•>• Journal of Magnetism and Magnetic Materials 157/158 (1996) 442-443 journal of magnetism and magnetic

~ i ~ materials ELSEVIER

Magnetometric sensors with improved functional parameters

A. Moldovanu, Elena-Despina Diaconu *, C. Ioan, Elena Moldovanu Institute of Technical Physics, Bd. D. Mangeron Nr. 47, P.O. 3, 6600 la~i, Romania

A b s t r a c t

A functional study of TFS-3 fluxgate sensors developed for the INTERBALL space missions is presented. Good sensitivity and thermostability, a low offset of 0 - 2 nT, a thermal null shift of 0.04 n T / ° C and a peak-to-peak magnetic noise level of < 0.2 nT were obtained for a field frequency range of 0-1 kHz in the - 7 0 to +70°C temperature interval using special thermostabilization procedures.

Keywords: Fluxgate sensors; Magnetometry; Offset thermostability

The technological and metrological requirements im- posed on the new generation of magnetometric sensors used for the exploration of magnetic fields in space are very severe, considering the fact that they should work without any mechanical or thermal protection. These re- quirements imply robustness against mechanical stress, vibrations and temperature variations and offset and sensi- tivity stability within the temperature range - 70 to + 70°C. The Romanian three-axis fluxgate magnetometer SGR-7, mounted onboard the Czech subsatellite MAGION-2 of the AKTIVNII space mission included in the Solar Terrestrial Energy Program launched in November 1989, was equipped with TFS-2 fluxgate sensors. The performance of the TFS-2 sensor has been reported previously [1]. The Czech subsatellite of the INTERBALL 'tail ' space mission launched on 2 August 1995 has on board Romanian SGR-8 three-axis magnetometers equipped with Romanian TFS-3 fluxgate sensors. Both the TFS-2 and TFS-3 sensors were developed at the Institute of Technical Physics, Iasi, Ro- mania.

This paper presents a functional study of TFS-3 sensors with respect to temperature and magnetic induction varia- tion. The studied TFS-3 sensors are Vaquier-F~Srster type sensors with two open magnetic paths and a simple design. The magnetic material used in these sensors is Vacoperm 100 (Vacuumschmelze, Hanau, Germany). The geometri- cal specifications are: length 50 mm, diameter 11 mm and weight 10 g. The sensor includes excitation, sense, calibra- tion and feedback (compensation) windings.

As part of the sensor technology, before assembly of

* Corresponding author. Email: rangh@dragon.uaic.ro; fax: + 40-32-231132.

the sensors the glass-textolite and textolite components are subjected to 'thermal ageing' in the temperature inter- val - 7 0 to +150°C. For assembly of the sensors a common reference position of pair components is used: all the main components (the main framework, the excitation winding coil, the core supports) are solidly attached at one end of the ferroprobe and can move freely at the other end. Two parallel Vacoperm 100 tapes (length 35 mm, width 2 mm and thickness 0.05 mm) are also attached at one extremity of the supports. This assembly procedure confers better thermal stability. For functional stabilization, the sensors were subjected to cycles of thermal shocks be- tween - 70 and + 80°C, vibrations of frequency between 5 and 2500 Hz on six occasions with an overload of 0 .2-15g (10 rain each) and 20 mechanical shocks of 20g amplitude and 5 ms duration each (where g is gravitational accelera- tion).

We measured the sensitivity, the offset and the mag- netic noise of 20 individual sensors. All the measurements were performed with the sensor situated in a five-layer permalloy P-80 shielding system [2] with a transversal shielding factor of 5 X 104. The shielding system allows a thermostatic enclosure and a mechanical 'flipper' device to be placed inside.

