A transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer

Abstract

The application provides a transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer, wherein a low-frequency piezoelectric vibrator is arranged in front, and a high-frequency piezoelectric vibrator is arranged in back, and the ultrasonic transducer specifically comprises a high-frequency matching layer, a low-frequency flexible circuit layer, an insulating layer and a high-frequency piezoelectric vibrator arranged in sequence, the low-frequency piezoelectric vibrator is of a multi-element structure, a cavity is etched on the lower surface of the high-frequency matching layer, the low-frequency piezoelectric vibrator is embedded in the cavity on the lower surface of the high-frequency matching layer and is attached to the low-frequency flexible circuit layer, the insulating layer is used for isolating the electrical signals between the high-frequency piezoelectric vibrator and the low-frequency piezoelectric vibrator, and the low-frequency flexible circuit layer is used for exciting the low-frequency piezoelectric vibrator. The application solves the problem that the longitudinal resolution and the sound energy transmission efficiency cannot be considered simultaneously in the field of air-coupled ultrasonic technology. The application belongs to the field of ultrasonic nondestructive testing technology.

Description

Technical Field

[0001] This invention relates to a transmitting, dual-frequency driven air-coupled piezoelectric ultrasonic transducer, belonging to the field of ultrasonic nondestructive testing technology. Background Technology

[0002] Air-coupled ultrasonic nondestructive testing technology is characterized by being completely non-contact and non-invasive, overcoming the limitation of traditional ultrasonic testing techniques that require the use of coupling agents (usually water or silicone oil). It shows great promise for applications in areas such as defect detection in precision optical glass in lithography machines, diagnosis of malignant tissue and skin burns, food and drug quality and safety testing, and defect detection in aerospace composite materials.

[0003] However, a severe acoustic impedance mismatch exists between the piezoelectric element and air, resulting in low sensitivity and narrow bandwidth of air-coupled piezoelectric transducers. Currently, improvements in the performance of air-coupled piezoelectric transducers are limited by the following issues: 1) Under good acoustic matching conditions, high sensitivity and large bandwidth of the transducer cannot be achieved simultaneously; 2) At a fixed center frequency, the transducer has a large pulse width and low longitudinal resolution.

[0004] Generally, the main methods to improve the longitudinal resolution of a transducer are: 1) increasing the center frequency of the transducer; and 2) reducing the pulse width of the ultrasonic transducer. High-frequency ultrasound sacrifices the penetration depth of the ultrasound waves, and the higher the frequency (f) of the ultrasound waves, the more severe the attenuation (α) in air (α∝f). 2 Therefore, it is more ideal to improve the longitudinal resolution of ultrasonic transducers by reducing their pulse width.

[0005] Studies have shown that dual-frequency ultrasound technology can not only expand the bandwidth of ultrasonic transducers but also improve their longitudinal resolution. Existing patents 1 (patent number: CN114190976A) and 2 (patent number: CN107843653A) both design contact-type dual-frequency ultrasonic transducer structures, with a high-frequency piezoelectric vibrator placed in front and a low-frequency piezoelectric vibrator placed behind. In patent 1, the high-frequency piezoelectric vibrator lacks good matching conditions, leading to low transmission efficiency when applied to an air-coupled ultrasonic transducer. In patent 2, the low-frequency piezoelectric vibrator has a three-layer matching structure, which sacrifices transducer sensitivity when applied to an air-coupled ultrasonic transducer. Furthermore, the outermost matching layer of this structure cannot be made of a material that meets the acoustic impedance requirement (<0.01 MRayl). To apply dual-frequency ultrasound technology in the field of air-coupled ultrasonic testing, this patent proposes a reasonable structural design for an air-coupled piezoelectric dual-frequency ultrasonic transducer. Summary of the Invention

[0006] The purpose of this invention is to provide a transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer to solve the problem of the inability to simultaneously achieve longitudinal resolution and acoustic energy transmission efficiency in the field of air-coupled ultrasonic technology.

