Method for measuring attitude angle of free throwing object in wind tunnel test through binocular system

Abstract

The invention discloses a method for measuring an attitude angle of a free throwing object in a wind tunnel test through a binocular system, and belongs to the field of aerodynamics tests based on a photogrammetry technology. Comprising the steps that a binocular system is arranged in a wind tunnel, and internal and external parameters of the binocular system are calibrated through a cross scale; reconstructing the surface mark points of the calibration plate by photogrammetry outside the wind tunnel and establishing a local coordinate system; placing a calibration plate in the wind tunnel, wherein each vector of a local coordinate system is parallel or vertical to an incoming vector, and calibrating a wind tunnel coordinate system by adopting a resection method; carrying out photogrammetry and initial pose installation on mark points on the surface of a thrown object; acquiring each state image of the thrown object and reconstructing the three-dimensional coordinates of the surface mark points; taking the attitude angle of the thrown object as 0 degree as a reference state, and comparing the rigid body transformation relationship between the blowing state point cloud and the reference state point cloud to obtain attitude angle data. Alignment of a wind tunnel coordinate system and a binocular system coordinate system can be achieved, limitation of the appearance of the thrown object is avoided, and attitude angle data of the thrown object are calculated in a non-contact and high-precision mode.

Description

Technical Field

[0001] The invention relates to a method for measuring the attitude angle of a freely dropped object in a wind tunnel test using a binocular system, and belongs to the field of aerodynamic tests. Background Art

[0002] The measurement of the attitude of objects in wind tunnel tests is a key basis for aircraft design, and visual measurement methods, as non-contact, high-precision and high-efficiency measurement methods, have gradually been applied to wind tunnel tests.

[0003] Wind tunnel droplet attitude measurement tests face various challenges. These include: These tests are often conducted in large wind tunnels, resulting in a wide camera field of view. The prevailing Zhang Zhengyou calibration method requires a large, high-precision checkerboard grid for such a large field of view. This large grid is difficult to manufacture and maintain, resulting in low calibration accuracy. Attitude measurement tests must be conducted in a wind tunnel coordinate system, which theoretically requires that each axial vector in the wind tunnel coordinate system be absolutely parallel or perpendicular to the wind tunnel flow vector. However, the visual measurement camera coordinate system is a virtual coordinate system, making alignment of the camera measurement coordinate system with the wind tunnel coordinate system challenging. Furthermore, existing research on visual attitude measurement in wind tunnels has primarily focused on angle-of-attack attitude measurement in step tests or continuously varying angle-of-attack tests, where attitude changes are relatively small. Measuring the complex motion attitude of free-flying objects, such as aircraft droplets, is challenging, particularly in eliminating the coupling effects between angles when the droplet attitude undergoes large angle changes. Summary of the Invention

[0004] In order to solve the above-mentioned problems, the present invention proposes a method for measuring the attitude angles of freely projected objects in wind tunnel tests using a binocular system. The present invention can complete high-precision camera calibration and wind tunnel coordinate system calibration without the need to manufacture high-precision, large-format targets; it is not restricted by the shape of the projected object and can realize attitude calculation including pitch angle, sideslip angle, and roll angle.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention provides a method for measuring the attitude angle of a freely-placed object in a wind tunnel test using a binocular system, comprising the following steps: S1: Arrange a binocular system in the wind tunnel and calibrate the internal and external parameters of the binocular system using a cross scale; the binocular system includes two industrial cameras. S2: Photogrammetry is used outside the wind tunnel to reconstruct the surface markers of the calibration plate and establish a local coordinate system , to obtain the coordinates in the local coordinate system ; S3: Place the calibration plate in the wind tunnel so that the vectors of the local coordinate system are parallel or perpendicular to the incoming flow vector, and use the resection method to calibrate the wind tunnel coordinate system ; S4: Photogrammetry of the marked points on the surface of the projectile to obtain the three-dimensional coordinates of all points on the surface of the projectile , and perform initial posture installation; S5: Collect images of various states of the object and reconstruct the three-dimensional coordinates of the surface markers ; S6: Taking the object posture angle of 0° as the reference state, compare the point cloud of the blowing state and baseline point cloud The rigid body transformation relationship is used to obtain the attitude angle data.

[0006] Preferably, step S1 specifically includes: The binocular camera's intrinsic and extrinsic parameters are calibrated using a crosshair. These parameters include ten intrinsic parameters: focal length, two principal point deviation parameters, three radial distortion parameters, two eccentricity distortion parameters, and two image plane distortion parameters. Extrinsic parameters include the rotation and translation matrices of each industrial camera relative to the world coordinate system.

