CN107727002B - Structure for shadow Moire parameter calibration, calibration method and offset error elimination method - Google Patents

Structure for shadow Moire parameter calibration, calibration method and offset error elimination method Download PDF

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CN107727002B
CN107727002B CN201710911338.6A CN201710911338A CN107727002B CN 107727002 B CN107727002 B CN 107727002B CN 201710911338 A CN201710911338 A CN 201710911338A CN 107727002 B CN107727002 B CN 107727002B
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grating
measurement
light source
phase shift
point light
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CN107727002A (en
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杜虎兵
宇建红
张少锋
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Xian Technological 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
    • G01B11/254—Projection of a pattern, viewing through a pattern, e.g. moiré

Abstract

The invention relates to a structure for shadow Moire parameter calibration, a calibration method and a maladjustment error elimination method. The structure comprises a point light source, a main camera CCD, an auxiliary camera CCD, a measuring grating and an electric displacement platform, wherein the measuring grating is horizontally arranged under the main camera CCD, the point light source and the auxiliary camera CCD are respectively arranged on two sides of the main camera CCD, a measured object is arranged on the electric displacement platform, the point light source is monochromatic, obliquely irradiates the measuring grating to project a deformed grating on the surface of the measured object, the main camera CCD penetrates through the measuring grating to observe, the measuring grating and the deformed grating are overlapped to generate a moire fringe pattern, the main camera CCD and the auxiliary camera CCD form a stereoscopic vision measuring system, and a world coordinate system is established at the optical center of the CCD. The invention is mainly applied to the coordinate measurement of space sparse points, so that the stereoscopic vision provides a convenient way.

