Power transmission device and power reception device

A foreign object detector includes a plurality of first coils and a plurality of second coils. The plurality of first coils are arranged along an upper surface of a power transmission coil. The plurality of second coils are provided to correspond to the plurality of first coils, with the second coils each being arranged to face a corresponding one of the first coils. An outer shape of each of the plurality of first coils and each of the plurality of second coils is smaller than an outer shape of the power transmission coil when the power transmission coil is viewed two-dimensionally from above the power transmission coil.

This nonprovisional application is based on Japanese Patent Application No. 2014-138791 filed on Jul. 4, 2014 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to power transmission devices and power reception devices, and more particularly to detection of a foreign object between a power transmission device and a power reception device that exchange electric power in a contactless manner.

Description of the Background Art

There have been proposed a variety of contactless electric power transfer systems in which electric power is transferred from a power transmission device to a power reception device in a contactless manner.

In such a contactless electric power transfer system, it is expected that a foreign object (an object that should not be present) will enter between the power transmission device and the power reception device, and it is necessary to properly detect the foreign object. For example, Japanese Patent Laying-Open No. 2013-27171 discloses a detection device capable of accurately detecting a metallic foreign object even during power transmission from a power transmission device to a power reception device. This detection device detects a metallic foreign object present between the power transmission device and the power reception device based on a change in Q factor of a resonant circuit including a coil. This coil for Q factor measurement may be provided separately from a power feeding coil (a power transmission coil or a power reception coil), or the power feeding coil may be used as this coil.

In the detection device described in Japanese Patent Laying-Open No. 2013-27171, the power feeding coil may be used as the coil for Q factor measurement, and if the coil for Q factor measurement is provided separately from the power feeding coil, the coil for Q factor measurement is expected to have a similar size to the power feeding coil. However, in a system handling high power such as a contactless power feeding system of a vehicle, for example, the power feeding coil increases in size to some extent which may result in inability to detect a relatively small foreign object compared to the power feeding coil.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is that a relatively small foreign object compared to a power transmission coil can be detected in a power transmission device that transmits electric power to a power reception device in a contactless manner.

Another object of the present invention is that a relatively small foreign object compared to a power reception coil can be detected in the power reception device that receives electric power output from the power transmission device in a contactless manner.

According to the present invention, a power transmission device that transmits electric power to a power reception device includes a power transmission coil and a foreign object detector. The power transmission coil is configured to transmit electric power to a power reception coil of the power reception device in a contactless manner. The foreign object detector is provided above the power transmission coil, with the power transmission coil being configured to transmit electric power to the power reception coil located above the power transmission coil. The foreign object detector includes a plurality of first coils and a plurality of second coils. The plurality of first coils are arranged along an upper surface of the power transmission coil. The plurality of second coils are arranged along the upper surface of the power transmission coil above or below the plurality of first coils, and provided to correspond to the plurality of first coils. The plurality of second coils are each arranged to face a corresponding one of the first coils. An outer shape of each of the plurality of first coils and of each of the plurality of second coils is smaller than an outer shape of the power transmission coil when the power transmission coil, the plurality of first coils and the plurality of second coils are viewed two-dimensionally from above the power transmission coil.

In this power transmission device, the foreign object detector includes the plurality of first coils and the plurality of second coils, with the plurality of first coils and the plurality of second coils being provided as a set in the power transmission device. Consequently, the distance between the first coils and the corresponding second coils can be reduced to decrease the size of the first coils and the second coils. In addition, according to this power transmission device, the outer shape of each first coil and each second coil is smaller than the outer shape of the power transmission coil, allowing the foreign object detector to detect a relatively small foreign object compared to the power transmission coil.

Preferably, the plurality of first coils and the plurality of second coils are arranged in a matrix, within a housing containing the power transmission coil, on an upper surface of the housing.

With this configuration, a small foreign object on the housing containing the power transmission coil can be detected. Further, the plurality of first coils and the plurality of second coils can be protected against breakage and contamination.

Preferably, the plurality of first coils and the plurality of second coils each have a shape formed by arranging at least one pair of coil elements having a same number of turns and wound in opposite directions on a same plane and by connecting the coil elements in series.

