3D laser contourgraph coordinate system conversion calibration method

Abstract

The invention discloses a coordinate system conversion calibration method for a 3D laser contourgraph. The method comprises the following steps: firstly, building a calibration device; the calibration device is provided with four calibration balls with adjustable spatial positions, the four calibration balls are enabled to intersect with a laser surface emitted by the contourgraph through adjustment, and the centers of the balls are not coplanar. Then, a world coordinate system is established through a laser tracker, and the center coordinates of all the calibration balls in the world coordinate system are obtained; and further, acquiring a circle center coordinate and a circular arc radius corresponding to the circular arc on the spherical surface of the calibration ball through a contourgraph to obtain a sphere center coordinate of each positioning ball under the contourgraph. And finally, according to the obtained coordinates of the sphere center under the two coordinate systems, carrying out transformation matrix parameter solving to obtain a transformation matrix, and selecting an optimal result. According to the method, the relative attitude of the 3D laser contourgraph can be accurately obtained, rigid body transformation parameters are completely solved, the parameter solving precision is high, and calibration of multiple coordinate systems can be achieved.

Description

Technical Field

[0001] The present invention relates to the field of automatic detection technology, and designs a 3D laser profiler coordinate system conversion calibration method, specifically a calibration method for establishing a conversion relationship between a 3D laser profiler measurement coordinate system and a world coordinate system. Technical Background

[0002] 3D laser profilers are often used in 3D inspection engineering applications to measure the 3D model of workpieces. The basic method is that the 3D laser profiler combines with the workpiece's transmission system to continuously collect contours and reconstruct the workpiece's 3D model. The rigid body transformation relationship between the 3D laser profiler's measurement coordinate system and the world coordinate system where the transmission system is located needs to be determined. That is, the point cloud data collected by the 3D laser profiler is converted to the world coordinate system, and multiple contour sections are spliced ​​together to reconstruct the workpiece's 3D model in combination with the motion model of the transmission system in the world coordinate system.

[0003] Among the current 3D laser profiler coordinate system conversion methods, most are based on fixed 3D laser profilers for application scenarios, have high requirements on the equipment installation posture, and have low method generalization. Summary of the invention

[0004] In view of the above problems, the present invention proposes a 3D laser profiler coordinate system conversion calibration method, which can convert the 3D laser profiler measurement coordinate system to any world coordinate system. The method of the present invention can solve the six degrees of freedom in the rigid body transformation matrix, and has no strict requirements on the installation method of the 3D laser profiler. The calibration process is simple and fast, and the calibration mechanism can be used in multiple application scenarios.

[0005] A 3D laser profiler coordinate system conversion calibration method, the steps are as follows:

[0006] Step 1: Calibration device construction and adjustment.

[0007] The calibration device is four calibration balls with adjustable spatial positions. According to the irradiation range of the laser surface emitted by the 3D laser profiler, the position of the 3D laser profiler and the positions of the four calibration parts are adjusted so that the four calibration balls intersect with the laser plane emitted by the 3D laser profiler; the centers of the calibration balls in the four calibration parts are not coplanar; and there is a spacing between the calibration balls in the longitudinal direction.

[0008] Step 2: Conversion between the 3D laser profiler measurement coordinate system and the world coordinate system.

[0009] (1) Establish the world coordinate system through the laser tracker.

[0010] (2) Using the target ball of the same size as the calibration ball in the laser tracker, replace the four calibration balls one by one, and measure the center coordinates of the four calibration balls in the world coordinate system in turn:

[0011] PW =(X W ,Y W ,Z W ).

[0012] (3) Control the 3D laser profiler to collect images as calibration images.

[0013] (4) Convert the calibration image collected in (3) into a point cloud, filter out the arc point cloud formed by the laser emitted by the 3D laser profiler on the calibration sphere, and fit the least squares algorithm in the 3D laser profiler measurement coordinate system to obtain the center coordinate P of the circle where the arc is located. r =(X C ,Z C ) and the arc radius r.

[0014] (5) According to the radius R of the sphere, the radius r of any cross-section circle on the sphere, and the distance d between the center of the cross-section circle and the center of the sphere, R 2 =d 2 +r 2 The relationship between the center of the positioning ball and the Y-axis coordinate value Y of the center of the positioning ball in the 3D laser profiler measurement coordinate system is obtained. C The absolute value of |Y C |=d;Y C The positive or negative value of is determined according to the quadrant of the center of the calibration ball in the calibration image obtained in step (3) in the 3D laser profiler measurement coordinate system. Finally, combined with step 4, the coordinates P of the center of each positioning ball in the 3D laser profiler measurement coordinate system are obtained. C =(X C ,Y C ,Z C ).

