A method for three-dimensional measurement of a moving object surface on a production line based on phase shift profilometry

Abstract

This invention provides a three-dimensional measurement method for the surface of a moving object on a production line based on phase-shifting profilometry. Employing a three-dimensional vision measurement system composed of a projector and a camera, and based on the measurement principle of phase-shifting profilometry, the method analyzes the phase error caused by the motion of the measured object and proposes a three-dimensional measurement method for the surface of a moving object that does not require phase error correction. By setting the installation position and angle of the projector and camera, the phase encoding direction in the projected stripe pattern is perpendicular to the direction of motion of the measured object, thereby eliminating the phase error caused by motion. This allows the three-dimensional surface measurement results of the measured object under motion conditions to essentially achieve the measurement accuracy of the measured object under static conditions, realizing high-precision online real-time measurement of the surface three-dimensional morphology of moving objects on actual production lines.

Description

Technical Field

[0001] This invention relates to the field of three-dimensional measurement technology, and more particularly to a three-dimensional measurement method for the surface of a moving object on a production line based on phase-shifting profilometry. Background Technology

[0002] Phase-shifting profilometry utilizes continuous phase information encoded in fringe structured light to achieve high-precision, non-contact, and rapid full-field 3D topographic measurement of object surfaces, and is widely used in various 3D measurement fields. To obtain the phase information in the fringe patterns on an object's surface, phase-shifting profilometry requires at least three fringe patterns with known phase shifts. During the acquisition of these fringe patterns, the object being measured must be stationary; otherwise, the phase information will be interfered with, greatly limiting the application of phase-shifting profilometry in 3D measurement of moving object surfaces.

[0003] Object motion introduces errors into phase extraction based on phase-shifting profilometry, directly affecting the accuracy of 3D surface topography measurement. In the published literature, 3D measurement of moving object surfaces based on phase-shifting profilometry is a major research focus, primarily addressing phase error correction caused by motion. Most of these studies are based on the assumption that the object's surface texture is monotonically uniform, utilizing the spatial or temporal changes in phase during motion to correct for phase errors. Some studies also consider changes in surface texture, using target tracking methods to correct phase errors. All these methods share a common characteristic: object motion introduces phase errors, and the effectiveness of error correction directly impacts the accuracy of 3D surface measurement, exhibiting problems such as poor adaptability, weak robustness, and low measurement precision. Summary of the Invention

[0004] To address the high demand for online 3D topography measurement of moving objects on actual production lines, which existing technologies cannot meet, this invention provides a 3D surface measurement method for moving objects on production lines based on phase-shifting profilometry. This invention primarily utilizes a 3D surface measurement method for moving objects based on phase-shifting profilometry, characterized by the following steps: Given the known motion law of the object to be measured, the object is translated within a fixed plane with a fixed direction of motion. By setting the installation angle and position of the projector, the phase encoding direction or phase change direction in the projected stripe pattern is perpendicular to the motion direction of the object, or the stripe direction in the projected pattern is the same as the motion direction of the object. The phase information of the projected stripes at the same point on the surface of the object remains unchanged during the motion, eliminating the influence of phase errors caused by motion. A camera installed at another viewpoint tracks the motion of the object's surface, and the 3D measurement information of the object's surface is obtained by combining the camera and projector stereo calibration.

[0005] Furthermore, the phase information encoded in the striped pattern projected by the projector is obtained through phase-shift profilometry; the phase-shift profilometry requires a striped pattern with known phase shift and multiple corresponding striped images captured by a camera to solve for the phase information in the striped pattern projected by the projector.

[0006] Furthermore, the phase-shifting profilometry is an N-step phase-shifting method.

[0007] Furthermore, within the projector pixel coordinate system, since the direction of motion of the measured object is parallel to the v-axis, and the phase encoding in equation (3) is independent of the v-direction, the phase information of the same point on the surface of the measured object remains unchanged during the motion, i.e.:

[0008] φ(p1,q1)=φ(p2,q2)=…=φ(p n ,q n )=φ(x,y); (4)

[0009] Therefore, combining (2) and (4), equation (1) can be transformed into

[0010] I k (x,y)=A(x,y)+B(x,y)cos[φ(x,y)+2(k-1)π / N]; (5)

[0011] Where k represents the k-th fringe pattern, k = 1, 2, ..., N; (x, y) represents the pixel coordinates of a point on the surface of the object being measured in the camera image when the projector projects the first fringe pattern; A(x, y) and B(x, y) represent the ambient light intensity and surface reflectivity of the object being measured corresponding to that point, respectively.

