Ž .Sensors and Actuators 78 1999 71–73www.elsevier.nlrlocatersna

Design of a fourth harmonic fluxgate magnetometer

Juan Carlos Cruz, Hector Trujillo )

( )Microelectronics Research Center, Electrical Engineering Faculty of the Superior Polytechnical Institute ‘ J.A. EcheÕerrıa’ ISPJAE , Km. 8.5 Vento´AÕenue, C.P. 10800, P.O. Box 8016, HaÕana, Cuba

Received 7 May 1998; received in revised form 22 January 1999; accepted 22 February 1999

Abstract

The second harmonic induced voltage in the pick-up coil of a fluxgate magnetometer is generally used as a measure of the externalŽ .magnetic field M.F. in applications where high stability and low noise are required. Fourth harmonic component advantages over the

Ž .second harmonic, such as improved sensitivity, fundamentally at weak fields are reported in this paper. Moreover it was obtained thatthe fourth harmonic induced voltage linearity in open loop is greater than that of the second harmonic. q 1999 Elsevier Science S.A. Allrights reserved.

Keywords: Fluxgate magnetometer; Fluxgate sensors

1. Introduction

Fluxgate magnetometers are used to measure weakM.F., such as those found in geomagnetic observations,mineral prospecting, navigation instruments and any otherwhere the M.F. flux density ranges from 0.1 mT to 2 mTw x1,2 . Majority of magnetometers designed up to now forthese applications use the second harmonic induced volt-age in the pick-up coil as a measure of the external M.F.because of its advantages over other detection principles

w xsuch as low noise and higher stability 1,4 ; although somemagnetometers use short-circuit currents output, where avirtual current in the secondary coil corresponding to theexternal magnetic field.

The measurement results of the output voltage of thesecond and fourth harmonics obtained using a simple

w xprobe 1 are shown in this work, where it is observed thegreater sensitivity of the 4th harmonic over the second onefor a range between 10 to 30 mT of the external M.F.

The processing electronics block diagram is also de-scribed, affording the possibility to obtain either the fourthor the second harmonic output voltage. Such scheme pre-

w xsents the novel approach reported previously 3 to elimi-nate the associated error due to the possible phase mis-match between the output signal and the reference signal atthe synchronous detector.

) Corresponding author. Fax: q53-7-272964; e-mail:trujillo@electrica.ispjae.edu.cu

2. Design of the magnetometer

A fluxgate magnetometer is an instrument composed ofa probe or sensor coupled to a processing electronicscircuit. The probe, whose core is of a soft ferromagneticmaterial, has in its simplest version two windings, anexcitation coil and a pick-up coil. The processing electron-ics must supply an excitation current to the premagnetiza-tion coil and perform the pick-up’s signal processingw x1,4,5 .

The 2nd or 4th harmonics output may be chosen throughthe implemented circuit. Its block diagram is shown in Fig.1, where the PLL is used with a double purpose: to feedthe premagnetization coil and to synchronize the second orfourth harmonics signals with the reference. This lastfunction of the PLL discards the error that would beintroduced if there were some mismatch between both

w xsignals 3,7 . The frequency divider by two is used to getthe excitation frequency f , depending upon which har-0

Ž .monic is under test. The active band-pass filter ABPF istuned to the second or fourth harmonic, and in the syn-chronous detector, the output signal of the ABPF is fullwave rectified.

3. Experimental

w xExperiments were carried out using a simple probe 1with a Mumetal core. The measuring circuit employed was

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00194-6

( )J.C. Cruz, H. TrujillorSensors and Actuators 78 1999 71–7372

Fig. 1. Block diagram of the fluxgate magnetometer with 2nd and 4th harmonics output.

constructed according to Fig. 1. It enables to get thesecond or the fourth harmonics output, setting or not afrequency divider by two, tuning the V.C.O. of the PLLand the ABPF to the given harmonic while the overall gainof the circuit remains constant.

The probe is fed with a square wave current signal andŽthe output voltages taken at the output of the synchronous

.detector are measured varying the external M.F. set by aŽHelmholtz coil this winding has a proportional constant

Kbs98.09 where BsKb= I, being I in mA and B in

Ž .Fig. 2. a 2nd harmonic output voltage vs. current through the HelmholtzŽ .coil. b 4th harmonic output voltage vs. current through the Helmholtz

coil.

.Gauss fed with an adjustable D.C. current to get thedesired M.F. magnitude. The sensor and Helmholtz coilwere located in a Faraday cage.

The tested probe is 32 mm length, 5 mm wide and 0.1mm in depth. It has a 660 turns excitation coil and a 1600turns pick-up coil.

Maximum peak excitation current employed was 400mA. The results are shown in Fig. 2. Fig. 2a belongs to thesecond harmonic output voltage vs. the current of theHelmholtz coil, while Fig. 2b shows the same results forthe 4th harmonic. The plotted results correspond to thelinear range found in each case.

