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CN-116147533-B - Light plane calibration method for multi-line laser three-dimensional measurement system

CN116147533BCN 116147533 BCN116147533 BCN 116147533BCN-116147533-B

Abstract

The invention discloses a light plane calibration method for a multi-line laser three-dimensional measurement system, which comprises the steps of obtaining a plurality of groups of checkerboard calibration pictures at different positions, collecting clear checkerboard images under normal exposure, collecting laser stripe images of all levels falling on the checkerboard planes under low exposure parameters, calibrating an internal parameter matrix and an external parameter matrix of a camera by collecting clear checkerboard image groups under normal exposure, generating line structure light through grating diffraction, adjusting the diffraction order of the laser stripe with a diaphragm width to select working, carrying out laser stripe brightness redistribution by using a phase plate, realizing system calibration by using diffraction equation constraint of multi-line laser, improving the calibration precision of the light plane of the multi-line laser system, and realizing the improvement of measurement precision and efficiency. The diffraction grating produces a plurality of diffraction line lasers on the object, so that three-dimensional information of the surface at a plurality of line positions on the surface of the object can be obtained simultaneously. Meanwhile, the selection of diffraction orders can be selected according to the surface type of the object.

Inventors

  • LIN BIN
  • WANG YAO

Assignees

  • 浙江大学

Dates

Publication Date
20260505
Application Date
20230303

Claims (6)

  1. 1. An optical plane calibration method for a multi-line laser three-dimensional measurement system, comprising the steps of: acquiring a plurality of groups of checkerboard calibration pictures at different positions, acquiring clear checkerboard images under normal exposure, and acquiring laser stripe images of each level falling on the checkerboard plane under low exposure parameters; Acquiring clear checkerboard image groups under the normal exposure to calibrate an internal parameter matrix and an external parameter matrix of the camera (5); Calculating a plane equation of a camera (5) coordinate system corresponding to the position checkerboard plane through an external parameter matrix of the camera (5); Collecting each level of laser stripe images falling on the checkerboard plane under the low exposure parameters, and calculating each level of laser stripe central line pixel coordinates by adopting a central line extraction algorithm; Calculating homogeneous coordinates of the laser stripe center line under a camera (5) coordinate system through the camera (5) internal reference matrix and pixel coordinates of each level of laser stripe center line; converting checkerboard plane equations under a world coordinate system at corresponding positions into plane equations under a camera (5) coordinate system through external parameters of the positions of the camera (5); The method comprises the steps of (1) converting the homogeneous coordinates of a camera (5) of a laser stripe center line into three-dimensional coordinates of the camera (5) of the laser stripe center line by using a plane equation of a checkerboard under the coordinate system of the camera (5) and the homogeneous coordinates of the camera (5) of the laser stripe center line; determining diffraction laser line orders corresponding to each laser stripe on a laser stripe image falling on a checkerboard plane, collected under low exposure parameters, substituting three-dimensional coordinates under a central line camera (5) coordinate system of the corresponding diffraction laser stripe into a laser stripe light plane equation of the corresponding order, and obtaining an initial value of a light plane equation coefficient of each diffraction laser stripe; selecting the square of the distance from the central point of the laser stripe to the light plane equation as an objective function, taking the diffraction angle relation of the grating equation as constraint, and obtaining the normal vector of each plane through a constrained least squares optimization algorithm Is a function of the optimization value of (a); And according to the fact that each diffraction light plane is intersected with a straight line in space, expressing and substituting the normal vector optimization value of each light plane through a space straight line equation, and completing optimization of the position parameters of each light plane.
  2. 2. The method for calibrating a light plane of a multi-line laser three-dimensional measurement system according to claim 1, wherein the steps of obtaining a plurality of groups of checkerboard calibration pictures at different positions are as follows: The camera (5) and the line laser (1) keep unchanged in position, the positions of the checkerboard are moved, two calibration images are collected when the checkerboard is at the same position, clear checkerboard photos are collected under normal exposure of the camera (5), laser stripe images falling on the plane of the checkerboard are collected under low exposure parameters of the camera (5), and after the two images are taken, the checkerboard is moved to collect multiple groups of images.
  3. 3. The method for calibrating the optical plane of the multi-line laser three-dimensional measurement system according to claim 1 or 2, wherein the substituting the three-dimensional coordinates of the corresponding diffraction laser fringes under the central line camera (5) coordinate system into the corresponding order of the laser fringe optical plane equation to obtain the initial value of the optical plane equation coefficient of each diffraction laser fringe is specifically: The center line point coordinates of the laser stripe on the light plane pi corresponding to the ith diffraction stripe are as follows ; In the above formula, m is the number of central points on each level of laser stripes, if the normal vector of the light plane is Averaging the directions of plane point clouds The plane equation can be expressed as ; Coordinates of all laser stripe center line points Substituted into the above formula and expressed as a matrix form ; Obtaining normal vector of each plane through SVD algorithm Is set to be a constant value.
  4. 4. The method for calibrating a light plane of a multi-line laser three-dimensional measurement system according to claim 3, wherein the square of the distance from the center point of the laser stripe to the light plane equation is selected as an objective function, the diffraction angle relation of the grating equation is a constraint, and the normal vector of each plane is obtained by a constrained least squares optimization algorithm The optimized values of (2) are specifically: selecting the coordinates of the central line points of the laser stripes The square of the distance to the light plane equation is the objective function, i.e ; At the same time, the included angle between the i-level light plane and the 0-level light plane Satisfy the following relation ; ; In the above formula, lambda is the laser center wavelength, Is a grating constant; Obtaining normal vector of each plane through constrained least square optimization algorithm Is used for the optimization of the values of (a).
  5. 5. The method for calibrating a light plane of a multi-line laser three-dimensional measurement system according to claim 1, 2 or 4, wherein the method is characterized in that the light plane is spatially intersected with a straight line according to each diffraction light plane, and the normal vector optimization value of each light plane is expressed and substituted by a spatial straight line equation, so as to complete the optimization of the position parameters of each light plane, and specifically comprises the following steps: the light planes of the diffraction laser stripes are collinear in space and can be expressed as a combination of two base planes ; The obtained plane normal vector The following matrix can be obtained by substituting the optimized value of (a) into the above formula: ; And obtaining a coefficient combination of a base plane equation and the light plane of each laser stripe by fitting, wherein the light plane equation of each laser stripe can be obtained by the corresponding combination coefficient and the two base plane equations, so that the optimization of the light plane calibration is completed.
  6. 6. The method for calibrating the optical plane of a multi-line laser three-dimensional measurement system according to claim 1, wherein the calibrating method is realized based on the following devices: the line laser (1) is used for emitting line laser stripes; The multi-line laser stripe is generated by a line laser (1) in a grating diffraction mode, a diffraction grating (2) positioned in front of the line laser (1) and used for dividing the line laser stripe into multi-level line laser stripes with certain spatial distribution, wherein the stripe distribution can be adjusted according to grating parameters; A rectangular diaphragm (3) positioned in front of the diffraction grating (2) and used for shielding high-order low-energy laser stripes so as to enable a plurality of generated linear laser stripes with specific orders to be projected onto the surface of an object; A camera (5) for acquiring deformed multi-line laser stripe images from another angle; A translation stage for placing an object and moving the object for scanning measurement; and the industrial personal computer (6) is respectively connected with the camera (5) and the driving motor of the translation.

