HYDRO-ACOUSTIC ANTENNA- a device that provides spatially selective or sound reception in the aquatic environment. Usually G. and. consists of electroacoustic transducers(antenna elements), acoustic screens, supporting structure acoustic. interchanges, shock absorbers and electrical communication lines. According to the method of formation of spatial selectivity of G. a. can be divided into interference, focusing, horn and parametric.
Spatial selectivity. G. a. due to acoustic interference. created at a certain point in space decomp. sections of the oscillating surface of the antenna (radiation mode) or electrical interference. at the exits antenna converters when a sound wave falls on it (reception mode). Interference G. a. subdivided into continuous, normal component fluctuations. the speed of the active surface to-rykh varies continuously from point to point (for example, antennas radiating through a common metal plate), and discrete, on the active surface of which discontinuities in the function describing the distribution of the normal component of the oscillations can be observed. speed. Discrete antennas are often called. antenna arrays
Spatial selectivity of focusing G. and. (cm. Sound focus) is formed with the help of reflective or refracting boundaries or media, producing focusing of sound energy, accompanied by the transformation of the wave front (for example, from spherical to flat).
Reflective surfaces are also used in horn antennas, however, the wave front does not transform and the role of the reflecting boundaries is reduced to limiting the part of the space in which the sound is emitted.
Active surfaces parametric. antennas oscillate at two close frequencies; spatial selectivity is formed as a result of the difference frequency arising from the nonlinear interaction of the primary radiated waves (the so-called pump waves).
Main parameters that determine the spatial selectivity of G. a., - directivity characteristic and coefficient. concentration (see Orientation acoustic emitters and receivers). G.'s ability and. convert energy (usually from electrical to acoustic when emitted and acoustic to electrical when received) is characterized by sensitivity, radiated power and sp. radiated power.
Antennas not only provide the formation of spatial selectivity, but also allow you to control it. In the case of most a common type of GA - gratings - such control is carried out by introducing an amplitude-phase distribution, that is, by creating a given distribution of amplitudes and phases of oscillations. velocities of the active surfaces of the transducers in the radiation mode. In the receive mode, the introduction of the amplitude-phase distribution is provided by the selection of complex coefficients. transmission of devices included in each antenna channel between the receiver and the adder. By introducing a phase distribution, it is possible to provide the summation of the sound pressures developed by the dep. G.'s converters and. in any given space direction, and thus control the direction of max. radiation (and in the reception mode - the direction of maximum sensitivity). Antennas, in the channels of which the specified phase distribution is introduced, called. compensated.
Managing the position of Ch. maximum directivity in space can be carried out not only by changing the phase distribution, but also by mechanical. turn G. a. or by changing the position of the compensated working section of a curved surface (for example, circular, cylindrical G. a.). The amplitude distribution allows you to change the shape of the directivity characteristic, obtaining the desired relationship between dec. elements of the directional characteristic, in particular between the width of its main. maximum and the level of additional.
Often the term "antenna" is used in a broader sense, covering both the antenna itself and the method of processing signals from its parts. elements. In this understanding, G. a. subdivided into additive, multiplicative, self-focusing, adapting, etc. Additive called. antennas, signals from elements to-rykh are subjected to linear operations (amplification, filtering, time or phase shift) and then added to the adder. In multiplicative G. a. signals in channels receivers are subjected not only to linear, but also to nonlinear operations (multiplication, exponentiation, etc.), which, with small interference, increases the accuracy of determining the position of the source. Self-focusing called. antennas, the receiving path to-rykh produces automatic. introduction of distributions that provide in-phase addition of signals at the antenna adder when the sound source is located at an arbitrary point in space. The receiving or emitting path of adaptive antennas produces automatic. the introduction of amplitude-phase distributions that ensure the maximization of some predetermined parameter (noise immunity, resolution, direction-finding accuracy, etc.).
The level of noise accompanying the useful signal, the efficiency of the antenna, the maximum depth that can practically be measured under existing sailing conditions, and sometimes the very possibility of making measurements depend on the choice of the location of the echo sounder antennas.
