A method for detecting defects on the surface of a ferromagnetic pipe

Abstract

This invention discloses a method for detecting surface defects in ferromagnetic pipes, comprising: a transmitting module, a transducer, and a receiving module. The transmitting module generates an excitation signal which is fed into the excitation coil of the excitation probe of the transducer. Under the action of mutually orthogonal dynamic and static magnetic fields, the torsional guided waves excited by the two excitation coils in the pipe undergo constructive interference, achieving peak-to-peak superposition. The guided waves are transmitted along the pipe axis to the receiving probe of the transducer, where two voltage signals are induced in the receiving coil. The receiving module filters and amplifies the signals, and then the signals are acquired and sent to a host computer for phase calibration and differential calculation. This invention can effectively increase the amplitude of the excited torsional guided waves, improve the detection distance, amplify the amplitude of the received signal, suppress noise, and more clearly distinguish defect echo signals.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology for pipelines, and in particular to a method for detecting surface defects in ferromagnetic pipelines. Background Technology

[0002] Currently, most testing institutions use piezoelectric ultrasonic methods, employing WAVEMAKER equipment from the British company, Waveguide. Piezoelectric ultrasonic transducers offer advantages such as small size, robustness, diverse dimensions and shapes, zero maintenance, long service life, and ease of control. However, they also have the following drawbacks: piezoelectric ultrasonic transducers use fixed-diameter retaining ring type transducers, meaning one transducer can only test oil and gas pipelines of a corresponding diameter; the transducer's inner diameter is not adjustable, resulting in poor flexibility.

[0003] Electromagnetic ultrasonic technology (EMAT) is an advanced technology in the field of modern nondestructive testing. It uses electromagnetic induction to generate ultrasonic waves and excites and receives ultrasonic waves through electromagnetic coupling. Compared with piezoelectric ultrasonic technology, electromagnetic ultrasonic technology has the following advantages: (1) No coupling agent required: It avoids the limitation of contact with the surface of the object being tested in traditional methods, and is suitable for complex environments and high-temperature testing; (2) Wide detection range: It is applicable to various conductive materials, including metals, non-metals and composite materials, and has a wide detection range; (3) High lift-off characteristics: It can penetrate paint and anti-corrosion coatings up to 6 mm thick, without grinding the surface of the test piece, and the influence of coating thickness on the measurement results does not need to be considered when measuring thickness; (4) High precision: Electromagnetic ultrasonic technology can excite various ultrasonic wave patterns, which improves the accuracy and flexibility of the test; (5) Electromagnetic ultrasonic technology is easy to operate, small in size, light in weight, and easy to carry. However, the existing electromagnetic ultrasonic ferromagnetic pipe surface defect detection methods have the disadvantages of low transducer transduction efficiency, short propagation distance, large noise in the received signal, and defect echo signals are easily submerged in noise and difficult to observe. Summary of the Invention

[0004] Purpose of the invention: This invention provides a method for detecting surface defects in ferromagnetic pipes, which can effectively increase the amplitude of the excitation torsional guided wave, improve the detection distance, amplify the amplitude of the received signal and suppress noise, and more clearly distinguish the defect echo signal.

[0005] Technical Solution: The present invention discloses a ferromagnetic pipe surface defect detection device, comprising: a transmitting module, a transducer, and a receiving module; the transmitting module generates an excitation signal which is fed into the excitation coil of the excitation probe of the transducer. Under the action of mutually orthogonal dynamic and static magnetic fields, the torsional guided waves excited by the two excitation coils in the pipe undergo constructive interference, achieving peak-to-peak superposition. The guided waves are transmitted along the pipe axis to the receiving probe of the transducer, where two voltage signals are induced in the receiving coil of the receiving probe. The receiving module filters and amplifies the signals, and then the signals are acquired and fed into a host computer for phase calibration and differential calculation.

