A monocular camera calibration device and method

Abstract

The present application relates to the field of computer vision and photogrammetry, and discloses a monocular camera calibration device and method, comprising a calibration arm; a rotation center component arranged on the calibration arm, used to provide a fixed spatial rotation center reference during rotation; at least two mark points arranged on the calibration arm and having a known, fixed spatial geometric relationship with the spatial center position of the rotation center component; and a base used to cooperate with the rotation center component to enable rotation around the fixed spatial rotation center reference. The present application simplifies the calibration process in the traditional method which relies on multi-view spatial intersection into motion around a single fixed rotation center, and can obtain multi-angle sample data at one time through continuous rotation of the calibration arm, thereby avoiding the cumbersome operation of repeatedly adjusting the pose of the calibration plate and fundamentally improving the degree of automation and overall efficiency of the calibration process.

Description

Technical Field

[0001] This invention belongs to the field of computer vision and photogrammetry technology, specifically relating to a monocular camera calibration device and method, and more specifically to the lightweight design of the monocular camera calibration device and the optimization of the calibration method. Background Technology

[0002] Camera calibration is a crucial step in computer vision. Currently, monocular camera calibration mainly relies on three-dimensional calibration objects (such as polyhedral calibration blocks) or planar calibration boards (such as checkerboard boards). However, three-dimensional calibration objects are bulky, inconvenient to carry, and require precise machining to ensure multi-angle geometric accuracy. Planar calibration boards, on the other hand, require frequent manual pose changes and strict maintenance of the board surface, making the operation cumbersome and susceptible to environmental occlusion. Both of these technologies suffer from poor spatial adaptability (difficult to deploy in confined spaces), low operational efficiency (relying on multiple pose adjustments to extend calibration time), and insufficient accuracy stability (planar calibration boards are prone to feature point extraction errors due to physical deformation or occlusion). The root cause lies in the fact that traditional methods must construct multi-view geometric constraints through feature points distributed in two-dimensional / three-dimensional space, leading to complex calibration device structures and redundant operation processes. Summary of the Invention

[0003] The purpose of this invention is to provide a monocular camera calibration device and method to solve the problems of large size and cumbersome operation of traditional calibration objects.

[0004] Based on the above concept, the technical solution adopted by this invention is as follows: According to a first aspect of the present invention, a monocular camera calibration apparatus is provided, comprising: Calibration arm; A rotation center component, mounted on the calibration arm, is used to provide a fixed spatial rotation center reference during rotation. Multiple marker points are distributed non-collinearly on at least two of the calibration arms and have a known and fixed spatial geometric relationship with the spatial center position of the rotation center component, wherein at least one marker point deviates from the straight line determined by the other marker points and the rotation center; The base is provided with a constraint mechanism and a driving device that cooperate with the rotation center component, for driving the multi-arm segment calibration structure to rotate controllably around the fixed spatial rotation center reference.

[0005] In some embodiments, the rotation center component includes a cooperating conical surface and a shaft system, and the spatial rotation center reference is the axis of rotation of the shaft system.

[0006] In some embodiments, the known, fixed spatial geometric relationship is that the center of the marker point and the rotation center reference together constitute a two-dimensional or three-dimensional non-collinear feature point set.

[0007] In some embodiments, the calibration arm is a rod-shaped, plate-shaped, or L-shaped structure, and can be expanded and folded in multiple sections via hinges or quick-release joints.

[0008] In some embodiments, the markers are programmable electronic ink screen markers, or are directly printed onto the surface of the calibration arm using a high-contrast coating.

[0009] In some embodiments, the driving device of the base is a stepper motor or a servo motor, and the constraint mechanism is an electromagnetic coupling drive device or a precision air bearing.

