Compact stripe monocular three-dimensional measurement system and method based on mechanical grating

Abstract

The invention discloses a compact stripe monocular three-dimensional measurement system and method based on a mechanical grating, and belongs to the technical field of measurement sensing. The invention aims to solve the problems that the existing three-dimensional measurement technology depends on expensive active devices, the structure is complex, and single-frame high-density fringe projection and dynamic measurement are difficult to realize. The system comprises a dense fringe projection device, a monocular acquisition module and a processing unit. The compact stripe projection device is composed of an active light source, a mechanical grating and an optical lens, and is used for projecting a single-frame compact stripe formed by the mechanical grating to the surface of an object to be measured. And the monocular acquisition module is used for acquiring a deformed stripe image which is subjected to height modulation by the surface of an object. And the processing unit is used for synchronously controlling projection and acquisition and resolving the acquired single-frame deformed stripe image based on a pre-established system calibration model so as to reconstruct a three-dimensional point cloud coordinate of the object. The passive mechanical grating is adopted to replace traditional digital projection elements such as DLP, MEMS and DOE, and monocular vision and a single-frame reconstruction algorithm are combined, so that the system has the advantages of low cost, small size, single camera and the like, and has wide application prospects in the fields of miniaturized 3D imaging modules, extremely small endoscopes, portable 3D imaging equipment, dynamic 3D imaging and the like.

Description

Technical Field

[0001] This invention relates to the field of sensor measurement, and more specifically, to a dense fringe monocular three-dimensional measurement system and method based on mechanical gratings. Background Technology

[0002] With the development of technology, the widespread use of industrial cameras, and the advancement of digital image processing technology, 3D measurement technology has also seen significant development. By performing digital 3D reconstruction of objects, their shape, size, and features can be effectively analyzed. 3D measurement technology has applications in industry...

[0003] Traditional projection-based 3D measurement technologies, especially those based on structured light, primarily rely on active optical components such as digital light processing (DLP), microelectromechanical systems (MEMS), or diffractive optical elements (DOE) to generate and project one or more sets of specific modulation patterns onto the surface of the object being measured. These components are not only expensive and have complex system structures, but their tooling size is also often limited, thus hindering their application in 3D measurement fields such as microscopic or extremely small endoscopic spaces. More importantly, due to limitations in manufacturing processes and digital image processing algorithms, these technologies typically struggle to generate extremely high-density fringe patterns (e.g., fringe count N < 40) in a single projection (single frame), limiting the efficiency of single-measurement operations. Furthermore, traditional methods such as phase-shifting structured light require projecting multiple frames of patterns, which is time-consuming and therefore ineffective for measuring dynamic objects. Additionally, most 3D cameras currently on the market are based on binocular models, and increasing the number of cameras also leads to increased cost and size.

[0004] Therefore, there is an urgent need in this field for a technical solution that is simple in structure, low in cost, small in size, and capable of single-frame high-density stripe projection and monocular dynamic three-dimensional measurement. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a compact fringe monocular 3D measurement system and method based on a mechanical grating. This solves the problems of existing 3D measurement equipment requiring separate optical components such as DLP, MEMS, and DOE, as well as limitations in tooling size, low fringe count per frame (N<40), and inability to perform dynamic measurements. By using a customized mechanical grating sheet, single-frame compact fringe projection (N>100) is achieved, while dynamic 3D measurement is realized through an innovative algorithm.

[0006] According to one aspect of the present invention, a three-dimensional scanning system is provided, characterized in that:

[0007] The system as a whole includes: a dense stripe projection device 1, a data acquisition module 2, an active light source 3, and a processing unit 4.

[0008] The dense stripe projection device 1 comprises an active light source 101, a mechanical grating 102, and an optical lens 103. The active light source 101 provides an illumination beam; the mechanical grating 102 is a physical optical element with a preset high-density periodic transparent / opaque or phase-modulated pattern engraved on it; the optical lens 103 images the dense stripe pattern formed by the mechanical grating and projects it onto the surface of the object to be measured.

[0009] Preferably, the mechanical grating is a high-precision physically engraved grating sheet or a chemically etched grating sheet. By customizing its grating constant and pattern, it can stably generate arbitrary dense sinusoidal or rectangular wave fringes with a single frame number N>100, or even hundreds or thousands. This fundamentally eliminates the dependence on digital projection elements and greatly simplifies the projection optical path structure.

