CN109765562B - Three-dimensional forward-looking acoustic image sonar system and method - Google Patents

Three-dimensional forward-looking acoustic image sonar system and method Download PDF

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CN109765562B
CN109765562B CN201811503846.1A CN201811503846A CN109765562B CN 109765562 B CN109765562 B CN 109765562B CN 201811503846 A CN201811503846 A CN 201811503846A CN 109765562 B CN109765562 B CN 109765562B
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CN109765562A (en
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刘晓东
张东升
程亚楠
王晏宾
吴明明
王凯
曹金亮
王舒文
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Institute of Acoustics CAS
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Abstract

The invention provides a three-dimensional forward-looking sound image sonar (3DFLS) system and a method, wherein the system comprises a sonar cabin, a transmitting linear array, a receiving array and a data processing module; the transmitting array adjusts the width of a vertical transmitting beam by changing a vertical transmitting aperture, so that shallow water environment detection is realized; the receiving array comprises a plurality of layers of receiving circular arrays, the layer spacing is the same, each layer of receiving circular array is provided with a plurality of receiving elements at equal intervals, and the receiving circular arrays are attached to the bottom of the sonar capsule; the data processing module is arranged in the sonar capsule body and used for processing detection data of the transmitting array and the receiving array. The method comprises the steps that the transmitting array sets transmitting beams through the data processing module, the transmitting beams are stably processed, the receiving array processes echo data, coordinates of scattering points in a three-dimensional space are calculated, and three-dimensional acoustic imaging is obtained. The invention adopts the transmitting scheme with adjustable beam width, and is more suitable for different water body environments, thereby improving the detection effect of the 3DFLS and improving the adaptability to the environment.

Description

Three-dimensional forward-looking acoustic image sonar system and method
Technical Field
The invention relates to the technical field of marine acoustic equipment, in particular to a three-dimensional forward-looking sound image sonar system.
Background
With the increasing number of human marine activities, the detection and imaging of underwater targets has become one of the main directions of research in recent years. The three-dimensional forward looking sonar (3DFLS) is mainly used for detecting obstacles, targets and seabed in a three-dimensional space in front of a carrier, and is an important acoustic imaging sonar installed on an underwater carrier. From the scanning form of beam forming, the existing 3DFLS can be divided into two main categories of mechanical scanning type and electronic scanning type: the mechanical scanning type 3DFLS completes scanning of horizontal and vertical dimensions in a mechanical rotation mode so as to complete underwater three-dimensional reconstruction, and the type is generally simple in structure and low in imaging efficiency; the electronic scanning type 3DFLS can be divided into a 3DFLS based on a one-dimensional receiving line array, a 3DFLS based on a two-dimensional area array and beam forming technology, and a 3DFLS based on a two-dimensional area array and a direction of arrival estimation technology. The sparse processing optimizes the design of a planar array, and the two-dimensional area array-based 3DFLS adopting the beam forming technology becomes the current mainstream product, but the data processing of the sonar adopts the conventional beam forming method, the angular resolution is influenced by the array aperture, namely the beam width, and the sonar with larger aperture is needed to obtain high-resolution acoustic imaging; in conventional beam forming, the beam widths in different directional directions are different, and in actual detection, each beam result corresponds to different physical dimensions at the same distance, which generally affects the final acoustic imaging effect.
Disclosure of Invention
The invention aims to solve the problems that the angular resolution is influenced by the array aperture, namely the beam width, and the beam widths in different directional directions are different when the conventional beam forming method is adopted for sonar data processing in the prior art, so that the detection effect of the 3DFLS is improved, and the adaptability of the sonar to the environment is improved. In order to achieve the purpose, the invention provides a high-resolution three-dimensional forward-looking sound image sonar system, which comprises a sonar cabin, a transmitting array and a receiving array, wherein each transmitting element in the transmitting array corresponds to a transmitter, and each receiving element in the receiving array corresponds to a receiver, and the system is characterized by further comprising a data processing module;
the transmitting elements of the transmitting array are arranged in a linear array along the axial direction of the sonar capsule, are tightly attached to the outer wall of the sonar capsule, and are used for adjusting the width of a vertical transmitting wave beam to realize shallow water environment detection;
the receiving array comprises a plurality of layers of receiving circular arrays, the layer spacing is the same, each layer of receiving circular array is provided with a plurality of receiving elements at equal intervals, and the receiving circular arrays are attached to the bottom of the sonar capsule;
the data processing module is arranged in the sonar capsule body and used for processing detection data of the transmitting array and the receiving array.
Based on the high-resolution three-dimensional forward-looking acoustic image sonar system, the invention also provides a three-dimensional forward-looking acoustic imaging method, which comprises the following steps:
step 1) the transmitting array sets transmitting beams through a data processing module, calculates the relative change of the positions of transmitting elements in real time according to the change of the attitude of a carrier by adopting a beam stabilizing method, converts the position change into time delay, compensates in a time domain, and stabilizes the transmitting center in a transmitting preset direction so as to finish the stabilizing treatment of the transmitting beams;
step 2) the receiving array carries out filtering and demodulation preprocessing on the received echo data through a data processing module, and a plurality of receiving beams oriented to the normal direction of a central element of the subarray are formed in the horizontal direction of each layer of elements; and performing pulse compression processing on the wave beams, performing source number estimation and direction of arrival estimation on wave beam signals in the same horizontal direction of each layer on a vertical plane, and calculating coordinates of scattering points in a three-dimensional space by combining the arrival time of echoes to obtain three-dimensional acoustic imaging.
