Single-frequency four-step phase-shift three-dimensional measurement method based on structured light

Abstract

The present application belongs to the technical field of three-dimensional measurement, and specifically discloses a single-frequency four-step phase shift three-dimensional measurement method based on structured light. The projector only needs to project four pictures, and the camera only needs to shoot four pictures, so that the detection of the fluctuation of a plane can be completed, the detection speed is greatly accelerated, the detection logic is simplified, and the acquisition efficiency of the substrate data before detection is improved. The fluctuation analysis of a plane can be obtained in a very short time (<2s), and the Z-direction resolution can be up to 3 mu m. The present application has a great effect on the industrial scene with high flatness requirements, such as additive manufacturing powder laying, large-area coating, flatness uniformity detection, etc. Especially for high-pixel large-format detection, the time of picture acquisition and calculation can be greatly reduced. Compared with the existing multi-frequency heterodyne method, the present application has the characteristics of small calculation amount and fast solving speed.

Description

Technical Field

[0001] This invention belongs to the field of three-dimensional measurement technology, specifically involving a single-frequency four-step phase-shift three-dimensional measurement method based on structured light. Compared with the existing multi-frequency heterodyne method, it has the characteristics of low computational load and fast solution speed, and is suitable for rapid detection of high-pixel and large-format objects. Background Technology

[0002] Fringe projection profilometry, due to its advantages such as high speed, high precision, non-contact operation, full-field measurement, and rapid information acquisition, is of great significance in 3D measurement and has been widely applied in many fields such as industrial manufacturing, cultural relic protection, and medicine. Referring to the detailed description of the 3D measurement method combining dual-frequency heterodyne and phase-shift coding disclosed in patent number CN201810470370.X, the existing 3D measurement system includes a DLP projector, a camera, a workstation, a measurement support, a reference plane, and the object to be measured. The DLP projector and CCD camera are placed on the measurement support; the DLP projector and CCD camera are connected to the workstation via data cables; the object to be measured is placed on the reference plane; the workstation contains an image acquisition card, projection software, and measurement software. The DLP projector focuses and projects fringes with feature information onto the surface of the object to be measured. The camera acquires the fringe information, which is then processed by the workstation to extract the feature information and perform 3D reconstruction according to a specific algorithm. The optical axis of the DLP projector and the optical axis of the camera intersect at point O. The DLP projector and the camera are at the same height, the distance between them is d, and the distance from them to the reference plane is l0. The formula for calculating the height of the object being measured is:

[0003]

[0004] Where f y For the vertical frequency division of the sine fringes on the reference plane, It represents the continuous phase difference between corresponding points on the object's surface image and the reference plane image.

[0005] Currently, there are many time-based unwrapping methods for large-format inspection, with multi-frequency heterodyne being a commonly used approach. For example, the 3D measurement method combining dual-frequency heterodyne and phase-shift coding disclosed in patent number CN201810470370.X is a good example. The basic idea of ​​multi-frequency heterodyne is to use a synthesized fringe pattern whose period covers the measured object, and then use the phase value of the synthesized phase pattern to solve for the phase of the original fringe. This method uses the synthesized low-frequency measurement results to determine the phase unfolding value of the original high-frequency measurement, thus obtaining the true phase. However, to accurately unfold the phase across the entire field, a suitable frequency value must be selected so that the sum of the frequency values ​​is unique. This imposes certain limitations in practical deployment, and the large number of images acquired each time consumes more computational resources, especially in large-format, high-pixel applications. Inspired by multi-frequency heterodyne, this invention proposes a four-step phase-shift 3D reconstruction method that relies on only one frequency. Compared to existing multi-frequency heterodyne methods, it has the advantages of low computational cost and fast solution speed, making it suitable for rapid inspection of high-pixel, large-format applications. Summary of the Invention

[0006] The present invention provides a single-frequency four-step phase-shift three-dimensional measurement method based on structured light, which has the advantages of low computational load and fast solution speed compared with the existing multi-frequency heterodyne method, and is suitable for rapid detection of high-pixel and large-format objects.

[0007] The specific technical solution of the present invention is as follows:

[0008] A single-frequency, four-step phase-shift three-dimensional measurement method based on structured light includes the following steps:

[0009] 1) Generate a single-frequency sinusoidal fringe pattern: Select the fringe frequency f according to experimental requirements, and set four-step phase-shifting sinusoidal fringes I1, I2, I3, and I4, whose intensities are expressed as:

[0010]

[0011]

[0012]

[0013]

[0014] Where A(x,y) is the background light intensity and B(x,y) is the modulation light intensity. The phase to be determined;

[0015] 2) Playing the sinusoidal fringe pattern: A sinusoidal fringe pattern I1, I2, I3, I4 with frequency f is projected onto the reference plane and the object under test using a projector. The modulated four-step phase-shifted sinusoidal fringe pattern I1, I2, I3, I4 on the object under test is acquired by a camera and transmitted to the computer. The computer calculates the wrapping phase using the following formula. and

[0016]

[0017]

[0018] in, The phase that must be obtained for each measurement; It serves as the base phase and is also responsible for completing phase stitching in the next step of unwrapping the phase, requiring only one image instead of acquiring it every time;

[0019] 3) Unwrapping the phase: Calculated using the following formula and Obtaining absolute phase

[0020]

[0021] 4) Using the following phase-height formula, the height information of each point on the surface of the object under test can be obtained:

[0022]

[0023] Where the projector and camera are at the same height, d is the distance between the projector and camera, l0 is the distance from the projector and camera to the reference plane, and f y The vertical frequency division of the sinusoidal fringes on the reference plane; It represents the continuous phase difference between corresponding points on the object's surface image and the reference plane image.

