Cross-shaped variable baseline magnetic flux gate array and method for measuring third-order magnetic gradient tensor

Abstract

The invention belongs to the technical field of magnetic field detection, and discloses a cross-shaped variable baseline magnetic flux gate array and method for measuring a three-order magnetic gradient tensor. The cross-shaped variable baseline magnetic flux gate array comprises a cross-shaped array rack, each supporting arm is provided with a sliding groove with the length of 1, and a sensor is installed on each sliding groove in a clamped mode. A sensor is mounted at the central point of the cross array rack; the baseline distances d between the adjacent sensors are equal, and the sensors slide in the corresponding sliding grooves. According to the adjusting method, the baseline distance is adjusted through the sliding groove. According to the rotation correction method, the surrounding sensors are corrected through the middle sensor, the measurement precision is improved, the zero-order magnetic gradient tensor and the first-order magnetic gradient tensor at the position of each sensor and the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the position of the center sensor can be measured at the same time, rich target magnetic field information is obtained, and the measurement accuracy is improved. The engineering practical value is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetic field detection, and in particular relates to a cross-shaped variable baseline fluxgate array and a method for measuring a third-order magnetic gradient tensor. Background Art

[0002] The measurement of the magnetic gradient tensor has wide applications in geophysical exploration, marine magnetic detection, and military target positioning. Conventional magnetic field detection equipment can only measure the magnetic induction intensity at a single point (i.e., the first-order magnetic gradient) and cannot accurately measure higher-order magnetic field gradients (such as the second-order and third-order magnetic gradients). The magnetic gradient tensor, on the other hand, can provide more spatial information about a target, such as its magnetic moment distribution and positional characteristics.

[0003] Traditional magnetic field detection equipment usually adopts a fixed baseline distance and a fixed sensor layout, resulting in fixed measurement accuracy and sensitivity, poor adaptability or limited measurement accuracy, and difficulty in adapting to the distance changes and magnetic moment changes of different measured targets. When faced with a complex magnetic field environment, it is impossible to effectively capture various target groups with large changes in magnetic moment, position and scale. Among them, the centimeter-level baseline distance fluxgate structure is more targeted at weak and small magnetic targets passing by the near end, and is suitable for tracking and capturing the trajectories of bullets, millimeter-level ferromagnetic targets, and tiny ferromagnetic particles. The meter-level baseline distance fluxgate structure is more suitable for large-scale, large magnetic moment, and distant targets such as large aircraft, ships, and underground mines. Moreover, the traditional magnetic gradient tensor measurement device has a complex calibration process and low calibration efficiency, and is easily affected by sensor measurement errors.

[0004] Currently, there is an urgent need to develop a cross-shaped variable-baseline fluxgate array and method for measuring the third-order magnetic gradient tensor. Summary of the Invention

[0005] One technical problem to be solved by the present invention is to provide a cross-shaped variable baseline fluxgate array for measuring the third-order magnetic gradient tensor. Another technical problem to be solved by the present invention is to provide a baseline distance adjustment method based on the cross-shaped variable baseline fluxgate array. Still another technical problem to be solved by the present invention is to provide a rotation correction method based on the cross-shaped variable baseline fluxgate array to overcome the defects of the prior art.

[0006] The cross-shaped variable-baseline fluxgate array and method for measuring the third-order magnetic gradient tensor of the present invention adopt a variable baseline design to adapt to different detection working conditions and environments, are suitable for detecting detection targets with different object distances and different magnetic moments, can adjust the magnetic gradient tensor measurement sensitivity according to the characteristics of the detection target, and realize the measurement of the zero-order magnetic gradient tensor (magnetic field intensity) and the first-order magnetic gradient tensor (magnetic induction intensity vector) at each sensor position, as well as the measurement of the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the central sensor position.

