Design and Optimization of Inductive Power Transmission for Implantable Sensor System

Transcription

1 010 XIh Inernaional Workshop on Symbolic and Numerical Mehods, Modeling and Applicaions o Circui Design (SMACD) Design and Opimizaion of Inducive Power Transmission for Implanable Sensor Sysem Enver G Kilinc, Caherine Dehollain FIC-Group Ecole Polyechnique Fédérale de Lausanne Lausanne, Swizerland enverkilinc@epflch, caherinedehollain@epflch Franco Maloberi Inegraed Microsysem Laboraory Universià degli Sudi di Pavia Pavia, Ialy francomaloberi@unipvi Absrac This paper presens a mehodology o design and o opimize inducive power link for biomedical applicaions The imporance of he operaion frequency on he applicaion is expressed A model of inducive link is presened The dimensions of he coils are compaible wih he size of a mouse and he mouse cage The simulaion resuls are in good agreemen wih he analysis I INTODUCTION The advances in microelecronic echnologies allow fabricaing very small circuis ha can be implanable as a sensor sysem for a body These sensor sysems help monioring paiens online The response of he body o he medicine is recorded, so ha, he paien can be reaed personally due o is meabolism Anoher imporan issue is invesigaing he side-effecs of he used ani-inflammaory drugs o rea paiens individually The monioring sysem measures he oxiciy of he drugs, he drug level, ATP (Adenosine Triphosphae), glucose level, emperaure, and ph level The developed biochip sysem will be implaned o a laboraory mouse, which can be used for biomedical and geneic researches and also for invesigaing new reamens [1] Fig 1 shows he miniaure concep of a ypical implanable biochip sysem One soluion is o use he cables for daa ransmission and powering he sensor sysem However, hese cables are limis he mobiliy of he subjec animal and may also cause infecions An alernaive soluion is o use baeries on he ag side By his way, a fully implanable sysem can be obained However, many surgeries may be necessary o replace he baeries a he end of heir life ime Addiionally, he baeries increase he oal weigh of he sysem and also creae oxic effec for he subjec To solve his, wireless power ransmission is he mos appropriae soluion in order o ge an infinie life ime wih respec o remoe powering There are differen ways o ransmi wireless power such as ulrasound, magneic and elecromagneic [1-3] Acousic waves propagae in media such as waer; however, hey are srongly aenuaed in air Henceforh, for his applicaion, he magneic and elecromagneic mehods are compared in his sudy The boundary beween magneic coupling and elecromagneic power harvesing is defined by d=λ/π, where, d and λ are he disance and he wavelengh of he signal, respecively For a chosen wavelengh, if he disance beween he coils is smaller han d, he magneic coupling gives more efficien wireless power ransmission [] In he nex secion, he selecion of inducive link frequency is discussed In Secion III, he design of inducive power link is summarized including analysis for increasing he efficiency Secion IV compares he hand calculaions and simulaion resuls Conclusions are given in Secion V II FEQUENCY SELECTION The frequency is one of he mos imporan parameers for efficien power ransmission in he sysem For an inducive link in he air, he induced volage a he ransponder side is given by [4]: V = μ A N ω H (1) IND 0 eff Figure 1 Miniaure concep of a ypical implanable biochip sysem where, μ 0, A, N, ω, and H eff are he permeabiliy consan of air, he loop area of he ag coil, he number of urns of he ag coils, he angular frequency of he magneic coupling, and he /10/$ IEEE

2 010 XIh Inernaional Workshop on Symbolic and Numerical Mehods, Modeling and Applicaions o Circui Design (SMACD) effecive field srengh of he magneic field The higher he frequency, he higher induced volage is In addiion, higher operaion frequencies allow higher daa raes due o higher bandwidh However, in he case of biomedical applicaions, he absorpion of he body issues increases wih frequency [5] Moreover, he parasiic effecs of he inducance such as skin effec, eddy currens, ec increase wih frequency [6] By considering all he aforemenioned issues, he frequency is chosen as 1356 MHz which is one of he inducive applicaion band deermined by regulaions [7] Moreover, i allows maximum magneic field srengh, higher daa bandwidh, and less inerference compared o oher opions, due o he sandards dedicaed o his band [4] III INDUCTIVE POWE LINK AND EFFICIENCY ANALYSIS In magneic coupling, he reader and ag coils mus be designed