Vehicle and wireless power transmission system

A vehicle to be driven by electric power which is wirelessly transmitted from a power transmitter having two transmission electrodes includes: two reception electrodes to receive AC power from the two transmission electrodes through capacitive coupling respectively with the two transmission electrodes; a power receiving circuit which is connected to the two reception electrodes to convert AC power received by the two reception electrodes into DC power or another form of AC power, and supply the DC power or other form of AC power to an electric motor which drives the vehicle; and a control circuit which, while the vehicle is traveling over the two transmission electrodes, increases an impedance of the vehicle as viewed from the power transmitter when a value of at least one of power, voltage and current in the power receiving circuit, or a change rate over time thereof, exceeds a threshold value.

BACKGROUND

1. Field of the Invention

The present disclosure relates to a vehicle that is driven by electric power which is wirelessly transmitted thereto, and relates also to a wireless power transmission system.

2. Description of the Related Art

In recent years, wireless power transmission techniques for wirelessly (contactlessly) transmitting electric power to devices that are capable of moving or being moved, e.g., mobile phones and electric vehicles, have been being developed. Wireless power transmission techniques include methods based on electromagnetic induction and methods based on electric field coupling. Among these, a wireless power transmission system based on the electric field coupling method is such that, while a pair of transmission electrodes and a pair of reception electrodes are opposed to each other, AC power is wirelessly transmitted from the pair of transmission electrodes to the pair of reception electrodes. A wireless power transmission system based on such an electric field coupling method may be used for the purpose of transmitting electric power to a load (e.g., a motor or a battery in a vehicle such as a mobile robot) from a pair of transmission electrodes that are provided on the road surface (or on the floor surface). International Publication No. 2015/037526 (hereinafter “Patent Document 1”) discloses an example of a wireless power transmission system based on such an electric field coupling method.

SUMMARY

In a wireless power transmission system based on the electric field coupling method, when a vehicle travels near a pair of transmission electrodes that are provided on the road surface, etc., electric power is transmitted from the transmission electrodes to the vehicle. However, if the power transmitter has already been transmitting electric power to another (“first”) vehicle, the efficiency of power transmission to this first vehicle will decrease as a second vehicle travels near the transmission electrode.

The present disclosure provides a novel technique which, even if a second vehicle happens to travel near transmission electrodes, does not allow much decrease in the efficiency of power transmission to a first vehicle which has already been being charged.

In order to solve the above problems, a vehicle according to one implementation of the present disclosure, to be driven by electric power which is wirelessly transmitted from a power transmitter having two transmission electrodes, comprises:

two reception electrodes to receive AC power from the two transmission electrodes through capacitive coupling respectively with the two transmission electrodes;

a power receiving circuit which is connected to the two reception electrodes to convert AC power received by the two reception electrodes into DC power or another form of AC power, and supply the DC power or other form of AC power to an electric motor which drives the vehicle or to a secondary battery which stores electric power for driving the vehicle; and

a first control circuit which, while the vehicle is moving with the two reception electrodes being opposed to the two transmission electrodes, increases an impedance of the vehicle as viewed from the power transmitter in response to an instruction that the electric power from the power transmitter is not to be received.

General or specific aspects of the present disclosure may be implemented using a system, a method, an integrated circuit, a computer program, or a storage medium, or any combination of a system, an apparatus, a method, an integrated circuit, a computer program, and/or a storage medium.

With the technique according to the present disclosure, it is possible to suppress decrease in the efficiency of power transmission from the transmission electrodes to a first vehicle even if a second vehicle happens to travel near the transmission electrodes.

DETAILED DESCRIPTION

Findings Providing the Basis of the Present Disclosure

Prior to describing embodiments of the present disclosure, findings providing the basis of the present disclosure will be described.

FIG. 1is a diagram schematically showing an example of a wireless power transmission system based on the electric field coupling method as conceived by the inventors. The wireless power transmission system shown in the figure may be a system which wirelessly transmits electric power to a transport robot (automated guided vehicle: AGV)10that is used in transporting articles in a factory, for example. The transport robot10is an example of a vehicle according to the present disclosure. In this system, a pair of transmission electrodes120aand120b, which are in plate shape, are disposed on the road surface (floor surface)30. The transport robot10includes a pair of reception electrodes (not shown) opposing the pair of transmission electrodes120aand120b. With the pair of reception electrodes, the transport robot10receives AC power which has been transmitted from the transmission electrodes120aand120b. The received electric power is supplied to a load in the transport robot10, e.g., a motor, a secondary battery, or a capacitor for electrical storage purposes. With this, the transport robot10may be driven or charged.

FIG. 1shows XYZ coordinates indicating the X, Y and Z directions which are orthogonal to one another. The following description will rely on XYZ coordinates as shown in the figures. The direction that the transmission electrodes120aand120bextend will be referred to as the Y direction; a direction which is perpendicular to the surface of the transmission electrodes120aand120bas the Z direction; and a direction which is perpendicular to the Y direction and the Z direction as the X direction. Note that the orientation of any structure that is shown in a drawing of the present application is so set for ease of description, and it shall not limit the orientation in which an embodiment of the present disclosure may actually be employed. Moreover, the particular shape and size with which the whole or a part of any structure may be presented in a drawing shall not limit its actual shape and size.

FIG. 2is a diagram showing a general construction for the wireless power transmission system shown inFIG. 1. This wireless power transmission system includes a power transmitter100and a transport robot (vehicle)10. The power transmitter100includes a pair of transmission electrodes120aand120b, and a power transmitting circuit110which supplies AC power to the transmission electrodes120aand120b. The power transmitting circuit110is, for example, an AC output circuit including an inverter circuit. The power transmitting circuit110converts DC power which is supplied from a DC power source not shown into AC power, and outputs it to the pair of transmission electrodes120aand120b.

The transport robot10includes a power receiver200and a load330. The power receiver200includes a pair of reception electrodes220aand220band a power receiving circuit210which converts the AC power that is received by the reception electrodes220aand220binto electric power as desired by the load330(e.g., DC voltage of a predetermined voltage or AC power of a predetermined frequency) and supplies it to the load330. The power receiving circuit210may include various circuits such as a rectifier circuit or a frequency conversion circuit. The load330may be any device that consumes or stores electric power, e.g., a motor, a capacitor for electrical storage purposes, or a secondary battery. Through electric field coupling (capacitive coupling) between the pair of transmission electrodes120aand120band the pair of reception electrodes220aand220b, electric power is wirelessly transmitted while the two pairs are opposed to each other.

With such a wireless power transmission system, the transport robot10is able to wirelessly receive electric power while moving along the transmission electrodes120aand120b. While the transmission electrodes120aand120band the reception electrodes220aand220bremain in a closely opposed state, the transport robot10moves in a direction that the transmission electrodes120aand120bextend (i.e., the Y direction inFIG. 1). As a result, the transport robot is able to move while allowing a means of electrical storage, e.g., a capacitor, to be charged.

However, the following problem may occur when a plurality of vehicles (e.g., transport robots) simultaneously move and are simultaneously charged in such a wireless power transmission system. While a first vehicle is being charged, if a second vehicle travels over the transmission electrodes120aand120b, part of the energy that has been sent out from the transmission electrodes120aand120bwill be received by the second vehicle, thereby lowering the efficiency of power transmission to the first vehicle.

The inventors have found the aforementioned problem, and sought for constructions for solving this problem. Consequently, the inventors have succeeded in solving the above problem based on embodiments of the present disclosure as will be described below.

A vehicle according to one implementation of the present disclosure is a vehicle to be driven by electric power which is wirelessly transmitted from a power transmitter having two transmission electrodes, the vehicle comprising:

two reception electrodes to receive AC power from the two transmission electrodes through capacitive coupling respectively with the two transmission electrodes;

a power receiving circuit which is connected to the two reception electrodes to convert AC power received by the two reception electrodes into DC power or another form of AC power, and supply the DC power or other form of AC power to an electric motor which drives the vehicle or to a secondary battery which stores electric power for driving the vehicle; and

a first control circuit which, while the vehicle is moving with the two reception electrodes being opposed to the two transmission electrodes, increases an impedance of the vehicle as viewed from the power transmitter in response to an instruction that the electric power from the power transmitter is not to be received.

In the above implementation, the vehicle includes a first control circuit which, while the vehicle is traveling near the two transmission electrodes, increases the impedance of the vehicle as viewed from the power transmitter in response to an instruction from the power transmitter that the electric power is not to be received.

