A method for underwater two-dimensional ac flow field sensing using tubular ipmc sensor

Abstract

The application discloses a method for underwater two-dimensional AC flow field sensing by using a tubular IPMC sensor. The method processes a tubular IPMC sensor by using Nafion tube, and performs an underwater one-dimensional AC flow field sensing characterization experiment on the tubular IPMC sensor, and uses the tubular IPMC sensor to perform underwater two-dimensional AC flow field sensing. The application solves the problem that existing IPMC AC flow speed sensors lack effective algorithms and models, and has important significance for promoting the application of intelligent flexible materials and the development of underwater sensing technology.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sensor sensing, and in particular to a method for sensing a two-dimensional alternating current flow field underwater by using a tubular IPMC sensor. BACKGROUND

[0002] IPMC (Ionic Polymer Metal Composites) is a representative EAP (Electro Active Polymer) smart material. The structure of IPMC is composed of "electrode-ion exchange film-electrode". The ion exchange film in the middle layer is usually selected from Nafion film of the American DuPont Company, which contains hydrophobic polymer chains, movable cations and a certain amount of solvent. The noble metal electrode (commonly known as palladium, platinum, gold, etc.) on both sides of the ion exchange film is usually prepared by surface plating process. When the IPMC material is bent and deformed by external force, the cations and water molecules in the film combine to form movable hydrated cations. These hydrated cations will migrate according to the bending direction, resulting in a stress gradient, which causes the charge distribution in the film to change. In the film, the area with low density expands due to the excess of positive charges and the decrease of negative charges, while the area with high density shrinks due to the accumulation of negative charges and the lack of positive charges. This uneven charge distribution leads to the polarization of charges, forming a potential difference on both sides of the electrode. Due to the existence of metal electrode, the electric signal can be detected. In addition, the change of potential difference can be used to detect and measure the size or direction of external force. By monitoring the change of potential difference on both sides of the electrode, the information of external force applied on the IPMC can be inferred, so that the IPMC smart material has application potential in sensors.

[0003] IPMC sensor has the characteristics of soft texture, fast response, simple structure, high sensitivity, strong plasticity and the ability to work in complex underwater environment, and can be used to sense and measure underwater flow rate, so it has great application potential in underwater robots, energy collection, environmental monitoring, ocean resource development and other fields.

[0004] However, most of the existing methods for sensing flow rate by using IPMC sensor are based on sheet-shaped IPMC material to design flow rate sensor, so the sensing ability of the single sensor is one-dimensional. Even if the sensor device with multi-dimensional sensing ability is assembled, it is difficult to meet the requirements of miniaturization, compact structure, high stability and simple calculation algorithm at the same time.

[0005] Flow rate sensing requires on one hand the sensor device to convert the measured physical quantity into a measurable electrical signal, and on the other hand the design and derivation of algorithms and models matching the physical characteristics of the sensor to give the calculation formula to quantitatively describe the corresponding relationship between the sensor output and input. At present, many researchers have done a lot of work on the design of the sensor device itself, but the related matching algorithms and models are relatively less.

[0006] In addition, the properties of IPMC material itself will cause the IPMC sensor to be naturally sensitive to the alternating external excitation response. The changing excitation will cause the changing sensing signal output, which can fully exert the advantages of the IPMC sensor on one hand, but on the other hand, it makes the modeling of the sensor more complex and brings some algorithmic challenges and difficulties. SUMMARY

[0007] To solve the above problems, the application provides a method for underwater two-dimensional alternating current flow field sensing using a tubular IPMC sensor, comprising the following steps:

[0008] Step one: use Nafion tube to process tubular IPMC, divide its outer surface electrode into four sub-electrodes along the axial direction, and the inner surface electrode as the common ground; the four sub-electrodes are sub-electrode p1, sub-electrode p2, sub-electrode p3 and sub-electrode p4 in turn along the radial direction, wherein the sub-electrode p1 and the sub-electrode p3, the sub-electrode p2 and the sub-electrode p4 are symmetrical along the axial line;

