Inverted Pendulum Type Bicomponent Airflow Wall Friction Resistance Sensor

Abstract

Provided is an inverted pendulum type two-component airflow wall friction resistance sensor, which is characterized by comprising a non-metallic housing (10), an inverted pendulum (20), induction electrodes (30), an electromagnet (40), a permanent magnet ring (50) and a metal base (60). And a measurement method of the sensor. Also provided is a control system for the inverted pendulum type two-component airflow wall friction resistance sensor, and an inverted pendulum balance control method based on differential voltage signals obtained therefrom. The sensor of the present invention has a wide range of applications and is applicable to both low-speed flow and high-speed flow; it can achieve two-component measurement; it has a high frequency response; it has a low cost; the measurement process is simple, and it has high practical popularization value.

Description

Technical Field

[0001] The present invention relates to the field of wall friction resistance sensing, and in particular to a sensor that combines a micro inverted pendulum structure and magnetic adaptive closed-loop control for measuring two-component airflow wall friction resistance. Background Art

[0002] In the field of aerospace, the diagnosis of wall friction resistance is of great significance for the research and development of aircraft. Wall friction resistance reflects the state of the near-wall boundary layer (laminar or turbulent), determines the cruise performance of the aircraft, and also affects the structural thermal load of high-speed aircraft. Existing wall friction resistance measurement methods can be roughly divided into three categories. One is the measurement method based on the near-wall velocity profile. The physical principle it relies on is Newton's viscosity law, and the main experimental equipment used includes hot wires, microscopic PIV, and laser Doppler anemometer LDA, etc. (CN105004466A, a high-precision non-contact pneumatic friction resistance measurement method and measurement device, Pan Chong, Wang Jianjie, Shen Junqi, Wang Jinjun, Li Zhibo, Li Lei). This method has low measurement efficiency. A near-wall velocity profile containing dozens of data points can only deduce one friction resistance component value. For three-dimensional wing flows, there are two components of wall friction resistance, and two velocity measurement sections need to be arranged to obtain the total wall friction resistance at one spatial position. In addition, limited by the spatial resolution of velocity measurement, it is difficult to obtain the velocity distribution in the near-wall viscous sublayer region in high-speed and high Reynolds number flows (the order of magnitude of the viscous sublayer thickness: 1-10 μm). Therefore, the velocity-based measurement device can only be applied to the measurement of friction resistance in low-speed quasi-two-dimensional flows. The second type of friction resistance measurement device is the MEMS (Micro-Electro-Mechanical System) (CN108467007A, a method for fabricating a MEMS friction resistance sensor based on visual alignment, Wang Xiong, Xu Xiaobin, Zhu Tao, Gao Yang, Chen Liguo, Wang Nantian, Qiu Huacheng, Shi Yunlong). This system is installed flush with the wall, and the wall friction resistance is obtained by converting the displacement signal of the suspended micro-platform into the strain signal or capacitance signal of the balance. Compared with the velocity-based measurement, this method has the advantages of high measurement efficiency and good dynamic characteristics. However, the MEMS friction resistance sensor requires complex preparation processes and extremely high costs. The micro-groove features on the surface are easily blocked by dust in the airflow. Therefore, it can only be applied to the measurement in a clean airflow environment. The third type of friction resistance measurement device is based on the oil film interference principle (CN 108007668 A, a three-dimensional boundary layer friction resistance measurement device and measurement method for a swept wing). This device consists of a monochromatic light source, a camera, etc. By converting the change in oil film thickness into the interference fringe spacing, this device can achieve multi-point simultaneous measurement on the entire surface of the aircraft model. The main problems of this method are long measurement preparation time and poor accuracy, and it cannot diagnose the dynamic friction resistance changes at the same spatial position. Summary of the Invention

[0003] The present invention provides an inverted pendulum type two-component airflow wall friction resistance sensor, which comprises a non-metallic outer shell 10, an inverted pendulum 20, induction electrodes 30, an electromagnet 40, a permanent magnet ring 50 and a metal base 60; wherein

[0004] The non-metallic outer shell 10 is integrally in the shape of a stepped cylinder, thicker at the bottom and thinner at the top, and is a two-stage cylindrical structure; the non-metallic outer shell 10 is a hollow structure, and its interior includes two interconnected cavities: a slender cylindrical cavity 103 located in the upper part and a square cavity 104 located in the lower part, and their heights and positions respectively correspond to the upper half cylinder and the lower half cylinder; the axes of the slender cylindrical cavity 103 and the square cavity 104 located in the lower part coincide with the axis of the non-metallic outer shell 10; the square cavity 104 is a cuboid structure, and its height is determined by the height of the lower half cylinder of the non-metallic outer shell 10; four threaded holes 101 and four electromagnet mounting holes 102 are arranged on the outer wall surface of the lower half cylinder, which are respectively used for the assembly of the induction electrodes 30 and the electromagnet 40; the threaded holes 101 are evenly distributed circumferentially on the outer wall surface of the lower half cylinder, and the included angle between adjacent hole positions is 90 degrees; the electromagnet mounting holes 102 are also evenly distributed circumferentially on the outer wall surface of the lower half cylinder, with an included angle of 90 degrees; each threaded hole 101 is directly above the corresponding electromagnet mounting hole 102, so the four threaded holes 101 and the four electromagnet mounting holes 102 can be divided into four groups, and the depth of the holes penetrates vertically from the outer wall surface of the lower half cylinder to the inner wall surface of the square cavity 104;

