Patent Publication Number: US-2022216739-A1

Title: Power feed system

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply system for wirelessly supplying power from a power transmission device to a small mobile device, such as a mobile information terminal or a smart watch. 
     BACKGROUND ART 
     Methods for supplying power wirelessly include an electromagnetic induction method, a magnetic resonance method, and a radio wave reception method, and an appropriate method is adopted according to conditions such as an inter-device distance between a power transmission device and a power reception device or power to be supplied. Regarding the efficiency of wireless power supply, for example, Patent Document 1 describes “power transmission device including an acquisition means for acquiring distance information indicating the distance between the power reception device and the power transmission device and a change means for changing the power supply area in wireless power transmission based on the distance information acquired by the acquisition means” in order to realize energy saving when the power transmission device supplies power to the power reception device by wireless power transmission. 
     In addition, Patent Document 2 describes a wearable power supply device for wirelessly supplying power to a wearable device worn by a user, and the wearable power supply device is configured to be attracted to the wearable device by the force of a magnet and to transmit power by an induction coil. At this time, it is described that the attractive force of the magnet aligns the induction coil to improve the efficiency of power transmission. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: JP 2015-126635 A 
     Patent Document 2: US 2017/0033567 A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     When charging a small mobile device (power reception device), such as a mobile information terminal or a smart watch, with a power transmission device such as a mobile battery, it is preferable that power can be supplied wirelessly from the power transmission device. In addition, since there are various usage conditions of small mobile devices, it is preferable that the positional relationship between the power reception device and the power transmission device is not fixed and it is possible to cope with various states, such as a state in which the devices are in close contact with each other and a state in which the devices are apart from each other. In addition, in mobile batteries, it is necessary to prevent wasteful power consumption during power supply. 
     The technique disclosed in Patent Document 1 changes the output of wireless power transmission based on the inter-device distance between the power reception device and the power transmission device to change the power supply area. However, if the inter-device distance increases or misalignment between the devices increases, the power supply efficiency is reduced and wasteful power supply is performed, but this is not taken into consideration. 
     In addition, the technique disclosed in Patent Document 2 is to attract the wearable device and the wearable power supply device with a magnet to align the wearable device and the wearable power supply device. That is, the technique is based on the assumption that the wearable device and the wearable power supply device are used in close contact with each other, and performing a power supply operation in a state in which the wearable device and the wearable power supply device are apart from each other is not taken into consideration. 
     It is an object of the present invention to provide a power supply system capable of appropriately performing wireless power supply by preventing a decrease in power supply efficiency by responding to an inter-device distance or misalignment between a power transmission device and a power reception device. 
     Solutions to Problems 
     The present invention is a power supply system for wirelessly supplying power between a power transmission device and a power reception device. The power transmission device includes: a power transmission unit for generating supply power and transmitting the generated supply power to the power reception device; a power transmission side magnet for alignment with the power reception device; a magnetic sensor for detecting a magnetic force from a power reception side magnet provided in the power reception device; and a control unit for controlling the power transmission device. The power reception device includes: a power reception unit for receiving the supply power supplied from the power transmission device; the power reception side magnet for alignment with the power transmission device; and a control unit for controlling the power reception device. When the power transmission device and the power reception device are in close contact with each other, power is wirelessly transmitted from the power transmission unit to the power reception unit in a state in which the power transmission unit and the power reception unit are aligned by an attractive magnetic force between the power transmission side magnet and the power reception side magnet. When the power transmission device and the power reception device are apart from each other, the control unit of the power transmission device starts or stops power transmission from the power transmission unit according to a magnetic force detection signal from the power reception side magnet detected by the magnetic sensor. 
     Effects of the Invention 
     According to the present invention, it is possible to realize a power supply system that appropriately performs wireless power supply by preventing a decrease in power supply efficiency without wasting power according to the inter-device distance between the power transmission device and the power reception device and by eliminating misalignment between the devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram showing a configuration example of a power supply system (power transmission device) according to a first embodiment. 
         FIG. 1B  is a block diagram showing a configuration example of a power supply system (power reception device) according to the first embodiment. 
         FIG. 2A  is a diagram showing a relationship between an inter-device distance and power supply efficiency. 
         FIG. 2B  is a diagram showing a relationship between an inter-device distance and a magnetic force detection signal of a magnetic sensor. 
         FIG. 3  is a diagram showing a state in which a power transmission device and a power reception device are closely arranged. 
         FIG. 4  is a diagram showing a state in which a power transmission device and a power reception device are arranged apart from each other. 
         FIG. 5A  is a flowchart when a power supply method is switched so as to maximize the power supply efficiency. 
         FIG. 5B  is a flowchart when the switching of the power supply method in  FIG. 5A  is simplified. 
