阅读说明：本技术 一种基于偏振成像的冰形三维测量方法 (Ice-shaped three-dimensional measurement method based on polarization imaging ) 是由 陈向成 陈颖 张娅荻 杨旭 罗杰 于 2019-09-30 设计创作，主要内容包括：本发明提出了一种基于偏振成像的冰形三维测量方法,采用线偏振光源对各种冰形进行照明,偏振相机拍摄激光光条,旋转台架对结冰表面进行扫描；对线激光视觉传感器进行标定；对旋转台进行标定；用图像处理方法提取激光光条中心线,计算激光光条中心线在世界坐标体系中的三维坐标；通过本发明的方法,为冰形在线三维测量奠定良好的理论和技术基础。(The invention provides an ice shape three-dimensional measurement method based on polarization imaging, which adopts a linearly polarized light source to illuminate various ice shapes, a polarization camera to shoot laser light bars, and a rotating platform to scan an icing surface; calibrating a line laser vision sensor; calibrating the rotating platform; extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system; by the method, a good theoretical and technical foundation is laid for ice shape online three-dimensional measurement.)

1. An ice-shaped three-dimensional measurement method based on polarization imaging is characterized by comprising the following steps:

step 1: a linear polarization polarized light source is adopted to illuminate the ice, a polarization camera shoots a laser light bar, and a rotating platform frame scans the frozen surface;

step 2: calibrating a line laser vision sensor;

and step 3: calibrating a rotating table;

and 4, step 4: and extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system.

2. The ice-shaped three-dimensional measurement method based on polarization imaging is characterized in that: in step 2, the camera and the line laser form a line laser vision sensor together; the working process is as follows: the line laser projects a light plane to a measured object, the light plane intersects with the surface of the measured object to generate a light bar, the light bar comprises height information of the object, and then the camera records images of the light bar and transmits the images to the computer for analysis and the height information of the object is calculated.

3. The ice-shaped three-dimensional measurement method based on polarization imaging as claimed in claim 2, wherein: set to O_W-X_WY_WZ_WAs a world coordinate system and simultaneously as a measurement coordinate system; o is_C-X_CY_CZ_CAs a camera coordinate system; o is_u-x_uy_uRepresenting an image coordinate system in pixels; pi represents a structured light plane; a point P on the arbitrary light-taking plane has a coordinate P in the measurement coordinate system_w＝[x_w,y_w,z_w]^TIn aThe homogeneous coordinate of the image of the projection point p on the image plane is p ═ u, v,1]^TThe camera imaging model is expressed as:

establishing the measurement coordinate system on the camera coordinate system, i.e. P_w＝P_c＝[x_c,y_c,z_c]^TThen the camera imaging model is expressed as:

wherein: s is a proportionality coefficient; a is a camera internal reference matrix, f_u,f_vAs focal length parameter, u₀,v₀Is the image coordinate of the principal point; meanwhile, since P is a point on the light plane, the following light plane equation is also satisfied:

ax_w+by_w+cz_w+d＝0 (3)

wherein: a, b, c and d are coefficients of the light plane equation respectively.

4. The ice-shaped three-dimensional measurement method based on polarization imaging is characterized in that: by calibrating a camera internal reference matrix A and a light plane equation under a camera coordinate system and combining formulas (2) and (3), the three-dimensional coordinate of a point P on the center of a light bar under the camera coordinate system is solved as follows:

5. the ice-shaped three-dimensional measurement method based on polarization imaging is characterized in that: and (4) taking the lens distortion of the camera into consideration, calibrating the internal reference matrix A of the camera by a Zhang-Zhengyou calibration method, and calibrating by adopting a checkerboard target.