[0007] The cross-shaped variable baseline fluxgate array for measuring third-order magnetic gradient tensor of the present application comprises a cross-shaped array rack, each branch arm is provided with a sliding slot with a length of , and each sliding slot is equipped with a sensor; a sensor is installed at the center point of the cross-shaped array rack; the baseline distance between adjacent sensors is d equal, and each sensor slides in the corresponding sliding slot; The sensors on the sliding slots are named as sensor 1, sensor 2, sensor 3 and sensor 4 in anticlockwise direction starting from 3 o'clock direction, and the sensor at the center point is named as sensor 5; sensor 5 provides component data for differential calculation of the third-order tensor, and also provides a reference correction signal for correcting the measurement data of sensors 1-4; The baseline distance d is adjusted according to the distance and magnetic moment of the detection target, and by changing the baseline distance d , the sensitivity and resolution are matched to the detection of near and far distance targets.

[0008] Further, the sensor is a three-axis fluxgate sensor, the cross-shaped array rack is made of non-magnetic material, and the detection target is a ferromagnetic target.

[0009] Further, the baseline distance d is manually or mechanically adjusted; the mechanism used for mechanical adjustment includes a locking screw, a gear sliding device or a stepping motor adjustment mechanism.

[0010] Further, the cross-shaped variable baseline fluxgate array simultaneously measures the zero-order tensor and the first-order tensor at the positions of sensors 1-5, and calculates the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the position of sensor 5 through differential operation between the sensors; wherein the zero-order tensor is named as total field intensity, and the first-order tensor is named as magnetic induction intensity vector. a. Zero-order tensor: sensors 1-5 independently measure the total field intensity at their respective positions : ; wherein i =1, 2, 3, 4, 5; b. First-order tensor: sensors 1-5 independently measure the magnetic induction intensity vector at their respective positions : ; wherein i =1, 2, 3, 4, 5; c. Second-order magnetic gradient tensor: For sensors 1~4, let b i j denote the readings of the sensors i in the axial direction, j i =1,2,3,4, j x , y , z For sensor 5, 9 second-order magnetic gradient tensor matrices are obtained: ; wherein is a second-order magnetic gradient tensor component; d. Third-order magnetic gradient tensor: for sensors 1~5, let b i j denote the readings of the sensors i in the axial direction, j i =1,2,3,4,5, j x , y , z For sensor 5, 6 third-order magnetic gradient tensor components among 27 third-order magnetic gradient tensor components are obtained: ; wherein is a third-order magnetic gradient tensor component.

[0011] The baseline distance adjustment method based on the cross-shaped variable baseline magnetic flux gate array of the application changes the baseline distance of the cross-shaped variable baseline magnetic flux gate array according to the detection target magnetic moment, size and distance d , so that the cross-shaped variable baseline magnetic flux gate array controls the tensor measurement noise within the pre-set range while capturing the magnetic gradient tensor signal; including the following contents: The baseline distance adjustment method is based on the calculation formula of the tensor measurement noise and the limit of the tensor measurement range; a. Tensor measurement noise; The solving method of the tensor measurement noise of the 9 second-order magnetic gradient tensor components is the same, wherein for the second-order magnetic gradient tensor component , if the sensor background noise variance is σ s 2 , the Gaussian distribution with expectation of 0, that is, satisfies s ~ N (0, σ s ​​​​2 ), then the second-order magnetic gradient tensor component of the structural error is: ; The above formula shows that also obeys the Gaussian distribution, and: 1) The expectation of the noise distribution is proportional to the square of the baseline distance d , and the proportional coefficient is related to the third-order gradient of the magnetic field; 2) The variance of the noise distribution is inversely proportional to the square of the baseline distance d , and is proportional to the sensor noise variance σ s 2 ; b. The limit of the tensor measurement range; If q is the measurement accuracy of each component reading of the magnetic field vector of the sensor (unit: ±nT), Q is the measurement accuracy of each component reading of the second-order magnetic gradient tensor (unit: ±nT / m), and q = Qd / 2, then the observation distance representing the limit of the tensor measurement range r The requirement for the cross-shaped variable baseline fluxgate array measurement to be reliable is: ; In the formula, M is the magnetic moment, α is the angle between the direction of the detected target magnetic moment and the observation position, μ 0 is the vacuum permeability; In comparison, when the detected target is close, the magnetic moment is weak, or the millimeter-scale small-scale detected target, the baseline distance d is shortened until the magnetic gradient tensor signal of the magnetic anomaly is not overwhelmed by the tensor measurement noise, and the sensitivity of the system is increased; when the detection distance is far, the magnetic moment is large, or the tens of meters-scale large-scale target, the baseline distance d is increased until the magnetic gradient tensor signal of the magnetic anomaly is measured by difference.

