Online detection method for thickness of insulating layer of power cable

Abstract

The invention relates to the technical field of thickness detection, in particular to an online detection method for the thickness of an insulating layer of a power cable. The method comprises the following steps: decomposing a signal into decomposed signals of different scales through a multi-stage decomposition technology, and obtaining a scale contribution degree through similarity of the decomposed signals under the scales; performing similarity analysis on two aspects of frequency domain and signal near a decomposition signal corresponding to the peak value group through a matching algorithm, and obtaining a consistency index by combining a scale contribution degree to screen out an echo peak value group; obtaining a correction time difference through the time difference of the echo peak value group and the consistency index; according to the change relation of the temperature and the sound velocity of the probe cooling liquid environment, the sound velocity is adjusted based on the current temperature change, and the insulation layer thickness is obtained by combining the correction time difference. According to the invention, on the basis of correcting the sound velocity according to the temperature, the echo time difference is corrected from the local similar feature conditions of the scale and the peak value through multi-stage decomposition, and the accuracy of insulation layer thickness detection is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of thickness detection, and particularly relates to an on-line detection method for the thickness of the insulating layer of a power cable. Background Art

[0002] A power cable is a cable used for transmitting and distributing electric energy, which mainly consists of a conductor core, an insulating layer, a shielding layer and a protective layer. Among them, the insulating layer, as the first line of defense for protecting the internal conductor and preventing current from entering the external environment, plays a crucial role in the power cable. An appropriate insulating layer thickness can effectively protect the internal conductor of the cable, and a uniform insulating layer thickness can make the electric field inside the conductor evenly distributed. Therefore, in order to ensure the safety and lifespan of the power cable, it is necessary to detect the thickness of the insulating layer of the cable during production. The current detection method for the insulating layer relies on the propagation characteristics of ultrasonic waves in the insulating layer, measures its reflection time, and calculates the thickness of the insulating layer.

[0003] The basic process of ultrasonic detection of the insulating layer thickness is to utilize the characteristic that the speed of ultrasonic waves is different when propagating in different media. When ultrasonic waves propagate from one layer of medium to another layer of medium, reflection and refraction will occur at the interface. Since the cable conductor and the insulating layer are different media, when ultrasonic waves encounter the interface between the insulating layer - air and the interface between the conductor - insulating layer, reflection and refraction occur, and the external probe receives two ultrasonic echo signals returned, and thus the thickness of the insulating layer is obtained.

[0004] However, during the production of power cables, since the cable conductor and the insulating layer are newly produced, the insulating layer is in a high-temperature state and needs to be cooled. In the case of temperature fluctuations, the propagation speed of ultrasonic waves at these temperatures is different, resulting in changes in the ultrasonic signals when encountering the cable interface and propagating in the medium, causing the detected echo signals to broaden. The existing method for determining the time difference when the echo is received is basically to analyze the wave peaks of the echo signals through methods such as envelope detection. However, because the echo signals are broadened, and due to the uneven surface of the stranded inner conductor of the cable causing diffuse reflection, the echo signals are further dispersed, resulting in multiple sets of wave peaks in the echo signals, affecting the determination of the corresponding relationship between the two echo signals, resulting in errors in the time difference of the two detected echo signals, and further making the ultrasonic detection thickness result inaccurate. Summary of the Invention

[0005] In order to solve the technical problems in the prior art that the echo signals are broadened and diffuse reflection occurs, further dispersing the echo signals, resulting in multiple sets of wave peaks in the echo signals and affecting the determination of the corresponding relationship between the two echo signals, the purpose of the present invention is to provide an on-line detection method for the thickness of the insulating layer of a power cable, and the specific technical solution adopted is as follows: The present invention provides an on-line detection method for the thickness of the insulating layer of a power cable. The method includes: Obtaining two echo signals at the detection point through a probe, and obtaining the peak values in the echo signals; forming each pair of different peak values into a peak value group, where the two peak values in the peak value group are respectively located in two echo signals; performing multi-level decomposition on the echo signals to obtain the decomposition signals at each scale; According to the degree of correlation between the two decomposition signals at each scale, obtaining the scale contribution degree of each scale; within the preset neighborhood range of each peak value group, matching the two decomposition signals at the scale to obtain a time matching group; according to the similarity of the energy distributions and the signal similarity of the two decomposition signals in each time matching group, obtaining the matchability of each peak value group at each scale; For each peak value group, combining the scale contribution degree and the matchability of each scale to obtain a consistency index; screening out the echo peak value groups based on the size of the consistency index; according to the difference between the corresponding times of the peak values in all echo peak value groups and the consistency index of the echo peak value groups, obtaining the current corrected time difference; Detecting the change relationship between the temperature and the sound speed of the environmental coolant through a probe, adjusting the calibrated sound speed in combination with the temperature deviation of the current detection environment to obtain the current corrected sound speed; combining the current corrected time difference and the corrected sound speed to obtain the thickness of the insulating layer at the current detection point.

