Abstract:
Power supply device is provided with the following: a power supply unit ( 121 ) that includes a spiral coil or a solenoid coil; a frequency characteristic acquisition unit ( 211 ) that acquires the frequency characteristics of the efficiency of the supply of power from the power supply unit to the power receiving device; a peak determining unit ( 212 ) that determines peaks in the frequency characteristics of the efficiency of power supply; and a drive frequency determination unit ( 213 ) that, if two peaks have been found, determines as the drive frequency used for supplying power a frequency near the frequency that is the lower of the frequencies of the two peaks, with the lower frequency being given priority over a frequency near the higher frequency.

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply apparatus provided on a ground side, a power receiving apparatus provided on a vehicle that receives power supplied from the power supply apparatus using an electromagnetic force in a wireless manner, and a charging system including these apparatuses. 
     BACKGROUND ART 
     In recent years, automobiles running on electric power such as an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), and a hybrid electric vehicle (HEV) (hereinafter simply referred to as “vehicle” or “vehicles”) have become widespread. Such vehicles are mounted with a large capacity storage battery, store electric energy supplied from outside in a storage battery and run using the stored electric energy. 
     As a method of supplying power to a storage battery of a vehicle from the outside, a method is known whereby power is supplied between a primary side coil of a power supply apparatus provided on a ground side and a secondary side coil of a power receiving apparatus provided on a vehicle side using an electromagnetic force in a wireless manner. According to this wireless power supply method, it is known that a frequency characteristic for efficiency of power supply (hereinafter, referred to as “power supply efficiency”) when an inter-coil coupling coefficient is high is a double hump characteristic having two peaks (resonance points) (e.g., see PTL 1). In wireless power supply for vehicles in particular, the shape of double hump characteristic varies depending on a gap, displacement and battery condition, and the peak positions may also constantly change, and so it is necessary to check these changes every time. 
     Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a technique of sequentially changing a frequency of power to be sent from a power transmission coil, detecting a frequency characteristic of power received by a power receiving coil, and transmitting power using a frequency corresponding to maximum received power. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1 
         Japanese Patent Application Laid-Open No. 2011-142769 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     When the aforementioned frequency characteristic of power supply efficiency has two peaks, and when power is supplied using frequencies at a peak on a high-frequency side and a peak on a low-frequency side, magnetic fields formed between the coils are known to have different shapes. When power supply from a primary side coil on the ground to a secondary side coil provided on a bottom surface of the vehicle is considered, a magnetic flux in the horizontal direction with respect to the ground may become unnecessary radiation. For this reason, suppression of radiation of the horizontal direction component of the magnetic flux with respect to the ground makes it possible to reduce unnecessary radiation leaking out of the vehicle. 
     However, the wireless power supply method disclosed in aforementioned PTL 1 selects a frequency to obtain high power supply efficiency, and PTL 1 neither discloses nor suggests which peak frequency should be used when the same power supply efficiency is obtained at two peaks. 
     An object of the present invention is to provide a power supply apparatus, a power receiving apparatus and a charging system that suppress radiation of a horizontal direction component of a magnetic flux with respect to the ground. 
     Solution to Problem 
     A power supply apparatus according to an aspect of the present invention is to be disposed on a ground side and supplies power using an electromagnetic force to a power receiving apparatus disposed on a vehicle, the power supply apparatus including: a power supply section including a spiral coil or a solenoid coil; a frequency characteristic acquiring section that acquires a frequency characteristic for efficiency of power supply performed from the power supply section to the power receiving apparatus; a peak determining section that determines a peak in the frequency characteristic for the efficiency of power supply; and a drive frequency determining section that determines, when two peaks are determined, a frequency in the vicinity of a lower frequency of frequencies at the two peaks to be a drive frequency preferentially over a frequency in the vicinity of a higher frequency, the drive frequency being used for the power supply. 
     A power receiving apparatus according to an aspect of the present invention is to be disposed on a vehicle and that receives power supplied using an electromagnetic force from the power supply apparatus according to claim  1  to be disposed on a ground side, the power receiving apparatus comprising a power receiving section including a spiral coil or solenoid coil that receives the power supplied from the power supply apparatus. 
