Patent Publication Number: US-2016226313-A1

Title: Wireless power transfer system and wireless power transfer method

Description:
TECHNICAL FIELD 
     The present invention relates to a wireless power transfer system and a wireless power transfer method. 
     BACKGROUND ART 
     Patent Literature 1 relates to a shielding technique in a power supply system that supplies power from a power source outside a vehicle to the vehicle in a wireless manner using a resonance method, and Patent Literature 1 describes that a shielding box is disposed such that an opening thereof is able to face a power supply unit, other five surfaces thereof reflect a resonant electromagnetic field (near field) generated around a power receiving unit when receiving power from the power supply unit, the power receiving unit is disposed in the shielding box, and power is received from the power supply unit through the opening portion of the shielding box. 
     Patent Literature 2 relates to an electrically powered vehicle which is capable of receiving power from a power supply device in a wireless manner by causing resonance between a resonator provided to each vehicle and the power supply device outside the vehicle through an electromagnetic field, and Patent Literature 2 describes that, in order to implement such a configuration, at low cost, that the leaked electromagnetic field generated when receiving power is shielded, a power reception unit, which includes a secondary self-resonant coil that receives power from a power transmission unit of a power supply device by resonating with a primary self-resonant coil included in the power transmission unit through an electromagnetic field, is disposed at the bottom of an engine room which stores a driving power generation unit including an engine and a motor generator; and a shielding material, made of a cloth, sponge, or the like, having an electromagnetic wave shielding effect, is provided to electromagnetically shield the engine room from inside and outside. 
     Patent Literature 3 relates to a method of shielding an electromagnetic wave in wireless power transfer system, and describes that unnecessary radiation of an electromagnetic wave is restrained and, in order to suppress reduction in power transport efficiency as much as possible, a power-transmission-side magnetic shielding member made of a magnetic material is disposed on the side opposite to the side of power transmission that is performed by a power transmission system coil in the power transmission system coil; an electric field shielding member made of a conductive material is disposed on the side orthogonal to a direction of power transmission performed by the power transmission system coil; and a power-reception-side magnetic shielding member made of a magnetic material is disposed on the side opposite to the side of power reception performed by a power reception system coil in the power reception system coil. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Laid-open Publication No. 2011-91999 
     [Patent Literature 2] Republication of PCT International Publication No. WO2010/041320 
     [Patent Literature 3] Japanese Patent Application Laid-open Publication No. 2011-45189 
     SUMMARY OF INVENTION 
     Wireless power transfer is a technique of transferring power in space through an electromagnetic wave (high-frequency electromagnetic field), and when power is supplied from a power transmission element to a power receiving element, the electromagnetic wave is inevitably radiated into space from the power transmission element (a coil, an antenna, or the like) serving as a wave source. This electromagnetic wave radiated from the power transmission element causes noise in electronic equipment, and might affect human bodies. Further, the strength in electromagnetic wave is stipulated by Radio Act, and the like. Thus, when wireless power transfer is performed, it is required that an unnecessary electromagnetic wave radiated from the power transmission element are restrained as much as possible. 
     One or more embodiments of the invention provide a wireless power transfer system and a wireless power transfer method capable of reliably transmitting (supplying) power from a power transmission element to a power receiving element, while shielding radiation of an unnecessary electromagnetic wave from the power transmission element, in wireless power transfer. 
     One or more embodiments of the present invention are directed to a wireless power transfer system including: a power receiving element provided to a mobile object and configured to receive power transmitted through wireless power transfer, the mobile object being configured to move along a movement surface; a power transmission element embedded on a movement surface side and configured to transmit the power; a shield provided in a manner slidable along the movement surface so as to shield an electromagnetic wave radiated from the power transmission element; a first magnet provided to the shield; an urging mechanism configured to urge the shield to slide in a direction in which a shielding effect for the power transmission element is increased; and a second magnet provided to the mobile object at a predetermined distance from the power receiving element, the second magnet being configured to, when the mobile object approaches the power transmission element, become coupled to the first magnet, to couple the mobile object and the shield together. 