As an improvement of the method, the step 1) specifically includes:
step 1-1) setting a wide beam coverage mode of a single pulse signal or a narrow beam coverage mode of a multi-pulse signal; determining the emission of beams with different widths by setting the number of emission elements used in working;
step 1-2) according to the carrier yaw angle in a horizontal plane at a certain time t
Figure BDA0001898912490000021
Pitch angle P (T), roll angle R (T), initial coordinates T of emission element0Calculating the coordinate T of the carrier after the attitude change exists;
the yaw angle
Figure BDA0001898912490000022
In the horizontal plane in the direction of the carrier headerThe angle deviating from the track direction is positive when the carrier head deviates from the track clockwise from the view from the upper side of the carrier;
the longitudinal inclination angle P (t) is an included angle between the longitudinal direction of the carrier and the horizontal plane, and the head of the carrier is positively lifted when viewed from the upper part of the carrier;
the transverse rolling angle R (t) is an included angle between the transverse direction of the carrier and the horizontal plane, and the left half part of the carrier is positively lifted when viewed from the upper part of the carrier;
the initial coordinate T of the transmitting array element0＝(X0,Y0,Z0)；
And the coordinate of the carrier after the posture change at the moment T is T ═ X, Y and Z:
Figure BDA0001898912490000023
Figure BDA0001898912490000024
Figure BDA0001898912490000025
Figure BDA0001898912490000026
T＝*T0(1)
Figure BDA0001898912490000031
a correction matrix representing the yaw angle at time t,P(t) a correction matrix representing the pitch angle at time t,R(t) a correction matrix for representing the roll angle at the time t, and (t) a coordinate conversion matrix under the posture at the time t;
step 1-3) calculating the time delay amount t of the num primitive relative to the reference pointn：
Figure BDA0001898912490000032
Wherein, the transmitting arrayThe element interval is d, the number of elements in the vertical direction of the linear array is Num, c is the sound velocity,
Figure BDA0001898912490000033
an included angle is formed between a preset direction vector of a transmitting wave beam and a direction vector of a transmitting array with posture change;
step 1-4) setting a reference point at the time t to transmit a signal as s0(t), the emission signal of the p-th primitive after delay compensation is:
snum(t)＝s0(t-tnum) (4)
step 1-5), windowing each transmitting element to form a transmitting beam:
Figure BDA0001898912490000034
wherein, wnumWindow function coefficients for the num primitive:
wnum＝chebwin(N,β) (5)
where β represents the main lobe above the side lobe dB value.
As a modification of the method, the step 2) includes:
step 2-1), filtering out-of-band noise of the original AD data acquired by the receiving element, and reserving signals near the central frequency to obtain anti-aliasing filtered signals;
step 2-2) demodulating the signals subjected to anti-aliasing filtering, and respectively and correspondingly multiplying the signals by carrier signals of one or N central frequencies according to different transmission modes;
step 2-3), low-pass filtering is carried out, redundant frequency band components in the demodulated frequency spectrum signal are filtered, frequency components near a baseband are reserved, and the baseband signal is obtained;
step 2-4), the baseband signals are subjected to beam forming, beam signals from a certain direction are enhanced, beam signals from other directions are attenuated, and a plurality of receiving beams in different directional directions are formed; if the transmission mode is the wide beam transmission mode, executing the step 2-5); if the emission mode is a narrow beam mode and high-resolution detection is needed to be carried out on the target in the narrow beam, executing the step 2-5); performing steps 2-6) if the transmit mode is a narrow beam mode but high resolution detection of the targets within the beam is not required;
step 2-5) solving the vertical direction incident angle through pulse compression processing of the beam and source number estimation and direction of arrival estimation of the vertical plane in the beam
Figure BDA0001898912490000035
Performing steps 2-7);
step 2-6) the directional angle of the emission beam is equivalent to the vertical direction incident angle
Figure BDA0001898912490000036
The directional angle of the received beam is equivalent to the azimuth angle theta0Executing the step 2-7);
and 2-7) calculating a three-dimensional coordinate to obtain three-dimensional acoustic imaging.
As an improvement of the method, the step 2-1) is specifically:
setting parameters required for generating the anti-aliasing filter according to the signal frequency bandwidth and the sampling frequency, wherein the parameters comprise the order M of the filterantiCut-off frequency omeganAnd obtaining MantiOrder anti-aliasing filter coefficient banti：
banti＝fir1(Manti,ωn) (7)
Inputting the original data acquired by the receiving primitive into an anti-aliasing filter to obtain an anti-aliasing filtered signal ADdata _ banti(n), where n denotes the nth sample point, banti(manti) Denotes the m-thantiFilter coefficients of order:
Figure BDA0001898912490000041
as an improvement of the method, the step 2-2) specifically comprises:
step 2-2-1) is to carry out anti-aliasing filtering on the signal ADdata _ banti(n) according to the transmission mode and the center frequency f of the transmission signalcTo carry outDemodulation results in:
Figure BDA0001898912490000042
Figure BDA0001898912490000043
Figure BDA0001898912490000044
in the formula, ωcFor normalizing the digital frequency of the received signal, fcFor receiving the center frequency of the signal, fsFor the current signal sampling frequency, Y (n) is the demodulated mixed signal, and Y (ω) is the spectral representation.
As an improvement of the method, the step 2-3) is specifically: filtering out unnecessary frequency band components in the demodulated mixing signal y (n) to obtain a baseband signal s (n):
Figure BDA0001898912490000045
wherein, blow(mlow) Is m atlowCoefficient of order low-pass filter, MlowIs the low pass filter order.
As an improvement of the method, the step 2-4) specifically comprises:
step 2-4-1) selecting a single-layer baseband signal of M adjacent elements after low-pass filtering output, taking the circle center of the circular arc array as a reference point, and expressing the received data of the mth received element at the t moment as sm(t)：
sm(t)＝Am*s0(t-τm) (16)
τm＝R*cos(θ-αm)/c (17)
Wherein A ismAmplitude response of element m, s0(t) is the output signal of the reference point after low-pass filtering, τmIs the time delay of No. m primitive relative to the reference point, theta is the signal echo direction, c is soundSpeed, alphamThe central angle corresponding to the position of the No. m primitive is as follows:
αm＝(m-1)*αper(18)
corresponding central angle alpha of adjacent elementsper：
αper＝2*arcsin(d/(2*R)) (19)
Wherein R is the radius of the arc array, and d is the distance between the center points of the adjacent elements; in the horizontal direction theta0Obtaining maximum directivity in a direction, i.e. forming a beam Rec _ BF directed in that directioni,j(t), delay compensation is needed to be carried out on the array element, and the received data moment of the mth element after compensation at the t moment is represented as:
sm(t)＝Am*s0(t-τm+τ′m) (20)
τ′m＝R*cos(θ0-αm)/c (21)
τ′mthe delay compensation amount of the m-th primitive is shown.
