Built-in electrical capacitance tomography sensor based on flexible electrode and installation method

Abstract

The invention discloses a built-in electrical capacitance tomography sensor based on a flexible electrode and an installation method. The sensor comprises a metal pipeline, a sampling signal line, a flexible electrode, a limiting tool and a fastening tool, wherein the metal pipeline comprises a straight-through pipeline and standard flanges arranged at the two ends of the straight-through pipeline; the flexible electrode integrates a sampling electrode, a shielding ring and a non-metal film, the flexible electrode is mounted in the sensor in a crimping manner, and a pole piece combination of the flexible electrode can be freely replaced according to use scenes and requirements. The cost of the electrical capacitance tomography sensor is reduced through the flexible electrode, the design and manufacturing efficiency is improved, the noise introduced by the front end of the sensor is greatly reduced, and therefore the sampling precision and stability are improved. According to the crimping fixing scheme of the flexible electrode, low temperature is not used, the installation steps and time of the sensor are reduced, the flatness of the sampling electrode is improved, deformation in the temperature changing process is reduced, and then the stability of the sampling front end is improved.

Description

Technical Field

[0001] The present invention relates to the fields of cryogenic refrigeration engineering technology, fluid flow process monitoring and sensor science and technology, and in particular to a built-in high-voltage and low-temperature capacitance tomography sensor and an installation method thereof. Background Art

[0002] Gas-liquid two-phase flow is a common flow form in cryogenic fluid devices. Determining the phase distribution of cryogenic two-phase flow is crucial to the cryogenic fluid industry. Due to the properties of cryogenic fluids and the working environment, the measurement of cryogenic fluid phase distribution remains a thorny issue.

[0003] In order to obtain the phase distribution of low-temperature two-phase flow, there are many methods, including the use of capacitance probe method, radio frequency sensor (RF-sensor), two-electrode capacitance sensor and particle image velocimetry (PIV). In contrast, capacitance tomography (ECT), as a non-invasive measurement technology, can reconstruct the phase distribution image based on the difference in dielectric constant between phases. It also has the advantages of easy assembly, fast imaging speed and low cost. Therefore, since the emergence of ECT technology in the 1980s, it has been widely used in the image reconstruction of normal temperature fluids, such as fluidized beds, water / oil flows, water / solid flows, aircraft engines and other multiphase flow application scenarios.

[0004] However, there are few experimental studies on electrical capacitance tomography applied to cryogenic two-phase flows. The difficulty is that the dielectric constant ratio of a cryogenic liquid to its saturated vapor is usually an order of magnitude smaller than that of a room temperature fluid pair. This requires the ECT system to have higher sensitivity and noise immunity.

[0005] In the practice of normal temperature application scenarios, flexible sensors made of flexible printed circuit boards (FPC) are significantly superior to traditional patch sensors in terms of design and manufacturing efficiency, installation accuracy, signal acquisition stability, etc., and are more suitable for low-temperature application scenarios with higher requirements. However, after migrating to low temperatures, it brings a series of problems: 1) The flexible electrode will curl along the direction of the long electrode after experiencing extremely low temperatures, and bubbles will be generated if it is not fixed; 2) The container of the low-temperature fluid is mostly made of metal to ensure strength, and the capacitance tomography system requires the sensor pipe to be non-metallic, so the capacitance tomography sensor cannot be directly attached to the outside of the metal pipe 3) If non-metallic materials are needed to replace metal pipes, non-metallic pipes are difficult to process and have extremely thick pipe walls, which greatly wastes the high-sensitivity measurement area close to the sensor, thereby reducing the overall imaging performance of the capacitance tomography system. Summary of the invention

[0006] The present invention provides a built-in capacitance tomography sensor based on flexible electrodes and an installation method, which can fully utilize the advantages of flexible electrodes in sampling accuracy, eliminate the deformation of flexible electrodes through limiting fixtures and fastening fixtures, and greatly reduce the ineffective medium between the electrodes and the fluid to be measured, further improving the sampling accuracy of the system. The electrodes of the sensor adopt a crimping solution, which greatly reduces the installation steps and difficulty, and reduces the installation materials. It has the characteristics of simple installation and high sampling accuracy.