[0005] The inverted pendulum 20 is overall spike-shaped and made of a conductive material, including a dome 201, a support cylinder 204, a force-sensing area 202, and a support cone 203; the dome 201 is an inverted frustum at the top of the inverted pendulum 20, and the diameter of the upper surface circle is larger than that of the lower surface circle; the diameter of the upper surface of the dome 201 is slightly smaller than the diameter of the cylindrical cavity 103 to ensure the existence of a gap between the two after nesting; an excessive gap will cause gas to flow into the sensor interior during the measurement process, resulting in uneven pressure on the left and right sides of the inverted pendulum 20 and increasing the measurement error; the support cylinder 204 is fixedly connected to the dome 201 and is located below it; the force-sensing area 202 is a cuboid structure, fixedly connected to the support cylinder 204 and located below it; the support cone 203 is a quadrangular pyramid structure, fixedly connected to the force-sensing area 202 and located below it; the axes of the dome 201, the support cylinder 204, the force-sensing area 202, and the support cone 203 in the vertical direction coincide with the axis of the non-metallic housing 10; the cross-sectional area of the frustum of the dome 201 shrinks sharply from top to bottom; the upper half of the four sides of the force-sensing area 202 are opposite to the positions of the four induction electrodes 30; the lower half of the force-sensing area 202 is nested with a square permanent magnet ring 50, that is, the lower half of the force-sensing area 202 is tightly nested into the permanent magnet ring 50; the permanent magnet ring 50 is a square enclosure structure, and its horizontal cross-section is a square ring. The inner side length of the inner ring of the permanent magnet ring 50 is equal to the side length of the side surface of the force-sensing area 202. When the lower half of the force-sensing area 202 is inserted into the permanent magnet ring 50, the two are in a tight fit structure; the vertical installation position of the permanent magnet ring 50 should be at the same height as the electromagnet mounting hole 103, so that the permanent magnet ring 50 and the four electromagnets 40 are in corresponding positions; the support cone 203 is in the shape of an inverted pyramid, that is, an inverted quadrangular pyramid, and the bottom side length of the quadrangular pyramid is the same as the bottom side length of the force-sensing area 202; the induction electrode 30 is in the shape of a stepped cylinder, including two parts: a slender cylinder and a short and thick cylinder, and their axes coincide; threads are engraved on the surface of the slender cylinder of the induction electrode 30, and the induction electrode 30 is assembled with the housing 10 through a threaded hole 101; the electromagnet 40 is in the shape of a cylinder and consists of an iron core and a spiral conductive coil; the electromagnet 40 is fixed inside the cylindrical electromagnet mounting hole 102, and the inner end face of its inserted part is kept at a certain distance from the permanent magnet ring 50; the metal base 60 is overall in the shape of a thin circular sheet, including a boss 601 and a cylindrical base 603 located below it; the diameter of the cylindrical base 603 is the same as the diameter of the lower half cylinder of the housing 10; the side length of the boss 601 is the same as the side length of the square cavity 104. By embedding the boss 601 into the square cavity 104 from bottom to top, the metal base 60 is tightly fitted with the housing 10; in addition, at the center of the boss 601, a conical pit 602 is provided; the angle of the bottom cone angle of the pit 602 is larger than the cone angle of the support cone 203; after the sensor is assembled, the vertex of the pit 602 contacts the vertex of the cone 203 to form the movement fulcrum of the inverted pendulum 20.

[0006] In an embodiment of the present invention, the total height of the non-metallic housing 10 is 30 - 40 mm, the diameter range of the upper cylindrical part is 4 - 8 mm, and the diameter range of the lower cylindrical part is 10 - 20 mm;

[0007] The height of the upper and lower cylindrical parts is 15 - 20 mm;

[0008] The bottom surface of the square cavity 104 is square or rectangular, and the maximum side length is 10 - 12 mm;

[0009] The aperture diameter of the threaded hole 101 is 1.5 - 3 mm;

[0010] The aperture diameter of the electromagnet mounting hole 102 is 2 - 4 mm.

[0011] In a specific embodiment of the present invention, the gap between the dome 201 and the cylindrical cavity 103 is less than 0.1 mm, and the contraction angle of the dome 201 from top to bottom is greater than 120 degrees.

[0012] In another embodiment of the present invention, the height range of the support tip cone 203 is 5 - 10 mm, and the full cone angle range of the tower top is 30 - 60 degrees.

[0013] In another specific embodiment of the present invention, the screwing-in distance of the induction electrode 30 in the threaded hole 101 is not less than 5 mm, and the distance between the four side surfaces of the force induction area 202 after the sensor is assembled and the end surface of the induction electrode 30 is less than 1 mm.

[0014] In yet another embodiment of the present invention, the thickness range of the metal base 60 is 2 - 4 mm; the depth range of the pit 602 is 1 - 2 mm, and the angle range of the bottom cone angle is 90 - 120 degrees.

[0015] In yet another specific embodiment of the present invention,

[0016] The diameter range of the upper cylindrical part of the non-metallic housing 10 is 6 mm, and the diameter range of the lower cylindrical part is 16 mm;

[0017] The heights of the upper and lower cylindrical parts are equal;

[0018] The bottom surface of the square cavity 104 is square, and the side length of the square is 10 mm;

[0019] The side wall thickness of the cylindrical cavity 103 is not less than 1 mm, and the wall thickness at the opening position of the square cavity 104 is not less than 3 mm;

[0020] The aperture diameter of the threaded hole 101 is 2 mm, the number is 4, and the included angle between adjacent hole positions is 90 degrees;

[0021] The lower limit of the aperture diameter of the electromagnet mounting hole 102 is 2 mm, the number is 4, and the included angle is 90 degrees;

[0022] The height of the permanent magnet ring 50 is 3 mm and the thickness is 1 mm;

[0023] The inner end face of the part of the electromagnet 40 inserted into the electromagnet mounting hole 102 is 1 mm away from the permanent magnet ring 50;

[0024] The thickness of the metal base 60 is 2 mm; the convex platform 601 and the cylindrical base 603 are integrally formed; the depth of the concave pit 602 is 1 mm and the angle of the bottom cone angle is 90 degrees.

