Ferromagnetic pipeline surface defect detection device and detection method

Abstract

The invention discloses a ferromagnetic pipeline surface defect detection device and a detection method. The ferromagnetic pipeline surface defect detection device comprises a transmitting module, a transducer and a receiving module, an excitation signal generated by the transmitting module is introduced into an excitation coil of an excitation probe of the transducer, constructive interference occurs to torsional guided waves excited by the two excitation coils in a pipeline under the action of mutually orthogonal dynamic and static magnetic fields, wave crest-wave crest superposition is realized, the guided waves are axially transmitted to a receiving probe of the transducer along the pipeline, and the ultrasonic wave is transmitted to the receiving probe of the transducer. Two paths of voltage signals are induced in a receiving coil of a receiving probe, the signals are filtered and amplified through a receiving module, and then the signals are collected into an upper computer for phase calibration and differential operation. According to the invention, the amplitude of the excited torsional guided wave can be effectively increased, the detection distance is increased, the amplitude of the received signal is amplified, the noise is suppressed, and the defect echo signal is distinguished more clearly.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline nondestructive testing, and in particular to a ferromagnetic pipeline surface defect detection device and a detection method. Background Art

[0002] At present, most inspection agencies use piezoelectric ultrasonic methods, using the WAVEMAKER equipment of the British Guided Wave Company. Piezoelectric ultrasonic transducers have the advantages of small size, firmness, various sizes, shapes and forms, zero maintenance, long service life and easy control. However, there are the following disadvantages: piezoelectric ultrasound uses a fixed inner diameter clamping ring transducer, one transducer can only detect one oil and gas pipeline of the corresponding diameter, the inner diameter of the transducer cannot be adjusted, and the flexibility is poor.

[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 ultrasound, electromagnetic ultrasound has the following advantages: (1) No coupling agent is required: it avoids the limitation of contacting the surface of the object to be tested in traditional methods and is suitable for complex environments and high temperature testing; (2) Wide detection range: it is suitable for 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 the need to polish the surface of the test piece, and when measuring thickness, it is not necessary to consider the influence of coating thickness on the measurement results; (4) High accuracy: electromagnetic ultrasonic technology can excite various ultrasonic waveforms, which improves the accuracy and flexibility of detection; (5) Electromagnetic ultrasonic technology is easy to operate, small in size, light in weight, and easy to carry. However, the existing electromagnetic ultrasonic ferromagnetic pipeline surface defect detection method has the disadvantages of low transducer transducer efficiency, short propagation distance, large received signal noise, and defect echo signals are easily submerged in noise and difficult to observe. Summary of the invention

[0004] Purpose of the invention: The present invention provides a ferromagnetic pipeline surface defect detection device and detection method, which can effectively increase the amplitude of the excited 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 ferromagnetic pipeline surface defect detection device described in the present invention includes: a transmitting module, a transducer and a receiving module; the transmitting module generates an excitation signal and passes it 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 pipeline undergo constructive interference, achieving peak-to-peak superposition, and the guided waves are transmitted to the receiving probe of the transducer along the axial direction of the pipeline, and two voltage signals are induced in the receiving coil of the receiving probe. The signal is filtered and amplified by the receiving module, and then the signal is collected into the host computer for phase calibration and differential operation.

[0006] Furthermore, the transmitting module includes an STM32 single-chip microcomputer, an isolation drive circuit, a full-bridge inverter circuit and an impedance matching circuit; the STM32 single-chip microcomputer generates two complementary PWM waves with dead zones as control signals, the dead zone time is set to 5% of the pulse signal period to prevent four MOS tubes from being turned on at the same time, the on and off of the MOS tubes are controlled by the isolation drive circuit, and the full-bridge inverter circuit composed of four MOS tubes realizes power amplification of the excitation signal. When the input signal controlling Q1 and Q4 is at a high level, Q1 and Q4 are turned on. At this time, the other input signal is at a low level, Q2 and Q3 are turned off, and the output voltage is +VCC. Otherwise, the output voltage is -VCC. The full-bridge inverter circuit can finally output an excitation signal with a frequency and a number of pulses that are the same as the control signal and a peak-to-peak value of 2VCC. The voltage peak-to-peak value of the AC signal is further increased by the impedance matching circuit to increase the power of the excitation signal.

