Fluxgate magnetometers for balloons and small satellites

\I)

Pergamon

Acta Astronautica Vol. 35. Suppl., pp. 89-98. 1995 Elsevier Science Ltd. Printed in Great Britain

0094-5765(94)00172-3

FLUXGATE MAGNETOMETERS FOR BALLOONS AND SMALL SATELLITES J.Rustenbach, H.U.Auster, A.Bogdanov, H.Bitterlich, K.H.Fornacon, O.Hillenmaier, RKrause, A.Lichopoj, M.Markwardt, H.J.Schenk, RSchrodter (MPE Garching, AuBenstelle Berlin, Rudower Chaussee 5, 12489 Berlin) A.Best, G.Scholz (GFZ Potsdam, LindenstaBe 7, 18240 Niemegk) Y.Yeroshenko, V.Stashkin (IZMIRAN Troizk, Moscow Region 142092) RGottfried-Gottfried (Fraunhofer Institut, GrenzstraBe 28,01109 Dresden) U. Lorreit (IMO Wetzlar, 1m Amtmann 6,6330 Wetzlar) P.Triska (Geophysical Institute of the CAS, Bocni 11,14131 Prague)

Abstract Referring to the magnetometer experiments onboard the Mars-96 balloon, and the magnetometer on EQUATOR-S we present the development of fluxgate magnetometers in the MPE-A Berlin for application on small satellites and balloons. Techniques to overcome limitations of field and time resolution by ranging, compensation and data compression are described. To reduce the costs of the magnetic cleanliness program a transportable magnetic cleanliness facility was developed. Results from the measurement of the INTERBALL subsatellite will be shown. The miniaturization of the fluxgate sensor and its electronic and the influence on noise and on offset drift are presented. The feasibility and first results of a near-sensor digitalization are discussed.

INTRODUCTION The ringcore fluxgate magnetometers FGMM 1 and 2 were designed for the russian Phobos missions (1988/89). Beside these magnetometers were applied in observatories and for magnetotellurical measurements. Today we are involved in the following magnetometer experiments: Mars94 - MAREMF (one of two single magnetometers) 1994 Interball - magnetic cleanliness 1994 Mars96 - MAGIBAL (one of two single magnetometers) 1996 Mars96 - MAREMF 2 (one of two single magnetometers) 1996 Equator S - (complete dual magnetometer) 1995 Relict - FGM-R (one of two single magnetometers) 1995 89

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Low-cost planetary missions

Based on two of these experiments, the magnetometer onboard the Equator-S satellite and the magnetometer onboard the Mars Balloon, we present some suggestions how to overcome metrological and technical problems and ideas how to miniaturise the sensor and electronics. The Equator S magnetometer uses a two sensor arrangement on a short boom. This instrument supplies online the direction of the magnetic field with an accuracy better than 1 degree to the electron beam experiment and measures field fluctuations up to 100Hz and O.01nT. An automatic ranging (between ±52000 nT and ±100 nT) in dependence of spacecraft position to the earth field is given controlled by the DPU. The MARS-94/96 mission provides the unique opportunity to perform simul-taneous magnetic field measurements aboard the orbiter and near the Martian surface with the help of surface stations and a balloon. The MAGIBAL magnetometer onboard the balloon consist of two triaxial fluxgate magnetometers. Two kinds of information are desired, namely data on the Martian static magnetic field (measured every 10 sec.) and on field fluctuations in the frequency range between 25 and 2 Hz. The following table shows the main parameters of the two dual magnetometers: EquatorS Earth orbiter macnetometer

Magibal Mars balloon macnetometer

2S00g 2x 250Q

200g 2xSOa

3W

0.1 W averaae

128Hz

0.1 Hz, (SOHz)

16bit

14bit

±100nT...±S1200nT

±200nT,±2000nT

0.01% + 0.1 nT O.Q1nT

0.5nT O.OSnT

mass electronic sensor powerconsumption time resolution resolution of the ADC ranaes expected accuracy dc-field ac-field data processing

data compression digital filtering health check programablequota control

data compression calculation of mean valueand standard deviation

Low-cost planetary missions

91

MEASUREMENT AND TECHNICAL PROBLEMS

Fjeld resolytion The ratio between necessary dynamic range and required resolution is in manycases higher than the resolution of the ADC (in most cases: 16 bit =±32768). The solution which we havechosen in the Equator S experiment is shown in Figure 1.