[0010] Acquisition module 2 consists of a monocular camera. Acquisition module 2 and dense fringe projection device 1 are arranged at a certain angle to synchronously acquire the height-modulated deformed fringe image of the surface of the object under test from another perspective.

[0011] Preferably, the camera is a high-resolution, high-frame-rate industrial camera to ensure that the details of the deformed dense stripes can be captured clearly and quickly, providing hardware support for dynamic measurement.

[0012] Processing unit 4, for example, a PC computer. The processing unit 4 is connected to the active light source 101 and the acquisition module 2 via control lines to achieve precise time synchronization control of the light source switching, stripe projection, and image acquisition process.

[0013] The processing unit 4 is also equipped with a memory and a processor, which are used to receive the deformed stripe image returned by the camera module, run the pre-established model algorithm, solve the acquired image data, and finally calculate the three-dimensional point cloud coordinates (X,Y,Z) of the surface of the object to be measured, thereby reconstructing its complete three-dimensional shape information.

[0014] According to another aspect of the present invention, a three-dimensional scanning measurement method is provided, which utilizes the aforementioned three-dimensional scanning device, characterized by comprising the following steps:

[0015] System calibration: Before use, the 3D scanning device is geometrically calibrated to accurately determine the internal parameters of the camera module, the lens distortion coefficient, and the relative position and attitude relationship (i.e., rotation matrix R and translation vector T) between the camera coordinate system and the projector coordinate system defined by the projection device, and to establish an accurate mathematical mapping model from two-dimensional pixels of the image to three-dimensional points in space.

[0016] Single-frame projection and acquisition: The active light source 101 emits light, and after passing through the mechanical grating 102, the optical lens 103 projects a single frame of dense fringe pattern onto the surface of the object under test. Simultaneously, through synchronous control, a monocular camera is triggered to acquire a frame of deformed fringe image modulated by the object surface. For dynamic objects, this process can be performed continuously at a high frame rate to obtain an image sequence of deformed fringe patterns.

[0017] 3D Reconstruction Calculation: The acquired single-frame (or each frame in the sequence) deformed stripe image is transmitted to processing unit 4. Processing unit 4 executes an innovative reconstruction algorithm that can accurately extract stripe height modulation information from a single-frame high-density stripe image and perform depth calculation in conjunction with system calibration parameters.

[0018] Specifically, the calculation process may include: preprocessing the deformed fringe image by filtering and normalization; extracting the coordinates of the fringe center point and obtaining the fringe level; applying a pre-set fringe level model to calculate the depth of the fringe center point coordinates; finally, converting the depth data to the world coordinate system to obtain accurate three-dimensional coordinates X, Y, and Z, performing the above steps on all the fringe in the entire image to obtain dense point cloud data and complete the reconstruction of the object's three-dimensional shape.

[0019] Compared with the prior art, the embodiments of the present invention have the following significant advantages:

[0020] 1. Simplified structure and low cost: The use of passive, customized mechanical grating sheets replaces expensive and complex core components of digital projection such as DLP, MEMS, and DOE, greatly simplifying the system's optical path and hardware structure, reducing manufacturing costs, and facilitating system miniaturization and weight reduction. Simultaneously, the adoption of a monocular measurement model significantly reduces the overall cost, resulting in higher commercial value.

[0021] 2. High density per frame, high measurement accuracy: The mechanical grating enables stable and clear projection of dense fringes with a single frame fringe count N>100, far exceeding the single-frame capability of traditional digital projection equipment. Higher fringe density means a higher spatial sampling rate within the same field of view, thus significantly improving the resolution and accuracy of 3D measurements.

[0022] 3. Achieving Dynamic Measurement: Since a complete 3D reconstruction can be completed by acquiring only a single frame of fringe image, the measurement speed is only limited by the camera's frame rate, completely eliminating the time delay constraints of traditional multi-step phase-shifting methods. Combined with an innovative single-frame image processing algorithm, this invention can perform high-speed, real-time 3D topography measurement of objects in motion or vibration, greatly expanding the application scenarios of structured light technology. Attached Figure Description

[0023] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0024] Figure 1 This is a schematic diagram of the structural composition of a dense fringe monocular three-dimensional measurement system and method based on a mechanical grating, according to an embodiment of this application.

