Technological Innovation in the Development of Underwater Acoustic Transducers(2)

     Technological Innovation in the Development of Underwater Acoustic Transducers(2)

Iron-gallium alloy (Galfenol) is a new type of magnetostrictive material that has emerged in recent years. Its magnetostrictive strain is between nickel and Terfenol-D, at 300ppm (ppm is a microvariable, representing ΔL/L=10- 6) Above, compared with Terfenol-D, it has the advantages of higher relative permeability (>100), good machinability, high temperature stability and high tensile strength. Because the iron-gallium alloy material has good machining performance and high mechanical strength, it can be used to design and process the flextensional transducer housing. Figure 2b is a research example of a concave barrel flextensional transducer with an iron-gallium alloy housing. The underwater acoustic transducer is driven The vibrator is composed of Φ20mm×40mm iron-gallium alloy elements and neodymium-iron-boron permanent magnet sheets, and forms a closed magnetic circuit with the radiating shell. The experimental results show that the transducer emission current response is 168.4dB (resonant frequency 1750Hz), which is better than duralumin of the same geometric size. The housing transducer (resonant frequency 1900Hz) is improved by nearly 5dB, which reflects the design advantages of the active housing.

Published in 2000, the research results of the magnetostrictive-piezoelectric joint excitation broadband longitudinal transducer. The longitudinal transducer is jointly driven by the Terfenol-D unit and the PZT stack, which realizes the broadband operation of 1.8KHz and 3.5KHz dual resonance peak coupling. Characteristics, the literature also reported that the 4×4 high-power planar array composed of this type of transducer, the sound source level of the array is greater than 225dB in the 1.5-6kHz frequency band.

Terfenol-D multi-unit drive longitudinal transducer, the author ingeniously designed the drive unit, its structure uses a permanent magnet sleeve to apply a bias magnetic field to separate the static magnetic field from the dynamic magnetic circuit, and the dynamic magnetic The permanent magnet elements with low permeability are avoided in the road, and the magnetic field energy driving effect is increased; is the physical diagram of the drive unit. 4 such drive units are mechanically connected in series to form a low-frequency longitudinal replacement with the front cover and the tail mass. The energy device, the central screw is prestressed; Fig. 3c is the actual picture of the transducer after packaging, the resonant frequency of the transducer is 1.6kHz, and the sound source level is 177bB.

The magnetic circuit design of the magnetostrictive transducer is very important. Butler took the concave barrel flextensional transducer as an example and compared the working effects of six magnetic circuit schemes through finite element analysis. The magnetic circuit structures of Fig. 4a-f are respectively .Continuous rare earth rod plus pure iron magnetic permeable accessory end cover and sleeve, continuous rare earth bar plus pure iron permeable accessory end cover, continuous rare earth bar without pure iron permeable accessory, combination of rare earth rod and permanent magnet piece plus pure iron permeable Attachment end cover and sleeve, rare earth rod and permanent magnet piece combination plus pure iron magnetic permeable accessory end cover, rare earth rod and permanent magnet piece combination without pure iron permeable magnetic accessory, the effective electromechanical coupling coefficients are calculated to be 0.33, 0.30, 0.27, respectively , 0.23, 0.21, and 0.20, indicating that the effective electromechanical coupling coefficient of the rare earth vibrator is changed from a continuous rare earth rod to a rare earth rod combined with a permanent magnet sheet. The end caps and sleeves of pure iron magnetic permeable accessories have a certain effect on improving the electromechanical coupling performance of the rare earth vibrator, but for driving materials with low relative permeability such as Terfenol-D, the improvement is small, and the effective electromechanical coupling coefficient is determined by 0.20 to 0.23 or 0.27 to 0.33.

2.A new generation of piezoelectric materials and their transducers

Until the first half of the 20th century, all piezoelectric materials were single crystals. Polycrystalline piezoelectric ceramic barium titanate was first discovered in the 1950s, followed by lead zirconate titanate (PZT) in the 1960s. The performance of these piezoelectric ceramics far exceeds that of early single crystals, and PZT has since become the main functional material of underwater acoustic transducers.

