... Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294, USA. E-mail: ... Abstractâ In this paper, a wireless power transfer system for.
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8th International Conference on Electrical and Computer Engineering 20-22 December, 2014, Dhaka, Bangladesh
A Miniaturized Spiral Antenna for Energy Harvesting of Implantable Medical Devices Qingyun Ma, Laxmi Ray, and Mohammad Rafiqul Haider Department of Electrical and Computer Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294, USA E-mail: {maq, laxmiray, mrhaider}@uab.edu
Abstract— In this paper, a wireless power transfer system for implantable medical devices using a miniaturized 3 mm diameter single arm circular spiral antenna is presented. The transmission characteristics, S21 between the transmitting and receiving antennas with air and tissue as a medium between antennas are analyzed and measured. The receiver antenna receives almost 2 mW average power with power transfer efficiency of 53%. The power transfer efficiency could still reach to 4% with 274 µW average received power, with placement of 1.5 cm tissue and 1 cm air in between two antennas. With one PN diode rectifier, the total DC power from the receiver side is 489 nW. Index terms— Wireless power transfer, implantable, spiral antenna, biomedical tissue, power transfer efficiency.
I. INTRODUCTION The implantable medical devices market is expected to exceed $50 billion in 2015 in the US alone due to its immense applications in cardiovascular, neurological, orthopaedic and ophthalmological disorders. For continuous operation, the implanted units could be powered up either from an external source or implantable batteries. The tethered cables for power and data transmission have potential risks of skin infections and irritation. The implantable batteries pose potential battery fluid leakage and biohazard. Wireless power transfer to the implanted medical units has become increasingly important because of the problems seen with less feasible battery power sources. The wireless power transfer shows the great promise by eliminating the risk from the skin irritation and biohazard from battery fluid leakage. Two most common forms of wireless power transmission are the inductive power link with magnetically coupled resonating coils and the antenna based power transmission through electromagnetic (EM) radiation. Inductive power link consists of two loosely coupled resonating coils separated by a certain distance. The coils in the inductive link must be in perfect alignment, and the distance should be less than one half of the coil diameter. The position of the receiving and transmitting coil could vary due to the body movement of the patients. Furthermore, to satisfy FCC standards the link frequency needs to be in between 5 to 10 MHz which results the inductor diameter over several centimetres [1]. The antenna based power transmission through electromagnetic field has high tolerance on the mismatch of the positions of the transmitting and the receiving antennas. The optimal EM frequency for biomedical tissue of 1.5 cm
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thickness with high link efficiency could be below 3 GHz [2]. Because of the higher frequency, the antenna diameter can be decreased into millimetre level. To accommodate the antenna for medical applications the antenna must be low profile and small size in order to make the system compact. Spiral antenna has widespread usage because of its lowprofile planar structure, radiation of circular polarization and flexibility of changing resonant and radiation characteristics for desired application by changing circular radius (r), number of turns (N), spacing between turns (s) and width of the spiral arm (w). The spiral antenna was investigated previously [3] and feeding of the antenna can be done either from the center or side end of the spiral antenna. The concept of side-fed spiral antenna [4, 5] and center-fed spiral antenna [6, 8] are well known. Several different possible configurations of spiral antenna have been presented lately like flexible double arm side fed circular spiral antenna with microstrip infinite balun for wearable applications [7]. In addition, a single arm circular spiral antenna has advantage on adaptability of changing polarization and beam characteristics using changes in the center and side feed [8]. In this paper we propose a 3 mm diameter compact single arm center-fed circular spiral antenna operating at 524.8 MHz. The communication between transmitting and receiving antenna is studied to analyse the power transfer efficiency. The general characteristics of antenna like return loss, transmission coefficient, and its transient response analysis, power transfer efficiency with air and tissue as a medium between the spiral antennas are shown. II.