ADU : CryWil C85102; MUX: HIm. 5411 CPU: TM8320 or RTX 2000. Prollrlmmlnllllnlluille : A_mbler, C, FORTH RAM: ,RAM 32kW, 2x IDT712511 or 2KH8115755 EEPROM: 32kW, 2K211C2511, 8EEQ FPOA : ACTEL, 1020 Power DlIIlpltlon IIOOmW

'.OK180 mm

anT

InT

UaM

Figure 1:

The Equator-S Control System

The balloon magnetometer is working only with two analog outputs (200nT/4.5V and 2000nT/4.5V) and without any compensation. Both outputs are parallel converted and the software decides which range will be transmitted.

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Time resQlutiQn TQ reduce the internal sample rate from 8192 vectors/min to the telemetry rate, the data are digitally filtered in the DPU. FQr increasing the effective data rate a version ot a nolselsss data compresslon Qriginating from image processinq and mQdified tor our purposes is used. The expected compression effect is 2-5. The data compression decreases the high redundancy Qf magnetometer data withQut any data loss. One ot the consequences ot data cornpresslon is that since the real compression effect is unknown there is no direct connection between the telemetry quota and the time resolunon (vectors per second), SQ with fixed telemetry quota the time resolution has to be controlled in dependence on the current data. A second consequence is that in the case ot noisy data transrnlsslon the data IQSS will be high. The first flight test ot the compression alqorithm will be during the Mars-94 rnisslon. Beside the uncomprsssed mode two kinds ot coder can be switched on. Upon using the best (maximum) coder one damaged bit may cause the IQSS ot a whole telemetry frame. FQr this case a simplified (minimum) coder with fixed bit positlons may be used (via digital command) SQ the IQSS Qf one bit results in the damage ot one vector only. We have an extreme situation on the Martian balloon. Since the power consurrpfion and telemetry rate are limited, the magnetometer will be switched on every 10 sec tor 1 sec with a sampling rate ot 20 msec. The data will be rated in twQ classes. First, the data with high prlorlty - these are the meanvalues ot the six data channels each 10 sec. second - the informatlon about the magnetic field in the frequency range between 2 and 25 Hz (orlqlnal data, standard devlatlon or FFT) and the nousekeeplnq data -with a lower prlorlty- which are compressed and added to the meanvalues up to the limit ot the transrnlsslon rate. Magnetic cleanliness An expensive magnetic cleanliness program like that tor Cluster was up to nQW not possible for Russian spacecraft. SQ we have had to develop a flexible, IQW cost measurement system to determine the magnetic properties ot pavload, SUbsystems and ot the whole spacecraft where- and whenever feasible. We use a differential sensor arrangement on a moveable guide bar to determine the magnetic disturbance sources also under disturbed condhlons. TWQ kinds ot apolicatlons are possible: TQ determine the magnetic properties ot the boom, ot the experiments and ot the spacecraft with the help ot a gradiQmeter sensor which is moveable over 2.500 (proflle mode). These measurements will be carried out both during the test periods as well as finally, during the launch preparation. TQ determine the magnetic fields caused by electric current with the help ot statlonary measurements and switching on and Qff ot experiments and service systems (see measurements ot the solar panels in Toulouse)

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Low-cost planetary missions

This measurement method has the following advantages: The possibility to measure profiles near the spacecraft and its boom in arbitrary direction. Nocompensation of theearth field and its variations with the help of a reference magnetometer is necessary. The independence from varying (spatial and temporal) sources of disturbances in the surrounding of the spacecraft. Theon-line dataevaluation, which supports the search for sources of magnetic disturbances. The measurement accuracy is listed in the following table. distance

resolution

moment of the dipole

10cm

10nT

O.1mAm 2

30cm

10nT

1.4mAm2

100cm

50nT

250mAm 2

Figure 2 shows the measurement of the magnetic moment of the Interball Subsatellite in Prague in a laboratory not specially prepared for magnetic measurements.

