Multifrequency capacitively coupled electrical impedance tomography system and method

Abstract

The application discloses a multi-frequency capacitive coupling electrical impedance tomography system and method. The system is composed of a 12-electrode non-contact electrical impedance sensor, six detection modules, a data acquisition module and a computer. A multi-frequency electrical impedance measurement model is established in the computer, including a mechanism model obtained by mechanism modeling on an inverting amplifier circuit and an empirical model obtained by empirical modeling on system demodulation results. Based on the multi-frequency capacitive coupling electrical impedance tomography system and the established multi-frequency electrical impedance measurement model, a multi-frequency capacitive coupling electrical impedance tomography method is disclosed. Tests show that the system and method can realize high-quality electrical impedance measurement and imaging within 20 MHz. Compared with existing electrical impedance tomography systems and methods, the application has the characteristics of non-contact, wide frequency band, simultaneous imaging of real part and imaginary part of electrical impedance and good imaging performance, and has a broader practical industrial application prospect.

Description

Technical Field

[0001] This invention relates to electrical impedance tomography, and more particularly to a multi-frequency capacitively coupled electrical impedance tomography system and method. Background Technology

[0002] Electrical Resistance Tomography (ERT) is a key technology in the field of gas-liquid two-phase flow parameter measurement. It acquires conductivity distribution information through conductivity / resistance measurements. While ERT offers advantages such as low cost, fast response, and no radiation, existing techniques cannot meet the measurement requirements of multiphase fluids in actual industrial pipelines. Its contact-based measurement method can lead to electrode polarization, electrochemical corrosion, and electrode contamination, negatively impacting measurements. Furthermore, current ERT techniques, based on conductivity / resistance measurements, often only utilize the real part of the impedance signal or simply treat the impedance amplitude signal as the equivalent conductivity of the gas-liquid two-phase flow, neglecting the imaginary part of the impedance and thus failing to fully utilize the fluid impedance. For gas-liquid two-phase flows, research has shown that the imaginary part of the impedance is closely related to the distribution and related parameters of the gas-liquid two-phase flow, containing rich information reflecting fluid flow characteristics. Therefore, the effective utilization of the imaginary part is also of great significance.

[0003] In recent years, the emergence of Capacitively Coupled Electrical Impedance Tomography (CCEIT) has provided an effective means to overcome the aforementioned problems and a new approach to impedance imaging of gas-liquid two-phase flows. Compared with traditional ERT technology, CCEIT technology has made the following two improvements: 1) By drawing on the techniques of Capacitively Coupled Contactless Conductivity Detection (CCEIT), it improves upon traditional ERT technology in the following ways: 4 D) The measurement concept of this technology enables non-contact measurement of fluid impedance, avoiding the problems associated with contact measurements; 2) It utilizes both the real and imaginary parts of the impedance signal in the measured gas-liquid two-phase fluid, solving the problem of insufficient attention to the imaginary part of impedance and incomplete utilization of impedance information in traditional ERT technology. Therefore, CCEIT technology has a broader prospect for industrial applications and has attracted the attention of researchers at home and abroad since its inception. However, although previous studies have demonstrated the effectiveness and development potential of CCEIT technology, as a developing new technology, related research is still insufficient, and current CCEIT systems still cannot meet the growing requirements of industrial applications.

[0004] Existing CCEIT systems are single-frequency systems, meaning they can only acquire the real and imaginary parts of fluid impedance at a single operating frequency. Previous research has shown that the choice of operating frequency significantly impacts impedance measurement and imaging, and the real and imaginary parts of impedance have different frequency characteristics. This means that single-frequency systems cannot simultaneously meet the measurement requirements of both the real and imaginary parts of impedance, thus failing to acquire the most valuable real and imaginary impedance information. Meanwhile, research indicates that multi-frequency measurements can obtain the spectral information of impedance, and utilizing multi-frequency impedance information holds promise for further improving imaging quality. However, research on multi-frequency CCEIT measurements is currently scarce, especially research on multi-frequency CCEIT systems, which remains a gap. Therefore, relevant research is needed to develop high-performance multi-frequency CCEIT systems to provide hardware support for future research on multi-frequency CCEIT technology. Summary of the Invention

