A method for underwater friction detection of hot film sensors based on pulsed thermal flux excitation

Abstract

The present invention discloses a method for underwater friction detection of a hot film sensor based on pulsed heat flux excitation, in the field of underwater friction detection technology. The method comprises the following steps: fabricating a flexible sensor element using a MEMS process, calculating the underwater friction stress by calculating the slope during the initial heating phase; and applying pulsed heat flux excitation to the hot film wall surface to control the hot film sensor's perception of surface friction changes. This method significantly improves the response speed of underwater friction measurement, overcomes the problem of excessively long thermal equilibrium time for conventional hot film sensors in underwater environments, and increases the friction detection range by adjusting pulsed heating parameters.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater friction detection of sensors, and in particular to an underwater friction detection method of a hot film sensor based on pulsed heat flow excitation. Background Art

[0002] Wall friction stress testing is crucial for optimizing the structural design of aircraft and ships, as well as improving their maneuverability and energy efficiency. The hot film method, commonly used in airborne friction stress testing, boasts high temporal and spatial resolution, capable of measuring friction stress over a wide range of speeds. This provides valuable insights into aircraft design, improving maneuverability, and the placement of thermal insulation materials. However, underwater wall friction stress testing remains at an exploratory stage due to the high specific heat capacity of water, phase change bubbles, and high viscosity.

[0003] When hot film sensors are operating, they need to provide a certain superheat ratio so that the surface temperature is higher than that of the surrounding fluid. The higher the temperature, the more heat is removed by convective heat transfer, and the higher the resolution of the measured wall friction resistance. Therefore, hot film sensors in air are generally overheated to above 20°C. This allows for stable friction resistance measurement in air. However, in underwater environments, if the superheat ratio is too high, bubbles will appear in the wall water. These bubbles attached to the surface will disrupt the wall flow field and increase measurement errors. Therefore, underwater hot film sensors are generally controlled to have a superheat ratio of less than 15°C. However, this treatment method not only has poorer resolution than hot film sensing in air, but also reduces the measurement range of the hot film.

[0004] Hot film sensors use the Joule heating effect generated by the passage of electric current, which raises the temperature of the sensitive element. The surface temperature of the hot film must be higher than the actual flow field temperature. When the incoming flow passes over the hot film, the temperature mismatch between the incoming flow and the hot film surface causes convective heat transfer, which cools the hot film surface temperature. By measuring the temperature of the sensitive element within the hot film, the relative velocity of the fluid at the wall is calculated. The specific wall friction stress is then derived from the velocity using theoretical formulas. Therefore, achieving a certain overheat ratio in the hot film sensor is crucial for measuring wall friction stress. However, in underwater environments, the high specific heat capacity of water leads to rapid heat absorption and a long temperature equilibration time, which reduces the real-time performance of the hot film method for detecting wall friction stress. As the flow field changes, the measured relationship often does not correspond to real-time measurements. Summary of the Invention

[0005] The purpose of the present invention is to provide an underwater friction detection method of a hot film sensor based on pulsed heat flow excitation, to solve the technical problems existing in the detection of wall friction stress of underwater hot film sensors, to improve the previous hot film measurement method so that it can be applied to underwater wall friction measurement, to overcome the problem of wall bubble generation through pulsed heating, and to achieve a wider range of wall friction measurement than traditional methods; at the same time, by detecting the slope of the temperature change edge, the wall friction stress can be quickly detected, and real-time measurement of the wall friction detection can be achieved.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] A method for detecting underwater friction of a hot film sensor based on pulsed heat flow excitation includes the following steps:

[0008] S1. Fabricate a flexible sensor element using MEMS technology and calculate underwater friction stress by calculating the slope during the initial heating phase.

[0009] S2. Use the pulsed heat flux excitation method to output pulsed heat flux excitation to the hot film wall surface, and control the hot film sensor to sense the change in surface friction.

[0010] Furthermore, the S1 includes the following steps:

[0011] S11. Using Ni or Pt as temperature-sensitive materials, a flexible sensing element is fabricated using MEMS technology.

[0012] S12. Calculate the underwater friction stress by calculating the slope in the initial stage of heating.