[0006] Further, the method for obtaining the scale contribution degree includes: For any scale, calculating the DTW distance between the two decomposition signals at this scale for negative correlation mapping and normalization processing to obtain the scale contribution degree of this scale.

[0007] Further, the method for obtaining the time matching group includes: For any peak value group, sequentially taking each scale as the analysis scale, within the preset neighborhood range of the corresponding time of the peak value in this peak value group, matching the two decomposition signals at the analysis scale through the DTW algorithm, and taking each pair of matched times as a time matching group.

[0008] Further, the method for obtaining the matchability includes: For any peak value group, within the preset neighborhood range of this peak value group, obtaining the frequency domain features of the two decomposition signals at the corresponding times in each time matching group respectively, and analyzing the similarity situation to obtain the frequency domain similarity index of the two decomposition signals in each time matching group; Performing negative correlation mapping on the difference between the signal values of the two decomposition signals at the corresponding times in each time matching group to obtain the signal similarity index of the two decomposition signals in each time matching group; Multiply the frequency-domain similarity index of each moment matching group by the signal similarity index as the similarity index of each moment matching group; Within the preset neighborhood range of the peak group, use the sum of the similarity indices of all moment matching groups of the two decomposed signals as the numerator, and use the sum of the frequency-domain similarity indices of all moment matching groups of the two decomposed signals as the denominator to obtain the matchability of the peak group at the corresponding scales of the two decomposed signals.

[0009] Furthermore, the method for obtaining the frequency-domain similarity index includes: Obtain the frequency-domain characteristics of the two decomposed signals in the time series through fast Fourier transform; For any moment matching group, take the decomposed signal corresponding to each moment in the moment matching group as the correlation signal for each moment; take the time period formed by each moment in the moment matching group and the two adjacent moments in the time series as the characteristic time period for each moment; Calculate the correlation of the frequency-domain characteristics between the corresponding correlation signals on the two characteristic time periods in the moment matching group as the frequency-domain similarity index of the two decomposed signals in the moment matching group.

[0010] Furthermore, the method for obtaining the consistency index includes: For any peak group, at this peak group, multiply the matchability at each scale by the scale contribution degree as the match possibility index for each scale; Use the sum of the match possibility indices of all scales as the numerator and the sum of the scale contributions of all scales as the denominator to obtain the consistency index of this peak group.

[0011] Furthermore, the method for obtaining the echo peak group includes: Take the peak group with a consistency index greater than the preset judgment threshold as the echo peak group.

[0012] Furthermore, the method for obtaining the corrected time difference includes: Calculate the difference between the corresponding moments of the two peaks in each echo peak group as the time difference of each echo peak group; Multiply the time difference of each echo peak group by the consistency index as the weighted time difference index of each echo peak group; Use the sum of the weighted time difference indices of all echo peak groups as the numerator and the sum of the consistency indices of all echo peak groups as the denominator to obtain the current corrected time difference.

[0013] Furthermore, the method for obtaining the corrected sound speed includes: Obtain the sound speed temperature coefficient through pre-experiment; Multiply the difference between the current detected temperature and the calibrated temperature by the sound speed temperature coefficient as the sound speed adjustment degree; Multiply the calibrated sound speed by the sound speed adjustment degree to obtain the sound speed adjustment value; take the sum of the calibrated sound speed and the sound speed adjustment value as the corrected sound speed.

[0014] Furthermore, the method for obtaining the thickness of the insulating layer includes: Divide the product of the current corrected time difference and the corrected sound speed by 2 to obtain the thickness of the insulating layer at the current detection point.

