Patent Publication Number: US-9841524-B2

Title: Metal object detection device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of International Application No. PCT/JP2013/007316 filed 12 Dec. 2013 which designated the U.S. and claims priority to Japanese Patent Application No. 2012-284971 filed on Dec. 27, 2012 and Japanese Patent Application No. 2012-284970 filed on Dec. 27, 2012, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a metal object detection device applied to a non-contact power feeding device. 
     BACKGROUND ART 
     Conventionally, as disclosed in PTL 1 described below, a metal detector detecting a metal object has been known. Specifically, the metal detector includes a first coil, a second coil magnetically coupled with the first coil and configuring a resonant circuit in cooperation with a capacitor, and a control circuit having a function of detecting the object on the basis of a voltage between both ends of the resonant circuit. The first coil is supplied with an exciting current. 
     The metal detector configured as described above is slid along a surface of an object embedded with the metal object. When the metal object comes close to the first coil that makes up the metal detector by sliding the metal detector, an eddy current loss is caused by an electromagnetic induction to change an inductance of the first coil. When the inductance of the first coil changes, because an amplitude of a voltage between terminals of the resonant circuit is reduced, the metal object can be detected on the basis of the voltage between the terminals. 
     In the technique disclosed in the above PTL 1, the metal detector is required to be slid for the purpose of detecting the metal object. Thus, when a region in which the metal object is assumed to be present is large, there is a concern that the metal object cannot be appropriately detected such that man-hours for detecting the metal object increases. 
     As the metal object detection device applied to a non-contact power feeding device that performs power transfer between the primary coil and the secondary coil in a non-contact manner, as disclosed in the following PTL 2, a device having plural temperature sensors on the primary coil has been known. When the metal object is present on the primary coil at the time of non-contact power feeding, the device detects the metal object by detecting a rise in temperature of the metal object through a temperature sensor. The rise in temperature is caused when the eddy current flows in the metal object. According to the metal object detection device, when the metal object is detected, the power supply of the non-contact power feeding can be lowered, or the power supply can stop. As a result, a rise in the temperature of the metal object present on the primary coil can be suppressed, and further the safety of the non-contact power feeding device can be enhanced. 
     However, in the metal object detection device disclosed in the above PTL 1, there is a concern about disadvantages described below. In detail, the temperature of the metal object detected by the temperature sensor becomes lower than a real temperature of the metal object due to the presence of the metal object on the primary coil, or an air layer between the respective temperature sensors. This leads to a concern that a detection precision of the metal object is lowered. If the metal object is large, a time until the metal object becomes high in temperature since the non-contact power feeding starts is shortened. Thus, there is a concern that the metal object cannot be detected since the temperature of the metal object starts to rise due to the non-contact power feeding until the temperature of the metal object arrives at the temperature to be detected. Further, there is a concern that the metal object cannot be detected only during the operation of the non-contact power feeding device. 
     PRIOR ART LITERATURES 
     Patent Literature 
     PTL 1: JP 4421532 B 
     PTL 2: US 2011/0074346 A1 
     SUMMARY OF INVENTION 
     An object of the present disclosure is to provide a metal object detection device that is capable of appropriately detecting the metal object. 
     A metal object detection device according to one aspect of the present disclosure is applied to a non-contact power feeding device that performs power transfer between a primary coil and a secondary coil in a non-contact manner. The metal object detection device includes a plurality of detection coils, a capacitor, a first series connection body, a second series connection body, a voltage applying unit, and a processing unit. The plurality of detection coils detects the metal object present in a path of a magnetic flux generated by at least one of the primary coil and the secondary coil. The capacitor configures a resonant circuit in each of at least two of the plural detection coils. 
     The first series connection body includes one of a series connection body having at least one of the resonant circuits and a first passive element, and a series connection body having the detection coil and the capacitor configuring one of the resonant circuits. The second series connection body includes one of a series connection body having at least one of the resonant circuits except for the resonant circuit configuring the first series connection body and a second passive element, and a series connection body having the detection coil and the capacitor configuring one of the resonant circuits except for the resonant circuit configuring the first series connection body. 
     The voltage applying unit applies an AC voltage to both ends of each of the first series connection body and the second series connection body. The processing unit performs a process for detecting the metal object on the basis of a potential difference between a connection point included in the first series connection body and a connection point included in the second series connection body. 
     The metal object detection device is capable of spreading a detection region of the metal object while enhancing a detection precision of the metal object. 
     A metal object detection device according to another aspect of the present disclosure is applied to a non-contact power feeding device that performs power transfer between a primary coil and a secondary coil in a non-contact manner. The metal object detection device includes a detection coil, a supply unit, and a processing unit. The detection coil detects the metal object present in a path of a magnetic flux generated by at least one of the primary coil and the secondary coil when the power transfer is performed in the non-contact manner. The supply unit supplies an AC power to the detection coil. The processing unit performs a process for detecting the metal object on the basis of a change in impedance of the detection coil when the AC power is supplied by the supply unit. The detection coil includes a portion in which an induced current flows in a predetermined direction, and a portion in which the induced current flows in a direction opposite to the predetermined direction when a main magnetic flux circulating between the primary coil and the secondary coil passes through the detection coil. 
     Even if the detection of the temperature rise of the metal object by the temperature sensor is insufficient, or the non-contact power feeding device does not operate, the metal object detection device can detect the metal object. The metal object detection device is capable of suppressing an influence of the main magnetic flux on the detection of the metal object and the non-contact power transfer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a configuration diagram of a non-contact power feeding system according to a first embodiment; 
         FIG. 2  is a perspective view of a power transmitting pad and a power receiving pad according to the first embodiment; 
         FIG. 3  is a circuit diagram of a metal object detection device according to the first embodiment; 
         FIG. 4  is a diagram illustrating the arrangement of detection coils in the power transmitting pad according to the first embodiment; 
         FIG. 5  is a graph illustrating the transition of a detection value of a voltage sensor according to the first embodiment; 
         FIG. 6  is a flowchart illustrating a procedure of a metal object detecting process according to the first embodiment; 
         FIG. 7  is a circuit diagram of a metal object detection device according to a second embodiment; 
         FIG. 8  is a diagram illustrating the shape of detection coils according to a third embodiment; 
         FIG. 9  is a diagram illustrating the arrangement of detection coils in the power transmitting pad according to the third embodiment; 
         FIG. 10  is a circuit diagram of a metal object detection device according to a fourth embodiment; 
         FIG. 11  is a circuit diagram of a metal object detection device according to a fifth embodiment; 
         FIG. 12  is a circuit diagram of a metal object detection device according to a sixth embodiment; 
         FIG. 13  is a diagram illustrating the arrangement of detection coils in a power transmitting pad according to the sixth embodiment; 
         FIG. 14  is a circuit diagram of a metal object detection device according to a seventh embodiment; 
         FIG. 15  is a circuit diagram of a metal object detection device according to an eighth embodiment; 
         FIG. 16  is a circuit diagram of a metal object detection device according to a ninth embodiment; 
         FIG. 17  is a graph illustrating a frequency characteristic of a system according to the ninth embodiment; 
         FIG. 18  is a flowchart illustrating a procedure of a metal object detecting process according to the ninth embodiment; 
         FIG. 19  is a circuit diagram of a metal object detection device according to a tenth embodiment; 
         FIG. 20  is a graph illustrating a frequency characteristic of a system according to a tenth embodiment; 
         FIG. 21  is a flowchart illustrating a procedure of a metal object detecting process according to the tenth embodiment; 
         FIG. 22  is a circuit diagram of a metal object detection device according to another embodiment; 
         FIG. 23  is a diagram illustrating the arrangement of detection coils in a power transmitting pad according to another embodiment; 
         FIG. 24  is a diagram illustrating the shape of detection coils according to another embodiment; 
         FIG. 25  is a circuit diagram of a metal object detection device according to an eleventh embodiment; 
         FIG. 26  is a diagram illustrating the shape of detection coils according to the eleventh embodiment; 
         FIG. 27  is a diagram illustrating the arrangement of detection coils in the power transmitting pad according to the eleventh embodiment; 
         FIG. 28  is a flowchart illustrating a procedure of a metal object detecting process according to the eleventh embodiment; 
         FIG. 29A  is a graph illustrating an output voltage of a metal object oscillator circuit when no metal object is present according to the eleventh embodiment; 
         FIG. 29B  is a graph illustrating an output voltage of a metal object oscillator circuit when a metal object is present according to the eleventh embodiment; 
         FIG. 30  is a flowchart illustrating a procedure of a metal object detecting process according to a twelfth embodiment; 
         FIG. 31A  is a graph illustrating an output voltage of a metal object oscillator circuit when no metal object is present according to the twelfth embodiment; 
         FIG. 31B  is a graph illustrating an output voltage of a metal object oscillator circuit when a metal object is present according to the twelfth embodiment; 
         FIG. 32  is a circuit diagram of a metal object detection device according to a thirteenth embodiment; 
         FIG. 33  is a graph illustrating the transition of an output voltage of an oscillator circuit according to the thirteenth embodiment; 
         FIG. 34  is a diagram illustrating the shape of detection coils according to a fourteenth embodiment; 
         FIG. 35  is a diagram illustrating the arrangement of detection coils in a power transmitting pad according to the fourteenth embodiment; 
         FIG. 36  is a diagram illustrating the shape of detection coils according to another embodiment; 
         FIG. 37  is a diagram illustrating the shape of detection coils according to another embodiment; 
         FIG. 38  is a diagram illustrating the arrangement of the detection coils illustrated in  FIG. 37  in a power transmitting pad; 
         FIG. 39  is a diagram illustrating the shape of detection coils according to another embodiment; 
         FIG. 40A  is a front view of a first substrate on which a first coil position is patterned; 
         FIG. 40B  is a cross-sectional view of the first substrate taken along a line XLB-XLB in  FIG. 40A ; 
         FIG. 40C  is a front view of a second substrate on which a second coil position is patterned; 
         FIG. 40D  is a cross-sectional view of the second substrate taken along a line XLD-XLD in  FIG. 40C ; 
         FIG. 40E  is a front view of a third substrate on which a third coil position is patterned; 
         FIG. 40F  is a cross-sectional view of the third substrate taken along a line XLF-XLF in  FIG. 40E ; 
         FIG. 40G  is a front view of a fourth substrate on which a fourth coil position is patterned; 
         FIG. 40H  is a cross-sectional view of the fourth substrate taken along a line XLH-XLH in  FIG. 40G ; 
         FIG. 40I  is a diagram of any one plate surface of the first to fourth substrates on which patterns formed on the other three substrates are projected; 
         FIG. 40J  is a cross-sectional view illustrating a state in which the first to fourth substrates are laminated; 
         FIG. 41A  is a diagram illustrating a detection coil formed on a first surface of a double-sided substrate; 
         FIG. 41B  is a diagram illustrating a detection coil formed on a second surface of the double-sided substrate; 
         FIG. 41C  is a cross-sectional view of the double-sided substrate taken along a line XLIC-XLIC of  FIGS. 41A and 41B ; 
         FIG. 42A  is a diagram illustrating detection coils formed on a first surface of a double-sided substrate; 
         FIG. 42B  is a diagram illustrating a detection coil formed on a second surface of the double-sided substrate; and 