[0005] Addressing the current state of CCEIT (Capacitively Coupled Interference Tomography) technology research, this invention discloses a multi-frequency capacitively coupled impedance tomography system and method. This system can measure and image the real and imaginary parts of the impedance in a gas-liquid two-phase flow within a wide frequency range, thereby enabling fluid state monitoring and parameter measurement. The system mainly consists of a 12-electrode non-contact impedance sensor, six identical detection modules, a data acquisition module, and a computer. Furthermore, for the aforementioned multi-frequency capacitively coupled impedance tomography system, this invention employs a combination of mechanistic and empirical modeling to establish a multi-frequency impedance measurement model. This model combines the mechanistic model of the inverting amplifier circuit in the detection module with the empirical model of the system demodulation results in the data acquisition module, resulting in more accurate and stable multi-frequency impedance measurement results, thus achieving high-quality multi-frequency impedance imaging. Compared to existing impedance tomography systems, this invention features non-contact operation, wide bandwidth, and superior imaging performance. It can simultaneously measure and image the real and imaginary parts of multi-frequency impedance, making fuller use of impedance information and possessing broader prospects for practical industrial applications.

[0006] The technical solution of the present invention is as follows:

[0007] The present invention first provides a multi-frequency capacitively coupled electrical impedance tomography system, characterized in that it includes a 12-electrode non-contact electrical impedance sensor, six identical detection modules, a data acquisition module, and a computer for multi-frequency electrical impedance calculation and imaging;

[0008] The 12-electrode non-contact impedance sensor consists of 12 metal electrodes of the same shape and size uniformly pasted in a ring array on the outer wall of an insulating pipe containing the measured medium. The metal electrodes do not directly contact the measured medium inside the pipe. The measurement path is constructed through the insulating pipe wall to realize non-contact impedance measurement.

[0009] Each detection module includes two channel selection units for channel switching, one I / V conversion unit for current-to-voltage conversion of the detection signal, and one CPLD. The two channel selection units are connected to two metal electrodes respectively, and the six detection modules are connected to twelve metal electrodes. The electrode states are selected and switched by high-frequency relays in the channel selection units, thereby realizing the switching of the measurement electrode pairs. The I / V conversion unit consists of an inverting amplifier circuit containing an operational amplifier, which performs current-to-voltage conversion on the detection signal for subsequent impedance measurement. The CPLD (Complex Programming Logic Device) is used to generate control logic to control the closing or opening of the corresponding relays.

[0010] The data acquisition module mainly includes a microprocessor unit, a configurable excitation source unit, a signal demodulation unit, a communication unit, and a power supply unit. The microprocessor unit is responsible for controlling the coordinated operation of each module. The configurable excitation source unit generates sinusoidal excitation signals of different frequencies. The signal demodulation unit demodulates the amplitude ratio and phase difference between the detection signal and the excitation signal. The communication unit sends the demodulation results to the computer. The power supply unit provides power to the entire system.

[0011] The computer has a multi-frequency impedance measurement model, which includes a mechanistic model obtained by modeling the mechanism of the inverting amplifier circuit and an empirical model obtained by modeling the demodulation results of the system. After receiving the demodulation results sent by the data acquisition module, the computer calculates the real and imaginary parts of the impedance to be measured in real time, and uses an image reconstruction algorithm to image the real and imaginary parts of the impedance respectively.

[0012] In a preferred embodiment of the present invention, the 12 metal electrodes are fabricated using flexible circuit board processing technology. The 12-electrode non-contact impedance sensor is based on C... 4 The D principle constructs a resistance measurement path, where the electrode does not directly contact the medium being measured inside the pipe, thus realizing non-contact resistance measurement.

[0013] As a preferred embodiment of the present invention, the I / V conversion unit of the detection module is connected to the feedback loop of the inverting amplifier circuit by constructing multiple parallel feedback resistors, and automatically selecting different feedback resistors according to the measurement frequency, so that the gain of the operational amplifier remains stable at different frequencies, providing a high-quality detection signal for subsequent impedance measurement.