Figure 2: The magnetic measurement of the Interball subsatellite

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Low-cost planetary missions

Although we know that this method doesn't substitute the real measurement of the magnetic moment and its induced part in a coil system, we believe that it can supply a sufficiently good estimate of the magnetic properties.

Sub.atallHe C2 Type

top view

X l-

e

mea.ured calculated

y

i ~\ 8

:

Z

I

500 nT

E.rlh Flald : Bx(a)

Bx- 20000 nT Bz - -40 000 nT

Figure 3: The magnetic moments of the Interball subsatellite The curves in Figure 3 show the magnetic field component perpendicular to the profile. The profiles were measured in two directions (X and Y) and at two distances (50cm and 80cm) from the subsatellite. The calculated fields of assumed dipoles (red arrows) of the Interball subsatellite in the position of the two sensors (the base length of the gradiometer is 20cm) are drawn in blackcolour, theirdifference in blue colour and the measured difference in red colour. All marked dipoles are on the order of 20 to 130mAm2. Other positions of the subsatellite with respect to the Earth field certify, that all these dipoles are of remanent nature. The residual field is less than 500mAm 2. The field in the point of the magnetic field sensor (on top of the satellite) is above 1000nT in all three components. The sources are the springs for deploying the four booms witha residual moment of (-8mAm 2,-5mAm2,-5mAm2) at a distance of gem. If the springs are substituted by a nonferromagnetic material the influence can be reduced to 500nT in X direction.

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Low-cost planetary missions

MINIATURISATION OF ELECTRONICS AND SENSOR Miniaturization of the electronics Applying the usual analog electronics which we have used for the MARS-94 Magnetometer we have reduced the electronics for the small satellite Equator-S and we have tried to substitute the whole analog electronics by using a MARS-94 DPU (Figure 4). preampll""

notch niter

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84Hz

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Figure 4: Three Variants of the fluxgate sense electronics In the first case, the field independent frequencies (fo and 3fo with an amplitude of 2V after the preamplifier) are diminished by notch filters and so the field-proportional 2fo signal (O.2mV/nT after the preamplifier) can be amplified by V1. In the second case (no filters at all) the requirements for the phase-sensitive detector are increasing by this factor. In the third case (the digital fluxgate magnetometer) the requirements for the sampling rate of the ADC and the DPU performance become extremely high. The 2fo signal is digitalized at its second harmonic and meanvalues are online phase-sensitively calculated. In all three cases the noise level of the electronics was low enough, so that the sensor noise of 5pT/%Hz limits the accuracy of the magnetometer. The offset temperature dependence of the analogue electronics of the magnetometers for MARS-94 and Equator-S measured over several temperature cycles is in both cases less than 10pT/K. The different behaviour is caused by different sources, the filter in the first and the phase-sensitive detector in the second case. Noise and temperature dependence are shown in Figure 5 and 6.

