CN115406960A - Digital multi-frequency eddy current signal processing method - Google Patents

Abstract

The invention discloses a digital multi-frequency eddy current signal processing method, which is characterized in that it comprises the following steps: a. setting several groups of eddy current detection frequency combinations; b. converting sinusoidal signals of frequencies generated by different frequency combinations into digital sine waves signal; c. superimpose the digitized sine wave signal through the formula ∑ _i cos(2πF _i t+φ _i ); d. carry out digital-to-analog conversion to the first digital signal that completes the superposition, and become an analog signal; e. Amplify the analog sine wave signal to form an excitation signal for driving the eddy current probe; f. use the generated excitation signal to act on the detection coil on the eddy current probe to generate a feedback signal; g. perform analog-to-digital conversion on the feedback signal to obtain the second digital signal, analyzing the second digital signal to obtain different frequency signals, and separately calculating the real part and imaginary part of each frequency signal; h. analyzing and evaluating according to the analyzed frequency signal. It has the advantages of low cost, low power consumption, high resolution and fast conversion speed.

Description

A digital multi-frequency eddy current signal processing method

technical field

The invention belongs to the field of nuclear power detection, in particular to a digital multi-frequency eddy current signal processing method.

Background technique

Eddy current testing is a non-destructive testing method based on the principle of electromagnetic induction, which is suitable for conductive materials. When a conductor is placed in an alternating magnetic field, an induced current exists in the conductor, that is, an eddy current is generated. Due to the change of various factors of the conductor itself (such as electrical conductivity, magnetic permeability, shape, size and defects, etc.), the change of eddy current will be caused. The detection method of using this phenomenon to determine the nature and state of the conductor is called eddy current detection. Eddy current testing technology is widely used in nuclear power plants, such as steam generator heat transfer tubes, condenser titanium tubes, reactor neutron flux measurement finger bushings, reactor pressure vessel main bolts/main nuts, reactor pressure vessel top penetrations, control Inspection of rod bundle assemblies (RCCA), control rod drive mechanism (CRDM) seal welds, etc.

Due to the limitations of single-frequency eddy current technology, for the detection of many complex and important components, such as the in-service detection of steam generator heat transfer tubes, adjacent support plates, tube sheets and other structural components will generate strong interference signals. It is difficult for eddy current to accurately detect the defects of the pipe. Multi-frequency eddy current detection technology uses several frequencies to excite the detection coil at the same time, and multiple sets of signals in the detection can be collected at the same time through the sensor. Mixing these signals can effectively suppress multiple interference factors. The eddy current inspection of nuclear power plant equipment and components adopts multi-frequency eddy current inspection technology. According to the actual situation of the detected object, two or more detection frequencies are usually used, and each frequency has its purpose in positioning or detection. In the common eddy current inspection of steam generator heat transfer tubes, five frequencies are generally used for data collection. In the process of data analysis, signals of different frequencies will also be processed comprehensively for some specific purposes.

As we all know, the traditional analog eddy current signal processing technology uses an oscillator to generate alternating current to flow through a coil placed on a conductor, forming an alternating magnetic field around the coil and forming an eddy current on the surface of the conductor; when the position of the detection coil changes, Due to the defect under the position of the coil, the change in the shape, size or electromagnetic characteristics of the conductor will cause the size of the eddy current to change and act on the detection coil through the secondary magnetic field, causing the coil impedance to change, thereby causing the detection signal to change; After the detection signal is amplified and filtered, it is transmitted to the hardware detection circuit. The hardware detection circuit completes the analysis of the frequency signal, and obtains the phase and amplitude of the detection signal on the detection coil, thereby completing the processing of the analog eddy current signal.

With the acceleration of modernization, nuclear power plants have higher and higher requirements for eddy current testing technology. In the development and integration of electronics, material science, computer technology, signal processing technology, and defect identification technology, eddy current testing technology is constantly making new developments and new applications. Currently in the digital information age, digital signal processing technology has become the most One of the cutting-edge development technologies, digital processing of eddy current signals is a more advanced eddy current signal processing technology developed under this background. Compared with the analog eddy current signal processing method, the digital eddy current signal processing method has the following advantages: the digital signal generator is easy to realize the superimposition of multiple frequency signals of different frequencies and different amplitudes to form a multi-frequency excitation signal; the digital method circuit structure Simple, requires few analog circuit components, low power consumption, small circuit temperature drift, higher stability and reliability; digital circuit has less noise, strong anti-interference ability, high signal-to-noise ratio, and can detect other similar Minor defects that cannot be detected by the product. How to digitally apply multi-frequency eddy current signals to the analysis and evaluation of eddy current flaw detection results is an urgent problem to be solved.