[0014] As a preferred embodiment of the present invention, the establishment of the mechanism model includes the following steps:

[0015] 1.1) Introduce an upper limit cutoff frequency f c The impact on the operational amplifier is determined based on the operational amplifier's DC open-loop gain A and the system's operating frequency f, establishing the operational amplifier's open-loop gain A. ol The model:

[0016]

[0017] Where j is the imaginary unit;

[0018] 1.2) Based on the structure of the inverting amplifier circuit, construct the closed-loop control system of the operational amplifier in the inverting amplifier circuit, and derive the forward coefficient α and reverse coefficient β of the closed-loop control system, as well as the closed-loop gain A of the closed-loop control system. cl The model:

[0019]

[0020] 1.3) Based on the closed-loop gain model of the closed-loop control system, the closed-loop gain A of the operational amplifier in the inverting amplifier circuit at the operating frequency f is derived. cl and the impedance Z to be measured x The mechanism model between them is:

[0021]

[0022] Among them, R f It is the feedback resistor connected to the operational amplifier feedback loop when the operating frequency is f.

[0023] As a preferred embodiment of the present invention, the establishment of the empirical model includes the following steps:

[0024] 2.1) Use multiple known resistor-capacitor combinations as the impedance Z to be measured. x The closed-loop gain A of the operational amplifier at different operating frequencies was calculated using the established mechanistic model for these impedances under test. cl ;

[0025] 2.2) The impedance Z to be measured in step 2.1) x As the detection object, the multi-frequency capacitively coupled impedance tomography system was used to measure the corresponding system demodulation results at different operating frequencies, i.e., the amplitude ratio V. mag and phase difference V phs ;

[0026] 2.3) For each operating frequency, based on a linear model, the least squares method is used to calculate the amplitude ratio V. magThe logarithm of the closed-loop gain of the operational amplifier |A cl Perform empirical modeling to obtain model parameters k1 and b1 for each operating frequency:

[0027] |A cl |=k1V mag +b1

[0028] Similarly, based on the linear model, the least squares method is used to calculate the phase difference V. phs The phase ∠A of the operational amplifier closed-loop gain cl Perform empirical modeling to obtain model parameters k2 and b2 at each operating frequency:

[0029] ∠A cl =k2V phs +b2.

[0030] The present invention also provides a multi-frequency capacitive coupling impedance tomography method based on the system, which includes the following steps:

[0031] (1) For an unknown distribution of the medium to be measured, the demodulation results of the distribution at different operating frequencies are obtained by measuring the multi-frequency capacitively coupled impedance tomography system described above, i.e., the amplitude ratio V at different frequencies. mag and phase difference V phs Based on the empirical model, the magnitude logarithm |A| of the corresponding operational amplifier closed-loop gain at different operating frequencies was calculated. cl | and phase ∠A cl The closed-loop gain A is obtained. cl ;

[0032] (2) Operational amplifier closed-loop gain A based on the distribution of the medium under test at different frequency points cl Based on the inverse function of the operational amplifier mechanism model The equivalent impedance Z of the medium under test at different operating frequencies was calculated. x ;

[0033] (3) Repeat steps (1) and (2) for all measurement electrode pairs until all independent measurement electrode pairs have obtained multi-frequency impedance data of the distribution of the medium under test, forming a frame of impedance data at each working frequency; then, at each working frequency, use an image reconstruction algorithm to reconstruct the real part and imaginary part of the frame of impedance data respectively, to obtain the real part image and imaginary part image of impedance reflecting the distribution of the medium under test at different working frequencies, thus realizing multi-frequency impedance imaging.

[0034] The advantages of this invention compared to the prior art are as follows:

[0035] 1) Compared with existing resistance tomography systems and methods, this invention can achieve non-contact measurement of the object under test, avoiding problems such as electrode polarization, electrochemical corrosion, and electrode contamination caused by contact measurement. It can also obtain complete impedance information and simultaneously measure and image the real and imaginary parts of the impedance, thus having better prospects for practical industrial applications.

[0036] 2) Compared with existing capacitively coupled impedance tomography (CCEIT) systems and methods, this invention can realize multi-frequency impedance measurement and imaging, obtain impedance spectrum information of the medium under test, and provide hardware support for subsequent research on impedance frequency characteristics and multi-frequency CCEIT technology, as well as further improvement of imaging performance.

