Patent Publication Number: US-2022239165-A1

Title: Position detection system and electric power transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2021-11687, filed on Jan. 28, 2021, the entire disclosure of which is incorporated by reference herein. 
     FIELD 
     This application relates generally to a position detection system and an electric power transmission system. 
     BACKGROUND 
     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. 
     In order to efficiently transmit electric power, a coil axis of the power transmission coil needs to be precisely aligned with a coil axis of the power reception coil. In order to precisely align the coil axes, a relative position between the power transmission coil and the power reception coil needs to be precisely detected. Unexamined Japanese Patent Application Publication No. 2020-198689 describes a technology for detecting a relative position between a power transmission coil and a power reception coil by using a radio wave in the low frequency (LF) band. 
     In the technology described in Unexamined Japanese Patent Application Publication No. 2020-198689, a relative position between the power transmission coil and the power reception coil is detected based on intensity of a radio wave acquired for each combination of a transmission antenna and a reception antenna at the time of detection of the relative position and predetermined reference data. The reference data are data indicating, for each combination of a transmission antenna and a reception antenna, a reference of a correspondence between a relative position between the power transmission coil and the power reception coil, and intensity of a radio wave received by the reception antenna. For example, the reference data are acquired by using a reference position detection system before executing detection of a relative position. The reference position detection system is basically a position detection system similar to a position detection system used for detection of a relative position. 
     SUMMARY 
     In order to precisely detect a relative position by using the reference data, a difference in the antenna characteristics of antennas including the transmission antenna and the reception antenna is required not to exist between the time of acquisition of the reference data and the time of detection of the relative position. However, the antenna characteristic of an antenna used when the reference data are acquired may differ from the antenna characteristic of an antenna used for detection of a relative position due to individual differences between antenna characteristics caused in a manufacturing process of antennas, a change in an environment around the antennas, and the like. 
     Therefore, calibration is preferably executed when a relative position is detected, in order to align the antenna characteristic of the antenna used for detection of the relative position with the antenna characteristic of the antenna used when the reference data are acquired. However, the technology described in Unexamined Japanese Patent Application Publication No. 2020-198689 does not have a mechanism for achieving such calibration and therefore may not precisely detect a relative position. Therefore, a position detection system precisely detecting a relative position between a power transmission coil and a power reception coil in wireless electric power transmission is desired. 
     The present disclosure has been made in view of the aforementioned problem, and an objective of the present disclosure is to precisely detect a relative position between a power transmission coil and a power reception coil in wireless electric power transmission. 
     In order to solve the aforementioned problem, a position detection system according to an embodiment of the present disclosure 
     is a position detection system for an electric power transmission system wirelessly transmitting electric power from a power transmission coil included in a power transmission device to a power reception coil included in a power reception device and includes: 
     at least one first antenna provided in one device from among the power transmission device and the power reception device; 
     a first transmission circuit driving the at least one first antenna; 
     a plurality of second antennas provided in the other device from among the power transmission device and the power reception device; 
     a radio wave detection circuit detecting intensity of a radio wave received by the plurality of second antennas; 
     a position detector detecting a relative position between the power transmission coil and the power reception coil, based on the intensity detected by the radio wave detection circuit; and 
     a second transmission circuit driving at least one second antenna from among the plurality of second antennas, 
     wherein the radio wave detection circuit detects intensity of a radio wave being transmitted from a second antenna driven by the second transmission circuit from among the plurality of second antennas and being received by another second antenna being a second antenna other than the second antenna driven by the second transmission circuit from among the plurality of second antennas. 
     The aforementioned configuration can precisely detect a relative position between a power transmission coil and a power reception coil in wireless electric power transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of this application can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  is a schematic configuration diagram of an electric power transmission system according to Embodiment 1; 
         FIG. 2  is a perspective view of a power transmission coil unit and a power reception coil unit according to Embodiment 1; 
         FIG. 3  is a placement diagram of components included in a position detection system according to Embodiment 1; 
         FIG. 4  is a circuit diagram of the position detection system according to Embodiment 1; 
         FIG. 5  is a configuration diagram of the position detection system according to Embodiment 1; 
         FIG. 6  is a graph illustrating a frequency characteristic of an RLC parallel resonant circuit; 
         FIG. 7  is a graph illustrating a frequency characteristic of an LC series resonant circuit; 
         FIG. 8  is a first graph illustrating a correspondence between difference voltage and VCr; 
         FIG. 9  is a second graph illustrating a correspondence between difference voltage and VCr; 
         FIG. 10  is a graph illustrating a correspondence between difference voltage and VRr; 
         FIG. 11  is a flowchart illustrating antenna calibration processing executed by the position detection system according to Embodiment 1; 
         FIG. 12  is a flowchart illustrating first calibration processing in  FIG. 11 ; 
         FIG. 13  is a flowchart illustrating second calibration processing in  FIG. 11 ; 
         FIG. 14  is a placement diagram of a transmission antenna and a reception antenna according to Embodiment 2; 
         FIG. 15  is a circuit diagram of a position detection system according to Embodiment 2; 
         FIG. 16  is a configuration diagram of the position detection system according to Embodiment 2; 
         FIG. 17  is a flowchart illustrating antenna calibration processing executed by the position detection system according to Embodiment 2; 
         FIG. 18  is a flowchart illustrating third calibration processing in  FIG. 17 ; and 
         FIG. 19  is a flowchart illustrating fourth calibration processing in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION 
     Electric power transmission systems according to embodiments of a technology according to the present disclosure will be described below referring to drawings. Note that, in the following embodiments, the same components are given the same sign. Further, the ratio in size between components and the shapes of the components that are illustrated in each diagram are not necessarily the same as those in implementation. 
     Embodiment 1 
     An electric power transmission system according to the present embodiment can be used for charging secondary batteries in various devices such as an electric vehicle (EV), mobile equipment such as a smartphone, and industrial equipment. An example of the electric power transmission system executing charging of a storage battery in an EV will be described below. 
       FIG. 1  is a diagram illustrating 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. The electric vehicle  700  is an example of a movable body. 
     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 a position detection system  100  detecting a relative position between a power transmission coil and a power reception coil, a 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 a 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. Further, details of the position detection system  100  will be described later. 
     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 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 . 
       FIG. 2  illustrates main parts of the power transmission coil unit  210  and main parts of a power reception coil unit  310 . As illustrated in  FIG. 2 , the power transmission coil unit  210  includes a power transmission coil  211  being supplied with AC power from the electric power supply device  220  and inducing alternating magnetic flux  1 , and a magnetic body plate  212  provided for improving the inductance value of the power transmission coil  211 . The power transmission coil  211  is formed by spirally winding a conducting wire around a coil axis  213  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. 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. 
     The magnetic body plate  212  has a plate shape with a hole in the central part and is formed of a magnetic body. For example, the magnetic body plate  212  is a plate-shaped member formed of ferrite being a composite oxide of iron oxide and metal. The magnetic body plate  212  may be formed of an aggregate of a plurality of segmented magnetic bodies, and the plurality of 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 the power reception coil unit  310  receiving AC power transmitted by the power transmission device  200  and a power rectifier circuit  320  converting AC power supplied from the 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  1  induced by the power transmission coil  211 , and a magnetic body plate  312  provided for improving the inductance value of the power reception coil  311 . The power reception coil  311  is formed by spirally winding a conducting wire around a coil axis  313  on the magnetic body plate  312 . The power reception coil  311  and a capacitor provided at each of two ends of the power reception coil  311  forms 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  1  by receiving electric power from the electric power supply device  220 , an induced electromotive force is induced at the power reception coil  311  by interlinkage of the alternating magnetic flux  1  with the power reception coil  311 . 
     The magnetic body plate  312  is a plate-shaped member with a hole in the central part and is formed of a magnetic body. For example, the magnetic body plate  312  is a plate-shaped member formed of ferrite being a composite oxide of iron oxide and metal. The magnetic body plate  312  may be formed of an aggregate of a plurality of segmented magnetic bodies, and the plurality of 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 at 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 . 
     The position detection system  100  is a system detecting a relative position between the power transmission coil  211  included in the power transmission device  200  and the power reception coil  311  included in the power reception device  300 . The position detection system  100  is incorporated into the electric power transmission system  1000  and is used for alignment of a coil axis of the power transmission coil  211  with a coil axis of the power reception coil  311 . The position detection system  100  detects a relative position between the power transmission coil  211  and the power reception coil  311  by using a radio wave in the LF band. The position detection system  100  is placed in such a way as to be split between the power transmission device  200  and the power reception device  300 . 
