A system and method for detecting bubble defects in cable insulation based on terahertz waves

Abstract

The application belongs to the technical field of nondestructive testing, and provides a cable insulation bubble defect detection system and method based on terahertz waves, and proposes a rotating and translating device suitable for hollow cylindrical samples such as cable insulation, and a matching rapid detection scheme to reduce detection time while ensuring accuracy, so as to realize rapid and comprehensive detection of XLPE cable samples; the scanning control module can control the rotating and translating device to move the sample according to a pre-input scanning step and a defect judgment result; the signal processing system can process data of a time domain reflection signal detected by a terahertz wave receiver, calculate time intervals between pulse signals, and obtain detailed information of internal defects of the sample, and proposes to calculate an average refractive index by using the pulse time intervals to verify the existence of the internal defects of the sample, and the method and system provided by the application can realize rapid and non-contact detection of bubble defects in XLPE cable defects.

Description

Technical Field

[0001] This invention belongs to the field of nondestructive testing technology, and in particular relates to a system and method for detecting bubble defects in cable insulation based on terahertz waves. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] The quality and reliability of the insulation protective layer of high-voltage cables in transmission lines directly affect the safe and economical operation of the power system and the daily lives of the people. However, during the cable insulation production process, factors such as material dampness and insufficient cooling can cause arc-shaped air bubble defects at equidistant locations from the fiber core. Under high-voltage conditions, these defects will generate a strong and uneven electric field around them, which may lead to partial discharge, affecting insulation performance and gradually developing into a factor affecting the stable and safe operation of the power system. Therefore, comprehensive and accurate factory defect testing of cable insulation is extremely important.

[0004] Currently, common non-destructive testing and evaluation methods for cable insulation include ultrasonic, infrared, X-ray, and terahertz wave methods. Among them, ultrasonic methods use ultrasonic waves for contact testing, thus requiring the use of a coupling agent. Furthermore, the diffraction of waves during propagation makes it difficult to accurately determine defects, hindering its direct industrial application. Infrared methods also have many shortcomings, such as the difficulty in producing suitable thermal excitation and improving the resolution of infrared thermal imagers, preventing their widespread industrial application. X-ray methods pose certain health risks to personnel, so their use should be avoided to protect the safety of testing personnel.

[0005] Commonly used terahertz nondestructive testing methods are limited by the sample translation device and cannot achieve a 360-degree circumferential scan. Therefore, the existing terahertz nondestructive testing methods are mostly used for micron-sized flat sheet samples. At the same time, the use of a single scanning point step size usually causes problems such as excessively long scanning time. Especially for millimeter-sized hollow cylindrical XLPE cable insulation samples, due to their special shape and size, it is difficult for commonly used terahertz nondestructive testing methods to accurately, comprehensively and quickly detect the internal defects of the sample. Summary of the Invention

[0006] To address at least one of the technical problems mentioned above, this invention provides a terahertz wave-based system and method for detecting bubble defects in cable insulation. This system combines a scanning control module with a rotating and translating device to achieve a comprehensive scan of the sample, overcoming the limitation of traditional terahertz detection methods that cannot perform comprehensive detection of cylindrical samples. Furthermore, a rapid detection scheme is proposed, employing a preliminary-fine scanning method with two different scanning lengths. This reduces the number of scanning points while maintaining accuracy, thus saving time.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] The first aspect of the present invention provides a terahertz wave-based system for detecting bubble defects in cable insulation, comprising: a terahertz wave transmitter, a terahertz wave receiver, a rotation and translation device, a scanning control module, and a signal processing system;

[0009] The terahertz wave transmitter is used to generate terahertz electromagnetic waves and irradiate the cable insulation sample to be tested.

[0010] The terahertz wave receiver is used to receive terahertz waves reflected from the cable insulation sample under test and internal bubble defects.

[0011] The scanning control module controls the rotation and translation device to perform a preliminary scan of the cable insulation sample to be tested based on the step size of the preliminary scan points, and obtains the time-domain spectral signal of the corresponding scan points.

[0012] The signal processing system compares the initially scanned time-domain spectral signal with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scan point. If so, the scan point is defined as a preliminary defect point. The scanning control module controls the rotation and translation device to perform a fine scan of the cable insulation sample to be tested based on each preliminary defect point. The signal processing system determines the location and size of the bubble defect based on the time-domain spectral signal obtained from the fine scan.

[0013] As one embodiment, the signal processing system includes a signal truncation module, a signal alignment module, a defect judgment module, a pulse interval module, and a defect calculation module;

[0014] The signal truncation module is used to truncate the time-domain spectral signal at each scanning point.

[0015] The signal alignment module is used to perform time-domain alignment of the first reflection pulse in the time-domain spectral signal of each scanning point;

[0016] The defect judgment module is used to compare the time-domain spectral signal received by the terahertz wave receiver after time-domain truncation and signal alignment with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scanning point.

