Downhole fluxgate sensor signal processing method

Abstract

The invention discloses an underground fluxgate sensor signal processing method, which comprises the following steps of: extracting a magnetic field voltage signal mainly comprising a second harmonic component by adopting a signal detection circuit, and further removing other harmonic components through a digital phase-sensitive rectification and smooth filtering algorithm to realize effective extraction of the magnetic field signal. The second harmonic extraction effect of the phase-sensitive rectification algorithm is verified through a time sequence simulation result, and an open-loop fluxgate sensor designed based on a second harmonic method is experimented. Compared with other types of fluxgate signal detection methods, the second harmonic method effectively improves key performance such as resolution and linearity of the sensor, and the fluxgate sensor designed based on the second harmonic method can realize detection and effective modulation of a detected magnetic field signal and shows a relatively high magnetic measurement level.

Description

Technical Field

[0001] The invention belongs to the technical field of sensor signal processing, and proposes a method for extracting downhole magnetic field signals based on a second harmonic method by analyzing a modulation process of a fluxgate sensor to realize a magnetic field signal. Background Art

[0002] As a natural physical phenomenon, the space magnetic field is widely present in the universe and is an important environmental factor affecting human production, life and aerospace engineering. As an important component of the space magnetic field, many natural phenomena on Earth are closely related to the changes in the geomagnetic field. [1] The study and measurement of the Earth's magnetic field can serve important production and life fields such as Earth structure research, natural disaster forecasting, and energy exploration.

[0003] The fluxgate is a sensor based on the magnetosensitive effect. It can help explore the physical structure of the earth's interior by observing changes in the geomagnetic field. Compared with other types of magnetic measuring instruments, the fluxgate has the advantages of wide range, high resolution, and high reliability. It is widely used in mineral resource exploration, earthquake prevention and disaster reduction, magnetic navigation and other fields.

[0004] The fluxgate sensor is mainly composed of a probe and an electronics system. As a derivative of the transformer effect, the probe uses the fluxgate phenomenon to convert the captured magnetic field signal into a voltage signal. The electronics unit works with the probe to extract and process the magnetic field information. [2] .

[0005] The present invention is a signal detection system circuit applied to underground fluxgate electronics units, which plays a vital role in detecting and processing magnetic field signals. [3] The induction coil of the fluxgate's annular core probe detects the ambient magnetic field signal, obtains a voltage signal containing magnetic field information according to Faraday's law of electromagnetic induction, and sends it to the signal detection circuit [4] The main function of the signal detection circuit designed based on the second harmonic method is to detect and amplify the second harmonic component in the voltage signal and filter out other noise interference. After being collected by the ADC, the digital signal is output and sent to the FPGA, so that the fluxgate signal can be processed by digital phase-sensitive detection and other processing through software algorithms.

[0006] In practical applications, due to the Barkhausen effect, the asymmetry of the external electromagnetic and core shape parameters of the fluxgate probe, noise interference is inevitably introduced into the voltage signal output by the fluxgate probe, and the amplitude of each even harmonic is proportional to the strength of the axial component of the measured magnetic field in the induction coil. [5], and the linearity and stability of the sensor are inversely proportional to the harmonic order. According to the characteristics of the second harmonic method, the fluxgate signal extraction of the present invention adopts a digital phase-sensitive rectification and smoothing filtering algorithm, which can effectively extract the second harmonic component in the magnetic field signal and filter out other harmonic components, thereby improving the accuracy and stability of magnetic field detection, thereby locating the downhole guide drill position.

[0007] [1] Yue Liangguang. Research on aviation full tensor magnetic gradient sensing system based on high temperature DC-SQUID magnetometer[D]. Jilin University, 2023.

[0008] [2] Zhi Menghui. Research on high-precision digital fluxgate sensor[D]. Suzhou University, 2017.

[0009] [3] Liu Xuanming. Design of miniaturized three-axis fluxgate magnetometer[D]. Suzhou University, 2017.

