Patent ID: 12199453

DESCRIPTION OF THE EMBODIMENTS

A foreign matter detection device according to one embodiment of the disclosure will be described below with reference to the drawings. The foreign matter detection device includes a substrate arranged between a transmission coil of a device on the power transmission side (hereinafter simply referred to as power transmission device) and a reception coil of a device on the power reception side (hereinafter simply referred to as power reception device), included in a power transmission system that transmits power in a contactless manner. The foreign matter detection device includes a power feeding coil formed on the substrate for supplying power for foreign matter detection, and a plurality of detection coils arranged at mutually different positions within the power feeding coil to be electromagnetically coupled with the power feeding coil and being smaller than the power feeding coil. Each of the plurality of detection coils is capable of resonating at a frequency, which is different from the frequency of the power supplied to the transmission coil of the power transmission device and at which neither the resonance circuit including the transmission coil (when the resonance circuit is provided on the power transmission side) nor the resonance circuit including the reception coil resonates. The foreign matter detection device supplies AC power having a frequency at which these detection coils resonate to the power feeding coil, and detects the voltage output from each of the plurality of detection coils. If a conductive foreign matter such as metal enters between the transmission coil and the reception coil, the resonance characteristics of any of the plurality of detection coils change, resulting in a change in the detected voltage. Thereby, the foreign matter detection device monitors the detected voltage, and determines that a foreign matter has entered between the transmission coil and the reception coil if the voltage deviates from a predetermined reference range corresponding to a case where no foreign matter is present. Furthermore, each of the plurality of detection coils is arranged so that adjacent two of the plurality of detection coils overlap by a predetermined amount that allows mutual electromagnetic coupling to be ignored when viewed from the normal direction of the substrate. As a result, the foreign matter entering between the transmission coil and the reception coil is positioned inside any of the detection coils when viewed from the normal direction of the substrate, and since the detection coil whose resonance characteristics are changed by the foreign matter is not affected by other detection coils, the accuracy of detecting the foreign matter is improved.

FIG.1is a schematic configuration diagram of a power transmission system including a foreign matter detection device according to one embodiment of the disclosure. As shown inFIG.1, the power transmission system1includes a power transmission device2, a power reception device3, and a foreign matter detection device4. The power transmission device2and the power reception device3constitute a contactless power feeding device. Power is transmitted from the power transmission device2to the power reception device3through space in a contactless manner. The power transmission device2includes a power supply circuit11and a transmission coil12while the power reception device3includes a reception coil21, a resonance capacitor22, and a power reception circuit23. The power transmission system1may be, for example, a contactless power feeding device of a so-called primary-series-secondary-series resonance capacitor system (SS system) or primary-series-secondary-parallel resonance capacitor system (SP system). Alternatively, the power transmission system1may be a contactless power feeding device of a system (NS system) in which the reception coil and the resonance capacitor resonate in series on the secondary side without using resonance on the primary side, or a system (NP system) in which the reception coil and the resonance capacitor resonate in parallel on the secondary side without using resonance on the primary side.

The power transmission device2will be described first. The power supply circuit11supplies AC power to the transmission coil12. Therefore, the power supply circuit11includes, for example, a DC power supply that supplies DC power, an inverter circuit that converts the DC power supplied from the DC power supply into AC power and supplies the AC power to the transmission coil12, and a control circuit that controls the inverter circuit. The inverter circuit may be a full-bridge inverter in which four switching elements (for example, MOSFETs) are connected in a full-bridge form, or a half-bridge inverter in which two switching elements are connected in a half-bridge form. The control circuit controls on/off switching of each switching element of the inverter circuit to set the frequency of the AC power supplied to the transmission coil12to a predetermined frequency (for example, the frequency at which the resonance circuit of the power reception device3resonates).

The power supply circuit11may further include a DC-DC converter between the DC power supply and the inverter circuit. Alternatively, the power supply circuit11may include a rectifier circuit that is connected to an AC power supply and rectifies the AC power from the AC power supply, and a power factor correction circuit that is connected to the rectifier circuit and converts pulsating flow power output from the rectifier circuit into DC power, instead of the DC power supply. In this case, for example, the control circuit may control the power factor correction circuit to adjust the voltage of the DC power supplied to the inverter circuit in order to keep the voltage of the power received by the power reception device3constant.

