Patent Publication Number: US-2023147563-A1

Title: Power feeding station

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-184261 filed on Nov. 11, 2021, the contents of which are incorporated herein by reference. 
     FIELD 
     The present disclosure relates to a power feeding station that feeds power contactlessly to an electric mobility vehicle. 
     BACKGROUND 
     Equipment has been developed for feeding power contactlessly to a two-wheeler to drive a motor that provides auxiliary power in the two-wheeler (refer to Patent Literatures 1 and 2). 
     Patent Literature 1 describes a power feeding stand including a power feeder circuit to feed power to a bicycle including a power receiver circuit. A wireless power feeding system described in Patent Literature 2 includes a transmitter and an electric bicycle including a receiver that receives and charges power wirelessly transmitted from the transmitter and uses the charged power for electric assistance. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: WO 2017/150713 
         Patent Literature 2: WO 2016/147295 
       
    
     SUMMARY 
     An example contactless power feeding system includes a device for feeding power including a coil, and a device for receiving power including another coil. The device for feeding power (hereafter referred to as a feeder) supplies alternating current (AC) power to the coil for feeding power (hereafter referred to as a feeder coil). The feeder coil thus generates a magnetic field. The coil for receiving power (hereafter referred to as a receiver coil) in the device for receiving power (hereafter referred to as a receiver) resonates with the magnetic field to allow contactless power feeding from the feeder to the receiver. 
     A contactless power feeding system may transmit power independently of slight changes in the positional relationship between the feeder coil in the power feeder and the receiver coil in the power receiver. However, the power transmission efficiency may be changed by the positional relationship between the feeder coil and the receiver coil. To increase the power transmission efficiency, a user of a two-wheeler may receive guidance about the stop position of the two-wheeler. 
     One or more embodiments are directed to a power feeding station that feeds power contactlessly to an electric mobility vehicle through a feeder coil and provides guidance about the stop position of the electric mobility vehicle to increase power transmission efficiency. 
     A power feeding station according to one or more embodiments feeds power to a receiver including a receiver coil included in an electric mobility vehicle. The power feeding station includes a feeder that feeds power to the electric mobility vehicle through the receiver coil, and a housing accommodating the feeder. The feeder includes a feeder coil that feeds power to the receiver through the receiver coil, a power supply circuit that supplies alternating current power to the feeder coil, and a control circuit that controls a frequency and a voltage of the alternating current power supplied to the feeder coil. The control circuit provides, through a notification source included in the power feeding station or in the electric mobility vehicle, a notification of guidance about a stop position of the electric mobility vehicle relative to a housing to increase power transmission efficiency from the feeder to the receiver in accordance with the frequency of the alternating current power supplied to the feeder coil with which the receiver outputs a constant voltage or in accordance with the voltage of the alternating current power supplied to the feeder coil with which the receiver outputs a constant and predetermined voltage. The power feeding station with the above structure feeds power contactlessly to the electric mobility vehicle through the feeder coil and reduces the likelihood that foreign objects enter between the receiver coil included in the electric mobility vehicle and the feeder coil. 
     In the above power feeding station, the control circuit may provide, through the notification source, the notification of guidance about the stop position of the electric mobility vehicle to allow the feeder coil and the receiver coil to be nearer each other when the alternating current power supplied to the feeder coil with which the receiver outputs a constant voltage has a frequency lower than a predetermined frequency threshold. The power feeding station with the above structure may guide the electric mobility vehicle to the stop position at which higher power transmission efficiency may be achieved. 
     In the above power feeding station, the control circuit may provide, through the notification source, the notification of guidance about the stop position of the electric mobility vehicle to allow the feeder coil and the receiver coil to be nearer each other when the alternating current power supplied to the feeder coil with which the receiver outputs a constant and predetermined voltage has a higher voltage than a predetermined voltage threshold. The power feeding station with the above structure may guide the electric mobility vehicle to the stop position at which higher power transmission efficiency may be achieved. 
     In the power feeding station according to one or more embodiments, the control circuit may record a temporal change in the frequency of the alternating current power supplied to the feeder coil with which the receiver outputs a constant voltage, determine, in accordance with the temporal change in the frequency, a direction in which the electric mobility vehicle is to be moved relative to the housing to increase power transmission efficiency from the feeder to the receiver, and provide, through the notification source, a notification to move the electric mobility vehicle in the direction. The power feeding station with the above structure determines the direction in which the electric mobility vehicle is to be moved toward the stop position at which higher power transmission efficiency may be achieved and guides the electric mobility vehicle in the determined direction. 
     In the power feeding station according to one or more embodiments, the control circuit may record a temporal change in the voltage of the alternating current power supplied to the feeder coil with which the receiver outputs a constant and predetermined voltage, determine, in accordance with the temporal change in the voltage, a direction in which the electric mobility vehicle is to be moved relative to the housing to increase power transmission efficiency from the feeder to the receiver, and provide, through the notification source, a notification to move the electric mobility vehicle in the direction. The power feeding station with the above structure determines the direction in which the electric mobility vehicle is to be moved toward the stop position at which higher power transmission efficiency may be achieved and guides the electric mobility vehicle in the determined direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram illustrating a contactless power feeding system including a power feeding station and a two-wheeler according to an embodiment or embodiments. 
         FIG.  2    is a diagram illustrating a schematic front view of a power feeding station as viewed from a two-wheeler to be parked. 
         FIG.  3    is a diagram illustrating a schematic side view of a power feeding station. 
         FIG.  4    is a diagram illustrating a schematic perspective view of a power feeding station with a two-wheeler parked at the power feeding station. 
         FIG.  5    is a diagram illustrating a partially enlarged view of a power feeding station and a two-wheeler describing power feeding from the power feeding station to the two-wheeler. 
         FIG.  6    is a diagram illustrating a schematic diagram of a feeder. 
         FIG.  7    is a diagram illustrating a schematic diagram of a receiver. 
