CN109580721B - Pulse eddy current detection method and detection device for conductivity of ferromagnetic metal material - Google Patents
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Abstract
The invention discloses a pulse eddy current detection method and a pulse eddy current detection device for the conductivity of a ferromagnetic metal material, and belongs to the technical field of electromagnetic nondestructive detection. On the basis of time domain analytic solution of induced voltage of a ferromagnetic flat plate pulse eddy current detection model, establishing a least square problem between an induced voltage time domain signal measured value and a theoretical calculated value by utilizing an induced voltage measurement curve to invert the conductivity and the magnetic permeability of a detected ferromagnetic component; obtaining a change curve of the conductivity inversion result under the excitation of the pulse currents with different amplitudes by gradually increasing the amplitude of the pulse excitation current; and finally, finding out the intersection point of the change curve of the conductivity inversion result and the ordinate axis by a curve fitting method, namely the conductivity of the detected ferromagnetic component. The method and the device provided by the invention can provide non-contact and nondestructive detection for the conductivity of the ferromagnetic metal material in the industry, weaken the influence of the nonlinear change of the permeability on the detection result of the conductivity and improve the precision and the reliability.
Description
Technical Field
The invention belongs to the technical field of electromagnetic nondestructive testing, and particularly relates to a pulse eddy current testing method and a pulse eddy current testing device for conductivity of a ferromagnetic metal material.
Background
The conductivity is an important parameter for characterizing the conductivity of the metal material, and is closely related to the material quality, microstructure and heat treatment state of the material. By detecting the conductivity of the ferromagnetic metal material, the raw material can be sorted and identified, the heat treatment states of annealing, quenching and the like are judged, the hardness and the mechanical property of the ferromagnetic component are evaluated, and the residual stress, the metallographic structure change and the like in the ferromagnetic component are detected. Therefore, the method has important application value in nondestructive detection of the conductivity of the ferromagnetic metal material.
For non-ferromagnetic metal materials, the conductivity of the materials can be reliably detected by a sinusoidal eddy current method, or the detection result is calibrated by a standard test block with known conductivity, or the absolute value of the conductivity is directly calculated by an analytical formula inversion. The method for detecting the conductivity of the non-ferromagnetic metal material by the sine eddy current method is mature, and has the advantages of realizing non-contact nondestructive detection without preprocessing a non-conductive coating on the surface of a component. However, ferromagnetic metal materials have strong magnetic permeability, and the magnetic permeability changes with remanence, excitation current amplitude and frequency, so that the sinusoidal eddy current method cannot accurately detect the electrical conductivity of ferromagnetic metal materials.
The direct current four-probe method is an important method for measuring the conductivity of materials with weak conductivity, such as soil, semiconductors and the like at present. The direct-current four-probe method is used for detecting the conductivity of the ferromagnetic metal material, and has the advantage that the conductivity detection result is not influenced by the magnetic conductivity of the material. The limitation is that the probe must have good electrical contact with the surface of the detected member, so the non-conductive coating, paint, oxide layer, corrosion layer, etc. on the surface of the detected member must be removed before detection. In addition, the ferromagnetic metal material is a good conductor, and a large current excitation is needed to generate a large enough voltage signal, so that the error is large when the conductivity of the ferromagnetic metal material is detected by a four-probe method due to the thermal effect of the excitation current and the measurement error of the weak voltage signal, and the repeatability of the detection result is poor.
The pulse eddy current method is an electromagnetic nondestructive testing method which can carry out detection on ferromagnetic metal materials. The method comprises the steps of exciting a pulse magnetic field outside a detected component by using pulse current excitation instead of sinusoidal current excitation, inducing pulse eddy currents in the detected component, and detecting the geometric dimension and electromagnetic parameters of the detected component by detecting the attenuation process of the pulse eddy current electromagnetic field.
Disclosure of Invention
The invention aims to provide a pulse eddy current detection method and a pulse eddy current detection device for the conductivity of a ferromagnetic metal material. On the basis of time domain analytic solution of induced voltage of a ferromagnetic flat plate pulse eddy current detection model, establishing a least square problem between an induced voltage time domain signal measured value and a theoretical calculated value by utilizing an induced voltage measurement curve to invert the conductivity and the magnetic permeability of a detected ferromagnetic component; obtaining a change curve of the conductivity inversion result under the excitation of the pulse currents with different amplitudes by gradually increasing the amplitude of the pulse excitation current; and finally, finding out the intersection point of the change curve of the conductivity inversion result and the ordinate axis by a curve fitting method, namely the conductivity of the detected ferromagnetic component.