The main sources of hydroacoustic interference affecting antennas are ship machinery and mechanisms, propellers, turbulent boundary layer, as well as other hydroacoustic systems simultaneously operating on the ship. Each of the interference sources creates noise of a certain spectrum, which enters the antenna, propagating directly along the ship's hull, in the water along the ship's hull, reflecting from scattering objects in the marine environment or from the bottom. Air bubbles scattered in the layer of water surrounding the antenna have a special effect on the operation of antennas. It has been repeatedly observed in practice that when the ship was moving in ballast, when intense bubble formation took place in its near-bottom area, the echo sounder stopped measuring even relatively small depths. With a decrease in the speed of movement of the vessel or its stop, the operation of the echo sounder was restored. This phenomenon can be explained by the fact that air bubbles, on the one hand, intensively dissipate and absorb energy, on the other hand, they change the physical properties of the medium in direct contact with the antennas, reducing its equivalent rigidity, which, in turn, affects the tuning of the antenna system - environment, reducing the efficiency of converting an electrical signal into a mechanical one and vice versa.
In order to reduce the influence of disturbing factors on the operation of the echo sounder, antennas should be installed in places that meet the following requirements:
the change in hydrodynamic pressure at the place of installation of the antenna with a change in the speed of the ship should be minimal;
the antenna installation site should be located as far as possible from the engine room, propellers, thrusters, as well as from rooms in which very noisy machines and mechanisms are located;
in the area where the antennas are located (at a distance of up to 3-5 m) there should not be water intake and drainage devices, as this can cause a sharp increase in gaps or a complete disappearance of echo sounder readings, especially those operating at low frequency;
in the immediate vicinity of the antenna, especially in the direction towards the bow of the ship, there should be no protruding ship structures and firing devices that could interfere with the operation of the echo sounder;
on the path of emission and reception of the signal, within 60° from the vertical, there should be no parts that can create a reflection of acoustic vibrations;
the place of installation of the antenna should be as far as possible from other hydro-acoustic systems available on the ship;
It is recommended to provide convenient access to the antenna for its inspection, maintenance and replacement.
After determining the possible installation locations of the echo sounder antennas, taking into account the above requirements, it is necessary, based on a preliminary assessment of the noise level at the intended location of the antennas, to choose a place with the lowest level of acoustic noise.
When choosing the location of the echo sounder antenna, it is necessary to take into account the direction of rotation of the ship's propeller. So, when using a right-hand propeller, the water flow disturbed by it hits the left side of the ship's stern contours. As a result, vibration of the plating occurs, spreading mainly in the left side of the bottom of the vessel. This vibration of the ship's hull causes additional acoustic noise. In this regard, with a right-hand pitch propeller, it is recommended to install the echo sounder antenna on the starboard side, and with a left-pitch propeller - on the port side.
The results of measurements of noise levels at various points on the hull indicate that the lowest noise level, as a rule, is observed in the bow of the vessel. Therefore, it is recommended to install the echosounder antenna as close as possible to the bow of the vessel (in the area of positive pressure), taking into account the inadmissibility of its exposure during rolling. It is recommended to install the antenna as close as possible to the centreline of the ship.
You can reduce the effect of interference by installing special screens.
Installation of other devices of the echo sounder set is carried out in accordance with the requirements of the technical documentation for the product and taking into account the convenience of working with the device.
and their specifications
Purpose of hydroacoustic antennas
Hydroacoustic antennas designed to emit or receive hydroacoustic signals using hydroacoustic transducers and to ensure spatial selectivity.
Hydroacoustic transducers
Hydroacoustic transducer is a technical device that converts electrical vibrations into mechanical vibrations, or, conversely, mechanical vibrations into electrical vibrations.
There are two main classes of hydroacoustic transducers:
a) magnetostrictive;
b) piezoelectric.
The principle of operation of magnetostrictive transducers
Magnetostrictive transducers use the phenomenon of magnetostriction. Phenomenon magnetostriction lies in the fact that in some ferromagnetic materials, under the influence of a magnetic field, a deformation occurs, characterized by a change in the length of the sample when it is located along magnetic field lines. This effect is called direct magnetostrictive effect.
If the length of the rod increases with increasing magnetic field strength, then the magnetostriction is called positive, and if the length of the rod decreases, then the magnetostriction is called negative.
A graph of the dependence of the relative elongation of various ferromagnetic materials on the magnetic field strength is shown in fig. five.
