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Journal of Magnetism and Magnetic Materials 320 (2008) 2466–2473

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Sensors based on bulk soft magnetic materials: Advances and challenges

Pavel Ripka�

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

Available online 12 May 2008

Abstract

Sensors with cores, yokes or field concentrators made of bulk magnetic material are more sensitive and stable than thin-film sensors.

Non-linearity and temperature dependence of sensitivity is often suppressed by a feedback. The common problem of these sensors is

remanence, cross-field sensitivity and temperature stability of offset. The long-time effort to miniaturize the fluxgates led only to a few

practical designs. For flat sensors (either pcb or CMOS) the core etched from amorphous tape gives better properties then

electrodeposited or sputtered core. We compare traditional miniature fluxgates using wire cores based on longitudinal fluxgate effect with

sensors using transverse fluxgate effect and GMI effect. Well-designed field concentrators or yokes can improve the parameters of any

magnetic sensor. The achievable stable amplification factor is 10–100. Having a means to demagnetize the field concentrator is desirable.

Overview of magnetic sensors for mechanical quantities is also given with special focus on torque sensors.

r 2008 Elsevier B.V. All rights reserved.

PACS: 85.75.Ss; 07.07.Df

Keywords: Magnetic sensor; Soft magnetic material

1. Introduction

Sensors based on thin magnetic films found manyindustrial and commercial applications in the past 20years. By far the most important of them are AMRsensors, which have reached 100 times higher fieldresolution than Hall sensors with the same size and powerconsumption.

However, in many other areas magnetic sensors based onbulk soft magnetic materials kept their dominance forvarious reasons:

1.

The magnetic properties of the bulk material are usuallybetter than those of the thin film.

2.

Or the magnitude of the sensor signal depends on thecross-section of the core.

3.

Or the sensor principle is based on the bulk or thickmagnetic component such as a yoke.

- see front matter r 2008 Elsevier B.V. All rights reserved.

/j.jmmm.2008.04.079

420 224353945; fax: +420 233339929.

ddress: ripka@fel.cvut.cz

The industry requires these sensors being smaller andcheaper, with lower power consumption and wide opera-tional range of temperatures.Bulk magnetic sensors include:

1.

precise sensors of magnetic field; 2. sensors of electric current; 3. sensors of linear and angular position and speed, and

position trackers;

4. sensors of force and torque.

We will review recent advances in some of these sensorssince [1] and discuss their main challenges connected tomaterial properties.

2. Soft magnetic materials for magnetic sensors

The requirements on magnetic materials for thesesensors are rather diverse: while minimum Bs and minimumls are usually required for fluxgate sensors, maximum Bs isneeded for most of other sensors and large ls is required for

ARTICLE IN PRESSP. Ripka / Journal of Magnetism and Magnetic Materials 320 (2008) 2466–2473 2467

magnetoelastic devices [2]. Minimum coercivity, minimumtemperature dependence of permeability and high electricalresistivity are general requirements.

Most of the soft magnetic materials used in sensors arecrystalline but very often amorphous alloys are used, whichdo not require high-temperature annealing.

Nanocrystalline soft magnetic alloys are rarely usedbecause of their brittleness. Nanocrystalline cores forinstrument current transformers are one of the favorableapplications [3]. Sixteen percent Si Finemet was used inRef. [4] for fluxgate sensor of relaxation type. The materialhas 600 1C Curie temperature compared to 210 1C forcommonly used Vitrovac 6025. The material has highsaturation magnetization of 1.12 T, which is normallyregarded as a disadvantage for fluxgates, as such materialrequires higher excitation power. However, in this case ofsensor working in open-loop (uncompensated) mode,higher saturation increases the sensor range.

3. Flux concentrators

Soft magnetic alloys are used in flux concentrators,which are designed to enhance the sensitivity of Hallsensors and magnetoresistors [5]. Shields of magneticallysoft materials are used in some GMR sensors.

The drawbacks of flux concentrators are mainly:

�

remanence, � non-linearity and danger of saturation, � temperature dependence of the gain factor.

