CN113206388B - Imaging system based on phase modulation active frequency selection surface and imaging method thereof - Google Patents
Imaging system based on phase modulation active frequency selection surface and imaging method thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
- H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
- H01Q15/0013—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective
- H01Q15/002—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices working as frequency-selective reflecting surfaces, e.g. FSS, dichroic plates, surfaces being partly transmissive and reflective said selective devices being reconfigurable or tunable, e.g. using switches or diodes
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- H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
- H01Q3/30—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture varying the relative phase between the radiating elements of an array
- H01Q3/34—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture varying the relative phase between the radiating elements of an array by electrical means
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Abstract
本发明公开了一种基于相位调制有源频率选择表面的成像系统及其成像方法。包括有源频率选择表面、发射接收天线、相位控制系统、射频收发系统、发射和接收功分网络，其中单个有源频率选择表面是由可独立编程调相单元均匀间隔排列组成的二维表面，对空间馈电的电磁波具有带通滤波特性，添加变容管针对空间馈电电磁波的相位进行随机二进制相位调制。本发明既用于对发射天线产生的入射场进行随机调制，也对成像目标产生的散射场进行随机调制，实现对成像区域的随机照射与散射回波的随机接收，降低了不同随机模式下测量之间的冗余，有利于提高成像质量，具有硬件成本低，集成度高，操作简单，成像快速，口径可扩展等优点。
The invention discloses an imaging system and an imaging method based on a phase modulation active frequency selective surface. Including active frequency selective surface, transmitting and receiving antenna, phase control system, radio frequency transceiver system, transmitting and receiving power division network, wherein a single active frequency selective surface is a two-dimensional surface composed of independently programmable phase modulation units evenly spaced, The electromagnetic wave of the space feeding has the characteristics of band-pass filtering, and a varactor is added to perform random binary phase modulation for the phase of the electromagnetic wave of the space feeding. The invention is not only used to randomly modulate the incident field generated by the transmitting antenna, but also randomly modulate the scattered field generated by the imaging target, so as to realize the random irradiation of the imaging area and the random reception of scattered echoes, and reduce the measurement of different random modes. The redundancy between them is beneficial to improve the imaging quality, and has the advantages of low hardware cost, high integration, simple operation, fast imaging, and scalable aperture.
Description
Technical Field
The invention relates to an imaging system and an imaging method, in particular to an imaging system based on a phase modulation active frequency selection surface and an imaging method thereof.
Background
Microwave, millimeter wave and terahertz imaging have been widely studied in the past decades, and their application scenarios include security inspection, medical imaging, geographic detection, through-wall imaging, and the like. In recent years, with the increase of terrorist attack events, the demand of large-scale security inspection equipment in public places such as airports, stations, markets and the like is increasing, and microwaves have good penetrability to clothes, do not cause harm to human bodies and are very suitable for security inspection systems. Researchers at home and abroad are continuously exploring low-cost and high-resolution microwave security inspection imaging systems.
The traditional microwave imaging system adopts phased array or synthetic aperture radar to carry out beam scanning imaging, and has higher cost and larger power consumption. In recent years, the proposal of the compressive sensing theory opens up a new path for microwave imaging. By utilizing the sparsity of an imaging scene and using the random wave beams changing along with time to sample scene information, the correlation among different samples is greatly reduced, and the surface scattering characteristic distribution of a target can be extracted by inverting the measured sample information. Compared with the traditional beam synthesis scanning imaging, the imaging method based on the random beam only needs a single radio frequency channel, and has the advantages of low hardware cost and high imaging speed. There are currently two main structures available for random beam generation: frequency diversity antennas and programmable phased array antennas. Frequency diversity antennas often need broadband frequency spectrum to generate enough random patterns, wasting spectrum resources; the programmable phased array antenna can generate a plurality of random modes under a single frequency by randomly encoding and controlling the phase state of each unit, and has the advantage of high spectrum efficiency.
Disclosure of Invention
In order to solve the problems existing in the background technology, the invention discloses an imaging system based on a phase modulation active frequency selection surface and an imaging method thereof, wherein the active frequency selection surface has a band-pass filtering characteristic on electromagnetic waves fed in space, and the phase modulation on the electromagnetic waves passing through the surface is realized by adding a varactor in a unit structure of the imaging system.
