An omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on Lorentz force

Abstract

The present invention discloses an omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on the Lorentz force, comprising a permanent magnet array consisting of a circular coil arranged in a certain pattern and a group of sector magnets with alternating polarization along the thickness direction. This structural form can excite 360° SH waves in all directions, and the uniquely distributed circular coils can simultaneously generate multiple SH wave sound sources. The coil distribution patterns of the excitation and receiving transducers are opposite, and the receiving transducer can achieve the pulse compression effect of the excitation sound wave. The pulse compression technology can improve the transduction efficiency and have a higher signal-to-noise ratio. Therefore, it is suitable for defect detection and health monitoring of various uniform, conductive, isotropic elastic plate structures, and greatly improves the monitoring accuracy and efficiency.

Description

Technical Field

[0001] The invention belongs to the field of non-destructive testing and relates to an omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on Lorentz force. Background Art

[0002] Plates are widely used in aerospace, automotive, and marine applications. As they age, their performance gradually declines, creating an urgent need for online monitoring methods to guide operational maintenance. Ultrasonic guided wave detection methods are attracting attention for their speed and efficiency.

[0003] The electromagnetic ultrasonic guided wave transducer is a commonly used device for online monitoring and non-destructive testing, generally composed of a permanent magnet and a coil. The electromagnetic-to-force conversion mechanism usually includes the Lorentz force mechanism and the magnetostrictive effect mechanism. In ferromagnetic materials, both effects usually exist, and the magnetostrictive effect is dominant. In conductive non-ferromagnetic materials, there is only the Lorentz force. The electromagnetic ultrasonic guided wave transducer relies on the electromagnetic effect to directly excite ultrasonic waves on the test piece without applying a coupling agent, and can be used for the detection of structural parts with coating materials on the surface. The multiple coils of the traditional electromagnetic ultrasonic transducer simultaneously excite the same stress wave, and the multiple stress waves are superimposed and propagated outward, and then sensed by the receiving transducer. The sensed signal is the delayed superposition of multiple sound sources, and its time domain length is longer than the excitation signal, which is not conducive to signal analysis. This patent designs the excitation and receiving transducers based on the pulse compression principle, which can achieve energy concentration of the received time domain signal. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to produce an omnidirectional electromagnetic guided wave transducer based on Lorentz force that can be applied to the detection of non-ferromagnetic flat panels and can produce a compressed pulse effect.

[0005] The present invention adopts the following technical solutions:

[0006] An electromagnetic guided wave transducer comprises a circumferentially periodic permanent magnet array and a PCB coil. The permanent magnet array consists of N NdFeB sector magnets arranged in a circular ring. Each magnet is polarized along its thickness, with adjacent magnets having opposite poles. The improvement lies in the combination of PCB coils of varying diameters, enabling the omnidirectional electromagnetic ultrasonic guided wave transducer based on the Lorentz force to produce a compressed pulse effect.

[0007] Furthermore, a toroidal coil wound with copper foil generates a Lorentz force under the action of a vertical magnetic field, thereby generating an omnidirectional SH guided wave to detect defects at various positions within the flat material.

[0008] Furthermore, when working, a half-cycle sinusoidal signal s(t) is fed into the equally spaced spiral coils. If the half wavelength of the input signal is equal to the spacing between the spiral coils, the mechanism is as follows: Figure 2 As shown. Figure 2 In the coil structure shown, the final received signal is as follows Figure 3 shown.

[0009] Furthermore, consider using a variable period coil to replace the equidistant spiral coil. The coil structure is as follows: Figure 4 As shown in the figure, the coil spacing is 1, 2, and 3 times the half wavelength of the excitation signal respectively; the spacing between the excitation coil and the receiving coil is arranged in opposite ways. From the representation of the receiving coil in the figure, it can be seen that the received signal presents a pulse compression effect, that is, the energy will be concentrated at a certain point in time, such as Figure 5 shown.

[0010] Furthermore, consider the case where the coil spacing is l x +l y =l z In the case of pulse compression, a small peak of energy will appear outside the location where the pulse compression energy is concentrated. To avoid this situation, the coil spacing is set to l1, l2, l3...l n , where l2>l1, l3>l1+l2, l4>l2+l3, and so on, can avoid the above situation. For example, the coil spacing can be set to a, 2a, 4a, 7a, 12a, ... respectively.

[0011] Furthermore, the coil adopts flexible printed circuit board technology, so that it can better fit the tested object and reduce unnecessary noise during detection.

