A non-contact measurement method of vehicle underframe lateral deflection based on line structured light

Abstract

The invention discloses a non-contact measurement method for the lateral deflection of a vehicle frame based on line structured light. A monocular single-line measurement unit is installed according to the matching of a field of view and a width of a frame; the positional relationship between the monocular single-line measurement unit and a global coordinate system is calibrated; by projecting a single-line structured light to the vehicle frame, each monocular single-line measurement unit three-dimensionally reconstructs a surface three-dimensional point cloud of a cross section, and converts it to a global coordinate system; a global point cloud fitting plane is used as an approximate surface of the cross section, all point clouds are projected to the approximate surface of the cross section, a local coordinate system is established on the approximate surface of the cross section, the normal vector direction of the global point cloud is used as the Z-axis direction of the local coordinate system, the X-axis direction is the cross section direction, and all projected point clouds are converted to the local coordinate system; according to the distribution of geometric features of the cross section, straight line segments of the measurement position are segmented; a reference plane is fitted with the point clouds of the first and last straight line segments, the average distances of other straight line segments to the reference plane are calculated, and the maximum positive value and the minimum negative value thereof are taken as the lateral deflection.

Description

Technical Field

[0001] The invention relates to a non-contact measurement method for the lateral deflection of a vehicle body underframe based on line structured light, and belongs to the technical field of subway underframe production and assembly. Background Art

[0002] The deflection of the car body chassis refers to the linear displacement of the axis in the direction perpendicular to the axis when the car body chassis is subjected to force or uniform temperature changes. In the manufacturing process of aluminum alloy subway vehicles, the deflection of the car body chassis is a key element affecting the quality of the car body chassis, and its measurement method has attracted more and more attention. Most of the current research is aimed at the deflection measurement in the longitudinal direction of the car body. In terms of lateral deflection measurement, due to the long car body and many measuring stations, the traditional manual measurement method greatly restricts the efficiency of production inspection.

[0003] The current mainstream is still based on manual judgment and standard blocks. By placing standard blocks at the measuring points, manual measurement of relative height is performed for detection and judgment. The repeatability is poor, the difference is large, and the efficiency is very low. In order to avoid the uncertainties brought by manual detection, some units use high-precision and large-range detection equipment, which has a high introduction cost. Commonly used methods include precision total station method or laser tracker method. The basic principle of its measurement is trigonometric height measurement, that is, measuring the horizontal distance between two points and the height difference between two points at the top of the vertical angle ball. This method requires manual layout of targets and then point-by-point measurement. The measurement efficiency is low and it is impossible to measure multiple points synchronously in real time. In terms of deflection measurement, some companies and institutions have explored it. The Hong Kong Polytechnic University obtains deflection information by installing targets at the measuring points and tracking targets through monocular vision measurement. Wang Shuo and others from Shanghai University have developed a deflection test technology based on image measurement. The surface texture feature points of the target object are identified by monocular vision and measured, but this can only be limited to surfaces with obvious textures. Zhao Xiaohua of China CNR Corporation started with the arrangement and analysis of measurement points and designed a method for calculating lateral deflection, but still did not provide a means to improve measurement efficiency. Summary of the invention

[0004] In order to reduce equipment costs, reduce manual participation in the process, improve production efficiency and improve measurement accuracy, the present invention proposes a non-contact measurement method for the lateral deflection of an aluminum alloy subway car body frame based on line structured light.

[0005] A non-contact measurement method for lateral deflection of a vehicle body underframe based on line structured light, the method comprising:

[0006] Install the monocular single-line measurement unit according to the field of view and chassis width, configure the laser tracker, install the hole calibration plate and calibration plate fixing tooling;

[0007] Use the laser tracker and the installation hole calibration plate to calibrate the position relationship between the monocular single-line measurement unit and the global coordinate system;

[0008] By projecting a single-line structured light onto the vehicle chassis, each monocular single-line measurement unit 3D reconstructs the surface 3D point cloud of the cross section and converts it to the global coordinate system;

[0009] The global point cloud fitting plane is used as the approximate surface of the cross section. All point clouds are projected onto the approximate surface of the cross section. A local coordinate system is established on the approximate surface of the cross section. The direction of its normal vector is used as the Z-axis direction of the local coordinate system. The X-axis direction is the cross section direction. All projected point clouds are converted to the local coordinate system.

