Online detection method and system based on 3PE anticorrosive coating

Abstract

The invention provides an online detection method and system based on a 3PE anti-corrosion coating, and belongs to the technical field of anti-corrosion pipe defect detection. A terahertz camera is erected on the downstream of a 3PE anti-corrosion steel pipe production line and located over a 3PE anti-corrosion steel pipe, a detection strip is constructed by scanning images, the rotating speed, the advancing speed and the scanning frequency of the terahertz camera are regulated and controlled based on the detection strip, and full-coverage detection is achieved. In the detection process, the quality of the epoxy powder coating is evaluated based on the time difference delta t of the returned terahertz wave signals, and the problem that in the prior art, real-time high-precision online detection cannot be conducted on the ultrathin epoxy powder coating located on the bottom layer is effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of anti-corrosion pipe defect detection, and in particular to an online detection method and system based on 3PE anti-corrosion coating. Background Art

[0002] 3PE anti-corrosion steel pipe is a high-performance anti-corrosion pipeline widely used in the fields of oil, natural gas, water supply and drainage, etc. It is mainly composed of two parts: anti-corrosion steel pipe base material and outer wall anti-corrosion layer. The outer wall anti-corrosion layer adopts a three-layer structure design, including an epoxy powder layer (FBE) at the bottom layer, an adhesive layer (AD) at the middle layer, and a high-density polyethylene layer (HDPE) at the surface layer.

[0003] In the production process of 3PE anti-corrosion steel pipes, the thickness of the epoxy powder layer (FBE) is one of the key indicators of its quality control. As the first line of defense of the entire anti-corrosion system, the epoxy powder layer is directly related to the anti-corrosion effect and service life of the anti-corrosion steel pipe. Because it can not only effectively isolate corrosive substances such as moisture and oxygen in the external environment, but also provide a good adhesion basis for the subsequent adhesive layer and high-density polyethylene layer. If the thickness of the epoxy powder layer is insufficient, it may lead to a decrease in anti-corrosion performance and shorten the service life of the pipeline; conversely, if the thickness exceeds the standard, it will not only cause material waste, but also may affect the overall performance of the coating. Therefore, strictly controlling the thickness of the epoxy powder layer is of great significance to ensure the quality and performance of 3PE anti-corrosion steel pipes.

[0004] However, the epoxy powder layer is located at the bottom layer of the anti-corrosion layer, and its thickness is relatively thin (in an environment with ordinary anti-corrosion requirements, the thickness of the epoxy powder layer is usually in the range of 100-150 microns; in projects with harsh corrosive environments or high anti-corrosion requirements, its thickness may exceed 150 microns, for example, in oil and gas pipelines, the thickness of the FBE layer is generally controlled in the range of 280-420 microns). This thin and bottom-layer feature makes the measurement of its thickness face great challenges. In early measurements, many attempts were made to achieve online measurement of the thickness of the epoxy powder layer. For example, Han Lihua et al. compared four methods, including direct measurement, sticker method, peeling method and stripping method, in the paper "Detection and Control of Thickness of Epoxy Powder Layer in WOPP Anti-corrosion Engineering". It was found that the first three methods had defects such as the probe not working, complicated operation, inaccurate measurement values, and large hysteresis. Finally, the stripping method was selected as a feasible solution. However, the stripping method is a destructive detection method, which will cause certain damage to the 3PE anti-corrosion steel pipe, which limits its application in actual production. With the development of technology, people have proposed to use coating thickness gauges such as DR5000S and DR6000 to measure the thickness of the anti-corrosion layer of 3PE anti-corrosion steel pipes. This type of thickness gauge uses the principle of electromagnetic induction, which will not damage the anti-corrosion steel pipes, and has very high measurement accuracy. Therefore, it has been widely used in practical applications. However, there are many constraints in the measurement process: First, the equipment requires the probe to be perpendicular and tightly fitted to the measured surface, while the 3PE anti-corrosion steel pipe is moving on the production line, and the surface of the anti-corrosion steel pipe is not flat. These factors undoubtedly bring challenges to online real-time detection. For example, it is difficult for the probe to maintain a stable contact state during online detection, which causes fluctuations or distortions in the measurement data. In fact, this type of probe is more often used for offline detection in actual production processes. Secondly, the electromagnetic induction method is highly dependent on the magnetic permeability of the substrate. When the anti-corrosion steel pipe is made of non-magnetic material (such as austenitic stainless steel) or has a surface hardening layer, the magnetic circuit closure effect is significantly reduced and the measurement error is greatly increased. In addition, the high resistivity of the outer polyethylene layer will form an electromagnetic shielding effect, resulting in probe signal attenuation, especially when the FBE layer thickness exceeds 300 microns, the problem of insufficient effective detection depth becomes more prominent. In addition, the operation mode of the DR5000S and DR6000 coating thickness gauges is to press the probe vertically against the surface of the object to be measured for single-point measurement. A single probe cannot achieve full coverage measurement unless a multi-probe array is deployed, which will lead to a significant increase in costs.

