Single-line laser method and system for measuring surface flatness of a medium-thick plate

Abstract

This invention discloses a method and system for measuring the surface flatness of a medium-thick plate using single-line laser scanning, belonging to the field of intelligent metallurgical equipment. The method includes: acquiring a laser line image of the medium-thick plate to be measured, wherein the laser line image is an image projected onto the plate by line lasers in the longitudinal and transverse directions; acquiring the intersection points of the projected laser lines in two sequentially adjacent laser line images, and performing correction based on these intersection points; performing image preprocessing on the laser line image; extracting the laser contour line from the preprocessed laser line image to obtain the area enclosed by the extracted laser contour line; and calculating the surface flatness of the medium-thick plate based on the area enclosed by the extracted laser contour line and a standard area, thereby improving the accuracy and stability of the flatness calculation for medium-thick plates.

Description

Technical Field

[0001] This invention belongs to the field of intelligent metallurgical equipment technology, and in particular relates to a method and system for measuring the surface flatness of medium-thick plates using single-line laser. Background Technology

[0002] Medium-thick plates are a common material used in construction, manufacturing, and shipbuilding. In these applications, the flatness of medium-thick plates is crucial to product quality and performance. Straightness is one of the key factors limiting the longitudinal flatness of medium-thick plates, including edge waves, center waves, and mixed waves. Currently, common methods for measuring the straightness of medium-thick plates include manual inspection at the center and edges using a level, which is inefficient and limited by the size of the level. Using technologies such as machine vision for straightness calculation can improve measurement accuracy and efficiency. For machine vision measurement systems, high-precision and reliable methods for detecting the straightness of medium-thick plates are essential.

[0003] In machine vision measurement methods, commonly used methods include vision measurement based on dual-line lasers and vision measurement based on structured light. The dual-line laser method essentially acquires laser points along the longitudinal direction of a medium-thick plate. Straightness can be obtained by measuring and fitting multiple positions on the plate. However, this method requires high calibration of the dual-line laser vision system and struggles to accurately extract the laser's centerline, directly impacting measurement accuracy. The structured light method involves segmenting the complete point cloud of the medium-thick plate surface into strip-shaped point clouds and analyzing them one by one. This method is time-consuming in its point cloud stitching process and struggles to address vibration issues during plate movement. Therefore, current methods cannot achieve robust and high-precision straightness measurement, severely hindering their practical application. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method and system for measuring the surface flatness of a medium-thick plate using single-line laser technology, thereby resolving the issues present in the prior art.

[0005] To achieve the above objectives, the present invention provides a method for measuring the surface flatness of a medium-thick plate using a single-line laser, comprising:

[0006] Acquire a laser line image of the medium-thick plate to be tested, wherein the laser line image is an image of the medium-thick plate projected by a line laser in the longitudinal and transverse directions;

[0007] The intersection of the projected laser lines in two adjacent laser line images is used for correction.

[0008] The laser line image is preprocessed, and the laser contour line is extracted from the preprocessed laser line image to obtain the area enclosed by the extracted laser contour line.

[0009] The surface flatness of the medium-thick plate is calculated based on the area enclosed by the extracted laser contour and the standard area.

[0010] The standard area is obtained by measuring according to a standard straight and thick plate.

[0011] Optionally, the image preprocessing process includes: cropping of the ROI region, grayscale conversion, and binarization.

[0012] Optionally, the laser contour extraction process includes: image filtering, edge detection, and selection of the maximum contour line.

[0013] Optionally, the process of obtaining the standard area includes:

[0014] A laser line image of a standard straight and thick plate is acquired. The laser line image of the standard straight and thick plate is preprocessed and the contour line is extracted to obtain the pixel area enclosed by the standard contour line. The pixel areas enclosed by the standard contour line are summed to obtain the standard area.

