A method for generating multi-frequency electromagnetic detection signals for defect detection in composite materials.

Abstract

This invention discloses a multi-frequency electromagnetic detection signal generation method for defect detection in composite materials, belonging to the field of electromagnetic detection technology. Its implementation includes: based on the layered structure of the composite material, acquiring the response voltage of a defect-free standard sample of the composite material at different excitation frequencies, under different structural layers, and in different directions; normalizing the response voltage and calculating the impedance characteristic curve of the composite material using Ohm's law; finding the optimal frequency excitation point from the impedance characteristic curve and calculating the impedance matching coefficient accordingly; calibrating the probe based on the impedance matching coefficient and generating a multi-frequency composite signal output to the excitation coil of the excitation probe. This achieves impedance matching between the composite material, the excitation coil, and the differential coil.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic detection technology, and more specifically to a method for generating multi-frequency electromagnetic detection signals for detecting defects in composite materials. Background Technology

[0002] Traditional methods for material testing based on electromagnetic technology include eddy current testing and magnetic flux leakage testing. Eddy current testing (ECT) works by using the principle of electromagnetic induction. An alternating current is passed through a coil to generate eddy currents on the surface of the material. When the eddy currents pass through cracked material, they are distorted, and the coil captures the eddy current magnetic field signal. Based on the phase, amplitude, and distribution of the signal, the location, shape, and size of the defect can be deduced.

[0003] Magnetic Flux Leakage Testing (MFL) works by utilizing the fact that when a material is magnetized, surface and near-surface defects cause changes in its permeability, which in turn causes changes in the magnetic flux and magnetic field lines in the magnetic circuit. This allows some of the magnetic flux to bypass the defects through the magnetic circuit in the air and then enter the material, forming a leakage magnetic field. The acquisition device extracts information such as the magnitude and direction of the leakage magnetic field, and after further processing, the defect information is obtained.

[0004] Based on the aforementioned traditional technologies, multi-frequency eddy current detection and multi-frequency excitation composite electromagnetic detection have been further developed.

[0005] Multi-frequency eddy current testing has two forms: time-division multi-frequency excitation, which applies different frequencies of excitation at different time periods; and simultaneous multi-frequency excitation, where multiple excitation signals of various frequencies exist on the coil at the same time. During the testing process, multi-frequency eddy current testing replaces the single-frequency excitation of traditional eddy current testing by employing multiple frequency excitations simultaneously. Utilizing the detection characteristics of different frequencies, it can acquire more defect information without increasing the structural complexity of the acquisition device, and can effectively suppress various noise signals, improving the signal-to-noise ratio.

[0006] Multi-frequency excitation composite electromagnetic testing is a non-destructive testing method that combines multi-frequency eddy current testing and magnetic flux leakage testing. It leverages the high sensitivity of multi-frequency eddy current testing to material surfaces and the near-surface detection capabilities of magnetic flux leakage testing to effectively and rapidly detect the two most common surface defects: fatigue diagonal cracks and near-surface buried defects. Based on different excitation and response signals, it enables omnidirectional testing from the surface to near-surface, and can analyze various parameter information of defects based on depth information.

[0007] When applying multi-frequency excitation electromagnetic detection technology to composite materials, the anisotropy, multilayer stacking, and non-uniform conductivity distribution of composite materials can cause electromagnetic field distortion in the excitation probe. Specifically, the anisotropy of the composite material can distort the normal electromagnetic field, leading to inaccurate or submerged defect data (making it impossible to determine whether the issue is caused by the defect or the anisotropy of the composite material). Furthermore, due to the anisotropy of the composite material, each layer has different impedance characteristics, and the impedance mismatch between the composite material and the excitation coil can also lead to misjudgments of defects. Summary of the Invention

[0008] The purpose of this invention is to provide a method for generating multi-frequency electromagnetic detection signals for defect detection in composite materials. The method involves impedance normalization of the anisotropic conductivity of the composite material and dynamic matching of the differential coil. By adjusting the differential output of the probe coil and the impedance matching coefficient, the method achieves adaptive coupling between the detection signal (multi-frequency composite signal) output by the probe and the impedance characteristics of the material, thereby significantly improving the accuracy of defect detection.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] Based on the layering of composite material structure, the response voltage of defect-free composite material standard sample was collected at different excitation frequencies, different structural layers, and different directions. The response voltage is correlated with the structural layer, excitation frequency, and direction.

[0011] The response voltage was normalized, and the impedance characteristic curve of the composite material was calculated using Ohm's law.

