Visual measurement system for inner and outer walls of small holes, and its calibration device and calibration method

Abstract

The present invention provides a calibration device for a visual measurement system of the inner and outer walls of a small hole. Among them, the calibration device uses a workpiece sample to be detected as the calibration piece; the workpiece sample to be detected is provided with three through-round holes with equal diameters at a set position, and the connecting lines between the three through-round holes form a right triangle; two through-round holes forming one of the right-angled sides of the right triangle are located on the same generatrix and are used as the calibration sample for determining the pixel size in the y-axis direction; two through-round holes forming the other right-angled side of the right triangle are located in the same circumferential direction and are used as the calibration sample for calibrating the pixel size in the x-axis direction. The present invention also provides a calibration method and a visual measurement system for the inner and outer walls of a small hole. The present invention can realize the simultaneous calibration of the inner wall and the outer wall and the simultaneous calibration of the circumferential direction and the axial direction, and directly perform on the existing platform of the system. It has the characteristics of strong anti-interference ability and good calibration effect, and can determine the conversion coefficient between the pixels and the size of the detection system.

Description

Technical Field

[0001] The present invention relates to the technical field of measurement and calibration, and in particular, to a visual measurement system for the inner and outer walls of a small hole, and a calibration device and a calibration method thereof. Background Art

[0002] Visual images of various parts of the inner and outer walls of a workpiece are collected by a visual measurement system. After being processed by digital image technology, a complete unfolded digital image of the inner and outer walls is presented. The basic unit of a digital image is a pixel. Therefore, to know the actual size of the workpiece to be measured, as well as the size and relative position information of defects, it is necessary to clarify the corresponding coefficient between pixels and actual physical dimensions, that is, to determine the specific physical dimension represented by each pixel. In machine vision inspection, calibration is a very important link and is the premise and basis of image measurement.

[0003] Currently, the most common method for calibrating the pixel equivalent of a visual measurement system is to use a calibration plate or a standard part method, that is, to compare the accurate physical dimensions of a calibration object with known features and the corresponding pixel dimensions in the camera to obtain the pixel equivalent at this object distance (see Wang Guohui, Qian Kemao. Review of linear array camera calibration methods [J / OL]. Acta Optica Sinica: 1-24 [2019-12-04]. http: / / kns.cnki.net / kcms / detail / 31.1252.O4.20191126.1528.014.html.).

[0004] For example, after retrieval, it is found that:

[0005] Bi Chao et al. in "Research on the Calibration Method of the Rotary Axis Based on Visual Measurement" selected a customized cuboid calibration block for calibration. This method also requires mechanical adjustment of the orientation of the calibration block, and the automation level is relatively low (see Bi Chao, Hao Xue, Liu Mengchen, Fang Jianguo. Research on the Calibration Method of the Rotary Axis Based on Visual Measurement [J / OL]. Infrared and Laser Engineering: 1-9 [2019-12-04]. http: / / kns.cnki.net / kcms / detail / 12.1261.tn.20191021.1456.002.html.).

[0006] Chen Yangguang and others from Xiamen University conducted robot vision calibration based on HALCON. The built-in calibration plate based on the calibration function of Halcon software has the characteristics of easy extraction of calibration points, high calibration accuracy, and unique direction. The calibration plate is an equilateral rectangular frame with 7 rows and 7 columns of black circular targets of the same size arranged in the frame. The center of each circle can be used as a calibration point. All black circular targets are arranged at equal intervals. The arrangement method is simple, the coordinates of the center of the circular targets are easy to extract, the camera calibration algorithm is simple, and the measurement accuracy is high. However, due to the small inner diameter of the small hole, the field of view that the 45-degree reflector can measure is also very small. How to mark the calibration points on the inner wall and image all the required calibration points within the effective field of view is a difficult problem (see Chen Yangguang, Wang Lei. Robot vision calibration based on HALCON [J]. Optical Instruments, 2016, 38(04): 320-324.).