Contents of the invention

The purpose of the present invention is to provide a digital multi-frequency eddy current signal processing method, which improves the frequency stability and accuracy of the signal generator to the same level as the reference frequency, and can perform fine frequency in a wide frequency range adjust.

In order to solve the above technical problems, the present invention adopts the following technical solutions: a digital multi-frequency eddy current signal processing method, characterized in that it comprises the following steps:

a. Set several groups of eddy current detection frequency combinations;

b. Convert sinusoidal signals of frequencies generated by different frequency combinations into digitized sinusoidal signals;

c. superimpose the digitized sine wave signal through the formula ∑ _i cos(2πF _i t+φ _i );

d. DAC conversion is performed on the first digital signal that has been superimposed to become an analog sine wave signal;

e. Amplify the analog sine wave signal to form an excitation signal for driving the eddy current probe;

f. Use the generated excitation signal to act on the detection coil on the eddy current probe to generate a feedback signal;

g. performing analog-to-digital conversion on the feedback signal to obtain a second digital signal, analyzing the second digital signal to obtain various frequency signals, and separately calculating the real part and imaginary part of each frequency signal;

h. Analyze and evaluate the results of eddy current testing based on the real and imaginary parts of the analyzed frequency signal.

Optimally, the excitation signal is filtered after step e.

Optimally, after the excitation signal is filtered, the filtered signal needs to be amplified.

Optimally, in step g, after obtaining the second digital signal, discrete Fourier transform is performed on the second digital signal to obtain the corresponding real part and imaginary part (ri, zi) of each frequency signal in the frequency domain, where (ri, zi) corresponds to the variation of detection coil impedance at different frequencies.

The beneficial effects of the present invention are:

1. This method has less electronic noise in the data, has a high signal-to-noise ratio, and can detect tiny defects;

2. This method adopts DDS (digital synthesis technology that converts a series of digital signals into analog signals through D/A converter) technology to generate excitation signals, which has the advantages of low cost, low power consumption, high resolution and fast conversion speed. It can improve the frequency stability and accuracy of the signal generator to the same level as the reference frequency, and can perform fine frequency adjustment in a wide frequency range;

3. This method can support a variety of excitation signals with different frequencies to work at the same time, realize the simultaneous detection of defects of different depths, and greatly improve the detection efficiency;

4. This method adopts DFT (Discrete Fourier Transform) technology to transform the detection signal from the time domain to the frequency domain, so as to realize the calculation of the real and imaginary parts of each different frequency in the multi-frequency composite signal. Among them, the DFT operation on the digital signal is realized through pure software algorithm, without hardware detection circuit. No hardware detection circuitry means less electronic noise in the data.

Description of drawings

Fig. 1 is a schematic diagram of a digital eddy current signal processing method;

Figure 2 is a schematic diagram of multi-frequency signal superposition.

Detailed ways

Below in conjunction with the embodiment shown in the accompanying drawings, the present invention is described in detail as follows:

The digital multi-frequency eddy current signal processing method specifically includes the following steps:

a. First, set the frequency combination required for eddy current testing through the CPU. According to different characteristics of the object to be inspected, different frequency combinations need to be set. The present invention supports setting 1 to 5 groups of frequency combinations at the same time, and each frequency can be independently configured with its frequency, phase and amplitude.

b. As shown in Figure 1, after the configuration of frequency signal parameters is completed, the present invention converts sinusoidal signals of different frequencies into digital sine waves through DDS technology, and completes the digital processing of each sine wave.

c. Then superimpose the digitized sine wave signal through the formula. It should be noted that when superimposing the digital signal, it needs to be adjusted to avoid the superimposition of different frequency signal peaks and cause the signal amplitude to exceed the range. The problem.

d. Then DAC converts the superimposed digital signal into a usable analog sine wave signal.

