Non-contact material electrical property measurement method and system based on STM32

Abstract

The present invention provides a contactless material electrical property measurement method and system based on STM32, the method including: using an eddy current sensor to approach different measured materials, measuring circuit response changes, to obtain inductance change data of the eddy current sensor; according to the inductance change data, performing electromagnetic field theoretical analysis, eddy current sensor theoretical analysis, and capacitance three-point oscillation circuit theoretical analysis, obtaining material magnetic field intensity distribution, to infer the electrical properties of the measured material; performing experiments and simulations on the measured material, obtaining and based on the experimental data, establishing and storing a circuit response database at different temperatures; based on the STM32 control chip, designing a capacitance three-point oscillation circuit of an eddy current sensor, collecting circuit response data in real time, and comparing with the circuit response database to obtain and display electrical property detection results. The present invention solves the technical problems of environmental interference, material complexity, and complex measurement models.

Description

Technical Field

[0001] The present invention relates to the field of non-contact material testing, and in particular to a non-contact material electrical property measurement method based on STM32. Background Art

[0002] Non-contact testing of material electrical properties is a testing technique based on eddy currents. Specifically, it measures electrical properties by placing an inductive coil close to a metal or conductive material. This technique avoids impacting or damaging the object being measured and is primarily used to measure parameters such as surface resistance, conductivity, and thickness of sensitive or flexible conductive surfaces. Non-contact testing of material electrical properties can also be used in a wide range of materials, including semiconductors, compound semiconductors, liquid crystals, and novel carbon-based materials. Its diverse applications and broad scope, along with its promising prospects, offer non-destructive, rapid, flexible, and multifunctional measurement capabilities, making it a highly valuable research topic. Since the test instrument does not require contact with the object being measured, it does not impact or damage it, preserving its pristine condition and performance. Due to the rapid response of eddy currents, measurements can be completed in a short time, improving efficiency and accuracy. Since the test range, accuracy, and sensitivity can be adjusted by varying parameters such as the coil's shape, size, position, angle, and the frequency of the applied alternating current, it can be adapted to varying measurement needs and conditions. Because the impedance changes of the coil can be analyzed to obtain a variety of material physical parameters, such as resistivity, magnetic permeability, hysteresis loops, film thickness, surface roughness, cracks, and defects, multifunctional measurement and detection can be achieved. This nondestructive testing technology has developed rapidly in recent years and is one of the five major nondestructive testing technologies, along with magnetic particle testing, penetrant testing, ultrasonic testing, and radiographic testing. For example, in the prior invention patent application document "Non-contact Eddy Current Sensor and Test Object Testing Method," published with publication number CN101052860A, an alternating current is passed through the sensor coil to generate an electromagnetic field. As a result, the sensor coil has a characteristic impedance, the impedance of which increases depending on the conductive material within the coil's magnetic field. If a conductive object approaches the sensor coil, the electromagnetic field generates eddy currents in the surface area of ​​the conductive object. These eddy currents, in turn, act on the sensor coil by increasing its impedance. The greater the impedance increment, the better the eddy currents can propagate within the conductive object. Large surfaces facing the sensor coil thus produce a particularly large impedance increment.

[0003] And the existing public document "Non-contact current detection and control system designed based on STM32 microcontroller" shows that the existing system is a non-contact current detection and control system designed based on STM32 microcontroller. The given arbitrary signal is amplified by the OPA548 chip, and the current-voltage conversion and amplification are performed by a 100Ω resistor and an INA128 chip. The obtained voltage signal is sampled at a frequency of 0.488μs using the STM32 microcontroller, and the harmonic information of the signal is obtained using the FFT library of the STM32 microcontroller.

[0004] Although the current non-contact testing technology for the electrical properties of materials has made great progress, there are still many problems, such as environmental interference, material complexity, and complex measurement models. Since the generation and detection of eddy currents rely on an external alternating magnetic field, if there are other magnetic or electric field interferences in the surrounding area, it will affect the accuracy and stability of the measurement. Since the distribution and intensity of eddy currents are related to factors such as the electrical conductivity, magnetic permeability, and geometric shape of the material, if the structure and properties of the material are relatively complex, such as multi-layer, multi-phase, non-uniform or nonlinear, the measurement difficulty and error will increase. Since the theoretical analysis and numerical calculation of eddy currents are relatively complex, it is necessary to establish a suitable measurement model to describe the electromagnetic coupling relationship between the coil and the material to be measured, as well as the relationship between the impedance change of the coil and the physical parameters of the material to be measured. This is one of the current research hotspots.

[0005] In summary, the existing technology has technical problems such as environmental interference, material complexity and complex measurement model. Summary of the Invention

[0006] The technical problem to be solved by the present invention is how to solve the technical problems of environmental interference, material complexity and measurement model complexity in the prior art.

[0007] The present invention adopts the following technical solutions to solve the above technical problems: a non-contact material electrical property measurement method based on STM32 includes:

[0008] S1. Use the eddy current sensor to approach different materials to measure the circuit response changes to obtain the inductance change data of the eddy current sensor;

[0009] S2. Based on the inductance change data, perform electromagnetic field theory analysis, eddy current sensor theory analysis, and capacitance three-point oscillation circuit theory analysis to obtain the material magnetic field intensity distribution to infer the electrical properties of the measured material;

[0010] S3. Conduct experiments and simulations on the tested materials, obtain and establish and store a database of circuit responses at different temperatures based on the experimental data;

[0011] S4. Based on the STM32 control chip, design a capacitive three-point oscillation circuit for the eddy current sensor, collect circuit response data in real time, and compare it with the circuit response database to obtain and display the electrical property detection results.

[0012] This invention, based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, combines theoretical analysis, simulation, and experimental research. It primarily considers parameters such as the rod's material, size, and shape, as well as its coupling with the coil and the entire circuit. The method explores a non-contact method for testing the electrical and magnetic properties of non-magnetic metal or ferromagnetic rods. By observing changes in the circuit response (resistance, inductance, and capacitance) when different rod types are inserted, and precisely measuring these changes, the magnetic and electrical properties of the inserted rod can be accurately inferred.

[0013] The present invention deduces the relationship between the inductance coil and the output voltage of the eddy current sensor, as well as the relationship between the parameters of the inductance coil during oscillation and the conductivity of the material, through theoretical analysis and simulation, and thus obtains the relationship between the material conductivity and the output voltage. The theory and simulation are verified experimentally, and the circuit responses of five different metal materials at five temperatures are collected to establish a comparative database. This provides data support for data collection and processing at the edge of the STM32 embedded device, quickly and accurately realizes non-contact and non-destructive measurement of the material's conductivity, and provides convenience for industrial production and scientific research measurements.

[0014] In a more specific technical solution, step S2 includes:

[0015] S21. Conduct electromagnetic field theoretical analysis, including deriving general formulas for eddy currents based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, including: expressions for alternating magnetic fields, equations satisfied by induced electromotive force, equations satisfied by induced current density, and the relationship between induced current density and induced electromotive force, from which the basic equations for eddy currents are derived;

[0016] S22. Solve the basic eddy current equation to obtain the eddy current distribution and eddy current intensity, and use them to obtain the material magnetic field intensity distribution;

[0017] S23. Calculate the reflection coefficient Γ and the transmission coefficient τ using the impedance matching principle.

[0018] S24. Perform theoretical analysis of the eddy current sensor, wherein, based on the equivalent circuit of the eddy current sensor, a set of equivalent circuit equations is listed, expressions for the complex impedance, equivalent inductance, and equivalent Q value of the coil of the eddy current sensor are obtained, and the nonlinear functional relationship is obtained by processing to measure the material under test;

[0019] S25. Conduct theoretical analysis of the capacitor three-point oscillation circuit.