Step 2-4-2) windowing the received beam formation, wherein the coefficient of the mth primitive window function is bm：
bm＝chebwin(Msub，β) (22)
Wherein M issubThe window length is beta, the main lobe is higher than the side lobe dB value, the received signals of each element after delay compensation and windowing are added are accumulated, and the beam result can be expressed as Rec _ BFi,j(t)：
Figure BDA0001898912490000051
Wherein i ∈ [1, L ]]Is the i-th layer receiving array, has L-layer receiving arrays in total, j is the beam number, Rec _ BFi,jThe jth receive beam formed for the ith receive array.
As an improvement of the method, the step 2-5) specifically comprises:
step 2-5-1) when the transmitting signal is a linear frequency modulation signal, performing pulse compression on the result after the receiving wave beam is formed, and setting an original transmitting signal sL(t) is:
Figure BDA0001898912490000052
b is the bandwidth of the original emission signal, and tau is the pulse width of the original emission signal;
step 2-5-2) Beam Rec _ BFi,jWith local transmission signals sL(t) performing cross-correlation operation to obtain the j wave beam Rec _ BF of the ith layer receiving circular arrayi,j(t) result after pulse compression Compi,j(t)：
Figure BDA0001898912490000053
Superscript "+" indicates conjugation;
step 2-5-3) selecting equivalent channel data after beam forming or pulse compression to carry out DOA (direction of arrival) estimation in the beam, specifically comprising the steps of carrying out vertical direction incident angle estimation by adopting a multi-signal classification algorithm based on characteristic decomposition or carrying out vertical direction incident angle estimation by adopting a rotation invariant subspace algorithm to obtain the vertical direction incident angle of an incident target signal
Figure BDA0001898912490000054
As a modification of the method, the steps 2 to 7) include: according to the pitch angle of the signal
Figure BDA0001898912490000061
Azimuth angle theta0Echo arrival time n0Sum signal sampling frequency fsAnd calculating to obtain two-dimensional plane coordinates x and y and a height value z of the target point, wherein the two-dimensional plane coordinates x and y and the height value z are respectively as follows:
Figure BDA0001898912490000062
Figure BDA0001898912490000063
Figure BDA0001898912490000064
the invention has the advantages that:
1. the three-dimensional forward-looking acoustic image sonar system adopts a transmitting scheme with adjustable beam width, and is more suitable for different water body environments;
2. the three-dimensional forward-looking acoustic image sonar system adopts circular arc array design for receiving, the width of each horizontal wave beam is constant, and the calculation amount of algorithm realization can be greatly reduced by forming the horizontal wave beams;
3. in the receiving process of the three-dimensional forward-looking acoustic image sonar system, the direction of arrival estimation technology is adopted in the vertical direction, so that the limitation of the dimension array aperture on the imaging resolution is reduced, and the number of elements is reduced;
4. the three-dimensional forward-looking sound image sonar system adopts a compact design, and the transmitting and receiving arrays and the acoustic electronic parts are uniformly distributed in the same cylindrical cabin unit, so that the volume is optimized, and the influence of water resistance is reduced.
Drawings
Fig. 1(a) is a perspective view of a sonar array configuration of the present invention;
fig. 1(b) is a top view of a sonar array configuration of the present invention;
fig. 1(c) is a partial enlarged view of a circular arc array of a sonar array according to the present invention;
fig. 2 is a schematic view of a sonar system of the present invention;
FIG. 3 is a signal processing flow diagram of a wide beam transmission mode of the present invention;
FIG. 4 is a signal processing flow diagram of a narrow beam transmit mode of the present invention;
fig. 5(a) is a perspective view of a sonar array embodiment of the present invention;
fig. 5(b) is a top view of a sonar array embodiment of the present invention;
fig. 5(c) is a partial enlarged view of a circular arc array of an embodiment of a sonar array of the present invention;
FIG. 6 is a flow chart of a beam stabilization algorithm of the present invention;
FIG. 7 is a diagram of a vertical multi-layer receive beam in an example of a receive circular array of the present invention;
fig. 8 is a schematic diagram of the estimation of the incoming wave direction in a beam.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
The 3DFLS applicable to the complex environment mainly comprises an emitting linear array, a receiving linear array and a data processing module based on array design.
As shown in fig. 1(a), fig. 1(b) and fig. 1(c), each transmission element in the transmission array linear array corresponds to one transmitter.
The transmitting linear array is composed of a plurality of arc-shaped elements at equal intervals, and the vertical transmitting beam width is adjusted by changing the vertical transmitting aperture, so that the detection of different coverage areas in a shallower water environment is realized.
The receiving array consists of a plurality of layers of circular arc arrays, and the layer intervals are the same; and a plurality of elements are arranged on each layer of circular arc array at equal intervals.
The high-resolution three-dimensional forward-looking sound image sonar system is streamline as a whole: the transmitting linear array is tightly attached to the outer wall of the sonar capsule, the receiving circular array is attached to the bottom of the sonar capsule, and the data processing module is arranged in the sonar capsule.
Based on the structure, the sonar system schematic diagram of the three-dimensional forward-looking sound image sonar system is shown in fig. 2, and the data processing module comprises a transmitting processing sub-module and a receiving processing sub-module;
and the transmission processing submodule is used for determining a transmission mode, realizing transmission beam stabilization and transmission beam forming. Wherein the transmission mode comprises a wide beam coverage mode of the single pulse signal and a narrow beam coverage mode of the multi-pulse signal. The device specifically comprises a transmitting mode determining unit, a transmitting beam stabilizing processing unit and a transmitting beam forming unit.