The transmission coil12transmits the AC power supplied from the power supply circuit11to the reception coil21of the power reception device3through space. The power transmission device2may include a capacitor connected in series with the transmission coil12between the transmission coil12and the inverter circuit of the power supply circuit11. This capacitor may be for blocking the DC power, or may be for configuring a resonance circuit that resonates together with the transmission coil12at the frequency of the AC power supplied to the transmission coil12.

The power transmission device2may further include a communication device that receives a signal indicating the state of power reception of the power reception device3. In this case, the control circuit of the power supply circuit11may change the on/off switching timing of each switching element of the inverter circuit so as to change the frequency of the AC power supplied to the transmission coil12according to the state of power reception.

Further, when receiving a signal indicating that a foreign matter has been detected between the transmission coil12and the reception coil21from the foreign matter detection device4, the control circuit of the power supply circuit11may stop the power supply from the power supply circuit11to the transmission coil12by turning off each switching element of the inverter.

Furthermore, the power supply circuit11may include a DC-AC converter instead of the inverter circuit. Such a DC-AC converter may include a coil that is connected in series between the DC power supply and the transmission coil12, a capacitor that has one end connected between the coil and the transmission coil12and is connected in parallel to the transmission coil12, and a switching element that is connected in parallel to the capacitor. Then, the switching element can be a field effect transistor made of gallium nitride (GaN). Such a configuration allows the power supply circuit11to switch the switching element on and off at a switching frequency included in the ISM band to supply the transmission coil12with AC power having the switching frequency included in the ISM band.

Next, the power reception device3will be described. The reception coil21forms a resonance circuit together with the resonance capacitor22and receives power from the transmission coil12by resonating with the alternating current flowing through the transmission coil12of the power transmission device2. Therefore, the resonance capacitor22may be connected in series with the reception coil21or may be connected in parallel to the reception coil21. Then, the AC power output from the resonance circuit formed by the reception coil21and the resonance capacitor22is output to the power reception circuit23. The number of turns of the reception coil21and the number of turns of the transmission coil12may be the same or may be different from each other.

The power reception circuit23converts the AC power from the resonance circuit formed by the reception coil21and the resonance capacitor22into DC power, and outputs the DC power to a load circuit (not shown) connected to the power reception circuit23. Therefore, the power reception circuit23includes, for example, a full-wave rectifier circuit that converts the AC power from the resonance circuit into pulsating flow power, and a smoothing capacitor for smoothing the pulsating flow power output from the full-wave rectifier circuit and outputting the same to the load circuit. The power reception circuit23may further include a voltmeter for measuring the voltage output to the load circuit, a communication device for transmitting a signal indicating the state of power reception, such as voltage measured by the voltmeter, to the power transmission device2, a switching element for switching between connection or disconnection between the load circuit and the power reception circuit23, and a control circuit controlling on/off switching of the switching element.

Next, the foreign matter detection device4according to this embodiment will be described. The foreign matter detection device4is arranged between the transmission coil12and the reception coil21in a case where the power transmission device2and the power reception device3are in a positional relationship that allows power transmission, that is, a positional relationship that allows the transmission coil12and the reception coil21to be electromagnetically coupled. Then, the foreign matter detection device4detects a conductive foreign matter such as metal that has entered between the transmission coil12and the reception coil21.

FIG.2is a schematic configuration diagram of the foreign matter detection device4.FIG.3is a schematic cross-sectional side view showing an example of the positional relationship between the substrate, on which the foreign matter detection device4, is provided and the transmission coil12. In addition,FIG.4is a schematic plan view showing an example of the arrangement of a plurality of detection coils and the power feeding coil of the foreign matter detection device4.

As shown inFIG.2, the foreign matter detection device4includes a power supply circuit41, a power feeding coil42, a plurality of detection coils43-1to43-n(n is an integer of 2 or more), a plurality of capacitors44-1to44-n, and a detection circuit45. The power supply circuit41, the power feeding coil42, the plurality of detection coils43-1to43-n, the plurality of capacitors44-1to44-n, and the detection circuit45are provided on a substrate46positioned between the transmission coil12and the reception coil21when the transmission coil12and the reception coil21are in a positional relationship that allows them to be electromagnetically coupled. In this embodiment, the substrate46is attached to the power transmission device2. Then, a signal indicating a foreign matter detection result from the detection circuit45is output to the power supply circuit11of the power transmission device2.

FIG.5is a circuit configuration diagram showing an example of the power supply circuit41. The power supply circuit41includes, for example, a DC power supply51that supplies DC power, a capacitor52, an inverter circuit53, and a control circuit54that controls the inverter circuit53. Then, the power supply circuit41supplies power for foreign matter detection to the detection coils43-1to43-nthrough the power feeding coil42.