The technical scheme adopted by the invention is as follows:
an imaging system based on a phase modulation active frequency selective surface:
the imaging system is composed of N active frequency selection surfaces with expandable quantity and phase modulation capability and feeding by space electromagnetic wave irradiation, N/2 transmitting antennas and N/2 receiving antennas, a phase control system for phase control of the active frequency selection surfaces, a radio frequency transceiving system with a transmission scattering parameter (S21) measuring function, a transmitting power distribution network for connecting ports of the transmitting antennas and transmitting channel ports in the radio frequency transceiving system, and a receiving power distribution network for connecting ports of the receiving antennas and receiving channel ports in the radio frequency transceiving system; n is an even number.
The N active frequency selection surfaces are arranged according to the interval array to form an active frequency selection surface array, and each active frequency selection surface is oppositely provided with an antenna; all transmitting antennas are connected with transmitting channel ports of the radio frequency transceiving system through a transmitting power distribution network, and all receiving antennas are connected with receiving channel ports of the radio frequency transceiving system through a receiving power distribution network; the active frequency selective surface array is connected to a phase control system. The transmitting power division network and the receiving power division network are connected with the radio frequency transceiving system through respective public ports.
All the active frequency selective surfaces are averagely divided into two types of transmission type active frequency selective surfaces and receiving type active frequency selective surfaces, and the transmission type active frequency selective surfaces and the receiving type active frequency selective surfaces are alternately arranged in the active frequency selective surface array, so that the periphery of each transmission type active frequency selective surface is a receiving type active frequency selective surface, and the periphery of each receiving type active frequency selective surface is a transmission type active frequency selective surface; each emission type active frequency selection surface is oppositely provided with a transmission antenna; each receiving type active frequency selection surface is oppositely provided with a receiving antenna, so that N/2 transmitting type active frequency selection surfaces are respectively arranged opposite to N/2 transmitting antennas in an oriented way, and N/2 receiving type active frequency selection surfaces are respectively arranged opposite to N/2 receiving antennas in an oriented way. The transmitting type active frequency selective surface and the receiving type active frequency selective surface have the same structure.
The specific implementation also comprises a main control chip (MCU) and a computer (PC), wherein the radio frequency transceiving system and the phase control system are both connected with the MCU, and the MCU is connected with the PC.
The single active frequency selection surface is composed of Ns phase modulation units, each phase modulation unit is arranged in a periodic array, and the arrangement distance between every two adjacent phase modulation units is larger than or equal to one half wavelength.
The N active frequency selective surfaces are arranged on a plane facing the imaging area or on a closed or non-closed curved surface surrounding the imaging area.
The working frequency of the active frequency selection surface selects a microwave, millimeter wave or terahertz frequency band according to the required imaging resolution.
The total number Ns of phase modulation units in each active frequency selective surface and the total number N of active frequency selective surfaces are determined by the geometric size of the imaging target in the imaging region and the required imaging resolution, so that the electromagnetic waves radiated from the active frequency selective surfaces can effectively cover the imaging region, and the measurement times M are equal to the number of pixels included in the imaging region.
Each phase modulation unit is designed according to the structure of the second-order band-pass filter in fig. 4, and comprises three metal layers and two dielectric layers, wherein the three metal layers and the two dielectric layers are alternately stacked, one dielectric layer is arranged between every two adjacent metal layers, and the dielectric layers adopt dielectric substrates; the three-layer metal sheet is divided into an upper metal layer, a middle metal layer and a lower metal layer, the upper metal layer and the lower metal layer are the same in structure and respectively comprise two metal patches which are arranged at intervals, a gap is formed between the two metal patches, a varactor is arranged in the gap, the two metal patches are electrically connected through the varactor, and the variable capacitor C is used in a simulation diagramv(ii) a The shape and size of the outer edge of the metal layer are the same as those of the outer edge of the dielectric layer, four rectangular hollow structures which are arrayed at intervals are arranged in the middle of the metal layer, and specifically, the metal layer is formed by etching four rectangles with the same size on a whole metal and is used for simulating an inductor L in a figure; the upper metal layer, the dielectric layer and the metal layer form a parallel flat transmission line, and the equivalent circuit is a parallel capacitor C in the figureTLAnd a series inductance LTL. The lower metal layer is a mirror image of the upper metal layer, and the size of the metal sheet and the bias voltage of the varactor are completely the same as the upper metal layer.
