Journal of Magnetism and Magnetic Materials 215}216 (2000) 735}739

New directions in #uxgate sensors

Pavel Ripka*

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

Abstract

Although #uxgates may have a resolution of 50 pT and an absolute precision of 1 nT, their accuracy is often degradedby cross"eld response, non-linearities, hysteresis and perming e!ects. The trends are miniaturization, lower powerconsumption and production cost, non-linear tuning and digital processing. New core shapes and signal-processingmethods have been suggested. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: Fluxgate; Magnetic sensors; Magnetometers

1. Introduction

Fluxgates are the most suitable vector magnetic "eldsensors for applications requiring a resolution up to0.1 nT and an absolute precision of 1 nT (for periodicallycalibrated and thermostated station magnetometers) to100nT (for low-cost portable units). Only SQUIDs aremore sensitive vector sensors [1].

AMR and recently also GMR magnetoresistors are thelatest competitors of small-size #uxgate sensors. AMRbridges using barber-pole geometry are linear devices;their performance can be improved by periodical #ip-ping. GMR magnetoresistors may be linearized only bybiasing; it was recently shown that AC bias may decreasetheir o!set, hysteresis and noise [2]. In general, commer-cial magnetoresistors have a resolution of 10 nT witha sensor size of several mm, but their precision is limitedby large temperature coe$cient of sensitivity (typically600ppm/3C, compared to 30 ppm/3C for #uxgate). Al-though magnetoresistors and #uxgates exploit on com-pletely di!erent e!ects, it is necessary to periodicallysaturate their cores if one wants to remove perming (theo!set caused by residual DC magnetization). As a result,the electronic circuits used are similar and there are nolarge di!erences in power consumption.

*Corresponding author. Tel.: #4202-2435-3945; fax:#4202-311-9929.

E-mail address: ripka@feld.cvut.cz (P. Ripka).

The general requirements for #uxgate core material arethe following:

(i) low losses at excitation frequency (typically 10 kHz):low H

#, high resistivity;

(ii) low B4

(resulting in low power consumption), lowmagnetostriction;

(iii) low noise: rotational magnetization reversals, norandom internal stresses, low number of structuralimperfections, smooth surface;

(iv) uniform cross section and large homogeneity of theparameters.

The materials used are 79}81% Ni permalloys andCo-based amorphous alloys. The nanocrystalline alloyshave shown no advantage [3]. Besides the traditionalvoltage output, some magnetometers use short-circuitedcurrent output; the main advantage is that the parasiticcapacitances are suppressed and the pick-up coil requireslower number of turns [4].

The present paper completes the existing review on#uxgate sensors [5] with the latest trends. It is based onmaterial collected for the book on magnetic sensors [6].

2. Miniaturization

A lot of applications, including magnetic ink reading,safety and security sensors and sensor arrays, requirea small sensor size. The process of the #uxgate sensor

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 7 3 - 0

miniaturization is rather complicated as the magneticnoise dramatically increases with decreasing sensorlength. Small-sized #uxgates are made of open or closedcores from amorphous or permalloy wires, tapes anddeposited structures. Up to now, the quality of sputteredor electrodeposited permalloy is not su$cient for low-noise #uxgate applications, so patterns etched ofamorphous tape are often used for the core of miniaturesensors.

The simple printed circuit board (PCB) design of the15mm long #uxgates is described in Ref. [7]. The an-nealed core made of amorphous foil is sandwiched be-tween two layers of PCB, which have outer metal layersforming the halves of the winding. The layers are thenconnected by electroplating.

Microelectronic technology has been used to lower theproduction cost of #uxgates.

Planar #uxgate sensor with #at excitation and pick-upcoils was described in Ref. [8]. The sensor core is in theform of two serially con"gured 1.4mm long strips ofsputtered 2lm thick permalloy "lm. The #at excitationcoil saturates the strips in opposite directions, the di!er-ential #ux is sensed by two antiserially connected #atpick-up coils. The maximum sensitivity of 73 V/T wasreached for a 1MHz/150mA p}p excitation current.A similar sensor having three #at excitation coils is de-scribed in Ref. [9]. The sensor response covers "elds upto 250lT without the feedback. In the $60lT range thelinearity and hysteresis error is below $1.2%. The errorof angular response to a 50lT "eld is $1.6%.

The orthogonal #uxgate with #at coils was introducedby Kejik [10]. The sensor 10-mm diameter ring core isetched from Vitrovac 6025 amorphous ribbon. The sen-sor resolution is 40 nT and the linearity error in the400lT range is 0.5%. A parallel-mode two-axis integ-rated #uxgate magnetometer made by the same authorswas developed for a low-power watch compass [11].

A common weak point of integrated #at coils is thatthey cannot su$ciently saturate the core to erase theperming e!ect. The two reasons are: (i) the weaker coup-ling to the core than in the case of solenoid coil and (ii)low metalization layer thickness resulting in high coilresistance and low limit for current amplitude. A highcoil resistance is also the reason that the sensor cannot betuned, neither in the excitation, nor at the output.

