CN117405044B - Workpiece three-dimensional measurement method and system based on multi-frequency polarization stripe technology - Google Patents

Abstract

The present disclosure relates to a three-dimensional measurement method and system for workpieces based on multi-frequency polarization fringe technology. The method includes the following steps: obtaining polarization information of the workpiece to be measured, and using a standard four-step phase shift method to generate multi-frequency encoding fringes based on the polarization information. image, the multi-frequency encoded fringe image is encoded into fringe images with three different frequencies: high, medium and low to generate a multi-frequency polarized fringe image, which eliminates the impact of highlight on the wrapping phase of the workpiece; the generated multi-frequency polarized fringe image is Sent to the projector and projected onto the surface of the workpiece, the warp polarizer is captured by the camera to obtain the deformation fringes of each frequency; the high, middle and low frequency wrapping phases are calculated based on the deformation fringes of each frequency, and the high frequency wrapping phase is expanded to obtain the absolute value through the high, middle and low frequency wrapping phases. Phase, to complete the three-dimensional measurement of the workpiece, the disclosed low-frequency polarization stripes are used to assist the high-frequency polarization stripes, expand the high-frequency wrapped phase, and improve the measurement accuracy and measurement range.

Description

Workpiece three-dimensional measurement method and system based on multi-frequency polarization fringe technology

Technical Field

The present disclosure relates to the field of visual sensor measurement technology, and in particular to a three-dimensional measurement method and system for a workpiece based on multi-frequency polarization fringe technology.

Background Art

With the development of modern industrial technology, a large number of metal workpieces of different shapes are used in various occasions. In order to ensure the applicability and accuracy of metal workpieces, the three-dimensional profile measurement of metal workpieces has become increasingly important. The most typical measurement method is fringe projection technology, in which a fringe image is generated in a computer using a certain coding algorithm, projected onto the surface of the metal workpiece to be measured by a projector, and then the camera collects the fringe image modulated by the metal workpiece from different angles. The fringe image contains the phase information of the metal workpiece, which is extracted from the fringe image by a certain phase solution algorithm. The three-dimensional profile information of the metal workpiece is measured by the phase information. Undoubtedly, in the entire measurement process, the accurate acquisition and precise expansion of the metal workpiece phase information is the most important, because it directly affects the accuracy and efficiency of three-dimensional measurement.

In fringe projection technology, commonly used phase unwrapping methods are divided into two categories: spatial domain unwrapping methods and time domain unwrapping methods. Among them, the time domain unwrapping method is further divided into two unwrapping methods: single-frequency phase shift and multi-frequency phase shift. Since our previous research was mainly based on the single-frequency phase shift method, although single-frequency polarization encoding can effectively eliminate the influence of highlights in the three-dimensional measurement of metal workpieces, when measuring metal workpieces with large surface slopes, the identifiable slope range is small. At the same time, considering the measurement speed, the number of projected fringe images should not be too large, so a low frequency of 16 is often used in single-frequency polarization encoding, which leads to increased measurement errors when measuring metal workpieces with complex surfaces.

Summary of the invention

The present disclosure provides a method and system for three-dimensional measurement of workpieces based on multi-frequency polarization stripe technology, which can solve the problem that the high reflectivity of the workpiece causes errors in the wrapped phase of the multi-frequency stripes, and the errors are transmitted layer by layer, resulting in an increase in the absolute phase error or even inability to unfold, thereby affecting the measurement accuracy and measurement range. In order to solve the above technical problems, the present disclosure provides the following technical solutions:

As one aspect of an embodiment of the present disclosure, a method for three-dimensional measurement of a workpiece based on a multi-frequency polarization fringe technology is provided, comprising the following steps:

Obtaining polarization information of the workpiece to be measured, generating a multi-frequency coded fringe image according to the polarization information using a standard four-step phase shift method, encoding the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image;

The generated multi-frequency polarization fringe image is sent to a projector and projected onto the surface of the workpiece, and the deformed fringe of each frequency is captured by a camera through a linear polarizer;

The high, medium and low frequency wrapping phases are calculated based on the deformation fringes of each frequency, and the high frequency wrapping phase is expanded by the high, medium and low frequency wrapping phases to obtain the absolute phase, thus completing the three-dimensional measurement of the workpiece;

The multi-frequency coded fringe image is represented as:

,

Where ( x , y ) is the pixel coordinate, is the average intensity, is the modulation intensity, f is the fringe frequency, n is the number of phase shift steps;

The multi-frequency polarization fringe image is expressed as:

,

in, is the average intensity, is the modulation intensity;

The frequency deformation fringes captured by the camera are expressed as:

,

In the formula, is the reflectivity of the workpiece and the reference plane, is the wrap phase for each frequency;

The wrapped phase of each frequency is unfolded to obtain the unfolded wrapped phase as:

,

in, is the wrapped phase after expansion, is the number of stripes.

