Three-dimensional pressure and surface type measurement method and device based on structured light and pressure-sensitive paint

Abstract

The invention discloses a three-dimensional pressure and surface type measurement method and device based on structured light and pressure-sensitive paint. The method comprises the following steps: spraying the pressure-sensitive paint on an object to be measured; building a dual-mode camera imaging system; burning a pre-generated structured light fringe pattern to a DLP projector; a first camera, a second camera and a DLP projector are calibrated; using a DLP projector to project structured light to the surface of the to-be-detected object coated with the pressure-sensitive paint in a wind tunnel environment to excite the pressure-sensitive paint to emit red light; wavelength separation is carried out by using a semi-transparent and semi-reflective mirror, and a surface type image and a pressure image are respectively acquired by using a first camera and a second camera; processing the surface type image and the pressure image to respectively obtain three-dimensional point cloud data and a two-dimensional pressure pseudo-color image; and based on the three-dimensional point cloud data and the two-dimensional pressure pseudo-color image, obtaining a three-dimensional coordinate and a pressure value of each pixel point on the surface of the to-be-measured object. Through the combination of the structured light and the pressure-sensitive paint, the non-contact measurement technology is adopted, and the pressure and surface type data of the to-be-measured object can be synchronously and accurately obtained.

Description

Technical Field

[0001] This invention relates to the field of non-contact mechanical measurement technology, and in particular to a method and apparatus for measuring three-dimensional pressure and surface shape based on structured light and pressure-sensitive paint. Background Technology

[0002] In the field of aircraft design, accurate measurement and analysis of surface pressure loads are crucial for validating and improving aerodynamic models. Wind tunnel experiments simulate various flight conditions to measure pressure on the aircraft surface, thereby obtaining detailed pressure distribution data. This data provides direct information on the aerodynamic forces experienced by the aircraft under different flight conditions and is an important basis for optimizing airfoil design, fuselage structure, and propulsion system layout.

[0003] Currently, three-dimensional measurement methods in wind tunnel testing environments are mainly divided into two categories: one is contact measurement, which uses a probe to make contact with the surface of the object under test for single-point measurement. This method has high measurement accuracy and reliability, but its disadvantage is that it is easy to damage the probe and the surface of the object under test; the other is non-contact measurement, which uses non-contact optical sensors in combination with the rapidly developing pressure-sensitive paint (PSP) technology to provide non-contact full-field pressure measurement on complex aerodynamic surfaces in a wind tunnel environment.

[0004] PSP technology mainly utilizes the property that pressure-sensitive coatings change their fluorescence characteristics when subjected to pressure. Combined with optical equipment, it measures surface pressure in a non-invasive manner. Its advantage is that it can be sprayed onto the surfaces of various complex geometric models. Complex curved surfaces that are difficult to measure with traditional contact probes can be measured using PSP technology.

[0005] Currently, methods that combine PSP technology with three-dimensional optical measurement technology include VMD (Visual Mark Detection), Stereo-photogrammetry, LF-3D-PSP (Light Field 3D-PSP), and DIC (Digital Image Correlation).

[0006] VMD (Virtual Machine Deposition) technology captures surfaces coated with PSP (Pressure Sensitive Material) and marked points using single or multiple cameras, analyzing pressure or deformation based on changes in the position of the marked points. This method has low spatial resolution, depends on the size of the marked points, requires additional marking coatings, and may interfere with surface properties. Stereo-photogrammetry technology uses multiple cameras to photograph surfaces with PSP and grid markings from different angles, generating a 3D model through parallax calculations. This method also has low spatial resolution, depends on grid density, requires complex multi-camera systems, and is costly. LF-3D-PSP technology uses a light field camera (including a microlens array) to directly capture light direction information, combining it with the PSP coating to achieve high-resolution 3D reconstruction. This method has high costs for custom light field cameras, complex data processing with low accuracy, and is only suitable for surface contour measurements. DIC (Distributed Injection) technology tracks the deformation of random speckle patterns on the PSP coating using multiple cameras to calculate surface displacement or strain. This requires the fabrication of complex, high-density speckle coatings and depends on high-performance multi-camera systems, resulting in high costs.

