Patent Publication Number: US-2023155421-A1

Title: Power transmission device and electric power transmission system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a power transmission device and an electric power transmission system. 
     BACKGROUND ART 
     A wireless electric power transmission technology for wirelessly transmitting electric power has been receiving attention. The wireless electric power transmission technology can wirelessly transmit electric power from a power transmission device to a power reception device, and therefore application of the technology to various products such as transportation equipment such as an electric train and an electric vehicle, a home appliance, wireless communication equipment, and a toy is expected. A power transmission coil and a power reception coil coupled to each other by magnetic flux are used for transmission of electric power in the wireless electric power transmission technology. 
     Existence of a foreign object such as a metal piece near the power transmission coil and the power reception coil may cause various problems. For example, such a foreign object may adversely affect electric power transmission from the power transmission coil to the power reception coil or may generate heat by eddy current. Accordingly, a technology for suitably detecting a foreign object existing near the power transmission coil and the power reception coil is desired. 
     For example, Patent Literature 1 discloses a technology for determining, in a power transmission device wirelessly transmitting electric power to a power reception device, a threshold value for foreign object detection in accordance with information defining electric power that can be transmitted by the power transmission device and information defining electric power that can be received by the power reception device. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 6671920 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the wireless electric power transmission as described above, density of magnetic flux generated by the power transmission coil is not uniform, and unevenness occurs depending on the location. Therefore, when a uniform reference is set to sensors as a reference for foreign object detection in a case of detecting a foreign object by placing the sensors side by side, a foreign object may not be precisely detected depending on the magnetic flux received by each sensor. 
     The present disclosure has been made in view of the problem described above, and an objective of the present disclosure is to enhance detection precision of a foreign object in wireless electric power transmission. 
     Solution to Problem 
     In order to solve the aforementioned problem, a power transmission device according to an embodiment of the present disclosure is 
     a power transmission device wirelessly transmitting electric power to a power reception device and includes: 
     a power transmission coil configured by winding a conductive wire; and 
     a foreign object detection device detecting a foreign object, wherein 
     the foreign object detection device includes:
         sensor coils placed to cover the power transmission coil; and   a detector executing determination processing of determining existence of the foreign object on each of the sensor coils, based on a comparison result between a comparison target value based on output voltage output from one sensor coil of the sensor coils and a threshold value set to the one sensor coil, and       

     the detector executes threshold value change processing of changing the threshold value set to the one sensor coil on each of the sensor coils, based on induced voltage induced in the one sensor coil by magnetic flux generated by the power transmission coil. 
     Advantageous Effects of Invention 
     The power transmission device with the aforementioned configuration can enhance detection precision of a foreign object in wireless electric power transmission. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic configuration diagram of an electric power transmission system according to Embodiment 1; 
         FIG.  2    is a placement diagram of a foreign object detection device according to Embodiment 1; 
         FIG.  3    is a plan view of the foreign object detection device according to Embodiment 1; 
         FIG.  4    is a plan view of a detection coil unit according to Embodiment 1; 
         FIG.  5    is an equivalent circuit of a resonant circuit included in the detection coil unit according to Embodiment 1; 
         FIG.  6    is a configuration diagram of a detector included in the foreign object detection device according to Embodiment 1; 
         FIG.  7    is a graph illustrating a correspondence between a measurement count and a comparison target value, according to Embodiment 1; 
         FIG.  8    is a diagram illustrating an example of voltage induced in sensor coils when electric power is transmitted by a power transmission device, according to Embodiment 1; 
         FIG.  9    is a diagram illustrating an example of a threshold value set to each of sensor coils, according to Embodiment 1; 
         FIG.  10    is a diagram illustrating a relation between the threshold values illustrated in FIG.  9  and a reference value; 
         FIG.  11    is a flowchart illustrating foreign object detection processing executed by the foreign object detection device according to Embodiment 1; 
         FIG.  12    is a flowchart illustrating threshold value change processing in  FIG.  11   ; 
         FIG.  13    is a flowchart illustrating determination processing in  FIG.  11   ; 
         FIG.  14    is a diagram illustrating an example of a frequency of determination processing set to each of sensor coils, according to Embodiment 2; 
         FIG.  15    is a diagram illustrating an example of an execution order of determination processing executed at the frequencies illustrated in  FIG.  14   ; 
         FIG.  16    is a diagram illustrating an example of an ordinal number for determination processing set to each of sensor coils, according to Embodiment 3; and 
         FIG.  17    is a flowchart illustrating threshold value change processing according to Embodiment 4. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described below with reference to drawings. Note that the same or equivalent parts are given the same sign in each diagram. 
     Embodiment 1 
     An electric power transmission system according to Embodiment 1 is a system wirelessly transmitting electric power to a movable body and charging a secondary battery included in the movable body. Examples of the movable body include an electric vehicle (EV), mobile equipment such as a smartphone, and industrial equipment. An example of the movable body being an EV and the electric power transmission system charging a storage battery included in the EV will be described below. 
       FIG.  1    illustrates a schematic configuration of an electric power transmission system  1000  used for charging of a storage battery  500  included in an electric vehicle  700 . The electric vehicle  700  travels with a motor driven by electric power charged in the storage battery  500  such as a lithium-ion battery or a lead storage battery as a power source. 
     As illustrated in  FIG.  1   , the electric power transmission system  1000  is a system wirelessly transmitting electric power from a power transmission device  200  to a power reception device  300  by magnetic coupling. The electric power transmission system  1000  includes the power transmission device  200  wirelessly transmitting electric power of an alternating-current (AC) or direct-current (DC) commercial power source  400  to the electric vehicle  700 , and the power reception device  300  receiving the electric power transmitted by the power transmission device  200  and charging the storage battery  500 . Note that the commercial power source  400  is an AC power source in the following description. 
     The power transmission device  200  is a device wirelessly transmitting electric power to the power reception device  300  by magnetic coupling. The power transmission device  200  includes a foreign object detection device  100  detecting a foreign object, a power transmission coil unit  210  transmitting AC power to the electric vehicle  700 , and an electric power supply device  220  supplying AC power to the power transmission coil unit  210 . 
     As illustrated in  FIG.  2   , the foreign object detection device  100  is placed on the power transmission coil unit  210 . In  FIG.  2   , an axis in an upward vertical direction is a Z-axis, an axis orthogonal to the Z-axis is an X-axis, and an axis orthogonal to the Z-axis and the X-axis is a Y-axis. Details of the foreign object detection device  100  will be described later. 
     As illustrated in  FIG.  2   , the power transmission coil unit  210  includes a power transmission coil  211  being supplied with AC power by the electric power supply device  220  and inducing alternating magnetic flux  1 , and a magnetic body plate  212  allowing passage of a magnetic force generated by the power transmission coil  211  and suppressing loss of the magnetic force. 
     The power transmission coil  211  is configured by spirally winding a conductive wire on the magnetic body plate  212 . The power transmission coil  211  and a capacitor provided at each of two ends of the power transmission coil  211  form a resonant circuit and induce alternating magnetic flux Φ by AC current flowing according to application of AC voltage. 
     The magnetic body plate  212  has a plate shape having a hole in the central part and includes a magnetic body. For example, the magnetic body plate  212  is a plate-shaped member including ferrite being a composite oxide of iron oxide and metal. The magnetic body plate  212  may be formed of an aggregate of segmented magnetic bodies, and the segmented magnetic bodies may be placed in a frame shape having an opening in the central part. 
     The electric power supply device  220  includes a power factor improvement circuit improving the power factor of commercial AC power supplied by the commercial power source  400  and an inverter circuit generating AC power to be supplied to the power transmission coil  211 . The power factor improvement circuit rectifies and boosts AC power generated by the commercial power source  400  and converts the power into DC power having a predetermined voltage value. The inverter circuit converts DC power generated by electric power conversion by the power factor improvement circuit into AC power at a predetermined frequency. For example, the power transmission device  200  is fixed on the floor surface of a parking lot. 