One of the metal patches in the upper metal layer and the lower metal layer is connected with a lead, and the direct current bias voltage is applied to the varactor through the lead.
The metal patches on the same side in the upper metal layer and the lower metal layer are electrically connected through a via hole, the metal patches belong to the same electrode of the varactor, and the via hole penetrates through the two dielectric layers and the middle metal layer.
The imaging method based on the phase modulation active frequency selection surface comprises the following steps of:
1) electromagnetic waves emitted by a transmitting channel of the radio frequency transceiving system are fed into the transmitting power distribution network through a public port of the transmitting power distribution network and then fed into N/2 transmitting antennas through an output port of the transmitting power distribution network;
the N/2 active frequency selection surfaces work in a transmission mode to form an emission type active frequency selection surface, the N/2 transmission antennas transmit electromagnetic waves fed from an output port of a transmission power division network and irradiate the electromagnetic waves on the emission type active frequency selection surface, all phase modulation units are subjected to random phase modulation before the electromagnetic waves pass through the emission type active frequency selection surface, transmission phases of all the phase modulation units in the emission type active frequency selection surface are randomly distributed, and the electromagnetic waves pass through a random wave beam generated by the emission type active frequency selection surface to irradiate an imaging area;
2) the rest N/2 active frequency selective surfaces work in a receiving mode to form a receiving type active frequency selective surface, electromagnetic waves scattered by an imaging target in an imaging area irradiate the receiving type active frequency selective surface, and random phase modulation is carried out on all phase modulation units on the surface before the electromagnetic waves pass through the receiving type active frequency selective surface, so that the transmission phases of all the phase modulation units in the receiving type active frequency selective surface are randomly distributed;
3) the electromagnetic wave passing through the receiving type active frequency selection surface is the electromagnetic wave after two times of random phase modulation, is received by N/2 receiving antennas, respectively enters N/2 input ports of a receiving power division network, is subjected to power synthesis through the receiving power division network, and then enters a receiving channel of a radio frequency transceiving system through a public port of the receiving power division network;
4) dividing the signal entering the receiving channel of the radio frequency transceiving system by the signal transmitted by the transmitting channel of the radio frequency transceiving system in the step 1) to obtain a transmission scattering parameter S21 of the imaging system under single random phase modulation, and finishing one measurement;
5) and (3) repeating the steps 1) to 4) by adopting different random phase modulation modes to carry out M times of measurement, and applying random phase modulation to the emission type active frequency selection surface and the receiving type active frequency selection surface again when the steps are repeated each time, so that the random phase distribution of each phase modulation unit of the emission type active frequency selection surface and the receiving type active frequency selection surface in the steps 1) to 4) is different each time, M transmission scattering parameters S21 are obtained, a measurement matrix is further formed, and the measurement matrix and an imaging system transmission matrix obtained by calibration measurement in advance are subjected to multiplication operation to complete the imaging of the measured target.
Performing binary phase modulation on the phase modulation unit with the phase modulation function to realize random phase distribution:
the phase modulation unit with the active frequency selection surface and the second-order band-pass filtering characteristic is added with a variable capacitance tube, binary direct-current voltage is applied to the variable capacitance tube through a lead and switched, the transmission phase of each phase modulation unit is controlled, and binary phase modulation of the transmission phase of the active frequency selection surface under the space feeding condition is achieved. In binary phasing, the resulting binary change in transmission phase is greater than zero degrees and less than 360 degrees. The binary dc voltage generated by the phase control system is applied to the conductors and thus to the varactors.
