Patent Publication Number: US-2022239162-A1

Title: Power transmission device, power reception device, wireless power transmission system, and method for driving power transmission device

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 17/161,413, filed Jan. 28, 2021, which is a U.S. Continuation of International Patent Application No. PCT/JP2019/028604, filed on Jul. 22, 2019, which in turn claims the benefit of Japanese Application No. 2018-143147, filed on Jul. 31, 2018, the entire disclosures of which applications are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a power transmission device that transmits power wirelessly, a power reception device that receives power wirelessly, and a wireless power transmission system including both the power transmission device and the power reception device. 
     2. Description of the Related Art 
     In a wireless power transmission system, there is a problem that if power is transmitted in a non-contact manner in a state where a metal foreign substance exists between a power transmission coil and a power reception coil, the metal foreign substance generates heat and causes a danger. Therefore, as shown in  FIG. 17 , in Patent Literature (PTL) 1 (Unexamined Japanese Patent Publication No. 2016-59236), metal foreign substance A on a surface of a power transmission device is dropped from the surface of the power transmission device to a peripheral portion of the power transmission device by a wiper so as to remove metal foreign substance A from an area between a power transmission coil (not shown) and power reception coil (not shown). 
     SUMMARY 
     However, in recent years, there has been an increasing demand for non-contact and short-time charging of electric vehicles (EVs), and non-contact and high-output power transmission is desired (for example, 120 kW or more). In such a case, leakage flux due to a power transmission coil becomes large, and a metal foreign substance may generate heat even in a peripheral portion of a power transmission device (outside of the power transmission coil) in which the metal foreign substance is dropped or moved by a wiper. 
     In order to solve the above problem, a power transmission device according to one aspect of the present disclosure includes a power transmission coil, a magnetically shielded space created by a power transmission-side cancel coil arranged outside the power transmission coil, a moving member configured to move a metal foreign substance, and a moving mechanism configured to move a part or all of an upper surface of the moving member from an area outside the magnetically shielded space into the magnetically shielded space. 
     The comprehensive or specific aspect described above may be implemented with a system, method, integrated circuit, computer program, or recording medium. Alternatively, the aspect may be implemented with any combination of the system, the device, the method, the integrated circuit, the computer program, and the recording medium. 
     According to one aspect of the present disclosure, it is possible to suppress heat generation of a metal foreign substance existing in a peripheral portion of the power transmission device even if high-output power transmission is performed in a non-contact manner. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a wireless power transmission system according to a first exemplary embodiment of the present disclosure. 
         FIG. 2  is a side view of the wireless power transmission system according to the first exemplary embodiment of the present disclosure. 
         FIG. 3  is a schematic view of a power transmission device according to the first exemplary embodiment of the present disclosure. 
         FIG. 4  is a diagram showing a modified example of a moving member according to the first exemplary embodiment of the present disclosure. 
         FIG. 5  is a flowchart showing an operation of the power transmission device according to the first exemplary embodiment of the present disclosure. 
         FIG. 6  is a schematic side view of a power transmission device according to a second exemplary embodiment of the present disclosure. 
         FIG. 7  is a flowchart showing an operation of the second exemplary embodiment of the present disclosure. 
         FIG. 8  is a schematic side view showing a modified example of the power transmission device according to the second exemplary embodiment of the present disclosure. 
         FIG. 9  is a schematic side view of a wireless power transmission system according to a third exemplary embodiment of the present disclosure. 
         FIG. 10  is a block diagram showing an example of a configuration of the wireless power transmission system according to the third exemplary embodiment of the present disclosure. 
         FIG. 11  is a schematic side view of a modified example of the wireless power transmission system according to the third exemplary embodiment of the present disclosure. 
         FIG. 12  is a schematic side view of a power transmission device according to a fourth exemplary embodiment of the present disclosure. 
         FIG. 13  is a flowchart showing an operation of the fourth exemplary embodiment of the present disclosure. 
         FIG. 14A  is a schematic configuration diagram of a power transmission-side cancel coil according to a fifth exemplary embodiment of the present disclosure. 
         FIG. 14B  is a circuit diagram of the power transmission-side cancel coil according to the fifth exemplary embodiment of the present disclosure. 
         FIG. 15  is a diagram showing an example of a negative resistance circuit according to the fifth exemplary embodiment of the present disclosure. 
         FIG. 16  is a diagram showing an example of a magnetically shielded space according to a sixth exemplary embodiment of the present disclosure. 
         FIG. 17  is a schematic configuration diagram of a power transmission device showing a conventional technique. 
     
    
    
     DETAILED DESCRIPTION 
     Findings Underlying the Present Disclosure 
     Before explaining exemplary embodiments of the present disclosure, findings underlying the present disclosure will be described. 
