7/27/2019 The Relay Effect on Wireless Power Transfer Using Witricity

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Abstract Witricity is a recent technology for wireless powertransfer over a limited distance. In this paper, we investigate a relayeffect to extend the distance. The concept of magnetic resonance instrongly coupled regime with more than two resonators is presented.Theoretical analysis is performed based on a set of differentialequations and experiments are conducted to demonstrate itseffectiveness. Our results show that the efficiency of power transfercan be improved significantly using one or more relay resonators.This approach enhances the performance of the present two-resonator witricity systems and allows transmitting power over alonger range, following a curved path, and using smaller resonators.

I. I NTRODUCTIONWireless electricity (in short, witricity) is a recently

developed technology for wireless power transfer [1]. It has beendemonstrated that it is feasible to transmit a significant amount of

power at a high efficiency that has never been achieved previously. Despite this significant development, its effectivetransmission distance is limited by the size of the resonator. Inmany applications [2], it is highly desirable to extend the distanceof transmission and allow the route of transmission to follow acurved path in space. In this work, we take advantage of the relayeffect in resonant physical systems and present an effectivesolution using more than two resonators. Both theoretical andexperimental results show that this approach overcomes thedrawbacks of two-resonator system, allowing much longer andmore flexible power transmission without sacrificing efficiency.

II. THEORY OF R ELAYED W ITRICITY

Fig.1 Scheme of relayed witricity systemOur relayed witricity power transfer system (Fig. 1) consists

of at least three resonators (source, relay(s) and device) with thesame resonant frequency, a driving loop, and an output loop. Thesource resonator is coupled inductively with the driving looplinked to an RF power source. Similarly, the device resonator iscoupled inductively with the output loop to supply power to anexternal load. This system is modeled by the followingdifferential equations based on the coupled mode theory:

)()()()()( t aik t aik t ai

dt

t da RSR DSDS S S

S ++G-= w (1)

)()()()()( t aik t aik t ai

dt

t da D RDS RS R R R

R ++G-= w (2)

)()()()()( t aik t aik t aidt

t da R DRS DS D D D

D ++G-= w (3)

Solving these equations under appropriate initial conductions,we found that, with the presence of the relay resonator, the deviceand source can interact more strongly, exchange energy morequickly and deliver power more efficiently when compared withthe system without the relay resonator (Fig.2a). We also foundthat the energy of the relay resonator is kept at a lower levelcompared to those of the source and device. However, when weremove the relay resonator (Fig. 2b), energy cannot be transferredas efficiently, quickly, and timely as in the previous case.

0.0 5.0x10 -6 1.0x10 -5 1.5x10 -5 2.0x10 -5 2.5x10 -50.0

0.3

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E n e r g y

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DeviceSystem Source

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0.25

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Device

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(a) (b)

Figure 2 Energy exchanges of source, relay and device for relayed (a) &conventional (b) witricity systems

III. EXPERIMENTS We have previously reported a thin film design of witricity

resonator with a number of attractive features for practicalapplications [2]. The same resonator with a radius of 8.1 cm wasused in our experiments for the source, relay and deviceresonators. Fig. 3a shows a functional 7 MHz conventional two-resonator system without relay. Due to large resonator separation,the LED could not be lit up. When we placed a relay resonator inthe system, the LED was lit up fully (Fig. 3b). Fig. 3c shows thatthe distance between source and device can be increased further if we put one additional relay resonator between them. Their efficiencies (Figs. 3a and 3b) are shown in Fig. 3d. Theconventional witricity could only achieve approximately 10%efficiency over a 30 cm distance, compared to the relayed systemof 46%. In addition, we can change the transmission directionusing relays. This is due to the attraction of the magnetic flux bythe resonant relay. As a result, wireless energy transfer can turn acorner like wire connection (e.g., a Z shaped route in Fig. 3e).

IV. CONCLUSION

We have shown that one or more relay resonators can be addedto the witricity system to extend power transfer range, increaseefficiency, and allow a curved transmission path in space. Thissystem has great potential in civic and military applications to

build up a wireless energy transfer network.

V. R EFERENCES [1] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic,

Wireless power transfer via strongly coupled magnetic resonances, Science, Vol. 317, pp. 83-86, Jul. 2007.

[2] F. Zhang, X. Liu, S.A. Hackworth, R.J. Sclabassi, and M. Sun, In Vitro and In VivoStudies on Wireless Powering of Medical Sensors and Implantable Devices, IEEE NIH

Lissa09 , Bethesa, MD, USA, Apr. 2009

The Relay Effect on Wireless Power Transfer Using WitricityFei Zhang 1, Steven A. Hackworth 1, Weinong Fu 2, and Mingui Sun 1

1Departments of Neurosurgery and Electrical Engineering, University of Pittsburgh, Pittsburgh, PA, USA2Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong

drsun@pitt.edu

30 40 50 60 70

0.0

0.1

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E f f i c i e

n c y

Separation distance (cm)

Original WitricityRelayed Witricity

(d)

(e) (a) (b) (c)

Figure 3. Experimental results of relayed and conventional witricity systems.

978-1-4244-7062-4/10/$26.00 2010 IEEE