[0007] Preferably, step S2 specifically includes: S21: calibrate a plate and spray circular feature marking points on the surface of the plate; adjacent boundaries of the calibration plate are perpendicular to each other, and the shape of the calibration plate is a rectangular parallelepiped.

[0008] S22: Use industrial close-range photogrammetry technology to perform 3D reconstruction of circular feature points on the calibration plate surface to obtain global point coordinates .

[0009] S23: Global point coordinates Establish a local coordinate system so that each vector of the local coordinate system is 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 .

[0010] Preferably, step S22 specifically includes: Before the experiment, a single-lens reflex camera was used outside the wind tunnel to take images of the calibration plate from all directions. The circular feature points on the calibration plate surface were reconstructed in three dimensions using industrial close-range photogrammetry technology to obtain the global point coordinates of each mark point on the calibration plate surface in the photogrammetric coordinate system. .

[0011] Preferably, step S3 specifically includes: In wind tunnel tests, the attitude angle of the projectile is calculated in the wind tunnel coordinate system. The establishment of the wind tunnel coordinate system only cares about the direction vector but not the origin position. After the camera calibration is completed, the binocular system measurement coordinate system is in a certain reference camera coordinate system, so the binocular system measurement coordinate system should be aligned with the wind tunnel coordinate system.

[0012] The local coordinate system vectors of the calibration plate placed in the wind tunnel are parallel or perpendicular to the incoming flow vector, and the three-dimensional coordinates of the calibration plate surface in the local coordinate system are and corresponding points of the left and right images 、 Construct the least squares equation and calibrate the wind tunnel coordinate system using the resection method for the binocular system .

[0013] Preferably, step S5 specifically includes: S51: Calculate the three-dimensional coordinates of each state mark point on the surface of the object using the least squares method :

[0014] Where:

[0015]

[0016] in, is the coordinate of the 3D point to be reconstructed; 、 are the pixel coordinates on the left and right images after distortion correction; 、 They are the elements of the extrinsic matrix of the left and right cameras of the binocular system, which are the transformation matrices that project the points in the wind tunnel coordinate system to the camera coordinate system, where i and j are the subscripts of the matrix elements. .

[0017] S52: 3D coordinates of all points on the surface of the projectile in the world coordinate system Three-dimensional coordinates of some points on the surface of the object under different states in the wind tunnel coordinate system Align to obtain the three-dimensional coordinates of all points on the surface of the object in each state .

[0018] Preferably, step S6 specifically includes: S61: Take the object posture angle of 0° as the reference state and calculate the point cloud reflecting the reference state Point cloud with blowing state The rotation matrix of the posture change between and translation matrix

[0019] S62: By rotation matrix Calculate the angle of attack of the projectile , in Indicates the X-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane:

[0020] S63: By rotation matrix Calculate the sideslip angle of the object , in Indicates the Y-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane:

[0021] S64: By rotation matrix Calculate the roll angle of the projectile :

[0022] in, 、 Respectively represent State rotation matrix Quantity.

[0023] This invention addresses the existing problem of camera vibration causing changes in external parameters and significantly reducing attitude angle measurement accuracy in actual wind tunnel environments. This method proposes a method for obtaining attitude angle data by comparing the rigid body transformation relationship between the wind-blown point cloud and the reference point cloud, using the attitude angle of the projectile at 0° as the baseline state. Experimental verification shows that in a laboratory environment, using the readings of a high-precision dual-axis turntable as the standard, the average error in calculating the angle of attack, sideslip angle, and roll angle of compound motion using the algorithm of the invention is within 0.01°, demonstrating high measurement accuracy. In the dynamic vibration environment of an actual wind tunnel, the attitude angle calculated by the method of the invention is significantly improved compared to direct attitude angle calculation methods, achieving an attitude angle measurement accuracy better than 0.1°. This method can effectively meet the requirements for measuring the attitude angles of aircraft and their projectiles in wind tunnel testing and is particularly suitable for attitude angle measurement in wind tunnel testing under vibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a flow chart of the present invention; Figure 2 It is a test scene diagram of the present invention; Figure 3 This is a schematic diagram of photogrammetry of marking points on the surface of a calibration plate according to the present invention; Figure 4 Schematic diagram of the local coordinate system of the calibration plate surface of the present invention; Figure 5 It is a schematic diagram of wind tunnel coordinate system calibration of the present invention; Figure 6It is a cross scale used for camera calibration in the present invention; Figure 7 This is a real-time status diagram disclosed in an embodiment of the present invention; Explanation of the symbols in the accompanying drawings: a1, incoming flow; a2, dropped object; a3, two cameras of the binocular system; c1, coded mark point; c2, non-coded mark point; c3, local coordinate system of the calibration plate; d1, calibration plate; d2, local coordinate system; d3, wind tunnel coordinate system; d4, two cameras of the binocular system. DETAILED DESCRIPTION