[0007] To this end, the present invention adopts the following technical solutions:

[0008] On the one hand, the present invention provides a built-in capacitance tomography sensor based on a flexible electrode, which includes a metal pipe, a sampling signal line, a limit fixture, a flexible electrode and a fastening fixture;

[0009] The two ends of the metal pipe are respectively provided with standard flanges, and the limiting tooling and the fastening tooling are both cylindrical structures; the limiting tooling is tightly arranged inside the metal pipe, the flexible electrode is arranged on the inner surface of the limiting tooling, and the fastening tooling is arranged in the innermost layer and tightly arranged on the flexible electrode, so that the flexible electrode is fastened between the limiting tooling and the fastening tooling;

[0010] The flexible electrode comprises a sampling electrode, a shielding ring and a non-metallic film for packaging; the shielding ring is arranged at both ends of the inner surface of the limiting tooling to shield the axial magnetic field outside the measurement domain, and the flexible electrode has multiple pieces, which are evenly arranged circumferentially in the area between the shielding rings on the inner surface of the fixed tooling; two non-metallic films are respectively covered on the inner and outer surfaces of the sampling electrode and the shielding ring to realize the packaging of the flexible electrode; wherein the sampling electrode and the shielding ring are made of metal, and the thickness is less than 0.2mm;

[0011] One end of the sampling signal line is connected to the sampling electrode, and the other end passes through a preset hole on the limit fixture and the metal pipe to be connected to an external capacitance tomography system.

[0012] In another aspect, the present invention provides a method for installing the above-mentioned sensor, which comprises the following steps:

[0013] 1) Arrange the sampling electrode and the shielding ring, and use two non-metallic films to encapsulate the arranged sampling electrode and the shielding ring to obtain a flexible electrode, and open a window in the non-metallic film at the connection point of the sampling electrode; connect one end of the sampling signal line to the connection point of the sampling electrode;

[0014] 2) Push the limit fixture into the interior of the external metal pipe, adjust the angle and position so that the holes for the sampling signal lines to pass through are aligned one by one;

[0015] 3) Pass the other end of the sampling signal line through the holes on the limit fixture and the metal pipe and lead it out; fit the flexible electrode to the inner surface of the limit fixture; insert the fastening fixture into the circular tube surrounded by the flexible electrode, so that the flexible electrode is fastened between the limit fixture and the fastening fixture;

[0016] 4) Use low-temperature glue to fill the holes on the limit fixture and metal pipes to complete the internal and external sealing;

[0017] 5) Connect the flanges at both ends of the metal pipe to the flanges of the section to be inspected, and connect the sampling signal line to the capacitance tomography system.

[0018] The beneficial effects that the present invention can produce include:

[0019] 1. The built-in capacitive tomography sensor based on flexible electrodes provided by the present invention applies flexible electrodes to metal pipes in low-temperature scenarios. Since the pipes are made of metal and both sides are standard flanges, there is no need to consider the connection between the metal flange of the incoming flow and the non-metallic pipe, so the installation and disassembly between the sensor and the pipe are convenient.

[0020] 2. The built-in capacitive tomography sensor based on flexible electrodes provided by the present invention places the flexible electrodes inside the metal pipe, reducing the distance between the sensor electrodes and the fluid to be measured, and spans the metal casing, effectively reducing the invalid imaging measurements of the thick-walled casing during the sampling process, and in principle improving the sampling accuracy at the forefront of the sampling process.

[0021] 3. The built-in capacitive tomography sensor based on a flexible electrode provided by the present invention firmly presses the flexible electrode between the fluid to be measured and the external metal pipe through a limiting fixture and a fastening fixture, strictly limits the deformation of the flexible electrode under temperature changes, improves the flatness of the flexible electrode, improves the stability of the front end of the capacitive tomography sampling, and further improves the imaging accuracy of the capacitive tomography system at the back end.

