A system and method for non-destructive testing of a cable based on electromagnetic induction

Abstract

The application discloses a kind of cable defect nondestructive testing system and method based on electromagnetic induction, method includes the following steps: S1, closing switch, make detection circuit become passage;S2, by transmission device makes that probe moves along cable line, in the process of moving, by oscilloscope obtains detection signal;S3, using Superlets signal processing method to detection signal is handled, obtains the time-frequency diagram of detection signal when probe at different positions.This application provides a kind of cable defect nondestructive testing system and method based on electromagnetic induction, it is non-injury to the object to be measured, will not cause secondary injury, is expected to be used in the nondestructive testing of cable.

Description

Technical Field

[0001] This invention belongs to the field of industrial testing technology, specifically relating to a non-destructive testing system for cable defects based on electromagnetic induction. Background Technology

[0002] Due to limited urban space and the need to improve the city's appearance, more and more overhead cables are being replaced by underground cables, which occupy less land and are less susceptible to severe weather. The construction of underground cables in cities has become a trend in the new era. Accurate and rapid detection of cable damage can greatly improve the speed of cable maintenance and operational safety. Currently, there are relatively few non-destructive testing methods for cables; the more commonly used methods are... X Radiographic nondestructive testing (NDT) is available, but this method requires offline operation, which is not only inconvenient but also expensive. Therefore, it is generally used for spot checks. It is difficult to adapt to continuous online inspection of underground cables. Therefore, there is an urgent need to research new NDT methods to adapt to the complex underground cable environment. Summary of the Invention

[0003] In view of the above-mentioned shortcomings in the prior art, the present invention provides a cable defect non-destructive testing system and method based on electromagnetic induction, which solves the problem that commonly used non-destructive testing methods cannot adapt to the complex underground cable environment.

[0004] To achieve the above-mentioned objectives, the technical solution adopted by this invention is: a non-destructive testing system for cable defects based on electromagnetic induction, the system comprising:

[0005] A function generator is used to generate an excitation voltage signal of a specific frequency.

[0006] Power amplifier, used to adjust the operating voltage of the system;

[0007] The transmission device is used to fix the positioning device and also to control the movement of the positioning device;

[0008] A positioning device for fixing two parallel probes;

[0009] A data processing device includes an oscilloscope and a computer, wherein the oscilloscope is used for visualizing and storing the detected signal, and the computer is used for processing and analyzing the detected signal.

[0010] Furthermore: the probe is located outside the cable and at the same distance from the cable axis, and an electromagnetic wave shield is provided around the probe;

[0011] The cable is located in the detection circuit, which is equipped with a switch.

[0012] A method for a non-destructive testing system for cable defects based on electromagnetic induction, the method comprising the following steps:

[0013] S1, closing the switch, making the detection circuit into a pass;

[0014] S2, moving the probe along the cable through the transmission device, and obtaining the detection signal through the oscilloscope during the movement;

[0015] S3, processing the detection signal using the Superlets signal processing method to obtain the time-frequency diagram of the detection signal at different positions of the probe.

[0016] Further, in S1, the voltage of the detection circuit is alternating current.

[0017] Further, in S2, the method for obtaining the detection signal is specifically:

[0018] S21, calculating the total magnetic flux of the measured point to obtain the magnetic induction intensity of the measured point;

[0019] S22, calculating the electric field intensity of the measured point according to the magnetic induction intensity of the measured point, and taking it as the detection signal.

[0020] Further, in S21, the total magnetic flux of the measured point is calculated The expression is specifically:

[0021]

[0022] In the formula, is the defect depth, is half of the damage length, is the vertical distance between the measured point and the cable, is the distance between the measured point and the damage center, is the vacuum permeability, and are functions of the detection time t The expression is specifically:

[0023]

[0024]

[0025] In the formula, is the current amplitude, is the angular frequency, is the initial phase.

