Anthony J. Peyton

An overview of electromagnetic inductance tomography: Description of three different systems

This paper presents a general overview of electromagnetic inductance tomography (EMT). A general introduction is given together with a description of the theoretical background of the technique. Three examples of different EMT systems are discussed and images produced using several different image reconstruction techniques are presented. A discussion of the main features of the techniques is included and some potential applications are suggested.

Hardware and software design for an electromagnetic induction tomography (EMT) system for high contrast metal process applications.

This paper presents the latest development of an EMT system designed for use in the metal production industry such as imaging molten steel flow profiles during continuous casting. The system that has been developed is based on a commercial data acquisition board residing in a PC host computer and programmed in the LabView graphical language. The paper reviews the new EMT hardware electronics and software. The noise effects and the detectability limits of the system are given in the paper followed by the system sensitivity map analysis. Optimal image reconstructions, including the simultaneous iterative reconstruction technique (SIRT) and non-iterative Tikhonov regularization, truncated singular value decomposition (TSVD), are also discussed and applied for the system. The system has been demonstrated in real time (10 frames s−1 for 5 kHz excitation) with test phantoms that represent typical metal flow profiles such as central, annular stream and multiple streams.

A planar EMT system for the detection of faults on thin metallic plates

This paper describes a novel planar electromagnetic tomography system for the detection of conductivity inhomogeneity on a metallic plate. The proposed system differs from traditional electromagnetic inductance tomography (EMT) systems in its spatial arrangements of coils. Sensor coils are distributed to form a circular array with their axes not parallel but perpendicular to the plate under inspection. The forward solution for the sensor array next to a homogeneous conductive plate is based on the analytical solution provided by Cheng. The sensitivity matrix for a prototype sensor was computed by numerical evaluation of the analytical solution. For the inverse solution, a modified Newton?Raphson method was used to adjust the conductivity distribution to fit a set of inductances measured from the sensor array in a least-squared sense. Frequency- dependent sensitivity analysis was performed to find an optimum testing frequency. The far-field and near-field effects in electrical tomography are discussed. Good estimates for the conductivity distribution were obtained at the optimum frequency. Experimental tests were performed by taking the difference in mutual inductance of the coil pairs when placed next to a homogeneous reference conductor and next to a conductor with faults. Inverse results based on experimental data verified this method.

Simultaneous measurement of distance and thickness of a thin metal plate with an electromagnetic sensor using a simplified model

This paper presents a simplified model which can describe the inductance change when an air-core coil is placed next to a thin nonmagnetic metallic plate. The model has two independent parameters and is valid for a range of thickness, conductivity, and lift-offs. Use of this new relationship provides a fast and accurate method to measure the distance and thickness simultaneously. Measurements made for a sample coil next to thin copper and aluminum plates of various thicknesses verified the theory and the proposed method.

Dynamic imaging in electrical capacitance tomography and electromagnetic induction tomography using a Kalman filter

Electrical capacitance tomography (ECT) and electromagnetic induction tomography (EMT) attempt to visualize the distributions of materials with different permittivity and conductivity/permeability, aiming to reveal electrical and magnetic characteristics of an object, by measuring electrical capacitance and electromagnetic inductance on the periphery of the object. In ECT, capacitances of pairs of electrodes placed around the periphery are measured and in EMT, mutual induction of pairs of coils is measured. In this paper, a dynamic imaging technique is developed for ECT and EMT with a linearized Kalman filter to improve the temporal resolution of images. The inverse problem is treated as a state estimate. A Kalman estimator is used to obtain the material distribution. Experimental results demonstrate that the dynamic imaging technique can improve the spatio-temporal resolution of both ECT and EMT.

