A broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor

Abstract

This invention discloses a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor, which is composed of an array of permanent magnets and variable-pitch folding coils. By adjusting the single permanent magnets into a Hallbach array arrangement to enhance the magnetic flux density inside the metal material, and by designing variable-pitch folding coils, the spacing between adjacent coils is perfectly matched to the change in half-wavelength of the linear frequency modulated excitation signal, thereby compressing the ultrasonic surface wave pulse. The broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor can achieve broadband, high-amplitude, and narrow-pulse ultrasonic surface wave excitation, effectively broadening the frequency information of the ultrasonic surface wave excited by the sensor, increasing the amplitude of the ultrasonic surface wave signal, and compressing the pulse width, thus giving the sensor better detection and evaluation capabilities for metal material surfaces.

Description

Technical Field

[0001] The invention relates to a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor for the detection and evaluation of metallic material surfaces, belonging to the field of non-destructive testing and structural health monitoring. This sensor can excite broadband pulse-compressed broadband ultrasonic surface waves. Compared with conventional single-frequency ultrasonic surface wave electromagnetic acoustic sensors, its ultrasonic surface wave signal has advantages such as rich frequency components, high amplitude and narrow pulse width, which is more conducive to the detection and evaluation of metallic material surfaces. Background Technology

[0002] High-strength metallic materials are widely used in aerospace, marine, thermal power, and petrochemical industries. Due to their long-term operation in harsh environments, these materials are prone to surface cracks and defects, posing serious safety hazards to critical components. Therefore, the inspection and evaluation of metallic material surfaces is of great significance. When ultrasonic surface waves propagate through metallic materials, their energy is mainly concentrated within a depth of one wavelength below the material surface. This method offers advantages such as low attenuation, long propagation distance, and high sensitivity, and is commonly used for the inspection and evaluation of metallic material surfaces.

[0003] Sensors used to excite ultrasonic surface waves mainly include piezoelectric sensors and electromagnetic acoustic transducers (EMATs). Piezoelectric sensors transmit ultrasonic energy into metallic materials through a coupler; however, they are less suitable for materials with poor surface conditions or when non-contact detection is required. Electromagnetic acoustic transducers (EMATs), based on the principle of electromagnetic induction, can excite ultrasonic surface waves inside metallic materials without direct contact, offering advantages such as non-contact operation, no need for couplers, and high-temperature detection.

[0004] Currently, EMAT (Electrical Excitation Mechanism) for exciting ultrasonic surface waves mainly consists of a single permanent magnet and equally spaced loop coils. The frequency of the excited single-frequency narrowband ultrasonic surface wave signal is determined by the spacing of the loop coil wires. However, when using narrowband ultrasonic surface wave signals for detection and evaluation, the ultrasonic surface wave signal contains relatively little frequency information, only enabling detection and evaluation within the wavelength depth corresponding to that frequency in metallic materials. Compared to piezoelectric ultrasonic sensors, ultrasonic surface wave EMATs using a single permanent magnet have lower transduction efficiency. When used for metal surface detection, they suffer from low signal-to-noise ratio due to low surface wave signal amplitude and signal aliasing due to large wave packet width. In summary, existing ultrasonic surface wave EMATs suffer from problems such as limited frequency information, low amplitude, and wide pulse width in the excited ultrasonic surface wave signal. To improve the detection and evaluation capabilities of ultrasonic surface wave EMATs for metal surfaces, adjustments are made to the sensor's magnet, coils, and excitation signal to excite wideband, high-amplitude, and narrow-pulse ultrasonic surface waves.

