High Frequency Eddy Currents Research Articles

Quantitative characterization of tensile stress in electroplated nickel coatings with a magnetic incremental permeability sensor

Quantitative characterization of tensile stress in electroplated nickel coatings on the copper substrate with the thickness range of 2.809 µm to 35.654 µm in the elastic stage was experimentally performed with a magnetic incremental permeability (MIP) sensor in the study. Firstly, a differential mutual-inductance sensor based on the parallel excitation of bifilar co-directional coil was proposed. With the proposed sensor, multiple signals including high-frequency eddy current (HFEC), tangential magnetic field strength, and magnetic incremental permeability butterfly curve (MIPBC) could be measured synchronously and eight typical magnetic parameters were obtained from experimental signals. Then, four parameters including the peak height of MIPBC, amplitude at cross point of MIPBC, difference frequency of HFEC, and sum frequency component of HFEC were selected to quantitatively characterize tensile stress. Experimental results showed that the four parameters depended on tensile stress and showed a parabolic trend. The parameters of the parabolic equation varied with nickel coating thickness. Finally, a polynomial surface fitting method based on the combination of the factors of thickness and tensile stress were derived with the selected four parameters and the coefficient of determination R2 was higher than 0.9. The further comparison between theoretical results and predicted stress confirmed that the proposed MIP sensor could realize the quantitative detection of tensile stress in different nickel coatings.

A Winding Structure of Air-Core Planar Inductors for Reducing High-Frequency Eddy Currents

High-frequency converters with high power density have shown promise in recent years. Air-core planar inductors built with printed circuit boards can eliminate problems associated with the use of magnetic materials, such as iron loss and magnetic saturation. However, conventional air-core planar inductors suffer from eddy current loss. Eddy currents affect the resistance and inductance of the inductor. This article proposes a stranded wire structure to reduce eddy currents more than that in the conventional spiral wire structure. Eddy currents can be reduced by splitting the windings as well as currents of each split winding are balanced in the proposed structure. Seven types of air-core planar inductors were built, and their resistances, inductances, and impedances were measured to evaluate the effects of different structures on eddy currents. Furthermore, the temperature distribution of the planar inductors was measured while supplying a sinusoidal voltage with a power amplifier. The experimental results demonstrate the usefulness of the proposed stranded wire structure. Furthermore, the experimental result shows that the stray capacitance of the inductor can be reduced as a by-product of the proposed stranded wire structure.

Rapid Temperature Control in Melt Extrusion Additive Manufacturing Using Induction Heated Lightweight Nozzle

An approach for improving and maintaining consistent fusion quality of the deposited material during FDM 3d-printing is proposed. This approach is based on the nozzle temperature control during the printing process to adjust the polymer extrusion temperature with a speed and accuracy adequate to the FDM process. High frequency induction heating of the lightweight nozzle (<1 g) was used. To control the temperature of a lightweight nozzle, the resonant temperature measurement method based on the analysis of the high frequency eddy currents is proposed. To determine the parameters of the nozzle and the inductor as a plant, a FEM model of the inductive heating of the nozzle and a simulated model of a serial-parallel resonant circuit containing inductor were developed. Linearization of the automatic control system was performed to ensure the equal quality of regulation when operating in a wide temperature range. The quality of regulation, stability of the system, and coefficients of the PID controller were evaluated using a simulated model of the control system. A number of test samples were printed from various materials, and tensile stress testing was carried out. The developed control method reduces the nozzle temperature control error from 20 to 0.2 °C and decreases control delay by more than six times.

Improved detection of surface defects at sample edges using high-frequency eddy current amplitude and phase measurements

ABSTRACT The detection of surface cracks at or close to sample edges is a challenging problem because the interaction of the eddy current with the sample edge can make it difficult to distinguish changes in the eddy current signal due to a defect. Samples with poor electrical conductivity such as titanium alloys used extensively in aerospace applications can be more difficult to inspect due to the low amplitude eddy currents induced in them and increased electromagnetic skin depths due to lower electrical conductivity. As fatigue surface cracks or manufacturing surface defects can often occur close to edges, the challenges of detecting small defects close to sample edges is an important research area to address. High-frequency eddy currents of over 10 MHz are used in a transmit-receive configuration using two solenoid type coils adjacent to each other. While conventional eddy current sensors are commonly designed for operating at frequencies into the low MHz region, the supporting electronics here will be positioned immediately behind the coils to improve electrical stability and reduce induced noise. The magnitude and phase of the voltage on transmit and receiver coils are measured, and finite element modelling is used to validate the experimental measurements and gain insight into the system behaviour. Small defects of down to 1 mm are easily detected, on the edge and at the corner of a titanium alloy sample with excellent signal-to-noise.