[0006] Furthermore, the transmitting module includes an STM32 microcontroller, an isolation drive circuit, a full-bridge inverter circuit, and an impedance matching circuit. The STM32 microcontroller generates two complementary PWM waves with dead time as control signals. The dead time is set to 5% of the pulse signal period to prevent all four MOSFETs from conducting simultaneously. The isolation drive circuit controls the conduction and cutoff of the MOSFETs. The full-bridge inverter circuit, composed of the four MOSFETs, amplifies the power of the excitation signal. When the input signals controlling Q1 and Q4 are high, Q1 and Q4 conduct. At this time, the other input signal is low, Q2 and Q3 are cut off, and the output voltage is +VCC. Conversely, the output voltage is -VCC. The full-bridge inverter circuit can ultimately output an excitation signal with the same frequency and number of pulses as the control signal and a peak-to-peak value of 2VCC. The impedance matching circuit further increases the peak-to-peak value of the AC signal, thereby increasing the power of the excitation signal.

[0007] Furthermore, the receiving module includes an RLC series resonant circuit and a preamplifier circuit, a second-order filter amplifier circuit and a multi-stage amplifier circuit, and a data acquisition card. The RLC series resonant circuit and the preamplifier circuit filter out noise and spurious components of other frequencies and amplify the signal. The second-order low-pass filter circuit filters out high-frequency components outside the signal passband. The multi-stage amplifier circuit adopts a combination of a fixed-gain second-stage amplifier circuit and a variable-gain third-stage amplifier circuit. The amplification factor is selected by a single-pole triple-throw switch K. The signal after the above filtering and amplification is acquired by the data acquisition card and phase calibration and differential calculation are performed by the host computer.

[0008] Accordingly, a method for detecting surface defects in ferromagnetic pipes includes the following steps:

[0009] Step 1: The transmitting module generates an excitation signal that is fed into the excitation coil of the transducer's excitation probe;

[0010] Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipe. According to the principle of wave interference, by controlling the distance between the two coils, the torsional guided waves excited in the two coils can have a fixed path difference, thereby enabling constructive interference, achieving wave crest-to-crest superposition, and increasing the amplitude of the torsional guided waves.

[0011] Step 3: The guided wave is transmitted along the axial direction of the pipe to the receiving probe of the transducer, and two voltage signals are induced in the receiving coil of the receiving probe.

[0012] Step 4: Filter and amplify the signal through the receiving module, then collect the signal into the host computer for phase calibration and differential calculation, and display the final echo signal to facilitate further analysis of pipeline surface defects.

[0013] Furthermore, in step 2, the coil width d affects the amplitude of the excited torsional guided wave. Within the range of 0 to λ, as d increases, the guided wave amplitude first increases and then decreases. When the amplitude reaches its maximum, the coil width is selected. Select two coils with opposite winding directions and λ is the wavelength corresponding to the design frequency.

[0014] Furthermore, in step 3, the relative distance between the two receiving coils is y. The size and position of the permanent magnet magnetized along the length direction vary with the size and position of the coils, and it is placed as evenly as possible on the coils, with a length equal to the width of the coil to ensure coverage. The relationship between the phase difference of the signals received by the two coils and y is:

[0015]

[0016] Due to the numerous combinations of the winding direction of the two coils and the wiring methods at both ends of the coils, the two received signals may have two situations: the positive and negative signals are the same or the positive and negative signals are opposite.

[0017] Since differential amplification can amplify differential signals and suppress common-mode signals, it is necessary to adjust the phase difference between the two acquired signals to meet the characteristics of differential signals, that is, the voltages are opposite at the same time. When the two signals are the same, their phase difference needs to be adjusted to 180°. When the two signals are opposite, their phase difference needs to be adjusted to 0°.

[0018] Furthermore, in step 4, two signals are acquired using a data acquisition card. Due to the relative distance between the two receiving coils, the phase difference between the direct wave and the defect reflection wave and the end face reflection wave of the two signals is different. Therefore, it is necessary to extract the defect echo and end face echo portions for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. By subtracting the phase value in the corresponding phase spectrum from the frequency, the phase difference between the two signals can be obtained. When the two received signals are the same polarity, one of the signals is phase-shifted (180° - phase difference) to automatically calibrate the phase difference between the two signals to 180°. Finally, the two signals are differentially analyzed and the final signal waveform is displayed, amplifying the differential signal and suppressing the common-mode signal. When the two received signals are opposite polarity, 180° is changed to 0°.

