Ž .Sensors and Actuators 85 2000 227–231www.elsevier.nlrlocatersna

Race-track fluxgate with adjustable feedthrough

Pavel Ripka)

Czech Technical UniÕersity, Faculty of Electrical Engineering, Department of Measurement, Czech Technical UniÕersity, Technicka 2,166 27 Prague 6, Czech Republic

Received 14 September 1999; received in revised form 18 January 2000; accepted 24 January 2000

Abstract

Improved shape of the race-track fluxgate sensor is suggested, which allows precise symetrization of the sensor and thus lower level ofŽ .the feedthrough signal. The sensor has high untuned sensitivity 16.7 VrT per turn and low crossfield response due to the high shape

anisotropy. The closed core and low cross-section guarantee low power consumption, low noise and stable offset. The sensor can be maderesistant to vibrations and mechanical and temperature shocks. The sensitivity obtained after the proper tuning was 1.1 VrmT. q 2000Elsevier Science S.A. All rights reserved.

Keywords: Fluxgate; Magnetic sensors; Magnetometers

1. Introduction

Fluxgate sensors serve for the measurement of DC andlow-frequency AC magnetic field in the range of approxi-mately 1 nT to 1 mT with possible resolution of 50 pT.Their principle is based on modulation of the flux in thepick-up coil by changing the permeability of the ferromag-

w xnetic core by means of the AC excitation field 1 . Largespurious component at odd harmonics coming from thetransformer effect may be highly suppressed by usingsymmetrical construction of the sensor. Most of the flux-gate magnetometers work in the feedback mode to im-prove the sensor linearity and increase the measurement

w xrange: the open-loop magnetometer described in Ref. 2has a frequency range of 300 Hz, while linearity error is0.5% for the field range of "1.2 mT. The sensor sensitiv-

w xity may be increased by tuning 3 . In case of voltage-out-put sensors the parasitic self-capacitance and the induc-tance of the pick-up coil form parallel resonant circuit: wemay find the multiple resonant peaks by changing theexcitation frequency. Because of the nonlinear character ofthe circuit, the resonant frequency depends also on the

) Corresponding author. Tel.: q42-2-2435-3945; fax: q42-2-311-9929;http:rrmeasure.feld.cvut.czrusrrstaffrripka.

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

excitation amplitude. Sometimes the circuit oscillates atsome harmonics even without external field. In general,this kind of resonance is unwanted as the coil self-capaci-tance is unstable and temperature dependent. Many studieson fluxgate sensitivity are influenced by such resonance,so they are valid only for the particular pick-up coil andtheir results should be generalized only with great care.

Some of the fluxgate magnetometers intentionally usethe parametric amplification by tuning the sensor voltageoutput by parallel capacitor. Increase of the sensitivity bythe factor of 10 to 100 is easily achievable; for high-qual-

Ž .ity factor low pick-up coil resistance the circuit mayagain oscillate.

w xPrimdahl at al. 4 short-circuited the fluxgate sensorand measured the output current. By using the gated

Žintegrator detector of the switching type with adjustable.gate width , the maximum sensitivity is achieved for spe-

cific phase delay and width of the reference voltage whenw xthe shape of the pulse is best fitted 6 . In digital magne-

tometers the signal detection is performed in a digital wayby correlating it with reference function which has arbi-trary shape so that it may be fitted even more precisely to

w xthe signal shape to be extracted 5 . The analysis of thecurrent-output feedthrough signal coming from the sensorgeometrical unsymmetry and core nonhomogeneity is given

w xin Ref. 7 . It was recently shown that the current fluxgatew xcould also be tuned by serial capacitor 8 .

0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 00 00394-0

( )P. RipkarSensors and Actuators 85 2000 227–231228

Two important unwanted effects are not often men-tioned in the fluxgate papers: perming and crossfield ef-fect. Perming is a change in the sensor offset after mag-netic shocks. In many cases the magnetometer works in theEarth’s field or occasionally in higher fields, and it changesits orientation. In such a case the magnetometer absoluteerror is given by offset unstability, perming and crossfielderrors and also by sensor head mechanical stability ratherthan by sensor noise.

