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Highlights Recognizing the critical demand for composite bolts and the damage‐mode complexity of composite bolt joints, this study introduces the concept of an intelligent carbon fiber reinforced polymer (CFRP) bolt, which achieves self‐monitoring of damage in small and complex CFRP bolted joints. The proposed eddy current sensor (ECS) is mainly composed of a coil made of carbon nanotube film and encapsulated with epoxy resin film. This achieves homogeneous embedding of the sensor by using the same material for both sensor and bolt. The CFRP bolt is fabricated by molding process, with the proposed ECS embedded in bolt shank. The numerical simulation results indicate that hole‐edge damages impact the distribution of induced current density and eddy current flow in CFRP plates. Additionally, the impedance amplitude of the ECS effectively monitors bearing damage propagation. Shear tests confirm that the integration method of the proposed ECS within the CFRP bolt introduces negligible intrusion to the host bolt's integrity. Experimental results demonstrate that the intelligent CFRP bolt can effectively monitor the damage propagation at the hole edge, with an accuracy for bearing damage exceeding 0.5 mm. At a working frequency of 10 MHz, the sensor's sensitivity in detecting a 0.5 mm bearing damage reaches 4.20%. An intelligent CFRP bolt was fabricated using the molding method. The embedding of the ECS did not cause adverse effects on the bolt. The proposed EC sensor is composed of carbon nanotube and epoxy resin adhesive films. The eddy current distribution at the hole edge of the CFRP bolted joints is analyzed. The ability of intelligent bolts to monitor hole edge damage is verified by experiments. [ABSTRACT FROM AUTHOR]

A displacement sensor with smaller volume and lower cost is needed for magnetic suspension control system. A transverse flux displacement sensor based on eddy current sensors is studied. The sensor analysis is based on its working principle. The parameters of excitation coil and induction coil are designed by particle swarm optimization, and the design process of the transverse flux sensor is simplified. The developed displacement sensor has a sensitivity of 7.2 V/mm, a linearity of 1.7%, and a detection range of ±0.5 mm. The sensor is applied in a magnetic levitation permanent magnet synchronous motor system. When the motor speed is 20 000 r/min, the high‑speed magnetic levitation rotor still maintains stable operation. The transverse flux displacement sensors developed in this paper can meet the requirements of magnetic suspension motor displacement detection and suspension control, and achieve the control effect of similar products. [ABSTRACT FROM AUTHOR]

Eddy current sensors are increasingly being used to measure the dynamic blade tip clearance in turbines due to their robust anti-interference capabilities and non-contact measurement advantages. However, the current research primarily focuses on enhancing the performance of eddy current sensors themselves, with few studies investigating the influence of turbine rotor parameters on the measurements taken by these sensors for dynamic blade tip clearance. Hence, this paper addresses this gap by using COMSOL Multiphysics 6.2 software to establish a finite model with circuit interfaces. Additionally, the model's validity was verified through experiments. This model is used to simulate the voltage output of the sensor and the measurement of dynamic blade tip clearance under various rotor parameters. The results indicate that the length and number of blades, as well as the hub radius, significantly affect the sensor voltage output in comparison to rotation speed. Furthermore, we show that traditional static calibration methods are inadequate for measuring dynamic blade tip clearance using eddy current sensors. Instead, it is demonstrated that incorporating rotor parameters into the calibration of eddy current sensors can enhance the accuracy of dynamic blade tip clearance measurements. [ABSTRACT FROM AUTHOR]

A new eddy current sensor with planar coils designed for noncontact distance measurement was applied in the inter electrode gap detection in the electrochemical machining (ECM). The sensor consists of two micro electroforming planar coils (driver coil and sensitive coil) stacked on the magnetic core. The magnetic model of the detection principle was built, and a new micro motion system was developed as a test and machining platform also. The size of the sensor was: Φ5 mm × 1.5 mm. The results show that the sensor is compact enough to nondestructively assess the electrode gap distance and could be easily integrated into a tool cathode for real-time electrolytic machining gap detection. [ABSTRACT FROM AUTHOR]