[0013] Furthermore, the S11 includes the following steps:

[0014] S111. Spin-coating a layer of PI on a glass substrate and curing it at high temperature to form a flexible PI substrate;

[0015] S112. A layer of copper is deposited on the flexible PI substrate as a first layer of heating elements, and the deposition process is covered by a mask;

[0016] S113. Spin-coating a PI film on the surface, and then curing at high temperature to cover the copper wire circuit;

[0017] S114. Surface etching is performed by laser to etch out areas requiring nickel or platinum plating, while exposing portions of the copper conductor circuit;

[0018] S115. Sputtering operation is performed on parts of the material that require temperature sensitivity by sputtering;

[0019] S116. Spin-coat a layer of PI, and after high-temperature curing, cover the entire surface. Simultaneously, perform laser etching to expose the actual external wiring location.

[0020] Furthermore, in the S115 process, a mask is used for covering to control the thickness of the sensitive material to be 50-200 nm, and the sensitive material is Pt or Ni.

[0021] Furthermore, the S12 includes the following steps:

[0022] S121. By directly applying pulsed heat flow to the non-overheated hot film wall, the hot film wall will experience a temperature jump. During the jump, different flow rates will result in different amounts of convective heat transfer, and the temperature rise slope of the hot film wall will vary under the same pulse.

[0023] S122. Take the values ​​of the sensitive element at the initial moment of current application and at the very short delay time Δt, calculate the slope, and obtain the approximate slope of the temperature rise process. The approximate slope reflects the actual wall friction stress.

[0024] Furthermore, the S2 includes the following steps:

[0025] S21. Pulsed heat flow excitation output to the external circuit;

[0026] S22. Perform slope detection on the external circuit.

[0027] Furthermore, the S21 includes the following steps:

[0028] S211. By dividing the DSP chip crystal oscillator frequency, extremely high frequency sampling and PWM square wave are achieved;

[0029] S212. By turning off the thyristor, a pulsed heat flow excitation output is achieved for the heating circuit part.

[0030] Furthermore, the S22 includes the following steps:

[0031] S221. After the thyristor generates pulsed heat flux, the temperature of the thermal film wall changes, causing the resistance of the sensitive material to change. A mA-level current is applied to the sensitive material resistance, and the DSP chip reads the analog voltage drop across the sensitive material resistance through rapid A / D conversion.

[0032] S222. The DSP chip calculates the resistance of the sensitive material in real time. After calculating a fixed number of sampling points, it compares the voltage drop signals at those sampling points. If the fixed number of sampling signals have good consistency and show an overall upward trend, they are considered to be useful signals. Otherwise, they are considered to be noise. The noise is distinguished from the useful signal and the noise is discarded.

[0033] S223. Analyze the practical signal and calculate the slope.

[0034] By adopting the above technical solution, the present invention has the following advantages:

[0035] The present invention provides a method for underwater friction detection of a hot film sensor based on pulsed heat flux excitation. The method adopts pulsed heat flux excitation technology to control the hot film sensor to perceive changes in surface friction, significantly improving the response speed of underwater friction measurement, overcoming the problem of long thermal equilibrium time of traditional hot film sensors in underwater environments, and achieving rapid response. By pulse heating, the pulse parameters are adjusted to increase the friction detection range. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 (a) is the measurement process in constant current mode, which reflects the actual wall friction resistance by measuring the resistance of the sensitive element;

[0037] Figure 1 (b) is the measurement process in constant current mode, which reflects the actual wall friction resistance by measuring the current output power;

[0038] Figure 2 Flowchart for preparing flexible sensing elements using Ni / Pt as temperature sensitive materials through MEMS processing;

[0039] Figure 3 Schematic diagram of the temperature rise slope of the hot film wall under the same pulse at different flow rates;

[0040] Figure 4 The block diagram shows the working principle of the pulsed heat flow excitation method for the external circuit. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. It should be noted that, in this document, relational terms such as first and second, etc., are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include," "comprise," or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or apparatus.

[0042] like Figure 1 (a) Figure 1 As shown in (b), Figure 1(a) is the measurement process in constant current mode. Every time the external flow velocity changes, the hot film measurement method is to wait for the temperature to reach equilibrium, then read the resistance R (R1, R2...R4) of the sensitive element to reflect the actual wall velocity change and thus obtain the actual wall friction stress. Figure 1 (b) shows the measurement process in constant flow mode. When the external flow velocity changes, the wall temperature fluctuates due to convection heat transfer. By adjusting the output current, the wall temperature of the hot film is kept constant. The actual wall friction is reflected by measuring the current output power P (P1...P4). Underwater, due to unstable heat transfer, temperature adjustment takes a long time, making it difficult to measure and reflect the actual wall friction stress changes in real time.

