Construction Method of Inner Wall Multi-Scale Thermal Field Pattern Based on Sector Ring Interpolation

Abstract

In order to achieve key monitoring of the thermal fatigue damage in the nose region of the cylinder head and obtain highly recognizable reconstructed images, the present invention proposes a construction method for the multi-scale thermal field pattern on the inner wall based on sector ring interpolation. First, the firing surface of the cylinder head is divided into multi-scale thermal field regions, the probabilities of thermal fatigue damage occurring in each region of the inner wall are analyzed, and its resolution is determined. Based on the idea of circular arc interpolation, the divided multi-scale thermal field regions are discretized into sector ring regions composed of multiple circular arc point groups. Then, interpolation is performed on each circular arc in the sector ring region to obtain the sector ring thermal field pattern, and then each sector ring thermal field pattern is combined into a multi-scale thermal field pattern. The contrast of the reconstructed inner wall images with and without cracks by the present invention is increased by 74.77% - 75.09% compared with the reconstructed images constructed by the bilinear interpolation method. The structural similarity and peak signal-to-noise ratio are also slightly improved, and it is more conducive to the detection of defects such as cracks caused by thermal fatigue damage and the improvement of the quality of the reconstructed images in non-key detection regions.

Description

Technical Field

[0001] The present invention relates to a construction method for the thermal field pattern on the inner wall of a cylinder head, specifically a construction method for the multi-scale thermal field pattern on the inner wall based on fan-ring interpolation. Background Art

[0002] As the current power machinery with the highest thermal efficiency, regular detection of an internal combustion engine is an important means to improve efficiency and extend service life. Due to the long-term exposure of the inner wall of the cylinder head of the internal combustion engine to an extreme working environment, thermal fatigue damages such as wear, cracks, and depressions are prone to occur on the inner wall of the cylinder head [1-3] , resulting in unnecessary property losses and casualties. Currently, there are two main types of methods for detecting the inner wall of the cylinder head: one is to disassemble the cylinder head and determine the fault area through visual inspection, hydrostatic pressure, etc. [4] ; the other is to use software such as finite element to [5] simulate and analyze the inner wall of the cylinder head to determine its thermal fatigue damage area. Although these two types of methods can detect the thermal fatigue damage area, due to time-consuming, labor-intensive, and the large influence of boundary conditions on the determination of the damage area, etc., the detection effect is not good. To solve this problem, the research group proposed a reconstruction method for the inner wall of the cylinder head based on correlation imaging [6] to achieve rapid detection of thermal fatigue damage. This method can not only visually display the inner wall situation through images, but also greatly save manpower, material resources, and financial resources.

[0003] The reconstruction principle of the cylinder head inner wall detection method based on correlation imaging is to reconstruct the inner wall image through the correlation operation of a large number of thermal field patterns on the inner wall of the cylinder head and the total thermal energy on the outer wall. In fact, due to the limitations of the cylinder head size and current measurement means, it is impossible to comprehensively detect the inner wall of the cylinder head during measurement to obtain the thermal field pattern of the inner wall of the cylinder head. Only based on the data of a few measurement points, the thermal field pattern can be constructed by data interpolation. However, when using traditional interpolation methods such as nearest neighbor interpolation and bilinear interpolation [7-9] to construct the thermal field pattern, obvious block effects or incomplete filling are prone to occur in the thermal field pattern, and these methods do not conform to the thermal field transfer law of the inner wall of the cylinder head, resulting in a low recognition rate of the final reconstructed image. Summary of the Invention

[0004] In view of the above analysis, the present invention proposes a construction method for the multi-scale thermal field pattern on the inner wall based on fan-ring interpolation from the perspectives of the thermal field transfer law of the inner wall of the cylinder head and the integrity of regional filling. This method constructs a multi-scale thermal field pattern by interpolating a discrete circular arc point group, and correlates it with the total thermal energy on the outer wall to reconstruct the inner wall image. This method can not only highlight the key monitoring areas, that is, it is easier to identify thermal fatigue areas such as cracks, but also improve the recognition rate of the images in non-key monitoring areas.

