Pair Of Coils Research Articles

Reducing crosstalk in optically-pumped magnetometer arrays

Optically pumped magnetometers (OPMs) operating in the spin-exchange relaxation-free regime are emerging as alternative sensors to superconducting quantum interference devices (SQUIDs) for magnetoencephalography (MEG). As the number of OPMs in a single imaging system increases to rival SQUID MEG systems, cross-talk between nearby sensors limits the measurement accuracy. We experimentally demonstrate a coil geometry, which generates an order of magnitude less cross-talk (less than 0.5%) than a Helmholtz coil (8%). The new coil design is simple and compact, requiring two coaxial coil pairs that add 1 mm to the 6 mm radius and is driven by a single current driver. The new design maintains a magnetic field homogeneity over the volume of the magnetometer of more than 94%, which is sufficient for the zero-field OPM to operate in a 200 nT ambient field environment. Our result increases the feasibility of high-spatial resolution OPM-based bio-magnetic imaging technology due to the reduction of cross-talk at high sensor density.

Effect of low frequency magnetic field on efficiency of chromosome break repair

ABSTRACTDNA repair is essential to maintain genome integrity. There is scientific evidence that exposure to magnetic fields (MF) can produce alterations in DNA repair without clear conclusions. This work aims to study the cellular response to and repair of a very deleterious type of DNA damage, the DNA double strand break (DSB), in S. cerevisiae, under MF exposure. In S. cerevisiae cells, pairs of DSB were induced enzymatically by HO endonuclease by plating the cells on Galactose-containing media. The repair processes took place under exposure to a 50Hz, 2.45mT sinusoidal MF during 21 days. MF was generated by a pair of Helmholtz coils. MF induced 1.29- and 1.5-fold increase in the number of colonies grown at day 21 of exposure in relation to untreated controls for Pho91 and Rmd5 strain, respectively. In relation to the kinetics of DSB repair during MF exposure, a higher increase (55.56-fold) in DNA reparation was observed at day 15 for Rmd5 strain in relation to the slight increment (1.18-fold) found for Pho91 strain. The results suggest that long-term MF exposure could increase the DNA repair activity and there may be a relationship between the position of the DSB and the distance to the centromere.

Feasibility of magnetic susceptibility tomography for shallow targets

We present an approach to obtain images of magnetic susceptibility of the subsurface using as data the surface magnetic fields of small dipole sources. The approach relies on measurements taken with the instrument known as EM-38 MK2. This device operates with a pair of coils, one operating as the source and the other as the receiver. Usually four measurements are taken, vertical magnetic dipoles at 0.5 and 1.0 m separations, and horizontal magnetic dipoles at the same two separations. To improve depth discrimination we introduce two new arrays using the same instrument but rotated vertically from its usual horizontal lay out. The different configurations are contrasted by analyzing the subsurface signatures generated by elementary ground contributions, in the way it is currently done for resistivity arrays. We model subsurface objects using continuous variations by assuming that they are a superposition of Hann functions. For the inverse problem the superposition is such that we can model uniform bodies with rounded boundaries. We use quadratic programming for optimizing the fit to the data with only positive magnetic susceptibilities, and standard Tikhonov regularization for stabilizing the models. To deal with the non-uniqueness of the inverse problem we compute the shallowest and deepest models that fit the data. This is accomplished by a simple modification of the standard objective function for regularized optimization. We present simulations with synthetic data to demonstrate the viability of the approach. A case with field data also demonstrates the feasibility of magnetic susceptibility tomography for shallow targets.

A capacitive-inductive dual modality imaging system for non-destructive evaluation applications

Abstract For many non-destructive evaluation (NDE) applications, more information can be obtained by using different techniques, especially when the techniques are sensitive to different types of defects. However separate inspections are not always practical due to time and cost constraints. Therefore, an inspection system combing more than one modalities would have many advantages. A dual modality imaging system is thus proposed which can automatically switch between capacitive imaging and inductive imaging modes. Instead of using a physical combination of two sensors, the proposed system employs a coplanar coil pair as a sensor, and the modality switching is done by changing the wiring schemes through program-controlled switch box. After a single scan over the specimen under test, two types of images, namely capacitive and inductive, can be obtained by the proposed system. For an insulated metallic structure, the capacitive image contains the defect information in the insulation layer and on the top surface of the conducting layer, while the inductive image contains the defect information within the conducting region. The proposed integration of the two imaging modalities in a single system does not introduce any interference between the modes and provides more information on the defects with a reduced testing time and production cost on hardware and software compared to using two NDE techniques separately. A theoretical explanation of the imaging mechanisms for the capacitive and inductive modes are provided. The results of finite element modelling show perturbation of the probing fields due to defects in the two imaging modes. Experimental results from a dual modality imaging system are also presented, demonstrating detection of defects in insulator-metal hybrid structures to verify the effectiveness of this approach.

