Dual-frequency grating fringe projection system

Abstract

The invention, which relates to the technical field of structured light measurement, discloses a dual-frequency grating fringe projection system comprising a light source modulation group, a grating modulation group, a beam splitter prism and a projection lens. The light source modulation group comprises a first LED light source, a second LED light source, a first collimating lens group and a second collimating lens group; the grating modulation group comprises a first grating sheet, a second grating sheet, a first piezoelectric ceramic and a second piezoelectric ceramic; the first piezoelectric ceramic in the grating modulation group is connected with the first grating sheet, and the second piezoelectric ceramic is connected with the second grating sheet, so that the positions of the grating sheets are moved, and the stripe patterns at the bottoms of the grating sheets are translated. The system adopts the two piezoelectric ceramics to drive the grating sheets and the beam splitter prisms with different widths for beam combination, the structure is simple, and a low-cost double-frequency four-step phase shift pattern can be realized. The fine grating stripes can improve the measurement precision of three-dimensional reconstruction, and the wide grating stripes can improve the measurement depth of field of three-dimensional reconstruction, thereby improving the application scene of the structured light 3D system.

Description

Technical Field

[0001] The present invention relates to the technical field of structured light measurement, and in particular to a dual-frequency grating fringe projection system. Background Art

[0002] The structured light 3D measurement technology is also known as active triangulation. Its measurement principle is to project the encoded grating pattern onto the surface of the object through a projector, and the acquisition camera obtains the deformed fringe image modulated by the object's morphology. The modulated image contains the height information of the object, so that the 3D shape of the object can be obtained. However, the measurement accuracy and depth of field of single-frequency grating fringes still cannot meet the measurement requirements of higher requirements. In order to improve the measurement accuracy and depth of field, researchers in this field need to develop a dual-frequency grating fringe projection system to meet the applicability of measurement scenarios under high requirements. Summary of the invention

[0003] The present invention provides a dual-frequency grating fringe projection system, comprising: A light source modulation group (1), a grating modulation group (2), a beam splitter prism (3) and a projection lens (4); the light source modulation group (1) comprises a first LED light source (11), a second LED light source (12), a first collimating lens group (13) and a second collimating lens group (14); the grating modulation group (2) comprises a first grating plate (21), a second grating plate (22), a first piezoelectric ceramic (23) and a second piezoelectric ceramic (24); the first piezoelectric ceramic (23) in the grating modulation group is connected to the first grating plate (21), and the second piezoelectric ceramic (24) is connected to the second grating plate (22). The light source emitted by the first LED light source (11) in the light source modulation group is modulated by the first collimating lens group (13) and then irradiated to the first grating piece (21); the light source emitted by the second LED light source (12) is modulated by the second collimating lens group (14) and then irradiated to the second grating piece (22); the structured light modulated by the first grating piece (21) and the second grating piece (22) is then combined by the beam splitter prism (3) and finally projected onto the object to be measured by the projection lens (4) to obtain a dual-frequency grating fringe modulation pattern.

[0004] A dual-frequency grating fringe projection system as described above, wherein the first collimating lens group (13) and the second collimating lens group (14) respectively comprise a plurality of collimating lenses, the plurality of collimating lenses in the first collimating lens group (13) are connected in a vertical direction, and their central axes are kept consistent in the vertical direction; and the plurality of collimating lenses in the second collimating lens group (14) are connected in a horizontal direction, and their central axes are kept consistent in the horizontal direction.

[0005] A dual-frequency grating stripe projection system as described above, wherein the first collimating lens group (13) includes an upper lens (131) and a lower lens (132), and the corresponding second collimating lens group (14) includes a left lens (141) and a right lens (142), wherein the upper lens (131) and the right lens (142) are of the same size, the lower lens (132) and the left lens (141) are of the same size, and the upper lens (131) and the right lens (142) are smaller than the lower lens (132) and the left lens (141).