[0010] According to the distribution of the cross-sectional geometric features, the straight line segments of the measurement positions are segmented;

[0011] The point cloud of the first and last straight line segments is used to fit the reference plane, and the average distance from other straight line segments to the reference plane is calculated. The maximum positive value and the minimum negative value are taken as the lateral deflection.

[0012] Furthermore, the perforated calibration plate is a ceramic calibration plate with standard through holes on all sides and a white background and black dots distributed thereon.

[0013] Furthermore, the single-line structured light is a line structured light emitted by a line laser, which is essentially a continuous spatial light plane with a certain thickness, and the light strip feature with a certain width formed on the target surface is the intersection line formed by the intersection of the light plane and the target surface. In the thickness direction of the spatial light plane, the light intensity approximately obeys a Gaussian distribution.

[0014] Furthermore, the three-dimensional point cloud of the surface of the cross section of each monocular single-line measurement unit is three-dimensionally reconstructed and converted into a global coordinate system, including:

[0015] The Steger algorithm is used to extract the center of the light stripe from the collected line structured light image, and the sub-pixel position of the center of the light stripe is calculated;

[0016] Calculate the pixel coordinates of the center point of the light strip in the normal direction according to the sub-pixel position of the center of the light strip;

[0017] The dedistorted normalized image coordinates are calculated using the intrinsic parameters of the camera calibration. Assume that point P is on the light plane and the image point corresponding to point P is p. Therefore, point P is also on the ray formed by the optical center of the camera and point p. The relative positions of the camera and the structured light projector remain unchanged, and the light plane equation remains unchanged. The spatial coordinates of P (x, y, z) and the coordinates of p (x c ,y c ,1), let the light plane equation under the camera be:

[0018] (1)

[0019] The ray formed by the camera optical center and p is expressed as follows:

[0020] (2)

[0021] Combined to get:

[0022] (3)

[0023] The point cloud under each monocular single-line structured light measurement unit is transformed into the global coordinate system through the corresponding transformation matrix.

[0024] Furthermore, segmenting the straight line segments of the measurement positions according to the distribution of the cross-sectional geometric features includes:

[0025] The reconstructed point clouds of the four groups of units are transferred to the point clouds in the global coordinate system for least square plane fitting to obtain the cross-sectional plane equation. All point clouds are projected onto the cross-sectional plane equation and a local coordinate system is established on the cross-sectional plane equation. The direction of the normal vector is the Z-axis direction of the local coordinate system, and the X-axis direction is the cross-sectional direction. At this point, the Z coordinates of the point clouds on the plane are all 0, so the point cloud has achieved dimensionality reduction processing, and can be analyzed from a two-dimensional perspective.

[0026] By traversing the point cloud from left to right, we can find the location of the steps. The location where the Y-axis coordinate suddenly changes is the location of the steps. The grooves are between the nearest steps, that is, the measurement area. The points in the measurement area are mapped to the three-dimensional space and the straight line segments are fitted, that is, the straight line segments in the grooves of the measurement area.

[0027] Furthermore, the method of fitting the reference plane with the point cloud of the first and last straight line segments, calculating the average distance from other straight line segments to the reference plane, and taking the maximum positive value and the minimum negative value as the lateral deflection includes:

[0028] Sort the straight line segments by position, take the first and last straight line segments to fit the plane by the least square method, and use it as the reference plane. Calculate the distance from the midpoint of each other straight line segment to the reference plane. The distance is a vector and has positive and negative values. That is, when the distance is greater than 0, the point is above the reference plane, when the distance is less than 0, the point is below the reference plane, and when the distance is equal to 0, the point is on the reference plane.

[0029] Sort the distances and take the largest positive value and the smallest negative value as the lateral deflection at that location.