         FIG.  8    is a diagram illustrating an equivalent circuit diagram of a feeder and a receiver. 
         FIG.  9    is a graph illustrating example simulation results of a frequency response of an output voltage from a receiver. 
         FIG.  10    is a graph illustrating example simulation results of a frequency response of an output voltage at varying voltages applied to a power feeder coil in accordance with a degree of coupling in a simulation, such as is shown in  FIG.  9   . 
         FIGS.  11 A and  11 B  are diagrams each illustrating an example positional relationship between a feeder coil in a power feeding station and a receiver coil in a two-wheeler and an example message appearing on a display in a positional relationship. 
     
    
    
     DETAILED DESCRIPTION 
     A power feeding station according to one or more embodiments will now be described with reference to the drawings. The power feeding station includes a feeder for supplying power to a two-wheeler in a housing to transmit power to the two-wheeler through a feeder coil in the feeder and a receiver coil in a receiver included in the two-wheeler being parked. The frequency of alternating current (AC) power supplied to the feeder coil is set to a predetermined frequency corresponding to the degree of coupling between the feeder coil and the receiver coil. The feeder in the power feeding station and the receiver in the two-wheeler can thus perform a constant voltage output operation in which the receiver outputs a constant voltage independently of the resistance of a load connected to the receiver. The power feeding station identifies the frequency of AC power supplied to the feeder coil that allows the feeder and the receiver to perform a constant voltage output operation, and provides, based on the identified frequency, a notification to guide the two-wheeler to a position at which higher power transmission efficiency is achieved. 
       FIG.  1    is a schematic diagram of a contactless power feeding system including a power feeding station and a two-wheeler according to the present embodiment. 
     As shown in  FIG.  1   , the contactless power feeding system  1  includes a power feeding station  2  and a two-wheeler  3 . The power feeding station  2  can feed power to the two-wheeler  3 . The power feeding station  2  includes a housing  10 , a feeder  11 , and a display  12 . The feeder  11  includes a feeder coil  32  in the housing  10 . 
     The two-wheeler  3  is an example of an electric mobility vehicle. The two-wheeler  3  includes a receiver  21  including a receiver coil  41 , and a battery  22  for storing power received by the receiver  21 . The two-wheeler  3  includes a front basket  24  above its front wheel  23 , and a box  25  attached to the bottom surface of the front basket  24 . The box  25  is formed from an insulating material such as a resin. The box  25  accommodates the receiver  21 . The receiver coil  41  in the receiver  21  between the front wheel  23  and the front basket  24  of the two-wheeler  3  is attached to the two-wheeler  3  with its winding axis extending ahead of the two-wheeler  3 . The battery  22  is installed on a rack located above the rear wheel of the two-wheeler  3  and is charged with power received through the receiver  21 . The power charged in the battery  22  is used to drive a motor (not shown) that provides auxiliary power for the two-wheeler  3  or is used to turn on a headlight  26 . When the parked two-wheeler  3  satisfies a predetermined positional relationship with the power feeding station  2 , the feeder coil  32  in the feeder  11  feeds power contactlessly to the receiver coil  41  in the receiver  21 . 
       FIG.  2    is a schematic front view of the power feeding station  2  as viewed from the two-wheeler  3  to be parked.  FIG.  3    is a schematic side view of the power feeding station  2 .  FIG.  4    is a schematic perspective view of the power feeding station  2  with the two-wheeler  3  parked at the power feeding station  2 .  FIG.  5    is a partially enlarged view of the power feeding station  2  and the two-wheeler  3  describing power feeding from the power feeding station  2  to the two-wheeler  3  parked at the power feeding station  2 . A surface of the housing  10  facing the two-wheeler  3  parked at the power feeding station  2  may hereafter be referred to as a front surface of the housing  10 . 
     The housing  10  in the power feeding station  2  defines a predetermined parking position for the two-wheeler  3 . In the present embodiment, as shown in  FIGS.  2  to  4   , the front surface of the housing  10  has an inverted U shape as a whole. The housing  10  includes two substantially quadrangular prism-shaped pillars  13  and  14 , which extend substantially vertically to a road surface on which the power feeding station  2  is installed, and an upper end  15  connecting upper portions of the two pillars  13  and  14 . When the two-wheeler  3  is parked at the power feeding station  2 , the front wheel of the two-wheeler  3  is placed in a space between the two pillars  13  and  14  of the housing  10 . More specifically, the space between the two pillars  13  and  14  corresponds to the position of the front wheel of the two-wheeler  3  to be parked. 
     Each of the pillars  13  and  14  and the upper end  15  is hollow and is formed from a metal such as aluminum or stainless steel, a resin, or a combination of these materials. One of the pillars  13  or  14  internally accommodates a power cable (not shown) for transmitting power from a utility or direct current (DC) power source to the feeder  11 . A board (not shown) is attached inside the pillar  13 , the pillar  14 , or the upper end  15 . A power supply circuit in the feeder  11  is located on the board. The pillar  13  may have a groove  13   a  on its surface facing the pillar  14 . The pillar  14  may have a groove  14   a  on its surface facing the pillar  13 . The grooves  13   a  and  14   a  may be engageable with the hub shaft of the front wheel of the two-wheeler  3  when the two-wheeler  3  is parked. Each of the grooves  13   a  and  14   a  may be gradually narrower from its portion near the front surface to its portion near a surface opposite to the front surface (hereafter referred to as a back surface). 
     To allow the receiver coil  41  in the receiver  21  in the two-wheeler  3  parked at the power feeding station  2  to face the feeder coil  32  in the feeder  11 , the upper end  15  of the housing  10  includes, on its front surface, a substantially rectangular compartment  16  protruding toward the parked two-wheeler  3 . The compartment  16  internally accommodates the feeder coil  32 . The feeder coil  32  is installed with its winding axis orthogonal to a surface  16   a  at the front (hereafter also referred to as a front surface) of the compartment  16 . The compartment  16  is formed from an insulating material, such as a resin, to avoid being affected by power fed from the feeder  11  to the receiver  21 . 