[0026] Further, in S22, the expression for calculating the electric field intensity of the measured point is specifically as follows:

[0027] .

[0028] Further, the S3 comprises the following steps:

[0029] S31, establishing a wavelet set with a fixed center frequency;

[0030] S32, obtaining the response of each wavelet in the wavelet set to the detection signal, to obtain the time-frequency diagram of the detection signal when the probe is at different positions.

[0031] The above further scheme has the beneficial effect that the Superlets signal processing method can simultaneously consider high time and high frequency resolution.

[0032] Further, in the S31, the expression of the wavelet set with a fixed center frequency is specifically as follows:

[0033]

[0034] wherein, n is the order of the SLs, is the period number of each wavelet in the wavelet set, and is the total period number of the wavelets, is an improved Morlet function, and the expression thereof is specifically as follows:

[0035]

[0036] wherein, n is the order of the SLs, is the period number of the mother wavelet, is a time expansion parameter, and the expression thereof is specifically as follows:

[0037]

[0038] wherein, n is the order of the SLs. is a control parameter.

[0039] Further, in the S32, the expression of the response of each wavelet in the wavelet set to the detection signal is specifically as follows:

[0040]

[0041] wherein, n is the order of the SLs, is the response of the wavelet i to the detection signal, x and the expression thereof is specifically as follows:

[0042]

[0043] wherein, n is the order of the SLs, is a complex convolution operator, ​​​For…

[0044] The application provides a cable defect nondestructive detection system and method based on electromagnetic induction, which is nondestructive to the measured object and cannot cause secondary damage, and is expected to be used for nondestructive detection of cables. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 A schematic diagram of the cable defect nondestructive detection system based on electromagnetic induction of the application Figure 1 ;

[0046] Figure 2 A schematic diagram of the cable defect nondestructive detection system based on electromagnetic induction of the application Figure 2 ;

[0047] Figure 3 A flow chart of the cable defect nondestructive detection method based on electromagnetic induction of the application

[0048] Figure 4 A schematic diagram of a magnetic field in a current-carrying (infinite length) straight wire under damage state

[0049] Figure 5 A schematic diagram of the influence of different cable depths on electric field intensity of a detection point

[0050] Figure 6 A schematic diagram of the influence of different cable lengths on electric field intensity of a detection point

[0051] Figure 7 A result diagram of a detection signal obtained by numerical solution

[0052] Figure 8 A result diagram of a detection signal obtained by processing the result in the Figure 7 application by Superlets

[0053] Figure 9 A schematic diagram of a signal collected by an oscilloscope

[0054] Figure 10 A result diagram of FFT processing of the signal in the Figure 9 application

[0055] Figure 11 A time-amplitude curve diagram of test data

[0056] Figure 12 A detection result diagram of a cable in a perfect state and a defect state DETAILED DESCRIPTION

[0057] The specific embodiments of the present application are described below to enable those skilled in the art to understand the present application, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions using the concept of the present application are within the scope of protection.

[0058] Embodiment 1:

[0059] As shown in Figure 1 and Figure 2 In one embodiment of the present application, an electromagnetic induction-based non-destructive testing system for cable defects, the system comprises:

[0060] a function generator for generating an excitation voltage signal of a specific frequency;

[0061] a power amplifier for adjusting the operating voltage of the system;

[0062] a transmission device for fixing the positioning device and also for controlling the movement of the positioning device;

[0063] a positioning device for fixing two parallel probes;

[0064] a data processing device comprising an oscilloscope for detecting the visualization and storage of signals, and a computer for processing and analyzing the detection signals.

[0065] The probes are located outside the cable and equidistant from the axis of the cable line, and an electromagnetic wave shield is provided around the probes;

[0066] The cable is located in a detection circuit, and the detection circuit is provided with a switch.

