A method for three-dimensional measurement of a moving object

Abstract

The application discloses a three-dimensional measurement method for a moving object, which combines a 2+1 phase shift method and a phase ordering method to correct phase errors caused by object movement, and has the advantages of high speed, high precision, high flexibility, wide applicability and easy implementation.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of three-dimensional measurement, and particularly relates to a three-dimensional measurement method of a moving object. BACKGROUND

[0002] The fringe projection technology is widely used in the field of three-dimensional measurement, and has the advantages of non-contact, high precision, fast speed, dense point cloud, and easy implementation. A commonly used fringe projection system includes two core modules of a camera and a projector. The projector is responsible for projecting a standard fringe image to a measured object, and the camera is responsible for collecting the fringe image modulated by the measured object. The phase distribution of the fringe is obtained through digital fringe analysis, and the three-dimensional topographic information of the measured object can be recovered in combination with the geometric calibration parameters. The Fourier transform method and the multi-step phase shift method are two commonly used digital fringe analysis methods. The multi-step phase shift method generally requires three or more fringe images, has high robustness, and is more widely used. However, when a moving object is measured, the phase shift between multiple fringe images will be offset, resulting in a periodic phase error in the recovered phase distribution. In view of this problem, a series of phase error correction methods have been proposed by domestic and foreign scholars, such as the object tracking method, the Fourier transform auxiliary method, and the Hilbert transform method, which can correct the phase error caused by object motion to different degrees (Optics and Lasers in Engineering, 2021, 141: 106573). The object tracking method needs additional characteristic marks (Optics Letters, 2014, 39(23): 6715-8; Optics Express, 2019, 27(16): 22100-15), color channels (Optics Express, 2020, 28(19): 28600-11), etc., which limits their scope of application. The Fourier transform auxiliary method (Optics Express, 2016, 24(20): 23289-303) and the Hilbert transform method (Optics Express, 2018, 26(26): 34224-35) both involve frequency domain operations, which also limit their scope of application. In summary, how to accurately recover the three-dimensional topography of a moving object still faces great challenges.

[0003] The three-dimensional measurement method of the moving object can use the triangular stereo model introduced in the reference (Optics and Lasers in Engineering, 2021, 143: 106622). The internal parameters of the camera and the projector include focal length, principal point, distortion, etc., and the external parameters include rotation matrix, translation vector, SUMMARY

[0004] The application provides a three-dimensional measurement method for a moving object to solve the problems in the background art.

[0005] To achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows: a three-dimensional measurement method for a moving object, specifically comprising the following steps:

[0006] Step S1: building a three-dimensional measurement system for a moving object, including a camera, a projector and a computer; the camera and the projector are synchronously triggered to start working, and are connected to the computer through a cable; the projector projects 2+1 phase-shifted stripes I sin ,I0,I cos ; the camera collects 2+1 phase-shifted stripes I sin ,I0,I cos ; the computer performs data analysis and processing; wherein I sin is a sinusoidal stripe, I0 is a uniform background, and I cos is a cosine stripe;

[0007] Step S2: according to a preset stripe projection strategy, the projector cyclically projects 2+1 phase-shifted stripes I sin ,I0,I cos onto the surface of the moving object, the camera synchronously collects 2+1 phase-shifted stripe images I sin ,I0,I cos of the moving object after modulation, and transmits them to the computer for digital stripe analysis;

[0008] Step S3: using a background difference method to detect the moving object frame by frame on 2+1 phase-shifted stripes I sin ,I0,I cos , and dividing the region of interest of the moving object according to the minimum circumscribed rectangle of the moving object; and zero-padded expansion is performed from inside to outside on the region of interest to a uniform size, so as to ensure that the size of the region of interest of each frame remains consistent;

[0009] Step S4: using a 2+1 phase-shift method to extract the truncated phase φ k of kth to k+2th phase-shifted stripes I ; similarly, using the 2+1 phase-shift method to extract the truncated phase φ k+1 of k+1th to k+3th phase-shifted stripes I , and using the 2+1 phase-shift method to extract the truncated phase φ k+2 of k+2th to k+4th phase-shifted stripes I ;

[0010] Step S5: using a phase sorting method to correct the phase error of the truncated phases φ k , φ k+1 , φ k+2 ... one by one, and obtaining the corrected truncated phases φ k ′, φk ′ +1 ,φ k ′ +2 ...; then, the phase expansion is performed using the geometric constraint method to obtain the corresponding absolute phase Φ′. k ,Φ′ k+1 ,Φ′ k+2 ...;

[0011] Step S6: Calibrate the 3D measurement system for the moving object to obtain the intrinsic and extrinsic parameters of the camera and projector; then, using the calibrated intrinsic and extrinsic parameters, frame by frame, measure the absolute phase Φ′ of the moving object. k ,Φ′ k+1 ,Φ′ k+2 ...convert to three-dimensional physical space to obtain three-dimensional point cloud data of the moving object.