[0005] This invention enhances the magnetic flux density within the tested metal material by replacing a single permanent magnet with a Hallbach array-type permanent magnet. It also adjusts the spacing between adjacent coils of a variable-pitch folding coil to perfectly match the wavelength variation of the linear frequency modulated excitation signal, thereby compressing the ultrasonic surface wave signal into a pulse. This structural configuration results in ultrasonic surface waves with wide bandwidth, high amplitude, and narrow pulse characteristics. Summary of the Invention

[0006] The purpose of this invention is to propose a method for exciting wide-bandwidth, high-amplitude, narrow-pulse ultrasonic surface wave (EMAT). Addressing the problems of limited frequency information, low amplitude, and large pulse width in conventional ultrasonic surface wave EMAT signals, this invention focuses on sensor structural design. By replacing the single permanent magnet with a Hallbach array type permanent magnet, the magnetic flux density inside the measured metal material is enhanced. Furthermore, the spacing between adjacent coils of the variable-pitch folding coil is adjusted to perfectly match the wavelength variation of the linear frequency modulated excitation signal, thereby compressing the ultrasonic surface wave signal into a pulse. This achieves wide-bandwidth, high-amplitude, narrow-pulse ultrasonic surface wave excitation.

[0007] To achieve the above objectives, the present invention adopts the following design scheme:

[0008] A broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor includes a Hallbach array type permanent magnet 1 and a variable-pitch folding coil 2; characterized in that: multiple permanent magnets are combined in a Hallbach array and placed on the variable-pitch folding coil.

[0009] The broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor is characterized in that: the Hallbach array magnet is composed of multiple permanent magnets with different magnetization directions, arranged in a certain order.

[0010] The aforementioned broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor is characterized in that: in the Hallbach array magnet, the two outermost permanent magnets are magnetized vertically upwards. The middle permanent magnet is magnetized vertically downwards, the permanent magnet to the left of the middle permanent magnet is magnetized horizontally to the right, and the permanent magnet to the right of the middle permanent magnet is magnetized horizontally to the left.

[0011] The broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor is characterized in that: the excitation signal passed through the variable-pitch folding coil is a linear frequency modulation signal, the frequency of the signal changes linearly after modulation, and the current direction of adjacent wires is opposite.

[0012] The broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor is characterized in that the wavelength of the linear frequency modulated signal passed through the variable-pitch folding coil changes linearly, and this trend is opposite to the frequency change trend of the linear frequency modulated excitation signal passed through the coil.

[0013] The broadband pulse compression ultrasonic surface wave electromagnetic acoustic sensor is characterized in that the spacing between adjacent conductors of the variable-pitch folding coil varies linearly, and the spacing between adjacent conductors of the variable-pitch folding coil is equal to half the half wavelength of the linear frequency modulation signal.

[0014] The present invention can achieve the following beneficial effects:

[0015] 1. The magnetic field lines of Hallbach-type array magnets are mainly distributed on the lower surface of the permanent magnet, and the magnetic field line density on the upper surface is greatly weakened. This unidirectionally enhances the magnetic field strength below the permanent magnet, thereby increasing the amplitude of the ultrasonic surface wave signal.

[0016] 2. The linear frequency modulated signal is fed into the variable-pitch folding coil. The change in the wire spacing of the variable-pitch folding coil is perfectly matched with the change in the half wavelength of the linear frequency modulated signal. The variable-pitch folding coil can perform matched filtering on the linear frequency modulated signal, so that the signals of different frequency components have peaks at a certain instant, thereby significantly compressing the pulse width of the ultrasonic surface wave signal, thus obtaining a high signal-to-noise ratio broadband pulse compressed ultrasonic surface wave signal. Attached Figure Description

[0017] Figure 1 A schematic diagram of a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor.

[0018] Figure 2 Schematic diagram of a two-dimensional Hallbach array-type permanent magnet structure;

[0019] Figure 3 Schematic diagram of linear frequency modulation excitation signal;

[0020] Figure 4 Schematic diagram of variable pitch folding coil;

[0021] Figure 5 Model of a conventional ultrasonic surface wave electromagnetic acoustic sensor;

[0022] Figure 6 Model of a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor;

[0023] Figure 7 A schematic diagram comparing the magnetic field strength of the two sensors.