[0067] As shown in Figure 3 A method for an electromagnetic induction-based non-destructive testing system for cable defects, the method comprises the following steps:

[0068] S1, close the switch to make the detection circuit a pass;

[0069] S2, move the probes along the cable line by the transmission device, and obtain the detection signal by the oscilloscope during the movement;

[0070] S3, process the detection signal using the Superlets signal processing method (Superlets Method) to obtain the time-frequency diagram of the detection signal at different positions of the probes.

[0071] In the S1, the voltage of the detection circuit is alternating current.

[0072] In step S2, the method for obtaining the detection signal is as follows:

[0073] S21. Calculate the total magnetic flux at the measured point to obtain the magnetic induction intensity at the measured point;

[0074] S22. Calculate the electric field strength at the measured point based on the magnetic induction intensity at the measured point, and use it as the detection signal.

[0075] In this embodiment, as Figure 4 The diagram shows the magnetic field under defect conditions of an infinitely long current-carrying straight conductor. P For the measured point, For defect depth, Half the length of the damage The perpendicular distance between the measured point and the cable. The distance between the measured point and the damage center. The angle between the line connecting the lower end of the intact upper section and the measured point and the direction of the cable axis. The angle between the line connecting the upper end of the damaged section and the measured point and the direction of the cable axis. The angle between the line connecting the lower end of the damaged section and the measured point and the direction of the cable axis. The angle between the line connecting the upper end of the intact lower section to the point being measured and the direction of the cable axis is given. Since the cable is considered to be of infinite length, the angles between the line connecting the point being measured to the upper and lower ends of the cable and the direction of the cable axis are 180° and 0°, respectively.

[0076] According to the Biot-Saffar law, the formula for calculating the magnetic flux of a long straight current-carrying conductor is:

[0077]

[0078] measured point P Measured magnetic flux For the magnetic flux of the intact upper section Magnetic flux in the intermediate damaged section Magnetic flux of the intact lower segment The sum of .

[0079]

[0080]

[0081]

[0082] In step S21, the total magnetic flux at the measured point is calculated. The specific expression is:

[0083]

[0084] In the formula, is the depth of the defect, is half of the length of the damage, is the vertical distance between the measured point and the cable, is the distance between the measured point and the center of the damage, is the vacuum permeability, and are functions of the detection time, whose expressions are as follows: t

[0085]

[0086]

[0087] wherein, is the current amplitude, is the angular frequency, is the initial phase.

[0088] In the S22, the expression of the electric field intensity of the measured point is as follows:

[0089] .

[0090] From the above formula, it can be seen that the electric field intensity is related to the depth, length of the defect and the speed of the detection moving device. Theoretical analysis is performed on each parameter to obtain the influence law of the parameters on the detection result, as shown in Figure 5~Figure 6 , and Figure 5 and Figure 6 show the influence of the depth and length of the damage on the detection result, respectively. In the case where other parameters are constant, it can be seen from Figure 5 that the electric field intensity is smaller with the increase of the depth. According to Figure 6 , the electric field intensity is smaller when the length of the damage is larger. Therefore, with the increase of the depth and length of the damage, the electric field intensity is smaller and smaller.

[0091] In addition to the influence of the depth and length of the defect on the detection, the moving speed of the detection device also has an influence. In the following, the moving condition of the probe along the cable is analyzed by using the numerical analysis method. When the cable has a defect, the position of the defect is set at the middle position of the cable, and the probe is moved at a constant speed from one section of the cable to the other end. The detection result can be obtained by solving the expression of the electric field intensity of the measured point as shown in Figure 7~8 ​As shown, when the defect depth and length are certain, the probe moves at a certain speed near the cable, and when the moving time is 50s, the electric field intensity near the cable changes obviously. The position corresponding to the lowest point between the two peaks corresponds to the center position of the defect. Accordingly, the defect position can be determined. In addition, in order to more accurately determine the accurate position of the defect, the Superlets method is used to process the signal, and the curve shown in the figure can be obtained. Figure 8 The physical meaning of the curve is the same as Figure 7 , except that the method can obtain more accurate defect position and length / depth parameters. In terms of experimental verification, the data processing is carried out by using the method.

