Search

Pulsed eddy current (PEC) is an electromagnetic non-destructive testing (NDT) technique mainly used to inspect corrosion in pipelines. Like many other NDT techniques, to quantify pipe wall thickness, a signal feature has to be extracted from the PEC response. The authors’ previous work has exploited the linear relationship of the time-derivative feature to propose an in-situ calibration routine. However, to extract the time-derivative feature, two configuration parameters need to be determined prior to feature extraction. To the authors’ knowledge, none of the previous works explores the determining factors of this configuration parameters, where the current technique is still limited to using brute force method or trial-and-error. This paper brings novelty by formulating the aforementioned problem as a minimisation problem, and solving it using a particle swarm optimisation (PSO) algorithm. The obtained results, compared against brute force method, demonstrate the feasibility and performance improvement of using PSO in determining the configuration parameters. The approach thus presented is validated and the results obtained are justified by analysing the underlying physical system theory.

This paper describes a two-coil eddy current sensor being used in a transmit-receive arrangement at 1 MHz, where the drive and amplification electronics are miniaturised and built directly behind the coil to reduce noise and the effects from cable length. Small, simulated defects are detected, less than 500 microns in length, on titanium and titanium aluminide, which is an increasingly important alloy for aerospace applications. Data is analysed quantitatively in a parametric approach. This experiment uses a transmitting coil driven by a constant current source and a separate receiving coil, where the magnitude and phase of the induced voltage signals on both coils are measured independently. Experimental measurements are validated using finite element modelling and the phase of the signal on the receiving coil in particular is less susceptible to variations caused by changes in lift-off. A combination of experimental and simulation data of 2D surface scans and lift-off measurements show the variation in the magnitude and phase of the eddy current signal with lift-off on Ti, TiAl (Ti-45Al-2Mn-2Nb-1B) and 316L stainless steel. It is also shown that the high-frequency lower noise approach can reliably detect defects of less than 500 microns in length in both Ti and TiAl.

Eddy current testing (ECT), is a non-destructive method widely applied in detecting defects of metal plates based on the principle of electromagnetic induction. The flexible eddy current testing (FECT) has been developed from ECT. What’s more, it’s also an effective method that is given a higher priority in defect detection of metal with complex surface structure due to its flexibility and small size. An arrayed differential flexible butterfly-shape eddy current sensor is proposed to detect the surface defect of iron screw thread in this paper, which has one butterfly-shape coil, four sets of differential receiver (RX) coils and one top RX coil, and the proposed sensor is designed as butterfly-shape with a certain folding angle to match with the curvature of the internal and external screw thread, which can solve the lift-off problem of screw thread defect detection at the bottom. The theoretical analysis illustrates the design principle for the proposed sensors. The simulation based on COMSOL Multiphysics is applied to illustrate the probability of the proposed sensor, and the experimental testing system is adopted to verify the high performance of the proposed sensors. The results of simulation and experiment are identical, which show the flexible differential butterfly-shape array eddy current sensor can not only improve the sensitivity and reduce the error rate, but also orientate the defect on the surface of the screw thread.

This paper proposes a measurement method for the rotation angle of the spherical joint based on the extreme learning machine (ELM) artificial neural network and four eddy current sensors. Aiming at the problems of small range and low accuracy in the early three-eddy-current angle measurement prototype, the position matching scheme of four eddy current sensors is researched, a new prototype is developed through simulation analysis, and ELM neural network substitutes the previous generalized regression neural network (GRNN) for building a new measurement model. The modelling training and comparison test are completed in the self-developed high-precision angle calibration device. Experimental results show that the new prototype not only covers a ±20° measurement range but also promotes measurement accuracy, and the standard deviation of the single-axis measurement drops to $3^{\prime }$ within the range of 5°–15°. It provides a relatively high-precision measurement method for real-time, multi-axis active detection of spherical joint space rotation angle error.

Displacement sensor is the key component of the magnetic levitation system. Reducing complexity and improving reliability of the sensor circuits are very important to mass production of magnetic levitation devices. In this article, an improved driver circuit of eddy current sensor is proposed. In order to generate a stable oscillating source with high stability and good precision of frequency, a crystal oscillator circuit with power boost is used to replace the traditional Colpitts oscillator circuit. The improved modulation circuit is composed of passive components with a simple structure for improving reliability. The parameters design method of the modulation circuit is analyzed in detail, which greatly affects the sensitivity and linearity of the sensor. The experimental results show that the designed eddy current sensor has good linearity (1.12%) and sensitivity (2.14 V/mm), and it can ensure the stable operation of the magnetic levitated turbomolecular pump at the rated speed.

Eddy current sensors can be used to test the characteristics and measure the parameters of the conductive samples. As the main obstacle of the multi-frequency eddy current sensor, the lift-off distance affects the effectiveness and accuracy of the measurement. In this paper, a material-independent algorithm has been proposed for the restoration of the lift-off distance when using the multi-frequency eddy current sensor, which is based on the approximation under the thin-skin effect. Experiment testing on the performance of the proposed method is presented. Results show that from the dual-frequency inductance, the lift-off distance could be restored with a maximum error of 0.24 mm for the distance up to 12 mm. Besides, the derived lift-off distance is used for the inversion of the magnetic permeability. Based on a lift-off insensitive inductance (LII) feature, the magnetic permeability of steels can be inversed in an iterative manner, with an error of less than 0.6 % for the lift-off distance up to 12 mm.

