Binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points

Abstract

The invention discloses a calibration board and wind tunnel feature point-based binocular system wind tunnel coordinate system calibration method, and belongs to the field of aerodynamic tests. Comprising the following steps: reconstructing surface mark points of a calibration plate by photogrammetry outside a hole and establishing a local coordinate system to obtain coordinates under the local coordinate system; a binocular system is arranged in the hole, and binocular internal and external parameters are calibrated through a special cross calibration ruler; arranging a plurality of circular feature mark points on the surface of the hole wall with overlapped visual fields of the binocular system; the calibration plate is correctly placed according to the geometrical relationship, so that each vector of the local coordinate system is parallel or perpendicular to an incoming vector, and the binocular system reconstructs global points of the calibration plate and registers the global points to achieve alignment of a measurement coordinate system of the binocular system and a wind tunnel coordinate system; reconstructing a three-dimensional coordinate of the fixed point in a wind tunnel coordinate system through a binocular system; and in a subsequent test, adopting resection to realize the recalibration of the wind tunnel coordinate system. According to the invention, high-precision and high-efficiency alignment requirements of a measurement coordinate system and a wind tunnel coordinate system of a binocular system can be met.

Description

TECHNICAL FIELD

[0001] The application relates to a wind tunnel coordinate system calibration method based on a calibration board and feature points of a wind tunnel, and belongs to the field of aerodynamic tests. BACKGROUND

[0002] As a non-contact, high-precision and high-efficiency measurement method, visual measurement technology has been gradually applied to a wind tunnel test section in China and related data is calculated. The visual measurement technology usually needs to be corresponded to a wind tunnel coordinate system, and existing wind tunnel coordinate system establishment has problems of calculation difficulty and poor precision, and the precision of the wind tunnel coordinate system establishment determines the subsequent data calculation precision.

[0003] Specifically, the wind tunnel coordinate system is the basis for data calculation, such as aircraft wing bending and torsional deformation and aircraft attitude angle calculation, which are all carried out in the wind tunnel coordinate system. Theoretically, the axial vectors of the wind tunnel coordinate system need to be absolutely parallel or perpendicular to the wind tunnel flow vector, and the visual measurement camera measurement coordinate system is a virtual coordinate system, so it can be seen that how to realize the alignment of the camera measurement coordinate system and the wind tunnel coordinate system. At present, the existing visual measurement related research in the wind tunnel has a complicated calibration process of the wind tunnel coordinate system, and almost all of them are special devices designed for specific types of wind tunnels, which have great installation difficulty, increase the construction time in the wind tunnel, and have low measurement precision. In addition, due to the change of light, focal length or other factors, the visual system needs to be recalibrated, so that the position relationship between the camera measurement coordinate system and the wind tunnel coordinate system changes, which undoubtedly increases the process and time of recalibrating the wind tunnel coordinate system again. SUMMARY

[0004] The purpose of the present application is to solve the above-mentioned problems, and a wind tunnel coordinate system calibration method based on a calibration board and immobile feature points in a wind tunnel is provided. The present application is not limited by the model shape and the wind tunnel type, and only needs to calibrate the initial wind tunnel coordinate system by using the calibration board for the first time. The wind tunnel coordinate system calibration is realized based on the immobile feature points on the wall of the wind tunnel in subsequent wind tunnel tests, which greatly simplifies the calibration process of the wind tunnel coordinate system.

[0005] In order to achieve the above-mentioned purpose, the following technical scheme is adopted: One embodiment of the present application discloses a wind tunnel coordinate system calibration method based on a calibration board and immobile feature points in a wind tunnel, which comprises the following steps: S1: the surface global coordinate points of the calibration board are reconstructed by photogrammetry outside the wind tunnel, and a local coordinate system is established, so that the coordinates in the local coordinate system are obtained. ; S2: a binocular system is arranged in the wind tunnel, and the internal and external parameters of the binocular system are calibrated by a specially designed cross calibration ruler. The binocular system comprises two ordinary industrial cameras.

[0006] S3: arranging several circular feature markers on the surface of the wind tunnel wall where the fields of view of the binocular system overlap; S4: correctly placing the calibration board according to the geometric relationship so that the vectors of the local coordinate system are parallel or perpendicular to the incoming flow vector, and the binocular system reconstructs the global points of the calibration board and registration to align the measurement coordinate system of the binocular system with the wind tunnel coordinate system; S5: reconstructing the three-dimensional coordinates of the fixed points in the wind tunnel coordinate system by the binocular system ; S6: in subsequent tests, according to , re-calibrating the wind tunnel coordinate system by resection.

