simulation wireless electricity打印

J.of Electromagn.Waves and Appl.,Vol.23,925–934,2009 SIMULATION AND EXPERIMENTAL COMPARISON OF DIFFERENT COUPLING MECHANISMS FOR THE WIRELESS ELECTRICITY TRANSFERX.C.Wei and E.P.LiComputational Electromagnetics and Electronic DivisionInstitute of High Performance Computing1Fusionopolis Way,#16-16Connexis,Singapore138632,Singapore Y.L.Guan and Y.H.ChongNanyang Technological University50Nanyang Drive,Research Techno PlazaLevel4Border X Block,Singapore637553Abstract—Wireless electricity transfer is an emerging technology for the future power supply system.In this paper,we simulate two different coupling mechanisms used in the wireless electricity transfer:the inductive coupling and resonant near-field coupling. Equivalent circuits are extracted to simulate their transfer efficiencies. To achieve the maximum transfer efficiency,the optimized frequencies are derived for two mechanisms.There is good agreement between simulation results and measurement results.By comparing their transfer efficiencies,it shows that the resonant near-field coupling is usually more efficient than the inductive coupling for the middle-distance electricity transfer.1.INTRODUCTIONThe wireless electricity transfer is an emerging technology for the future power supply system.It has distinct advantages over traditional electricity transfer.It provides a convenient way to run electrical devices,where the power-connecting wires are troublesome or even impossible,such as implanted electrical devices.It also helps to reduce Corresponding author:X.C.Wei(weixc@.sg).926Wei et al. the product cost,since it throws offthe connecting wires and gives a maintenance-free solution for power supply.The wireless electricity transfer is an interesting research topic with a long history.Probably the most famous effort is the Solar Power Satellite introduced by NASA at1968[1].In their proposed system, the solar energy is collected and converted to microwave power on the space satellite,and then this microwave radiation is transmitted to Earth where it would be captured by ground antenna and transformed to usable electricity.Some wireless electricity transfer technologies are also proposed for low-power applications.In[2],a contactless power delivery system is proposed for moving loads in mining applications. In[3],a contactless battery charger is designed for cellular phone.And in[4],the wireless technology is used to transfer power and information at the same time.All of above technologies are based on the inductive coupling.Because the inductive coupling decays quickly with the distance,the air-gap between the transmitter and receiver of published literatures is much small.Recently,Marin Soljacic of MIT and his team have demonstrated how a60Watt light bulb could be wirelessly powered up from a distance of2meters[5].In their experiment,the resonant coupling instead of inductive coupling is used.The major difference between inductive coupling and resonant coupling is that:in the inductive coupling,only the static magneticfield contributes to the energy coupling,while in the resonant coupling,both the electric and magnetic fields contribute to the energy coupling.At the same time,the working frequency of the resonant coupling is higher than the working frequency of the inductive coupling.However,for the resonant coupling,there are still some technical challenges,such as the low efficiency and safety issue.In this paper,we focus on the middle-distance(within few meters)applications.The efficiency of inductive coupling and resonant coupling is simulated and compared.An equivalent network model is proposed for inductive coupling and resonant coupling.To achieve the maximum transfer efficiency,we derive their optimized frequencies. This can serve as the guideline for the design of wireless electricity transfer system.Finally,simulations results are verified with the measurement results.Different coupling mechanisms for wireless electricity transfer927Figure 1.(a)Before and (b)after insertion of the transceiver network,and (c)the equivalent source.PARISON OF INDUCTIVE AND RESONANT COUPLING2.1.Equivalent Network of the Power Transceiver and Its Efficiency DefinitionThere are two different mechanisms which can be used for wireless electricity transfer:the inductive coupling and resonant coupling.They dominate at low and high frequency bands.For both mechanisms,the transceiver can be equivalent to a two-port network.Figs.1(a)and 1(b)show the source and load before and after the insertion of the transceiver network respectively.Z g and Z L are the source internal impedance and load impedance respectively.We define the power transfer efficiency of the transceiver asη=P La /P Lb (1)where P Lb is the received powers by the load before the insertion of the transceiver(i.e.,the traditional power supply system),while P La is the received powers by the load after the insertion of the transceiver.The cascade matrix of the transceiver network is defined as V 1I 1 = A B C D V 2I 2(2)with V 1,V 2,I 1,and I 2defined in Fig.1(b).By using this cascade matrix,the transfer efficiency can be written asη=|Z g +Z L |2/|AZ L +B +CZ g Z L +DZ g |2(3)The source together with the transceiver is equal to a new equivalent source as shown in Fig.1(c).V is the equivalent voltage with V =V/(Z g C +A ).Z gis the equivalent internal impedance with928Wei et al.Z g =(Z g D +B )/(Z g C +A ).When the load appears as the complex conjugate of the equivalent internal impedance as Z L =Z g ∗,the load will receive the maximum power P La,A (the available power of the equivalent source).It should be noted that the conjugate match for maximum received power does not necessarily results in the maximum power transfer efficiency.