[0020] In a more specific technical solution, S21 also includes:

[0021] S211. Place the material to be tested in an alternating magnetic field generated by a coil with a high-frequency current. The frequency of the high-frequency current is ω. Use the following logic to represent the alternating magnetic field:

[0022]

[0023] Where, is the complex amplitude of magnetic induction intensity, ω is the angular frequency, t is the time, j is the imaginary unit, σ represents the conductivity, and μ represents the magnetic permeability;

[0024] S212. According to Faraday's law of electromagnetic induction, the induced electromotive force in the material being tested Satisfies the following equation:

[0025]

[0026] Where, represents the curl operator;

[0027] S213. According to Ampere's circuit theorem, the induced current density in the material being tested Satisfies the following equation:

[0028]

[0029] Where, is the magnetic field strength, and the magnetic induction intensity satisfies

[0030] S214. Use the following logic to express the induced current density and induced electromotive force The relationship between:

[0031]

[0032] Combining equations (1), (2), and (3), we can obtain the basic equation of eddy current:

[0033]

[0034] Where, represents the Laplace operator. The above equation describes the propagation and attenuation of an alternating magnetic field in a conductive material and is also the fundamental equation for eddy currents. Depending on the boundary conditions and geometry, the eddy current distribution and intensity can be calculated for different situations.

[0035] In a more specific technical solution, step S22 further includes:

[0036] S221. Suppose a plane wave is incident on an infinitely large conductive plate with a thickness of d, a conductivity of σ, and a magnetic permeability of μ. The angular frequency of the incident wave is ω, and the angle of incidence is θ.

[0037] S222, suppose the magnetic field strength inside the conductive plate is Distribution along the x direction:

[0038]

[0039] Where H(x) is the complex amplitude, is a unit vector;

[0040] S223. According to the basic equation of eddy current, the ordinary differential equation is obtained:

[0041]

[0042] Where k 2 =jωμσ is the complex wave number;

[0043] S224. Find the general solution of ordinary differential equations:

[0044] H(x)=Ae -kx +Be kx (7)

[0045] Where A and B are arbitrary constants. The magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave are determined. Based on the magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave, the material magnetic field intensity distribution inside the conductive plate is obtained.

[0046] In a more specific technical solution, in S224, the boundary conditions and initial conditions of the basic eddy current equation are used to determine the arbitrary constants A and B:

[0047] H(0)=H i +H r (8)

[0048] H(d)=H t (9)

[0049] Where H i is the magnetic field intensity of the incident wave, H r is the magnetic field strength of the reflected wave, H t is the magnetic field intensity of the transmitted wave;

[0050] According to Snell's law, the following relationship can be obtained:

[0051] H i =H0cos θ (10)

[0052] H r =-H0cos θΓ (11)

[0053]

[0054] Where H0 is the maximum value of the magnetic field intensity of the incident wave, Γ is the reflection coefficient, τ is the transmission coefficient, is the wave number in vacuum, μ0 and ∈0 are the magnetic permeability and dielectric constant in vacuum respectively;

[0055] Substituting the magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave into the boundary conditions, we can obtain the magnetic field intensity equations:

[0056] A+B=H0cos θ(1+Γ) (13)

[0057]

[0058] Solving the magnetic field strength equations, we can obtain:

[0059]

[0060] Therefore, the magnetic field intensity distribution inside the plate can be obtained as follows:

[0061]

[0062] According to Ampere's circuit theorem, the current density distribution inside the plate can be obtained as follows:

[0063]

[0064] Substituting the magnetic field intensity distribution inside the plate, we can get the following formula:

[0065]

[0066] Where, As a unit vector, the eddy current distribution and eddy current intensity inside the conductive plate are obtained.

[0067] This invention directly derives a formula for calculating the distribution of eddy currents generated by a hollow excitation coil within a uniform flat conductor from the Biot-Savart law and Maxwell's equations. This calculation is faster and simpler than traditional methods. The invention uses MATLAB software to simulate and calculate the distribution of eddy currents, and the simulation takes only seconds to half a minute on a gaming laptop.

[0068] In a more specific technical solution, step S23 further includes:

[0069] S231. Define impedance using the following logic:

[0070]

[0071] Where E and H are the electric field intensity and magnetic field intensity respectively;

[0072] S232. For the case of a plane wave incident on an infinite plate, the following relationship can be obtained:

[0073]

[0074] Where Z1 is the impedance of the incident medium and Z2 is the impedance of the plate. For vacuum or air, is the free space impedance.

[0075] In a more specific technical solution, S24 includes:

[0076] S241. Use the following logic to list the equivalent circuit equations:

[0077]

[0078] Solve the equivalent circuit equations to obtain the preset frequency alternating current I1 and the self-closing current I2;

[0079] S242. Obtain expressions for the coil complex impedance, coil equivalent inductance, and coil equivalent Q value of the eddy current sensor;

[0080] S243. According to the expression and based on Maxwell's mutual inductance formula, a nonlinear functional relationship between the mutual inductance, the coil, and the distance x(H) between the metal conductors can be obtained;

[0081] S244. Determine the eddy current coil, metal eddy current sheet and excitation source, and maintain the ambient temperature. Through the preamplifier of the eddy current sensor, the changes in the complex impedance of the coil, the equivalent inductance of the coil and the equivalent Q value of the coil are converted into voltage and current change data and output to complete the measurement of the material under test; wherein, the preamplifier is a variable frequency amplitude modulation measurement circuit including: a capacitor three-point oscillator, a π-shaped filter detector and an emitter follower.

[0082] The present invention uses the PSPICE program to simulate a three-point capacitor oscillator circuit, allowing direct observation and adjustment of circuit parameters, allowing for rapid circuit design and optimization. The present invention utilizes Q2 to form an emitter follower. The emitter follower's function is to match input and output to achieve the largest possible undistorted output amplitude.

[0083] In a more specific technical solution, in S242:

[0084] The complex impedance of the coil is expressed using the following logic:

[0085]

[0086] Use the following logic to express the equivalent inductance of the coil:

[0087]

[0088] Use the following logic to express the equivalent Q value of the coil:

[0089]

[0090] Where Q0 represents the Q value of the coil without eddy current, satisfying: Q0 = ωL1 / R1; Z2 2 Represents the impedance of the part of the metal conductor that generates eddy current, satisfying:

[0091] In a more specific technical solution, step S25 includes:

[0092] S251. Setting the oscillation frequency of the capacitor three-point oscillator according to the oscillation starting conditions, wherein the eddy current sensor is connected to the oscillation circuit of the capacitor three-point oscillator, and the Q value change of the oscillation circuit caused by the displacement change is converted into high-frequency carrier signal amplitude change data;

[0093] When the capacitor three-point oscillator meets the start-up conditions, the oscillation frequency is expressed using the following logic:

[0094]

[0095] The oscillation frequency can be approximated using the following logic:

[0096]

[0097] S252, using a phase-sensitive detection circuit to perform signal extraction, signal frequency discrimination, and signal phase discrimination, wherein a π-shaped filter detector is used to extract a low-frequency signal from a high-frequency amplitude modulated signal;

[0098] S253. Use an emitter follower to match input and output to obtain an undistorted amplitude value.

[0099] The phase-sensitive detection circuit adopted in the present invention has frequency and phase detection functions for input signals, thereby improving the anti-interference ability of the circuit.

