CN114387228A - A method of automatic workpiece positioning and alignment based on line structured light - Google Patents

Abstract

The invention relates to the field of electronic detection methods, in particular to a method for automatic positioning and alignment of workpieces based on line structured light. It includes the following steps: S1. Fixing the workpiece; S2. Irradiating the line structured light; S3. Obtaining the light bar image; S4. Image processing; S5. Obtaining the three-dimensional coordinates of the point on the light bar; S6. The invention has high processing efficiency, low labor cost, flexible operation and high processing precision, can effectively shorten the processing cycle, is suitable for single-piece and small-batch mode, and can better meet the current market demand.

Description

A method of automatic workpiece positioning and alignment based on line structured light

technical field

The invention relates to the field of electronic detection methods, in particular to a method for automatic positioning and alignment of workpieces based on line structured light.

Background technique

In the field of machining, workpiece positioning is an important step in the workpiece machining process. After the workpiece is positioned, the workpiece clamping and workpiece machining operations are performed, which affect the efficiency of the entire workpiece machining process and the machining accuracy of the workpiece. In mass production mode, special high-precision fixtures can be used. However, in the processing of single and small batch workpieces, manual alignment is still mainly performed by the operator. In this case, the machining accuracy is greatly reduced, the machining efficiency is seriously reduced, the labor cost is greatly increased, and the utilization rate of the machining center is reduced. Therefore, this traditional method cannot well meet the current market demands. Therefore, this patent proposes an automatic alignment method. Different from the traditional workpiece positioning method, the automatic positioning and alignment method does not need to accurately align the initial posture of the workpiece, and adopts active positioning and adjustment of the pose instead of passive positioning. The automatic positioning and alignment method has high processing efficiency, low labor cost, flexible operation and high processing accuracy, which can effectively shorten the processing cycle and is suitable for single-piece small batch mode.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the defects of the prior art, and to provide a method for automatic positioning and alignment of workpieces based on linear structured light, which can be used for automatic positioning and pose adjustment of workpieces on horizontal machining centers.

The technical scheme that realizes the object of the present invention is:

A method for automatic workpiece positioning and alignment based on line structured light, comprising the following steps:

S1. Fix the workpiece: install the workpiece on the rotary table of the horizontal machining center with a fixture or a fixing element;

S2. Irradiate line structured light: use a laser to irradiate the line structured light on the surface of the workpiece, and bright stripes appear on the surface of the workpiece at this time;

S3. Acquire the light bar image: use a CCD camera to collect the light bar image to obtain a two-dimensional image of the light bar;

S4. Image processing: perform image filtering, image segmentation, and contour extraction processing on the obtained image;

S5. Obtain the three-dimensional coordinates of the points on the light bar: The two-dimensional image coordinates can be converted into three-dimensional actual coordinates by CCD camera calibration;

S6. Deviation between calculation and ideal position: The three-dimensional actual coordinates can accurately describe the position of the workpiece on the machining center, and the position deviation including rotation and translation can be obtained by comparing with its correct position;

S7. Adjust the pose of the workpiece: adjust the workpiece to the correct position through the rotary table.

Further, in the S3, the CCD camera is calibrated to obtain the conversion relationship between the three-dimensional coordinates and the two-dimensional coordinates. The CCD camera includes left and right cameras. In addition to the calibration of the internal and external parameters of the camera, it is also necessary to perform a positional relationship between the two cameras. To calibrate, first place the camera calibration target within the field of view of the two cameras, set the translation between the left and right camera coordinate systems and the world coordinate system as T ₁ , T ₂ , and the rotation matrices as R ₁ , R ₂ , on the calibration target The coordinates of a calibration point in the left camera coordinate system are P _l = (X _cl , Y _cl , Z _cl ) ^T , and the coordinates in the right camera coordinate system are Pr = (X _cr , Y _cr , Z _cr ) ^T , in the world The coordinates in the coordinate system are P=( _Xw ,Yw, _Zw ) ^T ; then there are:

can get

in,

Through the above formula, the conversion relationship between the left and right camera coordinate systems can be obtained;