[0005] The present invention is implemented by adopting the following technical solution: A construction method for an inner wall multi-scale thermal field pattern based on sector-ring interpolation, comprising the following steps:

[0006] Select a standard sector area and a non-standard sector area from the divided areas on the firing surface of the cylinder head. The radius of the standard sector area is a straight line, and the radius of the non-standard sector area is a curve;

[0007] For the standard sector area: (1) Establish a coordinate system. At this time, the starting and ending positions of the vectors formed by the two radii of the standard sector area are obtained. At this time, taking a point on one vector as the starting point and a corresponding point on the other vector as the ending point, substitute them into the circular arc interpolation method formula and iterate continuously. Finally, a sector-ring area composed of several discrete circular arc point groups is obtained;

[0008] (2) Take detection points at equal intervals on the two vectors, and measure the temperature values at a certain moment under the calibrated working condition. Put the values measured on the vectors into the discrete points of the vectors at equal intervals, and then fit these obtained into a function of the relationship between points and temperature values. After determining these two functions, interpolation can be performed on each discrete circular arc point group. The specific formula is: Where T is the temperature value of the point to be obtained on the circular arc, N is the total number of points on the circular arc, n is the number of the circular arc where the point to be obtained is located, f x (n) is the temperature value of the point on one vector of this circular arc, f y (n) is the temperature value of the point on the other vector of this circular arc, and m is the interval between the point to be obtained and the point on one vector of the circular arc where the point is located; Interpolate each discrete arc point group in the standard sector area in this way. After completing the above operations, the thermal field pattern of this area can be obtained;

[0009] For the non-standard sector area: (1) Establish a coordinate system. According to the dimensions of the cylinder head, the positions of the centers of the exhaust valve and the intake valve in this coordinate system can be obtained. Substitute the starting point, ending point, and the position of the center of the exhaust valve of a curve in the non-standard sector area into the circular arc interpolation method formula, and this curve will be discretized into a circular arc point group. Substitute the starting point, ending point, and the position of the center of the intake valve of another curve in the non-standard sector area into the circular arc interpolation method formula, and this curve will be discretized into a circular arc point group. Then, taking a point on one curve as the starting point and a point on the other curve as the ending point, substitute them into the circular arc interpolation method formula and iterate continuously to discretize the non-standard sector area into a sector-ring area composed of multiple scattered point circular arcs;

[0010] (2) Take detection points at equal intervals on the two curves, and measure the temperature values at a certain moment under the calibrated working condition. Put the values measured on the vectors into the discrete points of the vectors at equal intervals, and then fit these obtained into a function of the relationship between points and temperature values. After determining these two functions, interpolation can be performed on each discrete circular arc point group. The specific formula is: Where T is the temperature value of the point sought on the circular arc, N is the total number of points on the circular arc, n is the number of circular arcs where the point sought is located, and f x (n) is the temperature value of the point on the circular arc on one of the curves, and f y (n) is the temperature value of the point on the circular arc on the other curve. m is the interval between the point sought and the point on the curve where the circular arc to which the point belongs is located. Interpolate each discrete arc point group in the non-standard sector area in this way. After completing the above operations, the thermal field pattern of this area can be obtained;

[0011] Cut the remaining area into standard sector areas and non-standard sectors. The thermal field patterns of the cut standard sector areas and non-standard sectors are obtained by the above method;

[0012] Finally, combine all the obtained thermal field patterns of the fan rings to obtain a multi-scale thermal field pattern, thus completing the construction of the thermal field pattern of the divided area on the firing surface of the cylinder head.

[0013] For the above method for constructing the multi-scale thermal field pattern of the inner wall based on fan ring interpolation, since multiple groups of thermal field patterns are required for the reconstruction of the inner wall correlation image, it is necessary to construct the thermal field patterns at different times.

