Methods for inspecting spark plugs

Abstract

Method for inspecting a spark plug (10), comprising a plug imaging step for imaging a target part (17) to obtain a target image (24), the target part (17) being a measurement target of the spark plug (10); a candle measuring step for measuring a measured dimension (M) of the target part (17) in the target image (24) or a measured number of pixels included in the target part (17) in the target image (24); a reference standard imaging step for imaging a plurality of reference standards (30) to obtain a plurality of reference images, each reference standard having a reference part (31) with a predetermined known dimension (K) different from each other and imaged to include the corresponding reference part (31); a reference standard measuring step for measuring measured dimensions (L) of the respective reference parts (41) in the plurality of reference images (40) or measured numbers of pixels included in the respective reference parts in the plurality of reference images (40); a regression line derivation step for obtaining a regression line (51) by the least squares method from: the measured dimensions (L) of the plurality of reference standards (30) or the measured number of pixels included in the plurality of reference standards (30); and the known dimensions (K); and a determination step for determining whether the target part (17) is located within a predetermined range based on a correction value obtained by correcting the measured dimension (M) of the target part (17) or the measured number of pixels included in the target part (17) obtained in the candle measuring step using a relational expression describing the regression line (51).

Description

Field of invention

[0001] The present invention relates to a method of inspecting a spark plug, and more particularly to a method of inspecting a spark plug including a step of checking the dimensions of various parts of the spark plug. Background of the invention

[0002] A technique for inspecting a spark plug is known in which the dimensions of various sections that affect ignition performance, including the spark gap, are checked by image processing (Japanese Patent Application Laid-Open (kokai) JP 2002 - 313525 A). In this technique, an improvement in inspection accuracy is desired.

[0003] Further relevant prior art is disclosed in the following documents: US 2004 / 0 067 712 A1 and CN 1 03 615 987 A. In US 2004 / 0 067 712 A1, the alignment of the center and ground electrodes is determined using image processing. In CN 1 03 615 987 A, the electrode spacing is determined using subpixel analysis, an image acquisition. Summary of the invention

[0004] An advantage of the present invention is a method for inspecting a spark plug that improves inspection accuracy.

[0005] According to a first aspect of the present invention, there is provided a method for inspecting a spark plug, comprising: a plug imaging step of imaging a target part to obtain a target image, the target part being a measurement target of the spark plug; a plug measuring step of measuring a measured dimension of the target part in the target image or a measured number of pixels included in the target part in the target image; a reference standard imaging step of imaging a plurality of reference standards to obtain a plurality of reference images, each reference standard having a reference part with a predetermined known dimension different from each other and imaged to include the corresponding reference part;a reference standard measuring step for measuring measured dimensions of the respective reference parts in the plurality of reference images or measured numbers of pixels included in the respective reference parts in the plurality of reference images; a regression line deriving step for obtaining a regression line by the least squares method from: the measured dimensions of the plurality of reference standards or the measured number of pixels included in the plurality of reference standards; and the known dimensions;and a determining step of determining whether the target part is within a predetermined range based on a correction value obtained by correcting the measured dimension of the target part or the measured number of pixels included in the target part obtained in the candle measuring step using a relational expression describing the regression line;

[0006] According to the method for inspecting a spark plug or according to a first aspect, the measured dimension or the measured number of pixels in the target part measured in an image is corrected based on the correction value derived from the plurality of reference standards having different known dimensions. Therefore, compared with a case of using a correction equation derived from a reference standard, it is possible to improve the correction accuracy of the measured dimension or the measured number of pixels in the target part. This improves the inspection accuracy. According to a second aspect of the present invention, there is provided a method for inspecting a spark plug as described above, wherein in the plug measuring step, the correction value is displayed on a display device that displays the measurement result for the target part.In addition to the effect of the first aspect, the operator can therefore confirm the good or bad value based on the value shown on the display device.