[0015] The present invention has the following beneficial effects: The present invention first disassembles the signal into signals of different scales through a multi-level decomposition technique, which is beneficial to reducing the noise influence caused by the roughness of the conductor surface or the flow of the coolant. Based on the decomposed signals, the scale contribution degree is obtained through the similarity of the decomposed signals at different scales, and the credibility of the analysis at different scales is reflected by the echo similarity characteristics. Then, through a matching algorithm, two aspects of similarity analysis of the frequency domain and the signal are carried out near the decomposed signals corresponding to the peak group, and the peak matching degree of the peak group is reflected from the local detail similarity degree. Further, the consistency index is obtained by combining the scale contribution degree to screen out the echo peak group and reduce the multi-peak interference. By adjusting the time difference and the consistency index of the echo peak group, a more accurate corrected time difference is obtained. At the same time, according to the change relationship between the coolant temperature and the sound speed detected by the probe in real time, the calibrated sound speed is adjusted based on the change of the current temperature to eliminate the influence of the sound speed drift in the dynamic temperature environment on the thickness calculation and improve the adaptability to complex working conditions. Combining the corrected time difference and the corrected sound speed, the thickness of the insulating layer is obtained. Based on the temperature correction of the sound speed, the present invention corrects the echo time difference from the scale and the local similarity characteristics of the peak, effectively improving the accuracy of the insulating layer thickness detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0017] Figure 1 It is a flowchart of an on-line detection method for the thickness of the insulating layer of a power cable provided by an embodiment of the present invention; Figure 2 It is a schematic diagram of a partial structure of a probe and a cable provided by an embodiment of the present invention; Figure 3 It is a schematic diagram of two echo signals provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] In order to further elaborate on the technical means and effects adopted by the present invention to achieve the intended invention purpose, the following specifically describes, in conjunction with the accompanying drawings and preferred embodiments, a method for on-line detection of the thickness of the insulating layer of a power cable proposed according to the present invention, including its specific implementation manner, structure, features and effects, as follows. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. In addition, the specific features, structures or characteristics in one or more embodiments can be combined in any suitable form.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs.

[0020] The following specifically describes, in conjunction with the accompanying drawings, the specific solution of a method for on-line detection of the thickness of the insulating layer of a power cable provided by the present invention.

[0021] During the production of power cables, the cable copper wires are first straightened and stranded, and then in an extruder, a screw with a specific shape is used to melt the insulating layer plastic under high-temperature conditions, and then extruded forward to make the insulating layer plastic pass through a die to form an insulating layer around the cable conductor.

[0022] During this process, the plastic is heated to a relatively high temperature, and then cooled by coolant and air, and finally enters the next step of installing a shielding layer, a protective layer, etc. When detecting the thickness of the cable insulating layer by ultrasonic waves, the ultrasonic probe is located in the coolant. Since the insulating layer has just been produced and is at a relatively high temperature, and the cable is still in the coolant, the cable conductor and the insulating layer gradually dissipate heat, and at the same time, the temperature of the coolant gradually rises. In coolants and insulating layers at different temperatures, the speed of ultrasonic waves will change, which makes the time between the two echo signals received by the ultrasonic probe during detection affected by temperature.

[0023] Therefore, when performing ultrasonic detection correction, it is also necessary to consider the temperature influence on the detected insulating layer, and comprehensively correct the received ultrasonic echo signals and the temperature during the current time period to improve the accuracy of thickness detection. Please refer to Figure 1 , which shows a flowchart of a method for on-line detection of the thickness of the insulating layer of a power cable provided by an embodiment of the present invention. The method includes the following steps: S1: Obtain two echo signals at the detection point through the probe, and obtain the peaks in the echo signals; form each two different peaks into a peak group, and the two peaks in the peak group are respectively located in the two echo signals; perform multi-level decomposition on the echo signals to obtain the decomposed signals at each scale.

[0024] First, on the production line, the power cable is straightened by two rollers and immersed in the coolant. Then, ultrasonic probes and temperature probes are installed at the bottom of the production line to detect the reflection of ultrasonic waves at various parts of the power cable and the temperature of the cable. Please refer to Figure 2 , which shows a schematic diagram of the structure of a probe and a cable part provided by an embodiment of the present invention.

