A method for measuring the out-of-plane displacement of a curved object based on speckle shearing interferometry

Abstract

The present application relates to a kind of metrology curved surface object displacement methods based on speckle shear interference, steps include:1, the sinusoidal fringe of measured object modulation is obtained and carries out gray processing;2, obtain absolute phase information, obtain the three-dimensional coordinates of object relative to camera, obtain the three-dimensional coordinates of object relative to camera;3, calculate the angle of measured object surface normal vector relative to CCD camera optical axis;4, collect the speckle wrapped phase diagram of measured object surface before and after deformation and carry out gray processing and filtering processing;5, calculate the measured surface displacement information;6, establish the geometric relationship between the displacement measured by laser speckle and the actual displacement of the surface of measured object;7, obtain actual displacement;The present application combines shear speckle interference and digital stripe projection to detect the internal defects of curved surface (inclined surface) object, obtain its displacement information at the same time, analyze its defect attribute, which is beneficial to the evaluation of curved surface object internal defect attribute.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical measurement and detection, and particularly relates to a method for measuring the out-of-plane displacement of a curved object based on speckle shearing interferometry. BACKGROUND

[0002] As an optical measurement technology, laser speckle shearing interferometry has the characteristics of high precision, non-contact, full-field, real-time measurement, etc. Its excellent anti-shock performance makes it widely used in displacement measurement, strain measurement, vibration measurement, deformation measurement, temperature measurement, and internal defect detection of engineering materials. Using laser speckle shearing technology to measure the out-of-plane deformation caused by internal defects is one of the methods for judging the performance of materials.

[0003] Laser speckle shearing interferometry obtains the out-of-plane displacement by subtracting the speckle patterns before and after the deformation of the measured object. The out-of-plane displacement is along the optical axis of the camera. For the out-of-plane displacement measurement of curved (inclined) objects, the deformation direction of each point of the measured object is not necessarily parallel to the optical axis, and the measured out-of-plane displacement is not the actual deformation of each point of the curved surface. Li Xiaodong et al. from Shanghai University constructed three laser measurement paths with different wavelengths, and used a CCD color camera to record the speckle interference images obtained from the three paths to measure the three-dimensional deformation of the object. However, this method has a complex optical path and requires multiple laser sources, making the operation more cumbersome. Sun Yongming et al. from Shandong Normal University combined digital speckle correlation method with electronic speckle interferometry to measure the three-dimensional deformation of the object. This method can only measure the three-dimensional deformation of planar objects. Li Pengfei et al. from Shanghai Jiaotong University used laser speckle deflection method to obtain the three-dimensional morphology of curved objects, and combined the speckle patterns before and after the deformation of the curved surface to obtain the out-of-plane deformation information. However, this method requires high-precision laser deflection instruments and has poor anti-interference performance.

[0004] Therefore, it is of great significance to propose a method for measuring the out-of-plane deformation of curved and inclined surfaces by combining the three-dimensional morphology and out-of-plane deformation with strong anti-interference performance. SUMMARY

[0005] In view of the above problems, the present application provides a method for measuring the out-of-plane displacement of a curved object based on speckle shearing interferometry. The method combines shearing speckle interferometry and digital stripe projection to detect the internal defects of a curved (inclined) object and obtain its out-of-plane displacement information for analyzing the defect properties.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] A method for measuring the out-of-plane displacement of a curved object based on speckle shearing interferometry, comprising the following steps:

[0008] S1, the projector projects a sinusoidal fringe to the surface of the measured object, and the CCD camera collects the sinusoidal fringe pattern modulated by the measured object and performs gray scale processing;

[0009] S2, the wrapped phase of the modulated projection fringe image collected by the CCD camera is extracted by using the phase shifting method, and the absolute phase information is calculated by combining the multi-frequency extrapolation method to obtain the three-dimensional coordinates of the object relative to the camera, and the three-dimensional coordinates of the object relative to the camera are obtained by combining the camera and the calibrated projection system parameters;

[0010] S3, the normal vector of the three-dimensional model in the reconstructed point cloud dataset is estimated by using PCL (Point Cloud Learning), and the angle between the surface normal vector of the measured object and the optical axis of the CCD camera is calculated ;

[0011] S4, the laser projects laser to the surface of the measured object, and the speckle wrapped phase pattern before and after the deformation of the measured object is collected by the CCD camera and subjected to gray scale processing and filtering processing;

[0012] S5, the phase of the speckle wrapped phase pattern collected by the CCD camera is extracted by using the phase shifting method, and the phase unwrapping algorithm is used to expand the phase of the speckle wrapped phase pattern, and the unwrapped phase distribution information is obtained, and the wavelength, shear quantity and other parameters are calculated to obtain the measured surface out-of-plane displacement information (i.e. );