       FIG. 1  is a diagram schematically showing an example of wireless power transmission system  11  that wirelessly supplies power to mobile body  200 . In wireless power transmission system  11  here, power transmission coil  110  arranged along a road surface wirelessly transmits power to power reception coil  215  arranged on a bottom surface of mobile body  200 . Mobile body  200  is a vehicle driven by an electric motor in this example. Mobile body  200  may be, for example, a vehicle such as a bus, an automobile, a train, or an automatic guided vehicle (AGV), but may be a movable object other than the vehicle. 
       FIG. 1  shows XYZ coordinates indicating X, Y, and Z directions that are orthogonal to each other. In the following description, the XYZ coordinates that are shown are used. A traveling direction of mobile body  200  is the Y direction, a direction perpendicular to the road surface is the Z direction, and a direction perpendicular to both the Y direction and the Z direction is the X direction. Orientations of structures shown in the drawings of the present application are set in consideration of easy-to-understand explanation, and do not limit orientations when the exemplary embodiments of the present disclosure are actually implemented. Also, shapes and sizes of all or a part of the structures shown in the drawings do not limit actual shapes and sizes. 
     Wireless power transmission system  11  includes power transmission device  101  and power reception device  210 . Power transmission device  101  outputs power supplied from external power supply  300  from power transmission coil  110  to power reception coil  215 . Power reception device  210  is provided on mobile body  200 . In addition to power reception coil  215 , power reception device  210  includes components such as a rectifier circuit and a power reception control circuit, which are not shown. 
     In such a system, if metal foreign substance  400  is present directly above or in the vicinity of power transmission coil  110 , metal foreign substance  400  is heated during power transmission, which may cause a safety problem. Therefore, a technique for detecting such a metal foreign substance during power transmission and removing the metal foreign substance has been proposed so far. 
     For example, PTL 1 discloses a device that removes a metal foreign substance existing on the upper surface of a power transmission device by using a member such as a foreign substance removal plate or a brush. The foreign substance removal plate is a member similar to an automobile wiper. For reference, a part of FIG. 1 of PTL 1 is cited as  FIG. 17 . 
     The method disclosed in PTL 1 can certainly remove a foreign substance existing on the upper surface of a power transmission device, but some foreign substances may not be removed. A foreign substance that enters the upper surface of a power transmission coil includes, for example, a coin containing metals such as copper, zinc, or nickel, or a metal foreign substance such as a steel can or aluminum can, as well as a nonmetal foreign substance such as soil or mud, or an animal such as an insect or cat. Some foreign substances may slip between the upper surface (i.e., flat surface) of the power transmission device and the foreign substance removal member, and remain unremoved. As described above, it is difficult to reliably remove a metal foreign substance by a method of removing a metal foreign substance by some means. 
     In addition, when it clears up after rain, for example, soil and a metal foreign substance are mixed together and the soil dries. In such a case, the metal foreign substance is fixed with the soil on the upper surface of the power transmission device, and it is difficult to reliably remove the metal foreign substance. 
     The present inventors have found the above problems and examined a configuration for solving these problems. The present inventors have come up with an idea that heat generation of a metal foreign substance can be prevented by moving a moving member on which the metal foreign substance is placed to an outside of a power transmission-side cancel coil, instead of removing the metal foreign substance. 
     Further, as described above, the present inventors have come up with an idea that even when high output transmission (for example, 120 kW or more) is performed from the power transmission device in a non-contact manner, it is possible to prevent heat generation of a metal foreign substance existing in a peripheral portion of the power transmission device by changing a relative position between the moving member and the power transmission-side cancel coil. 
     A power transmission device according to one aspect of the present disclosure includes a power transmission coil, a magnetically shielded space created by a power transmission-side cancel coil arranged outside the power transmission coil, a moving member configured to move a metal foreign substance, and a moving mechanism configured to move a part or all of an upper surface of the moving member from an area outside the magnetically shielded space into the magnetically shielded space. 
     According to the above aspect, it is possible to suppress heat generation of a metal foreign substance existing in a peripheral portion of the power transmission device even if high output power transmission is performed in a non-contact manner. 
     A power transmission device according to another aspect of the present disclosure includes a power transmission coil, a power transmission-side cancel coil that is arranged outside the power transmission coil and generates a magnetic field opposite to a magnetic field generated by the power transmission coil, a housing provided with the power transmission coil and the power transmission-side cancel coil inside, an opening provided on a surface of the housing above the power transmission coil, a moving member that covers the opening and allows a metal foreign substance to be placed on top, a moving mechanism that moves a part or all of an upper surface of the moving member to an outside of the power transmission-side cancel coil, and a belt that is arranged in the housing and conveys the metal foreign substance that has entered through the opening to an outside of the power transmission-side cancel coil. 
     According to the above aspect, even if a metal foreign substance intrudes during non-contact power transmission, an operation can be continuously and safely performed. 
     A power reception device according to another aspect of the present disclosure is a power reception device arranged to face a power transmission device having a power transmission coil, and a moving mechanism that moves a part or all of an upper surface of a moving member on which a metal foreign substance can be placed to an outside of the power transmission coil. The power reception device includes a power reception coil that couples with a magnetic field generated by the power transmission coil, and a power reception-side cancel coil that is arranged outside the power reception coil and generates a magnetic field opposite to a magnetic field generated by the power reception coil. 