[0012] Furthermore, combining the pixel grayscale information I acquired by the camera k (x,y), setting the acquired image information N≥3, then the corresponding wrapping phase for:

[0013]

[0014] Among them, the wrapping phase described in equation (6) The calculation result is between -π and +π. Combined with the fringe order k(x,y), the phase φ(x,y) of the fringe image is obtained:

[0015]

[0016] Furthermore, the phase φ(x,y) obtained by the camera can be used to determine the coordinate u of the point in the projector pixel coordinate system according to equation (3); assuming the three-dimensional coordinates of the point are (X... w ,Y w Zw Based on the stereo calibration results of the camera and projector, the following relationship is satisfied.

[0017]

[0018] In equation (8), M c and M p The results show the calibration results of the intrinsic and extrinsic parameters of the camera and projector, respectively.

[0019] Using the camera pixel coordinates (x, y) and projector pixel coordinates u at the same point, after eliminating irrelevant terms v, s1, and s2 according to equation (8), and then combining like terms, we obtain:

[0020]

[0021] Solving the above equations simultaneously will give us the three-dimensional coordinates (X, Y, Z) of that point. w ,Y w Z w This allows for the measurement of the three-dimensional morphology of the surface of the object being measured.

[0022] Furthermore, in the N-step phase shift method:

[0023] When the object being measured moves, for the same point on the object's surface, when the projector projects the k-th fringe pattern, the fringe image captured by the camera is:

[0024] I k (p k ,q k ) = A k (p k ,q k )+B k (p k ,q k )cos[φ(p k ,q k )+2π(k-1) / N]; (1)

[0025] Among them, (p k ,q k ) represents the pixel coordinates of the same point on the surface of the measured object in the image captured by the camera when the k-th fringe pattern is projected by the projector. Taking the position of the point in the image captured by the camera when k=1 as the reference, let p1=x, q1=y; A k (p k ,q k ) and B k (p k ,q k ) represent the ambient light intensity and surface reflectance of the point at step k, respectively; φ(p k ,q k(p) represents the encoded phase projected by the projector at step k; k ,q k This represents the pixel position tracking result of the same point on the surface of the measured object within the camera image plane at different times;

[0026] During the projection of N stripe patterns by the projector, the object's movement is small, meaning the position of the same point on the object's surface (p) remains constant. k ,q k Since the changes are not significant and the ambient light is relatively stable, we can approximate A and B as constant, that is:

[0027]

[0028] Assuming the projector's pixel plane coordinate system is (u, v), and the direction of motion of the object being measured is parallel to the v-axis, then the encoding method for the projector's projected stripes is as follows:

[0029]

[0030] Where T is the stripe period.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. This invention eliminates phase errors caused by motion by setting the installation position and angle of the projector and camera so that the phase encoding direction in the stripe pattern projected by the projector is perpendicular to the direction of motion of the object being measured, thereby achieving high-precision real-time online measurement of the three-dimensional morphology of the surface of the moving object being measured. Attached Figure Description

[0033] 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 some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram illustrating the principle of the present invention for analyzing the three-dimensional coordinates of points on the surface of the object being measured.

[0035] Figure 2 This is a schematic diagram illustrating the phase error and image position change caused by the motion of the object in this invention.

[0036] Figure 3 This is a schematic diagram of the device in an embodiment of the present invention.

[0037] Figure 4 This invention provides a pixel motion tracking scheme for complex textures.