[0013] S6, the geometric relationship between the out-of-plane displacement measurement value (i.e. ) measured by the laser speckle and the actual out-of-plane displacement value of the measured object surface deformation ;

[0014] S7, according to the geometric relationship in S6, the angle between the surface normal vector of the measured object and the optical axis of the CCD camera The out-of-plane displacement information measured by the speckle shear interference is corrected to obtain the actual out-of-plane displacement 。

[0015] As a preferred technical solution:

[0016] A kind of measurement curved surface object out-of-plane displacement method based on speckle shear interference as described above, in step S2, the process of solving absolute phase information is as follows:

[0017] The mathematical expression of the phase fringe pattern collected by the camera is:

[0018]

[0019] In the formula, It is fringe intensity, is the image pixel coordinate, is the background light intensity distribution, For the sake of light, is the phase to be determined, is the phase shift, where .

[0020] Using the phase shift method to find the phase , and we get:

[0021] ;

[0022] The absolute phase is obtained by phase expansion using the multi-frequency extrapolation method. , the relationship between phase distribution and height is:

[0023] ;

[0024] Where: is the height information distribution of the object, is the phase change of the object being measured, is the distance from the camera optical center to the reference plane, is the distance from the projector light source to the camera optical center, is the projection fringe spacing.

[0025] As described above, in the method for measuring the off-plane displacement of curved surface objects based on speckle shearing interferometry, in step S3, the normal vector of the point cloud data set is estimated, and based on the local plane fitting, the covariance matrix of the adjacent elements of any point is created, and the eigenvector corresponding to the minimum eigenvalue is the local plane normal vector. ; For each point , and its corresponding covariance matrix for:

[0026] ;

[0027] ;

[0028] Where, Yes The number of neighboring points, represents the 3D centroid of the nearest neighbor, They are represented as the first eigenvalues ​​and eigenvectors, when the normal direction is toward the camera, satisfy , is the viewpoint of the point cloud dataset.

[0029] In the above-mentioned method for measuring the off-surface displacement of a curved surface object based on speckle shearing interferometry, in step S3, the angle between the surface normal vector of the object being measured and the optical axis of the CCD camera is calculated. :

[0030] ;

[0031] Where, is the surface normal vector of the object being measured, is the unit vector in the direction of the CCD camera optical axis.

[0032] In the above-mentioned method for measuring the off-plane displacement of a curved surface object based on speckle shearing interferometry, in step S5, the phase distribution information is obtained by performing unwrapping calculation on the speckle wrapped phase image. and out-of-plane displacement derivatives ( Directional shear):

[0033]

[0034] Integrate it to solve the surface displacement, that is, the optical path difference ;

[0035] Where, For the The shear amount in the direction is the angle between the CCD camera and the light source, is the laser wavelength.

[0036] As described above, in the method for measuring the out-of-plane displacement of a curved surface object based on speckle shearing interferometry, in step S6, the laser shearing speckle method measures the out-of-plane displacement information by subtracting the phase information before and after the change to obtain the out-of-plane displacement information in the direction of the optical axis (i.e., the optical path difference). When the object undergoes a slight out-of-plane deformation, its direction is along the surface normal vector direction, so the measured optical path difference is The actual deformation The relationship between them is:

[0037] .

[0038] In the above-mentioned method for measuring the off-plane displacement of a curved surface object based on speckle shearing interferometry, in step S5, the process of performing unwrapping calculation on the speckle wrapped phase image is as follows:

[0039] The light intensity expression of the speckle pattern before deformation is: ,

[0040] ;

[0041] Deformed light intensity expression for:

[0042] ;

[0043] Subtract the fringe patterns before and after deformation to obtain four phase wrapping images ,

[0044] ;

[0045] According to the four-part phase shift principle, the phase change before and after deformation is obtained:

[0046] ;

[0047] It needs to be unwrapped to obtain the unfolded phase map and the phase distribution information and out-of-plane displacement derivatives ( The relationship between the shear direction is:

[0048]

[0049] Integrate it to solve the surface displacement, that is, the optical path difference .

[0050] In the above-described method for measuring the off-plane displacement of a curved surface object based on speckle shearing interferometry, in steps S1 and S4, the modulated sinusoidal fringe pattern and the speckle package phase pattern are respectively acquired by the same CCD camera (or two groups of CCD cameras with parallel optical axes). A projector projects sinusoidal fringes onto the surface of the object to be measured. The light path passes through a lens and a shearing interferometer system and is captured by the CCD camera. At this time, the shearing interferometer system is not in an interference state. The computer module processes and obtains three-dimensional information of the object to be measured. The laser array emitted by the laser generator is irradiated onto the surface of the object to be measured after passing through a beam expander. The light path is divided into two beams by the shearing interferometer system to form interference in space. The speckle package phase pattern is captured by the CCD camera.

