532 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010 Double Coil-Less Fluxgate in Bridge Configuration M. Butta , P. Ripka , Joaquín Pérez Navarrete , and M. Vázquez Faculty of Electrical Engineering, Czech Technical University in Prague, Praha EU 16627, Czech Republic Universidad Politécnica de Valencia, ETSIT, Valencia 46022, Spain Institute for Materials Science of Madrid, CSIC, Madrid 28049, Spain In this paper, a new method for excitation of coil-less fluxgate is presented. The purpose of this method is to reduce the spurious component of the output voltage, allowing us to increase the amplification. The method is based on the employment of two coil-less fluxgates in a double bridge, which injects pulsing current in opposite direction in each wire. By taking the difference of the voltages on the two wires, we suppress the component of the voltages, which does not change under application of external measured field. The sensitive axes are in opposite direction, so the wire feels opposite field. As a result, we will obtain an output voltage with low peak value, including only the component of the voltage that changes when we apply external field. Finally, we propose an improved version of the double bridge to allow the employment of two sensing elements with difference characteristics. This is obtained by optimizing the suppression of the spurious voltages and, at the same time, setting independently chosen values of exciting current for each wire. Index Terms—Bridge, coil-less, double, fluxgate, orthogonal, pulse excitation. I. INTRODUCTION F LUXGATE sensors are usually classified in two cate- gories: parallel and orthogonal ones. The mainstream of research and applications is focused on parallel fluxgates be- cause of their better properties. However, orthogonal fluxgates have been recently rediscovered thanks to the employment of magnetic microwire as core, instead of bulk cylinders. One of the advantages of the orthogonal fluxgate mode is the absence of any output at pickup coil in case of no external field measured, which allows us to increase the gain of the amplifi- cation. On the other hand, the amplification of output voltage in single-core parallel-type fluxgate is limited by the peak value of the voltage, which is present even without external field. Coil-less fluxgate is a special type of orthogonal fluxgate. The sensor is composed of a single magnetic microwire excited by the current flowing through it. The output voltage is taken on the terminations of the wire itself, without need of any coil [1]. Even if such a structure reminds other types of magnetic sensors, it has been shown that it works in fluxgate mode. The output of coil- less fluxgate sensors is composed of an idle resistive component and a magnetic component that shifts under the application of external magnetic field. Coil-less fluxgate has simple structure (only one wire without coils), but contrary to traditional orthogonal fluxgate, it has nonzero output voltage for zero external field. In fact, it is defined as orthogonal fluxgate because of the orthogonality between the excitation field and the sensing direction. However, its output is characterized by a high spurious component, as typically found in parallel fluxgates. We define the spurious component of the output voltage as that part of the voltage that does not change when magnetic field is applied to the sensor. For instance, a spurious component is the resistive voltage drop Manuscript received June 20, 2009; revised September 13, 2009 and September 16, 2009; accepted September 16, 2009. Current version published January 20, 2010. Corresponding author: M. Butta (e-mail: buttam1@fel.cvut.cz). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2033339 on the wire, which does not depend on magnetic field. Such a component of the voltage increases the maximum value of the voltage, resulting as a limit to maximum amplification, without giving any information about the measured field. In this paper, we propose a technique that allows us to over- come the problem of such spurious components by using two sensing elements in a double-bridge configuration. In this way, we can take advantage of the simple structure of coil-less flux- gate, with much lower limitation on the amplification gain. II. SENSING ELEMENT AND SIGNAL EXTRACTION For this research, we used magnetic microwires produced in cooperation with the team led by M. Vazquez at the Institute for Materials Science of Madrid, CSIC, Madrid, Spain [2]. The microwires are composed of a glass-coated copper core (40 m diameter). On the glass coating, we sputter a few nanometers thick layer of gold, and finally we perform electrodeposition of 8- m permalloy layer on it. The ac excitation current is injected into the copper core, cre- ating a circumferential field that saturates the Permalloy layer in circumferential direction. The voltage measured at the ter- minations of the wire is composed of two components. The first is the resistive component, i.e., the voltage drop on the copper wire resistance caused by the excitation current. The second component is the inductive component, following the saturation process of the permalloy layer; in this case, we can consider a single-turn coil composed of the wire itself and the return wire, including the circumferential cross section of the permalloy wire as magnetic area. As shown in [1], in case of he- lical anisotropy, the induction component of the output voltage shifts in time with applied dc measured field. Pulse excitation has been proven to be suitable for this kind of sensor because is allows strong reduction of power consumption (down to tens of microwatts). Signal extraction is performed by integrating the output voltage for a short time interval on both positive and negative peak [3]. Even if the sum of the integrals on positive and negative peak is null for no applied field, the in- tegrated voltage still has a high peak, which limits the maximum amplification factor. 0018-9464/$26.00 © 2010 IEEE