L. A. P. Gusmão

High sensitivity giant magnetoimpedance (GMI) magnetic transducer: magnitude versus phase sensing

This paper presents the results of investigations recently done by our research group that lead to a new and much better approach to the design of GMI (giant magnetoimpedance) magnetic transducers, which considers the changes in the impedance phase characteristics of GMI sensors due to varying low-intensity magnetic fields, instead of the usual impedance magnitude characteristics considered in the GMI literature. The development process of this new class of magnetic field transducers is discussed, beginning with the definition of the ideal conditioning of the GMI sensor elements (the dc level and frequency of the excitation current and the sample length), proceeding to compare the differences observed between the impedance magnitude and phase of GMI sensors and closing with the electronic circuits that condition ribbon-shaped GMI sensors and read their phase or magnitude variation as a function of the longitudinal magnetic field. Simulation studies, including the full electronic circuit and based on the experimental data obtained from measured GMI curves, have shown that an improvement in the sensitivity of GMI magnetometers larger than ten times can be expected when phase-based transducers are used instead of magnitude-based transducers. Finally, it is also shown that phase-based transducers are highly adequate for miniaturization purposes.

Ring shaped magnetic field transducer based on the GMI effect

In this paper the design of a magnetic-field-to-voltage transducer based on the giant magnetoimpedance phenomenon (GMI) is proposed, characterized by an innovative geometric configuration. In order to attain the best near-field sensibility and far-field immunity, the transducers sensitive element and electronic circuit were planned and implemented. By thoroughly characterizing them it was possible to obtain an estimate of the transducers sensibility, which is approximately 12 V Oe−1. This value is comparable to those observed in two of the most important existing magnetic sensors: the fluxgate and the Hall effect sensor. The main application of the developed transducer is the localization of magnetic foreign bodies in humans, based on a previously developed and tested SQUID sensor technique. In order to provide a better interpretation of the experimental results, a theoretical model of the magnetic field associated with a needle and of the signal it generates in the transducer was created. Measurements with a needle were performed to analyze the behavior of the prototype, which has a high sensitivity, as expected, but presents strong hysteresis, lack of linearity and low immunity to uniform fields. However, despite the improvements that can still be done and have already been identified, the developed transducer has many promising applications, and has the advantage of reduced fabrication and operation costs.

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

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.

Medição não-invasiva de ondas de pulso arterial utilizando transdutor de pressão MIG

This manuscript presents a high sensitivity pressure transducer, developed at the Biometrology Laboratory of PUC-Rio for biomedical applications and examined in this article for the registration of arterial pulse waves. Such transducer is based on the Giant Magnetoimpedance (GMI) phenomenon and is an evolution of an unit previously developed at the same Laboratory. Knowing the main characteristics of the the GMI strips used as sensors, the configuration which should yield the highest possible sensitivity has been implemented and tested. The upgrade introduced to the original project has increased its sensitivity and enabled us to record not just the carotid arterial pulse wave (as the previous transducer configuration did), but also the radial and brachial arterial pulse.

Point matching: A new electronic method for homogenizing the phase characteristics of giant magnetoimpedance sensors

Recently, our research group at PUC-Rio discovered that magnetic transducers based on the impedance phase characteristics of GMI sensors have the potential to multiply by one hundred the sensitivity values when compared to magnitude-based GMI transducers. Those GMI sensors can be employed in the measurement of ultra-weak magnetic fields, which intensities are even lower than the environmental magnetic noise. A traditional solution for cancelling the electromagnetic noise and interference makes use of gradiometric configurations, but the performance is strongly tied to the homogeneity of the sensing elements. This paper presents a new method that uses electronic circuits to modify the equivalent impedance of the GMI samples, aiming at homogenizing their phase characteristics and, consequently, improving the performance of gradiometric configurations based on GMI samples. It is also shown a performance comparison between this new method and another homogenization method previously developed.

Progress Toward a Hundredfold Enhancement in the Impedance Phase Sensitivity of GMI Magnetic Sensors aiming at Biomagnetic Measurements

Over the last decade, Giant Magnetoimpedance (GMI) sensors have been intensively studied as a new possibility for measuring ultra-weak magnetic fields. Pursuing the measurement of biomagnetic fields, this paper presents a newly developed method that allows the achievement of about a hundredfold enhancement in the impedance phase sensitivity of GMI sensors and, consequently, increases the sensitivity of phase-based GMI magnetometers. Also, the electronic circuit implemented to realize the proposed method is presented and discussed, and the obtained experimental results are shown.

