A method for underwater two-dimensional direct current flow field sensing using a tubular IPMC sensor

Abstract

The application discloses a method for underwater two-dimensional direct current flow field sensing by using a tubular IPMC sensor. The method processes a tubular IPMC sensor by using Nafion tubes, and performs an underwater one-dimensional direct current flow field sensing characterization experiment on the tubular IPMC sensor, and uses the tubular IPMC sensor to perform underwater two-dimensional direct current flow field sensing. The application solves the problem that existing IPMC direct current 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, in particular to a method for sensing underwater two-dimensional direct current flow field 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 membrane-electrode", which is shaped like a "sandwich". The ion exchange membrane in the middle layer is usually selected from Nafion membrane of American DuPont Company, which contains hydrophobic polymer chain, movable cation and a certain amount of solvent. The noble metal electrode (commonly known as palladium, platinum, gold, etc.) on both sides of the ion exchange membrane is usually prepared by surface plating process.

[0003] IPMC sensor has the characteristics of soft texture, fast response, simple structure, high sensitivity, strong plasticity, and can work in complex underwater environment, etc., and can be used for sensing and measuring underwater flow rate, thus having great application potential in the fields of underwater robots, energy collection, environmental monitoring, ocean resource development, etc.

[0004] The existing method for sensing flow rate by using IPMC sensor is mostly based on the design of flow rate sensor by using sheet-shaped IPMC material, so that 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 measurement requirements of miniaturization, compact structure, high stability and simple calculation algorithm at the same time.

[0005] Flow rate sensing requires that the sensor device converts the measured physical quantity into a measurable electrical signal, and also needs to design and derive an algorithm and model matched with the physical characteristics of the sensor, and give a calculation formula to quantitatively describe the corresponding relationship between the sensor output and the input. At present, many researchers have done a lot of work on the design of the sensor device itself, but the related matching algorithm and model are relatively less.

[0006] In addition, the properties of IPMC material itself will cause the IPMC sensor to not generate sensing signals when it is kept in a certain fixed bending deformation state after being bent, and only generate sensing signals in the dynamic process of bending and recovery, that is, the IPMC material is naturally not responsive to the external excitation of direct current. Therefore, there are still great challenges and difficulties in using IPMC sensor to sense underwater excitation of direct current. SUMMARY

[0007] To solve the above problems, the application provides a method for underwater two-dimensional direct current flow field sensing by 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 use the inner surface electrode as the common ground; the four sub-electrodes are sequentially sub-electrode p1, sub-electrode p2, sub-electrode p3 and sub-electrode p4 along the radial direction, wherein sub-electrode p1 and sub-electrode p3, sub-electrode p2 and sub-electrode p4 are symmetrical along the axial line;

[0009] 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;

[0010] Step three: perform an underwater one-dimensional direct current flow field sensing characterization experiment on the tubular IPMC sensor, calibrate the zero point noise u10(t), u20(t), u30(t) and u40(t) of the four sensing signals in a static water environment, and 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 under different laminar flow velocities, wherein the scalar v represents the flow velocity, and the positive integer n represents the number of the sub-electrode n facing the water flow direction, and the value of n ranges from 1 to 4;

[0011] Step four: for each sub-electrode facing the water flow direction, a least square method is used to fit a calibration curve for describing the corresponding relationship between the four output signals of the tubular IPMC sensor and the change of the laminar flow velocity, wherein the horizontal axis variable of the calibration curve is the flow velocity v, and the vertical axis variable is the corrected mean value of the sub-electrode sensing signal minus the corrected mean value of the sensing signal of the sub-electrode symmetrical to the sub-electrode;

[0012] Step five: use the tubular IPMC sensor to perform underwater two-dimensional direct current flow field sensing: obtain one-dimensional direct current flow velocity information vx in the x direction through u1(t), u3(t) and the calibration curve, and obtain one-dimensional direct current flow velocity information vy in the y direction through u2(t), u4(t) and the calibration curve.