[0030] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: the method of the present invention provides a non-contact measurement method for the lateral deflection of a vehicle frame based on line structured light, which can quickly and accurately obtain the required lateral deflection information of the frame through non-contact measurement of multiple monocular single-line structured light measurement units; it can improve the robustness of the measurement algorithm by reducing the data dimension according to the geometric characteristics of the frame, and can realize automatic positioning of the measurement area, thereby overcoming the errors caused by manual participation in traditional manual measurement, improving measurement stability, and improving measurement efficiency; it can be applied to the fields of subway frame production, assembly and quality evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a flow chart of the method of the present invention;

[0032] Figure 2 Schematic diagram of point cloud distribution after dimensionality reduction of the method of the present invention. DETAILED DESCRIPTION

[0033] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0034] The present invention provides a non-contact measurement method of the lateral deflection of a vehicle body chassis based on line structured light, comprising the following steps:

[0035] Step 1: Install the monocular single-line measurement unit according to the field of view and the width of the chassis, configure the laser tracker, install the hole calibration plate and the calibration plate fixing tooling;

[0036] Step 2: Use a laser tracker and an installed perforated calibration plate to calibrate the positional relationship between the monocular single-line measurement unit and the global coordinate system. The perforated calibration plate is a ceramic calibration plate with standard through holes on all sides and a white background and black dots.

[0037] Step 3: Projecting a single-line structured light, each monocular single-line measurement unit collects a structured light image projected onto the cross section of the vehicle body chassis after surface modulation;

[0038] Step 4: Each monocular single-line measurement unit reconstructs a three-dimensional point cloud of the surface of the cross section and converts it into a global coordinate system;

[0039] Step 5: The global point cloud fitting plane is used as the cross-sectional approximate surface, and all point clouds are projected onto the cross-sectional approximate surface;

[0040] A local coordinate system is established on the cross-sectional approximate surface, and the direction of its normal vector is used as the Z-axis direction of the local coordinate system, and the X-axis direction is the cross-sectional direction. All projected point clouds are converted to the local coordinate system;

[0041] Step 6: Segment the straight line segments of the measurement positions according to the distribution of the cross-sectional geometric features;

[0042] Step 7: Fit the reference plane with the point cloud of the first and last straight line segments, calculate the average distance from other straight line segments to the reference plane, and take the maximum positive value and the minimum negative value as the lateral deflection;

[0043] The following are detailed descriptions of the above key steps:

[0044] In step 3, structured light refers to projecting a laser bar with structural information on the surface. Structural information means that the brightness of the laser bar in the vertical direction conforms to the Gaussian distribution. The camera captures the image of the structured light projected onto the surface of the object, and then the calibration relationship is used in advance. The calibration relationship refers to the spatial position relationship between the camera and the structured light generator to calculate the three-dimensional information of the object surface.

[0045] In step 4, each monocular single-line measurement unit 3D reconstructs the surface 3D point cloud of the cross section and converts it to the global coordinate system, including:

[0046] The Steger algorithm is used to extract the center of the light stripe from the collected line structured light image, and the sub-pixel position of the center of the light stripe is calculated;

[0047] Calculate the pixel coordinates of the center point of the light strip in the normal direction according to the sub-pixel position of the center of the light strip;

[0048] The dedistorted normalized image coordinates are calculated using the intrinsic parameters of the camera calibration. Assume that point P is on the light plane and the image point corresponding to point P is p. Therefore, point P is also on the ray formed by the optical center of the camera and point p. The relative positions of the camera and the structured light projector remain unchanged, and the light plane equation remains unchanged. The spatial coordinates of P (x, y, z) and the coordinates of p (x c ,y c ,1), let the light plane equation under the camera be:

[0049] (1)

[0050] The ray formed by the camera optical center and p is expressed as follows:

[0051] (2)

[0052] Combined to get:

[0053] (3)

[0054] The point cloud under each monocular single-line structured light measurement unit is transformed into the global coordinate system through the corresponding transformation matrix.