     A charging system according to an aspect of the present invention includes: a power supply apparatus to be disposed on a ground side; and a power receiving apparatus that is to be disposed on a vehicle and that receives power supplied from the power supply apparatus using an electromagnetic force, in which the power receiving apparatus includes: a power receiving section including a spiral coil or solenoid coil that receives the power supplied from the power supply apparatus; and a storage battery that stores the power received by the power receiving section, and the power supply apparatus includes: a power supply section that includes a spiral coil or solenoid coil; a frequency characteristic acquiring section that acquires a frequency characteristic for efficiency of power supply performed from the power supply section to the power receiving section; a peak determining section that determines a peak in the frequency characteristic for the efficiency of power supply; and a drive frequency determining section that determines, when two peaks are determined, a frequency in the vicinity of a lower frequency of frequencies at the two peaks to be a drive frequency preferentially over a frequency in the vicinity of a higher frequency, the drive frequency being used for the power supply. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to suppress radiation of a horizontal direction component with respect to the ground. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a charging system according to Embodiment 1 of the present invention; 
         FIG. 2  is a block diagram illustrating an internal configuration of the power supply apparatus shown in  FIG. 1 ; 
         FIGS. 3A to 3D  are diagrams illustrating a coil arrangement of the power supply section and the power receiving section shown in  FIG. 1 ; 
         FIG. 4  is a flowchart illustrating an operation of the power supply apparatus shown in  FIG. 1 ; 
         FIG. 5  is a flowchart illustrating a procedure of the frequency characteristic acquiring process shown in  FIG. 4 ; 
         FIG. 6  is a flowchart illustrating a procedure of the peak determining process shown in  FIG. 4 ; 
         FIG. 7  is a diagram provided for describing how a frequency characteristic over an entire operating frequency is acquired; 
         FIG. 8  is a flowchart illustrating a procedure of the drive frequency determining process shown in  FIG. 4 ; 
         FIG. 9  is a diagram provided for describing how a frequency characteristic on a low-frequency side of an operating frequency is acquired; 
         FIGS. 10A and 10B  are conceptual diagrams illustrating a magnetic field shape formed between a power supply coil and a power receiving coil; 
         FIG. 11  is a flowchart illustrating a procedure of a frequency characteristic acquiring process according to Embodiment 1 of the present invention; and 
         FIGS. 12A to 12D  are diagrams illustrating a coil arrangement when a solenoid coil is used for the power supply section and the power receiving section. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     (Embodiment 1) 
     &lt;Configuration of Charging System&gt; 
     A configuration of charging system  100  according to Embodiment 1 of the present invention will be described using  FIG. 1 . 
     Charging system  100  includes power supply apparatus  120  and vehicle  10 . Note that  FIG. 1  illustrates a state in which power supply coil  121   a  and power receiving coil  153   a  are placed opposite to each other and power can be supplied. 
     Power supply apparatus  120  is installed on or buried in the ground so that power supply section  121  is exposed from the surface of ground g. Power supply apparatus  120  is provided, for example, in a parking space, and faces power receiving section  153  and supplies power to power receiving apparatus  153  while vehicle  10  is parked. Here, the term “power supply” refers to supplying power from power supply coil  121   a  to power receiving coil  153   a . The power supply is classified into power supply to acquire a frequency characteristic for power supply efficiency while sequentially changing the frequency before supplying power to storage battery  170  (hereinafter described as “test power supply”) and power supply to supply power to storage battery  170  with greater power than that of the test power supply (hereinafter, described as “full-scale power supply”). Note that the term simply described as “power supply” in the following description includes both the test power supply and the full-scale power supply. The configuration of power supply apparatus  120  will be described later. 
     Vehicle  10  includes power receiving apparatus  150 , storage battery  170  and positioning camera  180 , and runs using storage battery  170  as a power source. Vehicle  10  is a vehicle that runs on power of storage battery  170  such as a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) or an electric vehicle (EV). 
     &lt;Configuration of Power Receiving Apparatus&gt; 
     Power receiving apparatus  150  supplies power which is supplied from power supply apparatus  120  to storage battery  170 . 
