Electromagnetic marine coating corrosion defect detection system
By using a three-coil differential metal layer thickness sensor and signal processing technology, the problem of distinguishing between point-like and weld-like corrosion in existing technologies has been solved, achieving efficient and accurate corrosion defect detection at a single frequency.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- DALIAN MARITIME UNIVERSITY
- Filing Date
- 2025-05-27
- Publication Date
- 2026-05-19
AI Technical Summary
Existing technologies make it difficult to distinguish between pitting corrosion and weld corrosion at a single frequency during metal thickness measurement. Furthermore, measurements of non-ferromagnetic metals are complex, require extensive data processing, and involve long testing times.
A three-coil differential metal layer thickness sensor, including a filter capacitor, an excitation coil, and a detection coil, is used. The signal is processed by a lock-in amplifier and an operational amplifier, and combined with a LabVIEW data acquisition unit, it can distinguish between point-like and weld-like corrosion.
It can accurately distinguish between pitting and weld-like corrosion under a single AC excitation frequency, improve detection speed and sensitivity, reduce interference signals, and is suitable for corrosion defect detection under marine coatings.
Smart Images
Figure CN224263140U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of non-destructive metal testing and sensor technology. Specifically, it designs an electromagnetic marine coating corrosion defect detection system. Background Technology
[0002] In the field of machinery manufacturing, various equipment components experience thinning due to corrosion, friction, and other factors during use, which in turn affects the safety and reliability of the equipment. Therefore, regular corrosion testing, especially for pitting and weld corrosion, is crucial for real-time monitoring of equipment status and reducing the risk of failure. In recent years, metal layer thickness measurement has largely relied on the principle that metals of different thicknesses have different impedance values at different frequencies, using frequency sweeping to differentiate metal thicknesses, resulting in complex overall experiments.
[0003] Current research on metal thickness measurement in lithium battery equalization technology faces the following challenges: While sweep frequency testing and phase characteristic analysis can ignore some lift-off effects, the testing time is long; single-frequency metal thickness measurement can mostly only measure one type of metal (ferromagnetic or non-ferromagnetic), and the analyzed features often require further extraction from the raw data; methods using algorithms to extract features from raw data to distinguish metals of different thicknesses often require a large amount of data and still necessitate complex post-processing. Utility Model Content
[0004] Based on the aforementioned technical problems, an electromagnetic marine coating corrosion defect detection system is proposed. This system can distinguish between pitting corrosion, weld seam corrosion, and weld seam corrosion of different widths and depths under a single AC excitation frequency. Furthermore, it can extract one or more thickness-related features of different metals with minimal impact from the lift-off distance using raw data at different lift-off distances.
[0005] The technical means adopted in this utility model are as follows:
[0006] This utility model provides an electromagnetic marine coating under-coating corrosion defect detection system, comprising: an AC signal generator, a three-coil differential metal layer thickness sensor, a signal processor, and a data acquisition unit. The AC signal generator and the three-coil differential metal layer thickness sensor are both connected to the signal processor, which is connected to the data acquisition unit.
[0007] The three-coil differential metal layer thickness sensor includes a filter capacitor, an excitation coil, a detection coil, and a PLA 3D-printed probe housing, wherein:
[0008] The filter capacitor, excitation coil, and detection coil are fixed inside the PLA 3D-printed probe housing. The detection coil consists of two 35-turn planar coils connected in series. The excitation coil is closely attached to the underside of the two planar coils for electromagnetic coupling with the detection coil to acquire information from the three-coil differential metal layer thickness sensor and transmit energy to it. The filter capacitor is connected in parallel with the two planar coils connected in series. This connection method effectively reduces interference signals in the measurement system, ensuring the accuracy and stability of the measurement results.
