Recent trends in electromagnetic non-destructive sensing

322 Ad ances in Elec ical and Elec onic Enginee ing RECENT TRENDS IN ELECTROMAGNETIC NON-DESTRUCTIVE SENSING K. ápo á, I. áp, L. Janoušek, M. Sme ana Uni e si y o Žilina, Facul y o Elec ical Enginee ing, Veký diel, 010 26 Žilina, Slo akia Tel.: +421-41-5132100, Fax: +421-41-513 1519, e-mail: kla a.capo a@ el.uniza.sk Summa y The pape deals wi h ma e ial elec omagne ic non-des uc i e es ing (eNDT) wi h emphasize on eddy cu en es ing (ECT). Va ious modi ica ions o ECT sensing a e compa ed and discussed om he desi ed de ec ed signal cha ac e is ics poin o iew. Excep o he op imiza ion o usual p obe coils a angemen s o he conc e e applica ions, he new magne ic senso s as gian magne o- esis ance (GMR) and spin dependen unneling (SDT) a e p esen ed. The ad anced ECT senso s a e cha ac e ized by hei sensi i i y, equency ange and senso dimensions. Keywo ds: elec omagne ic nondes uc i e es ing, eddy cu en sensing, su ace and subsu ace de ec s, de ec ion p obes, magne ic senso s, gian magne o- esis ance (GMR), spin dependen unneling (SDT). 1. INTRODUCTION Non-des uc i e elec omagne ic es ing (eNDT) is an e ec i e me hodology o diagnos ics in many echnical and scien i ic applica ions. The equi emen s o new e ec i e eNDT ools a e connec ed wi h he wide and s ill inc easing demands o high quali y and eliabili y s anda ds in indus ial p oduc ion and also wi h de elopmen s o o he echnical and scien i ic a eas, e.g. medicine, geology, ci il and en i onmen al enginee ing, e c. Va ious NDT echniques o m a wide g oup o a he di e en ools, which a e based on di e en physical phenomena and hey a e cha ac e ized by di e en and speci ic pe o mance and applica ion ields. F om he applica ion poin o iew he eNDT is used in many a eas - om he inspec ion o me allic pipes o he ae onau ical main enance and om he localiza ion o liquids in subsoil o he ho acic imaging o clinical diagnos ics. One o he mos popula eNDT me hods is he eddy cu en es ing (ECT) and e alua ion. The p inciple o ECT can be b ie ly desc ibed by he ollowing way. Eddy cu en coil ed by al e na ing sinusoidal cu en (AC), o equencies in he ange 50 Hz – 10 MHz, gene a es p ima y magne ic ield acco ding o he Ampe e´s law. This p ima y magne ic ield induces eddy cu en s in he es ed conduc i e ma e ial objec acco ding o he Fa aday´s law. Then eddy cu en s gene a e seconda y magne ic ield in he opposi e di ec ion in ag eemen wi h he Lenz´s law. Following om hese p ocesses he coil impedance changes in he case o ma e ial changes, e.g. in he p esence o impe ec ions - de ec s in he ma e ial objec . The impedance change is measu ed, analyzed and co ela ed wi h he de ec dimensions. The locus o impedance change o med du ing he mo emen o an eddy cu en p obe coil o e a es ma e ial ha ing de ec is called an eddy cu en signal. I s ampli ude p o ides in o ma ion abou he de ec size and i s phase angle wi h espec o li -o gi es in o ma ion abou de ec loca ion o dep h, Fig.1. Eddy cu en densi y in ma e ial is no uni o m in Fig. 1. Eddy cu en es ing a angemen . he ma e ial dep h di ec ion. I is g ea es on he su ace and dec eases mono onously wi h dep h (skin e ec ) acco ding o he ela ion o s anda d dep h o pene a ion which dec eases wi h inc easing equency, conduc i i y and pe meabili y. I means ha o measu ing hickness o hin su aces e y high equencies a e o be used and on he con a y o de ec ion o sub-su ace bu ied de ec s and o es ing highly conduc i e (magne ic) hick ma e ials low equencies a e o be employed. Usually he d