Presented as Thesis (Ph. D.)--University of Wisconsin--Madison, 1929. / Reprinted from Journal of the Franklin Institute, vol. 213, no. 1 (Jan. 1932). Includes bibliographical references.

Thesis (Ph. D.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains x, 144 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 117-120).

Magnetic Induction Tomography (MIT) is a non-invasive technique that utilises the passive electrical properties of a material to produce cross-sectional images. In MIT system, the signals detected by the sensors must be measured using a phase sensitive technique. A sub-millidegree phase stability is typically required for biological tissues, where the objects to be imaged have relatively low conductivity (< 2S/m). The phase noise and thermal related phase drift in the receiver's signal chain of currently available MIT systems are the major limiting factors of MIT performance for practical measurements. This thesis describes the development of a high precision DSP based signal measurement platform. It utilises multi-channel high speed digitisers to sample two or more signals simultaneously and phase differences between the signals are calculated by using FFT based algorithms. The algorithms are optimised for higher speed performance using parallel processing on both multi-core PC and graphic card processors. A faster approach based on a dedicated DSP processor for each MIT channel is later suggested to reduce data transfer speed limitations between the digitiser and the signal processing hardware. By formulating a phase noise estimation model to optimise the digitiser's setting, it is shown that better phase measurement precision and dynamic range can be achieved. To improve the phase drift for practical MIT measurements, a novel instrumentation amplifier was designed and it was incorporated into a new 5-channel annular array MIT prototype. The prototype was fully developed into a 14-channel Cardiff MIT-MKIIa system and both systems demonstrated sub-millidegree phase noise performance with a highly stable phase drift characteristic. To further investigate the MIT system for practical applications, phantom measurements were carried out to investigate the MIT system precision for detecting cerebral stroke and a single channel multi-frequency MIT system was built to perform spectroscopy measurements on biological samples.

A finite difference algorithm for solving the forward modelling problem of geo-electromagnetic induction in two-dimensional (2D) structures has been developed in this thesis. The governing equations have been modified to solve for the anomalous field by separating out the 'host' field which is assumed to be the field generated by the one-dimensional (1D) conductivity distribution on the left hand side of the model. This was done to prevent the small anomalous fields being masked by the much larger host field due to the finite length of the computer word. One of the most important features of this program is an automatic gridding subroutine which greatly reduces the amount of time required to design a suitable grid for a model and removes the human element from such grid design. Up to 20 periods can be submitted to the model at one time and specific locations (e.g. the locations at which field data are available) can be added to the automatically generated grid. Integral boundary conditions at the surface and bottom (z = d) of the model eliminate the need to extend the grid above the earth's surface or down into the half-space underlying the model. The program has been used to perform a 2D inversion of magnetoteliuric data from a NS profile in Sardinia. The magnetoteliuric responses from two sites along this profile indicated that the structure underneath them could not be considered to be solely 2D. To examine the conductivity anomalies perpendicular to the profile indicated that are affecting the results at these two sites, 2D inversions were performed on the data to obtain their EW conductivity models. The apparent resistivity curves from the models fit the data fairly well at both sites especially at short periods. Many features of the models were in agreement with the 2D model along the profile obtained by Peruzza et al. (1990) and they also provided insight into the geological structure of the area. A study was made of the behaviour of 2D induction arrows over a buried conductivity contrast. Although the general trend of in-phase arrows is to point towards the regions of high electrical conductivity, some investigators have found small amplitude in-phase arrows that point away from these same regions. Reversals such as these, which do not behave according to the general trend, can cause confusion and erroneous interpretation of the in-phase induction arrows. Using a model with two semi-infinite conducting plates, one at the surface and one buried at a depth d in a layered half space, it was found that the period at which a reversal in the in-phase induction arrow direction occurs was a function of the apparent resistivity of the layered host. Anomalous behaviour was found in the short period in-phase arrows from which the coast effect had been removed. The problems in interpretation of such arrows was discussed. Finally a 2D inversion scheme was discussed in which a 2D forward modelling program was incorporated with a minimization routine MTNDEF. First an investigation was made into the relative merits of using the impedances ZTE, ZTM, Zave and Zeff to calculate the ID inversions that are combined to form starting models for the 2D inversions. A subsequent 2D inversion of the North American Central Plains (NACP) anomaly results in a best fit model whose responses show good agreement with the field data from 20 sites. Tests have been performed to ensure that an oversimplification of the starting model is not responsible for the lack of certain features found by other authors. It is concluded that the incorporation of these features in the model is not required in order to obtain a good fit to the field data. / Graduate

No description available.