[0012] Find the optimal frequency excitation point from the impedance characteristic curve, and calculate the impedance matching coefficient based on it. The impedance matching coefficient includes the PWM duty cycle, excitation phase, and capacitor array capacitance value.

[0013] The probe calibration is performed with the impedance matching coefficient as the target. The probe calibration includes: adjusting the PWM duty cycle to generate a power control signal to control the output current of the excitation probe; reconfiguring the excitation phase of the excitation probe by configuring the phase register of the direct digital synthesizer (DDS); and calculating the capacitance value of the target capacitor array and controlling the switching of the differential probe capacitor array.

[0014] The multi-frequency composite signal generated by the direct digital synthesizer (DDS) is output to the excitation coil of the excitation probe.

[0015] As one specific implementation scheme, the noise baseline is also calculated using the response voltage, and the calculation method is as follows:

[0016] Without the composite material, at frequency The following data is collected: voltage signals generated by the electromagnetic system itself and environmental electromagnetic interference. The noise baseline was obtained by performing a sliding filter averaging process on the response voltage collected when placing a defect-free composite material standard sample, and then weighted and superimposed.

[0017] ;

[0018] In the formula, and These are the weighting coefficients, , Represents frequency The response voltages of each structural layer in the composite material along the fiber direction (x), perpendicular to the fiber direction (y), and thickness direction (z) are measured.

[0019] As a specific implementation scheme, the process of normalizing the response voltage and calculating the impedance characteristic curve of the composite material using Ohm's law is as follows:

[0020] The response voltages of each layer of the composite material were detected, and the layer response matrix was constructed.

[0021] Based on the anisotropic conductivity of composite materials and the structural layering of composite materials, a layer impedance matrix is ​​constructed. The theoretical impedance range of each layer of the composite material under different directions and frequencies is obtained by analyzing the layer impedance matrix.

[0022] The scaling factor matrix and offset matrix are calculated based on the theoretical impedance range. After normalizing the layer response matrix using the scaling factor matrix and offset matrix to obtain the response voltage, the current signal of the acquisition and detection system is used to calculate the equivalent impedance using Ohm's law, thereby obtaining the impedance characteristic curve.

[0023] As a specific implementation scheme, the process of finding the optimal frequency excitation point from the impedance characteristic curve and calculating the impedance matching coefficient based on it is as follows:

[0024] The impedance characteristic curve is divided into several sub-bands based on frequency bands;

[0025] Within each sub-frequency band, the optimal excitation point and optimal phase reference are found using interpolation fitting methods;

[0026] The amplitude modulation coefficient and phase compensation amount are calculated based on the optimal excitation point and the optimal phase reference.

[0027] The impedance matching coefficient is calculated based on the amplitude modulation coefficient and the phase compensation amount.

[0028] As a specific implementation scheme, the formula for calculating the amplitude modulation coefficient is as follows:

[0029] ;

[0030] in, For impedance mode matching, Voltage is the preferred option; The modulus of the probe impedance at the current moment; The modulus of the composite material impedance under the current i-th operating frequency band; Let be the voltage value at the optimal excitation point that maximizes energy transfer efficiency under the current i operating frequency band; It is a constant, which is the set reference excitation voltage value of the detection system;

[0031] The formula for calculating the phase compensation amount is as follows:

[0032] ;

[0033] in: These are conjugate matching terms; This is a resonance optimization term; The optimal impedance of composite materials in the current i-band The conjugate; This represents the complex impedance of the probe at the current moment; This is a high-frequency correction factor at the current excitation frequency; This represents the optimal phase offset for the current i-th operating frequency band; The constant represents the system's reference phase;

[0034] The high-frequency correction factor is:

[0035] ;

[0036] in: and These are the material's characteristic constants; This represents the optimal frequency for the current operating frequency band i.

[0037] As a specific implementation scheme, the method for obtaining the PWM duty cycle in the impedance matching coefficient, which is the calibration target, is as follows:

[0038] Based on amplitude modulation coefficient Calculate the target current and target PWM duty cycle;

[0039] ;

[0040] In the formula, The target current at the current moment; It is the preset probe safety operating reference drive current amplitude of the detection system;

[0041] ;

[0042] In the formula, It is the target duty cycle. This is the maximum allowable current of the detection system; It is the maximum allowed percentage of the preset value.