The power feeding coil42has one end connected to the inverter circuit53via the capacitor52and the other end grounded. Nevertheless, the connection order of the power feeding coil42and the capacitor52may be changed.

The inverter circuit53converts the DC power supplied from the DC power supply51into AC power and supplies the AC power to the power feeding coil42. In this example, the inverter circuit53is configured as a half-bridge inverter in which two switching elements (for example, MOSFETs) are connected in a half-bridge form, but the inverter circuit53may be a full-bridge inverter in which four switching elements are connected in a full-bridge form. The control circuit54controls on/off switching of each switching element included in the inverter circuit to set the frequency of the AC power supplied to the power feeding coil42to a predetermined frequency.

The frequency of the AC power supplied from the power supply circuit41to the detection coils43-1to43-nvia the power feeding coil42is different from the frequency of the AC power supplied to the transmission coil12of the power transmission device2, and is preferably set to a frequency at which neither the resonance circuit including the transmission coil12(when the resonance circuit is provided in the power transmission device2) nor the resonance circuit including the reception coil21resonates. For example, the frequency of the AC power supplied by the power supply circuit41can be higher than the frequency of the AC power supplied to the transmission coil12(for example, 85 kHz or 150 kHz), which prevents the AC power supplied from the power supply circuit41from affecting the power transmission from the power transmission device2to the power reception device3. Besides, by setting the frequency of the AC power supplied by the power supply circuit41in this way, the inductance of each of the plurality of detection coils43-1to43-nof the foreign matter detection device4can be made relatively small, so it is easy to make the size of each detection coil smaller than the size of the transmission coil12.

Referring toFIG.2toFIG.4again, each of the power feeding coil42and the plurality of detection coils43-1to43-nis configured as a wiring pattern of a conductor such as metal, which is provided on the substrate46. Then, each detection coil43-iand the corresponding capacitor44-i(i=1, 2, . . . , n) are connected to each other to form one resonance circuit. The power supply circuit41, the capacitor44-i, and the detection circuit45are omitted fromFIG.4. The inductance of each detection coil and the electrostatic capacitance of each capacitor are preferably set so that the resonance frequency of the resonance circuit composed of the detection coil and the capacitor is a frequency that does not resonate with the frequency of the AC power supplied to the transmission coil12. Since each resonance circuit of the foreign matter detection device4does not resonate with the AC power transmitted from the power transmission device2to the power reception device3, the foreign matter detection device4is capable of preventing the AC power transmitted from the power transmission device2to the power reception device3from affecting foreign matter detection. Further, the inductance of each detection coil and the electrostatic capacitance of each capacitor are preferably set so that the resonance frequency of the resonance circuit composed of the detection coil and the capacitor is a frequency that resonates with the frequency of the AC power supplied from the power supply circuit41via the power feeding coil42. Since the AC power supplied from the power supply circuit41via the power feeding coil42is suppressed from being lost until it reaches the detection circuit45, the foreign matter detection device4is capable of suppressing a decrease in foreign matter detection accuracy. It should be noted that the resonance frequency of each resonance circuit and the frequency of the AC power supplied from the power supply circuit41may not match each other as long as a voltage corresponding to the AC power supplied from the power supply circuit41is output from each detection coil to the detection circuit45.

In addition, the substrate46is arranged so that the central axis direction of the transmission coil12and the normal direction of the substrate46are substantially parallel. Then, it is preferable to provide the power feeding coil42so that the outer diameter of the power feeding coil42, viewed from the normal direction of the substrate46, that is, the central axis direction of the transmission coil12, is approximately the same as the outer shape of the transmission coil12or larger than the transmission coil12. Further, it is preferable to provide the power feeding coil42so that the transmission coil12is positioned inside the power feeding coil42when viewed from the normal direction of the substrate46. Each of the plurality of detection coils43-1to43-nis smaller than the power feeding coil42on one surface of the substrate46, and is arranged to be electromagnetically coupled with the power feeding coil42at a different position inside the power feeding coil42when viewed from the normal direction of the substrate46. Thus, the power feeding coil42is capable of reliably supplying power for foreign matter detection to each detection coil. Accordingly, when each detection coil resonates with the corresponding capacitor in response to the AC power supplied from the power supply circuit41via the power feeding coil42, a voltage corresponding to the AC power is output from each detection coil to the detection circuit45. It should be noted that one or two or more of the plurality of detection coils may be arranged outside the power feeding coil42or overlapping the power feeding coil42when viewed from the normal direction of the substrate46as long as each of the plurality of detection coils is allowed to receive power from the power feeding coil42.

