APPARATUS, METHOD, AND PROGRAM FOR MONITORING OPERATION OF HIGH FREQUENCY RESISTANCE WELDING AND INDUCTION HEATING WELDING OF ELECTRIC RESISTANCE WELDED STEEL PIPE

Abstract

The objective of the present invention is to enable accurate detection of a mismatch during electric resistance welding. This operation monitoring device for high-frequency resistance welding and induction heated welding of an electric resistance welded steel pipe, in which a strip-shaped metal sheet is continuously formed into a cylindrical shape by means of a group of rollers while being conveyed from an upstream side to a downstream side, and in which the two edge portions, in the circumferential direction, of the metal sheet, which are caused to converge into a V-shape, are caused to melt by the application of heat and are caused to abut one another, is characterized by being provided with a means for detecting a mismatch by recognizing a non-uniformity between light-emitting regions of a metal part, on both sides, in the circumferential direction, of the abutting position on an outer surface or an inner surface of the metal plate, on the basis of an image of a region including a V-convergence location, which is a location at which the two edge portions in the circumferential direction converge into said V-shape, and said metal part which is caused to flow out onto the surface of the metal plate by means of an electromagnetic force downstream of the V-convergence location, wherein said image is captured by means of an image capturing device from an outer surface side or an inner surface side of the metal plate that has been formed into said cylindrical shape.

Claims

1. An apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe where a strip of metal plate is continuously formed into a tubular shape by a group of rolls while being conveyed from an upstream side to a downstream side and two end parts of said metal plate in its circumferential direction made to converge to a V-shape are heated to melt and made to abut against each other, characterized in that said apparatus detects misalignment by obtaining a grasp of unevenness of light emitting regions of metal parts at two sides in the circumferential direction at abutting positions at an outside surface or inside surface of said metal plate based on an image, captured by an imaging device from the outside surface side or inside surface side of said metal plate being formed into the tubular shape, of a region including a V-convergence portion where said two end parts in the circumferential direction converge to a V-shape and said metal parts flowing out to the surface of said metal plate by electromagnetic force at a downstream side from said V-convergence portion.

2. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 1, characterized in that the apparatus comprises an input means to which an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction is input from said imaging device, an image processing means for performing image processing on the image input to said input means, a V-convergence point detecting means for detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to the V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the image processed by said image processing means, an area calculating means for finding a line passing through the geometric V-convergence point detected by said V-convergence point detecting means and parallel to the X-direction of the image in the image processed by said image processing means, using said line as the abutting position, and calculating an area S.sub.1 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at one side divided by said line and an area S.sub.2 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at the other side divided by said line, and a judging means for comparing the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of the abutting position calculated by said area calculating means to judge the occurrence of misalignment.

3. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 1, characterized in that the apparatus comprises an input means to which an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction is input from said imaging device, an image processing means for performing image processing on the image input to said input means, a V-convergence point detecting means for detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to a V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the image processed by said image processing means, an area calculating means for extending the approximation lines linearly approximating the two end parts in the circumferential direction to the downstream side over said geometric V-convergence point and calculating an area S.sub.1 of the light emitting region of said metal part at the outside from one of said extended approximation lines and an area S.sub.2 of the light emitting region of said metal part at the outside from the other of the extended approximation lines, and a judging means for comparing the areas S.sub.1, S.sub.2 of the light emitting regions calculated by said area calculating means to judge the occurrence of misalignment.

4. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 2, characterized in that said judging means finds a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side and judges whether said ratio is within predetermined upper and lower limit values.

5. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 3, characterized in that said judging means finds a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side and judges whether said ratio is within predetermined upper and lower limit values.

6. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 2, characterized in that said judging means judges whether said geometric V-convergence point is at an upstream side from a predetermined X-direction position in the image processed by said image processing means.

7. The apparatus for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 2, characterized in that said area calculating means finds a bisector of an angle of intersection of the approximation lines of said two end parts in the circumferential direction converging to the V-shape or a median line passing through said geometric V-convergence point in a triangular shape formed by the approximation lines of said end parts in the circumferential direction converging to the V-shape and the end part at the upstream side in the X-direction of said image in the image processed at said image processing means and corrects said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side calculated by said area calculating means.

8. A method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe continuously forming a strip of metal plate into a tubular shape by a group of rolls while conveying the strip of metal plate from an upstream side to a downstream side and heating two end parts of said metal plate in its circumferential direction made to converge to a V-shape to melt and making the two end parts of said metal plate abut against each other, characterized in that: said method comprises capturing an image, by an imaging device from an outside surface side or inside surface side of said metal plate being formed into the tubular shape, of a region including a V-convergence portion where said two end parts in the circumferential direction converge to a V-shape and metal parts flowing out to the surface of said metal plate by electromagnetic force at a downstream side from said V-convergence portion, and detecting misalignment by obtaining a grasp of unevenness of light emitting regions of said metal part at two sides in the circumferential direction at abutting positions at the outside surface or inside surface of said metal plate based on said image.

9. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 8, characterized by: capturing an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction by said imaging device, performing image processing on said captured image, detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to the V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the processed image, finding a line passing through the detected geometric V-convergence point and parallel to the X-direction of the image in the processed image, using said line as the abutting position, and calculating an area S.sub.1 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at one side divided by said line and an area S.sub.2 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at the other side divided by said line, and comparing the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of the abutting position to judge the occurrence of misalignment.

10. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 8, characterized by: capturing an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction by said imaging device, performing image processing on said captured image, detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to the V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the processed image, extending the approximation lines linearly approximating said two end parts in the circumferential direction to the downstream side of said conveyance direction over said geometric V-convergence point and calculating an area S.sub.1 of the light emitting region of said metal part at the outside from one of said extended approximation lines and an area S.sub.2 of the light emitting region of said metal part at the outside from the other of the extended approximation lines, and comparing said areas S.sub.1, S.sub.2 of the light emitting regions calculated to judge the occurrence of misalignment.

11. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 9, characterized by, in said judgment, finding a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side, and judging whether said ratio is within predetermined upper and lower limit values.

12. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 10, characterized by, in said judgment, finding a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side, and judging whether said ratio is within predetermined upper and lower limit values.

13. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 9, characterized by, in said judgment, judging whether said geometric V-convergence point is at an upstream side from a predetermined X-direction position in said processed image.

14. The method for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 9, characterized by, in said calculation of the areas S.sub.1, S.sub.2, finding a bisector of an angle of intersection of the approximation lines of said two end parts in the circumferential direction converging to the V-shape or a median line passing through said geometric V-convergence point in a triangular shape formed by the approximation lines of said end parts in the circumferential direction converging to the V-shape and the end part at the upstream side in the X-direction of said image in said processed image and correcting said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of light emitting region at the other side.

15. A program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe continuously forming a strip of metal plate into a tubular shape by a group of rolls while conveying the strip of metal plate from an upstream side to a downstream side and heating two end parts of said metal plate in its circumferential direction made to converge to a V-shape to melt and making two end parts of said metal plate abut against each other, characterized in that: said program makes a computer run processing for detecting misalignment by obtaining a grasp of unevenness of light emitting regions of metal parts at two sides in the circumferential direction at abutting positions at an outside surface or inside surface of said metal plate based on an image, captured by an imaging device from the outside surface side or inside surface side of said metal plate being formed into the tubular shape, of a region including a V-convergence portion where said two end parts in the circumferential direction converge to a V-shape and said metal parts flowing out to the surface of said metal plate by electromagnetic force at a downstream side from said V-convergence portion.

16. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 15, characterized in that said program makes said computer function as an input means to which an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction is input from said imaging device, an image processing means for performing image processing on the image input to said input means, a V-convergence point detecting means for detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to the V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the image processed by said image processing means, an area calculating means for finding a line passing through the geometric V-convergence point detected by said V-convergence point detecting means and parallel to the X-direction of the image in the image processed by said image processing means, using said line as the abutting position, and calculating an area S.sub.1 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at one side divided by said line and an area S.sub.2 of the light emitting region of said metal part at the downstream side from said geometric V-convergence point at the other side divided by said line, and a judging means for comparing the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of the abutting position calculated by said area calculating means to judge the occurrence of misalignment.

17. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 15, characterized in that said program makes said computer function as an input means to which an image having a conveyance direction of said metal plate as an X-direction and a circumferential direction of said metal plate as a Y-direction is input from said imaging device, an image processing means for performing image processing on the image input to said input means, a V-convergence point detecting means for detecting a geometric V-convergence point where said two end parts in the circumferential direction converging to a V-shape geometrically intersect by linearly approximating said two end parts in the circumferential direction and finding the intersecting point of the approximation lines of said two end parts in the circumferential direction in the image processed by said image processing means, an area calculating means for extending the approximation lines linearly approximating the two end parts in the circumferential direction to the downstream side over said geometric V-convergence point and calculating an area S.sub.1 of the light emitting region of said metal part at the outside from one of said extended approximation lines and an area S.sub.2 of the light emitting region of said metal part at the outside from the other of the extended approximation lines, and a judging means for comparing the areas S.sub.1, S.sub.2 of the light emitting regions calculated by said area calculating means to judge the occurrence of misalignment.

18. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 16, characterized in that said judging means finds a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side and judges whether said ratio is within predetermined upper and lower limit values.

19. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 17, characterized in that said judging means finds a ratio of either of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side with respect to the sum of said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side and judges whether said ratio is within predetermined upper and lower limit values.

20. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 16, characterized in that said judging means judges whether said geometric V-convergence point is at an upstream side from a predetermined X-direction position in the image processed by said image processing means.

21. The program for monitoring an operation of high frequency resistance welding and induction heating welding of an electric resistance welded steel pipe according to claim 16, characterized in that said area calculating means finds a bisector of an angle of intersection of the approximation lines of said two end parts in the circumferential direction converging to the V-shape or a median line passing through said geometric V-convergence point in a triangular shape formed by the approximation lines of said end parts in the circumferential direction converging to the V-shape and the end part at the upstream side in the X-direction of said image in the image processed at said image processing means and corrects said area S.sub.1 of the light emitting region at the one side and said area S.sub.2 of the light emitting region at the other side calculated by said area calculating means.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0069] FIG. 1 is a view showing the configuration of a manufacturing facility of an electric resistance welded steel pipe and an apparatus for monitoring an operation of electric resistance welding according to a first embodiment.

[0070] FIG. 2 is a flow chart showing a method for monitoring an operation by an apparatus for monitoring an operation of electric resistance welding according to the first embodiment.

[0071] FIG. 3 is a flow chart showing processing for detection of a V-convergence point of the flow chart of FIG. 2.

[0072] FIG. 4 is a view showing a state of occurrence of misalignment.

[0073] FIG. 5 is a schematic view showing an image captured by an imaging device.

[0074] FIG. 6 is a view for explaining a V-convergence point.

[0075] FIG. 7 is a schematic view showing images on which image processing has been performed and for which the V-convergence point has been detected.

[0076] FIG. 8 is a schematic view showing an example of a binary image in which a blob at a V-convergence portion is not extracted.

[0077] FIG. 9 is a schematic view showing images for which the area has been calculated in the first embodiment.