[0019] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: First, by using the interference principle of ultrasound, two excitation coils are used to simultaneously excite the torsional guided wave, and by rationally designing the relative distance between the two coils, the two guided waves undergo constructive interference, which can effectively increase the amplitude of the excited guided wave and thus improve the detectable distance; Second, two receiving coils are designed to simultaneously receive two echo signals, and the phase difference between the two signals is adjusted by software, so that the phase difference between signals with the same positive and negative signals is 180°, and the phase difference between signals with opposite positive and negative signals is 0°, making them differential mode signals. Through differential operation, the useful signal is amplified and noise is suppressed, which can effectively extract the echo signal of small defects and further identify small defects; Finally, by adding a part of phase calibration using software, the distance y between the receiving coils does not need to be fixed. The position can be adjusted according to the detection environment, and the phase difference of the processed signal is not affected by the distance y by the subsequent phase calibration using software. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the device structure of the present invention.

[0021] Figure 2 This is a schematic diagram of the transducer excitation probe structure of the present invention.

[0022] Figure 3 This is a schematic diagram of the automatic phase calibration method of the present invention.

[0023] Figure 4 This is a schematic diagram of the transducer receiving probe structure of the present invention.

[0024] Figure 5 This is a schematic diagram of the excitation circuit structure of the present invention.

[0025] Figure 6 This is a schematic diagram of the full-bridge inverter circuit structure of the present invention.

[0026] Figure 7 This is a schematic diagram of the receiving circuit structure of the present invention.

[0027] Figure 8 This is a schematic diagram of the series resonant circuit structure of the present invention. Detailed Implementation

[0028] like Figure 1As shown, a ferromagnetic pipe surface defect detection device includes: a transmitting module, a transducer, and a receiving module; the transmitting module generates an excitation signal which is fed into the excitation coil of the excitation probe of the transducer. Under the action of mutually orthogonal dynamic and static magnetic fields, the torsional guided wave excited by the excitation coil in the pipe undergoes peak-to-peak superposition. The guided wave is transmitted along the pipe axis to the receiving probe of the transducer, where two voltage signals are induced in the receiving coil of the receiving probe. The receiving module filters and amplifies the signals, and then the signals are acquired and sent to the host computer for phase calibration and differential calculation.

[0029] like Figure 5 and Figure 6 As shown, the transmitting module includes an STM32 microcontroller, an isolation drive circuit, a full-bridge inverter circuit, and an impedance matching circuit. The STM32 microcontroller generates two complementary PWM waves with dead time as control signals. The dead time is set to 5% of the pulse signal period to prevent all four MOSFETs from conducting simultaneously. The isolation drive circuit controls the on / off state of the MOSFETs. The four MOSFETs form a full-bridge inverter circuit to amplify the excitation signal. When the input signals for Q1 and Q4 are high, Q1 and Q4 conduct; when the other input signal is low, Q2 and Q3 are off, and the output voltage is +VCC. Conversely, the output voltage is -VCC. Therefore, the full-bridge inverter circuit can ultimately output an excitation signal with the same frequency and number of pulses as the control signal, and a peak-to-peak value of 2VCC. The power of the excitation signal can be increased by further increasing the peak-to-peak value of the AC signal.

[0030] like Figure 7 and Figure 8 As shown, the receiving module includes an RLC series resonant circuit, a preamplifier circuit, a second-order filter amplifier circuit, a multi-stage amplifier circuit, and a data acquisition card. First, the RLC series resonant circuit filters out noise and spurious components of other frequencies, achieving a certain amplification effect. According to series resonance theory, adding a capacitor C and an inductor L to the circuit creates a series resonance, which further increases the amplitude of the output signal.