Furthermore, adjacent two detection coils, among the plurality of detection coils43-1to43-n, are arranged to overlap by a predetermined amount that allows the electromagnetic coupling between the two detection coils to be ignored when viewed from the normal direction of the substrate46. At this time, an insulating layer (not shown) is provided between the two detection coils to keep the two detection coils from being electrically connected at the position where the two detection coils overlap each other.

In the example shown inFIG.4, each detection coil has a rectangular shape and the detection coils are arranged in a staggered pattern. In both the horizontal direction and the vertical direction, adjacent two detection coils are arranged to overlap by a predetermined amount that allows the mutual electromagnetic coupling to be ignored when viewed from the normal direction of the substrate46.

As the detection coils are arranged to make adjacent two detection coils partially overlap each other, a foreign matter entering between the transmission coil12and the reception coil21is positioned inside any one of the detection coils when viewed from the normal direction of the substrate46. Then, the detection coil having the foreign matter therein changes the resonance characteristics under the influence of the foreign matter. Besides, the electromagnetic coupling between the detection coils can be ignored. That is to say, the influence of the electromagnetic coupling between the detection coils on the resonance characteristics of the individual detection coil is sufficiently smaller than the change in resonance characteristics of the detection coil caused by the foreign matter. Therefore, the detection coil whose resonance characteristics are changed by the foreign matter is not affected by the other detection coils. Hence, the detection circuit45corresponding to the detection coil having the foreign matter therein is capable of accurately detecting the change in voltage due to the change in resonance characteristics. Details of the degree of overlap between adjacent two detection coils will be described later.

Moreover, in this example, the power feeding coil42and the detection coils43-1to43-nare arranged on the same surface of the substrate46, but the surface of the substrate46on which the power feeding coil42is arranged may be different from the surface of the substrate46on which the detection coils43-1to43-nare arranged. In addition, some of the detection coils43-1to43-nmay be arranged on the same surface as the power feeding coil42while some other detection coils43-1to43-nare arranged on a different surface from the power feeding coil42.

In the example shown inFIG.4, each of the plurality of detection coils43-1to43-nis formed in a rectangular shape, but the shape of each detection coil is not limited to a rectangular shape and may be a circular or elliptical shape, for example. The shapes and sizes of the plurality of detection coils43-1to43-nmay be the same as or different from each other. Furthermore, the size of each of the plurality of detection coils43-1to43-nviewed from the central axis direction of the transmission coil12is preferably smaller than the size of the transmission coil12. Thus, even if a foreign matter smaller than the transmission coil12enters between the transmission coil12and the reception coil21, any one of the detection coils43-1to43-nis easily affected by the foreign matter, which enables the foreign matter detection device4to accurately detect such a small foreign matter.

The detection circuit45detects the voltage of the AC power output from each of the plurality of detection coils43-1to43-n, and detects the foreign matter entering between the transmission coil12and the reception coil21based on the detected voltage.

In this embodiment, the AC power transmitted from the power transmission device2to the power reception device3does not affect transmission of the AC power from the power supply circuit41to the detection circuit45via the power feeding coil42and any of the plurality of detection coils43-1to43-n. However, if a foreign matter enters between the transmission coil12and the reception coil21, the resonance characteristics of the detection coil having the foreign matter therein, among the plurality of detection coils43-1to43-n, change due to the foreign matter. Therefore, the presence of the foreign matter affects transmission of the AC power from the power supply circuit41to the detection circuit45via the detection coil in which the foreign matter is located. As a result, the voltage that is output from the detection coil having the foreign matter therein and detected by the detection circuit45changes. For example, if metal approaches any of the detection coils through which a current flows due to the supplied AC power, a magnetic flux is generated near the detection coil by the current and causes an eddy current in the metal, resulting in a loss. In addition, the inductance of the detection coil decreases due to the magnetic flux resulting from the generated eddy current. In particular, when the metal is magnetic, the loss may be relatively large even though the change in inductance is small. As a result of these, the resonance characteristics of the resonance circuit containing the detection coil change. Thus, the detection circuit45determines whether the voltage output from the detection coil is within a predetermined reference range for each of the plurality of detection coils43-1to43-n. The predetermined reference range may be the same for the detection coils or may be set for each detection coil.