Impact of human and environmental factors on the probability of detection during NDT control by eddy currents

The Probability Of Detection (POD) curves are generally determined in laboratory conditions as the optimum case. These conditions are not the same as in a real in-service inspection. Build an experimental POD curve requires data from a wide range of inspector skills, defect types, test protocols, etc. This paper analyzes the impact of the maintenance conditions on the probability of defect detections in the High Frequency Eddy Currents (HFEC) method.After having described the experimental campaign on titanium specimens associated to HFEC method, we construct an experimental database based on three environments scenario (Laboratory, A321 and A400M aircrafts) and representative inspector pattern with different Non-Destructive Test (NDT) certification levels. Then, the POD using Berens methodology is provided for different data transformation. Comparisons between POD statistical parameters in different scenarios permit to quantify the impact of human and environmental factors. Some additional advices can be done to minimize the uncertainty on POD results.

Study of a Particle Based Films Cure Process by High-Frequency Eddy Current Spectroscopy

Particle-based films are today an important part of various designs and they are implemented in structures as conductive parts, i.e., conductive paste printing in the manufacture of Li-ion batteries, solar cells or resistive paste printing in IC. Recently, particle based films were also implemented in the 3D printing technique, and are particularly important for use in aircraft, wind power, and the automotive industry when incorporated onto the surface of composite structures for protection against damages caused by a lightning strike. A crucial issue for the lightning protection area is to realize films with high homogeneity of electrical resistance where an in-situ noninvasive method has to be elaborated for quality monitoring to avoid undesirable financial and time costs. In this work the drying process of particle based films was investigated by high-frequency eddy current (HFEC) spectroscopy in order to work out an automated in-situ quality monitoring method with a focus on the electrical resistance of the films. Different types of particle based films deposited on dielectric and carbon fiber reinforced plastic substrates were investigated in the present study and results show that the HFEC method offers a good opportunity to monitor the overall drying process of particle based films. Based on that, an algorithm was developed, allowing prediction of the final electrical resistance of the particle based films throughout the drying process, and was successfully implemented in a prototype system based on the EddyCus® HFEC device platform presented in this work. This prototype is the first solution for a portable system allowing HFEC measurement on huge and uneven surfaces.

Loss Separation Model: A Tool for Improvement of Soft Magnetic Materials

Loss separation has fundamental importance for optimizing the magnetic material for a given frequency of operation. The loss separation model assumes the existence of two main terms: one due to the hysteresis at the quasi-static situation with frequency less than 0.01 Hz and another dynamic, due to high frequency eddy currents. In this study, it is discussed the physical reasoning behind the loss separation model. Magnetic Barkhausen Noise can be a valuable tool for better understanding the physics of loss separation.

Revealing surface microcracks in metal articles by excitation of high-frequency eddy currents with subsequent infrared-camera imaging

A method has been developed for revealing microcracks in metal articles by excitation of high-frequency eddy currents with subsequent recording of the temperature relief with an infrared camera. The sensitivity, as measured by outside crack opening, is 8 μm; this corresponds to Level B of magnetic-powder testing. The method is technologically easier to implement than magnetic-powder one and can therefore replace the latter in many problems of testing.

Measuring and Imaging Permittivity of Insulators Using High-Frequency Eddy-Current Devices

This paper shows that the high-frequency eddy-current (HFEC) measurement devices can be used not only for characterizing conductivity and magnetic permeability related properties of electrically conductive materials, but also for permittivity characterization of insulators. Maxwell's equations, finite-element method simulations, and experimental research are applied to support this hypothesis. An industrial HFEC device is used to measure the change of dielectric properties during the curing process of the epoxy resin L20. The measurement results are in good agreement with the expected behavior of the parameters relative permittivity and tan δ during cure. Using a capacitive reference device, similar characteristics regarding the change of the complex permittivity of the resin can be observed. In addition, HFEC imaging results on polymethyl methacrylate are presented, discussed, and compared with capacitive imaging. HFEC permittivity mapping benefits from a high spatial resolution with a sensitivity and penetration depth that is at least comparable with those of capacitive imaging technology.