     For example, part of components of the position detection system  100  are placed in the power transmission coil unit  210  and the other components of the position detection system  100  are placed in the power reception coil unit  310 , as illustrated in  FIG. 3 . Specifically, an antenna  110  and a transmission circuit  120  are placed in the power reception coil unit  310 ; and four antennas  150 , a transmission circuit  160 , and a radio wave detection circuit  170  are placed in the power transmission coil unit  210 . The four antennas  150  are placed at four corners of the power transmission coil unit  210  having an almost rectangular shape in a plan view. The antenna  150  is a general name for an antenna  150 A, an antenna  150 B, an antenna  150 C, and an antenna  150 D. Note that  FIG. 3  illustrates only main components from among the components included in the position detection system  100 . 
     The antenna  110  is an antenna emitting a radio wave in the LF band. The antenna  110  converts a high-frequency signal supplied from the transmission circuit  120  into a radio wave and emits the radio wave. The antenna  110  according to the present embodiment is a coil formed by winding a conducting wire around a bar-shaped magnetic body. While the impedance of the antenna  110  includes resistance, inductive reactance, and capacitive reactance, inductive reactance is dominant. The antenna  110  may be hereinafter referred to as a transmission antenna. The antenna  110  is an example of a first antenna. 
     The transmission circuit  120  is a circuit feeding electric power to at least one antenna  110  and driving at least one antenna  110 . The transmission circuit  120  according to the present embodiment drives one antenna  110 . The transmission circuit  120  generates a high-frequency signal from power source voltage and supplies the generated high-frequency signal to the antenna  110 . The transmission circuit  120  includes an inverter circuit converting DC power into AC power and is pulse width modulation (PWM) controllable. The transmission circuit  120  includes a capacitive element forming an LC resonant circuit with the antenna  110 . The LC resonant circuit according to the present embodiment is an LC series resonant circuit. The frequency characteristic of the LC series resonant circuit may be hereinafter referred to as an antenna characteristic of the antenna  110 . The transmission circuit  120  is an example of a first transmission circuit. 
     The antenna  150  is an antenna for receiving a radio wave in the LF band. The antenna  150  captures a radio wave emitted by the antenna  110 , converts the captured radio wave into a high-frequency signal, and supplies the signal to the radio wave detection circuit  170 . The antenna  150  is basically formed similarly to the antenna  110 . In other words, the antenna  150  is a coil formed by winding a conducting wire around a bar-shaped magnetic body. Further, while the impedance of the antenna  150  includes resistance, inductive reactance, and capacitive reactance, inductive reactance is dominant. The antenna  150  may be hereinafter referred to as a reception antenna. The antenna  150  is an example of a second antenna. 
     The radio wave detection circuit  170  is a circuit detecting intensity of a radio wave received by a plurality of antennas  150 . The radio wave detection circuit  170  outputs voltage related to the amplitude of a high-frequency signal related to a radio wave received by each of the plurality of antennas  150 . The radio wave detection circuit  170  includes a second capacitive element and a first resistor forming an RLC resonant circuit with the antenna  150 . The RLC resonant circuit according to the present embodiment is an RLC parallel resonant circuit. The frequency characteristic of the RLC parallel resonant circuit may be hereinafter referred to as an antenna characteristic of the antenna  150 . 
     The position detection system  100  detects a relative position between the power transmission coil  211  and the power reception coil  311 , based on intensity detected by the radio wave detection circuit  170  and predetermined reference data. The reference data are data indicating, for each combination of the antenna  110  and the antenna  150 , a reference of a correspondence between a relative position between the power transmission coil  211  and the power reception coil  311 , and intensity of a radio wave received by the antenna  150 . For example, the reference data are acquired before detection of a relative position is executed. The reference data are data indicating a map of reference intensity and therefore may be referred to as map data. 
     In order to precisely detect a relative position by using the reference data, a difference in the antenna characteristics of the antenna  110  and the antenna  150  between the time of acquisition of the reference data and the time of detection of the relative position is required to be nonexistent. However, the antenna characteristics of the antenna  110  and the antenna  150  used when the reference data are acquired may differ from the antenna characteristics of the antenna  110  and the antenna  150  used for detection of the relative position due to individual differences between the antenna characteristics caused in a manufacturing process of the antenna  110  and the antenna  150 , a change in an environment around the antenna  110  and the antenna  150 , and the like. 
     Therefore, calibration is executed when a relative position is detected, in order to align the antenna characteristics of the antenna  110  and the antenna  150  used for detection of the relative position with the antenna characteristics of the antenna  110  and the antenna  150  used when the reference data are acquired, according to the present embodiment. In order to achieve such calibration, at least one antenna  150  from among a plurality of antennas  150  being reception antennas can not only receive a radio wave but also transmit a radio wave, according to the present embodiment. Specifically, the position detection system  100  according to the present embodiment includes the transmission circuit  160  driving the at least one antenna  150 . 
     The transmission circuit  160  is a circuit feeding electric power to at least one antenna  150  and driving at least one antenna  150 . Antennas  150  driven by the transmission circuit  160  according to the present embodiment are two antennas being the antenna  150 A and the antenna  150 B. The transmission circuit  160  generates a high-frequency signal from power source voltage and supplies the generated high-frequency signal to the antenna  150 . The transmission circuit  160  includes an inverter circuit converting DC power into AC power and is PWM controllable. The transmission circuit  160  includes a first capacitive element forming an LC resonant circuit with the antenna  150 . The LC resonant circuit according to the present embodiment is an LC series resonant circuit. The frequency characteristic of the LC series resonant circuit may be hereinafter referred to as an antenna characteristic of the antenna  150 . The transmission circuit  160  is an example of a second transmission circuit. 
     The radio wave detection circuit  170  detects intensity of a radio wave being transmitted from an antenna  150  driven by the transmission circuit  160  from among a plurality of antennas  150  and being received by another antenna  150  being an antenna  150  other than the antenna  150  driven by the transmission circuit  160  from among the plurality of antennas  150 . For example, a case of the antenna  150 A being driven by the transmission circuit  160  and the antenna  150 D receiving a radio wave emitted by the antenna  150 A is assumed. In this case, the radio wave detection circuit  170  detects intensity of the radio wave received by the antenna  150 D. 
     The position detection system  100  detects the difference between the intensity of the radio wave received by the antenna  150 D and predetermined reference intensity. The reference intensity is intensity to be detected by the radio wave detection circuit  170  when the antenna  150 D receives a radio wave emitted by the antenna  150 A in a case of the antenna characteristic of the antenna  150 A and the antenna characteristic of the antenna  150 D being suitable. For example, the reference intensity is acquired when the reference data are acquired, for each combination of an antenna  150  capable of transmitting a radio wave and an antenna  150  capable of receiving the radio wave. Since there are two antennas  150  capable of transmitting a radio wave and three antennas  150  capable of receiving a radio wave, there are six combinations, according to the present embodiment. 
     A relative position between the four antennas  150  is the same between the time of acquisition of the reference intensity and the time of detection of the relative position. Accordingly, intensity acquired when the relative position is detected is the same as the reference intensity as long as the antenna characteristic of the antenna  150  transmitting a radio wave and the antenna characteristic of the antenna  150  receiving the radio wave are the same between the time of acquisition of the reference intensity and the time of detection of the relative position. In other words, the antenna characteristics at the time of acquisition of the reference intensity are considered to be reproduced by adjusting the antenna characteristic of the antenna  150  transmitting a radio wave and the antenna characteristic of the antenna  150  receiving the radio wave in such a way that intensity acquired when the relative position is detected is the same as the reference intensity. 
     Therefore, the antenna characteristic of the antenna  150  transmitting a radio wave and the antenna characteristic of the antenna  150  receiving the radio wave are adjusted in such a way that the antenna characteristics at the time of acquisition of the reference intensity are reproduced, according to the present embodiment. Specifically, the frequency characteristic of an LC series resonant circuit including the antenna  150  transmitting a radio wave and the frequency characteristic of an RLC parallel resonant circuit including the antenna  150  receiving the radio wave are adjusted. Details of the adjustment method will be described later. 
     Next, connections between the antenna  150 , the transmission circuit  160 , and the radio wave detection circuit  170  will be described with reference to a circuit diagram of the position detection system  100  illustrated in  FIG. 4 . Note that  FIG. 4  illustrates a circuit diagram of part of the components included in the position detection system  100 . As illustrated in  FIG. 4 , the position detection system  100  includes two switches  151  and four switches  152 . The switch  151  is a general name for a switch  151 A and a switch  151 B. The switch  152  is a general name for a switch  152 A, a switch  152 B, a switch  152 C, and a switch  152 D. 