Each first coil and each second coil receive a magnetic flux generated by the power transmission coil during power transmission from the power transmission coil to the power reception coil. Since this magnetic flux penetrates the coil elements in the same direction, the induced voltages generated in the coil elements are canceled by each other by employing the above configuration. According to this power transmission device, therefore, the generation of induced voltage in the foreign object detector during power transmission from the power transmission coil to the power reception coil can be suppressed. It is noted that the at least one pair of coil elements includes one pair of coils forming a figure of eight, two pairs of coil elements forming a shape of petals, and the like.

Preferably, the foreign object detector further includes a switching device. The switching device is configured to switch among pairs of coils, the pairs each including one of the plurality of first coils to which an AC voltage for detection is applied (hereinafter referred to as an “energized coil”) and one of the second coils corresponding to the energized coil. The power transmission device further includes a control device. The control device controls the switching device so as to successively switch among the pairs each including the energized coil and the second coil corresponding to the energized coil, and determines presence or absence of a foreign object based on a power receiving state of the second coil corresponding to the energized coil.

With this configuration, a small foreign object can be detected over a broad range where the plurality of first coils and the plurality of second coils are arranged. It is noted that the power receiving state of the second coil indicates at least one of a receiving voltage, a receiving current and receiving power of the second coil.

More preferably, a resonant frequency of each of a first resonant circuit including the energized coil and a second resonant circuit including the second coil corresponding to the energized coil is higher than a frequency of an AC voltage supplied to the power transmission coil. More preferably, the resonant frequency is different from a harmonic frequency of the AC voltage supplied to the power transmission coil.

With this configuration, the effect of the magnetic field formed by the power transmission coil on the foreign object detector can be suppressed.

According to the present invention, a power reception device that receives electric power output from a power transmission device includes a power reception coil and a foreign object detector. The power reception coil is configured to receive electric power from a power transmission coil of the power transmission device in a contactless manner. The foreign object detector is provided below the power reception coil, with the power reception coil being configured to receive electric power from the power transmission coil located below the power reception coil. The foreign object detector includes a plurality of first coils and a plurality of second coils. The plurality of first coils are arranged along a lower surface of the power reception coil. The plurality of second coils are arranged along the lower surface of the power reception coil above or below the plurality of first coils, and provided to correspond to the plurality of first coils. The plurality of second coils are each arranged to face a corresponding one of the first coils. An outer shape of each of the plurality of first coils and of each of the plurality of second coils is smaller than an outer shape of the power reception coil when the power reception coil, the plurality of first coils and the plurality of second coils are viewed two-dimensionally from below the power reception coil.

In this power reception device, the plurality of first coils and the plurality of second coils are provided as a set in the power reception device. Consequently, the distance between the first coils and the corresponding second coils can be reduced to decrease the size of the first coils and the second coils. In addition, according to this power reception device, the outer shape of each first coil and each second coil is smaller than the outer shape of the power transmission coil, allowing the foreign object detector to detect a relatively small foreign object compared to the power reception coil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detail with reference to the drawings, in which the same or corresponding parts are designated by the same reference characters and description thereof will not be repeated. Although a plurality of embodiments will be described below, it is originally intended to appropriately combine configurations described in the embodiments. Unless otherwise specified, the scope of the present invention is not necessarily limited to numbers, amounts and the like mentioned.

First Embodiment

FIG. 1is a schematic configuration diagram of a contactless electric power transfer system to which a power transmission device according to a first embodiment of the present invention is applied. In the figure, an arrow D indicates a vertically downward direction, an arrow U indicates a vertically upward direction, an arrow F indicates a vehicle forward direction, and an arrow B indicates a vehicle backward direction. These directions also apply toFIGS. 2, 3, 8 and 12which will be described later.

Referring toFIG. 1, a contactless electric power transfer system1000includes a vehicle100and a power transmission device300. Vehicle100includes a vehicle body110and a power reception unit200. Vehicle body110includes a vehicle ECU (Electronic Control Unit)120, a rectifier130, a DC/DC converter140, a power storage device150, a power control unit160, a drive unit170, and a communication unit180.

