A rotation calibration method for non-orthogonality of a three-axis vector magnetometer

Abstract

The application provides a rotation calibration method for non-orthogonality of a three-axis vector magnetometer, comprising: starting a stable standard magnetic field environment, and placing a non-magnetic turntable in a uniform magnetic field area; fixing a sensor of the three-axis vector magnetometer on the non-magnetic turntable; adjusting a direction of the non-magnetic turntable so that an axial direction of a pitch axis of the non-magnetic turntable is east-west; adjusting an installation direction of the three-axis vector magnetometer so that a direction of an axis of the three-axis vector magnetometer is north; adjusting the pitch axis of the non-magnetic turntable so that a magnetic field measurement value of the axis of the three-axis vector magnetometer pointing to the north reaches a maximum value; adjusting angles of the pitch axis and a direction axis of the non-magnetic turntable according to a latitude uniformity and a longitude uniformity, and measuring magnetic field vector readings; and calibrating the magnetometer according to multiple magnetic field vector readings. The application has the advantages that the direction uniformity is greatly improved, and thus the accuracy of the non-orthogonality calibration test is improved.

Description

Technical Field

[0001] This application belongs to the field of magnetometer calibration technology, specifically relating to a non-orthogonal rotational calibration method for a three-axis vector magnetometer. Background Technology

[0002] The axes of the three magnetic field measurement axes of a triaxial vector magnetometer are usually non-orthogonal, requiring calibration experiments for correction. Existing non-orthogonal calibration techniques fall into two categories: one involves measuring the magnetic field projection values ​​at a specific attitude angle to calculate the non-orthogonal angle. This projection method typically involves adjusting the attitude to find the maximum or zero point of a certain axis, recording the magnetic field measurements for each axis. However, due to inherent errors in magnetic field measurement (including environmental fluctuations and other factors), the determination of this attitude angle has a certain error range, which is then passed on to downstream calculations, resulting in insufficient calibration accuracy. Another method uses rotation to obtain magnetic field measurements under multiple attitudes (usually continuous rotation measurement), and then calculates calibration parameters. This method suffers from uneven distribution of the magnetic field vector direction in the rotation space, leading to overfitting in some directions and underfitting in others, affecting calibration accuracy.

[0003] The projection method is intuitive and easy to understand, and subsequent calculations are also simple. However, it requires a small number of measurement data points (usually only 3 sets of data are needed for calculation), places high demands on calibration equipment and the environment, and measurement errors are amplified during implementation, resulting in lower calibration accuracy. The rotation method offers higher calibration accuracy compared to the projection method. This is because it avoids directly introducing measurement errors related to finding a specific direction during the measurement process, and because continuous rotation sampling results in a larger amount of fitted data, it improves the accuracy of subsequent data processing. However, the calibration results are still closely related to how the data is obtained through rotation, meaning there is an issue of uneven calibration direction, and there is still room for improvement in the calibration results. Summary of the Invention

[0004] The purpose of this application is to overcome the defect of uneven calibration direction in the existing technology.

[0005] To achieve the above objectives, this application proposes a non-orthogonal rotational calibration method for a triaxial vector magnetometer, comprising: Step S1: Start a stable standard magnetic field environment by placing the non-magnetic turntable in a uniform magnetic field region; Step S2: Fix the sensor of the triaxial vector magnetometer on the non-magnetic turntable; Step S3: Adjust the orientation of the non-magnetic turntable so that the pitch axis of the non-magnetic turntable is in the east-west direction; Step S4: Adjust the installation direction of the triaxial vector magnetometer so that the direction of one axis of the triaxial vector magnetometer is north; Step S5: Adjust the pitch axis of the non-magnetic turntable so that the magnetic field measurement value of the three-axis vector magnetometer pointing north reaches the maximum value; Step S6: Adjust the pitch and direction axes of the non-magnetic turntable according to the rules of latitude and longitude uniformity, and measure the magnetic field vector reading; Step S7: Calibrate the magnetometer based on multiple magnetic field vector readings.

[0006] As an improvement to the above method, when the calibration environment magnetic field is the laboratory coil magnetic field, the direction of the calibration environment magnetic field projected onto the horizontal plane is regarded as north, and the direction perpendicular to it in the horizontal plane is regarded as east-west.

[0007] As an improvement to the above method, when calibration is performed in the Northern Hemisphere, the pitch axis is tilted to the north, and the reading is taken as positive when the pitch axis is tilted to the north and negative when it is tilted to the south; when calibration is performed in the Southern Hemisphere, the pitch axis is tilted to the south, and the reading is taken as positive when the pitch axis is tilted to the south and negative when it is tilted to the north.

