RU2728512C1 - Method of mapping using circular antenna array - Google Patents

Abstract

FIELD: acoustics.

SUBSTANCE: invention relates to near-field location and can be used for acoustic waves tomography during monochromatic probing of surrounding space. In method of mapping by means of annular antenna array monochromatic probing signal is emitted, reflected signal is received. At that, the space scanning the focus is controlled by means of the fixed annular antenna array by setting the calculated initial phases on each radiating element of the annular antenna array, forming an image matrix by distributing a received signal by the range elements, in accordance with the reconstruction region, and matrix conversion into brightness signals to obtain section image of tomographed object.

EFFECT: invention simplifies the data recording system during tomography and high image quality.

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Description

The proposed invention relates to the field of near ranging and can be used for tomography on acoustic waves with monochromatic sensing of the surrounding space.

The known method of mapping the earth's surface, which is an analogue used in side-looking radar stations (radar), provides for the emission of a coherent pulsed sounding signal, gating of the reflected signal in time, that is, the distribution of the signal along the range channels due to gating, compression of the processed signal due to the synthesis of the aperture in each range channel, modulation of the optical beam in terms of brightness by the amplitude of the received signal and scanning the image on a photographic film in coordinates of distance from the projection of the trajectory to the ground and the distance along the trajectory.

This method is described in the book by A.P. Reutova, B.A. Mikhailova, G.S. Kondratenkova, B.V. Boyko. Side-looking radar stations. M .: Sov. radio, 1970, p. 98-107, and a digital device that implements the well-known method of mapping is described in detail in the book by V.N. Antipova, V.G. Goryainova et al. Radar stations with digital synthesis of the antenna aperture. M .: Radio and communication, 1988, p. 61, fig. 28. This device makes it possible to obtain an ultra-narrow beam due to the synthesis of the aperture and, due to the narrow beam, to examine in detail small-sized objects on the earth's surface. This device can be considered an analogue of the proposed method of mapping.

The known method works as follows. The pulse signal received by the receiver is distributed over the range channels using gating cascades. Then, in each channel, the signals are converted into digital samples using analog-to-digital converters (ADCs), and a sample from the N-samples is entered into memory. In each range channel, a Fast Fourier Transform (FFT) block converts the signal samples into spectrum samples.

Next, the samples of the same name of the input spectrum and the coefficients of the support function are multiplied. The result of multiplying samples of the same name is subjected to the inverse fast Fourier transform (IFFT) and the resulting complex samples of the convolution signal are taken modulo. Then the readings go to the digital display system (DSS), with the help of which the image is reconstructed.

A known method of mapping objects to obtain their internal structure for tomography (patent No. 2066060 dated February 16, 1993, taken as a prototype). This method uses a moving Doppler locator with a monochromatic sounding signal. Emitting and receiving antennas are located side by side, practically at the same point in space. With this arrangement of antennas, the location is called single-position. The signal reflected from the object with a Doppler frequency shift is mixed on a nonlinear element with a monochromatic probe signal and, after low-frequency filtering, a trajectory Doppler signal is extracted. The trajectory Doppler signal reflected from the object is processed by the correlation method using pre-calculated reference trajectory signals for point reflectors located at different distances with a certain step. In this way, range channels are organized. As a result of correlation processing, an antenna aperture with a tunable focal length is synthesized. With the help of a tunable focus, the received signal is distributed over the range channels. In the range channels, the amplitude of the correlation function is encoded into pixels that differ in brightness. These pixels unfold in focal length and distance coordinates along the trajectory. The result is a cross-sectional picture of the object.

If the signal propagates in the material of the object, then the internal structure of the object can be obtained.

For the purposes of tomography, ultrasonic vibration is most suitable, penetrating well into the human body. The advantage of a monochromatic probe signal is that it eliminates dispersion distortions that are necessarily present when using broadband pulsed probing signals. The essence of dispersion distortion is that the components of the spectrum of a broadband pulse signal propagate in the human body at an unequal speed, which leads to blurring of a short pulse and, ultimately, to image distortion. Let's list the features borrowed from the prototype: 1. Radiation of a monochromatic probe signal; 2. Reception of the reflected signal; 3. Formation by calculating reference signals for each range; 4. Distribution of the trajectory signal over K ranges due to the correlation of the received trajectory signal with the reference trajectory signals; 5. Formation of a matrix from a set of correlation functions for different ranges; 6. Converting the matrix into luminance signals to obtain a cross-sectional image of the tomographic object.