Most of the vector magnetic field sensors have nonlin-ear response to magnetic fields perpendicular to their

Ž .sensing direction ‘‘crossfields’’ . The crossfield effectexists in every magnetic field sensor containing ferromag-netic material. In general, the crossfield effect may be

Žsuppressed by core shape in rod-type or race-track sen-.sors or by total compensation of the measured field as in

the compact spherical coil used in Oersted magnetometerw x4 , not only the component in the sensing direction

w xBrauer et al. 9 made a theoretical analysis of thecurrent-output ring-core fluxgate sensor response to a per-pendicular field. They showed that a variation in magneticsusceptibility along the core may result in a nonlinearperpendicular field response. The shape of the derivedcurve fits the experimental data well. Another study per-formed on the voltage-output sensors confirmed Brauer’s

w xresults 10 . It was also shown that the crossfield responseis temperature dependent: errors as high as 2 nTr8C in theEarth’s field were observed for the low-cost sensor, whichare much higher than the sensor 0.1 nTr8C offset drift.The crossfield response may be reduced by decreasing thering-core diameter. The crossfield effect may be a domi-nant source of error when making three-axial measure-ments in the presence of the Earth’s field.

The ring-core fluxgates have in general low sensitivity,but they exhibit low noise in the input field units. Theymay be adjusted for minimum feedthrough by turning thesensor core with respect to the pick-up coil. Rod-coreŽ .Vacquier or Forster type fluxgate sensors are very sensi-tive to the measured fields and resistant against perpendic-ular fields owing to the shape anisotropy of their cores, butthey have constructional problems and disadvantages com-

w xing from the existence of the core ends. Diaconu et al. 11reached 3 nT offset change in the temperature range ofy758C to q258C and noise level of 200 pT p–p with

w x35-mm-long cores of Vacoperm 100. Moldovanu et al. 12used a similar sensor with stress-annealed cores of 1-mm-wide, 25-mm-thick stress-annealed tape of Vitrovac 6025.The sensor worked in the current-output mode. With 35-mm-long excitation coil and 30-mm-long core they mea-

Ž .sured 46 pT rms noise 64 mHz to 10 Hz ; for 60-mm-longcores they reached 11 pT noise. They have shown that:

1. the excitation coil should be longer than the core, sothat the core ends are properly saturated;

2. the optimum pick-up coil should be short in order toavoid the core end regions which were shown to bemore noisy.

2. Modified race-track core

Ž . w xOval-shaped race-track sensors 13 have high sensi-tivity and low crossfield error. They have in principle very

Žlow noise: 17 pT rms in the frequency range of 50 mHz to.10 Hz , which corresponds to 93 pT p–p, was measured on

sensors having 70-mm race-track core from as-cast Vit-w xrovac 6025x 14 . The race-track sensors wound from the

w xtape were reported in Ref. 15 . Such sensors have poten-tial problem in higher tape stress in the corners. Sensorsmade of etched sheets of race-track shape have been

w xdescribed in Ref. 13 . The core is made of eight layers ofŽ .the 35-mm-thick amorphous Co Fe Cr Si B material.67 4 7 8 14

These sensors had 8 pT rms noise in the 50 mHz to 10 HzŽ .band 50 pT p–p . The core length is 70 mm, width 12

mm and the track width is 2 mm. Even the closed-coresensors require high saturation values to have low noiseand low perming: in this case the excitation current was1.5 A p–p into 400 turns. Such excitation values arereached due to the resonant circuit described by Acunaw x16 .

The main disadvantage of the race-track sensors wasthat they could not be adjusted to balance them for theminimum feedthrough: a spurious output signal limits theirperformance so that they are rarely used. The suggestednew shape of race-track core was designed so that it allows

w xfine adjustment to suppress the sensor unbalance 17 . Thetrack width is slightly increasing along the core length,while the width of the opposite track is decreasing asshown in Fig. 1. By sliding the pick-up coil out of itssymmetrical position the effective cross-section of the corehalves changes, resulting in variation of the spurious out-put signal. The rough balance may be eventually first

Ž .Fig. 1. Modified race-track fluxgate. Shown is the excitation IexŽ .winding and the pick-up coil V . Notice the changing track width.out

( )P. RipkarSensors and Actuators 85 2000 227–231 229

adjusted by adding extra turns of the pick-up winding toonly one of the tracks, then final balance for minimum

Ž .output signal voltage or current may be reached byshifting the coil.