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

[0008] Accordingly, a method for detecting surface defects of a ferromagnetic pipeline comprises the following steps:

[0009] Step 1: The transmitting module generates an excitation signal which is passed into the excitation coil of the excitation probe of the transducer;

[0010] Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipeline. According to the principle of wave interference, by controlling the spacing between the two coils, the torsional guided waves excited in the two coils have a fixed path difference, so that constructive interference can occur, peak-to-peak superposition can be achieved, and the amplitude of the torsional guided waves can be increased;

[0011] Step 3: The guided wave is transmitted along the axial direction of the pipeline 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 operation and display the final echo signal to facilitate further analysis of pipeline surface defects.

[0013] Furthermore, in step 2, the coil width d has an effect on the amplitude of the excited torsional waveguide. In the range of 0 to λ, as d increases, the amplitude of the waveguide increases first and then decreases. When the amplitude reaches the 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 in the length direction vary with the size and position of the coil, and as many as possible are evenly placed on the coil, and the length is the coil width to ensure that the coil can be covered. 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 winding directions of the two coils and wiring methods at both ends of the coils, the two received signals may have the same positive and negative and opposite positive and negative.

[0017] Since the differential amplification method can amplify the differential signal and suppress the common signal, the phase difference of the two collected signals needs to be adjusted to meet the characteristics of the differential signal, that is, the voltage is opposite at the same time; when the two signals are the same, the phase difference needs to be adjusted to 180°, and when the two signals are opposite, the phase difference needs to be adjusted to 0°.

[0018] Furthermore, in step 4, two signals are collected respectively through a data acquisition card. Due to the relative distance between the two receiving coils, the phase differences between the direct waves and the defect reflection waves and the end face reflection waves of the two signals are different. Therefore, it is necessary to intercept the defect echo and the end face echo parts for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals respectively. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. The phase difference between the two signals can be obtained by subtracting the phase value in the corresponding phase spectrum found by frequency. When the positive and negative of the two received signals are the same, one of the signals is phase-shifted (180°-phase difference) to automatically calibrate the phase difference of the two signals to 180°. Finally, the two signals are differentially calculated and the final signal waveform is displayed. The differential mode signal is amplified and the common mode signal is suppressed. When the positive and negative of the two received signals are opposite, 180° is changed to 0°.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: First, through the interference principle of ultrasonic waves, two excitation coils are used to simultaneously excite the torsional waveguide, and the two waveguides are constructively interfered by reasonably designing the relative distance between the two coils, which can effectively increase the amplitude of the excited waveguide and thus increase the detectable distance; secondly, two receiving coils are designed to simultaneously receive two echo signals, and the phase difference of the two signals is adjusted by a software method, so that the phase difference of the positive and negative signals is 180°, and the phase difference of the positive and negative opposite signals is 0°, so that they become differential signals and the useful signals are amplified and the noise is suppressed by differential operations, which can effectively extract tiny defect echo signals and further identify tiny defects; finally, by adding a part for calibrating the phase using a software method, the distance y between the receiving coils does not need to be fixed, and the position is adjusted according to the detection environment. The phase is calibrated by a later software method so that the phase difference of the processed signal is not affected by the distance y. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0022] Figure 3 Schematic diagram of the automatic phase calibration method of the present invention.

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

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

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

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

[0027] Figure 8 It is a schematic diagram of the structure of the series resonant circuit of the present invention. DETAILED DESCRIPTION

[0028] like Figure 1 As shown, a ferromagnetic pipeline surface defect detection device includes: a transmitting module, a transducer and a receiving module; the transmitting module generates an excitation signal and passes it 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 pipeline generates a peak-to-peak superposition, and the guided wave is transmitted to the receiving probe of the transducer along the axial direction of the pipeline, and two voltage signals are induced in the receiving coil of the receiving probe. The signal is filtered and amplified by the receiving module, and then the signal is collected into the host computer for phase calibration and differential operation.

