A flexible electromagnetic ultrasonic elbow wall thickness measurement method and device

Abstract

This invention discloses a flexible electromagnetic ultrasonic elbow wall thickness measurement method and device, belonging to the field of ultrasonic non-destructive testing technology. The device includes a flexible probe array module, a signal transceiver module, and a processing module. The flexible probe array module comprises a flexible substrate, several electromagnetic ultrasonic transducer units, and a permanent magnet array, which can be bent to fit the surface of elbows of different diameters. The permanent magnet array provides a bias magnetic field for the transducers and fixes the probe array to the carbon steel elbow surface through magnetic attraction. During measurement, a time-division multiplexing method is used to sequentially select each transducer unit, collect the echo signal, calculate the original wall thickness value, and then perform temperature compensation correction, stress compensation correction, lift-off phase compensation correction, and surface geometry correction. The four correction factors are multiplied together to obtain the true wall thickness value. This invention does not require a coupling agent and can complete multi-point wall thickness measurement of the elbow back arc area in one operation, applicable to the inspection of carbon steel and stainless steel elbows.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic nondestructive testing technology, specifically to a method and apparatus for measuring the wall thickness of a flexible electromagnetic ultrasonic elbow. Background Technology

[0002] In industries such as petrochemicals, power, and shipbuilding, metal pipe elbows are critical components prone to corrosion and thinning. Due to fluid erosion and media corrosion, the wall thickness of the elbow's back arc region gradually decreases, potentially leading to pipe rupture and leakage, causing safety accidents. Therefore, regular wall thickness testing of elbows is an important measure to ensure industrial safety. Traditional methods for measuring elbow wall thickness mainly employ piezoelectric ultrasonic thickness measurement technology. This technology has the following drawbacks: the piezoelectric ultrasonic probe must achieve acoustic impedance matching with the workpiece surface through a coupling agent to transmit and receive ultrasonic waves; single-point measurement efficiency is low, as traditional methods require operators to hold the probe and measure point by point on the elbow surface; rigid probes are not suitable for curved surfaces, as the contact area between the rigid planar probe and the curved surface is small, resulting in unstable coupling; when working on vertical pipelines or at heights, operators need to hold the probe with one hand and operate the instrument with the other, which is inconvenient and poses safety hazards.

[0003] Electromagnetic ultrasonic (EMAT) technology is a non-contact ultrasonic testing technique that requires no coupling agent. It directly excites and receives ultrasonic waves on a metal surface through electromagnetic induction. While EMAT technology has been applied to the inspection of regularly shaped workpieces such as flat plates and straight pipes, the following technical challenges remain in its application to curved workpieces such as elbows: Existing EMAT probes are mostly rigid structures, unable to fit tightly against curved surfaces, resulting in uneven bias magnetic fields and low excitation efficiency; the curvature of elbows causes skew in the sound beam propagation path, which, if not corrected, introduces measurement errors; multi-point measurements require multiple independent probes, leading to large equipment size and high cost; furthermore, most correction schemes in existing wall thickness measurement technologies only consider a single factor, such as linear compensation for temperature or simple geometric cosine correction, without comprehensively considering the coupling effects of temperature field, residual stress field, electromagnetic lift-off effect, and surface geometry, resulting in limited correction accuracy.

[0004] Therefore, there is an urgent need for a elbow wall thickness detection technology that can adapt to the curved surface of elbows, requires no coupling agent, allows for multi-point measurement at one time, and has comprehensive correction capabilities. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a method and device for measuring the wall thickness of a flexible electromagnetic ultrasonic elbow.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for measuring the wall thickness of a flexible electromagnetic ultrasonic elbow includes the following steps:

[0008] Step S1: Attach the flexible probe array to the outer surface of the pipe bend, wherein the flexible probe array includes several electromagnetic ultrasonic transducer units arranged along the back arc direction of the bend.

[0009] Step S2: Using a time-division multiplexing method, select each electromagnetic ultrasonic transducer unit in sequence and send an excitation pulse to the currently selected electromagnetic ultrasonic transducer unit to excite longitudinal waves.

[0010] Step S3: Receive the bottom echo signal collected by each electromagnetic ultrasonic transducer unit, and calculate the original wall thickness value corresponding to each measurement point based on the longitudinal wave round-trip time.

[0011] Step S4: Perform temperature compensation correction, stress compensation correction, lift-off phase compensation correction, and surface geometry correction on the original wall thickness value to obtain the true wall thickness value and output it.

[0012] Furthermore, in step S3, the echo arrival time is picked up using the cross-correlation method, and 32 to 128 measurements are performed on the same measurement point and the average is taken to improve the signal-to-noise ratio.

[0013] Further, in step S4, the temperature compensation correction uses a temperature compensation factor to correct the original wall thickness. The temperature compensation factor is equal to 1 plus the product of the sound velocity temperature coefficient and the equivalent temperature deviation. The equivalent temperature deviation is the value obtained by adding the equivalent temperature difference coefficient to the outer wall temperature, multiplying it by the difference between the outer wall temperature and the ambient air temperature, and then subtracting the calibration temperature.