[0022] 4. The installation method provided by the present invention adopts a crimping method when fixing the flexible electrode, fastening the tooling, and limiting the tooling, and does not require the use of low-temperature glue, which simplifies the installation steps and does not require the bonding of the flexible electrode and the tooling. In addition, the amount of low-temperature glue used is reduced, which also reduces the material cost during installation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is an isometric view of the overall structure of a built-in capacitive tomography sensor based on a flexible electrode of the present invention;

[0024] Figure 2 It is a front view of the overall structure of a built-in capacitive tomography sensor based on a flexible electrode of the present invention;

[0025] Figure 3 is an isometric view of a metal pipe of a built-in electrical capacitance tomography sensor based on a flexible electrode of the present invention;

[0026] Figure 4 It is a front view of a metal pipe of a built-in capacitance tomography sensor based on a flexible electrode of the present invention;

[0027] Figure 5 It is an isometric view of a fastening tool, a flexible electrode, and a limiting tool of a built-in electrical capacitance tomography sensor based on a flexible electrode of the present invention after being combined;

[0028] Figure 6 It is an isometric view of a flexible electrode of a built-in electrical capacitance tomography sensor based on a flexible electrode of the present invention;

[0029] Figure 7 It is an isometric view of a fastening tool for a built-in capacitive tomography sensor based on a flexible electrode of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described and illustrated below in conjunction with specific embodiments. The embodiments are merely exemplary of the present disclosure and do not define the scope of limitation. The technical features of each embodiment of the present invention may be combined accordingly without conflicting with each other.

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] like Figure 1-7 As shown, the built-in capacitance tomography sensor based on the flexible electrode of the present invention mainly includes a metal pipe 1, a sampling signal line 2, a limit fixture 3, a flexible electrode 4 and a fastening fixture 5; the sensor of the present invention can be used to measure the gas content and bubble position of low-temperature fluids (such as liquid nitrogen-saturated nitrogen vapor two-phase flow).

[0033] The metal pipe 1 is provided with a standard flange at both ends, and the limiting fixture 3 and the fastening fixture 5 are both cylindrical structures; the limiting fixture 3 is closely arranged inside the metal pipe 1, the flexible electrode 4 is arranged on the inner surface of the limiting fixture 3, and the fastening fixture 5 is arranged in the innermost layer and closely arranged on the flexible electrode 4, so that the flexible electrode 4 is fastened between the limiting fixture 3 and the fastening fixture 5;

[0034] The flexible electrode 4 includes a sampling electrode 41, a shielding ring 42 and a non-metallic film 43 for packaging; the shielding ring 42 is arranged at both ends of the inner surface of the limiting fixture 3 to shield the axial magnetic field outside the measurement domain, and the flexible electrode 4 has multiple pieces, which are evenly arranged in the circumferential direction in the area between the shielding rings on the inner surface of the fixed fixture 3; two non-metallic films 43 respectively cover the inner and outer surfaces of the sampling electrode 41 and the shielding ring 42 to realize the packaging of the flexible electrode 4; wherein, the sampling electrode 41 and the shielding ring 42 are made of metal, and the thickness is less than 0.2mm; in a specific embodiment of the present invention, the sampling electrode 41 and the shielding ring 42 are made of copper. The shielding ring 42 is grounded when the sensor is working. The grounding method is to connect the pad above the shielding ring to the shielding layer of the sampling signal line, and the shielding layer is connected to the ground of the external device to achieve the effect of grounding the shielding ring.

[0035] One end of the sampling signal line 2 is connected to the sampling electrode 41 , and the other end passes through a preset hole on the limiting fixture 3 and the metal pipe 1 to be connected to an external capacitance tomography system.

[0036] In a specific embodiment of the present invention, an 8-electrode capacitance tomography scheme is adopted, the straight pipe in the external metal pipe 1 is a standard stainless steel pipe of standard DN65 GB2.5MPa, the standard flanges at both ends are standard stainless steel flanges of standard DN65 GB2.5MPa, and the diameter of the external through hole 11 is 5mm. The working fluid in the pipe is liquid nitrogen.