[0024] Preferably, f is set between 1 Hz and 1000 Hz.

[0025] The beneficial effects of this invention are as follows: This invention requires only four images from the projector and four images from the camera to complete the detection of planar undulations, significantly accelerating the detection speed, simplifying the detection logic, and improving the efficiency of acquiring substrate data before detection. This invention can obtain a precise analysis of a planar undulation within an extremely short time (<2 seconds), with a Z-direction resolution of up to 3μm. It is extremely useful in industrial scenarios with high flatness requirements, such as additive manufacturing powder spreading, large-area coating, and planar uniformity detection. Especially for high-pixel, large-format detection, it can greatly reduce the time for image acquisition and calculation. Compared with existing multi-frequency heterodyne methods, this invention features low computational load and fast solution speed. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is the four-step phase-shifting sinusoidal fringe pattern of the present invention;

[0028] Figure 2 This is a schematic diagram of the single-frequency heterodyne principle of the present invention. The broken line in the diagram represents a one-dimensional vector extracted from two-dimensional data.

[0029] Figure 3 For wrapping phase and the absolute phase solved using single-frequency heterodyne.

[0030] Figure 4 The flowchart of the single-frequency four-step phase shift three-dimensional measurement method provided by the present invention. Detailed Implementation

[0031] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0032] like Figure 1-4 As shown, the single-frequency four-step phase-shift three-dimensional measurement method based on structured light provided in this embodiment includes the following steps:

[0033] 1) Generate a single-frequency sinusoidal fringe pattern: Select the fringe frequency f according to experimental requirements, and set four-step phase-shifting sinusoidal fringes I1, I2, I3, and I4, whose intensities are expressed as:

[0034]

[0035]

[0036]

[0037]

[0038] Where A(x,y) is the background light intensity and B(x,y) is the modulation light intensity. The phase to be determined;

[0039] 2) Playing the sinusoidal fringe pattern: Project sinusoidal fringe patterns I1, I2, I3, and I4 with frequency f onto the reference plane and the object under test using a projector. Acquire the modulated four-step phase-shifted sinusoidal fringe patterns I1, I2, I3, and I4 on the object under test using a camera and transmit them to the computer. The computer calculates the desired pattern using the following formula. and

[0040]

[0041]

[0042] in, The phase that must be obtained for each measurement; It serves as the base phase and is also responsible for completing phase stitching in the next step of unwrapping the phase, requiring only one image instead of acquiring it every time;

[0043] 3) Unwrapping the phase: Calculated using the following formula and Obtaining absolute phase

[0044]

[0045] 4) Using the following phase-height formula, the height information of each point on the surface of the object under test can be obtained:

[0046]

[0047] Where the projector and camera are at the same height, d is the distance between the projector and camera, l0 is the distance from the projector and camera to the reference plane, and f y The vertical frequency division of the sinusoidal fringes on the reference plane; It represents the continuous phase difference between corresponding points on the object's surface image and the reference plane image.

[0048] Preferably, f is set between 1 Hz and 1000 Hz.

[0049] In this embodiment, the projector only needs to display four images, and the camera only needs to capture four images to complete the detection of planar undulations, greatly accelerating the detection speed, simplifying the detection logic, and improving the efficiency of acquiring substrate data before detection. This embodiment can obtain a precise analysis of the undulations of a plane in a very short time (<2 seconds), with a Z-direction resolution of up to 3μm. It is of great use in industrial scenarios with high flatness requirements, such as additive manufacturing powder laying, large-area coating, and planar uniformity detection. Especially for high-pixel, large-format detection, it can greatly reduce the time for image acquisition and calculation. Compared with existing multi-frequency heterodyne methods, it has the characteristics of low computational load and fast solution speed.

[0050] Example 1

[0051] See Figure 4 As shown, step S1: The projector generates sinusoidal fringes with a frequency f of 9. Figure 1 The projector (as shown) is projected onto the reference plane and the object under test. The relationship between the projector, camera, reference plane and other objects is shown in formula (8). These sinusoidal fringe patterns satisfy formulas (1)(2)(3)(4) in sequence. The camera captures the modulated four-step phase-shifted sinusoidal fringe pattern on the object under test. The phase expansion is explained below using planar simulation as an example.

