CN113932737A - Flexible and high-precision structured light system calibration method - Google Patents

Flexible and high-precision structured light system calibration method Download PDF

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CN113932737A
CN113932737A CN202111151755.8A CN202111151755A CN113932737A CN 113932737 A CN113932737 A CN 113932737A CN 202111151755 A CN202111151755 A CN 202111151755A CN 113932737 A CN113932737 A CN 113932737A
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calibration plate
calibration
camera
plane
phase shift
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CN113932737B (en
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伏燕军
杨吴训博
钟可君
陈建斌
肖玮
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Nanchang Hangkong University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
    • G01B11/25—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures by projecting a pattern, e.g. one or more lines, moiré fringes on the object
    • G01B11/2504—Calibration devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
    • G01B11/25—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures by projecting a pattern, e.g. one or more lines, moiré fringes on the object
    • G01B11/2518—Projection by scanning of the object
    • G01B11/2527—Projection by scanning of the object with phase change by in-plane movement of the patern

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Abstract

The invention discloses a flexible and high-precision calibration method of a structured light system, which comprises the steps of generating fringe patterns of several specific frequencies by using a computer to carry out four-step phase shift, and projecting the four-step phase shift pattern of each frequency onto a calibration plate with characteristic points; solving through a phase shift unwrapping algorithm to obtain a continuous phase value of the calibration plate; and finally, obtaining the pixel coordinates of the circle center on the calibration plate and the internal and external parameters of the camera at each position by using a Zhang friend calibration method so as to obtain the coordinates of the circle center on the calibration plate under a camera coordinate system, fitting the imaging plane of the calibration plate by using a plane fitting algorithm through the coordinates of the circle center camera, and then calibrating the measurement system through the distance between different imaging planes and the phase difference of the continuous phases of the different imaging planes. The method can effectively improve the calibration precision of the three-dimensional measurement system, is convenient to operate and small in budget due to the fact that a precise guide rail is not needed, and has potential application prospects and practical values in the field of optical three-dimensional measurement.

