Method for measuring surface topography of large structures

Abstract

This invention discloses a method for measuring the surface morphology of large-size structures, comprising the following steps: 1) Fixing a line laser on an electric rotary table, placing the line laser scanning module at a suitable distance from the large workpiece, connecting and adjusting the laser scanning module so that the normal of the line laser line lies in the plane formed by the laser and the camera, arranging the camera according to the triangulation principle, adjusting the focal length and exposure time, and acquiring a clear image of the laser line on the large workpiece; 2) Starting the line laser scanning module and controlling the line laser scanning module to scan the surface of the large workpiece at a suitable angular velocity, while the camera acquires scanned images of the workpiece surface at a fixed frame rate; 3) According to the width of the laser line, fusing the acquired sequence of images every n frames to obtain a fringe pattern containing a 2π / n phase shift; 4) Performing phase shift analysis on the fringe pattern obtained in step 3 to obtain a phase pattern of the entire field, and then calculating the three-dimensional morphology of the large workpiece according to the relationship between phase and height.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and specifically to a method for detecting the surface morphology of large-size structures in outdoor environments. Background Technology

[0002] With the rapid development of optical measurement technology, numerous techniques for detecting structural surface morphology have emerged. Currently, mainstream 3D optical measurement techniques include structured light fringe projection and linear or dot array scanning methods. In particular, the application of structured light-based 3D morphology measurement technology is becoming increasingly widespread. The projection grating phase method, with its advantages of non-contact and high precision, is widely used in the automatic 3D measurement of objects. Its principle is to calculate the phase value of each pixel on multiple fringe patterns with a certain phase difference, and then calculate the 3D information of the object based on these phase values. Although these methods are basically mature and have high accuracy, there are still technical bottlenecks in the measurement of large-size surface morphology. The most significant limitation lies in the limitation of the projection device's power, resulting in insufficient intensity of the projected structured light. Under outdoor conditions or when projecting onto large-area surfaces, image quality significantly degrades. Linear / dot array scanning technology's measurement range is limited by the geometric dimensions between the scan lines and the camera. It can only complete the morphology measurement of large surfaces at close range through stitching. Based on these reasons, developing measurement techniques suitable for outdoor large-size structural morphology measurement is of great significance. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies and the limitations of existing equipment, as well as the urgent need for surface morphology inspection of large-size workpieces. This invention proposes a method for measuring the surface morphology of large-size structures based on line laser beam projection. This method is a non-contact optical measurement method that can be applied to the morphology inspection of various large-size structural surfaces.

[0004] The present invention can be specifically solved by the following technical solutions:

[0005] 1) Fix a line laser on an electric rotary table, then place the line laser scanning module at a suitable distance from the large workpiece, connect and adjust the line laser scanning module so that the normal of the line laser line is located in the plane formed by the laser and the camera. Arrange the camera according to the principle of triangulation, adjust the focal length and exposure time, and obtain a clear image of the laser line on the large workpiece.

[0006] 2) Start the line laser scanning module and control the line laser scanning module to scan the surface of the large workpiece at a suitable angular velocity, while the camera acquires the scanned images of the workpiece surface at a fixed frame rate.

[0007] 3) Based on the width of the laser line, the acquired sequence images are fused every n frames to obtain a stripe pattern containing a 2π / n phase shift;

[0008] 4) Perform phase shift analysis on the fringe pattern obtained in step 3 to obtain the phase pattern of the entire field, and then calculate the three-dimensional shape of the large workpiece based on the relationship between phase and height.

[0009] Further, in step 2), to improve measurement accuracy, the distance P between two adjacent laser lines in the fused image needs to be twice the laser line width. Assume the camera captures one image every t seconds, and the pitch angle of the laser line structured light scanning module is α. Let the distance from the line laser scanning module to the object being measured be l. Let the angular velocity of the line laser scanning module be ω, and the number of phase shifts be n. Then, the distance d between the laser lines in two adjacent frames can be expressed as:

[0010] d=l[tan(α+ωt)-tanα] (1)

[0011] Let the linewidth of the laser line be w. Since P = 2w = nd, we can find ω by substituting it into Equation 1.

[0012]

[0013] Since the pitch angle α, the distance l from the laser line structured light scanning module to the object being measured, the time interval t for image acquisition, and the number of phase shift steps n are all known quantities, we only need to scan according to the calculated angular velocity ω to obtain a stripe image that meets the measurement requirements.

[0014] Step 3) specifically involves: after scanning, a set of sequential images is obtained, each image containing a laser line. The acquired sequential images are fused every 4 frames using an image fusion algorithm to obtain 4 stripe patterns containing a 2π / 4 phase shift.