     According to the above aspect, it is possible to suppress heat generation of a metal foreign substance existing in a peripheral portion of the power transmission device even if high output power transmission is performed in a non-contact manner. 
     First Exemplary Embodiment 
     Hereinafter, a more specific exemplary embodiment of the present disclosure will be described. However, a more detailed description than necessary may be omitted. For example, the detailed description of already well-known matters and the overlap description of substantially same configurations may be omitted. This is to avoid an unnecessarily redundant description below and to facilitate understanding of a person skilled in the art. It should be noted that the inventors provide the accompanying drawings and the following description in order for those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims. In the following description, the same or similar components are designated by the same reference numerals. 
     [Basic Configuration] 
     As shown in  FIG. 1 , power transmission device  101  is electrically connected to external power supply  300  via a cable or the like. Power transmission device  101  includes power transmission coil  110  inside housing  580 . Foreign substances of metal foreign substance  400  and nonmetal foreign substance  410  may be present on power transmission coil  110  in power transmission device  101 . 
       FIG. 2  is a schematic diagram showing a situation in which power transmission coil  110  and power reception coil  215  face each other, and power is transmitted from power transmission coil  110  to power reception coil  215  in a non-contact manner. 
     As shown in  FIG. 2 , power transmission coil  110  electromagnetically (or magnetically) couples with power reception coil  215  to output power to power reception coil  215 . Power reception coil  215  magnetically couples with power transmission coil  110  by a magnetic field generated from power transmission coil  110  to receive at least a part of the transmitted power (i.e., energy). Power reception coil  215  supplies the received power to a load (such as a secondary battery) in mobile body  200  via a rectifier circuit, which is not shown. As a result, mobile body  200  is charged and supplied with power. 
       FIG. 3  is a diagram schematically showing power transmission device  101 . Part (a) of  FIG. 3  shows a YZ cross section of power transmission device  101  (before moving member  510  is moved), part (b) of  FIG. 3  shows a plan view of power transmission device  101  (after moving member  510  is moved), and part (c) of  FIG. 3  shows the YZ cross section of power transmission device  101  (after moving member  510  is moved). Power transmission device  101  here includes housing  580 , moving member  510 , power transmission coil  110  arranged inside housing  580 , power transmission circuit  120 , and power transmission-side cancel coil  130 . 
     Power transmission-side cancel coil  130  is arranged outside power transmission coil  110 , and generates a magnetic field opposite to a magnetic field generated by power transmission coil  110  by induced electromotive force. 
     When power transmission coil  110  that generates a magnetic field is present inside power transmission-side cancel coil  130 , a magnetically shielded space is a space in which magnetic field strength outside the power transmission-side cancel coil is smaller than the magnetic field strength inside. In a first exemplary embodiment, as shown in  FIG. 3 , power transmission-side cancel coil  130  is formed in a concentric circle having a radius larger than that of power transmission coil  110 , and is arranged outside in a radial direction of power transmission coil  110 . However, as long as a magnetically shielded space can be formed, power transmission-side cancel coil  130  does not have to be a strictly concentric circle, and does not have to be strictly radial outside. 
     As shown in part (b) of  FIG. 3 , power transmission-side cancel coil  130  preferably has a shape such as a circle, an ellipse, or a rectangle that surrounds power transmission coil  110 . Power transmission-side cancel coil  130  is preferably a conductor such as a wire or plate made of copper, aluminum, enamel, or the like, and is a loop coil whose both ends are short-circuited. In the case of a wire, the wire may be wound once or multiple times. In the case of a plate, the plate shape may be stacked in a plurality of layers. 
     As shown in part (a) of  FIG. 3 , moving member  510  is arranged on a surface of housing  580 , and allows metal foreign substance  400  to be placed on top. It is preferable that stopper  610  is arranged on a side opposite to a traveling direction (right direction) of moving member  510  such that metal foreign substance  400  does not slip off. 
     Since metal foreign substance  400  enters the surface of housing  580 , the surface of moving member  510  is preferably larger than that of power transmission-side cancel coil  130  and housing  580  such that metal foreign substance  400  is not placed on housing  580 . As shown in  FIG. 4 , when there is a folded portion (folded portion  511 ) at both ends of moving member  510 , metal foreign substance  400  may not be able to enter the folded portion. In this case, metal foreign substance  400  can be placed on a part of an upper surface of moving member  510 . Metal foreign substance  400  can be placed on a part or all of the upper surface of moving member  510 . 
     As shown in part (c) of  FIG. 3 , a part or all of the upper surface of moving member  510  on which metal foreign substance  400  can be placed is moved to an outside (magnetically shielded space) of power transmission-side cancel coil  130  by moving mechanism  530  shown in  FIG. 10 . Therefore, moving mechanism  530  can move metal foreign substance  400  placed on moving member  510  to the magnetically shielded space and prevent heat generation of metal foreign substance  400 . 