     The compartment  16  may have a cover  17  protruding further toward the two-wheeler  3  than the front surface  16   a  of the compartment  16 . The cover  17  may surround at least a part of the outer periphery of the compartment  16  including its upper end. 
     The upper end  15  includes the display  12  on its front surface. The display  12  is located lateral to the compartment  16 . The display  12  is an example of a notification source and may be, for example, a liquid crystal display or an organic electroluminescent (EL) display. The display  12  displays a message for guidance about the parking position (or stop position) of the two-wheeler  3  received from the feeder  11 . The position of the display  12  is not limited to the present example. The display  12  may be located on the top surface of the upper end  15 . 
     The feeder  11  and the receiver  21  will now be described in detail. 
     In the present embodiment, the feeder  11  and the receiver  21  are included in a contactless power feeding apparatus that uses no resonance in the power feeder but uses a series resonance between the receiver coil  41  and the resonant capacitor  42  in the power receiver (an NS configuration). The feeder  11  and the receiver  21  are not limited to the present example and may be, for example, a contactless power feeding apparatus including primary series-secondary series resonant capacitors (an SS configuration) or primary series-secondary parallel resonant capacitors (an SP configuration). In another example, the feeder  11  and the receiver  21  may be included in a contactless power feeding apparatus that uses no resonance in the power feeder but uses a parallel resonance between the receiver coil  41  and the resonant capacitor  42  in the power feeder (an NP configuration). 
     The feeder  11  included in the power feeding station  2  will now be described.  FIG.  6    is a schematic diagram of the feeder  11 . The feeder  11  includes a power supply circuit  31 , the feeder coil  32 , a communicator  33 , and a control circuit  34 . 
     The power supply circuit  31  supplies AC power with an adjustable frequency and an adjustable voltage to the feeder coil  32 . The power supply circuit  31  includes a full-wave rectifying circuit  311  that converts the AC power supplied from a utility power source to pulsed current power, a power factor correction circuit  312 , and an inverter circuit  313 . 
     The full-wave rectifying circuit  311  is connected between the utility power source and the power factor correction circuit  312 , and converts the AC power supplied from the utility power source to pulsed current power. The full-wave rectifying circuit  311  includes four bridge-connected diodes. The full-wave rectifying circuit  311  outputs pulsed current power to the power factor correction circuit  312 . 
     The power factor correction circuit  312  is connected between the full-wave rectifying circuit  311  and the inverter circuit  313 , and improves the power factor of the pulsed current power output from the full-wave rectifying circuit  311  to convert the pulsed current power to DC power with a boosted voltage. The power factor correction circuit  312  includes, for example, a coil L and a diode D connected in series in the described order from the positive electrode output terminal of the full-wave rectifying circuit  311 , a switching element SW, which is an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET) having a drain terminal connected between the coil L and the diode D and a source terminal connected to the negative electrode output terminal of the full-wave rectifying circuit  311 , and a smoothing capacitor C connected in parallel to the switching element SW across the diode D. The switching element SW has a gate terminal connected to the control circuit  34 . The power factor correction circuit  312  includes two resistors R 1  and R 2  connected in series between the positive electrode output terminal and the negative electrode output terminal of the full-wave rectifying circuit  311 . The resistors R 1  and R 2  are connected in parallel to the smoothing capacitor C between the diode D and the smoothing capacitor C. The voltage across the resistors R 1  and R 2  is measured by the control circuit  34  as a voltage output from the diode D. 
     The control circuit  34  controls the on-off state of the switching element SW in accordance with the duty cycle indicated by the control circuit  34  to allow the waveform of the current output from the diode D to have the same waveform as the voltage of the pulsed current power supplied from the full-wave rectifying circuit  311 . The power factor correction circuit  312  thus performs power factor correction. As the duty cycle in which the switching element SW is on is higher, the diode D outputs a higher voltage. 
     The power output from the diode D is smoothed by the smoothing capacitor C into DC power and is output to the inverter circuit  313 . 
     The power factor correction circuit  312  is not limited to the above structure and may have another structure to output a voltage adjustable as controlled by the control circuit  34 . 
     The inverter circuit  313  connected between the power factor correction circuit  312  and the feeder coil  32  converts the DC power supplied from the power factor correction circuit  312  to AC power with a predetermined frequency and supplies the AC power to the feeder coil  32 . The inverter circuit  313  may be a full-bridge inverter with four switching elements (e.g., n-channel MOSFETs) forming a full-bridge connection. The inverter circuit  313  may be a half-bridge inverter with two switching elements forming a half-bridge connection. With the control circuit  34  controlling on-off switching of each switching element in accordance with the predetermined frequency, the inverter circuit  313  converts the DC power supplied from the power factor correction circuit  312  to AC power with a predetermined frequency. The predetermined frequency allows the feeder  11  and the receiver  21  to perform a constant voltage output operation and is determined in accordance with the degree of coupling between the feeder coil  32  and the receiver coil  41 . The predetermined frequency is hereafter referred to as a constant voltage frequency for ease of explanation. 
     The power supply circuit  31  may further include, between the power factor correction circuit  312  and the inverter circuit  313 , a DC-DC converter that increases or decreases the voltage of the DC power output from the power factor correction circuit  312 . The power supply circuit  31  may include an AC-DC converter that converts AC power supplied from a utility AC power source to DC power, instead of the full-wave rectifying circuit  311  and the power factor correction circuit  312 . The power supply circuit  31  may include a DC power source, such as a lithium-ion secondary battery or a lead-acid storage battery, to supply DC power and a DC-DC converter that increases or decreases the voltage of the DC power supplied from the DC power source, instead of the full-wave rectifying circuit  311  and the power factor correction circuit  312 . 