[0015] The sphere center coordinates of each positioning sphere in the world coordinate system and in the 3D laser profiler measurement coordinate system are taken as a set of coordinate pairs. Three sets of coordinate pairs are randomly selected from the four sets of coordinate pairs, the transformation matrix parameters are solved, the transformation matrix M is obtained, and the optimal transformation matrix is ​​determined.

[0016] The advantages of the present invention are:

[0017] 1. The 3D laser profiler coordinate system conversion calibration method of the present invention can accurately obtain the relative posture of the 3D laser profiler and completely solve the rigid body transformation parameters.

[0018] 2. The 3D laser profiler coordinate system conversion calibration method of the present invention uses a high-precision sphere, takes the center of the sphere as a feature point, and solves parameters with high accuracy.

[0019] 3. The 3D laser profiler coordinate system conversion and calibration method of the present invention can combine multiple 3D laser profilers for measurement to achieve multiple coordinate system calibrations, and can be applied to multiple industrial scenarios.

[0020] 4. The 3D laser profiler coordinate system conversion and calibration method of the present invention collects less data, has a simple and fast solution process, and does not require fixing the 3D laser profiler posture. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow chart of the 3D laser profiler coordinate system conversion and calibration method of the present invention;

[0022] Figure 2 It is a schematic diagram of the structure of a calibration device used in the 3D laser profiler coordinate system conversion calibration method of the present invention;

[0023] Figure 3 It is a schematic diagram of the structure of the calibration parts in the calibration device.

[0024] In the figure:

[0025] 1- Calibration ball 2- Magnetic base 3- Bend plate

[0026] 4- Bracket 5- Laser Profiler 301- Side Slide

[0027] 302-Front slide DETAILED DESCRIPTION

[0028] The 3D laser profiler coordinate system conversion calibration method of the present invention is as follows: Figure 1 As shown, the specific steps are as follows:

[0029] Step 1: Calibration device construction;

[0030] like Figure 2 , Figure 3 As shown, the calibration device includes four groups of calibration parts with the same structure, which are composed of a calibration ball 1, a magnetic base 2, a bending plate 3, and a bracket 4, a laser profiler 5 and an industrial computer.

[0031] First, fix the bracket 4 on the horizontal support surface; the bracket 4 is constructed of aluminum profiles, one of which is longitudinally arranged for installation of the calibration piece; the side wall of the aluminum profile has a longitudinal slide groove for installing bolts to connect the bent plate 3.

[0032] Then, the calibration part is installed. In the calibration part, the bent plate 3 is an L-shaped plate with a width of 60mm; two side slides 301 designed along the front and rear directions are designed longitudinally on one side surface. The two bolts installed in the groove of the aluminum profile are respectively passed through the two side slides 301 on the bent plate 3 and then connected with the nuts. The bent plate 3 can be fixed on the aluminum profile by tightening the nuts. The front end surface of the bent plate 3 has two front slides 302 in the horizontal direction; the two threaded columns on the back side of the magnetic base 2 are respectively passed through the two front slides 302 and then connected with the nuts. The magnetic base 2 can be fixed on the front end surface of the bent plate 3 by tightening the nuts. At the same time, a calibration ball 1 of matching size is placed in the spherical recess on the front side of each of the above-mentioned magnetic bases 2, and the calibration ball 1 is magnetically fixed to the magnetic base. The calibration ball 1 is a sphere with a diameter of 38.1mm and an accuracy of 0.01mm. The repeatability accuracy of the magnetic base in taking and placing the calibration ball 1 is 0.01mm.

[0033] Finally, the 3D laser profiler 5 is placed on the front side of the calibration piece and connected to the network port of the industrial computer using a network cable.