[0055] Step 5. Establish a local coordinate system on the cross-sectional approximate surface, with the normal vector direction as the Z-axis direction of the local coordinate system, and the X-axis direction as the cross-sectional direction. All projected point clouds are converted to the local coordinate system, including:

[0056] Project all 3D point clouds onto the cross-sectional plane equation and establish a local coordinate system on the cross-sectional plane equation. The coordinate system establishment process is as follows:

[0057] Take any point p1 of the projected point cloud on the cross-sectional plane as the origin of the local coordinate system;

[0058] Take the normal vector (m1, n1, p1) of the cross-section plane equation as the Z axis of the local coordinate system;

[0059] Take another point p2 of the projected point cloud on the cross-sectional plane as the X-axis of the local coordinate system, and calculate the unit direction vector (m2, n2, p2) of p1 and p2;

[0060] Construct the corresponding points of the local coordinate system and the global coordinate system. The corresponding point of p1 is q1, that is, q1 is the origin of the local coordinate system (0,0,0);

[0061] The corresponding point of p2 is q2, that is, q2 is a point on the X-axis of the local coordinate system, the coordinates are (dist, 0, 0), and dist is the distance between p1 and p2;

[0062] Take a point p3 on the normal vector of the global coordinate system, and the corresponding point is q3, that is, the coordinates of p3 in the global coordinate system are (x p1 +dist*m1,y p1 +dist*n1, z p1 +dist*p1), the coordinate of q3 in the local coordinate system is (0,0, dist), and dist is the distance between p1 and p2.

[0063] At this point, the transformation matrix between the global coordinate system and the local coordinate system can be calculated through rigid body transformation, as shown in formula (1), and the transformation matrix T can be obtained through SVD decomposition.

[0064]

[0065] At this point, the Z-axis direction of the local coordinate system is perpendicular to the cross section, and the X-axis direction is along the cross section. Therefore, the Z coordinates of the point clouds on the plane are all 0. Therefore, the point cloud is reduced in dimension and can be analyzed from a two-dimensional perspective.

[0066] Step 6. Segment the straight line segments at the measurement location based on the distribution of the cross-sectional geometric features, including:

[0067] like Figure 2, transfer the reconstructed point clouds of the four groups of units to the point clouds in the global coordinate system for least square plane fitting to obtain the cross-sectional plane equation, project all point clouds to the cross-sectional plane equation and establish a local coordinate system on the cross-sectional plane equation, with the normal vector direction as the Z-axis direction of the local coordinate system, and the X-axis direction as the cross-sectional direction. So far, the Z coordinates of the point clouds on the plane are all 0, so the point cloud has achieved dimensionality reduction processing, and can be analyzed from a two-dimensional perspective;

[0068] Traversing the point cloud from left to right, we can find the location of the steps. Since the height of the steps is 10mm, the location where the Y-axis coordinate suddenly changes is where the steps are. The grooves are between the nearest steps, which is the measurement area. The points in the measurement area are mapped to the three-dimensional space and the straight line segments are fitted, which are the straight line segments in the grooves of the measurement area.

[0069] The method of fitting the reference plane with the point cloud of the first and last straight line segments, calculating the average distance from other straight line segments to the reference plane, and taking the maximum positive value and the minimum negative value as the lateral deflection includes:

[0070] Sort the straight line segments by position, take the first and last straight line segments to fit the plane by the least square method, and use it as the reference plane. Calculate the distance from the midpoint of each other straight line segment to the reference plane. The distance is a vector and has positive and negative values. That is, when the distance is greater than 0, the point is above the reference plane, when the distance is less than 0, the point is below the reference plane, and when the distance is equal to 0, the point is on the reference plane.

[0071] Sort the distances and take the largest positive value and the smallest minimum value as the lateral deflection at that location.

[0072] In summary, the method of the present invention provides a non-contact measurement method for the lateral deflection of a vehicle underframe based on line structured light, which has the characteristics of high precision and high efficiency and is suitable for the fields of subway underframe production, assembly and quality evaluation.

[0073] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A non-contact measurement method for lateral deflection of vehicle underframe based on line structured light, characterized in that: The method comprises: Install the monocular single-line measurement unit according to the field of view and chassis width, configure the laser tracker, install the hole calibration plate and calibration plate fixing tooling; Use the laser tracker and the installation hole calibration plate to calibrate the position relationship between the monocular single-line measurement unit and the global coordinate system; By projecting a single-line structured light onto the vehicle chassis, each monocular single-line measurement unit 3D reconstructs the surface 3D point cloud of the cross section and converts it to the global coordinate system; The global point cloud fitting plane is used as the approximate surface of the cross section. All point clouds are projected onto the approximate surface of the cross section. A local coordinate system is established on the approximate surface of the cross section. The direction of its normal vector is used as the Z-axis direction of the local coordinate system. The X-axis direction is the cross section direction. All projected point clouds are converted to the local coordinate system. According to the distribution of the cross-sectional geometric features, the straight line segments of the measurement positions are segmented; The point cloud of the first and last straight line segments is used to fit the reference plane, and the average distance from other straight line segments to the reference plane is calculated. The maximum positive value and the minimum negative value are taken as the lateral deflection.