[0009] The signal processor includes a lock-in amplifier and an operational amplifier, wherein:
[0010] The lock-in amplifier has two signal channels, connected to a signal generator and a detection coil, respectively receiving the signal generated by the signal generator and the output signal of the detection coil. At the same time, it compares the phase and amplitude differences of the two input signals and extracts the response signal related to the excitation signal. The operational amplifier is connected to the lock-in amplifier, receives the stable DC signal from the lock-in amplifier, and amplifies the stable DC signal a second time.
[0011] The data acquisition device includes a data acquisition card and a LabVIEW data acquisition unit. The data acquisition card is connected to an operational amplifier to acquire the signal after secondary amplification, and then inputs the acquired signal after secondary amplification into the LabVIEW data acquisition unit.
[0012] Furthermore, the excitation coil is made of enameled copper wire with a diameter of 70μm, and the coil has a single layer of 79 turns, an inner diameter of 1mm, and an outer diameter of 15mm.
[0013] Furthermore, both planar coils of the detection coil are wound with enameled copper wire with a diameter of 70μm. The coils are single-layered with 35 turns, with an inner diameter of 1mm and an outer diameter of 5mm.
[0014] Furthermore, both ends of the excitation coil and the detection coil are connected with leads, and there is no electrical connection between the excitation coil and the detection coil. The leads are all located on one side of the three-coil differential metal layer thickness sensor.
[0015] Compared with the prior art, the present invention has the following advantages:
[0016] 1. The present invention provides an electromagnetic marine coating corrosion defect detection system that can distinguish between point defects and weld defects of different widths and depths under a single AC excitation frequency.
[0017] 2. The present invention provides an electromagnetic marine coating corrosion defect detection system that can extract the characteristics of defects of different shapes affected by the lift-off distance using raw data at different lift-off distances.
[0018] 3. The electromagnetic marine coating corrosion defect detection system provided by this utility model has high sensitivity and detection speed, and can accurately assess the shape of defects, making it particularly suitable for metal flaw detection and defect detection.
[0019] 4. The electromagnetic marine coating corrosion defect detection system provided by this utility model adopts a differential three-coil structure. The two pickup coils are wound in opposite ways, so that the signals generated by the coils cancel each other out. The initial value of the superimposed signal is 0, which increases its relative change value.
[0020] 5. This utility model provides an electromagnetic marine coating corrosion defect detection system with a unique double-vortex coil structure. It can distinguish between pitting corrosion, weld seam corrosion, and weld seam corrosion of different widths and depths at the same frequency, maintaining excellent detection performance even at a maximum lift-off distance of 5mm. It can detect and distinguish between pitting and weld seam corrosion using a single AC excitation frequency. Furthermore, it can extract relevant features of weld seam corrosion of different widths and depths from raw data at different lift-off distances.
[0021] Based on the above reasons, this utility model can be widely promoted in the fields of sensors. Attached Figure Description
[0022] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0023] Figure 1 This is a schematic diagram of the electromagnetic marine coating corrosion defect detection system of this utility model.
[0024] Figure 2 This is a schematic diagram of the electromagnetic marine coating corrosion defect detection system of this utility model.
[0025] Figure 3 This is a structural diagram of the three-coil differential metal layer thickness sensor of this utility model.
[0026] Figure 4 This is a schematic diagram of the signal processing circuit structure of the signal processor of this utility model.
[0027] Figure 5 This is a diagram showing the amplitude of the detection signal for test specimens with corrosion defects of different shapes according to this invention.
[0028] Figure 6This refers to the internal magnetic flux mode density of this utility model.
[0029] Figure 7 This is a simulation diagram of the coil signal of this utility model.
[0030] Figure 8 This is a graph showing the amplitude of the detection signals for pitting corrosion and weld seam corrosion samples of different widths according to this invention.
[0031] Figure 9 This is a graph showing the amplitude of the detection signals for pitting corrosion and weld corrosion samples of different depths according to this invention.
[0032] Figure 10 This is a diagram showing the detection limit of this utility model.
[0033] Figure 11 This is a diagram showing the detection signal results of this utility model at a vertical distance of 2mm to 5mm.
