Phase-height relationship calibration method based on phase-shifted fringe projection

Abstract

The application discloses a phase-height relation calibration method based on phase-shifted fringe projection, which comprises the following steps: determining a calibration body and a reference plane; assembling a projector and a CCD camera to form a measuring head, and placing the measuring head fixedly relative to the reference plane; calculating a phase-shifted fringe pattern of the reference plane and a phase-shifted fringe pattern of the calibration body by using a three-step phase shift method respectively; subtracting the phase of the phase-shifted fringe pattern of the reference plane from the phase of the phase-shifted fringe pattern of the calibration body to obtain a phase change of the standard body relative to the reference plane; and selecting two rows of data in the phase change, and calculating a phase-height relation coefficient according to the surface height of the calibration body corresponding to the two rows of data. The phase-height relation coefficient in the whole measuring range can be obtained by only one measurement and calculation process, and the method has the characteristics of rapidness and easy use.

Description

Technical Field

[0001] This invention relates to the field of calibration technology for three-dimensional measurement systems, and more specifically to a phase-height relationship calibration method based on phase-shifting fringe projection. Background Technology

[0002] Optical 3D shape measurement systems hold great potential for product inspection due to their ability to capture shape data from surfaces in the form of point clouds. This has enabled successful non-contact inspection of manufactured goods in various fields, such as the automotive industry, semiconductor inspection, food and pharmaceutical manufacturing. Optical 3D shape measurement systems can also allow manufacturers to inspect goods (silicon wafers, semiconductor chips, or painted vehicle surfaces) individually for defects, with the aim of controlling process parameters as soon as defects are detected. The flexibility, reliability, higher operating speed, consistency, and objectivity of this technology make it more competitive than traditional measurement systems.

[0003] Calibration is a fundamental requirement for vision-based measurement systems because it evaluates the vision system parameters necessary to infer three-dimensional information from two-dimensional images acquired from cameras. Due to its impact on the overall accuracy of the final measurements, and because many acquisition tasks must be performed in the field, often within limited timeframes, calibration has been the subject of extensive research.

[0004] Existing calibration methods often require adjusting the relative positions of the camera and projector to fit the surface being measured. However, the system must be recalibrated after each change in position and optical configuration, making the process cumbersome. Alternatively, a calibration plane parallel to the reference plane may need to be moved multiple times within the measurement range to different distances from the reference surface; or at least two parallel positions and an inclined plate may be required within the maximum measurement range. Therefore, the phase-height relationship coefficients need to be obtained through repeated measurements and calculations across the entire measurement range.

[0005] Therefore, how to provide a phase-height relationship calibration method that is fast, easy to use, and simplifies the measurement and calculation process is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a phase-height relationship calibration method based on phase-shifting fringe projection, which can obtain the phase-height relationship coefficients of the entire measurement range with only one measurement and calculation process, and has the characteristics of being fast and easy to use.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A phase-height relationship calibration method based on phase-shifting fringe projection includes:

[0009] Determine the calibration body and reference plane;

[0010] The projector and CCD camera are assembled into a measuring head and fixedly placed relative to the reference plane;

[0011] A set of sinusoidal fringe patterns is projected onto the reference plane using the projector. The fringe patterns on the reference plane are recorded by the CCD camera, and the phase-shifted fringe pattern φ of the reference plane is calculated using the three-step phase-shift method. R (x,y);

[0012] The calibration body is placed on the reference plane, and a set of sinusoidal fringe patterns is projected onto the calibration body using the projector. The fringe patterns on the calibration body are recorded by the CCD camera, and the phase-shifted fringe pattern φ of the calibration body is calculated using the three-step phase-shifting method. O (x,y);

[0013] φ O (x,y) and φ R Subtracting (x,y) yields the phase change p(x,y) of the standard body relative to the reference plane;

[0014] Take any two rows of data from the phase change p(x,y), and calculate the phase-height relationship coefficient based on the corresponding surface height of the calibrator body.

[0015] Furthermore, in the phase-height relationship calibration method based on phase-shifting fringe projection mentioned above, the calibration body has an inclined surface, and the height of the inclined surface covers the entire measurement height, and the width is greater than or equal to the maximum measurement width.

[0016] Furthermore, in the aforementioned phase-height relationship calibration method based on phase-shifting fringe projection, the shape of the calibration body is an inclined plane or a roof ridge.

[0017] Furthermore, in the phase-height relationship calibration method based on phase-shifting fringe projection described above, when the shape of the calibration body is a ridge surface, after placing the calibration body on the reference plane, the ridge line is adjusted so that it is parallel to the line connecting the optical centers of the projector and the CCD camera.

[0018] Furthermore, in the phase-height relationship calibration method based on phase-shifting fringe projection described above, each row of pixels in the phase change p(x,y) corresponds to a point on the calibration body surface with the same height.

