CN109115868B - Defect depth detection device and method based on pulse eddy current - Google Patents

Defect depth detection device and method based on pulse eddy current Download PDF

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CN109115868B
CN109115868B CN201811147005.1A CN201811147005A CN109115868B CN 109115868 B CN109115868 B CN 109115868B CN 201811147005 A CN201811147005 A CN 201811147005A CN 109115868 B CN109115868 B CN 109115868B
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module
coil
eddy current
voltage
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CN109115868A (en
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冯健
李铨
汪刚
刘金海
马大中
张化光
卢森骧
吴振宁
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Northeastern University China
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
    • G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
    • G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
    • G01N27/9046—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents by analysing electrical signals
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
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Abstract

The invention provides a defect depth detection device and method based on pulse eddy current, and relates to the technical field of nondestructive testing. The specific method comprises the following steps: the excitation signal generator generates periodic pulse signals, and the periodic pulse signals are added to two ends of the excitation coil after passing through the power amplification module; the detection coil receives a magnetic field signal above the test piece, converts the magnetic field signal into an analog voltage signal and outputs the analog voltage signal to the signal conditioning module; the signal conditioning module filters and amplifies the analog voltage signal and outputs the analog voltage signal to the A/D conversion module; the A/D conversion module performs analog/digital conversion of signals under the control of the acquisition triggering module, the converted digital signals are sent to the characteristic parameter identification module to be identified to obtain characteristic parameters, and then the characteristic parameters are sent to the defect depth detection module based on random forest to detect the depth information of the defects on the test piece. The physical model of the pulse eddy current detection system established by the device takes the influence of the induced eddy current on the test piece on the characteristic parameters into consideration, so that the modeling precision is improved, and the detection error of the defect depth is reduced.

Description

Defect depth detection device and method based on pulse eddy current
Technical Field
The invention relates to the technical field of nondestructive testing, in particular to a defect depth detection device and method based on pulse eddy current.
Background
Along with the gradual lengthening of oil and gas pipelines and the gradual increase of transportation volume at home and abroad, the high-efficiency and safe transportation of the oil and gas pipelines is widely regarded. Ferromagnetic oil and gas pipelines can suffer from various defects due to long-term corrosion, wear, and accidental mechanical damage. In order to prevent the leakage accident, it is necessary to detect the pipeline by using a pipeline detecting device. The eddy current detection technology is a unique and low-cost high-speed large-scale detection technology and has unique advantages; compared with an ultrasonic method and an ray method, the method does not need a coupling agent, can perform non-contact measurement, and has higher detection speed; compared with the magnetic powder method, the method is effective to both magnetic and non-magnetic materials and does not pollute the environment; compared with the penetration method, the method does not need to clean the test piece, and can realize the detection automation. At present, pulse eddy current detection probes and detection methods aiming at oil and gas pipeline defects are less in research, and related equipment is expensive and low in detection precision.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a defect depth detection device and method based on a pulse eddy current to reduce the detection error of the defect depth.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
in one aspect, the present invention provides a defect depth detection apparatus based on pulsed eddy current, including: the device comprises an excitation signal generator, a power amplification module, an excitation coil, a detection coil, a signal conditioning module, an A/D conversion module and a DSP central processing unit; the excitation signal generator is connected with the input end of the power amplification module, the output end of the power amplification module is connected with two ends of the excitation coil, the detection coil is coaxially arranged with the excitation coil, the input end of the signal conditioning module is connected with the output end of the detection coil, the output end of the signal conditioning module is connected with the input end of the A/D conversion module, and the output end of the A/D conversion module is connected with the input end of the DSP central processing unit;
the excitation signal generator is used for generating a periodic pulse signal and transmitting the periodic pulse signal to the power amplification module;
the power amplification module is used for amplifying voltage and current of a periodic pulse signal generated by the excitation signal generator and then adding the periodic pulse signal to two ends of the excitation coil;
the exciting coil is used for introducing the amplified periodic pulse signal to generate an alternating magnetic field;
the detection coil is used for detecting a magnetic field signal above the tested piece, converting the magnetic field signal into a voltage signal and outputting the voltage signal to the signal conditioning unit;
the signal conditioning module is used for filtering and amplifying a voltage signal detected by the detection coil and outputting the voltage signal to the A/D conversion unit;
the A/D conversion module is used for converting the voltage signal output by the signal conditioning module into a digital voltage signal and outputting the converted digital signal to the DSP data processing module;