[0037] 3) Compared with existing multi-frequency impedance tomography systems and methods, this invention can achieve measurement of the real and imaginary parts of multi-frequency impedance over a wider frequency range. Furthermore, the established multi-frequency impedance measurement model simultaneously considers the mechanism model of integrated circuits and the empirical model of system errors, enabling higher quality impedance measurement and imaging. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of a multi-frequency capacitively coupled electrical impedance tomography system;

[0039] Figure 2 Schematic diagram of a 12-electrode non-contact impedance sensor;

[0040] Figure 3 This is a schematic diagram of the detection module;

[0041] Figure 4 Schematic diagram of I / V conversion unit;

[0042] Figure 5 This is a schematic diagram of the data acquisition module;

[0043] Figure 6 This is a schematic diagram illustrating the process of establishing a multi-frequency impedance measurement model.

[0044] Figure 7 A schematic diagram of the forward and reverse voltages of the I / V conversion unit;

[0045] Figure 8 This is a schematic diagram of the closed-loop control system of the operational amplifier in an inverting amplifier circuit.

[0046] Figure 9 This is a schematic diagram of the multi-frequency capacitively coupled electrical impedance tomography method.

[0047] Figure 10 This is a diagram showing the system imaging results. Detailed Implementation

[0048] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0049] like Figure 1 The diagram shows the overall framework of the present invention. The multi-frequency capacitively coupled impedance tomography system mainly consists of a 12-electrode non-contact impedance sensor, six identical detection modules, a data acquisition module, and a computer for multi-frequency impedance calculation and imaging. The 12-electrode non-contact impedance sensor consists of 12 identical metal electrodes, fabricated using flexible circuit board technology, uniformly bonded in a ring array to the outer wall of an insulated pipe containing the object under test, thus constructing a impedance measurement path for the object. The detection module includes a channel selection unit, an I / V conversion unit, and a CPLD, used for channel switching and current-to-voltage conversion of the detection signal. The data acquisition module mainly includes an MCU, a configurable excitation source unit, a signal demodulation unit, a communication unit, and a power supply unit, used for generating multi-frequency excitation signals and demodulating detection signals, and responsible for communication with the computer. A multi-frequency impedance measurement model is established within the computer, including a mechanistic model obtained by modeling the inverting amplifier circuit and an empirical model obtained by empirically modeling the system demodulation results. After receiving the demodulation results from the data acquisition module, the computer calculates the real and imaginary parts of the impedance under test in real time and uses an image reconstruction algorithm to image the real and imaginary parts of the impedance separately.

[0050] like Figure 2The diagram illustrates the structure of the 12-electrode non-contact impedance sensor of this invention. Twelve electrodes of identical shape and size are uniformly attached to the outer wall of an insulating pipe in a circular array. Each electrode has a lead wire connected to a channel selection unit of the detection module. For each electrode, a coupling capacitor is formed between the electrode and the object being measured through the insulating pipe wall, thus constructing a measurement path for the impedance to be measured and achieving non-contact impedance measurement. When a pair of measuring electrodes is selected, one electrode serves as the excitation electrode, and the other as the detection electrode, realizing the impedance measurement of the distribution of the medium to be measured between the measuring electrode pairs. Within one measurement cycle of the system, measuring electrode pairs (a total of 66 pairs) are selected and switched sequentially, in the following order: first electrode and second electrode, first electrode and third electrode, ..., first electrode and twelfth electrode; second electrode and third electrode, ..., second electrode and twelfth electrode, ...; until the eleventh to twelfth electrode. The detection is performed sequentially according to the above order until the measurement of 66 independent impedances at each operating frequency is completed. This means that one frame of data required for imaging at each operating frequency is obtained, which includes 66 independent real part impedance measurements and 66 independent imaginary part impedance measurements, for subsequent real part image reconstruction and imaginary part image reconstruction.

