A method for separating vibration in strain measurement of wind tunnel tests

Abstract

The present invention proposes a method for separating jitter in wind tunnel strain measurement, which can eliminate measurement errors caused by jitter in wind tunnel strain measurement, thereby accurately measuring the surface strain of the object being measured in the wind tunnel test. When using this jitter separation method, N measurement targets are set on a support rod; after the wind tunnel test begins, a dual-camera measurement system performs image acquisition to obtain m sets of image sequence pairs with jitter; using the coordinate system of any camera in the dual-camera measurement system as the camera measurement coordinate system, the spatial coordinates of each measurement point and each measurement target in each set of image sequence pairs are calculated in the camera measurement coordinate system; then, the spatial coordinates of each measurement point in each set of image sequence pairs are transformed in the camera measurement coordinate system to obtain the coordinates of the measurement point after de-jittering.

Description

Technical Field

[0001] The present invention relates to a jitter separation method, in particular to a jitter separation method in wind tunnel test strain measurement, and belongs to the technical field of photoelectric measurement. Background Art

[0002] A wind tunnel is a pipe-type device used for aerodynamic testing. It is a fundamental piece of equipment for aerodynamics and aircraft development. Power equipment drives the airflow within its pipes, and the airflow velocity can be controlled to the speed required for aerodynamic testing on the model.

[0003] During wind tunnel testing, the aircraft model 3 under test typically uses a tail support configuration. The head of the aircraft model 3 faces the airflow outlet 6, while the tail is connected to a strut 4, which is then fixed to the scimitar. The wind tunnel model-strut system forms a typical cantilever beam structure. This support configuration has a minimal impact on the flow field surrounding the model. However, because the tail support strut 4 is typically 3 to 5 times the length of the model, the cantilever beam structure exhibits low system stiffness. During wind tunnel testing, the model is subjected to aerodynamic loads across a wide frequency range, causing the model-strut system to produce low-frequency, large-amplitude vibrations at the first-order natural frequency.

[0004] To study the performance characteristics of aircraft model 3 in wind tunnel 2, it is necessary to study the stress and strain generated by the airflow pressure on its surface. The non-contact three-dimensional optical measurement method based on the DIC (digital image correlation) principle is applied to the measurement of the surface strain of the aircraft in the wind tunnel test. Figure 1 The dual-camera measurement system shown has two measurement cameras 1, one for the left and one for the right. During the loading process, the surface of the aircraft model 3 under test is continuously photographed and measured. Based on DIC (digital image correlation) technology, the image analysis and strain resolution unit 5 analyzes and calculates each set of images. A large number of microfacets are obtained in each camera image, and the grayscale distribution of each facet is calculated. The exact position of each microfacet in all images is calculated. The three-dimensional spatial position of the microfacets at different loading stages is analyzed, and their displacements in the X, Y, and Z directions are accurately calculated, thereby achieving full-field strain measurement.

[0005] However, due to the vibrations present in wind tunnel testing, the vibration displacement of aircraft model 3, the strain displacement on its surface, and the vibration displacement of measurement camera 1 in the wind tunnel environment all coexist. These multiple displacements combine to directly affect the accuracy of strain measurements. While strain displacement is typically small, typically no greater than 0.1 mm, vibration displacement can reach several millimeters, overwhelming the strain measurement results. Therefore, it is necessary to separate the vibration displacement (i.e., jitter) of aircraft model 3 and measurement camera 1 to obtain accurate strain measurements. Summary of the Invention

[0006] In view of this, the present invention proposes a method for separating jitter in wind tunnel test strain measurement, which can eliminate the measurement error caused by jitter in wind tunnel test strain measurement, thereby accurately measuring the surface strain of the object being measured in the wind tunnel test.

[0007] A method for separating vibration in strain measurement during wind tunnel testing uses a dual-camera measurement system to measure strain on a test object. The test object is supported by a support rod and has multiple measurement points on it.

