阅读说明：本技术 一种基于脉冲涡流检测探头线圈间距交叉点的提离测量装置及方法 (Lift-off measuring device and method based on pulse eddy current detection probe coil interval cross point ) 是由 张卿 顾波 李晓光 李庆顺 吴世亮 朱悦铭 程婧婷 王海涛 于 2021-07-14 设计创作，主要内容包括：本发明公开了一种基于脉冲涡流检测探头线圈间距交叉点的提离测量装置及方法,具体包括：S1、分别获取不同提离下不同线圈间距处的差分信号曲线,分别提取差分信号曲线的交叉点时间,进一步得到差分信号交叉点时间与提离的关系曲线；S2、对被测试件的未知提离进行定量评估,分别获得被测试件在未知提离下不同线圈间距处差分信号曲线,提取该差分信号曲线的交叉点时间,代入S1所得差分信号交叉点时间与提离的关系曲线的表达式中,获得被测试件的未知提离。该方法可用于检测中的提离测量,为减小脉冲涡流提离效应,提高缺陷定量精度提供参考。(The invention discloses a lift-off measuring device and method based on a pulse eddy current detection probe coil interval cross point, which specifically comprise the following steps: s1, respectively obtaining differential signal curves at different coil distances under different liftoff, respectively extracting intersection point time of the differential signal curves, and further obtaining a relation curve of the intersection point time of the differential signal and the liftoff; and S2, carrying out quantitative evaluation on the unknown lift-off of the tested piece, respectively obtaining differential signal curves of the tested piece at different coil pitches under the unknown lift-off, extracting the intersection time of the differential signal curves, and substituting the intersection time of the differential signal curves into the expression of the relation curve between the intersection time of the differential signal and the lift-off obtained in the S1 to obtain the unknown lift-off of the tested piece. The method can be used for lift-off measurement in detection, and provides reference for reducing the pulse eddy current lift-off effect and improving the quantitative precision of defects.)

1. A lift-off measuring device based on pulse eddy current detection probe coil spacing crosspoint is characterized by comprising a signal generator, a power amplifier, a TR structure sensor, a data acquisition card and a computer;

the output end of the signal generator is connected with the external circulation input end of the power amplifier and the external trigger port of the data acquisition card;

TR structure sensor is a non-coaxial formula and receives coil, includes: a TR sensor exciting coil and a TR sensor receiving coil; the coil is formed by winding an enameled wire around a nylon material framework; the external circulation output end of the power amplifier is connected with an excitation coil of the TR structure sensor; the receiving coil of the TR structure sensor is connected with a computer through a data acquisition card;

the signal generator generates an excitation square wave signal, the signal forms stable excitation current after passing through the power amplifier and is loaded into an excitation coil of the TR sensor, and the current changed in the excitation coil of the TR sensor excites an eddy current field in a test piece; the eddy current field is converted into a voltage signal through a receiving coil of the TR sensor, captured by a data acquisition card, and finally displayed and stored by a computer to obtain the information of the tested piece.

2. A lift-off measurement method based on a pulse eddy current detection probe coil spacing intersection is characterized by comprising the following steps:

s1, selecting a non-defective test piece and a defective test piece as tested pieces, and respectively obtaining time domain curves of response signals of the tested pieces at different coil intervals under different lift-off heights; then, carrying out differential processing on the time domain curve of the obtained response signal to obtain differential signal curves at different coil intervals under different liftoff, respectively extracting intersection point time of the differential signal curves, and further obtaining a relation curve of the intersection point time of the differential signal and the liftoff;

s2, carrying out quantitative evaluation on the unknown lift-off of the tested piece, respectively obtaining time domain curves of response signals of the tested piece at different coil intervals under the unknown lift-off, carrying out differential processing on the obtained time domain curves of the response signals, obtaining differential signal curves at different coil intervals under the unknown lift-off, extracting the intersection time of the differential signal curves, and substituting the extracted intersection time of the differential signal curves into the expression of the relation curve between the intersection time of the differential signal and the lift-off obtained in the step S2 to obtain the unknown lift-off of the tested piece.