     Storage battery  170  stores the power supplied from power receiving apparatus  150 . 
     Positioning camera  180  captures an image of the outside of vehicle  10 , reads a marker placed in the vicinity of power supply apparatus  120  and outputs the read image to vehicle-side control section  151 . 
     Power receiving apparatus  150  includes vehicle-side control section  151 , vehicle-side communication section  152  and power receiving section  153 . 
     Vehicle-side control section  151  determines whether power supply section  121  and power receiving section  153  are positioned facing each other based on the image outputted from positioning camera  180 . When power supply section  121  and power receiving section  153  are positioned facing each other (aligned with each other), and a charging start instruction is received from an input section (not shown), vehicle-side control section  151  controls vehicle-side communication section  152  and power receiving section  153  so as to perform various types of processes associated with the charging start. Moreover, vehicle-side control section  151  detects power received by power receiving coil  153   a  and outputs the received power detection result to vehicle-side communication section  152  as received power information. Upon receiving a charging stop instruction from an input section (not shown), vehicle-side control section  151  controls vehicle-side communication section  152  and power receiving section  153  so as to perform various kinds of processes associated with the charging stop. 
     Vehicle-side communication section  152  transmits the received power information outputted from vehicle-side control section  151  to power-supply-side communication section  123 . Vehicle-side communication section  152  generates a power-supply-start signal or power-supply-stop signal under the control of vehicle-side control section  151  and transmits the generated power-supply-start signal or power-supply-stop signal to power-supply-side communication section  123 . 
     Power receiving section  153  includes power receiving coil  153   a  (secondary coil) on the bottom surface of vehicle  10 . Power receiving coil  153   a  is a planar spiral coil, receives power transmitted from power supply section  121  under the control of vehicle-side control section  151  and supplies the received power to storage battery  170 . Power receiving section  153  is provided while being exposed to the outside at the bottom of vehicle  10 . 
     &lt;Configuration of Power Supply Apparatus&gt; 
     Power supply apparatus  120  includes power supply section  121 , power-supply-side control section  122 , power-supply-side communication section  123  and storage section  124 . 
     When supplying power to power receiving section  153  of power receiving apparatus  150 , power supply section  121  faces power receiving section  153  in a wireless manner. Power supply section  121  includes power supply coil  121   a  (primary coil) and power supply coil  121   a  is a planar spiral coil. Power supply section  121  performs test power supply from power supply coil  121   a  while sequentially changing the frequency under the control of power-supply-side control section  122  and performs full-scale power supply at a determined drive frequency. This power supply is performed under, for example, an electromagnetic induction scheme, an electric field resonance scheme or a magnetic field resonance scheme. 
     When a power-supply-start signal is outputted from power-supply-side communication section  123 , power-supply-side control section  122  controls power supply section  121  so as to perform test power supply with respect to power receiving coil  153   a  while sequentially changing the frequency for power supply coil  121   a . Power-supply-side control section  122  calculates power supply efficiency for each frequency based on the supplied power subject to the test power supply from power supply coil  121   a  and received power information received from power-supply-side communication section  101  and causes storage section  124  to store the calculated power supply efficiency. Power-supply-side control section  122  selects a frequency corresponding to a peak where the power supply efficiency reaches a local maximum based on the power supply efficiency for each frequency stored in storage section  124  and controls power supply section  121  so as to start full-scale power supply using the selected frequency. Power-supply-side control section  122  also controls power supply section  121  to stop power supply according to a power-supply-stop signal received from power-supply-side communication section  123 . 
     Power-supply-side communication section  123  receives the received power information transmitted from vehicle-side communication section  152  and outputs the received power information to power-supply-side control section  122 . Power-supply-side communication section  123  receives a power-supply-start signal or a power-supply-stop signal from vehicle-side communication section  152  and outputs the received power-supply-start signal or power-supply-stop signal to power-supply-side control section  122 . 
     Storage section  124  stores the power supply efficiency for each frequency outputted from power-supply-side control section  122 . 