The phase control of each active frequency selective surface is realized by a phase control system, and the method comprises the following steps:
for each active frequency selection surface, arranging a plurality of serial-parallel converters, wherein one serial-parallel converter comprises a plurality of parallel output ports and can be connected and controlled with a plurality of phase modulation units, the plurality of serial-parallel converters are cascaded together and controlled by an MCU, and the output binary direct-current voltage is applied to a varactor of each phase modulation unit through a lead; the binary direct-current voltage output by the serial-parallel converter is divided into two types of high voltage and low voltage, two equivalent capacitance values of the varactor are corresponded, two phase states of binary phase modulation are further corresponded, and independent phase modulation aiming at each phase modulation unit is realized.
In specific implementation, firstly, according to the total quantity NxNs of all phase modulation units, a binary random array with the length of NxNs is generated by using a random number function, numbers 0 and 1 in the binary random array respectively correspond to binary direct-current voltages output by two serial-parallel converters, and the voltages of the serial-parallel converters are respectively biased to all the phase modulation units of each active frequency selection surface, so that the binary phase modulation aiming at all the active frequency selection surfaces and all the phase modulation units thereof can be realized at one time.
In specific implementation, the imaging process consists of two measurement processes with completely identical random phase modulation, wherein the first measurement process is system calibration measurement and aims to obtain a transmission matrix of the imaging system; the second measurement process is imaging measurement, and only imaging measurement is needed for the imaging system which has finished system calibration measurement.
The system calibration measurement comprises the following implementation steps:
s1, firstly, placing a target to be imaged with known scattering property distribution in an imaging area, repeating the steps 1) -4) to perform M times of measurement, so that the emission type active frequency selective surface and the receiving type active frequency selective surface perform M times of different random phase modulation on electromagnetic waves passing through the surface, the radio frequency transceiver system performs corresponding measurement to obtain M transmission scattering parameters S21, and all M transmission scattering parameters S21 form an M multiplied by 1 reference measurement matrix;
s2, repeating the step S1 using a plurality of objects to be imaged with known scattering characteristics, and obtaining a plurality of reference measurement matrices corresponding to the respective objects to be imaged, each time the transmission-type active frequency selective surface and the reception-type active frequency selective surface are subjected to the same random phase modulation as the processing in the step S1;
and S3, taking the obtained multiple reference measurement matrixes and the corresponding scattering characteristic distribution of the target to be imaged as a training data set, training the mapping relation between the reference measurement matrixes and the scattering characteristic distribution of the target to be imaged by using a machine learning method, and taking the mapping relation as a transmission matrix of an imaging system to finish calibration measurement.
The imaging measurement is realized by the following steps:
placing an imaging target to be measured with unknown scattering characteristic distribution in an imaging area, and using the same random phase modulation as that in calibration measurement on an active frequency selection surface to obtain an Mx 1 measurement matrix of the imaging target to be measured;
and multiplying the obtained measurement matrix of the imaging target to be detected by a transmission matrix of the imaging system to obtain the scattering characteristic distribution of the imaging target, and finishing the imaging of the target object.
The radio frequency transceiving system with the function of measuring the transmission scattering parameter S21 can be implemented by an instrument (such as a vector network analyzer), or by a radio frequency circuit with a receiving channel and a transmitting channel synchronized in phase.
The single active frequency selection surface is a two-dimensional surface formed by uniformly and alternately arranging independently programmable phase modulation units, has band-pass filtering characteristics on the space feed electromagnetic wave, and has the capability of performing random binary modulation on the phase of the space feed electromagnetic wave by adding the varactor in the unit structure. The active frequency selection surface can be used for randomly modulating an incident field generated by the transmitting antenna and a scattered field generated by an imaging target, so that random irradiation of an imaging area and random reception of scattered echoes are realized, the correlation among samples acquired in different random modes is further reduced, and the sampling times and the imaging time are favorably reduced.
The beneficial effects of the invention are:
for a traditional phased array antenna, each antenna in a general array needs independent phase shift control and a feed source, and for a large-scale array, a large number of phase shifters and power dividers need to be used, so that the hardware cost is high, and a control circuit is very complex; the active frequency selection surface can be used for phase modulation of electromagnetic waves of space feed by applying different direct current bias voltages to each phase modulation unit without using a large number of phase shifters and power dividers, can be manufactured by a PCB process, and has the advantages of low hardware cost, simple control circuit and low power consumption.
