CN109781041B - Electromagnetic ultrasonic thickness measurement method based on frequency domain analysis - Google Patents
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Abstract
The invention belongs to the field of electromagnetic ultrasonic nondestructive testing, and discloses an electromagnetic ultrasonic thickness measuring method based on frequency domain analysis. The method comprises the following steps: (a) selecting a test piece with known thickness and a test piece with thickness to be measured, and respectively carrying out the following steps: (a1) an exciting coil of the electromagnetic ultrasonic sensor sends out a signal, and an induced electrical signal of a receiving coil is collected to be used as a receiving signal; (a2) intercepting a section of echo signal from a received signal, and sequentially performing resampling, whole-period continuation and low-pass filtering processing; (a3) taking a positive semi-cycle signal from the processed signal, performing fast Fourier transform to obtain a spectrogram, selecting a frequency spectrum corresponding to a partial frequency band from the spectrogram, and calculating a frequency corresponding to a peak value of the frequency spectrum; (b) and calculating the thickness of the test piece to be measured by using a formula. According to the invention, the thickness information is converted from the period which is difficult to read in the time domain to the peak value which is easy to read in the frequency domain, the use lift-off is increased on the basis of not changing the original circuit, and the detection precision is improved.
Description
Technical Field
The invention belongs to the field of electromagnetic ultrasonic nondestructive testing, and particularly relates to an electromagnetic ultrasonic thickness measuring method based on frequency domain analysis.
Background
Electromagnetic ultrasound is a new technology appearing in the field of nondestructive testing, has the advantages of simple structure, no need of a coupling agent and capability of adapting to certain lifting, and utilizes an electromagnetic coupling method to excite and receive ultrasonic waves. Compared with the traditional ultrasonic detection technology, the ultrasonic detection device has the advantages of high precision, no need of couplant, non-contact, suitability for high-temperature detection, easiness in exciting various ultrasonic waveforms and the like. In industrial applications, electromagnetic ultrasound is receiving more and more attention and attention. Electromagnetic ultrasound is a metal conductor in an alternating magnetic field, eddy currents are generated inside the metal conductor, any current is acted by Lorentz force in the magnetic field, a metal medium is acted by alternating stress waves, the stress waves with the frequency in the ultrasonic wave range are ultrasonic waves, on the contrary, the effect is reversible, the vibration of mass points can change the voltage at two ends of an eddy current coil under the action of the magnetic field due to the returned sound pressure, so the electromagnetic ultrasound can be received and amplified by a receiving device for displaying, the ultrasonic waves excited and received by the method are called electromagnetic ultrasound, in the method, a transducer is not only a combination of the eddy current coil which is electrified by the alternating current and an external fixed magnetic field, the metal surface is also an important component of the transducer, and the conversion of electricity and sound is completed by the metal surface, electromagnetic ultrasound can only be generated on conductive media, and thus electromagnetic ultrasound can only find applications on conductive media.
At present, the invention patent application with the application number of 201610172021.0 discloses an electromagnetic ultrasonic thickness measuring method based on a digital acquisition circuit system; the invention patent application with application number 201510582587.6 discloses a method for measuring the thickness of a metal material by using two types of electromagnetic ultrasound for determining frequency pulses, the technology is mainly carried out around the acquisition of the time domain period of an electromagnetic ultrasonic echo signal, the problem of how to extract the time interval of the echo signal by various methods is intensively solved, because the electromagnetic ultrasonic transduction efficiency is low, the influence of lift-off is large, and circuit noise can influence the measurement of the echo period, so that the lift-off which can be realized by the existing electromagnetic ultrasonic instrument is small, and a signal processing circuit is complex.
Disclosure of Invention
Aiming at the defects or the improvement requirements of the prior art, the invention provides an electromagnetic ultrasonic thickness measuring method based on frequency domain analysis, which converts an acquired induction signal from a time domain to a frequency domain and converts the induction signal from a measurement time interval to a final measurement peak frequency mode by sequentially carrying out resampling, whole period continuation, low-pass filtering, positive half-cycle signal acquisition and fast Fourier transform on the induction signal, wherein the process does not change an induction signal acquisition way, but only converts a signal processing mode, and the obtained frequency domain signal analysis has low requirements on signal amplitude, so that the use lift can be increased on the basis of not changing an original circuit, and the detection precision is improved.
