Industrial camera ball target array calibration plate for telecentric lens and pixel calibration method

Abstract

The invention relates to the technical field of precision measurement, and discloses an industrial camera ball target array calibration plate and a pixel calibration method for a telecentric lens, and the calibration plate comprises a calibration plate which is a ball target array calibration plate and is suitable for the pixel size calibration of an industrial camera using the telecentric lens. The calibration plate is composed of a ball target array base and a high-precision ball target, a pixel size calibration method for the industrial camera using the telecentric lens is provided, the industrial camera collects a ball target fixed plate image, sub-pixel edge coordinates of the ball target are obtained through image processing, circle fitting is conducted on the sub-pixel edge of the ball target, and the diameter is calculated; the invention provides a high-precision calibration method for the pixel size of the industrial camera using the telecentric lens, the full-domain pixel size calibration of the field of view of the lens can be realized, and the calibration result is accurate and reliable.

Description

Technical Field

[0001] The present invention relates to the technical field of precision measurement, and particularly to an industrial camera spherical target array calibration plate and pixel calibration method for a telecentric lens. Background Technique

[0002] With the continuous development of intelligent manufacturing technology in China, new requirements have been put forward for product dimension measurement technology. The measurement speed and repeatability of traditional measurement methods need to be improved. The image measurement technology based on machine vision has the characteristics of high measurement efficiency, low cost, and quick deployment. Compared with manual measurement methods, its measurement reliability is higher, it can perform on-line measurement, data management is convenient, and it is easy to realize modern quality management.

[0003] In order to further improve the measurement accuracy of image measurement technology, telecentric lenses are increasingly used in industrial production practices. Compared with ordinary lenses, telecentric lenses have many advantages such as a fixed and consistent magnification ratio, large depth of field, and small distortion. Using a telecentric lens in industrial measurement practice can avoid the parallax problem caused by different magnification ratios due to the measurement object not being in the same plane. The edge imaging in the obtained image is clearer and the measurement result is more accurate.

[0004] In traditional camera calibration methods, mainly two-dimensional calibration plates such as dot patterns and checkerboards are used as calibration objects. By taking calibration plate images from multiple angles multiple times, then detecting the corner points or centers of the calibration plate, obtaining the coordinates of feature points, and based on the inherent geometric dimension relationships between the feature points, calculating the internal parameters, external parameters of the camera, and other camera parameters such as image distortion coefficients. However, this method is not applicable to industrial cameras using telecentric lenses. Telecentric lenses can perform precision measurement using front light and backlight. For example, when measuring, the light passes through the edge of the object (such as measuring the diameter of a workpiece), which is three-dimensional, while the traditional checkerboard calibration plate is planar, so it is not applicable to the calibration of industrial cameras using telecentric lenses.

[0005] In addition, the accuracy of the calibration plate will affect the accuracy of the calibration result in the traditional camera calibration process. Currently, the commonly used calibration plates in calibration mainly print patterns on substrates such as glass and other materials, and their accuracy is about 1μm. With the continuous improvement of the requirements for measurement accuracy, higher requirements are also placed on the accuracy of the calibration plate. Therefore, the present invention proposes an industrial camera spherical target array calibration plate and pixel calibration method for a telecentric lens. Summary of the Invention

[0006] The purpose of the present invention is to provide an industrial camera spherical target array calibration plate and pixel calibration method for a telecentric lens to solve the problems raised in the above background technique.

[0007] To achieve the above object, the present invention provides the following technical solution: An industrial camera ball target array calibration board for a telecentric lens, the ball target array calibration board comprising a ball target array calibration board base and a high-precision ball target.

[0008] Preferably: The diameter size of the high-precision ball target is selected based on the size of the ball target array calibration board base, and there is a space reserved between adjacent ball targets.

[0009] Preferably: The high-precision ball target is made of high-precision bearing steel balls, and the precision reaches the sub-micron level.

[0010] According to the above pixel calibration method for an industrial camera ball target array calibration board for a telecentric lens, it includes the following steps:

[0011] Step 1: The industrial camera collects images of the ball target calibration board;

[0012] Step 2: Obtain the sub-pixel edge coordinates of the ball target through image processing;

[0013] Step 3: Perform circle fitting on the sub-pixel edges of the ball target and calculate the diameter;

[0014] Step 4: Calibrate the pixel size of the camera.

[0015] Preferably: In the above Step 1, an industrial camera is used in combination with a telecentric lens, and the industrial camera and a coaxial parallel light source are placed on both sides of the ball target array calibration board to collect images of the ball target array calibration board.

