A method of processing signals from a downhole fluxgate sensor

Abstract

The application discloses a kind of downhole fluxgate sensor signal processing methods, the method extracts the magnetic field voltage signal mainly with second harmonic component using signal detection circuit, further eliminates other harmonic components after digital phase-sensitive rectification and smoothing filter algorithm, realizes the effective extraction of magnetic field signal.The extraction effect of phase-sensitive rectification algorithm on second harmonic is verified by time sequence simulation result, and the open-loop fluxgate sensor designed based on second harmonic method is experimented.Compared with other types of fluxgate signal detection methods, second harmonic method effectively improves the resolution, linearity and other key performances of the sensor, and the fluxgate sensor designed based on second harmonic method can realize the detection and effective modulation of the measured magnetic field signal, and shows higher magnetic measuring level.

Description

Technical Field

[0001] This invention belongs to the field of sensor signal processing technology. By analyzing the modulation process of magnetic field signals realized by fluxgate sensors, a method for extracting downhole magnetic field signals based on the second harmonic method is proposed. Background Technology

[0002] Space magnetic fields, as a natural physical phenomenon, are widespread in outer space and are an important environmental factor affecting human production, daily life, and aerospace engineering. The Earth's magnetic field, as a crucial component of the space magnetic field, is closely related to many of Earth's natural phenomena. [1] Researching and measuring the Earth's magnetic field can serve important areas of production and daily life, such as Earth structure research, natural disaster forecasting, and energy exploration.

[0003] A fluxgate magnetometer is a sensor based on the magnetic sensing effect. By observing changes in the Earth's magnetic field, it can help explore the Earth's internal physical structure. Compared with other types of magnetic measuring instruments, fluxgate magnetometers have advantages such as wide measurement range, high resolution, and high reliability, and are widely used in mineral resource exploration, earthquake disaster prevention and mitigation, and magnetic navigation.

[0004] A fluxgate sensor mainly consists of a probe and an electronic system. As a derivative of the transformer effect, the probe utilizes the fluxgate phenomenon to convert the captured magnetic field signal into a voltage signal. The electronic unit works in conjunction with the probe to extract and process the magnetic field information. [2] .

[0005] This invention relates to a signal detection system circuit applied in downhole fluxgate electronics units, playing a crucial role in detecting and processing magnetic field signals. [3] The induction coil of the fluxgate's toroidal core probe detects the ambient magnetic field signal. Based on Faraday's law of electromagnetic induction, it obtains a voltage signal containing the magnetic field information and sends it to the signal detection circuit. [4] The signal detection circuit designed based on the second harmonic method is mainly responsible for detecting and amplifying the second harmonic component in the voltage signal and filtering out other noise interference. After being acquired by the ADC, the output digital signal is sent to the FPGA so that the fluxgate signal can be processed by software algorithms for digital phase-sensitive detection and other processing.

[0006] In practical applications, due to the Barkhausen effect and the asymmetry of the external electromagnetic and magnetic core shape and size parameters of the fluxgate probe, noise interference is inevitably introduced into the voltage signal output by the fluxgate probe. Furthermore, the amplitude of each even harmonic is proportional to the intensity of the axial component of the measured magnetic field in the induction coil. [5]Furthermore, the linearity and stability of the sensor are inversely proportional to the harmonic order. Based on the characteristics of the second harmonic method, the fluxgate signal extraction of this invention adopts 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 and thus locating the orientation of the downhole guide drill bit.

[0007] [1] Yue Liangguang. Research on airborne 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]. Soochow University, 2017.

[0009] [3] Liu, Xuanming. Design of a miniaturized triaxial fluxgate magnetometer [D]. Soochow University, 2017.