     According to one or more embodiments of the present invention, when the mobile object approaches the power transmission element, the first magnet provided to the shield and the second magnet provided to the mobile object at the predetermined distance from the power receiving element are coupled to each other, which causes the shield to be coupled to the mobile object, and with the movement of the mobile object (due to the kinetic energy of the mobile object), the shield slides against the urging force, to automatically release shielding of the power transmission element. Thus, while the mobile object is at a location away from the power transmission element, an unnecessary electromagnetic wave radiated from the power transmission element can reliably be shielded, and while the mobile object approaches the power transmission element, shielding by the shield can automatically be released, to reliably supply power from the power transmission element to the power receiving element. 
     One or more embodiments of the present invention are directed to the wireless power transfer system, in which the first magnet is provided near a periphery of the shield, and the predetermined distance between the second magnet and the power receiving element is at least equal to or greater than a maximum diameter of an electromagnetic wave radiating surface of the power transmission element. 
     In a case where the first magnet is provided near the periphery of the shield, the predetermined distance between the second magnet and the power receiving element is at least equal to or greater than the maximum diameter of the electromagnetic-wave radiating surface of the power transmission element. With such a configuration, when the mobile object approaches the power transmission element from the direction of the second magnet, the second magnet and the first magnet are coupled to each other first, and when the power receiving element passes right over the power transmission element, the shielding effect by virtue of the shield is reduced at the highest possible degree, and thus power can efficiently be supplied from the power transmission element to the power receiving element. 
     One or more embodiments of the present invention further include a power transmission supply device configured to supply power to the power transmission element only when an amount of sliding of the shield exceeds a predetermined threshold, the shield being configured to slide against the urging force with a movement of the mobile object. 
     Accordingly, a configuration is made such that power is supplied to the power transmission element only when the amount of sliding of the shield exceeds the predetermined threshold, and thus radiation of an unnecessary electromagnetic wave from the power transmission element can be shielded more reliably. 
     One or more embodiments of the present invention further include a plurality of shields provided in a manner slidable in directions different from one another, the shield including the plurality of shields. 
     Accordingly, since the power transmission element is shielded with a plurality of shields that are provided in a manner slidable in the directions different from one another, such cases can be handled that the mobile object approaches power transmission element from a plurality of directions, and shielding can reliably be released in response to the mobile object approaching the power transmission element from various directions, and thus wireless power transfer can reliably be performed. 
     One or more embodiments of the present invention provide a plurality of first magnets near a periphery of the shield, the first magnet including the plurality of first magnets, a plurality of power receiving elements are provided around the second magnet of the mobile object, the power receiving element including the plurality of power receiving elements, and the predetermined distance between the second magnet and each of the power receiving elements is equal to or smaller than a maximum diameter of an electromagnetic wave radiating surface of the power transmission element. 
     With such a configuration, it is possible to release the shielding by reliably sliding the shield in a case where the mobile object approaches the power transmission element in any direction, thereby being able to reliably perform wireless power transfer. 
     Other aspects of one or more embodiments of the present invention will become clear from the description of embodiments and the drawings. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to one or more embodiments of the present invention, in wireless power transfer, power can reliably be supplied from a power transmission element to a power receiving element while radiation of an unnecessary electromagnetic wave from the power transmission element being shielded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of a wireless power transfer system  1  according to a first embodiment. 
         FIG. 2A  is a diagram explaining an operation of a shield  31 . 
         FIG. 2B  is a diagram explaining an operation of a shield  31 . 
         FIG. 2C  is a diagram explaining an operation of a shield  31 . 
         FIG. 2D  is a diagram explaining an operation of a shield  31 . 
         FIG. 2E  is a diagram explaining an operation of a shield  31 . 
         FIG. 3  is a diagram explaining a configuration around a power transmission element  21  and a shield  31  in a wireless power transfer system  1  according to a second embodiment. 
         FIG. 4  is a diagram explaining a configuration around a power receiving element  11  in a wireless power transfer system  1  according to the second embodiment. 
         FIG. 5  is a diagram explaining a configuration around a power transmission element  21  and a shield  31  in a wireless power transfer system  1  according to a third embodiment. 
         FIG. 6  is a diagram explaining a configuration around a power receiving element  11  in a wireless power transfer system  1  according to the third embodiment. 
         FIG. 7  is a diagram explaining a function of a wireless power transfer system  1  according to the third embodiment. 
         FIG. 8  is a diagram explaining a configuration around a power transmission element  21  and a shield  31  in a wireless power transfer system  1  according to a fourth embodiment. 