         FIG. 42C  is a cross-sectional view of the double-sided substrate taken along a line XLIIC-XLIIC of  FIGS. 42A and 42B . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a description will be given of a first embodiment in which a metal object detection device according to the present disclosure is applied to a non-contact power feeding system for a vehicle (hybrid vehicle or electric vehicle) having a rotary machine as an in-vehicle main equipment with reference to the drawings. 
     As illustrated in  FIG. 1 , a non-contact power feeding system includes a power transmitting system disposed outside of a vehicle  10 , and a power receiving system installed in the vehicle  10 . 
     The power transmitting system includes a power transmitting pad  20  as a power transmitting member, and a power transmitting circuit  30 . In detail, the power transmitting circuit  30  includes a power converter circuit (for example, a full bridge circuit) that converts a frequency of an AC external power supply  32  (system power supply) installed outside of the vehicle  10  into a predetermined high frequency (several kHz to tens of MHz, for example, 20 kHz), and a resonant circuit that supplies an electric power output from the power converter circuit into the power transmitting pad  20 . 
     The power transmitting pad  20  is a member including a primary core and a primary coil wound on the primary core, for transmitting the electric power to a power receiving pad  40  provided in the power receiving system by an electromagnetic induction. 
     On the other hand, the power receiving system includes the power receiving pad  40  as a power receiving member, a power receiving circuit  50 , a DCDC converter  52 , a main battery  54  as a power storage unit, and a control device  56 . In detail, the power receiving pad  40  is a member including a secondary core and a secondary coil wound on the secondary core, for receiving an electric power transmitted from the power transmitting pad  20 . The power receiving pad  40  is arranged on a lower portion (outside of a floor) of the vehicle  10 . 
     The electric power received by the power receiving pad  40  is supplied to the power receiving circuit  50 . The power receiving circuit  50  includes a resonant circuit that receives the electric power received by the power receiving pad  40 , a rectifier circuit that converts an AC current having a high frequency output from the resonant circuit into a DC current, and a power converter circuit that converts an output voltage of the rectifier circuit into a predetermined voltage, and applies the converted voltage to the main battery  54 . The output voltages from the power receiving circuit  50  and the main battery  54  are stepped down by the DCDC converter  52 , and applied to vehicle accessories  58  and an accessory battery  60 . 
     The main battery  54  is a battery having a terminal voltage of, for example, one hundred V or higher, specifically such as a nickel-hydrogen secondary battery or a lithium-ion secondary battery. Also, the accessory battery  60  is a battery having a terminal voltage sufficiently lower than the terminal voltage of the main battery  54 , specifically such as a lead-acid storage battery. Further, the vehicle  10  is equipped with an inverter  62  that converts a DC power of the main battery  54  into an AC power, and outputs the AC power, and a motor generator  64  rotationally driven by the AC power output from the inverter  62  as an in-vehicle main equipment, in addition to the power receiving system. 
     The control device  56  operates the inverter  62  for controlling the driving of the motor generator  64 , and operates the DCDC converter  52  for supplying the electric power to the vehicle accessories  58  and the accessory battery  60 . The control device  56  instructs the power receiving circuit  50  to execute a charging process for feeding the power to the vehicle. With this operation, the power receiving circuit  50  operates to charge the vehicle  10  while transferring information between the power receiving circuit  50  and the power transmitting circuit  30  through respective radio communication interfaces installed in the power receiving circuit  50  and the power transmitting circuit  30 . 
     Subsequently, the power transmitting pad  20  and the power receiving pad  40  according to the present embodiment will be described in detail with reference to  FIG. 2 .  FIG. 2  is a perspective view of components of the power transmitting pad  20  and the power receiving pad  40 . 
     As shown in the figure, the power transmitting pad  20  includes a primary core  21  and a primary coil  22  wound on the primary core  21 , and is formed in a flat shape. In detail, the primary core  21  is a member in which a pair of primary spacing parts  21   a  is formed integrally with a primary coupling part  21   b . The pair of primary spacing parts  21   a  is shaped into a rectangular plate and spaced from each other, and the primary coupling part  21   b  is shaped into a rectangular plate that couples the respective primary spacing parts  21   a  with each other. Each of the pair of primary spacing parts  21   a  has plate surfaces (plane), and a peripheral edge portion  21   c . The primary coupling part  21   b  is coupled to one of the paired plate surfaces of each of the primary spacing parts  21   a  on the opposite side of the power receiving pad  40  side. The primary coil  22  is wound on each peripheral edge portion  21   c  of those primary spacing parts  21   a  in plural rounds. In the present embodiment, ferrite is used as the primary core  21 . 
     On the other hand, the power receiving pad  40  includes a secondary core  41  and a secondary coil  42  wound on the secondary core  41 , and is formed in a flat shape. In detail, the secondary core  41  is a member in which a pair of secondary spacing parts  41   a  is formed integrally with a secondary coupling part  41   b . The pair of secondary spacing parts  41   a  is shaped into a rectangular plate and spaced from each other, and the secondary coupling part  41   b  is shaped into a rectangular plate that couples the respective secondary spacing parts  41   a  with each other. Each of the pair of secondary spacing parts  41   a  has plate surfaces (plane), and a peripheral edge portion  41   c . The secondary coupling part  41   b  is coupled to one of the paired plate surfaces of each of the secondary spacing parts  41   a  on the opposite side of the power transmitting pad  20  side. The secondary coil  42  is wound on each peripheral edge portion  41   c  of those secondary spacing parts  41   a  in plural rounds. In the present embodiment, ferrite is used as the secondary core  41  as with the primary core  21 . 
     When non-contact power feeding is performed, the primary core  21  and the secondary core  41  are arranged in such a manner that the plate surfaces of the primary spacing parts  21   a  and the plate surfaces of the secondary spacing parts  41   a  face each other, and become in parallel to each other. 
     In  FIG. 2 , an example of a flow direction of a current I 1  flowing in the primary coil  22  and a flow direction of a current I 2  flowing in the secondary coil  42  is indicated by arrows. When a high frequency current flows in the primary coil  22 , a magnetic field is generated inside of the primary coil  22 . As a result, as indicated by dashed arrows in the figure, a magnetic flux flows from one plate surface side of the paired primary spacing parts  21   a  into the opposed plate surface side of the secondary spacing parts  41   a , and an induced current flows into the secondary coil  42 . Thus, a magnetic field is generated inside of the secondary coil  42 , and a magnetic flux flows from the other plate surface side of the paired secondary spacing parts  41   a  into the opposed plate surface side of the primary spacing parts  21   a . As a result, a main magnetic flux circulating between the primary core  21  and the secondary coil  41  is formed. As a result, an electric power is transferred between the primary coil  22  and the secondary coil  42  in a non-contact manner. 
     Subsequently, a description will be given of the metal object detection device according to the present embodiment provided in the power transmitting system with reference to  FIGS. 3 to 6 . 
     The metal object detection device includes first to fourth detection coils  70   a  to  70   d , first to fourth capacitors  71   a  to  71   d , an AC power supply  72 , a filter circuit  73 , and a determination unit  74 . In detail, the first detection coil  70   a  and the first capacitor  71   a  configure a first resonant circuit Res 1  that is a series resonant circuit, and the second detection coil  70   b  and the second capacitor  71   b  configure a second resonant circuit Res 2  that is the series resonant circuit. The third detection coil  70   c  and the third capacitor  71   c  configure a third resonant circuit Res 3  that is a series resonant circuit, and the fourth detection coil  70   d  and the fourth capacitor  71   d  configure a fourth resonant circuit Res 4  that is the series resonant circuit. The first resonant circuit Res 1  and the second resonant circuit Res 2  are connected in series with each other, and the third resonant circuit Res 3  and the fourth resonant circuit Res 4  are connected in series with each other. 
     In the present embodiment, a series connection body of the first resonant circuit Res 1  and the second resonant circuit Res 2  corresponds to “first series connection body”, and a series connection body of the third resonant circuit Res 3  and the fourth resonant circuit Res 4  corresponds to “second series connection body”. The second resonant circuit Res 2  corresponds to “first passive element” connected in series with the first resonant circuit Res 1 , and the fourth resonant circuit Res 4  corresponds to “second passive element” connected in series with the third resonant circuit Res 3 . 
     The series connection body of the first resonant circuit Res 1  and the second resonant circuit Res 2  is connected in parallel to the series connection body of the third resonant circuit Res 3  and the fourth resonant circuit Res 4 . In other words, an AC bridge (Wheatstone bridge) is formed by the first to fourth resonant circuits Res 1  to Res 4 . 
       FIG. 4  illustrates an arrangement mode of the first to fourth detection coils  70   a  to  70   d . In  FIG. 4 , the illustration of the primary coil  22  is omitted. 
     As shown in the figure, the first to fourth detection coils  70   a  to  70   d  are coils for detecting a metal object as a foreign matter present on the power transmitting pad  20 , and in the present embodiment, circular coils of a flat shape are assumed. The first to fourth detection coils  70   a  to  70   d  are so arranged as to detect the metal object present on a path of a magnetic flux generated by at least one of the primary coil  22  and the secondary coil  42 . Specifically, those coils  70   a  to  70   d  are arranged so that radial directions of the coils  70   a  to  70   d  are in parallel to the plate surface (plate surface of the primary spacing parts  21   a ) of the power transmitting pad  20 . In  FIG. 4 , a pair of axes configuring a plane parallel to the plate surface of the primary spacing parts  21   a , and orthogonal to each other is indicated by an x-axis and a y-axis. 
     In the present embodiment, it is assumed that the power transmitting pad  20  is really configured so that the first to fourth detection coils  70   a  to  70   d  are arranged above the primary coil  22 , and the primary core  21 , the primary coil  22 , and the first to fourth detection coils  70   a  to  70   d  are molded with resin. That is, the first to fourth detection coils  70   a  to  70   d  are embedded in the power transmitting pad  20 . As with the power transmitting pad  20 , it is assumed that the power receiving pad  40  is configured so that the secondary core  41  and the secondary coil  42  are molded with resin. 
     As illustrated in  FIG. 3 , the AC bridge is equipped with a voltage sensor  75 . The voltage sensor  75  detects a potential difference between a connection point of the first resonant circuit Res 1  and the second resonant circuit Res 2 , and a connection point of the third resonant circuit Res 3  and the fourth resonant circuit Res 4 . A detection value (hereinafter referred to as “output voltage Vout”) of the voltage sensor  75  is input to the filter circuit  73 . In the present embodiment, the filter circuit  73  is a band-pass filter, and disposed to avoid a reduction in detection precision of the metal object which is caused by a metal object detecting process which will be described later. That is, as illustrated in  FIG. 5 , as indicated by dashed lines, a component of a use frequency fe of the non-contact power feeding (frequency of a voltage applied to the primary coil  22 , for example, 20 kHz) may be superimposed on the output voltage Vout. In this case, since a waveform of the output voltage Vout is distorted, there is a concern about a reduction in the detection precision of the metal object caused by the metal object detecting process. In order to avoid the above problem, the filter circuit  73  is provided. 
     The output value Vf of the filter circuit  73  is input to the determination unit  74  as a software processing unit. The determination unit  74  performs the metal object detecting process for detecting the metal object on the power transmitting pad  20  on the basis of the output value Vf. Hereinafter, the above processing will be described in association with setting of a resonant frequency fd of the first to fourth resonant circuits Res 1  to Res 4 , and a frequency fv of the output voltage of the AC power supply  72 . 
     First, the setting of the resonant frequency fd will be described. In the present embodiment, the resonant frequencies fd of the first to fourth resonant circuits Res 1  to Res 4  are set to the same frequency. In other words, the first to fourth detection coils  70   a  to  70   d  configuring the first to fourth resonant circuits Res 1  to Res 4  have the same specification, and the first to fourth capacitors  71   a  to  71   d  configuring the first to fourth resonant circuits Res 1  to Res 4  have the same specification. Thus, in  FIG. 3 , an inductance of the first to fourth detection coils  70   a  to  70   d  is indicated by “L”, and a capacitance of the first to fourth capacitors  71   a  to  71   d  is indicated by “C”. As a result, the resonant frequencies fd of the first to fourth resonant circuits Res 1  to Res  4  are represented by the following equation (eq1). 
     