GEOMETRY OF SPIRAL ANTENNA AND SYSTEM OF WIRELESS POWER TRANSFER
A center-fed 3 mm diameter circular spiral antenna operating at 524.8 MHz was designed and simulated. The substrate chosen for realizing the antenna has dielectric constant εr =4.6, loss tangent tanδ=0.01 and thickness of 1mm with 0.01 mm thick aluminium metal layer. The Agilent Advanced Design System (ADS) software was used to simulate and analyze the performance and functionality of the proposed spiral antenna. The configuration of the antenna is shown in Fig. 1. The radius(r), the spacing (s), the number of turns(N) and the width of the spiral arm(w) are important design parameters of this single arm spiral antenna. For tuning the desired resonant frequency a radius of 1.5 mm with 20 turns and the arm width of 0.015 mm and 0.05 mm spacing between each turn are
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Parameters Radius Number of turns Spacing between turns Width of spiral arm
Values r = 1.5 mm N=20 s = 0.05 mm w = 0.015 mm
(a)
(b) Fig. 2. Return Loss plot of transmitting spiral antenna. Fig . 1. (a) Geometry of center-fed Transmitting (port1) and Receiving antennas (port2) (b) 3D simulation setup of wireless power transfer.
maintained. The optimized design parameters of the proposed center-fed spiral antenna are summarized in Table I. III. SIMULATION RESULTS The simulated return loss (S11) plot of the transmitting spiral antenna is shown in Fig. 2. The proposed antenna satisfies 10 dB return loss characteristics from 520 MHz to 529 MHz for 524.8 MHz application. The transmitting antenna when excited at the center gives low return loss of about 30.931 dB at the operating frequency. The simulated gain and directivity of the transmitting spiral antenna at 524.8 MHz is shown in Fig.3. The average gain of the transmitting spiral antenna is about -39dBi and the directivity is 2 dBi at 524.8 GHz operating frequency. The simulated far-field radiation pattern for circular polarization of the transmitting spiral antenna at 524.8 MHz for θ = 0° and θ = 90° are depicted in Figs. 4 (a) and (b), respectively. For the study of wireless power transfer between transmitting and receiving antenna two identical spiral antennas are used. Fig. 5 shows transmission characteristics S21 between the transmitting and receiving antennas. TABLE I: OPTIMIZED PARAMETER OF THE SPIRAL ANTENNA
Fig. 3. Gain and directivity of the spiral antenna at 524.8 MHz.
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(a) Fig. 5.Transmission characteristics S21 between spiral antennas
Due to the coupling of the mutual impedance of two antennas, the frequency is shifted to 473.8 MHz with maximum transmission characteristics of -5.659 dB. This strong coupling shows the influence of nearby placed receiving spiral antenna on radiation characteristics of the antenna.
(b) Fig. 4. Circular polarization of the spiral antenna at (a) θ = 0° (b) θ = 90°.
IV. TRANSIENT SIMULATION RESULTS The wireless power transfer system structure is converted to a symbol and the performance of this system is analysed in the ADS circuit simulator by using the circuit schematic as shown in Fig. 6. In the transient simulation, a sinusoid source is directly connected to the transmitting antenna through a matching network. The result from this transient simulation is shown in Fig. 7. The received voltage signal is about 1.8 Vp-p with 2 Vp-p input signal. The transmitting antenna and the receiving antenna are on the two different substrates with 1.5 cm separation distance. With this configuration, the receiver side could receive almost 2 mW average power with power transfer efficiency of 53%. The power transfer efficiency reduces to 4% with 274 µW average received power, when we put 1.5 cm tissue and 1 cm air in between two antennas. The tissue layer acts as a lossy medium which absorbs most of the transmitted power and reduces power transfer efficiency. With one PN diode rectifier, the total DC power from the receiver side is 489 nW.
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V. CONCLUSION This paper presents a wireless power transfer link for medical applications. The proposed single arm center-fed spiral antenna covers 524.8 MHz frequency band. By properly selecting the design parameters of the spiral antenna tunable resonant and radiation characteristics for desired band can be achieved. The transmission characteristic between the transmitting and receiving spiral antennas gives maximum coupling. The transient simulation results shows that the power transfer efficiency is directly proportional to the distance between the center points of two antennas rather than the alignment condition of the two spiral antennas. This is well explained by simulation results with medium as air and tissue between transmitting and receiving antennas. The reduced power transfer efficiency when tissue is used as a medium shows that tissue acts as a lossy medium. From the above discussed result we confirmed that the antenna based wireless power transmission shows the greater promise on tolerance of the position mismatch and size than the inductive power link. Fig. 6. Circuit simulation setup of the wireless power transfer link system.
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(a)
(b) Fig. 7. (a)The input voltage signal (vin) from the transmitting antenna (b) The received voltage signal (vout) from the receiving antenna.