[0014] For the above method for constructing the multi-scale thermal field pattern of the inner wall based on fan ring interpolation, in order to obtain a relatively ideal reconstructed image of the inner wall thermal field, positive and negative correlation imaging is adopted. First, obtain the multi-scale thermal field pattern based on fan ring interpolation to obtain the thermal field patterns at N moments under the calibration condition. Then, use an NTC patch thermistor array to cover the outer wall area of the cylinder head, measure the temperatures of each area on the outer wall corresponding to the above N moments, and sum them to obtain the total heat energy of the outer wall. Finally, perform positive and negative correlation imaging on the thermal field pattern of the inner wall of the cylinder head and the total heat energy of the outer wall.

[0015] For the above method for constructing the multi-scale thermal field pattern of the inner wall based on fan ring interpolation, when dividing the firing surface of the cylinder head into areas, multi-scale division is performed, that is, division is carried out with circular rings of different radii.

[0016] The present invention proposes a correlation imaging method for constructing multi-scale thermal field patterns based on sector-ring interpolation. This method can not only improve the detail information of the key observation area, but also enhance the recognition of images in non-key detection areas. The sector-ring interpolation method not only conforms to the variation law of the in-cylinder firing surface, that is, centered on the fuel injection port and decreasing radially around, but also reasonably supplements the sector-ring area completely, providing more accurate data support for the image reconstruction of the inner wall of the cylinder head. Through the peak signal-to-noise ratio, structural similarity, and contrast, the reconstructed images with cracks and without cracks are quantitatively analyzed. The results show that the images reconstructed based on the thermal field pattern constructed by sector-ring interpolation are higher than those reconstructed based on the thermal field pattern constructed by bilinear interpolation, especially in terms of contrast, which is increased by 74.77%-75.09%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the division of the firing surface of the cylinder head

[0018] Figure 2 Schematic diagram of a partial area of the firing surface

[0019] Figure 3 Schematic diagram of area 4 of the firing surface

[0020] Figure 4 Schematic diagram of the coordinate system established for area 4

[0021] Figure 5 Schematic diagram for determining the position of the detection point

[0022] Figure 6 is an arc Schematic diagram of the scatter point group

[0023] Figure 7 Schematic diagram of the coordinate system establishment for area 5

[0024] Figure 8 Schematic diagram of the reconstructed image without cracks

[0025] Figure 9 Schematic diagram of the reconstructed image with cracks DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Existing research shows that [10-13] , the thermal field distribution law of the firing surface of the cylinder head is centered on the fuel injection port, with the characteristic of decreasing radially around, and affected by factors such as the positions of the intake port and exhaust port, and the cooling water cavity. The temperature changes in different areas of the firing surface in the cylinder head are different and show an annular change trend.

[0027] The division area of the firing surface of the cylinder head is as Figure 1As shown, since the four-valve cylinder head is symmetrical, the present invention takes the right half of the cylinder head as an example for analysis. Since the nose bridge area is between the exhaust valve and the fuel injection port, the temperature changes violently and is prone to failure, so it is necessary to divide it into four parts based on the influence of the exhaust valve and the intake valve, that is, the position between the exhaust valve and the intake valve is divided into areas 4, 5, and 6; the position between the exhaust valve and the exhaust valve is divided into areas 1 and 2; the position between the intake valve and the intake valve is divided into area 8; and the position of the outer edge is divided into areas 3, 7, and 9. The division rules are as follows: Figure 1 As shown: First, the center of the injection port is connected to the center of the exhaust valve and the center of the intake valve respectively and intersects at points D and E, and the line OD intersects the exhaust valve at point A; secondly, with O as the center, A as the starting point, OA as the radius, an arc is drawn and intersects with the intake valve to obtain area 4; then, the positions of 2 / 5 and 4 / 5 of the exhaust valve diameter are taken as points B and C (due to the influence of the thermal field of the inner wall of the cylinder head, the area obtained by taking these two points as the standard is more convenient for determining the resolution of each subsequent area. If the thermal field of the target cylinder head or other non- In order to avoid the influence of control factors, we can also take the center of the scheduling valve as point B, and get another point C where line OB intersects the exhaust valve. After experimental analysis, the quality of the reconstructed image obtained by dividing and constructing the thermal field pattern in this way is slightly reduced, but it does not affect the subsequent analysis) and draw a circle with O as the center and OB as the radius (the present invention uses the right half of the cylinder head for analysis, so it is displayed as a semicircle here), and we can get areas 1, 5, and 8; finally, with O as the center and OC as the radius, draw a circle until it intersects the intake valve at point F, and we can get the remaining areas. So far Figure 1 The above shows the division rules of each area.