In the mid-1990s, new piezoelectric single crystal lead magnesium niobate-lead titanate (PMN-PT) and lead zinc niobate-lead titanate (PZN-PT) were discovered, these two piezoelectric single crystal materials It has very high saturation strain (more than 1%), low loss, and high piezoelectric coupling coefficient (greater than 0.9), showing the potential advantages of increasing power and broadening the frequency band in the direction of the underwater acoustic transducer. In recent years, the ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT) and manganese-doped lead indium niobate-lead magnesium niobate-lead titanate (Mn: PIN-PMN-PT) piezoelectric single crystal material, which further improves the working characteristics under high electric field conditions.

The application of piezoelectric single crystal materials such as PMN-PT in the field of underwater acoustics started from the design and development of longitudinal transducers. Meyer and others have carried out a series of research work, including detailed analysis of 33-mode and 32-mode PMN-PT longitudinal transducers , And a comparative study with PZT-8. Figure 5a is a 33-mode longitudinal transducer driven by a stack of 10 PZT-8 wafers, Figure 5b is a 33-mode longitudinal transducer driven by a stack of 3 PMN-PT wafers, and Figure 5c is a 4 PMN-PT The long strips form a “mouth”-shaped 32-mode longitudinal transducer. The results show that when PMN-PT and PZT-8 are used to make longitudinal transducers with the same frequency and emission source level and other parameters, the PMN-PT crystal The stack length is only about 30% of PZT-8, which shows the technical advantages of piezoelectric single crystal materials to make small transducers; the 32 mode can make the single crystal materials be cut according to the best performance orientation, and at the same time use the combination of long strips It can avoid technical problems such as growing large-size single wafers, improve the reliability and consistency of the transducer, and has obvious advantages for medium and high frequency light-weight sonar array applications.

Single crystal has developed a cylindrical transmitting transducer composed of inlaid rings. Each ring is made up of 12 wedge-shaped strips, and 9 rings are tightly assembled in the axial direction to form a cylinder. The geometric size (Φ20.3mm×66mm) It is significantly smaller than the piezoelectric ceramic transducer of the same frequency, and realizes the broadband working characteristics of more than 2.5 octave. Another document uses PMN-PT single crystal to develop a concave barrel flextensional transducer. The drive vibrator of the transducer is composed of a stack of 16 axially polarized Φ28mm×Φ10mm×4.8mm elements, and a titanium alloy vibration shell. The emission voltage response is improved by more than 5dB compared with the same structure transducer of PZT-4 material.

The trigonal-tetragonal phase transition temperature of PMN-PT single crystal is relatively low, which limits its application range to a certain extent, especially for applications under high-power conditions. The ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT) and manganese doped single crystal (Mn: PIN-PMN-PT) make the phase transition temperature of the relaxor ferroelectric single crystal obvious Increase and greatly reduce the loss factor at the same time: the phase transition temperature is increased from 95°C to 125°C, the loss factor is reduced from 0.26 to 0.15, and the loss factor is only 1/2 of the usual PZT-4 piezoelectric ceramics. There is also literature using these two new formula single crystals, PMN-PT and PZT-4 to make longitudinal transducers and compare their high-power operating characteristics, which proves that the new formula single crystal material is more suitable for high-power and large duty cycle conditions. The sound source level of the PMN-PT transducer is 5dB higher than that of the PMN-PT transducer at the resonance frequency. Compared with the PZT-4 piezoelectric ceramics, the sound source level and power capacity at the resonance frequency are basically equivalent, and the working bandwidth Increased by 1 time, and the maximum sound source level outside the resonance frequency is increased by about 6dB.

The application research of PMN-PT single crystal material mostly focuses on medical high-frequency ultrasonic imaging system. Here is only one case of Cymbal hydro-acoustic transducer application research, using Φ12.7mm×1mm PMN-PT element to drive 0.25mm thick titanium The alloy bending vibration cap has developed a Cymbal-type small-size bending-tension transducer, which has a 6dB higher emission voltage response than the PZT-4 driven transducer with the same structure.