[0043] To solve the above technical problems, the present invention provides a method for underwater friction detection of a hot film sensor based on pulsed heat flow excitation, comprising the following steps:

[0044] S1. A flexible sensing element is fabricated using MEMS technology. The underwater friction stress is calculated by calculating the slope during the initial heating phase. The flexible sensing element includes a heating element made of copper and a sensitive element made of platinum or nickel.

[0045] S1 includes the following specific steps:

[0046] S11. In order to realize the wall friction stress detection, Ni or Pt is used as the temperature sensitive material and a flexible sensing element is prepared by MEMS process. The specific preparation process is as follows Figure 2 As shown:

[0047] S11 includes the following steps:

[0048] S111. Spin-coating a layer of PI on a glass substrate and curing it at high temperature to form a flexible PI substrate;

[0049] S112. A layer of copper is evaporated on the flexible PI substrate as the first layer of heating element. During the evaporation process, a mask is used to cover the surface to achieve the layout of the copper conductive wires.

[0050] S113. Spin-coating a PI film on the surface, and then curing at high temperature to cover the copper wire circuit;

[0051] S114. Surface etching is performed by laser to etch out areas requiring nickel or platinum plating, while exposing portions of the copper conductor circuit;

[0052] S115. Sputtering is performed on areas requiring temperature-sensitive material. A mask is used to cover the area, controlling the thickness of the sensitive material to 50-200 nm and the resistance of the thermal film within a 0.4 x 5 mm area to approximately 50-200 Ω. This improves repeatability and prepares for subsequent mass production. Sensitive materials can be selected from materials such as Pt or Ni.

[0053] S116. Spin-coat a layer of PI, and after high-temperature curing, cover the entire surface. Simultaneously, perform laser etching to expose the actual external wiring location.

[0054] S12. Calculate the underwater friction stress by calculating the slope in the initial stage of heating.

[0055] Among them, S12 includes the following specific steps: Figure 3 As shown:

[0056] S121. During the measurement process, by directly applying pulsed heat flow to the non-overheated hot film wall, the wall surface will experience a temperature jump. During the jump process, different flow rates will lead to different amounts of convective heat transfer, so the temperature rise slope of the hot film wall surface will be different under the same pulse.

[0057] S122. Take the value of the sensitive element at the initial moment of current application and at the very short delay time Δt, and calculate the slope to obtain the approximate slope of the temperature rise process. At this time, the slope will change with the flow rate. Figure 3 As shown, the actual wall friction stress can be reflected. This method does not incorporate the thermal equilibrium process, but instead measures during the unsteady heat transfer process. Because the Δt time is very short, rapid measurement is possible, allowing for the detection of the actual wall friction stress. By continuously applying pulses and adjusting the pulse parameters, real-time measurement of wall friction stress is achieved.

[0058] S2. Use the pulsed heat flux excitation method to output pulsed heat flux excitation to the hot film wall surface, and control the hot film sensor to sense the change in surface friction.

[0059] Among them, S2 includes the following steps: Figure 4 As shown:

[0060] S21. Pulsed heat flow excitation output to the external circuit;

[0061] Among them, S21 includes the following specific steps:

[0062] S211. Since the DSP chip F28335 has a 150MHz crystal oscillator frequency, very high-frequency sampling and PWM square wave are achieved by dividing the DSP chip crystal oscillator frequency.

[0063] S212. Use the DSP chip to output a specific PWM square wave to switch the thyristor on and off, thereby achieving pulsed heat flow excitation output for the heating circuit part. The heating circuit part generates heat to increase the temperature of the entire component, providing pulsed heat flow excitation for the entire component.

[0064] S22. Perform slope detection on the external circuit.

[0065] Wherein, S22 includes the following specific steps:

[0066] S221. After the thyristor generates pulsed heat flux, the temperature of the thermal film wall changes, causing the resistance of the sensitive material to change. A mA-level (1-10 mA) current is applied to the sensitive material resistor. The DSP F28335 reads the analog voltage drop across the sensitive material resistor through rapid A / D conversion.