[0005] Currently, non-contact measurements using ultrasonic thickness measurement, X-rays, etc. have also been considered, but they all have many deficiencies. For an ultrasonic thickness gauge, firstly, a coupling agent is required to maintain acoustic impedance matching, which is difficult to implement stably on a production line. Secondly, the measurement range of the ultrasonic thickness gauge cannot meet the measurement requirements. Taking the CTS-500 high-precision ultrasonic thickness gauge on the market as an example, its detection range is 0.6 - 500 mm, making it difficult to meet the detection requirements for ultra-thin layers such as the FBE layer. For an X-ray thickness gauge, although it can achieve high-precision measurement, it has a radiation risk, requires high protection costs, and moreover, the response speed of X-rays is slow, making it difficult to adapt to the rapid measurement requirements of on-line real-time. In addition, whether it is an ultrasonic thickness gauge or an X-ray thickness gauge, a large number of return signals will be generated during measurement. Echo aliasing at multiple-layer interfaces will lead to analytical errors in the time-domain signal, increasing the difficulty of signal processing.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] Aiming at the deficiencies of the prior art, the present invention proposes an on-line detection method and system based on a 3PE anti-corrosion coating, aiming to overcome at least one of the above defects to achieve on-line real-time detection of full coverage of the 3PE anti-corrosion coating.

[0008] To solve the above technical problems, the present invention provides the following technical solutions: An on-line detection method based on a 3PE anti-corrosion coating, the method comprising the following steps: Step S1, adjust the position of the terahertz camera so that it is directly above the 3PE anti-corrosion steel pipe, and the field of view of the terahertz camera scanning downward completely covers the 3PE anti-corrosion steel pipe in the transverse direction; Step S2, continue to use the terahertz camera to scan the 3PE anti-corrosion steel pipe conveyed on the production line, obtain a scanned image and based on the scanned image, obtain a detection strip corresponding to the 3PE anti-corrosion steel pipe on each frame of the image, and use it as the effective scanning area for a single sampling of the 3PE anti-corrosion steel pipe by the terahertz camera; Step S3, based on the detection strip obtained in Step S2, adjust the rotation speed, traveling speed of the 3PE anti-corrosion steel pipe and the scanning frequency of the terahertz camera to achieve full coverage scanning of the 3PE anti-corrosion steel pipe; during the scanning process, perform on-line quality detection of the 3PE anti-corrosion coating according to the terahertz wave signal returned by the scanning.

[0009] Among them, in step S3, the return terahertz signal of each point in the detection strip is obtained. When the number of returned terahertz signals is four, they are recorded in chronological order as the first return signal, the second return signal, the third return signal and the fourth return signal, and the time difference Δt between the third return signal and the fourth return signal is obtained to evaluate whether the thickness of the epoxy powder coating meets the standard; when the number of return signals is three times, it indicates that there is a leak and the epoxy powder coating may not be sprayed; when the number of return signals is other times, it is regarded as an abnormal situation.

[0010] An online detection system based on 3PE anti-corrosion coating, comprising a terahertz camera and a production line for conveying 3PE anti-corrosion steel pipes, on which the 3PE anti-corrosion steel pipes are conveyed along their axial direction while rotating; medium frequency heating equipment, powder spraying equipment, adhesive coating equipment, polyethylene winding equipment, coolant spraying equipment and drying isolation equipment are sequentially arranged along the production line, and the terahertz camera is located downstream of the drying isolation equipment; and wherein the drying isolation equipment is provided with an air blowing head on one side close to the terahertz camera, the air blowing head comprising a body, A first cavity and a second cavity are provided which surround the 3PE anti-corrosion steel pipe and are arranged side by side. The second cavity is located on the side close to the terahertz camera. The first cavity and the second cavity are respectively connected to the external gas source through gas pipelines. On the upstream side of the first cavity, a first gap is formed between the main body and the 3PE anti-corrosion steel pipe, and on the downstream side of the second cavity, a third gap is formed between the main body and the 3PE anti-corrosion steel pipe. Between the first cavity and the second cavity, a second gap is formed between the main body and the 3PE anti-corrosion steel pipe, and the first gap, the first cavity, the second gap, the second cavity and the third gap are connected in sequence.

[0011] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention proposes a technical solution for online detection of the quality of 3PE anti-corrosion coatings using a terahertz camera. The terahertz wave is unable to penetrate the anti-corrosion steel pipe parent material to form an effective reflection. By constructing a detection strip, the return number of the returned terahertz signal in the detection strip is counted, and the thickness of the epoxy powder coating is evaluated based on the time difference Δt between the third return signal and the fourth return signal. Whether it meets the standard, effectively overcomes the problem that the prior art cannot perform real-time high-precision online detection of ultra-thin epoxy powder coatings located at the bottom layer. Through the technical solution of the present invention, the detection accuracy can reach 1~2 microns, and the full coverage detection of epoxy powder coatings can be achieved during the production process of 3PE anti-corrosion steel pipes. In addition, through the detection system of the present invention, the reliability and stability of the terahertz camera for detection on the production line are effectively ensured. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings: Figure 1 It is a schematic diagram of the cross-sectional structure of 3PE anti-corrosion steel pipe; Figure 2 It is a schematic diagram of the detection state of scanning a 3PE anti-corrosion steel pipe using a terahertz camera in the present invention; Figure 3 1 is a comparison diagram of the scanning range of the terahertz camera of the present invention before and after adjustment, wherein (a) is a schematic diagram of the scanning before adjustment, and (b) is a schematic diagram of the scanning after adjustment; Figure 4 It is a schematic diagram of acquiring detection strips of a scanned image acquired by a terahertz camera; Figure 5 This is a schematic diagram of the effect of overlapping detection strips obtained by a terahertz camera when a 3PE anti-corrosion steel pipe is in motion; Figure 6 It is a schematic diagram of the geometric relationship between a single detection strip and the corresponding arc segment on the circumference of the 3PE anti-corrosion steel pipe; Figure 7 yes Figure 5 The effect diagram of the detection strips shown when they are partially overlapped in a two-dimensional plane layout (that is, arc segments are spread as straight line segments); Figure 8 It is a partial structural schematic diagram of a system involved in the online detection method of the present invention; Fig. 9 yes Figure 8 A schematic diagram of the structure of the air blowing head provided on the side of the drying isolation device close to the terahertz camera; Fig.10 It is a schematic diagram of the principle of transmitting and receiving signals when using terahertz signals for imaging; Fig.11 It is a schematic diagram of a signal peak formed when a terahertz camera receives a return terahertz signal under normal circumstances; Fig.12 It is a schematic diagram of a signal peak formed when a terahertz camera receives a return terahertz signal under abnormal circumstances; Fig.13 It is a schematic diagram of the coordination relationship between the 3PE anti-corrosion steel pipe, the conveying device, etc. and the ground when the terahertz camera is used to scan the 3PE anti-corrosion steel pipe in the present invention; The meanings of the reference numerals are as follows: 100-3PE anti-corrosion steel pipe, 1-anti-corrosion steel pipe base material, 2-epoxy powder layer, 3-adhesive layer, 4-polyethylene layer, 5-terahertz camera, 6-transmission device, 7-field of view, 8-scanning range, 9-marking busbar, 10-detection strip, 10'-detection strip acquired in the next rotation cycle, 11-medium frequency heating equipment, 12-powder spraying equipment, 13-adhesive coating equipment, 14-polyethylene winding equipment, 15-coolant spraying equipment, 16-drying isolation equipment, 17-first gap, 18-first cavity, 19-second gap, 20-second cavity, 21-third gap, 22-original terahertz signal, 23-return terahertz signal, 24-isolation cabinet, 25-ground, 26-center horizontal plane, 161-air blowing head, 241-table, 242-inner cavity, 801-first area, 802-second area, 803-third area, A-first return signal, B-second return signal, C-third return signal, D-fourth return signal, E-fifth return signal, F-sixth return signal. DETAILED DESCRIPTION