[0007] Preferably, step S1 specifically comprises: S11: spraying circular feature markers on the surface of the calibration board; the adjacent boundaries of the calibration board are perpendicular to each other, and the shape is a cuboid.

[0008] S12: three-dimensionally reconstructing the circular feature points on the surface of the calibration board to obtain global point coordinates .

[0009] S13: establishing a local coordinate system so that the vectors of the local coordinate system are perpendicular or parallel to the boundaries of the calibration board, and obtaining the three-dimensional coordinates of the global points in the local coordinate system .

[0010] Preferably, step S12 specifically comprises: Before the experiment, the calibration board is photographed from various directions outside the wind tunnel by a single-lens reflex camera, and the circular feature points on the surface of the calibration board are three-dimensionally reconstructed according to the industrial close-range photogrammetry technology to obtain the global point coordinates of each marker point on the surface of the calibration board in the photogrammetry coordinate system :

[0011] In the formula, represents the pixel coordinates (unit: pixel) of the marker point in the image coordinate system, represents the focal length of the lens, represents the coordinates of the marker point in the world coordinate system, , represents the coordinates of the origin of the camera coordinate system in the photogrammetry coordinate system, represents the rotation matrix component from the photogrammetry coordinate system to the camera coordinate system, represents the principal point deviation, represents the pixel coordinate deviation caused by lens distortion, mainly including radial distortion ​, tangential distortion , image plane distortion .

[0012] Preferably, step S2 specifically comprises: The internal and external parameters of the binocular camera are calibrated by a specially designed cross calibration ruler. The internal parameters include ten parameters: focal length, two principal point deviation parameters, three radial distortion parameters, two eccentric distortion parameters, and two image plane distortion parameters. The external parameters include the rotation matrix and translation matrix of each industrial camera with respect to the world coordinate system.

[0013] Preferably, step S4 specifically comprises: S41: Find the geometric constraint relationship between the calibration board and the wind tunnel, and place the calibration board so that the vectors of the local coordinate system are parallel or perpendicular to the incoming flow vector; S42: Reconstruct the circular feature points on the surface of the calibration board by the binocular system to obtain the three-dimensional coordinates of each marker point in the binocular system measurement coordinate system ; S43: By the three-dimensional coordinates in the local coordinate system and the three-dimensional coordinates in the binocular system measurement coordinate system , the point cloud and point cloud registration method is used to realize the alignment of the binocular system measurement coordinate system and the wind tunnel coordinate system.

[0014] Preferably, step S6 specifically comprises: By the three-dimensional coordinates of the fixed points on the surface of the wind tunnel wall in the wind tunnel coordinate system and the corresponding points of the left and right images , , a least squares equation is constructed to recalibrate the wind tunnel coordinate system:

[0015]

[0016] In the formula , f is the focal length of the left and right cameras corresponding to the pixel coordinates; are the internal parameter matrices of the left and right cameras, respectively; is the rotation and translation matrix of the right camera to the wind tunnel coordinate system; is the rotation and translation matrix of the left camera to the wind tunnel coordinate system; , , and

[0017] This invention replaces the stepped calibration target with a lightweight calibration plate, which is easy to carry and operate, while also meeting relatively low precision requirements and adapting to complex working conditions. It also utilizes the plate to acquire images from multiple angles to complete camera calibration. A large number of measurement steps are transferred to the wind tunnel debugging room, optimizing the process, avoiding overlapping operations, and improving efficiency. By combining photogrammetry with auxiliary equipment, a unified wind tunnel coordinate system is constructed based on the relationship between the model and the wind direction, providing a foundation for high-precision visual measurement and calculation of the attitude deformation of the measured object. Attached Figure Description

[0018] Figure 1 This is a flowchart of one embodiment of the present invention; Figure 2 This is a schematic diagram of the calibration plate and its circular feature markings. Figure 3 This is a schematic diagram of the three-dimensional coordinates of the circular feature markers on the surface of the scene calibration board using photogrammetry technology, as described in this invention. Figure 4 This is a schematic diagram of the cross-shaped calibration ruler of the present invention; Figure 5 This is a scene diagram of an embodiment of the present invention.

[0019] The markings in the attached diagram are as follows: L1, coded marker point; L2, non-coded marker point; L3, local coordinate system of the calibration plate; M1, binocular system; M2, incoming flow vector; M3, test model and angle of attack mechanism; M4, wind tunnel coordinate system; M5, local coordinate system of the calibration plate; M6, circular marker point on the wind tunnel wall surface. Detailed Implementation

[0020] The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.