The invention firstly provides a pulse eddy current detection method for the conductivity of a ferromagnetic metal material, which comprises the following specific steps:
step one, acquiring a pulse eddy current detection signal of the ferromagnetic component to be detected.
And step two, an inversion method of electromagnetic parameters of the ferromagnetic component to be detected.
And step three, eliminating the influence of the change of the magnetic conductivity on the detection result of the electric conductivity by utilizing curve fitting.
The method for detecting the conductivity of the ferromagnetic metal material by the pulse eddy current has the advantages that:
(1) the method can provide a non-contact and nondestructive detection method for the conductivity of the ferromagnetic metal material in the industry, and fills the technical blank. The hardness, the heat treatment state, the residual stress and the like of the material can be indirectly judged, the processing technology level of the material is favorably improved, and a novel nondestructive testing means is provided for the links of production, maintenance, monitoring and the like of the ferromagnetic component at the key part.
(2) The decoupling between the conductivity and the magnetic conductivity is realized, and the detection of the conductivity of the ferromagnetic metal material by the pulse eddy current method is possible. Theoretically, it can be proved that when the eddy current signal cannot penetrate through the ferromagnetic member to be detected, the conductivity and the magnetic conductivity are coupled together, and the eddy current detection signal can only be used for accurately inverting the ratio of the conductivity to the magnetic conductivity, so that the conductivity cannot be detected. In the invention, an eddy current signal can effectively penetrate through the ferromagnetic component to be detected by using a pulse eddy current method, and the product of the conductivity and the wall thickness of the ferromagnetic component to be detected and the product of the relative permeability and the wall thickness can be accurately inverted by using a time domain induced voltage signal, so that the decoupling of the conductivity and the permeability is realized, and the detection of the conductivity of the ferromagnetic metal material by using the pulse eddy current method is possible.
(3) The influence of the nonlinear change of the magnetic conductivity on the detection result of the electric conductivity is weakened, and the precision and the reliability of the detection result of the electric conductivity are improved. For ferromagnetic metal materials, the magnetic permeability of the material is difficult to directly measure, and the magnitude of the magnetic permeability is influenced by factors such as the microstructure, the remanence, the pulse excitation magnetic field strength and the like of the ferromagnetic material to be detected, so that the detection result of the electric conductivity can be influenced. On one hand, the magnetic permeability of the ferromagnetic component to be detected is set as an unknown parameter, and the value of the magnetic permeability detected each time is determined by inversion through the signal processing method provided by the invention; on the other hand, the corresponding conductivity inversion result when the external excitation magnetic field is zero is found as the detection result by performing curve fitting on the conductivity inversion result under the excitation of the pulse current with different amplitudes, so that the influence of the nonlinear change of the magnetic conductivity on the detection result is weakened, and the accuracy and the reliability of conductivity detection are improved.
Drawings
FIG. 1 is a structural diagram of a pulsed eddy current electromagnetic nondestructive testing device for conductivity of ferromagnetic metal materials.
Fig. 1A is a sectional structural view of a coil probe.
FIG. 2 shows the inversion results of the conductivity under different amplitude pulse current excitation and the fitting curve thereof.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention provides a pulse eddy current detection method and a detection device for conductivity of a ferromagnetic metal material, wherein the detection device comprises a coil probe, a host, a DA (digital-to-analog) converter, a power amplification circuit, a sampling resistor, a first AD (analog-to-digital) converter and a second AD (analog-to-digital) converter as shown in figure 1, the coil probe comprises a coil framework, an excitation coil and a detection coil, the coil probe (1 is the coil framework, 2 is the excitation coil and 3 is the detection coil) is arranged above a ferromagnetic component to be detected as shown in figure 1A, and the distance between the lower edge of the coil probe and the upper surface of the ferromagnetic component to be detected (called the lifting distance of the coil probe) is recorded asoAnd the wall thickness of the ferromagnetic component to be detected below the coil probe is recorded as d. The coil framework adopts a hollow cylindrical coil framework, an excitation coil and a detection coil are sequentially wound on the coil framework, and the positions of the excitation coil and the detection coil can be interchanged. The output end of the host is connected with the input end of the DA digital-to-analog converter, the output end of the DA digital-to-analog converter is connected with the input end of the power amplifying circuit, the output end of the power amplifying circuit is connected with the input end of the sampling resistor, the second output end of the sampling resistor is connected with the input end of the second AD analog-to-digital converter, the output end of the second AD analog-to-digital converter is connected with the second input end of the host, the first output end of the sampling resistor is connected with the two ends of the exciting coil, the two ends of the detecting coil are connected with the input end of the first AD analogThe host computer outputs an excitation digital signal to the DA digital-to-analog converter, the DA digital-to-analog converter converts the received excitation digital signal into an excitation analog signal and transmits the excitation analog signal to the power amplifying circuit, the power amplifying circuit amplifies the power of the received excitation analog signal and outputs pulse excitation current to the sampling resistor, the sampling resistor receives the pulse excitation current and supplies the pulse excitation current to the excitation coil, the detection coil transmits an induction voltage analog signal to the first AD analog-to-digital converter, and the first AD analog-to-digital converter converts the received induction voltage analog signal into an induction voltage discrete digital signal and transmits the induction voltage discrete digital signal to the host computer for result.