[0038] Figure 5 This is a schematic diagram of the three-dimensional morphology measurement results of the continuous casting billet surface of the present invention; wherein, (a) is a schematic diagram of the surface texture of the object under test; (b) is a schematic diagram of the three-dimensional measurement results of the surface under static conditions; and (c) is a schematic diagram of the three-dimensional measurement results of the surface under dynamic conditions. Detailed Implementation

[0039] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0040] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0041] Phase-shift profilometry involves projecting a series of fringe patterns with known phase shifts using a projector, and then capturing multiple corresponding fringe images with a camera. This allows for the construction of multiple equations to solve for the phase information in the fringe patterns projected by the projector.

[0042] For the standard N-step phase-shift method, when the object being measured moves, for the same point on the object's surface, when the projector projects the k-th fringe pattern, the fringe image captured by the camera is:

[0043] I k (p k ,q k ) = A k (p k ,q k )+B k (p k ,q k )cos[φ(p k ,q k )+2π(k-1) / N]; (1)

[0044] Among them, (pk ,q k ) represents the pixel coordinates of the same point on the surface of the measured object in the image captured by the camera when the k-th fringe pattern is projected by the projector. Taking the position of the point in the image captured by the camera when k=1 as the reference, let p1=x, q1=y; A k (p k ,q k ) and B k (p k ,q k ) represent the ambient light intensity and surface reflectance of the point at step k, respectively; φ(p k ,q k (p) represents the encoded phase projected by the projector at step k. Note that (p) k ,q k This represents the pixel position tracking result of the same point on the surface of the measured object in the camera image plane at different times.

[0045] When the measurement speed is fast enough, the amplitude of the object's motion is small, meaning the position of the same point on the object's surface (p) is... k ,q k Since the changes are not significant and the ambient light is relatively stable, we can approximate A and B as constant.

[0046]

[0047] Based on the phase-shifting profilometry measurement principle, the phase information of the projector's projected fringes only needs to be encoded along one direction. According to the design scheme of claim 1, the phase encoding direction of the projector's projected fringes is set to be perpendicular to the direction of motion of the object being measured. Assuming the projector pixel plane coordinate system is (u, v), and the direction of motion of the object being measured is parallel to the v-axis, the encoding method of the projector's projected fringes is designed as follows:

[0048]

[0049] Where T is the stripe period.

[0050] Within the projector pixel coordinate system, since the direction of motion of the measured object is parallel to the v-axis, and the phase encoding in equation (3) is independent of the v-direction, the phase information of the same point on the surface of the measured object remains unchanged during the motion process, i.e.

[0051] φ(p1,q1)=φ(p2,q2)=…=φ(p n ,q n )=φ(x,y); (4)

[0052] Therefore, combining (2) and (4), equation (1) can be transformed into

[0053] I k(x,y)=A(x,y)+B(x,y)cos[φ(x,y)+2(k-1)π / N]; (5)

[0054] Where k represents the k-th fringe pattern, k = 1, 2, ..., N; (x, y) represents the pixel coordinates of a point on the surface of the object being measured in the camera image when the projector projects the first fringe pattern; A(x, y) and B(x, y) represent the ambient light intensity and surface reflectivity of the object being measured corresponding to that point, respectively.

[0055] Combined with pixel grayscale information acquired by the camera I k (x,y), setting the acquired image information N≥3, then the corresponding wrapping phase for:

[0056]

[0057] Among them, the wrapping phase described in equation (6) The calculation result is between -π and +π. Combined with the fringe order k(x,y), the phase φ(x,y) of the fringe image is obtained:

[0058]

[0059] The coordinates u of the point in the projector pixel coordinate system can be determined by equation (3) using the phase φ(x,y) obtained from the camera. Assume the three-dimensional coordinates of the point are (X... w ,Y w Z w Based on the stereo calibration results of the camera and projector, the following relationship is satisfied.

[0060]

[0061] In equation (8), M c and M p These are the calibration results for the intrinsic and extrinsic parameters of the camera and projector, respectively.

[0062] Using the camera pixel coordinates (x, y) and projector pixel coordinates u at the same point, irrelevant terms v, s1, and s2 are eliminated according to equation (8), and then like terms are combined to obtain the result.

[0063]

[0064] Solving the above equations simultaneously will give us the three-dimensional coordinates (X, Y, Z) of that point. w ,Y w Z w This allows for the measurement of the three-dimensional morphology of the surface of the object being measured, such as... Figure 1 As shown.

