JPH07237094A - Detecting method for center position of target mark by image processing - Google Patents

Abstract

PURPOSE:To improve the detecting accuracy of a center by, when a cursor center is not in the center circle of the binary image of a target mark displayed on a monitor, investigating the gradation of the binary image in fixed process in the directions of X, Y axes to severally obtain the starting' point of the gradation in the directions of X, Y axes, and comparing the distance of a starting point with diametral reference value and fitting it to the above value. CONSTITUTION:In the case where a cursor center 13 is not in the center circle of the binary image of a target mark (b) displayed on a monitor 3, an image is radially investigated within an investigation reference point set up on the monitor 3 in fixed order at each fixed angle to obtain a starting point W being the same in gradation as the center circle (b1). The inside of a gradation part is investigated from the starting point W in the direction of X axis to obtain starting points X1, X2 being different in the gradation from the center circle (b1). Investigation in the direction of Y axis is carried out about the midpoint coordinate positions of the starting points X1, X2 to similarly decide starting points Y1, Y2 in the direction of Y axis. Starting points X'1, X'2 are similarly decided about the midpoint coordinate positions of the starting points Y1, T2. intersection point O of both the starting points is recognized as the target mark (b).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting the center position of a target mark by image processing.

[0002]

2. Description of the Related Art Since a reference hole is punched in a work such as a printed circuit board, target marks are provided at four corners, and the printed circuit board is moved by a constant quantity feeding device so that each target mark is located immediately below an image pickup section. At the same time, the center of the target mark is detected by a predetermined means, and the drill is corrected and moved by an amount of error from the detected center through the X and Y axis feed mechanism to center the center of the drill and the target mark. Then, the reference hole is drilled at the center of the target mark by raising the drill in the Z direction from that position. The predetermined means means that the target mark is received by the image pickup unit, the image signal is binarized by the image processing unit, the binarized image is enlarged and displayed on the monitor, and the center of gravity is measured by the dynamic window method. A method of detecting the center of the target mark (hereinafter referred to as the former) and a binarized image are enlarged and displayed on the monitor screen, and the target mark is searched in the X and Y directions from the center of the cursor to obtain different gradations. Search for the intersection of the perpendicular bisectors between each of the two X and Y points,
This is a method in which the intersection is the center of the target mark (hereinafter referred to as the latter).

[0003]

By the way, a target mark generally has a plan view shape in which a plurality of concentric circles having the same gradation as that in the central circle are alternately provided through outer rings having gradations different from those in the central circle. Is. However, in the former case, when a pattern other than the target mark is converted into black and white by binarization (negative / hosi conversion), the conversion coordinate system is greatly affected, and the center of the target mark cannot be accurately detected. In the latter case, the center of the target mark cannot be detected unless the cursor center on the monitor screen is located at the target mark, and only the diameters in the X and Y directions are searched to find the intersection point of the bisectors between the two points. Since this is a method of detecting as the center of the target mark, when a pattern having the same condition as the target mark is generated on the monitor screen, the center of the pattern is erroneously recognized as the center of the target mark and detected.

The present invention has been made in view of the conventional circumstances, and its object is to locate an outer ring outside the center circle of the target mark at the center of the cursor on the monitor screen (displaying a binarized image). It is an object of the present invention to provide a method capable of accurately detecting the center of a target mark even when a work is fed with such a feeding error. Another object of the present invention is to provide a method capable of accurately detecting the center position of the target mark regardless of whether the work feed error is large or small.

[0005]