[0006] A dual-frequency grating stripe projection system as described above, wherein the stripe pattern widths of the first grating sheet (21) and the second grating sheet (22) are different, the first grating sheet (21) has fine grating stripes at the bottom, which are used to improve the measurement accuracy of three-dimensional reconstruction, and the second grating sheet (22) has wide grating stripes at the bottom, which are used to improve the measurement depth of field of three-dimensional reconstruction.

[0007] The present invention also provides a height information measurement method based on a dual-frequency grating fringe projection system, comprising: Step S10: the first LED light source emits collimated light to the first grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the first grating sheet is pushed by the first piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the first grating sheet is collected; Step S20: The second LED light source emits collimated light to the second grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the second grating sheet is pushed by the second piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the second grating sheet is collected; Step S30: generating a dual-frequency height map of the object under test based on the two sets of collected grating fringe modulation patterns; Step S40: Perform three-dimensional reconstruction of the object to be measured according to the generated dual-frequency height map to obtain a final high-precision three-dimensional model.

[0008] The beneficial effects achieved by the present invention are as follows: two piezoelectric ceramics are used to drive grating sheets of different widths, and a beam splitter prism is used to combine the beams. The structure is simple, and a low-cost dual-frequency four-step phase shift pattern can be achieved. Fine grating stripes can improve the measurement accuracy of three-dimensional reconstruction, and wide grating stripes can improve the measurement depth of three-dimensional reconstruction, thereby improving the applicable scenarios of structured light 3D systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0010] Figure 1 and Figure 2 This is a schematic diagram of a dual-frequency grating fringe projection system provided in Example 1 of the present application; Figure 3 It is a schematic diagram of the thin stripe pattern at the bottom of the grating sheet provided in the first embodiment of the present application; Figure 4 It is a schematic diagram of a wide stripe pattern at the bottom of a grating sheet provided in the first embodiment of the present application; Figure 5 This is a flow chart of a height information measurement method based on a dual-frequency grating fringe projection system provided in Example 2 of the present application. DETAILED DESCRIPTION

[0011] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0012] Embodiment 1 like Figure 1 As shown, the first embodiment of the present application provides a dual-frequency grating fringe projection system, including: a light source modulation group 1, a grating modulation group 2, a beam splitter prism 3 and a projection lens 4; the light source modulation group 1 includes a first LED light source 11, a second LED light source 12, a first collimating lens group 13 and a second collimating lens group 14; the grating modulation group 2 includes a first grating piece 21, a second grating piece 22, a first piezoelectric ceramic 23 and a second piezoelectric ceramic 24; the first piezoelectric ceramic 23 in the grating modulation group is connected to the first grating piece 21, and the second piezoelectric ceramic 24 is connected to the first grating piece 21. The second grating sheet is connected to the first grating sheet 21 and the second grating sheet is connected to the first grating sheet 21 and the second grating sheet is connected to the first grating sheet 21 and the second grating sheet is connected to the first grating sheet 21 and the second grating sheet 22, and the structured light modulated by the first grating sheet 21 and the second grating sheet 22 is combined by the beam splitter prism 3, and finally projected onto the object to be measured through the projection lens 4 to obtain a dual-frequency grating stripe modulation pattern.

[0013] In the embodiment of the present application, the number of light source modulation groups (1) and grating modulation groups (2) is set as required to obtain a multi-frequency grating stripe modulation pattern.

[0014] The first collimating lens group 13 and the second collimating lens group 14 respectively include a plurality of collimating lenses, wherein the plurality of collimating lenses in the first collimating lens group 13 are connected in a vertical direction, and their central axes are kept consistent in the vertical direction; the plurality of collimating lenses in the second collimating lens group 14 are connected in a horizontal direction, and their central axes are kept consistent in the horizontal direction. It should be noted that the number of lenses in the collimating lens group can be set as required under the permission of cost, so as to improve the coupling efficiency and image quality of LED collimation.