[0014] Further, in step S4, the stress compensation correction uses a stress compensation factor to correct the original wall thickness. The stress compensation factor is equal to 1 minus the product of the stress compensation coefficient multiplied by the wall thickness correction factor and then multiplied by the cosine of the central angle of the measurement point. The wall thickness correction factor is equal to 1 plus the wall thickness correction coefficient multiplied by the ratio of the original wall thickness to the nominal radius of curvature of the elbow. The central angle of the measurement point is calculated by the arcsine function of the ratio of the arc distance from the measurement point to the apex of the back arc to the nominal radius of curvature of the elbow.

[0015] Further, in step S4, the lift-off phase compensation correction uses a lift-off phase compensation factor to correct the original wall thickness. The lift-off phase compensation factor is equal to 1 plus the lift-off phase comprehensive coefficient divided by the original wall thickness and then multiplied by the natural logarithm of the ratio of the echo signal main peak amplitude to the reference amplitude under standard bonding state.

[0016] Furthermore, in step S4, the surface geometry correction includes the following steps:

[0017] Calculate the radius of curvature of the inner wall of the elbow. The radius of curvature of the inner wall of the elbow is equal to the nominal radius of curvature of the elbow minus the original wall thickness.

[0018] Calculate the arc-directional drift of the sound beam at the reflection point on the inner wall relative to the incident point on the outer wall. The arc-directional drift is equal to the arcsine function value of the inner wall radius of curvature multiplied by the arc distance from the measurement point to the apex of the back arc and the inner wall radius of curvature, minus the arcsine function value of the outer wall radius of curvature multiplied by the arc distance from the measurement point to the apex of the back arc and the outer wall radius of curvature.

[0019] Calculate the surface geometry correction factor, which is equal to the ratio of the inner wall radius of curvature to the outer wall radius of curvature, multiplied by the square root of the square of the ratio of the equivalent arc distance of the outer wall to the outer wall radius of curvature, where the equivalent arc distance of the outer wall is equal to the arc distance from the measurement point to the vertex of the back arc minus the arc drift.

[0020] Further, in step S4, the true wall thickness value is obtained by multiplying the original wall thickness by the temperature compensation factor, stress compensation factor, lift-off phase compensation factor, and surface geometry correction factor. That is, the true wall thickness value is equal to the original wall thickness multiplied by the temperature compensation factor, the stress compensation factor, the lift-off phase compensation factor, and the surface geometry correction factor.

[0021] A flexible electromagnetic ultrasonic elbow wall thickness measuring device, used to implement any one of the flexible electromagnetic ultrasonic elbow wall thickness measuring methods, comprising:

[0022] A flexible probe array module is used to fit the outer surface of a pipe bend. The flexible probe array module includes a flexible substrate, a plurality of electromagnetic ultrasonic transducer units disposed on the flexible substrate, and a permanent magnet array corresponding to the plurality of electromagnetic ultrasonic transducer units. The flexible substrate can be bent to adapt to the curved surface of the bend.

[0023] The signal transceiver module is connected to the flexible probe array module and is used to select each electromagnetic ultrasonic transducer unit in a time-division manner, transmit excitation pulses to the selected electromagnetic ultrasonic transducer unit and receive echo signals.

[0024] The processing module, connected to the signal transceiver module, is used to calculate the original wall thickness value corresponding to each measurement point based on the echo signal, and to perform temperature compensation correction, stress compensation correction, lift-off phase compensation correction and surface geometry correction to obtain the true wall thickness value.

[0025] The permanent magnet array is used both to provide a bias magnetic field for the electromagnetic ultrasonic transducer unit and to fix the flexible probe array module to the surface of the carbon steel elbow by magnetic attraction.

[0026] Furthermore, the flexible substrate is made of polyimide or polyethylene naphthalate film with a thickness of 0.1 to 0.2 mm; the electromagnetic ultrasonic transducer unit is made of a spiral planar coil using flexible printed circuit board technology with a coil layer thickness of 0.05 to 0.1 mm; each permanent magnet in the permanent magnet array corresponds to one electromagnetic ultrasonic transducer unit, the permanent magnet material is neodymium iron boron, and the surface magnetic field strength is 0.5 to 1.0 Tesla.

[0027] Furthermore, the flexible probe array module consists of a protective layer, a permanent magnet array layer, a coil layer, a flexible substrate, and a backing layer, arranged sequentially from the side closest to the surface being measured to the outside, with a total thickness of 1.5 to 2.5 mm, which can accommodate elbows with pipe diameters from DN50 to DN300; several electromagnetic ultrasonic transducer units are arranged linearly along the back arc direction of the elbow, with a center distance of 15 to 25 mm between adjacent units.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. This invention does not require a coupling agent. It uses electromagnetic ultrasonic technology to directly excite and receive ultrasonic waves on the metal surface through electromagnetic induction, eliminating the step of applying a coupling agent. This makes the operation simpler and avoids measurement errors caused by inconsistent coupling agent coating thickness.