[0078] FIG. 10 are graphs finding the area ratio in actual operation and plotting it along with the elapse of time.

[0079] FIG. 11 is a flow chart showing a method for monitoring operation of an apparatus for monitoring operation of electric resistance welding according to a second embodiment.

[0080] FIG. 12 gives views for explaining the reason why the V-convergence point V.sub.1 shifts to the upstream side in the case where misalignment occurs compared to when misalignment does not occur.

[0081] FIG. 13 gives schematic views showing images for which the area is calculated in the second embodiment.

[0082] FIG. 14 is a view showing the configuration of a manufacturing facility of an electric resistance welded steel pipe and an apparatus for monitoring operation of electric resistance welding according to a third embodiment.

[0083] FIG. 15 gives schematic views showing images for which the area is calculated in the third embodiment.

[0084] FIG. 16 is a graph finding the area ratio in actual operation and plotting it along with the elapse of time.

[0085] FIG. 17 is a flow chart showing a method for monitoring operation of an apparatus for monitoring operation of electric resistance welding according to a fourth embodiment.

[0086] FIG. 18 gives schematic views showing images for which the area is calculated in the fourth embodiment.

[0087] FIG. 19 is a cross-sectional schematic view showing the directions of the high frequency current, the outflow of the melted portions due to electromagnetic force, and the discharge of the melted portions due to upset.

DESCRIPTION OF EMBODIMENTS

[0088] Below, referring to the attached drawings, preferred embodiments of the present invention will be explained.

First Embodiment

[0089] Referring to FIG. 1, a manufacturing facility of an electric resistance welded steel pipe will be explained in brief. As shown in FIG. 1, a strip of steel plate 1 is continuously formed into a tubular shape by a group of rolls (not shown) while being conveyed from an upstream side to a downstream side toward a direction 3. Further, an impeder 6 is arranged inside of the steel plate 1 being formed into the tubular shape and a pair of contact tips 7 (high frequency resistance welding) or induction coils (not shown) (induction heating welding) are used to supply high frequency current 5 while applying upset by the squeeze rolls 2. Due to this, two end parts 4, 4 of the steel plate 1 in the circumferential direction (below, also simply referred to as the end parts) can be heated to melt to make them abut and melt bond the steel plate 1 while being made to converge to a V-shape (electric resistance welding (ERW)).

[0090] Above the steel plate 1, an imaging device 8 is arranged. This captures the pattern of natural light (radiant pattern) of the region including the V-convergence portion where the outside surface of the steel plate 1 being formed into a tubular shape converges to a V-shape. The V-convergence portion includes a geometric V-convergence point V.sub.1 explained below, a portion where the two end parts 4, 4 of the steel plate 1 converge toward the geometric V-convergence point V.sub.1, and an abutment point V.sub.2 where the two end parts 4, 4 physically abut (contact). The portion where the two end parts 4, 4 of the steel plate 1 converge toward a geometric V-convergence point V.sub.1 preferably includes a region of 5 mm to 30 mm from the V-convergence point V.sub.1 toward the upstream side. The imaging device 8, for example, uses a 16001200 pixel 3CCD type color camera to capture images by an imaging field of a width of 30 mm or more and a length of 50 to 100 mm, an imaging resolution of 50 to 100 m/pixel, an imaging rate of 30 fps or more, and an exposure time of 1/5000 sec or less. The image data captured by the imaging device 8 is input to an apparatus 100 for monitoring an operation of electric resistance welding.

[0091] When a strip of steel plate 1 is continuously formed into a tubular shape by a group of rolls while being conveyed, sometimes a step difference occurs between one end part and the other end part of the steel plate, that is, misalignment occurs, as explained above. If misalignment occurs, as shown in FIG. 4, high frequency current 5 concentrates at the facing locations of the abutting end faces of the two end parts 4, 4 of the steel plate 1 (actual thicknesses h of abutting end faces). As a result, at the end part 4 offset upward (end part at left side in FIG. 4), the temperature rises at the inside surface side of the steel plate 1 and the degree of melting becomes higher. As opposed to this, at the end part 4 offset downward (end part at right side in FIG. 4), the temperature rises at the outside surface side of the steel plate 1 and the degree of melting becomes higher. Therefore, when misalignment occurs, unevenness occurs in the light emitting regions due to the melting or red heat at the two sides of the abutting position at the outside surface side and inside surface side of the steel plate 1. The misalignment is preferably within 10% of the plate thickness. For example, when the plate thickness is 10 mm, the misalignment is preferably made within 1.0 mm. Therefore, as explained in detail below, the imaging device 8 is used to capture the image of the region including the V-convergence portion and the metal parts flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence portion from the outside surface side of the steel plate 1 formed into a tubular shape. The inventors discovered that by obtaining a grasp of the unevenness of the light emitting regions of the metal parts at the two sides of the abutting position at the outside surface of the steel plate 1 based on that image, misalignment is detected. It is also possible to place the imaging device 8 at the inside surface side of the steel plate 1 formed into a tubular shape and, in the same way as the case of placing the imaging device 8 at the outside surface side of the steel plate 1, capture an image from the inside surface side of the steel plate 1 to detect misalignment. The metal parts flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence portion preferably include the metal parts at regions of 0 mm to 20 mm from the V-convergence portion toward the downstream side in the horizontal direction of the image obtained by the imaging device.