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

[0014] Step 1.1: use Nafion tube to process tubular IPMC;

[0015] 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 set an insulating isolation belt between adjacent sub-electrodes, and remove the metal deposition on the position of the isolation belt to achieve the effect of insulation;

[0016] Step 1.3: Connect a wire to the entire inner surface electrode as a common ground wire, and connect a wire to each of the 4 sub-electrodes as a signal wire, and each signal wire and the common ground wire form a sensor signal output;

[0017] Step 1.4: Thin film encapsulation is performed on the tubular IPMC, and after soaking in a water bath for a predetermined time, the tubular IPMC has moisture inside;

[0018] Step 1.5: Perform biomimetic encapsulation on the tubular IPMC using a colloidal resin material.

[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, the positions where the inner surface electrode and the outer surface sub-electrode are connected to the corresponding wires are located at the same height position of the tube.

[0021] Further, in step three, the tubular IPMC sensor is located at least 10 centimeters deep below the water surface, and the axis is perpendicular to the water surface.

[0022] Further, in step three, the duration of the collected sensor signal is at least 10 seconds, and the sampling frequency is at least 1000 Hz.

[0023] Further, in step four, according to the size of the amount of data obtained in the characterization experiment, the calibration curve can be fitted as a straight line or a parabola; or segmented to fit a combination of different curves.

[0024] Further, in step four, the calculation method of the corrected mean value is: the mean value of the sub-electrode sensor signal minus the mean value of the zero-point noise of the sub-electrode sensor signal.

[0025] Further, in step five, the axis of the tubular IPMC sensor is perpendicular to the plane formed by the x direction and the y direction, the axis forms a first plane with the x direction, and the first plane intersects the surface of the sub-electrode p1 and the sub-electrode p3 at the axial midline, and the axis forms a second plane with the y direction, and the second plane intersects the surface of the sub-electrode p2 and the sub-electrode p4 at the axial midline; when the water flow is along the x direction, the sub-electrode p3 is directly facing the water flow direction, and when the water flow is along the y direction, the sub-electrode p4 is directly facing the water flow direction.

[0026] Further, in step five, the vx value is obtained by: setting a time window length in advance, calculating the modified mean of u1(t) minus the modified mean of u3(t), and finding the corresponding horizontal axis coordinate point in the calibration curve of the sub-electrode p1 and the sub-electrode p3 according to the calculation result.

[0027] The vy value is obtained by: calculating the modified mean of u2(t) minus the modified mean of u4(t), and finding the corresponding horizontal axis coordinate point in the calibration curve of the sub-electrode p2 and the sub-electrode p4 according to the calculation result, that is, the value of vy.

[0028] The beneficial effects of the present application are:

[0029] The present application overcomes the shortcomings of the prior art, realizes the direct two-dimensional isotropic sensing capability of a single IPMC sensor by using a tubular symmetric structure, quantitatively calibrates and describes the direct current flow rate sensing characteristics of the sensor from the perspective of energy, solves the problem of lack of effective algorithm and model for existing IPMC direct current flow rate sensors, and has important significance for promoting the application of intelligent flexible materials and the development of underwater sensing technology. DETAILED DESCRIPTION

[0030] Before introducing the present application, the preparation method of the underwater two-dimensional sensing IPMC sensor is described.

[0031] The preparation method of the underwater two-dimensional sensing IPMC sensor comprises the following steps:

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

[0033] The specific steps of processing include Step1~Step5:

[0034] Step1. In the pretreatment stage: the Perma Pure Nafion dry tube (model: TT-110) is boiled in 2wt% dilute hydrochloric acid for 1 hour, and then heated in deionized water for 1 hour, to remove impurities and enable the Nafion tube to be freely bent.

[0035] Step2. In the ion adsorption stage: the Nafion tube is immersed in a tetraammine platinum chloride (molecular formula: [Pt(NH3)4]Cl2) solution, and the platinum ions in the platinum complex solution are exchanged with the cations in the Nafion.

[0036] Step 3. In the first chemical plating stage: the Nafion tube is cleaned with deionized water, and then placed in a 5 wt% sodium borohydride (NaBH4) solution. A heating magnetic stirrer is used to increase the temperature of the solution, 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 second chemical plating stage: in order to precipitate more platinum ions, the acid treatment and reduction reaction steps of Step 1-Step 3 are repeated to complete the secondary deposition of platinum, enhancing the conductivity of the platinum electrode.

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

[0039] It should be noted that the electrodes on both ends of the tubular IPMC need to be removed to prevent the inner and outer surface electrodes from shorting out.