[0007] In addition to the more conventional existing technologies mentioned above, research on 3D scanning technology based on structured light (SL) is also very active. PSP 2 (Phase-shift profilometry, PSP technology) integrates phase-shift profilometry and PSP technology through a single-camera, single-projector system to measure three-dimensional surface profiles and pressure. However, the single-camera, single-projector structured light system cannot eliminate the interference of ambient light. Under complex lighting conditions, ambient light interference will reduce the accuracy of pressure measurement. When the wavelengths of reflected light and PSP emitted light are close, it is difficult to completely separate them through filters, which can easily lead to measurement errors. Furthermore, a single camera needs to process the topography and pressure signals in a time-division manner, making it difficult to capture transient phenomena. Summary of the Invention

[0008] In view of the above-mentioned shortcomings in the prior art, the three-dimensional pressure and surface shape measurement method and device based on structured light and pressure-sensitive paint provided by the present invention solves the problems of high measurement cost, cumbersome measurement method and low measurement accuracy of the prior art.

[0009] To achieve the aforementioned objectives, the present invention employs the following technical solution: a three-dimensional pressure and surface shape measurement method based on structured light and pressure-sensitive paint, comprising: Spray pressure-sensitive paint onto the object to be tested; A dual-mode camera imaging system was constructed to ensure that the structured light of a specific wavelength projected by the dual-mode camera imaging system completely covers the object under test and can accurately collect data; the dual-mode camera imaging system includes a first camera, a second camera, a semi-transparent mirror, and a DLP projector. The pre-generated structured light fringe pattern is burned into the DLP projector, enabling the DLP projector to project structured light; the timing trigger control is adjusted so that the camera is triggered to synchronously acquire data while the DLP projector projects structured light. The calibration parameters of camera 1, camera 2, and DLP projector were obtained by calibrating them. In a wind tunnel environment, structured light is projected onto the surface of a test object coated with pressure-sensitive paint using a DLP projector, which excites the pressure-sensitive paint to emit red light. The light reflected after the structured light is projected onto the surface of the test object coated with pressure-sensitive paint carries surface shape information, while the red light emitted by the pressure-sensitive paint carries pressure information. Wavelength separation was performed using a semi-transparent and semi-reflective mirror, and surface images and pressure images were acquired using camera No. 1 and camera No. 2, respectively. Process the surface image and the pressure image to obtain three-dimensional point cloud data and two-dimensional pressure pseudo-color image, respectively; The three-dimensional pressure skin of the object under test is obtained by using three-dimensional point cloud data and two-dimensional pressure pseudo-color image, that is, the three-dimensional coordinates and pressure values ​​of each pixel on the surface of the object under test are obtained.

[0010] In addition, the present invention also provides a three-dimensional pressure and surface shape measurement device based on structured light and pressure-sensitive paint, comprising: a test object, a dual-mode imaging system, wherein the test object is located in a wind tunnel, and a light window is provided below the wind tunnel; the dual-mode imaging system is located below the light window; the dual-mode imaging system includes a first camera, a second camera, a semi-transparent mirror, a DLP projector, and a computer; the lenses of the first camera and the second camera are respectively facing the left side and bottom of the semi-transparent mirror; the DLP projector is located on the right side of the semi-transparent mirror, and the light source of the DLP projector is located below the light window; the first camera, the second camera, and the DLP projector are connected to the computer.

[0011] The beneficial effects of this invention are as follows: 1. By combining structured light with pressure-sensitive paint technology, a dual-mode camera imaging system is used to measure the pressure and surface data of the object under test. This not only solves the problem of mapping from two dimensions to three dimensions, but also has high spatial resolution under low structural complexity, reducing the cost of non-contact measurement and dependence on the system.

[0012] 2. By using dual cameras to collaboratively acquire pressure and surface shape data, the difficulty of capturing transient data of the object under test caused by time-division processing of pressure and surface shape signals is avoided. At the same time, the dual cameras use a semi-transparent and semi-reflective mirror to separate wavelengths for data acquisition, avoiding the problems of light source crosstalk and ambient light pollution during the measurement process. This enables synchronous and accurate measurement of three-dimensional surface shape and pressure signals in high dynamic scenes.