[0008] Set N measurement targets on the support rod, N ≥ 5;

[0009] After the wind tunnel test begins, the dual-camera measurement system acquires images and obtains m sets of image sequence pairs with jitter measurements;

[0010] Taking the coordinate system of any camera in the dual-camera measurement system as the camera measurement coordinate system, calculate the spatial coordinates of each measurement point and each measurement target in each set of image sequence pairs in the camera measurement coordinate system;

[0011] For the same measurement target, its spatial coordinates Q1 in the camera measurement coordinate system calculated by the first image sequence pair and its spatial coordinates Q in the camera measurement coordinate system calculated by the jth image sequence pair are j Satisfaction: Q j =R j Q1+t j ; where R j is the rotation matrix, t j is the translation matrix; solve R j and t j ;

[0012] The following coordinate transformation is performed on the spatial coordinates of each measurement point in each set of solved image sequence pairs in the camera measurement coordinate system:

[0013] (x j ,y j ,z j )=(x cj ,y cj ,z cj )R j +t j

[0014] Where: j = 2...m; (x cj ,y cj ,z cj ) is the spatial coordinate of the measurement point calculated from the j-th image sequence pair in the camera measurement coordinate system; (x j ,y j ,z j) is the spatial coordinate of the measurement point in the camera measurement coordinate system after the j-th image sequence pair is de-jittered.

[0015] As a preferred embodiment of the present invention, when solving the rotation matrix and translation matrix:

[0016] First, the calculated spatial coordinates of each measurement target in the camera measurement coordinate system are de-centered to obtain the de-centered coordinates of each measurement target. The de-centered coordinates are spatial coordinates containing only rotational components.

[0017] According to the following parametric equation, the rotation matrix R is solved by optimizing iteration j :

[0018]

[0019] In the above system of equations: r j1 …r j9 is the rotation matrix element; (x 1i ,y 1i ,z 1i is the decentering coordinate of the i-th measurement target in the first image sequence, i = 1, 2…N, x ji ,y ji ,z ji is the de-centering coordinate of the i-th measurement target in the j-th image sequence pair, j = 2...m;

[0020] Add the following constraints to each element of the rotation matrix:

[0021] Optimize the iteration to get the rotation matrix R j After that, through Q j =R j Q1+t j Find the translation matrix.

[0022] As a preferred embodiment of the present invention, the process of de-centering is as follows: calculating the average value of the spatial coordinates of the N measurement targets solved for each image sequence in the camera measurement coordinate system, that is, the center of gravity coordinates; then subtracting the center of gravity coordinates from the spatial coordinates of each measurement target in the camera measurement coordinate system to obtain the de-centered coordinates of the measurement target.

[0023] As a preferred embodiment of the present invention, the method is integrated into the image analysis and strain calculation unit of the wind tunnel test dual-camera measurement system.

[0024] As a preferred embodiment of the present invention, the measurement target is a circular reflective target, a coded target, a cross-line target, or a checkerboard target.

[0025] Beneficial effects:

[0026] (1) The jitter separation method of the present invention can be used to eliminate the measurement error caused by the jitter in the strain measurement of the wind tunnel test, thereby effectively improving the accuracy of the strain measurement in the vibration environment.

[0027] (2) This jitter separation method directly uses the dual-camera measurement system used in DIC strain measurement. By reusing the same dual-camera measurement system, it is only necessary to set the measurement target on the support rod. The jitter separation calculation can be performed while measuring the surface strain of the object to be measured.

[0028] (3) The calculation process of the jitter separation method is simple. It only requires adding the vibration transformation matrix solution step and the measurement point coordinate conversion step in the image analysis and strain solution unit, and has strong versatility. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the dual-camera measurement system mentioned in the background technology;

[0030] Figure 2 is a flow chart of the jitter amount separation method of the present invention;

[0031] Figure 3 Schematic diagram of the four coordinate systems involved in the wind tunnel test.

[0032] Among them: 1- measurement camera; 2- wind tunnel; 3- aircraft model; 4- support rod; 5- image analysis and strain calculation unit; 6- airflow outlet. DETAILED DESCRIPTION

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0034] This embodiment provides a method for separating jitter in wind tunnel test strain measurement. This embodiment takes the wind tunnel test of aircraft model 3 as an example (i.e., the object to be measured is aircraft model 3). Through this jitter separation method, the vibration displacement (i.e., the jitter) of aircraft model 3 and measurement camera 1 in the strain measurement results can be separated, thereby obtaining accurate surface strain measurement results of aircraft model 3, and improving the accuracy of aircraft model surface strain measurement in a vibration environment.