[0007] To address the aforementioned problems, this invention provides a transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer. In this transducer, a low-frequency piezoelectric vibrator is positioned at the front, and a high-frequency piezoelectric vibrator is positioned at the rear. Specifically, it includes a high-frequency matching layer, a low-frequency flexible circuit layer, an insulating layer, and a high-frequency piezoelectric vibrator arranged sequentially. The low-frequency piezoelectric vibrator has a multi-element structure. Cavities are etched on the lower surface of the high-frequency matching layer. The low-frequency piezoelectric vibrator is embedded within the cavities on the lower surface of the high-frequency matching layer and is attached to the low-frequency flexible circuit layer. The insulating layer isolates the electrical signals between the high-frequency and low-frequency piezoelectric vibrators. The low-frequency flexible circuit layer excites the low-frequency piezoelectric vibrator.

[0008] In the aforementioned ultrasonic transducer, the high-frequency piezoelectric vibrator uses PZT series piezoelectric ceramic material, and the low-frequency piezoelectric vibrator uses PVDF piezoelectric material. The PZT piezoelectric ceramic has the advantage of a high piezoelectric coefficient (~300 pC / N), but a high acoustic impedance (~30 MRayl); the PVDF has the advantage of low acoustic impedance (3.9 MRayl), but a small piezoelectric coefficient (~25 pC / N). Under the same excitation signal, the amplitudes of the acoustic pressure signals excited by the PZT and PVDF differ by two orders of magnitude. To ensure effective superposition of the acoustic pressure signals from the high-frequency and low-frequency piezoelectric vibrators, the low-frequency piezoelectric vibrator is designed as a multi-element structure.

[0009] In the aforementioned ultrasonic transducer, the high-frequency piezoelectric vibrator has a single-matching layer structure and good matching conditions to ensure that the ultrasonic transducer has high sensitivity.

[0010] In the aforementioned ultrasonic transducer, the insulating layer is made of epoxy resin. In order not to affect the propagation of high-frequency signals, its thickness is less than λ / 4, where λ is the wavelength corresponding to high-frequency ultrasound.

[0011] In the aforementioned ultrasonic transducer, the diameter of the high-frequency piezoelectric vibrator is 5 to 10 times the diameter of the low-frequency piezoelectric vibrator.

[0012] In the aforementioned ultrasonic transducer, both the high-frequency piezoelectric vibrator and the low-frequency piezoelectric vibrator are excited by a single-frequency ultrasonic pulse signal.

[0013] In the aforementioned ultrasonic transducer, the center frequency of the high-frequency piezoelectric vibrator excitation signal is 700kHz to 1MHz, and the center frequency of the low-frequency piezoelectric vibrator excitation signal is 200kHz to 500kHz. Based on the frequency and relative position between the high-frequency and low-frequency piezoelectric vibrators, the amplitude and delay of the dual-frequency ultrasonic pulse excitation signal are adjusted to obtain the maximum amplitude superposition of the dual-frequency pulse signals in the air domain.

[0014] Compared with existing technologies, this invention effectively combines air-coupled ultrasound technology with dual-frequency ultrasound technology, effectively reducing the pulse width of ultrasound and achieving higher longitudinal resolution. It solves the problem that longitudinal resolution cannot be effectively improved in the field of air-coupled ultrasound technology and has extremely high practical application value. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the ultrasonic transducer structure used in the simulation of this invention.

[0016] Figure 2 It is the time-domain waveform of the dual-frequency excitation signal simulation;

[0017] Figure 3 It is the time-domain sound pressure waveform of an ultrasonic transducer.

[0018] Figure 4 It is the simulated sound pressure signal spectrum of an ultrasonic transducer.

[0019] Among them, 1-high frequency piezoelectric vibrator, 2-insulating layer, 3-high frequency matching layer, 4-low frequency piezoelectric vibrator, and 5-low frequency flexible circuit layer. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0021] Example 1: This example improves the air-coupled dual-frequency piezoelectric ultrasonic transducer. In the improved ultrasonic transducer, the low-frequency piezoelectric vibrator 4 is placed in front and the high-frequency piezoelectric vibrator 1 is placed behind. It includes a high-frequency matching layer 3, a low-frequency flexible circuit layer 5, an insulating layer 2 and a high-frequency piezoelectric vibrator 1 arranged in sequence. The high-frequency piezoelectric vibrator 1 has a single matching layer structure and good matching conditions to ensure that the ultrasonic transducer has high sensitivity.

