Single-excitation double-output multi-parameter measurement eddy current sensor

Abstract

The invention discloses a single-excitation dual-output multi-parameter measurement eddy current sensor, which is characterized in that after the eddy current sensor is arranged on the surface of a tested piece, a sine excitation signal with certain frequency and power is introduced into an excitation coil, and a magnetic field generated by the excitation coil generates eddy current on the surface of the tested piece; in the actual detection process, the lift-off value of the eddy current sensor and the surface of the tested piece is changed due to mechanical vibration or random working conditions, impedance in the excitation coil is changed, and then the current value of a loop where the excitation coil and the capacitor are located is changed. The impedance change caused by lift-off can be represented by acquiring the voltage values at the two ends of the capacitor; the differential receiving coil receives a combined magnetic field of a primary magnetic field and an eddy current field under the combined action of the primary magnetic field and the eddy current field, and has a lift-off suppression effect to a certain extent, and the balance of the receiving coil is broken only when a defect is encountered, so that the defect is detected and represented by measuring a voltage signal of the receiving coil.

Description

Technical Field

[0001] The invention belongs to the technical field of eddy current nondestructive testing, and more specifically, relates to a multi-parameter measurement eddy current sensor with single excitation and dual output. Background Art

[0002] Eddy current testing technology is one of the most important nondestructive testing methods. Its working principle is to generate alternating eddy currents and secondary magnetic fields in the metal parts by applying alternating current to the excitation coil, and to use the electromagnetic induction effect of the alternating magnetic field generated around it and the metal parts to be tested. By capturing the superposition field of the primary magnetic field and the secondary magnetic field in the detection unit, the information related to the integrity of the metal parts to be tested is reflected, and the integrity of the metal parts is evaluated. Eddy current testing has the characteristics of non-contact detection with the working surface, and is sensitive to multiple parameters such as conductivity and magnetic permeability. It is an important technology for the safety and maintenance of energy production, transportation or other infrastructure. Compared with ultrasonic testing technology and electromagnetic ultrasonic testing technology, it does not require the use of coupling agents and is not limited by conversion efficiency; compared with magnetic flux leakage testing technology, it does not require a strong magnetization device. However, a major challenge facing eddy current testing is that it is extremely susceptible to various noise interferences, such as lift-off, vibration or inclusions. At this time, the information related to the defect is masked in these noises, resulting in reduced sensitivity or recognition accuracy of defect detection. Therefore, it is crucial to achieve lift-off measurement and defect separation.

[0003] At present, the research related to lift-off and defect detection includes the following aspects: response characteristic research, theoretical analysis modeling, probe design and optimization, frequency characteristics and signal processing methods, etc. Among them, the structural design and system implementation of the sensor are important ways to improve the detection performance, because their optimization is directly based on the principles of physical fields and affects the quality of the signal. The shape of the coil, the configuration of the probe and the operation mode are strongly correlated. For example, rectangular / square coils have better suppression of the lift-off effect. Probe configuration refers to the arrangement and packaging of one or more coils in different ways to achieve the best "coupling" with the detection area of ​​interest. The operation mode refers to the wiring method of the coil, which is generally divided into: absolute, differential, reflection and hybrid. An absolute probe refers to a single coil that generates eddy currents and is used to sense changes in the eddy current field. This common mode type is very sensitive to common mode factors (such as lift-off and temperature), so it can be used for displacement measurement. A differential probe refers to the use of two coils with opposite winding methods. When this coil is located in the defect area, the differential output signal can directly reflect the difference between the defect information and the background information, thereby increasing the response to the defect area. The reflection probe is also composed of two coils, one of which is used as a transmitter to generate eddy currents, and the other is used as a receiver to sense changes in magnetic field signals. A hybrid configuration probe refers to the use of any combination of the above designs to achieve lift-off suppression or defect detection. For example, a rectangular transverse pickup coil is placed in the center of a circular excitation coil. At this time, the output signal only reflects the disturbing eddy currents along the pickup coil, and the axisymmetric lift-off noise will not affect the pickup coil. Orthogonal axial eddy current probes can effectively solve welding defects on uneven surfaces on carbon steel plates. The double-layer differential probe design can generate a symmetrical background magnetic field, which can achieve defect detection under a higher lift-off state.