i ing cu en is kep cons an ( ew hund eds o mA) and he impedance changes occu ed due o pe u ba ion o eddy cu en s a de ec egions a e o be measu ed. Since hese changes a e e y small (), high p ecision AC b idge is used, Fig. 2. The b idge imbalance is co ela ed wi h he de ec o ma e ial cha ac e is ic esponsible. The ECT ins umen consis s usually o an oscilla o ( o exci ing equency), cons an AC supply, AC (Maxwell) b idge ci cui , ampli ie and sc een ( o display he changes in a 2D g aph o as a ec o ). In mode n sys ems he e a pe sonal compu e wi h he necessa y ha dwa e (plug-in ca d) and so wa e is used o he measu emen s, AC SUPPLY R L PRIMARY FIELD INDUCED FIELD EDDY CURRENTS Li o Dep h o pene a ion Recen ends in elec omagne ic non-des uc i e es ing 323 Fig. 2. AC b idge o ECT signal measu emen . adjus men , da a s o age, analysis and managemen . The ECT is he mos ly used echnique o de ec ing a igue c acks and co osion in conduc i e ma e ials. The cos o using echnology is low and i is possible o moni o subsu ace de ec s and de ec s unde insula ing coa ings wi hou ouching he su ace specimen. One o he mos impo an pa s o ECT de ice is he sensing pa called p obe which is c ea ed ob iously by p obe coils o o he senso s. Because sa e y-c i ical sys ems depend on ea ly de ec ion o a igue c acks o a oid majo ailu es, he e is an inc easing need o eddy cu en p obes ha can eliably de ec e y small de ec s. Also he e a e inc easing demands o p obes ha can de ec deeply bu ied de ec s o a oid disassembling s uc u es. The e a e also many o he applica ions whe e ECT is success ully used, e.g. ma e ial hickness measu emen s, coa ing hickness measu emen s and conduc i i y measu emen s o he ma e ial iden i ica ion, hea damage de ec ion, case dep h de e mina ion o hea ea men moni o ing. Acco ding o he applica ion a eas he measu ing se -up is designed a ealized. 2. EDDY CURRENT PROBES CONFIGURATIONS As i was men ioned abo e he app op ia e selec ion o p obe coil is e y impo an in ECT in o de o ge he igh (desi ed) in o ma ion om i . The mos common p obes used in ECT a e su ace o pancake p obes (wi h he axis no mal o he su ace) which a e chosen o inspec ion o pla es and bol -holes ei he as a single elemen o an a ay, in bo h absolu e and di e en ial modes. The enci cling p obes a e used o inspec ion o ods, ba s and ubes wi h ou side access and he Bobbin p obes o p e- and in- se ice inspec ion o hea exchange , s eam gene a o , condense and o he s wi h inside access, Fig. 3, [1]. These h ee ypes can also ope a e in he send- ecei e mode wi h he sepa a e coils o sending and ecei ing o signal and also in absolu e o di e en ial mode. The absolu e EC p obe consis s o a single sensing coil o signal exci a ion and ecep ion. I is de e mined o de ec ion o c acks as well as g adual Fig. 3. Con igu a ions o ECT P obes. a ia ions. Bu absolu e p obes a e also sensi i e o li -o , p obe il , empe a u e changes, e c. Di e en ial p obes ha e wo sensing coils wound in opposi e di ec ion and in es iga ing wo di e en egions o he ma e ial. These p obes a e good o high sensi i i y de ec ion o small de ec s and hey a e mo e immune o changes in empe a u e and p obe wobble. The e a e many ac o s which in luence eddy cu en esponse om a p obe. Success ul assessmen o laws elies on holding he o he s cons an , o somehow elimina ing hei e ec on he esul s. The main ac o s a e ma e ial conduc i i y, pe meabili y, equency, geome y and he li -o . As o he ma e ial conduc i i y is seen he g ea e he conduc i i y he g ea e he low o eddy cu en s on he su ace. F