[0034] In the figure: 1. AC signal generator (RIGOL DG812 10MHZ dual channel); 2. Three-coil differential metal layer thickness sensor; 3. Filter capacitor; 4. Excitation coil; 5. Detection coil; 6. PLA 3D printed probe housing; 7. Signal processor; 8. Lock-in amplifier; 9. Operational amplifier; 10. Data acquisition card (DAQ) (NI USB_6211, National Instruments, USA); 11. LabVIEW data acquisition unit; 12. Test sample support platform; 13. Dual-axis linear synchronous belt sliding test bench. Detailed Implementation
[0035] It should be noted that, where there is no conflict, the embodiments and features in the embodiments of this utility model can be combined with each other. The present utility model will now be described in detail with reference to the accompanying drawings and embodiments.
[0036] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the technical solutions of the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this utility model or its application or use. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this utility model.
[0037] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.
[0038] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values of the components and steps described in these embodiments do not limit the scope of this invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following drawings denote similar items; therefore, once an item is defined in one drawing, it need not be further discussed in subsequent drawings.
[0039] In the description of this utility model, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is usually based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this utility model and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this utility model. The directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.
[0040] For ease of description, spatial relative terms such as "above," "over," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation besides the orientation of the device as described in the figures. For example, if the device in the figures is inverted, a device described as "above" or "above" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.
[0041] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore cannot be construed as limiting the scope of protection of this utility model.
[0042] like Figure 1 As shown, this utility model provides an electromagnetic marine coating under-coating corrosion defect detection system, including: an AC signal generator 1, a three-coil differential metal layer thickness sensor 2, a signal processor 7, and a data acquisition unit. The AC signal generator 1 and the three-coil differential metal layer thickness sensor 2 are both connected to the signal processor 7, and the signal processor 7 is connected to the data acquisition unit.
[0043] The three-coil differential metal layer thickness sensor 2 includes a filter capacitor 3, an excitation coil 4, a detection coil 5, and a PLA 3D-printed probe housing 6, wherein:
[0044] The filter capacitor 3, excitation coil 4, and detection coil 5 are fixed inside the PLA 3D printed probe housing 6. The detection coil 5 consists of two 35-turn planar coils connected in series. The excitation coil 4 is tightly fitted below the two planar coils for electromagnetic coupling with the detection coil 5 to acquire information from the three-coil differential metal layer thickness sensor and to transmit energy to the three-coil differential metal layer thickness sensor. Figure 2 As shown, the filter capacitor 3 is connected in parallel with two series-connected planar coils; this connection method effectively reduces interference signals in the measurement system to achieve specific filtering and adjustment functions in the circuit, ensuring the accuracy and stability of the measurement results.
[0045] The signal processor 7 includes a lock-in amplifier 8 and an operational amplifier 9, wherein:
[0046] The lock-in amplifier 8 has two signal channels, connected to the signal generator 1 and the detection coil 5, respectively receiving the signal generated by the signal generator 1 and the output signal of the detection coil 5. At the same time, it compares the phase and amplitude differences of the two input signals and extracts the response signal related to the excitation signal. The operational amplifier 9 is connected to the lock-in amplifier 8, receives the stable DC signal from the lock-in amplifier 8, and amplifies the stable DC signal a second time.
[0047] The data acquisition device includes a data acquisition card 10 and a LabVIEW data acquisition unit 11. The data acquisition card 10 is connected to an operational amplifier 9 to acquire the signal after secondary amplification, and then inputs the acquired signal after secondary amplification to the LabVIEW data acquisition unit 11. In this embodiment, the test sample support platform 12 and the dual-axis linear synchronous belt sliding test bench 13 are prior art and are not within the protection scope of this application; therefore, these two components will not be described in detail.
