Method for detecting underwater friction resistance of hot film sensor based on pulse type heat flow excitation

Abstract

The invention discloses a hot film sensor underwater friction resistance detection method based on pulse type heat flow excitation in the technical field of sensor underwater friction resistance detection, and the method comprises the following steps: processing and preparing a flexible sensing element through an MEMS technology; the underwater friction resistance stress is calculated by calculating the slope of the initial heating stage; a pulse type heat flow excitation method is adopted, pulse type heat flow excitation output is conducted on the wall face of the hot film, and the hot film sensor is controlled to sense the surface friction resistance change. According to the hot film sensor underwater friction resistance detection method based on pulse type heat flow excitation, the response speed of underwater friction resistance measurement is remarkably increased, the problem that the heat balance time of a traditional hot film sensor in the underwater environment is too long is solved, and the friction resistance detection range is widened through pulse type heating and pulse parameter adjustment.

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 thermal film sensor based on pulsed heat flow excitation. Background Art

[0002] The detection of wall friction stress is of great significance for the structural optimization design of aircraft and ships, as well as improving their maneuverability and energy efficiency. The hot film method commonly used for friction stress detection in the air has high temporal and spatial resolution, and can measure friction stress under a wide range of speeds, providing great reference value for the design of aircraft shape, improving maneuverability and laying out thermal insulation materials. However, due to the large specific heat capacity of water, phase change bubbles and high viscosity of water in the underwater environment, underwater wall friction stress detection is still in the exploratory stage.

[0003] When the hot film sensor is working, it needs to provide a certain overheat ratio so that the surface temperature is higher than the surrounding fluid temperature. The higher the temperature, the more temperature is taken away by the convective heat transfer, and the higher the resolution of the measured wall friction resistance. Therefore, the temperature of the hot film sensor in the air is generally overheated to more than 20°C, which can be used for stable friction resistance measurement in the air. However, when the overheat ratio in the underwater environment is too high, bubbles will appear in the wall water. The bubbles attached to the surface will disrupt the wall flow field and increase the measurement error. Therefore, the overheat ratio of underwater hot film sensors is generally controlled within 15°C, but this processing method is not only poorer in resolution than the hot film sensing effect in the air, but also the hot film measurement range will be correspondingly reduced.

[0004] The hot film sensor generates Joule heating effect through the passage of electric current, which causes the temperature of the sensitive element to rise. The surface temperature of the hot film needs to be higher than the actual flow field temperature. When the incoming flow flows through the hot film surface, convective heat transfer occurs due to the inconsistency between the incoming flow temperature and the surface temperature of the hot film, which causes the surface temperature of the hot film to decrease. The relative velocity of the fluid at the wall is calculated by detecting the temperature of the sensitive element in the hot film, and then the specific wall friction stress is derived from the velocity through a theoretical formula. Therefore, achieving a certain overheat ratio of the hot film sensor is the key to measuring the wall friction stress. However, in an underwater environment, the large specific heat capacity of water causes a fast heat absorption rate and a long temperature equilibrium time, which will reduce the real-time detection of the wall friction stress by the hot film method. When the flow field changes, the measurement often does not obtain a real-time correspondence. 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 edge slope of temperature change, rapid detection of wall friction stress can be achieved, thereby realizing real-time measurement of wall friction detection.

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

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

[0008] S1. The flexible sensing element is prepared by MEMS process, and the underwater friction stress is calculated by calculating the slope in the initial stage of heating;

[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 of surface friction.

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

[0011] S11. Using Ni or Pt as temperature sensitive material, a flexible sensing element is prepared by MEMS process;

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

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

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

[0015] S112. Evaporating a layer of copper on the flexible PI substrate as a first layer of heating element, and covering it with a mask during the evaporation process;

[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 the areas that need to be plated with nickel or platinum, while exposing part of the copper wire circuit;

[0018] S115. Perform sputtering operation on the parts requiring temperature sensitive materials by sputtering;

[0019] S116. Spin-coat a layer of PI, and cover the entire surface after high-temperature curing. 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] Further, the S12 comprises the following steps:

[0022] S121. By directly performing pulsed heat flow excitation on the hot film wall surface that is not overheated, the hot film wall surface 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 surface will be different under the same pulse;

[0023] S122. Take the value of the sensitive element from the initial moment of current application and the extremely short delay time Δt, calculate its slope, and obtain the approximate slope of the temperature rise process. The actual wall friction stress is reflected by the approximate slope.

[0024] Further, the S2 comprises the following steps:

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

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

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

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

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

[0030] Further, the S22 comprises the following steps:

[0031] S221. After the thyristor performs pulsed heat flow excitation output, the temperature of the thermal film wall changes, the resistance of the sensitive material changes, 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;

[0032] S222. The DSP chip calculates the resistance of the sensitive material in real time, and compares the voltage drop signal at the sampling point after calculating a fixed number of sampling points. If the fixed number of sampling signals have good consistency and show an overall upward trend, they are practical signals; otherwise, they are noise. The noise and practical signals are distinguished 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 an underwater friction detection method for 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 sense surface friction changes, 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 pulsed heating, 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 by measuring the resistance of the sensitive element;

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

[0038] Figure 2 Flow chart for preparing flexible sensing elements by MEMS process using Ni / Pt as temperature sensitive materials;

[0039] Figure 3 It is a schematic diagram of the change of the temperature rise slope of the hot film wall under the same pulse at different flow rates;

[0040] Figure 4 The working principle block diagram 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 in conjunction with the drawings in the specification. It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including 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 device.