Optionally, the polarization degree of the linear polarizer is expressed as:

,

In the formula, is the degree of linear polarization, It represents the maximum light intensity obtained when the polarizer is rotated 0°. Indicates the minimum light intensity obtained when the polarizer is rotated 90°.

Optionally, the absolute phase obtained by unfolding the high-frequency wrapped phase is expressed as:

,

In the formula, It represents the absolute phase obtained after the high-frequency wrapped phase is unfolded. is the low-frequency wrapping phase synthesized by the wrapping phases of three frequencies, Indicates rounding down. is a constant.

As another aspect of the embodiments of the present disclosure, a workpiece three-dimensional measurement system based on multi-frequency polarization fringe technology is provided, comprising:

A multi-frequency polarization fringe image generation module is used to obtain polarization information of the workpiece to be measured, generate a multi-frequency coded fringe image according to the polarization information using a standard four-step phase shift method, encode the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image;

The frequency deformation fringe capture module sends the generated multi-frequency polarization fringe image to the projector and projects it onto the surface of the workpiece. The camera captures each frequency deformation fringe through the linear polarizer.

The three-dimensional measurement module calculates the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expands the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thus completing the three-dimensional measurement of the workpiece.

As another aspect of an embodiment of the present disclosure, an electronic device is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the above-mentioned three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology when executing the computer program.

As another aspect of an embodiment of the present disclosure, a computer-readable storage medium is provided, on which a computer program is stored, and when the program is executed by a processor, the three-dimensional measurement method of a workpiece based on the multi-frequency polarization stripe technology is implemented.

Compared with the prior art, the beneficial effects of the present disclosure are:

1. The traditional multi-frequency grayscale coding (MFGC) mode of the present invention is replaced by the multi-frequency polarization coding (MFPC) mode. The polarization information is encoded into stripes of three different frequencies: high, medium and low. This eliminates the influence of the high light on the metal workpiece wrapping phase, and the error transmission is blocked from the source.

2. In the present invention, since the higher frequency wrapped phase has a higher signal-to-noise ratio, the low-frequency polarization stripes are used to assist the high-frequency polarization stripes to expand the high-frequency wrapped phase. The advantages of the large-scale accuracy of the low-frequency fringe phase image and the accurate details of the high-frequency fringe phase image are fully utilized, and the measurement range and measurement accuracy of metal workpieces are effectively improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a method for three-dimensional measurement of a workpiece based on multi-frequency polarization fringe technology in Example 1;

FIG2 is a schematic diagram of a metal workpiece to be measured in Example 1;

FIG3 is a schematic diagram of the PSC image contrast comparison in Example 1;

FIG4 is a schematic diagram of a wrapped phase and a 370th row pixel curve in Example 1;

FIG5 is a schematic diagram of a difference frequency phase and a 370th row pixel curve in Example 1;

FIG6 is a schematic diagram of the absolute phase and the 370th row of pixel curves in Example 1;

FIG7 is a schematic diagram of a three-dimensional profile of a metal workpiece in Example 1;

FIG8 is a block diagram of a workpiece three-dimensional measurement system based on multi-frequency polarization fringe technology in Example 2.

DETAILED DESCRIPTION

Various exemplary embodiments, features and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise specified.

The word “exemplary” is used exclusively herein to mean “serving as an example, example, 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" herein is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and/or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the term "at least one" herein represents any combination of at least two of any one or more of a plurality of. For example, including at least one of A, B, and C can represent including any one or more elements selected from the set consisting of A, B, and C.

In addition, in order to better illustrate the present disclosure, numerous specific details are given in the following specific embodiments. It should be understood by those skilled in the art that the present disclosure can also be implemented without certain specific details. In some examples, methods, means, components and circuits well known to those skilled in the art are not described in detail in order to highlight the subject matter of the present disclosure.

It can be understood that the above-mentioned various method embodiments mentioned in the present disclosure can be combined with each other to form a combined embodiment without violating the principle logic. Due to space limitations, the present disclosure will not go into details.

In addition, the present disclosure also provides a three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology and a system thereof, which can be used to implement any three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology provided by the present disclosure. The corresponding technical solutions and descriptions can be found in the corresponding records in the method section and will not be repeated here.

The execution subject of the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology can be a computer or other workpiece three-dimensional measurement device that can implement the multi-frequency polarization stripe technology. For example, the method can be executed by a terminal device or a server or other processing device, wherein the terminal device can be a user equipment (User Equipment, UE), a mobile device, a user terminal, a terminal, a cellular phone, a cordless phone, a personal digital assistant (Personal Digital Assistant, PDA), a handheld device, a computing device, a vehicle-mounted device, a wearable device, etc. In some possible implementations, the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology can be implemented by a processor calling a computer-readable instruction stored in a memory.

