New Type of Underwater Acoustic Transducer and New Technology of Transducer

Acoustic waves are the only carrier that humans have that can transmit information and energy over long distances in the vast ocean. On the land, people use electromagnetic waves to develop radars. Similarly, people use acoustic waves as information carriers to develop underwater targets for detection,and electronic equipment for positioning, identification and communication-sonar. Facing the vast ocean, sonar shoulders an important mission which reach all corners of the vast ocean, identify various things in it,which tell people the true face of the underwater world, and assist people in exploring the mysteries of the ocean,in order to become underwater communication navigation , aquatic fisheries, marine resource development, marine geology and geomorphology exploration, The reason why sound waves become the best underwater information carrier is that in water media, sound waves have the smallest attenuation coefficient compared with other physical fields such as electromagnetic waves, and can be transmitted over long distances. This advantage makes sonar by using ultrasonic waves to observe underwater from the beginning. The goal begins and continues to develop. At present, the working frequency band of sonar has been extended to a wide range. Active sonar has been extended from tens of Hz to tens of MHz, and the low frequency end of passive sonar has been extended to the infrasound range. In such a wide frequency band, according to regulations, the important device that stimulates and generates sound waves in the form of signals and senses and receives sound waves in water without distortion is called sonar transducer or sonar array. These devices are the front-end equipment of the sonar system, as well as the "window" for the sonar system to interact with the water medium and exchange information, and are the "realizers" of the sonar system's functions, for sonar transducers or sonar arrays ,It is vividly referred to as the "eyes and ears" of the sonar system. With the continuous expansion of the application field of sonar technology and the ever-increasing demand for military confrontation and combat, new principles, new technologies, and new sonar equipment have emerged one after another, one after another. The development requirements of new sonar technology have driven the rapid development of transducer technology, and the same technological breakthroughs in the field of transducers and the development of new materials, new mechanisms, and new structures of underwater acoustic transducershave also made the sonar system "refreshing". Here is a brief overview of the development status of transducer technology in recent years based on the information that the author has and the limited level of understanding. It mainly includes new material hydroacoustic transducer, new structure and new mechanism hydroacoustic transducer,new type of hydrophone, broadband transducer technology and so on.

2 New material underwater acoustic transducer

The transducer is a device that realizes energy conversion in the sonar system. In the transducer, there is a special material that has the ability to convert energy. This material is called a functional material. The functional materials used to make transducers mainly include piezoelectric materials (such as piezoelectric crystals, piezoelectric ceramics, piezoelectric polymers, etc.) and magnetostrictive materials (such as nickel, cobalt, nickel-iron alloys, ferrites, rare earth iron alloys, etc.) Etc.), they use piezoelectric effect and magnetostrictive effect to realize the mutual conversion between electric field energy or magnetic field energy and mechanical energy. The breakthrough of transducer technology is fundamentally determined by the technological breakthrough of functional materials. In recent years, various technical achievements in the field of functional materials have also brought dawn to the development of transducer technology. The doctor discovered that the lanthanide rare earth materials have amazing magnetostrictive properties, but they have not been used in practice because the curie point is lower than room temperature. Later, it was discovered that binary, ternary or quaternary alloys composed of rare earth elements and iron also have giant magnetostrictive properties at room temperature. The most representative rare earth alloy is Terfenol-D (the composition is Tb0.27Dy0.73Fe1). 95), it has become a new type of functional material that has attracted much attention since the 1980s. Relaxation ferroelectric single crystal lead magnesium niobate-lead titanate (PMN-PT) and lead zinc niobate-lead titanate (referred to as PZN-PT) are new types of composite perovskite crystal materials, and they are also emerging A class of new functional materials with great application prospects. Prior to this, nickel was commonly used as a material for transducers. In 1917, French scientist Langevin made a sonar transducer with a quartz crystal, setting a precedent for the application of piezoelectric materials in the sonar in the 1940s , BaTiO3 pzt ceramics with strong piezoelectric properties were successfully developed and were widely used in sonar systems . The PZT piezoelectric ceramics developed in the 1950s have a wide operating temperature range and excellent electromechanical properties. The conversion efficiency makes up for the insufficiency of Ba TiO3 ceramics and once became the material of choice for underwater acoustic transducers. Among them, the piezoelectric ceramic material with high energy density is PZT-8. A simple comparison of the above materials: Terfenol-D, PMN-PT, PZN-PT can produce strain about 5 times that of PZT-8 and 50 times that of nickel; the piezoelectric constants of PMN-PT and PZN-PT are d33. It is 6-8 times that of PZT-8 material. The use of these pzt materials to develop new underwater acoustic transducers is one of the current hot topics.