[0004] Although there are many ways to suppress lift-off noise, it is still difficult to completely eliminate this interference factor in practical applications. How to achieve multi-parameter measurement in a single coil for lift-off suppression or defect separation is a future research trend. Existing work uses the relative change of magnetic flux to obtain lift-off information in pulsed eddy current according to the relative change of the detection signal and the reference signal. In addition, the three-coil sensor is used as two coil pairs to work in multi-frequency mode. By analyzing the difference in peak frequency, the lift-off effect can be reduced. The non-coaxial transmitting-receiving pulsed eddy current sensor has the conductive intersection feature to invert the lift-off and the thickness of the specimen. In pulsed eddy current testing, the distributed capacitance change is used as the lift-off function, and the lift-off is determined by measuring the leakage current, and defect detection can be achieved at the same time. If the coil is set to a configuration of one transmitting end and two different receiver coils, the larger coil is used for lift-off measurement, and the smaller coil can be used for defect detection. In summary, establishing multi-parameter measurement of the probe is crucial to achieving a higher signal-to-noise ratio and accurate detection results. However, current research considers the structure of the probe, and does not analyze the relationship between the probe structure and the conditioning circuit, as well as the relationship between the conditioning circuit and the characteristic parameters. Because physical information can be converted into circuit parameters, different signal conditioning circuits can measure different physical parameters, so conditioning circuits are also important in parameter measurement of nondestructive testing and evaluation technology. It is crucial to use the combination of conditioning circuits under a specific coil structure to achieve multi-feature measurement and separation of lift-off and defects. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a single-excitation dual-output multi-parameter measurement eddy current sensor, which utilizes two coaxial hollow rectangular coils located on different layers of a PCB and a differential receiving coil structure placed in the hollow excitation coil, and adopts an LC resonant conditioning circuit formed by the excitation coil and the differential coil to realize simultaneous measurement of lift-off and defects; a lift-off function is constructed by extracting the voltage value on the resonant capacitor to realize the estimation of lift-off, and the separated lift-off information is used to assist in the identification of defects, thereby improving the identification of defects under the influence of lift-off and increasing the reliability of the quantification process.

[0006] To achieve the above-mentioned purpose of the invention, the present invention provides a single excitation dual-output multi-parameter measurement eddy current sensor, characterized in that it comprises: an excitation coil, a receiving coil and a capacitor;

[0007] The excitation coil is wound in series in two layers in the same direction. The two layers of coils have the same size and parameters, but are hollowed out in the center and placed coaxially.

[0008] The receiving coil is wound in series with two coils in opposite directions. The two coils are equal in size and have the same parameters and are arranged in the same plane. The receiving coil is placed in the hollow position of the excitation coil, and its two ends are connected to a differential amplifier for measuring voltage signals.

[0009] The capacitor is connected in series with the excitation coil and then connected to a sinusoidal excitation source, and measurement endpoints are drawn from both ends of the capacitor to collect the voltage across the capacitor;

[0010] The object of the invention of the present invention is achieved in this way:

[0011] The present invention discloses a multi-parameter measurement eddy current sensor with single excitation and dual output, comprising: an excitation coil, a receiving coil and a capacitor; the eddy current sensor is placed on the surface of the test piece, and then a sinusoidal excitation signal of a certain frequency and power is introduced into the excitation coil. The magnetic field generated by the excitation coil generates eddy currents on the surface of the test piece. In the actual detection process, such as pipeline and rail detection, the relative distance (lift-off) between the probe and the surface of the test piece changes due to mechanical vibration or random working conditions, which causes the change of impedance in the excitation coil, thereby affecting the current value of the loop where the excitation coil and the capacitor are located. Since the capacitance value is a fixed value, the impedance change caused by the lift-off can be characterized by obtaining the voltage value at both ends of the capacitor. Under the joint action of the primary magnetic field and the eddy current field, the differential receiving coil receives the combined magnetic field of the two, and has the effect of suppressing lift-off to a certain extent. Only when a defect is encountered, the balance of the differential coil is broken, so the voltage signal of the differential coil that measures the change can be used to detect and characterize the defect.