Flaw Detection in Aluminum Plates Using a Rotating Uniform Eddy Current Probe with Two Pairs of Excitation Coils

The use of eddy currents for detecting flaws in specimens is of considerable significance in the industrial sector. In this study, a new design of a rotating uniform eddy current (UEC) probe, termed the rotating butterfly probe, is presented. The probe consists of two pairs of excitation coils arranged perpendicular to each other, positioned in two layers, and in a detection coil. The excitation and detection coils were installed the pancake orientation, which provides larger induction and enhances the sensitivity of flaws detection. In addition, to generate a rotating UEC distribution with same amplitude in all directions, the number of turns between first and second layers of the excitation coils and the amplitude of excitation current were arranged. Finite element simulations were conducted to confirm that rotating UEC distribution has the same amplitude in all directions. The experiment with the rotating butterfly probe was then conducted. In the experiment, the measured results with the probe was indicated the self-differential and self-nulling properties. Moreover, the probe was successful in detecting flaws in all directions on an aluminum plate. This attribute can be used for the effective inspection of test pieces.

A geometrically decoupled, twisted solenoid single-axis gradient coil set for TRASE.

TRASE uses phase gradients in the RF transmit field to encode MRI data. A highly efficient twisted solenoid coil was proposed recently for TRASE imaging for transverse geometries. This novel coil can be rotated to generate a phase gradient in any transverse direction, therefore, combining two such coils would double k-space coverage for single-axis encoding, resulting in higher spatial resolution. However, the strong inductive coupling between a pair of coaxial twisted solenoids must be overcome. Here, we demonstrate that two concentric twisted solenoids, designed using previously described Biot-Savart calculations, can be geometrically decoupled by attaching to each a regular solenoid in series. The regular solenoid geometry resulting in minimization of mutual inductance was determined from simulations using the FastHenry2 tool. The effects on TRASE encoding performance due to the regular solenoids were assessed from simulations and experiments. The maximum resulting magnitude and phase distortions were 3.7% and , while a good isolation dB between the coil pair was obtained. TRASE experiments confirmed the double k-space coverage, and achieved a rapid spin echo train with 128 k-space points collected within 80 ms, allowing short samples to be accurately imaged. This study demonstrates that a pair of twisted solenoid phase gradient RF coils can be geometrically decoupled. Advantages over active PIN diode decoupling include faster switching, lower hardware complexity, and scalability.

Optimum Design of Decoupled Concentric Coils for Operation in Double-Receiver Wireless Power Transfer Systems

Wireless power transfer is becoming ubiquitous in consumer electronics and electrified transportation, providing a convenient method to supply power or charge various devices, such as cell phones, watches, laptops, electric vehicles, and so on. Delivering power from one transmitter unit to more than one receiver can save space and reduce the cost of the overall system. For applications where the sizes of transmitter and receiver units are commeasurable, a frequently used design is the one with carefully overlapped D-shaped coils (also named bipolar pad) to achieve decoupling between coils. In this paper, an alternative optimized design of two concentric decoupled coils is proposed. The design allows the receiver coils to operate at close proximity, collecting power from a shared magnetic field generated by a single transmitter pad. The design methodology is based on a bucking coil layout where the coils’ decoupling is achieved by means of flux cancellation caused by alternating the winding direction of turns in one of the coils. Apart from eliminating coupling between the two receivers, the proposed design maximizes the coupling between the transmitter and the second receiver coil and consequently increases the power transfer capability, offering the equivalent coupling coefficient similar to or better than a pair of bipolar DD coils. Finally, the number and space between turns are selected to improve the quality factor of the proposed receiver. The proposed solution is designable analytically and the second receiver can be added even after the first receiver is already built. Moreover, the proposed design demonstrates superior misalignment tolerance, particularly in the lateral direction, compared to other decoupled coil designs. The proposed methodology is validated through simulations and experimental tests and has proved to be accurate and easy for implementation.

