A dynamic deformation and strain measurement method based on digital speckle interferometry

Abstract

This invention discloses a method for dynamically measuring deformation and strain using digital speckle interferometry, relating to the field of optical detection. First, a speckle interferometry optical path is constructed. Then, the area of ​​the object to be measured is selected. Four speckle interferograms with different phase shifts are recorded before and after deformation using a polarization camera. The recorded speckle interferograms are then subtracted after a four-step phase shift to obtain a deformation-wrapped phase map. After sine and cosine filtering and unwrapping processing of the deformation-wrapped phase map, the true phase map is obtained, thus yielding the deformation and strain of the object. This invention enables real-time dynamic detection of the deformation and strain of the object, while simplifying the measurement optical path, making it a high-precision, full-field, non-destructive measurement method.

Description

Technical Field

[0001] This invention pertains to photoelectric detection technology, and specifically relates to a dynamic strain measurement method based on digital speckle interferometry. Background Technology

[0002] With the continuous development of the defense industry and cutting-edge science and technology, high-precision large-aperture optical components have become key components in aerospace, astronomical observation, inertial confinement fusion, lithography machines, and other major projects. If a large-aperture optical component experiences even a small strain during use, its surface shape will change, deviating from its original design parameters, thus affecting the performance and stability of the entire system. Real-time dynamic detection of the strain of large-aperture optical components is beneficial for adjustments during assembly, avoiding assembly and adjustment deviations.

[0003] Digital speckle interferometry (DII), as a non-contact optical detection technique, not only possesses the general characteristics of optical measurement methods, such as real-time operation, full field of view, non-contact operation, and high sensitivity, but also boasts wide applicability, simple structure, rapid measurement, and intuitive results, leading to its widespread application in aerospace and other fields. DII primarily measures deformation; by calculating its first derivative, the strain of the corresponding object can be determined. After acquiring the speckle interferogram, phase de-phase processing is typically required to obtain a phase map. Following this, phase de-phase calculation is performed to obtain the true phase, thus revealing the object's true deformation and strain. In this process, interferometric systems generally use piezoelectric ceramics (PZT) for time phase shifting or employ multiple cameras and glass slides for spatial phase shifting to calculate the phase. However, because PZT requires time shifting, real-time phase calculation is impossible, compromising the real-time performance of strain detection. Furthermore, spatial phase shifting methods require multiple cameras to acquire images, complicating the device, making it difficult to move, and unsuitable for real-world measurement environments. Summary of the Invention

[0004] This invention proposes a dynamic strain measurement method based on digital speckle interferometry. By constructing a speckle interferometry optical path, a polarization camera is used to record four speckle interferograms with different phase shifts at once. The method employs the concept of polarization phase shifting, combined with four-step phase shifting for phase calculation, and then uses sine and cosine filtering methods for image denoising. Finally, the phase is unwrapped using the minimum cost flow method, and the deformation and strain of the object are obtained. This method is real-time, simple, and can achieve the goal of rapid dynamic measurement.

[0005] The technical solution for achieving this invention is: a dynamic deformation and strain measurement method based on digital speckle interferometry, comprising the following steps:

[0006] Step 1: Construct the speckle interference optical path:

[0007] The speckle interference optical path includes a laser, a first lens, a second lens, a first quarter-glass slide, a beam splitter, a mirror, an attenuator, the object under test, an aperture, a third lens, a fourth lens, a polarizing beam splitter, a second quarter-glass slide, and a polarizing camera.

[0008] The laser, first lens, second lens, first quarter-glass slide, beam splitter, and reflector are arranged sequentially along the first coaxial axis. The polarized light emitted by the laser is at 45° to the fast axis of the first quarter-glass slide. The reflector is at an angle of 20-30° to the first optical axis, and the beam direction is adjusted so that it can illuminate the object under test. The object under test, aperture, third lens, fourth lens, polarizing beam splitter, second quarter-glass slide, and polarizing camera are arranged along the second optical axis, which is parallel to the first optical axis. The attenuator and polarizing beam splitter are located on the reflected light path of the beam splitter. The fast axis of the second quarter-glass slide is at 45° to the polarization direction of the light emitted from the polarizing beam splitter.

