Photoelastic effect measuring device and measuring method

Abstract

The invention discloses a photoelastic effect measuring device and a photoelastic effect measuring method, particularly relates to the technical field of optical measurement, and solves the technical problems that a traditional method in the prior art cannot record the change of a refractive index in a stretching process in real time, lacks dynamic analysis capability and is difficult to quantify the photoelastic effect. According to the technical scheme, the refractive indexes corresponding to different thicknesses after the PET plastic film is uniformly stretched are quantitatively measured by utilizing the high sensitivity advantage and the unique interference phenomenon of a Michelson interferometer, a refractive index-thickness curve is fitted by utilizing a least square method, an experimental phenomenon is presented through a visual interface, and the measurement accuracy is improved. Automatic data processing and uncertainty analysis are carried out, and a user is helped to deeply understand the photoelastic effect principle, namely the birefringence phenomenon of the photoelastic effect; according to the invention, automatic, quantitative, high-precision and visual measurement can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical measurement technology, in particular to a photoelastic effect measuring device and a measuring method. BACKGROUND

[0002] As an important phenomenon in the fields of material mechanics, aerospace, construction and industry, the visualization and quantitative analysis of the birefringence characteristics of the photoelastic effect are of great significance to teaching and research. However, there are many defects in the traditional Michelson interferometer measurement of the refractive index of transparent thin films. First, using white light as the light source for the experiment, since the white light has poor coherence (the coherence length is only microns), the moving mirror needs to be precisely adjusted to the equal optical path position to observe the color fringes, which not only leads to low success rate of operation, but also puts high requirements on the operator. Second, this method relies on the naked eye to judge the critical state of interference, which makes the measurement repeatability poor and the measurement not rigorous. At the same time, the process of manually measuring the film thickness and mirror displacement also introduces reading errors. In addition, the traditional method cannot record the change of the refractive index in real time during the stretching process, and lacks dynamic analysis capability, making it difficult to quantify the photoelastic effect. Moreover, the calculation and error analysis of the refractive index need to be done manually, and the data processing is very tedious. Finally, the abstract physical phenomenon lacks visual interaction, which makes it difficult for students to understand in the teaching process, and thus causes limitations in teaching. SUMMARY

[0003] Therefore, the present application solves the technical problems of the prior art that the Michelson interferometer measurement is difficult to operate, relies on subjective judgment, has low measurement accuracy, low efficiency and lacks dynamic visual analysis capability. The photoelastic effect measuring device and measuring method provided by the present application organically combines high coherence light source, machine vision, automatic control and real-time data processing technology to realize automatic, quantitative, high-precision and visual measurement of the photoelastic effect of transparent samples.

[0004] The present application provides a photoelastic effect measuring device, comprising a Michelson interferometer, the interferometer comprising a movable mirror; a high coherence monochromatic light source configured to produce an interference pattern via the Michelson interferometer; an image capture device arranged at a position to capture the interference pattern; a processor in communication with the image capture device, and the processor is configured to:

[0005] processing image data from the image capture device to identify a predefined geometric interference pattern composed of a plurality of substantially circular and mutually tangent fringes; generating an output signal when the captured interference pattern matches the predefined geometric interference pattern; and calculating a refractive index n of the sample based on a first position at which the predefined geometric interference pattern is identified when no stress is applied to the sample and a second position at which the predefined geometric interference pattern is re-identified after stress is applied to the sample, wherein the calculation of the refractive index n is based on the physical principle that the displacement of the movable mirror determined by the first and second positions, Δd, compensates for the optical path difference of 2t(n-1) introduced by the sample with thickness t due to the change in refractive index.

[0006] Further, the high-coherence monochromatic light source is a helium-neon laser. The high monochromaticity and high coherence (coherence length can reach more than 100 meters) of the laser are the premise of stably generating a clear and large-field interference pattern, which fundamentally solves the operation difficulty problem brought by the white light source.

[0007] Further, transparent samples are provided with clamps on both sides for clamping the samples, and the bottom of each clamp is provided with a support rod, wherein the bottom of one support rod is provided with an automatic numerical control module linear guide rail; the guide rail can control the movement of the support rod, accurately control the elongation of the sample, ensure the uniformity and repeatability of stress application, and eliminate the uncertainty introduced by manual operation.