According to a first aspect of the present invention, there is provided a technical means for achieving the above object. According to claim 1, an image pickup section receives a target mark, and the image processing section binarizes the image signal. The binarized image is displayed on the monitor screen linked to the image processing unit, and when the cursor center of the monitor screen is not located within the center circle of the binarized target mark, the binarized image is displayed at a predetermined angle from the cursor center. Radial exploration is performed every time to find a starting point W having the same gradation as the central circle, and the starting point W
To search for the starting points X1 and X2 of the gradation different from that of the central circle by further searching the inside of the gradation part in the X or Y direction, and then the midpoint coordinate position between the starting points X1 and X2 of the gradation. Searching the upper part in the other axial direction, the starting point Y of the gradation different from that of the central circle
1 and Y2 are searched for, and the midpoint coordinate position between the starting points Y1 and Y2 is searched in the direction parallel to the starting points X1 and X2 directions, and the starting points X1 ′ and X2 of the gradation different from the central circle are searched. '
For the diameter reference values in the X and Y directions stored in the storage unit, the distance between the start points Y1 and Y2, and the start points X1 ′ and X.
The point is to check the distance between 2'and match the diameter reference value and recognize the cross-shaped intersection of each of the starting points Y1, Y2, starting points X1 ', X2' as the center of the target mark. And According to a second aspect of the present invention, the target mark is received by the imaging unit, the image signal is binarized by the image processing unit, the binarized image is displayed on the monitor screen linked to the image processing unit, and the image is displayed on the monitor screen. The processing unit sets a large number of search reference points at vertical and horizontal intervals, and the image processing unit radially searches the binarized image at predetermined angles from the search reference point in a predetermined order to form the central circle. A starting point W having the same gradation is sought, and X is extracted from the starting point W within the gradation part.
Alternatively, by further exploring in the Y direction, the starting points X1 and X2 of the gradation different from the central circle are searched, and then the starting point X of the gradation is obtained.
The center point coordinate position between 1 and X2 is searched in the other axis direction to search for the start points Y1 and Y2 of the gradation different from that of the central circle, and the center point coordinate position between the start points Y1 and Y2 is searched. Is searched in a direction parallel to the starting points X1 and X2 to search for starting points X1 ′ and X2 ′ having gradations different from the center circle, and the diameter reference values in the X and Y directions stored in the storage unit and the start. Points Y1 and Y2
The distance between the starting points Y1 and Y2 and the starting points X1 'and X2' are matched with the distance reference and the distances between the starting points X1 'and X2' and the target mark Accreditation as the center of Further, in claim 3, after detecting the center of the center circle of the target mark in claim 1 or 2, the inside of the center circle is searched in an X shape or a radial shape with a desired inclination angle intersecting the detection center, and the center circle is searched. The starting points Z1 and Z2 having different gradations are searched for, and the distance between the starting points Z1 and Z2 is adapted to the diameter reference values in the X and Y directions of the center circle taught in the storage unit, and the detection center is detected. The main point is to certify as the center of the target mark.

[0006]

According to the above technical means, the following actions are achieved. (Claim 1) The image signal of the target mark received by the image pickup unit is binarized by the image processing unit, and the binarized image is displayed on the monitor. Then, when the cursor center is not located within the center circle of the binarized image of the target mark, the binarized image is radially searched at predetermined angles from the cursor center,
First, the starting point W having the same gradation as that of the central circle is searched. Next, from the starting point W, the inside of the gradation portion is searched in, for example, the X direction to search for the starting points X1 and X2 of the gradation different from the central circle, and the midpoint coordinate position is calculated. Then, the center point coordinate position is used as a reference point to search in the Y direction, and start points Y1 and Y2 of gradations different from the central circle are obtained. The distance between the starting points Y1 and Y2 is the diameter in the Y-axis direction of a certain location that is searched assuming the center circle. Then, the center point coordinate position is searched in the X-axis direction this time, and start points X1 ′ and X2 ′ of gradations different from the central circle are obtained. The distance between the starting points X1 ′ and X2 ′ is the diameter in the X-axis direction of a certain portion that is searched assuming the center circle. Lastly, the diameter reference values in the X-axis and Y-axis directions of the target mark stored in the storage unit are collated with the distance between the starting points Y1 and Y2 and the distance between the starting points X1 ′ and X2 ′, and the diameter is matched with the diameter reference value. Only after that, it is confirmed that the circle is the center circle of the target mark of the regular area, and the starting points Y1, Y2, and the starting point X
The cross-shaped intersection of 2 points each of 1'and X2 'is determined as the actual center. Therefore, even if there is a feed error (feed error of a work such as a printed circuit board) such that the cursor center of the monitor screen is located on the outer ring outside the center circle of the target mark, only the center of the target mark can be detected. When the center of the cursor is located within the center circle of the binarized image of the target mark, the gradation center is searched with the cursor center as the starting point W without radially searching for one pixel pitch at every predetermined angle. , The center of the target mark is surely detected. (Claim 2) When there is a feeding error (feeding error of a work such as a printed circuit board) larger than the cursor center of the monitor screen is located on the outer ring outside the center circle of the target mark, the center circle of the claim 1 The center cannot be detected. Therefore, a large number of search reference points are set on the monitor screen at vertical and horizontal intervals including the cursor center, and search is performed from the search reference points in a predetermined order in the same manner as in claim 1. . This makes it possible to detect the center of the central circle by searching from any of the search reference points.
The search is performed in a predetermined predetermined search reference point order until the center of the center circle of the target mark is detected. When the center is detected, the search ends and the search from the remaining search reference points is not performed. (Claim 3) The taught X and Y according to claims 1 and 2.
Even if the portion other than the central circle has a condition that matches the reference diameter in the direction, the portion other than the true circle is not recognized as the central circle.
The center of the center circle can be reliably detected.