Bolt links and riveted joints, although widely used in aerospace and other mechanical systems, are susceptible to stress concentrations and fatigue cracks, which are undesirable. The flexible rosette eddy current sensor is typically used for the structural monitoring of bolt connections and riveted joints. This study proposes a method to improve the sensitivity of a flexible rosette eddy current sensor by reducing the cloud zenith angle of the pickup coil. First, the effect of the cloud zenith angle upon the sensitivity of the flexible rosette eddy current sensor is analyzed theoretically; the results show that a larger cloud zenith angle corresponds to a larger background signal. This diminishes the relative rate of change of the crack signal when the sensitivity of the sensor is evaluated based upon the maximum rate of change of the output. Therefore, the sensitivity is optimized by reducing the cloud zenith angle. Subsequently, through simulation and experimental methods, the relationship between sensor sensitivity and cloud zenith angle was studied. The results indicate that the cloud zenith angle significantly affects the sensitivity and that reducing the cloud zenith angle effectively reduces the background and improves the sensitivity. [ABSTRACT FROM AUTHOR]

In this research, an experimental study is presented, which extends the usage of eddy current and load sensors in progressive stamping tools to optimize and continuously monitor the stamping process. The purpose of the research was to automatically detect material scrap before it leaves imprints on the part and based on the sensor’s readings, determine the optimal tool bottom position. The scrap thickness that needs to be detected was established in an experiment by visual evaluation of the result. To determine the optimal bottom position, a linear regression method was used, and the results were evaluated by part quality parameter. The research results consist of separate detection steps and the conclusion was made only after the serial type production. Overall results of scrap detection were influenced by the design of the existing tool. The bottom position detection consists of various readings interpretations and multi-step method descriptions. Based on the acquired results of both methods, implementing the in–die sensors was considered successful and applicable to new tools. [ABSTRACT FROM AUTHOR]

The aim of this article is to determine a similar point to the Lift-Off intersection point (LOI) obtained from a geometric configuration of a Pulsed Eddy Current (PEC) sensor. This configuration enables easy determination of the LOI point through a single measurement, thus avoiding measurement noise. The sensor consists of an external excitation coil and two internal probes that overlap and have similar characteristics for reception. The similar point LOI is determined by the intersection of the signals from the transient response of the sensor's internal probes. Importantly, this point is independent of lift-off variation. Moreover, the amplitude and time measurement of this point vary with the variation of the physical and geometric properties of the sample. To assess the effectiveness of this similar point, a numerical code based on electromagnetic phenomena was developed. This code investigates the impact of thickness and electrical conductivity of metal sheets on the response of the PEC sensor, both with and without a ferrite core at this point. Additionally, a practical sensor prototype was constructed to validate the results obtained from the numerical model. The obtained results demonstrated a strong correlation between the numerical and experimental outcomes, thus confirming the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

This paper provides a comparative analysis between a force sensor and an eddy current sensor, focusing on their usability to determine the position of a circular levitating permanent magnet (PM) mover within an axially symmetric magnetic levitation platform (MLP) with an exceptionally large air gap. The sensors enable closed-loop control, which is essential for accurately and stably maintaining the mover's radial position. For the considered MLP, a change in radial position in principle results in a tilting of the mover, i.e., a deviation from the parallel alignment relative to the stator. As both the radial position and the tilting angle affect the sensors' (force and eddy current) output voltage, an observer must deduce the radial position from the output sensor's voltage, requiring a comprehensive MLP dynamic model and calibration of the models for both sensing approaches. The paper discusses the advantages and weaknesses of each sensor concept, exploring operational principles and performance in levitation tests. The force sensor exhibits versatility, proving functional across various application scenarios, such as when the mover is sealed in a conductive, non-magnetic chamber. In contrast, due to its high-frequency operation, the eddy current sensor is more straightforward to characterize, simplifying its behavior relative to the mover's slower dynamics. Measurements are conducted to validate the models, showing the eddy current sensor's robustness against disturbances and imperfections in the MLPs and its immunity to cross-axis interference. Conclusively, in levitation experiments where the mover is vertically distanced at 104 mm from the stator, the eddy current sensor achieves a position tracking precision about ten times better and a signal-to-noise ratio (SNR) ten times higher compared to the off-the-shelf force sensor, confirming its better performance and reliability; however, it cannot be used in applications where conductive objects are present in the air gap. Furthermore, additional experiments are conducted on the MLP using the eddy current sensor to show the controller's robustness and dynamic reference tracking capability, with and without a payload. [ABSTRACT FROM AUTHOR]