[0025] A measurement method for an inverted pendulum type two-component airflow wall friction resistance sensor is also provided, specifically:

[0026] For the two-dimensional flat plate boundary layer flow:

[0027] The top of the sensor is flush-mounted with the wall 70. In the ideal two-dimensional case, the inverted pendulum 20 is not affected by force in the direction perpendicular to the plane. Therefore, only two electromagnets can be used to keep the inverted pendulum in balance. Assume the left side is L1 and the right side is L2; when the boundary layer on the flat plate surface flows from left to right over the dome of the inverted pendulum 20, due to viscous friction, a frictional resistance F to the right will be generated on the dome of the inverted pendulum 20 f ; Under the action of this frictional resistance, the inverted pendulum 20 deviates from the central position and begins to tilt to the right; to restore the inverted pendulum to the neutral state, it is necessary to increase the repulsive force F2 of the right electromagnet L2 and decrease the repulsive force F1 of the left electromagnet L1 through an automatic control system; when the force balance is reached again, the moment of all forces about the movement fulcrum of the inverted pendulum 20 should be equal to 0, which is expressed by the formula as follows:

[0028] F f ·(s1 + s2)+(F1 - F2)·s2 = 0 (1)

[0029] Where, s1 and s2 respectively represent the vertical distances between the center of the electromagnet and the wall and the fulcrum; further simplified, the expression of the frictional resistance is as follows:

[0030]

[0031] Because the force of the electromagnet 40 on the magnetic ring 50 is proportional to the DC voltage applied to both ends of the electromagnet, the magnetic forces F1 and F2 can be written as linear expressions of the voltages at both ends of L1 and L2, that is

[0032]

[0033] Where, k is the proportionality coefficient; U de and U fgrespectively represent the voltages at both ends of the electromagnets L1 and L2; Substitute formula (3) into formula (2) to obtain the relationship between the frictional resistance and the supply voltage of the electromagnet in the equilibrium state:

[0034]

[0035] In addition, a control system for an inverted pendulum type two-component airflow wall friction resistance sensor is also provided. The electrical connection of this system is as follows: The positive pole of the DC power supply is connected to terminal a led out from the left induction electrode, and the negative pole is connected to terminal c led out from the right induction electrode; Terminal b is led out from the metal base, and this terminal is connected to the inverted pendulum 20 through the electrical contact of the fulcrum; Since both the induction electrode 30 and the inverted pendulum 20 are made of metal materials and there is a certain air gap between them, therefore, an equivalent capacitor can be formed between terminals a and b, and the capacitance value C ab is inversely proportional to the gas gap; Similarly, an equivalent capacitor can also be formed between terminals c and b, and the capacitance value C bc is inversely proportional to the gas gap between the inverted pendulum 20 and the right induction electrode; Two resistors R1 and R2 are also connected in series between the positive and negative poles of the DC power supply; The resistance values of these two resistors are equal, and the purpose is to provide a reference voltage of 1 / 2 times the DC power supply at the midpoint h.

[0036] In addition, an inverted pendulum balance control method based on differential voltage signals is also provided, specifically:

[0037] Based on formula (4)

[0038]

[0039] When the inverted pendulum 20 is strictly in the neutral position, the distances between the force sensing area 202 and the left and right induction electrodes are equal. Therefore, the capacitance C ab = C bc , and the voltage difference U bh between terminals b and h is 0; When the inverted pendulum is subjected to frictional resistance and deviates to the right from the equilibrium position, the right gas gap becomes smaller and the capacitance value increases; The left gas gap becomes larger and the capacitance value decreases, that is, C ab < C bc ; According to the capacitance voltage division principle, at this time, the voltage of terminal b is higher than half of the DC power supply voltage, U bh > 0; This voltage difference is used as the input signal of the automatic control system and enters the PID controller. The PID controller outputs a control voltage U o ; Based on this voltage U o and the output U1 of the electromagnet at both ends under the reference frictionless resistance state, the real-time supply voltages of the electromagnets L1 and L2 can be obtained as follows:

[0040]

[0041] where U de and U fg are the supply voltages of the two electromagnets on the left and right sides respectively, that is, the input voltage between terminals d and e and the input voltage between terminals f and g;

[0042] Since the automatic control system increases the supply voltage of the right electromagnet L2 and decreases the supply voltage of the left electromagnet L1, the repulsive force F2 on the right side increases and the repulsive force F1 on the left side decreases. The inverted pendulum 20 swings to the left and gradually returns to the neutral position. After reaching the neutral position, the input voltage U bh is 0, and the output voltage U o of the control system is proportional to the frictional resistance received by the dome of the inverted pendulum 20. Substituting formula (5) into formula (3) gives:

[0043]

[0044] The present invention can solve the disadvantages of the traditional frictional resistance measurement system, such as complexity, high cost, and the ability to measure only a single frictional resistance component. In addition, the invented sensor also has advantages in measurement efficiency and dynamic characteristics, and has a wide range of applications, being applicable to both low-speed flow and high-speed flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Showing the frictional resistance sensor, where Figure 1 (a) shows the assembly diagram, Figure 1 (b) shows the exploded view, Figure 1 (c) shows the central cross-sectional view;

[0046] Figure 2 Showing the housing 10, where Figure 2 (a) shows the central cross-sectional view, Figure 2 (b) shows the three-dimensional view, Figure 2 (c) shows the perspective view;

[0047] Figure 3 Showing the metal base 60, where Figure 3 (a) shows the three-dimensional view, Figure 3 (b) shows the central cross-sectional view;

[0048] Figure 4 Showing the force balance analysis diagram of the sensor;

[0049] Figure 5 Showing the sensor wiring terminals, where Figure 5 (a) shows the structural marking diagram of the sensor wiring terminals, Figure 5 (b) shows the electrical connection diagram;

[0050] Figure 6 Shows the automatic balancing control block diagram of the inverted pendulum.

