Parachute image scale factor solving method

Abstract

The invention provides a parachute image scale factor solving method. The scale factor is the ratio of the actual size of the parachute structure to the size of the parachute structure in the image. The method comprises the following steps: firstly, selecting a canopy top hole as a reference object, extracting the outer contour of the canopy top hole by using a sub-pixel edge detection technology, counting the number of pixel points occupied by the outer contour, and solving to obtain a scale factor corresponding to a top hole plane; then, establishing an image coordinate system and a camera coordinate system in a parachute system, and solving according to a camera imaging model to obtain a relationship between scale factors corresponding to different canopy sections; finally, a canopy deformation theoretical model in the parachute unfolding process is established, the optical distances from different sections of the canopy to the camera are obtained, and therefore scale factors corresponding to key planes such as the maximum projection section of the canopy are obtained through calculation. According to the method, the parachute image scale factor solving efficiency and precision are improved, and a foundation is laid for high-precision parameter identification of the parachute image shot by the monocular camera.

Description

Technical Field

[0001] The invention relates to the field of image measurement, and in particular to a method for solving a parachute image scale factor. Background Art

[0002] As a pneumatic deceleration device, parachutes are widely used in spacecraft recovery and landing processes because of their simple structure, efficient deceleration, reliable operation, and low cost. The working process of the parachute system is extremely complex, and its performance directly affects the safety and reliability of space missions. There are many cases in history where missions failed due to parachute system failures. Therefore, it is very important to analyze the performance of parachutes. The analysis and evaluation methods of parachute performance can be divided into experimental methods and simulation analysis methods. Among them, the airdrop test method is widely used because it can restore the actual working conditions of the parachute to a greater extent.

[0003] The relevant parameters of parachutes in airdrop tests are usually obtained by mechanical measurement, electrical measurement, optical measurement and other means. The optical measurement method is a non-contact measurement method. The working process of the parachute is recorded by optical equipment, and information is extracted from the acquired optical images or video data using image measurement technology. Compared with the other two methods, there is no need to install sensors on the canopy or parachute ropes during the measurement process, which will not affect the working state and dynamic characteristics of the parachute. Therefore, the optical measurement method is increasingly valued in the development of parachutes.

[0004] For the parachute parameter identification using image measurement technology, solving the scale factor is a crucial step, which determines the accuracy of the identification parameters. Therefore, the present invention proposes an accurate and efficient method for solving the parachute image scale factor. Summary of the invention

[0005] The present invention proposes a method for solving the parachute image scale factor. The method steps of the present invention are as follows:

[0006] Step 1, solve the proportional factor corresponding to the canopy top hole plane:

[0007] (1) The top hole of the canopy is introduced as a reference, and the outer contour of the top hole of the canopy in the image is extracted using sub-pixel edge detection technology.

[0008] (2) The number of pixels occupied by the outer contour is n, and the circumference of the top hole of the canopy is l. a , solve the proportional factor k1 corresponding to the top hole plane as:

[0009]

[0010] Step 2, solve the relationship between the proportional factors corresponding to different canopy cross sections:

[0011] (1) Establish the camera coordinate system and image coordinate system in the parachute system, and clarify the geometric position relationship between the parachute canopy and the two coordinate systems;

[0012] (2) Considering that the distance change of the canopy cross section along the camera optical axis direction cannot be ignored, the weak perspective model is no longer valid. Based on the camera imaging model, the relationship between the corresponding proportional factors of different canopy cross sections is solved as follows:

[0013]

[0014] where h x1 With h x2 is the distance between the optical center of the camera and the two different planes, k x1 With k x2 is the proportional factor corresponding to the two planes.

[0015] Step 3: Calculate the proportional factors corresponding to key positions such as the maximum projection cross section of the canopy

[0016] (1) Establish a theoretical model for the shape change of the parachute canopy during the parachute deployment process, and solve the distance from the top hole plane to the camera light as h a , the distance from the maximum projection surface to the optical center h p .

[0017] (2) Based on the relationship in step 2, the scale factor k2 corresponding to the maximum projection surface is solved as follows:

[0018]

[0019] The proportional factors corresponding to other canopy cross-sections can also be obtained using the above formula.