As a result, even if the vehicle travels near the transmission electrodes, decrease in the efficiency of power transmission from the transmission electrodes to another vehicle is suppressed.

The vehicle may further comprise a second control circuit which acquires information on at least one of a status of electric power transmission from the power transmitter to the vehicle, location of the vehicle, and remaining power of the secondary battery, determines whether or not electric power from the power transmitter is to be received based on the information, and sends the instruction to the first control circuit when determining that the electric power is not to be received.

By comprising the second control circuit, the vehicle in itself is able to determine whether electric power is to be received or not, and when necessary, increases the impedance of the vehicle as viewed from the power transmitter. As a result, even if the vehicle travels near the transmission electrodes, decrease in the efficiency of power transmission from the transmission electrodes to another vehicle is suppressed.

The “status of electric power transmission from the power transmitter to the vehicle” may include, for example, information on a value of at least one of power, voltage and current in the power receiving circuit of the vehicle, or a change rate over time thereof.

The second control circuit may send the instruction to the first control circuit when a value of at least one of power, voltage and current in the power receiving circuit, or a change rate over time thereof, exceeds a threshold value.

In one embodiment, the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, is greater than the threshold value when the power transmitter is transmitting power to another vehicle distinct from the vehicle. The value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, is not greater than the threshold value when the power transmitter is not transmitting power to another vehicle distinct from the vehicle.

While the two reception electrodes are opposed respectively to the two transmission electrodes, if a predetermined amount of time has elapsed before the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, exceeds the threshold value, the first control circuit may request the power transmitter for power transmission.

When a predetermined amount of time has elapsed before the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, exceeds a threshold value, it is presumable that no other vehicle exists which is receiving electric power supplied from the power transmitter. Therefore, in such a case, the first control circuit may request the power transmitter for power transmission. In response to this request, the power transmitter may increase the transmission power.

The present disclosure encompasses a wireless power transmission system (also referred to as a “vehicle system”) that includes a power transmitter, one or more vehicles, and a central controller which controls the power transmitter and the one or more vehicles. The central controller performs wireless communication between the power transmitter and the one or more vehicles, thereby controlling them. The vehicle may perform the aforementioned impedance control in response to an instruction from the central controller.

In other words, upon receiving the aforementioned instruction from the central controller which controls the vehicle(s) and the power transmitter, the first control circuit may increase the impedance of the vehicle as viewed from the power transmitter.

In one embodiment, the central controller acquires information on at least one of a status of electric power transmission from the power transmitter to the vehicle, location of the vehicle, and remaining power of the secondary battery, determines whether or not the vehicle is to receive the electric power from the power transmitter based on the information, and when determining that the electric power from the power transmitter is not to be received from the vehicle, sends the instruction to the first control circuit.

In such an embodiment, from the power transmitter and/or the vehicle, the central controller may acquire information on at least one of a status of electric power transmission, location of the vehicle, and remaining power of the secondary battery, via wireless communication, for example.

In the present disclosure, a “vehicle” is not limited to a wheeled vehicle such as the aforementioned transport robot, but encompasses any movable object that is driven by electric power. Examples of vehicles may include an electric vehicle that includes an electric motor and one or more wheels. Such a vehicle may be an automated guided vehicle (AGV) such as the aforementioned transport robot, an electric vehicle (Electric Vehicle: EV), or an electric cart, for example. The “vehicle” within the meaning of the present disclosure also encompasses any movable object that lacks wheels. For example, bipedal robots, unmanned aerial vehicles (UAV, or so-called drones) such as multicopters, and manned electric aircraft are also examples of “vehicles”.

Hereinafter, more specific embodiments of the present disclosure will be described. Note however that unnecessarily detailed descriptions may be omitted. For example, detailed descriptions on what is well known in the art or redundant descriptions on what is substantially the same construction may be omitted. This is to avoid lengthy description, and facilitate the understanding of those skilled in the art. The accompanying drawings and the following description, which are provided by the present inventors so that those skilled in the art can sufficiently understand the present disclosure, are not intended to limit the scope of claims. In the following description, identical or similar constituent elements are denoted by identical reference numerals.

First, an illustrative first embodiment of the present disclosure will be described. In the present embodiment, when a status of electric power transmission from a power transmitter to a vehicle matches a predetermined condition, the vehicle determines that electric power from the power transmitter is not to be received. More specifically, when a value of at least one of power, voltage and current in a power receiving circuit, or a change rate over time thereof, exceeds a threshold value, the vehicle increases the impedance of the vehicle as viewed from the power transmitter, so that electric power from the power transmitter will not be received. This suppresses decrease in the efficiency of power transmission to another vehicle.

FIG. 3is a diagram showing a construction for a power transmitter100and a transport robot (as one example of a vehicle)10in a wireless power transmission system according to an embodiment of the present disclosure.

The power transmitter100includes a power transmitting circuit110, a pair of transmission electrodes120aand120b, a transmission detector190, and a power transmission control circuit150. The external AC power source20supplies AC power to the power transmitting circuit110. The power transmitting circuit110may include a converter circuit and an inverter circuit. The AC power which is supplied to the power transmitting circuit110is converted by the converter circuit into DC power. Thereafter, the DC power is converted by the inverter circuit into another form of AC power. The pair of transmission electrodes120aand120bare connected to the power transmitting circuit110. The pair of transmission electrodes120aand120bwirelessly transmit the AC power which is output from the power transmitting circuit110. The transmission detector190detects a power, voltage, current, or the like at a specific site in the power transmitting circuit110. In the present embodiment, as one example, the transmission detector190detects a current which is output from the inverter circuit in the power transmitting circuit110. The transmission detector190sends data representing the detected value thereof to the power transmission control circuit150. The power transmission control circuit150sends a command based on this information to the power transmitting circuit110.

On the other hand, the transport robot10includes a power receiving circuit210, a pair of reception electrodes220aand220b, a reception detector290, a power reception control circuit250, an electric motor330(which may hereinafter be simply referred to as the “motor330”), a motor control circuit340, a charge-discharge control circuit350, and a secondary battery320(which may hereinafter be simply referred to as the “battery320”). The pair of reception electrodes220aand220bestablish capacitive coupling with their respective counterparts in the pair of transmission electrodes120aand120b. The pair of reception electrodes220aand220breceive AC power from the pair of transmission electrodes120aand120b. The power receiving circuit210is connected to the pair of reception electrodes220aand220b. The power receiving circuit210may include a rectifier circuit, with which the power receiving circuit210converts the AC power which is received by the pair of reception electrodes220aand220binto DC power. The power receiving circuit210may convert the received AC power into another form of AC power.

The power receiving circuit210supplies the converted electric power to the electric motor330and/or the battery320, which drive the transport robot10. Charging and discharging of the battery320is controlled by the charge-discharge control circuit350. The reception detector290detects power, voltage, current, or the like at a specific site in the power receiving circuit210(e.g., immediately after a rectifier circuit). In the present embodiment, as one example, the reception detector290detects a current which is output from the rectifier circuit in the power receiving circuit210. The power receiving circuit210sends data representing the detected value thereof to the power reception control circuit250. The power reception control circuit250sends a command based on this information to the power receiving circuit210and the motor control circuit340. In accordance with this command, the motor control circuit340starts or stops the electric motor350. In accordance with the driving method of the electric motor350, the motor control circuit340may include a converter circuit or an inverter circuit. In the present embodiment, the motor control circuit340corresponds to a first control circuit, whereas the power reception control circuit250corresponds to a second control circuit.

The power reception control circuit250acquires information on the status of electric power transmission from the power transmitter100to the vehicle10and remaining power of the battery320, and based on this information, determines whether electric power from the power transmitter100is to be received or not. When determining that electric power is not to be received, the power reception control circuit250sends to the motor control circuit340an instruction that electric power is not to be received. Upon receiving this instruction, the motor control circuit340stops the motor330, thereby increasing the impedance of the vehicle10as viewed from the power transmitter100.

The power transmission control circuit150of the power transmitter100and the power reception control circuit250of the transport robot10are able to communicate with each other wirelessly.

In the following description, the transmission electrodes120aand120bmay be indiscriminately expressed as “transmission electrodes120”. Similarly, the reception electrodes220aand220bmay be indiscriminately expressed as “reception electrodes220”. Moreover, a current/voltage/power as taken at a specific site in the power transmitting circuit110or the power receiving circuit210may simply be referred to as a “current/voltage/power in the power transmitting circuit110or the power receiving circuit210”.

Hereinafter, a fundamental operation according to the present embodiment will be described. First, an exemplary operation will be described where, while the transport robot travels over the transmission electrodes120, there is no other transport robot above the transmission electrodes120.