[0029] like Figure 5 and Figure 6 As shown, the transmitting module includes an STM32 single-chip microcomputer, an isolation drive circuit, a full-bridge inverter circuit and an impedance matching circuit; the STM32 single-chip microcomputer generates two complementary PWM waves with dead zones as control signals, and the dead zone time is set to 5% of the pulse signal period to prevent four MOS tubes from being turned on at the same time, and the on and off of the MOS tubes are controlled by the isolation drive circuit. The full-bridge inverter circuit composed of four MOS tubes realizes the power amplification of the excitation signal. When the input signal of controlling Q1 and Q4 is high level, Q1 and Q4 are turned on, and the other signal input is low level, Q2 and Q3 are cut off, and the output voltage is +VCC, otherwise, the output voltage is -VCC. Therefore, the full-bridge inverter circuit can finally output an excitation signal with the same frequency and pulse number as the control signal and a peak-to-peak value of 2VCC, and the power of the excitation signal can be increased by further increasing the peak-to-peak value of the voltage of the AC signal.

[0030] like Figure 7 and Figure 8 As shown, the receiving module includes an RLC series resonant circuit and 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 is used to filter out clutter and noise components of other frequencies and achieve a certain amplification effect. According to the series resonance theory, adding capacitor C and inductor L to the circuit to form a series resonance can further increase the output signal amplitude.

[0031] The preamplifier stage uses an op amp to form a common-phase amplifier circuit, which first amplifies the signal 1000 times. Next, a second-order low-pass filter circuit is used to filter out high-frequency components outside the signal passband. Since the signal after preamplification still contains high-frequency noise, it can be weakened by a second-order low-pass filter circuit. The capacitor connects the positive input and output of the op amp to introduce positive feedback. By setting a reasonable cutoff frequency, the high-frequency band of the amplitude-frequency characteristic of the second-order low-pass filter circuit is rapidly attenuated, and only the low-frequency band is allowed to pass. After the EMAT receiving coil detection signal passes through the RLC series resonant frequency selection circuit, the preamplifier circuit and the filter circuit, its amplitude still cannot meet the subsequent data processing requirements, so the signal needs to be amplified again. However, considering the different specifications of the objects under test, there is a certain gap in the strength of the received signal. And for defect detection of different sizes, the amplitude of the echo signal is also different. In order to avoid signal distortion caused by excessive amplification or invalid amplification caused by too small amplification, the multi-stage amplifier circuit adopts a combination circuit of a fixed-gain second-stage amplifier circuit and a variable-gain third-stage amplifier circuit. The amplification factor is selected by controlling the single-pole three-throw switch K, which not only ensures the basic amplification factor of the signal, but also allows flexible amplification processing for received signals of different amplitudes. The signal after the above filtering and amplification can be collected by the data acquisition card and phase-calibrated and differentially calculated by the host computer.

[0032] Accordingly, a method for detecting surface defects of a ferromagnetic pipeline comprises the following steps:

[0033] Step 1: The transmitting module generates an excitation signal which is passed into the excitation coil of the excitation probe of the transducer;

[0034] Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipeline. According to the principle of wave interference, by controlling the spacing between the two coils, the torsional guided waves excited in the two coils have a fixed path difference, so that constructive interference can occur, peak-to-peak superposition can be achieved, and the amplitude of the torsional guided waves can be increased;

[0035] Step 3: The guided wave is transmitted along the axial direction of the pipeline 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 operation and display the final echo signal to facilitate further analysis of pipeline surface defects.

[0037] like Figure 2 The figure shows the schematic diagram of the transducer excitation probe structure of the present invention. The relationship between the torsional waveguide wavelength and the wave velocity is:

[0038]

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

[0040] Two transmitting coils, magnetized strips and permanent magnets are used to form the excitation probe of the transducer. Based on the reversed Widman effect, the permanent magnet 2 is magnetized in the length direction and placed along the circumference of the pipe 3. The cross-sectional size of the permanent magnet is 10x10mm. 2 , the length covers the entire transducer excitation probe, and is placed as evenly as possible in the axial direction of the pipeline. The enameled wire 1 with a diameter of D is wound axially on two pure nickel magnetostrictive belts 4, with the number of turns n, and the current I passed through the coil. The length of the nickel belt is equal to the outer circumference of the pipeline to ensure that it can cover the outer surface of the pipeline. The wound nickel belt 4 and coil 1 are pasted on the circumferential surface of the pipeline 3 through epoxy resin, and the center points of the coils on the two nickel belts 4 are L apart. The excitation signal induces a circumferential dynamic magnetic field in the magnetostrictive belt 4 through the two transmitting coils 1, and then excites two torsional guided waves in the pipeline 3. The relationship between the amplitude of the torsional guided wave and the above parameters is:

[0041]

[0042] That is, the more turns the coil has, the greater the current passed through, the greater the torsional waveguide amplitude; the larger the coil diameter, the lower the ultrasonic transmittance, the smaller the torsional waveguide amplitude; the higher the excitation signal frequency, the torsional waveguide amplitude decreases exponentially.