Description

Structure for shadow Moire parameter calibration, calibration method and offset error elimination method
Technical Field
The invention relates to the technical field of optical precision measurement, in particular to a structure for shadow Moire parameter calibration, a calibration method and a maladjustment error elimination method.
Background
The shadow Moire three-dimensional profile measurement technology is a full-field non-contact optical three-dimensional profile measurement method, which is proposed since 1970 and widely applied to the fields of human body measurement, wafer surface detection, BGA coplanarity test, automatic production, aviation industry and the like.
Shadow moire based on phase technique has become its mainstream demodulation method at present. However, due to the inherent phase-height non-linear relationship of shaded Moire, classical phase-shifting techniques cannot achieve an accurate solution in shaded Moire; on the other hand, in the phase-height mapping process, the problems of difficult geometric parameter calibration and poor precision also exist.
Disclosure of Invention
The invention provides a structure and a calibration method for shadow moire parameter calibration, provides a solution for the problem of geometric structure parameter calibration of a traditional phase-shift shadow moire object three-dimensional profile measuring device, and further provides a method for eliminating offset errors, so that phase-height nonlinear errors in the method are eliminated, and the measurement precision is effectively improved.
In order to overcome the existing problems, the solution of the invention is as follows:
a shadow Moire device geometric structure parameter calibration structure comprises a point light source, a main camera CCD, an auxiliary camera CCD, a measurement grating and an electric displacement platform, wherein the measurement grating is horizontally arranged under the main camera CCD, the point light source and the auxiliary camera CCD are respectively arranged on two sides of the main camera CCD, a measured object is arranged on the electric displacement platform, the point light source is monochromatic, obliquely irradiates the measurement grating, a deformation grating is projected on the surface of the measured object, when the main camera CCD penetrates through the measurement grating for observation, the measurement grating and the deformation grating are overlapped to generate a Moire fringe pattern, the main camera CCD and the auxiliary camera CCD form a stereo vision measurement system, and a world coordinate system is established at the optical center of the CCD.
Firstly, calibrating a camera, establishing internal reference and external reference of the camera, projecting mark points with the number larger than that of the mark points on a grating surface, measuring the left sides of the mark points by using a stereoscopic vision system, and further fitting a grating plane, wherein at the moment, the distance between the optical center of a CCD and the grating surface, namely a parameter h, can be obtained by using a geometric principle;
the point light source can rotate around the optical center of the CCD, the point light source is controlled to rotate 90 degrees anticlockwise to enter a visual field of the binocular stereoscopic vision measuring system, then the point light source is used as a characteristic point, the world coordinate of the point light source is measured, and the ordinate of the point light source is the parameter d.
A method for eliminating the misadjustment error of the geometric structure parameters of the shadow Moire device comprises the following specific steps:
the first step is as follows: building a measuring structure, placing a measuring object in a measuring space after parameter calibration, and moving a measuring grating by using an electric displacement table in a small unit to obtain a plurality of frames of phase-shift fringe images;
the second step is that: processing the fringe pattern to determine the grating movement amount so as to obtain an initial estimation phase shift;
the third step: calculating a phase according to the shot fringe pattern, and performing phase unwrapping on the extracted measurement phase by using a unwrapping method so as to obtain an initial estimated measurement height;
the fourth step: the phase shift and hence the measurement height is updated.
The fourth step is specifically as follows:
during measurement, a workpiece is placed in a measurement space, a light source is started to irradiate a measurement grating, a moire fringe pattern is generated on the surface of an object, then the grating is controlled to move twice relative to an initial position and perpendicular to the grating surface, and the light field change is captured and stored by a camera, so that the expression formula of the phase shift fringe pattern for demodulating the surface appearance of the object is obtained as follows.
Assuming a (x, y) as background, b (x, y) as modulation term, phi (x, y) as phase, delta as phase shift, and n as time domain stripe number, the light intensity of the captured stripe pattern can be expressed as:
In(x,y)＝A(x,y)+B(x,y)cos[φ(x,y)+nδ(x,y)],(n＝0,1,2) (1)
for clarity, the following derivation process omits the coordinate terms (x, y), and the phase demodulation method proposed by this patent is as follows:
firstly, subtracting a background removal term by using stripes, and expressing the obtained new stripes as follows:
Figure BDA0001425040290000021
and further performing addition and subtraction operation on the new stripes to obtain:
Figure BDA0001425040290000022
ensuring that the number of stripes in the taken stripe pattern is more than 1, neglecting the nonlinear relation in phase shift, obtaining approximate phase shift deltaeAnd replacing the phase shift δ with it:
Figure BDA0001425040290000023
in the above formula
Figure BDA0001425040290000024
And then determining the grating movement amount as follows:
Δh＝phδe/2πd (5)
using delta simultaneouslyeThe estimated phase (unwrapped) and measured height are:
Figure BDA0001425040290000031
the phase shift is updated as follows:
Figure BDA0001425040290000032
substituting the updated phase shift into equations (5), (6), and iterating until the updated phase shift is satisfied
max(|zq-zq-1|)＜ε (8)
In the formulaqRepresenting the iteration times, wherein epsilon is the preset precision, and finally obtaining a high-precision result.
The invention has the advantages that:
1. the structure is simple: the measurement structure provided by the invention does not increase the complexity of a shadow Moire measurement structure, and an auxiliary camera CCD is not needed after the structural parameter calibration is finished. Therefore, the measuring device of the invention has simple structure and low price.