3. The lift-off measurement method based on the pulse eddy current test probe coil pitch intersection point as claimed in claim 2, wherein said step S1 includes:

s11, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s12, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s13, the exciting coil of TR sensor generates exciting magnetic field, the coil is placed above the tested piece to obtain a group of signals with known lift-off height L₀The time domain curves of the response signals at the certain fixed coil interval are obtained, and the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

then at a known lift-off height L₀On the basis of the distance delta d of the coils, the known lift-off height L of the tested piece under not less than 3 different coil distances is obtained₀Time domain curves of the response signal of (a);

s14, carrying out difference processing on the reference signal time domain curve and the detection signal time domain curve in each group of time domain curves to obtain the known lift-off height L₀Differential signal curves at different coil spacings;

s15, repeating the steps S13 and S14 to obtain the lift-off height L at different known heights₁、L₂、L₃、L₄Differential signal curves at different coil spacings;

s16, extracting the known lift-off height L obtained in the step S15 respectively₀、L₁、L₂、L₃、L₄The time value corresponding to the point of the differential signal curve crossing the sink in the same coordinate system under different coil spacing is the crossing point time T₀、T₁、T₂、T₃、T₄；

S17, fitting the intersection time of the differential signal curve obtained in the step S16 and the corresponding known lift-off height into a linear function curve, wherein the linear function curve is as follows: l is_o1＝aT_DIP+ b, wherein, saidL_o1Is the lift-off height; t is_DIPFor the differential signal crossing point time, a, b are the coefficients of the linear function curve, respectively, and the known lift-off height L is₀、L₁、L₂、L₃、L₄Sum-difference signal crossing point time T₀、T₁、T₂、T₃、T₄Substituting the linear function curve to obtain corresponding values of a and b, and substituting the obtained values of a and b into the linear function curve to obtain a relation curve of the time of the cross point of the differential signal and the lift-off: l is_o1＝aT_DIP+b。

4. The method for measuring lift-off based on the probe coil pitch intersection of the pulsed eddy current inspection as claimed in claim 3, wherein each set of known lift-off heights L is obtained in step S13₀The time domain plot of the response signal at a fixed coil spacing of (a) is as follows:

s131, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and obtaining the known lift-off height L on the test piece₀The reference signal time domain plot of (a);

s132, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a known lift-off height L on the defect-containing test piece₀The time domain curve of the detection signal.

5. The method of claim 3, wherein the known lift-off height L is determined by a method of measuring lift-off at a cross point of a pitch between coils of a probe based on pulsed eddy current inspection₀、L₁、L₂、L₃、L₄Wherein, the lifting height value of one position is 0, namely no lifting is carried out.

6. The lift-off measurement method based on the pulse eddy current test probe coil pitch intersection point as claimed in claim 3, wherein said step S2 includes:

s21, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s22, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s23, enabling an excitation coil of the TR sensor to generate an excitation magnetic field, placing the excitation coil above a tested piece, and acquiring a group of time domain curves of response signals at a certain fixed coil interval under an unknown lift-off height Lx; the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

sequentially increasing the coil spacing delta d on the basis of the unknown lift-off height Lx to obtain a time domain curve of response signals of the unknown lift-off height Lx of the tested piece under not less than 3 different coil spacings;

s24, carrying out differential processing on the detection time domain curve in each group of response signals obtained in the step S23 and the time domain curve of the reference signal to obtain differential signal curves at different coil distances at an unknown lift-off height Lx; the intersection time T of the differential signal curve is extracted and substituted into the relationship curve L obtained in the step S17_o1＝aT_DIP+ b, the lift-off height of Lx in step S23, which is unknown lift-off, is obtained.

