CN117405044B - Workpiece three-dimensional measurement method and system based on multi-frequency polarization stripe technology - Google Patents
Workpiece three-dimensional measurement method and system based on multi-frequency polarization stripe technology Download PDFInfo
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Abstract
The present disclosure relates to a method for three-dimensional measurement of a workpiece based on a multi-frequency polarization fringe technique and a system thereof, the method comprising the steps of: the method comprises the steps of obtaining polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies of high, medium and low, generating a multi-frequency polarization stripe image, and eliminating influence of high light on a wrapping phase of the workpiece; transmitting the generated multi-frequency polarized stripe image to a projector, projecting the multi-frequency polarized stripe image onto the surface of a workpiece, and capturing the multi-frequency polarized stripe image by a camera through a linear polarizer to obtain each frequency deformation stripe; according to each frequency deformation stripe, a high-frequency wrapping phase, a medium-frequency wrapping phase and a low-frequency wrapping phase are calculated, the high-frequency wrapping phase is unfolded to obtain an absolute phase through the high-frequency wrapping phase, three-dimensional measurement of a workpiece is completed, the low-frequency vibration stripe is used for assisting the high-frequency vibration stripe, the high-frequency wrapping phase is unfolded, and measurement accuracy and measurement range are improved.
Description
Technical Field
The disclosure relates to the technical field of vision sensor measurement, in particular to a workpiece three-dimensional measurement method and system based on a multi-frequency polarization stripe technology.
Background
With the development of modern industrial technology, a large number of metal workpieces of different shapes are used in various applications. To ensure the applicability and accuracy of metal workpieces, three-dimensional profile measurement of metal workpieces is becoming increasingly important. The most typical measurement method is a fringe projection technology, in which fringe images are generated by adopting a certain coding algorithm in a computer, projected onto the surface of a measured metal workpiece by a projector, then a camera acquires fringe images modulated by the metal workpiece from different angles, the fringe images contain phase information of the metal workpiece, the phase information is extracted from the fringe images by a certain dephasing algorithm, three-dimensional contour information of the metal workpiece is measured by the phase information, and it is of no doubt that the phase information of the metal workpiece is accurately acquired and accurately unfolded in the whole measurement process, because he directly influences the accuracy and efficiency of three-dimensional measurement.
In the fringe projection technology, the common phase unwrapping method is divided into two kinds of spatial unwrapping methods and time domain unwrapping methods, wherein the time domain unwrapping method is further divided into two unwrapping methods of single-frequency phase shift and multi-frequency phase shift. As the previous research is mainly based on single-frequency phase shift method development, although single-frequency vibration encoding can effectively eliminate the influence of high light in three-dimensional measurement of metal workpieces, the identifiable slope range is smaller when measuring metal workpieces with larger surface slopes. Meanwhile, the number of projected fringe images is not too large in consideration of the measurement speed, so that low frequency with frequency of 16 is often adopted in single frequency vibration encoding, and measurement errors are increased when a metal workpiece with a complex surface is measured.
Disclosure of Invention
The invention provides a workpiece three-dimensional measurement method and a workpiece three-dimensional measurement system based on a multi-frequency polarization stripe technology, which can solve the problems that errors are caused by high reflectivity of a workpiece to wrap phases of multi-frequency stripes, absolute phase errors are increased or even cannot be spread due to layer-by-layer transmission of the errors, and measurement accuracy and measurement range are affected. In order to solve the technical problems, the present disclosure provides the following technical solutions:
as one aspect of an embodiment of the present disclosure, there is provided a workpiece three-dimensional measurement method based on a multi-frequency polarization fringe technique, including the steps of:
acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
transmitting the generated multi-frequency polarized stripe image to a projector, projecting the multi-frequency polarized stripe image onto the surface of a workpiece, and capturing the multi-frequency polarized stripe image by a camera through a linear polarizer to obtain each frequency deformation stripe;
calculating high-frequency, medium-frequency and low-frequency wrapping phases according to each frequency deformation stripe, and expanding the high-frequency wrapping phases through the high-frequency, medium-frequency and low-frequency wrapping phases to obtain absolute phases, so that three-dimensional measurement of a workpiece is completed;
the multi-frequency encoded fringe image is represented as:
，
in the middle of (a)x,y) For the pixel coordinates,for average intensity +.>In order to modulate the intensity of the light,ffor the fringe frequency to be a function of the fringe frequency,nis the phase shift step number;
the multi-frequency polarization fringe image is represented as:
，
wherein,for average intensity +.>Is the modulation intensity;
the various frequency distortion fringes captured by the camera are represented as:
，
in the method, in the process of the invention,for the reflectivity of the workpiece and the reference plane, +.>Wrapping phases for each frequency;
the unwrapped wrapped phase of each frequency is expressed as:
，
wherein,for the unwrapped wrapping phase, +.>Is a fringe order.
Optionally, the polarization degree of the linear polarizer is expressed as:
，
in the method, in the process of the invention,for linear polarization degree, +.>Indicating the maximum light intensity obtained by rotating the polarizer by 0 °o +.>Representing the minimum light intensity obtained by rotating the polarizer by 90.