[0012] At the same time, the single-excitation dual-output multi-parameter measurement eddy current sensor of the present invention also has the following beneficial effects:

[0013] (1) The present invention adopts a novel coil structure, an LC resonance conditioning circuit and a differential transformer conditioning circuit, and realizes simultaneous measurement of multiple signals of a single coil only by connecting lines without introducing extra volume and space.

[0014] (2) The present invention uses an LC resonant conditioning circuit to achieve lift-off measurement and a differential transformer to achieve defect measurement. The combination of the two and the combination of the coil structure and the conditioning circuit simultaneously achieves the separation of lift-off and defect.

[0015] (3) The present invention integrates the eddy current sensor on the same PCB board and uses a multi-layer PCB to perform different routing methods to achieve high integration.

[0016] (4) The present invention utilizes the circuit characteristics of inductance and capacitance without introducing additional complex components to achieve the output and collection of two-way signals and features in one sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a single-excitation dual-output multi-parameter measurement eddy current sensor of the present invention;

[0018] Figure 2 Schematic diagram of the eddy current probe based on LC resonance and transformer conditioning circuit proposed by the present invention

[0019] Figure 3 The schematic diagram of the connection between the structure of the excitation coil and the LC resonance conditioning circuit in the present invention is

[0020] Figure 4 It is a schematic diagram of the connection between the structure of the receiving coil and the equivalent transformer in the present invention;

[0021] Figure 5 is the signal output of the excitation coil and the LC resonance conditioning circuit in this embodiment when measuring lift-off;

[0022] Figure 6 is the response output of the receiving coil and the differential transformer to the defect in this example;

[0023] Figure 7 It is the signal outputted simultaneously by the eddy current sensor in this embodiment when detecting a composite defect;

[0024] Figure 8 is the inversion result of the eddy current sensor for lift-off in this embodiment;

[0025] Fig. 9 This is the defect characterization result of the eddy current sensor in this embodiment. DETAILED DESCRIPTION

[0026] The specific implementation of the present invention is described below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0027] Example

[0028] Figure 1 It is a schematic diagram of a single-excitation dual-output multi-parameter measurement eddy current sensor of the present invention.

[0029] In this embodiment, if Figure 1As shown in the figure, the actual sensor is printed on the PCB board. The entire sensor consists of 4 layers of PCB. The excitation coil is a hollow rectangular planar spiral coil located on the first and fourth layers of the PCB. The first and fourth layers are connected by vias and are wound in the same direction. The detection coil is a pair of differential rectangular planar spiral coils wound in opposite directions. It is located on the 234th layer of the PCB. Similar to the excitation coil, each layer is connected by vias, and each layer has the same winding direction.

[0030] The PCB coil has good consistency, thus avoiding the error caused by manual winding. In addition, the wiring can be arbitrarily arranged in the PCB, thus reducing the connection of the leads and making it easier to realize the combination of the two conditioning circuits.

[0031] The following is a detailed description of a single excitation dual-output multi-parameter measurement eddy current sensor according to the present invention, which specifically includes: an excitation coil, a receiving coil and a capacitor;

[0032] The excitation coil is wound in series with two layers in the same direction. The two layers of coils have the same size and parameters, but the center is hollowed out and placed coaxially.

[0033] The receiving coil is wound in series with two coils in opposite directions. The two coils are equal in size and have the same parameters and are arranged in the same plane. The receiving coil is placed in the hollow position of the excitation coil, and its two ends are connected to a differential amplifier for measuring voltage signals.

[0034] The capacitor and the excitation coil are connected in series and then connected to a sinusoidal excitation source. The two ends of the capacitor are connected to measurement endpoints to collect the voltage across the capacitor.