For the electromagnetic eddy current (EC) testing, various methods have been proposed for reducing the lift-off error on the measurement of samples. In this article, instead of eliminating the measurement error caused by the lift-off effect, an algorithm has been proposed to directly measure the lift-off distance between the sensor and nonmagnetic conductive plates. The algorithm is based on a sample-independent inductance (SII) feature. That is, under high working frequencies, the inductance is found to be sensitive to the lift-off distance and independent of the test piece under an optimal single high working frequency (43.87 kHz). Furthermore, the predicted lift-off distance is used for the thickness prediction of the nonmagnetic conductive samples using an iterative method. Considering the EC skin depth, the thickness prediction is operated under a single lower frequency (0.20 kHz). As the inductance has different sensitivities to the lift-off and thickness, the prediction error of the sample thickness is different from that of the lift-off distance. From the experiments on three different nonmagnetic samples—aluminum, copper, and brass, the maximum prediction error of the lift-off distance and sample thickness is 1.1 mm and 5.42%, respectively, at the lift-off of 12.0 mm.

Aluminum alloys are commonly used in commercial aircraft due to their low density and excellent mechanical properties. However, they need to become increasingly competitive in order that aluminum plate manufacturers maintaining market share. One way to ensure this is to improve or control parameters used in heat treatments. It is well known that electrical conductivity of metal alloys varies with microstructural change. Therefore, it is believed that In-Situ control of microstructural evolution can lead to optimization of treatment time, reduced energy consumption, increased production and final product quality, as well as reduction of costs, nonconformities and rework, having low implementation cost. In this way, eddy current sensors can be coupled to the furnace controller to optimize time and quality of heat treatment through the analysis of electrical conductivity with low implementation cost. Therefore, the objective of this work is to design, model and validate an eddy current sensor to monitor heat treatment of precipitation hardenable aluminum alloys in real time. Samples of 2024 alloy were prepared by solution treatment at 495 °C for 3 h and monitorization of aging treatment was performed at 190 °C for 9 h. Hardness, metallography, electrical conductivity (outside furnace) and relative electrical conductivity (inside furnace) were analyzed. The results were compared with the literature. There is a lot of discrepancy in the results of other authors. The eddy current sensor showed that behavior is similar to electrical conductivity through four-terminal method outside furnace.

A simple-to-implement technique for measuring radial clearances with controlled precision due to decreasing random components of errors is considered. The quantitative estimates of precision, performance, and efficiency of the proposed technique are provided. This technique is stochastic, is based on characteristics of random errors and on periodicity of blade wheel rotation, and allows obtaining the measurement results with uniform precision in the entire range of rotation speeds of the turbomachine rotor. The considered technique makes it possible to avoid procedures for approximating numerical readouts.

This article proposes a self-temperature compensation method based on an analog multiplier, which reduces the circuit thermal drift by more than an order of magnitude. An ac bridge eddy current sensor (ECS) is used as an example to analyze the thermal drift of the circuit in detail, and the basic principle of the compensation method is expounded. A high-precision ECS prototype is manufactured and tested. Results show that the temperature coefficient of the circuit in the prototype is decreased from 243 to 5 ppm/°C with this technique. This self-compensation method is simple, effective, and universal. Furthermore, this technique has a unique advantage for high-bandwidth and high-precision sensors and can work efficiently in complex and harsh temperature environments.

In the field of massive forged components the mechanical engineering industry searches for processes with increasing energy and resource efficiency. The new generation bainitic steels are promising for such application because of the high strength, toughness and fatigue properties. In order to achieve the desired mechanical properties, the development of the bainitic microstructure depending on the parameters of the thermomechanical process and on the cooling procedure must be well-known. In the present work diverse experimental techniques were applied for the investigation of the microstructural development during thermomechanical treatment and subsequent continuous cooling through the bainitic transformation range. The thermomechanical processes were simulated using dilatometers and at the same time, the specimens were analyzed using an eddy current sensor or using in-situ X-ray diffraction measurements at synchrotron (DESY). The results show that the eddy current sensor is suitable for the monitoring of the microstructural development during cooling and during deformation. From the investigations suitable process parameters were deduced for achieving a possibly fine bainitic microstructure. The main factors are a relatively low deformation temperature in austenitic range, a fast cooling (> 2 K/s) into the bainitic range, bainitic transformation and/or a short deformation in the lower bainite range, and finally a slower cooling until room temperature.

The classification of conductivity is significant for recycling metallic scraps. The eddy current sensor is distinct from other classification methods by its merits of non-contact, economical set-up, fast measuring, and so forth. By introducing a set-up that could ensure the single-valued mutual inductance trajectory on the complex plane, we propose the circle fitting method to extract the global features of different trajectories. It is subsequently observed that the fitting circle centers-one of the global features-for the same conductivity with various tilting angles are distributed longitudinally close to each other. The feature line, which is parallel to the abscissa axis, is introduced to represent this gathering distribution behavior. The feature lines for different conductivity are distinguishable. Thus, we construct a simple classification method with only two steps. The test results show great fidelity of the proposed classification technique which can successfully classify the tilting metallic samples within 11.3°.

Electromagnetic eddy current sensors are commonly used to identify and quantify the surface notches of metals. However, the unintentional tilt of eddy current sensors affects results of size profiling, particularly for the depth profiling. In this paper, based on the eddy current thin-skin regime, a revised algorithm has been proposed for the analytical voltage or impedance of a tilted driver–pickup eddy current sensor scanning across a long ideal notch. Considering the resolution of the measurement, the bespoke driver–pickup, also termed as transmitter–receiver (T-R) sensor is designed with a small mean radius of 1 mm. In addition, the T-R sensor is connected to the electromagnetic instrument and controlled by a scanning stage with high spatial travel resolution, with a limit of 0.2 μm and selected as 0.25 mm. Experiments were conducted for imaging of an aluminium sheet with seven machined long notches of different depths using T-R sensor under different tilt angles. By fitting the measured voltage (both real and imaginary part) with proposed analytical algorithms, the depth profiling of notches is less affected by the tilt angle of sensors. From the results, the depth of notches can be retrieved within a deviation of 10% for tilt angles up to 60 degrees.