[0012] Furthermore, in step S2, the 2+1 phase-shifted stripes I modulated by the moving object sin ,I0,I cos It can be represented as:

[0013] I sin (x sin ,y sin ) = A sin (x sin ,y sin )+B sin (x sin ,y sin )sin[φ(x sin ,y sin )-δ sin ];

[0014] I0(x0,y0)=A0(x0,y0);

[0015] I cos (x cos ,y cos ) = A cos (x cos ,y cos )+B cos (x cos ,y cos )cos[φ(x cos ,y cos )+δ cos ];

[0016] In the formula: (x sin ,y sin ),(x0,y0),(x cos ,y cos ) represent the coordinates of a pixel in a 2+1 phase-shifted fringe image; A sin ,A0,Acos respectively represent the background intensity of the phase-shifting fringe image; B sin , cos respectively represent the modulation intensity of the phase-shifting fringe image; δ sin , cos respectively represent the additional phase shift caused by the motion of the object.

[0017] Further, in the step S3, by the segmentation of the region of interest and the zero padding expansion, the size of the region of interest of the 2+1 phase-shifting fringe I sin , I0, I cos is kept consistent, and thus the motion object in the 2+1 phase-shifting fringe I sin , I0, I cos is corresponded pixel by pixel; in addition, assuming that the reflectivity of each point on the motion object in the adjacent 2+1 phase-shifting fringe I sin , I0, I cos changes little, it can be considered that A sin ≈A0≈A cos and B sin ≈B cos , and thus the 2+1 phase-shifting fringe I sin , I0, I cos corresponding to the region of interest of the motion object can be represented as:

[0018] I sin (x, y) = A(x, y) + B(x, y) sin [φ(x, y) - δ sin ];

[0019] I0(x, y) = A(x, y);

[0020] I cos (x, y) = A(x, y) + B(x, y) cos [φ(x, y) + δ cos ];

[0021] In the formula, (x, y) represents a pixel coordinate in the region of interest; A represents the background intensity of the phase-shifting fringe image; and B represents the modulation intensity of the phase-shifting fringe image.

[0022] Further, in the step S4, the truncated phase φ of the phase-shifting fringe I sin , I0, I cos is extracted by using the 2+1 phase-shifting method, and the calculation formula is as follows:

[0023]

[0024] In the formula, the calculation formula involves the arctangent operation, and thus the value range of the truncated phase φ is [0, 2π), and the absolute phase Φ needs to be recovered by further phase unwrapping; because the phase-shifting fringe Isin and I cos An additional phase shift amount δ exists in sin ,δ cos , resulting in a phase error in the calculated truncated phase φ.

[0025] Further, in the step S5, the basic idea of the phase ordering method is to arrange the effective phase values of all pixels on the moving object in the truncated phase φ in ascending order from small to large. Assuming that the total number of pixels on the moving object is equal to M, the phase value of the mth pixel (x m ,y m ) is equal to φ(x m ,y m ), and the order corresponding to the phase value φ(x m ,y m ) after ascending arrangement is equal to n, then the corresponding corrected phase value is equal to φ'(x m ,y m )=2πm / M. The corrected truncated phase φ' can be obtained by correcting the truncated phase φ pixel by pixel.

[0026] The beneficial effects of the above technical solution are:

[0027] 1. The three-dimensional measurement method of the moving object provided by the application combines the 2+1 phase shift method and the phase ordering method to correct the phase error caused by the movement of the object, and has the advantages of fast speed, high precision, strong flexibility, wide applicability, and easy implementation. BRIEF DESCRIPTION OF DRAWINGS

[0028] Fig. 1 is a 2+1 phase shift fringe image I sin ,I0,I cos ;

[0029] Fig. 2 is the truncated phase φ before and after correction and φ';

[0030] Fig. 3 is the absolute phase Φ before and after correction and φ'; DETAILED DESCRIPTION

[0031] The specific embodiments of the application will be further described in detail below with reference to the drawings, and the purpose is to help those skilled in the art to have a more complete, accurate and in-depth understanding of the concept and technical solution of the application, and to facilitate its implementation.