[0021] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as "connected to" another component, it can be directly connected to or indirectly connected to that other component. Furthermore, a connection can be used for both fixing and circuit / signal connectivity.

[0022] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.

[0023] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0024] like Figure 1 As shown, one embodiment of the present invention discloses a binocular system wind tunnel coordinate system calibration method based on a calibration plate and stationary feature points inside the wind tunnel, comprising the following six parts: S1: outside the wind tunnel, photogrammetry is used to reconstruct the marked points on the surface of the calibration plate and establish a local coordinate system to obtain the coordinates in the local coordinate system. S2: A binocular system is installed in the wind tunnel, and its internal and external parameters are calibrated using a specially made crosshair calibration ruler (the specific algorithm is as follows). Figure 2 (As shown); S3: Arrange several circular feature markers on the wind tunnel wall surface where the binocular system's field of view overlaps; S4: Correctly position the calibration plate according to geometric relationships so that each vector in the local coordinate system is parallel or perpendicular to the incoming flow vector, and the binocular system reconstructs the global points of the calibration plate. and Registration is performed to align the measurement coordinate system of the binocular system with the wind tunnel coordinate system; S5: Reconstruct the three-dimensional coordinates of the fixed point in the wind tunnel coordinate system using the binocular system. S6: Subsequent experiments, based on The wind tunnel coordinate system was recalibrated using resection.

[0025] The specific implementation process is as follows: Step 1: Global point in local coordinate system of calibration board Get like Figure 2 The diagram illustrates the calibration plate and its coordinate system of this invention. The calibration plate is entirely black, with white coded points L1 and non-coded points L2 sprayed on it. The two adjacent boundaries of the calibration plate should be perpendicular to each other, forming a cuboid shape, so that subsequent constraints such as the shape of the calibration plate and the wind tunnel wall ensure that the vectors of the local coordinate system L3 of the calibration plate are perpendicular or parallel to the vectors of the wind tunnel coordinate system. Several marker points are affixed to the surface of the calibration plate, not limited to coded marker points L1 or non-coded marker points L2. The distribution of the marker points should be as uniform and regular as possible, and each row or column of marker points should be parallel or perpendicular to the boundaries of the calibration plate.

[0026] Before the experiment, prepare several calibrated rulers and several coded markers and place them around the calibration plate. Take images of the calibration plate and rulers from various angles using a DSLR camera outside the wind tunnel. Figure 3As shown, the calibration plate has a black background with white markers sprayed on it. Industrial close-range photogrammetry was used to reconstruct the circular feature points on the surface of the calibration plate in three dimensions, obtaining the global point coordinates of each marker on the calibration plate surface in the photogrammetric coordinate system. :

[0027] In the formula, Indicates the marker point Pixel coordinates in the image coordinate system Indicates the lens focal length. Indicates the marker point Coordinates in the world coordinate system , The coordinates of the camera coordinate system origin in the photogrammetric coordinate system. This represents the rotation matrix components from the photographer's coordinate system to the camera's coordinate system. Indicates the principal point deviation. This indicates the pixel coordinate deviation caused by lens distortion, mainly including radial distortion. Tangential distortion Image plane distortion ...

[0028] like Figure 2 As shown, the global point coordinates of each marked point on the surface of the calibration plate. Located in a photogrammetric coordinate system, which is typically situated in the camera coordinate system used to capture the first frame of the image, rather than on the calibration plate itself. Therefore, a local coordinate system needs to be established for the global points on the calibration plate surface based on the constraint that the marked points in each row or column are parallel or perpendicular to the calibration plate boundary. M5 makes the local coordinate system Each vector of M5 is perpendicular or parallel to the boundary of the calibration plate, thus obtaining the local coordinate system. Global point 3D coordinates under M5 . Step 2: Deploy the binocular system in the wind tunnel and calibrate its internal and external parameters. like Figure 5As shown, before the test, according to the wind tunnel test measurement requirements and the wind tunnel site installation conditions, the binocular system installation position is reasonably arranged, so that the two camera M1 optical axes of the binocular system are about 30°, and the public field of view is directly opposite the model surface M3: during the test, the high-speed airflow M2 flows into from the air inlet, and when the wing bending and twisting deformation of the aircraft model M3 is measured, the binocular system M1 should be arranged on the upper wall of the wind tunnel to view the whole wing; if the aircraft model M3 is measured in the attitude, the binocular system M1 should be arranged on the side wall of the wind tunnel and as high as possible to avoid the binocular system calculating the depth of field direction motion data, thereby improving the attitude measurement accuracy. In addition, if the wind tunnel test section has an optical window, the binocular system should be arranged outside the optical window first.