[0009] Step two: convert the current output of each sub-electrode into a measurable bipolar voltage signal, respectively u1(t), u2(t), u3(t) and u4(t), wherein t represents the time variable;

[0010] Step three: perform an underwater one-dimensional alternating current flow field sensing characterization experiment on the tubular IPMC sensor: under different oscillation frequencies, respectively calibrate the sensing signals u1(t, v, n), u2(t, v, n), u3(t, v, n) and u4(t, v, n) of each sub-electrode facing the water wave oscillation direction, wherein v represents the flow rate, and the positive integer n represents the number of the sub-electrode pn facing the water wave oscillation direction, and the value of n ranges from 1 to 4;

[0011] Step four: for each sub-electrode facing the water wave oscillation direction, first calculate the frequency response of the output signal of the sub-electrode under different oscillation frequencies, and then use the least square method to fit a transfer function Hn(s) corresponding to the sub-electrode to describe the corresponding relationship between the output signal and the alternating current flow rate;

[0012] Step five: when using the tubular IPMC sensor to sense the underwater two-dimensional alternating current flow field, first obtain the one-dimensional alternating current flow rate vx(t) in the x direction through u1(t), u3(t) and the transfer function Hn(s), and obtain the one-dimensional alternating current flow rate vy(t) in the y direction through u2(t), u4(t) and the transfer function Hn(s), so as to realize the two-dimensional alternating current flow field sensing.

[0013] Further, in step one, the method for processing the tubular IPMC using the Nafion tube is as follows:

[0014] Step 1.1: processing the tubular IPMC using the Nafion tube;

[0015] Step 1.2: dividing the outer surface electrode into four equal parts along the axial direction of the tubular IPMC, forming four sub-electrodes, and setting an insulating isolation belt between adjacent sub-electrodes, and removing the metal deposition on the position of the isolation belt to achieve the effect of insulation;

[0016] Step 1.3: connecting a wire to the entire inner surface electrode as a common ground wire, and connecting a wire to each of the four sub-electrodes as a signal line, and each signal line and the common ground wire constitute a sensor signal output;

[0017] Step 1.4: encapsulating the tubular IPMC with a thin film capsule, and placing it in a water bath for a predetermined time, and then taking it out, and the tubular IPMC has moisture inside;

[0018] Step 1.5: using a colloidal resin material to encapsulate the tubular IPMC.

[0019] Further, after step 1.4, a thin film coating is formed on the surface of the tubular IPMC, and the thickness of the thin film coating is between 20 microns and 50 microns.

[0020] Further, in step three, the characterization experiment for the tubular IPMC sensor to sense the underwater one-dimensional alternating current flow field is as follows: placing an underwater vibrating sphere in a static water environment, the vibration direction being parallel to the water surface, the tubular IPMC sensor being placed below the water surface, the axis of the tubular IPMC sensor being perpendicular to the water surface, the midpoint of the axis of the tubular IPMC sensor and the center of the sphere being at the same depth, the plane formed by the axis of the tubular IPMC sensor and the vibration direction of the sphere intersecting the surface of the sub-electrode facing the water wave oscillation direction at the midline of the sub-electrode in the axial direction, and the plane also intersecting the surface of the sub-electrode facing away from the water wave oscillation direction at the midline of the sub-electrode in the axial direction; the vibrating sphere generates a dipole source and generates a potential flow in the water, so that the water flow velocity function v(t)=Asin(2πft)a at the position of the tubular IPMC sensor can be calculated. 3 / r 3 , A is the vibration amplitude, and f is the vibration frequency.

[0021] Furthermore, the corresponding relationship between the output signals of each sub-electrode and the AC flow velocity is un(s)=Hn(s)v(s), where s represents the Laplace operator; v(s) is obtained by performing a Laplace transform on the water flow velocity function v(t); un(s) is obtained by performing a Laplace transform on the sub-electrode output signal un(t); and n is the sub-electrode number.

[0022] Furthermore, the diameter 'a' of the underwater vibrating sphere is 2 cm, and the center of the sphere is 10 cm from the water surface; the axis of the tubular IPMC sensor is 10 cm from the center of the sphere.