The receiving processing submodule is used for preprocessing the received data such as filtering and demodulation, forming the received data through a conventional beam forming technology, forming a plurality of receiving beams oriented to different horizontal directions for each layer of receiving array, obtaining an azimuth angle, obtaining a pitch angle through pulse compression processing of the beams and source number estimation and arrival direction estimation technologies of a vertical plane in the beams, and calculating coordinates of scattering points in a three-dimensional space by combining the arrival time of echoes to obtain three-dimensional acoustic imaging.
The data processing module mainly comprises echo data processing aiming at two different emission modes.
As shown in fig. 3, for a wide beam pattern of a single pulse signal, a beam of a certain width is transmitted through the transmitter array using a chirp signal. In the receiving process, received data of each primitive is first preprocessed. Then, through conventional beam forming, only selecting the received data of M adjacent elements of a single layer at a time to form a beam oriented to the normal direction of the central element; sequentially selecting received data of M adjacent elements at different positions to form a plurality of directional beams in different directions; in different receiving beam forming, the time delay and the weighting coefficient are the same. And performing pulse compression processing on each receiving beam result by a matched filtering method. And the vertical angle is obtained in the multilayer receiving beams with the same horizontal angle and the vertical direction through the source number estimation and direction of arrival estimation technologies. And calculating a three-dimensional coordinate to obtain three-dimensional acoustic imaging by combining the azimuth angle and the pitch angle obtained in the process and the arrival time of the echo.
As shown in fig. 4, for a narrow beam pattern of a multi-pulse signal, a beam with a certain width is transmitted through the transmitter array using a simple pulse signal. In the received data preprocessing, the pulse signals with different frequencies are respectively demodulated and filtered, and the pulse signals with different frequencies in the received signals are separated. The beamforming process is as above. The method for estimating the number of information sources and the direction of arrival is the same as above, and the step can be omitted when high-resolution detection on the target in the narrow beam is not needed.
The three-dimensional forward-looking sound image sonar processing system provided by the invention is combined with specific examples to further explain the signal processing process respectively.
As shown in fig. 1(a), which is a schematic diagram of a sonar array structure of the present invention, the array is mainly used for acoustic detection in a small range, the detection range of the transmitting linear array in the horizontal direction is about 120 °, and the coverage ranges of the receiving circular array beams in the horizontal direction are all larger than 90 °.
In actual detection, the array structure can be adjusted as required to realize detection in a wider range, and as shown in fig. 5(a), 5(b) and 5(c), when 3 transmitting linear arrays (attached to the outer wall of the sonar capsule at equal intervals) and transmitting and receiving circular arc array elements cover the central angle of 360 degrees, 360-degree omnibearing water body detection can be realized.
The transmitting processing module mainly comprises a transmitting beam stabilizing processing unit. Aiming at the sonar attitude problem caused by external factors such as water body resistance and the like, a transmitted beam stabilizing algorithm is adopted to enhance the applicability of the sonar array.
As shown in fig. 6, the algorithm flowchart is to calculate the position change of the transmitting element caused by the attitude change in real time, and compensate each transmitting channel by converting the position change into the time delay, so that the center of the transmitting beam is stabilized in the transmitting direction. By using
Figure BDA0001898912490000081
The yaw angle at the moment t is represented, the angle of the direction of the carrier head deviating from the track direction in the horizontal plane is represented, and the clockwise direction of the carrier head deviating from the track direction is set to be positive when viewed from the upper side of the carrier; p (t) represents the trim of the moment t, represents the included angle between the longitudinal direction of the carrier and the horizontal plane, and the lifting of the carrier head is positive when viewed from the upper part of the carrier; and R (t) represents the rolling at the time t, represents the included angle between the transverse direction of the carrier and the horizontal plane, and the left half part of the carrier is positively lifted when viewed from the upper part of the carrier. When the carrier has attitude change, the initial coordinate T of the transmitting array element0＝(X0,Y0,Z0) The pose coordinate T can be transformed by the following equation:
Figure BDA0001898912490000082
Figure BDA0001898912490000083
Figure BDA0001898912490000084
Figure BDA0001898912490000091
T＝*T0(1)
wherein
Figure BDA0001898912490000092
A correction matrix representing the yaw angle at time t,P(t) a correction matrix representing the pitch angle at time t,Rand (t) represents a correction matrix of the roll angle at the time t, and (t) represents a coordinate transformation matrix in the posture at the time t. Each transmitting element after the transmitting beam stabilizing treatment is carried out, and the time delay of each transmitting element is composed of two parts: the delay added by the transmitting beam directivity and the delay introduced by the position change of the element caused by the attitude change of the carrier. The method has the advantages that the change of the position of the element caused by the change of the attitude of the carrier is converted into time delay, and the time delay correction is carried out on the transmitting channel so as to achieve the purpose of stabilizing the transmitting beam.
Calculating the included angle between the expected direction vector of the transmitting wave beam and the direction vector of the transmitting array with the changed posture by using the posture data at the transmitting moment
Figure BDA0001898912490000095
And obtaining the time delay of each element relative to the reference point according to the linear relation. If the interval of the transmitting array elements is d, the number of the elements in the vertical direction of the linear array is N, and the delay amount of the num element is tnum：
Figure BDA0001898912490000093
When the transmit beam is formed, time domain compensation should be performed on each transmit element based on the desired angle of the transmit beam. If the reference point emission signal is s (t):
s(t)＝s0(t) (3)
of the nth primitiveThe transmission signal is sn(t)：
snum(t)＝s0(t-tnum) (4)
Windowing is adopted in the transmission beam forming, and side lobes are reduced. Taking the Chebyshev window as an example, the window function coefficient is w:
wnum＝chebwin(N,β) (5)
where β represents the main lobe above the side lobe dB value.
In conjunction with transmit beam stabilization, windowing, transmit beam forming can be expressed as:
Figure BDA0001898912490000094
the beam width is smaller as the aperture (ratio of the size of the matrix to the wavelength) of the matrix is larger. The transmitting aperture is adjusted by setting the number of elements actually used in work so as to realize the transmission of beams with different widths; in practical work, the number of the used elements can be set to be 4-80, and the width range of the emitted beam is about 1.5-40 degrees.
The data processing of the received echoes is slightly different for different transmit modes. The processing methods used in the signal processing process are further described below.