[0009] The linearly polarized light emitted from the laser is expanded by the first and second lenses and then enters the first quarter-glass slide, becoming expanded parallel light. The expanded parallel light is split into transmitted light and 90° reflected light by a beam splitter. The transmitted light is reflected by a mirror and shines onto the object under test to form diffuse reflected light. The 90° reflected light passes through an attenuator and enters the polarizing beam splitter, and is then reflected by the polarizing beam splitter to obtain S-light with a vertical polarization state as reference light. The diffused light passes sequentially through an aperture, a third lens, a fourth lens, and the polarizing beam splitter to obtain P-light with a horizontal polarization state as object light. The reference light and object light then pass through the second quarter-glass slide and reach the target surface of the polarizing camera to form a speckle interference pattern.

[0010] Step 2: Select the area S to be measured on the object, and record four speckle interference patterns before deformation using a polarization camera. Then, after the object undergoes deformation, obtain four more speckle interference patterns after deformation using the polarization camera.

[0011] Step 3: After performing a four-step polarization phase shifting process on the four speckle interferograms before and after deformation, the corresponding speckle wrapping phase maps before deformation are obtained. Phase diagram of speckle wrapping after deformation

[0012] Step 4: Wrap the deformed speckle pattern around the phase map. Subtracting the speckle wrapping phase map before deformation Obtain the deformation-wrapped phase map

[0013] Step 5: Perform deformation-wrapped phase mapping. Perform sine and cosine filtering to obtain the filtered phase diagram.

[0014] Step six: Unwrap the filtered phase map to obtain the unwrapped phase map, i.e., the true phase map. Then, perform calculations on the true phase map to obtain the deformation and strain of the measured object.

[0015] Compared with existing technologies, the significant advantages of this invention are as follows: This invention incorporates a polarization phase-shifting method into digital speckle interferometry. Using a polarization camera with a polarization mask, four speckle interferometry images with different phase differences can be directly obtained, and the enclosed phase can be solved in four steps by phase shifting. Compared with the time-shifting method using piezoelectric ceramics, this significantly shortens the phase calculation time and enables real-time dynamic measurement. This invention uses a polarization camera as the tool to achieve the polarization phase-shifting effect. Compared with the method of using multiple cameras for spatial phase shifting, only one camera is needed, which greatly simplifies the optical path design, making the entire measurement optical path more compact, easier to adjust, easier to operate and maintain, and improving the reliability of the measurement system. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the speckle interference optical path of the present invention.

[0017] Figure 2 This is a flowchart of a dynamic deformation and strain measurement method based on digital speckle interferometry according to the present invention.

[0018] In the figure: laser 1, first lens 2, second lens 3, first quarter glass plate 4, beam splitter 5, mirror 6, attenuator 7, object under test 8, aperture 9, third lens 10, fourth lens 11, polarizing beam splitter 12, second quarter glass plate 13, polarizing camera 14. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0021] In this invention, the use of terms such as "first," "second," etc., is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0022] The following section will further introduce the specific implementation method, as well as the technical difficulties and inventive points of this invention, using examples from this design.

[0023] Combination Figure 2 The present invention discloses a dynamic deformation and strain measurement method based on digital speckle interferometry, comprising the following steps:

[0024] Step 1, combined Figure 1 Constructing the speckle interference optical path:

[0025] The speckle interference optical path includes a laser 1, a first lens 2, a second lens 3, a first quarter glass plate 4, a beam splitter 5, a reflector 6, an attenuator 7, an aperture 9, a third lens 10, a fourth lens 11, a polarizing beam splitter 12, a second quarter glass plate 13, and a polarizing camera 14.

[0026] Laser 1, first lens 2, second lens 3, first quarter glass plate 4, beam splitter 5, and reflector 6 are arranged sequentially along the first optical axis. The polarized light emitted by laser 1 forms a 45° angle with the fast axis of the first quarter glass plate 4. The reflector 6 forms a 20-30° angle with the first optical axis to adjust the beam direction so that it can illuminate the object under test 8. The object under test 8, aperture 9, third lens 10, fourth lens 11, polarizing beam splitter 12, second quarter glass plate 13, and polarizing camera 14 are arranged along the second optical axis, which is parallel to the first optical axis. Attenuator 7 and polarizing beam splitter 12 are located on the reflected light path of beam splitter 5. The fast axis of the second quarter glass plate 13 forms a 45° angle with the polarization direction of the light emitted from polarizing beam splitter 12.