Permalloy
Cobalt
– Nickel
Rice. 5. Plot of dependence of relative deformation on field strength
The nature and degree of deformation depends on the material of the sample, the method of its processing, the amount of pre-magnetization and temperature. From the materials shown in Fig. 5, permalloy has a positive magnetostriction, nickel has a negative one, and cobalt has a variable magnetostriction sign, depending on the magnetic field strength.
The deformation of any sample is limited by a limit called magnetostrictive saturation. The amount of saturation strain and the magnetic field strength at which saturation occurs depends on the material. For example, the magnetostrictive saturation of nickel is much greater than that of cobalt, and nickel saturation occurs at a lower field strength than cobalt saturation.
Heat treatment has a great influence on the properties of magnetostrictive materials. Annealing of any material leads to an increase in the magnitude of magnetostriction.
As the temperature rises, the magnetostrictive effect weakens until it disappears completely.
From the molecular kinetic point of view, the phenomenon of magnetostriction is explained as follows:
The crystallographic axes of small homogeneous crystals of a ferromagnetic material have a random orientation in space. However, individual crystals are combined into so-called domains. The magnetic moments of each domain have a certain orientation. For example, in nickel, the magnetic moments of the domains are oriented in eight directions - along the four diagonals of the cube. These directions are called directions of the easiest magnetization. If the sample is not magnetized, then the magnetic moments of the domains are randomly oriented, and the total magnetic moment is zero.
Under the influence of an external magnetic field, the magnetic domains are reoriented. They are oriented in those directions that coincide with the direction of the external field. In this case, the deformation of the crystal lattice occurs, which leads to a change in the dimensions of the sample.
Along with the direct magnetostrictive effect, there is also inverse magnetostrictive effect, the essence of which is to change the magnetic state of the sample under the influence of mechanical stress. Under mechanical action on a ferromagnetic material, the crystal lattice is deformed, as a result of which the orientation of the magnetic moments of the domains with respect to the external magnetic field changes.
Magnetostriction is an even effect. This means that when the polarity of the magnetic field changes, the sign of the deformation does not change. Thus, if an alternating electric current is passed through the solenoid, inside which the rod is located, then the rod will perform periodic oscillations with a frequency equal to twice the frequency of the exciting electromagnetic field. This effect can be eliminated by applying preliminary magnetization of the transducer. In transducers of search hydroacoustic instruments, magnetization is carried out by installing permanent magnets or by introducing a special direct current source.
The characteristic of the operation of a magnetostrictive transducer without bias is shown in fig. 6, and with bias - in fig. 7.
–H+H
Rice. 6. Job Description
magnetostrictive transducer without bias
Rice. 7. Job Description
magnetostrictive transducer with bias
To increase the efficiency of the converters, the frequency of the external excitation must be equal to the frequency of its own oscillations. The frequency of natural elastic oscillations of the rod depends on its length and the material from which it is made.
The natural frequency of the rod is determined by the formula:
where n- harmonic number (usually n= 1);
l - rod length, cm;
E- modulus of elasticity of the material, n/m 2 ;
ρ is the density, kg/m 3 .
Designs of magnetostrictive transducers
Any magnetostrictive transducer is a core made of magnetostrictive material, on which there is a winding made of flexible copper wire with waterproof insulation. The core is recruited from thin stamped plates. After stamping, the plates are annealed. The oxide layer formed on the surface of the plates during annealing is a good insulator. The insulation between the plates prevents the appearance of eddy currents in the core, and thus reduces the energy loss for heating the core.
In search instruments, rod magnetostrictive transducers are most widely used. The plates from which the rod transducers are assembled have a rectangular shape with slots. The plates are assembled into a package, which is a closed magnetic circuit, on the rods of which the winding is laid. To install permanent magnets, with the help of which the constant magnetization of the converter is carried out, longitudinal grooves are provided in the core. The design of the rod magnetostrictive transducer is shown in fig. 8.
and their specifications
Purpose of hydroacoustic antennas
Hydroacoustic antennas designed to emit or receive hydroacoustic signals using hydroacoustic transducers and to ensure spatial selectivity.
Hydroacoustic transducers
Hydroacoustic transducer is a technical device, ĸᴏᴛᴏᴩᴏᴇ converts electrical vibrations into mechanical ones, or, conversely, mechanical vibrations into electrical ones.