[0020] The present invention uses sub-pixel edge detection technology to extract the outer contour of the top hole, so that the outer contour can be extracted at the sub-pixel level, and the calculation accuracy of the proportional factor is improved. The top hole of the canopy will not be blocked during the parachute deployment process, and the shape of the top hole is almost unchanged, which improves the stability of the proportional factor solution process. At the same time, since the top hole of the canopy is located at the special position of the top of the parachute, the solution process of the proportional factor is effectively simplified. The influence of the distance change of the canopy cross section along the optical axis of the camera on the proportional factor is considered, and the proportional factors corresponding to different canopy cross sections are solved, which lays the foundation for the accurate identification of parachute parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a block diagram of the overall concept of the present invention.

[0022] Figure 2 For all sub-pixel outline images.

[0023] Figure 3 It is the sub-pixel outer contour image of the top hole of the canopy.

[0024] Figure 4 Schematic diagram of the parachute system structure.

[0025] Figure 5 are the camera coordinate system and image coordinate system in the parachute system.

[0026] Figure 6 It is the geometric relationship image of the camera imaging model.

[0027] Figure 7 This is a theoretical model of canopy deformation during the parachute deployment process. DETAILED DESCRIPTION

[0028] The present invention is described in detail below with reference to the accompanying drawings and embodiments. It should be understood that this example is only used to illustrate the present invention and is not used to limit the scope of the present invention. The overall idea of ​​the method for solving the parachute image scale factor provided by the present invention is as follows: Figure 1 As shown, the scale factor is solved in combination with a specific parachute. Taking the image of the parachute as an example, the image processing process involved in the scale factor solution process is shown. The specific implementation method includes the following steps:

[0029] Step 1, solve the proportional factor corresponding to the canopy top hole plane:

[0030] (1) The top hole of the canopy is introduced as a reference, and the outer contour of the top hole of the canopy in the image is extracted using sub-pixel edge detection technology, such as Figure 2 As shown;

[0031] (2) At this time, there are multiple sub-pixel outer contours in the image. By using the outer contour perimeter as the selection condition, the sub-pixel outer contour of the top hole of the umbrella canopy is extracted, such as Figure 3 As shown;

[0032] (3) The number of pixels occupied by the outer contour of the canopy top hole is n, and the circumference of the canopy top hole is l. a , solve the proportional factor k1 corresponding to the top hole plane as:

[0033]

[0034] Step 2, solve the relationship between the proportional factors corresponding to different canopy cross sections:

[0035] (1) Parachute system structure Figure 4 As shown in the figure, the camera coordinate system and the image coordinate system in the parachute system are established, and the geometric position relationship between the parachute canopy and the two coordinate systems is clarified, as shown in the figure. Figure 5 As shown;

[0036] (2) Considering that the distance change of the canopy cross section along the camera optical axis direction cannot be ignored, the weak perspective model is no longer valid. Based on the camera imaging model, the following is established: Figure 6 The geometric relationship image shown above solves the relationship between the corresponding scale factors of different planes:

[0037] Assume that the coordinates of a point in the image coordinate system are (u, v) and the coordinates of a point in the camera coordinate system are (X c ,Y c ,Z c ), let the equivalent focal length of the camera be α x =α y , then the coordinate transformation relationship between the camera coordinate system and the image coordinate system is:

[0038]

[0039] Assume that the coordinates of two points in the image coordinate system are (u1, v1) and (u2, v2), and their coordinates in the camera coordinate system are (X c1 ,Y c1 ,Z c1 ) and (X c2 ,Y c2 ,Z c2 ). From the above formula, we can get that the distance between two points in the image and the distance in the physical space satisfy the following relationship:

[0040]

[0041] Depend on Figure 5 As shown, A, B and C, D are located in plane Z respectively. c ′1 and plane Z c The coordinates of the four points on ′2 in the camera coordinate system are Figure 6 The coordinates of A and C in the image coordinate system are (u1′, v1′), and the coordinates of B and D in the image coordinate system are (u′2, v′2), that is, the projection lengths of the two line segments AB and CD in the image coordinate system are the same, which is l MN , however, their actual lengths are different.