[0030] 2. This invention employs four independent corrections to improve measurement accuracy. The original measured values ​​are corrected from four physical dimensions: thermal, elasticity, electromagnetic, and geometric optics. In temperature compensation, an equivalent temperature difference coefficient is introduced to correct the single surface temperature to the average temperature along the wall thickness to compensate for the temperature difference between the inner and outer walls. In stress compensation, a wall thickness correction term is introduced to reflect the influence of the neutral layer offset of the thick-walled elbow on the residual strain distribution. In phase compensation, the echo amplitude attenuation is quantized into acoustic time error for deduction. In surface geometry correction, the thin-wall assumption is broken to strictly distinguish the curvature radii of the inner and outer walls and the sound beam drift is introduced for accurate correction. Each correction factor is calculated independently based on real-time measurement data and is uncoupled, avoiding the parameter calibration difficulties caused by complex coupled models and improving measurement accuracy.

[0031] 3. This invention can achieve simultaneous measurement at multiple points. The probe array contains several electromagnetic ultrasonic transducer units arranged linearly along the back arc direction of the bend, covering the back arc area of ​​the bend in one go, and the measurement time is shortened to less than 1 minute.

[0032] 4. The permanent magnet in this invention has a dual function: it provides a bias magnetic field for the electromagnetic ultrasonic transducer and fixes the probe array on the surface of the carbon steel elbow through magnetic attraction. This eliminates the need for operators to hold the probe or use external clamps in traditional testing, simplifies the probe structure and fixing method, and improves the convenience and safety of on-site operation.

[0033] 5. This invention is flexible and adaptable to curved surfaces and is applicable to a wide range of materials. The multi-layer composite flexible probe array can be bent to fit the curved surfaces of bends of different pipe diameters. One set of probes can cover the range from DN50 to DN300, eliminating the need to equip multiple sets of curved probes for different pipe diameters, thus reducing equipment costs and carrying burden. Attached Figure Description

[0034] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0035] Figure 1 This is a flowchart illustrating an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the overall structure of the device according to an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of the multi-layer structure of the flexible probe array according to an embodiment of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] like Figure 1 As shown, a method for measuring the wall thickness of a flexible electromagnetic ultrasonic elbow includes the following steps:

[0040] Step S1: Attach the flexible probe array to the outer surface of the pipe elbow and arrange several electromagnetic ultrasonic transducer units along the back arc direction of the elbow; fix the carbon steel elbow with the magnetic attraction of a permanent magnet; fix the stainless steel elbow with clamps or vacuum suction cups.

[0041] Step S2: Using a time-division multiplexing method, select each electromagnetic ultrasonic transducer unit in sequence and send a high-voltage excitation pulse to the currently selected unit. The voltage is 100~500V, the frequency is 1~5MHz, and the pulse width is 1~3 cycles, so that it excites longitudinal waves on the metal surface.

[0042] Step S3: Receive the bottom echo signal collected by each electromagnetic ultrasonic transducer unit; the EMAT echo signal is extremely weak, first through a 40~60dB low-noise preamplifier, then through a 0~40dB programmable gain amplification, and then converted into a digital signal by a high-speed ADC with a sampling rate of 100~200MHz and a resolution of 12~14bit; the digital signal is bandpass filtered with a passband of 0.5 to 2 times the excitation frequency, Hilbert envelope detection, cross-correlation precise pickup, and multiple averagings, 32~128 times, and then the original wall thickness value of each measurement point is calculated;

[0043] Step S4: Perform four corrections on the original wall thickness value, including temperature compensation correction, stress compensation correction, lift-off phase compensation correction, and surface geometry correction, to obtain the true wall thickness value and output it to the terminal for display.

[0044] Longitudinal waves propagate at a constant speed of sound through the wall thickness being measured. The ultrasonic wave travels from the probe position on the outer wall to the inner wall, where it is reflected, and then returns to the outer wall to be received by the same probe. The total distance traveled by the ultrasonic wave is twice the wall thickness. Therefore, multiplying the speed of sound by the round-trip time and dividing by 2 gives the original wall thickness. The formula for calculating the original wall thickness is as follows:

[0045]

[0046] in, This represents the original wall thickness measurement without any correction. This represents the reference longitudinal wave velocity at a calibration temperature of 20℃, obtained through calibration under standard conditions before shipment. For carbon steel, it is approximately 5900 m / s. The round-trip time of the ultrasonic wave from emission to reception of the bottom surface echo is accurately extracted from the echo signal by the cross-correlation method in step S3.

[0047] The original wall thickness value is the reference input for the subsequent four corrections. However, due to factors such as temperature changes, residual stress, probe lifting, and the geometric effects of the elbow surface, the original value has systematic errors, which need to be eliminated through the four corrections in step S4.

[0048] This invention performs four independent physical corrections on the original wall thickness measurement value. Elbow wall thickness measurement faces multiple error sources such as temperature changes, residual stress, probe lift-off, and curved surface geometry. If all error sources are coupled into a complex model for solution, not only is the computational load large, but the cross-coupling between the physical quantities is also difficult to model accurately. Therefore, this invention constructs independent correction factors for each physical effect. Each correction factor is a dimensionless multiplier, and finally multiplies it with the original wall thickness to obtain the true wall thickness value.