There are two basic classes of hydroacoustic transducers:
a) magnetostrictive;
b) piezoelectric.
The principle of operation of magnetostrictive transducers
Magnetostrictive transducers use the phenomenon of magnetostriction. Phenomenon magnetostriction essentially consists in the fact that in some ferromagnetic materials under the influence of a magnetic field a deformation occurs, characterized by a change in the length of the sample when it is located along magnetic field lines. This effect is called direct magnetostrictive effect.
If the length of the rod increases with increasing magnetic field strength, then the magnetostriction is called positive, and if the length of the rod decreases, then the magnetostriction is called negative.
A graph of the dependence of the relative elongation of various ferromagnetic materials on the magnetic field strength is shown in fig. five.
Permalloy
Cobalt
– Nickel
Rice. 5. Plot of dependence of relative deformation on field strength
The nature and degree of deformation depends on the material of the sample, the method of its processing, the amount of pre-magnetization and temperature. From the materials shown in Fig. 5, permalloy has a positive magnetostriction, nickel has a negative one, and cobalt has a variable magnetostriction sign, depending on the magnetic field strength.
The deformation of any sample is limited by a limit, which is commonly called magnetostrictive saturation. The amount of saturation strain and the magnetic field strength at which saturation occurs depends on the material. For example, the magnetostrictive saturation of nickel is much greater than that of cobalt, and nickel saturation occurs at a lower field strength than cobalt saturation.
Heat treatment has a great influence on the properties of magnetostrictive materials. Annealing of any material leads to an increase in the magnitude of magnetostriction.
As the temperature rises, the magnetostrictive effect weakens until it disappears completely.
From a molecular kinetic point of view, the phenomenon of magnetostriction is explained as follows:
The crystallographic axes of small homogeneous crystals of a ferromagnetic material have a random orientation in space. In this case, individual crystals are combined into so-called domains. The magnetic moments of each domain have a certain orientation. For example, in nickel, the magnetic moments of the domains are oriented in eight directions - along the four diagonals of the cube. These directions are called directions of the easiest magnetization. If the sample is not magnetized, then the magnetic moments of the domains are randomly oriented, and the total magnetic moment is zero.
Under the influence of an external magnetic field, the magnetic domains are reoriented. Οʜᴎ are oriented in those directions that coincide with the direction of the external field. In this case, the deformation of the crystal lattice occurs, which leads to a change in the dimensions of the sample.
Along with the direct magnetostrictive effect, there is also inverse magnetostrictive effect, the essence of which is to change the magnetic state of the sample under the influence of mechanical stress. Under mechanical action on a ferromagnetic material, the crystal lattice is deformed, due to which the orientation of the magnetic moments of the domains with respect to the external magnetic field changes.
Magnetostriction is an even effect. This means that when the polarity of the magnetic field changes, the sign of the deformation does not change. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, if an alternating electric current is passed through the solenoid inside which the rod is located, then the rod will perform periodic oscillations with a frequency equal to twice the frequency of the exciting electromagnetic field. This effect can be eliminated if the pre-magnetization of the transducer is applied. In transducers of search hydroacoustic instruments, magnetization is carried out by installing permanent magnets or by introducing a special DC source.
The characteristic of the operation of a magnetostrictive transducer without bias is shown in fig. 6, and with bias - in fig. 7.
–H+H
Rice. 6. Job Description
magnetostrictive transducer without bias
Rice. 7. Job Description
magnetostrictive transducer with bias
To increase the efficiency of the converters, the frequency of the external excitation must be equal to the frequency of its natural oscillations. The frequency of natural elastic oscillations of the rod depends on its length and the material from which it is made.
The natural frequency of the rod is determined by the formula:
where n- harmonic number (usually n= 1);
l - rod length, cm;
E- modulus of elasticity of the material, n/m 2 ;
ρ is the density, kg/m 3 .
Designs of magnetostrictive transducers
Any magnetostrictive transducer is a core made of magnetostrictive material, on which there is a winding made of flexible copper wire with waterproof insulation. The core is recruited from thin stamped plates. After stamping, the plates are annealed. The oxide layer formed on the surface of the plates during annealing is a good insulator. The insulation between the plates prevents the appearance of eddy currents in the core, and thus reduces the energy loss for heating the core.