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Low-cost planetary missions

Figure 5

signal (1 nT) noise1 -

noise ratio of three variants of sense electronic analogue electronics for the MARS-94 mission

reduced analogue electronics for the Equator-S mission noise3 - digitolizotion ofter the preamplifier

noise2 -

0'0'

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,

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[lte] NOISEI

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[sed NOISE2

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s [sec] NOISEJ

Figure 6

temperature drift of the two variants of sense electronic templ :offset 1 -

temp2:offset2 -

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~

analogue electronics for the MARS-94 mrsaron

reduced analogue electronics for the Equator-S mission

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0.3

0.25

0.25

0.2

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0.15

0.1'

0.1

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0.05

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-0.06

-0.05

-0.1

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-10

10

20

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['C]' temol :OffSET'

40

so

ee

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10

20

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In

Low-cost planetary missions

Miniaturization of the sensor The first step to reduce sensor weight , power and dimension for the Mars balloon experiment was the modification of the measurement of the three components by using the fully tested ringcore sensors. The two single sensors are located so that the ringcore is placed in the plane defined by the measurement direction of the single sensor and by the z direction of the sensor system. Thus the outside windings measure the field-proportional signal in the z direction over both ringcores . The three axes are therefore very close to each other (25mm), and only two ringcores need to be driven to obtain a three component measurement. Furthermore , this sensor system has the advantage that fields in the ring core plane perpendicular to the measurement direction of the two single sensors will be compensated automatically by the feedback field of the z component. Figure 7 shows the Equator-S sensor compared with the balloon sensor .

The Equator S Sensor

The Balloon Sensor

mass: noise: offset:

mass: noise: offset:

250g <5pTIv'Hz <1 nT/1 0 days

50g <10pTIv'Hz <3nT/10 days

Figure 7: Two variants of fluxgate ringcore sensors

The second step was, looking for new commercial developments in the magnetic field measurement (for instant for highway control , orientation systems or medical applications). We found in Germany two institutes are developing magnetic field sensors , the Frauenhofer Institute in Dresden and the Institute of Micro structur technology and Optoelectronics (IMO) in Wetzlar. The Frauenhofer Institute produces Fluxgate sensors in a C-MOS technology (see Figure 8). The IMO Wetzlar has developed a magneto-resistive sensorsystems with an integrated thin film compensation line for dynamic range linearisation.

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Low-cost planetary missions

The main parameters are the following : Sensitivity : Noise: Zero offsett drift: Chip size: Power consumption:

3,3 mVNlkAlm 0,5 nTI/Hz 1 IJT/K reducing by a factor of 100 is possible 0,9 x 1,0 mm2 10mW

We are testing now both sensor types for space applications. If such a system satisfies the metrological requirements it is extremely suitable for small satellites and surface stations because weight and power consumption of the sensor itself are negligible. Excitation coi ls Figure 8:Fluxgate Magnetic-Field Sensor Monolithic Integrated on Silicon Permalloy

Pic:k-up coils

Cross-sectlon of one solenoid with ferromagnetic core

main parameters : Sensitivity: Noise: Core dimension :

Planardouble coref1uxgale sensor

0.15V/(kHz*T)

Excitation coil: 2*55 turns,280 Ohm 30nT/v'Hz Pick-up coil: 39 turns, 100 Ohm 1000(L)*100(W)*0.5(H)lJm 3 Excitation current: >2mA (pp, sinus)

CONCLUSIONS

We choose as our principal goal the miniaturisation of the sensor and its electronics without loss of accuracy accounting for the following arguments: A further increase of accuracy seems difficult because it is limited by the noise of the f1uxgate sensors to the lever of a few p'I. The efforts in the last 15 years to reduce this noise have given only modest results. Furthermore, the basic shortcomings of magnetic field experiments are poor time resolution, data lags, spacecraft interferences, the drift of offsets or bad coordination with other plasma instruments, but in no case the insufficient accuracy. With the help of more sensors the reliability of the experiment and the possibility to isolate spacecraft interferences are increasing . In this way the requirements on the magnetic properties of the spacecraft can be reduced. This is one of the ways to realize low cost projects. Developments of magnetic field sensors apply ing a new technology , for instance monolithic integrated fluxgate sensors or magnetoresistive sensors, applied in space research seem to have good prospect. A huge saving in weight and power consumption is guaranteed , a high accuracy for frequencies above 0.1 Hz is possible, and only an offset stability on the order of the usual fluxgate sensors remains a problem.