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An improved data acquisition system for contactless conductivity imaging

Çolak, İlyas Evrim
The previous data acquisiton system developed for the electrical impedance imaging via contactless measurements is improved to obtain measurements with a faster scanning speed of 0.15 sec/mm2. This system uses magnetic excitation to induce currents inside the body and measures the magnetic fields of the induced currents with an axial gradiometer. Gradiometer consists of two differentially connected 10000-turn coils with diameter of 30 mm and a transmitter coil of 100-turn coil of diameter 30 mm placed and magnetically coupled between them. Transmitter coil is driven by a sinusoidal current of 200 mA (peak) whose frequency is 14.1 kHz. A Data Acquisition Card (DAcC) is designed and constructed on PCB, thus elliminates the use of the Lock-In Amplifier Instrument (LIAI) in the phase sensitive measurements. User interface programs to control the scanning experiments via PC (MATLAB Scanner 1.0, HP VEE Scanner 1.0) and to analyze the acquired data (Data Observer 1.0) are prepared. System performance tests for the DAcC are made. Error in the phase sensitive measurements is measured to be 0.6% of the test signals. Minimum magnetic field density that can be detected is found to be 7 DT. Output stage performance of the DAcC is improved by using an integrator instead of an amplifier in the output stage. In this manner, maximum linearity error is measured as 6.60*10-4 % of the full scale for the integrator circuit. Thermally generated voltage drift at the sensor output is measured to be 0.5 mV/minute in the ambient temperature. Overall normalized standard deviation at the output of the data acquisition system is observed as to be in the order of 10-4. Mathematical relation between the resistive rings and conductive phantoms is studied. It is derived that maximum resistor value that can be distinguished in the resistive ring experiment which is 461 F, corresponds to the phantom