[0008] Further, a high-precision electronic measuring device (such as an electronic digital display micrometer) is further included and is in communication connection with the processor, and is used to measure the thickness t of the transparent sample. The thickness data can be automatically read or conveniently input into the system, avoiding the error of manual reading.

[0009] Further, the processor is further configured to generate and display a refractive index-thickness relationship curve in real time according to the refractive indexes calculated by multiple measurements and the corresponding sample thicknesses. This function converts discrete measurement data points into intuitive charts, clearly reveals the inherent law of the photoelastic effect, and greatly enhances the analysis capability and teaching value of the device.

[0010] The application also provides a measurement method based on the above-mentioned measuring device, comprising the following steps:

[0011] Emitting light from a high-coherence monochromatic light source and passing it through a Michelson interferometer including a movable mirror to generate an interference pattern;

[0012] The interferometer is adjusted to form a predefined geometric pattern consisting of multiple approximately circular and mutually tangent fringes, which serves as the reference state for measurement.

[0013] The image of the predefined geometric pattern is captured by an image capture device, and its formation is confirmed by an algorithm run by a processor. The first position of the movable reflector at this time is recorded.

[0014] A controllable mechanical stress is applied to the transparent sample placed in the optical path of the interferometer to stretch it; the movable mirror is readjusted until the predefined geometric pattern is confirmed to be restored again by the algorithm, and the second position of the movable mirror at this time is recorded; and the refractive index n of the sample is calculated based on the first position, the second position and the thickness t of the transparent sample; wherein the calculation step is based on the following physical principle: the displacement Δd of the movable mirror determined by the first position and the second position compensates for the optical path difference of 2t(n-1) introduced by the change in refractive index of the sample with thickness t.

[0015] The technical effects and advantages provided by the present invention in the above technical solution are as follows:

[0016] 1. This invention provides a photoelastic effect measurement device and method that minimizes multiple major error sources (subjective judgment error, reading error, and operational error) by employing a highly coherent laser light source, machine vision for objective judgment of critical states, and a high-precision automated measurement module. Experimental data shows that the repeatability error of refractive index measurement can be reduced from 0.005 in traditional methods to 0.001 or less, improving accuracy by at least 5 times.

[0017] 2. The photoelastic effect measurement device and method provided by this invention achieve semi-automation or full automation of the entire measurement process, with data processing completed in real time. The time for a single complete measurement is significantly reduced, and efficiency is improved.

[0018] 3. The photoelastic effect measurement device and method provided by this invention automatically complete the complex critical state judgment by computer. Users only need to perform simple operations according to the clear instructions given by the system (such as LED light signals). This greatly reduces the requirements for the professional skills and experience of the operator, enabling ordinary students and technicians to easily obtain reliable measurement results.

[0019] 4. The photoelastic effect measurement device and method provided by the present invention can quickly perform multiple measurements and plot the relationship curve between refractive index and thickness (stress) in real time, so as to dynamically and intuitively present the abstract photoelastic effect.

[0020] 5. The photoelastic effect measurement device and method provided by the present invention integrates optical system, mechanical control, image processing and data analysis into an organic whole, realizes closed-loop control of the measurement process, and ensures the synergy of the whole process and the high reliability of the measurement results. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0022] Figure 1 : Schematic diagram of the overall structure of the present invention.

[0023] Figure 2 Physical sample of the present invention Figure 1 .

[0024] Figure 3 : A schematic diagram of the "four-ring intersection" state interference image used as a measurement reference in this invention.

[0025] Figure 4 : A schematic diagram of the processor main interface of the automatic photoelastic effect testing device of the present invention.

[0026] Figure 5 : A schematic diagram illustrating the improvements of this invention in its core concept compared to existing technologies.

[0027] Figure 6 Physical sample of the present invention Figure 2 .

[0028] Figure 7 The graph shows the relationship between refractive index and thickness of five sets of data for a PET film with an initial thickness of 0.1 mm.

[0029] Figure 8 The graph shows the relationship between refractive index and thickness of five sets of data for a PET film with an initial thickness of 0.2 mm.

[0030] Figure 9 The graph shows the relationship between refractive index and thickness of five sets of data for a PET film with an initial thickness of 0.3 mm.