The disadvantage of the known method is that the synthesis of the aperture requires a trajectory Doppler signal, which is formed as a result of the movement of the locator. In addition, the moving locator must be in contact with the patient in order to carry out penetrating sensing. This opportunity appears if the patient is placed in an aqueous environment. The locator, when moving in an aquatic environment, provokes surface waves, from which ultrasonic sounding vibrations are reflected. This noise leads to distortion in the reconstructed image.

The second significant drawback is that the patient must be placed in an aquatic environment. This is a very uncomfortable situation for the patient.

These disadvantages are eliminated if the movement of the locator is eliminated and the synthesis of the aperture is abandoned. It is known that by synthesizing the antenna aperture, it is possible not only to make an infinitely thin antenna beam, but also to focus the energy of the emitter at a given distance, just as it is done with a lens. In addition, the synthesized aperture allows you to control the position of the focus in space. This is done with reference path signals calculated mathematically for point features. Thus, by controlling the focus, you can scan the surrounding space around the aperture synthesis path. By placing the focus inside the tomographic object, you can scan the insides, build a picture of the internal structure of the object.

The task (technical result) of the proposed invention is:

1. Elimination of movement of registration elements, that is, elimination of a moving locator, which allows replacing the aqueous medium with a gel.

2. Exclusion of placing the patient in the aquatic environment.

3. Simplification of the data registration system during tomography due to the absence of moving elements of the registration apparatus.

4. Improving the quality of the image, due to the elimination of surface waves left by a moving locator in the aquatic environment.

The task is achieved by the fact that in the known method, which consists in the fact that a monochromatic probe signal is emitted, the reflected signal is received, focus is controlled using a synthesized aperture, the image matrix is formed by distributing the signal received by the focus among the range elements, the matrix is converted into luminance signals to obtain a cross-sectional image of the tomographic object. The difference of the proposed method is that in order to eliminate image distortions due to the formation of surface waves when the locator moves, to eliminate the patient's placement in the aquatic environment, which creates inconveniences for the patient when collecting data for tomography, the spatial position of the focus is controlled using a ring antenna array by setting the calculated initial phases on each radiating element of the annular antenna array.

The proposed method also assumes the use of a monochromatic probe signal and focus control for scanning the surrounding space, but this scanning is done not using an aperture synthesis, but using a real ring antenna array. The energy of the even elementary emitters of the annular antenna array is concentrated inside the annular antenna of FIG. 1. If the phases of the waves emitted by each element of the antenna array are the same, then the focus of energy concentration will be in the center of the circle. At the center of the circle in FIG. 1, the arriving waves from individual antenna elements of the array will add up in phase (1) at the central focus and give a burst of amplitude in Fig. 2. Mathematically, this can be explained as follows.

Where ϕ _i = ϕ ₁ = ϕ ₂ = ϕ ₃ = ϕ ₄ =…, Phases of waves (signals) at the output of individual elements of the annular antenna array.

s _i = s ₁ = s ₂ = s ₃ =… Amplitudes of waves (signals) at the output of individual elements of the annular antenna array.

Suppose there is a reflective point in focus. Then the wave concentrated in focus will be reflected and in the form of a spherical wave will simultaneously reach all odd elements of the antenna array. After summing up all the signals received by the odd elements, we get a burst of the amplitude of the reflected signal coming from the focus.

FIG. 1 denoted ϕ _i = ϕ ₁ = ϕ ₂ = ϕ ₃ = ϕ ₄ =…, phase shifters, U _y is the voltage that controls the phase of the phase shifter.

The annular antenna array contains an even number of elements. The even elements of the antenna array with phase shifters emit waves, and the odd ones receive reflected signals from the tomographic objects located inside the annular antenna array.

If the phases of the emitted waves by each element are different and are calculated and set in accordance with expression (2), then the focus of energy concentration will be shifted from the center of the circle in FIG. 1 and FIG. 3. In this case, the phases of the waves from the individual antenna emitters will be phased not in the center of the circle, but elsewhere in FIG. 3 and will give an amplitude burst at the shifted focus of FIG. 3. The phase distribution in each i-th element of the antenna array for the off-center focus is calculated by the formula (2)

R_i - расстояние от k-го элемента матрицы изображения до i-го элемента кольцевой антенной решетки, s_i - амплитуда волны на выходе i-го элемента кольцевой антенной решетки, R₀ - радиус кольцевой антенной решетки, δ_k - расстояние от центра кольцевой антенной решетки до k-го элемента матрицы изображения, λ - длина ультразвуковой волны, θ_i - меняющийся угол между δ_k и R_i при смене i-го элемента кольцевой антенной решетки.Where

R _i is the distance from the k-th element of the image matrix to the i-th element of the annular antenna array, s _i is the wave amplitude at the output of the i-th element of the annular antenna array, R ₀ is the radius of the annular antenna array, δ _k is the distance from the center of the annular antenna array to the kth element of the image matrix, λ is the ultrasonic wavelength, θ _i is the changing angle between δ _k and R _i when the i-th element of the annular antenna array is changed.

