A calibration method for a laser profilometer

Abstract

The present application discloses a calibration method for a large field of view laser profilometer, which includes parameterizing the line laser profilometer system model into a polynomial mapping function B = AX, and mapping the pixel coordinates A of the laser light plane or surface on the image plane to the spatial coordinates B in the camera coordinate system through the polynomial coefficients X. By obtaining the observed values of A and B through the calibration of the system, X can be solved to achieve the calibration of the laser profilometer. This method does not involve the calibration of the laser light plane equation, so it is not affected by the distortion of the laser plane and is applicable to the precise calibration of line laser systems with and without distortion under a large field of view.

Description

Technical Field

[0001] The present invention relates to the calibration technology of a laser profilometer, and particularly to a calibration method for a laser profilometer. Background Art

[0002] With the rapid development of China's manufacturing industry, line laser profilometers are increasingly widely used in the field of 3D industrial intelligent manufacturing. A line laser profilometer is an optical precision system composed of a laser emitter and a 2D camera sensor. The laser emitter projects a standard light plane, and the equation parameters of the light plane are obtained through system calibration, so as to calculate the spatial 3D coordinates of the laser line projected on the surface of an object.

[0003] In industrial large-scale scene applications, due to cost limitations in manufacturing, the laser emitter projects a non-standard distorted curved surface in a large field of view projection range (for example, 1.5m - 3m), making the conventional light plane calibration method no longer applicable. Summary of the Invention

[0004] The purpose of the present invention is to provide a calibration method for a laser profilometer. This method is based on polynomial transformation, independent of the system geometric model, and does not involve the solution of the laser light plane equation. Therefore, it is not affected by laser plane distortion and is applicable to the precise calibration of line laser systems with and without distortion in a large field of view.

[0005] The following gives a brief overview of one or more aspects to provide a basic understanding of these aspects. This overview is not an exhaustive survey of all contemplated aspects, and is neither intended to identify key or decisive elements of all aspects nor to define the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description to follow.

[0006] According to one aspect of the present invention, there is provided a calibration method for a laser profilometer, including:

[0007] S1. Parameterize the system model of the line laser profilometer into a polynomial mapping function B = AX, and map the pixel coordinates A of the laser light plane or curved surface on the image plane to the spatial coordinates B in the camera coordinate system through the polynomial coefficients X;

[0008] S2. Obtain the observed values of A and B through the calibration of the system;

[0009] S3. Solve for X to achieve the calibration of the laser profilometer.

[0010] In one embodiment, the S1 includes:

[0011] S11. Establish a mapping function model xyz = f(u, v) between the pixel coordinates (u, v) of the laser light plane or surface on the image plane and the spatial coordinates xyz in the camera coordinate system;

[0012] S12. Parameterize the mapping function model f(u, v) into a K - order linear polynomial:

[0013] x = a1u k + a2v k + a3u k-1 v + a4uv k-1 + … + a n-2 u + a n-1 v + a n

[0014] y = b1u k + b2v k + b3u k-1 v + b4uv k-1 + … + b n-2 u + b n-1 v + b n

[0015] z = c1u k + c2v k + c3u k-1 v + c4uv k-1 + … + c n-2 u + c n-1 v + c n

[0016] s.t. (u, v) ∈ P(π l );

[0017] where P(π l ) is the projection of the laser light plane or surface on the camera image plane;

[0018] S13. Let B = [x, y, z], A = [u k v k u k-1 v uv k-1 … u v],

[0019]

[0020] Rewrite the above formula as B = AX.

[0021] In one embodiment, the S2 includes:

[0022] S21. Place the calibration board in N different poses and capture N calibration board images;

[0023] S22. After each calibration board image is captured, keep the current position of the calibration board unchanged, project the laser line clearly onto the calibration board, and then capture a laser calibration board image. A total of N laser calibration board images are captured;

[0024] S23. Based on the N calibration board images, calculate the internal parameters M of the camera, the lens distortion coefficient D, and the external parameters [R, T] from the calibration board to the camera;

[0025] S24. According to the camera internal parameters M and the lens distortion coefficient D, undistort the N laser calibration board images;

[0026] S25. Extract the pixel coordinates (u i , v i ) of the laser line from the undistorted laser calibration board images, and transform (u i , v i ) into the form of [u k v k u k-1 v uv k-1 … u v], that is, obtain the observed value of A;

[0027] S26. According to the homography matrix H between the calibration board plane and the camera imaging plane obtained during the calibration process, calculate the coordinates (x i , y i , z w ) of the homogeneous coordinates (u w , v w , 1) of the laser line in the world coordinate system,

[0028] (x w , y w , z w ) = H(u i , v i , 1) T ;

[0029] S27. According to the external parameter transformation matrix [R, T] from the calibration board coordinates to the camera coordinates obtained during the calibration process, convert (x w , y w , z w ) to the coordinates (x c , y c , z c ) in the camera coordinate system:

[0030] (x c , y c , z c ) = R(x w , y w , z w ) + T

[0031] That is, the observed value of B is obtained.