[0023] Furthermore, in step three, the duration of the acquired sensor signal is at least 10 seconds, and the sampling frequency is at least 1000 Hz.

[0024] Furthermore, in step four, the frequency response includes the ratio of the output and input amplitudes at different oscillation frequencies and the output and input phase difference. The output refers to the output signal of the sub-electrode, and the input refers to the flow velocity at the location of the sensor. The output and input in the time domain are respectively subjected to fast Fourier transform to obtain their respective amplitudes and phases.

[0025] Furthermore, in step five, the axis of the tubular IPMC sensor is perpendicular to the plane formed by the x and y directions. The axis and the x direction form a first plane, which intersects the surfaces of sub-electrodes p1 and p3 at their axial centerline. The axis and the y direction form a second plane, which intersects the surfaces of sub-electrodes p2 and p4 at their axial centerline.

[0026] Furthermore, in step five, the specific method for calculating vx(t) is as follows: calculate the inverse transfer function H1_inv(s) = 1 / H1(s) of the transfer function H1(s) of the sub-electrode p1, and then input u1(t) into the inverse transfer function H1_inv(s) to obtain the one-dimensional AC velocity vx1(t) in the x direction. Similarly, the one-dimensional AC velocity vx2(t) in the x direction can be obtained through u2(t). Take the average value of vx1(t) and vx2(t) as vx(t).

[0027] The specific method for calculating vy(t) is as follows: calculate the inverse transfer function H2_inv(s) = 1 / H2(s) of the transfer function H2(s) of the sub-electrode p2, and then input u2(t) into the inverse transfer function H2_inv(s) to obtain the one-dimensional AC velocity vy2(t) in the y direction. Similarly, the one-dimensional AC velocity vy4(t) in the y direction can be obtained through u4(t). Take the average value of vy2(t) and vy4(t) as vy(t).

[0028] The beneficial effects of this invention are as follows:

[0029] This invention overcomes the shortcomings of existing technologies by utilizing a tubular symmetrical structure to achieve direct two-dimensional isotropic sensing capability of a single IPMC sensor. It quantitatively defines and describes the AC flow velocity sensing characteristics of the sensor from the frequency domain dimension, establishes the sensor's transfer function model, and solves the problem of the lack of effective algorithms and models in existing IPMC AC flow velocity sensors. This invention is of great significance for promoting the application of intelligent flexible materials and the development of underwater sensing technology. Detailed Implementation

[0030] Before introducing the present invention, the preparation method of the underwater two-dimensional sensing IPMC sensor will be explained.

[0031] The fabrication method of an underwater two-dimensional sensing IPMC sensor includes the following steps:

[0032] Step 1.1: Select Nafion tubing and process it into tubular IPMC.

[0033] The specific processing steps include Step 1 to Step 5:

[0034] Step 1. In the pretreatment stage: Boil the Perma Pure Nafion drying tube (model: TT-110) in 2wt% dilute hydrochloric acid for 1 hour, and then heat it in deionized water for 1 hour to remove impurities and make the Nafion tube bend freely.

[0035] Step 2. In the ion adsorption stage: Immerse the Nafion tube in a solution of tetraammineplatinum chloride (molecular formula: [Pt(NH3)4]Cl2), where platinum ions in the platinum complex solution exchange with cations in the Nafion.

[0036] Step 3. In the first chemical plating stage: the Nafion tube is cleaned with deionized water and placed in a 5 wt% sodium borohydride (molecular formula: NaBH4) solution. The temperature of the solution is increased by heating with a magnetic stirrer, and 1 ml of sodium borohydride solution is added every 15 minutes. Through the reduction reaction, platinum electrodes are formed on the inner and outer surfaces of the Nafion tube.

[0037] Step 4. In the secondary electroless plating stage: In order to precipitate more platinum ions, repeat the acid treatment to reduction reaction steps of Step 1-Step 3 to complete the secondary deposition of platinum and enhance the conductivity of the platinum electrode.