Since the present invention is configured as described above, it has the following advantages. (Claim 1) Even if a work such as a printed circuit board is misaligned so that the center of the cursor on the monitor screen for displaying a binarized image by the image processing unit is located outside the center circle of the target mark, the target mark It is possible to reliably detect the center position of the. (Claim 2) A large number of search reference points are set on the monitor screen at vertical and horizontal intervals, and the set reference points are searched radially at a predetermined angle in a predetermined order and have the same gradation as the central circle. This is a method of searching for the same as in claim 1, and if the center of the target mark is located within the monitor screen, whether the work feed error is large or small The center position of the target mark can be reliably detected by the search from the point. (Claim 3) Even if the same conditions as the diameters of the central circle in the X and Y directions detected by the first and second cases occur in the outer ring surrounding the central circle, etc. Since it is determined that the object is not the central circle, there is no fear of erroneously detecting anything other than the center of the central circle of the target mark.

[0008]

Embodiments of the present invention will now be described with reference to the drawings. 1 to 5 show a first embodiment of a target mark center position detecting method according to the image processing of the present invention, and FIGS. 6 to 8 show a second embodiment thereof. First, the first embodiment will be described.

The punching device A is a work B such as a printed circuit board.
An image pickup unit 1 for receiving a transmission image obtained by illuminating the image from below or a reflected image obtained by illumination from above, and image processing for image-processing the image signal received by the image pickup unit 1 and binarizing the image signal. Section 2, a monitor 3 for displaying a binarized image thereof, a drill 4 provided to face the image pickup section 1, an X, Y axis feed mechanism 5 and a Z axis feed mechanism of the drill 4. It is composed of 6 etc.

The image processing unit 2 includes an image processing device 12 for performing arithmetic processing such as binarization on the image signal received by the image pickup unit 1, an image memory 22 for storing the binarized data, and a predetermined program. Central processing unit 32 to be executed, RAM (storage unit) 42 for storing necessary data to the central processing unit 32, ROM
(Storing unit) 52, etc., and a feeding device (so that the center of the target mark b of the work B such as a printed circuit board having the target marks b attached to the four corners thereof may coincide with the center of the cursor 13 on the monitor 3 screen ( The target mark b is received by the image pickup unit 1 every time it is automatically transferred (not shown), the image signal thereof is binarized by image processing in the image processing device 12, and the binarized data is stored in the image memory 22. And remember it in part 2
The binarized image is displayed on the monitor 3, the binarized image is searched from the center of the cursor 13 on the monitor 3, the center of the target mark b is detected, and the center of the cursor 13 and the center of the detected target mark b are detected. The configuration is such that the amount of error is detected.

The central processing unit 32 of the punching device A is arranged so that the center of the target mark b of the work B is moved by a constant amount feeding device (not shown) so that the center of the target mark b coincides with the center of the cursor 13. It is designed to execute a control program for detecting. In the image processing device 12, a threshold value is set for black and white discrimination, and a level higher than the threshold value is judged to be white or black, and conversely, a lower level is judged to be black or white. Further, the control program is a plan view shape in which a plurality of target marks (concentric rings whose gradation changes in the same state as in the central circle are alternately provided through an outer ring whose gradation changes in a state different from the central circle). ) B, the feed error of the constant amount feeding device (not shown) for the work B is relatively large, and the outer ring b2 outside the center circle b1 of the target mark b is located at the center of the cursor 13 on the monitor 3 screen. The center of the target mark b is detected and is incorporated in the ROM 52.