     The switches  151  are switches changing a connection between at least one antenna  150  from among a plurality of antennas  150  and the transmission circuit  160 . The switch  151 A is a switch changing a connection between the antenna  150 A and the transmission circuit  160 . When the switch  151 A is turned on, an LC series resonant circuit is formed by the antenna  150 A and a capacitive element  161 A included in the transmission circuit  160 . The switch  151 B is a switch changing a connection between the antenna  150 B and the transmission circuit  160 . When the switch  151 B is turned on, an LC series resonant circuit is formed by the antenna  150 B and a capacitive element  161 B included in the transmission circuit  160 . Each of the capacitive element  161 A and the capacitive element  161 B is a variable capacitance element having a variable capacitance value. A capacitive element  161  is a general name for the capacitive element  161 A and the capacitive element  161 B. The capacitive element  161 A and the capacitive element  161 B are examples of the first capacitive element. The switch  151  is an example of a first switch. 
     The switches  152  are switches changing connections between a plurality of antennas  150  and the radio wave detection circuit  170 . The switch  152 A is a switch changing a connection between the antenna  150 A and the radio wave detection circuit  170 . When the switch  152 A is turned on, an RLC parallel resonant circuit is formed by the antenna  150 A, and a capacitive element  171 A and a resistor  172 A that are included in the radio wave detection circuit  170 . The switch  152 B is a switch changing a connection between the antenna  150 B and the radio wave detection circuit  170 . When the switch  152 B is turned on, an RLC parallel resonant circuit is formed by the antenna  150 B, and a capacitive element  171 B and a resistor  172 B that are included in the radio wave detection circuit  170 . 
     The switch  152 C is a switch changing a connection between the antenna  150 C and the radio wave detection circuit  170 . When the switch  152 C is turned on, an RLC parallel resonant circuit is formed by the antenna  150 C, and a capacitive element  171 C and a resistor  172 C that are included in the radio wave detection circuit  170 . The switch  152 D is a switch changing a connection between the antenna  150 D and the radio wave detection circuit  170 . When the switch  152 D is turned on, an RLC parallel resonant circuit is formed by the antenna  150 D, and a capacitive element  171 D and a resistor  172 D that are included in the radio wave detection circuit  170 . The switch  152  is an example of a second switch. 
     Each of the capacitive element  171 A, the capacitive element  171 B, the capacitive element  171 C, and the capacitive element  171 D is a variable capacitance element having a variable capacitance value. Each of the resistor  172 A, the resistor  172 B, the resistor  172 C, and the resistor  172 D is a variable resistance element having a variable resistance value. A capacitive element  171  is a general name for the capacitive element  171 A, the capacitive element  171 B, the capacitive element  171 C, and the capacitive element  171 D. A resistor  172  is a general name for the resistor  172 A, the resistor  172 B, the resistor  172 C, and the resistor  172 D. The capacitive element  171 A, the capacitive element  171 B, the capacitive element  171 C, and the capacitive element  171 D are examples of the second capacitive element. The resistor  172 A, the resistor  172 B, the resistor  172 C, and the resistor  172 D are examples of the first resistor. 
     The antenna  110  and the transmission circuit  120  are always connected. Accordingly, an LC series resonant circuit is always formed by the antenna  110  and a capacitive element  121 C included in the transmission circuit  120 . The capacitive element  121 C is a capacitive element having a fixed capacitance value. 
     When the antenna  150 A is connected to the transmission circuit  160 , one of the antenna  150 B, the antenna  150 C, and the antenna  150 D is connected to the radio wave detection circuit  170 . When the antenna  150 B is connected to the transmission circuit  160 , one of the antenna  150 A, the antenna  150 C, and the antenna  150 D is connected to the radio wave detection circuit  170 . The antenna  150 C and the antenna  150 D are not connected to the transmission circuit  160 . The antenna  150 A and the antenna  150 B are not simultaneously connected to the transmission circuit  160 . The antenna  150 A and the antenna  150 B are not simultaneously connected to the radio wave detection circuit  170 . Each of the antenna  150 A and the antenna  150 B is not simultaneously connected to both the transmission circuit  160  and the radio wave detection circuit  170 . 
     Next, the configuration of the position detection system  100  will be described in detail with reference to  FIG. 5 . Note that description of a component that has been already described is omitted or simplified. The position detection system  100  includes the antenna  110 , the transmission circuit  120 , a power source circuit  125 , a controller  135 , a storage  141 , a communicator  142 , the antennas  150 , the switches  151 , the switches  152 , the transmission circuit  160 , a power source circuit  165 , the radio wave detection circuit  170 , a controller  180 , a storage  191 , and a communicator  192 . 
     The antenna  110  emits a radio wave related to a high-frequency signal supplied from the transmission circuit  120 . The transmission circuit  120  supplies a high-frequency signal generated from power source voltage to the antenna  110 . The power source circuit  125  supplies power source voltage to the transmission circuit  120 . The controller  135  controls operation of components placed in the power reception device  300  from among the components included in the position detection system  100 . For example, the controller  135  causes the transmission circuit  120  to emit a radio wave from the antenna  110  by controlling the transmission circuit  120 . The controller  135  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), a real time clock (RTC), an analog/digital (A/D) converter, and the like. 
     The storage  141  stores various types of information used by the controller  135  or various types of information acquired through operation of the controller  135 . For example, the storage  141  includes a flash memory. The communicator  142  communicates with the communicator  192  in accordance with control by the controller  135 . The communicator  142  includes a communication interface conforming to a well-known wireless communication standard such as Wi-Fi (registered trademark), Bluetooth (registered trademark), Long Term Evolution (LTE), the 4th Generation (4G), or the 5th Generation (5G). 
     The antenna  150  receives a radio wave emitted by the antenna  110 . The switch  151  changes the connection between the antenna  150  and the transmission circuit  160 . The switch  152  changes the connection between the antenna  150  and the radio wave detection circuit  170 . The transmission circuit  160  supplies a high-frequency signal generated from power source voltage to the antenna  150 . The power source circuit  165  supplies power source voltage to the transmission circuit  160 . The radio wave detection circuit  170  detects intensity of a radio wave received by the antenna  150 . 
     The controller  180  controls operation of components placed in the power transmission device  200  from among the components included in the position detection system  100 . For example, the controller  180  adjusts the antenna characteristic of the antenna  150 , based on intensity detected by the radio wave detection circuit  170 . The controller  180  includes a CPU, a ROM, a RAM, an RTC, an A/D converter, and the like. Details of the controller  180  will be described later. 
     The storage  191  stores various types of information used by the controller  180  or various types of information acquired through operation of the controller  180 . For example, the storage  191  stores the reference data used for detection of a relative position and the reference intensity used for adjustment of an antenna characteristic. For example, the storage  191  includes a flash memory. The communicator  192  communicates with the communicator  142  in accordance with control by the controller  180 . The communicator  192  includes a communication interface conforming to a well-known wireless communication standard, similarly to the communicator  142 . 
     Next, the function of the controller  180  will be described in detail. The controller  180  functionally includes a switch controller  181 , a position detector  182 , a difference detector  183 , and an adjuster  184 . For example, the functional units included in the controller  180  are provided by the CPU executing an operation program stored in the ROM. 
     The switch controller  181  controls the switches  151  and the switches  152 . The switch controller  181  controls the switches  152  in such a way as to connect the radio wave detection circuit  170  to an antenna  150  other than an antenna  150  connected to the transmission circuit  160  by the switch  151  from among a plurality of antennas  150 . 
     The position detector  182  detects a relative position between the power transmission coil  211  and the power reception coil  311 , based on intensity detected by the radio wave detection circuit  170 . For example, the position detector  182  estimates a relative position, based on reference data previously acquired for each of four combinations of one antenna  110  being a transmission antenna and four antennas  150  being reception antennas, and intensity detected by the radio wave detection circuit  170  for each of the four combinations. For example, for each of the four combinations, the position detector  182  specifies a relative position where the difference between intensity based on an intensity distribution indicated by the reference data and detected intensity is small as a candidate position. Then, the position detector  182  specifies a relative position repeatedly specified as a candidate position for each of the four combinations. Alternatively, the position detector  182  retrieves similar coordinates from statistics by using four detected intensity values, based on the reference data. Alternatively, the position detector  182  generates a function acquiring an output for an input from compressed reference data and calculates a relative position by using the function. In the power transmission device  200 , positions where the power transmission coil  211  and the antenna  150  are installed, respectively, are predetermined, and a positional relation between the power transmission coil  211  and the antenna  150  is predetermined. Further, in the power reception device  300 , positions where the power reception coil  311  and the antenna  110  are installed, respectively, are predetermined, and a positional relation between the power reception coil  311  and the antenna  110  is predetermined. Therefore, when a positional relation between the antenna  110  and the antenna  150  can be specified from detected intensity values, a positional relation between the power transmission device  200  and the power reception device  300  can also be specified. 