     The feeder coil  32  transmits, through a space, AC power supplied from the power supply circuit  31  to the receiver coil  41  in the receiver  21 . The feeder  11  may include a capacitor connected in series to the feeder coil  32  between the feeder coil  32  and the inverter circuit in the power supply circuit  31 . The capacitor may be used to cut DC power or to form a resonant circuit that resonates with the feeder coil  32  at the frequency of the AC power supplied to the feeder coil  32 . 
     The communicator  33  extracts, from every radio signal received from a communicator in the receiver  21 , a signal indicating the power reception state of the receiver  21 , and outputs the signal to the control circuit  34 . The communicator  33  includes, for example, an antenna that receives a radio signal in accordance with a predetermined wireless communication standard and a communication circuit that demodulates the radio signal. The predetermined wireless communication standard is, for example, ISO/IEC 15693, ZigBee (registered trademark), or Bluetooth (registered trademark). 
     The control circuit  34  includes, for example, nonvolatile and volatile memory circuits, an arithmetic circuit, and an interface circuit for connection to another circuit. 
     Based on the signal received from the receiver  21  through the communicator  33  indicating the power reception state of the receiver  21 , the control circuit  34  controls on-off switching of each switching element included in the inverter circuit  313  to allow the feeder  11  and the receiver  21  to perform a constant voltage output operation. More specifically, the control circuit  34  controls on-off switching of each switching element included in the inverter circuit  313  to cause the AC power supplied to the feeder coil  32  to have a constant voltage frequency. The control circuit  34  may control the on-off state of the switching element SW in the power factor correction circuit  312  to adjust the voltage of the DC power supplied to the inverter circuit  313  to retain a constant voltage of the power received by the receiver  21 . 
     The control circuit  34  determines whether the two-wheeler  3  is at an appropriate parking position by referring to the constant voltage frequency. When determining that the parking position is inappropriate, the control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  to an appropriate parking position. 
     The operation of the control circuit  34  will be described in detail later. 
     The receiver  21  included in the two-wheeler  3  will now be described.  FIG.  7    is a schematic diagram of the receiver  21 . The receiver  21  includes the receiver coil  41 , a resonant capacitor  42 , a power receiver circuit  43 , and a communicator  44 . The receiver coil  41  forms a resonant circuit with the resonant capacitor  42 . 
     The receiver coil  41  forming the resonant circuit with the resonant capacitor  42  resonates with the AC flowing through the feeder coil  32  in the feeder  11  to receive power from the feeder coil  32 . The resonant capacitor  42  is connected in series to the receiver coil  41 . The resonant capacitor  42  may be connected in parallel to the receiver coil  41 . The resonant circuit including the receiver coil  41  and the resonant capacitor  42  outputs AC power, which is output to the power receiver circuit  43 . The receiver coil  41  and the feeder coil  32  may have the same number or different numbers of turns. 
     The power receiver circuit  43  converts the AC power from the resonant circuit including the receiver coil  41  and the resonant capacitor  42  to DC power. The DC power is output to the battery  22  that is connected to the power receiver circuit  43  through a power cable (not shown) and a charger (not shown) and is located above the rear wheel. The power receiver circuit  43  determines the state of power reception from the feeder  11 , or more specifically, determines whether the output voltage from the power receiver circuit  43  is constant. The power receiver circuit  43  includes a rectifier-smoothing circuit  431 , a voltage detection circuit  432 , a switching element  433 , and a determination circuit  434 . 
     The rectifier-smoothing circuit  431 , which is an example of a rectifier circuit, includes a full-wave rectifying circuit including four bridge-connected diodes or switching elements (e.g., MOSFETs), and a smoothing capacitor. The rectifier-smoothing circuit  431  rectifies and smooths the power received through the receiver coil  41  to convert the power to DC power. The rectifier-smoothing circuit  431  outputs the resultant DC power to the battery  22  through the charger. 
     The voltage detection circuit  432  measures an output voltage across the rectifier-smoothing circuit  431  (specifically, an output voltage from the power receiver circuit  43 , hereafter simply referred to as an output voltage) at predetermined intervals. The output voltage across the rectifier-smoothing circuit  431  corresponds one-to-one to the output voltage from the resonant circuit including the receiver coil  41  and the resonant capacitor  42 . The measurement value of the output voltage across the rectifier-smoothing circuit  431  thus indirectly represents the measurement value of the output voltage from the resonant circuit. The voltage detection circuit  432  may be any known voltage detection circuit that can detect, for example, a DC voltage. The voltage detection circuit  432  outputs a voltage detection signal representing the measurement value of the output voltage to the determination circuit  434 . 
     The switching element  433  is, for example, a MOSFET and is connected between the rectifier-smoothing circuit  431  and the battery  22 . The switching element  433  does not allow a current to flow from the rectifier-smoothing circuit  431  to the battery  22  in the off state (specifically, the AC equivalent resistance of the battery  22  and the charger Rac=00) and allows a current to flow from the rectifier-smoothing circuit  431  to the battery  22  in the on state. 
     The determination circuit  434  determines, based on the measurement value of the output voltage received from the voltage detection circuit  432 , whether the feeder  11  and the receiver  21  are performing a constant voltage output operation and whether the measurement value of the output voltage is within the range of allowable voltages. The determination circuit  434  provides the determination result to the communicator  44 . The determination circuit  434  includes, for example, a memory circuit that stores the allowable range of voltages, an arithmetic circuit that compares the measurement value of the output voltage with the allowable range of voltages, and the control circuit that controls the on-off state of the switching element  433 . 
     The determination circuit  434  turns on and off the switching element  433  at predetermined intervals while the measurement value of the output voltage is out of the allowable range of voltages, which changes the resistance of the entire circuit including the battery  22  connected to the rectifier-smoothing circuit  431  at predetermined intervals. The determination circuit  434  can thus determine whether the feeder  11  and the receiver  21  are performing a constant voltage output operation by determining whether the measurement value of the output voltage is substantially constant while turning on and off the switching element  433 . The determination circuit  434  notifies the communicator  44  that the feeder  11  and the receiver  21  are performing a constant voltage output operation when the measurement value of the output voltage remains substantially constant while the switching element  433  is being turned on and off at predetermined intervals. 