In order to estimate the sensor output signal, selective measurements of the second harmonic component were used. The sensor was untuned and not included in the feedback magnetometric circuit. The measurements were performed using a pulse excitation field (f0 = 20.6 kHz). The excitation current was 600 mApp. The sensitivity was determined from the transfer characteristics obtained in the field range from - 500 to + 500 nT with 50 nT step in the temperature interval from - 7 0 to + 70°C. All the transfer characteristics are linear outside the field range from - 5 0

0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0304-8853(95)01053-X

A. Moldovanu et a l . / Journal of Magnetism and Magnetic Materials 157/158 (1996) 442-443 443

10

"SB ~ x T=+60 F_ , . ~

I , , I , t , - # • I - I • I -

0 -500 -t,O0 -~0 -200 -iO0 0 I00 ~ 3~0 tOO 500

B(nT)

Fig. 1. Fluxgate output voltage versus applied field at various temperatures.

to + 5 0 nT. The room-temperature sensitivity was (17 _+ 0.2) /xV/nT. The variation in the - 7 0 to + 70°C interval was only 3% over the whole temperature range. Fig. 1 presents the transfer characteristics of the sensor. The second harmonic signal does not cancel near the null field (Fig. 1), probably due to a residual signal on 2f0 present in the excitation signal.

The two lines which approximate the transfer character- istic intercept on the abscissa (field axis) at a point which corresponds to a field value equal to the sensor offset. The room-temperature offset values of the individual sensors do not exceed _+ 5 nT, with most of the sensors having an offset of < 2 nT. The temperature drift of the offset is

0.04 nT / °C . Fig. 2 presents the variation of the offset of the sensor over the ( + 70 to - 7 0 to + 70°C) thermic cycle. The weak hysteresis in the offset versus temperature curve is the effect of the spread of the measured values. This effect presumably originates either in the small me- chanical deformations undergone by some sensor compo- nents (core supports, excitation winding coil) with temper- ature variation, or in the condensation and solidification of water vapour from the air on the sensor components. It is

@ OF

--101 L t -70 -50 -30 -1 ; 1; 30 50 70

TO[

Fig. 2. Offset variation versus temperature.

Fig. 3. Fluxgate noise; 45 s time plot of the sensor output.

difficult to control for these effects in our experiment. The maximum variation of the offset in the + 50 to - 4 0 ° C temperature interval corresponds to a drift of 0.044 nT/°C . The temperature stability of the sensitivity and offset of TFS-3 sensors in the specified temperature range is better than those reported for both the ring core 6-81 MoPe fluxgate Sensors, which equipped the EXOS-D magne- tometer of the Akebono satellite launched in 1989 [3], and the fluxgate sensors reported by Ripka [4].

The magnetic noise of the studied sensors was mea- sured using the method described by Acuna [5]. The p - p noise level does not exceed 200 pT. A 45 s time plot of the sensor output is shown in Fig. 3. The magnetometer 's pass band is 0 - 1 0 Hz and the p - p noise level of the magne- tometer (itself) together with the recorder does not exceed 200 pT.

Our conclusion is that the ' thermal ageing' of the components, the common reference position of the pair components used in the assembly of the sensors and their thermal stabilization have improved the functional parame- ters of TFS-3 fluxgate sensors.

Although the TFS-3 sensor Was designed for dc magne- tometers, we have tested its utility for 0 - 5 kHz alternating field measurements using a sample and hold detection method. The experiments show that the sensor has the capability of measuring fields with frequencies between 0 and 5 kHz in the 0 - 1 0 0 /~T range.

The functional characteristics and performance of the TFS-3 sensor place it among the best fluxgate sensors with open magnetic paths. The use of the TFS-3 sensor for the Romanian SGR-8 magnetometers of the INTERBALL space missions confirm this.

R e f e r e n c e s

[1] E. Moldovanu, H. Chiriac, A. Moldovanu, E.D. Diaconu and G. Pop, 13th Gen. Conf. of the European Physical Society, Regensburg, Germany, ECA 17A, 1993, p. 1206.

[2] A. Moldovanu, H. Chiriac, C. Ioan, E. Moldovanu, M. Lozo- vanu and V. Apetrei, Appl. Electromagn. and Mechan. (1996), to appear.

[3] H. Fukunishi et al., J. Geomagn. Geoelectron. 42 (1990) 385. [4] P. Ripka, Sensors and Actuators A 33 (1992) 129. [5] M.H. Acuna, IEEE Trans. Magn. 10 (1974) 519.