FIG.6is a circuit configuration diagram showing an example of the detection circuit45. Since the detection circuit45can have the same configuration for each detection coil,FIG.6shows the configuration of the detection circuit45for one detection coil.

The detection circuit45includes a resonance circuit63having a reception coil61and a resonance capacitor62, a high-pass filter64, an amplifier65, a half-wave rectifier circuit66, a low-pass filter67, a voltage detection circuit68, a determination circuit69, and a memory circuit70. However, the circuit configuration of the detection circuit45is not limited to the one shown inFIG.6, and can be any of a variety of circuits capable of determining whether the detected voltage is outside the predetermined reference range.

The resonance circuit63detects the AC power that is supplied from the power supply circuit41and transmitted via the power feeding coil42and the corresponding detection coil43-i(i=1, 2, . . . , n) among the plurality of detection coils43-1to43-n. Therefore, the reception coil61of the resonance circuit63is arranged to be electromagnetically coupled with the corresponding detection coil among the plurality of detection coils43-1to43-n. Then, the inductance of the reception coil61and the electrostatic capacitance of the resonance capacitor62are set so that the resonance circuit63resonates with the AC power supplied from the power supply circuit41. AlthoughFIG.6shows that two resonance capacitors62are connected in parallel to the reception coil61, the number of resonance capacitors62included in the resonance circuit63is not necessarily two and may be one or three or more. Besides, the reception coil61and the resonance capacitor62may be connected in series. Furthermore, according to a modified example, the resonance circuit63itself may be formed by the corresponding detection coil43-iand capacitor44-i. That is, the detection coil43-iand the capacitor44-imay be provided in place of the reception coil61and the resonance capacitor62to simplify the configuration of the detection circuit45.

The high-pass filter64is connected between the resonance circuit63and the amplifier65for attenuating a noise component, which has a frequency lower than the frequency of the AC power supplied from the power supply circuit41, in the AC power received by the resonance circuit63. The amplifier65is connected between the high-pass filter64and the half-wave rectifier circuit66, and amplifies the AC power output from the high-pass filter64.

The half-wave rectifier circuit66is connected between the amplifier65and the low-pass filter67, and half-wave rectifies the amplified AC power output from the amplifier65and converts it into pulsating flow power. The low-pass filter67is connected between the half-wave rectifier circuit66and the voltage detection circuit68, and smooths the pulsating flow power output from the half-wave rectifier circuit66and converts it into DC power.

The voltage detection circuit68is connected to the low-pass filter67and detects the voltage of the DC power output from the low-pass filter67. The voltage detection circuit68then outputs the detected voltage to the determination circuit69. The voltage detection circuit68can be any voltage detection circuit for detecting a DC voltage.

The determination circuit69determines whether the detected voltage is within a predetermined reference range. If the detected voltage is within the predetermined reference range, the determination circuit69determines that there is no foreign matter between the transmission coil12and the reception coil21. However, if the detected voltage deviates from the predetermined reference range, the determination circuit69determines that there is a foreign matter entering between the transmission coil12and the reception coil21. The determination circuit69then outputs a signal indicating the foreign matter detection result to the power supply circuit11of the power transmission device2.

In this embodiment, the determination circuit69includes a conversion circuit that converts the voltage received from the voltage detection circuit68into a signal value, an arithmetic circuit for determining whether the signal value is within a predetermined reference range, and a communication circuit for outputting the signal indicating the foreign matter detection result to the power supply circuit11of the power transmission device2.

The memory circuit70is an example of a memory unit, and has, for example, a non-volatile semiconductor memory or a volatile semiconductor memory and stores information representing the predetermined reference range.

AlthoughFIG.6shows an example in which the detection circuit45is provided with the determination circuit69and the memory circuit70for each detection coil, the detection circuit45may be provided with one determination circuit69and one memory circuit70that are commonly used for each of the plurality of detection coils43-1to43-n.

The degree of overlap between adjacent two detection coils will be described below.FIG.7is a diagram for illustrating the degree of overlap between two detection coils that are adjacent in the horizontal direction. In the example shown inFIG.7, the two detection coils43-kand43-(k+1) have the same size and shape. Each of the two detection coils43-kand43-(k+1) is formed in a rectangular shape. In regard to each of the two detection coils43-kand43-(k+1), two horizontal sides w1and w2facing each other have a length a and two vertical sides h1and h2facing each other have a length c. In addition, the two detection coils43-kand43-(k+1) are each formed by a winding of a conductor that has a width t. Then, the detection coil43-(k+1) is displaced to the right in the horizontal direction by a distance (a-b-2t) with respect to the detection coil43-k(where b<a). That is, the width of the region sandwiched between the right side h2of the detection coil43-kand the left side h1of the detection coil43-(k+1) is b. The position of the detection coil43-kand the position of the detection coil43-(k+1) are the same in the vertical direction.