         FIG. 9  is a diagram explaining a configuration around a power receiving element  11  in a wireless power transfer system  1  according to the fourth embodiment. 
         FIG. 10  is a diagram explaining a function of a wireless power transfer system  1  according to the fourth embodiment. 
         FIG. 11  is a diagram explaining another configuration around a power transmission element  21  and a shield  31  in a wireless power transfer system  1  according to the fourth embodiment. 
         FIG. 12  is a diagram explaining another configuration around a power receiving element  11  in a wireless power transfer system  1  according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will hereinafter be described in detail with reference to drawings for purposes of illustration only. Note that, in the following descriptions, the same or similar components are given the same reference numerals, and descriptions thereof may be omitted. 
     First Embodiment 
       FIG. 1  illustrates a schematic configuration of a wireless power transfer system  1  which will be described as a first embodiment. The wireless power transfer system  1  includes: a mobile object  10  configured to move along a floor surface, a ground surface, or the like, (hereinafter, referred to as a movement surface  5 ); power receiving equipment provided to the mobile object  10  and configured to receive power through wireless power transfer; power transmission equipment provided on the movement surface  5  side and configured to transmit power to the power receiving equipment through wireless power transfer; and shielding equipment provided to the movement surface  5  side and configured to shield an electromagnetic wave radiated from the power transmission equipment. 
     The mobile object  10  is, for example, an electric vehicle, a cargo transportation vehicle, a vacuum cleaner, a robot, or the like. The mobile object  10  is provided with electric/electronic equipment and mechanical equipment that are configured to be operated using power transmitted through wireless power transfer. Note that a wireless power transfer system includes an electromagnetic wave type, a magnetic resonance type, an electromagnetic induction type, etc., and a mechanism which will be described hereinafter can be applied to any type of the wireless power transfer system. 
     The aforementioned power receiving equipment configuring the wireless power transfer system  1  includes a power receiving element  11 , a charging circuit  12 , a secondary battery  13 , a load  14 , and at least one magnet  18  (second magnet) (e.g., permanent magnet (alnico magnet, ferrite magnet, neodymium magnet, etc.,)). The power receiving element  11  is configured to convert energy in the form of an electromagnetic wave, which propagates in space, into electric energy, and is, for example, a coil or an antenna. The charging circuit  12  includes, for example, a rectifier circuit configured to rectify power received by the power receiving element  11 , a control circuit configured to control charging/discharging of a secondary battery, or the like. The secondary battery  13  is, for example, a lithium-ion secondary battery, a lithium-ion polymer secondary battery, an electric double layer capacitor, or the like. The load  14  is, for example, an electric/electronic circuit, a mechanical device, a motor, or the like, and is operated utilizing power stored in the secondary battery  13 . A magnet  18  is provided at a location facing the movement surface  5  of the mobile object  10 . The magnet  18  is configured to, when the mobile object  10  approaches a power transmission element  21  which will be described later, become coupled to a magnet  33  (first magnet) that is provided on the shield  31  side which will be described later, to couple the mobile object  10  and the shield  31  together. 
     The aforementioned power transmission equipment configuring the wireless power transfer system  1  includes the power transmission element  21  and a power transmission supply device  22 . The power transmission element  21  is configured to transfer electric energy into energy in the form of an electromagnetic wave, which propagates in space, and is, for example, a coil, an antenna, or the like. The power transmission supply device  22  is configured to supply power (power required to radiate an electromagnetic wave from the power transmission element  21 ) to the power transmission element  21 . The power transmission supply device  22  includes: for example, a rectifier device configured to rectify an alternating current supplied from a commercial power source  23 ; an inverter circuit configured to generate a high-frequency current that is to be supplied to the power transmission element  21  based on a direct current rectified by a rectifier device; and the like. Note that at least the power transmission element  21  out of components of the power transmission equipment is embedded closer to the movement surface  5  so as to be reliably coupled to the magnet  18  provided on the mobile object  10  side when the mobile object  10  approaches the power transmission element  21 . 