[0032] As shown in Figs. 1 to 3 , the application is a three-dimensional measurement method of a moving object, which combines the 2+1 phase shift method and the phase ordering method to correct the phase error caused by the movement of the object, and has the advantages of fast speed, high precision, strong flexibility, wide applicability, and easy implementation.

[0033] The specific working manner is described below with specific examples:

[0034] Example 1:

[0035] Step S1: build a three-dimensional measurement system of a moving object, including a camera, a projector and a computer; the camera and the projector are synchronously triggered to start working, and are connected to the computer through a cable; the projector projects 2+1 phase shift stripes I sin ,I0,I cos , the camera collects 2+1 phase shift stripes I sin ,I0,I cos , and the computer performs data analysis and processing; wherein I sin is a sinusoidal stripe, I0 is a uniform background, and I cos is a cosine stripe;

[0036] Step S2: according to a preset stripe projection strategy, the projector cyclically projects 2+1 phase shift stripes I sin ,I0,I cos onto the surface of the moving object, the camera synchronously collects 2+1 phase shift stripe images I sin ,I0,I cos of the moving object after modulation, and transmits them to the computer for digital stripe analysis; Fig. 1 2+1 phase shift stripe images I sin ,I0,I cos are shown.

[0037] Step S3: use the background difference method to detect the moving object from 2+1 phase shift stripes I sin ,I0,I cos frame by frame, and segment the region of interest of the moving object according to the minimum circumscribed rectangle of the moving object; and expand the region of interest from inside to outside to a uniform size, to ensure that the size of the region of interest of each frame remains consistent;

[0038] Step S4: use the 2+1 phase shift method to extract the truncated phase φ k of the kth to k+2th phase shift stripes I ; similarly, use the 2+1 phase shift method to extract the truncated phase φ k+1 of the k+1th to k+3th phase shift stripes I , and use the 2+1 phase shift method to extract the truncated phase φ k+2 of the k+2th to k+4th phase shift stripes I ;

[0039] Step S5: use the phase sorting method to correct the truncated phases φ k , φ k+1 , φ k+2phase error of..., obtain the corrected truncated phase φ k ′,φ k ′ +1 ,φ k ′ +2 ...; then use geometric constraint method to phase unwrapping, obtain the corresponding absolute phase Φ′ k ,Φ′ k+1 ,Φ′ k+2 ...; Fig. 2 The truncated phase φ and φ′ before and after correction are shown, Fig. 3 The absolute phase Φ and φ′ before and after correction are shown.

[0040] Step S6: calibrate the three-dimensional measurement system of the moving object, obtain the internal and external parameters of the camera and the projector; then use the calibrated internal and external parameters to convert the absolute phase Φ′ k ,Φ′ k+1 ,Φ′ k+2 ... of the moving object to the three-dimensional physical space frame by frame, and obtain the three-dimensional point cloud data of the moving object.

[0041] The above describes the present application by way of example with reference to the drawings, and it is obvious that the specific implementation of the present application is not limited to the above described manner, and any non-essential improvement using the method concept and technical solution of the present application, or direct application of the above described concept and technical solution of the present application to other occasions without improvement, all fall within the protection scope of the present application.

