RU2617795C1 - Method for formation of cardioid response of a wide band gidroacustical receiver channel for an uninhabited underwater apparatus - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to methods for solving the problem of broadband reception the narrow-band (relative to the reception band frequency) acoustic signals with a priori unknown center frequency of the spectrum by a compact receiver cardioid (CNs) in a wide range of working angles amid interfering noise centered on corner. The main result achieved when using the proposed method is a significant increase in depth of the dip in the characteristic of the receiver orientation in a direction of interfering noise in the operating band of the reception channel frequencies. Additional results include: reduction in requirements for the identity of the active elements of the characteristics that make up a cardioid receiver, reducing the requirements for receiver assembly in the manufacture precision and simplification of the whole procedure of tuning the receiver while maintaining a large depth of the dip in the CN. The method is based on splitting by the procedure complex FFT receiving broad band signals at the output of the first and second elements forming the cardioid receiver into a plurality of narrowband channels. When setting up the receiver for the direction in which the failure should be provided in the CN, formed a table containing the complex coefficients for the center frequency of each narrowband channel, equal treatment of complex values of the output signals of the first and second channels. When a signal of the failure XH multiplication received second channel signal to the corresponding complex factor ensures precise alignment of the amplitude and phase cardioid receiver outputs of the first and second channels for the center frequency of each narrow frequency band, which is divided into the source a wide range of operating frequencies.

EFFECT: resulting in a cardioid XH shaper for calculating difference values of output signals of the first and second channels is achieved a substantial increase in depth of the dip at any frequency XH broadband reception channel.

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Description

The proposed method relates to hydroacoustics, in particular to methods for solving the problem of broadband reception of hydroacoustic signals using a small receiver in a wide range of working angles against a background of disturbing noise concentrated in the corner. To solve this problem, sonar receivers with a cardioid directivity characteristic (CI) are often used, in which the dip in the CI is oriented in the direction of the interference source.

The method of forming a cardioid directivity pattern of a receiver consisting of a pair of active elements is widely known (see, for example, Smaryshev M.D., Dobrovolsky Yu.Yu. Hydroacoustic antennas. Reference, ed. Sudostroenie, L., 1986). The known "Method for forming the directivity characteristics of a hydroacoustic antenna" (see patent No. 2169439, published 06/20/2001), in which the use of cardioid directivity characteristics of a group of elements of a translucent multielement antenna provides good directivity of the main (narrow) lobe of the diagram by suppressing the sensitivity of the antenna in the back direction and eliminates the use of large-sized acoustic screens that cover the rear direction of reception.

Known "Hydroacoustic antenna and a method of processing signals in it" (see patent No. 2466420, published November 10, 2012), in which the cardioid directivity of the elements are used to suppress structural interference from the carrier on which the antenna is located.

The cited patents refer to the characteristics of narrowly targeted multi-element (large) antennas designed to operate in relatively narrow frequency ranges. In such antennas, the depth of dip in the cardioid directivity characteristic is practically independent of the spread in the characteristics of individual active elements, since the sensitivity spreads of individual elements even in a fairly wide frequency band are averaged over the set of active elements used in the antenna.

The closest prototype in essence of the proposed method for the formation of a cardioid directivity is the classical method described in the book Smaryshev MD, Dobrovolsky Yu.Yu. Hydroacoustic antennas. Directory. In self-propelled uninhabited underwater vehicles, the receiving channel is often made in the form of a towed two-element receiver and two channels for processing the output signals of the receiver elements, which provide signals from the rear hemisphere. The dip in the XI is oriented towards the underwater vehicle towing the receiver. The implemented method is illustrated in FIG. 1 and includes the following operations:

- amplification in two channels for processing output signals from two active elements forming the receiver and spaced in space by a distance equal to a quarter of the wavelength of the acoustic signal at the selected frequency of the operating range;

- phase shift of the signal of the 2nd processing channel by delaying it by a quarter of the period of the signal frequency at the selected frequency of the operating range;

- equalization of signals in amplitude in the processing channels in the working frequency band;

- the formation of a differential signal on a subtracting device;

- amplitude-frequency correction of the sensitivity of the entire receiving channel by the output (by the difference signal), excluding the frequency dependence of the channel sensitivity in the working frequency range.