the DSP central processing unit comprises an acquisition triggering module, a characteristic parameter identification module and a defect depth detection module based on a random forest; the acquisition triggering module is used for controlling the A/D conversion module to perform digital/analog conversion of voltage; the characteristic parameter identification module is used for extracting a digital voltage signal output by the A/D conversion module after being controlled by the acquisition triggering module, identifying characteristic parameters of the signal through a constructed physical model of the pulse eddy current detection system, and outputting the identified characteristic parameters to the defect depth detection module based on the random forest; the defect depth detection module based on the random forest is used for taking the characteristic parameters output by the characteristic parameter identification module as input and outputting the depth information of the detected test piece defects;
in another aspect, the present invention provides a defect depth detection method based on pulsed eddy current, which is implemented by a defect depth detection device based on pulsed eddy current, and includes the following steps:
step 1: collecting a magnetic field signal above a tested piece by using a detection coil, converting the magnetic field signal into a voltage signal, filtering and amplifying the voltage signal, and further converting the voltage signal into a digital voltage signal;
step 2: in the characteristic parameter identification module, constructing a physical model of the pulse eddy current detection system, establishing a complex frequency domain response function of a voltage response signal on a detection coil, and obtaining a detection coil voltage time domain equation after inverse Laplace transform; then, taking the digital voltage signal in the step 1 as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method; the physical model of the constructed pulse eddy current detection system has 4 characteristic parameters, and the defect depth information of a tested piece is reflected through the characteristic parameters;
and step 3: in a defect depth detection module based on a random forest, characteristic parameters are used as input, and depth information for detecting the defects of a tested piece is obtained by using a defect depth detection method based on the random forest.
The step 2 comprises the following steps:
step 2.1: constructing a physical model of the pulsed eddy current detection system;
step 2.1.1: solving a change equation of the current on the exciting coil; pulse signals are applied to two ends of the exciting coil, and the signal voltage changes from 0 to the amplitude U of the pulse signalssThen, the current I on the exciting coil is obtainedL1The change equation of (a) is:
Figure BDA0001817023870000021
wherein R isL1Is the equivalent resistance of the exciting coil, r is the internal resistance of the exciting source,τeis the current decay time constant, t is the time, e is the natural constant;
step 2.1.2: calculating the induced electromotive force on the detection coil; the detection coil detects a magnetic field signal above the tested piece and converts the magnetic field signal into a voltage signal to obtain the induced electromotive force on the detection coil:
Figure BDA0001817023870000022
wherein N is1、N2The number of turns of the exciting coil and the detecting coil are respectively, and k is the ratio of the single-turn magnetic flux of the detecting coil to the single-turn magnetic flux of the exciting coil;
step 2.1.3: solving a voltage frequency domain response function at two ends of the detection coil; a loop consisting of the induced electromotive force epsilon, the inductance, the resistance and the parasitic capacitance of the detection coil is used for obtaining a voltage complex frequency domain response function U at the two ends of the detection coil0(s) is:
Figure BDA0001817023870000031
wherein
Figure BDA0001817023870000032
R2e、L2e、C2eRespectively representing the equivalent resistance, the inductance and the parasitic capacitance of the detection coil, wherein s represents the independent variable of a complex frequency domain;
step 2.1.4: solving a physical model of the pulse eddy current detection system; and performing inverse Laplace transform on the obtained frequency domain response function to obtain a detection coil voltage time domain equation:
Figure BDA0001817023870000033
order to
Figure BDA0001817023870000034
Obtaining:
Figure BDA0001817023870000035
Uo(t) is the physical model of the pulse eddy current detection system obtained, B, C, D, taueCharacteristic parameters of the model;
step 2.2: taking a digital voltage signal of the A/D conversion module as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method;
step 2.2.1: to be inputted digital voltage signal UDiConverted into a voltage value Ui；
Figure BDA0001817023870000036
Where i is T,2T, …, nT, T is the sampling period of the a/D conversion module, UDiN digital voltages collected at the same position and different time on the tested piece, N is the bit number of the A/D conversion module, VrefIs the reference voltage of the A/D conversion module;
step 2.2.2: determining a weight coefficient of a variable-weight multi-parameter fitting method; calculating UiMaximum value ofUmaxWhen the voltage value U is taken into consideration for the rapidity of the decay of the pulse eddy currentiGreater than 0.368UmaxThe characteristics of the eddy current can be embodied to obtain the weight coefficient wiThe expression of (a) is as follows:
Figure BDA0001817023870000041
wherein n is1For all voltage values to satisfy Ui＞0.368UmaxThe number of (2);
step 2.2.3: physical model U using pulsed eddy current inspection systemo(t) calculating Uo(i) The value of (i) ═ T,2To(i) And voltage value UiMaking difference values one by one, and calculating the difference value Ui-Uo(i) Is multiplied by the corresponding weight coefficient wiAnd summing again to obtain an error function E:
Figure BDA0001817023870000042
step 2.2.4: error function E pairs B, C, D and tau respectivelyeCalculating partial derivative to obtain
Figure BDA0001817023870000043
And solving the system of equations:
Figure BDA0001817023870000044
obtaining characteristic parameters B, C, D and tau of the physical modeleThe value of (c).