FIG. 4Ais a diagram schematically showing change over time in the relative positioning of a transport robot10aand transmission electrodes120. Upward arrows schematically represent electric power which is supplied to the transport robot10a. The number of upward arrows corresponds to the amount of received power. Downward blank arrows indicate lapse of time. Even when the transport robot10ais not situated over the transmission electrodes120as shown in the uppermost illustration, i.e., the reception electrodes220of the transport robot10are not opposed to the transmission electrodes120, the power transmitter100is constantly transmitting weak electric power from the transmission electrodes120. The level of weak electric power is such that it does not affect the environment, but is also detectable by the circuitry on the reception side. When the traveling transport robot10abegins to move over the transmission electrodes120as shown in the middle illustration, the transport robot10abegins to receive the weak electric power from the transmission electrodes120. At this time, since the transmitted power is weak, the power, voltage, and current that are output from the rectifier circuit in the power receiving circuit210of the transport robot10ado not increase much from zero.

Therefore, if a predetermined amount of time has elapsed before the value of at least one of power, voltage and current in the power receiving circuit210, or the change rate over time thereof, exceeds a threshold value, the power reception control circuit250of the transport robot10adetermines that no other transport robot exists over the transmission electrodes120that is receiving power.

Furthermore, on the basis of the current status of the load, the power reception control circuit250of the transport robot10adetermines whether or not there is need to receive power. “Need to receive power” exists when, for example, the remaining power in the battery320of the transport robot10ais smaller than a predetermined value, etc. When the transport robot10ahas placed itself completely over the transmission electrodes120as indicated in the lowermost illustration ofFIG. 4A, and if the power reception control circuit250of the transport robot10adetermines that there is need to receive power and that no other transport robot exists over the transmission electrodes120that is receiving power, the power reception control circuit250requests the power transmission control circuit150of the power transmitter100to give full transmission. Full transmission means transmission of relatively large electric power, through which the transport robot is provided with the power that it needs. The power under full transmission is larger than the power under weak transmission. Based on a command from the power transmission control circuit150having received a request for full transmission from the power reception control circuit250, the power transmitting circuit110switches from weak transmission to full transmission.

FIG. 4Bis a diagram showing an example change over time of a current in the power receiving circuit210of the transport robot10a. As one example, a case of utilizing a detected value of a current that flows in the power receiving circuit210will be described. Instead of a current, a detected value of voltage or power in the power receiving circuit210may be utilized. In the example shown in the figure, the current to be detected in the power receiving circuit210is a DC current. In the case where the current to be detected in the power receiving circuit210is an AC current, the presence or absence of another transport robot may be determined based on change over time in the amplitude of the AC current. For simplicity, in the following description, a current in the power receiving circuit210will be depicted as linear changes. In actuality, the current in the power receiving circuit210may undergo changes that represent a curve, due to noise or transient response.

First, an exemplary operation where the transport robot10adoes not need to receive power will be described. In this case, the transport robot10acontinues to receive weak electric power from the transmission electrodes120. When a portion of the reception electrodes220of the traveling transport robot10abecomes opposed to the transmission electrodes120, a current C0in the power receiving circuit210begins to increase from zero. As the transport robot10amoves along the transmission electrodes120, the move causes a gradual increase in the area of overlap between the transmission electrodes120and the reception electrodes220, thereby also increasing the detected current. Once the entire reception electrodes220become opposed to the electrodes120, the current C0in the power receiving circuit210has a value Iweakwhich is governed by the status of the load and the transmission efficiency at that moment. When there is no need to receive power, the transport robot10adoes not request the power transmitter100for full transmission; therefore, as is indicated by a broken line inFIG. 4B, the current C0in the power receiving circuit210stays at the constant value Iweak.

Next, an exemplary operation in the case where the transport robot10aneeds to receive power (e.g., when there is little remaining battery power) will be described. In this case, the transport robot10arequests the transmission electrodes120for full transmission. As the traveling transport robot10atravels over the transmission electrodes120, similarly to the case where there is no need to receive power, the output current C1from the power receiving circuit210increases from zero and reaches Iweak. At this time, if another transport robot exists over the transmission electrodes120that is being charged, this transmission power will also be received by the transport robot10a. As a result of this, unlike in the example shown inFIG. 4B, the current C1in the power receiving circuit210of the transport robot10adrastically increases to a value which is much higher than Iweak. On this account, when the value of the current C1exceeds a threshold value (e.g., a value which is equal to or greater than Iweak), it can be determined that another transport robot is being charged. Conversely, if a predetermined amount of time has elapsed before the value of the current C1exceeds the threshold value, the power reception control circuit250of the transport robot10amay determine that no other transport robot exists over the transmission electrodes120that is receiving power. After making this determination, the power reception control circuit250of the transport robot10arequests the power transmission control circuit150to begin full transmission. Once the power transmitting circuit110of the power transmitter100begins full transmission, the current C1in the power receiving circuit210drastically increases to reach a constant value Ifull. For the sake of distinction between Iweakand Ifull, Ifullmay be set to five times greater than Iweak, or even more, for example.

FIG. 4Cis a diagram showing another example where a current in the power receiving circuit210of the transport robot10aundergoes change over time. If a predetermined amount of time has elapsed before the change rate over time of a current C′1in the power receiving circuit210exceeds a threshold value (e.g., the change rate over time concerning an amount of increase of the current C0from zero), the power reception control circuit250of the transport robot10amay determine that no other transport robot exists over the transmission electrodes120that is receiving power. After making this determination, the power reception control circuit250of the transport robot10arequests the power transmission control circuit150to begin power transmission. Once the power transmitting circuit110of the power transmitter100begins full transmission, the current C′1in the power receiving circuit210drastically increases. Thereafter, the current C′1in the power receiving circuit210undergoes a proportional increase, until reaching a constant value Ifull. As compared to the case of making the determination based on a current value, determining the presence or absence of another transport robot based on the change rate over time of a current allows a signal requesting the power transmission control circuit150to begin full transmission to be sent more quickly.

After full transmission has begun, the transport robot10awhich is receiving power goes past the transmission electrodes120. Then, the current in the power receiving circuit210begins to decrease from Ifull, until finally reaching zero. Similarly, the current in the power transmitting circuit110also begins to decrease. As a result of this, the power transmission control circuit150is able to determine that the transport robot10ahas passed. After making this determination, with a command from the power transmission control circuit150, the power transmitting circuit110switches from full transmission to weak transmission. In another method, while the transport robot10ais passing over the transmission electrodes120, the power reception control circuit250of the transport robot10amay send a command to the power transmission control circuit150to switch from full transmission to weak transmission.

Next, with reference toFIG. 5A, an exemplary operation where another transport robot10bexists on the transmission electrodes120while the transport robot10ais traveling over the transmission electrodes120will be described. Once electric power begins to be supplied in part to the transport robot10a, the electric power that is supplied to the other transport robot10bthat is also receiving power decreases, whereby the efficiency of power transmission to the transport robot10bmay be deteriorated. In order to avoid this situation, the transport robot10aperforms control to suppress power reception from the transmission electrodes120.

Suppressing supply of electric power to the transport robot10acorresponds to decreasing the current in the power receiving circuit210of the transport robot10a. This is equivalent to increasing the impedance of the transport robot10aas viewed from the power transmitter100. One possible method of controlling the impedance of the transport robot10aas viewed from the power transmitter100may be a method of controlling driving of the electric motor330in the transport robot10a(which will be referred to as the motor drive controlling method).

FIG. 5Ais a diagram schematically showing change over time in the relative positioning of two transport robots10aand10b, which perform control under the motor drive controlling method, and the transmission electrodes120. As is indicated in the uppermost illustration ofFIG. 5A, another transport robot10bthat is receiving power exists over the transmission electrodes120. When the transport robot10atravels over the transmission electrodes120as indicated in the middle illustration ofFIG. 5A, the transport robot10areceives electric power which is sent from the transmission electrodes120. Since full transmission is being conducted, the current in the power receiving circuit210of the transport robot10adrastically increases from zero. The current value and the change rate over time thereof are remarkably greater than those in the examples shown inFIG. 4BandFIG. 4C. Therefore, if the current value in the power receiving circuit210of the transport robot10aor the change rate over time thereof exceeds a threshold value, the power reception control circuit250of the transport robot10amay determine that another transport robot10bexists over the transmission electrodes120that is receiving power. In that case, the power reception control circuit250of the transport robot10asends to the motor control circuit340a command to stop the electric motor330. As is indicated in the lowermost illustration ofFIG. 5A, with the command to stop the motor, the transport robot10amakes a stop near the entry into the transmission electrodes120. This makes it possible to suppress decrease in the efficiency of power transmission from the transmission electrodes120to another transport robot10b.