[0015] Furthermore, the image fusion algorithm is a pixel-maximum-value-based image fusion algorithm. Assume there are m frames (m is a multiple of 4) of source images to be fused. The grayscale value of each frame can be represented as:

[0016] I k (i,j)(k=1,2,3,L,m) (3)

[0017] The grayscale values ​​of the fused stripe pattern can then be represented as:

[0018]

[0019] Applying the same processing to the reference plane also yields four fringe patterns containing a 2π / 4 phase shift.

[0020] Furthermore, the fringe pattern in step 3) is processed using a four-step phase-shifting method, and the principal phase value φ(i,j) of the object under test can be obtained according to equation (4).

[0021]

[0022] The solved φ(i,j) has a range of (-π,π), and the image exhibits a jagged, step-like grayscale distribution. However, the deformation of a real object should be continuous, and the corresponding phase change should also be continuous. Therefore, to restore the phase distribution of the enclosed object to a continuous phase distribution, the absolute phase must be obtained.

[0023] Furthermore, the unwrapping algorithm is a global phase unwrapping algorithm based on least squares.

[0024] The relationship between absolute phase and enveloping phase is as follows:

[0025]

[0026] Where k(i,j) are integers, if there is a matrix consisting of M×N data points, x and y correspond to its indices, 0≤i≤M-1, 0≤j≤M-1. Let... The wrap-around phase in the x-direction, The wrap-around phase in the y-direction can be represented as:

[0027]

[0028] Where W represents the wrapping operator, whose purpose is to add or subtract 2π to the partial derivative of the wrapping image, making... and The range of is [-π, π]. The least squares unwrapping method minimizes the difference between the partial derivative of the actual phase and the wrapped phase, i.e.:

[0029]

[0030] Let the above formula be Since the derivative is 0, we can derive the Poisson equation:

[0031]

[0032] in Because the phase differentiation method is invalid for the edges that enclose the phase, the Nenmann boundary conditions for the Poisson equation are:

[0033]

[0034] The Gauss-Seidel iterative method can be used as an iterative algorithm for unwrapping, and its expression is:

[0035]

[0036] Where n is the number of iterations.

[0037] Similarly, the absolute phase of the reference plane can be obtained.

[0038] After obtaining the absolute phase, the protrusion height of the defect can be calculated based on the relationship between the phase and the height.

[0039]

[0040] in The distance between the image acquisition system and the reference plane is l, the distance between the optical axis of the image acquisition system and the optical axis of the line laser scanning module is d, and the period of the fringe pattern is p.

[0041] The present invention has the following obvious and prominent substantive features:

[0042] This invention proposes a method for detecting the surface morphology of large-size structures based on line laser beam projection. It overcomes many technical bottlenecks of traditional fringe projection methods when measuring the surface morphology of large-size structures. Compared to traditional visual methods, the proposed method can perform surface morphology detection on large-area objects outdoors. The measuring equipment is simple to install, small in size, and achieves efficient non-contact measurement. The method proposed in this invention has advantages such as high speed, high precision, and high automation. As a non-contact optical measurement technology, this invention is simple to operate and highly accurate. Therefore, it is very suitable for surface morphology detection of various large-size structures. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the projection grating phase method.

[0044] Figure 2 This is a diagram of the measurement object in Implementation Case 1.

[0045] Figure 3 This is the line laser scan image collected in Implementation Case 1.

[0046] Figure 4 This is a reconstruction diagram of the shape of the foam board from Case Study 1. Detailed Implementation

[0047] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0048] The method for measuring the surface morphology of large-size structures according to the present invention specifically includes the following steps:

[0049] 1) Fix a line laser on an electric rotary table, then place the line laser scanning module at a suitable distance from the large workpiece. Connect and adjust the line laser structured light scanning module so that the normal of the line laser line lies within the plane formed by the laser and the camera. Arrange the camera according to the triangulation principle and record the geometric parameters between the camera, the object being measured, and the laser. Adjust the camera parameters to ensure a clear image of the object's surface is obtained. Figure 1 This is a schematic diagram of the projection grating phase method. The optical axis PO of the line laser scanning module intersects the optical axis EO of the image acquisition system (CCD) at a point O on the reference plane R. The distance between the image acquisition system and the reference plane is l, and the distance between the optical axis of the image acquisition system and the optical axis of the line laser scanning module is d. The reference plane R is perpendicular to EO. During measurement, the light rays originally projected onto point a on the reference plane can only illuminate point c due to the presence of the surface being measured. Therefore, point b on the reference plane and point c on the object plane are phased at the same point on the CCD. That is, due to the phase modulation caused by the surface height, point a is phase-shifted to point b, and the phase shift value is... If the period of the grating on the reference plane is p, then the period T = 1 / p.