Optionally, the absolute phase obtained by unwrapping the high-frequency wrapped phase is expressed as:
，
in the method, in the process of the invention,represents the absolute phase obtained after unwrapping the high-frequency wrapped phase, < >>For a low-frequency wraparound phase composed of wraparound phases of three frequencies, < >>Representing a rounding down, a +.>Is a constant.
As another aspect of an embodiment of the present disclosure, there is provided a workpiece three-dimensional measurement system based on a multi-frequency polarization fringe technique, including:
the multi-frequency polarization stripe image generation module is used for acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
the frequency deformation fringe capturing module is used for transmitting the generated multi-frequency polarization fringe image to the projector and projecting the multi-frequency polarization fringe image onto the surface of the workpiece, and capturing each frequency deformation fringe by the camera through the linear polarizer;
and the three-dimensional measurement module calculates high-middle-low frequency wrapping phases according to the deformation stripes of each frequency, and expands the high-frequency wrapping phases to obtain absolute phases through the high-middle-low frequency wrapping phases so as to finish three-dimensional measurement of the workpiece.
As another aspect of the embodiments of the present disclosure, an electronic device is provided, including a memory, a processor, and a computer program stored on the memory and executable on the processor, where the processor implements the method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique described above when the processor executes the computer program.
As another aspect of the embodiments of the present disclosure, there is provided a computer-readable storage medium having stored thereon a computer program which, when executed by a processor, implements the above-described multi-frequency polarization fringe technique-based workpiece three-dimensional measurement method.
Compared with the prior art, the beneficial effects of the present disclosure are:
1. the traditional multi-frequency gray scale coding (MFGC) mode is replaced by a multi-frequency polarization coding (MFPC) mode, polarization information is coded into three stripes with different frequencies, namely high, medium and low, so that the influence of high light on the wrapping phase of a metal workpiece is eliminated, and the transmission of errors is blocked from the source;
2. in the method, as the wrapping phase with higher frequency has higher signal to noise ratio, the low-frequency vibration stripes are used for assisting the high-frequency vibration stripes, the high-frequency wrapping phase is unfolded, the advantages of large-scale accuracy of the low-frequency stripe phase diagram and accurate details of the high-frequency stripe phase diagram are fully exerted, and the measuring range and measuring precision of the metal workpiece are effectively improved.
Drawings
FIG. 1 is a flow chart of a three-dimensional measurement method of a workpiece based on the multi-frequency polarization stripe technique in example 1;
FIG. 2 is a schematic diagram of a metal workpiece to be tested in example 1;
FIG. 3 is a PSC image contrast ratio diagram of example 1;
FIG. 4 is a schematic diagram of the wrapping phase and 370 row pixel curve in example 1;
FIG. 5 is a graph showing the difference frequency phase and the line 370 pixels in example 1;
FIG. 6 is a graph of absolute phase and line 370 of the pixel in example 1;
FIG. 7 is a schematic view of three-dimensional contours of a metal workpiece in example 1;
fig. 8 is a block diagram of a three-dimensional measurement system for a workpiece based on the multi-frequency polarization stripe technique in example 2.
Detailed Description
Various exemplary embodiments, features and aspects of the disclosure will be described in detail below with reference to the drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Although various aspects of the embodiments are illustrated in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The word "exemplary" is used herein to mean "serving as an example, embodiment, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
The term "and/or" is herein merely an association relationship describing an associated object, meaning that there may be three relationships, e.g., a and/or B, may represent: a exists alone, A and B exist together, and B exists alone. In addition, the term "at least one" herein means any one of a plurality or any combination of at least two of a plurality, for example, including at least one of A, B, C, and may mean including any one or more elements selected from the group consisting of A, B and C.
Furthermore, numerous specific details are set forth in the following detailed description in order to provide a better understanding of the present disclosure. It will be understood by those skilled in the art that the present disclosure may be practiced without some of these specific details. In some instances, methods, means, elements, and circuits well known to those skilled in the art have not been described in detail in order not to obscure the present disclosure.
It will be appreciated that the above-mentioned method embodiments of the present disclosure may be combined with each other to form a combined embodiment without departing from the principle logic, and are limited to the description of the present disclosure.
In addition, the disclosure also provides a workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology and a system thereof, and the above can be used for realizing any one of the workpiece three-dimensional measurement methods based on the multi-frequency polarization stripe technology provided by the disclosure, and corresponding technical schemes and descriptions and corresponding descriptions referring to method parts are omitted.
The execution subject of the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology may be a computer or other workpiece three-dimensional measurement device capable of implementing the multi-frequency polarization stripe technology, for example, the method may be executed by a terminal device or a server or other processing device, where the terminal device may be a User Equipment (UE), a mobile device, a User terminal, a cellular phone, a cordless phone, a personal digital processing (Personal Digital Assistant, PDA), a handheld device, a computing device, a vehicle-mounted device, a wearable device, or the like. In some possible implementations, the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology can be implemented by a mode that a processor calls computer readable instructions stored in a memory.