[0100] In a more specific technical solution, the STM32-based non-contact material electrical property measurement system includes:

[0101] An inductance change acquisition module is used to use the eddy current sensor to approach different materials to measure the circuit response change to obtain the inductance change data of the eddy current sensor;

[0102] The magnetic field distribution analysis module is used to perform electromagnetic field theory analysis, eddy current sensor theory analysis, and capacitance three-point oscillation circuit theory analysis based on the inductance change data to obtain the material's magnetic field intensity distribution to infer the electrical properties of the measured material. The magnetic field distribution analysis module is connected to the inductance change acquisition module;

[0103] A response database construction module is used to conduct experiments and simulations on the tested materials, obtain and establish and store a circuit response database at different temperatures based on the experimental data. The response database construction module is connected to the magnetic field distribution analysis module;

[0104] The material property detection module is used to design a capacitive three-point oscillation circuit for the eddy current sensor based on the STM32 control chip, collect circuit response data in real time, and compare it with the circuit response database to obtain and display the electrical property detection results. The material property detection module is connected to the response database construction module.

[0105] Compared with the prior art, the present invention has the following advantages:

[0106] This invention, based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, combines theoretical analysis, simulation, and experimental research. It primarily considers parameters such as the rod's material, size, and shape, as well as its coupling with the coil and the entire circuit. The method explores a non-contact method for testing the electrical and magnetic properties of non-magnetic metal or ferromagnetic rods. By observing changes in the circuit response (resistance, inductance, and capacitance) when different rod types are inserted, and precisely measuring these changes, the magnetic and electrical properties of the inserted rod can be accurately inferred.

[0107] The present invention deduces the relationship between the inductance coil and the output voltage of the eddy current sensor, as well as the relationship between the parameters of the inductance coil during oscillation and the conductivity of the material, through theoretical analysis and simulation, and thus obtains the relationship between the material conductivity and the output voltage. The theory and simulation are verified experimentally, and the circuit responses of five different metal materials at five temperatures are collected to establish a comparative database. This provides data support for data collection and processing at the edge of the STM32 embedded device, quickly and accurately realizes non-contact and non-destructive measurement of the material's conductivity, and provides convenience for industrial production and scientific research measurements.

[0108] This invention directly derives a formula for calculating the distribution of eddy currents generated by a hollow excitation coil within a uniform flat conductor from the Biot-Savart law and Maxwell's equations. This calculation is faster and simpler than traditional methods. The invention uses MATLAB software to simulate and calculate the distribution of eddy currents, and the simulation takes only seconds to half a minute on a gaming laptop.

[0109] The present invention uses the PSPICE program to simulate a three-point capacitor oscillator circuit, allowing direct observation and adjustment of circuit parameters, allowing for rapid circuit design and optimization. The present invention utilizes Q2 to form an emitter follower. The emitter follower's function is to match input and output to achieve the largest possible undistorted output amplitude.

[0110] The phase-sensitive detection circuit adopted in the present invention has frequency and phase detection functions for input signals, thereby improving the anti-interference ability of the circuit.

[0111] The present invention solves the technical problems of environmental interference, material complexity and measurement model complexity existing in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0112] Figure 1 This is a schematic diagram of the functional principle of the non-contact material electrical property measurement method based on STM32 according to Example 1 of the present invention;

[0113] Figure 2 Schematic diagram of the basic steps of the non-contact material electrical property measurement method based on STM32 according to Example 1 of the present invention;

[0114] Figure 3 Schematic diagram of specific steps of theoretical analysis of Example 1 of the present invention;

[0115] Figure 4 This is a schematic diagram of the eddy current sensor according to embodiment 1 of the present invention;

[0116] Figure 5 This is a block diagram of the eddy current sensor displacement measurement experiment of Example 1 of the present invention;

[0117] Figure 6 Schematic diagram of the reaction of eddy current in the measured object in Example 1 of the present invention;

[0118] Figure 7 This is a data flow processing diagram of the STM32-based non-contact material electrical property measurement method according to Example 1 of the present invention;

[0119] Figure 8 This is a block diagram of the principle of the eddy current non-contact resistance measurement characteristic experiment according to an embodiment of the present invention;

[0120] Figure 9 Schematic diagram of specific steps for theoretical analysis of a capacitor three-point oscillation circuit according to Example 1 of the present invention;

[0121] Figure 10 Schematic diagram of the capacitor three-point oscillator circuit structure according to Example 1 of the present invention;

[0122] Figure 11 Schematic diagram of the phase-sensitive detection circuit structure of Example 1 of the present invention;

[0123] Figure 12 Schematic diagram of specific steps of simulation of Example 1 of the present invention;

[0124] Figure 13 Schematic diagram of specific steps of electromagnetic simulation in Example 1 of the present invention;

[0125] Figure 14 This is a simulation effect diagram of Example 1 of the present invention;

[0126] Figure 15 This is a flow density component effect diagram of Example 1 of the present invention;

[0127] Figure 16 This is a trend chart of experimental data of Example 1 of the present invention;

[0128] Figure 17 This is a point-line graph of the experimental data of Example 1 of the present invention;

[0129] Figure 18 This is the experimental verification scene diagram of Example 1 of the present invention;

[0130] Figure 19 This is a diagram of the experimental instruments and materials of Example 1 of the present invention;

[0131] Figure 20 This is a linear relationship diagram between voltage and distance according to Example 1 of the present invention;

[0132] Figure 21 This is a connection diagram of the eddy current sensor module according to embodiment 2 of the present invention;

[0133] Figure 22 This is a schematic diagram of an eddy current sensor circuit according to embodiment 2 of the present invention;

[0134] Figure 23 This is a schematic diagram of the STM32F373 chip pins according to Example 2 of the present invention;

[0135] Figure 24 This is a pin diagram of an external high-precision analog-to-digital converter such as LTC2414 and a preamplifier LTC6078 according to embodiment 2 of the present invention;

[0136] Figure 25 The figure is a schematic diagram of data flow processing of an STM32 microcontroller according to embodiment 2 of the present invention. DETAILED DESCRIPTION

[0137] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0138] Example 1

[0139] like Figure 1 and Figure 2 As shown, the non-contact material electrical property measurement method based on STM32 provided by the present invention includes the following basic steps:

[0140] S1. Conduct theoretical analysis based on Faraday's law of electromagnetic induction and Ampere's circuit theorem;

[0141] like Figure 3 As shown, in this embodiment, the theoretical analysis step S1 further includes the following specific steps:

[0142] S11. Electromagnetic field theory analysis;

[0143] In this embodiment, the theoretical basis for eddy currents is Faraday's law of electromagnetic induction and Ampere's circuit theorem. Faraday's law of electromagnetic induction states that when the magnetic flux in a closed circuit changes, an induced electromotive force is generated in the circuit, the magnitude of which is equal to the negative value of the rate of change of the magnetic flux. Ampere's circuit theorem states that the line integral of the magnetic induction intensity B along any closed path is equal to the algebraic sum of the currents enclosed by the closed path multiplied by the magnetic permeability. Based on these two laws, the general formula for eddy currents can be derived as follows:

[0144] In this embodiment, a conductive material with electrical conductivity σ and magnetic permeability μ is placed in an alternating magnetic field generated by a coil passing a high-frequency current with a frequency of ω. The alternating magnetic field can be written as:

[0145]

[0146] in, is the complex amplitude of the magnetic induction intensity, ω is the angular frequency, t is the time, and j is the imaginary unit.