The source of interference that interferes with reception is usually the apparatus itself, which tows the receiver and, very often, emitters towed directly behind it. The failure in the HN, aimed at the source of interference, simultaneously with the separation in space of the receiver and the source of interference in this case provides the possibility of simultaneous non-directional radiation and reception of signals from the rear hemisphere during the movement of the underwater vehicle. An increase in the depth of the dip in the HN makes it possible to reduce the necessary distance between the receiver and emitters while maintaining the necessary margin for the stability of the apparatus to self-excitation, i.e. reducing the required cable length. In turn, reducing the length of the cable cable improves the overall dimensions of the device, increases its reliability and, most importantly, reduces the necessary towing forces and makes it possible to improve the speed characteristics of the device as a whole. Thus, the depth of dip in the receiver XN directly affects the main performance characteristics of the device. The required width of the working frequency band in which reception and radiation are made is often very large and reaches 2 or more octaves. In this frequency band, the receiver is required to implement the maximum achievable dip depth in the XN in the direction of the interference source. The aforementioned confirms the relevance of the task of finding methods for synthesizing a cardioid CN receiver with a maximum dip depth provided in a wide range of operating frequencies.

When implementing the known method for the synthesis of a cardioid CN of a broadband receiver in the operating frequency range, a frequency is selected for which the value of the necessary signal delay in the 2nd processing channel corresponding to a quarter of the period of this frequency is determined. At this frequency, after performing the operation of equalizing the sensitivity of the channels by adjusting the gain in the channels, the maximum dip depth in the XI is achieved due to the complete compensation of the output signal after subtraction.

A significant drawback of the traditional method for the synthesis of cardioid CN is that when the input signal frequency deviates from the selected frequency, which provides full compensation of the differential output signal, the achieved dip depth decreases due to the non-identical frequency characteristics of the processing channel electronics and, mainly, for non-identity in the working band of the frequency characteristics of the sensitivity of the active elements forming the receiver. The differences in the sensitivity of the receiver elements in a wide frequency band are random and vary from sample to sample. As a result, in order to obtain acceptable CVs of cardioid broadband receivers, it is necessary to carefully select the sensitivity of the elements making up the receiver in the manufacture of the receiver itself and use sophisticated methods for additional sensitivity correction in the working frequency band, which evens out the sensitivity of the elements during tuning. In the serial production of receivers using these operations at the edges of the operating range with a width of 2.4 octaves, the depth of the dip in the XN decreases by 8-10 dB compared to the depth of the dip at the selected compensation frequency.

The proposed method for the synthesis of cardioid CN is illustrated in FIG. 2.

The signals supplied to the inputs of the 1st and 2nd ADCs are signals from the outputs of a cardioid receiver formed by two receiving elements spaced apart by a distance equal to 1/4 of the wavelength for the selected frequency of the operating range. After performing the ADC and complex FFT signals from the outputs of the receiver elements for an arbitrary spectral component of the signal in the first and second processing channels, we have:

,(1) at the exit. 1 can -

,

,(2) at the exit. 2 can -

,

where A is the amplitude, ω is the circular frequency, r is the distance between the elements of the receiver, α is the angle of arrival of the signal, C is the speed of sound.

To form a dip in the optic wave in a given direction, it is necessary to equalize the signals in amplitude and turn the phase of the signal of the second processing channel to an angle that provides a zero result when subtracting the signals of the 1st and 2nd channels in a given direction. The required equalization and phasing of the signals in the channels for a selected compensation direction (at α = 180 °) is achieved by multiplying the complex spectral component of the signal of the second processing channel by a complex coefficient equal to the ratio of (1) to (2), i.e.

Performing the multiplication of the spectral component (2) by the coefficient (3) and subtracting the product from (1), we obtain the signal at the output of the difference channel:

From (4) we obtain for XN:

) не зависит от частоты) ХН в линейном масштабе для диапазона частот с шириной в 2,4 октавы. Расстояние между элементами приемника выбрано равным 8 мм, что соответствует

периода частоты, равной 46,875кГц. Коэффициенты вида (3) для заданного направления компенсации формируются в ПЗУ при настройке в виде таблицы для центральных частот каждого канала БПФ. Т.е. выполнение БПФ входного сигнала 2-го канала обработки необходимо производить сразу с домножением результата БПФ на набор коэффициентов типа (3).In FIG. Figure 3 shows an example of calculating idealized (when the sensitivity of the first receiver element is Α ₁ (

) does not depend on frequency) ХН on a linear scale for a frequency range with a width of 2.4 octaves. The distance between the elements of the receiver is chosen equal to 8 mm, which corresponds to

a frequency period of 46.875 kHz. Coefficients of the form (3) for a given direction of compensation are generated in the ROM when configured in the form of a table for the center frequencies of each FFT channel. Those. FFT of the input signal of the 2nd processing channel must be performed immediately with the multiplication of the FFT result by a set of coefficients of type (3).