     &lt;Detailed Configuration of Power Supply Apparatus&gt; 
     Next, more detailed configurations of power supply section  121  and power-supply-side control section  122  in the internal configuration of power supply apparatus  120  will be described.  FIG. 2  is a block diagram illustrating the internal configuration of power supply apparatus  120  shown in  FIG. 1 . 
     Power supply section  121  includes power supply section  201 , switching section  202 , voltage detection section  203 , power-supply-side inverter  204 , current detection section  205  and power supply coil  121   a.    
     Power supply section  201  supplies DC power with a predetermined voltage and current to power-supply-side inverter  204  via switching section  202 . 
     Switching section  202  opens and/or closes connection between power supply section  201  and inverter  203  under the control of power supply control section  214 . 
     Voltage detection section  203  detects a voltage value of DC power supplied from power supply section  201  to power-supply-side inverter  204  and outputs the detected voltage value to frequency characteristic acquiring section  211 . 
     When performing test power supply, power-supply-side inverter  204  converts DC power supplied from power supply section  201  to AC power while sequentially changing the frequency under the control of power supply control section  214  and supplies the AC power to power supply coil  121   a . On the other hand, when performing full-scale power supply, power-supply-side inverter  204  converts DC power supplied from power supply section  201  to AC power under the control of power supply control section  214  and supplies the AC power to power supply coil  121   a.    
     Current detection section  205  detects a current value of the AC power supplied from power-supply-side inverter  204  to power supply coil  121   a  and outputs the detected current value to frequency characteristic acquiring section  211 . 
     Power supply coil  121   a  receives supply of AC power from power-supply-side inverter  204 , thereby supplying power to power receiving coil  153   a.    
     Note that, although voltage detection section  203  is provided between power supply section  201  and power-supply-side inverter  204 , voltage detection section  203  may be provided between power-supply-side inverter  204  and power supply coil  121   a . Although current detection section  205  is provided between power-supply-side inverter  204  and power supply coil  121   a , current detection section  205  may also be provided between power supply section  201  and power-supply-side inverter  204 . 
     Power-supply-side control section  122  includes frequency characteristic acquiring section  211 , peak determining section  212 , drive frequency determining section  213  and power supply control section  214 . 
     Frequency characteristic acquiring section  211  sequentially calculates supplied power in the test power supply according to the voltage value outputted form voltage detection section  203  and the current value outputted from current detection section  205 . Frequency characteristic acquiring section  211  sequentially calculates power supply efficiency based on the calculated supplied power and the received power information outputted from power-supply-side communication section  123 , causing storage section  124  to store the calculated power supply efficiency, and sequentially outputting the power supply efficiency to peak determining section  212 . Note that, the frequency characteristic acquiring method will be described later in more details. 
     Peak determining section  212  performs a peak determining process in a power supply efficiency frequency characteristic based on the result of a comparison between the power supply efficiency stored in storage section  124  that is calculated earlier than the last time and the power supply efficiency of this time that is outputted from frequency characteristic acquiring section  211 . Peak determining section  212  outputs the peak determining result to drive frequency determining section  213 . Note that details of the peak determining method will be described later. 
     When a power-supply-start signal is outputted from power-supply-side communication section  123 , drive frequency determining section  213  outputs a start value and an end value of a preset frequency and a step value to change the frequency to power supply control section  214 . Among the frequencies at two peaks shown in the peak determining result outputted from peak determining section  212 , drive frequency determining section  213  determines one with a lower frequency as a drive frequency for full-scale power supply and notifies power supply control section  214  of the determined drive frequency. 
     When a power-supply-start signal is outputted from power-supply-side communication section  123 , power supply control section  214  closes switching section  202  and causes power supply section  201  to be connected to inverter  203  to start test power supply, and controls power-supply-side inverter  204  so as to change the frequency of AC power supplied to power supply coil  121   a  using the start value, end value and step value of the frequency outputted from drive frequency determining section  213 . When a drive frequency for full-scale power supply is outputted from drive frequency determining section  213 , power supply control section  214  keeps switching section  202  closed, keeps power supply section  201  connected to power-supply-side inverter  204  and controls power-supply-side inverter  204  so as to change an AC power frequency supplied to power supply coil  121   a  based on the drive frequency. 