[0032] In one embodiment, in S2, the calibration plate coordinate system is used as the world coordinate system.

[0033] In one embodiment, in S3, X is solved by the least squares method:

[0034] X = (A T A) -1 A T B.

[0035] In one embodiment, in S3, X is solved by minimizing an objective function L:

[0036]

[0037] X * = argmin X L(X).

[0038] The beneficial effects of the embodiments of the present invention are as follows: By adopting a new calibration method based on polynomial transformation, which is independent of the system geometric model and does not involve solving the laser light plane equation, it is not affected by laser plane distortion and is applicable to the precise calibration of line laser systems with and without distortion under a large field of view. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0040] After reading the detailed description of the embodiments of the present disclosure in conjunction with the following drawings, the above features and advantages of the present invention can be better understood. In the drawings, the components are not necessarily drawn to scale, and components with similar related characteristics or features may have the same or similar reference numerals.

[0041] Figure 1 is the flowchart of the method of the embodiment of the present application;

[0042] Figure 2 is the schematic diagram of the calibration process of the embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The following will describe the present invention in detail with reference to the drawings and specific embodiments. Note that the aspects described in conjunction with the following drawings and specific embodiments are only exemplary and should not be construed as imposing any limitation on the protection scope of the present invention.

[0044] As shown Figure 1 in the figure, an embodiment of the present application provides a calibration method for a laser profilometer, including:

[0045] S1. Parameterize the system model of the line laser profilometer into a polynomial mapping function B = AX, and map the pixel coordinates A of the laser light plane or surface on the image plane to the spatial coordinates B in the camera coordinate system through the polynomial coefficients X;

[0046] S1 specifically includes:

[0047] S11. Establish a mapping function model xyz = f(u, v) between the pixel coordinates (u, v) of the laser light plane or surface on the image plane and the spatial coordinates xyz in the camera coordinate system, where (u, v) is constrained by the laser plane or surface and is the projection of the laser plane or surface on the image plane of the camera.

[0048] S12. Parameterize the mapping function model f(u, v) into a K-order linear polynomial:

[0049] x = a1u k + a2v k + a3u k-1 v + a4uv k-1 + … + a n-2 u + a n-1 v + a n

[0050] y = b1u k + b2v k + b3u k-1 v + b4uv k-1 + … + b n-2 u + b n-1 v + b n

[0051] z = c1u k + c2v k + c3u k-1 v + c4uv k-1 + … + c n-2 u + c n-1 v + c n

[0052] s.t. (u, v) ∈ P(π l );

[0053] where P(π l ) is the projection of the laser light plane or surface on the image plane of the camera;

[0054] S13. Let B = [x, y, z], A = [uk v k u k-1 v uv k-1 … u v],

[0055] Thus, the above K - order linear polynomial can be rewritten as B = AX.

[0056] In a possible embodiment, when K = 3, the system can be expressed as:

[0057] B = [X, Y, Z], A = [u 3 v 3 u 2 v uv 2 u 2 v 2 uv u v 1],

[0058]

[0059] That is, the system equation is:

[0060] x = a1u 3 + a2v 3 + a3u 2 v + a4uv 2 + a5u 2 + a6v 2 + a7uv + a8u + a9v + a 10

[0061] y = b1u 3 + b2v 3 + b3u 2 v + b4uv 2 + b5u 2 + b6v 2 + b7uv + b8u + b9v + b 10

[0062] z = c1u 3 + c2v 3 + c3u 2 v + c4uv 2 + c5u 2 + c6v 2 + c7uv + c8u + c9v + c 10

[0063] s.t.(u, v) ∈ P(π l ).

[0064] S2. Obtain the observed values of A and B through the calibration of the system;

[0065] S2 specifically includes:

[0066] S21, as Figure 2 shown, a checkerboard calibration plate is used as the calibration tool for the system. The laser line is projected onto the calibration plate, and the calibration plate is placed in N different poses to capture N calibration plate images;

[0067] S22. After each calibration plate image is captured, keep the current position of the calibration plate unchanged, clearly project the laser line onto the calibration plate, and then capture another laser calibration plate image. A total of N laser calibration plate images are captured;

[0068] S23. Based on the N calibration plate images, calculate the internal parameters M of the camera, the lens distortion coefficient D, and the external parameters [R, 7] from the calibration plate to the camera;

[0069] S24. Based on the camera internal parameters M and the lens distortion coefficient D, undistort the N laser calibration plate images;

[0070] S25. Extract the pixel coordinates (u i , v i ) of the laser line from the undistorted laser calibration plate images. Transform (u i , v i ) into the form of [u k v k u k-1 v uv k-1 ... u v], that is, obtain the observed value of A;

[0071] S26. According to the homography matrix H between the calibration plate plane and the camera imaging plane obtained during the calibration process, calculate the coordinates (x i , y i , 1) of the homogeneous coordinates of the laser line in the world coordinate system (x w , y w , z w ),

[0072] (x w , y w , z w ) = H(u i , v i , 1) T ;

[0073] S27. According to the external parameter transformation matrix [R, 7] from the calibration plate coordinates to the camera coordinates obtained during the calibration process, convert (x w , y w , z w ) to the coordinates (x c , y c , z c ) in the camera coordinate system:

[0074] (x c , y c , z c ) = R(x w , y w , z w ) + T, that is, the observed value of B is obtained.