[0007] According to a third aspect of the present invention, there is provided a method for inspecting a spark plug as described above, wherein the target part is a spark gap formed between two electrodes. Thus, in addition to the effects of the first or second aspect, it is possible to improve the accuracy of inspecting the spark gap between the two electrodes.

[0008] According to a fourth aspect of the present invention, there is provided a method for inspecting a spark plug as described above, wherein the number of pixels included in the reference part in one of the plurality of reference images is counted before the reference standard measurement step, and a pixel size per pixel is calculated from the known dimension and the number of pixels. The reference standard measurement step and the plug measurement step are performed using the pixel dimension. This can improve the correlation of a regression analysis, so that the inspection accuracy can be further improved in addition to the effects of any of the first to third aspects. Brief description of the drawings Fig. 1 is a cross-sectional view of one side of a spark plug in one embodiment. Fig. 2 is a schematic representation of an inspection device. Fig. 3A is a front view of a reference standard, and Fig. Figure 3B is a schematic representation of an image element that depicted the reference standard. Fig. Figure 4 is a correlation diagram showing a relationship between known dimensions and measured dimensions of reference standards. Detailed description of the invention

[0009] In the following, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view of one side of a spark plug 10 in one embodiment. In Fig. 1, the lower side of the drawing sheet is referred to as the upper end side of the spark plug 10, and the upper side of the drawing sheet is referred to as the rear end side of the spark plug 10. The spark plug 10 includes a center electrode 13 supported by an insulator 11 and a ground electrode 16 connected to a metal shell 15. The insulator 11 is a cylindrical member made of alumina or the like with excellent mechanical and insulating properties at high temperatures and has an axial bore 12 along an axial line O. The center electrode 13 is arranged on the upper end side of the axial bore 12.

[0010] The center electrode 13 is a rod-shaped member extending along the axial line O and is obtained by plating a core material made of copper or a core material containing copper as a main component with nickel or a nickel-based alloy. The core may be omitted. The center electrode 13 is supported by the insulator 11, and an upper end side of the center electrode 13 protrudes from the upper end of the insulator 11.

[0011] A metal terminal 14 is a rod-shaped member to which a high-voltage cable (not shown) made of a conductive metal material (e.g., low-carbon steel) is connected. When the upper end of the metal terminal 14 is inserted into the axial hole 12, the metal terminal 14 is disposed at the rear end of the insulator 11. The metal terminal 14 is electrically connected to the center electrode 13 within the axial hole 12.

[0012] The metal housing 15 is attached to the upper front side of the outer circumference of the insulator 11. The metal housing 15 is an essentially cylindrical element made of a conductive metal material (e.g., low-carbon steel). The ground electrode 16 is connected to the upper end of the metal housing 15. The ground electrode 16 is a rod-shaped metallic element (e.g., nickel-based alloy) with a bent tip end surface. The tip end surface of the ground electrode 16 forms a spark gap 17 with the center electrode 13.

[0013] The metal shell 15 includes a body 60 having an external thread 61 on its outer peripheral surface, a seat 62 adjacent to the rear end side of the body 60, a connecting portion 63 adjacent to the rear end side of the seat 62, a tool engagement portion 64 adjacent to the rear end side of the connecting portion 63, and a crimping portion 65 adjacent to the rear end side of the tool engagement portion 64. The external thread 61 on the shell 60 screws into a threaded hole of an internal combustion engine (not shown). The seat 62 is a portion for closing a gap between the threaded hole of the internal combustion engine (not shown) and the external thread 61. The connecting portion 63 is a portion that is plastically deformed into a curved shape when the metal shell 15 is mounted to the insulator 11 by means of the crimping portion 65.The tool engagement portion 64 is a portion for engaging a tool, such as a wrench, when the external thread 61 is screwed into the threaded hole of the internal combustion engine. The crimp portion 65 is a portion that undergoes plastic deformation and is bent radially inward when the metal shell 15 is assembled with the insulator 11. A gasket 66 is disposed between the seat 62 and the external thread 61. When the spark plug 10 is installed in the internal combustion engine, the gasket 66 is clamped between the seat 62 and the internal combustion engine to ensure an airtight seal.