     After starting power supply, when a power-supply-stop signal is outputted from power-supply-side communication section  123 , power supply control section  214  opens switching section  202  and causes power supply section  201  to be disconnected from power-supply-side inverter  204 . 
     &lt;Arrangement of Coil&gt; 
     Next, an arrangement of aforementioned power supply coil  121   a  and power receiving coil  153   a  will be described using  FIGS. 3A to 3D .  FIGS. 3A to 3D  illustrate a state in which power supply coil  121   a  is aligned with power receiving coil  153   a . The x-axis represents a lateral direction of vehicle  10  (+x direction indicating the right direction of vehicle  10 , −x direction indicating the left direction of vehicle  10 ), the y-axis represents a longitudinal direction of vehicle  10  (+y direction indicating the backward direction of vehicle  10 , the −y direction indicating the forward direction of vehicle  10 ), and the z-axis represents a vertical direction with respect to the ground (+z direction indicating the upward direction of vehicle  10 , the −z direction indicating the downward direction of vehicle  10 ).  FIG. 3A ,  FIG. 3B  and  FIG. 3C  illustrate an xy plane, a yz plane and an xz plane respectively.  FIG. 3D  illustrates a perspective view of power supply coil  121   a  and power receiving coil  153   a.    
     Thus, planar spiral coils are used for power supply coil  121   a  and power receiving coil  153   a  respectively and the spiral coils are arranged so that the respective planar surfaces become parallel to the surface of ground g. 
     &lt;Operation of Power Supply Apparatus&gt; 
     Next, an operation of aforementioned power supply apparatus  120  will be described using  FIG. 4 . 
     In step (hereinafter abbreviated as “ST”)  301 , power-supply-side communication section  123  of power supply apparatus  120  determines whether or not a power-supply-start signal has been received from vehicle-side communication section  152 , and proceeds to ST 302  when a power-supply-start signal has been received (YES) or repeats the process in ST 301  when no power-supply-start signal has been received (NO). 
     In ST 302 , frequency characteristic acquiring section  211  performs a process of acquiring a frequency characteristic for power supply efficiency. Note that details of the frequency characteristic acquiring method will be described later. 
     In ST 303 , peak determining section  212  performs a peak determining process based on the power supply efficiency stored in storage section  124  calculated earlier than the last time and power supply efficiency this time acquired from frequency characteristic acquiring section  211 . Note that details of the peak determining method will be described later. 
     In ST 304 , peak determining section  212  determines whether or not two peaks appear in the power supply efficiency frequency characteristic, and proceeds to ST 305  when two peaks appear (YES) or ends the operation of power supply apparatus  120  when two peaks do not appear (NO). 
     In ST 305 , drive frequency determining section  213  determines the lower frequency of the frequencies at the two peaks determined by peak determining section  212  as drive frequency Fk for full-scale power supply. Note that details of the drive frequency determining method will be described later. 
     In ST 306 , power supply control section  214  determines whether or not the charging start requirements are satisfied. Here, the “charging start requirements” refer to detection of a displacement between the engaging coils, leakage detection, equipment search, equipment authentication, detection of foreign substance between the coils or the like, and suppose the charging start requirements are satisfied when there is no abnormality in the requirements. When the charging start requirements are satisfied (YES), the process proceeds to ST 307  or when the charging start requirements are not satisfied (NO), the operation of power supply apparatus  120  is ended. 
     In ST 307 , power supply control section  214  starts power-supply-side inverter  204  based on drive frequency Fk for full-scale power supply and determines in ST 308  whether or not power-supply-side communication section  123  has received a power-supply-stop signal from vehicle-side communication section  152 . Power supply control section  214  proceeds to ST 309  when a power-supply-stop signal has been received (YES) or returns to ST 306  when no power-supply-stop signal has been received (NO). 
     In ST 309 , power supply control section  214  stops power-supply-side inverter  204  and ends the operation of power supply apparatus  120 . 
