Large field of view tire full groove depth measurement method based on line structured light

Abstract

The application relates to a large-view-field tire full-circumferential groove depth measuring method based on line structured light, and relates to the field of tire pattern measurement. The application is aimed at solving the problems of low efficiency, great influence of human factors and incapability of automation of the existing tire pattern depth detection method. In the process of rotation of the measured tire, the application collects continuous line laser images on the surface of the tire, generates a depth map by using the line laser images, positions all main grooves on the surface of the measured tire by using the depth map, and calculates the depth of each main groove according to the position of each main groove. The large-view-field tire full-circumferential groove depth measuring method based on line structured light can automatically find the position of the tire and calculate the depth of the tire pattern by driving control of a visual sensor, can quickly find the position of the tire, and can calculate the depth of the main grooves of the tire full circle.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of tire pattern measurement. BACKGROUND

[0002] Braking, acceleration, steering and other operations between the tire and the ground are realized by the pattern on the tire, and the tire pattern has a very huge impact on the tire. The main function of the tire pattern of the automobile is to increase the friction between the tire and the road surface, to remove water, and to prevent the wheels from slipping. The depth of the tire pattern of the vehicle is not less than the limit value, which is one of the important guarantee conditions for the safe driving of the automobile. Therefore, it is of great significance to detect the depth of the tire pattern with high precision.

[0003] At present, there are two methods for detecting the depth of the tire pattern, which are visual inspection according to the tire wear limit mark and measurement of the depth of the tire pattern by using a tire pattern depth gauge. The efficiency of these two methods is very low, and the precision is greatly affected by human factors, and the automation of tire pattern detection cannot be realized. SUMMARY

[0004] The present application is to solve the problem of low efficiency, great influence of human factors and inability to realize automation in the existing method for detecting the depth of the tire pattern. The present application provides a large field of view tire full groove depth measurement method based on line structured light.

[0005] The large field of view tire full groove depth measurement method based on line structured light comprises the following steps:

[0006] collecting a continuous line laser image of the surface of the measured tire during the rotation of the measured tire,

[0007] generating a depth map using the line laser image,

[0008] positioning the positions of all main grooves on the surface of the measured tire using the depth map,

[0009] calculating the depth of each main groove according to the position of each main groove.

[0010] Further, the line laser image of the surface of the measured tire is collected by using a line laser vision sensor.

[0011] Further, before collecting the line laser image, the relative position between the line laser vision sensor and the measured tire is positioned, and the specific method is as follows:

[0012] Step one: respectively under the conditions of having and not having laser irradiation, the line laser vision sensor is used to take pictures of the static measured tire, and tire images containing line laser stripes and not containing line laser stripes are obtained,

[0013] Step two: subtract the tire image with linear laser stripe from the tire image without linear laser stripe, obtain the light bar image with only linear laser stripe,

[0014] Step three: calculate the distance from both ends of the linear laser stripe in the light bar image to its adjacent image boundary respectively,

[0015] Step four: judge whether the ratio of the two distances is between 0.5 and 1,

[0016] Yes, the relative position between the linear laser vision sensor and the measured tire is unchanged, and the positioning of the relative position between the linear laser vision sensor and the measured tire is completed.

[0017] Otherwise, adjust the linear laser vision sensor to move towards the measured tire along the axial direction of the measured tire by a fixed step length, and then return to step one.

[0018] Further, a depth map is generated by intercepting any continuous sub-image in the linear laser image, and the sub-image accounts for 10% of the linear laser image.

[0019] Further, the specific method for positioning the positions of all main grooves on the surface of the measured tire using the depth map is:

[0020] The depth map is taken as a gray-scale image,

[0021] The gray-scale values in the same column of the gray-scale image are accumulated to obtain a row of gray-scale data,

[0022] The row of gray-scale data is plotted in a coordinate system with the horizontal axis as the pixel value and the vertical axis as the gray-scale value,

[0023] The image plotted in the coordinate system is subjected to Gaussian smoothing processing, and then the gray-scale minimum value of the image after Gaussian smoothing processing is solved, the number of gray-scale minimum values is taken as the number of all main grooves on the surface of the measured tire, and the pixel coordinates corresponding to each gray-scale minimum value are taken as the position coordinates of the corresponding main groove.