[0008] As an improvement to the above method, the latitude uniformity refers to the equal latitude difference between measurement points on each longitude circle; the longitude uniformity refers to the equal ratio of the number of measurement points on each latitude circle to the length of that latitude circle; and the difference between the spherical arc length between latitude circles and the spherical arc length between measurement points within each latitude circle is within a set threshold range.

[0009] Among them, the pitch axis reading of the non-magnetic turntable corresponds to latitude, and the direction axis reading corresponds to longitude.

[0010] As an improvement to the above method, the pitch axis of the non-magnetic turntable was adjusted 6 times during the measurement.

[0011] As an improvement to the above method, the angles of the pitch axis and azimuth axis of the non-magnetic turntable are adjusted as follows: Pitch axis: a-15° or a+165°, corresponding directional axis: b, b+180°; Pitch axis: a-45° or a+135°, corresponding direction axis: b, b+60°, b+120°, b+180°, b+240°, b+300°; Pitch axis: a-75° or a+105°, corresponding direction axis: b, b+45°, b+90°, b+135°, b+180°, b+225°, b+270°, b+315°; Pitch axis: a-105° or a+75°, corresponding direction axis: b, b+45°, b+90°, b+135°, b+180°, b+225°, b+270°, b+315°; Pitch axis: a-135° or a+45°, corresponding direction axis: b, b+60°, b+120°, b+180°, b+240°, b+300°; Pitch axis: a-165° or a+15°, corresponding directional axis: b, b+180°; Where a and b are the pitch angle and azimuth angle of the non-magnetic turntable after step 5 is executed, respectively; the pitch angle ranges from -90° to 90°.

[0012] Compared with existing technologies, the advantages of this application are: Triaxial vector magnetometers inevitably suffer from non-orthogonality issues. High-precision measurements necessitate non-orthogonality calibration tests. Among calibration methods, the rotation method generally offers better accuracy than the projection method, but it suffers from uneven rotation directions, leading to overfitting in some directions and underfitting in others. This application proposes a calibration method based on the rotation method. It first determines the initial direction of the external magnetic field, then calculates the distribution of several measurement points based on latitude and longitude, significantly improving the direction uniformity and thus enhancing the accuracy of non-orthogonality calibration tests. Attached Figure Description

[0013] Figure 1 The diagram shows the flowchart of the non-orthogonal rotation calibration method for a three-axis vector magnetometer. Detailed Implementation

[0014] The technical solution of this application will be described in detail below with reference to the accompanying drawings.

[0015] This application proposes a non-orthogonal rotational calibration method for a triaxial vector magnetometer. It innovatively proposes the concept and specific method of first determining the directional relationship between the external magnetic field direction and the directional relationship between the device to be calibrated and the non-magnetic turntable before rotational calibration. A 32-point measurement scheme based on a specific rotation angle is proposed and validated. This application employs a simple and easy-to-implement method that does not rely on high-precision calibration equipment, significantly improving the directional uniformity and calibration accuracy of the rotational calibration scheme.

[0016] The environment for non-orthogonal rotational calibration of a triaxial vector magnetometer requires a stable standard magnetic field environment, which can be an open field environment or a magnetically shielded room, a standard magnetic field coil, or other equipment that can provide a stable standard magnetic field.

[0017] The equipment used for non-orthogonal rotational calibration of triaxial vector magnetometers includes: Non-magnetic turntable: A non-magnetic turntable has two rotation axes, one is the pitch axis and the other is the directional axis.

[0018] A three-axis vector magnetometer device awaiting calibration.

[0019] The non-orthogonal rotational calibration methods for triaxial vector magnetometers include: Step 1: Start a stable standard magnetic field environment by placing the non-magnetic turntable in the uniform magnetic field region at its center.

[0020] Step 2: Fix the sensor of the triaxial vector magnetometer on the non-magnetic turntable, start the device to begin measurement and data reading.

[0021] Step 3: Adjust the direction of the non-magnetic turntable so that its pitch axis is in the east-west direction.

[0022] Step 4: Adjust the installation orientation of the magnetometer so that the direction of one of its axes (using the X-axis as an example in this embodiment, but in other embodiments it could be the Y-axis or Z-axis) is north. When the calibration environment magnetic field is not the Earth's magnetic field, but the laboratory coil magnetic field, consider the projection of the calibration environment magnetic field direction onto the horizontal plane as north, and the direction perpendicular to it on the horizontal plane as east-west, and then repeat steps 3 and 4.

[0023] Step 5: Adjust the pitch axis of the non-magnetic turntable so that the magnetic field along the X-axis reaches the maximum measurement value.