Example 1

This embodiment provides a method for three-dimensional measurement of a workpiece based on multi-frequency polarization fringe technology, comprising the following steps:

S10, obtaining polarization information of the workpiece to be measured, generating a multi-frequency coded fringe image according to the polarization information by using a standard four-step phase shift method, encoding the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image;

S20, sending the generated multi-frequency polarization stripe image to a projector and projecting it onto the surface of the workpiece, and capturing the deformed stripes of each frequency by a camera through a linear polarizer;

S30, calculating the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expanding the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thereby completing the three-dimensional measurement of the workpiece.

In this embodiment, the implementation of the workpiece three-dimensional measurement method based on the multi-frequency polarization stripe technology specifically includes the following steps, and its algorithm flow chart is shown in Figure 1. The following is a detailed description of each step of the embodiment of the present disclosure.

S10, obtaining polarization information of the workpiece to be measured, generating a multi-frequency coded fringe image according to the polarization information using a standard four-step phase shift method, encoding the multi-frequency coded fringe image into fringe images of three different frequencies, high, medium and low, to generate a multi-frequency polarization fringe image.

The high reflectivity of metal workpieces causes the intensity of the multi-frequency fringe image to be oversaturated, resulting in the loss of some phase information in the fringe image captured by the camera, making the extracted wrapped phase information incomplete. The slight error of the wrapped phase will be passed layer by layer to the unfolded high-frequency phase, resulting in the "hole" phenomenon after reconstruction. Considering the advantage of multi-frequency projection fringes over single-frequency projection fringes in terms of measurement range, a multi-frequency polarization encoding strategy is adopted in this embodiment to obtain the polarization information of the workpiece to be measured, and a multi-frequency encoded fringe image is generated using a standard four-step phase shift method based on the polarization information.

The multi-frequency coded fringe image is represented as:

,

Where ( x , y ) is the pixel coordinate, is the average intensity, is the modulation intensity, f is the fringe frequency, and n is the number of phase shift steps.

The Stokes vector is used to describe different polarization states. The Stokes expression of the multi-frequency coded fringe image is:

,

in, represents the total intensity of the fringe image with frequency f , It represents the component of linear polarized light in the horizontal direction, and its value is equal to the intensity difference of the polarization grating image when the polarizer is rotated at 0° and 90°. It represents the linear polarized light component in the 45° direction, and its value is the intensity difference between the 45° and 135° polarization grating images. Represents the component of circularly polarized light, and its value is equal to the difference between the intensities of left and right circularly polarized images. It represents the intensity of 0° linear polarized light with frequency f , It represents the intensity of 90° linear polarized light with frequency f , It represents the intensity of 45° linear polarized light with frequency f , It represents the intensity of 135° linear polarized light with frequency f , It represents the intensity of right-hand circularly polarized light with frequency f , Represents the intensity of left-handed circularly polarized light with a frequency of f .

A beam of natural light can be decomposed into two beams of linearly polarized light with non-fixed phases, perpendicular to each other, and equal amplitudes. From the above formula, we can see that the total intensity of the sinusoidal grating image with a frequency of f can be the superposition of 0° linear polarized light and 90° linear polarized light, that is:

,

At this time, the polarization degree of light can be described only by the polarization states of 0° and 90°.

Furthermore, the multi-frequency polarization fringe image is represented as:

,

in, is the average intensity, is the modulation intensity.

Compared with traditional multi-frequency grayscale coding, in frequency polarization coding, horizontally polarized light and vertically polarized light are encoded into stripe images of three different frequencies: high, medium and low. The characteristic that the polarization state of light remains unchanged during the transmission of light is utilized to eliminate the influence of high light on metal workpieces, and the transmission of errors layer by layer is prevented at the source.

S20, sending the generated multi-frequency polarization stripe image to a projector and projecting it onto the surface of the workpiece, and capturing the deformed stripes of each frequency by a camera through a linear polarizer.

The projector projects multi-frequency polarized structured light onto the metal workpiece. After reflection from the metal workpiece, a composite light mainly composed of diffuse scattered light and specular reflected light is formed. The specular reflected light still maintains the linear polarization state without change, while the diffuse reflected light becomes unpolarized light. The composite light beam is captured by the camera and then filtered after passing through the linear polarizer in front of the camera. Finally, the intensity of the diffuse reflected light reaching the camera does not change, and the specular reflected light is filtered out, thereby achieving the effect of removing highlights. However, due to the filtering of the polarizer, the intensity of the modulated image captured by the camera is attenuated, resulting in reduced image contrast and increased phase error.

The accuracy of the phase value depends on the number of phase shift steps, fringe frequency and fringe contrast. The variance of the phase error can be expressed as:

,

In the formula, is the variance of a Gaussian distributed additive noise, N is the number of phase shift steps, is the modulation fringe intensity.

In this embodiment, considering that the polarization degree has the function of enhancing the contrast of polarization images, the linear polarization degree is introduced to solve the above problem. The polarization degree of the linear polarizer is expressed as:

,

In the formula, is the degree of linear polarization, It represents the maximum light intensity obtained when the polarizer is rotated 0°. Indicates the minimum light intensity obtained when the polarizer is rotated 90°.