     In addition, although moving member  510  is moved to a right side in part (c) of  FIG. 3 , moving member  510  may be moved to a left side. Further, moving member  510  may be divided into two at substantially a center and moved from a central portion by opening both left and right (double doors). 
     [Operation] 
       FIG. 5  is a flowchart showing a basic operation of the power transmission device. 
     First, moving mechanism  530  is operated to move a part or all of the upper surface of moving member  510  to the outside of power transmission-side cancel coil  130  (S 101 ). It is preferable that mobile body  200  covers power transmission device  101  as shown in  FIG. 2  before moving member  510  moves (the state shown in part (a) of  FIG. 3 ). 
     Next, power is output from power transmission coil  110  to the outside of power transmission coil (for example, power reception coil  215 ) (S 102 ). 
     The details of step S 102  will be described. As shown in  FIG. 3 , power transmission circuit  120  converts the power supplied from external power supply  300  into alternating current (AC) power having a frequency and voltage suitable for power transmission, and outputs the power. Power transmission coil  110  is connected to power transmission circuit  120  and transmits the AC power supplied from power transmission circuit  120  to power reception coil  215 . Power transmission circuit  120  includes components such as an inverter circuit and a power transmission control circuit, which are not shown in  FIG. 3 . As a result, it is possible to prevent heat generation of metal foreign substance  400  placed on a part or all of the upper surface of moving member  510 , and it is possible to safely output power in a non-contact manner. 
     Second Exemplary Embodiment 
     The difference from the first exemplary embodiment is that, as shown in  FIG. 6 , after moving member  510  has moved (after a metal foreign substance has entered the magnetically shielded space), while moving member  510  is moving metal foreign substance  400 , moving member  510  moves metal foreign substance  400  to the outside of power transmission-side cancel coil  130  on a side opposite to a moving direction so as to remove metal foreign substance  400 . 
     Part (a) of  FIG. 6  is a diagram corresponding to part (c) of  FIG. 3 . Part (b) of  FIG. 6  is a diagram in a case where metal foreign substance  400  enters a top of housing  580  inside power transmission-side cancel coil  130  when power is output from power transmission coil  110  in a non-contact manner. In this case, an eddy current is generated in metal foreign substance  400  and heat is generated. 
     Therefore, as shown in part (c) of  FIG. 6 , moving member  510  is returned so as to move metal foreign substance  400  to the outside of power transmission-side cancel coil  130  on the opposite side of the moving direction. 
     As for the timing of returning moving member  510 , moving member  510  may be returned at regular intervals. Alternatively moving member  510  may be returned through detection of metal foreign substance  400  by a sensor such as a camera or a temperature sensor. Further, after metal foreign substance  400  is detected, the output of power from power transmission coil  110  to the outside of the power transmission coil (for example, power reception coil  215 ) may be reduced or stopped, and then moving member  510  may be returned. As a result, heat generation of metal foreign substance  400  can be prevented. 
     As shown in  FIG. 6 , metal foreign substance  400  is preferably stored in storage container  600  arranged on a side surface of power transmission device  102  or the like. 
     In order to output power in a non-contact manner again, as shown in part (a) of  FIG. 6 , a part or all of the upper surface of moving member  510  is moved to the outside of power transmission-side cancel coil  130 , and power is output from power transmission coil  110  in a non-contact manner. By repeating parts (a) to (c) in  FIG. 6 , power can be continuously and safely output from power transmission coil  110  in a non-contact manner. 
     [Operation] 
       FIG. 7  is a flowchart showing an operation of the power transmission device shown in  FIG. 6 . 
     First, a moving mechanism ( FIG. 10 ) not shown in  FIG. 3  is operated to move a part or all of the upper surface of the moving member to the outside of the power transmission-side cancel coil (S 201 ). It is preferable that mobile body  200  covers the power transmission device before the moving member moves. 
     Next, power is output from the power transmission coil to the outside of the power transmission coil (for example, the power reception coil) (S 202 ). 
     Next, when a metal foreign substance enters the upper surface of the housing, the power output from the power transmission coil is reduced or stopped (S 203 ). 
     Next, the moving member is returned, and the metal foreign substance is moved to the outside of the power transmission-side cancel coil on the opposite side of the moving direction (S 204 ). 
     Whether to continue the power output from the power transmission coil is determined (S 205 ). If the power output is continued (Yes), the process returns to step S 201 . If the power output is not continued (No), the process ends. 
     As shown in  FIG. 8 , power transmission device  102  may be provided with removal member  620  for metal foreign substance  400  in which an end portion of the moving member on a side opposite to the moving direction is in contact with a surface of housing  580 . In that case, there may be a gap between the surface of housing  580  and moving member  510 . As a result, a contact area between the surface of housing  580  and moving member  510  is reduced, so that a load on the moving mechanism can be reduced. 