In order to prevent electromagnetic coupling between the detection coil43-kand the detection coil43-(k+1), the interlinkage magnetic flux emitted from each side of one of the detection coil43-kand the detection coil43-(k+1) and passing through the inside of the other of the detection coil43-kand the detection coil43-(k+1) cancels each other, and as a result, the strength of the interlinkage magnetic flux becomes zero. Here, inside the detection coil43-(k+1), a region where the detection coil43-kand the detection coil43-(k+1) overlap is defined as a region A, and a region where the detection coil43-kand the detection coil43-(k+1) do not overlap is defined as a region B. Then, the currents flowing through the sides h1, h2, w1, and w2of the detection coil43-kare i1, i2, i3, and i4, respectively. At this time, if t<<a, b, c, then the condition under which the strength of the interlinkage magnetic flux emitted from each side of the detection coil43-kand passing through the inside of the detection coil43-(k+1) becomes 0 is expressed by the following formula.

ϕ1⁢A+ϕ2⁢A+ϕ3⁢A+ϕ4⁢A=ϕ2⁢B-ϕ1⁢B⁢ϕ1⁢A=μ0·i1·c2·π·ln⁡(aa-b)⁢ϕ1⁢B=μ0·i1·c2·π·ln⁡(2⁢a-ba)⁢ϕ2⁢A=μ0·i2·c2·π·ln⁡(bt/2)⁢ϕ2⁢B=μ0·i2·c2·π·ln⁡(a-bt/2)⁢ϕ3⁢A=μ0·i3·b2·π·ln⁡(ct/2)⁢ϕ4⁢A=μ0·i4·b2·π·ln⁡(ct/2)[Formula⁢1]
Here, Φpqis the magnetic flux generated by the current ip(p=1, 2, 3, 4) flowing through the corresponding side of the detection coil43-k, and passing through the region q (q=A or B). Also, μ0is the magnetic permeability of a vacuum. When the current flowing through each side of the detection coil43-kis equal to the current flowing through each side of the detection coil43-(k+1) due to symmetry and the formula (1) is satisfied, the strength of the interlinkage magnetic flux emitted from each side of the detection coil43-(k+1) and passing through the inside of the detection coil43-kalso becomes zero.

For example, it is assumed that the length of each side of the two detection coils is2010 mm (that is, a=c=20 mm), the width t of the winding of each detection coil is 0.1 mm, and the minimum spacing between the two windings at the portion where the winding of the detection coil43-kand the winding of the detection coil43-(k+1) overlap is 0.1 mm. Then, it is assumed that the current flowing through each side of the detection coil43-kis equal (that is, i1=i2=i3=i4). In this case, the value of b that satisfies the formula (1) based on theoretical calculation is 2.2 mm. That is, when the detection coil43-kand the detection coil43-(k+1) overlap by approximately 11% of the length of one side of the detection coil43-k, the two detection coils are no longer electromagnetically coupled to each other and the coupling coefficient is zero. In addition, assuming that the length of each side of the two detection coils is 10 mm and other conditions are the same as above, the value of b that satisfies the formula (1) based on theoretical calculation is 1.18 mm.

Furthermore, even though the coupling coefficient between adjacent two detection coils may not become completely zero, if the change in resonance characteristics due to electromagnetic coupling between the two detection coils is sufficiently smaller than the change in resonance characteristics due to the presence of a foreign matter, the mutual electromagnetic coupling can be ignored in foreign matter detection. Thus, each detection coil is arranged so that the two detection coils overlap to an extent that the coupling coefficient between the adjacent two detection coils is less than 0.02 to less than 0.03. For example, in the above example where the length of each side of the detection coil is 20 mm, a predetermined amount of overlap between adjacent two detection coils may be set so that the value of b falls within the range of 1.9 mm to 2.5 mm.

FIG.8is a diagram for illustrating the degree of overlap between two detection coils that are adjacent in the oblique direction. In the example shown inFIG.8, like the example shown inFIG.7, three detection coils43-k,43-(k+1), and43-(k+2) have the same size and shape. Each detection coil is formed in a rectangular shape. In regard to each detection coil, two horizontal sides w1and w2facing each other have a length a and two vertical sides h1and h2facing each other have a length c. In addition, each detection coil is formed by a winding of a conductor that has a width t. Then, the detection coil43-(k+1) is displaced to the right in the horizontal direction by a distance (a-b-2t) with respect to the detection coil43-k(where b<a). This b is a distance at which the detection coil43-kand the detection coil43-(k+1) are not electromagnetically coupled.

