Evaluation of the planar inductive magnetic field sensors for metallic crack detections

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<ul><li><p>Sensors and Actuators A 162 (2010) 1319</p><p>Contents lists available at ScienceDirect</p><p>Sensors and Actuators A: Physical</p><p>journa l homepage: www.e lsev ier .co</p><p>Evalua lddetecti</p><p>Yu-Jung a,</p><p>a Department o eab Department o lic of K</p><p>a r t i c l</p><p>Article history:Received 4 JanReceived in reAccepted 3 JunAvailable onlin</p><p>Keywords:Soft magneticThin lm deviNondestructivMagnetic eldInductive sens</p><p>nsorss andgneticrackstraiduceivingred o</p><p>were</p><p>1. Introdu</p><p>In magnetic eld sensors, the practical limit of the resolutiondepends on the possibility of achieving the noise oor. In compari-son to various magnetic eld sensors, Prance et al. [13] estimatedthese noisefor inductiooptically pusistive sensmodern senbeen widelrecording htion coils hdesign,widto optimizemagnetic coinduction cof changeoffrom rapidlsor were decoil, their trthe voltage</p><p>Vsignal = d</p><p>d</p><p> CorresponE-mail add</p><p> isof 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 eld intensity around the coil which is proportional tothe operating frequency, f [9]. From Eq. (1), that high amplitude of</p><p>0924-4247/$ doi:10.1016/j.levels as 50100 fTHz1/2 for SQUID, </p></li><li><p>14 Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 1319</p><p>plana</p><p>and magneinductive setolithograpsensitivity fmetallic spe</p><p>2. Experim</p><p>The planposed of thcoil for driwas placedplanar induthe sensorscoil turns (sized plana(1mm1mturns) usingcess. The players-stacksion of 3003m(w) netic lmasby electrolygap betweeand top maproperties oing sampleexhibited 2</p><p>The plalayers-stacksion of 4001.5m(w)with 120 tthat of the</p><p>en bognetproc. Thee shoive sy thec frehe p/s wFig. 1. Schematic of the fabrication processes for the</p><p>tic specimens. Therefore, we have fabricated the planarnsorswith planar coil andmagnetic core using the pho-hyprocess.And the sensorswereevaluatedby the signalor thedetectionof cracks onmagnetic andnonmagneticcimens.</p><p>ental procedure</p><p>ar inductive magnetic eld sensor probe was com-</p><p>betwetop macation(a)(i)sors arinductated bspecithen, t34mme planar inductive sensor and the straight single Cuving ac eld to the specimens, in which driving coilin the edge side of the magnetic core gap of thective sensor. In order to evaluate the sensitivity of, the planar inductive sensors with different pickup42 turns and 120 turns) were prepared. The micro-r inductive sensors were fabricated on AlTiC substratem2mm for 120 turns, 0.5mm1mm2mm for 42sputtering, electroplating and photolithography pro-</p><p>lanar inductive sensor with the total 42 turns (threestructure) of planar pickup coil has a total dimen-m350m21m (cross-sectional area of coil:</p><p>2.7m(t)). The Cu pickup coil and Co24Ni37Fe39 mag-amagnetic core (top and bottom layer)were depositedte process. The magnetic core has a 0.6m-spacedn bottom magnetic layer (120m170m4m)gnetic layer (120m170m3m). The magneticf Co24Ni37Fe39 magnetic lmweremeasured by vibrat-magnetometer, in which magnetization and coercivity1.5 kG and 4.5Oe, respectively.nar inductive sensor with total 120 turns (three</p><p>structure) of planar pickup coil has a dimen-m380m35m (cross-sectional area of coil:</p><p> 3.5m(t)). The area of the planar inductive sensorurns of the coil is about twice in comparison with42 turns. The magnetic core has a 0.8m-spaced gap</p><p>The sensormeasured oby the preanar inductianalyze thesensitivitierical speciof the picku</p><p>In orderthick nonmwere prepaarticial cra</p><p>3. Results</p><p>Fig. 4 sh120 turns (in which c(c and d)-wdepth of 15weremeasuwas appliedmagnitudesment of thecrack positr inductive sensor (a)(i).</p><p>ttom magnetic layer (207m280m6.5m) andic layer (157m250m9m). The typical fabri-ess of the planar inductive sensor was shown in Fig. 1optical images of the fabricated planar inductive sen-wn in Fig. 2. To detect the induced signal using planarensors at crack positions, the driving coil was gener-alternative current with the range of 0.11.4A at the</p><p>quency in the range of 0.52.0MHz, respectively. Andlanar inductive sensor probe was scanned the speed ofith the ying height of 0.20.3mm from specimens.</p><p>position was controlled by x-, y-, z-axis scanner. Theutput signals from sensors were amplied and lteredmplier. And the resistance and inductance the pla-ve sensors were measured by impedance analyzer torelations between the electromagnetic properties and</p><p>s with the change of sensors geometry. These geomet-cations of the planar inductive sensors with the changep coil turns were summarized in Table 1.to detect the metallic cracks, the 20mm-thick, 5mm-agnetic Al and 5mm-thick magnetic FeC specimensredwith the different size of the circular and slit shapedcks, respectively, as shown in Fig. 3.