Coil Impedance Research Articles

Boundary Element Calculation of Eddy Currents in Cylindrical Structures Containing Cracks

Electromagnetic fields due to excitation coils in boreholes containing cracks have been calculated using a boundary element method formulated in terms of a scalar decomposition of the field. By using a Green's kernel that ensures continuity of the tangential electric and magnetic field at the cylindrical surface, it is only necessary to render the flaw region in a discrete form. The effect of an ideal crack, one with zero opening but acting as a perfect barrier to eddy currents, is here represented by a current dipole layer orientated normal to the crack surface. An integral equation determines the source density on the crack and a numerical solution is found using boundary elements. The coil impedance change due to the crack is then computed using an expression based on a reciprocity principle.

Equivalent Magnetization Current Method Applied to the Design of Gradient Coils for Magnetic Resonance Imaging

A new method is described for the design of gradient coils for magnetic resonance imaging systems. The method is based on the known equivalence between a magnetized volume surrounded by a conducting surface and its equivalent representation by a surface current density. The curl of a vertical magnetization vector of a magnetized thin volume is equivalent to a surface current density whose stream line defines a coil current pattern. This concept is applied to the design of gradient coils of arbitrary shape. The thin magnetized volume is discretized in small triangular elements. By calculating the contribution of each magnetized block at target points a field source matrix is obtained. The equivalent magnetization current concept is applied to obtain the equivalent coil impedance, force and torques. A quadratic programming optimization algorithm is used to obtain the stream-magnetization-thickness function value at each node such that coils of optimal performance are obtained. This method can be used for gradient coils wound on arbitrary surface shapes and can be applied to hybrid current/iron solutions. A variety of examples are shown to demonstrate the versatility of the method. A novel partially shielded 3-D biplanar gradient coil for open MRI magnets is presented.

The Effect of Ultrasonic Resonance on Impedance in Magnetic Rods

When an ultrasonic wave is applied to the end of a magnetic rod and a standing wave is generated, high frequency stress will be generated at the standing wave nodes according to the frequency of the ultrasonic wave. The change in magnetic moment generated by this high frequency stress can be easily observed with a detector coil. On the other hand, since stress is not generated at the positions of the antinodes, an induced electromotive force is not produced in the detector coil. Therefore, observation of the stress distribution in the sample rod with the detector coil is possible. Since this phenomenon is reversible, a standing wave may also be generated by applying high frequency voltage, using a detector coil as an excitation coil. In that case, the impedance of the excitation coil changes from the node to the antinode positions. Therefore, it is possible to measure the stress distribution in a sample using only an excitation coil. This paper is a report of research into the effect that the standing wave in a magnetic rod exerts on the impedance of an excitation coil.

Eddy-current effects in an electromagnetic flowmeter

A new two-dimensional analytical model is developed for the effect of eddy currents in the metal parts of an electromagnetic flowmeter under ac excitation. This model considers the influence of both the inner stainless-steel meter tube and the outer iron magnet core, treating them as thin sheets. Appropriate boundary conditions are derived across these sheets. The model predicts the coil impedance Z and the magnetic field B as complex numbers (magnitude and phase) for a given frequency f . The model is validated by extensive experiments on a commercial 2 inch meter using excitation frequencies in the range 10 Hz to 1 kHz. After first fitting some unknown physical parameters from the data, such as the effective core permeability μ and the conductivity σ of each metal component, the resulting model has a typical accuracy of better than 1% for the magnitude and phase of Z and B .

A Compact, Low-Power, and Electromagnetically Actuated Microspeaker for Hearing Aids

In this letter, we present an electromagnetically actuated microspeaker with microelectromechanical systems (MEMS) technology to reduce form factor, cost, and power consumption in hearing aid applications. The microspeaker has multilayer copper coils, a NiFe soft magnet on a polyimide membrane, and a NdFeB permanent magnet on the perimeter. The coil impedance is measured at 1.5 Omega and shows a very flat response across the audio frequency range. The device operates at a very low power, the lowest in MEMS speakers, comparable to that of the macro- size counterparts. A single-turn microspeaker with a diameter of 2.5 mm consumes 11.6 and 0.13 mW to generate a sound pressure level of 129 and 106 dB at 1 kHz, respectively. The measurement uncertainty is less than 10%, and the reproducibility is within 36% among the tested devices.

Towards a microcoil for intracranial and intraductal MR microscopy.