The optical axes of the two cameras are parallel, the optical axes are perpendicular to the image plane, and the distance between the two cameras is d; C _l and C _r are the left and right camera coordinate systems, and the three-dimensional point P in the left camera coordinate system is (X _cl , Y _cl ) , Z _cl ), in the right camera coordinate system (X _cr , Y _cr , Z _cr ), the relationship between the pixel coordinate system and the image coordinate system is:

Where u0, v0 are camera internal parameters, (u _l , v _l ), (u _r , v _r ) are the coordinates of P in the left and right pixel coordinate systems, (x _l , y _l ), (x _r , y _r ) is the coordinate of P in the image coordinate system;

Combining the relationship between the image coordinate system and the camera coordinate system, we can get:

Among them, f is the focal length of the camera, α=f/dx, and β=f/dy are the internal parameters of the camera, and the coordinate conversion between the image coordinate system and the world coordinate system can be realized by the above formula.

Further, in the S4, after the light bar is obtained, the center of the light bar is extracted by the gray-scale centroid method, and the calculation formula of the gray-scale centroid method is:

where (X _i ,Y _i ) is the coordinate of a pixel in the light bar plane, and the grayscale value of the corresponding pixel is I(X _i ,Y _i ),(i=1,2,...,N), N is the number of pixels in the section, and (X _c , Y _c ) is the center coordinate of the desired light bar. So far, the coordinates of the center point of the light bar can be obtained.

Further, in the S6, it is projected on the plane of Y=0, and the coordinates of any two points A _i and B _j on the projection are respectively A _i (X _Ai , Z _Ai ), Bj (X _Bj , Z _Bj ); make line segment A _i C _i,j parallel to X _w axis through A, make line segment B _j C _{i,j through B j parallel} to Z _w axis, A _i C _i ,j intersect B _i C _i,j For C _i,j , the coordinates of C i, _j are C _i,j (X _Bj ,Z _Ai ), and there is an angle α _i, j between A _i C _i,j and A _i B _j ; then there are:

You can get:

The final required rotation angle is:

Among them, N is the number of coordinate points obtained;

Assuming that the measured coordinate point is (xi,yi), and the corresponding coordinate point of the correct position is (Xi,Yi), the translation amount of the position deviation is:

After adopting above-mentioned technical scheme, the present invention has following positive effect:

(1) The invention has high processing efficiency, low labor cost, flexible operation and high processing precision, can effectively shorten the processing cycle, is suitable for single-piece and small-batch mode, and can better meet the current market demand.

Description of drawings

In order to make the content of the present invention easier to understand clearly, the present invention will be described in further detail below according to specific embodiments and in conjunction with the accompanying drawings, wherein

Figure 1 is a work flow chart;

Figure 2 is a diagram of a positioning system;

Figure 3 is a model diagram of the camera system;

FIG. 4 is a schematic diagram of the rotation angle.

Detailed ways

A method for automatic workpiece positioning and alignment based on line structured light, comprising the following steps:

S1. Fix the workpiece: install the workpiece on the rotary table of the horizontal machining center with a fixture or a fixing element;

S2. Irradiate line structured light: use a laser to irradiate the line structured light on the surface of the workpiece, and bright stripes appear on the surface of the workpiece at this time;

S3. Acquire the light bar image: use a CCD camera to collect the light bar image to obtain a two-dimensional image of the light bar;

S4. Image processing: perform image filtering, image segmentation, contour extraction and other processing on the obtained image;

S5. Obtain the three-dimensional coordinates of the points on the light bar: The two-dimensional image coordinates can be converted into three-dimensional actual coordinates by CCD camera calibration;

S6. Deviation between calculation and ideal position: The three-dimensional actual coordinates can accurately describe the position of the workpiece on the machining center, and the position deviation including rotation and translation can be obtained by comparing with its correct position;

S7. Adjust the pose of the workpiece: adjust the workpiece to the correct position through the rotary table.