om he conduc i i y measu emen s we can ge he in o ma ion abou he ma e ial composi ion, hea ea men and wo k ha dening, e c. Fo he non- e ous me als he pe meabili y is he same as o he “ ee space”, he ela i e pe meabili y is equal o one, and o he e ous me als i has alues se e al hund eds o mo e. Pe meabili y is a ying s ongly wi hin he me al pa due o localized s esses, hea ing e ec s, e c. F equency g ea ly a ec ed he eddy cu en esponse bu i can be con olled wi hou p oblems. Geome ical ea u es such as cu a u e, edges, g oo es, e c. a ec he eddy cu en esponse. The used echniques mus ecognize his, e.g. in es ing an edge o c acks he p obe will no mally be mo ed along pa allel o he edge so ha small changes may be easily seen. Whe e he ma e ial hickness is less han he e ec i e dep h o pene a ion his will also a ec he eddy cu en esponse. As o he p oximi y o li -o he close a p obe coil is o he su ace he g ea e will be he e ec on ha coil. I means ha he li -o signal a ises as he p obe is mo ed on and o he su ace and he sensi i i y will be educed as he coil p oduc spacing inc eases. Fig. 4. Posi ions o ECT p obes abo e es ed objec . U R 1 R 2 R 3 L 3 Tes ed body Inspec ion coil Z 4 = R 4 + j ω L 4 U ou pu ∼ 324 Ad ances in Elec ical and Elec onic Enginee ing Fig. 4 shows a ious posi ions o he p obe abo e he es ed objec acco ding o changes o li o , il and geome y o edge e ec s. 3. ADVANCES OF EDDY CURRENT PROBES Eddy cu en es ing p obes usually combine an exci a ion coil ha induces eddy cu en s in a specimen and a de ec ion elemen ha iden i ies he pe u ba ion o he cu en s by c acks o o he de ec s. In o de o de ec deepe de ec s in ma e ial objec i is necessa y o p opose he ad anced coils con igu a ion and ollowing da a p ocessing, as i was ealized and published e.g. in he pape s[2], [3]. The op imiza ion and c ea ion o a new p obe modi ica ion is connec ed wi h he used ECT echnique. The new coils p obe de elopmen used in he emo e ield eddy cu en es ing (RFECT) de o ed o he inspec ion o pipes ha e been desc ibed e.g. in he pape [4]. Bu excep o coils and hei a ious a angemen s he ECT de ec ion elemen s can be also supe conduc ing quan um in e e ence (SQUID) de ec o s, o solid-s a e magne ic senso s, such as Hall e ec , luxga e o magne o- esis ance (AMR o GMR) and spin- dependen - unneling (SDT) senso s. The use o low- ield solid-s a e magne ic senso s ep esen s a signi ican ad ance o e mo e adi ional induc i e p obes in use oday, [5]. Two key a ibu es will open oppo uni ies o inc eased use o eddy cu en p obes: senso cons an sensi i i y o e a wide ange o equencies and de elopmen o smalle senso s. P obes ha de ec eddy cu en ields using induc i e coils ha e less sensi i i y a low equencies. Un o una ely, his is whe e he de ice would ha e o ope a e o de ec deep laws. Small sensing coils which a e equi ed o de ec small de ec s, also ha e low sensi i i y. In con as , small, high-sensi i i y hin ilm senso s can locally measu e a magne ic ield o e an a ea compa able o he size o he senso i sel / ens o mic ome e s/. Limi a ion o con en ional eddy cu en p obes is he di icul y o de ec ing small c acks o igina ing a he edges o a specimen. This de ec is he mos common ype encoun e ed in p ac ice. An example is he c acks ha appea a ound he as ene o i e holes in ai c a mul ilaye s uc u es. Mos induc i e coil p obes a e sensi i e o bo h he edge and he c acks ini ia ing om o nea he edge. The edge c ea es a la ge signal ha obscu es he small signal om he c ack. GMR and SDT magne ic senso s can be o ien ed o elimina e he edge