     Removal member  620  is preferably in the form of a plate or a brush, and the material is preferably a metal such as stainless steel or aluminum, resin, wood, rubber, cloth, thread or the like. 
     Stopper  610  may be arranged at both end portions of moving member  510 . 
     Third Exemplary Embodiment 
     The difference from the first and second exemplary embodiments is that power reception device  210  is arranged so as to face power transmission device  103 , as shown in  FIG. 9 . 
       FIG. 9  shows wireless power transmission system  13  that outputs power from a power transmission device to power reception device  210  in a non-contact manner. Power transmission device  103  shown in  FIG. 9  differs from power transmission device  101  of the first exemplary embodiment in that power transmission device  103  does not have power transmission-side cancel coil  130 . Part (a) of  FIG. 9  shows a state before moving member  510  is moved, and part (b) of  FIG. 9  shows a state after moving member  510  is moved. As shown in  FIG. 9 , power transmission coil  110  included in power transmission device  103  and power reception coil  215  included in power reception device  210  face each other, and power transmission coil  110  and power reception coil  215  are electromagnetically coupled. 
     As shown in  FIG. 9 , power reception-side cancel coil  220  may be arranged outside power reception coil  215  and may generate a magnetic field opposite to a magnetic field generated by power reception coil  215  by induced electromotive force. 
     Power reception-side cancel coil  220  can weaken the magnetic field generated by power reception coil  215  in a space outside power reception-side cancel coil  220 , and creates a magnetically shielded space. In a third exemplary embodiment, power reception-side cancel coil  220  is formed in a concentric circle having a radius larger than that of power reception coil  215  and is arranged outside in a radial direction of power reception coil  215 . 
     However, as long as a magnetically shielded space can be formed, power reception-side cancel coil  220  does not have to be a strictly concentric circle, and does not have to be strictly radial outside. 
     Power transmission device  103  makes an adjustment by moving moving member  510  such that a part or all of the upper surface of moving member  510  on which metal foreign substance  400  can be placed is located outside power reception-side cancel coil  220 . As a result, it is possible to prevent heat generation of metal foreign substance  400  placed on moving member  510 . The method for removing a metal foreign substance described in the second exemplary embodiment can also be applied. 
       FIG. 10  is a block diagram showing an example of a configuration of wireless power transmission system  13  of  FIG. 9 . Power transmission device  103  includes above-mentioned power transmission coil  110 , power transmission circuit  120 , position sensor  140 , moving member  510 , moving mechanism  530 , and communication circuit  170 . Power transmission circuit  120  includes inverter circuit  160  and power transmission control circuit  150 . Inverter circuit  160  is connected between external power supply  300  and power transmission coil  110 . 
     Inverter circuit  160  converts direct current (DC) power supplied from power supply  300  into AC power, and supplies the AC power to power transmission coil  110 . Power transmission control circuit  150  controls inverter circuit  160 , communication circuit  170 , position sensor  140 , and moving mechanism  530 . Power transmission control circuit  150  controls conduction/non-conduction of a plurality of switching elements in inverter circuit  160 , for example, to output AC power having a desired frequency and voltage. Power transmission control circuit  150  further controls moving mechanism  530  to change the position of moving member  510 . 
     Communication circuit  170  transmits and receives signals to and from communication circuit  270  in mobile body  200 . Position sensor  140  plays a role of measuring a relative position between power reception device  210  (mobile body  200 ) and power transmission device  103 . 
     Moving mechanism  530  may change a relative position between housing  580  and moving member  510 . Moving mechanism  530  may employ for example, a mechanical type in which, for example, a part of mobile body  200  comes into contact to push moving member  510 , an electric type such as a linear motor, a combination of the mechanical type and the electric type such as a linear motion mechanism provided with an electric motor and a plurality of gears (including a rack and pinion), or the like. 
     Mobile body  200  (power reception device  210 ) includes power reception coil  215 , power reception-side cancel coil  220 , rectifier circuit  225 , power reception control circuit  230 , secondary battery  240 , communication circuit  270 , electric motor  260 , and motor inverter  250 . Rectifier circuit  225  is connected to power reception coil  215 , converts AC power output from power reception coil  215  into DC power, and outputs the DC power. 
     Electric motor  260  is a motor for driving mobile body  200 , and is driven by, for example, three-phase AC power. Motor inverter  250  converts supplied DC power into three-phase AC power, and supplies the three-phase AC power to electric motor  260 . Power reception control circuit  230  controls secondary battery  240  to be charged by DC power output from rectifier circuit  225 , and controls motor inverter  250  and communication circuit  270 . 
     For example, mobile body  200  in the present exemplary embodiment approaches power transmission device  103  for charging when a storage amount of secondary battery  240  becomes low. 
     Power transmission control circuit  150  drives inverter circuit  160  to start power transmission. Power transmitted by a magnetic field coupling between power transmission coil  110  and power reception coil  215  is stored in secondary battery  240 . When the charging of secondary battery  240  is completed, mobile body  200  drives electric motor  260  by the power stored in secondary battery  240  and resumes traveling. 
