CN113916154A - Self-calibration structured light measuring method based on constant half width of modulation degree - Google Patents
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- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
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Abstract
The invention discloses a self-calibration structured light measuring method based on constant half width of modulation degree, which is universally applicable to improving the measurement precision, the measurement reliability and the measurement repeatability of a structured light illumination measuring method. The method adopts a digital micromirror array (DMD) to generate a coded light field, the coded light field is projected to the surface of an object through an optical system, the height information of the surface of the object can be coded in the stripe modulation degree information, the mapping relation between the modulation degree information and the height information of the object can be established by combining a phase shift algorithm and Gaussian curve fitting, and a step self-calibration algorithm is further used for carrying out accurate self-calibration on an actual scanning step so as to obtain accurate three-dimensional shape information of the object. The method has the advantages of simple system structure, strong stability and high universality, and can effectively improve the effect of the structured light illumination measurement and expand the application range of the structured light illumination measurement.
Description
Technical Field
The invention belongs to the technical field of optical measurement engineering, and particularly relates to a self-calibration structured light measurement method based on constant half width of modulation degree.
Background
The micro-nano device is used as a core component in a plurality of fields and is industrial grain in the information age. The method has a huge application prospect in the fields of aerospace, military, biomedicine, semiconductors and the like, and the wide application of the method greatly promotes the development of related science, wherein the micro-nano detection technology is the leading edge and hot spot of international research.
The non-contact optical measurement method has the characteristics of no damage, high precision, high efficiency and the like, and becomes a main measurement means in the field of micro-nano structure detection at present, and the existing optical measurement method mainly comprises a white light interference method, a laser confocal method, a structured light measurement method and the like. The white light interference method utilizes the principle that two beams of coherent light from a measured surface and a reference surface interfere to form interference fringes, and the position of the maximum light intensity value of the interference fringes corresponds to the position where the optical path difference of the two beams of coherent light is zero, so that the position of the zero-order interference fringes is positioned, and the appearance of an object is recovered. The method has extremely high measurement precision and is widely applied to the measurement of the micro-nano structure with a smooth surface, but when the method is applied to the surface detection with larger surface roughness and larger surface curvature, the method fails because interference fringes cannot be formed and the problems of surface characterization and phase ambiguity are difficult to carry out. The laser confocal method utilizes a point detector to search the maximum light intensity value to carry out point-by-point measurement on the surface of an object, has strong adaptability and wide application, belongs to point measurement, and has low detection efficiency.
Therefore, the scholars propose the structured light measurement method, a white light source is used for illuminating a digital micromirror array (DMD) to generate a structured light field, the structured light field is projected to the surface of an object to be measured through an optical system, the stripes projected to the surface of the object are modulated by the height information of the surface of the object, the modulation degree information of the stripes changes correspondingly, and the position of the maximum value of the modulation degree corresponds to the focal plane position of a micro-lens. The modulation degree value of each scanning position is obtained by using a high-precision displacement table to longitudinally move the object to be detected, a modulation degree response curve of which the modulation degree changes along with the scanning position is obtained, and the focusing position of the modulation degree response curve is further obtained through a peak positioning algorithm, so that the three-dimensional appearance of the object is restored. The measurement method based on the structural light modulation degree analysis does not have the problems of phase ambiguity and the like in a white light interferometry, has wide application scenes, belongs to surface measurement, and has high measurement efficiency, so the method is considered to be a measurement means with great potential.
However, in the existing structured light measurement method research, no solution is provided for solving the problem of errors caused by the scanning system. No matter the space domain phase shift or Fourier transform method, different height modulation degree distributions are obtained through longitudinal scanning, and therefore height measurement is achieved. The positioning precision of the workpiece table directly determines the final measurement precision. However, in the actual measurement process, because the scanning optical path cannot be strictly perpendicular to the surface of the object, the surface of the object is inclined, and other factors, a certain difference exists between the step pitch of the workpiece table and the actual optical path change, and the final measurement precision is seriously affected.
Disclosure of Invention
The invention provides a self-calibration structured light measuring method based on constant half width of modulation degree, which can effectively correct scanning step distance and improve the measuring precision and repeatability of the structured light measuring method.
In order to achieve the purpose, the invention adopts the technical scheme that: a self-calibrating structured light measurement method based on constant half-width modulation, the method comprising the steps of:
step S1: controlling piezoelectric ceramic micro-step pitch to vertically scan an object to be detected through an upper computer program, sequentially projecting eight sinusoidal grating stripes with pi/4 phase difference by utilizing a DMD (digital micromirror device) in each step of scanning, collecting eight reflection images by adopting a CCD (charge coupled device) and storing data;
step S2: each time of scanning, respectively analyzing the modulation degree distribution of the collected image by a phase shift algorithm, completing longitudinal scanning to obtain a phase shift modulation degree curve of the modulation degree value of each pixel point along with the change of the scanning position, and performing peak positioning on the modulation degree curve by using a Gaussian curve fitting algorithm;
step S3: calculating a theoretical modulation half-width value through a formula, and carrying out accurate self-calibration on an actual scanning step by combining with an actually obtained modulation half-width value, so as to obtain an accurate scanning step, wherein the formula is as follows;
wherein, Δ z is the scanning step after correction, λ is the light source central wavelength, v is the projection fringe normalization frequency, NA is the micro-lens numerical aperture, n is the refractive index of the medium where the system is located, X is1And X2Are respectively asAnd actually acquiring two corresponding modulation degree curve full width at half maximum positions.
