A system and method for capturing the structural state of a multi-rotor blade tip vortex system under near-ground effect

Abstract

The present invention discloses a system and method for capturing the structural state of a multi-rotor blade tip vortex system under near-ground effect, belonging to the technical field of near-ground effect capture, and the captured vortex structure has high precision. The system of the present invention comprises: a box, a shooting system, a laser transmitter, a rotor experimental platform, and a mechanical arm; a groove is provided at the bottom of the box to place the mechanical arm; the rotor and the rotor force measuring platform should be placed at the center of the box; a rectangular through hole is provided at the center of the bottom of the box, and optical glass is installed, and a laser transmitting device is installed under the optical glass; a shooting system is installed on the side and top of the box, and the camera in the shooting system has two degrees of freedom of movement in the plane where it is located, and has a degree of freedom of rotation; in the flow field reconstruction method, the TrackFit method and the FlowFit method are used to interpolate and reduce noise for data points in the flow field in the process of constructing the B-spline curve, so as to improve the accuracy and convergence of the calculation results.

Description

Technical Field

[0001] The present invention belongs to the technical field of ground proximity effect capture, and in particular relates to a system and method for capturing the structural state of a multi-rotor blade tip vortex system under ground proximity effect. Background Art

[0002] For rotor helicopters, the influence of the ground proximity effect is very significant. When the rotor is close to the ground, the vortex in the lower part of the rotor tip vortex system structure will be broken up by the ground, and the downwash of the rotor tip airflow will be significantly hindered, which will cause the induced drag of the rotor to drop significantly, the lift to increase significantly, and the lift-to-drag ratio of the rotor to increase. For general helicopters, when the height of the rotor from the ground is the rotor's span, the induced drag of the rotor will decrease by about 1%. When the height of the rotor from the ground is 1 / 10 of the rotor's span, the induced drag of the rotor will increase by more than 48%. According to statistics, when the height of a multi-rotor helicopter from the ground is 1,000 meters, the ground proximity effect can still have a significant impact on the static ceiling of the helicopter. Therefore, studying and making full use of the ground proximity effect of the rotor is of great significance to increasing the load capacity and static ceiling of the rotorcraft.

[0003] The lower part of the rotorcraft's wingtip vortex structure being broken up by the ground is an important reason for the proximity effect. Accurately capturing the state of the rotor's wingtip vortex structure near the ground can enable researchers to more fully understand and study the proximity effect. Summary of the invention

[0004] The present invention provides a system and method for capturing the structural state of a multi-rotor blade tip vortex system under the near-ground effect, which can accurately capture the structural state of the wing tip vortex system of a near-ground rotor and facilitate subsequent analysis of the properties of the flow field.

[0005] To achieve the above objectives, the present invention adopts the following technical solutions:

[0006] A system for capturing the structural state of a multi-rotor blade tip vortex system under a near-ground effect, comprising: a box, a shooting system, a laser transmitter, a rotor test platform, and a mechanical arm;

[0007] The bottom of the box is provided with a groove for placing the mechanical arm; the groove has a groove cover, and when the rotor rotates, the mechanical arm is placed in the groove, and the groove cover is closed to reduce the interference of the groove on the experimental results; the mechanical arm is a foldable six-degree-of-freedom mechanical arm, and the length of a single mechanical arm after full extension should be greater than 4R to meet the requirements of spraying, given laser position, and hot wire anemometer speed measurement; the working part of the head of the mechanical arm is replaceable;

[0008] The rotor and rotor force measuring platform should be placed at the center of the box, and a motor should be installed at the bottom of the box below the rotor. The motor controls the rotation of the rotor and the pitch of the blades through a connecting rod. The size of the box is determined by the radius R of the rotor disc in the box. In order to reduce the interference of the inner wall of the box on the experimental results, the length, width and height of the box should be greater than 7.2R, 5.2R, and 5.2R respectively.

[0009] A rectangular through hole is opened at the center of the bottom of the box body, and an optical glass is installed. A laser emitting device is installed below the optical glass, so that the rotor and the flow field near it are completely covered by the laser; the rectangular through hole is 3.9R long and 3.2R wide;

[0010] The side and top of the box are both equipped with shooting systems, in which the camera has two degrees of freedom of movement in the plane where it is located, and has a degree of freedom of rotation.