[0147] In this embodiment, according to Faraday's law of electromagnetic induction, the induced electromotive force in the conductive material Satisfies the following equation:

[0148]

[0149] in, represents the curl operator. According to Ampere's circuit theorem, the induced current density in the conductive material Satisfies the following equation:

[0150]

[0151] in, is the magnetic field strength, and the magnetic induction intensity satisfies

[0152] In this embodiment, since the material is conductive, the induced current density and induced electromotive force The following relationship exists:

[0153]

[0154] Combining the above three equations (1), (2), and (3), we can obtain the following equation:

[0155]

[0156] in, represents the Laplace operator. The above equation describes the propagation and attenuation of an alternating magnetic field in a conductive material and is also the fundamental equation for eddy currents. Depending on the boundary conditions and geometry, the eddy current distribution and intensity can be calculated for different situations.

[0157] In this embodiment, to simplify the problem, some special cases can be considered, including but not limited to: plane wave incidence, infinite plate, cylinder, and sphere. In these cases, some analytical solutions or approximate solutions can be obtained for analysis and calculation.

[0158] In this embodiment, a plane wave incident on an infinite plate is used as an example to illustrate:

[0159] In this embodiment, it is assumed that a plane wave is incident on an infinite conductive plate with a thickness of d, a conductivity of σ, a magnetic permeability of μ, an angular frequency of ω, and an incident angle of θ. Figure 1 .

[0160] It can be assumed that the magnetic field strength inside the plate is Distributed along the x direction, that is Where H(x) is the complex amplitude, is a unit vector. According to the basic equation of eddy current, the following ordinary differential equation can be obtained:

[0161]

[0162] Among them, k 2=jωμσ is the complex wave number. The general solution of this equation is:

[0163] H(x)=Ae -kx +Be kx (7)

[0164] Where A and B are arbitrary constants.

[0165] In this embodiment, to determine the aforementioned arbitrary constants A and B, the boundary conditions and initial conditions of the equation are used. At the two surfaces of the plate, i.e., at x = 0 and x = d, the tangential component of the magnetic field intensity should be continuous, as shown in the following equation:

[0166] H(0)=H i +H r (8)

[0167] H(d)=H t (9)

[0168] Among them, H i is the magnetic field intensity of the incident wave, H r is the magnetic field strength of the reflected wave, H t is the magnetic field strength of the transmitted wave.

[0169] In this embodiment, according to Snell's law, the following relationship can be obtained:

[0170] H i =H0cos θ (10)

[0171] H r =-H0cos θΓ (11)

[0172]

[0173] Where H0 is the maximum magnetic field intensity of the incident wave, Γ is the reflection coefficient, τ is the transmission coefficient, is the wave number in vacuum, μ0 and ∈0 are the magnetic permeability and dielectric constant in vacuum, respectively.

[0174] Substituting the above parameters into the boundary conditions, we can obtain the following set of equations:

[0175] A+B=H0cos θ(1+Γ) (13)

[0176]

[0177] Solving, we can get:

[0178]

[0179] Therefore, the magnetic field intensity distribution inside the plate can be obtained as follows:

[0180]

[0181] In this embodiment, according to Ampere's circuit theorem, the current density distribution inside the plate can be obtained as follows:

[0182]

[0183] Substituting the magnetic field intensity distribution inside the plate, we can get the following formula:

[0184]

[0185] in, is a unit vector. From this, the eddy current distribution and intensity inside the plate can be obtained.

[0186] In this embodiment, in order to calculate the reflection coefficient Γ and the transmission coefficient τ, it is necessary to use the principle of impedance matching. Impedance refers to the ratio of reflection to transmission when an electromagnetic wave encounters an interface in a propagation medium. In this embodiment, impedance is defined as:

[0187]

[0188] Where E and H are the electric field intensity and magnetic field intensity respectively. For the case of a plane wave incident on an infinite plate, the following relationship can be obtained:

[0189]

[0190] Where Z1 is the impedance of the incident medium and Z2 is the impedance of the plate. For vacuum or air, is the impedance of free space. For a conductive plate, Z2=Z s / cosθ, where Z s =1 / (jωσd) is the surface impedance of the plate.

[0191] S12, Theoretical analysis of eddy current sensor;

[0192] like Figure 4As shown, in this embodiment, the eddy current sensor is a sensor based on the principle of eddy current effect. The eddy current sensor includes: a sensor coil, and a measured object (conductor - metal eddy current sheet). According to the principle of electromagnetic induction, when the sensor coil, such as a flat coil, is passed through an alternating current and the frequency is set to, for example, I1 of 1MHz to 2MHz, an alternating magnetic field H1 will be generated in the space around the coil. When the plane of the coil is close to a certain conductor surface, the magnetic flux chain of the coil passes through the conductor, causing the surface layer of the conductor to induce a current I2 that is self-closed in a spiral shape, and the magnetic flux chain formed by I2 passes through the sensor coil. In this way, the coil and the eddy current coil form a mutual inductance with a certain coupling, and finally the original coil feeds back an equivalent inductance, which causes the impedance Z of the sensor coil to change. The eddy formed on the conductor to be measured can be equivalent to a short-circuit loop, so that the following can be obtained: Figure 5 The equivalent circuit of the sensor coil is shown in Figure 1. In the figure, R1 and L1 represent the resistance and inductance of the sensor coil. The short-circuit loop can be considered a single turn of a short-circuited coil, with resistance R2 and inductance L2. There is a mutual inductance M between the coil and the conductor, which increases as the distance between the coil and the conductor decreases.

[0193] In this embodiment, the short-circuit ring can be considered as a short-circuit coil with a resistance of R2 and an inductance of L2. In this embodiment, there is a mutual inductance M between the coil and the conductor, which increases as the distance between the coil and the conductor decreases.

[0194] In this embodiment, the circuit equations can be listed based on the equivalent circuit:

[0195]

[0196] By solving the equations, we can get I1 and I2. Therefore, the complex impedance of the sensor coil is as follows:

[0197]

[0198] In this embodiment, the equivalent inductance of the coil is:

[0199]

[0200] In this embodiment, the equivalent Q value of the coil is:

[0201]

[0202] Among them, Q0 represents the Q value of the coil without eddy current, satisfying: Q0=ωL1 / R1; Z2 2 Represents the impedance of the part of the metal conductor that generates eddy current, satisfying:

[0203] In this embodiment, the aforementioned expressions for the sensor coil's impedance Z, the coil's equivalent inductance L, and the coil's equivalent quality factor Q indicate that the impedance Z, equivalent inductance L, and quality factor Q of the coil-metal conductor system are all functions of the square of the system's mutual inductance. Based on Maxwell's basic formula for mutual inductance, the mutual inductance is a nonlinear function of the distance x(H) between the coil and the metal conductor. Therefore, Z, L, and Q are all nonlinear functions of x. Although the entire function is nonlinear, characterized by an S-shaped curve, a segment can be selected that approximates linearity. In reality, the variations in Z, L, and Q are related to the conductor's electrical conductivity, magnetic permeability, geometry, coil geometry, excitation current frequency, and the distance between the coil and the conductor being measured. If one of these parameters is controlled to change while the others remain unchanged, the impedance becomes a single-valued function of that variable parameter. Once the eddy current coil, metal eddy current sheet, and excitation source are determined and the ambient temperature remains constant, the impedance is solely dependent on the distance conductivity. In this embodiment, the sensor's conditioning circuit (preamplifier) ​​converts changes in the coil's impedances Z, L, and Q into voltage or current outputs. The magnitude of the output signal varies with the distance between the probe and the surface of the object being measured. Eddy current sensors use this principle to measure parameters such as displacement and vibration of metal objects.

