Abstract:
Disclosed herein is a foreign object detector to detect a foreign object while distinguishing metal from water. The foreign object detector includes a detection coil; a transmitting circuit generating RF power of a predetermined frequency; a directional coupler outputting the RF power supplied from the transmitting circuit to the detection coil, and extracting reflected power that is a power component reflected by the detection coil; and a detection circuit receiving the reflected power extracted by the directional coupler, and detecting the foreign object by sensing a change in the frequency characteristic of the reflected power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application No. PCT/JP2014/000831 filed on Feb. 18, 2014, which claims priority to Japanese Patent Application No. 2013-029601 filed on Feb. 19, 2013. The entire disclosures of these applications are incorporated by reference herein. 
    
    
     BACKGROUND 
     The present disclosure relates to a foreign object detector, a method of detecting a foreign object, and a non-contact charging system. 
     In recent years, wireless power transmission systems (i.e., non-contact charging systems) have been, and are being, developed to charge, for example, electric vehicles via a non-contact method. In each wireless power transmission system, a transmitting coil and an RF oscillation source are provided for a charger, and a receiving coil is provided for an electric vehicle. Some wireless power transmission systems using an electromagnetic induction method enable high-efficiency non-contact power transmission. 
     Such a wireless power transmission system using the electromagnetic induction method is designed to transmit a large quantity of electric power. Therefore, if a metallic foreign object enters the gap between its transmitting and receiving coils and/or its environment, there is a risk that the system would generate heat. In view of this consideration, it is thus important, for safety reasons, to detect such a metallic foreign object before or during charging. 
     As a conventional method of detecting a foreign object in a wireless power transmission system, there is a method of detecting a change in the inductance of a detection coil to be caused by electromagnetic induction between the metal and the detection coil (see, e.g., Japanese Unexamined Patent Publication No. 2012-16125). 
     SUMMARY 
     Since an electric vehicle is sometimes charged while being parked outside, such a foreign object needs to be detected regardless of the weather. Specifically, even if it rains, it is necessary to detect the foreign object without being affected by the rainwater, that is, to distinguish the metal from the water. 
     In the method of Japanese Unexamined Patent Publication No. 2012-16125, however, influence of rainwater in detecting such a foreign object is not considered. 
     The present inventor found this problem and made the present disclosure. The present disclosure provides a foreign object detector detecting such a foreign object while distinguishing the metal from the water, a method of detecting the foreign object, and a non-contact charging system. 
     In order to solve the problem described above, a foreign object detector according to the present disclosure includes a detection coil; a transmitting circuit generating RF power of a predetermined frequency; a directional coupler outputting the RF power supplied from the transmitting circuit to the detection coil, and extracting reflected power that is a power component reflected by the detection coil; and a detection circuit receiving the reflected power extracted by the directional coupler and detecting a foreign object by sensing a change in frequency characteristic of the reflected power. 
     A method of detecting a foreign object according to the present disclosure includes outputting RF power generated at a predetermined frequency to a detection coil; and detecting the foreign object by sensing a change in frequency characteristic of reflected power that is a power component reflected by the detection coil. 
     The present disclosure allows for detecting a foreign object by sensing a change in the frequency characteristic of reflected power while distinguishing metal from water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a basic configuration of a foreign object detector according to the present disclosure. 
         FIG. 2  illustrates a detailed configuration of a foreign object detector according to a first embodiment of the present disclosure. 
         FIG. 3  is a front view illustrating an example where a wireless power transmission system is employed in a vehicle. 
         FIG. 4  is an enlarged plan view of the transmitting coil case shown in  FIG. 3 . 
         FIG. 5  is an enlarged cross-sectional view of the transmitting coil case shown in  FIG. 3 . 
         FIG. 6  illustrates a change in the frequency characteristic of the reflected power caused by a foreign object in the present disclosure. 
         FIG. 7  illustrates a detailed configuration of a foreign object detector according to a second embodiment of the present disclosure. 
         FIG. 8  illustrates a detailed configuration of a foreign object detector according to a third embodiment of the present disclosure. 
         FIG. 9  illustrates a detailed configuration of a foreign object detector according to a variation of  FIG. 8 . 
         FIG. 10  illustrates an equivalent circuit model of the foreign object detector of  FIG. 9 . 
         FIGS. 11A-11C  illustrate the frequency characteristic of reflected power of a coil L 1  where there is no mutual coupling between the coils L 1  and L 2  shown in  FIG. 10 .  FIG. 11A  illustrates a case where there is neither a foreign object nor rainwater.  FIG. 11B  illustrates a case where there is a foreign object (metal).  FIG. 11C  illustrates a case where there is rainwater. 