[0027] The linearly polarized light emitted from laser 1 is expanded by the first lens 2 and the second lens 3 and then enters the first quarter-glass plate 4, becoming expanded parallel light. The expanded parallel light is split into transmitted light and 90° reflected light by the beam splitter prism 5. The transmitted light is reflected by the reflector 6 and shines on the test object 8 to form diffuse reflected light. The 90° reflected light is reflected by the attenuator 7 and enters the polarization beam splitter prism 12, and is then reflected by the polarization beam splitter prism 12 to obtain S-light with vertical polarization as reference light. The diffused light passes through the aperture 9, the third lens 10, the fourth lens 11, and the polarization beam splitter prism 12 in sequence to obtain P-light with horizontal polarization as object light. The reference light and the object light then pass through the second quarter-glass plate 13 and reach the target surface of the polarization camera 14 to form a speckle interference pattern.

[0028] The beam-expanding system composed of the first lens 2 and the second lens 3, and the imaging system composed of the third lens 10 and the fourth lens 11, have the same numerical aperture.

[0029] The polarization camera 14 is a polarization camera with a pixel-level polarization mask. The polarization mask covers the photodiode of the camera and is arranged in groups of four pixels with polarization angles of 0 degrees, 45 degrees, 90 degrees and 135 degrees, respectively. It can output images with four different polarization angles at the same time.

[0030] The above-mentioned speckle interferogram is formed by polarization interference. After the reference light and the object light pass through the second quarter glass plate 13, they form left-handed circularly polarized light and right-handed circularly polarized light, respectively. The polarization directions of the two beams are orthogonal. After the two orthogonally polarized circularly polarized beams pass through the polarization mask of the polarization camera 14, they interfere on each pixel to form a speckle interferogram. The polarization camera 14 can obtain four speckle interferograms required for phase resolution at one time.

[0031] Step 2: Select the test area S of the test object 8, and record four speckle interference patterns before deformation using the polarization camera 14. Then, after the test object 8 undergoes deformation, obtain four speckle interference patterns after deformation using the polarization camera.

[0032] The light intensity I in the speckle interference patterns before and after deformation in step two above. n It can be obtained from the Stokes vector and the Mueller matrix of the polarization device:

[0033]

[0034] In the formula, S1 is the Stokes matrix of the combined beam of the reference beam and the object beam after passing through the polarizing beam splitter 12, and E p E is the amplitude of the object light. s As a reference light amplitude, This provides phase information for the surface of the object being measured (8).

[0035] The Stokes vector S2 of the beam exiting after passing through the second quarter glass slide 13 is:

[0036]

[0037] In the formula, M Q γ is the Mueller matrix of the second quarter slide 13, γ is the fast axis azimuth angle of the second quarter slide 13, and δ is the retardation of the second quarter slide 13.

[0038] After passing through the polarization mask of polarization camera 14, the Stokes vector S3 of the beam finally received on the target surface of the polarization camera is:

[0039]

[0040] In the formula M P θ is the Mueller matrix of the polarization mask of polarization camera 14, and θ is the transmission angle of the polarization mask.

[0041] Combining the above two equations, according to the definition of the Stokes vector, the intensity I of the light detected by the polarization camera in the speckle interferogram is... n for:

[0042]

[0043]

[0044] Substituting γ = 45° and δ = π / 2 into equation (4), we get:

[0045]

[0046] Substituting θ = 0°, 45°, 90°, and 135° into equation (5), we obtain the light intensity expressions for the four interference patterns:

[0047]

[0048] Step 3: After performing a four-step polarization phase shifting process on the four speckle interferograms before and after deformation, the corresponding speckle wrapping phase maps before deformation are obtained. Phase diagram of speckle wrapping after deformation

[0049] The four-step phase-shifting algorithm described in step three above is used to calculate the speckle envelopment phase map of the test object before deformation.

[0050]

[0051] Similarly, the speckle wrapping phase diagram after deformation is obtained.

[0052] Step 4: Wrap the deformed speckle pattern around the phase map. Subtracting the speckle wrapping phase map before deformation Obtain the deformation-wrapped phase map

[0053] Deformation-wrapped phase diagram

[0054] Step 5: Perform deformation-wrapped phase mapping. Perform sine and cosine filtering to obtain the filtered phase diagram.

[0055] Deformation-wrapped phase diagram Perform multiple sine and cosine transforms, divide the results of the sine and cosine transforms, and then perform an arctangent transform to obtain a smoothed, denoised phase, i.e., the filtered phase diagram. The specific steps are as follows:

[0056]

[0057] In the formula, a and b are the selected filter window sizes, w is the total number of data in a×b, n is the number of filtering iterations, and (x,y) are the coordinates of a point in w.