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

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

[0045] P=A 2 ·f·ρ

[0046] Among them, f is the waveguide frequency and ρ is the medium density, both of which are constant values ​​in this 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 when Among them, 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 when Among them, n = 0, 1, 2..., that is, the coil spacing L should be selected as At this time, the torsional guided wave constructively interferes, resulting in a positive superposition of the peaks and peaks, thereby increasing the amplitude of the torsional guided wave.

[0050] Since the coil width d has an influence on the amplitude of the excited torsional waveguide, in the range of 0 to λ, as d increases, the amplitude of the waveguide increases first and then decreases. The amplitude reaches the maximum when For the best.

[0051] In the actual system detection process, the lifting distance of the magnetostrictive belt relative to the pipeline surface affects the amplitude of the torsional waveguide, and there is an ultrasonic transmittance problem when the two coils overlap, so the spacing L should not be 0. At the same time, since the waveguide will attenuate when propagating in the medium, when the two coils are too far apart, the amplitude difference is too large and the superposition effect is not good. Therefore, the two coils are wound in opposite directions and Best time.

[0052] like Figure 4 As shown in the figure, the specific structure of the transducer receiving probe used in the detection device of the present invention is shown. The receiving coil 1 is wound around two magnetized strips 4, and as many permanent magnets 2 magnetized in the length direction are evenly placed thereon as possible. The relative distance between the magnetized strips of the two winding coils is y. The relationship between the phase difference of the signals received by the two coils and y is:

[0053]

[0054] Since there are many combinations of winding directions of the two coils and wiring methods at both ends of the coils (signal end and reference voltage end), the two received signals may have the same positive and negative and opposite positive and negative.

[0055] Since the differential amplification method can amplify the differential signal and suppress the common signal, it is necessary to adjust the phase difference of the two signals collected to meet the characteristics of the differential signal, that is, the voltage is opposite at the same time. When the two signals are the same, the phase difference needs to be adjusted to 180°, and when the two signals are opposite, the phase difference needs to be adjusted to 0°.

[0056] The present invention adopts LabVIEW to design the host computer program to perform data acquisition, automatic phase calibration and differential amplification of two-way 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 describes the design method of the host computer program by taking the automatic calibration of the phase difference to 180° as an example.

[0058] First, two signals are collected separately through the 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 intercept the defect echo and the end face echo for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals respectively. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. The phase difference of the two signals can be obtained by subtracting the phase value in the corresponding phase spectrum through the frequency. When the positive and negative of the two received signals are the same, one of the signals is phase-shifted (180°-phase difference) to automatically calibrate the phase difference of the two signals to 180°. Finally, the two signals are differentially operated and the final signal waveform is displayed to amplify the differential signal and suppress the common mode signal. When the positive and negative of the two received signals are opposite, 180° can be changed to 0°.

Claims

1. A ferromagnetic pipeline surface defect detection device, characterized in that: include: Transmitter module, transducer and receiver module; The transmitting module generates an excitation signal which is passed 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 pipeline undergo constructive interference, achieving peak-to-peak superposition. The guided waves are transmitted along the axial direction of the pipeline to the receiving probe of the transducer, and two voltage signals are induced in the receiving coil of the receiving probe. The signal is filtered and amplified by the receiving module, and then the signal is collected into the host computer for phase calibration and differential operation.

2. The ferromagnetic pipeline surface defect detection device according to claim 1, characterized in that: The transmitting module comprises an STM32 single-chip microcomputer, an isolation driving circuit, a full-bridge inverter circuit and an impedance matching circuit; the STM32 single-chip microcomputer generates two complementary PWM waves with dead zones as control signals, the dead zone time is set to 5% of the pulse signal cycle to prevent four MOS tubes from being turned on at the same time, the on and off of the MOS tubes are controlled by the isolation driving circuit, and the full-bridge inverter circuit composed of four MOS tubes realizes power amplification of the excitation signal; when the input signal controlling Q1 and Q4 is at a high level, Q1 and Q4 are turned on, at this time, the other input signal is at a low level, Q2 and Q3 are turned off, and the output voltage is +VCC, otherwise, the output voltage is -VCC, and the full-bridge inverter circuit can finally output an excitation signal with a frequency and a pulse number that are the same as the control signal and a peak-to-peak value of 2VCC, and the voltage peak-to-peak value of the AC signal is further increased by the impedance matching circuit to improve the power of the excitation signal.