Electromagnetic sensing has been used for diverse applications of non-destructive testing, including the surface inspection, measurement of properties, object characterization. However, the measurement accuracy could be significantly influenced by the lift-off between sensors and samples. To address the issue caused by lift-offs, various strategies have been proposed for the permeability measurement of ferromagnetic steels, which mainly involves different sensor designs and signal features (e.g., the zero-crossing feature). In this paper, a single high-frequency scenario for the permeability retrieval is introduced. By combining the signal of two sensing pairs, the retrieval of magnetic permeability is less affected by the lift-off of sensors. Unlike the previous strategy on reducing the lift-off effect (directly taking the phase term out of the integration) using the Dodd-Deeds analytical method, the proposed method is based on a high-frequency linear feature of the phase term. Therefore, this method has the merit of high accuracy and fast processing for the permeability retrieval (a simplified version of Dodd-Deeds analytical formulas after the integration). Experimental measurement has been carried out on the impedance measurement of designed sensors interrogating ferromagnetic dual-phase steels. For sensor lift-offs of up to 10 mm, the error of the permeability retrieval is controlled within 4 % under the optimal frequency.

Funding Information: This research was funded by the 19ENG07 Met4Wind project, which has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme . Additionally, the research was funded by the Business Finland as part of the Reboot IOT project (grant number 4356/31/2019 ) and by ABB Motors and Generators, Finland . Publisher Copyright: © 2021 The Authors Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Several standards for vibration testing of rotating machinery define limits for relative shaft displacement during rotation. With requirements for relative shaft displacement, limits are given for combined electrical and mechanical runout, which is the eddy probe reading error and the error caused by geometry of the measured shaft. Some standards allow for compensation of the mechanical and electrical runout by vectorially subtracting them from the eddy current probe values. This paper presents a measurement procedure applying multi-probe error separation to accurately determine the roundness profile and center point motion of electric motor shafts. The roundness error and center point movement obtained from multi-probe measurement with tactile displacement probes are compared with eddy current sensor displacement measurements to separate the total displacement measured with an eddy current probe into three components: center point motion, roundness error and electrical runout. Additionally, a transform for the measured eccentric component is presented to obtain an estimate for permanent rotor bow regardless of the rotor support arrangement during measurement. The relevant industrial vibration testing standards were reviewed. Several issues were found in the measurement methods required by the standards. The requirements of the standards and the used terminology are assessed critically and improvement proposals are presented.

With the development of a new generation of high-integrated semiconductors, a need for improvement of technology to monitor processes on a nanometer scale is increased. A detection technology that accurately measures the thickness of thin films and informs the end-point of the chemical mechanical polishing process is also required. In this study, the thickness of thin copper films with sub-micron scales was measured by fabricated eddy current sensors with increased sensitivity. The sensitivity of the sensor was improved by aligning the resonant frequency of the circuit with the frequency of the supplied AC voltage and increasing the Q factor of the circuit. The thickness of the wafer was changed through the polishing process and output voltages depending on the thickness of the copper layer were observed by the eddy current sensor. The thickness with a sub-micron scale of the copper film was precisely measured within ±1.5 nm of an average error.

The traditional pressure sensors are mostly based on piezoresistive, piezoelectric effect, and strain gauge, which are easily affected by ambient temperature. The structure of some sensors is rather complex to effectively acquire the signal or measure distribution force. Therefore, a pressure sensor based on the planar coil and elastic element was designed to solve this problem. The newly designed sensor has a spiral planar coil made by a printed circuit board and a polydimethylsiloxane (PDMS) thin film as the elastic element. The conductive layer is attached on the upper surface of PDMS to realise the eddy current effect. Meanwhile, a simulation model was established to find the relationship between deformation displacement and the pressure of the elastic element. By generating the experimental curves of the deformation displacement of the elastic element and the output frequency of the eddy current sensor, the pressure measurement on the surface of the elastic element can be identified. The preliminary experimental results showed that the eddy current pressure sensor based on the plane coil has good linearity and repeatability, and it can realise large area distributed pressure measurement, which is also suitable for accurate measurement of distribution force.

Array eddy current testing (AECT) sensor is capable of inspecting a large area in a single probe pass resulting in a C-scan image, which is beneficial for customizing the profile of the part being inspected. This paper presents a novel AECT sensor with three-phase excitation. The sensor has two rows of excitation coils and a row of pickup coils. The excitation coils are driven by three-phase currents that are 120° apart in phase. The induced eddy current in the conductive sample shifts electrically along the phase variation direction of the excitation currents. As a result, the sensor does not need multiplexer resulting in advantages of lower cost, less noise and faster inspection speed. The background signal measured by the sensor is small if there is no defect in the sample. Therefore, the sensor has high relative sensitivity to the presence of defect. A 3D finite element method (FEM) model is utilized to study the operating principle of the sensor. A prototype sensor has been developed and tested, with which aluminum sample and carbon fiber reinforced polymer (CFRP) sample with machined defects are inspected. The experiment results demonstrate the feasibility of the sensor for fast imaging of conductive materials.

This article presents a study of a Multi-coils circular eddy current non-destructive testing sensor for determining the fibers orientation as well as the detection of defect in multidirectional car...

Deep defect detection for ferromagnetic materials is a challenging task for Eddy Current testing (ECT) due to the skin effect of the low penetration depth. Pulsed Eddy Current testing (PECT) is a potential and effective method for detecting deep subsurface defects as it can obtain multiple depth information owing to its wide spectrum range. A novel weak coupling sensing structure of pulsed eddy current is proposed. In particular, the structure improves the ratio of the indirect coupling energy by reducing the direct coupling energy between the excitation and the detection coils. It obviously improves the ability to detect deep subsurface defects of ferromagnetic materials. The principle of penetration and the analysis of its equivalent circuit are presented. In addition, both experiments and simulations on different defects detection have been studied. The results have confirmed that all types of defects can be detected and it has shown the relatively monotonic linear relationships and reliability.