[0017] The pulse interval module is used to calculate the time interval between reflected terahertz wave pulses in a time-domain spectral signal containing defects;

[0018] The defect calculation module is used to calculate the average refractive index and detailed information about the defect at that location using the time interval between reflected pulse peaks calculated in the pulse interval module.

[0019] As one embodiment, the scanning control module controls the rotation and translation device to perform a fine scan of the cable insulation sample to be tested based on each preliminary defect point, including:

[0020] Each initial defect point is sequentially scanned clockwise in a fixed axial position. The defect condition is determined based on the measured time-domain spectral signal, and the result is transmitted to the scanning control module. When there are no internal defects below the scan point, the scanning control module controls the rotation and translation device to end the clockwise circumferential scan at that axial position and controls it to return to the original circumferential position for a reverse circumferential scan. The counterclockwise circumferential scan ends when there are no internal defects at the scan point, thus completing the fine circumferential scan of that axial scan point.

[0021] After the scanning control module drives the rotation and translation device back to the original circumferential position of the initial defect point, it performs an axial movement with the circumferential position fixed, so that the terahertz wave transmitter is aligned with the next axial scanning point. The circumferential scanning is repeated at this point until the time-domain spectral signals of the circumferential scanning in both clockwise and counterclockwise directions do not reflect the internal defect, thus completing the fine circumferential scanning of the next axial scanning point. Until the time-domain spectral signals of the axial scanning points on both sides of the initial defect point do not reflect the internal defect, the scanning control module controls the rotation and translation device to end the fine axial and circumferential scanning, thus completing the fine scanning of the cable insulation sample to be tested at the initial defect point.

[0022] As one embodiment, the rotation and translation device includes a sample fixing device, a rotation axis, and a horizontal axis;

[0023] One end of the rotating shaft is connected to the sample fixing device, and the other end is connected to the horizontal shaft. The sample fixing device is used to fix the sample, and the sample is rotated and translated by the cooperation of the rotating shaft and the horizontal shaft.

[0024] The rotating shaft is used to rotate the sample circumferentially while keeping its horizontal axial position unchanged, and the horizontal shaft is used to move the sample horizontally while keeping its circumferential position unchanged.

[0025] As one embodiment, the signal processing system determines whether there is a bubble defect below the scanning point based on the following: defect judgment is performed based on the time-domain spectral signal. If a pair of reflected pulses with opposite phases appear between the reflected pulses of the air and the inner and outer interfaces of the upper part of the sample, it is the terahertz wave reflection caused by the upper and lower interfaces of the internal insulation defect, and it is determined that there is a defect below the scanning point.

[0026] As one embodiment, the method for obtaining the time-domain spectral signal at the defect-free location is as follows: the cable insulation sample to be tested is fixed by a rotation and translation device, the distance between the cable insulation sample to be tested and the terahertz wave transmitter probe is recorded, the probe is always kept perpendicular to the plane where the sample is to be tested, the transmitted pulse waveform of the terahertz wave transmitter is modulated, the terahertz wave receiver and the terahertz wave transmitter are in the same vertical direction, a defect-free location is selected as the initial detection point at the beginning, and the initial position of the cable insulation sample to be tested and the time-domain spectral signal received by the terahertz wave receiver are recorded as the defect-free reference signal.

[0027] A second aspect of the present invention provides a method for detecting bubble defects in cable insulation based on terahertz waves, comprising the following steps:

[0028] The step size of the initial scanning points is determined based on the defect size of the cable insulation sample to be tested;

[0029] Based on the step size of the preliminary scanning points, a preliminary scan is performed on the cable insulation sample to be tested to obtain the time-domain spectral signal of the corresponding scanning point;

[0030] The time-domain spectral signal obtained from the initial scan is compared with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scan point. If so, the scan point is defined as the initial defect point. Based on each initial defect point, a fine scan is performed on the cable insulation sample to be tested. The location and size of the bubble defect are determined based on the time-domain spectral signal obtained from the fine scan.

[0031] As one embodiment, the detailed scanning of the cable insulation sample to be tested based on each preliminary defect point includes:

[0032] The clockwise circumferential scan is performed sequentially at each preliminary defect point with the axial position fixed. The defect situation is judged based on the measured time-domain spectral signal. When there is no internal defect below the scan point, the clockwise circumferential scan at that axial position ends, and the original circumferential position is returned to perform a reverse circumferential scan. The counterclockwise circumferential scan ends when there is no internal defect at the scan point, thus completing the fine circumferential scan of that axial scan point.