[0014] The spark plug 10 is manufactured, for example, according to the following method. First, the center electrode 13 is inserted into the axial bore 12 of the insulator 11 and is arranged so that the upper end of a center electrode 13 protrudes from the upper end of the insulator 11. Then, while ensuring electrical conductivity between the metal terminal 14 and the center electrode 13, after inserting the metal terminal 14 into the axial bore 12, the metal shell 15, to which the ground electrode 16 is connected in advance, is crimped and mounted on the outer periphery of the insulator 11. The ground electrode 16 is bent so that the spark gap 17 of a predetermined size is formed between the ground electrode 16 and the center electrode 13, thereby obtaining the spark plug 10.

[0015] Fig. 2 is a schematic diagram of an inspection device 20. The inspection device 20 checks the dimensions of various parts of the spark plug 10, including the spark gap 17. Hereinafter, the spark gap 17 is used as an example of a target part for measuring the spark plug 10.

[0016] The inspection device 20 is a device for checking whether the spark gap 17 (target part) is within a predetermined range by imaging the center electrode 13 and the ground electrode 16, including the spark gap 17, of the spark plug 10. The inspection device 20 includes a camera 21 with an imaging element 23, such as a CCD, and a lens 22 for adjusting the size of the field of view imaged by the camera 21. In the imaging element 23, the field of view (range) of the center electrode 13 and the ground electrode 16, including the spark gap 17, adjusted by the lens 22 is imaged as a target image 24.

[0017] The inspection device 20 includes: a calculation device 25 for performing calculation processing based on the measured dimension M of the spark gap 17 in the target image 24 acquired by the imaging element 23; an input device 26 for inputting the dimension of a pixel of the imaging element 23 (ie, a pixel size, which will be described later) and the like to the calculation device 25; and a display device 27 for displaying the measurement result for the spark gap 17 from the calculation device 25. The inspection device 20 images the center electrode 13 and the ground electrode 16, including the spark gap 17, with the imaging element 23 to obtain the target image 24 (candle imaging step).

[0018] Furthermore, the calculation device 25 measures the measured dimension M of the spark gap 17 in the target image 24 (plug measurement step). As described below, the inspection device 20 corrects the measured dimension M of the spark gap 17 (target part), thereby improving the inspection accuracy.

[0019] With reference to the Fig. 3A, Fig. 3B and Fig. 4 describes a method for correcting the measured dimension M with the inspection device 20 using reference standards 30. Fig. Figure 3A is the front view of a Reference Standard 30.

[0020] As in Fig. 3A, each reference standard 30 is a plate-shaped member made of a material that does not transmit light (e.g., metal or ceramic) and includes a reference part 31 having a predetermined known dimension K. Each reference standard 30 has a first portion 32, a second portion 34 disposed at a predetermined distance from the first portion 32, and a connecting portion 36 connecting the first portion 32 and the second portion 34. The edges 33, 35 of the first portion 32 and the second portion 34 are both straight, and the edges 33, 35 are parallel. The reference part 31 refers to an area between the edges 33, 35. The known dimension K of the reference part 31 is a distance between the edges 33, 35 (i.e., the length of the straight line perpendicular to the edges 33, 35).The known dimension K is a dimension inherent in the reference standard 30, measured with a calibrated "distance gauge" or other measuring device. A variety of reference standards 30 with different known dimensions K (e.g., known dimensions K that differ in 0.1 mm increments) are manufactured. Fig. Figure 3B is a schematic representation of the imaging element 23 imaging a reference standard 30. In the drawing, the X and Y arrows represent directions in the plane of the image element 23. The imaging element 23 is a group of pixels 23a for converting brightness intensities into electrical signals. The reference standards 30 are imaged by the imaging element 23 as reference images 40, each reference image 40 including a reference part 41. Each reference image 40 includes a first portion 42, a second portion 44 arranged at a predetermined distance in the Y direction from the first portion 42, and a connecting portion 46 connecting the first portion 42 and the second portion 44.

