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Wireless power transfer

Wireless power transfer(Wireless power,British: contactless power transmission,British: wireless power transfer) IsCordless phone,Electric shaver,electric toothbrushIt is a technology for transmitting electric power without using metal contacts or connectors in such devices.Wireless charging,Wireless charging,Non-contact power transmissionAlso called.Secondary batteryWhen sending power to a device with a built-inContactless charging(British: inductive charging) And so on.

Among these, the technology that uses electromagnetic inductionElectromagnetics OfMutual inductionThis is based on the action ofBritish: highly elevated) Is introduced.


The concept was in the early 20th centuryNikola TeslaDevised byTesla coilUsingWorld systemThere was a plan to send power called. This is to utilize the reflection of the ionosphere.Schumann resonanceIt is thought that he was trying to utilize, but at that time Schumann resonance was not discovered yet, and it failed because the frequency being tested was too high. Since then, various studies have been advanced, and now radiant energy (Microwave) Is used to study power generation satellites.

To enumerate the ones that use the magnetic field, which is non-radiative energy,

In 1891, Barton R. Shover attempted to put it into practical use as an electric train (Electric Railway).[1]

In 1974 by American inventor John George BorgerElectric carWas attempted to supply power to[2][3].

John George Borger invented the electric power control of the electric vehicle by the switched capacitor method in 1979.[4].

In 1989, Eight Electronics applied for a circuit that has exactly the same principle as the magnetic resonance of WiTricity.[5] Based on the same principle, and now mobileFeliCaVarious methods have been realized, starting with. In both cases, the transmitted energy is low.

In 1993Auckland UniversityWorld's First Wireless Power Transfer System Based on Theory of John Boys et al.[6] Is a corporationDaifukuRealized by

In 1994Murata ManufacturingDevelopers announced "Magnetic field resonance technology"[7][8].

200611ToMassachusetts Institute of Technology (MIT)'s Marine Sorachic isWiTricityAnnounced the possibility of practical application of magnetic field resonance technology based on the coupled mode theory[9].

In July 2010, the Wireless Power Consortium (WPC) introduced the international standard "Qi] Was formulated. Although it is a standard for mobile terminals of 5 W or less, it is expected to become popular after 2011 due to the establishment of an international standard.[10], Standards for up to 120W will be set up for laptops in the future[10].


As of 2017, there are six types of technologies that enable non-contact power supply, and they can be broadly classified into radiative and non-radiative types. Out of non-radiativeElectromagnetic inductionIs an improvement of the electromagnetic induction method, which utilizes the phase modulation phenomenon of the magnetic field generated when the coil resonates.Magnetic field resonanceMethod is regarded as influential[11].. In addition, the radiation type converts electric power into electromagnetic waves.antennaTo send and receive viaMicrowaveThe "method" is being researched as a method that reaches a long distance.

In principle, the method using simple electromagnetic induction is electromagnetic induction itself, and the magnetic flux is transmitted as a medium to the receiving side coil. At this timeCoupling coefficient kIs small, the efficiency is low. k isMutual inductanceDepends on the distance, which in the end depends on the distanceThe parameter, The mutual inductance becomes small between the separated coils, and most of the coils areLeakage inductanceIs caused by this leakage inductanceShort circuit inductanceReduces the efficiency by increasing the reactive current and increasing the copper loss. Therefore, if you use a small coil, you can only transmit power at a distance that is close to non-contact, mainly for charging cordless phones and electric toothbrushes,IHIt was used at most for short-distance power transmission applications such as cookers. In order to improve this in the electromagnetic induction method, resonance (broadly defined magnetic field resonance) that combines short-circuit inductance and resonance capacitance was adopted from an early stage,Suica,iDUsed forFeliCaIs increasing the transmission distance.

In either case, if the transmission power is increased even a little, the position shift of the transmitting/receiving device and the magnetic substance of the receiving device approachSkin effectThis is a problem in high-power wireless power transfer because the efficiency is reduced due to the loss due to the phenomenon (proximity effect) similar to the above.