A method of forming a table of complex coefficients of the form (3) is illustrated in FIG. 4. To create a table of complex coefficients that take into account all the deviations of the amplitude-phase characteristics of the 2nd processing channel relative to the 1st, the following operations are performed. Using the tuning equipment, a sequence of pulsed tones is generated with a repetition period set during tuning and with a discrete change in frequency (sequentially from pulse to pulse). The discreteness of the frequency change must strictly correspond to the discreteness in the frequency of the FFT performed in the processing channels during the operation of the receiving channel (i.e., the central frequencies of the FFT channels must correspond to the frequencies of the generated pulses). This sequence is used as input for acoustic equipment irradiating a tunable cardioid receiver. The tunable receiver is oriented in space in the direction where the dip in the XN should be. In the process of irradiating the receiver in the tuning mode for time-gated signals of the 1st and 2nd processing channels (for each received pulse), FFTs of the same length as in real work are performed, a complex division of the spectral samples of the 1st processing channel by spectral samples of the 2nd channel and store in the memory a table of complex coefficients of the form (3). In real mode, this table is used to adjust the results of the FFT for the signal of the 2nd processing channel.

). Кроме того, из фиг. 3 видно, что, как и в прототипе, чувствительность кардиоидного приемника в рабочем секторе углов зависит от частоты. При заданной геометрии приемника для частот ниже частоты, для которой выбрано расстояние между элементами приемника r, его чувствительность на максимуме ХН (в направлении 0°) с приближением к нижней границе частотного диапазона изменяется примерно на минус 6 дБ по закону:From (5) it is seen that, in contrast to the prototype, the non-uniformity of the sensitivity of the proposed cardioid receiver in the frequency range depends on the non-uniformity of the sensitivity of only the first element A ₁ (

) In addition, from FIG. 3 shows that, as in the prototype, the sensitivity of the cardioid receiver in the working sector of the angles depends on the frequency. For a given receiver geometry for frequencies below the frequency for which the distance between the receiver elements r is chosen, its sensitivity at the maximum ХН (in the direction of 0 °) with approaching the lower boundary of the frequency range changes by about minus 6 dB according to the law:

This additional unevenness of the cardioid channel in output, in contrast to the prototype, is compensated in the frequency domain by multiplying by the material coefficients of the correction table (K ₂ coefficients).

Cardioid CN is formed in fact in the frequency domain. At the same time, the output of the cardioid channel is most often used in the time domain. Therefore, after performing correction of the spectrum of the signal of the 2nd channel using complex coefficients and subtracting the values of the spectral samples of the 2nd channel from the values of the samples of the 1st channel, it is necessary to perform the IFFT operation for the difference signal in order to form the output signal of the cardioid channel in the time domain. The FFT of the input signals and the IFFT of the output difference signal must be performed with time overlap by half the length of the array of time samples processed in one FFT cycle and by weighting the array of input samples with a time window in the form of a half-period of the Sin ² (x) function. This procedure eliminates periodic (with a period equal to the FFT length) distortion of the output signal in the form of clicks.

Using the proposed method allows to obtain the following technical results:

- automatic compensation is achieved for differences in the sensitivity of the active elements forming the cardioid receiver and the electronic elements of the first and second processing channels in a wide frequency band; this fact allows the manufacture of the receiver to exclude the selection of the sensitivity of the active elements of the receiver while maintaining a large depth of dip XN;