Example 1
The embodiment provides a workpiece three-dimensional measurement method based on a multi-frequency polarization stripe technology, which comprises the following steps:
s10, obtaining polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
s20, sending the generated multi-frequency polarization stripe image to a projector, projecting the multi-frequency polarization stripe image onto the surface of a workpiece, and capturing the multi-frequency polarization stripe image by a camera through a linear polarizer to obtain each frequency deformation stripe;
s30, calculating high-frequency, medium-frequency and low-frequency wrapping phases according to the deformation stripes of each frequency, and unfolding the high-frequency wrapping phases through the high-frequency, medium-frequency and low-frequency wrapping phases to obtain absolute phases, so that three-dimensional measurement of the workpiece is completed.
In this embodiment, the implementation of the workpiece three-dimensional measurement method based on the multi-frequency polarization fringe technology specifically includes the following steps, an algorithm flow chart of which is shown in fig. 1, and each step of the embodiments of the disclosure is described in detail below.
S10, obtaining polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, and coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, to generate the multi-frequency polarization stripe image.
The high reflectivity of the metal workpiece results in oversaturation of the multi-frequency fringe image intensity, so that partial phase information of the fringe image captured by the camera is lost, and the extracted wrapping phase information is incomplete. The minor errors of the wrapping phase can be transferred to the unfolded high-frequency phase layer by layer, so that a cavity phenomenon can occur after reconstruction. In consideration of the advantage of the multi-frequency projection fringes over the single-frequency projection fringes in the measurement range, in the embodiment, a multi-frequency polarization coding strategy is adopted to obtain polarization information of a workpiece to be measured, and a standard four-step phase shift method is adopted to generate a multi-frequency coding fringe image according to the polarization information.
The multi-frequency encoded fringe image is represented as:
，
in the middle of (a)x,y) For the pixel coordinates,for average intensity +.>In order to modulate the intensity of the light,ffor the fringe frequency to be a function of the fringe frequency,nis the number of phase shift steps.
The stokes vector is used for describing different polarization states, and the stokes expression of the multi-frequency coding stripe image is as follows:
，
wherein,representing the frequency asfFringe image total intensity of +.>Representing the component of linearly polarized light in the horizontal direction, whose value is equal to the rotation of the polarizing plateIntensity difference to polarization grating image with angles of 0 ° and 90 °, respectively>Represents the linearly polarized light component in the 45 DEG direction, which has a value of 45 DEG and 135 DEG polarization grating image intensity difference,/and>representing the component of circularly polarized light with a value equal to the difference between the intensities of the left and right circularly polarized images, +.>Representing the frequency asf0℃linear polarized light intensity, +.>Representing the frequency asf90 DEG linear polarized light intensity,>representing the frequency asf45℃linear polarized light intensity,>representing the frequency asfIs a light intensity of 135 deg. linearly polarized light,representing the frequency asfRight-hand circularly polarized light intensity, < >>Representing the frequency asfLeft-handed circularly polarized light intensity of (c).
One natural light beam can be decomposed into two linearly polarized light beams with unfixed phase, mutually perpendicular and equal amplitude, and the frequency is as followsfThe total intensity of the sinusoidal grating image of (2) can be the superposition of 0 degree linear polarized light and 90 degree linear polarized light, namely:
，
the degree of polarization of the light can now be described by the polarization states of only 0 ° and 90 °.
Further, the multi-frequency polarization fringe image is represented as:
，
wherein,for average intensity +.>For modulating intensity.
Compared with the traditional multi-frequency gray scale coding, in the frequency offset coding, horizontal polarized light and vertical polarized light are coded into stripe images with three different frequencies, namely high frequency, medium frequency, low frequency and the like, and the characteristic that the polarization state of light is kept unchanged in the light transmission process is utilized, so that the influence of high light on a metal workpiece is eliminated, and layer-by-layer transmission of errors is prevented from the source.
And S20, the generated multi-frequency polarization fringe image is sent to a projector and projected onto the surface of a workpiece, and each frequency deformation fringe is obtained through capturing by a camera through a linear polarizer.
The projector projects the multi-frequency polarized structured light to the metal workpiece, after the multi-frequency polarized structured light is reflected by the metal workpiece, composite light mainly composed of diffuse scattered light and specular reflected light is formed, wherein the specular reflected light still keeps the linear polarization state unchanged, the diffuse reflected light becomes unpolarized, the composite light is filtered after being captured by a camera through a linear polarizer in front of the camera, the diffuse reflected light finally reaching the camera does not change, and the specular reflected light is filtered, so that the effect of removing high light is achieved, however, due to the filtering of the polarizer, the intensity of a modulated image captured by the camera is attenuated, the contrast of the image is reduced, and the phase error is increased.
The accuracy of the phase value depends on the number of phase shift steps, fringe frequency and fringe contrast, and the variance of the phase error can be expressed as:
，
in the method, in the process of the invention,is the variance of additive noise of a gaussian distribution,Nfor phase shift step number>To modulate the fringe intensity.
In the present embodiment, in view of the effect of the degree of polarization in enhancing the contrast of polarized images, the degree of linear polarization is introduced to solve the above-described problem. The degree of polarization of the linear polarizer is expressed as:
，
in the method, in the process of the invention,for linear polarization degree, +.>Indicating the maximum light intensity obtained by rotating the polarizer by 0 °o +.>Representing the minimum light intensity obtained by rotating the polarizer by 90.
