A micrometer-level visual displacement calibration method and device

Abstract

This invention relates to a micrometer-level visual displacement calibration method. It employs an object-side telecentric lens / or dual telecentric lenses as the image acquisition tool, and selects two objects to be calibrated, named a moving calibration object and a fixed calibration object, respectively. The lens acquires images of the calibration objects, calculates the center pixel coordinates of the marker points, and combines this with the displacement of the moving calibration object and the offset of the center pixel coordinates of the marker points caused by temperature variations in the fixed calibration object to calculate the actual displacement value of the center pixel coordinates of the marker points after the moving calibration object has been moved. This invention also relates to a micrometer-level visual displacement calibration device, comprising: a camera, an object-side telecentric lens / or dual telecentric lenses, a moving calibration object, and a fixed calibration object. This invention solves the technical problem of decreased measurement accuracy under the influence of ambient temperature and camera self-heating, meeting the high-precision measurement requirements of visual displacement calibration in normal temperature environments, achieving a measurement accuracy of ±1 micrometer.

Description

Technical Field

[0001] This invention belongs to the field of displacement calibration technology, specifically relating to a micron-level visual displacement calibration method and apparatus. Background Technology

[0002] By comparing two images of the same object (the object to be calibrated) before and after the movement, the displacement of the object can be obtained using a two-dimensional digital imaging method. However, in actual measurements, deformation of the object's surface, changes in ambient temperature, or slight changes in the position of the camera sensor plane due to camera self-heating may cause the previously assumed linear correspondence between the object plane and image plane displacements to no longer hold strictly, resulting in non-negligible measurement errors and making it difficult to achieve the accuracy requirements of micrometer-level visual displacement calibration. Summary of the Invention

[0003] To address the technical problem of decreased accuracy in visual displacement calibration under normal, non-constant temperature environments due to temperature variations or camera self-heating, this invention provides a micrometer-level visual displacement calibration method and apparatus. The technical solution adopted by this invention is as follows:

[0004] A micrometer-level visual displacement calibration method includes the following steps:

[0005] Step 1: Select an object-side telecentric lens or a dual telecentric lens as the image acquisition tool for the object to be calibrated;

[0006] Step 2: Select two objects with the same structure to be calibrated. Name the two objects with the same structure as the moving calibration object and the fixed calibration object, respectively.

[0007] Step 3: Place both the moving and fixed calibration objects simultaneously within the image acquisition area of ​​the object-side telecentric lens or dual telecentric lens;

[0008] Step 4: The object-side telecentric lens or dual telecentric lenses acquire images of the moving and fixed calibration objects in real time. The center pixel coordinates of the markers at the current position and temperature of the moving calibration object are obtained as (x1, y1) and the center pixel coordinates of the markers at the current position and temperature of the fixed calibration object are obtained as (x2, y2) through the image recognition algorithm.

[0009] Step 5: Move the position of the moving calibration object while keeping the position of the fixed calibration object unchanged. Use a telecentric lens or dual telecentric lenses to acquire images of the moving and fixed calibration objects in real time. Use an image recognition algorithm to obtain the center pixel coordinates of the marker points after the position of the moving calibration object has been moved and after the temperature change as (x1'+δx1,y1'+δy1), where x1' and y1' are the center pixel coordinates of the marker points after the position of the moving calibration object has been moved, and δx1 and δy1 are the offset values ​​of the center pixel coordinates of the marker points caused by the temperature effect of the moving calibration object.

[0010] The image recognition algorithm obtains the current position of the fixed calibration object and the center pixel coordinates of the marker point after temperature change as (x2+δx2,y2+δy2), where δx2 and δy2 are the offset values ​​of the center pixel coordinates of the marker point caused by the temperature effect on the fixed calibration object.

[0011] Step 6: Perform displacement calibration of the moving calibration object, and calculate the displacement value of the center pixel coordinates of the marker point after the moving calibration object has been moved. The calculation formula is:

[0012] ,

[0013] Calculate the offset of the center pixel coordinates of the marker point after the temperature change of the fixed calibration object. The calculation formula is:

[0014] ;

[0015] Step 7: Move the position of the calibration object. The actual displacement value of the center pixel coordinates of the marker point after the movement. .

[0016] In the above method of the present invention, the offset value of the fixed calibration material after temperature change is used. As a correction value, micron-level visual displacement calibration of the moving calibration object was achieved.

