Design, Development and Application of Broadband Combined Underwater Acoustic Transducer

introduction

The ocean is not only an important treasure of fishery and mineral resources, but also an important position for countries to maintain national security and military struggles. Therefore, underwater acoustic technology has become an important means for the current exploration and development of marine resources, underwater communication and navigation of ships, underwater target detection and recognition, as well as marine environmental monitoring and natural disaster forecasting. The underwater acoustic transduceris the carrier of sound wave emission and reception in the underwater acoustic technology, and its technical level directly affects or even determines the final realization effect of the underwater acoustic technology. Active sonar detection and marine resource exploration require transducers with low frequency, high power, and small size. Noise simulation and sonar calibration requires underwater acoustic transducers with ultra-low frequency and ultra-wideband characteristics. In the field of underwater acoustic communication, underwater acoustic transducers are required to have the characteristics of high efficiency, ultra-wideband, high sensitivity, and flat in-band. In general, underwater acoustic transducers are developing towards low frequency, broadband, high power, small size and deep water. The deep-water transducer adopts the internal flushing method to work at a depth of up to 11,000m, and uses the coupling of the internal oil cavity and structural parts to form multi-mode vibration, which broadens the frequency band of the transducer. A multi-resonant cavity is formed by overflowing round tubes of different sizes, and the working frequency can be adjusted by changing the size of the round tubes to obtain a wider transducer.

The bandwidth of frequency range is 200Hz~2kHz. The diameter of the underwater hydrophone transducer is 250mm and the length is 500mm. The coverage band is 7~15kHz, the sound source level is 200dB, the receiving sensitivity is -176dB, and the working underwater depth is 11000m. The recently developed transducer has a size of The diameter is 240mm, the length is 420mm, the coverage frequency band is 1.8～8.0kHz, the transmission response is 144dB, and the in-band fluctuation is less than 6dB. In summary, overseas underwater acoustic transducers have covered the entire working frequency band, even covering the entire water area, and have formed a certain scale in engineering, serialization and generalization, representing the advanced level of the industry. Domestic research institutes and other related units have conducted a lot of research and experiments, and have achieved certain results. However, there is still a certain gap in the key technology and processing technology of underwater acoustic transducers compared with foreign countries, especially in The ever-increasing requirements for ultra-wideband, small size, and high performance in underwater acoustic detection require in-depth research. Development requirements .With the development of noise reduction technology of ships in various countries, the noise level of ships and underwater targets have been gradually reduced. Underwater weapons and equipment such as torpedoes mostly use broadband underwater acoustic transducers to expand the detection range and improve complex underwater acoustics. The detection ability and hit accuracy under the reverberation background enhance the underwater target recognition ability. In addition, in response to various navies, intelligence agencies, economic entities, and even international terrorist organizations, In the deployment of frogmen, autonomous underwater vehicles (AUVs), and micro-submarines for reconnaissance, sabotage, explosions, and mine-laying operations are often carried out in small-scale operations. Remotely controlled unmanned submersibles (ROV) and other underwater vehicles are equipped with various detection equipment for safety protection, and specific requirements are put forward for the main technical indicators of their sonar. In this paper, aiming at the requirements of acoustic detection of the wake bubbles of surface ships, a model is designed and developed with 3～100kHz ultra-wideband receiving and transmitting functions, which can conduct real-time underwater acoustic measurement of the wake bubbles of ships at a large opening angle, and requires that the receiving and sending functions are independent of each other. And controllable, the overall structure needs to be compact, the physical size is small, and it is easy to install and use on a small ROM. Considering the actual requirements and actual working conditions, the main technical indicators of the transducer described in this article are as follows: 1) The transmitting frequency is 3~100kHz, and the receiving frequency is 1~100kHz. 2) The emission sound source level ≥189dB. 3) Receiving sensitivity ≥ -180dB. 4) In-band fluctuation ≤6dB. 5) Beam width (horizontal) ≥90° (-3dB). 6) Beam width (vertical) ≥ 70° (-3dB). 7) Working water depth ≥500m. 8) Dimensions ≤ 350mm×150mm×250mm. 9) Mass ≤10kg. Among them, ROV is a small detection structure, and its carrying capacity is limited, so the transducer must be as small as possible, light in weight, and easy to implement under the premise of meeting performance indicators.