A Design Study on a Multibillet HTS Induction Heater With REBCO Racetrack Coils

Recently an induction heating system comprising a pair of no-insulation high-temperature superconductor (HTS) racetrack coils was built and now it is commercially available. It generates a direct current (dc) magnetic field in which a conductive metallic billet rotates at a high speed. Most dc induction machines accommodate a “single” billet per operation. This paper proposes an alternative concept of a “multiple-billet” induction heater, where the multiple billets can be <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">simultaneously</i> heated. The system consists of eight pairs of HTS racetrack coils that are placed in a circular pattern. An arbitrary number of billets may be placed and rotate at the same time in the space between racetrack coil pairs. The main goal in this paper is to theoretically demonstrate the principle of operation of the new multibillet system. The paper presents: first, overall system configuration and the principle of operation; second, electromagnetic design of the eight-pole HTS magnet; third, analyses with key operation parameters to estimate the multibillet heater performance. Due to the “positive” mutual inductance among neighbor racetrack coils, the multibillet system is estimated to deliver a larger heating power per billet than that of its single-billet counterpart for given operating conditions. Also, due to the eddy-currents induced in each billet, the power per billet reduces when the number of billets increases beyond an optimal value.

The Method to Diagnose Soundness of the High Temperature Superconducting Coil by Pick-Up Coil Pairs

We have proposed a new system using pick-up coil pairs to detect normal transitions in the high temperature superconducting (HTS) coils which are applied to several applications, high field magnets, SMES, transformer, and so on. In the previous study, we have demonstrated that our system can detect local normal transitions in the winding of the test transformer wound by Bi-2223 multifilamentary tapes even if the operation of the transformer is ongoing. However, it is not sufficient to detect normal transitions for operations of practical devices because sudden breaks are not allowable for practical operation. It is important that soundness of the coil, such as critical currents, are monitored and diagnosed for actual operation of practical superconducting devices. In this paper, the external magnetic fields and temperature dependences of signals measured by our system are investigated, in order to make possible to diagnose soundness of the windings from the signals. And then it is shown that critical current of the windings, as an indicator of soundness, can be estimated under ac operation. The diameter of the sample coil is 70 mm. The results indicate that the critical currents of the winding can be estimated from our measured signals.

Compact high performance hybrid reluctance actuated fast steering mirror system

This work presents the design, analysis, control and evaluation of a novel compact and highly integrated fast steering mirror (FSM) system, which is based on a hybrid reluctance tip/tilt actuator. The actuator design employs a permanent magnet for biasing the magnet circuit with a constant flux and two pairs of coils to generate a steering flux for rotating the mover around the two system axes. The system, designed to be used as scanning unit in compact optical metrology systems, uses a custom made eddy current sensor system for position measurement and a titanium flexure, which compensates the negative actuator stiffness, for suspending the mover. The FSM provides a mechanical angular range of ± 3° ( ± 52.4 mrad) and a small signal closed-loop bandwidth of 1.5 kHz in the tip and tilt axis, has a small diameter of 32 mm and a system height of only 30 mm. Compared to a state-of-the-art hybrid reluctance actuated FSM system the volume of the system is reduced by one order of magnitude, while the product of range times bandwidth, representing a measure for the system performance, is concurrently improved by 50%.

Modeling and Configuration Design of Electromagnetic Actuation Coil for a Magnetically Controlled Microrobot

Non-contact actuated microbeads have attracted a lot of attention in recent years because of its enormous potential in medical, biological, and industrial applications. Researchers have proposed a multitude of electromagnetic actuation (EMA) systems consisting of a variety of coil pairs. However, a unified method to design and optimize a coil pair according to technical specifications still does not exist. Initially, this paper presented the modeling of an untethered ferromagnetic particle actuated by externally applied magnetic field. Based on the models, a simple method of designing and optimizing the EMA coil pair according to technical specifications, was proposed. A loop-shaped coil pair generating uniform magnetic and gradient fields was chosen to demonstrate this method clearly and practically. The results of the optimization showed that the best distance to radius ratio of a loop-shaped coil pair is 1.02 for a uniform magnetic field and 1.75 for a uniform gradient field. The applicability of the method to other shapes of coil configuration was also illustrated. The best width to distance ratio for a square-shaped coil pair is 0.558 and 0.958 for uniform magnetic and gradient fields, respectively. The best height to width ratio and distance to width ratio for a rectangle-shaped coil pair is h/w = [0.9,1.1], d/w = [0.5,0.6] for uniform magnetic field and h/w = [1.0,1.2], d/w = [0.9,1.1] for uniform gradient field. Furthermore, simulations of a microparticle tracking the targeted trajectory were conducted to analyze the performance of the newly designed coils. The simulations suggested the ability of manipulating microparticles via the coils designed by our proposed method. The research mainly proposed a unified design and optimization method for a coil pair, which can support researchers while designing a specific coil pair according to the technical requirements. This study is aimed at researchers who are interested in EMA system and microrobots.