-60

-40

-20

0

20

40

60

-50

Line

arity

err

or [n

T]3AmodIIAIIB

-40 -30 -20 -10 10 20 30 40 50

Measured field [μT]

0

Fig. 1. Non-linearity error of race-track pcb fluxgate for three feedback

coils: (a) wire-wound, length 80% of the core (3A), (b) pcb, 80% (2A), (c)

pcb, 50% (2B) (from Ref. [13]).

Remanence of field concentrators is a very serious issue,although it is ignored by majority of publications. Only fewconcentrators are equipped with demagnetization coils.The same coils can be used for the feedback compensationto increase the linearity and avoid saturation. In order notto compromise the sensor bandwidth, the feedback mayonly reduce the DC component of the field.

For the conservatively designed concentrators, the gainfactor is reasonably low and the core permeability high,thus the gain is mainly given by the geometry and it is verystable. For high gains the temperature coefficient ofpermeability plays an important role. This is a well-knownproblem especially for ferrites at low temperatures: avai-lable materials typically experience 50% permeability dropbetween +40 and �20 1C. The mechanical design shouldguarantee that the airgap variations due to the temperaturedilatations are small. Concentrators 20-cm long with100 mm air gap has a gain of 600 and the achieved noiselevel is 100 pT/OHz at 1Hz with special low-noise Hallsensor [6]. Much smaller magnetometer described inQuasimi paper uses modulation of the permeability of the5-mm-long wire concentrator. This shifts the signal fre-quency out of the 1/f noise of the Hall sensor. The achievednoise level is 8 nT/OHz at 1Hz [7].

4. Fluxgate sensors

Fluxgates may have resolution of 1 nT–10 pT andlinearity better than 10 ppm. The measured field createsmagnetic flux in the sensor’s core, which is inside themultiturn pick-up coil. Periodical excitation field modu-lates core permeability and induces voltage in the pick-upcoil. The output voltage has double excitation frequency asflux gating occurs twice in each period. The sensor core ismade of low-noise, low-magnetostriction magnetically softmaterial—either crystalline Permalloy or amorphous Co-based alloy. Although the sensor size is usually about 2 cm,it can be used to measure inhomogeneous fields [8].Fluxgates have non-linear field response, thus they areusually compensated by feedback. In such a case, the non-linearity error can be below 10 ppm. Care should be takenthat the field of the feedback coil is homogeneous. Tooshort coil degrades linearity, as shown in Fig. 1.Fluxgates are expensive, bulky and power-consuming

devices. Recent effort is to develop miniaturized fluxgatesensors to fill the gap between fluxgate and AMR. Threebasic paths for this development are:

1.

CMOS-based devices with flat coils, 2. PCB-based devices with solenoids made by tracks and

vias and,

3. sensors with thin-film or microfabricated solenoids.

Cores etched from amorphous alloy (such as Vitrovac6025 from Vacuumschmelze) are preferred as they havemuch better magnetic properties than thin-film cores madeby sputtering, laser ablation or electroplating [9,10].CMOS microfluxgate may have low-power sensor

electronics integrated on the same chip. Two-axis sensorfor watch compass reached 15 nT/OHz at 1Hz noise with10mW power consumption and 4� 4mm chip size [11].Even if the coil axis is perpendicular to the core, pro-perly positioned coil pair can magnetize the core strip

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perpendicularly to that axis. The challenge of this design isthat the flat coil has in principle poor magnetic couplingwith the core. Solenoid is much better in this aspect, but itis more difficult to manufacture. PCB-based fluxgatesachieved high sensitivity (240V/T for 30 turns of the pick-up coil), low noise (25 pT/OHz at 1Hz) and good tempe-rature stability (20 nT in the �20 to +70 1C range stabilityshown in Fig. 2), but the minimum size achievable with thislow-cost technology is about 10mm [12]. The disadvantageof these sensors is their high-power consumption; it may belowered by using short-current pulses for excitation, butthis brings challenges to the signal processing [13]. Smallerfluxgate sensors with solenoid coils can be made by thin-film technology [14].

Besides the mainstream a lot of modified fluxgate sensorshave appeared. One of them uses orthogonal (or trans-verse) fluxgate effect, which may find application inminiature sensors. The main advantage of this type ofsensor is that it needs no excitation coil—the sensor is exci-ted by the current flowing through the core. This, however,needs high current levels for saturation. As the magneticfield in the inner part of the conductor is low, which causesperming, the favorable design is a non-magnetic conductorcovered by magnetically soft thick layer. The diametershould be kept low in order to reduce the requiredexcitation current: sensor having 0.3-mm-diameter coppercore requires 250mA excitation [15].