[0010] [4] Ge Hanlin, Zhang Yiming, Zhang Chenhao, et al. Design and implementation of high temperature fluxgate sensor [J]. Instrumentation Technology and Sensors, 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] With the shrinking size of downhole exploration directional drill bits, circuit size and selection are limited. Addressing the issues of low acquisition accuracy and poor stability encountered by fluxgate sensors in extracting magnetic field signals, a magnetic field signal extraction method based on the second harmonic method is proposed, based on the proportional relationship between even-order harmonic components in the effective fluxgate signal and the measured magnetic field. A signal detection circuit extracts the magnetic field voltage signal dominated by the second harmonic component. This signal is further processed by digital phase-sensitive rectification and smoothing filtering algorithms to remove other harmonic components, achieving effective extraction of the magnetic field signal. Timing simulation results verify the effectiveness of the phase-sensitive rectification algorithm in extracting the second harmonic, and experiments are conducted on an open-loop fluxgate sensor designed based on the second harmonic method. Compared to other types of fluxgate signal detection methods, the second harmonic method effectively improves the sensor's resolution, linearity, and other key performance characteristics. Furthermore, the fluxgate sensor designed based on the second harmonic method can detect and effectively modulate the measured magnetic field signal, exhibiting a high level of magnetic measurement accuracy.

[0013] The technical solution adopted in this invention is a signal processing method for a downhole fluxgate sensor. It uses a signal detection circuit to extract the magnetic field voltage signal dominated by the second harmonic component, and then uses a digital phase-sensitive rectification and smoothing filtering algorithm, i.e., a phase-sensitive detection algorithm, to remove other harmonic components, thereby achieving effective extraction of the magnetic field signal. The fluxgate sensor uses a toroidal magnetic core, which is approximated as a dual-core structure.

[0014] Based on the relationship between magnetic flux density, permeability, and magnetic field strength, the total magnetic flux density inside the induction coil is the superposition of the external magnetic fields on the two magnetic cores. Using the magnetic flux formula, the magnetic flux in the induction coil can be calculated. for:

[0015]

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

[0017]

[0018] Where, Φ 0m for The fundamental component of These are harmonic components;

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

[0020]

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

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

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

[0024] Furthermore, the induction coil of the toroidal magnetic core probe is a wound inductor, and its output impedance is mainly inductive. To maximize the extraction of the second harmonic component of the output signal, a resonant capacitor needs to be connected in parallel with the induction coil. At the same time, matching resistors R1 and R2 are added in the resonant network. The differential amplifier circuit adjusts the gain range by adjusting the resistance values. Due to the differential structure of the differential amplifier circuit, the next stage sampling circuit adopts a differential input ADC. In the design, the ADC sampling point is near the peak and trough of each cycle of the second harmonic signal to achieve high-resolution sampling of the second harmonic signal 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 intensity of the measured magnetic field in the axial component of the induction coil, and the linearity and stability of the sensor are inversely proportional to the harmonic order. The fluxgate signal extraction uses digital phase-sensitive rectification and smoothing filtering algorithms to extract the harmonic components in the induction signal other than the second harmonic component. This second harmonic component is used to characterize the measured magnetic field information.

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

[0027]

[0028] In the formula, A is the amplitude, and 2f is the second harmonic frequency. To account for the phase difference with the reference signal, a phase-sensitive signal with the same frequency as the second harmonic to be processed is selected. Let the given reference signal be:

[0029] s(t)=Bcos(2πft)(5)

[0030] In the formula, B is the amplitude of the reference signal; by product-to-sum conversion, the output value of multiplier MUL1 can be expressed as:

[0031]

[0032] Similarly, the output expression of MUL2 can be obtained as follows:

[0033]

[0034] The low-pass filter outputs are as follows: Combining the two equations, we obtain the amplitude of the second harmonic component of the measured signal as: phase

[0035] Furthermore, the phase-sensitive detector 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 range and -1 in the negative range, and has an instantaneous jump from positive to negative.