7. The method for measuring lift-off based on the probe coil pitch intersection of the pulsed eddy current inspection as claimed in claim 6, wherein the step of obtaining a set of time domain curves of the response signal at a certain fixed coil pitch at an unknown lift-off height Lx in step S23 comprises the steps of:

s231, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and acquiring a reference signal time domain curve of unknown lift-off height Lx on the test piece;

s232, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a detection signal time domain curve of unknown lift-off height Lx on the test piece.

8. The method for measuring lift-off based on the intersection of the pitch between the coils of the probe for pulsed eddy current inspection as claimed in claim 6, wherein Δ d is 3mm in steps S13 and S23.

9. The method for lift-off measurement based on the probe coil pitch intersection of the pulsed eddy current inspection as claimed in claim 6, further comprising an amplification filtering process of the obtained response signal before the steps S14 and S24.

Technical Field

The invention belongs to the technical field of nondestructive testing, and particularly relates to a lift-off measuring device and method based on a pulse eddy current testing probe coil interval intersection point.

Background

The aluminum alloy material has the characteristics of high strength, small density, good corrosion resistance and the like, and is widely applied to the fields of aerospace, petrochemical industry and rail transit. Steel and aluminum alloy components are used extensively to support loads or to transport liquid media in order to reduce corrosion, increase strength, and simultaneously reduce component weight.

For structural components, on one hand, the structural components bear high-strength alternating load in the use process, on the other hand, the structural components can be exposed in extreme environments for a long time, corrosion of different degrees is easily caused, the wall thickness of the components is reduced, the quality safety is seriously threatened, and serious safety accidents and casualties are even caused. Therefore, the nondestructive testing of the structural component can be accurately carried out in time, and the nondestructive testing method has great significance for improving the structural performance of the component and ensuring the safety.

The pulse eddy current testing technology breaks through the limitation that the traditional eddy current testing technology can only detect the surface defects of the components, has the characteristics of high testing speed, strong penetrability and the like, and can detect metal parts and structural parts in service outside the coating layer. The square wave current is used as excitation to be led into the exciting coil, the eddy current induced in the component can generate a secondary electromagnetic field and is coupled into the receiving coil, voltage is induced in the receiving coil at the moment, the eddy current on the component can be disturbed due to the existence of defects, the magnetic field induced by the eddy current is changed, the induced voltage of the receiving coil is changed, and the corrosion thinning degree of the wall thickness of the component can be evaluated through analysis of the induced voltage.

However, any factor causing the eddy current to change during the detection process affects the detection result, wherein the probe lift-off effect is the main problem faced by the pulse eddy current. Since the mutual inductance between the coil and the measured member is rapidly reduced along with the increase of the lifting distance from the probe to the surface of the measured member, the eddy current density in the measured body is also significantly changed along with the slight change of the lifting distance, and the effect is called the lifting-off effect. The change of the thickness of the insulating coating, the irregular surface of the tested body, the change of the pressure applied to the probe by an operator, the expansion caused by heat and the contraction caused by cold of the tested body and the like can cause the lift-off change, thereby covering the real detection information. Therefore, the inhibition and elimination of lift-off interference are always an important link in the research of the pulse eddy current detection technology, the probe lift-off during detection is accurately obtained, and a method for inhibiting the lift-off effect and improving the quantitative precision of defects can be provided.

The coil spacing cross point is used as a novel ideal signal characteristic, can effectively detect the changed lift-off height and inhibit the lift-off effect, and can also effectively inhibit the lift-off effect while detecting the wall thickness reduction, thereby eliminating the lift-off influence caused by the coating change and the sediment on the pipe wall and further improving the detection precision of the pulse eddy current in the field of wall thickness reduction; the method is widely applied to the fields of wall thickness reduction measurement, defect and thickness measurement and the like caused by corrosion. The problem of lift-off effect caused by the change of the lift-off of the probe due to different coating thicknesses when the corrosion thinning detection is carried out on the structural component with the coating is solved, and the purpose of the invention is achieved.