Orthogonal fluxgate works in second-harmonic mode,which is more sensitive than ‘‘off-diagonal GMI’’ (or betterIWE) mode—see Section 6 [16]. Also here the output coilmay be tuned to enhance the sensitivity by parametricamplification. Notice that essential condition for para-metric amplification is non-linearity.

Orthogonal fluxgate with rod core made by Permalloylayer electrodeposited on rectangular copper conductor isreported in Ref. [17]. The sensor core is only 1-mm-longand the sensor has two flat 60-turn pick-up coils. Theoverall dimension of the sensor chip is 1.8� 0.8mm. With100mAp, 100 kHz excitation current the sensitivity was510 mV/mT and noise 95 nT/OHz at 1Hz with 8mW netexcitation power consumption.

-10

-5

0

5

10

15

-30

T [°C]

Offs

et [n

T]

Double-core sensorfexc = 5 kHz, sineIexc,p-p = 300 mA

A

BC

DA-B-C-D cycle

-20 -10 0 10 20 30 40 50 60 70

Fig. 2. Temperature offset stability of pcb fluxgate (from Ref. [12]).

‘‘Fundamental mode’’ transverse fluxgate uses unipolarexcitation; the output is on the excitation frequency. Asthis sensor is saturated only in one polarity, the offsetstability is poor. This was improved by periodical switchingof the excitation bias [18]. By increasing the bias amplitudethe noise was as low as 20 pT/OHz at 1Hz [19].

5. Induction magnetometers

These devices are based on Faraday induction law,which means that the voltage sensitivity is proportional tofrequency. In order to obtain flat frequency characteristics,one can use integrator at their output. Another possibilityis to use current output (either open-loop or feedback).Current-output induction coils have flat frequency char-acteristics for frequencies larger than R/2pL. Inductioncoils are used for AC magnetic fields in the frequency rangeof 110mHz–1MHz. At low frequencies, ferromagneticcores are used to increase the sensitivity. Optimum shape ofsuch core is a long rod and the coils are slim solenoids [20].Rod made of amorphous laminations is preferable core forlarge coils working to about 100 kHz. Cores made of ferritemay have more complicated shape, which gives higher fieldhomogeneity for the coil. Thus the optimum length of thecoil is up to 90% of the core length, while for simple cylin-drical core the optimum length may be only 75%. Smallsensor of this type described in Ref. [21] is 10-cm-long andweighs only 11 g; its noise is 2 pT/OHz at 1Hz. Inductionmagnetometers were recently reviewed in Ref. [22].

6. GMI

Giant magnetoimpedance (GMI) is based on field-dependent change of the penetration depth [23]. The effecthas only few practical applications as it gives weak,temperature-dependent signals and the characteristics arenon-linear and unipolar. The noise values are rarelyavailable. In Ref. [24], the lowest noise level is 20 pT/OHz at 100Hz. For 1/f noise it would correspond to200 pT/OHz at 1Hz. Similarly, we may extrapolate thenoise level of 100 pT/OHz at 1Hz from the data given for10-mm-long device in Ref. [25].The temperature change of GMI sensitivity is large, but

it can be compensated by feedback. The large temperatureoffset drift is usually due to the temperature dependence ofDC resistivity. Using proper alloy composition, thiscomponent was nulled and achieved offset drift of 30 nT/1C was caused mainly by temperature dependence ofcircular permeability [26]. The temperature dependence ofthe dc resistance and total impedance is shown in Fig. 3.Fig. 4 shows a single-peak GMI characteristic at 1MHz

on high-quality GMI amorphous tape—same as in Fig. 3[26]. In order to achieve bipolar response, we should makeabout 5A/m biasing. The sensitivity is then about 8%/A/mi.e. 6.5%/mT. The zero-field impedance of the 10-cm-longsensor is 23O, which means that the sensitivity inimpedance units is 0.065� 23 ¼ 1.5O/mT. With 10mA

ARTICLE IN PRESS

Impedance and resistance dependenceon temperature

5.75

5.76

5.77

5.78

5.79

5.8

-2518

19

20

21

22

23

|Z| (

Ω)

Rdc|Z|

RD

C (Ω

)

0 5025 75temp (°C)

Fig. 3. Temperature dependence of GMI sensor dc resistance and total

AC impedance at 1MHz. (from Ref. [26]).