[0007] Hao Yongping et al. collected commonly used pixel equivalent calibration methods in their paper "Pixel Equivalent Calibration Method for Visual Measurement" and designed a new type of pixel equivalent calibration object. The calibration object consists of two black solid circles of equal diameter. The precise physical size of the center distance is D, and the center distance to the pixel distance is d. The calibration object is printed at a ratio of 1:1 on good quality photo paper and pasted on a hard and thin board to ensure high flatness. The measurement system is calibrated by calibrating the coefficient of the actual size and the number of pixels. Compared with the traditional pixel equivalent calibration object, this calibration object does not require precise calibration auxiliary equipment. The required pixel equivalent calibration parameters can be obtained, and the production cost is low, there is no direction marking requirement, the operation process is simple, and the anti-interference ability is strong. However, the accuracy and measurement repeatability of this method are not high enough, and the line between the two circles can only be calibrated in one direction. If the inner surface, outer surface and different directions on each surface of the workpiece are to be calibrated, the operation needs to be repeated many times (see Hao Yongping, Wang Yongjie, Zhang Jiayi, Liu Zhoulin. Pixel equivalent calibration method for visual measurement [J]. Nanotechnology and Precision Engineering, 2014, 12(05): 373-380.).

[0008] At present, no description or report of similar technology to the present invention has been found, and similar information at home and abroad has not been collected yet. Summary of the invention

[0009] In view of the particularity of the existing calibration methods and detection systems, the present invention provides a feasible calibration device and calibration method for a visual measurement system of the inner and outer walls of a small hole. The calibration device and calibration method use a workpiece sample as a calibration piece, drill three through holes on the sample to mark the actual size as calibration holes, calculate the number of pixels between the holes through an algorithm, determine the proportionality coefficient, and thus calibrate the measurement of the actual workpiece. It not only solves the problem of marking on the inner surface of a small-diameter hole part, but also can realize simultaneous calibration in the circumferential and axial directions and simultaneous calibration of the inner wall and the outer wall. By positioning the positions of the three calibration holes, the corresponding position relationship of the measurement unfolded images of the inner and outer walls can be determined, the calibration steps are more streamlined, and the use and operation are more simple and efficient.

[0010] The present invention is realized through the following technical solutions.

[0011] According to one aspect of the present invention, there is provided a calibration device for a visual measurement system of the inner and outer walls of a small hole. The calibration device includes one or more calibration pieces, and each calibration piece uses a workpiece sample to be detected as a base piece, and three through-round holes with equal diameters are provided at set positions on the workpiece sample to be detected. The connection lines between the three through-round holes form a right triangle; wherein:

[0012] Two through-round holes forming one right-angled side of the right triangle are located on the same generatrix and are used as a standard sample for determining the pixel size in the y-axis direction;

[0013] Two through-round holes forming the other right-angled side of the right triangle are located in the same circumferential direction and are used as a standard sample for calibrating the pixel size in the x-axis direction.

[0014] Preferably, all three through-round holes are within the measurement field of view of the visual measurement system.

[0015] Preferably, the center distances between the two groups of through-round holes forming the right-angled sides are equal.

[0016] According to another aspect of the present invention, there is provided a calibration method for a visual measurement system of the inner and outer walls of a small hole. Using the calibration device described in any one of the above, it includes:

[0017] S1, setting the calibration piece of the calibration device on the rotating platform of the system;

[0018] S2, adjusting the focal length of the CCD camera lens, the attitude of the reflector and the brightness of the light source of the system to make the system image clearly;

[0019] S3, manually rotating the rotating platform of the system to make the three through-round holes appear in the lens imaging area;

[0020] S4, calculating, through the computer of the system, the number of pixels occupied by the connecting line of the centers of the right-angled sides of the three through-round holes;

[0021] S5. Mark the direction of the generatrix of the calibration piece for the angle between the connecting line of the centers of the right-angled sides of the three pre-calibrated through-round holes and the generatrix.

[0022] S6. Calculate the pixel equivalent according to the calibrated actual size and the number of pixels.