[0025] Figure 2 This is a schematic diagram of the composition of a dense stripe projection module provided according to an embodiment of this application;

[0026] Figure 3 This is a flowchart illustrating a compact fringe monocular three-dimensional measurement system and method based on a mechanical grating, according to an embodiment of this application.

[0027] The attached figures include the following reference numerals:

[0028] Dense stripe projection device 1;

[0029] Active light source 101, mechanical grating 102, optical lens 103;

[0030] 2. Acquisition module; 3. Power supply; 4. Processing unit; Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0032] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0033] Example 1

[0034] This embodiment provides a compact stripe monocular three-dimensional measurement system and method based on mechanical gratings.

[0035] System hardware composition

[0036] See Figure 1 The three-dimensional scanning device described in this embodiment includes: a dense stripe projection device 1, an acquisition module 2, a processing unit 4, and a power supply 3 for powering the system.

[0037] See Figure 2The dense stripe projection device 1 is composed of an active light source 101, a mechanical grating 102 and an optical lens 103 connected in series.

[0038] Active light source 101: In this embodiment, a high-brightness LED light source is preferably used, such as a white LED or a specific monochromatic LED (e.g., blue LED). LED light sources have advantages such as low heat generation, long lifespan, and fast response speed, and are easy to control precisely by means of a control line for switching and brightness control. In other embodiments, a laser diode in conjunction with a beam expander can also be used to form a uniform sheet-like or planar illumination beam.

[0039] Mechanical grating 102: This is the core component of the present invention. It is a physical optical element, and in this embodiment, it is a grating sheet fabricated on an optical-grade glass substrate using high-precision photolithography and chemical etching processes. The grating pattern consists of a series of parallel, equally spaced transparent and opaque fringes (i.e., a rectangular wave grating). By using a customized photolithographic mask, the period of the grating (grating constant) can be precisely controlled, thereby achieving a total number of projected bright and dark fringes N > 100, for example, N = 200 or N = 500. This customized mechanical grating sheet structure is stable, does not produce thermal deformation, and can generate dense fringes with high contrast and clear edges. In other embodiments, the mechanical grating can also be a physical grating with a sinusoidal transmittance distribution to project sinusoidal fringes.

[0040] Optical lens 103: This is a set of imaging lenses, such as standard C-mount or F-mount industrial lenses. Its function is to image the physical pattern on the mechanical grating 102 and clearly focus and project it onto the measurement space containing the object to be measured, which is tens of centimeters to several meters away. The focal length and aperture of the lens can be selected and adjusted according to the measurement distance and depth of field requirements.

[0041] Acquisition module 2 consists of only one monocular camera.

[0042] In this embodiment, the monocular camera is preferably a high-resolution (e.g., 5 megapixels or higher) and high-frame-rate (e.g., 60fps or higher) global shutter CMOS industrial camera. High resolution ensures clear resolution of the densely deformed stripes modulated by the object's surface, while the global shutter avoids image blurring or rolling shutter effects when measuring moving objects. The camera is equipped with its own optical lens, and its field of view must completely cover the stripe area projected by the projection device.

[0043] like Figure 1 As shown, the acquisition module 2 and the dense fringe projection device 1 are arranged at a certain angle (baseline distance) to form a triangulation geometry. The existence of this angle is the physical basis for realizing depth measurement, because it allows changes in the height of the object's surface to be converted into lateral displacements of the stripes in the camera image.

[0044] Processing unit 4 is a high-performance PC computer.

[0045] The PC is equipped with a high-speed processor (CPU), a large-capacity memory (RAM), and preferably a graphics processing unit (GPU) to accelerate parallel image processing and point cloud computing. The PC is connected to the acquisition module 2 via a high-speed data interface (such as USB 3.0 or GigE Vision) to receive image data; and is connected to the trigger ports of the dense stripe projection device 1 and the acquisition module 2 via I / O control lines (such as GPIO trigger lines) to achieve microsecond-level precise synchronization control.

[0046] Example 2

[0047] Please see Figure 3 The three-dimensional scanning measurement method described in this embodiment, implemented using the aforementioned device, specifically includes the following steps:

[0048] Step S1: System Calibration

[0049] Before taking measurements, the entire system must be precisely geometrically calibrated.

[0050] Camera intrinsic parameter calibration: Using the standard Zhang Zhengyou calibration method, the intrinsic parameter matrix K of camera 2 (including focal lengths fx, fy and principal points cx, cy) and lens distortion coefficients (such as radial distortion k1, k2, k3 and tangential distortion p1, p2) are calculated by photographing a planar checkerboard calibration board in different poses.