[0033] After returning to the original circumferential position of the initial defect point, perform an axial movement with the circumferential position fixed, align with the next axial scanning point, and repeat the circumferential scanning at this point until the time-domain spectral signals of the circumferential scanning in both clockwise and counterclockwise directions do not reflect the internal defect. This completes the fine circumferential scanning of the next axial scanning point. This process continues until the time-domain spectral signals of the axial scanning points on both sides of the initial defect point do not reflect the internal defect. This completes the fine scanning of the cable insulation sample to be tested at the initial defect point.

[0034] As one embodiment, the basis for determining whether there is a bubble defect below the scanning point is:

[0035] Defect judgment is based on time-domain spectral signals. If a pair of reflected pulses with opposite phases appear between the reflected pulses at the inner and outer interfaces of the air and the upper part of the sample, and there is a significant difference from the same position of the defect-free signal, it is the terahertz wave reflection caused by the upper and lower interfaces of the internal insulation defect. It is determined that there is a defect below the scanning point, and the existence of the defect is verified by calculating the average refractive index.

[0036] As one embodiment, the time-domain spectral signal at the defect-free location is obtained by terahertz wave detection of the cable insulation sample at the defect-free location based on the electromagnetic wave propagation theory model.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] (1) The present invention combines a rotation and translation device to achieve comprehensive testing of the sample, overcoming the shortcomings of traditional terahertz wave testing that cannot comprehensively test cylindrical cable insulation samples.

[0039] (2) This invention proposes a matching rapid detection scheme, which adopts a preliminary-fine scanning method to formulate two different scanning schemes with different timings, thereby reducing the number of scanning points while ensuring accuracy and saving time.

[0040] (3) In the present invention, the signal processing system performs time-domain truncation on the time-domain spectral signal to reduce the influence of noise in the millimeter-sized large sample, performs signal alignment on the time-domain spectral signal to reduce the influence caused by the change in the distance between the probe and the sample in the actual experiment, and proposes a method to feed back the defect judgment result to the scanning control module to complete the fine scanning. Finally, the average refractive index can be calculated based on the time-domain spectral signal to verify the existence of the defect.

[0041] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0042] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0043] Figure 1 This is a schematic diagram of the overall structure of the bubble defect detection system according to Embodiment 1 of the present invention;

[0044] Figure 2 This is a schematic diagram of the interaction between terahertz electromagnetic waves and defective samples in Embodiment 1 of the present invention;

[0045] Figure 3 This is a schematic diagram of the signal processing system structure according to Embodiment 1 of the present invention;

[0046] Figures 4(a)-4(b) This is the defective sample model used in Embodiment 2 of the present invention;

[0047] Figure 5 This is a flowchart of the method for detecting internal bubble defects in XLPE insulation according to Embodiment 2 of the present invention;

[0048] Figure 6 This is a comparison of time-domain spectral signals at the detection points with and without internal bubble defects in Embodiment 2 of the present invention. Detailed Implementation

[0049] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0050] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0051] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0052] Example 1

[0053] See Figure 1 This embodiment provides a terahertz wave-based system for detecting bubble defects in cable insulation, comprising: a terahertz wave transmitter 1, a terahertz wave receiver 2, a rotation and translation device 3, a scanning control module 4, and a signal processing system 5;

[0054] The terahertz wave transmitter 1 is used to generate the terahertz electromagnetic waves required for non-destructive testing and irradiate the XLPE cable insulation sample to be tested.

[0055] The terahertz wave receiver 2 is used to receive terahertz wave signals reflected from the XLPE cable insulation sample to be tested and internal bubble defects.

[0056] The rotation and translation device 3 is used to realize the rotation and translation of the XLPE cable insulation sample, and can be used in conjunction with the terahertz wave transmitter 1 and the terahertz wave receiver 2 to realize the full scanning of the XLPE cable insulation sample.

[0057] The scanning control module 4 controls the rotation and translation device 3 to perform a preliminary scan of the cable insulation sample to be tested based on the pre-input preliminary scan point step size to obtain the time-domain spectral signal of the corresponding scan point.

[0058] The signal processing system 5 is used to compare the time-domain spectral signal initially scanned by the terahertz wave receiver 2 with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scan point. If there is, the scan point is defined as the initial defect point and the average refractive index is calculated to verify the existence of the defect.

[0059] The scanning control module 4 controls the rotation and translation device 3 to perform fine scanning of the cable insulation sample to be tested based on the fine scanning point step size and the defect results fed back by the signal processing system 5, and the fine scanning signal obtained by the terahertz wave receiver 2 is processed by the signal processing system 5. The signal processing system 5 determines the position and size of the bubble defect based on the time-domain spectral signal obtained from the fine scanning.

[0060] It should be noted that the terahertz wave transmitter 1 and the terahertz wave receiver 2 are located on the same side of the XLPE cable insulation sample to be tested.

[0061] In this embodiment, the terahertz pulse frequency range of the terahertz wave transmitter 1 is within 0.1-5THz.