[0017] A micrometer-level visual displacement calibration device, using the aforementioned method, includes: a camera, an object-side telecentric lens or a dual telecentric lens, a movable calibration object, and a fixed calibration object. The camera is fixed on a camera bracket, the object-side telecentric lens or the dual telecentric lens is mounted on the camera, the movable calibration object is placed on a first displacement stage, and the fixed calibration object is fixed on a second displacement stage.

[0018] In the micron-level visual displacement calibration device of the present invention, a telecentric lens or a dual telecentric lens and two calibration objects (one moving and one fixed) are used to achieve micron-level visual displacement calibration of the moving calibration object.

[0019] The beneficial effects of this invention are:

[0020] In non-constant temperature environments, by designing moving and fixed calibration objects and using object-side telecentric lenses or dual telecentric lenses, the technical problem of decreased measurement accuracy under the influence of ambient temperature and camera self-heating was solved. This meets the high-precision measurement requirements of visual displacement calibration in normal temperature environments, and the measurement accuracy can reach ±1 micrometer. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some specific embodiments of the present invention. For those skilled in the art, other drawings falling within the scope of protection of this application can be obtained based on these drawings without any creative effort.

[0022] Figure 1 This is a schematic diagram of the structure of the first calibration plate according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic diagram showing the pixel change trend of the center coordinates of the marker points on the first calibration board according to an embodiment of the present invention.

[0024] Figure 3 This is a schematic diagram showing the pixel change trend of the center coordinates of the marker points on the second calibration board according to an embodiment of the present invention.

[0025] Figure 4 This is a schematic diagram of the pixel difference in the center coordinates of the marker points between the first calibration board and the second calibration board according to an embodiment of the present invention;

[0026] Figure 5 This is a schematic diagram of the assembly structure of the micron-level visual displacement calibration device according to an embodiment of the present invention;

[0027] In the figure, 1-camera, 2-object-side telecentric lens, 3-coaxial point light source, 4-first calibration plate, 5-second calibration plate, 6-reflector, 7-first displacement stage, 8-second displacement stage, 9-dual-frequency laser interferometer. Detailed Implementation

[0028] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0029] A micrometer-level visual displacement calibration method includes the following steps:

[0030] Step 1: Select object-side telecentric lens 2 as the image acquisition tool for the object to be calibrated;

[0031] In this embodiment of the invention, an object-side telecentric lens 2 is chosen instead of a regular lens to address the deformation changes on the surface of the object being measured. Alternatively, a dual telecentric lens (object-side telecentric lens + image-side telecentric lens) can be used. Telecentricity is a description of the optical imaging characteristics of a lens. In optics, the side where light enters the optical system is called the object side. When the entrance pupil is located near infinity from the optical system, the optical system is object-side telecentric (the opposite is image-side telecentric). The advantage of an object-side telecentric lens is that it has a fixed magnification on the object side, ensuring minimal error during image acquisition.

[0032] Step 2: Select two objects with the same structure to be calibrated. Name the two objects with the same structure as the first calibration plate 4 and the second calibration plate 5, respectively.

[0033] In this embodiment of the invention, the first calibration plate 4 and the second calibration plate 5 are selected as the moving calibration object and the fixed calibration object, respectively. Figure 1 The diagram shown is a structural schematic of the first calibration plate according to an embodiment of the present invention. The first calibration plate has the same structure as the second calibration plate. Figure 1 The image shows 21×21 markers on the first calibration plate.

[0034] Step 3: Place the first calibration plate 4 and the second calibration plate 5 simultaneously within the image acquisition area of ​​the object-side telecentric lens 2;

[0035] In this embodiment of the invention, the first calibration plate 4 is placed on the first displacement stage 7, and the second calibration plate 5 is fixed on the second displacement stage 8. The first calibration plate 4 and the second calibration plate 5 are located below the object-side telecentric lens 2, and the object-side telecentric lens 2 can acquire images of the first calibration plate 4 and the second calibration plate 5 in real time.

[0036] Step 4: The telecentric lens 2 acquires images of the first calibration plate 4 and the second calibration plate 5 in real time. The center pixel coordinates of the current position and current temperature markers of the first calibration plate 4 are (x1, y1) and the center pixel coordinates of the current position and current temperature markers of the second calibration plate 5 are (x2, y2) respectively, using an image recognition algorithm.