[0013] 3. Through non-contact measurement technology, there is no need to treat the model surface or formulate a specific pressure-sensitive paint with feature points. It can achieve complete physical isolation between the measuring device and the measured object, effectively eliminating errors caused by mechanical contact, thereby significantly improving the accuracy of synchronous measurement of dynamic pressure field and three-dimensional morphology. Attached Figure Description

[0014] Figure 1 A flowchart illustrating a three-dimensional pressure and surface shape measurement method based on structured light and pressure-sensitive paint, provided for an embodiment; Figure 2 A structural diagram of a three-dimensional pressure and surface shape measurement device based on structured light and pressure-sensitive paint is provided for an embodiment. Figure 3 A schematic diagram of the optical path separation for a dual-camera system with a semi-transparent, semi-reflective mirror. Figure 4 This is a schematic diagram of the three-dimensional reconstruction principle of a structured light system based on triangulation. Figure 5 This is a flowchart for pressure data analysis based on cross-section display; Among them, 1 is the wind tunnel, 2 is the light window, 3 is the object to be tested, 4 is the dual-mode imaging system, 5 is the first camera, 6 is the second camera, 7 is the semi-transparent mirror, 8 is the DLP projector, and 9 is the computer. Detailed Implementation

[0015] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0016] like Figure 1 As shown, in one embodiment of the present invention, the three-dimensional pressure and surface profile measurement method based on structured light and pressure-sensitive paint includes the following steps: S1. Spray a uniform pressure-sensitive paint onto the object to be tested (in this embodiment, the object to be tested is an airplane model).

[0017] S2. Build a dual-mode camera imaging system to ensure that the structured light of a specific wavelength projected by the dual-mode camera imaging system completely covers the object under test and that the camera can accurately collect data.

[0018] like Figure 2 As shown, the object to be tested is placed and fixed in a wind tunnel. A light window 2 is provided below the wind tunnel 1. The dual-mode imaging system 4 is located below the light window 2. The dual-mode imaging system 4 includes a first camera 5, a second camera 6, a semi-transparent mirror 7, a DLP projector 8, and a computer 9. The lenses of the first camera 5 and the second camera 6 are respectively facing the left and bottom of the semi-transparent mirror 7. The DLP projector 8 is located on the right side of the semi-transparent mirror 7, and the light source of the DLP projector 8 is located below the light window. The first camera 5, the second camera 6, and the DLP projector 8 are connected to the computer 9.

[0019] S3. The pre-generated structured light stripe pattern is burned into the DLP projector 8 by the computer 9, so that the DLP projector 8 projects structured light.

[0020] Forty-three images, including a pre-generated structured light fringe pattern and a full-white image required for camera calibration, are burned into the DLP projector 8. Based on these 43 images, the camera exposure time and corresponding trigger sequence are set sequentially. A high-level signal is sent to trigger the camera each time an image is projected, while simultaneously ensuring that the camera's exposure time is greater than or equal to the projection time of one image (the goal being to acquire a complete structured light fringe pattern). This allows the DLP projector 8 to project structured light while simultaneously triggering the camera for synchronous acquisition. The camera parameters are set based on the projected structured light, and the aperture and exposure time are adjusted to ensure that the captured images are neither overexposed nor underexposed.

[0021] S4. Calibrate the first camera 5, the second camera 6, and the DLP projector 8 to obtain the calibration parameters of the first camera 5, the second camera 6, and the DLP projector 8.

[0022] The specific method for calibrating camera 5 (number 1) and camera 6 (number 2) is as follows: Multiple images of a checkerboard calibration board in different poses were simultaneously captured using camera 5 (first camera) and camera 6 (second camera). The checkerboard calibration board has a regular distribution of corner points, which can be accurately detected in the images. Corner point feature data were extracted from the checkerboard calibration board images. The corresponding eigenvalue matrix was calculated based on the corner point feature data using a calibration algorithm, and a matching relationship between two-dimensional pixels and three-dimensional point clouds was established. The rotation matrix and translation vector for dual-target calibration were calculated based on the corresponding corner points in the checkerboard calibration board images captured by camera 5 and camera 6, and the pixel transformation relationship between camera 5 and camera 6 was determined.