[0016] Preferably: In the above Step 2, a contour detection algorithm is used to extract the target region of interest in the image, a suitable filtering algorithm is selected to filter the segmented and extracted region of interest, an edge detection algorithm is used to perform edge detection on the region of interest to obtain the pixel edges of the ball target, and a sub-pixel detection algorithm is used to further subdivide the pixel edges of the ball target to obtain the sub-pixel edge coordinates of the ball target.

[0017] Preferably: In the above Step 3, the sub-pixel edge coordinates of each ball target in the obtained ball target array calibration board are inspected, the outliers are deleted, and then a circle fitting algorithm or the least squares method is selected to fit the inspected sub-pixel edge coordinates to obtain the center pixel coordinates of each ball target in the ball target array calibration board and the pixel diameter of the ball target.

[0018] Preferably, in the fourth step, according to the nominal diameter D of each ball target in the ball target array and the pixel diameter P of each ball target obtained by detection, calculate the pixel equivalent K = D / P of each ball target, and sort according to the pixel coordinates of the ball target center to form a pixel equivalent matrix. Select a suitable surface fitting or interpolation algorithm to fit or interpolate the data points in the pixel equivalent matrix, and the actual length represented by each pixel point of the camera can be obtained, completing the final calibration of the camera pixel size.

[0019] The beneficial effects of the present invention compared with the prior art are as follows:

[0020] The present invention uses a ball target array calibration plate as a calibration object. The projected shape of the ball target is closer to an ideal circle, which can reduce the error caused by the non-perpendicularity between the calibration plane and the camera optical axis. At the same time, high-precision bearing steel balls are selected for the ball targets, and their dimensional accuracy reaches the sub-micron level to ensure the accuracy of the final calibration result. Description of the Drawings

[0021] Figure 1 It is the design drawing of the ball target array calibration plate of the present invention;

[0022] Figure 2 It is the schematic diagram of the ball target array calibration plate of the present invention;

[0023] Figure 3 It is the flow block diagram of a method for calibrating the pixel size of an industrial camera using a telecentric lens according to an embodiment of the present invention;

[0024] Figure 4 It is the schematic diagram for obtaining the grayscale image of the ball target array calibration plate placed in the present invention;

[0025] Figure 5 It is the partial enlarged schematic diagram of the edge of the ball target in the ball target array calibration plate of the present invention;

[0026] Figure 6 It is the partial enlarged physical diagram of the edge of the ball target in the ball target array calibration plate of the present invention.

[0027] Description of the Reference Numerals:

[0028] 1 - Base of the ball target array calibration plate, 2 - High-precision ball target, 3 - Industrial camera, 4 - Telecentric lens, 5 - Light source. Detailed Embodiments

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0030] Embodiment

[0031] Manufacture a calibration plate base 1 according to the field of view size of the telecentric lens, and its size should be close to the maximum inscribed rectangle of the lens.

[0032] Reasonably select the diameter size of the spherical targets 2 according to the size of the calibration plate base 1, ensure that there is a certain space between the spherical targets 2, and ensure that the imaging of the boundaries of the backlight calibration target balls 2 does not overlap and does not interfere with each other.

[0033] Select high-precision bearing steel balls as the spherical targets 2, and the size of the calibration plate base 1, the diameter size of the spherical targets 2, and the number of spherical targets 2 can be adjusted according to the camera field of view and processing conditions.

[0034] See Figure 2 , and install the spherical targets 2 on the calibration plate base 1 to form a spherical target array calibration plate.

[0035] See Figure 3 , a method for calibrating the pixel size of an industrial camera using a telecentric lens, the main steps are as follows:

[0036] Step 1: Collect an image of the spherical target calibration plate by the industrial camera 3;

[0037] See Figure 4 , use the industrial camera 3 equipped with a telecentric lens 4, place the industrial camera 3 and the light source 5 on both sides of the spherical target array calibration plate, make the spherical target array calibration plate within the depth of field of the telecentric lens, and make the spherical target array calibration plate cover the entire field of view of the lens. The optical axis of the lens 4 and the light source 5 are perpendicular to the spherical target array calibration plate, and collect an image of the spherical target array calibration plate. See Figure 5 and Figure 6 are the schematic diagram and the physical diagram of the edges of the spherical targets in the spherical target array calibration plate.