         FIG. 5C  is a flowchart when the user&#39;s selection is included in the switching of the power supply method in  FIG. 5B . 
         FIG. 6A  is a bird&#39;s-eye view showing a configuration example of a power transmission unit of a power transmission device. 
         FIG. 6B  is a cross-sectional view when a power transmission device and a power reception device are closely arranged. 
         FIG. 6C  is a cross-sectional view when a power transmission device and a power reception device are arranged apart from each other. 
         FIG. 7A  is a bird&#39;s-eye view showing another configuration example of the power transmission unit of the power transmission device. 
         FIG. 7B  is a cross-sectional view when a power transmission device and a power reception device are closely arranged. 
         FIG. 8  is a flowchart showing a pairing process between a power transmission device and a power reception device (second embodiment). 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the diagrams. 
     First Embodiment 
     In a first embodiment, the basic configuration and operation of a power supply system of the present invention will be described. 
       FIGS. 1A and 1B  are block diagrams showing a configuration example of a power supply system  1  according to the first embodiment. The power supply system  1  is configured to include a power transmission device  100  and a power reception device  200 , and power wirelessly transmitted from the power transmission device  100  is received by the power reception device  200 .  FIG. 1A  shows the internal configuration of the power transmission device  100 , and  FIG. 1B  shows the internal configuration of the power reception device  200 . The power transmission device  100  shown in  FIG. 1A  shows a case of a mobile battery that has a built-in battery charged by an external AC power supply and wirelessly supplies power stored in the built-in battery to the power reception device  200 . In addition, the power reception device  200  shown in  FIG. 1B  shows a case of a smart watch that is a multifunctional wristwatch-type wearable device. The smart watch is powered by a built-in battery, has a display with a small touch screen, a speaker, a microphone, a vibrator, operation buttons, and a human body sensor such as a heart rate sensor, and can communicate various kinds of information and make voice calls through wireless communication. Hereinafter, the configuration of each device will be described in detail. 
     In  FIG. 1A , the power transmission device  100 , which is a mobile battery, includes a power transmission antenna  101 , a power-to-radio wave conversion unit  102 , a power transmission coil  103 , and a power-to-current conversion unit  104  as a power transmission unit  105 . In addition, the power transmission device  100  includes power transmission side magnets  106  and  107 , a magnetic sensor  108 , an optical sensor  109 , a control unit  110 , a memory unit  111 , a display unit  113  having an operation input unit  112 , and a short-range wireless communication unit  115  including a transmission and reception antenna  114 . In addition, the power transmission device  100  includes a DC converter  132  for converting external AC power  2  into DC power, a battery  131 , and a switch  130 . The components excluding the battery  131  and the power transmission side magnets  106  and  107  are connected to each other through a bus  140 . 
     In  FIG. 1B , the power reception device  200 , which is a smart watch, includes a power reception antenna  201 , a radio wave power reproduction unit  202 , a power reception coil  203 , and a coil power reproduction unit  204  as a power reception unit  205 . In addition, the power reception device  200  includes power reception side magnets  206  and  207 , a light emitting element  209 , a control unit  210 , a memory unit  211 , a display unit  213  having an operation input unit  212 , a short-range wireless communication unit  215  including a transmission and reception antenna  214 , a sound input and output unit  216 , and a vibrator  217 . In addition, the power reception device  200  includes a human body sensor  218  such as a heart rate sensor, a wearing detection unit  219 , a position measurement unit  220 , a base station communication unit  222  including a transmission and reception antenna  221 , a switch  230 , a charge processing unit  231 , and a battery  232 . The components excluding the power reception side magnets  206  and  207  and the battery  232  are connected to each other through a bus  240 . 
     Next, the operation of the power supply system  1  will be described from the power transmission device  100  side. DC power is supplied from the battery  131  to the power transmission unit  105  through the switch  130 . The power-to-radio wave conversion unit  102  in the power transmission unit  105  converts the DC power supply power supplied from the battery  131  into radio waves, and the power transmission antenna  101  radiates the converted radio waves to the outside. The radiated radio waves are converted into power by the power reception antenna  201  on the power reception device  200  side. In this case, wireless power supply is performed by a so-called radio wave reception method. Therefore, the power supply efficiency is not high, but relatively large power can be transmitted over a long distance. At that time, in the radio wave band, microwaves or millimeter waves having a wavelength of 1 meter to 1 millimeter are used. 
