CN114119747B - Three-dimensional flow field flow display method based on PMD wave front detection - Google Patents
Three-dimensional flow field flow display method based on PMD wave front detection Download PDFInfo
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Abstract
The invention provides a three-dimensional flow field flow display method based on PMD wavefront detection according to the inverse Hartmann wavefront detection principle. The method utilizes 11 measuring cameras and 4 displays to form a PMD wavefront detection system, orthogonal stripe patterns are displayed on the displays to serve as structured light sources, the stripe patterns acquired by the cameras are distorted due to the existence of flow field disturbance, phase information is obtained through a space-carrier-based orthogonal stripe phase extraction algorithm, then coordinates of the displays are obtained, and high-precision and high-resolution measurement on density distribution of a three-dimensional flow field can be achieved by combining the cameras and system calibration parameters. Meanwhile, a monitoring system based on binocular stereoscopic vision is formed by 4 monitoring cameras, the posture of the display is monitored in real time, and calibration parameters of the system are adjusted. Therefore, the method has the advantages of simple device, low detection cost, high precision and high resolution, and has better robustness on the influence of system vibration in the detection process.
Description
Technical Field
The invention relates to a three-dimensional flow field flow display method based on Phase Measurement Deflection (PMD) wavefront detection, in particular to a three-dimensional flow display technical scheme of a wind tunnel flow field in the field of aerodynamics.
Background
The flow display technology is widely applied to aerodynamic layout design of turbines, engines, aircrafts and the like as a method for realizing flow visualization of a flow field. The traditional flow display technology includes a shadow method, a schlieren method, an interference method and a particle image velocimetry method. The shadow method is characterized in that rays emitted by a point light source are converted into parallel rays by utilizing a convex lens, the rays reach an observation screen through a measured flow field, and then a flow field structure is obtained through a shadow map on the observation screen, and the method is mainly used for qualitative measurement; the schlieren method is to change the light from point light source into parallel light by spherical reflector or convex lens, to converge the emergent light from the measured flow field again, and to place the opaque knife edge near the convergence point (focus) to obtain shadow image. The interference method utilizes the phase change of interference fringes formed by a measuring beam and a reference beam emitted by a measured flow field to obtain the density change of the flow field, and although the method can complete quantitative measurement, the detection cost is high and the method is easily influenced by environmental noise. The particle image velocimetry method is characterized in that tracer particles are added into the airflow of a measured flow field, and the flow field distribution is obtained by measuring the motion form of the tracer particles, so that the particle image velocimetry method not only can be used for quantitative measurement, but also can realize three-dimensional flow display, but has strict requirements on the tracer particles, and not only needs the tracer particles to have better tracing performance, but also needs to ensure that the influence of the existence of the tracer particles on the airflow flow is small.
In recent years, in order to further realize three-dimensional flow display of a flow field, atcheson et al have proposed a three-dimensional Background guided schlieren (3 d Background Oriented schlieren,3d BOS), which measures the deflection amount of light passing through a measured flow field from different angles by using a plurality of cameras, and further obtains three-dimensional distribution of the density or refractive index of the flow field. However, in the method, a paraxial imaging formula is adopted to calculate the deflection angle of the light, a random dot diagram is used as a characteristic pattern in the measurement process, and an image correlation point matching algorithm which is time-consuming in operation and low in precision is required to be used when the deflection amount is calculated.
Disclosure of Invention
In order to overcome the problems in the prior art, the invention provides a novel three-dimensional flow field non-contact quantitative flow display method based on a PMD wavefront detection method. The method utilizes 11 measuring cameras and 4 displays to form a PMD wavefront detection system, orthogonal stripe patterns are displayed on the displays to serve as structured light sources, the 11 measuring cameras synchronously acquire the orthogonal stripe patterns emitted by a measured flow field, the stripe patterns acquired by the cameras are distorted due to flow field disturbance, phase information is obtained by solving based on a space carrier orthogonal stripe phase extraction algorithm, then coordinates of the displays are obtained, and high-precision and high-resolution measurement on density distribution of the three-dimensional flow field can be completed by combining the cameras and system calibration parameters. Meanwhile, considering that the instability of the posture of the display is caused by the existence of system vibration in the actual detection process, a monitoring system based on binocular stereo vision is formed by 4 monitoring cameras to monitor the posture of the display in real time. Therefore, the method has the advantages of simple device, low detection cost, high precision and high resolution, and has better robustness on the influence of system vibration in the detection process.