The active frequency selection surface can simultaneously carry out random phase modulation on the electromagnetic wave irradiating an imaging area and the electromagnetic wave scattered by an imaging target to obtain completely random measurement, greatly reduces redundancy among measurements in different random modes, and is beneficial to improving the imaging quality.
The imaging system based on the phase modulation active frequency selection surface can conveniently expand the caliber of the system according to the size of an imaging target and the required resolution, and has high system flexibility and rich application scenes.
The imaging system has the advantages of low hardware cost, high integration level, simple operation, quick imaging, expandable caliber and the like, and is suitable for security inspection of airports, stations, malls and other public places.
Drawings
FIG. 1 is a schematic view of an imaging system of the present invention.
Figure 2 is a typical second order coupled bandpass filter structure.
Figure 3 is the modulation result of the transmission phase and amplitude after loading a variable capacitor in a typical second order coupled bandpass filter structure.
Fig. 4 is a second order coupled bandpass filter structure that actually implements a phase modulating cell reference.
Fig. 5 is a phase modulating unit multi-layer PCB structure used in an embodiment of the present invention.
Fig. 6 is a single active frequency selective surface of an embodiment of the present invention.
Fig. 7 is a block diagram of the imaging system hardware circuit based on a phase modulated active frequency selective surface of an embodiment of the present invention.
Fig. 8 is a diagram of the radiation field in the imaging area after random modulation by the emissive active frequency selective surface at different times.
Fig. 9 is a graph of experimental imaging results for an embodiment of the present invention.
In the figure: 1. the array comprises an active frequency selective surface array, 2 an emission type active frequency selective surface, 3 a receiving type active frequency selective surface, 4 a transmitting antenna, 5 a receiving antenna, 6 an imaging area, 7 an imaging target, 8 an upper metal layer, a lower metal layer, 9 a conducting wire, 10 a varactor, 11 a gap, 12 a via hole, 13 a dielectric layer, 14 a metal layer and 15 a rectangular hollow structure.
Detailed Description
An embodiment of the present invention is described in detail below with reference to the accompanying drawings and examples.
As shown in fig. 1, the imaging area 6 is located on the xoy plane, N active frequency selective surfaces are disposed on a plane parallel to the imaging area, and N/2 transmitting antennas 4 and N/2 receiving antennas 5 are respectively disposed right behind the N/2 transmitting active frequency selective surfaces 2 and the N/2 receiving active frequency selective surfaces 3, wherein a single active frequency selective surface includes Ns units. The N/2 transmitting antennas 4 transmit and irradiate the electromagnetic waves fed from the output port of the transmitting power division network onto the transmitting active frequency selection surface 2, all phase modulation units on the surface are subjected to random phase modulation before the electromagnetic waves pass through the transmitting active frequency selection surface 2, so that the transmission phases of all the phase modulation units in the transmitting active frequency selection surface 2 are randomly distributed, and the electromagnetic waves pass through a random beam generated by the transmitting active frequency selection surface 2 to irradiate an imaging area 6; incident random wave beams interact with an imaging target 7 to generate scattered electromagnetic waves, the scattered electromagnetic waves irradiate an N/2 receiving type active frequency selection surface 3, and all phase modulation units on the surface are subjected to random phase modulation before the electromagnetic waves pass through the receiving type active frequency selection surface 3, so that the transmission phases of all the phase modulation units in the receiving type active frequency selection surface 3 are randomly distributed. The electromagnetic waves which are subjected to the two random phase modulations and carry the scattering characteristic distribution of the imaging region are finally received by the receiving antenna and enter a receiving channel of the radio frequency transceiving system, and are divided by signals transmitted by a transmitting channel of the radio frequency transceiving system to obtain a transmission scattering parameter S21 of the imaging system under the single random phase modulation, and one measurement is completed.
In a specific implementation process, the emission-type active frequency selective surface 2 and the reception-type active frequency selective surface 3 need to perform M different random phase modulations, and M measurements of an imaging region are completed, where the measurement times M are equal to the number of pixels included in the imaging region.