e. The driving ability of the analog signal converted by DAC is weak, and the driving ability needs to be enhanced through a power amplifier (Power Amplifier), so as to form an excitation signal capable of driving the eddy current probe.

f. The excitation signal acts on the detection coil on the eddy current probe. When the detection coil is close to the workpiece to be inspected, the eddy current is induced on the surface of the workpiece and a magnetic field opposite to the direction of the original magnetic field is generated at the same time, which partially offsets the original magnetic field, resulting in changes in the resistance and inductance of the detection coil. If there is a defect in the metal workpiece, the intensity and distribution of the eddy current field will be changed, and the impedance of the coil will change, which will lead to a change in the feedback signal. These changes can be detected by monitoring changes in coil impedance. The feedback signal generated on the detection coil itself contains a lot of high-frequency clutter. After passing through the low-pass filter (LPF) shown in Figure 2, these interference signals can be effectively filtered out to improve the quality of the detection signal. The detection signal amplitude is generally only a dozen mv, which is prone to attenuation. When the detection signal decays to a certain extent, it will become an invalid signal. Therefore, as shown in FIG. 1 , the present invention adds a suitable amplitude amplifier (Amplifier) to the detection circuit to realize the amplification of the detection signal amplitude, thereby further improving the detection signal quality and anti-interference ability.

g. The amplified detection signal becomes a digital signal after analog-to-digital (AD) conversion. The digital signal is a multi-frequency superimposed signal, which contains defect information of the inspected object. It is necessary to analyze this digital signal, that is, to separate each different frequency signal, and calculate the real part and imaginary part of each frequency signal separately. The analysis method is to perform DFT discrete Fourier transform on the multi-frequency detection digital signal at the set frequency points (F1~F5), and obtain the corresponding real part and imaginary part (ri, zi) of each frequency signal in the frequency domain. where (ri, zi) corresponds to the variation of detection coil impedance at different frequencies.

h. Send the analyzed (ri, zi) values to the CPU for professional eddy current testing analysts to complete the analysis and evaluation of the eddy current testing results. The real part and the imaginary part correspond to the amplitude and phase of the eddy current testing coil. The amplitude and phase of the analysis flaw detection. When the probe is in a conductor with a uniform medium and no damage, the amplitude and phase of the detection coil are relatively stable. When the eddy current probe passes through a defective part, it will cause a change in the amplitude and phase of the detection coil, and the larger the defect, the greater the change. It is through the change of the real part and the imaginary part that analysts can analyze and evaluate the results of eddy current flaw detection. The analysis and evaluation process can refer to the book "Eddy Current Testing" published by Machinery Industry Press. The analysis and evaluation process is not the original The key points of the invention are not repeated here.

The above-mentioned embodiments are only to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and not to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (4)

1. A digital multifrequency eddy current signal processing method is characterized by comprising the following steps:

a. setting a plurality of groups of eddy current detection frequency combinations;

b. converting sinusoidal signals of different frequencies generated by combination into digital sinusoidal signals;

c. the digitized sine wave signal is processed by the formula sigma _i cos(2πF _i t+φ _i ) Performing superposition processing;

d. performing digital-to-analog conversion on the superposed first digital signal to obtain an analog sine wave signal;

e. amplifying the analog sine wave signal to form an excitation signal for driving the eddy current probe;

f. the generated excitation signal acts on a detection coil on the eddy current probe to generate a feedback signal;

g. performing analog-to-digital conversion on the feedback signal to obtain a second digital signal, analyzing the second digital signal to obtain different frequency signals, and independently calculating a real part and an imaginary part of each frequency signal;

h. and analyzing and evaluating the eddy current flaw detection result according to the real part and the imaginary part of the analyzed frequency signal.

2. The method of processing digital multifrequency eddy current signals as defined in claim 1, wherein: and e, filtering the excitation signal after the step e.

3. The method of claim 2, wherein the method further comprises: and amplifying the filtered signal after the excitation signal is filtered.

4. The method of processing digital multifrequency eddy current signals as defined in claim 1, wherein: in step g, after the second digital signal is obtained, performing discrete fourier transform on the second digital signal to obtain real parts and imaginary parts (ri, zi) corresponding to each frequency signal in a frequency domain, wherein (ri, zi) correspond to the change of the impedance of the detection coil at different frequencies.

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