The position of the detection coil43-kand the position of the detection coil43-(k+1) are the same in the vertical direction. Further, the detection coil43-(k+2) is arranged to be positioned at the middle point between the detection coil43-kand the detection coil43-(k+1) in the horizontal direction. That is, the detection coil43-(k+2) is displaced respectively from the detection coil43-kand the detection coil43-(k+1) along the horizontal direction by a distance (a-b-2t)/2. Also, in the vertical direction, the detection coil43-(k+2) is displaced downward by a distance (c-d-2t) with respect to each of the detection coil43-kand the detection coil43-(k+1) (where d<c). That is, the width of the region sandwiched between the lower side w2of the detection coil43-kand the upper side w1of the detection coil43-(k+2) is d. In order to prevent electromagnetic coupling between the detection coil43-kand the

detection coil43-(k+2), the interlinkage magnetic flux emitted from each side of one of the detection coil43-kand the detection coil43-(k+2) and passing through the inside of the other of the detection coil43-kand the detection coil43-(k+2) cancels each other, and as a result, the strength of the interlinkage magnetic flux becomes zero. Here, inside the detection coil43-(k+2), a region where the detection coil43-kand the detection coil43-(k+2) overlap is defined as a region A, and a region where the detection coil43-kand the detection coil43-(k+2) do not overlap is defined as a region B. Then, the currents flowing through the sides h1, h2, w1, and w2of the detection coil43-kare i1, i2, i3, and i4, respectively. At this time, if t<<a, b, c, d, then the condition under which the interlinkage magnetic flux emitted from each side of the detection coil43-kand passing through the inside of the detection coil43-(k+1) becomes 0 is expressed by the following formula.

ϕ1⁢A+ϕ2⁢A+ϕ3⁢A+ϕ4⁢A=ϕ2⁢B+ϕ4⁢B-ϕ1⁢B-ϕ3⁢B⁢ϕ1⁢A=μ0·i1·d2·π·ln(a-3⁢t/2a2-b2-t2)=μ0·i1·d2·π·ln⁡(2⁢aa-b)⁢ϕ1⁢B=μ0·i1·d2·π·ln⁡(3⁢a2-b2-5⁢t2a-t2)=μ0·i1·d2·π·ln⁡(3⁢a-b2⁢a)⁢ϕ2⁢A=μ0·i1·d2·π·ln(3⁢a2+b2-t2t/2)=μ0·i2·d2·π·ln⁡(a+bt)⁢ϕ2⁢B=μ0·i2·d2·π·ln(a2-b2-3⁢t2t/2)=μ0·i2·d2·π·ln⁡(a-bt)⁢ϕ3⁢A=μ0·i3·(a+b-2⁢t)/22·π·ln⁡(c-3⁢t2c-d-3⁢t2)=μ0·i3·(a+b)/24·π·ln⁡(cc-d)⁢ϕ3⁢B=μ0·i3·(a+b-2⁢t)/22·π·ln⁡(2⁢c-d-7⁢t2c-t2)=μ0·i3·(a+b)/24·π·ln⁡(2⁢c-dc)⁢ϕ4⁢A=μ0·i4·(a+b-2⁢t)/22·π·ln⁡(dt/2)=μ0·i4·(a+b)/24·π·ln⁡(2⁢dt)⁢ϕ4⁢B=μ0·i4·(a+b-2⁢t)/22·π·ln(c-d-5⁢t2t/2)=μ0·i4·(a+b)/24·π·ln⁡(2⁢(c-d)t)[Formula⁢2]
Here, Φpqis the magnetic flux generated by the current ip(p=1, 2, 3, 4) flowing through the corresponding side of the detection coil43-k, and passing through the region q (q=A or B). Also, μ0is the magnetic permeability of a vacuum. Accordingly, in order to prevent electromagnetic coupling between the detection coil43-(k+1) and the detection coil43-(k+2), the distance d may be set to satisfy the formula (2). Further, the detection coil43-(k+2) is positioned at the center of the detection coil43-kand the detection coil43-(k+1) in the horizontal direction, and the positional relationship between the detection coil43-(k+2) and the detection coil43-kis the same as the positional relationship between the detection coil43-(k+2) and the detection coil43-(k+1) in the vertical direction. Accordingly, by setting the distance d not to satisfy the formula (2), the detection coil43-(k+1) and the detection coil43-(k+2) are not electromagnetically coupled. Furthermore, the detection coils are arranged so that the detection coil43-(k+2) and the detection coil43-koverlap in the vertical direction by a predetermined amount that corresponds to a range of about several percent (%) of the length of one side of each detection coil, centered on the distance d by which the formula (2) is satisfied, thereby allowing electromagnetic coupling between these detection coils to be ignored in foreign matter detection.