[0015] Figure 1 Take two collimating lenses as an example, see Figure 2 The first collimating lens group 13 includes an upper lens 131 and a lower lens 132, and the corresponding second collimating lens group 14 includes a left lens 141 and a right lens 142, wherein the upper lens 131 and the right lens 142 are of the same size, the lower lens 132 and the left lens 141 are of the same size, and the upper lens 131 and the right lens 142 are smaller than the lower lens 132 and the left lens 141.

[0016] like Figure 3 , Figure 4 As shown, the stripe pattern widths of the first grating sheet 21 and the second grating sheet 22 are different. The first grating sheet 21 has thin grating stripes at the bottom to improve the measurement accuracy of three-dimensional reconstruction, and the second grating sheet 22 has wide grating stripes at the bottom to improve the measurement depth of field of three-dimensional reconstruction.

[0017] The working principle of the system is: use two grating sheets of different widths, combine the beams through a beam splitter prism, and project them onto the projection object surface through a projection lens. The system can project two grating stripe patterns of different widths. Fine grating stripes can improve the measurement accuracy of three-dimensional reconstruction, and wide grating stripes can improve the measurement depth of field of three-dimensional reconstruction, thereby improving the applicability of structured light 3D systems.

[0018] During measurement, the first LED light source 11 is turned on first. The light emitted by the first LED light source 11 is collimated by the first collimating lens group 13 and then irradiated to the first grating piece 21. The stripe pattern at the bottom of the first grating piece 21 passes through the dichroic prism 3 and is then projected onto the object to be measured through the projection lens 4. At this time, the acquisition camera collects the mapped stripe pattern once, and then the first piezoelectric ceramic 23 pushes the first grating piece 21 to move three times, each time moving 1 / 2 of the grating stripe width. The acquisition camera synchronously collects the projected stripe pattern after each movement of the first grating piece 21. At this time, the fine grating stripe modulation pattern has been collected; the first LED light source 11 is turned off, and the second LED light source is turned on 12. The light emitted by the second LED light source 12 is collimated by the second collimating lens group 14 and then irradiated to the second grating piece 22. The stripe pattern at the bottom of the second grating piece 22 passes through the dichroic prism 3 and is then projected onto the object to be measured through the projection lens 4. The mapped stripe pattern is collected once using the acquisition camera. Subsequently, the second grating piece 22 is pushed to move three times by the second piezoelectric ceramic 24, each time by 1 / 2 of the grating stripe width. The acquisition camera synchronously collects the projected stripe pattern after each movement of the second grating piece 22. At this time, the wide grating stripe modulation pattern is also collected. The 8 collected images are used for three-dimensional reconstruction to obtain more accurate height information of the object to be measured.

[0019] Embodiment 2 like Figure 5 As shown, the second embodiment of the present application provides a height information measurement method based on a dual-frequency grating fringe projection system, comprising: Step S10: the first LED light source emits collimated light to the first grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the first grating sheet is pushed by the first piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the first grating sheet is collected; First, turn on the first LED light source. The light emitted by the first LED light source is collimated by the first collimating lens group 13 and then irradiated onto the first grating sheet. The stripe pattern at the bottom of the first grating sheet is projected onto the object to be measured by the projection lens after passing through the dichroic prism. At this time, the acquisition camera acquires the mapped stripe pattern once, and then the first piezoelectric ceramic is used to push the first grating sheet to move three times, each time by 1 / 2 of the grating stripe width. The acquisition camera synchronously acquires the projected stripe pattern after each movement of the first grating sheet. At this point, the fine grating stripe modulation pattern has been acquired, and the first LED light source is turned off.

[0020] Step S20: The second LED light source emits collimated light to the second grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the second grating sheet is pushed by the second piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the second grating sheet is collected; The second LED light source is turned on. The light emitted by the second LED light source is collimated by the second collimating lens group and then irradiated onto the second grating piece. The stripe pattern at the bottom of the second grating piece passes through the dichroic prism and is projected onto the object to be measured by the projection lens. The mapped stripe pattern is collected once using the acquisition camera. Subsequently, the second grating piece is pushed to move three times by the second piezoelectric ceramic, each time by 1 / 2 of the grating stripe width. The acquisition camera synchronously collects the projected stripe pattern after each movement of the second grating piece. At this point, the acquisition of the wide grating stripe modulation pattern is completed.

