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Sensors and Actuators A 162 (2010) 13–19

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

valuation of the planar inductive magnetic field sensors for metallic cracketections

u-Jung Chaa, Baekil Nama, Jongryoul Kimb, Ki Hyeon Kima,∗

Department of Physics, Yeungnam University, 214-1, Dae-dong, Gyeongsan 712-749, Republic of KoreaDepartment of Metallurgical and Materials Engineering, Hanyang University, Ansan 425-791, Republic of Korea

r t i c l e i n f o

rticle history:eceived 4 January 2010eceived in revised form 1 May 2010ccepted 3 June 2010

a b s t r a c t

The planar inductive magnetic field sensors were fabricated by photolithography process. The sensor wascomposed of the planar coils (42 turns and 120 turns) with magnetic thin film as a core. To evaluate thesensor for crack detection, the nonmagnetic Al and magnetic FeC specimens were prepared with the cir-cular and slit shaped artificial surface cracks. The alternative current (0.1 A to 1.4 A) with frequency range

vailable online 10 June 2010

eywords:oft magnetic thin filmhin film devices

of 1.0–2.0 MHz was generated by the straight shaped driving Cu coil. In the case of 120 turns of planarinductive magnetic field sensor, the induced signals on crack positions exhibited the high intensities andstable signal to ratio as varying the driving current and frequency in comparison with those of 42 turnsof planar inductive sensor. The measured output signals on FeC specimen with the micro-sized surface

ed scaly.

ondestructive testingagnetic field sensors

nductive sensor

crack by the non-contactoptical image, respective

. Introduction

In magnetic field sensors, the practical limit of the resolutionepends on the possibility of achieving the noise floor. In compari-on to various magnetic field sensors, Prance et al. [1–3] estimatedhese noise levels as ∼50–100 fT Hz−1/2 for SQUID, <100 fT Hz−1/2

or induction coil, ∼500 fT Hz−1/2 for flux gates, ∼1 pT Hz−1/2 forptically pumped magnetometers, ∼100 pT Hz−1/2 for magnetore-istive sensors and ∼10 nT Hz−1/2 for Hall sensors. Among theseodern sensors and applications, the inductive coil sensors have

een widely used for positions and distance detection, magneticecording heads, as well as nondestructive testing (NDT). Induc-ion coils have the characteristics for simplicity of operation andesign, wide frequency bandwidth and large dynamics, etc. In ordero optimize the inductive sensor, coil shape, number of turns and its

agnetic core can be determined according to each application. Annduction coil outputs a voltage drop proportional to the time ratef change of the magnetic field. Hence the large responses will comerom rapidly changing magnetic fields. The properties of a coil sen-or were described in detail many years ago [4]. In inductive searchoil, their transfer function results from fundamental Faraday’s law,

he voltage induced in the coil can be simplified [5–8]:

signal = −d�

dt= d(NA�0�rH)

dt∝ NAf (1)

∗ Corresponding author. Tel.: +82 53 810 2334; fax: +82 53 810 4616.E-mail address: kee1@ynu.ac.kr (K.H. Kim).

924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2010.06.003

nning were converted to the image which was compared with that of the

© 2010 Elsevier B.V. All rights reserved.

where � is the magnetic flux in the coil, N is the number of turnsof wire, A is the effective area of the loops, �0 is the permeabilityof vacuum, �r is the relative permeability of the core and H is themagnetic field intensity around the coil which is proportional tothe operating frequency, f [9]. From Eq. (1), that high amplitude ofsignal from the coil can be obtained by increasing the number ofturns, N of wire in the coil. However, the increasing total lengthof wire leads to additional resistance noise. The sensitivity of aninduction coil is limited by Johnson noise [5]. In practice, the coilmust have enough windings to ensure sufficient signal amplitude sothat the performance is not impacted by amplifier noise. However,the optimization process for coil performance, in many cases, is notas easy [1].

The electromagnetic-based nondestructive evaluation (NDE)of metallic components is increasing in engineering industry. Ithas been mainly applied for inspection of metal constructions,pipes, and nuclear power plants. Among the electromagnetic-basednondestructive evaluation, induction coil probes combine an alter-native current (ac) excitation coil that induces the changing ofmagnetic fields in a specimen and a detection element that iden-tifies the changing of magnetic fields caused by cracks or otherdefects [7,10]. In order to detect the small size cracks and to enhancethe sensitivity of the sensor, the geometrical conditions of theinductive sensors should be optimized and reduce the size by using

high permeable magnetic core.