[0036] Furthermore, the digital phase-sensitive detection and rectification algorithm performs the same operation on induced signals with different frequencies. For an input signal with a fundamental frequency f1, the induced signal in the first half of the cycle is divided into two parts by the phase-sensitive signal, both of which are positive voltages. Since both the input signal and the phase-sensitive signal are positive voltages in the first half, the input signal remains unchanged after phase-sensitive rectification. In the second half, the phase-sensitive signal is in the negative voltage range, causing the input signal in the same range to change from positive to negative voltage. In the first half of the cycle, the harmonic signal voltage at frequency f1 is alternating between positive and 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 of the cycle. It can be seen that the input signal at frequency f1 is completely canceled out after phase-sensitive rectification and integration. The input signal at frequency f3 is also completely canceled out after the same processing.

[0037] Furthermore, when the input signal and the phase-sensitive signal have the same frequency, the second harmonic component is processed. Because the frequency period is the same, 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 positive voltage. After being processed by a low-pass filter, the second harmonic component signal is completely preserved and contains the information of the measured magnetic field.

[0038] Furthermore, the average number of filtering points 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 module, and the target magnetic field value is obtained after the smoothing filtering digital signal is calibrated. Attached Figure Description

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

[0040] Figure 2 This is a circuit diagram for signal detection.

[0041] Figure 3 This is a flowchart of phase-sensitive detection.

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

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

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

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

[0046] Figure 8 It uses an open-loop method to sense the signal waveform using FFT.

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

[0048] Figure 10 This is a fluxgate field test diagram. Detailed Implementation

[0049] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

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

[0051] Figure 2 This is a circuit diagram for signal detection.

[0052] Figure 3 This is a flowchart of phase-sensitive detection.

[0053] Figure 4 This is a schematic diagram of a 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 fluxgate sensor probe mainly consists of a magnetic core, excitation coil, induction coil, feedback coil, frame, and housing. Depending on the structure, the magnetic core inside the probe can be classified as single-core, dual-core, toroidal, or racetrack-core. In practical applications, fluxgate signals are weak signals. To effectively filter out noise signals caused by the transformer effect and improve the efficiency of the sensor system in extracting the second harmonic signal, this invention's fluxgate sensor adopts a toroidal core structure. This is because the closed structure of the toroidal core reduces leakage flux and improves system resolution, and the parameter consistency effectively filters out noise interference generated by the transformer effect.

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

[0057] The induction coil of the toroidal magnetic core probe is essentially a wire-wound inductor, and its output impedance is predominantly inductive. To maximize the extraction of the second harmonic component of the output signal, a resonant capacitor needs to be connected in parallel with the induction coil. Simultaneously, matching resistors R1 and R2 are added within the resonant network to prevent oscillations in the output signal and to increase bandwidth to some extent, reducing the measurement impact caused by probe differences. The differential amplifier circuit adjusts the gain range by adjusting the resistor values. This circuit has a high common-mode rejection ratio, ensuring effective amplification of the second harmonic while avoiding waveform distortion caused by noise disturbances. Due to the differential structure of the differential amplifier circuit, the next stage sampling circuit uses a differential input ADC. During the design, the ADC sampling point is designed to be near the peaks and troughs of each cycle of the second harmonic signal, enabling high-resolution sampling of the second harmonic signal's second harmonic frequency and outputting a serial digital signal to the FPGA chip.

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

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

[0060]

[0061] In the formula, A is the amplitude, and 2f is the second harmonic frequency. To account for the phase difference with the reference signal, a phase-sensitive signal with the same frequency as the second harmonic to be processed is selected. Let the given reference signal be:

[0062] s(t)=Bcos(2πft) (5)

[0063] In the formula, B is the amplitude of the reference signal. From the product-to-sum-to-difference method, the output value of multiplier MUL1 can be expressed as:

[0064]

[0065] Similarly, the output expression of MUL2 can be obtained as follows:

[0066]

[0067] The low-pass filter outputs are as follows: By combining the two equations, we can obtain the amplitude of the second harmonic component of the measured signal as follows: phase

[0068] To illustrate the rectification effect of phase-sensitive detection on each harmonic, a schematic diagram of the phase-sensitive rectification algorithm is shown below. Figure 4 As shown. The phase-sensitive detector 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 range and -1 in the negative range, and has an instantaneous jump from positive to negative.