     &lt;Frequency Characteristic Acquiring Method&gt; 
     Next, the aforementioned frequency characteristic acquiring method will be described using  FIG. 5 . 
     In ST 401 , drive frequency determining section  213  sets start value Fa, end value Fb and step value Fs of drive frequency F for test power supply, and in ST 402 , power supply control section  214  sets start value Fa as drive frequency F. 
     In ST 403 , power supply control section  214  starts power-supply-side inverter  204  at drive frequency F and predetermined power Ws. Here, since predetermined power Ws is power in test power supply, Ws is power lower than supplied power in full-scale power supply. In ST 404 , frequency characteristic acquiring section  211  receives received power information on received power Wj from vehicle-side communication section  152  via power-supply-side communication section  123 . 
     In ST 405 , frequency characteristic acquiring section  211  calculates power supply efficiency η based on predetermined power (supplied power) Ws and power receiving power Wj received in ST 404 . For example, frequency characteristic acquiring section  211  divides received power Wj by predetermined power Ws (Wj/Ws) to calculate power supply efficiency η. Frequency characteristic acquiring section  211  causes storage section  124  to store calculated power supply efficiency η together with drive frequency F. 
     In ST 406 , power supply control section  214  adds step value Fs to drive frequency F, and power supply control section  214  determines, in ST 407 , whether or not drive frequency F is equal to or higher than end value Fb. When drive frequency F is equal to or higher than end value Fb (YES), power supply control section  214  proceeds to ST 408  or returns to ST 403  when drive frequency F is less than end value Fb (NO). 
     In ST 408 , power supply control section  214  opens switching section  202 , causes power supply section  201  to be disconnected from power-supply-side inverter  204 , stops power-supply-side inverter  204  and ends the frequency characteristic process. 
     Note that in  FIG. 5 , in ST 406 , step value Fs is added to drive frequency F by setting end value Fb to a higher frequency side than start value Fa, but step value Fs may be subtracted from drive frequency F in ST 406  by setting end value Fb to a lower frequency side than start value Fa. Alternatively, drive frequency F may be increased or decreased. Furthermore, the pitch width of step value Fs may be an irregular interval instead of a regular interval. 
     &lt;Peak Determining Method&gt; 
     Next, the aforementioned peak determining method will be described using  FIG. 6 . 
     The peak determining process is performed using the frequency characteristic for power supply efficiency that is acquired by the aforementioned frequency characteristic acquiring method. 
     In ST 501 , peak determining section  212  starts a search, and reads, in ST 502 , power supply efficiency η earlier than the last time stored in storage section  124 . 
     In ST 503 , peak determining section  212  determines whether or not a peak is detected. For example, when power supply efficiency calculated last time is higher than power supply efficiency calculated the time before the last time and power supply efficiency calculated this time is lower than power supply efficiency calculated last time, peak determining section  212  determines that the power supply efficiency is at a peak. Peak determining section  212  proceeds to ST 504  upon determining that a peak is detected (YES), or ends the search upon determining that no peak is detected (NO) (step ST 505 ). 
     In ST 504 , peak detection section  212  adds “1” as the number of peaks and causes storage section  124  to store frequency Fpl at which the power supply efficiency reaches a peak. Note that when a second peak is detected in the repetition process in ST 501  to ST 505 , frequency Fph (where, Fpl&lt;Fph is assumed) is stored in storage section  124  and the search is ended (step ST 505 ). 
     Note that in the peak determining process, the search in ST 501  to ST 505  is performed in the same range as the range of the frequency when a frequency characteristic is acquired or in a range narrower than that. 
     Thus, in the peak determining process, peak determining section  212  selects a frequency at a peak at which the power supply efficiency reaches a local maximum. Note that the frequency characteristic acquiring process and the peak determining process may be performed simultaneously. 
     Here,  FIG. 7  illustrates a situation in which a frequency characteristic is acquired over the entire operating frequency. In  FIG. 7 , peaks are detected in areas enclosed by a circle and an ellipse. Note that  FIG. 7  illustrates four frequency characteristics which differ depending on a gap, a positional displacement or a battery condition. 