[0025] In the embodiment of the present invention, the coolant can be preset as water. A set of probes, including an ultrasonic probe and a temperature probe, is set every 10 cm. The temperature probe can adopt an infrared temperature sensor. The ultrasonic probe needs to be perpendicular to the surface of the cable. The preset center frequency of the ultrasonic wave is , and the sampling rate of the ultrasonic receiver is . The frequencies of the ultrasonic wave pulses emitted by the ultrasonic wave and the detection frequency of the infrared temperature sensor are both set to 10 times per second. The specific acquisition settings can be adjusted by the implementer according to the specific implementation scenario and are not limited here.

[0026] At the detection point of the probe on the cable insulation layer, ultrasonic waves are emitted. The ultrasonic waves propagate in the coolant. When they encounter the surface of the insulation layer, the first reflection occurs and is received by the probe, which is recorded as the first echo signal. At the same time, part of the ultrasonic waves are refracted into the insulation layer and continue to propagate. When they encounter the surface of the conductor, they are reflected again and are received by the probe after propagation, which is recorded as the second echo signal. Since the surfaces of the newly produced cable insulation layer and the stranded conductor bundle are slightly rough, and the insulation layer and the coolant are in a moving state, diffuse reflection occurs when the two interfaces reflect ultrasonic signals, which causes the wave peaks to become wider when the received echo signals are received, resulting in multiple peaks. Please refer to Figure 3 , which shows a schematic diagram of two echo signals provided by an embodiment of the present invention. M1 represents the first echo signal, and M2 represents the second echo signal.

[0027] In the embodiment of the present invention, first, the upper and lower envelopes of the two echo signals are constructed through the Hilbert transform, and then the peaks of the two echo signals, that is, the local maxima, are determined through the upper and lower envelopes. The time difference between the peaks is used for thickness analysis. However, due to the influence of multiple peaks, the situation of incorrect selection will occur and errors will be generated, so further correction is required.

[0028] Each two different peaks in the two echo signals form a peak group. The two peaks in the peak group are respectively located in the two echo signals, that is, one peak in the peak group is located in the first echo signal, and the other peak is located in the second echo signal.

[0029] Further perform multi-level decomposition on the echo signals to obtain the decomposed signals at each scale. In the embodiment of the present invention, wavelets similar to the waveform of the echo signals are used to perform multi-level decomposition on the two echo signals. The preset number of decomposition layers is 6, and there are decomposed signals at 6 scales. It should be noted that the multi-level decomposition performed by wavelet decomposition is a well-known technical means for those skilled in the art. For example, wavelets, etc., will not be elaborated and restricted here.

[0030] S2: Obtain the scale contribution degree of each scale according to the correlation degree between the two decomposed signals at each scale; within the preset neighborhood range of each peak group, match the two decomposed signals at the scale to obtain the time matching group; according to the similarity of the energy distribution and the signal similarity of the two decomposed signals in each time matching group, obtain the matchability of each peak group at each scale.

[0031] First, considering that there are noise signals in the echo signals, these signals are not very helpful for determining the accurate echo time and may even have a negative effect. On the contrary, the effective non-noise signals are more helpful for determining the echo time. When detecting the insulation layer thickness, since the ultrasonic signals emitted are from the same ultrasonic transmitting probe, the reflection and refraction on the surface of the insulation layer and the surface of the cable conductor do not change the ultrasonic signals much. Therefore, the waveform information of the effective echo signals is relatively consistent. So, the higher the consistency of the signals in the two echo signals at the scale, the greater the contribution of the signal analysis at the scale to determining the echo time.

[0032] Preferably, in the embodiment of the present invention, the method for obtaining the scale contribution degree includes: for any scale, calculate the DTW distance between the two decomposed signals at the scale, perform negative correlation mapping and normalization processing to obtain the scale contribution degree of the scale. When the DTW distance is smaller, it indicates that the decomposed signals are more similar, indicating that they contain less noise and have a higher contribution to determining the echo time.

[0033] It should be noted that the DTW algorithm, negative correlation mapping, and normalization algorithm corresponding to the DTW distance are well-known technical means for those skilled in the art. The negative correlation mapping can adopt negative exponential power or inverse proportion form, etc. The choice of normalization can be linear normalization or standard normalization, etc., which will not be elaborated here.

[0034] Secondly, in the decomposed signals at each scale, since the two echo signals are emitted at the same time and are directly reflected at the same position, in the two received echo signals, the reflection of the same ultrasonic signal does not change the signal much, that is, the frequency and other signal energy distributions of the same ultrasonic signal in the two echo signals are the same. Further, aiming at the more local similarity of the decomposed signals at each scale, it reflects the possibility that the peaks in the peak group correspond to the same echo.