[0051] like Figure 3 As shown, the detection module includes a channel selection unit, an I / V conversion unit, and a CPLD. This invention has six identical detection modules, each containing two channel selection units, resulting in a total of 12 channel selection units, each connected to one of 12 metal electrodes. The channel selection units switch channels by closing or opening high-frequency relays (HF355) at different positions, thus enabling the selection and switching of measurement electrode pairs, and consequently, the measurement of the impedance distribution of the medium under test between different measurement electrode pairs. The CPLD receives control information from the MCU, generates control logic, and controls the closing or opening of the corresponding relays. When a measurement electrode pair is selected, one electrode switches to excitation mode, the other to detection mode, and the remaining 10 electrodes not in that measurement electrode pair switch to idle mode. Therefore, each electrode has three modes: excitation mode, detection mode, and idle mode. Figure 3 As shown, when the electrode is in idle mode, relays S1, S2, and S3 of the connected channel selection unit all switch to port 2; when the electrode is in excitation mode, relays S1 and S2 of the connected channel selection unit switch to port 1, and relay S3 switches to port 2. At this time, the excitation signal V... in The current will be applied to the electrode; when the electrode is in detection mode, relays S1 and S3 of the connected channel selection unit switch to port 1, and relay S2 switches to port 2. At this time, the current detection signal I... outThe current will flow from the electrode into the detection module, where the I / V conversion unit will perform current-to-voltage conversion to obtain the voltage detection signal V. out This is used for subsequent impedance measurements.

[0052] like Figure 4 The image shows the I / V conversion unit in the detection module. This unit mainly consists of an inverting amplifier circuit based on the OPA659 operational amplifier. It constructs a feedback loop with multiple parallel feedback resistors connected to the operational amplifier, automatically selecting different feedback resistors R according to the operating frequency. f By incorporating a feedback loop, the operational amplifier's gain remains stable over a wide frequency range, ensuring its performance and avoiding measurement errors caused by the gain-bandwidth product limitation. This provides a high-quality detection signal for subsequent impedance measurements. The inverting amplifier circuit measures the distribution of the dielectric under test, i.e., the equivalent impedance Z... x As the impedance of the inverting input terminal, when the excitation signal V in When applied to the excitation electrode, a current detection signal I carrying the equivalent resistivity information of the measured medium distribution is obtained on the detection electrode. out The current is converted into a voltage signal V by an I / V conversion unit. out This is used for subsequent impedance measurements. The automatic selection and switching of the feedback resistor is also controlled by a high-frequency relay (HF355) S controlled by a CPLD. f1 S f2 S f3 accomplish.

[0053] like Figure 5 As shown, the data acquisition module mainly includes an MCU, a configurable excitation source unit, a signal demodulation unit, a communication unit, and a power supply unit. The MCU uses an STM32F103 and is responsible for issuing control commands to each module and unit, enabling them to work collaboratively. The configurable excitation source unit consists of a Direct Digital Synthesizer (DDS) (AD9958) and a Low Pass Filter (LPF), responsible for generating sinusoidal excitation signals V at different frequencies. in The signal demodulation unit consists of a gain phase detector (GPD) (AD8302) and an analog-to-digital converter (ADC) (ADS8328). The GPD converts the excitation signal V... in and detection signal V out As input, the signal is demodulated internally and two DC demodulated signals are output. After sampling by the ADC, the demodulation result, i.e., the amplitude ratio V, is obtained. mag and phase difference Vphs The communication unit consists of a Universal Serial Bus (USB) converter chip (CH340N), which converts the serial port to a USB interface and sends the demodulation results to the computer for multi-frequency impedance calculation and imaging; the power supply unit is responsible for providing power to the entire system.

[0054] like Figure 6 The diagram illustrates the process of establishing the multi-frequency impedance measurement model of this invention, comprising two parts: mechanistic modeling of the inverting amplifier circuit and empirical modeling of the system demodulation results. Through the mechanistic model and the empirical model, the measured impedance Z is established. x Demodulation result V from the system mag and V phs A relationship model is established. For the inverting amplifier circuit in the I / V conversion unit, a mechanistic model is created to describe the measured impedance Z. x and the closed-loop gain A of the operational amplifier cl The relationship between A cl =h3(Z x For the demodulation results output by the system, two empirical models are established to describe the amplitude ratio V. mag The logarithm of the magnitude of the closed-loop gain |A cl The relationship between | and the phase difference V phs Phase ∠A of the closed-loop gain cl The relationship between them, i.e., |A cl |=h1(V mag ) and ∠A cl =h2(V phs ).