Magnetostrictive material of cylinder underwater acoustic transducer is rare ,which earth giant magnetostrictive material uses magnetostrictive effect to realize the mutual conversion between magnetic field energy and mechanical energy, and is mainly used to develop low-frequency and high-power underwater acoustic emission transducers. It is a kind of "complex" structure of the transducer-high temperature super conducting magnetostrictive hydroacoustic transducer. From the perspective of the structure of the transducer, its structure is very simple. It is an ordinary dual-radiator longitudinal transducer. The so-called "complex" here refers to its rich physical connotation. The magnetostrictive power of rare earth alloy materials at low temperature is greater than that at room temperature. For example, the maximum magnetostrictive strain of Tb0.6Dy0.4 at 77K temperature is 0.65%, while Terfenol-D is at room temperature. The highest magnetostrictive strain is 0.25%. Developed a Tb0.6Dy0.4 material magnetostrictive hydroacoustic transducer with a temperature range of 50-60K: the rare-earth alloy rod-shaped material is placed in the air-conditioning room, and the cooling tower of the refrigerator is cyclically cooled, and the air-conditioning room is provided by a superconducting material coil. The bias magnetic field and the excitation magnetic field excite the magnetostrictive rod to produce stretching vibration, which is transmitted to the piston-type radiating surface through the mechanical transition piece, and the piston-type radiating surface pushes the water medium to generate pressure wave radiation. A vacuum chamber is designed in the structure to isolate heat conduction. The outer wall of the vacuum chamber is a dome-shaped pressure-resistant cover, which can withstand a pressure of 10 atmospheres. The main technical parameters are as follows: resonance frequency 430 Hz, maximum sound source level 181. 4db, the efficiency is about 25%. The manufacturing process of this kind of transducer is complicated. In recent years, people are still willing to use terfenol-D material that works at room temperature, abandon some magnetostrictive strain, and replace it with a new structure to achieve excellent radiation performance. The following is a brief introduction to the research progress of several structural magnetostrictive materials in underwater acoustic transducers. The longitudinal transducer has a simple structure. The magnetostrictive rod is combined with the front radiating head and tail mass to form a similar one-dimensional vibration system. The front radiating head is generally made of lightweight materials, and the tail mass is generally made of dense materials to achieve a radiating surface. Output greater vibration displacement. Two longitudinal transducers developed with Terfenol-D materials are introduced. One is a general longitudinal transducer with a resonance frequency of 1200 Hz, a sound power of 3 kW, and a transducer weight of 60 kg; the other is the rare earth rod at both ends. Designed as a horn-shaped double-ended radiating longitudinal transducer, the resonance frequency is 400 Hz, the sound power is 1.5 kW, and the transducer weight is up to 100 kg. Ring-shaped transducer: a regular polygon surrounded by a number of rare earth rods, and a series of circular arc surfaces are excited by the transition piece to make radial vibrations to achieve high-power acoustic radiation. A series of rare earth low-frequency high-power toroidal transducers have been developed including transducers with a resonant frequency of 200 Hz (inner diameter 0.56m, outer diameter 0.94m, height 0.37m, sound source level 193dB, weight 410kg) And a transducer with a resonance frequency of 30 Hz (diameter 2m, height 1.1m, sound source level 195dB, weight 5t). Flextensional transducer is a kind of transducer that uses the longitudinal vibration of piezoelectric ceramic stack or magnetostrictive rod to excite the shell (or barrel beam) radiation surface with amplitude amplification effect for bending vibration. Several commonly used transducers are listed Types of flextensional transducers, among which I, II, and III have the same characteristics. The longitudinal vibrating rod excites a rotationally symmetrical curved shell. The shell can be a continuous structure or a structure that is cut into a group of beams. Purcell used Terfenol-D material to develop a concave barrel beam bending-tension transducer (type III), with a resonance frequency of 1300 Hz, a sound source level of 188.7 dB, and a bandwidth of 600 Hz. Due to the use of a single rod open magnetic circuit, the resonance frequency is The maximum AC electroacoustic efficiency is only 7%, and the weight of the transducer is 2.7kg. Fish-lip flextensional transducers have common characteristics. The transducer is excited by a longitudinal vibrating rod to flex the convex or concave elliptical shell to achieve high-power radiation. The fish-lip flextensional transducer adopts the amplitude amplification effect. The effect of weighting with area increases the sound radiation power.Reported this new type of low-frequency high-power underwater acoustic transducer, including the resonant frequency of 210Hz, 450Hz, 800Hz and 1200Hz, the research results of this new type of transducer is currently used in low-frequency active sonar arrays, acoustics Acoustic systems such as target sound sources and noise simulators.