[0034] Step 2: Calibration device debugging;

[0035] According to the irradiation range of the laser surface emitted by the 3D laser profiler 5, adjust the displacement of the 3D laser profiler and the position of the four calibration parts. The position of the four calibration parts can be adjusted in the longitudinal and front-to-back directions by loosening the nuts. After adjustment, it is necessary to ensure that:

[0036] (1) The calibration spheres 1 in the four calibration pieces all intersect with the laser plane emitted by the 3D laser profiler;

[0037] (2) The centers of calibration balls 1 among the four calibration parts are not coplanar. Here, the four calibration parts need to be adjusted in the front-to-back direction so that the four calibration balls are arranged in a stepped manner. At the same time, the magnetic base is adjusted to slide along the front slide groove to different positions.

[0038] (3) There is a certain distance between the calibration balls in the four calibration pieces in the longitudinal direction. In this embodiment, the distance is set to about 70 mm to ensure that there is enough distance to distinguish the arc part of the collected laser surface irradiated on each calibration ball.

[0039] Step 3: Conversion between the 3D laser profiler measurement coordinate system and the world coordinate system.

[0040] 301. Establish a world coordinate system through a laser tracker.

[0041] 302. Use the target ball with a diameter of 38.1 mm selected by the laser tracker to replace the calibration ball 1 on the four calibration parts one by one, and replace the center of the calibration ball 1 with the center of the target ball. Then measure and obtain the center coordinates P of the calibration ball 1 on the four calibration parts in the world coordinate system in turn. W =(X W,Y W ,Z W ).

[0042] 303. Turn on the industrial computer, set the acquisition mode to free acquisition, and control the 3D laser profiler to acquire tif format images as calibration images.

[0043] 304. Convert the calibration image into a point cloud, and select the arc point cloud formed by the laser emitted by the 3D laser profiler on the calibration ball 1; for a single arc point cloud, use the least squares algorithm to fit the center coordinates P of the circle where the arc is located in the measurement coordinate system of the 3D laser profiler 5. r =(X C ,Z C ), and get the arc radius r.

[0044] 305. According to the radius R of the sphere, the radius r of any cross-section circle on the sphere, and the distance d between the center of the cross-section circle and the center of the sphere, R 2 =d 2 +r 2 The relationship between the Y axis coordinate value Y of the center of the positioning ball 1 in the measurement coordinate system of the 3D laser profiler 5 can be obtained. C The absolute value of |Y C |=d; and Y C The positive or negative value of is determined according to the quadrant of the center of the calibration ball 1 in the calibration image obtained in step 303 in the measurement coordinate system of the 3D laser profiler 5; if it is in quadrants 1 and 4, record Y C If the value is positive, if it is in quadrants 2 or 3, record Y C The value is negative. Finally, the coordinates obtained in step 304 can be combined to obtain the center coordinates P r =(X C ,Z C ), and obtain the spherical center coordinates P of the four positioning balls 1 in the measurement coordinate system of the 3D laser profiler 5 C =(X C ,Y C ,Z C ).

[0045] The center coordinates of each positioning ball 1 in the world coordinate system and in the laser profiler 5 measurement coordinate system are taken as a set of coordinate pairs. Three sets of coordinate pairs are randomly selected from the four sets to solve the transformation matrix parameters. The rigid change between the coordinate systems satisfies P W =M*P C , M is the transformation matrix; from the four sets of spherical center coordinates [P W ,P C ] to select any three groups for combination, and obtain four combination results. For each combination, the following process is carried out in sequence:

[0046] a. Solve the transformation matrix parameters to obtain the transformation matrix M;

[0047] b. Set the unselected sphere center coordinates P C Use the obtained transformation matrix M to transform to the world coordinate system;

[0048] c. The coordinates of the center of the sphere after transformation are P T =(X T ,Y T ,Z T ).

[0049] d. According to the obtained P T and the truth value P W Calculate the coordinate similarity θ:

[0050]

[0051] Finally, 4 θ values ​​are obtained, among which the transformation matrix M corresponding to the minimum θ value is the optimal transformation matrix.

[0052] Therefore, the conversion matrix obtained by the above method can realize the conversion of the point cloud data of the measured object obtained by scanning the 3D laser profiler into the world coordinate system. In addition, multiple 3D laser profilers can be set up at the same time to correspond to multiple calibration parts respectively, and the combined measurement can be performed by the method of the present invention to realize the calibration of multiple coordinate systems, which can be applied to multiple industrial scenarios.