Optionally, each frequency deformation fringes captured by the camera is expressed as:

,

In the formula, is the reflectivity of the workpiece and the reference plane, is the wrap phase for each frequency.

The above formula can be simplified as:

,

Furthermore, the wrapping phase and modulation intensity can be obtained as:

,

,

The wrapped phase is truncated at The interval needs to be expanded into a continuous absolute phase to facilitate the next step of reconstruction.

S30, calculating the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expanding the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thereby completing the three-dimensional measurement of the workpiece.

The wrapped phase of each frequency is unfolded to obtain the unfolded wrapped phase as:

,

in, is the wrapped phase after expansion, is the number of stripes.

The multi-frequency phase unwrapping is calculated pixel _by pixel. ( x , y ) is the pixel coordinate. For the convenience of description, it is omitted below. In this embodiment _, three frequencies are taken as an example. _{Assume that the fringe frequencies are f1} , f2 _{, and f3} , and f1>f2 > _f3 . and There are the following relationships between them:

,

Where W represents the field of view of the measurement.

Based on the above formula, the high, medium and low frequency package phases can be calculated as follows: , , In order to make the phase unfold unambiguously in the whole field, it is necessary to superimpose the three wrapped phases and three frequencies. The frequency superposition must satisfy:

,

,

,

Through the above frequency superposition, the measurement field of view will eventually contain only one period of sinusoidal grating. The wrapped phase superposition must meet the following requirements:

,

,

,

By wrapping the phase , , Take the difference and superimpose the phase into a phase with a lower frequency , and then according to the superimposed frequency and Phase , the high-frequency wrapped phase in the three frequencies can be further expanded.

The absolute phase obtained by unfolding the high-frequency wrapped phase is expressed as:

,

In the formula, It represents the absolute phase obtained after the high-frequency wrapped phase is unfolded. is the low-frequency wrapping phase synthesized by the wrapping phases of three frequencies, Indicates rounding down. is a constant.

,

The higher frequency wrapping phase has a higher signal-to-noise ratio. , , Perform step-by-step synthesis to transform the low-frequency wrapped phase into a unique period in the measurement field , and then wrap the low frequency phase , As the reference phase, the high frequency wrapping phase Expanding into absolute phase gives full play to the advantages of large-scale accuracy of low-frequency fringe phase image and accurate details of high-frequency fringe phase image, effectively improving the measurement range and accuracy of metal workpieces.

Furthermore, in order to verify the effectiveness of the workpiece three-dimensional measurement method based on multi-frequency polarization fringe technology, a three-dimensional shape measurement system based on polarization state superposition sinusoidal coding technology was built for measurement. It includes a 3LCD projector (EPSON-CB965) with a resolution of 1024*768, a monochrome camera (FLIRFLIR BFS-U3-23-23-3M-C) with a resolution adjusted to 1024*768, and a 1/4 wave plate horizontal polarizer. The projector and the camera are placed left and right, with an angle of less than 10° between them. The metal workpiece to be measured is about 1000 cm away from the camera lens. The system uses the eight-parameter method for system calibration, and uses four-step phase shift and three-frequency heterodyne for phase unfolding.

In order to compare the performance of PSC multi-frequency heterodyne method and GSC multi-frequency heterodyne method in measuring high gloss metal workpieces, we measured a metal workpiece with a glossy surface, as shown in Figure 2 (a). The red marked area in the figure has quite serious saturation. , , Three groups of frequencies were used to generate 12 projection fringe images, 4 for each frequency, with a lateral resolution of 768 pixels and a longitudinal resolution of 1024 pixels. The system was calibrated under the same conditions, keeping the camera and projector fixed, and projecting PSC and GSC coded images onto the metal workpiece. The images captured by the camera are shown in Figure 2 (b) and (c). It can be seen from the figure that the overexposure in Figure 2 (b) has been resolved, but the contrast is much lower than that in Figure 2 (c).

In order to improve the contrast reduction caused by polarizer attenuation, the fringe image after the metal workpiece is modulated by the camera, and the linear polarization degree needs to be calculated to enhance the contrast, so as to achieve the purpose of improving the signal-to-noise ratio. The image intensity captured in the vertical polarization state is recorded as , the image intensity captured in the horizontal polarization state is recorded as ,The DoLP image is obtained by calculation, as shown in Figure 3.,From the experimental results, it can be seen that the contrast of the enhanced DoLP image is greatly improved.

In order to accurately evaluate the degree of image contrast enhancement, the contrast of the image before and after enhancement was calculated. According to the commonly used four-nearest neighbor ratio calculation method, the contrast calculation formula is:

,

In the formula, is the pixel difference between adjacent pixels m and n , that is , The grayscale difference between adjacent pixels is The distribution probability of .

The experiment selected four phase shift images of each frequency, each with a phase shift of 0 and The contrast statistics are shown in Table 1.

