A low-power, highly stable electronic sensor circuit for measuring a small change in the ambient magnetic field with high sensitivity exploits the giant magneto-impedance (GMI) effect, in which the radio-frequency (RF) impedance of a fiber made of a suitably formulated material varies with the externally generated magnetic field to which it is exposed. The GMI effect has been observed in fibers thinner than a human hair made of amorphous (in the sense of lacking crystalline structure) alloys of cobalt, iron, silicon, and boron.

Prior GMI-based magnetic-field sensors have been built around GMI fibers as short as a few millimeters. In a typical prior GMI-based sensor, the reactive portion of the magnetic-field-dependent impedance is used as one of the frequency- determining elements of an oscillator. Unfortunately, oscillator frequency drift attributable to causes other than a change in the magnetic field can be large enough to be significant with respect to the frequency change associated with the magnetic-field change that one seeks to measure. Hence, there is a need for a GMI-based magnetic-field sensor that is not significantly affected by oscillator frequency drift. The present sensor circuit was invented to satisfy this need.

In this Magnetometer Circuit, a stable RF oscillator is used to excite the GMI effect, which is measured at DC by an operational amplifier stabilized by an input reference voltage. Values of inductance, resistance and capacitance shown here are only typical; other values could be better suited to specific applications.
The circuit (see figure) can be characterized as consisting of six functional blocks. The first functional block is a voltage regulator that serves as the source of a stable voltage reference potential, Vref. The second functional block is the combination of a very-lowfrequency operational amplifier (U1) and a feedback resistor (R1). The positive terminal of the reference potential source is connected to the noninverting input terminal of U1. Vref is chosen to bias U1 to a highly stable zero-magneticfield operating point that affords maximum allowable dynamic range for output voltage swings for the range of magnetic fields expected or desired to be measured.

The third functional block consists of a GMI fiber and a magnetic-bias coil wrapped around the GMI fiber. One end of the GMI fiber is grounded; the other end is connected via inductor L1 to the inverting input terminal of U1. When the impedance of the GMI fiber varies in response to a change in the magnetic field applied to the fiber, the resistance between the inverting input terminal and ground changes, resulting in a change in the DC amplifier output voltage, Vout. The magnetic-bias coil is used to fix the operating point of the GMI fiber at the level of applied magnetic field that affords the greatest change in impedance per unit change in applied magnetic field, thereby maximizing sensitivity.

The fourth functional block is a crystal oscillator, U2, connected to the ungrounded end of the GMI fiber via the series combination of capacitor C2 and resistor R2. The oscillator U2 generates a 0-to-5-volt RF square-wave signal to be applied to the GMI fiber to excite the magnetic-fielddependent impedance. Because the crystal oscillator is many times more stable than is an oscillator of the type used in prior GMI-based magnetic sensors, frequency drift of the oscillator is small enough to be considered insignificant.

The fifth functional block is a decoupling network comprising inductor L1 and capacitor C1. This network keeps the RF excitation signal out of the DC paths of amplifier U1: In slightly oversimplified terms, L1 acts as a short circuit for DC and an open circuit for the RF signal, while C1 acts as short circuit for RF between ground and the inverting input terminal of U1. Care must be taken so that the impedance of the GMI fiber does not become capacitive enough to resonate with inductor L1. If the impedance of the GMI fiber unavoidably becomes significantly capacitive, a damping resistor can be added in series with the GMI fiber to suppress the resonance.

Capacitor C2 passes the RF excitation while preventing DC coupling from the oscillator to the GMI fiber. Resistor R2 provides damping to prevent resonance of the combination of capacitor C2 and inductor L1. R2 can also be used to bias the RF excitation of the GMI fiber to the correct level (typically, a few milliamperes).

The sixth functional block is an analog- to-digital (A/D) converter. This block receives the analog output voltage (Vout) from amplifier U1 and converts it to a digital signal for further processing. Because the Vout signal has low frequency content, the A/D converter can be chosen to be of a low-bandwidth type to keep power consumption low.

This work was done by James D. Hagerty of the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. NRL-0006


This Brief includes a Technical Support Package (TSP).
Improved Magnetic Sensor Based on Giant Magneto-Impedance

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This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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