     When the measurement value of the output voltage indicates that the feeder  11  and the receiver  21  are performing a constant voltage output operation for a predetermined period longer than the predetermined interval, the determination circuit  434  stops turning on and off the switching element  433  and retains the on state. The determination circuit  434  determines whether the measurement value of the output voltage is within the allowable range of voltages and provides the determination result to the communicator  44 . 
     When the measurement value of the output voltage is within the allowable range of voltages for the predetermined period longer than the predetermined interval, the determination circuit  434  provides the communicator  44  with the determination result indicating that the feeder  11  and the receiver  21  are performing a constant voltage output operation and the measurement value of the output voltage is within the allowable range of voltages. 
     In one modification, the power receiver circuit  43  may include a resistor connected in parallel to the battery  22  to the rectifier-smoothing circuit  431 . Accordingly, the switching element  433  may be connected in series to the resistor and in parallel to the battery  22 . The determination circuit  434  turns off the switching element  433  while the measurement value of the output voltage is within the allowable range of voltages. When the measurement value of the output voltage is out of the allowable range of voltages, the determination circuit  434  may turn on and off the switching element  433  at predetermined intervals in the same manner as in the above embodiment. In the present modification, the battery  22  continuously receives power while the feeder  11  and the receiver  21  are not performing a constant voltage output operation. 
     In another modification, a second switching element, such as a MOSFET, may be connected in parallel to the above resistor and in series to the battery  22 . Accordingly, while the measurement value of the output voltage is within the allowable range of voltages, the determination circuit  434  retains the on state of the second switching element to supply power to the battery  22 . When the measurement value of the output voltage is out of the allowable range of voltages, the determination circuit  434  may turn off the second switching element to stop supplying power to the battery  22 . The above described structure prevents an unexpectedly high voltage from being applied to the battery  22 , although the voltage of received power rises unexpectedly during adjustment of the frequency of the AC power applied to the feeder coil  32  in the feeder  11 . 
     The communicator  44  generates, at predetermined transmission intervals, a signal indicating the power reception state based on the determination result received from the determination circuit  434 . The signal indicating the power reception state includes determination information indicating whether the feeder  11  and the receiver  21  are performing a constant voltage output operation and whether the measurement value of the output voltage is within the allowable range of voltages. The communicator  44  then generates a radio signal including the signal indicating the power reception state and transmits the radio signal to the communicator  33  in the feeder  11 . The communicator  44  includes, for example, a communication circuit that generates a radio signal in accordance with a predetermined wireless communication standard and an antenna that outputs the radio signal. As in the communicator  33 , the predetermined wireless communication standard is, for example, ISO/IEC 15693, ZigBee (registered trademark), or Bluetooth (registered trademark). 
     The operation of the control circuit  34  in the feeder  11  will be described in detail. A constant voltage output operation will be described first. 
       FIG.  8    is an equivalent circuit diagram of the feeder  11  and the receiver  21 . In an equivalent circuit  100 , the feeder coil  32  is coupled with the receiver coil  41  to form an ideal transformer of n:1. In the equivalent circuit  100 , Lr is the leakage inductance of the feeder coil  32 , and Lm is the magnetizing inductance of the feeder coil  32 . An inductance Lp of the feeder coil  32  is equal to (Lm+Lr), and Lr=(1−k)Lp and Lm=kLp, where k is the degree of coupling between the feeder coil  32  and the receiver coil  41 . Ri is the coil resistance in the feeder  11 , and Ris is the coil resistance in the receiver  21 . Cp is the capacitance of the resonant capacitor  42  in the receiver  21 . Rac is the AC equivalent resistance of a resistance Ro of the battery  22  and the charger, and Rac=(8/π2)×Ro. 
       FIG.  9    is a graph showing example simulation results of the frequency response of an output voltage from the receiver  21  calculated in accordance with the output gain from the equivalent circuit. In  FIG.  9   , the horizontal axis indicates the frequency of AC power applied to the feeder coil  32 , and the vertical axis indicates an output voltage from the receiver  21 . A line  901  represents the frequency response of an output voltage for the degree of coupling k=0.15, and the AC equivalent resistance of the battery  22  and the charger being Rac. A line  902  represents the frequency response of an output voltage for the degree of coupling k=0.15, and the AC equivalent resistance of the battery  22  and the charger being (10*Rac). A line  903  represents the frequency response of an output voltage for the degree of coupling k=0.3, and the AC equivalent resistance of the battery  22  and the charger being Rac. A line  904  represents the frequency response of an output voltage for the degree of coupling k=0.3, and the AC equivalent resistance of the battery  22  and the charger being (10*Rac). A line  905  represents the frequency response of an output voltage for the degree of coupling k=0.6, and the AC equivalent resistance of the battery  22  and the charger being Rac. A line  906  represents the frequency response of an output voltage for the degree of coupling k=0.6, and the AC equivalent resistance of the battery  22  and the charger being (10*Rac). In the simulation, Lp=174 pH, Cp=20 nF, Ri=Ris=0.1Ω, n=1, Vin=300 V, and Ro=10Ω(Rac 8.1Ω). 
     As indicated with the points  911  to  913  in  FIG.  9   , the graph includes, for each degree of coupling k, the combination of the frequency and the output voltage that causes an output voltage to be substantially constant against a varying AC equivalent resistance of the battery  22  under a constant degree of coupling k (or to be a constant voltage output under a constant degree of coupling k). Accordingly, it becomes apparent that appropriately adjusting the frequency of the AC power applied to the feeder coil  32  allows the feeder  11  and the receiver  21  to perform a constant voltage output operation independently of the varying resistance of the battery  22  and the charger. Further, although the output voltage, which is constant against a varying resistance of the battery  22  and the charger, differs depending on the degree of coupling as indicated at the points  911  to  913 , adjusting the voltage applied to the feeder coil  32  can eliminate the difference in the output voltage. The output voltage thus can be substantially constant at any degree of coupling. 