[0012] The rotation correction method based on the cross-shaped variable baseline fluxgate array of the application comprises the following steps: S10. Send the sensor 5 to the calibration center for correction, and obtain accurate calibration error parameters, including three-axis non-orthogonal error, three-axis zero deviation, and three-axis linear sensitivity error; S20. Install sensors 1-5 in a cross-shaped array platform to obtain a cross-shaped variable baseline magnetic fluxgate array; install the cross-shaped variable baseline magnetic fluxgate array on a three-axis non-magnetic rotating platform, with sensor 5 located at the center of rotation of the three-axis non-magnetic rotating platform; S30. Install the cross-shaped variable baseline magnetic fluxgate array and the three-axis non-magnetic rotating platform in a uniform magnetic field environment by adjusting the platform base, so that the three-axis non-magnetic rotating platform is tilted; rotate each axis of the three-axis non-magnetic rotating platform to ensure that the rotation trajectory of each sensor is in a different rotation plane; S40. Obtain spatial rotation magnetic field data of sensors 1-5 in the same uniform magnetic field environment by three-axis rotation of the three-axis non-magnetic rotating platform; calibrate all three-axis data of sensor 5 using the calibration error parameters of sensor 5 to obtain ideal data of sensor 5 as a calibration reference; S50. Solve the three-axis non-orthogonal error, three-axis zero deviation and three-axis linear sensitivity error of sensors 1-4 respectively by Levenberg-Marquardt algorithm, nonlinear least squares method and ideal data of sensor 5, with the relationship equation being: ; In the formula, I 0=( i x , i y , i z ) T is a three-axis zero deviation vector; c x 、 c y 、 c z is a three-axis linear sensitivity, representing the output weighting factor of X-axis, Y-axis and Z-axis respectively; is a three-axis non-orthogonal angle error, representing the non-orthogonal angle error of X-axis, Y-axis and Z-axis respectively; S60. Transmit the signals of sensors 1-5 to the processing system through the data acquisition system for real-time calculation, analysis and display of multi-order magnetic gradient tensors; calibrate the three-axis output of sensors 1-4 using the solved error parameters of sensors 1-4.

[0013] The cross-shaped variable baseline magnetic fluxgate array and method for measuring third-order magnetic gradient tensors have the following characteristics: a. Variable baseline design is adopted; the geometric structure of the cross-shaped variable baseline magnetic fluxgate array can be flexibly adjusted to adapt to the needs of different detection targets and improve detection capability and sensitivity; b. The multi-order magnetic gradient tensor measurement can be carried out; the zero-order magnetic gradient tensor and the first-order magnetic gradient tensor at each sensor position can be measured simultaneously, and the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the central sensor position can be measured, so that abundant target magnetic field information is provided; c. The correction reference is provided; the cross-shaped symmetric structure is adopted, so that the cross-shaped variable baseline magnetic flux gate array can be fixed on the three-axis non-magnetic rotating platform to carry out the space measurement rotation data calibration, the correction reference is directly provided by the central sensor, the measurement consistency of the surrounding sensors is ensured, and the overall measurement precision is improved.