[0015] Optionally, the process of obtaining the area enclosed by the extracted laser contour line includes:

[0016] Based on the mapping relationship between pixel position and pixel area, the area of ​​the pixels enclosed by the extracted laser contour line is calculated, and the areas of the enclosed pixels are summed to obtain the area enclosed by the extracted laser contour line.

[0017] The mapping relationship between pixel position and pixel area is obtained by fitting the pixel area enclosed by the standard contour line to the pixel position enclosed by the standard contour line.

[0018] Optionally, the process of correcting two laser line images acquired in adjacent sequences includes:

[0019] The intersection points of the projected laser lines in two sequentially adjacent laser line images are obtained. The intersection points are divided into two groups of three. The positions of the intersection points in the latter image are adjusted so that the positions of the intersection points of the first group in the former image are the same as the positions of the intersection points of the second group in the latter image, in order to correct the laser image.

[0020] Optionally, the process of obtaining the surface flatness of medium-thick plates includes:

[0021] The difference between the area enclosed by the extracted laser contour line and the standard area is calculated, and the ratio of the difference to the standard area is calculated to obtain the surface flatness of the medium-thick plate.

[0022] On the other hand, the present invention provides a single-line laser surface flatness measurement system for medium-thick plates, including a vision system, a laser velocimeter, and a medium-thick plate flatness calculation system; wherein the vision system and the laser velocimeter are both connected to the medium-thick plate flatness calculation system;

[0023] The vision system includes a line laser and a camera. The line laser is used to project a laser line onto the medium-thick plate under test, and the camera is used to acquire the laser line image of the medium-thick plate under test.

[0024] The laser velocimeter is used to measure the speed of the medium-thick plate under test, and sets the interval of the projected laser lines according to the speed of the medium-thick plate under test and the time interval of the laser line image capture.

[0025] The medium-thickness plate straightness calculation system is used to acquire the laser line image of the medium-thickness plate to be tested and execute the above method.

[0026] Optionally, the laser includes three longitudinal lasers and three transverse lasers. The three longitudinal lasers are used to transmit laser lines in the longitudinal direction into the medium-thick plate, and the three transverse lasers are used to transmit laser lines in the transverse direction into the medium-thick plate.

[0027] Optionally, the system also includes a metal switch, wherein the metal switch is used to detect the movement of the medium-thick plate to the measurement position and control the measurement system to perform the measurement.

[0028] Compared with the prior art, the present invention has the following advantages and technical effects:

[0029] This invention proposes a single-line laser method and system for measuring the surface flatness of medium-thick plates. It reintroduces a method for measuring flatness. The flatness value of medium-thick plates obtained by this method can be used to further evaluate the plate shape effect of rolling mills, leveling machines, and straightening machines. At the same time, this method can also be input into rolling equipment, which helps to improve the closed-loop correction of the rolling process of medium-thick plate rolling equipment.

[0030] This invention proposes a new method for characterizing and measuring the flatness of medium-thick plates. The flatness of medium-thick plates obtained by this method is more intuitive. At the same time, this method eliminates the error caused by the vibration of medium-thick plates, further improving the calculation accuracy and stability of flatness. Attached Figure Description

[0031] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0032] Figure 1 This is a flowchart of the single-line laser surface flatness measurement method for medium-thick plates according to an embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram illustrating an example measurement of an embodiment of the present invention;

[0034] Figure 3 This is a schematic diagram of laser line-related images according to an embodiment of the present invention;

[0035] Figure 4 This is a structural diagram of a single-line laser surface flatness measurement system for medium-thick plates according to an embodiment of the present invention. Detailed Implementation

