394 Sensors and Actuators A, 41-42 (1994) 394-397

Magnetic sensors for industrial and field applications

Pave1 Rlpka Czech Techntcal Unrvemrty, EIectroiechmcai Faculty, Techmchw 2, 16427 Prague 6 (Czech Repubhc)

Abstract

Vector-type magnetic-field sensors sudable for use m a heavy mdustnal environment and for field measurements mclude Hall sensors, magnetoresistive sensors and fluxgates The basic propertles of these sensors are &cussed Hysteresis and offset of the permalloy amsotroplc magnetoreslstance sensors can be lowered by penodlcally fllppmg the sensor charactenstlcs Whde low-noise fluxgate sensors may work m the sub-nanotesla range and muuature fluxgates may still have nanotesta (nT) resolutron, the typical measunng range of pennalloy anlsotroplc magnetoreslstors starts at tens of nT and the resolution of ordinary Hall sensors IS of the order of 10 FT

Introduction

Magnetic-field sensors explort a broad range of pbys- lcal prmclples This paper tivers the sensor types smtable for use m a heavy mdustnal environment and for field measurements The scope IS concentrated on vector sensors with a sensltivlty around or below the value of the Earth’s field

Typical applications of the magnetic sensors m the mentioned field are

displacement, and proxlmlty switches, position sensors, non-contact tachometers, non-contact current measurements, magnetic compass, detection of and search for metal oblects, mmeral prospectmg, paleomagnetic measurements, momtonng of the anomalies and varlatlons of the

Earth’s field, biomagnek measurements The mam demands for mdustrlal magnetic sensors

are low power, ruggedness, envuonmental resistance, noise immunity, rehatnllty and low price, plus tem- perature and tnne stabdlty

The mentioned reqmrements exclude sensors havmg movmg parts (such as rotatmg cods), superconducting and resonant devices and magnetoelastlc sensors m- cludmg optical-fibre magnetometers The most common sensor types for the apphcations mentioned are Hall and magnetoreslstwe sensors and fluxgates

Magnetoresistive sensors

Sensors based on the anisotropic magnetoreslstance effect m thm films of permalloy are much more ad-

vantageous than traditional semiconductor magneto- resistors [l] They have larger sensitivity, lower nose level and they may be fabncated m such a way that the sensor charactenstlc 1s lmear The maxnnum relative change of the permalloy resistivlty urlth the measured field 1s about 2% Multilayer structures such as C&u and FeCr are very prormsmg for field sensors, as they may display a so-called gant magnetoresistance change as large as 6% [2]

Commercially available permalloy magnetoreslstors (such as Phdips KMZlO) are made as a meander pattern of anisotropic permalloy strips on a sdlcon substrate connected m a bridge configuration [3, 41 The alu- mmmm ‘barber pole’ structure changes the current direction by 90” so that the sensor characteristic 1s lmear and the field orientation may also be detected The sensltlve dIrection lies m the film plane and It 1s perpendicular to the strip length The sensor sensitivity may be simply controlled by the layer thickness There are two prmclpal disadvantages of tlus type of sensor the fippmg effect and hysteresis Each sensor has to be magnetized m the direction of the strips before use m order to have well-defined charactenstics If a large dlsturtnng field m the opposite dIrection appears, the sensor characterlsttcs may be distorted and m an extreme case completely reversed (‘flipped’) This danger may be elnnmated by penodlc magnetization of the sensor, as discussed later

A prototype compass usmg KMZlO sensors with a resolution of 0 1” was developed m the Fachhochschule Hannover [5] The sensor sensltlvlty may be further increased usmg permalloy thin-film flux concentrators Smith et al [6] have reached a 20-tunes magnification of the field measured and a shleldmg factor of 10 for

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395

the orthogonal component The noise level of then sensor was about 2nT p-p for 0 l-2 Hz bandmdth