[0035] The jitter separation method is directly based on the dual-camera measurement system used in DIC strain measurement (i.e. Figure 1 The dual-camera measurement system shown in FIG), an improved design is made on the system (specifically, a measurement target is pasted on the support rod 4 of the dual-camera measurement system), and the same dual-camera measurement system is reused, such as Figure 1 As shown, the method includes the following steps:

[0036] Step 1: Construct the measurement coordinate system:

[0037] There are four coordinate systems involved in the wind tunnel test: camera measurement coordinate system O C -X C Y C Z C , support coordinate system O t -X t Y t Z t , aircraft model coordinate system O P -X P Y P Z P and the world coordinate system O W -X W Y W Z W ; In this example, the camera measurement coordinate system O C -X C Y C Z C Defined as the left camera coordinate system of the dual-camera measurement system, the relationship between the four coordinate systems is as follows Figure 2 As shown. The support coordinate system O t -X t Y t Z t The center of mass of the support rod 4 is the coordinate origin O t , the axial direction of the support rod 4 is X t The vertical diameter of the support rod 4 is Y t Direction, upward is positive; aircraft model coordinate system O P -X P Y P Z P The center of mass of the aircraft model 3 is the coordinate origin O P , the axis of aircraft model 3 is X P direction, and with the support coordinate system O t -X t Y t Z t Medium X t The vertical diameter of aircraft model 3 is Y P Direction: upward is positive.

[0038] In the wind tunnel test, the strut 4 is fixedly connected to the aircraft model 3 and vibrates together, which can be regarded as the strut coordinate system O t -X t Y t Z t With the aircraft model coordinate system O P -X P Y P ZP The spatial angle and position relationship of the rod coordinate system O t -X t Y t Z t Relative to the world coordinate system O W -X W Y W Z W Vibration occurs; the camera measures the coordinate system O C -X C Y C Z C Also relative to the world coordinate system O W -X W Y W Z W Vibration occurs.

[0039] Let the spatial point P be a measurement point on the surface of the aircraft model 3 (i.e., a microfacet, a microfacet is a measurement point on the surface of the aircraft model 3, and there are multiple measurement points on the surface of the aircraft model 3), and the measurement point is in the aircraft model coordinate system O P -X P Y P Z P The displacement change under is the strain displacement of the point. By measuring a large number of measurement points, the strain field on the surface of the aircraft model 3 can be obtained.

[0040] Step 2: Paste the measurement target on the support rod 4:

[0041] Paste N measurement targets on the support rod 4, where N ≥ 5; the measurement targets are randomly distributed on the support rod 4, but cannot be arranged in a straight line; the measurement targets can be in any form, such as circular reflective targets, coded targets, cross-line targets, and checkerboard targets, and they are firmly pasted so that they do not fall off or move during the wind tunnel test.

[0042] Step 3: Collect the image containing the jitter:

[0043] The measurement ranges of the two measurement cameras in the dual-camera measurement system must simultaneously cover the measured strain area of the aircraft model 3 (with several measurement points set in the measured strain area) and the measurement target on the support rod 4.

[0044] After the wind tunnel test begins, the two measurement cameras start to collect images, and the left camera and the right camera are used to obtain m sets of image sequence pairs with jitter (T L1 , T R1 ), (T L2 , T R2 )……(T Lm , T Rm ); let j=1,2……m, then T Ljis the jth image measured by the left camera, T Rj is the jth image measured by the right camera.

[0045] Step 4: The left camera and the right camera send the measured image sequence pairs to the image analysis and strain calculation unit, which respectively calculates the micro-facet coordinates containing jitter, calculates the measurement target coordinates, and calculates the vibration transformation matrix.

[0046] (1) Calculate the spatial coordinates of the jittered measurement point in the camera measurement coordinate system:

[0047] After receiving the image sequence pairs measured by the left and right cameras, the image analysis and strain calculation unit calculates the exact position of each measurement point in all images using the internal preset strain calculation method, analyzes the three-dimensional spatial position of the measurement point at different loading stages, and accurately calculates its X coordinate in the camera measurement coordinate system. C 、Y C , Z C The displacement in the direction includes the strain of the measurement point, the vibration displacement of the aircraft model 3, and the vibration displacement of the camera. The vibration displacement of the aircraft model 3 and the vibration displacement of the camera are the jitter amounts that need to be separated. The solution method can adopt a general measurement method, such as a strain measurement method based on DIC, a strain measurement method based on speckle interferometry, etc.

