Six-degree-of-freedom measurement precision calibration method for double-camera photogrammetry system

Abstract

The invention relates to a six-degree-of-freedom measurement precision calibration method for a double-camera photogrammetry system, which can effectively solve the problems of six-degree-of-freedom precision test and calibration of the existing double-camera photogrammetry system, and adopts the technical scheme that the method comprises the following steps of: 1) erecting the double-camera photogrammetry system; 2) calibrating course angle measurement precision; 3) roll angle measurement precision calibration; 4) calibrating the pitch angle measurement precision; 5) calibrating the length measurement precision in three directions; according to the method, the course angle, the pitch angle and the roll angle serve as evaluation indexes of attitude measurement of the double-camera measurement system, the moving lengths in the horizontal direction, the depth direction and the vertical direction serve as the evaluation indexes of attitude measurement of the double-camera measurement system, and the attitude of the double-camera measurement system is evaluated by comparing the difference value between the measured value and the standard value. Therefore, the calibration of the six-degree-of-freedom measurement capability of the double-camera industrial photogrammetry system is realized, and the method is an innovation of a six-degree-of-freedom measurement precision method.

Description

Technical Field

[0001] The invention relates to the field of industrial photogrammetry, and in particular to a six-degree-of-freedom measurement accuracy calibration method for a dual-camera photogrammetry system. Background Art

[0002] The dual-camera measurement system is widely used in industrial measurement, aerospace, and intelligent manufacturing. It has the advantages of high precision, non-contact, and dynamic tracking measurement, and can accurately measure the three-dimensional coordinates and posture of space objects. However, the dual camera may produce position and angle errors during measurement. Especially under high-precision requirements, it is difficult for an uncalibrated dual camera system to achieve the ideal measurement accuracy. Therefore, in order to ensure the accuracy and reliability of the measurement data of the photogrammetry system, it is necessary to study its calibration method and device and solve its value traceability problem.

[0003] The measurement accuracy calibration of the dual-camera measurement system is a process that reduces system errors and improves measurement accuracy and consistency through a calibration procedure. At present, the calibration of camera photogrammetry systems abroad mainly refers to the VDI / VDE 2634 standard proposed by the German Engineers Association and the German Electrical Engineers Association, but this method is mainly used for the calibration of single-camera measurement systems. The American GSI company completes the calibration of various indicators of the photogrammetry system by shooting the company's calibration wall. Its calibration idea is to use the high-accuracy STARS system to evaluate the low-precision measurement system, which does not involve the dual-camera six-degree-of-freedom measurement accuracy calibration. The domestic dual-camera measurement system calibration method is also mainly aimed at point coordinate accuracy calibration, length measurement accuracy and camera repeatability calibration.

[0004] The existing technology still lacks a method that can systematically and accurately calibrate the six-degree-of-freedom measurement accuracy of dual cameras, especially the lack of effective calibration means for the accuracy of posture angles. Therefore, an efficient and reliable calibration method is urgently needed to solve such precision error problems. Summary of the invention

[0005] In view of the above situation, in order to solve the defects of the prior art, the purpose of the present invention is to provide a six-degree-of-freedom measurement accuracy calibration method for a dual-camera photogrammetry system, which can effectively solve the six-degree-of-freedom accuracy testing and calibration problems of the current dual-camera measurement system.