[0049] 1. Temperature compensation correction

[0050] Temperature changes can alter the elastic modulus of a material, which in turn can lead to changes in the speed of sound. When the temperature rises, the lattice vibrations of the metal material intensify and the interatomic bonding forces weaken, resulting in a corresponding decrease in the speed of sound. Conversely, when the temperature decreases, the opposite occurs. Without temperature compensation, different wall thickness measurements will be obtained at the same measuring point under different temperatures.

[0051] There is often a temperature difference between the inner and outer walls of an elbow. The inner wall is in contact with a high-temperature medium, while the outer wall is in contact with ambient air. When ultrasonic waves pass through the wall thickness, they encounter a non-uniform temperature field, and it is insufficient to use only the outer wall surface temperature for correction. This invention uses an NTC temperature sensor mounted on the probe array surface to measure the outer wall temperature in real time, while the host's built-in sensor measures the ambient air temperature. Considering that the temperature distribution along the wall thickness is linear, this invention introduces an equivalent temperature difference coefficient to convert the outer wall temperature and ambient temperature into an average temperature along the wall thickness, thereby correcting the shortcomings of measuring only the surface temperature. The formula for calculating the temperature compensation factor is as follows:

[0052]

[0053] in, This represents the temperature compensation factor, used to correct the original wall thickness based on temperature. The temperature coefficient of sound velocity reflects the sensitivity of sound velocity to temperature changes. It is determined by the inherent properties of the material; for carbon steel, it is typically taken as -5.5 × 10⁻⁶. -4 / ℃, the negative sign indicates that the speed of sound decreases as the temperature increases. The outer wall temperature is measured in real time by an NTC temperature sensor mounted on the probe array surface. This represents the equivalent temperature difference coefficient, characterizing the degree of deviation between the average internal temperature and the external temperature of the wall. It is obtained from factory calibration and ranges from 0.3 to 0.7. The ambient air temperature is measured by the temperature sensor built into the main unit. This indicates the calibration temperature, taken as 20℃.

[0054] With the calibration temperature as the benchmark, when the current average temperature deviates from the calibration temperature, the sound velocity changes linearly, and the wall thickness measurement value needs to be corrected in the opposite direction. When the average temperature is higher than the calibration temperature, the sound velocity decreases and the original measurement value is too large. When the temperature compensation factor is less than 1, it is corrected downward; otherwise, it is corrected upward.

[0055] 2. Stress compensation correction

[0056] During the manufacturing process of the elbow, the back arc region is subjected to tensile plastic deformation, generating residual tensile stress. The residual tensile stress causes the material lattice to be elongated along the stress direction, increasing the lattice spacing. As a result, the ultrasonic waves are scattered more during propagation, which macroscopically manifests as a decrease in sound velocity, leading to an overestimation of the measured value. This change in sound velocity exhibits a regular distribution along different positions of the elbow's back arc, with the residual strain being the greatest at the apex of the back arc and gradually decreasing as it moves away from the apex.

[0057] This invention introduces the acoustoelastic effect and performs independent correction based on the position of the measurement point on the back arc. The residual strain at each point on the back arc can be approximated as the cosine function of the central angle at that point, i.e., the strain is greatest at the vertex and decreases with deviation. Simultaneously, for thick-walled elbows, increasing the wall thickness causes the neutral layer to shift outwards, altering the actual distribution of residual strain. Therefore, a wall thickness correction term is introduced to fine-tune the correction amplitude. The formula for calculating the stress compensation factor is:

[0058]

[0059] in, This represents the stress compensation factor, used to correct residual stress in the original wall thickness. This represents the stress compensation coefficient, characterizing the relative change in sound velocity caused by the residual tensile stress at the apex of the back arc. It is obtained from the factory calibration and ranges from 0.02 to 0.08. This represents the wall thickness correction factor, characterizing the effect of wall thickness on the residual strain distribution. It is obtained from factory calibration and ranges from 0.1 to 0.5. This indicates the nominal radius of curvature of the elbow, which is directly obtained from the elbow specifications. The central angle corresponding to the measurement point is obtained by using the arcsine function, which is the ratio of the arc distance from the measurement point to the vertex of the back arc to the nominal radius of curvature of the bend.

[0060] At the apex of the back arc, the central angle is 0, the residual tensile stress is the greatest, and the measured value is the most severely overestimated. The minimum value is taken, resulting in the largest correction amount; the correction amount decreases after deviating from the vertex; when the deviation reaches 90° from the central angle... Take 1, no correction needed.

[0061] 3. Lift-off phase compensation correction

[0062] Electromagnetic ultrasonic transducers rely on electromagnetic induction to excite and receive ultrasonic waves. Their transduction efficiency is extremely sensitive to the distance between the probe coil and the metal surface being measured. Flexible probe arrays rely on the magnetic attraction of permanent magnets to adhere to the elbow surface. However, due to the possibility of microscopic unevenness, oxide scale, or slight surface mismatch on the elbow surface, the actual lift-off distance of each transducer unit is not exactly equal to the thickness of the protective layer, but there are slight differences.