[0058] Step six: Unwrap the filtered phase map to obtain the unwrapped phase map, i.e. the true phase map. Then, perform calculations on the true phase map to obtain the deformation and strain of the test object 8.

[0059] In step six above, the filtered phase map is unwrapped to obtain the wrapped phase map, which is the true phase map, as follows:

[0060] When calculating the true phase, a reference point (i,j) is selected within the test area S of the object 8. The phase gradient changes in two directions, namely (i,j+1) and (i+1,j), are considered to obtain the true phase. The phase gradient is then wrapped. The relationship between the unwrapped phase gradient Δφ and the gradient residual k is as follows:

[0061]

[0062] The gradient residuals k1 and k2 in the two directions are obtained by the following formula:

[0063]

[0064] In the formula, c1 and c2 are weighting coefficients, and M and N are the maximum number of terms in the region S to be measured in the i and j directions, respectively. When the above formula reaches its minimum value, the gradient residuals k1 and k2 in the two directions are obtained, and the unwrapped phase gradients Δφ1 and Δφ2 in the two directions are calculated. The true phase φ is calculated by the following formula:

[0065]

[0066] Where i' represents the order of unwrapping the phase gradient Δφ1 in the i-direction, and j' represents the order of unwrapping the phase gradient Δφ2 in the j-direction.

[0067] After unpacking in step six above, the true phase φ of the surface deformation of the tested object 8 is obtained. Therefore, the deformation of the tested object 8... strain λ is the wavelength of laser 1, and L is the thickness of the object 8 before deformation.

[0068] In summary, this invention incorporates a polarization phase-shifting method into digital speckle interferometry. Using a polarization camera with a polarization mask, four speckle interferometry images with different phase differences can be directly obtained, and the enclosed phase can be solved through a four-step phase shifting process. Compared with the time-shifting method using piezoelectric ceramics, this significantly shortens the phase calculation time and enables real-time dynamic measurement. This invention uses a polarization camera to achieve the polarization phase-shifting effect. Compared with spatial phase shifting methods using multiple cameras, this invention only requires one camera, greatly simplifying the device, making the system more compact, easier to adjust, easier to maintain, and improving the reliability of the device.

[0069] The above description is merely a specific embodiment of the present invention and is not intended to limit it. Although, with reference to this embodiment, those skilled in the art should readily understand that they can still make equivalent substitutions for some of the technical features in the foregoing solution. These modifications and substitutions do not change the essence of the solution and should all be included within the protection scope of the present invention.

Claims

1. A method for measuring dynamic deformation and strain based on digital speckle interferometry, characterized in that, The steps are as follows: Step 1: Construct the speckle interference optical path: The speckle interference optical path includes a laser (1), a first lens (2), a second lens (3), a first quarter glass plate (4), a beam splitter (5), a mirror (6), an attenuator (7), an object under test (8), an aperture (9), a third lens (10), a fourth lens (11), a polarizing beam splitter (12), a second quarter glass plate (13), and a polarizing camera (14). The laser (1), first lens (2), second lens (3), first quarter glass plate (4), beam splitter (5), and reflector (6) are arranged sequentially along the first coaxial axis; the polarized light emitted by the laser (1) forms a 45° angle with the fast axis of the first quarter glass plate (4); the reflector (6) forms a 20-30° angle with the first optical axis, and the beam direction is adjusted so that it can illuminate the object under test (8); the object under test (8), aperture (9), third lens (10), fourth lens (11), polarizing beam splitter (12), second quarter glass plate (13), and polarizing camera (14) are arranged along the second optical axis, which is parallel to the first optical axis. The attenuator (7) and the polarizing beam splitter (12) are located on the reflected light path of the beam splitter (5); the fast axis of the second quarter glass plate (13) forms a 45° angle with the polarization direction of the light emitted from the polarizing beam splitter (12). The linearly polarized light emitted from the laser (1) is expanded by the first lens (2) and the second lens (3) and then enters the first quarter glass plate (4) to become expanded parallel light. The expanded parallel light is split into transmitted light and 90° reflected light by the beam splitter (5). The transmitted light is reflected by the mirror (6) and shines on the test object (8) to form diffuse reflected light. The 90° reflected light is passed through the attenuator (7) and enters the polarization beam splitter (12), and is then reflected by the polarization beam splitter (12) to obtain S light with vertical polarization state as reference light. The diffuse light passes through the aperture (9), the third lens (10), the fourth lens (11), and the polarization beam splitter (12) in sequence to obtain P light with horizontal polarization state as object light. The reference light and the object light then pass through the second quarter glass plate (13) to reach the target surface of the polarization camera (14) to form a speckle interference pattern. Step 2: Select the area S to be measured of the object (8), and record four speckle interference patterns before deformation using a polarization camera (14). After the object (8) undergoes deformation, obtain four speckle interference patterns after deformation using a polarization camera. Step 3: After performing a four-step polarization phase shifting process on the four speckle interferograms before and after deformation, the corresponding speckle wrapping phase maps before deformation are obtained. Phase diagram of speckle wrapping after deformation Step 4: Wrap the deformed speckle pattern around the phase map. Subtracting the speckle wrapping phase map before deformation Obtain the deformation-wrapped phase map Step 5: Perform deformation-wrapped phase mapping. Perform sine and cosine filtering to obtain the filtered phase diagram; Step 6: Unwrap the filtered phase map to obtain the unwrap phase map, i.e. the true phase map. Then, calculate the deformation and strain of the test object (8) by processing the true phase map.