[0010] [4] Ge Hanlin, Zhang Yiming, Zhang Chenhao, et al. Design and implementation of high temperature fluxgate sensor[J]. Instrument Technology and Sensor, 2023, (11): 23-28.

[0011] [5] Wei Yanlin, Wang Yandong, Liu Zhiwei, et al. Design of measurement accuracy detection method for dual-core fluxgate magnetometer[J]. Ship Electronic Engineering, 2023, 43(02): 182-184+195. Summary of the invention

[0012] As the size of downhole exploration guide drill bits is reduced, the size and selection of circuits are limited. In view of the problems of low acquisition accuracy and poor stability encountered by fluxgate sensors in the process of extracting magnetic field signals, a magnetic field signal extraction method based on the second harmonic method is proposed according to the relationship that the even harmonic components in the fluxgate effective signal are proportional to the measured magnetic field. The signal detection circuit is used to extract the magnetic field voltage signal dominated by the second harmonic component, and other harmonic components are further eliminated through digital phase-sensitive rectification and smoothing filtering algorithms to achieve effective extraction of magnetic field signals. The extraction effect of the phase-sensitive rectification algorithm on the second harmonic is verified by timing simulation results, and the open-loop fluxgate sensor designed based on the second harmonic method is experimented. Compared with other types of fluxgate signal detection methods, the second harmonic method effectively improves the key performance of the sensor such as resolution and linearity, and the fluxgate sensor designed based on the second harmonic method can realize the detection and effective modulation of the measured magnetic field signal, and shows a high level of magnetic measurement.

[0013] The technical solution adopted by the method of the present invention is a downhole fluxgate sensor signal processing method, which uses a signal detection circuit to extract a magnetic field voltage signal mainly composed of a second harmonic component, and removes other harmonic components through a digital phase-sensitive rectification and smoothing filter algorithm, i.e., a phase-sensitive detection algorithm, to achieve effective extraction of the magnetic field signal; the fluxgate sensor uses an annular magnetic core, which is approximately a double-core structure;

[0014] According to the relationship between magnetic induction intensity, magnetic permeability, and magnetic field intensity, the total magnetic induction intensity inside the induction coil is formed by the superposition of the external magnetic fields on the two magnetic cores. Using the magnetic flux formula, the magnetic flux in the induction coil is for:

[0015]

[0016] Where: S is the cross-sectional area of ​​the core, μ1 and μ2 are the magnetic permeabilities of the equivalent double core, H0 is the measured magnetic field strength; H m cosωt is the excitation magnetic field strength in the magnetic core; ω is the angular frequency of the excitation signal; the magnetic flux Performing Fourier expansion gives:

[0017]

[0018] Among them, Φ 0m for The fundamental component of is the harmonic component;

[0019] Using trigonometric functions, limit method and Faraday's law of electromagnetic induction, when the size and magnetic permeability of the dual cores are exactly equal, the induced electromotive force E nm for:

[0020]

[0021] Wherein, μ0 is the magnetic permeability of the core in the unsaturated state, and W is the number of turns of the induction coil;

[0022] According to formula (3), the induced electromotive force output by the annular core probe contains even harmonic signals of the magnetic field H0. Due to the Barkhausen effect, the external electromagnetic properties of the fluxgate probe, and the asymmetry of the core shape and size parameters, noise interference is inevitably introduced into the voltage signal output by the fluxgate probe. According to the measurement, the induced voltage signal output by the fluxgate probe has the largest amplitude with the harmonic components of the first three frequencies f1, f2, and f3, where f1 is the fundamental frequency of the excitation signal.

[0023] Furthermore, the signal detection circuit of the fluxgate sensor is composed of an LC resonant circuit, a differential amplifier, and an ADC sampling circuit; its function is to frequency-selectively amplify the output signal of the fluxgate probe, and then use the ADC to realize analog-to-digital conversion, and send it to the FPGA chip for processing.