In order to achieve the above object, according to the present invention, there is provided an electromagnetic ultrasonic thickness measuring method based on frequency domain analysis, comprising the steps of:
(a) selecting a test piece with known thickness and a test piece with to-be-measured thickness of the same material, and respectively carrying out the following steps:
(a1) installing an excitation coil and a receiving coil of an electromagnetic ultrasonic sensor on a test piece with known thickness/a test piece with thickness to be tested, wherein the excitation coil of the electromagnetic ultrasonic sensor sends out signals, acquiring induced electric signals of the receiving coil, taking the induced electric signals as receiving signals, and recording the receiving signals on the test piece with known thickness and the test piece to be tested as calibration signals and detection signals respectively;
(a2) intercepting a section of echo signal from the received signal as a sample signal, and sequentially performing resampling, whole period extension and low-pass filtering on the sample signal so as to reduce the data processing difficulty and improve the frequency domain resolution and precision;
(a3) further taking a positive semi-cycle signal from the signal processed in the step (a2) to obtain a sample signal, separating the frequency of the excitation signal and the frequency of the echo signal which are coupled with each other on a frequency domain, then carrying out fast Fourier transform to obtain a spectrogram corresponding to the signal, selecting a frequency spectrum corresponding to a partial frequency band in the spectrogram, and calculating the frequency corresponding to a peak value of the frequency spectrum, wherein the frequency obtained by the test piece with the known thickness is recorded as f0The frequency obtained from the thickness specimen to be measured is f1；
(b) Calculating the thickness of the test piece to be measured by using the following formula:
D1＝Df0/f1
further preferably, in the step (a2), when a section of echo signal is intercepted as the sample signal on the received signal, it is preferable to intercept the echo signal which changes periodically as the sample signal.
Further preferably, in the step (a2), when a section of echo signal is intercepted as the sample signal on the received signal, the time length of the intercepted echo signal satisfies the following relationship:
T1＞2D/V
wherein, T1Is the time length of the intercepted echo signal, D is the thickness of the test piece of known thickness, 2000m/s<V<8000m/s。
Further preferably, in the step (a2), the resampled sampling rate Fs satisfies the following relationship:
Fs＞2fe
wherein f iseIs the excitation frequency.
Further preferably, in step (a2), to ensure signal integrity, the low-pass filter cut-off frequency satisfies the following relation:
fc＞fe
wherein f iscIs the low pass filter cut-off frequency.
Further preferably, in the step (a2), the whole period extension number and the required frequency domain resolution satisfy the following relation:
where Δ f is the frequency domain resolution and n is the whole period extension.
Further preferably, in step (a3), the frequency f of the test piece of known thickness is acquired0Then, the propagation velocity of the electromagnetic ultrasonic wave is calculated according to the following expression:
V0＝2Df0
wherein, V0Is the propagation velocity of electromagnetic ultrasound.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
1. the invention uses the principle that when the transverse wave generated in a test piece by an electromagnetic ultrasonic sensor is reflected on the upper surface and the lower surface of the test piece, the periodic vibration of the surface of the test piece can cut a magnetic induction line to generate induction voltage, the induction voltage is coupled into a receiving coil, a receiving signal with the same period is generated in the receiving coil, and the period of the receiving signal is related to the thickness of the test piece and the wave speed of ultrasonic waves in the test piece to measure the test piece with unknown thickness;
2. the method provided by the invention is characterized in that a section of echo signal with a determined length is intercepted, the signal is resampled and filtered in a time domain, a positive half cycle signal is taken, then FFT is carried out, the frequency corresponding to a peak value is analyzed in a frequency domain, and the wave velocity or the thickness information of a test piece is obtained.
Drawings
FIG. 1 is a flow chart of a method for electromagnetic ultrasonic thickness measurement based on frequency domain analysis, constructed in accordance with a preferred embodiment of the present invention;
FIG. 2 is a diagram of a test piece electromagnetic ultrasonic thickness measurement calibration signal for acquiring a known thickness constructed in accordance with a preferred embodiment of the present invention;
FIG. 3 is an echo signal intercepted from a calibration signal for a test piece of known thickness constructed in accordance with a preferred embodiment of the present invention;
FIG. 4 is the echo signal of FIG. 3 after resampling and extending to obtain the positive half cycle signal;
FIG. 5 is a frequency domain signal within a certain frequency band after FFT of the signal shown in FIG. 4, and its peak value and corresponding frequency;
FIG. 6 is a block diagram illustrating the acquisition of electromagnetic ultrasonic thickness measurement signals of a test piece of unknown thickness constructed in accordance with a preferred embodiment of the present invention;