50

100

150

200

250

-50H (A/m)

GM

I (%

)

Decreasing HIncreasing H

-30 -10 10 30 50

Fig. 4. Detail of the GMI curve (from Ref. [26]).

P. Ripka / Journal of Magnetism and Magnetic Materials 320 (2008) 2466–2473 2469

measuring current the voltage sensitivity is 0.01�1.5 ¼ 15mV/mT.

The sensitivity of GMI sensors is roughly linearlyproportional to the sensor length. This gives the potentialadvantage of GMI over fluxgate, as the fluxgate sensitivitydrops more rapidly with downsizing.

The only available GMI sensors on the market aremanufactured by Aichi Micro Intelligent. These sensors areused in low-end compass for mobile phone. They havesputtered feedback coils. The non-linearity of the high-linear type in the 300 mT range is 1%, but no data abouttemperature stability and noise are available from themanufacturer [27]. Aichi MGM-1DS Milli-Gaussmeter has72% accuracy and 10 nT resolution.

Asymmetric GMI gives linear response, but the effect isbased on potentially unstable self-biasing [23].

Inverse Wiedemann effect (IWE) is of different originthan GMI, the output is voltage induced into the pick-upsolenoid [28]. The effect is antisymmetrical and requireshelical or circular anisotropy and dc component in the acexcitation. Although the output is voltage, some authorscall this effect ‘‘off-diagonal magnetoimpedance’’ [29]. Thesensor is exactly the same as for transverse fluxgate, the

only difference is that in IWE mode the current does notsaturate the core and the output is on the same frequencyas the excitation. Using helical anisotropy and 23mA dcbias in double-core 8-cm-long IWE sensor having 400-turnpick-up coil, sensitivity with 100 kHz/5mA excitation was28mV/mT. The achieved sensitivity temperature coefficientwas 0.5%/K, which is significantly worse than with similarGMI sensor. However, this can be suppressed by feedback.The most important advantage is reasonable offset stabilityof 5 nT/K, more than 10 times better than for GMI sensormade of the same core. The offset long-term stability was3 nT/12 h [28]. The sensitivity can be increased by tuningthe coil with parallel capacitor, or just by using the self-resonance caused by parasitic capacitance. However, theuse of resonance can degrade the temperature stability.The common disadvantage of IWE and GMI sensors is

the perming effect, because the ferromagnetic core is notdemagnetized. This problem is often ignored in literature.Contrary to IWE sensors, in fluxgate the current shouldsaturate the core and the output is usually on the secondharmonics of the excitation frequency. The perming isremoved by excitation field.

7. Other magnetic field sensors

Another effect with possible potential for magneticsensing is Colossal magnetoresistance. Magnetostaticdevices [30], and magnetooptical sensors based on Faradayeffect may be used to measure very large fields.Also magnetostrictive magnetic field sensors did not

reach parameters of above described classical sensors.Table 1 gives an overview of high-resolution magnetic

field sensors.

8. Magnetic position and speed sensors

Traditional sensors like LVDT, Inductosyns, Synchrosand resolvers, eddy current sensors, variable reluctancesensors, magnetic encoders, and PLCD sensors [1] are wellknown and widely used in industry. In these cases theimprovements were mainly made by redesigning the sensorgeometry based on improved CAD models rather than bydevelopment of the used materials. This also applies tomagnetic flowmeters, which measure the speed of theconducting liquid by electromagnetic induction. Thesesensors usually use AC excitation field and measureinduced voltage between two electrodes immersed in theliquid. Contactless sensing of the induced electrical fieldusing two plates outside the non-conductive pipe isalso possible.Magnetostrictive position sensors use sonic waveguide

made of magnetostrictive wire or tube. The length of thesesensors is limited by attenuation to about 4–6m. Resolu-tion can be as low as 0.4 mm and uncorrected non-linearity0.02% FS. Magnetostrictive delay lines allow measuringalso other physical variables at multiple points [31].