Table 1

It can be seen from Table 1 that after the linear polarization degree is enhanced, the contrast of the image is increased by two times, which greatly improves the image quality and enhances the image's anti-noise ability.

Substitute the enhanced PSC image and the captured GSC image into the wrapping phase calculation formula to obtain the PSC wrapping phase and the GSC wrapping phase, as shown in (d)-(f) and (j)-(l) in Figure 4, respectively. In order to more clearly illustrate the impact of highlights on the phase of metal workpieces and the process of removing highlights by PSC encoding combined with multi-frequency heterodyne, any row of pixels in the phase can be selected for observation. Here, the cross-sectional image of the 370th row in the phase is uniformly selected for observation, as shown in (a)-(c) and (g)-(i) in Figure 4.

When the sinusoidal coded stripes are projected onto the metal workpiece, each pixel has a unique phase value to characterize it. Due to the influence of highlights, the brightness value of some pixels exceeds 255, and the phase information recorded by the pixel is lost. For the PSC coding method, since the polarization states of 0° and 90° are superimposed on the sinusoidal coding, the polarization state will remain unchanged after the polarized light is reflected by the metal workpiece, and the polarization state is not affected by parameters such as the reflection coefficient. Therefore, the PSC coding method effectively filters out the influence of high reflection light and restores the phase information. It can be seen from (g)-(i) in Figure 4 that the influence of highlights causes the three wrapped phases of GSC coding to be , , There is an error in the phase value of the pixel interval from 800 to 900 columns in row 370, and the curve is disturbed. However, the PSC encoding method effectively removes the influence of highlights, and the curve is smoother.

Wrap Phase , , Take the difference between the two and get the difference frequency phase , , , and finally the global phase is synthesized using the wrapped phase , it can be seen from the marked areas (g)-(i) in Figure 5 that the highlight has an impact on the wrapped phase , , The influence of does not disappear after the wrapping phase difference, but is transferred to the difference frequency phase , , However, (a)-(c) in Figure 5 show that there is no error between the pixels in row 370 and columns 800 to 900.

As can be seen from (c) in Figure 5, the global phase It is still a wrapped phase, which needs to be further unfolded. Considering that the high-frequency phase has a higher signal-to-noise ratio, the experiment selects a wrapped phase with a frequency of 70 for phase unfolding. First, the sum frequency and difference frequency phases are obtained by the calculation formula, the fringe order of the high-frequency phase is calculated, and then the high-frequency wrapped phase is unfolded into the absolute phase. As can be seen from Figures 4 and 5, in the frequency difference process, there is obvious background noise in the phase diagram, which needs to be filtered out. A mask is set here to filter out the noise. The mask is expressed as:

,

in, represents the modulation intensity, is a threshold value, which can be adjusted freely. It is verified in the experiment that Setting it to 0.1 can basically filter out all background noise.

The absolute phase after unwrapping is shown in (c) and (d) in Figure 6. As can be seen from (b) in Figure 6, the GSC method causes errors in the wrapped phase due to highlights. After multi-frequency heterodyne layer-by-layer difference, the 370th row cross-sectional curve of the absolute phase obtained has a phase jump in the 800 to 900 column interval, resulting in the failure of phase unwrapping in this part, and a hole appears in the absolute phase diagram, as shown in (d) in Figure 6. As can be seen from (a) in Figure 6, the 370th row cross-sectional curve of the proposed PSC combined with multi-frequency heterodyne phase unwrapping method is very smooth, the unwrapping phase is successful, and the absolute phase is obtained without a hole.

Finally, in order to verify the quality of the absolute phases unfolded by the two methods, a 3D data reconstruction experiment was carried out on the two unfolded absolute phases, as shown in Figure 7. It can be seen from (a) and (b) in Figure 7 that due to the influence of highlights, the point cloud is missing at the highlights, holes appear in the packaged metal workpiece, the point cloud density becomes weak at the highlights, and scratches appear on the packaged metal workpiece at the highlights. This once again proves that the adopted PSC method is superior to the traditional GSC method.

In the disclosed embodiment, a three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology is proposed. Different from the traditional MFGC method, in this method, the traditional multi-frequency grayscale coding (MFGC) mode is replaced by the multi-frequency polarization coding (MFPC) mode, and the polarization information is encoded into stripes of three different frequencies, high, medium and low, to eliminate the influence of high light on the metal workpiece wrapping phase, and the transmission of errors is blocked from the source. Since the higher frequency wrapping phase has a higher signal-to-noise ratio, the low-frequency polarization stripes are used to assist the high-frequency polarization stripes to unfold the high-frequency wrapping phase, and the advantages of the large-scale accuracy of the low-frequency stripe phase map and the accurate details of the high-frequency stripe phase map are fully utilized. The measurement range and measurement accuracy of metal workpieces are effectively improved. After experimental comparison with the traditional multi-frequency grayscale coding (MFGC), it is proved that through this method, 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 can be obtained.