     &lt;Drive Frequency Determining Method&gt; 
     Next, the aforementioned drive frequency determining method will be described using  FIG. 8 . 
     In ST 601 , drive frequency determining section  213  acquires frequency Fpl which is the lower frequency at the two peaks determined by peak determining section  212 . 
     In ST 602 , drive frequency determining section  213  determines the frequency where the power supply efficiency becomes equal to or higher than predetermined power supply efficiency η in the vicinity of frequency Fpl as drive frequency Fk for full-scale power supply. Here, drive frequency Fk is assumed to be frequency Fpl or in the vicinity of frequency Fpl. 
     Thus, drive frequency determining section  213  determines the lower frequency of the frequencies at the two peaks as drive frequency Fk. On the other hand, a frequency corresponding to a local minimum value located between peaks (value taken when the function reaches a local minimum) never changes depending on conditions. From this, the frequency characteristic acquiring process may be performed within a range equal to or lower than a frequency of a valley shape region in the frequency characteristic acquiring process.  FIG. 9  illustrates this situation. It is thereby possible to shorten the time required to obtain a frequency characteristic. 
     &lt;Direction of Magnetic Flux&gt; 
       FIGS. 10A and 10B  are conceptual diagrams each illustrating magnetic field shapes formed between power supply coil  121   a  and power receiving coil  153   a  when frequencies at two peaks are used. These diagrams illustrate the yz plane or xz plane shown in  FIG. 3 .  FIG. 10A  illustrates a magnetic field shape when the higher frequency of those at the two peaks and  FIG. 10B  illustrates a magnetic field shape when the lower frequency of those at the two peaks. 
     It is clear from  FIG. 10A  that a magnetic flux (thick arrows in the diagram) is in the horizontal direction and it is clear from  FIG. 10B  that a magnetic flux (thick arrow in the diagram) is in the +z direction, that is, the direction of vehicle  10 . Thus, using the lower frequency causes the magnetic flux to be directed toward vehicle  10 , and can thereby prevent unnecessary radiation from leaking outside. 
     &lt;Effects of Embodiment&gt; 
     Thus, charging system  100  according to Embodiment 1 uses planar spiral coils for power supply coil  121   a  and power receiving coil  153   a  respectively and performs full-scale power supply using a lower frequency or a frequency in the vicinity thereof among the frequencies at two peaks where power supply efficiency reaches a local maximum that are obtained from a frequency characteristic for power supply efficiency at a position where these coils are placed opposite to each other. Thus, it is possible to direct the magnetic flux formed between power supply coil  121   a  and power receiving coil  153   a  toward the vehicle  10  and thus to suppress unnecessary radiation. 
     Embodiment 2 
     In Embodiment 1, the case has been described where power supply efficiency is calculated when the frequency characteristic is acquired as shown in  FIG. 5 . In Embodiment 2 of the present invention, a case will be described where a current value of power supply coil  121   a  is detected while the voltage of power supply coil  121   a  is kept constant and the frequency characteristic of the current value is acquired. Note that since a configuration of a charging system according to Embodiment 2 of the present invention is similar to the configuration shown in  FIG. 1  and  FIG. 2  of Embodiment 1, only functions different from those in Embodiment 1 will be described using  FIG. 1  and  FIG. 2 . 
     &lt;Frequency Characteristic Acquiring Method&gt; 
     A frequency characteristic acquiring method according to Embodiment 2 of the present invention will be described using  FIG. 11 . Note that in  FIG. 11 , the processes identical to those in  FIG. 5  will be assigned identical reference numerals and a duplicate description thereof will be omitted. 
     In ST 701 , power supply control section  214  sets a voltage value of power supply coil  121   a  to predetermined voltage Vs. Here, predetermined voltage Vs is a voltage in test power supply, so that it may be lower than a voltage in full-scale power supply. 
     In ST 702 , power supply control section  214  starts power-supply-side inverter  204  at drive frequency F and predetermined voltage Vs, and frequency characteristic acquiring section  211  detects, in ST 703 , current value Ik of power supply coil  121   a  and causes storage section  124  to store detected current value Ik together with drive frequency F. 