P2‐047: EFFECT OF WEAK COMBINED MAGNETIC FIELDS ON SPATIAL MEMORY IN AN ANIMAL MODEL OF HERITABLE ALZHEIMER'S DISEASE

In this work the effect of the weak combined magnetic fields (MFs) was studied in vivo in heritable animal model of AD: well characterized mice transgenic for mutant APPswe and mutant presenilin 1 (PS1dE9) that cause early onset familial Alzheimer's disease (AD). Transgenic mice (Tg) and control C3H mice were exposed to the same MF at age of 8 months. Mice were also exposed by MF in two groups at the same time (Tg+MF and C3H+MF). A setup for generating a MFs consisted of two pairs of coaxial Helmhollz coils oriented along the geomagnetic field vector. A direct current was supplied to one of the pairs of coils to generate a constant component of MFs with induction of 42 mcT. An alternating current signal produced by a programmable sinusoidal current generator was fed to other pair of coils to create a variable component of MFs with induction of the amplitude of 80 nT. The current signal was the sum of two frequencies of 4.38 and 4.88 Hz. These frequencies correspond to the cyclotron frequencies of glutamic and aspartic acids. After exposure to the MFs, the mice were trained in a Morris water maze for 18 days (four trials per day). Spatial memory was tested on the following day after completion of training with the hidden platform removed. During the test period (60 s), occupancy time spent in each sector was recorded. The MFs improved the memory in Tg animals. The results of the factor analysis demonstrate that factors of the sector preference became statistically significant for Tg groups after MFs exposure. Thus, the data suggest that MFs can be applied for preventive purposes in a group of risk of AD.

The MR Cap: A single-sided MRI system designed for potential point-of-care limited field-of-view brain imaging.

The size, cost, and siting requirements of conventional MRI systems limit their availability and preclude usage as monitoring or point-of-care devices. To address this, we developed a lightweight MRI for point-of-care brain imaging over a reduced field of view (FOV). The B0 magnet was designed with a genetic algorithm optimizing homogeneity over a 3 × 8 × 8 cm FOV and a built-in gradient for slice selection or readout encoding. An external pair of gradient coils enables phase encoding in the other two directions and a radiofrequency (RF) coil provides excitation and detection. The system was demonstrated with high-resolution 1D "depth profiling" and 3D phantom imaging. The lightweight B0 magnet achieved a 64-mT average field over the imaging region at a materials cost of <$450 USD. The weight of the magnet, gradient, and RF coil was 8.3 kg. Depth profiles were obtained at high resolution (0.89 mm) and multislice rapid acquisition with refocused echoes (RARE) images were obtained with a resolution ~2 mm in-plane and ~6-mm slice thickness, each in an imaging time of 11 min. The system demonstrates the feasibility of a lightweight brain MRI system capable of 1D to 3D imaging within a reduced FOV. The proposed system is low-cost and small enough to be used in point-of-care applications.

Study of a magnetically driven reconnection platform using ultrafast proton radiography

A novel magnetically driven reconnection platform was created by a pair of U-shaped Cu coils that connect two parallel Cu plates irradiated at a focused laser intensity of ∼3 × 1016 W/cm2 and characterized using ultrafast proton radiography. The proton data show two prolate voids, each corresponding to the coil current, with an inferred maximum magnitude of 57 ± 4 kA. A center “flasklike” feature was also observed in the proton radiographs. By prescribing electromagnetic fields associated with magnetic reconnection in proton ray tracing simulations, characteristics of this center feature were reproduced. These results demonstrate the robustness of the laser-driven capacitor coils for generating strong magnetic fields and provide promise of using such coils as a viable platform for studying magnetically driven reconnection in the laboratory.