         FIGS. 12A-12C  illustrate the frequency characteristic of reflected power of a coil L 1  where mutual coupling between the coils L 1  and L 2  shown in  FIG. 10  is taken into consideration and where no short circuit is used.  FIG. 12A  illustrates a case where there is neither a foreign object nor rain.  FIG. 12B  illustrates a case where there is a foreign object (metal).  FIG. 12C  illustrates a case where there is rainwater. 
         FIGS. 13A-13C  illustrate the frequency characteristic of reflected power of a coil L 1  where mutual coupling between the coils L 1  and L 2  shown in  FIG. 10  is taken into consideration and where a short circuit is used.  FIG. 13A  illustrates a case where there is neither a foreign object nor rain.  FIG. 13B  illustrates a case where there is a foreign object (metal).  FIG. 13C  illustrates a case where there is rainwater. 
         FIG. 14  is a circuit diagram illustrating a detailed exemplary configuration of the short circuit shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a foreign object detector, method of detecting a foreign object, and non-contact charging system according to the present disclosure will now be described with reference to the drawings. These embodiments are not intended to limit the scope of the present disclosure. Those skilled in the art would readily understand that those embodiments may also be expressed in a similar language or depicted similarly in the same or similar technical fields. 
       FIG. 1  illustrates a basic configuration of a foreign object detector  100  according to the present disclosure. A transmitting circuit  101  generates RF power of a predetermined frequency using a voltage supply Vg, and outputs the RF power to a detection coil  103  via a directional coupler  107 . The directional coupler  107  outputs the power supplied from the transmitting circuit  101  to the detection coil  103 , extracts a power component reflected by the detection coil  103 , and outputs the reflected power component to a detection circuit  108 . The detection circuit  108  receives the reflected power extracted by the directional coupler  107 , and detects a foreign object by sensing a change in the frequency characteristic of the reflected power. Specifically, the detection circuit  108  detects a foreign object based on the amount of change in the matching frequency of the reflected power. 
     This configuration allows for detecting a metallic foreign object  201  while distinguishing the foreign object from water. 
     First Embodiment 
       FIG. 2  illustrates a detailed configuration of a foreign object detector  100  according to a first embodiment. A transmitting circuit  101  outputs RF power while changing the frequency within a predetermined sweeping frequency range, and outputs the RF power to balanced-to-unbalanced transformers (baluns)  104  via respective coaxial cables  102  and a directional coupler  107 . Each coaxial cable  102  transmits an unbalanced RF signal. Each coaxial cable  102  is a transmission line including a center conductor shielded with an external conductor. Each detection coil  103  generates an RF magnetic field to detect the metallic foreign object  201 . The detection coils  103  form an arrangement of n coils L 1 , L 2 , . . . , and Ln (where n is an integer equal to or greater than two). 
     Each balun  104  is connected to a matching circuit  105 , which will be described later, and the coaxial cable  102 . The balun  104  transforms the unbalanced signal, which is RF power supplied from the coaxial cable  102 , to a balanced signal, and outputs the balanced signal to the matching circuit  105 . 
     Each matching circuit  105  performs impedance matching between the associated detection coil  103  and balun  104 . The matching circuit  105  converts the impedance of the detection coil  103  at a predetermined matching frequency f 0  to match the impedance with the balanced impedance of the balun  104 . 
     The closer the point of connection between the balun  104  and the matching circuit  105  to the detection coil  103  is, the better. As will be described later, the detector according to the present disclosure detects a foreign object by sensing a change in the frequency characteristic responding to a change in the inductance of the detection coil  103  that has been caused by a target to be detected. However, if the detection coil  103  is connected to the balun  104  and the matching circuit  105  via a long wire, the wire comes to have an inductance component to reduce the amount of change in the inductance of the detection coil  103  caused by the target to be detected. Thus, the balun  104  and the matching circuit  105  are connected at a closest possible point to the detection coil  103 , which allows for reducing deterioration in the foreign object detecting performance. 
     A parasitic capacitor  106  is parasitic capacitance generated by each detection coil  103 . In the equivalent circuit, capacitance components are defined in parallel with the respective detection coils  103 . If a dielectric exists near the detection coils  103 , the capacitance value of the parasitic capacitor  106  increases. Examples of the dielectric include a transmitting coil case, which will be described later, and rainwater on the transmitting coil case. 