[0051] Dense Fringe Projection Device 2 - Camera Extrinsic Parameter Calibration: Here, the dense fringe projection device 2 can be assumed to be a projector model. This step is crucial for establishing the transformation relationship between the projection space and the camera space. Since the projector cannot "photograph" the calibration board, we adopt an indirect calibration method. The dense fringes are projected onto the checkerboard calibration board, which is also described above, and the camera simultaneously acquires a checkerboard image with deformed fringes.

[0052] Through image processing, we can obtain both the two-dimensional coordinates of the checkerboard corner points in the image and the deformation stripe information at the locations of these corner points. Since the world coordinates of the checkerboard are known, combined with the calibrated camera intrinsic parameters, we can determine the relative pose between the camera coordinate system and the world coordinate system (calibration board coordinate system).

[0053] For the projector, we consider the center line of each projected bright (or dark) fringe as a projection plane. By analyzing the images of the fringes on different pose calibration plates, we can solve for the spatial plane equation (in the camera coordinate system) corresponding to each fringe center line. This establishes a mapping model from the fringe series to the spatial plane. Ultimately, the calibration results of the entire system are expressed as camera intrinsic parameters, distortion coefficients, and the relative rotation matrix R and translation vector T between the camera coordinate system and the projector coordinate system (defined by a series of fringe planes).

[0054] Step S2: Single-frame projection and acquisition

[0055] Place the object to be measured within the measurement field of view.

[0056] Processing unit 4 sends a synchronization trigger signal through the control line.

[0057] The signal simultaneously triggers the active light source 101 to light up instantly. The beam of light, through the mechanical grating 102 and the optical lens 103, projects a fixed, high-density stripe pattern onto the surface of the object under test.

[0058] At the same time, the acquisition module 2 receives the same trigger signal, performs a global exposure, and acquires a frame of deformed stripe image modulated by the surface morphology of the object.

[0059] For static objects, this process only needs to be performed once. For dynamic objects, the processing unit 4 can continuously send trigger signals at high speed (e.g., at a frequency of 60Hz), while the acquisition module 2 synchronously and continuously acquires the image sequence of the deformed stripes and sends the image data stream back to the processing unit 4 in real time.

[0060] Step S3: 3D Reconstruction Calculation

[0061] Processing unit 4 performs the following calculations on each received frame of deformed stripe image to reconstruct the 3D point cloud:

[0062] Image preprocessing: The original image is preprocessed by filtering, normalization and other methods to remove noise.

[0063] Fringe center point extraction and series determination:

[0064] Center point extraction: For each column (or each row, depending on the stripe direction) in the image, scan its grayscale value distribution. Employ sub-pixel center point extraction algorithms such as grayscale centroid method, Steger method, and Gaussian fitting to accurately locate the center pixel coordinates (u,v) of each bright (or dark) stripe.

[0065] Stripe Order Determination: Establish a reference. During the calibration phase, a stripe image projected onto a reference plane (such as a flat white board) can be acquired. On this reference image, each stripe is assigned a unique index number (order) sequentially from left to right (or from top to bottom), for example, 1, 2, 3, ..., N. For newly acquired deformed stripe images, by tracing the continuity and topological relationships of the stripes and comparing them with the stripe positions in the reference image, the correct order can be assigned to the center line of each stripe in the deformed stripe image.

[0066] Depth calculation and 3D coordinate calculation:

[0067] For any fringe center point p(u,v) of a given order k on the image, we already know two sets of key information:

[0068] Camera ray: Based on the camera intrinsic parameters and the coordinates of point p(u,v), the equation of a three-dimensional spatial ray originating from the camera optical center and passing through the pixel can be determined.

[0069] Projection plane: Based on the fringe series-space plane mapping model obtained from step S1, we know that a fringe of series k corresponds to a space plane with a known equation in the camera coordinate system.

[0070] The point P(X,Y,Z) to be measured must be the unique intersection of the camera ray and the projection plane. By solving the simultaneous equations of the ray and the plane, the three-dimensional coordinates (X,Y,Z) of this point can be accurately calculated.

[0071] Point cloud generation: Repeat the depth calculation steps described above for all the stripe center points extracted from the image to obtain a series of discrete 3D coordinate points, which constitute a dense point cloud of the object's surface. At this point, a single-frame 3D topography measurement is complete.