[0023] Preferably, in S3, it further includes: manually rotating the rotating platform of the system to make the axis of the calibration piece parallel to the horizontal plane of the rotating platform, and making the 3 through-round holes face the CCD camera. Adjust the position of the CCD camera lens of the system until the images of the three through-round holes appear in the center of the screen.

[0024] Preferably, in S4, the method for calculating the number of pixels occupied by the connecting line of the centers of the right-angled sides of the three through-round holes is: select at least 3 points on the edge of each through-round hole to form an envelope circle, and obtain the center of the circle by the least squares method.

[0025] Preferably, the number of points selected on the edge of each through-round hole is greater than or equal to 10.

[0026] Preferably, in S5, the method for marking the direction of the generatrix of the calibration piece is:

[0027] Taking the generatrix of the calibration piece as the reference, intercept a rectangular area for image stitching to ensure that the obtained image is a rectangular image of the outer wall unfolded within a certain height range; if the intercepted rectangular area is not parallel to the generatrix of the calibration piece, determine the angle between the connecting line of the centers of the two through-round holes that need to be on the same generatrix and the generatrix of the calibration piece, and determine the direction of the generatrix of the calibration piece according to this connecting line and the calibrated angle.

[0028] Preferably, S6 is specifically:

[0029] Assume that the distance between the centers of the right-angled sides of the three through-round holes is length A, and the corresponding number of pixels is N. Then the actual physical size corresponding to each pixel is:

[0030]

[0031] where K is the calibration coefficient;

[0032] Multiply the measured actual number of pixels by K to obtain the required actual physical size.

[0033] According to the third aspect of the present invention, a visual measurement system for the inner and outer walls of small holes is provided, and the system uses the calibration method described in any one of the above to calibrate the pixel equivalent.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The visual measurement system for the inner and outer walls of small holes, its calibration device and calibration method proposed by the present invention. Among them, the calibration device uses the workpiece sample to be detected as the calibration piece, drills three through-round holes on the workpiece sample to be detected as calibration hole marks for the actual size, and determines the calibration coefficient (scale coefficient) by calculating the number of pixels between the holes, so as to calibrate the measurement of the actual workpiece to be measured. The design of the through-round holes not only solves the problem of how to mark the inner surface of small-diameter and micro-depth hole parts, but also can realize the simultaneous calibration of the inner wall and the outer wall. By positioning the positions of the three calibration holes, the corresponding position relationship of the unfolded images of the inner and outer wall measurements can be determined. By combining any two of the three round holes, the circumferential and axial directions of the small hole surface can be calibrated simultaneously. The calibration steps of the calibration method are more concise, the operation is simple and efficient, and it is more suitable for calibration under measurement conditions with a relatively small field of view, and the accuracy and measurement repeatability are also relatively high. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] By reading the following detailed description of the non-limiting embodiments with reference to the accompanying drawings, other features, objects and advantages of the present invention will become more apparent:

[0037] Figure 1 Schematic diagram of the structure of a small-diameter workpiece provided in an embodiment of the present invention;

[0038] Figure 2 Schematic diagram of the structure of the visual measurement system for the inner and outer walls of small holes provided in an embodiment of the present invention;

[0039] Figure 3 Schematic diagram of the principle of inner wall measurement of the workpiece provided in an embodiment of the present invention;

[0040] Figure 4 Schematic diagram of the principle of outer wall measurement of the workpiece provided in an embodiment of the present invention;

[0041] Figure 5 Schematic diagram of workpiece calibration provided in an embodiment of the present invention;

[0042] Figure 6 Image obtained by the system for pixel equivalent calibration of the visual measurement system using the calibration device and calibration method provided in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The following is a detailed description of the embodiments of the present invention: This embodiment is implemented on the premise of the technical solution of the present invention, and gives detailed implementation manners and specific operation processes. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

[0044] An embodiment of the present invention provides a calibration device for a visual measurement system of the inner and outer walls of a small hole. The calibration device uses a workpiece sample to be detected as a calibration piece. Three through holes with equal diameters are provided at set positions on the workpiece sample to be detected, and the connection lines between the three through holes form a right triangle. Among them:

[0045] Two through holes that form one of the right-angled sides of the right triangle are located on the same generatrix and are used as a standard sample for determining the pixel size in the y-axis direction.