Optionally, each of the frequency-warping fringes captured by the camera is expressed as:
，
in the method, in the process of the invention,for the reflectivity of the workpiece and the reference plane, +.>Wrapping the phase for each frequency.
The above can be abbreviated as:
，
further, the wrapping phase and the modulation intensity can be obtained as follows:
，
，
the wrapping phase is truncated atThe interval needs to be spread out into a continuous absolute phase to facilitate the next reconstruction.
S30, calculating high-frequency, medium-frequency and low-frequency wrapping phases according to the deformation stripes of each frequency, and unfolding the high-frequency wrapping phases through the high-frequency, medium-frequency and low-frequency wrapping phases to obtain absolute phases, so that three-dimensional measurement of the workpiece is completed.
The unwrapped wrapped phase of each frequency is expressed as:
，
wherein,for the unwrapped wrapping phase, +.>Is a fringe order.
The multi-frequency phase expansion is calculated pixel by pixelx,y) For the pixel coordinates, for convenience of description, the three frequencies are taken as examples in this embodiment, and the stripe frequencies are respectivelyf 1 、f 2 、f 3 And is also provided withf 1 >f 2 >f 3 ，And->The following relation exists:
，
in the method, in the process of the invention,Wrepresenting the field of view range of the measurement.
Based on the above formula, the high-middle-low frequency wrapping phases can be respectively calculated as、、In order to enable the phase to be unfolded in a full-field range without ambiguity, three wrapping phases and three frequencies need to be overlapped, and the frequency overlapping needs to be satisfied:
，
，
，
through the frequency superposition, the measuring field of view contains only one period of sinusoidal grating. The wrapping phase superposition needs to satisfy:
，
，
，
by wrapping the phase、、Taking the difference, superimposing the phases as less frequent phases +.>Then according to the superimposed frequency +.>And phase->The high frequency wrapping phase of the three frequencies can be further spread out.
The absolute phase obtained by unwrapping the high-frequency wrapped phase is expressed as:
，
in the method, in the process of the invention,represents the absolute phase obtained after unwrapping the high-frequency wrapped phase, < >>For a low-frequency wraparound phase composed of wraparound phases of three frequencies, < >>Representing a rounding down, a +.>Is a constant.
，
Higher frequency wrapped phases have higher signal-to-noise ratios by wrapping phases for multiple different frequencies、、Step-by-step synthesis is performed to become the low-frequency wrapping phase which is unique in period in the measurement view field>Then wrap the low frequency by phase +.>、As reference phase, high frequency wrapped phase +.>The method is unfolded into absolute phases, the advantages of large-scale accuracy of the low-frequency fringe phase diagram and accurate details of the high-frequency fringe phase diagram are fully exerted, and the measuring range and measuring accuracy of the metal workpiece are effectively improved.
Further, in order to verify the effectiveness of the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology, a three-dimensional shape measurement system based on a polarization state superposition sinusoidal coding technology is built for measurement, the three-dimensional shape measurement system comprises a 3LCD projector (EPSON-CB 965) with 1024 x 768 resolution, a monochromatic camera (FLIRFLIR BFS-U3-23-23-3M-C) with 1024 x 768 resolution, a horizontal polarizer of a 1/4 wave plate, the projector and the camera are placed left and right, the included angle between the projector and the camera is smaller than 10 degrees, the measured metal workpiece is about 1000cm away from a camera lens, and the system adopts an eight-parameter method for system calibration and adopts four-step phase shift and three-frequency heterodyne for phase expansion.
To compare the performance of PSC and GSC multifrequency heterodyne methods in measuring high gloss metal workpieces, we measured a metal workpiece with a glossy surface, as shown in FIG. 2 (a), where the red marked area has significant saturation. Taking out、、Three sets of frequencies, each 12 projection fringe images are generated by adopting PSC and GSC coding methods, wherein each frequency is 4, the transverse resolution of the projection fringe images is 768 pixels, and the longitudinal resolution of the projection fringe images is 1024 pixels. The system is calibrated under the same condition, the camera and the projector are kept unchanged, the PSC coding image and the GSC coding image are projected to the tested metal workpiece successively, the images captured by the camera are shown as (b) and (c) in fig. 2, the overexposure in fig. 2 is solved, but the contrast is greatly reduced compared with that in fig. 2.
In order to improve contrast reduction caused by polarizer attenuation, a stripe image modulated by a metal workpiece is captured by a camera, and the linear polarization degree is calculated to enhance the contrast, so that the aim of improving the signal to noise ratio is fulfilled, and the intensity of the image captured in a vertical polarization state is recorded asThe intensity of the image captured in the horizontal polarization state is recorded as +.>The DoLP image is obtained through calculation, and as shown in fig. 3, the contrast of the enhanced DoLP image is greatly improved as can be seen from the experimental result.
In order to accurately evaluate the degree of image contrast enhancement, the contrast of the images before and after enhancement is calculated, and according to a common four-neighbor method contrast calculation method, the calculation formula of the contrast is as follows:
，
in the method, in the process of the invention,for adjacent pixelsmAndnpixel difference between, i.e. +.>，Representing the gray level difference between adjacent pixels as +.>Is a probability of distribution of (a).
Experiment from four phase-shifted images of each frequency, each phase shift was selected to be 0 andthe two images of (2) were calculated and the contrast statistics are shown in table 1.