[0013] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0014] like Figures 1 to 13 As shown, the present invention provides an online detection method based on 3PE anti-corrosion coating, which is implemented by means of an online detection system, wherein the online detection system at least comprises a terahertz camera 5 and a production line for conveying 3PE anti-corrosion steel pipes 100. On the production line, the 3PE anti-corrosion steel pipes 100 are conveyed along their axial direction while rotating. The method comprises the following steps: Step S1: Initialize camera position The position of the terahertz camera 5 is adjusted so that it is located directly above the 3PE anti-corrosion steel pipe 100, and at this position, the field of view 7 of the terahertz camera 5 scanning downward completely covers the 3PE anti-corrosion steel pipe 100 in the lateral direction. Specifically, the lateral direction is Figure 2 The ox direction shown is perpendicular to the axis extension direction of the 3PE anti-corrosion steel pipe 100 and is located in the horizontal plane. More specifically, when the terahertz camera 5 scans downward, the range of its field of view 7 in the lateral direction should at least ensure that the outer edges of both sides of the 3PE anti-corrosion steel pipe 100 are completely included. Figure 2Taking the position relationship shown as an example, when the terahertz camera 5 scans downward, the terahertz camera 5 can scan at least part of the area outside the left and right sides of the 3PE anti-corrosion steel pipe 100, thereby ensuring that the outer edges on both sides of the 3PE anti-corrosion steel pipe 100 are located within the field of view 7, thereby achieving that the 3PE anti-corrosion steel pipe 100 is completely covered by the field of view 7.

[0015] Preferably, the adjustment of the position of the terahertz camera 5 includes at least adjusting the position of the terahertz camera 5 along the vertical direction ( Figure 2 The vertical movement in the oy direction) and the horizontal movement in the direction perpendicular to the axis of the 3PE anti-corrosion steel pipe 100 ( Figure 2 The vertical movement is mainly to adjust the scanning range 8 of the terahertz camera 5 to ensure the above-mentioned complete coverage; and the horizontal movement is mainly to make the terahertz camera 5 located directly above the 3PE anti-corrosion steel pipe 100. Figure 3 Initially, the terahertz camera 5 may not be located directly above the 3PE anti-corrosion steel pipe 100. At the same time, its scanning range 8 is also small and cannot cover the entire 3PE anti-corrosion steel pipe 100. When the field of view angle of the terahertz camera 5 remains unchanged, by changing the vertical distance between it and the 3PE anti-corrosion steel pipe 100, the size of its scanning range 8 can be adjusted, and by horizontal lateral movement, the terahertz camera 5 is ensured to be directly above the 3PE anti-corrosion steel pipe 100. It should be noted that the position adjustment of the terahertz camera 5 can be done manually, or an additional control mechanism can be set to automatically adjust, or by a combination of man and machine. The specific adjustment method is not the innovation point of the present invention and will not be further elaborated here.

[0016] Further preferably, when adjusting the position of the terahertz camera 5, the final position of the terahertz camera 5 is determined based on the scanning result of the terahertz camera 5 on the 3PE anti-corrosion steel pipe 100 on the production line. The scanning result is preferably a scanned image. Specifically, see Figure 3As shown in Figure (b), when the position of the terahertz camera 5 is adjusted, the terahertz camera 5 can scan and obtain the corresponding image. When it is about to be adjusted in place, the image obtained by scanning is divided into three areas, namely the first area 801, the second area 802 and the third area 803, wherein the second area 802 represents the covered 3PE anti-corrosion steel pipe 100, and the first area 801 and the third area 803 represent the at least part of the area outside the anti-corrosion steel pipe; wherein the first area 801 and the third area 803 are respectively located on both sides of the 3PE anti-corrosion steel pipe 100. Then, when the areas of the first area 801 and the third area 803 are equal, the position of the terahertz camera 5 is its final position. It should also be noted that, since the terahertz wave has penetrability, the images obtained by scanning in the present invention are all drawn by receiving the terahertz wave signal reflected from the surface of the object, for example, the terahertz wave signal reflected back to the terahertz camera 5 from the surface of the 3PE anti-corrosion steel pipe 100.

