair of shaft supports 18, 18 are provided on both sides of the table 14 and support both ends of the support shaft 16. Therefore, the table 14 is provided between the pair of left and right shaft supports 18, 18.

[0031] The measuring device 10 is provided with a load applying means for applying a tensile force to the pair of shaft supports 18, 18 to pull the support shaft 16 downward, thereby applying a load to the tire 1. In this example, the load applying means includes a pair of extension mechanisms 52, 52 that can extend or retract the length of the pair of shaft supports 18, 18, and a load adjustment unit 54 that adjusts the load by extending or retracting the pair of extension mechanisms 52, 52 in the vertical direction. By shortening the length of the pair of shaft supports 18, 18, the support shaft 16 is pulled downward, and the load applied to the tire 1 can be increased. In addition, by extending the length of the pair of shaft supports 18, 18, the support shaft 16 moves upward, and the load applied to the tire 1 can be reduced.

[0032] In detail, each shaft support 18 is configured by vertically connecting a rotatable lower column member 56 having at least at its upper end a male threaded portion 56A with a thread cut on the outer periphery, and an upper column member 58 having at its lower end a female threaded portion 58A that screws into the male threaded portion 56A. By rotating the lower column member 56, the screwing position of the male threaded portion 56A and the female threaded portion 58A changes, causing each shaft support 18 to expand and contract. Therefore, each expansion mechanism portion 52 is configured by the rotatable male threaded portion 56A and the female threaded portion 58A that screws into it.

[0033] The load adjustment unit 54 extends and retracts the pair of telescopic mechanism units 52, 52 by controlling the rotation of the lower pillar member 56, and in this example is configured with a manual handle 54A. The telescopic mechanism units 52, 52 of the pair of shaft supports 18, 18 are configured to extend and retract synchronously. Therefore, when the handle 54A is rotated, the lower pillar member 56 rotates in sync with the pair of shaft supports 18, 18, which causes the pair of shaft supports 18, 18 to extend and retract in sync, and the force applied to the shaft supports 18, 18 becomes equal on the left and right.

[0034] The measuring device 10 is provided with a pair of load cells 60, 60 as force detection devices that detect the tensile force acting on the pair of shaft supports 18, 18. The load cells 60 are respectively incorporated into each shaft support 18, and more specifically, into the upper pillar members 58. The load cells 60 detect the force acting on the shaft supports 18 in the vertical direction.

[0035] 1, a pair of left and right support rods 61, 61 are provided on both sides of the load cell 60. To enable the load cell 60 to detect tensile force, both ends of the support rod 61 are not fixed to upper and lower members, i.e., the support rod 61 is configured to be free to move up and down.

[0036] The measuring device 10 is provided with a load calculation unit 62 that calculates the load applied to the tire 1 based on the tensile forces detected by the pair of load cells 60, 60. The load calculation unit 62 sums up the left and right tensile forces generated by shortening the length of the pair of shaft supports 18, 18, and calculates the load applied to the tire 1 due to the tensile forces, and can be configured by a computer such as a simple microprocessor.

[0037] The measuring device 10 is provided with a load display unit 64 that displays the load on the tire 1 calculated by the load calculation unit 62. The load display unit 64 can be configured with a general display.

[0038] The measuring device 10 includes wheels 66 that movably support the measuring device 10. The wheels 66 are attached to the lower surface of the base stand 20, and are provided at each of the four corners of the rectangular base stand 20, for example.

[0039] In the above-described measuring device 10, the tire 1 is supported by a pair of left and right shaft supports 18, 18, but the tire load may be supported by a cantilever beam. In that case, a load cell may be installed at the base of the beam to detect the load.

[0040] In addition, in the above-described measuring device 10, the load cells 60 for measuring the load applied to the tire 1 are incorporated inside the left and right shaft supports 18, 18, but the load cells may be incorporated, for example, into the bottom surface of the pseudo road plate 12 or the bottom surface of the device body. In that case, the load cells detect the compression force, thereby making it possible to measure the load applied to the tire.