Example 2

As another aspect of the embodiment of the present disclosure, a workpiece three-dimensional measurement system 100 based on multi-frequency polarization fringe technology is also provided, as shown in FIG8 , including:

The multi-frequency polarization fringe image generation module 1 obtains polarization information of the workpiece to be measured, generates a multi-frequency coded fringe image according to the polarization information by using a standard four-step phase shift method, and encodes the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image;

The frequency deformation fringe capturing module 2 sends the generated multi-frequency polarization fringe image to the projector and projects it onto the surface of the workpiece, and each frequency deformation fringe is captured by the camera through the linear polarizer;

The three-dimensional measurement module 3 calculates the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expands the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thereby completing the three-dimensional measurement of the workpiece.

The following is a detailed description of each module in the embodiment of the present disclosure.

The multi-frequency polarization fringe image generation module 1 obtains polarization information of the workpiece to be measured, generates a multi-frequency coded fringe image according to the polarization information by using a standard four-step phase shift method, and encodes the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image;

The high reflectivity of metal workpieces causes the intensity of the multi-frequency fringe image to be oversaturated, resulting in the loss of some phase information in the fringe image captured by the camera, making the extracted wrapped phase information incomplete. The slight error of the wrapped phase will be passed layer by layer to the unfolded high-frequency phase, resulting in the "hole" phenomenon after reconstruction. Considering the advantage of multi-frequency projection fringes over single-frequency projection fringes in terms of measurement range, a multi-frequency polarization encoding strategy is adopted in this embodiment to obtain the polarization information of the workpiece to be measured, and a multi-frequency encoded fringe image is generated using a standard four-step phase shift method based on the polarization information.

The multi-frequency coded fringe image is represented as:

,

Where ( x , y ) is the pixel coordinate, is the average intensity, is the modulation intensity, f is the fringe frequency, and n is the number of phase shift steps.

The Stokes vector is used to describe different polarization states. The Stokes expression of the multi-frequency coded fringe image is:

,

in, represents the total intensity of the fringe image with frequency f , It represents the component of linear polarized light in the horizontal direction, and its value is equal to the intensity difference of the polarization grating image when the polarizer is rotated at 0° and 90°. It represents the linear polarized light component in the 45° direction, and its value is the intensity difference between the 45° and 135° polarization grating images. Represents the component of circularly polarized light, and its value is equal to the difference between the intensities of left and right circularly polarized images. It represents the intensity of 0° linear polarized light with frequency f , It represents the intensity of 90° linear polarized light with frequency f , It represents the intensity of 45° linear polarized light with frequency f , It represents the intensity of 135° linear polarized light with frequency f , It represents the intensity of right-hand circularly polarized light with frequency f , Represents the intensity of left-handed circularly polarized light with a frequency of f .

A beam of natural light can be decomposed into two beams of linearly polarized light with non-fixed phases, perpendicular to each other, and equal amplitudes. From the above formula, we can see that the total intensity of the sinusoidal grating image with a frequency of f can be the superposition of 0° linear polarized light and 90° linear polarized light, that is:

,

At this time, the polarization degree of light can be described only by the polarization states of 0° and 90°.

Furthermore, the multi-frequency polarization fringe image is represented as:

,

in, is the average intensity, is the modulation intensity.

Compared with traditional multi-frequency grayscale coding, in frequency polarization coding, horizontally polarized light and vertically polarized light are encoded into stripe images of three different frequencies: high, medium and low. The characteristic that the polarization state of light remains unchanged during the transmission of light is utilized to eliminate the influence of high light on metal workpieces, and the transmission of errors layer by layer is prevented at the source.

The frequency deformation fringe capturing module 2 sends the generated multi-frequency polarization fringe image to the projector and projects it onto the surface of the workpiece, and each frequency deformation fringe is captured by the camera through the linear polarizer;

The projector projects multi-frequency polarized structured light onto the metal workpiece. After reflection from the metal workpiece, a composite light mainly composed of diffuse scattered light and specular reflected light is formed. The specular reflected light still maintains the linear polarization state without change, while the diffuse reflected light becomes unpolarized light. The composite light beam is captured by the camera and then filtered after passing through the linear polarizer in front of the camera. Finally, the intensity of the diffuse reflected light reaching the camera does not change, and the specular reflected light is filtered out, thereby achieving the effect of removing highlights. However, due to the filtering of the polarizer, the intensity of the modulated image captured by the camera is attenuated, resulting in reduced image contrast and increased phase error.

The accuracy of the phase value depends on the number of phase shift steps, fringe frequency and fringe contrast. The variance of the phase error can be expressed as:

,

In the formula, is the variance of a Gaussian distributed additive noise, N is the number of phase shift steps, is the modulation fringe intensity.

In this embodiment, considering that the polarization degree has the function of enhancing the contrast of polarization images, the linear polarization degree is introduced to solve the above problem. The polarization degree of the linear polarizer is expressed as:

,

In the formula, is the degree of linear polarization, It represents the maximum light intensity obtained when the polarizer is rotated 0°. Indicates the minimum light intensity obtained when the polarizer is rotated 90°.

