Two-Dimensional Wireless Power Supply to Ubiquitous Robots Using Microwaves

Paper: Two-Dimensional Wireless Power Supply to Ubiquitous Robots Using Microwaves Hiroyuki Sinoda, Yasutosi Makino, Naosi Yamaira 3,andHirototai 4 Te University of Tokyo 7-3- Hongo, Bunkyo-ku, Tokyo 3-8656, Japan E-mail: sino@alab.t.u-tokyo.ac.jp Keio University 3 JFE Steel Corporation 4 Cellcross Co. Ltd. [Received January 8, 00; accepted January 8, 00] Tis paper proposes a wireless power supply metod to ubiquitous small robots using microwaves propagated in a two-dimensional waveguide DW). A robot working anywere on a DW seet receives te power from te seet. Te communication signal is also transmitted troug te identical seet. Te structure of DW seet is simple and realized wit various materials at low cost. Since te microwave power is confined inside te DW seet, it enables safe power transmission and communication witout strong interference wit te space outside te seet. Keywords: wireless power transmission, ubiquitous robot, ambient mecatronics, surface LAN, twodimensional communication. ntroduction Ubiquitous small robots ave great potential for supporting our everyday activities []. Tey can keep te environment clean and tidy and watc people for timely assistance bot in omes and public spaces. A crucial problem for practical use of suc robots is te power supply to te robots. Since carging te batteries by and is not acceptable in practical scenes, wireless power transmission is desired for expanding te possibility. Current wireless power transmission tecnology is classified into tree types: conventional induction coupling [], electromagnetic wave connection [3], and recently proposed resonant coupling [4]. n te inductive coupling metod, a coupling coil located under a robot supplies te power to te robot. Tus large scale coil arrays over floors and walls are necessary for powering ubiquitous robots. On te oter and, te metods based on electromagnetic wave connection and resonant coupling provide te freedom of te robot location relative to te power source. nstead, strong electromagnetic fields are produced around te robots working wit people, wic poses a considerable problem of electromagnetic compatibility EMC). Te fourt possibility, wic we propose, is a power DW Fig.. Powering small ubiquitous robots using a DW medium. Power supplied to individual robots is assumed to be W in te present system. supply using two-dimensional waveguide DW) sown in Fig.. Robots working on te seet receive power and excange signals troug electromagnetic waves propagating in a tin DW seet. Power to a robot is assumed to be less tan W in te present system. Te structure of te DW seet is simple and implemented at low cost on large areas of walls and floors. Since te DW seet confines microwave power, it enables safe power transmission and communication witout strong interference wit te space outside te seet. Te concept of DW system using microwaves as been reported since 006 [5 7]. Some commercial products for signal transmission and RFD-tag reading ave already been available as LAN Seet and @CELL RFD. n te previous reports, owever, no application to ubiquitous robots was mentioned and te power transmission mecanism was not fully disclosed. We move beyond te previous reports to demonstrate te basic principle of D power transmission and te experimental results.. Non-Contact Connection to DW Seet Figure sows two possible types of DW wic trap microwaves and provide nearby robots wit power troug couplers. n tis paper, we assume using a frequency band of.4.5 GHz, called ndustry-science- Medical SM) band based on te following reason. Assuming tat a ubiquitous robot uses a coupler 0 cm square, it sould use a microwavelengt less tan 0 cm Journal of Robotics and Mecatronics Vol. No.6, 00 777

Sinoda, H. et al. a) Tree-layer DW seet Fig. 3. Coordinates and seet parameters. b) Connection by simple dielectric layer Fig.. Cross-sections of possible DW seets. a) is studied in tis work. for igly efficient power transmission using convergent waves. For relative dielectric constant ε r =.5 ofte medium, te 0 cm wavelengt corresponds to a frequency of.4 GHz, wic means tat te frequency used sould be.4 GHz. From te aspects of te transmission loss and robustness of te nearfield connection, we sould minimize te frequency, wic leads to te conclusion tat we sould use.4 GHz. Assume SM band to be used, te DW seet in Fig. b) cannot be used because it does not confine electromagnetic waves sufficiently. Most electromagnetic