</p><p>and discussions</p><p>ows the measured signals using 42 turns (a and c) andb and d) of the planar inductive sensor on Al specimen,rack of diameters are 800m (a and b) and 500mide circular shaped articial cracks with the differentmm, 10mm, and 5mm, respectively. When the signalsredby theplanar inductive sensor, theacmagneticeldto the specimen by driving coil (0.3A at 1.6MHz). Theof the detected signals were decreasedwith the decre-diameter and depth of cracks. The detected signals on</p><p>ion by 120 turns of planar inductive sensor exhibited</p></li><li><p>Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 1319 15</p><p>Fig. 2. The fab ection120 turns, resp</p><p>the higherthe 42 turnby inductanis changedpermeabilitnetic core dthe 120 turabout 20 ti(2H, 4of inductanand volumelimited by Jwhere, KB if is frequenbe increasethe increme</p><p>ithp rad42 trderenmecimhowb annciesl speiving</p><p>Table 1The geometric</p><p>No. of turns</p><p>Magnetic th(Co24Ni37Fe3</p><p>Measuring c</p><p>Specimensricated inductive sensors; the top view of planar inductive sensors and the cross-sectively.</p><p>intensities over 25 times in comparison with those ofs. These enhanced signal intensities could be governedce of the planar inductive sensors, in which inductanceby number of coil turns, coil loop area and magneticy including demagnetizing effect bymeans of the mag-imension. The measured inductance and resistance ofns (40H, 317) of planar inductive sensor aremes and 6.5 times higher than those of the 42 turns8), respectively, as shown in Table 1. The incrementce was basically caused by the increment of coil turnsof magnetic layer. The sensitivity of an inductive coil is </p><p>tance wthe loothat of</p><p>In ospecimFeC spFig. 5 sturns (frequenetic AThe drohnson noise [9] given by Vnoise = 4KBTRf N/ r,s Boltzmann constant, T is temperature, R is resistance,cy and r is loop radius. Although the Johnson noise willd with the proportion to the increment of resistance,nt ratio of inductance ismuch higher than that of resis-</p><p>imens are to 1.2mm.were not deplanar indufrom 0.5MH</p><p>al specications of the planar inductive sensors with the change of pickup coil turns and</p><p>42 tu</p><p>AlTiC substrate (w l t : mm) 0.5Loop area of pickup coil (mm2) 0.10</p><p>Cross-section of pickup coil (w t : m) 32.</p><p>in lm core9)</p><p>Top (w l t : m) 120Bottom (w l t : m) 120Magnetic head gap (m) 0.6Magnetization (kG) 21.5Coercivity (Oe) 4.5</p><p>Electrical resistance () 48Inductance (H) 2.1</p><p>onditions Driving ac current (A) 0.11Driving freq. (MHz) 0.52Scan speed (mm/s) 34Flying height (mm) 0.20</p><p>Materials Al (noArticial crack shapes Slit tyal view of pickup coil on AlTiC substrates (a), (c) 42 turns and (b), (d)</p><p>the change of coil turns from 42 turns to 120 turns. Andius of 120 turns of pickup coil increased about 50% thanurns.to verify the detected signals with the change of theaterials dependency, the nonmagnetic Al andmagneticens were prepared with the slit shaped articial cracks.s the detected signals using 40 turns (a and c) and 120d d) pickup coils of planar inductive sensors at typicalof 0.5MHz (a and b) and 1.6MHz (c and d) on nonmag-cimenwith the articial slit shaped cracks, respectively.ac current was xed at 0.3A. The crack depths on spec-</p><p>xed with 1mm and widths are changed from 0.5mmIn the case of 0.5MHz driving frequency, the signalstected on crack positions by the 42 turns pickup coil ofctive sensor. When the driving frequency was changedz to 1.6MHz, the signals by 42 turns pickup coil were</p><p>measuring conditions.</p><p>rns 120 turns</p><p>12 1120.15</p><p>7 1.53.51704 15725091703 2072806.5</p><p>0.8</p><p>31840</p><p>.4</p><p>.3</p><p>nmagnetic), FeC (magnetic)pe, circular type</p></li><li><p>16 Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 1319</p><p>Fig. 3. The top view of the optical images of the articial surface cracks: (a) thecircular typed cracks, (b) the slit typed cracks on nonmagnetic Al plate, and (c) theslit typed cracks on magnetic FeC plate, respectively.</p><p>detected on the crack positions shown in Fig. 5(a) and (c).When theplanar pickup coil turns were increased from 42 turns to 120 turnspickupcoils at 0.5MHzdriving frequency, the signalsweredetectedon the crackpositions as shown inFig. 5(a) and (b). These results canbe easily prsignal by th0.5MHzdrinals were inthe change120 turns oincreased a</p><p>portional to the frequency based on Eq. (1). In the case of 1.6MHzdriving 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.