Description

Flexible and high-precision structured light system calibration method
Technical Field
The invention relates to an optical three-dimensional measurement method, belongs to the technical field of photoelectric detection, and particularly relates to a flexible and high-precision calibration method for a structured light system.
Technical Field
The structured light three-dimensional imaging technology is an information acquisition mode mainly based on optical sensing, and an information processing means mainly based on computer graphic image processing, and is one of important research contents in the fields of computational imaging and geometric measurement. Because of its advantages of high precision, high speed and non-contact measurement, it is widely used in the fields of industrial detection, biomedicine and machine vision. Along with the industrial upgrading of the production structure in China, the requirement on the three-dimensional measurement precision of a large structural part is gradually increased, and the three-dimensional precision measurement cannot be separated in the links of workpiece manufacturing, equipment and detection.
In phase profilometry, a conventional phase-height mapping calibration method generally utilizes a precision guide rail to drive a standard plane to move longitudinally within the depth of field range of a camera, each translation position represents a different calibration plane, and system calibration is performed according to the position of the calibration plane and the continuous phase distribution of the calibration plane. However, in practical measurement, it takes long time to move the standard plane by the guide rail for many times, and the error accumulation effect is generated in the moving process of the standard plane. Therefore, in order to solve the problems of long time consumption, accumulated errors, difficulty in carrying equipment and the like, a simple, quick and flexible high-precision calibration method is also a key problem to be solved urgently in the field of three-dimensional measurement. The invention provides a flexible high-precision phase-height mapping relation calibration method by fully considering the problems. The method can effectively improve the calibration precision of the three-dimensional measurement system, and is convenient to operate and small in budget because a precise guide rail is not needed. Has potential application prospect and practical value in the field of optical three-dimensional measurement.
Disclosure of Invention
The invention aims to provide a flexible and high-precision structured light system calibration method, which is realized by the following technical scheme:
a flexible and high-precision structured light system calibration method is characterized in that a computer is used for generating fringe patterns of several specific frequencies to carry out four-step phase shift, and the four-step phase shift pattern of each frequency is projected onto a calibration plate with characteristic points; because the calibration plate needs to be moved randomly for many times when the camera calibration is carried out, after the position is moved once and a group of pictures with phase information are taken, a picture without stripes is taken for fitting an imaging plane of the picture; then, respectively solving by using a phase shift unwrapping algorithm to obtain a continuous phase of each position of the calibration plate; finally, obtaining the pixel coordinates of the circle center on the calibration plate and the internal and external parameters of the camera at each position by using a Zhang-friend calibration method, thereby obtaining the coordinates of the circle center on the calibration plate under a camera coordinate system, fitting the imaging plane of the calibration plate by using a plane fitting algorithm through the coordinates of the circle center camera, and then calibrating the measurement system through the distance between different imaging planes and the phase difference of the continuous phases of the different imaging planes; the method specifically comprises the following steps:
s1, establishing a three-dimensional measurement system: the system comprises a DLP projector, a CCD camera and a circle calibration plate; the DLP projector and the CCD camera can be placed at will;
s2, generating fringe patterns of several specific frequencies by using a computer to perform four-step phase shift, and projecting the four-step phase shift patterns of the frequencies onto a circular calibration plate with characteristic points;
s3, using a camera calibration program to move the circular calibration plate forward for multiple times to calibrate the camera to obtain the internal and external parameters of the camera; a standard pinhole lens model that does not take into account lens distortion can be described mathematically as:
S[u v 1]T＝A[R T][Xw Yw Zw 1]T (1)
for each calibration plate position, the world coordinate system is on the calibration plate, so its Z can be defined on the calibration plate w0, camera coordinate system [ X [ ]c Yc Zc]Can be expressed as:
Figure RE-GDA0003400508390000031
obtaining the three-dimensional coordinate X under the camera coordinate system of the center of a circle on the calibration plate at each positionc、Yc、 ZcAnd fitting the calibration plate by using a plane fitting algorithm to obtain an imaging plane of the calibration plate at each position in a camera coordinate system. Similarly, the firstThe calibration plate for one position is a reference plane, and the fitting plane can be expressed as:
Figure RE-GDA0003400508390000032
wherein (X)C YC ZC)TCamera coordinates representing the center of a circle on the calibration plate,
Figure RE-GDA0003400508390000033
and i represents the calibration board at the ith position. The plane parameters A, B, C and D are solved by the least squares method, so the height of the point on the calibration plate at each position can be expressed as:
Figure RE-GDA0003400508390000034
subtracting the z value of the reference plane from the z value of each plane to obtain the height difference hi(u，v)；
S4, phase information of each position of the circular calibration plate is obtained by using a phase shift method; using the circle calibration plate at the first position as a reference plane, and subtracting the phase value of the corresponding point on the reference plane from the phase value of the circle calibration plate at each position to obtain several sets of phase difference information, which are marked as delta phii(u，v)；