Two magnetic resonance methods

Practical application of magnetic resonance method started in Japan in 1993Auckland UniversityMethod[6] When,200611ToMassachusetts Institute of Technology (WITH) Announced the feasibility of large gap power transmission[9] There is a method. The major difference between these methods lies in whether the resonator is arranged on the primary side or the secondary side, or on both the primary side and the secondary side.

WITHWas announced by two sets of coilsCapacitorSince the resonators of (XNUMX) and (XNUMX) are resonated with each other and are coupled to each other, they are also referred to as an “electromagnetic resonance method” and a “resonant coupling method”. Developer Marin Sorachic(Marin Soljačić) Uses this technologywireless (wireless) WhenElectrical (electricity) Is a coined wordWiTricity[9] A corporation with the same name was established.WiTricityThen about this technologyHighly Resonant Wireless Power TransferIs explained[12].. This coupling is sometimes called electromagnetic coupling or electromagnetic resonance, but to be exact,electric fieldmagnetic fieldIs a different thing, and electric field coupling using only an electric field and magnetic field coupling using only a magnetic field are different ideas. Alternatively, both electric field and magnetic field are used and analyzed by interpreting them as electromagnetic field coupling, but it has been found that when electrolysis and magnetic field coexist, they adversely affect each other, so this name is inappropriate. As for the current magnetic field resonance (British: magnetic resonance,MR) Is said to be appropriate.

According to WiTricity's theoretical explanation and concept, the principle of magnetic field resonance is far awaytuning forkBy the same resonance frequency共鳴It is said that the non-radiative energy transfer between two resonators that resonate with a coil and a capacitor is used.Evanescent・Combination of tails[13][14][15] And the non-radiative electromagnetic resonance energy tunnel (non-radiative electromagnetic energy emissions tunneling) Occurs, and when power is exchanged through this non-radiative electromagnetic resonance energy tunnel, power can be transmitted with high efficiency even in a state of considerable loose coupling with a coupling coefficient k of 0.1 or less, so it is longer than electromagnetic induction. Described as capable of transmitting distance. In this regard, MIT researchers think they have discovered a new way to transfer power wirelessly [16] [17].. In addition, MIT researchers microscopically explain the wireless power transfer by the electromagnetic energy resonance tunneling effect of quantum mechanical electromagnetic field radiation (microscopic field of quantum mechanics radiated electromagnetic energy residency tunneling effect) I tried to explain [18] But this was criticized. MIT's Malin Thorachic initially thought that the theoretical maximum efficiency was 50% because the resonant evanescent tail coupling was assumed to be the transmission line. Then, based on this theory, he announced that he succeeded in transmitting power by lighting a light bulb 2 m away with 25% efficiency. However, after that, after noticing an error in this theory, the theory was revised, and under the new theory that the theoretical maximum efficiency depends on the kQ product, an efficiency of about 1% at a gap of 90 m and about 2% at 45 m was realized. .. This is a resonance circuit composed of a coil and a capacitor.Q valueIncrease (highly elevated) Is realized. It is said that the higher the Q value, the better. However, if the Q value is too high, high frequency accuracy is required, which makes it difficult to design the transmission system. It is said that the theoretical efficiency of a transmission system depends on the product kQ of k and Q. In the magnetic field resonance method of MIT, it is common to arrange a coil for power supply and a coil for power extraction separately from the two sets of resonance coils, close to each resonator. The MIT method is sensitive to the displacement of the transmitting and receiving devices, but it is possible to increase the tolerance of the displacement of the transmitting and receiving devices and to supply power to multiple devices at the same time by sacrificing the efficiency. It is difficult to transmit power to multiple devices if high efficiency is required, but it has been evaluated that high efficiency and wireless power transmission with a large gap can be realized.IEEESelected as "7 technologies that change the world" by[19], And the market size after its completionBlue light emitting diodeIt is said to greatly exceed.

There is also a technology that can transmit power and data at the same time.[20].. This is non-radiative power transmission using an evanescent field that appears in a region below the wavelength of electromagnetic waves.

