IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-22, NO. 4, JULY 1986 299

A Fluxgate Magnetometer with a Metallic Glass Core A. ENGELTER

Abstract-A square-wave excited pulse-height fluxgate circuit is de- scribed, using a metallic glass core fluxgate and a minimum amount of active components. High reliability and linearity have been achieved.

I. CIRCUIT DESCRIPTION

T HE MAGNETOMETER circuit (Fig. 1) is powered by two voltage regulators (7806 and 7906), delivering

plus and minus 6 V from an 18-65-V dc supply. A diode (Dl) provides polarity protection; A voltage follower (74 1), connected by two equal resistors to the supply lines, ensures that the center tap is at half of the supply voltage.

The excitation current is derived from a precise 50-per- cent duty-cycle square-wave generator (CD 4047) buff- ered by two complementary transistors (T1 and T2). R6 and C8 act as a current amplifier as C8 is kept discharged while the fluxgate is in its high-impedance state and charges rapidly when the core saturates [ 11.

Due to the lack of grain boundaries and insensitivity to mechanical stresses, a ferromagnetic glass strip (Met- glas* 2826) seemed attractive as a core material. The movement of the grain boundaries of an ordinary high- permeability material, such as Permalloy, may cause some undesirable noise (Barkhausen effect). The fluxgate con- sists of a coil of 2300 turns of 0.2-mm + enameled Cu wire wound on a 70-mm X 4-mm + former, containing a 50-mm X 1.8-mm X 0.05-mm core.

Under the zero-field condition the current in the fluxgate will rise, determined by R6, C8, and the fluxgate imped- ance Lf. It will then fall off once C8 is fully charged. This produces two pulses of equal magnitude but opposite sign in each cycle.

When these pulses are applied to the bridge (BR1, R7, and C9), the smoothed (C10) output will be the mean be- tween the pulse voltage (zero) but practically without the ac components [2]. On the application of a magnetic field in the direction of the fluxgate axis, the core will saturate

Manuscript received June 16, 1985; revised February 12, 1986. The author was with the Institute for Marine Technology, P.O. Box

4874, Cape Town, 8000, South Africa. He is now at 9 Dalmore Road, 7945 Tokai, South Africa.

IEEE Log Number 8608363. @Registered service mark of Allied Chemical, Metglas Products.

faster in one excitation half-cycle and slower in the other one. This results, due to the finite rise time, in a differ- ence of the pulse heights, leading to an equivalent dc out- put of the bridge, roughly proportional to the external field (Fig. 2). One note of caution may be appropriate here: the discrete diodes in the power rectifier (type WM48) are not completely matched, and a bridge may have selected which had the minimum offset and temperature effects.

The feedback is used to improve stability and linearity and to select a convenient scale factor. This is achieved by connecting the bridge output to a noninverting integra- tor (CA3130). The output of this IC delivers a current to the fluxgate through R10, R1 1 , and P1, thus generating a magnetic field which compensates the external field. The output voltage of the integrator is, after suppressing spikes (R12, C14, and C15), a direct measure of the external field (Fig. 3).

11. PERFORMANCE The magnetometer showed good linearity, low noise,

and good stability, even when the critical components were heated.

111. SPECIFICATION The supply voltage V, is equal to 24 V (18-65 V); the

supply current Z, is 20 mA (at V, = 24 V); the sensitivity V,JB is 200 mV/,uT (20 mV/mG); the linearity is <0.2 percent. The operating frequency f is 2.3 kHz; the tem- perature stability is <0.2 percent (10-50°C); the noise (measured under laboratory conditions) is e 1 mV,, (5 nT,,), and the frequency response is 0-22 Hz ( - 3 dB). The sensitivity and linearity were measured in a .Helm- holtz coil calibrated at the Magnetic Observatory, Her- manus, Republic of South Africa.

ACKNOWLEDGMENT The author thanks the management of IMT for the per-

mission to publish this paper.

REFERENCES [ l ] M. H . Acuna, IEEE Trans. Mugn., vol. MAG-10, p. 521, 1974. [2 ] A. Engelter, Electron. Design, no. 9, Apr. 1975.

0018-9464/86/0700-0299$01 .OO O IEEE

300 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-22, NO. 4, JULY 1986

1 CO 4047

L

Fig. 1. Circuit diagram of magnetometer. All values are kQ and nF unless otherwise noted

mV METGLASS OPEN LOOP

l o o o m I

I 10

Fig. 2 . Voltage at C10 versus external field (feedback removed).

*"i---; CLOSED L O P

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Fig. 3 . Output voltage versus external field.