     The difference detector  183  detects the difference between intensity of a radio wave received by another antenna  150  and predetermined reference intensity. The another antenna  150  is an antenna  150  other than an antenna  150  driven by the transmission circuit  160  from among the four antennas  150 . The antenna  150  driven by the transmission circuit  160  is the antenna  150 A, and the another antenna  150  is the antenna  150 D. For example, the reference intensity is intensity of a radio wave being emitted from an antenna  150  placed at the same position as the antenna  150 A and being received by an antenna  150  placed at the same position as the antenna  150 D when the reference data are acquired. A larger difference detected by the difference detector  183  indicates a larger difference in the antenna characteristic between the time of acquisition of the reference intensity and the time of execution of calibration. 
     The adjuster  184  adjusts each element in such a way that the difference detected by the difference detector  183  decreases. For example, the adjuster  184  adjusts the frequency characteristic of an RLC parallel resonant circuit including an antenna  150  receiving a radio wave. For example, the adjuster  184  adjusts the capacitance value of a capacitive element  171  included in the RLC parallel resonant circuit in such a way that the aforementioned difference decreases. Further, the adjuster  184  adjusts the resistance value of a resistor  172  included in the RLC parallel resonant circuit in such a way that the aforementioned difference decreases. 
     Further, for example, the adjuster  184  adjusts the frequency characteristic of an LC series resonant circuit including an antenna  150  transmitting a radio wave. For example, the adjuster  184  adjusts the capacitance value of a capacitive element  161  included in the LC series resonant circuit in such a way that the aforementioned difference decreases. Further, the adjuster  184  adjusts a duty ratio in PWM control of the transmission circuit  160  in such a way that the aforementioned difference decreases. PWM information indicating the duty ratio is transmitted from the communicator  192  to the communicator  142 . Then, when a relative position is detected, PWM control of the transmission circuit  120  is executed at the duty ratio indicated by the PWM information in accordance with control by the controller  135 . 
       FIG. 6  illustrates a frequency characteristic of an RLC parallel resonant circuit including an antenna  150  receiving a radio wave. In  FIG. 6 , VCr denotes a capacitance value of a capacitive element  171  included in the RLC parallel resonant circuit, and VRr denotes a resistance value of a resistor  172  included in the RLC parallel resonant circuit. As illustrated in  FIG. 6 , the resonance frequency of the RLC parallel resonant circuit is adjusted by adjusting the capacitance value of the capacitive element  171 . Further, a peak value at the resonance frequency of the RLC parallel resonant circuit is adjusted by adjusting the resistance value of the resistor  172 . Intensity of a radio wave received by the antenna  150  is adjusted by adjustment of the resonance frequency of the RLC parallel resonant circuit or the peak value at the resonance frequency of the RLC parallel resonant circuit. 
       FIG. 7  illustrates a frequency characteristic of an LC series resonant circuit including an antenna  150  transmitting a radio wave. In  FIG. 7 , VCt denotes a capacitance value of a capacitive element  161  included in the LC series resonant circuit, and Wt denotes a duty ratio in PWM control of the transmission circuit  160 . As illustrated in  FIG. 7 , the resonance frequency of the LC series resonant circuit is adjusted by adjusting the capacitance value of the capacitive element  161 . Further, a peak value at the resonance frequency of the LC series resonant circuit is adjusted by adjusting the duty ratio. Intensity of a radio wave transmitted by the antenna  150  is adjusted by adjustment of the resonance frequency of the LC series resonant circuit or the peak value at the resonance frequency of the LC series resonant circuit. 
     Next, a method of adjusting each element by the adjuster  184  will be described with reference to  FIG. 8 ,  FIG. 9 , and  FIG. 10 .  FIG. 8  is a first graph illustrating a correspondence between difference voltage and VCr.  FIG. 9  is a second graph illustrating a correspondence between difference voltage and VCr.  FIG. 10  is a graph illustrating a correspondence between difference voltage and VRr. A main objective of the present embodiment is basically adjustment of the antenna characteristic of an antenna  150  receiving a radio wave. Accordingly, the adjuster  184  mainly adjusts the frequency characteristic of the RLC parallel resonant circuit including the antenna  150  receiving a radio wave. 
     However, when the antenna characteristic of an antenna  150  transmitting a radio wave is not suitably adjusted, the antenna characteristic of an antenna  150  receiving the radio wave may not be suitably adjusted. Therefore, the adjuster  184  additionally adjusts the frequency characteristic of the LC series resonant circuit including the antenna  150  transmitting a radio wave. Further, the frequency characteristic of an RLC parallel resonant circuit heavily depends on the capacitance value of the capacitive element  171  rather than the resistance value of the resistor  172 . Therefore, the adjuster  184  adjusts the resistance value of the resistor  172  after precisely adjusting the capacitance value of the capacitive element  171 . 
     Difference voltage is voltage related to the difference between the reference intensity and intensity detected by the radio wave detection circuit  170  and is voltage related to the difference detected by the difference detector  183 . V 1  denotes a first threshold value, and V 2  denotes a second threshold value. The first threshold value is an upper limit of allowable difference voltage. In other words, when the difference voltage is adjusted to the first threshold value or less, calibration is considered to be suitably executed. The second threshold value is an upper limit of the difference voltage at which a big-step search is changed to a small-step search in a search for VCr minimizing the difference voltage. The second threshold value is a value larger than the first threshold value. 
     A big-step search has a larger step width being a shift amount of VCr compared with a small-step search. In a big-step search, speed of bringing VCr close to C 0  being VCr minimizing the difference voltage is high but precision is low. In a small-step search, speed of bringing VCr close to C 0  is low but precision is high. Therefore, the adjuster  184  rapidly brings VCr close to C 0  by a big-step search and then precisely brings VCr close to C 0  by a small-step search. 
       FIG. 8  illustrates a scene in which VCr is rapidly brought close to C 0  by a big-step search. C 1  is the current value of VCr. C 2  is a value less than C 1  by one first step width being a large step width. C 3  a value less than C 1  by two first step widths. C 4  is a value greater than C 1  by one first step width. C 5  is a value greater than C 1  by two first step widths. The adjuster  184  acquires difference voltage while changing VCr on a per first step width basis with the current value of VCr as a reference. Then, the adjuster  184  employs a value of VCr minimizing the acquired difference voltage as a new value of VCr. The adjuster  184  repeats the big-step search with a newly employed value of VCr as a current value of VCr. When the difference voltage is equal to or less than the second threshold value, the adjuster  184  completes the big-step search. For example, the big-step search is completed at the time point when the value of VCr becomes C 5 . 
       FIG. 9  illustrates a scene in which VCr is precisely brought close to C 0  by a small-step search. C 6  is a value less than C 5  by one second step width. The second step width is smaller than the first step width. C 7  is a value less than C 5  by two second step widths. C 8  is a value greater than C 5  by one second step width. C 9  is a value greater than C 5  by two second step widths. The adjuster  184  acquires difference voltage while changing VCr on a per second step width basis with the current value of VCr as a reference. Then, the adjuster  184  employs a value of VCr minimizing the acquired difference voltage as a new value of VCr. The adjuster  184  repeats the small-step search with a newly employed value of VCr as a current value of VCr. When the difference voltage is equal to or less than the first threshold value, the adjuster  184  completes the small-step search. For example, the small-step search is completed at the time point when the value of VCr becomes C 9 . 
       FIG. 10  illustrates a scene in which VRr is brought close to R 0  by a step search. R 0  is VRr minimizing difference voltage when the value of VCr is C 9 . R 1  is a value of VRr after completion of a small-step search. R 2  is a value less than R 1  by one third step width. The third step width is a shift amount of VRr. R 3  is a value less than R 1  by two third step widths. R 4  is a value greater than R 1  by one third step width. R 5  is a value greater than R 1  by two third step widths. The adjuster  184  acquires difference voltage while changing VRr on a per third step width basis with the current value of VRr as a reference. Then, the adjuster  184  employs a value of VRr minimizing the acquired difference voltage as a new value of VRr. The adjuster  184  repeats the step search with a newly employed value of VRr as a current value of VRr. When the difference voltage becomes a minimum value, the adjuster  184  completes the step search. 