[0038] Step 5. In the displacement reaction stage: After the reduction reaction is completed, the Nafion tube is cleaned and then placed in dilute hydrochloric acid and boiled in a constant temperature water bath to allow the cations in the Nafion tube to exchange with the hydrogen ions in the hydrochloric acid.

[0039] It is important to note that the electrodes at both ends of the tubular IPMC cross-section need to be removed to prevent short circuits between the inner and outer surface electrodes.

[0040] Step 1.2: Fabricating the sub-electrodes on the outer surface. The specific method is as follows: By removing the surface electrode to create an insulating band, the outer electrode is divided into four mutually insulated sections along the axial direction of the tubular IPMC, forming four uniformly symmetrical sub-electrodes on the outer surface. It is important to note that, while ensuring sufficient insulation between adjacent sub-electrodes, the width and depth of the insulating band should be as narrow as possible to minimize the impact of this step on the overall material and mechanical properties of the tubular IPMC.

[0041] When manufacturing sub-electrodes, 3D printing can be used to create molds. The tubular IPMC is placed and fixed in the mold. The mold acts as a mask, covering the sub-electrode portion while exposing the isolation zone portion to be processed, thereby ensuring that the division of the sub-electrodes is uniform and symmetrical.

[0042] Step 1.3: Create a common ground wire. Connect the entire inner surface electrode with a wire as a common ground wire, and connect four wires to the four sub-electrodes on the outer surface as four signal lines. Each signal line and the common ground wire constitute one sensing signal output of the sensor. In this step, the connection method between the wires and the electrodes can be welding, bonding, or other methods that can ensure good long-term conductivity.

[0043] The four signal lines of the sub-electrodes on the outer surface can also be paired to form multiple sensing signal outputs.

[0044] Step 1.4: Thin-film encapsulation. The tubular IPMC with connected wires is encapsulated in a polymeric material using vacuum physical vapor deposition (VPV). It is then immersed in a high-temperature water bath (typically 60-80 degrees Celsius) for a period of time before being removed, thus locking sufficient water molecules inside the encapsulated tubular IPMC. The polymeric material used for encapsulation forms a thin-film coating on the surface of the tubular IPMC through the VPV process.

[0045] After the thin film is deposited, the temperature and duration of the water bath are determined based on the thickness of the film. The thicker the film, the higher the water bath temperature and the longer the soaking time are required.

[0046] The thickness of the thin film coating is between 20 and 50 micrometers, ensuring extremely low permeability to water molecules at room temperature, while significantly increasing permeability to water molecules in a high-temperature water bath. The polymer material includes, but is not limited to, Parylene.

[0047] In this embodiment, the film thickness is 25 micrometers, the temperature of the high-temperature water bath is 80 degrees Celsius, and the film is removed after immersion for 48 hours. Changing the thickness of the film capsule, the temperature of the high-temperature water bath, and the length of the immersion time will directly change the number of water molecules that enter the tubular IPMC through osmosis, thereby affecting the sensing characteristics of the tubular IPMC.

[0048] Step 1.5: Bionic Encapsulation. The processed tubular IPMC is biomimetically encapsulated with a resin material using injection molding to amplify the water flow excitation. A mold is fabricated using 3D printing or machining. The tubular IPMC from step four is placed and fixed in the center of the mold. The resin material used for biomimetic encapsulation encapsulates the tubular IPMC and fills the internal space through the injection molding process, forming a shell-like encapsulation shape similar to a bullet casing. The axis of the encapsulation shell coincides with the axis of the tubular IPMC, and the cross-sectional radius of the main body is larger than the outer radius of the tubular IPMC. Preferably, the difference in radius is approximately 1-3 mm. In this embodiment, the cross-sectional radius of the main body is approximately two millimeters larger than the outer radius of the tubular IPMC. The resin material includes, but is not limited to, polydimethylsiloxane (PDMS).

[0049] Preferably, the method further includes step 1.6: fabricating the base of the aforementioned processed tubular IPMC using 3D printing, wherein the axis of the tubular IPMC is perpendicular to the plane of the base. The shape of the base is not limited, as long as it serves a fixing function.