[0011] A method for capturing the structural state of a multi-rotor blade tip vortex system under a near-ground effect comprises the following steps:

[0012] S1: Open the groove cover at the bottom of the box controlled by the cylinder, and unfold the robot arm into the box. At this time, the working part of the head of the robot arm is the spray device, which performs the spray operation until the air in the box fills the HFSB, then the robot arm can be retracted and the groove can be closed;

[0013] S2: Capturing vortex structure: Turn on the laser beam light source at the bottom of the box, turn on the rotor motor, and use a high-speed camera to take pictures of the flow field environment in the box at a given position in the specific experiment according to the specific experimental requirements. The operation method of high-speed camera shooting is: operate the motor that controls the ball screws on the two linear guides and the motor that controls the rotation of the camera interface to achieve the three-degree-of-freedom movement of the camera;

[0014] S3: Turn off the laser beam light source at the bottom of the box, change the working part of the head of the robot arm to a laser emitting device, open the groove cover, and use the robot arm to irradiate a specific plane in the box with laser and take pictures;

[0015] S4: retract the robotic arm, close the groove cover, replace the working part of the robotic arm head with the hot wire anemometer probe, let it stand, wait for the soap bubbles HFSB in the flow field to dissipate, open the groove cover, extend the robotic arm into the box again, and use the hot wire anemometer probe to detect the flow field velocity at a given point in the box;

[0016] S5: Finally, the robotic arm is retracted and the groove cover is closed. The photos taken by the device are interpolated with B-spline curves and filtered for noise reduction through the TrackFit method and the FlowFit method to obtain a three-dimensional flow field vector diagram. The calculated results are then compared with the measurement results of the hot wire anemometer at a given point to verify the accuracy of the high-speed camera photography and post-processing methods.

[0017] Beneficial effects: The present invention provides a system and method for capturing the structural state of a multi-rotor blade tip vortex system under the near-ground effect. The measurement system has the following advantages over the prior art:

[0018] 1. The high-speed camera in the shooting system has two degrees of freedom in its plane, which allows the high-speed camera to move freely in the plane, and the camera has one degree of rotational freedom;

[0019] 2. The working part of the robot head can be replaced under the box;

[0020] 3. When reconstructing the flow field, the point information captured by the high-speed camera is passed through the cost function minimizer, and the velocity, acceleration and pressure field are calculated based on the trajectory of the noise particles. The photos taken by the device are interpolated with B-spline curves and filtered for noise reduction through the TrackFit method and the FlowFit method. The vector distribution map of the flow field at a given time can be obtained. The distribution map will show the rotor wingtip vortex system structure broken up by the ground, which is convenient for the subsequent analysis of the properties of the flow field. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the structure of a measurement system in an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the structure of a shooting system in an embodiment of the present invention;

[0023] Figure 3 This is a schematic diagram of the structure of the box in an embodiment of the present invention;

[0024] Figure 4 It is a structural schematic diagram of a rotor system in an embodiment of the present invention;

[0025] Figure 5 Schematic diagram of the structure of the robotic arm in an embodiment of the present invention.

[0026] In the figure, 1-box, 2-shooting system, 3-laser transmitter, 4-rotor experimental platform, 5-groove cover, 6-mechanical arm, 7-motion linear guide a, 8-motion linear guide b, 9-camera bracket, 10-camera rotation mechanism. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments:

[0028] like Figure 1 As shown, a system for capturing the structural state of a multi-rotor blade tip vortex system under near-ground effect comprises: a box 1, a shooting system 2, a laser transmitter 3, a rotor experimental platform 4, and a robotic arm 6.