TABLE 1
As can be seen from Table 1, the contrast of the image is doubled after the linear polarization degree is enhanced, the quality of the image is greatly improved, and the noise immunity of the image is enhanced.
The enhanced PSC image and the captured GSC image are substituted into a wrapping phase calculation formula to obtain a PSC wrapping phase and a GSC wrapping phase, which are shown as (d) - (f) and (j) - (l) in fig. 4, respectively, and in order to more clearly illustrate the influence of high light on the phase of a metal workpiece and the process of removing high light by combining PSC coding with multi-frequency heterodyne, any row of pixels in the phase can be selected for observation, and here, the 370 th row of sectional views in the phase are uniformly selected for observation, as shown as (a) - (c) and (g) - (i) in fig. 4.
When sinusoidal code stripes are projected onto goldAfter the PSC encoding method is applied to a workpiece, each pixel point has a unique phase value to represent the PSC encoding method, the brightness value of part of pixels exceeds 255 due to the influence of high light, the phase information recorded by the pixels is lost, for the PSC encoding method, the polarization state is unchanged after the polarized light is reflected by the metal workpiece and is not influenced by parameters such as reflection coefficient, the PSC encoding method effectively filters out the influence caused by the high reflected light, the phase information is recovered, and as can be seen from (g) -i in fig. 4, the influence of the high light leads to GSC encoding to wrap three phases、、Errors occur in the phase values between the 800 th row and the 900 th row of the 370 th row of pixels, the curves are disturbed, the PSC coding mode effectively removes the influence of high light, and the curves are smoother.
Will wrap the phase、、Taking difference two by two to obtain difference frequency phase +.>、、The global phase is finally synthesized using the wrapped phase>As can be seen from the marked areas (g) - (i) in FIG. 5, the highlight vs. wrap phase +.>、、The effect of (2) is not lost after the wrapped phase difference is taken, but is transferred to the difference frequency phase +.>、、. Whereas (a) - (c) in fig. 5 show that no error occurs between the pixels in row 370 and columns 800 to 900.
As can be seen from FIG. 5 (c), the global phase at this timeStill being wrapping phase, the wrapping phase with the frequency of 70 is selected for phase unwrapping in an experiment in consideration of the fact that the high-frequency phase has a higher signal-to-noise ratio, the combined frequency phase and the difference frequency phase are obtained through a calculation formula, the fringe order of the high-frequency phase is calculated, and then the high-frequency wrapping phase is unwrapped into an absolute phase->As can be seen from fig. 4 and 5, in the difference frequency process, there is significant background noise in the phase diagram, which needs to be filtered, and a mask is providedmaskFiltering is performed. Mask filmmaskExpressed as:
，
wherein,representing modulation intensity +.>Can be freely adjusted to a threshold value, and is obtained by verification in experiments, the +.>Setting to 0.1 can filter out substantially all background noise.
As can be seen from fig. 6 (b), the GSC method generates an error in wrapping the phase due to high light, and after multi-frequency heterodyne layer-by-layer subtraction, the 370 th row cross section curve of the obtained absolute phase generates phase jump in the interval of 800 to 900 columns, which results in failure of phase unwrapping of the part, and a void appears in the absolute phase diagram, as shown in fig. 6 (d). In the method of PSC combining multi-frequency heterodyne unwrapping phase, as can be seen from (a) in FIG. 6, the 370 th line cross section curve is very smooth, unwrapping phase is successful, and absolute phase is obtained without voids.
Finally, in order to test the quality of the absolute phases developed by the two methods, three-dimensional data reconstruction experiments are performed on the two developed absolute phases, as shown in fig. 7, as can be seen from (a) and (b) in fig. 7, due to the influence of high light, point clouds at the high light are lost, cavities appear in the packaged metal workpiece, the point clouds at the high light are weakened, scratches appear in the packaged metal workpiece at the high light, and therefore, the adopted PSC method is proved to be superior to the traditional GSC method again.
In the embodiment of the disclosure, a workpiece three-dimensional measurement method based on a multi-frequency polarization fringe technology is provided, unlike a traditional MFGC method, in the method, a traditional multi-frequency gray scale coding (MFGC) mode is replaced by a multi-frequency polarization coding (MFPC) mode, polarization information is coded into fringes with three different frequencies, namely high, medium and low, so that influence of high light on wrapping phases of metal workpieces is eliminated, and transmission of errors is blocked from the source. Because the wrapping phase with higher frequency has higher signal to noise ratio, the low-frequency vibration stripes are used for assisting the high-frequency vibration stripes, the high-frequency wrapping phase is unfolded, and the advantages of large-scale accuracy of the low-frequency stripe phase diagram and accurate detail of the high-frequency stripe phase diagram are fully exerted. The measuring range and the measuring precision of the metal workpiece are effectively improved. Through experimental comparison with the traditional multi-frequency gray scale coding (MFGC), the method has the advantages that the phase of the metal workpiece can be reliably unfolded under the influence of high light, and the complete three-dimensional information of the metal workpiece is obtained.