[0036] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0037] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0038] This invention relates to a method and system for measuring the surface flatness of a medium-thick plate using a single-line laser. The method and system include: a flatness measurement method and a flatness measurement system. The detailed process is as follows: three longitudinal laser lines from a line laser fixed on a gantry are projected onto the edge and middle of the medium-thick plate, respectively. Three transverse lasers on the gantry project three transverse laser lines onto the steel plate surface, which are captured by a camera. The acquired color images undergo grayscale processing and contour extraction. The distance the steel plate travels is controlled by the length measured by a laser velocimeter, which is the spacing between the transverse laser lines. The image is divided into multiple window blocks along the laser lines using square windows of fixed sizes for segmentation and extraction. The relationship between the window block position and the window block pixel area is analyzed. By establishing the mapping relationship between the position of each window block and its pixel area, the system calibration parameters can be obtained. The flatness value of the medium-thick plate can be calculated based on the relative value between the window block pixel area calculated online and the system calibration parameters, thus achieving flatness measurement. This invention can further evaluate the plate shape correction effect of rolling, leveling and straightening equipment. At the same time, the method can also feed back to the corresponding straightening equipment to realize closed-loop correction of the straightening equipment.

[0039] The purpose of this invention is to address the shortcomings of the prior art by proposing a method and system for measuring the surface flatness of a medium-thick plate using a single-line laser. This method studies the laser line striking the medium-thick plate, segments and extracts the wide-area deformed laser region in the image, divides it into multiple pixel blocks, analyzes the relationship between pixel position and pixel block area, establishes a mapping relationship between the position and area of ​​each region of the laser line, and finally obtains the flatness information of the plate, thus achieving flatness measurement.

[0040] To achieve the above objectives, the present invention provides the following solution:

[0041] A single-line laser surface flatness measurement system for medium-thick plates includes:

[0042] The system comprises a vision system consisting of six line lasers and three cameras, a laser velocimeter, metal switches, a gantry frame for installing the detection system, and a flatness calculation system for the medium-thick plate. The three lasers and one camera are mounted as a unit on both sides and in the middle of the roller conveyor, with adjustable positions in the width direction. The other three lasers project along the width direction of the medium-thick plate to determine the sliding position of the window. The laser velocimeter is vertically mounted above the steel plate entrance of the gantry frame. The metal switches are installed between the roller conveyors. During the movement of the medium-thick plate, the laser lines emitted by the line lasers are projected perpendicularly to the width direction of the plate onto its surface. The positions of the lasers and cameras in the width direction of the plate are adjusted according to its width. When the metal switch detects the plate, it triggers the detection system to begin detection. The laser velocimeter controls the forward distance of the plate and the operation of the detection system, performing position-by-position detection as the plate moves until the detection is complete.

[0043] A method for measuring the surface flatness of medium-thick plates by characterizing the area difference of line laser pixel blocks includes:

[0044] Calculation of system calibration parameters: Before online measurement, the vision measurement system needs to be calibrated. Three lasers are projected onto the surface of the medium-thick plate along the longitudinal direction of the standard straight medium-thick plate. A camera above the medium-thick plate captures the laser line image. The captured image is preprocessed and the laser line contour is extracted. A window of fixed size is selected along the laser line at fixed intervals, and the pixel area enclosed by the contour is calculated. The relationship between the laser point position of the laser line in the selected window and the pixel area in the corresponding position window is fitted using a polynomial fitting method. The coefficients of the fitted quadratic curve are the calibration parameters of the measurement system.

[0045] Optionally, three lasers are projected in parallel onto the surface of a standard medium-thick plate. Due to the limited length of the laser line, it can only cover a portion of the plate. As the plate moves forward, the laser line gradually scans its entire length. The actual distance the plate moves is measured in real time by a laser velocimeter and provided to the controller, which then stops the plate and triggers a camera to capture the laser line image of the plate.

[0046] Optionally, the captured image needs to be preprocessed first. The preprocessing process of the medium-thick plate laser line image includes cropping of the ROI region, grayscale conversion, and binarization. Cropping of the ROI region is to remove the area outside the laser line to obtain the region of interest containing the complete laser line. By converting the image to grayscale, the color image can be transformed into a grayscale image, thereby enhancing the overall grayscale contrast of the captured image and making the laser line features more obvious in the image. The global threshold binarization method is used to transform the grayscale image into a binary image, and finally obtain an image containing only the laser line.

