advances in fluxgate sensors

Sensors and Actuators A 106 (2003) 8–14

Advances in fluxgate sensorsPavel Ripka∗

Department of Measurement, Faculty of Electrical Engineering, Czech Technical University, Technicka 2, 166 27 Praha 6, Czech Republic

Abstract

This paper reviews recent achievements in the technology and design of fluxgate sensors and magnetometers. The major recent trendswere decreasing of the sensor size, power consumption and price, and, on the other hand, increasing of the precision in the large range ofthe measured fields. The potential frequency range was increased up to units of kHz. Present fluxgate sensors have a resolution comparablewith high-temperature superconducting quantum interference devices (SQUIDs), while their precision is the best of vectorial field sensors.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Fluxgate sensor; Fluxgate magnetometer

1. Introduction

Fluxgate sensors measure dc or low-frequency ac mag-netic fields. They are vector devices, i.e. sensitive to the fielddirection in the range of up to 1 mT with achievable resolu-tion down to 10 pT. Although first fluxgate sensors appearedin early 1930s, these sensors are still being used in manyapplications. Recent development of magnetoresistors, es-pecially anisotropic magnetoresistance sensors (AMR),limit the market for classical fluxgate to applications requir-ing high precision and resolution. This paper reviews recentadvances and development in the fluxgate technology sincethe last review[1]. Some recent trends were already com-mented in[2]. Broader overview of applications of fluxgatesand other magnetic field sensors and magnetometers ismade in[3]. Recent comparative study of uncooled magne-tometers and superconducting quantum interference devices(SQUIDs) with proposed hybrid systems can be found in[4].

Fluxgate principle was first used in magnetic modulators.The excitation currentIexc through the excitation coil pro-duces field that periodically saturates (in both directions) thesoft magnetic material of the sensor core (Fig. 1). In satura-tion the core permeability drops down and the dc flux asso-ciated with the measured dc magnetic fieldB0 is decreased.The name of the device comes from this “gating” of the fluxthat occurs when the core is saturated. When the measuredfield is present, the voltageVi is induced into the sensing(pick-up) coil at the second (and also higher even) harmonicsof the excitation frequency. This voltage, proportional to the

∗ Tel.: +420-2-2435-3945; fax:+420-2-3333-9929.E-mail address: [email protected] (P. Ripka).

measured field, is usually the sensor output, but some flux-gates also work in the short-circuited mode (current-output).

Although the fluxgate effect is known for decades, it stillattracts interest of theoretical work. Kaplan and Suissa hadshown a duality between electric antenna and gapped flux-gate[5,6]. They also newly derived the sensor response tonon-uniform fields. Although their results are not directlyapplicable for commonly used “short” fluxgates, they helpto understand fluxgate effect.

Trujillo et al. analyze fluxgate using simple spice model[7]. This tool allows to evaluate the influence of the coreBH loop shape and excitation waveform on the open-loopperformance of the sensor.

Fluxgate magnetometers are used in geophysics and forspace applications. Space applications of fluxgate sensorswere recently reviewed by Acuna[8]. Fluxgate compassesare exploited for aircraft and vehicle navigation. The flux-gate principle is also employed in current sensors and pre-cise current comparators and for remote measurement of dccurrents. For reading magnetic marks and labels and for de-tection of ferromagnetic objects, compact fluxgate magne-tometers are used.

Fluxgate sensors are reliable solid-state devices, work-ing in a wide temperature range. Resolution of 100 pT and10 nT absolute precision is standard in commercially pro-duced devices, but they can reach 10 pT resolution and 1 nTlong-term stability. Many dc fluxgate magnetometers have acut-off frequency of several Hertz, but when necessary, theycan work up to kilohertz frequencies[9]. The temperaturestability is the following: the offset drift may be well be-low 0.1 nT/◦C, and the sensitivity tempco is usually around50–30 ppm/◦C, but some fluxgates are compensated up to

0924-4247/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0924-4247(03)00094-3

P. Ripka / Sensors and Actuators A 106 (2003) 8–14 9

Fig. 1. Fluxgate principle.