[0019] Furthermore, in the aforementioned phase-height relationship calibration method based on phase-shifting fringe projection, the phase-shifting fringe pattern φ of the reference plane... R (x,y) and the phase shift fringe pattern φ of the calibratorO The calculation method for (x, y) is the same, including the following steps:

[0020] Three fringe patterns were recorded using a three-step phase-shifting method;

[0021] Calculate the wrap-around phase of the reference plane or the calibration body based on the three fringe patterns;

[0022] By adding or subtracting a multiple of 2π to the wrapped phase, the discontinuity of the arctan function at 2π is eliminated, thus obtaining the phase shift fringe pattern of the reference plane or the calibration body.

[0023] Furthermore, in the aforementioned phase-height relationship calibration method based on phase-shifting fringe projection, the formula for calculating the phase-height relationship coefficient is as follows:

[0024]

[0025]

[0026] Where p(x,i) and p(x,j) represent the i-th and j-th rows of data taken from p(x,y) on the surface of the calibration body, respectively; h(i) and h(j) represent the heights of the corresponding calibration body surfaces of the i-th and j-th rows of data in p(x,y), respectively.

[0027] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a phase-height relationship calibration method based on phase-shifting fringe projection. It utilizes a calibration body with an inclined surface that can cover the entire measurement height along the y-direction. The phase-height relationship coefficients a(x,y) and b(x,y) of the entire measurement range can be obtained in only one measurement and calculation process. The entire measurement process is fast, accurate and easy to use, and can meet the requirements of moving the measuring equipment to different positions, covering the entire scene or the entire workpiece. Attached Figure Description

[0028] 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 embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0029] Figure 1 A flowchart of the phase-height relationship calibration method based on phase-shifting fringe projection provided by the present invention;

[0030] Figure 2 A geometrical diagram illustrating the relationship between phase shift p and height h provided by the present invention;

[0031] Figure 3(a) is a front view of the optical principle diagram of the phase-height relationship calibration method based on phase-shifting fringe projection provided by the present invention;

[0032] Figure 3(b) is a right-hand view of the optical principle of the phase-height relationship calibration method based on phase-shifting fringe projection provided by the present invention;

[0033] Figures 4(a)-4(b) This is a schematic diagram of the structure of the calibrator provided by the present invention;

[0034] Figure 5 Phase shift fringe pattern of the reference plane provided for this invention;

[0035] Figure 6 The phase shift fringe pattern of the calibration body provided by the present invention;

[0036] Figure 7(a) is a phase distribution diagram of the surface of the calibrator provided by the present invention;

[0037] Figure 7(b) is a schematic diagram of the phase variation along the X direction at different heights of the quasi-body surface provided by the present invention;

[0038] Figure 8 A schematic diagram of the phase-height relationship coefficients provided by this invention;

[0039] Figure 9 The phase shift fringe pattern of the verification body provided by the present invention;

[0040] Figure 10 Phase diagram of the verification body provided by the present invention;

[0041] Figure 11 The measurement results of the verification body provided by the present invention. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 are within the scope of protection of the present invention.

[0043] like Figure 1 As shown in Figures 3(a) and 3(b), this embodiment of the invention discloses a phase-height relationship calibration method based on phase-shifting fringe projection, comprising:

[0044] S1. Determine the calibration body and reference plane;

[0045] S2. Assemble the projector and CCD camera to form a measuring head, and fix it relative to the reference plane;

[0046] S3. Project a set of sinusoidal fringe patterns onto the reference plane using a projector. Record the fringe patterns on the reference plane using a CCD camera. Calculate the phase-shifted fringe pattern φ of the reference plane using the three-step phase-shifting method. R (x,y);

[0047] S4. Place the calibrator on the reference plane, project a set of sinusoidal fringe patterns onto the calibrator using a projector, record the fringe patterns on the calibrator using a CCD camera, and calculate the phase-shifted fringe pattern φ of the calibrator using the three-step phase-shift method. O (x,y);

[0048] S5, φ O (x,y) and φ R Subtracting (x,y) gives the phase change p(x,y) of the standard body relative to the reference plane.

[0049] S6. Take any two rows of data from the phase change p(x,y), and calculate the phase-height relationship coefficient based on the corresponding surface height of the calibrator according to the two rows of data.

[0050] The following is combined Figure 2 The relationship between phase shift and altitude will be further explained.

[0051] Figure 2 In the diagram, Dc is the image plane of the CCD camera, and Prj is the object plane of the projector; I1 and I2 are the principal points of the projector and the CCD camera, respectively; α is the angle between the optical axes of the projector and the CCD camera. The direction of the projected fringes is parallel to the Y-axis.

[0052] Original geometric relations:

[0053] a and D are points on plane P1 with height h. On the image plane of camera Dc, they coincide with point C on the reference plane Pref. Let the coordinates of point C be (X, 0).