[0022] The high-frequency piezoelectric vibrator 1 uses PZT series piezoelectric ceramic material, while the low-frequency piezoelectric vibrator 4 uses PVDF piezoelectric material. The PZT piezoelectric ceramic has the advantage of a high piezoelectric coefficient (~300 pC / N), but a high acoustic impedance (~30 MRayl); the PVDF has the advantage of low acoustic impedance (3.9 MRayl), but a small piezoelectric coefficient (~25 pC / N). Under the same excitation signal, the amplitudes of the acoustic pressure signals excited by the PZT and PVDF differ by two orders of magnitude. To ensure that the acoustic pressure signals of the high-frequency piezoelectric vibrator 1 and the low-frequency piezoelectric vibrator 4 can be effectively superimposed, the low-frequency piezoelectric vibrator 4 is designed as a multi-element structure.

[0023] The diameter of the high-frequency piezoelectric vibrator 1 is 5 to 10 times the diameter of the low-frequency piezoelectric vibrator 4. The lower surface of the high-frequency matching layer 3 is etched with cavities that match the size of the low-frequency piezoelectric vibrator 4. The low-frequency piezoelectric vibrator 4 is embedded in the cavity on the lower surface of the high-frequency matching layer 3 and is attached to the low-frequency flexible circuit layer 5. The low-frequency flexible circuit layer 5 is used to excite the low-frequency piezoelectric vibrator 4.

[0024] The insulating layer 2 is made of epoxy resin and is used to isolate the electrical signal between the high-frequency piezoelectric vibrator 1 and the low-frequency piezoelectric vibrator 4. In order not to affect the propagation of the high-frequency signal, its thickness is less than λ / 4, where λ is the wavelength corresponding to the high-frequency ultrasound, and λ / 4 is approximately 0.085 to 0.122 mm.

[0025] use Figure 2 The ultrasonic pulse signal shown excites the high-frequency piezoelectric transducer 1 and the low-frequency piezoelectric transducer 4. Considering the severe attenuation of high-frequency ultrasound in air, the center frequency of the excitation signal for the high-frequency piezoelectric transducer 1 is set to 700 kHz to 1 MHz. Considering the spectral superposition of the dual-frequency ultrasonic transducers, the center frequency of the excitation signal for the low-frequency piezoelectric transducer 4 is set to 200 kHz to 500 kHz. Based on the frequency and relative position between the high-frequency piezoelectric transducer 1 and the low-frequency piezoelectric transducer 4, the amplitude and delay of the dual-frequency ultrasonic pulse excitation signal are adjusted to obtain the maximum amplitude superposition of the dual-frequency pulse signals in the air domain.

[0026] Example 2: This example uses a high-frequency piezoelectric vibrator 1 with a center frequency of 800kHz and a low-frequency piezoelectric vibrator 4 with a center frequency of 350kHz for simulation, as follows... Figures 2 to 4 As shown:

[0027] use Figure 2The ultrasonic pulse signals shown excite both a high-frequency piezoelectric transducer and a low-frequency piezoelectric transducer. Considering the severe attenuation of high-frequency ultrasound in air, the center frequency of the high-frequency piezoelectric transducer excitation signal is set to 700 kHz–1 MHz. Considering the spectral superposition of the dual-frequency ultrasonic transducers, the center frequency of the low-frequency piezoelectric transducer excitation signal is set to 200 kHz–500 kHz. Based on the frequencies and relative positions of the high-frequency and low-frequency piezoelectric transducers, the amplitude and delay of the dual-frequency ultrasonic pulse excitation signals are adjusted to achieve maximum amplitude superposition of the dual-frequency pulse signals in the air domain.