[0021] The imaging element 23 detects an edge 43 where the contrast of light and dark changes in the first portion 42, and an edge 45 where the contrast of light and dark changes in the second portion 44. The measured dimension L of the reference part 41 (bright portion) in the reference image 40, measured with the inspection device 20, is a distance between the edges 43, 45 (i.e., a length of a straight line extending in the Y direction perpendicular to the edges 43, 45). When the known dimension K is compared with the measured dimension L, a difference (error) may occur between the measured dimension L and the known dimension K, which is caused by the influence of the field of view of the inspection device 20, its halation, the accuracy of the lens 22, calculation errors, and the like.

[0022] To reduce this error, the inspection device 20 first images one of the plurality of reference standards 30 with different known dimensions K with the imaging element 23 to obtain a reference image 40. The known dimension K (mm) of the reference standard 30 is divided by the number of pixels 23a in the Y direction contained in the reference part 41 of the reference image 40 to calculate a dimension per pixel (hereinafter referred to as "pixel size"). The calculated pixel dimension (mm) is inputted by an input device 26 (see Fig. ) is transferred to the calculation device 25. In this way, the calculation device 25 calculates the measured dimension L by multiplying the calculated pixel dimension by the number of pixels of the reference part 41 captured by the imaging element 23. This makes it possible to minimize the calculation error of the calculation device 25. Next, the inspection device 20 images the plurality of reference standards 30 having different known dimensions K with the imaging element 23 to capture the reference part 31, and obtains a plurality of reference images 40, each reference image 40 including the reference part 41 (reference standard imaging step).

[0023] Subsequently, the measured dimension L of the reference part 41 is measured in each reference image 40 (reference standard measurement step). Subsequently, a regression analysis is performed based on the known dimensions K of the reference standards 30 and the measured dimensions L from the reference images 40.

[0024] Fig.4 is a correlation diagram illustrating the relationship between the known dimensions K in the reference part 31 of the plurality of reference standards 30 and the measured dimensions L in the reference part 41 of the reference images 40. The operator of the inspection device 20 plots a plurality of points 50 indicating the measured dimensions L and the known dimensions K of the plurality of reference standards 30 on a diagram whose vertical axis represents the measured dimensions L and whose horizontal axis represents the known dimension K. Subsequently, a regression line 51 can be obtained by the least squares method from the relationship between the known dimensions K and the measured dimensions L (regression line derivation step).

[0025] The calculation device 25 uses a relational expression describing the regression line 51 to correct the measured dimension M of the spark gap 17 in the target image 24 and obtain a correction value for the measured dimension M. Based on the correction value, the inspection device 20 determines whether the spark gap 17 of each manufactured spark plug 10 is within a predetermined range (determination step). In the determination step, the spark plug 10 whose spark gap 17 is determined to be within the predetermined range proceeds to the next step. For the spark plug 10 whose spark gap 17 is determined not to be within the predetermined range, the spark gap 17 is reset by adjusting the curvature of the ground electrode 16.

[0026] Since the inspection device 20 inspects the dimension of the spark gap 17 based on the correction value for the measured dimension M, the inspection accuracy can be improved compared to the case where the measured dimension M is not corrected. The correction value for the measured dimension M is obtained from a regression analysis on the plurality of known dimensions K and the plurality of measured dimensions L. In this way, the correction accuracy for different measured dimensions M with different values ​​can be improved compared to the case of using a correction equation derived by relying on only a single reference standard 30, for example. This contributes to improving the inspection accuracy.

[0027] The inspection device 20 displays the correction value for the measured dimension M on the display device 27, not the measured dimension M of the spark gap 17 before the correction. Thus, the operator of the inspection device 20 can confirm the pass or fail of the spark gap 17 based on the value displayed on the display device 27.