signal. Wi h his o ien a ion he p esence o he edge enhances he signal om he c ack. To achie e high esolu ion o de ec ing small su ace and nea -su ace de ec s i is necessa y o educe he dimensions o he exci a ion coil. The minimum leng h o a de ec able c ack is oughly equal o he mean adius o he coil. The e ha e been de eloped and es ed p obes inco po a ing small, la , pancake coils o plana exci a ion coils deposi ed on he senso subs a e. Recen de elopmen o hin ilm magne ic echnology has esul ed in ilms exhibi ing a la ge change in esis ance wi h magne ic ield, [5]. This phenomenon is called gian magne o- esis ance o dis inguish i om con en ional aniso opic magne o- esis ance (AMR). Whe eas AMR esis o s exhibi a change o esis ance o up o 3%, a ious GMR ma e ials achie e abou a 10% - 20% change in esis ance. GMR ilms ha e wo o mo e so magne ic laye s o i on, nickel and cobal alloys sepa a ed by a nonmagne ic conduc i e laye such as coppe . Because o spin-dependen sca e ing o conduc ion elec ons, he esis ance has maximum alue when he magne ic momen s o he laye s a e an i-pa allel and minimum when hey a e pa allel, [5 ]. SDT s uc u es a e a ecen addi ion o he ma e ials exhibi ing a la ge change in esis ance. In hese s uc u es an insula ing laye sepa a es wo magne ic laye s. Quan um unneling h ough he insula o allows conduc ion. The angle be ween he magne iza ion ec o s in he wo magne ic laye s modula es he magni ude o he unneling cu en be ween he wo laye s. The e we e obse ed changes o esis ance o 10% o 40% in SDT s uc u es. The ield equi ed o maximum change in esis ance depends on he composi ion o he magne ic laye s and he me hod o achie ing an i- pa allel alignmen . Values o he sa u a ion ield ange om 100 o 10000 A/m. A he low end, his o e s he possibili y o ex emely sensi i e magne ic senso s. Wi hin he ame o senso s p og ess he e ha e been adap ed SDT ma e ials o c ea e highly sensi i e magne ic ield senso s o use in low- ield applica ions ha p esen ly equi e luxga e magne ome e s. These senso s a e e y small, equi e li le powe , and a e easily combined wi h o he elec onics. The insula ing unnelling laye p o ides high- esis ance senso s sui able o ba e y ope a ion. The e can be ab ica ed ex emely small SDT de ices (se e al ens o mic ome e on a side) wi h high esis ance using pho oli hog aphy, allowing dense packing o magne ic senso s in small a eas. The main componen s o an eddy cu en p obe o non-des uc i e es ing a e pancake- ype coil and an AC b idge o GMR o SDT senso s. A angemen o coil and GMR senso o eddy cu en de ec ion o de ec s in conduc o s is shown in he Fig. 5, [5]. When measu ing he sensing axis, i mus be kep he GMR p obe coplana wi h he su ace o specimen. The exci a ion ield on he coil axis, being pe pendicula o he sensing axis o he GMR, has no e ec on he senso . In his way, he de ec ed ield, which is he esul o he pe u ba ion o he eddy cu en low pa hs caused by he c ack, is sepa a ed om he exci a ion ield. Eddy cu en induced in he su ace o a de ec - ee specimen a e ci cula because o he ci cula symme y o he ield p oduced by he coil. Recen ends in elec omagne ic non-des uc i e es ing 325 Fig. 5. Se up o ECT wi h GMR senso . The angen ial componen o he ield c ea ed by he eddy cu en s is ze o a he loca ion o he senso . In p esence o de ec s, he eddy cu en s a e no longe symme ical and he p obe p o ides a measu e o he pe u bed eddy cu en s caused by unde lying laws. The size o he coil is ela ed o he esolu ion necessa y o de ec he de ec s. Fo la ge de ec s and o deep de ec s, la ge