FIG. 7 is an echo signal intercepted from a test signal of unknown thickness constructed in accordance with a preferred embodiment of the present invention;
FIG. 8 is the echo signal of FIG. 7 after resampling and extension, and taking the positive half cycle of the echo signal as the echo signal;
fig. 9 is a frequency domain signal within a certain frequency band after FFT of the signal of fig. 8 and its peak and corresponding frequency.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Fig. 1 is a flowchart of an electromagnetic ultrasonic thickness measuring method based on frequency domain analysis according to a preferred embodiment of the present invention, and as shown in fig. 1, the method includes the following steps:
(1) installing an excitation coil and a receiving coil of an electromagnetic ultrasonic sensor on a test piece with known thickness, and acquiring an induced electrical signal of the receiving coil to obtain a group of calibration signals X0;
(2) taking a certain time T for starting excitation in a calibration signal X00The echo signal is used as the starting point of data, and the interception time length is T1Preferably, the echo signal of (2) is taken as a sample signal X2, and the echo signal of periodically varying amplitude is preferably taken as a sample signal T0The selection of the test piece is determined by the excitation signal frequency and the material of the test piece, and if the T is selected according to the detection requirement0Is 30 to 150 μ s. Truncating the time length T1>2D/V, D is the thickness of the calibrated test piece, generally, 2000m/s<V<8000m/s, e.g. according to detection requirements, T1The temperature can be taken for 10 to 500 mu s;
(3) the sampling rate after resampling the sample signal is determined by the excitation frequency, Fs>2feFs is the sampling rate after resampling, feIs the excitation frequency. The resampled signal is extended for a full period, low pass filtered and the positive half cycle signal is taken (negative signal is zeroed) to get X4. Number of period extensions n and frequency domain resolution (1/nT)1) The higher the requirement on the frequency domain resolution is, the larger the period continuation number is, and the lower the low-pass filtering cut-off frequency is to be smaller than the excitation frequency;
(4) performing Fast Fourier Transform (FFT) on X4 to obtain spectrogram, selecting frequency band, and reading frequency point f corresponding to spectral peak in frequency band0And the frequency is used for calibrating the echo frequency of the test piece. The frequency band is selected such that the maximum frequency does not exceed the excitation frequency and the minimum frequency must be greater than 0. According to f0And the thickness D of the test piece, the wave velocity V can be obtained0＝2Df0；
(5) Placing an excitation coil and a receiving coil of an electromagnetic ultrasonic sensor on a piece to be tested with unknown thickness, collecting an induced voltage signal X1 in the receiving coil, carrying out the same treatment as X0 on X1 to obtain X3 and X5, carrying out FFT on X5, selecting a frequency band, readingFrequency spectrum peak value corresponding to frequency point f1And (4) selecting the frequency band as the echo frequency of the test piece to be tested, wherein the frequency band is the same as that in the step (4), and the thickness of the test piece can be obtained by adopting the following two modes:
a. according to f1And V0The thickness D of the tested part can be obtained1＝V0/2f1。
b. If the wave velocity V is known0The calibration process can be omitted and D can be directly obtained1＝V0/2f1。
One embodiment is provided below in connection with the present disclosure.
The thickness of the standard test piece is 8mm, the thickness of the test piece to be tested is 5.35mm, and the material of the test piece is 16MnR steel. The excitation voltage of the sensor is 600V, the excitation frequency is 1MHz, and the distance from the bottom end of the sensor to the surface of the test piece is 2 mm. Firstly, the sensor is arranged on an 8mm standard test piece, and as shown in figure 2, an induced electric signal of the receiving coil can be obtained. Taking T0 as 70 mus and T1 as 30 mus, obtaining echo signal X2 as sample signal as shown in figure 3, the original sampling rate of data is 2.5GHz, in order to reduce the data processing amount, resampling is carried out, the sampling rate is reduced to 25MHz, after resampling, carrying out 10-cycle extension on the obtained signal and taking positive half-cycle signal, obtaining signal as shown in figure 4. The FFT is performed on the signal shown in fig. 4, and the frequency band is selected to be 0.1MHz to 0.35MHz, and the frequency spectrum and the frequency corresponding to the peak of the spectrum obtained in the frequency band are shown in fig. 5. According to V0＝2Df0In the figure, f can be read out0At 0.2MHz, the wave velocity V is calculated0Is 3200 m/s. And then the sensor is placed on a piece to be tested with the thickness of 5.35mm, and the same excitation voltage, excitation frequency and lift-off are adopted. The resulting receive coil signal is shown in fig. 6. Also take T0Is 70 mus, T1At 30 μ s, an echo signal X3 was obtained as a sample signal as shown in fig. 7, and the signal shown in fig. 8 was obtained by adjusting the data sampling rate to 25MHz, similarly performing 10-cycle extension on the resampled signal and taking the positive half-cycle signal. The FFT is performed on the signal shown in fig. 8, and the frequency band is selected to be 0.1MHz to 0.35MHz, and the spectrum signal is obtained as shown in fig. 9, from which it can be seen that the signal spectrum obtained in the frequency band and the point corresponding to the peak of the spectrum are f10.2997MHz, according to D1＝V0/2f1Obtaining D1The measurement result is accurate when the thickness is 5.339 mm.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (7)