[0075] In the above calibration process of S2, the calibration plate coordinate system is used as the world coordinate system.

[0076] S3. Solve to obtain X to achieve the calibration of the laser profile measuring instrument.

[0077] In S3, X can be solved by the least squares method:

[0078] X = (A T A) -1 A T B.

[0079] It can also be solved by minimizing an objective function L and iteratively solving X through L - M nonlinear optimization:

[0080]

[0081] X * = argmin X L(X).

[0082] After obtaining the solution X ＊ of X, the calibration of the system is completed.

[0083] In summary, the embodiments of the present application provide a new calibration method for a laser profile measuring instrument based on polynomial transformation, which is independent of the system geometric model, and this method does not involve solving the laser light plane equation, so it will not be affected by laser plane distortion, and is particularly suitable for accurate calibration of line laser systems with and without distortion under a large field of view. It should be particularly noted that this method is not limited to the calibration of laser profile measuring instruments under a large field of view, and is also applicable to the calibration of laser profile measuring instruments under a small field of view.

[0084] The various embodiments in this specification are described in a progressive manner, and the key points of each embodiment are the differences from other embodiments. The same or similar parts among the various embodiments can be referred to each other.

[0085] The foregoing description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0086] The above are only the preferred examples of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. A calibration method for a laser profilometer, characterized in that, Including: S1. Parameterize the system model of the line laser profiler into a polynomial mapping function B = AX, and map the pixel coordinates A of the laser light plane or surface on the image plane to the spatial coordinates B in the camera coordinate system through the polynomial coefficients X; S2. Obtain the observed values of A and B through the calibration of the system; S3. Solve for X to achieve the calibration of the laser profiler; The S1 includes: S11. Establish a mapping function model xyz = f(u, v) between the pixel coordinates (u, v) of the laser light plane or surface on the image plane and the spatial coordinates xyz in the camera coordinate system; S12. Parameterize the mapping function model f(u, v) into a K-order linear polynomial: x = a1u k + a2v k + a3u k-1 v + a4uv k-1 +...+ a n-2 u + a n-1 v + a n y = b1u k + b2v k + b3u k-1 v + b4uv k-1 +... + b n-2 u + b n-1 v + b n z = c1u k + c2v k + c3u k-1 v + c4uv k-1 +... + c n-2 u + c n-1 v + c n s.t. (u, v) ∈ P(π l ) where P(π l ) is the projection of the laser light plane or surface on the camera image plane; S13. Let B = [x, y, z], A = [u k v k u k-1 v uv k-1 … u v], Rewrite the above K-order linear polynomial as B = AX; The S2 includes: S21. Place the calibration board in N different poses and capture N calibration board images; S22. After capturing each calibration board image, keep the current position of the calibration board unchanged, project the laser line clearly onto the calibration board, and then capture one laser calibration board image, for a total of N laser calibration board images; S23. Calculate the internal parameters M of the camera, the lens distortion coefficient D, and the external parameters [R, T] from the calibration board to the camera based on the N calibration board images; S24. Undistort the N laser calibration board images according to the camera internal parameters M and the lens distortion coefficient D; S25. Extract the pixel coordinates (u i , v i ) of the laser line from the undistorted laser calibration plate image, and transform (u i , v i ) into the form of [u k v k u k-1 v uv k-1 … u v], that is, obtain the observed value of A; S26. Calculate the coordinates (x i , y i , z w ) of the homogeneous coordinates (u w , v w , 1) of the laser line in the world coordinate system according to the homography matrix H between the calibration plate plane and the camera imaging plane obtained during the calibration process. (x w , y w , z w ) = H(u i , v i , 1) T ; S27. According to the external parameter transformation matrix [R, 7] from the calibration board coordinates to the camera coordinates obtained during the calibration process, convert (x w , y w , z w ) to the coordinates (x c , y c , z c ) in the camera coordinate system: (x c ,y c ,z c ) = R(x w ,y w ,z w ) + T, that is, the observed value of B is obtained.

2. The calibration method of the laser profile measuring instrument according to claim 1, characterized in that, In the S2, the calibration board coordinate system is used as the world coordinate system.

3. The calibration method of the laser profile measuring instrument according to claim 2, characterized in that, In the S3, solve for X by the least squares method: X = (A T A) -1 A T B.

4. The calibration method of the laser profilometer according to claim 2, characterized in that, In the S3, solve for X by minimizing an objective function L: X * = arg min X L(X).

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