[0041] In one embodiment of the tire ground contact characteristic evaluation method, the simulated road surface plate 12 is fixed to a mounting stand 22, and the fluorescent liquid 2 is poured onto the simulated road surface 12A. As the fluorescent liquid 2, for example, an aqueous solution containing a hydrophilic fluorescent dye (e.g., pyranine) whose peak wavelength difference between the excitation spectrum and the fluorescence spectrum is 100 nm or more is used.

[0042] Next, the tire 1 mounted on the wheel 3 is fixed to the support shaft 16, and the support shaft 16 is attached so as to span between a pair of shaft supports 18, 18, and the tire 1 is grounded on the simulated road surface 12A. As a result, the tire 1 is grounded on the simulated road surface 12A with the fluorescent liquid 2 interposed between the tire 1 and the simulated road surface 12A. The tire 1 is filled with internal pressure. The internal pressure may be an air pressure determined for each tire by each standard in a standard system including the standard on which the tire is based. For example, in the case of tires for trucks and buses and tires for light trucks, the maximum air pressure in the case of JATMA, the maximum value described in the table "TIRE LOAD LIMITS AT VARIOUS COLD INFLATION PRESSURES" in the case of TRA, and "INFLATION PRESSURE" in the case of ETRTO, and in the case of tires for passenger cars, 180 kPa, 220 kPa, etc. are usually used, and the air pressure set in the actual vehicle is also used.

[0043] Next, by turning the handle 54A of the load adjustment unit 54, the lower pillar members 56, 56 of the pair of shaft supports 18, 18 rotate, and the upper pillar members 58, 58 move downward in sync on the left and right, and the pair of shaft supports 18, 18 contract. As a result, a force (tensile force) that pulls the support shaft 16 downward acts on the pair of shaft supports 18, 18, and a load is applied to the tire 1. At that time, the load cell 60 incorporated in each shaft support 18 detects the tensile force acting on the shaft support 18, and the load calculation unit 62 calculates the load applied to the tire 1 based on the detected tensile force, and the calculated load is displayed on the load display unit 64. In this way, a specified load can be applied to the tire 1 while checking the load displayed on the load display unit 64.

[0044] The load applied to the tire 1 may be a normal load, a load equivalent to an actual vehicle load, or a value 0.2 to 1.5 times these loads, or any other value, depending on the purpose of the ground contact characteristic evaluation. The load may be set to a plurality of stages, the ground contact state may be measured at each load, and the relationship between the load and the ground contact characteristic may be evaluated. Here, the normal load is a load determined for each tire by each standard in the standard system including the standard on which the tire is based, and may be, for example, the maximum load capacity in the JATMA standard, the maximum value listed in the above table in the TRA standard, or the "LOAD CAPACITY" in the ETRTO standard, or a load equivalent to 88% of these loads when the tire is for a passenger car.

[0045] After applying a predetermined load to the tire 1, excitation light is irradiated from the light source 38 of the irradiation device 28 as shown in FIG. 2. The excitation light irradiated from the light source 38 is separated by a filter 40 into light having a predetermined wavelength (e.g., 400 nm) or less. The separated excitation light is reflected by a dichroic mirror 42 and irradiated from the lower surface of the pseudo road surface plate 12 onto the fluorescent liquid 2 interposed between the tire 1 and the pseudo road surface 12A. This causes the fluorescent dye contained in the fluorescent liquid 2 to transition from the ground state to an excited state. The excited fluorescent dye then returns to the ground state, emitting fluorescence at that time.

[0046] The emitted fluorescence passes through the pseudo road surface plate 12 and then through the dichroic mirror 42, whereupon the fluorescence of a predetermined wavelength or longer (e.g., 480 nm or longer) is separated by the filter 46, and the separated fluorescence is photographed by the camera 48 of the photographing device 32.