[0010] According to a second aspect of the present invention, a calibration method for a monocular camera calibration device is provided, comprising the following steps: The rotation center component of the calibration device is placed in the constraint mechanism of the base to establish a fixed spatial rotation center; The base is driven to rotate the calibration arm continuously by at least 180 degrees around the fixed spatial rotation center, while a fixed monocular camera is used to acquire a continuous frame image sequence containing non-collinearly distributed marker points. Extract the subpixel-level coordinates of each marker point on the image plane from each frame of the image sequence; Based on the known non-collinear spatial geometric constraints between the marker points and the fixed spatial rotation center, and the elliptical or circular arc motion trajectory formed by each marker point on the image plane, a target optimization function that integrates multi-view geometry and rotational kinematics model is constructed. The objective optimization function is solved using a nonlinear optimization algorithm, and the complete intrinsic parameter matrix and lens distortion coefficients of the monocular camera are obtained in one step.

[0011] In some embodiments, during the model building step, the objective function of the optimization model is constructed by using the characteristic that the trajectory of the marker point on the image plane is a circular arc or an ellipse, through a least squares fitting algorithm.

[0012] In some embodiments, the physical position and optical focal length of the monocular camera remain unchanged during image acquisition.

[0013] In some embodiments, the known spatial geometric constraint is that the marker point is collinear with a fixed spatial rotation center.

[0014] The beneficial effects of this invention are as follows: 1. This invention simplifies the calibration process, which relies on multi-view spatial intersection in traditional methods, into motion around a single fixed rotation center. By continuously rotating the calibration arm, multi-angle sample data can be acquired at one time, thereby avoiding the tedious operation of repeatedly adjusting the pose of the calibration plate, and fundamentally improving the automation level and overall efficiency of the calibration process.

[0015] 2. This invention, based on the strict collinear geometric constraint between the marker point and the rotation center, constructs the parameter solution as a one-dimensional optimization problem. This fundamentally eliminates errors introduced by physical deformation or instantaneous jitter of the planar calibration plate and effectively suppresses the influence of partial occlusion on feature extraction. This design significantly improves the accuracy and stability of the calibration results, making it highly applicable in complex scenarios such as confined spaces, vibration environments, and on-site operations. Attached Figure Description

[0016] Figure 1 This is a top view schematic diagram of the monocular camera calibration device of the present invention; Figure 2 This is a front view schematic diagram of the monocular camera calibration device of the present invention; Figure 3 This is a schematic diagram illustrating the implementation of the monocular camera target method of the present invention.

[0017] Figure label: Rotation center component 1, calibration arm 2, marker point 3, base 4, monocular camera 5. Detailed Implementation

[0018] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention as detailed in the appended claims.

[0019] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, the technical or scientific terms used in this application should be understood in their ordinary sense by one of ordinary skill in the art to which this invention pertains. The words “a” or “one” and similar terms used in this application specification and claims do not indicate a limitation of quantity, but rather indicate the presence of at least one. “A plurality” means two or more. The words “comprising” or “including” and similar terms mean that the element or object preceding “comprising” or “including” covers the element or object listed following “comprising” or “including” and its equivalents, and does not exclude other elements or objects. The words “connected” or “linked” and similar terms are not limited to physical or mechanical connections and can include electrical connections, whether direct or indirect. The words “above” and / or “below” and similar terms are for ease of description only and are not limited to a location or spatial orientation. The singular forms “a,” “the,” and “the” used in this application specification and appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0020] The technical concept of this invention includes: Traditional monocular camera calibration methods rely on two-dimensional planar calibration boards or polyhedral three-dimensional calibration objects, which suffer from three major bottlenecks: cumbersome calibration procedures, poor environmental adaptability, and limited accuracy. Specifically, the need for multiple manual adjustments to the calibration board pose leads to low operational efficiency; the calibration board is susceptible to physical deformation and occlusion, introducing errors in feature point extraction; and it is difficult to deploy effectively in confined spaces or complex scenes.

[0021] To address the aforementioned issues, this solution proposes a novel calibration paradigm centered on a "fixed rotation center + collinear marker points." By transforming multi-view spatial pose acquisition into rotational motion around a fixed axis, and utilizing the rigid geometric constraints between marker point 3 and the rotation center to construct a one-dimensional optimization model, the traditional discrete pose acquisition is optimized into continuous rotational sampling, significantly improving data acquisition efficiency. Collinear constraints reduce the dimensionality of the 3D reconstruction problem, fundamentally suppressing planar deformation errors. A lightweight rod-type structure overcomes spatial deployment limitations.