[0014] The cross-shaped variable baseline magnetic flux gate array for measuring the third-order magnetic gradient tensor has the variable baseline and simple and flexible structure, the cross-shaped symmetric structure is adopted, the sensor layout is changed by adjusting the baseline distance between the sensors in the array, the object distance and the magnetic moment of different detection targets are adapted, and the detection effect is improved; and the cross-shaped variable baseline magnetic flux gate array can be fixed on the three-axis non-magnetic rotating platform to carry out the rotation correction through the central sensor, and the detection capability for the target is enhanced. The baseline distance adjustment method based on the cross-shaped variable baseline magnetic flux gate array adjusts the baseline distance through the sliding groove. The rotation correction method based on the cross-shaped variable baseline magnetic flux gate array corrects the surrounding sensors through the intermediate sensor, the measurement precision is improved, the zero-order magnetic gradient tensor and the first-order magnetic gradient tensor at each sensor position can be measured simultaneously, and the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the central sensor position can be measured, so that abundant target magnetic field information is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 FIG. 1 is a structural schematic diagram of the cross-shaped variable baseline magnetic flux gate array for measuring the third-order magnetic gradient tensor; Figure 2 FIG. 2 is a position relationship schematic diagram of the cross-shaped variable baseline magnetic flux gate array for measuring the third-order magnetic gradient tensor and the three-axis non-magnetic rotating platform.

[0016] In the figure, 1. Sensor 1; 2. Sensor 2; 3. Sensor 3; 4. Sensor 4; 5. Sensor 5; 6. Cross-shaped array rack; 7. Sliding groove; 8. Three-axis non-magnetic rotating platform; 9. Platform base.

[0017] In the figure, X , Y , Z FIG. 5 is a coordinate axis of the cross-shaped variable baseline magnetic flux gate array and the sensor 5, O FIG. 6 is a coordinate origin of the sensor 5; X 1 , Y 1 , Z 1 FIG. 7 is a coordinate axis of the sensor 1, O1 is the coordinate axis of sensor 2, X 2 , Y 2 , Z 2 is the coordinate axis of sensor 2, O 2 is the coordinate origin of sensor 2; X 3 , Y 3 , Z 3 is the coordinate axis of sensor 3, O 3 is the coordinate origin of sensor 3; X 4 , Y 4 , Z 4 is the coordinate axis of sensor 4, O 4 is the coordinate origin of sensor 4. DETAILED DESCRIPTION

[0018] The present application will be described in detail below with reference to the accompanying drawings and examples.

[0019] As shown in Figure 1 , the cross-shaped variable baseline fluxgate array for measuring third-order magnetic gradient tensor of the present application comprises a cross-shaped array rack 6, one sliding slot 7 with a length of l is arranged on each branch, and one sensor is respectively clamped on each sliding slot 7; one sensor is installed on the center point of the cross-shaped array rack 6; the baseline distance d between adjacent sensors is equal, and each sensor slides in the corresponding sliding slot 7; The sensors on the sliding slots 7 are named as sensor 1, sensor 2, sensor 3 and sensor 4 in anticlockwise direction from 3 o'clock direction, and the sensor at the center point is named as sensor 5; the sensor 5 provides component data for differential calculation of the third-order tensor, and also provides a reference correction signal for correcting the measurement data of the sensors 1-4, reducing the measurement deviation caused by sensor error; at the same time, the uniform magnetic field interference in the environment is compensated by using the differential calculation between sensors, and the detection precision is improved; The baseline distance d is adjusted according to the distance of the detection target and the magnetic moment size, and by changing the baseline distance d , the appropriate sensitivity and resolution are matched when detecting near and far distance targets.

[0020] Further, the sensor is a three-axis fluxgate sensor, the cross-shaped array pedestal 6 is made of non-magnetic material, and the detection target is a ferromagnetic target.

[0021] Further, the baseline distance d Manual or mechanical adjustment is performed; the mechanism used in mechanical adjustment includes a locking screw, a gear slide device, or a stepping motor adjustment mechanism.