[0048] Take one of the measurement points (such as Figure 2 The spatial position change of point P in the camera measurement coordinate system O is taken as an example to illustrate the solution result. C -X C Y C Z C The spatial coordinates under the condition that the coordinates of the point P calculated by the m sets of image sequences are (x C1 ,y C1 , z C1 ), (x C2 ,y C2 , z C2 )……(x Cm ,y Cm , z Cm ); let j=1,2……m, then (x Cj ,y Cj , z Cj ) is the spatial coordinate of point P in the camera measurement coordinate system calculated by the j-th group of image sequence pairs.

[0049] (2) Calculate the spatial coordinates of the measurement target in the camera measurement coordinate system:

[0050] After receiving the image sequence pairs measured by the left camera and the right camera, the image analysis and strain calculation unit calculates the spatial coordinates of each measurement target using the internal preset coordinate calculation method; the calculation result is the spatial coordinates of each measurement target in the camera measurement coordinate system O C -X C Y C Z C The spatial coordinates of .

[0051] Assume that the spatial coordinate sequence of the i-th measurement target in the camera measurement coordinate system calculated by m sets of image sequence pairs is: (x qi1 ,y qi1 , z qi1 ), (x qi2 ,y qi2 , z qi2 )……(x qim ,y qim , z qim ); let j=1,2……m,i=1,2……N,then (x qij ,y qij , z qij ) is the spatial coordinate of the i-th measurement target in the camera measurement coordinate system calculated by the j-th image sequence pair; thus, corresponding to N measurement targets and m-th image sequence pairs, N spatial coordinate sequences are calculated, each of which includes m spatial coordinates.

[0052] The coordinate calculation of the measurement point containing jitter and the coordinate solution of the measurement target can be performed simultaneously.

[0053] (3) Calculate the vibration transformation matrix:

[0054] The image analysis and strain solution unit calculates the vibration transformation matrix after completing the coordinate calculation of the measurement point containing jitter and the solution of the spatial coordinates of the measurement target in the camera measurement coordinate system.

[0055] Among the m sets of image sequence pairs with jitter, for the same measurement target point, its spatial coordinate Q1 in the camera measurement coordinate system solved by the first image sequence pair and its spatial coordinate Q2 in the camera measurement coordinate system solved by the second image sequence pair satisfy the following relationship Q2=R2Q1+t2, its spatial coordinate Q1 in the camera measurement coordinate system solved by the first image sequence pair and its spatial coordinate Q3 in the camera measurement coordinate system solved by the third image sequence pair satisfy the following relationship Q3=R3Q1+t3, ..., and so on, its spatial coordinate Q1 in the camera measurement coordinate system solved by the first image sequence pair and its spatial coordinate Q in the camera measurement coordinate system solved by the mth image sequence pair satisfy the following relationship Q3=R3Q1+t3, ..., and so on. m Satisfies the following relationship Q m =Rm Q1+t m ; Among them R2, R3...R m is the rotation matrix, t2, t3...t m is the translation matrix.

[0056] Each image has N measurement targets, corresponding to N measurement target points. The spatial coordinates of the measurement targets in the camera measurement coordinate system are calculated and de-centered (the de-centering process is to calculate the average spatial coordinates of the N measurement targets in each image, that is, the center of gravity coordinates, and then subtract the center of gravity coordinates from each measurement target point coordinate to obtain the de-centering measurement target point coordinates, which only contain rotational components). The coordinates containing only rotational components are thus obtained.

[0057] The following parametric equations are listed, and the rotation matrix is solved through optimization iteration (for example, iterative solution can be performed using general algorithms such as orthogonal matrix method and singular value decomposition method).

[0058] Taking the rotation matrix R2 as an example,

[0059]

[0060] In the above system of equations: r 21 …r 29 is the rotation matrix element; x 1i ,y 1i ,z 1i is the decentering coordinate of the i-th measurement target at the initial moment (i.e., the first image sequence pair), x 2i ,y 2i ,z 2i is the de-centering coordinate of the i-th measurement target in the second image sequence pair (i.e. after the measurement target moves), i = 1, 2…N.

[0061] Add the following constraints to the rotation matrix elements:

[0062] After the rotation matrix R2 is obtained through optimization iteration, the rotation angles around the three axes can be obtained by the following formula (i.e., the rotation angle of the support rod 4 around the camera coordinate system at the moment corresponding to the second image sequence pair relative to the initial moment, that is, the rotation angle of the image pair of the second image sequence pair relative to the image pair at the initial moment in the camera coordinate system):

[0063]

[0064]

[0065] β=-arcsin(r 27 )

[0066]

[0067] The translation matrix t2 is obtained according to Q2=R2Q1+t2.