[0043] As a specific implementation scheme, the method for obtaining the capacitance value of the capacitor array in the impedance matching coefficient, which is the calibration target, is as follows:

[0044] Calculate the target capacitance value of the composite material and the target reactance of the differential probe capacitor array:

[0045] ;

[0046] ;

[0047] ;

[0048] In the formula, This indicates the target impedance of the differential probe. Indicates the equivalent impedance of the composite material; Indicates the amplitude modulation coefficient; Indicates the target reactance of the differential probe; This indicates the target capacitance value of the differential probe's capacitor array.

[0049] As a specific implementation scheme, the method for obtaining the excitation phase in the impedance matching coefficient, which is the calibration target, is as follows:

[0050] The real-time phase compensation amount and the preset reference phase are obtained and added together to obtain the phase setting value, which is used as the excitation phase.

[0051] The phase setting value is obtained as follows:

[0052] ;

[0053] In the formula, Let be the phase compensation amount at time t. This is the reference phase.

[0054] As a specific implementation scheme, the process of generating a multi-frequency composite signal based on a direct digital synthesizer (DDS) and outputting it to the excitation coil of the excitation probe is as follows:

[0055] Multiple signals at different frequencies are generated by a direct digital synthesizer (DDS), and then the signals are superimposed to obtain a multi-frequency composite signal. :

[0056] ;

[0057] In the formula, N represents the total number of frequencies in the composite signal. This represents the real-time amplitude modulation coefficient at the k-th frequency. Let t represent the k-th frequency and t represent time. This represents the reference phase at the k-th frequency. This represents the real-time phase compensation amount at the k-th frequency.

[0058] As a specific implementation plan, the process of probe calibration with impedance matching coefficient as the target is as follows:

[0059] Using the capacitance value of the capacitor array in the impedance matching coefficient as the target capacitance value, output control commands to control the switching of each capacitor unit in the capacitor array of the differential probe, so that the capacitor array is as close as possible to the target capacitance value.

[0060] The PWM duty cycle in the impedance matching coefficient is used as the target duty cycle. The target duty cycle is used as a comparison value and output to the detection system. The detection system generates a square wave signal with the new duty cycle as the power control signal.

[0061] Using the excitation phase in the impedance matching coefficient as the target, the phase setting value is written into the phase register, and the excitation phase of the excitation probe is rematched.

[0062] Compared with the prior art, the present invention has the following advantages:

[0063] In this invention, response voltages at different frequencies, different structural layers, and different directions are collected from a defect-free composite material standard sample through structural layering. Based on these response voltages, an impedance curve of the composite material with a mapping relationship to frequency, structural layer, and direction can be obtained. The optimal frequency excitation point is found from the impedance characteristic curve, and the amplitude modulation coefficient and phase compensation amount are calculated accordingly. The excitation signal of the excitation probe can be generated based on the amplitude modulation coefficient and phase compensation amount. Furthermore, the target capacitance in the corresponding direction of the corresponding structural layer at the corresponding frequency is found based on the impedance characteristics of the composite material, thereby adjusting the differential coil. Ultimately, impedance matching among the excitation probe, composite material, and differential coil is achieved, improving the accuracy of the detection results.

[0064] A noise baseline can be constructed by using response voltages at different frequencies, in different structural layers, and in different directions. This overcomes the response voltage variations caused by natural conductivity disturbances in different regions of the composite material. By comparing the defect response voltage with the noise baseline, voltage variations caused by natural conductivity disturbances and those caused by defects can be distinguished. Attached Figure Description

[0065] Figure 1 This is the process of the present invention. Detailed Implementation

[0066] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be briefly introduced below in conjunction with the accompanying drawings and descriptions of the embodiments or the prior art. Obviously, the following description of the structure of the accompanying drawings is only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. It should be noted that the description of these embodiments is for the purpose of helping to understand the present invention, but does not constitute a limitation of the present invention.

[0067] It should be understood that although the terms first, second, etc., may be used herein to describe various modules, these modules should not be limited by these terms. These terms are only used to distinguish one module from another. For example, a first module may be referred to as a second module, and similarly, a second module may be referred to as a first module, without departing from the scope of the exemplary embodiments of the invention.

[0068] It should be understood that the term "and / or" that may appear in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, B exists alone, and A and B exist simultaneously. The term " / and" that may appear in this document describes another relationship between related objects, indicating that two relationships can exist. For example, A / and B can mean: A exists alone, and A and B exist alone. In addition, the character " / " that may appear in this document generally indicates that the related objects before and after it are in an "or" relationship. Example

[0069] like Figure 1 As shown, the method for generating multi-frequency electromagnetic detection signals for defect detection in composite materials includes the following steps:

[0070] Step S100: Based on the layering of the composite material structure, collect the response voltages of the defect-free standard sample of the composite material at different frequencies and under different structural layers in the fiber direction x, perpendicular fiber direction y, and thickness direction z. The response voltage is correlated with the structural layer, excitation frequency, and direction mapping.