Disclosure of Invention

The invention aims to provide a lift-off measuring device and method for detecting a distance intersection point between a probe coil based on pulse eddy current, which solve the problem of lift-off effect caused by probe lift-off change caused by uncertain coating thickness when a structural component with a coating is subjected to corrosion thinning detection.

The invention adopts the following technical scheme:

a lift-off measuring device based on pulse eddy current detection probe coil interval cross points comprises a signal generator, a power amplifier, a TR structure sensor, a data acquisition card and a computer;

the output end of the signal generator is connected with the external circulation input end of the power amplifier and the external trigger port of the data acquisition card;

TR structure sensor is a non-coaxial formula and receives coil, includes: a TR sensor exciting coil and a TR sensor receiving coil; the coil is formed by winding an enameled wire around a nylon material framework; the external circulation output end of the power amplifier is connected with an excitation coil of the TR structure sensor; the receiving coil of the TR structure sensor is connected with a computer through a data acquisition card;

the signal generator generates an excitation square wave signal, the signal forms stable excitation current after passing through the power amplifier and is loaded into an excitation coil of the TR sensor, and the current changed in the excitation coil of the TR sensor excites an eddy current field in a test piece; the eddy current field is converted into a voltage signal through a receiving coil of the TR sensor, captured by a data acquisition card, and finally displayed and stored by a computer to obtain the information of the tested piece.

A lift-off measurement method based on pulse eddy current detection probe coil spacing intersection comprises the following steps:

s1, selecting a non-defective test piece and a defective test piece as tested pieces, and respectively obtaining time domain curves of response signals of the tested pieces at different coil intervals under different lift-off heights; then, carrying out differential processing on the time domain curve of the obtained response signal to obtain differential signal curves at different coil intervals under different liftoff, respectively extracting intersection point time of the differential signal curves, and further obtaining a relation curve of the intersection point time of the differential signal and the liftoff;

s2, carrying out quantitative evaluation on the unknown lift-off of the tested piece, respectively obtaining time domain curves of response signals of the tested piece at different coil intervals under the unknown lift-off, carrying out differential processing on the obtained time domain curves of the response signals, obtaining differential signal curves at different coil intervals under the unknown lift-off, extracting the intersection time of the differential signal curves, and substituting the extracted intersection time of the differential signal curves into the expression of the relation curve between the intersection time of the differential signal and the lift-off obtained in the step S2 to obtain the unknown lift-off of the tested piece.

Further, the step S1 includes:

s11, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s12, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s13, the exciting coil of TR sensor generates exciting magnetic field, the coil is placed above the tested piece to obtain a group of signals with known lift-off height L₀The time domain curves of the response signals at the certain fixed coil interval are obtained, and the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

then at a known lift-off height L₀On the basis of the distance delta d of the coils, the known lift-off height L of the tested piece under not less than 3 different coil distances is obtained₀Time domain curves of the response signal of (a);

s14, carrying out difference processing on the reference signal time domain curve and the detection signal time domain curve in each group of time domain curves to obtain the known lift-off height L₀Differential signal curves at different coil spacings;

s15, repeating the steps S13 and S14 to obtain the lift-off height L at different known heights₁、L₂、L₃、L₄Differential signal curves at different coil spacings;

s16, extracting the known lift-off height L obtained in the step S15 respectively₀、L₁、L₂、L₃、L₄The time value corresponding to the point of the differential signal curve crossing the sink in the same coordinate system under different coil spacing is the crossing point time T₀、T₁、T₂、T₃、T₄；