The disc cutter is the main rock cutting tool of a tunnel boring machine (TBM), and its wear directly affects the cutter change decision. Therefore, real-time monitoring of disc cutter wear has become an urgent technical problem for boring machines. In this study, an online monitoring system based on an eddy current sensor is designed to monitor disc cutter wear in real time. The effects of three types of rock and muddy water mixture on the measurement data are compared. Results show that measuring the eddy current distance can solve the problem of inaccurate measurement after the disc cutter is covered by rocks. Besides, the error data of this method in measuring disc cutter wear are analyzed. A function test is performed for similar disc cutters on a multi-function disc cutter performance test bench, and a real-time continuous test is conducted for the wear of two disc cutters in a certain TBM project. With a comparison of monitored and measured results, it shows that the error is less than 1 mm, which indicates that the system exhibits stable transmission and high accuracy in measurement and meets engineering requirements.

Aiming at the problem of low resolution of traditional eddy current sensor for buried defects detection in metal materials, a null-offset planar eddy current sensor array is designed, realized, and tested. The arrayed sensor is composed of a spatially periodic excitation coil and multiple differential detection coils, which are skillfully designed and can effectively suppress the common mode component in the induced voltage and improve the signal-noise-ratio of the sensor. The null-offset eddy current sensor is fabricated and utilized to detect the buried defects 0.5-mm depth beneath the surface of aluminum alloy test pieces. The simulation and experimental results show that the null-offset eddy current detection unit improves the detection resolution of buried defects by more than 1000 times.

The disturbance caused by actuator is an important factor affecting the performance of the high-precision payload, and a new actuator is urgently required for the non-contact satellite. First, a flat voice coil actuator (VCA) with high precision and high bandwidth is proposed for the non-contact ultra-quiet satellite, and the principle of the eddy current sensor is analysed. Second, an optimised integrated structure is proposed with the flat actuator to output high-precision forces and the eddy current sensor to measure the distance of the displacement. The mathematical model of the VCA is established, and the precision of output force is studied. Third, the attitude control using six actuators in Stewart configuration is discussed, and the compensation method of unbalanced torque is provided. Finally, physical test is taken on the principle prototype, and numerical simulations are implemented to verify the effectiveness of the integrated structure and the control methodology.

In this paper, an eddy current loop coupled with the tunneling magneto resistor (EC-TMR) is used as a displacement sensor to detect the micromotion of an orthopaedic implant. The high sensitivity and signal to noise ratio of the TMR sensor are used to achieve high resolution at a large standoff distance. First, a small three-turn rectangular eddy current loop of dimension $2.5\,\,\text {mm} \times 10$ mm is designed and simulated inside the human body using a full-wave EM simulator. Then, it is fabricated and tested using vector network analyzer. The magnetic tunnel junction stack is optimized and a six-element TMR sensor is fabricated and characterized. The eddy current and tunnelling magneto resistive sensor are integrated and heterodyne detection technique is used to obtain the high-resolution micromotion detection at an extended standoff range. This technique can be used for in-vivo detection of the micromotion of the orthopaedic implant which will be useful in reducing the revision surgeries due to the mechanical failures of the implant.

The radial and axial vibration signals of a rotating machine are crucial information to understand the machine operation and to diagnose potential faults. Instead of using three single sensors to measure the horizontal, vertical and axial displacements of rotating shafts, a novel non-projection vision-based system was proposed to realize simultaneous measurement of the radial and axial displacements with high accuracy and good reliability using an tailored artificial constant density sinusoidal fringe pattern (CDSFP), which was pasted around the shaft surface and worked as a tri-axial (i.e. the horizontal, vertical and axial) displacement sensor. The measurement principle and setup of the proposed measurement system were well established. The horizontal displacement could be correctly obtained from the fringe period density changes of the CDSFP image sequence recorded by a high-speed camera. Simultaneously, the vertical displacement could be acquired by tracking the centerline of shaft whilst the axial displacement could be obtained by locating the peaks of the cross-correlation sequence of the fringe intensities. A sub-pixel method was employed to improve the displacement resolution of the developed system. The performance of the proposed system was demonstrated by the comparison of the experiments using eddy current sensors. It showed that the proposed method was an effective and accurate technique for real-time tri-axial vibration monitoring of rotating shaft. Experimental results verified the feasibility, effectiveness and good robustness of the proposed methodology, which demonstrated that the proposed system was capable of achieving accurate tri-axial vibration displacements of rotating shaft compared to the commercial eddy current sensor which could only measure one dimensional displacement at each measurement. Therefore, the vision-based tri-axial vibration monitoring system could be recommended for real engineering applications in condition monitoring of rotating shafts.

In the scope of this paper, a first exemplary eddy current sensor for seawater conductivity measurement is developed, based on the derived sensor theory of a previous work. By high-frequency excitation, eddy currents are induced in the fluid and are counter-fields measured with a sensing coil. The coil’s resonance point is used for amplification. The developed prototype is analyzed based on a derived transfer function and FEM simulations. The theory is validated using a prototype implementation. With conducted experiments on a sensor test bench, the characteristics are confirmed and disturbances identified. It is shown that frequencies exist where temperature influence is minimal. This work gives a perspective for a novel sensor to allow seawater conductivity measurement.

Eddy current sensors have been widely applied to various measurements, whereas it is still obscure if these measurement techniques are workable for sloping samples. We start from a modified Dodd and Deeds’s analytical solution for finite-size samples and find that the pseudo-linearity exists in the magnitude-phase curve of the theoretical mutual inductance. The curves for different conductivities have no intersections. The experiments for verifying the pseudo-linearity are conducted at multiple frequencies from 20 kHz to 100 kHz. We subsequently involve the sloping samples in our simulations and experiments at 20 kHz. The pseudo-linearity preserves in both the simulated and experimental results. To characterize this pseudo-linearity, we resort to the method of least squares. The obtained intercepts for the same conductivity at different tilting angles are almost the same. Hence the intercept is independent of the tilting angle. The intercepts for different conductivities are clearly separated. Thus, the intercepts for non-sloping samples can be directly utilized as the criterion to classify sloping samples. We then test the classification process at multiple frequencies, which works properly at all the frequencies. Our classification rates are advanced compared to those in the literature. This sloping-invariance (that is the tilting-angle-independent intercept) might make the eddy current sensors find wider applications.