[0024] Further, the method for calculating the depth of the main groove is:

[0025] The position coordinates of the main groove on the surface of the measured tire are subjected to cubic spline interpolation processing to obtain a point cloud image,

[0026] The curvatures of each point in the point cloud image are calculated respectively, and the point corresponding to the maximum curvature is selected as the vertex of the main groove,

[0027] All points in the point cloud image are fitted into a straight line, and the distance from the vertex of the main groove to the straight line is the depth of the main groove.

[0028] Further, the cubic spline interpolation function expression is:

[0029] Si (x) = a i +b i (x-x i )+c i (x-x i ) 2 +d i (x-x i ) 3 ,

[0030]

[0031] wherein, S i (x) is the i-th interpolation result, a i is the constant term of the cubic spline interpolation function, b i , c i and d i are the one, two and three order term coefficients of the cubic spline interpolation function respectively, i = 1, 2,..., n, n is the total number of results, x i is the horizontal coordinate of the i-th interpolation point, x is the horizontal coordinate of the center point of the line laser,

[0032] in the formula, the omitted in the formula is 0,

[0033] a i = y i , m i = S″ i (x i ) = 2c i ,

[0034] S″ i (x i ) is the second derivative of S i (x), the intermediate variable h i = x i+1 -x i .

[0035] Further, three consecutive points (x1, y1), (x2, y2), (x3, y3) in the point cloud diagram are selected, and the coordinates of the three points are substituted into the curve parameter equation to obtain the parameters m1, m2, m3, n1, n2, n3 of the curve parameter equation,

[0036] The expression of the curve parameter equation is:

[0037]

[0038] Wherein, j=1, 2, 3, m1 and n1 are constant terms of horizontal and vertical coordinates in the parametric equation of the curve respectively, m2 and m3 are the coefficients of the first order term and the second order term of the horizontal coordinate in the parametric equation of the curve respectively, n2 and n3 are the coefficients of the first order term and the second order term of the vertical coordinate in the parametric equation of the curve respectively, t is the independent variable of the parametric equation of the curve, when j=1, t is the Euclidean distance between the points (x1, y1) and (x2, y2), when j=3, t is the Euclidean distance between the points (x3, y3) and (x2, y2), when j=2, t=0,

[0039] Then the curvature k of the intermediate point (x2, y2) is:

[0040]

[0041] The beneficial effects of the present application are as follows:

[0042] 1. By driving the laser vision sensor to move, the limitation of the field of view of a single vision sensor is overcome, the tire pattern depth measurement under a large field of view is realized, and the sensor is controlled to move to the position directly opposite the tire tread quickly and accurately.

[0043] 2. The positioning accuracy of the main groove of the tire pattern can be significantly improved, and the interference of the fine lines of the tread is avoided.

[0044] 4. The problem of data loss caused by the blocking of the laser light strip by the groove is overcome, the lowest point of the tire groove can be accurately positioned, and the groove depth can be accurately calculated.

[0045] In summary, the large field of view tire full groove depth measurement method based on line structured light according to the present application can quickly find the position of the tire by driving and controlling the vision sensor to automatically find the position of the tire and calculate the tire pattern depth, and then the main groove depth of the tire full circle can be calculated. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 The flowchart of the large field of view tire full groove depth measurement method based on line structured light according to the present application is shown in the figure;

[0047] Figure 2 The schematic diagram of the automatic tire position finding process according to the present application is shown in the figure;

[0048] Figure 3 The depth map obtained from the first 10% of the line laser map is shown in the figure;

[0049] Figure 4 The principle diagram of the depth map acquisition process is shown in the figure;