[0062] like Figure 1 As shown, the rotation and translation device 3 includes a sample fixing device 31, a rotation axis 32, and a horizontal axis 33;

[0063] One end of the rotating shaft 32 is connected to the sample fixing device 31, and the other end is connected to the horizontal shaft 33. The sample fixing device 31 is used to fix the sample. The rotating shaft 32 and the horizontal shaft 33 cooperate with each other to realize the rotation and translation of the sample.

[0064] Specifically, the rotating shaft 32 is used to rotate the sample circumferentially while keeping its horizontal axial position unchanged, so as to achieve circumferential scanning of the sample.

[0065] The horizontal axis 33 is used to perform horizontal axial movement of the sample with the circumferential position unchanged, so as to achieve axial scanning of the sample.

[0066] like Figure 3 As shown, the signal processing system 5 includes a signal truncation module 51, a signal alignment module 52, a defect judgment module 53, a pulse interval module 54, and a defect calculation module 55;

[0067] The signal truncation module 51 is used to truncate a time-domain spectral signal of a certain time length;

[0068] Considering that terahertz waves will attenuate and disperse during propagation in insulation and air, resulting in indistinct long-distance reflected signals and difficulty in distinguishing noise and defects, the signal truncation module 51 truncates the signal waveform in the time domain at a certain point to ensure that the reflected pulse is not drowned out by noise.

[0069] The signal alignment module 52 is used to align the first reflection pulse (i.e. the reflection pulse at the interface between air and sample) of the time-domain spectral signal at each scanning point in the time domain to eliminate errors caused by the change in distance between the terahertz wave transmitter and the sample surface during the experiment.

[0070] Because the distance between the terahertz wave transmitter 1 and the sample surface may change due to factors such as changes in the height of the rotating translation device 3 and vibration of the platform during the experiment, the change in distance will cause the reflected pulse to be ahead or delayed in time, thus affecting the judgment of the defect location. Therefore, the signal alignment module 52 performs time-domain alignment on the first reflected pulse in the time-domain spectral signal of each scanning point to reduce the impact of distance change on the detection results.

[0071] The defect judgment module 53 is used to compare the time-domain spectral signal after signal truncation and alignment with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scanning point. If a bubble defect is determined to exist in the preliminary scan, the corresponding scanning point is defined as the preliminary defect point. Based on the preliminary defect point, the scanning control module 4 controls the rotation and translation device 3 to perform a fine scan on the preliminary defect point according to the defect information fed back by the defect judgment module 53 to determine the accurate information of the defect.

[0072] In the defect judgment module 53, the time-domain spectral signal of each scan point after processing is compared with the signal at the defect-free location. If a pair of pulses with opposite waveforms appear in the middle of the scan point signal, and there is a significant difference from the signal at the same position as the defect-free location, then it is considered that there is a bubble defect below the scan point.

[0073] Reference Figure 2The specific principle is as follows: When there is a bubble defect below the scanning point, the terahertz wave will be reflected once at the upper and lower interfaces of the defect. According to the half-wave loss, when the wave is incident from the optically less dense medium to the optically denser medium, the vibration phase of the reflected wave when it leaves the reflection point is π different from the vibration phase of the incident wave when it reaches the incident point. Conversely, no phase difference will occur. Therefore, when the terahertz wave propagates to the lower interface of the bubble defect, the reflected signal will experience half-wave loss, and the phase difference between the reflected waveform and the incident waveform is π. When it propagates to the upper interface of the bubble defect, the reflected signal will not experience half-wave loss. That is, the reflected pulses generated at the upper and lower interfaces of the defect are out of phase. The defect judgment module 53 uses the reflected signals of each scanning point to judge the defect. If a pair of reflected pulse signals with opposite phases appear between the reflection peaks of the air and the inner and outer interfaces of the sample, it is the reflection of the terahertz wave caused by the upper and lower interfaces of the bubble defect inside the insulation. It is judged that there is a defect below the scanning point.

[0074] The pulse interval module 54 is used to calculate the time interval between reflected terahertz wave pulses in a time-domain spectral signal containing defects.

[0075] The defect calculation module 55 is used to calculate the average refractive index to verify the existence of defects and the specific information of internal defects of the sample by using the pulse time interval obtained by the pulse interval module 54.

[0076] In this embodiment, the scanning control module controls the rotation and translation device to perform a preliminary scan of the cable insulation sample to be tested based on the preliminary scan point step size, as follows:

[0077] First, a suitable initial scanning point interval is selected based on the size of the test sample and its internal bubble defects to perform an initial scan. The information is then imported into the scanning control module 4, which controls the rotation and translation device 3 to move the sample to complete the detection of each initial scanning point. The signal is received by the terahertz wave receiver 2 and then transmitted to the signal processing system 5. After signal processing, the initial defect point is determined by the defect judgment result in the defect judgment module 53, and the approximate location of the bubble defect is located.

