ELSEVIER Sen~ors and Aclualorg A 60 (1997) 76-79 A

PHYSICAL

Fh ×gate sensor for magnetopneumometry

Pavel Ripka *, Petr Navratil C:ec!: Tc"ht:i:a! U;'_iver.~it.~, Eh','irolt'Chltit'ol I"aeulty. Department ~ Meaxurement, 166 27 Prague 6, ~r':~ '~'1~ Republic

Abstract

A race-track single-core fluxgate magnetic gradient sensor has achieved a noise level of 30 pT cm ~ r.m.s. (50 mHz--10 ttz) with a gradiometric base of only 3,75 cm. Such a sensor has sensitivity high enough to replace SQUIDs in the detection o1' ferromagnetic particles deposited in human lungs. Measurements on tissue samples, dust from respiration lilters and experiments with phantoms have proved that the to,al amount and spatial distribution of du~t in lhe lungs may be measured in the case of welders, grinders and other metal workers. Measuremcnts on coal miners and asbestos workers are less specific. The construction of the magnetization device for in rive experiments is in the progress.

Keywords: Flu~tgale sensors: Magnetopneumom¢lry

1. Introduction

Magnetic methods for respiratory diagnosis were first used by Cohen [ I l. The standard instrument for magnetopneu- mographic measurements is a SQUID magnetometer work- ing in a magnetically shielded room. Such equipment is costly and cannot be made portable [2], Thus magnetometry has never become a routine diagnostic method. High-temperature SQUIDs have noise of about 0.2 pT Hz 1/2 at I Hz, which is low enough for magnetopneumometric measurements, but they ~,till need liquid nitrogen and tkeir lifetime is limited 13 I. The present paper describes a fluxgate gradiometric sensor wlqch is sensitivc enough to be used for the detection of ferromagnetic particles. Such an instrument could replace a Y, QUID in magnetometric systems and potentially replace X- ray screening of exposed workers.

2. Magnetic properties of respirable dusts

A study of magnetic properties of the coal dust deposited in human lungs was performed on tissue samples from the lungs of black-coal miners who cried of silicosis [4] and on dust chemically extracted f~om the lungs [5]. MOssbauer ~pectroscopy in eo',bination with a vibrz~ting sample mag- netometer have shown that the ferromagnetic component may be attributed to large ,',0gnetite particles, very fine particles

* Corresponding author. Tel.: +422 2435 2188. Fax: +422 311 9929. E-mail: ripk~t@feld.c~ut cz

0924-4247/'07/$i7.00 © 1997 Elsevier Science S.A. All fights reser'.,ed PII S0924-4247 ( 96 ) 014 39-2

are responsibie for the superparamagnetic component and the paramagnetic component is probably created by the blood proteins. Magnetic remanence of the d,c. magnetized samples was measured using a rotating sample magnetometer (JR-4 Rockgenerator). The dependence of the specific remaLlcnt moment on the amplitude of magnetizing lield has shown that the d.c. flux density required for the saturation of most of the samples is 200 roT; 80% of the remanent m~gnetization is reached even for a lield of 100 mT. The variavce of the magnetic properties of individual samples of coal dust was high: the saturated specific remanent moments ranged from 0.5 to 40 nT m ~ kg -

Measurements on dust samples collected fiom respiratory filters in the workplace of welders and grinders have shown much larger and less spread values. This indicates that the magnetic detection method would be much more sensitive and specific in the case of metal workers.

3. Fluxgate gradiometer

The race :rack geometry of the tluxgate ,~ensor core decreases the demagnetizatie, n over the traditional ring cores and thus increases tile open-loop sensor sensitivity [ 6 I- A 7 cm long flux~,ate made of oval shcc:s e!cbe,.t fi am amorphous cobalt-based low-mag,'tetostricti,~n material has reached a noise level of 3 pT Hz- t t~ at I Hz, which is the lowest noise levet for a solid-state room-temperature vector magnetic-field sensor reported in open literature.