carefully because here are many parameers which affec he power efficiency in an inducive link Furhermore, hese parameers are applicaion dependen which means ha he inducive link mus be designed and opimized for each applicaion The disance beween he coils is one of he mos imporan parameers o have efficien power ransmission, since he power ransmission efficiency is srongly proporional o inverse of he disance (1/d 6 ) in magneic coupling [] Hence, he disance beween he reader coil and he ag coil should be minimized Addiionally, he coil geomery has a significan effec on power efficiency [8] For a defined boundary box, he square spiral coil is preferable o ge maximized power ransfer area The addiional dead-zones for he circular spiral coil are displayed as red in Fig Figure Square and circular shaped coils The efficiency of he power ransmission is mosly relaed o inducive link efficiency Fig 3 shows he simplified model of he inducive power link In his figure, L i, i, and C pi represen he self inducance, series resisance, and parasiic capaciance, respecively In addiion, he muual inducance and load resisance are represened as M and, respecively C1 and C are he capaciances o une he link a resonance frequency A Modeling of Inducive Link The magneic field is formed by curren flowing hrough he reader coil and his magneic field induces curren flowing hrough ag coil The value of he induced curren is relaed o he inducance values of L 1 and L The inducance value of he square spiral coil in Fig can be calculaed as [9]: Figure 3 Simplified model of he inducive power link 17 μ nd 0 avg 07 L = ln( ) + 018ρ + 013ρ ρ where, n is he number of urns of he spiral coil d i and d o are he inner and he ouer diameer of he coil d avg is he average diameer ((d i +d o )/) ρ is he fill raio and defined as ρ=(d o -d i )/(d i +d o ) Qualiy facor of he inducor is also anoher imporan parameer ha affecs he power efficiency, and is relaed o he parasiic resisance and capaciance of he inducor The parasiic resisance also increases wih frequency due o he skin and proximiy effecs By considering skin effec, he resisance can be calculaed as [10]: =, ac dc / δ ( 1 e ) δ δ = ρc, π μ f dc () l = ρ (3) c w where,, w, and ρ c are hickness, widh, and resisiviy of he conducor, respecively l, f, and δ, are he oal lengh of he conducor line, operaion frequency, and he skin deph, respecively μ is he permeabiliy consan and defined as μ=μ 0 μ r where μ r is relaive permeabiliy of he conducor Finally, he parasiic capaciance is required o model he coil and o define he self-resonance frequency in order o choose opimum inducance The capaciance is defined by [11]: C p = ( αεrc + βε rs ) ε0 l (4) s where,, s, l, and ε 0 are he hickness of he conducor, he spacing beween wo conducors, he oal lengh of he conducor line, and he dielecric consan of he air, respecively The oal capaciance is he sum of he coaing capaciance and subsrae capaciance; hence, ε rc and ε rs are he relaive dielecric consans of coaing and subsrae maerials Moreover, (α,β) is assumed as (09,01) in he case of air and prined circui board subsrae (F4) [11] The muual inducance is he key parameer in he link efficiency Assuming a perfec alignmen, he oal muual inducance beween he reader and he ag coils is expressed by [1]:

3 010 XIh Inernaional Workshop on Symbolic and Numerical Mehods, Modeling and Applicaions o Circui Design (SMACD) N1 N M θ M( x, y, d ) = (5) 1 i j 1 i= 1 j= 1 where, θ is a consan and changes wih he shape of he coil I is found empirically as 13 for square-shaped coils [11] Addiionally, N 1, N, x i, y j and d 1 are he number of urns in reader coil, he number of urns in ag coil, he radius of i h urn in reader coil, he radius of j h urn in ag coil and disance beween coils, respecively M(x, y, d 1 ) is defined as [1]: ) Laeral Misalignmen: In laeral misalignmen case, he ag coil is moved by a disance Δ and he coils are siuaed in parallel planes M( x, y, d ) = μ x y γ K( γ) E( γ) 1 0 γ γ where, γ is (6) γ = 4xy ( x + y) + d 1 (7) B Link Power Efficiency The efficiency is maximized when LC-anks are uned a he working frequency (ω) In an inducive power link, L 1 and L coils are uned wih C o1 and C o, respecively Assuming ω=ω 01 =ω 0, he efficiency of he ransmied power from source o load is dominaed by receiver side efficiency (η 1 ) and ag side efficiency (η ) and can be expressed by [13]: η = η η = 1 1 X X (1 + + )(1 + ) where, X is M ω / 1 or k Q 1 Q k is he coupling coefficien and, Q 1 and Q are he qualiy facors of unloaded reader coil and unloaded ag coil, respecively