[0022] Further, the cross-shaped variable baseline fluxgate array simultaneously measures the zero-order tensor and the first-order tensor at the positions of sensors 1-5, and calculates the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the position of sensor 5 through differential operation between sensors; wherein the zero-order tensor is named total field intensity, and the first-order tensor is named magnetic induction intensity vector. a. Zero-order tensor: sensors 1-5 independently measure the total field intensity at their respective positions : ; wherein, i =1,2,3,4,5; b. First-order tensor: sensors 1-5 independently measure the magnetic induction intensity vector at their respective positions : ; wherein, i =1,2,3,4,5; c. Second-order magnetic gradient tensor: For sensors 1-4, let b i j represent the readings of sensors i in the j axis direction, i =1,2,3,4, j = x , y , z ; for sensor 5, 9 second-order magnetic gradient tensor matrices are obtained: ; wherein, is a second-order magnetic gradient tensor component; d. Third-order magnetic gradient tensor: for sensors 1-5, let b i j represent the readings of sensors i in the j axis direction, i =1,2,3,4,5, j =x , y , z ; for sensor 5, 6 of 27 third-order magnetic gradient tensor components are obtained: ; wherein, is a third-order magnetic gradient tensor component.

[0023] The baseline distance adjustment method based on the cross-shaped variable baseline magnetic flux gate array of the application changes the baseline distance of the cross-shaped variable baseline magnetic flux gate array according to the detection target magnetic moment, size and distance d , so that the cross-shaped variable baseline magnetic flux gate array controls the tensor measurement noise within the pre-set range while capturing the magnetic gradient tensor signal; including the following contents: The baseline distance adjustment method is based on the calculation formula of the tensor measurement noise and the tensor measurement range limit; a. Tensor measurement noise; The solving method of the tensor measurement noise of the 9 second-order magnetic gradient tensor components is the same, wherein, for the second-order magnetic gradient tensor component , if the sensor background noise variance is σ s 2 , the Gaussian distribution with expectation of 0, that is, satisfies s ~ N (0, σ s 2 , the structural error of the second-order magnetic gradient tensor component is: ; The above formula shows that also obeys the Gaussian distribution, and: 1) The expectation of the noise distribution is proportional to the square of the baseline distance d , and the proportional coefficient is related to the third-order gradient of the magnetic field; 2) The variance of the noise distribution is inversely proportional to the square of the baseline distance d , and is proportional to the sensor background noise variance σ s 2 ; b. Tensor measurement range limit; If q is the measurement accuracy of each component reading of the magnetic field vector of the sensor (unit: ±nT), Q is the measurement accuracy of each component reading of the second-order magnetic gradient tensor (unit: ±nT / m), and qQd / 2, represents the observation distance of the limit of the tensor measurement range r The requirement for making the cross-shaped variable baseline fluxgate array measurement reliable is: ; In the formula, M is the magnetic moment, α is the angle between the direction of the detected target magnetic moment and the observation position, μ 0 is the vacuum permeability; Therefore, changing the baseline distance d can greatly enhance the adaptability of the cross-shaped variable baseline fluxgate array to different magnetic moments, detected target scales and observation distances, can flexibly adapt to different observation distances and magnetic moment sizes of the detected target according to the detection task requirements, and widen the measurement capability boundary of the cross-shaped variable baseline fluxgate array; for the sensor array whose tensor measurement noise obeys Gaussian noise distribution, the baseline distance d should comprehensively consider the sensor background noise and the magnetic moment size of the detected target; when the baseline distance d is small, the measurement error is dominated by the sensor background noise, and the structural error caused by the difference process has little effect, at this time, it is more suitable for measuring the near-end target with small magnetic moment; when the baseline distance d is large, the influence of the tensor measurement noise on the measurement accuracy gradually becomes insignificant, and the influence of the high-order term of the Taylor series ignored in the difference process on the measurement accuracy gradually dominates, at this time, it is more suitable for observing the target with large observation distance and large magnetic moment, the larger the baseline distance d and the larger the magnetic moment, the farther the range limit of the cross-shaped variable baseline fluxgate array can detect.

[0024] By comparison, when the detected target is close, the magnetic moment is weak, or it is a millimeter-scale small-scale detected target, the baseline distance d is shortened d , until the magnetic gradient tensor signal of the magnetic anomaly is not submerged by the tensor measurement noise, and the sensitivity of the system is increased; when the detection distance is far, the magnetic moment is large, or it is a tens of meters-scale large-scale target, the baseline distance d is increased d , until the magnetic gradient tensor signal of the magnetic anomaly is measured by difference.