[0035] To analyze similar situations from local details, the two decomposed signals at the peak group are matched in terms of time correspondence, and the similar situations are analyzed from the time matching. Preferably, in the embodiments of the present invention, the method for obtaining the time matching group includes: for any peak group, each scale is sequentially used as the analysis scale, and within the preset neighborhood range corresponding to the peak time in the peak group, the two decomposed signals at the analysis scale are matched by the DTW algorithm, and each pair of matched times is used as a time matching group. In this embodiment, the preset neighborhood range is a range centered on the time with a radius of 10 times. The implementer can adjust it by himself.

[0036] For example, the echo signals where the peaks are located in the peak group are M1 and M2, and the corresponding times are t1 and t2. For the two decomposed signals at the analysis scale, the part of the decomposed signal corresponding to M1 within the neighborhood range of t1 is DTW-matched with the part of the decomposed signal corresponding to M2 within the neighborhood range of t2, and the completed matching times are used as each time matching group.

[0037] Furthermore, the similar situations are analyzed from each time matching group. When the decomposed signals at the matching times are more similar in terms of energy distribution and signal, it indicates that the peak group under the time analysis is more likely to be the reflection of two matching interfaces. Preferably, in the embodiments of the present invention, the method for obtaining the matchability of each peak group at each scale includes: First, for any peak group, within the preset neighborhood range of the peak group, the two decomposed signals respectively obtain frequency domain features at the corresponding times in each time matching group, and the similar situations are analyzed to obtain the frequency domain similarity index of the two decomposed signals in each time matching group, and the similarity analysis is carried out from the energy distribution situation in the frequency domain space. In the embodiments of the present invention, the method for obtaining the frequency domain similarity index includes: The frequency domain features of the two decomposed signals in time series are obtained through the fast Fourier transform. The frequency domain feature is a quantization index used to describe the spectral characteristics of the signal after the signal is transformed from the time domain to the frequency domain through the Fourier transform, mainly including the center frequency, mean square frequency, root mean square frequency, frequency variance, and frequency standard deviation, etc. In the embodiments of the present invention, the frequency variance in the frequency domain space can be used as the frequency domain feature. The larger the variance, the more dispersed the energy distribution, which can intuitively reflect the concentration characteristics of the spectral energy distribution. It should be noted that the method of obtaining the frequency domain feature through the Fourier transform is a well-known technical means for those skilled in the art and will not be elaborated and limited here.

[0038] For any moment matching group, the decomposed signals corresponding to each moment in the moment matching group are used as the correlation signals for each moment, and the moments in the moment matching group are correspondingly marked with the corresponding decomposed signals. The time periods formed by each moment in the moment matching group and the two adjacent moments in time sequence are used as the characteristic time periods for each moment, and the time periods formed by each moment and the two adjacent moments before and after are used as the characteristic time periods to realize local characteristic correlation analysis.

[0039] On the two characteristic time periods in the moment matching group, calculate the correlation of the frequency domain characteristics between the corresponding correlation signals as the frequency domain similarity index of the two decomposed signals in the moment matching group, and reflect the similarity of the energy distribution through the correlation degree of the time sequence change of the frequency domain characteristics corresponding to the two decomposed signals on the corresponding time periods of the two moments in the moment matching group. It should be noted that the calculation of the data correlation in the time sequence is a well-known technical means for those skilled in the art, such as the sampling DTW algorithm or the Pearson correlation coefficient, etc., which are not limited here.

[0040] Furthermore, perform a negative correlation mapping on the difference in the signal values of the two decomposed signals at the corresponding moments in each moment matching group to obtain the signal similarity index of the two decomposed signals in each moment matching group, and analyze the similarity between moments in terms of signal amplitude.

[0041] Finally, use the frequency domain characteristic similarity index as the weight for weighted normalization, and take the product of the frequency domain similarity index and the signal similarity index of each moment matching group as the similarity index of each moment matching group, and analyze the similarity degree through frequency domain similarity weighted analysis.

[0042] Within the preset neighborhood range of the peak group, take the sum value of the similarity indexes of all moment matching groups of the two decomposed signals as the numerator, and take the sum value of the frequency domain similarity indexes of all moment matching groups of the two decomposed signals as the denominator to obtain the matchability of the peak group at the corresponding scales of the two decomposed signals, and perform weighted normalization processing through the ratio to ensure that the regions with high frequency domain consistency contribute more to the final matching score. The higher the matchability, the higher the possibility that the peak group is the corresponding echo relationship at a single scale.