Example 2
As another aspect of the embodiments of the present disclosure, there is also provided a workpiece three-dimensional measurement system 100 based on a multi-frequency polarization fringe technique, as shown in fig. 8, including:
the multi-frequency polarization stripe image generation module 1 is used for acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
the frequency deformation fringe capturing module 2 is used for transmitting the generated multi-frequency polarization fringe image to a projector, projecting the multi-frequency polarization fringe image onto the surface of a workpiece, and capturing each frequency deformation fringe by a camera through a linear polarizer;
and the three-dimensional measurement module 3 calculates high-middle-low frequency wrapping phases according to the frequency deformation stripes, and expands the high-frequency wrapping phases to obtain absolute phases through the high-middle-low frequency wrapping phases so as to finish three-dimensional measurement of the workpiece.
The following describes each module of the embodiments of the present disclosure in detail.
The multi-frequency polarization stripe image generation module 1 is used for acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
the high reflectivity of the metal workpiece results in oversaturation of the multi-frequency fringe image intensity, so that partial phase information of the fringe image captured by the camera is lost, and the extracted wrapping phase information is incomplete. The minor errors of the wrapping phase can be transferred to the unfolded high-frequency phase layer by layer, so that a cavity phenomenon can occur after reconstruction. In consideration of the advantage of the multi-frequency projection fringes over the single-frequency projection fringes in the measurement range, in the embodiment, a multi-frequency polarization coding strategy is adopted to obtain polarization information of a workpiece to be measured, and a standard four-step phase shift method is adopted to generate a multi-frequency coding fringe image according to the polarization information.
The multi-frequency encoded fringe image is represented as:
，
in the middle of (a)x,y) For the pixel coordinates,for average intensity +.>In order to modulate the intensity of the light,ffor the fringe frequency to be a function of the fringe frequency,nis the number of phase shift steps.
The stokes vector is used for describing different polarization states, and the stokes expression of the multi-frequency coding stripe image is as follows:
，
wherein,representing the frequency asfFringe image total intensity of +.>A component representing linearly polarized light in the horizontal direction having a value equal to the difference in intensity to the polarization grating image of the polarizing plate rotation angles of 0 DEG and 90 DEG, & lt/EN & gt>Representing linearly polarized light component in 45 ° direction, whichPolarization grating image intensity difference of 45 DEG and 135 DEG,>representing the component of circularly polarized light with a value equal to the difference between the intensities of the left and right circularly polarized images, +.>Representing the frequency asf0℃linear polarized light intensity, +.>Representing the frequency asf90 DEG linear polarized light intensity,>representing the frequency asf45℃linear polarized light intensity,>representing the frequency asfIs a light intensity of 135 deg. linearly polarized light,representing the frequency asfRight-hand circularly polarized light intensity, < >>Representing the frequency asfLeft-handed circularly polarized light intensity of (c).
One natural light beam can be decomposed into two linearly polarized light beams with unfixed phase, mutually perpendicular and equal amplitude, and the frequency is as followsfThe total intensity of the sinusoidal grating image of (2) can be the superposition of 0 degree linear polarized light and 90 degree linear polarized light, namely:
，
the degree of polarization of the light can now be described by the polarization states of only 0 ° and 90 °.
Further, the multi-frequency polarization fringe image is represented as:
，
wherein,for average intensity +.>For modulating intensity.
Compared with the traditional multi-frequency gray scale coding, in the frequency offset coding, horizontal polarized light and vertical polarized light are coded into stripe images with three different frequencies, namely high frequency, medium frequency, low frequency and the like, and the characteristic that the polarization state of light is kept unchanged in the light transmission process is utilized, so that the influence of high light on a metal workpiece is eliminated, and layer-by-layer transmission of errors is prevented from the source.
The frequency deformation fringe capturing module 2 is used for transmitting the generated multi-frequency polarization fringe image to a projector, projecting the multi-frequency polarization fringe image onto the surface of a workpiece, and capturing each frequency deformation fringe by a camera through a linear polarizer;
the projector projects the multi-frequency polarized structured light to the metal workpiece, after the multi-frequency polarized structured light is reflected by the metal workpiece, composite light mainly composed of diffuse scattered light and specular reflected light is formed, wherein the specular reflected light still keeps the linear polarization state unchanged, the diffuse reflected light becomes unpolarized, the composite light is filtered after being captured by a camera through a linear polarizer in front of the camera, the diffuse reflected light finally reaching the camera does not change, and the specular reflected light is filtered, so that the effect of removing high light is achieved, however, due to the filtering of the polarizer, the intensity of a modulated image captured by the camera is attenuated, the contrast of the image is reduced, and the phase error is increased.
The accuracy of the phase value depends on the number of phase shift steps, fringe frequency and fringe contrast, and the variance of the phase error can be expressed as:
，
in the method, in the process of the invention,is a high oneThe variance of the additive noise of the gaussian distribution,Nfor phase shift step number>To modulate the fringe intensity.
In the present embodiment, in view of the effect of the degree of polarization in enhancing the contrast of polarized images, the degree of linear polarization is introduced to solve the above-described problem. The degree of polarization of the linear polarizer is expressed as:
，
in the method, in the process of the invention,for linear polarization degree, +.>Indicating the maximum light intensity obtained by rotating the polarizer by 0 °o +.>Representing the minimum light intensity obtained by rotating the polarizer by 90.