[0023] During the calibration of a DLP projector 8, it is typically viewed as a "reverse camera." The image captured by the camera reveals the object's imaging position on its pixel plane. The DLP projector 8 projects coded structured light stripes onto the object's surface, and decoding this information reveals the object's "imaging position" on the DLP projector 8. Then, using camera calibration methods, the various parameters of the DLP projector 8 can be calculated.

[0024] The specific method is as follows: Three sets of sinusoidal structured light of different frequencies are projected onto the surface of the object under test by a DLP projector 8; and structured light fringe patterns of the surface of the object under test at different frequencies are captured by a camera 5. The wrapping phase of the structured light fringe pattern corresponding to each frequency group is calculated using the seven-step phase-shifting method, and its expression is as follows:

[0025] in, To wrap the phase, It is the arctangent function. For the first n The light intensity of the structured light fringe pattern. It is a sine function. It is a cosine function. Pi; The wrapped phase is expanded into absolute phase using the three-frequency heterodyne method; The correspondence between camera pixels and 8 pixels of DLP projector is established by using absolute phase; The intrinsic parameters and distortion coefficients of the DLP projector 8 were solved using the world coordinate system and the image coordinate system; Based on the internal parameters of the DLP projector 8, the known internal parameters of the camera, and the correspondence between the camera pixels and the pixels of the DLP projector 8, stereo calibration is performed to obtain the external parameters of the DLP projector 8.

[0026] S5. In the wind tunnel 1 environment, structured light (striped blue light 460nm) is projected onto the surface of the test object 3 coated with pressure-sensitive paint using a DLP projector 8 to excite the pressure-sensitive paint to emit red light (640nm). The structured light, after being projected onto the surface of the test object 3 coated with pressure-sensitive paint, carries surface shape information, while the red light emitted by the pressure-sensitive paint carries pressure information.

[0027] S6. Wavelength separation is performed using a semi-transparent mirror 7, and surface images and pressure images are acquired using camera 5 and camera 6 respectively.

[0028] The surface of the semi-transparent mirror 7 is coated with a thin film. The structured light reflectivity of the thin film is greater than a preset value, and the red light transmittance of the thin film is also greater than a preset value. This causes the light reflected from the surface of the object under test 3 to be reflected to camera 5, and the red light emitted by the pressure-sensitive paint to be transmitted to camera 6. To further eliminate interference from ambient light, a 460±10nm bandpass filter is installed on camera 5, allowing only reflected blue light to pass through while blocking the red light emitted by the PSP and ambient stray light. A 640±10nm bandpass filter is installed on camera 6 to block reflected blue light and other wavelength interference.

[0029] Taking the aircraft model under test as an example, the DLP projects striped structured light with a wavelength of 460nm. A dual-camera system equipped with a semi-transparent mirror (7) separates the wavelengths of the reflected blue light and the excited red light. Figure 3 As shown, camera 6 collects the PSP red light signal transmitted through the semi-transparent mirror 7, while camera 5 is used to collect the reflected structured light morphology signal.

[0030] S7. Process the surface image and pressure image to obtain three-dimensional point cloud data and two-dimensional pressure pseudo-color image, respectively; The specific method for creating a two-dimensional pressure pseudo-color image is as follows: The response characteristics of the pressure-sensitive paint sprayed on the test object 3 under different pressure conditions are calibrated to obtain a calibration curve; two images taken by the camera that collects pressure images in the wind tunnel 1 environment before and after wind blowing are obtained, and the images are filtered, background subtracted and ratio calculated to extract the grayscale information of the processed images; combined with the calibration curve and the grayscale information, the pressure value on the surface of the test object 3 is calculated using the Stern-Volmer equation, and a two-dimensional pressure pseudo-color image is output.

[0031] The specific method for obtaining 3D point cloud data is as follows: based on the surface image, a binocular vision method is used to calculate the 3D point cloud information of the surface of the object under test using absolute phase, calibration parameters, and triangulation principles. For example... Figure 4 As shown, the ray formed by a point on the projection plane of the DLP projector 8 and its optical center intersects with the ray formed by a point on the camera plane and its optical center at a spatial point on the aircraft model. Based on the principle of triangulation and calibration parameters, the three-dimensional coordinates of the corresponding point can be obtained.