3. The measurement

The sensor core is made of four modified race-tracksheets etched from 40-mm-thick amorphous Vitrovac 8116Ž .produced by Institute of Physics SAV, Bratislava . Thecore is 30 mm long and 8 mm wide, the track width is 1.5mm. The sensor is excited by 15 kHz triangle waveform.The excitation winding of N s128 turns, B 0.3 mm, is1

parallely tuned by C s0.6 mF. All the measurements1

were performed with voltage output and without feedback.

3.1. Low-capacitance coils

Two 16-mm-long cylindrical movable pick-up coilswere tested:

Ž .i single-layer coil a1: 45 turns of B 0.28 mm wireŽ .ii multilayer coil a2: 470 turns of B 0.1 mm.

The measured sensitivities at the first three even har-monics are shown in Table 1 both for untuned and tunedsensors. Due to low parasitic capacitances the sensitivity isproportional to the number of turns. The quality factor ofboth coils is low: the sensitivity increase by tuning are 3and 5, respectively. Fig. 2 shows the excitation current andthe voltage output of the untuned sensor with the 45-turnpick-up coil. The parasitic capacitance is low and it causesringing spikes at high odd harmonics. Fig. 3 shows thesame sensor tuned to the 2nd harmonics by parallel capaci-tor. The voltage output for zero measured field is shown inthree positions of the pick-up coil. The feedthrough signalin the middle position is 20 mV p–p. This signal may besuppressed by linear displacement of the pick-up coil: theminimum value was 4 mV p–p for 1 mm; further 1-mmshift causes the appearance of the feedthrough signal withreversed phase.

Table 1Sensitivity for low capacitance 45- and 470-turn coils

Ž .Coil turns 45 470

Tuning Cs0 Cs1.3 mF Cs0 Cs13 nFŽ .Harmonic Sensitivity mVrmT

2nd 0.755 2 7.2 38.34th 0.6 0.44 6.5 11.46th 0.215 0.09 2.9 5.2

Ž .Minimum feedthrough mV p–p100 4 2200 25

Ž .Fig. 2. The excitation current upper track, 200 mArdiv , and theŽ .untuned output voltage lower track, 50 mVrdiv of the 45-turn pick-upŽ .coil for central position 0 mm .

3.2. High-sensitiÕity coil

The high-sensitivity pick-up coil a3 has N s20002

turns of B 0.1 mm wire. The coil length is 25 mm; itsthickness is decreasing toward the ends to approximate theelipsoid. The pick-up coil high parasitic self-capacitancetunes the sensor close to the second harmonics: precisetuning requires parallel capacitor C of only 330 pF.2

Adding a serial damping resistor R improved the sensitiv-s

ity and offset stability, while the 2nd harmonic sensitivitywas only slightly reduced; sensitivities at higher even

Ž .Fig. 3. The excitation current upper track, 200 mArdiv, and the tunedŽ .output voltage 10 mVrdiv of the 45-turn pick-up coil for central

Ž . Ž .position 0 mm , minimum feedthrough at 1 mm shift y1 mm andŽ .unbalanced position of y2 mm .

( )P. RipkarSensors and Actuators 85 2000 227–231230

Table 2Sensitivity for 2000-turn pick-up coil: untuned and tuned without andwith damping resistor R

Ž .Harmonic Sensitivity mVrmT

Cs0 Cs330 pF

Rs0 Rs0 Rs30 V

2nd 51 1100 7304th 28 132 1786th 9.6 25 34

Ž .Minimum feedthrough mV p–p1000 700 700

harmonics were increased after adding the damping resis-tor, which may be explained by broadening the resonancecurve. The sensor sensitivity and feedthrough levels areshown in Table 2. The maximum sensitivity of 1.1 VrmTis probably the highest value reported for fluxgate sensors.The balancing is much more efficient in this case: whilethe sensitivity was increased 550 times over the value for45-turn coil, the feedthrough level increased only by afactor of 15. Figs. 4 and 5 show the untuned and tuned

Ž .balanced sensor output voltage for Bs0 middle traceŽ .and Bs1 mT lower trace .