[0063] Increased lift-off has a dual impact: firstly, echo amplitude attenuates, reducing the signal-to-noise ratio; secondly, changes in eddy current density distribution cause a drift in the initial phase of acoustic wave excitation, manifesting as a slight acoustic time shift on the time axis, which is directly converted into wall thickness error. This invention utilizes the negative exponential relationship between echo signal amplitude and lift-off distance—the larger the lift-off, the more severe the amplitude attenuation—to infer the equivalent acoustic time error caused by the current lift-off through measured amplitude and deduct it. The formula for calculating the lift-off phase compensation factor is as follows:

[0064]

[0065] in, This represents the lift-off phase compensation factor, used to correct the acoustic time error caused by the lift-off of the original wall thickness. The lift-off phase composite coefficient is determined by the probe's structural and electromagnetic parameters, obtained from factory calibration, and ranges from 0.5 to 3.0 mm. This indicates the amplitude of the main peak of the echo signal in this channel, which is obtained by envelope detection after sampling the echo signal using a high-speed ADC. This represents the reference amplitude under standard bonding conditions, i.e., the echo amplitude measured when the probe is fully bonded to the smooth flat surface, calibrated before leaving the factory;

[0066] The attenuation of the echo amplitude relative to the reference amplitude is logarithmically mapped and multiplied by a comprehensive coefficient to obtain the relative correction amount for the wall thickness. This correction factor is applied when the probe is fully in contact with the target area, thus removing the phase compensation factor. Set to 1, no compensation required; lift-off phase compensation factor increases as lift-off increases. If the value is less than 1, the oversized measurement value will be corrected downwards.

[0067] 4. Surface geometric correction

[0068] The influence of curved surfaces on wall thickness measurement of elbows is reflected in two aspects. When the measurement point deviates from the apex of the back arc, the ultrasonic wave is incident along the normal direction of the outer wall. However, the true wall thickness of the elbow is defined as the radial direction, that is, the direction pointing to the center of the elbow. There is an angle between the normal direction and the radial direction, which is the central angle corresponding to the measurement point. The projection of the sound path in the radial direction is less than the actual propagation distance in the normal direction, resulting in an overestimation of the measured value. When the ultrasonic wave propagates to the inner wall and is reflected, due to the difference in the curvature radii of the inner and outer walls, the reflection point of the sound beam on the inner wall will drift in an arc direction relative to the incident point on the outer wall. The amount of drift increases with the increase of wall thickness. Traditional thin-walled systems that approximate and ignore this drift will introduce errors.

[0069] This invention breaks through the thin-wall assumption, strictly distinguishes the curvature radii of the inner and outer walls, and introduces sound beam drift for correction, specifically including:

[0070] Calculate the radius of curvature of the inner wall of the elbow. The radius of curvature of the inner wall of the elbow is equal to the nominal radius of curvature of the elbow minus the original wall thickness.

[0071] The formula for calculating the arc-shaped drift of the sound beam at the reflection point on the inner wall relative to the incident point on the outer wall is:

[0072]

[0073] in, This represents the arc-shaped drift of the reflection point on the inner wall of the sound beam relative to the incident point on the outer wall. Indicates the nominal radius of curvature of the elbow. This indicates the radius of curvature of the inner wall of the elbow. Indicates the original wall thickness. The arc-shaped distance from the measurement point to the apex of the back arc is determined by the geometric arrangement of the transducers in the flexible array. Represents the arcsine function;

[0074] The surface geometry correction factor is equal to the ratio of the inner wall radius of curvature to the outer wall radius of curvature, multiplied by a factor minus the square root of the ratio of the equivalent arc distance of the outer wall to the outer wall radius of curvature. The equivalent arc distance of the outer wall is equal to the arc distance from the measurement point to the vertex of the back arc minus the arc drift. The specific formula is as follows:

[0075]

[0076] in, This represents the surface geometry correction factor, used to correct the original wall thickness for the geometric effects of the elbow surface. The first part of the formula is the ratio of the inner and outer wall curvature radii, reflecting the basic correction caused by the curvature difference itself; the second part is the normal projection correction after deducting the drift, at the apex of the back arc. If we set the value to 0, the drift is 0, and the second half is equal to 1, but the first half is still less than 1, indicating that even at the vertex position, the difference in curvature between the inner and outer walls will still cause the measured value to be too large; after deviating from the vertex, the second half is also less than 1, and the correction amount increases.

[0077] Multiplying the above four correction factors by the original wall thickness yields the final true wall thickness value, as shown in the following formula:

[0078]

[0079] in, The true wall thickness value at the measurement point is the final measurement result after four corrections: temperature compensation, stress compensation, lift-off phase compensation, and surface geometry correction.

[0080] The four correction factors mentioned above correct the original measurement values ​​from four different physical dimensions: thermal, elasticity, electromagnetic, and geometric optics. Each factor is calculated independently based on real-time measurement data and is not coupled with each other. Therefore, the order of multiplication does not affect the final result. This avoids the difficulties in parameter calibration and unstable solutions caused by establishing complex coupled models. It has the outstanding advantages of simple engineering implementation and clear physical meaning.