An enhanced electronic topology aimed at improving the phase sensitivity of GMI sensors

The giant magnetoimpedance effect (GMI) is used in the most recent technologies developed for the detection of magnetic fields, showing potential to be applied in the measurement of ultra-weak fields. GMI samples exhibit a huge dependency of their electrical impedance on the magnetic field, which makes them excellent magnetic sensors. In spite of GMI magnetometers being mostly based on magnitude impedance characteristics, it was previously verified that sensitivity could be significantly increased by reading the impedance phase. Pursuing this idea, a phase-based GMI magnetometer has been already developed as well as an electronic configuration capable of improving the phase sensitivity of GMI samples. However, when using this topology, it was noted that the sensitivity improvement comes at the cost of reduced voltage levels in the reading terminal, degrading the signal-to-noise ratio. Another drawback of the electronic configuration was that it was not capable of enforcing a linear behavior of the impedance phase in the function of the magnetic field in a given operation region. Aiming at overcoming those issues and then optimizing the behavior of the circuit developed to improve the phase sensitivity, this paper mathematically describes a completely new methodology, presents an enhanced newly developed electronic topology and exemplifies its application.

Modelagem da sensibilidade de amostras GMI por redes neurais

Over the past few years, several studies have been developed in order to quantitatively model the GMI effect (Giant Magnetoimpedance). However, these models adopt simplifications that significantly affect its theoretical-experimental performance and its generalization capability, and models that incorporate parameters that generate asymmetry - AGMI (asymmetric GMI) - such as the DC level of the excitation current of the GMI samples are still rare. This work aims to develop a new model, sufficiently general, which also incorporates the asymmetry induced by the DC level of the excitation current, capable of guiding the experimental procedures of characterization of the GMI samples. Thus, this paper proposes, presents and discusses the use of a computational model based on feedforward Multilayer Perceptron Neural Networks to model the impedance magnitude sensitivity and impedance phase sensitivity, of the GMI effect, as functions of the magnetic field, for Co70Fe5Si15B10 ferromagnetic amorphous alloys. The proposed model allows obtaining these sensitivities based on some of the main parameters that affect it: length of the samples, DC level and frequency of the excitation current and the external magnetic field.

Transdutor de pressão, baseado nas características de fase do efeito GMI, destinado a aplicações biomédicas

Neste trabalho e apresentada a evolucao de um transdutor de pressao para aplicacoes biomedicas, desenvolvido pelo Laboratorio de Biometrologia da PUC-Rio. O mesmo utiliza-se do fenomeno da Magnetoimpedância Gigante (GMI) como elemento intermediario no processo de transducao, o qual compreende a conversao da grandeza pressao em campo magnetico e, posteriormente, deste em tensao. Dentre as possiveis aplicacoes biomedicas destacam-se sua utilizacao no registro da onda de pulso arterial e na medicao da velocidade de propagacao da onda de pulso (VOP). Ao longo do texto, destacam-se os fatores que propiciaram o aumento da sensibilidade do transdutor para cerca de 50 mV/Pa. Isto representa um aumento de 50 vezes na sensibilidade do transdutor, quando comparado a prototipos previamente desenvolvidos. Essa melhoria deve-se, basicamente, a utilizacao da fase da impedância do efeito GMI, ao inves das caracteristicas de modulo da impedância, e a nova configuracao estrutural

Biomedical comparison of magnetometers for non-ferromagnetic metallic foreign body detection

The location and surgical removal of foreign bodies in patients is still challenging, especially for firearm projectiles, which are small and non-ferromagnetic. Conventional location techniques use ionizing radiation, posing health risks while the procedures often last several hours and end unsuccessfully. The use of high sensitivity magnetometers provides a noninvasive and innocuous alternative for metallic foreign body location. The developed technique consists of a primary AC magnetic field generator (a solenoid) inducing eddy currents in nonferromagnetic metallic foreign bodies, which results in an ultra-low secondary magnetic field that can be measured. This work compares the initially developed theoretical technique using Superconducting Quantum Interference Device (SQUID) magnetometers with the developed prototypes using lower cost alternatives, namely Giant Magnetoresistance (GMR) and Giant Magnetoimpedance (GMI). The comparison is based on biomedical device requirements for widespread clinical application. The proposed GMI location system is deemed the most qualified