[0025] The following is a detailed description of the embodiments of the present invention. It should be emphasized that the following description is only exemplary and is not intended to limit the scope of the present invention and its application.

[0026] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element. In addition, connection can be used for both fixing and circuit / signal communication.

[0027] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and 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, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0028] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of such features. In the description of the embodiments of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0029] like Figure 1 As shown, one embodiment of the present invention discloses a method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system, comprising the following six steps: S1: arranging the binocular system in the wind tunnel and calibrating the internal and external parameters of the binocular system using a cross-scale; S2: reconstructing the surface markers of the calibration plate using photogrammetry outside the wind tunnel and establishing a local coordinate system. , to obtain the three-dimensional coordinates of the global point in the local coordinate system ; S3: Place the calibration plate in the wind tunnel so that the vectors of the local coordinate system are parallel or perpendicular to the incoming flow vector, and use the resection method to calibrate the wind tunnel coordinate system ; S4: Photogrammetry of the marked points on the surface of the projected object to obtain the three-dimensional coordinates of all points on the surface of the projected object , and perform initial posture installation; S5: Collect images of each state of the object and reconstruct the three-dimensional coordinates of the surface marker points ; S6: Take the object posture angle of 0° as the reference state and compare the point cloud of the blowing state and baseline point cloud The rigid body transformation relationship is used to obtain the attitude angle data.

[0030] The specific implementation process is: Step 1: Arrange the binocular system in the wind tunnel and calibrate the internal and external parameters of the binocular system: like Figure 2 As shown, before the test, based on the wind tunnel test measurement requirements and on-site wind tunnel installation conditions, appropriate camera models and lenses were selected and the installation positions were rationally arranged so that the optical axes of the two cameras a3 in the binocular system formed an angle of approximately 30°, and the public field of view should include the spatial range of the model's motion. In wind tunnel experiments, the object a2 moves extremely fast under the influence of the incoming flow a1, with its fall time typically ranging from several hundred milliseconds. Therefore, the two cameras a3 in the binocular system should have a high frame rate and high resolution.

[0031] In addition, if there is an optical window in the wind tunnel test section, the binocular system should first be considered to be arranged outside the optical window.

[0032] After installing the two cameras a3 in the binocular system, adjust the camera aperture and focus and set an appropriate exposure time to ensure a clear image. Place the crosshairs at at least eight different locations within the wind tunnel and collect calibration images. The placement should encompass the motion space of the projectile a2.

[0033] According to the three-dimensional coordinates of the known mark point on the cross scale in the world coordinate system And the marker pixels detected by the left and right images of the binocular system , the least squares method is used to calibrate the internal and external parameters of the two cameras a3 of the binocular system. The cross scale used for camera calibration is as follows Figure 6 As shown, the entire scale is black with coding dots sprayed on it.

[0034] Step 2: Photogrammetry of 3D points on the calibration plate surface and establishment of the local coordinate system like Figure 4Figure 1 shows a schematic diagram of the local coordinate system on the calibration plate surface. Several markers are affixed to the surface of the calibration plate. These markers are not limited to coded markers c1 or non-coded markers c2. The distribution of these markers should be as uniform as possible, with each row or column of markers parallel or perpendicular to the plate boundary. Adjacent boundaries of the calibration plate should be perpendicular to each other, forming a rectangular parallelepiped. This allows for subsequent shape constraints between the calibration plate and the wind tunnel wall to ensure that the vectors in the local coordinate system c3 of the calibration plate are perpendicular or parallel to the vectors in the wind tunnel coordinate system.

[0035] Before the experiment, a single-lens reflex camera was used outside the wind tunnel to take images of the calibration plate from various directions, such as Figure 3 As shown. Based on the industrial close-range photogrammetry technology, the three-dimensional reconstruction of the mark points c1c2 on the calibration plate surface is performed to obtain the global point three-dimensional coordinates of each mark point p on the calibration plate surface in the photogrammetry coordinate system. .