6. The ice-shaped three-dimensional measurement method based on polarization imaging is characterized in that: the calibration of the light plane equation by adopting the checkerboard target comprises the following four steps:

(1) acquiring a target posture; placing a designed target in the visual field of a camera, shooting a target image by the camera, extracting characteristic points of the target, establishing a mapping equation of image coordinates and world coordinates, and solving external parameters of the target, namely a rotation matrix R and a translation vector T between a camera coordinate system and a target coordinate system;

(2) extracting the centers of the light bars; keeping the target posture unchanged, projecting line laser to the surface of the target, shooting to obtain a light strip image, and obtaining an image coordinate of the light strip center through a light strip center extraction algorithm;

(3) solving the three-dimensional coordinates of the optical strips; determining three-dimensional coordinates of the light bar center under a camera coordinate system through target attitude parameters R and T and image coordinates of the light bar center;

(4) solving a light plane; and repeating the operation, shooting the target and the light strip image at different postures, obtaining light strip center three-dimensional coordinates at different postures, wherein the coordinates meet a light plane equation, and determining the coefficient of the light plane equation by a least square method.

7. The ice-shaped three-dimensional measurement method based on polarization imaging as claimed in claim 6, wherein: the light strip center extraction algorithm adopts a gray scale gravity center method.

8. The ice-shaped three-dimensional measurement method based on polarization imaging is characterized in that: in step 3, the camera and the line laser are fixed on the same support, and then the turntable drives the support to rotate so as to enable the relative position of the camera and the line laser to be unchanged, namely the coefficient of the light plane under the coordinate system of the camera is fixed and unchanged; however, since the camera rotates along with the support, the camera coordinate system is not fixed, and the attitude change of the camera at different rotation angles needs to be calibrated to perform the calibration of the rotating table.

9. The ice-shaped three-dimensional measurement method based on polarization imaging as claimed in claim 8, wherein: the rotary table calibration process comprises the following steps:

the target marking position is fixed in the support rotating process, two target images are obtained by shooting at an angle 1 and an angle 2 respectively, and an external reference equation between the polarization camera and the target is obtained according to the internal reference calibration of the camera:

the positional relationship of the polarization camera at angles 1 and 2 is:

at the moment, the camera coordinate systems under different rotation angles are unified under the same global coordinate system.

Technical Field

The invention relates to the field of optical three-dimensional measurement and computer vision, in particular to an ice-shaped three-dimensional measurement method based on polarization imaging.

Background

Aircraft icing widely exists in flight practice and poses a serious threat to flight safety, and the icing can not only damage smooth flow of air and increase flight resistance, but also change the aerodynamic characteristics of wings and cause failure or adverse effect of control surfaces. Icing on various components of an aircraft may affect the safe flight of the aircraft. Almost all icing studies now begin with measuring the icing profile of the aircraft surface in an icing wind tunnel.

Up to now, the methods for measuring the icing profile of an aircraft mainly include: cross-sectional profiling, photogrammetry, casting, laser line scanning. The above methods all have their advantages but also have their limitations. The continuous growth of the three-dimensional shape of the frozen ice is firstly measured by utilizing the characteristic that the ice presents Lambert radiation in a specific infrared band, and an infrared laser three-dimensional scanning method is developed, but the measuring method is essentially a heating method, and has influence on the growth process of the ice and is unknown.

From the technical development trend at home and abroad, the optical non-contact on-line ice shape measurement is the future development trend, however, no suitable method can realize the on-line measurement of the ice shape at present. Scholars at home and abroad make a great deal of research on improvement of calibration precision of a laser cutting method, the projection error of a calibration point in camera calibration reaches one hundredth pixel level, and a laser plane equation can be accurately obtained by an optimization method. However, the measurement accuracy is still limited by the extraction error of the laser central line, because the ice body has high transparency and smooth surface, and the laser penetration capacity is strong, the line laser is projected on the surface of the ice body, most light rays project the ice body, and only a small part of light rays are reflected by the surface of the ice body, so that the laser band area of the collected image is seriously diffused, and the laser band area is a bright spot area on the image. Therefore, how to filter the influence of scattered stray light is an urgent problem to be solved in the process of realizing ice shape online measurement.