power runs along te surface outside te seet tinner tan te wavelengt, wit leakage typically reacing 0 cm in its eigt from te seet surface, wic may adversely affect te surroundings, including uman beings. n contrast, te DW in Fig. a) as minimal leakage outside te seet, maintaining ig-efficiency connection using an especially designed resonance coupler... Electromagnetic Wave Around DW Seet Witin te coordinates in Fig. 3,tezcomponent of te electric field above te seet surface is written as follows: = k V exp k z)exp jkx)exp jωt) z > 0) ) k k =µε µ 0 ε 0 )ω jzεω k = jzεω k = µεω jzεω using j =, for te wave traveling toward +x, were µ is te seet s magnetic permeability, ε its dielectric constant, µ 0 te atmosperic magnetic permeability, ε 0 its dielectric constant, and parameter V voltage between top layer S and bottom layer B at x = t = 0. Te Eq. ) is an approximation assuming k and k, were is dielectric layer tickness. Parameter Z is te seet impedance of te top layer defined as follows: Z R + jx E x [Ω].......... ) i x E x is electric field [V/m] along te x-axis at top layer S, i x is current density [A/m] along te x-axis at te same layer, bottom layer B an ideal conductor, and top layer S sufficiently tin. f top layer S is an ideal continuous conductor, Z equals 0, yielding E x = 0. A top layer wose seet impedance is inductive as X > 0 is called an inductive layer, realized by a conductive mes wit a period sorter tan te wavelengt. Wen S is a mes wose period is sorter tan te wavelengt, E x and i x are teir averages witin tis period. We can easily confirm tat te solution of Eq. ) satisfies Maxwell s equations [8]. Based on Eq. ), we calculate leakage ratio r defined as te ratio of electromagnetic energy flow J running in te +x direction outside of te seet to energy flow J inside te seet: r J = πε 0 J ε γ 3 + γ λ λ0.... 3) λ 0 and λ are equal to te electromagnetic wavelengt in air and tat in a medium wit ε and µ. Dimensionless parameter γ is defined as follows: ε γ X εµ ε 0 µ 0 )ω.......... 4) i.e., normalized seet inductance determining electromagnetic leakage and connection. f ε/ε 0 =.4, µ = µ 0, = mm, and γ =.0, r is 0.4% at.4 GHz. Electromagnetic waves are sent to or absorbed from te DW seet wen a conductive plate is placed near S, as explained later, wile electromagnetic energy outside te seet remains small witout conductive or dielectric objects near te seet... Nearfield Connection Assume conductive plate P is placed near te seet were an electromagnetic wave runs in te +x direction as sown in Fig. 4. Let DW seet tickness be, dielectric constant ε,andµ magnetic permeability inside seet 0< z < ). A dielectric layer wit tickness H, dielectric constant ε, and magnetic permeability µ exists under P. Solving Maxwell s equations in x > 0 yields two modes running in te +x direction as follows. n two extreme cases as ε i) jωx H + ε ) A ε ii) jωx H + ε ) A 778 Journal of Robotics and Mecatronics Vol. No.6, 00

DW Fig. 4. Simple coupler consisting of a conductive plate and a dielectric layer. Electromagnetic energy between S and B is absorbed in te space between S and P. mode expressions are simplified, were A µ ε µ ε )ω. n tis paper, we utilize te modes in case i) wic are written as follows. Mode a): k a = Mode b): k b ωx C ε µ ω k a ε µ ω k a exp jk ax)... 5) ε ε /H + ε / H µ ε + ε ) ) µ ε ω C ε H + ε H ε µ ω k b H ε µ ω k b ) + µ ε ω ) exp jk bx). 6) Ez and are z components of te electric field inside and outside te DW seet. B y and B y are y components of magnetic flux density inside and outside te DW seet, and C is a constant. n derivation, we assume tat electromagnetic field variation along z is small, equivalent to ε ωx H + ε ) ε H and ωx H + ε ). Since te wavenumber of Mode a) k a and tat of Mode b) k b differ, tey produce a beat along x wit a period of L = π k a K b............. 7) An electromagnetic wave runs in te +x direction weaving between regions 0 < z < ) and < z < + H) wit period L. f we assume ) ) E z 0 Ez =........... 8) p at x = 0, we obtain te solution in x > 0as Fig. 5. Simulation results. Tick line: absolute electric field in region. Tin line: absolute electric field in region. E z E z ) [ = p + ε /ε +H/ +ε H/ε ) ε /ε +H/ +ε /ε H) ) exp jk a x) ) ] exp jk b x). 9) Wen + ε H/ε ) = + ε /ε H) tat is, H =............... 