Optionally, each frequency deformation fringes captured by the camera is expressed as:

,

In the formula, is the reflectivity of the workpiece and the reference plane, is the wrap phase for each frequency.

The above formula can be simplified as:

,

Furthermore, the wrapping phase and modulation intensity can be obtained as:

,

,

The wrapped phase is truncated at The interval needs to be expanded into a continuous absolute phase to facilitate the next step of reconstruction.

The three-dimensional measurement module 3 calculates the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expands the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thereby completing the three-dimensional measurement of the workpiece.

The wrapped phase of each frequency is unfolded to obtain the unfolded wrapped phase as:

,

in, is the wrapped phase after expansion, is the number of stripes.

The multi-frequency phase unwrapping is calculated pixel _by pixel. ( x , y ) is the pixel coordinate. For the convenience of description, it is omitted below. In this embodiment _, three frequencies are taken as an example. _{Assume that the fringe frequencies are f1} , f2 _{, and f3} , and f1>f2 > _f3 . and There are the following relationships between them:

,

Where W represents the field of view of the measurement.

Based on the above formula, the high, medium and low frequency package phases can be calculated as follows: , , In order to make the phase unfold unambiguously in the whole field, it is necessary to superimpose the three wrapped phases and three frequencies. The frequency superposition must satisfy:

,

,

,

Through the above frequency superposition, the measurement field of view will eventually contain only one period of sinusoidal grating. The wrapped phase superposition must meet the following requirements:

,

,

,

By wrapping the phase , , Take the difference and superimpose the phase into a phase with a lower frequency , and then according to the superimposed frequency and Phase , the high-frequency wrapped phase in the three frequencies can be further expanded.

The absolute phase obtained by unfolding the high-frequency wrapped phase is expressed as:

,

In the formula, It represents the absolute phase obtained after the high-frequency wrapped phase is unfolded. is the low-frequency wrapping phase synthesized by the wrapping phases of three frequencies, Indicates rounding down. is a constant.

,

The higher frequency wrapping phase has a higher signal-to-noise ratio. , , Perform step-by-step synthesis to transform the low-frequency wrapped phase into a unique period in the measurement field , and then wrap the low frequency phase , As the reference phase, the high frequency wrapping phase Expanding into absolute phase gives full play to the advantages of large-scale accuracy of low-frequency fringe phase image and accurate details of high-frequency fringe phase image, effectively improving the measurement range and accuracy of metal workpieces.

Based on the description of the above embodiments, it can be seen that the embodiments of the present disclosure can achieve the following technical effects:

(1) The traditional multi-frequency grayscale coding (MFGC) mode of the present invention is replaced by a multi-frequency polarization coding (MFPC) mode. The polarization information is encoded into stripes of three different frequencies: high, medium and low. This eliminates the influence of high light on the wrapping phase of the metal workpiece, and the transmission of errors is blocked at the source.

(2) In the present invention, since the higher frequency wrapped phase has a higher signal-to-noise ratio, the low-frequency polarization stripes are used to assist the high-frequency polarization stripes to expand the high-frequency wrapped phase. The advantages of the large-scale accuracy of the low-frequency stripe phase image and the detailed accuracy of the high-frequency stripe phase image are fully utilized, and the measurement range and measurement accuracy of the metal workpiece are effectively improved.

Example 3

This embodiment provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the three-dimensional measurement method for workpieces based on the multi-frequency polarization stripe technology in Embodiment 1 is implemented.

Embodiment 3 of the present disclosure is merely an example and should not bring any limitation to the functions 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, for example, it may be a server device. The components of the electronic device may include, but are not limited to: at least one processor, at least one memory, and a bus connecting different system components (including the memory and the processor).

The bus includes data bus, address bus and 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 a program tool having a set (at least one) of program modules, such program modules including but not limited to: an operating system, one or more application programs, other program modules and program data, each of which or some combination may include the 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., keyboards, pointing devices, etc.). Such communication may be performed through an input/output (I/O) interface. Furthermore, the electronic device may also communicate with one or more networks (e.g., local area networks (LANs), wide area networks (WANs), and/or public networks, such as the Internet) through a network adapter. The network adapter communicates with other modules of the electronic device through a bus. It should be understood that, although not shown in the figure, other hardware and/or software modules may be used in conjunction with the electronic device, including but not limited to: microcode, device drivers, redundant processors, external disk drive arrays, RAID (disk array) systems, tape drives, and data backup storage systems, etc.

It should be noted that although several units/modules or sub-units/modules of the electronic device are mentioned in the above detailed description, this division is merely exemplary and not mandatory. In fact, according to the embodiments of the present application, the features and functions of two or more units/modules described above can be embodied in one unit/module. Conversely, the features and functions of one unit/module described above can be further divided into multiple units/modules to be embodied.