[0051] Explanation of the reference numerals in the attached drawings:

[0052] 10 Non-metallic housing, 101 Threaded hole, 102 Electromagnet mounting hole, 103 Cylindrical cavity, 104 Square cavity

[0053] 20 Inverted pendulum, 201 Dome, 202 Force sensing area, 203 Support tip

[0054] 30 Induction electrode

[0055] 40 Electromagnet

[0056] 50 Permanent magnet ring

[0057] 60 Metal base, 601 Boss, 602 Pit

[0058] 70 Wall surface Detailed implementation manners

[0059] The present invention will be described in detail below with reference to the accompanying drawings.

[0060] As Figures 1 - 3 shown, the invented inverted pendulum type two-component airflow wall friction resistance sensor is composed of a non-metallic housing 10, an inverted pendulum 20, an induction electrode 30, an electromagnet 40, a permanent magnet ring 50 and a metal base 60.

[0061] The non-metallic housing 10 can be made of insulating materials such as nylon, bakelite or ceramic, and is in an overall stepped cylindrical shape, thicker at the bottom and thinner at the top, being a two-stage cylindrical structure. The total height of the non-metallic housing 10 is about 30 - 40 mm, the diameter range of the upper half cylinder is 4 - 8 mm (preferably 6 mm), and the diameter range of the lower half cylinder is 10 - 20 mm (preferably: 16 mm). In an embodiment of the present invention, the heights of the upper and lower parts of the cylinder are approximately equal, both being half of the total height of the non-metallic housing 10, that is, 15 - 20 mm (preferably 20 mm). The non-metallic housing 10 is a hollow structure, and internally includes two interconnected cavities: the slender cylindrical cavity 103 located in the upper part and the square cavity 104 located in the lower part, whose heights and positions respectively correspond to the upper half cylinder and the lower half cylinder; the axes of the slender cylindrical cavity 103 and the square cavity 104 located in the lower part coincide with the axis of the non-metallic housing 10. The square cavity 104 is a cuboid structure, and its main function is to provide an installation space for the permanent magnet ring 50. The bottom surface of the square cavity 104 can be square or rectangular, preferably square. In an embodiment of the present invention, the side length of the square is 10 - 12 mm (preferably 10 mm), and the height is determined by the height of the lower half cylinder of the non-metallic housing 10.

[0062] From the perspective of structural strength, the thickness of the side wall of the cylindrical cavity 103 is not less than 1 mm, and the thickness of the wall at the opening position of the square cavity 104 is not less than 3 mm. Four threaded holes 101 and four electromagnet mounting holes 102 are provided on the outer wall of the lower half of the cylinder, which are used for the assembly of the induction electrode 30 and the electromagnet 40 respectively. The four threaded holes 101 are evenly distributed circumferentially on the outer wall of the lower half of the cylinder, with a hole diameter of 1.5 - 3 mm (preferably 2 mm), and the included angle between adjacent hole positions is 90 degrees. The four electromagnet mounting holes 102 are also evenly distributed circumferentially on the outer wall of the lower half of the cylinder, with a hole diameter of 2 - 4 mm (preferably the lower limit of 2 mm for the consideration of overall size compactness), and the included angle is 90 degrees. Each threaded hole 101 is directly above the corresponding electromagnet mounting hole 102. Therefore, the four threaded holes 101 and the four electromagnet mounting holes 102 can be divided into four groups, and the depth of the holes penetrates vertically from the outer wall of the lower half of the cylinder to the inner wall of the square cavity 104.

[0063] The inverted pendulum 20 is overall in the shape of a long nail and is made of a conductive metal material (such as steel, iron, copper, aluminum), preferably an aluminum alloy material with low density and high structural strength. The inverted pendulum 20 includes a dome 201, a support cylinder 204, a force sensing area 202, and a support tip 203. The dome 201 is an inverted frustum at the top of the inverted pendulum 20, and the diameter of the upper surface circle is larger than that of the lower surface circle. At the same time, to ensure that the inverted pendulum 20 does not contact the outer shell 10 in the upright state, the diameter of the upper surface of the dome 201 should be slightly smaller than the diameter of the cylindrical cavity 103, ensuring that there is a gap between the two after nesting but controlling it below 0.1 mm. An excessive gap will cause gas to flow into the sensor interior from the gap during the measurement process, resulting in uneven pressure on both sides of the inverted pendulum 20 and increasing the measurement error. The support cylinder 204 is fixedly connected to the dome 201 and is located below it. The force sensing area 202 is in a cuboid structure, fixedly connected to the support cylinder 204 and located below it. The support tip 203 is in a quadrangular pyramid structure, fixedly connected to the force sensing area 202 and located below it. The axes of the dome 201, the support cylinder 204, the force sensing area 202, and the support tip 203 in the vertical direction coincide with the axis of the non-metallic outer shell 10. The cross-sectional area of the frustum of the dome 201 shrinks rapidly from top to bottom, and the contraction angle is greater than 120 degrees. This conical design can, on the one hand, increase the pressure loss of the airflow passing through the gap at the top of the inverted pendulum 20; on the other hand, the diameter of the support cylinder 204 below the frustum is smaller than the diameter of the lower surface circle of the frustum, making the upper half of the inverted pendulum 20 lighter in weight and the overall center of mass move downward, improving the stability of the inverted process. The four side surfaces of the upper half of the force sensing area 202 are opposite to the positions of the four induction electrodes 30, and the attitude angle of the inverted pendulum 20 is reflected by the change in the capacitance of the induction electrodes. A square permanent magnet ring 50 is nested in the lower half of the force sensing area 202, that is, the lower half of the force sensing area 202 is tightly nested into the permanent magnet ring 50.