[0072] Beneficial effects

[0073] This embodiment of the solution simplifies the system's optical path and hardware structure, and reduces manufacturing costs by replacing expensive and complex digital projection core components such as DLP and MEMS with a passive, customized mechanical grating sheet. Simultaneously, the monocular camera configuration further reduces the overall cost and size compared to binocular or multi-view systems, making it more commercially valuable and portable. The mechanical grating can stably project dense fringes with N>100, significantly improving the spatial resolution and accuracy of a single measurement. Most importantly, reconstruction can be completed with only a single frame image, and the measurement speed is only limited by the camera frame rate, thus successfully achieving high-speed 3D measurement of dynamic objects and overcoming the application limitations of existing technologies in dynamic scenes. The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A compact fringe monocular three-dimensional measurement system based on a mechanical grating, characterized in that, include: A. A dense stripe projection device, the projection device comprising: a. An active light source used to provide an illumination beam; b. A mechanical grating, which is a physical optical element having a pre-defined periodic pattern engraved thereon for forming a single frame of dense fringes when the illumination beam passes through it; c. An optical lens for imaging the single frame of dense stripes and projecting it onto the surface of the object to be measured; B. Acquisition module, characterized in that the acquisition module consists of a monocular camera and is arranged at a preset angle with the dense stripe projection device for acquiring deformed stripe images that are highly modulated by the surface of the object under test; C. A processing unit, connected to the active light source and the acquisition module, is used to synchronously control stripe projection and image acquisition, and is configured to receive the deformed stripe image, and calculate the three-dimensional coordinate data of the object under test from a single frame of the deformed stripe image based on preset system calibration parameters and reconstruction algorithm.

2. The system according to claim 1, characterized in that, The mechanical grating is a high-precision physically engraved grating sheet or a chemically etched grating sheet, on which the periodic patterns can generate dense sinusoidal or rectangular wave stripes with a single frame stripe number N>100.

3. The system according to claim 1, characterized in that, The monocular camera in the acquisition module is a high-resolution, high-frame-rate industrial camera.

4. The system according to claim 1, characterized in that, The processing unit is connected to the active light source and the acquisition module via a control line, and is used to achieve precise time synchronization control of the switching on and off of the active light source, stripe projection, and image acquisition process.

5. A method for monocular three-dimensional measurement of dense fringes based on mechanical gratings, characterized in that, This method is implemented using the system described in any one of claims 1 to 4, and includes the following steps: System calibration steps: Before use, the system is geometrically calibrated to determine the internal parameters of the acquisition module, the lens distortion coefficient, and the relative position and attitude relationship between the acquisition module and the dense stripe projection device, thereby establishing a mapping model between the two-dimensional pixel coordinates of the image and the three-dimensional coordinates in space. Single-frame projection and acquisition steps: control the active light source to emit light, and the beam is projected onto the surface of the object under test through the mechanical grating and optical lens; at the same time, the monocular camera is synchronously triggered to acquire a frame of deformed stripe image modulated by the surface of the object under test; 3D reconstruction calculation steps: The acquired deformed stripe image is transmitted to the processing unit. The processing unit executes the reconstruction algorithm and, in conjunction with the parameters obtained in the system calibration step, solves the deformed stripe image to calculate the 3D point cloud coordinates of the object under test, thus completing the 3D topography reconstruction.

6. The method according to claim 5, characterized in that, The system calibration steps specifically include: determining the internal parameters of the camera module, the lens distortion coefficient, and the rotation matrix R and translation vector T from the camera coordinate system to the projector coordinate system.

7. The method according to claim 5, characterized in that, The specific steps of the three-dimensional reconstruction calculation include: The deformed stripe image is preprocessed by filtering, normalization, etc. Extract the coordinates of the stripe center point and obtain the stripe order; Based on the parameters obtained from the system calibration steps, the depth of the fringe center point coordinates is calculated to obtain three-dimensional coordinate (X,Y,Z) data, thereby reconstructing the three-dimensional shape of the object under test.

8. The method according to claim 5, characterized in that, The single-frame projection and acquisition steps can be performed continuously at a high frame rate to obtain the deformation stripe image sequence of the object under test in the dynamic process, thereby realizing the three-dimensional measurement of the dynamic object.

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