Disclosure of Invention

In order to solve the technical problem, the invention provides a three-dimensional ice shape measuring method based on polarization imaging, which adopts a linear polarization light source to illuminate various ice shapes so as to carry out three-dimensional ice shape measurement. The method specifically comprises the following steps:

step 1: a linear polarization polarized light source is adopted to illuminate the ice, a polarization camera shoots a laser light bar, and a rotating platform frame scans the frozen surface;

step 2: calibrating a line laser vision sensor;

and step 3: calibrating a rotating table;

and 4, step 4: and extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system.

In the step 2, the camera and the line laser jointly form a line laser vision sensor; the working process is as follows: the line laser projects a light plane to a measured object, the light plane intersects with the surface of the measured object to generate a light bar, the light bar comprises height information of the object, and then the camera records images of the light bar and transmits the images to the computer for analysis and the height information of the object is calculated.

Wherein, set to O_W-X_WY_WZ_WAs a world coordinate system and simultaneously as a measurement coordinate system; o is_C-X_CY_CZ_CAs a camera coordinate system; o is_u-x_uy_uRepresenting an image coordinate system in pixels; pi represents a structured light plane; a point P on the arbitrary light-taking plane has a coordinate P in the measurement coordinate system_w＝[x_w,y_w,z_w]^TThe image homogeneous coordinate of the projection point p on the image plane is p ═ u, v,1]^TThe camera imaging model is expressed as:

establishing the measurement coordinate system on the camera coordinate system, i.e. P_w＝P_c＝[x_c,y_c,z_c]^TThen the camera imaging model is expressed as:

wherein: s is a proportionality coefficient; a is a camera internal reference matrix, f_u,f_vAs focal length parameter, u₀,v₀Is the image coordinate of the principal point; meanwhile, since P is a point on the light plane, the following light plane equation is also satisfied:

ax_w+by_w+cz_w+d＝0 (3)

wherein: a, b, c and d are coefficients of the light plane equation respectively.

The three-dimensional coordinate of a point P on the center of a light bar in the camera coordinate system is solved by calibrating a camera internal reference matrix A and a light plane equation under the camera coordinate system and combining formulas (2) and (3), and the three-dimensional coordinate is as follows:

the method comprises the steps of taking the lens distortion of a camera into consideration, calibrating a camera internal reference matrix A by a Zhang-Yongyou calibration method, and calibrating by adopting a checkerboard target.

The calibration of the optical plane equation by adopting the checkerboard target comprises the following four steps:

(1) acquiring a target posture; placing a designed target in the visual field of a camera, shooting a target image by the camera, extracting characteristic points of the target, establishing a mapping equation of image coordinates and world coordinates, and solving external parameters of the target, namely a rotation matrix R and a translation vector T between a camera coordinate system and a target coordinate system;

(2) extracting the centers of the light bars; keeping the target posture unchanged, projecting line laser to the surface of the target, shooting to obtain a light strip image, and obtaining an image coordinate of the light strip center through a light strip center extraction algorithm;

(3) solving the three-dimensional coordinates of the optical strips; determining three-dimensional coordinates of the light bar center under a camera coordinate system through target attitude parameters R and T and image coordinates of the light bar center;

(4) solving a light plane; and repeating the operation, shooting the target and the light strip image at different postures, obtaining light strip center three-dimensional coordinates at different postures, wherein the coordinates meet a light plane equation, and determining the coefficient of the light plane equation by a least square method.

Wherein, the light strip center extraction algorithm adopts a gray scale gravity center method.

In the step 3, the camera and the line laser are fixed on the same support, and then the turntable drives the support to rotate, so that the relative positions of the camera and the line laser are unchanged, namely the coefficient of the light plane under the coordinate system of the camera is fixed and unchanged; however, since the camera rotates along with the support, the camera coordinate system is not fixed, and the attitude change of the camera at different rotation angles needs to be calibrated to perform the calibration of the rotating table.