2. The dynamic deformation and strain measurement method based on digital speckle interferometry as described in claim 1, characterized in that, In step three, after performing a four-step polarization phase shifting process on the four speckle interferograms before and after deformation, the corresponding speckle-wrapped phase maps before and after deformation are obtained, as follows: The speckle interferogram obtained by the polarization camera (14) is denoted as I. n : In the formula, F p E is the amplitude of the object light. s As a reference light amplitude, The phase information of the surface of the object under test (8) is given by θ, which is the transmission angle of the polarization mask of the polarization camera (14). Substituting θ = 0°, 45°, 90°, and 135° into equation (5), we obtain four speckle interference patterns before deformation, denoted as I1, I2, I3, and I4, respectively. Then, using a four-step phase-shifting algorithm, we calculate the speckle-wrapped phase diagram of the object under test (8) before deformation. Similarly, the speckle wrapping phase diagram after deformation is obtained.

3. The dynamic deformation and strain measurement method based on digital speckle interferometry as described in claim 1, characterized in that, In step five, the deformation-wrapped phase diagram is processed. After performing sine and cosine filtering, the filtered phase diagram is obtained as follows: Deformation-wrapped phase diagram Deformation-wrapped phase diagram Perform multiple sine and cosine transforms, divide the results of the sine and cosine transforms, and then perform an arctangent transform to obtain a smoothed, denoised phase, i.e., the filtered phase diagram. The specific steps are as follows: In the formula, a and b are the selected filter window sizes, w is the total number of data in a×b, n is the number of filtering iterations, and (x, y) are the coordinates of a point in w.

4. The dynamic deformation and strain measurement method based on digital speckle interferometry as described in claim 1, characterized in that, In step six, the filtered phase map is unwrapped to obtain the unwrapped phase map, which is the true phase map, as follows: When calculating the true phase, a reference point (i, j) is selected within the test area S of the object (8). The phase gradient changes in two directions, namely (i, j+1) and (i+1, j), are considered to obtain the true phase. The phase gradient is then wrapped. The relationship between the unwrapped phase gradient Δφ and the gradient residual k is as follows: The gradient residuals k1 and k2 in the two directions are obtained by the following formula: In the formula, c1 and c2 are weighting coefficients, and M and N are the maximum number of terms in the region S to be measured in the i and j directions, respectively. When the above formula reaches its minimum value, the gradient residuals k1 and k2 in the two directions are obtained, thereby calculating the unwrapped phase gradients Δφ1 and Δφ2 in the two directions. The true phase φ is calculated by the following formula: Where i′ represents the order of unwrapping the phase gradient Δφ1 in the i direction, and j′ represents the order of unwrapping the phase gradient Δφ2 in the j direction.

5. The dynamic deformation and strain measurement method based on digital speckle interferometry as described in claim 1, characterized in that, Step six: Calculate and process the true phase diagram to obtain the deformation and strain of the tested object (8), as follows: After unpacking, the true phase φ of the surface deformation of the test object (8) is obtained. Then the deformation of the test object (8) is... strain λ is the wavelength of the laser (1), and L is the thickness of the object under test (8) before deformation.

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