[0029] The size of the box 1 is determined by the radius R of the rotor disc in the box. Existing studies have found through numerical calculations that when the rotor is close to the wall, the wall only has aerodynamic interference on the rotor close to the wall, and the closer to the wall, the more serious the lift fluctuation and lift loss of the rotor. The lift fluctuation of the rotor is mainly caused by the wall interference during the process of the rotor approaching the wall, which reduces the strength of the high-pressure area on the lower surface of the rotor. At the same time, the strength of the low-pressure area on the upper surface of the rotor does not change. The minimum flight distance near the wall is 1.6R. In order to reduce the interference of the inner wall of the box on the experimental results, the length, width and height of the box 1 are greater than 7.2R, 5.2R, and 5.2R respectively; a groove is opened at the bottom of the box 1 to place the mechanical arm 6, and the groove has a groove cover 5. When the rotor rotates, the mechanical arm should be placed in the groove, and the groove cover 5 is closed to reduce the interference of the groove on the experimental results.

[0030] A rectangular through hole with a length of 3.9R and a width of 3.2R is opened at the center of the bottom of the box 1, and an optical glass is installed. A laser emitter 3 is installed under the optical glass. The laser emitter 3 is a laser array that can emit a stable straight pulse light beam at a given frequency, providing a reliable light source for the display of streamlines in the flow field and the gas vortex structure, so that the rotor and the flow field near it are completely covered by the laser.

[0031] The rotor test platform 4 is installed at the center of the box 1, and the motor is installed on the rotor test platform 4. The rotor is installed on the rotating shaft of the motor. The rotor is connected to the rotating shaft of the motor through a collective swash plate. The motor controls the rotation of the rotor and the pitch of the blades through the rotating shaft; the rotor can be a double propeller or a multi-propeller, etc. The rotor speed is adjusted and monitored by a laser grating, and the laser grating is fixed above the through-hole glass plate. The laser grating is a measuring device using the principle of laser interference. By emitting a laser beam, the laser passes through a specific grating structure to produce an interference phenomenon. When the rotor rotates, it will cause a change in the optical path, thereby causing the movement of the interference fringes. By continuously detecting the rotor speed and comparing it with the target speed, the laser grating system and the motor control system form a closed-loop feedback system to continuously adjust the rotor speed until the actual speed reaches or approaches the target speed. The laser grating continuously emits lasers and detects changes in the interference fringes. According to the movement frequency or phase change of the interference fringes, the rotation speed of the rotor can be accurately calculated. The geometric blade pitch angle is adjusted by the collective swash plate setting; the rotor thrust is measured by a piezoelectric balance connected to the lower end of the model bracket.

[0032] The robot arm 6 is a foldable six-degree-of-freedom robot arm, which consists of a robot arm body and a working part at the head of the robot arm, wherein the working part can be replaced with working parts of different functions, and the operating motor controls the robot arm. The length of a single robot arm after full extension should be greater than 4R, and various working parts can be installed on the head of the robot arm, such as a hot wire anemometer probe, a laser emitting device, a spray device, etc.; when the robot arm is placed in the groove, the head working part of the robot arm can be replaced by the staff at the bottom of the box, and this design allows the above three working parts to play a role, namely: the laser emitting device is used to emit scattered laser beams to the specified plane, the spray device can spray soap bubbles (HFSB) for flow field trajectory display, and the hot wire anemometer probe can measure the flow velocity at any position in the flow field in the box, and transmit the measurement results wirelessly to the computer; when in use, the cylinder below the bottom surface retracts the groove cover 5 to the bottom of the bottom surface through the linear guide 501, and then the robot arm 6 will unfold and enter the experimental space, and the working parts of the head of the robot arm 6 will be sent to the specified area in the box for work.