       FIG.  10    is a graph showing example simulation results of the frequency response of an output voltage at varying voltages applied to the feeder coil  32  in accordance with the degree of coupling in the simulation shown in  FIG.  9   . In  FIG.  10   , the horizontal axis indicates the frequency of the AC power applied to the feeder coil  32 , and the vertical axis indicates an output voltage from the receiver  21 . A line  1001  represents the frequency response of an output voltage for the degree of coupling k=0.15, the AC equivalent resistance of the battery  22  and the charger being Rac, and a voltage applied to the feeder coil  32  being Vin. A line  1002  represents the frequency response of an output voltage for the degree of coupling k=0.15, the AC equivalent resistance of the battery  22  and the charger being (10*Rac), and a voltage applied to the feeder coil  32  being Vin. A line  1003  represents the frequency response of an output voltage for the degree of coupling k=0.3, the AC equivalent resistance of the battery  22  and the charger being Rac, and a voltage applied to the feeder coil  32  being ( 0 . 5 *Vin). A line  1004  represents the frequency response of an output voltage for the degree of coupling k=0.3, the AC equivalent resistance of the battery  22  and the charger being (10*Rac), and a voltage applied to the feeder coil  32  being ( 0 . 5 *Vin). A line  1005  represents the frequency response of an output voltage for the degree of coupling k=0.6, the AC equivalent resistance of the battery  22  and the charger being Rac, and a voltage applied to the feeder coil  32  being ( 0 . 25 *Vin). A line  1006  represents the frequency response of an output voltage for the degree of coupling k=0.6, the AC equivalent resistance of the battery  22  and the charger being (10*Rac), and a voltage applied to the feeder coil  32  being ( 0 . 25 *Vin). 
     The combinations of the frequency and the output voltage at three points  1011  to  1013  correspond to the combinations at the three points  911  to  913  shown in  FIG.  9    that cause an output voltage to be substantially constant (or to be a constant voltage output) against a varying AC equivalent resistance of the battery  22  and the charger under the constant degree of coupling k. The output voltages at the points  1011  to  1013  are substantially equal to one another. 
     Accordingly, it becomes apparent that appropriately adjusting the frequency and the voltage of the AC power applied to the feeder coil  32  allows the output voltage to remain substantially constant independently of the varying resistance of the battery  22  and the charger or the varying degree of coupling. 
     The control circuit  34  thus controls the frequency of the AC power applied to the feeder coil  32  (hereafter, simply referred to as a frequency) and the voltage of the AC power applied to the feeder coil  32  in the manner described below to perform a constant voltage output operation. 
     When the determination information included in the signal indicating the power reception state received from the receiver  21  through the communicator  33  indicates that the feeder  11  and the receiver  21  are not performing a constant voltage output operation, the control circuit  34  increases the frequency from the lowest limit within a predetermined frequency range to the highest limit within the frequency range. 
     To allow the determination circuit  434  in the receiver  21  to determine whether the output voltage is substantially constant, the control circuit  34  may change the frequency in a stepwise manner to retain a constant frequency for a period longer than the interval at which the determination circuit  434  turns on and off the switching element  433 . 
     The control circuit  34  may lower the voltage applied to the feeder coil  32  to the lowest value while adjusting the frequency. Lowering the voltage, while adjusting the frequency reduces the likelihood that power with an unexpectedly high voltage is supplied to the receiver  21 . 
     When the determination information included in the signal indicating the power reception state received from the receiver  21  through the communicator  33  indicates that the measurement value of the output voltage is out of the allowable range of voltages but remains substantially constant against a varying resistance of the battery  22 , or more specifically, a constant voltage output operation is being performed, the control circuit  34  subsequently retains a constant frequency. The control circuit  34  selects the duty cycle by referring to a reference table indicating the correspondence between each frequency and the duty cycle that controls the on-off state of the switching element SW in the power factor correction circuit  312  to perform a constant voltage output at the corresponding frequency at any degree of coupling. The control circuit  34  turns on and off the switching element SW in the power factor correction circuit  312  in accordance with the duty cycle. Thus, the voltage to be applied to the feeder coil  32  is adjusted to allow an output voltage from the receiver  21  to be within the allowable range of voltages, or more specifically, to allow a constant voltage to be output at any degree of coupling. When the determination information included in the signal indicating the power reception state received from the receiver  21  through the communicator  33  indicates that the measurement value of the output voltage is within the allowable range of voltages, the control circuit  34  retains a constant frequency and a constant voltage of AC power supplied to the feeder coil  32 . 
     The control circuit  34  may gradually change the duty cycle until the determination information indicates that the measurement value of the output voltage is within the allowable range of voltages, instead of referring to the above reference table to select the duty cycle. 
     The control circuit  34  detects the frequency (constant voltage frequency) of the AC power applied to the feeder coil  32  that allows the feeder  11  and the receiver  21  to perform a constant voltage output operation and determines whether the parking position of the two-wheeler  3  is to be changed based on the frequency. Based on the determination result, the control circuit  34  provides a notification of guidance about the stop position of the two-wheeler  3  relative to the housing  10  through the display  12 . 