FIG. 5Bis a diagram showing an example change over time of a current in the power receiving circuit210of the transport robot10a, in the case where control under the motor drive controlling method is performed. In this example, a threshold value for the current in the power receiving circuit210is set to a value which is equal to or greater than Iweak.

A current C0inFIG. 5Bdepicts an example change over time in the current in the power receiving circuit210in the case where there is no need to receive power. This current corresponds the current C0shown inFIG. 4B, except for being represented in different scales therefrom as to time and current. As shown inFIG. 5B, when the traveling transport robot10atravels over the transmission electrodes120, a current C2in the power receiving circuit210undergoes a large increase as compared to the case where no other transport robot10bexists. When this current exceeds the threshold value, the power reception control circuit250of the transport robot10asends to the motor control circuit340a command to stop the electric motor330. In this case, the geometric area in which the transmission electrodes120and the reception electrodes220of the transport robot10aare opposed ceases to increase and remains constant at a level which is sufficiently small relative to the maximum possible area. As a result of this, the current C2in the power receiving circuit210maintains at a constant value. This constant value is sufficiently smaller than the Ifullvalue which pertains to full transmission, and thus is expected to make the influences on the transport robot10bsufficiently small. Making the current in the power receiving circuit210of the transport robot10asufficiently small is equivalent to increasing the impedance of the transport robot10aas viewed from the power transmitter100.

FIG. 5Cis a diagram showing another example change over time of a current in the power receiving circuit210of the transport robot10ain the case where control under the motor drive controlling method is performed. In this example, a change rate over time of the current C0existing before the current value in the power receiving circuit210reaches Iweakis set as a threshold value. When the change rate over time of the current exceeds the threshold value, a command to stop the electric motor330is sent to the motor control circuit340of the transport robot10a. As a result, a current C′2in the power receiving circuit210has a final value which is smaller than the value in the example ofFIG. 5B. By making a determination based on the change rate over time of a current, a signal to stop the motor can be sent to the power receiving circuit210more quickly than in the case of making the determination based on a current value.

Next, an exemplary operation where the transport robot10bgoes past the transmission electrodes120after the motor in the transport robot10ais stopped will be described. In this case, the power transmitter100switches from full transmission to weak transmission. At the same time, the current in the power receiving circuit210of the transport robot10abegins to decrease from the constant value. For example, when the current C2inFIG. 5Bhas decreased to Iweak, the motor control circuit340of the transport robot10asends a command to the electric motor330to start. As a result of this, the transport robot10abegins to travel again. In this case, it is evident that no transport robot10bthat is receiving power exists over the transmission electrodes120. Therefore, only when power reception is necessary, the power reception control circuit250of the transport robot10asends a command to the power transmission control circuit150of the power transmitter100to switch from weak transmission to full transmission. A decrease in the current in the power receiving circuit210may be determined based on whether the current value in the power receiving circuit210or the change rate over time thereof has become lower than the threshold value or not. This threshold value may be different from the threshold value which is used in determining the presence or absence of another transport robot10bover the transmission electrodes120that is receiving power.

FIG. 6is a flowchart showing an example of motor drive controlling for the transport robot10a. The power reception control circuit250determines whether a predetermined amount of time has elapsed before the current value in the power receiving circuit210or the change rate over time thereof exceeds a threshold value (S101). If a predetermined amount of time has elapsed before the current value in the power receiving circuit210or the change rate over time thereof exceeds the threshold value, the power reception control circuit250determines whether there is need to receive power or not (S102). If there is need to receive power, the power reception control circuit250sends to the power transmission control circuit150a command to switch from weak transmission to full transmission (S103). On the other hand, when the current value in the power receiving circuit210or the change rate over time thereof has exceeded the threshold value, the power reception control circuit250sends a command to the electric motor330to stop via the motor control circuit340(S104). Thereafter, the power reception control circuit250determines whether the current in the power receiving circuit210has begun to decrease or not (S105). If the current in the power receiving circuit210has begun to decrease, the power reception control circuit250sends a command to the electric motor330to start (S106). Thereafter, the power reception control circuit250determines whether there is need to receive power or not (S102).

Other than the aforementioned motor drive controlling, as a method of controlling the impedance of the transport robot10aas viewed from the power transmitter100, a method which turns electrical connections ON or OFF in the power receiving circuit210of the transport robot10a(which will be referred to as the “power-receiving circuit controlling method”) may also be possible.

FIG. 7Ais a diagram showing the construction of the power transmitter100and the transport robot10in a wireless power transmission system according to a variant of the present embodiment. The transport robot10includes a switch control circuit255. The power receiving circuit210includes switches whose electrical connections can be turned ON or OFF. Upon receiving an instruction from the power reception control circuit250, the switch control circuit255sends a command to turn ON or OFF the switch connections in the power receiving circuit210. Although the power reception control circuit250and the switch control circuit255are illustrated as separate circuits in the present embodiment, a single circuit containing these circuits may be provided. In the example ofFIG. 7A, the switch control circuit255corresponds to the first control circuit, whereas the power reception control circuit250corresponds to the second control circuit.

FIG. 7Bis a diagram schematically showing an exemplary arrangement of switch circuits270. In this example, the power receiving circuit210includes a matching circuit280, a rectifier circuit260, and a plurality of switch circuits270. The switch circuits270are connected in at least one of the following places: between the reception electrodes220and the impedance matching circuit (hereinafter referred to as the “matching circuit”)280; between the matching circuit280and the rectifier circuit260; between the rectifier circuit260and the charge-discharge control circuit350; and between the charge-discharge control circuit350and the electric motor330. In the present specification, the term “power receiving circuit” may allude to any circuitry that includes constituent elements between the reception electrodes220and the electric motor330.

FIG. 8Ais a diagram schematically showing an example change over time in the relative positioning of two transport robots10aand10band transmission electrodes120, in the case where control under the power-receiving circuit controlling method is performed. As is indicated in the uppermost illustration ofFIG. 8A, a transport robot10bthat is receiving power exists over the transmission electrodes120. When the traveling transport robot10atravels over the transmission electrodes120, as is indicated in the middle illustration ofFIG. 8A, the transport robot10abegins to receive electric power from the transmission electrodes120. In this case, since full transmission to another transport robot10bis being conducted, the current in the power receiving circuit210of the transport robot10adrastically increases from zero. Therefore, when the current value in the power receiving circuit210of the transport robot10aor the change rate over time thereof exceeds a threshold value, the transport robot10amay determine that another transport robot10bthat is receiving power exists over the transmission electrodes120. At this time, the power reception control circuit250of the transport robot10asends to the power receiving circuit210a command to turn OFF the electrical connection of at least one switch, via the switch control circuit255. Thus cuts the current path in the power receiving circuit210, so that there is zero current in the power receiving circuit210. As is indicated in the lowermost illustration ofFIG. 8A, based on the command to the power receiving circuit210, the transport robot10astops power reception. However, supply of power to its motor330can be continued. Thus, the transport robot10ais able to continue traveling, while not affecting supply of power to the transport robot10b. Unlike in the motor drive controlling method, without causing the transport robot10ato stop, the power-receiving circuit controlling method is able to suppress decrease in the efficiency of power transmission from the transmission electrodes120to another transport robot10b.

FIG. 8Bis a diagram showing an example change over time of a current in the power receiving circuit210of the transport robot10a, in the case where control under the power-receiving circuit controlling method is performed. In this example, a threshold value for the current is set to a value which is equal to or greater than Iweak. The current C0represents change over time of the current in the power receiving circuit210during weak transmission. This current corresponds to the current C0in the power receiving circuit210inFIG. 5B. As shown inFIG. 8B, as the transport robot10atravels over the transmission electrodes120, the current C3in the power receiving circuit210greatly increases from zero, as compared to how it was during weak transmission. When the current C3in the power receiving circuit210exceeds the threshold value, the switch control circuit255of the transport robot10asends to the power receiving circuit210a command to turn OFF the electrical connection of at least one switch, so that there is zero current in the power receiving circuit210. There being zero current flowing in the power receiving circuit210is equivalent to the impedance of the transport robot10aas viewed from the power transmitter100being increased to infinity.