[0022] Therefore, through structural principle innovation and method reconstruction, this invention achieves a synergistic improvement in the portability, operational efficiency, and measurement accuracy of the calibration device, providing key technical support for the precise application of monocular cameras in fields such as industrial inspection and mobile robots.

[0023] This application provides a monocular camera calibration device and method, including Calibration arm 2; Rotation center component 1 is disposed on the calibration arm 2 and is used to provide a fixed spatial rotation center reference during rotation; Multiple marker points 3 are distributed non-collinearly on at least two of the calibration arms 2 and have a known and fixed spatial geometric relationship with the spatial center position of the rotation center component 1, wherein at least one marker point 3 deviates from the straight line determined by the other marker points 3 and the rotation center; The base 4 is provided with a constraint mechanism and a driving device that cooperate with the rotation center component 1, for driving the calibration arm 2 to rotate controllably around the fixed spatial rotation center reference.

[0024] This invention simplifies the calibration process, which relies on multi-view spatial intersection in traditional methods, into motion around a single fixed rotation center. By continuously rotating the calibration arm 2, multi-angle sample data can be acquired at once, thus avoiding the tedious operation of repeatedly adjusting the pose of the calibration plate. This fundamentally improves the automation level and overall efficiency of the calibration process. Based on the strict collinear geometric constraint between the marker point 3 and the rotation center, this invention constructs the parameter solution as a one-dimensional optimization problem, which eliminates the error introduced by the physical deformation or instantaneous jitter of the planar calibration plate in principle, and effectively suppresses the influence of partial occlusion on feature extraction.

[0025] The calibration method for a monocular camera calibration device described in this application includes the following steps: The rotation center component 1 of the calibration device is placed in the constraint mechanism of the base 4 to establish a fixed spatial rotation center; The base 4 is driven to rotate the calibration arm 2 continuously by at least 180 degrees around the fixed spatial rotation center, while a fixed monocular camera is used to acquire a continuous frame image sequence containing non-collinearly distributed marker points. Extract the subpixel-level coordinates of each marker point on the image plane from each frame of the image sequence; Based on the known non-collinear spatial geometric constraints between the marker point 3 and the fixed spatial rotation center, and the elliptical or circular arc motion trajectory formed by each marker point on the image plane, a target optimization function that integrates multi-view geometry and rotational kinematics model is constructed. The objective optimization function is solved using a nonlinear optimization algorithm, and the complete intrinsic parameter matrix and lens distortion coefficients of the monocular camera are obtained in one step.

[0026] The following is in conjunction with the appendix Figures 1 to 3 This application provides a detailed description of a monocular camera calibration device and method.

[0027] like Figures 1 to 3As shown, this embodiment provides a basic form of a monocular camera 5 calibration device. The device includes four core components: a calibration arm 2, a rotation center component 1, a marker point 3, and a base 4.

[0028] In this embodiment, the calibration arm 2 adopts a cylindrical rod structure, made of aerospace-grade aluminum alloy through precision machining, ensuring sufficient structural rigidity while achieving lightweight design. The rod surface is anodized to form a uniform dark background, facilitating the identification and extraction of the marker points 3. The rod is 300mm long and 15mm in diameter; this optimized design provides sufficient space for the distribution of marker points 3 while ensuring the overall portability of the device.

[0029] In this embodiment, the rotating center component 1 is a high-precision spherical probe made of tungsten carbide hard alloy with a sphere diameter of 10mm. The spherical probe is connected to one end of the calibration arm 2 via a thread, and coaxiality after installation is ensured by a locating pin. The center of the spherical probe constitutes the spatial rotation center reference of the device, and its high hardness ensures that the stability of the rotation center is not affected by wear during repeated use.

[0030] In this embodiment, the marker points 3 are arranged in a black and white concentric circle pattern and are directly fabricated on the surface of the calibration arm 2 using photolithography. There are five marker points 3, evenly spaced along the axial direction of the calibration arm 2, with a center-to-center distance of 50mm between adjacent marker points 3. The centers of all marker points 3 are strictly collinear with the center of the spherical probe, and the straightness error is controlled within ±0.02mm. The marker points 3 have a diameter of 8mm and clear edge features, facilitating sub-pixel-level precise positioning by image processing algorithms.