[0047] Optionally, after image preprocessing, the laser lines in the image need to be extracted. The laser contour extraction process includes image filtering, edge detection, and selection of the maximum contour line. After binarization, some bright spots in the image still interfere with the extraction of the laser contour. Noise points can be eliminated by filtering the image. Then, the Canny edge detection method is used to extract the single-pixel edges of the laser lines, and the maximum contour line detection method is used to select the maximum contour line of the laser lines to remove redundant contour lines. The remaining closed contour line is the laser contour in the image.

[0048] Optionally, a fixed-size window with a width of W and a height of H is used, starting from a vertical edge of the maximum outline of the laser line, and the window is extended at fixed intervals from the first pixel of the laser line. By performing a single slide, the window can capture the covered laser outline within the window, thereby calculating the area enclosed by the laser line within the window. .

[0049] Optionally, the distance between the laser lines is set according to the time interval between two photos taken by a single camera. Multiplied by the speed of the roller conveyor Determined, that is:

[0050] ;

[0051] The location of the segmented window in the acquired image is achieved using three additional lasers that project transverse laser lines. These transverse and longitudinal laser lines intersect at nine points on the surface of the medium-thick plate. These nine intersection points cover the entire width of the steel plate, as well as the deformation measurement areas in the middle and at the edges.

[0052] Optionally, the measurement system is robust to the vibration of the steel plate. The single-direction three-laser-point longitudinal laser window sliding method adopted by the system can eliminate the window pixel area error caused by the vertical jump caused by the vibration of the medium-thick plate during the forward movement. The system uses three laser intersection points as a group. Each measurement is completed by moving from one laser point to the next laser point, that is, advancing one window distance on the surface of the medium-thick plate. After obtaining two sets of laser point data, the positions of the last two laser points of the previous group are always the same as the positions of the first two laser points of the next group. That is, two of the three laser points in the two sets of measurement data will be repeated, that is, the last two points of the previous group and the first two points of the next group are repeated. By moving the laser point with vertical deviation, it can be ensured that the laser point position is on the first set of measurement reference plane, thereby eliminating the influence of vibration.

[0053] The system consists of three vertical laser lines intersecting with three horizontal laser lines, creating three sets of laser intersection points in the vertical direction. The three horizontal laser lines are spaced 50mm apart, and the camera can simultaneously capture these intersection points within its field of view. These intersection points are calculated using corner point recognition in the image to ensure that two points from the previous and next sets coincide during the steel plate's movement, allowing for vertical movement to eliminate vibration. This operation does not require contour extraction; it processes the image directly at a macroscopic level, eliminating the need for microscopic window sliding. This vertical deviation is calibrated and becomes part of the system parameters, ultimately used to calculate the flatness of the entire steel plate. This calculation is completed before the system calibration parameters in the flowchart.

[0054] Optionally, after the system completes the measurement of the entire steel plate, the position of each measurement window block and the area of ​​the window pixel block are calculated, and the width-direction coordinates of all laser point pixel positions in the width direction of the medium-thick plate are constructed using a quadratic curve fitting method. Area enclosed by laser points within the corresponding window The mapping relationship. Among them, the constant coefficients of the fitted curve. These are the standard system parameters for the flat, medium-thick plate to be calibrated.

[0055] ;

[0056] Calculation of laser line area for a single-position window in medium-thick plates: After each measurement, the measurement window is moved to a different position, and the area of ​​all laser points within that window is calculated along the laser line using the following formula:

[0057] ;

[0058] Online measurement process: Similar to the system calibration process described above, after the metal switch detects the actual medium-thick plate, the measurement system starts measuring. This involves all lasers projecting longitudinal laser lines onto the middle and edges of the medium-thick plate, and preprocessing, binarizing, and extracting the contour lines from the laser line images captured by the camera, using the same step size. Same size The window is contoured along the laser line, and all laser points inside the corresponding window are obtained. area .