[0065] According to the standard N-step phase shift method, since the object being measured is in motion, the position of the same point on the object's surface changes continuously not only in the camera image but also in the phase information of the fringes projected onto the surface of the object by the projector. If the phase of the first fringe pattern is taken as the reference according to equation (1), then φ(p k ,q k )=φ(x,y)+Δφ(p k ,q k Thus, equation (5) becomes

[0066] I k (p k ,q k )=A(x,y)+B(x,y)cos[φ(x,y)+△φ(p k ,q k )+2π(k-1) / N]; (10)

[0067] Comparing equation (10) and equation (5), we can see that there are two main differences: First, due to motion, the position of the same point in the images acquired at different times is different, that is, (p1,q1)≠(p2,q2)≠…≠(p k ,q k Secondly, the phase φ(p) caused by motion. k ,q k The phase error Δφ(p) is constantly changing and exists. k ,q k This means that the corresponding encoding phase has deviated, making it impossible to calculate the accurate phase of the projector's projected stripes according to equation (6) and thus reliably obtain the position of the corresponding projector pixel coordinates. Figure 2 As shown.

[0068] During the production process, products on the production line are generally transported by conveyor belts or tracks, and the products translate within a fixed plane with a fixed direction of motion. According to the principle of phase profilometry, only one direction needs to be encoded for the phase. If the encoding direction of the phase information or the phase change direction is perpendicular to the direction of motion of the measured object, i.e., equation (3), then the phase error caused by the motion of the object in equation (10) can be completely eliminated. Therefore, the implementation scheme proposed in this invention is as follows: Figure 3 As shown, the phase encoding direction in the projected pattern is perpendicular to the direction of object motion, or the stripe direction in the projected pattern is the same as the direction of motion of the measured object. Therefore, the phase φ(p) in equation (1) k ,q k If the phase error Δφ(p) caused by the motion of the object remains constant, then in equation (10)... k ,q k ) can be eliminated.

[0069] Therefore, according to the proposed embodiment of the present invention, by setting the installation position and angle of the projector and camera so that the phase encoding direction in the stripe pattern projected by the projector is perpendicular to the direction of motion of the object being measured, the phase error caused by motion can be eliminated, and high-precision real-time online measurement of the three-dimensional morphology of the surface of the moving object being measured can be realized.

[0070] Furthermore, as a preferred implementation, if the complex texture of the object being measured is taken into account, pixel-by-pixel motion tracking is required on the fringe pattern captured by the camera. However, the projected fringes during the measurement process... Figure 1 The constant change is detrimental to high-precision and reliable motion tracking of pixels.

[0071] To address this problem, this application proposes a fringe projection scheme to solve the reliability issue of pixel tracking for moving objects. Based on the original projected fringe sequence, a projected all-white pattern is added, and pixel motion tracking is performed using this all-white pattern. Motion tracking is performed on the pixels of the all-white patterns before and after the projected fringe sequence. Based on the assumption that the speed of the measured object remains constant during the projection time period, the tracking position of pixels in the fringe pattern can be determined by interpolation based on the pixel tracking results of the preceding and following all-white images according to a time-proportional relationship. Figure 4 As shown.

[0072] The implementation effect of the solution proposed in this invention is as follows: Figure 5 As shown. The object under test is the online measurement of the three-dimensional morphology of the surface of a continuously cast billet in the iron and steel metallurgical industry. The surface texture of the continuously cast billet is very complex, such as... Figure 5 As shown in (a), the three-dimensional surface measurement results of the continuously cast billet under static conditions are as follows: Figure 5 As shown in (b), the three-dimensional measurement results under motion conditions are as follows: Figure 5 As shown in (c), the proposed solution has achieved excellent results. The three-dimensional surface measurement results of the measured object under motion conditions are basically the same as those of the measured object under static conditions. This breakthrough overcomes the limitations of phase-shifting profilometry in the three-dimensional surface measurement of moving objects and realizes high-precision online real-time measurement of the three-dimensional surface morphology of moving objects on actual production lines.