A fluxgate sensor made on a similar core but also being able to measure the field gradient has been proposed in Ref. [ 7 ]. The sen: ~r construction is shown in Fig. !. The magneti,:

P, Rq~ka, P Nm'raltl I ~'+'n~'m-~ mui Actuators ,'t 60 (1907) 76 -79 77

3 , ~ 4 10 '

Fig. I. l)ace-track fluxgate gradiornetnc sensor.

o u t p u t vo l t age (V) 2

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Fig. 2. Impotaant waveforms of the rnagnetometric sensor: (a,bl induced output voltage for B= 0 in the untuned pick-up coil (a) and coil tuned by parallel capacitor ( b ); ( c ) PSD output for B = O; ( d ) excitation current: [ e ) unstable output for undamped resonator; (f) output stabilized by damping resistor for B = 2 IsT.

core consists of eight layers of Vitrovac 6025 (produced by VAC Hanau, Germany) etched from 30 l, tm thick tape. The core length is 70 mm and the track width is 2 ram. The core is wound by Nz = 80 turns of the excitation winding tuned by parallel capacitor Ct = 1 l.tF. Fig. 2(d) shows the excitation current of 15 kHz frequency and 1.5 A amplitude which brings the core into deep saturation and thus eliminates even- tual memory effects.

A centrally located pick-up coil of No= 1000 turns serves for the measurement of the homogeneous field component (zero-order gradient). The raw induced voltage waveform of this coil lbr zero licld is shown in F;g. 2(a}: parasitic capac- itances cause ringing at higher harmoni::s. A 13 nF capacitor p3rallel to the pick-up coil creates a non-linear resonant cir- cuit tuned to the second harmonic of the excitation frequency: Fig. 2(b,c) shows the corresponding induced voltage and phase-sensitive detector (PSD) output waveforms, respec- tively. The circuit was damped by a I0 k~ loading resistor; very strong parametric amplification leads to instabilities of the unloaded circuit even for zero rid& as shown in Fig. 2(e). The stability condition is field dependent: a loading resistor of 13 kl~ guarantees the stability up to 100 FT. Fig. 2(f)

shows the induced voltage for a measured field of 2 I, tT+ The gradiomctric sensing coils Nz to N7 of 900 turns are

connected ~ntiserially: the gradiometric base of 3.75 cm is given by the distance t,f their axes. An unusually short base is advantageous for the measurement of the near-field sources in the presence of large interference, which is the case for pneumographic measurements without the use of magnetic shielding. Such a short gradiometric base cannot be achieved using the traditional gradiomctric configuration using two separate ring-core Ituxgate ";ensors. Fig. 3(b) shows the

excntat iot~ cu r ren t (A)

, r i i • : I

-, v + . . . . . . . . . . . . . . . . v : : ....... ........ . . . . . ; ........ .i....+-kl . . . . . . + . . . . . . . + . . . . . . + . . . . . + .. . . . . . ÷ ........ i . . . . .... i ......... i ......... i-,-,"-,:,,I output vo l tage(V) : : ~ ;: ! gra~ B = 0 I

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output vo l tage (V) " ~ i !~ grad B- - t rn~/cm I

• : i . . . . . . . . . i . . . . . . . : ; . . ,~ . . . . . i . . . . . . . . i . . . . . . . . . i . . . . .

.... ........

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2; i i + u ~ no n2n 16o 200

time, tits) Fig. 3. Important waveforms of the gradiornetric sensor: excivation current i a) and output v oltage for grad B = 0 t b ) and grad B = i mT cm- t j. c ).

78 P. Ripka. P. Navratil I Sensors and Act, awrs A 60 { 1997~ 76-79

induced voltage waveform for zero field gradient. This spu- rious signal is caused by the s,:,,~,r non-symmetry and by capacitive coupling from the excitation winding, and it is practically eliminated by the PSD. The signal for a gradient of l m T c m ' is shown in Fig, 3(c).