It is noted that the conventional planar induction coil by eddycurrent test (ECT) method is very difficult to detect the cracks ofmagnetic specimens; however, the planar inductive magnetic fieldsensor which we suggested can detect the cracks on nonmagnetic

14 Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 13–19

cesses

aitsm

2

pcwptcs(tcls3nbgapie

ls1wt

Fig. 1. Schematic of the fabrication pro

nd magnetic specimens. Therefore, we have fabricated the planarnductive sensors with planar coil and magnetic core using the pho-olithography process. And the sensors were evaluated by the signalensitivity for the detection of cracks on magnetic and nonmagneticetallic specimens.

. Experimental procedure

The planar inductive magnetic field sensor probe was com-osed of the planar inductive sensor and the straight single Cuoil for driving ac field to the specimens, in which driving coilas placed in the edge side of the magnetic core gap of thelanar inductive sensor. In order to evaluate the sensitivity ofhe sensors, the planar inductive sensors with different pickupoil turns (42 turns and 120 turns) were prepared. The micro-ized planar inductive sensors were fabricated on AlTiC substrate1 mm × 1 mm × 2 mm for 120 turns, 0.5 mm × 1 mm × 2 mm for 42urns) using sputtering, electroplating and photolithography pro-ess. The planar inductive sensor with the total 42 turns (threeayers-stack structure) of planar pickup coil has a total dimen-ion of 300 �m × 350 �m × 21 �m (cross-sectional area of coil:�m(w) × 2.7 �m(t)). The Cu pickup coil and Co24Ni37Fe39 mag-etic film as a magnetic core (top and bottom layer) were depositedy electrolyte process. The magnetic core has a 0.6 �m-spacedap between bottom magnetic layer (120 �m × 170 �m × 4 �m)nd top magnetic layer (120 �m × 170 �m × 3 �m). The magneticroperties of Co24Ni37Fe39 magnetic film were measured by vibrat-

ng sample magnetometer, in which magnetization and coercivityxhibited 21.5 kG and 4.5 Oe, respectively.

The planar inductive sensor with total 120 turns (three

ayers-stack structure) of planar pickup coil has a dimen-ion of 400 �m × 380 �m × 35 �m (cross-sectional area of coil:.5 �m(w) × 3.5 �m(t)). The area of the planar inductive sensorith 120 turns of the coil is about twice in comparison with

hat of the 42 turns. The magnetic core has a 0.8 �m-spaced gap

for the planar inductive sensor (a)–(i).

between bottom magnetic layer (207 �m × 280 �m × 6.5 �m) andtop magnetic layer (157 �m × 250 �m × 9 �m). The typical fabri-cation process of the planar inductive sensor was shown in Fig. 1(a)–(i). The optical images of the fabricated planar inductive sen-sors are shown in Fig. 2. To detect the induced signal using planarinductive sensors at crack positions, the driving coil was gener-ated by the alternative current with the range of 0.1–1.4 A at thespecific frequency in the range of 0.5–2.0 MHz, respectively. Andthen, the planar inductive sensor probe was scanned the speed of3–4 mm/s with the flying height of 0.2–0.3 mm from specimens.The sensor position was controlled by x-, y-, z-axis scanner. Themeasured output signals from sensors were amplified and filteredby the preamplifier. And the resistance and inductance the pla-nar inductive sensors were measured by impedance analyzer toanalyze the relations between the electromagnetic properties andsensitivities with the change of sensors geometry. These geomet-rical specifications of the planar inductive sensors with the changeof the pickup coil turns were summarized in Table 1.

In order to detect the metallic cracks, the 20 mm-thick, 5 mm-thick nonmagnetic Al and 5 mm-thick magnetic FeC specimenswere prepared with the different size of the circular and slit shapedartificial cracks, respectively, as shown in Fig. 3.