2 Transducer design and development

2.1 Transducer design and simulation analysis

The underwater cylinderical transducer belongs to a separate structure of receiving and transmitting. The transmitting end is realized by using three composite rod structure transmitting transducers, and the corresponding frequency bands are 3～18kHz, 18～45kHz, 45～100kHz; the receiving end is realized by using 2 piezoelectric ceramic ring series hydrophones, and the frequency bands are respectively It is 1-40kHz, 40-100kHz. The above-mentioned transmitting and receiving transducer base is packaged as a whole, and an anti-acoustic baffle is designed inside. After the package is integrated, the total mass is about 9kg. The overall shape of the transducer is an irregular cuboid. The basic size is about 310mm×150mm×220mm. The appearance is shown in Figure 1. The main cable can be connected to external sonar electronic equipment in the form of connectors.

Aiming at the main technical index requirements of the underwater acoustic transducer in this article, combined with the above design scheme, simulation analysis of its transmitting and receiving performance is carried out. Due to the complex structure of the transducer designed in this paper and the wide frequency band coverage, theoretical analysis methods are not suitable for calculation and simulation. As we all know, the finite element method is a numerical simulation method widely used in current engineering practice. Use ANSYS software to simulate a free-field water area and establish a simplified model of the transducer. Select a point in the far-field unit directly in front of the front cover to calculate the sound pressure, and then the transducer's transmit voltage response can be converted. In the far-field unit, select the sound pressure in each direction at a certain distance along the center of the transducer to calculate the open angle of the transducer's emission directivity. Since the composite rod transducer has axial symmetry, a 2D axisymmetric transducer finite element model is selected for finite element analysis. When using ANSYS calculation, it is necessary to consider the influence of water on the transducer. Usually the equivalent effect is a water polo, and then the load is applied to calculate the solution. The model of the transducer in the water is shown in Figures 2 and 3..

It can be seen from Figures 2 and 3 that the transmitting transducers are designed with dual-resonance peak broadband. The resonant frequencies of the 3~18kHz unit of the transmitting transducer are 5kHz, 14kHz, and the resonant frequencies of the unit of 18~45kHz are 20kHz, 40kHz, and the resonant frequencies of 45~100kHz, 55k. The 1-40kHz unit of the receiving hydrophone uses a piezoelectric ring, and the single-ring resonant frequency is greater than 40kHz to ensure a flat working frequency band. The internal two-series and two-parallel structure improves sensitivity and stability; the 40-100kHz unit of the receiving hydrophone uses piezoelectric Composite material, the resonance frequency is greater than 100kHz to ensure flatness in the band. In this paper, the finite element equation is used as MU¨ + CU · +KU = F (1) where: M is the mass matrix; C is the damping matrix; K is the stiffness matrix; U is the nodal displacement vector; F is the load vector. The emission voltage response level TVR is TVR = 20lg p R V + 120 (2) where: p is the sound pressure of the node; R is the distance from the node to the equivalent center of the sound source; V is the applied voltage. Extract the sound pressure p of the node on the acoustic axis in ANSYS, and calculate the emission response curve of the transducer. In the actual design, the transmitting part of the underwater acoustic transducer is composed of three kinds of composite rod transmitting transducers, which realizes broadband directional emission and suppresses the rear radiation at the same time. The transmitting transducer covers a wide frequency range and is mainly used for underwater acoustic measurement. It needs to have good in-band flatness to ensure the accuracy of underwater acoustic measurement. In engineering, methods such as optimizing the size of the radiating head of the transducer, or controlling the phase optimization to reduce the fluctuations in the band, and the series resistance on the piezoelectric ceramic stack before and after the dual-resonance (or "dual excitation") emission transducer are often used. , To further reduce the fluctuation of the transducer's transmit voltage response in the working frequency band. This paper considers the size and quality of the transducer mounted on the small ROM, as well as the overall installation structure, and mainly adopts the method of literature to suppress the in-band fluctuation of the transmitting transducer, that is, the method of adjusting the resistance of the matching resistor. Assuming that the series resistance of the front and rear piezoelectric ceramic stacks inside the transmitting transducer are R1 and R2, respectively, the resistance values of R1 and R2 are adjusted to control the flatness of the transmitting transducer in the band. Through finite element analysis, the emission response of the transmitting transducer under different resistance values is simulated. Taking the designed 18~45kHz double-resonance transmitting transducer as an example, the simulation analysis shows that the transmitting response varies with the resistance value curve as shown in Figure 4. It can be seen from the figure that adjusting R1 and R2 can basically control the flatness in the frequency band of the transmitting transducer. By optimizing the resistances R1 and R2, it can be concluded that when R1=940Ω, R2=330Ω, it has better in-band flatness. (Shown by the dotted line in Figure 4), and the overall in-band emission response does not change much,