The invention adopts the technical scheme that the invention is a three-dimensional flow field flow display method based on PMD wavefront detection, and establishes a PMD-based flow field wavefront distortion detection system according to the inverse Hartmann wavefront detection principle. From the back ray tracking point of view, the rays are considered to emanate from the center of the entrance pupil of the camera imaging lens, and are refracted by the measured flow field to reach the display. Before and after the flow field disturbance, the emergent direction of the light corresponding to the same pixel point on the camera through the measured flow field is changed, and then the light reaches different positions on the display, and the deflection quantity of the light is calculated according to the calibration parameters of the camera and the system, so that the density distribution of the three-dimensional flow field is calculated. The method comprises the following specific steps:
the method comprises the following steps: calibration of cameras and systems
The display is used as a plane calibration target, orthogonal stripe patterns displayed on the display are used as characteristic patterns, and all cameras with adjacent relations are simultaneously calibrated pairwise by changing the posture of the calibration target by using a stereoscopic vision camera calibration method; in order to obtain an accurate camera calibration result, when any two adjacent cameras are calibrated, images of 25 different postures are shot, internal parameters, external parameters and distortion coefficients of the cameras are solved by utilizing a pinhole camera model and a lens distortion model, and the solved parameters and world coordinates of control points are optimized by utilizing beam adjustment; after the optimization is completed, one camera coordinate system is selected as a world coordinate system, and the pose relations between all other camera coordinate systems and the world coordinate system are obtained by using external parameters between two adjacent cameras; all displays are fixed on a display fixing frame of a detection system bracket, and the postures of the displays are determined by using binocular stereo vision, so that the calibration of a camera and a system is realized; meanwhile, 4 monitoring cameras monitor the postures of the display in real time in the measuring process, so that system calibration parameters are adjusted;
step two: acquisition of light deflection angle
Displaying orthogonal stripe patterns on a display, synchronously acquiring stripe images by all cameras under the condition that a measured flow field flows, solving by utilizing a space-carrier-based orthogonal stripe phase extraction algorithm to obtain phase information, and then combining system calibration parameters to obtain a display coordinate C (x) s ,y s ,z s ) (ii) a Establishing a light ray tracing model by utilizing a camera and system calibration parameters to obtain the coordinates M (x) of the emergent point of the light ray passing through the measured flow field m ,y m ,z m ) (ii) a Unit vector of emergent ray
Comprises the following steps:
unit vector of incident ray obtained by camera and system calibration parameters
Comprises the following steps:
in the formula R i For the extrinsic parameters of the ith measuring camera,
normalizing the undistorted image coordinates of the corresponding image point on the ith measuring camera for the incident ray; the deflection angle epsilon (epsilon) of the light ray is obtained by calculation x ,ε y ,ε z )：
Step three: calculation of three-dimensional flow field Density
The deflection angle of the ray is the integral of the density gradient over the path s, so the model after deflection of any ray path of the ith measurement camera is:
wherein K is the Gladstone Dale constant; n is a radical of an alkyl radical 0 The refractive index of air around the measured flow field is a constant value in the measuring process; rho is the density of the measured flow field; μ e (x, y, z); epsilon μ The deflection angle of the light ray corresponding to the pixel at (m, n) on the ith measurement camera in the mu direction; then the above formula is discretized into matrix to obtain:
in the formula, a matrix A is the product of a space finite difference matrix D and a chromatography projection matrix T; the three-dimensional distribution of the measured flow field density is obtained by minimizing an objective function:
wherein, the first and the second end of the pipe are connected with each other,
is a regularization term, delta is a discrete Laplace operator, and lambda is a regularization parameter; the objective function can be optimized by using a Conjugate gradient (CG-Conjugate Gradients) algorithm, and then the distribution of the three-dimensional flow field density is obtained.
Drawings
FIG. 1 is a schematic two-dimensional schematic of the system apparatus of the present invention;
FIG. 2 is a schematic three-dimensional illustration of the system apparatus of the present invention;
fig. 3 is an orthogonal fringe pattern displayed on the display in the present invention.
Detailed Description
In order to make the objects and aspects of the invention more clear, the invention is described in detail below by way of example with reference to the accompanying drawings. It should be noted that the following examples are only for illustrative purposes and should not be construed as limiting the scope of the present invention, and that the skilled person in the art may make modifications and adaptations of the present invention without departing from the scope of the present invention.