[0006] The technical solution provided by the present invention is that the six-degree-of-freedom measurement accuracy calibration method of the dual-camera photogrammetry system specifically comprises the following steps: 1) Set up a dual-camera measurement system Choose a suitable location to set up the dual cameras. The camera height is 1.6m, the distance between the two cameras is 3m, and the distance from the calibrator is 2.5m. Use a reference ruler to orient the cameras, determine the relative position relationship between the cameras, and give the dual cameras a length reference. 2) Calibration of heading angle measurement accuracy An electronic theodolite is set up in the middle of the field of view of the dual-camera measurement system, at the same height as the dual cameras. A photographic tool is fixed on the side of the electronic theodolite. The electronic theodolite is rotated to a suitable position as the 0 position, and the dual-camera measurement system takes pictures. The picture is taken after rotating 5° and then taken again. The picture is rotated until the dual-camera measurement cannot completely measure the mark points on the photographic tool. The software is used to calculate the fitting plane of the data taken by the dual-camera measurement system, and the angle between the planes is calculated and compared with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's heading angle measurement capability. 3) Roll angle measurement accuracy calibration After the heading angle calibration is completed, remove the photographic tooling and install it in front of the electronic theodolite eyepiece. Rotate the electronic theodolite eyepiece cylinder to a suitable position as the 0 position, and use the dual-camera measurement system to take pictures. Rotate 5° and take pictures again. Continue rotating until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Use software to calculate the rotation angle taken by the dual-camera measurement system and compare it with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's ability to measure the roll angle. 4) Pitch angle measurement accuracy calibration After the roll angle calibration is completed, remove the photographic tooling and install it on the side of the eyepiece rotating cylinder of the electronic theodolite. Rotate the eyepiece rotating cylinder of the electronic theodolite to a suitable position as the 0 position, and use the dual-camera measurement system to take pictures. Rotate 5° and take pictures again. Continue rotating until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Use software to calculate the rotation angle taken by the dual-camera measurement system and compare it with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's ability to measure pitch angles. 5) Calibration of length measurement accuracy in three directions After the three-angle calibration is completed, the length measurement accuracy calibration in three directions (vertical, horizontal, and depth) is carried out to keep the position of the dual-camera measurement system unchanged. A high-precision three-dimensional mobile platform is used as a calibrator. By calibrating the dual-camera measurement system one by one in the horizontal, depth, and vertical directions, the displacement measurement accuracy of the dual-camera system in different directions can be ensured, thereby ensuring the overall six-degree-of-freedom measurement accuracy.

[0007] The horizontal measurement accuracy calibration method is as follows: the three-dimensional mobile platform is controlled to perform a precise single displacement in the horizontal direction, the starting position is set to 0, and the displacement is 20 mm each time, which is increased to 40 mm, 60 mm and 80 mm in sequence. After each displacement, the dual-camera system captures and records the position coordinates of the object, and the moving distance is calculated by the coordinates of the point after the displacement and the initial point coordinates, and compared with the standard displacement value of the three-dimensional mobile platform to evaluate the measurement accuracy of the system in the horizontal direction; the purpose of the horizontal direction (X-axis) calibration is to ensure that the dual-camera system can accurately measure the changes of the object in the horizontal displacement.

[0008] The depth measurement accuracy calibration method is as follows: use a three-dimensional mobile platform to move the object in the depth direction (far and near direction), and gradually increase the displacement in units of 20 mm. The dual-camera system takes pictures after each displacement, records the depth displacement of the object, calculates the moving distance by the point coordinates after the movement and the initial point coordinates, and compares it with the reference value of the mobile platform to verify the depth measurement accuracy of the system; the depth direction (Z-axis) calibration evaluates the dual-camera system's ability to accurately measure the depth direction of the object.

[0009] The vertical measurement accuracy calibration method is as follows: when the vertical movement range is small, a three-dimensional mobile platform is used to perform a small displacement in the vertical direction, such as 2 mm each time. The dual-camera system takes pictures after each displacement to record the vertical position change of the object. The moving distance is calculated by the coordinates of the point after the movement and the initial point coordinates, and compared with the standard value of the platform to evaluate the measurement error of the vertical displacement; the calibration of the vertical direction (Y axis) is used to evaluate the measurement accuracy of the vertical displacement of the object.

[0010] The electronic theodolite is a CT105 electronic theodolite.

[0011] The three-dimensional mobile platform is the XYZMT118H-15 three-dimensional mobile platform.

[0012] The photographic tooling is based on an acrylic plate, on which at least 3 coding points and 10-15 photographic marking points are pasted.

[0013] The method of the present invention uses the heading angle, pitch angle and roll angle as evaluation indicators of the attitude measurement of the dual-camera measurement system, and uses the moving lengths in the horizontal direction, depth direction and vertical direction as evaluation indicators of the attitude measurement of the dual-camera measurement system. By comparing the difference between the measured value and the standard value, the six-degree-of-freedom measurement capability of the dual-camera industrial photogrammetry system is calibrated, which is an innovation in the six-degree-of-freedom measurement accuracy method. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of the installation position of the heading angle photography tooling of the present invention.