[0024] Figure 8 A schematic diagram comparing the ultrasonic surface wave signals excited by the two sensors;

[0025] Figure 9 Time-frequency analysis diagram of broadband pulse compressed ultrasonic surface wave signal;

[0026] Figure 1 In the middle, 1. Hallbach array type permanent magnet, 2. Variable pitch folding coil;

[0027] Figure 5 In the middle, 3, air domain, 4, aluminum plate, 5, permanent magnet (two-dimensional), 6, 0.5MHz equidistant folding coil (two-dimensional), 7, aluminum plate mesh refinement area, 8, ultrasonic surface wave signal sampling point, 9, 1.5MHz equidistant folding coil (two-dimensional).

[0028] Figure 6 Among them, 10 is a Hallbach array permanent magnet (two-dimensional), and 11 is a variable-pitch folding coil (two-dimensional). Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments. The following embodiments are descriptive and not limiting, and should not be construed as limiting the scope of protection of the present invention.

[0030] A schematic diagram of a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor is shown below. Figure 1 As shown, it includes a Hallbach array type permanent magnet 1 and a variable-pitch folding coil 2. The two-dimensional structure of the Hallbach array type permanent magnet is as follows: Figure 2 As shown, the array-type permanent magnet consists of five permanent magnets. The magnetic field of the central permanent magnet is vertically downward, and its length D1 is 14mm. The magnetic fields of the permanent magnets located to the left and right of the central permanent magnet are horizontally to the right and horizontally to the left, respectively, and their lengths D2 and D2 are 7mm. The magnetic fields of the two outermost permanent magnets are vertically upward, and their lengths D3 and D3 are 7mm. The height H1 of all five permanent magnets is 20mm. A schematic diagram of the linear frequency modulation excitation signal is shown below. Figure 3 As shown, the starting frequency of the linear frequency modulated excitation signal is 0.5MHz, the ending frequency is 1.5MHz, and the pulse width of the excitation signal is 10μs. Figure 4 This is a schematic diagram of a variable-pitch foldback coil. The spacing between adjacent conductors of the variable-pitch foldback coil is equal to... Figure 3 The wavelength of the signal is half that of the coil, and the coil spacing is d1 = 2.73 mm, d2 = 2.36 mm, d3 = 2.11 mm, d4 = 1.93 mm, d5 = 1.79 mm, d6 = 1.67 mm, d7 = 1.57 mm, d8 = 1.49 mm, d9 = 1.42 mm, d 10 =1.36mm, d 11 =1.31mm, d 12 =1.26mm, d 13 =1.22mm, d 14 =1.18mm, d 15 =1.14mm, d 16 =1.11mm, d 17 =1.08mm, d 18 =1.06mm, d19 =1.03mm, d 20 =1.01mm.

[0031] A finite element simulation model was established to compare and analyze a conventional surface wave sensor and a broadband pulse-compressed ultrasonic surface wave sensor. The two-dimensional simulation model of the conventional ultrasonic surface wave sensor is shown below. Figure 5 As shown, the rectangular permanent magnet is 42mm long and 20mm high. Equally spaced folding coils are placed below the permanent magnet, with coil spacing of 3mm and 1mm, corresponding to excitation signals with frequencies of 0.5MHz and 1.5MHz, respectively. The sensor is placed on a steel plate 200mm long and 30mm high. Five-cycle sinusoidal signals modulated by a Hanning window with center frequencies of f1 = 0.5MHz and f2 = 1.5MHz and amplitude of 1A are passed through the coils, with the current directions on adjacent folding coils being opposite. The signal acquisition point is set 30mm to the right of the center of the upper surface of the steel plate to acquire the ultrasonic surface wave signal excited by the sensor.