       
         
           
             
               
                 
                   
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     In the present embodiment, the frequency fv of the output voltage of the AC power supply  72  is set to the same frequency as the above resonant frequency fd under a situation where no metal object is present on the power transmitting pad  20 . Thus, when no metal object is present on the power transmitting pad  20 , since a potential of a connection point between the first resonant circuit Res 1  and the second resonant circuit Res 2  becomes equal to a potential of a connection point between the third resonant circuit Res 3  and the fourth resonant circuit Res 4 , the output voltage Vout becomes “0”. On the contrary, when the metal object is present on the power transmitting pad  20 , and the metal object comes close to any one of the first to fourth detection coils  70   a  to  70   d , an eddy current loss is caused by electromagnetic induction. Thus, a resonance level of the resonant circuit having the detection coil to which the metal object comes close as a component changes due to a change in impedance of the detection coil to which the metal object comes close, and the output voltage Vout largely changes from a reference value “0”. In view of this phenomenon, the metal object on the power transmitting pad  20  can be detected on the basis of the output voltage Vout. 
     A configuration having the first to fourth detection coils  70   a  to  70   d  as a component of the resonant circuit to detect the metal object on the basis of the potential difference between the respective connection points of the resonant circuit is employed for the purpose of enhancing the detection precision of the metal object. That is, as the metal object on the power transmitting pad  20  is smaller, a change in the inductance of the detection coils becomes smaller. According to the resonant circuit, even when a change in the inductance of the detection coils is small, because this change is amplified to enable the higher sensitivity, the detection precision of the metal object can be enhanced. 
     Further, the resonant frequency fd is set to a frequency (for example, 200 kHz) higher than the use frequency fe of the non-contact power feeding, and more particularly, set to a frequency twice or more as large as the use frequency fe. This setting is intended to avoid a reduction in reliability of the metal object detection device. That is, when the main magnetic flux circulating between the primary coil  22  and the secondary coil  42  passes through the detection coils, a current flows into the detection coils. When the resonant frequency fd is sufficiently separated from the use frequency fe, since the impedance of the resonant circuit at the use frequency fe becomes sufficiently large, even if the main magnetic flux passes through the detection coils, the current can be restrained from flowing into the detection coils. As a result, the reduction in the reliability of the metal object detection device is avoided. 
     Subsequently,  FIG. 6  illustrates a procedure of the metal object detecting process according to the present embodiment. This process is repetitively executed by the determination unit  74 , for example, in a predetermined period. 
     In the series of processes, it is first determined in S 10  whether a value of a detection flag F is “0”, or not. The detection flag F of “0” indicates that no metal object is present, and the detection flag F of “1” indicates that the metal object is present on the power transmitting pad  20 . An initial value of the detection flag F is set to “0”. 
     If an affirmative determination is made in S 10 , it is determined that no metal object is present, and the process proceeds to S 12 . In S 12 , an output value Vf of the filter circuit  73  is acquired. 
     Subsequently in S 14 , it is determined whether an absolute value of the difference between the output value Vf and a specified value Vα exceeds a threshold value Δ, or not. In more detail, it is determined whether a latest maximum value of the absolute value exceeds the threshold value Δ, or not. This process is intended to determine whether the metal object is present on the power transmitting pad  20 , or not. That is, as described above, when the metal object is present on the power transmitting pad  20 , the output value Vf largely changes from “0”. The specified value Vα may be set to, for example, the output value Vf (for example, nearly “0”) assumed when no metal object is present on the power transmitting pad  20 . The threshold value Δ may be set to, for example, a minute value. 
     If an affirmative determination is made in S 14 , it is determined that the metal object is present on the power transmitting pad  20 , and the process proceeds to S 16 . In S 16 , the value of the detection flag F is set to “1”. 
     When the process in S 16  is completed, or when a negative determination is made in S 10 , the process proceeds to S 18 . In S 18 , stop of the power supply to the power receiving pad  40  from the power transmitting pad  20 , and notification of a fact that the metal object is present to the user are instructed. As a result, the power supply from the power transmitting pad  20  to the power receiving pad  40  stops, and the user is notified of the fact that the metal object is present by some notification unit. 
     Incidentally, when it is determined that the metal object is present, the value of the detection flag F is held at “1” until it is then determined that the metal object has been removed from the power transmitting pad  20 . 
     When the negative determination is made in S 14 , or when the process in S 18  is completed, the series of processes is terminated once. 
     According to the present embodiment described above in detail, the following advantages are obtained. 
     (i) The metal object detecting process for detecting the metal object on the power transmitting pad  20  on the basis of the potential difference between the connection point of the first resonant circuit Res 1  and the second resonant circuit Res 2 , and the connection point of the third resonant circuit Res 3  and the fourth resonant circuit Res 4  is performed. Accordingly, the detection precision of the metal object can be enhanced. In particular, in the present embodiment, the respective resonant frequencies fd of the first to fourth resonant circuits Res 1  to Res 4 , and the frequency fv of the output voltage of the AC power supply  72  are set to the same frequency. This contributes to an improvement in the detection precision of the metal object. 
     Further, according to the present embodiment, since the plural detection coils as the components of the first to fourth resonant circuits Res 1  to Res 4  are provided, the detection region of the metal object can be spread while the detection precision of the metal object is enhanced. 
     (ii) The first to fourth resonant circuits Res 1  to Res 4  are configured so that the impedances at the frequency fv of the output voltage of the AC power supply  72  are identical with each other. In particular, in the present embodiment, the detection coils and the capacitors configuring the respective first to fourth resonant circuits Res 1  to Res 4  have the same specifications in the respective first to fourth resonant circuits Res 1  to Res 4 . According to this configuration, the components configuring the metal object detection device can be commonalized. 
     (iii) The AC bridge is formed by the first to fourth resonant circuits Res 1  to Res 4 . Thus, the detection of the metal object by the first to fourth detection coils  70   a  to  70   d  can be performed with the detection value of the single voltage sensor  75 . 
     (iv) The resonant frequencies fd of the first to fourth resonant circuits Res 1  to Res 4  are set to the frequency higher than the use frequency fe. According to this setting, even when the main magnetic flux passes through the detection coils at the time of non-contact power feeding, a current can be restrained from flowing through the detection coils. Accordingly, the reliability of the metal object detection device can be prevented from being lowered. 
     (v) The filter circuit  73  is provided. Accordingly, an influence of the use frequency fe can be removed from the output voltage Vout, and further a reduction in the detection precision of the metal object can be avoided. 
     Second Embodiment 
     Hereinafter, a description will be given of a second embodiment, mainly differences from the first embodiment described above with reference to the drawings. 
     In the present embodiment, a circuit configuration of a metal object detection device is changed. In detail, as illustrated in  FIG. 7 , the first to fourth resonant circuits Res 1  to Res 4  are configured by parallel resonant circuits. In  FIG. 7 , detection coils configuring those resonant circuits Res 1  to Res 4  are indicated by “ 70   e  to  70   h ”, and capacitors configuring those resonant circuits Res 1  to Res 4  are indicated by “ 71   e  to  71   h ”. In  FIG. 7 , the same members as the members illustrated in  FIG. 3  described above are denoted by the same symbols for convenience. 
     Likewise, in the present embodiment described above, the same advantages as the advantages (i) to (iv) of the first embodiment can be obtained. 
     Third Embodiment 
     Hereinafter, a description will be given of a third embodiment, mainly differences from the first embodiment described above with reference to the drawings. In the present embodiment, the shape of the detection coils is changed. 
       FIG. 8  illustrates an arrangement mode of the detection coils according to the present embodiment.  FIG. 8  is a diagram of the detection coils when viewed from a direction orthogonal to a plane configured by the x-axis and the y-axis illustrated in  FIG. 4  described above. 
     As shown in the figure, in the present embodiment, axisymmetric coils (coils formed on a single plane) are used as first to fourth detection coils  70   i  to  70   l , and specifically, 8-shaped coils are employed. 
       FIG. 9  illustrates an arrangement mode of the first to fourth detection coils  70   i  to  70   l . In  FIG. 9 , the same members as the members illustrated in  FIG. 4  described above are denoted by the same symbols for convenience. 
     As shown in the figure, the respective first to fourth detection coils  70   i  to  70   l  are formed on a plane parallel to plate surfaces of primary spacing parts  21   a . Thus, when a main magnetic flux circulating between a primary coil  22  and a secondary coil  42  passes through the first to fourth detection coils  70   i  to  70   l , each of the first to fourth detection coils  70   i  to  70   l  has a portion in which an induced current flows in a predetermined direction, and a portion in which the induced current flows in a direction opposite to the predetermined direction. With the above configuration, a current can be restrained from flowing through the first to fourth detection coils  70   i  to  70   l  by the main magnetic flux. A situation in which the reliability of the metal object detection device is lowered can be avoided, and a reduction in the power transmission efficiency from the primary coil  22  to the secondary coil  42  can be suppressed. 
     Fourth Embodiment 
     Hereinafter, a description will be given of a fourth embodiment, mainly differences from the first embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 10  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 10 , the same members as the members illustrated in  FIG. 3  described above are denoted by the same symbols for convenience. 
     As shown in the figure, in the present embodiment, a second capacitor  71   b  and a fourth capacitor  71   d  are removed. In  FIG. 10 , a resonant circuit described as the third resonant circuit in  FIG. 3  described above is denoted by “Res 2 ”. 
     Incidentally, in the present embodiment, a series connection body of the first resonant circuit Res 1  and the second detection coil  70   b  corresponds to “first series connection body”, and a series connection body of the second resonant circuit Res 2  and the fourth detection coil  70   d  corresponds to “second series connection body”. The second detection coil  70   b  connected in series with the first resonant circuit Res 1  corresponds to “first passive element”, and the fourth detection coil  70   d  connected in series with the second resonant circuit Res 2  corresponds to “second passive element”. The respective resonant frequencies fd of the series connection body of the first resonant circuit Res 1  and the second detection coil  70   b , and the series connection body of the second resonant circuit Res 2  and the fourth detection coil  70   d  are represented by the following equation (eq2). 
     