     The power reception device  300  is a device wirelessly receiving electric power from the power transmission device  200  by magnetic coupling. The power reception device  300  includes a power reception coil unit  310  receiving AC power transmitted by the power transmission device  200  and a rectifier circuit  320  converting AC power supplied from the power reception coil unit  310  into DC power and supplying the DC power to the storage battery  500 . 
     As illustrated in  FIG.  2   , the power reception coil unit  310  includes a power reception coil  311  inducing an electromotive force according to a change in the alternating magnetic flux Φ induced by the power transmission coil  211 , and a magnetic body plate  312  allowing passage of a magnetic force generated by the power reception coil  311  and suppressing loss of the magnetic force. The power reception coil  311  and a capacitor provided at each of two ends of the power reception coil  311  configure a resonant circuit. 
     The power reception coil  311  faces the power transmission coil  211  when the electric vehicle  700  is at a standstill at a preset position. When the power transmission coil  211  induces the alternating magnetic flux Φ by receiving electric power from the electric power supply device  220 , an induced electromotive force is induced in the power reception coil  311  by interlinkage of the alternating magnetic flux Φ with the power reception coil  311 . 
     The magnetic body plate  312  has a plate shape having a hole in the central part and includes a magnetic body. For example, the magnetic body plate  312  is a plate-shaped member including ferrite being a composite oxide of iron oxide and metal. The magnetic body plate  312  may be formed of an aggregate of segmented magnetic bodies, and the segmented magnetic bodies may be placed in a frame shape having an opening in the central part. 
     The rectifier circuit  320  generates DC power by rectifying an electromotive force induced in the power reception coil  311 . The DC power generated by the rectifier circuit  320  is supplied to the storage battery  500 . The power reception device  300  may include, between the rectifier circuit  320  and the storage battery  500 , a charging circuit converting DC power supplied from the rectifier circuit  320  into DC power suitable for charging the storage battery  500 . For example, the power reception device  300  is fixed to the chassis of the electric vehicle  700 . 
     A terminal device  600  is a device receiving notification of existence of a foreign object from the foreign object detection device  100 . For example, the terminal device  600  is a smartphone carried by an owner of the electric vehicle  700 . When receiving notification of existence of a foreign object from the foreign object detection device  100 , the terminal device  600  informs a user of the existence of the foreign object by a screen display, a voice output, or the like. 
     The foreign object detection device  100  detects a foreign object existing in a detection target area. The detection target area is a target area of foreign object detection and is an area in which a foreign object can be detected. The detection target area is an area near the power transmission coil unit  210  and the power reception coil unit  310  and is an area including an area between the power transmission coil unit  210  and the power reception coil unit  310 . A foreign object is an object or a living body not required for electric power transmission. 
     When placed in the detection target area during electric power transmission, a foreign object may adversely affect electric power transmission or may generate heat. Therefore, the foreign object detection device  100  detects a foreign object existing in the detection target area and notifies a user of the detection of the foreign object. The user receives the notification and may remove the foreign object. Various objects such as a metal piece, a person, and an animal may be assumed as foreign objects. 
     As illustrated in  FIG.  2   , the foreign object detection device  100  includes a detection coil unit  110 , a detector  120 , a pulse generator  140 , and a notifier  150 . 
     The detection coil unit  110  is a unit detecting a foreign object. As illustrated in  FIG.  3   , the detection coil unit  110  is formed in a flat plate shape and is placed on the power transmission coil unit  210  in such a way as to overlap the power transmission coil  211  in a plan view. The detection coil unit  110  includes a detection coil substrate  113  made of a magnetically permeable material typified by resin. Sensor coils  111  placed in a matrix shape in the X-axis direction and the Y-axis direction to cover the power transmission coil  211 , and an external connection connector  112  connecting each sensor coil  111  to the detector  120  and the pulse generator  140  are implemented on the detection coil substrate  113 . 
     Note that the number and placement of the sensor coils  111  illustrated in  FIG.  3    are exemplifications. In other words, the number and placement of sensor coils  111  included in the detection coil unit  110  are not limited to those illustrated in  FIG.  3   . The same holds for succeeding diagrams. 
     The detector  120  determines whether a foreign object exists in the detection target area, based on output voltage output from each of the sensor coils  111  included in the detection coil unit  110 . Details of the detector  120  will be described later. 
     The pulse generator  140  includes a pulse generator generating a pulse and generates pulse voltage being a pulse-shaped voltage signal as input voltage for foreign object detection. The pulse generator  140  applies the generated pulse voltage to each of the sensor coils  111  included in the detection coil unit  110 . 
     The notifier  150  includes a communication interface for communicating with equipment outside the foreign object detection device  100  in accordance with a well-known communication standard such as a wireless local area network (LAN). When a foreign object is detected by the detector  120 , the notifier  150  notifies a user of the detection of the foreign object. For example, the notifier  150  transmits information indicating that the foreign object is detected to a terminal device  600  carried by the user. 
     Next, a configuration of the sensor coil  111  will be described in detail with reference to  FIG.  4    and  FIG.  5   . The sensor coil  111  is a general name for sensor coils  111 A to  111 L. The sensor coils  111  practically have similar configurations. The sensor coil  111  includes a coil  114 , a capacitor  115 , a switch  116 , and a switch  117 . In consideration of viewability of the drawing, only the sensor coil  111 A is given signs in  FIG.  4   . 
     The coil  114  has a conductor pattern wound one or more turns around an axis parallel to the Z-axis on the top surface of the detection coil substrate  113 . One terminal of the coil  114  is connected to one terminal of the switch  116  and a first connection wiring  118 . The first connection wiring  118  is placed on the top surface of the detection coil substrate  113  and is connected to the external connection connector  112 . The other terminal of the coil  114  is connected to one terminal of the capacitor  115  and one terminal of the switch  117 . The other terminal of the switch  117  is connected to a second connection wiring  119 . The other terminal of the capacitor  115  is connected to the other terminal of the switch  116 . The second connection wiring  119  is placed on the back surface of the detection coil substrate  113  and is connected to the external connection connector  112 . 
     The switch  116  and the switch  117  are controlled to an on-state or an off-state in accordance with control from the detector  120  through an unillustrated control line. The on-state is a conducting state, and the off-state is a nonconducting state. The switch  116  has a function of changing the state between the coil  114  and the capacitor  115 . When the switch  116  is turned on, the coil  114  and the capacitor  115  form a resonant circuit. The switch  117  has a function of changing the state between the resonant circuit and the pulse generator  140 . The resonance frequency of the resonant circuit is designed to be several MHz as an example. 
     When both the switch  116  and the switch  117  are in the on-state, the coil  114  and the capacitor  115  form the resonant circuit. The resonant circuit is electrically connected to the detector  120  and the pulse generator  140  through the first connection wiring  118  and the second connection wiring  119 . Therefore, voltage between both ends of the resonant circuit, that is, voltage between both ends of the coil  114  is introduced to the detector  120  through the first connection wiring  118  and the second connection wiring  119 . 
     On the other hand, when the switch  116  is in the off-state, the coil  114  and the capacitor  115  do not form the resonant circuit. Further, when the switch  117  is in the off-state, the resonant circuit is electrically disconnected from the detector  120  and the pulse generator  140 . 
       FIG.  5    illustrates an equivalent circuit of the resonant circuit formed by the coil  114  and the capacitor  115 . When pulse voltage is input from the pulse generator  140  in a state of the switches  116  and  117  being closed and the coil  114  and the capacitor  115  forming the resonant circuit, pulse-shaped voltage is applied between both ends of the resonant circuit. 
     When pulse voltage is input, energy is accumulated in the coil  114  and the capacitor  115 , and voltage between both ends of the resonant circuit oscillates while attenuating with a fall of the pulse voltage. Therefore, the resonant circuit outputs oscillating voltage the peak value of which gradually attenuates as time progresses to the detector  120  as response voltage to the pulse voltage. 
     When a foreign object  10  exists near the resonant circuit, a change in the inductance of the coil  114  occurs. Therefore, the frequency of the oscillating voltage changes and a degree of attenuation of the oscillating voltage changes when a foreign object  10  exists, compared with a case that the foreign object  10  does not exist. The detector  120  determines existence of the foreign object  10  by detecting a change in the frequency of the oscillating voltage, a change in a degree of attenuation of the oscillating voltage, or the like. 