As shown in fig. 2, the anti-aliasing filtering unit mainly passes the original AD data acquired by the receiving primitive through an anti-aliasing filter to retain signals near the signal center frequency and filter out-of-band noise; the anti-aliasing filter coefficients can be set and generated as required in matlab, where the filter coefficient is assumed to be bantiIn total of MantiStep (2):
banti＝fir1(Manti,ωn) (7)
wherein M isantiOrder of the anti-aliasing filter, ωnIs a cut-off frequency set according to the signal frequency and the sampling frequency. After the filter is used for filtering the original ADdata data, an anti-aliasing filtered signal is obtained, and the method comprises the following steps:
Figure BDA0001898912490000101
in the formula, ADdata is original AD data, ADdata _ bantiIs the anti-aliasing filtered output.
The demodulation unit functions to acquire a baseband signal. If the center frequency of the received signal is fc，fsFor the current signal sampling frequency, omegacFor the digital frequency of the normalized received signal, the anti-aliasing filtered output signal can be regarded as the baseband signal s (n) and the center frequency fcThe result of the multiplication of the carrier signals of (a) is:
Figure BDA0001898912490000102
Figure BDA0001898912490000103
if S (n) spectrum is S (ω), ω represents the digital domain frequency. According to the frequency shift characteristic of Fourier transform, ADdata _ banti(n) the frequency spectrum X (ω) is:
Figure BDA0001898912490000104
theoretically, the filtered received signal is multiplied by
Figure BDA0001898912490000105
The signal spectrum can be shifted to the vicinity of zero frequency to obtain a mixed signal y (n):
Figure BDA0001898912490000106
spectrum of Y (n) is Y (ω):
Figure BDA0001898912490000107
different transmission modes, corresponding to carrier signals of different center frequencies: and for the wide beam transmission mode, corresponding to one central frequency, but for the narrow beam transmission mode, corresponding to N central frequencies, and in the demodulation process, correspondingly multiplying the carrier signals of the N central frequencies respectively to obtain the demodulation result of the corresponding signal frequency.
The mixed signal comprises a baseband signal and a high-frequency signal, and a required frequency band signal can be obtained after filtering processing.
The low-pass filtering unit is used for filtering out redundant frequency band components, only frequency components near a baseband are reserved, a filtering coefficient can be directly generated in matlab according to the basic requirements of a required filter, and the filtering coefficient is assumed to be blowIn total of MlowAnd (4) carrying out step. The filtered output is the required baseband data s (n):
Figure BDA0001898912490000108
wherein, blow(mlow) Is m atlowCoefficient of order low-pass filter, MlowIs the low pass filter order.
The beam forming unit is used for enhancing signals from a certain direction and attenuating signals from other directions, so that the beam orientation is realized. The beam forming is equivalent to a filter in a spatial domain, for a uniform receiving array, a far-field signal from a theta direction is considered to be incident to the receiving array in parallel, a single-time transmitting and receiving signal is taken as a simple pulse as an example, the circle center of an arc array is taken as a reference point, and if a receiving signal s (t) of the reference point is:
s(t)＝s0(t) (15)
the received signal s of the mth primitivem(t) is:
sm(t)＝Am*s0(t-τm) (16)
τm＝R*cos(θ-αm)/c (17)
wherein A ismFor amplitude response of element No. m, τmIs the time delay of the No. m element relative to a reference point, c is the sound velocity, R is the radius of the circular arc array, theta is the signal echo direction, alphamCorresponding to the position of the No. m primitiveCenter angle (direction with cells numbered 1,2, … … clockwise and with the direction through the center of the circle and passing through cell 1 set to 0 °):
αm＝(m-1)*αper(18)
corresponding central angle alpha of adjacent elementsperRelated to cell pitch, αperComprises the following steps:
αper＝2*arcsin(d/(2*R)) (19)
wherein d is the distance between the center points of adjacent elements;
in the horizontal direction theta0Obtaining maximum directivity in a direction, i.e. forming a beam Rec _ BF directed in that directioni,j(t), delay compensation is needed to be carried out on the array element, and the received data moment of the mth element after compensation at the t moment is represented as:
sm(t)＝Am*s0(t-τm+τ′m) (20)
τ′m＝R*cos(θ0-αm)/c (21)
τ′mthe delay compensation amount of the m-th primitive is shown.
In order to suppress the influence of side lobes in the beam forming process, windowing can be carried out on the primitive received data; selecting different window functions according to different main and side lobe requirements, wherein only Chebyshev window weighting and weighting coefficient b are discussedmComprises the following steps:
bm＝chebwin(Msub，β) (22)
where β represents the main lobe above the side lobe dB value.
Under the condition of not controlling the incoming wave direction, the received signals of all the receiving elements are weighted and then directly summed, so that the directivity in the center direction of the corresponding circular arcs of the M elements is the largest. If desired to be in a certain direction theta0Obtaining the maximum directivity, adding a time delay term to the received signal of each element, compensating the acoustic path difference, adjusting the amplitude of the main and side lobes through a window function, and then accumulating the signals to obtain the directional theta0The directional beam output is Rec _ BFi,j(t)：
Figure BDA0001898912490000121
At this time, the receiving circular arc array is equivalently rotated, so that theta is enabled0The maximum directivity is obtained in the direction of the center of the arc. Wherein i ∈ [1, L ]]Denotes the i-th layer receiving array, j is the beam number, BFi,j(t) is the jth receive beam, τ ', formed by the ith receive array'mAnd the delay compensation quantity of the array element m is shown. To meet the 1.5 degree open angle requirement for the receive beam, M56 was used in the simulation as the number of elements used for a single beamforming.