To measure the micro-displacement reliably with high precision, a single-ended eddy current sensor based on temperature compensation was studied in detail. At first, the principle of the eddy current sensor was introduced, and the manufacturing method of the probe was given. The overall design plan for the processing circuit was induced by analyzing the characteristics of the probe output signal. The variation in the probe output signal was converted to pulses with different widths, and then it was introduced to the digital phase discriminator along with a reference signal. The output from the digital phase discriminator was processed by a low-pass filter to obtain the DC component. At last, the signal was amplified and compensated to reduce the influence of temperature. The selection criteria of the frequency of the exciting signal and the design of the signal conditioning circuit were described in detail, as well as the design of the temperature-compensating circuit based on the digital potentiometer with an embedded temperature sensor. Finally, an experimental setup was constructed to test the sensor, and the results were given. The results show that nonlinearity exists in the single-ended eddy current sensor with a large range. When the range is 500 μm, the resolution can reach 46 nm, and the repeatability error is ±0.70% FR. Within the temperature range from +2 °C to +58 °C, the voltage fluctuation in the sensor is reduced to 44 mV after temperature compensation compared to the value of 586 mV before compensation. The proposed plan is verified to be feasible, and the measuring range, precision, and target material should be considered in real-world applications. [ABSTRACT FROM AUTHOR]

Eddy current sensors are increasingly being used to measure the dynamic blade tip clearance in turbines due to their robust anti-interference capabilities and non-contact measurement advantages. However, the current research primarily focuses on enhancing the performance of eddy current sensors themselves, with few studies investigating the influence of turbine rotor parameters on the measurements taken by these sensors for dynamic blade tip clearance. Hence, this paper addresses this gap by using COMSOL Multiphysics 6.2 software to establish a finite model with circuit interfaces. Additionally, the model’s validity was verified through experiments. This model is used to simulate the voltage output of the sensor and the measurement of dynamic blade tip clearance under various rotor parameters. The results indicate that the length and number of blades, as well as the hub radius, significantly affect the sensor voltage output in comparison to rotation speed. Furthermore, we show that traditional static calibration methods are inadequate for measuring dynamic blade tip clearance using eddy current sensors. Instead, it is demonstrated that incorporating rotor parameters into the calibration of eddy current sensors can enhance the accuracy of dynamic blade tip clearance measurements.

It is shown that when performing surgical operations to remove foreign metal particles from the human body, it is advisable to use metal detectors of various types: flux-gate detectors for localizing ferromagnetic particles and eddy current detectors for localizing nonferromagnetic metal particles. The sensitivity of medical equipment must be sufficient to detect small ferromagnetic fragments and particles from a distance of at least 10 mm. The feasibility of preliminary magnetization of the search area with a strong permanent magnet is shown. Methods for setting up metal detectors are given. The need to minimize extraneous electromagnetic fields in the operating room is shown. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Shanghai Jiaotong University (Science) is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper provides a comparative analysis between a force sensor and an eddy current sensor, focusing on their usability to determine the position of a circular levitating permanent magnet (PM) mover within an axially symmetric magnetic levitation platform (MLP) with an exceptionally large air gap. The sensors enable closed-loop control, which is essential for accurately and stably maintaining the mover’s radial position. For the considered MLP, a change in radial position in principle results in a tilting of the mover, i.e., a deviation from the parallel alignment relative to the stator. As both the radial position and the tilting angle affect the sensors’ (force and eddy current) output voltage, an observer must deduce the radial position from the output sensor’s voltage, requiring a comprehensive MLP dynamic model and calibration of the models for both sensing approaches. The paper discusses the advantages and weaknesses of each sensor concept, exploring operational principles and performance in levitation tests. The force sensor exhibits versatility, proving functional across various application scenarios, such as when the mover is sealed in a conductive, non-magnetic chamber. In contrast, due to its high-frequency operation, the eddy current sensor is more straightforward to characterize, simplifying its behavior relative to the mover’s slower dynamics. Measurements are conducted to validate the models, showing the eddy current sensor’s robustness against disturbances and imperfections in the MLPs and its immunity to cross-axis interference. Conclusively, in levitation experiments where the mover is vertically distanced at 104 mm from the stator, the eddy current sensor achieves a position tracking precision about ten times better and a signal-to-noise ratio (SNR) ten times higher compared to the off-the-shelf force sensor, confirming its better performance and reliability; however, it cannot be used in applications where conductive objects are present in the air gap. Furthermore, additional experiments are conducted on the MLP using the eddy current sensor to show the controller’s robustness and dynamic reference tracking capability, with and without a payload.