       
         
           
             
               
                 
                   
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                     f 
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                             L 
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                             C 
                           
                         
                       
                     
                   
                 
               
               
                 
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     According to the present embodiment described above, an advantage analogous to the advantage (i) of the above first embodiment, and the same advantages as the advantages (ii) to (v) of the first embodiment can be obtained. 
     Fifth Embodiment 
     Hereinafter, a description will be given of a fifth embodiment, mainly differences from the first embodiment described above with reference to the drawings. 
     In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 11  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 11 , the same members as the members illustrated in  FIG. 3  described above are denoted by the same symbols for convenience. 
     As shown in the figure, in the present embodiment, it is assumed that a series resonant circuit including a first detection coil  70   m  and a first capacitor  71   m  is a first resonant circuit Res 1 , and a series resonant circuit including a second detection coil  70   n  and a second capacitor  71   n  is a second resonant circuit Res 2 . The first resonant circuit Res 1  and the second resonant circuit Res 2  are connected in parallel to each other. In the present embodiment, the first resonant circuit Res 1  corresponds to “first series connection body”, and the second resonant circuit Res 2  corresponds to “second series connection body”. 
     In the above configuration, a potential difference between a connection point (connection point included in the first resonant circuit Res 1 ) of the first detection coil  70   m  and the first capacitor  71   m , and a connection point (connection point included in the second resonant circuit Res 2 ) of the second detection coil  70   n  and the second capacitor  71   n  is detected by a voltage sensor  75 . 
     According to the present embodiment described above, an advantage analogous to the advantage (i) of the above first embodiment, and the same advantages as the advantages (ii) to (v) of the first embodiment can be obtained. 
     Sixth Embodiment 
     Hereinafter, a description will be given of a sixth embodiment, mainly differences from the first embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 12  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 12 , the same members as the members illustrated in  FIG. 3  described above are denoted by the same symbols for convenience. 
     As shown in the figure, the metal object detection device according to the present embodiment includes a series resonant circuit (hereinafter referred to as “input side resonant circuit Res 1 ”) including an input side coil  80   a  and an input side capacitor  80   b , and a series resonant circuit (hereinafter referred to as “load side resonant circuit ResO”) including a load side coil  81   a , a load side capacitor  81   b , and a resistor  81   c . The input side resonant circuit ResI is applied with an AC voltage by an AC power supply  82 . 
     First to sixth relay coils  90   a  to  90   f  for supplying the electric power from the input side resonant circuit ResI to the load side resonant circuit ResO in the non-contact manner are provided between the input side resonant circuit ResI and the load side resonant circuit ResO. The first to sixth relay coils  90   a  to  90   f  configure first to sixth resonant circuits ResA to ResF in cooperation with first to sixth relay capacitors  91   a  to  91   f , respectively. Specifically, the electric power of the input side resonant circuit ResI is supplied to the load side resonant circuit ResO through the first to third resonant circuits ResA to ResC. The electric power of the input side resonant circuit ResI is supplied to the load side resonant circuit ResO through the fourth to sixth resonant circuits ResD to ResF.  FIG. 13  illustrates an arrangement mode of the first to sixth relay coils  90   a  to  90   f  on the power transmitting pad  20  side.  FIG. 13  corresponds to  FIG. 4  described above. 
     Incidentally, in the present embodiment, the second resonant circuit ResB corresponds to “first series connection body”, and the fifth resonant circuit ResE corresponds to “second series connection body”. The AC power supply  82  corresponds to the “voltage applying unit”. 
     As illustrated in  FIG. 12 , in the present embodiment, the respective inductances of the input side coil  80   a , the load side coil  81   a , and the first to sixth relay coils  90   a  to  90   f  are set to the same value “L”, and the respective capacitances of the input side capacitor  80   b , the load side capacitor  81   b , and the first to sixth relay capacitors  91   a  to  91   f  are set to the same value “C”. That is, the respective resonant frequencies fd of the input side resonant circuit ResI, the load side resonant circuit ResO, and the first to sixth resonant circuits ResA to ResF are identical with each other. The frequency fv of the output voltage of the AC power supply  82  is set to the same frequency as the above resonant frequency fd under a situation where no metal object is present on the power transmitting pad  20 . 
     According to this setting, with the application of the AC voltage to both ends of the input side resonant circuit ResI by the AC power supply  82 , the input side coil  80   a  is magnetically coupled with the load side coil  81   a  through the first to third relay coils  90   a  to  90   c . Also, the input side coil  80   a  is magnetically coupled with the load side coil  81   a  through the fourth to sixth relay coils  90   d  to  90   f . As a result, the electric power is transferred between the input side resonant circuit ResI and the load side resonant circuit ResO in the non-contact manner. 
     A first connection point N 1  which is one of a pair of connection points of the second relay coil  90   b  and the second relay capacitor  91   b  in a closed circuit having the second relay coil  90   b  and the second relay capacitor  91   b  is grounded. A second connection point N 2  which is one of a pair of connection points of the fifth relay coil  90   e  and the fifth relay capacitor  91   e  in a closed circuit having the fifth relay coil  90   e  and the fifth relay capacitor  91   e  is grounded. Thus, the first connection point N 1  and the second connection point N 2  have the same potential. 
     In the present embodiment, a potential difference between a third connection point N 3  (corresponding to “a portion not having the same potential in the closed circuit having the first series connection body) on the opposite side of the first connection point N 1  with respect to both ends of the second relay capacitor  91   b , and a fourth connection point N 4  (corresponding to “a portion not having the same potential in the closed circuit having the second series connection body) on the opposite side of the second connection point N 2  with respect to both ends of the fifth relay capacitor  91   e  is detected by the voltage sensor  75 . 
     In the present embodiment, when no metal object is present on the power transmitting pad  20 , the metal object detection device is configured so that a potential of the third connection point N 3  is identical with a potential of the fourth connection point N 4  in phase. That is, when no metal object is present on the power transmitting pad  20 , the output voltage Vout becomes “0”. On the contrary, when the metal object is present on the power transmitting pad  20 , and the metal object comes close to any one of the first to sixth relay coils  90   a  to  90   f , a resonance level of the resonant circuit having the relay coil to which the metal object comes close as a component changes due to a change in impedance of the relay coil to which the metal object comes close. The resonance levels of all the resonant circuits configuring a set including the resonant circuit of which the resonance level changes, of a set of the first to third resonant circuits ResA to ResC and a set of the fourth to sixth resonant circuits ResD to ResF, change. As a result, the output voltage Vout largely changes from the reference value “0”. Thus, the metal object on the power transmitting pad  20  can be detected on the basis of the output voltage Vout. 
     According to the present embodiment described above, the following advantages are obtained in addition to the same advantages as the advantages (i), (ii), (iv), and (v) of the above first embodiment. 
     (vi) Because there is no need to connect the first to sixth relay coils  90   a  to  90   f  by wiring, the degree of freedom of the arrangement of those relay coils  90   a  to  90   f  as the detection coils can be enhanced. 
     Seventh Embodiment 
     Hereinafter, a description will be given of a seventh embodiment, mainly differences from the sixth embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 14  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 14 , the same members as the members illustrated in  FIG. 12  described above are denoted by the same symbols for convenience. 
     As shown in the figure, in the present embodiment, the first connection point N 1  and the second connection point N 2  are short-circuited to each other. Thus, the first connection point N 1  and the second connection point N 2  can be identical in potential with each other. 
     Likewise, in the present embodiment described above, the same advantages as those obtained in the above sixth embodiment can be obtained. 
     Eighth Embodiment 
     Hereinafter, a description will be given of an eighth embodiment, mainly differences from the seventh embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 15  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 15 , the same members as the members illustrated in  FIG. 14  described above are denoted by the same symbols for convenience. 
     As shown in the figure, in the present embodiment, first to fourth connection points N 1  to N 4  are changed. Specifically, it is assumed that in a closed circuit having a third relay coil  90   c  and a third relay capacitor  91   c , a first connection point N 1  is one of a pair of connection points of the third relay coil  90   c  and the third relay capacitor  91   c . It is assumed that in a closed circuit having a sixth relay coil  90   f  and a sixth relay capacitor  91   f , a second connection point N 2  is one of a pair of connection points of the sixth relay coil  90   f  and the sixth relay capacitor  91   f . It is assumed that a third connection point N 3  is on the opposite side of the first connection point N 1  with respect to both ends of the third relay capacitor  91   c , and a fourth connection point N 4  is on the opposite side of the second connection point N 2  with respect to both ends of the sixth relay capacitor  91   f . Further, a potential difference between the third connection point N 3  and the fourth connection point N 4  is detected by the voltage sensor  75 . 
     In the present embodiment, the third relay coil  90   c  and the sixth relay coil  90   f  used for detection of the above potential difference by the voltage sensor  75  are arranged outside of the power transmitting pad  20 . That is, the third relay coil  90   c  and the sixth relay coil  90   f  are arranged outside of a path of a main magnetic flux circulating between the primary coil  22  and the secondary coil  42 . 
     According to such an arrangement technique, wiring for connecting the voltage sensor  75  and the third connection point N 3  as well as the fourth connection point N 4  becomes easy, and an influence of the main magnetic flux on the detection of the potential difference by the voltage sensor  75  can be removed. 
     Ninth Embodiment 