[0037] like Figure 3-Figure 5 As shown, the metal pipe 1 is a straight metal pipe, and an outer through hole 11 is provided on the pipe wall of the metal pipe 1 for the sampling signal line 2 to pass through. The outer through holes 11 on the metal pipe 1 are evenly arranged along the circumference of the same cross section of the metal pipe, and the diameter of the outer through hole 11 is not less than the diameter of the sampling signal line 2. The main body of the limit fixture 3 is a non-metallic circular tube, on which an inner through hole 31 is provided for the sampling signal line 2 to pass through. The number of the inner through holes 31 is equal to the number of the outer through holes 11 of the metal pipe 1, the number of the sampling signal lines 2, and the number of the sampling electrodes 41, and they correspond one to one; the inner surface of the limit fixture 3 is smooth without dents or protrusions, and the material is an insulating material; the inner and outer circles of the limit fixture 3 are coaxial, and the outer circle diameter is equal to the inner surface diameter of the metal pipe 1; the limit fixture 3 and the metal pipe 1 adopt an interference fit. The diameter of the inner through hole 31 on the limit fixture 3 is not greater than the diameter of the outer through hole 11.

[0038] In a specific embodiment of the present invention, an RG316 coaxial cable is used as the sampling signal line 2 , and the wire is welded on the flexible electrode 4 .

[0039] like Figure 5 As shown, in a specific embodiment of the present invention, the material of the limiting tool is polytetrafluoroethylene, the thickness is 7 mm, and the length is 120 mm.

[0040] like Figure 6 As shown, in a specific embodiment of the present invention, the finished board thickness of the flexible electrode is 0.11mm, the number of board layers is 1, the non-metallic film material is also polyimide, the thickness is 25 microns, the internal electrode material is copper, the copper thickness is 1oz, and the overall length of the flexible electrode is 115mm. Both non-metallic films 43 are complete films, and the non-metallic film on the outer surface of the flexible electrode has a window at the connection point of the sampling electrode so that the sampling signal line 2 can be welded to the connection point of the sampling electrode.

[0041] like Figure 7 As shown, in a specific embodiment of the present invention, the material of the fastening fixture is polytetrafluoroethylene, with a thickness of 2 mm, a length of 120 mm, and an inner through hole diameter of 5 mm. The fastening fixture 5 is a non-metallic round tube, and its outer diameter is equal to the inner diameter of the limiting fixture 3, and the two are interference fit; the inner and outer surfaces of the fastening fixture 5 are smooth surfaces, and its inner diameter is the same as the inner diameter of the incoming flow pipeline, and its circumferential length is not less than the length of the sampling electrode 41 in the length direction.

[0042] like Figure 1 As shown, in a specific embodiment of the present invention, epoxy resin is used as a sealing material to fill the gap between the signal wire 2 and the outer through hole 11 and the inner through hole 31 to achieve sealing.

[0043] In particular, for standard electrical capacitance tomography applications, the circumferential coverage of the sensing electrode is 0.75-0.85, and the circumferential width is Where a is the circumferential coverage of the pole piece, n is the number of pole pieces, and d is the outer diameter of the fastening fixture 5. The sampling electrode 41 and the shielding ring 42 are made of copper, and the axial gap between the sampling electrode 41 and the shielding ring 42 is the same as the circumferential gap between each sampling electrode 41. The length of the sampling electrode 41 in the longitudinal direction is not greater than the axial length of the limiting fixture 3.

[0044] In a specific embodiment, the sensor is installed using the following steps:

[0045] 1) Arrange the sampling electrode 41 and the shielding ring 42, and use two non-metallic films 43 to encapsulate the arranged sampling electrode 41 and the shielding ring 42 to obtain a flexible electrode, and open a window in the non-metallic film 43 at the connection point of the sampling electrode; connect one end of the sampling signal line 2 to the connection point of the sampling electrode;

[0046] 2) Push the limiting tool 3 into the interior of the external metal pipe 1, and adjust the angle and position so that the holes therein for the sampling signal lines 2 to pass through are aligned one by one;

[0047] 3) Pass the other end of the sampling signal line through the holes on the limiting fixture 3 and the metal pipe 1 and lead out; fit the flexible electrode 4 to the inner surface of the limiting fixture 3; insert the fastening fixture 5 into the circular tube surrounded by the flexible electrode 4, so that the flexible electrode 4 is fastened between the limiting fixture 3 and the fastening fixture 5;

[0048] 4) Use low-temperature glue to fill the holes on the limiting fixture 3 and the metal pipe 1 to complete the internal and external sealing;

[0049] 5) Connect the flanges at both ends of the metal pipeline to the flanges of the section to be inspected, and connect the sampling signal line 2 to the capacitance tomography system.