[0092] The S3 comprises the following steps:

[0093] S31, establishing a wavelet set with a fixed center frequency;

[0094] S32, obtaining the response of each wavelet in the wavelet set to the detection signal to obtain the time-frequency diagram of the detection signal when the probe is at different positions.

[0095] In this embodiment, the principle of the Superlets signal processing method is to establish a wavelet set with a fixed center frequency and a series of different periods, and the processing method is similar to wavelet transform. The advantage of this method is that high time and high frequency resolution can be considered at the same time.

[0096] In the S31, the expression of the wavelet set with a fixed center frequency is as follows:

[0097]

[0098] In the formula, n is the order of SLs, is the period number of each wavelet in the wavelet set, and , is the total period number of the wavelet, is the improved Morlet function, and its expression is as follows:

[0099]

[0100] In the formula, n is the order of SLs, is the period number of each wavelet in the wavelet set, and is the time expansion parameter, and its expression is as follows:

[0101]

[0102] In the formula, n is the order of SLs, ​​is a control parameter, and its value is generally set to 5, which controls the time variance of the wavelet.

[0103] In the S32, the expression of the response of each wavelet in the wavelet set to the detection signal is

[0104]

[0105] is the response of the wavelet to the detection signal, and its expression is specifically as follows: i x

[0106]

[0107] is a complex convolution operator,

[0108] In this embodiment, the response is not the actual intensity of the signal, and after processing by the Superlets signal processing method, only half of the actual intensity of the signal can be recovered; the Superlets signal processing method is similar to the CWT, and only the wavelet set is used instead of the wavelet, so the first-order SL transform is the CWT.

[0109] Embodiment 2:

[0110] This embodiment is directed to the specific experiment of embodiment 1.

[0111] In actual detection of cable defects, the moving speed of the probe has an influence on the electric field intensity, and therefore, the experimental method can be carried out in two ways, i.e., the positioning device is kept in a stationary state (speed is 0) and a moving state (speed is not 0).

[0112] The moving state of the positioning device is analyzed, and a copper core is used as a wire to replace the actual cable. From the inside to the outside, there are a copper core, an aluminum pipe, an insulating pipe, a shielding aluminum pipe and an insulating tape, wherein the defect in the copper core is a man-made defect.

[0113] Prepare the experimental instruments, and the experimental instruments include a function generator, a power amplifier, a detection device, an oscilloscope and a transmission device controller and the like. The detection device includes a positioning device and a transmission device, and the transmission device controller is mainly used for the movement control of the positioning device.

[0114] After the experimental instruments are prepared, the online nondestructive testing experiment of the cable is preliminarily carried out. In order to avoid the interference of external electromagnetic signals, the frequency of the excitation signal of the function generator is set to 20 Hz. The positioning device moves at a certain speed from one side of the cable to the other side at a constant speed, and the obtained signal is displayed and collected by the oscilloscope, and the signal is as follows​​​​​​Figure 9 As shown. For Figure 9 The signal shown can be processed using FFT to obtain its amplitude-frequency curve, as shown below. Figure 10 As shown, it can be clearly seen that the cable operates at a frequency of 20Hz.

[0115] To further pinpoint the exact location of the cable defect, the Superlet signal processing method was used. Figure 9 The data shown is processed, and the result is as follows: Figure 11 The results are shown. When the positioning device moves at a constant speed to the defect location, a peak will be generated at the end of the defect opening, and the location of the cable defect is between the two peaks. Under the same detection parameters, the error between the experimental results and the theoretical results is about 2.6 cm. It should be noted that the probe moves at a constant speed along the cable axis and must ensure that the distance to the cable axis is equal.