The three-dimensional image information of the workpiece surface of the CCD camera is obtained by binocular structured light measurement technology. The measurement system is composed of a laser, a CCD camera and a computer system. CCD cameras include left and right cameras.

The software part consists of structured light scanning control, image processing, contour extraction and so on. When the laser projects the line structured light on the surface of the workpiece, a light bar will be formed on the surface of the workpiece, and the light bar contains the three-dimensional coordinate information of the measured surface. Further, the locating system uses two CCD cameras that shoot the workpiece surface from different directions to imitate the human eye to obtain the three-dimensional information of the workpiece surface, thereby obtaining the coordinate information of multiple points on the workpiece surface. The three-dimensional coordinate information on the light bar becomes the two-dimensional image coordinate system information after the transformation of the camera. In this step, to obtain the conversion relationship between the actual three-dimensional coordinates and the two-dimensional coordinates of the image, the calibration of the camera is essential. The parameter matrix obtained by the camera calibration can realize the conversion of three-dimensional coordinates and two-dimensional coordinates.

For the obtained light bar image with coordinate information, image preprocessing is performed and the center of the light bar is extracted. In order to obtain high-quality clear and complete photos, the original image must be preprocessed such as filtering and image segmentation to improve the accuracy. Then, edge detection and feature point extraction are performed on the image to obtain the required center information of the light stripe.

The obtained fringe center coordinates are converted into three-dimensional world coordinates, and the position deviation from the correct position is calculated through certain mathematical operations. This positional deviation specifically includes a rotation amount and a displacement amount.

Example 1

The specific work flow of the method is shown in Figure 1, and the positioning system is shown in Figure 2. First, the workpiece fixtures or fixing elements are mounted on the rotary table of the horizontal machining center without placing the workpiece in its correct position. A laser is used to irradiate the surface of the workpiece with linear structured light, and bright streaks appear on the surface of the workpiece. The point on the light bar contains the three-dimensional coordinate information of the object point on the workpiece. A CCD camera is used for image acquisition of the light bar to obtain a two-dimensional image of the light bar. The obtained image is processed by image filtering, image segmentation, contour extraction, etc. The matrix obtained by the camera calibration can convert the two-dimensional image coordinates into three-dimensional actual coordinates. A series of three-dimensional coordinates can accurately describe the position of the workpiece on the machining center, and the position deviation including rotation and translation can be obtained by comparing with its correct position. Finally, the workpiece is adjusted to the correct position by the rotary table.

The purpose of camera calibration is to obtain the conversion relationship between the three-dimensional coordinates and the two-dimensional coordinates. In addition to the calibration of the internal and external parameters of the camera, it is also necessary to calibrate the positional relationship between the two cameras. First, the camera calibration target is placed within the field of view of the two cameras. Let the translations between the left and right camera coordinate systems and the world coordinate system be T ₁ , T ₂ , and the rotation matrices be R ₁ , R ₂ . The coordinates of a calibration point on the calibration target are P _l = (X _cl , Y _cl , Z _cl ) ^T in the left camera coordinate system, and Pr = (X _cr , Y _cr , Z _cr ) ^T in the right camera coordinate system , and the coordinates in the world coordinate system are P=(Xw, Y _w , Z _w ) ^T . Then there are:

can get

in,

Through the above formula, the conversion relationship between the left and right camera coordinate systems can be obtained.

The camera system is shown in Figure 3. The optical axes of the two cameras are parallel, and the optical axes are perpendicular to the image plane. The distance between the two cameras is d. C _l and C _r are left and right camera coordinate systems. Let the three-dimensional point P be (X _cl , Y _cl , Z _cl ) in the left camera coordinate system, and (X _cr , Y _cr , Z _cr ) in the right camera coordinate system. The relationship between the pixel coordinate system and the image coordinate system is:

Where u0, v0 are camera internal parameters, (u _l , v _l ), (u _r , v _r ) are the coordinates of P in the left and right pixel coordinate systems, (x _l , y _l ), (x _r , y _r ) is the coordinate of P in the image coordinate system.