     When adjustment of the antenna characteristic of the antenna  150  receiving a radio wave is completed, the adjuster  184  adjusts the antenna characteristic of the antenna  150  transmitting a radio wave. In other words, the adjuster  184  adjusts the frequency characteristic of the LC series resonant circuit including the antenna  150  transmitting a radio wave. The adjuster  184  adjusts the frequency characteristic of the LC series resonant circuit by a procedure similar to that for adjustment of the frequency characteristic of the RLC parallel resonant circuit. 
     Specifically, the adjuster  184  repeats processing of rapidly adjusting VCt by a big-step search until the difference voltage becomes equal to or less than the second threshold value. Then, when the difference voltage becomes equal to or less than the second threshold value, the adjuster  184  repeats processing of precisely adjusting VCt by a small-step search until the difference voltage becomes equal to or less than the first threshold value. When the difference voltage becomes equal to or less than the first threshold value, the adjuster  184  repeats processing of adjusting Wt by a step search until the difference voltage becomes a minimum value. 
     Next, antenna calibration processing executed by the position detection system  100  will be described with reference to  FIG. 11 . For example, when receiving a start instruction for position detection processing from the power transmission device  200 , the position detection system  100  executes the antenna calibration processing prior to the position detection processing. The position detection processing is processing of detecting a relative position between the power transmission coil  211  and the power reception coil  311 . 
     First, the controller  180  included in the position detection system  100  selects a reception antenna capable of transmission (Step S 101 ). For example, the controller  180  selects either antenna  150  of the antenna  150 A and the antenna  150 B. When completing the processing in Step S 101 , the controller  180  selects a reception antenna (Step S 102 ). For example, the controller  180  selects one antenna  150  other than the antenna  150  selected in Step S 101  from among the four antennas  150 . 
     When completing the processing in Step S 102 , the controller  180  executes first calibration processing (Step S 103 ). The first calibration processing will be described in detail with reference to a flowchart illustrated in  FIG. 12 . The first calibration processing is processing of adjusting the antenna characteristic of an antenna  150  receiving a radio wave. 
     First, the controller  180  zeros n (Step S 201 ). Note that n denotes a counter variable for counting the number of repetitions of search processing. When completing the processing in Step S 201 , the controller  180  acquires reception intensity (Step S 202 ). For example, the controller  180  causes the antenna  150  selected in Step S 101  to emit a radio wave and causes the antenna  150  selected in Step S 102  to receive the radio wave. The controller  180  acquires reception intensity being intensity of the radio wave received by the antenna  150  from the radio wave detection circuit  170 . 
     When completing the processing in Step S 202 , the controller  180  determines whether an intensity difference is equal to or less than the first threshold value (Step S 203 ). The intensity difference is the difference between reference intensity being previously acquired and being stored in the storage  191  and the reception intensity acquired in Step S 202 . When determining that the intensity difference is not equal to or less than the first threshold value (Step S 203 : NO), the controller  180  determines whether the intensity difference is equal to or less than the second threshold value (Step S 204 ). When determining that the intensity difference is not equal to or less than the second threshold value (Step S 203 : NO), the controller  180  executes a big-step search on VCr (Step S 205 ). Specifically, the controller  180  searches for VCr minimizing the intensity difference while adjusting VCr on a per first step width basis. 
     When determining that the intensity difference is equal to or less than the second threshold value (Step S 203 : YES), the controller  180  executes a small-step search on VCr (Step S 206 ). Specifically, the controller  180  searches for VCr minimizing the intensity difference while adjusting VCr on a per second step width basis. When completing the processing in Step S 206 , the controller  180  executes a step search on VRr (Step S 207 ). Specifically, the controller  180  searches for VRr minimizing the intensity difference while adjusting VRr on a per third step width basis. 
     When completing the processing in Step S 205  or Step S 207 , the controller  180  increments n by one (Step S 208 ). When completing the processing in Step S 208 , the controller  180  determines whether n exceeds N (Step S 209 ). When determining that n does not exceed N (Step S 209 : NO), the controller  180  returns the processing to Step S 202 . When determining that the intensity difference is equal to or less than the first threshold value (Step S 203 : YES) or determining that n exceeds N (Step S 209 : YES), the controller  180  completes the first calibration processing. 
     When completing the first calibration processing in Step S 103 , the controller  180  executes second calibration processing (Step S 104 ). The second calibration processing will be described in detail with reference to a flowchart illustrated in  FIG. 13 . The second calibration processing is processing of adjusting the antenna characteristic of an antenna  150  transmitting a radio wave. 
     First, the controller  180  zeros n (Step S 301 ). When completing the processing in Step S 301 , the controller  180  acquires reception intensity (Step S 302 ). When completing the processing in Step S 302 , the controller  180  determines whether an intensity difference is equal to or less than the first threshold value (Step S 303 ). When determining that the intensity difference is not equal to or less than the first threshold value (Step S 303 : NO), the controller  180  determines whether the intensity difference is equal to or less than the second threshold value (Step S 304 ). When determining that the intensity difference is not equal to or less than the second threshold value (Step S 303 : NO), the controller  180  executes a big-step search on VCt (Step S 305 ). When determining that the intensity difference is equal to or less than the second threshold value (Step S 303 : YES), the controller  180  executes a small-step search on VCt (Step S 306 ). When completing the processing in Step S 306 , the controller  180  executes a step search on Wt (Step S 307 ). 
     When completing the processing in Step S 305  or Step S 307 , the controller  180  increments n by one (Step S 308 ). When completing the processing in Step S 308 , the controller  180  determines whether n exceeds N (Step S 309 ). When determining that n does not exceed N (Step S 309 : NO), the controller  180  returns the processing to Step S 302 . When determining that the intensity difference is equal to or less than the first threshold value (Step S 303 : YES) or determining that n exceeds N (Step S 309 : YES), the controller  180  completes the second calibration processing. 
     When completing the second calibration processing in Step S 104 , the controller  180  determines whether an unselected reception antenna exists (Step S 105 ). Specifically, the controller  180  determines whether an antenna  150  not being selected in Step S 102  and being an antenna  150  other than the antenna  150  selected in Step S 101  from among the four antennas  150  exists. When determining that an unselected reception antenna exists (Step S 105 : YES), the controller  180  returns the processing to Step S 102  and selects the unselected reception antenna. 
     When determining that an unselected reception antenna does not exist (Step S 105 : NO), the controller  180  determines whether an unselected reception antenna capable of transmission exists (Step S 106 ). Specifically, the controller  180  determines whether an antenna  150  not selected in Step S 101  from among the antenna  150 A and the antenna  150 B exists. When determining that an unselected reception antenna capable of transmission exists (Step S 106 : YES), the controller  180  returns the processing to Step S 101  and selects the unselected reception antenna capable of transmission. 
     When determining that an unselected reception antenna capable of transmission does not exist (Step S 106 : NO), the controller  180  transmits PWM information (Step S 107 ). For example, the controller  180  transmits PWM information including Wt finally acquired by the second calibration processing to the communicator  142  through the communicator  192 . On the other hand, the controller  135  acquires the PWM information from the communicator  142 . In the position detection processing, the controller  135  controls the transmission circuit  120  in such a way that PWM at a duty ratio indicated by the PWM information is executed. When completing the processing in Step S 107 , the controller  180  completes the antenna calibration processing. 
     As described above, the transmission circuit  160  driving the antenna  150  being a reception antenna is provided, and intensity of a radio wave being transmitted from a driven antenna  150  and being received by another antenna  150  is detected by the radio wave detection circuit  170 , according to the present embodiment. Accordingly, a basis for determination of whether the antenna characteristic of the another antenna  150  is suitable can be acquired, according to the present embodiment. Further, the difference between the intensity of the radio wave received by the another antenna  150  and the predetermined reference intensity is detected, according to the present embodiment. Accordingly, whether the antenna characteristic of the another antenna  150  is suitable can be determined, according to the present embodiment. 
     Further, the frequency characteristic of an RLC parallel resonant circuit including the another antenna  150  is suitably adjusted by adjustment of the capacitance value of the capacitive element  171  and adjustment of the resistance value of the resistor  172 , according to the present embodiment. Accordingly, the antenna characteristic of the another antenna  150  is suitably adjusted, and a relative position between the power transmission coil  211  and the power reception coil  311  can be precisely detected in wireless electric power transmission, according to the present embodiment. 