[0040] Step 1.2: Making the outer surface sub-electrodes. The specific method is as follows: by removing the surface electrode to make an isolation strip, the outer electrode is divided into four equal parts along the axis of the tubular IPMC, forming four uniform and symmetrical sub-electrodes on the outer surface. It should be noted that the width of the isolation strip should be as thin as possible, and the depth of the isolation strip should be as shallow as possible, so as to minimize the impact of this step on the overall material properties and mechanical properties of the tubular IPMC.

[0041] When making sub-electrodes, a 3D printing method can be used to make a mold. The tubular IPMC is placed and fixed in the mold, which acts as a mask to cover the sub-electrode part while exposing the isolation strip part to be processed, thereby ensuring that the sub-electrode division is uniform and symmetrical.

[0042] Step 1.3: Making the common ground wire. A wire is connected to the entire inner surface electrode as a common ground wire, and four wires are connected to the four sub-electrodes on the outer surface as four signal lines. Each signal line and the common ground wire form a sensor signal output. In this step, the connection method between the wire and the electrode can be welding or adhesion, or other methods that can ensure long-term good conductivity.

[0043] The four signal lines of the outer surface sub-electrodes can also form multiple sensor signal outputs in pairs.

[0044] Step 1.4: Thin film encapsulation. The tubular IPMC with connected wires is encapsulated with a thin film of polymer material using vacuum physical vapor deposition. The encapsulated tubular IPMC is then immersed in a high temperature (typically 60-80 degrees Celsius) water bath for a period of time, after which it is removed. The thin film encapsulation traps a sufficient amount of water molecules inside the tubular IPMC. The polymer material used for encapsulation is capable of forming a thin film coating on the surface of the tubular IPMC using the process of vacuum physical vapor deposition.

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

[0046] The thin film coating has a thickness of between 20 microns and 50 microns, which ensures that the coating has a very low permeability to water molecules at room temperature, while the permeability of the coating to water molecules is significantly increased in a high temperature water bath. The polymer material includes, but is not limited to, Parylene.

[0047] In this embodiment, the thin film has a thickness of 25 microns, the high temperature water bath has a temperature of 80 degrees Celsius, and the tubular IPMC is removed after 48 hours of immersion. Changing the thickness of the thin film encapsulation, the temperature of the high temperature water bath, and the length of the immersion time directly changes the amount of water molecules that enter the tubular IPMC through osmosis, which in turn affects the sensing characteristics of the tubular IPMC.

[0048] Step 1.5: Biomimetic encapsulation. The tubular IPMC after the foregoing processing is encapsulated with a biomimetic encapsulation of a gel resin material using injection molding. A mold is made using 3D printing or mechanical processing, and the tubular IPMC after step four is placed and fixed in the center of the mold. The gel resin material used for biomimetic encapsulation is capable of wrapping the tubular IPMC and filling the space inside the tube using the process of injection molding, forming an encapsulation shape that resembles a bullet shell. The axis of the encapsulation shell coincides with the axis of the tubular IPMC, and the radius of the main cross section is larger than the outer radius of the tubular IPMC. Preferably, the difference between the two radii is about 1-3 millimeters. In this embodiment, the radius of the main cross section is about two millimeters larger than the outer radius of the tubular IPMC. The gel resin material includes, but is not limited to, polydimethylsiloxane (PDMS).

[0049] Step 1.6: A base for the tubular IPMC after the foregoing processing is made using 3D printing. The axial direction of the tubular IPMC is perpendicular to the plane of the base.

[0050] The method described in the present application is described below.

[0051] Step one: according to the foregoing, using Nafion tube to process tubular IPMC, the outer surface electrode is divided into four mutually insulated, orthogonal symmetrically distributed sub-electrodes in the axial direction, which form four-way sensing signal output with the inner surface electrode as the common ground, the four sub-electrodes are numbered as p1, p2, p3, 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 axis, and the sub-electrode p1 is adjacent to the sub-electrode p2 and the sub-electrode p4.

[0052] Step two: using signal conditioning circuit based on photoelectric coupling and signal amplification circuit based on operational amplifier to convert the current output of each sensing signal into measurable bipolar voltage signal u1(t), u2(t), u3(t), u4(t) corresponding to sub-electrode p1, sub-electrode p2, sub-electrode p3, sub-electrode p4 respectively, and t in the voltage signal represents time variable.