We have tested Phlhps magnetoreslstors KMZlOA and KMZlOAl Both sensors were powered from a stabilized 5 V voltage source If the sensors are supplied from a 3 mA current source, the temperature depen- dence of the sensltity is slgmficantly reduced, the unprovement of the hneanty IS less pronounced and no change m sensor noise IS observed

The measurmg range for 1% hneanty error was f 300 PT for both sensors The low-field sensltlvlty was 70 pV/pT for KMZlOA and 80 pV/pT for KMZlOAl The sensor offset (after magnetlzatlon) was well below 50 PT for each of the tested sensor umts The KMZlO sensor hysteresis was 1% for the f300 /-CT range

All the measurements were performed after sensor pre-magnetlzatlon by an 8 kA/m field The sensor offset was determmed and the noise level was measured m a SK-layer permalloy magnetic shield Hnth the rest field below 2 nT The sensor charactenstics were determined using a 50 cm diameter Helmholtz toll pan The static charactenstlcs of the KMZlO are shown m Fig 1 Both the curves for dtierent dlrectlons of the sensor pre-

-65

(a) magnetic f?eld ( pT)

15

(b)

Fig 1

1 I~~~“~I~““‘,“““,~‘IIII,

-7 -4 mag& field (‘pT)

4 7

Static charactenstlcs of the Phhps KMZlOAl magneto-

magnetlzahon are shown the sensltivlty and hysteresis are both slgmticantly dtierent while m the f50 PT range for non-mverted (mcreasmg) characterlstlcs the sensltlvlty was 82 pV/pT and the hysteresis was 6 1 IV, the correspondmg values for the Inverted curve were 78 pV/pT and 4 5 pV, respectively In the &5 PT range the sensltlvlties for both slopes were identical and the hysteresis was below 1%

Noise measurements were made using a Kelthley 181 nanovoltmeter and the noise-analysis system described m ref 7 The sensor output m the tune domain 1s shown m Fg 2(a), and the power spectral density (PSD) as a function of frequency m Fig 2(b) The PSD spectrum was calculated from 1024 samples and smoothed using the N=50 linear average m the fre- quency domam The measured noBe spectral density was08nT/&atlHzandtherms valuewas nT m the 0 01-2 Hz frequency band

Most of the hysteresis and offset can be removed by penodxally reversing the sensor charactenstxs by passing successive negative and positive current peaks mto the premagnetlzatlon co11 In such a case the sensor output could be processed by a sunple a c amphfier, but the use of a gated integrator to ehmmate the spikes from the magnetlzmg pulses would be more sophlstl-

I I fre\:ency

(Hz) 1

Fig 2 MagnetoresIstor noise (a) time plot, (b) power spectral . aensq resIstwe sensor (a) for B= *SO pT, (b) for B= f5 PT

cated Problems associated wth the power consumption of such a call should be solved m our experunental set-up an additional 100 mW IS dissipated m the mag- netlzmg cod resistance

Fluxgate sensors

Fluxgates offer the best sensitmty of the room- temperature solid-state vector-type sensors the noBe level of the large race-track or ring-core sensors IS below 10 pT [8] Traditional sensors have the core wound from low-noise low-magnetostnctlon permalloy tape The production, selection and adJustment of the permalioy sensors 1s costly, as they have to be wound on cerauuc bobbms and annealed m order to remove the stresses and to obtam lllgh permeablhty Such sensors are sensitive to vlbratlons and envuonmental shocks The mtroductlon of low-magnetostnctlou cobalt-based amorphous materials allowed the productlon of low- cost rugged sensors mth sumlar senslttvlty parameters

A lot of apphcations, mcludmg magnetic mk reading, safety and security sensors and sensor arrays, require very small sensor size The process of sensor mmla- turlzatlon IS rather complicated as the magnetic noise dramatically mcreases with decreasmg sensor length New miniature sensor types have appeared m the form of a thm-film sensor mth flat toll [9] and an open- core sensor for the multmibrator-type magnetometer