     The directional coupler  107  outputs an unbalanced signal, which is RF power supplied from the transmitting circuit  101 , to the balun  104 , extracts a power component reflected by the detection coil  103 , and outputs the power component to the detection circuit  108 , which will be described later. 
     The detection circuit  108  receives the reflected power extracted by the directional coupler  107 , and detects a foreign object based on the amount of change in the matching frequency at which the minimum power is reflected. 
     Out of the coils L 1 , L 2 , . . . , and Ln arranged as the detection coils  103 , the switch circuit  109  turns itself by selecting one of the coils to be supplied with power. 
     The foreign object detector  100  described above is combined with a transmitting coil  302  and an RF oscillation source  305  to form a non-contact charging system. The RF oscillation source  305  supplies RF power of a predetermined frequency to the transmitting coil  302  to start non-contact charging. 
       FIG. 3  is a front view illustrating an example where a wireless power transmission system is employed in a vehicle  301 . In the example of  FIG. 3 , the transmitting coil  302  is placed on the ground, and a receiving coil  303  is mounted on the vehicle  301 . The transmitting coil case  304  is, for example, a resin dielectric, and houses the transmitting coil  302 . The detection coils  103  are arranged above the transmitting coil  302 , and housed in the transmitting coil case  304 . 
       FIG. 4  is an enlarged plan view of the transmitting coil case  304 . In order to detect a foreign object on and/or around the transmitting coil  302 , the detection coils  103  are arranged so as to cover the transmitting coil  302  and its surroundings without leaving any space between them. These detection coils  103  are switched by the switch circuit  109  to detect the metallic foreign object  201  on and/or around the transmitting coil  302 . 
     In  FIG. 2 , magnetic field coupling occurs not only between the detection coils  103  and the metallic foreign object  201  but also between the detection coils  103  and the transmitting coil  302 . The detection coils  103  induce currents in the transmitting coil  302 . If magnetic field coupling occurs between the detection coils  103  and the transmitting coil  302 , a change in received power caused by the metallic foreign object  201  decreases so much as to deteriorate the foreign object detecting performance. 
       FIG. 5  is an enlarged cross-sectional view of the transmitting coil case  304 . Assume that the distance between the metallic foreign object  201  and the detection coil  103  (e.g., the distance between the (external) surface of the transmitting coil case  304  and (the top surface of) the detection coils  103 ) is “a,” and the distance between the transmitting coil  302  and the detection coils  103  (e.g., the distance between the upper surface of the transmitting coil  302  and the lower surface of the detection coils  103 ) is “b.” The detection coils  103  are suitably arranged so that the distance “a” is shorter than the distance “b” to reduce the influence of magnetic field coupling between the detection coils  103  and the transmitting coil  302 . This configuration allows for reducing influence of magnetic field coupling between the detection coils  103  and the transmitting coil  302 , thereby reducing deterioration in the foreign object detecting performance. 
     A method of detecting a foreign object using the foreign object detector  100  configured as described above will now be outlined. RF power generated at a predetermined frequency is output to the detection coils  103 . A foreign object is detected by sensing a change in the frequency characteristic of the power reflected by the detection coil  103 . 
       FIG. 6  illustrates a change in the frequency characteristic of the reflected power caused by a foreign object. If the metallic foreign object  201  exists near the detection coils  103 , electromagnetic induction is generated between the detection coils  103  and the metallic foreign object  201  to cause a decrease in the inductance of the detection coils  103 . As a result, the matching frequency increases as compared to a case where there is no metallic foreign object  201 . 
     On the other hand, if rainwater exists near the detection coil  103 , capacitance coupling occurs between the detection coils  103  and the rainwater to increase the capacitance value of the parasitic capacitors  106  of the detection coils  103 , because rainwater has a high dielectric constant. As a result, the matching frequency decreases as compared to a case where there is no rainwater. 
     The detection circuit  108  detects the metallic foreign object  201  distinctively from rainwater by sensing such a change in matching frequency. A matching frequency f 0  when neither a metallic foreign object  201  nor rainwater exists is regarded to as a reference value. If the matching frequency of the reflected power is higher than the reference matching frequency f 0 , the detection circuit  108  determines that there is the metallic foreign object  201  near the detection coils  103 . On the other hand, if the matching frequency of the reflected power is lower than the reference matching frequency f 0 , the detection circuit  108  determines that there is rainwater near the detection coils  103 . 