Relaxation ferroelectric material underwater acoustic transducer

Relaxor ferroelectric materials are a kind of potential functional materials, which can be divided into electrostrictive ceramic types and relaxor ferroelectric single crystal types. The manufacturing process of relaxor ferroelectric single crystals is much more complicated than that of electrostrictive ceramic materials. Researchers have used these materials to make many types of transducers, such as flexural transducers, longitudinal transducers and so on. The transducer manufacturing technology of this type of material is more complicated, and it is necessary to add a DC bias electric field, apply prestress, and control the temperature of the process. The use of PMN-PT-BT (lead magnesium niobate-lead titanate-barium titanate) electrostrictive ceramics developed the IV type flextensional transducer. The research results show that the developed transducer has not maximized the potential of the material. This work will still be one of the hot spots that need to be explored in the field of underwater acoustic transducers for a period of time. Using PMN-PT relaxor ferroelectric single crystal material to study 64 channels of 3.5MHz ultrasound probe, used in medical B-ultrasound and Doppler color ultrasound imaging equipment, suggesting that relaxor ferroelectric single crystal material in high-frequency image sonar.

Piezoelectric polymer film of spherical underwater acoustic transducer can be made into a flexible membrane, and the transducer can be designed into any shape when making the transducer, and the acoustic impedance of the material is low, and it is easy to achieve impedance with water and other fluid media and biological tissues. Matching, often used to make high-frequency standard hydrophones, high-frequency transducers, medical ultrasound transducers, conformal arrays and diversified composite transducer arrays,The commonly used piezoelectric polymer for making transducers is mainly polyvinylidene fluoride (PVDF). At present, the more eye-catching piezoelectric polymer material film EMFi (abbreviation of electro mechanicalfil), is a kind of polypropylene foam flexible film, its piezoelectric constant is about 10 times that of PVDF, which can be used to make high-sensitivity transducers. The structure of the EMFi thin-film transducer has a receiving surface diameter of 35mm, and the receiving sensitivity of the transducer is greater than -190dB (reference value is 1V/μPa). This kind of transducer can also be used in the air to receive or emit sound waves.

Introduction of the new structure of underwater acoustic transducer and various transduction mechanisms. Functional materials are important in the transducer, but they need to be played by a suitable structure. Therefore, the structural design of the transducer appears to be particularly important in the development of transducer technology. important. According to different application fields and various technical requirements, or according to the characteristics of different transduction mechanisms and functional materials, various types of transducers have come out one after another, many of which combine multi-disciplinary technologies to jointly break new ground Technical difficulties to meet some special technical requirements. The high-temperature superconducting magnetostrictive hydroacoustic transducer is a typical example. In the foregoing content of this article and the types of transducers to be introduced later, many are also new structures and new mechanisms of underwater acoustic transducers. In order not to repeat, this section only cite two other design examples of new structures.

Cymbal type (cymbal) transducer is a kind of new structure transducer similar to flextensional transducer. Each cymbal type transducer consists of a pair of PZT piezoelectric ceramic disc and one The metal cap is bonded together. The PZT piezoelectric ceramic disc applies an alternating voltage to generate radial vibration to excite the metal cap for bending vibration, and the raised metal cap of the transducer produces alternating vibration of "expansion-shrinkage". Radiation sound waves. When the same alternating pressure wave acts on the metal cap, the pressure will be transmitted to the PZT piezoelectric ceramic disc, and the alternating voltage is output at the two poles of the ceramic disc, which is used as a receiving transducer. The resonant frequency of the cymbal-type transducer in water is 16.1kHz, and the emission voltage response is 130dB (reference value is 1μPa/V, at 1m). Figure 5 also shows the photo of the 9-element matrix composed of this type of transducer. . In the coil spring type low frequency piezoelectric transducer, the piezoelectric ceramic is processed into a coil spring shape (as shown in Figure 6), thepiezoelectric ceramic transducer is polarized in the tangential direction, and then an excitation electrode pair is constructed. The neutral section with no electrodes in the middle is separated to form an outer ring electrode pair 1 and an inner ring electrode pair 2 (see the enlarged schematic diagram of a small fragment in Fig. 6). In this way, the excitation voltage V is applied to the electrode pair, the part of the piezoelectric ceramic controlled by the outer ring electrode pair and the inner ring electrode pair will produce opposite vibrations (extension or contraction), and the expansion and contraction movement of the spring system will drive the piston working surface to vibrate sound energy. Due to the low stiffness of this structure, it has a low resonant frequency and can be used as a low-frequency transmitting transducer. When used as a receiver, it also has high sensitivity in low frequency bands. Starting from the piezoelectric equation,electromechanical conversion relationship of this type of transducer was obtained, and some exploratory research work was carried out.

Introduction to various energy conversion mechanisms in underwater acoustic transducers From the perspective of energy conversion, transducers can be mainly divided into piezoelectric transducers that use piezoelectric effect to achieve energy conversion and magnetics that use magnetostrictive effect to achieve energy conversion. Retractable transducers, the transducers involved in the foregoing content belong to these two types.