[0024] Step 6: Record the pitch axis reading of the turntable as 'a' and the azimuth axis reading (the angle of the non-magnetic turntable in the horizontal direction) as 'b'. (When located in the Northern Hemisphere, the pitch axis is tilted to the north. The reading 'a' is defined as positive when the pitch axis is tilted to the north and negative when it is tilted to the south. When located in the Southern Hemisphere, the pitch axis is tilted to the south. The reading 'a' is defined as positive when the pitch axis is tilted to the south and negative when it is tilted to the north.) Step 7: Adjust the pitch axis and azimuth axis to the readings shown in Table 1, and measure the magnetic field vector reading: Table 1 Pitch and Taymight Adjustment Angles

[0025] The range of pitch axis readings is [-90°, 90°]. When the value calculated in the table above exceeds the range, it can be replaced with +180°.

[0026] Step 8: Calibrate the magnetometer using the ellipsoid fitting calibration method.

[0027] Steps 3, 4, and 5 of this application determine that the pitch axis of the non-magnetic turntable is oriented east-west, the initial X-axis direction is north, and the adjusted X-axis direction corresponds to the direction of the external magnetic field. This means that the axial direction of the pitch axis is perpendicular to the direction of the external magnetic field. In the measurement space, the initial magnetic field vector direction is considered the polar axis direction, corresponding to 90° latitude and 0° longitude. Adjusting the pitch axis reading is equivalent to adjusting the latitude line where the magnetic field vector direction is located in the measurement space, and adjusting the direction axis reading is equivalent to adjusting the longitude line where the magnetic field vector direction is located in the measurement space. Adjusting the direction axis one full rotation at a specific pitch axis means that the magnetic field vector direction rotates one full rotation along the latitude line across various longitudes in the measurement space at that specific latitude.

[0028] In the measurement space, the longitude and direction axis readings are the same, and the initial pitch axis reading 'a' is regarded as the 90° latitude polar axis direction. The correspondence between latitude and pitch axis readings is shown in Table 2: Table 2 Correspondence between Latitude and Pitch Axis Readings

[0029] Considering that these six latitude lines are evenly spaced and uniform in the latitudinal direction, and the length ratio of these six latitude lines is cos(75°):cos(45°):cos(15°):cos(-15°):cos(-45°):cos(-75°)≈2:6:8:8:6:2, the number of measurement points on the latitude lines, that is, the number of points per revolution when adjusting the direction axis, is also in this ratio. When the direction angle intervals are 180°, 60°, and 45°, the number of measurement points per revolution is exactly 2, 6, and 8 respectively, which ensures that the distribution is also uniform in the longitude direction.

[0030] The method for calculating directional uniformity in this application can be summarized as follows: directional uniformity can be decomposed into latitude uniformity and longitude uniformity. Latitude uniformity refers to the equal latitude difference between measurement points on each longitude circle. Longitude uniformity refers to the equal ratio of the number of measurement points on each latitude circle to the length of that latitude circle, while the difference between the spherical arc length between latitude circles and the difference between the spherical arc length between measurement points within each latitude circle is within an allowable range. The 32-point measurement scheme presented in this application is an example that satisfies this calculation scheme. The rules for setting latitude and longitude in this type of scheme can be summarized as follows:

[0031] in, Latitude Longitude Let D be the longitude interval and be a constant. Latitude values ​​are taken at equal intervals, and longitude values ​​at the same latitude are also taken at equal intervals. The aforementioned 32-point scheme is a special case of D=0.12. The accurate solution for D=0.12 is shown in the table below. Considering that latitude values ​​are taken at equal intervals, within a certain allowable error range, latitude can be approximated as ±15°, ±45°, and ±75°.

[0032] Table 3. List of latitude and longitude intervals when D=0.12

[0033] Because the method in this application is appropriately designed, it greatly improves the directional uniformity of the magnetic field vector in the measurement space, thus naturally reducing the requirements for other related measurements. The higher the accuracy of the initial orientation alignment in steps 3, 4, and 5, and the higher the control accuracy of the attitude adjustment in step 7, the higher the accuracy of the subsequent calibration results. However, since this application's method solves the biggest source of error in the rotation method, the calibration accuracy has been significantly improved, and the requirements for the above two items can be appropriately reduced. This also reflects the advantages of this scheme—its simplicity, ease of implementation, and reduced reliance on high-precision calibration equipment. The reading accuracy of a high-precision non-magnetic turntable can reach 0.001°, while this scheme only requires a non-magnetic turntable reading accuracy of 1°.

[0034] Steps 3, 4, and 5 of this application's method determine the relationship between the direction of the external magnetic field, the direction of the magnetic field measuring equipment, and the direction of the turntable. The method is effective, simple, and easy to implement, applicable to most regions within my country. The 32-point measurement scheme provided in step 7 is established based on the initial position of the turntable; ignoring various rotation schemes based on the initial position would lead to a significant decrease in directional uniformity. In the measurement space, the 32-point measurement scheme establishes measurement points at specific latitude and longitude coordinates, ensuring high directional uniformity through calculation. Using a similar approach, changing the latitude and longitude coordinates and the number of measurement points can produce similar measurement schemes with different numbers of measurement points.