[0055] The mechanism modeling of the inverting amplifier circuit is derived in detail below:

[0056] like Figure 7 The diagram shows a simplified form of the I / V conversion unit in this invention. The inverting amplifier circuit in the I / V conversion unit can be considered as a closed-loop control system of an operational amplifier, since the voltage V at the non-inverting input terminal... + Since the voltage is 0V, the open-loop input voltage of the operational amplifier is -V. - The inverting input voltage V - It comes from two aspects, one of which is the excitation signal V. in The generated forward input voltage V f The other is determined by the detection signal V. out The generated reverse input voltage V b Therefore, the open-loop input voltage of the operational amplifier is:

[0057] V + -V - =-V - =-(V f +Vb (1)

[0058] like Figure 8 The diagram shows the closed-loop control system of the operational amplifier in the inverting amplifier circuit of the I / V conversion unit of this invention. In the figure, α represents V. f Forward propagation coefficient, β is V b The reverse transfer coefficient, which can be expressed as the impedance Z under test. x and feedback resistor R f To indicate:

[0059]

[0060]

[0061] A ol The open-loop gain of the operational amplifier is expressed by the following formula:

[0062]

[0063] Where A is the DC open-loop gain of the operational amplifier, j is the imaginary unit, f is the system operating frequency, and fj is the operating frequency. c This is the upper cutoff frequency of the operational amplifier, and its value is approximately equal to the bandwidth of the operational amplifier. It can be expressed as the ratio of the operational amplifier's gain-bandwidth product GBP to A:

[0064]

[0065] according to Figure 8 In a closed-loop control system, the closed-loop gain A of the operational amplifier... cl Described as:

[0066]

[0067] Finally, according to the above formula, the closed-loop gain A of the operational amplifier is... cl and the impedance Z to be measured x The mechanism model between them can be described as follows:

[0068]

[0069] Where f c A and A are known parameters of the operational amplifier, which can be obtained from the datasheet. The feedback resistor R... f It can also be determined based on the current measurement frequency. Therefore, for each operating frequency, the closed-loop gain A of the operational amplifier... cl and the impedance Z to be measured x They can be mutually determined by mechanistic models.

[0070] The derivation and steps for empirical modeling of the system demodulation results are as follows:

[0071] Step 1: Use multiple known resistor-capacitor combinations as the impedance Z to be measured. x The closed-loop gain A of the operational amplifier at different operating frequencies was calculated using the mechanistic model (7). cl .

[0072] Step Two: Using the multi-frequency capacitively coupled impedance tomography system of this invention, the multiple sets of impedances to be measured in Step One are used as detection objects to obtain the demodulation results of each impedance to be measured at different operating frequencies, including the amplitude ratio V. mag and phase difference V phs The demodulation result is compared with the excitation signal V. in Detection signal V out The relationship is as follows:

[0073]

[0074] V phs =V Φ (∠V out -∠V in (9)

[0075] Among them, V SLP It is the amplitude-to-slope ratio of GPD, V Φ It is the phase difference slope of GPD.

[0076] Step 3: For each operating frequency, use the least squares method to calculate the amplitude ratio V. mag Logarithm of the closed-loop gain of the operational amplifier Perform linear fitting to determine the coefficients k1 and b1; for the phase difference V phs The phase ∠A of the operational amplifier closed-loop gain cl =∠V out -∠V in Perform linear fitting to determine the coefficients k2 and b2, and obtain an empirical model of the system demodulation output:

[0077] |A cl |=h1(V max )=k1V mag +b1 (10)

[0078] ∠A cl =h2(V phs )=k2V phs +b2 (11)

[0079] Figure 9 The diagram shows a multi-frequency capacitively coupled electrical impedance tomography method based on this system, which mainly includes the following steps:

[0080] Step 1: For any unknown distribution of the test medium in practical applications, the multi-frequency capacitively coupled impedance tomography system of this invention is used to measure and obtain the system demodulation results at different operating frequencies, i.e., the amplitude ratio V at different operating frequencies. mag and phase difference V phs Based on empirical models (10) and (11), the magnitude logarithm of the operational amplifier closed-loop gain |A| at different operating frequencies was calculated. cl | and phase ∠A cl The closed-loop gain A is obtained. cl .