1 ppm/◦C. If they work in the feedback mode, the result-ing magnetometer linearity error may be as low as 10−5

[10].If resolution in the nanotesla range is required, fluxgates

are the best selection. Compared to high-temperature su-perconducting quantum interference device they may havesimilar noise level, but the measurement range of fluxgateis much larger. If pT or even smaller fields are measured,a low-temperature SQUID should be used. Magnetoresis-tors, mainly anisotropic magnetoresistance sensors, are themain competitors of fluxgate sensors. Commercially avail-able AMR magnetoresistors such as Philips KMZ have a res-olution worse than 10 nT, but they are smaller and cheaperand may consume less energy. Linearity of the best presentcompensated AMR sensors is 0.05%[11,12].

The mostly used modern low-noise fluxgate sensor is the“parallel” type with ring-core. “Parallel” type means that theexcitation and the measured field have the same direction.Orthogonal type is rarely used, mostly in thin-film devices.The second harmonic in the induced voltage is extracted bya phase-sensitive detector, and the pick-up coil often servesalso for the feedback. Current-output is also used in somedesigns. Other designs are used for special purposes, suchas rod-type sensors for non-destructive testing or positionsensing[13].

2. Core shapes of fluxgates

The main problem of using the basic single-core designis the large signal on the excitation frequency at the sen-sor output, because the sensor acts as a transformer. Thus,the single-core design is used mainly for simple devicesand special applications. Pulse-position type sensors alsohave single-core[14]. Some simple magnetometers such as[15] are based on autooscillation circuits. These devices aresmall, low-power and cheap; however, they have strong com-petitors in AMR magnetoresistors. Orthogonal single-corefluxgate sensor for defectoscopy was developed by Sasada[16]. For precise fluxgates, double cores (either double-rodor ring-core) are normally used.

Moldovanu et al. developed a number of double-rod(Vacquier—Foerster type) core sensors. They report 120 pTp–p noise and 0.42 nT/K offset in the temperature range of−20 to +70◦C for tensile-stress annealed amorphous core[17,18].

Fig. 2. Race-track fluxgate.

2.1. Ring-core sensors

While the pick-up coil is a straight solenoid with thering-core in its center, the excitation coil is toroidally woundaround it. Ring-core sensors can be regarded as a form ofbalanced double sensor. The closed magnetic circuit is con-sisting of two half-cores. The core is usually made of severalturns of thin tape of soft magnetic material. The ring-coregeometry is advantageous for the low-noise sensors, eventhough that the ring-core sensors have low sensitivity, dueto the large demagnetization. Ring-core sensors also allowfine balancing of the core symmetry by rotating the corewith respect to the sensing coil.

2.2. Race-track sensors

Their sensitivity is higher and the race-track sensor isless sensitive to perpendicular fields, due to the lower de-magnetization factor (Fig. 2). Race-tracks, on the other side,still have the advantages of the closed-type sensors, mainlylow-noise—6 pT/

√Hz@1 Hz was reported for sensor hav-

ing 65 mm long race-track amorphous core[19]. Sensitivityand noise for smaller sensors is studied in[20]. Modifiedrace-track sensor design allows final adjustment of the sen-sor balance by sliding the pick-up coil along the core[21].

3. The effect of demagnetization

If the constant pick-up coil area in the general inductionsensor equation is assumed, we get:

−Vi = dΦ

dt= NAµ0µ dH(t)

dt+ NAµ0H dµ(t)

dt

whereµ is relative permeability.The basic induction effect (first term) is still present in

fluxgate sensors, and causes interference. But the most im-portant component is the second term caused by fluxgateeffect. The core permeability is periodically changing withthe excitation field. The given formula can be used for longrod-type sensors, but for the more often used ring-cores,the demagnetization effect should be considered. Demag-netization means thatH in the core material is lower than

10 P. Ripka / Sensors and Actuators A 106 (2003) 8–14

the measured fieldH0 in the open air. Thus, the flux densitywithin the core must be written:

B = µ0µH0

[1 + D(µ − 1)]= µ0µaH0

whereD is the effective demagnetization factor andµa isthe apparent permeability,µa = µ/[1+D(µ−1)], for veryhigh µ, µa → 1/D.

If demagnetization is considered, the equation for fluxgateoutput voltages becomes more complex:

Vi = NAdB

dt= NAµ0H0

1 − D

{1 + Dµ(t) − 1}2

dµ(t)

dt

Demagnetisation of ring-cores was studied by Clarke[22],study of the ring-core internal field was also performed byPrimdahl et al.[23].

From this equation, and also from practical experience,general practical rules for achieving high sensitivity can bededuced:

1. Voltage sensitivity increases with number of turnsN (ifN is very high, other factors, such as coil parasitic capac-itance, limits the sensitivity).