S17, fitting the intersection time of the differential signal curve obtained in the step S16 and the corresponding known lift-off height into a linear function curve,the linear function curve is: l is_o1＝aT_DIP+ b, wherein, said L_o1Is the lift-off height; t is_DIPFor the differential signal crossing point time, a, b are the coefficients of the linear function curve, respectively, and the known lift-off height L is₀、L₁、L₂、L₃、L₄Sum-difference signal crossing point time T₀、T₁、T₂、T₃、T₄Substituting the linear function curve to obtain corresponding values of a and b, and substituting the obtained values of a and b into the linear function curve to obtain a relation curve of the time of the cross point of the differential signal and the lift-off: l is_o1＝aT_DIP+b。

Further, each set of known lift-off heights L is obtained in the step S13₀The time domain plot of the response signal at a fixed coil spacing of (a) is as follows:

s131, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and obtaining the known lift-off height L on the test piece₀The reference signal time domain plot of (a);

s132, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a known lift-off height L on the defect-containing test piece₀The time domain curve of the detection signal.

Further, the known lift-off height L₀、L₁、L₂、L₃、L₄Wherein, the lifting height value of one position is 0, namely no lifting is carried out.

Further, the step S2 includes:

s21, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s22, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s23, enabling an excitation coil of the TR sensor to generate an excitation magnetic field, placing the excitation coil above a tested piece, and acquiring a group of time domain curves of response signals at a certain fixed coil interval under an unknown lift-off height Lx; the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

sequentially increasing the coil spacing delta d on the basis of the unknown lift-off height Lx to obtain a time domain curve of response signals of the unknown lift-off height Lx of the tested piece under not less than 3 different coil spacings;

s24, carrying out differential processing on the detection time domain curve in each group of response signals obtained in the step S23 and the time domain curve of the reference signal to obtain differential signal curves at different coil distances at an unknown lift-off height Lx; the intersection time T of the differential signal curve is extracted and substituted into the relationship curve L obtained in the step S17_o1＝aT_DIP+ b, the lift-off height of Lx in step S23, which is unknown lift-off, is obtained.

Further, the step of obtaining a set of time domain curves of the response signals at a certain fixed coil distance at the unknown lift-off height Lx in step S23 is as follows:

s231, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and acquiring a reference signal time domain curve of unknown lift-off height Lx on the test piece;

s232, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a detection signal time domain curve of unknown lift-off height Lx on the test piece.

Further, Δ d in step S13 and step S23 is 3 mm.

Further, amplification filtering processing of the obtained response signal is further included before the steps S14 and S24.

The invention has the beneficial effects that: the invention relates to a lift-off measuring device and a method based on pulse eddy current detection probe coil interval cross points, which process differential signal results generated under different known coil intervals of lift-off on a non-defective test piece and a test piece containing defects, change the coil intervals to obtain differential signal characteristic values, fit a curve, and determine the lift-off distance of a detection environment with unknown lift-off distance through the curve, thereby being beneficial to accurate quantitative detection of defects of a system. Thereby bringing great economic benefit and solving the core problem of the development of the pulse eddy current testing technology for many years. The method can accurately detect the position lift-off, effectively inhibit the lift-off effect caused by the change of the lift-off distance between the probe and the test piece in the pulse eddy current detection process, and further expand the application of the novel characteristic quantity coil spacing cross point signal characteristic of the pulse eddy current.

Drawings

Fig. 1 is a structural diagram of a lift-off measurement apparatus according to an embodiment of the present invention.

Fig. 2 is a flow chart of a lift-off measurement method according to an embodiment of the present invention.

FIG. 3 is a graph of the detection signal, reference signal and differential signal for pulsed eddy currents of the present invention at different coil spacings at 6mm lift-off.

FIG. 4(a) is a graph of the normalized result of the time-domain differential signal of the pulsed eddy current test of the present invention under different coil spacing without lift-off.

FIG. 4(b) is a graph of the normalized result of the time-domain difference signal of the pulsed eddy current test of the present invention at different coil pitches at 2mm lift-off.