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Table 1

Magnetic field sensors

Principle Size (mm) Noise (pT/

OHz@1Hz)

Temperature offset

drift (nT/K)

Excitation power

(mW)

Fluxgate

Ring-core 25 3.8 0.1 50

Race-track 70 2.5 0.5 70

PCB 30�8�1.8 17 2 20

Thin-film 3�3 1 000 ? 10

CMOS 1.5 15 000 ? 10

Induction 100 2 AC only 0

GMI 10 100 30 5

IWE 80 ? 5 20

AMR 4�11�1.7 200 10 55

P. Ripka / Journal of Magnetism and Magnetic Materials 320 (2008) 2466–24732470

9. Magnetic force and torque sensors

Many force and pressure sensors are using deformableelastic elements together with magnetic position sensor.Magnetic load cells (pressductors) were developed byASEA (later ABB) in 1950s. These sensors are based ontransformer with perpendicular windings. In unloadedstate the coupling between primary and secondary windingis zero, and there is no output voltage. In loaded statestress-induced anisotropy causes asymmetry of the fluxlinesand output voltage appears.

Magnetoelastic torque sensors become very compact andinexpensive, so that in many applications they are morepopular than traditional torque meters based on straingauges measuring the shaft deformation. They are used inmass applications: to measure cyclist effort in power-assisted bicycles and exercise bikes, in electric powersteering systems, to measure engine and transmissiontorque in automatic car transmissions, and individualwheel torques in 4-wheel vehicles with automaticallycontrolled differential and also in handheld tools tomeasure the tightening torque.

Magnetoelastic torque sensors are based on stress-induced anisotropy of the magnetic shaft or magneticlayer on nonmagnetic shaft. The torque on rotating shaftcan be measured without contact by external coils. Thereare two basic types of these sensors:

(1)

‘‘Permeability based’’ sensors, which uses changes ofpermeability of a surface layer of magnetic shaft causedby stress. The permeability is sensed by couplingbetween the source and pick-up coil.

(2)

‘‘Polarized band’’ sensors.

9.1. Cross-type permeability-based torque sensors

The traditional sensor of this type uses two orthogonalU-shaped cores (with excitation and sensing windings)directed towards the shaft. If the permeability of the shaftbecomes anisotropic, the symmetry is broken and voltage

appears on the sensing coil. The phase of this voltagedepends on the torque direction.A new thin pick-up head was proposed in Ref. [32]. The

new head consists of a stacked pair of figure-eight coils, inwhich one of the coils is rotated in the coil plane by 901from the other. These coils are facing the shaft through anair gap and they are directed in7451 direction with respectto the shaft axis. The applied torque can be detectedfrom the difference in self-inductances of these coils, aseach figure-eight coil sees the permeability of the shaftalong 7451 direction in which the applied torque createssurface stress.The disadvantage of the cross design is that the local

variations in the magnetic properties of the shaft, which areunavoidable, as well as the geometrical imperfections(causing variations of the airgap), cause distortions in theoutput signal. This may be partially averaged by multiplecoil systems. Frequency dependence of the permeabilitycauses dependence of the torque sensor on the rotationspeed, even if the excitation frequency is stable. The reasonfor that are the mentioned AC signals caused by rotationalimperfections, which are modulated on the excitationfrequency. Even though these signals are averaged bymultiple coil pairs positioned around the shaft diameter,the frequency dependence of the permeability will causedecrease of the sensor sensitivity with increasing rotationalspeed. That is why these sensors were almost replaced bythe solenoidal type ones, in which the field has axialdirection. However, precise cross-type torque sensors areused to monitor the radial vibrations of the shaft.

9.2. Solenoidal permeability-based torque sensors

These sensors use shaft with 7451 grooves. The axialpermeability increases when the easy axis of the stress-induced magnetic anisotropy is parallel to these groovesand decreases when it is perpendicular. The changes of theaxial permeability are therefore proportional to themeasured torque. Axial permeability is measured bysolenoidal axial excitation/sensing coils. The grooves canbe machined or formed, or they are made by electro-

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deposited strips of copper. The advantage of the coaxial(solenoidal) design is that the output is free of rotationalartifacts. Sensor of this type can be made differential, sothe output for zero torque may be zero, but to maintain thesymmetry is even more difficult.

