New developments piezo ceramics used in the underwater sonar transducers

Transducer is an important part of sonar. From the perspective of the history of hydroacoustics, every step of the development of underwater acoustics is inseparable from the development of transducer technology. Since hydroacoustic transducers play a key role in underwater acoustic engineering, many developed countries have invested enormously in the research. From the history of the development of underwater acoustic equipment, from the beginning of the First World War, the Langevin type transducer consisting of piezoelectric ceramics crystals and metal masses was used. After many product replacements, the transducers were used. The performance has been greatly improved. In the past two or three decades, due to military demand, the rapid development of science and technology, the continuous development and application of new transducer materials, and the application of finite piezo element analysis in transducer design, transducer research. Based on the classical theory and analytical methods, many new concepts and methods have emerged. The underwater acoustic transducer is facing a new round of product replacement. Giant magnetostrictive dilute transducer, high performance electrostrictive piezo ceramic transducer, vector hydrophone, piezoelectric composite transducer, low frequency large area PVDF hydrophone, fiber optic hydrophone, etc. It represents the latest developments in transducer research. From this aspect, this article looks forward to the latest developments of underwater sonar transducers.

giant magnetostrictive transducer

In the 1970s, AE Clark discovered that rare earth alloys have super magnetostrictive properties. These maximum strains caused by magnetostrictive effects are 6 to 20 times larger than those of piezoelectric tube transducer used in underwater acoustic transducers, and the energy density is about 10 to 20 times, and the sound velocity is only 2/3 to 3/4 of the piezoelectric ceramic, and the performance comparison between the rare earth material Terfenol2D and the conventional piezoelectric material PZT-8 and nickel is given. Therefore, under the same volume conditions, the Terfenol2D hydroacoustic transducer has a resonance frequency that is 2/3 to 3/4 lower than the resonant frequency of the piezoelectric ceramic hydroacoustic transducer. Because the transducer made of rare earth giant magnetostrictive material Terfenol2D has the characteristics of large transmission power, small volume, light weight and high temperature environment, it is obtained in the development of low frequency-very low frequency high power underwater acoustic transducer. Adequate attention and application. In the 1980s, developed countries have developed various rare earth transducers and applied them to the military field. Sweden has successfully developed curved rare earth transducers with a sound power of 151 kW for minesweeping. China began research in the 1990s, but it has made rapid progress. They have successfully developed rare earth flexural transducers, rare earth inlaid transducers and longitudinal composite rod rare earth transducers, etc. The main features of Terfenol2D materials are (1) brittle materials and difficult machining. (2) Since rare earth materials are not only a magnetic conductive material but also a conductive material, when the external magnetic field changes, the rare earth transducer.Eddy current losses will be generated inside, and losses will be large at high frequencies. Compared with piezoelectric ceramic transducers, giant magnetostrictive transducers need to solve problems such as magnetic bias, prestress, eddy current loss and deep water compensation. At present, there are some solutions to these shortcomings. For the purpose of solving the problem of brittle material and large eddy current loss, it is studied the giant magnetostrictive material GMPC, powdered Terfenol 2D, mixed the bonding materials, and pressed and formed by powder metallurgy. For the design of giant magnetostrictive transducers, it is an advantageous solution to make full use of its large deformation and high energy density as the excitation source of the flexural transducer. It can be used to make a variety of low frequency, small volume and high power conversion. Correct selection of the magnetic bias, pre-stress and application techniques of rare earth giant magnetostrictive materials have a great impact on the performance of the transducer. Generally, the magnetic bias is selected at 1/3 of the magnetostrictive saturation value, and the pre-stress is selected from 7 MPa to 10 MPa to obtain a large output power.

Fiber optic hydrophone

Fiber optic hydrophone technology began in the US Naval Laboratory in the late 1970s. The fiber optic hydrophone has the advantages of high sensitivity, strong anti-electromagnetic interference capability, large dynamic range, small size and light weight Pzt4 piezoelectric hemisphere . Therefore, the technology was highly valued as soon as it was born, and it was regarded as one of the key technologies of national defense. After more than 20 years of development, fiber-optic hydrophone technology has made great progress in developed countries, and various fiber-optic hydrophones have been introduced. They have completed the all-fiber hydrophone submarine sound monitoring system, towed line array, submarine conformal array and so on. In particular, the successful development of solid-state lasers has opened up a vast world of applications for optical fibers. The fiber-optic hydrophone technology also has a good start. The performance of the unit prototype has been close to or reached the international level, and the fiber-optic hydrophone array technology research has been carried out. The penetration of sonar research into laser technology will undoubtedly open a new page in sonar research. All kinds of fiber optic hydrophones are designed according to the effect of sound wave to make the phase modulation or intensity modulation of the fiber light. The fiber is divided into multimode fiber and single mode fiber. The fiber optic hydrophone is mostly made of single mode fiber. Interferometer type and light intensity modulation type. Under the action of sound pressure, stress is generated in the core of the optical fiber to cause changes in refractive index and length. These two changes cause phase modulation of the laser propagating in the optical fiber. The interferometer type fiber optic hydrophone is to use the fiber that is affected by the sound field as the sensitive fiber, and the other is separated from the sound field. The fiber with a fixed phase difference is used as the reference fiber, which is placed on the arms of the interferometer, and the photoelectric converter .After the synthesis, interference is formed on the surface of the received photomultiplier, and acoustic information is detected. Because the wavelength of light is very small, the slight strain of the signal caused by the sound pressure is not a small change with respect to the wavelength of the light, and thus causes a large change in the output light intensity, so the sensitivity of the fiber interference type hydrophone is particularly high. The technical performance achieved by the fiber optic hydrophone of the fiber interferometer is as follows: receiving voltage sensitivity: - 140dB (0dB = 1V/μPa) Phase sensitivity: 2. 56 × 10 - 8 rad / μPa .Frequency response: 16Hz ~ 10kHz (undulation ≤ 3dB) directionality: omnidirectional (undulation ≤ 2. 5dB) Among all intensity type hydrophones, the grating type hydrophone is a new, proven and effective underwater acoustic detector. 

The output is expressed as a direct intensity modulation of the incident sound field. Its main working principle is to cause the relative displacement of the two gratings between the constant light source and the light receiver under the action of the sound field, and the receiving intensity is a function of the relative displacement of the two gratings, so that the sound field can be transformed . For intensity modulation. The grating hydrophone itself consists essentially of two axially aligned optical waveguides (or fibers) with a small gap and the apertures in the gap that control the transmittance provide the required intensity modulation. The hydrophone provides all the advantages of a direct intensity modulation device and is inexpensive. The grating method can achieve a relatively high sensitivity, and the device is simple to manufacture, without any advanced optical technology, and has a dynamic range of up to 160 dB, and has the ability to detect a displacement caused by sound less than 0.01 A. In addition, because of the flexible choice of grating density, offset, optical power and hydrophone structure, there is more flexibility in the design of sensitivity, dynamic range, size and operating frequency range. For fiber optic hydrophones, shot noise caused by current fluctuations on the photodiode is its main source of noise and is often referred to as the theoretical noise limit. In addition, beam alignment, reference beam isolation, and source vibration isolation have a direct impact on performance. The biggest disadvantage of fiber optic hydrophones is the large temperature effect.