[0046] Two through holes that form the other right-angled side of the right triangle are located in the same circumferential direction and are used as a standard sample for calibrating the pixel size in the x-axis direction.

[0047] Specifically:

[0048] Prepare the workpiece sample to be detected, make through holes on the workpiece sample, and complete the calibration of the workpiece by the visual measurement system by establishing the proportional relationship between the accurate physical dimensions and the number of pixels between the center distances of the through holes. In this way, the calibration can be directly carried out on the existing visual measurement system platform. This calibration device has strong anti-interference ability and good calibration effect. It can not only determine the conversion coefficient between the pixels and the dimensions of the visual measurement system, but also determine the generatrix of the workpiece, providing a basis for subsequent image capture and splicing.

[0049] The preparation process of the calibration device is mainly divided into three steps:

[0050] 1) Design and manufacture the workpiece sample to be detected as a calibration piece.

[0051] 2) Calibrate the calibration piece; the calibration process is to pre-calibrate the calibration piece using existing technologies, which is a common technical means in the art and will not be elaborated here.

[0052] 3) Calibrate the visual measurement system with the calibrated calibration piece.

[0053] First, design and process the calibration piece. Drill 3 through holes with equal diameters at the set positions (such as the middle thin neck position) of the workpiece sample to be detected. Among them, the upper and lower 2 holes are on the same generatrix and are used as a standard sample for determining the pixel size in the y-axis direction, and the left and right 2 holes are in the same circumferential direction and are used as a standard sample for calibrating the pixel size in the x-axis direction.

[0054] Because the inner wall detection method based on a mirror has a relatively small field of view through a 45° planar mirror. Therefore, when making the calibration piece, the effective imaging area needs to be considered, and when measuring the pixel distance (number of pixels) of the circle, all 3 holes need to be within the field of view of the visual measurement system.

[0055] Based on the calibration device provided by the embodiments of the present invention, the embodiments of the present invention also provide a calibration method for a visual measurement system of the inner and outer walls of a small hole. By using the calibration device described in any one of the above, through the visual measurement system, the center distances between the upper and lower two holes and the left and right two holes, as well as the angle between the center connection line and the axis, are obtained. Furthermore, the direction of the generatrix of the calibration piece and the proportional relationship between the actual size between the holes and the number of pixels are marked to determine the calibration coefficient.

[0056] Specifically, it includes:

[0057] S1, Set the calibration piece of the calibration device on the rotating platform of the system;

[0058] S2, Adjust the lens focal length, mirror attitude, and light source brightness of the system to make the system image clearly;

[0059] S3, Manually rotate the rotating platform of the system so that three through-round holes appear in the imaging area of the lens;

[0060] S4, Calculate the number of pixels occupied by the center connection line of the right-angle sides of the three through-round holes through the computer of the system;

[0061] S5, Mark the direction of the generatrix of the calibration piece according to the angle between the center connection line of the right-angle sides of the three through-round holes and the generatrix pre-calibrated;

[0062] S6, Calculate the pixel equivalent according to the calibrated actual size and the number of pixels.

[0063] Furthermore, in S3, it also includes: Manually rotate the rotating platform of the system so that the axis of the calibration piece is parallel to the horizontal plane of the rotating platform, and make the three through-round holes face the CCD camera. Adjust the position of the CCD camera lens of the system until the images of the three through-round holes appear in the center of the screen.

[0064] Furthermore, in S4, the method for calculating the number of pixels occupied by the center connection line of the right-angle sides of the three through-round holes is: At least select 3 points (preferably, more than 10 points can be selected) on the edge of each through-round hole to form an envelope circle, and obtain the center of the circle through the least squares method. Since the distances between the center of the circle and the points selected on the edge of the round hole are all equal, the least squares solutions of the center coordinates and the radius can be obtained. Furthermore, in S5, using the existing technology to pre-calibrate the angle between the center connection line of the right-angle sides of the three through-round holes and the generatrix is a common technical means in the art and will not be elaborated here.