Implantable RF-coils have enabled sub-mm resolution magnetic resonance images (MRI) of deep structures. Scaling down the size of RF coils has similarly provided a gain in signal-to-noise ratio in nuclear-magnetic-resonance spectroscopy. By combining both approaches we designed, fabricated, and imaged with an implantable microcoil catheter. While typical implantable catheters use a transverse magnetization, the axial magnetization of the microcoil provides improved sensitivity and allows visualization of the tissue beyond the distal end of the catheter. The microcoil catheter was designed with a diameter of 1 mm for future integration with intracranial devices, and for intraductal use in breast oncology. We modified the NMR-microcoil design to allow implantation of the RF coil, by winding the microcoil on medical-grade silicone tubing and incorporating leads on the catheter to connect circuit components. In order to achieve proper turn spacing, we coated copper wire with 25 microm of biocompatible polymer (Parylene C). Tuning and matching circuitry insured that the impedance of the RF coil was approximately 50 ohm at the operating frequency for 3-T proton MR applications. A duplexer was used to enable use of the microcoil catheter as a transceiver. Experimental verification of the coil design was achieved through ex vivo imaging of neural tissue. As expected, the microcoil catheter provided microscale images with 20-microm in-plane-resolution and 170-microm-thick slices. While 3-T MRI typically provides 1 to 30 voxels per-cubic-millimeter, in this paper we report that the MRI microcoil can provide hundreds, and even thousands of voxels in the same volume.

Coil impedance calculation of an eddy current sensor by the finite element method

An eddy current sensor utilizes the variation of coil impedance to perform the measurement of physical parameters, and the coil impedance is an important parameter for investigating the properties of an eddy current sensor. Therefore, the study of approaches to calculation of coil impedance is very important. In this paper, an integral expression for coil impedance with a magnetic substance is given directly. For the convenience of calculation of the coil impedance, this integral expression is further expanded in series form and the coil impedance is computed using Mathematica. Then, two different finite element models of the eddy current measurement system are built on the basis of the finite element modeling theory. The infinite boundary of model 1 is truncated. On the contrary, the infinite boundary of model 2 is simulated completely by an infinite element. The coil impedance and the magnetic distribution are obtained by the finite element method, respectively. The comparison shows that the result obtained by the finite element method is in good agreement with that of the theoretical calculation approach, which indicates the correctness and reasonability of modeling of the eddy current measurement system and calculation of the coil impedance by the finite element method.

Two approaches to coil impedance calculation of eddy current sensor

Eddy current (EC) sensor detects measurand by the change of coil impedance, which is introduced by coupling the magnetic energy between coil and measurement target. Therefore, it is of great significance for both the forward and inverse eddy current problems to study on the approaches to coil impedance calculation. In the current paper, two approaches are presented to calculate coil impedance of EC sensor. First, the theoretical calculation model of coil impedance is simplified as an air-cored cylindrical coil, over an infinite measurement target. And the integral expression of coil impedance is derived according to the sensor operation principle and electromagnetic field theory. Then, the integral expression is expanded as series form in cylindrical coordinate system in order to compute the coil impedance easily, and computed using MathematicaTM. Next, based on the finite element modeling theory, two different finite element models of eddy current measurement system are built. The infinite boundary in mode1 is truncated, while in model 2, it is simulated completely by an infinite element. Furthermore, the appropriate location where the air domain is truncated at is determined. Finally, the coil impedance and the magnetic distribution of eddy current testing system are obtained by finite element method, respectively. By comparison, the results from the theoretical calculation approach and finite element method are in very good agreement, and the accuracy of two approaches is validated with each other.

Impedance of an induction coil at the opening of a borehole in a conductor

The electromagnetic field of a cylindrical eddy current probe coil near the open end of a borehole in a conductor has been calculated analytically accounting for edge effects. Calculations of the coil impedance as a function of position and excitation frequency have been made allowing theoretical results to be compared with experimental measurements. Comparisons have been carried out for special cases in which a cylindrical coil has its axis either perpendicular or parallel to the axis of the hole. In the approach used, the field is expressed in terms of transverse electric and transverse magnetic potentials defined with respect to the axis of the hole. The domain of the problem is truncated in the axial direction in order to express the solution in the form of eigenfunction expansions. The truncation modifies the original unbounded domain problem, but the additional boundaries can be made as remote from the coil as desired so that they have a negligible effect on numerical estimates of the coil field. The truncated region approach has proved to be accurate and computationally efficient but more significantly, it allows new solutions to be found for problems that are otherwise analytically intractable. The results model eddy current inspections of boreholes including edge effects at the opening of the hole.