Defining the i-th phase modulation unit coordinate on the emission-type active frequency selective surface as r in the imaging processi(xi,yi,zi) The j th phase modulation element on the receiving type active frequency selection surface has the coordinate rj(xj,yij,zj) For the sake of calculation convenience, the imaging plane is divided into N 'meshes, wherein the coordinates of the k-th mesh are defined as r'k(xk,yk,zk). Without loss of generality, the field of the transmission antenna impinging on the i-th phase modulation unit on the emitting active frequency selective surface is defined as Einc(ri) For the m-th modulation, regardless of mutual coupling between phase modulation units, the field passing through the jth phase modulation unit of the receiving-type active frequency selective surface and subjected to two random phase modulations can be expressed as:
wherein
And
the random phase controlled by binary data of the ith phase modulation unit of the transmitting active frequency selection surface and the jth phase modulation unit of the receiving active frequency selection surface is taken as
Or
SRO(rj,r′k) Representing the transmission coefficient between the kth grid of the imaging area and the jth phase modulation unit of the receiving active frequency selective surface, and, for the same reason, SOT(r′k,ri) Representing the transmission coefficient, f (r'k) And representing the surface reflectivity of the kth grid, wherein the field subjected to the random phase modulation twice is finally received by the receiving antenna and enters a radio frequency transceiving system to be divided by a transmitting signal, so that a measurement of the transmission scattering coefficient S21 is obtained.
For all M measurements, the resulting imaging model is:
g＝Hf
where g denotes a measurement matrix of size M × 1, H denotes a perception matrix of size M × N ', where the (M, k) th element of the H matrix corresponds to S21 measured when a point scatterer is present only at the k-th pixel point within the imaging region at the M-th measurement, and f denotes a scattering property distribution within the imaging region of size N' × 1. The direct solution of the above-mentioned relation is a general method based on random beam imaging, and the imaging principle is to invert the scattering characteristic distribution in the imaging region based on the relation, i.e. f, and the commonly used algorithms include a least square method, a matched filter method, pseudo-inversion, a two-step iterative shrinkage threshold algorithm (TwIST), and the like.
The H matrix is required to be obtained in advance by using the imaging model, a common method is to measure or calculate the transmission scattering coefficient at each pixel point in an imaging area point by using a near field scanning or point scatterer calibration method, however, the size of the H matrix is correspondingly increased along with the increase of the size of the imaging area, and the time for calibration measurement, the time for inversion calculation and the storage requirement are rapidly increased. Therefore, in order to avoid calculation of the H matrix and directly apply the imaging algorithm to the measurement matrix g, the invention combines a deep learning method to complete calibration measurement.
The method for realizing the calibration measurement of the system comprises the following steps:
s1, firstly, placing a target to be imaged with known scattering property distribution in an imaging area, performing M times of different random phase modulation on electromagnetic waves passing through the target by an emission type active frequency selection surface and a receiving type active frequency selection surface, correspondingly measuring by a radio frequency transceiver system to obtain M transmission scattering parameters S21, and forming an Mx 1 reference measurement matrix by all M transmission scattering parameters S21;
s2, repeating the step S1 using a plurality of objects to be imaged with known scattering characteristics, and obtaining a plurality of reference measurement matrices corresponding to the respective objects to be imaged, each time the transmission-type active frequency selective surface and the reception-type active frequency selective surface are subjected to the same random phase modulation as the processing in the step S1;
and S3, taking the obtained multiple reference measurement matrixes and the corresponding scattering characteristic distribution of the target to be imaged as a training data set, training the mapping relation between the reference measurement matrixes and the scattering characteristic distribution of the target to be imaged by using a machine learning method, and taking the mapping relation as a transmission matrix of an imaging system to finish calibration measurement.
After the system calibration measurement is completed, the imaging measurement can be performed, and the specific steps are as follows:
(1) placing an imaging target to be measured with unknown scattering characteristic distribution in an imaging area, and using the same random phase modulation as that in calibration measurement on an active frequency selection surface to obtain an Mx 1 measurement matrix of the imaging target to be measured;
(2) and multiplying the obtained measurement matrix of the imaging target to be detected by the transmission matrix of the imaging system to obtain the scattering characteristic distribution of the imaging target, and finishing imaging of the target object.