S5, phase-height calibration, also called system geometric parameter calibration; the corresponding relation between the phase difference delta phi (u, v) of the measured object and the height h (u, v) of the measured object is as follows:
Figure RE-GDA0003400508390000035
when the measuring system is fixed, the parameters a (u, v), b (u, v) and c (u, v) are regarded as system constants, and h is obtained by taking a plurality of different positionsi(u, v) and Δ φi(u, v), establishing an equation system, solving three parameters of a (u, v), b (u, v) and c (u, v) by a least square method, and finishing the operationAnd (5) calibrating the system.
The invention has the advantages that:
1. compared with the traditional calibration method that a displacement table is used for moving the standard plane for multiple times, the calibration method simplifies the calibration procedure without losing the rigor and improves the calibration speed of the measurement system;
2. the imaging plane of the calibration plate is fitted by using the characteristic point information on the calibration plate for calibration, so that the random error caused by moving the standard plane in the traditional method is reduced, and the calibration precision is effectively improved.
3. Since the precision guide rail is no longer required, the operation is convenient and the budget can be reduced.
Drawings
FIG. 1 is a schematic diagram of a three-dimensional measurement system of the present invention.
Fig. 2 is a three-dimensional measurement system geometry of the present invention.
FIG. 3 is a plot of the pixel coordinates of a marker point of the present invention.
FIG. 4 is a landmark pixel coordinate fitting plane of the present invention.
Detailed Description
The following description will be provided in detail with reference to the accompanying drawings, which are not intended to limit the present invention, and all similar structures and similar variations using the present invention shall fall within the scope of the present invention.
A flexible and high-precision structured light system calibration method is characterized in that a computer is used for generating fringe patterns of several specific frequencies to carry out four-step phase shift, and the four-step phase shift pattern of each frequency is projected onto a calibration plate with characteristic points; solving through a phase shift unwrapping algorithm to obtain a continuous phase value of the calibration plate; finally, obtaining the pixel coordinates of the circle center on the calibration plate and the internal and external parameters of the camera at each position by using a Zhang-friend calibration method, thereby obtaining the coordinates of the circle center on the calibration plate under a camera coordinate system, fitting the imaging plane of the calibration plate by using a plane fitting algorithm through the coordinates of the circle center camera, and then calibrating the measurement system through the distance between different imaging planes and the phase difference of the continuous phases of the different imaging planes; the method specifically comprises the following steps:
s1, establishing a three-dimensional measurement system: the system comprises a DLP projector, a CCD camera and a circle calibration plate; the DLP projector and the CCD camera can be placed at will;
s2, generating fringe patterns of several specific frequencies by using a computer to perform four-step phase shift, and projecting the four-step phase shift patterns of the frequencies onto a circular calibration plate with characteristic points;
and S3, using a camera calibration program to move the circular calibration plate forward for any time to calibrate the camera, and obtaining the internal and external parameters of the camera. A standard pinhole lens model that does not take into account lens distortion can be described mathematically as:
S[u v 1]T＝A[R T][Xw Yw Zw 1]T (1)
for each calibration plate position, the world coordinate system is on the calibration plate, so its Z can be defined on the calibration plate w0, camera coordinate system [ X [ ]c Yc Zc]Can be expressed as:
Figure RE-GDA0003400508390000051
obtaining the three-dimensional coordinate X under the camera coordinate system of the center of a circle on the calibration plate at each positionc、Yc、 ZcAnd fitting the calibration plate by using a plane fitting algorithm to obtain an imaging plane of the calibration plate at each position in a camera coordinate system. Similarly, the calibration plate at the first position is a reference plane, and the fitting plane thereof can be expressed as:
Figure RE-GDA0003400508390000052
wherein (X)C YC ZC)TCamera coordinates representing the center of a circle on the calibration plate,
Figure RE-GDA0003400508390000061
friend indicates the camera coordinates of any point on the calibration plate, and i indicates the calibration plate at the ith position. The plane parameters A, B, C and D are solved by the least squares method, so the height of the point on the calibration plate at each position can be expressed as:
Figure RE-GDA0003400508390000062
subtracting the z value of the reference plane from the z value of each plane to obtain the height difference hi(u，v)；
S4, obtaining phase information of each position of the circular calibration plate by using a phase shift method. Using the circle calibration plate at the first position as a reference plane, and subtracting the phase value of the corresponding point on the reference plane from the phase value of the circle calibration plate at each position to obtain several sets of phase difference information, which are marked as delta phii(u，v)。
S5, phase-height calibration, also called system geometry calibration. The corresponding relation between the phase difference delta phi (u, v) of the measured object and the height h (u, v) of the measured object is as follows:
Figure RE-GDA0003400508390000063
when the measuring system is fixed, the parameters a (u, v), b (u, v) and c (u, v) are regarded as system constants, and h is obtained by taking a plurality of different positionsi(u, v) and Δ φi(u, v), establishing an equation system, and solving three parameters of a (u, v), b (u, v) and c (u, v) by a least square method, thereby completing system calibration.
Without being limited thereto, any changes or substitutions that are not thought of through the inventive work should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope defined by the claims.