[0050] In this field, a capacitance tomography system is composed of three subsystems: sensor, capacitance data acquisition system, and host computer.

[0051] The design key points of the present invention lie in the sensor subsystem. The capacitance data acquisition system and the host computer in the capacitance tomography system can be implemented using existing technologies. Among them, the built-in capacitance tomography sensor based on flexible electrodes of the present invention is the measurement front end of the capacitance tomography system, which determines the upper limit of the system accuracy. The key to the design of the sensor is the working fluid to be measured and the pole piece, and other components are all developed around these two cores. The shielding device of the sensor is to prevent external electromagnetic signals from interfering with the measurement process; the transmission wire of the sensor connects the pole piece to the capacitance data acquisition subsystem for transmitting the measurement signal.

[0052] In this field, the role of the capacitance data acquisition subsystem is to collect effective capacitance data. It is the core part of the capacitance tomography system and is responsible for collecting the capacitance data between the electrode array and the measured medium. These data are then used to calculate the dielectric constant distribution inside the medium through the image reconstruction algorithm. The main functions and components of the capacitance data acquisition subsystem usually include:

[0053] ①Signal generator: generates AC signals for exciting electrodes. These signals are usually high frequency to ensure the measurement accuracy of capacitance. Signal collector: receives the response signals on the electrodes, which contain information about the changes in capacitance inside the medium.

[0054] ②Analog-to-digital converter (ADC): converts analog signals into digital signals for computer processing.

[0055] ③Data processing unit: usually a microprocessor or digital signal processor (DSP), which performs amplification, filtering, digitization and preliminary processing of the signal to extract capacitance data.

[0056] ④Interface: transfer the processed data to a computer or other data processing equipment for image reconstruction.

[0057] The capacitance data acquisition subsystem requires high precision and stability to ensure the reliability of capacitance data and imaging quality.

[0058] The host computer subsystem usually needs to have the following functions:

[0059] ① Data reception: Receive capacitance data from the data acquisition system. These data are measured by capacitance sensors.

[0060] ② Image reconstruction: Using the received capacitance data, an image of the medium distribution is calculated through an image reconstruction algorithm.

[0061] ③ Real-time processing: Support real-time or near real-time data processing and image reconstruction to meet the needs of industrial process monitoring.

[0062] ④ User Interface: Provides an intuitive user interface that allows users to set parameters, start measurements, monitor the process, and view reconstructed images.

[0063] ⑤Data storage and playback: Able to store processed data and reconstruct images, and support playback and analysis of historical data.

[0064] According to the searched information, the host computer subsystem can be implemented using different programming languages ​​and software platforms. For example, some systems use the C++ programming language combined with the Qt software platform to develop the host computer software, obtain projection data from the capacitive data collector through serial communication, and call the image reconstruction algorithm to display the reconstructed image. In addition, as needed, the host computer subsystem may also integrate a variety of image reconstruction algorithms, such as linear back projection method, iterative reconstruction method, etc., to provide different imaging quality and speed options. The host computer subsystem also needs to consider the system's stability, anti-interference ability, and compatibility with hardware. In actual applications, the host computer system may also need to be integrated with other industrial control systems to achieve more complex monitoring and control functions.

[0065] The present invention relates to the design of a sensor subsystem. After the parameters such as the diameter of the measuring domain, the length and width of the measuring electrode, and the dielectric constant of the fluid to be measured are determined, the change in the sensor shape will not affect the size of the capacitance to be measured. Therefore, the peripheral capacitance data acquisition system can regard the sensor of the present invention as a conventional sensor under the same geometric parameters and can be directly connected to the sensor without debugging the parameters.

