Method and system for measuring six-degree-of-freedom pose of launch test model target

Abstract

The invention discloses a projection test model target six-degree-of-freedom pose measurement method and system. The method comprises the following steps: (1) camera internal reference calibration: calibrating a focal length, a distortion coefficient, a pixel size and an image center point coordinate of a camera; (2) estimating external parameters of the camera; (3) estimating a model target pose in a camera coordinate system; and (4) estimating a model target pose in the wind tunnel coordinate system: inputting the model target pose in the camera coordinate system in the step (3) and the camera external parameters in the step (2), and calculating the model target pose in the wind tunnel coordinate system. The method can measure the pose information of the target at each moment, and has high accuracy.

Description

Technical Field

[0001] The invention belongs to the technical field of visual measurement, and in particular relates to a 6-DOF posture measurement method and system for a test model target. Background Art

[0002] During the development of the aircraft embedded suspension delivery system, obtaining the displacement, posture and other motion characteristics of the target in a simulated release test environment is of great significance to the design verification and performance evaluation of the delivery system and to ensure flight safety.

[0003] In assembly release tests, pose estimation can usually be achieved using methods based on inertial measurement units (IMUs), vision, and multiple sensor fusion. Since the inertial measurement unit is unstable and has accumulated errors, the pose accuracy will decrease over time. Some researchers have used compensation algorithms to readjust the weights of sensors during pose calculation to solve the error accumulation problem, but the performance of the compensation algorithm is not stable. Summary of the invention

[0004] The purpose of the present invention is to provide a method and system for measuring the 6-DOF position and posture of a test model target. The present invention can measure the position and posture information of the target at each moment with high accuracy.

[0005] The technical solution of the present invention is a method for measuring the 6-DOF posture of a test model target, comprising the following steps:

[0006] (1) Camera intrinsic calibration: calibrate the camera’s focal length, distortion coefficient, pixel size, and image center coordinates;

[0007] (2) Camera extrinsic parameter estimation;

[0008] (3) Estimate the model target pose in the camera coordinate system;

[0009] (4) Estimate the model target pose in the wind tunnel coordinate system: Input the model target pose in the camera coordinate system of step (3) and the camera external parameters of step (2) to calculate the model target pose in the wind tunnel coordinate system.

[0010] In the aforementioned 6-DOF pose measurement method for the test model target, step (1) is as follows: by using a calibration plate of fixed size and dimension to calibrate the camera intrinsic parameters, the focal length f, distortion coefficient K, pixel size (S x , S y ) and the image center coordinates (C x , C y ).

[0011] In the aforementioned method for measuring the 6-DOF pose of the test model target, the estimation method of step (2) is as follows: fix the calibration block on the release mechanism in the wind tunnel; input the digital-analog 3D data of the calibration block, the motion angle of the release mechanism, the image sequence captured by the camera and the camera intrinsic parameters, and calculate the camera extrinsic parameters;

[0012] In the aforementioned method for measuring the 6-DOF pose of the test model target, step (2) is specifically as follows:

[0013] Assume that the digital-analog 3D coordinate of the calibration block is O and the pixel coordinate is I. The transformation relationship between the target coordinate system of the calibration block and the camera coordinate system is: c T o The transformation relationship between the calibration block target coordinate system and the release mechanism and wind tunnel coordinate system is r T o and b T o ; c T o Using the PNP method, we can estimate:

[0014] I=K× c T o ×O;

[0015] Among them, K is the distortion coefficient. r T o The transformation relationship of the release mechanism relative to the wind tunnel coordinate system after the i-th movement remains unchanged. If known, then the corresponding calculation is

[0016]

[0017] Transformation relationship between the wind tunnel coordinate system and the camera coordinate system during the movement of the release mechanism c T b has remained unchanged, according to Solution c T b .

[0018] In the aforementioned 6-DOF pose measurement method for the test model target, step (3) is as follows: first, the flight process of the aircraft is divided into three different stages: extravehicular, stable flight, and abnormal flight, and the digital model 3D data is input to create a corresponding 2D template set; then, the pose of the previous frame is used to determine the three rotation angles and Z-axis displacement of the initial pose, and then the target detection is used to locate the aircraft in the image, and its unknown X-axis and Y-axis displacements are calculated according to the 2D-3D coordinate pair of the center point, the camera intrinsic parameters, and the four parameters determined by the initial pose, so as to realize the initial pose estimation.

[0019] In the aforementioned 6-DOF pose measurement method for the test model target, when the initial pose estimation is completed, edge detection is performed on the current frame to extract the edge points in the target detection positioning frame. According to the X-axis and Y-axis displacement changes of the initial pose of the current frame relative to the pose of the previous frame, the template set to be searched is determined, and the best matching pose is searched in the selected template set.