[0031] Figure 10 The graph shows the relationship between refractive index and thickness of five sets of data for a PET film with an initial thickness of 0.5 mm.

[0032] Figure 11 The graph shows the relationship between refractive index and thickness of five sets of data for a PET film with an initial thickness of 1 mm.

[0033] Explanation of reference numerals in the attached figures:

[0034] 1. CCD camera; 2. Support rod; 3. Helium-neon laser; 4. Metal rod; 5. Transparent thin film sample; 6. Movable mirror; 7. Gripper; 8. Beam splitter; 9. Automated CNC module linear guide; 10. Light screen. Detailed Implementation

[0035] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0036] Example 1:

[0037] This embodiment provides a photoelastic effect measurement device, such as... Figure 1 As shown, it mainly consists of the following parts: Optical system: The core is a Michelson interferometer. The light source is a helium-neon laser 3 (e.g., wavelength 632.8nm, power 15mW, coherence length greater than 100m), which is fixed by a support rod 2 to ensure that the emitted laser beam can accurately enter the center of the beam splitter of the interferometer. Image capturing device: A CCD camera 1 faces the screen 10 of the Michelson interferometer to capture the image formed on the screen in real time.

[0038] The image is involved, and the image data is transferred to a processor (e.g., a personal computer) via a USB interface.

[0039] Sample Manipulation and Measurement System: The transparent thin film sample 5 to be tested is fixed by two grippers 6 and a metal rod 4. The grippers 7 are connected to an automated CNC module linear guide rail 9. By rotating the handwheel of the guide rail 9 or by motor drive, uniform and controllable stretching can be applied to the thin film sample 5. Simultaneously, a high-precision digital micrometer is provided for accurately measuring the thickness of the sample under different stretching conditions. Data Processing and Control System: A computer running specific software acts as the processor. This software integrates image processing algorithms and data analysis functions. After a monochromatic laser beam passes through beam splitter 8 of a Michelson interferometer, part of the light is reflected to the moving mirror, then reflected again through beam splitter 8 to reach the screen; the other part is refracted, reflected by the compensating mirror and the fixed mirror, passes back through the compensating mirror, and is reflected by beam splitter 8 to the screen. The two beams, due to their identical frequency, consistent vibration direction, and constant phase difference, form equally inclined interference fringes. Adjusting the tilt screw of the moving mirror so that it tilts only slightly along the x-axis (or y-axis) allows the fringes to form a phase gradient in the y-axis (or x-axis) direction. Simultaneously turning the two fine-tuning screws causes the "linear phase gradients" in the two directions to superimpose, resulting in a total phase of _____. When they are in phase, they satisfy the condition that, where , For total phase, The optical path difference between the two arms, The "phase gradient" is caused by tilting around the x-axis. The "phase gradient" is caused by the tilt around the y-axis, where t is the film thickness and n is the film refractive index.

[0040] By adjusting the tilt screw of the moving mirror, the tilt in both directions is made "symmetrical," shrinking the equiphase surface and allowing the "linear equiphase surface" on the screen to close into a circle. Moving the coarse wheel to the critical value makes the first... When the optical path is clearly visible and its center is at the intersection boundary, four concentric circles that "tangent to each other" are formed. This "four-circle intersection" state is used to determine the equioptic path position. After adding a transparent film to the optical path, the coarse adjustment handwheel and fine adjustment knob are adjusted to bring the critical state to the surface, the additional optical path difference is determined, and then the refractive index of the transparent film is calculated.

[0041] The measurement method and workflow are as follows:

[0042] Step 1: System Initialization and Baseline State Setting

[0043] 1. Start the device and turn on the helium-neon laser 3. The laser beam will generate interference through the Michelson interferometer.

[0044] 2. Without placing a sample, the operator adjusts the tilt screw of the interferometer's moving mirror. Because a highly coherent monochromatic laser is used, clear, equal-tilt interference rings will form on the screen. Through fine adjustment, a special interference pattern can be formed on the screen, consisting of four approximately tangent concentric circles, i.e. Figure 3 The "four-ring intersection" state shown is a distinctive geometric feature that is well-suited for recognition by machine vision algorithms and is therefore defined as the "predefined geometric pattern" or baseline state for this measurement.