In search instruments, rod magnetostrictive transducers are most widely used. The plates from which the rod transducers are assembled have a rectangular shape with slots. The plates are assembled into a package, which is a closed magnetic circuit, on the rods of which the winding is laid. To install permanent magnets, with the help of which the constant magnetization of the converter is carried out, longitudinal grooves are provided in the core. The design of the rod magnetostrictive transducer is shown in fig. 8.
 |
Rice. 8. Rod magnetostrictive transducer
Radiation and reception of acoustic vibrations is carried out by the end surfaces of the package. A porous rubber screen is usually glued onto one of the end surfaces. In this case, the emission and reception of acoustic vibrations is carried out by the second end surface in contact with water. In order to decouple the oscillatory system from the antenna housing, rubber cuffs are laid between the package and the housing. The antenna housing is hermetically sealed with a lid, through which the winding wires are led out with the help of glands.
Sometimes cylindrical magnetostrictive transducers with a toroidal winding are used in hydroacoustic instruments. The cylindrical transducer package is also assembled from thin annealed rings with holes. The winding wire passes through the holes and the inner cavity of the package. An alternating current in the winding creates a magnetic field, the lines of force of which are located in a circle centered on the axis of the ring. As a result, forces appear in the ring, directed along the tangents to the lines of force and causing radial oscillations of the ring. In order to direct vibrations in a given direction, the package is installed in the center of the reflector, which has the shape of a cone with an opening angle of 45º.
The device of the ring magnetostrictive transducer and the method of its installation are shown in fig. nine.
 |
Rice. 9. Ring magnetostrictive transducer with a reflector
Specifications for magnetostrictive transducers
Magnetostrictive transducers are widely used in hydroacoustic fish-finding equipment due to their simplicity and reliability. These transducers have high mechanical strength and do not corrode in sea water. In the manufacture of converters, the necessary insulation of the windings is easily provided, since their operation does not require the use of high voltages.
The disadvantages of magnetostrictive transducers include the following:
a) the impossibility of using high operating frequencies: the upper limit of radiated frequencies is limited to 60 kHz;
b) relatively low efficiency (20% - 30%);
c) low sensitivity in receive mode;
d) dependence of natural frequency on temperature.
The principle of operation of piezoelectric transducers
The operation of piezoelectric transducers is based on the use of direct and inverse piezoelectric effect.
direct piezoelectric effect essentially consists in the fact that under the action of mechanical forces applied to the crystals of certain substances, electric charges appear on the surfaces of these crystals, the magnitude of which is proportional to the degree of deformation.
If the crystal is placed between two electrodes connected to an alternating voltage source, then it will undergo deformation, the magnitude and sign of which depends on the electric field strength and its polarity. The appearance of mechanical deformation under the action of an electric field is commonly called reverse piezoelectric effect.
Many substances have a piezoelectric effect, both from among those existing in nature and those obtained artificially. Of natural materials, quartz crystals have the most pronounced piezoelectric effect ( SiO 2).
For the manufacture of antennas for hydroacoustic devices, barium titanate ( BaTio 3). This material is a piezoceramic obtained by firing a mixture of powders of titanium dioxide and barium carbonate at a temperature of 1400º.
Then the samples are polished, and electrodes are applied to them by burning silver into the working edges of ceramics. The ceramics are then polarized.
In unpolarized ceramics, individual randomly arranged crystals have regions (domains) with different directions of electric moments. Under the influence of a strong electric field (with a strength of 15–20 kV/cm2), the electric moments of individual crystallite domains are reoriented and the resultant polarization of the sample appears. This polarization persists after the removal of the external electric field.
In addition to barium titanate, lead zirconate-titanate, as well as synthetic crystals of Rochelle salt ( NaKC 4 H 4 O 6 4 H 2 O) and ammonium dihydrogen phosphate ( NH 4 H 2 PO 4).
The shape of a natural quartz crystal is shown in fig. 10. Axis z–z passing through the vertices of the crystal, it is customary to call it optical axis.
z z
Rice. 10. Quartz crystal
In addition to the optical axis, crystals have electrical and mechanical axles.