[0078] The scanning control module controls the rotation and translation device to perform fine scanning based on each preliminary defect point, the fine scanning point step size, and the defect judgment result. This includes:

[0079] (1) The scanning control module 4 drives the rotation and translation device 3 to move the initial defect point to below the terahertz wave transmitter 1, and then performs a clockwise circumferential scan with the axial position fixed. The measured time-domain spectral signal is transmitted to the signal processing system 5 for further processing and the defect judgment result is fed back to the scanning control module 4. The scanning control module 4 controls the rotation and translation device 3 to end the clockwise circumferential scan at the axial position and controls the rotation and translation device 3 to return to the original circumferential position to start the reverse circumferential scan. Similarly, the counterclockwise circumferential scan ends when the scan point does not reflect internal defects, which is considered as the completion of the fine circumferential scan of the axial scan point.

[0080] (2) The scanning control module 4 controls the rotation and translation device 3 to return to the original circumferential position of the initial defect point, and then performs an axial movement with the circumferential position fixed, so that the terahertz wave transmitter 1 is aligned with the next axial scanning point. The previous circumferential scan is repeated at this point until the time domain spectral signal of the circumferential scan in both clockwise and counterclockwise directions does not reflect the internal defect. Then the fine circumferential scan of the next axial scanning point is completed. The rotation axis 32 returns to the circumferential position of the original initial defect point, and the horizontal axis 33 moves to the next axial scanning point to repeat the fine circumferential scan until the time domain spectral signal of the axial scanning points on both sides of the initial defect point does not reflect the internal defect. Only then will the scanning control module 4 control the rotation and translation device 3 to end the fine scan of the initial defect point and move to the next initial defect point for fine scan. If there are other initial defect points in the fine scan area of ​​this initial defect point, they can be ignored and there is no need to repeat the fine scan of other initial defect points in this area.

[0081] Initially, the sample should be fixed and a defect-free location should be selected as the initial detection point to determine the defect-free reference signal. Specifically:

[0082] The test sample is fixed by the sample fixing device 31 of the rotation and translation device 3. The distance between the test sample and the probe of the terahertz wave transmitter 1 is recorded. During the experiment, the probe should always be perpendicular to the plane where the test position of the sample is located. The emission pulse of the transmitter 1 is modulated. The terahertz wave receiver 3 and the transmitter 1 are in the same vertical direction. At the beginning, a defect-free position is selected as the initial detection point. The initial position of the sample and the time-domain spectral signal received by the terahertz wave receiver 2 are recorded as the defect-free reference signal.

[0083] Example 2

[0084] This embodiment provides a method for detecting bubble defects in cable insulation based on terahertz waves. In the experiment, the sample is fixed by a fixing device of a rotation and translation mechanism. The fixing device is connected to both the rotation axis and the horizontal axis to achieve rotation and translation of the sample. This example uses COMSOL Multiphysics 5.6 simulation software to build a sample model and sets different rotation angles and positions of the sample to simulate the function of the rotation and translation mechanism in actual experiments. Combined with a matching rapid detection scheme, comprehensive detection of the sample and defect calculation are achieved. Figures 4(a)-4(b) Figure 4(a) is a front view of a cable insulation sample containing bubble defects, and Figure 4(b) is a schematic diagram of a cross-section containing defects.

[0085] like Figure 5 As shown, the specific steps include the following:

[0086] Step 1: Select a commonly used 110kV cable insulation model for industrial applications and perform software modeling. Specific details are as follows:

[0087] The hollow radius of the XLPE insulation sample is 15 mm, and the refractive index is 1.

[0088] The XLPE insulation sample has an insulation thickness of 16.5 mm and a refractive index of 2.25.

[0089] A bubble defect layer with a refractive index of 1 is set at a distance of 23 mm from the center of the sample, equidistant from the center, with a height of 0.5 mm, a thickness of 1 mm, and an angle of 60 degrees.

[0090] To reduce simulation calculation time, the upper surface of the sample is 2 cm away from the terahertz wave transmitter and 5 mm away from the terahertz wave receiver (slightly larger than the terahertz pulse wavelength is acceptable);

[0091] Step 2: Set the excitation pulse E0 to a Gaussian modulated sine pulse.

[0092]

[0093] Where ω0=2πf, in this experiment we take 0.4THz as the pulse frequency f, and t0 determines the time domain position of the pulse. The pulse waveform can be adjusted by changing the frequency f and the pulse width Δt.

[0094] Step 3: Establish a terahertz wave reflection model based on the electromagnetic wave propagation theory of interaction between the terahertz wave and the sample;

[0095] Step 4: At the start of the detection, select a defect-free location as the initial detection point and record the reflected time-domain spectral waveform as the defect-free reference signal;

[0096] Step 5: Based on the size of the test sample and its internal bubble defects, select an appropriate initial scanning point interval for initial scanning. To avoid skipping defects due to excessively large scanning point spacing, the axial interval of scanning points should not be greater than half the defect thickness.