     Referring back to  FIG.  9   , when the AC power applied to the feeder coil  32  has a constant voltage, the output voltage with the feeder  11  and the receiver  21  performing a constant voltage output operation varies depending on the degree of coupling between the feeder coil  32  and the receiver coil  41 . More specifically, the output voltage increases as the degree of coupling between the feeder coil  32  and the receiver coil  41  increases. The power transmission efficiency thus increases as the degree of coupling increases. The degree of coupling between the feeder coil  32  and the receiver coil  41  changes in accordance with the positional relationship between the feeder coil  32  and the receiver coil  41 . With the structure of the housing  10  in the power feeding station  2  in the present embodiment, the two-wheeler  3  parked at the power feeding station  2  moves to cause the receiver coil  41  to approach the front of the feeder coil  32 . More specifically, when the two-wheeler  3  is parked, the distance between the feeder coil  32  and the receiver coil  41  changes in the direction parallel to the winding axis of the feeder coil  32 , but the feeder coil  32  and the receiver coil  41  remain in mostly the same positional relationship in the direction orthogonal to the winding axis of the feeder coil  32 . In the present embodiment, the distance between the feeder coil  32  and the receiver coil  41  is smaller when the two-wheeler  3  is nearer the housing  10 , thus increasing the degree of coupling between the feeder coil  32  and the receiver coil  41 . 
     The control circuit  34  thus compares the constant voltage frequency with a predetermined frequency threshold. As shown in  FIG.  9   , the feeder  11  and the receiver  21  in the present embodiment have higher power transmission efficiency when the constant voltage frequency is higher. The predetermined frequency threshold is thus set as a minimum constant voltage frequency with which the power transmission efficiency from the feeder  11  to the receiver  21  satisfies a predetermined power efficiency condition. When the constant voltage frequency is lower than the predetermined frequency threshold, the control circuit  34  determines that the feeder coil  32  is farther than intended from the receiver coil  41 . The control circuit  34  then causes the display  12  to display a message prompting movement of the two-wheeler  3  further toward the power feeding station  2 . 
     When the constant voltage frequency is higher than or equal to the predetermined frequency threshold, the control circuit  34  determines that the feeder coil  32  is sufficiently near the receiver coil  41  to achieve sufficiently high power transmission efficiency. The control circuit  34  then causes the display  12  to display a message prompting stopping of the two-wheeler  3  at the current position. 
       FIGS.  11 A and  11 B  are diagrams each showing an example positional relationship between the feeder coil  32  in the power feeding station  2  and the receiver coil  41  in the two-wheeler  3  and an example message appearing on the display  12  in the positional relationship. 
     In the example shown in  FIG.  11 A , the receiver coil  41  is not sufficiently near the feeder coil  32 . The constant voltage frequency is thus lower than the frequency threshold. The display  12  displays a message such as “Please move your vehicle a little forward.” prompting movement of the two-wheeler  3  further toward the power feeding station  2 . 
     In the example shown in  FIG.  11 B , the receiver coil  41  is sufficiently near the feeder coil  32 . The constant voltage frequency is thus higher than or equal to the frequency threshold. The display  12  displays a message such as “Please park here.” prompting stopping of the two-wheeler  3  at the current position. 
     As described above, the power feeding station includes the feeder for feeding power to a two-wheeler and transmits power to the two-wheeler through the feeder coil in the feeder and the receiver coil in the receiver included in the parked two-wheeler. The power feeding station detects the frequency of AC power supplied to the feeder coil that allows the feeder and the receiver to perform a constant voltage output operation and provides, based on the frequency, a notification to guide the two-wheeler to a position at which higher power transmission efficiency is achieved. The power feeding station thus can feed power contactlessly to the two-wheeler through the feeder coil and can also provide guidance about the parking position of the two-wheeler to increase the power transmission efficiency. 
     In one modification, the control circuit  34  in the feeder  11  may determine whether a parking position of a two-wheeler is appropriate based on the voltage of the AC power applied to the feeder coil  32  with which the receiver  21  outputs a constant voltage. 
     Referring back to  FIGS.  9  and  10   , each constant voltage frequency corresponds one-to-one to the voltage of the AC power applied to the feeder coil  32  to allow the receiver  21  to output a predetermined voltage. When the feeder coil  32  and the receiver coil  41  have a higher degree of coupling, the AC power applied to the feeder coil  32  is set to have a lower voltage. The control circuit  34  thus compares the voltage of the AC power applied to the feeder coil  32 , which allows the receiver  21  to output the predetermined voltage when the feeder  11  and the receiver  21  are performing a constant voltage output operation, with a predetermined voltage threshold. When the voltage of the AC power applied to the feeder coil  32  is higher than the predetermined voltage threshold, the control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  further toward the power feeding station  2 . When the voltage of the AC power applied to the feeder coil  32  is lower than or equal to the predetermined voltage threshold, the control circuit  34  causes the display  12  to display a message prompting stopping of the two-wheeler  3  at the current position. 
     In the present modification, the power feeding station can feed power contactlessly to a two-wheeler through the feeder coil and can provide guidance about the parking position of the two-wheeler to increase the power transmission efficiency. 
     In another modification, the receiver coil  41  included in the receiver  21  in the two-wheeler  3  may be attached to the two-wheeler  3  with its winding axis extending in the lateral direction of the two-wheeler  3 . For example, the receiver coil  41  may be installed lateral to the front wheel  23  of the two-wheeler  3  with its winding axis orthogonal to the surface of rotation of the front wheel  23  of the two-wheeler  3 . Accordingly, the feeder coil  32  included in the feeder  11  in the power feeding station  2  may be installed to face the receiver coil  41  when the two-wheeler  3  is parked at an appropriate position for power transmission. For example, in the above embodiment, the feeder coil  32  may be installed in one of the pillars  13  or  14  in the housing  10  to be adjacent to the front wheel  23  of the parked two-wheeler  3 . Accordingly, the feeder coil  32  is installed with its winding axis orthogonal to the surface of the pillar in the housing  10  that faces the front wheel  23  of the two-wheeler  3 . 
     The above-described structure may cause insufficient power transmission efficiency when the two-wheeler  3  is parked to have its receiver coil  41  located beyond the feeder coil  32  or before the feeder coil  32 . To place the receiver coil  41  near the feeder coil  32 , the two-wheeler  3  is to be moved in a different direction depending on whether the receiver coil  41  is located beyond the feeder coil  32  or before the feeder coil  32 . 