[0031] The preamplifier stage uses an operational amplifier to form a non-inverting amplifier circuit, amplifying the signal by 1000 times. Next, a second-order low-pass filter circuit is used to filter out high-frequency components outside the signal's passband. Since the signal still contains high-frequency noise after preamplification, it can be attenuated by the second-order low-pass filter circuit. A capacitor connects the positive input and output of the operational amplifier, introducing positive feedback. By setting a reasonable cutoff frequency, the high-frequency band of the second-order low-pass filter circuit's amplitude-frequency characteristic is rapidly attenuated, allowing only the low-frequency band to pass. The EMAT receiving coil detection signal, after passing through the RLC series resonant frequency selection circuit, preamplifier circuit, and filter circuit, still does not meet the requirements for subsequent data processing; therefore, the signal needs to be amplified again. However, considering the different specifications of the measured object, the received signal strength varies. Furthermore, the amplitude of the echo signal also differs for defects of different sizes. To avoid signal distortion due to excessive amplification or ineffective amplification due to insufficient amplification, the multi-stage amplifier circuit employs a combination of a fixed-gain second-stage amplifier and a variable-gain third-stage amplifier. The amplification factor is selected by a single-pole triple-throw switch K, ensuring both the basic signal amplification and flexible amplification for received signals of varying amplitudes. The filtered and amplified signal is then acquired by a data acquisition card and subjected to phase calibration and differential calculations by a host computer.

[0032] Accordingly, a method for detecting surface defects in ferromagnetic pipes includes the following steps:

[0033] Step 1: The transmitting module generates an excitation signal that is fed into the excitation coil of the transducer's excitation probe;

[0034] Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipe. According to the principle of wave interference, by controlling the distance between the two coils, the torsional guided waves excited in the two coils can have a fixed path difference, thereby enabling constructive interference, achieving wave crest-to-crest superposition, and increasing the amplitude of the torsional guided waves.

[0035] Step 3: The guided wave is transmitted along the axial direction of the pipe to the receiving probe of the transducer, and two voltage signals are induced in the receiving coil of the receiving probe.

[0036] Step 4: Filter and amplify the signal through the receiving module, then collect the signal into the host computer for phase calibration and differential calculation, and display the final echo signal to facilitate further analysis of pipeline surface defects.

[0037] like Figure 2 The diagram shown is a schematic of the transducer excitation probe structure of the present invention. The relationship between the wavelength and wave velocity of the torsional guided wave is as follows:

[0038]

[0039] Where f is the frequency of the excitation signal, i.e. the design frequency of the torsional guide wave; λ is the wavelength corresponding to the design frequency; v is the wave velocity of the torsional guide wave; L is the distance between the center points of the two excitation coils; and d is the width of the magnetostrictive band.

[0040] The excitation probe of the transducer consists of two transmitting coils, a magnetized strip, and a permanent magnet. Based on the inverted Widmann effect, the permanent magnet 2 is magnetized along its length and placed circumferentially along the pipe 3. The cross-sectional size of the permanent magnet is 10x10mm. 2 The excitation probe of the transducer is covered in length and is placed as evenly as possible along the axial direction of the pipe. Enameled wire 1 of diameter D is wound axially on two pure nickel magnetostrictive strips 4 with n turns. The current I flowing through the coils is equal to the outer circumference of the pipe to ensure it covers the outer surface. The wound nickel strips 4 and coils 1 are then bonded to the circumferential surface of the pipe 3 using epoxy resin. The center points of the coils on the two nickel strips 4 are L apart. The excitation signal induces a circumferential dynamic magnetic field in the magnetostrictive strips 4 through the two transmitting coils 1, thereby exciting two torsional guided waves in the pipe 3. The relationship between the amplitude of the torsional guided waves and the above parameters is as follows:

[0041]

[0042] That is, the more turns of the coil, the greater the current, the greater the amplitude of the torsional guided wave; the larger the coil diameter, the lower the ultrasonic transmittance, the smaller the amplitude of the torsional guided wave; the more the excitation signal frequency increases, the more the amplitude of the torsional guided wave decreases exponentially.

[0043] According to the principle of wave interference, since the distance L between the two coils is fixed, the two torsional guided waves have a constant path difference L, and their phase difference can be obtained as follows: Also constant:

[0044] Assuming the amplitudes of the two torsional guided waves are A1 and A2, and their power has a square relationship with their amplitude, that is:

[0045] P = A 2 ·f·ρ

[0046] Where f is the waveguide frequency and ρ is the medium density, both of which are constant values ​​throughout the process.