[0054] b. I1 and I2 are the optical centers (principal points) of the projection and the camera, respectively, with coordinates I1(Xp, Zp) and I2(0, Zc). The camera optical axis is perpendicular to the reference plane.

[0055] c. The X' axis is perpendicular to the projection optical axis. Therefore, the phase shift of point D relative to point C is proportional to the length of C'A', denoted as p.

[0056] Depend on Figure 2 The geometric relationship in the middle can be obtained as follows:

[0057] p=Lp(tan(α-β1)-tan(α-β2)) (1);

[0058] in:

[0059] tan(β1)=((Xp-X)) / Zp (2);

[0060] tan(β2)=(Xp-Xd) / (Zp-h)

[0061] tan(β2)=(ZcXp-(Zc-h)X) / Zc(Zp-h) (3)

[0062]

[0063] From equations (2), (3), and (4), we can obtain:

[0064]

[0065]

[0066] Substituting equations (5) and (6) into equation (1), we can obtain the relationship between phase and height:

[0067]

[0068] Or the relationship between altitude and phase change:

[0069]

[0070] Equations (7) and (8) neglect a coefficient related to the projected fringes.

[0071] It can be seen from the above equations that:

[0072] (1) After calculating the phase distribution caused by the height modulation of the measured object, the height distribution of the object is related to the configuration parameters of the measurement system, such as Zc, Zp, Xp, Lp; where (Xp, Zp) are the coordinates of the optical center of the projector; Zc is the z-coordinate of the optical center of the CCD camera (i.e., the vertical distance from the optical center of the camera to the reference plane); Lp is the distance from the optical center of the projector to point O (the origin of the reference plane coordinate system).

[0073] (2) After the positioning phase is given, the height of the measured point is still related to the X coordinate of the measuring point, but not to the Y coordinate of the measuring point.

[0074] Equation (8) can be rewritten in the following form:

[0075]

[0076] Therefore, by using appropriate calibration methods to determine the coefficients a(x) and b(x) under a given measurement system, the phase and height correspondence of each spatial point within the measurement volume of the system can be obtained.

[0077] Based on this, it can be found that the phase-height coefficients a(x,y) and b(x,y) are independent of y. A calibration body with an inclined surface covering the entire measurement height along the y-direction can be used. Only one measurement and calculation process is needed to obtain the a(x,y) and b(x,y) coefficients over the entire measurement range.

[0078] Specifically, the calibration device in this embodiment of the invention consists of a reference plane and a calibration body. The width W of the calibration body should be greater than or equal to the designed maximum measurement width, and the height h... max The measurement height should be close to the designed maximum. The direction of the inclined plane of the calibration body should be consistent with the direction of the projected fringes; the inclined plane angle α has no special requirements. Figures 4(a)-4(b) As shown, the shape of the calibration body can be a slope or a ridge, and the upper surface of the calibration body can be sprayed with a regular pattern in order to determine the height of the upper surface of the calibration body corresponding to each pixel in the two-dimensional image.

[0079] In one specific embodiment, in S3, when performing phase calculation on the reference plane, a three-step phase shift method is used to calculate the reference plane.

[0080] Capture the fringe pattern of a sinusoidal function with three-step phase shift on the reference plane, such as... Figure 5 As shown.

[0081] For the three-step phase-shift method, taking the reference plane as an example, the recorded fringe pattern can be represented as:

[0082] I R1 (x,y)=I0(x,y)+I mod (x,y)cos[φ R [(x,y)-2π / 3] (10);

[0083] I R2 (x,y)=I0(x,y)+I mod (x,y)cos[φ R (x,y)] (11);

[0084] I R3 (x,y)=I0(x,y)+I mod (x,y)cos[φ R (x,y)+2π / 3] (12).

[0085] Where I0(x,y) is the DC component (background), I mod (x,y) is the amplitude of the modulation signal, φ R (x, y) represents the phase of the reference plane, and 2π / 3 is the phase shift angle. The wrap-around phase of the reference plane can be calculated using the following equation:

[0086]

[0087] Because the arctangent operation folds the phase between 0 and 2π, it is called a "wrapped phase". In φ' R Adding or subtracting a multiple of 2π to the (x,y) values ​​can eliminate the discontinuity of the arctan function at 2π, a process known as phase unwrapping, resulting in the phase shift fringe pattern φ on the reference plane. R (x,y).

[0088] In S4, taking a ridge-shaped calibration body as an example, when the calibration body is shaped like a ridge, after placing the calibration body on the reference plane, adjust the ridge line to make it parallel to the line connecting the optical centers of the projector and the CCD camera. Then, repeat the three-step phase-shifting method in S3 to calculate the phase-shifted fringe pattern φ of the calibration body. O (x,y), such as Figure 6 As shown.