[0097] In this embodiment, 1 / 4 of the defect thickness, i.e. 0.25 mm, is taken as the initial axial scanning step size, and 5 degrees is taken as the initial circumferential scanning step size. That is, there is a set of axial scanning points every 5 degrees on the surface of the insulating sample, which are defined as the initial scanning points and used to detect the approximate location of the bubble defect.

[0098] Step 6: Simulate the function of the rotation and translation device using a rotation model, rotate and move each preliminary scanning point to below the terahertz wave transmitter for preliminary scanning, and have the receiver collect the time-domain spectral signal at each scanning point;

[0099] Step 7: Input the time-domain spectral signals of each preliminary scan point from Step 6 into the signal processing system for data processing and defect judgment, and calculate the time interval between reflected pulse peaks, such as... Figure 6 As shown, pulse numbers are defined based on the time-domain spectral signals reflected by each scanning point of the sample to facilitate the next step of calculation. In the time-domain spectral signal, reflection pulse 1 is the reflection at the interface between air and the outer layer of the sample, reflection pulses 2 and 3 are the reflections at the upper and lower interfaces between the sample and the internal air defects, and reflection pulse 4 is the reflection at the interface between the inner layer of the sample and the air. For scanning points without defects, only reflection pulses 1 and 4 exist in their time-domain spectral signals.

[0100] The data processing includes the following steps:

[0101] First, the time-domain spectral signal is truncated at 260 ps, ​​and then time-domain alignment is performed based on the first reflected pulse of the signal to reduce the influence of noise and the change in distance between the terahertz wave transmitter and the sample.

[0102] Defect identification includes the following steps:

[0103] Defect judgment is performed on the time-domain spectral signal after data processing. If a pair of reflection pulses with opposite phases appear between the reflection pulses at the inner and outer interfaces of the air and the upper part of the sample (reflection pulse peaks 1 and 4), it is the terahertz wave reflection caused by the upper and lower interfaces of the internal defects of the insulation, and it is judged that there is a defect below the scanning point.

[0104] Finally, the time interval between reflected pulse peaks in the defective signal was calculated.

[0105] Step 8: Based on the results in Step 7, define the preliminary scan point with defects as the preliminary defect point, and calculate the average refractive index of the preliminary defect point to verify that there is a defect below it.

[0106] The scan points containing defect features in the preliminary scan signal are designated as preliminary defect points. The average refractive index at these preliminary defect points is calculated and compared with the refractive index when there are no defects to verify the existence of the defects below. The specific steps include the following:

[0107] The average refractive index n at each preliminary defect point is calculated based on the time interval between reflection pulse peaks 1 and 4 in the time-domain spectral signal. AVG :

[0108]

[0109] Where c is the speed of light, Δt1 is the time interval between reflection pulses 1 and 4 calculated by the signal processing system, D is the thickness of the insulating sample, and n AVG If the refractive index is less than that of the original insulating material, it verifies that there is an air defect below the scanning point.

[0110] Step 9: Perform a more detailed scan around the initial defect point to accurately determine the location and size of the defect.

[0111] A fine scan with smaller scanning intervals is performed centered on the initial defect point until no defect can be detected. During the fine scan, scanning points located in the same circumferential position as the initial defect point are defined as axial scanning points, and scanning points in the same axial position are defined as circumferential scanning points. In this embodiment, 0.1 mm and 1 degree are used as the axial and circumferential scanning step sizes for the fine scan.

[0112] The detailed scanning process is as follows:

[0113] In this embodiment, simulation software is used to simulate the movement process of the sample in the experiment. First, the simulation scanning control module controls the rotation and translation device to move the initial scanning point to below the terahertz transmitter. Then, a clockwise circumferential scan with a fixed horizontal axis and a circumferential step of 1 degree is performed until the time-domain spectral signal of the circumferential scanning point does not reflect the internal defect after being judged by the signal processing system. The clockwise circumferential scan of the axial position ends, and the rotation and translation device returns to the original circumferential position of the initial defect point. Then, a counterclockwise circumferential scan with a step of 1 degree is performed until the signal does not reflect the internal defect. This is considered as the completion of the fine circumferential scan of the axial scanning point.

[0114] The simulation scanning control module then controls the rotating axis to return to the circumferential position of the initial defect point. Then, the rotating axis moves axially in a fixed step of 0.1 mm to align the transmitter with the next axial scanning point and perform a fine circumferential scan. After completion, the rotating axis returns to the original circumferential position of the initial defect point. The horizontal axis moves to the next axial scanning point and this step is repeated until the time-domain spectral signals of the axial scanning points on both sides of the initial defect point no longer reflect the internal defect. This is considered as completing the fine scan near the initial defect point. Then, the same fine scan is performed on the next initial defect point. If there are other initial defect points in the fine scan area of ​​this initial defect point, they can be ignored, and there is no need to repeat the fine scan on other initial defect points in this area.