[0017] It should also be noted that when adjusting the scanning range 8, the scanning range 8 can be increased or decreased, which is mainly achieved by adjusting the distance between the terahertz camera 5 and the 3PE anti-corrosion steel pipe 100 in the vertical direction, wherein preferably, the distance between the two in the vertical direction is controlled within the range of 1m~3m.

[0018] It should also be noted that in the actual production process, the production line usually continuously transports multiple (at least two) 3PE anti-corrosion steel pipes 100, and these 3PE anti-corrosion steel pipes 100 are from the same batch, and their sizes are almost the same. During transportation, the positions of these 3PE anti-corrosion steel pipes 100 remain basically unchanged. Therefore, when initializing the camera position, it is only necessary to adjust the position of the terahertz camera 5 according to the scanning result of the first 3PE anti-corrosion steel pipe 100. That is, when adjusting the position of the terahertz camera 5, the final position of the terahertz camera 5 is determined based on the scanning result of the first 3PE anti-corrosion steel pipe 100 transported from the production line, and after the position is adjusted in place, the terahertz camera 5 is fixed, and the fixed terahertz camera 5 is used to scan and detect the subsequent 3PE anti-corrosion steel pipes 100 to be inspected that are transported from the production line.

[0019] Further preferably, the first 3PE anti-corrosion steel pipe 100 is a 3PE standard anti-corrosion steel pipe of the same model as the subsequent 3PE anti-corrosion steel pipe 100 to be tested. This is to achieve the detection of all 3PE anti-corrosion steel pipes 10 produced on the production line. If the first 3PE anti-corrosion steel pipe 100 is not a 3PE standard anti-corrosion steel pipe, the position of the terahertz camera 5 must be adjusted during the detection process, resulting in the inability to perform a comprehensive and accurate detection.

[0020] Step S2, after the camera position is initialized, the terahertz camera 5 is used to continue scanning the 3PE anti-corrosion steel pipe 100 to be detected that is transported from the production line, and a scanned image is obtained. Based on the scanned image, a detection strip 10 corresponding to the 3PE anti-corrosion steel pipe 100 to be detected on each frame of the image is obtained, and it is used as the effective scanning area for the terahertz camera 5 to perform a single sampling of the 3PE anti-corrosion steel pipe 100. Preferably, the acquisition of the detection strip 10 starts from the second 3PE anti-corrosion steel pipe 100. On the production line, the 3PE anti-corrosion steel pipe 100 to be detected that is transported is the first, second, ... Nth 3PE anti-corrosion steel pipe 100 in a front-to-back order, wherein the first 3PE anti-corrosion steel pipe 100 is mainly used for initializing the camera position, for example, it can be the 3PE standard anti-corrosion steel pipe, and from the second 3PE anti-corrosion steel pipe 100, a formal scanning and detection is carried out, and the detection content covers multiple indicators such as the thickness of the epoxy powder coating.

[0021] The detection strip 10 is obtained by the following steps: Step S21, performing denoising and other processing on the single-frame image obtained by scanning with the terahertz camera 5, and then using edge detection algorithms such as Canny to identify the left and right edge lines of the 3PE anti-corrosion steel pipe 100 from the image (the two edge lines are parallel to the axis of the 3PE anti-corrosion steel pipe 100), and based on the identified edge lines, generating a marking busbar 9 parallel to the edge lines and located exactly in the middle of the two edge lines through linear interpolation; Step S22: On the single-frame image, with the marked generatrix 9 as the reference line, a preset range X is symmetrically extended toward the two edge lines to generate a detection strip 10, wherein the value of the range X satisfies: X=kl*kj*D / 2 Wherein, D is the diameter of the 3PE anti-corrosion steel pipe 100, kl is an empirical coefficient, and its value is preferably 0.05~0.1, and kj is the ratio coefficient between the object on the single-frame image and the actual object, which is used to convert the size of the actual object into the size on the image. For example, kj*D represents the diameter of the 3PE anti-corrosion steel pipe 100 on the single-frame image. It should be noted that the value of kl should not be too large. An excessively large kl value is likely to cause a large measurement error in the thickness of the epoxy powder coating (especially at a position far away from the marking busbar 9), thereby affecting the detection result. Practice has proved that within the range of the above-mentioned preferred values, the error of the thickness of the epoxy powder coating detected is small, which meets the production requirements. In addition, it should be noted that the reason for setting the detection strip 10 is that, on the one hand, this setting can avoid the detection signal beating caused by the vibration of the production line or the mechanical vibration of the equipment, thereby improving the stability of the detection. On the other hand, it can ensure comprehensive coverage of the detection and avoid missed detection due to a small detection range. It should also be noted that, usually, for the same batch of 3PE anti-corrosion steel pipes 100 on the same production line, the value of kl does not need to be adjusted after it is set. Therefore, for each frame of image, the arc length of the corresponding detection strip 10 (along the circumferential direction of the anti-corrosion steel pipe) is constant, and the length L of the detection strip 10 in the axial direction of the 3PE anti-corrosion steel pipe 100 is determined by the scanning range of the terahertz camera 5. When the terahertz camera 5 is fixed, the length L is also a constant.