     Further, the frequency characteristic of an LC series resonant circuit including the antenna  150  transmitting a radio wave is suitably adjusted by adjustment of the capacitance value of the capacitive element  161  and adjustment of a duty ratio in PWM control, according to the present embodiment. Accordingly, the antenna characteristic of the another antenna  150  is precisely adjusted even when the antenna characteristic of the antenna  150  transmitting a radio wave changes from that at the time of acquisition of the reference data and the reference intensity, according to the present embodiment. 
     Further, PWM control of the transmission circuit  120  is executed at a duty ratio adjusted in PWM control of the transmission circuit  160 , according to the present embodiment. Accordingly, the antenna characteristic of an antenna  110  being a transmission antenna can be suitably adjusted without providing a function of detecting suitability of the antenna characteristic of the antenna  110  on the power reception device  300  side where the antenna  110  is placed, according to the present embodiment. 
     Further, transmission circuit  160  according to the present embodiment drives two or more antennas  150  from among the four antennas  150 . Accordingly, the antenna characteristic of every antenna  150  can be adjusted, according to the present embodiment. 
     Embodiment 2 
     An example of providing the function of transmitting a radio wave for a reception antenna and adjusting antenna characteristics between reception antennas has been described in Embodiment 1. An example of providing a function of receiving a radio wave for a transmission antenna and adjusting antenna characteristics between transmission antennas will be described in the present embodiment. Note that description of a configuration and processing similar to those according to Embodiment 1 is omitted or simplified. 
     According to the present embodiment, a part of components of a position detection system  101  are placed in a power transmission coil unit  210  and the other components of the position detection system  101  are placed in a power reception coil unit  310 , as illustrated in  FIG. 14 . Specifically, two antennas  110 , a transmission circuit  120 , and a radio wave detection circuit  130  are placed in the power reception coil unit  310 , and an antenna  150  and a radio wave detection circuit  170  are placed in the power transmission coil unit  210 . The two antennas  110  are placed at positions distant from each other on the power reception coil unit  310 . The antenna  110  is a general name for an antenna  110 A and an antenna  110 B. Note that  FIG. 14  illustrates only main components from among the components included in the position detection system  101 . 
     The antenna  110  is an antenna emitting a radio wave in the LF band. There are two antennas  110 , according to the present embodiment. The antenna  110  may be hereinafter referred to as a transmission antenna. The antenna  110  is an example of a second antenna. The transmission circuit  120  is a circuit driving a plurality of antennas  110 . The transmission circuit  120  is an example of a transmission circuit. The transmission circuit  120  includes a first capacitive element forming an LC resonant circuit with the antenna  110 . The LC resonant circuit according to the present embodiment is an LC series resonant circuit. The frequency characteristic of the LC series resonant circuit may be hereinafter referred to as an antenna characteristic of the antenna  110 . 
     According to the present embodiment, calibration is executed when a relative position is detected, in order to align the antenna characteristic of an antenna  110  used for detection of the relative position with the antenna characteristic of an antenna  110  used when reference data are acquired. In order to achieve such calibration, at least one antenna  110  from among a plurality of antennas  110  being transmission antennas is capable of not only transmitting a radio wave but also receiving a radio wave, according to the present embodiment. Specifically, the position detection system  100  according to the present embodiment includes the radio wave detection circuit  130  detecting intensity of a radio wave received by the at least one antenna  110 . 
     The radio wave detection circuit  130  is a circuit detecting intensity of a radio wave received by at least one antenna  110 . For example, the radio wave detection circuit  130  detects intensity of a radio wave being transmitted from the antenna  110 A and being received by the antenna  110 B. The radio wave detection circuit  130  includes a second capacitive element and a first resistor forming an RLC resonant circuit with the antenna  110 . The RLC resonant circuit according to the present embodiment is an RLC parallel resonant circuit. The frequency characteristic of the RLC parallel resonant circuit may be hereinafter referred to as an antenna characteristic of the antenna  110 . The radio wave detection circuit  130  is an example of a second radio wave detection circuit. 
     The position detection system  101  detects the difference between intensity of a radio wave received by the antenna  110 B and predetermined reference intensity. The reference intensity is intensity to be detected by the radio wave detection circuit  130  when the antenna  110 B receives a radio wave emitted by the antenna  110 A in a case of the antenna characteristic of the antenna  110 A and the antenna characteristic of the antenna  110 B being suitable. For example, the reference intensity is acquired for each combination of a plurality of antennas  110  transmitting a radio wave and an antenna  110  capable of receiving a radio wave when the reference data are acquired. Since there are two antennas  110  transmitting a radio wave and one antenna  110  capable of receiving the radio wave, there are two combinations, according to the present embodiment. 
     A relative position between the two antennas  110  when the reference intensity is acquired and that when the relative position is detected are the same. Accordingly, intensity acquired when the relative position is detected is the same as the reference intensity as long as the antenna characteristic of the antenna  110  transmitting a radio wave and the antenna characteristic of the antenna  110  receiving the radio wave at the time of acquisition of the reference intensity and those at the time of detection of the relative position are the same. In other words, the antenna characteristic at the time of acquisition of the reference intensity is considered to be reproduced by adjusting the antenna characteristic of the antenna  110  transmitting a radio wave and the antenna characteristic of the antenna  110  receiving the radio wave in such a way that intensity acquired when the relative position is detected and the reference intensity are the same. 
     Therefore, the antenna characteristic of the antenna  110  transmitting a radio wave and the antenna characteristic of the antenna  110  receiving the radio wave are adjusted in such a way that the antenna characteristics at the time of acquisition of the reference intensity is reproduced, according to the present embodiment. Specifically, the frequency characteristic of an LC series resonant circuit including the antenna  110  transmitting a radio wave and the frequency characteristic of an RLC parallel resonant circuit including the antenna  110  receiving the radio wave are adjusted. Details of the adjustment method will be described later. 
     The antenna  150  is an antenna receiving a radio wave in the LF band. There is one antenna  150  in the present embodiment. The antenna  150  may be hereinafter referred to as a reception antenna. The antenna  150  is an example of a first antenna. The radio wave detection circuit  170  is a circuit detecting intensity of a radio wave received by at least one antenna  150 . The radio wave detection circuit  170  detects intensity of a radio wave received by the antenna  150 . 
     Next, connections between the antenna  110 , the transmission circuit  120 , and the radio wave detection circuit  130  will be described with reference to a circuit diagram of the position detection system  101  illustrated in  FIG. 15 . Note that  FIG. 15  illustrates a circuit diagram of part of components included in the position detection system  101 . As illustrated in  FIG. 15 , the position detection system  101  includes two switches  111  and two switches  112 . The switch  111  is a general name for a switch  111 A and a switch  111 B. The switch  112  is a general name for a switch  112 A and a switch  112 B. 
     The switches  111  are switches changing a connection between at least one antenna  110  from among a plurality of antennas  110  and the transmission circuit  120 . The switch  111 A is a switch changing a connection between the antenna  110 A and the transmission circuit  120 . When the switch  111 A is turned on, an LC series resonant circuit is formed by the antenna  110 A and a capacitive element  121 A included in the transmission circuit  120 . The switch  111 B is a switch changing a connection between the antenna  110 B and the transmission circuit  120 . When the switch  111 B is turned on, an LC series resonant circuit is formed by the antenna  110 B and a capacitive element  121 B included in the transmission circuit  120 . Each of the capacitive element  121 A and the capacitive element  121 B is a variable capacitance element having a variable capacitance value. The capacitive element  121 A and the capacitive element  121 B are examples of the first capacitive element. 
     The switches  112  are switches changing connections between a plurality of antennas  110  and the radio wave detection circuit  130 . The switch  112 A is a switch changing a connection between the antenna  110 A and the radio wave detection circuit  130 . When the switch  112 A is turned on, an RLC parallel resonant circuit is formed by the antenna  110 A, and a capacitive element  131 A and a resistor  132 A that are included in the radio wave detection circuit  130 . The switch  112 B is a switch changing a connection between the antenna  110 B and the radio wave detection circuit  130 . When the switch  112 B is turned on, an RLC parallel resonant circuit is formed by the antenna  110 B, and a capacitive element  131 B and a resistor  132 B that are included in the radio wave detection circuit  130 . 
     Each of the capacitive element  131 A and the capacitive element  131 B is a variable capacitance element having a variable capacitance value. Each of the resistor  132 A and the resistor  132 B is a variable resistance element having a variable resistance value. A capacitive element  131  is a general name for the capacitive element  131 A and the capacitive element  131 B. A resistor  132  is a general name for the resistor  132 A and the resistor  132 B. The capacitive element  131 A and the capacitive element  131 B are examples of the second capacitive element. The resistor  132 A and the resistor  132 B are examples of the first resistor. 