The preliminary noise and temperature stability mea-Ž .surements long pick-up coil, tuned, damped had shown

similar results as received for 22-mm-diameter ring-coresensors: noise of 100 pT p–p and temperature offset driftsbelow 0.5 nTr8C. We observed perming effect of 1–2 nTwhen the sensor was exposed to magnetic fields of up to10 mT. It should be noted that the sensor structuralmaterials were low-cost plastic for coil support and alu-minum for the core holder, and also that the magnetic corewas made from as-cast amorphous material. As the excita-tion amplitude was large enough to completely eraseperming in ring-core sensors with the same core materialsbut with ceramic core holder, we explain the observed

Ž .Fig. 4. Excitation current upper trace, 1 Ardiv and untuned voltageŽ .induced in the 2000-turn coil for Bs0 middle trace, 1 Vrdiv and

Ž .Bs1 mT lower trace, 1 Vrdiv .

Ž .Fig. 5. Excitation current upper trace,1 Ardiv and tuned voltageŽ .induced in the 2000-turn coil for Bs0 middle trace, 1 Vrdiv and

Ž .Bs1 mT lower trace, 1 Vrdiv .

perming effect by ferromagnetic residual components inthe aluminum core holder. The use of dimensional stablematerials such as machinable ceramics with low andmatched temperature coefficients of expansion would re-duce not only the temperature coefficient of the sensitivity,but also the offset drift. Furthermore the annealing andsurface polishing of the core tape may also significantlyimprove the sensor properties, especially lower the noise

w xand perming effect 18 .The response to the perpendicular fields at room tem-

perature was below the resolution of our crossfield mea-surement, which was 0.5 nT.

4. Discussion

If we compare the measured untuned sensitivities of the45-T and 2000-T coils, we should keep in mind that whilethe ratio in number of turns is 45, the actual coil geometryis different. The sensitivity is decreasing with increasingcoil mean diameter, so that the actual increase in sensitiv-ity between the 45 turns and large 2000 turns should belower than 45. However, the measured increase in sensitiv-ity was 67 at 2nd harmonics, 46 at 4th harmonics and 45 at6th harmonics, which indicates the tuning effect of theparasitic capacitance of the large pick-up coil. This is anexample of how the sensitivity studies of fluxgate sensorsshould be taken with great reservations.

The coil with higher number of turns had much higherquality factor: tuning it to resonance further increased thesensor sensitivity 20 times, while in case of the 45-turncoil the sensitivity improvement was only 3. High reso-nance increase of the sensor sensitivity would have prob-lems in the poor long-term stability of both the sensitivityand offset. There are two ways of stabilizing the sensor

Ž .performance: 1 decreasing the parasitic capacitance byŽusing the coil construction splitted coil or cross-wound

.coil and eventually by reducing the number of turns. Then

( )P. RipkarSensors and Actuators 85 2000 227–231 231

the more temperature-stable external parallel capacitorwould represent the major part of the total resonant capaci-

Ž .tance; 2 decreasing the resonant circuit quality factor byusing thinner wire or external damping resistor.

5. Conclusions

Modified shape of the race-track fluxgate sensor coreallows adjusting the symmetry of the sensor, thus loweringthe level of the feedthrough signal. The described 35-mm-

Žlong sensor has high untuned sensitivity 16.7 VrT per.turn and low crossfield response thanks to the high shape

anisotropy. The closed core and low cross-section guaran-tee low power consumption, low noise and stable offset.The sensor can be made resistant to vibrations and me-chanical and temperature shocks. The sensitivity obtainedafter the proper tuning may be as high as 1.1 VrmT, but toensure the stable operation the sensor output was slightlydamped by adding the serial resistor.

Acknowledgements

This work was supported by Research Project of theŽMinistry of Education CR J04r98:21 23 00 016 Pollution

.control and Monitoring of the Environment .