[0020] In the aforementioned 6-DOF posture measurement method for the test model target, the 2D template set generation method is as follows: given the posture change range, the 2D template set of the 3D model at various rotation angles and distances can be generated.

[0021] In the aforementioned method for measuring the 6-DOF pose of the test model target, step (4) is specifically as follows:

[0022] Assume that the transformation relationship of the target estimated in the i-th frame relative to the camera coordinate system is according to Transform the target of each frame relative to the camera coordinate system Transformation relationship converted to the target relative wind tunnel coordinate system b T o (i) ; Further, according to Will Transformed into 6-DOF pose [t x , t y , t z , α, β, γ], where:

[0023] R=R x (α)R y (β)R z (γ),t=[t x t y t z ] T ;

[0024]

[0025] A measurement system based on the aforementioned 6-DOF position and posture measurement method for a test model target includes:

[0026] Camera calibration module, used to obtain camera intrinsic and extrinsic parameters;

[0027] The aircraft pose estimation module in the camera coordinate system is used to estimate the 6-DOF pose of the aircraft in the camera coordinate system, including the angles and displacements of pitch, yaw and roll;

[0028] The aircraft pose estimation module in the wind tunnel coordinate system is used to transform the aircraft pose in the camera coordinate system into the aircraft pose in the wind tunnel coordinate system.

[0029] Beneficial effects of the present invention:

[0030] 1. The 6DOF pose measurement method for a model target provided by the present invention can measure the pose information of the target at each moment with high accuracy when the model target moves at high speed, such as in a wind tunnel launch experiment.

[0031] 2. The present invention obtains the image of the target motion process in the test through a high-speed camera. On the basis of known target three-dimensional data and camera intrinsic parameters (3D-2D projection model taken by the camera), the relative posture relationship between the target coordinate system and the camera coordinate system can be solved, and then the camera extrinsic parameters (the conversion relationship between the camera and the wind tunnel coordinate system) can be used to estimate the 6DOF posture of the target in the wind tunnel coordinate system.

[0032] 3. Compared with the existing measurement methods, the model target 6DOF pose measurement method of the present invention has the following advantages:

[0033] (1) High precision: By establishing a segmented template set, the accuracy of posture measurement during the extravehicular phase is significantly improved;

[0034] (2) High robustness: The initial pose estimation is used to constrain the best matching pose search, reducing mismatches caused by multiple solutions;

[0035] (3) Large measurement range: Able to calculate the position of the target in various postures and under partial occlusion.

[0036] Simulation experiments show that the pitch, yaw and roll angle measurement accuracy of the present invention can reach 0.05 degrees, 0.11 degrees and 0.5 degrees, with high accuracy.

[0037] 4. The present invention uses the pose of the previous frame to constrain the initial pose estimation of the next frame, thus avoiding the problem that traditional template matching easily falls into the local optimal solution;

[0038] 5. Since rotation and translation estimation have different properties and are affected by different factors, the initial pose of each frame is implemented in two steps.

[0039] 6. The present invention analyzes the flight process of the experimental aircraft in the wind tunnel and creates corresponding template sets for different flight stages, which can balance the accuracy and measurement range of posture estimation. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is the calibration plate used in the present invention;

[0041] Figure 2 This is a schematic diagram of camera extrinsic parameter measurement in the present invention;

[0042] Figure 3 Schematic diagram of the camera extrinsic measurement coordinate system transformation relationship in the present invention. DETAILED DESCRIPTION

[0043] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] Example 1. A method for measuring the 6-DOF position and posture of a test model target, see Figure 1-Figure 3 The equipment conditions required for the specific implementation of the present invention are: ① an ordinary microcomputer; ② a high-speed camera; ③ the position of the camera: the camera is set on the side of the target; ④ a calibration board, the checkerboard calibration board is used to calibrate the camera and obtain internal and external parameters.

[0045] The environmental conditions required for the specific implementation of the present invention are: a model target motion scene, such as a wind tunnel launch test.

[0046] The present invention is a 6DOF pose measurement method for a model target, and the method steps are as follows:

[0047] Step 1: Camera intrinsic calibration

[0048] The high-speed camera intrinsic calibration is achieved by using a calibration plate of fixed size and dimensions. Using the open source single-target calibration toolbox, the calibration plate is moved according to the camera calibration requirements and the corresponding images are collected to obtain the camera's focal length (f), distortion coefficient (K), pixel size (S x , S y ) and the image center coordinates (C x , C y ) and other parameters.