[0064] The permanent magnet ring 50 has a square enclosure structure, and its horizontal cross-section is a square ring. The size of the inner ring of the square ring is adapted to the outer size of the lower half of the force sensing area 202 (that is, the side length of the inner ring of the permanent magnet ring 50 is equal to the side length of the side surface of the force sensing area 202). When the lower half of the force sensing area 202 is inserted into the permanent magnet ring 50, the two form a tight fit structure. The height range of the permanent magnet ring 50 is 2-4 mm (preferably 3 mm), and the thickness range is 1-2 mm (preferably 1 mm). The vertical installation position of the permanent magnet ring 50 should be at the same height as the electromagnet installation hole 103, so that the permanent magnet ring 50 and the four electromagnets 40 are in corresponding positions, ensuring that the force between the electromagnet 40 and the permanent magnet ring 50 is always along the horizontal direction. The mutual repulsive magnetic force between the permanent magnet ring 50 and the four electromagnets 40 is the restoring force after the inverted pendulum 20 becomes unstable. The present invention does not limit the magnetic material and polarity configuration used for the permanent magnet ring 50, as long as it can generate a mutual repulsive force when facing the electromagnet.

[0065] The support tip cone 203 is in the shape of an inverted pyramid, that is, an inverted quadrangular pyramid. The side length of the bottom surface of the quadrangular pyramid (that is, the upper surface of the support tip cone 203) is the same as the side length of the bottom surface of the force sensing area 202. The height range of the support tip cone 203 is 5-10 mm, and the full cone angle at the top of the tower ranges from 30 to 60 degrees.

[0066] The induction electrode 30 is in a stepped cylindrical shape, including two parts: a slender cylinder and a short and thick cylinder. Their axes coincide, and the whole is processed from a metal material, and the material is preferably copper with good electrical conductivity. Threads are engraved on the surface of the slender cylinder of the induction electrode 30, and the induction electrode 30 is assembled with the housing 10 through the threaded hole 101. The screwing distance of the induction electrode 30 in the threaded hole 101 should be no less than 5 mm to ensure that the distance between the four side surfaces of the force sensing area 202 and the end surface of the induction electrode 30 is less than 1 mm after the sensor is assembled.

[0067] The electromagnet 40 is in a cylindrical shape and consists of an iron core and a spiral conductive coil. Its specific structure and usage method are well-known to those skilled in the art and will not be elaborated here. During the assembly process, the electromagnet 40 is fixed inside the cylindrical electromagnet installation hole 102 through hot melt adhesive or other types of glue, and the inner end surface of its inserted part is about 0.5-2 mm away from the permanent magnet ring 50 (preferably 1 mm for the sake of structural compactness).

[0068] The metal base 60 is made of a metal material with good electrical conductivity (preferably copper), and is in the shape of a thin circular sheet as a whole, with a thickness ranging from 2 to 4 mm (preferably 2 mm). The metal base 60 includes a boss 601 and a cylindrical base 603 located below it, and the two are integrally formed. The diameter of the cylindrical base 603 is the same as the diameter of the lower semi-cylinder of the housing 10. The side length of the boss 601 is the same as the side length of the square cavity 104. By embedding the boss 601 into the square cavity 104 from bottom to top, the metal base 60 is tightly fitted with the housing 10, playing a role in sealing the square cavity 104. In addition, at the center of the boss 601, a conical pit 602 is provided. The depth of the pit 602 ranges from 1 to 2 mm (preferably 1 mm), and the angle of the bottom cone needs to be greater than the cone angle of the support tip 203, and the typical range is 90 - 120 degrees (preferably 90 degrees). After the sensor is assembled, the vertex of the pit 602 contacts the vertex of the tip 203, forming the movement fulcrum of the inverted pendulum 20.

[0069] The working principle and measurement method of the inverted pendulum type two-component airflow wall friction resistance sensor provided by the present invention will be introduced from the aspects of the force balance analysis of the inverted pendulum, the electrical connection method of the electromagnet, and the typical system control frame Figure 3 as follows. Taking the two-dimensional flat plate boundary layer flow as an example for illustration.

[0070] Figure 4 It is a schematic diagram of the force balance analysis of the sensor. The top of the sensor is flush-mounted with the wall surface 70. In the ideal two-dimensional case, the inverted pendulum 20 is not subjected to force in the direction perpendicular to the plane. Therefore, only two electromagnets can be used to maintain the balance of the inverted pendulum (assuming the left one is L1 and the right one is L2). When the boundary layer on the flat plate surface flows from left to right over the dome of the inverted pendulum 20, due to the viscous friction effect, a rightward frictional resistance F will be generated on the dome of the inverted pendulum 20 f . Under the action of this frictional resistance, the inverted pendulum 20 deviates from the central position and begins to tilt to the right. In order to restore the inverted pendulum to the neutral state, it is necessary to increase the repulsive force F2 of the right electromagnet L2 and decrease the repulsive force F1 of the left electromagnet L1 through the automatic control system. When the force balance is reached again, the moment of all forces about the movement fulcrum of the inverted pendulum 20 should be equal to 0, which is expressed by the formula as follows:

[0071] F f ·(s1 + s2)+(F1 - F2)·s2 = 0 (1)

[0072] Where s1 and s2 respectively represent the vertical distances between the centers of the electromagnets and the wall surface and the fulcrum. Further simplification can obtain the expression of the frictional resistance as follows:

[0073]

[0074] Because the force exerted by the electromagnet 40 on the permanent magnet ring 50 is proportional to the DC voltage applied across the electromagnet, the magnetic forces F1 and F2 can be written as linear expressions of the voltages across L1 and L2, that is

[0075]