References

w x1 Pavel Ripka, Review of fluxgate sensors, Sens. Actuators, A 33Ž .1992 129–141.

w x2 C. Aroca, J.L. Prieto, P. Sanchez, E. Lopez, C. Sanchez, SpectrumŽ . Ž .analyzer for low magnetic-field, Rev. Sci. Instrum. 66 11 1995

5355–5359.w x3 P. Ripka, W. Billingsley, Fluxgate: tuned vs. untuned output, IEEE

Ž .Trans. Magn. 34 1998 1303–1305.w x4 F. Primdahl, J.R. Petersen, C. Olin, K. Harbo Andersen, The short-

Ž .circuited fluxgate output current, J. Phys. E: Sci. Instrum. 22 1989349–353.

w x5 E.B. Pedersen, F. Primdahl, J.R. Petersen, J.M.G. Merayo, P. Brauer,

O.V. Nielsen, Digital fluxgate magnetometer for the Astrid-2 satel-Ž .lite, Meas. Sci. Technol. 10 1999 124–129.

w x6 O.V. Nielsen, J.R. Petersen, A. Fernandez, B. Hernando, P. Spisak,F. Primdahl, N. Moser, Analysis of a fluxgate magnetometer based

Ž .on metallic glass sensor, Meas. Sci. Technol. 2 1991 435–440.w x7 J.R. Petersen, F. Primdahl, B. Hernando, A. Fernandez, O.V. Nielsen,

The ring core fluxgate sensor null feed-through signal, Meas. Sci.Ž .Technol. 3 1992 1149–1154.

w x8 P. Ripka, F. Primdahl, Tuning the current-output fluxgate,Proc.Transducers,1999, pp. 590–593.

w x9 P. Brauer, J.M.G. Merayo, O.V. Nielsen, F. Primdahl, J.R. Petersen,Transverse field effect in fluxgate sensors, Sens. Actuators, A 59Ž .1977 70–74.

w x10 P. Ripka, W. Billingsley, Crossfield effect at fluxgate, Sens. Actua-Ž .tors, A 81 2000 176–179.

w x11 E.D. Diaconu, H. Chiriac, C. Ioan, E. Moldovanu, A. Moldovanu, C.Macovei, F. Barariu, Offset thermostability of the TFS-3 magneto-

Ž .metric sensors, Sens. Actuators, A 59 1997 109–112.w x12 C. Moldovanu, P. Brauer, O.V. Nielsen, J.R. Petersen, The noise of

the Vacquier type sensors referred to changes of the sensor geomet-Ž .rical dimensions, Sens. Actuators, A 81 2000 197–199.

w x13 P. Ripka, Race-track fluxgate sensors, Sens. Actuators, A 37–38Ž .1993 417–421.

w x14 F. Primdahl, P. Ripka, J.R. Petersen, O.V. Nielsen, The sensitivityparameters of the short-circuited fluxgate, Meas. Sci. Technol. 2Ž .1991 1039–1045.

w x15 D.I. Gordon, R.E. Brown, Recent advances in fluxgate magnetome-Ž .try, IEEE Trans. Magn. 8 1972 76–82.

w x16 M.H. Acuna, Fluxgate magnetometers for outer planet exploration,Ž .IEEE Trans. Magn. MAG-10 1974 519–523.

w x17 P. Ripka, Race-track fluxgate sensor, Int. pat. appl. PCTrCZ00r00005.

w x18 G. Vertesy, A. Gasparics, Z. Vertesy, Improving the sensitivity offluxset magnetometer by processing of the sensor core, JMMM

Ž .167–169 1999 333–334.

Biography

Ž .PaÕel Ripka received an Ing degree in 1984, a CSc equivalent to PhD in1989 and Doc degree in 1996 at the Czech Technical University, Prague,Czech Republic. He works at the Department of Measurement, Faculty ofElectrical Engineering, Czech Technical University as a lecturer, teachingcourses in electrical measurements and instrumentation, engineering mag-netism and sensors. His main research interests are magnetic measure-ments and magnetic sensors, especially fluxgate. He is a member ofElektra Society, Czech Metrological Society, Czech National IMEKOCommittee and Eurosensors Steering Committee.