2. The structural parameter calibration can be carried out: an effective structural parameter calibration scheme is not available so far, so that the invention provides a way for solving the problem. The patent discloses a method for independently calibrating structural parameters, which provides a new way for manufacturing a Moire device with precise structural parameters and for precise phase demodulation.
3. The application requirement is low: the method does not need the consistency of the light source, has low requirement on the environment and is suitable for the measurement requirement of industrial occasions.
4. The method of the invention applies iteration technology to eliminate the inherent phase-height nonlinear error of phase shift shadow Moire.
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FIG. 1 is a schematic diagram of the implementation of the measurement method of the present invention.
The reference numerals are explained below: 1, a linear light source; 2, camera CCD 1; 3 auxiliary camera CCD 2; 4, an electric displacement table; 5, an object stage; and 6, measuring the grating.
Detailed Description
The stereoscopic vision measurement technology belongs to a passive optical measurement method, is one of the most mature three-dimensional coordinate measurement methods applied at present, and can be divided into a monocular vision system, a binocular vision system and a multi-ocular vision system. Unlike active optical measurement methods, the stereoscopic vision technique uses only the coordinate information of the pixel points of the sampling pattern, rather than the grayscale information of the sampling pattern. The realization of stereoscopic vision three-dimensional measurement is a very complex measurement process, which mainly comprises camera calibration, image acquisition, image processing, main reconstruction operation, data output and the like. But the invention mainly applies space sparse point coordinate measurement, so that stereoscopic vision provides a convenient way.
The shadow Moire system only uses one camera, and the monocular vision system is convenient to measure the coordinates, but the monocular distance measurement process is complicated, so the method of the common binocular distance measurement is adopted in the invention.
The present invention will be described in detail below with reference to specific examples.
Referring to fig. 1:
the invention provides a structure for calibrating geometric structure parameters of a shadow Moire device, which is to be copied.
The measuring gratings 6 are arranged in parallel by a grating holder on the motor 4.
Referring to fig. 1, a method for calibrating geometric parameters of a shadow moire device using the device is to be copied.
The structural parameters of the experimental device of shadow moire include: p, h, d. The grating period p is generally considered to have higher precision and does not need to be calibrated. However, the other two parameters need to be calibrated, and since the centers of the camera and the light source are virtual points, the actual calibration is difficult. The patent proposes to introduce the binocular stereo vision technology into a shadow Moire measurement system, and utilizes the powerful coordinate measurement function thereof to realize the precise and convenient calibration of geometric parameters in a Moire measurement structure so as to improve the phase-height mapping precision and provide a way for the design of the Moire measurement structure.
A method for eliminating the misadjustment error of the geometric structure parameters of the shadow Moire device comprises the following specific steps:
the first step is as follows: constructing an image measuring structure, placing a measuring object in a measuring space after parameter calibration, and moving a grating by a precision displacement table in a small unit (such as 0.01mm) to obtain a 3-frame phase shift fringe image;
the second step is that: processing the fringe pattern to determine the grating movement amount so as to obtain an initial estimation phase shift;
the third step: calculating a phase according to the shot fringe pattern, and performing phase unwrapping on the extracted measurement phase by using a unwrapping method so as to obtain an initial estimated measurement height;
the fourth step: updating the phase shift and further updating the measurement height, and the specific method comprises the following steps:
during measurement, a workpiece is placed in a measurement space, a light source is started to irradiate a measurement grating, a moire fringe pattern is generated on the surface of an object, then the grating is controlled to move twice relative to an initial position and perpendicular to the grating surface, and the light field change is captured and stored by a camera, so that the expression formula of the phase shift fringe pattern for demodulating the surface appearance of the object is obtained as follows.
Assuming a (x, y) as background, b (x, y) as modulation term, phi (x, y) as phase, delta as phase shift, and n as time domain stripe number, the light intensity of the captured stripe pattern can be expressed as:
In(x,y)＝A(x,y)+B(x,y)cos[φ(x,y)+nδ(x,y)],(n＝0,1,2)(1)
for clarity of presentation, the following derivation process omits the coordinate terms (x, y). According to the above formula, the phase demodulation method proposed by this patent is:
firstly, subtracting a background removal term by using stripes, and expressing the obtained new stripes as follows:
Figure BDA0001425040290000051
and further performing addition and subtraction operation on the new stripes to obtain:
Figure BDA0001425040290000052
ensuring that the number of stripes in the taken stripe pattern is more than 1, neglecting the nonlinear relation in phase shift, obtaining approximate phase shift deltaeAnd replacing the phase shift δ with it:
Figure BDA0001425040290000053
in the above formula
Figure BDA0001425040290000054
And then determining the grating movement amount as follows:
Δh＝phδe/2πd (5)
using delta simultaneouslyeThe estimated phase (unwrapped) and measured height are:
Figure BDA0001425040290000055
the phase shift is updated as follows:
Figure BDA0001425040290000056
substituting the updated phase shift into equations (5), (6), and iterating until the updated phase shift is satisfied
max(|zq-zq-1|)＜ε (8)
In the formulaqRepresenting the number of iterations, epsilon is a preset precision. And finally obtaining a high-precision result.
And if the measurement is needed to be continued, repeating the first step to the fourth step. The calibration process can be omitted.
The present invention has been described in terms of specific examples, which are provided to aid understanding of the invention and are not intended to be limiting. For a person skilled in the art to which the invention pertains, several simple deductions, modifications or substitutions may be made according to the idea of the invention.