The pulse compression unit is used for matched filtering; for chirp signal s (t), its impulse response to matched filtering h (t) is:
h(t)＝s*(t0-t) (24)
wherein t is0Is the additional time delay that the filter can achieve, the upper corner "+" indicates the conjugate. Therefore, pulse compression of the received chirp signal is equivalent to receiving the signal sr(t) and a transmission signal st(t) performing a cross-correlation operation Comp (t):
Figure BDA0001898912490000122
in practical applications, the demodulation makes the processed signals all baseband data, so the local transmission signal should be regenerated as the transmission signal s in the above formulat(t) pulse compressing with the processed data. Generating a local chirp transmitting signal s according to a signal bandwidth B and a signal pulse width tau in an original transmitting signalL(t) is:
Figure BDA0001898912490000123
using the resultant data after beam forming as a received signal sr(t), and a local chirp signal sL(t) performing a cross-correlation operation to obtain a compressed pulse beam Compi,j(t)：
Figure BDA0001898912490000124
Result Compi,jAnd (t) is the result of pulse compression of the j beam of the ith receiving circular array.
In the actual processing, when the transmitting signal is a linear frequency modulation signal, pulse compression processing is required to improve the signal-to-noise ratio and the distance resolution of the transmitting signal; if the transmitted signal is a simple pulse signal, this step can be omitted, and the direction of arrival estimation is directly performed on the result after the beam forming. Only pulse compression in the wide beam mode is required.
After the beam forming is completed in the horizontal direction, the number of simultaneously arriving information sources can be greatly reduced by extremely narrow beams, the direction-of-arrival estimation technology is adopted in the vertical plane, and the vertical direction incident angle is obtained by adopting a subspace algorithm based on the feature decomposition, wherein only a multiple signal classification (MUSIC) algorithm and a rotation invariant subspace (ESPRIT) algorithm based on the feature decomposition are briefly introduced here.
The principle of the MUSIC algorithm is: and dividing the space for parameter estimation based on the orthogonality of the signal subspace and the noise subspace. Assuming that a mathematical model based on a far-field narrow-band independent target signal source is X:
X＝AS+N (28)
wherein X is snapshot data received by the antenna, A represents a manifold matrix of the array, S is a space incident target signal, and N represents an array received noise data vector.
The covariance matrix R of the received data of the antenna array can be obtained and characterized by the following formula (24):
Figure BDA0001898912490000131
wherein U isSIs formed by a large eigenvalue lambdaiI-1, 2, …, M corresponding to the feature vector spans the target signal subspace, and UNThen by the small eigenvalue λiI is M +1, M +2, …, N corresponds to the feature vector spanned noise subspace. In actual data processing, however, the covariance matrix is selected
Figure BDA0001898912490000132
Instead of the data covariance matrix, i.e.:
Figure BDA0001898912490000133
due to the incident target signal subspace USThe space spanned by the array manifold matrix A of the antennas is the same space, and under ideal conditions, the signal subspace USSum noise subspace UNAre orthogonal to each other; i.e. the array flow pattern matrix A equivalent to an antenna is orthogonal to the noise subspace UN. Thus, any one guide vector in the antenna array flow pattern matrix A can be obtained
Figure BDA0001898912490000134
Same noise subspace UNAre all orthogonal to each other, i.e.:
Figure BDA0001898912490000135
the difference between the actual condition and the ideal condition makes the guide vector in practice
Figure BDA0001898912490000136
And noise subspace UNNot completely orthogonal. So a minimal optimization search can be used to accomplish the DOA estimation in practice, namely:
Figure BDA0001898912490000137
thereby obtaining the spatial spectrum estimation of the MUSIC algorithm;
Figure BDA0001898912490000138
by scanning all angles of the space, if there is an incident target signal source at a certain angle, the subspace is noisy due to the steering vectorThe orthogonal characteristic of (2) is that a sharp spectral peak is shown on the curve of the spatial spectrum function. Angle corresponding to spectrum peak
Figure BDA0001898912490000139
I.e. the incident angle of the incoming wave of the incident target signal in the vertical direction
Figure BDA00018989124900001310
The example 4-layer receiving circular array has the same horizontal receiving beam as that in fig. 7. And selecting equivalent 4 channel data after beam forming or pulse compression to estimate the direction of arrival in the beam according to different transmission modes.
The principle of the ESPRIT algorithm is: it is believed that a fixed spacing exists between adjacent subarrays, which reflects the rotationally invariant characteristics of each adjacent subarray. Assuming that there are two identical sub-arrays and the spacing between the sub-arrays is known, the outputs of the two sub-arrays have only one phase difference phi for the same signal1Then, the received signals of the two sub-arrays are:
Figure BDA00018989124900001311
in the formula: s is a transmitting signal, A is a flow pattern matrix of a space array, and the rotation is invariable
Figure BDA00018989124900001312
Figure BDA00018989124900001313
The array flow pattern A of the subarray 11Array pattern A of subarray 22＝Aφ，
Figure BDA00018989124900001314
The method is a merging form of two subarray array flow patterns, and the noise N is generally zero-mean white Gaussian noise and is uncorrelated with signals.
The covariance matrix R of the received signal is subjected to eigen decomposition to obtain:
Figure BDA0001898912490000141
in the formula: e [. C]Represents a computational mathematical expectation, {. The }HRepresenting a conjugate transpose operation, Σ s being a diagonal matrix formed by large eigenvalues, UsIs a signal subspace spanned by the feature vectors corresponding to the large feature values; sigma n being a diagonal matrix of small eigenvalues, UNIs a noise subspace spanned by the feature vectors corresponding to the small feature values. There is a unique non-singular matrix T such that UsAT, relationship A of flow patterns by arrays2＝A1φ, one can deduce:
Us2＝Us1T-1φT＝Us1ψ (36)
so that the rotation invariant relation matrix phi is only obtained as T psi T-1Then the incident angle of the signal in the vertical direction can be calculated
Figure BDA0001898912490000142
The calculation formula is as follows:
Figure BDA0001898912490000143
as shown in fig. 7, in the embodiment, the 4-layer receiving circular array, a receiving beam diagram in the same horizontal direction, and for different transmission modes, equivalent 4 channel data after beam forming or pulse compression are selected for estimation of the direction of arrival in the beam. If oriented at θ for the horizontal direction as shown in FIG. 80The multiple beam results are subjected to direction-of-arrival estimation to obtain an incident angle in the vertical direction of the signal of
Figure BDA0001898912490000144
Combining echo arrival times n0Sum signal sampling frequency fsThe two-dimensional plane coordinates x, y and the height value z of the target point can be calculated as follows:
Figure BDA0001898912490000145
Figure BDA0001898912490000146
Figure BDA0001898912490000147
finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and are not limited. Although the present invention has been described in detail with reference to the embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (10)

1. A three-dimensional forward-looking sound image sonar system comprises a sonar cabin, a transmitting array and a receiving array, wherein each transmitting element in the transmitting array corresponds to a transmitter, and each receiving element in the receiving array corresponds to a receiver;
the emission elements of the emission array are arranged in a linear array along the axial direction of the sonar capsule and are tightly attached to the outer wall of the sonar capsule;
the receiving array comprises a plurality of layers of receiving circular arrays, the layer spacing is the same, each layer of receiving circular array is provided with a plurality of receiving elements at equal intervals, and the receiving circular arrays are attached to the bottom of the sonar capsule;
the data processing module is arranged in the sonar capsule body and used for setting the number of transmitting elements actually used in work so as to adjust the width of a vertical transmitting beam and processing the transmitting beam by adopting a beam stabilizing method; the method is used for forming a plurality of receiving beams oriented to the normal direction of a central element of a subarray in the horizontal direction of each layer of elements, then carrying out source number estimation and arrival direction estimation on beam signals of each layer in the same horizontal direction in a vertical plane, and finally obtaining three-dimensional acoustic imaging.