[0028] The inspection device 20 calculates the dimension per pixel (i.e., the pixel size) based on a reference image 40. This is done before measuring the measured dimensions L in the plurality of reference images 40 for the purpose of performing regression analysis based on the known dimensions K and the measured dimensions L. The calculated pixel dimension is transferred from the input device 26 to the calculation device 25, and the calculation device 25 multiplies the number of pixels of the reference part 41 by the pixel dimension to calculate a measured dimension L for performing regression analysis. This can improve the correlation of the regression analysis and thus further improve the correction accuracy. This can further improve the inspection accuracy. Note that this process may be omitted.

[0029] Each reference standard 30 includes the first portion 32 and the second portion 34 separated by the gap (i.e., the reference part 31). Likewise, the spark plug 10, which is the measurement target, includes the center electrode 13 and the ground electrode 16 separated by the spark gap 17. The reference part 31 of the reference standard 30 is detected as a bright portion in the reference image 40. Likewise, the spark gap 17 is detected as a bright portion in the target image 24. Since the structure of the reference part 31 of the reference standard 30 is similar to the structure of the center electrode 13 and the ground electrode 16 separated by the spark gap 17 to be measured, it is possible to minimize the occurrence of an error by the structure of the reference standard 30.

[0030] Although the present invention has been described above based on one embodiment, the present invention is by no means limited to the above-mentioned embodiment, and it is easily understood that various improvements and modifications can be made within the scope. The scope of protection is determined by the claims. In the embodiment, a case was described in which the calculating device 25 obtains the correction value of the measured dimension M based on the regression analysis of the known dimensions K and the measured dimensions L of the plurality of reference standards 30, but the present invention is not necessarily limited thereto.Of course, instead of the measured dimensions L in the reference part 31 of the reference standards 30, it is possible to obtain the number of pixels 23a in the Y direction included in the reference part 41 in the reference images 40 of the plurality of reference standards 30 (hereinafter referred to as "measured number of pixels"), and obtain the correction value for the measured number of pixels based on a regression analysis of the measured number of pixels and the known dimensions K of the reference standards 30. The inspection device 20 compares the correction value for the measured number of pixels with a predetermined range of the measured number of pixels for the spark gap 17 and determines whether the spark gap 17 is within the predetermined range. In this case, too, the inspection accuracy can be improved similarly to the embodiment.

[0031] The correction value for the measured dimension M can be calculated by multiplying the correction value for the measured number of pixels, which is obtained from the regression analysis of the measured number of pixels and the known dimensions K of the reference standards 30, by the dimension per pixel (i.e., the pixel dimension). In this case, the inspection device 20 compares the calculated correction value of the measured dimension M with a predetermined range for the spark gap 17 and determines whether the spark gap 17 is within the predetermined range. In this case, too, the inspection accuracy can be improved similarly to the embodiment.

[0032] In this case, the display device 27 can display the correction value of the measured dimension M or the correction value for the measured number of pixels. In both cases, the operator of the inspection device 20 can confirm the quality of the spark gap 17 using the correction value displayed on the display device 27.

[0033] Although the embodiment was described based on a case where a tip for suppressing the occurrence of quenching or the like is not disposed on the center electrode 13 and the ground electrode 16, the present invention is not necessarily limited thereto. It is of course possible to dispose such a tip on at least one of the center electrodes 13 and the ground electrode 16 of the spark plug 10. Even in this case, the inspection device 20 can perform a dimensional inspection of the spark gap 17 between the center electrode 13 and the ground electrode 16.

[0034] In the embodiment, a case of using reference standards 30 was described in which a gap is the reference part 31, but the present invention is not necessarily limited to this. Of course, it is possible to use reference standards of various shapes, such as rectangular plate shapes, needle shapes, and columnar shapes. In this case, any portion of the reference standard whose known dimension can be measured can be used as the reference part.

[0035] In the embodiment, a case where the ground electrode 16 connected to the metal shell 15 is curved was described, but the present invention is not necessarily limited to this. Instead of using a curved ground electrode 16, it is of course possible to use a straight ground electrode. In this case, the upper end side of the metal shell 15 is extended in the axial direction, and a straight ground electrode is connected to the metal shell 15 so that the ground electrode faces the center electrode 13.