[0047] The power of the torsional guided wave after interference is:

[0048]

[0049] Therefore, when the two coils are wound in the same direction, A1 and A2 have the same direction. Constructive interference occurs at this time. Where n = 0, 1, 2..., that is, the coil spacing L should be selected as... An even multiple of; when the two coils are wound in opposite directions, A1 and A2 are in opposite directions. Constructive interference occurs at this time. Where n = 0, 1, 2..., that is, the coil spacing L should be selected as... An odd multiple of the amplitude. At this point, constructive interference occurs in the torsional guided wave, resulting in positive superposition of wave crests, thereby increasing the amplitude of the torsional guided wave.

[0050] Since the coil width d affects the amplitude of the excited torsional guided wave, within the range of 0 to λ, the guided wave amplitude first increases and then decreases as d increases. When the amplitude reaches its maximum, the coil width is selected. That would be the best.

[0051] Because the lift-off distance of the magnetostrictive band relative to the pipe surface affects the amplitude of the torsional guided wave during actual system testing, and because there are ultrasonic transmission issues when the two coils overlap, the spacing L should not be zero. Furthermore, since guided waves attenuate as they propagate through the medium, if the two coils are too far apart, the amplitude difference becomes too large, resulting in poor superposition. Therefore, two coils with opposite winding directions are selected. The best time is when.

[0052] like Figure 4 The diagram shows the specific structure of the transducer receiving probe used in the detection device of this invention. Receiving coils 1 are wound around two magnetized strips 4, and permanent magnets 2, magnetized along their length, are placed on them as evenly as possible. The relative distance between the magnetized strips of the two wound coils is y. The relationship between the phase difference of the signals received by the two coils and y is as follows:

[0053]

[0054] Due to the numerous combinations of winding directions of the two coils and wiring methods at both ends of the coils (signal end and reference voltage end), the received two signals can have two possible outcomes: either the positive and negative signals are the same, or the positive and negative signals are opposite.

[0055] Since differential amplification can amplify differential-mode signals and suppress common-mode signals, it is necessary to adjust the phase difference between the two acquired signals to meet the characteristics of differential-mode signals, i.e., the voltages are opposite at the same time. When the two signals are of the same sign, their phase difference needs to be adjusted to 180°; when the two signals are of opposite signs, their phase difference needs to be adjusted to 0°.

[0056] This invention uses LabVIEW to design a host computer program for data acquisition, automatic phase calibration, and differential amplification of two signals. The design method is as follows: Figure 3 As shown:

[0057] Since the signal processing methods for adjusting the phase to 180° and 0° are the same, the following section uses the automatic calibration of the phase difference to 180° as an example to introduce the design method of the host computer program.

[0058] First, two signals are acquired using a data acquisition card. Due to the relative distance between the two receiving coils, the phase difference between the direct wave and the defect reflection and end-face reflection of the two signals is different. Therefore, the defect echo and end-face echo portions need to be extracted for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. By subtracting the corresponding phase value in the phase spectrum from the frequency, the phase difference between the two signals can be obtained. When the two received signals are of the same sign, shifting the phase of one of the signals by (180° - phase difference) automatically calibrates the phase difference between the two signals to 180°. Finally, differential operations are performed on the two signals, and the final signal waveform is displayed, amplifying the differential signal and suppressing the common-mode signal. When the two received signals are of opposite signs, 180° is changed to 0°.

Claims

1. A method for detecting surface defects in ferromagnetic pipes, characterized in that, include: Transmitter module, transducer, and receiver module; The transmitting module generates an excitation signal that is fed into the excitation coil of the transducer's excitation probe. Under the influence of mutually orthogonal dynamic and static magnetic fields, the torsional guided waves excited by the two excitation coils in the pipe undergo constructive interference, achieving peak-to-peak superposition. The guided waves are transmitted axially along the pipe to the transducer's receiving probe, where two voltage signals are induced in the receiving coil. The receiving module filters and amplifies the signals, and then the signals are acquired and sent to the host computer for phase calibration and differential calculation. The specific steps include the following: Step 1: The transmitting module generates an excitation signal that is fed into the excitation coil of the transducer's excitation probe; Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipe. According to the principle of wave interference, by controlling the distance between the two coils, the torsional guided waves excited in the two coils have a fixed path difference, thereby causing constructive interference, realizing the superposition of wave crests, and increasing the amplitude of the torsional guided waves. Step 3: The guided wave is transmitted along the axial direction of the pipe to the receiving probe of the transducer, and two voltage signals are induced in the receiving coil of the receiving probe. Step 4: Filter and amplify the signal using the receiving module, then input the signal into the host computer for phase calibration and differential calculation, and display the final echo signal for further analysis of pipe surface defects. Acquire two signals using a data acquisition card. Due to the relative distance between the two receiving coils, the phase difference between the direct wave, defect reflection wave, and end-face reflection wave of the two signals is different. Therefore, it is necessary to extract the defect echo and end-face echo portions for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. Find the corresponding phase value in the phase spectrum by subtracting the phase values ​​to obtain the phase difference between the two signals. When the two received signals are of the same sign, shift one signal by 180° - the phase difference to automatically calibrate the phase difference between the two signals to 180°. Finally, perform differential calculation on the two signals and display the final signal waveform, amplifying the differential signal and suppressing the common-mode signal. When the two received signals are of opposite signs, adjust the phase difference to 0°.