3. Results and discussions

Fig. 4 shows the measured signals using 42 turns (a and c) and120 turns (b and d) of the planar inductive sensor on Al specimen,in which crack of diameters are 800 �m (a and b) and 500 �m(c and d)-wide circular shaped artificial cracks with the differentdepth of 15 mm, 10 mm, and 5 mm, respectively. When the signals

were measured by the planar inductive sensor, the ac magnetic fieldwas applied to the specimen by driving coil (0.3 A at 1.6 MHz). Themagnitudes of the detected signals were decreased with the decre-ment of the diameter and depth of cracks. The detected signals oncrack position by 120 turns of planar inductive sensor exhibited

Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 13–19 15

F d the1

ttbipnta(oalwfbt

TT

ig. 2. The fabricated inductive sensors; the top view of planar inductive sensors an20 turns, respectively.

he higher intensities over 25 times in comparison with those ofhe 42 turns. These enhanced signal intensities could be governedy inductance of the planar inductive sensors, in which inductance

s changed by number of coil turns, coil loop area and magneticermeability including demagnetizing effect by means of the mag-etic core dimension. The measured inductance and resistance ofhe 120 turns (∼40 �H, ∼317 �) of planar inductive sensor arebout 20 times and 6.5 times higher than those of the 42 turns∼2 �H, ∼48 �), respectively, as shown in Table 1. The incrementf inductance was basically caused by the increment of coil turnsnd volume of magnetic layer. The sensitivity of an inductive coil isimited by Johnson noise [9] given by Vnoise =

√4KBTR�f ∝ N/

√r,

here, KB is Boltzmann constant, T is temperature, R is resistance,is frequency and r is loop radius. Although the Johnson noise wille increased with the proportion to the increment of resistance,he increment ratio of inductance is much higher than that of resis-

able 1he geometrical specifications of the planar inductive sensors with the change of pickup

No. of turns

AlTiC substrate (w × l × t : mm)Loop area of pickup coil (mm2)

Cross-section of pickup coil (w × t : �m)

Magnetic thin film core(Co24Ni37Fe39)

Top (w × l × t : �m)Bottom (w × l × t : �m)Magnetic head gap (�m)Magnetization (kG)Coercivity (Oe)

Electrical resistance (�)Inductance (�H)

Measuring conditions Driving ac current (A)Driving freq. (MHz)Scan speed (mm/s)Flying height (mm)

Specimens MaterialsArtificial crack shapes

cross-sectional view of pickup coil on AlTiC substrates (a), (c) 42 turns and (b), (d)

tance with the change of coil turns from 42 turns to 120 turns. Andthe loop radius of 120 turns of pickup coil increased about 50% thanthat of 42 turns.

In order to verify the detected signals with the change of thespecimen materials dependency, the nonmagnetic Al and magneticFeC specimens were prepared with the slit shaped artificial cracks.Fig. 5 shows the detected signals using 40 turns (a and c) and 120turns (b and d) pickup coils of planar inductive sensors at typicalfrequencies of 0.5 MHz (a and b) and 1.6 MHz (c and d) on nonmag-netic Al specimen with the artificial slit shaped cracks, respectively.The driving ac current was fixed at 0.3 A. The crack depths on spec-imens are fixed with 1 mm and widths are changed from 0.5 mm

to 1.2 mm. In the case of 0.5 MHz driving frequency, the signalswere not detected on crack positions by the 42 turns pickup coil ofplanar inductive sensor. When the driving frequency was changedfrom 0.5 MHz to 1.6 MHz, the signals by 42 turns pickup coil were

coil turns and measuring conditions.

42 turns 120 turns

0.5 × 1 × 2 1 × 1 × 20.10 0.153 × 2.7 1.5 × 3.5

120 × 170 × 4 157 × 250 × 9120 × 170 × 3 207 × 280 × 6.50.6 0.821.54.5

48 3182.1 40

0.1–1.40.5–23–40.2–0.3

Al (nonmagnetic), FeC (magnetic)Slit type, circular type

16 Y.-J. Cha et al. / Sensors and Actu

Fcs

dppobs0nt1i

ble values in comparison with those of the 42 turns with the change

Fb

ig. 3. The top view of the optical images of the artificial surface cracks: (a) theircular typed cracks, (b) the slit typed cracks on nonmagnetic Al plate, and (c) thelit typed cracks on magnetic FeC plate, respectively.

etected on the crack positions shown in Fig. 5(a) and (c). When thelanar pickup coil turns were increased from 42 turns to 120 turnsickup coils at 0.5 MHz driving frequency, the signals were detectedn the crack positions as shown in Fig. 5(a) and (b). These results cane easily predicted by Eq. (1) in general. Except the non-detectedignal by the 42 turns pickup coil of planar inductive sensor at.5 MHz driving frequency, the intensities of the other detected sig-

als were increased with the increment of the width of cracks. Byhe change of driving frequency from 0.5 MHz to 1.6 MHz in the20 turns of pickup coil, the intensities of the detected signal were

ncreased about twice. It implies that the induced voltage is pro-

ig. 4. The comparison of the detected signals using 42 turns (a and c) and 120 turns (b a) and 500 �m (c and d) artificial circular shaped cracks, respectively.