Claims (4)

1. A shadow Moire device geometric structure parameter calibration structure is characterized by comprising a point light source (1), a main camera CCD1, an auxiliary camera CCD2, a measurement grating (6) and an electric displacement table (5), wherein the measurement grating (6) is horizontally arranged right below a main camera CCD1, the point light source (1) and the auxiliary camera CCD2 are respectively arranged on two sides of the main camera CCD1, a measured object is arranged on the electric displacement table (5), the point light source (1) is monochromatic and obliquely irradiates the measurement grating (6) to project a deformation grating for the surface of the measured object, when the main camera CCD1 is observed through the measurement grating (6), the measurement grating and the deformation grating are overlapped to generate a Moire fringe image, a stereoscopic vision measurement system is formed by the main camera CCD1 and the auxiliary camera CCD2, and a world coordinate system is established at the optical center of the CCD 1.
2. The calibration method of the structure according to claim 1, wherein the camera is calibrated first, the internal reference and the external reference of the camera are established, the number of the mark points is larger than 3, then a stereoscopic vision system is used for measuring the left side of the mark points, and then a grating plane is fitted, and at the moment, the distance between the optical center of the CCD1 and the grating plane, namely the parameter h, can be obtained by using a geometric principle;
the point light source can rotate around the optical center of the CCD1, the point light source (1) is controlled to rotate 90 degrees anticlockwise to enable the point light source to enter a visual field of a binocular stereo vision measuring system, then the point light source (1) is used as a characteristic point, the world coordinate of the point light source is measured, and the ordinate of the point light source is the parameter d.
3. The method for removing the misalignment error of the structure according to claim 1, comprising the following steps:
the first step is as follows: building a measuring structure, after parameter calibration, placing a measuring object in a measuring space, and moving a measuring grating (6) by a small unit by using an electric displacement table (4) to obtain a plurality of frames of phase shift fringe images;
the second step is that: processing the fringe pattern to determine the grating movement amount so as to obtain an initial estimation phase shift;
the third step: calculating a phase according to the shot fringe pattern, and performing phase unwrapping on the extracted measurement phase by using a unwrapping method so as to obtain an initial estimated measurement height;
the fourth step: the phase shift and hence the measurement height is updated.
4. The misalignment error elimination method according to claim 3,
the fourth step is specifically as follows:
during measurement, a workpiece is placed in a measurement space, a light source is started to irradiate a measurement grating, a moire fringe pattern is generated on the surface of an object, then the grating is controlled to move twice relative to the initial position and perpendicular to the grating surface, and the light field change is shot and stored by a camera, so that the expression formula of the phase shift fringe pattern for demodulating the surface appearance of the object is obtained as follows:
assuming a (x, y) as background, b (x, y) as modulation term, phi (x, y) as phase, delta as phase shift, and n as time domain fringe sequence number, the light intensity of the captured fringe pattern can be expressed as:
In(x,y)＝A(x,y)+B(x,y)cos[φ(x,y)+nδ(x,y)],(n＝0,1,2) (1)
for clarity, the following derivation process omits the coordinate terms (x, y), and the phase demodulation method proposed by this patent is as follows:
firstly, subtracting a background removal term by using stripes, and expressing the obtained new stripes as follows:
Figure FDA0001425040280000021
and further performing addition and subtraction operation on the new stripes to obtain:
Figure FDA0001425040280000022
ensuring that the number of stripes in the taken stripe image is more than 1, neglecting the nonlinear relation in phase shift, obtaining approximate phase shift deltaeAnd replacing the phase shift δ with it:
Figure FDA0001425040280000023
in the above formula
Figure FDA0001425040280000024
And then determining the grating movement amount as follows:
Δh＝phδe/2πd (5)
using delta simultaneouslyeThe estimated phase (unwrapped) and measured height are:
Figure FDA0001425040280000025
Figure FDA0001425040280000026
the phase shift is updated as follows:
Figure FDA0001425040280000027
substituting the updated phase shift into equations (5), (6), and iterating until the updated phase shift is satisfied
max(|zq-zq-1|)＜ε (8)
In the formula, q represents the iteration times, epsilon is preset precision, and finally, a high-precision result is obtained.
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