coils su ounding he senso s a e equi ed. Small coils loca ed close o he specimen a e necessa y o esol e small de ec s. Eddy cu en s shield he in e io o he conduc ing ma e ial wi h he skin dep h ela ed o he conduc i i y and he equency. By changing he equency i is possible o p obe di e ing dep hs o he ma e ial. GMR and SDT senso s wi h hei wide equency esponse, om DC o he MHz ange, a e well sui ed o his applica ion. The small size o he sensing elemen inc eases he esolu ion o de ec loca ion while he de ec o is as e -scanned o e he su ace. Mo e apid scans can be p e o med using an a ay o de ec o s, [6]. Wi hin he ecen de elopmen he e we e buil he op imized EC p obe p o o ypes o de ec and map di e en ypes o de ec s encoun e ed in p ac ice. They a e e alua ed p obe pe o mance on calib a ed slo s o di e en leng hs, wid hs, and heigh s machined in o he op su ace, bo om su ace, o edges o specimen. The esul s a e combined wi h he esul s ob ained on a specimen ha con ained eal c acks a i icially g own a ound a hole. Finally, hey demons a ed magne ic p o ile imaging by scanning a gi en objec using a high- esolu ion p obe, [6]. EC p obes we e es ed on su ace c acks longe han he exci a ion coil diame e . Quali y o he maps p oduced when scanning his ype o de ec depend on he ela i e o ien a ion o he sensi i e axis o he SDT o GMR sensing elemen s wi h espec o he c ack o ien a ion. Sho su ace c acks can also be eliably de ec ed using small exci a ion coils. The unidi ec ional sensi i i y o GMR and SDT senso s enables he de ec ion o c acks a and pe pendicula o he edge o a specimen. This disc imina ion is possible because he sensi i e axis o he senso can be o ien ed pa allel o he edge. Consequen ly, he ou pu signal o he senso is caused only by he c ack. SDT senso s a e pa icula ly a ac i e o nondes uc i e e alua ion, low- equency applica ions, such as he de ec ion o deeply bu ied laws. In con as , induc i e p obes ha e poo sensi i i y a low equencies because hey a e sensi i e o he ime de i a i e o he magne ic ield a he han o he magni ude o he magne ic ield c ea ed by he law. To de ec deep c acks, i is necessa y o use la ge diame e exci a ion coils o inc ease he pene a ion o he eddy cu en s in he ma e ial unde es . 4. CONCLUSION The main cha ac e is ics o ECT p obes we e desc ibed and compa ed in he pape . Acco ding o he a ious equi emen s on he de ec ed signals and o he used ECT echniques he applica ions o usual and selec ed ad anced p obe de ices we e discussed and mu ually compa ed. As o he las ECT sensing ends he emphasis was pu mainly on he solid s a e GMR and SDT senso s and hei p ope ies such as he cons an sensi i i y in a wide equency ange, hei small size and he possibili y o low magne ic ields measu emen . The examples o he wide u iliza ion o hese ypes o ECT senso s we e p esen ed mainly o he p ac ical cases whe e he ECT coil p obes applica ions ha e some limi a ions. Acknowledgemen This wo k was suppo ed by g an o he Slo ak G an Agency VEGA, P ojec No. 1/0308/08. REFERENCES [1] Bli z, J.: Elec ical and Magne ic Me hods o Nondes uc i e Tes ing, Edi ed By Chapman and Hall, 1997, ISBN 0-412-79150-1. [2] Janoušek, L., Gombá ska, D., ápo á, K.: A new app oach o enhancing eddy- cu en non- des uc i e e alua ion. Elec oScope, on-line jou nal, h p://elec oscope.zcu.cz, Vol. 2007, No. 4, ISSN 1802-4564, 6 p. [3] Janoušek, L., ápo á, K., Yusa, N., Miya, K.: Ad anced p obe wi h a ay o pick-up coils o imp o ed c ack e alua ion in eddy-cu en non- des uc i e es ing. 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