The gradiomctric sensor has an unwanted response to a homogeneous magnetic field caused by non-homogeneity of the core and non-symmetry in the geometry of the coil sec- tions. The sensor was ~'urther astatized using extra balancing turns serially connected to the gradient coil and a small bal- ancing capacitor C2. The achieved sensitivity is 7894 nT m ' V ', while the cross-sensitivity is 0.026 nT (nT m ~) '

Fig. 4(a) shows the gradiometer response to a gradient step of 20 nT cm- i created by an antiserially connected Helmholtz calibration coil pair; slow variations are caused by the field changes in the laboratory as the lower trace shows the gradiometer signal for the sei~sor placed in magnetic shielding. The gradiometer noise is of the 1/f type (Fig. 5) with a power spectral density o f 0.3 mV r.m.s. Hz- ~/-~ at I

Hz; the band noise is 0.15 mV r.m.s. (50 mHz to I0 Hz); the corresponding field gradient values ,are 10 pT cm-~ noise density and 40 pT cm - ~ band noise.

i o . . . . . . . . . . . . . . . . . i . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . i . . . . . . . i . . . . . . .

0 1 C 2f, 30 40 5D t i m e ( p s )

Fig. 4. Gradiometer response to a gradient step of 20 nT cm - t. Lower trace

shows the gradiometer signal for the sensor placed in enagnetic shielding,

to I ' ~ n a - o i ~ , ~ , . a ~ i i ~i i i i i i i i i i : i i ! i

, i - ! ~. ] i i . . . . . . . . . . . i . . . . ~ ! . . . . . . . i - - - - , - , - I . . . . . . . . . . i:~'i'~ i ~ ~: i i ~ : ~ i ~ :, i i l ;;~,~!i I .... ~, ~ ! , ! - i i i . . . . . i ..... ; : : . ~ [ : I i : i i i f : ' !

__~.~: : ~ r :~ i i l ......... i ....... : ? ~ : ! ' i i : : " : : : : : : ~ l ] : ! i ! i : : ~ : : : : : : . .

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Fig. 5. The gradiometer no,se power spectral density.

. . . . . . j:2~2:F_-2:U_ ....... ' ...... ~" •

[ . . . . l r - - -~ I

Fig. 6. l'hc schematic diagram of the computer-controlled fluxgale gradi- omelet. DV is a digital voltmeter and PSi) is a please-sensitive detector.

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tO

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Fig. 7. The main panel of the virtual gradiometer showing the response to a

gradient step of 20 nT m '

The schematic diagram of the computer-controlled flux- gate gradiometer is shown in Fig. 6. The excitation generator also produces a synchronized 30 kHz squarewave reference signal for the PSD. The d.c. output is measured by a voltmeter connected to a PC. The instrument software wrilten under LabWindows CVI allows automatic calibration of the sensor sensitivity to be performed before each measurement (because the sensor works with no feedback) and also even- tual compensation of the Earth's iield using a 20-bit ADC [81.

Fig. 7 shows the main instrument panel with the response to a gradient step of 20 nT ill ~ generated by the gradient coil. The measurement was perlormed inside the six-layer shield.

4. Measurements on phantoms

A physical model of contaminated human lungs was made from 576 2 cm × 2 cm × 2 cm cubes. The individual cubes were made from gypsum containing known amount of mag- netite. The first phantom had in total 4.67 g of magnetite homog,:nically spread in the whole volume. The cubes were individually magnetized in the air gap of the electromagnet and their remanent volume was measured using the rotating sample magnetometer. After assembling, the remanent field of the phantom was measured by the described fluxgate gra- diometer. The senso, was mounted stably and the phantom was moving during the scanning. This was necessary because

P. Ripka. P Navratil / S~',t~',,rs and Acr'.mt,,r~" A 60!1997) 76-79 79

Grad (nTlm)

i::13~00-33~L1 3~(]0' 0 31~.320G0

X(rn) o~ o3~

Fig. 8. The magnetic-field-gradient distribution map in a verticam plane 6 cm away from the front side of a lung model containing 4 g of magnetite.

Z : i Lung ctistanee

i l e , .