     Hereinafter, a description will be given of a ninth embodiment, mainly differences from the seventh embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 16  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 16 , the same members as the members illustrated in  FIG. 14  are denoted by the same symbols for convenience. In the present embodiment, the third resonant circuit Res 3  and the sixth resonant circuit Res 6  are removed. 
     In the present embodiment, in a system including the input side resonant circuit ResI, the first, second, fourth, and fifth resonant circuits ResA, ResB, ResD, and ResE, and the load side resonant circuit ResO, those resonant circuits are magnetically coupled with each other whereby the system has plural resonant frequencies.  FIG. 16  exemplifies a state in which magnetic coupling is performed between the first resonant circuit ResA and the fifth resonant circuit ResE, and between the second resonant circuit ResB and the fourth resonant circuit ResD, in addition to magnetic coupling in the path extending from the input side resonant circuit ResI to the load side resonant circuit ResO through the first resonant circuit ResA and the second resonant circuit ResB, and magnetic coupling in the path extending from the input side resonant circuit ResI to the load side resonant circuit ResO through the fourth resonant circuit ResD and the fifth resonant circuit ResE. In the present embodiment, as illustrated in  FIG. 17 , the system has three resonant frequencies f 1 , f 2 , and f 3 . In  FIG. 17 , the axis of abscissa represents a frequency f, and the axis of ordinate represents a value (hereinafter referred to as “amplitude ratio Ap”) obtained by dividing the amplitude of a fluctuating potential at the third connection point N 3  (or the fourth connection point N 4 ) by the amplitude of an AC voltage output from the AC power supply  82 . Hereinafter, the respective three resonant frequencies are called a first frequency f 1 , a second frequency f 2 , and a third frequency f 3  in ascending order. 
     In the present embodiment, it is assumed that the first to third frequencies f 1  to f 3  are sufficiently higher than the use frequency fe of the non-contact power feeding. The filter circuit  73  is a band-pass filter including frequencies close to the first to third frequencies f 1  to f 3  in a passband, and the use frequency fe in a stopband for the purpose of removing the component of the use frequency fe which is superimposed on the output voltage Vout. 
     Subsequently, the metal object detecting process according to the present embodiment will be described. 
     In the present embodiment, as the above process, a process for detecting the metal object is performed on the basis of the respective output voltages Vout when the frequencies of the AC voltage to be applied to the input side resonant circuit ResI by the AC power supply  82  are the respective first to third frequencies f 1  to f 3 . 
     Subsequently,  FIG. 18  illustrates a procedure of the metal object detecting process according to the present embodiment. This process is repetitively executed by the determination unit  74 , for example, in a predetermined period. In the processes illustrated in  FIG. 18 , the same processes as the processes illustrated in  FIG. 6  described above are denoted by the same step numbers for convenience. 
     In the series of processes, it is first determined in S 20  whether all of values of the first to third detection flags F 1  to F 3  are “0”, or not. The first to third detection flags F 1  to F 3  of “0” indicate that no metal object is present on the power transmitting pad  20 , and the first to third detection flags F 1  to F 3  of “1” indicate that the metal object is present thereon. Initial values of the first to third detection flags F 1  to F 3  are set to “0”. 
     When an affirmative determination is made in S 20 , the process proceeds to S 22 , and the frequency fv of the output voltage of the AC power supply  82  is set to the first frequency f 1 . 
     Subsequently in S 24 , it is determined whether an absolute value of the output value Vf and a specified value Vα of the filter circuit  73  exceeds a threshold value Δ, or not. In more detail, it is determined whether a latest maximum value of the absolute value exceeds the threshold value Δ, or not. This process is intended to determine whether the metal object is present on the power transmitting pad  20 , or not. That is, when the metal object comes close to the detection coil that largely affects the first frequency f 1  among the detection coils configuring the system, the output value Vf largely changes from “0”. 
     If an affirmative determination is made in S 24 , it is determined that the metal object is present, and the process proceeds to S 26 . In S 26 , the value of the first detection flag F 1  is set to “1”. 
     When the process in S 26  is completed, or when a negative determination is made in S 24  described above, the process proceeds to S 28 , and the frequency fv of the output voltage of the AC power supply  82  is set to the second frequency f 2 . 
     Subsequently, in S 30 , the same process as that in S 24  described above is performed. When the affirmative determination is made in S 30 , the process proceeds to S 32 , and the value of the second detection flag F 2  is set to “1”. 
     When the process in S 32  is completed, or when a negative determination is made in S 30  described above, the process proceeds to S 34 , and the frequency fv of the output voltage of the AC power supply  82  is set to the third frequency f 3 . 
     Subsequently, in S 36 , the same process as that in S 24  described above is performed. When the affirmative determination is made in S 36 , the process proceeds to S 38 , and the value of the third detection flag F 3  is set to “1”. 
     When the process in S 38  is completed, or when a negative determination is made in S 36  described above, the process proceeds to S 40 , and it is determined whether a logical sum of a condition that the value of the first detection flag F 1  is “1”, a condition that the value of the second detection flag F 2  is “1”, and a condition that the value of the third detection flag F 3  is “1” is true, or not. When the affirmative determination is made in S 40 , or when the negative determination is made in S 20  described above, the process proceeds to S 18 . 
     When the negative determination is made in S 40 , or when the process in S 18  is completed, the series of processes is terminated once. 
     Likewise, in the present embodiment described above, the metal object can be detected with high precision. 
     Tenth Embodiment 
     Hereinafter, a description will be given of a tenth embodiment, mainly differences from the ninth embodiment described above with reference to the drawings. In the present embodiment, a circuit configuration of a metal object detection device is changed. 
       FIG. 19  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 19 , the same members as the members illustrated in  FIG. 16  are denoted by the same symbols for convenience. 
     As shown in the figure, in the present embodiment, a switch  92  is disposed in the load side resonant circuit ResO. The switch  92  has a function of switching the load side resonant circuit ResO to an open state or a closed state. 
     In the present embodiment, the resonant frequency of the system is different depending on an operation state of the switch  92 .  FIG. 20  illustrates an example in which the resonant frequency changes depending on the operation state of the switch  92 . In the present embodiment, when the switch  92  is turned on, it is assumed that the resonant frequency of the system is identical with that illustrated in  FIG. 17  described above. On the other hand, when the switch  92  is turned off, it is assumed that the resonant frequency of the system is a fourth frequency f 4  between the first frequency f 1  and the second frequency f 2 , and a fifth frequency f 5  between the second frequency f 2  and the third frequency f 3 . 
     Subsequently, the metal object detecting process according to the present embodiment will be described. 
     In the present embodiment, as the above process, a process for detecting the metal object is performed on the basis of the respective output voltages Vout when the operation state of the switch  92  is changed to two kinds of states. Such a detection technique is employed for the following reasons. 
     That is, in the system (system illustrated in  FIG. 19  described above) when the switch  92  is turned on, since the resonant frequency of the system becomes the first to third frequencies f 1  to f 3 , the amplitude ratio Ap of the system at the fourth and fifth frequencies f 4  and f 5  becomes small. In this situation, when the metal object comes close to the detection coil that largely affects, for example, the fifth frequency f 5  on the power transmitting pad  20 , even if the frequency fv of the output voltage of the AC power supply  82  is set to the fifth frequency f 5 , there is a concern that the metal object cannot be detected with high precision since the amplitude ratio Ap is small. That is, there is a concern that the detection precision of the metal object is lowered depending on a magnetic coupling state between the plural resonant circuits configuring the system. 
     To cope with the above problem, the operation state of the switch  92  is changed to change a magnetic coupling state between the resonant circuits configuring the system. In the present embodiment, with a change in the magnetic coupling state, for example, the amplitude ratio Ap of the fifth frequency f 5  increases. This makes it possible to avoid a reduction in the detection precision of the metal object. 
       FIG. 21  illustrates a procedure of the metal object detecting process according to the present embodiment. This process is repetitively executed by the determination unit  74 , for example, in a predetermined period. In the processes illustrated in  FIG. 21 , the same processes as the processes illustrated in  FIG. 18  described above are denoted by the same step numbers for convenience. 
     In the series of processes, it is first determined in S 42  whether all of the value of the fourth detection flag F 4  and the value of the fifth detection flag F 5  are “0”, or not. The fourth detection flag F 4  of “0” indicates that no metal object is present when the switch  92  is turned on, and the fourth detection flag F 4  of “1” indicates that the metal object is present. The fifth detection flag F 5  of “0” indicates that no metal object is present when the switch  92  is turned off, and the fifth detection flag F 5  of “1” indicates that the metal object is present. Initial values of the fourth and fifth detection flags F 4  and F 5  are set to “0”. 
     When the affirmative determination is made in S 42 , the process proceeds to S 44 , and the switch  92  is turned on. 
     Subsequently in S 46 , it is determined whether an absolute value of a difference between the output value Vf and the specified value Vα of the filter circuit  73  exceeds a threshold value Δ, or not, at at least one frequency when the frequencies fv of the output voltage of the AC power supply  82  correspond to the respective first to third frequencies f 1  to f 3 . When the affirmative determination is made in S 46 , the process proceeds to S 48 , and a value of the fourth detection flag F 4  is set to “1”. 
     When the process in S 48  is completed, or when a negative determination is made in S 46  described above, the process proceeds to S 50 , and the switch  92  is turned off. 
     Subsequently in S 52 , it is determined whether an absolute value of a difference between the output value Vf and the specified value Vα of the filter circuit  73  exceeds a threshold value Δ, or not, at at least one frequency when the frequencies fv of the output voltage of the AC power supply  82  correspond to the respective fourth and fifth frequencies f 4  to f 5 . When the affirmative determination is made in S 52 , the process proceeds to S 54 , and the value of the fifth detection flag F 5  is set to “1”. 
     When the process in S 54  is completed, or when a negative determination is made in S 52  described above, the process proceeds to S 56 , and it is determined whether a logical sum of a condition that the value of the fourth detection flag F 4  is “1”, and a condition that the value of the fifth detection flag F 5  is “1” is true, or not. When the affirmative determination is made in S 56 , or when the negative determination is made in S 42  described above, the process proceeds to S 18 . 