[0048] In a specific implementation, as a preferred embodiment of this utility model, the electromagnetically coupled three-coil differential metal layer thickness sensor further includes an AC power supply, such as... Figure 3 As shown, in the three-coil differential metal layer thickness sensor, the detection coil 5 is connected in parallel with the filter capacitor 3. The excitation coil 4 is connected to the same side lead and then to an AC signal generator. In this embodiment, the AC signal generator produces two identical AC signals with the same frequency, amplitude, and phase. Figure 4 As shown, one signal is connected to the excitation coil 4, while another signal, along with the detection coil 5, is connected to the lock-in amplifier 8. The lock-in amplifier 8 compares, processes, and amplifies the input signal, then outputs a DC signal to the operational amplifier 7 for secondary amplification. The final amplified signal is collected by the data acquisition card 10 and input to the LabVIEW data acquisition unit 11. The LabVIEW data acquisition unit 11 is software for data acquisition and instrument control, capable of controlling the application of excitation to the coils and simultaneously acquiring their signal changes. The entire process, from signal generation from the AC signal generator to the final signal being written from the data acquisition card (DAQ), is connected via RF radio frequency lines, resulting in minimal signal interference and eliminating the need for filtering.
[0049] In a specific implementation, as a preferred embodiment of this utility model, the excitation coil 4 is made of enameled copper wire with a wire diameter of 70μm, and the coil has a single layer of 79 turns, an inner diameter of 1mm, and an outer diameter of 15mm.
[0050] In a specific implementation, as a preferred embodiment of this utility model, the two planar coils in the detection coil 5 are wound with enameled copper wire with a wire diameter of 70μm, and the coils are single-layered with 35 turns, with an inner diameter of 1mm and an outer diameter of 5mm.
[0051] In a specific implementation, as a preferred embodiment of this utility model, both ends of the excitation coil 4 and the detection coil 5 are connected with leads, but there is no electrical connection between the excitation coil 4 and the detection coil 5, and the leads are all located on one side of the three-coil differential metal layer thickness sensor 2.
[0052] This utility model also provides a method for manufacturing an electromagnetic marine coating under-coating corrosion defect detection system, including:
[0053] S1. Fabrication of a differential three-coil defect detection sensor: The large-diameter coil is located in the lower layer as the excitation coil, and the two small-diameter coils are located in the upper layer as detection coils. Connect one end of the two detection coils together and connect a capacitor in parallel at the other end to ensure that the detection coil and the excitation coil are in close contact and that there are no wires connecting the excitation coil and the detection coil.
[0054] S2. Fabrication of the excitation unit: Clean and inspect the excitation coils to ensure they meet requirements. Adjust the distance between the defect detection sensor probe and the metal defect, and fix it at the center of the first layer of the PLA 3D printed probe for optimal measurement results. The fabrication of the excitation unit is now complete.
[0055] S3. Fabrication of the Detection Unit: Clean and inspect the detection coil to ensure it meets requirements. Fix the capacitor inside the PLA 3D printed probe. Place the two detection coils horizontally opposite each other, ensuring they are in close contact with the excitation coil. To obtain a more stable circuit, design and fabricate a PCB board using Altium Designer (AD) software for the SMA shielding terminals and capacitor components, excluding the coils. Then, fix the coils to the probe end, and attach the PCB board to the second layer of the probe for assembly.
[0056] Complete the fabrication of the detection unit;
[0057] S4. Construct a signal processing unit: Use a signal generator to generate two AC signals with identical frequency, amplitude, and phase. One signal is input to the excitation coil, and the other, along with the detection coil, is connected to the two signal channels of a lock-in amplifier. The lock-in amplifier processes and amplifies these two signals, outputting a stable DC signal to an operational amplifier. After secondary amplification, the signal is sent to a data acquisition card for data collection.