[0043] S3: For each peak group, obtain the consistency index by combining the scale contribution degree and the matchability of each scale; screen out the echo peak groups based on the size of the consistency index; obtain the current corrected time difference according to the difference between the corresponding moments of the peaks in all echo peak groups and the consistency index of the echo peak groups.

[0044] If a single peak group is relatively matched at each scale, then it can be considered that these two peaks are obtained by the reflection of the same ultrasonic signal through two interfaces. At the same time, considering the contribution degree of the effective situation and combining the matchability of each scale of the single peak group, the consistency index is obtained.

[0045] In an embodiment of the present invention, the method for obtaining the consistency index includes: for any peak group, at this peak group, the product of the matchability and the scale contribution degree at each scale is used as the match possibility index for each scale, and is weighted by the scale contribution degree. The scale with a higher contribution degree has a more reliable match analysis.

[0046] Furthermore, the sum value of the match possibility indexes of all scales is used as the numerator, and the sum value of the scale contributions of all scales is used as the denominator to obtain the consistency index of this peak group. The consistency index is obtained through weighted normalization. The larger the consistency index, the greater the possibility that the peak of this peak group is the same ultrasonic signal. As an example, the expression of the consistency index is: , where represents the consistency index of the th peak group, represents the matchability of the th peak group at the th scale, represents the scale contribution degree of the th scale, represents the total number of scales. represents the match possibility index of the th peak group at the th scale.

[0047] By threshold judgment, peak groups with relatively high possibility are screened. In an embodiment of the present invention, the peak groups with a consistency index greater than the preset judgment threshold are used as echo peak groups. The preset judgment threshold is set to 0.5. When it is greater than the threshold, it indicates that the peaks in the peak group are reflections of the same ultrasonic wave, and subsequent time difference analysis is performed as echo peak groups.

[0048] However, there are certain errors in determining the time when the ultrasonic wave penetrates the insulating layer based on a single echo signal point, and not every echo peak group is relatively accurate. That is, the more consistent the two echo signals in the echo peak group are, the more reliable the obtained time difference is. Therefore, the time difference is weighted by the consistency index of each echo peak group to obtain the corrected time difference between the two echo signals.

[0049] Preferably, in an embodiment of the present invention, the method for obtaining the corrected time difference includes: Calculate the difference between the corresponding moments of the two peaks in each echo peak group as the time difference of each echo peak group, which is used as the acceptable time difference between echoes. The product of the time difference of each echo peak group and the consistency index is used as the weighted time difference index of each echo peak group, and is weighted by the consistency index. For peak groups with higher consistency, the credibility in the comprehensive time difference is higher.

[0050] Furthermore, taking the sum of the weighted time difference indicators of all echo peak groups as the numerator and the sum of the consistency indicators of all echo peak groups as the denominator, the current corrected time difference is obtained. Through weighted normalization correction, a more accurate echo time difference value is obtained.

[0051] S4: Detect the change relationship between the temperature and the sound speed of the ambient coolant through a probe, and adjust the calibrated sound speed in combination with the temperature deviation of the current detection environment to obtain the current corrected sound speed; combine the current corrected time difference and the corrected sound speed to obtain the insulation layer thickness at the current detection point.

[0052] Regarding the influence of temperature on the sound wave speed, by pre-testing the sound speed-temperature change relationship of the insulation layer plastic in the coolant environment, the sound speed can be corrected in a timely manner by monitoring the real-time temperature deviation change. Preferably, in the embodiment of the present invention, the method for obtaining the corrected sound speed includes: Obtain the sound speed temperature coefficient through pre-experiments. The sound speed temperature coefficient refers to the ratio of the sound speed change with temperature when the sound wave propagates in the medium. In the embodiment of the present invention, the sound speed-temperature relationship between the coolant and the insulation layer plastic is pre-detected in the laboratory. In the coolant environment, the resonance interference method is preset to measure the corresponding sound speed at different temperatures to obtain a large number of sound speed temperature data points, and then the sound speed at various temperatures is fitted by the least square method. Then every Select a sound speed data point to form a temperature-sound speed sequence, and then use the difference method to calculate the difference sequence of the temperature-sound speed sequence to describe the influence of temperature change on the sound speed in the medium. Calculate the average value of the elements in the difference sequence to obtain the sound speed temperature coefficient corresponding to the coolant.

