CN114705216B - Secondary calibration method for three-dimensional vision measurement system - Google Patents

Abstract

The invention relates to the technical field of calibration of vision measurement systems, and discloses a secondary calibration method for a three-dimensional vision measurement system, which comprises the following steps of S1: performing primary calibration on the system according to the world coordinate value and the image coordinate value; step S2: three-dimensional reconstruction is carried out on the standard component by using a system for primary calibration, and calibration errors are evaluated according to reconstruction results; step S3: optimizing world coordinate values when the primary system is calibrated according to the calibration error evaluation result, and carrying out secondary calibration on the primary calibrated system by utilizing the optimized world coordinate values; step S4: and reconstructing the standard component by using the system after secondary calibration until the result of reconstructing the standard component and the theoretical value error thereof meet the preset condition, thereby obtaining a high-precision calibration result. The method is based on an optimization algorithm, and performs deviation analysis on the reconstruction result and the theoretical value of the standard component so as to obtain an optimization target with the minimum error of the reconstruction standard component, and performs high-precision secondary calibration on the system.

Description

A secondary calibration method for 3D vision measurement system

Technical Field

The present invention relates to the technical field of visual measurement system calibration, and in particular to a secondary calibration method for a three-dimensional visual measurement system.

Background Art

Visual measurement has the advantages of non-contact, fast measurement speed, high accuracy, and flexible measurement methods. It is widely used in many fields such as defect detection and three-dimensional surface measurement. The three-dimensional visual measurement method has become one of the best methods for three-dimensional measurement due to its fast speed, high accuracy, and strong practicality. The key to three-dimensional visual measurement is to obtain the relationship between the pixel coordinates of the camera image and its corresponding coordinates in the world coordinate system. In order to accurately describe this relationship, it is necessary to calibrate the parameters of the measurement system composed of cameras, that is, system calibration. In three-dimensional visual measurement, system calibration is the basic step and key link of the system operation. The accuracy of system calibration will directly affect the measurement accuracy of the three-dimensional visual measurement system.

The calibration of the 3D vision measurement system first requires obtaining the coordinates of the world coordinate system and the image coordinate system. The world coordinates of the world coordinate system are obtained by establishing the world coordinate system. The coordinate system established at this time is called the ideal coordinate system. Due to the axis system error or the calibration target posture error, there will be a certain deviation between the actual world coordinate system and the ideal coordinate system. Therefore, there is also a deviation between the actual value and the ideal value obtained in the world coordinate system. This deviation will affect the accuracy of the system calibration. On the other hand, after the 3D vision measurement system is calibrated, if the system is placed for a long time or bumps during the transportation of the system, the internal structure of the system will drift slightly. With the change of the internal structure of the system, the parameters of the measurement system will also change, and the results of the initial system calibration will no longer apply. Therefore, it is necessary to calibrate the system twice to ensure that the system has a high measurement accuracy.

Summary of the invention

In view of the deficiencies in the prior art, the purpose of the present invention is to provide a secondary calibration method for a three-dimensional vision measurement system with important practical value, so as to improve the calibration accuracy and measurement accuracy of the three-dimensional vision measurement system.

In order to achieve the above object, the present invention provides the following technical solutions:

A secondary calibration method for a three-dimensional vision measurement system, comprising:

Step S1: Initially calibrate the system model according to the world coordinate value and the image coordinate value;

Step S2: Use the initially calibrated system model to perform three-dimensional reconstruction of the standard part, and evaluate the calibration error based on the reconstruction result;

Step S3: according to the calibration error evaluation result, the world coordinate value is optimized during the initial system calibration, and the system model of the initial calibration is recalibrated using the optimized world coordinate value;

Step S4: Reconstruct the standard part using the system model after the secondary calibration, until the error between the result of the reconstructed standard part and its theoretical value meets the preset condition, thereby obtaining a high-precision calibration result.

In the present invention, further, it also includes: Step S5: verifying the accuracy of the calibration result in Step S4.

In the present invention, further, step S4 includes:

Step S41: Reconstruct the standard parts and obtain the deviation Δ _i between the actual value and the ideal value of each standard part;

Step S42: Sum the deviations between all actual values and ideal values to obtain the value of:

Step S43: Determine whether the preset conditions are met, if not, return to step S31, if yes, go to step S44;

Step S44: Obtain the deflection angle with the highest accuracy for reconstructing the standard component, calculate the world coordinate value at this time, and use the coordinate to calibrate the system to obtain a high-precision calibration result.