Combining the relationship between the image coordinate system and the camera coordinate system, we can get:

Among them, f is the focal length of the camera, α=f/dx, and β=f/dy are the internal parameters of the camera. The coordinate conversion between the image coordinate system and the world coordinate system can be achieved through the above formula.

After a light stripe image is acquired by the CCD camera, the acquired image should be preprocessed and the center of the light stripe extracted. Among them, the image preprocessing mainly includes the elimination of image noise and other operations to make the image better reflect the real light bar and prepare for the subsequent light bar center extraction. This part is mainly divided into image filtering, image segmentation and strip center extraction.

Firstly, the median filter is used to eliminate the impulse noise without affecting the edge information of the image. Then use histogram processing to eliminate irrelevant information in the image and make the important feature information in the image stand out, so as to improve the image quality. Next, image segmentation is performed, the purpose of which is to extract bright areas in the image to facilitate subsequent feature extraction. The threshold segmentation method is used here. Threshold segmentation is mainly based on the grayscale difference between the target and the background in the image, select a reasonable threshold, and compare the pixels in the image with the selected threshold to determine whether the point is the target or the background. After image segmentation, a clear light bar image can be obtained, and then the image thinning operation can be performed to extract the light bar from the image. The center of light stripe was extracted by gray barycentric method. The gray-scale centroid method takes the center of the gray-scale distribution of the pixels in the image as the center point of the light-stripe, which can reduce the error of uneven gray-scale distribution of the light-stripe. The calculation formula of the gray-scale centroid method is:

where (X _i ,Y _i ) is the coordinate of a pixel in the light bar plane, and the grayscale value of the corresponding pixel is I(X _i ,Y _i ),(i=1,2,...,N), N is the number of pixels in the section, and (X _c , Y _c ) is the center coordinate of the desired light bar. So far, the coordinates of the center point of the light bar can be obtained.

After the above steps, the workpiece surface coordinate information can be obtained, and the position deviation from the correct position will be calculated next.

As shown in Figure 4, the workpiece surface coordinate points obtained through the above steps are projected on the plane of Y=0, and the coordinates of any two points A _i and B _j on the projection are respectively A _i (X _Ai , Z _Ai ) ), Bj(X _Bj , Z _Bj ). Make a line segment A _i C _i,j parallel to the X _w axis through A, and make a line segment B _j C _{i,j through B j parallel} to the Z _w axis, A _i C _i,j and B _i C _i,j intersect at C _{i ,j} , then the coordinates of C i, _j are C _i,j (X _Bj ,Z _Ai ), and there is an angle α _i, j between A _i C _i,j and A _i B _j . Then there are:

You can get:

The final required rotation angle is:

Among them, N is the number of coordinate points obtained.

Assuming that the measured coordinate point is (xi,yi), and the corresponding coordinate point of the correct position is (Xi,Yi), the translation amount of the position deviation is:

Finally, the position deviation is sent to the machining center, and the workpiece is placed in the correct position through the rotary table.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims (4)