Claims

1. A method of three-dimensional measurement of a moving object, characterized by: Specifically comprising the following steps: Step S1: building a three-dimensional measurement system of moving objects, including a camera, a projector, and a computer; Camera and projector are triggered to work synchronously, and connected to computer through cable; projector projects 2+1 phase shift fringe I sin ,I0,I cos , camera collects 2+1 phase shift fringe I sin ,I0,I cos , computer carries out data analysis and processing; wherein I sin is sinusoidal fringe, I0 is uniform background, I cos is cosine fringe; Step S2: According to the preset stripe projection strategy, the projector projects 2+1 phase shift stripes I sin ,I0,I cos to the surface of the moving object, and the camera synchronously collects the 2+1 phase shift stripe images I sin ,I0,I cos modulated by the moving object, and transmits them to the computer for digital stripe analysis; Step S3: using background subtraction method to frame 2+1 phase shift fringe I sin ,I0,I cos Motion object detection is performed, and a region of interest of the motion object is segmented according to a minimum circumscribed rectangle of the motion object; and the region of interest is zero-padded and expanded to a uniform size from inside to outside, so that the size of the region of interest of each frame remains consistent; Step S4: Extract the truncated phase φ of the k to k+2 frames phase shifting fringe by 2+1 phase shifting method k ; and so on, extract the truncated phase φ of the k+1 to k+3 frames phase shifting fringe by 2+1 phase shifting method k+1 , extract the truncated phase φ of the k+2 to k+4 frames phase shifting fringe by 2+1 phase shifting method k+2 ;​​​ Step S5: Correct the phase error of φ k ,φ k+1 ,φ k+2 ... one by one to obtain the corrected truncated phase φ' k ,φ′ k+1 ,φ′ k+2 ... by using the phase ordering method; then use the geometric constraint method to perform phase unwrapping to obtain the corresponding absolute phase Φ' k ,Φ′ k+1 ,Φ′ k+2 ... Step S6: calibrate the three-dimensional measurement system of the moving object to obtain the internal and external parameters of the camera and the projector; then use the calibrated internal and external parameters to convert the absolute phase Φ' k ,Φ' k+1 ,Φ' k+2 ... of the moving object to the three-dimensional physical space frame by frame to obtain the three-dimensional point cloud data of the moving object; In the step S5, the basic idea of the phase ordering method is to arrange the effective phase values of all pixels on the moving object in the truncated phase φ in ascending order from small to large. Assuming that the total number of pixels on the moving object is equal to M, the phase value of the mth pixel (x m ,y m ) is equal to φ(x m ,y m ), the order corresponding to the phase value φ(x m ,y m ) after ascending arrangement is equal to n, and the corresponding corrected phase value is equal to φ'(x m ,y m )=2πm / M. The corrected truncated phase φ' can be obtained by correcting the truncated phase φ pixel by pixel.

2. The method of claim 1, wherein: The step S2, the 2+1 phase shift stripe I modulated by the moving object sin I0, I cos Can be expressed as: I sin (x sin ,y sin )=A sin (x sin ,y sin )+B sin (x sin ,y sin )sin[φ(x sin ,y sin )-δ sin ] I0(x0, y0) = A0(x0, y0); I cos (x cos ,y cos )=A cos (x cos ,y cos )+B cos (x cos ,y cos )cos[φ(x cos ,y cos )+δ cos ] where (x sin ,y sin ),(x0,y0),(x cos ,y cos ) are the coordinates of a pixel in the 2+1 phase-shifting fringe pattern; A sin ,A0,A cos are the background intensities of the phase-shifting fringe patterns; B sin ,B cos are the modulation intensities of the phase-shifting fringe patterns; δ sin ,δ cos are the additional phase shifts caused by the motion of the object.

3. The method of claim 2, wherein: The step S3, by the region of interest segmentation and zero padding, can guarantee that the 2+1 phase shift fringe I sin I0, I cos The region of interest size is consistent, and the moving object in the 2+1 phase shift fringe I sin I0, I cos Corresponds to each pixel; in addition, assuming that the reflectivity of each point on the moving object in the adjacent 2+1 phase shift fringe I sin I0, I cos Changes little, A sin ≈A0≈A cos and B sin ≈B cos , therefore the 2+1 phase shift fringe I sin I0, I cos Corresponding to the region of interest of the moving object can be expressed as: I sin (x,y) = A(x,y) + B(x,y) sin [φ(x,y) - δ sin ]; I0(x, y) = A(x, y); I cos (x,y) = A(x,y) + B(x,y) cos [φ(x,y) + δ cos ]; In the formula: (x, y) represents a pixel coordinate in a region of interest; A represents the background intensity of the phase shift fringe image; and B represents the modulation intensity of the phase shift fringe image, respectively.

4. The method of claim 1, wherein: In the step S4, the phase shift fringes I are extracted by using the 2+1 phase shift method sin I0, I cos The cut-off phase φ of the fringes is calculated as follows: In the formula, the calculation formula involves arctangent operation, so the value range of the truncated phase φ is [0, 2π), and the absolute phase Φ needs to be recovered by further phase unwrapping; because there is an additional phase shift δ sin and I cos in the phase shift fringes I sin , δ cos , the calculated truncated phase φ has a phase error.

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