An industrial example of this type of torque meter isTorductor S by ABB. Sensors of this type achieve mea-suring error o2% and repeatability error o0.1%.

The common disadvantage of both types of thepermeability-based sensors is the fact that permeability isnot an intrinsic property of the material. It changes withtemperature, frequency and it is also sensitive to theamplitude of the AC field and presence of external DCmagnetic field. Moreover, permeability is irreversiblychanged each time after the sensor is overloaded. Inautomotive environment, which is characterized by high-temperature changes and mechanical vibrations, thepermeability gradually decreases in time due to micro-structural changes of the material.

9.3. Polarized band sensors

These sensors use thin ring of magnetoelastic materialrigidly attached to the shaft (or a part of the shaft materialitself). The ring is circumferentially magnetized. Theoperation is based on rotation of the ring magnetizationtowards the direction of stress induced by the torque. Thisis DC-type sensor and works without excitation. Themagnetization rotation is measured by DC magnetic fieldsensor. The torque sensor usually consists of two ringspolarized in opposite directions and one or more magneticsensors (Hall, AMR, but most often fluxgate). Sensors ofthis type are manufactured by Magna-Lastic Devices, Inc.(www.mdi-sensor.com), Magnova, Inc. (www.magnova.com) and by Siemens VDO [33]. The Siemens sensor(Fig. 5) has shaft covered by 0.5mm layer of magnetos-trictive material made by thermal spray. The radialmagnetic field is sensed by a large fluxgate sensor, whichis encompassing the shaft, which is very innovativegeometry. This brings high shielding of external fields.

FluxConcetrators

MagneticPickup S1

Magnetic pickup S2

MagnetoelasticElement

Inner Flux Guide Ring

Outer Flux Guide Shell

Fig. 5. Siemens torque sensor with polarized rings. Outer fluxgate sensor

senses the radial field and shields the bands from external fields (after Ref.

[33]).

These sensors achieved high-temperature stability of70.04% FS/1C, while the hysteresis/nonlinearity error isbelow 0.5% FS and repeatability below 0.25% FS. Sensorof this type applied on 0.8mm shaft had mNm resolution[34]. For even thinner shafts, torsion impedance effect canbe employed: high-frequency impedance of magnetostric-tive Fe-based wire changes with applied torsion by up to200% with negligible hysteresis [35].Diagnostics of motors require monitoring very small

variations in the torque. This is possible simply byreplacing DC magnetic sensor by induction coil [36].Amorphous tapes and wires can be used to sense

force and strain. However to the author’s knowledgeno such device reached market; temperature dependence,nonlinearity and sensitivity to magnetic field are obviousreasons.

10. Electric current sensors

Many contactless current sensors use Hall sensors(usually in the air gap of a magnetic yoke) and morerecently AMR sensors. However, more precise contactlesscurrent sensors use bulk magnetic material as a sensingelement [37].Precise current transformers use high-permeability ring

cores to scale down the measured current and convert it tovoltage drop on load resistor. They are also made withopenable core as AC current clamps. Current transformersare used in electronic energy meters, but it turned out thatthey could easily be disabled by saturation—either fromstrong permanent magnet, or by DC component in themeasured current. One way to avoid this is to use flat-loopmagnetic material, which has very high saturation field Hs,another approach is to use Rogowski coil, without anymagnetic core. Some manufacturers use composite cores,consisting of high-permeability ring which gives lowangular and amplitude error and low-permeability ring,which is DC-resistant. However, some cheap low-perme-ability rings can make the job as well: they have someamplitude error and phase shift, but these are constant overthe wide range of measured currents and therefore can becompensated [38]. Nanocrystalline alloys are prospectivematerials for very precise small-size current transformers[3].

11. Temperature sensors

Materials with low Curie temperatures are used in simpleWeller soldering irons to control the temperature of the tip.Thermo-reversible permanent magnets as well as low-Tc

soft magnetic alloys may be used as powerless remotetemperature sensors [39].

12. Special applications of magnetic sensors

The applications of magnetic sensors in industry, homeappliances, transport and many other areas are countless.