A novel pulsed eddy current method for high-speed pipeline inline inspection

High-speed inspection of inner diameter (ID) and outer diameter (OD) surface defects on thick-wall steel pipes is an important aspect to advance the pipeline inline inspection (ILI) in the oil and gas industry. The state-of-the-art ILI methods including magnetic flux leakage (MFL), ultrasonic testing (UT), electromagnetic acoustic transducer (EMAT), eddy current (EC) and pulsed eddy current (PEC) are hardly applicable to practical high-speed pipeline ILI due to the reasons of long inspection time required for sensor response and low detection sensitivity caused by severe motion. New sensing techniques that offer faster inspection speed, deeper signal penetration depth, better detection sensitivity and linearity, as well as capability of ID/OD discrimination are imperatively needed. This paper proposes a novel PEC sensing method to detect and discriminate ID/OD defects by utilizing the conductivity-dependent and permeability-dependent distribution patterns of induced eddy current at the ID surface of steel pipes. For ID defects, the pattern is caused by the discontinuous conductivity distribution, while for OD defects, the pattern is caused by the non-uniform incremental permeability distribution. A pulse-excited current with short width (2.5 μs), low duty cycle (1%) and fast-falling edge (100 ns) is injected into an excitation coil, so that the secondary magnetic field at the fast-falling edge will produce a transient oscillation in a pair of differential pick-up coils. Then, the time-domain transient oscillatory pick-up signal is extended, filtered, amplified, and extracted to be one feature by an envelope detector and an average sample method, which is processed in real-time by the developed probe for facilitating the back-end data recording. Meanwhile, a novel high-speed pipeline inspection gauge (PIG) with a sensor array is developed for field testing to validate the effectiveness of the proposed PEC method that achieved high inspection speed, deep detection depth, superior sensitivity, good linearity, low power consumption, easy implementation, ID/OD discrimination and crack detection capability.

Accurate Measurement of Characteristic Response for Unexploded Ordnance With Transient Electromagnetic System

Characteristic response parameterized in time-dependent target intrinsic polarizabilities has been widely used to create feature libraries for unexploded ordnance (UXO) discrimination. This paper discusses the accurate measurement of characteristic response for UXOs with transient electromagnetic (TEM) system. First, a specially designed TEM system, which utilizes a modified solenoid and a pair of rectangular coils as transmitting coils to ensure the uniformity of primary field up to 96% throughout the target, is described. Then, the characteristic responses of 12 UXOs are measured from directions vertical or parallel to the axis at distances from 40 to 120 cm. The measured characteristic responses show a power law decay function at an early time and an exponential decay function at a later time. The characteristic responses parallel to the target axis are larger, and the decay is considerably slower than those vertical to the axis, leading to one to two orders of magnitude difference at a later time. Results also show that the characteristic response measured from the direction vertical to the axis of the target is more precise than that from the direction parallel to the axis at the same distance. The portion of the target that is close to the system accounts for a large proportion of the response. A target can be approximated as a single dipole only when it is located no less than twice its length from the system. Finally, the characteristic responses of 12 targets are parameterized with the Pasion–Oldenburg expression for feature library creation.

Prototype Coil Evaluation for NSTX-U Replacement Inner Poloidal Field Coils

The National Spherical Torus eXperiment Upgrade (NSTX-U) is an innovative magnetic fusion device constructed at the Princeton Plasma Physics Laboratory (PPPL). In 2016, because of the failure of the PF-1a upper divertor coil, which experienced a coolant blockage, the NSTX-U operation was suspended. A postmortem investigation indicated that an undetected gradual deterioration of coil inductance preceded the coolant blockage leading up to the operational suspension. The project team decided that all inner poiloidal field (PF) upper and lower coil pairs, denoted PF-1a, PF-1b, and PF-1c, shall be replaced with new coils of improved design and manufacture. The new prototype inner PF coils from four suppliers across the globe were evaluated at PPPL following a prototype technical evaluation procedure. Mechanical inspection and electrical testing were performed to qualify each supplier.This paper discusses the details of the mechanical and electrical tests and measurements performed on the complete coils. The test results were used to assess quality of turn-to-turn and turn-to-ground insulations of the prototype coils. Two prototype coils were power tested at PPPL for five pulses to reach its rated current and maximum temperature following the completion of low-power electrical testing. During pulses, the conductors experience a near adiabatic temperature rise and hoop stress. Between pulses, cold water enters the inlet, and a cooling wave propagates through the coils as slugs of cold water heat up to the conductor temperature and then pass through the coil to the outlet. Results show that full power testing did not change coil electrical characteristics. Each prototype coil was then sectioned into two halves to permit examination of the internal insulation, conductor spacing, and vacuum pressure impregnation quality. The high-voltage breakdown test of sectioned coils was performed to evaluate turn and ground insulation breakdown voltage. The estimate for the production coils is based largely on the experience learned from the prototype coil program. The first production coil will be delivered to PPPL for testing by January 2020.