[0022] Step S3, based on the detection strip 10 obtained in step S2, adjust the rotation speed w of the 3PE anti-corrosion steel pipe 100, the travel speed v of the 3PE anti-corrosion steel pipe 100 along its axial direction, and the scanning frequency of the terahertz camera 5 to achieve full coverage scanning of the 3PE anti-corrosion steel pipe 100; during the scanning process, the quality of the 3PE anti-corrosion coating is detected online according to the terahertz wave signal returned by the scanning.

[0023] Specifically, see Figures 4 to 7 Since the detection strip 10 is determined on a single frame image, the scanning width size X' corresponding to the preset range X on the 3PE anti-corrosion steel pipe 100 can be determined by the ratio coefficient kj between the object on the single frame image and the actual object, and the scanning arc segment corresponding to the detection strip 10 on the 3PE anti-corrosion steel pipe 100 can be obtained. The length S. Figure 6 , which has the following relationship: , in, is the angle, R is the radius of the 3PE anti-corrosion steel pipe 100, then the scanning arc segment The corresponding angles are: , Thus, the scanning arc segment can be obtained The corresponding arc length is: .

[0024] In the actual production process, if you want to ensure full coverage of the scan, there needs to be overlap between the effective scanning areas of each scan, such as Figure 5 and Figure 7 As shown, for example, in the circumferential direction, there is an overlapping portion of arc length △s, then the following relationship exists: S=s+2*△s, Where S is the arc segment scanned on the circumference of the 3PE anti-corrosion steel pipe 100 The corresponding arc length, △s is the arc length of the overlapping area of ​​the effective scanning area of ​​two adjacent scans on the circumference of the 3PE anti-corrosion steel pipe 100, and s is the arc length of the non-overlapping area.

[0025] To achieve full coverage and uniform scanning, the terahertz camera 5 needs to scan at least the following times during one rotation: , In the formula, n is a positive integer, and △s is set manually according to actual needs; According to the scanning time T and the number of scans n for one rotation, the scanning frequency of the terahertz camera 5 can be obtained; In addition, in order to achieve full coverage scanning, the 3PE anti-corrosion steel pipe 100 also needs to be stacked in the axial direction, for example, see Figure 5 , between each single-frame image obtained by scanning, the detection strip 10 needs to be overlapped with the detection strip 10' obtained by the corresponding next rotation cycle; therefore, when the 3PE anti-corrosion steel pipe 100 rotates one circle, the travel distance of the 3PE anti-corrosion steel pipe 100 in its axial direction (that is, the pitch of the rotational conveying) should be less than the length of the corresponding detection strip 10 on the 3PE anti-corrosion steel pipe 100, that is, L>v*T, where L is the length of a single detection strip 10 on the 3PE anti-corrosion steel pipe 100, in m, which is obtained by combining the length of the detection strip 10 with the proportional coefficient kj, and v is the speed of the 3PE anti-corrosion steel pipe 100 along its axial direction, in m / s. In addition, since the time for the 3PE anti-corrosion steel pipe 100 to rotate one circle satisfies: , Therefore, the following relationship can be obtained: , From this we get: , According to this relationship, the rotation speed w and the travel speed v of the 3PE anti-corrosion steel pipe 100 can be controlled to achieve this. Of course, the control of the rotation speed w and the travel speed v also needs to comprehensively consider the coating uniformity, time, cost, etc. For example, too fast a travel speed will lead to insufficient coating thickness and affect the anti-corrosion performance, while too slow a travel speed will increase production costs and time.

[0026] For further information, see Figures 10 to 12 During operation, the terahertz camera 5 mainly acquires information about the object by emitting the original terahertz signal 22 and receiving the returned terahertz signal 23. Different return signals may exist between different layers of the 3PE anti-corrosion steel pipe 100. Figure 1 , Figures 10 to 12 For example, the 3PE anti-corrosion steel pipe 100 is composed of an anti-corrosion steel pipe base material 1, an epoxy powder layer 2, an adhesive layer 3 and a polyethylene layer 4 from the inside to the outside. Among them, due to the poor penetration ability of terahertz waves on metal materials, the terahertz wave signal from the terahertz camera 5 cannot form a return signal after penetrating the anti-corrosion steel pipe base material 1. Therefore, under normal circumstances, when the original terahertz signal 22 is emitted to the surface of the anti-corrosion steel pipe base material 1, the return terahertz signal 23 will include the first return signal A, the second return signal B, the third return signal C and the fourth return signal D in sequence. Among them, the first return signal A is formed by reflection from the outer surface of the polyethylene layer 4, the second return signal B is formed by reflection at the layered interface between the adhesive layer 3 and the polyethylene layer 4, the third return signal C is formed by reflection at the layered interface between the epoxy powder layer 2 and the adhesive layer 3, and the fourth return signal D is formed by the original terahertz signal 22 reflected from the outer surface of the anti-corrosion steel pipe base material 1. Since there is a time difference between each returned terahertz signal 23, within a scanning time period Tk of the terahertz camera 5 (taking n scans as an example, Tk=T / n), the return time difference Δt between the third return signal C and the fourth return signal D can be obtained. Through this time difference Δt, the thickness of the epoxy powder layer 2 can be calculated, thereby realizing the online detection of the quality of the 3PE anti-corrosion coating. It should be understood that the above returned terahertz signal 23 comes from the detection strip 10. By analyzing the returned terahertz signal 23 of each point in the detection strip 10, when the number of return signals is four, the time difference Δt is calculated to evaluate whether the coating thickness meets the standard; when the number of return signals is three, it indicates that there is a leak and the epoxy powder coating may not be sprayed (usually not other layers); when the number of return signals is other times, it is regarded as an abnormal situation. It should be noted that the size of Δt directly reflects the coating thickness. The larger Δt is, the thicker the coating is; the smaller Δt is, the thinner the coating is; if Δt is close to 0 or the fourth return signal D cannot be detected, it indicates that there is no epoxy powder coating here, forming a leakage point. The system will trigger the alarm mechanism to prompt the inspection personnel to pay attention to the integrity of the coating; thereby, the quality inspection of the epoxy powder coating at the corresponding position can be achieved.