Optionally, each of the frequency-warping fringes captured by the camera is expressed as:
，
in the method, in the process of the invention,for the reflectivity of the workpiece and the reference plane, +.>Wrapping the phase for each frequency.
The above can be abbreviated as:
，
further, the wrapping phase and the modulation intensity can be obtained as follows:
，
，
the wrapping phase is truncated atThe interval needs to be spread out into a continuous absolute phase to facilitate the next reconstruction.
And the three-dimensional measurement module 3 calculates high-middle-low frequency wrapping phases according to the frequency deformation stripes, and expands the high-frequency wrapping phases to obtain absolute phases through the high-middle-low frequency wrapping phases so as to finish three-dimensional measurement of the workpiece.
The unwrapped wrapped phase of each frequency is expressed as:
，
wherein,for the unwrapped wrapping phase, +.>Is a fringe order.
The multi-frequency phase expansion is calculated pixel by pixelx,y) For the pixel coordinates, for convenience of description, the three frequencies are taken as examples in this embodiment, and the stripe frequencies are respectivelyf 1 、f 2 、f 3 And is also provided withf 1 >f 2 >f 3 ，And->The following relation exists:
，
in the method, in the process of the invention,Wrepresenting the field of view range of the measurement.
Based on the above formula, the high-middle-low frequency wrapping phases can be respectively calculated as、、In order to enable the phase to be unfolded in a full-field range without ambiguity, three wrapping phases and three frequencies need to be overlapped, and the frequency overlapping needs to be satisfied:
，
，
，
through the frequency superposition, the measuring field of view contains only one period of sinusoidal grating. The wrapping phase superposition needs to satisfy:
，
，
，
by wrapping the phase、、Taking the difference, superimposing the phases as less frequent phases +.>Then according to the superimposed frequency +.>And phase->The high frequency wrapping phase of the three frequencies can be further spread out.
The absolute phase obtained by unwrapping the high-frequency wrapped phase is expressed as:
，
in the method, in the process of the invention,represents the absolute phase obtained after unwrapping the high-frequency wrapped phase, < >>For a low-frequency wraparound phase composed of wraparound phases of three frequencies, < >>Representing a rounding down, a +.>Is a constant.
，/>
Higher frequency wrapped phaseWith a high signal-to-noise ratio by wrapping the phase for a plurality of different frequencies、、Step-by-step synthesis is performed to become the low-frequency wrapping phase which is unique in period in the measurement view field>Then wrap the low frequency by phase +.>、As reference phase, high frequency wrapped phase +.>The method is unfolded into absolute phases, the advantages of large-scale accuracy of the low-frequency fringe phase diagram and accurate details of the high-frequency fringe phase diagram are fully exerted, and the measuring range and measuring accuracy of the metal workpiece are effectively improved.
Based on the description of the above embodiments, the embodiments of the present disclosure can achieve the following technical effects:
(1) The traditional multi-frequency gray scale coding (MFGC) mode is replaced by a multi-frequency polarization coding (MFPC) mode, polarization information is coded into stripes with three different frequencies, namely high frequency, medium frequency and low frequency, the influence of high light on the wrapping phase of a metal workpiece is eliminated, and the transmission of errors is blocked from the source.
(2) In the method, as the wrapping phase with higher frequency has higher signal to noise ratio, the low-frequency vibration stripes are used for assisting the high-frequency vibration stripes, the high-frequency wrapping phase is unfolded, the advantages of large-scale accuracy of the low-frequency stripe phase diagram and accurate details of the high-frequency stripe phase diagram are fully exerted, and the measuring range and measuring precision of the metal workpiece are effectively improved.
Example 3
The present embodiment provides an electronic device including a memory, a processor, and a computer program stored on the memory and executable on the processor, the processor implementing the multi-frequency polarization fringe technique-based workpiece three-dimensional measurement method of embodiment 1 when executing the computer program.
Embodiment 3 of the present disclosure is merely an example, and should not be construed as limiting the functionality and scope of use of the embodiments of the present disclosure.
The electronic device may be in the form of a general purpose computing device, which may be a server device, for example. Components of an electronic device may include, but are not limited to: at least one processor, at least one memory, a bus connecting different system components, including the memory and the processor.
The buses include a data bus, an address bus, and a control bus.
The memory may include volatile memory such as Random Access Memory (RAM) and/or cache memory, and may further include Read Only Memory (ROM).
The memory may also include program means having a set (at least one) of program modules including, but not limited to: an operating system, one or more application programs, other program modules, and program data, each or some combination of which may include an implementation of a network environment.
The processor executes various functional applications and data processing by running computer programs stored in the memory.
The electronic device may also communicate with one or more external devices (e.g., keyboard, pointing device, etc.). Such communication may be through an input/output (I/O) interface. And, the electronic device may also communicate with one or more networks such as a Local Area Network (LAN), a Wide Area Network (WAN), and/or a public network, such as the Internet, through a network adapter. The network adapter communicates with other modules of the electronic device via a bus. It should be appreciated that although not shown, other hardware and/or software modules may be used in connection with an electronic device, including but not limited to: microcode, device drivers, redundant processors, external disk drive arrays, RAID (disk array) systems, tape drives, data backup storage systems, and the like.