[0033] The shooting system 2 is installed on the four sides and the top of the box 1. The shooting system 2 consists of a fixed plate, two sets of superimposed moving linear guides a7 and moving linear guides b8 driven by ball screws, a camera rotation mechanism 10 and a camera bracket 9; wherein the moving linear guide a7 is fixed above the fixed plate, the moving linear guide b8 is fixed above the moving linear guide a7 and is perpendicular to the moving linear guide a7, the camera bracket 9 is installed above the moving linear guide b8, and the camera rotation mechanism 10 is fixed above the camera bracket 9. When working, the high-speed camera is mounted on the camera bracket 9. The high-speed camera should be able to ensure that the flow field intercepted by any vertical section in the box 1 can be photographed or calculated, and the flow field intercepted by any horizontal plane can be calculated; the shooting system 2 includes five high-speed pulse cameras, such as the "Phantom VEO 640L" camera and the "LaVision Imager HS 4M" camera. The focal length and shooting frequency of the camera can be wirelessly adjusted by inputting external signals into the camera's built-in processor. The bracket of each high-speed camera is mounted on two vertical motion linear guides a7 and b8, so that the high-speed camera has two degrees of freedom of movement in the plane where it is located. At the same time, the camera bracket 9 has a camera rotation mechanism 10, which enables the camera to have a rotational degree of freedom and can rotate around an axis parallel to the linear guide, so that the camera has the following functions: if the camera is a horizontal camera, it can rotate around the vertical axis and has a horizontal rotation function. If the camera is a vertical camera, the camera can be made to shoot top views of the flow field at different angles, which is convenient for subsequent image flow field reconstruction.

[0034] The working process of the system for capturing the structural state of the multi-rotor blade tip vortex system under the near-ground effect is as follows:

[0035] 1. Check whether the whole mechanism has any faults, open the box cover, install the rotor on the connecting rod in the center of the box, fix the base on the rotor force measuring platform, and close the box cover after confirming that the installation is good;

[0036] 2. Open the groove cover at the bottom of the box controlled by the cylinder, and unfold the robot arm into the box. At this time, the working part of the head of the robot arm is the spray device, which performs the spray operation until the air in the box fills the HFSB, then the robot arm can be retracted and the groove closed;

[0037] 3. Capture vortex structure: Turn on the laser beam light source at the bottom of the box, turn on the rotor motor, and use a high-speed camera to take pictures of the flow field environment in the box at a given position of the specific experiment according to the specific experimental requirements. The operation method of high-speed camera shooting is: operate the motor that controls the ball screws on the two linear guides and the motor that controls the camera rotation mechanism to achieve the three-degree-of-freedom movement of the camera;

[0038] 4. Turn off the laser beam light source at the bottom of the box, change the working part of the robot arm head to a laser emitting device, open the groove cover, use the robot arm to irradiate a specific plane in the box with a laser, and take a photo;

[0039] 5. Retract the robotic arm, close the groove cover, replace the working part of the robotic arm head with the hot wire anemometer probe, let it stand, wait for the soap bubbles HFSB in the flow field to dissipate, open the groove cover, extend the robotic arm into the box again, and use the hot wire anemometer probe to detect the flow field velocity at a given point in the box;

[0040] 6. Finally, retract the robotic arm, close the groove cover, and use the TrackFit method and FlowFit method to perform B-spline curve interpolation and filtering noise reduction on the photos taken by the device to obtain a three-dimensional flow field vector diagram. Then compare the calculation results with the measurement results of the hot wire anemometer at a given point to verify the accuracy of the high-speed camera photography and post-processing methods.

[0041] The above method can be divided into two parts:

[0042] (1) The position data of particles at different time points are collected by the TrackFit method. Due to various interference factors in the actual measurement environment, these position data are often affected by noise. The TrackFit method may involve steps such as feature extraction of particles, establishment of motion models, and position prediction and matching to determine the position of particles as accurately as possible and calculate the B-spline curve of the trajectory. This step includes denoising by Wiener / Kalman filtering. The denoising process includes the following steps: Establish a mathematical model of particle trajectory: regard the motion of particles as a linear process, and determine the corresponding state equation and observation equation; the state equation and observation equation are expressed as:

[0043] ,

[0044] in, is the state vector, is the observation vector, A is the state transfer matrix, H is the observation matrix, and They are process noise and observation noise respectively. Initialize the filter: According to the known particle position data, initialize the state estimate of the filter and the error covariance matrix ,

[0045] ,

[0046] in represents the position and velocity state variables of the particle at the initial time, and E represents expected value.

[0047] At each time point, the next position and velocity of the particle are predicted based on the state equation, and the error between the observed and predicted values ​​is calculated based on the observation equation. The state estimate and the error covariance matrix are updated based on the error using the filter gain matrix to obtain a more accurate position and velocity estimate.