[0067] S222. The DSP chip calculates the resistance of the sensitive material in real time. After calculating a fixed number of sampling points, it compares the voltage drop signals at the sampling points. If the fixed number of sampling signals have good consistency and show an overall upward trend, it is a useful signal; otherwise, it is noise. The noise is distinguished from the useful signal and the noise is discarded.

[0068] S223. Analyze the practical signal and calculate the slope, thereby realizing the measurement of the wall friction slope.

[0069] Finally, it should be pointed out that although the present invention has been described with reference to the current specific embodiments, ordinary technicians in this technical field should realize that the above embodiments are only used to illustrate the present invention and are not used to limit the present invention. Various equivalent changes or substitutions can be made without departing from the concept of the present invention. Therefore, as long as the changes and modifications to the above embodiments are within the scope of the essential spirit of the present invention, they will fall within the scope of the claims of the present invention.

Claims

1. A method for underwater friction detection of a hot film sensor based on pulsed heat flow excitation, characterized in that: The following steps are involved: S1. Fabricate a flexible sensor element using MEMS technology and calculate underwater friction stress by calculating the slope during the initial heating phase. Said S1 comprises the following steps: S11. Using Ni or Pt as temperature-sensitive materials, a flexible sensing element is fabricated using MEMS technology. S12. Calculate underwater friction stress by calculating the slope in the initial heating stage; The S12 includes the following steps: S121. By directly applying pulsed heat flow to the non-overheated hot film wall, the hot film wall will experience a temperature jump. During the jump, different flow rates will result in different amounts of convective heat transfer, and the temperature rise slope of the hot film wall will vary under the same pulse. S122. Take the values ​​of the sensitive element at the initial moment of current application and at the very short delay time ∆t, and calculate the slope of the two values ​​to obtain the approximate slope of the temperature rise process. The approximate slope reflects the actual wall friction stress.

2. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 1 is characterized in that: The S11 includes the following steps: S111. Spin-coating a layer of PI on a glass substrate and curing it at high temperature to form a flexible PI substrate; S112. A layer of copper is deposited on the flexible PI substrate as a first layer of heating elements, and the deposition process is covered by a mask; S113. Spin-coating a PI film on the surface, and then curing at high temperature to cover the copper wire circuit; S114. Surface etching is performed by laser to etch out areas requiring nickel or platinum plating, while exposing portions of the copper conductor circuit; S115. Sputtering operation is performed on parts of the material that require temperature sensitivity by sputtering; S116. Spin-coat a layer of PI, and after high-temperature curing, cover the entire surface. Simultaneously, perform laser etching to expose the actual external wiring location.

3. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 2 is characterized in that: In the S115 process, a mask is used for covering, and the thickness of the sensitive material is controlled to be 50-200 nm. The sensitive material is Pt or Ni.

4. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 1 is characterized in that: The method also includes using a pulsed heat flow excitation method to output a pulsed heat flow excitation to the hot film wall surface, and controlling the hot film sensor to sense the change in surface friction resistance, including the following steps: (1) Pulse heat flow excitation output to the external circuit; (2) Perform slope detection on the external circuit.

5. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 4 is characterized in that: Said (1) comprises the following steps: (11). By dividing the DSP chip crystal oscillator frequency, extremely high frequency sampling and PWM square wave can be achieved; (12) By turning off the thyristor, a pulsed heat flow excitation output is achieved for the heating circuit.

6. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 4 is characterized in that: Said (2) comprises the following steps: (21). After the thyristor performs pulsed heat flow excitation output, the temperature of the thermal film wall changes, and the resistance of the sensitive material changes. A mA-level current is applied to the resistance of the sensitive material, and the DSP chip reads the analog value of the voltage drop generated on the resistance of the sensitive material through fast AD conversion; (22) The DSP chip calculates the resistance of the sensitive material in real time. After calculating a fixed number of sampling points, the voltage drop signal at the sampling point is compared. If the fixed number of sampling signals have good consistency and show an overall upward trend, it is a practical signal; otherwise, it is noise. The noise and practical signal are distinguished and the noise is discarded. (23). Analyze the practical signal and calculate the slope.

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