[0076] where k is the proportionality coefficient, which is related to the number of turns, resistance, magnetic materials, etc. of the electromagnet coil winding and can be obtained through experimental calibration; U de and U fg represent the voltages across the electromagnets L1 and L2 respectively. Substituting Equation (3) into Equation (2), the relationship between the frictional resistance and the supply voltage of the electromagnet in the equilibrium state can be obtained:

[0077]

[0078] Based on Equation (4), the present invention provides an inverted pendulum balance control method based on differential voltage signals. Figure 5 and Figure 6 are the electrical connection diagram of the sensor and the control system block diagram. The positive pole of the DC power supply is connected to the terminal a led out from the left induction electrode, and the negative pole is connected to the terminal c led out from the right induction electrode. The terminal b is led out from the metal base, and this terminal is connected to the inverted pendulum 20 through the electrical contact of the fulcrum. Since both the induction electrode 30 and the inverted pendulum 20 are made of metal materials and there is a certain air gap between them, therefore, the terminals a and b can be equivalent to a capacitor, and the capacitance value C ab is inversely proportional to the gas gap. Similarly, the terminals c and b can also be equivalent to a capacitor, and the capacitance value C bc is inversely proportional to the gas gap between the inverted pendulum 20 and the right induction electrode. Two resistors R1 and R2 are also connected in series between the positive and negative poles of the DC power supply. The resistances of these two resistors are equal, and the purpose is to provide a reference voltage of 1 / 2 times the DC power supply at the midpoint h. As Figure 5 shown, when the inverted pendulum 20 is strictly in the neutral position, the distances between the force sensing area 202 and the left and right induction electrodes are equal. Therefore, the capacitance C ab = C bc , and the voltage difference U bh between the terminal b and h is 0.

[0079] When the inverted pendulum is subjected to Figure 4 the frictional resistance in and deviates to the right from the equilibrium position, the right gas gap becomes smaller and the capacitance value increases; the left gas gap becomes larger and the capacitance value decreases, that is, C ab < C bc . According to the principle of capacitance voltage division, at this time, the voltage of the terminal b is higher than half of the DC power supply voltage, U bh> 0. This voltage difference serves as the input signal for the automatic control system and enters the PID controller. After operations such as proportional, derivative, and integral calculations, a control voltage U is output. o , which is well-known to those skilled in the art and will not be elaborated here. Based on this voltage U o and the voltage output U1 across the electromagnets in the reference frictionless state, the real-time supply voltages of the electromagnets L1 and L2 can be obtained as follows:

[0080]

[0081] where U de , U fg are respectively Figure 5 the supply voltages of the two electromagnets on the left and right sides in

[0082] , that is, the input voltage between terminals d and e and the input voltage between terminals f and g. bh is 0. The output voltage U o of the control system is proportional to the frictional resistance received by the dome of the inverted pendulum 20. Substituting formula (5) into formula (3) gives:

[0083]

[0084] From the above working principle and structural description, it is not difficult to obtain the main advantages and effects of the present invention as follows:

[0085] 1. Wide application range. According to formula (6), the measurement range of the invented inverted pendulum type wall friction resistance sensor is determined by k, s1, and s2. For low-speed flow, the measurement range can be reduced and the force measurement resolution of the sensor can be improved by increasing s1, reducing the number of turns of the electromagnet, and increasing the internal resistance of the electromagnet winding. In high-speed flow, the measurement range can be increased by the opposite method. The applicable speed range of the sensor is not limited.

[0086] 2. Two-component measurement. In the Figure 4 implementation case, the sensor only needs to use two electromagnets and two induction electrodes to achieve the measurement of two-dimensional airflow wall friction resistance. For three-dimensional flow, the wall friction resistance can be decomposed into two components. The measurement process and principle of each component are exactly the same as those in the above implementation case. This is also the reason why the induction electrodes 40 and the electromagnets 30 in the present invention are arranged in an axially uniform distribution with an included angle of 90 degrees.

[0087] 3. High frequency response. Through the parameter optimization of the PID controller, the inverted pendulum can be restored to the neutral position within milliseconds after deviating from the equilibrium position. That is, the frequency response of this sensor can easily reach above 100 Hz, enabling real-time measurement of dynamic frictional resistance under complex airflow conditions.

[0088] 4. Low cost. The main components of this sensor, such as the electromagnet 40 and the PID controller, are all mature products, and the processing costs of other components such as the inverted pendulum 20 and the metal base 60 are low. Therefore, the overall usage cost is much lower than that of the MEMS microelectromechanical frictional resistance sensor.

[0089] 5. Simple measurement process. This sensor has good linearity. After simple calibration, the frictional resistance can be directly obtained through voltage acquisition conversion. The frictional resistance measurement device based on the velocity profile measurement and optical interference principle requires complex optical equipment and calibration processes.