[0042] like Figure 1 (a) Figure 1 (b) Figure 1(a) is the constant current mode measurement process. Every time the external flow velocity changes, the measurement method of the hot film is to wait for the temperature to be balanced, and 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) is the measurement process in constant current mode. When the external flow velocity changes, the wall temperature changes due to convection heat transfer on the wall. By adjusting the output current, the temperature of the hot film wall is kept constant. The actual wall friction resistance is reflected by measuring the current output power P (P1...P4). Underwater, due to unstable heat transfer, the temperature adjustment time is too long, and it is difficult to measure and feedback the actual wall friction stress changes in real time.

[0043] In order 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. The flexible sensing element is manufactured by MEMS process, and the underwater friction stress is calculated by calculating the slope in the initial stage of heating; the flexible sensing element includes a heating element and a sensitive element, the heating element is made of copper, and the sensitive element is made of Pt or Ni;

[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 the flexible sensor 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 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, and a mask is used to cover it during the evaporation process 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 the areas that need to be plated with nickel or platinum, while exposing part of the copper wire circuit;

[0052] S115. Sputtering is performed on the parts that need temperature-sensitive materials. In this process, the mask is used to cover the parts, and the thickness of the sensitive material is controlled to be 50-200nm. The thermal film resistance of 0.4*5mm area is controlled to be around 50-200Ω, so as to improve repeatability and prepare for subsequent mass production. The sensitive material can be selected from materials such as Pt or Ni.

[0053] S116. Spin-coat a layer of PI, and cover the entire surface after high-temperature curing. 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 excitation to the hot film wall without overheating, the wall will experience a temperature jump. During the jump process, different flow rates will lead to different amounts of convective heat transfer, so that the temperature rise slope of the hot film wall will be different under the same pulse.

[0057] S122. Take the value of the sensitive element from the initial moment of current flow and the extremely short delay time Δt, calculate the slope, and obtain the approximate slope of the temperature rise process. At this time, the size of the slope will change due to the flow velocity. Figure 3 As shown, the actual wall friction stress can be reflected. This method does not include the thermal equilibrium process, but measures in the non-steady-state heat transfer process. And because the Δt time is very short, it can achieve rapid measurement, thereby detecting the wall friction stress at the real moment. By continuously applying pulses and adjusting the pulse parameters, the real-time measurement of the wall friction stress can be 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 of surface friction.

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

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

[0061] Wherein, S21 includes the following specific steps:

[0062] S211. Since the DSP chip F28335 has a 150MHz crystal frequency, extremely high frequency sampling and PWM square wave are achieved by dividing the DSP chip crystal 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, thereby 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 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 (1-10 mA) current is applied to the resistance of the sensitive material, and the DSP F28335 reads the analog voltage drop generated on the resistance of the sensitive material through fast AD conversion;

[0067] S222. 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. Distinguish between noise and practical signals and discard the noise;

[0068] S223. Analyze the practical signal and calculate the slope, so as to achieve 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 as a limitation of 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 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. The flexible sensing element is prepared by MEMS process, and the underwater friction stress is calculated by calculating the slope in the initial stage of heating; 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 of surface friction.

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 S1 comprises the following steps: S11. Using Ni or Pt as temperature sensitive material, a flexible sensing element is prepared by MEMS process; S12. Calculate the underwater friction stress by calculating the slope in the initial stage of heating.

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: The S11 comprises the following steps: S111. Spin-coating a layer of PI on a glass substrate, and curing at high temperature to form a flexible PI substrate; S112. Evaporating a layer of copper on the flexible PI substrate as a first layer of heating element, and covering it with a mask during the evaporation process; 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 the areas that need to be plated with nickel or platinum, while exposing part of the copper wire circuit; S115. Perform sputtering operation on the parts requiring temperature sensitive materials by sputtering; S116. Spin-coat a layer of PI, and cover the entire surface after high-temperature curing. Simultaneously, perform laser etching to expose the actual external wiring location.

4. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 3 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, and the sensitive material is Pt or Ni.

5. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 2 is characterized in that: The S12 comprises the following steps: S121. By directly performing pulsed heat flow excitation on the hot film wall surface that is not overheated, the hot film wall surface 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 surface will be different under the same pulse; S122. Take the value of the sensitive element from the initial moment of current application and the extremely short delay time Δt, calculate its slope, and obtain the approximate slope of the temperature rise process. The actual wall friction stress is reflected by the approximate slope.

6. 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 S2 comprises the following steps: S21. Pulse heat flow excitation output to the external circuit; S22. Perform slope detection on the external circuit.

7. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 6 is characterized in that: The S21 comprises the following steps: S211. By dividing the DSP chip crystal frequency, extremely high frequency sampling and PWM square wave are achieved; S212. By turning off the thyristor, pulsed heat flow excitation output is achieved for the heating circuit part.

8. The underwater friction detection method of a hot film sensor based on pulsed heat flow excitation according to claim 6 is characterized in that: The S22 comprises the following steps: S221. After the thyristor performs pulsed heat flow excitation output, the temperature of the thermal film wall changes, the resistance of the sensitive material changes, 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; S222. The DSP chip calculates the resistance of the sensitive material in real time, and compares the voltage drop signal at the sampling point after calculating a fixed number of sampling points. If the fixed number of sampling signals have good consistency and show an overall upward trend, they are practical signals; otherwise, they are noise. The noise and practical signals are distinguished and the noise is discarded. S223. Analyze the practical signal and calculate the slope.

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