[0038] Step 2: Obtain the sub-pixel edge coordinates of the spherical targets through image processing:

[0039] Adopt a contour detection algorithm to extract the target area of interest in the image, select a suitable filtering algorithm to filter the segmented and extracted area of interest, use an edge detection algorithm to detect the edges of the area of interest to obtain the pixel edges of the spherical targets, and use a sub-pixel detection algorithm to further subdivide the pixel edges of the spherical targets to obtain the sub-pixel coordinates of the spherical target edges.

[0040] Step 3: Fit the sub-pixel edges of the spherical targets with a circle and calculate the diameter:

[0041] Check the sub-pixel edge coordinates of each spherical target in the obtained spherical target array calibration plate, delete the outliers among them, and then select a circle fitting algorithm or the least squares method to fit the checked sub-pixel edge coordinates to obtain the center pixel coordinates of each spherical target in the spherical target array calibration plate and the pixel diameter of the spherical targets.

[0042] Step 4: Calibration of the pixel size of the industrial camera using a telecentric lens:

[0043] According to the nominal diameter D of each ball target in the ball target array and the pixel diameter P of each ball target obtained by detection, calculate the pixel equivalent K = D / P of each ball target, and sort according to the pixel coordinates of the ball target center to form a pixel equivalent matrix. Select a suitable surface fitting or interpolation algorithm to fit or interpolate the data points in the pixel equivalent matrix, and the actual length represented by each pixel point of the camera can be obtained, completing the final calibration of the camera pixel size.

[0044] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variation thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device.

[0045] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An industrial camera spherical target array calibration board for a telecentric lens, characterized in that: The spherical target array calibration board includes a spherical target array calibration board base (1) and high-precision spherical targets (2).

2. The industrial camera ball target array calibration board for a telecentric lens according to claim 1, characterized in that: The diameter size of the high-precision spherical target (2) is selected based on the size of the spherical target array calibration board base (1), and there is a space reserved between adjacent spherical targets.

3. The industrial camera spherical target array calibration board for a telecentric lens according to claim 2, characterized in that: The high-precision spherical target (2) is made of high-precision bearing steel balls, and the precision reaches the sub-micron level.

4. A pixel calibration method for an industrial camera ball target array calibration plate for a telecentric lens according to any one of claims 1-3, characterized in that, It includes the following steps: Step 1: The industrial camera (3) collects images of the spherical target calibration board; Step 2: Obtain the sub-pixel edge coordinates of the spherical target through image processing; Step 3: Perform circle fitting on the sub-pixel edge of the spherical target and calculate the diameter; Step 4: Calibrate the pixel size of the camera.

5. A pixel calibration method for an industrial camera spherical target array calibration plate for a telecentric lens according to claim 4, characterized in that: In the above Step 1, the industrial camera (3) is paired with a telecentric lens (4), and the industrial camera (3) and the coaxial parallel light source (5) are placed on both sides of the spherical target array calibration board to collect images of the spherical target array calibration board.

6. A pixel calibration method for an industrial camera spherical target array calibration plate for a telecentric lens according to claim 5, characterized in that: In the above Step 2, a contour detection algorithm is used to extract the target area of interest in the image, a suitable filtering algorithm is selected to filter the segmented and extracted area of interest, an edge detection algorithm is used to detect the edge of the area of interest to obtain the pixel edge of the spherical target, and a sub-pixel detection algorithm is used to further subdivide the pixel edge of the spherical target to obtain the sub-pixel edge coordinates of the spherical target.

7. A pixel calibration method for an industrial camera ball target array calibration plate for a telecentric lens according to claim 6, characterized in that: In the above Step 3, the sub-pixel edge coordinates of each spherical target in the obtained spherical target array calibration board are inspected, the outliers are deleted, and then a circle fitting algorithm or the least squares method is selected to fit the inspected sub-pixel edge coordinates to obtain the center pixel coordinates of each spherical target in the spherical target array calibration board and the pixel diameter of the spherical target.

8. A pixel calibration method for an industrial camera spherical target array calibration board for a telecentric lens according to claim 7, characterized in that: In the above Step 4, according to the nominal diameter D of each spherical target in the spherical target array and the pixel diameter P of each detected spherical target, calculate the pixel equivalent K = D / P of each spherical target, and sort according to the pixel coordinates of the spherical target center to form a pixel equivalent matrix. Select a suitable surface fitting or interpolation algorithm to fit or interpolate the data points in the pixel equivalent matrix, and the actual length represented by each pixel point of the camera can be obtained, and the final calibration of the camera pixel size is completed.

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