[0081] For stainless steel elbows made of non-ferromagnetic materials, permanent magnets cannot provide sufficient attraction force, and EMAT detection of stainless steel relies on the Lorentz force effect, requiring a stronger bias magnetic field. Therefore, the permanent magnet array was replaced with an electromagnet, and an external power supply was added to provide the bias magnetic field. The probe fixation was changed from magnetic suction to clamp or vacuum chuck. The excitation voltage and pulse width were appropriately increased, and the average number of receptions was increased to 128 to compensate for the signal loss due to the weak Lorentz force effect.

[0082] For elbows with pipe diameters of DN300 and above, the back arc length is relatively large. The back arc length of a DN400 90° elbow can reach 942mm. Fabricating a long flexible probe array that covers the entire back arc at once is not only costly, but also makes it difficult to ensure that each unit is in close contact with the curved surface during the bonding process. Therefore, a flexible probe array covering a 60° arc length of about 420mm is used, consisting of 7 units. The array moves step by step along the circumference of the elbow, moving one step at a time, covering 50% to 80% of the arc length. The position is recorded by mechanical positioning pins or encoders, and the data from each measurement are spliced ​​and interpolated according to coordinates to generate a complete wall thickness distribution map.

[0083] like Figure 2 As shown, the present invention provides a flexible electromagnetic ultrasonic elbow wall thickness measuring device, including a flexible probe array module, a signal transceiver module, and a processing module.

[0084] The flexible probe array module is the core physical execution component of this invention. It is used to fit the outer surface of the pipe bend. Existing rigid EMAT probes cannot form good acoustic coupling with the curved surface of the bend, resulting in uneven distribution of the bias magnetic field and low excitation efficiency. To address this, this invention designs a multi-layer composite flexible structure, including a flexible substrate, several electromagnetic ultrasonic transducer units disposed on the flexible substrate, and a permanent magnet array corresponding to the several electromagnetic ultrasonic transducer units.

[0085] like Figure 3As shown, the flexible probe array module consists of six functional layers from the side closest to the measured surface to the outside. The first layer is a protective layer, made of polyurethane or wear-resistant rubber material, with a thickness of 0.5~1mm, which serves to resist wear, oil, and scratches. Its acoustic impedance is also close to that of metal, facilitating ultrasonic wave transmission into the measured workpiece. The second layer is a permanent magnet array layer, composed of several small neodymium iron boron (NdFeB) magnetic sheets, with a thickness of 0.5~1mm. The magnetization direction is perpendicular to the tube wall surface. This layer is the core layer in this invention, providing a bias magnetic field for EMAT transduction and a magnetic attraction force for the probe. The third layer is a coil layer, made of a spiral planar coil using flexible printed circuit board (FPC) technology, with a copper foil thickness of 0.05~0.1mm. This layer is the energy conversion layer for electromagnetic-acoustic conversion. When a high-frequency alternating current passes through the spiral coil, the measured metal... The first layer consists of a surface-induced eddy current layer, which, under the influence of a bias magnetic field, experiences a Lorentz force, thereby generating ultrasonic waves. The second layer is a flexible substrate, made of polyimide (PI) or polyethylene naphthalate (PEN) film with a thickness of 0.1~0.2mm. This layer provides mechanical support for the entire probe array and, thanks to its excellent flexibility, allows for overall bending, which is key to achieving compatibility with different pipe diameters. The third layer is a backing layer, made of epoxy resin or rubber material with a thickness of 0.3~0.5mm. It is used to absorb the acoustic wave energy radiated from the back of the coil, suppress interlayer reflection and ringing effects, and improve the signal time domain resolution. All the above layers are combined into one piece through hot pressing or bonding processes, with a total thickness of only 1.5~2.5mm, which can be bent to fit the curved surface of elbows with pipe diameters of DN50~DN300.

[0086] Several electromagnetic ultrasonic transducer units are linearly arranged on a flexible substrate along the back arc of the bend, with a center distance of 15~25mm between adjacent units. Each unit has an independent coil pattern and electrode lead-out end, and the units are interconnected by flexible wires and converge to the main lead-out end. This array layout allows several measurement points within a 120° range of the back arc of the bend to be covered with a single attachment, which is the structural basis for achieving efficient measurement.

[0087] The signal transceiver module is connected to the flexible probe array module and is used for time-division multiplexing of each electromagnetic ultrasonic transducer unit, transmitting excitation pulses to the selected unit and receiving echo signals. The signal transceiver module includes an analog switch matrix and a shared set of pulse transmitting circuit and signal receiving circuit. The sequential selection of each electromagnetic ultrasonic transducer unit is achieved through time-division switching of the analog switch matrix. Taking N transducer units as an example, the traditional solution requires N sets of independent transmitters and receivers, and the hardware cost increases linearly with the number of channels. However, the present invention only requires one set of transmitters and receivers, which are shared by N units, resulting in a significant reduction in cost.