The second harmonic results give a 5 VrA sensitivityand 2.9% nonlinearly error while those of the 4th give 17.4VrA sensitivity and 1.5% nonlinearly error.

From the results in Fig. 2 it is appreciated that thefourth harmonic is more linear than the second one, afford-ing a minor relative error.

4. Analysis of the results

From Fig. 2 it is noticed that the fourth harmonicsensitivity is higher than the second one. This result agrees

w xwith those reported in Fig. 8 of Ref. 6 , results that weresimulated using a sinusoidal excitation current and a ‘Z’shaped magnetization curve of the core From Fig. 2a,b it isappreciate that the linear range of the second harmonicsignal is greater than the 4th one. So in non fed backamplifiers 2nd harmonic should be employed for the great-est external M.F. range. In spite of the different waveshapes employed in the simulations and these experimentalresults, the comparison was found valuable because of thefollowing reasons.

Ž .a For a simple probe it should be considered thew xapparent permeability m given as 5 :a

m sm r 1qD m y1Ž .Ž .a r r

( )J.C. Cruz, H. TrujillorSensors and Actuators 78 1999 71–73 73

where m is the relative permeability and D the demagne-r

tization factor of the given probe. This leads to a drop inw xthe differential permeability 1 , but this does not imply a

great error in doing the said comparison, because themagnetization curve will continue being approximately ‘Z’type.

Ž . w xb Although simulation data in Fig. 8 of Ref. 6 wereobtained exciting the probe with a sinusoidal current andthose of Fig. 2 were obtained using a square wave excita-tion current, the comparison is possible because a square-wave waveform can be substituted by their Fourier compo-

Ž .nents odd harmonics only affecting the output voltage ofthe second and fourth fluxgate harmonics only the funda-mental signal, hence the behavior of the probe is similar inboth cases.

5. Conclusions

From the above results the following conclusions maybe drawn.

1. When the coil is excited with a square wave currentwaveform the 4th harmonic sensitivity is higher than thatof the 2nd one, thus it is advantageous to use a 4thharmonic magnetometer in the range 10 to 30 mT.

2. Second harmonic magnetometers should be used ifwe want a linear range greater for the measurement of theM.F. flux density.

3. The 4th harmonic output voltage is more linear thanw xthe second one in agreement with simulation results 6

w xand as mentioned in Ref. 8 .w x4. The use of a PLL in the processing electronics 3 ,

may be employed also in the construction of a 4th har-monic magnetometer.

5. It is confirmed that a 4th harmonic magnetometerbrings some advantages over traditional second harmonicones.

References

w x1 W. Bornhofft, G. Trenkler, Magnetic field sensors, in: R. Boll, K.J.Ž .Overshott Eds. , Sensors, Vol. 2, Chap. 5, VCH, Veiden, Germany

Ž .1989 153–205.w x2 B. Travis, Electromagnetic sensors put a spin on compasses, E.D.N.

Ž .1996 77–82.w x3 J.C. Cruz, H. Trujillo, M. Rivero, New kind of fluxgate magnetome-

ter probe with enhanced electronic processing, Sensors and ActuatorsŽ .A 71 1998 167–171.

w x4 P. Ripka, Review of fluxgate sensors, Sensors and Actuators A 33Ž .1992 129–141.

w x5 F. Primdahl, The fluxgate magnetometer, J. Phys. E: Sci. Instrum. 12Ž .1979 241–253.

w x6 H. Trujillo, J.C. Cruz, M. Rivero, M. Barrios, Analysis of the fluxgateresponse through a simple SPICE model, Sensors and Actuators,November 1997, to be published.

w x7 M. Rivero, Design of Probes and Different Electronic ProcessingŽ .Circuits for Fluxgate Magnetometers, in Spanish , MSc Thesis,

Microelectronics Research Center, Havana Cuba, 1997.w x8 P. Ripka, Improved fluxgate for compasses and position sensors,

Ž .Journal of Magnetism and Magnetic Materials 83 1990 543–544.

Juan Carlos Cruz, graduated in electronic engineering from the InstitutoŽ .Superior Politecnico Jose Antonio Escheverria ISPJAE , Havana, Cuba,

in 1980. He received an MSc in microelectronics in 1990. He has beenprofessor of the Electrical Faculty of ISPJAE since 1984. In addition, he

Ž .has worked in the Microelectronics Research Center CIME of ISPJAE,Havana, Cuba, in 1984.

Hector Trujillo, graduated in electronic engineering from the InstitutoŽ .Superior Politecnico Jose Antonio Escheverria ISPJAE , Havana, Cuba,

in 1969. He received an MSc in microelectronics in 1976. He received aDrSc in magnetic sensors in 1994. He has been a professor of theElectrical Faculty of ISPJAE since 1970. He has also worked as anassistant researcher at the CIME of ISPJAE, Havana, Cuba, in 1972.