Next, an enlarged view of a monitor screen showing a state in which the control program is searching for a binarized image image-processed by the image processing unit of FIGS. 2 to 4, the center of the punching device of FIG. Description will be given based on a flowchart of a program controlled by the processing device. The image of the target mark B is received by the image pickup unit 1, the image signal thereof is subjected to image processing by the image processing device 12 to be binarized, and the image is enlarged and displayed on the monitor 3 screen. First, it is determined whether or not the center of the cursor 13 is located on the center circle b1 of the target mark B (step 1). And
When the center of the cursor 13 is located on the center circle b1 of the target mark B, as shown in FIG. 3, the cursor 13 center is set as the starting point W, and a search is performed for one pixel pitch in the X direction from that position, which is different from the center circle b1. The starting points X1 and X2 of the gradation are searched, the midpoint coordinate position (point) of the points X1 and X2 is calculated, and this point is temporarily stored in the RAM 42 (steps 2, 3, 4). Then, this time, the middle point coordinate position (point) is searched for one pixel pitch in the Y direction, and the start points Y1 and Y of the gradation different from the central circle b1 are obtained.
Two points are searched for, the midpoint coordinate position (point) of the Y1 point and the Y2 point is calculated, and this point is stored in the RAM 42 (steps 5, 6, 7). Then, this intermediate coordinate position (point) is searched in the X direction for one pixel pitch this time, and the starting points X1 'and X2' of the gradation different from the central circle b1 are searched (steps 8 and 9). Further, the center circle b of the taught target mark B in which the distances between the Y1 point and the Y2 point and between the X1 ′ point and the X2 ′ point are stored in the ROM 52.
It is recognized as a diameter when matched with the diameter reference value of 1, and a cross-shaped intersection O (virtual center) of two Y1, Y2, X1 ′, and X2 ′ points is obtained and stored in the RAM 42 (steps 10 and 11). ). The diameter in the X and Y directions is R
Even if it matches the diameter reference value taught to the OM52, the central circle b1 is further searched in an X shape or a radial shape with a desired inclination angle intersecting the intersection point O (virtual center).
The starting points Z1 and Z2 of different gradations are searched for, and when the distance between the searching points meets the taught diameter reference value, the circle is recognized as a true circle, and the virtual center that is the intersection O is the target mark b for the first time. It is recognized as the center of the center circle b1 of (steps 12 and 13).

When the center of the cursor 13 is located on the outer ring b2 of the target mark center circle b1 as shown in FIG.
The mm range is searched every 30 degrees to find the starting point W in the X direction having the same gradation as the central circle b1 (steps 14, 15, 1).
6 and 17), from the starting point W, the steps 2 to 1
Execute 3 respectively. As a result, if the center circle of the target mark and its center are not detected even on the last search line, error processing is performed (step 18).

Therefore, the center circle b1 of the target mark b
Indicates a feed error such that the center of the cursor is positioned or the cursor 13 on the outer ring b2 outside the center circle b1 of the target mark b.
Even if the work B is fed with a feed error such that the center is located, the center of the target mark center circle b1 can be detected reliably and at high speed.

Reference numeral 7 is an I / O port, and its I
The X and Y axis feed mechanism 5 is controlled by the X and Y axis driver circuit 8 connected to the / O port 7, and the Z axis feed mechanism 6 is composed of a solenoid 6a and an air cylinder 6b.

The second embodiment will be described. In this embodiment, if the center of the target mark is located within the monitor screen, the feeding error (feeding error of the printed circuit board or the like) may be large or small. This is a method capable of detecting the center position of the target mark.

In this embodiment, the image processing apparatus 12 sets a large number of search reference points C ... On the screen of the monitor 3 displaying the binarized image at vertical and horizontal intervals including the center of the cursor 13, and the search reference points C ... The configuration is the same as that of the above-described embodiment except that the central processing unit 32 executes a control program for searching in a predetermined order from the points C. By the way, if the computer control finds the same gradation position as in the central circle b1 of the target mark B during the search of the binarized image due to its function, the search does not proceed any further and the search in that position is not performed. The search ends just by itself, and the center cannot be detected. As described above, the center circle b of the target mark B is located at the center of the cursor 13 on the monitor 3 screen (displaying a binarized image).
When 1 is located or when the outer ring b2 outside the center circle b1 of the target mark B is located at the center of the cursor 13, the center can be detected by exploring from one location (center of the cursor 13). However, if the feed error is larger than that, the central circle of the target mark cannot be detected by the method of the first embodiment. In order to prevent this, a large number of search reference points C ... Are set on the screen of the monitor 3 at vertical and horizontal intervals including the center of the cursor 13. This exploration reference point C is
In this embodiment, there are nine points on the drawing, but more points are of course arbitrary, and it is effective when the shape of the target mark becomes more complicated as the number of search reference points C increases.