     On the other hand, the power-to-current conversion unit  104  in the power transmission unit  105  converts the DC power supply power supplied from the battery  131  into an alternating current, and the converted alternating current flows through the power transmission coil  103 . Due to the alternating current flowing through the power transmission coil  103 , a magnetic flux passing through the power transmission coil  103  is generated. At this time, a magnetic flux is also generated in the power reception coil  203  on the power reception device  200  side arranged close to the power transmission coil  103  to induce an alternating current. As a result, power is wirelessly supplied to the power reception coil  203 . In this case, the wireless power supply is performed by a so-called electromagnetic induction method, and stable wireless power supply is possible when the power transmission device  100  and the power reception device  200  are closely aligned with each other. However, if the inter-device distance increases or misalignment between the coils occurs, the power supply efficiency decreases. 
     In addition, by arranging the coils so as to face each other as in the electromagnetic induction method, it is possible to operate the coils in a magnetic resonance method. In the magnetic resonance method, the vibration of the magnetic field generated by the flow of an alternating current through the power transmission coil  103  is transmitted to a resonance circuit configured to include the power reception coil  203  that resonates at the same frequency, so that power is wirelessly supplied to the power reception coil  203 . In the case of the magnetic resonance method, there is some margin in the alignment between the coils as compared with the case of the electromagnetic induction method. However, in order to transmit the vibration of the magnetic field, it is necessary to arrange the power reception coil  203  at a distance sufficiently smaller than the wavelength of the frequency at which the power reception coil  203  is used, and it is necessary to shorten the inter-device distance as in the case of the electromagnetic induction method. 
     The power transmission side magnets  106  and  107  are magnets arranged near the surface of the power transmission device  100 . When the power transmission device  100  and the power reception device  200  are closely arranged, the power transmission device  100  and the power reception device  200  are attracted to each other by the magnetic force between the power reception side magnets  206  and  207  arranged at opposite positions in the power reception device  200 . As a result, the power transmission coil  103  and the power reception coil  203  can be accurately aligned, and the efficiency of power supply from the power transmission coil  103  to the power reception coil  203  can be maximized. 
     The magnetic sensor  108  detects the magnetic force from the power reception side magnets  206  and  207  and generates a magnetic force detection signal. The level of the generated magnetic force detection signal depends on the inter-device distance between the power transmission device  100  and the power reception device  200 . The shorter the distance, the higher the signal level, and the longer the distance, the lower the signal level. Therefore, the inter-device distance can be determined by the level of the magnetic force detection signal. As the magnetic sensor  108 , for example, a Hall element that outputs a voltage according to the magnetic flux density or the direction when a current flows through a semiconductor thin film or the like can be used. Since the Hall element can detect even a static magnetic field in which there is no magnetic flux density change, the Hall element is suitable for detecting the magnetic force from the magnet. 
     The optical sensor  109  detects light from the light emitting element  209  on the power reception device  200  side. For example, a light inlet and a light outlet are provided on the facing surfaces of the power transmission device  100  and the power reception device  200  with the power transmission unit  105  (power transmission coil  103 ) and the power reception unit  205  (power reception coil  203 ) aligned. Then, the alignment of the power transmission unit  105  and the power reception unit  205  is determined by detecting the light emitted from the light emitting element  209  with the optical sensor  109 . 
     The control unit  110  is configured to include a CPU (Central Processing Unit) and the like, and executes a program such as an OS (Operating System) or an operation control application stored and stored in the memory unit  111 . Then, the operation of the entire power transmission device  100  is controlled according to the operation information input to the operation input unit  112 , the magnetic force detection signal from the magnetic sensor  108 , or the like. 
     For example, in a state in which the power transmission device  100  and the power reception device  200  are closely arranged and the power transmission coil  103  and the power reception coil  203  are aligned by the attractive force of the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207 , the switch  130  is operated to supply the DC power from the battery  131  to the power-to-current conversion unit  104 . Then, control is performed so that power is wirelessly supplied from the power transmission coil  103  using the electromagnetic induction method or the magnetic resonance method. In addition, when the power transmission device  100  and the power reception device  20  are not closely arranged, the switch  130  is operated to supply the DC power from the battery  131  to the power-to-radio wave conversion unit  102 . Then, control is performed so that power is wirelessly supplied from the power transmission antenna  101  using the radio wave reception method. In addition, the control unit  110  determines the inter-device distance between the power transmission device  100  and the power reception device  200  based on the magnetic force detection signal from the magnetic sensor  108 . When the magnetic force detection signal is less than a predetermined level (when the inter-device distance is larger than a predetermined value), the control unit  110  performs control to stop the wireless power supply using the radio wave reception method. 
     The memory unit  111  is a flash memory or the like, and stores not only various programs used by the control unit  110  but also data such as information set and input by the operation input unit  112 , a magnetic force detection signal detected by the magnetic sensor  108 , and communication information between the wireless communication units  115  and  215 . 