[0088] The signal transceiver module also includes: a high-voltage pulse circuit, which generates high-voltage spike pulses with an amplitude of 100~500V, a frequency of 1~5MHz, and a width of 1~3 cycles under FPGA timing control; a preamplifier circuit with a fixed gain of 40~60dB, used to amplify the microvolt-level echo signal received by the EMAT transducer to the millivolt level; a programmable gain amplifier circuit with a gain range of 0~40dB, which can automatically adjust the gain according to the echo amplitude; and a high-speed analog-to-digital converter (ADC) with a sampling rate of 100~200MHz and a resolution of 12~14bit, used to digitize the analog echo signal.

[0089] The processing module, connected to the signal transceiver module, is the core of data calculation and correction. The processing module includes a Field Programmable Gate Array (FPGA) and a Digital Signal Processor (DSP) or Microcontroller (MCU). The FPGA is responsible for precise timing control, controlling the switching timing of the analog switch matrix, the transmission time of the high-voltage pulse, and the data acquisition time of the ADC. The DSP or MCU is responsible for the signal processing algorithm, performing digital signal processing on the echo signal, calculating the original wall thickness value at each measurement point, performing four corrections, and outputting the final result.

[0090] In the flexible probe array of this invention, the permanent magnet array performs a dual function:

[0091] The first function is to provide a bias magnetic field: the working principle of the Electromagnetic Ultrasonic Transducer (EMAT) dictates that it must be under an external bias magnetic field to function properly. When a high-frequency excitation current is passed into the spiral coil, alternating eddy currents are induced in the skin layer of the metal surface being tested. These eddy currents are subjected to Lorentz force in the static bias magnetic field provided by the permanent magnet, thereby generating ultrasonic waves inside the metal. In this invention, the surface magnetic field strength of each permanent magnet is 0.5~1.0T, and the magnetization direction is perpendicular to the surface of the pipe wall being tested, which can provide a sufficient and uniform bias magnetic field for each transducer unit.

[0092] The second function is magnetic fixation: For the ferromagnetic material of the carbon steel elbow, the permanent magnet not only provides a bias magnetic field, but also uses its magnetic attraction to firmly attract and fix the flexible probe array module to the elbow surface; the magnetic attraction force of each unit is about 3N / cm², and the combined effect of several units is sufficient to fix the entire probe array to the vertical or inverted elbow surface; this design eliminates the need for operators to hold the probe or use external clamps in traditional testing, greatly simplifying the on-site testing process.

[0093] The integrated design with the aforementioned dual functions combines the originally two separate components, the bias magnet and the fixing device, into one, which reduces the size and weight of the probe array and improves the convenience and safety of on-site operation.

[0094] Several electromagnetic ultrasonic transducer units are connected to several input terminals of an analog switch matrix. The output terminals of the analog switch matrix are connected to a pulse transmitting circuit and a signal receiving circuit in sequence. In terms of operation sequence, the FPGA first controls the analog switch matrix to turn on the first channel. The high-voltage pulse circuit emits an excitation pulse, which is transmitted to the first transducer unit through the analog switch matrix, exciting ultrasonic waves at the first measurement point. The sound wave propagates inside the wall of the elbow under test and is reflected back to the first transducer unit. The echo signal is then transmitted to the preamplifier circuit through the analog switch matrix, completing the transmission-reception process of the first channel. Subsequently, the analog switch matrix switches to the second channel and repeats the above process, thus completing the measurement of the first to Nth channels in sequence. When all N channels have been scanned, a complete array scanning cycle is completed, and then the next round of scanning begins.

[0095] Taking 11 transducer units as an example, the complete measurement cycle of each channel, including transmission, reception, signal stabilization, and switching, is about 5ms. Therefore, the total scanning time of 11 channels is about 55ms, and about 18 full array scans can be completed per second. This speed fully meets the real-time requirements for industrial field testing, while the hardware cost is only one transmitter and one receiver, which is about N times lower than the cost of traditional solutions.

[0096] The examples described herein are merely preferred embodiments of the invention and are not intended to limit the concept and scope of the invention. Any modifications and improvements made by those skilled in the art to the technical solutions of the invention without departing from the design concept of the invention should fall within the protection scope of the invention.

Claims

1. A method for measuring the wall thickness of a flexible electromagnetic ultrasonic elbow, characterized in that, Includes the following steps: Step S1: Attach the flexible probe array to the outer surface of the pipe bend, wherein the flexible probe array includes several electromagnetic ultrasonic transducer units arranged along the back arc direction of the bend. Step S2: Using a time-division multiplexing method, select each electromagnetic ultrasonic transducer unit in sequence and send an excitation pulse to the currently selected electromagnetic ultrasonic transducer unit to excite longitudinal waves. Step S3: Receive the bottom echo signal collected by each electromagnetic ultrasonic transducer unit, and calculate the original wall thickness value corresponding to each measurement point based on the longitudinal wave round-trip time. Step S4: Perform temperature compensation correction, stress compensation correction, lift-off phase compensation correction, and surface geometry correction on the original wall thickness value to obtain the true wall thickness value and output it.

2. The method according to claim 1, characterized in that, In step S3, the echo arrival time is picked up using the cross-correlation method, and 32 to 128 measurements are taken at the same measurement point and the average is taken to improve the signal-to-noise ratio.