[0024] Furthermore, the induction coil of the ring core probe is a winding inductor, and its output impedance is mainly inductive. In order to extract the second harmonic component of the output signal to the maximum extent, a resonant capacitor needs to be connected in parallel with the induction coil; at the same time, matching resistors R1 and R2 are added to the resonant network; the differential amplifier circuit adjusts the range of circuit gain by adjusting the resistance value; the differential amplifier circuit has a differential structure, so the next-level sampling circuit adopts a differential input ADC; when designing, the ADC sampling point is near the peak and trough of each cycle of the second harmonic signal, so as to achieve high-resolution sampling of the second harmonic signal at twice the frequency, and output a serial digital signal to the FPGA chip.

[0025] Furthermore, the implementation process of the phase-sensitive detection algorithm is as follows: the amplitude of each even harmonic is proportional to the strength of the axial component of the measured magnetic field in the induction coil, and the linearity and stability of the sensor are inversely proportional to the harmonic order. The fluxgate signal extraction uses a digital phase-sensitive rectification and smoothing filter algorithm to extract the harmonic components in the induction signal except the second harmonic component, and use this second harmonic component to characterize the measured magnetic field information;

[0026] The second harmonic component of the input signal is x(t), then the expression is:

[0027]

[0028] Where A is the amplitude, 2f is the second harmonic frequency, is the phase difference with the reference signal; the phase-sensitive signal is selected to have the same frequency as the second harmonic to be processed, and the given reference signal is assumed to be:

[0029] s(t)=Bcos(2πft) (5)

[0030] Where B is the reference signal amplitude. By converting the product to the difference, the output value of the multiplier MUL1 is expressed as:

[0031]

[0032] Similarly, the output expression of MUL2 is:

[0033]

[0034] The low-pass filter outputs are, Combining the two equations, we get the amplitude of the second harmonic component of the measured signal is Phase

[0035] Furthermore, the phase-sensitive detection uses a square wave signal with the same frequency as the second harmonic as the phase-sensitive signal. The phase-sensitive signal is 1 in the positive interval and -1 in the negative interval, and has an instantaneous jump from positive to negative.

[0036] Furthermore, the digital phase-sensitive detection and rectification algorithm performs the same operation on the induced signals with different input frequencies; for the input signal of the base frequency f1, the induced signal in the first half cycle is divided into two parts by the phase-sensitive signal, and both are positive voltages; in the first half, since the input signal and the phase-sensitive signal are both positive voltages, the input signal remains unchanged after phase-sensitive rectification; the phase-sensitive signal in the second half is in the negative voltage interval, so that the input signal in the same interval changes from positive voltage to negative voltage. In the first half cycle, the harmonic signal voltage with a frequency of f1 is one positive and one negative after phase-sensitive rectification; the processed signals are then accumulated and integrated in half a cycle, and the input signals in this half cycle cancel each other out; the same processing is performed in the second half cycle, and it can be seen that the input signal with a frequency of f1 is completely canceled out after phase-sensitive rectification and integration; the input signal with a frequency of f3 is also completely canceled out after the same processing.

[0037] Furthermore, when the frequency of the input signal is consistent with that of the phase-sensitive signal, the second harmonic component is processed; because the frequency period is consistent, the input signal and the phase-sensitive signal will be in the positive interval and the negative interval at the same time; according to the working principle of the phase-sensitive rectification algorithm, it can be obtained that the waveform of the target signal is completely converted into a periodic waveform of the forward voltage, and then after processing by the low-pass filter, the second harmonic component signal can be completely retained and contains the measured magnetic field information.

[0038] Furthermore, the number of points for average filtering processing is designed to be 256; the signal-to-noise ratio of the fluxgate signal is improved after being processed by the phase-sensitive rectification and smoothing filtering modules, and the digital signal after smoothing filtering is calibrated to obtain the target magnetic field value. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the structural schematic diagram of the fluxgate system.