Hole-edge crack quantification of bolt joints is critical for monitoring and estimating structural integrity of aircraft. The paper proposes a new triangle eddy current sensor array for the purpose of increasing the level of quantifying hole-edge crack parameters, especially, the crack angle. The new senor array consists of triangular coils instead of planar rectangular coils. The configuration of the novel sensor array, including the excitation current directions and the excitation winding shape, is optimized by simulation. The ability of the proposed sensing film to identify the crack parameters has been verified by finite element simulations and experiments. Results shows that triangular coils with same current directions in circumferentially adjacent coils and opposite current directions in axially adjacent coils achieve better performance in sensor linearity and resolution compared to rectangular coils. In addition, it has also been proved that the sensing film has a good potential to identify the crack depth and length.

Health monitoring of mechanical transmission systems is an important area of research. Mechanical transmission systems, especially gear boxes in aircraft, automobiles, and wind turbines etc. account for many of the maintenance costs due to repairs, replacements and downtime. Gear boxes can experience high level of failure due to varied load conditions and harsh environments. Replacing the gear box is quite an expensive process and has significant downtime. Current gear box monitoring involves mainly measuring vibrations, however vibrations occur when the fault in the gear has already progressed significantly. Gear teeth monitoring lacks sensor technology to successfully detect tooth damage and misalignment. This paper presents a new concept gear teeth damage detection using eddy current sensors fitted on to the teeth of an idler gear at various locations. These sensors detect various faults encountered in a gear such as micro and macro pitting of the tooth surface, contact wear etc. Eddy current sensors are already being used to detect turbomachinery blade vibrations and tip clearance as they are robust and immune to contamination. In the present case, we use an idler gear that incorporates miniature eddy current sensors and state of the art electronics with wireless data transmission to enable the device to operate remotely and in harsh environments. A rotating rig with gears (spur and helical) and oil supply was built to test and validate the sensor by seeding various faults on the tooth surface. The results show that the idler sensor gear was able to detect various faults. The new eddy current sensor idler gear concept will enable health monitoring of the gearbox and ensure timely maintenance and reduction in operation costs.

In order to reduce the complexity of software and hardware design in multi-channel eddy current testing, a multi-channel real-time eddy current testing system based on lock-in amplifier technology and self- balancing technology is designed. The system uses graphical programming language platform (LabVIEW), Myrio1900 data acquisition control card and lock-in amplifier as key components. The lock-in amplifier adopts the lock-in technology to detect the amplitude and phase of the weak eddy current signal. The Myrio1900 card outputs multiple analog switch control signals and collects the amplitude component of the weak eddy current signal. The LabVIEW platform uses self-balancing technology to eliminate the imbalance of the bridge itself. The detection system realizes the real-time and reliable display of the amplitude and XY components of the 6-channel eddy current signal. Experimental results show that the designed system has a detection width of 56.00 mm at one time, and can detect artificial grooves with a covering layer of 2.90 mm. The artificial grooves have a width of 1.00 mm, a depth of 1.00 mm and a length of 50.00 mm. The system design process described in the article can be used as a reference for equipment developers and can also be used for educational purposes.

Defect detection in ferromagnetic substrates is often hampered by non-magnetic coating thickness variation when using conventional eddy current testing technique. The lift-off distance between the sample and the sensor is one of the main obstacles for the thickness measurement of non-magnetic coatings on ferromagnetic substrates when using the eddy current testing technique. Based on the eddy current thin-skin effect and the lift-off insensitive inductance (LII), a simplified iterative algorithm is proposed for reducing the lift-off variation effect using a multi-frequency sensor. Compared to the previous techniques on compensating the lift-off error (e.g., the lift-off point of intersection) while retrieving the thickness, the simplified inductance algorithms avoid the computation burden of integration, which are used as embedded algorithms for the online retrieval of lift-offs via each frequency channel. The LII is determined by the dimension and geometry of the sensor, thus eliminating the need for empirical calibration. The method is validated by means of experimental measurements of the inductance of coatings with different materials and thicknesses on ferrous substrates (dual-phase alloy). The error of the calculated coating thickness has been controlled to within 3 % for an extended lift-off range of up to 10 mm.

Real-time structural health monitoring (SHM) of engineering components exposed to high temperature and pressure is an utmost need. The hostile operating conditions such as elevated temperatures, contraction/expansion, vibrations, etc., may lead to catastrophic failure due to creep, thermo-mechanical fatigue or environmental attack (oxidation and hot corrosion). Non-availability of sensors susceptible at high temperature (HT) is a reason to handicap assessment and monitoring of such degradation during operation. Sensors using micro-fabricated sensing element are the promising, non-contact technology that have significant potential in structural health monitoring of critical engineering components operated at elevated temperatures. The present paper addresses this issue by developing application specific high-temperature sensors for real-time condition monitoring of the components operated at high temperature. The goal is to provide superior performance for in situ material condition monitoring (material degradation, flaw detection, stress relaxation, and/or creep monitoring) and through-wall temperature measurement. The sensor consists of a micro-fabricated primary (drive) winding and a secondary winding adjacent to the primary for sensing the response to a material under test. Multiple coils are cascaded/stacked together to increase the SNR and sensitivity of the sensor. The effectiveness of the proposed technique is first demonstrated on synthetic dataset from an eddy current simulation model. Further work is being underway for addressing the issues at elevated temperature.