Wherein, the revolving stage demarcation's process includes:

the target marking position is fixed in the support rotating process, two target images are obtained by shooting at an angle 1 and an angle 2 respectively, and an external reference equation between the polarization camera and the target is obtained according to the internal reference calibration of the camera:

the positional relationship of the polarization camera at angles 1 and 2 is:

at the moment, the camera coordinate systems under different rotation angles are unified under the same global coordinate system.

Different from the prior art, the ice shape three-dimensional measurement method based on polarization imaging adopts a linearly polarized light source to illuminate various ice shapes, a polarization camera shoots laser light bars, and a rotating platform scans the icing surface; calibrating a line laser vision sensor; calibrating the rotating platform; extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system; by the method, a good theoretical and technical foundation is laid for ice shape online three-dimensional measurement.

Drawings

FIG. 1: is a flow chart of the method of the present invention

FIG. 2: is a structural block diagram of a measuring system;

FIG. 3: the method is a schematic diagram of line laser vision sensor calibration;

FIG. 4: is a schematic diagram of the principle of rotary calibration.

Detailed Description

In order to facilitate the understanding and implementation of the present invention for those of ordinary skill in the art, the present invention is further described in detail with reference to the accompanying drawings and examples, it is to be understood that the embodiments described herein are merely illustrative and explanatory of the present invention and are not restrictive thereof.

The experiment of the invention is to carry out the three-dimensional ice shape on-line measurement in an icing wind tunnel of 3 meters multiplied by 2 meters. The main equipment comprises: computer, polarization camera, rotary rack, line laser, 3 m 2 m icing wind tunnel and ice block. The ice-cube is placed in freezing wind tunnel, and in order to reduce the interference of wind tunnel experiment and avoid in freezing wind tunnel low temperature, steam, the influence of the phenomenon of desublimation to measuring equipment, camera, laser instrument and corresponding optical instrument all install outside freezing wind tunnel test section, measure through the printing opacity glass window. The laser and the polarization camera are fixed on the rotating rack, the relative positions of the laser and the polarization camera are kept unchanged, the icing surface is scanned through the rotating rack, and then the three-dimensional shape data of the whole icing shape is obtained through measurement. During the measurement process of the system, various devices need to be synchronized, such as the camera and the rotating platform. The invention adopts FPGA to read the rotation angle (code disc reading) of the rotating platform, and transmits the rotation angle (code disc reading) as a trigger signal to the camera for position trigger measurement, and the computer stores the image information of the corresponding position.

Embodiments of the present invention will be described below with reference to fig. 1 to 3. Fig. 1 is a schematic flow chart of a method according to the invention, and fig. 2 is a schematic structural diagram of a device for implementing the method. The specific steps of the embodiment of the invention are as follows:

step 1: a linear polarization polarized light source is adopted to illuminate the ice, a polarization camera shoots a laser light bar, and a rotating platform frame scans the frozen surface;

step 2: calibrating a line laser vision sensor;

step (ii) ofIn 2, the camera and the line laser together form a line laser vision sensor. As shown in fig. 3, the working process is as follows: the line laser projects a light plane to a measured object, the light plane intersects with the surface of the measured object to generate a light bar, the light bar comprises height information of the object, and then the camera records images of the light bar and transmits the images to the computer for analysis and the height information of the object is calculated. Suppose O_W-X_WY_WZ_WRepresenting a world coordinate system, also a measurement coordinate system; o is_C-X_CY_CZ_CRepresenting a camera coordinate system; o is_u-x_uy_uRepresenting an image coordinate system in pixels; and pi represents a structured light plane. A point P on the arbitrary light-taking plane has a coordinate P in the measurement coordinate system_w＝[x_w,y_w,z_w]^TThe image homogeneous coordinate of the projection point p on the image plane is p ═ u, v,1]^T。