[0040] Figure 2 It is the signal detection circuit diagram.

[0041] Figure 3 It is the phase-sensitive detection flow chart.

[0042] Figure 4 It is a schematic diagram of the phase-sensitive rectification algorithm.

[0043] Figure 5 This is the timing simulation diagram of phase-sensitive detection.

[0044] Figure 6 It is the FFT of the induced signal waveform in the shielding barrel.

[0045] Figure 7 It is the FFT of the induced signal waveform outside the shielding barrel.

[0046] Figure 8 It uses open-loop sensing signal waveform FFT.

[0047] Fig. 9 It uses closed-loop feedback to sense the signal waveform FFT.

[0048] Fig.10 This is the fluxgate field test diagram. DETAILED DESCRIPTION

[0049] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0050] Figure 1 This is the structural schematic diagram of the fluxgate system.

[0051] Figure 2 It is the signal detection circuit diagram.

[0052] Figure 3 It is the phase-sensitive detection flow chart.

[0053] Figure 4 It is a schematic diagram of the phase-sensitive rectification algorithm.

[0054] like Figure 1 As shown, the fluxgate sensor system mainly includes a three-component sensor probe, an FPGA main control circuit, an excitation drive circuit, an induction detection circuit, a feedback circuit, and a communication and interface circuit.

[0055] The three-component probe of the fluxgate sensor mainly consists of a magnetic core, an excitation coil, an induction coil, a feedback coil, a skeleton, a shell, etc. According to different structures, the magnetic core inside the probe can be divided into a single magnetic core type, a double magnetic core type, a ring magnetic core type, and a runway magnetic core type. In practical applications, the fluxgate signal is a weak signal. In order to effectively filter out the noise signal caused by the transformer effect and improve the efficiency of the sensor system in extracting the second harmonic signal, the closed structure of the ring magnetic core can reduce the leakage flux and improve the system resolution, and the consistency of the parameters effectively filters out the noise interference caused by the transformer effect. The fluxgate sensor of the present invention adopts a ring magnetic core structure.

[0056] Figure 2 It is the signal detection circuit of the fluxgate sensor, which is mainly composed of LC resonant circuit, differential amplifier, ADC sampling, etc. Its main function is to frequency-selectively amplify the output signal of the fluxgate probe, and then use ADC to realize analog-to-digital conversion, and send it to the FPGA chip for processing.

[0057] The induction coil of the ring core probe is essentially still a winding inductor, and its output impedance is mainly inductive. In order to extract the second harmonic component of the output signal to the maximum extent, a parallel resonant capacitor is required in the induction coil. At the same time, matching resistors R1 and R2 are added to the resonant network to prevent the output signal from oscillating, and the bandwidth can be increased to a certain extent, reducing the impact on the measurement caused by probe differences. The differential amplifier circuit adjusts the range of circuit gain by adjusting the resistance value. The circuit has a very high common mode rejection ratio, which not only ensures the effective amplification of the second harmonic, but also avoids the waveform distortion caused by noise disturbance. Due to the differential structure of the differential amplifier circuit, the next level sampling circuit adopts a differential input ADC. When designing, try to ensure that the ADC sampling point is near the peak and trough of each cycle of the second harmonic signal, so that high-resolution sampling of the second harmonic signal can be achieved, and a serial digital signal can be output to the FPGA chip.

[0058] Figure 3 This is the principle diagram of the phase-sensitive detection algorithm. Since the amplitude of each even harmonic is proportional to the strength of the axial component of the measured magnetic field in the induction coil, and the linearity and stability of the sensor are inversely proportional to the harmonic order, the fluxgate signal extraction of the present invention adopts a digital phase-sensitive rectification and smoothing filtering algorithm for the harmonic components in the induction signal except the second harmonic component, and uses this second harmonic component to characterize the measured magnetic field information.