[0015] Figure 2 It is a schematic diagram of the installation position of the rolling angle photography tooling of the present invention.

[0016] Figure 3 It is a schematic diagram of the installation position of the pitch angle photography tooling of the present invention.

[0017] Figure 4 This is a schematic diagram of the use status of the three-dimensional mobile platform of the present invention.

[0018] Figure 5It is the work flow chart of the present invention.

[0019] Figure 6 This is a schematic diagram of the accuracy of posture measurement (three angles) of the present invention.

[0020] Figure 7 It is a schematic diagram of the photographic tooling of the present invention.

[0021] Figure 8 It is a schematic diagram of the three-dimensional mobile platform of the present invention. DETAILED DESCRIPTION

[0022] The specific implementation of the present invention is further described in detail below with reference to the accompanying drawings and examples. Example 1

[0023] The present invention, when implemented, specifically comprises the following steps: 1) Set up a dual-camera measurement system Choose a suitable location to set up the dual cameras. The camera height is 1.6m, the distance between the two cameras is 3m, and the distance from the calibrator is 2.5m. Use a reference ruler to orient the cameras, determine the relative position relationship between the cameras, and give the dual cameras a length reference. 2) Calibration of heading angle measurement accuracy Set up an electronic theodolite in the middle of the field of view of the dual-camera measurement system, with the height being the same as that of the dual cameras, and fix the photographic tooling on the side of the electronic theodolite, such as Figure 1 As shown, the electronic theodolite is rotated to a suitable position as the 0 position, and the dual-camera measurement system is used to take pictures. The pictures are rotated 5° and taken again until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. The software is used to calculate the data fitting plane taken by the dual-camera measurement system, and the angle between the planes is calculated and compared with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system in terms of heading angle measurement capability. 3) Roll angle measurement accuracy calibration After the heading angle calibration is completed, remove the photographic tooling and install it in front of the electronic theodolite eyepiece. Figure 2 As shown, rotate the electronic theodolite eyepiece cylinder to a suitable position as the 0 position, and take photos with the dual-camera measurement system. Rotate 5° and take photos again until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Compare the rotation angle taken by the dual-camera measurement system with the rotation angle of the electronic theodolite by software to calibrate the dual-camera measurement system's ability to measure roll angles. 4) Pitch angle measurement accuracy calibration After the roll angle calibration is completed, remove the photographic tooling and install it on the side of the eyepiece rotating cylinder of the electronic theodolite. Figure 3As shown, rotate the electronic theodolite eyepiece to a suitable position as the 0 position, and use the dual-camera measurement system to take pictures. Rotate 5° and take pictures again until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Use software to calculate the rotation angle taken by the dual-camera measurement system and compare it with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's ability to measure pitch angles. 5) Calibration of length measurement accuracy in three directions After the three-angle calibration is completed, the three-direction (vertical, horizontal, and depth) length measurement accuracy calibration is performed, keeping the position of the dual-camera measurement system unchanged, and using a high-precision three-dimensional mobile platform as a calibrator, such as Figure 4 As shown, by calibrating the dual-camera measurement system one by one in the horizontal direction, depth direction and vertical direction, the displacement measurement accuracy of the dual-camera system in different directions can be ensured, thereby ensuring the overall six-degree-of-freedom measurement accuracy.

[0024] The horizontal measurement accuracy calibration method is as follows: the three-dimensional mobile platform is controlled to perform a precise single displacement in the horizontal direction, the starting position is set to 0, and the displacement is 20 mm each time, which is increased to 40 mm, 60 mm and 80 mm in sequence. After each displacement, the dual-camera system captures and records the position coordinates of the object, and the moving distance is calculated by the coordinates of the point after the displacement and the initial point coordinates, and compared with the standard displacement value of the three-dimensional mobile platform to evaluate the measurement accuracy of the system in the horizontal direction; the purpose of the horizontal direction (X-axis) calibration is to ensure that the dual-camera system can accurately measure the changes of the object in the horizontal displacement.