     The reference matching frequency f 0  may be initially set when the foreign object detector  100  according to the present disclosure is installed. At that time, it may be confirmed visually, or checked in advance by another detector, that there is neither a metallic foreign object, for example, nor rainwater there. Alternatively, the reference matching frequency f 0  may be set at the time of shipment of the foreign object detector  100  according to the present disclosure. 
     Next, it will be described how to reduce the influence of the length of the wires. During charging, an AC magnetic field generated by the transmitting coil  302  induces a high voltage in the detection coils  103 . This high induced voltage could cause breakdown in the circuits connected to the detection coils  103 . If the frequency of the RF power output from the transmitting circuit  101  to detect a foreign object is increased to avoid such a breakdown, the wavelength of the RF power becomes shorter. As a result, the length of the wires supplying the power to the detection coils  103  increases electromagnetically. 
     At this time, the wires function as a linear antenna such as a dipole antenna. Then, the radiation resistance of the detection coils  103  increases, and the Q factor of the detection coils  103  decreases, so much as to deteriorate the foreign object detecting performance. That is, since the detection coils  103  do not function as inductors, no foreign objects are detectable, which is a problem. 
     In particular, the transmitting coil  302  utilized for charging an electric vehicle has so large a size (e.g., diameter if the coil is in a circular shape) that there is a need to arrange a lot of detection coils  103 . In this case, the length of the wires increases physically. 
     In order to address this problem, the method employed in the foreign object detector  100  of  FIG. 2  according to the present disclosure is to connect the detection coils  103  to the respective circuits via the coaxial cables  102 , which greatly reduces leakage electromagnetic fields, and to detect a foreign object based on the frequency characteristic of the power reflected by the detection coils  103 . Furthermore, balanced power is supplied to the detection coils  103  via the respective balun  104  to reduce leakage currents induced on outer conducting shields of the coaxial cables  102 . With this configuration, the power being supplied to the detection coils  103  is transmitted through the outer conducting shields of the coaxial cables  102  to prevent the coaxial cables  102  from functioning as a linear antenna even if the coaxial cables  102  are electromagnetically long. 
     Second Embodiment 
     Now, it will be described how to prevent the circuits from causing breakdown in a foreign object detector  200  shown in  FIG. 7 . 
     As shown in  FIG. 2 , the foreign object detector  100 , the transmitting coil  302 , and the RF oscillation source  305 , which have been described in the first embodiment, are combined to form a non-contact charging system. The RF oscillation source  305  supplies RF power of a predetermined frequency to the transmitting coil  302  to start non-contact charging. 
     The RF oscillation source  305  transmits a large quantity of electric power. Thus, at occurrence of magnetic field coupling between the detection coils  103  and the transmitting coil  302 , that large quantity of electric power is supplied to the transmitting circuit  101 , the detection circuit  108 , and the switch circuit  109  to incur the risk of causing breakdown in these circuits. In order to address this problem, the frequency of the RF power supplied from the transmitting circuit  101  is set to be higher than the frequency of the RF power supplied from the RF oscillation source  305 . 
     As shown in the foreign object detector  200  of  FIG. 7 , a circuit protection filter  110  is arranged, for example, between each matching circuit  105  and its associated parasitic capacitor  106  and connected to its associated detection coil  103 . The circuit protection filter  110  is a filter circuit with the characteristic of allowing the frequency of the RF power supplied from the transmitting circuit  101  to pass, and cutting the frequency of the RF power supplied from the RF oscillation source  305 . This configuration protects the transmitting circuit  101 , the detection circuit  108 , the switch circuit  109 , and other circuits and elements.  FIG. 7  illustrates an example where capacitors functioning as high-pass filters are connected to both terminals of the detection coils  103 . 
     As shown in  FIG. 4 , the size of each detection coil  103  (e.g., its diameter if the coil is in a circular shape) is set smaller than the size of the transmitting coil  302  (e.g., its diameter if the coil is in a circular shape). This configuration reduces the degree of the magnetic field coupling between the detection coils  103  and the transmitting coil  302  and thereby reduces the power supplied to the transmitting circuit  101 , the detection circuit  108 , and the switch circuit  109 . 
     Third Embodiment 
     Now, it will be described how to reduce mutual coupling between the respective detection coils  103  in a foreign object detector  300  shown in  FIG. 8 . 
     As shown in  FIG. 4 , since the plurality of detection coils  103  are arranged close to each other, mutual coupling occurs between them. In  FIG. 2 , when the coil L 1  is selected among the detection coils  103 , the non-selected coils other than L 1  are coupled to the coil L 1 , which induces currents. As a result, a change in received power caused by the metallic foreign object  201  decreases so much as to deteriorate the foreign object detecting performance. 