[0059] Assuming that the second harmonic component of the input signal is x(t), the expression is:

[0060]

[0061] Where A is the amplitude, 2f is the second harmonic frequency, is the phase difference with the reference signal. The phase-sensitive signal is selected to have the same frequency as the second harmonic to be processed, and the given reference signal is assumed to be:

[0062] s(t)=Bcos(2πft) (5)

[0063] Where B is the reference signal amplitude. By converting the product to the difference, the output value of the multiplier MUL1 is expressed as:

[0064]

[0065] Similarly, the output expression of MUL2 is:

[0066]

[0067] The low-pass filter outputs are, Combining the two equations, we can get the amplitude of the second harmonic component of the measured signal is Phase

[0068] To illustrate the rectification effect of phase-sensitive detection on each harmonic, the schematic diagram of the phase-sensitive rectification algorithm is shown in the figure below: Figure 4 Phase-sensitive detection uses a square wave signal with the same frequency as the second harmonic as the phase-sensitive signal. The phase-sensitive signal is 1 in the positive interval and -1 in the negative interval, and has an instantaneous jump from positive to negative.

[0069] The digital phase-sensitive detection and rectification algorithm performs the same operation on the induction signals with different input frequencies. For the input signal of the base frequency f1, the induction signal of the first half cycle is divided into two parts by the phase-sensitive signal, and both are positive voltages. In the first half, since the input signal and the phase-sensitive signal are both positive voltages, this part of the input signal remains unchanged after phase-sensitive rectification. The phase-sensitive signal of the second half is in the negative voltage interval, which changes the input signal in the same interval from positive voltage to negative voltage. Therefore, in the first half cycle, the harmonic signal voltage with a frequency of f1 is one positive and one negative after phase-sensitive rectification. Then, the processed signal is accumulated and integrated in half a cycle, and it can be obtained that the input signals in this half cycle cancel each other out. Similarly, the same processing is performed in the second half cycle, and it can be seen that the input signal with a frequency of f1 is completely canceled out after phase-sensitive rectification and integration. Similarly, the input signal with a frequency of f3 is also completely canceled out after the same processing.

[0070] When the input signal and the phase-sensitive signal frequency are consistent, the second harmonic component is processed, such as Figure 4 The input signal has an intermediate frequency of f2. Because the frequency cycle is consistent, the input signal and the phase-sensitive signal will be in the positive and negative intervals at the same time. According to the working principle of the phase-sensitive rectification algorithm, the waveform of the target signal is completely converted into a periodic waveform of the forward voltage, and then after being processed by the low-pass filter, the second harmonic component signal is completely retained and contains the measured magnetic field information.

[0071] According to the principle of phase-sensitive rectification, the periodic mean of the pulsating DC waveform output contains the target magnetic field strength information, but there is still some high-frequency noise interference with a small amplitude, which interferes with the accuracy of the fluxgate magnetic measurement signal. The main function of smoothing filtering is to eliminate the instantaneous noise in the rectified waveform, thereby improving the signal-to-noise ratio of the signal. The smoothing filtering algorithm adopts a mean filtering method to perform mean smoothing processing on points within a certain neighborhood of the second harmonic data. The number of points processed by the average filtering in the present invention is designed to be 256. The signal-to-noise ratio of the fluxgate signal is effectively improved after being processed by the phase-sensitive rectification and smoothing filtering modules, and the digital signal after the smoothing filtering processing can obtain the target magnetic field value after calibration.

[0072] Figure 5The signal simulation diagram of the phase-sensitive detection module. The phase-sensitive detection algorithm is modularly designed, and the module is time-constrained. The HDL language simulation software Modelsim is used to complete the signal simulation after the constraints. The second harmonic component to be processed is a digital quantity, and the phase-sensitive signal is a square wave with the same frequency as the second harmonic digital quantity divided by the main clock. In the simulation process, the original digital signal is represented as an analog signal, and the rectification effect can be observed more intuitively. In the timing simulation diagram, the sine signal to be processed is multiplied by the square wave reference signal with the same frequency, completing the rectification operation of the amplitude of the second harmonic component.