An enlarged view of a monitor screen showing a state in which the control program is searching for a binarized image image-processed by the image processing device 12 of FIG. 6, and the central processing unit of the punching device of FIGS. 7 and 8. A description will be given based on the flowchart of the program controlled by. This control program is the same as the control program of the first embodiment, but continues the search in the order of the search reference points until the center of the center circle of the target mark is detected. Each step will be described by adding the step NO of the first embodiment. First, the target mark B is received by the image pickup unit 1, and its image signal is received by the image processing device 12.
Image processing is carried out with to binarize, and it is enlarged and displayed on the monitor 3 screen. Exploration from the exploration reference point C1 has started,
The search reference point C1 is the center circle b1 of the target mark B.
It is determined whether or not the search reference point C1 is located at the same gradation as (step 1). In FIG. 6, the exploration reference point C1
Is located at the same gradation as the center circle b1 of the target mark B, the steps 2 to 10 are executed from the search reference point C1. However, the distance between X and Y is ROM52
Since it does not match the diameter reference value of the center circle b1 of the taught target mark B stored in, it is determined that it is not the center circle. Then, the search from the search reference point C2 starts. Each of the search reference points C2 to C9 also includes steps 1 to 17 in FIG. Since the search reference point C2 is located on the outermost outer ring b3 having a gradation different from that of the center circle b1 of the target mark B, a range of, for example, 2 mm is 30 degrees from the search reference point C2 to one pixel pitch. Central circle b searched for each
The starting point W (not shown) having the same gradation as that of 1 is searched (steps 14, 15, 16, 17), and the steps 2 to 10 are executed from the starting point W (not shown). If the same gradation position as that of the center circle b1 of the target mark B is found during the search, the search will not proceed any further, because of the controllability, the concentric ring b5 between the outermost outer ring b3 and the inner outer ring b4. Then, steps 2 to 10 are executed, and as a result, it is determined that the circle is not the central circle. Then, the search from the search reference point C3 starts. Since the search reference point C3 is located on the concentric ring b5 having the same gradation as that of the center circle b1 of the target mark B, the steps 2 to 10 are executed as they are, and the center circle is the same as the search from the search reference point C1. Is judged not to be. The search from the search reference point C4 has the same result as the search from the search reference point C1. Further, since the search from the search reference point C5 is located on the outer ring b4 having a gradation different from that of the center circle b1 of the target mark B, as in the search from the search reference point C2, it is addressed to one pixel pitch. For example, a range of 2 mm is searched every 30 degrees to search for a starting point W (not shown) having the same gradation as that of the central circle b1 (steps 14, 15, 16, 17), and the starting point W (not shown). ), The steps 2 to 10
However, it is judged that it is not the central circle. Then, the search from the search reference point C6 starts. The search reference point C6 is located on the outer ring b2 having a different gradation on the outer side of the center circle b1, and the search reference point C6 is searched for a 1-pixel pitch, for example, a range of 2 mm every 30 degrees. The starting point W of the same gradation as that of the central circle b1 (steps 14, 15, 16 and 17), and from the starting point W, steps 2 to 13 are executed respectively, and the central circle b
Detect the center of 1. If the center of the central circle b1 is not detected in the search from the search reference point C6, the subsequent search from the search reference points C7, C8, and C9 is continued. (Feed error)
Is processed as an error (step 18). This is displayed on the monitor 3 or gives a warning.

As described above, in the second embodiment, the search reference point C
1, C2, C3, C4, C5, C6, C7, C8, C9
On the monitor 3 screen, a large number are set at vertical and horizontal intervals including the center of the cursor 13, and the respective search reference points C1, C2, C3,
Since it is a method of radially searching at predetermined angles in order from C4, C5, C6, C7, C8, C9, if the center of the target mark B is located on the monitor screen 3, any of the searching criteria It becomes possible to detect the center by exploring from the point. In addition, the search reference points C1, C2, C
3, C4, C5, C6, C7, C8, and C9 are not limited to those having constant intervals in the vertical and horizontal directions as illustrated.

[Brief description of drawings]

FIG. 1 is a schematic view of a punching device for carrying out a target mark center position detecting method according to a first embodiment.