[0048] ,

[0049] where R is the covariance matrix of the observation noise, is the Kalman gain, is the prior state estimate at time k, is the prior error covariance matrix, is the posterior state estimate at time k, is the posterior error covariance matrix;

[0050] Repeat the above steps until all particle position data are processed to obtain the particle trajectory after noise reduction. B-spline curve is a commonly used curve fitting method. A smooth curve can be defined by a set of control points. In particle trajectory tracking, B-spline curve can be used to fit the particle position data after noise reduction to obtain a smooth trajectory curve. The process of calculating B-spline curve usually includes the following steps: determine the order of B-spline curve and the number of control points. The higher the order, the smoother the curve, but the higher the calculation complexity; the number of control points can be determined according to the number and distribution of particle position data; according to the particle position data, determine the control points of B-spline curve, use the least squares method to fit the particle position data, and obtain the coordinates of the control points. The least squares method aims to minimize the sum of squares of errors. The formula is:

[0051] ,

[0052] in, is the observed particle position data, are the coordinates of the control points, is the B-spline basis function; to calculate the position of the B-spline curve at any time point, the mathematical expression of the B-spline curve can be used to calculate the position of the curve at any time point based on the time parameter and the coordinates of the control point. The B-spline curve can be calculated by the following mathematical expression:

[0053] ,

[0054] in, is the B-spline basis function after fitting, are the coordinates of the control points after fitting.

[0055] Calculate the first and second order derivatives of the B-spline curve. The first order derivative represents the slope of the curve at a certain point, and the second order derivative represents the curvature of the curve. Once the B-spline curve is obtained, the position of the particle and its first and second order derivatives can be calculated at any time point. Position calculation: Substitute the time parameter into the mathematical expression of the B-spline curve to obtain the position coordinates of the particle at that time point; first order derivative calculation: Derivate the mathematical expression of the B-spline curve to obtain the expression of the first order derivative:

[0056] ,

[0057] in, It is the first-order derivative of the B-spline basis function. Substituting the time parameter into the expression of the first-order derivative, the velocity of the particle at that time point (first-order derivative) can be obtained. Second-order derivative calculation: The expression of the first-order derivative is differentiated again to obtain the expression of the second-order derivative:

[0058] ,

[0059] in, is the second-order derivative of the B-spline basis function. Substituting the time parameter into the expression of the second-order derivative, the acceleration (second-order derivative) of the particle at that time point can be obtained. Through the above steps, a smooth particle trajectory curve can be obtained from the noise particle position data, and the position, velocity and acceleration of the particle can be calculated at any time point.

[0060] (2) The calculation results of the TrackFit method can be further processed by the FlowFit method to obtain more accurate particle motion information, and a three-dimensional B-spline curve that meets specific constraints can be calculated by minimizing the cost function.

[0061] The calculation results of the TrackFit method are expanded in the form of three-dimensional convolution. Three-dimensional convolution can be regarded as a signal processing technology. It extracts features from the data and performs smoothing by convolving a three-dimensional convolution kernel with the input data. The specific steps are as follows: Design a three-dimensional convolution kernel that can capture the local features of the particle trajectory and perform smoothing. Convolve the three-dimensional convolution kernel with the input data to extract features from the data and perform smoothing. The particle trajectory data obtained by the TrackFit method is usually discrete points in time and space. By expanding it in the form of three-dimensional convolution, these discrete data can be expanded to a continuous three-dimensional space to better analyze the movement of particles. Position acquisition: Using the expanded results, the position of the particle at any specific time point is determined by interpolation and other methods. For example, methods such as cubic spline interpolation can be used to estimate the particle position at a specific time point based on the particle position data at a known time point. Cubic spline interpolation function: Define the function In each small area is a cubic polynomial, and at the nodes Given function value , and satisfy the second-order derivative continuity condition, the three-bending moment equations of the cubic spline interpolation function are:

[0062] ,

[0063] x is in the interval The horizontal coordinate of any point in The nodes are the locations of known data points through which the spline will interpolate or near these points. It is at the node The function value at the known data point is the value of the function at the known data point. These values ​​are the points that the spline curve needs to pass through or approach, which is crucial for constructing the interpolation function. i is the distance between adjacent nodes, , Indicates that S(x) is at the node The second derivative value at .