</p><p>Fig. 6 shows thedetected signals using 42 turns (a and c) and120turns (b and d) pickup coils of the planar inductive sensor at typicalfrequency 0.5MHz (a and b) and 1.6MHz (c and d) onmagnetic FeCspecimen, respectively. In the case of 0.5MHz 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 0.5MHz to 1.6MHz, 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 0.5MHz, 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 0.5MHz to 1.6MHz 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 1.6MHz drivingfrequency, the intensities of the detected signalswere increased upto about 230 times.</p><p>In order to evaluate the sensing quality of the planar inductivesensor, themeasured signal-to-noise ratios (SNRs) were calculatedfrom the measured signals with the change of the driving current(0.11.4A) and frequency (1.02.0MHz) on slit shaped nonmag-</p><p>l and magnetic FeC specimens as shown in Fig. 7. The SNRs120ues indrivif theremerespe</p><p>Fig. 4. The comb) and 500medicted by Eq. (1) in general. Except the non-detectede 42 turns pickup coil of planar inductive sensor atving frequency, the intensities of the other detected sig-creased with the increment of the width of cracks. Byof driving frequency from 0.5MHz to 1.6MHz in thef pickup coil, the intensities of the detected signal werebout twice. It implies that the induced voltage is pro-</p><p>netic Aby theble valof theturns othe incmens,parison of the detected signals using 42 turns (a and c) and 120 turns (b and d) inducti(c and d) articial circular shaped cracks, respectively.turns of the planar inductive sensor exhibited very sta-comparisonwith those of the 42 turnswith the change</p><p>ng current and frequency. However, the SNRs of the 40planar inductive sensor were linearly increased withnt of the driving current for the nonmagnetic Al speci-ctively, as shown in Fig. 7(a) and (b). As increasing the</p><p>ve planar sensor on Al specimen with the diameter of 800m (a and</p></li><li><p>Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 1319 17</p><p>Fig. 5. The comparison of the detected signals using 42 turns (a and c) and 120 turns (b and d) planar inductive sensor in typical frequency 0.5MHz (a and b) and 1.6MHz (cand d) on Al specimen with the articial slit shaped cracks (xed depth: 1mm, width: 0.51.2mm), respectively.</p><p>frequency from 1.0MHz to 2.0MHz at 0.3A driving current, theSNRs for the 42 turns of the planar inductive sensor were not lin-early changed as shown in Fig. 7(c) and (d). The reason of unstableSNR values for the 42 turns of the planar inductive sensor can bededuced thenough to dcracks on splanar indu</p><p>In ordersensor withinterval scaspecimen u</p><p>articial surface crack size is 0.3mm20mm. And then the mea-sured signalswere converted to the image proles. 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 converted</p><p>exhibeal crit lonsed bide oremethecrac</p><p>Fig. 6. The com(c and d) on mat the inductance of the planar inductive sensor is notetect the small change of the magnetic elds at near</p><p>pecimen in comparison with those of 120 turns of thective sensors.to verify the detected crack shapes, the inductive planar120 turns pickup coil was scanned by one axis 50m-n with the speed of 3mm/s at 0.3mm height from FeCnder driving current of 1A at 1.6MHz. The slit typed</p><p>imagewide rlittle bbe cauedge smeasualso onsurfaceparison of the detected signals using 42 turns (a and c) and 120 turns (b and d) planar indagnetic FeC specimen with the articial slit shaped cracks (xed depth: 1mm, width: 0.5ited the 0.6mm in comparisonwith that of the 0.3mm-ack. The length of the converted crack image also was ager than that of the real crack. These differences couldy the wide distribution of the magnetic elds at thef crack. Unlike the optical method, the magnetic eldnt gives information not only on the actual prole butshape of cracks. As results, the positions and shapes ofk on various metallic specimens were easily detecteductive coil sensor in typical frequency 0.5MHz (a and b) and 1.6MHz1.2mm), respectively.</p></li><li><p>18 Y.-J. Cha et al. / Sensors and Actuators A 162 (2010) 1319</p><p>Fig. 7. The me1.0MHz to 2.0</p><p>Fig. 8. The opwith the 120 t</p><p>with high snetic eldproles.</p><p>4. Conclus</p><p>In ordersurface crachave fabricanumber ofweredetectAll of the oposition. Thasured signal-to-noise ratios (SNR) were summarized with the change of the driving cMHz (c and d) with current of 0.3A on Al and FeC specimens with slit shaped cracks, res</p><p>tical image (a and b) and the converting image (c and d) from the induced voltages by onurns of pickup coil was scanned with the speed of 3mm/s at 0.3mm height from the spe</p><p>ensitivity. Also, the micro-sized planar inductive mag-sensor makes it possible to apply in imaging metallic</p><p>ion</p><p>to detect the positions, various sizes and shapes of theks on nonmagnetic Al andmagnetic FeC specimens, weted the planar inductivemagnetic eld sensorwith the42 turns and 120 turns planar pickup coil. The signalsedwith thechangeof thedriving frequencyandcurrent.utput signals were in good...</p></li></ul>