2. Technical innovation of underwater acoustic transducer structure and technology

⒈Technical innovation to improve beam characteristics

In modern sonar, various basic arrays are generally used to achieve the required beam characteristics. However, when the installation aperture of the transducer is limited and there are special requirements for the beam characteristics, technical measures need to be taken to control the beam characteristics of the transducer. The main technical approaches for improvement include: baffle application, modal superposition technology using dipoles and multipoles, etc. This section selects some typical research examples, focusing on the analysis and summary of the use of modal superposition methods to improve the beam characteristics of the transducer Technical achievements.

⑴Using the baffle to improve the beam characteristics of the transducer

In the early sonar system, an independent transducer was generally used. When the directivity cannot meet the requirements, the reflection of the baffle is used to control the transmission beam, which mainly includes passing through a flat baffle, a cylindrical baffle, and a spherical baffle. Plate and cone baffle to change the directivity of cylindrical transducers, piston transducers, spherical transducers, etc., to a certain extent meet the needs of one-way transmission beam control, as shown in Figure 6, the use of double cones The reflective baffle adjusts the directivity of the magnetostrictive toroidal transducer, and realizes the single-sided beam radiation characteristic.

There is a literature that the 3kHz type IV flextensional transducer is placed near the focus of the parabolic reflector baffle, so that the type IV flextensional transducer with its own non-directionality can achieve unidirectional radiation characteristics. The experiment obtains a single-angle opening angle of 83°. To the beam, the front and back response difference is 21dB.

⑵ Modal combination directional transducer

Various structural transducers have different multi-order vibration modes. Resonant transducers generally work based on the fundamental frequency vibration mode. Different vibration modes will correspond to their effective excitation methods, so a combination of excitation methods can be used Realize the superposition driving of multiple vibration modes, so as to achieve the purpose of changing the characteristics of the transmitting beam. The main modes that can change the beam characteristics of the transducer through combination include monopole mode, dipole mode and quadrupole mode, etc. These basic modes can achieve a variety of directivity patterns through weighted combination . In this section, combined with specific literature results, a brief analysis and summary of the processing technology and excitation methods of different structural transducers to achieve modal superposition are made.

Excitation multi-mode work generally adopts the partition excitation method, such as: piezoelectric ceramic tube or spherical shell often adopts the split electrode method, see Figure 7a, b; magnetostrictive polygon (ring) transducer, adopts independent edge excitation the way.

Butler et al. designed and developed a "modal transducer", still using the design idea of partition excitation, but breaking through the limitation of the division of independent components, using 8 independent 1/4 longitudinal vibrators to share the tail mass, each transducer The radiating surface is a cylindrical arc surface close to 45°, and they collectively enclose a partitioned and independently driven cylindrical emitting transducer. The geometrical size of the transducer is not restricted by the process conditions of the independent elements, and the longitudinal direction of the prestressed structure is adopted at the same time. The vibrator has technical advantages for the design of low-frequency and high-power directional transmitting transducers. Figure 8 shows the basic modal vibration shapes of the "modal transducer". Modal transducers based on PZT-8 piezoelectric ceramics, PMN-PT single crystal and Terfenol-D giant magnetostrictive materials have been designed and developed respectively. It has obtained a cardioid directional transmitting beam with a directivity index of 6dB and a 25dB difference in front-to-back response.

It is another type of low-frequency and high-power directional emission transducer—a zone-excited flextensional transducer. In the design, the piezoelectric stack (or magnetostrictive vibrator) of the flexion-tension transducer is subjected to zone excitation, using The combination of monopole and dipole modes is superimposed to form a cardioid directional emission beam. Figure 9a is a 900Hz directivity type IV flextensional transducer, and Figure 9b is a 3kHz directivity type VII flextensional transducer.

The literature studies a wideband multimode cylindrical transducer with a baffle plate (shown in Figure 10). The electrodes of the piezoelectric ceramic cylindrical tube are equally divided into two groups, and independently excited to obtain a monopole (0 mode) and a dipole (1 mode), and then cooperate with the baffle to realize the unilateral directional emission. The research work also uses the phase relationship between the modes to design an independent power amplifier and tuning circuit, through the low frequency "0+1" and the high frequency "0 + 1". －1” Modal combination control realizes broadband working characteristics. The transducer adopts 4 PZT-4 piezoelectric round tubes of Φ38.2mm×Φ31.8mm×19mm in the height direction, and the size after packaging is Φ48mm×79mm. The baffle is made of two pieces of cork rubber laminated to form a semicircle. The cylindrical surface has a thickness of 6mm, and the emission voltage response fluctuates by 6dB in the 26-46kHz frequency band.