     On the other hand, even in a case of pulse voltage not being input from the pulse generator  140 , induced voltage is induced in the coil  114  by alternating magnetic flux Φ generated by the power transmission coil  211  when electric power is transmitted from the power transmission device  200  to the power reception device  300 . 
     Specifically, when AC current is supplied to the power transmission device  200  from the electric power supply device  220  and the power transmission coil  211  induces the alternating magnetic flux Φ, induced voltage according to a change in the alternating magnetic flux Φ is generated between both ends of the coil  114 . The induced voltage is voltage generated by such a magnetic force during electric power transmission. The induced voltage induced in the coil  114  is output to the detector  120 , similarly to response voltage to pulse voltage. 
     Next, the detector  120  will be described in detail. As illustrated in  FIG.  6   , the detector  120  includes a controller  121 , a storage  122 , and a measurer  123 . 
     The controller  121  includes a central processing unit (CPU). The CPU includes a microprocessor or the like and is a central processing unit executing various types of processing and operations. In the controller  121 , the CPU reads a control program stored in a ROM and controls operation of the entire detector  120  while using a RAM as a work memory. 
     The storage  122  includes a read only memory (ROM), a random access memory (RAM), a flash memory, and the like. The storage  122  stores a program and data used by the controller  121  for performing various types of processing. Further, the storage  122  stores data generated or acquired by performing various types of processing by the controller  121 . 
     The measurer  123  measures output voltage output from each of the sensor coils  111  included in the detection coil unit  110 . Specifically, the measurer  123  includes an analog/digital (A/D) conversion circuit, a peak hold circuit, and the like. The measurer  123  converts an analog signal output from each sensor coil  111  into a digital signal by the A/D converter and measures a voltage value of the digital signal after the A/D conversion. 
     More specifically, when pulse voltage is input from the pulse generator  140 , the measurer  123  measures response voltage to the pulse voltage. Further, even in a case of pulse voltage not being input from the pulse generator  140 , the measurer  123  measures induced voltage induced by the alternating magnetic flux Φ when electric power is transmitted from the power transmission device  200 . Thus, the measurer  123  measures two types of voltage signals being response voltage and induced voltage. 
     When response voltage is measured, the switch  116  is set to the on-state, and a resonant circuit is formed by the coil  114  and the capacitor  115 . In this state, the measurer  123  measures, as response voltage, voltage between both ends of the resonant circuit when pulse voltage is input from the pulse generator  140 . On the other hand, when induced voltage is measured, the switch  116  is set to the off-state, and the capacitor  115  is detached from the circuit. Therefore, the resonant circuit is not formed. In this state, the measurer  123  measures, as induced voltage, voltage between both ends of the coil  114  when electric power is transmitted by the power transmission device  200 . 
     Further, as illustrated in  FIG.  6   , the detector  120  functionally includes a power transmission information acquirer  131 , a selector  132 , a driver  133 , a determiner  134 , a changer  135 , a result output device  136 , and a power transmission controller  137 . The components represent functions of the controller  121 . Specifically, in the controller  121 , the CPU functions as each component by reading a program stored in the ROM into the RAM and performing control by executing the program. 
     The power transmission information acquirer  131  acquires power transmission information. The power transmission information is information about electric power transmission by the power transmission device  200  and is specifically information indicating whether the power transmission device  200  is transmitting electric power to the power reception device  300 . When the power transmission device  200  starts electric power transmission to the power reception device  300 , the electric power supply device  220  notifies the detector  120  that electric power is being transmitted. Further, when the power transmission device  200  ends electric power transmission to the power reception device  300 , the electric power supply device  220  notifies the detector  120  that electric power is not being transmitted. The power transmission information acquirer  131  acquires information thus notified from the electric power supply device  220  as power transmission information. 
     Alternatively, as power transmission information, the power transmission information acquirer  131  may acquire information indicating transmitted power of the power transmission device  200  from the power transmission device  200  or may acquire information indicating received power of the power reception device  300  from the power reception device  300  through wireless communication. When the transmitted power or the received power is practically zero, electric power can be determined to be not being transmitted from the power transmission device  200  to the power reception device  300 ; and when the transmitted power or the received power is not practically zero, electric power can be determined to be being transmitted from the power transmission device  200  to the power reception device  300 . 
     The selector  132  selects one sensor coil  111  being a target of foreign object detection processing of the sensor coils  111  included in the detection coil unit  110  in accordance with a predetermined selection rule. Specifically, the selector  132  individually and sequentially selects sensor coils  111 A to  111 L in an order of the sensor coil  111 A, the sensor coil  111 B, . . . , the sensor coil  111 L, the sensor coil  111 A, the sensor coil  111 B, . . . . 
     When selecting one sensor coil  111 , the selector  132  controls the switch  116  and the switch  117  in the selected sensor coil  111  in order to detect existence of a foreign object  10  close to the selected sensor coil  111 . Specifically, when measuring induced voltage, the selector  132  sets the switch  116  in the selected sensor coil  111  to the on-state and sets the switch  117  to the off-state. Further, when measuring response voltage, the selector  132  sets the switch  116  and the switch  117  in the selected sensor coil  111  to the on-state. The selector  132  sets the switch  116  and the switch  117  in an unselected sensor coil  111  to the off-state. 
     The driver  133  drives the pulse generator  140  and causes the pulse generator  140  to generate single pulse voltage after execution of selection and on-state control by the selector  132 . Thus, the driver  133  inputs the pulse voltage generated in the pulse generator  140  to a sensor coil  111  selected by the selector  132  as input voltage. 
     The pulse voltage generated in the pulse generator  140  is applied to a resonant circuit formed in the sensor coil  111  selected by the selector  132  through the external connection connector  112 , the first connection wiring  118 , the second connection wiring  119 , and the like. Voltage between both ends of the resonant circuit is introduced to the measurer  123  through the external connection connector  112 , the first connection wiring  118 , the second connection wiring  119 , and the like. 
     When the pulse voltage is input as input voltage, the sensor coil  111  selected by the selector  132  outputs oscillating voltage representing the voltage between both ends of the resonant circuit as response voltage. The measurer  123  measures the response voltage output from the sensor coil  111 . 
     Based on output voltage output from a sensor coil  111 , the determiner  134  determines whether a foreign object  10  exists near the sensor coil  111 . The output voltage is specifically response voltage output from a sensor coil  111  as a response to pulse voltage input from the pulse generator  140 , according to Embodiment 1. The determiner  134  determines existence of a foreign object  10 , based on a comparison result between a comparison target value based on response voltage output from a sensor coil  111  and a threshold value set to the sensor coil  111 . 
     The comparison target value is a target value compared with the threshold value. Specifically, the comparison target value is a difference value between a value indicating a characteristic of response voltage measured by the measurer  123  and a reference value, or a value based on the difference value. A case of the comparison target value being a difference value between a value indicating a characteristic of response voltage and a reference value will be described below as an example. 
     Examples of a value indicating a characteristic of response voltage include a frequency of oscillating voltage, a convergence time of oscillating voltage, and magnitude of amplitude of oscillating voltage. For example, the convergence time of oscillating voltage refers to a time period from application of pulse voltage until the amplitude of the oscillating voltage converges to a predetermined amplitude or less. For example, the magnitude of amplitude of oscillating voltage refers to the magnitude of the amplitude of the oscillating voltage when a predetermined time elapses from application of pulse-shaped voltage. A value indicating a characteristic indicating response voltage for acquiring a comparison target value may be adjusted as appropriate. 
     The reference value is a value indicating a characteristic of response voltage when a foreign object  10  does not exist close to a sensor coil  111 . The reference value is preset based on an experiment, a simulation, or the like and is stored in the storage  122 . 