[0092] Returning the explanation to FIG. 1, in the apparatus 100 for monitoring an operation of electric resistance welding, 101 indicates an input part by which image data captured by the imaging device 8 is input. From the imaging device 8, an image having the conveyance direction of the steel plate 1 as the X-direction and the abutment direction of the steel plate 1 as the Y-direction is input. FIG. 5 is a schematic view illustrating the image captured by the imaging device 8. In the image captured by the imaging device 8, a light emitting region 51 (high heat region with high luminance level) appears along the two end parts 4, 4 of the steel plate 1. At the downstream side of the conveyance direction (X-direction), the melted portions of the two end parts 4, 4 flow out to the surface of the metal plate due to the electromagnetic force resulting in wave-like patterns. As shown in FIG. 1 and FIG. 19, the high frequency current 5 flows in opposite directions at facing locations of the abutting end faces of the two end parts 4, 4 of the steel plate 1, so repulsive force is generated between the two end parts 4, 4. The melted portions of the two end parts 4, 4 flow out to the surface of the metal plate due to the electromagnetic force. After that, the molten steel is pushed out in the upward and downward directions due to the upset of the strong pressure applied from the left and right. In the present invention, the unevenness of the light emitting regions of the metal parts flowing out to the surface of the metal plate due to this electromagnetic force is grasped to detect the misalignment. If misalignment occurs, current concentrates and melting is accelerated at the outside surface side of the steel plate 1 at the end part (end part at right side of FIG. 4) 4 offset downward, while current is reduced and melting becomes more difficult at the outside surface side of the end part 4 offset upward (end part at left side of FIG. 4), so the areas of outflow of molten metal to the outside surface of the metal plate at the two end parts 4, 4 differ. Similarly, if misalignment occurs, current concentrates and melting is accelerated at the inside surface side of the steel plate 1 at the end part 4 offset upward (end part at right side of FIG. 4), while current is reduced and melting becomes more difficult at the inside surface side of the end part 4 offset downward (end part at left side of FIG. 4), so the areas of outflow of molten metal to the inside surface of the metal plate at the two end parts 4, 4 differ. The change can be clearly discriminated even with small misalignment of less than 5% of the plate thickness, so it is possible to obtain a grasp of the unevenness of the light emitting regions of the metal parts flowing out to the surface of the metal plate due to the electromagnetic force so as to detect misalignment more precisely than in the past.

[0093] 102 is an image processing part which processes the image input to the input part 101 by red component extraction, binarization, and other image processing.

[0094] 103 is a V-convergence point detecting part which detects the geometric V-convergence point V t on the image processed by the image processing part 102. The geometric V-convergence point V.sub.1, as shown in FIG. 6 by the broken line, is the point where the two end parts 4, 4 converging to a V-shape geometrically intersect. By capturing the pattern of natural light of the portion where the two end parts 4, 4 of the steel plate 1 converge toward the geometric V-convergence point V t (radiant patterns), it is possible to detect the geometric V-convergence point V t based on the approximation lines of the two edges of the two end parts 4, 4. Note that in actuality, as shown in FIG. 6, a two-stage convergence phenomenon is observed of there being an abutment point V.sub.2 where the two end parts 4, 4 physically abut (contact each other) at the downstream side of the geometric V-convergence point V.sub.1, instead of the two end parts 4, 4 abutting at the geometric V-convergence point V.sub.1. Further, the welding point (point where solidification starts) is present at the further downstream side from the abutment point V.sub.2. Note that in the following explanation, the geometric V-convergence point V.sub.1 will sometimes be simply called the V-convergence point V.sub.1.

[0095] 104 is an area calculating part which finds the line L.sub.1 passing through the V-convergence point V t detected by the V-convergence point detecting part 103 and parallel to the X-direction of the image on the image processed at the image processing part 102. The imaging device 8 is set so that the horizontal direction of the image obtained by the imaging device 8 becomes parallel to the conveyance direction (X-direction) of the steel plate 1. The line passing through the V-convergence point V.sub.1 and parallel to the horizontal direction of the image obtained at the imaging device 8 is defined as the line L.sub.1. Further, this line L.sub.1 is deemed as the abutting position and an area S t of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V t at one side divided by the line L.sub.1 and an area S.sub.2 of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 at the other side divided by the line L.sub.1 are respectively calculated. The metal parts flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 preferably include the metal parts at regions of 0 mm to 20 mm from the V-convergence point V.sub.1 toward the downstream side in the horizontal direction of the image obtained at the imaging device. Note that details of the calculation of the areas S.sub.1, S.sub.2 will be explained at the later explained FIG. 9.

[0096] 105 is a judging part which compares the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of the abutting position calculated at the area calculating part 104 to judge the occurrence of any misalignment.

[0097] 106 is an output part which, for example, displays the images handled by the parts 101 to 105 and the results of comparison of the areas S.sub.1, S.sub.2 at the judging part 105 on a not shown display device. Further, when the judging part 105 judges misalignment, for example, it outputs an alarm.

[0098] Next, referring to FIG. 2, the method for monitoring operation according to the apparatus 100 for monitoring an operation of electric resistance welding according to the first embodiment will be explained in detail. The image capture operation by the imaging device 8 is performed continuously at certain time intervals. One image captured at the same timing is called a frame. If the image data is input from the imaging device 8 through the input part 101 (step S1), the image processing part 102 extracts the red component (wavelength 590 to 680 nm) from the image data to clarify the contrast (step S2).

[0099] The image processing part 102 binarizes (inverts) the image data from which the red component has been extracted at step S2 (step S3). Here, 0 is entered for a pixel with a luminance level of a predetermined threshold value or more and 1 is entered for a pixel of less than a certain value. The threshold value here is made the level of a disturbance factor, such as the noise level of the camera or reflection from the top roll, or more and is adjusted in the range where the shapes of the melted parts or end parts of the steel material can be grasped. For example, if a melted region is the 160 level by 255 gradations and the disturbance factor is the 30 level, about the 40 level is selected. By setting this threshold value, the range of a light emitting region for which the area is calculated in the present application is determined. FIG. 7(a) is a schematic view illustrating a binary image.