1. a workpiece automatic locating method based on line structured light, is characterized in that: comprise the following steps: S1. Fix the workpiece: install the workpiece on the rotary table of the horizontal machining center with a fixture or a fixing element; S2. Irradiate line structured light: use a laser to irradiate the line structured light on the surface of the workpiece, and bright stripes appear on the surface of the workpiece at this time; S3. Acquire the light bar image: use a CCD camera to collect the light bar image to obtain a two-dimensional image of the light bar; S4. Image processing: perform image filtering, image segmentation, and contour extraction processing on the obtained image; S5. Obtain the three-dimensional coordinates of the points on the light bar: The two-dimensional image coordinates can be converted into three-dimensional actual coordinates by CCD camera calibration; S6. Deviation between calculation and ideal position: The three-dimensional actual coordinates can accurately describe the position of the workpiece on the machining center, and the position deviation including rotation and translation can be obtained by comparing with its correct position; S7. Adjust the pose of the workpiece: adjust the workpiece to the correct position through the rotary table. 2. The workpiece automatic positioning and alignment method based on line structured light according to claim 1, is characterized in that: in the S3, the CCD camera is calibrated to obtain the conversion relationship between the three-dimensional coordinates and the two-dimensional coordinates, and the CCD camera comprises: For the left and right cameras, in addition to the calibration of the internal and external parameters of the camera, it is also necessary to calibrate the positional relationship between the two cameras. First, place the camera calibration target within the field of view of the two cameras, and set the left and right camera coordinate systems and the world coordinate system. The amount of translation between is T ₁ , T ₂ , the rotation matrices are R ₁ , R ₂ , the coordinates of a calibration point on the calibration target in the left camera coordinate system are P _l =(X _cl , Y _cl , Z _cl ) ^T , at The coordinates under the right camera coordinate system are Pr=(X _cr , Y _cr , Z _cr ) ^T , and the coordinates under the world coordinate system are P=(Xw, Y _w , Z _w ) ^T ; then there are:

can get

in,

Through the above formula, the conversion relationship between the left and right camera coordinate systems can be obtained; The optical axes of the two cameras are parallel, the optical axes are perpendicular to the image plane, and the distance between the two cameras is d; C _l and C _r are the left and right camera coordinate systems, and the three-dimensional point P in the left camera coordinate system is (X _cl , Y _cl ) , Z _cl ), in the right camera coordinate system (X _cr , Y _cr , Z _cr ), the relationship between the pixel coordinate system and the image coordinate system is:

Where u0, v0 are camera internal parameters, (u _l , v _l ), (u _r , v _r ) are the coordinates of P in the left and right pixel coordinate systems, (x _l , y _l ), (x _r , y _r ) is the coordinate of P in the image coordinate system; Combining the relationship between the image coordinate system and the camera coordinate system, we can get:

Among them, f is the focal length of the camera, α=f/dx, and β=f/dy are the internal parameters of the camera, and the coordinate conversion between the image coordinate system and the world coordinate system can be realized by the above formula. 3. The workpiece automatic positioning method based on line structured light according to claim 1, characterized in that: in the S4, after obtaining the light bar, the gray barycenter method is used to extract the center of the light bar, and the gray barycenter The calculation formula of the method is:

Where (X _i ,Y _i ) is the coordinate of a pixel in the light bar plane, and the gray value of the corresponding pixel is I(X _i ,Y _i ),(i=1,2,...,N), N is the number of pixel points in the section, and (X _c , Y _c ) is the center coordinate of the desired light bar. So far, the coordinates of the center point of the light bar can be obtained. 4. The workpiece automatic positioning method based on line structured light according to claim 1, characterized in that: in the S6, it is projected on the plane of Y=0, and any two points A _i are set on the projection. , the coordinates of B _j are respectively A _i (X _Ai , Z _Ai ), Bj (X _Bj , Z _Bj ); the line segment A _i C _{i, j} is made parallel to the X _w axis through A, and the line segment B _j C is made through B _{j i,j} is parallel to the Z _w axis, A _i C _i,j and B _i C _i,j intersect at C _i,j , then the coordinates of C i, _j are C _i,j (X _Bj ,Z _Ai ), and have The included angle α _i,j of A _i C _i,j and A _i B _j ; then there are:

You can get:

The final required rotation angle is:

Among them, N is the number of coordinate points obtained; Assuming that the measured coordinate point is (xi,yi), and the corresponding coordinate point of the correct position is (Xi,Yi), the translation amount of the position deviation is:

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