2. A three-dimensional forward looking acoustic imaging method implemented based on the three-dimensional forward looking acoustic image sonar system of claim 1, the method comprising:
step 1) the transmitting array sets transmitting beams through a data processing module, calculates the relative change of the positions of transmitting elements in real time according to the change of the attitude of a carrier by adopting a beam stabilizing method, converts the position change into time delay, compensates in a time domain, and stabilizes the transmitting center in a transmitting preset direction so as to finish the stabilizing treatment of the transmitting beams;
step 2) the receiving array carries out filtering and demodulation preprocessing on the received echo data through a data processing module, and a plurality of receiving beams oriented to the normal direction of a central element of the subarray are formed in the horizontal direction of each layer of elements; and performing pulse compression processing on the wave beams, performing source number estimation and direction of arrival estimation on wave beam signals in the same horizontal direction of each layer on a vertical plane, and calculating coordinates of scattering points in a three-dimensional space by combining the arrival time of echoes to obtain three-dimensional acoustic imaging.
3. The three-dimensional anterior optoacoustic imaging method of claim 2, wherein the step 1) specifically comprises:
step 1-1) setting a wide beam coverage mode of a single pulse signal or a narrow beam coverage mode of a multi-pulse signal; determining the emission of beams with different widths by setting the number of emission elements used in working;
step 1-2) according to the carrier yaw angle in a horizontal plane at a certain time t
Figure FDA0002594746560000011
Pitch angle P (T), roll angle R (T), initial coordinates T of emission element0Calculating the coordinate T of the carrier after the attitude change exists;
the yaw angle
Figure FDA0002594746560000012
The angle of the carrier head direction deviating from the track direction in the horizontal plane is shown, and the carrier head clockwise deviates from the track direction and is positive when viewed from the upper side of the carrier;
the longitudinal inclination angle P (t) is an included angle between the longitudinal direction of the carrier and the horizontal plane, and the head of the carrier is positively lifted when viewed from the upper part of the carrier;
the transverse rolling angle R (t) is an included angle between the transverse direction of the carrier and the horizontal plane, and the left half part of the carrier is positively lifted when viewed from the upper part of the carrier;
the initial coordinate T of the transmitting array element0＝(X0,Y0,Z0)；
And the coordinate of the carrier after the posture change at the moment T is T ═ X, Y and Z:
Figure FDA0002594746560000021
Figure FDA0002594746560000022
Figure FDA0002594746560000023
Figure FDA0002594746560000024
T＝*T0(1)
Figure FDA0002594746560000025
a correction matrix representing the yaw angle at time t,P(t) a correction matrix representing the pitch angle at time t,R(t) a correction matrix for representing the roll angle at the time t, and (t) a coordinate conversion matrix under the posture at the time t;
step 1-3) calculating the time delay amount t of the num primitive relative to the reference pointnum：
Figure FDA0002594746560000026
Wherein, the interval of the transmitting array elements is d, the number of the elements in the vertical direction of the linear array is Num, c is the sound velocity,
Figure FDA0002594746560000027
an included angle is formed between a preset direction vector of a transmitting wave beam and a direction vector of a transmitting array with posture change;
step 1-4) setting a reference point at the time t to transmit a signal as s0(t), the emission signal of the p-th primitive after delay compensation is:
snum(t)＝s0(t-tnum) (4)
step 1-5), windowing each transmitting element to form a transmitting beam:
Figure FDA0002594746560000028
wherein, wnumWindow function coefficients for the num primitive:
wnum＝chebwin(N,β) (5)
where β represents the main lobe above the side lobe dB value.
4. The three-dimensional anterior optoacoustic imaging method of claim 2, wherein the step 2) comprises:
step 2-1), filtering out-of-band noise of the original AD data acquired by the receiving element, and reserving signals near the central frequency to obtain anti-aliasing filtered signals;
step 2-2) demodulating the signals subjected to anti-aliasing filtering, and respectively and correspondingly multiplying the signals by carrier signals of one or N central frequencies according to different transmission modes;
step 2-3), low-pass filtering is carried out, redundant frequency band components in the demodulated frequency spectrum signal are filtered, frequency components near a baseband are reserved, and the baseband signal is obtained;
step 2-4), the baseband signals are subjected to beam forming, beam signals from a certain direction are enhanced, beam signals from other directions are attenuated, and a plurality of receiving beams in different directional directions are formed; if the transmission mode is the wide beam transmission mode, executing the step 2-5); if the emission mode is a narrow beam mode and high-resolution detection is needed to be carried out on the target in the narrow beam, executing the step 2-5); performing steps 2-6) if the transmission mode is a narrow beam mode but high resolution detection of the target in the beam is not required;
step 2-5) solving the vertical direction incident angle through pulse compression processing of the beam and source number estimation and direction of arrival estimation of the vertical plane in the beam
Figure FDA0002594746560000031
Performing steps 2-7);
step 2-6) the directional angle of the emission beam is equivalent to the vertical direction incident angle
Figure FDA0002594746560000032
The directional angle of the received beam is equivalent to the horizontal incident angle theta0Executing the step 2-7);
and 2-7) calculating a three-dimensional coordinate to obtain three-dimensional acoustic imaging.