[0064] Velocity and acceleration calculation: By performing differential operations on the position data, the velocity and acceleration of the particle can be obtained. Specifically, the position change between adjacent time points can be calculated, and then divided by the time interval to obtain the velocity;

[0065] ,

[0066] By performing a differential operation on the velocity data again, we can get the acceleration:

[0067] ,

[0068] and computing three-dimensional B-spline curves of velocity, acceleration and / or pressure by minimizing a cost function that may optionally take into account constraints such as divergent degrees of freedom for incompressible flow; ,in, is the B-spline basis function, are the coordinates of the control points, and the result is a continuous function that depends on a finite number of degrees of freedom. The final result is a continuous function, which means that the position, velocity, acceleration and other information of the particle can be calculated at any point in time and space. At the same time, this continuous function depends on a finite number of degrees of freedom, which means that the shape and behavior of the function can be controlled by adjusting these degrees of freedom. For example, in a B-spline curve, the degrees of freedom can be the coordinates of the control points. By adjusting the position of the control points, the shape and fitting effect of the curve can be changed.

[0069] Finally, the calculation results of the TrackFit method and the FlowFit method were compared with the measurement results of the hot wire anemometer at a given point. The two results were in good agreement, which proved the accuracy of the high-speed camera photography and post-processing method of the present invention.

[0070] The above are only preferred embodiments of the present invention, which cannot limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical ideas proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for capturing the structural state of a multi-rotor blade tip vortex system under a near-ground effect, characterized in that: The following steps are involved: S1: Open the groove cover at the bottom of the box controlled by the cylinder, and unfold the robot arm into the box. At this time, the working part of the head of the robot arm is the spray device, which performs the spray operation until the air in the box fills the HFSB, then the robot arm can be retracted and the groove can be closed; S2: Capturing vortex structure: Turn on the laser beam light source at the bottom of the box, turn on the rotor motor, and use a high-speed camera to take pictures of the flow field environment in the box at a given position in the specific experiment according to the specific experimental requirements. The operation method of high-speed camera shooting is: operate the motor that controls the ball screws on the two linear guides and the motor that controls the rotation of the camera interface to achieve the three-degree-of-freedom movement of the camera; S3: Turn off the laser beam light source at the bottom of the box, change the working part of the head of the robot arm to a laser emitting device, open the groove cover, and use the robot arm to irradiate a plane in the box with laser and take pictures; S4: retract the robotic arm, close the groove cover, replace the working part of the robotic arm head with the hot wire anemometer probe, let it stand, wait for the soap bubbles HFSB in the flow field to dissipate, open the groove cover, extend the robotic arm into the box again, and use the hot wire anemometer probe to detect the flow field velocity at a given point in the box; S5: Finally, the robotic arm is retracted and the groove cover is closed; the photos taken by the device are interpolated with B-spline curves and filtered for noise reduction by TrackFit method and FlowFit method to obtain a three-dimensional flow field vector diagram, which specifically includes the following steps: S501: Obtain the noisy particle position data of the particle trajectory through the TrackFit method and calculate the B-spline curve of the trajectory. This step includes denoising by Wiener / Kalman filtering and calculates the particle position and its first-order and second-order derivatives at any time point within the observed particle trajectory time interval; S502: Through the FlowFit method: expand the calculation result of the TrackFit method in the form of three-dimensional convolution, then obtain the position of the particle at any time point and its velocity and / or acceleration, and calculate the three-dimensional B-spline curve of velocity, acceleration and / or pressure by minimizing the cost function.