[0051] The present invention also provides a multiplexed optical path system for implementing the above method, such as Figure 2 As shown, the system includes a beam expander, a projector, a shearing interferometer, a CCD camera, a laser generator, a computer module, a reflector, an adjustment mirror, and a phase shifter. The computer can control the projector and the laser generator to project phase-shifted coded stripes and lasers onto the surface of the object to be measured; the CCD camera is used to capture the image radiated from the surface of the object to be measured, and send it to the computer to generate a modulated image group and sort it. The computer is also used to perform phase expansion on the modulated image to achieve point cloud data reconstruction of the surface of the object to be measured and reconstruction of the out-of-plane deformation of the surface of the object to be measured, and use the point cloud data to obtain the angle between the surface normal vector of the object to be measured and the optical axis to correct the out-of-plane displacement value.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] The present invention combines the three-dimensional measurement of surface structured light and the out-of-plane displacement measurement technology of shear speckle interferometry, controls the adjustment mirror to realize the switching between the surface structured light and shear speckle systems, and can realize the combined measurement of three-dimensional shape and out-of-plane deformation, introduce the normal vector information of the three-dimensional shape into the measurement of out-of-plane displacement, and realize the measurement of out-of-plane displacement information of curved surface objects. The present invention has a simple structure, can realize fast measurement, has strong anti-interference ability, and can realize full-field, non-contact measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 Schematic diagram of the method flow of the present invention;

[0055] Figure 2 Schematic diagram of the device and optical path system in the out-of-plane displacement measurement method of the present invention;

[0056] Figure 3 Schematic diagram of the geometric relationship between the out-of-plane displacement of the measured object when it is deformed and the out-of-plane displacement measured by the CCD camera;

[0057] Figure 4 The fringe grating image is captured by the CCD camera;

[0058] Figure 5 To reconstruct the three-dimensional point cloud of the object being measured;

[0059] Figure 6 The phase image of the speckle wrapper for loading deformation;

[0060] Figure 7 The phase map obtained by unwrapping the speckle wrapped phase map;

[0061] Figure 8 It is the off-plane displacement information measured by laser speckle;

[0062] Figure 9 is the corrected off-plane displacement information;

[0063] In the figure: 1-laser generator, 2-beam expander, 3-surface of the object to be measured, 4-projector, 5-computer module, 6-CCD camera, 7-shearing interferometer, 8-reflecting mirror, 9-adjusting mirror, 10-phase shifter. DETAILED DESCRIPTION

[0064] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0065] A method for measuring out-of-plane displacement of a curved object based on speckle shearing interferometry, as shown in the accompanying drawings, the steps are as follows: Figure 1

[0066] S1, the projector 4 projects grating fringe on the measured object 3, adjusts the adjusting mirror 9 to make the shearing interferometer 7 not in the interference state, the CCD camera 6 collects the sinusoidal fringe diagram modulated by the measured object and carries out gray scale processing;

[0067] S2, the computer module 5 extracts the wrapped phase of the modulated projection fringe image collected by the CCD camera through the phase shift method, and calculates the unwrapped phase by combining the multi-frequency extrapolation method, obtains the absolute phase information, and the computer module 5 combines the camera and the calibrated projection system parameters to obtain the three-dimensional coordinates of the object relative to the camera;

[0068] Wherein, the process of solving the absolute phase information is as follows:

[0069] The mathematical expression of the phase fringe diagram collected by the camera is:

[0070]

[0071] In the formula, I is the fringe light intensity, is the image pixel coordinate, is the background light intensity distribution, is the light intensity regulation, is the phase to be solved, is the phase shift amount, wherein ;

[0072] The phase is solved by using the phase shift method , and the solution is:

[0073] ;

[0074] The absolute phase is obtained by using the multi-frequency extrapolation method to expand the phase , and the relationship between the phase distribution and the height is:

[0075] ;

[0076] In the formula: is the height information distribution of the object, is the phase change amount of the measured object, is the distance from the camera optical center to the reference plane, is the distance from the projector light source to the camera optical center, is the projection fringe spacing;

[0077] ​S3, using PCL (Point Cloud Learning) to estimate the normal vector of the three-dimensional model in the reconstructed point cloud dataset, and calculating the included angle of the surface normal vector of the measured object relative to the optical axis of the CCD camera ;