[0073] The digital phase-sensitive detection and smoothing filter algorithm are applied to the fluxgate signal detection system. During the experiment, the magnetic field inside and outside the high-precision shielding barrel is measured by comparison, and FFT spectrum analysis is performed to verify the effect of the second harmonic method on the magnetic field signal extraction of different magnetic field strengths. The accuracy of the signal extraction method for the actual measurement of the magnetic field signal of the fluxgate is verified by calibration experiments and field addition tests on the fluxgate sensor device designed based on the second harmonic method.

[0074] Figure 6 , 7 The waveforms and FFT spectra of the induced signals under the magnetic field conditions inside and outside the shielded barrel correspond to the approximate zero field and the ambient field. The experiment uses an open-loop fluxgate sensor to measure the target magnetic field, and the fluxgate probe is fixed in a shielded barrel that can isolate the external ambient magnetic field noise. The excitation frequency f1 of the fluxgate probe coil designed based on the second harmonic method is set to 9.6kHz.

[0075] Through FFT spectrum analysis, it can be obtained that in the open-loop state of the fluxgate system, due to the small magnetic field strength in the shielding barrel, the voltage amplitude of the induced signal at 19.2kHz is small. When the probe is in the ambient magnetic field, the magnetic field strength increases, and the voltage amplitude of the second harmonic component of the induced signal increases accordingly, which is about 9.4 times the voltage in the shielding barrel. It can be seen that the second harmonic component of the magnetic field signal is positively correlated with the measured magnetic field strength, and the fluxgate sensor that uses the second harmonic method to extract the magnetic field signal can sensitively reflect the changes in the measured magnetic field and effectively extract the measured magnetic field information.

[0076] Figure 8 , 9 The open-loop and closed-loop sensing signal waveforms and FFT spectra show that the sensing signal in the open-loop state of the fluxgate sensor designed based on the second harmonic method contains magnetic field information, that is, the second harmonic component has the largest amplitude. In theory, a feedback link is introduced in the design of the fluxgate sensor system, and a compensation magnetic field corresponding to the measured magnetic field is generated on the three-axis orthogonal feedback coil to offset the measured signal, so that the inside of the probe is in an approximately zero magnetic state, that is, the second harmonic component of the sensing signal will be completely offset.

[0077] From the FFT spectrum analysis, it can be seen that the second harmonic component amplitude is the largest in the induction signal waveform in the open-loop state, and the second harmonic in the closed-loop state is fed back to about 3% of the open-loop state. It can be seen that the fluxgate sensor designed based on the second harmonic method has the largest second harmonic component amplitude in the open-loop state because the induction signal contains magnetic field information. After adding feedback control, the second harmonic component is offset.

[0078] The fluxgate system designed based on the second harmonic method provided by the present invention is subjected to linear calibration and the sensitivity is tested. The calibration work is divided into two steps. First, the high-performance fluxgate Magson is placed in a shielding barrel for comparison. The current is adjusted by a high-precision shielding barrel controller so that the Magson reading is 0, and the control current value at this time is recorded. At the same time, the current value is increased in both positive and negative directions so that the magnetic field in the barrel changes evenly from -30000nT to 30000nT every 5000nT to simulate the movement of the downhole drill guide, and the current value corresponding to the target magnetic field value is recorded. Then the self-developed fluxgate probe is placed in the shielding barrel, the control current is adjusted to apply the specified target magnetic field, and the calibrated magnetic field value is read from the upper computer interface. Table 1 shows the calibration results of the self-developed fluxgate in the open-loop state. By comparing the maximum absolute error of Magson and the calibration value, it can be seen that the linearity of the open-loop fluxgate is 0.05%.

