Magnetic field transducers based on the phase characteristics of GMI sensors and aimed at biomedical applications

Abstract

For the last four years the Laboratory of Biometrology of PUC-Rio has been working in the development of magnetic field transducers to be used in biomedical applications - especially in the three-dimensional localization of needles inserted in the human body and in the measurment of arterial pulse waves. While previous investigations were based on the behavior of the magnitude of the impedance of Giant Magnetoimpedance (GMI) ribbon-shaped sensors, this manuscript presents the preliminary results of a new research that considers the changes in the phase characteristics of GMI sensors due to varying low-intensity magnetic fields. In spite of being less explored in the literature, the work carried out so far indicates that the sensitivity of the phase can lead to more promising results than the ones already obtained with transducers based on the variation of the impedance magnitude. By means of examples showing that the sensitivity of the phase is affected by parameters (amplitude, frequency and DC level) of the AC biasing current that flows throught the sensor, this manuscript discusses how an ideal stimulation condition was derived in order to obtain more sensitive transducers. It is also examined the influence of the ribbon length in the sensitivity. A new conditioning electronic circuit - responsible for the excitation and measurement of the GMI sensor, and designed to work in the 100kHz to 5Mhz range - has been developed and is presented in the manuscript. Simulation studies of the complete transducer, including the conditioning circuit and based on data obtained from measured curves, have shown that an improvement of 10 to 100 times can be expected when compared to the sensitivity of previous magnitude-based transducers.