FIG. 8Cis a diagram showing another example change over time of a current in the power receiving circuit210of the transport robot10ain the case where control under the power-receiving circuit controlling method is performed. In this example, the change rate over time of the current C0during weak transmission, before the current value reaches Iweak, is set as a threshold value. When the change rate over time of this current exceeds the threshold value, the power reception control circuit250of the transport robot10asends to the power receiving circuit210a command to turn OFF electrical connection, via the switch control circuit255. As a result, the current C′3in the power receiving circuit210becomes zero. By making a determination based on the change rate over time of a current, a signal to turn OFF electrical connection can be sent to the power receiving circuit210of the transport robot10amore quickly than in the case of making the determination based on a current value.

After a predetermined amount of time has elapsed since electrical connection was turned OFF, the switch control circuit255of the transport robot10asends a command to turn ON electrical connection in the power receiving circuit210. The aforementioned predetermined amount of time may be set to a period of time at which the transport robot10bwill presumably have gone past the transmission electrodes120so that the power transmitter100will have returned to weak transmission from full transmission. With the above command, the transport robot10atraveling over the transmission electrodes120again begins to receive electric power from the transmission electrodes120. Thereafter, the power reception control circuit250of the transport robot10aagain determines the presence or absence of any transport robot10bover the transmission electrodes120that is receiving power.

FIG. 9is a flowchart showing an example of power-receiving circuit controlling in the transport robot10a. Instead of Steps S104, S105and S106in the flowchart ofFIG. 6, Steps S107, S108and S109are executed. If the current value in the power receiving circuit210or the change rate over time thereof is higher than the threshold value, the power reception control circuit250sends to the power receiving circuit210a command to turn OFF electrical connection via the switch control circuit255(S107). Thereafter, the power reception control circuit250determines whether the predetermined amount of time has elapsed (S108). If the predetermined amount of time has already elapsed, the power reception control circuit250sends to the power receiving circuit210a command to turn ON electrical connection, via the switch control circuit255(S109). Thereafter, the power reception control circuit250again determines whether or not the current value in the power receiving circuit210or the change rate over time thereof is equal to or less than the threshold value (S101).

Below is a comparison of behavior between the motor drive controlling method and the power-receiving circuit controlling method described above, in the case where another transport robot10bthat is receiving power exists over the transmission electrodes120.

Under the motor drive controlling method, the transport robot10astops near the entry of the transmission electrodes120(FIG. 5A). On the other hand, under the power-receiving circuit controlling method, the transport robot10atravels over the transmission electrodes120without receiving power (FIG. 8A). In other words, under power-receiving circuit controlling, travel of the transport robot10adoes not incur loss of time.

Under the motor drive controlling method, the current in the power receiving circuit210of the transport robot10ahas a constant value which is sufficiently smaller than Ifull(FIG. 5BorFIG. 5C). On the other hand, under the power-receiving circuit controlling, the current in the power receiving circuit210of the transport robot10abecomes zero (FIG. 8BorFIG. 8C). In other words, under the power-receiving circuit controlling method, the transport robot10anever affects another transport robot10bover the transmission electrodes120that is receiving power.

Under the motor drive controlling method, after the motor is started (S106inFIG. 6), it is not necessary to again determine whether another transport robot10bthat is receiving power exists over the transmission electrodes120or not (S102inFIG. 6). On the other hand, under the power-receiving circuit controlling method, after electrical connection in the power receiving circuit210is turned ON (S109inFIG. 9), it is determined again as to whether another transport robot10bthat is receiving power exists over the transmission electrodes120(S101inFIG. 9).

In a variant, the motor drive controlling method and the power-receiving circuit controlling method described above may be used in combination.

FIG. 10is a flowchart showing another example of the motor drive controlling method in the transport robot10a. Instead of Steps S107to S109in the flowchart ofFIG. 9, Steps S110to S112are executed. If the current value in the power receiving circuit210or the change rate over time thereof exceeds a threshold value, the power reception control circuit250sends a command to the electric motor330to stop (S110). Thereafter, the power reception control circuit250determines whether a predetermined amount of time has elapsed or not (S111). If the predetermined amount of time has elapsed, the power reception control circuit250sends to the electric motor330a command to start, via the motor control circuit340(S112). Thereafter, the power reception control circuit250again determines whether or not the current value in the power receiving circuit210or the change rate over time thereof is equal to or less than the threshold value (S101). As the predetermined amount of time, a period of time which is obtained by dividing the length of the transmission electrodes120by the velocity of travel of the transport robot10bmay be used. In this case, after the motor in the transport robot10ais started (S112), there will be no other transport robot10bover the transmission electrodes120that is receiving power. The power reception control circuit250may determine whether there is need to receive power or not next (S102).

FIG. 11is a flowchart showing another example of the power-receiving circuit controlling method in the transport robot10a. Instead of Steps S104to S106inFIG. 6, Steps S113to S115are executed. If the current value in the power receiving circuit210or the change rate over time thereof exceeds a threshold value, the power reception control circuit250sends to the power receiving circuit210a command to increase impedance (S113). At this time, the current in the power receiving circuit210is sufficiently small than Ifull. However, let it be assumed that the current in the power receiving circuit210never reaches zero, unlike in the aforementioned power-receiving circuit controlling method. Thereafter, the power reception control circuit250determines whether the current in the power receiving circuit210has begun to decrease or not (S114). If the current in the power receiving circuit210has begun to decrease, the power reception control circuit250sends to the power receiving circuit210a command to decrease impedance, via the switch control circuit255(S115). Thereafter, the power reception control circuit250determines whether there is need to receive power or not (S102). Adjustment of the impedance of the power receiving circuit210to be higher or lower may be achieved by, for example, changing the value of at least one of the resistance, inductance, and capacitance of the power receiving circuit210.

Next, effects of the present embodiment will be described in contrast to the technique of Patent Document 1. In Patent Document 1, a voltage monitoring section in the power transmitter monitors voltage. When the voltage exceeds a predetermined threshold value, a power transmission stop section in the power transmitter stops power transmission to the power receiver. On the other hand, in the present embodiment, the power reception control circuit250of the transport robot10adetermines the presence or absence of another transport robot10bover the transmission electrodes120. If another transport robots10bexists over the transmission electrodes120, the transport robot10asuppress power reception from the transmission electrodes120. That is, in the present embodiment, power transmission can be controlled on the reception side. Such control of power transmission on the reception side is applicable not only to a system including two vehicles, but also to a system including three or more vehicles.

In the aforementioned embodiment, based on the prerequisite that the transport robot10ais traveling over the transmission electrodes120, the power reception control circuit250of the transport robot10arequests the power transmission control circuit150to begin full transmission. However, even while the transport robot10ais not traveling over the transmission electrodes120, due to noise influences, or due to the transport robot10aapproaching the transmission electrodes120, a weak current which is much smaller than Iweakmay flow in the power receiving circuit210. In this case, at the lapse of a predetermined amount of time, the power reception control circuit250of the transport robot10amay erroneously request the power transmission control circuit150to begin power transmission.

In order to prevent such malfunctioning, in addition to the threshold value (first threshold value) that is used for determining the presence or absence of a transport robot10bover the transmission electrodes120that is receiving power, a threshold value (second threshold value) to be satisfied in making a request for power transmission may be introduced. The second threshold value may be set to a smaller value than the first threshold value. For example, after the lapse of a predetermined amount of time, the power reception control circuit250of the transport robot10amay request the power transmission control circuit150to begin power transmission only if the current value or the change rate over time thereof has exceeded the second threshold value, while not exceeding the first threshold value. On the other hand, if the current value or the change rate over time thereof does not exceed the second threshold value even after the lapse of the predetermined amount of time during which the current value or the change rate over time thereof has remained below the first threshold value, then the power reception control circuit250of the transport robot10acan determine that the transport robot10aitself is not traveling over the transmission electrodes120. In that case, the power reception control circuit250of the transport robot10amay not request the power transmission control circuit150to begin power transmission.

When the current value is relied upon, the second threshold value may be Iweak, or a value which is slightly smaller than that, for example. When the change rate over time of current is relied upon, the second threshold value may be a change rate over time of the amount of increase in the current C0from zero, or a value which is slightly smaller than that, for example.

In the above embodiment, the power reception control circuit250requests the power transmitter100for full transmission, through a determination based on change in the current or power flowing in the power receiving circuit210of the transport robot10a. Instead of this operation, while the power transmitter100is conducting weak transmission, the power transmission control circuit150may determine arrival of the transport robot10aover the transmission electrodes120based on change in the current or power flowing in the power transmitting circuit110. The power transmitter100may request the transport robot10ato decide whether or not to conduct full transmission. When this request is received, upon determining that full transmission is needed, the power reception control circuit250of the transport robot10amay request the power transmitter100for full transmission.