The two-layer metalization process can form a sol-enoid around the core [12]; the noise of such a sensorwas 40 nT p}p for a 5mm long core. A similar sensor wasdeveloped at Fraunhofer Institute and became a part ofa CMOS integrated magnetometer [13].

3. Suppressing the cross5eld response

Many of the vector magnetic "eld sensors have a non-linear response to magnetic "elds perpendicular to their

sensing direction (`cross"eldsa). The cross"eld e!ect in#uxgates is less dramatic than in anisotropic mag-netoresistors [14], but still it may be a dominant sourceof error when making 3-axial measurements in the pres-ence of the Earth's "eld. In general, the cross"eld e!ectmay be suppressed by core shape (in rod-type or race-track sensors) or by omnidirectional compensation of themeasured "eld as in compact spherical coil (CSC) mag-netometer [15].

Although the cross"eld e!ect was known for years, itsorigin and mechanism was analyzed recently: Brauer hadshown that the e!ect is caused by non-uniformity of thesensor core susceptibility [16]. Another study performedon the voltage-output sensors con"rmed Brauer's results;it also showed that the cross"eld response is temperaturedependent: errors as high as 2 nT/3C in the Earth's "eldwere observed on low-cost sensors, which should becompared with their 0.1 nT/3C o!set drift [17]. It isinteresting that the cross"eld response may be reducedby decreasing the ring-core diameter; unfortunately, thissimultaneously increases the sensor noise.

A typical example of 20 mm ring-core #uxgateresponse to perpendicular "eld is shown in Fig. 1.

4. Novel core shapes

Rod-type sensors (either Vacquier-type with one com-mon pick-up coil or Foerster-type with two separatepick-up coils) are traditionally made of permalloywires. Unfortunately, most of the amorphous wiresexhibit large noise due to the long domains in theircentral part.

Vacquier-type sensors made of stress-annealed Vit-rovac 6025 amorphous tape exhibited a minimumnoise of 11 and 46 pT rms (60 mHz}10Hz) for 65and 35mm long sensors, respectively. It was shownthat the cores and detection coils should be 10% shorterthan the excitation coils to avoid noise caused by un-saturated ends [18]. However, there are indications thatVacquier- and Foerster-type sensors have worse o!setstability and noise than the ring-core sensors of the samelength.

Racetrack sensors may have low noise and good o!setstability similar to ring cores, but they are inherentlyresistant to cross"eld. The main problem of the racetracksensors is the large spurious signals: they cannotbe simply balanced as the ring cores, by rotation tominimize the spurious feedthrough. A modi"ed racetrackcore, which allows for a "nal adjustment was suggestedin Ref. [19]. The track width is slightly changing alongthe core length. By sliding the pick-up coil out of itssymmetrical position the e!ective cross section of thecore halves is changing, which allows to adjust for theunbalance caused by core or excitation winding non-homogeneity.

736 P. Ripka / Journal of Magnetism and Magnetic Materials 215}216 (2000) 735}739

Fig. 1. Cross"eld response of 20 mm diameter ring-core #uxate sensor (the imperfect orthogonality of the test "eld was numericallycorrected).

Fig. 2. Waveforms in current output #uxgate: Upper trace:excitation current (tuned to resonance), 0.5 A/div. Middle trace:untuned output current for 5lm, measured "eld, 200lA/div.Lower trace: the same current output tuned by serial capacitorto secons-harmonic resonance.

5. Lowering the power consumption

Excitation circuits usually consume most of the #ux-gate magnetometer power. The amplitude of the excita-tion current must be large enough to fully saturate thesensor core in each cycle in order to remove any re-manent e!ect. High narrow peaks of the excitation cur-rent may be achieved by using a tuning capacitor eitherparallel to the excitation winding (for current-mode orhigh source impedance) or connected in series (for volt-age-mode excitation). Tuning may also decrease the sec-ond-harmonic distortion of the excitation current.

If the circuit is properly tuned, the generator deliversonly the energy dissipated by the hysteresis and ohmiclosses; the circuit is very energy e$cient, as the ratiobetween the peak sensor current and generator outputrms current may be as high as 50. The parallel tunedexcitation circuit was analyzed by Nielsen et al. [20].Current waveforms for both parallel and serial tuningmodes are similar; an example is shown in Fig. 2 (uppertrace).

The tuned excitation circuit is highly non-linear andthe average excitation winding inductance is much lowerthan the small signal inductance. In some cases, thepreferred excitation working point is quasistable [21].Tuning the excitation may degrade the sensor temper-ature stability; the tuning capacitor should have a verylow-temperature coe$cient.

It is more di$cult to guarantee complete saturation athigher frequencies because of the eddy currents. Theexcitation frequency for #uxgate magnetometer is usuallybetween 1 and 20 kHz. Current comparators and somestation #uxgates are excited at 400Hz, while miniature

#uxgates sometimes have excitation frequencies higherthan 1MHz to increase the low sensitivity caused bysmall core and low number of turns of the pick-up coil.