To measure the micro-displacement reliably with high precision, a single-ended eddy current sensor based on temperature compensation was studied in detail. At first, the principle of the eddy current sensor was introduced, and the manufacturing method of the probe was given. The overall design plan for the processing circuit was induced by analyzing the characteristics of the probe output signal. The variation in the probe output signal was converted to pulses with different widths, and then it was introduced to the digital phase discriminator along with a reference signal. The output from the digital phase discriminator was processed by a low-pass filter to obtain the DC component. At last, the signal was amplified and compensated to reduce the influence of temperature. The selection criteria of the frequency of the exciting signal and the design of the signal conditioning circuit were described in detail, as well as the design of the temperature-compensating circuit based on the digital potentiometer with an embedded temperature sensor. Finally, an experimental setup was constructed to test the sensor, and the results were given. The results show that nonlinearity exists in the single-ended eddy current sensor with a large range. When the range is 500 μm, the resolution can reach 46 nm, and the repeatability error is ±0.70% FR. Within the temperature range from +2 °C to +58 °C, the voltage fluctuation in the sensor is reduced to 44 mV after temperature compensation compared to the value of 586 mV before compensation. The proposed plan is verified to be feasible, and the measuring range, precision, and target material should be considered in real-world applications.

Conventional eddy-current sensors have the advantages of being contactless and having high bandwidth and high sensitivity. They are widely used in micro-displacement measurement, micro-angle measurement, and rotational speed measurement. However, they are based on the principle of impedance measurement, so the influence of temperature drift on sensor accuracy is difficult to overcome. A differential digital demodulation eddy current sensor system was designed to reduce the influence of temperature drift on the output accuracy of the eddy current sensor. The differential sensor probe was used to eliminate common-mode interference caused by temperature, and the differential analog carrier signal was digitized by a high-speed ADC. In the FPGA, the amplitude information is resolved using the double correlation demodulation method. The main sources of system errors were determined, and a test device was designed using a laser autocollimator. Tests were conducted to measure various aspects of sensor performance. Testing showed the following metrics for the differential digital demodulation eddy current sensor: nonlinearity 0.68% in the range of ±2.5 mm, resolution 760 nm, maximum bandwidth 25 kHz, and significant suppression in the temperature drift compared to analog demodulation methods. The tests show that the sensor has high precision, low temperature drift and great flexibility, and it can instead of conventional sensors in applications with large temperature variability. [ABSTRACT FROM AUTHOR]

Nowadays, 3D eddy current nondestructive characterization of crack and corrosion defects while using ECA remains an industrial challenge because the obtained image permits to determine only the 2D defect shape. Consequently, this article is devoted to determine directly the crack length and width by eddy current images through sensor array. Afterwards, we extract the maximal impedance amplitude to estimate the crack depth while using the deterministic algorithm that we have recently developed. In fact, the obtained results have demonstrated the effectiveness and the reliability of the proposed method. [ABSTRACT FROM AUTHOR]

This study proposes a force plate with a planar spring and an eddy current displacement sensor to measure the ground reaction force (GRF) of a small insect and reveal its motion characteristics. The proposed force plate comprises a circular aluminum plate, four aluminum springs symmetrically connected to the plate, and an eddy current displacement sensor under the plate. The diameter and thickness of the fabricated plate were 8 and 0.1 mm, respectively. The spring width was 0.4 mm. When a force is applied to the plate, the plate moves vertically downward. Then, an eddy current displacement sensor detects the plate displacement without contact. The applied force can be measured using Hooke's law. The proposed force plate has the advantages of ease of fabrication and cost-effectiveness. The central displacement variation and resonant frequency of the designed springs were evaluated by simulation. Then, we calibrated the fabricated force plate to obtain the sensitivity variation and resonant frequency. The experimental results suggest that the proposed force plate can effectively measure the GRF of a small insect. [ABSTRACT FROM AUTHOR]