[0069] The digital phase-sensitive detection and rectification algorithm performs the same operation on induced signals with different frequencies. For an input signal with a fundamental frequency f1, the induced signal in the first half of the cycle is divided into two parts by the phase-sensitive signal, both of which are positive voltages. Since both the input signal and the phase-sensitive signal are positive voltages in the first half, this part of the input signal remains unchanged after phase-sensitive rectification. The phase-sensitive signal in the second half is in the negative voltage range, causing the input signal in the same range to change from positive to negative voltage. Therefore, in the first half of the cycle, the harmonic signal voltage at frequency f1 is alternating between positive and negative after phase-sensitive rectification. By summing and integrating the processed signals over half a cycle, we can see that the input signals in this half-cycle cancel each other out. Similarly, the same processing is performed in the second half of the cycle, showing that the input signal at frequency f1 is completely canceled out after phase-sensitive rectification and integration. Similarly, the input signal at frequency f3 is also completely canceled out after the same processing.

[0070] When the input signal and the phase-sensitive signal have the same frequency, the second harmonic component is processed, such as... Figure 4 The input signal has a frequency of f2. Because the frequency periods are consistent, the input signal and the phase-sensitive signal will be in the positive and negative intervals simultaneously. 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 positive voltage. After being processed by a low-pass filter, the second harmonic component signal is completely preserved and contains the information of the measured magnetic field.

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

[0072] Figure 5This is a signal simulation diagram of the phase-sensitive detection module. The phase-sensitive detection algorithm is modularly designed, and timing constraints are applied to the module. The constrained signal simulation is completed using the HDL simulation software Modelsim. The second harmonic component to be processed is a digital quantity. The phase-sensitive signal is a square wave with the same frequency as the second harmonic digital quantity, divided by the main clock. During the simulation, the original digital signal is represented as an analog signal, allowing for a more intuitive observation of the rectification effect. In the timing simulation diagram, the sinusoidal signal to be processed undergoes a dot product operation with its corresponding square wave reference signal, thus completing the rectification operation on the amplitude of the second harmonic component.

[0073] Digital phase-sensitive detection and smoothing filtering algorithms were applied to a fluxgate signal detection system. During the experiment, the magnetic fields inside and outside a high-precision shielded container were measured comparatively, and FFT spectrum analysis was performed to verify the effectiveness of the second harmonic method in extracting magnetic field signals of different magnetic field intensities. Calibration experiments and field tests were conducted on a fluxgate sensor device designed based on the second harmonic method to verify the accuracy of this signal extraction method for actual magnetic field signal measurements by a fluxgate.

[0074] Figure 6 , 7 The figures show the induced signal waveforms and FFT spectra under magnetic field conditions inside and outside the shielded container, corresponding to approximately zero field and ambient field, respectively. An open-loop fluxgate sensor was used to measure the target magnetic field. The fluxgate probe was fixed inside a shielded container that isolated it from external ambient magnetic field noise. The excitation frequency f1 of the fluxgate probe coil, designed based on the second harmonic method, was set to 9.6 kHz.

[0075] FFT spectrum analysis revealed that in the open-loop state of the fluxgate system, the voltage amplitude at 19.2kHz of the induced signal is small due to the relatively low magnetic field strength inside the shielding barrel. 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, approximately 9.4 times that inside the shielding barrel. This indicates a positive correlation between the second harmonic component of the magnetic field signal and the strength of the measured magnetic field. Furthermore, the fluxgate sensor using the second harmonic extraction method can sensitively reflect changes in the measured magnetic field and effectively extract the measured magnetic field information.

[0076] Figure 8 , 9 The open-loop and closed-loop induction signal waveforms and FFT spectra are shown. In the open-loop state, the induction signal of the fluxgate sensor designed based on the second harmonic method contains magnetic field information, meaning the second harmonic component has the largest amplitude. Theoretically, a feedback loop is introduced into the fluxgate sensor system design. A compensating magnetic field corresponding to the measured magnetic field is generated on the triaxially orthogonal feedback coil, thereby canceling the measured signal and placing the probe in a near-zero magnetic state, meaning the second harmonic component of the induction signal is completely canceled out.