[0071] Step S200: Based on the response voltage in step S100, obtain the noise baseline, the anisotropic conductivity of the composite material, and the impedance characteristic curve of the composite material;

[0072] Step S300: Find the optimal frequency excitation point from the impedance characteristic curve, and use it to calculate the amplitude modulation coefficient, phase compensation amount, and impedance matching coefficient.

[0073] Step S400: Calibrate the probe. Probe calibration includes adjusting the PWM duty cycle, generating a power control signal to control the output current of the excitation probe, reconfiguring the excitation phase of the excitation probe by configuring the DDS phase register, calculating the target capacitance value, and controlling the switching of the differential probe capacitor array.

[0074] Step S500: The calibrated DDS generates a multi-frequency composite signal output to the excitation coil of the excitation probe.

[0075] Specifically, the implementation process in step S100 is as follows:

[0076] During scanning, a probe matrix identical to that used in the formal inspection is employed. The probes scan along the defect-free composite material standard sample at multiple frequencies and in multiple directions, according to the structural layers. Specifically:

[0077] A scanning grid is established in the xy plane, with each grid point serving as a measurement point. The structure is layered in the thickness direction, and the scanning is repeated once at the center plane of each layer.

[0078] Specifically, the composite material is co-discrete along the thickness direction. Layer, the thickness of the lth layer is , .

[0079] Record the response voltage at each measuring point along the fiber direction (x), perpendicular to the fiber direction (y), and in the thickness direction (z). , , And construct the response voltage and corresponding excitation frequency. Grid coordinate index, corresponding thickness The mapping relationship between the directions x, y, and z; This represents the frequency value of the kth excitation frequency.

[0080] Specifically, in step S200, the process of obtaining the anisotropic conductivity of the composite material based on the response voltage is as follows:

[0081] Based on the collected response voltage, it is converted into a relative conductivity value after being processed by a normalization formula, as follows:

[0082] ;

[0083] in, Representing frequency Below, the normalized relative values ​​of conductivity in the fiber direction x, perpendicular to the fiber direction y, and thickness direction z are obtained; The response voltage values ​​collected along the fiber direction (x), perpendicular to the fiber direction (y), and thickness direction (z) represent the values ​​obtained along the fiber direction (x), perpendicular to the fiber direction (y), and thickness direction (z). , , ; Represents frequency Minimum response voltage; Represents frequency The maximum value of the response voltage.

[0084] Based on the above process, a set of electrical conductivity parameters characterizing the anisotropy of the composite material is obtained. Each electrical conductivity parameter in this set corresponds to a specific excitation frequency. Grid coordinate index, corresponding thickness Directional signs There is a mapping relationship between them.

[0085] Specifically, in step S200, the process of obtaining the noise baseline based on the response voltage is as follows;

[0086] Without placing any composite material, at frequency The following data is collected: voltage signals generated by the electromagnetic system itself and environmental electromagnetic interference. The noise baseline is obtained by performing a sliding filter averaging process on the response voltage collected from the defect-free composite material standard sample, and then weighting and superimposing the results.

[0087] ;

[0088] In the formula, and These are the weighting coefficients, , Represents frequency The response voltages of each structural layer in the composite material along the fiber direction (x), perpendicular to the fiber direction (y), and thickness direction (z) are measured.

[0089] A noise baseline is constructed by collecting the response voltage generated by a defect-free standard sample of the composite material, thereby overcoming the response voltage variation caused by the natural conductivity disturbance in different regions of the composite material. The voltage variation caused by the natural conductivity disturbance and the voltage variation caused by the defect can be distinguished by the defect response voltage and the noise baseline.

[0090] Specifically, in step S200, the process of obtaining the impedance characteristic curve of the composite material based on the response voltage is as follows:

[0091] Step S210: Based on the response voltage of each layer of the composite material under structural delamination, construct the layer response matrix. The specific implementation process is as follows:

[0092] At the j-th measurement depth, the response voltage measured by the probe can be expressed as a linear superposition of the defect-free adaptive response voltages:

[0093] ;

[0094] ;

[0095] in, Indicates the j-th measured depth position, with θ in the direction. Response voltage at a given frequency; Indicates the θ direction, The independent response of the l-th layer at a given frequency; For interlayer coupling weights; It is the distance from the probe excitation side to the l-th layer. It is the eddy current penetration depth of the l-th layer at the probe excitation frequency; Indicates the position at the j-th measurement depth, in the θ direction, Additive noise components at a given frequency.