[0052] Step S2: Process the four-step phase-shift sinusoidal fringe pattern captured by the camera using a computer, and obtain the wrapping phase using formula (5). And the basis phase is obtained by formula (6). like Figure 2 As shown.

[0053] Step S3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full and The absolute phase is obtained by computer calculation using formula (7). like Figure 2 and Figure 3 As shown.

[0054] Step S4: Final Absolute Phase The height information of the object is calculated using formula (8).

[0055] In the above process, besides initially capturing the substrate image to obtain the substrate phase... In addition, the detection of plane undulations can be completed by projecting four photos and taking four photos with the camera each time the plane is measured, which shortens the operation time, greatly speeds up the detection speed, and simplifies the detection logic.

[0056] Example 2

[0057] See Figure 4 As shown, in step S1: the projector generates sinusoidal fringes with a frequency of 25 and projects them onto the reference plane and the object under test. The relationship between the projector, camera, reference plane, and other objects is shown in formula (8). These sinusoidal fringe patterns sequentially satisfy formulas (1)(2)(3)(4). The camera captures the modulated four-step phase-shifted sinusoidal fringe pattern on the object under test. The phase expansion is explained below using planar simulation as an example.

[0058] Step S2: Process the four-step phase-shift sinusoidal fringe pattern captured by the camera using a computer, and obtain the wrapping phase using formula (5). And the basis phase is obtained by formula (6). like Figure 2As shown.

[0059] Step S3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full and The absolute phase is obtained by computer calculation using formula (7). like Figure 2 and Figure 3 As shown.

[0060] Step S4: Final Absolute Phase The height information of the object is calculated using formula (8).

[0061] In the above process, besides initially capturing the substrate image to obtain the substrate phase... In addition, each time a plane is measured, only four photos need to be played by the projector and four photos need to be taken by the camera to complete the detection of plane undulations, which effectively shortens the operation time in the case of high-precision mass production of circuit boards.

[0062] Example 3

[0063] See Figure 4 As shown, in step S1: the projector generates sinusoidal fringes with a frequency of 30 and projects them onto the reference plane and the object under test. The relationship between the projector, camera, reference plane, and other objects is shown in formula (8). These sinusoidal fringe patterns sequentially satisfy formulas (1)(2)(3)(4). The camera captures the modulated four-step phase-shifted sinusoidal fringe pattern on the object under test. The phase expansion is explained below using planar simulation as an example.

[0064] Step S2: Process the four-step phase-shift sinusoidal fringe pattern captured by the camera using a computer, and obtain the wrapping phase using formula (5). And the basis phase is obtained by formula (6). like Figure 2 As shown.

[0065] Step S3: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full and The absolute phase is obtained by computer calculation using formula (7). like Figure 2 and Figure 3 As shown.

[0066] Step S4: Final Absolute Phase The height information of the object is calculated using formula (8).

[0067] In the above process, besides initially capturing the substrate image to obtain the substrate phase... Furthermore, each measurement of a plane only requires the projector to display four photos, and the camera to take only four photos to complete the detection of the plane's undulations. The entire device is deployed in a powder bed-based 3D printer, and the 3D reconstruction of the layer-by-layer printed powder surface can still produce a reconstruction result in less than 1 second even with a camera with up to 65 megapixels.

[0068] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A single-frequency four-step phase-shift three-dimensional measurement method based on structured light, characterized in that, The measurement method is used for powder plane detection in additive manufacturing powder spreading, and includes the following steps: 1) Generate a single-frequency sinusoidal fringe pattern: Select the fringe frequency f according to experimental requirements, and set a four-step phase-shifting sinusoidal fringe pattern I1, I2, I3, I4, whose intensity is expressed as: (1) (2) (3) (4) Where A(x,y) is the background light intensity and B(x,y) is the modulation light intensity. (x,y) is the phase to be determined; 2) Playing the sinusoidal fringe pattern: A sinusoidal fringe pattern I1, I2, I3, I4 with frequency f is projected onto the reference plane and the object under test using a projector. The modulated four-step phase-shifted sinusoidal fringe pattern I1, I2, I3, I4 on the object under test is acquired by a camera and transmitted to the computer. The computer calculates the wrap-around phase using the following formula. 1 and 2: （5） （6） in, 1 represents the phase that must be obtained for each measurement; 2 is the base phase, which is also responsible for completing phase stitching in the next step of unwrapping phase. It only needs to be captured once and does not need to be captured every time. 3) Unwrapping the phase: Calculated using the following formula 1 and 2. Obtain the absolute phase Δ : （7） 4) Using the following phase-height formula, the height information of each point on the surface of the object under test can be obtained: （8） In this context, the projector and camera are at the same height, and d represents the distance between the projector and camera. 0 represents the distance from the projector and camera to the reference plane, f y The vertical frequency component of the sine fringes on the reference plane; Δ It represents the continuous phase difference between corresponding points on the object's surface image and the reference plane image.

2. The single-frequency four-step phase-shift three-dimensional measurement method based on structured light according to claim 1, characterized in that, The frequency f is set between 1 Hz and 1000 Hz.

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