     When the negative determination is made in the above S 56 , or when the process in S 18  is completed, the series of processes is terminated once. 
     Likewise, in the present embodiment described above, the metal object can be detected with high precision. 
     The above first to tenth embodiments may be implemented as follows. 
     The arrangement technique of “the detection coils” is not limited to the techniques exemplified in the respective embodiments. For example, if the power transmitting pad  20  is installed in a power equipment arranged above the vehicle  10 , because it is conceivable that the power receiving pad  40  is disposed on an upper part of the vehicle, the detection coils may be arranged on the power receiving pad  40 . Even in this case, the metal object present in the path of the magnetic flux generated by the primary coil  22  and the secondary coil  42  can be intended to be detected by the detection coils. For example, in the power equipment in which the power transmitting pad  20  is arranged behind the vehicle  10 , because it is conceivable that the power receiving pad  40  is disposed at the rear of the vehicle  10 , the detection coil may be arranged on at least one of the power transmitting pad  20  and the power receiving pad  40 . Even in this case, since there is a risk that the metal object adheres to the power transmitting pad  20  or the power receiving pad  40 , the application of the present disclosure is effective. 
     The number of resonant circuits having the relay coils as components is not limited to the number exemplified in the sixth embodiment.  FIG. 22  illustrates a case in which the number of resonant circuits having the relay coils as the components is two. In  FIG. 22 , the same members as the members illustrated in  FIG. 12  are denoted by the same symbols for convenience. 
     The shapes of the primary core  21  and the secondary core  41  are not limited to that illustrated in  FIG. 2  described above. For example, the shape may be a circular core.  FIG. 23  exemplifies a circular primary core  23 . In  FIG. 23 , the primary coil is indicated by “24”. In  FIG. 23 , the same members as the members illustrated in  FIG. 4  are denoted by the same symbols for convenience. 
     The AC power supply as “the voltage applying unit” is not limited to a single AC power supply that is connected in parallel to the parallel connection body of “the first series connection body” and “the second series connection body”. For example, the AC power supply may be connected in parallel to each of “the first series connection body” and “the second series connection body”. 
     “The first passive element” and “the second passive element” are not limited to the coils (detection coils) described in the above fourth embodiment, but may be capacitors. Also, “the first passive element” and “the second passive element” are not limited to reactance elements such as coils or capacitors, but may be resistors. 
     The setting of the resonant frequency fd of the first to fourth resonant circuits Res 1  to Res 4 , and the frequency fv of the output voltage of the AC power supply  72  is not limited to the setting exemplified in the above first embodiment. As the frequency fv of the output voltage of the AC power supply  72  is more away from the resonant frequency fd of the first to fourth resonant circuits Res 1  to Res 4 , a variation in the impedance of the resonant circuit per unit variation of the resonant frequency fd is smaller. Therefore, the frequency fv may be appropriately set according to a request for the detection precision of the metal object. 
     In the above first embodiment, the first to fourth resonant circuits Res 1  to Res 4  are configured so that the impedances at the frequency fv of the output voltage of the AC power supply  72  are identical with each other. However, the present disclosure is not limited to this configuration, and the first to fourth resonant circuits Res 1  to Res 4  may be configured so that the impedances may be different from each other. In this case, in each of the first to fourth resonant circuits Res 1  to Res 4 , the specifications of the detection coil and the capacitor are made different in each of the resonant circuits in order to make the resonant frequency fd and the frequency fv of the output voltage of the AC power supply  72  identical with each other. 
     In the above first embodiment, a first voltage sensor for detecting the potential of the connection point between the first resonant circuit Res 1  and the second resonant circuit Res 2 , and a second voltage sensor for detecting the potential of the connection point between the third resonant circuit Res 3  and the fourth resonant circuit Res 4  may be provided, and the metal object detecting process may be performed on the basis of a difference between the respective detection values of those voltage sensors. 
     The resonant frequency fd of the resonant circuit may be set to a frequency lower than the frequency of the voltage applied to the primary coil  22  when the non-contact power feeding is performed. Specifically it is desirable that the resonant frequency fd is set to half or lower of the frequency of the above voltage. Even in this case, the current can be restrained from flowing through the detection coils by allowing the main magnetic flux to pass through the detection coils. 
     In the above first embodiment, a circuit in which two series connection bodies of the series resonant circuits each including the detection coil and the capacitor are connected in parallel to each other is installed in the metal object detection device. However, the present disclosure is not limited to this configuration, but a circuit in which three or more series connection bodies are connected in parallel to each other may be provided. In this case, one series connection body arbitrarily selected from the series connection bodies configuring the circuits in which the series connection bodies are connected in parallel is set as “a first series connection body”, and one series connection body arbitrarily selected from the remaining series connection bodies is set as “a second series connection body”. The metal object detecting process may be performed on the basis of a difference between a potential of the connection point between the series connection bodies of a pair of resonant circuits configuring the first series connection body, and a potential of the connection point between the series connection bodies of a pair of resonant circuits configuring the second series connection body. 
     “The filter unit” is not limited to the band-pass filter, but may be a high-pass filter. 
     “The detection coil” including the portion in which the induced current flows in the predetermined direction, and the portion in which the induced current flows in the direction opposite to the predetermined direction when the main magnetic flux passes through the detection coil is not limited to the detection coil described in the above third embodiment. The coil of this type may be configured, for example, as illustrated in  FIG. 24 . In detail, a detection coil  76  is configured by a series connection body of a first winding part  76   a  wound around a first axis P 1  in a specified direction, and a second winding part  76   b  wound around a second axis P 2  in a direction opposite to the direction of the first winding part  76   a . The first axis P 1  and the second axis P 2  are in parallel to each other, and spaced from each other. The first winding part  76   a  and the second winding part  76   b  are formed on a single plane (on a plane parallel to the plate surface of the primary core  21 ) orthogonal to the first axis P 1  and the second axis P 2 . That is, the detection coil  76  is a planar coil having a winding start and a winding end on both ends of the series connection body of the first winding part  76   a  and the second winding part  76   b . Even in this shape, the current can be restrained from flowing through the detection coils by allowing the main magnetic flux to pass through the detection coils. 
       FIG. 24  illustrates the detection coil  76  in which each of the first winding part  76   a  and the second winding part  76   b  configuring the detection coil  76  is formed in a spiral shape when the winding parts  76   a  and  76   b  are viewed from the first axis P 1  direction and the second axis P 2  direction, respectively. 
     The metal object detection device may be equipped with plural sets of first series connection bodies and second series connection bodies for the purpose of spreading the detection region of the metal object. 
     In the above ninth embodiment, the resonant frequencies provided in the system are not limited to three if plural resonant frequencies are provided. Likewise, in this case, the metal object detecting process can be performed on the basis of the respective output voltages Vout when the frequencies of the output voltage of the AC power supply  82  are the respective resonant frequencies of the system. 
     In the above tenth embodiment, switches may be disposed in a part and two or more of the first to sixth resonant circuits ResA to ResF and the load side resonant circuit ResO. Likewise, in this case, the magnetic coupling state between the plural resonant circuits configuring the system changes according to the operation state of the plural switches, and the resonant frequencies of the system can change. Thus, the metal object detecting process can be performed on the basis of the respective output voltages Vout when the operation state of the plural switches is changed to plural kinds of states. 
     Eleventh Embodiment 
     Subsequently, a description will be given of the metal object detection device according to an eleventh embodiment of the present disclosure with reference to  FIGS. 25 to 28, 29A, and 29B . The metal object detection device according to the present embodiment is also provided in the power transmitting system of the non-contact power feeding system. 
     As illustrated in  FIG. 25 , the metal object detection device includes an oscillator circuit  170 , a detector circuit  180 , and an output circuit  182 . In the present embodiment, the oscillator circuit  170  is a colpitts oscillator circuit, and includes an inverting amplifier circuit  171 , a resistor  172 , a first capacitor  173   a , a second capacitor  173   b , and plural detection coils Lij (i=1 to 3, j=1 to 3). In detail, an output terminal of the inverting amplifier circuit  171  is connected to one end of the resistor  172 , and the other end of the resistor  172  is connected to respective one ends of the plural detection coils Lij through a first switch  174   a . The respective other ends of the plural detection coils Lij are connected to an inverting input terminal side of the inverting amplifier circuit  171  through a second switch  174   b . The first switch  174   a  and the second switch  174   b  are operated by a switching circuit  175  to connect any one of the plural detection coils Lij between the other end of the resistor  172  and the inverting input terminal side of the inverting amplifier circuit  171 . 
     A connection between the other end of the resistor  172  and the first switch  174   a  is grounded through the first capacitor  173   a . A connection between the inverting input terminal side of the inverting amplifier circuit  171  and the second switch  174   b  is grounded through the second capacitor  173   b.    
     In the present embodiment, the oscillator circuit  170  corresponds to a supply unit. The first switch  174   a  and the second switch  174   b  configure a connection part. 
     In the figure, a capacitance of the first capacitor  173   a  is indicated by “C 1 ”, and a capacitance of the second capacitor  173   b  is indicated by “C 2 ”. In the present embodiment, the respective detection coils Lij are formed of coils having the same specification (shape, material, etc.). 
     When the oscillation frequency fd of the oscillator circuit  170  is represented by the following equation (eq3) assuming that the respective inductances of the detection coils Lij are “L”. 
     