[0045] 3. At this time, the operator is in such a situation as Figure 4 In the software interface shown, the command system records the current state. The system captures an image through the CCD camera 1, runs an image processing algorithm (for example, first converting the image to grayscale, then performing edge detection, and finally using the Hough circle transform algorithm to accurately identify the position and center of the four rings), confirms that the "four-ring intersection" state has been reached, and automatically records the initial position at which the interferometer moving mirror M1 can move the reflector 6, denoted as the first position d1.

[0046] Step 2: Applying stress and restoring the baseline state

[0047] 1. Clamp the transparent film sample 5 (e.g., PET plastic film) to be tested with the gripper 7 and place it in one of the optical paths of the interferometer.

[0048] 2. Rotate the linear guide 9 of the automated CNC module to apply a certain tensile stress to the thin film sample 5. Due to the photoelastic effect, the refractive index n and thickness t of the thin film change, resulting in a change in the optical path difference. The "four-ring intersection" state on the screen is disrupted, and the interference fringes shift or deform.

[0049] 3. Use a high-precision digital micrometer to measure the thickness t of the film at this time, and input the value into the software system.

[0050] 4. The operator begins to slowly adjust the moving mirror of the Michelson interferometer (by using the coarse and fine adjustment handwheels) to change the optical path of the reference optical path and compensate for the additional optical path difference introduced by the sample variation.

[0051] 5. During this process, the software system continuously monitors the interference pattern in real time through CCD camera 1. The image processing algorithm continuously analyzes each frame of the image and calculates the matching degree between the pattern and the preset "four-ring intersection" reference state (for example, by calculating the offset between the centroid of the quadrilateral formed by the tangents of the four rings and the reference centroid).

[0052] 6. When the algorithm determines that the interference pattern has been restored to the "four-ring intersection" state (i.e., the matching degree is within the set threshold), the system will generate an output signal, such as lighting up the LED indicator on the software interface (e.g., ...). Figure 4 (As shown).

[0053] 7. Once the operator sees the indicator light illuminate, immediately stop adjusting the moving mirror. The system automatically records the position of the moving mirror at this moment, designating it as the second position d2.

[0054] Step 3: Automatic Calculation and Visualization

[0055] 1. The system calculates the displacement Δd = |d2-d1| of the moving mirror based on the recorded first position d1 and second position d2.

[0056] 2. According to physical principles, the optical path 2Δd compensated by the displacement Δd of the moving mirror is equal to the additional optical path 2t(n-1) introduced by the sample. Therefore, the processor automatically calculates the refractive index n of the thin film sample in the current state according to the formula n=1+Δd / t.

[0057] 3. The calculation results will be displayed in real time in the data area of ​​the software interface.

[0058] 4. Repeat steps two and three to apply different stretching amounts to the sample and obtain a series of refractive index n data points at different thicknesses t.

[0059] 5. The system plots all measured data points in real time on a graph such as... Figure 4 The curve region shown forms a refractive index-thickness relationship curve.

[0060] Meanwhile, the system can also automatically calculate the Type A uncertainty of multiple measurements, providing reliable error analysis for experimental results.

[0061] Example 2:

[0062] This embodiment provides the test data for this application:

[0063] Test group 1:

[0064] Experimental data for unstretched PET with an initial thickness of 0.1mm:

[0065]

[0066] Experimental data of PET with an initial thickness of 0.1 mm after stretching.

[0067]

[0068] like Figure 7 The relationship between the refractive index and thickness of a PET film with an initial thickness of 0.1 mm is given.

[0069] Test group 2:

[0070] Experimental data for unstretched PET with an initial thickness of 0.2mm:

[0071]

[0072] Experimental data of PET with an initial thickness of 0.2mm after stretching:

[0073]

[0074] like Figure 8 The relationship between the refractive index and thickness of a PET film with an initial thickness of 0.2 mm.

[0075] Test group 3:

[0076] Experimental data for unstretched PET with an initial thickness of 0.3mm:

[0077]

[0078] Experimental data of PET with an initial thickness of 0.3mm after stretching:

[0079]

[0080] like Figure 9 The relationship between the refractive index and thickness of a PET film with an initial thickness of 0.3 mm is given.