Modeling Microinductors With Thin-Film Alloy Magnetic Cores

This paper presents a compact nonlinear frequency-dependent circuit model for microinductors with magnetic thin-film cores. The model captures saturation, high-frequency eddy currents in the conductors and core, and capacitive effects. A modeling extraction algorithm is proposed that generates a passive and accurate circuit from current-biased small-signal impedance measurements. These measurements are wafer-level compatible and can readily be made using an impedance analyzer or vector network analyzer. The model is applicable to the most common microinductor topologies, including toroidal, stripline, and racetrack used for integrated power electronics applications. We demonstrate the model performance of these three different microinductor geometries with commercially available field solvers as well as actual device measurements for switched mode power supply applications. Field-solver simulations show that the model captures ac power loss significantly better than traditional models and captures the current waveforms within 4.5% error. Comparison of the model to device measurements results in an RMS error less than 7.1% in current waveform for a device transitioning in and out of saturation due to a 10-MHz rectangular pulse.

Reduced-Order Modeling of High-Fidelity Magnetic Equivalent Circuits

Magnetic components, such as inductors and transformers, enable short-term energy storage and transfer and are essential for many power electronic converters. Physics-based models of magnetic systems, such as finite element-based models and/or high-fidelity magnetic equivalent circuit (HFMEC) models, accurately represent the magnetic device. However, these models are computationally intensive and hard to formulate. In this paper, an HFMEC approach for laminated and solid magnetic cores is set forth that avoids conventional geometrical simplifications and assumptions of uniform flux density. The proposed dynamic HFMEC model accurately captures the effects of magnetic saturation, high-frequency eddy currents, corner effects, and 3-D effects. The resulting full-order HFMEC models introduce a large set of state variables. Automated linear and nonlinear order-reduction techniques are introduced to extract the desired essential system dynamics, thus preserving both model accuracy and computational efficiency. The resulting reduced-order models are validated with hardware measurements and the original full-order HFMECs in both time and frequency domains.

Conductivity profile determination by eddy current for shot-peened superalloy surfaces toward residual stress assessment

This paper demonstrates a swept high frequency eddy current (SHFEC) methodology that can determine near-surface conductivity deviation profiles of shot-peened superalloy surfaces, from which residual stress state can be assessed nondestructively. Our methodology is built around a laboratory-grade SHFEC hardware and a model-based SHFEC data inversion software, both described in this article. For the demonstration, a series of shot-peened Inconel 718 block specimens is prepared and examined by the proposed SHFEC inversion technique. The conductivity depth profiles of the samples under various shot peening intensities have been obtained by the inversion. Several sensitivity and consistency test results are given to support the reliability of the inverted conductivity profiles. The extreme near-surface regions (10–20μm) of the shot-peened surfaces are also examined by various microstructural characterization methods such as scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction, to examine our inversion results microscopically.

Surface impedances, BIEM and FEM coupled with 1D non-linear solutions to solve 3D high frequency eddy current problems

The BIEM coupled with surface impedances gives good results at a low cost for 3D high frequency eddy currents calculation with linear materials. We review the possible extensions to non-linear materials. We propose (and test in 3D) an original method. Based on 1D computations of the complex surface impedance. Results are applied to induction hardening processes. This paper presents essentially validations of the method of non-linear surface impedances.

A boundary integral equation method for high frequency eddy currents

The authors present a new method to compute the current distribution at the surface of a conducting piece in a high frequency varying field. This method uses boundary integral equation techniques and allows at a very low computing cost to define in three dimensions the hot and cold parts of such a piece before case hardening. The integral equations have to be solved only on the boundary, so the number of dimensions of the mathematical problem is reduced from three to two. Results of current distribution on the surface of a complicated shape piece as a toothed gear are given as an example.

An Analysis of Solid Rotor Machines Part IV. An Approximate Nonlinear Analysis

One of the most important problems in the theory of rotating electric machinery is that of accounting for saturation effects. This paper adapts the results of the analysis of eddy current effects in semi-infinite solid saturated iron to develop the high-frequency operational and equivalent circuit impedances of a solid rotor machine. Parts I, II, and III of this series have presented a development of the basic theory, an investigation of the effects of curvature and a consideration of end effects, respectively. 1-3 Another paper has investigated the effects of field leakage. 4 This paper utilizes the electromagnetic model of Part I, neglecting the minor effects of curvature and finite length, except that the rotor iron is assumed to have a rectangular saturation curve.1 With the aid of this model, operational-and equivalent-circuit impedance equations are developed for conditions with relatively high-frequency eddy currents wherein the air gap fields are essentially one dimensional. Per-unit numerical data are given which illustrate the theoretical effect of saturation in increasing the resistive rotor impedance components over those obtained from a linear analysis.