     The determiner  134  calculates, as a comparison target value, a difference value between a value indicating a characteristic of response voltage measured by the measurer  123  and the reference value. Thus, the determiner  134  calculates a variation between a value indicating a characteristic of response voltage output from the sensor coil  111  as a response to pulse voltage input from the pulse generator  140  and the value when a foreign object  10  does not exist. A small difference value means a high likelihood that a foreign object  10  does not exist, and a large difference value means a high likelihood that a foreign object  10  exists. 
     When calculating the difference value as a comparison target value, the determiner  134  compares the comparison target value with the threshold value. Then, when an excess count of the comparison target value exceeding the threshold value reaches a predetermined threshold count, the determiner  134  determines that a foreign object  10  exists. 
     The threshold value is a threshold value for determining a comparison target value. The threshold value is individually set to each of the sensor coils  111  included in the detection coil unit  110  and is stored in the storage  122 . For example, the threshold value is preset in consideration of predicted magnitude of noise, a degree of change in response voltage due to existence of a foreign object  10 , and the like. 
     The threshold count is a threshold value for determining an excess count. When an excess count reaches the threshold count, a foreign object  10  is determined to exist. For example, the threshold count is predetermined in consideration of a likelihood of noise generation, the magnitude of risk due to existence of a foreign object  10 , and the like and is stored in the storage  122 . An increased threshold count allows suppression of erroneous determination and enhanced reliability of foreign object detection. Further, a decreased threshold count allows increased response speed of foreign object detection. 
     An example of a foreign object  10  being determined to exist by a comparison target value consecutively exceeding the threshold value a number of times equal to or greater than the threshold count will be described below with reference to  FIG.  7   .  FIG.  7    illustrates a correspondence between a measurement count of response voltage by the measurer  123  and a comparison target value based on response voltage. The graph illustrated in  FIG.  7    illustrates a scene in which the comparison target value does not exceed the threshold value from the first measurement to the twentieth measurement and the comparison target value exceeds the threshold value from the twenty-first measurement onward. For example, when the threshold count is 5, the determiner  134  determines that a foreign object  10  exists when the twenty-fifth measurement is completed. 
     The determiner  134  executes such determination processing of determining existence of a foreign object  10  on sensor coils  111  individually and sequentially selected by the selector  132 , based on a comparison result between a comparison target value and a threshold value. Specifically, the determiner  134  executes the determination processing of determining existence of a foreign object  10  on each of the sensor coils  111 , based on a comparison result between a comparison target value based on response voltage output from one sensor coil  111  as a response to input voltage input to the one sensor coil  111  of the sensor coils  111 , and a threshold value set to the one sensor coil  111 . Thus, the determiner  134  determines whether a foreign object  10  exists close to each of the sensor coils  111 . 
     Returning to  FIG.  6   , based on induced voltage induced in a sensor coil  111  by magnetic flux generated by the power transmission coil  211 , the changer  135  executes threshold value change processing of changing a threshold value set to the sensor coil  111  on each of the sensor coils  111 . The threshold value is a value used for determining existence of a foreign object  10  in the aforementioned determination processing by the determiner  134 . 
     As described above, when electric power is transmitted from the power transmission device  200  to the power reception device  300 , induced voltage is induced in each sensor coil  111  by alternating magnetic flux Φ induced by the power transmission coil  211 . The density of the alternating magnetic flux Φ induced by the power transmission coil  211  is not uniform in the detection target area, and unevenness depending on the location in the detection target area occurs based on characteristics, shapes, placement, and the like of the power transmission coil  211 , the power reception coil  311 , and the magnetic body plate  312 . Therefore, the magnitude of induced voltage induced in each sensor coil  111  is not the same across the sensor coils  111 , and a difference between the sensor coils  111  occurs. 
     When such a difference in magnitude of induced voltage between the sensor coils  111  occurs in a case that threshold values set to the sensor coils  111  are the same, a foreign object  10  may not be precisely detected when electric power is transmitted by the power transmission device  200 . Therefore, in order to enhance detection precision of a foreign object  10  when electric power is transmitted, the changer  135  individually changes the threshold values set to the sensor coils  111 , based on the magnitude of induced voltage induced in each sensor coil  111 . 
     When electric power is transmitted by the power transmission device  200 , the changer  135  refers to a voltage value of induced voltage being output from a sensor coil  111  selected by the selector  132  and being measured by the measurer  123 . Then, the changer  135  uses the voltage value as the magnitude of induced voltage induced in the selected sensor coil  111 . 
       FIG.  8    illustrates an example of the magnitude of induced voltage induced in each of sensor coils  111 . As an example,  FIG.  8    illustrates  36  sensor coils  111  placed in 6×6. For ease of understanding, the following description assumes that induced voltage of the same magnitude is induced in each of sensor coils  111  in the same color. In the example in  FIG.  8   , an induced voltage of 10 V is induced in each of four sensor coils  111  in the central part, an induced voltage of 20 V is induced in each of 12 sensor coils  111  in the surrounding area of the central part (hereinafter referred to as an “intermediate part”), and an induced voltage of 15 V is further induced in each of 20 sensor coils  111  in the surrounding area of the intermediate part (hereinafter referred to as an “outer part”). In other words, magnetic flux density of alternating magnetic flux Φ is relatively high and high induced voltage is induced in an area where the 12 sensor coils  111  in the intermediate part are placed. 
     In general, the temperature at a location where magnetic flux density is relatively high tends to rise rapidly when a foreign object  10  exists, compared with a location where magnetic flux density is relatively low. Therefore, it is desirable that a foreign object  10  existing at a location where magnetic flux density is relatively high be more promptly and more reliably detected compared with a foreign object  10  existing at a location where magnetic flux density is relatively low. Therefore, the changer  135  changes a threshold value for a sensor coil  111  in which a relatively high induced voltage is induced to a value smaller than a threshold value for a sensor coil  111  in which a relatively low induced voltage is induced. 
     When a threshold value decreases, the determiner  134  determines that a foreign object  10  exists even when the difference between a comparison target value and a reference value is small. Therefore, the changer  135  changes a threshold value for a sensor coil  111  in which a relatively high induced voltage is induced to a value sufficiently small to the extent that a foreign object  10  is not erroneously detected. Thus, when a foreign object  10  exists at a location where magnetic flux density is relatively high, the foreign object  10  can be more promptly and reliably detected. 
     More specifically, when induced voltage induced in a first sensor coil  111  of the sensor coils  111  is higher than induced voltage induced in a second sensor coil  111  of the sensor coils  111 , the changer  135  makes a threshold value set to the first sensor coil  111  smaller than a threshold value set to the second sensor coil  111 . 
     The first sensor coil  111  and the second sensor coil  111  correspond to two or more sensor coils  111  with varying magnitude of induced voltage of the sensor coils  111  included in the detection coil unit  110 . For example, the first sensor coil  111  corresponds to 12 sensor coils  111  in the intermediate part in  FIG.  8    where a relatively high induced voltage is induced, and the second sensor coil  111  corresponds to four sensor coils  111  in the central part in  FIG.  8    where a relatively low induced voltage is induced. Alternatively, the first and second sensor coils  111  may be considered to correspond to sensor coils  111  in the intermediate part and the outer part, respectively, or may be considered to correspond to sensor coils  111  in the outer part and the central part, respectively. 
       FIG.  9    illustrates an example of a threshold value changed by the changer  135  in a case of the induced voltage illustrated in  FIG.  8    being induced in each sensor coil  111 . A threshold value set to each sensor coil  111  is represented by a ratio (%) to a reference value in  FIG.  9   . In the example in  FIG.  9   , the changer  135  changes a threshold value for a sensor coil  111  in the central part where induced voltage is relatively low to a relatively large value of 50%, changes a threshold value for a sensor coil  111  in the intermediate part where induced voltage is relatively high to a relatively small value of 20%, and changes a threshold value for a sensor coil  111  in the outer part where the magnitude of induced voltage is moderate to a moderate value of 30%. 