[0050] Figure 5 The tire light strip map is shown in the figure;

[0051] Figure 6 a graph of the groove bottom positions derived from the depth map;

[0052] Figure 7 a graph of the minimum values derived from the curvature;

[0053] Figure 8 a point cloud graph derived by cubic spline interpolation;

[0054] Figure 9 a graph of the curvature of a pattern groove;

[0055] Figure 10 a pattern depth graph of a pattern groove. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0057] The technology of machine vision measurement has gradually begun to be applied in tire pattern depth detection. The patent application with the patent number CN213274135U and the name of a line laser wheel rotating type full surface tire pattern depth measuring device designs a whole frame and a tire positioning roller arranged on the plane of the whole frame and a pattern measuring device at the bottom. It controls the position of the pattern measuring sensor through the sliding table movement, but does not elaborate how the sensor automatically tracks the tire position and the pattern measurement method. The patent application with the publication number CN106996750A and the name of a tire pattern depth measuring device and a tire pattern depth calculation method measures the tire pattern depth through a laser and a CCD camera when the tire rolls, and can only obtain the pattern depth corresponding to a laser scanning line.

[0058] In summary, the existing machine vision measurement technology still has many defects. In order to solve these defects, the following solutions are given in the present embodiment.

[0059] In combination with Figure 1 To specifically describe the present embodiment, the large field of view tire full groove depth measurement method based on line structured light described in the present embodiment includes the following steps:

[0060] The line laser vision sensor is placed on the platform driven by the motor, and the relative position between the line laser vision sensor and the measured tire is positioned, as shown in Figure 2 the specific method is:

[0061] Step one: take pictures of the static measured tire with and without laser irradiation respectively using the line laser vision sensor, and obtain tire images containing and not containing line laser stripes.

[0062] Step two: subtract the tire images containing and not containing line laser stripes to obtain light strip images containing only line laser stripes, as shown in Figure 5 .

[0063] Step three: calculate the distance from the left end of the line laser stripe in the light strip image to the left edge of the light strip image (i.e. left edge distance), and the distance from the right end of the line laser stripe to the right edge of the light strip image (i.e. right edge distance) respectively.

[0064] Step four: determine whether the ratio of the left edge distance to the right edge distance is between 0.5 and 1,

[0065] Yes, the relative position between the line laser vision sensor and the measured tire is unchanged, and the positioning of the relative position between the line laser vision sensor and the measured tire is completed;

[0066] No, adjust the line laser vision sensor to move towards the measured tire along the axial direction of the measured tire by a fixed step, and then return to step one.

[0067] After positioning, the line laser vision sensor is used to collect a continuous line laser image of the surface of the measured tire during rotation.

[0068] A depth map is generated by intercepting any continuous sub-image in the line laser image, and the sub-image accounts for 10% of the line laser image, as shown in Figure 3 and Figure 4 .

[0069] The depth map is taken as a grayscale image, and the grayscale values in the same column of the grayscale image are accumulated to obtain a row of grayscale data.

[0070] The row of grayscale data is plotted in a coordinate system with the horizontal axis as the pixel value and the vertical axis as the grayscale value, as shown in

[0071] The image plotted in the coordinate system is subjected to Gaussian smoothing processing, and then the grayscale minimum value of the Gaussian smoothed image is solved to obtain the number of grayscale minimum values as the number of all major grooves on the surface of the measured tire, and the pixel coordinates corresponding to each grayscale minimum value as the position coordinates of the corresponding major groove.

[0072] The depth of each major groove is calculated according to the position of each major groove, and the specific method is:

[0073] The position coordinates of the major grooves on the surface of the measured tire are subjected to cubic spline interpolation processing to obtain a point cloud map.