FIG. 4(c) is a graph of the normalized result of the time-domain difference signal of the pulsed eddy current test of the present invention at different coil pitches at 4mm lift-off.

FIG. 4(d) is a graph of the normalized result of the time-domain difference signal of the pulsed eddy current test of the present invention at different coil pitches at 6mm lift-off.

FIG. 4(e) is a graph of the normalized result of the time-domain difference signal of the pulsed eddy current test of the present invention at different coil pitches at 8mm lift-off.

FIG. 5 is a time variation diagram of the intersection of the distances between the coils under different lifts.

Detailed Description

The embodiments of the present invention will be further described with reference to the accompanying drawings.

Fig. 1 is a block diagram of a lift-off measuring device for detecting a crossing point of a probe coil pitch based on a pulse eddy current according to an embodiment of the present invention, the device including: the device comprises a signal generator, a power amplifier, a TR structure sensor, a data acquisition card and a computer;

the output end of the signal generator is connected with the external circulation input end of the power amplifier and the external trigger port of the data acquisition card; the external circulation output end of the power amplifier is connected with an excitation coil of the TR structure sensor; the receiving coil of the TR structure sensor is connected with a computer through a data acquisition card;

TR structure sensor is a non-coaxial formula and receives coil, includes: a TR sensor exciting coil and a TR sensor receiving coil; the coil is formed by winding an enameled wire around a nylon material framework; the excitation square wave signal is generated by a signal generator, stable excitation current is formed after the excitation square wave signal passes through a power amplifier and is loaded into an excitation coil, and the current changed in the coil excites an eddy current field in a test piece; the eddy current field is converted into a voltage signal through a receiving coil, the voltage signal is captured by a capture card, and finally data display and storage are carried out through a computer to obtain the information of the tested piece.

Fig. 2 is a flow chart of a pulsed eddy current detection method for measuring lift-off according to the present invention, which specifically includes the steps of:

s1, selecting a defect-free test piece and a defect-containing test piece as tested pieces, wherein the tested pieces are made of aluminum alloy 7075;

in the embodiment, five different lift heights are selected, namely, the lift height is 0mm, the lift height is 2mm, the lift height is 4mm, the lift height is 6mm and the lift height is 8 mm; 6 different coil pitches are selected, namely 40mm, 43mm, 46mm, 49mm, 52mm and 55 mm; respectively acquiring time domain curves of response signals of a tested piece at different coil intervals under different lift-off heights; then, carrying out differential processing on the time domain curve of the obtained response signal to obtain differential signal curves at different coil intervals under different liftoff, respectively extracting intersection point time of the differential signal curves, and further obtaining a relation curve of the intersection point time of the differential signal and the liftoff;

s11, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s12, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s13, enabling an excitation coil of the TR sensor to generate an excitation magnetic field, placing the coil above a tested piece, and acquiring a group of time domain curves of response signals at the position of 40mm of coil space under the known lift-off height of 0mm, wherein the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

the step of obtaining the time domain curve of the response signal at a certain fixed coil distance with each group of known lift-off heights of 0mm in the step S13 is as follows:

s131, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and acquiring a reference signal time domain curve with the known lift-off height of 0mm on the test piece;

s132, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a detection signal time domain curve with the known lift-off height of 0mm on the test piece.

Then, the coil pitches of 43mm, 46mm, 49mm, 52mm and 55mm are adopted to obtain time domain curves of response signals of the known lift-off height 0mm of the tested piece under the coil pitches of 43mm, 46mm, 49mm, 52mm and 55 mm;

s14, carrying out differential processing on the reference signal time domain curves and the detection signal time domain curves in each group of time domain curves to obtain differential signal curves of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm coil distances at the known lift-off height of 0 mm; as shown in figure 4(a) of the drawings,

s15, repeating the step S13 and the step S14, and acquiring differential signal curves of coil pitches of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm at known different lift-off heights of 2mm, 4mm, 6mm and 8 mm; as shown in FIGS. 4(b) to 4(e)