[0047] In this embodiment, when photographing with such an imaging device 32, the contact portion 5 of the tire 1 with the pseudo road surface 12A is divided into multiple parts and photographed to obtain multiple images. That is, the contact portion 5 is divided and photographed locally enlarged. This makes it possible to obtain an image of the contact portion 5 between the tire 1 and the pseudo road surface 12A with high accuracy. The contact portion 5 is an area where the tire 1 is considered to be in contact with the pseudo road surface plate 12 from the appearance, and is also called the tire contact surface.

[0048] Such divided photography can be realized by moving the irradiation device 28 and the imaging device 32 in the horizontal direction as described above to change the irradiation position of the excitation light on the ground part 5 and, in synchronization with this, change the location photographed by the imaging device 32. Note that the divided photography described above may be realized by moving only the imaging device 32 in the horizontal direction without moving the irradiation device 28.

[0049] The method of dividing the ground contact portion 5 is not particularly limited, and for example, the ground contact portion 5 may be divided into a plurality of portions in the tire axial direction in order to obtain a composite image over the entire width of the tire tread, which will be described later. The ground contact portion 5 may also be divided into a plurality of portions in each of the tire axial direction and the tire circumferential direction in order to obtain a composite image of the entire ground contact portion 5. For example, the ground contact portion 5 may be divided into three portions in the tire axial direction and three portions in the tire circumferential direction, for a total of nine portions. Note that when the ground contact portion 5 is divided and photographed, it is not necessary to divide the entire ground contact portion 5.

[0050] When dividing the ground contact portion 5 into multiple parts in this way, it is preferable that each divided and captured image has some overlap between adjacent images. This allows multiple images to be combined without gaps, resulting in a highly accurate combined image.

[0051] FIG. 3 is a diagram showing an example of the ground contact shape of a tire 1, showing the contour shape of a ground contact portion 5. A plurality of (four in this example) main grooves 6 extending in the tire circumferential direction are provided on the surface of the tire tread. A plurality of tread lands 7 are defined and formed in the tire tread by the main grooves 6. In detail, a center land 7A located in the center, a pair of left and right quarter lands 7B, 7B located on both sides of the center land 7A, and a pair of left and right shoulder lands 7C, 7C located outside the quarter land 7B are provided. In this example, these tread lands 7 are provided as continuous ribs over the entire circumference in the tire circumferential direction, but they may be provided as block rows by providing lateral grooves that cross the ribs.

[0052] As shown in FIG. 3, in this example, the ground contact portion 5 across the entire width of the tire tread is divided into three parts in the tire axial direction and photographed. That is, the left part 70L surrounded by a dashed line frame, the center part 70C surrounded by a dotted line frame, and the right part 70R surrounded by a dashed line frame are photographed separately, and "image G1", "image G2", and "image G3" shown in FIG. 4 are obtained. The left part 70L and the center part 70C overlap in the tire axial direction, and the center part 70C and the right part 70R overlap in the tire axial direction. Therefore, the obtained images also have overlapping parts between the image G1 and the image G2, and between the image G2 and the image G3.

[0053] Next, the above-mentioned multiple images are synthesized to obtain a composite image of the ground contact portion 5. The synthesis of images can be performed by a computer program using a predetermined algorithm. Preferably, the multiple images are synthesized using a feature amount calculation algorithm. Generally, there is variation in background luminance between images captured in a divided manner due to the constraints of the measuring device. Development of a shooting system that satisfies the robustness required to resolve this has disadvantages such as increased costs. In contrast, synthesis using a feature amount calculation algorithm can improve robustness while suppressing cost increases, has excellent alignment accuracy between divided images, and can automatically correct any slight positional deviations using a computer program.

[0054] The feature amount calculation algorithm may be a known algorithm, such as SIFT (Scale-Invariant Feature Transform), SURF (Speed-Up Robust Features), FAST (Features from Accelerated Segment Test), HOG (Histograms of Oriented Gradients), etc. Among these, SIFT or SURF is preferred.

[0055] SIFT is an algorithm that detects image feature points and describes feature quantities, and multiple 128-dimensional feature quantities are assigned. SIFT is characterized by being invariant to rotation and scale changes, and robust to changes in lighting. Therefore, it is suitable for synthesizing images of tire contact areas where there are differences in lighting intensity, and is also highly robust to camera position.