End-point detection (EPD) is important for controlling the chemical mechanical polishing process to prevent underpolishing and overpolishing. One EPD method using the relative current density on a Cu wafer involves an eddy current, which has advantages of noncontact and a fast response speed. However, the eddy current method has a large mean error at a low relative current density. To overcome the disadvantage, composition signal filtration uses a high mean error and long delay time using a moving average and 1-D Kalman filtration. After applying composition filtration, the mean error improved by approximately 94.7 %. Still, it is difficult to measure a thickness of 4000 Å or less with existing systems. A digital potentiometer was developed that changed the resistance in real time and improved the detection range. The step number for changing the resistance was 120 steps in the resistance range of 100–5000 Ω at approximately 40 Ω/step. When a digital potentiometer was applied, the EPD detection range improved by 85.8 % over the existing system. In other words, the developed system could measure a Cu wafer thickness of 586 Å. [ABSTRACT FROM AUTHOR]

Eddy current sensors are an attractive choice due to their high resolution, reliability, and durability in harsh environments while being able to measure in a noncontact manner. This article presents a novel design to realize a thin eddy-current-based angle sensor. It is realized by converting the shaft, whose rotation angle is to be measured, into the sensing element. The modification to the shaft is minimal; a small surface groove is introduced without affecting the mechanical strength. The stationary part of the sensor consists of two layers of flexible square-planar coils. Depending on the angular position of the shaft, the inductances of the planar coils get modified. These are measured using a specially designed circuitry, optimized for this sensor. The output for the entire circle range (360°) is derived from the inductance values of each coil using a successive approximation algorithm developed for this purpose. Finite-element analysis was employed to design the sensor and analyze the axial/radial misalignment of the rotor. A sensor prototype was built and tested. The output showed a resolution of 0.1° and the worst case linearity error of 0.9%. The prototype sensor dimensions are designed to fit in a standard steering column. The proposed sensor is thin, easy to manufacture at low cost, tolerant to axial vibration by design, and has a 360° sensing range.

Position sensing is one of the crucial parts of the active magnetic bearing (AMB) system. The printed circuit board (PCB) eddy current position sensor is a new type of position sensor for the AMB system, which makes a compact structure, high sensing quality, and is low cost. In this article, an improved driver circuit is proposed for this new sensor. The driver circuit includes an excitation circuit and signal conditioning circuits. A crystal oscillator circuit with a power stage is used to provide the excitation coil with a stable excitation source of high stability and good precision of frequency. The analog demodulation circuit is designed for signal conditioning circuits to extract the rotor displacement information from the sensing coil outputs. Compared with the state-of-art driver schemes, the proposed method reduces the circuit complexity and cost. Accordingly, the experimental results show that the designed sensor has good linearity and sensitivity, and it can ensure AMB stable operation at the rated speed. [ABSTRACT FROM AUTHOR]

In the radial displacement measurement of a small-sized cylindrical target, coupling interference between eddy current sensors reduces the accuracy of the measurement. In this study, finite element method (FEM) simulation based on ANSYS Maxwell was adopted to investigate the relationships between the coupling coefficient of the sensors and different parameters including the lift-off, cylinder diameter, axis angle, material, and excitation frequency. The experimental results were consistent with the simulation results. The coupling interference between the sensors increased with the decrease in the lift-off and cylinder diameter. The coupling effect decreased significantly when the probe axis angle increased to 120°, and the decrease in the sensor sensitivity was acceptable. A polynomial fitting function fitted the output signal well. A compensation method was given based on the compensation necessity evaluation. The results showed that the error of the rotor motion track was significantly decreased after compensation. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Conventional eddy-current sensors have the advantages of being contactless and having high bandwidth and high sensitivity. They are widely used in micro-displacement measurement, micro-angle measurement, and rotational speed measurement. However, they are based on the principle of impedance measurement, so the influence of temperature drift on sensor accuracy is difficult to overcome. A differential digital demodulation eddy current sensor system was designed to reduce the influence of temperature drift on the output accuracy of the eddy current sensor. The differential sensor probe was used to eliminate common-mode interference caused by temperature, and the differential analog carrier signal was digitized by a high-speed ADC. In the FPGA, the amplitude information is resolved using the double correlation demodulation method. The main sources of system errors were determined, and a test device was designed using a laser autocollimator. Tests were conducted to measure various aspects of sensor performance. Testing showed the following metrics for the differential digital demodulation eddy current sensor: nonlinearity 0.68% in the range of ±2.5 mm, resolution 760 nm, maximum bandwidth 25 kHz, and significant suppression in the temperature drift compared to analog demodulation methods. The tests show that the sensor has high precision, low temperature drift and great flexibility, and it can instead of conventional sensors in applications with large temperature variability.