The explanatory geometry for deriving formula (2) is shown in FIG. 4.

To calculate by what angle you need to rotate the phase of the signal in the i-th phase shifter (ϕ _i , in order to focus on the desired element of the image matrix, you should discard an integer number of waves (wave periods) determined by the formula

where n ₁ is an integer number of waves λ, lying on the segment R _i , Digit 4 means that the wave travels twice the distance R _i , that is, from the focus to the element of the annular antenna array and back.

It remains to show on the model of a point-to-point object whether other reflective points located in the reconstruction area, that is, inside the ring of the antenna array, will not interfere with the reconstruction. When modeling the reconstruction of a two-point object, it should be borne in mind that since only points that are in focus can reflect the probe signal, neighboring points that are out of focus will not significantly affect the reconstruction of the image of a point that is in focus. If there is concentrated focusing, covering one element of the image matrix (Fig. 4), then the result of the reconstruction of the image of the two-point object will look ideal Fig. 5. In FIG. 5, a shows the result of the reconstruction in the form of two signal amplitudes at the output of the adder, that is, in the form of two delta pulses, and barely noticeable two points are visible in the top view. The top view of these delta functions of Fig. 5b is a reconstructed image of a two-point object.

Thus, the simulation results confirm the possibility of reconstruction, that is, tomography using a ring antenna array.

The influence of the surface wave on the results of reconstruction is considered in the article [Yushchenko, V.P. Circular aperture synthesis for tomography / V.P. Yushchenko // Autometry. - 2002. - T. 38, No. 6. - S. 28-33.]. The results taken from this work are shown in FIG. 6 In FIG. 6, a shows the result of the reconstruction of a point object without the influence of reflections from the surface wave. FIG. 6, b and c, the result of the influence of reflections from the surface wave is presented: b) with a commensurate ratio, the signal is interference; c) with a significant predominance of reflections from the surface wave over the signal from a point object.

The idea of the proposed method is illustrated by the example of one of the devices shown in FIG. 1.

FIG. 1 shows an example of a device that allows you to control the spatial position of the focus using a circular antenna array. The main element of the device is a circular antenna array 1. The even radiating elements of the antenna array are fed through phase shifters ϕ ₁ … ϕ ₈ from a monochromatic wave source 3. The odd elements of the antenna array receive the reflected signal from the focusing area. The received signals by odd elements are summed in the adder 2. The elements of the antenna array have a weakly directional pattern and are very small in size. Such antenna elements are easily implemented in the ultrasonic range. The controlled focus sequentially occupies a number of spatial positions that correspond to the positions of the image matrix elements of FIG. 4. At these moments of time in the computer memory the amplitudes of the received signals from the outputs of the adder 2 are recorded in digital form. Thus, a set of digital readings of the received summed amplitudes is obtained in the form of a two-dimensional digital matrix. Next, the amplitudes of the two-dimensional matrix of amplitudes are converted into luminance or color signals (pixels). As a result, the two-dimensional matrix turns into a tomographic cross-section.

The technical result of the distinctive actions is a positive effect that the prototype does not have. The positive effect is that the absence of movement of the locator does not provoke a surface wave that interferes with image reconstruction and does not require placing the patient in an aquatic environment, which increases the comfort of tomography.

Claims (4)

1. A method of mapping using a circular antenna array, which consists in emitting a monochromatic probe signal, receiving a reflected signal, controlling a focusing scanning space, forming an image matrix by distributing the signal received by the focus among range elements, in accordance with the reconstruction area, converting the matrix into brightness signals for obtaining a cross-sectional image of the tomographic object, characterized in that the spatial position of the focus is controlled using a fixed annular antenna array by setting the calculated initial phases on each radiating element of the annular antenna array. 2. The method according to claim 1, characterized in that the initial phases of the radiating antenna elements of the annular array are set using controlled phase shifters and are calculated by the formulas

Where

R _i is the distance from the k-th element of the image matrix to the i-th element of the annular antenna array, s _i is the wave amplitude at the output of the i-th element of the annular antenna array, R ₀ is the radius of the annular antenna array, δ _k is the distance from the center of the annular antenna array up to the kth element of the image matrix, λ is the ultrasonic wavelength, θ _i is the changing angle between δ _k and R _i when the i-th element of the annular antenna array is changed, n _i is an integer number of λ waves that fit into the segment R _i , digit 4 means that the wave travels twice the distance R _i , that is, from the focus to the element of the annular antenna array and back.

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