[0066] The advantages of the present invention over other copper plate solutions or external solutions are:

[0067] ① Built-in type: Due to the extreme characteristics of cryogenic fluids, their transmission pipelines or other containers need to maintain a certain structural strength, which will inevitably increase the thickness. If an external solution is used, the increased thickness will encroach on the measurement domain. The pipe wall thickness area is also the area close to the pole piece, where the measurement sensitivity is the highest. If this area is filled with a thicker pipe wall instead of the working fluid to be measured, it will undoubtedly waste the effective measurement range, thereby reducing the measurement accuracy. Therefore, for low-temperature application scenarios, the measurement accuracy of the external pole piece solution is significantly lower than that of the built-in solution.

[0068] ② Flexible electrode: Currently, even built-in solutions mostly use copper plates. The reason is that the copper plate is thicker and not easily deformed at low temperatures, so it can better maintain the preset measurement shape. However, the disadvantages of the copper plate are that the arc-shaped copper plate is more difficult to process, and because it is a separate component, there are assembly steps, and the installation accuracy can easily affect the measurement accuracy. The flexible electrode solution adopted in this patent integrates the shielding ring and the pole piece into an integrated design and manufacturing. At the installation level, it is only necessary to wrap the flexible electrode around the outside of the fastening tooling, and no additional installation steps are required. And because it is an integrated manufacturing, it can ensure that the relative position between the pole piece and the pole piece, and the pole piece and the shielding ring are consistent with the design, and the error caused by the relative position placement during the installation process is eliminated, further increasing the measurement accuracy.

[0069] ③ Fastening fixture: Although there is a fastening fixture inside the electrode for support in this solution, it is located inside the pipeline and does not need to bear the thermal stress caused by the temperature difference between the inside and outside. There is no excessive requirement for structural strength, so the thickness is relatively small and will not have a visible impact on the measurement results. The existence of the fastening fixture directly limits the deformation of the flexible electrode, and at the same time isolates the working fluid to be measured and the flexible electrode with a small thickness, avoiding physical damage to the flexible electrode caused by factors such as fluid impact, and further ensuring the integrity of the sensor measurement structure.

[0070] The specific implementation methods described above further illustrate the structure, technical solutions and operation methods of the device of the present invention in actual use. It should be understood that the above is only a specific implementation example of the present invention and is not intended to limit the present invention. However, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention, such as: the number and arrangement of flexible measuring electrode arrays, insulating measuring pipe materials, etc.

[0071] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A built-in electrical capacitance tomography sensor based on a flexible electrode, characterized in that: It comprises a metal pipe (1), a sampling signal line (2), a limit fixture (3), a flexible electrode (4) and a fastening fixture (5); The metal pipe (1) is provided with a standard flange at each end, and the limiting tool (3) and the fastening tool (5) are both cylindrical structures; the limiting tool (3) is arranged closely inside the metal pipe (1), the flexible electrode (4) is arranged on the inner surface of the limiting tool (3), and the fastening tool (5) is arranged in the innermost layer and closely attached to the flexible electrode (4), so that the flexible electrode (4) is fastened between the limiting tool (3) and the fastening tool (5); The flexible electrode (4) comprises a sampling electrode (41), a shielding ring (42) and a non-metallic film (43) for packaging; the shielding ring (42) is arranged at both ends of the inner surface of the limiting fixture (3) to shield the axial magnetic field outside the measurement domain; the flexible electrode (4) has multiple pieces, which are evenly arranged along the circumferential direction in the area between the shielding rings on the inner surface of the fixed fixture (3); two non-metallic films (43) are respectively covered on the inner and outer surfaces of the sampling electrode (41) and the shielding ring (42) to realize the packaging of the flexible electrode (4); wherein the sampling electrode (41) and the shielding ring (42) are made of metal material, and the thickness is less than 0.2 mm; One end of the sampling signal line (2) is connected to the sampling electrode (41), and the other end passes through a preset hole on the limiting tooling (3) and the metal pipe (1) to be connected to an external capacitance tomography system.