[0029] After the binocular system is installed, the camera aperture and focus are adjusted, and the appropriate exposure time is set, so that the image is in a clear state. A specially designed crosshair scale is placed in at least 8 different positions in the wind tunnel, and the three-dimensional coordinates of the known marker points in the world coordinate system and the marker point pixels detected by the left and right images of the binocular system are obtained. The camera internal and external parameters include ten internal parameters: focal length, two principal point deviation parameters, three radial distortion parameters, two eccentric distortion parameters and two image plane distortion parameters. The external parameters include the rotation matrix and translation matrix of each industrial camera and the world coordinate system. The whole crosshair scale is black with white marker points sprayed on the top, as shown in Figure 4 . Step 3: Arrangement of circular feature marker points on the surface of the wind tunnel wall According to the overlapping field of view area of the binocular system, a plurality of circular feature marker points M6 are randomly arranged on the surface of the wind tunnel wall. Among them, the radius of the circular marker point M6 in the image occupied by the binocular system should be about 10-15 pixels, but it is not limited to the coded marker points and non-coded marker points. In addition, the number of circular marker points should be not less than 3 and not collinear.

[0030] Step 4: Wind tunnel coordinate system calibration Under the premise of obtaining the coordinates of the global points in the local coordinate system of the calibration board described in step 1, the calibration board is placed so that the vectors of its local coordinate system are parallel or perpendicular to the flow vector through the geometric constraint relationship between the calibration board and the wind tunnel. For example, if the shape of the calibration board is a cuboid, the calibration board can be tightly attached to the side wall and the bottom wall of the wind tunnel to realize that the vectors of its local coordinate system are parallel or perpendicular to the flow vector.

[0031] The circular feature points on the surface of the calibration board are reconstructed by the binocular system to obtain the three-dimensional coordinates of each marker point in the measurement coordinate system of the binocular system ; the point cloud and the point cloud The registration method can align the measurement coordinate system of the binocular system with the wind tunnel coordinate system and calculate the rotation matrix. Translation matrix .

[0032] Step 5: Fixed points on the wind tunnel wall surface 3D Reconstruction After camera calibration, the binocular system's measurement coordinate system is aligned parallel to the wind tunnel coordinate system. Based on the pixel coordinates of circular feature markers placed on the wind tunnel wall surface within the overlapping field of view of the binocular system, these markers are mapped onto the left and right camera images. , First, increase the distortion parameters of the left and right cameras respectively to adjust the distortion coordinates. , Distortion is removed, and 3D reconstruction is performed on the distortion-free coordinates to obtain the 3D coordinates of each circular marker point in the current wind tunnel coordinates. .

[0033] Step 6: Based on fixed points Wind tunnel coordinate system alignment To meet the different testing scenarios or even the different testing requirements within the same scenario, the camera's installation position, focal length, exposure time, and aperture need to be adjusted accordingly, resulting in changes to both the camera's internal and external parameters. If the camera were to be calibrated again using the aforementioned or other conventional wind tunnel coordinate systems, the calibration process would inevitably be cumbersome and time-consuming.

[0034] However, the position of the circular feature markers on the wind tunnel wall surface within the overlapping field of view of the binocular system remains unchanged, and their three-dimensional coordinates in the initial wind tunnel coordinate system remain the same. All have been determined, and the alignment of the binocular system measurement coordinate system with the wind tunnel coordinate system under this condition can be achieved by using the resection method.

[0035]

[0036]

[0037] In the formula The focal lengths of the left and right cameras corresponding to the pixel coordinates; These are the intrinsic parameter matrices for the left and right cameras, respectively. Let be the rotation and translation matrix from the right camera to the wind tunnel coordinate system; Let be the rotation and translation matrix from the left camera to the wind tunnel coordinate system; , Let i and j represent the extrinsic parameter matrices from the left and right camera coordinate systems to the wind tunnel coordinate system after the binocular system has been recalibrated, respectively, where i and j are the subscripts of the matrix elements. .