[0037] In this embodiment of the invention, the center pixel coordinates (x1, y1) and (x2, y2) of the marker points of the first calibration plate 4 and the second calibration plate 5 at the current position and the current temperature are initial values, which are the basic values ​​for realizing the micrometer-level visual displacement calibration method.

[0038] Step 5: Move the position of the first calibration plate 4 while keeping the position of the second calibration plate 5 unchanged. Use a telecentric lens or dual telecentric lenses to acquire images of the first calibration plate 4 and the second calibration plate 5 in real time. Use an image recognition algorithm to obtain the center pixel coordinates of the marker points after the position of the first calibration plate 4 has been moved and after the temperature change as (x1'+δx1,y1'+δy1), where x1' and y1' are the center pixel coordinates of the marker points after the position of the first calibration plate 4 has been moved, and δx1 and δy1 are the center pixel coordinate offsets of the marker points of the first calibration plate 4 caused by the temperature effect. Use an image recognition algorithm to obtain the current position of the second calibration plate 5 and the center pixel coordinates of the marker points after the temperature change as (x2+δx2,y2+δy2), where δx2 and δy2 are the center pixel coordinate offsets of the marker points of the second calibration plate 5 caused by the temperature effect.

[0039] In this embodiment of the invention, the center pixel coordinates of the marker point after the position of the first calibration plate 4 is moved, and the center pixel coordinates of the marker point after the temperature change of the second calibration plate 5 (x1'+δx1,y1'+δy1) and (x2+δx2,y2+δy2) are numerical values ​​used for comparison calculation, which are the calculation values ​​used to realize the micron-level visual displacement calibration method.

[0040] Step 6: Perform displacement calibration of the first calibration plate 4, and calculate the displacement value of the center pixel coordinates of the marker point after the position of the first calibration plate 4 has been moved. The calculation formula is:

[0041] ,

[0042] Calculate the offset of the center pixel coordinates of the marker points after the temperature change of the second calibration plate 5. The calculation formula is:

[0043] ;

[0044] In this embodiment of the invention, the calculated displacement value after moving the position of the first calibration plate 4 is... The offset value of the center pixel coordinates of the marker points after the temperature change of the second calibration plate 5. As the actual displacement value of the first calibration plate 4 is calculated The basis values ​​for displacement calibration are determined because, as measured at room temperature, the offset values ​​of the center pixel coordinates of the marker points in the first calibration plate 4 and the second calibration plate 5 are correlated. This correlation characteristic is used to obtain the temperature-induced offset values ​​of the center pixel coordinates of the marker points in the calibration plates. By using the corresponding calculation formula, the effects of ambient temperature and camera self-heating can be resolved.

[0045] The actual displacement value of the calibration object is affected by factors such as ambient temperature and camera self-heating. Equal to the offset value of the center pixel coordinates calculated from the two images. Subtract the offset of the center pixel coordinates caused by temperature. Therefore, how to obtain the offset value caused by temperature? This is the key to solving the problem.

[0046] The correlation test process for the offset values ​​of the center pixel coordinates of the calibration points caused by temperature changes in the first calibration board 4 and the second calibration board 5 is as follows:

[0047] Under room temperature conditions, keeping the positions of the first calibration plate 4 and the second calibration plate 5 unchanged, select a point from either the first calibration plate 4 or the second calibration plate 5 as a marker point, and test the offset of the center pixel coordinates of the marker point from its initial position. Figure 2 and Figure 3 As shown, without corrective measures, the marker points are affected by ambient temperature and their own self-heating. The center pixel coordinates of the marker points in the first calibration plate 4 and the second calibration plate 5 deviate from their initial positions, sometimes by as many as several pixels, failing to meet the accuracy requirements of micrometer-level displacement calibration. However, analysis reveals that the changing trends of the center pixel coordinates of the two marker points in the first calibration plate 4 and the second calibration plate 5 are basically consistent. For example... Figure 4 As shown, the offset of the center pixel coordinates of the two marker points is less than ±0.1 pixels. A single pixel is 4.5 μm, and the offset is less than ±0.5 μm. Therefore, it can be considered that the changing trends of the center pixel coordinates of the marker points in the first calibration plate 4 and the second calibration plate 5 are basically the same, i.e. This is the offset value of the first calibration plate 4 caused by temperature changes. This is the offset value of the second calibration plate 5 caused by temperature changes. We can use its good correlation to solve the impact of temperature changes on measurement accuracy.