[0021] Step S30: generating a dual-frequency height map of the object under test based on the two sets of collected grating fringe modulation patterns; There are two groups of grating fringe modulation patterns collected, each with four pictures. The four-step phase shift method is used to represent the phase changes on the four pictures. The height information of the measured object is solved by using the relationship between phase and height, and the dual-frequency height map of the measured object is constructed. Specifically: Step S31: creating a phase value solving equation group according to the grayscale values ​​of pixels in the grating stripe adjustment pattern; The gray value corresponding to the object point (x, y) in the grating fringe adjustment pattern is expressed as :

[0022] in, is the intensity of ambient light, is the light wave amplitude value related to the projected light intensity, Point for the object The corresponding phase value, is the phase shift of the phase shift fringes.

[0023] When collecting grating stripes to adjust the pattern, the structured light stripes move each time cycles, which means the phase shift each time is , then the phase value solution equation group is expressed as:

[0024] Among them I 1 ,I 2 ,I 3 ,I 4 , respectively represent the grayscale value corresponding to the object point (x, y) in a set of grating fringe modulation patterns.

[0025] Step S32: Solving the equations in the equation group by simultaneous phase value calculation to respectively solve the phase value of each point of the measured object in the two groups of grating fringe modulation patterns; Using the formula: Solve the points of the object The corresponding phase value, where I 1 ,I2 ,I 3 ,I 4 , respectively represent the grayscale value corresponding to the object point (x, y) in a set of grating fringe modulation patterns.

[0026] Since the inverse tangent function is used in the phase value calculation formula, the phase value range is , such a phase is called a wrapped phase or a truncated phase. In order to reconstruct a continuous phase distribution, we need to unfold the wrapped phase, usually by comparing the truncated phase values ​​of two adjacent pixels, by adding and subtracting to restore the continuous phase.

[0027] Step S33: Calculate the height of the object surface according to the two sets of phase values ​​at each point of the measured object to generate a dual-frequency height map; Using formula To calculate the height h of each point of the measured object, AC represents the distance between the calculated point A and the adjacent point C, and β is the angle between the point C and the light projected by the projection lens. is the stripe interval, is the phase difference between point A and point C.

[0028] After calculation, two sets of phase values ​​can be obtained to obtain two sets of height data, namely the height data presented at the two stripe frequencies of fine grating stripes and wide grating stripes. The height map made based on these height data is stored as a dual-frequency height map.

[0029] Step S40: performing three-dimensional reconstruction of the object to be measured according to the generated dual-frequency height map to obtain a final high-precision three-dimensional model; Fine grating stripes can improve the measurement accuracy of 3D reconstruction, and wide grating stripes can improve the measurement depth of 3D reconstruction. In this step, the dual-frequency height map generated by the two grating stripes is combined to perform 3D reconstruction of the object to be measured, and the final high-precision 3D model is obtained to meet the measurement requirements of the application scenario to the greatest extent. It is divided into the following sub-steps: Step S41: Calculate the fusion coefficient of measurement accuracy and measurement depth of field in combination with the measurement requirements of the current application scenario; Take n expected measurement data of the object under test as standard data, and bring them into the fusion coefficient calculation formula together with the current n dual-frequency height data: The fusion coefficient of measurement accuracy and measurement depth of field is obtained, which is recorded as and ,in is the estimated value of the j-th measurement accuracy fusion coefficient, is the estimated value of the jth measured depth of field fusion coefficient, j ranges from 1 to m, m is the total number of estimated values ​​of fusion coefficients, and the subscript j is iterated after each summation formula is calculated. is the height data of the i-th point measured by the fine grating stripes in the dual-frequency height map, is the height data of the i-th point measured by the wide grating stripes in the dual-frequency height map, Y is the expected measurement data of the i-th point, i ranges from 1 to n, and n is the total number of points involved in the calculation. Returns the value of the expression in parentheses that minimizes the result. and .