[0058] Example 1
[0059] like Figure 5The image shows the signal amplitude of the electromagnetic marine coating corrosion defect detection system of this invention for testing specimens with corrosion defects of different shapes (weld-like and pitting corrosion). The high magnetic permeability of ferromagnetic specimens significantly affects the penetration depth of eddy current detection. Magnetic flux leakage detection methods reduce the permeability of the specimen by subjecting it to magnetic saturation treatment. However, when a ferromagnetic specimen is excited but not yet magnetically saturated, it undergoes magnetization, thus acquiring a certain level of magnetization. In this state, the surface of the ferromagnetic specimen exhibits a multi-source effect. When the electromagnetic sensor approaches the defective specimen, the excitation magnetic field it generates causes magnetization on the specimen surface, while eddy current effects also exist on the surface. The presence of defects disrupts the continuity of the specimen surface, leading to conductivity distortion in the defect and its surrounding area. As the sensor approaches the defect, the magnetization at the defect edge cannot be canceled out by the magnetization in adjacent domains, creating a reverse resistance on both sides of the defect. This leads to magnetic charge accumulation near the defect edge, altering the magnetic field around the crack and its vicinity, resulting in non-uniformity in the permeability distribution. The induced electromotive force change amplitude of pitting defects is relatively small, but its peak value and recovery rate are significantly different from weld-like corrosion defects. When the sensor scans along the length of the weld defect, the signal characteristics show two troughs followed by two peaks. The signal difference between the peaks and troughs can reach 1.87V. When the sensor scans along the width of the defect, the signal characteristics show one trough followed by one peak, with a signal difference of up to 2.84V. Two pitting defects, however, show two peaks and two troughs. The signal difference between the peaks and troughs can reach 0.43V.
[0060] Example 2
[0061] like Figure 6 The figure shows the internal magnetic flux modulus of the coil in the electromagnetic marine coating corrosion defect detection system of this utility model. The sensor parameters are as follows: the excitation coil is a single-layer planar coil with 79 turns, with an outer diameter of 15mm and an inner diameter of 1mm; the two planar coils in the detection unit are both single-layer planar coils with 35 turns, with an outer diameter of 5mm and an inner diameter of 1mm.
[0062] Example 3
[0063] like Figure 7 The diagram shown is a simulation of the coil signal in the electromagnetic marine coating corrosion defect detection system of this invention. When using a single coil, the initial value of both coil 1 and coil 2 in the detection unit is not 0, resulting in a small relative change in the signal. Therefore, we use a three-coil structure with the two pickup coils wound in opposite ways, causing the signals generated by the coils to cancel each other out, and the initial value of the superimposed signal is 0. Because the order of the signals is different, the time of change of the superimposed signal is different, which in turn causes the superimposed signal to change, increasing its relative change value.
[0064] Example 4
[0065] like Figure 8 , 9 The diagram shows the detection of different types of defects by the electromagnetic marine coating under-coating corrosion defect detection system of this invention. This invention tested pitting corrosion and weld corrosion of different widths and depths. To ensure the stability of the sample properties, 45 steel specimens of the same specifications, with a length of 100mm, a width of 100mm, and a thickness of 4mm, were selected for surface processing. The defect types, thicknesses, widths, and numbers are shown in the table below.
[0066]
[0067] Example 5
[0068] like Figure 10 As shown, the detection limit of the electromagnetic marine coating under-coating corrosion defect detection system of this utility model was tested. Because CNC machine tools have difficulty processing defects with a width of less than 1 mm, pitting defects were processed manually. For ease of processing, 1 mm thick 45 steel was used. The processed steel plate is shown in the figure. Figure 3 As shown in Figure 21, a cylindrical pitting defect with a diameter of 0.5 mm and a depth of 0.5 mm was machined on a 45 steel plate. The experiment was conducted with an excitation frequency of 1.2 MHz, under the control of a three-axis precision micro-motion platform, and the distance between the sensor probe and the defect was maintained at 1 mm. The detection results show that the sensor's detection characteristic at the defect is a peak followed by a trough, consistent with the principle of ferromagnetic metal plate defect detection in Chapter 2. Furthermore, at locations not close to the defect, there is a drift in the detection signal amplitude. This is because the pitting defect on the specimen was artificially created, and the specimen is relatively thin, which may have caused a certain degree of deformation during processing, resulting in the original magnetic permeability of the specimen becoming similar to that of air, thus reducing the sensor's impedance value.