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In most cases the sensors used are just the off-shelfcomponents. Here we give examples of some high-endapplications, which require specialized sensors.

12.1. Position tracking

Magnetic position trackers use multiaxial artificialmagnetic field created by coil system. Three-axial magneticsensors (usually induction coils, AMR sensors of fluxgates)are attached to the tracked object. Trackers are used invirtual reality and entertainment, but they also have manyindustrial applications—one example is drilling and miningindustry, where trackers measure distance and positionbetween drilling paths and underground tunnels. Sophis-ticated techniques are used to detect and correct fielddistortion caused by metal objects. Another possibility is touse passive targets such as LC resonant marker using 10-mm-long Ni–Zn ferrite core [40]. The system consists ofone excitation and multiple detection coils. It is able tolocalize multiple targets, which have different resonantfrequencies with accuracy below 1mm for a distance of100mm between the marker and sensor array.

12.2. Navigation

Traditional magnetic compass has moving needle leveledin horizontal plane. It is possible to detect the position ofthis needle by Hall sensors, however this is very inaccurate.Precise electronic magnetic compass traditionally usesfluxgate sensors and they may reach 0.11 accuracy. Keepingsensors in the horizontal plane is not practical especially forfast moving platforms. In this case it is possible to use tri-axial magnetic sensors and two inclinometers and recalcu-late the correct azimuth from known pitch and roll. This iscalled strap-down compass. The inclinometer should have atleast double precision than the required azimuth error. Afterall corrections 11 azimuth error is typical for AMR compass[41], but it is sufficient for many portable application, as thecompass is usually used together with GPS and the Earth’sfield is often distorted by ferromagnetic bodies anyway.

12.3. Antitheft systems

They use permanent magnets or other targets attached tothe monitored objects. The most popular are magnetos-trictive targets: the strip of magnetostrictive material hashigh absorption of AC magnetic field at its mechanicalresonance frequency. Some systems use pulse mode: afterdecay of the excitation pulse the signal from vibratingmagnetostrictive labels is detected. The label can be de-activated by demagnetization of the attached strip ofmagnetically hard material [42].

12.4. Detection of vehicles

Magnetic sensors (fluxgates and AMRs) are being usedto detect and recognize vehicles. This is used for traffic

control and for security and military purposes [43]. Multi-axis gradiometers are used to detect submarines.

12.5. Location of UXO and mines

Magnetometers can detect large ferromagnetic bombs asdeep as 6m. They either measure vertical gradient usingtwo fluxgates, or scalar vertical gradient using two Over-hauser or Cesium magnetometers. Extremely precise tri-axial fluxgate gradiometer is described in Ref. [44].Magnetic sensors can also be used to find small ferromag-netic objects. Vectorial sensors give more informationabout the target, but they are sensitive to angular mismatchand positioning errors [45].

12.6. Space research and geophysics

Space DC magnetometers use three orthogonallymounted fluxgate sensors together with resonant magnet-ometer [46]. AC fields are usually measured by inductioncoils with ferromagnetic core. Geophysical and archeolo-gical prospecting methods include DC magnetometry andmeasurement of magnetic properties of samples. Remanentmagnetization of rock, soil samples and archeologicalartefacts is measured either by fluxgate gradiometer or byinduction coil when the sample is rotating.

12.7. Medical distance and position sensors

Magnetic trackers are used to navigate catheters insidethe body. 1mm precision is achievable for 2-mm-diametersensors. The main operational principle is simple wirewound induction coil. As the body liquids are highly con-ductive, the field frequency should be small typically 1 kHz.Simple system measuring a distance between two coils wasused to monitor the movements and size of the stomach[47]. The pick-up coil has ferrite core to increase itssensitivity. Using core for the source coil is contraproductive—the only effect is that the coil impedance isincreased, which can increase the necessary excitationvoltage.Glass-covered magnetic wires were used as targets to

monitor the movement of heart valve [48]. Magneticbiscuits are used to monitor the digestion tract. Thesebiscuits are swallowed and their movement is monitored byexternal magnetic sensors. They are based on the sametechnologies as magnetic trackers with passive marker,which may be hard magnet, soft magnetic material,Wiegand wire, LC resonator [40] or RF transponder.

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