[0053] Multiply the difference between the current detected temperature and the calibrated temperature by the sound speed temperature coefficient as the sound speed adjustment degree, which reflects the sound speed change rate caused by the temperature difference change. Therefore, multiply the calibrated sound speed by the sound speed adjustment degree as the sound speed adjustment value, which characterizes the sound speed adjustment amount. In the embodiment of the present invention, the calibrated temperature is the preset standard temperature, set to , and the calibrated sound speed is the sound speed value at the calibrated temperature. The specific value can be controlled by the implementer according to the specific implementation situation.

[0054] Finally, take the sum of the calibrated sound speed and the sound speed adjustment value as the corrected sound speed for more accurate correction of the ambient temperature fluctuation.

[0055] Compensate the echo signal according to the temperature information of the cable insulation layer, and the insulation layer thickness can be obtained in combination with the time difference. In the embodiment of the present invention, divide the product of the current corrected time difference and the corrected sound speed by 2 to obtain the insulation layer thickness at the current detection point. As an example, the expression of the insulation layer thickness is: , where represents the insulation layer thickness, Indicates the corrected time difference; Is expressed as the corrected sound velocity.

[0056] In the embodiments of the present invention, in the ultrasonic detection of the thickness of the power cable insulation layer, repeated measurement is a key step to ensure the accuracy and reliability of the data. To obtain reliable measurement results, equidistant multiple groups of probes are set in the whole production process, and the same temperature compensation method in the previous steps is adopted to calculate the thickness of the insulation layer at the cable detection point. , etc. Since the production speed of the power cable is constant and the interval distance of the probe setting is fixed, the time interval for each probe to detect the same position of the cable is determined. Then, the average value of the thicknesses of the insulation layer detected multiple times is used as the final thickness of the insulation layer at the detection point.

[0057] In summary, the present invention first disassembles the signal into signals of different scales through a multi-level decomposition technique, which is beneficial to reducing the noise influence caused by the rough surface of the conductor or the flow of the coolant. Based on the decomposed signals, the scale contribution degree is obtained through the similarity of the decomposed signals at different scales, and the credibility of the analysis at different scales is reflected through the echo similarity characteristics. Furthermore, through a matching algorithm, two-aspect similarity analysis of the frequency domain and the signal is performed near the decomposed signals corresponding to the peak group, and the peak matching degree of the peak group is reflected from the local detail similarity degree. Further combined with the scale contribution degree, a consistency index is obtained to screen out the echo peak group and reduce the multi-peak interference. Through the adjustment of the time difference of the echo peak group and the consistency index, a more accurate corrected time difference is obtained. At the same time, according to the change relationship between the coolant temperature and the sound velocity detected by the probe in real time, the calibrated sound velocity is adjusted based on the change of the current temperature to eliminate the influence of the sound velocity drift in the dynamic temperature environment on the thickness calculation and improve the adaptability to complex working conditions. By combining the corrected time difference and the corrected sound velocity, the thickness of the insulation layer is obtained. Based on the temperature correction of the sound velocity, the present invention corrects the echo time difference from the scale and the local similarity characteristics of the peak, effectively improving the accuracy of the insulation layer thickness detection.

[0058] It should be noted that: the above sequence of the embodiments of the present invention is only for description and does not represent the advantages and disadvantages of the embodiments. The processes depicted in the drawings do not necessarily require the specific order or continuous order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0059] Each embodiment in this specification is described in a progressive manner. The same or similar parts among the embodiments can be referred to each other, and the key points of each embodiment are the differences from other embodiments.