[0050] 2) Start the line laser scanning module and control it to scan the blade at a suitable angular velocity. To improve measurement accuracy, the distance P between two adjacent laser lines in the fused image needs to be twice the laser line width. Assume the camera captures one image every t seconds, and the pitch angle of the laser line structured light scanning module is α. Let the distance from the line laser scanning module to the object be l. The angular velocity of the line laser scanning module is ω, and the number of phase shifts is n. Then, the distance d between the laser lines in two adjacent frames can be expressed as:

[0051] d=l[tan(α+ωt)-tanα] (1)

[0052] Let the linewidth of the laser line be w. Since P = 2w = nd, we can find ω by substituting it into Equation 1.

[0053]

[0054] Since the pitch angle α, the distance l from the laser line structured light scanning module to the object being measured, the time interval t for image acquisition, and the number of phase shift steps n are all known quantities, we only need to scan according to the calculated angular velocity ω to obtain a stripe image that meets the measurement requirements.

[0055] 3) After scanning, a sequence of images is obtained, each containing a laser line. The acquired image sequence is then fused every 4 frames using an image fusion algorithm to obtain 4 stripe patterns with a 2π / 4 phase shift. A typical image fusion algorithm is based on the maximum pixel value. Let there be m frames of source images (m is a multiple of 4). The grayscale value of each frame can be represented as:

[0056] I k (i,j)(k=1,2,3,L,m) (3)

[0057] The grayscale values ​​of the fused stripe pattern can then be represented as:

[0058]

[0059] Applying the same processing to the reference plane also yields four fringe patterns containing a 2π / 4 phase shift.

[0060] 4) The fringe pattern in step 3 is processed using a four-step phase-shifting method. The principal phase value φ(i,j) of the object under test can be obtained according to equation (4).

[0061]

[0062] The solved φ(i,j) has a range of (-π,π), and the image exhibits a jagged, step-like grayscale distribution. However, the deformation of a real object should be continuous, and the corresponding phase change should also be continuous. Therefore, to restore the phase distribution of the enclosed object to a continuous phase distribution, the absolute phase must be obtained. A typical unwrapping algorithm is the global phase unwrapping algorithm based on least squares.

[0063] The relationship between absolute phase and enveloping phase is as follows:

[0064]

[0065] Where k(i,j) are integers, if there is a matrix consisting of M×N data points, x and y correspond to its indices, 0≤i≤M-1, 0≤j≤M-1. Let... The wrap-around phase in the x-direction, The wrap-around phase in the y-direction can be represented as:

[0066]

[0067] Where W represents the wrapping operator, whose purpose is to add or subtract 2π to the partial derivative of the wrapping image, making... and The range of is [-π, π]. The least squares unwrapping method minimizes the difference between the partial derivative of the actual phase and the wrapped phase, i.e.:

[0068]

[0069] Let the above formula be Since the derivative is 0, we can derive the Poisson equation:

[0070]

[0071] in Because the phase differentiation method is invalid for the edges that enclose the phase, the Nenmann boundary conditions for the Poisson equation are:

[0072]

[0073] The Gauss-Seidel iterative method can be used as an iterative algorithm for unwrapping, and its expression is:

[0074]

[0075] Where n is the number of iterations.

[0076] Similarly, the absolute phase of the reference plane can be obtained.

[0077] After obtaining the absolute phase, the protrusion height of the defect can be calculated based on the relationship between the phase and the height.

[0078]

[0079] in The distance between the image acquisition system and the reference plane is l, the distance between the optical axis of the image acquisition system and the optical axis of the line laser scanning module is d, and the period of the fringe pattern is p.

[0080] Preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings:

[0081] Implementation Case 1:

[0082] Figure 2-4 As shown, this invention is used for morphological measurement of a foam board with circular protrusions. According to the measurement method provided by this invention, the measurement process is as follows:

[0083] a) Place the foam board 2.5 meters away from the laser line scanning module, connect and adjust the laser line structured light scanning module to ensure it is ready. Adjust the camera aperture and focus to ensure the object being measured is clearly imaged within the field of view. The object being measured is as follows: Figure 4 As shown;

[0084] b) The control line laser scanning module scans the foam board from bottom to top at a suitable angular velocity. While the line laser scanning module scans the foam board, the camera captures images of the foam board at a fixed frame rate of 10 frames per second, acquiring a total of 96 frames.

[0085] c) After the image acquisition is completed, the shape of the foam board is output using software.