[0025] The depth measurement accuracy calibration method is as follows: use a three-dimensional mobile platform to move the object in the depth direction (far and near direction), and gradually increase the displacement in units of 20 mm. The dual-camera system takes pictures after each displacement, records the depth displacement of the object, calculates the moving distance by the point coordinates after the movement and the initial point coordinates, and compares it with the reference value of the mobile platform to verify the depth measurement accuracy of the system; the depth direction (Z-axis) calibration evaluates the dual-camera system's ability to accurately measure the depth direction of the object.

[0026] The vertical measurement accuracy calibration method is as follows: when the vertical movement range is small, a three-dimensional mobile platform is used to perform a small displacement in the vertical direction, such as 2 mm each time. The dual-camera system takes pictures after each displacement to record the vertical position change of the object. The moving distance is calculated by the coordinates of the point after the movement and the initial point coordinates, and compared with the standard value of the platform to evaluate the measurement error of the vertical displacement; the calibration of the vertical direction (Y axis) is used to evaluate the measurement accuracy of the vertical displacement of the object.

[0027] The electronic theodolite is a CT105 electronic theodolite.

[0028] The three-dimensional mobile platform is the XYZMT118H-15 three-dimensional mobile platform.

[0029] The photographic tooling is based on an acrylic plate, on which at least 3 coding points and 10-15 photographic marking points are pasted.

[0030] The present invention is based on the current development of dual-camera measurement system. There is no relevant literature on the calibration of the six-degree-of-freedom measurement accuracy of the dual-camera measurement system. Therefore, according to the relevant parameter indicators of the dual-camera measurement system, a set of calibration methods for the six-degree-of-freedom measurement system of the dual-camera measurement system is proposed to solve the current six-degree-of-freedom accuracy test and calibration problems of the dual-camera measurement system. In order to calibrate the six-degree-of-freedom measurement accuracy of the dual-camera measurement system, the CT105 electronic theodolite (with an angle measurement accuracy of 0.5") and the photographic tooling (such as Figure 7 As shown), a three-dimensional mobile platform (distance measurement accuracy is ±1um) that provides standard angle values ​​and standard length values ​​is used for measurement (such as Figure 8 As shown in the figure), the heading angle, pitch angle and roll angle are used as the evaluation indicators of the attitude measurement of the dual-camera measurement system, and the moving lengths in the horizontal direction, depth direction and vertical direction are used as the evaluation indicators of the attitude measurement of the dual-camera measurement system. By comparing the difference between the measured value and the standard value, the six-degree-of-freedom measurement capability of the dual-camera industrial photogrammetry system is calibrated (as shown in the figure). Figure 5 As shown in the figure, practice has proved that when calibrating the M05 dual-camera measurement system, the posture measurement accuracy of the present invention reaches 0.04° and the position measurement accuracy reaches 0.02mm. Compared with the length measurement and point coordinate calibration method of the dual-camera measurement system in the prior art, the posture measurement accuracy is clearly given to be 0.04°, which fully proves that the measurement and calibration method using the present invention has high accuracy and small error, and can be widely used in industrial photogrammetry.

[0031] The present invention can provide accurate standard values ​​for the dual-camera system by using a high-precision electronic theodolite (angle measurement accuracy of 0.5″) and a three-dimensional mobile platform (position measurement accuracy of ±1μm). By analyzing the error of the calibration data, the method can identify the error sources of different degrees of freedom and provide a data correction mechanism. The calibration process is simple and easy to operate. It is an innovation in the six-degree-of-freedom measurement accuracy method and has good economic and social benefits.

[0032] It should be pointed out that the above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the profession can make changes or modify the technical contents disclosed above into equivalent embodiments with equivalent changes without departing from the scope of the technical solution of the present invention, which all fall within the protection scope of the present invention.

