Dynamic deformation and strain measurement method based on digital speckle interference

Abstract

The invention discloses a method for dynamically measuring deformation and strain through digital speckle interference, and relates to the field of optical detection. The method comprises the following steps: firstly, building a speckle interference light path, secondly, selecting a to-be-detected area of a detected object, recording four speckle interference patterns with different phase shifts before and after deformation through a polarization camera, performing four-step phase shift on the recorded speckle interference patterns, and then performing subtraction to obtain a deformation wrapped phase diagram; and carrying out sine and cosine filtering and unwrapping processing on the deformation wrapped phase diagram to obtain a real phase diagram, thereby obtaining the deformation and strain of the measured object. According to the invention, the deformation and strain of the measured object can be dynamically detected in real time, the measurement light path is simplified, and the method is a high-precision full-field nondestructive measurement method.

Description

Technical Field

[0001] The invention belongs to the technology of photoelectric detection, and in particular relates to a dynamic strain measurement method based on digital speckle interferometry technology. Background Art

[0002] With the continuous development of the national 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 and other fields and major projects. If a large-aperture optical component has a small strain during use, its surface shape will change and deviate from the 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 conducive to adjustment during assembly and avoids installation deviation.

[0003] As a non-contact optical detection technology, digital speckle interferometry not only has the general characteristics of optical measurement, such as real-time, full field of view, non-contact and high sensitivity, but also has wide applicability, simple structure, rapid measurement, and intuitive results. It has been widely used in aerospace and other fields. Digital speckle interferometry is mainly used for deformation measurement. The strain of the corresponding object can be calculated by calculating its first-order derivative. After obtaining the speckle interferogram, it is usually necessary to perform phase deconvolution on the speckle interferogram to obtain the phase diagram, and then perform phase unwrapping calculation to obtain the real phase, thereby obtaining the real deformation and strain of the object. In this process, the general interference system uses piezoelectric ceramics (PZT) to perform time phase shift or uses multiple cameras and glass slides for spatial phase shift, and then solves the phase. However, because PZT requires time movement, it is impossible to solve the phase in real time and the real-time nature of strain detection cannot be guaranteed. The spatial phase shift method requires the use of multiple cameras to collect images, which makes the device complicated and difficult to move, and cannot be used in a real measurement environment. Summary of the invention

[0004] The present invention proposes a dynamic strain measurement method based on digital speckle interferometry technology. By building a speckle interferometry optical path, a polarization camera is used to record four speckle interferograms with different phase shifts at one time. The idea of ​​polarization phase shifting is adopted, and phase solution is performed in combination with four-step phase shifting. Then, a sine-cosine filtering method is used for image denoising. The minimum cost flow method is used to unwrap the phase, and finally the deformation and strain of the object are obtained. The measurement is real-time, the method is simple, and the purpose of rapid dynamic measurement can be achieved.

[0005] The technical solution to realize the present invention is: a dynamic deformation and strain measurement method based on digital speckle interferometry, the steps are as follows:

[0006] Step 1: Build the speckle interferometer optical path:

[0007] The speckle interference optical path includes a laser, a first lens, a second lens, a first quarter glass, a beam splitter prism, a reflector, an attenuation plate, an object to be measured, an aperture, a third lens, a fourth lens, a polarization beam splitter prism, a second quarter glass, and a polarization camera.

[0008] The laser, the first lens, the second lens, the first quarter glass, the beam splitter prism and the reflector are arranged in sequence along the first coaxial line; the polarized light emitted by the laser forms an angle of 45° with the fast axis of the first quarter glass; the reflector forms an angle of 20-30° with the first optical axis, and the direction of the light beam is adjusted so that it can irradiate the object to be measured; the object to be measured, the aperture, the third lens, the fourth lens, the polarization beam splitter prism, the second quarter glass and the polarization camera are arranged along the second optical axis, and the second optical axis is parallel to the first optical axis, wherein the attenuation plate and the polarization beam splitter prism are located on the reflected light path of the beam splitter prism; the fast axis of the second quarter glass forms an angle of 45° with the polarization direction of the light emitted from the polarization beam splitter prism.

[0009] The linear polarized light emitted by the laser is expanded by the first lens and the second lens and enters the first quarter glass slide to become expanded parallel light; the expanded parallel light is divided into transmitted light and 90° reflected light by the beam splitter prism; the transmitted light is irradiated onto the object to be measured through the reflector to form diffuse reflected light; the 90° reflected light enters the polarization beam splitter prism through the attenuation plate, and is then reflected by the polarization beam splitter prism to obtain S light with a vertical polarization state as the reference light; the diffuse light passes through the aperture, the third lens, the fourth lens, and the polarization beam splitter prism in sequence to obtain P light with a horizontal polarization state as the object light; the reference light and the object light then pass through the second quarter glass slide to reach the target surface of the polarization camera to form a speckle interference pattern.