Power reception unit200is configured to receive electric power from a power transmission unit400of power transmission device300in a contactless manner while facing power transmission unit400. Power reception unit200includes a power reception coil having a core with a conducting wire wound therearound, and a resonant capacitor (neither shown). The number of turns of the conducting wire of the power reception coil is appropriately designed in consideration of the distance between the power reception coil and a power transmission coil of power transmission unit400, and so as to increase a Q factor (for example, Q≥100) showing the intensity of resonance between the power reception coil and the power transmission coil and a coupling coefficient κ showing the degree of coupling therebetween.

Power transmission device300includes a high-frequency power device310, a power transmission ECU320, a communication unit322, power transmission unit400, and a control device485. High-frequency power device310is connected to an AC power supply330. Power transmission unit400includes the power transmission coil having a core with a conducting wire wound therearound, and a resonant capacitor (neither shown). It is intended that a coil winding axis O1of the power reception coil of power reception unit200and a coil winding axis O2of the power transmission coil of power transmission unit400be parallel with each other when vehicle100is parked in a position where electric power can be transferred from power transmission unit400to power reception unit200of vehicle100.

The power transmission coil of power transmission unit400forms a magnetic field when high-frequency power (an AC voltage) is applied thereto from high-frequency power device310, and transmits electric power to the power reception coil of power reception unit200in a contactless manner through the formed magnetic field. The number of turns of the conducting wire of the power transmission coil is also appropriately designed in consideration of the distance between the power transmission coil and the power reception coil, and so as to increase the Q factor (for example, Q≥100) and the coupling coefficient κ. Control device485will be described later with reference toFIG. 4.

FIG. 2is an exploded perspective view of power transmission unit400shown inFIG. 1. Referring toFIG. 2, power transmission unit400includes a power transmission coil410, a resonant capacitor420, a housing430, and a foreign object detector460. Power transmission coil410includes a core440, and a conducting wire450wound around core440. As an example, core440is made of ferrite, and has lengths L1and L2of about 400 mm. Power transmission coil410and resonant capacitor420are contained in housing430. Housing430includes a shield432and a cover member434.

When there is a foreign object (an object that should not be present) between power transmission unit400and power reception unit200, the foreign object generates heat or power transfer efficiency is lowered during power transfer from power transmission coil410to the power reception coil of power reception unit200. When there is a foreign object between power transmission unit400and power reception unit200(FIG. 1), foreign object detector460detects such an object. Possible foreign objects include, for example, a piece of metal such as a beverage can or a coin, an animal, and the like.

Foreign object detector460includes a plurality of first coils468and a plurality of second coils478. The plurality of first coils468and the plurality of second coils478are provided above power transmission coil410, and arranged on an inner surface of cover member434of housing430in the first embodiment. The plurality of second coils478are provided to correspond to the plurality of first coils468, and first coils468and second coils478each have the same size and shape. Each of second coils478is arranged to face a corresponding one of first coils468, and forms a pair of coils together with the corresponding first coil468. A plurality of pairs of coils corresponding to the number of the plurality of first coils468(the plurality of second coils478) are arranged in a matrix on the inner surface of cover member434.

That is, the plurality of pairs of coils are arranged in a matrix along an upper surface of power transmission coil410above power transmission coil410. An outer shape (L3×L4) of each pair of coils (each first coil468and each second coil478) is smaller than an outer shape (L1×L2) of power transmission coil410. With the pairs of coils of this size, when there is a small foreign object that cannot be detected based on a change in power receiving state of the power reception coil (for example, a reduction in induced voltage generated in the power reception coil) between power transmission unit400and power reception unit200, the foreign object can be detected.

More specifically, when there is a foreign object between power transmission unit400and power reception unit200, the foreign object may be detected during power transmission from power transmission coil410to the power reception coil of power reception unit200by detecting a change in power receiving state of the power reception coil (for example, a reduction in induced voltage) which exhibits itself as a change (reduction) in coupling coefficient between power transmission coil410and the power reception coil. However, if the foreign object has a small size relative to power transmission coil410and the power reception coil, the effect of the foreign object on the coupling coefficient between power transmission coil410and the power reception coil is small, resulting in inability to detect the foreign object based on a change in power receiving state of the power reception coil.