The magnetometer power can also be reduced by sim-plifying the analog signal processing circuits and inter-face: non-linear tuning at the sensor output increases thesensitivity and allows to construct magnetometers with-out input "lters [22]. Newly described serial tuning of thecurrent output may bring this advantage to miniaturesensors as the necessary number of turns is low [23].

P. Ripka / Journal of Magnetism and Magnetic Materials 215}216 (2000) 735}739 737

Non-selective methods are being used in low-cost,low-power instruments. Autooscillation magnetometersare based on magnetic multivibrator [24]. Phase-delaymethod suggested by Heinecke was further developedinto `Fluxset sensora by Vertesy [25].

6. Increasing the bandwidth

There are three basic approaches to increasing thefrequency range of the #uxgate: (i) increasing the excita-tion frequency, (ii) exploitation of the direct inductione!ect in the pick-up coil and (iii) using the AC errorsignal of the DC loop [26].

(i) The limiting factors of the excitation frequency arethe velocity of the domain wall motion and eddycurrents, which attenuate the "eld inside the core.The e!ect of eddy currents is complex: The coresurface is magnetized "rst into the saturation; thedecrease of the core permeability gradually increasesthe "eld penetration into the deeper parts of the core.Thus, the core may be saturated even if the initialpenetration depth is only a fraction of the tape thick-ness. The amorphous materials are advantageousover permalloys as they have higher resistivity andlower thickness: The maximum excitation frequencyfor a 20lm amorphous tape core is about 100 kHz.Cores made of ferrites, thin layers or amorphoustapes may work at MHz frequencies.

(ii) AC magnetic "elds induce a voltage (or short-cir-cuited current) of the same frequency in the pick-upcoil; while the induced voltage should be integrated,then the short-circuited induced current ideally fol-lows the waveform of the magnetic #ux.

(iii) The AC error signal of the slow DC feedback loopwas used to measure AC "elds in the Thunderstormrocket experiment. The frequency band of the ACoutput was 3 kHz, while the upper frequency limit ofthe DC feedback loop was 10Hz [27].

7. Digital magnetometers

The feasibility of digital signal processing of the #ux-gate output was shown in Ref. [28]. The "rst real-time#uxgate magnetometer was built in Max-Planck Insti-tute in Berlin [29].

Fully digital #uxgate magnetometer performs the ana-log-to-digital conversion of the sensor output signal rightafter the pre-ampli"cation and eventual analog pre-"lter-ing to suppress the unwanted signals.

The harmonic distortion in the ADC would causea false signal output. As the feedthrough changes withtemperature, this e!ect could degrade the o!set stability.The phase-sensitive detection and further "ltration isperformed numerically in digital signal processor (DSP).

Digital #uxgate magnetometer was on board theAstrid-2 satellite [30]. The signal-processing electronicsfor each axis consisted of a 12-bit ADC with a samplingrate of 128 kHz (16 samples per excitation period), a DSPand an 18-bit DAC controlling the feedback currentsource. The instrument range was $61 000nT, resolu-tion was increased to 20 bits by interpolation. The powerconsumption was 2W, which is still about double that ofa similar analog-feedback magnetometer with &* con-verter at the output [20]. The weak point of the instru-ment was the audio DAC which has high o!set, o!setdrift (&1 nT/3C) and non-linearity. The instrument noisewas 50 pT rms in 16 Hz bandwidth (about 5 times higherthan the sensor noise) and the residual calibration error(after the correction for DAC non-linearities) was only1.3 nT rms.

The advantage of using the digital detection is thatthe reference signal may have an arbitrary shape sothat it can perfectly match the measured signal * evenbetter than the variable-width detector of the switchingtype.

Another approach was suggested by Kawahito et al.[31]. They used an analog switching-type synchronousdetector followed by an analog integrator and a second-order *& modulator. Magnetic feedback loop is closedvia a 1-bit DAC with current output, followed by ananalog low-pass "lter which guarantees 1-bit linearity;the problem was the excessive noise of the device whichhad magnetic circuit integrated on the same chip with theprocessing electronics.

It would be theoretically possible to combine the twomentioned principles in magnetometer employing digitaldetection and *& modulation in the feedback; in order toachieve the high dynamic range, high oversamplingwould be necessary.

In any case, the feedback of the digital magnetometersshould include voltage-to-current converter to eliminatethe in#uence of the changing resistance of the feedbackwinding.

It should be noted that the increased noise level of thementioned instruments is not the property of digitalprocessing, but it is because the design compromises tolower the power consumption of the magnetometer elec-tronics: digital laboratory lock-in ampli"ers such as SR830 have very low noise so that they can be used fortesting of high-performance #uxgate sensors.

Acknowledgements

This work was partly supported by Ministry of Educa-tion of the Czech Republic under No. J04/98:212300016.Artech publishers are acknowledged for the permissionto use the material collected for Ref. [6]. Fig. 1 is basedon data measured by Billingsley Magnetics.

738 P. Ripka / Journal of Magnetism and Magnetic Materials 215}216 (2000) 735}739

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