[0036] In the embodiment, the ground electrode 16 is arranged so that the tip end portion of the ground electrode 16 faces the center electrode 13 in the axial direction. However, the present invention is not necessarily limited to this, and the positional relationship between the ground electrode 16 and the center electrode 13 can be adjusted accordingly. Other positional relationships between the ground electrode 16 and the center electrode 13 include, for example, disposing the ground electrode so that the spark gap is formed between the side surface of the center electrode 13 and the upper end surface of the ground electrode 16.

[0037] In the embodiment, a spark gap 17 is used as an example of the target part to be subjected to dimensional inspection of the spark plug 10, but the present invention is not necessarily limited thereto. Of course, it is possible to check the dimensions of other target parts of the spark plug 10 by image processing. The dimensions of other target parts include, for example, the outer diameter of the connecting portion 63 of the metal shell 15.

[0038] In the embodiment, a case of checking the dimensions of various parts of the spark plug 10 for igniting an air-fuel mixture by spark discharge was described, but the present invention is not necessarily limited to this. Of course, it is possible to check the dimensions of the various parts of a spark plug that ignite the air-fuel mixture, for example, by corona discharge, dielectric barrier discharge, or pulsed arc discharge.

[0039] Although the spark plug 10 in which the gasket 66 is disposed between the seat 62 and the external thread 61 is illustrated in the embodiment, the present invention is not necessarily limited thereto. It is of course possible to apply the present invention to a so-called conical disk type spark plug in which the gasket 66 is omitted, the upper end surface of the seat 62 is a conical surface, and the conical surface of the seat 62 is pressed against the internal combustion engine to ensure airtight sealing. Description of reference symbols 10 Spark plug 13 Center electrode 16 Ground electrode 17 Spark gap (target part) 23a pixels 24 Target image 27 Display device 30 Reference standard 31 Reference section 40 Reference image 51 Regression line K Known dimension of the reference part L Measured dimension of the reference part: M Measured dimension of the target part:

Claims

[1] A method for inspecting a spark plug (10), comprising a plug imaging step for imaging a target part (17) to obtain a target image (24), the target part (17) being a measurement target of the spark plug (10); a candle measuring step for measuring a measured dimension (M) of the target part (17) in the target image (24) or a measured number of pixels included in the target part (17) in the target image (24); a reference standard imaging step for imaging a plurality of reference standards (30) to obtain a plurality of reference images, each reference standard having a reference part (31) with a predetermined known dimension (K) different from each other and imaged to include the corresponding reference part (31); a reference standard measuring step for measuring measured dimensions (L) of the respective reference parts (41) in the plurality of reference images (40) or measured numbers of pixels included in the respective reference parts in the plurality of reference images (40); a regression line derivation step for obtaining a regression line (51) by the least squares method from: the measured dimensions (L) of the plurality of reference standards (30) or the measured number of pixels included in the plurality of reference standards (30); and the known dimensions (K); and a determination step of determining whether the target part (17) is located within a predetermined range based on a correction value obtained by correcting the measured dimension (M) of the target part (17) or the measured number of pixels included in the target part (17) obtained in the candle measuring step using a relational expression describing the regression line (51). [2] The method of inspecting the spark plug (10) according to claim 1, wherein, in the plug measuring step, the correction value is displayed on a display device that displays the measurement result for the target part (17). [3] A method of inspecting the spark plug (10) according to claim 1 or 2, wherein the target part (17) is a spark gap formed between two electrodes (13, 16). [4] A method for inspecting the spark plug (10) according to any one of claims 1 to 3, wherein a number of pixels contained in the reference part (41) in one of the plurality of reference images (40) is counted before the reference standard measuring step, a pixel size per pixel is calculated from the known size (K) and the number of pixels, and the reference standard measurement step and the candle measurement step are performed using the pixel size.

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