3. The method according to claim 2, characterized in that, In step S4, the temperature compensation correction uses a temperature compensation factor to correct the original wall thickness. The temperature compensation factor is equal to 1 plus the product of the sound velocity temperature coefficient and the equivalent temperature deviation. The equivalent temperature deviation is the value obtained by adding the equivalent temperature difference coefficient to the outer wall temperature, multiplying it by the difference between the outer wall temperature and the ambient air temperature, and then subtracting the calibration temperature.

4. The method according to claim 3, characterized in that, In step S4, the stress compensation correction uses a stress compensation factor to correct the original wall thickness. The stress compensation factor is equal to 1 minus the product of the stress compensation coefficient multiplied by the wall thickness correction factor and then multiplied by the cosine of the central angle of the measurement point. The wall thickness correction factor is equal to 1 plus the wall thickness correction coefficient multiplied by the ratio of the original wall thickness to the nominal radius of curvature of the elbow. The central angle of the measurement point is calculated by the arcsine function of the ratio of the arc distance from the measurement point to the apex of the back arc to the nominal radius of curvature of the elbow.

5. The method according to claim 4, characterized in that, In step S4, the lift-off phase compensation correction uses a lift-off phase compensation factor to correct the original wall thickness. The lift-off phase compensation factor is equal to 1 plus the lift-off phase comprehensive coefficient divided by the original wall thickness and then multiplied by the natural logarithm of the ratio of the echo signal main peak amplitude to the reference amplitude under standard bonding state.

6. The method according to claim 5, characterized in that, The surface geometry correction includes the following steps: Calculate the radius of curvature of the inner wall of the elbow. The radius of curvature of the inner wall of the elbow is equal to the nominal radius of curvature of the elbow minus the original wall thickness. Calculate the arc-directional drift of the sound beam at the reflection point on the inner wall relative to the incident point on the outer wall. The arc-directional drift is equal to the arcsine function value of the inner wall radius of curvature multiplied by the arc distance from the measurement point to the apex of the back arc and the inner wall radius of curvature, minus the arcsine function value of the outer wall radius of curvature multiplied by the arc distance from the measurement point to the apex of the back arc and the outer wall radius of curvature. Calculate the surface geometry correction factor, which is equal to the ratio of the inner wall radius of curvature to the outer wall radius of curvature, multiplied by the square root of the square of the ratio of the equivalent arc distance of the outer wall to the outer wall radius of curvature, where the equivalent arc distance of the outer wall is equal to the arc distance from the measurement point to the vertex of the back arc minus the arc drift.

7. The method according to claim 6, characterized in that, In step S4, the true wall thickness value is obtained by multiplying the original wall thickness by the temperature compensation factor, stress compensation factor, lift-off phase compensation factor, and surface geometry correction factor. That is, the true wall thickness value is equal to the original wall thickness multiplied by the temperature compensation factor, the stress compensation factor, the lift-off phase compensation factor, and the surface geometry correction factor.

8. A flexible electromagnetic ultrasonic elbow wall thickness measuring device, used to implement the flexible electromagnetic ultrasonic elbow wall thickness measuring method as described in any one of claims 1-7, characterized in that, include: A flexible probe array module is used to fit the outer surface of a pipe bend. The flexible probe array module includes a flexible substrate, a plurality of electromagnetic ultrasonic transducer units disposed on the flexible substrate, and a permanent magnet array corresponding to the plurality of electromagnetic ultrasonic transducer units. The flexible substrate can be bent to adapt to the curved surface of the bend. The signal transceiver module is connected to the flexible probe array module and is used to select each electromagnetic ultrasonic transducer unit in a time-division manner, transmit excitation pulses to the selected electromagnetic ultrasonic transducer unit and receive echo signals. The processing module, connected to the signal transceiver module, is used to calculate the original wall thickness value corresponding to each measurement point based on the echo signal, and to perform temperature compensation correction, stress compensation correction, lift-off phase compensation correction and surface geometry correction to obtain the true wall thickness value. The permanent magnet array is used both to provide a bias magnetic field for the electromagnetic ultrasonic transducer unit and to fix the flexible probe array module to the surface of the carbon steel elbow by magnetic attraction.

9. The apparatus according to claim 8, characterized in that, The flexible substrate is made of polyimide or polyethylene naphthalate film with a thickness of 0.1 to 0.2 mm; the electromagnetic ultrasonic transducer unit is made of a spiral planar coil using flexible printed circuit board technology with a coil layer thickness of 0.05 to 0.1 mm; each permanent magnet in the permanent magnet array corresponds to one electromagnetic ultrasonic transducer unit, the permanent magnet material is neodymium iron boron, and the surface magnetic field strength is 0.5 to 1.0 Tesla.

10. The apparatus according to claim 9, characterized in that, The flexible probe array module consists of a protective layer, a permanent magnet array layer, a coil layer, a flexible substrate, and a backing layer, arranged sequentially from the side closest to the surface being measured to the outside. The total thickness is 1.5 to 2.5 mm, which can accommodate elbows with pipe diameters from DN50 to DN300. Several electromagnetic ultrasonic transducer units are arranged linearly along the back arc direction of the elbow, with a center distance of 15 to 25 mm between adjacent units.

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