[0069] Example 6
[0070] like Figure 11 The image shows the detection signal results of the electromagnetic marine coating corrosion defect detection system of this invention at vertical distances ranging from 2mm to 5mm. Due to the small radiation range and rapid attenuation of the coil's electromagnetic field, the vertical distance between the sensor and the specimen has a significant impact on the detection results of the electromagnetic non-destructive testing sensor (lift-off effect). To truly verify the lift-off effect of the designed sensor, a defect detection lower limit specimen was used for the extreme lift-off distance experiment. Using a three-axis precision micro-motion platform, the vertical distance between the sensor and the specimen was increased from 2mm to 10mm. The experiment revealed that when the vertical distance exceeded 5mm, the signal value could not be distinguished from the noise as a defect at the detection lower limit.
[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this utility model, and are not intended to limit it. Although the utility model has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this utility model.
Claims
1. An electromagnetic marine coating corrosion defect detection system, characterized in that, include: An AC signal generator (1), a three-coil differential metal layer thickness sensor (2), a signal processor (7), and a data acquisition unit are included. The AC signal generator (1) and the three-coil differential metal layer thickness sensor (2) are both connected to the signal processor (7), and the signal processor (7) is connected to the data acquisition unit. The three-coil differential metal layer thickness sensor (2) includes a filter capacitor (3), an excitation coil (4), a detection coil (5), and a PLA 3D printed probe housing (6), wherein: The filter capacitor (3), excitation coil (4) and detection coil (5) are fixed inside the PLA3D printed probe housing (6). The detection coil (5) includes two planar coils connected in series. The excitation coil (4) is closely attached to the bottom of the two planar coils and is used for electromagnetic coupling with the detection coil (5) to obtain information from the three-coil differential metal layer thickness sensor and transmit energy to the three-coil differential metal layer thickness sensor. The filter capacitor (3) is connected in parallel with the two planar coils connected in series. The signal processor (7) includes a lock-in amplifier (8) and an operational amplifier (9), wherein: The lock-in amplifier (8) has two signal channels, which are connected to the signal generator (1) and the detection coil (5). It receives the signal generated by the signal generator (1) and the output signal of the detection coil (5) respectively. At the same time, it compares the phase and amplitude differences of the two input signals and extracts the response signal related to the excitation signal. The operational amplifier (9) is connected to the lock-in amplifier (8), receives the stable DC signal from the lock-in amplifier (8), and amplifies the stable DC signal a second time. The data acquisition device includes a data acquisition card (10) and a LabVIEW data acquisition unit (11). The data acquisition card (10) is connected to an operational amplifier (9) to acquire the signal after secondary amplification, and then inputs the acquired signal after secondary amplification to the LabVIEW data acquisition unit (11).
2. The electromagnetic marine coating under-coating corrosion defect detection system according to claim 1, characterized in that, The excitation coil (4) is made of enameled copper wire with a diameter of 70μm. The coil has a single layer of 79 turns, an inner diameter of 1mm, and an outer diameter of 15mm.
3. The electromagnetic marine coating under-coating corrosion defect detection system according to claim 1, characterized in that, The two planar coils of the detection coil (5) are both made of enameled copper wire with a diameter of 70μm. The coils are single-layered with 35 turns, with an inner diameter of 1mm and an outer diameter of 5mm.
4. The electromagnetic marine coating under-coating corrosion defect detection system according to claim 1, characterized in that, Both ends of the excitation coil (4) and the detection coil (5) are connected to leads, and there is no electrical connection between the excitation coil (4) and the detection coil (5). The leads are all located on one side of the three-coil differential metal layer thickness sensor (2).