[0077] FFT spectrum analysis shows that the second harmonic component has the largest amplitude in the open-loop state of the induced signal waveform, while in the closed-loop state, the second harmonic is fed back to about 3% of the open-loop state amplitude. It can be seen that in the open-loop state, the fluxgate sensor designed based on the second harmonic method has the largest amplitude of its second harmonic component because the induced signal contains magnetic field information; after adding feedback control, the second harmonic component is canceled out.

[0078] The fluxgate system designed based on the second harmonic method provided in this invention underwent linear calibration, followed by sensitivity testing. The calibration process consisted of two steps. First, a high-performance fluxgate Magson was placed in a shielded container for comparison. The current was adjusted using a high-precision shielded container controller until the Magson reading was 0, and the control current value was recorded. Simultaneously, the current value was increased in both directions to uniformly vary the magnetic field within the container from -30000nT to 30000nT in 5000nT increments, simulating downhole drilling movement, and the current value corresponding to the target magnetic field value was recorded. Then, the self-developed fluxgate probe was placed in the shielded container, and the control current was adjusted to apply the specified target magnetic field. The calibrated magnetic field value was then read from the host computer interface. Table 1 shows the calibration results of the self-developed fluxgate in open-loop mode. By comparing the maximum absolute error between the Magson and calibrated values, the linearity of the open-loop fluxgate is 0.05%.

[0079] Table 1 Calibration Data

[0080]

[0081] Figure 10 This section describes the performance of an open-loop fluxgate magnetometer in a field test. The sensitivity of a magnetic measuring instrument refers to its ability to detect changes in magnetic field strength, typically expressed as the change in instrument reading caused by a unit change in magnetic flux density. Higher sensitivity means the instrument can more accurately detect changes in the target magnetic field. To verify the sensitivity of the open-loop fluxgate magnetometer to changes in the downhole target magnetic field, the calibrated magnetometer was subjected to further field measurements from -30000 nT to 30000 nT, with extended measurement times and corresponding magnetic field data recorded. Plotting the magnetic field data transformation using Matlab shows that the open-loop fluxgate magnetometer accurately reflects magnetic field changes, and the magnetic field values ​​after smoothing, filtering, and calibration exhibit low noise fluctuations, indicating high sensitivity.

[0082] The novel fluxgate sensor signal processing method designed in this invention can sensitively reflect changes in the measured magnetic field and effectively extract magnetic field information. Within the range of -30000nT to 30000nT, the linearity of the open-loop fluxgate is 0.05%. Furthermore, through field testing, the fluxgate sensor exhibits low noise fluctuation and high sensitivity.