In the present invention, further, step S3 includes: optimizing the deflection angle between the coordinate systems by using an optimization algorithm, and substituting the optimized result into the world coordinate system to obtain the optimized world coordinate value.

In the present invention, further, the relationship between the deflection angle between the coordinate systems and the actual world coordinate value is as follows:

X＝X ₀ +Z ₀ ·sin α ·cos β

Y＝Y ₀ +Z ₀ ·sin α ·sin β

Z＝Z ₀ ·cos α

Where XYZ are the coordinate values of the three axes of the world coordinate system, α is the angle between the target moving direction and the ideal moving direction, and β is the angle between the projection of _{the OwZw} axis on the surface and _{the OwXw} axis.

In the present invention, further, step S5 includes: using the system model after secondary system calibration to reconstruct different standard parts or different postures of the same standard part, comparing the reconstructed results with the theoretical values of the standard parts, and verifying the accuracy of system measurement.

In the present invention, further, step S0 is provided before step S1, and step S0: obtaining the world coordinate values of feature points at different positions under the world coordinate system and the image coordinate values and phase values under the image coordinate system.

In the present invention, further, the step S0 includes:

S0-1: Establish the conversion relationship between world coordinates and camera pixels, where the conversion relationship can be expressed as:

Among them, u and v are the coordinates of a point on the camera image plane, and _xw , _yw , and _zw are the world coordinates in the corresponding world coordinate system;

S0-2: Obtain the coordinate value of each target point in the world coordinate system;

S0-3: Obtain the coordinate value and phase value of each target point in the image coordinate system.

In the present invention, further, a method for obtaining image coordinate values and phase values in the image coordinate system is as follows:

The image of the target taken by the camera is obtained, and the image is processed to obtain the pixel value of the center of each target point; the target with periodic stripes projected by the projector is photographed by the camera, the photographed image is solved to obtain the phase size corresponding to each pixel, and the phase value corresponding to the center of each target is obtained by interpolation operation.

In the present invention, preferably, the optimization algorithm includes but is not limited to the Levenberg-Marquardt algorithm.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention adopts an optimized iterative method to perform secondary calibration on the three-dimensional vision measurement system. There is no need to perform repetitive experiments on the system calibration process, and efficient system calibration can be performed while reducing the workload.

2. The present invention performs secondary calibration on the system which has been placed for a long time or has micro-movement, thereby solving the problem that the system hardware drifts and the system parameters change. The system parameters can be corrected without recalibrating the system.

3. Under the premise that there is no requirement for the calibration target position and posture, the present invention ensures that the system has high measurement accuracy, performs secondary calibration on the original calibration results, and improves the measurement accuracy of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

FIG1 is a flow chart of a secondary calibration method for a three-dimensional vision measurement system according to the present invention;

FIG2 is a schematic diagram of the relationship between the ideal and actual target positions during calibration of a three-dimensional vision measurement system;

3 is a schematic diagram of a measurement model of a three-dimensional vision measurement system of the present invention;

FIG4 is a schematic diagram of a three-dimensional model of a standard step of the present invention and its reconstruction result;

FIG5 is a schematic diagram of the workflow of step S0 of the present invention;

FIG6 is a schematic diagram of the workflow of step S4 of the present invention;

FIG7 is the results of reconstructing standard parts before and after optimization by the method of the present invention;

FIG8 is a schematic diagram of system reprojection error.

DETAILED DESCRIPTION

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only 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 ordinary technicians in this field without creative work are within the scope of protection of the present invention.

It should be noted that when a component is referred to as being "fixed to" another component, it may be directly on the other component or there may also be a component centered. When a component is considered to be "connected to" another component, it may be directly connected to the other component or there may also be a component centered. When a component is considered to be "set on" another component, it may be directly set on the other component or there may also be a component centered. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and/or" used herein includes any and all combinations of one or more related listed items.

As shown in FIG2 , the present invention provides a three-dimensional vision measurement system, which includes a camera 1 and a projector 2 , and a planar target 4 is placed on a precision translation stage 3 so that its movement direction is perpendicular to the target plane.

The accuracy of the target point center coordinate value in the world coordinate system used in the system calibration process will affect the accuracy of the system calibration. Therefore, it is necessary to obtain the world coordinate value of each target point in the world coordinate system as the coordinate value in the actual coordinate system to improve the accuracy of the system calibration; at the same time, the measurement system with drift in the internal structure is recalibrated to improve the system measurement accuracy.