       
         
           
             
               
                 
                   
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                     f 
                     d 
                   
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                       1 
                       
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                         π 
                       
                     
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                             C 
                             1 
                           
                           + 
                           
                             C 
                             2 
                           
                         
                         
                           L 
                           · 
                           
                             C 
                             1 
                           
                           · 
                           
                             C 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
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     In the present embodiment, the oscillation frequency fd of the oscillator circuit  170  is set to a frequency higher than the use frequency fe (frequency of a voltage to be applied to the primary coil  22 , for example, several kHz to tens of MHz) of the non-contact power feeding. 
     In the present embodiment, the detection coils Lij can be patterned on, for example, a flexible substrate. Further, it is assumed that the power transmitting pad  20  is really configured so that the detection coils Lij are arranged above the primary coil  22 , and the primary core  21 , the primary coil  22 , and the detection coils Lij are molded with resin. That is, the detection coils Lij are embedded in the power transmitting pad  20 . As with the power transmitting pad  20 , it is assumed that the power receiving pad  40  is configured so that the secondary core  41  and the secondary coil  42  are molded with resin. 
     As illustrated in  FIG. 26 , the detection coils Lij are configured to detect the metal object as a foreign matter present on the power transmitting pad  20 , and are 8-shaped coils in the present embodiment. In detail, the detection coils Lij each include a portion in which an induced current flows in a predetermined direction, and a portion in which the induced current flows in a direction opposite to the predetermined direction when a main magnetic flux circulating between the primary coil  22  and the secondary coil  42  passes through the detection coils Lij. In more detail, as the two portions, the detection coils Lij are each configured by a series connection body of a first winding part Laij wound around a first axis P 1  in a specified direction, and a second winding part Lbij wound around a second axis P 2  in a direction opposite to the direction of the first winding part Laij. The first axis P 1  and the second axis P 2  are in parallel to each other, and spaced from each other. The first winding part Laij and the second winding part Lbij are formed on a single plane orthogonal to the first axis P 1  and the second axis P 2 . That is, each of the detection coils Lij is a planar coil having a winding start and a winding end on both ends of the series connection body of the first winding part Laij and the second winding part Lbij. 
     As illustrated in  FIG. 27 , the detection coils Lij are so arranged as to detect the metal object present on a path of a magnetic flux generated by at least one of the primary coil  22  and the secondary coil  42 . In the present embodiment, the detection coils Lij are arrayed so that the plane on which the coils are formed is in parallel to the plate surface (plate surface of the primary spacing parts  21   a ) of the power transmitting pad  20 . Incidentally, in the arrangement of the detection coils Lij, the detection of the metal object present in the path of the magnetic flux generated by at least one of the primary coil  22  and the secondary coil  42  is enabled by the presence of a leakage magnetic flux from one of those coils  22  and  42  in addition to the main magnetic flux circulating between those coils  22  and  42 . 
     In the present embodiment, the oscillation frequency fd is set under a situation where no object is present on the power transmitting pad  20 . Thus, when the metal object is present on the power transmitting pad  20 , and the metal object comes close to the detection coils Lij, an eddy current loss is caused by electromagnetic induction. As a result, the impedance of the detection coils Lij reduces due to an increase in the resistance and an increase or decrease in the inductance of the detection coils Lij, and the output voltage Vout of the oscillator circuit  170  decreases. Therefore, according to the output voltage Vout, the metal object on the power transmitting pad  20  can be detected. 
     A configuration in which the metal object on the power transmitting pad  20  is detected with the detection coils Lij as the components of the oscillator circuit  170  is employed for the purpose of enhancing the detection precision of the metal object. That is, as the metal object on the power transmitting pad  20  is smaller, a change in the impedance of the detection coils Lij becomes smaller. With the adjustment of the gain of the oscillator circuit  170 , because the sensitivity caused by a change in the impedance can be improved, the detection precision of the metal object can be enhanced. 
       FIG. 28  illustrates a procedure of the metal object detecting process according to the present embodiment. This process is repetitively executed by the detector circuit  180 , for example, in a predetermined period with turning on of the power supply of the oscillator circuit  170  as a trigger. In the present embodiment, the detector circuit  180  corresponds to a processing unit and a detection unit. 
     In the series of processes, first in S 110 , the detection coils Lij as the components of the oscillator circuit  170  are selected. This process is performed by instructing the switching circuit  175  to operate the first switch  174   a  and the second switch  174   b . According to this process, the detection coils configuring the oscillator circuit  170  are sequentially switched every process period. 
     In subsequent S 112 , it is determined whether a logical sum of a condition in which a latest maximum value Vmax (also called “peak value”) of the output voltage Vout of the oscillator circuit  170  is lower than an upper limit voltage Vα (&gt;0), and a condition in which a latest minimum value Vmin of the output voltage Vout is lower than a lower limit voltage Vβ (&gt;0) lower than the upper limit voltage Vα is true, or not. This process is intended to determine whether the metal object is present on the power transmitting pad  20 , or not. In other words, as illustrated in  FIGS. 29A and 29B , when the metal object is present on the power transmitting pad  20 , the amplitude of the output voltage Vout of the oscillator circuit  170  is reduced with the results that the maximum value Vmax of the output voltage becomes lower than the upper voltage Vα, or the minimum value Vmin of the output voltage exceeds a lower limit voltage Vβ. 
     As illustrated in  FIG. 28 , if an affirmative determination is made in S 112 , it is determined that the metal object is present on the power transmitting pad  20 , and the process proceeds to S 114 . In S 114 , stop of the power supply to the power receiving pad  40  from the power transmitting pad  20 , and notification of a fact that the metal object is present to the user are instructed to the output circuit  182  illustrated in  FIG. 25  described above. As a result, the output circuit  182  instructs the power transmitting circuit  30  to stop the power supply from the power transmitting pad  20  to the power receiving pad  40 . Also, the user is notified of the fact that the metal object is present by some notification unit. 
     When it is determined that the metal object is present, a stop instruction of power feeding or a notification instruction to the output circuit  182  is continued until it is then determined that the metal object has been removed from the power transmitting pad  20 . 
     When the negative determination is made in the above S 112 , or when the process in S 114  is completed, the series of processes is terminated once. 
     According to the present embodiment described above in detail, the following advantages are obtained. 
     The detection coils Lij are each configured as a series connection body of the first winding part Laij wound around the first axis P 1  in the specified direction, and the second winding part Lbij wound around the second axis P 2  in the direction opposite to the specified direction. Thus, a current can be restrained from flowing through the detection coils Lij by the main magnetic flux circulating between the primary coil  22  and the secondary coil  42 . The situation in which the reliability of the metal object detection device is lowered can be avoided, and the reduction in the power transmission efficiency from the primary coil  22  to the secondary coil  42  can be suppressed. Likewise, in the non-contact power feeding, the metal object on the power transmitting pad  20  can be detected by the metal object detecting process. 
     In particular, in the present embodiment, the first winding part Laij and the second winding part Lbij are formed on a signal plane parallel to the surface of the power transmitting pad  20  which faces the power receiving pad  40 . That is, when the non-contact power feeding is performed, the detection coils Lij are arranged so that the main magnetic flux passing through the first winding part Laij becomes equal to the main magnetic flux passing through the second winding part Lbij. Thus, the effect of restraining the current from flowing through the detection coils Lij by the main magnetic flux circulating between the primary coil  22  and the secondary coil  42  can increase. 
     The detection coils Lij configuring the oscillator circuit  170  are sequentially changed by the operation of the first switch  174   a  and the second switch  174   b  every process period of the detector circuit  180 . Thus, for example, as compared with a configuration having the oscillator circuit  170  and the detector circuit  180  corresponding to the respective detection coils Lij, the numbers of oscillator circuits  170  and detector circuits  180  required for detecting a change in the impedance of the detection coils Lij can be reduced. 
     The metal object detecting process based on the amplitude of the output voltage Vout is performed. Accordingly, the metal object can be appropriately detected. 
     Twelfth Embodiment 
     Hereinafter, a description will be given of a twelfth embodiment, mainly differences from the eleventh embodiment described above with reference to the drawings. In the present embodiment, a technique for the metal object detecting process is changed. 
       FIG. 30  illustrates a procedure of the metal object detecting process according to the present embodiment. This process is repetitively executed by the detector circuit  180 , for example, in a predetermined period with turning on of the power supply of the oscillator circuit  170  as a trigger. In  FIG. 30 , the same processes as the processes illustrated in  FIG. 28  described above are denoted by the same step numbers for convenience. 
     In the series of processes, after the process in S 110  has been completed, the process proceeds to S 112   a , and it is determined whether an absolute value of a difference between the frequency fr and the oscillation frequency fd of the output voltage Vout of the oscillator circuit  170  exceeds a specified value Δ, or not. This process is intended to determine whether the metal object is present on the power transmitting pad  20 , or not, as with the process in S 112  of  FIG. 28  described above. That is, when the metal object is present on the power transmitting pad  20 , and the metal object comes close to the detection coils Lij, a resistance of the detection coils Lij increases, and an inductance of the detection coils Lij increases or decrease. As a result, a real oscillation frequency of the oscillator circuit  170  deviates from the original oscillation frequency fd.  FIGS. 31A and 31B  illustrate a state in which the oscillation frequency is lowered with an increase in the inductance. The frequency fr of the output voltage Vout may be detected by, for example, a frequency counter. 
     If an affirmative determination is made in S 112   a , it is determined that the metal object is present on the power transmitting pad  20 , and the process proceeds to S 114 . 
     When the negative determination is made in the above S 112   a , or when the process in S 114  is completed, the series of processes is terminated once. 
     Likewise, in the present embodiment described above, the same advantages as those obtained in the above eleventh embodiment can be obtained. 
     Thirteenth Embodiment 
     Hereinafter, a description will be given of a thirteenth embodiment, mainly differences from the eleventh embodiment described above with reference to the drawings. In the present embodiment, a configuration of a metal object detection device is changed. 
       FIG. 32  is a circuit diagram of the metal object detection device according to the present embodiment. In  FIG. 32 , the same members as the members illustrated in  FIG. 25  described above are denoted by the same symbols for convenience. 
     As shown in the figure, the output voltage Vout of the oscillator circuit  170  is input to a filter circuit  184 . In the present embodiment, the filter circuit  184  is formed of a high-pass filter including a resistor and a capacitor. An output signal of the filter circuit  184  is captured into the detector circuit  180 . 
     Subsequently, a technical significance for the provision of the filter circuit  184  will be described with reference to  FIG. 33 . 
     As shown in the figure, a use frequency component of the non-contact power feeding may be superimposed on the output voltage Vout of the oscillator circuit  170  as indicated by dashed lines. In this case, since a waveform of the output voltage Vout is distorted, there is a concern about a reduction in the detection precision of the metal object. In order to avoid the above problem, the filter circuit  184  is provided. 
     According to the present embodiment described above, an influence of the use frequency can be removed from the output voltage Vout. 
     Fourteenth Embodiment 
     Hereinafter, a description will be given of a fourteenth embodiment, mainly differences from the eleventh embodiment described above with reference to the drawings. 
     In the present embodiment, the shape of the detection coils Lij is changed. In detail, as illustrated in  FIG. 34 , a first winding part Lcij and a second winding part Ldij configuring the detection coils Lij are formed in a spiral shape when the first winding part Lcij and the second winding part Ldij are viewed from a direction of the first axis P 1  and a direction of the second axis P 2 . This shape is intended to suppress a variation in the detection precision of the metal object on the power transmitting pad  20 . 