     Thus, the changer  135  changes a threshold value for a sensor coil  111  with a relatively high induced voltage of the sensor coils  111  to a relatively small value and changes a threshold value for a sensor coil  111  with a relatively low induced voltage to a relatively large value. As a result, threshold values, such as 20%, 30%, and 50%, that are nonuniform relative to the reference value are set depending on the sensor coil  111 , as illustrated in  FIG.  10   . The determiner  134  determines existence of a foreign object  10  near each of the sensor coils  111 , based on the threshold value changed by the changer  135 . 
     Returning to  FIG.  6   , the result output device  136  outputs a foreign object detection result by the determiner  134 . For example, when a foreign object  10  is determined to exist by the determiner  134 , the result output device  136  instructs the notifier  150  to notify that a foreign object  10  exists. When receiving the notification from the determiner  134 , the notifier  150  transmits information indicating the detection of the foreign object  10  to a terminal device  600  carried by a user. When receiving the information, the terminal device  600  informs the user of the detection of the foreign object  10  by a screen display, a voice output, or the like. When receiving the information about existence of the foreign object  10  from the terminal device  600 , the user removes the foreign object  10 . 
     The power transmission controller  137  controls electric power transmission to the power reception coil unit  310  by the power transmission coil unit  210 . Specifically, when a foreign object  10  is determined to exist by the determiner  134 , the power transmission controller  137  causes the electric power supply device  220  to restrict supply of AC power to the power transmission coil  211  in order to avoid occurrence of a problem caused by the foreign object  10 . 
     Restricting supply of AC power means stopping supply of AC power or decreasing the supply to the extent that a problem does not occur. A case of the power transmission controller  137  giving an instruction to stop supply of AC power as restriction on supply of AC power will be described below as an example. When being instructed to stop supply of AC power, the electric power supply device  220  stops supply of AC power to the power transmission coil  211 . Thus, electric power transmission by the power transmission device  200  stops. 
     Next, foreign object detection processing executed by the foreign object detection device  100  will be described with reference to  FIG.  11   . For example, the foreign object detection processing illustrated in  FIG.  11    is started when power to the foreign object detection device  100  is turned on. 
     First, the detector  120  determines whether a start instruction for the foreign object detection processing is received (Step S 101 ). Specifically, when electric power is determined to be being transmitted from the power transmission device  200  to the power reception device  300 , based on power transmission information acquired by the power transmission information acquirer  131 , the detector  120  determines that a start instruction is received. 
     When determining that a start instruction for the foreign object detection processing is received (Step S 101 : YES), the detector  120  executes initial setting for the foreign object detection processing (Step S 102 ). For example, the switches  116  and the switches  117  included in the detection coil unit  110  are set to the off-state in the initial setting. 
     When executing initial setting, the detector  120  functions as the selector  132  and selects one sensor coil  111  of the sensor coils  111  (Step S 103 ). Specifically, the detector  120  selects one sensor coil  111  being a target of the determination processing of the sensor coils  111  included in the detection coil unit  110  in accordance with a predetermined selection rule. 
     When selecting one sensor coil  111 , the detector  120  controls states of the switches  116  and  117  (Step S 104 ). Specifically, the detector  120  controls switches  116  and  117  in each sensor coil  111  included in the detection coil unit  110  in such a way that the switch  117  included in the selected sensor coil  111  is set to the on-state and the switch  116  included in the selected sensor coil  111  and all switches  116  and  117  included in unselected sensor coils  111  are set to the off-state. 
     When controlling the states of the switches  116  and  117 , the detector  120  functions as the changer  135  and executes the threshold value change processing on the selected sensor coil  111  (Step S 105 ). Details of the threshold value change processing will be described with reference to  FIG.  12   . 
     When starting the threshold value change processing illustrated in  FIG.  12   , the detector  120  reads a threshold value set to the selected sensor coil  111  of threshold values for the sensor coils  111  stored in the storage  122  (Step S 201 ). Then, the detector  120  measures induced voltage induced in the selected sensor coil  111  by the measurer  123  (Step S 202 ). 
     When measuring induced voltage, the detector  120  changes the threshold value set to the selected sensor coil  111 , based on the measured induced voltage (Step S 203 ). Specifically, when the magnitude of the measured induced voltage is relatively large, the detector  120  changes the threshold value set to the selected sensor coil  111  to a relatively small value. Further, when the magnitude of the measured induced voltage is relatively small, the detector  120  changes the threshold value set to the selected sensor coil  111  to a relatively large value. When changing the threshold value, the detector  120  saves the changed threshold value into the storage  122 . Thus, the threshold value change processing illustrated in  FIG.  12    ends. 
     Returning to  FIG.  11   , when executing the threshold value change processing, the detector  120  controls the states of the switches  116  and  117  (Step S 106 ). Specifically, while keeping the switch  117  included in the sensor coil  111  selected in Step S 103  in the on-state, the detector  120  changes the state of the switch  116  to the on-state. Thus, a resonant circuit is formed by the coil  114  and the capacitor  115 . The detector  120  continues to keep the switches  116  and  117  included in the unselected sensor coils  111  in the off-state. 
     When controlling the states of the switches  116  and  117 , the detector  120  functions as the determiner  134  and executes the determination processing on the selected sensor coil  111  (Step S 107 ). Details of the determination processing will be described with reference to  FIG.  13   . 
     When starting the determination processing illustrated in  FIG.  13   , the detector  120  inputs pulse voltage to the selected sensor coil  111  (Step S 301 ). Specifically, the detector  120  causes the pulse generator  140  to generate pulse voltage by the driver  133  and inputs the pulse voltage to the selected sensor coil  111  as input voltage. 
     When inputting the input voltage, the detector  120  measures response voltage output from the selected sensor coil  111  by the measurer  123  (Step S 302 ). Then, the detector  120  calculates a difference value between a value indicating a characteristic of the measured response voltage and a reference value (Step S 303 ). 
     When calculating the difference value, the detector  120  determines whether the calculated difference value exceeds a threshold value set to the selected sensor coil  111  (Step S 304 ). The threshold value refers to the threshold value changed based on induced voltage induced in the selected sensor coil  111  in the threshold value change processing in Step S 105 . 
     When determining that the difference value exceeds the threshold value (Step S 304 : YES), the detector  120  increments an excess count (Step S 305 ). In other words, the detector  120  increases the excess count by one. When determining that the difference value does not exceed the threshold value (Step S 304 : NO), the detector  120  resets the excess count (Step S 306 ). In other words, the detector  120  sets the excess count to 0. 
     When incrementing or resetting the excess count, the detector  120  determines whether the excess count reaches a threshold count (Step S 307 ). When the excess count does not reach the threshold count (Step S 307 : NO), the detector  120  determines that a foreign object  10  does not exist near the selected sensor coil  111 . In this case, the detector  120  skips processing in Steps S 308  and  309  and ends the determination processing illustrated in  FIG.  13   . 
     On the other hand, when the excess count reaches the threshold count (Step S 307 : YES), the detector  120  determines that a foreign object  10  exists near the selected sensor coil  111 . In this case, the detector  120  functions as the result output device  136  and notifies a user of the foreign object detection through the notifier  150  (Step S 308 ). 
     When notifying the foreign object detection, the detector  120  functions as the power transmission controller  137  and instructs the electric power supply device  220  to stop electric power transmission (Step S 309 ). Specifically, the detector  120  transmits, to the electric power supply device  220 , an instruction to stop supply of AC power to the power transmission coil  211 . The processing of giving an instruction to stop electric power transmission in Step S 309  may be performed prior to the processing of notifying foreign object detection in Step S 308 . 
     When giving the instruction to stop electric power transmission, the detector  120  exits the determination processing illustrated in  FIG.  12    and returns to Step S 101  in the foreign object detection processing illustrated in  FIG.  11   . Specifically, the detector  120  exits a loop of the threshold value change processing and the determination processing repeatedly executed on sensor coils  111  and stands by until receiving a start instruction for the foreign object detection processing again. 
     When determining that the excess count does not reach the threshold count in Step S 107  in the foreign object detection processing illustrated in  FIG.  11   , the detector  120  determines whether an end instruction for the foreign object detection processing is received (Step S 108 ). For example, when electric power is determined to be not being transmitted from the power transmission device  200  to the power reception device  300 , based on power transmission information acquired by the power transmission information acquirer  131 , the detector  120  determines that an end instruction for the foreign object detection processing is received. 