[0204] like Figure 6 As shown in the figure, in this embodiment, a dedicated measurement circuit is required to achieve eddy current non-contact resistance measurement. This measurement circuit (called a preamplifier, also called an eddy current transformer) should include a stable oscillator with a certain frequency and a detection circuit.

[0205] According to the basic principle of eddy current sensor, the distance between the sensor and the measured object is converted into three parameters: Q value, equivalent impedance Z and equivalent inductance L of the sensor, and measured using the corresponding measurement circuit (preamplifier).

[0206] S13, Theoretical analysis of capacitor three-point oscillation circuit;

[0207] like Figure 7 As shown, in this embodiment, the eddy current converter of this paper is a variable frequency amplitude modulation measurement circuit. The circuit includes but is not limited to: a capacitor three-point oscillator, a π-shaped filter detector and an emitter follower.

[0208] like Figure 8 and Figure 9 As shown, in this embodiment, the step S13 of theoretical analysis of the capacitor three-point oscillation circuit further includes the following specific steps:

[0209] S131. Apply and analyze capacitor three-point oscillator;

[0210] In this embodiment, the capacitor three-point oscillator provides an excitation voltage for the inductive displacement sensor on the one hand, and provides a reference signal for the phase-sensitive detection circuit on the other hand. The excitation voltage signal is the key to the eddy current sensor measuring the conductivity of the material. Its frequency and amplitude require very high stability and are important factors in the performance of the sensor measurement system. In the design and manufacturing process of most actual circuits, since the single-chip microcomputer can only generate a simple square wave signal, it is necessary to obtain a triangular wave through integration and then use a low-pass filter circuit to remove the high-frequency components in the signal in order to obtain the sinusoidal signal required for the test. Compared with the capacitor three-point oscillator adopted in the present invention, the existing technology is suitable for filtering methods of any frequency. When the frequency is high, the waveform distortion is serious and a large error will be generated.

[0211] like Figure 10 As shown, in this embodiment, the capacitor three-point oscillator comprises, but is not limited to, Q2, C1, C2, and C8, generating a sinusoidal carrier signal with a frequency of approximately 1 MHz. The eddy current sensor is connected to the oscillation circuit, with the sensor coil serving as an inductive element in the oscillation circuit. The oscillator converts the Q-value changes of the oscillation circuit caused by displacement changes into amplitude changes of the high-frequency carrier signal.

[0212] In this embodiment, when the circuit satisfies both the amplitude balance condition and the phase balance condition, the circuit can start oscillating. When the circuit satisfies the oscillation conditions, the oscillation frequency is approximately equal to the resonant frequency, which can be expressed as follows:

[0213]

[0214] because:

[0215] Therefore, the oscillation frequency is approximately expressed as follows:

[0216]

[0217] From the above expression, it can be seen that the oscillation conditions of the circuit can be met by properly selecting C8, C2, R3 and the transistor amplification factor β value.

[0218] In this embodiment, when the inductor L and capacitors C8 and C2 are appropriately sized, the resonant frequency is independent of the transistor parameters. However, changing the C8 / C2 ratio will alter the feedback coefficient, affecting the oscillation amplitude and even causing the oscillator to stall. Therefore, when adjusting the oscillation frequency, the C8 / C2 ratio cannot be changed. When the oscillation frequency is very high, the capacitances of C8 and C2 are very small, and the transistor's junction capacitance is directly connected in parallel with C8 and C2, affecting the stability of the oscillation frequency. Using the PSPICE program to simulate a three-point capacitor oscillator circuit allows for direct observation and adjustment of circuit parameters, enabling rapid circuit design and optimization.

[0219] S132, using a phase-sensitive detection circuit to perform signal extraction, signal frequency detection, and signal phase detection;

[0220] like Figure 11 As shown, in this embodiment, the sensor output signal is mixed with other waveforms, affecting the accuracy and stability of the signal processing system. A detection circuit is required to extract the desired signal. Envelope detection is simple and easy to implement, but it cannot discriminate signals or rectify all signals. In contrast, a phase-sensitive detection circuit performs both frequency and phase discrimination on the input signal, improving the circuit's anti-interference capabilities. The phase-sensitive detection circuit structure is shown in the figure.

[0221] In this embodiment, D1, C5, L2, and C6 form a π-shaped filter detector formed by a diode and LC. The function of the detector is to extract the low-frequency signal detected by the sensor from the high-frequency amplitude modulated signal.

[0222] S133. Use emitter followers to perform input and output matching.

[0223] In this embodiment, Q2 forms an emitter follower, which matches input and output to obtain the largest possible undistorted output amplitude.

[0224] In this embodiment, the eddy current sensor changes the physical parameters of the inductor coil when it is close to the material, causing the oscillation frequency to change, resulting in different output voltages. There is no direct mechanical contact between the eddy current sensor and the object being measured, and it has a wide operating frequency range (from 0 to 10 Hz). When there is no conductor to be measured, the oscillator circuit resonates at f0, the sensor end coil Q0 is a constant and highest value, and the corresponding detection output voltage Vo is the largest. When the conductor to be measured approaches the sensor coil, the coil Q value changes, the oscillator's resonant frequency changes, the resonance curve becomes flat, and the detected amplitude Vo decreases. The higher the conductivity of the material, the greater the eddy current, and thus the greater the secondary magnetic field generated, the greater the impact on the eddy current sensor inductor coil, causing the output voltage value to decrease. Therefore, the change in Vo reflects the change in material conductivity. Eddy current sensors are used in non-contact resistance, vibration, speed, flaw detection, and thickness measurement.

[0225] S2, simulation;

[0226] like Figure 12 As shown, in this embodiment, the simulation step S2 further includes the following specific steps:

[0227] S21, electromagnetic simulation;

[0228] In this embodiment, the purpose of electromagnetic simulation is to use numerical calculation methods to solve the governing equations of the electromagnetic field, thereby obtaining the electromagnetic coupling effect between the coil and the sample, as well as the impedance change of the coil. Traditional methods first introduce the vector magnetic potential and then use the finite element method to calculate the distribution of eddy currents excited by various coils, which is computationally intensive. However, this paper directly derives the calculation formula for the distribution of eddy currents generated by a hollow excitation coil in a uniform flat conductor from the Biot-Savart law and Maxwell's equations, which is faster and simpler than traditional calculation methods. This paper uses MATLAB software to simulate and calculate the distribution of eddy currents, and the run time on a gaming laptop is only a few seconds to half a minute.

[0229] like Figure 13 As shown, in this embodiment, the electromagnetic simulation step S21 further includes the following specific steps:

[0230] S211, establish a model;

[0231] In this embodiment, appropriate simulation software is selected based on the actual measurement system, including but not limited to: COMSOL Multiphysics, Ansys Maxwell, CST Studio Suite, and Matlab. A three-dimensional model including a coil, a sample, and a medium such as air is established, and parameters such as the model's geometric dimensions, material properties, boundary conditions, and excitation source are defined.

[0232] S212, grid division;

[0233] In this example, the model is divided into several small cells, such as tetrahedrons, hexahedrons, and prisms, so that the electromagnetic field within each cell can be approximated by a simple function. The quality and density of the meshing affect the accuracy and efficiency of the simulation. Generally, it is best to minimize the number and irregularity of cells while maintaining accuracy.

[0234] S213, solve equations;

[0235] In this embodiment, based on the selected electromagnetic field simulation software, a suitable solver is selected, such as the finite element method, finite difference method, finite volume method, and boundary element method, to convert the control equations of the electromagnetic field into a set of algebraic equations, and then these equations are solved using an iterative method or a direct method to obtain the electromagnetic field distribution and intensity within each unit.