The main topics of this dissertation are the hardware and the software developments in magnetic induction tomography imaging techniques. In the hardware sections, all the tomography systems developed by the author will be presented and discussed in detail. The developed systems can be divided into two categories, according to the property of the target imaging materials: high conductivity materials and low conductivity materials. Each system has its own suitable application, and each will thus be tested under different circumstances. In terms of the software development, the forward and inverse problems have been studied, including the eddy current problem modeling, sensitivity map formulae derivation and iterative/non-iterative inverse solvers equations. The Biot-Savart Theory was implemented in the ‘two-potential’ method that was used in the eddy current model in order to improve the system’s flexibility. Many different magnetic induction tomography schemes are proposed for the first time in this field of research, their aim being to improve the spatial and temporal resolution of the final reconstructed images. These novel schemes usually involve some modifications of the system hardware and forward/inverse calculations. For example, the rotational scheme can improve the ill-posedness and edge detectability of the system; the volumetric scheme can provide extra spatial resolution in the axial direction; and the temporal scheme can improve the temporal resolution by using the correlation between the consecutive datasets. Volumetric imaging requires an intensive amount of extra computational resources. To overcome the issue of memory constraints when solving large-scale inverse problems, a matrix-free method was proposed, also for the first time in magnetic induction tomography. All the proposed algorithms are verified by the experimental data obtained from suitable tomography systems developed by the author. Although magnetic induction tomography is a new imaging technique, it is believed that the technique is well developed for real-life applications. Several potential applications for magnetic induction tomography are suggested. The initial proof-of-concept study for a challenging low conductivity two-phase flow imaging process is provided. In this thesis, a range of contributions have been made in the field of magnetic induction tomography, which will help the magnetic induction tomography research to be carried on further.

Near-field magnetic induction (NFMI) is a short range wireless technology that uses loop antennas coupled by a magnetic field. NFMI antennas are electrically small and thus extremely inefficient and narrow band, making system design for multi-user and high-bitrate applications challenging. The goals of this thesis are to develop a test platform suitable for NFMI antenna testing, to model, design and test NFMI antennas that have high bandwidth-efficiency, and to explore the possibility of using MIMO (multiple-input multiple-output) to increase the capacity of the NFMI channel. This thesis provides system implementations, test results, and channel modeling to aid in the design of future NFMI systems. Implementation of a multi-channel discrete-time wireless system are provided for PC-based software and FPGA-based firmware as a platform for antenna testing. Optimized antenna designs in terms of efficiency and bandwidth are presented, achieving the theoretical bandwidth-efficiency bound for small antennas. Preliminary modeling and simulation results for the NFMI-MIMO channel are included, which show that the information-theoretic capacity of the NFMI-MIMO channel is approximately double the standard single-antenna NFMI capacity at 10 bits/s/Hz.

The master thesis is focused on a remanent magnetism of electromagnets of DC contactors. In the first part of the thesis deals with the formation of a magnetic field in different kinds of materials, description of their magnetization and sorting. The next part is focused on the principle and history of an electromagnet. Negative effects and remanent induction of the contactor are described in the end of the theoretical part. The second part is a practical part. First, the reaction times were measured on a pair of DC contactors. Using the obtained data, an electronic circuit was designed to minimize the difference in switching times of the given DC contactors.

Electrical capacitance tomography (ECT) is a technology that images the dielectric permittivity distribution of materials under test. ECT has been used as a tool for process monitoring in particular for two-phase flow measurement. These applications mainly focus on the dielectric samples, whose conductivity is negligibly small. This thesis studies ECT imaging with conductivity considerations. The conductive materials will affect the capacitance measurements and introduce difficulties in the ECT image reconstruction. This thesis presents solutions based on ECT to image material of different values of conductivity in different practical process or monitoring scenarios: the conductivity within materials under test is considered to be higher than 10^6 S/m, or less than 10 S/m. This work consists of the following innovative steps. (i) Through an ECT monitoring, floating (i.e., electrically non-grounded) metallic samples are imaged as dielectric illusions and the analysis of capacitance measurements over the conductors is delivered. (ii) Magnetic induction tomography (MIT) is firstly used for locating grounded metallic samples, thereafter as an assistant method to guide ECT to image the dielectric components. (iii) In low conductivity case MIT, as an indicator of conductive material again, helps ECT to solve multiphase flow problems. (iv) The multi-frequency complex ECT measurement provides a potential method to improve the ECT imaging ability for both conductive and dielectric materials. The first three ideas have been testified by both simulated and experimental results, while the fourth part is simulation-based results only on current stage.

Present demagnetization methods for large power system transformers are time consuming and can be dangerous to persons performing demagnetization. The work of this thesis was to develop improved demagnetization methods and to construct an automated instrument that would implement the methods developed. One previously developed method was analyzed for effectiveness. Then, two new methods for demagnetization were developed and also analyzed for effectiveness. An automated test instrument prototype was redesigned to be able to accommodate these methods and to improve the safety of the user. The previously developed method attempts demagnetization based on current flow behavior characteristics. The first new method is a magnetic flux estimation based on saturation time. The second new method is also based on measuring saturation time, modified to account for the variable voltage loss due to wire resistance. The second of the two new methods developed proved to be the most effective for demagnetization and was able to demagnetize a transformer within an error margin of 2%. The instrument designed to perform the demagnetization with this new routine is now in early production stages for an expanded field trial with transformer maintenance teams.