[0059] Flatness calculation: The area of ​​the window laser line obtained during online measurement. Corresponding position coordinates The area of ​​the laser line in the window is obtained by substituting the system calibration parameters into the mapping relationship fitted during offline calibration. The actual window laser line area Area with calibration system parameters By performing the subtraction, we can obtain the difference in the area enclosed by the laser line contours within the corresponding window position. By plotting a coordinate graph with the pixel coordinates of each window's detection position as the horizontal axis and the area difference as the vertical axis, the flatness deformation of the medium-thick plate can be presented. The flatness value... Represented as:

[0060] ;

[0061] The above technical solution is described in detail below:

[0062] like Figure 1 As shown, this invention discloses a method for measuring the surface flatness of a medium-thick plate using a single-line laser, including an offline system parameter calibration process and online flatness measurement. The offline system parameter calibration process includes the following steps:

[0063] Step 1: Place the straight steel plate flat on the roller conveyor. Multiple line lasers and cameras are installed in a specific arrangement on the gantry frame above the roller conveyor. A laser velocimeter is installed at the steel plate inlet of the gantry frame to measure the steel plate's forward distance. A metal switch is installed below the roller conveyor to detect the steel plate. Three longitudinal line lasers are arranged according to the width of the steel plate, projecting onto both sides and the middle of the fixed steel plate.

[0064] Step 2: After the roller conveyor starts running, the steel plate begins to move. When it moves to the gantry, the metal switch will detect the steel plate and reverse the signal. This signal is transmitted to the controller, which controls the measurement system to start measuring. That is, it controls all lasers to start projecting and synchronously acquire images. After the measurement is completed, the roller conveyor is controlled to continue moving forward and the steel plate continues to move forward. The laser velocimeter starts measuring the distance. After moving forward a distance d, the roller conveyor is controlled to stop and a second measurement is performed on the steel plate.

[0065] Step 3: Determine the measurement rules of the measurement system. The lasers of the three horizontal laser lines are projected onto a flat steel plate. The projected horizontal laser lines and vertical laser lines have 9 intersection points on the surface of the medium-thick plate. The images captured by the camera contain these intersection points, which are the marker points for calculating the area of ​​the laser line contour when the window slides.

[0066] Using three laser intersection points as a group, the medium-thick plate is moved forward one unit distance via a laser velocimeter after each image acquisition. This ensures that the positions of the last two laser points in the previous group are always the same as the positions of the first two laser points in the next group. By subtracting the positional deviation, the laser point positions are guaranteed to be on the same horizontal plane. This method can eliminate the area deviation caused by steel plate vibration. A schematic diagram of the laser intersection points is shown below. Figure 2 As shown, the intersection points of the first group and the intersection points of the next group need to coincide. The intersection points of the previous group are 1, 2, and 3; 2 and 3 are the last two intersection points of the previous group, while the intersection points 2 and 3 in the next group are the first two intersection points, and intersection point 4 is a newly appearing intersection point in the next group.

[0067] Step 5: After the measurement system completes all measurements, the acquired laser line image of the medium-thick plate is preprocessed, including ROI region cropping, grayscale conversion, and binarization. Cropping the ROI region removes areas outside the laser line to obtain the region of interest containing the complete laser line. Grayscale conversion transforms the color image into a grayscale image, enhancing the overall grayscale contrast of the captured image and making the laser line features more prominent. Global threshold binarization transforms the grayscale image into a binary image, finally obtaining only the laser line with the highest light intensity in the image.

[0068] Step 6: Extract the laser contour line from the preprocessed image, including image filtering, edge detection, and selection of the maximum contour line. After binarization, some bright spots in the image still interfere with the extraction of the laser contour line. Filtering the image can eliminate noise points. Then, the Canny edge detection method is used to extract the single-pixel edges of the laser line, and the maximum contour line detection method is used to select the maximum contour line of the laser line. An exemplary display of the binary image of the laser line, the laser line contour image, and the window sliding diagram image is shown below. Figure 3 As shown.