Claims

1. A method for online detection of the thickness of the insulation layer of a power cable, characterized in that: The method comprises: Acquire two echo signals at the detection point through the probe, and obtain the peak value in the echo signal; form a peak group with every two different peak values, and the two peak values ​​in the peak group are respectively located in the two echo signals; perform multi-level decomposition on the echo signal to obtain the decomposed signal at each scale; According to the correlation between the two decomposed signals at each scale, the scale contribution of each scale is obtained; within the preset neighborhood range of each peak group, the two decomposed signals at the scale are matched to obtain a moment matching group; according to the similarity of the energy distribution and signal similarity of the matching group of the two decomposed signals at each moment, the matchability of each peak group at each scale is obtained; For each peak group, the consistency index is obtained by combining the scale contribution and matchability of each scale; the echo peak group is selected based on the consistency index; the current corrected time difference is obtained based on the difference between the corresponding times of the peaks in all echo peak groups and the consistency index of the echo peak group; The probe detects the changing relationship between the temperature and sound velocity of the ambient coolant, and adjusts the calibrated sound velocity in combination with the temperature deviation of the current detection environment to obtain the current corrected sound velocity; the thickness of the insulation layer at the current detection point is obtained in combination with the current corrected time difference and corrected sound velocity.

2. According to claim 1, a method for online detection of the thickness of the insulation layer of a power cable is characterized in that: The method for obtaining the scale contribution includes: For any scale, the DTW distance between two decomposed signals is calculated at the scale for negative correlation mapping and normalization to obtain the scale contribution of the scale.

3. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the time matching group includes: For any peak group, each scale is used as the analysis scale in turn. Within the preset neighborhood of the peak corresponding to the moment in the peak group, the two decomposed signals under the analysis scale are matched by the DTW algorithm, and each pair of matching moments is regarded as a moment matching group.

4. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the matchability includes: For any peak group, within the preset neighborhood of the peak group, the two decomposed signals obtain frequency domain features at the corresponding time in the matching group at each moment, and analyze the similarity to obtain the frequency domain similarity index of the two decomposed signals matching the group at each moment; Negative correlation mapping is performed on the difference in signal values ​​of the two decomposed signals at the corresponding time in the matching group at each time, so as to obtain the signal similarity index of the two decomposed signals at each time matching group; The product of the frequency domain similarity index and the signal similarity index of the matching group at each moment is used as the similarity index of the matching group at each moment; Within the preset neighborhood of the peak group, the sum of the similarity indices of the matching groups of the two decomposition signals at all times is taken as the numerator, and the sum of the frequency domain similarity indices of the matching groups of the two decomposition signals at all times is taken as the denominator to obtain the matching degree of the peak group at the corresponding scales of the two decomposition signals.

5. The method for online detection of the thickness of the insulation layer of a power cable according to claim 4, characterized in that: The method for obtaining the frequency domain similarity index includes: The frequency domain characteristics of the two decomposed signals in time series are obtained by fast Fourier transform; For any moment matching group, the decomposed signal corresponding to each moment in the moment matching group is taken as the relevant signal of each moment; the period consisting of each moment in the moment matching group and two adjacent moments in time sequence is taken as the characteristic period of each moment; In the two characteristic time periods in the matching group at this moment, the correlation of the frequency domain features between the corresponding correlation signals is calculated as the frequency domain similarity index of the matching group of the two decomposed signals at this moment.

6. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the consistency index includes: For any peak group, at the peak group, the product of the matchability at each scale and the scale contribution is used as the matching possibility index of each scale; The sum of the matching possible indicators of all scales is used as the numerator, and the sum of the scale contributions of all scales is used as the denominator to obtain the consistency indicator of the peak group.

7. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for acquiring the echo peak group includes: The peak group whose consistency index is greater than the preset judgment threshold is regarded as the echo peak group.

8. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the corrected time difference includes: Calculate the difference between the corresponding moments of two peaks in each echo peak group as the time difference of each echo peak group; The product of the time difference of each echo peak group and the consistency index is used as the weighted time difference index of each echo peak group; The sum of the weighted time difference indices of all echo peak groups is used as the numerator, and the sum of the consistency indices of all echo peak groups is used as the denominator to obtain the current corrected time difference.

9. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the corrected sound speed comprises: Obtain the temperature coefficient of the speed of sound through preliminary experiments; The product of the difference between the current detection temperature and the calibration temperature and the sound velocity temperature coefficient is taken as the sound velocity adjustment degree; The product of the calibrated sound speed and the sound speed adjustment degree is taken as the sound speed adjustment value; the sum of the calibrated sound speed and the sound speed adjustment value is taken as the corrected sound speed.

10. The method for online detection of the thickness of the insulation layer of a power cable according to claim 1, characterized in that: The method for obtaining the thickness of the insulating layer includes: Divide the product of the current corrected time difference and the corrected sound speed by 2 to obtain the insulation layer thickness at the current detection point.

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