[0049] Step 2: Camera extrinsic estimation

[0050] Design and process a calibration block, as shown in the attached Figure 1 As shown, it is fixed to the release mechanism in the wind tunnel, and the camera is fixed at a position outside the wind tunnel to shoot, as shown in the attached figure. Figure 2 As shown. Control the release mechanism to move and collect the release mechanism movement angle data. After the release mechanism moves to a fixed position, control the camera to capture the calibration block image.

[0051] Input the calibration block digital-analog 3D data, the release mechanism motion angle, the image sequence captured by the camera, and the camera intrinsic parameters calibrated in step 1 to estimate the camera extrinsic parameters.

[0052] The measurement coordinate system transformation relationship is as follows: Figure 3 As shown, let the digital-analog 3D coordinate of the calibration block be O, the pixel coordinate of the calibration block in the image be I, and the transformation relationship between the calibration block target coordinate system and the camera coordinate system is:c T o The transformation relationship between the calibration block target coordinate system and the release mechanism and wind tunnel coordinate system is r T o and b T o .

[0053] c T o It can be estimated using the PNP (Perspective-n-Point) method:

[0054] I=K× c T o ×O (1)

[0055] During the calibration process r T o The transformation relationship of the release mechanism relative to the wind tunnel coordinate system after the i-th movement remains unchanged. Known, we can calculate:

[0056]

[0057] Transformation relationship between the wind tunnel coordinate system and the camera coordinate system during the movement of the release mechanism c T b has remained unchanged, according to

[0058]

[0059] It can be solved c T b .

[0060] Step 3: Estimation of the model target pose in the camera coordinate system

[0061] The motion images of the model target collected in the wind tunnel launch experiment are processed to obtain the position and pose of the target in the camera coordinate system at each acquisition moment.

[0062] Input digital and analog 3D data, image sequences captured by the camera, and camera intrinsic parameters, and estimate the model target pose through template matching based on the initial estimation.

[0063] (1) Create multiple template sets based on digital model files

[0064] Given the 3D coordinates of the aircraft, the camera intrinsic parameters and the position of the aircraft in the camera coordinate system, the corresponding 2D pixel coordinates can be calculated using formula (1), which is called a 3D contour template. Given the range of position variation, a set of 2D contour templates of the 3D model at different rotation angles and distances can be generated.

[0065] Obviously, the larger the range of posture change and the smaller the change step, the larger the template set and the slower the subsequent template matching speed. Therefore, the aircraft flight process is divided into three different stages: extravehicular, stable flight and abnormal flight, and the corresponding template sets are created.

[0066] The exit from the cabin is the initial stage when the aircraft has just separated from the release mechanism. The target is less affected by the flow field, and the posture is slightly different from the launch posture. The template posture change range and step length used in this stage are set to be very small. Due to the influence of aircraft design and flow field speed, abnormal flight stages may occur. At this time, the template posture change range and step length are set to be larger.

[0067] (2) Initial pose estimation

[0068] The 6 variables of the initial pose of each frame are determined in two steps:

[0069] ① The pose of the previous frame determines the four parameters of the initial pose.

[0070] The origin of the camera coordinate system is defined as the optical center of the camera, the X-axis and Y-axis are parallel to the X-axis and Y-axis of the image coordinate system, and the Z-axis is the optical axis of the camera. The three rotation angles and Z-axis displacement of the pose estimated in the previous frame are used as the rotation angle and Z-axis displacement of the initial pose of the current frame.

[0071] ②Target detection estimates the X-axis and Y-axis displacement of the initial pose.

[0072] First, the aircraft in the image is located by using target detection to obtain its 2D center point. Then, the unknown X-axis and Y-axis displacements are calculated using equation (1) based on the 2D-3D coordinate pair of the center point, the camera intrinsic parameters, and the four parameters of the initial pose.

[0073] (3) Template matching to achieve pose estimation

[0074] First, edge detection is performed on the current frame to extract edge points in the target detection positioning frame. Then, the template set to be searched is determined based on the X-axis and Y-axis displacement changes of the initial pose of the current frame relative to the pose of the previous frame. Finally, given the initial pose, the best matching pose is searched in the selected template set.

[0075] Step 4: Estimation of model target pose in wind tunnel coordinate system

[0076] The model target pose in the camera coordinate system calculated in step 3 and the camera extrinsic parameters estimated in step 2 are input to calculate the model target pose in the wind tunnel coordinate system.

[0077] Assume that the transformation relationship of the target estimated in the i-th frame relative to the camera coordinate system is According to formula (3)

[0078]

[0079] The transformation relationship of each frame target relative to the camera coordinate system can be Transformation relationship converted to the target relative wind tunnel coordinate system

[0080] Further, according to You can Transformed into 6-DOF pose [t x , t y , t z , α, β, γ], which is the position of the target in the wind tunnel coordinate system at each acquisition moment.