[0010] Step 2: Select the area S to be tested of the object to be tested, record four speckle interferograms before deformation by a polarization camera, and then obtain four speckle interferograms after deformation by the polarization camera again after the object to be tested is deformed.

[0011] Step 3: After performing polarization four-step phase shift processing on the four speckle interferograms before deformation and the four speckle interferograms after deformation, the corresponding speckle wrapped phase images before deformation are obtained. and the phase image of the speckle wrapping after deformation

[0012] Step 4: Wrap the deformed speckle in the phase image Speckle wrapping phase image before deformation subtraction Obtaining deformation wrapping phase image

[0013] Step 5: Wrap the deformation in the phase map Perform sine and cosine filtering to obtain the filtered phase diagram.

[0014] Step six, unpacking the filtered phase image to obtain an unpacked phase image, that is, a real phase image, and then calculating and processing the real phase image to obtain the deformation and strain of the object under test.

[0015] Compared with the prior art, the present invention has the following significant advantages: the present invention adds a polarization phase shifting method to the digital speckle interferometry, and uses a polarization camera including a polarization mask to directly obtain four speckle interference images with different phase differences, and performs four-step phase shifting to resolve the wrapped phase. Compared with the time phase shifting method using piezoelectric ceramic phase shifting, the phase resolution time is greatly shortened, and real-time dynamic measurement can be achieved; the present invention uses a polarization camera as a 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, makes the entire measurement optical path more compact, easy to adjust, convenient to operate and maintain, and improves the reliability of the measurement system. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0017] Figure 2 The present invention is a flow chart of a dynamic deformation and strain measurement method based on digital speckle interferometry.

[0018] In the figure: laser 1, first lens 2, second lens 3, first quarter glass 4, beam splitter prism 5, reflector 6, attenuation plate 7, object to be measured 8, aperture 9, third lens 10, fourth lens 11, polarization beam splitter prism 12, second quarter glass 13, polarization camera 14. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

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

[0021] In the present invention, the descriptions of "first", "second", etc. are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0022] The following will further introduce the specific implementation method, as well as the technical difficulties and inventive points of this invention in combination with this design example.

[0023] Combination Figure 2 The method for measuring dynamic deformation and strain based on digital speckle interferometry described in the present invention comprises the following steps:

[0024] Step 1: Combine Figure 1 , build the speckle interferometer optical path:

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

[0026] The laser 1, the first lens 2, the second lens 3, the first quarter glass 4, the beam splitter 5, and the reflector 6 are arranged in sequence along the first coaxial line; the polarized light emitted by the laser 1 is 45° to the fast axis of the first quarter glass 4; the reflector 6 is at an angle of 20-30° to the first optical axis, and the direction of the light beam is adjusted so that it can irradiate the object to be measured 8; the object to be measured 8, the aperture 9, the third lens 10, the fourth lens 11, the polarization beam splitter prism 12, the second quarter glass 13, and the polarization camera 14 are arranged along the second optical axis, and the second optical axis is parallel to the first optical axis, wherein the attenuation plate 7 and the polarization beam splitter prism 12 are located on the reflected light path of the beam splitter prism 5; the fast axis of the second quarter glass 13 is 45° to the polarization direction of the light emitted from the polarization beam splitter prism 12.

[0027] The linear polarized light emitted by the laser 1 is expanded by the first lens 2 and the second lens 3 and then enters the first quarter glass 4 to become expanded parallel light; the expanded parallel light is divided into transmitted light and 90° reflected light by the beam splitter prism 5; the transmitted light is irradiated onto the object 8 to form diffuse reflected light through the reflector 6; the 90° reflected light enters the polarization beam splitter prism 12 through the attenuation plate 7, and is then reflected by the polarization beam splitter prism 12 to obtain S light with a vertical polarization state as the reference light; the diffuse 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 a horizontal polarization state as the object light; the reference light and the object light then pass through the second quarter glass 13 to reach the target surface of the polarization camera 14 to form a speckle interference pattern.

[0028] The numerical apertures of the beam expansion 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 are consistent.

[0029] The polarization camera 14 is a polarization camera with a pixel-level polarization mask. The polarization mask covers the photodiode of the camera. Four pixels are grouped together, and the polarization angles are 0 degrees, 45 degrees, 90 degrees, and 135 degrees, respectively. Images with four different polarization angles can be output simultaneously.