[0028] The present invention matches the resolution from large to small according to the probability of cracks appearing in each area of ​​the inner wall of the cylinder head, that is, between rows, between rows, between entrances and the peripheral area, so as to achieve the purpose of improving the recognition of the thermal fatigue area of ​​the inner wall reconstructed image. In order to avoid the disadvantage of the smaller specifications of the four-valve cylinder head, the method sequentially determines the pixel size of the resolution filled in each area as 256×256 for areas 1 and 4; 128×128 for areas 5 and 8; 64×64 for areas 2 and 6; and 32×32 for areas 3, 7 and 9. Due to the influence of the shape of the area and the pixel size of the corresponding area, some edge positions of a certain area cannot be completely divided by the pixel size of the area. At this time, the resolution needs to be expanded layer by layer until the area can be completely divided.

[0029] The construction of the fan ring thermal field pattern

[0030] Determination of arc point group

[0031] Circular interpolation

[14] By giving the position information between two endpoints, a point group approximating the actual circular arc is calculated to obtain the circular arc curve. In this way, the present invention can discretize each region on the inner wall of the cylinder head into a fan-shaped ring region composed of multiple scattered-point circular arcs, and then interpolate these discrete circular arc point groups to construct a fan-shaped ring thermal field pattern. Let the starting point be (x s , y s ), the ending point be (x e , y e ), and the center of the circle be (x o , y o ). The coordinate formula of the circular arc scattered points is as follows:

[0032] where x T , y T are the iterative coordinates, with the initial values of x s , y s . The result L of the matrix calculation is the coordinate of the next interpolation point, and it is re-substituted into x T , y T to complete the iterative calculation of formula (1). α is the angle between the starting point vector and the x-axis.

[0033] Constructing the fan-shaped ring thermal field pattern

[0034] Limited by the current process means, it is often impossible to use thermocouples or other detection means to detect the firing surface of the cylinder head in all directions and determine the temperature values of each point in each region. Only by measuring a few special points and using the interpolation method can the thermal field pattern of each region on the inner wall of the cylinder head be constructed. In view of the temperature change trend of the firing surface on the inner wall of the cylinder head, that is, the temperature gradually decreases outward in a ring shape, the present invention uses the fan-shaped ring interpolation method to construct the thermal field pattern of each region in the cylinder. Here, the region shown in Figure 2 is used to specifically illustrate the construction of the thermal field pattern.

[0035] First, the thermal field of region 4 is constructed. As shown in Figure 3 , since the resolution pixel size for dividing this region is 256×256, the pixel point size of this region can be determined, and region 4 is discretized into a fan-shaped ring region composed of a group of circular arc points. Then, by determining the temperature change functions on two vectors and interpolating the fan-shaped ring region, the thermal field pattern of region 4 can be constructed. The specific method is as follows:

[0036] (1) First, discretize region 4 into a fan-shaped ring region composed of circular arc point groups corresponding to the resolution size, establish the coordinate system as shown in Figure 4 . At this time, the starting point and ending point positions of the two vectors and can be obtained. The starting point A of is (8, 8), and the ending point B is (17, 17). The starting point C is (8, -8), and the ending point D is (17, -17). At this time, by substituting the coordinates of points A and C into equations (1) and (2) and continuously iterating, an arc composed of an arc point group can be obtained. Then, using the points on as the starting points and the corresponding points on

[0037] as the ending points, substituting them into equations (1) and (2), a fan-shaped ring area composed of 10 discrete arc point groups can be obtained. Figure 5 (2) Interpolate the fan-shaped ring area by using the fan-shaped ring interpolation method. Specifically, first use a thermocouple to measure area 4, and place the measuring points as shown in . Take 6 equally spaced detection points on and vectors respectively, and measure the temperature values at a certain moment under the calibration working condition. Among them, the temperature values in the direction are 195.92, 206.87, 214.15, 225.61, 236.83, 259.6 in sequence, and the temperature values in the direction are 184.15, 189.67, 186.33, 176.7, 162.65, 156.24 in sequence (where the unit of temperature is °C). Also, since is composed of 10 discrete points, put the measured values on