The invention provides a pulse eddy current detection method for conductivity of a ferromagnetic metal material, which comprises the following steps:
the method comprises the following steps: acquisition of pulsed eddy current detection Signal (SAP) of the ferromagnetic component under examination:
in the first step of the invention, the acquisition step of the pulse eddy current detection signal is as follows:
step SAP-1, vertically placing a coil probe (as shown in FIG. 1A, 1 is a coil frame, 2 is an excitation coil, and 3 is a detection coil) above the ferromagnetic component to be detected, and keeping the coil probe lifted away by a distance lo(0-20mm) unchanged as shown in FIG. 1;
step SAP-2, winding exciting coil and detecting coil on the coil skeleton, the both ends of the exciting coil connect the first output end of the sampling resistor, the input end of the sampling resistor connects the output end of the power amplifier circuit, the both ends of the detecting coil connect the input end of the first AD analog-to-digital converter, the second output end of the current sampling resistor connects the input end of the second AD analog-to-digital converter;
SAP-3, programming and outputting an excitation digital signal with a continuous pulse width of 10-500 ms and an amplitude of 0.1-1V by using a host; after passing through a DA digital-to-analog converter, the signal is converted into an excitation analog signal with the continuous pulse width of 10-500 ms and the amplitude of 0.1-1V, and the excitation analog signal is output to a power amplifying circuit; after power is amplified by the power amplifying circuit, pulse excitation current with continuous pulse width of 10-500 ms and amplitude of 0.1-5A is output, the pulse excitation current flows through the sampling resistor and is supplied to the excitation coil, I0Is the pulse excitation current amplitude;
step SAP-4, meanwhile, the first AD analog-to-digital converter collects the induced voltage time domain signals u (t) (unit V) at the two ends of the detection coil, and stores the collected induced voltage signals u (t) into the host; and a second AD analog-to-digital converter is used for collecting pulse excitation current signals i (t) (unit A) at two ends of the sampling resistor and storing the pulse excitation current signals in the host.
In the invention, the collection of the induced voltage time domain Signal and the excitation current Signal of the ferromagnetic component to be detected by using the pulse eddy current detection device is called as a pulse eddy current detection Signal collection step, Signal Acquisition process, SAP.
Step two: inversion method (PIP) of electromagnetic parameters of ferromagnetic component to be detected:
after the induced voltage at the two ends of the detection coil is acquired according to the SAP step, the key of the signal processing is how to determine the conductivity and the relative permeability of the detected ferromagnetic component by the induced voltage. The authors have said to be equal to 2014, and a paper "Excitation current waveform for edge current testing on the ferromagnetic of magnetic plates" published in volume 66 of NDT & E International, and a ferromagnetic plate pulse eddy current testing model is provided, and when a pulse Excitation current i (t) is introduced into an Excitation coil, a time domain expression of an induced voltage at two ends of a detection coil is:
pi is 3.14;
e is the base of the natural logarithm, and the value is 2.72;
sigma is the conductivity of the ferromagnetic component to be detected, and the unit is S/m;
d is the wall thickness of the ferromagnetic component to be detected, and the unit is m;
μ0the value is 4 pi x 10 for vacuum magnetic permeability-7H/m；
μrIs the relative permeability of the ferromagnetic member to be inspected;
b is a truncation boundary of the ferromagnetic flat plate pulse eddy current detection model, and the unit is m;
λirepresenting a first order Bessel function J1(bλi) The ith positive root of 0;
J0(x) Representing a first class of 0 th order Bessel functions;
ξkis the transcendental equation
The kth positive root;
coefficient of performance
Time constant
t represents time;
i (t) is a pulse excitation current signal collected by a sampling resistor, and the unit is A;
i' (t) represents the derivative of the pulsed excitation current signal over time t;
"+" denotes convolution operation
CdTo excite the coil coefficient, CpFor detecting the coil coefficient of the coil, the calculation formula can refer to Chenxingle equal to 2014, published in NDT&"circulation current fashion for the implementation of current testing on the same physical tables" on volume 66 of E International ", volume 66.