[0086] Implementation Case 2:

[0087] This invention is used to measure a prefabricated wind turbine blade model containing protruding defects. According to the measurement method provided by this invention, the measurement process is as follows:

[0088] a) Fix the blade with three pre-fabricated protrusions onto the simulation system support. Connect and adjust the laser line structured light scanning module to make it ready. Adjust the camera aperture and focus to ensure the object under test is clearly imaged within the field of view;

[0089] b) Start the system to scan and detect the target blade from the root to the tip;

[0090] c) Output the size and location of the detected protrusion defect.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for measuring the surface morphology of large-size structures, characterized in that, Includes the following steps: 1) Fix a line laser on an electric rotary table, then place the line laser scanning module at a suitable distance from the large workpiece, connect and adjust the line laser scanning module so that the normal of the line laser line is located in the plane formed by the laser and the camera. Arrange the camera according to the principle of triangulation, adjust the focal length and exposure time, and obtain a clear image of the laser line on the large workpiece. 2) Start the line laser scanning module and control the line laser scanning module to scan the surface of the large workpiece at a suitable angular velocity, while the camera acquires the scanned images of the workpiece surface at a fixed frame rate. 3) Based on the width of the laser line, the acquired sequence of images is fused every n frames to obtain a result containing... Phase-shifted fringe pattern; 4) Perform phase shift analysis on the fringe pattern obtained in step 3 to obtain the phase pattern of the entire field, and then calculate the three-dimensional shape of the large workpiece based on the relationship between phase and height; In step 2), in order to improve the measurement accuracy, the distance P between two adjacent laser lines in the fused image satisfies twice the laser linewidth; Assume that the camera takes one image every t seconds, and the pitch angle of the laser scanning module is... Let the distance from the line laser scanning module to the object being measured be l, and the angular velocity of the line laser scanning module be ω. If the number of phase shift steps is n, then the distance d between the laser lines in two adjacent frames of the acquired images can be expressed as: (1) Let the linewidth of the laser line be w, because Substituting into equation 1, we can obtain , (2) Because of the pitch angle The distance *l* from the laser line structured light scanning module to the object being measured, the time interval *t* between image acquisitions, and the number of phase shifts *n* are all known quantities. Therefore, it is only necessary to calculate the angular velocity... A stripe image that meets the measurement requirements can be obtained by scanning.

2. The method for measuring the surface morphology of large-size structures according to claim 1, characterized in that, Step 3) specifically involves: After scanning, a sequence of images is obtained, each containing a laser line. The acquired sequence of images is then fused every 4 frames using an image fusion algorithm to obtain 4 images containing a laser line. A fringe pattern indicating phase shift.

3. The method for measuring the surface morphology of large-size structures according to claim 2, characterized in that, The image fusion algorithm is based on the maximum pixel value. Let there be m frames of source images to be fused, where m is a multiple of 4. The grayscale value of each frame can be represented as: (3) The grayscale values ​​of the fused stripe pattern can then be represented as: (4) Applying the same processing to the reference plane can also yield four images containing... Phase-shifted fringe pattern, The fringe pattern in step 3) is processed using a four-step phase-shifting method, and the principal phase value of the object under test can be obtained according to equation (4). (5) The solution The range of values ​​is The image exhibits a jagged, step-like grayscale distribution. However, the deformation of a real object should be continuous, and the corresponding phase change should also be continuous. Therefore, to restore the phase distribution of the object to a continuous phase distribution and obtain the absolute phase, the following steps are required. .

4. The method for measuring the surface morphology of large-size structures according to claim 3, characterized in that, The unwrapping algorithm is a global phase unwrapping algorithm based on least squares. The relationship between absolute phase and enveloping phase is as follows: (6) in If it is an integer, then... A matrix consisting of 10 data points, where x and y correspond to their indices. , ,set up The wrap-around phase in the x-direction, The wrap-around phase in the y-direction can be represented as: (7) Where W represents the wrapping operator, which aims to add or subtract the partial derivatives of the wrapping image. ,make and The range of values ​​is The least squares unwrapping method minimizes the difference between the partial derivative of the actual phase and the wrapped phase, i.e.: (8) Let the above formula be Since the derivative is 0, we can derive the Poisson equation: (9) in Since the phase differentiation method is invalid for the edges that enclose the phase, the Nenmann boundary conditions for the Poisson equation are: (10) The Gauss-Seidel iterative method can be used as an iterative algorithm for unwrapping, and its expression is: (11) Where n is the number of iterations. Similarly, the absolute phase of the reference plane can be obtained. , After obtaining the absolute phase, the protrusion height of the defect can be calculated based on the relationship between the phase and the height. (12) in The distance between the image acquisition system and the reference plane is l, the distance between the optical axis of the image acquisition system and the optical axis of the line laser scanning module is d, and the period of the fringe pattern is p.

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