[0100] The V-convergence point detecting part 103 detects the geometric V-convergence point V.sub.1 on the binary image generated at step S3 (step S4). FIG. 3 shows a specific example of the processing for detection of the V-convergence point of step S4. First, as shown in FIG. 7(b), labeling is performed for labeling each blob (step S41), then it is judged if a blob matching predetermined conditions has been extracted (step S42). The blob referred to here means a region forming a clump at the 255 level in a binary image (at 8 bit, 0 or 255 level), more specifically means an individual region where any of the eight pixels adjoining a pixel of 1, including the four pixels to the top, bottom, left, and right and the four pixels in the diametrical directions, is 1 and are connected to form a clump. Further, labeling indicates assigning the same label numbers to individual blobs to extract specific blobs and perform processing for extracting positions inside the image (maximum points and minimum points of X-coordinates and maximum points and minimum points of Y-coordinates) and widths, lengths, areas, etc. together. For example, at FIG. 7(b), the three blobs are labeled 1, 2, and 3. If at step S42 there is a blob matching predetermined conditions, that blob (here, the label 2) is extracted as the blob 52 of the V-convergence portion of the portion where the two end parts 4, 4 converge in a V-shape (see FIG. 7(c)) and its coordinates, area, or other shape information is acquired. For example, in the binary image shown in FIG. 7(a), if there is a blob contiguous with the left end and having a predetermined area condition, that is extracted as the blob 52 of the V-convergence portion. As the condition of the predetermined area, for example, the condition of the actual dimension of the area of the blob being 15 to 150 mm.sup.2 and/or the condition of the actual dimension of the circumscribing rectangular shape being 25 to 320 mm.sup.2 may be set.

[0101] If at step S42 a blob matching predetermined conditions is extracted, the two end parts 4, 4 of the steel plate 1 are searched for at the extracted blob 52 of the V-convergence portion (step S43). As shown in FIG. 7(d) enlarging FIG. 7(c), points where 1 becomes 0 are searched for in the +Y-direction and Y-direction from the line passing through the downstream most point of the blob 52 of the V-convergence portion in the conveyance direction and parallel to the X-direction (shown by one-dot chain line in figure) and the points are deemed end parts 4 of the steel plate 1. This is performed in a predetermined range in the direction converging to the V-shape (X-direction), for example, in a range of from the left end of the range from the left end of the binary image (downstream side of conveyance direction) to the front end of the blob 52 of the V-convergence portion. Further, the end parts 4, 4 of the steel plate 1 are linearly approximated in this predetermined range (step S44) and the intersecting point of approximation lines of the same is detected as the geometric V-convergence point V.sub.1 (step S45). Note that, the above predetermined range is not always made a range of from the left end. If the position of the V-convergence point V.sub.1 moves to the upstream side of the conveyance direction due to the operating conditions, it is preferable to set this to a smaller value, for example, , or other suitable value.

[0102] When searching for the end parts 4 of the steel plate 1, for example, it is also possible to search for the points where 0 becomes 1 from the top and bottom positions of the image shown in FIG. 7(d) in the Y-direction toward the inside. However, it is learned that the blob 52 of the V-convergence portion appears near the center of the image in the Y-direction. If the search is started from the topmost position and bottommost position of the image, the processing becomes wasteful. Therefore, as explained above, the points where 1 becomes 0 are searched for from the inside of the blob 52 of the V-convergence portion in the +Y-direction and Y-direction to thereby shorten the processing time. Further, even if searching for the points where 0 becomes 1 from the top and bottom positions of the image toward the inside, it is possible to learn the Y-direction position of the broad part (left end of image) of the blob 52 at the V-convergence portion by labeling, so if searching for the points where 0 becomes 1 from the Y-direction position or near it toward the inside, it is possible to shorten the processing time.

[0103] If at step S42 a blob matching predetermined conditions is not extracted, an abnormal flag is set (step S46). For example, if the amount of input heat is low, as shown in FIG. 8, a blob at the V-convergence portion is not extracted, so the routine proceeds to step S46. Further, it is judged if the abnormal flag has been continuously set for exactly a predetermined number of frames (step S47). If the abnormal flag has been continuously set for exactly a predetermined number of frames, an abnormality alarm is output (step S48).

[0104] Returning the explanation to FIG. 2, the area calculating part 104, as shown in FIG. 9(a), finds the line L.sub.1 passing through the V-convergence point V.sub.1 detected at step S4 and parallel to the X-direction of the image on the binary image generated at step S3 (step S5). Note that, in FIG. 9, considering ease of viewing, the black and white such as shown in FIG. 7 is omitted. Further, the area calculating part 104 uses the line L.sub.1 as the abutting position and, by labeling, respectively calculates the area S.sub.1 of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V t at one side divided by the line L.sub.1 and the area S.sub.2 of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 at the other side divided by the line L.sub.1 (step S6).