If an octahedral plate is cut out of a quartz crystal perpendicular to its optical axis, then the axis x–x, perpendicular to the axis through z–z and passing through mutually opposite vertical edges of the crystal, it is customary to call electric axis. Axis y–y perpendicular to the axis z–z and two opposite side faces of the crystal, it is customary to call mechanical axis. The octagonal plate thus obtained has three electrical and three mechanical axes.
If now a rectangular plate is cut out of the obtained octahedral plate in such a way that its faces are perpendicular to the three indicated axes, and the largest face is perpendicular to the axis x–x, then such a plate will have a piezoelectric effect. This plate is called the plate X-cut or Curie cut.
When subjected to mechanical force F x , on faces perpendicular to the axis x–x, a direct longitudinal piezoelectric effect arises (the pressure direction coincides with the electrical axis). In this case, a voltage appears between these faces:
where l, b, h length, width and thickness of the plate;
ε is the permittivity of the material;
d x is the coefficient of proportionality, which is commonly called
piezoelectric module.
If a mechanical force is applied F y to faces perpendicular to the axis y–y, then a direct transverse piezoelectric effect arises (the direction of the external force is perpendicular to the axis x–x). In this case, a voltage of opposite polarity appears:
U = -;
To obtain the reverse piezoelectric effect, the same plate X-slice is extremely important to place in an electric field so that the axis x– x coincided with the direction of the field lines. In this case, the plate is deformed both in the direction of the axis x– x, and in the direction of the axis y–y. Under the influence of the reverse longitudinal piezoelectric effect, the thickness of the plate h will increase by:
Δ h = d x· U;
At the same time, under the influence of the reverse transverse piezoelectric effect, the length of the plate l will decrease by:
Δ l = – d xU.
In any case, the frequency of mechanical vibrations is equal to the frequency of electrical vibrations.
The surface area of the world's oceans is about 71% of the Earth's surface. Most of it has not yet been studied.
The need to explore the world's oceans in the face of ever-increasing human needs for cheap fuel and the need to control civil navigation led to the emergence of hydroacoustic sensor systems capable of exploring hydrocarbons on the sea shelf and identifying and locating civilian ships in water areas.
Today, high requirements are imposed on such systems in order to provide optimal parameters, and the use of optical waveguides as transmitting and sensitive elements can significantly increase the efficiency of such systems and reduce the cost of studying the oceans and monitoring water areas.
The main factors for replacing traditional hydroacoustic sensors with piezoelectric transducers are lower cost, high reliability, smaller weight and size parameters, ease of manufacturing a distributed sensor and high sensitivity in the low frequency region, and the absence of electromagnetic interference on the sensitive fiber part.
Reconnaissance is carried out using active sonar. The ship source emits broadband acoustic radiation. Seabed areas with different densities, such as an oil and gas field and normal soil, will reflect acoustic radiation with different spectral components. An outboard fiber-optic antenna registers these signals. The on-board equipment processes the data received from the antenna and, based on the time delay of the useful signal, gives the direction to the desired object.
The principle of operation of an acousto-optic cable, the sensitive element of which is an optical fiber, is based on the effect of changing the refractive index of the fiber, and hence the phase of optical radiation under the action of an acoustic field. By calculating the phase change, information about the acoustic impact can be obtained.
There are many optical circuits and designs of sensitive elements, but all of them allow multiplexing a large number of sensors on a single fiber, placing several fibers in an acousto-optic cable can increase the number of sensors in the antenna by slightly increasing the thickness of the acousto-optic cable. This method of multiplexing a large number of sensors at the moment can only be provided by the use of optical fibers.
Work on the subject of this project began in 2011 together with the Central Research Institute "Electropribor Concern". In 2011-2013, preparatory work was carried out, the basic concepts for creating acousto-optic cables were worked out, various signal processing methods were tested. In 2014-2016, and implemented several layouts of passive acousto-optic cables and electronic signal processing units.
To determine the dynamic range, sensitivity, noise floor and other parameters, a series of tests were carried out on each antenna. The tests included studies of the antenna in an anechoic chamber (the acousto-optic cable is located on tripods around the source of the acoustic field) and in open water (the acousto-optic cable is wound on a sound-transparent test basket, in the center of which a spherical source of the acoustic field is placed). Below are photos from the tests.
The creation and study of extended hydroacoustic fiber-optic antennas is a young branch of science in Russia, which opens up great prospects in the field of hydroacoustic measurements.