[0025] The rotation correction method based on the cross-shaped variable baseline fluxgate array of the application comprises the following steps: S10. The sensor 5 is sent to a metering center for correction, and accurate calibration error parameters are obtained, the calibration error parameters including three-axis non-orthogonal error, three-axis zero deviation and three-axis linear sensitivity error; S20. As shown in Figure 2 , the sensors 1-5 are all installed in the cross-shaped array rack 6 to obtain a cross-shaped variable baseline fluxgate array; the cross-shaped variable baseline fluxgate array is installed on a three-axis non-magnetic rotating platform 8, and the sensor 5 is located at the rotation center of the three-axis non-magnetic rotating platform 8; ​S30. The cross-shaped variable baseline fluxgate array and the three-axis non-magnetic rotating platform 8 are installed in a uniform magnetic field environment by adjusting the platform base 9, and the platform base 9 is adjusted so that the three-axis non-magnetic rotating platform 8 is tilted; the axes of the three-axis non-magnetic rotating platform 8 are rotated to ensure that the rotation trajectories of the sensors are in different rotation planes; S40. Spatial rotation magnetic field data of the sensors 1-5 in the same uniform magnetic field environment is obtained by three-axis rotation of the three-axis non-magnetic rotating platform 8; all three-axis data of the sensor 5 is calibrated using the calibration error parameters of the sensor 5 to obtain ideal data of the sensor 5 as a calibration reference; S50. The three-axis non-orthogonal error, three-axis zero deviation and three-axis linear sensitivity error of the sensors 1-4 are respectively solved by the Levenberg-Marquardt algorithm, the nonlinear least squares method and the ideal data of the sensor 5, and the relationship equation is: ; In the formula, I 0=( i x , i y , i z ) T is a three-axis zero deviation vector; c x 、 c y 、 c z is a three-axis linear sensitivity, which respectively represents the output weighting factor of the X-axis, Y-axis and Z-axis; is a three-axis non-orthogonal angle error, which respectively represents the non-orthogonal angle error of the X-axis, Y-axis and Z-axis; S60. The signals of the sensors 1-5 are transmitted to the processing system by the data acquisition system for real-time calculation, analysis and display of the multi-order magnetic gradient tensor; the three-axis output of the sensors 1-4 is calibrated using the error parameters of the sensors 1-4.

[0026] Embodiment: The cross-shaped variable baseline fluxgate array of the embodiment can adjust the baseline distance d in the range of 0.1m-2m; The detection targets of the embodiment are: 1) a small car 10 meters away; 2) a fast-moving small iron ball with a diameter of 3cm within 1m; 3) a ferromagnetic shell buried in the ground at a depth of about 3m and a length of 50cm.

[0027] For a small car 10 meters away, the baseline distance d is continuously adjusted from small to large while observing the magnetic gradient tensor signal data, and when the baseline distance d is increased to about 1.5 m, the magnetic gradient tensor component data can be clearly distinguished from noise; for a small iron ball with a diameter of 3 cm moving at a speed of 1 m, the baseline distance d is continuously adjusted from large to small, and when the baseline distance d is reduced to about 15 cm, the magnetic gradient tensor component data can be clearly distinguished from noise; for a ferromagnetic shell buried in the ground at a depth of about 3 m and a length of 50 cm, the baseline distance d is continuously adjusted from small to large, and when the baseline distance d is adjusted to about 50 cm, the target magnetic anomaly tensor component signal data is obtained.

[0028] Although the embodiments of the present application have been disclosed as above, they are not limited to the uses listed in the specification and embodiments, and all features disclosed by the present application, or steps in all methods or processes disclosed by the present application, except for mutually exclusive features and / or steps, can be combined in any manner, without departing from the principles of the present application. The present application is not limited to specific details and figures shown and described herein.