[0032] S8. Based on the 3D point cloud data and the 2D pressure pseudo-color image, obtain the 3D pressure skin of the surface of the object under test 3, that is, obtain the 3D coordinates and pressure value of each pixel on the surface of the object under test 3: Based on the matching relationship between 3D point cloud data and surface image, and the pixel conversion relationship between camera 5 and camera 6, a mapping relationship between the pixels of the pressure image and the 3D point cloud data is established to realize the pixel-by-pixel mapping from 2D pressure value to 3D digital model, thereby obtaining a 3D point cloud with pressure field; the 3D point cloud with pressure field is heterogeneously registered with the standard digital model to generate a 3D pressure skin on the model surface.

[0033] Furthermore, in this embodiment, the wing area of ​​the aircraft model can be selected, and the pressure distribution in that area can be integrated to further convert the pressure (scalar) into a mechanical load (vector), quantify the overall and local stress state, and analyze performance indicators more accurately.

[0034] Specifically, experiments are conducted by changing the aircraft's angle of attack. The lift is calculated by integrating the pressure on the wing surface, and then the lift coefficient can be plotted as a function of different angles of attack. This provides data support for further aerodynamic performance analysis, stall characteristic analysis, and diagnosis of flow problems.

[0035] like Figure 5 As shown, a cutting direction is defined along a section of the wing of the aircraft model to generate a cutting plane. The intersection of the section and the pressure field point cloud is calculated, and the pressure values ​​of all points on the intersection line are extracted. The pressure coefficient distribution map can be drawn using the extracted pressure values ​​and chord length, which can provide support for further analysis of performance indicators and identification of flow separation zones.

[0036] In summary, this invention combines structured light with pressure-sensitive paint, employing non-contact measurement technology to simultaneously and accurately acquire pressure and surface shape data of the object under test. Furthermore, this invention eliminates the need for complex high-density layer preparation using photosensitive printing equipment and avoids contact with the model surface coated with PSP, thus preventing interference with the PSP coating. The entire measurement process is non-contact, eliminating interference caused by human contact. By adding a semi-transparent, semi-reflective mirror to separate the pressure and surface shape signals, crosstalk between the two signals (pressure and surface shape) is avoided, resulting in higher work efficiency and measurement accuracy.

Claims

1. A three-dimensional pressure and surface shape measurement method based on structured light and pressure-sensitive paint, characterized in that, include: Spray pressure-sensitive paint onto the object to be tested; A dual-mode camera imaging system was constructed to ensure that the structured light of a specific wavelength projected by the dual-mode camera imaging system completely covers the object under test and can accurately collect data; the dual-mode camera imaging system includes a first camera, a second camera, a semi-transparent mirror, and a DLP projector. The pre-generated structured light fringe pattern is burned into the DLP projector, enabling the DLP projector to project structured light; the timing trigger control is adjusted so that the camera is triggered to synchronously acquire data while the DLP projector projects structured light. The calibration parameters of camera 1, camera 2, and DLP projector were obtained by calibrating them. In a wind tunnel environment, structured light is projected onto the surface of a test object coated with pressure-sensitive paint using a DLP projector, which excites the pressure-sensitive paint to emit red light. The light reflected after the structured light is projected onto the surface of the test object coated with pressure-sensitive paint carries surface shape information, while the red light emitted by the pressure-sensitive paint carries pressure information. Wavelength separation was performed using a semi-transparent and semi-reflective mirror, and surface images and pressure images were acquired using camera No. 1 and camera No. 2, respectively. Process the surface image and the pressure image to obtain three-dimensional point cloud data and two-dimensional pressure pseudo-color image, respectively; The three-dimensional pressure skin of the object under test is obtained by using three-dimensional point cloud data and two-dimensional pressure pseudo-color image, that is, the three-dimensional coordinates and pressure values ​​of each pixel on the surface of the object under test are obtained.

2. The method according to claim 1, characterized in that, The specific method for adjusting the timing trigger control is as follows: Set the exposure time and corresponding trigger timing according to the burned structured light stripe pattern. Send a high-level signal to trigger the camera when each image is projected, and at the same time ensure that the camera's exposure time is greater than or equal to the projection time of one image.