Disney research TheAnnular solenoid内 のMagnetic flux densityThe quasi-static cavity resonance method that can be charged anywhere in the room by using the principle thatQSCR: Quasi-static Cavity Resonance) Is published[21][22].. This is also a technology classified as non-radiative magnetic resonance.

Radial type

On the other hand, for power transmissionlaserMethod using light[23] There is, but this is classified as a radial type.As a radial type, about several wattsMicrowaveCota and Wattup using the above have been proposed, and are attracting attention as a technology that can transmit power farther than magnetic field resonance.To control the radiated beamArray antennaIs used.In addition, it generates electricity in space and microwavesLaser lightSend power to the groundSpace solar powerHas also been studied.

UltrasoundUBeam, which performs power transmission with, has also been proposed.[24].. This technology is also classified as a radial type.


Generally, in both the electromagnetic induction method and the magnetic field resonance method, electric power is exchanged in the near field in order to utilize the non-radiated energy, so that it is difficult to transmit over a distance defined in the near field. Further, since the size of the coil, the coupling coefficient k, and the Q value of the resonance circuit greatly influence the transmission distance, it is difficult to perform long-distance transmission with a small coil or capacitor.

In addition, any of these methods is vulnerable to positional deviation between the transmitting and receiving devices and has a large loss. The dominant loss isCopper lossAndSkin effectSince there is also a loss due to transmission, it is not possible to transmit at near 100% efficiency even at short distances.

When considering a power feeding system in the electromagnetic induction method, it is necessary to detect a receiving device, and therefore an apparatus using a plurality of smaller coils than one using a large coil has been put into practical use.

WiTricityThe magnetic resonance method of is (British: Coupled Mode TheoryIt is said that it is based on ), but it is difficult to explain, and it is said that conventional electromagnetics and electrical engineering may be sufficient to explain the principle. Further, there are resonators for both transmission and reception, and it is necessary to accurately match the resonance frequencies of those resonators. Furthermore, an impedance matching network (IMN) is essential, but the specific circuit configuration is not clear.[25].. Further, in the coupled mode theory, there is no mention of the fact that the resonance frequency changes due to the positional deviation between the coils. However, in reality, the resonance frequency changes due to the change in the distance between the coils, so there are problems such as how to incorporate that problem into the coupled mode theory and how to solve the bimodal characteristics that appear when the distance between the coils is close. It is a pile up, and the difficulty is that the theory construction and concrete circuit design to solve them cannot be done easily.

These problems, that is, the degree of freedom regarding misalignment is called robustness, but efficiency is sacrificed to increase robustness based on the current coupling theory proposed by MIT. While it has already been confirmed that this can be solved, whether or not both efficiency and robustness can be solved simultaneously under the coupled mode theory is a major issue.

Term problem

Magnetic field resonance using magnetic field for power transmission (British: magnetic resonance,MRThe term)Nuclear magnetic resonance(British: nuclear magnetic resonance,NMR) OrMagnetic resonance(British: magnetic resonance,MR) Is said to be the same and misleading. Therefore, more appropriate terms are being considered.

Movement toward commercialization for high power applications

Wireless power transmission in the small power sector since the early 1960sInductive power transmission using resonanceImplantable medical devices, including devices such as pacemakers and artificial hearts[26] Has started to be used in and has been one success. Resonance was adopted only on the receiving coil side in the early systems, but later systems[27] Then, resonance was also adopted on the transmission coil side. These medical devices are designed to achieve relatively high efficiency in low-power electronic devices, and effectively adjust the coil displacement and twist. The spacing between coils in implantable applications is less than 20 cm in most cases. Resonance-based power transfer is now widely used to power many commercial medical implantable devices[28] .. In addition, fields such as electric toothbrushes and electric shavers, in which the exposure of terminals is not preferred because waterproofness is particularly required, have also been adopted, but in other fields as well, contactless IC cards and[29] Or cordless phone[30] And so, at least 2006-around 2007 has already become widely used.