     The antenna  150  and the radio wave detection circuit  170  are always connected. Accordingly, an RLC parallel resonant circuit is always formed by the antenna  150 , and a capacitive element  171 E and a resistor  172 E that are included in the radio wave detection circuit  170 . The capacitive element  171 E is a capacitive element having a fixed capacitance value. The resistor  172 E is a resistance element having a fixed resistance value. 
     When the antenna  110 A is connected to the transmission circuit  120 , the antenna  110 B is connected to the radio wave detection circuit  130 . When the antenna  110 B is connected to the transmission circuit  120 , the antenna  110 A is connected to the radio wave detection circuit  130 . The antenna  110 A and the antenna  110 B are not simultaneously connected to the transmission circuit  120 . The antenna  110 A and the antenna  110 B are not simultaneously connected to the radio wave detection circuit  130 . Each of the antenna  110 A and the antenna  110 B is not simultaneously connected to both the transmission circuit  120  and the radio wave detection circuit  130 . 
     Next, components of the position detection system  101  will be described in detail with reference to  FIG. 16 . Note that description of a component that has been already described is omitted or simplified. The position detection system  101  includes the antennas  110 , the switches  111 , the switches  112 , the transmission circuit  120 , a power source circuit  125 , the radio wave detection circuit  130 , a controller  135 , a storage  141 , a communicator  142 , the antenna  150 , the radio wave detection circuit  170 , a controller  180 , a storage  191 , and a communicator  192 . 
     The antenna  110  emits a radio wave related to a high-frequency signal supplied by the transmission circuit  120 . The switch  111  changes the connection between the antenna  110  and the transmission circuit  120 . The switch  112  changes the connection between the antenna  110  and the transmission circuit  120 . The transmission circuit  120  supplies a high-frequency signal generated from power source voltage to the antenna  110 . The power source circuit  125  supplies power source voltage to the transmission circuit  120 . The radio wave detection circuit  130  detects intensity of a radio wave received by the antenna  110 . The radio wave detection circuit  130  is an example of the second radio wave detection circuit. 
     The controller  135  controls operation of components placed in a power reception device  300  from among the components included in the position detection system  101 . For example, the controller  135  causes the antenna  110  to emit a radio wave by controlling the transmission circuit  120 . For example, the controller  135  adjusts the antenna characteristic of the antenna  110 , based on intensity detected by the radio wave detection circuit  130 . The controller  135  includes a CPU, a ROM, a RAM, an RTC, an A/D converter, and the like. Details of the controller  135  will be described later. 
     The storage  141  stores various types of information used by the controller  135  or various types of information acquired through operation of the controller  135 . For example, the storage  141  stores reference data used for detection of a relative position and reference intensity used for adjustment of an antenna characteristic. For example, the storage  141  includes a flash memory. The communicator  142  communicates with the communicator  192  in accordance with control by the controller  135 . The communicator  142  includes a communication interface conforming to a well-known wireless communication standard. 
     The antenna  150  receives a radio wave emitted by the antenna  110 . The radio wave detection circuit  170  detects intensity of a radio wave received by the antenna  150 . The radio wave detection circuit  170  is an example of a first radio wave detection circuit. The controller  180  controls operation of components placed in a power transmission device  200  from among the components included in the position detection system  100 . The controller  180  functionally includes a position detector  182 . The position detector  182  detects a relative position between a power transmission coil  211  and a power reception coil  311 , based on intensity detected by the radio wave detection circuit  170 . The controller  180  includes a CPU, a ROM, a RAM, an RTC, an A/D converter, and the like. 
     The storage  191  stores various types of information used by the controller  180  and various types of information acquired through operation of the controller  180 . For example, the storage  191  includes a flash memory. The communicator  192  communicates with the communicator  142  in accordance with control by the controller  180 . The communicator  192  includes a communication interface conforming to a well-known wireless communication standard, similarly to the communicator  142 . 
     Next, the function of the controller  135  will be described in detail. The controller  135  functionally includes a switch controller  136 , a difference detector  137 , and an adjuster  138 . For example, the functional units included in the controller  135  are provided by the CPU executing an operation program stored in the ROM. 
     The switch controller  136  controls the switches  111  and the switches  112 . The switch controller  136  controls the switches  112  in such a way as to connect the radio wave detection circuit  130  to an antenna  110  other than an antenna  110  connected to the transmission circuit  120  by a switch  111  from among a plurality of antennas  110 . 
     The difference detector  137  detects the difference between intensity of a radio wave received by another antenna  110  and predetermined reference intensity. The another antenna  110  is an antenna  110  other than an antenna  110  driven by the transmission circuit  120  from among the two antennas  110 . The antenna  110  driven by the transmission circuit  120  here is the antenna  110 A, and the another antenna  110  is the antenna  110 B. For example, the reference intensity is intensity of a radio wave being emitted from an antenna  110  placed at the same position as the antenna  110 A and being received by an antenna  110  placed at the same position as the antenna  110 B when the reference data are acquired. A larger difference detected by the difference detector  137  indicates a larger difference in the antenna characteristic between the time of acquisition of the reference intensity and the time of execution of calibration. 
     The adjuster  138  adjusts each element in such a way that the difference detected by the difference detector  137  decreases. For example, the adjuster  138  adjusts the frequency characteristic of an LC series resonant circuit including an antenna  110  transmitting a radio wave. For example, the adjuster  138  adjusts the capacitance value of a capacitive element  121  included in the LC series resonant circuit in such a way that the aforementioned difference decreases. Further, the adjuster  138  adjusts a duty ratio in PWM control of the transmission circuit  120  in such a way that the aforementioned difference decreases. 
     Further, for example, the adjuster  138  adjusts the frequency characteristic of an RLC parallel resonant circuit including an antenna  110  receiving a radio wave. For example, the adjuster  138  adjusts the capacitance value of a capacitive element  131  included in the RLC parallel resonant circuit in such a way that the aforementioned difference decreases. Further, the adjuster  138  adjusts the resistance value of a resistor  132  included in the RLC parallel resonant circuit in such a way that the aforementioned difference decreases. 
     Next, antenna calibration processing executed by the position detection system  101  will be described with reference to  FIG. 17 . For example, when receiving a start instruction for position detection processing from the power transmission device  200 , the position detection system  101  executes the antenna calibration processing prior to the position detection processing. 
     First, the controller  135  included in the position detection system  101  selects a transmission antenna capable of reception (Step S 401 ). For example, the controller  135  selects either antenna  110  of the antenna  110 A and the antenna  110 B. When completing the processing in Step S 401 , the controller  135  selects a transmission antenna (Step S 402 ). For example, the controller  135  selects an antenna  110  other than the antenna  110  selected in Step S 401  from among the two antennas  110 . 
     When completing the processing in Step S 402 , the controller  135  executes third calibration processing (Step S 403 ). The third calibration processing will be described in detail with reference to a flowchart illustrated in  FIG. 18 . The third calibration processing is processing of adjusting the antenna characteristic of an antenna  110  transmitting a radio wave. 
     First, the controller  135  zeros n (Step S 501 ). When completing the processing in Step S 501 , the controller  135  acquires reception intensity (Step S 502 ). For example, the controller  135  causes the antenna  110  selected in Step S 402  to emit a radio wave and causes the antenna  110  selected in Step S 401  to receive the radio wave. The controller  135  acquires reception intensity being intensity of the radio wave received by the antenna  110  from the radio wave detection circuit  130 . 
     When completing the processing in Step S 502 , the controller  135  determines whether an intensity difference is equal to or less than a first threshold value (Step S 503 ). The intensity difference is the difference between reference intensity being previously acquired and being stored in the storage  191 , and the reception intensity acquired in Step S 502 . When determining that the intensity difference is not equal to or less than the first threshold value (Step S 503 : NO), the controller  135  determines whether the intensity difference is equal to or less than a second threshold value (Step S 504 ). When determining that the intensity difference is not equal to or less than the second threshold value (Step S 504 : NO), the controller  135  executes a big-step search on VCt (Step S 505 ). Specifically, the controller  135  searches for VCt minimizing the intensity difference while adjusting VCt on a per first step width basis. 