It can meet the design requirements, combined with the actual physical size and broadband impedance matching, comprehensive simulation can get 3 ~ 18kHz, 18 ~ 45kHz and 45 ~ 100kHz transmitter transmitter voltage response simulation results, as shown in Figure 5-7. It can be seen from Figs. 5-7 that the transmitter voltage response of the transducer is not less than 140dB in the frequency band, which meets the requirements of design input related technical indicators, and can provide a larger sound source level for long-distance underwater acoustic detection.

The receiving part of the hydroacoustic transducer is realized by the combination of two sets of hydrophone arrays, each of which adopts a series and parallel connection of piezoelectric ceramic rings to achieve directional reception. Among them, the 1-40kHz frequency band hydrophone is made in the form of two piezoelectric ceramic rings connected in series. The sensitivity of a single hydrophone is not less than -193dB, and the sensitivity of the hydrophone after series connection is not less than -178dB. The sensitivity simulation analysis results are shown in Figure 8. The hydrophone has no horizontal directivity (baffle-adjustable directivity can be applied), and the 3kHz vertical directivity is about 130°. The simulation results are shown in Figure 9. The 40kHz vertical directivity is about 73°, and the simulation results are shown in Figure 

11. The receiving part of the hydrophone in the 40~100kHz frequency band adopts two piezoelectric ceramic ring series structure. The working frequency can meet the use of 40~100kHz, but the sensitivity is low. After the series connection, the sensitivity of the hydrophone is not less than -180dB. The sensitivity simulation results are as follows As shown in Figure 11. The level of the hydrophone has no directivity (a baffle can be applied to adjust the directivity), and the vertical directivity at 100kHz is about 77°. The simulation results are shown in Figure 12

According to the simulation analysis based on the finite element method, the combined transducer designed in this paper can meet the design input requirements in terms of transmitting and receiving, and the main technical indicators are satisfied.

 2.2 Transducer development

The broadband combined spherical underwater acoustic transducer is installed on a small ROM for use. On the basis of meeting the needs of broadband acoustic detection, it focuses on small size and light weight design. In this paper, combined with the overall structure design of a small ROM, the final developed transducer is shown in Figure 13. The specific design structure is shown in Figure 14. The broadband combined underwater acoustic transducer designed and developed in this paper covers the transmitting frequency range of 3~100kHz, the receiving frequency band of 1~100kHz, and the total mass of the physical object is 9.4kg (in the air, including the bracket and connection cable), the size is 328.5mm×140mm×240mm, which is smaller than the size and quality requirements in the design input, reducing the ROM carrying capacity requirements. The transducer is matched and installed on the ROM body, and the actual object after installation is shown in Figure 15. The simulation analysis results can be used as design reference input, but in the subsequent actual development and debugging process, it needs to be adjusted according to the actual measurement situation to meet the actual use requirements.