[0030] Step S42: fusing the height data in the dual-frequency height map based on the calculated fusion coefficient; Substitute the height data of each point in the dual-frequency height map into the height fusion formula: In the process, the fused point height data is obtained and collected into a set to provide data support for subsequent 3D reconstruction. is the fused point height data, It is the height data measured by the fine grating stripes in the dual-frequency height map. It is the height data measured by the wide grating stripes in the dual-frequency height map. and are the fusion coefficients of measurement accuracy and measurement depth of field, is the error caused by a one-unit change in the fringe spacing, is the preset standard stripe spacing, is the spacing of the fine grating fringes, is the spacing of the wide grating stripes.

[0031] Step S43: reconstructing a three-dimensional model of the object under test using the fused height data; The spatial coordinates (X, Y, Z) of each sampling point of the measured object are obtained based on the fused height data. The resulting set of points is called a point cloud. The point cloud is further simplified into a mesh form. The mesh consists of a collection of vertices and polygons, usually triangles, quadrilaterals or other simple convex polygons. The point cloud data is converted into a polyhedron shape, and then the image is covered on the surface of the polyhedron to achieve texture mapping, which increases the details and realism of the model. The final three-dimensional model is the result of the combination of the polyhedron and the mapping or rendering.

[0032] Those skilled in the art will appreciate that in one or more of the above examples, the functions described in the present invention can be implemented using a combination of hardware and software. When software is used, the corresponding functions can be stored in a computer-readable medium or transmitted as one or more instructions or codes on a computer-readable medium. Computer-readable media include computer storage media and communication media, wherein communication media include any media that facilitates the transmission of computer programs from one place to another. Storage media can be any available media that can be accessed by general or special-purpose computers.

[0033] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made on the basis of the technical solution of the present invention should be included in the scope of protection of the present invention.

Claims

1. A dual-frequency grating fringe projection system, characterized in that: include: A light source modulation group (1), a grating modulation group (2), a beam splitter prism (3) and a projection lens (4); the light source modulation group (1) comprises a first LED light source (11), a second LED light source (12), a first collimating lens group (13) and a second collimating lens group (14); the grating modulation group (2) comprises a first grating plate (21), a second grating plate (22), a first piezoelectric ceramic (23) and a second piezoelectric ceramic (24); the first piezoelectric ceramic (23) in the grating modulation group is connected to the first grating plate (21), and the second piezoelectric ceramic (24) is connected to the second grating plate (22). The light source emitted by the first LED light source (11) in the light source modulation group is modulated by the first collimating lens group (13) and then irradiated to the first grating piece (21); the light source emitted by the second LED light source (12) is modulated by the second collimating lens group (14) and then irradiated to the second grating piece (22); the structured light modulated by the first grating piece (21) and the second grating piece (22) is then combined by the beam splitter prism (3) and finally projected onto the object to be measured by the projection lens (4) to obtain a dual-frequency grating fringe modulation pattern.

2. A dual-frequency grating fringe projection system according to claim 1, characterized in that: The number of light source modulation groups (1) and grating modulation groups (2) is set as required, and is used to obtain a multi-frequency grating stripe modulation pattern.

3. A dual-frequency grating fringe projection system according to claim 1, characterized in that: The first collimating lens group (13) and the second collimating lens group (14) respectively comprise a plurality of collimating lenses, wherein the plurality of collimating lenses in the first collimating lens group (13) are connected in a vertical direction, and their central axes are kept consistent in the vertical direction; and the plurality of collimating lenses in the second collimating lens group (14) are connected in a horizontal direction, and their central axes are kept consistent in the horizontal direction.

4. A dual-frequency grating fringe projection system according to claim 3, characterized in that: The first collimating lens group (13) comprises an upper lens (131) and a lower lens (132), and the corresponding second collimating lens group (14) comprises a left lens (141) and a right lens (142), wherein the upper lens (131) and the right lens (142) are of the same size, the lower lens (132) and the left lens (141) are of the same size, and the upper lens (131) and the right lens (142) are smaller in size than the lower lens (132) and the left lens (141).