The phase modulation unit of the active frequency selective surface is designed according to the theory of a second-order band-pass filter, as shown in fig. 2, and is a typical second-order coupling resonance band-pass filter structure by adjusting a variable capacitor CvThe value of (2) can change the passband of the filter, realize phase modulation at a single frequency, and for a second-order bandpass filter, realize 180-degree phase modulation within 3dB of insertion loss. To further illustrate the binary phase modulation principle of a second order band-pass filter, assume Z in the band-pass filter structure0＝377Ω，L＝0.38nH，LmThe transmission characteristic of the second-order band-pass filter obtained by simulation is shown in fig. 3 when the variable capacitor C is equal to 1.9nHvFrom 0.1097pF to 0.1681pFIn the process, the passband of the filter moves towards low frequency, 180-degree phase modulation capability is realized at 24GHz, and the transmission loss in two transmission phase states is the same and is-2 dB, so that the phase modulation based on the band-pass filter structure is feasible, the binary phase modulation can be realized only by switching two equivalent capacitance values of the varactor, and higher energy transmission efficiency can be realized.
The circuit structure shown in fig. 2 can be transformed to obtain the structure shown in fig. 4 more suitable for PCB process, and a specific implementation structure is shown in fig. 5, where a single phase modulation unit includes three metal layers and two dielectric layers, the three metal layers and the two dielectric layers 13 are alternately stacked, a dielectric layer is arranged between every two adjacent metal layers, and the dielectric layer is a dielectric substrate; the three-layer metal sheet is divided into an upper metal layer, a middle metal layer and a lower metal layer, the upper metal layer 8 and the lower metal layer 8 are identical in structure and respectively comprise two metal patches which are arranged at intervals, a gap 11 is formed between the two metal patches, a varactor 10 is arranged in the gap 11, and the two metal patches are electrically connected through the varactor 10 and used for simulating a variable capacitor Cv in the graph 4; the outer edge shape and size of the metal layer 14 are the same as those of the dielectric layer, four rectangular hollow structures 15 arranged at intervals in an array are arranged in the middle of the metal layer 14, and specifically, the four rectangular hollow structures are formed by etching four rectangles with the same size on a complete metal and are used for simulating the inductor L in FIG. 4; the metal layer 8, the dielectric layer 13 and the metal layer 14 on the upper layer together form a parallel flat transmission line, and the equivalent circuit of the parallel transmission line is the parallel capacitor CTL and the series inductor LTL in fig. 4. The lower metal layer is a mirror image of the upper metal layer, and the size of the metal sheet and the bias voltage of the varactor are completely the same as the upper metal layer.
One of the metal patches of the upper metal layer 8 and the lower metal layer 8 is connected to a lead 9, and a dc bias voltage is applied to the varactor 10 through the lead 9.
The metal patches on the same side in the upper metal layer 8 and the lower metal layer 8 are electrically connected through a via hole 12, the metal patches belong to the same electrode of the varactor, and the via hole 12 penetrates through two dielectric layers 13 and a middle metal layer 14. It should be understood that this is only an illustration, the structure of the phase modulation unit can be designed according to the required operating frequency and phase modulation requirement, and the adjustable device and metal structure that can be used are not limited to this.
To further illustrate the operating principles of an imaging system based on a phase modulated active frequency selective surface, an example is now provided for illustration.
As shown in fig. 6, a single active frequency selective surface comprises 100 cells with a spacing of λ/2, where λ is the wavelength in free space and the operating frequency is 24Ghz, each cell comprises two varactors, MAVR-011020-. A total of 4 active frequency selective surfaces are used, two of which are active frequency selective surfaces 2 of the transmitting type, after which the transmitting antenna 4 is placed, and two of which are active frequency selective surfaces 3 of the receiving type, after which the receiving antenna 5 is placed, all of which are placed on a plane parallel to the imaging area, it being understood that this is merely an illustration, the number of cells contained by a single active frequency selective surface and the number of active frequency selective surfaces used can be flexibly adjusted according to the size of the imaging area, and the active frequency selective surfaces can be placed on a plane facing the imaging area, or on a closed or non-closed curved surface surrounding the imaging area.