[0096] Rewrite the above equation in matrix form:

[0097]

[0098] in, Indicates the θ direction, Response voltage vector at frequency; Interlayer coupling matrix; Indicates the θ direction of each layer, Independent response vectors at different frequencies; Additive complex Gaussian noise vector.

[0099] Tikhonov regularized least squares estimation of layer response is used :

[0100]

[0101] The analytical solution is:

[0102]

[0103] in Indicates the θ direction, The response matrices of each layer obtained by estimation at the frequency; , is the first-order difference regularization matrix; λ>0, is the regularization parameter; Represents the conjugate transpose of the interlayer coupling matrix. This represents the transpose of the regularized matrix.

[0104] Step S220: Based on the anisotropic conductivity and the structural layering of the composite material, construct the layer impedance matrix, and obtain the theoretical impedance range of each layer of the composite material in different directions and frequencies by analyzing the layer impedance matrix.

[0105] The specific implementation process of step S220 is as follows:

[0106] Obtain the anisotropic electrical conductivity σ of the composite material along the fiber direction, perpendicular to the fiber direction, and in the thickness direction. x σ y σ z ; Obtain the thickness d of each layer of the composite material z Width d w and length d l Using the geometry of the materials and the excitation frequency, the theoretical minimum and maximum impedance values ​​of each layer under different directions and frequencies are calculated to form a theoretical impedance range.

[0107] In this step, a local coordinate system is established for each layer of composite material, such that the x-axis of the coordinate system is along the fiber direction, the y-axis is perpendicular to the fiber direction, and the z-axis is the thickness direction.

[0108] Based on Maxwell's equations, the impedance matrix of each layer of the composite material is derived in the frequency domain. For the l-th layer, its impedance matrix... It is given by the following formula:

[0109] ;

[0110] ;

[0111] ;

[0112] ;

[0113] ;

[0114] ;

[0115] ;

[0116] In the formula, Z xx Z xy Z xz Z yx Z yy Z yz Z zx Z zy Z zz These correspond to the impedance components along the fiber direction, perpendicular to the fiber direction, and in the thickness direction, respectively; j is the imaginary unit. ω is the angular frequency; μ0 is the free permeability, σ x σ is the conductivity along the fiber direction. y Conductivity along the direction perpendicular to the fiber, σ z Conductivity along the thickness direction, d z d represents the thickness of the corresponding layer in the composite material. l d represents the length of the corresponding layer in the composite material.w The width of the corresponding layer in the composite material; l x , l y , l z These are the wave fractions in the fiber direction x, perpendicular to the fiber direction y, and thickness direction z, respectively.

[0117] Under different excitations in each direction, the value of each impedance component is calculated to obtain the set of impedance values ​​in each direction. For each direction, the minimum and maximum impedance values ​​at all frequencies are found to form the theoretical impedance range in that direction. The theoretical impedance ranges in all directions are combined to form the theoretical impedance range of the entire composite material in different directions and frequencies.

[0118] S230: Based on the theoretical impedance range, the scaling factor matrix and offset matrix are calculated. After normalizing the layer response matrix using the scaling factor matrix and offset matrix to obtain the response voltage, the current signal of the detection system is collected and the equivalent impedance is calculated using Ohm's law, thereby obtaining the impedance characteristic curve. The impedance characteristic curve is plotted with the voltage value corresponding to the time domain on the horizontal axis and the relative value of the equivalent impedance on the vertical axis, which maps to different frequencies and directions.

[0119] The specific implementation process of step S230 is as follows:

[0120] Calculate scaling factor and offset: Using a linear mapping method, the theoretical impedance range is mapped to the target numerical domain, and the scaling factor and offset are calculated. The scaling factor is the difference between the impedance ranges, and the offset is the minimum value of the impedance range.

[0121] The calculated scaling factor and offset are represented as scaling factor matrices and offset matrices, respectively. The response voltage for each layer is then normalized using these matrices. The normalized response voltage undergoes a moving average filtering operation to remove interference spikes or noise, ultimately yielding a continuous and stable normalized response voltage signal. Based on the normalized response voltage signal and the current signal from the detection system, the equivalent impedance is calculated using Ohm's law, resulting in a continuous and stable impedance characteristic curve. Ohm's law states that impedance is the ratio of voltage to current; this is a fundamental technique and will not be elaborated upon further.