Qualiy facor (Q) is defined as Q= ωl/ ac for low frequencies The opimal can be found by [13]: (8) = 1 + X (9) By aking as opimum, he maximum efficiency can be calculaed as: kqq 1 η = 1 ( kqq) 1 (10) The maximum power efficiency can be obained by maximizing no only he coupling facor, bu also he qualiy facors of he coils Hence he coils mus be designed carefully o ge higher coupling facor as well as higher qualiy facors C Misalignmen Analysis The previous equaions are done by aking ino accoun he perfec alignmen case However, here are wo imporan misalignmen cases: 1) Angular Misalignmen: In angular misalignmen case, he ag coil is urned by an angle α and he ceners of he coils are aligned Figure 4 a) Perfec alignmen case, b) Angular misalignmen case, c) Laeral misalignmen case, d) Boh angular and laeral misalignmen case Fig 4 represens perfec alignmen, angular misalignmen, laeral misalignmen, and boh angular and laeral misalignmen cases of he reader and ag coils The link power efficiency will be reduced by boh angular and laeral misalignmen cases Hence, hese cases should be considered during he coil design In he angular misalignmen case, he magneic field is changing wih respec o cos α [14] When he ag coil is parallel o he reader coil (α=0), he magneic field is maximized However, he magneic field is minimized when he ag coil is siuaed perpendicular o he reader coil (α=90) In he laeral misalignmen case, he magneic field is relaed o displacemen Δ [14] When he displacemen Δ is increased, he link efficiency decreases IV CASE STUDY: EMOTE POWEING OF A MOUSE IMPLANT FO BIOCHIP SYSTEMS To find he opimum inducive power link, he coils are designed in erms of geomerical parameers In order o do his, a MATLAB code is developed o find he opimum inducive link by using he expressions presened in Secion III Furhermore, he resuls are verified and uned by using HFSS program Some parameers are limied due o he applicaion such as he disance (d 1 ) beween he coils, he ouer diameer of ag coil (d o ) Oher parameers have a degree of freedom They should be chosen carefully o ge opimum inducive power link Table I presens he fixed parameers for his applicaion The opimal value of he ouer diameer of he reader coil (d o1 ) which maximizes he magneic srengh is defined by [4]: do1 = d1 (11)

4 010 XIh Inernaional Workshop on Symbolic and Numerical Mehods, Modeling and Applicaions o Circui Design (SMACD) maximum link efficiency in erms of coil geomery parameers η 1 (%) Figure 5 3D model of he inducive power link on HFSS program d o1 (cm) Figure 6 Efficiency (η 1) versus ouer diameer of he reader coil (d o1) TABLE I DESIGN PAAMETES LIMITED BY APPLICATION Parameer Link operaion frequency (f) Disance beween coils (d 1) Tag coil ouer diameer (d o) Minimum spacing beween conducors a (s) Minimum widh of conducor a (w) a The conducor is copper on F4 prined circui board Value 1356 MHz 30 mm 0 mm 150 μm 150 μm By placing he coils under he cage o minimize he disance (d 1 =3 cm), he opimal ouer diameer (d o1 ) can be derived as 848cm Wih respec o power efficiency, he opimal d o1 is calculaed as 95 cm as shown in Fig 6 In he projec, he sensor sysem is implaned inside a laboraory mouse which can move in he cage and he ag coil posiion will be changed compared o he reader coil In conclusion, he laeral and also angular misalignmen problems can be occurred The angular misalignmen may no be significan issue under assumpion of he mouse is moving parallel o he basemen of he cage mos of he ime However, he laeral misalignmen will be more hazardous for link efficiency Increasing d o1 oo much may no be an opimum soluion, because he laeral misalignmen is also increasing which decreases he link efficiency Hence, d o1 is chosen as 8cm due o he cage geomery which is depiced in Fig 7 and also minimizing he misalignmen problem However, o cover all he basemen of he cage, here should be an array of coils On he oher hand, powering all of he coils a he same ime creaes inerferences o each oher and also increases he oal power consumpion To solve his, a magne is placed on he ag side and a sensor is added each of he reader coils [15] When he mouse comes on he coil, he magne is deeced and he coil is powered There are sill several oher parameers which affec he power efficiency For a chosen conducor widh, he selfinducance value increases wih increasing he number of urns bu he self-resonance frequency decreases Moreover, he qualiy facor increases proporional o conducor widh However, he parasiic capaciances increase, and herefore, he self-resonance