[0079] Table 1 Calibration data

[0080]

[0081] Fig.10 The performance of the open-loop fluxgate in the added field test. The sensitivity of a magnetic measuring instrument refers to its ability to detect changes in magnetic field strength, usually expressed by the change in instrument indication caused by a unit change in magnetic induction intensity. The higher the sensitivity, the more accurately the magnetic measuring instrument can detect changes in the target magnetic field. In order to verify the sensitivity of the open-loop fluxgate to changes in the target magnetic field downhole, the calibrated fluxgate continued to perform added field measurements from -30000nT to 30000nT, while extending the measurement time and recording the corresponding magnetic field data. By drawing the magnetic field data transformation diagram with Matlab, it can be concluded that the open-loop fluxgate can accurately reflect the magnetic field transformation, and the magnetic field value after smoothing filtering and calibration has small noise fluctuations and high sensitivity.

[0082] The novel fluxgate sensor signal processing method designed by the present invention can sensitively reflect the changes in the measured magnetic field and effectively extract the magnetic field information. In the range of -30000nT to 30000nT, the linearity of the open-loop fluxgate is 0.05%, and through the field addition test, the fluxgate sensor has small noise fluctuation and high sensitivity.

Claims

1. A method for processing downhole fluxgate sensor signals. As the size of downhole exploration guide drill bits is reduced, the circuit size and selection are limited. In view of the low acquisition accuracy and poor stability encountered by the fluxgate sensor in the process of extracting magnetic field signals, a magnetic field signal extraction method based on the second harmonic method is adopted according to the relationship that the even harmonic components in the fluxgate effective signal are proportional to the measured magnetic field; characterized in that: The signal detection circuit is used to extract the magnetic field voltage signal with the second harmonic component as the main component, and other harmonic components are eliminated through the digital phase-sensitive rectification and smoothing filter algorithm, namely the phase-sensitive detection algorithm, so as to realize the effective extraction of the magnetic field signal; the fluxgate sensor adopts a ring core, which is approximated as a double core structure; According to the relationship between magnetic induction intensity, magnetic permeability, and magnetic field intensity, the total magnetic induction intensity inside the induction coil is formed by the superposition of the external magnetic fields on the two magnetic cores. Using the magnetic flux formula, the magnetic flux in the induction coil is for: Where: S is the cross-sectional area of ​​the core, μ1 and μ2 are the magnetic permeabilities of the equivalent double core, H0 is the measured magnetic field strength; H m cosωt is the excitation magnetic field strength in the magnetic core; ω is the angular frequency of the excitation signal; the magnetic flux Performing Fourier expansion gives: Among them, Φ 0m for The fundamental component of is the harmonic component; Using trigonometric functions, limit method and Faraday's law of electromagnetic induction, when the size and magnetic permeability of the dual cores are exactly equal, the induced electromotive force E nm for: Wherein, μ0 is the magnetic permeability of the core in the unsaturated state, and W is the number of turns of the induction coil; According to formula (3), the induced electromotive force output by the annular core probe contains even harmonic signals of the magnetic field H0. Due to the Barkhausen effect, the external electromagnetic properties of the fluxgate probe, and the asymmetry of the core shape and size parameters, noise interference is inevitably introduced into the voltage signal output by the fluxgate probe. According to the measurement, the induced voltage signal output by the fluxgate probe has the largest amplitude with the harmonic components of the first three frequencies f1, f2, and f3, where f1 is the fundamental frequency of the excitation signal.

2. A method for processing downhole fluxgate sensor signals according to claim 1, characterized in that: The signal detection circuit of the fluxgate sensor consists of an LC resonant circuit, differential amplifier, and ADC sampling. Its function is to frequency-selectively amplify the output signal of the fluxgate probe, then use the ADC to achieve analog-to-digital conversion, and send it to the FPGA chip for processing.