[0056] SURF is an algorithm that speeds up the feature processing part of SIFT and achieves SIFT-level accuracy (however, SIFT generally has higher accuracy in feature recognition), and multiple 64-dimensional features are assigned. When dealing with large images, such as compositing images of tire contact areas, SURF is more preferable from the perspective of calculation time.

[0057] 5(A) shows a composite image G4 obtained by combining the above images G1 to G3 using SURF. The composite image G4 is an image of a ground contact portion 5 across the entire width of the tire tread. In the composite image G4, image information such as brightness is averaged for each pixel in the overlapping portion between the images G1 and G2, and in the overlapping portion between the images G2 and G3.

[0058] Next, the composite image obtained above is converted into a luminance image (fluorescence intensity image) that represents the luminance distribution of the fluorescence. The conversion into a luminance image can be performed using commercially available numerical analysis software such as MATLAB (registered trademark). The conversion into a luminance image may be performed before the divided and captured images are combined. That is, the divided and captured images may each be converted into a luminance image, and then the converted images may be combined using a predetermined algorithm as described above.

[0059] Next, the composite image converted into the luminance image is binarized to obtain a binarized image. The composite image can be binarized by a computer program using a binarization algorithm. Preferably, the composite image is binarized using a local adaptive threshold algorithm. Generally, in a composite image, there is variation in background luminance between captured images, and if the local adaptive threshold algorithm is not used, it is necessary to manually reset the threshold value by checking the luminance histogram one by one, which increases the number of steps. In contrast, by using the local adaptive threshold algorithm, the number of steps required for manual work can be reduced and consistent processing can be achieved.

[0060] The adaptive threshold algorithm is a method of calculating a threshold for each small region in an image. For an image in which the brightness of the image varies from region to region, a threshold is calculated for each region, and the image is binarized using the threshold, so that the contact region and the non-contact region with respect to the simulated road surface 12A can be clearly separated. For example, a region below the threshold is determined as a "contact region" because the brightness is low and the film thickness of the fluorescent liquid 2 is small, and a region above the threshold is determined as a "non-contact region" because the brightness is high and the film thickness of the fluorescent liquid 2 is large, and is binarized. The adaptive threshold algorithm is described, for example, in Bradley, D., G. Roth, "Adapting Thresholding Using the Integral Image," Journal of Graphics Tools. Vol. 12, No. 2, 2007, pp.13-21, and a known algorithm can be used.

[0061] 5B shows a binarized image G5 obtained by binarizing the above-mentioned composite image G4 using an adaptive threshold algorithm. In the binarized image G5, the black parts are areas that are in contact with the simulated road surface 12A (contact areas), and the white parts are areas that are not in contact with the simulated road surface 12A (non-contact areas).

[0062] After the binarized image is obtained in this manner, the ground contact characteristics are evaluated based on the binarized image. For example, the number of black pixels in the binarized image is counted, and the ratio of the number of black pixels to the total number of pixels in the binarized image is calculated to obtain the contact area occupancy rate described below, thereby making it possible to evaluate the true contact area of ​​the tire 1 with the pseudo road surface 12A having projections and recesses that simulate an actual road surface. Contact area occupancy rate = [number of black pixels] / [total number of pixels in the image]

[0063] 6 is a flowchart showing a flow of a tire ground contact characteristic evaluation method according to one embodiment. In the evaluation method, first, in step S1, the tire 1 is brought into contact with the simulated road surface 12A with the fluorescent liquid 2 interposed between the tire 1 and the simulated road surface 12A.

[0064] Next, in step S2, a predetermined load is applied to the tire 1 using the load adjustment unit 54. In this example, the load is set in a plurality of stages, and is applied in order from the smallest load, for example. Therefore, the smallest load is applied first.

[0065] Next, in step S3, the irradiation device 28 irradiates excitation light and the imaging device 32 images the fluorescence (see FIG. 2). At that time, the ground contact portion 5 is divided into multiple parts and images are taken (see FIG. 3) as described above, thereby obtaining multiple images as shown in FIG.