     The aforementioned shielding equipment configuring the wireless power transfer system  1  includes the shield  31 , a slide mechanism  32 , and the magnet  33 . The shield  31  is made of a material such as an aluminum plate having properties of shielding an electromagnetic wave radiated from the power transmission element  21 . The shield  31  is provided such that the surface thereof is made parallel to the movement surface  5  so as to be able to efficiently attenuate the electromagnetic wave which is radiated from the power transmission element  21  embedded on the movement surface  5  side toward a space (a space above the movement surface  5 ). The slide mechanism  32  is configured to slide the shield  31  along the movement surface  5 , as well as includes an urging mechanism configured to urge the shield  31  to slide in a predetermined direction. The slide mechanism  32  is implemented, for example, by a rail mechanism configured to slidably support both sides of the shield  31 . For example, an elastic body such as spring, rubber, etc., is used to implement the urging mechanism. An urging force acting on the shield  31  increases with an increase in the amount of sliding of the shield  31 . The magnet  33  is provided, near the periphery of the shield  31 , at a location facing the side on which the mobile object  10  passes. The magnet  33  is configured to, when the mobile object  10  approaches, become coupled to the magnet  18  on the mobile object  10  side, thereby coupling the shield  31  and the mobile object  10  with each other. Thus, the surfaces, facing each other, of the magnet  33  on the shield  31  side and the magnet  18  on the mobile object  10  side have opposite polarities (the north pole and the south pole, or the south pole and the north pole). The magnet  18  and the magnet  33  are magnets having such coupling forces as not to excessively attenuate the kinetic energy of the mobile object  10  with the magnets being coupled together. 
     When the mobile object  10  passes the power transmission element  21 , the shield  31 , which is a component of the shielding equipment, is coupled to the mobile object  10 , with the magnet  18  and the magnet  33  being coupled, thereby sliding automatically (utilizing the kinetic energy of the mobile object  10 ) in association with the movement of the mobile object  10 . Hereinafter, a sliding operation of the shield  31  will be described with reference to  FIGS. 2A to 2E . Note that  FIGS. 2A to 2E  illustrate only the components necessary for explanations among the components of the wireless power transfer system  1 . Further, unless otherwise noted, the power transmission supply device  22  supplies power to the power transmission element  21  and the electromagnetic wave is radiated from the power transmission element  21 . 
       FIG. 2A  illustrates a state where the mobile object  10  moves on the movement surface  5  to come close to the power transmission element  21 . As illustrated in the drawing, the shield  31  is completely closed at this stage, and the electromagnetic wave radiated from the power transmission element  21  is shielded at the highest possible degree by the shield  31 . 
     Subsequently, as illustrated in  FIG. 2B , when the mobile object  10  approaches the power transmission element  21 , the magnet  18  on the mobile object  10  side and the magnet  33  on the power transmission element  21  side are coupled with each other, which causes the shield  31  to be coupled to the mobile object  10 , and with the movement of the mobile object  10 , the shield  31  starts to slide against the urging force of the urging mechanism. Then, as illustrated in  FIG. 2C , shielding of the power transmission element  21  is gradually released with the shield  31  sliding, thereby starting to supply power from the power transmission element  21  to the power receiving element  11 . Note that a configuration may be made such that a sensor is provided which detects that the amount of sliding of the shield  31  has exceeded the predetermined threshold, and that when it is detected that the amount of sliding has exceeded the predetermined threshold, the radiation of the electromagnetic wave from the power transmission element  21  (power supply to the power transmission element  21  by the power transmission supply device  22 ) is started. 
     Subsequently, as illustrated in  FIG. 2D , when the shield  31  further slides with the movement of the mobile object  10 , coupling between the magnet  18  and the magnet  33  is released at the point at which the urging force has exceeded the coupling force between the magnet  18  and the magnet  33 . As a result, as illustrated in  FIG. 2E , the shield  31  returns to its original position, and again, the electromagnetic wave radiated from the power transmission element  21  is shielded at the highest possible degree by the shield  31 . 
     Note that, in a case where the power receiving element  11  of the mobile object  10  passes over the power transmission element  21  at high speed, a time period during which the shield  31  is open is short. However, even if the time period during which shielding is released is short, for example, a battery having a small capacity is used as the secondary battery  13 , or an electric double layer capacitor is employed as the secondary battery  13 , thereby being able to supply the necessary amount of power from the power transmission element  21  to the power receiving element  11 . 