[0236] S214, analysis results;

[0237] In this embodiment, based on the obtained electromagnetic field data, the impedance change of the coil, as well as parameters such as the resistivity, permeability, and film thickness of the sample are calculated, and their relationship with factors such as the material properties, position, and angle of the sample is analyzed. The accuracy and sensitivity of the measurement are evaluated, and the design and performance of the measurement system are optimized.

[0238] like Figure 14 and Figure 15 As shown, in the Matlab simulation of this embodiment, basic parameters are defined. These basic parameters include but are not limited to: the coil radius, current, frequency, and the sample position, angle, resistivity, magnetic permeability, and film thickness. The above basic parameters can be modified according to actual conditions. Assume that the coil is a circular planar coil, the sample is a cylindrical metal film, and air is a uniform medium.

[0239] S22. Circuit simulation.

[0240] The entire signal processing circuit was simulated on the OrCAD simulation platform in this embodiment. Each functional submodule was simulated separately to test its performance. After successful debugging, the modules were connected together to form a simulation model of the eddy current sensor signal processing circuit system. We primarily analyzed the relationship between the inductance value and the output voltage.

[0241]

[0242] Inductive sensor measurement data

[0243] Assume the functional relationship of the curve is:

[0244] y=a0+a1x

[0245] Available

[0246]

[0247] When fitting the curve, the minimum is required, and we can get:

[0248]

[0249]

[0250] The curve relationship is:

[0251] y=-0.728x+5.324

[0252] like Figure 16 and Figure 17 As shown, in this embodiment, in order to facilitate the observation of the variation range and trend of the test data, a dotted line graph of the data is drawn.

[0253] Using simulation software for simulation tests can basically meet all conditions under ideal conditions. However, errors may occur during the circuit design process due to minor problems in the circuit design structure, but these errors can be ignored.

[0254] like Figure 18 and Figure 19 As shown, in this embodiment, S3, experimental verification and data collection are carried out, and the conductivity of some common metal rods at 10°C, 20°C, 30°C, 40°C, and 50°C is measured by a non-contact method and a database is established and stored to provide data support for the STM32 measurement system.

[0255] In this embodiment, the experimental instruments include but are not limited to: an eddy current sensor, a DC regulated power supply, a digital DC voltmeter, a micrometer, an oscilloscope, a circuit board, a ruler, a vernier caliper, and a micrometer screw.

[0256] In this embodiment, the metal materials used for the experiment include but are not limited to: aluminum sheets of different specifications (radius, thickness), duralumin (solid solution state), duralumin (annealed), copper sheets, No. 45 steel, and stainless steel.

[0257] like Figure 20 As shown, in this embodiment, a ruler and a vernier caliper are used to measure the specifications (radius and thickness) of the metal sheet; the power is turned on, a high-frequency current is passed through the coil, and the voltage in the coil is measured; iron rods, cobalt rods, etc. of different specifications (radius, height) are brought close to the coil at 1 cm, 2 cm, 3 cm, 5 cm, and 10 cm, and the voltage in the coil is measured; copper rods, aluminum rods, steel rods, alloy rods, etc. of different specifications (radius, height) are brought close to the coil at 1 cm, 2 cm, 3 cm, 5 cm, and 10 cm, and the voltage in the coil is measured; and the change in the coil impedance is obtained according to Ohm's law.

[0258] In this embodiment, the experimental data are recorded in Table 1 below:

[0259] Table 1 Specifications and Measurements

[0260]

[0261]

[0262] In this embodiment, when there is no metal in the distance test (15V power supply) and Uo=3.00V, the experimental data obtained at this time are shown in Table 2 below:

[0263] Table 2

[0264]

[0265] In this embodiment, different metal material tests (12V power supply) were conducted. When no metal was close, Uo=1.626 and the distance was 1mm, the experimental data obtained were shown in Table 3 below:

[0266] Table 3

[0267]

[0268] In this embodiment, the experimental conclusions are drawn based on the above experiments:

[0269] The output voltage is linearly related to the distance between the inductor coil and the metal sheet;

[0270] The output voltage has little to do with the thickness of the metal sheet. This is because the skin effect of high-frequency current causes eddy currents to only appear on the surface of the metal material.

[0271] Through experimental testing, we determined the impedance change caused by the substance under test, which is close to the value obtained in the simulation. Therefore, the theoretical analysis is correct.

[0272] Example 2

[0273] Currently, in the field of main control chips for eddy current sensors, common choices include ARM7 and ARM9. These chips are known for their powerful processing capabilities, complex hardware and software structures, and high costs, and are usually used in large embedded instrument designs. However, in 2007, ST Micro introduced a new microprocessor

[0274] STM32. This microprocessor, based on the Cortex-M core, not only optimizes the instruction set structure and hardware architecture, but also improves performance and reduces power consumption, all at a reasonable price. These features make the STM32 microprocessor ideal for portable instrument design, and as a result, the STM32 series chips are often used as the instrument's main control chip.

[0275] like Figure 21As shown, in this embodiment, the eddy current sensor includes: a main control module 1, a human-computer interaction module 2, an SD card storage module 3, an eddy current signal acquisition module 4, a PT1000 temperature sensor module 5, a high-precision data acquisition module 6 and a power supply module 7. The core of the main control module 1 adopts the ARM-Cortex processor STM32F373V8 of ST company, which is responsible for the functions of signal acquisition, processing, display and storage. The human-computer interaction module 2 is implemented by a TFT display module and a keyboard module, so that the user can perform key operations and information viewing. The SD card storage module 3 not only stores the measurement data and standard database of the material, but also can continuously record the eddy current response curve of the material. This module uses the znFAT file system. This is an open source file system developed by Zhennan. Compared with other file systems, it has significant advantages in RAM usage efficiency, data writing speed, stability, physical layer interface compatibility and platform portability. The LTC2414 or LTC2418, used in the high-precision data acquisition module 6, is an analog-to-digital converter that connects to the processor via a 4-wire SPI interface. It supports multi-channel inputs and is renowned for its excellent temperature stability, low noise, and extremely low nonlinearity. The stability of the power supply module is crucial to the overall performance of the sensor. A single power supply is used, and surge protection is provided by an inductor and diode to ensure power input stability. Furthermore, careful PCB layout design for the power supply and ground planes is crucial for improving the overall system's interference immunity and measurement accuracy.

[0276] like Figure 22 、 Figure 23 、 Figure 24As shown, in this embodiment, the eddy current sensor circuit is used to detect eddy current reactions within a material. It consists of an induction coil. When an alternating magnetic field passes through a conductive material, the probe responds to the eddy currents generated within the material. To improve the precision and accuracy of the probe signal measurement, the pre-processing circuit requires an amplifier with high input impedance and a high common-mode rejection ratio. In this design, a rail-to-rail op amp with high input impedance, such as the LTC6078, can be used. Its input uses a CMOS structure, resulting in extremely low input bias current and high common-mode rejection ratio. The preamplifier performs initial signal amplification, first-order RC filtering, and voltage following, before transmitting the processed signal to the analog-to-digital conversion module. Regarding the choice of analog-to-digital converter, although the STM32F373 chip has a high-performance SDADC module, additional calibration is typically required to improve measurement accuracy. Therefore, the design may choose an external high-precision analog-to-digital converter, such as the LTC2414, to improve measurement accuracy while leaving room for future upgrades to the LTC2418 chip. The overall eddy current probe circuit is shown in the figure. The probe is connected via a well-shielded interface, such as a BNC connector. To enable multi-element measurement, the circuit design can include multiple BNC connectors to connect multiple eddy current probes in parallel. The eddy current probe signal is first filtered by the LTC6078 preamplifier before being fed into the LTC2414 analog input channels for differential acquisition. Before transmission, each channel signal may be superimposed with a standard voltage reference value. This reference value is generated by the high-precision signal source LT1461 and divided by a precision resistor network.