[0078] The normal vector estimation method is: estimating the normal vector of the point cloud dataset, creating a covariance matrix of the adjacent elements of any point based on local plane fitting, and the eigenvector corresponding to the minimum eigenvalue is the local plane normal vector ; for each point , the covariance matrix corresponding thereto is:

[0079] ;

[0080] ;

[0081] In the formula, is the number of adjacent points of point , represents the three-dimensional centroid of the nearest element, respectively represent the first eigenvalue and eigenvector of the covariance matrix, when the normal direction is towards the camera direction, it satisfies , , is the viewpoint of the point cloud dataset;

[0082] The included angle of the surface normal vector of the measured object relative to the optical axis of the CCD camera is calculated as:

[0083] ;

[0084] In the formula, is the surface normal vector of the measured object, is the unit vector in the direction of the optical axis of the CCD camera;

[0085] S4, the laser generator 1 projects laser to the measured object surface 3, and through applying external load to the measured object, the speckle wrapped phase diagram before and after the deformation of the measured object surface is collected by the CCD camera and subjected to gray processing and filtering processing;

[0086] S5, the computer module 5 extracts the phase of the speckle wrapped phase diagram collected by the CCD camera by using the phase shift method, and uses the least square unwrapping algorithm to unwrap the phase of the speckle wrapped phase diagram, obtains the unwrapped phase distribution information, and substitutes the wavelength, shear quantity and other parameters to calculate the measured surface out-of-plane displacement information (i.e. optical path difference );

[0087] ​Among them, by unwrapping the speckle wrapped phase image,

[0088] The process of unwrapping the speckle wrapped phase image is as follows:

[0089] The light intensity expression of the speckle pattern before deformation is: ,

[0090] ;

[0091] Deformed light intensity expression for:

[0092] ;

[0093] Subtract the fringe patterns before and after deformation to obtain four phase wrapping images ,

[0094] ;

[0095] According to the four-part phase shift principle, the phase change before and after deformation is obtained:

[0096] ;

[0097] It needs to be unwrapped to obtain the unfolded phase map and the phase distribution information and out-of-plane displacement derivatives ( The relationship between the shear direction is:

[0098]

[0099] Integrate it to solve the surface displacement, that is, the optical path difference ;

[0100] Where, For the The shear amount in the direction is the angle between the CCD camera and the light source, is the laser wavelength;

[0101] S6. Establish the off-plane displacement measurement value (i.e., optical path difference) measured by laser speckle ) and the out-of-plane displacement of the actual deformation of the surface of the measured object The geometric relationship between them;

[0102] Among them, the laser shearing speckle method measures the out-of-plane displacement information by subtracting the phase information before and after the change to obtain the out-of-plane displacement information in the direction of the optical axis (i.e., optical path difference). When the object undergoes a small out-of-plane deformation, its direction is along the surface normal vector direction, so the measured optical path difference is The actual deformation The relationship between them is:

[0103] ;

[0104] S7, according to the geometric relationship in S6, the angle between the measured object surface normal vector and the CCD camera optical axis is The off-plane displacement information measured by the speckle shearing interferometer is corrected to obtain the corrected off-plane displacement

[0105] In steps S1 and S4, the modulated sinusoidal fringe pattern and the speckle wrapped phase pattern are obtained by the same CCD camera (or two sets of CCD cameras with parallel optical axes) respectively. The projector projects the sinusoidal fringe onto the surface of the measured object. The light path is collected by the CCD camera via the lens and the shearing interference system. At this time, the shearing interference system is not in the interference state. The computer module processes to obtain the three-dimensional information of the measured object. The laser array emitted by the laser generator is irradiated on the surface of the measured object after the beam expander. The light path is divided into two beams in space to form interference by the shearing interference system. The speckle wrapped phase pattern is collected by the CCD camera.

[0106] Although the embodiments of the present application are disclosed as above, the content described is only for the purpose of facilitating understanding of the present application, and is not intended to limit the present application. Any person skilled in the art of the present application can make any modification and change in the form and details without departing from the spirit and scope of the present application. The patent protection scope of the present application shall be subject to the scope defined by the appended claims.