Noncontact Characterization of Carbon-Fiber-Reinforced Plastics Using Multifrequency Eddy Current Sensors

The characterization of carbon-fiber-reinforced plastics (CFRPs) using multifrequency eddy current sensors is presented in this paper. Three sensors are designed for bulk conductivity measurements, directionality characterization, and fault detection and imaging of unidirection, cross-ply, and impact-damaged CFRP samples. Analytical and finite-element (FE) models describing the interaction of the sensors with the CFRP plate samples are developed to provide an explanation of, and physical insights into, the measured results and observed phenomena. A signal processing method is developed to compensate for the variation in lift-off during the measurements.

Simultaneous Non-contact Measurement of Water Level and Conductivity

Measurement of water level and conductivity is important in many applications, including geophysics, ocean engineering, and the oil industry, with one example being gas/oil/water separation. Noncontact measurement is ideal for those applications. Capacitance, radar, ultrasonic, and laser techniques have been used in measuring the level, but simultaneous measurements of level and conductivity have not been reported. In this paper, we developed a novel inductive sensor, which uses two coils of different sizes to simultaneously infer conductivity and level based on a simplified model deduced from an analytical solution. The simplified model is verified with both simulation and experimental data. The experimental results have shown that the measurement accuracy for both level and conductivity is within 3%.

Analysis of the Lift-off Effect of Phase Spectra for Eddy Current Sensors

This paper presents an analytical model that describes the inductance change when a double air-cored coil sensor is placed next to a conducting plate. Analysis of the analytical model reveals that the phase signature of such a sensor is virtually liftoff independent. This finding is verified by numerical evaluations. This paper also finds that the phase signature of a ferrite U-cored sensor can be approximated by that of a double air-cored sensor of similar size and, therefore, possesses a similar liftoff-independent property. Measurements made with a sample U-cored sensor next to plates of nonmagnetic and magnetic materials verified the theoretical results.

Electromagnetic techniques for imaging the cross-section distribution of molten steel flow in the continuous casting nozzle

Control of molten steel delivery through the pouring nozzle is critical to ensure an optimum laminar flow pattern in continuous casting, which influences the surface quality, cleanliness, and hence the value of the cast product. A nonintrusive and nonhazardous visualization technique, which uses rugged and noninvasive sensors, would be highly desirable in such harsh industrial production environments. This paper presents an electromagnetic approach for tomographically visualizing the molten steel distribution within a submerged entry nozzle (SEN). The tomographic system consists of an eight-coil sensor array, data acquisition unit, associated conditioning circuitry, and a PC computer, which have been purposely designed and constructed for hot trials. The paper starts with an overview of electromagnetic imaging techniques. The construction of the sensor array and associated electronics are then discussed, followed by sensitivity map analysis and a description of the applied image reconstruction algorithm. Image results, as reconstructed from cold sample measurements and hot pilot plant trials, are also presented. Despite a low frame acquisition rate (1.35s per frame), the images generated from the prototype system are capable of providing an adequate representation of the changes of real molten steel flow profiles within the SEN. The paper demonstrates that the application of electromagnetic tomographic technique to this problem shows significant promise for future industrial processes.

Imaging the continuous conductivity profile within layered metal structures using inductance spectroscopy

This paper presents an inverse method for determining the conductivity distribution of a flat, layered conductor using a multifrequency electromagnetic sensor. Eddy-current sensors are used in a wide range of nondestructive testing applications. Single-frequency sensors are very common; however, the potential of an eddy-current sensor with spectroscopic techniques offers the ability to extract depth profiles and examine more fully the internal structure of the test piece. In this paper, the forward solution for a small right-cylindrical air-cored coil placed next to a layered conductor is based on the analytic solution provided by the transfer matrix approach. For an inverse solution, a modified Newton-Raphson method was used to adjust the conductivity profile to fit a set of multifrequency inductances in a least-squared sense. The approximate Jacobian matrix (sensitivity matrix) was obtained by the perturbation method. Numerical results of the forward solution are provided for cases of step, continuous conductivity profiles. Good estimates for the conductivity profile were obtained. Experimental eddy-current tests are performed by taking the difference in inductance of the coil when placed next to a reference conductor and next to a layered conductor over the range 100 kHz - 1 MHz. Inverse results based on experimental and simulated data verified this method.