[0122] The specific implementation of step S300 is as follows:

[0123] Step S310: Divide the impedance characteristic curve into several sub-bands based on frequency bands.

[0124] Specifically, in this step, we find the frequency point corresponding to the minimum impedance value of the impedance characteristic curve. These frequency points satisfy the condition that the first derivative of the impedance curve is zero and the second derivative is greater than zero. Based on these points, we divide the frequency band.

[0125] Specifically, the operating frequency band Divide into N non-uniform sub-intervals:

[0126]

[0127] Where F is the operating frequency band; f min f max These are the minimum and maximum frequencies of the operating frequency band, respectively; B i For the i-th frequency band; , These are the lower and upper limits of the i-th frequency band, respectively.

[0128] Step S320: Find the optimal excitation point in each sub-band using an interpolation fitting method;

[0129] Specifically, bicubic spline interpolation is used for each frequency band B. i Constructing a 3D response surface:

[0130]

[0131] in: Let f be the three-dimensional response surface of the i-th frequency band, representing the relationship between the impedance characteristics of the composite material and the frequency f and voltage V. The constraint condition is represented by f; the frequency variable is represented by V; the voltage variable is represented by V. This represents the k-th frequency point, which is the operating frequency band. A specific frequency value; This represents the m-th voltage value; Indicates the frequency of composite materials ,Voltage The impedance below; It represents a small positive number that constrains the smoothness of the surface.

[0132] Solve for the operating point that maximizes efficiency in each frequency band:

[0133]

[0134] in: It is a function of energy transfer efficiency; Let be the real part of the impedance of the composite material; The resistance of the probe; Let be the optimal frequency for the i-th frequency band. This represents the optimal voltage for the i-th frequency band. Indicates the minimum voltage value; Indicates the maximum voltage value; This represents the optimal phase of the i-th frequency band.

[0135] The optimal phase reference is obtained as follows:

[0136] .

[0137] Step S330: Calculate the amplitude modulation coefficient and phase compensation amount based on the optimal excitation, and construct the impedance matching coefficient through the amplitude modulation coefficient and phase compensation.

[0138] The formula for calculating the amplitude modulation coefficient is as follows:

[0139]

[0140] in, For impedance mode matching, Voltage is the preferred option; The modulus of the probe impedance at the current moment; The modulus of the composite material impedance under the current i-th operating frequency band; Let be the voltage amplitude at the optimal excitation point that maximizes energy transfer efficiency under the current i operating frequency band; It is a constant, which is the set reference excitation voltage amplitude of the detection system.

[0141] The formula for calculating the phase compensation amount is as follows:

[0142]

[0143] in: These are conjugate matching terms; This is a resonance optimization term; The optimal impedance of composite materials in the current i-band The conjugate; This represents the complex impedance of the probe at the current moment; This is a high-frequency correction factor at the current excitation frequency; This represents the optimal phase offset for the current i-th operating frequency band; is a constant, representing the system's reference phase.

[0144] The high-frequency correction factor is:

[0145]

[0146] in: and These are the material's characteristic constants; This represents the optimal frequency for the current operating frequency band i.

[0147] The impedance matching coefficient is a vector that includes the PWM duty cycle, excitation phase, and capacitance value of the capacitor array.

[0148] The method for obtaining the capacitance value of the capacitor array is as follows:

[0149] Obtain the target impedance of the composite material and the target reactance of the differential probe capacitor array; calculate the target capacitance value of the differential probe capacitor array.

[0150] ;

[0151] ;

[0152] ;

[0153] In the formula, This indicates the target impedance of the differential probe. Indicates the equivalent impedance of the composite material; Indicates the amplitude modulation coefficient; Indicates the target reactance of the differential probe; This indicates the target capacitance value of the differential probe.

[0154] S412: Based on the target capacitance value, output control commands to control the switching of each capacitor unit in the capacitor array, so that the capacitor array is closest to the target capacitance value.

[0155] The method for obtaining the PWM duty cycle is as follows:

[0156] Based on amplitude modulation coefficient Calculate the target current and target PWM duty cycle;

[0157] ;

[0158] In the formula, The target current at the current moment; It is the preset probe safety operating reference drive current amplitude of the detection system.

[0159] ;

[0160] In the formula, It is the target duty cycle. This is the maximum allowable current of the detection system; It is the maximum allowed percentage of the preset value.

