Design and implementation of magnetic field sensors for biomedical applications /

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2015
İnan, Ulaş Can
In this work, firstly the magnetic sensor types and their feasibility for biomedical applications are investigated. Then the air-cored induction coil sensor is chosen due to its advantages. Afterwards the usage of induction coils combined with amplifiers and connection types are studied. The biomedical applications requiring the use of magnetic field sensors are introduced. One of them, Lorentz Field Electrical Impedance Tomography (LFEIT) is explained in detail and experimental work is done for this application. In the second part, formulation of the magnetic sensor and the amplification circuitry, its frequency related components and frequency response is introduced. An optimization algorithm using MATLAB software is prepared to be able to determine the most sensitive disk shaped air-cored induction coil by its size parameters. This software is tested before moving further forward, according to the formulations given. After explaining the discrepancies caused by the problems encountered through the experimental process, an optimum induction coil sensor is designed for LFEIT application. The sensitivity and linearity of the system are analyzed. Two different amplifier configurations are used. First one includes AD797 as voltage amplifier and the sensitivity of this system is measured to be 17.1 (V m/A). The amplification factor obtained is 20. The maximum deviation from linearity for this case is computed as %15.3. Second one includes two cascaded amplifiers namely, AD600 and AU-1291. The sensitivity of this configuration is measured as 5.45 (V m/A). The maximum deviation from linearity is computed as %2.7. Second configuration is preferred since with the use of cascaded amplifiers, the amplification is increased to 160000. Although the coil with the best sensitivity using the present equipment is wound, the performance of the sensor is still inadequate and it is concluded that better sensitivity is required. There are also other promising coil designs obtained using the software developed, however they are not wound yet. Still, the performance is not expected to increase significantly. A sensitivity of about 10^7 is itself a great challenge which cannot be overcome by adjusting the amplifier since the signal to noise ratio (SNR) does not improve and the signal to be observed remains smaller than the noise level. However the software providing the optimum coil parameters for a specific case is useful in more than one application. Furthermore it can be modified to optimize other coil types such as rectangular coil, flat-spiral coil etc. allowing a very wide range of possible sensor designs. Other than LFEIT, wireless power transfer application of magnetic sensors is also investigated. The conditions that maximizes the power transfer efficiency are studied. The same algorithm used in determining the coil size parameters with maximum sensitivity is also applied to find the coil with the optimum quality factor, which is the most important condition for obtaining the best efficiency in power transfer. Four coil wireless power transfer is chosen, designed and implemented to compare with the analytical results. The efficiency of the designed system is measured as %7.6. This value is increased to %39 by using litz wire at the windings instead of enamel copper wire.

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Citation Formats
U. C. İnan, “Design and implementation of magnetic field sensors for biomedical applications /,” M.S. - Master of Science, Middle East Technical University, 2015.