2.Technical innovation to improve frequency characteristics

With the multi-directional extension of the application direction of underwater acoustic technology, the working frequency range of active sonar systems has been continuously expanded. Among them, the working frequency of high-resolution image sonar has been increased to 106Hz, and the working frequency band of ultra-long-distance detection and communication sonar is even lower. Below 100 Hz; on the other hand, the development of sonar information processing requires that the working frequency band of the transducer is as wide as possible. Therefore, low-frequency transducers and broadband transducers have attracted much attention in the underwater acoustic field in recent years, and the research results are quite rich. However, there are still many theoretical and technical problems that have not been solved well. This aspect will still be the research hotspot and focus of future development. This section selects the research work in the direction of low-frequency transducers and broadband transducers, and analyzes and summarizes them. Innovative ideas and new technological achievements.

⑴ Innovative design of low frequency transducer

①Bending vibration low-frequency transducer

The first technical problem faced by the development of low-frequency transducers is geometric size. Generally, the working frequency of resonant transducers is inversely proportional to the geometric size, that is, the lower the frequency of the transducer, the larger the geometric size, such as 500Hz longitudinal conversion. The length of the energy device is about 3m. Bending vibration can effectively reduce the geometric size of low-frequency transducers. Among them, transducers whose functional devices directly participate in bending vibration mainly include bending beam transducers, bending disk transducers, etc.

Figure 11a shows a typical three-stacked bending beam structure. A piece of piezoelectric ceramic strips are pasted on the top and bottom of the bending beam. When one of the piezoelectric ceramic strips stretches and the other contracts when excited, the metal beam in the middle will produce bending vibration. This kind of energy conversion The device needs to be exposed to water on one side to radiate sound waves, so usually several curved beams are combined to form an air cavity, as shown in Figure 11b, each radiating surface vibrates in phase.

A similar working principle is called a curved disc transducer with a disc structure, which also includes a three-layered and double-laminated structure. Figure 11c shows a compact curved disc transducer composed of a pair of double-laminated sheets. (Bender). Delany system analysis researched Bender's low-frequency, small-size and high-power operating characteristics.

The development of flexural vibration low-frequency transducers also includes a new structure-split toroidal transducer (shown in Figure 12). The split toroidal transducer can be considered as a special bending beam transducer. The original structure was proposed by Harris in 1957. The composite ring beam was composed of an inner piezoelectric ceramic ring and an outer metal ring. The modeling and analysis of the transducer was based on the "tuning fork model" shown in Figure 12b, and the driving element was adjusted to a split structure. The split ring transducer can be designed with a larger size, and the mass can be adjusted through the thickness distribution-stiffness to achieve the optimization of the operating frequency and radiation characteristics, as shown in Figure 12c.

②Bending-tension transducer

The concept of the flextensional transducer started from Hayes's patent in 1936. After Toulis published the patent of the IV type flextensional transducer in 1966, the research and application of the flextensional transducer began to be active, and there have been more than half of them so far. In the century of development history, various structural forms of flextensional transducers have been born, and their working principles and structural processes are full of innovative design ideas. We cannot introduce them one by one in the chronological order of their development, only the flextensional transducers. The structure and incentive methods of the company are divided into the following three categories, which are briefly analyzed and summarized.

△Bending-tension transducer with cylindrical structure. This type of transducer is driven by a longitudinal telescopic vibrator to translate the flexural vibration shell, as shown in Figure 13. The vibrating shell of the transducer is a translational structure, that is, a cylindrical surface of various shapes, driven by one or more longitudinally telescopic vibrators, a is type IV flextensional transducer, b is type VII flextensional transducer Energy device, c is a "star-shaped" bending-tension transducer driven by an orthogonal piezoelectric stack, and a "star-shaped" bending-tension transducer driven by a quadrilateral magnetostrictive vibrator. Since this type of transducer is easy to design a partitioned excitation vibrator, the directional flextensional transducer described above generally chooses this type of structure.