Next, the construction concerning power transmission by the wireless power transmission system according to the present embodiment will be described in more detail. Note that the system construction as described below is only exemplary, and may be modified as appropriate, depending on the required function and performance.

FIG. 12is block diagram generally showing a construction involved in the power transmission in the wireless power transmission system according to the present embodiment. InFIG. 12, the power reception control circuit250is omitted from illustration. The power transmitter100includes an inverter circuit160which converts DC power that is supplied from an external DC power source310into AC power, two transmission electrodes120aand120bwhich transmit the AC power, and a matching circuit180which is connected between the inverter circuit160and the transmission electrodes120aand120b. The “DC power source310” also encompasses a power source that results from converting externally-supplied AC power into DC power with a converter circuit. In the present embodiment, the inverter circuit160are electrically connected to the first and second transmission electrodes120aand120bvia the matching circuit180, and outputs AC power to the first and second transmission electrodes120aand120b. The transport robot10includes a power receiver200and a load330.

The power receiver200includes two reception electrodes220aand220bwhich establish capacitive coupling with the two transmission electrodes120aand120bto receive electric power, a matching circuit280connected to the two reception electrodes220aand220b, and a rectifier circuit260which is connected to the matching circuit280and converts the received AC power into DC power and outputs it. When opposed to the first transmission electrode120a, the first reception electrode220aestablish capacitive coupling with the first transmission electrode120a. When opposed to the second transmission electrode120b, the second reception electrode220bestablish capacitive coupling with the second transmission electrodes. Via these two sites of capacitive coupling, AC power is contactlessly transmitted from the power transmitter100to the power receiver200.

Although not particularly limited, the respective sizes of the housing of the transport robot10according to the present embodiment, the transmission electrodes120aand120b, and the reception electrodes220aand220bmay be set to the following sizes, for example. The length (i.e., the size along the Y direction) of each of the transmission electrodes120aand120bmay be set in a range from 50 cm to 20 m, for example. The width (i.e., the size along the X direction) of each of the transmission electrodes120aand120bmay be set in a range from 5 cm to 2 m, for example. The sizes along the traveling direction and the lateral direction of the housing of the transport robot10may be set in a range from 20 cm to 5 m, for example. The length (i.e., the size along the traveling direction) of each reception electrode220amay be set in a range from 5 cm to 2 m, for example. The width (i.e., the size along the lateral direction) of each reception electrode220amay be set in a range from 2 cm to 2 m, for example. However, these numerical ranges are not limiting.

The load330may include an electric motor for driving purposes and a capacitor for electrical storage purposes, for example, and may be driven or charged by DC power which is output from the power receiving circuit210.

The electric motor may be any type of motor, such as a DC motor, a permanent magnet synchronous motor, an induction motor, a stepping motor, or a reluctance motor. The motor rotates the wheels of the transport robot10via shafts, gears, and the like, thus causing the transport robot10to move. Depending on the kind of motor used, the power receiving circuit210may include various circuits, e.g., a rectifier circuit(s), an inverter circuit(s), and/or an inverter control circuit(s).

The capacitor may be a high-capacitance and low-resistance capacitor, such as an electric double layer capacitor or a lithium-ion capacitor. Using such a capacitor as a means of electrical storage will allow for more rapid charging than when a battery (secondary battery) is used. Instead of a capacitor, a secondary battery (e.g., a lithium-ion battery) may be used. In that case, although more time will be required for charging, a greater amount of energy can be stored. The motor is driven with the electric power that is stored in the capacitor or secondary battery, whereby the transport robot10moves.

As the transport robot10moves, the amount of stored electricity (charge amount) in the capacitor or secondary battery will decrease. Therefore, recharging will be required in order to continue the movement. Therefore, when the charge amount falls below a predetermined threshold value during movement, the transport robot10will perform charging from the power transmitter100.

FIG. 13is a circuit diagram showing a more detailed exemplary construction for the wireless power transmission system. In the example shown in the figure, the matching circuit180of the power transmitter100includes a series resonant circuit130swhich is connected to the power transmitting circuit110, and a parallel resonant circuit140pwhich is connected to the transmission electrodes120aand120band establishes inductive coupling with the series resonant circuit130s. The matching circuit180has the function of matching the impedance of the inverter circuit160with the impedance of the transmission electrodes120aand120b. The series resonant circuit130sof the power transmitter100includes a first coil L1and a first capacitor C1being connected in series. The parallel resonant circuit140pof the power transmitter100includes a second coil L2and a second capacitor C2being connected in parallel. The first coil L1and the second coil L2constitute a transformer whose coupling is based on a predetermined coupling coefficient. The turns ratio between the first coil L1and the second coil L2is set to a value that realizes a desired transformation ratio (a step-up ratio or a step-down ratio).

The matching circuit280of the power receiver200includes a parallel resonant circuit230pwhich is connected to the reception electrodes220aand220band a series resonant circuit240swhich is connected to the rectifier circuit260and establishes inductive coupling with the parallel resonant circuit230p. The matching circuit280has the function of matching the impedance of the reception electrodes220aand220bwith the impedance of the power receiving circuit210. The parallel resonant circuit230pincludes a third coil L3and a third capacitor C3being connected in parallel. The series resonant circuit240sof the power receiver200includes a fourth coil L4and a fourth capacitor C4being connected in series. The third coil L3and the fourth coil L4constitute a transformer whose coupling is based on a predetermined coupling coefficient. The turns ratio between the third coil L3and the fourth coil L4is set to a value that realizes a desired transformation ratio.

Note that the construction of the matching circuits180and280is not limited to what is shown inFIG. 13. For example, a parallel resonant circuit may be provided instead of each of the series resonant circuits130sand240s. Moreover, a series resonant circuit may be provided instead of each of the parallel resonant circuits140pand230p. Furthermore, one or both of the matching circuits180and280may be omitted. In the case of omitting the matching circuit180, the inverter circuit160and the transmission electrodes120aand120bare directly connected. In the case of omitting the matching circuit280, the rectifier circuit260and the reception electrodes220aand220bare directly connected. In the present specification, a construction where the matching circuit180is provided also qualifies as a construction in which the inverter circuit160and the transmission electrodes120aand120bare electrically connected. Similarly, a construction where the matching circuit280is provided also qualifies as a construction in which the rectifier circuit260and the reception electrodes220aand220bare electrically connected.

FIG. 14Ais a diagram schematically showing an exemplary construction for the inverter circuit160. In this example, the inverter circuit160includes a full-bridge inverter circuit that contain four switching elements (e.g., transistors such as IGBTs or MOSFETs) and the power transmission control circuit150. The power transmission control circuit150includes a gate driver which outputs a control signal to control the ON (conducting) or OFF (non-conducting) state of each switching element and a processor which causes the gate driver to output a control signal, e.g., a microcontroller. Instead of the full-bridge inverter circuit that is shown in the figure, a half-bridge inverter circuit, or any other oscillation circuit, e.g., that of class E, may also be used. The inverter circuit160may include modulation/demodulation circuitry for communication purposes and various sensors for measuring voltage, current, etc. In the case where modulation/demodulation circuitry for communication purposes is included, data may be superposed onto the AC power so as to be sent to the power receiver200.

Note that the present disclosure also embraces implementations where a weak AC signal (e.g., a pulse signal) is sent to the power receiver200for the purpose of data transmission, rather than power transmission. In such an implementation, too, it can be said that weak electric power is being transmitted; therefore, transmission of a weak AC signal (e.g., a pulse signal) is also encompassed under the notion of “power transmission”. Moreover, such a weak AC signal is also encompassed under the notion of “AC power”.

FIG. 14Bis a diagram schematically showing an exemplary construction for the rectifier circuit260. In this example, the power receiving circuit210is a full-wave rectifier circuit including a diode bridge and a smoothing capacitor. The rectifier circuit260may have any other rectifier construction. Other than the rectifier circuit260, various circuits may also be included, such as constant voltage/constant current control circuitry, and/or modulation/demodulation circuitry for communication purposes. The rectifier circuit260converts the received AC energy into DC energy which is available for use by the load330. Various sensors for measuring the voltage and current, etc., being output from the series resonant circuit240smay also be included in the rectifier circuit260.