Step S4: accurately recovering the three-dimensional shape of the object according to a related formula, wherein the formula is as follows:
h＝zmax×Δz
wherein h is the relative height of the pixel point, zmaxAnd the accurate focusing position of the pixel point is obtained, and the delta z is the corrected scanning step distance.
In the step 2, the structured light illumination microscopic measurement method shows that the modulation degree of the collected image reflects the defocusing degree of the object, and the position of the maximum modulation degree is the accurate focusing position.
And reconstructing three-dimensional shape information of the object by extracting the position of the peak value of the modulation curve and combining the scanning step pitch.
Firstly, demodulating the modulation degree by adopting a phase shift algorithm, and further obtaining the accurate focusing position of a pixel point, wherein the precision is extremely high; then, the actual scanning step is accurately solved through a step self-calibration algorithm, so that the measurement precision, the measurement reliability and the measurement repeatability are further improved; after two key parameters are obtained, the three-dimensional shape of the object can be reconstructed according to a formula. The method can be suitable for the structured light measurement method based on the phase shift technology and the Fourier transform technology, so that the structured light measurement method can play a more important role in the fields of smooth surface detection, rough surface detection and the like.
The basic principle of the invention is as follows: a self-calibration structured light measurement method based on modulation half-width constant vertically scans a standard plane object by micro-step of piezoelectric ceramics under the control of an upper computer program, scans each step, sequentially projects sinusoidal grating stripes with a certain phase difference by utilizing DMD projection, collects a deformed stripe pattern through a CCD (charge coupled device), converts the deformed stripe pattern into a digital signal and stores the digital signal in a computer. And after the scanning is finished, processing the obtained image sequence by using a phase shift algorithm to obtain a phase shift modulation degree curve of which the modulation degree of each pixel point changes along with the scanning position, and combining Gaussian curve fitting to obtain the accurate peak position. And further combining the theoretical modulation half-width and the actual acquired modulation half-width to realize accurate self-calibration of the scanning step. After the accurate focusing position and the actual scanning step distance of the pixel point are obtained, the relative height information of the pixel point of the object can be obtained according to a formula, and therefore the three-dimensional appearance of the object is restored.
The characteristics and advantages of the invention are as follows:
(1) compared with the interference measurement, the method is a non-interference measurement method, so that the influence between layers is less, and the method is suitable for detection of three-layer or even more-layer structures.
(2) Compared with a confocal measurement method, the method has the advantages that a surface measurement mode is adopted, the efficiency is higher, and the system structure is simpler.
(3) Compared with the traditional structured light measuring method, the method realizes the correction of the introduced error of the scanning system, and further improves the measuring precision and repeatability.
(4) The invention has the advantages of non-contact, full view field, high precision, wide applicability and the like.
Drawings
Fig. 1 is a schematic structural diagram of a measurement system, wherein 1 is a white light source, 2 is a digital micromirror array DMD, 3 is a Tube lens i, 4 is a black-and-white CCD camera, 5 is a Tube lens ii, 6 is a spectroscope, 7 is a microscope objective, and 8 is a high-precision displacement stage;
FIG. 2 is a real-time image acquisition during an experiment;
FIG. 3 is a schematic diagram of a repeatability measurement;
fig. 4 is a flow chart of a self-calibration structured light measurement method based on constant half-width modulation of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer, the present invention is described in further detail below with reference to the accompanying drawings in combination with specific examples.
As shown in fig. 1, the measurement system used in the present invention mainly includes: the device comprises a white light source 1, a DMD digital micromirror array 2, a Tube lens I3, a black and white CCD camera 4, a Tube lens II 5, a spectroscope 6, a microscope objective 7 and a high-precision displacement table 8. The white light source 1 is used for providing a light source for the measuring system, the black-and-white CCD camera 4 is located at the focal plane position of the Tube lens II 5 and used for collecting images, the DMD digital micromirror array 2 is located at the focal plane position of the Tube lens I3 and used for generating grating images, the microscope objective 7 is used for imaging an object to be measured, the spectroscope 6 is used for separating light beams, and the high-precision displacement table 8 is used for driving the object to be scanned.