ators A 162 (2010) 13–19

portional to the frequency based on Eq. (1). In the case of 1.6 MHzdriving frequency, the intensities of the detected signals as thechange of pickup coil turns from 42 turns to 120 turns were greatlychanged although the turns of the pickup coil were increased from42 turns to 120 turns, as shown in Fig. 5(c) and (d). The signal inten-sities of the slit typed cracks were highly increased in comparisonwith those of the circular typed cracks of Fig. 4. It means that thesignal intensities are deeply related with the crack shapes, size andspecimen materials as well as the geometric conditions of sensors.

Fig. 6 shows the detected signals using 42 turns (a and c) and 120turns (b and d) pickup coils of the planar inductive sensor at typicalfrequency 0.5 MHz (a and b) and 1.6 MHz (c and d) on magnetic FeCspecimen, respectively. In the case of 0.5 MHz driving frequencyand 42 turns of pickup coil, the signals on crack positions were notdetected on the same results of Al specimen. When the driving fre-quency was changed from 0.5 MHz to 1.6 MHz, the signals by 42turns of pickup coil were detected on the crack positions shown inFig. 5(a) and (c). Except the non-detected signals for the 40 turnspickup coil of the planar inductive sensor at 0.5 MHz, the intensitiesof signal on crack positions were increased a little with the incre-ment of the width of cracks on magnetic FeC specimen. When thedriving frequency is changed from 0.5 MHz to 1.6 MHz using the120 turns pickup coil of the planar inductive sensor, the intensitiesof signal were increased up to about 38 times. As increasing theturns of pickup coil from 42 turns to 120 turns at 1.6 MHz drivingfrequency, the intensities of the detected signals were increased upto about 230 times.

In order to evaluate the sensing quality of the planar inductivesensor, the measured signal-to-noise ratios (SNRs) were calculatedfrom the measured signals with the change of the driving current(0.1–1.4 A) and frequency (1.0–2.0 MHz) on slit shaped nonmag-netic Al and magnetic FeC specimens as shown in Fig. 7. The SNRsby the 120 turns of the planar inductive sensor exhibited very sta-

of the driving current and frequency. However, the SNRs of the 40turns of the planar inductive sensor were linearly increased withthe increment of the driving current for the nonmagnetic Al speci-mens, respectively, as shown in Fig. 7(a) and (b). As increasing the

nd d) inductive planar sensor on Al specimen with the diameter of 800 �m (a and

Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 13–19 17

F s (b ana th: 0.5

fSeSdecp

sis

F(

ig. 5. The comparison of the detected signals using 42 turns (a and c) and 120 turnnd d) on Al specimen with the artificial slit shaped cracks (fixed depth: 1 mm, wid

requency from 1.0 MHz to 2.0 MHz at 0.3 A driving current, theNRs for the 42 turns of the planar inductive sensor were not lin-arly changed as shown in Fig. 7(c) and (d). The reason of unstableNR values for the 42 turns of the planar inductive sensor can beeduced that the inductance of the planar inductive sensor is notnough to detect the small change of the magnetic fields at nearracks on specimen in comparison with those of 120 turns of the

lanar inductive sensors.

In order to verify the detected crack shapes, the inductive planarensor with 120 turns pickup coil was scanned by one axis 50 �m-nterval scan with the speed of 3 mm/s at 0.3 mm height from FeCpecimen under driving current of 1 A at 1.6 MHz. The slit typed

ig. 6. The comparison of the detected signals using 42 turns (a and c) and 120 turns (b anc and d) on magnetic FeC specimen with the artificial slit shaped cracks (fixed depth: 1 m

d d) planar inductive sensor in typical frequency 0.5 MHz (a and b) and 1.6 MHz (c–1.2 mm), respectively.

artificial surface crack size is 0.3 mm × 20 mm. And then the mea-sured signals were converted to the image profiles. Fig. 8 shows theoptical real crack image (a) and the converted crack image (c) andtheir enlarged crack images (b) and (d). The width of the convertedimage exhibited the 0.6 mm in comparison with that of the 0.3 mm-wide real crack. The length of the converted crack image also was alittle bit longer than that of the real crack. These differences could

be caused by the wide distribution of the magnetic fields at theedge side of crack. Unlike the optical method, the magnetic fieldmeasurement gives information not only on the actual profile butalso on the shape of cracks. As results, the positions and shapes ofsurface crack on various metallic specimens were easily detected

d d) planar inductive coil sensor in typical frequency 0.5 MHz (a and b) and 1.6 MHzm, width: 0.5–1.2 mm), respectively.