Each coil in the resonant circuits130s,140p,230pand240smay be, for example, a planar coil or a laminated coil that is formed on a circuit board, or a wound coil of a copper wire, a litz wire, a twisted wire, or the like. Each capacitor in the resonant circuits130s,140p,230pand240smay be any type of capacitor having a chip shape or a lead shape, for example. It may also be possible to allow the capacitance between two wiring lines, with air interposed therebetween, to function as a capacitor. The self-resonance property of each coil may also be utilized to replace any such capacitor.

The DC power source310may be any kind of power source, e.g., a mains supply, a primary battery, a secondary battery, a photovoltaic cell, a fuel cell, a USB (Universal Serial Bus) power source, a high-capacitance capacitor (e.g., an electric double layer capacitor), a voltage converter that is connected to a mains supply, or the like.

The resonant frequency f0of the resonant circuits130s,140p,230pand240sis typically set equal to the transmission frequency f during power transmission. The resonant frequency f0of each of the resonant circuits130s,140p,230pand240smay not be exactly equal to the transmission frequency f. Each resonant frequency f0may be set to a value in a range from about 50% to about 150% of the transmission frequency f, for example. The power transmission frequency f may be set to e.g. 50 Hz to 300 GHz, more preferably 20 kHz to 10 GHz, still more preferably 20 kHz to 20 MHz, and further more preferably 20 kHz to 7 MHz.

In the present embodiment, an air gap exists between the transmission electrodes120aand120band the reception electrodes220aand220b, with a relatively long distance therebetween (e.g., about 10 mm). Therefore, the capacitances Cm1and Cm2between these pairs of electrodes are very small, while the impedances of the transmission electrodes120aand120band the reception electrodes220aand220bare very high (e.g., on the order of several kΩ). On the other hand, the impedances of the power transmitting circuit110and the power receiving circuit210are as low as several Ω, for example. In the present embodiment, the parallel resonant circuits140pand230pare disposed closer to the transmission electrodes120a,120band the reception electrodes220a,220b, respectively, whereas the series resonant circuits130sand240sare disposed closer to the power transmitting circuit110and the power receiving circuit210, respectively. With such a construction, impedance matching can be easily attained. A series resonant circuit has zero (0) impedance during resonance, and therefore allows for matching with low impedance. On the other hand, a parallel resonant circuit has infinite impedance during resonance, and therefore allows for matching with high impedance. Thus, as in the construction shown inFIG. 13, impedance matching can be easily achieved by disposing a series resonant circuit on the power source side, which is low in impedance, and a parallel resonant circuit on the electrode side, which is high in impedance. Similarly, impedance matching in the power receiver200can be suitably achieved by disposing a parallel resonant circuit on the electrode side and a series resonant circuit on the load side.

In a construction where the distance between the transmission electrodes120aand120band the reception electrodes220aand220bis made shorter, or a dielectric is interposed therebetween, the impedances of the electrodes will be lowered, so that the aforementioned asymmetric resonant circuit construction is unnecessary. When there is no issue of impedance matching, the matching circuits180and280may themselves be omitted.

Next, an illustrative second embodiment of the present disclosure will be described.

In the present embodiment, the determination as to whether a vehicle is to receive power or not is made by an external apparatus, which is distinct from the vehicle. The external apparatus notifies the vehicle of the result of determination. Upon receiving this notice, the vehicle alters its own impedance.

FIG. 15is a block diagram showing the construction of a wireless power transmission system according to the present embodiment. This wireless power transmission system includes at least one power transmitter100, at least one vehicle10, and a central controller300which manages traveling operations of the vehicle10. The central controller300controls the power transmitter100and the at least one vehicle10. The central controller300is connected, in a wireless or wired manner, to the power transmission control circuit150of the power transmitter100and the power reception control circuit250of the vehicle10.

The central controller300may be implemented as a computer having a control circuit such as a CPU and a storage device such as a memory, for example. As the control circuit executes a computer program which is stored in the storage device, the operation described may be achieved.

To the power transmission control circuit150, the central controller300sends a command to stop or begin power transmission. During power transmission, the power transmission control circuit150sends information concerning the status of electric power transmission, e.g., a value of at least one of power, voltage and current in the power transmitting circuit110, to the central controller300. Transmission of this information from the power transmitter100to the central controller300may be performed every predetermined period of time, for example.

Moreover, the central controller300sends to the power reception control circuit250a command to begin or stop power reception. The power reception control circuit250sends information representing the location of the vehicle10and information representing the remaining power in the battery320to the central controller300. Transmission of this information from the power receiver10to the central controller300may be performed every predetermined period of time, for example.

The power transmitter100and the vehicle10shown inFIG. 15are similar in construction to those shown inFIG. 3. Alternatively, the vehicle10may have a construction as shown inFIG. 7A.

Based on at least one of the information representing the status of electric power transmission, the information representing the location of the vehicle320, and the information representing the remaining power in the battery320, the central controller300determines the timing to begin or stop power reception in the vehicle10. When the vehicle10is about to arrive over the transmission electrodes120, the central controller300determines whether the vehicle10is to receive power or not, on basis of the above information. Based on the result of determination, the central controller300sends a command to begin power reception or a command to stop power reception to the power reception control circuit250. In accordance with the command to begin power reception or the command to stop power reception, the power reception control circuit250of the vehicle10adjusts its own impedance.

FIG. 16is a flowchart showing an operation of the vehicle10according to the present embodiment. The power reception control circuit250of the vehicle10executes the following operation.

Step S201: The power reception control circuit250determines whether a command to begin power reception has been received or not. If Yes, control proceeds to Step S202. If No, control proceeds to Step S203.

Step S202: Upon receiving a command to begin power reception, the power reception control circuit250sends to the power transmission control circuit150a command to switch from weak transmission to full transmission.

step S203: The power reception control circuit250determines whether a command to stop power reception has been received or not. If Yes, control proceeds to Step S204. If No, control returns to Step S201.

Step S204: Upon receiving a command to stop power reception, the power reception control circuit250sends to the motor control circuit340a command to increase impedance. Upon receiving this command, the motor control circuit340stops the motor330.

Step S205: Next, the power reception control circuit250determines whether a command to begin power reception has been received again. If Yes, control proceeds to Step S206. If No, Step S205is performed again.

Step S206: Upon receiving a command to begin power reception again, the power reception control circuit250sends to the motor control circuit340a command to decrease impedance. Upon receiving this command, the motor control circuit340drives the motor330again.

In the example ofFIG. 16, the motor control circuit340increases the impedance of the vehicle10as viewed from the power transmitter100by stopping the motor330. Alternatively, the power reception control circuit250may introduce an increased impedance through control of a switch(es), resistance, capacitance, or inductance in the power receiving circuit210. In the present embodiment, the power reception control circuit250and the motor control circuit340together function as the “first control circuit”.

The central controller300may make a determination to begin or stop power reception based on the following prerequisites, for example.

(1) Location information of all transmission electrodes120within the system is retained.

(2) For each transmission electrode120, information as to whether the power transmitter is transmitting power is constantly being acquired.

(3) For each and every vehicle10within the system, location information and the remaining charge information on the battery320are constantly being acquired.

(4) As a given vehicle10approaches the transmission electrodes120, if another vehicle10distinct from that vehicle10happens to exist over the transmission electrodes120, a determination to “begin or stop power reception” is made for these vehicles10, and a command is sent thereto.

For each vehicle10, the central controller300makes a determination to begin or stop power reception based on any one of the following Criteria 1 to 3, for example.

Criterion 1: First Come, First Served

The vehicle that was the first to exist over the transmission electrodes is given priority, and a command to “stop power reception” is sent to a later-approaching vehicle.

Criterion 2: Priority Based on Remaining Charge

Whichever vehicle has less remaining charge is given priority, and a command to “stop power reception” is sent to the vehicle that has comparatively more remaining charge.

Criterion 3: Priority to the Latter

The later-approaching vehicle is given priority, and a command to “stop power reception” is sent to the vehicle that was the first to exist over the transmission electrodes.

Hereinafter, with reference toFIG. 17, an exemplary operation of the central controller300will be described.FIG. 17is a flowchart showing an exemplary operation of the central controller300. It is assumed herein that, as shown inFIG. 5A, two vehicles10aand10bare vying to receive power from the pair of transmission electrodes120. The vehicle10bthat was the first to exist over the transmission electrodes will be referred to as “the first vehicle10b”, whereas the vehicle that later approached the transmission electrodes will be referred to as “the latter vehicle10a”. In this example, the central controller300performs the following operation.

S301: Transmission status information is acquired from the power transmitter100.

S302: Mobile unit location information and charging state information are acquired from the latter vehicle10a.