[0068] Through m sets of image sequence pairs with jitter, m-1 rotation matrices and translation matrices can be obtained: (R2, t2), (R3, t3)…(R m , t m ).

[0069] Step 5: Measurement point coordinate conversion

[0070] Perform the following coordinate transformation on the spatial coordinates of the jittered measurement point calculated in step 4 above in the camera measurement coordinate system:

[0071] (x1,y1,z1)=(x c1 ,y c1 ,z c1 )

[0072] (x2,y2,z2)=(x c2 ,y c2 ,z c2 )R2+t2 ......

[0074] (x m ,y m ,z m )=(x cm ,y cm ,z cm )R m +t m

[0075] (x1, y1, z1), (x2, y2, z2)……(x m ,y m , z m ) is the coordinate of the measurement point after de-jittering. The same method is used for the coordinates of the remaining measurement points on the aircraft model 3. After obtaining a large number of micro-facet point coordinates and analyzing their displacements, the strain field distribution of the aircraft can be constructed.

[0076] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A method for separating jitter in wind tunnel strain measurement employs a dual-camera measurement system to measure strain on a test object. The test object is supported by a support rod and has multiple measurement points on the test object. The method is characterized by: Set N measurement targets on the support rod, N ≥ 5; After the wind tunnel test begins, the dual-camera measurement system acquires images and obtains m sets of image sequence pairs with jitter measurements; Taking the coordinate system of any camera in the dual-camera measurement system as the camera measurement coordinate system, calculate the spatial coordinates of each measurement point and each measurement target in each set of image sequence pairs in the camera measurement coordinate system; For the same measurement target, its spatial coordinates Q1 in the camera measurement coordinate system calculated by the first image sequence pair and its spatial coordinates Q in the camera measurement coordinate system calculated by the jth image sequence pair are j Satisfaction: Q j =R j Q1+t j ; where R j is the rotation matrix, t j is the translation matrix; solve R j and t j ; The following coordinate transformation is performed on the spatial coordinates of each measurement point in each set of solved image sequence pairs in the camera measurement coordinate system: (x j ,y j ,z j )=(x cj ,y cj ,z cj )R j +t j Where: j = 2...m; (x cj ,y cj ,z cj ) is the spatial coordinate of the measurement point calculated from the j-th image sequence pair in the camera measurement coordinate system; (x j ,y j ,z j ) is the spatial coordinate of the measurement point in the camera measurement coordinate system after the j-th image sequence pair is de-jittered.

2. The method for separating jitter in wind tunnel test strain measurement according to claim 1, characterized in that: When solving the rotation matrix and translation matrix: First, the calculated spatial coordinates of each measurement target in the camera measurement coordinate system are de-centered to obtain the de-centered coordinates of each measurement target. The de-centered coordinates are spatial coordinates containing only rotational components. According to the following parametric equation, the rotation matrix R is solved by optimizing iteration j : In the above system of equations: r j1 …r j9 is the rotation matrix element; (x 1i ,y 1i ,z 1i ) is the decentering coordinate of the i-th measurement target in the first image sequence, i = 1, 2…N, x ji ,y ji ,z ji is the de-centering coordinate of the i-th measurement target in the j-th image sequence pair, j = 2...m; Add the following constraints to each element of the rotation matrix: Optimize the iteration to get the rotation matrix R j After that, through Q j =R j Q1+t j Find the translation matrix.

3. The method for separating jitter in wind tunnel test strain measurement according to claim 2, characterized in that: The process of de-centering is as follows: the average value of the spatial coordinates of the N measurement targets solved for each image sequence in the camera measurement coordinate system is calculated, i.e., the center of gravity coordinate; and then the center of gravity coordinate is subtracted from the spatial coordinates of each measurement target in the camera measurement coordinate system to obtain the de-centered coordinates of the measurement target.

4. The method for separating jitter in wind tunnel test strain measurement according to claim 1 or 2, characterized in that: This method is integrated into the image analysis and strain calculation unit of the dual-camera measurement system of the wind tunnel test.

5. The method for separating jitter in wind tunnel test strain measurement according to claim 1 or 2, characterized in that: The measurement target is a circular reflective target, a coding target, a cross-line target, or a checkerboard target.

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