0) ε ε tere exist points at wic te electric field becomes 0 inside te DW seet region ). n case difference k a k b is muc smaller tan k a and k b, electromagnetic energy density vanises at te zero points, meaning electromagnetic power is completely absorbed in region out of te DW seet. Lengt L is called te power excange lengt, so a conductive plate larger tan L absorbs all incident energy from te seet in a D problem setting, as simulated in Fig. 5. Te bold line sows te absolute electric field in region and te tin line tat in region. Parameters are set to ε = ε =.5ε 0 and H = = mm. Te inductive layer is a mes at 7 mm intervals wit mm line widt. Te power excange lengt is 70 mm. Te igfrequency fluctuations in te 4 mm period come from numerical simulation imperfectness. Waves in regions and are partially reflected at te rigt end of te simulation model. For power transmission not powering unwanted objects, L sould be maximized wile maintaining te coupler connection to te DW seet. To make connection between special couplers and te DW seet efficient, couplers use resonance as detailed below. Journal of Robotics and Mecatronics Vol. No.6, 00 779

Sinoda, H. et al. a) a) b) Fig. 6. Example of nearfield coupler design. a) Axisymmetric coupler details and b) principle illustrated..3. Nearfield Coupler Design An example of design concept on DW-based nearfield coupler is explained in Fig. 6. Connection assumes electromagnetic flow from te coupler cable to te seet. Receiving efficiency is given by te reciprocity teorem. Putting te coupler on te DW seet causes microwave from te coaxial cable to follow te winding axisymmetric pat sown by te broken-line arrow. Reflections occur at,, and 3 in te coupler due to discontinuous impedance cange. Te reflected signal back to te coaxial cable is te mixture of te reflected signals at eac point. Coosing coupler radius R to cancel te reflected signals, we reduce te reflected power. deally, te input signal is fed to te seet witout reflection back to te cable. n oter words, we transfer power from te cable to te DW seet using resonance. Even if coupler radius R is smaller tan power excange lengt L, input power is transferred to te DW seet repeating reflections in te coupler. Practically, omic and dielectric loss in te coupler and radiation loss from te gap between te coupler and te seet are considerable. Tus reducing tem is a key in te future coupler design. 3. Simulations and Results To confirm te above points, we modeled te DW seet using MW-Studio software AET Japan nc.) using conductive bottom layer B 35 µm tick, an insulator wit relative permittivity ε r = 4.9 and.6 mm tick, inductive rectangular mes layer S of conductive material 35 µm tick and a mes lattice period of 5 mm wit a lattice line 0.6 mm wide. Te seet was 95 mm square and we assumed no reflection at te seet edge. Figure 7 sows simulation results for electromagnetic fields induced around te DW seet as electric field intensity in a) and b) sowing tat energy density decreases 30 db by 3 mm from layer S. Te decreasing b) Fig. 7. Simulation results. a) Electric field around DW seet. b) Plots of electromagnetic energy density along vertical dased line above. Solid line: teoretical slope for attenuation coefficient. cm. curve is not purely exponential because orizontal mes spacing d is comparable to te vertical observation range of 3 mm. Mes pattern periodicity details te electromagnetic field as follows: f x,z) =Aexp jkx) B n z)exp j πn ) ) n= d x AC 0 exp jkx)exp k z) +A C n exp j πn ) n 0 d x exp πn ) d z, were d π/k. ) Term B n exp j πn ) n= d x is te general expression of a function wit period d. As Eq. ) sows, te electric field around te seet is a sum of multiple exponential curves along z wit different attenuation lengts, tus te line in Fig. 7b) is inflected several times wen te major term in Eq. ) switces. n Eq. ) we omitted te term n 0 by averaging te electromagnetic field in te mes period. Te term proportional to AC 0 in Eq. ) corresponds to Eq. ). Te teoretical attenuation constant along z axis for n = 0 is equal to k,or.cm. Line of attenuation constant k =. cm is sown by te solid line in Fig. 7b). Note tat te attenuation coefficient outside te grap z > 3 mm) is k. We also simulated te penomenon in a nearfield cou- 780 Journal of Robotics and Mecatronics Vol. No.6, 00