[0032] Figure 6 The image shows a model of a broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor, featuring a Hallbach-type array of permanent magnets and... Figure 2 The configuration is the same as described above, with variable pitch foldback coils and Figure 4 The configuration is the same as described above. A variable-pitch foldback coil is energized with... Figure 3 The linear frequency modulated excitation signal in the signal. Through... Figure 7 A comparison diagram of the magnetic flux density beneath the permanent magnets of the two sensors shows that, compared to a traditional single permanent magnet, the Hallbach array permanent magnet exhibits a superior unilateral magnetic field enhancement effect, significantly increasing the magnetic flux density beneath the permanent magnet. A comparison diagram of the ultrasonic surface wave signals excited by the two sensors is shown below. Figure 8 As shown, by extracting the direct and end-face reflected signals of ultrasonic surface waves from the three models, it can be found that the surface wave signal excited by the conventional 1.5MHz sensor has a narrow pulse width and weak signal amplitude. The ultrasonic surface wave signal excited by the conventional 0.5MHz sensor has a higher amplitude, but a wider pulse width and weaker time-domain resolution. The broadband pulse compression of the ultrasonic surface wave signal from 0.5MHz to 1.5MHz results in a larger amplitude, while the pulse width is compressed into a narrower range, significantly enhancing the time-domain resolution. The time-frequency analysis diagram of the broadband pulse-compressed ultrasonic surface wave signal is shown below. Figure 9 As shown, the energy of the excited ultrasonic surface wave signal is basically distributed between 0.5MHz and 1.5MHz, and the excitation signal contains rich frequency information. The comparison demonstrates that, compared to conventional ultrasonic surface wave sensors, the broadband pulse-compressed ultrasonic surface wave sensor can excite wide-bandwidth, high-amplitude, narrow-pulse ultrasonic surface wave signals.

[0033] This invention presents a wideband pulse-compressed ultrasonic surface wave (SSW) electromagnetic acoustic sensor. This sensor combines a Hallbach array of permanent magnets with variable-pitch folding coils. While enhancing the internal magnetic field strength of the tested metallic material, it also perfectly matches the coil spacing to the wavelength variation of the linearly modulated excitation signal, thus compressing the ultrasonic surface wave signal into a pulse. Comparison with conventional single-frequency SSW electromagnetic acoustic sensors reveals that the wideband pulse-compressed sensor significantly increases the amplitude of the ultrasonic surface wave signal and compresses the pulse width. Furthermore, the signal contains rich frequency information, indicating that the wideband pulse-compressed sensor can excite wide-bandwidth, high-amplitude, narrow-pulse ultrasonic surface wave signals, which is more beneficial for the detection and evaluation of metallic material surfaces.

Claims

1. A broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor, comprising a Hallbach array permanent magnet and a variable-pitch folding coil; characterized in that: A Hallbach array-type permanent magnet is placed directly above the variable-pitch folding coil; The Hallbach array type permanent magnet is formed by arranging 5 permanent magnets together. The magnetization direction of the magnets from left to right is vertically upward, horizontally to the right, vertically downward, horizontally to the left, and vertically upward. The excitation signal passed through the variable-pitch folding coil is a linear frequency modulated signal. The frequency of the signal changes linearly after modulation, and the current direction of adjacent wires is opposite. By combining Hallbach array permanent magnets with the variable-pitch folding coil, the magnetic flux density inside the metal material being tested is enhanced, and the coil spacing is made to perfectly match the wavelength change of the linear frequency modulated excitation signal. The ultrasonic surface wave signal is pulse compressed, so that the sensor excites a wide-bandwidth, high-amplitude, narrow-pulse ultrasonic surface wave, thereby improving the detection capability of the metal material surface.

2. The broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor according to claim 1, characterized in that: The wavelength of the linear frequency modulated signal passed through the variable-pitch folding coil changes linearly, and the trend of this linear change is opposite to the trend of the frequency change of the linear frequency modulated excitation signal passed through the coil.

3. The broadband pulse-compressed ultrasonic surface wave electromagnetic acoustic sensor according to claim 1, characterized in that: The spacing between adjacent conductors of a variable-pitch foldback coil d It exhibits a linear variation, and the spacing between adjacent conductors is equal to half the wavelength of the linear frequency modulated signal.

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