2009(21) March 5,Japan OfMinistry of Internal Affairs and Communications TheWireless powerAs a study on the practical application of the above, after investigating the effects on other home appliances and the human body, we started to study the issues for practical use such as radio frequency band allocation and radio wave interference, and in July of the same year. Included in the report content of the radio policy conference announced in2015Aiming for practical use[31][32] However, many problems could not be solved and they were not realized.

In the field of AGV (Unmanned Transporter)1993Currently has the top share in this fieldDAIFUKU[6] Practical application has begun, centered on This is when a resonant capacitor is combined with the power receiving side of electromagnetic induction to resonate.Magnetic field phasing phenomenonThe power factor on the power transmission side is improved due to the occurrence of the power transmission, and high efficiency power transmission can be performed. It is widely used as a semiconductor carrier in automated warehouses and other clean rooms, because it is a great advantage not to use sliding electrodes for power supply.

CurrentlyAuckland UniversityNora's proposal[34] Based on thehighly elevatedAttempts have been made to greatly extend the transmission distance by applying (increasing the Q value of resonance).[35].. This can be interpreted as an extension of electromagnetic induction, unlike the theory advocated by MIT's Marin Sorachic. Neither the coupled mode theory nor the non-radiative electromagnetic resonance energy tunnel is used, but it is said to be the essence of coupling of magnetic field resonancehighly elevatedIt is considered that the concept of is newly introduced.

Superconducting linearFor the on-board power source ofgas turbineAlthough it was equipped with a generator, it is an original induction current collection method that has been researching since the beginning of superconducting linear development.Magnetic field phasing phenomenonUsing a method that can be said to be magnetic field resonance in a broad sense, which uses power supply, performs precise frequency/phase control to supply power over a long distance (10 cm or more) and at high efficiency.Induction current collection[36] Technology is established[37],2027It has been decided that it will be put into practical use by the time of commercial operation. This method is alsoResonance transformerIt is based on the principle of, and cannot be explained based on the coupled mode theory. Omron Amusement also used the phenomenon of magnetic field phasing at Techno Frontier 2017, which is also not based on the coupled mode theory2.nd-resonance method[38][39] We demonstrated that it is possible to achieve both efficiency and robustness in medium-distance transmission.

WiTricity is a switched capacitor method in December 2016[4] Announced TMN (Tunable Matching Network), which showed improved efficiency and compatibility of voltage regulation between different coil systems.[40].. As a result, the WiTricity method has greatly come close to the method proposed by John Boys et al., which uses the phase-modulating phenomenon of the magnetic field, apart from the original Marin-Sorachic coupling mode theory. And in February 2019, WiTricityQualcommEV wireless charging departmentQualcomm HaloWiTricity technology will be integrated with technology from the University of Auckland[41].

In July 2018, Associate Professor Toshihiro Maki of the Institute of Industrial Science, University of Tokyo announced that wireless power supply technology for underwater robots (underwater robots) was developed to enable long-term autonomous operation. By adopting the magnetic field resonance method by infrared synchronization for the coupling between power transmission and reception, stable power transmission and reception can be performed even with changes in parameters caused by changes in temperature and water pressure.[42].

Practical example

19844, B&Plus Co., Ltd. (formerly Nippon Balluff Co., Ltd.) has succeeded in developing a sensor that enables both non-contact power supply and signal transmission using electromagnetic induction (resonance circuit method). Commercialized[43].

In 1993, Daifuku Co., Ltd.Auckland UniversityWorld's first contactless power transfer system based on Nora's proposal[6] Was put to practical use.

1998 years,RFUsed contactless power transferCapsule endoscopeannounced[44].. Since the conventional capsule endoscope uses a battery, there is a concern that the electrolyte may leak inside the body when the capsule endoscope is damaged.

200612/4,Tokyo Universitygraduate SchoolGraduate School of EngineeringAt the joint press conference of the University of Tokyo, Graduate School of Engineering, the University of TokyoAssociate Professor OfSomeo TakaoAnd a research team led by the University of TokyoTransistorSucceeded in realizing a sheet-type wireless power transmission system combining[45].