[0161] The method for obtaining the excitation phase is as follows:

[0162] The real-time phase compensation value and the preset reference phase are obtained, and then added together to obtain the phase setting value. The phase setting value is written into the phase register to realize the reconfiguration of the excitation phase of the excitation probe.

[0163] The phase setting value is obtained as follows:

[0164]

[0165] In the formula, Let be the phase compensation amount at time t. This is the reference phase.

[0166] Step S400: Probe calibration is performed based on the impedance matching coefficient. Probe calibration includes adjusting the PWM duty cycle, generating a power control signal to control the output current of the excitation probe, reconfiguring the excitation phase of the excitation probe by configuring the DDS phase register, and controlling the switching of the differential probe capacitor array based on the target capacitance value. The specific implementation process of this step is as follows:

[0167] Based on the target capacitance value, output control commands to control the switching of each capacitor unit in the capacitor array, so that the capacitor array is as close as possible to the target capacitance value.

[0168] Specifically, since the target capacitance value may not be an integer, the target capacitance value is set to an integer. After calculating the capacitance of each capacitor unit, the corresponding capacitor unit is controlled to work, so that the capacitor array can approach the total capacitance value.

[0169] In this step, the process of adjusting the PWM duty cycle to generate a power control signal to control the output current of the excitation probe is as follows:

[0170] Target duty cycle The comparison value is output to the detection system, which then generates a square wave signal with a new duty cycle as a power control signal.

[0171] In this step, the process of reconfiguring the excitation probe's excitation phase by configuring the DDS phase register is as follows:

[0172] The real-time phase compensation value and the preset reference phase are obtained, and then added together to obtain the phase setting value. The phase setting value is written into the phase register to realize the reconfiguration of the excitation phase of the excitation probe.

[0173] The specific implementation process of generating the multi-frequency composite signal output to the excitation coil of the excitation probe in step S500 is as follows:

[0174] Multiple signals at different frequencies are generated by a calibrated multi-channel DDS, and the signals are superimposed to obtain a multi-frequency composite signal. :

[0175]

[0176] In the formula, N represents the total number of frequencies in the composite signal. This represents the real-time amplitude modulation coefficient at the k-th frequency. Let t represent the k-th frequency and t represent time. This represents the reference phase at the k-th frequency. This represents the real-time phase compensation amount at the k-th frequency.

[0177] The composite signal is converted by a digital-to-analog converter, amplified by a power amplifier, and then drives the excitation coil to finally generate the electromagnetic field for electromagnetic detection.

[0178] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for the generation of multi-frequency electromagnetic inspection signals for composite material defect detection, characterized by, The implementation comprises the following steps: Based on the layered structure of the composite material, the response voltage of the composite material standard sample without defects is collected under different excitation frequencies, different structural layers, fiber direction x, perpendicular fiber direction y and thickness direction z, and the response voltage is associated with the structural layer, excitation frequency and direction mapping; The response voltage is normalized, and the impedance characteristic curve of the composite material is calculated through Ohm's law; The optimal frequency excitation point is found from the impedance characteristic curve, and the impedance matching coefficient is calculated therefrom, which includes the PWM duty cycle, excitation phase and capacitance array capacitance value; The impedance matching coefficient satisfying the target is used for probe calibration, which comprises adjusting the PWM duty cycle, generating a power control signal to control the output current of the excitation probe, reconfiguring the excitation phase of the excitation probe by configuring the phase register of the direct digital synthesizer DDS, calculating the target capacitance array capacitance value to control the differential probe capacitance array switching; A multi-frequency composite signal is generated based on the direct digital synthesizer DDS and output to the excitation coil of the excitation probe; The implementation process of the normalized response voltage and the impedance characteristic curve of the composite material calculated through Ohm's law is as follows: The response voltage of each layer of the composite material is detected, and a layer response matrix is constructed; Based on the anisotropic conductivity of the composite material and the structural layering of the composite material, a layer impedance matrix is constructed, and the theoretical impedance interval of each layer of the composite material in different directions and frequencies is obtained by analyzing the layer impedance matrix; The scaling coefficient matrix and the offset matrix are calculated based on the theoretical impedance interval, and the response voltage is obtained after the layer response matrix is normalized using the scaling coefficient matrix and the offset matrix. The current signal of the detection system is collected and the equivalent impedance is calculated using Ohm's law, thereby obtaining the impedance characteristic curve; The implementation process of finding the optimal frequency excitation point from the impedance characteristic curve and calculating the impedance matching coefficient therefrom is as follows: The impedance characteristic curve is layered based on the frequency band to obtain several sub-frequency bands; The optimal excitation point and the optimal phase reference are found in each sub-frequency band by an interpolation fitting method; The amplitude modulation coefficient and the phase compensation amount are calculated based on the optimal excitation point and the optimal phase reference; The impedance matching coefficient is calculated based on the amplitude modulation coefficient and the phase compensation amount; The calculation formula of the amplitude modulation coefficient is as follows: ； wherein, is an impedance module matching term, is a voltage preference term; is a module of the current moment probe impedance; is a module of the composite material impedance under the current i working frequency band; is a voltage value of the optimal excitation point that maximizes the energy transmission efficiency under the current i working frequency band; is a constant, which is a set reference excitation voltage value of the detection system; The calculation formula of the phase compensation amount is as follows: ； wherein: is a conjugate matching term; is a resonance optimization term; is the optimal impedance of the composite material under the current i frequency band is the conjugate of the optimal impedance of the composite material under the current i frequency band represents the complex impedance of the probe at the current time; is a high frequency correction factor under the current excitation frequency; is the optimal phase shift under the current i working frequency band; is a constant, representing the system reference phase; The high frequency correction factor is: ； wherein: and is a material-specific constant; represents the optimal frequency under the current i operating frequency band.

2. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The noise baseline is also calculated based on the response voltage, and the calculation method is as follows: In the absence of a standard sample, at frequency The following data is collected: voltage signals generated by the electromagnetic system itself and environmental electromagnetic interference. The noise baseline was obtained by performing a sliding filter averaging process on the response voltage collected when placing a defect-free composite material standard sample, and then weighted and superimposed. ； wherein and are weight coefficients, , denotes the frequency The response voltage of each structural layer in the fiber direction x, the perpendicular fiber direction y, and the thickness direction z is as follows.

3. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The method for obtaining the PWM duty cycle in the impedance matching coefficient as the calibration target is as follows: Based on amplitude modulation coefficient Carrying out target current, target PWM duty cycle calculation; ； In the formula, is the target current at the current time; is the probe safety working reference drive current amplitude preset by the detection system. ； In the formula, is the target duty cycle, is the maximum current allowed by the detection system; is the preset maximum allowed duty cycle.

4. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The method for obtaining the capacitance array capacitance value in the impedance matching coefficient as the calibration target is as follows: The target impedance of the composite material and the target reactance of the differential probe capacitance array are obtained to calculate the capacitance array target capacitance value: ； ； ； In the formula, represents the target impedance of the differential probe; represents the equivalent impedance of the composite material; represents the amplitude modulation coefficient; represents the target reactance of the differential probe; represents the target capacitance value of the differential probe.

5. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The method for obtaining the excitation phase in the impedance matching coefficient as the calibration target is as follows: The real-time phase compensation amount and the preset reference phase are obtained, and the sum thereof is the phase setting value, which is used as the excitation phase; The method for obtaining the phase setting value is as follows: ； In the formula, is the phase compensation amount at time t, is the reference phase.

6. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The implementation process of generating a multi-frequency composite signal based on the direct digital synthesizer DDS and outputting it to the excitation coil of the excitation probe is as follows: A plurality of signals at different frequencies are generated by a direct digital synthesizer DDS, and the signals are superimposed to obtain a multi-frequency composite signal : ； where N represents the total number of frequencies in the composite signal, A(k) represents the real-time amplitude modulation coefficient of the kth frequency, A(k) represents the real-time amplitude modulation coefficient of the kth frequency, φ(k) represents the reference phase of the kth frequency, φ(k) represents the real-time phase compensation amount of the kth frequency.

7. The method for multi-frequency electromagnetic inspection signal generation for composite material defect detection according to claim 1, characterized in that, The process of calibrating the probe with the impedance matching coefficient as the target is as follows: Taking the capacitance value of the capacitance array in the impedance matching coefficient as the target capacitance value, outputting a control instruction to control the switch of each capacitor unit in the capacitance array of the differential probe, so that the capacitance array is closest to the target capacitance value; Taking the PWM duty cycle in the impedance matching coefficient as the target duty cycle, and outputting the target duty cycle to the detection system as a comparison value, the detection system generates a square wave signal with a new duty cycle as a power control signal; Taking the excitation phase in the impedance matching coefficient as the target, and writing the phase setting value into the phase register to reconfigure the excitation phase of the excitation probe.

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