длины волны на выбранной частоте диапазона) при его изготовлении; это существенно с учетом того, что жесткое крепление элементов приемника перед вулканизацией недопустимо из-за необходимости обеспечения независимых друг от друга колебаний элементов приемника (акустической развязки), а при мягком креплении при вулканизации возможны смещения элементов, т.е. погрешность в величине расстояния между элементами, требующая подстройки величины задержки сигналов второго канала, обеспечивающей максимальную глубину провала ХН;- automatic compensation of the error in setting the required distance between the elements of the receiver (equal to

wavelength at the selected frequency of the range) during its manufacture; this is significant given that the rigid fastening of the receiver elements before vulcanization is unacceptable due to the need for independent oscillations of the receiver elements (acoustic isolation), and with soft fastening during vulcanization, element displacements are possible, i.e. an error in the magnitude of the distance between the elements, requiring adjustment of the delay value of the signals of the second channel, providing the maximum depth of the dip of the CN;

- from the composition of the apparatus that implements the proposed method, the adjustable delay line is excluded, providing the adjustment of the delay value of the signal of the 2 processing channels for 1/4 period;

- the method of correction of the final sensitivity of the cardioid receiving channel allows you to take into account the individual characteristics of changes in a wide frequency band of the sensitivity of the active elements used in the acoustic receiver.

Setting up the receiving channel that implements the proposed method is performed in two stages:

first stage: the receiver is guided in the pool by a dip in the CN in the direction of the broadband emitter; the receiver's electronic equipment is switched on in the mode of forming a table of complex coefficients; using the control equipment and the emitter, the receiver is irradiated with a sequence of tonal pulses with frequencies equal to the center frequencies of the bands into which the entire operating range is divided using FFT; as a result, the required table of complex coefficients is formed in the ROM of the receiver (table K ₁ );

second stage: the receiver focuses on the emitter with a maximum of ХН (direction 0 °); in the receiver sensitivity correction table (table K ₂ ) uniform sensitivity values are recorded; the receiver is again irradiated with a sequence of pulses at different frequencies (as in the first stage) and at the output of the receiver the sensitivity table of the receiver is removed in the working frequency range, on the basis of which a correction table is formed, which is entered in table K ₂ , with which the final sensitivity of the receiver is aligned in the working frequency range.

The proposed tuning technology eliminates the very time-consuming procedures for adjusting the correctors and the delay lines used when setting up the prototype with a significantly higher final result.

In FIG. 5 presents typical examples of the towed cardioid receiver CN in a wide frequency range obtained using the proposed method in real equipment. The dip in the CN was formed in the direction of 0 °, the scale grid was plotted after 5 dB, the frequencies are indicated in the right corner of the diagrams. It can be seen from the diagrams that in a wide frequency band the achieved dip depth in the direction of the interference source does not exceed minus 20 dB. In equipment that implements the classical method of forming CN selected as a prototype, at the edges of the frequency range, the depth of the dip decreases to minus 10 ÷ 12 dB.

List of drawings

FIG. 1. A known method for the implementation of cardioid CN.

Fig 2. The proposed method for the synthesis of cardioid CN.

Fig 3. The results of the calculation of idealized CNs on a linear scale.

Fig 4. The method of forming a table of complex coefficients.

FIG. 5. The directional characteristics obtained during the implementation of the method.

Claims (1)

A method of forming a cardioid directivity characteristic of a broadband sonar receiving channel for an uninhabited underwater vehicle, namely, that the output signals from two active elements of a cardioid receiver are amplified in two processing channels, the signal phase of one of the processing channels is shifted by a quarter of the period for the selected frequency using the delay line operating range and equalize the amplitudes of the signals in two processing channels, form a differential signal and perform amplitude-often the output sensitivity correction of the entire cardioid receiving channel, characterized in that, performing the operations of analog-to-digital conversion and fast Fourier transform of signals in the processing channels, the entire working frequency range of each processing channel is divided into many frequency channels, the number of which is determined by the length performed in both the processing channels of the Fourier transform procedure, the operation of equalizing the amplitudes of the signals in two processing channels and the phase shift of the signal of one of the processing channels in In each frequency channel of the selected processing channel, the result of the Fourier transform is adjusted using a table of constants recorded in the equipment when setting complex coefficients equal to the ratio of the complex frequency samples of the signals received during tuning by two processing channels when the cardioid receiver is irradiated with signals at discrete frequencies due to the length the Fourier transform procedure used, from the direction in which the dip in the response direction should be nnosti reception channel and before terminal procedures inverse Fourier transform and digital to analog converting the output signal form the differential signal processing channels and using a table of valid coefficients corrected sensitivity wideband sonar receiving channel.

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