[0031] In this embodiment, the base 4 is made of stainless steel and has a precision circular hole at the top as a constraint mechanism. The bottom of the base 4 is equipped with adjustable support feet, which can be adjusted to achieve precise leveling of the plane of the base 4, ensuring that the center of rotation is perpendicular to the direction of gravity.

[0032] During calibration, first place the base 4 on a stable platform and adjust it with a level to ensure the upper surface of the base 4 is horizontal. Then, gently insert the spherical probe into the circular hole of the base 4, ensuring full contact between the probe and the hole wall. Start the monocular camera 5, positioning its optical axis approximately perpendicular to the rotation plane of the calibration arm 2, keeping the camera position fixed. Manually or via a low-speed motor, rotate the calibration arm 2 around the spherical probe at a uniform speed, controlled within the range of 2-5° / second. During rotation, the camera continuously acquires image sequences containing the marker point 3 at a rate of 10 frames / second, with a total acquisition angle range of no less than 180°.

[0033] After acquiring the image sequence, the improved Canny operator edge detection algorithm is used to extract the contour of marker point 3 in each frame. Then, the center coordinates of marker point 3 are calculated using an ellipse fitting method. Based on the collinearity constraint between all marker points 3 and the rotation center, and combined with the motion trajectory characteristics of marker points 3 on the image plane, a projection geometric model is established. Specifically, the trajectory of each marker point 3 on the image plane during rotation forms an ellipse. By fitting these ellipses using the least squares method, the camera's intrinsic parameters, including focal length, principal point coordinates, and the lens's radial distortion coefficient, can be inversely calculated.

[0034] The advantages of this basic embodiment are its simple structure and low cost, while meeting the calibration requirements of most industrial vision systems. Through strict collinearity constraints and an optimized rotation sampling strategy, calibration data equivalent to 20-30 different poses using traditional methods can be obtained in a single rotation, significantly improving calibration efficiency.

[0035] This embodiment provides another basic form of monocular camera 5 calibration device. In this embodiment, the calibration device is extended from the basic rod structure. The calibration arm 2 adopts a cross-shaped structure, consisting of four mutually perpendicular arm segments, each 200mm long. The cross-shaped calibration arm 2 is fixed to the rotating center component 1 through a central connecting block. The central connecting block has a weight-reducing chamber inside, which minimizes the weight of the device while ensuring structural rigidity.

[0036] In this embodiment, the rotation center component 1 is still a spherical probe, but its diameter is increased to 15mm to provide more stable rotational support. Regarding the placement of the marker points 3, three marker points 3 are distributed on each arm segment, with a spacing of 60mm between adjacent marker points 3, for a total of twelve marker points 3 across the four arm segments. These marker points 3 are not only collinear with the rotation center, but more importantly, the marker points 3 on different arm segments form multiple collinear constraint groups, greatly enhancing the geometric constraint strength of the calibration system.

[0037] In this embodiment, the structure of the base 4 is adjusted accordingly, and the constraint mechanism adopts a cross-shaped groove design with a groove width of 15.05 mm and a depth of 8 mm. The spherical probe is located in the hemispherical positioning socket at the intersection of the grooves. This design not only ensures the accurate positioning of the rotation center, but also allows the calibration device to be tilted within a range of ±5°, thus expanding the spatial adaptability of the calibration device.

[0038] During calibration, the calibration device can rotate a full circle around the vertical axis or swing within a limited angle. This multi-arm structure allows for the acquisition of more spatially distributed marker point images during a single calibration process, which is particularly beneficial for the accurate calculation of camera distortion parameters.

[0039] This embodiment provides another basic form of monocular camera 5 calibration device. This embodiment is specifically designed for applications requiring high portability, such as field surveying and on-site services. The calibration arm 2 adopts a three-section foldable structure connected by high-precision hinges. When unfolded, its total length is 400mm, and when folded, its dimensions are only 150mm × 50mm × 40mm, allowing it to be placed in a dedicated carrying case.