a) a) b) b) Fig. 9. DW seet used in experiment left) and te nearfield coupler rigt). Te coupler is 60 mm in diameter for.4 GHz signals. Parameters R and W in Fig. 6a) are set at 0 mm and 0 mm. Fig. 8. Simulation results. a) Electric field intensity crosssection at te nearfield coupler and b) top view. pler on te DW seet, applying.4 GHz microwave troug te SMA connector sown in Fig. 8a). Coupler dimensions are optimized so tat it operates at.4 GHz. Fig. 8a) sows an electric field in te coupler wit W = 5 mm, R = 7 mm, and te oter dimensions sown in Fig. 6a). Fig. 8b) sows an overead view of te electric field in te DW seet. Applied energy is propagated concentrically in te seet wit te rectangular mes layer. 4. Experiments We fabricated a 80 mm square DW seet wit oter parameters identical to tose in te simulation and te nearfield coupler, prototype of Fig. 6a) as sown in Fig. 9. Parameters R and W in Fig. 6a) are 0 mm and 0 mm. Te insulator between te seet and coupler was paper 0. mm tick. Te purpose of experiments is to confirm tat sufficient power is transmitted from te seet to te coupler on te seet. 4.. S-Parameter Observation As sown in Fig. 9, te DW seet as a subminiature version A SMA) connector on te side edge connected to a 50 Ω cable. Altoug some reflection occurs at te connection, we neglect tis effect because our objective is to rougly estimate te connection between te coupler and seet. One network analyzer port was connected to te DW seet s connector and te oter to te nearfield coupler and performance evaluated by measuring transmission coefficient S from to 5 GHz. Fig. 0. Results for S measurement by a network analyzer. S is dimensionless, e.g., 40.0m = S = 0.4. Transmission coefficient S was measured between te nearfield coupler and te SMA connector connected to te DW seet. Te definition of S is detailed in Section 4. Figure 0 sows an example of measured S = S. Dimensionless parameter S mu at eac frequency is te output signal amplitude received troug te 50 Ω cable connected to te DW seet for te unit amplitude U input signal applied to te nearfield coupler troug te oter 50 Ω cable. Te communication bandwidt is 300 MHz at.4 GHz, indicating tat coupler resonance quality factor Q = 8=.4/300 MHz). Peak S = 30 mu = 0.30 at.4 GHz sowing tat 9% of input power was received by te oter port at best. We next looked at te relationsip between te vertical distance above te seet and S for.4 GHz, as sown in Fig. a). Te proximity connection rapidly decreases for d > 0. mm. We also looked at te relationsip be- Journal of Robotics and Mecatronics Vol. No.6, 00 78

Sinoda, H. et al. a) Demonstration of ig-speed video transmission a) b) LED c) Motor d) Circuit used in experiments b) and c) b) Fig.. Measured S versus coupler position. a) S versus vertical coupler displacement and b) S versus orizontal coupler displacement. tween te orizontal seet location and S, measuring S at.4 GHz moving te nearfield coupler orizontally straigt on te seet. As sown in Fig. b), S canged wit te period of 4 cm reflecting te standing wave induced in te DW seet. Data in Fig. 0 is te result obtained at te peak of te standing wave. 4.. Demonstration Experiments Communication by EEE 80.b based on a.4 GHz signal was demonstrated in an experiment in wic a personal computer transmitted a video signal to anoter PC using commercially available wireless LAN cards wose antennas were replaced by nearfield couplers te only difference from conventional wireless peer-to-peer communication, as sown in Fig. a). Good signal transmission was possible troug te DW seet and two nearfield couplers. Since te DW seet size was finite, standing waves induced in te seet degrade communication quality, but te adverse effect was negligible in Mbps connection. We also confirmed experimentally tat a LED and a small motor operated on te power caugt by nearfield couplers as sown in Figs. b) and c). We supplied W microwaves at.4 GHz to te DW seet, and te electromagnetic wave te coupler caugt was rectified by a full-wave rectifying circuit as sown in Fig. d). More efficient rectifying circuits are studied in [9], for example. Te LED brigtness and te motor rotation depended on Fig.. Views of demonstrations. a) Movie transmission using EEE 80.b. Te PC at left plays a video transmitting te video signal to te next computer. b) and c) Power transmission to a LED and a motor. d) Full-wave rectifying circuit for power reception. te location in te DW seet, wic was due to te standingwaveinteseet. Fora50Ω resister as te load in Fig. d), instead of te LED or te motor, te maximum consumed power was measured to be 80 mw for W microwave input. 5. Conclusions We ave proposed a D power supply based on a proximity nearfield) connection, analyzing and simulating signal transmission troug a mes DW seet to clarify an evanescent wave trapped at te seet propagating over te seet. We fabricated prototypes of a nearfield coupler and a 80 mm 80 mm DW seet based on numerical simulations and evaluated teir performance. We confirmed experimentally tat te coupler s communication bandwidt is 300 MHz at.4 GHz and 9% of power supplied to te DW seet was absorbed at best by te nearfield coupler troug te seet. n te experiment we input W into te DW seet, a load connected to a single coupler could consume 80 mw DC power. Troug tis study, we tus sowed tat D power transmission and communication are feasible. ncreasing transmission efficiency and decreasing electromagnetic radiation into te atmospere are te next tecnical callenges we face. 78 Journal of Robotics and Mecatronics Vol. No.6, 00