18 Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 13–19

Fig. 7. The measured signal-to-noise ratios (SNR) were summarized with the change of the driving current from 0.1 A to 1.4 A (a and b) at 1.6 MHz and frequency from1.0 MHz to 2.0 MHz (c and d) with current of 0.3 A on Al and FeC specimens with slit shaped cracks, respectively.

F ucedw height

wnp

4

shnwAp

ig. 8. The optical image (a and b) and the converting image (c and d) from the indith the 120 turns of pickup coil was scanned with the speed of 3 mm/s at 0.3 mm

ith high sensitivity. Also, the micro-sized planar inductive mag-etic field sensor makes it possible to apply in imaging metallicrofiles.

. Conclusion

In order to detect the positions, various sizes and shapes of theurface cracks on nonmagnetic Al and magnetic FeC specimens, we

ave fabricated the planar inductive magnetic field sensor with theumber of 42 turns and 120 turns planar pickup coil. The signalsere detected with the change of the driving frequency and current.ll of the output signals were in good agreement with the crackosition. The intensities of signal were enhanced by the increas-

voltages by one axis scanning on FeC specimen, which the planar inductive sensorfrom the specimen under 1 A at 1.6 MHz conditions.

ing pickup coil turns, frequency and currents. As results of thedetected signals, the SNRs of the planar inductive sensors exhibitedhighly stable values with the increase of the turns of pickup coil.As results, the optimized planar inductive magnetic field sensorswith 120 turns of pickup coil have good sensitivity for applicable tonondestructive and non-contact surface crack detections on mag-netic specimens as well as nonmagnetic specimens, which make itpossible to apply in imaging metallic profiles.

Acknowledgements

This work was supported by a grant from the Materials & Com-ponents Technology Development Program funded by the Ministry

d Actu

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University, Korea. From 2001 to 2003, Research fellow at Research Institute Electri-cal Communications (RIEC), Japan. From 2003 to 2006, he was an associate professorin the Dept. of Electrical Communications Engineering in Tohoku University, Japan.Since 2006, he is an associate professor in the Dept. of Physics at the Yeungnam Uni-versity, Korea. His interests are the development of magnetic materials & devices

Y.-J. Cha et al. / Sensors an

f Commerce, Industry and Energy (MOCIE), Republic of Korea.uthors thank Nova Magnetics Inc. for their support of the fab-ication facilities.

eferences

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ators A 162 (2010) 13–19 19

Biographies

Yu-Jung Cha received her PhD degree in Physics from Myongji University, Korea,in 2008. Since 2008, she is a post-doctoral fellow at the Institute of Photonics &Nanotechnology (IPNT) in Yeungnam University, Korea. Her interests are the char-acterization of amorphous materials and development of magnetic sensors.

Baekil Nam received his PhD degree in Physics from Myongji University, Korea, in1998. From 2001 to 2002, he was a post-doctoral fellow at Vanderbilt University,USA. From 2002 to 2008, he was a Research Fellow at Dongguk University, MyongjiUniversity, and Korea Advanced Institute of Science and Technology (KAIST), Korea.Since 2008, he was Research fellow at Institute of Photonics & Nanotechnology(IPNT) in Yeungnam University, Korea. His interests are the theoretical analysis ofnanomagnetic materials, electronic passive devices and metamaterials.

Prof. Jongryoul Kim received his PhD degree in Materials Engineering from Uni-versity of Illinois, USA in 1991. From 1991 to 1993, he was a post-doctoral fellowat University of Illinois, USA. From 1993 to 1996, he was an associate professor atthe University of Cincinnati, USA. From 1996 to 1997, he was a senior researcher atKorea Institute of Science and Technology (KIST), Korea. Since 1997, he is a profes-sor in the Dept. of Metallurgical and Materials Engineering at Hanyang University,Korea. His interests are the magnetism and TEM characterizations.

Prof. Ki Hyeon Kim received his PhD degree in Physics from the Myongji Univer-sity, Korea, in 1999. From 2000 to 2001, he was a research professor in Hanyang

and the research of metamaterials for IT and Bio-applications.