Claims

1. A cross-shaped variable baseline fluxgate array for measuring third-order magnetic gradient tensors, characterized in that: The cross-shaped variable baseline fluxgate array comprises a cross-shaped array stand (6), each arm of which is provided with a length of A sensor is mounted on each chute (7); a sensor is mounted on the center point of the cross array stand (6); and the baseline distance between adjacent sensors is d Equal, each sensor slides in the corresponding slide groove (7); The sensors on the slide (7) are named sensor 1 (1), sensor 2 (2), sensor 3 (3) and sensor 4 (4) in the counterclockwise direction, starting from the 3 o'clock direction, and the sensor at the center point is named sensor 5 (5); sensor 5 (5) provides component data for differential calculation of the third-order tensor and also provides a reference correction signal for correcting the measurement data of sensor 1 (1) to sensor 4 (4); Baseline distance d Adjust according to the detection target distance and magnetic moment size by changing the baseline distance d , matching appropriate sensitivity and resolution when detecting both close-range and long-range targets.

2. The cross-shaped variable baseline fluxgate array for measuring the third-order magnetic gradient tensor according to claim 1, characterized in that: The sensor is a three-axis fluxgate sensor, the cross-shaped array stand (6) is made of non-magnetic material, and the detection target is a ferromagnetic target.

3. The cross-shaped variable baseline fluxgate array for measuring the third-order magnetic gradient tensor according to claim 1, characterized in that: The baseline distance d Adjustments are made manually or mechanically; mechanical adjustments may use locking screws, gear slides, or stepper motor adjustments.

4. The cross-shaped variable baseline fluxgate array for measuring the third-order magnetic gradient tensor according to claim 1, characterized in that: The cross-shaped variable baseline fluxgate array simultaneously measures the zero-order tensor and the first-order tensor at the positions of sensor 1 (1) to sensor 5 (5), and calculates the second-order magnetic gradient tensor and the third-order magnetic gradient tensor at the position of sensor 5 (5) by differential operation between the sensors; wherein the zero-order tensor is named as the total field intensity, and the first-order tensor is named as the magnetic induction intensity vector; a. Zero-order tensor: Sensors 1 (1) to 5 (5) independently measure the total field strength at their respective locations : ； in, i =1,2,3,4,5; b. First-order tensor: Sensor 1 (1) to Sensor 5 (5) independently measure the magnetic induction intensity vector at their respective positions : ； in, i =1,2,3,4,5; c. Second-order magnetic gradient tensor: For sensor 1 (1) to sensor 4 (4), let b i j Indicates sensor i exist j Readings in the axis direction, i =1,2,3,4, j = x , y , z ; For sensor 5 (5), obtain 9 second-order magnetic gradient tensor matrices : ； in, is the second-order magnetic gradient tensor component; d. Third-order magnetic gradient tensor: For sensor 1 (1) to sensor 5 (5), let b i j Indicates sensor i exist j Readings in the axis direction, i =1,2,3,4,5, j = x , y , z ; For sensor 5 (5), 6 third-order magnetic gradient tensor components out of 27 third-order magnetic gradient tensor components are obtained: ； in, is the third-order magnetic gradient tensor component.