In order to detect such a small foreign object, a pair of small coils needs to be used for foreign object detection separately from power transmission coil410and the power reception coil. By using the pair of small coils, the effect of the foreign object on the coupling coefficient between a coil on the power transmission side and a coil on the power reception side is relatively increased, allowing the detection of even a small foreign object based on a change in power receiving state of the coil on the power reception side. When the coil size is reduced, however, the distance between the coil on the power reception side and the coil on the power transmission side needs to be reduced so as to couple the coils together. In power transmission device300according to the first embodiment, therefore, the plurality of first coils468and the plurality of second coils478which are arranged to face each other in proximity to each other are provided as a set in power transmission device300above power transmission coil410, allowing the detection of even a small foreign object that has entered between power transmission unit400and power reception unit200.

Although a foreign object will be present above the pair of coils, not between first coil468and second coil478forming the pair of coils, the coupling coefficient between first coil468and second coil478still changes due to the presence of the foreign object to thereby change the power receiving state of second coil478, thereby allowing the detection of the foreign object. It is noted that the power receiving state of second coil478is typically an induced voltage generated in second coil478, but may be an induced current, induced power or the like generated in second coil478.

FIG. 3is a perspective view of a pair of coils formed of first coil468and second coil478. Referring toFIG. 3, first coil468and second coil478each have a rectangular shape. Second coil478is arranged to face corresponding first coil468. A distance L5between first coil468and second coil478is small relative to the outer shape (L3×L4) of first coil468and second coil478. As an example, lengths L3and L4are 20 mm, and distance L5is several mm. Such an arrangement of a pair of coils can be readily fabricated, for example, by patterning metal interconnect lines on opposite surfaces of a print substrate.

When an AC voltage for detection is applied to first coil468, first coil468forms a magnetic field for detection AR1. Consequently, an induced voltage is generated by magnetic field for detection AR1in second coil478arranged to face first coil468. Here, when there is a foreign object in the vicinity of the pair of coils, magnetic field for detection AR1is affected by the foreign object to change the coupling coefficient between first coil468and second coil478, causing a change in power receiving state of second coil478. The presence or absence of a foreign object is determined based on this change in power receiving state.

It is noted that the vertical positional relationship between first coil468and second coil478is not particularly limited. Second coil478may be arranged above first coil468(U direction) as shown, or may be arranged below first coil468(D direction).

FIG. 4is a diagram showing an electrical configuration of foreign object detector460shown inFIG. 2. Referring toFIG. 4, foreign object detector460includes, in addition to the plurality of first coils468and the plurality of second coils478, an oscillator461, a power amplifier462, a resonant capacitor463, multiplexers464,465, and a plurality of common interconnect lines466,467. Foreign object detector460further includes a signal processing circuit471, a resonant capacitor472, a resonant resistor473, multiplexers474,475, and a plurality of common interconnect lines476,477.

The plurality of first coils468and the plurality of second coils478are shown as being away from each other for purposes of illustration, where in reality, each of second coils478is arranged to face a corresponding one of first coils468, and forms a pair of coils together with the corresponding first coil468, as was described with reference toFIG. 2. Although the first embodiment describes a configuration where sixteen pairs of coils are provided as an example, the number of pairs of coils is not limited thereto.

In the power transmission device according to the first embodiment, multiplexers464,465,474and475function as a “switching device.” This switching device successively switches among pairs (pairs of coils) each including one of the plurality of first coils468to which the AC voltage for detection is applied and one of second coils478corresponding to this first coil468to which the AC voltage for detection is applied. This will be specifically described below.

Oscillator461generates a signal having an arbitrary frequency (for example, 13.56 MHz), and the signal is amplified by power amplifier462. An AC voltage for foreign object detection which is output from power amplifier462is input to multiplexer464through resonant capacitor463. Multiplexer464is connected to resonant capacitor463, control device485and four common interconnect lines466. On the other hand, multiplexer465is connected to a ground line, control device485and four common interconnect lines467.

Each common interconnect line466is connected to one terminal of each of four first coils468. Each common interconnect line467is connected to the other terminal of each of four first coils468. Multiplexer464outputs an AC voltage to any one of four common interconnect lines466in accordance with a switching command from control device485. Multiplexer465establishes electrical continuity between any one of four common interconnect lines467and the ground line in accordance with a switching command from control device485.