[0116] The static state of the positioning device was analyzed. The probe spacing was set to 15mm, and the oscilloscope sampling frequency was 500Hz. The device was kept stationary at both the gap and intact parts of the cable. The cable was inspected, and the data was processed and analyzed using the Superlet method. The results are as follows: Figure 12 As shown.

[0117] Depend on Figure 12 It can be seen that when there is a gap in the cable, because the probe is relatively far from the intact cable, the electric field strength at the measured point is... As can be seen from the expression, the electric field strength when there is a defect is less than the electric field strength when there is a perfect defect.

[0118] The beneficial effects of this invention are as follows: This invention provides a cable defect non-destructive testing system and method based on electromagnetic induction, which causes no damage to the object being tested and will not cause secondary harm, making it promising for non-destructive testing of cables. It enables online testing with high efficiency; the portable device allows for on-site testing.

[0119] In the description of this invention, it should be understood that the terms "center," "thickness," "upper," "lower," "horizontal," "top," "bottom," "inner," "outer," and "radial," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying the relative importance or the number of technical features implicitly specified. Therefore, a feature defined by "first," "second," and "third" may explicitly or implicitly include one or more of that feature.

Claims

1. A method of electromagnetic induction based non-destructive testing of a cable defect, the electromagnetic induction based non-destructive testing of a cable defect comprising: A function generator for generating an excitation voltage signal of a specific frequency; A power amplifier for adjusting the operating voltage of the system; A transmission device for fixing the positioning device and for controlling the movement of the positioning device; A positioning device for fixing two parallel probes; The probes are located outside the cable and equidistant to the axis of the cable, and the probes are surrounded by electromagnetic wave shielding covers; The cable is located in a detection circuit, and the detection circuit is provided with a switch; A data processing device including an oscilloscope for detecting the visualization and storage of signals and a computer for processing and analyzing the detection signals, characterized in that the method comprises the following steps: S1, closing the switch to make the detection circuit a pass; S2, moving the probes along the cable line through the transmission device, and obtaining the detection signal through the oscilloscope during the movement; In S2, the method for obtaining the detection signal is specifically: S21, calculating the total magnetic flux of the measured point to obtain the magnetic induction intensity of the measured point; S22, calculating the electric field intensity of the measured point according to the magnetic induction intensity of the measured point, and taking it as the detection signal; The total magnetic flux of the measured point is calculated The expression is specifically: wherein is the defect depth, is half the length of the damage, is the vertical distance of the measured point from the cable, is the distance of the measured point from the center of the damage, is the vacuum permeability, and are functions of the detection time t and are expressed as follows: wherein is the current amplitude, is the angular frequency, is the initial phase, v is the probe's moving speed; S3, processing the detection signal by using the Superlets signal processing method to obtain the time-frequency diagram of the detection signal when the probes are at different positions; S3 includes the following steps: S31, establishing a wavelet set with a fixed center frequency; S32, obtaining the response of each wavelet in the wavelet set to the detection signal to obtain the time-frequency diagram of the detection signal when the probes are at different positions; In the S31, the expression of the wavelet set with a fixed center frequency is specifically​ In the formula, is the order of the SLs, is the number of periods of each wavelet in the wavelet concentration, and , is the total number of periods of the wavelets, is an improved Morlet function, and its expression is specifically as follows: In the formula, is a time expansion parameter, whose expression is specifically as follows: In the formula, is a control parameter; In the S32, the response of each wavelet of the wavelet set to the detection signal The expression is specifically: wherein is a wavelet i in response to the detection signal x whose expression is given by the following equation: In the formula, is a convolution operator.

2. The method of electromagnetic induction based non-destructive testing system for defects in a cable as claimed in claim 1, wherein, In S1, the voltage of the detection circuit is alternating current.

3. The method of electromagnetic induction based non-destructive testing system for defects in a cable of claim 1, wherein, In the S22, the electric field intensity of the measured point is calculated The expression of the electric field intensity is specifically as follows: 。

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