[0027] In order to better achieve the purpose of the present invention, during the online detection process, the upper and lower limits of the return time difference Δt can be set to control Δt within a preset range, so that the quality of the epoxy powder coating can be ensured. Fig.11 In the process of processing the return signal, the upper and lower limits of the signal strength are also set. The signal between the upper and lower limits of the signal strength will not be regarded as a return signal. Only when the strength of the return signal exceeds the upper and lower limits of the signal strength will it be regarded as a return signal. The purpose of this is to effectively filter out noise and improve the accuracy and reliability of detection.

[0028] It should also be noted that the acquisition of the return signal is crucial for the quality inspection of the epoxy powder coating thickness. In some areas of the 3PE anti-corrosion steel pipe 100, such as near the edge, the terahertz signal may not be transmitted to the surface of the anti-corrosion steel pipe base material 1, resulting in Fig.12 The abnormal situation shown, for example, the fifth return signal E and the sixth return signal F also appear. In this case, the thickness information of the epoxy powder coating cannot be effectively obtained, which is one of the reasons why the detection strip 10 is set in the previous text. The setting of the detection strip 10 can eliminate this situation and ensure that within a scanning time period Tk, the returned terahertz wave signals are four or three received in sequence. In addition, whether in normal or abnormal conditions, when the original terahertz signal 22 is emitted to the 3PE anti-corrosion steel pipe 100, a first return signal A will be formed at various positions on the surface of the 3PE anti-corrosion steel pipe 100, which is why the scanning images in the previous text are drawn by receiving the terahertz wave signal reflected from the surface of the object.

[0029] It should also be noted that the present invention may also involve the acquisition of the return time difference between the first return signal A and the second return signal B, the return time difference between the second return signal B and the third return signal C, etc., and corresponding analysis of each layer can be performed through these return time differences and their historical data, which is similar to the analysis of the epoxy powder layer and will not be introduced in detail here.

[0030] It should be understood that the present invention also relates to an online detection system, which at least includes a terahertz camera 5 and a production line for conveying 3PE anti-corrosion steel pipes 100, on which the 3PE anti-corrosion steel pipes 100 are conveyed along their axis while rotating. Figure 8, the online detection system also includes a medium frequency heating device 11, a powder spraying device 12, an adhesive coating device 13, a polyethylene winding device 14, a coolant spraying device 15 and a drying isolation device 16 which are sequentially arranged along the production line, wherein the terahertz camera 5 is located downstream of the drying isolation device 16, that is, away from the coolant spraying device 15. It should be noted that in the production process, the coolant spraying device 15 usually uses water as a cooling medium, and since the terahertz wave is very sensitive to the moisture content (water has a strong absorption of the terahertz wave), the presence of water will seriously affect the detection quality of the terahertz camera 5. Therefore, before using the terahertz camera 5 for detection, it is necessary to set a drying isolation device 16 upstream thereof to ensure the detection quality.

[0031] In a preferred embodiment, the drying isolation device 16 is provided with at least one Fig. 9 The air blowing head shown, wherein the air blowing head includes a body, a first cavity 18 and a second cavity 20 are arranged side by side around the 3PE anti-corrosion steel pipe 100, the second cavity is located near the side of the terahertz camera 5, the first cavity 18 and the second cavity 20 are respectively connected to the external gas source through the gas pipeline, and a first gap 17 is formed between the body and the 3PE anti-corrosion steel pipe 100 on the upstream side of the first cavity 18, and a third gap 21 is formed between the body and the 3PE anti-corrosion steel pipe 100 on the downstream side of the second cavity 20, and a second gap 19 is formed between the body and the 3PE anti-corrosion steel pipe 100 between the first cavity 18 and the second cavity 20, and the first gap 17, the first cavity 18, the second gap 19, the second cavity 20 and the third gap 21 are connected in sequence. Through the setting of this air blowing head, it is possible to prevent moisture from passing through and entering the downstream end of one side of the terahertz camera 5, thereby affecting the detection quality of the terahertz camera 5.

[0032] Preferably, the sizes of the first gap 17, the second gap 19 and the third gap 21 decrease in sequence. This facilitates the airflow to flow toward the interior of the drying isolation device 16, thereby achieving the barrier of moisture on the upstream side. Further preferably, the gas pressure injected into the second cavity 20 is higher than the gas pressure of the first cavity 18.

[0033] It should be noted that there are various ways to dry the surface moisture of the 3PE anti-corrosion steel pipe 100, such as wiping, heating, freezing, and vacuuming. Among them, the wiping method requires direct contact with the surface of the anti-corrosion steel pipe, which is likely to cause scratches or damage to the surface of the anti-corrosion steel pipe, and usually requires manual operation, which is time-consuming and laborious, and the drying effect is not ideal. For the methods of heating and freezing, there are disadvantages such as high cost and slow drying speed. In addition, only through the vacuum method, the drying effect is limited. Therefore, the present invention adopts the above-mentioned air blowing head to achieve this. Through the above-set air blowing head, on the one hand, the moisture can be completely isolated on the upstream side of the air blowing head, and on the other hand, it will not contact the surface of the 3PE anti-corrosion steel pipe 100 and will not have any adverse effects on the 3PE anti-corrosion steel pipe 100.