     When determining that the intensity difference is equal to or less than the second threshold value (Step S 504 : YES), the controller  135  executes a small-step search on VCt (Step S 506 ). Specifically, the controller  135  searches for VCt minimizing the intensity difference while adjusting VCt on a per second step width basis. When completing the processing in Step S 506 , the controller  135  executes a step search on VRt (Step S 507 ). Specifically, the controller  135  searches for VRt minimizing the intensity difference while adjusting VRt on a per third step width basis. 
     When completing the processing in Step S 505  or Step S 507 , the controller  135  increments n by one (Step S 508 ). When completing the processing in Step S 508 , the controller  135  determines whether n exceeds N (Step S 509 ). When determining that n does not exceed N (Step S 509 : NO), the controller  135  returns the processing to Step S 502 . When determining that the intensity difference is equal to or less than the first threshold value (Step S 503 : YES) or determining that n exceeds N (Step S 509 : YES), the controller  135  completes the third calibration processing. 
     When completing the third calibration processing in Step S 403 , the controller  135  executes fourth calibration processing (Step S 404 ). The fourth calibration processing will be described in detail with reference to a flowchart illustrated in  FIG. 19 . The fourth calibration processing is processing of adjusting the antenna characteristic of an antenna  110  receiving a radio wave. 
     First, the controller  135  zeros n (Step S 601 ). When completing the processing in Step S 601 , the controller  135  acquires reception intensity (Step S 602 ). When completing the processing in Step S 602 , the controller  135  determines whether the intensity difference is equal to or less than the first threshold value (Step S 603 ). When determining that the intensity difference is not equal to or less than the first threshold value (Step S 603 : NO), the controller  135  determines whether the intensity difference is equal to or less than the second threshold value (Step S 604 ). When determining that the intensity difference is not equal to or less than the second threshold value (Step S 604 : NO), the controller  135  executes a big-step search on VCr (Step S 605 ). When determining that the intensity difference is equal to or less than the second threshold value (Step S 604 : YES), the controller  135  executes a small-step search on VCr (Step S 606 ). When completing the processing in Step S 606 , the controller  135  executes a step search on VRr (Step S 607 ). 
     When completing the processing in Step S 605  or Step S 607 , the controller  135  increments n by one (Step S 608 ). When completing the processing in Step S 608 , the controller  135  determines whether n exceeds N (Step S 609 ). When determining that n does not exceed N (Step S 609 : NO), the controller  135  returns the processing to Step S 602 . When determining that the intensity difference is equal to or less than the first threshold value (Step S 603 : YES) or determining that n exceeds N (Step S 609 : YES), the controller  135  completes the fourth calibration processing. 
     When completing the fourth calibration processing in Step S 404 , the controller  135  determines whether an unselected transmission antenna exists (Step S 405 ). Specifically, the controller  135  determines whether an antenna  110  being not selected in Step S 402  and being an antenna  110  other than the antenna  110  selected in Step S 401  from among the two antennas  110  exists. When determining that an unselected transmission antenna exists (Step S 405 : YES), the controller  135  returns the processing to Step S 402  and selects the unselected transmission antenna. 
     When determining that an unselected transmission antenna does not exist (Step S 405 : NO), the controller  135  determines whether an unselected transmission antenna capable of reception exists (Step S 406 ). Specifically, the controller  135  determines whether an antenna  110  not selected in Step S 401  from among the antenna  110 A and the antenna  110 B exists. When determining that an unselected transmission antenna capable of reception exists (Step S 406 : YES), the controller  135  returns the processing to Step S 401  and selects the unselected transmission antenna capable of reception. When determining that an unselected transmission antenna capable of reception does not exist (Step S 406 : NO), the controller  135  completes the antenna calibration processing. 
     As described above, the radio wave detection circuit  130  detecting intensity of a radio wave received by an antenna  110  being a transmission antenna is provided, and intensity of a radio wave being transmitted from a driven antenna  110  and being received by another antenna  110  is detected by the radio wave detection circuit  130 , according to the present embodiment. Accordingly, a basis for determination of whether the antenna characteristic of the antenna  110  transmitting the radio wave is suitable can be acquired, according to the present embodiment. Further, the difference between the intensity of the radio wave received by the another antenna  110  and the predetermined reference intensity is detected, according to the present embodiment. Accordingly, whether the antenna characteristic of the antenna  110  transmitting the radio wave is suitable can be determined, according to the present embodiment. 
     Further, the frequency characteristic an LC series resonant circuit including an antenna  110  transmitting a radio wave is suitably adjusted by adjustment of the capacitance value of the capacitive element  121  and adjustment of a duty ratio in PWM control, according to the present embodiment. Accordingly, the antenna characteristic of the antenna  110  transmitting the radio wave is suitably adjusted, and a relative position between the power transmission coil  211  and the power reception coil  311  can be precisely detected in wireless electric power transmission, according to the present embodiment. 
     Further, the frequency characteristic of an RLC parallel resonant circuit including the another antenna  110  is suitably adjusted by adjustment of the capacitance value of the capacitive element  131  and adjustment of the resistance value of the resistor  132 , according to the present embodiment. Accordingly, the antenna characteristic of the antenna  110  transmitting a radio wave is precisely adjusted even when the antenna characteristic of the antenna  110  receiving the radio wave changes from that at the time of acquisition of the reference data and the reference intensity, according to the present embodiment. 
     Further, the radio wave detection circuit  130  according to the present embodiment detects intensity of a radio wave received by two or more antennas  110  from among a plurality of antennas  110 . Accordingly, the antenna characteristic of every antenna  110  can be adjusted, according to the present embodiment. 
     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, further configurations, functions, and operations may be employed in addition to the aforementioned configurations, functions, and operations. Further, the aforementioned embodiments may 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 providing the function of transmitting a radio wave for a reception antenna and adjusting the antenna characteristics of reception antennas between the reception antennas has been described in Embodiment 1. Further, an example of providing the function of receiving a radio wave for a transmission antenna and adjusting the antenna characteristics of transmission antennas between the transmission antennas has been described in Embodiment 2. The antenna characteristics of reception antennas may be adjusted between the reception antennas by providing the function of transmitting a radio wave for a reception antenna, and the antenna characteristics of transmission antennas may be adjusted between the transmission antennas by providing the function of receiving a radio wave for a transmission antenna. 
     An example of automatically adjusting the antenna characteristic of a reception antenna or a transmission antenna when the antenna characteristic differs from a predetermined antenna characteristic has been described in Embodiments 1 and 2. The antenna characteristic of a reception antenna or a transmission antenna may not be adjusted and an error notification may be made when the antenna characteristic differs from the predetermined antenna characteristic. 
     An example of adjusting a duty ratio in PWM control when adjusting the frequency characteristic of an LC series resonant circuit has been described in Embodiments 1 and 2. Power source voltage of a high-frequency signal in PWM control may be adjusted when adjusting the frequency characteristic of the LC series resonant circuit. In this case, output voltage of the inverter included in the transmission circuit  120  or the transmission circuit  160  is adjusted. 
     An example of not only adjusting the frequency characteristic of an RLC parallel resonant circuit including a reception antenna receiving a radio wave but also adjusting the frequency characteristic of an LC series resonant circuit including a reception antenna transmitting a radio wave has been described in Embodiment 1. The frequency characteristic of the LC series resonant circuit including the reception antenna transmitting a radio wave may not be adjusted. Further, an example of not only adjusting the frequency characteristic of an LC series resonant circuit including a transmission antenna transmitting a radio wave but also adjusting the frequency characteristic of an RLC parallel resonant circuit including a transmission antenna receiving a radio wave has been described in Embodiment 2. The frequency characteristic of the RLC parallel resonant circuit including the transmission antenna receiving a radio wave may not be adjusted. 
     An example of placing a reception antenna in the power transmission device  200  and placing a transmission antenna in the power reception device  300  has been described in Embodiments 1 and 2. A transmission antenna may be placed in the power transmission device  200 , and a reception antenna may be placed in the power reception device  300 . 
     Applying an operation program defining the operation of the position detection system  100  or  101  according to the present disclosure to a computer such as an existing personal computer or an information terminal device may cause the computer to also function as the position detection system  100  or  101  according to the present disclosure. Further, any method may be employed as a distribution method of such a program, and for example, the program may be stored and distributed in a non-transitory computer-readable recording medium such as a compact disk ROM (CD-ROM), a digital versatile disk (DVD), a magneto optical disk (MO), or a memory card and may be distributed through a communication network such as the Internet. 
     The foregoing describes some example embodiments for explanatory purposes. Although the foregoing discussion has presented specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. This detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is defined only by the included claims, along with the full range of equivalents to which such claims are entitled.