[0065] Furthermore, in S5, the method for marking the direction of the generatrix of the calibration piece is:

[0066] Taking the busbar of the calibration piece as the reference, intercept a rectangular area for image stitching to ensure that the obtained image is a rectangular image of the outer wall unfolded within a certain height range; if the intercepted rectangular area is not parallel to the busbar of the calibration piece, then determine the angle between the connection line of the centers of two through holes that need to be on the same busbar and the busbar of the calibration piece, and determine the direction of the busbar of the calibration piece based on this connection line and the calibrated angle.

[0067] Further, the specific content of S6 is as follows:

[0068] Assume that the center distance of the right-angled sides between three through holes is length A, and the corresponding number of pixels is N. Then the actual physical size corresponding to each pixel is:

[0069]

[0070] where K is the calibration coefficient;

[0071] By multiplying the measured actual number of pixels by K, the required actual physical size can be obtained.

[0072] Based on the calibration method provided by the embodiments of the present invention, the embodiments of the present invention also provide a visual measurement system for the inner and outer walls of small holes. The system uses the calibration method described in any of the above to perform pixel equivalent calibration.

[0073] The visual measurement system for the inner and outer walls of small holes, its calibration device and calibration method provided by the embodiments of the present invention drill three through holes with right-angled connections on the workpiece sample, calculate the coefficient relationship between the actual size and the pixels to calibrate the system, and can realize the simultaneous calibration of the inner wall and the outer wall as well as the simultaneous calibration in the circumferential and axial directions.

[0074] Next, in combination with the accompanying drawings and specific applications, the technical solutions provided by the above embodiments of the present invention will be further described in detail.

[0075] The calibration device and calibration method provided by the embodiments of the present invention can be applied to an existing visual measurement system platform to directly calibrate workpieces.

[0076] Such as Figure 2As shown in the figure, the existing visual measurement system platform includes: an area array CCD camera, a telecentric lens, an annular light source, a spherical mirror, a rotating platform, and a computer; assemble the telecentric lens with the front lens of the CCD camera, and fix them on the optical bracket in sequence with the annular light source and the workpiece to be detected from top to bottom. Adjust the position of the optical bracket so that the lens axis and the axis of the workpiece to be detected are on the same vertical axis. Place and fix the spherical mirror on the mirror bracket, and horizontally fix the mirror bracket on the rotating platform to achieve precise adjustment of the spatial position of the spherical mirror. Adjust the height of the rotating platform so that the spherical mirror penetrates into the workpiece hole from the bottom of the workpiece to be detected, and adjust the rotating platform so that the center of the spherical mirror is located on the vertical axis of the workpiece to be detected. Connect the CCD camera to the computer through a gigabit network cable. (For the specific structure, please refer to the Chinese patent application for invention "Optical Detection System for Inner Wall of Small Aperture Workpiece" with the publication number CN 103575748A).

[0077] As Figure 1 shown, the workpiece to be detected is as follows. The optical principle used for measuring the inner wall of the small hole of the workpiece to be detected is as Figure 3 shown, and the measurement principle of the outer wall of the workpiece to be detected is as Figure 4 shown.

[0078] First, design and process the calibration piece. Drill 3 through-round holes with equal diameters at the middle thin neck position of the sample workpiece to be detected, as Figure 5 shown. Among them, the upper and lower 2 holes are on the same generatrix, which are used as the standard samples for determining the pixel size in the y-axis direction, and the left and right 2 holes are in the same circumferential direction, which are used as the standard samples for calibrating the pixel size in the x-axis direction.

[0079] Since the inner wall detection method based on the mirror has a relatively small field of view through a 45° plane mirror. Therefore, when making the calibration piece, the effective imaging area needs to be considered, and it is ensured that all 3 holes are within the field of view of the vision system when measuring the pixel distance of the circle.