     A distance between the magnet  18  provided to the mobile object  10  and the power receiving element  11  is at least equal to or greater than the maximum diameter of the electromagnetic-wave radiating surface of the power transmission element  21  (for example, a diameter of a circle when the radiating surface is in a circular shape, and a length of a diagonal line when in a rectangular shape). With this distance such a configuration is enabled that the mobile object  10  comes close to the power transmission element  21 , to couple the magnet  18  and the shield  31  together in advance, and then at the time when the light receiving element  11  approaches right above the power transmission element  21 , shielding for the power transmission element  21  by the shield  31  has already been released without fail. Thus, power can efficiently be efficiently supplied from the power transmission element  21  to the power receiving element  11 . 
     As described hereinabove, with the wireless power transfer system  1  according to the first embodiment, when the mobile object  10  approaches the power transmission element  21  from a predetermined direction, the magnet  18  for the mobile object  10  and the magnet  33  for the shield  31  are coupled to each other to automatically slide the shield  31 , thereby releasing the shielding of the electromagnetic wave radiated from the power transmission element  21 , to start wireless power transfer from the power transmission element  21  to the power receiving element  11 . Furthermore, with a further movement of the mobile object  10 , the shield  31  automatically returns to its original position with the urging force, and again, the electromagnetic wave radiated from the power transmission element  21  is shielded at highest possible degree by the shield  31 . Thus, at normal times (while the mobile object  10  is away from the power transmission element  21 ), unnecessary electromagnetic wave radiated from the power transmission element  21  can reliably be shielded, and when the mobile object  10  approaches the power transmission element  21 , power can reliably be supplied from the power transmission element  21  to the power receiving element  11 . Further, since the shield  31  is slid utilizing the kinetic energy of the mobile object  10 , a complicated mechanism for sliding the shield  31  is not required to be provided separately, and a mechanism configured to reliably perform wireless power transfer while shielding the electromagnetic wave can be implemented with reduced energy and a simple configuration. Note that the above described power transmission equipment and the shielding equipment provided therewith may be provided at a plurality of locations in an area, where the mobile object  10  moves, in the movement surface  5 . 
     Second Embodiment 
     In a configuration of the wireless power transfer system  1  according to the first embodiment, in cases where the mobile object  10  approaches the power transmission element  21  in a direction different from a direction in which the shield  31  can slide, shielding cannot always be released. Then, in a wireless power transfer system  1  according to a second embodiment, a configuration is made such that shielding of a power transmission element  21  is released, even when a mobile object  10  approaches a power transmission element  21  from various (arbitrary) directions. 
       FIG. 3  is a diagram of a configuration, seen from above a movement surface  5 , around the power transmission element  21  and shields  31  in the wireless power transfer system  1  which will be described as the second embodiment. 
     As illustrated in the drawing, in the wireless power transfer system  1  according to the second embodiment, four shields  31   a - 31   d , each having a substantially square shape, which are arranged adjacent to one another to have a substantially square shape (this square shape is hereinafter referred to as the entire square shape) are provided in such a manner as to cover an electromagnetic-wave radiating surface of the single power transmission element  21 . These four shields  31   a - 31   d  can be slid in directions (directions illustrated in arrows in the drawing) toward the outer contour lines of the entire square shape by slide mechanisms, not shown, which are respectively provided to the shields. Further, these four shields  31   a - 31   d  are urged in such a manner as to return in directions (directions opposite to the directions illustrated in the arrows in the drawing) toward the center of the entire square shape by an urging mechanism not shown, respectively. 
     Magnets  33  are embedded in (or fixed to the undersides of) the four shields  31   a - 31   d , respectively. In  FIG. 3 , magnets  33   a - 33   h  are embedded in the four shields  31   a - 31   d  along the periphery of the entire square shape. Note that the number of the magnets  33 , the locations at which the magnets  33  are provided, and the polarities of the magnets  33  on the side facing a mobile object  10  are not necessarily limited to the embodiment illustrated in the drawing, but are set in an appropriate state in view of the relationship with a configuration of the mobile object  10 , which will be described later. 