△Bending-tension transducer with long rotating body. This type of transducer is driven by a longitudinal telescopic vibrator to drive a rotationally symmetrical bending vibration shell, as shown in Figure 14. The vibrating shell of the transducer is a rotationally symmetric structure, including a series of barrel beams distributed along the circumference, which are generally driven by a longitudinally telescopic vibrator. Figures 14a and b are the convex shapes of the type I flextensional transducer Structure and concave structure; as shown in Figure 14c, the longitudinal excitation vibrator of the transducer is lengthened in the axial direction to increase the volume of the functional material to develop into a type II flextensional transducer; as shown in Figure 14d, the flexural vibration shell is designed In the form of two or more sections, it is developed into a type III flextensional transducer. Both type II and type III flextensional transducers have corresponding concave structures.

△Bending-tension transducer with flat rotating body. This type of transducer is driven by a radially expanding vibrator to drive a rotationally symmetrical bending vibration shell, as shown in Figure 15. The vibrating shell of the transducer is a rotationally symmetrical structure, generally a pair of convex or concave spherical crowns (or spherical crowns), driven by a radially expanding ring or disc vibrator, Figure 15a shows the ring Drive V-type flextensional transducer, b is a wafer-driven V-type flextensional transducer, c is a type VI flextensional transducer, d and e are small flextensional transducers developed on the basis of structure b The device is called Cymbal transducer.

△ Cavity structure low frequency transducer. Helmholtz resonator is the basic form of cavity structure underwater acoustic transducer, as shown in Figure 16. a, b, and c are the three basic structures of Helmholtz resonators, which use piezoelectric ceramic tube excitation, bending disk excitation, and piezoelectric ceramic ball excitation. Helmholtz resonators generally have a narrow working frequency band, and d is used on the basis of b The double working surfaces of the curved disc excite the resonant cavities of different volumes to realize the double resonance operation. The literature established a more complete Helmholtz resonator analysis model, and analyzed the relationship between the working characteristics and structural parameters of the 300HZ Helmholtz resonator. Morozov et al. designed an underwater pipe organ sound source (shown in Figure 17). The design of Figure 17a realizes frequency tuning by moving the sleeve to change the impedance of the resonance system. The tuning frequency ranges from 225 to 325 Hz, and the efficiency is up to 80% or more, reflecting the high-Q (quality factor) system with high efficiency characteristics; Figure 17b The design uses a double-tube structure with a built-in spherical sound source to achieve dual-frequency resonance. The low-frequency resonance is a cavity resonance composed of a double-section sleeve. The high-frequency resonance is only the resonance corresponding to the inner resonance tube. The outer sleeve and the inner resonance tube can Use metallic aluminum or non-metallic carbon fiber materials.

⑵ Innovative design of broadband transducer

In the history of the development of underwater acoustic technology, a variety of structural forms of underwater acoustic transducers have been produced, each with working characteristics determined by its structural characteristics. In order to adapt to the engineering needs of broadband applications, almost every structural transducer is faced with the technical problems of broadband design and process improvement. Among them, the longitudinal transducer is one of the most common structural forms of transducers in the field of underwater broadband transducer. The research results of broadband design and application are quite rich. The technical principles of the broadband design of other structural transducers are basically similar. This section focuses on a series of new design ideas based on longitudinal transducers to achieve broadband characteristics.

① Band combination broadband longitudinal transducer

The application of frequency band combination has already begun in the early stage of the development of sonar technology. Early work was seen in the 1940s. Three magnetostrictive longitudinal transducers with different resonance frequencies were used to drive a rectangular radiating plate and six transducers in a ladder arrangement. Driven by a common winding coil (shown in Figure 18), the independent resonance frequencies of the transducer are respectively 21.5, 23 and 24.5kHz, Q=12, and Q=4 after the combination. Although this frequency band combination method is not strictly a wideband transducer, it is still widely used in the field of underwater acoustics, especially in acoustic systems such as noise simulation and acoustic decoys. The device combination realizes ultra-wideband emission characteristics.