5. A baseline distance adjustment method based on a cross-shaped variable baseline fluxgate array, which is used for a cross-shaped variable baseline fluxgate array for measuring a third-order magnetic gradient tensor according to any one of claims 1 to 3, characterized in that: The baseline distance adjustment method changes the baseline distance of the cross-shaped variable baseline fluxgate array while observing the change of the test signal according to the magnetic moment, scale and distance of the detected target. d , so that the cross-shaped variable baseline fluxgate array can capture the magnetic gradient tensor signal while controlling the tensor measurement noise within a preset range; Includes the following: The baseline distance adjustment method is based on a calculation formula of tensor measurement noise and tensor measurement range limit; a. Tensor measurement noise; The solution method for the tensor measurement noise of the nine second-order magnetic gradient tensor components is the same, among which, for the second-order magnetic gradient tensor component , if the sensor background noise variance is σ s 2 , a Gaussian distribution with an expectation of 0, that is, satisfying s ~ N (0, σ s 2 ), then the second-order magnetic gradient tensor component Structural error for: ； The above formula shows that It also obeys the Gaussian distribution, and: 1) Expected distance from the noise distribution to the baseline d Proportional to the square of Related to the third-order gradient of the magnetic field; 2) Variance of noise distribution and distance from baseline d Inversely proportional to the square of the sensor noise floor variance σ s 2 proportional to; b. Limits of tensor measurement range; like q is the measurement accuracy of each component of the sensor's magnetic field vector reading (unit: ±nT), Q is the measurement accuracy of each component of the second-order magnetic gradient tensor reading (unit: ±nT / m), q = QD / 2, which represents the observation distance at the limit of the tensor measurement range r The requirements for making cross-shaped variable baseline fluxgate array measurements reliable are: ； Where, M is the magnetic moment, α To detect the angle between the target magnetic moment direction and the observation position, μ 0 is the vacuum permeability; In comparison, when the detection target is close, the magnetic moment is weak, or the detection target is small in the millimeter range, shortening the baseline distance d , until the magnetic gradient tensor signal of the magnetic field anomaly is not overwhelmed by the tensor measurement noise, increasing the sensitivity of the system; when the detection distance is far, the magnetic moment is large, or it is a large-scale target of tens of meters, increase the baseline distance d , until the magnetic gradient tensor signal of the magnetic anomaly is achieved through differential measurement.

6. A rotation correction method based on a cross-shaped variable baseline fluxgate array, which is used for the cross-shaped variable baseline fluxgate array for measuring the third-order magnetic gradient tensor according to any one of claims 1 to 3, characterized in that: The rotation correction method comprises the following steps: S10. Send the sensor 5 (5) to the metrology center for calibration and obtain accurate calibration error parameters, which include three-axis non-orthogonality error, three-axis zero deviation and three-axis linear sensitivity error; S20. Install sensors 1 (1) to 5 (5) in a cross-shaped array stand (6) to obtain a cross-shaped variable baseline fluxgate array; install the cross-shaped variable baseline fluxgate array on a three-axis non-magnetic rotating platform (8), with sensor 5 (5) located at the rotation center of the three-axis non-magnetic rotating platform (8); S30. Install the cross-shaped variable baseline fluxgate array and the three-axis non-magnetic rotating platform (8) in a uniform magnetic field environment by adjusting the platform base (9), and adjust the platform base (9) to tilt the three-axis non-magnetic rotating platform (8); rotate each axis of the three-axis non-magnetic rotating platform (8) to ensure that the rotation trajectory of each sensor is in a different rotation plane; S40. Obtain spatial rotating magnetic field data of sensors 1 (1) to 5 (5) in the same uniform magnetic field environment by rotating the three-axis non-magnetic rotating platform (8); calibrate all three-axis data of sensor 5 (5) using the calibration error parameter of sensor 5 (5) to obtain ideal data of sensor 5 (5) as a calibration reference; S50. Using the Levenberg-Marquardt algorithm, the nonlinear least squares method, and the ideal data of sensor 5 (5), the three-axis non-orthogonality error, three-axis zero deviation, and three-axis linear sensitivity error of sensor 1 (1) to sensor 4 (4) are solved respectively. The relationship equations are: ； Where, I 0=( i x , i y , i z ) T is the three-axis zero deviation vector; c x 、 c y 、 c z is the three-axis linear sensitivity, which represents the output weighting factors of the X-axis, Y-axis, and Z-axis respectively; is the three-axis non-orthogonal angle error, which represents the non-orthogonal angle error of the X axis, Y axis, and Z axis respectively; S60. The signals of sensor 1 (1) to sensor 5 (5) are transmitted to the processing system through the data acquisition system for real-time calculation, analysis and display of multi-order magnetic gradient tensors; the three-axis outputs of sensor 1 (1) to sensor 4 (4) are calibrated using the calculated error parameters of sensor 1 (1) to sensor 4 (4).

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