In addition, in a case where the detection coils are formed in a circular shape and the same size, electromagnetic field simulations were performed assuming that the width of the winding of each detection coil is sufficiently small relative to the length of the perimeter of each detection coil. As a result, when adjacent two detection coils overlap by approximately 25% of the diameter, the coupling coefficient between the two detection coils is zero. Then, if the degree of overlap between adjacent two detection coils is within a range of about several percent (%) of the diameter centered at approximately 25% of the diameter, the influence of electromagnetic coupling between the two detection coils on the change in resonance characteristics when a foreign matter is present can be ignored. Accordingly, each of the plurality of detection coils may be arranged such that the predetermined amount of overlap between adjacent two detection coils is approximately 20% to approximately 30% of the diameter of the detection coils.

Furthermore, in a case where each detection coil has a shape other than that shown in the above example, an electromagnetic field simulation may be performed to determine the predetermined amount of overlap that allows electromagnetic coupling between adjacent two detection coils to be ignored.

As described above, the foreign matter detection device includes a plurality of detection coils provided on a substrate that is arranged between a transmission coil and a reception coil when a power transmission device and a power reception device are in a positional relationship which allows power to be transmitted from the power transmission device to the power reception device. The foreign matter detection device supplies AC power to each of the plurality of detection coils via a power feeding coil arranged to be electromagnetically coupled with the plurality of detection coils, and detects the voltage output according to the supplied AC power for each detection coil by a detection circuit. Then, the foreign matter detection device determines that a foreign matter has entered between the transmission coil and the reception coil when the voltage output from any of the detection coils deviates from a predetermined reference range. Furthermore, each of the plurality of detection coils is arranged so that adjacent two detection coils, among the plurality of detection coils, overlap by a predetermined amount that allows mutual electromagnetic coupling to be ignored when viewed from the normal direction of the substrate. Thus, the foreign matter entering between the transmission coil and the reception coil is positioned inside any one of the detection coils when viewed from the normal direction of the substrate, and the detection coil whose resonance characteristics are changed by the foreign matter is not affected by other detection coils. Accordingly, even if a foreign matter smaller than either of the transmission coil and the reception coil enters at any position between the transmission coil and the reception coil, the foreign matter detection device is still capable of detecting the foreign matter. Hence, the foreign matter detection device is able to improve the accuracy of detecting the foreign matter entering between the transmission coil and the reception coil.

According to a modified example, the foreign matter detection device4may be attached to the power reception device3. In this case, the power reception device3may further include a switching element (not shown) in parallel to the reception coil21for switching whether to short-circuit both ends of the reception coil21, and a control circuit (not shown) for switching whether to turn on or off the switching element. Then, a signal indicating the foreign matter detection result from the detection circuit45is output to the control circuit of the power reception device3, and when the signal indicates that a foreign matter has been detected between the transmission coil12and the reception coil21, the control circuit turns on the switching element to short-circuit both ends of the reception coil21. As a result, the power transmission from the power transmission device2to the power reception device3is interrupted to prevent the foreign matter from causing a failure.

Further, as described above, the power transmission device2and the power reception device3may each have a communication device (not shown) for communicating with each other. In this case, when the signal received from the detection circuit45of the foreign matter detection device4indicates that a foreign matter has been detected between the transmission coil12and the reception coil21, the control circuit of the power reception device3may send a signal instructing to stop the power transmission to the power transmission device2via the communication device. The control circuit of the power supply circuit11of the power transmission device2may stop supplying power from the power supply circuit11to the transmission coil12when receiving the signal instructing to stop the power transmission via the communication device.

Furthermore, the foreign matter detection device4may be attached to each of the power transmission device2and the power reception device3. That is, two foreign matter detection devices4may be arranged between the transmission coil12and the reception coil21.

Thus, those skilled in the art can make various modifications within the scope of the disclosure according to the embodiments.