5. A dual-frequency grating fringe projection system according to claim 1, characterized in that: The stripe patterns at the bottom of the first grating sheet (21) and the bottom of the second grating sheet (22) have different widths. The first grating sheet (21) has thin grating stripes at the bottom, which is used to improve the measurement accuracy of three-dimensional reconstruction, and the second grating sheet (22) has wide grating stripes at the bottom, which is used to improve the measurement depth of field of three-dimensional reconstruction.

6. A dual-frequency grating fringe projection system according to claim 1, characterized in that: During measurement, the first LED light source (11) is first turned on, and the light emitted by the first LED light source (11) is collimated by the first collimating lens group (13) and then irradiated onto the first grating sheet (21). The stripe pattern at the bottom of the first grating sheet (21) passes through the beam splitter prism (3) and is then projected onto the object to be measured by the projection lens (4). At this time, the acquisition camera collects the mapped stripe pattern once, and then the first piezoelectric ceramic (23) is used to push the first grating sheet (21) to move three times, each time by 1 / 2 of the grating stripe width. The acquisition camera synchronously collects the projected stripe pattern after each movement of the first grating sheet (21), and obtains four fine grating stripe modulation patterns. Then, the first piezoelectric ceramic (23) is turned off. An LED light source (11) is used, and a second LED light source (12) is turned on. The light emitted by the second LED light source (12) is collimated by a second collimating lens group (14) and then irradiated onto a second grating sheet (22). The stripe pattern at the bottom of the second grating sheet (22) passes through a beam splitter prism (3) and is then projected onto a measured object by a projection lens (4). A collection camera is used to collect the projected stripe pattern once. Subsequently, the second grating sheet (22) is pushed to move three times by a second piezoelectric ceramic (24), each time by 1 / 2 of the grating stripe width. The collection camera synchronously collects the projected stripe pattern after each movement of the second grating sheet (22), thereby obtaining four wide grating stripe modulation patterns.

7. A height information measurement method based on a dual-frequency grating fringe projection system, characterized in that: include: Step S10: the first LED light source emits collimated light to the first grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the first grating sheet is pushed by the first piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the first grating sheet is collected; Step S20: The second LED light source emits collimated light to the second grating sheet, which is then projected to the object to be measured by the projection lens after passing through the beam splitter prism, and the second grating sheet is pushed by the second piezoelectric ceramic, and the grating fringe modulation pattern generated during the movement of the second grating sheet is collected; Step S30: generating a dual-frequency height map of the object under test based on the two sets of collected grating fringe modulation patterns; Step S40: Perform three-dimensional reconstruction of the object to be measured according to the generated dual-frequency height map to obtain a final high-precision three-dimensional model.

8. The height information measurement method based on the dual-frequency grating fringe projection system according to claim 7 is characterized in that: Generate a dual-frequency height map of the object under test based on the two sets of grating fringe modulation patterns collected, which specifically includes the following sub-steps: A system of equations for solving phase values ​​is created based on the grayscale values ​​of pixels in the grating stripe adjustment pattern; Solve the equations in the equation group by simultaneous phase value calculation to respectively solve the phase value of each point of the measured object in the two groups of grating fringe modulation patterns; The height of the object surface is calculated according to the two sets of phase values ​​at each point of the measured object to generate a dual-frequency height map.

9. The height information measurement method based on the dual-frequency grating fringe projection system according to claim 7, characterized in that: The three-dimensional reconstruction of the object to be measured is performed according to the generated dual-frequency height map to obtain the final high-precision three-dimensional model, which specifically includes the following sub-steps: Calculate the fusion coefficient of measurement accuracy and measurement depth of field based on the measurement requirements of the current application scenario; fusing the height data in the dual-frequency height map based on the calculated fusion coefficient; The fused height data is used to reconstruct the 3D model of the object being measured.

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