As shown in fig. 4, a self-calibrating structured-light measurement method based on constant half-width modulation, the method comprises the steps of:
step S1: controlling piezoelectric ceramic micro-step pitch to vertically scan an object to be detected through an upper computer program, sequentially projecting eight sinusoidal grating stripes with pi/4 phase difference by utilizing a DMD (digital micromirror device) in each step of scanning, collecting eight reflection images by adopting a CCD (charge coupled device) and storing data;
step S2: each time of scanning, respectively analyzing the modulation degree distribution of the collected image by a phase shift algorithm, completing longitudinal scanning to obtain a phase shift modulation degree curve of the modulation degree value of each pixel point along with the change of the scanning position, and performing peak positioning on the modulation degree curve by using a Gaussian curve fitting algorithm;
after the scanning is completed, the light intensity distribution of a certain single pixel point in the CCD collected image sequence can be expressed as:
wherein, I0C (x, y, z) is the information of the degree of modulation of the fringes, f0Is the period of the sinusoidal light field, L is the number of phase shift steps,
is a phase shift of, wherein0C (x, y, z) is the information of the degree of modulation of the fringes, f0Is the period of the sinusoidal light field, L is the number of phase shift steps,
is a phase shift. According to the phase shift algorithm, after a plurality of phase shift pictures are collected at the same position, the modulation degree information can be calculated to obtain:
wherein, Ii(x, y, z) is the intensity of the ith picture, and L is the number of phase shift steps.
Step S3: calculating a theoretical modulation half-width value through a formula, and performing accurate self-calibration on an actual scanning step pitch by combining with an actually obtained modulation half-width value, so as to obtain an accurate scanning step pitch;
the accurate focusing position z of the pixel point can be obtained by combining the above formula with Gaussian function fittingmax. Further, in the case of optical system parameter determination, the modulation degree curve half-width may be expressed as:
wherein lambda is the central wavelength of the light source, v is the normalized frequency of the projection stripes, NA is the numerical aperture of the micro-lens, and n is the refractive index of the medium where the system is located.
Meanwhile, two X values of the actually obtained modulation degree curve at the half-peak position can be calculated by the formula (2)2，X1The actual scan step after calibration can be expressed as:
step S4: accurately recovering the three-dimensional appearance of the object according to a correlation formula;
after the accurate focusing position and the scanning step distance of the pixel point are obtained, the relative height information of the pixel point can be obtained, so that the three-dimensional appearance of the object can be restored:
h＝zmax×Δz (5)
as shown in FIG. 3, the self-calibration structured light measurement method based on constant half-width modulation can effectively realize three-dimensional detection of an object and keep extremely high measurement repeatability, and as can be seen in FIG. 3, the measurement repeatability of the method is better than 2 nm.
Claims (4)
1. A self-calibration structured light measuring method based on modulation degree half-width constant is characterized in that: the method comprises the following steps:
step S1: controlling piezoelectric ceramic micro-step pitch to vertically scan an object to be detected through an upper computer program, sequentially projecting eight sinusoidal grating stripes with pi/4 phase difference by utilizing a DMD (digital micromirror device) in each step of scanning, collecting eight reflection images by adopting a CCD (charge coupled device) and storing data;
step S2: each time of scanning, analyzing the modulation degree distribution of the collected image by using a phase shift algorithm, completing longitudinal scanning to obtain a phase shift modulation degree curve of the modulation degree value of each pixel point along with the change of the scanning position, and performing peak positioning on the modulation degree curve by using a Gaussian curve fitting algorithm;
step S3: calculating a theoretical modulation half-width value through a formula, and performing accurate self-calibration on an actual scanning step by combining with an actually obtained modulation half-width value, so as to obtain an accurate scanning step, wherein the formula is as follows:
wherein, Δ z is the scanning step after correction, λ is the light source center wavelength, ν is the projection fringe normalization frequency, NA is the micro-lens numerical aperture, n is the refractive index of the medium where the system is located, X1And X2Two corresponding to the half-height-width position of the actually obtained modulation degree curve are respectively obtained;
step S4: accurately recovering the three-dimensional shape of the object according to a related formula, wherein the formula is as follows:
h＝zmax×Δz
wherein h is the relative height of the pixel point, zmaxAnd the accurate focusing position of the pixel point is obtained, and the delta z is the corrected scanning step distance.
2. The self-calibrating structured-light measurement method based on constant half-width modulation according to claim 1, wherein: in the step 2, the structured light illumination microscopic measurement method shows that the modulation degree of the collected image reflects the defocusing degree of the object, and the position of the maximum modulation degree is the accurate focusing position.
3. The self-calibrating structured-light measurement method based on constant half-width modulation according to claim 1, wherein: and reconstructing three-dimensional shape information of the object by extracting the position of the peak value of the modulation curve and combining the scanning step pitch.
4. The self-calibrating structured-light measuring method based on the constant half-width modulation according to claim 1, 2 or 3, wherein: firstly, demodulating the modulation degree by adopting a phase shift algorithm, and further obtaining the accurate focusing position of a pixel point, wherein the accuracy is extremely high; then, the actual scanning step is accurately solved through a step self-calibration algorithm, so that the measurement precision, the measurement reliability and the measurement repeatability are further improved; after two key parameters are obtained, the three-dimensional shape of the object can be reconstructed according to a formula; the method can be suitable for the structured light measurement method based on the phase shift technology and the Fourier transform technology, so that the structured light measurement method can play a more important role in the fields of smooth surface detection and rough surface detection.
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