Based on the time domain analytic solution of the induced voltage of the ferromagnetic flat plate pulse eddy current detection model, the invention establishes the least square problem between the measured value of the induced voltage time domain signal and the theoretical calculated value, and inverts the conductivity and the magnetic conductivity at the detection point, and the specific steps are as follows:
step PIP-1, lifting the coil probe away fromoThe wall thickness d of the ferromagnetic component to be detected and the pulse excitation current signal i (t) are substituted as known quantities in formula (1), and the electrical conductivity sigma of the ferromagnetic component to be detected is addedRelative permeability mu of ferromagnetic member to be inspectedrSet as unknown parameters, i.e. the parameter vector to be inverted x ═ x1,x2)T＝(σ,μr)T；
Step PIP-2, according to the SAP step, the induction voltage time domain signal measurement data at two ends of the detection coil acquired by the first AD analog-to-digital converter is (t)1,u1),(t2,u2),…,(tm,um) The theoretical calculation value u (x, t) of the induced voltage calculated by the formula (1) is added to the calculated valuei) Comparing the measured value u of the time domain signal of the induced voltageiAnd inverting the parameter x by the minimum sum of squared errors from the theoretical calculated value, namely establishing a least square problem:
residual function ri(x)＝ui-u(x,ti) I is 1,2, …, m, and the residual function vector r (x) is recorded as (r)1(x),r2(x),...,rm(x))T，R2Is a two-dimensional real number space, m is the number of measurements, x1And x2Representing two elements in the vector x.
Step PIP-3, in the host computer, using iterative algorithm, solving the optimal solution x of least square problem (2)*The iterative algorithm comprises the following calculation steps:
(1) given initial point
(where σ(1)＝106～107S/m，
Tolerance > 0 (typically 10)-3) Setting k as 1;
(2) the parameter vector of the k step
Substituted into the formula (1) to calculate the secondEach time point t of k stepsiTheoretical calculation value u (x) of induced voltage of(k),ti) Then is compared with the measured value u of the induced voltage time domain signaliMaking difference to calculate residual function value of k step
ri(x(k))＝ui-u(x(k),ti),i＝1,2,…,m，
And obtaining the residual function vector r of the k step(k)(ii) a Then, a first-order partial derivative of the induction voltage theoretical curve to the conductivity sigma of the ferromagnetic component to be detected is further calculated by the formula (1)
And the relative permeability mu of the induced voltage theoretical curve to the ferromagnetic component to be detectedrFirst partial derivative of
Obtain a matrix A of m × 2k＝(aij)m×2；j＝1,2。
(3) Solving a system of equations
Obtaining a direction vector b;
(4) from the parameter vector x of the k step(k)Starting from a direction vector b in the k-th step(k)One-dimensional search is carried out to obtain step length alphakSo that
I.e. a function f (x) with alpha as argument(k)+αb(k)) When the minimum value is taken, the value of the independent variable alpha is alphak。
And order
x(k+1)＝x(k)+αkb(k)；
(5) If | | | x(k+1)-x(k)If | | is less than or equal to the maximum value, stopping the calculation to obtain the optimal solution x of the least square problem (2)*＝x(k+1)(ii) a Otherwise, k is set to k +1, and the step (2) is returned.
Step PIP-4, solving the optimal solution of the least square problem (2) by the iterative algorithm
Then, the inversion result sigma of the electrical conductivity of the detected ferromagnetic component and the inversion result of the relative magnetic permeability of the detected ferromagnetic component are obtained
The inversion result of the conductivity of the detected ferromagnetic component, the inversion result of the relative permeability of the detected ferromagnetic component and the pulse current excitation amplitude I0And correspondingly storing the data in the host.