     In other words, an electromagnetic field generated by a current flowing into a conductor wire is weakened with distance from the conductor wire configuring the detection coils Lij. Thus, even if the metal object is present within a region partitioned by a contour of each detection coil Lij in a front view of the plate surface of the power transmitting pad  20 , a variation in the impedance of the detection coils Lij may be different depending on a position of the metal object in the above region. This leads to a concern that a variation occurs in the detection precision of the metal object based on the output voltage Vout. In order to avoid the above problem, the first winding part Lcij and the second winding part Ldij are formed in a spiral shape, and the electromagnetic field in the region partitioned by the contour of the detection coil Lij in the front view of the plate surface of the power transmitting pad  20  is equalized as much as possible. As a result, a variation in the amount of change of the impedance of the detection coils Lij when the metal object comes close to the detection coils Lij can be suppressed, and further a variation in the detection precision of the metal object can be suppressed. 
     Further, in the present embodiment, as illustrated in  FIG. 34 , the respective contours of the first winding part Lcij and the second winding part Ldij are formed in a rectangular shape symmetrical with respect to a plane S passing through a center O between the pair of axes P 1  and P 2 , and being in parallel to the pair of axes P 1  and P 2  when those winding parts Lcij and Ldij are viewed from the direction of the first axis P 1  and the direction of the second axis P 2 . With the above configuration, as illustrated in  FIG. 35 , when the detection coils Lij are arrayed within the power transmitting pad  20 , gaps between the adjacent detection coils can be reduced. Even with the above configuration, a variation in the detection precision of the metal object on the power transmitting pad  20  can be suppressed.  FIG. 35  corresponds to  FIG. 27  described above. 
     The above eleventh to fourteenth embodiments may be changed and implemented as follows. 
     The shape of “the detection coils” is not limited to the shape exemplified in the above eleventh embodiment. For example, as illustrated in  FIG. 36 , the respective contours of a first winding part Leij and a second winding part Lfij may be formed into a rectangular shape. For example, as illustrated in  FIG. 37 , the respective contours of a first winding part Lgij and a second winding part Lhij may be formed into a regular hexagon. Even in those cases, since the gaps between the adjacent detection coils can be reduced, a variation in the detection precision of the metal object on the power transmitting pad  20  can be suppressed.  FIG. 38  illustrates an example of the arrangement mode of the detection coils illustrated in  FIG. 37  described above within the power transmitting pad  20 .  FIG. 38  corresponds to  FIG. 27  described above. 
     As “the detection coils”, the first winding part and the second winding part are not limited to the same shape, but may be different in shape from each other. Likewise, in this case, since the winding directions of the first winding part and the second winding part are opposite to each other, the current flowing through the detection coils can be suppressed by allowing the main magnetic flux circulating between the primary coil  22  and the secondary coil  42  to pass through the detection coils. 
     “The detection coils” are not limited to the configuration in which the winding start and the winding end are located on both ends of the series connection body of the first winding part and the second winding part. For example, the winding part may be further connected in series with at least one of both ends of the series connection body. Even in this case, since a portion of the series connection body is present, the effects of suppressing the current flowing into the detection coils can be obtained by allowing the main magnetic flux to pass through the detection coils. 
     “The detection coils” are not limited to a configuration in which the first winding part and the second winding part are formed on the single plane that is in parallel to the planes of the spacing parts  21   a  and  41   a . For example, those winding parts may be formed on the single plane that is not in parallel to the planes of the spacing parts  21   a  and  41   a . Even in this case, the current flowing through the detection coils can be suppressed by allowing the main magnetic flux to pass through the detection coils. 
     “The detection coils” are not limited to the configuration in which the series connection body of the first winding part and the second winding part is provided, but may be, for example, 8-shaped coils illustrated in  FIG. 39 . Likely, in this case, the detection coils Lij each include the portion in which the induced current flows in the predetermined direction, and the portion in which the induced current flows in the direction opposite to the predetermined direction when the main magnetic flux passes through the detection coils. As a result, the current can be restrained from flowing into the detection coils Lij by the main magnetic flux. 
     “The detection coils” are not limited to the planar coils. For example, as illustrated in  FIGS. 40A to 40J , the detection coils may be patterned on a multilayer substrate.  FIGS. 40A to 40J  exemplify the detection coils patterned on first to fourth substrates CB 1  to CB 4 . Insulating layers between the respective substrates are omitted from illustration. 
     Each of the first to fourth substrates CB 1  to CB 4  forms a rectangular shape in a front view of a plate surface thereof. In detail, a first coil part W 1  connected to a first terminal T 1  is patterned on the first substrate CB 1  illustrated in  FIGS. 40A and 40B . A first through-hole SH 1  connected on the opposite side of the first terminal T 1  at both ends of the first coil part W 1  is formed. A second coil part W 2  having one end connected to the first coil part W 1  through the first through-hole SH 1  is patterned on the second substrate CB 2  illustrated in  FIGS. 40C and 40D . A second through-hole SH 2  connected to the opposite side of the first through-hole SH 1  side at both ends of the second coil part W 2  is formed on the second substrate CB 2 . 
     A third coil part W 3  having one end connected to the second coil part W 2  through the second through-hole SH 2  is patterned on the third substrate CB 3  illustrated in  FIGS. 40E and 40F . A third through-hole SH 3  connected to the opposite side of the second through-hole SH 2  side at both ends of the third coil part W 3  is formed on the third substrate CB 3 . A fourth coil part W 4  having one end connected to the third coil part W 3  through the third through-hole SH 3  is patterned on the fourth substrate CB 4  illustrated in  FIGS. 40G and 40H . A second terminal T 2  is connected to the opposite side of the third through-hole SH 3  side at both ends of the fourth coil part W 4 . 
     The first substrate CB 1 , the second substrate CB 2 , the third substrate CB 3 , and the fourth substrate CB 4  are laminated as illustrated in  FIG. 40J . According to the above configuration, as illustrated in  FIG. 40I , the detection coil Lij having the winding start on one of the first terminal T 1  and the second terminal T 2 , and the winding end on the other terminal is formed. The detection coil Lij of this type obtains an advantage to easily manufacture the detection coils in mass production. 
     “The detection coils” may be patterned on a double-sided substrate as illustrated in  FIGS. 41A to 41C, and 42A to 42C .  FIGS. 41A to 41C, and 42A to 42C  exemplify 8-shaped detection coils patterned on a double-sided substrate CB having a first surface Sa, and a second surface Sb which is a rear surface of the first surface Sa. 
     First, an example illustrated in  FIGS. 41A to 41C  will be described. An 8-shaped first coil part Wa connected to a first terminal Ta is patterned on the first surface Sa. An opposite side of the first terminal Ta at both ends of the first coil part Wa is connected to one end of an 8-shaped second coil part Wb patterned on the second surface Sb through a first through-hole SHa. 
     An opposite side of the first through-hole SHa side at both ends of the second coil part Wb is connected to one end of a third coil part Wc patterned on the first surface Sa through a second through-hole SHb. The third coil part Wc is patterned on the first surface Sa over the second coil part Wb. An opposite side of the second through-hole SHb side at both ends of the third coil part Wc is connected to one end of a fourth coil part Wd patterned on the second surface Sb through a third through-hole SHc. 
     The fourth coil part Wd is patterned on the second surface Sb over the first coil part Wa. An opposite side of the third through-hole SHc at both ends of the fourth coil part Wd is connected to one end of a fifth coil part We patterned on the first surface Sa through a fourth through-hole SHd. A second terminal Tb is connected to the opposite side of the fourth through-hole SHd at both ends of the fifth coil part We. 
     Subsequently, an example illustrated in  FIGS. 42A to 42C  will be described. The first coil part Wa connected to the first terminal Ta is patterned on the first surface Sa. An opposite side of the first terminal Ta at both ends of the first coil part Wa is connected to one end of an 8-shaped second coil part Wb patterned on the second surface Sb through a first through-hole SHa. 
     An opposite side of the first through-hole SHa side at both ends of the second coil part Wb is connected to one end of a third coil part We patterned on the first surface Sa through a second through-hole SHb. A second terminal Tb is connected to the opposite side of the second through-hole SHb side at both ends of the third coil part Wc. 
     Likewise, with the configuration described above, the detection coil having the winding start on one of the first terminal Ta and the second terminal Tb, and the winding end on the other terminal can be formed. 
     The arrangement technique of “the detection coils” is not limited to the techniques exemplified in the respective embodiments. For example, if the power transmitting pad  20  is installed in a power equipment arranged above the vehicle  10 , because it is conceivable that the power receiving pad  40  is disposed on an upper part of the vehicle, the detection coils may be arranged on the power receiving pad  40 . Even in this case, the metal object present in the path of the magnetic flux generated by the primary coil  22  and the secondary coil  42  can be intended to be detected by the detection coils. For example, in the power equipment in which the power transmitting pad  20  is arranged behind the vehicle  10 , because it is conceivable that the power receiving pad  40  is disposed at the rear of the vehicle  10 , the detection coil may be arranged on at least one of the power transmitting pad  20  and the power receiving pad  40 . Even in this case, since there is a risk that the metal object adheres to the power transmitting pad  20  or the power receiving pad  40 , the application of the present disclosure is effective. 
     The number of arrangement of “detection coils” is not limited to plurality, but may be one if the detection coils have a size that can cover an area in which the metal object is heated by the non-contact power feeding, and allows the impedance change of the detection coil caused by the metal object to satisfy the detection precision of the metal object. 
     The “connection part” is not limited to the configuration exemplified in the above eleventh embodiment. For example, two or more detection coils which are a part of the plural detection coils Lij may be sequentially selected as the components of the oscillator circuit  170 . Likewise, in this case, the numbers of oscillator circuits  170  and detector circuits  180  can be reduced. In the above configuration, since the oscillation conditions of the oscillator circuit  170  change according to the number of selection of the detection coils, the capacitance of the capacitor configuring the oscillator circuit  170  may be changeable according to the number of selection of the detection coils. 
     The oscillator circuit as “the supply part” is not limited to the configuration exemplified in the above eleventh embodiment. For example, the oscillator circuit may be a Hartley oscillator circuit. In this case, for example, one of a pair of coils configuring the Hartley oscillator circuit may function as the detection coil. 
     The “supply unit” is not limited to the oscillator circuit, but may be, for example, a power supply that outputs an AC current having a predetermined amplitude. In this case, when it is determined that a real phase difference deviates from a reference phase difference of a potential difference (hereinafter referred to as “inter-terminal voltage”) between both ends of the detection coil to the AC current output from the power supply, it may be determined that the metal object is present on the power transmitting pad  20 . This technique uses a phenomenon that a phase of the inter-terminal voltage to the AC current output from the power supply changes due to a reduction in the impedance of the detection coil caused by the approximation of the metal object. 
     The process of  FIG. 28  in the above eleventh embodiment, and the process in  FIG. 30  in the above twelfth embodiment can be also used as an initial operation check routine for determining whether the metal object detection device normally operates, or not. That is, in the situation in which the affirmative determination is made in S 112  of  FIG. 28  or S 112   a  of  FIG. 30 , a metal foreign matter is present on the power transmitting pad  20 , and also the detection coils and the circuits configuring the metal object detection device may be abnormal. 
     The oscillation frequency fd of the oscillator circuit  170  may be set to be lower than the use frequency fe of the non-contact power feeding. 
     “The primary coil” and “the secondary coil” are not limited to the configurations exemplified in the above eleventh embodiment, but may be, for example, spiral circular planar coils. 
     In the above eleventh embodiment, the oscillator circuit  170 , the detector circuit  180 , and the output circuit  182  may be embedded in the power transmitting pad  20 . 
     “The filter unit” is not limited to the high-pass filter, but may be a band-pass filter. “The filter unit” is not limited to the analog filter, but may be a digital filter. 
     In S 112  of  FIG. 28  in the above eleventh embodiment, any one of the conditions for the latest maximum value Vmax of the output voltage Vout, or the conditions for the latest minimum value Vmin of the output voltage Vout may be removed. 
     In S 114  of  FIG. 28  in the above eleventh embodiment, the stop of the power supply is instructed, but a decrease in the power supply may be instructed without being limited to this configuration. 
     The vehicle to which the present disclosure is applied is not limited to an electric vehicle, but may be, for example, an automatic transport vehicle in a factory.