[0073] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments. In the above embodiments of the present invention, the descriptions of each embodiment have their own emphasis; parts not described in detail in a certain embodiment can be referred to in the relevant descriptions of other embodiments. It should be understood that the disclosed technical content in the several embodiments provided in this application can be implemented in other ways.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for three-dimensional measurement of the surface of a moving object based on phase-shifting profilometry, characterized in that, Includes the following steps: Given the known motion law of the object under test, the object is translated within a fixed plane with a fixed direction of motion. By setting the installation angle and position of the projector, the phase encoding direction or phase change direction in the projected fringe pattern is made perpendicular to the motion direction of the object, or the fringe direction in the projected pattern is the same as the motion direction of the object. The phase information of the projected fringe at the same point on the surface of the object remains unchanged during the motion, eliminating the influence of phase errors caused by motion. A camera mounted at another viewpoint tracks the motion of the object's surface, and three-dimensional measurement information of the object's surface is obtained by combining camera and projector stereo calibration. The phase information encoded in the projected fringe pattern is obtained through phase-shift profilometry. Phase-shift profilometry requires a fringe pattern with known phase shift and multiple corresponding fringe images acquired by the camera to solve for the phase information in the projected fringe pattern. N The step phase shift method; N In the step-shift method: When the object being measured moves, for the same point on the object's surface, the projection of the first... When creating a stripe pattern, the stripe image captured by the camera is as follows: ；（1） in, This indicates that the same point on the surface of the object being measured will appear as the first point projected onto the projector. When creating a striped pattern, the camera captures the pixel coordinates in the image. When = 1, the position of this point in the image captured by the camera is taken as the reference, let , ; and These represent the points at the [number]th [time]. The ambient light intensity and surface reflectivity at the time of step; This indicates that the point is at the th... The coded phase projected by the time-lapse projector; This represents the pixel position tracking result of the same point on the surface of the measured object within the camera image plane at different times. During the projection of N stripe patterns, the object's movement is minimal, meaning the position of the same point on the object's surface remains constant. The changes are not significant, and the ambient light is relatively stable, therefore it is approximately assumed that... and Unchanged, that is: ；（2） Assume the projector pixel plane coordinate system is ( u , v The direction of motion of the object being measured is parallel to v If the axis is specified, then the encoding method for the projected stripes is as follows: ；（3） in, T The stripe period.

2. The method for three-dimensional measurement of the surface of a moving object based on phase-shifting profilometry according to claim 1, characterized in that, Within the projector's pixel coordinate system, since the direction of motion of the measured object is parallel to... v The axis, and the phase encoding in equation (3) is related to v Since the direction is independent, the phase information of the same point on the surface of the measured object remains unchanged during the motion, that is: ；（4） Therefore, combining (2) and (4), equation (1) is transformed into ； （5） in, k Indicates the first k Zhang striped pattern, k = 1, 2, .., N ;( x , y () represents the pixel coordinates of a point on the surface of the object being measured in the camera image when the projector projects the first fringe pattern; and These represent the ambient light intensity and surface reflectivity of the object at that point, respectively.

3. The method for three-dimensional measurement of the surface of a moving object based on phase-shifting profilometry according to claim 1, characterized in that, Combined with pixel grayscale information acquired by the camera Set the image information to be acquired The corresponding wrap phase for: ；（6） Among them, the wrapping phase mentioned in equation (6) The calculation results are in arrive Between, combined with stripe levels Obtain the phase of the stripe image : （7）。 4. The method for three-dimensional measurement of the surface of a moving object based on phase-shifting profilometry according to claim 1, characterized in that, Phase acquired by the camera That is, the coordinates of the point in the projector pixel coordinate system are determined according to equation (3). u Assume the three-dimensional coordinates of this point are The following relationship is satisfied based on the stereo calibration results of the camera and projector. ；（8） In equation (8), and The results show the calibration results of the intrinsic and extrinsic parameters of the camera and projector, respectively. Camera pixel coordinates at the same point and projector pixel coordinates u Eliminate irrelevant terms according to equation (8) v , and After combining like terms, we get: ； （9） Solving the above equations simultaneously yields the three-dimensional coordinates of the point. This enables the measurement of the three-dimensional morphology of the surface of the object being measured.

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