Force plates have been widely used in various scientific fields to measure ground reaction forces experienced by animals and humans. A force plate that incorporates mechanical springs and non-contact displacement sensors offers the advantages of facilely modifying its shape and performance. However, the design protocol was limited to adjusting the plate size and spring constants of the springs supporting the plate at the four corners. Herein, we propose an optimization method for determining the spring constant distribution of a force plate. The proposed design enables the use of simulations to optimize the mechanical properties of the force plate, such as resonant frequency and error due to position. The experimental results demonstrated that the resonant frequency of the fabricated force plate was 133 Hz and the error due to position was within ±5 %. These results confirm that the proposed force plate can be effectively adjusted through optimization. [Display omitted] • Optimization method for spring constant distribution is proposed for a force plate. • The fabricated force plate realizes a resonant frequency of 133 Hz, error due to position within ±5%. • The ground reaction force measured by the fabricated force plate matches that of a commercially available force plate. [ABSTRACT FROM AUTHOR]

In this research, the design of non-destructive eddy current evaluation systems was improved using Harris Hawks optimization (HHO). To determine the orientation of cracks from eddy current data, a numerical approach was developed based finit elements methode FEM and on an iterative inversion technique that minimizes a non-linear least squares objective function that evaluates the overall difference between forecasts and observationsin axisymmetric model. In this paper the HHO optimization and steepest descent methods were compared and studied in order to minimize the objective function. To measure the precision of the estimated crack orientation, the lower bounds of the HHO optimization on crack orientation were obtained. This work explores the concept of an HHO algorithm that searches for the optimal solution within a large solution space for a specific crack orientation problem.The algorithm starts its search at any point in the solution space and looks for better solutions by searching in the "local" area surrounding the start point. The algorithm ends when it finds what it believes to be the best solution or when predetermined stopping criteria are met. • The damage (micro-cracks) is detected by NDT-EC using 2D-Axisymmetric FEM. • The effect of micro-crack orientation was studied using an eddy current test. • The proposed HHO algorithm is tested for crack-orientation detection. • The performance of HHO is also examined on several parametric problems. [ABSTRACT FROM AUTHOR]

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.

Lift-off can cause significant errors in the measurement of metallic ball thickness using the eddy current method. In this paper, a triple-coil sensor, which is composed of two receiving coils and one excitation coil, is applied to address the problem of lift-off variation. There exists a peak frequency feature, which is the frequency where the peak value of the imaginary part of the inductance change occurs. The difference in peak frequencies of two receiving coils is linearly proportional to the thickness of the metal balls, with the advantage of reducing the effect of the lift-off during the measurements. The simulation results agree well with the experimental results and therefore a look-up table can be built for different thicknesses by simulation and the actual thickness can be found through comparing the measured results with the look-up table. This thickness measurement approach is effective and the error of the measurement is within 6.1% based on the experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study proposes a force plate with a planar spring and an eddy current displacement sensor to measure the ground reaction force (GRF) of a small insect and reveal its motion characteristics. The proposed force plate comprises a circular aluminum plate, four aluminum springs symmetrically connected to the plate, and an eddy current displacement sensor under the plate. The diameter and thickness of the fabricated plate were 8 and 0.1 mm, respectively. The spring width was 0.4 mm. When a force is applied to the plate, the plate moves vertically downward. Then, an eddy current displacement sensor detects the plate displacement without contact. The applied force can be measured using Hooke’s law. The proposed force plate has the advantages of ease of fabrication and cost-effectiveness. The central displacement variation and resonant frequency of the designed springs were evaluated by simulation. Then, we calibrated the fabricated force plate to obtain the sensitivity variation and resonant frequency. The experimental results suggest that the proposed force plate can effectively measure the GRF of a small insect.