[0053] The signal conditioning circuit and the signal amplification circuit contain RC low-pass filter circuit based on resistance-capacitance combination, the low-pass cutoff frequency of which is set to 20 Hz, and the output range of the bipolar voltage signal is between positive and negative 5 volts.

[0054] Step three: under the experimental environment that can provide stable laminar flow, the tubular IPMC sensor is characterized for one-dimensional DC flow field sensing underwater, the zero point noise of the four-way sensing signal u10(t), u20(t), u30(t), u40(t) is calibrated in static water environment, and the sensing signal u1(t, v, n), u2(t, v, n), u3(t, v, n), u4(t, v, n) of each sub-electrode is calibrated in different laminar flow velocities, wherein the scalar v represents the flow velocity, and the positive integer n represents the number of the sub-electrode pn facing the water flow direction; n takes the value of 1 or 2 or 3 or 4.

[0055] A circulating water tank is used to build an experimental environment that can provide stable laminar flow, the flow velocity of the circulating water tank can be adjusted, and multiple honeycomb filters are arranged in the water tank to reduce the occurrence of turbulent flow. Third-party flow rate sensing devices are used to obtain the true value of the flow rate as a reference value for calibration, including but not limited to using impeller type water flow Hall sensor and Doppler flowmeter.

[0056] The tubular IPMC sensor is placed at least 10 cm deep below the water surface, its axis is perpendicular to the water surface, and the plane formed by its axis and the water flow direction intersects the surface of the sub-electrode facing the water flow direction at the midline of the sub-electrode in the axial direction, and the plane also intersects the surface of the sub-electrode facing away from the water flow direction at the midline of the sub-electrode in the axial direction. It should be noted that the sensing signal is collected after the experimental environment reaches a steady state, and the signal duration is at least 10 seconds, and the sampling frequency is above 1000 Hz.

[0057] Step four: based on the data in the characterization experiment, for each sub-electrode facing the water flow direction, a least square fitting is used to fit a calibration curve for describing the corresponding relationship between the four-way output signal of the tubular IPMC sensor and the change of the laminar flow velocity, the horizontal axis variable of the calibration curve is the flow velocity v, and the vertical axis variable is the corrected mean value of the sub-electrode sensing signal minus the corrected mean value of the sensing signal of the sub-electrode symmetrical to the sub-electrode.

[0058] The corrected mean value refers to the mean value of the sub-electrode sensing signal minus the mean value of the zero-point noise of the sub-electrode sensing signal.

[0059] Step five: when using the tubular IPMC sensor for underwater two-dimensional direct current flow field sensing, first obtain the one-dimensional direct current flow velocity information vx in the x direction through u1(t) and u3(t) and the aforementioned calibration curve, and simultaneously obtain the one-dimensional direct current flow velocity information vy in the y direction through u2(t) and u4(t) and the aforementioned calibration curve, the x direction and the y direction are orthogonal to each other, and the vx and vy together realize two-dimensional direct current flow field sensing.

[0060] The length of the sensing signal acquisition time window needs to be set in advance, the longer the time window, the worse the real-time performance of the calculated flow velocity information and the smaller the error; the shorter the time window, the better the real-time performance but the higher the error. Through experiments, the time window of the present embodiment is set to 5 seconds, which can better balance the real-time performance and error requirements

[0061] The axis of the tubular IPMC sensor is perpendicular to the plane formed by the x direction and the y direction, the plane formed by the axis and the x direction intersects the midline of the surface of the sub-electrodes p1 and p3 in the axial direction, and the plane formed by the axis and the y direction intersects the midline of the surface of the sub-electrodes p2 and p4 in the axial direction; when the water flow is along the x direction, p3 faces the water flow direction, and when the water flow is along the y direction, p4 faces the water flow direction.

[0062] The corrected mean value of u1(t) is calculated minus the corrected mean value of u3(t), and according to the calculation result, the corresponding horizontal axis coordinate point is found in the calibration curve of p1 and p3, which is the value of vx; the corrected mean value of u2(t) is calculated minus the corrected mean value of u4(t), and according to the calculation result, the corresponding horizontal axis coordinate point is found in the calibration curve of p2 and p4, which is the value of vy.