     When an end instruction for the foreign object detection processing is not received (Step S 108 : NO), the detector  120  returns the processing to Step S 103 . Then, the detector  120  changes a sensor coil  111  selected of the sensor coils  111  and executes the processing in Steps S 103  to S 108  again. Thus, the detector  120  repeats the processing of determining existence of a foreign object  10  by each of the sensor coils  111  until receiving end processing or detecting a foreign object  10  in Step S 105 . 
     On the other hand, when an end instruction for the foreign object detection processing is received (Step S 108 : YES) or when a start instruction for the foreign object detection processing is not received (Step S 101 : NO), the detector  120  returns the processing to Step S 101 . Then, the detector  120  stands by until a start instruction for the foreign object detection processing is received again. When a start instruction for the foreign object detection processing is received again, the detector  120  executes the processing in Steps S 101  to S 108  again. 
     As described above, the foreign object detection device  100  and the power transmission device  200  according to Embodiment 1 individually change a threshold value set to each sensor coil  111 , based on induced voltage induced in the sensor coil  111 , in the device determining existence of a foreign object  10 , based on a comparison result between a comparison target value based on response voltage output from each sensor coil  111  in response to input of pulse voltage, and a threshold value. Thus, decline in detection precision caused by difference between magnetic flux densities received by sensor coils  111  can be suppressed, and detection precision of a foreign object  10  can be enhanced. 
     In particular, when electric power is wirelessly transmitted to the power reception device  300  included in the electric vehicle  700  or the like, the detection target area may be widespread, and a difference between magnetic flux densities depending on the location tends to increase. Even when the detection target area is widespread as described above, the foreign object detection device  100  according to Embodiment 1 can suppress decline in detection precision caused by a difference between magnetic flux densities received by sensor coils  111  by individually changing a threshold value set to each sensor coil  111 . 
     Embodiment 2 
     Next, Embodiment 2 of the present disclosure will be described. Description of a configuration and processing similar to those according to Embodiment 1 is omitted or simplified. 
     The changer  135  changes a threshold value set to each of sensor coils  111 , according to Embodiment 1. On the other hand, a changer  135  changes a frequency of determination processing executed on each of sensor coils  111  in place of or in addition to changing a threshold value, according to Embodiment 2. Specifically, the changer  135  according to Embodiment 2 changes a frequency of the determination processing executed on each of the sensor coils  111 , based on induced voltage induced in each of the sensor coils  111 . 
       FIG.  14    illustrates an example of a frequency changed by the changer  135  when induced voltage illustrated in  FIG.  8    is induced in each sensor coil  111 . A frequency set to each sensor coil  111  corresponds to the number of times determination processing is executed per time span predetermined by a determiner  134 . When the frequency of the determination processing is increased, the determination processing is executed at shorter intervals on average; and when the frequency of the determination processing is decreased, the determination processing is executed at longer intervals on average. A frequency of the determination processing is set to an appropriate frequency in an initial setting. 
     When induced voltage induced in a first sensor coil  111  of the sensor coils  111  is higher than induced voltage induced in a second sensor coil  111  of the sensor coils  111 , the changer  135  makes the frequency of the determination processing executed on the first sensor coil  111  higher than the frequency of the determination processing executed on the second sensor coil  111 . The first sensor coil  111  and the second sensor coil  111  correspond to two or more sensor coils  111  with varying magnitude of induced voltage of the sensor coils  111  included in a detection coil unit  110 , similarly to Embodiment 1. 
     Specifically, the changer  135  changes the frequency for a sensor coil  111  in a central part where induced voltage is relatively low to a relatively low value being 5, changes the frequency for a sensor coil  111  in an intermediate part where induced voltage is relatively high to a relatively high value being 20, and changes the frequency for a sensor coil  111  in an outer part where the magnitude of induced voltage is moderate to a moderate value being 10, as illustrated in  FIG.  14   . In other words, the changer  135  changes the frequency of the determination processing executed on a sensor coil  111  with a relatively high induced voltage of the sensor coils  111  to a relatively high value and changes the frequency of the determination processing executed on a sensor coil  111  with a relatively low induced voltage to a relatively low value. 
     After the frequencies are changed by the changer  135 , a selector  132  selects sensor coils  111  being targets of the determination processing of the sensor coils  111  included in the detection coil unit  110  at the frequencies changed by the changer  135 . Specifically, the selector  132  repeats a cycle of selecting sensor coils  111  in an order of the intermediate part, the outer part, the intermediate part, the intermediate part, the outer part, the central part, the intermediate part, . . . , as illustrated in  FIG.  15   . In other words, the selector  132  makes the number of times sensor coils  111  in the intermediate part to which a relatively high frequency is set are selected largest and the number of times sensor coils  111  in the central part to which a relatively low frequency is set are selected smallest. 
     When a sensor coil  111  is selected by the selector  132 , the determiner  134  executes the determination processing of determining existence of a foreign object  10  near the selected sensor coil  111 , based on response voltage to pulse voltage input to the sensor coil  111 . Thus, a sensor coil  111  placed at a location where magnetic flux density is relatively high is selected at a high frequency, and therefore a foreign object detection device  100  according to Embodiment 2 can more promptly and more reliably detect a foreign object  10  existing at a location where magnetic flux density is relatively high. 
     Embodiment 3 
     Next, Embodiment 3 of the present disclosure will be described. Description of a configuration and processing similar to those according to Embodiments 1 and 2 is omitted or simplified. 
     The changer  135  changes a threshold value set to each of sensor coils  111 , according to Embodiment 1. On the other hand, a changer  135  changes ordinal numbers for determination processing executed on sensor coils  111  in place of or in addition to changing a threshold value, according to Embodiment 3. Specifically, the changer  135  according to Embodiment 3 changes ordinal numbers for the determination processing executed on the sensor coils  111 , based on induced voltage induced in each of the sensor coils  111 . 
       FIG.  16    illustrates an example of ordinal numbers changed by the changer  135  when the induced voltage illustrated in  FIG.  8    is induced in each sensor coil  111 . Numbers 1 to 36 indicated at the sensor coils  111  in  FIG.  16    represent ordinal numbers for the determination processing executed by the determiner  134 . Ordinal numbers for the determination processing are set to appropriate ordinal numbers in an initial setting. 
     When induced voltage induced in a first sensor coil  111  of the sensor coils  111  is higher than induced voltage induced in a second sensor coil  111  of the sensor coils  111 , the changer  135  makes an ordinal number for the determination processing executed on the first sensor coil  111  lower than an ordinal number for the determination processing executed on the second sensor coil  111 . The first sensor coil  111  and the second sensor coil  111  correspond to two or more sensor coils  111  with varying magnitude of induced voltage of the sensor coils  111  included in the detection coil unit  110 , similarly to Embodiments 1 and 2. 
     Specifically, the changer  135  sets ordinal numbers of sensor coils  111  in a central part where induced voltage is relatively low to relatively high ordinal numbers of 33rd to 36th, changes ordinal numbers of sensor coils  111  in an intermediate part where induced voltage is relatively high to relatively low ordinal numbers of first to 12th, and changes ordinal numbers of sensor coils  111  in an outer part where the magnitude of induced voltage is moderate to moderate ordinal numbers of 13th to 32nd, as illustrated in  FIG.  16   . In other words, the changer  135  changes an ordinal number for the determination processing executed on a sensor coil  111  in which induced voltage is relatively high of the sensor coils  111  to a relatively low ordinal number and changes an ordinal number for the determination processing executed on a sensor coil  111  in which induced voltage is relatively low to a relatively high ordinal number. 
     After ordinal numbers are changed by the changer  135 , the selector  132  selects a sensor coil  111  being a target of the determination processing of the sensor coils  111  included in the detection coil unit  110 , based on the ordinal numbers changed by the changer  135 . Specifically, the selector  132  selects a sensor coil  111  placed at a location where magnetic flux density is relatively high at an ordinal number lower than that for a sensor coil  111  placed at a location where magnetic flux density is relatively low. 