Claims

1. A six-degree-of-freedom measurement accuracy calibration method for a dual-camera photogrammetry system, characterized in that: The following steps are involved: 1) Set up a dual-camera measurement system Choose a suitable location to set up the dual cameras. The camera height is 1.6m, the distance between the two cameras is 3m, and the distance from the standard is 2.5m. Use a reference ruler to orient the cameras, determine the relative position relationship between the cameras, and give the dual cameras a length reference. 2) Calibration of heading angle measurement accuracy An electronic theodolite is set up in the middle of the field of view of the dual-camera measurement system, at the same height as the dual cameras. A photographic tool is fixed on the side of the electronic theodolite. The electronic theodolite is rotated to a suitable position as the 0 position, and the dual-camera measurement system takes pictures. The picture is taken after rotating 5° and then taken again. The picture is rotated until the dual-camera measurement cannot completely measure the mark points on the photographic tool. The software is used to calculate the fitting plane of the data taken by the dual-camera measurement system, and the angle between the planes is calculated and compared with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's heading angle measurement capability. 3) Roll angle measurement accuracy calibration After the heading angle calibration is completed, remove the photographic tooling and install it in front of the electronic theodolite eyepiece. Rotate the electronic theodolite eyepiece cylinder to a suitable position as the 0 position, and use the dual-camera measurement system to take pictures. Rotate 5° and take pictures again. Continue rotating until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Use software to calculate the rotation angle taken by the dual-camera measurement system and compare it with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's ability to measure the roll angle. 4) Pitch angle measurement accuracy calibration After the roll angle calibration is completed, remove the photographic tooling and install it on the side of the eyepiece rotating cylinder of the electronic theodolite. Rotate the eyepiece rotating cylinder of the electronic theodolite to a suitable position as the 0 position, and use the dual-camera measurement system to take pictures. Rotate 5° and take pictures again. Continue rotating until the dual-camera measurement cannot completely measure the mark points on the photographic tooling. Use software to calculate the rotation angle taken by the dual-camera measurement system and compare it with the rotation angle of the electronic theodolite to calibrate the dual-camera measurement system's ability to measure pitch angles. 5) Calibration of length measurement accuracy in three directions After the three-angle calibration is completed, the three-direction length measurement accuracy calibration is carried out to keep the position of the dual-camera measurement system unchanged. A high-precision three-dimensional mobile platform is used as a standard. By calibrating the dual-camera measurement system one by one in the horizontal, depth and vertical directions, the displacement measurement accuracy of the dual-camera system in different directions can be ensured, thereby ensuring the overall six-degree-of-freedom measurement accuracy.

2. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The horizontal measurement accuracy calibration method is as follows: the three-dimensional mobile platform is controlled to perform a precise single displacement in the horizontal direction, the starting position is set to 0, and the displacement is 20 mm each time, which is increased to 40 mm, 60 mm and 80 mm in sequence. After each displacement, the dual-camera system captures and records the position coordinates of the object, and the moving distance is calculated by the coordinates of the point after the displacement and the initial point coordinates, and compared with the standard displacement value of the three-dimensional mobile platform to evaluate the measurement accuracy of the system in the horizontal direction.

3. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The depth measurement accuracy calibration method is as follows: use a three-dimensional mobile platform to move the object in the depth direction, and gradually increase the displacement in units of 20 mm. The dual-camera system takes pictures after each displacement, records the depth displacement of the object, calculates the moving distance by comparing the point coordinates after the movement with the initial point coordinates, and compares it with the reference value of the mobile platform to verify the depth measurement accuracy of the system.

4. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The vertical measurement accuracy calibration method is as follows: when the vertical movement range is small, a three-dimensional mobile platform is used to perform a small displacement in the vertical direction, such as 2 mm each time. The dual-camera system takes pictures after each displacement to record the vertical position change of the object. The moving distance is calculated by the coordinates of the point after the movement and the initial point coordinates, and compared with the standard value of the platform to evaluate the measurement error of the vertical displacement.

5. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The electronic theodolite is a CT105 electronic theodolite.

6. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The three-dimensional mobile platform is the XYZMT118H-15 three-dimensional mobile platform.

7. The six-degree-of-freedom measurement accuracy calibration method of a dual-camera photogrammetry system according to claim 1, characterized in that: The photographic tooling is based on an acrylic plate, on which at least 3 coding points and 10-15 photographic marking points are pasted.

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