2007,Sanwa SupplyHas released wireless power supply. this is,USBConnected withMouse padBy generating a magnetic field in the mouse, it has a structure that supplies power to the circuit inside the mouse.[46]

2008XNUM X Month X NUM X Day,Ministry of Land, Infrastructure, Transport and TourismIs a hybrid bus that uses contactless power to rapidly charge a large amount of battery on the vehicle side as a part of driving force by electromagnetic induction from a power supply device embedded in the road surface etc. as a free contact bus between terminals at Haneda Airport Announced that[47].

2008XNUM X Month X NUM X Day,IntelOf MIT, which was announced in 2006physicistBased on the theory ofWireless Resonant Energy Link: WREL)[48],San FranciscoPresented research results at the 2008 Intel Developer Forum held atWattSucceeded in generating electricity[49].. IntelChief Technology Officer (CTO) has also released a movie that lights a light bulb with 60 watts of electricity actually generated at this lecture[50].

2008 years,Seiko Epson CorporationMurata ManufacturingMade a prototype of a "portable charger" that supplies power to mobile devices without contact, 200811/19 - 21 DayToPacifico YokohamaExhibited at "Embedded Technology 2008"[51].

Sony2009On October 10, announced that it has developed a "wireless power supply system" that can supply power to digital home appliances such as flat-screen TVs from a remote location without using a power cord.[52].

2010,Showa Aircraft IndustryJust stop at the charging spotEVWe have successfully commercialized a wireless power supply technology that can be charged to an EV bus. This uses the electromagnetic induction method,CirculationThe electric power required for running this bus, which is about 1 km per lap, can be covered by stopping at the charging spot for a total of 5 minutes.[53].

2010, KoreanKISTIt is,Online electric car We developed (Online Electric Vehicle, OLEV) and put the bus using contactless power transmission into practical use.

2016In March, Toyohashi University of Technology and Taisei Corporation, with the cooperation of the Ministry of Internal Affairs and Communications, laid an electrified road on the university campus. An electrified road is a road in which two steel plates are buried in the shape of a rail under asphalt pavement. It is a mechanism to feed electricity to the vehicle via tires by electric field coupling (no magnetic field is used). Succeeded in an experiment in which all driving batteries were removed from a commercially available electric vehicle and the vehicle was driven only by power supply from the electrified road.[54][55].

Commercialized products


[How to use footnotes]