     When a sensor coil  111  is selected by the selector  132 , the determiner  134  executes the determination processing of determining existence of a foreign object  10  near the selected sensor coil  111 , based on response voltage to pulse voltage input to the sensor coil  111 . Thus, a sensor coil  111  placed at a location where magnetic flux density is relatively high is preferentially selected, and therefore a foreign object detection device  100  according to Embodiment 3 can more promptly and more reliably detect a foreign object  10  existing at a location where magnetic flux density is relatively high. 
     Embodiment 4 
     Next, Embodiment 4 of the present disclosure will be described. Description of a configuration and processing similar to those according to Embodiments 1 to 3 is omitted or simplified. 
     The foreign object detection device  100  detects a foreign object  10  existing in the detection target area, according to Embodiment 1. On the other hand, a foreign object detection device  100  detects an abnormality other than a foreign object  10  in a power transmission device  200  in addition to detecting a foreign object  10 , according to Embodiment 4. 
     A determiner  134  according to Embodiment 4 determines whether induced voltage induced in any of sensor coils  111  included in a detection coil unit  110  is abnormal. An abnormality determined by the determiner  134  is an abnormality based on a phenomenon different from existence of a foreign object  10 , such as generation of overcurrent or a failure of a sensor coil  111 . 
     The determiner  134  measures, by a measurer  123 , induced voltage induced in a sensor coil  111  selected by a selector  132  when electric power is transmitted from the power transmission device  200  to a power reception device  300  and pulse voltage is not input from a pulse generator  140 . Then, when the magnitude of the induced voltage measured by the measurer  123  is greater than a predetermined upper limit, the determiner  134  determines that the induced voltage is abnormal. For example, the above corresponds to a case that an abnormally high induced voltage is induced due to overcurrent. The upper limit is preset to a maximum value of the magnitude of induced voltage assumed under normal operation. 
     Further, the determiner  134  also determines that induced voltage is abnormal when the magnitude of the induced voltage is less than a predetermined lower limit. The above corresponds to a case that induced voltage is not output from a sensor coil  111  selected by the selector  132  or an output value of the induced voltage is excessively small, such as a case of a failure of the sensor coil  111 . The lower limit includes 0 and is preset to a minimum value of the magnitude of induced voltage assumed under normal operation. 
     When induced voltage is determined to be abnormal by the determiner  134 , a result output device  136  instructs a notifier  150  to notify the abnormality. When receiving the notification from the determiner  134 , the notifier  150  transmits information indicating occurrence of the abnormality to a terminal device  600  carried by a user. When receiving the information, the terminal device  600  informs the occurrence of the abnormality to the user by a screen display, a voice output, or the like. 
     When induced voltage is determined to be abnormal by the determiner  134 , a power transmission controller  137  causes an electric power supply device  220  to restrict supply of AC power to a power transmission coil  211 . For example, the power transmission controller  137  instructs the electric power supply device  220  to stop supply of AC power to the power transmission coil  211 . When receiving the instruction to stop supply of AC power, the electric power supply device  220  stops supply of AC power to the power transmission coil  211 . Thus, electric power transmission by the power transmission device  200  stops. 
     Threshold value change processing in Embodiment 4 will be described with reference to  FIG.  17   . The foreign object detection device  100  according to Embodiment 4 executes threshold value change processing illustrated in  FIG.  17    in place of the threshold value change processing illustrated in  FIG.  12    in Embodiment 1. Note that processing other than the threshold value change processing in Embodiment 4 is similar to the processing illustrated in  FIG.  11    and  FIG.  13    in Embodiment 1, and therefore description thereof is omitted. 
     When starting the threshold value change processing illustrated in  FIG.  17   , a detector  120  reads a threshold value set to a selected sensor coil  111  of threshold values for the sensor coils  111  stored in a storage  122  (Step S 401 ). Then, the detector  120  measures induced voltage induced in the selected sensor coil  111  by the measurer  123  (Step S 402 ). 
     When measuring induced voltage, the detector  120  determines whether the induced voltage is abnormal (Step S 403 ). For example, the detector  120  determines that the induced voltage is abnormal when the magnitude of the induced voltage is greater than a predetermined upper limit or less than a predetermined lower limit. 
     When the induced voltage is abnormal (Step S 403 : YES), the detector  120  functions as the result output device  136  and notifies a user of the abnormality through the notifier  150  (Step S 404 ). 
     When notifying the abnormality, the detector  120  functions as the power transmission controller  137  and instructs the electric power supply device  220  to stop electric power transmission (Step S 405 ). Specifically, the detector  120  transmits, to the electric power supply device  220 , an instruction to stop supply of AC power to the power transmission coil  211 . The processing of giving an instruction to stop electric power transmission in Step S 405  may be performed prior to the processing of notifying foreign object detection in Step S 404 . 
     When giving the instruction to stop electric power transmission, the detector  120  exits the threshold value change processing illustrated in  FIG.  17    and returns to Step S 101  in the foreign object detection processing illustrated in  FIG.  11   . Specifically, the detector  120  exits a loop of the threshold value change processing and determination processing repeatedly executed on the sensor coils  111  and stands by until receiving a start instruction of the foreign object detection processing again. 
     On the other hand, when the induced voltage is not abnormal (Step S 403 : NO), the detector  120  executes processing similar to Step S 203  and beyond in Embodiment 1. Specifically, the detector  120  changes the threshold value set to the selected sensor coil  111 , based on the measured induced voltage (Step S 404 ). When changing the threshold value, the detector  120  saves the changed threshold value into the storage  122 . Thus, the threshold value change processing illustrated in  FIG.  17    ends. 
     As described above, the foreign object detection device  100  according to Embodiment 4 detects a foreign object  10 , based on induced voltage induced in each sensor coil  111  by alternating magnetic flux Φ and further detects an abnormality other than a foreign object  10  in the power transmission device  200 . The induced voltage used for detection of a foreign object  10  is also used for detection of an abnormality other than a foreign object  10 , and therefore a new configuration does not need to be added. Therefore, an abnormality occurring in the power transmission device  200  can be detected with a simple configuration. 
     Furthermore, the foreign object detection device  100  according to Embodiment 4 uses induced voltage and therefore can detect an abnormality on the power transmission device  200  side rather than on the power reception device  300  side. Assuming that a state abnormality is detected in the power reception device  300 , it takes time for the power reception device  300  to notify the power transmission device  200  of an abnormality after detecting the abnormality. For example, when instructing, from the power reception device  300 , the power transmission device  200  to stop power feed by using periodic communication such as wireless fidelity (Wi-Fi) in a case of occurrence of output overvoltage, it takes several tens of milliseconds. On the other hand, Embodiment 4 enables the detector  120  to directly instruct the electric power supply device  220  to stop power feed through a signal line. Therefore, power feed can be far more rapidly stopped compared with periodic communication such as Wi-Fi. 
     Embodiment 5 
     Next, Embodiment 5 of the present disclosure will be described. Description of a configuration and processing similar to those according to Embodiments 1 to 4 is omitted or simplified. 
     The determiner  134  determines whether a foreign object  10  exists near a sensor coil  111 , based on response voltage output from the sensor coil  111  as a response to pulse voltage input from the pulse generator  140 , according to Embodiments 1 to 4. In other words, the determiner  134  determines existence of a foreign object  10  by a self-excitation method. On the other hand, a determiner  134  determines existence of a foreign object  10  by a separate excitation method, according to Embodiment 5. 
     Specifically, the determiner  134  determines whether a foreign object  10  exists near a sensor coil  111 , based on induced voltage induced in the sensor coil  111  by magnetic flux generated by a power transmission coil  211 . In other words, output voltage used for determining existence of a foreign object  10  is induced voltage induced in a sensor coil  111  by alternating magnetic flux Φ generated by the power transmission coil  211  rather than response voltage output from the sensor coil  111  as a response to pulse voltage, according to Embodiment 5. 