[0105] The judging part 105 judges if the ratio of the area S.sub.1 or S.sub.2 of the light emitting region of either side designated in advance with respect to the sum of the area S.sub.1 of the light emitting region at one side and the area S.sub.2 of light emitting region at the other side calculated at step S6 is within the upper and lower limit values (step S7). If as a result the area ratio S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2) is within the upper and lower limit values, it is judged that misalignment is not occurring, while if it is over the upper limit value or lower limit value, it is judged that misalignment is occurring. As explained in FIG. 4, when misalignment occurs, unevenness occurs in the light emitting regions at the two sides of the abutting position at the outside surface side or inside surface side of the steel plate 1 and the state becomes one shown in FIG. 9(b). Therefore, it may be judged that misalignment has not occurred if the area ratio is near , that is, for example, is in a range of 40% to 60%, and that misalignment has occurred if it is under 40% or over 60%. Further, it is also possible to judge normality/misalignment based on a calibration curve correlating the step difference with an area ratio determined by changing the misalignment. If the area ratio is over the upper limit value or lower limit value, an alarm is output or other indication of abnormality is output (step S8).

[0106] At step S7, it is also possible to calculate the ratio of the area S.sub.1 and the area S.sub.2 or the absolute value of the difference of the area S.sub.1 and the area S.sub.2 and judge if this exceeds a predetermined threshold value. However, if swinging or twisting etc. of the steel plate 1 at the time of conveyance causes one area of the areas S.sub.1, S.sub.2 to fluctuate, that fluctuation affects the other area as is, so if just finding the ratio or difference of the area S.sub.1 and area S.sub.2, the judgment will tend to become excessively sensitive. As opposed to this, as shown in the present embodiment, by making the judgment based on the ratio of one area to the overall area (S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2)), more stable judgment becomes possible.

[0107] FIGS. 10(a) and (b) are graphs finding one area ratio with respect to the overall area in actual operation (S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2)) and plotting it along with the elapse of time. As shown in FIG. 10(a), as a result of plotting while monitoring it constantly during operation, at the time 15:03:21 on, the state of the area ratio exceeding the upper limit value continues. After that, if tracing and inspecting an actual material, it was confirmed that misalignment actually occurred at that point of time. From this result as well, it is learned that the technique of detecting misalignment using the present invention is effective.

[0108] FIG. 10(b) is a view replotting the data of FIG. 10(a) by obtaining the moving average of seven points by a time series. In FIG. 10(a), even before 15:03:21, sometimes the area ratio temporarily exceeds the lower limit value, but there is a high possibility of this being a noise component. Therefore, by taking the moving average of several points or so of data to smooth the graph and remove the noise component, it is possible to more clearly judge the occurrence of misalignment.

[0109] As explained above, misalignment is detected by obtaining a grasp of the unevenness of the light emitting regions of metal parts flowing out to the surface of the metal plate due to electromagnetic force at the two sides of an abutting position at the outside surface or inside surface of a steel plate 1 formed into a tubular shape, so it is possible to precisely detect misalignment in electric resistance welding without being affected by the end faces becoming mirror surfaces. By applying the present invention, it becomes possible to detect misalignment even if a step difference of a small step difference of about 2 to 3 mm occurs.

Second Embodiment

[0110] The second embodiment is an example configured to compare at the judging part 105 the areas S.sub.1, S.sub.2 of the light emitting regions of the metal parts flowing out to the metal plate due to the electromagnetic force at the two sides of the abutting position as explained in the first embodiment and also judge if the V-convergence point V.sub.1 is at the upstream side from a predetermined X-direction position on the image processed by the image processing part 102 to judge if any misalignment has occurred. Note that, below, the points of difference from the first embodiment will be focused on in the explanation and overlapping explanations will be omitted.

[0111] In the process of conveyance of the steel plate 1, the steel plate 1 sometimes swings or twists to the left and right of the conveyance direction. As explained in the first embodiment, the line L.sub.1 passing through the V-convergence point V.sub.1 and parallel to the X-direction of the image is found to calculate the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of this line L.sub.1, but if the steel plate 1 swings or twists to the left and right of the conveyance direction, sometimes the actual abutting position becomes slanted with respect to the line L.sub.1 (see FIG. 15(a)). In this case, regardless of the occurrence of any misalignment, a difference ends up occurring in the areas S.sub.1, S.sub.2 of the light emitting regions at the two sides of the line L.sub.1.

[0112] It was learned that if misalignment occurs, compared to if misalignment does not occur, the V-convergence point V.sub.1 detected by the V-convergence point detecting part 103 shifts to the upstream side. If misalignment does not occur, drawn to show the extreme case, as shown in FIG. 12(a), the inside surface sides and outside surface sides of the two end parts 4, 4 melt substantially evenly. On the other hand, if misalignment occurs, drawn to show the extreme case, as shown in FIG. 12(b), the degree of melting becomes higher at the inside surface side of the steel plate 1 at the end part 4 offset upward (end part at left side of FIG. 12), while the degree of melting becomes higher at the outside surface side of the steel plate 1 at the end part 4 offset downward (end part at right side of FIG. 12). For this reason, as shown in FIGS. 12(a) and (b), even in the state where the distances between the two end parts 4, 4 are equal, if the melted portions 120 flow out to the surface of the metal plate due to the electromagnetic force, when observed from the arrow Z-direction, the distance l.sub.1 of viewing when misalignment occurs becomes shorter compared with the distance l.sub.2 of viewing when no misalignment occurs. In other words, when observed from the arrow Z-direction, if misalignment occurs, compared to if no misalignment occurs, the V-convergence point V.sub.1 is detected at an early stage, that is, at the upstream side. Therefore, in the present embodiment, it is also judged if the V-convergence point V t is at the upstream side from a predetermined X-direction position.