Specifically, referring to FIG. 1 , a preferred embodiment of the present invention provides a secondary calibration method for a three-dimensional vision measurement system, comprising:

Step S0: obtaining the world coordinate values of feature points at different positions in the world coordinate system and the image coordinate values and phase values in the image coordinate system.

Step S1: Initially calibrate the system according to the world coordinate value and the image coordinate value;

Step S2: Use the initially calibrated system to perform three-dimensional reconstruction of the standard part, and evaluate the calibration error based on the reconstruction result;

Step S3: according to the calibration error evaluation result, the world coordinate value is optimized during the initial system calibration, and the initially calibrated system is recalibrated using the optimized world coordinate value;

Step S4: Reconstructing the standard part using the system after the secondary calibration until the error between the result of the reconstructed standard part and its theoretical value meets a preset condition, thereby obtaining a high-precision calibration result;

Step S5: verify the accuracy of the calibration result in step S4.

Based on the above embodiments, the present invention first performs an initial calibration on the system according to the world coordinate values and the image coordinate values to obtain a system model, and then creates a three-dimensional standard part according to the result of the initial system calibration, that is, the obtained system model, and initially evaluates the calibration accuracy of the system. Then, a new optimization method is used to optimize the initial calibration result of the three-dimensional visual measurement system, and the system model is recalibrated using the optimized world coordinates to obtain a new system model, and is verified using standard parts different from those used for optimization, and the reprojection error of the system is obtained, which ultimately proves that the invention has a very effective improvement in the accuracy of the reconstruction results.

Based on the above embodiments, the present invention has the following characteristics:

The present invention adopts an optimized iterative method to perform secondary calibration on the three-dimensional vision measurement system, and does not need to perform repetitive experiments on the system calibration process. It can perform efficient system calibration while reducing the workload.

The present invention performs secondary calibration on a system that has been placed for a long time or has micro-movements, thereby solving the problem of system hardware drifting and system parameter changes, and the system parameters can be corrected without the need for recalibration experiments on the system.

Under the premise that there is no requirement on the calibration target position and posture, the present invention ensures that the system has high measurement accuracy, performs secondary calibration on the original calibration results, and improves the measurement accuracy of the system.

Further, in the present invention, as shown in FIG5 , step S0 includes:

S0-1: Establish the conversion relationship between world coordinates and camera pixels;

Exemplarily, as shown in FIG3 , a schematic diagram of the conversion relationship between the established world coordinates and camera pixels is shown, a world coordinate system is established, wherein O _w is the origin of the world coordinate system, and X _w , Y _w , and Z _w are the three axes of the world coordinate system.

The world coordinate system is transformed into the camera coordinate system through the RT rotation matrix, where _Oc is the origin of the camera coordinate system, and _Xc , _Yc , and _Zc are the three axes of the camera coordinate system; where _Op is the origin of the projector coordinate system, and _Xp , _Yp , and _Zp are the three axes of the projector coordinate system.

Among them, the conversion relationship can be expressed as:

Among them, u and v are the coordinates of a point on the camera image plane, and _xw , _yw , and _zw are the world coordinates in the corresponding world coordinate system.

In contrast, the camera coordinate imaging model can be expressed as:

At the same time, considering the projector as an "inverse camera" can obtain the projector imaging model:

In this way, based on the above embodiment, a conversion relationship between world coordinates and camera pixels is established. It should be noted that the coordinate system established at this time is an ideal coordinate system, which lays the foundation for subsequent system calibration.

S0-2: Obtain the coordinate value of each target point in the world coordinate system;

Exemplarily, _a world coordinate system is established so that the _{OwXw axis} , _OwYw axis and _OwZw axis of the world coordinate system are perpendicular to each other, and the world coordinate value of the feature point of the world coordinate system at this time is obtained. Specifically, as shown in _FIG2 , the point at the upper left corner of the target is the origin _Ow of the world coordinate system, the axis passing through the center of the horizontal row of target points to the right, the axis passing through the telecenter of the vertical row of target points downward is _{the OwXw} axis, and the axis perpendicular to the target plane is _{the OwZw} axis. Since the center distance of each target point is known, the distance moved along the axis each time is known, so the world coordinate value of each target point of the target _OwYw in the world coordinate system can be obtained, which is specifically calculated based on the stepping distance _of the target and the spacing of the target points.

S0-3: Obtain the coordinate value and phase value of each target point in the image coordinate system.

Specifically, a planar target is photographed by camera 1 to obtain a picture of the target photographed by camera 1, and the picture is processed to extract the pixel value of the center of each target point; then, the target on which periodic stripes are projected by projector 2 is photographed by camera 1, the photographed picture is solved to obtain the phase size corresponding to each pixel, and the phase value corresponding to the center of each planar target is obtained by interpolation operation.