[0089] In S5, φ R Subtracting (x,y) from (x,y) yields the phase change p(x,y) of the standard body relative to the reference plane, as shown below. Figures 7(a)-7(b) As shown.

[0090] In S6, since the calibration body is a standard body with known geometric parameters, the height of the point on the calibration body surface corresponding to any pixel in the phase shift fringe pattern of the calibration body is easily determined. In this invention, the points on the calibration body surface corresponding to each row of pixels in the phase change p(x,y) of the standard body relative to the reference plane have the same height. According to the height-phase relationship equation (14):

[0091]

[0092] Take any two rows of data (e.g., row i and row j) p(x,i) and p(x,j) from the above phase calculation results; and obtain their corresponding calibration body surface heights h(i) and h(j); then the height phase relationship coefficients a(x) and b(x) can be calculated according to the following formula.

[0093]

[0094]

[0095] The coefficients a(x) and b(x) calculated from the above calibration body are as follows: Figure 8As shown. The ultimate goal of this invention is to quickly obtain the coefficients a(x) and b(x) (actually a(x,y) and b(x,y). Since equation (8) shows that the phase-height relationship is independent of the y-coordinate on the image plane, the phase-height relationship within the entire measurement range is determined by a(x) and b(x). Therefore, after calculating the phase distribution, the height distribution can be directly obtained from equation (14). This avoids the need to accurately measure system parameters such as Zc, Zp, Xp, and Lp to calculate the height distribution using equation (8). The structural parameters Zc, Zp, Xp, and Lp of the measurement system can be easily changed to adapt to different measurement environments.

[0096] To verify the effectiveness of the calibration method described above, a truncated square with a flat top height of 140 mm was measured. The three-step phase-shift sine function fringe pattern is shown below. Figure 9 As shown, the phase distribution diagram is as follows: Figure 10 As shown, the final measurement results are obtained using the phase-height relationship coefficient and equation (14). Figure 11 As shown.

[0097] Therefore, the phase-height relationship coefficients a(x) and b(x) obtained by this method can accurately reflect the phase-height relationship and obtain accurate measurement results.

[0098] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the method section.

[0099] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A phase-height relationship calibration method based on phase-shifting fringe projection, characterized in that, include: Determine the calibration body and reference plane; the calibration body has an inclined surface, and the height of the inclined surface covers the entire measurement height, and the width is greater than or equal to the maximum measurement width; the direction of the inclined surface of the calibration body should be consistent with the direction of the projected fringes, and the direction of the projected fringes should be parallel to the Y-axis; The projector and CCD camera are assembled into a measuring head and fixedly placed relative to the reference plane; A set of sinusoidal fringe patterns is projected onto the reference plane using the projector. The fringe patterns on the reference plane are recorded by the CCD camera, and the phase-shifted fringe pattern of the reference plane is calculated using the three-step phase-shift method. ; The calibration body is placed on the reference plane, and a set of sinusoidal fringe patterns is projected onto the calibration body using the projector. The fringe patterns on the calibration body are recorded by the CCD camera, and the phase-shifted fringe pattern of the calibration body is calculated using the three-step phase-shift method. ; Will and Subtraction yields the phase change of the calibrator relative to the reference plane. ; The upper surface of the calibration body is sprayed with a regular pattern in order to determine the height of the upper surface of the calibration body corresponding to each pixel in the two-dimensional image; Phase changes Take any two rows of data and calculate the phase-height relationship coefficient based on the corresponding surface height of the calibrator. , The formula for calculating the phase-height relationship coefficient is as follows: Where p(x,i) and p(x,j) represent the values ​​at x,i and x,j respectively. Take any two rows of data located on the surface of the calibration body, row i and row j; h(i) and h(j) represent respectively The height of the corresponding calibrator surface in rows i and j; The calibration body is a standard body with known geometric parameters.

2. The phase-height relationship calibration method based on phase-shifting fringe projection according to claim 1, characterized in that, The shape of the calibrator is a slope or a ridge surface.

3. The phase-height relationship calibration method based on phase-shifting fringe projection according to claim 1, characterized in that, Phase change Each row of pixels corresponds to a point on the surface of the calibration body with the same height.

4. The phase-height relationship calibration method based on phase-shifting fringe projection according to claim 1, characterized in that, Phase shift fringe pattern of the reference plane and the phase shift fringe pattern of the calibrator The calculation method is the same, including the following steps: Three fringe patterns were recorded using a three-step phase-shifting method; Calculate the wrap-around phase of the reference plane or the calibration body based on the three fringe patterns; By adding or subtracting a multiple of 2π to the wrapped phase, the discontinuity of the arctan function at 2π is eliminated, thus obtaining the phase shift fringe pattern of the reference plane or the calibration body.

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