[0035] The eddy current sensor is placed on the surface of the test piece, and then a sinusoidal excitation signal of a certain frequency and power is passed into the excitation coil. The magnetic field generated by the excitation coil generates eddy currents on the surface of the test piece. In the actual detection process, such as pipeline and rail inspection, the relative distance (lift-off) between the probe and the surface of the test piece changes due to mechanical vibration or random working conditions, which causes the impedance in the excitation coil to change, thereby affecting the current value of the loop where the excitation coil and the capacitor are located. Since the capacitance value is a fixed value, the impedance change caused by lift-off can be characterized by obtaining the voltage value across the capacitor. Under the joint action of the primary magnetic field and the eddy current field, the differential receiving coil receives the combined magnetic field of the two, and has the effect of suppressing lift-off to a certain extent. Only when a defect is encountered, the balance of the differential coil is broken, so the voltage signal of the differential coil that measures the change can be used to detect and characterize the defect.

[0036] Based on the above principle, we connect the excitation coil and capacitor in series with the excitation source to form a conditioning circuit. By selecting a suitable capacitance value, the capacitor and the excitation coil are resonated. We can measure the voltage value U on the capacitor.o1 , the inductance change at the LC resonant end can be obtained. The differential receiving coil itself is not directly connected, and their coupling depends on the mutual inductance between the excitation coil, forming a differential transformer mode, and the output is U o2 Therefore, the design includes two sets of conditioning circuits and two output signals.

[0037] Figure 2 This is a schematic diagram of an eddy current sensor based on an LC resonance conditioning circuit and a transformer conditioning circuit.

[0038] In this embodiment, the L of the excitation coil 1 It is connected in series with a capacitor with a capacitance of C and then connected to a sinusoidal excitation source. The voltage obtained on the capacitor is U o1 At the same time, L 1 constitutes the primary side of the transformer, and L 2 and L 3 Therefore, we can get the secondary side of the transformer from L 2 and L 3 The differential receiving coil composed of o2 .

[0039] In order to analyze the mapping relationship between the signal characteristics and the lift-off of the proposed probe under the change of lift-off, we established the system output response based on the equivalent circuit model. For the excitation coil and LC resonant conditioning circuit, it can be equivalent to the inductor L 1 and resistor R 1 The series connection of the resonant capacitor is C. Under the excitation voltage U and the resonant frequency f, the current flowing through the coil is i 1 When the excitation coil and the test piece interact to generate eddy current, the test piece can also be equivalent to the inductor L t and resistor R t Assume that the eddy current generated on the test piece is i e， The mutual inductance between the specimen and the excitation coil is M 1t . According to Kirchhoff's law, the following equation can be obtained:

[0040]

[0041] Where i is the excitation current and ω is the angular frequency;

[0042] The resonant frequency can be obtained by the formula:

[0043]

[0044] Output voltage U o1 for:

[0045]

[0046] The mutual inductance M between the coil and the sample under test 1t The coupling coefficient k can be used 1t It means that since the geometric parameters of the coil are unchanged, the coupling coefficient k 1t It depends only on the lift-off x, that is:

[0047]

[0048] When the lift-off changes, the change in the strength of the eddy current causes a change in impedance, which in turn leads to a change in current. Therefore, the measured voltage change can reflect the lift-off situation.

[0049]

[0050] Among them, F(x) is a function related to lift-off.

[0051] For the receiving coil, the differential rectangular coil L 2 and L 3 Located in the hollow center of the excitation coil, the coils are wound in opposite directions, thus reducing the impact of background noise such as lift-off on the detection. Under the action of mutual inductance, the excitation coil and the receiving coil form a differential transformer mode. Assume that the total inductance and internal resistance of the differential receiver are L d and R d According to Lenz's law, the direction of the secondary magnetic field generated by eddy current is opposite to that of the primary magnetic field. The presence of defects will interfere with the eddy current, resulting in changes in the magnetic flux density and induced electromotive force in the differential transformer. Therefore, the time-varying primary magnetic flux is directly or indirectly coupled to the receiving coil through the test piece. Assume that the current flowing into the transmitter and receiver is i 1 and i 2 . R L is the load resistance. Based on the transformer circuit model, the following equation can be derived:

[0052]

[0053] Among them, E 1 、E 2 is the induced electromotive force between the exciting coil and the receiving coil, M c0 is the direct mutual inductance between the excitation coil and the receiving coil, M c1 is the indirect mutual inductance between the excitation coil and the receiving coil through the test piece, s 1 and 2 It can be regarded as the self-inductance change of the excitation coil and the receiving coil through the test piece, Δr c and Δr d is the change in internal resistance in the test piece. Therefore, the voltage U o2 for:

[0054] U02 =jω(L d +M c1 +s 2 )i 3 +(R d +Δr d )i 3 -M c0 i 1 (7)

[0055] If a defect is present, the coupling between the test piece and the probe will change, causing the impedance on the receiving coil to change as well. c1 The receiving coils can therefore be used for defect measurement.

[0056] Figure 3 It is a schematic diagram of the connection between the structure of the excitation coil and the LC resonance conditioning circuit in the present invention;

[0057] In this embodiment, the excitation coil is a hollow rectangular planar spiral coil, which is wound by a copper wire with a wire diameter of 50 mil. The size of the upper and lower coils is 60 mm x 40 mm, and the turn spacing is 25 mil. The excitation coil is connected in series with a capacitor, and the resonant frequency is calculated by measuring the inductance of the excitation coil and selecting a suitable capacitance value. Therefore, the entire circuit constitutes an LC resonant conditioning circuit mode, one end of the excitation coil is connected to the sinusoidal excitation source, and the other end is connected to the capacitor, and the other end of the capacitor is grounded. The measurement endpoints are brought out from both ends of the capacitor, and the voltage U at both ends of the capacitor is collected using an acquisition card. o1 .

[0058] Figure 4 It is a schematic diagram of the connection between the structure of the receiving coil and the equivalent transformer in the present invention;

[0059] The receiving coil is a pair of differential rectangular planar spiral coils, wound with copper wire of 3.5 mil in diameter. The size of the left and right coils is 17x26 mm, and the turn spacing is 8 mil. The receiving coil is regarded as a whole, placed in the hollowed-out position of the excitation coil, and the two ends of the receiving coil are connected to the differential amplifier to increase the detection capability of defects. Therefore, the entire excitation coil is the primary side of the transformer, and the differential receiving coil is regarded as a whole as the secondary side of the transformer. The two form a differential transformer conditioning circuit through mutual inductance.

[0060] Figure 5 is the signal output of the excitation coil and the LC resonance conditioning circuit in this embodiment when measuring lift-off;

[0061] In this embodiment, if Figure 5As shown in the figure, the eddy current sensor is fixed on a workbench marked with XYZ axes, and the eddy current sensor is slowly moved along the Z direction to collect the voltage on the LC resonant circuit. For the output of the LC resonant conditioning circuit, as the lift-off increases, the amplitude of the output signal decreases in inverse proportion until it stabilizes. This change trend is consistent with formula (5), indicating that a relationship with the lift-off can be established in a limited range to measure the lift-off.

[0062] Figure 6 is the response output of the receiving coil and the differential transformer to the defect in this example;

[0063] In this embodiment, a defect is scanned under different lift-off states to obtain a voltage signal output by a differential transformer. Figure 6 It can be seen that the voltage signal output by the differential transformer clearly reflects the abnormal signal of the defect, and the baseline value does not change significantly with the increase of lift-off, which also shows that the receiving coil has a certain lift-off suppression ability, and its change can be used as a basis for defect measurement.