[0113] Next, referring to FIG. 11, a method for monitoring an operation according to an apparatus 100 for monitoring an operation of electric resistance welding according to the second embodiment will be explained in detail. Steps S1 to S6 and S8 are similar to FIG. 2 of the first embodiment. Here, the explanation will be omitted. At step S17, the judging part 105 judges if the ratio of the area S.sub.1 or S.sub.2 of the light emitting region of either side designated in advance with respect to the sum of the area S.sub.1 of the light emitting region at one side and the area S.sub.2 of light emitting region at the other side calculated at step S6 is within the upper and lower limit values. In addition, it judges if the V-convergence point V.sub.1 detected at step S4 is at the upstream side from the predetermined X-direction position X.sub.S. As a result, if the area ratio S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2) is over the upper limit value or lower limit value and the V-convergence point V.sub.1 is at the upstream side from a predetermined X-direction position X.sub.S, it is judged that misalignment is occurring (see FIG. 13(b)), while otherwise, it is judged that misalignment is not occurring (see FIG. 13(a)). For example, even if the area ratio S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2) exceeds the upper limit value or lower limit value, unless the V-convergence point V.sub.1 is at the upstream side of the predetermined X-direction position X.sub.S, it is deemed that there is a high possibility that this is caused by swinging or twisting etc. of the steel plate 1 and it is judged that no misalignment has occurred. By setting the position of the geometric V-convergence point V.sub.1 as the condition for judgment of normality/abnormality, it is possible to obtain broader upper and lower limit values of the area ratio than the first embodiment, so, as explained above, even if the steel plate 1 swings or twists to the left and right in the conveyance direction, a high precision, stable judgment becomes possible.

[0114] As explained above, it is possible to eliminate the effect of swinging or twisting of the steel plate 1 etc. to detect misalignment, so it is possible to precisely detect misalignment in electric resistance welding.

Third Embodiment

[0115] The third embodiment, as shown in FIG. 14, is an example where the area calculating part 104 is provided with a correcting part 104a and has the function of correcting the calculated areas S.sub.1, S.sub.2. Note that, below, the points of difference from the first embodiment will be focused on in the explanation and overlapping explanations will be omitted.

[0116] As stated in the second embodiment as well, in the process of conveyance of steel plate 1, if the steel plate 1 swings and twists to the left and right of the conveyance direction, the actual abutting position will become slanted with respect to the line L.sub.1. For this reason, as shown in FIG. 15(a), without regard as to occurrence of any misalignment, sometimes a difference will arise in the areas S.sub.1, S.sub.2 of the light emitting region at the two sides of the line L.sub.1.

[0117] Therefore, the area calculating part 104, as shown in FIG. 15(a), first, in the same way as the first embodiment, finds the line L.sub.1 passing through the V-convergence point V.sub.1 and parallel to the X-direction of the image and calculates the area S.sub.1 of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 at one side divided by the line L.sub.1 and the area S.sub.2 of the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 at the other side divided by the line L.sub.1.

[0118] Next, the correcting part 104a, as shown in FIG. 15(b), finds the bisector L.sub.2 of the angle formed by the intersection of approximation lines of the two end parts 4, 4 of the steel plate 1 found at step S44 of FIG. 3. Further, the region surrounded by the line L.sub.1 and the bisector L.sub.2 in the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 is used as the correction region and an area S.sub.3 of the correction region is calculated.

[0119] Next, the correcting part 104a, as shown in FIGS. 15(b) and (c), adds the area S.sub.3 to the area of the region which the bisector L.sub.2 does not pass through in the light emitting region at the downstream side from the V-convergence point V.sub.1 (in the case of the illustrated example, the area S.sub.1) and subtracts the area S.sub.3 from the area of the region which the bisector L.sub.2 passes through to calculate the corrected area S.sub.1(=S.sub.1+S.sub.3), S.sub.2(=S.sub.2-S.sub.3). In other words, the area S.sub.1 of the light emitting region at the downstream side from the V-convergence point V.sub.1 at one side divided by the bisector L.sub.2 and an area S.sub.2 of the light emitting region at the downstream side from the V-convergence point V.sub.1 at the other side divided by the bisector L.sub.2 are respectively calculated.

[0120] After that, in the same way as the first embodiment, it is judged if the ratio of the corrected area S.sub.1 or S.sub.2 of the light emitting region of either side designated in advance with respect to the sum of the corrected area S.sub.1 of the light emitting region at one side and the corrected area S.sub.2 of the light emitting region at the other side is within the upper and lower limit values. If as a result the area ratio S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2) is within the upper and lower limit values, it is judged that no misalignment has occurred, while if it is over the upper limit value or lower limit value, it is judged that misalignment has occurred.

[0121] As explained above, it is possible to detect misalignment while eliminating the effect of swinging or twisting etc. of the steel plate 1, so it is possible to precisely detect misalignment in the electric resistance welding.

[0122] FIG. 16 is a graph finding the uncorrected area ratio (S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2)) and the corrected area ratio (S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2)) when twisting actually occurs in steel plate 1 in actual operation and plotting it along with the elapse of time. In the figure, the fine line shows the uncorrected characteristic, while the bold line shows the corrected characteristic. It is confirmed that up to the time t.sub.1, no misalignment occurs, but with no correction, the result becomes lower than the lower limit value, while with correction, the result is kept within the upper and lower limit values after removing the noise component. It was confirmed that misalignment can be precisely detected even in the state where twisting occurs in the steel plate 1.

[0123] In the present embodiment, the bisector L.sub.2 of the angle formed by the intersection of the approximation lines of the two end parts 4, 4 of the steel plate 1 is found, but the invention is not limited to this. For example, it is also possible to find the median line passing through the V-convergence point V.sub.1 at the triangular shape formed by the approximation lines of the two end parts 4, 4 of the steel plate 1 and the X-direction end part of the image X.sub.0.