Exemplarily, the pixel coordinates corresponding to the center of the target circle are extracted by using image processing technology, and specifically, ellipse fitting and center extraction can be used. In addition, the phase value can be obtained by phase solution and interpolation operation.

Further, in the present invention, step S1: the system is initially calibrated according to the world coordinate value and the image coordinate value. Specifically, the system is initially calibrated according to the world coordinate value and the image coordinate value, and the coordinates in the world coordinate system and the coordinates in the image coordinate system and the phase value are substituted into the known system model, and the system parameters can be obtained by solving with the least square method, that is, the initial calibration of the known system model is completed, and the system model is obtained.

Furthermore, after the initial calibration is completed, the calibration accuracy needs to be evaluated, that is, step S2 is performed: the standard part is 3D reconstructed using the initially calibrated system, and the calibration error is evaluated according to the reconstruction result.

Preferably, the standard parts of the present invention include but are not limited to standard steps. As shown in FIG4 , the system reconstructs the standard steps after the initial calibration, and the reconstruction accuracy of the system after the initial calibration is obtained by comparing the theoretical value with the measured value.

In order to improve the system calibration accuracy and measurement accuracy, this solution also performs a secondary calibration on the system.

As shown in FIG2 , ideally, the target moving direction is perpendicular to the plane target 4, but due to the system error, the target moving direction changes, and there is an angle with the ideal moving direction. At this time, the angle between the projection of the axis on the plane and the O _w Z _w axis is α, and the angle between the projection of the O w Z w axis on the plane and the O _w X _w axis is β. The relationship between the deflection angle (α and/or β) between the coordinate systems and the actual world coordinate value is as follows:

X＝X ₀ +Z ₀ ·sin α ·cos β

Y＝Y ₀ +Z ₀ ·sin α ·sin β

Z＝Z ₀ ·cos α

Where XYZ are the coordinate values of the three axes of the world coordinate system, α is the angle between the target moving direction and the ideal moving direction, and β is the angle between the projection of _{the OwZw} axis on the surface and _{the OwXw} axis.

Since α and β are unknown, an optimization algorithm is used to optimize the deflection angle between the coordinate systems, and the optimized result is substituted into the world coordinate system to obtain the optimized world coordinate value. The system that was initially calibrated is recalibrated using the optimized world coordinate value to obtain new system parameters.

Preferably, the optimization algorithm of this implementation adopts the Levenberg-Marquardt algorithm, but is not limited to this algorithm.

After the secondary calibration is completed, a new system model can be obtained, and step S4 is executed: the standard parts are reconstructed using the system after the secondary calibration until the error between the result of the reconstructed standard parts and its theoretical value meets the preset condition, thereby obtaining a high-precision calibration result.

Exemplarily, as shown in FIG6 , step S4 includes:

Step S41: Reconstruct the standard steps and obtain the deviation Δ _i between the actual value and the ideal value of each standard step; wherein the deviation Δ _i is the difference between the actual value h _0i of the standard step and the ideal value h _i .

Step S42: summing up the deviations between all actual values and ideal values to be Δ, and the calculation formula is as follows:

Step S43: Determine whether the preset condition is met. If not, return to step S31. If met, go to step S44. Specifically, the preset condition is the selected loop condition. The loop condition is Δ minimum, which can also be set according to actual conditions. When the loop condition is not met, return to step S31 and execute until the loop condition is met.

Step S44: Obtain the deflection angle with the highest reconstruction accuracy of the standard component, calculate the world coordinate value at this time, and use the coordinate to calibrate the system to obtain a high-precision calibration result. As shown in Figure 8, the highest reconstruction accuracy is obtained at this time, and a smaller reprojection error is obtained.

In this way, the secondary calibration of the system is completed. Based on the optimization algorithm, the present invention analyzes the deviation between the reconstruction results of the standard parts and their theoretical values, with the optimization goal of minimizing the error of the reconstructed standard parts and making it close to the actual value, thereby realizing high-precision secondary calibration of the system.

In the present invention, further, after the calibration is completed, the system measurement accuracy needs to be evaluated. Specifically, the system model after the secondary system calibration is used to reconstruct different standard parts or different postures of the same standard part, and the reconstructed results are compared with the theoretical values of the standard parts to obtain the system measurement accuracy. In this embodiment, as shown in Figure 7, the measurement error is reduced from 12 to within 2, so the measurement accuracy of the three-dimensional visual measurement system can be effectively improved by using the present invention.