       FIG. 4  is a diagram of a configuration, seen from above the mobile object  10 , around a power receiving element  11  in the wireless power transfer system  1  which will be described as the second embodiment. As illustrated in the drawing, a plurality of magnets  18   a - 18   h  are provided around the single power receiving element  11 . The power receiving element  11  and the power transmission element  21  are substantially the same in shape and size, and the magnets  18   a - 18   h  are provided to the mobile object  10  at such locations that the magnets  18  are superposed on the magnets  33 , respectively, (the magnets  18   a - 18   h  correspond the magnets  33   a - 33   h , respectively) when the power receiving element  11  is superposed on the power transmission element  21  such that the entire shapes thereof are superposed on each other. 
     In the wireless power transfer system  1  having the above configuration, when a mobile object  2  approaches the power transmission element  21 , at least any of the magnets  18  on the mobile object  10  side and at least any of the magnets  33  on the power transmission element  21  side are coupled together, and coupled one or coupled ones of the shields  31   a - 31   d  slide with the movement of the mobile object  10 , and as a result, shielding of the power transmission element  21  is released. Further, the coupled shield(s)  31  further move(s) and when coupling between the magnet(s)  18  and the magnet(s)  33  is released at last, the shield(s)  31  automatically return(s) to the original position(s) with the urging force, and the electromagnetic wave radiated from the power transmission element  21  is shielded again at the highest possible degree by the shield  31 . 
     As such, with the wireless power transfer system  1  according to the second embodiment, it is possible to automatically release the shield  31 &#39;s shielding of the electromagnetic wave when the mobile object  10  approaches the power transmission element  21  from various directions, and it is possible to reliably perform wireless power transfer when the mobile object  10  approaches the power transmission element  21 , while, at normal times, reliably shield the electromagnetic wave. 
     In the second embodiment, the shapes and the sizes of the power receiving element  11  and the power transmission element  21  are not limited to those illustrated in the drawing. Further, the numbers, shapes, sizes, and arrangements of the magnets  18  and the magnets  33  are not limited to those illustrated in the drawing. 
     Third Embodiment 
     A wireless power transfer system  1  illustrated as the third embodiment is configured for the purpose of improving power supply efficiency in a case where a power transmission element and a power receiving element  11  have a predetermined radiating surface and a predetermined receiving surface, respectively, and the wireless power transfer system  1  is based on the configuration of the wireless power transfer system  1  according to the second embodiment. 
     As illustrated in  FIG. 5 , in the wireless power transfer system  1  according to the third embodiment, the power transmission element  21  is configured with power transmission elements  21   a ,  21   b  (at least electromagnetic-wave radiating surfaces thereof are in a circular shape) of circular shapes, respectively, of the same size having a diameter about half the length of one side of the aforementioned entire square shape. Further, as illustrated in  FIG. 6 , the power receiving element  11  is configured with power receiving elements  11   a ,  11   b  (at least electromagnetic-wave receiving surfaces hereof are in a circular shape) of circular shapes, respectively, of the same size having a diameter about half the length of one side of the aforementioned entire square shape. 
     In a case where the power transmission element  21  and the power receiving element  11  are configured as such, they are functioned, for example, as illustrated in  FIG. 7 . That is to say, with the movement of a mobile object  10 , the power receiving elements  11   a ,  11   b , arranged side by side, of the power receiving equipment move in a direction from +X toward −X so as to become superposed on the power transmission elements  21   a ,  21   b  of the power transmission equipment, thereby approaching the power transmission element  21 . And, firstly, the magnets  18   d ,  18   e  provided around the power receiving element  11   b , which is one of the power receiving elements, of the mobile object  10  are coupled to the magnets  33   d ,  33   e  on the power transmission element  21  side, respectively (at this time, since the magnets  18   d ,  18   e  have the same polarity as that of the magnets  33   a ,  33   h , the magnets  18   d ,  18   e  are not coupled thereto). Thereby, the shield  31  starts to slide with the movement of the mobile object  10 . Then, at the position at which the power receiving element  11   a , which is the other of the power receiving elements, is superposed right over the other power transmission element  21   a , the shielding effect for the electromagnetic wave by the shield  31  is released at the highest possible degree. As such, according to the present configuration, wireless power transfer can efficiently be performed from the power transmission element  21  to the power receiving element  11 . 
     Note that the shapes and sizes of the power receiving element  11  and the power transmission element  21  are not limited to those illustrated in the drawing. For example, the power transmission elements  21  having radiating surfaces of the same shape and size may be provided to the shields  31   a - 31   d , respectively (i.e., a total of four of the power transmission elements  21  are provided). With such a configuration, in both the X-axis direction and the Y-axis direction, the operation and effect similar to those in the above description can be achieved. Further, in this third embodiment, the numbers, shapes, sizes, and arrangements of the magnet  18  and the magnet  33  are not limited to those illustrated in the drawing. 