[0080] The calibration of the calibration piece is completed by the visual measurement system. The parameters to be calibrated include: the center distance A between the two vertical holes, the distance B from the lower round hole to the bottom surface of the workpiece, the center distance C between the two horizontal holes, the angle α between the connecting line of the centers of the two vertical holes and the generatrix of the workpiece, and the angle β between the connecting line of the centers of the two circumferential holes and the generatrix of the workpiece.

[0081] Place the workpiece horizontally on the rotating platform of the visual measurement system through a fixture, and adjust it through a two-axis rotation device so that the axis of the calibration workpiece is parallel to the horizontal plane and the 3 through-round holes are facing the CCD camera. Adjust the position of the optical probe of the CCD camera until the image of the round hole appears in the center of the screen.

[0082] Select at least 3 points (preferably more than 10 points) at the edge of each through-hole to form an envelope circle, and obtain the center of the circle by the least squares method.

[0083] Calculate the coordinates of the center of the through-hole, and the corresponding center distance can be solved.

[0084] When the vision measurement system performs image processing, it is necessary to use the generatrix of the workpiece as a reference, intercept a rectangular area for image stitching to ensure that the obtained image is a rectangular image of the outer wall unfolded within a certain height range. If the intercepted rectangular area is not parallel to the generatrix, then during the process of image registration and fusion, the image stitching will deviate, affecting the effect of vision detection. Therefore, when calibrating the calibration part, it is necessary to determine the angle between the line connecting the centers of the two holes in the vertical direction and the generatrix of the workpiece. When calibrating the system, the direction of the generatrix of the workpiece can be determined based on this line and the calibrated angle. The generatrix of the workpiece is a straight line parallel to the axis of the workpiece and perpendicular to the bottom surface. Taking the bottom surface of the workpiece as the reference plane, and then making a perpendicular line can obtain the direction of the generatrix. The contact measurement of the vision measurement system is not affected by the curvature of the workpiece surface. Cooperating with a computer, it can quickly and accurately measure the basic geometric shape of an object, with high accuracy and reliability. Finally, the reference plane of the bottom of the workpiece can be constructed by the way of the contact probe entering and exiting point by point, so as to determine the direction of the generatrix of the workpiece.

[0085] Before each operation of the vision measurement system, it is necessary to calibrate the pixel equivalent to establish the corresponding relationship between the pixels of the camera image and the actual size of the workpiece under the current conditions. The calibration of the system pixel equivalent is completed by the method of experimental calibration.

[0086] The calibration of the system is divided into the following steps:

[0087] 1. Turn on the system and place the calibration workpiece on the platform of the measured object;

[0088] 2. Adjust the focal length, the attitude of the mirror and the brightness of the light source to make the system image clearly;

[0089] 3. Manually rotate the rotating platform to make 3 round holes appear in the imaging area;

[0090] 4. Calculate the number of pixels occupied between the centers of the small holes through the system software;

[0091] 5. Mark the direction of the generatrix of the workpiece according to the angle between the line connecting the centers of the pre-calibrated small holes and the generatrix.

[0092] 6. Calculate the pixel equivalent according to the calibrated actual size and pixels.

[0093] Assume that the center distance is length A, and the corresponding number of pixels is N, then the actual physical size corresponding to each pixel is

[0094]

[0095] Multiply the measured actual pixel size by this calibration coefficient to obtain the required actual size.

[0096] Calibrate the pixel equivalent of the vision measurement system through a calibration part, and the image obtained by the system is as Figure 6 shown.

[0097] A vision measurement system for the inner and outer walls of a small hole, its calibration device and calibration method provided by the above embodiments of the present invention relate to the calibration technology of a vision measurement system for the inner and outer walls of a workpiece with a small-size aperture. By making three through holes with right-angled connections on the sample of the workpiece to be detected, establishing the proportional relationship between the precise physical size and the pixel distance between the centers of the round holes, and completing the calibration of the system, it is possible to achieve simultaneous calibration of the inner wall and the outer wall, as well as simultaneous calibration in the circumferential and axial directions. This calibration method can be directly carried out on the existing platform of the system, and has the characteristics of strong anti-interference ability and good calibration effect, and can determine the conversion coefficient between the pixels and the size of the detection system.