The invention provides a three-dimensional flow field flow display method based on PMD wave-front detection, the basic devices of which comprise 15 cameras 1-15, 4 displays 16-19, a measured flow field 20, a detection system bracket 21, an optical platform 22 and a computer for controlling and processing data; the method utilizes 11 measuring cameras 1-11 and 4 displays 16-19 to form a PMD wavefront detection system, an orthogonal stripe pattern shown in figure 3 is displayed on the displays 16-19 as a structured light source, the 11 measuring cameras 1-11 synchronously acquire the orthogonal stripe pattern emitted by a measured flow field 20, the orthogonal stripe pattern acquired by the measuring cameras 1-11 is distorted due to the disturbance of the measured flow field 20, phase information is obtained by solving based on a space carrier orthogonal stripe phase extraction algorithm, and then the coordinates of the displays are obtained, and high-precision and high-resolution measurement of the density distribution of the measured flow field 20 can be completed by combining the cameras and system calibration parameters. Meanwhile, considering that the instability of the display posture is caused by the existence of system vibration in the actual detection process, a monitoring system based on binocular stereo vision is formed by the 4 monitoring cameras 12-15 to monitor the posture of the display in real time; wherein the camera 12 and the camera 13 are used for monitoring the posture change of the display 16 on the display fixing frame, and the camera 14 and the camera 15 are used for monitoring the posture change of the display 19 on the display fixing frame. The specific implementation process is as follows:
the method comprises the following steps: calibration of cameras and systems
Taking the display 16 as a plane calibration target, taking an orthogonal stripe pattern shown in figure 3 displayed on the display 16 as a characteristic pattern, and calibrating every two of all the cameras 1-15 with adjacent relations simultaneously by utilizing a stereoscopic vision camera calibration method by changing the postures of the calibration target; in order to obtain an accurate camera calibration result, when any two adjacent cameras are calibrated, images of 25 different postures are shot, internal parameters, external parameters and distortion coefficients of the cameras are solved by utilizing a pinhole camera model and a lens distortion model, and the solved parameters and world coordinates of control points are optimized by utilizing beam adjustment; after the optimization is completed, one camera coordinate system is selected as a world coordinate system, and the pose relations between all other camera coordinate systems and the world coordinate system are obtained by using external parameters between two adjacent cameras; all displays 16-19 are fixed on a display fixing frame of a detection system bracket, and the postures of the displays are determined by using binocular stereo vision, so that the calibration of the cameras 1-15 and the system is realized; meanwhile, 4 monitoring cameras 12-15 monitor the posture of the display in real time in the measuring process, so that the calibration parameters of the system are adjusted;
step two: acquisition of light deflection angle
The orthogonal fringe pattern shown in fig. 3 displayed on the display 16-19, under the condition that the measured flow field 20 flows, all the cameras 1-15 synchronously acquire orthogonal fringe images, the phase information is obtained by solving based on a space carrier orthogonal fringe phase extraction algorithm, and then the display coordinate C (x) is obtained by combining with the system calibration parameters s ,y s ,z s ) (ii) a Establishing a ray tracing model by using camera and system calibration parameters to obtain coordinates M (x) of the emergent point of the ray passing through the measured flow field 20 m ,y m ,z m ) (ii) a Unit vector of emergent ray
Comprises the following steps:
obtaining unit vector of incident ray by camera and system calibration parameters
Comprises the following steps:
in the formula R i For the extrinsic parameters of the ith measuring camera,
normalizing the undistorted image coordinates of the corresponding image point on the ith measuring camera for the incident ray; the deflection angle epsilon (epsilon) of the ray is obtained by calculation x ,ε y ,ε z )：
Step three: calculation of three-dimensional flow field Density
The deflection angle of the ray is the integral of the density gradient over the path s, so the model after deflection of any ray path of the ith measurement camera is:
wherein K is a Gladstone Dale constant; n is 0 The refractive index of the air around the measured flow field 20 is a fixed value in the measurement process; ρ is the measured flow field 20 density; μ e (x, y, z); epsilon μ The deflection angle of the light ray corresponding to the pixel at (m, n) on the ith measurement camera in the mu direction; then the above formula is discretized into a matrix to obtain:
in the formula, a matrix A is the product of a space finite difference matrix D and a chromatography projection matrix T; the three-dimensional distribution of the measured flow field 20 density is obtained by minimizing an objective function:
wherein the content of the first and second substances,
is a regularization term, delta is a discrete Laplace operator, and lambda is a regularization parameter; the objective function can be optimized by using a Conjugate gradient (CG-Conjugate Gradients) algorithm, and then the distribution of the three-dimensional flow field density is obtained. />
Claims (1)
1. A three-dimensional flow field flow display method based on PMD wavefront detection is characterized in that: the measuring device comprises 15 cameras, 4 displays, a measured flow field, a detection system bracket, an optical platform and a computer for controlling and processing data; the method comprises the steps that 11 measuring cameras and 4 displays form a PMD wavefront detection system, orthogonal stripe patterns are displayed on the displays to serve as structured light sources, the 11 measuring cameras synchronously acquire the orthogonal stripe patterns emitted by a measured flow field, phase information is obtained through solving based on a space carrier orthogonal stripe phase extraction algorithm, then coordinates of the displays are obtained, and then the cameras and the system calibration parameters are combined to complete measurement of density distribution of the measured flow field; meanwhile, a monitoring system based on binocular stereo vision is formed by 4 monitoring cameras to monitor the posture of the display in real time; the method comprises the following specific steps:
the method comprises the following steps: calibration of cameras and systems
The display is used as a plane calibration target, the orthogonal stripe pattern displayed on the display is used as a characteristic pattern, and all the cameras with adjacent relations are simultaneously calibrated pairwise by a stereoscopic vision camera calibration method by changing the posture of the calibration target; the method comprises the steps of shooting 25 images in different postures in the process of calibrating any two adjacent cameras, solving internal parameters, external parameters and distortion coefficients of the cameras by using a pinhole camera model and a lens distortion model, and optimizing the solved parameters and world coordinates of control points by using beam adjustment; after the optimization is completed, one camera coordinate system is selected as a world coordinate system, and the pose relations between all other camera coordinate systems and the world coordinate system are obtained by using external parameters between two adjacent cameras; fixing all displays on a display fixing frame of a detection system bracket, and determining the postures of the displays by using binocular stereo vision so as to finish the calibration of a camera and a system; meanwhile, 4 monitoring cameras monitor the postures of the display in real time in the measuring process, so that system calibration parameters are adjusted;
step two: acquisition of light deflection angle
The method comprises the steps that under the condition that a measured flow field flows, all cameras synchronously acquire orthogonal stripe images, phase information is obtained by solving an orthogonal stripe phase extraction algorithm based on a space carrier, and then a display coordinate C (x) is obtained by combining system calibration parameters s ,y s ,z s ) (ii) a Establishing a light ray tracing model by utilizing a camera and system calibration parameters to obtain the coordinates M (x) of the emergent point of the light ray passing through the measured flow field m ,y m ,z m ) (ii) a Unit vector of emergent ray
Comprises the following steps:
obtaining unit vector of incident ray by camera and system calibration parameters
Comprises the following steps:
in the formula R i For the extrinsic parameters of the ith measuring camera,
normalizing the undistorted image coordinates of the corresponding image point on the ith measuring camera for the incident ray; whereby the light is calculatedDeflection angle epsilon (epsilon) x ,ε y ,ε z )：
Step three: calculation of three-dimensional flow field Density
The deflection angle of the ray is the integral of the density gradient over the path s, so the model after deflection of any ray path of the ith measurement camera is:
wherein K is the Gladstone Dale constant; n is a radical of an alkyl radical 0 The refractive index of air around the measured flow field is a constant value in the measuring process; rho is the density of the measured flow field; μ e (x, y, z); epsilon μ The deflection angle of the light ray corresponding to the pixel at (m, n) on the ith measurement camera in the mu direction; and discretizing the first formula in the third step to obtain:
in the formula, a matrix A is the product of a space finite difference matrix D and a chromatography projection matrix T; the three-dimensional distribution of the measured flow field density is obtained by minimizing an objective function:
wherein the content of the first and second substances,
is a regularization term, delta is a discrete Laplace operator, and lambda is a regularization parameter; the objective function can be optimized by using a Conjugate gradient (CG-Conjugate Gradients) algorithm, and then the distribution of the three-dimensional flow field density is obtained. />
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