The plurality of first coils468are arranged in a 4×4 matrix. By multiplexer464and multiplexer465, the AC voltage from multiplexer464is applied to any one of sixteen first coils468at certain timing. The one of first coils468to which the AC voltage is to be applied is determined in accordance with the switching commands provided to multiplexers464and465from control device485. Each first coil468forms a magnetic field for detection when the AC voltage for detection is applied thereto.

On the other hand, the plurality of second coils478are also arranged in a 4×4 matrix. Multiplexer474is connected to resonant capacitor472, control device485and four common interconnect lines476. Multiplexer475is connected to the ground line, control device485and four common interconnect lines477. Each common interconnect line476is connected to one terminal of each of four second coils478. Each common interconnect line477is connected to the other terminal of each of four second coils478. Signal processing circuit471is connected to resonant capacitor472and resonant resistor473. It is noted that resonant resistor473is provided so as to realize robust power transfer with respect to a shift in frequency.

When an AC voltage is applied to any one of sixteen first coils468, this first coil468forms a magnetic field for detection. Second coil478arranged to face this first coil468generates an induced voltage by the magnetic field for detection formed by this first coil468. This second coil478is specified in advance in accordance with switching commands transmitted to multiplexers474and475from control device485.

Here, for each of a capacitance C1of resonant capacitor463and an inductance L1of each first coil468, these circuit constants are predetermined to satisfy, regarding an oscillating frequency f1of oscillator461, f1=1/(2π×(L1×C1)1/2). Likewise, for a capacitance C2of resonant capacitor472and an inductance L2of each second coil478, these circuit constants are predetermined so as to satisfy f1=1/(2π×(L2×C2)1/2). That is, a resonant circuit formed of first coil468and resonant capacitor463as well as a resonant circuit formed of second coil478and resonant capacitor472are each designed such that its resonant frequency matches oscillating frequency f1of oscillator461.

The induced voltage generated in second coil478corresponding to first coil468to which the AC voltage for detection has been applied is transmitted through multiplexer474, resonant capacitor472and resonant resistor473to signal processing circuit471(for example, an AC/DC circuit). Signal processing circuit471converts the voltage received from resonant capacitor472into a signal suitable for reception by control device485, and outputs the signal to control device485.

Control device485determines the presence or absence of a foreign object based on the signal from signal processing circuit471, that is, based on the power receiving state of second coil478corresponding to first coil468to which the AC voltage for detection has been applied. When there is a foreign object near first coil468and second coil478, a magnetic field formed around first coil468and second coil478links the foreign object to thereby change the power receiving state of second coil478. Control device485has a comparison value in advance, which corresponds to the power receiving state of second coil478without a foreign object, and determines that there is a foreign object when the level of the signal received from signal processing circuit471is lower than this comparison value.

The foreign object detection by foreign object detector460is basically performed before the start of power transmission from power transmission unit400to power reception unit200, however, the detection can also be performed during the power transmission from power transmission unit400to power reception unit200. During the power transmission from power transmission unit400to power reception unit200, each first coil468and each second coil478are exposed to the magnetic field formed by power transmission coil410. Accordingly, the resonant frequency of the above resonant circuits formed in foreign object detector460should be set to a value different from that of the frequency of the magnetic field formed by power transmission coil410. That is, the resonant frequency (oscillating frequency f1of oscillator461) should be set to a value different from that of a frequency f2of the AC voltage supplied to power transmission unit400from high-frequency power device310.

Preferably, frequency f1should be set to a value significantly higher (for example, at least 30 times) than that of frequency f2, the value also being shifted from the frequency of the harmonic of frequency f2. By setting frequency f1in this manner, the generation of induced voltage in first coil468and second coil478caused by the magnetic field formed by power transmission coil410can be suppressed during power transmission from power transmission unit400to power reception unit200, thereby realizing highly accurate detection of a foreign object. As an example, when frequency f2is 85 kHz, frequency f1may be set to 13.56 MHz.

As has been described, in the first embodiment, foreign object detector460includes the plurality of first coils468and the plurality of second coils478, with the plurality of first coils468and the plurality of second coils478being provided as a set in power transmission unit400. Consequently, the distance between first coils468and corresponding second coils478can be reduced to decrease the size of first coils468and second coils478. In addition, according to the first embodiment, the outer shape of each first coil468and each second coil478is smaller than the outer shape of power transmission coil410, allowing foreign object detector460to detect a relatively small foreign object compared to power transmission coil410.