Claims

1. An inverted pendulum type two-component air flow wall friction resistance sensor, characterized in that, It includes a non-metallic outer shell (10), an inverted pendulum (20), induction electrodes (30), an electromagnet (40), a permanent magnet ring (50), and a metal base (60); among them The non-metallic outer shell (10) is in the shape of a stepped cylinder, thicker at the bottom and thinner at the top, with a two-stage cylindrical structure; the non-metallic outer shell (10) is a hollow structure, and its interior includes two interconnected cavities: an elongated cylindrical cavity (103) located in the upper part and a square cavity (104) located in the lower part, whose heights and positions correspond to the upper half cylinder and the lower half cylinder respectively; the axes of the elongated cylindrical cavity (103) and the square cavity (104) located in the lower part coincide with the axis of the non-metallic outer shell (10); the square cavity (104) is in a cuboid structure, and its height is determined by the height of the lower half cylinder of the non-metallic outer shell (10); four threaded holes (101) and four electromagnet mounting holes (102) are provided on the outer wall surface of the lower half cylinder, which are used for the assembly of the induction electrodes (30) and the electromagnet (40) respectively; the threaded holes (101) are evenly distributed circumferentially on the outer wall surface of the lower half cylinder, and the included angle between adjacent hole positions is 90 degrees; the electromagnet mounting holes (102) are also evenly distributed circumferentially on the outer wall surface of the lower half cylinder, with an included angle of 90 degrees; each threaded hole (101) is directly above the corresponding electromagnet mounting hole (102), so the four threaded holes (101) and the four electromagnet mounting holes (102) can be divided into four groups, and the depth of the holes penetrates vertically from the outer wall surface of the lower half cylinder to the inner wall surface of the square cavity (104); The inverted pendulum (20) is in the shape of a long nail, made of a conductive material, and includes a dome (201), a support cylinder (204), a force sensing area (202), and a support tip cone (203); the dome (201) is an inverted frustum at the top of the inverted pendulum (20), and the diameter of the upper surface circle is larger than that of the lower surface circle; the diameter of the upper surface of the dome (201) is slightly smaller than the diameter of the cylindrical cavity (103) to ensure the existence of a gap between the two after nesting; an excessive gap will cause gas to flow into the interior of the sensor during the measurement process, resulting in uneven left and right pressures on the inverted pendulum (20) and increasing the measurement error; the support cylinder (204) is fixedly connected to the dome (201) and is located below it; the force sensing area (202) is in a cuboid structure, fixedly connected to the support cylinder (204) and located below it; the support tip cone (203) is in a quadrangular pyramid structure, fixedly connected to the force sensing area (202) and located below it; the axes of the dome (201), the support cylinder (204), the force sensing area (202), and the support tip cone (203) in the vertical direction coincide with the axis of the non-metallic outer shell (10); The frustum cross-sectional area of the dome (201) shrinks sharply from top to bottom; the upper half of the four sides of the force-sensing area (202) are opposite to the positions of the four induction electrodes (30); the lower half of the force-sensing area (202) is nested with a square permanent magnet ring (50), that is, the lower half of the force-sensing area (202) is tightly nested into the permanent magnet ring (50); the permanent magnet ring (50) is of a square enclosure structure, its horizontal cross-section is a square ring, the inner side length of the permanent magnet ring (50) is equal to the side length of the side surface of the force-sensing area (202), and when the lower half of the force-sensing area (202) is inserted into the permanent magnet ring (50), the two are in a tight-fitting structure; the vertical installation position of the permanent magnet ring (50) should be at the same height as the electromagnet mounting hole (103), so that the permanent magnet ring (50) and the four electromagnets (40) are in corresponding positions; The support sharp cone (203) is in the shape of an inverted pyramid, that is, an inverted quadrangular pyramid, and the bottom side length of the quadrangular pyramid is the same as the bottom side length of the force-sensing area (202); The induction electrode (30) is in the shape of a stepped cylinder, including two parts: a slender cylinder and a short and thick cylinder, and their axes coincide; threads are engraved on the surface of the slender cylinder of the induction electrode (30), and the induction electrode (30) is assembled with the outer shell 10 through a threaded hole (101); The electromagnet (40) is in the shape of a cylinder and is composed of an iron core and a spiral conductive coil; the electromagnet (40) is fixed inside the cylindrical electromagnet mounting hole (102), and the inner end face of its inserted part is kept at a certain distance from the permanent magnet ring (50); The metal base (60) is in the shape of a thin circular sheet as a whole, including a boss (601) and a cylindrical base (603) located below it; the diameter of the cylindrical base (603) is the same as the diameter of the lower half cylinder of the non-metallic outer shell (10); the side length of the boss (601) is the same as the side length of the square cavity (104), and by embedding the boss (601) into the square cavity (104) from bottom to top, the metal base (60) is tightly fitted with the non-metallic outer shell (10); in addition, at the center of the boss (601), a conical pit (602) is provided; the angle of the bottom cone angle of the pit (602) is greater than the cone angle of the support sharp cone (203); after the sensor is assembled, the vertex of the pit (602) is in contact with the vertex of the support sharp cone (203) to form the movement fulcrum of the inverted pendulum (20).

2. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The total height of the non-metallic outer shell (10) is 30 - 40 mm, the diameter range of the upper half cylinder is 4 - 8 mm, and the diameter range of the lower half cylinder is 10 - 20 mm; The height of the upper and lower parts of the cylinder is 15 - 20 mm; The bottom surface of the square cavity (104) is square or rectangular, and the maximum side length is 10 - 12 mm; The aperture of the threaded hole (101) is 1.5 - 3 mm; The aperture of the electromagnet mounting hole (102) is 2 - 4 mm.

3. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The gap between the dome (201) and the cylindrical cavity (103) is less than 0.1 mm, and the contraction angle of the dome (201) from top to bottom is greater than 120 degrees.

4. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The height range of the support sharp cone (203) is 5 - 10 mm, and the full cone angle range of the top of the tower is 30 - 60 degrees.

5. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The screwing-in distance of the induction electrode (30) in the threaded hole (101) is not less than 5 mm, and the distance between the four side faces of the force induction area (202) and the end face of the induction electrode (30) is less than 1 mm after the sensor is assembled.

6. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The thickness of the metal base (60) ranges from 2 to 4 mm; the depth of the pit (602) ranges from 1 to 2 mm, and the angle of the bottom cone ranges from 90 to 120 degrees.