[0042] According to the geometric relationship and the above formula, the length of AB is:

[0043]

[0044] Similarly, the length of CD is:

[0045]

[0046] Therefore, the actual length ratio of the two is:

[0047]

[0048] Then plane Z c ′1 and plane Z c ′2 corresponds to the scaling factor k c1 and k c2 The following relationship exists:

[0049]

[0050] Conclusion: Assuming that all different canopy cross sections are parallel to the imaging plane, the scale factor corresponding to different canopy cross sections is proportional to the distance from the canopy cross section to the optical center of the camera.

[0051] Step 3: Calculate the proportional factors corresponding to key positions such as the maximum projection cross section of the canopy

[0052] (1) Establish a theoretical model for the shape change of the parachute canopy during the parachute deployment process, such as Figure 7 The following assumptions are made for this model:

[0053] a. During the inflation phase, the canopy is regarded as a combination of a hemisphere and an inverted truncated cone, and after being filled, the canopy is regarded as a spherical crown;

[0054] b. The parachute ropes coincide with the sides of the canopy during the inflation phase;

[0055] c. The optical axis of the camera is consistent with the symmetry axis of the canopy, and the angle between the optical axis of the camera lens and the symmetry axis of the canopy is ignored; d. The shape of the recovery cabin is simplified to a cube, and the optical center of the camera is located on the upper surface of the recovery cabin;

[0056] e. Ignore the tensile deformation of the straps, lines, canopy and top hole during inflation.

[0057] The distance from the top hole plane to the camera light is obtained as h a , the distance from the maximum projection surface to the optical center h p :

[0058]

[0059] To simplify the solution process, we assume:

[0060] x p1 =r p =pk2

[0061] x p2 =0.92r p =0.92pk2

[0062] where r p is the maximum projection radius of the canopy, and p is the number of pixels corresponding to the maximum projection radius in the image.

[0063] (2) The proportional factor k2 corresponding to the maximum projection cross section of the canopy is obtained as follows:

[0064]

[0065] Through the above steps, the proportional factors corresponding to the remaining key canopy cross sections can be solved.

Claims

1. A method for solving the scale factor of a parachute image, characterized in that: The following steps are involved: Step 1, introduce the canopy top hole as a reference, and solve the proportional factor corresponding to the canopy top hole plane; Step 2, establishing the camera coordinate system and the image coordinate system in the parachute system, and solving the relationship between the corresponding proportional factors of different canopy cross sections according to the camera imaging model; Step 3, establish a theoretical model of the canopy shape change during the parachute deployment process, and based on the relationship obtained in step 2, solve the proportional factors corresponding to the canopy's key cross-sections such as the canopy's maximum projected cross-section.

2. The method according to claim 1, characterized in that Since the top hole of the canopy is always visible during the parachute deployment process, and the shape of the top hole is almost unchanged, the top hole is introduced as a reference. The steps for solving the proportional factor corresponding to the plane of the top hole of the canopy include: a. In order to improve the accuracy of the scale factor solution, the sub-pixel edge detection technology is introduced to achieve sub-pixel level extraction of the outer contour of the canopy top hole; b. Count the number of pixels occupied by the sub-pixel outer contour of the top hole. Knowing the actual size of the circumference of the canopy top hole, the proportional factor corresponding to the plane of the canopy top hole can be solved.

3. The method according to claim 1, characterized in that Considering that the distance change of the canopy cross section along the camera optical axis direction cannot be ignored, the weak perspective model is no longer valid, and the proportional factors corresponding to different canopy cross sections are not the same. The steps to solve the relationship between the proportional factors corresponding to different canopy cross sections include: a. Establish a theoretical model for the shape change of the parachute canopy during the parachute deployment process, and clarify the geometric position relationship between the parachute canopy and the two coordinate systems; b. Considering that the distance change of the canopy cross section along the camera optical axis direction cannot be ignored, the conclusion is drawn based on the camera imaging model: assuming that different canopy cross sections are all parallel to the imaging plane, the proportional factors corresponding to different cross sections are proportional to their distances from the optical center of the camera.

4. The method according to claim 1, characterized in that The steps for solving the proportional factors corresponding to key positions such as the maximum projection cross-section of the canopy include: a. Establish a theoretical model of canopy deformation during the parachute deployment process and solve the distance from different canopy sections to the optical center of the camera; b. Based on the relationship obtained in step 2(b), solve the proportional factors corresponding to key positions such as the maximum projected cross-section of the canopy.

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