[0069] Step 7: As Figure 2 The parameters are measured along the laser line, and the covered laser contour line is intercepted within the window. The initial position of the steel plate and the position where the window stops are recorded, thereby calculating the area enclosed by the horizontal and vertical edges of the laser contour line within the window. The pixel position coordinates were constructed using a quadratic curve fitting method. With corresponding area The mapping relationship. Among them, the constant coefficients of the fitted curve. These are the standard system parameters for the flat, medium-thick plate to be calibrated:

[0070] ;

[0071] Calculation of laser line area for a single-position window in medium-thick plates: The area of ​​all laser points within the window is calculated for each sliding position of the window, using the same step size. Same size The window slides across the laser line profile and calculates all laser points within the corresponding window position. area That is, it can be obtained through the following formula:

[0072] ;

[0073] Step 8: Online flatness calculation. After the offline system calibration parameters are calibrated, during the online flatness measurement process, simply perform the operations from Steps 3 to 7 above on the actual laser line image acquired. Substitute the system calibration parameters obtained in Step 7 into the calculation of the area of ​​the laser point within the sliding window at each position in the actual medium-thick plate to be measured. Calculate the area of ​​the laser line within the window of the actual medium-thick plate. Area with calibration system parameters By performing the subtraction, the difference in the area enclosed by the laser line contours within the corresponding window position can be obtained. Detecting the pixel coordinates of the position in each window The horizontal axis represents the area difference. Plotting a coordinate graph on the vertical axis will show the flatness deformation of a medium-thick plate, and the flatness value. Represented as:

[0074] ;

[0075] Using the above method, a new approach for measuring the flatness of medium-thick plates using the area of ​​line laser pixel blocks was established. The flowchart of this method is as follows: Figure 1 As shown in the diagram, the straightness calculation is illustrated below. Figure 2 As shown, the specific outline image is illustrated as follows: Figure 3 As shown, this method can solve the problems of difficult calibration, low accuracy, and poor stability in the visual measurement of plate shape. It can measure the edge waviness, middle waviness, and mixed waviness of medium and thick plates respectively. It can be applied to the flatness measurement of metal surfaces such as thick plates and magnesium-aluminum alloy plates in the metallurgical industry, and has important theoretical significance and significant practical application value.

[0076] The example uses the flatness measurement of a medium-thick plate. Figure 4The diagram shows the actual measurement structure for the flatness of medium-thick plates based on the pixel area representation of line lasers. It includes a production line roller conveyor, gantry frame, sliding track, metal switch, laser velocimeter, the steel plate being measured, three line lasers projecting laterally and longitudinally, three cameras, a controller, a computer, and necessary power cables. The line lasers used are green line lasers with a 30-degree fan angle; the computer has an i7 processor and 32GB of RAM; the laser velocimeter is a Doppler laser velocimeter; and the controller is a PLC 1500 controller. After obtaining the system calibration parameters offline, during online measurement, the line lasers project line lasers onto the steel plate on the roller conveyor, and the cameras acquire line laser images. By preprocessing the line laser images, extracting contours, and calculating the window area, the flatness of the actual steel plate surface is calculated using the calibrated system parameters and the actual calculated window area. This embodiment is a preferred implementation of the present application, but the scope of protection of the present application is by no means limited thereto. It includes the types of metal medium and heavy plates, the fitting method and polynomial degree of the system calibration parameters, the hardware configuration, and any easily conceivable changes, all of which are covered within the scope of protection of the present application.