3. The ferromagnetic pipeline surface defect detection device according to claim 1, characterized in that: The receiving module includes an RLC series resonant circuit and a preamplifier circuit, a second-order filtering amplifier circuit, a multi-stage amplifier circuit and a data acquisition card; the RLC series resonant circuit and the preamplifier circuit filter out clutter and noise components of other frequencies and amplify the signal, the second-order low-pass filter circuit filters out high-frequency components outside the signal passband, and the multi-stage amplifier circuit uses a combination circuit of a second-stage amplifier circuit with a fixed gain and a third-stage amplifier circuit with a variable gain. The amplification factor is selected by a single-pole three-throw switch K control. The signal after the above filtering and amplification is collected by the data acquisition card and phase calibration and differential operation are performed by the host computer.

4. A detection method based on the ferromagnetic pipeline surface defect detection device according to claim 1, characterized in that: The steps include: Step 1: The transmitting module generates an excitation signal which is passed into the excitation coil of the excitation probe of the transducer; Step 2: Under the action of mutually orthogonal dynamic and static magnetic fields, both excitation coils can excite torsional guided waves in the pipeline. According to the principle of wave interference, by controlling the spacing between the two coils, the torsional guided waves excited in the two coils have a fixed path difference, so that constructive interference can occur, peak-to-peak superposition can be achieved, and the amplitude of the torsional guided waves can be increased; Step 3: The guided wave is transmitted along the axial direction of the pipeline 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 through the receiving module, then collect the signal into the host computer for phase calibration and differential operation and display the final echo signal to facilitate further analysis of pipeline surface defects.

5. The method for detecting ferromagnetic pipeline surface defects according to claim 4, characterized in that: In step 2, the coil width d has an effect on the amplitude of the excited torsional waveguide. In the range of 0 to λ, as d increases, the amplitude of the waveguide increases first and then decreases. When the amplitude reaches the maximum, the coil width is selected Select two coils with opposite winding directions and λ is the wavelength corresponding to the design frequency.

6. The method for detecting ferromagnetic pipeline surface defects according to claim 4, 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 in the length direction vary with the size and position of the coil. As many as possible are evenly placed on the coil. The length is the coil width to ensure that the coil can be covered. The relationship between the phase difference of the signals received by the two coils and y is: Due to the numerous combinations of winding directions of the two coils and wiring methods at both ends of the coils, the two received signals may have the same positive and negative and opposite positive and negative. Since the differential amplification method can amplify the differential signal and suppress the common signal, the phase difference of the two collected signals needs to be adjusted to meet the characteristics of the differential signal, that is, the voltage is opposite at the same time; when the two signals are the same, the phase difference needs to be adjusted to 180°, and when the two signals are opposite, the phase difference needs to be adjusted to 0°.

7. The method for detecting ferromagnetic pipeline surface defects according to claim 4, characterized in that: In step 4, two signals are collected respectively through the data acquisition card. Due to the relative distance between the two receiving coils, the phase differences between the direct waves and the defect reflection waves and the end face reflection waves of the two signals are different. Therefore, it is necessary to intercept the defect echo and the end face echo parts for FFT spectrum analysis to obtain the amplitude spectrum and phase spectrum of the two signals respectively. The frequency corresponding to the point with the largest amplitude in the amplitude spectrum is the signal frequency to be processed. The phase difference between the two signals can be obtained by subtracting the phase value in the corresponding phase spectrum found by frequency. When the positive and negative of the two received signals are the same, one of the signals is phase-shifted (180°-phase difference) to automatically calibrate the phase difference of the two signals to 180°. Finally, the two signals are differentially operated and the final signal waveform is displayed. The differential mode signal is amplified and the common mode signal is suppressed. When the positive and negative of the two received signals are opposite, 180° is changed to 0°.

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