Fig 9. D.c. gradiometer oulput as a function of the distance from a model containing 200 mg of magnetite.

any shift of the sensor with respect to the Earth's lieid during the measurement would cause large errors due to cross-sen- sitivities. The magnetic-tieid-gradient distribution map in a vertical plane 6 cm away from the lung front side is shown in Fig. 8. Further experiments using phantoms modelling a non-homogeneous distribution of ferromagnetic particles over the lung-volume are in the progress. To illustrate the potential sensitivity of the instrument, the d.c, gradiometer output as a function of the distance between the centrally located mode! and the sensor edge is shown in Fig. 9. rhe phantom consisted of a localized magnetically active area with a total amount of 200 mg of magnetite.

5. Coitelusions

Measurements on lung tissue samples, extra¢ ~ed dust and dust collected from respiratory filters have si',~wn that the fluxgate gradiometric sensor developed is sensitive enough to detect O. 1 g of magnetite or to measure the spatial distri- bution of larger amounts. A 100 mT magnetizing field is sufficient and a large magnetizing device for m vivo meas- urements may be constructed irom hard ferrite.

The instrument sensitivity may be further increased usin~ a light magnetic shielding made of amorphous material to suppress disturbing magnetic lields during the r~',,'.'t~rcment.

Preliminary experiments have shown that a shielding factor of 25 to 35 may be ,achieved using three-layer shielding made of 6 kg of amorphr,us magnetic material.

Acknowledgements

This work was supported by the Grant Agency oftheCzech Republic, grant no. 102/96/! 25 I.

References

[ I ] D. Cohen, Measurements of the magnetic fields produced by the human heart, brmn and lungs, IEEE Tran,~. Magn.. II (1975) 694--700.

[ 21 P. Ripka, Noise and stability of magnetic senso~,l Magn. Mag. Mater.. 157/158 (1996) 424-~27.

131 P Ripka, J. Blaha and J. Jerabek, Fluxgate gradk, meter for magnetopneumography, Proc. IMEKO W, rld Congress. Turin, llaly, 1994, pp. 1527~1531.

14] P Ripka. P. Navra61, T Zak, O Schneeweis, J. Blaha and J. Jeral~k. Magnetic properties of the respired particles, 3rd Jpn.-Cz.-Slov. Joint S~',mnar Apf,hed Eledro~mzgnetics, Prague, Czech Republic. J~[y 1995.

[5] P, Ripka, Review of fluxgale sensors, Sensors and Actuators A, 33 (1992) 129-141.

f 6 ] F. Ripka. Race-track fluxgate sensors, Sensors and Actuators A. 37-38 1,19931 417-421.

17] P. Ripka, K. Draxler and P_ KMpar, Race-track fluxgate gradiometer, Electron. Left.. 29 t 1093) 1193-1194.

181 J. Rozto.~il, V. Haasz and P. Ripka, Labwindows Projects at CTU Prague. National hzstruments Eur. User Syrup.. Munich, Germany, 1994 (no page numbers).

Biographies

Pavel Ripka received an lng. degree in 1984, a C.Sc, (equivalent to Ph.D.) in 1989 and a Doc. degree in 1996 at the Czech Technical University, Prague, Czech Republic. He works at the Faculty of Electrical Engineering, Czech Tech- nical University, as a lecturer. He was with the Danish Tech- nical University in 1992 and 1993 as a visiting researcher. lie is an author and co-author of 55 technical papers and three patents, He is a member of Elektra Society, Czech Metrolog- ical Society, Czech National IMEKO TC-14 Commlttee and the Eurosensors Steering Committee. His main research inter- ests are magnetic measurements and magnetic sensors, espe- cially fluxgate sensors. He teaches electrical measureme.-,',:, and instrumentation, engineering magnetism and sensors.

Petr Narratil received an Ing. degree in t995 at the Czech Technical University. He has been a Ph.D. student at the Dep',,u'tment of i~rca::u~'_mcr.:, there si,.::e ~095. His re,c~arch field is fluxgate magnetometry and bi,.~magnetic modelling and measurements.