② Modal coupling broadband longitudinal transducer

The front cover of the longitudinal transducer is usually assumed to vibrate in the manner of a piston in the analysis of the one-dimensional model, that is, no bending vibration occurs. When the horn of the radiating surface of the transducer is relatively wide, it must be accompanied by bending vibration, which is reasonable Using the bending vibration mode of the front cover to effectively couple it with the longitudinal vibration mode, a broadband longitudinal transducer can be designed. Literature has studied the coupling effect of flexural vibration and longitudinal vibration of the square radiating cover plate, and designed a broadband transducer. In another literature, a vibrating and bending disk is embedded in the radiation cover, and the bending disk is coupled with the vibration mode of the longitudinal transducer, and the broadband transducer is designed and developed as shown in Figure 19a. The piezoelectric stack of the longitudinal transducer can be designed in multiple groups. As shown in Figure 19b, it is the basic structure of the transducer that uses dual excitation modal coupling to achieve broadband operation. Butler is based on the structure of the dual excitation longitudinal transducer. In-depth development, such as the use of magnetostrictive and piezoelectric hybrid double excitation to design a broadband longitudinal transducer, and the front cover to paste a 1/4 wavelength matching layer, and design a third-order resonance mode coupling ultra-wideband longitudinal transducer The device, as shown in Figure 19c, has a working frequency band of 13 to 37kHz.

③Broadband longitudinal transducer coupled with liquid cavity

The typical design of the coupling between the longitudinal transducer and the liquid cavity is the Janus-Helmholtz transducer (shown in Figure 20). The longitudinal transducer adopts a double-ended radiating structure, called Janus, with a cylindrical sleeve designed to form a Helmholtz resonant cavity between Janus’s double radiating heads; the general liquid cavity resonant transducer has a narrow working frequency band. In Janus joint application, broadband transmission can be realized through optimized design of modal coupling.

Gall designed two Janus-Helmholtz transducers, 300Hz and 160Hz, and studied in depth the effect of adding a compliant tube in the Helmholtz resonant cavity on the broadband operating characteristics of the transducer.

⒊Technical innovation to improve the power of emitted sound

The direct way to increase the sound power of an underwater acoustic transducer is to increase the volume of the transducer, increase the number, and form a close-packed matrix. The most effective method is to use high-energy-density functional materials. The previous chapters have explained the application of high-energy density functional materials. This section focuses on the technical innovations in the structure and process of small-volume high-power transducers.

In describing the advantages and disadvantages of the small size and high power characteristics of the transducer, the volume figure of merit is generally used to measure, namely

FOMv=Wa/V/f0/Q ⑴

Formula ⑴ defines the volume merit factor of a certain type of transducer, where: Wa is the sound power (W), V is the volume of the transducer (m3), f0 is the resonance frequency (Hz), Q is the quality factor, The volume merit factor of the device is closely related to the structure and functional materials. Delany designed and developed a compact curved disc transducer (Bender), and systematically analyzed and studied the working characteristics of Bender's low-frequency, small-size and high-power operation.

There are literatures designing the concave structure type I (concave barrel type) bending-tension transducer into a more compact combination, which enables multiple transducer clusters in a limited volume to maximize volume displacement and achieve large Power characteristics, as shown in Figure 21, the apex of 6 type I flextensional transducers are clustered together to form a "three-dimensional six-pointed star" flextensional transducer, which has the characteristics of compact structure, low frequency, high power and wide frequency band: fundamental resonance frequency The transmit voltage response at 1.15kHz is 127dB, omnidirectional, and the transmit voltage response from 800Hz to 10kHz is greater than 120dB. The FOMv parameter is not given in the literature, and it is expected to be equivalent to or higher than the "star-shaped" flextensional transducer .

The above design and analysis for the pursuit of small size and high power basically start from the electrical and mechanical limits, and only consider the energy density of functional materials and the vibration limit of the structure. When the transducer requires long pulse or continuous operation, the heat and heat dissipation of the transducer will be the biggest problem under high power conditions. At this time, the thermal limit is the main factor restricting the ultimate power of the transducer. The thermal limit of the transducer is one of the important basic issues that are concerned in engineering. Just like the process details of the transducer, there are not many publicly reported research papers. There are literatures to model and analyze the thermal problems of low-frequency and high-power transducers, and discuss the thermal conduction problems of Janus-Helmholtz and Type IV flextensional transducers. When the transducer is working in shallow water, especially low frequency and high power transmission, increasing the sound power will also be restricted by the acoustic limit of the cavitation factor. Under this background, the method of increasing the power of a single transducer is no longer effective. The base array will also be restricted, so that there is only one way to form a sparse base array.