2. Sensitivity decreases with demagnetization factorD.3. The sensitivity is high for materials having rectangular

shape of the hysteresis loop, as they have a steep changeof permeability dµ/dt, when the core is coming into sat-uration. But these materials cannot be used because oftheir high noise level.

4. Until eddy currents (which change the shape of the hys-teresis loop) become important, voltage sensitivity in-creases with excitation frequency (because(dHexc/dt) ∼f ).

The voltage output is often tuned. Tuning may be inten-tional by parallel capacitance to utilize parametric amplifi-cation or unintentional (by parasitic coil capacitance).

4. Core materials

High permeability and low coercivity, but non-rectangularshape of the magnetization curve is preferred for the corematerial. The material should have low number of structuralimperfections, low internal stresses, uniform cross-section,smooth surface and high homogeneity of the parameters.Low saturation magnetization (for low-power) and high elec-trical resistivity (for low eddy current losses) are advanta-geous. The minimum noise is achieved for alloys possessingvery low magnetostriction. Materials suitable for fluxgatecores are permalloys (with 78–81% of nickel) and amor-phous alloys. Ferrites are used only exceptionally, as theygive low sensor sensitivity.

Amorphous magnetic materials, whose use for fluxgatecores started from the early 1980s, are magnetic “metallicglasses” produced by rapid quenching. Cobalt-based amor-phous alloys with low magnetostriction are particularly suit-

Fig. 3. Noise of Billingsley Magnetics fluxgate sensor. The sensor coreis 17 mm diameter amorphous ring (from[32]).

able for fluxgate applications. Annealing may further de-crease the noise level of a tape for fluxgate core. Using amor-phous 17 mm ring-core, Nielsen et al. reached noise levelof 4.2 pT/vHz@1 Hz, which corresponds to 11.1 nT rms inthe frequency range of 60 mHz–10 Hz[24]. It was recentlyshown that also the tape surface treatment such as chemicaletching may improve the core properties[25]. Fig. 3 showsthe typical noise spectrum and time plot measured on flux-gate sensor manufactured by Billingsley Magnetics.

Single-domain fluxgates proposed by Koch are theoret-ically free of magnetic noise[26]. Noise level achievedso far was 1.4 pT/vHz@1Hz for 25 mm ring-core and3.5 pT/vHz@1Hz for 13 cm long rod-core, but predictedvalues are even lower.

5. Principles of fluxgate magnetometers

The most frequently used principle of fluxgate magne-tometers is second-harmonic detection of the output volt-age. The other principles also appeared, but until now theywere not fully proved to bring substantial advantages exceptsimplification of the circuitry. We give only three recentexamples of these devices. Robertson presented a 1 mmlong single-core sensor. Using differential peak detection, asimilar sensor excited at 40 MHz had 250 pT/

√Hz@10 Hz

noise [27,28]. The relaxating-type magnetometer uses asingle-core saturated by unipolar pulses and measures thelength of the relaxation pulse after the excitation field isswitched-off. The instrument has+/−200 mT range, 5%linearity error and about 0.5 nT p–p noise[29]. Dimitropou-los suggests a new sensor principle combining fluxgate withMateucci effect[30]. The amorphous 6 cm long wire isexcited by flat coil pair. Although the precision of the firstprototype is reported to be 60 nT, the device can be scaleddown to 5 mm and further optimised.


Fig. 4. Analog fluxgate magnetometer.

Fluxgate may also work in the short-circuited mode (withcurrent-output)[24]. The main difference from voltage out-put is that the sensitivity is higher for lower number of turns,which may be advantage for miniature devices.

5.1. Second-harmonic analog magnetometer

The common feedback-type fluxgate magnetometer isshown inFig. 4. The measured field causes second-harmonicvoltage at the sensor output, which is demodulated byphase-sensitive detector back to dc or near-zero frequency.Integrator gives large feedback gain. The feedback currentis sensed on resistor by the differential amplifier and servesas the magnetometer output.

The usual excitation frequency is between 400 Hz and100 kHz, for permalloy cores typically about 5 kHz.

Increasing the frequency of the excitation current in-creases the sensitivity and improves the dynamic perfor-mance, but leads to a situation where the eddy currents inthe core material become important. The excitation currentshould have large amplitude and a low second-harmonicdistortion, as such distortion causes (through sensor unbal-ance) a false output signal. Tuned circuit in the excitationcan help to reduce the power consumption[31].