Typically, we establish the measurement coordinate system on the camera coordinate system, i.e. P_w＝P_c＝[x_c,y_c,z_c]^TThen, the camera imaging model is:

wherein: s is a proportionality coefficient; a is a camera internal reference matrix, f_u,f_vAs focal length parameter, u₀,v₀Is the principal point image coordinates. Meanwhile, since P is a point on the light plane, the following light plane equation is also satisfied:

ax_w+by_w+cz_w+d＝0 (3)

wherein: a, b, c and d are coefficients of the light plane equation respectively. Therefore, we can solve the three-dimensional coordinate of a certain point P in the center of the optical strip in the camera coordinate system by only calibrating the camera internal reference matrix a and the light plane equation in the camera coordinate system and combining the two formulas, as follows:

typically, the lens distortion of the camera also needs to be taken into account. The camera internal reference matrix A can be obtained by calibration through a famous Zhang Zhengyou calibration method, and usually a checkerboard target is adopted for calibration. The optical plane equation calibration is solved through the following four steps:

(1) and acquiring the target posture. The designed target is placed in the visual field of a camera, the camera shoots a target image, characteristic points of the target are extracted, then a mapping equation of image coordinates and world coordinates is established, and external parameters of the target, namely a rotation matrix R and a translation vector T between a camera coordinate system and a target coordinate system, can be solved.

(2) Light bar center extraction. Keeping the target posture unchanged, projecting line laser to the target surface, shooting to obtain a light strip image, and obtaining the image coordinate of the light strip center through a light strip center extraction algorithm, such as a gray scale gravity center method and the like.

(3) And solving the three-dimensional coordinates of the light bars. And determining the three-dimensional coordinates of the light bar center under the camera coordinate system through the target attitude parameters R and T and the image coordinates of the light bar center.

(4) And solving the light plane. And repeating the operation, shooting the target and the light strip image at different postures, obtaining the light strip center three-dimensional coordinates at different postures, wherein the coordinates all meet the light plane equation, and determining the coefficient of the light plane equation by a least square method.

And step 3: and calibrating the rotating table.

As shown in fig. 4, in step 3, the camera and the line laser are fixed on the same support, and then the turntable rotates with the support, so that the relative positions of the camera and the line laser are not changed, that is, the coefficients of the light plane in the camera coordinate system are fixed, so that the light plane does not need to be repeatedly calibrated at different rotation angles. However, the camera rotates along with the support, so the coordinate system of the camera is not fixed, and therefore, the attitude change of the camera at different rotation angles needs to be calibrated, which is called as rotation table calibration.

The target marking position is fixed in the rotation process, two target images are obtained by shooting at an angle 1 and an angle 2 respectively, and according to the internal reference calibration of the camera, an external reference equation between the polarization camera and the target can be obtained:

the positional relationship of the polarization camera at angles 1 and 2 is:

therefore, the camera coordinate systems under different rotation angles can be unified under the same global coordinate system.

And 4, step 4: and extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system.

Different from the prior art, the ice shape three-dimensional measurement method based on polarization imaging adopts a linearly polarized light source to illuminate various ice shapes, a polarization camera shoots laser light bars, and a rotating platform scans the icing surface; calibrating a line laser vision sensor; calibrating the rotating platform; extracting the central line of the laser light bar by using an image processing method, and calculating the three-dimensional coordinate of the central line of the laser light bar in a world coordinate system; by the method, a good theoretical and technical foundation is laid for ice shape online three-dimensional measurement.

It should be understood that although the description is made in terms of embodiments, not every embodiment includes only a single embodiment, and such description is for clarity only, and the present disclosure will be directed to those skilled in the art as a whole, and the embodiments may be suitably combined to form other embodiments as will be appreciated by those skilled in the art.

The above-listed detailed description is only a specific description of a possible embodiment of the present invention, and they are not intended to limit the scope of the present invention, and equivalent embodiments or modifications made without departing from the technical spirit of the present invention should be included in the scope of the present invention.