It should be pointed out that, for those skilled in the art, several improvements and modifications may be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Those of ordinary skill in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

It should be understood that in the embodiments of the present application, the various embodiments and features can be combined with each other to solve the aforementioned technical problems.

If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk.

The above description is a detailed description of the preferred feasible embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modified changes completed under the technical spirit suggested by the present invention should fall within the patent scope covered by the present invention.

Claims (10)

1. A secondary calibration method for a three-dimensional vision measurement system, comprising:

Step S1: performing primary calibration on the system according to the world coordinate value and the image coordinate value;

Step S2: three-dimensional reconstruction is carried out on the standard component by using a system for primary calibration, and calibration errors are evaluated according to reconstruction results;

Step S3: optimizing world coordinate values when the primary system is calibrated according to the calibration error evaluation result, and carrying out secondary calibration on the primary calibrated system by utilizing the optimized world coordinate values;

Step S4: and reconstructing the standard component by using the system after secondary calibration until the result of reconstructing the standard component and the theoretical value error thereof meet the preset condition, thereby obtaining a high-precision calibration result.

2. The secondary calibration method for a three-dimensional vision measurement system of claim 1, further comprising:

Step S5: and (3) performing accuracy verification on the marked result in the step S4.

3. A secondary calibration method for a three-dimensional vision measurement system according to claim 2, characterized in that said step S4 comprises:

step S41: reconstructing the standard components, and solving the deviation delta _i of the actual value and the ideal value of each standard component;

Step S42: the sum of all the deviations of the actual values from the ideal values is delta, and the delta is calculated as follows:

Step S43: judging whether the delta meets the preset condition, returning to the step S31 if the delta does not meet the preset condition, and if the delta does not meet the preset condition, returning to the step S44;

step S44: and obtaining the deflection angle with the highest accuracy of the reconstruction standard component, calculating the world coordinate value at the moment, and calibrating the system by using the coordinate to obtain a high-accuracy calibration result.

4. A secondary calibration method for a three-dimensional vision measurement system according to claim 2, characterized in that said step S3 comprises: and optimizing deflection angles between coordinate systems by adopting an optimization algorithm, and substituting the optimized result into the world coordinate system to obtain the optimized world coordinate value.

5. The secondary calibration method for a three-dimensional vision measurement system according to claim 4, wherein the representation relationship between the deflection angle between the coordinate systems and the real world coordinate values is as follows:

X＝X₀+Z₀·sinα·cosβ

Y＝Y₀+Z₀·sinα·sinβ

Z＝Z₀·cosα

Wherein XYZ is the coordinate values of three axial directions of a world coordinate system, alpha is the included angle between the target moving direction and the ideal moving direction, and beta is the included angle between the projection of the O _wZ_w axis on the plane and the O _wX_w axis.

6. A secondary calibration method for a three-dimensional vision measurement system according to claim 2, characterized in that said step S5 comprises: and reconstructing different standard components or different poses of the same standard component by using the system calibrated by the secondary system, comparing the reconstructed result with the theoretical value of the standard component, and verifying the measurement precision of the system.

7. A secondary calibration method for a three-dimensional vision measurement system according to claim 1, wherein step S0 is provided before step S1,

Step S0: world coordinate values under world coordinates of feature points at different positions and image coordinate values and phase values under an image coordinate system are obtained.

8. The secondary calibration method for a three-dimensional vision measurement system according to claim 7, wherein the step S0 comprises:

s0-1: establishing a conversion relation between world coordinates and camera pixels, wherein the conversion relation can be expressed as:

wherein u and v are coordinates of a certain point of the camera image plane, and x _w、y_w、z_w is a world coordinate under a corresponding world coordinate system;

S0-2: obtaining coordinate values of each target point under a world coordinate system;

S0-3: and obtaining coordinate values and phase values of each target point under the image coordinate system.

9. The secondary calibration method for a three-dimensional vision measurement system according to claim 8, wherein the coordinate values and phase values under the image coordinate system of each target point are obtained by:

acquiring a shooting picture of a target shot by a camera, and processing the picture to obtain a pixel value of the circle center of each target point; shooting targets projected by a projector through a camera, resolving the shot pictures to obtain the phase value corresponding to each pixel, and obtaining the phase value corresponding to each target circle center through interpolation operation.

10. A secondary calibration method for a three-dimensional vision measurement system as defined in claim 4, wherein the optimization algorithm includes, but is not limited to, the Levenberg-Marquardt algorithm.