Acknowledgements We tank Cellcross Co. Ltd. Dr. Naoya Asamura, Dr. Mitsuiro Hakozaki, and Tetsuro Kiyomatsu for teir advice and cooperation in device fabrication. References: [] H. siara and T. Fukuda, Micro Autonomous Robotic System, J. of Robotics and Mecatronics, Vol., No.5, pp. 443-447, 999. [] T. Sekitani, M. Takamiya, Y. Noguci, S. Nakano, Y. Kato, T. Sakurai, and T. Someya, A large-area wireless power-transmission seet using printed organic transistors and plastic MEMS switces, Nature Materials, Vol.6, pp. 43-47, 007. [3] N. Sinoara, H. Matsumoto, and K. Hasimoto, Solar Power Station/Satellite SPS) wit Pase Controlled Magnetrons, ECE Trans. Electron, Vol.E86-C, No.8, pp. 550-555, 003. [4] A. Karalis, J. D. Joannopoulos, and M. Soljačić, Efficient wireless non-radiative mid-range energy transfer, Annals of Pysics, Vol.33, No., pp. 34-48, 008. [5] N. Yamaira, Y. Makino, H. tai, and H. Sinoda, Proximity Connection in Two-Dimensional Signal Transmission, Proc. SCE- CASE nt. Joint Conf. 006, Oct., Busan, Korea, pp. 735-740, 006. [6] A. Noda and H. Sinoda, Power Transmission Coupler for Low Leakage D-Communication Seet, Proc. NSS009, Pittsburg, June 7-9, pp. 4-30, 009. [7] Y. Monnai and H. Sinoda, Converting D Microwave into 3D Beam Using Dielectric Grating Antenna, Proc. NSS009, Pittsburg, June 7-9, pp. 83-87, 009. [8] J. D. Kraus and D. A. Fleisc, Electromagnetics wit Applications, 5t Edition, McGraw-Hill, pp. 480-487, 999. [9] T. Miura,, N. Sinoara, and H. Matsumoto, Experimental Study of Rectenna Connection for Microwave Power Transmission, Electronics and Communications in Japan, Part, Vol.84, No., pp. 7-36, 00. Yasutosi Makino Keio University 4-- Hiyosi, Kooku-ku, Yokoama-si, Kanagawa 3-856, Japan Brief Biograpical History: 007- Received P.D. degree of nformation Science and Tecnology from Te University of Tokyo 008- Researcer in Te University of Tokyo 009- Assistant Professor in Keio University Y. Makino and H. Sinoda, Myoelectric Pattern Measurement Based on Two-Dimensional Communication Tecnology, Proc. SCE Annual Conf. 007, pp. 45-49, 007. Membersip in Academic Societies: Te Society of nstrument and Control Engineers SCE) Naosi Yamaira Researc Engineer, Steel Researc Laboratory Hiroyuki Sinoda Associate Professor, Department of nformation Pysics and Computing, Graduate Scool of nformation Science and Tecnology, Te University of Tokyo 7-3- Hongo, Bunkyo-ku, Tokyo 3-8656, Japan Brief Biograpical History: 995- Lecturer, Tokyo University of Agriculture and Tecnology 997- Associate Professor, Tokyo University of Agriculture and Tecnology 999- Visiting Researcer, UC Berkeley 000-Associate Professor, Te University of Tokyo nformation pysics, tactile/aptic interfaces, sensor systems and devices, sensor networks, two-dimensional communication, uman interfaces, and optical/acoustic measurement. Membersip in Academic Societies: Te Society of nstrument and Control Engineers SCE) Te Japan Society of Mecanical Engineers JSME) Te nstitute of Electrical Engineers of Japan EEJ) Te Robotics Society of Japan RSJ) Te Virtual Reality Society of Japan VRSJ) Te nstitute of Electrical and Electronics Engineers EEE) - Minamiwatarida-co, Kawasaki-ku, Kawasaki-si, Kanagawa 0-0855, Japan Brief Biograpical History: 008- Joined JFE Steel Corp. Proximity Connection in Two-Dimensional Signal Transmission, Proc. SCE-CASE nt. Joint Conf. 006, pp. 005-009, Oct. 006. Hiroto tai Senior Researcer, Researc Department, Cellcross Co.,Ltd. Entrepreneur Plaza 04, 7-3-, Hongo, Bunkyo-ku, Tokyo 3-0033, Japan Brief Biograpical History: 004- Joined Cellcross Co.,Ltd. nteractive Window Based on D Communication Tecnology: ntegrating Ubiquitous Devices on Transparent Medium, Pervasive 009, pp. 93-96, May 009 Membersip in Academic Societies: Te Society of nstrument and Control Engineers SCE) Journal of Robotics and Mecatronics Vol. No.6, 00 783