[0115] After performing a more detailed scan with denser scanning points near the initial defect point, the resulting time-domain spectral signal is processed by a signal processing system to obtain the time interval between reflected pulse peaks, and the location and size of air defects inside the sample are further calculated.

[0116] For the defect-containing scanning points in the fine scan, the distance d from the upper surface of the air defect to the outer layer of the sample is calculated based on the time interval between reflected pulses 1 and 2:

[0117]

[0118] Where Δt1 is the time interval between the first and second reflected pulses, and n XLPE The refractive index of the sample;

[0119] Calculate the bubble defect height h based on the time interval between reflected pulses 2 and 3:

[0120]

[0121] Where Δt2 is the time interval between the second and third reflected pulses, n air The refractive index of air is 0.4969 mm, and the calculated result in this example is approximately 1.35%.

[0122] The width w and angle θ of the bubble defect can be calculated based on the number of axial and circumferential scanning points containing the defect.

[0123] w = N axial ·d step1

[0124] θ=N cir ·d step2

[0125] Where N axial N cir d represents the number of axial and circumferential scan points containing defects. step1 d step2 The axial and circumferential scanning step sizes are used for fine scanning.

[0126] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A terahertz wave-based system for detecting bubble defects in cable insulation, characterized in that, include: Terahertz wave transmitter, terahertz wave receiver, rotation and translation device, scanning control module and signal processing system; The terahertz wave transmitter is used to generate terahertz electromagnetic waves and irradiate the cable insulation sample to be tested. The terahertz wave receiver is used to receive terahertz waves reflected from the cable insulation sample under test and internal bubble defects. The rotation and translation device includes a sample fixing device, a rotation axis, and a horizontal axis; One end of the rotating shaft is connected to the sample fixing device, and the other end is connected to the horizontal shaft. The sample fixing device is used to fix the sample, and the sample is rotated and translated by the cooperation of the rotating shaft and the horizontal shaft. The rotating shaft is used to rotate the sample circumferentially while keeping its horizontal axial position unchanged, and the horizontal shaft is used to move the sample horizontally while keeping its circumferential position unchanged. The scanning control module controls the rotation and translation device to perform a preliminary scan of the cable insulation sample to be tested based on the step size of the preliminary scan points, and obtains the time-domain spectral signal of the corresponding scan points. Defect judgment is based on time-domain spectral signals. If a pair of reflected pulses with opposite phases appear between the reflected pulses at the inner and outer interfaces of the air and the upper part of the sample, and there is a significant difference from the same position of the defect-free signal, it is the terahertz wave reflection caused by the upper and lower interfaces of the internal defects of the insulation. It is determined that there is a defect below the scanning point. The signal processing system compares the initially scanned time-domain spectral signal with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scan point. If so, the scan point is defined as a preliminary defect point. The scanning control module controls the rotation and translation device to perform a fine scan of the cable insulation sample under test based on each preliminary defect point, and the scanning control module controls the rotation and translation device to perform a fine scan based on each preliminary defect point, the fine scan point step size, and the defect judgment result. This includes: Each initial defect point is sequentially scanned clockwise in a fixed axial position. The defect condition is determined based on the measured time-domain spectral signal, and the result is transmitted to the scanning control module. When there are no internal defects below the scan point, the scanning control module controls the rotation and translation device to end the clockwise circumferential scan at that axial position and controls it to return to the original circumferential position for a reverse circumferential scan. The counterclockwise circumferential scan ends when there are no internal defects at the scan point, thus completing the fine circumferential scan of that axial scan point. After the scanning control module drives the rotation and translation device back to the original circumferential position of the initial defect point, it performs an axial movement with the circumferential position fixed, so that the terahertz wave transmitter is aligned with the next axial scanning point. The circumferential scanning is repeated at this point until the time-domain spectral signals of the circumferential scanning in both clockwise and counterclockwise directions do not reflect the internal defect, thus completing the fine circumferential scanning of the next axial scanning point. Until the time-domain spectral signals of the axial scanning points on both sides of the initial defect point do not reflect the internal defect, the scanning control module controls the rotation and translation device to end the fine axial and circumferential scanning, thus completing the fine scanning of the cable insulation sample to be tested at the initial defect point. The signal processing system determines the location and size of the bubble defect based on the time-domain spectral signal obtained from the fine scanning.