[0036] The three-dimensional coordinates of the global points on the calibration plate surface obtained by photogrammetry The photogrammetric coordinate system is usually located in the coordinate system of the camera that takes the first frame of the image, not on the calibration plate. Therefore, it is necessary to establish a local coordinate system for the global point on the calibration plate surface according to the constraint that the marking points in each row or column on the calibration plate surface are parallel or perpendicular to the calibration plate boundary, so that each vector in the local coordinate system is perpendicular or parallel to the calibration plate boundary, and the local coordinate system c3 of the calibration plate in Figure 4 is obtained. Step 3: Wind tunnel coordinate system calibration Obtain the global point of the local coordinate system d2 of the calibration plate d1 as described in step 2 Under the premise of the coordinates, through the geometric constraint relationship between the calibration plate d1 and the wind tunnel, the calibration plate d1 is placed so that the vectors of the local coordinate system d2 are parallel or perpendicular to the incoming flow vector (such as Figure 5 If the calibration plate is in the shape of a cuboid, the calibration plate can be placed in close contact with the side walls and bottom wall of the wind tunnel to ensure that the vectors of its local coordinate system are parallel or perpendicular to the incoming flow vector.

[0037] According to the three-dimensional point coordinates in the local coordinate system of the calibration plate surface And the corresponding points of the images taken by the two cameras d4 of the binocular system 、 The wind tunnel coordinate system d3 can be calibrated by constructing the least squares equation and using the resection method for the binocular system.

[0038] Determining the projectile's posture using visual measurement requires at least three pairs of non-collinear, identically named points in the point cloud reconstructed in each state and the point cloud reconstructed in the initial state frame. Due to the limited range of binocular vision measurement, only the 3D coordinates of a subset of markers on the projectile's surface can be reconstructed during each state frame. When the projectile's posture undergoes significant changes, there is often no overlap with the reference point cloud, making it impossible to determine the posture angle.

[0039] In order to ensure that each state has overlapping point clouds with the reference state of the attitude angle solution in real time, before the test, the three-dimensional coordinates of all the marking points on the surface of the projectile in the photogrammetric coordinate system are reconstructed using industrial close-range photogrammetry technology as described in step 2. .

[0040] Step 5: Reconstruction of surface point clouds of objects in different states A binocular system is used to collect images of each test state. The binocular camera acquisition process must use synchronized hard triggering to ensure the synchronization of the binocular system.

[0041] First, the binocular system is used to reconstruct the three-dimensional coordinates of each state mark point on the surface of the object in the overlapping area. :

[0042] Where:

[0043]

[0044] in, is the coordinate of the 3D point to be reconstructed; 、 are the pixel coordinates on the left and right images after distortion correction; 、 They are the elements of the extrinsic matrix of the left and right cameras of the binocular system, which are the transformation matrices that project the points in the wind tunnel coordinate system to the camera coordinate system, where i and j are the subscripts of the matrix elements. .

[0045] However, as mentioned above, the binocular system has a limited overlapping field of view and can only reconstruct the three-dimensional coordinates of some points on the surface of the projected object. In order to obtain the three-dimensional coordinates of all points on the surface of the projected object in each state, the three-dimensional coordinates of all points on the surface of the projected object in the photogrammetric coordinate system are used. Three-dimensional coordinates of some points on the surface of the object under different states in the wind tunnel coordinate system Align to obtain the three-dimensional coordinates of all points on the surface of the object in each state .

[0046] Step 6: Numerical calculation of attitude angle Figure 7The real-time state diagram of the falling object is shown. During the test, the attitude angle of the object is 0° as the reference state, and the point cloud reflecting the reference state is calculated. Point cloud with blowing state The rotation matrix of the posture change between and translation matrix .

[0047] By the rotation matrix Calculate the angle of attack of the projectile , in Indicates the X-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane:

[0048] By the rotation matrix Calculate the sideslip angle of the object ,in Indicates the Y-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane:

[0049] By the rotation matrix Calculate the roll angle of the projectile :

[0050] in, 、 Respectively represent State rotation matrix Quantity.