  1. ^ US461,057 Electric Railway Patented. Oct. 13, 1891
  2. ^ Vehicles used on roads that combine power sources that generate magnetic fields Japanese Patent Application Sho 54-51344
  3. ^ U.S. 3,914,562 Supplying power to vehicles Patented Oct. 21,1975 John G. Bolger
  4. ^ a b U.S. 4,331,225 Power control system for efficiently driven vehicle Patented May 25,1982 John G. Bolger
  5. ^ Power supply by electromagnetic induction Japanese Patent Application 1-235399
  6. ^ a b c d Contactless power transfer system technology DAIFUKU 1993
  7. ^ What is the "DC resonance" method?
  8. ^ Application of new physical phenomenon! Development of DC resonance wireless power transmission system
  9. ^ a b c MIT prototyped a resonance type wireless power transmission device and succeeded in lighting a 2.1W light bulb 60m away Nikkei Electronics, June 2007, 6
  10. ^ a b EE Times Japan (February 2011, 2). “Wireless power supply that gathers in the "Qi" standard, from 5W or less mobile to spread". 2011/7/5Browse.
  11. ^ Overview of wireless charging technology -Shunsuke Takahashi
  12. ^ Highly Resonant Wireless Power Transfer
  13. ^ Title: Wireless energy transfer device Special table 2012-502602
    Abstract: Disclosed is a device for wireless energy transfer, the device configured to radiate energy to and from a second resonant structure over a distance greater than a characteristic size of the second resonant structure. A first resonant structure. thisFor non-radiative energy transfer,Resonance field evanescent tail of the first resonance structureCoupling with the resonant field evanescent tail of the second resonant structure Intervene.
  14. ^ Title: Wireless non-radiative energy transfer Special table 2009-501510
    SUMMARY An electromagnetic energy transfer device includes a first resonator structure that receives energy from an external power source. The first resonator structure has a first Q factor. The second resonator structure is located distal to the first resonator structure and supplies useful operating power to an external load. The second resonator structure has a second Q factor. The distance between the two resonators can be larger than the characteristic size of each resonator.Non-radiative energy transfer between the first and second resonator structures is established through the coupling of their resonant field evanescent tails..
  15. ^ Ajey Kumar; Gayathri. H. R; Bette Gowda. R; Yashwanth. B (May 2014). “WiTricity: Wireless Power Transfer By Non-radiative Method”. International Journal of Engineering Trends and Technology (Thanjavur Seventh Sense Research Group) 11 (6): 291. ISSN 2231-5381. http://www.ijettjournal.org/volume-11/number-6/IJETT-V11P255.pdf. 
    b) Resonance Coupling: The idea of ​​such mid-range induction was given by Marin Soljacic for efficient wireless transfer.The reason behind it is that, if two such resonant objects are brought in mid-range proximity, their near fields (consisting of so-called'evanescent waves') and can allow the energy to transfer from one object to the other within times much shorter than all loss times, which were designed to be long, and thus with the maximum possible energy-transfer efficiency.Electromagnetic resonance induction works on the principle of a primary coil generating a predominantly magnetic field and a secondary coil being within that field so a current is induced within its coils, when both of these are made to resonate at same frequency they become much efficient.Fig. 3. Resonant Magnetic Coupled system.
  16. ^ Er. Manish Kumar; Dr. Umesh Kumar (13 December 2016). “WIRELESS POWER TRANSMISSION: A REVIEW”. Global Journal of Engineering Science and Researches. Pp. 120. http://www.gjesr.com/Issues%20PDF/Technology%20&%20Environment/20.pdf 
  17. ^ Pragati S. Chawardol; Deepali R. Badre; Mithul S. There (December 2014). “Wireless Power Transmission”. International Journal of Engineering Sciences & Research Technology. Pp. 150. http://www.ijesrt.com/issues%20pdf%20file/Archives-2014/December-2014/Wireless%20Power%20Transmission.pdf 
  18. ^ Alice Peng (6 August 2013). "PROCEEDINGS OF THE 2013 INTERNATIONAL CONFERENCE ON ENERGY". Conference on Energy (Energy2013) is a multidisciplinary international conference. DEStech Publications, Inc. pp. 337. https://books.google.co.jp/books?id=9TrFAQAAQBAJ&pg=PA337&lpg=PA337&dq=MIT+uses+microscopic+field+of+quantum+mechanics+radiated+electromagnetic+energy+resonant+tunneling+effect&source=bl&ots=1RDaqK7rAr&sig=N7EbWbwi2WEL--uoIfyvQBXPjOY&hl=ja&sa=X&ved=0ahUKEwiZz5q81NHWAhUMl5QKHQp5DG0Q6AEIJzAA#v=onepage&q=MIT%20uses%20microscopic%20field%20of%20quantum%20mechanics%20radiated%20electromagnetic%20energy%20resonant%20tunneling%20effect&f=false 