Claims

1. A signal processing method for a downhole fluxgate sensor, characterized in that, The magnetic field voltage signal, which is dominated by the second harmonic component, is extracted by a signal detection circuit. Other harmonic components are eliminated by digital phase-sensitive rectification and smoothing filtering algorithm, i.e., phase-sensitive detection algorithm, so as to achieve effective extraction of magnetic field signal. The fluxgate sensor adopts a toroidal magnetic core, which is approximated as a dual magnetic core structure. Based on the relationship between magnetic flux density, permeability, and magnetic field strength, the total magnetic flux density inside the induction coil is the superposition of the external magnetic fields on the two magnetic cores. Using the magnetic flux formula, the magnetic flux in the induction coil can be calculated. for: In the formula: S is the cross-sectional area of ​​the magnetic core, μ1 and μ2 are the permeability of the equivalent double magnetic core, and H0 is the strength of the measured magnetic field; H m cosωt is the excitation magnetic field strength in the magnetic core; ω is the angular frequency of the excitation signal; and the magnetic flux is... Performing a Fourier expansion yields: Where, Φ 0m for The fundamental component of These are harmonic components; Using trigonometric functions, the limit method, and Faraday's law of electromagnetic induction, when the dimensions and permeability of the two magnetic cores are exactly equal, the induced electromotive force E nm for: Where μ0 is the permeability of the magnetic core in its unsaturated state and W is the number of turns of the induction coil; According to equation (3), the induced electromotive force output by the toroidal magnetic core probe contains even harmonic signals of the magnetic field H0. Due to the Barkhausen effect, the asymmetry of the external electromagnetic and magnetic core shape and size parameters of the fluxgate probe, noise interference is inevitably introduced into the voltage signal output by the fluxgate probe. According to the measurement, the amplitude of the harmonic components of the first three frequencies f1, f2, and f3 of the induced voltage signal output by the fluxgate probe is the largest, where f1 is the fundamental frequency of the excitation signal. The induction coil of the toroidal magnetic core probe is a wire-wound inductor, and its output impedance is mainly inductive. To maximize the extraction of the second harmonic component of the output signal, a resonant capacitor needs to be connected in parallel with the induction coil. At the same time, matching resistors R1 and R2 are added in the resonant network. The differential amplifier circuit adjusts the gain range by adjusting the resistance values. Due to the differential structure of the differential amplifier circuit, the next stage sampling circuit uses a differential input ADC. In the design, the ADC sampling point is near the peak and trough of each cycle of the second harmonic signal to achieve high-resolution sampling of the second harmonic signal and output a serial digital signal to the FPGA chip.

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

3. The signal processing method for a downhole fluxgate sensor 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 measured magnetic field in the axial component of the induction coil, and the linearity and stability of the sensor are inversely proportional to the harmonic order. The fluxgate signal extraction uses digital phase-sensitive rectification and smoothing filtering algorithms to extract the harmonic components in the induction signal other than the second harmonic component. This second harmonic component is used to characterize the measured magnetic field information. If the second harmonic component of the input signal is x(t), then the expression is: In the formula, A is the amplitude, and 2f is the second harmonic frequency. To account for the phase difference with the reference signal, a phase-sensitive signal with the same frequency as the second harmonic to be processed is selected. Let the given reference signal be: s(t)=Bcos(2πft) (5) In the formula, B is the amplitude of the reference signal; by product-to-sum conversion, the output value of multiplier MUL1 can be expressed as: Similarly, the output expression of MUL2 can be obtained as follows: The low-pass filter outputs are as follows: Combining the two equations, we obtain the amplitude of the second harmonic component of the measured signal as: phase 4. The downhole fluxgate sensor signal processing method according to claim 3, characterized in that, The phase-sensitive detector 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 range and -1 in the negative range, and has an instantaneous jump from positive to negative.

5. The downhole fluxgate sensor signal processing method according to claim 4, characterized in that, The digital phase-sensitive detection and rectification algorithm performs the same operation on induced signals with different frequencies. For an input signal with a fundamental frequency f1, the induced signal in the first half of the cycle is divided into two parts by the phase-sensitive signal, both of which are positive voltages. Since both the input signal and the phase-sensitive signal are positive voltages in the first half, the input signal remains unchanged after phase-sensitive rectification. In the second half, the phase-sensitive signal is in the negative voltage range, causing the input signal in the same range to change from positive to negative voltage. In the first half of the cycle, the harmonic signal voltage at frequency f1 is alternating between positive and 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 of the cycle. It can be seen that the input signal at frequency f1 is completely canceled out after phase-sensitive rectification and integration. The input signal at frequency f3 is also completely canceled out after the same processing.

6. The signal processing method for a downhole fluxgate sensor according to claim 5, characterized in that, When the input signal and the phase-sensitive signal have the same frequency, the second harmonic component is processed. Because the frequency period is the same, 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 positive voltage. After being processed by a low-pass filter, the second harmonic component signal is completely preserved and contains the information of the measured magnetic field.

7. The downhole fluxgate sensor signal processing method according to claim 6, characterized in that, The average number of filtering points 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 module, and the target magnetic field value is obtained after the smoothing filtering digital signal is calibrated.

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