2. The terahertz wave-based bubble defect detection system for cable insulation as described in claim 1, characterized in that, The signal processing system includes a signal truncation module, a signal alignment module, a defect judgment module, a pulse interval module, and a defect calculation module; The signal truncation module is used to truncate the time-domain spectral signal at each scanning point. The signal alignment module is used to perform time-domain alignment of the first reflection pulse in the time-domain spectral signal of each scanning point; The defect judgment module is used to compare the time-domain spectral signal received by the terahertz wave receiver after time-domain truncation and signal alignment with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scanning point. The pulse interval module is used to calculate the time interval between reflected terahertz wave pulses in a time-domain spectral signal containing defects; The defect calculation module is used to calculate the average refractive index and detailed information about the defect at that location using the time interval between reflected pulse peaks calculated in the pulse interval module.

3. The terahertz wave-based bubble defect detection system for cable insulation as described in claim 1, characterized in that, The basis for the signal processing system to determine whether there is a bubble defect below the scanning point is: based on the time-domain spectral signal, if a pair of reflected pulses with opposite phases appear between the reflected pulses of the air and the inner and outer interfaces of the upper part of the sample, it is the terahertz wave reflection caused by the upper and lower interfaces of the internal insulation defect, and it is determined that there is a defect below the scanning point.

4. The terahertz wave-based bubble defect detection system for cable insulation as described in claim 1, characterized in that, The method for obtaining the time-domain spectral signal at the defect-free location is as follows: The cable insulation sample to be tested is fixed by a rotation and translation device. The distance between the cable insulation sample and the terahertz wave transmitter probe is recorded. The probe is always kept perpendicular to the plane where the sample is to be tested. The transmitted pulse waveform of the terahertz wave transmitter is modulated. The terahertz wave receiver and the terahertz wave transmitter are in the same vertical direction. At the beginning, a defect-free position is selected as the initial test point. The initial position of the cable insulation sample to be tested and the time-domain spectral signal received by the terahertz wave receiver are recorded as the defect-free reference signal.

5. A method for detecting bubble defects in cable insulation based on terahertz waves, applied to the terahertz wave-based cable insulation bubble defect detection system according to any one of claims 1-4, characterized in that, Includes the following steps: The step size of the initial scanning points is determined based on the defect size of the cable insulation sample to be tested; Based on the step size of the preliminary scanning points, a preliminary scan is performed on the cable insulation sample to be tested to obtain the time-domain spectral signal of the corresponding scanning point; The time-domain spectral signal obtained from the initial scan is compared with the time-domain spectral signal at the defect-free location to determine whether there is a bubble defect below the scan point. If there is, the scan point is defined as the initial defect point. Based on each initial defect point, a fine scan is performed on the cable insulation sample to be tested. The location and size of the bubble defect are determined based on the time-domain spectral signal obtained from the fine scan. Based on the initial defect points, the rotation and translation device is controlled to perform fine scanning according to the fine scanning point step size and defect judgment results.

6. The method for detecting bubble defects in cable insulation based on terahertz waves as described in claim 5, characterized in that, The detailed scanning of the cable insulation sample to be tested based on each preliminary defect point includes: The clockwise circumferential scan is performed sequentially at each preliminary defect point with the axial position fixed. The defect situation is judged based on the measured time-domain spectral signal. When there is no internal defect below the scan point, the clockwise circumferential scan at that axial position ends, and the original circumferential position is returned to perform a reverse circumferential scan. The counterclockwise circumferential scan ends when there is no internal defect at the scan point, thus completing the fine circumferential scan of that axial scan point. After returning to the original circumferential position of the initial defect point, perform an axial movement with the circumferential position fixed, align with the next axial scanning point, and repeat the circumferential scanning at this point until the time-domain spectral signals of the circumferential scanning in both clockwise and counterclockwise directions do not reflect the internal defect. This completes the fine circumferential scanning of the next axial scanning point. This process continues until the time-domain spectral signals of the axial scanning points on both sides of the initial defect point do not reflect the internal defect. This completes the fine scanning of the cable insulation sample to be tested at the initial defect point.

7. The method for detecting bubble defects in cable insulation based on terahertz waves as described in claim 5, characterized in that, The basis for determining whether there are bubble defects below the scanning point is: Defect judgment is based on time-domain spectral signals. If a pair of reflected pulses with opposite phases appear between the reflected pulses at the inner and outer interfaces of the air and the upper part of the sample, and there is a significant difference from the same position of the defect-free signal, it is the terahertz wave reflection caused by the upper and lower interfaces of the internal insulation defect. It is determined that there is a defect below the scanning point, and the existence of the defect is verified by calculating the average refractive index.

8. The method for detecting bubble defects in cable insulation based on terahertz waves as described in claim 5, characterized in that, The time-domain spectral signal at the defect-free location was obtained by terahertz wave detection of the cable insulation sample at the defect-free location based on the electromagnetic wave propagation theory model.

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