3 Experimental test

The transmitting part of the broadband combined underwater acoustic transducer adopts 3 vertical units to form a working frequency band covering 3~100kHz, and the receiving part adopts 2 independent units to form a working frequency band covering 1~100kHz. The overall layout of transmitting at both ends and receiving in the middle is adopted to ensure the opening angle of the transducer. An anti-acoustic baffle is designed inside the transducer to reduce the internal reflection and superposition of the acoustic signal. At the same time, an adjustable support mechanism is adopted in the receiving part, and the height of the receiving transducer is limitedly adjusted according to the actual test situation to further expand the receiving opening angle to avoid the occlusion and reflection of the transducer shell and the ROV body. After the development is completed, in order to further obtain the actual working performance of the transducer, which is different from the independent transceiver test method usually used in the laboratory, the overall acoustic performance index test of the transducer is used here. That is, after the whole is installed on the ROV, the tank test of the transducer is carried out under simulating actual working conditions to further confirm that the transducer is installed on the ROV and is affected by the ROV structure, so as to obtain the actual working condition of the transducer. Real performance parameters. A comprehensive test was carried out in an anechoic pool to verify the realization of its performance indicators. The test conditions of the anechoic water pool. the ambient room temperature is 25 ℃, the test cable length is 3 m, the water depth is 3 m, the ambient water temperature is 20 ℃, the insulation resistance is 500 MΩ, the static capacitance is 51,000 pF, and the test distance is 6.2 m. The actual measurement results are shown in Figures 16

The ROV is used to mount a broadband combined underwater acoustic transducer to perform broadband underwater acoustic detection of the wake bubbles of a surface ship, and obtain the relevant acoustic characteristics of the wake bubbles and the physical size of the wake. In the specific lake test, the surface ship was used to make high-speed direct navigation on the water surface. The ship was 7.5m long, 3m wide, and had a draft of 0.35m. The propeller of the external engine was 0.8m underwater. The test water area is an open area of a lake, the average depth of the area is 35m, and the speed of the ship is 10 knots when passing the measuring point. The ROV is equipped with a broadband combined underwater acoustic transducer in this article for continuous measurement. In repeated measurements, different acoustic frequency combinations are used for detection, and the measurement results of wake bubble distribution are obtained, as shown in Figure

It can be seen from Figure 18 that the actual measurement of the ship wake bubble size is concentrated in the high density of 10-20μm. The measurement result is consistent with the highest bubble number density in the wake given by the literature with a radius of 10-20μm, which proves that the transducer .The device meets the test requirements in the actual working environment. At the same time, the transducer is used to continuously measure the wake bubble layer formed after the surface ship sails, and according to the obtained wake bubble acoustic target intensity information, combined with the current underwater acoustic environment (such as sound speed, water depth, etc.) and prior data (such as transducer sensitivity, emission sound source-level circuit gain, etc.), estimated according to the corresponding processing algorithm, and obtained the bubble strength curve with depth and time as shown in Figure 19. It can be seen from Figure 19 that the wake bubble duration is about 173 s, and the actual measuring middle wake bubble thickness is 1.46 m, which is basically consistent with the empirical formula given by the conventional wake calculation formula. In summary, through the overall measurement test in the anechoic pool, the measurement results show that the actual performance of the transducer is basically consistent with the simulation results. It is installed on the ROV platform and verified by the actual navigation test on the lake. The test results show that the transducer covers a wide frequency band, has a small structure, and the measurement results are basically consistent with empirical formulas. The measurement data is credible and can meet the requirements of surface ship wake bubbles.

 4 Conclusion

 This paper proposes a combined integrated transducer design method, with a low-frequency to high-frequency broadband operating frequency band, which is characterized in that the transmitting end can cover 3~100kHz, the receiving end covers 1~100kHz, and the opening angle is not less than 70° ; Adopting a separate transceiver layout, transmitting at both ends, receiving concentrated in the center, internal acoustic baffle structure design; the internal components of the transducer are integrated and output through a watertight connector, reducing the complexity of external connections; Through the center support structure of the transducer, the overall center of gravity of the transducer can be adjusted, which is convenient for the adaptation and installation of small underwater vehicles such as ROV; the open layout of the transducer, the mechanical load-bearing through the metal support, reduces the entire transducer The quality and size of the device improve the fit. This transducer has the advantages of wide working frequency band, larger opening angle and lighter weight under the restriction of small size. It has been successfully applied to a small ROM, which solves the problem of ultra-wideband underwater acoustic testing on a small ROM platform. Has high military and civilian value.