[0077] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A single-line laser method for measuring surface flatness of a medium-thick plate, characterized by, include: Acquire laser line images of the medium-thick plate to be tested, wherein the laser line images are images projected onto the medium-thick plate by line lasers in the longitudinal and transverse directions; for each image acquired, the medium-thick plate is controlled to move forward one unit distance by a laser velocimeter, so that the positions of the last two laser points in the previous group are always the same as the positions of the first two laser points in the next group. By subtracting the positional deviation, it can be ensured that the laser point positions are on the same horizontal plane. The intersection points of the projected laser lines in two adjacent laser line images are collected, and the system is corrected based on these intersection points. The relationship between the laser point position of the selected window and the pixel area within the corresponding window is fitted using a quadratic curve fitting method. The coefficients of the fitted quadratic curve are the calibration parameters of the measurement system. The laser line image is preprocessed, and the laser contour line is extracted from the preprocessed laser line image to obtain the area enclosed by the extracted laser contour line. The surface flatness of the medium-thick plate is calculated based on the area enclosed by the extracted laser contour and the standard area. The standard area is obtained by measuring according to a standard straight and thick plate; The process of obtaining the standard area includes: A laser line image of a standard straight and thick plate is acquired. The laser line image of the standard straight and thick plate is preprocessed and the contour line is extracted to obtain the pixel area enclosed by the standard contour line. The pixel areas enclosed by the standard contour line are summed to obtain the standard area. The process of obtaining the area enclosed by the extracted laser contour line includes: Based on the mapping relationship between pixel position and pixel area, the area of ​​the pixels enclosed by the extracted laser contour line is calculated, and the areas of the enclosed pixels are summed to obtain the area enclosed by the extracted laser contour line. The mapping relationship between pixel position and pixel area is obtained by fitting the pixel area enclosed by the standard contour line and the pixel position enclosed by the standard contour line. The process of correcting two laser line images acquired in adjacent sequences includes: The intersection points of the projected laser lines in two sequentially adjacent laser line images are obtained. The intersection points are divided into two groups of three. The position of the intersection points in the latter image is adjusted so that the position of the intersection points of the first group in the former image is the same as the position of the intersection points of the second group in the latter image, in order to correct the laser image. The laser contour extraction process includes: image filtering, edge detection, and selection of the maximum contour line; The process of obtaining the surface flatness of medium-thick plates includes: The difference between the area enclosed by the extracted laser contour line and the standard area is calculated, and the ratio of the difference to the standard area is calculated to obtain the surface flatness of the medium-thick plate.

2. The method for measuring the surface flatness of a medium-thick plate using single-line laser as described in claim 1, characterized in that, The image preprocessing process includes: cropping of the ROI region, grayscale conversion, and binarization.

3. A single-line laser surface flatness measurement system for medium-thick plates, characterized in that, It includes a vision system, a laser velocimeter, and a medium-thick plate straightness calculation system; wherein the vision system and the laser velocimeter are both connected to the medium-thick plate straightness calculation system; The vision system includes a line laser and a camera. The line laser is used to project a laser line onto the medium-thick plate under test, and the camera is used to acquire the laser line image of the medium-thick plate under test. The laser velocimeter is used to measure the speed of the medium-thick plate under test, and sets the interval of the projected laser lines according to the speed of the medium-thick plate under test and the time interval of the laser line image capture. The medium-thickness plate straightness calculation system is used to acquire the laser line image of the medium-thickness plate to be tested and to perform the method described in any one of claims 1-2.

4. The single-line laser surface flatness measurement system for medium-thick plates according to claim 3, characterized in that, The laser includes three longitudinal lasers and three transverse lasers. The three longitudinal lasers are used to transmit laser lines in the longitudinal direction into the medium-thick plate, and the three transverse lasers are used to transmit laser lines in the transverse direction into the medium-thick plate.

5. The single-line laser surface flatness measurement system for medium-thick plates according to claim 3, characterized in that, It also includes a metal switch, which is used to detect the movement of the medium-thick plate to the measurement position and control the measurement system to perform the measurement.

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