     Position sensor  140  measures a relative position between power transmission device  103  and mobile body  200  by using, for example, light, radio waves, pressure, sound waves, and the like. Position sensor  140  may be, for example, a normal image sensor or a distance measuring device such as a time of flight (TOF) sensor. Position sensor  140  detects the position of mobile body  200  with respect to power transmission device  103 . Based on information output from position sensor  140 , power transmission control circuit  150  can grasp the relative positional relationship between mobile body  200  and power transmission device  103  (for example, the distance between mobile body  200  and power transmission device  103 ). When mobile body  200  covers moving member  510 , mobile body  200  serves as a barrier, so that metal foreign substance  400  is unlikely to come into contact with moving member  510 . Therefore, when mobile body  200  covers moving member  510 , it is preferable that the power transmission control circuit starts moving member  510 . 
       FIG. 11  shows wireless power transmission system  11  which is a modified example of  FIG. 9 . Power transmission device  103  shown in  FIG. 9  applies to a case where power transmission-side cancel coil  130  as shown in  FIG. 3  is not arranged. Power transmission device  101  shown in  FIG. 11  is the same as power transmission device  101  of  FIG. 3 , and power transmission-side cancel coil  130  is arranged. 
     Fourth Exemplary Embodiment 
       FIG. 12  is a cross-sectional view schematically showing a configuration of a power transmission device according to a fourth exemplary embodiment. 
     Power transmission device  104  in the present exemplary embodiment is different from power transmission device  101  in the first exemplary embodiment in that a belt conveyor is provided in housing  580 . Other than that, the configuration is basically the same as the configuration of power transmission device  101  in the first exemplary embodiment. 
     In power transmission device  104  of the present exemplary embodiment, after moving member  510  is moved from an initial position (the state in which moving member  510  covers opening  515 ), when a foreign substance (for example, metal foreign substance  400 ) passes through opening  515  and enters an inside of housing  580  during power supply, the foreign substance is removed by the belt conveyor. 
     The belt conveyor includes belt  570 , roller  540 , drive roller  520 , drive motor  525 , and blade  560 . 
     Power transmission circuit case  180  is a case for protecting power transmission coil  110 . Power transmission circuit case  180  may include power transmission circuit  120 . If it is not necessary to protect power transmission coil  110 , power transmission circuit case  180  may not be provided. 
     Belt  570  is a flexible strip-shaped member, which is arranged so as to surround power transmission circuit case  180 . Belt  570  is held by roller  540  and drive roller  520  arranged on both sides of power transmission circuit case  180 . Drive roller  520  is connected to drive motor  525 . In the present exemplary embodiment, a rotation direction of belt  570  is clockwise. Drive motor  525  rotates drive roller  520  in response to an instruction from power transmission control circuit  150 . 
     Blade  560  is arranged close to an outer circumference of drive roller  520 . Blade  560  is fixed to, for example, housing  580 . It is preferable that blade  560  and belt  570  on the outer circumferential portion of drive roller  520  are in contact with each other. There may be a slight gap between blade  560  and belt  570  as long as a foreign substance can be removed. Blade  560  is installed so as to drop a foreign substance that moves with the rotation of belt  570 . The foreign substance dropped by blade  560  is stored in storage container  590  included in power transmission device  104 . 
     Belt  570  is arranged in housing  580  and rotates to convey metal foreign substance  400  that has entered through opening  515  to the outside of power transmission-side cancel coil  130 . Belt  570  may be made of, for example, a material that is less likely to generate heat due to power output from power transmission coil  110 . For example, it is desirable that belt  570  is made of a non-metal such as resin, rubber, or cloth, or a material containing a non-magnetic material. Belt  570  is held between drive roller  520  and roller  540  and is rotated by drive motor  525 . When moving member  510  moves from opening  515  and opening  515  is open, belt  570  moves the foreign substance that has entered through opening  515  to the position of storage container  590 , and stores the foreign substance. 
     Blade  560  may have a function of removing metal foreign substance  400  from belt  570 , and the material and shape are not particularly limited. Blade  560  can be, for example, an aluminum or stainless steel plate, or a rubber member such as a wiper used in an automobile windshield. In the cross-sectional view shown in  FIG. 12 , when considering a coordinate plane with the position of the rotation axis of drive roller  520  as the origin, blade  560  is preferably attached in the fourth quadrant of the coordinate plane. If the direction of rotation of drive roller  520  (moving direction of belt  570 ) is opposite to the direction shown in  FIG. 12 , blade  560  is preferably attached in the third quadrant in the coordinate plane with the position of the rotation axis of the roller  540  as the origin. That is, blade  560  is preferably provided in the vicinity of a portion of belt  570  in contact with drive roller  520  or roller  540 , which is located below the rotation axis of each roller. 
     Moving member  510  in the present exemplary embodiment may be arranged so as to close opening  515  and may have a structure for preventing metal foreign substance  400  from entering housing  580 , and the material is not particularly limited. 