[0081] Test group 4:

[0082] Experimental data for PET film with an initial thickness of 0.5 mm:

[0083]

[0084] Experimental data on PET with an initial thickness of 0.5 mm after stretching:

[0085]

[0086] like Figure 10 The relationship between the refractive index and thickness of a PET film with an initial thickness of 0.3 mm is given.

[0087] Test group 5:

[0088] Experimental data for PET film with an initial thickness of 1 mm:

[0089]

[0090] Experimental data on the stretching of PET with an initial thickness of 1 mm:

[0091]

[0092] like Figure 11 The relationship between the refractive index and thickness of a PET film with an initial thickness of 0.3 mm is given.

[0093] In summary, under stress, PET films exhibit photoelasticity, and their refractive index is linearly related to their thickness.

[0094] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A device for measuring the photoelastic effect, characterized in that it comprises: The device comprises a Michelson interferometer provided with a movable mirror (6), a transparent film sample (5) is provided transversely in the light path of the Michelson interferometer, a helium-neon laser (3) is provided beside the Michelson interferometer, which is configured to generate a light source forming an interference pattern via the Michelson interferometer, and an image capturing device is provided at a position for capturing the interference image.

2. The photoelastic effect measuring apparatus according to claim 1, characterized by The device further comprises a processor connected with the image capturing device, which is configured to process the image data of the image capturing device to identify a predefined geometric interference pattern composed of a plurality of circular and mutually intersecting fringes, generate an output signal when the captured interference pattern matches the predefined geometric interference pattern, and calculate the refractive index n of the sample based on a first position of the movable mirror (6) when the predefined geometric interference pattern is identified without applying stress to the sample and a second position of the movable mirror (6) when the predefined geometric interference pattern is re-identified after stress is applied to the sample, wherein the calculation of the refractive index n is based on the displacement Δd of the movable mirror (6) determined by the first position and the second position, which compensates for the optical path difference of 2t(n-1) introduced by the sample with a thickness t.

3. The photoelastic effect measuring apparatus according to claim 2, characterized by The processor is configured to use a Hough circle transformation algorithm to identify the circular fringes in the predefined geometric interference pattern.

4. The photoelastic effect measuring apparatus according to claim 3, characterized by The transparent film sample (5) is provided with a clamping sample gripper (7) on both sides, and the bottom of each clamping sample gripper (7) is provided with a metal rod (4), and one of the metal rods (4) is provided at the bottom with an automatic numerical control module linear guide rail (9).

5. The photoelastic effect measuring apparatus according to claim 4, characterized by The processor is connected with a high-precision electronic measuring device for measuring the thickness t of the transparent film sample (5), and the processor calculates the refractive index n based on the thickness t.

6. The photoelastic effect measuring apparatus according to claim 5, wherein The processor is configured to generate and display a refractive index-thickness relationship curve in real time according to the refractive indices calculated by multiple measurements and the corresponding sample thicknesses.

7. A measuring method based on the optical effect measuring apparatus according to any one of claims 1 to 6, characterized by, The device comprises the following steps: S1: a light source is emitted from a helium-neon laser (3), and an interference image is formed through a Michelson interferometer comprising a movable mirror (6); S2: the Michelson interferometer is adjusted so that a predefined geometric image composed of a plurality of circular and mutually intersecting fringes appears on a light screen (10); S3: the image of the predefined geometric image is captured by an image capturing device, and an algorithm run by a processor confirms its formation, and the first position of the movable mirror (6) at this time is recorded; S4: the transparent film sample (5) is uniformly stretched through an automatic numerical control module linear guide rail (9); the movable mirror (6) is re-adjusted until the predefined geometric image is confirmed again by the algorithm, and the second position of the movable mirror at this time is recorded; S5: the refractive index n of the sample is calculated based on the first position, the second position, and the thickness t of the transparent film sample (5); the displacement Δd of the movable mirror (6) determined by the first position and the second position compensates for the optical path difference of 2t(n-1) introduced by the sample with a thickness t.

8. The measurement method according to claim 7, characterized in that, The above steps are repeated several times to obtain a series of data points of the refractive index and the sample thickness, and a graph of the data points is generated. The above steps are repeated several times to obtain a series of data points of the refractive index and the sample thickness, and a graph of the data points is generated.

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