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: according to the defect depth detection device and method based on the pulse eddy current, the physical model of the pulse eddy current detection system is established, and the influence of the induced eddy current on the test piece on the characteristic parameters is considered, so that the modeling precision is improved, and the detection error of the defect depth is reduced.
Drawings
FIG. 1 is a block diagram of an overall structure of a defect depth detection device based on pulsed eddy current according to an embodiment of the present invention;
FIG. 2 is a diagram of an apparatus for testing a test piece, an excitation coil and a detection coil according to an embodiment of the present invention; wherein, 1, a tested piece; 2, exciting a coil; 3, detecting the coil;
fig. 3 is a voltage curve diagram of a detection coil according to an embodiment of the present invention;
fig. 4 is a voltage curve diagram of a portion a of a voltage curve diagram of a detection coil provided by an embodiment of the present invention;
FIG. 5 is a general flowchart of a method for detecting defect depth based on pulsed eddy current according to an embodiment of the present invention;
fig. 6 is a flowchart illustrating a working process of a feature parameter identification module according to an embodiment of the present invention.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The method of this example is as follows:
in one aspect, the present invention provides a defect depth detection apparatus based on pulsed eddy current, as shown in fig. 1 and 2, including: the device comprises an excitation signal generator, a power amplification module, an excitation coil, a detection coil, a signal conditioning module, an A/D conversion module and a DSP central processing unit; the excitation signal generator is connected with the input end of the power amplification module, the output end of the power amplification module is connected with two ends of the excitation coil, the detection coil is coaxially arranged with the excitation coil, the input end of the signal conditioning module is connected with the output end of the detection coil, the output end of the signal conditioning module is connected with the input end of the A/D conversion module, and the output end of the A/D conversion module is connected with the input end of the DSP central processing unit;
the excitation signal generator is used for generating a periodic pulse signal and transmitting the periodic pulse signal to the power amplification module;
the power amplification module is used for amplifying voltage and current of a periodic pulse signal generated by the excitation signal generator and then adding the periodic pulse signal to two ends of the excitation coil;
the exciting coil is used for introducing the amplified periodic pulse signal to generate an alternating magnetic field;
the detection coil is used for detecting a magnetic field signal above the tested piece, converting the magnetic field signal into a voltage signal and outputting the voltage signal to the signal conditioning unit;
the signal conditioning module is used for filtering and amplifying a voltage signal detected by the detection coil and outputting the voltage signal to the A/D conversion unit;
the A/D conversion module is used for converting the voltage signal output by the signal conditioning module into a digital voltage signal and outputting the converted digital signal to the DSP data processing module;
the DSP central processing unit comprises an acquisition triggering module, a characteristic parameter identification module and a defect depth detection module based on a random forest; the acquisition triggering module is used for controlling the A/D conversion module to perform digital/analog conversion of voltage; the characteristic parameter identification module is used for extracting a digital voltage signal output by the A/D conversion module after being controlled by the acquisition triggering module, identifying characteristic parameters of the signal through a constructed physical model of the pulse eddy current detection system, and outputting the identified characteristic parameters to the defect depth detection module based on the random forest; the defect depth detection module based on the random forest is used for taking the characteristic parameters output by the characteristic parameter identification module as input and outputting the depth information of the detected test piece defects;
wherein the excitation signal generator generates a periodic pulse signal with the frequency of 100Hz, the duty ratio of 50 percent and the amplitude of 200mv, and sends the periodic pulse signal into the power amplification module; the power amplification module selects a structural form that an operational amplifier TL071 is connected with a Darlington tube in series, amplifies the voltage of an input pulse signal, and a push-pull circuit of the Darlington tube amplifies the current and outputs a driving exciting coil (the parameter is that a resistor R is 12.95 omega, a turn number N is 500, and the wire diameter d isl0.21mm, inner diameter di15.6mm, outside diameter do20.8mm, 10mm for coil height h); detection coil (parameters are resistance R is 4.32 omega, turn number N is 400, and wire diameter dl0.21mm, inner diameter di3mm, outer diameter do＝85mm, coil height h ═ 10mm) of the magnetic field above the test piece, and converted into analog voltage signals, which are shown in fig. 3 and 4; then the signal is output to a signal conditioning module, and a conditioning circuit carries out filtering and unit amplification on the signal and finally outputs the signal to an A/D conversion module; the A/D conversion module adopts ADS7844 chip, its I/O port is connected with DSP collecting trigger module, TMS320F28335 used by DSP.