[0066] Next, in step S4, it is determined whether imaging has been completed at all of the preset loads for multiple load levels, and if not, the process returns to step S2, the next load is applied in step S2, and imaging is performed at that load in step S3. Then, steps S2 to S4 are repeated until it is determined in step S4 that imaging has been completed at all loads. Then, when imaging has been completed at all loads, the process proceeds to step S5.

[0067] In step S5, the captured images are synthesized for each load to obtain a synthetic image such as that shown in FIG. 5(A). The synthesis of the images is performed automatically by a computer program using, for example, a feature amount calculation algorithm as described above. As a result, a synthetic image of the contact portion 5 is obtained for each load.

[0068] Next, in step S6, the obtained composite image is converted into a luminance image, and then the converted composite image is binarized to obtain a binarized image, for example, as shown in FIG. 5(B). The binarization is performed automatically by a computer program, for example, using a locally adaptive threshold algorithm, as described above. This results in a binarized image of the contact portion 5 for each load.

[0069] Next, in step S7, the contact area occupancy rate is calculated for each load from the obtained binarized image.

[0070] Fig. 7 is a graph showing an example of the relationship between the contact area occupation ratio thus obtained and the load [N] applied to the tire 1. This makes it possible to evaluate, as the tire ground contact characteristics, the change in the contact area of ​​the tire with the road surface due to differences in load, that is, the sensitivity of the contact area to the load.

[0071] In Fig. 7, the relationship between the contact area occupation rate and the tire load is obtained for multiple types of tires (i.e., tire A, tire B, and tire C). Therefore, it is possible to evaluate the ground contact characteristics between different types of tires.

[0072] In the example of FIG. 7, the contact area is relatively evaluated based on the contact area occupancy rate. 2 The contact area can also be absolutely evaluated using physical quantities such as . That is, for example, by acquiring an image captured by placing a scale with a known length and calibrating the length per pixel, the contact area can be calculated in physical quantities.

[0073] 7, the contact area occupancy rate over the entire width of the contact portion 5 is obtained from the binarized image, and the ground contact characteristics over the entire width are evaluated. Alternatively, the contact area occupancy rate for a local region in the tire tread may be obtained, and the ground contact characteristics in the local region may be evaluated.

[0074] Specifically, for an area in the binarized image that corresponds to a specific tread land (hereinafter referred to as a land-equivalent area), the contact area occupancy rate in the land-equivalent area may be calculated, and the sensitivity of the contact area occupancy rate in the land-equivalent area to the load may be obtained. This makes it possible to compare and evaluate the ground contact characteristics of different parts of the same tire. Alternatively, it is possible to compare and evaluate the ground contact characteristics of specific parts between different types of tires.

[0075] FIG. 8 shows local areas of interest in the binarized image, focusing on the center land area Ce surrounded by a rectangular frame in the center land 7A and the quarter land area Qu surrounded by a rectangular frame in the quarter land 7B. For the center land area Ce, the contact area occupancy rate of the center land 7A with respect to the pseudo road surface 12A can be obtained by calculating the ratio of the number of black pixels to the total number of pixels in the area. Also, for the quarter land area Qu, the contact area occupancy rate of the quarter land 7B with respect to the pseudo road surface 12A can be obtained by calculating the ratio of the number of black pixels to the total number of pixels in the area.

[0076] Fig. 9 is a graph showing an example of the relationship between the contact area occupation ratio thus obtained and the load applied to the tire 1, and allows evaluation of the ground contact characteristics due to the difference between the center land 7A and the quarter land 7B. As shown in Fig. 9, in this example, it can be seen that the quarter land 7B has a higher contact area occupation ratio than the center land 7A over the entire load range of 1000 to 5000N.

[0077] When calculating the contact area occupancy rate in a specific land-equivalent region as described above, in order to obtain the total number of pixels in the land-equivalent region, it is necessary to detect the range of the land-equivalent region in the binarized image. The following detection methods (1) to (3) can be used for this purpose.