The sensor voltage sensitivity increases with a numberof turns of the pick-up coil (to the limitation caused byparasitic self-capacitances that create a resonant circuit),and with tuning of the voltage output[32]. Short-circuited(current-output) fluxgate was introduced by Primdahl[24].Current-output fluxgate may also be tuned at the output byserial capacitor to suppress unwanted odd harmonics andincrease the sensitivity[33].

The pick-up coil often serves for the feedback, but withsome compromises. Pick-up coil should be close to the sen-sor core to keep the air flux low. Because of homogeneity,larger feedback coil is better. Therefore, two separated coilsare used in precise magnetometers.

The beginnings of digital magnetometers were describedin [2]. Astrid-2 magnetometer was based on DAC, DSP andADC in the feedback[34]. Kawahito employed delta–sigmamodulator and analog demodulator in his magnetometer[35]. Newly redesigned device based on switched detector,

analog integrator and second-order delta–sigma modulatorin the sensor feedback is reported in[36]. Present trend is toemploy fully digital concept and pulse-width modulator. De-spite the continuous effort, digital magnetometers still havedrawbacks over analog instruments: increased noise level,limited linearity and higher power consumption.

6. Miniature fluxgates

It is complicated to miniaturize the fluxgate, because themagnetic noise rapidly increases with decreasing sensorlength. Small-size fluxgates are needed and used in manyapplications, such as magnetic ink reading or sensor arrays.Small-size fluxgate sensors are made of cores from amor-phous materials or Permalloy and have simple electronics[37]. Integrated fluxgate sensors do not have wound coilsand therefore they can be very small and cheap. Their coreis made by sputtering or electrodeposition[38–40]. Forlow-noise integrated sensors, the cores of amorphous tapeare used, as they have better magnetic properties[41].

The compensation sensor manufactured by Siemens-VACHanau (Germany) which has a permalloy wire core mayalso work as a single-core fluxgate sensor[42]. A numberof simple multivibrator-type fluxgate magnetometers wasreported from Japan.

A 15 mm long hairpin sensor was made up of a strip withhelical anisotropy[43]. This classical sensor has 5 nT/vHznoise (averaged in the 64 mHz–10 Hz band) and 4 nT/◦Ctemperature drift in the 25–50◦C interval.

The simple “PCB” construction of the 15 mm long flux-gates is described in[44]. The annealed core made of amor-phous foil is sandwiched inside multilayer printed circuitboard. Outer metal layers of PCB connected by vias formthe winding. Resistance of the winding can be decreased byCu-electroplating after patterning of the winding[45].

Planar fluxgate sensor with flat coils was described in[46].The sensor core is in the form of two serially configured1.4 mm long strips of sputtered permalloy 2�m film. Theflat excitation coil saturates the strips in opposite directions,the differential flux is sensed by two anti-serially connectedflat pick-up coils. The maximum sensitivity of 73 V/T was


Fig. 5. Microfluxgate with flat coils and symmetrical core (from[38]).

reached for a 1 MHz/150 mA p–p excitation current. Similarstructure which has two permalloy strips on both sides ofthe flat coils is shown inFig. 5 [38].

A similar sensor having three flat excitation coils wasdescribed in[47]. The sensor response covers fields up to250 mT. In the±60 mT range the linearity and hysteresis er-ror is below±1.2%. The error of angular response to a 50�Tfield is±1.6%. Further improvement was achieved with sim-ilar sensor having two layers of ferromagnetic core[38,39].

The orthogonal fluxgate with flat excitation and pick-upcoil was described in[41]. The sensor 10 mm diameterring-core is also etched from Vitrovac 6025 amorphous rib-bon. The sensor resolution is 40 nT and the linearity error inthe 400 mT range is 0.5%. A parallel-mode two-axis inte-grated fluxgate magnetometer by the same authors was de-veloped for a low-power watch compass[48,49].

Solenoid coils have much better efficiency than flat coils,as they are ideally coupled with the core. However, micro-machining of solenoids in difficult. Early devices were de-veloped by Kawahito[50] and Gottfried[51]. The technol-ogy was further developed by Liakopoulos and Ahn[52,53].UV-LIGA based thick photoresist process was used to cre-ate electroplated permalloy core and copper coils. Maximumsensitivity for 5 mm long, 0.7 mm wide and 20�m thicksensor core was 900 V/T for excitation frequency of 5 kHz.Kuchenbrand et al. fabricated 7.3 mm long race-track sen-sors with solenoid coils by rf sputtering and argon ion etch-ing [54].