[0048] The first calibration plate 4 is moved 10 micrometers (based on the dual-frequency laser interferometer 9), and this movement is repeated a total of 20 times. The center pixel coordinates of the marked points on the first calibration plate 4 are recorded, as are the center pixel coordinates of the marked points on the second calibration plate 5. To ensure accurate movement of the first calibration plate 4 by 10 micrometers, the dual-frequency laser interferometer 9 and a reflector 6 are used for precise positional movement. A reflector 6 is placed on one side of the platform where the moving calibration object (first calibration plate 4) is placed. The laser emitted by the dual-frequency laser interferometer 9 is directed towards the reflector 6. The dual-frequency laser interferometer 9 has sub-micrometer level measurement accuracy. The laser emitted by the dual-frequency laser interferometer 9 shines on the reflector 6, and the reflected signal returns to the dual-frequency laser interferometer 9 to obtain the positional movement value of the moving calibration object (first calibration plate 4). The positional movement information obtained by the dual-frequency laser interferometer 9 serves as the reference data for the movement of the first calibration plate 4. By combining the center pixel coordinates of the marker points on the first calibration plate 4 after each recorded movement with the offset values ​​of the center pixel coordinates of the marker points on the second calibration plate 5 caused by temperature effects, the actual displacement value of the first calibration plate 4 can be calculated by comparing it with the initial point coordinates. The test results are as follows:

[0049]

[0050] Analysis of the test results shows that the measurement error using the displacement calibration method of this invention mainly comes from approximations ( The correlation test results show that the deviation is less than ±0.1 pixels, and the size of a single pixel is 4-8 μm. In this embodiment, the deviation is ±0.5 μm. Therefore, the error of the final displacement test result is less than ±0.1 pixels. The feasibility of this method is verified by the results recorded by the dual-frequency laser interferometer 9. The accuracy of the measurement results is at the micrometer level, so the overall error is within ±1 micrometer. This solves the problem of measurement accuracy under the influence of ambient temperature and camera self-heating, and meets the calibration requirements of micrometer-level high precision under normal temperature conditions.

[0051] Step 7: The actual displacement value of the center pixel coordinates of the marker point after the position of the first calibration plate 4 has been moved. .

[0052] like Figure 5The diagram shown is an assembly structure schematic of a micrometer-level visual displacement calibration device according to an embodiment of the present invention. A micrometer-level visual displacement calibration device, applying the aforementioned method, includes: a camera 1, an object-side telecentric lens 2, a coaxial point light source 3, a first calibration plate 4, and a second calibration plate 5. The camera 1 is fixed on a camera bracket and has the following parameters: 2 megapixels, 1 / 1.8 inch resolution, and a pixel size of 4.5 × 4.5 μm. The object-side telecentric lens 2 is mounted on the camera 1 and has the following parameters: 1.0X magnification, a field of view of 8.8 × 6.6 mm, and distortion less than 0.006%. The coaxial point light source 3 is fixed on the object-side telecentric lens 2. The coaxial point light source 3 provides supplementary light and ensures the clarity of the acquired image. The coaxial point light source 3 is a blue light source with an 8 mm aperture and a maximum power of 3 W. The first calibration plate 4 is placed on the first displacement stage 7, and the second calibration plate 5 is fixedly placed on the second displacement stage 8. A reflector 6 is placed on one side of the first displacement stage 7 as the test target of the dual-frequency laser interferometer 9 to verify the test results. The dual-frequency laser interferometer 9 is located on the side of the first displacement stage 7, and the laser emitted by the dual-frequency laser interferometer 9 is directly facing the reflector 6. The first calibration plate 4 and the second calibration plate 5 are 6×6mm in size, with a 7×7 dot array, a dot diameter of 0.375mm, and a center distance of 0.75mm.

[0053] The camera 1 is connected to a data processing device, which is a desktop or laptop computer. The data processing device includes an image recognition module and a data calculation module. The image recognition module is used to recognize images, and the data calculation module is used to calculate data. The methods for image recognition and data calculation have been described in detail in the preceding description and will not be repeated here.

[0054] Finally, it should be noted that the above embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit them. The scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention.