Claims

1. A 3D laser profiler coordinate system conversion and calibration method, characterized in that: The specific steps are: Step 1: Calibration device construction and adjustment; The calibration device is four calibration balls with adjustable spatial positions; according to the irradiation range of the laser surface emitted by the 3D laser profiler, the position of the 3D laser profiler and the positions of the four calibration parts are adjusted so that the four calibration balls intersect with the laser plane emitted by the 3D laser profiler; and the centers of the calibration balls in the four calibration parts are not coplanar; and there is a spacing between the calibration balls in the longitudinal direction; Step 2: Conversion between the 3D laser profiler measurement coordinate system and the world coordinate system; (1) Establishing the world coordinate system through the laser tracker; (2) Using the target ball of the same size as the calibration ball in the laser tracker, replace the four calibration balls one by one, and measure the center coordinates of the four calibration balls in the world coordinate system in turn: P W =(X W ,Y W ,Z W )。 (3) Control the 3D laser profiler to collect images as calibration images. (4) Convert the calibration image collected in (3) into a point cloud, filter out the arc point cloud formed by the laser emitted by the 3D laser profiler on the calibration sphere, and fit the least squares algorithm in the 3D laser profiler measurement coordinate system to obtain the center coordinate P of the circle where the arc is located. r =(X C ,Z C ) and the arc radius r; (3) According to the radius R of the sphere, the radius r of any cross-section circle on the sphere, and the distance d between the center of the cross-section circle and the center of the sphere, R 2 =d 2 +r 2 The relationship between the center of the positioning ball and the Y-axis coordinate value Y of the center of the positioning ball in the 3D laser profiler measurement coordinate system is obtained. C The absolute value of |Y C |=d;Y C The positive or negative value of is determined according to the quadrant of the center of the calibration ball in the calibration image obtained in step (3) in the 3D laser profiler measurement coordinate system; finally, the coordinates P of the center of each positioning ball in the 3D laser profiler measurement coordinate system are obtained. C =(X C ,Y C ,Z C ); (4) The coordinates of the center of each positioning ball in the world coordinate system and in the 3D laser profiler measurement coordinate system are taken as a set of coordinate pairs. Three sets of coordinate pairs are randomly selected from the four sets, and the transformation matrix parameters are solved to obtain the transformation matrix M, and the optimal transformation matrix is ​​determined.

2. A 3D laser profiler coordinate system conversion and calibration method as claimed in claim 1, characterized in that: The bracket is constructed of aluminum profiles, one of which is longitudinally arranged for installation of the calibration part; the side wall of the aluminum profile has a longitudinal slide groove for installing bolts to connect the bent plate to achieve front and rear and up and down position adjustment of the calibration part.

3. A 3D laser profiler coordinate system conversion and calibration method as claimed in claim 1, characterized in that: The calibration ball is a sphere with a diameter of 38.1mm and an accuracy of 0.01mm; the repeatability accuracy of the magnetic base in taking and placing the calibration ball is 0.01mm.

4. A 3D laser profiler coordinate system conversion and calibration method as claimed in claim 1, characterized in that: The front end surface of the bent plate is laterally provided with two front slideways; two threaded columns on the back side of the magnetic base respectively pass through the two front slideways and are connected with nuts.

5. A 3D laser profiler coordinate system conversion and calibration method as claimed in claim 1, characterized in that: In step 1, the calibration device consists of four groups of calibration parts with the same structure, namely a calibration ball, a magnetic base, a bent plate and a bracket, a laser profiler and an industrial computer; wherein the bracket is fixed on a horizontal support surface; four groups of calibration parts are installed on the bracket; among the calibration parts, the bent plate is an L-shaped plate, and one side surface is installed on the bracket in a sliding fit with the bracket; the other side is slidably fitted with a magnetic base, and a spherical recess is designed on the magnetic base for fixing the calibration ball, and the calibration ball is fixed by adsorption with the positioning ball; the 3D laser profiler is arranged on the front side of the calibration part, and is connected to the network port of the industrial computer using a network cable.

6. A 3D laser profiler coordinate system conversion and calibration method as claimed in claim 1, characterized in that: In step (4), the optimal transformation matrix solution method is: A. In each group of solutions, the unselected sphere center coordinates P C Use the obtained transformation matrix M to transform to the world coordinate system; c. The coordinates of the center of the sphere after transformation are P T =(X T ,Y T ,Z T ); d. According to the obtained P T and the truth value P W Calculate the coordinate similarity θ: Finally, 4 θ values ​​are obtained, among which the transformation matrix M corresponding to the minimum θ value is the optimal transformation matrix.

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