The ADS7844 converts the analog signals into digital signals under the control of the DSP acquisition trigger module and sends the digital signals to the DSP central processing unit. In the DSP central processing unit, the flow of the defect depth detection method based on the pulse eddy current is shown in FIG. 5; and the converted digital signals are identified by a characteristic parameter identification module to obtain characteristic parameters, and then the characteristic parameters are sent to a defect depth detection module based on random forests, and the depth information of the defects of the test piece is obtained by using a defect depth detection method based on random forests.
In another aspect, the present invention provides a defect depth detection method based on pulsed eddy current, which is implemented by a defect depth detection device based on pulsed eddy current, and includes the following steps:
step 1: collecting a magnetic field signal above a tested piece by using a detection coil, converting the magnetic field signal into a voltage signal, filtering and amplifying the voltage signal, and further converting the voltage signal into a digital voltage signal;
step 2: in the characteristic parameter identification module, constructing a physical model of the pulse eddy current detection system, establishing a complex frequency domain response function of a voltage response signal on a detection coil, and obtaining a detection coil voltage time domain equation after inverse Laplace transform; then, taking the digital voltage signal in the step 1 as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method; the physical model of the constructed pulse eddy current detection system has 4 characteristic parameters and can reflect the defect depth information of a tested piece;
and step 3: in a defect depth detection module based on random forest, characteristic parameters are used as input, and depth information of the detected test piece defects is obtained by using a defect depth detection method based on random forest.
As shown in fig. 6, the step 2 includes the following steps:
step 2.1: constructing a physical model of the pulsed eddy current detection system;
step 2.1.1: solving a change equation of the current on the exciting coil; pulse signals are applied to two ends of the exciting coil, and the signal voltage changes from 0 to the amplitude U of the pulse signalssThen, the current I on the exciting coil is obtainedL1The change equation of (a) is:
Figure BDA0001817023870000061
wherein R isL1Is the equivalent resistance of the exciting coil, r is the internal resistance of the exciting source,τethe current decay time constant is a current decay time constant, and the value of the current decay time constant is related to the resistance and the inductance of the coil and the eddy current induced above the tested piece; t is time and e is a natural constant, about 2.71828.