It should be noted that although several units/modules or sub-units/modules of an electronic device are mentioned in the above detailed description, such a division is merely exemplary and not mandatory. Indeed, the features and functionality of two or more units/modules described above may be embodied in one unit/module according to embodiments of the present application. Conversely, the features and functions of one unit/module described above may be further divided into ones that are embodied by a plurality of units/modules.
Example 4
The present embodiment provides a computer-readable storage medium storing a computer program which, when executed by a processor, implements the steps of the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technique in embodiment 1.
More specifically, among others, readable storage media may be employed including, but not limited to: portable disk, hard disk, random access memory, read only memory, erasable programmable read only memory, optical storage device, magnetic storage device, or any suitable combination of the foregoing.
In a possible embodiment, the disclosure may also be implemented in the form of a program product comprising program code for causing a terminal device to carry out the steps of implementing the method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique described in example 1, when said program product is run on the terminal device.
Wherein the program code for carrying out the present disclosure may be written in any combination of one or more programming languages, which program code may execute entirely on the user device, partly on the user device, as a stand-alone software package, partly on the user device, partly on the remote device or entirely on the remote device.
Although embodiments of the present disclosure have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the disclosure, the scope of which is defined in the appended claims and their equivalents.
Claims (6)
1. The workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology is characterized by comprising the following steps of:
acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
transmitting the generated multi-frequency polarized stripe image to a projector, projecting the multi-frequency polarized stripe image onto the surface of a workpiece, and capturing the multi-frequency polarized stripe image by a camera through a linear polarizer to obtain each frequency deformation stripe;
calculating high-frequency, medium-frequency and low-frequency wrapping phases according to each frequency deformation stripe, and expanding the high-frequency wrapping phases through the high-frequency, medium-frequency and low-frequency wrapping phases to obtain absolute phases, so that three-dimensional measurement of a workpiece is completed;
the multi-frequency encoded fringe image is represented as:
，
in the middle of (a)x,y) For the pixel coordinates,for average intensity +.>In order to modulate the intensity of the light,ffor the fringe frequency to be a function of the fringe frequency,nis the phase shift step number;
the multi-frequency polarization fringe image is represented as:
，
wherein,for average intensity +.>Is the modulation intensity;
the various frequency distortion fringes captured by the camera are represented as:
，
in the method, in the process of the invention,for the reflectivity of the workpiece and the reference plane, +.>Wrapping phases for each frequency;
the unwrapped wrapped phase of each frequency is expressed as:
，
wherein,for the unwrapped wrapping phase, +.>Is a fringe order.
2. The method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique according to claim 1, wherein the degree of polarization of the linear polarizer is expressed as:
，
in the method, in the process of the invention,for linear polarization degree, +.>Indicating the maximum light intensity obtained by rotating the polarizer by 0 °o +.>Representing the minimum light intensity obtained by rotating the polarizer by 90.
3. The method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique according to claim 1, wherein the absolute phase obtained by unwrapping the high-frequency wrapping phase is expressed as:
，
in the method, in the process of the invention,represents the absolute phase obtained after unwrapping the high-frequency wrapped phase, < >>For a low-frequency wraparound phase composed of wraparound phases of three frequencies, < >>Representing a rounding down, a +.>Is a constant.
4. A workpiece three-dimensional measurement system based on a multi-frequency polarization fringe technique, comprising:
the multi-frequency polarization stripe image generation module is used for acquiring polarization information of a workpiece to be measured, generating a multi-frequency coding stripe image by adopting a standard four-step phase shift method according to the polarization information, coding the multi-frequency coding stripe image into stripe images with three different frequencies, namely high, medium and low, and generating a multi-frequency polarization stripe image;
the frequency deformation fringe capturing module is used for transmitting the generated multi-frequency polarization fringe image to the projector and projecting the multi-frequency polarization fringe image onto the surface of the workpiece, and capturing each frequency deformation fringe by the camera through the linear polarizer;
the three-dimensional measurement module calculates high-middle-low frequency wrapping phases according to the deformation stripes of each frequency, and expands the high-frequency wrapping phases to obtain absolute phases through the high-middle-low frequency wrapping phases so as to finish three-dimensional measurement of the workpiece;
the multi-frequency encoded fringe image is represented as:
，
in the middle of (a)x,y) For the pixel coordinates,for average intensity +.>In order to modulate the intensity of the light,ffor the fringe frequency to be a function of the fringe frequency,nis the phase shift step number;
the multi-frequency polarization fringe image is represented as:
，
wherein,for average intensity +.>Is the modulation intensity;
the various frequency distortion fringes captured by the camera are represented as:
，
in the method, in the process of the invention,for the reflectivity of the workpiece and the reference plane, +.>Wrapping phases for each frequency;
the unwrapped wrapped phase of each frequency is expressed as:
，
wherein,for the unwrapped wrapping phase, +.>Is a fringe order.
5. An electronic device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, characterized in that the processor implements the method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique according to any of claims 1 to 3 when executing the computer program.
6. A computer-readable storage medium, on which a computer program is stored, characterized in that the program, when being executed by a processor, implements the method for three-dimensional measurement of a workpiece based on the multi-frequency polarization stripe technique as claimed in any one of claims 1 to 3.
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