Therefore, when designing low-frequency and high-power transducers, it is necessary to rationally choose the structural form and driving function materials, taking into account factors such as electrical limit, mechanical limit, thermal limit, and acoustic limit, and make overall analysis and comprehensive optimization. There is an optimal relationship between the limit power and the volume of the transducer. In-depth research on this will be one of the technical directions of low-frequency and high-power transducers in the future.

⒋Technological innovation to increase hydrostatic pressure resistance

At present, the academic community has proposed development ideas such as transparent oceans and informatized oceans. The goal is to allow underwater information technology to cover all corners of the ocean, including polar regions and abyssal trenches. Therefore, they put forward requirements for the use of underwater acoustic transducers in greater depth. Even challenge the ability to work in deep seas. The hydrostatic pressure resistance capacity of the transducer is closely related to the structure of the transducer, especially for low-frequency emission transducers with low structural rigidity. Solving the hydrostatic pressure resistance structure technology has become an important topic in the current transducer technology field one. The current effective methods and means to solve the working depth mainly include fluid filling, compliant tube matching fluid filling, natural structural support, high-pressure gas cylinder compensation, airbag compensation, etc., at working depths above 1000m, the only effective technical method is fluid filling technology, including The free overflow type directly uses seawater as the filling fluid or fills some oil impedance media to achieve self-static pressure balance; within 1000m, the pressure-resistant compliance tube can be used in the liquid cavity at the same time to improve the compliance of the liquid cavity; Within 200m, the natural support of the structure can withstand hydrostatic pressure. Some transducers with very low structural rigidity (such as moving coil transducers) can use high-pressure air cylinders to provide pressure compensation. Generally, within 100m, airbag compensation can be used. The cavity structure transducer introduced above can generally be designed as a fluid-filled working mode to achieve deep-water work. In this section, several application examples of oil-filled structure design are given.

Kendig’s research work published in 1965, combined application of 4 PZT-4 piezoelectric ceramic disc-driven longitudinal transducers, filled with silicone oil to protect the void formed between the steel shell (including the sound-transmitting rubber plate) and the transducer The cavity is connected with the rear fluid chamber. The front end sound-permeable rubber and the rear end rubber window are in contact with seawater to achieve internal and external pressure balance. The working bandwidth of the transducer is 30-50kHz, and the work within the hydrostatic pressure range of 0-6.9MPa has been experimentally studied. Characteristic, this pressure balance method is still used in many deep-water sonar arrays. Figure 22b shows a free overflow toroidal transducer with an oil-filled structure. The piezoelectric ceramic ring is suspended in a polyurethane rubber sleeve, and the inside is filled with silicone oil to achieve pressure balance with the outside world. The polyurethane rubber sleeve is ideal Sound-transmitting material, this kind of transducer has similar working characteristics as the direct infusion coating form of polyurethane rubber. For PZT-4 round tube Φ150mm×Φ140mm×50mm, the simulation analysis and experiment study of polyurethane rubber in the frequency range of 5～10kHz The material of the sleeve is replaced with titanium alloy or steel. As a result, the titanium alloy reduces the emission voltage response by about 6dB, and the steel reduces the emission voltage response by about 12dB.

3. Conclusion

Looking at the hundred-year development history of transducer technology, from the birth of the first piezoelectric transducer to the vigorous development of modern transducer technology, technological innovations in underwater acoustic transducers have frequently emerged. The main goals of the innovation and development of transducer technology include: simplifying complex processes, breaking through technical bottlenecks, rewriting technical limits, improving comprehensive technical performance, proposing new concepts and new mechanisms, generating and developing new technical directions, and deepening and perfecting the theory of transducer disciplines System and so on. This article introduces some research cases that reflect the innovative design and exquisite craftsmanship of the transducer from the aspects of new material application, new transducer structure and technology, etc.