[0030] The formation principle of the above-mentioned speckle interference pattern is polarization interference. After the reference light and the object light pass through the second quarter glass 13, they form left-handed circularly polarized light and right-handed circularly polarized light respectively. The polarization directions of the two light beams are orthogonal. After passing through the polarization mask of the polarization camera 14, the two circularly polarized lights with orthogonal polarization directions interfere at each pixel to form a speckle interference pattern. The polarization camera 14 can obtain four speckle interference patterns required for phase solution at one time.

[0031] Step 2: Select the test area S of the object 8 to be tested, record four speckle interferograms before deformation through the polarization camera 14, and then obtain four speckle interferograms after deformation through the polarization camera again after the object 8 to be tested is deformed.

[0032] The light intensity I of the speckle interference pattern before and after deformation in the above step 2 n It can be obtained by normalizing the Stokes vector and the Mueller matrix of the polarization device:

[0033]

[0034] Where S1 is the Stokes matrix of the combined light after the reference light and the object light pass through the polarization beam splitter prism 12, and E p is the amplitude of the object light, E s is the amplitude of the reference light, is the phase information of the surface of the object 8 under test.

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

[0036]

[0037] Where M Q is the Mueller matrix of the second quarter glass 13 , γ is the fast axis azimuth of the second quarter glass 13 , and δ is the delay amount of the second quarter glass 13 .

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

[0039]

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

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

[0043]

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

[0045]

[0046] Substituting θ = 0°, 45°, 90°, 135° into equation (5), we can get the intensity expressions of the four interference patterns:

[0047]

[0048] Step 3: After performing polarization four-step phase shift processing on the four speckle interferograms before deformation and the four speckle interferograms after deformation, the corresponding speckle wrapped phase images before deformation are obtained. and the phase image of the speckle wrapping after deformation

[0049] The four-step phase shift algorithm described in step 3 above is used to calculate the speckle wrapping phase image of the object 8 before deformation.

[0050]

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

[0052] Step 4: Wrap the deformed speckle in the phase image Speckle wrapping phase image before deformation subtraction Obtaining deformation wrapping phase image

[0053] Deformation Wrapping Phase Map

[0054] Step 5: Wrap the deformation in the phase map Perform sine and cosine filtering to obtain the filtered phase diagram.

[0055] Deformation Wrapping Phase Image Perform multiple sine and cosine transforms, divide the sine and cosine transform results, and then perform inverse tangent transform to obtain a smooth denoised phase, that is, the filtered phase map The specific steps are as follows:

[0056]

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

[0058] Step six, unpacking the filtered phase image to obtain an unpacked phase image, that is, a real phase image, and then calculating and processing the real phase image to obtain the deformation and strain of the object 8 under test.

[0059] In the above step 6, the filtered phase image is unwrapped to obtain a wrapped phase image, that is, a real phase image, as follows:

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

[0061]

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

[0063]

[0064] Where c1 and c2 are weighting coefficients, M and N are the maximum number of terms in the measured area S in the i and j directions respectively. When the above formula takes the minimum value, the gradient residuals k1 and k2 in the two directions are solved, thereby calculating the unwrapped phase gradients Δφ1 and Δφ2 in the two directions. The true phase φ is calculated by the following formula:

[0065]

[0066] Wherein, i' is the order of the unwrapped phase gradient Δφ1 in the i direction, and j' is the order of the unwrapped phase gradient Δφ2 in the j direction.

[0067] After the unwrapping process in step 6, the true phase φ of the surface deformation of the object 8 is obtained. strain λ is the wavelength of the laser 1 , and L is the thickness of the object 8 before deformation.

[0068] In summary, the present invention adds a polarization phase shifting method to digital speckle interferometry. A polarization camera including a polarization mask can be used to directly obtain four speckle interference images with different phase differences, and four-step phase shifting is performed to solve the wrapped phase. Compared with the time phase shifting method using piezoelectric ceramic phase shifting, the phase solution time is greatly shortened, and real-time dynamic measurement can be achieved. The present invention uses a polarization camera 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 device, makes the system more compact, easy to adjust, and convenient to maintain, and improves the reliability of the device.

[0069] The above is only a specific implementation scheme of the present invention, and not a limitation thereof. Although referring to this implementation scheme, ordinary researchers in the field should be able to easily understand that they can still make equivalent substitutions for some technical features in the aforementioned scheme. These modifications and substitutions do not change the essence of the scheme and should be included in the protection scope of the present invention.