2. The built-in capacitive tomography sensor based on flexible electrodes according to claim 1, characterized in that: The metal pipe (1) is a straight metal pipe, and an external through hole (11) for the sampling signal line (2) to pass through is arranged on the pipe wall of the metal pipe (1).

3. The built-in capacitive tomography sensor based on flexible electrodes according to claim 2, characterized in that: The external through holes (11) on the metal pipe (1) are evenly arranged along the circumference of the same cross section of the metal pipe, and the diameter of the external through holes (11) is not less than the diameter of the sampling signal line (2).

4. The built-in capacitive tomography sensor based on flexible electrodes according to claim 2, characterized in that: The main body of the position limiting fixture (3) is a non-metallic circular tube, on which an inner through hole (31) is provided for the sampling signal line (2) to pass through, and the number of the inner through holes (31) is equal to and corresponds to the number of the outer through holes (11) of the metal pipe (1), the number of the sampling signal lines (2), and the number of the sampling electrodes (41); the inner surface of the position limiting fixture (3) is smooth without any dents or protrusions, and the material is an insulating material; the inner and outer circles of the position limiting fixture (3) are coaxial, and the outer circle diameter is equal to the inner surface diameter of the metal pipe (1); the position limiting fixture (3) and the metal pipe (1) adopt an interference fit.

5. The built-in capacitive tomography sensor based on flexible electrodes according to claim 4, characterized in that: The diameter of the inner through hole (31) on the limiting tool (3) is not greater than the diameter of the outer through hole (11).

6. The built-in capacitive tomography sensor based on flexible electrodes according to claim 1, characterized in that: The sampling electrode (41) and the shielding ring (42) are made of red copper, and the shielding ring is grounded when the sensor is working; the axial gap between the sampling electrode (41) and the shielding ring (42) is the same as the circumferential gap between each sampling electrode (41); and the length of the sampling electrode (41) in the longitudinal direction is not greater than the axial length of the limiting fixture (3).

7. The built-in capacitive tomography sensor based on flexible electrodes according to claim 1, characterized in that: The two non-metallic films (43) are both complete films, and the non-metallic film on the outer surface of the flexible electrode has a window at the connection point of the sampling electrode so that the sampling signal line (2) can be connected to the connection point of the sampling electrode.

8. The built-in capacitive tomography sensor based on flexible electrodes according to claim 1, characterized in that: The fastening fixture (5) is a non-metallic circular tube, and its outer diameter is equal to the inner diameter of the limiting fixture (3), and the two are interference-fitted; the inner surface and the outer surface of the fastening fixture (5) are both smooth surfaces, its inner diameter is the same as the inner diameter of the incoming flow pipeline, and its circumferential length is not less than the length of the sampling electrode (41) in the length direction.

9. A method for installing a built-in electrical capacitance tomography based on a flexible electrode as claimed in claim 1, characterized in that: The following steps are involved: 1) Arranging the sampling electrode (41) and the shielding ring (42), and using two non-metallic films (43) to encapsulate the arranged sampling electrode (41) and the shielding ring (42) to obtain a flexible electrode, and opening a window in the non-metallic film (43) at the connection point of the sampling electrode; and connecting one end of the sampling signal line (2) to the connection point of the sampling electrode; 2) Push the limiting tool (3) into the interior of the external metal pipe (1), and adjust the angle and position so that the holes therein for the sampling signal lines (2) to pass through are aligned one by one; 3) Passing the other end of the sampling signal line through the holes on the limiting fixture (3) and the metal pipe (1) and leading out; fitting the flexible electrode (4) to the inner surface of the limiting fixture (3); Inserting the fastening tool (5) into the circular tube surrounded by the flexible electrode (4) so ​​that the flexible electrode (4) is fastened between the limiting tool (3) and the fastening tool (5); 4) Use low-temperature glue to fill the holes on the limiting fixture (3) and the metal pipe (1) to complete the internal and external sealing; 5) Connect the flanges at both ends of the metal pipeline to the flanges of the section to be inspected, and connect the sampling signal line (2) to the capacitance tomography system.

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