2. The method for capturing the structure state of a multi-rotor blade tip vortex system under the ground proximity effect according to claim 1, characterized in that: S501 specifically includes the following steps: A mathematical model of particle trajectory is established to determine the corresponding state equation and observation equation, which are expressed as: x k+1 =Ax k +w k z k =Hx k +v k , Among them, x k is the state vector, z k is the observation vector, A is the state transfer matrix, H is the observation matrix, w k and v k are process noise and observation noise respectively; At each time point, the next position and velocity of the particle are predicted based on the state equation, and the error between the observed and predicted values ​​is calculated based on the observation equation. The state estimate and the error covariance matrix are updated based on the error using the filter gain matrix to obtain a more accurate position and velocity estimate. Where R is the covariance matrix of the observation noise, K k is the Kalman gain, is the prior state estimate at time k, P k|k- 1 is the prior error covariance matrix, is the posterior state estimate at time k, P k|k is the posterior error covariance matrix; Repeat the above steps until all particle position data are processed to obtain the denoised particle trajectory; According to the particle position data, the control points of the B-spline curve are determined, and the least squares method is used to fit the particle position data to obtain the coordinates of the control points. The least squares method aims to minimize the sum of squares of errors. The formula is: Among them, y i is the observed particle position data, c j are the coordinates of the control points, B j (x i ) is the B-spline basis function; The mathematical expression of the B-spline curve is used to calculate the position of the curve at any time point according to the time parameter and the coordinates of the control point. The mathematical expression of the B-spline curve is: Among them, B i (t) is the B-spline basis function, P i are the coordinates of the control points; Calculate the first-order and second-order derivatives of the B-spline curve, substitute the time parameter into the expression of the first-order derivative to get the velocity of the particle at that time point, and substitute the time parameter into the expression of the second-order derivative to get the acceleration of the particle at that time point.

3. The method for capturing the structure state of a multi-rotor blade tip vortex system under near-ground effect according to claim 1 or 2, characterized in that: S502 specifically includes the following steps: The calculation results of the TrackFit method are expanded in the form of three-dimensional convolution. The expanded results are used to estimate the particle position at any time point based on the particle position data at known time points through the cubic spline interpolation method. The cubic spline interpolation function is defined as follows: The function S(x) is defined in each small interval [x i , x i+1 ] is a cubic polynomial, and at the node x i Given a function value y i =f(x i ), and satisfy the condition of second-order derivative continuity, the three-bending moment equations of the cubic spline interpolation function are: where h i =x i+1 -x i ,M i =S"(x i ), Calculate the position change between adjacent time points and then divide it by the time interval to get the speed; Perform a differential operation on the velocity data again to obtain the acceleration:

4. A system for capturing the structural state of a multi-rotor blade tip vortex system under near-ground effect, characterized in that: Used to implement the method described in any one of claims 1 to 3, comprising: a box, a shooting system, a laser transmitter, a rotor experimental platform, and a robotic arm; the bottom of the box is provided with a groove to place the robotic arm; the center of the box should be equipped with a rotor and a rotor force measuring platform; the center of the bottom of the box is provided with a rectangular through hole, and optical glass is installed, and a laser transmitting device is installed under the optical glass; the side and top of the box are both equipped with shooting systems, and the camera in the shooting system has two degrees of freedom of movement in the plane where it is located; and has a degree of freedom of rotation.

5. The system for capturing the structure state of the multi-rotor blade tip vortex system under the near-ground effect according to claim 4, characterized in that: The groove is provided with a groove cover. When the rotor rotates, the mechanical arm is placed in the groove and the groove cover is closed.

6. The system for capturing the structure state of the multi-rotor blade tip vortex system under the near-ground effect according to claim 4 or 5, characterized in that: The robotic arm is a foldable six-degree-of-freedom robotic arm, the working part of the head is replaceable, and the length of a single robotic arm after being fully extended should be greater than 4R, where R is the rotor blade radius.

7. The system for capturing the structure state of the multi-rotor blade tip vortex system under the near-ground effect according to claim 4, characterized in that: A motor is installed at the bottom of the box below the rotor, and the motor controls the rotation of the rotor and the pitch of the blades through a connecting rod.

8. The system for capturing the structure state of the multi-rotor blade tip vortex system under the near-ground effect according to claim 4, characterized in that: The length, width and height of the box are respectively greater than 7.2R, 5.2R and 5.2R, where R is the radius of the rotor blade.

9. The system for capturing the structure state of the multi-rotor blade tip vortex system under the near-ground effect according to claim 4 or 8, characterized in that: The rectangular through hole is 3.9R long and 3.2R wide, wherein R is the radius of the rotor blade.

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