[0034] Further preferably, the drying and isolation device 16 is a drying chamber, which is connected to the vacuum suction device. Through such a setting, on the one hand, the air blowing head can block the moisture on the surface of the 3PE anti-corrosion steel pipe 100 to prevent it from entering the downstream side, and on the other hand, it blows the moisture on the surface of the 3PE anti-corrosion steel pipe 100 into the air and is blown away under the action of the vacuum suction device, so as to achieve rapid non-contact drying treatment.

[0035] To better achieve the purpose of the present invention, the on-line detection system further includes a steel plate provided on the ground 25 at the position of the terahertz camera 5, so that the area other than the 3PE anti-corrosion steel pipe 100 in the field of view of the terahertz camera 5 falls on the steel plate. The purpose of this is to reduce the interference of terahertz wave signals. Since the ground is often uneven and there may be many other substances, this is likely to cause a large number of terahertz wave signals to return, affecting data processing. Therefore, a steel plate is provided. When the terahertz wave reaches the steel plate, it cannot penetrate, so there will be no excessive return signals.

[0036] As an alternative technical solution, the on-line detection system further includes an isolation cabinet 24 fixedly provided on the ground 25. The isolation cabinet 24 is open in the axial extension direction of the 3PE anti-corrosion steel pipe 100 to facilitate the 3PE anti-corrosion steel pipe 100 to enter and pass through the isolation cabinet 24 smoothly. The interior of the isolation cabinet 24 forms an inner cavity 242, and a conveying device 6 for conveying the 3PE anti-corrosion steel pipe 100 is provided in the inner cavity 242. The conveying device 6 is arranged along the axis direction of the 3PE anti-corrosion steel pipe 100 to ensure the stable conveying of the 3PE anti-corrosion steel pipe 100. The top of the isolation cabinet 24 forms a horizontally arranged tabletop 241, and at least a part of the 3PE anti-corrosion steel pipe 100 is located above the tabletop 241 (see Fig.13, the middle of the tabletop 241 is a notch to facilitate the 3PE anti-corrosion steel pipe 100 to extend from the inner cavity 242), so as to facilitate the scanning of the 3PE anti-corrosion steel pipe 100 by the terahertz camera 5. Through such a setting, a conveying device 6 can still be arranged at the position below the terahertz camera 5. If the isolation cabinet 24 is not arranged, it is often necessary to suspend the 3PE anti-corrosion steel pipe 100 below the terahertz camera 5 to avoid scanning the conveying device 6 etc. into the image during the scanning process, which will affect the subsequent data processing. And this suspension method is not conducive to the stable conveying of the 3PE anti-corrosion steel pipe 100. It should be noted that since edge detection algorithms such as Canny are needed to identify the left and right edge lines of the 3PE anti-corrosion steel pipe 100 from the scanned image, if there are images of the conveying device 6 etc. in the image, it will affect the extraction of the edge lines, and thus affect the final recognition result. In addition, the isolation cabinet 24 itself can play a certain protective role to prevent the influence of external interference on the detection process and ensure the accuracy and stability of the detection. Preferably, the tabletop on the top of the isolation cabinet 24 is also made of steel material.

[0037] In a preferred embodiment, the tabletop 241 is slightly lower than the central horizontal plane 26 of the 3PE anti-corrosion steel pipe 100. Such a setting can optimize the detection result and avoid scanning the gap between the tabletop 241 and the 3PE anti-corrosion steel pipe 100 during the scanning process.

[0038] In order to better achieve the purpose of the present invention, the coolant spraying device 15 is arranged in an isolated water room, and the 3PE anti-corrosion steel pipe 100 passes through the water room. Another terahertz camera 5 (not shown in the figure) is additionally arranged between the water room and the polyethylene winding device 14. The other terahertz camera 5 is connected to the control system of the powder spraying device 12 (it may also be the adhesive coating device 13 and the polyethylene winding device 14). The control system adjusts the operating parameters of the corresponding powder spraying device 12 according to the signal data detected by it, such as adjusting the speed of powder spraying. In this embodiment, by arranging the other terahertz camera 5 between the water room and the polyethylene winding device 14, the situation of the epoxy powder layer can be grasped in time, which is convenient for real-time feedback adjustment. The reason why the terahertz camera 5 arranged on the downstream side of the drying isolation device 16 is not used here is mainly because it is located at the end of the production line, and the data lag of its detection is too much. When the epoxy powder layer is found to have quality problems, it is too late to adjust. In addition, it should be noted that although quality problems such as insufficient coating thickness can be discovered in time here, the 3PE anti-corrosion steel pipe 100 here has not been cooled yet. Therefore, the detection data is not the final result. It can only be used as the basis for regulation and not as the basis for judging the final quality. In order to facilitate regulation based on it, the compensation coefficient can be set according to actual needs to correct its results (this is a prior art and will not be repeated). In addition, it should be understood that a blowing head can also be set on the upstream side of the water room to prevent water vapor in the water room from entering the upstream side and affecting the detection quality of the other terahertz camera 5.