[0038] equally spaced into these discrete points, and finally obtain the point-value relationship, that is, (0, 195.92), (2, 206.87), (4, 214.15), (6, 225.61), (8, 236.83), (10, 259.6). Then fit these obtained values into a continuous curve, and the point-temperature value relationship curve obtained is as follows: -bx +c (3)

[0039] where a = 24.31, b = -0.1251, c = 173.5, x is the position of the discrete points on and the value range is (0, 1, 2... 10), and f(x) is the temperature value at this point. In the same way, the point-temperature value relationship on can be obtained as follows:

[0040] f(y) = a0 + a1cos(xy) + b1sin(wy) (4)

[0041] where a0 = 172.8, a1 = 11.21, b1 = 13.13, w = 0.76, y ∈ (0, 1, 2... 10) is the The position of the upper discrete points is at a distance, and f(y) is the temperature value at that point. After determining these two functions, interpolation can be performed on a group of discrete circular arc points. The specific formula is:

[0042]

[0043] where T is the temperature value of the point sought on the circular arc, N is the total number of points on the circular arc, n is the number of the circular arc where the point sought is located (starting from 0), f x (n) is the temperature value of the point on this circular arc at , f y (n) is the temperature value of the point on this circular arc at , and m is the interval between the point sought and the point on the circular arc where the point is located at .

[0044] Next, take the sector ring as an example for illustration. As shown in Figure 6 , points A and C are the intersection points of the circular arcs and and of the two vectors, and the temperature values are f x (0) = 195.92 and f y (0) = 184.15 respectively, and the total number of points is 16, that is, N = 16. Then the temperature values of each point on it are When m = 0, it is the temperature value at point A, that is, T A = 195.92. When m = 1, it is the temperature value at a, that is, T a = 195.184. Until m = 16, it is the temperature value at point C, that is, T C = 184.15. In this way, the interpolation of the point group is completed. By interpolating each discrete arc point group in this area in this way, the thermal field pattern of this area can be obtained after the above operations are completed.

[0045] Secondly, it is to construct the thermal field pattern of region 5. When constructing the thermal field pattern of region 5, the basic idea is similar to the above content. Considering that region 5 belongs to an irregular sector ring, the specific method is as follows. As shown in Figure 7 , according to the dimensions of the cylinder head, the positions of the centers of the exhaust valve and the intake valve in this coordinate system can be obtained, that is, O 排 (36, 36), O 进 (40, -40). First, and need to be discretized into a circular arc point group with a pixel size of 128×128. The starting point B is (17, 17), and the ending point E is (27, 7). Substituting points B, E, and O 排 into equations (1) and (2), Discretize it into a point group, and in the same way, discretize it into a point group. After that, it is the same as constructing the thermal field pattern of region 4. Starting from the points on and ending with the points on , discretize region 5 into a fan-shaped ring region composed of multiple arcs of scattered points. Then obtain and the points on and the temperature numerical function at the points, and then insert the temperature values into the point group of the arcs, so that the thermal field pattern of region 5 can be obtained.

[0046] Finally, it is to construct the thermal field pattern of the remaining regions. The thermal field pattern of region 6 is similar to that of region 5 and belongs to an irregular fan-shaped ring. The thermal field pattern of region 7 is similar to that of region 5 in the first half in the form of an irregular fan-shaped ring and similar to that of region 4 in the second half in the form of a regular fan-shaped ring. Then combine these fan-shaped ring thermal field patterns to obtain a multi-scale thermal field pattern, so that the construction of the regional thermal field pattern can be completed.

[0047] Since multiple sets of thermal field patterns are required for the reconstruction of the inner wall correlation image, it is necessary to construct the thermal field patterns at different times according to the above method.