Claims

1. A method for calibrating a wind tunnel coordinate system using a binocular system based on a calibration plate and wind tunnel feature points, characterized in that, Includes the following steps: S1: Obtain the global coordinates on the calibration plate outside the wind tunnel using photogrammetry, construct a local coordinate system, and obtain the coordinates in the local coordinate system. ; S2: A binocular system is installed in the wind tunnel and the binocular internal and external parameters are calibrated using a crosshair calibration ruler. The binocular system includes two industrial cameras. S3: Arrange several circular feature markers on the surface of the cave wall where the binocular system's field of view overlaps; S4: Correctly position the calibration board according to the geometric relationship between the wind tunnel and the camera, ensuring that all vectors in the local coordinate system are parallel or perpendicular to the incoming flow vector. The binocular system then reconstructs the global points of the calibration board. and with Registration is performed to align the binocular system's measurement coordinate system with the wind tunnel's coordinate system; S5: Reconstructing the three-dimensional coordinates of the fixed point in the wind tunnel coordinate system using a binocular system. ; S6: Subsequent experiments, based on three-dimensional coordinates The wind tunnel coordinate system was recalibrated using resection.

2. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 1, characterized in that, Step S1 specifically includes: S11: Fabricate a calibration plate and spray circular feature marks onto the surface of the calibration plate; the adjacent boundaries of the calibration plate are perpendicular to each other, and the shape is a cuboid. S12: The global point coordinates are obtained by 3D reconstruction of the circular feature points on the calibration plate surface using industrial close-range photogrammetry technology. , S13: Global point coordinates Establish a local coordinate system such that all vectors in the local coordinate system are perpendicular or parallel to the boundary of the calibration plate, and obtain the three-dimensional coordinates of the global point in the local coordinate system. .

3. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 2, characterized in that, Step S11 specifically includes spraying coded points and non-coded points, wherein the coded points use 12-bit encoding.

4. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 2, characterized in that, Step S12 specifically includes: Before the experiment, images of the calibration plate were taken from various angles outside the wind tunnel using a camera. Using industrial close-range photogrammetry, 3D reconstruction of the circular feature points on the calibration plate surface was performed to obtain the global point coordinates of each marker point on the calibration plate surface in the photogrammetric coordinate system. : In the formula, Indicates the marker point Pixel coordinates in the image coordinate system Indicates the lens focal length. Indicates the marker point Coordinates in the world coordinate system , The coordinates of the camera coordinate system origin in the photogrammetric coordinate system. This represents the rotation matrix components from the photographer's coordinate system to the camera's coordinate system. Indicates the principal point deviation. This indicates the pixel coordinate deviation caused by lens distortion, mainly including radial distortion. Tangential distortion Image plane distortion .

5. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 1, characterized in that, Step S2 specifically includes: The binocular camera's internal and external parameters are calibrated using a crosshair calibration ruler. The internal parameters include ten parameters: focal length, two principal point deviation parameters, three radial distortion parameters, two eccentric distortion parameters, and two image plane distortion parameters. The external parameters include the rotation and translation matrices of each industrial camera relative to the world coordinate system.

6. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 1, characterized in that, In step S3, the binocular system includes two cameras, left and right, and the two cameras are installed at a certain angle. When arranging the circular feature markers, it is necessary to ensure that both the left and right cameras can capture the circular feature markers.

7. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 1, characterized in that, Step S4 specifically includes: S41: Find the geometric constraint relationship between the calibration plate and the wind tunnel, and position the calibration plate so that each vector in its local coordinate system is parallel or perpendicular to the incoming flow vector; S42: The binocular system reconstructs the circular feature points on the calibration board surface, obtaining the three-dimensional coordinates of each marker point in the binocular system's measurement coordinate system. ; S43: Three-dimensional coordinates in the local coordinate system and three-dimensional coordinates in the binocular system measurement coordinate system Point cloud With point clouds The registration method achieves alignment between the measurement coordinate system of the binocular system and the wind tunnel coordinate system.

8. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 5, characterized in that, Step S43 specifically includes: using point cloud registration to reconstruct the three-dimensional coordinates at different locations. and Alignment, calculation of rotation matrix Translation matrix .

9. The binocular system wind tunnel coordinate system calibration method based on calibration plate and wind tunnel feature points according to claim 1, characterized in that, Step S6 specifically includes: The three-dimensional coordinates of a fixed point on the wind tunnel wall in the wind tunnel coordinate system and corresponding points in the left and right images , Construct least-squares equations to recalibrate the wind tunnel coordinate system: In the formula The focal lengths of the left and right cameras corresponding to the pixel coordinates; These are the intrinsic parameter matrices for the left and right cameras, respectively. Let be the rotation and translation matrix from the right camera to the wind tunnel coordinate system; Let be the rotation and translation matrix from the left camera to the wind tunnel coordinate system; , Let i and j represent the extrinsic parameter matrices from the left and right camera coordinate systems to the wind tunnel coordinate system after the binocular system has been recalibrated, respectively, where i and j are the subscripts of the matrix elements. .

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