[0050] The method described in this invention will now be explained.

[0051] Step 1: Following the aforementioned method, a tubular IPMC is fabricated using Nafion tubes. The outer surface electrode is divided into four mutually insulated and orthogonally symmetrically distributed sub-electrodes along the axial direction. These sub-electrodes form four sensing signal outputs with the inner surface electrode. The inner surface electrode serves as a common ground wire. The four sub-electrodes are named sub-electrode p1, sub-electrode p2, sub-electrode p3, and sub-electrode p4 in the radial direction. Sub-electrode p1 and sub-electrode p3, and sub-electrode p2 and sub-electrode p4 are symmetrical along the axial direction. Sub-electrode p1 is adjacent to sub-electrode p2 and sub-electrode p4, respectively.

[0052] Step 2: Use an optocoupler-based signal conditioning circuit and an operational amplifier-based signal amplification circuit to convert the current output of each sensing signal into measurable bipolar voltage signals u1(t), u2(t), u3(t), u4(t), corresponding to sub-electrodes p1, p2, p3, and p4, respectively. t in the voltage signal represents the time variable.

[0053] The signal conditioning circuit and signal amplification circuit use existing technology. The RC low-pass filter circuit contained therein has a low-pass cutoff frequency of 100 Hz, and the output range of the bipolar voltage signal is between ±5 volts.

[0054] Step 3: Characterization experiment of one-dimensional AC flow field sensing of tubular IPMC sensor: The sensing signals u1(t,v,n), u2(t,v,n), u3(t,v,n), and u4(t,v,n) of each sub-electrode facing the direction of water wave oscillation are calibrated at different oscillation frequencies, where v represents the flow velocity and the positive integer n represents the number of the sub-electrode pn facing the direction of water wave oscillation. The value of n is 1, 2, 3, or 4.

[0055] In this step, the method to achieve water flow oscillation is as follows: An underwater vibrating sphere is placed in a static water environment, with the vibration direction parallel to the water surface. The sphere has a diameter *a* of 2 cm and its center is 10 cm from the water surface. The sphere is driven by a resonator to generate vibration and oscillating water waves. The amplitude *A* and frequency *f* of the vibration can be adjusted. A tubular IPMC sensor is placed below the water surface, with its axis perpendicular to the water surface and 10 cm from the center of the sphere. The midpoint of its axis is at the same depth as the center of the sphere. The plane formed by its axis and the vibration direction of the sphere intersects the surface of the sub-electrode facing the oscillation direction of the water waves at the midline of the sub-electrode's axis. This plane also intersects the surface of the sub-electrode facing away from the oscillation direction of the water waves at the midline of the sub-electrode's axis. The vibrating sphere generates a dipole source and produces potential flow in the water. From this, the flow velocity v(t) = Asin(2πft)a at the sensor's location can be calculated. 3 / r 3 The true value of the vibration amplitude A is obtained by a laser displacement sensor.

[0056] In this step, the sensor signal needs to be collected after the experimental environment reaches a steady state, the signal duration is at least 10 seconds, and the sampling frequency is above 1000 Hz.

[0057] Step 4: Based on the data from the aforementioned characterization experiment, for each sub-electrode facing the direction of water wave oscillation, first calculate the frequency response of the sub-electrode's output signal at different oscillation frequencies. Then, use the least squares method to fit a transfer function Hn(s) corresponding to the sub-electrode, which describes the relationship between the output signal and the AC flow velocity: un(s) = Hn(s)v(s), where s represents the Laplace operator, v(s) is obtained by performing a Laplace transform on the water flow velocity function v(t), and un(s) is obtained by performing a Laplace transform on the sub-electrode output signal un(t).

[0058] The frequency response includes the ratio of the output and input amplitudes at different oscillation frequencies, as well as the output and input phase difference. The output refers to the output signal of the sub-electrode, and the input is the flow velocity at the location of the sensor. The amplitude and phase of the output and input in the time domain are obtained by performing fast Fourier transforms on them respectively.