A novel eddy current angle sensor for electrohydraulic rotary valves

We present an eddy current angle sensor for high-pressure electrohydraulic rotary valves. An oblique ring-shaped sensing coil and a symmetric semi-cylindrical rotor are utilized, making the coil impedance and the output voltage proportional to the angular displacements. Analysis based on an electrical equivalent circuit and the finite-element method is provided, and good agreement is obtained between the simulated and measured results. Extensive experiments have shown that the sensor has a linear working range of about 30° with nonlinearity 0.8% and temperature drift less than 1% over 30 °C–90 °C.

Mutual Impedance of Cylindrical Coils at an Arbitrary Position and Orientation Above a Planar Conductor

We derive a closed-form expression for the mutual impedance due to eddy-current induction for a pair of cylindrical air-core coils with arbitrary position and orientation above a planar conductor. By extending a recently devised model for individual coils with an arbitrary tilt with respect to the surface, we obtain a remarkably simple result. We validated our model with measurements on a conductive plate. The results should be useful for designing new probe configurations and for evaluating the signals in eddy-current inspections when driver-pickup coil configurations are utilized.

Method to Determine Tensile Stress Alterations in Prestressing Steel Strands by Means of an Eddy-Current Technique

The increased demand for reinforced concrete elements, for example, in bridge construction, requires long-term monitoring using multisensor techniques. Dynamic load inspection and maximum load detection is now an important requirement to avoid overstressing in light of past building failures in Germany, Austria, Poland, and Russia. Magnetoelastic force measurement of prestressed reinforcing concrete cables is also an investigative method for detecting steel failures (McMaster, 1986; Cullity, 1972; Fabo, 2002). An examination of coil impedance measurements made using eddy-current sensors placed on mechanically stressed steel specimens demonstrates a high dependency on the magnetoelastic effect. This stress measurement on prestressings or other reinforced concrete cables can be achieved by calibration of the material properties and stabilization of the magnetization state at the working point.

Eddy current flaw testing probe with high performance in detecting flaws during in‐service inspection of tubing

AbstractEddy current flaw testing that uses changes in the impedance of the test coil as an electromagnetic field interacts with the test material is a nondestructive testing method. It can be used to inspect test material at high speed without bringing the test coil into contact with the test material. Therefore, it is widely used for product inspection. In the inspection of heat exchanger tubing in power plants and chemical plants, eddy current flaw testing is performed by using an inner probe, because of the structural restrictions of such plants. However, high levels of wobble noise and support plate noise occur in the conventional method using a rotating pancake coil. The authors therefore propose new minimal‐noise eddy current inner probes. Two new types of probes have been developed: one for detecting axial flaws and the other for detecting circumference flaws. These probes can detect flaws with a low level of noise. The phase of the flaw signal by the new probes changes according to the flaw depth. As a result, the signal phase by the new probes can be used for evaluating the depth of surface flaws. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 161(2): 52–61, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20605

Numerical modelling and implementation of ferrite cored eddy current probes

This paper focuses on the development of a magnetic moment method of calculating vector field quantities for a highly permeable ferrite cored eddy current probe. Basis functions are used in this method to replace the scattered field caused by the probe core in accordance with the surface equivalence theorem. These functions are further developed and tested for accuracy and convergence. An efficient material profile equation, independent of probe coil and basis function properties, is also designed and verified. Collocation point selection and optimisation is finally undertaken leading to the accurate determination of probe source coil impedance. The accuracy of calculation is verified using an industry standard finite element solver.

Determination of Thickness of Silver Coatings on Brass by Measuring the Impedance of a Thin Elliptic Coil

To measure the impedance of a thin elliptically shaped coil, in presence of a flat plate with a coat of metal, can be an instrument for determination of the cladding thickness. An electromagnetic field from the coil is then forced to the object, producing eddy currents inside the object. These are influenced by the characteristics of the object and the coil and give rise to an impedance change, which can be detected and correlated to the thickness of the coating. An electromagnetic model accounting for the impedance of the elliptic coil with different values on the numerical eccentricity and the coating thickness is described. The model is based on a dyadic Green function formulation of the problem from which the electric field and hence the impedance is evaluated by utilizing the method of scattering super position. Numerical calculations based on the model and experimental measurements have been taken. An example shows how the model can be used to model a brass surface with a coat of silver to find expected impedance as function of the coating thickness

Fault Current Limitation Characteristics of Bi-2212 Bulk Coil With Different Types of Shunt Coils