Example 4

This embodiment provides a computer-readable storage medium, which stores a computer program. When the program is executed by a processor, the steps of the workpiece three-dimensional measurement method based on the multi-frequency polarization fringe technology in Embodiment 1 are implemented.

The readable storage medium may include but is not limited to: a portable disk, a hard disk, a random access memory, a read-only memory, an erasable programmable read-only memory, an optical storage device, a magnetic storage device or any suitable combination of the above.

In a possible implementation, the present disclosure can also be implemented in the form of a program product, which includes a program code. When the program product is run on a terminal device, the program code is used to enable the terminal device to execute the steps of the three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology described in Example 1.

Among them, the program code for executing the present disclosure can be written in any combination of one or more programming languages, and the program code can be executed completely on the user device, partially on the user device, as an independent software package, partially on the user device and partially on a remote device, or completely on the remote device.

Although embodiments of the present disclosure have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present disclosure, and the scope of the present disclosure is defined by the appended claims and their equivalents.

Claims (6)

1. A three-dimensional measurement method for a workpiece based on multi-frequency polarization fringe technology, characterized in that it comprises the following steps: Obtaining polarization information of the workpiece to be measured, generating a multi-frequency coded fringe image according to the polarization information using a standard four-step phase shift method, encoding the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image; The generated multi-frequency polarization fringe image is sent to a projector and projected onto the surface of the workpiece, and the deformed fringe of each frequency is captured by a camera through a linear polarizer; The high, medium and low frequency wrapping phases are calculated based on the deformation fringes of each frequency, and the high frequency wrapping phase is expanded by the high, medium and low frequency wrapping phases to obtain the absolute phase, thus completing the three-dimensional measurement of the workpiece; The multi-frequency coded fringe image is represented as: , Where ( x , y ) is the pixel coordinate, is the average intensity, is the modulation intensity, f is the fringe frequency, n is the number of phase shift steps; The multi-frequency polarization fringe image is expressed as: , in, is the average intensity, is the modulation intensity; The frequency deformation fringes captured by the camera are expressed as: , In the formula, is the reflectivity of the workpiece and the reference plane, is the wrap phase for each frequency; The wrapped phase of each frequency is unfolded to obtain the unfolded wrapped phase as: , in, is the wrapped phase after expansion, is the number of stripes. 2. The method for three-dimensional measurement of workpieces based on multi-frequency polarization stripe technology according to claim 1, wherein the polarization degree of the linear polarizer is expressed as: , In the formula, is the degree of linear polarization, It represents the maximum light intensity obtained when the polarizer is rotated 0°. Indicates the minimum light intensity obtained when the polarizer is rotated 90°. 3. The three-dimensional measurement method of a workpiece based on multi-frequency polarization fringe technology according to claim 1, characterized in that the absolute phase obtained by unfolding the high-frequency wrapped phase is expressed as: , In the formula, It represents the absolute phase obtained after the high-frequency wrapped phase is unfolded. is the low-frequency wrapping phase synthesized by the wrapping phases of three frequencies, Indicates rounding down. is a constant. 4. A workpiece three-dimensional measurement system based on multi-frequency polarization fringe technology, characterized in that it includes: A multi-frequency polarization fringe image generation module is used to obtain polarization information of the workpiece to be measured, generate a multi-frequency coded fringe image according to the polarization information using a standard four-step phase shift method, encode the multi-frequency coded fringe image into fringe images of three different frequencies, namely, high, medium and low, to generate a multi-frequency polarization fringe image; The frequency deformation fringe capture module sends the generated multi-frequency polarization fringe image to the projector and projects it onto the surface of the workpiece. The camera captures each frequency deformation fringe through the linear polarizer. The three-dimensional measurement module calculates the high, medium and low frequency wrapping phases according to the deformation fringes of each frequency, and expands the high frequency wrapping phase through the high, medium and low frequency wrapping phases to obtain the absolute phase, thus completing the three-dimensional measurement of the workpiece; The multi-frequency coded fringe image is represented as: , Where ( x , y ) is the pixel coordinate, is the average intensity, is the modulation intensity, f is the fringe frequency, n is the number of phase shift steps; The multi-frequency polarization fringe image is expressed as: , in, is the average intensity, is the modulation intensity; The frequency deformation fringes captured by the camera are expressed as: , In the formula, is the reflectivity of the workpiece and the reference plane, is the wrap phase for each frequency; The wrapped phase of each frequency is unfolded to obtain the unfolded wrapped phase as: , in, is the wrapped phase after expansion, is the number of stripes. 5. An electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the three-dimensional measurement method for workpieces based on multi-frequency polarization stripe technology as described in any one of claims 1 to 3 when executing the computer program. 6. A computer-readable storage medium having a computer program stored thereon, wherein when the program is executed by a processor, the method for three-dimensional measurement of a workpiece based on multi-frequency polarization fringe technology according to any one of claims 1 to 3 is implemented.