[0074] Select three consecutive points (x1, y1), (x2, y2), (x3, y3) in the point cloud diagram, and substitute the coordinates of the three points into the curve parameter equation to obtain the parameters m1, m2, m3, n1, n2, n3 of the curve parameter equation,

[0075] The curve parameter equation expression is:

[0076]

[0077] Where j = 1, 2, 3, m1 and n1 are constant terms of the horizontal and vertical coordinates in the curve parameter equation, m2 and m3 are the coefficients of the first and second terms of the horizontal coordinate in the curve parameter equation, n2 and n3 are the coefficients of the first and second terms of the vertical coordinate in the curve parameter equation, t is the independent variable of the curve parameter equation, when j = 1, t is the Euclidean distance between points (x1, y1) and (x2, y2), when j = 3, t is the Euclidean distance between points (x3, y3) and (x2, y2), when j = 2, t = 0,

[0078] Then the curvature k of the middle point (x2, y2) is:

[0079]

[0080] Select the point corresponding to the maximum curvature as the main groove vertex.

[0081] Fit all points in the point cloud diagram to a straight line, then the distance from the main groove vertex to the straight line is the depth of the main groove.

[0082] The expression of the above cubic spline interpolation function is:

[0083] S i (x)=a i +b i (x-x i )+c i (x-x i ) 2 +d i (x-x i ) 3 ,

[0084]

[0085] Where S i (x) is the i-th interpolation result, a i is the constant term of the cubic spline interpolation function, b i , c i and d i are the coefficients of the first, second and third terms of the cubic spline interpolation function, i = 1, 2,..., n, n is the total number of results, xi Let x be the x-coordinate of the i-th interpolation point, and let x be the x-coordinate of the center point of the line laser.

[0086] The ellipsis in the text indicates that all values ​​are 0.

[0087] By S i (x i )=y i We can obtain: a i =y i .

[0088] By S i (x i+1 )=y i+1 We can obtain: In the formula h i =x i+1 -x i .

[0089] By S′ i (x i+1 )=S′ i+1 (x i+1 We can obtain:

[0090] By S″ i (x i+1 )=S″ i+1 (x i+1 We can obtain: 2c i +6h i d i =2c i+1 .

[0091] m i =S″ i (x i ) = 2c i From the above four equations, we can obtain:

[0092]

[0093]

[0094] The above (a) i ,b i ,c i ,d i Substitute into the formula We can obtain:

[0095]

[0096] Then, m can be calculated using the above formula. i Then, we can find a. i b i ci and d i .

[0097] In this embodiment, such as Figure 4 As shown, a roller is used to drive the tire to rotate at a constant speed, denoted as v. Triangulation is used to obtain the point cloud coordinates of the tire cross-section at different times. The point cloud coordinates are represented by P. | The laser scans the area from time t1 to t2. The point cloud data obtained during this time is then unfolded along the normal direction of the laser plane, based on time t and the tire's rotational speed v, to obtain new point cloud coordinates P. These updated point cloud coordinates are then projected onto an image, where the grayscale values ​​represent the depth of the tire surface, thus generating a tire depth unfolded map. Figure 6 In this process, since the generated depth map represents the actual depth of the tire, the gray values ​​at the bottom of the tire tread are darker, while the gray values ​​at the surface of the tire tread are brighter. Based on this characteristic, let the obtained depth map be a w×r matrix, and G(α,β) represent the gray values ​​in the α-th row and β-th column of the depth map. The gray values ​​are obtained by summing them row by row and column by column. It can be obtained Figure 6 The diagram shows the location of the bottom of the trough.

[0098] The core of tire tread depth calculation is locating the tire tread positions. Besides the main treads, the tire surface has many smaller, slanted grooves. Therefore, traditional tire tread location methods are easily affected by these smaller grooves, leading to misidentification. Furthermore, in a two-dimensional optical plane coordinate system, the vertical axis representing depth indicates the actual tire depth, and directly processing this data can introduce significant errors. Therefore, this implementation uses a depth map to process the image. The minimum grayscale value in the depth map represents the tire tread position, which can accurately locate the tire tread position without being affected by smaller grooves. The number of minimum values ​​represents the number of tire tread patterns.