ηalso can be written by using the [S ]matrix of the transceiver’s network asη=|S 21(1−ΓL Γg )|2/|(1−S 11Γg )(1−ΓL Γout )|2(4)where Γg is the source reflection coefficient with Γg =(Z g −Z 0)/(Z g +Z 0),ΓL is the load reflection coefficient with ΓL =(Z L −Z 0)/(Z L +Z 0),Γout is the transferred source reflection coefficient with Γout =S 22+S 12S 21Γg /(1−S 11Γg ),and Z 0is the reference impedance.When the transceiver is lossless,[S ]+[S ]=[U ],where +denotes the conjugate transpose and [U ]is the 2by 2unit matrix.Substituting this condition into (4),we can prove that the available power of the equivalent source of Fig.1(c)is equal to the available power of the original source.Therefore,P La,A =|V |2/Re(Z g )/4.In the following,the optimized frequencies for the maximum efficiency are derived for inductive coupling and resonant coupling.2.2.Efficiency of Inductive CouplingAt the low frequency,where the size of transmitter and receiver and their distance are much smaller than the working wavelength,they can be equivalent to the self and mutual inductors shown in Fig.2by using the quasi-static approximation.Its cascade matrix is A B C D =1M L 1jω(L 1L 2−M 2)1/jωL 2(5)Different coupling mechanisms for wireless electricity transfer 929Substituting (5)into (3),the power transfer efficiency can be obtained as η= M (Z g +Z L )L 1Z L +L 2Z g +j [ω(L 1L 2−M 2)−Z g Z L /ω]2(6)In many applications the source internal impedance Z g and load impedance Z L can be taken as pure resistances.Now for the given transmitter,receiver,source,and load,we need find the optimized frequency ω,at which ηachieves its maximum value.Let L 1,L 2,M ,Z g ,and Z L be constants in (6),by minimizing the imaginary part of the denominator of (6),we get the optimized frequency asω= Z g Z L L 1L 2(1−τ2)(7)with τ=M/√L 1L 2.At this optimized frequency,ηgets its maximum value asηm =[(Z g +Z L )M/(L 1Z L +L 2Z g )]2(8)2.3.Efficiency of Resonant Near-field CouplingIt is well-known that two objects with the same resonant frequency tend to exchange energy efficiently,and energy exchanging between off-resonant objects is much weaker.Based on this mechanism,we can use a power transmitter to fill the space around it (near-field region)with an evanescent electromagnetic field,and then the receivers specially designed to have the same resonant frequency will efficiently pick up this energy.For the high transfer efficiency and safety issue,the power transmitter should be an antenna with a small radiation resistance.This is just different from the traditional antennas used in radio communications,where the efficient radiation is required.The derivation of the efficiency of the resonant near-field coupling is not as straightforward as that of the inductive coupling,because the electromagnetic field distribution is very complex at high frequencies.We can not use the simple equivalent circuits of Fig.2to model the high-frequency coupling.Instead,the method of moments developed by authors [6–9]are used to simulate the transfer efficiency.The transfer efficiency achieves its maximum values at the resonant frequencies of the transceiver.For the classic geometry,such as the coil and helix shown in Fig.3and Fig.5,the resonant frequencies can be analytically derived.We take that coil/helix as two conductors with their ends shorted.Therefore,it is equivalent to a transmission930Wei et al.line with the length π∗r ,where r is the radius of the coil/helix.This transmission line is series resonant when its length is equal to the multiple of half working wavelength.Accordingly,for a given coil/helix,its series resonant frequencies are calculated asf n =n ∗c/(N ∗2∗π∗r )with n =1,2,3,...(9)where c is the speed of light and N is the turns of the coil/helix.3.SIMULATION AND MEASUREMENT RESULTSIn this section,the coil and helix are used to study the wireless electricity transfer efficiency.The reason to use such structures is that they are the non-efficient antenna,so that the power loss due to the radiation is reduced and hence much power can be transferred between the transceiver.In the following measurement,signal generator and spectrum analyzer are used as the source and load respectively,so that Z g =Z L =Z 0=50Ohm.Substituting these into (4),we get η=|S 21|2.Therefore,the measured S 21is used to validate the simulation results of the efficiency.In order to avoid the unwanted emission,the connecting cables are not used and the signal generator and spectrum analyzer are directly connected to the coils/helices.The first example is two coaxial copper coils as shown in Fig.3.In Fig.4,the S 21parameter is measured and compared with those from the derived quasi-static formula (6)and the method of moments.For the quasi-static formula,the self and mutual inductors are [10]L 1,2=μ0r (ln (r/a )+0.32944)(10)M =μ0r [(2/k −k )K (k )−2/kE (k )](11)where K and E are complete elliptic integrals of the first and second kinds,k =2r/√d 2+4r 2,r and a are the radius of the coil and copper wire respectively,and d is the distance between two coils.Different coupling mechanisms for wireless electricity transfer931|S21|(dB)Frequency (MHz) Figure4.Magnitude of S21for two coaxial coils.932Wei et al.|S 21| (d B )Frequency (MHz)Figure 6.Magnitude of S 21for two coaxial copper helices.|S 21| (d B )Distance between two coils (Meter)Figure 7.Efficiency comparison between the inductive coupling and resonant coupling for coils shown in Fig.3.measurement,quasi-static formula and method of moments.Good agreement can be observed.The optimized frequencies at low-frequency band and the 1st resonant frequency of the bigger helix are also plotted.Figure 7plots the change of the maximum |S 21|with the distance between the transmitter and receiver for the inductive coupling and resonant coupling.The |S 21|of the inductive coupling is calculated by using (8).The |S 21|of the resonant coupling is calculated at theDifferent coupling mechanisms for wireless electricity transfer933 1st resonant frequencies shown in Fig.4.Thisfigure reveals that the resonant near-field coupling can give a higher efficiency than the inductive coupling.And the inductive coupling of two coils drops more quickly than the resonant coupling when the distance increases.Until now,the inductive couplingfinds more applications than the resonant coupling.But all of these applications are limited to the very short distance[2–4].The resonant coupling provides a more efficient way for the middle distance power transfer.4.CONCLUSIONIn this paper,the efficiency of the wireless electricity transfer is simulated and compared with the measurement results.People had successfully used wireless technology to transfer information for centuries[11,12].However,the usage of wireless technology to transfer power for long distance still faces some challenges.One of the challenges is the low transfer efficiency.Fortunately,due to the technology advancement,nowadays the trend of the power consumption of electrical product 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