Claims

1. A method for underwater two-dimensional DC flow field sensing using a tubular IPMC sensor, characterized in that, 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 use the inner surface electrode as the common ground; the four sub-electrodes are sequentially sub-electrode p1, sub-electrode p2, sub-electrode p3 and sub-electrode p4 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 a measurable bipolar voltage signal, respectively u1(t), u2(t), u3(t) and u4(t), wherein t represents the time variable; Step three: perform a characterization experiment of underwater one-dimensional direct current flow field sensing on the tubular IPMC sensor, calibrate the zero point noise u10(t), u20(t), u30(t) and u40(t) of the four-way sensing signals in a static water environment, and 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 under different laminar flow velocities, wherein the scalar v represents the flow velocity, and the positive integer n represents the number of the sub-electrode n facing the water flow direction, and the value range of n is 1-4; Step four: for each sub-electrode facing the water flow direction, a calibration curve is fitted by using the least square method, which is used to describe the corresponding relationship between the four-way output signals of the tubular IPMC sensor and the change of the laminar flow velocity, and the horizontal axis variable of the calibration curve is the flow velocity v, and the vertical axis variable is the corrected mean value of the sub-electrode sensing signal minus the corrected mean value of the sensing signal of the sub-electrode symmetrical to the sub-electrode; Step five: use the tubular IPMC sensor to perform underwater two-dimensional direct current flow field sensing: through u1(t), u3(t) and the calibration curve, one-dimensional direct current flow velocity information vx in the x direction is obtained, and through u2(t), u4(t) and the calibration curve, one-dimensional direct current flow velocity information vy in the y direction is obtained. 2.The method of underwater two-dimensional DC flow field sensing 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, and form 4 sub-electrodes, and an insulating separation belt is arranged between adjacent sub-electrodes; Step 1.3: connect the entire inner surface electrode with a wire as a common ground, and connect each of the four sub-electrodes with a wire as a signal line, and each signal line and the common ground constitute one-way sensing signal output of the sensor; Step 1.4: encapsulate the tubular IPMC with a thin film capsule, 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 gum resin material to perform biomimetic encapsulation on the tubular IPMC. 3.The method of underwater two-dimensional DC flow field sensing 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 DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized in that, The positions where the inner surface electrode and the outer surface sub-electrode are connected with the corresponding wires are located at the same height position of the tube body.

5. The method for sensing a two-dimensional DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized in that, In step three, the tubular IPMC sensor is located at least 10 cm below the water surface with its axis perpendicular to the water surface.

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

7. The method for sensing a two-dimensional DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step four, the calibration curve is fitted as a straight line or a parabola according to the size of the amount of data obtained in the characterization experiment, or is fitted as a combination of different curves in sections.

8. The method for sensing a two-dimensional DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized by, In step four, the calculation method of the corrected mean value is: the mean value of the sub-electrode sensing signal minus the mean value of the zero-point noise of the sub-electrode sensing signal. 9.The method for sensing a two-dimensional DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized in that, In step five, the axis of the tubular IPMC sensor is perpendicular to the plane formed by the x direction and the y direction, the axis forms a first plane with the x direction, the first plane intersects the surface of the sub-electrode p1 and the sub-electrode p3 at the axial midline, the axis forms a second plane with the y direction, the second plane intersects the surface of the sub-electrode p2 and the sub-electrode p4 at the axial midline; when the water flow is along the x direction, the front surface of the sub-electrode p3 faces the water flow direction, and when the water flow is along the y direction, the front surface of the sub-electrode p4 faces the water flow direction.

10. The method for sensing a two-dimensional DC flow field underwater using a tubular IPMC sensor according to claim 2 or 3, characterized in that, In step five, the vx value is obtained by: setting the time window length in advance, calculating the corrected mean value of u1(t) minus the corrected mean value of u3(t), and finding the corresponding abscissa point in the calibration curve of the sub-electrode p1 and the sub-electrode p3 according to the calculation result. The vy value is obtained by: calculating the corrected mean value of u2(t) minus the corrected mean value of u4(t), and finding the corresponding abscissa point in the calibration curve of the sub-electrode p2 and the sub-electrode p4 according to the calculation result, which is the value of vy.