     A weight sensor may be arranged under storage container  590 . By providing the weight sensor, the weight of a foreign substance stored in storage container  590  can be measured. When the total weight of foreign substances measured by the weight sensor exceeds a threshold value, power transmission control circuit  150  may transmit a signal indicating the total weight exceeding the threshold value to another device (for example, a smartphone or a server computer) via communication circuit  170 . This allows, for example, an administrator to dispatch a cleaner to remove the foreign substances in storage container  590  or replace storage container  590  with new storage container  590 . 
     As shown in  FIG. 12 , brush  550  is arranged upstream of power transmission coil  110  to prevent a foreign substance from entering from a counterclockwise direction opposite to the rotation direction of belt  570 . 
     Although moving member  510  is moved to the right in  FIG. 12 , moving member  510  may be moved to the left. Further, moving member  510  may be divided into two at substantially a center and moved from a central portion by opening both left and right (double doors). 
     [Operation] 
       FIG. 13  is a flowchart showing an operation of  FIG. 12 . 
     First, belt  570  is rotated before moving member  510  opens opening  515  (S 301 ). 
     Next, moving mechanism  530 , which is not shown in  FIG. 12 , is operated to move a part or all of the upper surface of moving member  510  to the outside of power transmission-side cancel coil  130  (S 302 ). It is preferable that mobile body  200  covers power transmission device  104  before moving member  510  moves. 
     Next, power is output from power transmission coil  110  to the outside of the power transmission coil (for example, power reception coil  215 ) (S 303 ). 
     Next, when metal foreign substance  400  enters the housing, metal foreign substance  400  is conveyed by belt  570  and metal foreign substance  400  is removed by blade  560  (S 304 ). 
     Next, when a signal for stopping the power is received, for example, when the charging of the secondary battery is completed, the power is stopped and moving member  510  is returned to its original position (S 305 ). 
     Next, belt  570  is stopped (S 306 ). 
     In step S 301 , belt  570  may be rotated after moving member  510  opens opening  515 . 
     By the above operation, metal foreign substance  400  can be removed without being detected by rotating belt  570  while the power is being output from power transmission coil  110  to the outside of the power transmission coil. 
     The solutions and methods in the first to third exemplary embodiments can be applied to the present exemplary embodiment as long as there is no contradiction. 
     Fifth Exemplary Embodiment 
     The difference from the first to fourth embodiments is that power transmission-side cancel coil  730  is connected to power supply circuit  710  as shown in  FIG. 14A . 
     In order to explain an operation of this exemplary embodiment, first, an operation of power transmission-side cancel coil  130  composed of a loop coil with both ends short-circuited, as in the first, second, and fourth exemplary embodiments, will be described.  FIG. 14B  is a diagram showing an equivalent circuit when both ends of power transmission-side cancel coil  130  are short-circuited. Note that in principle, the operation of power transmission-side cancel coil  130  described here and the operation of power reception-side cancel coil  220  are exactly the same. Therefore, when the common operation is explained below, power transmission-side cancel coil  130  is used for the explanation. As shown in  FIG. 14B , power transmission-side cancel coil  130  is equivalently composed of a series circuit of inductance L 1  and DC internal resistance R 4 . 
     Induced electromotive voltage Vc 740  is generated in power transmission-side cancel coil  130  by mutual induction with power transmission coil  110  and/or power reception coil  215 . Then, current Ic flows due to induced electromotive voltage Vc 740 , but since the DC circuit has inductance L 1  and internal resistance R 4  as impedance, voltage of self-induced electromotive voltage Vs=L 1 ×dIc/dt+R 4 ×Ic is generated. In order for power transmission-side cancel coil  130  to operate ideally and total magnetic flux generated inside and outside power transmission-side cancel coil  130  to be zero, mutual induction electromotive voltage Vc due to an external magnetic field and self-induced electromotive voltage Vs due to current Ic flowing through power transmission-side cancel coil  130  need to have the same magnitude in opposite phases and be balanced. That is, in  FIG. 14B , when virtual voltage measurement point  760  is provided between inductance L 1  and internal resistance R 4 , a potential difference between first end point  770  of power transmission-side cancel coil  130  and virtual voltage measurement point  760  is required to be zero. 
     However, there is internal resistance R 4  in parallel between these two points, and as long as self-induced electromotive force is generated in power transmission-side cancel coil  130 , current Ic that does not become zero flows. Therefore, there is always a potential difference corresponding to R 4 ×Ic between first end point  770  of power transmission-side cancel coil  130  and virtual voltage measurement point  760 . Therefore, in order for power transmission-side cancel coil  130  to operate well, it is better to make a value of internal resistance R 4  as small as possible and bring a potential of virtual voltage measurement point  760  as close to zero as possible. For this reason, in power transmission-side cancel coil  130  using a loop coil with both ends short-circuited, the electric conductivity of the coil material is preferably made as large as possible (ideally infinite using a superconducting material) so as to secure a coil cross-sectional area that is as large as possible. 