[0064] Figure 7 is the signal output simultaneously when the sensor in this embodiment detects a composite defect;

[0065] In this embodiment, two artificially etched metal loss defects are scanned by an eddy current sensor, and two output signals are collected simultaneously, namely, the voltage signal at both ends of the capacitor and the voltage signal output by the differential transformer; Figure 7 As shown in the figure, comparing the two, the output signal of the LC resonance has only two large concave areas, and the area is the edge of the defect, and the depth of the metal loss is considered as a lift-off change. At the same position, the differential transformer also has an abnormal signal, indicating that this is a defect. By combining the two, the possibility of abnormal signals occurring at the location can be clearly evaluated, which in turn reflects the number of defects.

[0066] Figure 8 is the inversion result of the lift-off of the sensor in this embodiment;

[0067] In this embodiment, the value of the output signal of the LC resonance conditioning circuit is used to establish a function related to the lift-off, and the specific method is to fit it to obtain a fitting curve, such as Figure 8 As shown, different lift-off values ​​and voltage values ​​correspond one to one. By inversely solving the function, the lift-off value corresponding to the amplitude can be calculated, thereby realizing the calculation and evaluation of the lift-off.

[0068] Fig. 9 is the defect characterization result of the sensor in this embodiment;

[0069] In this embodiment, the output signal of the differential transformer is analyzed, and after the differential transformer is connected to the differential amplifier, the signal at both ends of the differential transformer is obtained, and the real and imaginary signals of the signal are solved to construct a two-dimensional Lissajous curve for characterizing the defect. Fig. 9 It can be seen that the two-dimensional Lissajous curve presents the typical shape of a differential coil, namely the figure of "8", indicating that there is a defect here. In addition, the defects of different sizes can also be effectively reflected from this figure, which can be used to distinguish the relative sizes of the defects. Therefore, the output signal and Figure 8 The signal in the sample is used to realize the separation of lift-off and defects.

[0070] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A single excitation dual output multi-parameter measurement eddy current sensor, characterized in that: include: Excitation coil, receiving coil and capacitor; The excitation coil is wound in series in two layers in the same direction. The two layers of coils have the same size and parameters, but are hollowed out in the center and placed coaxially. The receiving coil is wound in series with two coils in opposite directions. The two coils are equal in size and have the same parameters and are arranged in the same plane. The receiving coil is placed in the hollow position of the excitation coil, and its two ends are connected to a differential amplifier for measuring voltage signals. The capacitor is connected in series with the excitation coil and then connected to a sinusoidal excitation source, and measurement endpoints are drawn from both ends of the capacitor to collect the voltage across the capacitor; When the eddy current sensor is used for defect detection, the eddy current sensor is placed on the surface of the test piece, and a sinusoidal excitation signal of a certain frequency and power is passed through the excitation coil. The magnetic field generated by the excitation coil generates eddy currents on the surface of the test piece, and the impedance change caused by lift-off is characterized by obtaining the voltage value across the capacitor. The defect is detected and characterized by measuring the voltage signal across the receiving coil.

2. The single excitation dual output multi-parameter measurement eddy current sensor according to claim 1, characterized in that: The capacitance of the capacitor satisfies that the capacitor resonates with the excitation coil.

3. The single excitation dual output multi-parameter measurement eddy current sensor according to claim 1, characterized in that: The excitation coil is a hollow rectangular planar spiral coil, which is wound with a copper wire with a wire diameter of 50 mil. The size of the upper and lower coils is 60 mm x 40 mm, and the turn spacing is 25 mil.

4. The single excitation dual output multi-parameter measurement eddy current sensor according to claim 1, characterized in that: The receiving coil is a pair of differential rectangular planar spiral coils, which are wound with copper wires with a wire diameter of 3.5 mils. The size of the left and right coils is 17x26 mm, and the turn spacing is 8 mils.

5. The single excitation dual output multi-parameter measurement eddy current sensor according to claim 1, characterized in that: The eddy current sensor is printed on the same PCB board, and the entire eddy current sensor is distributed on a 4-layer PCB, wherein the excitation coil is located on the first and fourth layers of the PCB, the first layer and the fourth layer are connected through vias, and are wound in the same direction; the receiving coil is located on the second, third, and fourth layers of the PCB, each layer is connected through vias, and the winding direction of each layer is the same.

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