[0040] The rotating center component 1 is detachable. The spherical probe is connected to the calibration arm 2 via a quick connector. When not in use, it can be stored separately in a protective box to avoid collision damage. The marker point 3 uses flexible electronic ink screen technology, which can change the pattern of the marker point 3 through program control and maintain the display state after power failure, greatly enhancing the environmental adaptability of the device.

[0041] Base 4 employs a magnetic adsorption design, integrating a strong magnet at the bottom, allowing it to be directly attached to steel structure surfaces for calibration. A vacuum suction cup option is also available for fixing to non-ferromagnetic surfaces. The constraint mechanism on base 4 is an automatic centering structure; when the spherical probe is inserted, three sets of spring-loaded ceramic balls automatically center and fix it, ensuring repeatability and accuracy of the rotation center.

[0042] This embodiment is particularly suitable for field or high-altitude work environments such as wind turbine blade inspection, bridge monitoring, and large equipment installation. Workers can easily carry the device to the work site, quickly deploy it, and complete camera calibration, solving the pain point of traditional calibration equipment being difficult to use in complex environments.

[0043] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. The invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0044] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A monocular camera calibration device, characterized in that, include: Calibration arm; A rotation center component, mounted on the calibration arm, is used to provide a fixed spatial rotation center reference during rotation. Multiple marker points are distributed non-collinearly on at least two of the calibration arms and have a known, fixed spatial geometric relationship with the spatial center position of the rotation center component, wherein at least one marker point deviates from the straight line determined by the other marker points and the rotation center; The base is provided with a constraint mechanism and a driving device that cooperate with the rotation center component, for driving the calibration arm to perform controllable active rotation around the fixed spatial rotation center reference.

2. The monocular camera calibration device according to claim 1, characterized in that, The rotation center component includes a cooperating conical surface and a shaft system structure, and the spatial rotation center reference is the rotation axis of the shaft system structure.

3. The monocular camera calibration device according to claim 1, characterized in that, The known, fixed spatial geometric relationship is that the center of the marker point and the rotation center reference together form a two-dimensional or three-dimensional non-collinear feature point set.

4. The monocular camera calibration device according to claim 1, characterized in that, The calibration arm is a rod-shaped, plate-shaped, or L-shaped structure, and can be expanded and folded in multiple sections through hinges or quick-release joints.

5. A monocular camera calibration device according to claim 1, characterized in that, The markers are programmable electronic ink screen markers, or they can be directly printed on the surface of the calibration arm using high-contrast coating.

6. The monocular camera calibration device according to claim 1, characterized in that, The driving device of the base is a stepper motor or a servo motor, and the constraint mechanism is an electromagnetic coupling drive device or a precision air bearing.

7. A calibration method for a monocular camera calibration device as described in any one of claims 1-6, characterized in that, Includes the following steps The rotation center component of the calibration device is placed in the constraint mechanism of the base to establish a fixed spatial rotation center; The base is driven to rotate the calibration arm continuously by at least 180 degrees around the fixed spatial rotation center, while a fixed monocular camera is used to acquire a continuous frame image sequence containing non-collinearly distributed marker points. Extract the subpixel-level coordinates of each marker point on the image plane from each frame of the image sequence; Based on the known non-collinear spatial geometric constraints between the marker points and the fixed spatial rotation center, and the elliptical or circular arc motion trajectory formed by each marker point on the image plane, a target optimization function that integrates multi-view geometry and rotational kinematics model is constructed. The objective optimization function is solved using a nonlinear optimization algorithm, and the complete intrinsic parameter matrix and lens distortion coefficients of the monocular camera are obtained in one step.

8. The monocular camera calibration method according to claim 7, characterized in that, In the model construction step, the objective function of the optimization model is constructed by using the characteristic that the trajectory of the marker point on the image plane is a circular arc or an ellipse, through the least squares fitting algorithm.

9. The monocular camera calibration method according to claim 7, characterized in that, The physical position and optical focal length of the monocular camera remain unchanged during image acquisition.

10. The monocular camera calibration method according to claim 7, characterized in that, The known spatial geometric constraint is that the marker point is collinear with a fixed spatial rotation center.

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