PO1 Sensors made from etched sheets of amorphous ma-

terials have been reported m ref 11 This technology increased the sensor homogeneity due to the absence of tape ends The low-demagnetization oval (race-track) sensors etched from 35 pm thick amorphous (Co6,Fe4Cr&B14) nbbon have 8 pT r m s noise m the 50 ma-10 Hz band The core length IS 70 mm, the width 12 mm and the track width 2 mm

A sandwich-hke miniature fluxgate was developed using this technology 9 mm diameter, 35 pm thick amorphous rmgs were glued between two plastic covers of the same shape and directly wound by the excitation toll

When the sensor was used m the traditional voltage- output mode, the poor sensltlvlty had to be increased by tuning the sensmg (pick-up) co11 by a parallel ca- pacitor The current-output (short-circuited) mode m- traduced by Pnmdahl et al [12] makes it possible to increase the sensltmty by decreasing the number of turns of the sensing coil, this slmphfies the electronics and allows the pulse-detection method to be used, exploiting the mformatlon contamed m the fourth and higher even harmonics [12] Figure 3 (upper trace, 0 5 A/dlv) shows the typical waveform of the excltatlon current, the non-linear resonance excltatlon mode m-

I I 01 , ( , , , ( , , ; ,

Cl 40 60 120 160 200 ttme (ms)

Fig 3 Input and output current waveforms of the shortcuauted sandwich fluxgate sensor (top) exatatlon current, (bottom) output current for BO

-0020

(a) 0 ,11,,,,,,,,,,,,,,,,,,,,,,,,,,,,/,,,,,,,,, 50 100 150 zoo

Time (set)

01

G

s 2 0 01

E a

0 001

01 PI frequency ‘(Hz)

10

Fig 4 Noise of the Siemens FC 32 Hall sensor (a) bme plot, (b) power spectral density

creases the current peaks up to 4 A p-p, while the r m s value remams low The lower trace shows the sensor (short-arcmted) output current peaks (75 mA p-p) for a measured field of 50 PT

Hall sensors

Hall sensors have large offset (2 to 100 mT), mamly due to the stress m the package and geometrlcal errors [13] Stoessel and Resch suppressed the offset by three

391

TABLE 1 Sensmwty, noise and hnear range of the mdustnal-type magnetic-field sensors

Parameter Race-track Sandunch fluxgate fluxgate Sensor sensor

Pennalloy magnetoreslstor sensor

Hall sensor

SensltnUy Norse p-p (0 2-10 Hz) Noise rms Nmse. PSD at 1 Hz Range

(open-loop)

340 nV/pT 025 cLA/nT 80 /LV//LT 130 mVfl 50 pT 1 nT 15 nT 10 NT

?5$,&i 02 nT 25 nT 25 /.LT 008 nT/& 08 nT/@ 05 @T/G

1 PT 50 /L.T 50 ~LT 1T

orders of magmtude and removed the flicker noise by using the spmmng-current method [14]

We have made noise measurements of the popular Stemens FC32 Hall sensor The sensor was connected to a Systron Donner 3102 Teslameter and placed inside a magnetic shield The tnne plot of the sensor nose IS shown 1x1 Fig 4(a) and the noise spectrum m Fig 4(b) The noise spectral density was 0 5 pT/& at 1 Hz and the r m s value was 2 5 PT m the 0 01-2 Hz frequency band

Conclusions

Basic properties of the magnetoresistive, fluxgate and Hall magnetic-field sensors were discussed with special emphasis on the noise level and spectrum, as the noBe properties were not discussed m the recent review by Heremans [15] While fluxgate sensors may work in the sub-nanotesla range, the typical measurmg range of permalloy amsotropic magnetoresistors starts at tens of nT and the resolution of the ordinary Hall sensors IS about 10 PT The noise propertres of the mentioned sensor types are compared m Table 1

Acknowledgements

The author thanks Mr C van Antwerpen and Mr J L.ont from Phllrps for provldmg samples of mag- netoreslstlve sensors This work was supported by CVUT grant 8098 and GACR grant 1127

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