[0081] Step 2: Based on the distribution of the dielectric under test, the closed-loop gain A of the operational amplifier at different operating frequencies cl The equivalent electrical impedance Z of the medium under test at different operating frequencies is calculated from the inverse function of the mechanism model (7). x The inverse function can be expressed as:

[0082]

[0083] Step 3: Repeat steps 1 and 2 for all measurement electrode pairs until all independent measurement electrode pairs have obtained multi-frequency impedance data of the medium under test, forming a frame of impedance data at different operating frequencies. Then, at each operating frequency, an image reconstruction algorithm is used to reconstruct the real and imaginary parts of the impedance data frame, obtaining real and imaginary impedance images reflecting the distribution of the medium under test at different operating frequencies, thus realizing multi-frequency impedance imaging.

[0084] To verify the effectiveness of this invention, the system was tested at frequencies up to 20MHz. Figure 10 The imaging results of the system at six operating frequencies are shown, where white represents water and black represents a rubber rod, simulating the liquid and gas phases in a two-phase flow, respectively. The linear back projection (LBP) algorithm was used to reconstruct the real and imaginary parts of the impedance at different operating frequencies. The results demonstrate that the multi-frequency capacitively coupled impedance tomography system of this invention is effective; the real and imaginary impedance images obtained by the system at different operating frequencies are essentially consistent with the true distribution images.

[0085] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A multi-frequency capacitively coupled electrical impedance tomography system, characterized in that, It includes a 12-electrode non-contact impedance sensor, six identical detection modules, a data acquisition module, and a computer for multi-frequency impedance calculation and imaging; The 12-electrode non-contact impedance sensor consists of 12 metal electrodes of the same shape and size uniformly pasted in a ring array on the outer wall of an insulating pipe containing the measured medium. The metal electrodes do not directly contact the measured medium inside the pipe. The measurement path is constructed through the insulating pipe wall to realize non-contact impedance measurement. Each detection module includes two channel selection units for channel switching, one I / V conversion unit for current-to-voltage conversion of the detection signal, and one CPLD. The two channel selection units are connected to two metal electrodes respectively, and the six detection modules are connected to twelve metal electrodes. The electrode states are selected and switched by high-frequency relays in the channel selection units, thereby realizing the switching of the measurement electrode pairs. The I / V conversion unit consists of an inverting amplifier circuit containing an operational amplifier, which performs current-to-voltage conversion on the detection signal for subsequent impedance measurement. The CPLD is used to generate control logic to control the closing or opening of the corresponding relays. The data acquisition module mainly includes a microprocessor unit, a configurable excitation source unit, a signal demodulation unit, a communication unit, and a power supply unit. The microprocessor unit is responsible for controlling the coordinated operation of each module. The configurable excitation source unit generates sinusoidal excitation signals of different frequencies. The signal demodulation unit demodulates the amplitude ratio and phase difference between the detection signal and the excitation signal. The communication unit sends the demodulation results to the computer. The power supply unit provides power to the entire system. The computer has a multi-frequency impedance measurement model, which includes a mechanistic model obtained by modeling the mechanism of the inverting amplifier circuit and an empirical model obtained by modeling the demodulation results of the system. After receiving the demodulation results sent by the data acquisition module, the computer calculates the real and imaginary parts of the impedance to be measured in real time and uses an image reconstruction algorithm to image the real and imaginary parts of the impedance respectively. The establishment of the aforementioned mechanistic model includes the following steps: 1-1) Introducing an upper limit cutoff frequency Impact on operational amplifiers, based on the DC open-loop gain of operational amplifiers System operating frequency Establish the open-loop gain of the operational amplifier. The model: ； in, The imaginary unit; It is the DC open-loop gain of the operational amplifier. It is the imaginary unit. It is the system operating frequency. This is the upper cutoff frequency of the operational amplifier, and its value is approximately equal to the bandwidth of the operational amplifier. It can be expressed as the gain-bandwidth product of the operational amplifier. and The ratio: ； 1-2) Based on the structure of the inverting amplifier circuit, construct a closed-loop control system for the operational amplifier in the inverting amplifier circuit and derive the forward coefficients of the closed-loop control system. and reverse coefficient and the closed-loop gain of the closed-loop control system The model: ； 1-3) Based on the closed-loop gain model of the closed-loop control system, the following is derived for the operating frequency: At that time, the closed-loop gain of the operational amplifier in the inverting amplifier circuit and the impedance to be measured The mechanism model between them is: ； in, The operating frequency is The feedback resistor of the operational amplifier feedback loop connected to the system; The establishment of the empirical model includes the following steps: 2-1) Use multiple known resistor-capacitor combinations as the impedance to be measured. The closed-loop gain of the operational amplifier at different operating frequencies was calculated using the established mechanistic model for these impedances under test. ; 2-2) The impedance to be measured in step 2-1) As the detection object, the multi-frequency capacitively coupled impedance tomography system was used to measure the corresponding system demodulation results at different operating frequencies, i.e., amplitude ratio. and phase difference ; 2-3) For each operating frequency, based on the linear model, the least squares method is used to calculate the amplitude ratio. Logarithm of the closed-loop gain of the operational amplifier Perform empirical modeling to obtain model parameters at each operating frequency. and : ； Similarly, based on the linear model, the least squares method is used to calculate the phase difference. Phase with operational amplifier closed-loop gain Perform empirical modeling to obtain model parameters at each operating frequency. and : 。 2. The multi-frequency capacitively coupled electrical impedance tomography system according to claim 1, characterized in that, The I / V conversion unit of the detection module is connected to the feedback loop of the inverting amplifier circuit by constructing multiple parallel feedback resistors, and automatically selects different feedback resistors according to the measurement frequency, so that the gain of the operational amplifier remains stable at different frequencies, providing a high-quality detection signal for subsequent impedance measurement.