[0028] Figure 3 Based on simulation results obtained using the high-frequency piezoelectric vibrator 1 with a center frequency of 800kHz and the low-frequency piezoelectric vibrator 4 with a center frequency of 350kHz, the method for analyzing narrow pulse signals in the literature (Cai Y, Fan M, Sun P, et al. Axial Super-Resolution Ultrasound Imaging With Quasi-Monopolar Pulses From a Dual-Frequency Transducer[J]. IEEE Transactions on Instrumentation and Measurement, 2023, 72: 1-10.) is applied. The ratio of the absolute value of the negative pulse sound pressure amplitude to the positive pulse amplitude is defined to evaluate the dual-frequency superposition effect. A larger ratio indicates a better effect and higher longitudinal resolution. In this embodiment, the ratio of the absolute value of the negative pulse sound pressure amplitude to the positive pulse amplitude is approximately 1.82 (<1.2 for a single-frequency transducer). The pulse width at -6dB of the air-coupled dual-frequency transducer is 0.552μs, verifying that the air-coupled ultrasonic transducer can effectively reduce the pulse width, thereby improving the longitudinal resolution of the air-coupled ultrasonic transducer.

[0029] Figure 4 The simulated signal sound pressure spectrum is shown. Under simulation conditions, the -6dB bandwidths of the low-frequency piezoelectric vibrator, the high-frequency piezoelectric vibrator, and the air-coupled dual-frequency ultrasonic transducer are 78.2%, 57.5%, and 123.7%, respectively. This verifies that the present patent can effectively extend the bandwidth of the air-coupled ultrasonic transducer.

[0030] Example 3: The difference between this example and Example 2 is that a high-frequency piezoelectric vibrator 1 with a center frequency of 700kHz and a low-frequency piezoelectric vibrator 4 with a center frequency of 200kHz are used.

[0031] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer, characterized in that: The ultrasonic transducer comprises a high-frequency piezoelectric vibrator (1) and a low-frequency piezoelectric vibrator (4), wherein the low-frequency piezoelectric vibrator (4) is arranged in front of the high-frequency piezoelectric vibrator (1), and specifically comprises a high-frequency matching layer (3), a low-frequency flexible circuit layer (5), an insulating layer (2) and the high-frequency piezoelectric vibrator (1) arranged in sequence; the low-frequency piezoelectric vibrator (4) is of a multi-element structure, the lower surface of the high-frequency matching layer (3) is distributed with cavities, and the low-frequency piezoelectric vibrator (4) is embedded in the cavities of the lower surface of the high-frequency matching layer (3) and adheres to the low-frequency flexible circuit layer (5); the insulating layer (2) is used for isolating the electrical signals between the high-frequency piezoelectric vibrator (1) and the low-frequency piezoelectric vibrator (4). The low-frequency flexible circuit layer (5) is used for exciting the low-frequency piezoelectric vibrator (4).

2. The transmitting air-coupled dual-frequency piezoelectric ultrasonic transducer of claim 1, wherein: The high-frequency piezoelectric vibrator (1) adopts a PZT series piezoelectric ceramic material, and the low-frequency piezoelectric vibrator (4) adopts a PVDF piezoelectric material.

3. The transmitting air coupled dual frequency piezoelectric ultrasonic transducer of claim 1, wherein: The high-frequency piezoelectric vibrator (1) has a single matching layer structure.

4. The transmitting air coupled dual frequency piezoelectric ultrasonic transducer of claim 1, wherein: The insulating layer (2) is made of epoxy resin, and the thickness is less than λ / 4, wherein λ is the wavelength corresponding to the high-frequency ultrasonic.

5. The transmitting air coupled dual frequency piezoelectric ultrasonic transducer of claim 1, wherein: The diameter of the high-frequency piezoelectric vibrator (1) is 5-10 times of the diameter of the low-frequency piezoelectric vibrator (4).

6. The transmitting air coupled dual frequency piezoelectric ultrasonic transducer of claim 1, wherein: Both the high-frequency piezoelectric vibrator (1) and the low-frequency piezoelectric vibrator (4) are excited by a single-frequency ultrasonic pulse signal.

7. The transmitting air coupled dual frequency piezoelectric ultrasonic transducer of claim 6, wherein: The center frequency of the excitation signal of the high-frequency piezoelectric vibrator (1) is 700 kHz-1 MHz, and the center frequency of the excitation signal of the low-frequency piezoelectric vibrator (4) is 200 kHz-500 kHz; according to the frequency and relative position between the high-frequency piezoelectric vibrator (1) and the low-frequency piezoelectric vibrator (4), the amplitude and delay of the dual-frequency ultrasonic pulse excitation signal are adjusted to obtain the maximum amplitude superposition of the dual-frequency pulse signal in the air domain.

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