2. The non-contact measurement method of vehicle underframe lateral deflection based on line structured light according to claim 1 is characterized in that: The perforated calibration plate is a ceramic calibration plate with standard through holes on all sides and a white background and black dots distributed thereon.

3. The non-contact measurement method of vehicle underframe lateral deflection based on line structured light according to claim 1, characterized in that: The single-line structured light is a line structured light emitted by a line laser, which is essentially a continuous spatial light plane with a certain thickness. The light strip feature with a certain width formed on the target surface is the intersection line formed by the intersection of the light plane and the target surface. In the thickness direction of the spatial light plane, the light intensity approximately obeys Gaussian distribution.

4. The non-contact measurement method of vehicle underframe lateral deflection based on line structured light according to claim 1, characterized in that: The surface three-dimensional point cloud of the cross section of each monocular single-line measurement unit is three-dimensionally reconstructed and converted to a global coordinate system, including: The Steger algorithm is used to extract the center of the light stripe from the collected line structured light image, and the sub-pixel position of the center of the light stripe is calculated; Calculate the pixel coordinates of the center point of the light strip in the normal direction according to the sub-pixel position of the center of the light strip; The dedistorted normalized image coordinates are calculated using the intrinsic parameters of the camera calibration. Assume that point P is on the light plane and the image point corresponding to point P is p. Therefore, point P is also on the ray formed by the optical center of the camera and point p. The relative positions of the camera and the structured light projector remain unchanged, and the light plane equation remains unchanged. The spatial coordinates of P (x, y, z) and the coordinates of p (x c ,y c ,1), let the light plane equation under the camera be: (1) The ray formed by the camera optical center and p is expressed as follows: (2) Combined to get: (3) The point cloud under each monocular single-line structured light measurement unit is transformed into the global coordinate system through the corresponding transformation matrix.

5. The non-contact measurement method of vehicle underframe lateral deflection based on line structured light according to claim 1, characterized in that: The step of segmenting the straight line segments at the measurement positions according to the distribution of the cross-sectional geometric features comprises: The reconstructed point clouds of the four groups of units are transferred to the point clouds in the global coordinate system for least square plane fitting to obtain the cross-sectional plane equation. All point clouds are projected onto the cross-sectional plane equation and a local coordinate system is established on the cross-sectional plane equation. The direction of the normal vector is the Z-axis direction of the local coordinate system, and the X-axis direction is the cross-sectional direction. At this point, the Z coordinates of the point clouds on the plane are all 0, so the point cloud has achieved dimensionality reduction processing, and can be analyzed from a two-dimensional perspective. By traversing the point cloud from left to right, we can find the location of the steps. The location where the Y-axis coordinate suddenly changes is the location of the steps. The grooves are between the nearest steps, that is, the measurement area. The points in the measurement area are mapped to the three-dimensional space and the straight line segments are fitted, that is, the straight line segments in the grooves of the measurement area.

6. The non-contact measurement method of vehicle underframe lateral deflection based on line structured light according to claim 1, characterized in that: The method of fitting the reference plane with the point cloud of the first and last straight line segments, calculating the average distance from other straight line segments to the reference plane, and taking the maximum positive value and the minimum negative value as the lateral deflection includes: Sort the straight line segments by position, take the first and last straight line segments to fit the plane by the least square method, and use it as the reference plane. Calculate the distance from the midpoint of each other straight line segment to the reference plane. The distance is a vector and has positive and negative values. That is, when the distance is greater than 0, the point is above the reference plane, when the distance is less than 0, the point is below the reference plane, and when the distance is equal to 0, the point is on the reference plane. Sort the distances and take the largest positive value and the smallest negative value as the lateral deflection at that location.

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