5. The three-dimensional anterior opthalmic imaging method according to claim 4, wherein the step 2-1) is specifically:
setting parameters required for generating the anti-aliasing filter according to the signal frequency bandwidth and the sampling frequency, wherein the parameters comprise the order M of the filterantiCut-off frequency omeganAnd obtaining MantiOrder anti-aliasing filter coefficient banti：
banti＝fir1(Manti,ωn) (7)
Inputting the original data acquired by the receiving primitive into an anti-aliasing filter to obtain an anti-aliasing filtered signal ADdata _ banti(n), where n denotes the nth sample point, banti(manti) Denotes the m-thantiFilter coefficients of order:
Figure FDA0002594746560000033
wherein, ADdata is the raw data obtained by the receiving primitive, ADdata (n-m)anti) Is the (n-m) thanti) Raw data of each sample point.
6. The three-dimensional anterior opthalmic imaging method according to claim 5, characterized in that the step 2-2) comprises in particular:
step 2-2-1) is to carry out anti-aliasing filtering on the signal ADdata _ banti(n) according to the transmission mode and the center frequency f of the transmission signalcAnd demodulating to obtain:
Figure FDA0002594746560000034
Figure FDA0002594746560000035
Figure FDA0002594746560000041
in the formula, ωcFor normalizing the digital frequency of the received signal, fcFor receiving the center frequency of the signal, fsFor the current signal sampling frequency, Y (n) is the demodulated mixed signal, whose spectrum is denoted as Y (ω).
7. The three-dimensional anterior opthalmic imaging method according to claim 6, wherein the step 2-3) is specifically: filtering out unnecessary frequency band components in the demodulated mixing signal y (n) to obtain a baseband signal s (n):
Figure FDA0002594746560000042
wherein, blow(mlow) Is m atlowCoefficient of order low-pass filter, MlowIs the low pass filter order.
8. The three-dimensional foresight acoustic imaging method of claim 7, wherein the steps 2-4) specifically include:
step 2-4-1) selecting single-layer baseband signals of M adjacent elements after low-pass filtering outputThe number is delayed and compensated by taking the center of the circular arc array as a reference point, and the received data of the mth receiving element at the time t is expressed as sm(t)：
sm(t)＝Am*s0(t-τm) (16)
τm＝R*cos(θ-αm)/c (17)
Wherein A ismAmplitude response of element m, s0(t) is the output signal of the reference point after low-pass filtering, τmIs the time delay of the No. m element relative to a reference point, theta is the signal echo direction, c is the sound velocity, alphamThe central angle corresponding to the position of the No. m primitive is as follows:
αm＝(m-1)*αper(18)
corresponding central angle alpha of adjacent elementsper：
αper＝2*arcsin(d/(2*R)) (19)
Wherein R is the radius of the arc array, and d is the distance between the center points of the adjacent elements; in the horizontal direction theta0Obtaining maximum directivity in a direction, i.e. forming a beam Rec _ BF directed in that directioni,j(t), delay compensation is needed to be carried out on the array element, and the received data moment of the mth element t after the delay compensation is represented as:
sm(t)＝Am*s0(t-τm+τ′m) (20)
τ′m＝R*cos(θ0-αm)/c (21)
τ′mthe delay compensation quantity of the No. m primitive is represented;
step 2-4-2) windowing the received beam formation, wherein the coefficient of the mth primitive window function is bm：
bm＝chebwin(Msub，β) (22)
Wherein M issubThe window length is beta, the main lobe is higher than the side lobe dB value, the received signals of each element which is added with delay compensation and subjected to window processing are accumulated, and the processed beam result can be expressed as Rec _ BFi,j(t)：
Figure FDA0002594746560000051
Wherein i ∈ [1, L ]]Is the i-th layer receiving array, has L-layer receiving arrays in total, j is the beam number, Rec _ BFi,jThe jth receive beam formed for the ith receive array.
9. The three-dimensional anterior opthalmic imaging method according to claim 8, characterized in that said step 2-5) comprises in particular:
step 2-5-1) when the transmitting signal is a linear frequency modulation signal, performing pulse compression on the result after the receiving wave beam is formed, and setting an original transmitting signal sL(t) is:
Figure FDA0002594746560000052
b is the bandwidth of the original emission signal, and tau is the pulse width of the original emission signal;
step 2-5-2) Beam Rec _ BFi,jWith local transmission signals sL(t) performing cross-correlation operation to obtain the j wave beam Rec _ BF of the ith layer receiving circular arrayi,j(t) result after pulse compression Compi,j(t)：
Figure FDA0002594746560000053
Superscript "+" indicates conjugation;
step 2-5-3) selecting equivalent channel data after beam forming or pulse compression to carry out DOA (direction of arrival) estimation in the beam, specifically comprising the steps of carrying out vertical direction incident angle estimation by adopting a multi-signal classification algorithm based on characteristic decomposition or carrying out vertical direction incident angle estimation by adopting a rotation invariant subspace algorithm to obtain the vertical direction incident angle of an incident target signal
Figure FDA0002594746560000054
10. According to the rightThe three-dimensional anterior optoacoustic imaging method of claim 9, wherein the steps 2-7) comprise: according to the pitch angle of the signal
Figure FDA0002594746560000055
Azimuth angle theta0Echo arrival time n0Sum signal sampling frequency fsAnd calculating to obtain two-dimensional plane coordinates x and y and a height value z of the target point, wherein the two-dimensional plane coordinates x and y and the height value z are respectively as follows:
Figure FDA0002594746560000056
Figure FDA0002594746560000057
Figure FDA0002594746560000058
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