Blade tip timing (BTT) technology is the most effective means for real-time monitoring of blade vibration. Accurately extracting the time of blade tip reaching the sensors is the key to ensure the accuracy of the BTT system. The tip clearance changes due to various complex forces during high-speed rotation. The traditional BTT signal extraction method does not consider the influence of tip clearance change on timing accuracy and introduces large timing errors. To solve this problem, a quadratic curve fitting timing method was proposed. In addition, based on the measurement principle of the eddy current sensors, the relationship among the output voltage of the eddy current sensor, tip clearance, and the blade cutting magnetic line angle was calibrated. A multisensor vibration parameter identification algorithm based on arbitrary angular distribution was introduced. Finally, the experiments were conducted to prove the effectiveness of the proposed method. The results show that in the range of 0.4 to 1.05 mm tip clearance change, the maximum absolute error of the timing values calculated by the proposed method is 26.0359 us, which is much lower than the calculated error of 203.7459 us when using the traditional timing method. When the tip clearance changed, the constant speed synchronous vibration parameters of No. 0 blade were identified. The average value of the vibration amplitude is 1.0881 mm. Compared with the identification results without changing tip clearance, the average value error of the vibration amplitude is 0.0017 mm. It is proved that within the blade tip clearance variation of 0.4 to 0.9 mm, the timing values obtained by the proposed timing method can accurately identify the vibration parameters of the blade.

The rotating shafts of automotive turbochargers are commonly supported in floating ring bearings with two fluid films in serial connection. They are – due to the high rotational shaft speed – often designed as full-floating ring bearings with a free rotating ring between shaft and housing. The ring is driven by the shear stresses on the inner and outer surface of the fluid films, which depend on the rotational speed gradient in the fluid and the pressure distribution in the fluid films. If this frequency meets a natural frequency of the system the oil-whirl locks in an oil-whip and causes a circular motion of the shaft in the bearing with very high amplitudes. Finding an optimal bearing design as a result of these nonlinear relations is a sophisticated task. Several parameters of the floating rings could be tuned to avoid such oil-whip effects. Hence, a holistic simulation approach is used to calculate the run-up of an investigated turbocharger by using the simulation software EMD, which contains solution methods for the nonlinear fluid film equations in order to solve them in every time step. However, the simulation has to be validated against measurements. For this sake, the ring speed is measured using a miniature eddy current sensor during an electric driven run-up on a test rig, yielding an integral quantity containing all relevant effects.

As the main part of the bearing, the steel ball has a vital impact on the accuracy, running performance and service life of the bearing. Therefore, the bearing steel ball must be fully tested for the surface quality of the steel ball before leaving the factory. In this study, the one-dimensional guide-way unfolding mechanism is used to realize the complete unfolding of steel ball surface, and the eddy current sensor is used to detect the surface quality of the steel ball. Empirical mode decomposition (EMD) algorithm was used to eliminate the trend term of detection signal. Three kinds of bearing steel balls with natural defects were tested to compare with the good ones. results show that the detection system can effectively distinguish between qualified and defective steel balls.

This paper presents the development of a multi-coil eddy current (EC) sensor that uses an axial rotating magnetic field for the measurement of electrical resistance to determine the electrical conductivity tensor of stratified carbon fiber reinforced polymer (CFRP) materials. The sensor consists of an identical planar racetrack multi-coil, excited by two-phase sinusoidal current sources that are 90° apart in phase to generate an axial rotating magnetic field and eliminate the need for mechanical rotation. Each sensor's coil's resistance variation is measured using a developed experimental prototype unit and computed using a 3D finite element method (FEM) based on the (A, V–A) formulation. The inverse problem technique that minimizes the difference between the calculated and measured resistances is then used to identify the electrical conductivity tensor components using the particle swarm optimization (PSO) algorithm. The comparison between the computed resistances and the measured ones shows an excellent concordance.

This paper investigates the design of an Eddy Current Sensor (ECS) for position measurement of a moving conductive target located behind a fixed conductive shielding surface. Such a sensor can e.g. be used in completely sealed actuators with magnetically levitated rotor or mover for high purity applications. A detailed analysis of the sensor is conducted, starting from the conventional ECS and its equivalent circuit model, which is then further extended to an ECS looking through a conductive wall. With the aid of Finite Element Methods (FEM) simulations, a deeper understanding of the sensor operating principle is provided, together with guidelines to select its optimal excitation frequency, based on the properties of the selected materials for the target and the shielding surface. A sensitivity analysis is then conducted and the results are finally verified with measurements on a hardware sensor prototype, showing that the sensor can be used to capture the mover position in an active magnetic bearing control structure.

The lift-off spacing distance between the eddy current sensor and test piece will influence the detected signals and accuracy of the measurement. Various techniques including novel sensor designs, features (lift-off point of intersection, lift-off invariance phenomenon), and algorithms have been proposed for the compensation of error caused by the lift-off effect using the eddy current sensor. However, few of these have directly measured the lift-off spacing distance, particularly for the distance up to 15 mm. In this paper, a lift-off tolerant pancake sensor has been designed. By analysing the sensitive region of the magnetic vector potential change (due to the test piece), the receiver of the sensor is designed as a circular spiral pancake coil with a large mean radius and span length (the difference between inner and outer radius). Experiments on the inductance measurement of three different non-magnetic samples have been carried out using both the designed pancake sensor and the previous triple-helix sensor. From the experiment result, the detected signal of the designed sensor has been proved much larger than that of the triple-helix sensor. Besides, simplified algorithms have been proposed for the measurement of the lift-off spacing and thickness of non-magnetic samples when using the proposed pancake sensor. Results show that the lift-off spacing and thickness can be measured with a small error of 0.14 mm (absolute error under 209.66 kHz), and 1.35 % (relative error, under low working frequencies of 142, 238, and 338 Hz) for the lift-off spacing from 1 to 15 mm.

Estimations of electrical conductivity are essential for biological object detection. This paper presents a distributed current source (DCS) based method to develop a bio-magnetic/eddy-current (Bio-M/EC) sensor for biological object detection. The DCS method directly calculates magnetic flux densities (MFDs) generated by the eddy-current induced in the biological object, which accounts for the effects of displacement current. The electromotive force (EMF) of the differential coil system is formulated in a closed-form solution to numerically investigate the effects of sweep frequency analysis applied on the Bio-M/EC sensor for biological object classification and tumor detection. The proposed method, along with a prototype of Bio-M/EC sensor, has been experimentally compared using sweep frequency analysis for detecting meat and bone.