Claims

1. A binocular system method for measuring the attitude angle of a freely dropped object in a wind tunnel test, characterized in that The following steps are involved: S1: Arrange a binocular system in the wind tunnel and calibrate the internal and external parameters of the binocular system using a crosshair. The binocular system includes two industrial cameras. S2: Use photogrammetry to reconstruct the surface markers of the calibration plate outside the wind tunnel and build a local coordinate system , so as to obtain the coordinate value of the marker point in the local coordinate system ; S3: Place the calibration plate in the wind tunnel so that the vectors of the local coordinate system of the calibration plate are parallel or perpendicular to the incoming flow vector; then, use the resection method to realize the wind tunnel coordinate system Calibration; S4: Photogrammetry of the marked points on the surface of the projectile to obtain the three-dimensional coordinates of all points on the surface of the projectile , and install the initial posture of the launch object; S5: Collect images of various states of the object and reconstruct the three-dimensional coordinates of the surface markers ; S6: Taking the object posture angle of 0° as the reference state, compare the point cloud of the blowing state and baseline point cloud The rigid body transformation relationship is used to obtain the attitude angle data.

2. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S1 specifically includes: The internal and external parameters of the binocular camera are calibrated using a cross-scale, including ten internal parameters: focal length, two principal point deviation parameters, three radial distortion parameters, two eccentricity 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.

3. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S2 specifically includes: S21: calibrating a plate and spraying circular feature marking points on the surface of the plate; the adjacent boundaries of the calibration plate are perpendicular to each other and the shape of the calibration plate is a rectangular parallelepiped; S22: Use industrial close-range photogrammetry technology to perform 3D reconstruction of circular feature points on the calibration plate surface to obtain global point coordinates ; S23: Global point coordinates Establish a local coordinate system so that each vector of the local coordinate system is 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 .

4. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 3, characterized in that: Step S22 specifically includes: Before the experiment, a camera was used outside the wind tunnel to capture images of the calibration plate from all directions. The circular feature points on the calibration plate surface were reconstructed in three dimensions using industrial close-range photogrammetry technology to obtain the global point coordinates of each mark point on the calibration plate surface in the photogrammetric coordinate system. .

5. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S3 specifically includes: The local coordinate system vectors of the calibration plate placed in the wind tunnel are parallel or perpendicular to the incoming flow vector, and the three-dimensional coordinates of the calibration plate surface in the local coordinate system are and corresponding points of the left and right images 、 Construct the least squares equation to recalibrate the wind tunnel coordinate system, and use the resection method to calibrate the wind tunnel coordinate system for the binocular system .

6. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S4 specifically includes: First, the marking points are reasonably arranged on the surface of the placed object to ensure that the number and distribution cover the entire surface of the placed object; then, the marking points are photographed from multiple angles and shooting distances with a camera to obtain multiple sets of image data containing marking point information; because the placed object is three-dimensional, it does not affect the fact that all marking points cannot be photographed from some angles, and the same-name marking points will be found from multiple sets of photos later.

7. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S5 specifically includes: S51: Calculate the three-dimensional coordinates of each state mark point on the surface of the object using the least squares method : Where: in, is the coordinate of the 3D point to be reconstructed; 、 are the pixel coordinates on the left and right images after distortion correction; 、 They are the elements of the extrinsic matrix of the left and right cameras of the binocular system, which are the transformation matrices that project the points in the wind tunnel coordinate system to the camera coordinate system, where i and j are the subscripts of the matrix elements. ; S52: Reconstruct the three-dimensional coordinates of the surface marking points of the projectile in the wind tunnel coordinate system, use the SVD decomposition algorithm to achieve point cloud alignment, and obtain the three-dimensional coordinates of all surface marking points of the projectile in the wind tunnel coordinate system in the initial test frame.

8. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 7, characterized in that: Step S52 specifically includes: Three-dimensional coordinates of all points on the surface of the projectile in the world coordinate system Three-dimensional coordinates of some points on the surface of the object under different states in the wind tunnel coordinate system Align to obtain the three-dimensional coordinates of all points on the surface of the object in each state .

9. The method for measuring the attitude angle of a freely projected object in a wind tunnel test using a binocular system according to claim 1, characterized in that: Step S6 specifically includes: S61: Take the object posture angle of 0° as the reference state and calculate the point cloud reflecting the reference state Point cloud with blowing state The rotation matrix of the posture change between and translation matrix ; S62: By rotation matrix Calculate the angle of attack of the projectile , in Indicates the X-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane: S63: By rotation matrix Calculate the sideslip angle of the object ,in Indicates the Y-axis vector of the model body coordinate system when the model is in the reference state; Indicates the State machine body axis vector On the airflow axis Projection vector to the plane: S64: By rotation matrix Calculate the roll angle of the projectile : in, 、 Respectively represent State rotation matrix Quantity.

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