[0048] Correlation imaging based on fan-shaped ring interpolated thermal field pattern

[0049] In order to obtain an ideal reconstructed image of the inner wall thermal field, the present invention adopts positive and negative correlation imaging

[15] , and relevant literature proves that

[16] the quality of the reconstructed image by this method is much better than some other methods. The basic principle of reconstruction is: first, obtain a multi-scale thermal field pattern based on fan-shaped ring interpolation, and obtain the thermal field patterns at N moments under the calibration working conditions, denoted as I a (x, y; N).

[0050] Then, use an NTC patch-type thermistor array to cover the outer wall area of the cylinder head, measure the temperature of each area on the outer wall at the above-mentioned N moments, and sum them to obtain the total thermal energy of the outer wall, denoted as I b (N).

[0051] Finally, perform positive and negative ghost imaging on the thermal field pattern of the inner wall of the cylinder head and the total thermal energy of the outer wall, that is, divide the thermal field pattern of the inner wall into two parts, namely:

[0052]

[0053] Then reconstruct the two parts of the data respectively to obtain a positive image and a negative image, that is

[0054]

[0055] Subtract the positive image from the negative image to obtain the final reconstructed image, that is

[0056] T(x, y) = T + (x, y) - T - (x, y)(8)

[0057] Where T(x, y) is the final reconstructed image, T + (x, y) is the positive reconstructed image, T - (x, y) is the negative reconstructed image, <I b (t)> is the average total energy value at the outer wall at time N, I a + is the positive thermal field pattern, I a - is the negative thermal field pattern, <.> is the relevant operator.

[0058] In order to objectively evaluate the quality of the reconstructed image, the present invention introduces three quantitative indicators for its analysis, namely peak signal-to-noise ratio, structural similarity, and contrast. Their calculation methods are as follows:

[0059] (1) Peak Signal to Noise Ratio (PSNR)

[0060]

[0061] Where, is the mean square error, x and y are the dimensions of the reconstructed image; M 2 is the maximum gray value of the reconstructed image.

[0062] (2) Structural Similarity Index (SSIM)

[0063]

[0064] Where, I represents the original image information; represents the reconstructed image; c1 = (K1L) 2 , c2 = (K2L) 2 and K1 = 0.01, K2 = 0.03, L = 255; μ I and are the averages of I and ; σ I and are the variances of I and ; is the covariance of I and .

[0065] (3) Contrast

[0066] C = ∑δ δ(i, j) 2 P δ (i, j) (11)

[0067] where δ(i, j) = |i - j|, i.e., the gray level difference between adjacent pixels, and P δ (i, j) is the pixel probability distribution with a gray level difference of δ between adjacent pixels.

[0068] Analysis of Experimental Results

[0069] To verify the effectiveness of the method proposed in the present invention, 500 groups of thermal field patterns of the inner wall of the cylinder head with and without cracks at intervals of 0.5 s under calibrated working conditions and the total energy values of the outer wall at the corresponding moments were obtained respectively for correlation reconstruction. Based on the evaluation index, the reconstructed images of the inner wall with cracks and the reconstructed images of the inner wall without cracks were analyzed and compared with the reconstructed images of the inner wall constructed by bilinear interpolation to construct the thermal field pattern.

[0070] Figure 8 The images reconstructed by the two methods when the cylinder head has no cracks are shown. (a) is the reconstructed image corresponding to the thermal field pattern constructed by bilinear interpolation, and (b) is the reconstructed image corresponding to the thermal field pattern constructed by sector ring interpolation.

[0071] For the reconstructed image constructed by bilinear interpolation to construct the thermal field pattern, the edge area of the inner wall image is darker and it is not easy to reflect the corresponding details. While for the thermal field pattern constructed by sector ring interpolation, the edge brightness is higher, it is easy to distinguish the shape and size of the inner wall, and the reconstructed image can highlight the detail information in the key detection areas such as the nose area of the cylinder head and improve the information in the non-key detection areas at the same time.

[0072] To more easily reflect the superiority of the method of the present invention, the peak signal-to-noise ratio, contrast, and structural similarity of these two reconstructed images were calculated, as shown in Table 1 specifically.