S16, respectively extracting time values corresponding to the points of the differential signal curves of the known lift-off heights of 0mm, 2mm, 4mm, 6mm and 8mm obtained in the step S15 and with the coil distances of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm, which intersect and converge in the same coordinate system, namely intersection point time T₀、T₁、T₂、T₃、T₄；

S17, fitting the intersection time of the differential signal curve obtained in the step S16 and the corresponding known lift-off height into a linear function curve, wherein the linear function curve is as follows: l is_o1＝aT_DIP+ b, wherein, said L_o1Is the lift-off height; t is_DIPFor the time of the cross point of the differential signal, a and b are respectively the coefficients of the linear function curve, and the known lift-off heights are 0mm, 2mm, 4mm, 6mm and 8mm and the time T of the cross point of the differential signal₀、T₁、T₂、T₃、T₄Substituting the linear function curve to obtain corresponding values a and b, and substituting the obtained values a and b into the linear function curve to obtain a relation curve of the time of the crossing point of the differential signal and the lift-off as shown in fig. 5: l is_o1＝aT_DIP+b。

S2, carrying out quantitative evaluation on the unknown lift-off of the tested piece, respectively obtaining time domain curves of response signals of the tested piece at coil pitches of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm under the unknown lift-off, carrying out differential processing on the time domain curves of the obtained response signals, obtaining differential signal curves at the coil pitches of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm under the unknown lift-off, extracting the intersection time of the differential signal curves, and substituting the intersection time of the extracted differential signal curves into the relation curve expression of the intersection time and the lift-off of the differential signal obtained in the step S2 to obtain the unknown lift-off of the tested piece.

S21, generating a pulse signal with adjustable frequency and duty ratio by a signal generator to serve as external excitation of the power amplifier;

s22, the power amplifier generates pulse current with adjustable frequency and duty ratio under the action of external excitation and loads the pulse current on the TR sensor excitation coil;

s23, enabling an excitation coil of the TR sensor to generate an excitation magnetic field, placing the excitation magnetic field above a tested piece, and acquiring a group of time domain curves of response signals at the coil interval of 40mm under the unknown lift-off height Lx; the time domain curve of each group of response signals comprises a reference signal time domain curve and a detection signal time domain curve;

the step of acquiring a group of time domain curves of response signals at a coil distance of 40mm under an unknown lift-off height Lx in the step S23 is as follows:

s231, simulating a defect-free test piece by using an aluminum alloy test piece with the thickness of 10mm, and acquiring a reference signal time domain curve of unknown lift-off height Lx on the test piece;

s232, simulating a defect-containing test piece by using an aluminum alloy test piece with the thickness of 8mm, and acquiring a detection signal time domain curve of unknown lift-off height Lx on the test piece.

Then, acquiring time domain curves of response signals of unknown lift-off heights Lx of the tested piece under the coil intervals of 43mm, 46mm, 49mm, 52mm and 55mm by adopting the coil intervals of 43mm, 46mm, 49mm, 52mm and 55 mm;

s24, carrying out difference processing on the detection time domain curve in each group of response signals obtained in the step S23 and the time domain curve of the reference signal to obtain difference signal curves at coil pitches of 40mm, 43mm, 46mm, 49mm, 52mm and 55mm at an unknown lift-off height Lx; the intersection time T of the differential signal curve is extracted and substituted into the relationship curve L obtained in the step S17_o1＝aT_DIP+ b, the lift-off height of Lx in step S23, which is unknown lift-off, is obtained.

Since the output response signal is weak, in order to obtain a better detection result, the detection signal and the reference signal need to be amplified and filtered, so the step S14 is preceded by an amplification and filtering process for the response signal, that is, the detection signal and the reference signal, and similarly, the step S24 is preceded by an amplification and filtering process for the obtained response signal, that is, the detection signal and the reference signal.