     Thus, according to this embodiment, as shown in  FIG. 8 , the two terminals of each of those non-selected detection coils  103  are short-circuited together by an associated short circuit  111 . In the embodiment illustrated in  FIG. 8 , each short circuit  111  is provided between its associated matching circuit  105  and parasitic capacitor  106 . 
     Then, the impedance of the non-selected detection coils  103  changes so greatly as to cause impedance mismatch between the detection coils  103  and the respective baluns  104 . This allows for preventing currents from being induced in the non-selected detection coils  103  due to their mutual coupling. 
     Alternatively, as in the foreign object detector  300  shown in  FIG. 9 , each short circuit  111  may be connected to an associated detection coil  103  with a circuit protection filter  110  interposed therebetween. That is, the short circuit  111  may be provided between the circuit protection filter  110  and an associated matching circuit  105 . This configuration allows for preventing the short circuits  111  from being broken down due to a large voltage induced in the detection coils  103  by an AC magnetic field generated by the transmitting coil  302 . 
     Effective improvement in foreign object detecting performance owing to such reduction in mutual coupling will now be described based on the results of calculation obtained by an equivalent circuit. 
       FIG. 10  is an equivalent circuit model of the foreign object detector  300  of  FIG. 9 . The elements falling in the range from the detection coil  103  to the matching circuit  105  are represented by their equivalent circuit. The metallic foreign object  201  is represented by a series circuit comprised of a resistor and an inductor. The calculation was performed based on the assumption that if there was the metallic foreign object  201  near the coil L 1 , magnetic field coupling with a coupling coefficient k (=−0.1) would occur between the inductor of the metallic foreign object  201  and the coil L 1 . On the other hand, the calculation was also performed based on the assumption that if there was rainwater near the coil L 1  the parasitic capacitor  106  associated with the coil L 1  would increase via capacitance coupling. 
     The matching circuit  105  is comprised of capacitors connected in series/parallel. The calculation was performed based on the assumption that balanced impedance of the balun  104  was 50Ω, and the detection coils  103  were comprised of the coils L 1  and L 2 . The reference matching frequency f 0  where there was no foreign object was adjusted to 170 MHz. 
     Now, the result of calculation will be shown.  FIGS. 11A-11C  illustrate the frequency characteristic of the power reflected by the coil L 1  when there is no mutual coupling between the respective detection coils  103 . The matching frequency of  FIG. 11B  when there is a foreign object (metal) is higher than the reference matching frequency f 0  of  FIG. 11A  when there is neither a foreign object nor rainwater. The matching frequency of  FIG. 11C  when there is rainwater is lower than the reference matching frequency f 0  of  FIG. 11A  when there is neither a foreign object nor rainwater. 
       FIGS. 12A-12C  illustrate the frequency characteristic of the power reflected by the coil L 1  when there is mutual coupling between the respective detection coils  103 . The calculation was performed based on the assumption that the coupling coefficient k 12  of a magnetic field between the coils L 1  and L 2  was −0.1. It can be seen that the amount of change in the matching frequency was smaller than those shown in  FIGS. 11A-11C . 
       FIGS. 13A-13C  illustrate the frequency characteristic of the power reflected by the coil L 1  when there is mutual coupling between the respective detection coils  103  and the short circuit  111  of the coil L 2  is short-circuited. It can be seen that the amount of change in the matching frequency was almost equal to those shown in  FIGS. 11A-11C . 
       FIG. 14  illustrates a short circuit  111  using a diode. A diode  120  is connected in parallel to both terminals of a detection coil  103 . DC voltages V 1  and V 2  are applied to both terminals of the diode  120  via bias resistors  121  and  122 , respectively. The potential difference between the DC voltages V 1  and V 2  is changed to switch the diode  120  between an ON state (short-circuited state) and an OFF state (opened state). 
     While both of the terminals of the detection coil  103  are supposed to be short-circuited together in the third embodiment, some capacitors of the matching circuit  105  in  FIG. 10  may be short-circuited to cause impedance mismatch in the non-selected detection coils  103 . 
     The foreign object detector and method of detecting a foreign object described above allow for detecting the foreign object while distinguishing metal from water. Even if the wire between a detection coil and a detection circuit is long, the accuracy in detecting a foreign object is maintained while preventing the wire from functioning as a linear antenna. 
     The foreign object detector and method of detecting a foreign object according to the present disclosure are applicable for use in a non-contact charger for mobile electronic devices, electric propulsion vehicles, and other devices and machines to be charged.