Claims (1)

1. A flexible and high-precision structured light system calibration method is characterized in that: the method uses a computer to generate fringe patterns of several specific frequencies to carry out four-step phase shift, and the four-step phase shift pattern of each frequency is projected on a calibration plate with characteristic points; solving through a phase shift unwrapping algorithm to obtain a continuous phase value of the calibration plate; finally, obtaining the pixel coordinates of the circle center on the calibration plate and the internal and external parameters of the camera at each position by using a Zhang-friend calibration method, thereby obtaining the coordinates of the circle center on the calibration plate under a camera coordinate system, fitting the imaging plane of the calibration plate by using a plane fitting algorithm through the coordinates of the circle center camera, and then calibrating the measurement system through the distance between different imaging planes and the phase difference of the continuous phases of the different imaging planes; the method specifically comprises the following steps:
s1, establishing a three-dimensional measurement system: the system comprises a DLP projector, a CCD camera and a circle calibration plate; the DLP projector and the CCD camera can be placed at will;
s2, generating fringe patterns of several specific frequencies by using a computer to perform four-step phase shift, and projecting the four-step phase shift patterns of the frequencies onto a circular calibration plate with characteristic points;
s3, using a camera calibration program to move the circular calibration plate forward for multiple times to calibrate the camera to obtain the internal and external parameters of the camera; a standard pinhole lens model that does not take into account lens distortion can be described mathematically as:
S[u v 1]T＝A[R T][Xw Yw Zw 1]T (1)
for each calibration plate position, the world coordinate system is on the calibration plate, so its Z can be defined on the calibration platew0, camera coordinate system [ X [ ]c Yc Zc]Can be expressed as:
Figure RE-FDA0003400508380000011
obtaining the three-dimensional coordinate X under the camera coordinate system of the center of a circle on the calibration plate at each positionc、Yc、ZcFitting the calibration plate by using a plane fitting algorithm to obtain an imaging plane of the calibration plate at each position in a camera coordinate system; similarly, the calibration board at the first position is a reference plane, and the fitting plane thereof can be representedComprises the following steps:
Figure RE-FDA0003400508380000021
wherein (X)C YC ZC)TCamera coordinates representing the center of a circle on the calibration plate,
Figure RE-FDA0003400508380000022
the camera coordinates of any point on the calibration plate are represented, and i represents the calibration plate at the ith position; the plane parameters A, B, C and D are solved by the least squares method, so the height of the point on the calibration plate at each position can be expressed as:
Figure RE-FDA0003400508380000023
subtracting the z value of the reference plane from the z value of each plane to obtain the height difference hi(u，v)；
S4, phase information of each position of the circular calibration plate is obtained by using a phase shift method; using the circle calibration plate at the first position as a reference plane, and subtracting the phase value of the corresponding point on the reference plane from the phase value of the circle calibration plate at each position to obtain several sets of phase difference information, which are marked as delta phii(u，v)；
S5, phase-height calibration, also called system geometric parameter calibration; the corresponding relation between the phase difference delta phi (u, v) of the measured object and the height h (u, v) of the measured object is as follows:
Figure RE-FDA0003400508380000024
when the measuring system is fixed, the parameters a (u, v), b (u, v) and c (u, v) are regarded as system constants, and h is obtained by taking a plurality of different positionsi(u, v) and Δ φi(u, v), establishing an equation set, solving three parameters of a (u, v), b (u, v) and c (u, v) by a least square method, and finishing system calibration until the three parameters are obtained。
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CN104729429A (en) * 2015-03-05 2015-06-24 深圳大学 Calibration method of telecentric imaging three-dimension topography measuring system
CN112598747A (en) * 2020-10-15 2021-04-02 武汉易维晟医疗科技有限公司 Combined calibration method for monocular camera and projector
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US20070115484A1 (en) * 2005-10-24 2007-05-24 Peisen Huang 3d shape measurement system and method including fast three-step phase shifting, error compensation and calibration
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CN115127481A (en) * 2022-06-29 2022-09-30 苏州赫芯科技有限公司 Stripe projection 3D measuring method, terminal device and computer readable storage medium

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