     Fourth Embodiment 
     A fourth embodiment is another configuration of a wireless power transfer system  1  which is configured such that shielding of a power transmission element  21  can be released even in cases where a mobile object  10  approaches a power transmission element  21  from various directions, similarly to the second embodiment. 
       FIG. 8  is a diagram illustrating a schematic configuration of the wireless power transfer system  1  which will be described as the fourth embodiment, and is the diagram of a configuration, seen from above, around the power transmission element  21  and a shield  31 . 
     As illustrated in the drawing, the shield  31  in this wireless power transfer system  1  is in a substantially circular shape, and a plurality of magnets  33  are provided along the periphery thereof. In this example, four magnets  33   a - 33   d  are provided at locations, in the periphery of the shield  31 , at the same distance from the center of the shield  31  (the center of the circle) toward four directions in straight lines. An urging mechanism not shown is provided to the shield  31 , the shield  31  is configured to be urged in a direction in which the electromagnetic wave radiated from the power transmission element  21  is shielded at the highest possible degree (a direction in which the center of the shield  31  is directed to the center of the power transmission element  21 ). 
       FIG. 9  is a diagram of a configuration, when seen from above the mobile object  10 , around a power receiving element  11  in the wireless power transfer system  1  illustrated as the fourth embodiment. 
     As illustrated in the drawing, a plurality of the power receiving elements  11  are provided around a magnet  18  of the mobile object  10 . In this example, four power receiving elements  11   a - 11   d  are provided at locations at the same distance from the center of the magnet  18  toward four directions in straight lines (the distances between the center of the magnet  18  and the centers of the four power receiving elements  11   a - 11   d  are the same). Further, the distances between the center of the magnet  18  and the centers of the four power receiving elements  11   a - 11   d  are equal to or smaller than the maximum diameter of the electromagnetic-wave radiating surface in the power transmission element  21 . 
     As illustrated in  FIG. 10 , when the mobile object  10  approaches the power transmission element  21 , the magnet  18  provided to the mobile object  10  and the magnet  33   a  provided to the shield  31  are coupled to each other, to couple the shield  31  with the mobile object  10 , and thereafter, the shield  31  starts to slide with the movement of the mobile object  10 , thereby releasing shielding of the electromagnetic wave near the power receiving element  11   a , and wireless power transfer is performed from the power transmission element  21  to the power receiving element  11   a . Note that, although received power is smaller than that in the power receiving element  11   a , the power receiving elements  11   b ,  11   d  can also receive wireless power transfer from the power transmission element  21 , since parts of the power receiving surfaces thereof are overlapped with the radiating surface of the power transmission element  21 . 
     As described above, with the wireless power transfer system  1  according to the fourth embodiment, shielding of the electromagnetic wave by the shield  31  can automatically be released, in cases where the mobile object  10  approaches the power transmission element  21  from various directions. Thus, it is possible to reliably shield the electromagnetic wave at normal times, and reliably perform wireless power transfer when the mobile object  10  approaches the power transmission element  21 . 
     Note that the shapes and sizes of the power receiving element  11  and the power transmission element  21  are not limited to those illustrated in the drawing. Further, the numbers, the shapes, the sizes, and the arrangements of the magnet  18  and the magnets  33  are not limited to those illustrated in the drawing. For example, as illustrated in  FIG. 11 , a shield  31  may have a rectangular shape. Further, in this case, as illustrated in the drawing, magnets  33  may be arranged along the periphery of the shield  31  so as to form a rectangular shape as a whole. Further, in this case, for example, as illustrated in  FIG. 12 , a plurality of power receiving elements  11   a - 11   h  may be arranged around a magnet  18  so as to forma rectangular shape as a whole. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  wireless power transfer system,  5  movement surface, mobile object,  11  power receiving element,  12  charging circuit,  13  secondary battery,  14  load,  18  magnet,  21  power transmission element,  22  power transmission supply device,  31  shield,  32  slide mechanism,  33  magnet 
           
         
       
    
     Although the disclosure has been described with respect only to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.