[0277] In temperature sensors, temperature affects the conductivity of the material by affecting the thermal motion of the material's particles. In a smaller range, the relationship between the resistivity of the conductor and the temperature can be expressed as:

[0278] ρ1=ρ0[1+α(t1-t0)]

[0279] Where ρ1 is the resistivity at temperature t1, ρ0 is the resistivity at temperature t0, and α is the temperature coefficient of resistance within a given temperature range.

[0280] As can be seen from the above equation, the material conductivity varies with temperature, so the signal measured by the eddy current sensor also needs to be appropriately compensated for changes at different temperatures. Therefore, accurately measuring the temperature of the test environment is a critical aspect of the eddy current sensor system. This article utilizes the following temperature compensation measures for eddy current sensors: First, using multi-strand wires and a bundle of insulated thin metal wires instead of single solid wires directly reduces the AC resistance of the coil, thereby minimizing the effect of temperature on the coil impedance. Second, the sensor parameters are optimized, and the PT1000, with its excellent linearity and wide temperature measurement range, is selected as the temperature sensor. This selection of parameters allows automatic compensation for temperature drift to be achieved within a certain range and to a certain extent. The most critical step is to utilize software methods within the MCU to compensate for sensor nonlinearity using fitting, table lookup, and interpolation.

[0281] In this embodiment, the PT1000 platinum resistor uses a four-wire system and is connected to the circuit through a 3.5mm headphone jack. The voltage on R18 is 1V (marked as V1 in the figure), and the current flowing through R18 is approximately 0.5mA, which can avoid the self-heating effect of the platinum resistor. Due to the high input impedance of the LTC6078 (marked as U8 in the figure), it can be considered that the current flowing through the R18 resistor is equal to the current flowing through the PT1000 sensor. Therefore, the voltage ratio of R18 and PT1000 is equal to the resistance ratio. The PT1000 signal is sent to the LTC2414 differential channel for acquisition after first-order filtering and voltage following. The R(PT) resistance value can be calculated using the formula V(PT1000) = V1R(PT1000) / (R18). The R18 resistor in the figure can use a high-precision 2K resistor or measure the R18 resistance value in advance and solidify it into the C program using a macro definition as a constant. The measured platinum resistance value can be obtained by direct formula calculation or table lookup method.

[0282] 4.2 System Software Design

[0283] The design of an eddy current sensor involves both hardware and software design. Hardware design not only incorporates software design concepts, but also plays a key role in supporting system operation and improving hardware performance. During the design process, the software adopted a modular programming approach.

[0284] like Figure 25 As shown, in this embodiment, multiple key modules are defined, such as the STM32 firmware library, menu, keyboard scanning, temperature measurement, eddy current sensor, SD card storage, etc. In software design, the design of algorithms and processes is the most critical because they directly affect the efficiency and performance of the system.

[0285] Although the STM32 microcontroller is equipped with internal FLASH storage space, in order to avoid losing calibration information when updating the program, these calibration data can be saved as files on the SD card. In this way, these calibration files can be loaded directly from the SD card when performing non-contact conductivity measurement.

[0286] In summary, this invention, based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, combines theoretical analysis, simulation, and experimental research. This method primarily considers parameters such as the rod's material, size, and shape, as well as its coupling with the coil and the entire circuit. The method explores a non-contact method for testing the electrical and magnetic properties of non-magnetic metal or ferromagnetic rods. By observing changes in the circuit response (resistance, inductance, and capacitance) when different rod types are inserted, and precisely measuring the changes in these parameters, the magnetic and electrical properties of the inserted rod can be accurately inferred.

[0287] The present invention deduces the relationship between the inductance coil and the output voltage of the eddy current sensor, as well as the relationship between the parameters of the inductance coil during oscillation and the conductivity of the material, through theoretical analysis and simulation, and thus obtains the relationship between the material conductivity and the output voltage. The theory and simulation are verified experimentally, and the circuit responses of five different metal materials at five temperatures are collected to establish a comparative database. This provides data support for data collection and processing at the edge of the STM32 embedded device, quickly and accurately realizes non-contact and non-destructive measurement of the material's conductivity, and provides convenience for industrial production and scientific research measurements.

[0288] This invention directly derives a formula for calculating the distribution of eddy currents generated by a hollow excitation coil within a uniform flat conductor from the Biot-Savart law and Maxwell's equations. This calculation is faster and simpler than traditional methods. The invention uses MATLAB software to simulate and calculate the distribution of eddy currents, and the simulation takes only seconds to half a minute on a gaming laptop.

[0289] The present invention uses the PSPICE program to simulate a three-point capacitor oscillator circuit, allowing direct observation and adjustment of circuit parameters, allowing for rapid circuit design and optimization. The present invention utilizes Q2 to form an emitter follower. The emitter follower's function is to match input and output to achieve the largest possible undistorted output amplitude.

[0290] The phase-sensitive detection circuit adopted in the present invention has frequency and phase detection functions for input signals, thereby improving the anti-interference ability of the circuit.

[0291] The present invention solves the technical problems of environmental interference, material complexity and measurement model complexity existing in the prior art.

[0292] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A non-contact material electrical property measurement method based on STM32, characterized in that: The method comprises: S1. Use the eddy current sensor to approach different materials to measure the circuit response changes to obtain the inductance change data of the eddy current sensor; S2. Based on the inductance change data, perform electromagnetic field theory analysis, eddy current sensor theory analysis, and capacitance three-point oscillation circuit theory analysis to obtain the material magnetic field intensity distribution to infer the electrical properties of the measured material; S2 includes: S21. Conduct electromagnetic field theoretical analysis, including deriving general formulas for eddy currents based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, including: expressions for alternating magnetic fields, equations satisfied by induced electromotive force, equations satisfied by induced current density, and the relationship between induced current density and induced electromotive force, from which the basic equations for eddy currents are derived; S22. Solve the basic eddy current equation to obtain the eddy current distribution and eddy current intensity, and use them to obtain the material magnetic field intensity distribution; S23. Calculate the reflection coefficient Γ and the transmission coefficient τ using the impedance matching principle. S24. Perform theoretical analysis of the eddy current sensor, wherein, based on the equivalent circuit of the eddy current sensor, a set of equivalent circuit equations is listed, expressions for the complex impedance, equivalent inductance, and equivalent Q value of the coil of the eddy current sensor are obtained, and the nonlinear functional relationship is obtained by processing to measure the material under test; S25. Conduct theoretical analysis of capacitor three-point oscillation circuit; S3. Conduct experiments and simulations on the tested materials, obtain and establish and store a database of circuit responses at different temperatures based on the experimental data; S4. Based on the STM32 control chip, design a capacitive three-point oscillation circuit for the eddy current sensor, collect circuit response data in real time, and compare it with the circuit response database to obtain and display the electrical property detection results.