In the invention, a host computer analyzes and processes the induced voltage detection signal, and inverts the process of the conductivity of the detected ferromagnetic component and the relative permeability of the detected ferromagnetic component, which is called a parameter Inversion step, Parameters Inversion Procedure, PIP.
Step three: and (3) eliminating the influence of the change of the magnetic conductivity on the detection result of the electric conductivity by utilizing curve fitting:
for ferromagnetic metal materials, the magnetic permeability of the material is difficult to directly measure, and the magnitude of the magnetic permeability is influenced by factors such as microstructure, remanence, pulse excitation magnetic field strength and the like of the ferromagnetic material to be detected, so that the magnetic permeability of the ferromagnetic component to be detected may change every time of detection, thereby influencing the detection result of the electric conductivity. On one hand, the magnetic permeability of the ferromagnetic component to be detected is set as an unknown parameter, and the value of the magnetic permeability is determined by inversion through the signal processing method provided by the invention; on the other hand, the inversion result of the conductivity of the detected ferromagnetic component under the excitation of the pulse currents with different amplitudes is subjected to curve fitting, and the inversion result of the conductivity of the detected ferromagnetic component corresponding to the situation that the external excitation magnetic field is zero is found as the detection result, so that the influence of the nonlinear change of the magnetic permeability on the detection result is weakened. The specific implementation steps are as follows:
step 1: measuring the wall thickness d of the ferromagnetic component to be detected by a vernier caliper or an ultrasonic thickness gauge, vertically placing the coil probe above the ferromagnetic component to be detected, and measuring the lifting distance l of the coil probe by a ruler or a vernier calipero；
Step 2: lift-off distance l of fixed coil probeoThe amplitude I of the pulse excitation current is unchanged0Gradually increasing from 0.2A to 3.0A in steps of 0.2A, respectively acquiring induction voltage time domain signals at two ends of the detection coil and pulse excitation current signals at two ends of the sampling resistor corresponding to different pulse excitation current amplitudes according to the SAP (System on chip) step, and storing the signals in a host;
and step 3: respectively inverting the conductivity of the detected ferromagnetic member corresponding to the pulse excitation current with different amplitudes and the relative permeability of the detected ferromagnetic member according to the PIP step, and drawing the inversion result of the conductivity of the detected ferromagnetic member along with the amplitude I of the pulse excitation current0As shown by the solid dots in fig. 2;
and 4, step 4: using exponential functions
Performing curve fitting on the inversion result σ of the electrical conductivity of the detected ferromagnetic component in fig. 2, and obtaining the coefficient σ in the exponential function by using the electrical conductivity fitting curve as shown by the solid line in fig. 20And fitting results
And
wherein
Namely the conductivity detection result of the ferromagnetic component to be detected.
Example 1
An example of the conductivity measurement of a ferromagnetic component using the method of the present invention is given below.
The ferromagnetic component to be detected is three cylindrical steel test blocks with the diameter of 100mm and the thickness of 6mm, and the materials of the ferromagnetic component to be detected are 20# steel, 45# steel and Q235 steel respectively.
Sequentially placing the coil probe above the center positions of the three steel test blocks to be detected, keeping the lifting distance of the coil probe unchanged at 5mm, and keeping the pulse excitation current amplitude I0The method comprises the steps of taking 0.2A as a step, gradually increasing from 0.2A to 3.0A, firstly obtaining a pulse eddy current detection signal according to the SAP step in the invention, then establishing a least square problem by using the detection signal according to the PIP step, obtaining inversion results of the conductivity and the relative permeability of a detected ferromagnetic component under the excitation of pulse currents with different amplitudes, and drawing a conductivity inversion result change curve shown in figure 2. And fitting the change curve of the inversion result of the conductivity by using an exponential function to obtain the conductivity of the tested steel test block. 10 groups of data are measured on the conductivity of each test block according to the steps of the invention, and the conductivity detection results of the test blocks of different materials are shown in Table 1. And calculating the average value and the standard deviation of the conductivity detection result of each test block to be 5.09 +/-0.02 MS/m, 6.98 +/-0.04 MS/m and 6.95 +/-0.03 MS/m respectively.