We investigated fault current limitation characteristics of a resistive superconducting fault current limiter (SFCL) which consists of a Bi-2212 bulk coil and a shunt coil. For the experiments, different types of shunt-coils which had the same impedance were prepared. One group had high resistance, and the others were designed to have high inductance. The effects of the impedance component variation on current limitation characteristics of the SFCL were compared with each other. The test results showed that the SFCLs of both groups successfully limited the fault current of 7 kA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> to below 5 kA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> right after the fault occurred. The SFCLs with a shunt coil of higher inductance quenched more uniformly, but had a high voltage spike by Ldi/dt at the first half cycle after fault. On the other hand, the ones with higher resistance quenched comparatively less uniformly, but the voltage spike was much smaller. The impedance of the shunt coil is being optimized, taking into consideration stability, insulation, its effect on the circuit and other factors.

A New Type of Highly Compact Planar Inductor

The impedance of a planar coil suspended on a torsion beam and placed in a strong magnetic field is analyzed. It exhibits an inductive behavior with high inductance values. Numerical examples are provided for this galavanometer type inductor. An experimental demonstration of a magneto-mechanical system with high inductance is also shown. Potential applications and limitations are discussed

Theoretical Prediction of the Quench Behavior of a SFCL Module Having a BSCCO-2212 Bulk Coil and a Shunt Coil

This paper deals with theoretical prediction of the quench behavior for a superconducting fault current limiter (SFCL) module. The SFCL module consists of a monofllar type BSCCO-2212 bulk and a shunt coil made of copper or brass. The BSCCO-2212 bulk coil is placed inside the shunt coil, and they are connected in parallel. In order to analyse the quench behavior of the module, the equivalent circuit equation was first derived from the experimental circuit structure and then the partial differential equation was solved. The inductance values of the BSCCO bulk coil and impedance of the shunt coil are calculated by Bio-Savart and Ohm's formula, respectively. We computed the voltage and current behavior during quenches using those values, and compared the results with experimental short-circuit data for the SFCL module. Both computation and test results agreed well. Therefore, we conclude that the analytic result can be applied effectively to design of a SFCL module.

Do electrical stun guns (TASER-X26®) affect the functional integrity of implantable pacemakers and defibrillators?

High voltage electric current can adversely affect pacemakers (PM) and implantable cardioverter-defibrillator (ICD). The standard shock from an electrical stun gun (TASER- X26, TASER International, Scottsdale, AZ) consists of a 5-s long application of high voltage, low current pulses at 19 pulses per second. Its effect on the functional integrity of PM and ICDs is unknown. We tested the functional integrity of nine PMs and seven ICDs in a swine model after a standard stun gun shock. A transvenous, dual coil, bi-polar ICD lead (St Jude-SP01) and a PM lead were placed in the right ventricular (RV) apex and connected to pulse generators buried in the pre-pectoral pocket. The two darts were placed at the sternal notch (SN) and apex of the heart bracketing the device pocket. Standard neuromuscular incapacitating (NMI) discharges were delivered. Functional parameters of the devices and leads were checked before and after the shocks. The mean pacing thresholds, sensing thresholds, pacing impedances, and defibrillation coil impedances of the ICD lead were similar before and after the shocks. Similarly, pacing thresholds, sensing thresholds, and impedances of the PM lead were not significantly different before and after the shocks. No significant change was noted in battery voltage and projected longevity. Implantable cardioverter-defibrillator generators detected the NMI impulses at a mean cycle length of 176 +/- 20 ms with detection to charge time of 5.9 +/- 1.5 s. Shock delivery was aborted in all tests as tachycardia detection abruptly terminated at the end of the 5 s NMI application. None of the devices exhibited power on reset (POR), elective replacement indicator (ERI), or noise mode behaviour after the shock. Pacemakers and ICD generators and leads functions were not affected by the tested standard 5 s stun gun shocks.

Measurement Method for Ferrite Core Permeability Dispersion Using a Microstrip Line

This paper describes a method for measuring magnetic permeability dispersion of ferrite cores widely used in RF devices such as splitters in CATV systems. The present method has a step of measuring reflection coefficients for three kinds of microstrip line circuits, one having a test sample (ferrite core) mounted on it, one with a non-magnetic core instead of the ferrite, and one with an impedance matching resistor only. From the three reflection coefficients derived, the impedances of the coils with the ferrite and non-magnetic cores, respectively, are calculated and the complex permeability is determined by dividing the impedance of the ferrite core coil by the reactance component of the non-magnetic core coil. In this paper, the accuracy of the proposed method is discussed through a electromagnetic simulation and then the calculated results are compared with the experiments, indicating the validity of the method.