[0098] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A calibration method for the visual measurement system of the inner and outer walls of a small hole, characterized in that, a calibration device for the visual measurement system of the inner and outer walls of a small hole is adopted. The calibration device includes one or more calibration parts. Each calibration part uses a sample of the workpiece to be detected as the basic part, and three through holes with equal diameters are set at the set positions on the sample of the workpiece to be detected. The connection lines between the three through holes form a right triangle; where: two through holes that form one right side of the right triangle are on the same generatrix, and are used as the standard sample for determining the pixel size in the y-axis direction; two through holes that form the other right side of the right triangle are in the same circumferential direction, and are used as the standard sample for calibrating the pixel size in the x-axis direction. The calibration method includes: S1, setting the calibration part of the calibration device on the rotating platform of the system; S2, adjusting the focal length of the CCD camera lens, the attitude of the reflector and the brightness of the light source of the system to make the system clearly image; S3, manually rotating the rotating platform of the system to make the three through holes appear in the imaging area of the lens; S4, calculating the number of pixels occupied by the connection line of the centers of the right sides of the three through holes through the computer of the system; S5, marking the direction of the generatrix of the calibration part according to the angle between the connection line of the centers of the right sides of the three through holes and the generatrix that has been pre-calibrated; S6, calculating the pixel equivalent according to the actual size and the number of pixels of the calibration.

2. The calibration method for the visual measurement system of the inner and outer walls of a small hole according to claim 1, characterized in that, in S3, it further includes: manually rotating the rotating platform of the system to make the axis of the calibration part parallel to the horizontal plane of the rotating platform, and making the 3 through holes face the CCD camera, and adjusting the position of the CCD camera lens of the system until the images of the three through holes appear in the center of the screen.

3. The calibration method for the visual measurement system of the inner and outer walls of a small hole according to claim 1, characterized in that, in S4, the method for calculating the number of pixels occupied by the connection line of the centers of the right sides of the three through holes is: at least 3 points are selected on the edge of each through hole to form an envelope circle, and the center of the circle is obtained by the least square method.

4. The calibration method for the visual measurement system of the inner and outer walls of a small hole according to claim 3, characterized in that, the number of points selected on the edge of each through hole is greater than or equal to 10.

5. The calibration method for the visual measurement system of the inner and outer walls of a small hole according to claim 1, characterized in that, in S5, the method for marking the direction of the generatrix of the calibration part is: taking the generatrix of the calibration part as the reference, intercepting a rectangular area for image stitching to ensure that the obtained image is a rectangular image of the outer wall unfolded within a certain height range; if the intercepted rectangular area is not parallel to the generatrix of the calibration part, then determine the angle between the connection line of the centers of the two through holes that need to be on the same generatrix and the generatrix of the calibration part, and determine the direction of the generatrix of the calibration part based on this connection line and the calibrated angle.

6. The calibration method for the visual measurement system of the inner and outer walls of a small hole according to claim 1, characterized in that, S6 is specifically: Assume that the center distance of the right-angled sides between three through-round holes is length A, and the corresponding number of pixels is N. Then the actual physical size corresponding to each pixel is: where K is the calibration coefficient; Multiply the measured actual number of pixels by K to obtain the required actual physical size.

7. The calibration method of the visual measurement system for the inner and outer walls of a small hole according to any one of claims 1-6, characterized in that when the calibration device is in the working state, all three through-round holes are within the measurement field of view of the visual measurement system.

8. The calibration method of the visual measurement system for the inner and outer walls of a small hole according to any one of claims 1-6, characterized in that the center distance between the two groups of through-round holes forming the right-angled sides in the calibration device is equal.

9. A visual measurement system for the inner and outer walls of a small hole, characterized in that the system performs pixel equivalent calibration by using the calibration method according to any one of claims 1 to 8.

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