Description

Light plane calibration method for multi-line laser three-dimensional measurement system Technical Field The invention relates to the field of optical measurement, in particular to a light plane calibration method for a multi-line laser three-dimensional measurement system. Background The line structure light three-dimensional measurement based on the laser triangulation principle has the advantage of high precision measurement, and when the structure light scanning measurement is carried out, how to improve the measurement efficiency becomes a research key point. In order to achieve the aim of rapid measurement, multi-line structured light three-dimensional measurement is the most studied, and the measurement efficiency is improved while the measurement accuracy is ensured. Researchers put forward a three-dimensional measurement technology for simultaneously projecting a plurality of light planes to the surface of an object to be measured by using a semiconductor laser module, and provide a mathematical model and a related algorithm for measuring a multi-line structured light system. In this way the sensor can be mounted in a relatively random position, so that the installation and adjustment of the system becomes very easy and flexible. By independently calibrating each light plane, the multiple line lasers can be regarded as independently working. A new method for non-contact measurement of multi-structure linear illumination (or multi-optical knife) based on laser scanning measurement technology is proposed by scholars. In order to improve the precision of the developed multi-optical cutter measuring device, the virtual network mapping and the least square method are used for calibrating the multi-optical cutter measuring device in the whole measuring field respectively, each line is calibrated respectively, the precision is low, and the calibrating process is complex. There are researchers modifying existing calibration plates and transforming the center line of each stripe into a three-dimensional point cloud fit to a plane using random sample consensus algorithm (RANSAC). The relative error of the integral fitting result of the multi-line structure light planes can be reduced, and the integral calibration accuracy of the light planes is improved in terms of angle error among the calibration light planes. Also, this method is also independent of the calibration of the light plane, which is prone to errors compared to the calibration of the light plane. Besides the calibration of the system by means of a light plane, researchers also put forward a measurement model of a multi-parallel line laser system, and a multi-base-plane concept deduction measurement system formula is introduced, so that the calibration process is simplified, but the calibration mode based on the mathematical model of the system depends on the installation precision of the system. The current calibration method of the multi-line laser plane is characterized in that each laser plane is calibrated independently or based on a mathematical model of a system, and the calibration precision is often reduced due to an optical path or a mechanical error. The grating is used as a precision optical element, various structural light illumination objects can be generated through the grating, the three-dimensional reconstruction process is realized by utilizing the generated fine structural light distribution, and higher measurement precision can be realized based on a calibration mode of diffraction constraint of the grating. Disclosure of Invention In order to solve the defects in the prior art, the invention aims to provide a light plane calibration method for a multi-line laser three-dimensional measurement system, which is used for generating line structure light through grating diffraction and adjusting the width of a diaphragm to select the diffraction order of a laser stripe to work; the method comprises the steps of carrying out laser stripe brightness redistribution by using a phase plate, realizing system calibration by using diffraction equation constraint of the multi-line laser, improving the calibration precision of the light plane of the multi-line laser system, and realizing the improvement of measurement precision and efficiency. In order to achieve the above object, the technical method adopted by the invention is as follows: the invention discloses a light plane calibration method for a multi-line laser three-dimensional measurement system, which comprises the following steps: acquiring a plurality of groups of checkerboard calibration pictures at different positions, acquiring clear checkerboard images under normal exposure, and acquiring laser stripe images of each level falling on the checkerboard plane under low exposure parameters; Acquiring clear checkerboard image groups under the normal exposure to calibrate the internal and external parameter matrixes of the camera; Calculating a plane equation of a camera coordinat