[0035] The following explains the proper nouns used in this application.

[0036] A three-axis vector magnetometer is an instrument that measures the vector of a spatial magnetic field. When the directions of the three measuring axes are orthogonal, it can accurately describe the magnitude and direction of the spatial magnetic field vector.

[0037] Non-orthogonality: Due to the measurement principle and manufacturing process of magnetometers, the directions of their three measuring axes cannot be accurately known in advance, and therefore cannot be guaranteed to be orthogonal, thus exhibiting non-orthogonality.

[0038] Orientation uniformity: The uniformity of the magnetic field vector direction distribution on the surface of a sphere. It consists of latitudinal uniformity and longitude uniformity.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application do not depart from the spirit and scope of the technical solutions of this application, and should all be covered within the scope of the claims of this application.

Claims

1. A non-orthogonal rotational calibration method for a triaxial vector magnetometer, comprising: Step S1: Start a stable standard magnetic field environment by placing the non-magnetic turntable in a uniform magnetic field region; Step S2: Fix the sensor of the triaxial vector magnetometer on the non-magnetic turntable; Step S3: Adjust the orientation of the non-magnetic turntable so that the pitch axis of the non-magnetic turntable is in the east-west direction; Step S4: Adjust the installation direction of the triaxial vector magnetometer so that the direction of one axis of the triaxial vector magnetometer is north; Step S5: Adjust the pitch axis of the non-magnetic turntable so that the magnetic field measurement value of the three-axis vector magnetometer pointing north reaches the maximum value; Step S6: Adjust the pitch and direction axes of the non-magnetic turntable according to the rules of latitude and longitude uniformity, and measure the magnetic field vector reading; Step S7: Calibrate the magnetometer based on multiple magnetic field vector readings.

2. The non-orthogonal rotation calibration method for a triaxial vector magnetometer according to claim 1, characterized in that, When the calibration environment magnetic field is the laboratory coil magnetic field, the direction of the calibration environment magnetic field projected onto the horizontal plane is regarded as north, and the direction perpendicular to it in the horizontal plane is regarded as east-west.

3. The non-orthogonal rotation calibration method for a triaxial vector magnetometer according to claim 1, characterized in that, When calibration is performed in the Northern Hemisphere, the pitch axis is tilted to the north. The reading is positive when the pitch axis is tilted to the north and negative when it is tilted to the south. When calibration is performed in the Southern Hemisphere, the pitch axis is tilted to the south. The reading is positive when the pitch axis is tilted to the south and negative when it is tilted to the north.

4. The non-orthogonal rotation calibration method for a triaxial vector magnetometer according to claim 1, characterized in that, The latitude uniformity refers to the equal latitude difference between measurement points on each longitude circle; the longitude uniformity refers to the equal ratio of the number of measurement points on each latitude circle to the length of that latitude circle; and the difference between the spherical arc length between latitude circles and the spherical arc length between measurement points within each latitude circle is within a set threshold range. Among them, the pitch axis reading of the non-magnetic turntable corresponds to latitude, and the azimuth axis reading corresponds to longitude. After executing step 5, the pitch axis reading and azimuth axis reading correspond to latitude 90° and longitude 0°.

5. The non-orthogonal rotation calibration method for a triaxial vector magnetometer according to claim 4, characterized in that, The pitch axis of the non-magnetic turntable was adjusted 6 times during the measurement.

6. The non-orthogonal rotation calibration method for a triaxial vector magnetometer according to claim 5, characterized in that, Adjust the angles of the pitch and azimuth axes of the non-magnetic turntable as follows: Pitch axis: a-15° or a+165°, corresponding directional axis: b, b+180°; Pitch axis: a-45° or a+135°, corresponding direction axis: b, b+60°, b+120°, b+180°, b+240°, b+300°; Pitch axis: a-75° or a+105°, corresponding direction axis: b, b+45°, b+90°, b+135°, b+180°, b+225°, b+270°, b+315°; Pitch axis: a-105° or a+75°, corresponding direction axis: b, b+45°, b+90°, b+135°, b+180°, b+225°, b+270°, b+315°; Pitch axis: a-135° or a+45°, corresponding direction axis: b, b+60°, b+120°, b+180°, b+240°, b+300°; Pitch axis: a-165° or a+15°, corresponding directional axis: b, b+180°; Where a and b are the pitch angle and azimuth angle of the non-magnetic turntable after step 5 is executed, respectively; the pitch angle ranges from -90° to 90°.

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