[0039] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An online detection method based on 3PE anti-corrosion coating, characterized in that: The method comprises the following steps: Step S1, adjusting the position of the terahertz camera (5) so that it is located directly above the 3PE anti-corrosion steel pipe (100), and the field of view (7) of the terahertz camera (5) scanning downwards completely covers the 3PE anti-corrosion steel pipe (100) in the lateral direction; Step S2, continue to use the terahertz camera (5) to scan the 3PE anti-corrosion steel pipe (100) transported from the production line, obtain a scanned image, and based on the scanned image, obtain a detection strip (10) corresponding to the 3PE anti-corrosion steel pipe (100) on each frame of the image, and use it as an effective scanning area for the terahertz camera (5) to perform a single sampling on the 3PE anti-corrosion steel pipe (100); Step S3: Based on the detection strip (10) obtained in step S2, the rotation speed and the travel speed of the 3PE anti-corrosion steel pipe (100) and the scanning frequency of the terahertz camera (5) are adjusted to achieve full coverage scanning of the 3PE anti-corrosion steel pipe (100); during the scanning process, the quality of the 3PE anti-corrosion coating is detected online according to the terahertz wave signal returned by the scanning.

2. An online detection method based on 3PE anti-corrosion coating as claimed in claim 1, characterized in that: In step S1, for a plurality of 3PE anti-corrosion steel pipes (100) continuously conveyed on a production line, the position of a terahertz camera (5) is adjusted according to a scanning result of a first 3PE anti-corrosion steel pipe (100); after the position is adjusted to the right position, the terahertz camera (5) is fixed, and the fixed terahertz camera (5) is used to scan and inspect a subsequent 3PE anti-corrosion steel pipe (100) to be inspected that is conveyed from the production line.

3. An online detection method based on 3PE anti-corrosion coating as claimed in claim 2, characterized in that: The first 3PE anti-corrosion steel pipe (100) is a 3PE standard anti-corrosion steel pipe of the same model as the subsequent 3PE anti-corrosion steel pipe (100) to be tested.

4. An online detection method based on 3PE anti-corrosion coating as claimed in claim 1, characterized in that: In step S2, the detection strip (10) is obtained by the following steps: Step S21, performing denoising processing on the single-frame image obtained by scanning the terahertz camera (5), then using an edge detection algorithm to identify the left and right edge lines of the 3PE anti-corrosion steel pipe (100) from the image, and based on the identified edge lines, generating a marking generatrix (9) parallel to the edge lines and located exactly in the middle of the two edge lines by linear interpolation; Step S22: On the single frame image, taking the marked main line (9) as a reference line, symmetrically extending a preset range X in the direction of the two edge lines to generate a detection strip (10).

5. An online detection method based on 3PE anti-corrosion coating as claimed in claim 4, characterized in that: The value of the range X satisfies: X=kl*kj*D / 2, Where D is the diameter of the 3PE anti-corrosion steel pipe (100), kl is the empirical coefficient, and kj is the ratio coefficient between the object in the single-frame image and the actual object.

6. An online detection method based on 3PE anti-corrosion coating as claimed in claim 1, characterized in that: In step S3, the rotation speed and the travel speed of the 3PE anti-corrosion steel pipe (100) satisfy the following relationship: ， In the formula, w is the rotation speed, v is the travel speed, and L is the length corresponding to a single detection strip (10) on the 3PE anti-corrosion steel pipe (100).

7. An online detection method based on 3PE anti-corrosion coating according to any one of claims 1 to 6, characterized in that: In step S3, the return terahertz signal (23) of each point in the detection strip (10) is obtained. When the number of the return terahertz signal (23) is four, they are recorded in chronological order as the first return signal (A), the second return signal (B), the third return signal (C) and the fourth return signal (D). The time difference Δt between the third return signal (C) and the fourth return signal (D) is obtained to evaluate whether the thickness of the epoxy powder coating meets the standard. When the number of return signals is three, it indicates that there is a leak and the epoxy powder coating may not be sprayed. When the number of return signals is other times, it is regarded as an abnormal situation.

8. An online detection system based on 3PE anti-corrosion coating, which is used to implement the online detection method described in any one of claims 1 to 7, characterized in that: The online detection system comprises at least a terahertz camera (5) and a production line for conveying 3PE anti-corrosion steel pipes (100); on the production line, the 3PE anti-corrosion steel pipes (100) are conveyed along their axial direction while rotating.

9. The online detection system according to claim 8, characterized in that: The online detection system further comprises a medium frequency heating device (11), a powder spraying device (12), an adhesive coating device (13), a polyethylene winding device (14), a coolant spraying device (15) and a drying isolation device (16) which are sequentially arranged along the production line, wherein the terahertz camera (5) is located downstream of the drying isolation device (16), that is, away from the coolant spraying device (15).

10. The online detection system according to claim 9, characterized in that: The drying and isolating device (16) is provided with an air blowing head on a side close to the terahertz camera (5), the air blowing head comprising a main body, a first cavity (18) and a second cavity (20) which surround the 3PE anti-corrosion steel pipe (100) and are arranged side by side are arranged in the main body, the second cavity is located on a side close to the terahertz camera (5), the first cavity (18) and the second cavity (20) are respectively connected to an external gas source through an air pipeline, a first gap (17) is formed between the main body and the 3PE anti-corrosion steel pipe (100) on the upstream side of the first cavity (18), a third gap (21) is formed between the main body and the 3PE anti-corrosion steel pipe (100) on the downstream side of the second cavity (20), a second gap (19) is formed between the first cavity (18) and the second cavity (20), and between the main body and the 3PE anti-corrosion steel pipe (100), and the first gap (17), the first cavity (18), the second gap (19), the second cavity (20) and the third gap (21) are sequentially connected.

Images