According to the first embodiment, the plurality of first coils468and the plurality of second coils478are arranged in a matrix within housing430on the upper surface of the housing (on the inner surface of cover member434), allowing the detection of a small foreign object on power transmission unit400. Further, the plurality of first coils468and the plurality of second coils478can be protected against breakage and contamination.

In the first embodiment, the use of multiplexers464,465,474and475provides partial commonality of the interconnect lines to suppress an increase in the amount of interconnect lines. According to the first embodiment, therefore, the number of first coils468and second coils478arranged can be increased to improve the accuracy of foreign object detection. From another perspective, an interconnect line is a member that tends to function as a metal shield, and an increase in the amount of interconnect lines may cause lowering of the power transfer efficiency. According to the first embodiment, an increase in the amount of interconnect lines can be suppressed to prevent the lowering of the power transfer efficiency.

In the first embodiment, the resonant frequency of the resonant circuits formed in foreign object detector460is higher than the frequency of the AC voltage supplied to power transmission coil410. More preferably, the resonant frequency is different from the harmonic frequency of the AC voltage supplied to power transmission coil410. With this configuration, according to the first embodiment, the effect of the magnetic field formed by power transmission coil410on foreign object detector460can be suppressed.

Variation of First Embodiment

Although first coils468and second coils478have a rectangular shape in the first embodiment above (FIG. 3), first coils468and second coils478may have a triangular shape as shown inFIG. 5. Alternatively, as shown inFIG. 6, first coils468and second coils478may have another polygonal shape such as a hexagonal shape. According to the coils having a polygonal shape, the detection sensitivities can be rendered uniform by making the coils to have the same size and shape.

Alternatively, as shown inFIG. 7, first coils468and second coils478each including a coil portion having a circular outer shape may be employed. InFIG. 7, a plurality of pairs of coils formed of first coils468and second coils478are concentrically arranged.

Second Embodiment

In first coils468and second coils478shown inFIGS. 3, and 5 to 7, an induced voltage is generated under the influence of the magnetic field formed by power transmission coil410, which may result in lowering of the power transfer efficiency between power transmission coil410and power reception unit200. For this reason, the first embodiment described preferably setting the resonant frequency of the resonant circuits formed in foreign object detector460to be different from the frequency (including the harmonic) of the AC voltage supplied to power transmission coil410.

In a second embodiment, a coil having a shape less likely to be affected by the magnetic field formed by power transmission coil410is employed for each coil forming the foreign object detector.

FIG. 8is a perspective view of a pair of coils used in the foreign object detector in the second embodiment. Referring toFIG. 8, in the second embodiment, a pair of coils formed of a first coil468A and a second coil478A is used instead of the pair of coils in the first embodiment shown inFIG. 3.

First coil468A has a shape formed by arranging a pair of coil elements468B and468C having the same number of turns and wound in opposite directions on the same plane and connecting coil elements468B and468C in series (a figure of eight). Second coil478A has a similar coil shape to first coil468A, and is arranged to face corresponding first coil468A.

When an AC voltage for detection is applied to first coil468A, first coil468A forms a magnetic field for detection AR2. Consequently, an induced voltage is generated by magnetic field for detection AR2in second coil478A arranged to face first coil468A. While power transfer is being carried out between power transmission unit400and power reception unit200, the coil elements each receive a magnetic flux (an arrow AR3) in the same direction and in the same amount. Although an induced voltage may be generated in each coil element by this magnetic flux AR3, the induced voltage generated in coil element468B is canceled by the induced voltage generated in coil element468C, leading to no generation of induced voltage in total in first coil468A. The same applies to second coil478A. Even if second coil478A receives magnetic flux AR3, there is no generation of induced voltage in total in second coil478A.

Therefore, according to the second embodiment having such a coil shape, the effect of the magnetic field formed by power transmission coil410during power transfer from power transmission unit400to power reception unit200on the foreign object detector can be suppressed. By suppressing the generation of induced voltage by the magnetic field caused by the power transfer, the entry of noise into the value of the induced voltage caused by the magnetic field for detection output from second coil478A can be suppressed, thereby improving the detection sensitivity.