     In other words, induced voltage is used in both determination processing executed by the determiner  134  and threshold value change processing executed by a changer  135 , according to Embodiment 5. Existence of a foreign object  10  is highly likely to change on a relatively short time scale. Therefore, the determiner  134  determines existence of a foreign object  10  according to a minute change in induced voltage in a relatively short period. On the other hand, density of alternating magnetic flux Φ for determining a threshold value depends on a state, an installation environment, and the like of a power transmission device  200  and therefore is less likely to change on a short time scale. Therefore, the changer  135  changes a threshold value according to a dynamic change in induced voltage in a relatively long period. Thus, induced voltages induced in different time periods are used in the determination processing and the threshold value change processing, respectively, according to Embodiment 5. 
     More specifically, in the determination processing, the determiner  134  determines existence of a foreign object  10 , based on a comparison result between a comparison target value based on induced voltage output from a sensor coil  111  in a first period and a threshold value set to the sensor coil  111 . The first period is a period with a predetermined length in the past relative to the present moment and, for example, is a period with a length of roughly several minutes to several hours. The determiner  134  determines whether a foreign object  10  exists near a sensor coil  111 , based on the mean value of magnitudes of induced voltage induced in the sensor coil  111  in the first period. 
     In the threshold value change processing, the changer  135  changes a threshold value set to a sensor coil  111 , based on induced voltage induced in the sensor coil  111  by alternating magnetic flux Φ in a second period longer than the first period. The second period is a period with a predetermined length in the past relative to the present moment and, for example, is a period with a length of roughly several hours to several days. The changer  135  changes a threshold value set to a sensor coil  111 , based on the mean value of magnitudes of induced voltage induced in the sensor coil  111  in the second period. 
     The other items in Embodiment 5 can be similarly described by replacing “response voltage” and “induced voltage” in Embodiment 1 with “induced voltage in the first period” and “induced voltage in the second period,” respectively. Note that, in the case of the self-excitation method in Embodiment 1, the resonance frequency of a resonant circuit formed by the coil  114  and the capacitor  115  is designed to be several MHz. Therefore, the measurer  123  measures a voltage value between both ends of the coil  114  as induced voltage in a state of the switch  116  being turned off, according to Embodiment 1. On the other hand, in the case of the separate excitation method in Embodiment 5, the resonance frequency is designed to be about 85 kHz. Therefore, a measurer  123  measures a voltage value between both ends of a resonant circuit as induced voltage in a state of a switch  116  being turned on, according to Embodiment 5. 
     Specifically, in Step S 104  in the foreign object detection processing illustrated in  FIG.  11   , a detector  120  controls switches  116  and  117  in each sensor coil  111  included in a detection coil unit  110  in such a way that both switches  116  and  117  included in a sensor coil  111  selected in Step S 103  are in an on-state, and all switches  116  and  117  included in unselected sensor coils  111  are in an off-state. The detector  120  executes the threshold value change processing in Step S 105  and the determination processing in Step S 107  in such a state of a resonant circuit being formed in the selected sensor coil  111 . The switch control processing in Step S 106  is omitted. 
     Further, pulse voltage is not used in Embodiment 5, and therefore the foreign object detection device  100  may not include a pulse generator  140 . The processing in Step S 301  in the determination processing illustrated in  FIG.  13    is omitted. 
     Thus, even when the separate excitation method is used, existence of a foreign object  10  can be determined based on output voltage output from each sensor coil  111 . Then, by changing a threshold value set to each sensor coil  111 , based on induced voltage, an effect of enhancing detection precision of a foreign object  10  can be acquired, similarly to Embodiments 1 to 4. 
     Modified Examples 
     While the embodiments of the present disclosure have been described above, modifications and applications in various forms can be made in implementation of the present disclosure. In the present disclosure, any part of the configurations, functions, and operations described in the aforementioned embodiments can be employed. Further, in the present disclosure, more configurations, functions, and operations may be employed in addition to the aforementioned configurations, functions, and operations. Further, the aforementioned embodiments can be combined in any way as appropriate. Further, the number of components described in the aforementioned embodiments may be adjusted as appropriate. Further, it is apparent that materials, sizes, electric characteristics, and the like employable in the present disclosure are not limited to those described in the aforementioned embodiments. 
     An example of a comparison target value compared with a threshold value being a difference value between a value indicating a characteristic of response voltage output from a sensor coil  111  and a reference value has been described in the aforementioned embodiments. However, a comparison target value may not be a difference value itself as long as the comparison target value is a value based on a difference value. For example, a comparison target value may be a value calculated by performing a predetermined operation on a difference value or may be a value determined from a difference value with reference to a predetermined table. 
     The determiner  134  determines that a foreign object  10  exists when an excess count of a comparison target value exceeding a threshold value reaches a threshold count, according to the aforementioned embodiments. However, the determiner  134  may determine that a foreign object  10  exists when a comparison target value exceeds the threshold value only once. Alternatively, threshold values may be set and the determiner  134  may determine existence of a foreign object  10 , based on an excess count of a comparison target value exceeding each of the threshold values. When threshold values are set, the changer  135  may execute the aforementioned threshold value change processing on each of the threshold values or may execute the aforementioned threshold value change processing on part of the threshold values. 
     The measurer  123  measures induced voltage output from a sensor coil  111 , according to the aforementioned embodiments. Then, the changer  135  executes the threshold value change processing, based on the induced voltage measured by the measurer  123 . However, the measurer  123  may include a circuit for current measurement and measure induced current output from a sensor coil  111 . Then, the changer  135  may execute the threshold value change processing, based on the induced current measured by the measurer  123 . 
     Detection of a foreign object  10  is notified to a user of a terminal device  600  by transmitting information indicating the detection of the foreign object  10  to the terminal device  600  by the notifier  150 , according to the aforementioned embodiments. However, a method of notifying a user of detection of a foreign object  10  is not limited to the above. For example, the notifier  150  may directly notify a user of detection of a foreign object  10  by a screen display, a voice output, or the like. 
     In the controller  121 , the CPU functions as components being the power transmission information acquirer  131 , the selector  132 , the driver  133 , the determiner  134 , the changer  135 , the result output device  136 , and the power transmission controller  137  by executing a program stored in the ROM or the storage  122 , according to the aforementioned embodiments. However, the controller  121  may include dedicated hardware such as an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or various control circuits in place of the CPU, and the dedicated hardware may function as the components. In this case, the function of each component may be provided by individual piece of hardware, or the functions of the components may be collectively provided by a single piece of hardware. Further, part of the functions of the components may be provided by dedicated hardware and the other part may be provided by software or firmware. 
     While several embodiments of the present disclosure have been described, the embodiments are presented as examples and do not intend to limit the scope of the invention. The new embodiments may be implemented in various other forms, and various omissions, substitutions, and changes may be made without departing from the spirit of the invention. The embodiments and modifications thereof are included in the scope and spirit of the invention and are also included in the invention described in the claims and equivalents thereof. 
     REFERENCE SIGNS LIST 
     
         
           10  Foreign object 
           100  Foreign object detection device 
           110  Detection coil unit 
           111 ,  111 A,  111 B,  111 C,  111 D,  111 E,  111 F,  111 G,  111 H,  111 I,  111 J,  111 K,  111 L Sensor coil 
           112  External connection connector 
           113  Detection coil substrate 
           114  Coil 
           115  Capacitor 
           116 ,  117  Switch 
           118  First connection wiring 
           119  Second connection wiring 
           120  Detector 
           121  Controller 
           122  Storage 
           123  Measurer 
           131  Power transmission information acquirer 
           132  Selector 
           133  Driver 
           134  Determiner 
           135  Changer 
           136  Result output device 
           137  Power transmission controller 
           140  Pulse generator 
           150  Notifier 
           200  Power transmission device 
           210  Power transmission coil unit 
           211  Power transmission coil 
           212  Magnetic body plate 
           220  Electric power supply device 
           300  Power reception device 
           310  Power reception coil unit 
           311  Power reception coil 
           312  Magnetic body plate 
           320  Rectifier circuit 
           400  Commercial power source 
           500  Storage battery 
           600  Terminal device 
           700  Electric vehicle 
           1000  Electric power transmission system