[0073] Table 1 Evaluation Indexes of the Reconstructed Images of the Cylinder Head without Cracks

[0074]

[0075] As can be seen from Table 1, for the image reconstructed by sector ring interpolation to construct the thermal field pattern, all indexes are better than those of the image reconstructed by bilinear interpolation to construct the thermal field pattern. Especially in terms of contrast, it has increased by 75.09%. It can be seen that the reconstructed image based on the sector ring thermal field pattern is more conducive to the reflection of the detail information in the key monitoring areas of the inner wall of the cylinder head and the improvement of the image quality.

[0076] Figure 9 The images reconstructed by the two methods when the cylinder head has cracks are shown. (a) is the reconstructed image corresponding to the thermal field pattern constructed by bilinear interpolation, and (b) is the reconstructed image corresponding to the thermal field pattern constructed by sector ring interpolation.

[0077] Under the same conditions, the image reconstructed based on the fan-ring interpolation thermal field pattern can clearly distinguish the cracks and their locations compared with that based on the bilinear interpolation. This shows that the effect of the reconstructed image under the fan-ring interpolation thermal field pattern is better than that of the traditional interpolation thermal field pattern. The peak signal-to-noise ratio, structural similarity, and contrast of the reconstructed image are shown in Table 2.

[0078] Table 2 Evaluation indexes of the reconstructed image of the cracked cylinder head

[0079]

[0080] As can be seen from Table 1, the image reconstructed under the fan-ring interpolation thermal field pattern is significantly better than the image reconstructed from the traditional interpolation speckle thermal field pattern in terms of contrast and structural similarity, with increases of 74.77% and 8.56% respectively. It can be seen that the reconstructed image of the fan-ring interpolation constructed thermal field pattern is more conducive to the detection of defects on the inner wall of the cylinder head.

[0081] Conclusion

[0082] In order to achieve key monitoring of the thermal fatigue damage in the nose area of the cylinder head and obtain a highly recognizable reconstructed image, the present invention proposes a multi-scale thermal field pattern construction method based on fan-ring interpolation. First, in combination with the thermal field distribution law of the inner wall of the cylinder head, the firing surface of the cylinder head is divided into multi-scale thermal field regions, the probability of thermal fatigue damage in each inner wall region is analyzed, and its resolution is determined; secondly, based on the idea of circular arc interpolation, the divided multi-scale thermal field regions are discretized into fan-ring regions composed of multiple circular arc point groups; then, interpolation is performed on each circular arc in the fan-ring region to obtain the fan-ring thermal field pattern, and then each fan-ring thermal field pattern is combined into a multi-scale thermal field pattern; finally, the constructed multi-scale thermal field pattern is correlated with the thermal energy on the outer wall of the cylinder head to reconstruct the image of the inner wall of the cylinder head. In order to verify the rationality of the method of the present invention, the reconstructed inner wall image is compared and analyzed with the reconstructed image under the thermal field pattern constructed by bilinear interpolation. The analysis results show that the contrast of the reconstructed inner wall images with cracks and without cracks by the method adopted in the present invention is increased by 74.77% - 75.09% compared with the reconstructed image of the thermal field pattern constructed by the bilinear interpolation method, and the structural similarity and peak signal-to-noise ratio are also slightly improved, and it is more conducive to the detection of defects such as cracks caused by thermal fatigue damage and the improvement of the quality of the reconstructed image in non-key detection areas.