The conductivity of three steel test blocks is respectively detected by a four-probe method, and the conductivity is respectively 5.16 +/-0.05 MS/m, 7.03 +/-0.11 MS/m and 6.86 +/-0.08 MS/m. Therefore, the detection result of the conductivity of the ferromagnetic metal material by using the pulse eddy current conductivity detection method is basically consistent with the detection result of the conductivity of the ferromagnetic metal material by using the four-probe method, and the deviation between the detection result and the conductivity of the ferromagnetic metal material is 0.09MS/m at most. The feasibility and the reliability of detecting the conductivity of the ferromagnetic metal material by the method are verified.
Table 1 unit of detection results of different material test blocks by the conductivity pulse eddy current method: MS/m
| Test block material | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Mean value | Standard deviation of |
| 20# Steel | 5.11 | 5.09 | 5.11 | 5.08 | 5.09 | 5.03 | 5.07 | 5.08 | 5.10 | 5.11 | 5.09 | 0.02 |
| 45# Steel | 7.00 | 6.95 | 6.98 | 6.96 | 6.97 | 6.94 | 6.98 | 6.97 | 6.98 | 7.08 | 6.98 | 0.04 |
| Q235 steel | 6.92 | 6.96 | 6.95 | 6.93 | 6.89 | 6.97 | 6.96 | 6.96 | 7.00 | 6.97 | 6.95 | 0.03 |
Claims (1)
1. A pulse eddy current detection device for the conductivity of a ferromagnetic metal material comprises a coil probe, a host and a sampling resistor;
the coil probe comprises a coil framework, an excitation coil and a detection coil;
placing a coil probe on a ferromagnetic member to be inspectedThe distance between the lower edge of the coil probe and the upper surface of the ferromagnetic component to be detected is recorded as lo，lo0-20 mm; the wall thickness of the ferromagnetic component to be detected below the coil probe is recorded as d, and an excitation coil and a detection coil are wound on the coil framework;
the digital-to-analog converter is characterized by further comprising a DA analog-to-digital converter, a power amplifying circuit, a first AD analog-to-digital converter and a second AD analog-to-digital converter;
the output end of the host is connected with the input end of the DA digital-to-analog converter, the output end of the DA digital-to-analog converter is connected with the input end of the power amplifying circuit, the output end of the power amplifying circuit is connected with the input end of the sampling resistor, the second output end of the sampling resistor is connected with the input end of the second AD analog-to-digital converter, the output end of the second AD analog-to-digital converter is connected with the second input end of the host, the first output end of the sampling resistor is connected with two ends of the exciting coil, two ends of the detecting coil are connected with the input end of the first AD analog-;
the system comprises a host, a DA digital-to-analog converter, a power amplifying circuit, a sampling resistor, an exciting coil, a detection coil, a first AD analog-to-digital converter, a second AD analog-to-digital converter and a second AD analog-to-digital converter, wherein the host outputs an excitation digital signal to the DA digital-to-analog converter, the DA digital-to-analog converter converts the received excitation digital signal into an excitation analog signal and transmits the excitation analog signal to the power amplifying circuit, the power amplifying circuit amplifies the power of the received excitation analog signal and outputs pulse excitation current to the sampling resistor, the sampling resistor receives the pulse excitation current and supplies the pulse excitation current to the;
programming and outputting an excitation digital signal with a continuous pulse width of 10-500 ms and an amplitude of 0.1-1V by using a host; after passing through a DA digital-to-analog converter, the signal is converted into an excitation analog signal with the continuous pulse width of 10-500 ms and the amplitude of 0.1-1V, and the excitation analog signal is output to a power amplifying circuit; after power is amplified by the power amplifying circuit, pulse excitation current with continuous pulse width of 10-500 ms and amplitude of 0.1-5A is output, the pulse excitation current flows through the sampling resistor and is supplied to the excitation coil, I0Is the pulse excitation current amplitude;
the first AD analog-to-digital converter collects the induced voltage time domain signals u (t) at two ends of the detection coil and stores the collected induced voltage signals u (t) into the host;
the second AD analog-to-digital converter collects pulse excitation current signals i (t) at two ends of the sampling resistor and stores the pulse excitation current signals in the host;
utilizing exponential functions in a host
Inversion result sigma of electrical conductivity of detected ferromagnetic component*Performing curve fitting to obtain coefficient sigma in exponential function0And fitting results
And
wherein
Namely the conductivity detection result of the ferromagnetic component to be detected.
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