Step 2.1.2: calculating the induced electromotive force on the detection coil; the detection coil detects a magnetic field signal above the tested piece and converts the magnetic field signal into a voltage signal to obtain the induced electromotive force on the detection coil:
Figure BDA0001817023870000071
wherein N is1、N2The number of turns of the exciting coil and the detecting coil are respectively, and k is the ratio of the single-turn magnetic flux of the detecting coil to the single-turn magnetic flux of the exciting coil;
step 2.1.3: solving a voltage frequency domain response function at two ends of the detection coil; a loop consisting of the induced electromotive force epsilon, the inductance, the resistance and the parasitic capacitance of the detection coil is used for obtaining a voltage complex frequency domain response function U at the two ends of the detection coil0(s) is:
Figure BDA0001817023870000072
wherein
Figure BDA0001817023870000073
R2e、L2e、C2eThe equivalent resistance, the inductance and the parasitic capacitance of the detection coil are respectively related to induced eddy current, and s represents independent variable of a complex frequency domain;
step 2.1.4: solving a physical model of the pulse eddy current detection system; and performing inverse Laplace transform on the obtained frequency domain response function to obtain a detection coil voltage time domain equation:
Figure BDA0001817023870000074
order to
Figure BDA0001817023870000075
Obtaining:
Figure BDA0001817023870000076
Uo(t) is the physical model of the pulse eddy current detection system obtained, B, C, D, taueCharacteristic parameters of the model;
step 2.2: taking a digital voltage signal of the A/D conversion module as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method;
step 2.2.1: to be inputted digital voltage signal UDiConverted into a voltage value Ui；
Figure BDA0001817023870000077
Where i is T,2T, …, nT, T is the sampling period of the a/D conversion module, UDiN digital voltages collected at the same position and different time on the tested piece, N is the bit number of the A/D conversion module, VrefIs the reference voltage of the A/D conversion module;
step 2.2.2: determining a weight coefficient of a variable-weight multi-parameter fitting method; calculating UiMaximum value of UmaxWhen the voltage value U is taken into consideration for the rapidity of the decay of the pulse eddy currentiGreater than 0.368UmaxThe eddy current characteristics can be better reflected, so the weight coefficient w is obtainediIs expressed as follows
Figure BDA0001817023870000081
Wherein n is1For all voltage values to satisfy Ui＞0.368UmaxThe number of (2);
step 2.2.3: physical model U using pulsed eddy current inspection systemo(t) calculating Uo(i) The value of (i) ═ T,2To(i) And voltage value UiMaking difference values one by one, and calculating the difference value Ui-Uo(i) Is multiplied by the corresponding weight coefficient wiAnd summing again to obtain an error function E:
Figure BDA0001817023870000082
step 2.2.4: error function E pairs B, C, D and tau respectivelyeCalculating partial derivative to obtain
Figure BDA0001817023870000083
And solving the system of equations:
Figure BDA0001817023870000084
obtaining characteristic parameters B, C, D and tau of the physical modeleThe value of (c).
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; such modifications and substitutions do not depart from the spirit of the corresponding technical solutions and scope of the present invention as defined in the appended claims.

Claims (1)

1. A defect depth detection method based on pulse eddy current is realized by a defect depth detection device based on pulse eddy current;
the defect depth detection device based on the pulse eddy current comprises: the device comprises an excitation signal generator, a power amplification module, an excitation coil, a detection coil, a signal conditioning module, an A/D conversion module and a DSP central processing unit; the excitation signal generator is connected with the input end of the power amplification module, the output end of the power amplification module is connected with two ends of the excitation coil, the detection coil is coaxially arranged with the excitation coil, the input end of the signal conditioning module is connected with the output end of the detection coil, the output end of the signal conditioning module is connected with the input end of the A/D conversion module, and the output end of the A/D conversion module is connected with the input end of the DSP central processing unit;
the excitation signal generator is used for generating a periodic pulse signal and transmitting the periodic pulse signal to the power amplification module;
the power amplification module is used for amplifying voltage and current of a periodic pulse signal generated by the excitation signal generator and then adding the periodic pulse signal to two ends of the excitation coil;
the exciting coil is used for introducing the amplified periodic pulse signal to generate an alternating magnetic field;
the detection coil is used for detecting a magnetic field signal above the tested piece, converting the magnetic field signal into a voltage signal and outputting the voltage signal to the signal conditioning unit;
the signal conditioning module is used for filtering and amplifying a voltage signal detected by the detection coil and outputting the voltage signal to the A/D conversion unit;
the A/D conversion module is used for converting the voltage signal output by the signal conditioning module into a digital voltage signal and outputting the converted digital signal to the DSP data processing module;