[0099] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A large field of view tire full circumference groove depth measurement method based on line structured light, characterized in that, The method comprises the following steps: Collecting a continuous line laser image of the surface of the measured tire during rotation of the tire, Generating a depth map using the line laser image, Locating positions of all main grooves on the surface of the measured tire using the depth map, Calculating depths of the main grooves respectively according to the positions of the main grooves, The specific method for locating the positions of all main grooves on the surface of the measured tire using the depth map is as follows: Taking the depth map as a grayscale image, Cumulatively adding grayscale values of the same column in the grayscale image to obtain a row of grayscale data, Plotting the row of grayscale data in a coordinate system with the horizontal axis as a pixel value and the vertical axis as a grayscale value, Performing Gaussian smoothing on the plotted image in the coordinate system, and then solving a grayscale minimum value of the Gaussian-smoothed image to obtain a number of grayscale minimum values as a number of all main grooves on the surface of the measured tire, and a pixel coordinate corresponding to each grayscale minimum value as a position coordinate of a corresponding main groove, The method for calculating the depth of the main groove is as follows: Performing cubic spline interpolation on the position coordinates of the main groove on the surface of the measured tire to obtain a point cloud image, Calculating curvatures of points in the point cloud image respectively, and selecting a point corresponding to a maximum curvature as a main groove vertex, Fitting all points in the point cloud image as a straight line, and a distance from the main groove vertex to the straight line is the depth of the main groove, The expression of the cubic spline interpolation function is as follows: ， ， wherein, is the first interpolation result, is the constant term of the cubic spline interpolation function, , and are the first, second and third order term coefficients of the cubic spline interpolation function, respectively, , is the total number of results, is the horizontal coordinate of the interpolation point, is the horizontal coordinate of the center point of the line laser. The dots indicate omitted zeros. ， ， ， ， For second derivative of the function f(x), the intermediate variable ; Selecting three continuous points in the point cloud image , , , and substituting the coordinates of the three points into the curve parameter equation to obtain the parameters of the curve parameter equation , , , , , , The expression of the curve parameter equation is as follows: ， wherein , and are constant terms of the horizontal and vertical coordinates in the parametric equation of the curve, and are the linear and quadratic coefficients of the horizontal coordinate in the parametric equation of the curve, and are the linear and quadratic coefficients of the vertical coordinate in the parametric equation of the curve, is the argument of the parametric equation of the curve, when , is the Euclidean distance between the points and , when , is the Euclidean distance between the points and , when , , then the intermediate point curvature of the is: 。 2. The large field of view tire full groove depth measurement method based on line structured light according to claim 1, characterized in that, The line laser image of the surface of the measured tire is collected by using a line laser vision sensor.

3. The large field of view tire full groove depth measurement method based on line structured light according to claim 2, characterized in that, Before collecting the line laser image, the relative position between the line laser vision sensor and the measured tire is located in the following specific manner: Step 1: photographing the static measured tire by using the line laser vision sensor under the conditions of having and not having laser irradiation to obtain tire images containing and not containing line laser stripes, Step 2: subtracting the tire images containing and not containing line laser stripes to obtain a light stripe image containing only line laser stripes, Step 3: calculating distances from both ends of the line laser stripe in the light stripe image to the adjacent image boundaries respectively, Step 4: judging whether a ratio of the two distances is between 0.5 and 1, Yes, the relative position between the line laser vision sensor and the measured tire is unchanged, and the location of the relative position between the line laser vision sensor and the measured tire is completed; No, adjusting the line laser vision sensor to move towards the measured tire along the axial direction of the measured tire by a fixed step, and then returning to step 1.

4. Large field of view tire full groove depth measurement method based on line structured light according to claim 1, 2 or 3, characterized in that, A depth map is generated by intercepting a continuous sub-image in the line laser image, and the sub-image accounts for 10% of the line laser image.

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