It is desirable that coil elements468B and468C (coil elements478B and478C) each receive the magnetic flux from power transmission coil410in amounts as equal as possible. It is thus preferable to arrange the pair of coils such that coil elements468B and468C (coil elements478B and478C) are aligned along a direction in which conducting wire450(FIG. 2) of power transmission coil410extends (a vehicle width direction).

Variation of Second Embodiment

Although first coil468A and second coil478A each have the figure of eight by a pair of coil elements in the second embodiment above, the first coil and the second coil may each have a shape of petals by two pairs of coil elements as shown inFIG. 9. That is, a first coil468D, which is an alternative to first coil468A, is formed of a pair of coil elements468E and468F having the same number of turns and wound in opposite directions, and a pair of coil elements468G and468H having the same number of turns and wound in opposite directions. Coil elements468E to468H have a shape formed by arranging coil elements468E to468H on the same plane and successively connecting them in series. A second coil478D, which is an alternative to second coil478A, has the same configuration as first coil468D.

Alternatively, as shown in a plan view ofFIG. 10and a cross-sectional view ofFIG. 11, a pair of coils may be formed of a first coil468I and a second coil478I instead of first coil468A and second coil478A, with a distance L6provided between a coil element468J (478J) and a coil element468K (478K). By increasing distance L6between the coil elements, a magnetic field for detection AR4formed by the application of an AC voltage for detection to first coil468I can be increased to thereby improve the detection accuracy.

Third Embodiment

Although the foreign object detector is provided in power transmission device300in each of the embodiments above, the foreign object detector may be provided in the vehicle (the power reception device).

FIG. 12is an exploded perspective view of a power reception unit200A mounted on the vehicle in a third embodiment. Referring toFIG. 12, power reception unit200A includes a power reception coil210, a resonant capacitor220, a housing230, and a foreign object detector260. Power reception coil210includes a core240, and a conducting wire250wound around core240. Housing230includes a shield232and a cover member234.

Foreign object detector260includes a plurality of first coils268and a plurality of second coils278. The plurality of first coils268and the plurality of second coils278are provided below power reception coil210, and arranged on an inner surface of cover member234of housing230in the third embodiment. The plurality of second coils278are provided to correspond to the plurality of first coils268, and first coils268and second coils278each have the same size and shape. Each of second coils278is arranged to face a corresponding one of first coils268, and forms a pair of coils together with the corresponding first coil268. A plurality of pairs of coils corresponding to the number of the plurality of first coils268(the plurality of second coils278) are arranged in a matrix on the inner surface of cover member234.

The plurality of pairs of coils are arranged in a matrix along a lower surface of power reception coil210below power reception coil210. An outer shape (L3×L4) of each pair of coils (each first coil268and each second coil278) is smaller than an outer shape (L1×L2) of power reception coil210. With the pairs of coils of this size, when there is a small foreign object that cannot be detected based on a change in power receiving state of power reception coil210(for example, a reduction in induced voltage generated in power reception coil210) between power transmission unit400and power reception unit200, the foreign object can be detected.

Although not particularly shown, first coils268and second coils278can be formed of coils having the shapes such as shown inFIGS. 5 to 11in the third embodiment as well.

A similar effect to that of each of the embodiments above can be achieved in the third embodiment as well.

It is noted that the plurality of first coils and the plurality of second coils are not necessarily required to be arranged in a matrix in each of the embodiments above. With the matrix arrangement, multiplexers can be employed to reduce the number of interconnect lines.

Although foreign object detector460is provided on the inner surface of housing430of power transmission unit400(the inner surface of cover member434) or on the inner surface of housing230of power reception unit200(the inner surface of cover member234) so as to protect foreign object detector460against breakage and contamination in the above description, this arrangement is not a requirement, and foreign object detector460may be provided outside of the housing.

Although power transmission coil410and power reception coil210are each configured such that its coil winding axis extends in a front-back direction of the vehicle in the above description, the configuration of each coil is not limited thereto. For example, a helical coil having a coil winding axis extending in a vertical direction of the vehicle, or a spiral coil having a coil winding axis extending in the vertical direction of the vehicle may be employed as each of power transmission coil410and power reception coil210.

It is intended to appropriately combine and practice the embodiments disclosed herein. It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.