1. An electromagnetic ultrasonic thickness measuring method based on frequency domain analysis is characterized by comprising the following steps:
(a) selecting a test piece with known thickness and a test piece with to-be-measured thickness of the same material, and respectively carrying out the following steps:
(a1) installing an excitation coil and a receiving coil of an electromagnetic ultrasonic sensor on a test piece with known thickness/a test piece to be tested with unknown thickness, wherein the excitation coil of the electromagnetic ultrasonic sensor sends out signals, acquiring induced electric signals of the receiving coil, taking the induced electric signals as receiving signals, and recording the receiving signals on the test piece with known thickness and the test piece to be tested as calibration signals and detection signals respectively;
(a2) intercepting a section of echo signal from the received signal as a sample signal, resampling the sample signal, adjusting the sampling rate of the signal in a proper range to reduce the difficulty of data processing, and then performing whole-period continuation and low-pass filtering to prolong the time sequence and reduce the signal noise;
(a3) further performing positive semi-cycle signal acquisition on the signal processed in the step (a2), so as to separate the excitation signal frequency and the echo signal frequency which are originally coupled with each other in the frequency domain, then performing fast Fourier transform processing on the obtained positive semi-cycle signal, obtaining a spectrogram corresponding to the detection signal, selecting a frequency spectrum corresponding to a partial frequency band in the spectrogram, and calculating the frequency corresponding to the peak value of the frequency spectrum, wherein the frequency acquired by the test piece with the known thickness is recorded as the frequencyf 0 The frequency obtained from the thickness specimen to be measured isf 1 ；
(b) Calculating the thickness of the test piece to be measured by using the following formula:
D 1=Df 0 / f 1
wherein the content of the first and second substances,Dis the thickness of a test piece of known thickness,D 1is the thickness of the test piece to be measured.
2. The method according to claim 1, wherein in the step (a2), when a section of echo signal is intercepted from the received signal as the sample signal, the echo signal that changes periodically is intercepted as the sample signal.
3. The electromagnetic ultrasonic thickness measuring method based on frequency domain analysis according to claim 1 or 2, wherein in the step (a2), when a section of echo signal is intercepted from the received signal as a sample signal, the time length of the intercepted echo signal satisfies the following relation:
T 1 > 2D / V
wherein the content of the first and second substances, T 1is the length of time the echo signal is intercepted,D is the thickness of a test piece of known thickness,Vis the speed of the echo signal, 2000m/s<V<8000m/s。
4. The method of claim 1, wherein in step (a2), the re-sampled sampling rate is determined by a frequency domain analysis-based electromagnetic ultrasonic thickness measurement methodFsThe following relationships are satisfied:
Fs>2 f e
wherein the content of the first and second substances,f e is the excitation frequency.
5. The electromagnetic ultrasonic thickness measurement method based on frequency domain analysis according to claim 4, wherein in step (a2), in order to ensure signal integrity, the low-pass filter cut-off frequency satisfies the following relation:
f c >2 f e
wherein the content of the first and second substances,f cis the low pass filter cut-off frequency.
6. The electromagnetic ultrasonic thickness measuring method based on frequency domain analysis as claimed in claim 1, wherein in step (a2), the whole period extension number and the required frequency domain resolution satisfy the following relation:
Δf=1/（nT 1）
wherein the content of the first and second substances,Δfin order to be the frequency domain resolution,nis the whole cycle extension number.
7. The electromagnetic ultrasonic thickness measurement method based on frequency domain analysis as claimed in claim 1, wherein in step (a3), the frequency of the test piece with known thickness is obtainedf 0Then, the propagation velocity of the electromagnetic ultrasonic wave is calculated according to the following expression:
V 0 = 2Df 0
wherein the content of the first and second substances,V 0is the propagation velocity of electromagnetic ultrasound.
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| CN111256630B (en) * | 2020-02-27 | 2021-05-11 | 西北大学 | Method for rapidly measuring thickness of metal plate by utilizing electromagnetic ultrasonic guided wave frequency dispersion characteristic |
| CN113311073B (en) * | 2021-04-21 | 2022-12-02 | 武汉科技大学 | Electromagnetic ultrasonic sound time measuring method and system |
| CN113587866B (en) * | 2021-07-12 | 2022-10-28 | 西安交通大学 | Method for nondestructive measurement of thickness of thin film coating based on grating laser ultrasonic acoustic spectrum |
| CN113739728A (en) * | 2021-08-31 | 2021-12-03 | 华中科技大学 | Electromagnetic ultrasonic echo sound time calculation method and application thereof |
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| CN101614533B (en) * | 2008-06-26 | 2012-01-11 | 中国科学院金属研究所 | Method and instrument capable of accurately measuring thickness of ultrathin workpieces |
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