Precision spherical joint is a spherical motion pair that can realize rotation with three degrees of freedom. This joint is widely used in robots, parallel mechanisms, and high-end medical equipment, as well as in aerospace and other fields. However, the rotation orientation and angle cannot be determined when the joint is in passive motion. The real-time determination of the rotation orientation and angle is crucial to the improvement of the motion control accuracy of the equipment where the joint is installed in. In this study, a new measurement method that utilizes eddy current sensors is proposed to identify the special features of the joint ball and realize angle measurements indirectly. The basic idea is to manufacture the specific shape features on the ball without affecting its movement accuracy and mechanical performance. An eddy current sensor array is distributed in the ball socket. When the ball head rotates, the features on the ball opposite to the sensor, as well as the output signal of every eddy current sensor, change. The measurement model that establishes the relationship between the output signal of the eddy current sensor array and the rotation direction and angle of the ball head is constructed by learning and training an artificial neural network. A prototype is developed using the proposed scheme, and the model simulation and feasibility experiment are subsequently performed. Results show that the root mean square angular error of a single axis within a range of &plusmn, 14&deg, is approximately 20 min, which suggests the feasibility of the proposed method.

Mechanical transmission systems require online health monitoring for several reasons. Gearboxes account for many of the maintenance costs due to repairs, replacements and downtime. Transmission systems feature in many applications including rotor aircraft and wind turbines. The wind energy industry since its inception has experienced high levels of failure rates. Principal gearbox design defects and structural problems have been notably significant issues of wind turbines and have had to be addressed by the wind turbine manufacturers. Reliability and safety of conventional wind turbines has improved although rates of failures are still disappointingly high. There still exists a lack of sufficient technology to enable reliable monitoring of the structural and operational conditions of wind turbines and this is currently a significant area of research. Gearbox monitoring in particular lacks sensor technology to successfully detect tooth damage, high speed and low speed shaft faults. Typically, vibration measurement and spectrum analysis are chosen for gearbox monitoring but, these methods are not able to detect the faults until failure is imminent. More recently acoustic emissions sensors are being developed for early detection of stress and surface defects. This paper presents a new concept that employs an eddy current sensor fitted to the teeth of an idler gear to detect early micro and macro pitting of the gear tooth surface. A rotating bench test has been carried out to validate the technique where simple eddy current sensors have been fitted to an idler gear. Seeded faults of three different types on an actual gear have been shown to be detectable using this technique. Eddy current sensors are used for their immunity to dust, dirt and oil contamination. Thus this technique is targeted for in-service operation where sensors have not previously been deployed to access the tooth face, flank and root shank.

In this paper, the phase gradient is taken as the characteristic quantity of defect testing signal by using the phase spectrum analysis method of spatial domain, and the phase gradient of testing signal is extracted by using the digital phase- sensitive demodulation technology to detect and classify different types of defects in the plate metal sample. Dual air core coil eddy current sensor and 1-10kHz multi-frequency excitation signal are used to identify surface defects, lower surface defects and internal defects of the sample. The experimental results show that this experimental system can effectively detect defects, and visual detection results can be obtained through phase spectrum analysis of spatial domain. The phase gradient is used as the characteristic quantity to effectively suppress the influence of lift-off effect, making the detection results more accurate.

This paper presents a novel, non-contact displacement sensor based on the eddy current sensing technique. The moving part of the sensor is a conductive sheet with a simple surface groove. This part is easy to fabricate as the machining process required is simple. The moving part is not electrically connected to the measurement system, like the eddy current proximity sensor. The displacement of the moving part is determined by the change in the inductance of four identical stationary planar coils kept underneath the moving part. A signal conditioning circuit is presented in this paper which measures the difference in inductance between two coils. A suitable algorithm has been developed to calculate the displacement using the corresponding inductance values. A prototype has been fabricated in the laboratory to evaluate the performance. The test result shows that the maximum error is less than 1.65%. The output of the prototype sensor was linear for the full range, as expected. It is well suited for application where the vertical space available for the installation is limited. The overall thickness of the developed sensor is less than 5 mm.

This paper is focused on the real-time characterization of weld in high speed high-end drawn metallic wires using encircling coil eddy current sensor. Emphasis was laid on the evaluation of weld-length and its span of stability with a special reference to signal pattern recognition through the improvised non-linear regression tool. A 3-D CIVA simulation electromagnetic testing module was implemented to evaluate the theoretical expected response of the system. The qualitative and quantitative analysis of welds was studied using signal processing of EC data. The percentage deviations in weld-span between simulated and experimental results were found to be around 1.6% and 12.5% at welding location of 1st and 8th pass respectively. This demonstrates the non- uniformity of the weld on relatively smaller diameter wire (8th pass) as compared to the larger diameter wire (1st pass). A good agreement was observed between the theoretical and experimental results. From the applications perspective, these findings indicate the importance on achieving higher efficiency combined with reliable quality control. On-line implementation of the proposed method satisfies both the requirements; detection accuracy and detection speed.

This paper describes a measurement method for the quality control of cylinders for printing machines based on roll-to-roll presses. If the surface finishing of the cylinders is not adequate, the printing is unacceptable, and the defective cylinders must be reworked. The performed quality check of the cylinder surface roughness by means of contact methods was unable to identify the cylinder defects, and acceptance of the manufactured cylinders before integration was demanded to the visual inspection performed by trained operators. In this work a contactless measurement method based on the eddy current displacement sensor was proposed and validated as a tool for quality check as an alternative to optical roughness measurements. A test bench for the characterization of printer cylinders was designed and manufactured, allowing for the validation of the proposed method on different batches of cylinders and the identification of a threshold to guide the acceptance of tested cylinders prior to mounting on the roll-to-roll press.