     In the present modification, the control circuit  34  records temporal changes in the constant voltage frequency in a memory included in the control circuit  34 . The control circuit  34  then refers to the temporal changes in the constant voltage frequency to determine the direction in which the two-wheeler  3  is to be moved. 
     For example, the receiver coil  41  located beyond the feeder coil  32  passes a position nearest the feeder coil  32 . The constant voltage frequency changes to be high and then to be low again. In contrast, the receiver coil  41  located before the feeder coil  32  does not reach the position nearest the feeder coil  32 . Thus, the constant voltage frequency is not very high. The control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  rearward from the power feeding station  2  when the temporal changes in the constant voltage frequency for a latest predetermined period include a maximum value that is higher than or equal to a predetermined frequency threshold and the current constant voltage frequency is lower than the frequency threshold. In contrast, the control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  further toward the power feeding station  2  when the temporal changes in the constant voltage frequency for the latest predetermined period do not include reaching the predetermined frequency threshold or higher. When the current constant voltage frequency is higher than or equal to the frequency threshold, the control circuit  34  causes the display  12  to display a message prompting stopping of the two-wheeler  3  at the current position. 
     Similarly, the control circuit  34  may record, in a memory included in the control circuit  34 , the temporal changes in the voltage of AC power applied to the feeder coil  32  that allows an output voltage from the receiver  21  to be a predetermined voltage while the feeder  11  and the receiver  21  are performing a constant voltage output operation. The control circuit  34  may thus determine the direction in which the two-wheeler  3  is to be moved by referring to the temporal changes in the voltage of the AC power. Accordingly, the control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  rearward from the power feeding station  2  when the temporal changes in the voltage of the AC power for the latest predetermined period include a minimum voltage that is lower than or equal to a predetermined voltage threshold and the current voltage of the AC power is higher than the voltage threshold. In contrast, the control circuit  34  causes the display  12  to display a message prompting movement of the two-wheeler  3  further toward the power feeding station  2  when the temporal changes in the voltage of the AC power for the latest predetermined period do not include reaching the predetermined voltage threshold or lower. When the current voltage of the AC power is lower than or equal to the voltage threshold, the control circuit  34  causes the display  12  to display a message prompting stopping of the two-wheeler  3  at the current position. 
     In the present modification, the power feeding station can appropriately determine the direction in which the two-wheeler is to be moved relative to the power feeding station to increase the power transmission efficiency. 
     In the embodiment or the modifications described above, the power feeding station  2  may include one or more light sources instead of the display  12 . Accordingly, the light source is another example of the notification source. The control circuit  34  changes the lighting state of the light source depending on whether the constant voltage frequency is lower than the predetermined frequency threshold or the constant voltage frequency is higher than or equal to the predetermined threshold. For example, when the constant voltage frequency is lower than the predetermined frequency threshold, the control circuit  34  may cause the light source to flash or to emit red light to prompt movement of the two-wheeler  3  further forward. When the constant voltage frequency is higher than or equal to the predetermined frequency threshold, the control circuit  34  may cause the light source to light continuously or to emit blue or green light to prompt parking of the two-wheeler  3  at the current position. In some embodiments, the power feeding station  2  may include a speaker instead of the display  12  or in addition to the display  12 . The speaker is still another example of the notification source. The control circuit  34  may use a sound from the speaker to provide a notification of guidance about the parking position of the two-wheeler  3 . 
     In still another modification, the display  12  may be installed on the two-wheeler  3 . For example, the display  12  may be installed on the handle of the two-wheeler  3  to face upward. Accordingly, the control circuit  34  in the feeder  11  transmits a signal including a notification of guidance about the stop position of the two-wheeler  3  through the communicator  33  to the communicator  44  in the receiver  21  installed on the two-wheeler  3 . Upon receiving the signal from the communicator  33 , the communicator  44  outputs a notification of guidance about the stop position of the two-wheeler  3  included in the signal to the display  12 . The display  12  may display the notification. The power feeding station  2  and the two-wheeler  3  may each include the display  12 . The display  12  in the power feeding station  2  and the display  12  in the two-wheeler  3  may each output a notification of guidance about the stop position of the two-wheeler  3 . Similarly to the above modification, instead of the display  12 , or in addition to the display  12 , the two-wheeler  3  may include a light source or a speaker. The light source or the speaker may output a notification of guidance about the stop position of the two-wheeler  3  included in the signal received through the communicator  44 . The present modification also allows the power feeding station  2  to achieve the advantageous effects similar to the above embodiment. 
     In still another modification, the control circuit  34  in the feeder  11  may detect the constant voltage frequency by monitoring a current flowing through the feeder coil  32  as described in Japanese Unexamined Patent Application Publication No. 2018-207764. Accordingly, the feeder  11  includes a current detection circuit (not shown) that detects a current flowing through the feeder coil  32 . The power receiver circuit  43  in the receiver  21  includes a constant load circuit (not shown). The constant load circuit provides a constant load connected to the resonant circuit including the receiver coil  41  and the resonant capacitor  42 . The current detection circuit measures a current flowing through the feeder coil  32 . The control circuit  34  may determine, as the constant voltage frequency, the frequency of the AC power applied to the feeder coil  32  and causing a maximum current to flow through the feeder coil  32 . 
     In still another modification, the power feeding station may feed power to an object other than a two-wheeler. For example, the power feeding station may feed power to an electric scooter. The electric scooter is another example of an electric mobility vehicle. When the electric scooter is placed at the power feeding station with the receiver coil installed on the electric scooter approaching the feeder coil in a predetermined direction, the power feeding station can guide the electric scooter to a position at which higher power transmission efficiency is achieved as in the embodiments or the modifications described above. 
     As described above, those skilled in the art can make various changes in accordance with embodiments implemented within the scope of the present invention.