S303: Mobile unit location information and charging state information are acquired from the first vehicle10b.

Note that Steps S301through S303may be executed in any arbitrary order.

S304: It is determined whether or not the distance between the latter vehicle10aand the transmission electrodes120has become equal to or less than a designated value. If No, control returns to Steps S301through S303. If Yes, control proceeds to Step S305. Note that the distance between the vehicle10aand the transmission electrodes120may be calculated from the location information of each transmission electrode and each vehicle.

S305: It is determined whether the power transmitter100is transmitting power or not. If Yes, control proceeds to Step S306. If No, control proceeds to Step S309.

S306: It is determined whether the first vehicle10bis located the transmission electrodes or not. If Yes, control proceeds to Step S307. If No, control proceeds to Step S309.

S307: It is determined whether or not the remaining battery power of the latter vehicle10ais greater than a designated value. If Yes, control proceeds to Step S308. If No, control proceeds to Step S312.

S308: A command to stop power reception is sent to the latter vehicle10a.

S309: A command to stop power reception is sent to the first vehicle10b.

S310: A command to begin power reception is sent to the latter vehicle10a.

S311: A command to begin power transmission is sent to the power transmitter100.

S312: It is determined whether or not the remaining battery power of the latter vehicle10ais greater than the remaining battery power of the first vehicle10b. If Yes, control proceeds to Step S313. If No, control proceeds to Step S309.

S313: A command to stop power reception is sent to the latter vehicle10a.

Through the above operation, it becomes possible to supply electric power as appropriate, in accordance with the remaining battery powers of the first vehicle10band the latter vehicle10a.

Although the pair of transmission electrodes120are installed on the ground in the above embodiments, the pair of transmission electrodes120may instead be installed on a lateral surface, e.g., a wall, or an overhead surface, e.g., a ceiling. Depending on the place and orientation in which the transmission electrodes120are installed, the arrangement and orientation of the reception electrodes220of the vehicle10are to be determined.

FIG. 18Ais a diagram showing an example where the transmission electrodes120are installed on a lateral surface e.g., a wall. In this example, the reception electrodes220are provided on a lateral side of the vehicle10.FIG. 18Bis a diagram showing an example where the transmission electrodes120are installed on a ceiling. In this example, the reception electrodes220are provided on the top of the vehicle10. As demonstrated by these examples, there may be a variety of arrangements for the transmission electrodes110and the reception electrodes210.

A wireless power transmission system according to an embodiment of the present disclosure may be used as a system of transportation for articles within a factory, as mentioned above. The transport robot10functions as a cart having a bed on which to carry articles, and autonomously move in the factory to transport articles to necessary places. However, without being limited to such purposes, the wireless power transmission system and the vehicle according to the present disclosure are also usable for various other purposes. For example, without being limited to an AGV, the vehicle may be any other industrial machine, a service robot, an electric vehicle, a multicopter (drone), or the like. Without being limited to being used in a factory, the wireless power transmission system may be used in shops, hospitals, households, roads, runways, or other places, for example.

As described above, the present disclosure encompasses vehicles and wireless power transmission systems as recited in the following Items.

Item 1. A vehicle to be driven by electric power which is wirelessly transmitted from a power transmitter having two transmission electrodes, the vehicle comprising:

two reception electrodes to receive AC power from the two transmission electrodes through capacitive coupling respectively with the two transmission electrodes;

a power receiving circuit which is connected to the two reception electrodes to convert AC power received by the two reception electrodes into DC power or another form of AC power, and supply the DC power or other form of AC power to an electric motor which drives the vehicle or to a secondary battery which stores electric power for driving the vehicle; and

a first control circuit which, while the vehicle is moving with the two reception electrodes being opposed to the two transmission electrodes, increases an impedance of the vehicle as viewed from the power transmitter in response to an instruction that the electric power from the power transmitter is not to be received.

Item 2. The vehicle of item 1, further comprising a second control circuit which acquires information on at least one of a status of electric power transmission from the power transmitter to the vehicle, location of the vehicle, and remaining power of the secondary battery, determines whether or not electric power from the power transmitter is to be received based on the information, and sends the instruction to the first control circuit when determining that the electric power is not to be received.

Item 3. The vehicle of item 1 or 2, wherein the second control circuit sends the instruction to the first control circuit when a value of at least one of power, voltage and current in the power receiving circuit, or a change rate over time thereof, exceeds a threshold value.

Item 4. The vehicle of item 3, wherein,

the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, is greater than the threshold value when the power transmitter is transmitting power to another vehicle distinct from the vehicle; and

the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, is not greater than the threshold value when the power transmitter is not transmitting power to another vehicle distinct from the vehicle.

Item 5. The vehicle of item 3 or 4, wherein, while the two reception electrodes are opposed respectively to the two transmission electrodes, if a predetermined amount of time has elapsed before the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, exceeds the threshold value, the first control circuit requests the power transmitter for power transmission.

Item 6. The vehicle of any of items 1 to 5, wherein, when receiving the instruction from a central controller which manages traveling operations of the vehicle, the first control circuit increases the impedance of the vehicle as viewed from the power transmitter.

Item 7. The vehicle of item 6, wherein, the central controller acquires information on at least one of a status of electric power transmission from the power transmitter to the vehicle, location of the vehicle, and remaining power of the secondary battery, determines whether or not the vehicle is to receive the electric power from the power transmitter based on the information, and when determining that the electric power from the power transmitter is not to be received from the vehicle, sends the instruction to the first control circuit.

Item 8. The vehicle of any of items 1 to 7, wherein the first control circuit includes a motor control circuit which controls the electric motor, and the motor control circuit increases the impedance of the vehicle as viewed from the power transmitter by sending the electric motor a command to stop.

Item 9. The vehicle of item 8, wherein, after sending the electric motor the command to stop, at the lapse of a predetermined amount of time, the first control circuit sends the electric motor a command to start again.

Item 10. The vehicle of item 9 or 10, wherein, after sending the electric motor the command to stop, if the value of at least one of power, voltage and current in the power receiving circuit, or the change rate over time thereof, becomes lower than a second threshold value which is equal to or smaller than the threshold value, the first control circuit sends the electric motor a command to start again.

Item 11. The vehicle of any of items 1 to 10, wherein,

the power receiving circuit includes a switch to turn ON or OFF electrical connection between the two reception electrodes and the electric motor; and

the first control circuit increases the impedance of the vehicle as viewed from the power transmitter by sending the switch a command to turn OFF the electrical connection.

Item 12. The vehicle of item 11, wherein, after sending the command to the switch, at the lapse of a predetermined amount of time, the control circuit sends the switch a command to turn ON the electrical connection.

Item 13. The vehicle of item 11 or 12, wherein,

the power receiving circuit includes

a rectifier circuit, and

a switch circuit which is connected between the two reception electrodes and the rectifier circuit, the switch circuit including the switch.

Item 14. The vehicle of item 11 or 12, wherein,

the power receiving circuit includes

a rectifier circuit, and

a switch circuit which is connected between the rectifier circuit and the electric motor, the switch circuit including the switch.

Item 15. The vehicle of item 12 or 13, wherein,

the power receiving circuit includes

a rectifier circuit,

an impedance matching circuit which is connected between the two reception electrodes and the rectifier circuit, and

a switch circuit which is connected between the two reception electrodes and the impedance matching circuit, or between the impedance matching circuit and the rectifier circuit, the switch circuit including the switch.

Item 16. The vehicle of any of items 1 to 15, wherein the first control circuit increases the impedance of the vehicle as viewed from the power transmitter by sending the power receiving circuit a command to change a value of at least one of resistance, inductance and capacitance of the power receiving circuit.

Item 17. The vehicle of any of items 1 to 16, further comprising the electric motor.

Item 18. The vehicle of any of items 1 to 17, further comprising a detector to detect the at least one of power, voltage and current in the power receiving circuit.

Item 19. The vehicle of any of items 1 to 18, wherein the vehicle is an automated guided vehicle.

Item 20. A wireless power transmission system comprising:

the vehicle of any of items 1 to 19; and

the power transmitter.

Item 21. A wireless power transmission system comprising:

the vehicle of item 6 or 7;

the power transmitter; and

the central controller.

The technique according to the present disclosure is applicable to any device that is driven with electric power. For example, it is applicable to transport robots or electric vehicles, such as automated guided vehicles (AGV) that are used in a factory.

This application is based on Japanese Patent Applications No. 2016-249101 filed Dec. 22, 2016, and No. 2017-219214 filed Nov. 14, 2017, the entire contents of which are hereby incorporated by reference.