[0012] The transducer described in this invention offers the advantages of omnidirectional ultrasonic excitation and reception, and achieves energy concentration through the pulse compression effect, thereby improving transduction efficiency. Most importantly, the coil structure offers a significant advantage over existing designs in achieving a higher signal-to-noise ratio when exposed to random ambient noise. This makes it suitable for defect detection and health monitoring of various uniform, isotropic elastic plate structures, including those made of conductive and non-conductive materials, significantly improving monitoring accuracy and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the omnidirectional excitation principle based on Lorentz force;

[0014] Figure 2 This is a schematic diagram of the principle of the transducer under the structure of equidistant spiral coils;

[0015] Figure 3Schematic diagram of the signal collected under the transducer's equally spaced spiral coil structure;

[0016] Figure 4 It is the principle diagram of the variable spacing coil structure of the transducer;

[0017] Figure 5 Schematic diagram of the signals collected under the transducer's variable spacing coil structure. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] like Figure 1 As shown in the figure, a circumferentially periodic permanent magnet array consisting of 12 sector magnets with alternating polarization along the thickness generates a vertical magnetic field perpendicular to the plate surface. Simultaneously, a coil carrying an alternating current generates eddy currents within the plate's skin depth. Based on the principle of Lorentz force generation, the induced eddy currents and the static magnetic field act on the aluminum plate, generating a Lorentz force in the radial direction of the transducer, which in turn produces omnidirectional shear deformation, thereby exciting omnidirectional SH guided waves.

[0020] like Figure 2 As shown in the figure, an equidistant spiral coil structure is used, one coil is used for excitation and the other coil is used for reception. The spacing of the spiral coils is equal to half the wavelength of the SH waveguide of the excitation signal (half-cycle sine wave signal). Among them, the wavelength λ is equal to the wave velocity C divided by the frequency f, that is:

[0021]

[0022] exist Figure 2 The three coils shown in the figure excite a half-cycle sine wave almost simultaneously and propagate outward. As time goes by, the excitation signal is received in sequence starting from the outside of the receiving coil. The final received signal is as follows: Figure 3 As shown, the signal energy is relatively dispersed.

[0023] The coil structure of the proposed omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on Lorentz force is as follows: Figure 4 As shown in the figure, a variable pitch coil structure is used, and the spacing arrangement of the excitation coil and the receiving coil is opposite. Figure 5 It can be seen in At time n, each coil receives a corresponding signal. That is, the received signal at that time is n*s(t), where n is the number of coil turns. The received signal at all other times is l*s(t). As we can see, as n increases, the pulse compression effect of this structure becomes more pronounced.

[0024] At the same time, in order to make the received signal at other times l*s(t), consider setting the coil spacing to l1, l2, l3...l n , where l2>l1, l3>l1+l2, l4>l2+l3, and so on. For example, the coil spacing is set to a, 2a, 4a, 7a, 12a, … respectively.

[0025] In the above, l is the distance between the two transducers, and C represents the velocity of the horizontal shear wave generated by the transducer. The velocity of the horizontal shear wave is related to the plate density, Young's modulus, Poisson's ratio, and plate thickness. When using SH0 modal guided wave for detection, C is a constant.

Claims

1. An omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on Lorentz force, characterized in that: The device comprises a receiving transducer and an excitation transducer. The excitation transducer comprises a circumferentially periodically arranged permanent magnet array and a PCB coil composed of multiple coils of different diameters arranged in a certain pattern. The permanent magnet array in the excitation transducer is composed of N sector-shaped NdFeB magnets forming a ring. The sector-shaped NdFeB magnets are all polarized along the thickness direction, and the polarities of adjacent sector-shaped NdFeB magnets are opposite. The PCB coil generates a Lorentz force under the action of the perpendicular magnetic field generated by the permanent magnet array, thereby generating omnidirectional SH guided waves. The receiving transducer can produce a compressed pulse effect to detect defects at various locations within the flat material. The PCB coil is a ring coil with unequal spacing. The PCB coil in the excitation transducer serves as the excitation coil. The receiving transducer uses one of the PCB coils as the receiving coil. The excitation coil and the receiving coil are arranged in opposite directions, and the spacing between the excitation coils is an integer multiple of half the wavelength of the omnidirectional SH waveguide. The spacings of the excitation coils are l1, l2, l3...l n , where l2>l1, l3>l1+l2, l4>l2+l3, and its expression is: Where a is the half wavelength of the omnidirectional SH waveguide; f n is the pitch factor of the nth coil.

2. The omnidirectional pulse compression electromagnetic ultrasonic guided wave transducer based on Lorentz force according to claim 1, characterized in that: The PCB coil adopts flexible printed circuit board technology, which enables it to better fit the tested object and reduce noise during detection.

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