frequency decreases Hence, he opimum reader and ag coil design mus be invesigaed o ge Figure 7 The array of coils under basemen of he cage Due o he previous equaions, he link power efficiency is relaed o no only coupling coefficien (k), bu also qualiy facors of he coils (Q 1, Q ) Hence, he inducive link geomery is opimized in MATLAB using he aforemenioned equaions considering k, Q 1, Q ogeher by changing he coil parameers such as number of urns (n 1, n ), conducor widh (w 1, w ) and, spacing beween conducors (s 1, s ) The resuls of he opimizaion are summarized in Table II Addiionally, he opimized coils are simulaed in HFSS The comparison beween he analysis and simulaion are shown in Table III V CONCLUSION In his paper, he differences beween differen power harvesing mehods are discussed The magneic coupling is more efficien wih respec o elecromagneic power harvesing, in erms of ransmied power due o he shor disance beween he coils Moreover, he imporance of he operaion frequency is emphasized Addiionally, an inducive link is presened and analyzed in erms of power efficiency Besides, he laeral and angular misalignmen condiions are focused A mehod of geomeric opimizaion for inducive links is described Finally, a case sudy for remoe powering of a mouse implan for biochip sysems using inducive power link is presened For he fuure work, he array of reader coils and ag coil will be fabricaed including he magne and he sensors and will be characerized comparing wih analysis and simulaion In addiion, a conrol loop mechanism will be invesigaed o minimize laeral misalignmen issue

5 010 XIh Inernaional Workshop on Symbolic and Numerical Mehods, Modeling and Applicaions o Circui Design (SMACD) TABLE II OPTIMAL INDUCTIVE LINK COIL DESIGNS Parameer eader Coil Tag Coil Ouer Diameer (d o) 8 cm cm Inner Diameer (d i) 10 mm 11 mm Number of urns (n) 5 6 Widh of conducor (w) 1 mm 50 μm Spacing beween conducors (s) 75 mm 600 μm TABLE III COMPAISON BETWEEN ANALYSIS AND HFSS SIMULATION ESULTS Parameer Analysis Simulaion eader Coil Inducance (L 1) 1075 μh 100 μh Tag Coil Inducance (L ) 09 μh 075 μh eader Coil esisance ( 1) 0597 Ω 056 Ω Tag Coil esisance ( ) 0715 Ω 074 Ω eader Coil Qualiy Facor (Q 1) Tag Coil Qualiy Facor (Q ) Muual Inducance (M 1) 3604 nh 371 nh Power Efficiency (η1) 655 % 667 % ACKNOWLEDGMENT The auhors are graeful o Prof J Mosig from LEMA Laboraory, EPFL and his group for heir suppor of he HFSS simulaion ool This projec is suppored by Swiss Naional Foundaion (SNF) hrough Sinergia Iniiaive EFEENCES [1] P Cong e al, Novel long-erm implanable blood pressure monioring sysem wih reduced baseline drif, Proc IEEE EMBC '06, pp , 006 [] D C Yae e al, Opimal ransmission frequency for ulralow-power shor-range radio links, IEEE Trans Circuis Sys I, vol 51, no 7, pp , July 004 [3] F Mazzilli e al, In-viro plaform o sudy ulrasound as source for wireless energy ransfer and communicaion for implaned medical devices, o be presened in Proc IEEE EMBC '10 [4] K Finkenzeller, "FID Handbook," John Wiley and Sons Ld, 1999 [5] P Vaillancour e al, EM radiaion behaviour upon biological issues in a radio-frequency power ransfer link for a corical visual implan, Proc IEEE EMBC '97, pp , 1997 [6] S Mei and Y I Ismail, "Modeling skin and proximiy effec wih he reduced realizable L circuis", IEEE Trans VLSI Sys, vol 1, no 4, pp , 004 [7] EC-EC 70-03, Inducive Applicaions, in Annex 9, Oc 009 [8] K M Silay e al, Improvemen of power efficiency of inducive links for implanable devices, Proc PIME 08 Conf, 008, pp 9 3 [9] S S Mohan e al, Simple accurae expressions for planar spiral inducances, IEEE J Solid-Sae Circ, vol 34, pp , Oc 1999 [10] O Aasoy and C Dehollain, A sudy for remoe powering of a knee proshesis hrough inducive link, o be presened in Proc PIME 10 Conf, 010 [11] U- M Jow, and M Ghovanloo, Modeling and opimizaion of prined spiral coils in air, saline, and muscle issue environmens, IEEE Trans Biomed Circuis Sys, vol 3, pp , 009 [1] C M Zierhofer and E S Hochmair, Geomeric approach for coupling enhancemen of magneically coupled coils, IEEE Trans Biomed Eng, vol 43, pp , July 1996 [13] B Lenaers and Puers, Omnidirecional Inducive Powering for Biomedical Implans, Springer, 009 [14] K Foopoulou and B W Flynn, Wireless powering of implaned sensors using F inducive coupling, IEEE Sensors 006 EXCO, Korea, Oc 006 [15] H G Lim e al, A mehod for reducing body exposure o elecromagneic field of pillow ype wireless charger in fully implanable middle ear hearing device, IEICE Elecronics Express, vol 6, no 18 pp , 009