     However, when a material having finite electric conductivity is used as the material of power transmission-side cancel coil  130 , internal resistance R 4  cannot be made zero. Therefore, in a fifth exemplary embodiment, supplying a predetermined current from power supply circuit  710  located outside to power transmission-side cancel coil  730  is considered so as to actively control the potential of virtual voltage measurement point  760  to zero. Power transmission-side cancel coil  730  includes at least a loop coil connected to power supply circuit  710  capable of supplying a DC and/or AC. Specifically the potential of virtual voltage measurement point  760  can be actively controlled to zero by using negative resistance circuit  750  as shown in the rectangular portion of the alternate long and short dash line in  FIG. 15 . 
     The dotted rectangular portion in  FIG. 15  is an equivalent circuit of power transmission-side cancel coil  730 . Internal resistance R 4 , inductance L 1 , and AC power supply  740  showing mutual induction electromotive force are connected in series. 
     In this case, power supply circuit  710  includes negative resistance circuit  750 . In negative resistance circuit  750 , resistor R 1 , resistor R 2 , and resistor R 3  are arranged around amplifier circuit  720  having a large amplification factor such as an operational amplifier. When this circuit is used, for apparent resistance Rin of negative resistance circuit  750  as seen from a terminal to which second end point  780 , which is different from first end point  770  of power transmission-side cancel coil  730 , a formula below is established. 
         Rin=−R 3× R 1/ R 2  (Formula 1)
 
     Therefore, when this negative resistance Rin is connected in series with power transmission-side cancel coil  730 , an antiphase potential corresponding to a potential difference generated in internal resistance R 4  can be generated at second end point  780  of power transmission-side cancel coil  730 . For that purpose, it is preferable to adjust resistance values of resistors R 1  to R 3  so as to satisfy the following Formula 2 according to internal resistance R 4  of power transmission-side cancel coil  730  to be used. 
         R 2/ R 1= R 3/ R 4  (Formula 2)
 
     According to the above aspect, the potential of virtual voltage measurement point  760  can be made zero or close to zero, and preferably mutual induction electromotive voltage Vc due to the external magnetic field and self-induced electromotive voltage Vs due to current Ic flowing through power transmission-side cancel coil  730  can be adjusted to have the same magnitude and be balanced in opposite phases. Therefore, power transmission-side cancel coil  730  can operate ideally and the total magnetic flux generated inside and outside power transmission-side cancel coil  730  can be made or close to zero. 
     Although  FIG. 15  shows only negative resistance circuit  750  necessary for the operation explanation, power supply circuit  710  is not limited to this configuration. For example, power supply circuit  710  may include other accompanying circuits such as a power supply circuit of an amplifier circuit, a current booster circuit, an impedance detection circuit of power transmission-side cancel coil  730 , a phase compensation circuit for preventing transmission, and a circuit for adjusting resistance values of resistors R 1  to R 3 . Further, negative resistance circuit  750  is not limited to the circuit shown in  FIG. 15 , and may be another circuit showing a tendency that a predetermined terminal potential decreases with an increase in a current flowing into a predetermined terminal. Further, first end point  770  and second end point  780  of power transmission-side cancel coil  730  may be replaced with each other. Further, power reception-side cancel coil  220  can be configured as a cancel coil connected to power supply circuit  710 , similarly to power transmission-side cancel coil  730 . 
     The solutions and methods in the first to fourth exemplary embodiments can be applied to the present exemplary embodiment as long as there is no contradiction. 
     Sixth Exemplary Embodiment 
     As shown in  FIG. 16 , the difference from the first to fifth exemplary embodiments is that metal foreign substance  400  is put into magnetically shielded space  800  by moving member  510 . 
     When power transmission coil  110  is arranged outside power transmission-side cancel coil  130 , the magnetic field strength inside power transmission-side cancel coil  130  is weaker than the magnetic field strength outside power transmission-side cancel coil  130 . That is, the inside of power transmission-side cancel coil  130  is magnetically shielded space  800 . In a sixth exemplary embodiment, as shown in  FIG. 16 , power transmission-side cancel coil  130  has a rectangular shape having rounded corners, and power transmission coil  110  is formed in a circular shape so as to have a central axis on an outer side in a plane direction of power transmission-side cancel coil  130 . However, as long as a magnetically shielded space can be formed inside power transmission-side cancel coil  130 , the shapes of power transmission-side cancel coil  130  and power transmission coil  110  are not limited to the shapes shown in  FIG. 16 . 
     By putting metal foreign substance  400  into magnetically shielded space  800  by moving member  510 , it is possible to suppress heat generation of metal foreign substance  400 . 
     A moving mechanism capable of moving a part or all of the upper surface of a moving member from an area outside a magnetically shielded space to inside the magnetically shielded space may be provided. 
     As described above, the present disclosure can be applied to applications such as a power transmission device, a power reception device, and a wireless power transmission device that transmit or receive power in a non-contact manner.