3. The method according to claim 2, characterized in that, The specific method for calibrating camera number one and camera number two is as follows: Multiple images of a checkerboard calibration board in different poses were simultaneously captured using camera 1 and camera 2. Corner feature data were extracted from the checkerboard calibration board images. The corresponding eigenvalue matrix was calculated based on the corner feature data using a calibration algorithm, and the matching relationship between two-dimensional pixels and three-dimensional point clouds was established. The rotation matrix and translation vector of the dual-calibration were calculated based on the corresponding corner points in the checkerboard calibration board images captured by camera 1 and camera 2, and the pixel transformation relationship between camera 1 and camera 2 was determined.

4. The method according to claim 3, characterized in that, The specific method for calibrating a DLP projector is as follows: Three sets of sinusoidal structured light of different frequencies are projected onto the surface of the object under test by a DLP projector; the structured light fringe patterns of the object under test at different frequencies are captured by a camera. The wrapping phase of the structured light fringe pattern corresponding to each frequency group is calculated using the seven-step phase-shifting method, and its expression is as follows: in, To wrap the phase, It is the arctangent function. For the first n The light intensity of the structured light fringe pattern. It is a sine function. It is a cosine function. Pi; The wrapped phase is expanded into absolute phase using the three-frequency heterodyne method; The correspondence between camera pixels and DLP projector pixels is established by using absolute phase; Solve for the intrinsic parameters and distortion coefficients of a DLP projector using the world coordinate system and the image coordinate system; Based on the internal parameters of the DLP projector, the known intrinsic parameters of the camera, and the correspondence between camera pixels and DLP projector pixels, stereo calibration is performed to obtain the extrinsic parameters of the DLP projector.

5. The method according to claim 1, characterized in that, The surface of the semi-transparent mirror is coated with a thin film. The structure light reflectivity of the thin film is greater than the preset value, and the red light transmittance of the thin film is greater than the preset value. This causes the light reflected from the surface of the object under test to be reflected to camera number one, and the red light emitted by the pressure-sensitive paint to be transmitted to camera number two.

6. The method according to claim 4, characterized in that, The specific method for obtaining a two-dimensional pressure pseudo-color image is as follows: The response characteristics of the pressure-sensitive paint to be sprayed onto the test object under different pressure conditions were calibrated to obtain calibration curves; Two images, taken by a camera capturing pressure images, are obtained before and after wind blowing in a wind tunnel environment. These images are then filtered, have their background subtracted, and are subjected to ratio calculations to extract the grayscale information of the processed images. Combining the calibration curve and the grayscale information, the pressure value on the surface of the object under test is calculated using the Stern-Volmer equation, and a two-dimensional pressure pseudo-color image is output.

7. The method according to claim 6, characterized in that, The specific method for obtaining 3D point cloud data is as follows: based on the surface image, the binocular vision method is used to calculate the 3D point cloud information of the surface of the object to be measured based on absolute phase, calibration parameters and triangulation principles.

8. The method according to claim 7, characterized in that, The specific steps for obtaining the three-dimensional pressure skin of the surface of the object under test are as follows: Based on the matching relationship between 3D point cloud data and surface image, as well as the pixel conversion relationship between camera 1 and camera 2, a mapping relationship between the pixels of the pressure image and the 3D point cloud data is established to obtain a 3D point cloud with a pressure field. The 3D point cloud with the pressure field is then heterogeneously registered with the standard digital model to generate a 3D pressure skin on the model surface.

9. An apparatus for implementing the three-dimensional pressure and surface shape measurement method based on structured light and pressure-sensitive paint as described in any one of claims 1 to 8, characterized in that, include: The test object and the dual-mode imaging system are located in a wind tunnel with a light window at the bottom. The dual-mode imaging system is located below the light window and includes a first camera, a second camera, a semi-transparent mirror, a DLP projector, and a computer. The lenses of the first and second cameras are positioned facing the left and bottom of the semi-transparent mirror, respectively. The DLP projector is located on the right side of the semi-transparent mirror, with its light source positioned below the light window. The first camera, the second camera, and the DLP projector are connected to the computer.

10. The apparatus according to claim 9, characterized in that, Camera 1 was fitted with a bandpass filter of 460±10nm; Camera 2 was fitted with a bandpass filter of 640±10nm.

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