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Claims

1. A construction method for an inner-wall multi-scale thermal field pattern based on fan-ring interpolation, characterized in that: It includes the following steps: Select a standard sector area and a non-standard sector area from the divided areas on the firing surface of the cylinder head. The radius of the standard sector area is a straight line, and the radius of the non-standard sector area is a curve; For the standard sector area: (1) Establish a coordinate system. At this time, the starting and ending positions of the vectors formed by the two radii of the standard sector area are obtained. At this time, taking a point on one vector as the starting point and a corresponding point on the other vector as the ending point, substitute them into the formula of the circular arc interpolation method and iterate continuously. Finally, a fan-shaped ring area composed of several discrete circular arc point groups is obtained; (2) Detecting points are taken at equal intervals on two vectors, and the temperature values at a certain moment under the calibration condition are measured. The values measured on the vectors are placed at equal intervals into the discrete points of the vectors, and then these obtained values are fitted into a function of the relationship between points and temperature values. After determining these two functions, interpolation can be performed on each discrete arc point group. The specific formula is as follows: Where T is the temperature value of the point sought on the arc, N is the total number of points on the arc, n is the number of the arc where the point sought is located, and f x (n) is the temperature value of the point on the arc on one of the vectors, and f y (n) is the temperature value of the point on the arc on the other vector, and m is the interval between the point sought and the point on the arc where the point is located on one of the vectors; interpolate each discrete arc point group in the standard sector area in this way. After completing the above arc point group interpolation operation, the thermal field pattern of this area can be obtained; For non-standard fan-shaped regions: Establish a coordinate system. According to the dimensions of the cylinder head, the positions of the centers of the exhaust valve and the intake valve in this coordinate system can be obtained. Substitute the starting point, ending point, and the position of the exhaust valve center of a curve in the non-standard fan-shaped region into the formula of the circular arc interpolation method, and this curve will be discretized into a group of circular arc points. Substitute the starting point, ending point, and the position of the intake valve center of another curve in the non-standard fan-shaped region into the formula of the circular arc interpolation method, and this curve will be discretized into a group of circular arc points. Then, take the points on one curve as the starting points and the points on the other curve as the ending points, and substitute them into the formula of the circular arc interpolation method for continuous iteration to discretize the non-standard fan-shaped region into a fan-shaped ring region composed of multiple scattered-point circular arcs; (2) Take detection points at equal intervals on the two curves, and measure the temperature values at a certain moment under the calibrated working conditions. Put the values measured on the vector into the discretized points of the vector at equal intervals, and then fit these obtained points and the temperature value relationship function. After determining these two functions, interpolation can be performed on each discrete group of circular arc points. The specific formula is: where T is the temperature value of the point sought on the circular arc, N is the total number of points on the circular arc, n is the number of the circular arc where the point sought is located, f x (n) is the temperature value of the point of this circular arc on one of the curves, f y (n) is the temperature value of the point of this circular arc on the other curve, and m is the interval between the point sought and the point of this circular arc on one of the curves. Interpolate each discrete group of arc points in the non-standard fan-shaped region in this way. After completing the above interpolation operation of the group of arc points, the thermal field pattern of this region can be obtained; Cut the remaining area into standard sectors and non-standard sectors. The heat field patterns of the cut standard sector areas and non-standard sectors are obtained according to the above method; Finally, combine all the obtained fan-shaped ring heat field patterns to obtain a multi-scale heat field pattern, thus completing the construction of the heat field pattern of the divided area on the firing surface of the cylinder head.

2. The construction method of the inner wall multi-scale thermal field pattern based on fan-shaped ring interpolation according to claim 1, characterized in that: Since multiple groups of heat field patterns are required for the reconstruction of the inner wall correlation image, it is necessary to construct the heat field patterns at different times.

3. The construction method of the inner wall multi-scale thermal field pattern based on fan-ring interpolation according to claim 1 or 2, characterized in that: In order to obtain an ideal reconstructed image of the inner wall heat field, positive and negative correlation imaging is adopted. First, a multi-scale heat field pattern based on fan-shaped ring interpolation is obtained, and the heat field patterns at N moments under the calibration working conditions are obtained. Then, an NTC patch type thermistor array is used to cover the outer wall area of the cylinder head, measure the temperatures of each area on the outer wall corresponding to the above N moments, and sum them to obtain the total heat energy of the outer wall. Finally, positive and negative correlation imaging is performed on the heat field pattern of the inner wall of the cylinder head and the total heat energy of the outer wall.

4. The construction method of the inner wall multi-scale thermal field pattern based on fan-ring interpolation according to claim 1 or 2, characterized in that: When dividing the area on the firing surface of the cylinder head, multi-scale division is performed, that is, division is carried out with concentric circles of different radii.

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