7. The inverted pendulum type two-component air flow wall friction resistance sensor according to claim 1, characterized in that, The diameter of the upper half cylinder of the non-metallic housing (10) ranges from 6 mm, and the diameter of the lower half cylinder ranges from 16 mm; The heights of the upper and lower cylinders are equal; The bottom surface of the square cavity (104) is square, and the side length of the square is 10 mm; The wall thickness of the side wall of the cylindrical cavity (103) is not less than 1 mm, and the wall thickness at the opening position of the square cavity (104) is not less than 3 mm; The aperture of the threaded hole (101) is 2 mm, the number is 4, and the included angle between adjacent hole positions is 90 degrees; The lower limit of the aperture of the electromagnet mounting hole (102) is 2 mm, the number is 4, and the included angle is 90 degrees; The height of the permanent magnet ring (50) is 3 mm and the thickness is 1 mm; The distance between the inner end face of the part of the electromagnet (40) inserted into the electromagnet mounting hole (102) and the permanent magnet ring (50) is 1 mm; The thickness of the metal base (60) is 2 mm; the boss (601) and the cylindrical base (603) are integrally formed; the depth of the pit (602) is 1 mm, and the angle of the bottom cone is 90 degrees.

8. The measuring method of the inverted pendulum type two-component air flow wall friction resistance sensor, which is based on the inverted pendulum type two-component air flow wall friction resistance sensor according to any one of claims 1 to 7, characterized in that, For two-dimensional flat plate boundary layer flow: The top of the sensor is flush-mounted with the wall surface (70). In an ideal two-dimensional situation, the inverted pendulum (20) is not subjected to forces in the direction perpendicular to the plane. Therefore, only two electromagnets can be used to maintain the balance of the inverted pendulum. Assume that the left side is L1 and the right side is L2. When the boundary layer on the flat plate surface flows past the dome of the inverted pendulum (20) from left to right, due to viscous friction, a frictional resistance force F to the right will be generated on the dome of the inverted pendulum (20). f Under the action of this frictional resistance force, the inverted pendulum (20) deviates from the central position and begins to tilt to the right. To restore the inverted pendulum to the neutral state, it is necessary to increase the repulsive force F2 of the right electromagnet L2 and decrease the repulsive force F1 of the left electromagnet L1 through the automatic control system. When the force balance is reached again, the moment of all forces about the movement fulcrum of the inverted pendulum (20) should be equal to 0, which is expressed by the formula as follows: F f ·(s1 + s2)+(F1 - F2)·s2 = 0 (1) Wherein, s1 and s2 respectively represent the vertical distances between the electromagnet center and the wall surface and the fulcrum; further simplified, the expression of the frictional resistance is as follows: Since the force of the electromagnet (40) on the permanent magnet ring (50) is proportional to the DC voltage applied across the two ends of the electromagnet, the magnetic forces F1 and F2 can be written as linear expressions of the voltages at both ends of L1 and L2, that is where k is the proportionality coefficient; U de and U fg respectively represent the voltages across the electromagnets L1 and L2; Substituting Equation (3) into Equation (2), the relationship between the frictional resistance and the supply voltage of the electromagnet in the equilibrium state is obtained:

9. An inverted pendulum type two-component air flow wall friction resistance sensor control system, which is based on the inverted pendulum type two-component air flow wall friction resistance sensor according to any one of claims 1 to 7, and is characterized in that, The electrical connection of the system is as follows: The positive pole of the DC power supply is connected to terminal a led out from the left induction electrode, and the negative pole is connected to terminal c led out from the right induction electrode; Terminal b is led out from the metal base, and this terminal is connected to the inverted pendulum (20) through the electrical contact of the finger fulcrum; Since both the induction electrode (30) and the inverted pendulum (20) are made of metal materials and there is a certain air gap between them, therefore, an equivalent capacitance can be formed between terminals a and b, and the capacitance value C ab is inversely proportional to the gas gap; Similarly, an equivalent capacitance can also be formed between terminals c and b, and the capacitance value C bc is inversely proportional to the gas gap between the inverted pendulum (20) and the right induction electrode; Two resistors R1 and R2 are also connected in series between the positive and negative poles of the DC power supply; The resistance values of these two resistors are equal, and the purpose is to provide a reference voltage of 1 / 2 times the DC power supply at the midpoint h.

10. An inverted pendulum balance control method based on differential voltage signals, which is based on the inverted pendulum type two-component air flow wall friction resistance sensor control system according to claim 9, and is characterized in that, Based on formula (4) Among them, F f is the frictional resistance; k is the proportionality coefficient; s1 and s2 respectively represent the vertical distances between the center of the electromagnet and the wall surface and the fulcrum; U de and U fg respectively represent the voltages at both ends of the electromagnets L1 and L2; When the inverted pendulum (20) is strictly in the neutral position, the distance between the force sensing area (202) and the left and right induction electrodes is equal. Therefore, the capacitance C ab = C bc , and the voltage difference U bh between terminals b and h is 0; when the inverted pendulum is subjected to frictional resistance and deviates to the right from the equilibrium position, the right gas gap becomes smaller and the capacitance value increases; the left gas gap becomes larger and the capacitance value decreases, that is, C ab < C bc ; according to the principle of capacitive voltage division, at this time, the voltage of terminal b is higher than half of the high-voltage DC power supply voltage, U bh > 0; this voltage difference is used as the input signal of the automatic control system and enters the PID controller. The PID controller outputs a control voltage U o ; based on this voltage U o and the voltage output U1 across the electromagnet in the reference frictionless resistance state, the real-time supply voltages of the electromagnets L1 and L2 can be obtained as follows: where U de and U fg are the supply voltages of the two electromagnets on the left and right respectively, that is, the input voltage between terminals d and e and the input voltage between terminals f and g; Since the automatic control system increases the supply voltage of the right electromagnet L2 and decreases the supply voltage of the left electromagnet L1, the repulsive force F2 on the right increases and the repulsive force F1 on the left decreases, causing the inverted pendulum (20) to swing to the left and gradually return to the neutral position. After reaching the neutral position, the input voltage U of the automatic control system bh is 0, and the output voltage U of the control system o is proportional to the frictional resistance received by the dome of the inverted pendulum (20). Substituting Equation (5) into Equation (3) gives:

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