Claims

1. A dynamic deformation and strain measurement method based on digital speckle interferometry, characterized in that: Here are the steps: Step 1: Build the speckle interferometer optical path: The speckle interference optical path comprises a laser (1), a first lens (2), a second lens (3), a first quarter glass (4), a beam splitter prism (5), a reflector (6), an attenuation plate (7), a measured object (8), an aperture (9), a third lens (10), a fourth lens (11), a polarization beam splitter prism (12), a second quarter glass (13), and a polarization camera (14); The laser (1), the first lens (2), the second lens (3), the first quarter glass (4), the beam splitter (5), and the reflector (6) are sequentially arranged along a first coaxial line; the polarized light emitted by the laser (1) is at an angle of 45° to the fast axis of the first quarter glass (4); the reflector (6) is at an angle of 20-30° to the first optical axis, and the direction of the light beam is adjusted so that it can illuminate the object to be measured (8); the object to be measured (8), the aperture (9), the third lens (10), the fourth lens (11), the polarization beam splitter (12), the second quarter glass (13), and the polarization camera (14) are arranged along a second optical axis, and the second optical axis is parallel to the first optical axis, wherein the attenuation plate (7) and the polarization beam splitter (12) are located on the reflected light path of the beam splitter (5); the fast axis of the second quarter glass (13) is at an angle of 45° to the polarization direction of the light emitted by the polarization beam splitter (12); The linear polarized light emitted by the laser (1) is expanded by the first lens (2) and the second lens (3) and then enters the first quarter glass (4) to become expanded parallel light; the expanded parallel light is divided into transmitted light and 90° reflected light by the beam splitter prism (5); the transmitted light is irradiated onto the object to be measured (8) through the reflector (6) to form diffuse reflected light; the 90° reflected light enters the polarization beam splitter prism (12) through the attenuation plate (7) and is then reflected by the polarization beam splitter prism (12) to obtain S light with a vertical polarization state as reference light; the diffuse light sequentially passes through the aperture (9), the third lens (10), the fourth lens (11) and the polarization beam splitter prism (12) to obtain P light with a horizontal polarization state as object light; the reference light and the object light then pass through the second quarter glass (13) to reach the target surface of the polarization camera (14) to form a speckle interference pattern; Step 2, selecting a test area S of the test object (8), recording four speckle interferograms before deformation by using a polarization camera (14), and then obtaining four speckle interferograms after deformation by using the polarization camera again after the test object (8) is deformed; Step 3: After performing polarization four-step phase shift processing on the four speckle interferograms before deformation and the four speckle interferograms after deformation, the corresponding speckle wrapped phase images before deformation are obtained. and the phase image of the speckle wrapping after deformation Step 4: Wrap the deformed speckle in the phase image Speckle wrapping phase image before deformation subtraction Obtaining deformation wrapping phase image Step 5: Wrap the deformation in the phase map Perform sine and cosine filtering to obtain the filtered phase diagram; Step six, performing unwrapping processing on the filtered phase image to obtain an unwrapped phase image, that is, a real phase image, and then performing calculation processing on the real phase image to obtain the deformation and strain of the object under test (8).

2. The dynamic deformation and strain measurement method based on digital speckle interferometry according to claim 1, characterized in that: In step 3, after the polarization four-step phase shifting process is performed on the four speckle interferograms before deformation and the four speckle interferograms after deformation, the speckle wrapped phase diagrams before deformation and the speckle wrapped phase diagrams after deformation are obtained respectively, as follows: The speckle interference pattern obtained by the polarization camera (14) is denoted as I n : In the formula, F p is the amplitude of the object light, E s is the amplitude of the reference light, is the phase information of the surface of the object (8) being measured, and θ is the light transmission angle of the polarization mask of the polarization camera (14); Substituting θ = 0°, 45°, 90°, 135° into equation (5), the four speckle interference patterns before deformation are recorded as I1, I2, I3, and I4 respectively. Then, the speckle wrapping phase pattern of the object under test (8) before deformation is solved by the four-step phase shift algorithm. Similarly, the speckle wrapping phase image after deformation is obtained 3. The dynamic deformation and strain measurement method based on digital speckle interferometry according to claim 1, characterized in that: In step 5, the deformation is wrapped in the phase map Perform sine and cosine filtering to obtain the filtered phase diagram, as follows: Deformation Wrapping Phase Map Deformation Wrapping Phase Image Perform multiple sine and cosine transforms, divide the sine and cosine transform results, and then perform inverse tangent transform to obtain a smooth denoised phase, that is, the filtered phase map Specific steps: Where a and b are the selected filter window sizes, w is the total number of data in a×b, n is the number of filters, and (x, y) is the coordinates of the point in w.

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

5. The method for dynamic deformation and strain measurement based on digital speckle interferometry according to claim 1, characterized in that: Step 6, calculating and processing the real phase image to obtain the deformation and strain of the object (8) to be measured, as follows: After the unwrapping process, the true phase φ of the surface deformation of the object (8) is obtained. Then, the deformation of the object (8) is strain λ is the wavelength of the laser (1), and L is the thickness of the object (8) before deformation.

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