Claims

1. A method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry, characterized in that: The steps include: Step S1: A projector projects sinusoidal fringes onto the surface of the object being measured, and a CCD camera captures the sinusoidal fringes modulated by the object being measured and performs grayscale processing; Step S2: extracting the wrapped phase from the modulated projected fringe image captured by the CCD camera using a phase shift method, and performing unwrapping calculations in combination with a multi-frequency extrapolation method to obtain absolute phase information and the three-dimensional coordinates of the object relative to the camera; Step S3: Use PCL to estimate the normal vector of the 3D model in the reconstructed point cloud dataset and calculate the angle between the normal vector of the surface of the object being measured and the optical axis of the CCD camera. ; Step S4: The laser projects laser light onto the surface of the object to be measured. By applying an external load to the object to be measured, the CCD camera collects speckle wrapping phase images of the surface of the object to be measured before and after deformation and performs grayscale processing and filtering. Step S5: extracting the phase of the speckle wrapped phase image acquired by the CCD camera using the phase shift method, and performing phase unwrapping on the speckle wrapped phase image using the least squares unwrapping algorithm to obtain the unwrapped phase distribution information, substituting the wavelength, shear amount and other parameters into the calculated surface out-of-plane displacement information; Step S6: Establish the off-plane displacement measured by the laser speckle pattern and the off-plane displacement of the actual deformation of the surface of the measured object. The geometric relationship between them; Step S7: Based on the geometric relationship in S6, the angle between the surface normal vector of the object being measured and the optical axis of the CCD camera is used. Correct the out-of-plane displacement measured by speckle shearing interferometry to obtain the actual out-of-plane displacement .

2. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 1, characterized in that: In step S2, the process of obtaining the absolute phase information is as follows: The mathematical expression of the phase fringe pattern collected by the camera is: Where, is the fringe light intensity, is the image pixel coordinate, is the background light intensity distribution, For the sake of light, is the phase to be determined, is the phase shift, where ; Using the phase shift method to find the phase , and we get: ； The absolute phase is obtained by phase expansion using the multi-frequency extrapolation method. , the relationship between phase distribution and height is: ； Where: is the height information distribution of the object, is the phase change of the object being measured, is the distance from the camera optical center to the reference plane, is the distance from the projector light source to the camera optical center, is the projection fringe spacing.

3. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 2, characterized in that: In step S3, the normal vector of the point cloud dataset is estimated. Based on the local plane fitting, the covariance matrix of the neighboring elements of any point is created, and the eigenvector corresponding to the minimum eigenvalue is the local plane normal vector. ; For each point , and its corresponding covariance matrix for: ； ； Where, Yes The number of neighboring points, represents the 3D centroid of the nearest neighbor, They are represented as the first eigenvalues ​​and eigenvectors, when the normal direction is toward the camera, satisfy , is the viewpoint of the point cloud dataset.

4. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 3, characterized in that: In step S3, the angle between the surface normal vector of the object being measured and the optical axis of the CCD camera is calculated. : ； Where, is the surface normal vector of the object being measured, is the unit vector in the direction of the CCD camera optical axis.

5. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 1, characterized in that: In step S5, the phase distribution information is obtained by performing unwrapping calculation on the speckle wrapped phase image. and out-of-plane displacement derivatives exist Relationship under directional shear: Integrate it to solve the surface displacement, that is, the optical path difference ; Where, For the The shear amount in the direction is the angle between the CCD camera and the light source, is the laser wavelength.

6. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 1, characterized in that: In step S6, the laser shearing speckle method measures the out-of-plane displacement information by subtracting the phase information before and after the change to obtain the out-of-plane displacement information in the direction of the optical axis. When the object undergoes a small out-of-plane deformation, its direction is along the surface normal vector direction, so the measured optical path difference is The actual deformation The relationship between them is: 。 7. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 5, characterized in that: In step S5, the process of performing unwrapping calculation on the speckle wrapped phase image is as follows: The light intensity expression of the speckle pattern before deformation is: , ； Deformed light intensity expression for: ； Subtract the fringe patterns before and after deformation to obtain four phase wrapping images , ； According to the four-part phase shift principle, the phase change before and after deformation is obtained: ； It needs to be unwrapped to obtain the unfolded phase map and the phase distribution information and out-of-plane displacement derivatives exist The relationship under directional shear is: Integrate it to solve the surface displacement, that is, the optical path difference .

8. The method for measuring the off-surface displacement of curved surface objects based on speckle shearing interferometry according to claim 1, characterized in that: In steps S1 and S4, the modulated sinusoidal fringe pattern and the speckle wrapping phase image are respectively acquired by the same CCD camera. A projector projects sinusoidal fringes onto the surface of the object under test. The light path passes through a lens and a shearing interferometer system and is captured by the CCD camera. At this time, the shearing interferometer system is not in an interference state. The computer module processes and obtains three-dimensional information of the object under test. The laser array emitted by the laser generator passes through a beam expander and irradiates the surface of the object under test. The light path is split into two beams by the shearing interferometer system, forming interference in space. The speckle wrapping phase image is acquired by the CCD camera.

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