The microfluxgate technology is improving, but at presentAMR sensors are winners: they have better parameter thanfluxgates smaller than 5 mm.

7. Multiaxis magnetometers

Two-axial fluxgate magnetometers are used in com-passes. Popular dual-axis sensor has ring-core with dou-ble cross-shaped pick-up coil. If used as a compass, the

short-time angular accuracy may be 5 min of arc, long-termprecision 0.1◦. The main disadvantage is that such com-pass should be gimbaled. Modern compasses therefore usethree-axial fluxgate magnetometer together with inclinome-ter and proper azimuth is calculated from five readings:pitch, roll and three components of magnetic field. Flux-gate compasses are still superior, as they can easily achieve0.1◦ accuracy in the wide temperature range, while AMRdevices require complicated error compensation to reachaccuracy of 0.5◦.

7.1. Three-axial compensation system

For three-axial magnetometers, three single axis sensorsare usually used. To compensate measured fields three or-thogonal circular or rectangular Helmholtz coils or morecomplex coil systems are used. The spherically shapedthree-axial coil system consisting of three coils of identicalcenter points has been constructed for rocket and satelliteapplications by Primdahl and Jensen[55]. Each coil con-sisted of nine sections approximating the ideal sphericalcoil, which generates a uniform field. For either long-termstability or low-noise operation it is very important to keepthe sensors in a very low field. Placing all three orthogonalsensors in the center of three-dimensional feedback coilprovides that. Therefore, the system is free of errors causedby cross-field effect (non-linear sensitivity to magneticfield perpendicular to the sensing axis[56,57]. The mea-suring axes are defined only by the feedback coil system(the exact position of the individual sensors is not critical)and thus may be easily determined and kept very stable.The CSC system was successfully used for the Oerstedsatellite magnetometer[58]. The Oersted fluxgate mag-netometer linearity error was below 1 ppm in the earth’sfield, the temperature coefficient of the deviation angleswas 0.07 arcsec/◦C. Temperature coefficient of sensitivitywas reduced to 10 ppm/K. The in-flight offset stability wastypically 0.3 nT for 50 days.


7.2. Individually compensated sensors

Although these systems have many problems arisingfrom the cross-field effect, they are very popular becauseof their simplicity and low price. Individual sensors shouldbe mounted symmetrically and at a maximum distanceto avoid crosstalk. If possible, their excitation windingsare connected serially and they are excited from the samegenerator. An example of advanced magnetometer of thistype is device developed for the Swedish satellite Astrid-2,which has three closely mounted fluxgates with 17-mmring-cores made of amorphous alloy[34].

8. Fluxgate gradiometers

Gradiometric sensors are used in many applications, suchas biomagnetic measurement or magnetic testing, wheremeasured field source is in a very short distance. Althoughsingle-core fluxgate gradiometric sensor was developed[59], it turned out that for measuring the field gradient us-ing of two separate sensors and subtracting their readinggives better stability of the device. When two top-qualitysensors are used, dynamic range of 130 dB (i.e. 0.4 ppmresolution) for measurements in the earth’s field can bereached. The 50 000 nT calibration residuals were 2 nT p–p[60,61].

9. Calibration of fluxgate magnetometers

Three methods are used for calibration of precise vectorialmagnetometers:

1. thin-shell with random positioning;2. calibrated positioning by non-magnetic theodolite;3. coil systems.

The first two methods use the earth’s field as reference.The unknown parameters are sensor offsets, gains and anglesbetween the individual sensors in multi-axis systems. An-gular deviation from the reference coordinate system shouldalso be known.

Theodolite method is traditionally used at magnetic ob-servatories[62]. Coil systems are expensive and they shouldbe periodically recalibrated, but they allow to perform fullyautomatic tests[63]. Thin-shell method with random posi-tioning can be performed at any magnetically quiet locationand it requires only absolute instrument such as proton mag-netometer; surprisingly, experience shows that this methodgives most reliable results for estimate of gains and anglesbetween the sensors[64]. Calibration problems are furtherdiscussed in[65].

Calibration in strongly inhomogeneous field is importantfor application in non-destructive testing[66].

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