     Thus, when the voltage of power supply coil  121   a  is set to be constant, the present embodiment takes advantage of the fact that the current of power supply coil  121   a  has a frequency characteristic similar to the power supply efficiency. 
     Note that although the voltage of power supply coil  121   a  is set to be constant in the present embodiment, the current of power supply coil  121   a  may be set to be constant so that a voltage value of power supply coil  121   a  is detected. In this case, the voltage of power supply coil  121   a  has a frequency characteristic similar to the power supply efficiency as well. Without being limited to these methods, any method may be adopted as long as a frequency characteristic for the power supply efficiency can be acquired with the method. 
     Note that the case has been described in the embodiments where a lower frequency is used for the entire drive time, but the present invention is not limited to this. For example, a lower frequency may be used for a time period equal to or longer than half the entire drive time and a higher frequency may be used for the remaining time period. In other words, the lower frequency may be used preferentially over the higher frequency. 
     A case has been described in the embodiments where a planar spiral coil is used. However, the present invention is not limited to this, and a solenoid coil as shown in  FIGS. 12A to 12D  may be used.  FIGS. 12A to 12D  as well as  FIG. 3  illustrate a state in which alignment is made between power supply coil SC and power receiving coil RC.  FIGS. 12A, 12B and 12C  illustrate the xy, xz and yz planes, respectively.  FIG. 12D  illustrates a perspective view of power supply coil SC and power receiving coil RC. Thus, the solenoid coils used for power supply coil SC and power receiving coil RC respectively are arranged with the respective central axes placed parallel to the surface of ground g. 
     The case has been described in the embodiments where power supply apparatus  120  includes storage section  124 . However, the present invention is not limited to this, and power supply apparatus  120  may not include storage section  124 . In this case, power-supply-side control section  122  changes drive frequency F, stops changing drive frequency F when the current value, voltage value, efficiency, and coil current phase difference reach predetermined values, and the frequency at this time is assumed to be a drive frequency. 
     Drive frequency Fk may be determined based on a phase difference between currents flowing through the primary side coil and the secondary side coil. When the current flowing through the primary side coil is in-phase with the current flowing through the secondary side coil, both the spiral coil and the solenoid coil are driven at a peak on the low-frequency side, whereas when they are in opposite phases, both coils are driven at a peak on the high-frequency side. For this reason, when both currents become in-phase while the frequency is changed from a certain value, the change of drive frequency F is stopped and the frequency at this time is assumed to be drive frequency Fk. 
     When drive frequency Fk is determined according to a phase difference between the currents flowing through the primary side coil and the secondary side coil, current detection section  205  detects the phase of the current flowing through power supply coil  121   a  and vehicle-side control section  151  detects the phase of the current flowing through power receiving coil  153   a . Furthermore, the peak determining process in peak determining section  212  “reads a coil current phase difference” in ST 502  of  FIG. 6  and reads this, in ST 503 , as “determine whether or not the read coil current phase difference is in-phase.” As a result, drive frequency determining section  213  determines the frequency at which the coil current phase difference becomes in-phase to be drive frequency Fk. 
     The disclosure of Japanese Patent Application No. 2013-066137, filed on Mar. 27, 2013, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The power supply apparatus, the power receiving apparatus and the charging system according to the present invention are useful in suppressing radiation of a horizontal direction component with respect to the surface of the ground. 
     REFERENCE SIGNS LIST 
     
         
           10  Vehicle 
           120  Power supply apparatus 
           121  Power supply section 
           121   a  Power supply coil 
           122  Power-supply-side control section 
           123  Power-supply-side communication section 
           124  Storage section 
           150  Power receiving apparatus 
           151  Vehicle-side control section 
           152  Vehicle-side communication section 
           153  Power receiving section 
           153   a  Power receiving coil 
           170  Storage battery 
           180  Positioning camera 
           201  Power supply section 
           202  Switching section 
           203  Voltage detection section 
           204  Power-supply-side inverter 
           205  Current detection section 
           211  Frequency characteristic acquiring section 
           212  Peak determining section 
           213  Drive frequency determining section 
           214  Power supply control section