[0059] Step 5: When using the tubular IPMC sensor for underwater two-dimensional AC flow field sensing, firstly, the one-dimensional AC velocity vx(t) in the x-direction is obtained through u1(t), u3(t), and the transfer function Hn(s), and then the one-dimensional AC velocity vy(t) in the y-direction is obtained through u2(t), u4(t), and the transfer function Hn(s), thus realizing two-dimensional AC flow field sensing. The axis of the tubular IPMC sensor is perpendicular to the plane formed by the x and y directions. The plane formed by the axis and the x-direction intersects the surfaces of sub-electrodes p1 and p3 at their axial centerline, and the plane formed by the axis and the y-direction intersects the surfaces of sub-electrodes p2 and p4 at their axial centerline.

[0060] The specific method for calculating vx(t) is as follows: calculate the inverse transfer function H1_inv(s) = 1 / H1(s) of the transfer function H1(s) of the sub-electrode p1, and then input the sensing signal u1(t) of the sub-electrode p1 into the inverse transfer function H1_inv(s) to obtain the one-dimensional AC velocity vx1(t) in the x direction. Similarly, the one-dimensional AC velocity vx2(t) in the x direction can be obtained through u3(t). Take the average value of vx1(t) and vx2(t) as vx(t).

[0061] The specific method for calculating vy(t) is as follows: calculate the inverse transfer function H2_inv(s) = 1 / H2(s) of the transfer function H2(s) of the sub-electrode p2, and then input u2(t) into the inverse transfer function H2_inv(s) to obtain the one-dimensional AC velocity vy2(t) in the y direction. Similarly, the one-dimensional AC velocity vy4(t) in the y direction can be obtained through u4(t). Take the average value of vy2(t) and vy4(t) as vy(t).

Claims

1. A method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor, characterized by, The method comprises the following steps: Step one: use Nafion tube to process tubular IPMC, divide its outer surface electrode into four sub-electrodes along the axial direction, and the inner surface electrode as the common ground; the four sub-electrodes are sub-electrode p1, sub-electrode p2, sub-electrode p3 and sub-electrode p4 in turn along the radial direction, wherein sub-electrode p1 and sub-electrode p3, sub-electrode p2 and sub-electrode p4 are symmetrical along the axis; Step two: convert the current output of each sub-electrode into measurable bipolar voltage signals, respectively u1(t), u2(t), u3(t) and u4(t), wherein t represents the time variable; Step three: perform characterization experiments of underwater one-dimensional direct current flow field sensing on the tubular IPMC sensor: under different oscillation frequencies, respectively calibrate the sensing signals u1(t, v, n), u2(t, v, n), u3(t, v, n) and u4(t, v, n) of each sub-electrode facing the water wave oscillation direction, wherein v represents the flow rate, and the positive integer n represents the number of the sub-electrode pn facing the water wave oscillation direction, and n ranges from 1 to 4; Step four: for each sub-electrode facing the water wave oscillation direction, first calculate the frequency response of the output signal of the sub-electrode under different oscillation frequencies, and then use the least square method to fit a transfer function Hn(s) corresponding to the sub-electrode, which is used to describe the corresponding relationship between the output signal of the sub-electrode and the change of the alternating current flow rate; Wherein s represents the Laplace operator; Step five: when using the tubular IPMC sensor to perform underwater two-dimensional alternating current flow field sensing, first obtain the one-dimensional alternating current flow rate vx(t) in the x direction through u1(t), u3(t) and the transfer function Hn(s), and obtain the one-dimensional alternating current flow rate vy(t) in the y direction through u2(t), u4(t) and the transfer function Hn(s), to realize two-dimensional alternating current flow field sensing.

2. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 1, wherein, In step one, the method for processing tubular IPMC using Nafion tube is as follows: Step 1.1: use Nafion tube to process tubular IPMC; Step 1.2: divide the outer surface electrode into 4 equal parts along the axial direction of the tubular IPMC, forming 4 sub-electrodes, and an insulating separation belt is arranged between adjacent sub-electrodes, and the outer surface metal deposition at the position of the separation belt is removed to achieve the effect of insulation; Step 1.3: connect a wire to the entire inner surface electrode as a common ground, and connect a wire to each of the four sub-electrodes as a signal line, and each signal line and the common ground constitute a sensor signal output; Step 1.4: encapsulate the tubular IPMC with a thin film, and take it out after soaking in a water bath for a predetermined time, and the tubular IPMC has moisture inside; Step 1.5: use a colloidal resin material to encapsulate the tubular IPMC.

3. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2, wherein, After step 1.4, a thin film coating is formed on the surface of the tubular IPMC, and the thickness of the thin film coating is between 20 microns and 50 microns.

4. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step three, a characterization experiment of underwater one-dimensional DC flow field sensing is performed on the tubular IPMC sensor. Specifically, an underwater vibrating sphere is placed in a static water environment with the vibration direction parallel to the water surface. The tubular IPMC sensor is placed below the water surface with its axis perpendicular to the water surface. The midpoint of its axis is at the same depth as the center of the sphere. The plane formed by its axis and the vibration direction of the sphere intersects the surface of the sub-electrode facing the direction of water wave oscillation at the centerline of the sub-electrode's axis. The plane also intersects the surface of the sub-electrode facing away from the direction of water wave oscillation at the centerline of the sub-electrode's axis. The vibrating sphere generates a dipole source and generates potential flow in the water. The water flow velocity function v(t) = Asin(2πft)a3 / r3 at the location of the tubular IPMC sensor is calculated, where A is the vibration amplitude, f is the vibration frequency, a is the diameter of the underwater vibrating sphere, and r is the distance from the axis of the tubular IPMC sensor to the center of the sphere. 5.The method of underwater two-dimensional AC flow field sensing using a tubular IPMC sensor according to claim 4, wherein, The corresponding relationship between the output signals of each sub-electrode and the AC flow velocity is un(s)=Hn(s)v(s), where s represents the Laplace operator; v(s) is obtained by performing a Laplace transform on the water flow velocity function v(t); un(s) is obtained by performing a Laplace transform on the sub-electrode output signal un(t); and n is the sub-electrode number.

6. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 4, wherein, The diameter 'a' of the underwater vibrating sphere is 2 cm, and the center of the sphere is 10 cm from the water surface; the axis of the tubular IPMC sensor is 10 cm from the center of the sphere.

7. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step three, the duration of the acquired sensor signal is at least 10 seconds, and the sampling frequency is at least 1000 Hz.

8. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step four, the frequency response includes the ratio of output and input amplitudes at different oscillation frequencies and the output and input phase difference. The output refers to the output signal of the sub-electrode, and the input refers to the flow velocity at the location of the sensor. The output and input in the time domain are respectively subjected to fast Fourier transform to obtain their respective amplitudes and phases.

9. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step five, the axis of the tubular IPMC sensor is perpendicular to the plane formed by the x and y directions. The axis and the x direction form a first plane, which intersects the surfaces of sub-electrodes p1 and p3 at their axial midline. The axis and the y direction form a second plane, which intersects the surfaces of sub-electrodes p2 and p4 at their axial midline.

10. The method for sensing a two-dimensional AC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step five, the specific method for calculating vx(t) is as follows: calculate the inverse transfer function H1_inv(s) = 1 / H1(s) of the transfer function H1(s) of the sub-electrode p1, and then input u1(t) into the inverse transfer function H1_inv(s) to obtain the one-dimensional AC velocity vx1(t) in the x direction. Similarly, the one-dimensional AC velocity vx3(t) in the x direction can be obtained through u3(t). Take the average value of vx1(t) and vx3(t) as vx(t). The specific method for calculating vy(t) is as follows: the inverse transfer function H2_inv(s)=1 / H2(s) of the transfer function H2(s) of the sub-electrode p2 is calculated, and then u2(t) is input into the inverse transfer function H2_inv(s) to obtain the one-dimensional alternating current velocity vy2(t) in the y direction, and the one-dimensional alternating current velocity vy4(t) in the y direction can be obtained through u4(t) in the same way, and the average value of vy2(t) and vy4(t) is taken as vy(t).