In the hardware circuit implementation, as shown in fig. 7, the phase control system mainly includes an active frequency selective surface, a radio frequency transceiving system, a transmission power division network, and a reception power division network.
The phase control system is implemented as follows: providing a plurality of cascaded serial-to-parallel converters, such as the 32-channel serial-to-parallel converter HMC504LC4B from Analog, one serial-to-parallel converter being connectable to control 32 phase modulation units, at least 13 serial-to-parallel converters being required for an active frequency selective surface array with 4 × 100 to 400 phase modulation units, all serial-to-parallel converters being cascaded together, controlled by the MCU, and outputting a binary dc voltage at their parallel output terminals, applied to the varactor 10 of each phase modulation unit via the conductor 9; the binary direct-current voltage output by the serial-parallel converter is divided into a high voltage and a low voltage, wherein specific values of the high voltage and the low voltage can be controlled by a simple resistance voltage division circuit, and the two equivalent capacitance values of the varactor correspond to two phase states of binary phase modulation, so that the independent phase modulation for each phase modulation unit is realized.
As shown in fig. 8, in order to irradiate the field in the imaging area after the field generated by the transmitting antenna is subjected to different random binary phase modulations by the transmitting active frequency selective surface at different times, it can be seen that, as long as the different random binary phase modulations are performed on the active frequency selective surface, a plurality of radiation fields with random characteristics can be generated at a single frequency.
In this embodiment, the 24GHz rf transceiver system is designed based on a single millimeter wave integrated circuit, which is BGT24MTR11, and a 24GHz carrier generated by a transmitting end of the rf transceiver system enters a transmission power distribution network after being amplified by an amplifier, is further fed into a transmitting antenna, and irradiates an emission-type active frequency selective surface, an electromagnetic wave modulated at random phase by the surface is irradiated into an imaging region, and a scattered field generated by interaction with an imaging target is randomly modulated by a receiving-type active frequency selective surface, and then is received by a receiving antenna disposed at the rear end of the receiving-type active frequency selective surface. The received electromagnetic waves enter a receiving power division network, enter the receiving channel of a radio frequency transceiving system after power synthesis of the network, enter the receiving channel of the radio frequency transceiving system from the public end of the receiving power division network, are amplified and then are divided by a transmitting signal of the radio frequency transceiving system, two paths of I/Q data are demodulated, the data are read by an MCU and transmitted to a computer for processing, and S21 parameters of single measurement are obtained. In order to improve the integration level of the system and reduce the section, patch antennas are adopted for the transmitting antenna and the receiving antenna, simple microstrip binary networks are adopted for the transmitting power dividing network and the receiving power dividing network, the whole system is highly integrated, and the calibration measurement and the imaging measurement of the system can be completed without an expensive test instrument such as a vector network analyzer.
Fig. 9 shows the imaging results of different targets, where the size of the whole imaging plane is 8 λ × 8 λ, and the whole imaging plane is divided into 8 × 8 — 64 grids, where λ is 12.5mm, and is the wavelength in free space corresponding to 24GHz, and a total of 64 different random phase modulation modes are used to sample the imaging plane. It can be seen that the distribution of the scattering properties within the imaged region is accurately restored and the position and shape of all the original targets can be clearly resolved, demonstrating the effectiveness of the system.
As can be seen from the above embodiments, the present invention discloses an imaging system based on an active frequency selective surface and an imaging method thereof. The active frequency selection surface phase modulation unit designed according to the second-order band-pass filter structure can realize phase modulation on electromagnetic waves of space feed, all units on the active frequency selection surface can have random phase distribution through binary data control, random beams are generated, a large number of phase shifters and power dividers are not needed, the circuit structure is simple, the hardware cost is low, and the operation is easy. By utilizing the structure, the caliber of the imaging system can be conveniently expanded according to the size of an imaging target, the system has high flexibility and rich application scenes.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
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