Claims

1. A micrometer-level visual displacement calibration method, characterized in that, Includes the following steps: Step 1: Select an object-side telecentric lens or a dual telecentric lens as the image acquisition tool for the object to be calibrated; Step 2: Select two objects with the same structure to be calibrated. Name the two objects with the same structure as the moving calibration object and the fixed calibration object, respectively. Step 3: Place both the moving and fixed calibration objects simultaneously within the image acquisition area of ​​the object-side telecentric lens or dual telecentric lens; Step 4: The telecentric lens or dual telecentric lens acquires images of the moving and fixed calibration objects in real time. The center pixel coordinates of the marker point of the current position and current temperature of the moving calibration object are obtained as (x1, y1) through the image recognition algorithm. The center pixel coordinates of the marker point of the current position and current temperature of the fixed calibration object are obtained as (x2, y2) through the image recognition algorithm. Step 5: Move the position of the moving calibration object while keeping the position of the fixed calibration object unchanged. Use a telecentric lens or dual telecentric lenses to acquire images of the moving and fixed calibration objects in real time. Use an image recognition algorithm to obtain the center pixel coordinates of the marker points after the position of the moving calibration object has been moved and after the temperature change as (x1'+δx1,y1'+δy1), where x1' and y1' are the center pixel coordinates of the marker points after the position of the moving calibration object has been moved, and δx1 and δy1 are the offset values ​​of the center pixel coordinates of the marker points caused by the temperature effect of the moving calibration object. The image recognition algorithm obtains the current position of the fixed calibration object and the center pixel coordinates of the marker point after temperature change as (x2+δx2,y2+δy2), where δx2 and δy2 are the offset values ​​of the center pixel coordinates of the marker point caused by the temperature effect on the fixed calibration object. Step 6: Perform displacement calibration of the moving calibration object, and calculate the displacement value of the center pixel coordinates of the marker point after the moving calibration object has been moved. The calculation formula is: ， Calculate the offset of the center pixel coordinates of the marker point after the temperature change of the fixed calibration object. The calculation formula is: ； Step 7: Move the position of the calibration object. The actual displacement value of the center pixel coordinates of the marker point after the movement. .

2. The micrometer-level visual displacement calibration method according to claim 1, characterized in that, In step 5, when the position of the moving calibration object is moved, the precise position movement is achieved by using a dual-frequency laser interferometer and a reflector. A reflector is set on one side of the platform where the moving calibration object is placed. The dual-frequency laser interferometer emits a laser beam that shines on the reflector, and the reflected signal returns to the dual-frequency laser interferometer to obtain the value of the position movement of the moving calibration object.

3. A micrometer-level visual displacement calibration device, characterized in that, To implement the method as described in claim 1 or 2, the method includes: a camera (1), an object-side telecentric lens or a dual telecentric lens, a movable calibration object and a fixed calibration object, wherein the camera (1) is fixed on a camera bracket, the object-side telecentric lens or the dual telecentric lens is set on the camera (1), the movable calibration object is placed on a first displacement stage (7), and the fixed calibration object is fixed on a second displacement stage (8).

4. The micrometer-level visual displacement calibration device according to claim 3, characterized in that, It also includes a coaxial point light source (3), which is fixed on the object-side telecentric lens or a dual telecentric lens.

5. The micrometer-level visual displacement calibration device according to claim 3, characterized in that, The camera (1) is connected to a data processing device, which is a desktop computer or a laptop computer. The data processing device is equipped with an image recognition module and a data calculation module. The image recognition module is used to recognize images, and the data calculation module is used to calculate data.

6. The micrometer-level visual displacement calibration device according to claim 3, characterized in that, The camera (1) has the following parameters: 2 million pixels, 1 / 1.8 inch, and a pixel size of 4.5×4.5μm.

7. The micrometer-level visual displacement calibration device according to claim 3, characterized in that, The object-side telecentric lens uses the following parameters: 1.0X magnification, field of view of 8.8×6.6mm, and distortion of less than 0.006%.

8. The micrometer-level visual displacement calibration device according to claim 4, characterized in that, The coaxial point light source (3) is a blue light point light source with an output aperture of 8mm and a maximum power of 3W.

9. The micrometer-level visual displacement calibration device according to claim 3, characterized in that, It also includes a reflector (6) and a dual-frequency laser interferometer (9), with the reflector (6) set on one side of the platform where the moving calibration object is placed.

Images