the DSP central processing unit comprises an acquisition triggering module, a characteristic parameter identification module and a defect depth detection module based on a random forest; the acquisition triggering module is used for controlling the A/D conversion module to perform digital/analog conversion of voltage; the characteristic parameter identification module is used for extracting the digital voltage signal output by the A/D conversion module after being controlled by the acquisition triggering module, identifying the characteristic parameter of the digital voltage signal through a constructed physical model of the pulse eddy current detection system, and outputting the identified characteristic parameter to the defect depth detection module based on the random forest; the defect depth detection module based on the random forest is used for taking the characteristic parameters output by the characteristic parameter identification module as input and outputting the depth information of the detected test piece defects; the method is characterized by comprising the following steps of:
step 1: collecting a magnetic field signal above the test piece by using a detection coil, converting the magnetic field signal into a voltage signal, filtering and amplifying the voltage signal, and further converting the voltage signal into a digital voltage signal;
step 2: in the characteristic parameter identification module, constructing a physical model of the pulse eddy current detection system, establishing a complex frequency domain response function of a voltage response signal on a detection coil, and obtaining a detection coil voltage time domain equation after inverse Laplace transform; then, taking the digital voltage signal in the step 1 as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method; the physical model of the constructed pulse eddy current detection system has 4 characteristic parameters, and the defect depth information of a tested piece is reflected through the characteristic parameters;
step 2.1: constructing a physical model of the pulsed eddy current detection system;
step 2.1.1: solving a change equation of the current on the exciting coil; pulse signals are applied to two ends of the exciting coil, and the signal voltage changes from 0 to the amplitude U of the pulse signalssThen, the current I on the exciting coil is obtainedL1The change equation of (a) is:
Figure FDA0003478602090000021
wherein R isL1Is the equivalent resistance of the exciting coil, r is the internal resistance of the exciting source,τeis the current decay time constant, t is the time, e is the natural constant;
step 2.1.2: calculating the induced electromotive force on the detection coil; the detection coil detects a magnetic field signal above the test piece and converts the magnetic field signal into a voltage signal to obtain induced electromotive force on the detection coil:
Figure FDA0003478602090000022
wherein N is1、N2The number of turns of the exciting coil and the detecting coil are respectively, and k is the ratio of the single-turn magnetic flux of the detecting coil to the single-turn magnetic flux of the exciting coil;
step 2.1.3: solving a voltage frequency domain response function at two ends of the detection coil; a loop consisting of the induced electromotive force epsilon, the inductance, the resistance and the parasitic capacitance of the detection coil is used for obtaining a voltage complex frequency domain response function U at the two ends of the detection coil0(s) is:
Figure FDA0003478602090000023
wherein
Figure FDA0003478602090000024
R2e、L2e、C2eRespectively representing the equivalent resistance, the inductance and the parasitic capacitance of the detection coil, wherein s represents the independent variable of a complex frequency domain;
step 2.1.4: solving a physical model of the pulse eddy current detection system; and performing inverse Laplace transform on the obtained frequency domain response function to obtain a detection coil voltage time domain equation:
Figure FDA0003478602090000025
order to
Figure FDA0003478602090000026
Obtaining:
Figure FDA0003478602090000027
Uo(t) is the physical model of the pulse eddy current detection system obtained, B, C, D, taueCharacteristic parameters of the model;
step 2.2: taking a digital voltage signal of the A/D conversion module as input, and identifying characteristic parameters of a physical model of the pulse eddy current detection system by using a variable weight multi-parameter fitting method;
step 2.2.1: to be inputted digital voltage signal UDiConverted into a voltage value Ui；
Figure FDA0003478602090000031
Where i is T,2T, …, nT, T is the sampling period of the a/D conversion module, UDiN digital voltages collected at the same position and different time on the tested piece, N is the bit number of the A/D conversion module, VrefIs the reference voltage of the A/D conversion module;
step 2.2.2: determining a weight coefficient of a variable-weight multi-parameter fitting method; calculating UiMaximum value of UmaxWhen the voltage value U is taken into consideration for the rapidity of the decay of the pulse eddy currentiGreater than 0.368UmaxThe characteristics of the eddy current can be embodied to obtain the weight coefficient wiIs expressed as follows
Figure FDA0003478602090000032
Wherein n is1For all voltage values to satisfy Ui＞0.368UmaxThe number of (2);
step 2.2.3: physical model U using pulsed eddy current inspection systemo(t) calculating Uo(i) The value of (i) ═ T,2To(i) And voltage value UiDifference value by difference valueDifference value Ui-Uo(i) Is multiplied by the corresponding weight coefficient wiAnd summing again to obtain an error function E:
Figure FDA0003478602090000033
step 2.2.4: error function E pairs B, C, D and tau respectivelyeCalculating partial derivative to obtain
Figure FDA0003478602090000034
And solving the system of equations:
Figure FDA0003478602090000035
obtaining characteristic parameters B, C, D and tau of the physical modeleA value of (d);
and step 3: in a defect depth detection module based on random forest, characteristic parameters are used as input, and depth information of the detected test piece defects is obtained by using a defect depth detection method based on random forest.
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