Fourth Embodiment

[0124] In the first embodiment to the third embodiment, to detect the unevenness of the area at the downstream side from the geometric V-convergence point V.sub.1, a horizontal line passing through the geometric V-convergence point V.sub.1 and further a bisector of the angle formed by the intersection of the approximation lines were used. As opposed to this, in the present embodiment, the example is shown of using just the approximation lines for finding the geometric V-convergence point V.sub.1 to try to detect unevenness of the area. Note that, below, the points of difference from the first embodiment will primarily be explained and overlapping explanations will be omitted.

[0125] Referring to FIG. 17, a method for monitoring an operation according to the apparatus 100 for monitoring an operation of electric resistance welding according to the fourth embodiment will be explained in detail. Instead of step S5, the processing of step S15 is performed. Steps S1 to S4 and S6 to S8 are similar to FIG. 2 of the first embodiment, so here the explanation will be omitted. At step S15, the approximation lines of the end parts 4, 4 of the steel plate 1 obtained at step S4 (step S44) are extended to the downstream side of the V-convergence point V.sub.1. Further, at step S6, the area calculating part 104, as shown in FIG. 18(a), calculates the area S.sub.1 at the outside of the extended approximation line in the circumferential direction in the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at one side at the downstream side from the V-convergence point V.sub.1 and the area S.sub.2 at the outside of the extended approximation line in the circumferential direction in the light emitting region of the metal part flowing out to the surface of the metal plate due to the electromagnetic force at the other side at the downstream side from the V-convergence point V.sub.1. The metal parts flowing out to the surface of the metal plate due to the electromagnetic force at the downstream side from the V-convergence point V.sub.1 preferably include the metal parts at regions of 0 mm to 20 mm from the V-convergence point V.sub.1 toward the downstream side in the horizontal direction of the image obtained at the imaging device.

[0126] At step S7, the judging part 105, in the same way as the first embodiment, judges if the area ratio (S.sub.1/(S.sub.1+S.sub.2) or S.sub.2/(S.sub.1+S.sub.2)) or the absolute value of the difference of areas (|S.sub.1S.sub.2|) is within the upper and lower limit values. When misalignment occurs, unevenness occurs in the light emitting regions between the two sides of the abutting position at the outside surface side or inside surface side of the steel plate 1 and the state becomes one such as shown in FIG. 18(b). By doing this, even if the steel plate 1 swings or twists to the left and right of the conveyance direction, misalignment can be precisely and stably detected. Further, it is also enough to just extend the approximation lines of the two end parts in the circumferential direction converging to a V-shape, so it is not necessary to calculate the horizontal line passing through the geometric V-convergence point V.sub.1 and, further, the bisector of the angle formed by intersection of the approximation lines, and the processing becomes simpler.

[0127] Even if finding the areas S.sub.1, S.sub.2 like in this embodiment, as explained in the second embodiment, it is also possible to together judge if the geometric V-convergence point V.sub.1 is at the upstream side of a predetermined X-direction position.

[0128] Above, the present invention was explained together with various embodiments, but the present invention is not limited to these embodiments. Changes etc. are possible within the scope of the present invention. For example, in the above embodiments, a 3CCD type camera was used, but even if attaching an optical filter passing 570 to 740 nm or so to a monochrome camera, an image equivalent to the R-component of a color camera is obtained. For example, if using a camera provided with a type CCD (XGA size) and the distance up to the V-convergence portion is 1.2 m, a lens with a focal distance f=75 mm and a brightness F8 is set above the V-convergence portion to capture an image. Preferably -correction is performed to enable regions with low luminance at the end parts 4 of the steel plate 1 to be accurately detected.

[0129] The apparatus for monitoring an operation of electric resistance welding of the present invention specifically can be achieved by a computer system provided with a CPU, ROM, RAM, etc. and is realized by a CPU running a program. The apparatus for monitoring an operation of electric resistance welding of the present invention may be comprised of a single apparatus or may be comprised of a plurality of pieces of equipment.

[0130] Further, the object of the present invention is also achieved by providing a storage medium storing program codes of software for realizing the above-mentioned function of monitoring an operation of electric resistance welding in a system or an apparatus. In this case, the program codes read out from the storage medium themselves realize the functions of the above-mentioned embodiment, and the program codes themselves and the storage medium storing the program codes constitute the present invention. As the storage medium for providing the program codes, for example, a flexible disk, hard disk, optical disk, magneto optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM, etc. can be used.

REFERENCE SIGNS LIST

[0131] 1: steel plate [0132] 2: squeeze roll [0133] 3: conveyance direction [0134] 4: circumferential direction end part [0135] 5: high frequency current [0136] 6: impeder [0137] 7: contact tip [0138] 8: imaging device [0139] 100: apparatus for monitoring operation of electric resistance welding [0140] 101: input part [0141] 102: image processing part [0142] 103: V-convergence point detecting part [0143] 104: area calculating part [0144] 104a: correcting part [0145] 105: judging part [0146] 106: output part [0147] h: actual thickness of abutting end faces [0148] L.sub.1: line parallel to X-direction of image [0149] L.sub.2: bisector of angle formed by intersection of approximation lines of two end parts of steel plate [0150] l.sub.1: distance of viewing when misalignment occurs [0151] l.sub.2: distance of viewing when no misalignment occurs [0152] t: thickness of steel plate [0153] S1: area of light emitting region at one side [0154] S2: area of light emitting region at other side [0155] S3: correction region surrounded by line L.sub.1 and bisector L.sub.2 [0156] S.sub.1: S.sub.1+S.sub.3 [0157] S.sub.2: S.sub.2S.sub.3 [0158] V.sub.1: V-convergence point [0159] V.sub.2: abutment point [0160] X.sub.0: X-direction end part of image [0161] X.sub.S: predetermined X-direction position