[0078] (1) The binarized image is integrated in the tire circumferential direction to obtain a brightness integration graph in the tire axial direction. The area corresponding to the main groove 6 has a brightness distribution of almost zero, while the area corresponding to the tread land 7 has a certain brightness distribution. Based on the brightness distribution graph integrated in the tire circumferential direction, the binarized image can be divided into the land area and the groove area by discriminating with a threshold value.

[0079] (2) Instead of the pseudo road plate 12, a road plate with a flat upper surface is used, and the tire 1 is placed on the flat surface, and otherwise a binary image is obtained in the same manner as in the above embodiment. The binary image of the flat surface obtained in this manner is aligned with the binary image of the pseudo road surface 12A of the embodiment, and it is determined where each part of the binary image of the pseudo road surface 12A corresponds in the binary image of the flat surface. Since the land-equivalent region and the groove-equivalent region can be clearly distinguished in the binary image of the pseudo road surface 12A, the land-equivalent region and the groove-equivalent region are distinguished in the binary image of the pseudo road surface 12A from the above determination.

[0080] (3) By aligning the binarized image with a tire design pattern drawing, land areas and groove areas are distinguished in the binarized image.

[0081] By evaluating the ground contact characteristics in localized regions within the tire tread in this manner, it is possible to accurately evaluate differences in ground contact characteristics with the actual road surface due to differences in design, such as tread profile, tread pattern, tread rubber compound, reinforcing fibers, and belts, thereby obtaining knowledge that is useful for design.

[0082] In the embodiment described above, when the tire 1 is grounded on the pseudo road plate 12 and the electromagnetic waves transmitted through the pseudo road plate 12 are detected and optically photographed, the contact portion 5 with respect to the pseudo road plate 12 is divided into multiple parts and optically photographed, the multiple images obtained are synthesized, and the synthesized image is binarized to evaluate the contact characteristics. Therefore, it is possible to obtain a high-precision image of the contact portion 5 over a wide range, and the tire contact characteristics can be evaluated with high precision. Moreover, it is possible to obtain such high-precision images while suppressing an increase in cost.

[0083] In the above embodiment, the tire 1 is placed on the simulated road surface 12A for evaluation, but instead, a rubber test piece may be placed on the simulated road surface for evaluation. Such a rubber test piece may be a tread test piece cut out from a tire, or a tread test piece produced by vulcanizing and molding a rubber sample corresponding to the tread. Moreover, the rubber test piece may be one in which the tread test piece is attached to a test jig with an adhesive.

[0084] Although the embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0085] 1... tire, 2... fluorescent liquid, 5... ground contact portion, 10... measuring device, 12... simulated road surface plate, 12A... simulated road surface, 28... irradiation device, 32... imaging device

Claims

1. A step of grounding a tire or a rubber test piece on a simulated road surface of a simulated road plate having an unevenness equivalent to that of an actual road surface; a step of obtaining a plurality of images by optically photographing the contact portion of the tire or rubber test piece with the simulated road surface by using an imaging device that detects electromagnetic waves transmitted through the simulated road surface plate and performs the optical photographing to obtain a plurality of images; combining the images to obtain a combined image of the ground contact portion; binarizing the composite image to obtain a binarized image; and evaluating ground contact characteristics based on the binarized image; A tire ground contact characteristic evaluation method comprising:

2. The tire ground contact characteristic evaluation method according to claim 1 , wherein the plurality of images are synthesized using a feature amount calculation algorithm.

3. The method of claim 1 or 2, further comprising binarizing the composite image using a locally adaptive thresholding algorithm.

4. 2. The tire ground contact characteristic evaluation method according to claim 1, further comprising the steps of: placing a fluorescent liquid between the pseudo road surface and the tire or rubber test piece, grounding the tire or rubber test piece on the pseudo road surface, irradiating the fluorescent liquid with excitation light from the side of the pseudo road surface plate opposite the pseudo road surface, and detecting fluorescence emitted from the fluorescent liquid and transmitted through the pseudo road surface plate to perform the optical imaging.

Images