The creation and exploitation of gas turbine engines (GTE) often involve two mutually exclusive tasks related to ensuring the highest reliability while achieving a good economic and environmental performance of the power plant. The value of the radial clearance between the blade tips of the compressor or turbine and the stator is a parameter that has a significant impact on the efficiency and safety of the GTE. However, the radial displacements that form tip clearances are only one of the components of the displacements made by GTE elements due to the action of power loads and thermal deformations during engines’ operation. The impact of loads in conjunction with natural aging is also the reason for the wear of the GTE’s structural elements (for example, bearing assemblies) and the loss of their mechanical strength. The article provides an overview of the methods and tools for monitoring the dangerous states of the GTE (blade tips clearances, impellers and shafts displacements, debris detecting in lubrication system) based on the single-coil eddy current sensor, which remains operational at the temperatures above 1200 °C. The examples of practical application of the systems with such sensors in bench tests of the GTE are given.

Gas turbine engine health monitoring systems play an active role to ensure timely maintenance and prevention of failures. Tip-timing and tip clearance measurements form a major part of gas turbine health monitoring systems. They are used to assess turbomachinery blade vibrations using non-contact systems such as optical, capacitive, Hall effect, eddy current etc. Most of these sensors are prone to contamination, non-linearity and cannot measure both tip-timing and tip-clearance together. Eddy current sensors are found to be robust and can measure both tip-timing and tip-clearance simultaneously. They are already being used in gas turbine health monitoring systems to assess compressor and turbine blade vibrations. Apart from assessing blade vibrations, it will be quite beneficial to predict and prevent surge and stall of compressors in an engine. Surge and stall can be disastrous for an aircraft during flight as it can cause severe damage to the engine. Pressure sensors are generally used to study the variations in the inlet flow for surge and stall protection in an engine and play an important role in health monitoring. A new combined sensor that can measure tip-timing, tip-clearance and dynamic pressure was developed at the University of Oxford for use in gas turbine engine health monitoring. The combined sensor uses a pressure sensor in the centre and is enclosed by an eddy current sensor forming a compact single package. The pressure sensor used here is a fast response optical based sensor that is known to work at high temperatures and is less noisy compared to piezo based pressure sensors. The pressure sensor can also measure the steady state temperature of the casing. The combined sensor is found to be quite robust and is able to operate in harsh environments without any loss in accuracy. Due to the combined package, the space occupied is much less compared to that required by two separate sensors. The sensor has many applications that include measuring vibrations, active flow control, stall/surge of compressors etc. The paper presents the design and development of this combined sensor along with experimental results on tip timing and unsteady pressures from gas turbine engine fan blades. The engine tests included looking at the effects of squeezing the inlet casing and to study the effect of distorting the inlet flow on blade vibrations by placing varying number of bars in the inlet duct of the engine.

Large adaptive deformable mirror is becoming a trend for ground base telescope, at present almost all the large adaptive deformable mirrors in use are driven by voice coil, simultaneously the capacitive sensor is used as a feedback component. While the Eddy Current Sensor could have a lot of advantages over the capacitive one in large deformable mirror. In this research a prototype of 150mm deformable mirror is built, which is driven by 7 voice coil actuators, the ECSs are used in this prototype instead of capacitive ones. In the prototype experiment some standard low-order aberrations applied on the mirror could be corrected as expected, so it could be proved that the ECS could be used in large adaptive deformable mirror to take place of the capacitive one

Online monitoring of high-speed rotating blades has always been a hot topic. Of the various methods, the blade tip timing (BTT) technique, based on eddy current sensors, is considered to be the most promising. However, BTT signals are easily influenced by various factors, which means that the accurate extraction of BTT signals remains a challenge. To try to solve this problem, the causes of measurement error were analyzed. The three main reasons for the error were established: the variation in blade tip clearance, the interference of background noise, and the asymmetry of the blade tip shape. Further, pertinent improvement methods were proposed, and a compensation method was proposed for the errors caused by the variation of tip clearance. A new denoising and shaping algorithm based on ensemble empirical mode decomposition (EEMD) was introduced for the errors caused by background noise. Additionally, an optimal installation position of the sensor was also proposed for the errors caused by the asymmetry of the blade tip shape. Finally, simulations and experiments were used to demonstrate the improved methodology. The results show that the measurement error on vibration amplitude and vibration frequency using the proposed method is less than 2.89% and 0.17%, respectively, which is much lower than the traditional method (24.44% and 0.39%, respectively).

The article describes a hardware and software complex designed for defectoscopy and study of material properties. Sections of high-strength steel 08G2B pipes were scanned in the course of the research. The purpose of this study is to determine dependency of eddy current sensor signal and mechanical and operational properties of the base metal (K65 steel grade) and demonstrate the ability of developed eddy current probe to detect continuity defects and corrosion in this type of metal. The steel is ultra-low-carbon, which makes it possible to study their properties using the eddy current method. The eddy current method is a non-contact one being used both for cracks identification and determination of their characteristics. A special superminiature eddy current probe was developed, which includes three coils - stabilizing, measuring and exciting. The coils were placed on a pyramid-shaped core made of alloy 81NMA. The eddy-current transducer runs under the control of the developed hardware-software complex with a system for suppression of noise and amplification of signal received from the eddycurrent transducer. Signal processing makes it possible to separate the effects of closely spaced cracks on the eddy-current transducer signal and evaluate the contribution of each crack separately. As a parameter containing information about the presence of defects in the object of control, the voltage arrived at the measuring coil of the eddy current probe signal was used. The work studied sections of pipes with model continuity defects – cracks and holes. A research that allows to simulate the presence of corrosion in the metal was also conducted. The extreme values of the eddy current probe signal over defects of different types are obtained at various velocity, that allows to identify the type of defect, while changing the frequency of the signal on the probe allows to determine the depth of the defect.