3. The multi-frequency capacitively coupled electrical impedance tomography system according to claim 1, characterized in that, The CPLD receives control information sent by the microprocessor unit, generates control logic, and controls the closing or opening of the corresponding relays; thereby putting the electrodes in different modes; each electrode has three modes, namely excitation mode, detection mode and idle mode; when a pair of measurement electrodes is selected, one electrode in the measurement electrode pair switches to excitation mode, the other electrode switches to detection mode, and the remaining 10 electrodes not in the measurement electrode pair switch to idle mode.

4. The multi-frequency capacitively coupled electrical impedance tomography system according to claim 1, characterized in that, In the data acquisition module, the configurable excitation source unit generates sinusoidal AC signals of different frequencies as excitation signals by combining direct digital synthesis technology and low-pass filtering technology. The signal demodulation unit uses amplitude-phase detection technology to obtain the amplitude ratio and phase difference between the detection signal and the excitation signal for subsequent impedance calculation.

5. The multi-frequency capacitively coupled electrical impedance tomography system according to claim 1, characterized in that, The mechanistic model is used to describe the input-output characteristics of the inverting amplifier circuit; the empirical model is used to correct the measurement error of the system and improve the measurement and imaging accuracy of the impedance.

6. A multi-frequency capacitive coupling impedance tomography method based on the system of claim 1, characterized in that, The steps include the following: (1) For an unknown distribution of the medium to be measured, the demodulation results of the distribution at different operating frequencies are obtained by measuring the multi-frequency capacitively coupled impedance tomography system, i.e., the amplitude ratio at different frequencies. and phase difference ; Based on the aforementioned empirical model, the magnitude logarithm of the closed-loop gain of the operational amplifier at different operating frequencies was calculated. and phase The closed-loop gain is obtained. ; (2) Operational amplifier closed-loop gain based on the distribution of the medium under test at different frequency points Based on the inverse function of the operational amplifier mechanism model The equivalent electrical impedance of the medium under test at different operating frequencies was calculated. ; (3) Repeat steps (1) and (2) for all measurement electrode pairs until all independent measurement electrode pairs have obtained multi-frequency impedance data of the medium under test, forming a frame of impedance data at each working frequency; then, at each working frequency, use an image reconstruction algorithm to reconstruct the real part and imaginary part of the frame of impedance data respectively, to obtain the real part image and imaginary part image of impedance reflecting the distribution of the medium under test at different working frequencies, thus realizing multi-frequency impedance imaging.

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