A Novel Tip-positioning Control of a Magnetically Steerable Guidewire in Sharply Curved Blood Vessel for Percutaneous Coronary Intervention

This paper suggests a novel method for the positioning control of a magnetically steerable guidewire without needing to detect a tip angle, which can pass through sharply curved blood vessels with potentially reduced contact between the guidewire end tip and the blood vessel for Percutaneous Coronary Intervention (PCI). The proposed tip-positioning control consists of four parts: 1) Path-regeneration, 2) Feeding control, 3) EMA (Electromagnetic Actuator) control, and 4) Vision tracking. Path-regeneration provides an improved path based on the properties of the guidewire and the size of the blood vessel when the guidewire cannot follow the pre-planned path. Based on the desired path, the feeding control drives the translational motion of the guidewire while the EMA system steers the guidewire’s end tip, which is made of a permanent magnet. A pair of Helmholtz coils is employed for the EMA system, and a vision-tracking algorithm is used to detect the position of the magnet tip in real time. The system was integrated for the preliminary study, and the magnet tip could be controlled so as to track a desired path without any shape or curvature information, which requires the use of an additional catheter model or angle estimation algorithm. The proposed catheter tip position control method was verified by experiments on both smooth and sharply curved blood vessel phantoms. The results showed that the proposed method could accomplish the tip position control of the guidewire with high accuracy in real time.

Automated Device for Magnetic Control of Products of Complex Forms Made from Electrotechnical Steels

This study proposes the device for indirect measurement of magnetic induction, with the given value of magnetic field strength, in electrotechnical steel samples of complex form with application of two permeameters. The testing of enclosed form samples requires preliminary labor-intensive coiling of magnetization and measuring windings, which is not suitable for automation of express testing. Express testing of products from electrotechnical steels is usually related to the measurement of not absolute values, but the values of deviation, taking into account the sign of the selected conventional «standard» value. Therefore, for such measurements implementation it is necessary to apply methods of comparison with the measure. The most suitable for this purpose is a differential measurement method. Comparing the increase of accuracy of testing in comparison with analogues, the design of a permeameter implies coiling of the measuring winding of four sections and their placement on the magnetic circuit on both sides of the zones of contact with the controlled pro-duct.Besides that, the windings in each pair of sections are connected in series, and the pairs of coils themselves — in series in accordance. This allows to measure the voltage on the terminals of the measuring winding, which only depends on the working magnetic flux passing through the cross section of the sample. This does not take into account magnetic dispersion flows from the magnetic circuit, which are sources of errors. This constructive technique achieves one more objective — increasing the sensitivity of the permeameter, which is adjustable since its value depends on the number of turns of the measuring winding. Obtained results allow to solve the problem of automation of the routine magnetic control of products of complex form from thin-sheet electrical steel in conditions of its mass production.

Mutual Impedance of Eddy-Current Coils Above a Second-Layer Crack of Finite Length

A model is presented to determine the change in mutual impedance between a driver and pickup coil due to eddy-current interaction with a crack in a conductive two-layer plate system where the plates are separated by a non-conducting layer of constant thickness. Here, the impedance change of interest arises from a through-thickness crack of finite length located in the hidden lower plate. The model assumes that both conducting plates are electromagnetically thin and is therefore valid for large electromagnetic skin-depth. The model predictions are compared with experimental measurements using a pair of cylindrical air-cored coils arranged to simulate an eddy-current sliding probe. Agreement between the calculated and measured impedance change is obtained over the frequency range 10 Hz–3 kHz for which the plates can be treated as electromagnetically thin, and reasonable qualitative agreement was obtained at higher frequencies. A method for computing the induced current streamlines in the presence of the crack is also described and visualisation of the computed streamlines is used to assist in the interpretation of the impedance change for different probe orientations. These results provide a valuable initial insight into the interaction between a buried crack in a multilayer structure and the electromagnetic fields induced and detected by eddy-current sliding probes.