2. The method for detecting surface defects in ferromagnetic pipes as described in claim 1, characterized in that, The transmitting module includes an STM32 microcontroller, an isolation driver circuit, a full-bridge inverter circuit, and an impedance matching circuit. The STM32 microcontroller generates two complementary PWM waves with dead time as control signals. The dead time is set to 5% of the pulse signal period to prevent all four MOSFETs from conducting simultaneously. The isolation driver circuit controls the on / off state of the MOSFETs. The full-bridge inverter circuit, composed of the four MOSFETs, amplifies the excitation signal. When the input signals for MOSFETs Q1 and Q4 are high, Q1 and Q4 conduct. At the same time, when the other input signal is low, MOSFETs Q2 and Q3 are off, and the output voltage is +VCC. Conversely, the output voltage is -VCC. The full-bridge inverter circuit ultimately outputs an excitation signal with the same frequency and number of pulses as the control signal, and a peak-to-peak value of 2VCC. The impedance matching circuit further enhances the peak-to-peak value of the AC signal, increasing the power of the excitation signal.

3. The method for detecting surface defects in ferromagnetic pipes as described in claim 1, characterized in that, The receiving module includes an RLC series resonant circuit and a preamplifier circuit, a second-order filter amplifier circuit and a multi-stage amplifier circuit, and a data acquisition card. The RLC series resonant circuit and the preamplifier circuit filter out noise and spurious components at other frequencies and amplify the signal. The second-order filter amplifier circuit filters out high-frequency components outside the signal passband. The multi-stage amplifier circuit uses a combination of a fixed-gain second-stage amplifier circuit and a variable-gain third-stage amplifier circuit. The amplification factor is selected by a single-pole triple-throw switch K. The signal after the above filtering and amplification is acquired by the data acquisition card and phase calibration and differential calculation are performed by the host computer.

4. The method for detecting surface defects in ferromagnetic pipes as described in claim 1, characterized in that, In step 2, the coil width d affects the amplitude of the excited torsional guided wave. Within the range of 0 to λ, the guided wave amplitude first increases and then decreases as d increases. When the amplitude reaches its maximum, the coil width is selected. Select two coils with opposite winding directions and λ is the wavelength corresponding to the design frequency, and L is the distance between the center points of the two excitation coils.

5. The method for detecting surface defects in ferromagnetic pipes as described in claim 1, characterized in that, In step 3, the relative distance between the two receiving coils is y. The size and position of the permanent magnet magnetized along the length direction vary with the size and position of the coils. The permanent magnets are placed as evenly as possible on the coils, with a length equal to the width of the coil to ensure complete coverage. The relationship between the phase difference Δθ of the signals received by the two coils and y is: Due to the numerous combinations of winding directions and wiring methods at both ends of the two coils, the received two signals can have two possible outcomes: either the signals are of the same polarity or opposite polarity; λ is the wavelength corresponding to the design frequency. Since differential amplification can amplify differential signals and suppress common-mode signals, it is necessary to adjust the phase difference between the two acquired signals to meet the characteristics of differential signals, that is, the voltages are opposite at the same time. When the two signals are the same, their phase difference needs to be adjusted to 180°, and when the two signals are opposite, their phase difference needs to be adjusted to 0°.

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