2. The non-contact material electrical property measurement method based on STM32 according to claim 1, characterized in that, The step S21 further includes: S211. Place the material under test in the alternating magnetic field generated by a coil passing a high-frequency current. The frequency of the high-frequency current is ω. The alternating magnetic field is represented by the following logic: Where, is the complex amplitude of magnetic induction intensity, ω is the angular frequency, t is the time, j is the imaginary unit, σ represents the conductivity, and μ represents the magnetic permeability; S212, according to the Faraday's law of electromagnetic induction, the induced electromotive force in the measured material Satisfies the following equation: Where, represents the curl operator; S213, according to the Ampere circuit theorem, the induced current density in the measured material Satisfies the following equation: Where, is the magnetic field strength, and the magnetic induction intensity satisfies S214. Use the following logic to express the induced current density: With the induced electromotive force The relationship between: Combining the above equations (1), (2) and (3), we can obtain the basic equation of eddy current: Where, represents the Laplace operator. The above equation describes the propagation and attenuation law of the alternating magnetic field in the conductive material. It is also the basic equation of eddy current. According to different boundary conditions and geometric shapes, the eddy current distribution and intensity in different situations can be solved.

3. The non-contact material electrical property measurement method based on STM32 according to claim 1, characterized in that, The step S22 further includes: S221. Suppose a plane wave is incident on an infinitely large conductive plate with a thickness of d, a conductivity of σ, and a magnetic permeability of μ. The angular frequency of the incident wave is ω, and the angle of incidence is θ. S222, set the magnetic field strength inside the conductive plate Distribution along the x direction: Where H(x) is the complex amplitude, is a unit vector; S223. According to the basic eddy current equation, an ordinary differential equation is obtained: Where k 2 =jωμσ is the complex wave number; S224. Obtain the general solution of the ordinary differential equation: H(x)=Ae -kx +Be kx (7) Wherein A and B are arbitrary constants; the magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave are determined; and the magnetic field intensity distribution of the material inside the conductive plate is obtained based on the magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave.

4. The non-contact material electrical property measurement method based on STM32 according to claim 3, characterized in that, In S224, the arbitrary constants A and B are determined using the boundary conditions and initial conditions of the eddy current basic equation: H(0)=H i +H r (8) H(d)=H t (9) Where H i is the magnetic field intensity of the incident wave, H r is the magnetic field strength of the reflected wave, H t is the magnetic field intensity of the transmitted wave; According to Snell's law, the following relationship can be obtained: H i =H0cosθ (10) H r =-H0cos θΓ (11) Where H0 is the maximum value of the magnetic field intensity of the incident wave, Γ is the reflection coefficient, τ is the transmission coefficient, is the wave number in vacuum, μ0 and ∈0 are the magnetic permeability and dielectric constant in vacuum respectively; Substituting the magnetic field intensity of the incident wave, the magnetic field intensity of the reflected wave, the magnetic field intensity of the transmitted wave, and the maximum magnetic field intensity of the incident wave into the boundary conditions, we can obtain the magnetic field intensity equations: A+B=H0cos θ(1+Γ) (13) Solving the magnetic field strength equations, we can obtain: Therefore, the magnetic field intensity distribution inside the plate can be obtained as follows: According to Ampere's circuit theorem, the current density distribution inside the plate can be obtained as follows: Substituting the magnetic field intensity distribution inside the plate, we can get the following formula: Where, is a unit vector, and the eddy current distribution and the eddy current intensity inside the conductive plate are obtained.

5. The non-contact material electrical property measurement method based on STM32 according to claim 1, characterized in that, The step S23 further includes: S231. Define impedance using the following logic: Where E and H are the electric field intensity and magnetic field intensity respectively; S232. For the case of a plane wave incident on an infinite plate, the following relationship can be obtained: Where Z1 is the impedance of the incident medium, Z2 is the impedance of the plate, for vacuum or air, is the free space impedance.

6. The non-contact material electrical property measurement method based on STM32 according to claim 1, characterized in that, The S24 includes: S241. Use the following logic to list the equivalent circuit equations: Solving the equivalent circuit equations to obtain the preset frequency alternating current I1 and the self-closing current I2; S242, obtaining expressions for the coil complex impedance, coil equivalent inductance, and coil equivalent Q value of the eddy current sensor; S243. According to the expression and based on Maxwell's mutual inductance formula, the nonlinear functional relationship between the mutual inductance, the distance x(H) between the coil and the metal conductor can be obtained; S244. Determine the eddy current coil, the metal eddy current sheet and the excitation source, and maintain the ambient temperature. Through the preamplifier of the eddy current sensor, convert the changes in the complex impedance of the coil, the equivalent inductance of the coil and the equivalent Q value of the coil into voltage and current change data and output them to complete the measurement of the material under test; wherein, the preamplifier is a variable frequency amplitude modulation measurement circuit including: a capacitor three-point oscillator, a π-shaped filter detector and an emitter follower.

7. The non-contact material electrical property measurement method based on STM32 according to claim 6, characterized in that, In said S242: The complex impedance of the coil is expressed using the following logic: The equivalent inductance of the coil is expressed using the following logic: The equivalent Q value of the coil is expressed using the following logic: Where Q0 represents the Q value of the coil without eddy current, satisfying: Q0 = ωL1 / R1; Z2 2 Represents the impedance of the part of the metal conductor that generates eddy current, satisfying:

8. The non-contact material electrical property measurement method based on STM32 according to claim 1, characterized in that, The step S25 includes: S251. Setting an oscillation frequency of a capacitor three-point oscillator according to a starting condition, wherein the eddy current sensor is connected to an oscillation circuit of the capacitor three-point oscillator, and a change in the Q value of the oscillation circuit caused by a displacement change is converted into high-frequency carrier signal amplitude change data; When the capacitor three-point oscillator meets the starting conditions, the oscillation frequency is expressed using the following logic: The oscillation frequency is approximately expressed using the following logic: S252, using a phase-sensitive detection circuit to perform signal extraction, signal frequency discrimination, and signal phase discrimination, wherein a π-shaped filter detector is used to extract a low-frequency signal from a high-frequency amplitude modulated signal; S253. Use an emitter follower to match input and output to obtain an undistorted amplitude value.

9. The non-contact material electrical property measurement system based on STM32 is characterized by: The system comprises: An inductance change acquisition module is used to use the eddy current sensor to approach different materials to measure the circuit response change to obtain the inductance change data of the eddy current sensor; a magnetic field distribution analysis module, configured to perform electromagnetic field theory analysis, eddy current sensor theory analysis, and capacitance three-point oscillation circuit theory analysis based on the inductance change data to obtain the material magnetic field intensity distribution, thereby inferring the electrical properties of the measured material. The magnetic field distribution analysis module is connected to the inductance change acquisition module; Conduct electromagnetic field theory analysis, including deriving general formulas for eddy current based on Faraday's law of electromagnetic induction and Ampere's circuit theorem, including: expressions for alternating magnetic fields, equations satisfied by induced electromotive force, equations satisfied by induced current density, and the relationship between induced current density and induced electromotive force, from which the basic equations for eddy current are derived; Solve the basic eddy current equation to obtain the eddy current distribution and eddy current intensity, and then calculate the material magnetic field intensity distribution; Using the impedance matching principle, calculate the reflection coefficient Γ and transmission coefficient τ; Conduct theoretical analysis of eddy current sensors. According to the equivalent circuit of the eddy current sensor, the equivalent circuit equations are listed to obtain the expressions of the complex impedance, equivalent inductance and equivalent Q value of the coil of the eddy current sensor. The nonlinear function relationship is processed to measure the material under test. Conduct theoretical analysis of capacitor three-point oscillation circuit; a response database construction module, configured to conduct experiments and simulations on the material under test, obtain experimental data, and establish and store a circuit response database at different temperatures based on the experimental data; the response database construction module is connected to the magnetic field distribution analysis module; The material property detection module is used to design the capacitive three-point oscillation circuit of the eddy current sensor based on the STM32 control chip, collect circuit response data in real time, and compare it with the circuit response database to obtain and display the electrical property detection results. The material property detection module is connected to the response database construction module.

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