  19. ^ Seven technologies that change the world IEEE (Institute of Electrical and Electronics Engineers)
  20. ^ Power and data can be transmitted simultaneously by microwave! Two-dimensional communication system "Surface LAN"
  21. ^ Gigazine (February 2017, 02) Technology that turns the entire room into a chargerQuasistatic Cavity Resonance (QSCR)Developed by Disney
  22. ^ Magnetic Field inside a Toroid — Collection of Solved Problems in Physics Image
  23. ^ Laser power transmission
  24. ^ UBeam, an up-and-coming startup company, finally demonstrates ultrasonic completely wireless charging technology!! 2017/2/5
  25. ^ January 2016WiTricityThe tunable matching network (TMN), released by Press, is similar in name, but has no theoretical or technical relationship with the impedance matching network.
  26. ^ JC Schuder, “Powering an artificial heart: Birth of the inductively coupled-radio frequency system in 1960,” Artificial Organs, vol. 26, no. 11, pp. 909–915, 2002.
  27. ^ SCHWAN MA and PR Troyk, "High efficiency driver for transcutaneously coupled coil" IEEE Engineering in Medicine & Biology Society 11th Annual International Conference, November 1989, pp. 1403-1404.
  28. ^ "What is a cochlear implant?”. Cochlearamericas.com (January 2009, 1). As of December 30, 2008オ リ ジ ナ ルMore archives.2009/6/4Browse.
  29. ^ Daisuke Okada, @IT editorial department "A contactless IC card that can be absolutely understood in 5 minutes" @IT, May 2006, 5 (viewed April 18, 2011).
  30. ^ Finally the power supply is also wireless Nikkei Electronics March 2007, 3 issue
  31. ^ Home appliances: Wireless power supply, Ministry of Internal Affairs and Communications seriously consider Mainichi Newspapers, January 2009, 5
  32. ^ Announcement of "Radio New Industry Creation Strategy -Radio Policy Roundtable Report-" and result of solicitation 21
  33. ^ Realizes medium-distance power transmission by resonance on secondary side only Japan Tecmo
  34. ^ CERV 2015 Wireless power Transfer: Introduction and History-Tutorial, John Boys
  35. ^ Wireless Power Transfer High efficiency, high power, large gap.-Auckland UniServices Limited
  36. ^ On-board power supply by induction current collection system
  37. ^ Practical technology evaluation of superconducting magnetic levitation railway for on-board power supply by induction current collection -Superconducting Magnetic Levitation Railway Practical Technology Evaluation Committee July 2009, 7
  38. ^ Realizes high power supply efficiency 2nd-resonance method The power supply area is wide and 80% or more high-efficiency power supply is possible even if the position shifts.
  39. ^ "A new method of wireless power transfer, which finally found a breakthrough, was finely corrected to the problem of magnetic field resonance theory to improve efficiency and robustness."Green electronics』No.19,CQ publishing, November 2017, pp. 10-52, ISBN 9784789848503.
  40. ^ Nissan charges wirelessly, efficiency up to 94%
  41. ^ WiTricity acquires Qualcomm Halo to accelerate wireless charging for electric vehicles
  42. ^ Wireless power supply under the sea, enabling long-term autonomous operation of underwater robots Institute of Industrial Science, University of Tokyo Nikkan Kogyo Shimbun July 2018, 7
  43. ^ Full-scale wireless power supply-Tech-On SPECIAL
  44. ^ "17 Patent Application Technology Trend Survey Report Endoscope (Summary Version) (PDF)"(Japanese). Patent Office. pp. 22-23 (March 2006). 2013/2/19Browse.
  45. ^ World's first wireless power transmission sheet (PDF)
  46. ^ http://pc.watch.impress.co.jp/docs/2007/0523/sanwa.htm
  47. ^ Non-contact power supply hybrid bus operates at Haneda Airport Ministry of Land, Infrastructure, Transport and Tourism 2008(20) April 2
  48. ^ Intel Press Room June 2008, 8
  49. ^ Intel develops "wireless power" that does not require a power cord, releases demonstration movie - GIGAZINEAug 2008, 08 23:15:52
  50. ^ Intel CTO: No more power cords - YouTube
  51. ^ [ET2008] Seiko Epson and Murata Manufacturing prototype a "portable charger" that supplies power without contact Nikkei Electronics 2008/11/25
  52. ^ Sony, power supply without power cord system for digital home appliances
  53. ^ http://rikunabi-next.yahoo.co.jp/tech/docs/ct_s03600.jsp?p=001677
  54. ^ On the day when the road becomes a "power source", will it be partially used in 2020? Nikkei Electronics, May 2016, pp.5-48
  55. ^ Conducted the world's first batteryless electric vehicle and open road experiment with electrified road Tokai Communication Bureau, Ministry of Internal Affairs and Communications, viewed May 2016, 5


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