FIG. 2 is an enlarged view of a monitor screen showing an exploration state when a cursor center is located on an outer ring outside one center circle of a binarized image of a target mark.

FIG. 3 is an enlarged view of a monitor screen showing an exploration state when a cursor center is located on a center circle of a target mark.

FIG. 4 is an enlarged view of a monitor screen showing a state where a center circle of a target mark is searched in a direction intersecting a virtual center (intersection point O).

FIG. 5 is a flowchart of a program controlled by the central processing unit of the punching device.

FIG. 6 is an enlarged view of a monitor screen showing a state in which a state in which the center of the center circle of the target mark is detected in the second embodiment is being searched.

FIG. 7 is a flowchart of a program controlled by the central processing unit of the punching device, showing the time of exploration from the exploration reference point C1.

FIG. 8 is a flowchart showing the search reference points C2 to C9.
Indicates the time of exploration from.

[Explanation of symbols]

A: Punching device 2: Image processing unit 3: Monitor 13: Cursors 42, 52: Storage unit b: Target mark B: Work b1: Central circle b2: Outer ring of central circle C, C1, C2, C3, C4, C5, C6, C7, C
8, C9: Exploration reference point

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G01B 11/00 C G06T 7/60 // B23B 49/00 A

Claims (3)

[Claims] 1. A target mark is received by an image pickup unit, the image signal is binarized by an image processing unit, and the binarized image is displayed on a monitor screen linked to the image processing unit and binarized. When the cursor center of the monitor screen is not located within the center circle of the target mark, the binarized image is radially searched from the cursor center at every predetermined angle to search for the starting point W having the same gradation as the center circle. From the starting point W, the inside of the gradation part is further searched in the X or Y direction to find the starting points X1 and X2 of the gradation different from the central circle, and then the starting point X of the gradation.
The center point coordinate position between 1 and X2 is searched in the other axis direction to search for the start points Y1 and Y2 of the gradation different from that of the central circle, and the center point coordinate position between the start points Y1 and Y2 is searched. Is searched in a direction parallel to the starting points X1 and X2 to search for starting points X1 ′ and X2 ′ having gradations different from the center circle, and the diameter reference values in the X and Y directions stored in the storage unit and the start. Points Y1 and Y2
The distance between the starting points Y1 and Y2 and the starting points X1 'and X2' are matched with the distance reference and the distances between the starting points X1 'and X2' and the target mark A method for detecting the center position of a target mark by image processing, which is characterized by recognizing the center of the target mark. 2. A target mark is received by an image pickup section, the image signal is binarized by an image processing section, the binarized image is displayed on a monitor screen linked to the image processing section, and an image is displayed on the monitor screen. The processing unit sets a large number of search reference points at vertical and horizontal intervals, and the image processing unit radially searches the binarized image at predetermined angles from the search reference point in a predetermined order to form the central circle. A starting point W having the same gradation is sought, and the gradation part is further searched in the X or Y direction from the starting point W to start points X1 and X of gradations different from the central circle.
2 is searched for in the other axial direction on the midpoint coordinate position between the starting points X1 and X2 of the gradation, and the starting points Y1 and Y2 of the gradation different from the central circle are searched, and Its starting point Y
The midpoint coordinate position between 1 and Y2 is searched in the direction parallel to the starting points X1 and X2, and the starting point X of a gradation different from that of the center circle is searched.
1 ′, X2 ′ is searched, and the diameter reference values in the X and Y directions stored in the storage unit are compared with the distance between the start points Y1 and Y2 and the distance between the start points X1 ′ and X2 ′, and the diameter reference A method for detecting the center position of a target mark by image processing, characterized in that a cross-shaped intersection of each of the starting points Y1 and Y2 and the starting points X1 ′ and X2 ′ corresponding to the value is recognized as the center of the target mark. . 3. After detecting the center of the center circle of the target mark, the inside of the center circle is searched in an X shape or a radial direction with a desired inclination angle intersecting the detection center, and a gradation different from that of the center circle is obtained. To find the starting points Z1 and Z2, and the distance between the starting points Z1 and Z2 conforms to the diameter reference values in the X and Y directions of the center circle taught in the storage unit, and the detection center is set to the center of the target mark. 3. The method for detecting the center position of a target mark by image processing according to claim 1, wherein the center position is detected.