[0081] R=R x (α)R y (β)R z (γ),t=[t x t y t z ] T .

[0082]

[0083] The above is only a specific embodiment of the present invention, and the present invention is described in detail. The unexplained part is a conventional technology. However, the protection scope of the present invention is not limited to this. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. The protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for measuring the 6-DOF posture of a test model target, characterized in that: The steps include: (1) Camera intrinsic calibration: calibrate the camera’s focal length, distortion coefficient, pixel size, and image center coordinates; (2) Camera extrinsic parameter estimation; (3) Estimate the model target pose in the camera coordinate system; (4) Estimate the model target pose in the wind tunnel coordinate system: Input the model target pose in the camera coordinate system of step (3) and the camera external parameters of step (2) to calculate the model target pose in the wind tunnel coordinate system.

2. The method for measuring the 6-DOF position and posture of a test model target according to claim 1, characterized in that: Step (1) is as follows: by using a calibration plate of fixed size and dimension to calibrate the camera internal parameters, the focal length f, distortion coefficient K, pixel size (S x , S y ) and the image center coordinates (C x , C y ).

3. The method for measuring the 6-DOF posture of a test model target according to claim 1, characterized in that: The estimation method of step (2) is as follows: fix the calibration block on the release mechanism in the wind tunnel; input the digital-analog 3D data of the calibration block, the movement angle of the release mechanism, the image sequence captured by the camera and the camera intrinsic parameters, and calculate the camera extrinsic parameters.

4. The method for measuring the 6-DOF posture of a test model target according to claim 3, characterized in that: Step (2) is as follows: Assume that the digital-analog 3D coordinate of the calibration block is O and the pixel coordinate is I. The transformation relationship between the target coordinate system of the calibration block and the camera coordinate system is: c T o The transformation relationship between the calibration block target coordinate system and the release mechanism and wind tunnel coordinate system is r T o and b T o ; c T o Using the PNP method, we can estimate: I=K× c T o ×O; Among them, K is the distortion coefficient. r T o The transformation relationship of the release mechanism relative to the wind tunnel coordinate system after the i-th movement remains unchanged. If known, then the corresponding calculation is Transformation relationship between the wind tunnel coordinate system and the camera coordinate system during the movement of the release mechanism c T b has remained unchanged, according to Solution c T b .

5. The method for measuring the 6-DOF position and posture of a test model target according to claim 1, characterized in that: Step (3) is as follows: first, the flight process of the aircraft is divided into three different stages: extravehicular, stable flight, and abnormal flight, and the digital model 3D data is input to create a corresponding 2D template set; then, the three rotation angles and Z-axis displacement of the initial pose are determined using the pose of the previous frame, and then the target detection is used to locate the aircraft in the image, and the unknown X-axis and Y-axis displacements are calculated based on the 2D-3D coordinate pair of the center point, the camera intrinsic parameters, and the four parameters determined by the initial pose, to achieve initial pose estimation.

6. The method for measuring the 6-DOF position and posture of a test model target according to claim 5, characterized in that: When the initial pose estimation is completed, edge detection is performed on the current frame to extract the edge points in the target detection positioning frame. According to the X-axis and Y-axis displacement changes of the initial pose of the current frame relative to the pose of the previous frame, the template set to be searched is determined, and the best matching pose is searched in the selected template set.

7. The method for measuring the 6-DOF position and posture of a test model target according to claim 5, characterized in that: The method for generating a 2D template set is as follows: given a range of posture changes, a 2D template set for a 3D model at various rotation angles and distances can be generated.

8. The method for measuring the 6-DOF position and posture of a test model target according to claim 1, characterized in that: Step (4) is as follows: Assume that the transformation relationship of the target estimated in the i-th frame relative to the camera coordinate system is according to Transform the target of each frame relative to the camera coordinate system Transformation relationship converted to the target relative wind tunnel coordinate system Further, according to Will Transformed into 6-DOF pose [t x , t y , t z , α, β, γ], where: R=R x (a)R y (b)R z (c),t=[t x t y t z ] T ; 9. A measurement system based on the 6-DOF position and posture measurement method of a launch test model target according to any one of claims 1 to 8, characterized in that: include: Camera calibration module, used to obtain camera intrinsic and extrinsic parameters; The aircraft pose estimation module in the camera coordinate system is used to estimate the 6-DOF pose of the aircraft in the camera coordinate system, including the angles and displacements of pitch, yaw and roll; The aircraft pose estimation module in the wind tunnel coordinate system is used to transform the aircraft pose in the camera coordinate system into the aircraft pose in the wind tunnel coordinate system.

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