3. A method for processing downhole fluxgate sensor signals according to claim 1, characterized in that: The induction coil of the ring core probe is a winding inductor, and its output impedance is mainly inductive. In order to extract the second harmonic component of the output signal to the maximum extent, a resonant capacitor needs to be connected in parallel with the induction coil; at the same time, matching resistors R1 and R2 are added to the resonant network; the differential amplifier circuit adjusts the range of circuit gain by adjusting the resistance value; the differential amplifier circuit has a differential structure, so the next-level sampling circuit adopts a differential input ADC; during the design, the ADC sampling point is near the peak and trough of each cycle of the second harmonic signal, realizing high-resolution sampling of the second harmonic signal at twice the frequency, and outputting a serial digital signal to the FPGA chip.

4. A method for processing downhole fluxgate sensor signals according to claim 1, characterized in that: The implementation process of the phase-sensitive detection algorithm is as follows: the amplitude of each even harmonic is proportional to the strength of the axial component of the measured magnetic field in the induction coil, and the linearity and stability of the sensor are inversely proportional to the harmonic order. The fluxgate signal extraction uses a digital phase-sensitive rectification and smoothing filter algorithm to extract the harmonic components in the induction signal except the second harmonic component, and use this second harmonic component to characterize the measured magnetic field information; The second harmonic component of the input signal is x(t), then the expression is: Where A is the amplitude, 2f is the second harmonic frequency, is the phase difference with the reference signal; the phase-sensitive signal is selected to have the same frequency as the second harmonic to be processed, and the given reference signal is assumed to be: s(t)=Bcos(2πft) (5) Where B is the reference signal amplitude. By converting the product to the difference, the output value of the multiplier MUL1 is expressed as: Similarly, the output expression of MUL2 is: The low-pass filter outputs are, Combining the two equations, we get the amplitude of the second harmonic component of the measured signal is Phase 5. A method for processing downhole fluxgate sensor signals according to claim 4, characterized in that: Phase-sensitive detection uses a square wave signal with the same frequency as the second harmonic as the phase-sensitive signal. The phase-sensitive signal is 1 in the positive interval and -1 in the negative interval, and has an instantaneous jump from positive to negative.

6. A method for processing downhole fluxgate sensor signals according to claim 5, characterized in that: The digital phase-sensitive detection and rectification algorithm performs the same operation on the induction signals with different input frequencies; for the input signal of the base frequency f1, the induction signal in the first half cycle is divided into two parts by the phase-sensitive signal, and both are positive voltages; in the first half, since the input signal and the phase-sensitive signal are both positive voltages, the input signal remains unchanged after phase-sensitive rectification; the phase-sensitive signal in the second half is in the negative voltage interval, so that the input signal in the same interval changes from positive voltage to negative voltage. In the first half cycle, the harmonic signal voltage with a frequency of f1 is one positive and one negative after phase-sensitive rectification; the processed signals are then accumulated and integrated in half a cycle, and the input signals in this half cycle cancel each other out; the same processing is performed in the second half cycle, and it can be seen that the input signal with a frequency of f1 is completely canceled out after phase-sensitive rectification and integration; the input signal with a frequency of f3 is also completely canceled out after the same processing.

7. A method for processing downhole fluxgate sensor signals according to claim 6, characterized in that: When the frequency of the input signal is consistent with that of the phase-sensitive signal, the second harmonic component is processed; because the frequency period is consistent, the input signal and the phase-sensitive signal will be in the positive and negative intervals at the same time; according to the working principle of the phase-sensitive rectification algorithm, the waveform of the target signal is completely converted into a periodic waveform of the forward voltage, and then after processing by the low-pass filter, the second harmonic component signal can be completely retained and contains the measured magnetic field information.

8. A method for processing downhole fluxgate sensor signals according to claim 7, characterized in that: The number of points for average filtering processing is designed to be 256; the signal-to-noise ratio of the fluxgate signal is improved after being processed by the phase-sensitive rectification and smoothing filtering modules, and the digital signal after smoothing filtering is calibrated to obtain the target magnetic field value.

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