In the radial displacement measurement of a small-sized cylindrical target, coupling interference between eddy current sensors reduces the accuracy of the measurement. In this study, finite element method (FEM) simulation based on ANSYS Maxwell was adopted to investigate the relationships between the coupling coefficient of the sensors and different parameters including the lift-off, cylinder diameter, axis angle, material, and excitation frequency. The experimental results were consistent with the simulation results. The coupling interference between the sensors increased with the decrease in the lift-off and cylinder diameter. The coupling effect decreased significantly when the probe axis angle increased to 120°, and the decrease in the sensor sensitivity was acceptable. A polynomial fitting function fitted the output signal well. A compensation method was given based on the compensation necessity evaluation. The results showed that the error of the rotor motion track was significantly decreased after compensation.

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' 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The reliable classification of coin sets from different countries and counterfeit coins is an essential means of securing cash circulation. Especially bimetallic coins, such as 2-Euro coins, are often subjected to counterfeit by mixing them with coins of other countries or by imitations. In this article, a real-time embedded sensor system is proposed, based on inductance spectroscopy to characterize and identify bimetallic coins having similar geometric properties and looking similar at a first view. The system is based on inductance spectroscopy varying the penetration depth of the magnetic field in the bimetallic structure of the coin, which generally contains buried layers of other metals. The experimental evaluation shows that the bimetallic coins from different countries can be identified and classified by the use of the support vector machine, a machine learning algorithm. Experimental results showed that the system reached a classification accuracy up to 100% within a response rate of 36.62 ms per five bimetallic coins. [ABSTRACT FROM AUTHOR]

A suitable framework for the development of artificial neural networks is important because it decides the level of accuracy, which can be reached for a certain dataset and increases the certainty about the reached classification results. In this paper, we conduct a comparative study for the performance of four frameworks, Keras with TensorFlow, Pytorch, TensorFlow, and Cognitive Toolkit (CNTK), for the elaboration of neural networks. The number of neurons in the hidden layer of the neural networks is varied from 8 to 64 to understand its effect on the performance metrics of the frameworks. A test dataset is synthesized using an analytical model and real measured impedance spectra by an eddy current sensor coil on EUR 2 and TRY 1 coins. The dataset has been extended by using a novel method based on interpolation technique to create datasets with different difficulty levels to replicate the scenario with a good imitation of EUR 2 coins and to investigate the limit of the prediction accuracy. It was observed that the compared frameworks have high accuracy performance for a lower level of difficulty in the dataset. As the difficulty in the dataset is raised, there was a drop in the accuracy of CNTK and Keras with TensorFlow depending upon the number of neurons in the hidden layers. It was observed that CNTK has the overall worst accuracy performance with an increase in the difficulty level of the datasets. Therefore, the major comparison was confined to Pytorch and TensorFlow. It was observed for Pytorch and TensorFlow with 32 and 64 neurons in hidden layers that there is a minor drop in the accuracy with an increase in the difficulty level of the dataset and was above 90% until both the coins were 80% closer to each other in terms of electrical and magnetic properties. However, Pytorch with 32 neurons in the hidden layer has a reduction in model size by 70% and 16.3% and predicts the class, 73.6% and 15.6% faster in comparison to TensorFlow and Pytorch with 64 neurons.

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. [ABSTRACT FROM AUTHOR]

The study of 3D eddy current non destructive testing system for cracks characterization using finite element method requires a great amount of computing time and memory space. In this article, we have validated the developed model and then determined directly the crack length by analyzing the complete signal. Afterwards, we have extracted from the complete sensor sweep signal the maximal amplitude that we have exploited to estimate the crack depth. [ABSTRACT FROM AUTHOR]

This paper presents the design and development of a novel, linear, eddy current-based noncontact angle sensor with 360° range. Although the eddy current proximity sensors are known for high reliability, resolution, and insensitivity to moisture and oil, the approach has not been exploited to develop full-circle range angle sensors with such features. The proposed sensor consists of a rotary conducive hollow tube (e.g., made of aluminum) part and a stationary part with four identical flexible coils. The tube has a simple but special groove. The position of the groove modifies the value of the inductance of the stationary coil as a function of the sensing angle. The design is such that the inductance of each coil varies linearly for a specified range of the angle. From this piecewise linear characteristic, an output that is linear for the full-circle range is obtained using a simple yet effective algorithm. A suitable signal conditioning circuit is developed to obtain an output that is proportional to the change in the inductance of the coil. In order to optimize the design, first, the sensor structure was studied using finite-element analysis. Then, a prototype of the sensor was built and tested in the laboratory. The prototype sensor has a resolution of 0.08° and a maximum nonlinearity of 0.25%. The possible sources of error of the sensor have been analyzed and quantified. The sensor has no electrical contact to the rotary part and its output is immune to moisture, dust, and oil. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

With the development of power plants towards high power and intelligent operation direction, the vibrations or failures of blades, especially the last stage blades in steam turbines, happen more frequently due to the unstable operating conditions brought by flexible operation. A vibration measuring method for the shrouded blades of a steam turbine based on eddy current sensors with high frequency response is proposed, meeting the requirements of non-contact heath monitoring. The eddy current sensors produce the signals which are related to the area changing of every blade's shroud resulting from the rotation of stator. Then an improved blade tip timing (BTT) technique is proposed to detect the vibrations of shrouded blades by measuring the arrival time of each area changing signal. A structure of eddy current sensors is developed in steam turbines and an amplitude modulation/demodulation circuit is designed to improve the response bandwidth up to 250 kHz. Vibration tests for the last stage blades of a steam turbine were carried out and the results validate the efficiency of the improved BTT technique and the high frequency response of the eddy current sensors presented. [ABSTRACT FROM AUTHOR]

This paper presents an axial flux eddy current sensor with a compact and simple structure for measuring the rotating speed of iron shafts. The sensor structure is optimized for high sensitivity with a novel configuration of the coils positioned perpendicularly to each other. The sensor comprises two D-shaped excitation coils and two D-shaped pick-up coils in a double-layer structure. A disc-shaped magnetic shield or core shields the coils. Two iron shafts with different material properties are considered. A new cup-shaped configuration of nonmagnetic copper and aluminum caps mounted on iron shafts improves sensitivity and suppresses susceptibility to shaft material properties and airgap variation. 2D and 3D finite element methods are utilized for the performance analysis of the sensor. The measurements are performed at speeds up to ± 3000 rpm and different excitation frequencies from 400 Hz to 4 kHz. The eddy current speed sensor has excellent linearity characteristics with a nonlinearity error of 0.15%. The geometry of the coils is further optimized and improved for maximum sensitivity and compactness. The fault-tolerant capability of the sensor is also evaluated. [Display omitted] • Axial flux eddy current speed sensor for rotating speed measurement. • Novel configuration of perpendicular coils. • Optimization of the eddy current speed sensor. • 2D and 3D finite element method analysis. [ABSTRACT FROM AUTHOR]

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.

Eddy-current sensors are the most commonly used position sensors in a magnetic bearing application. Most commercial eddy current sensor outputs analog voltage of 0–2 V or 0–5 V in their measurement range. Such eddy current sensors are quite expensive and are in the price range of $2500 (USD). This paper presents a design of a $20 (USD) low-cost eddy current sensor capable of resolving displacements of 7 micron which is suitable for high-speed position control in active magnetic bearings. A novel method of directly converting the time period of the square wave signal from the eddy current sensor output for digital servo position feedback is employed in this design. This paper presents a scheme to implement digital position servo control algorithm on Texas Instruments C2000 real-time microcontroller to control the position of magnetically levitated object by controlling the magnetic actuator's coil current. The magnetic actuators are electromagnetic coils that generate the magnetic force in the bearing. [ABSTRACT FROM AUTHOR]

Lift-off of sensor affects the prediction of electromagnetic properties for both ferrous and non-ferrous steel plates. In this paper, we developed a strategy to address this issue for ferrous plates. With increased lift-off, the phase of the measured impedance for steel plates reduces. Meanwhile, the magnitude of the impedance signal decreases. Based on these facts, a phase compensation algorithm is developed which corrects the phase change due to lift-off considering the magnitude of the impedance signal. Further, a new magnetic permeability prediction technique is presented, which has been validated by analytical and measured results. With this new technique, the error in permeability prediction is less than 2% within the range of lift-offs tested. [ABSTRACT FROM AUTHOR]