     The operation input unit  112  is a user input means using, for example, a keyboard or key buttons.  FIG. 1A  shows a case where an input operation screen is displayed within the display screen of the display unit  113 . For example, the operation input unit  112  is a touch pad type input means such as a capacitance type input means, and detects an approach or contact operation using a finger or a touch pen as an operation input. In addition, an operation input unit such as a keyboard may be provided separately from the display unit  113 , or may be separated from the main body of the power transmission device  100  and connected by wire or wirelessly. In addition, the user may give an instruction for the operation input by voice, and a microphone may collect the sound to acquire the operation input information. 
     The display unit  113  is configured by using a liquid crystal panel or the like, and displays notifications to the user and various kinds of data information. For example, the power transmission operation state of the power transmission device  100 , the remaining amount or charge state of the battery  131 , information of communication with the power reception device  200 , and the like are displayed. 
     The short-range wireless communication unit  115  performs wireless communication of information regarding the power supply operation with the wireless communication unit  215  of the power reception device  200 . In addition, when the power transmission device  100  and the power reception device  200  are connected to each other, information necessary for performing a pairing operation, which is the work of authenticating each other, is communicated. In addition, for example, an electronic tag can be used for the short-range wireless communication units  115  and  215 . In addition, if wireless communication is possible in a short distance, it is possible to use a wireless LAN, such as Bluetooth (registered trademark), IrDA (Infrared Data Association, registered trademark), Zigbee (registered trademark), HomeRF (Home Radio Frequency, registered trademark), or Wi-Fi (registered trademark). 
     The switch  130  of the power transmission device  100  switches to the power-to-radio wave conversion unit  102  or the power-to-current conversion unit  104  according to the instruction from the control unit  110  to supply the DC power from the battery  131 . The battery  131  drives each component in the power transmission device  100 , and is charged by the DC power obtained as a result of conversion from the external AC power  2  by the DC converter  132  when the battery capacity decreases. 
     Next, an operation on the power reception device  200  side will be described. The power reception unit  205  receives the power transmitted from the power transmission unit  105  of the power transmission device  100 . In the case of the radio wave reception method, the radio wave radiated from the power transmission antenna  101  is received by the power reception antenna  201 , and the radio wave power reproduction unit  202  converts the received radio wave into a direct current to reproduce DC power supply power. On the other hand, in the case of the electromagnetic induction method or the magnetic resonance method, an alternating current is induced in the power reception coil  203  from the alternating current flowing through the power transmission coil  103 , and the coil power reproduction unit  204  converts the induced alternating current into a direct current to reproduce DC power supply power. 
     The power reception side magnets  206  and  207  are magnets arranged near the surface of the power reception device  200 . When the power transmission device  100  and the power reception device  200  are closely arranged, the power reception side magnets  206  and  207  and the power transmission side magnets  106  and  107  facing each other attract each other by magnetic force. Therefore, the power transmission coil  103  and the power reception coil  203  can be accurately aligned, and the efficiency of power supply from the power transmission coil  103  to the power reception coil  203  can be maximized. 
     A light emitting element, such as an LED, is used as the light emitting element  209 . The light emitted from the light emitting element  209  is detected by the optical sensor  109  on the power transmission device  100  side, and is used for determining the alignment between the power transmission unit  105  and the power reception unit  205 . 
     Similarly to the control unit  110 , the control unit  210  is configured to include a CPU and the like, and executes a program such as an OS or an operation control application stored and stored in the memory unit  111 . Then, the operation of the entire power reception device  200  is controlled according to the operation information input by the operation input unit  212  or the like. Similarly to the memory unit  111 , the memory unit  211  is a flash memory or the like, and stores not only various programs used by the control unit  210  but also data such as information set and input by the operation input unit  212 , signal information detected by various sensors, and communication information between the wireless communication units  115  and  215 . 
     Similarly to the operation input unit  112 , the operation input unit  212  is a user input means using a keyboard, key buttons, or the like.  FIG. 1B  shows a case where an input operation screen is displayed within the display screen of the display unit  213 . For example, the operation input unit  212  is a touch pad type input means such as a capacitance type input means. 
     Similarly to the display unit  113 , the display unit  213  is configured by using a liquid crystal panel or the like, and displays notifications to the user and various kinds of data information. For example, the power reception operation state of the power reception device  200 , the remaining amount or charge state of the battery  232 , information of communication with the power transmission device  100 , and the like are displayed. 
     The sound input and output unit  216  includes a microphone for receiving sound from the outside and a speaker for outputting sound to the outside, and processes sound that is input and output. The vibrator  217  generates vibration according to a control signal from the control unit  210 . When the vibrator  217  is worn on the user&#39;s body, the generated vibration can be reliably transmitted to the user. As a result, the notification information to the user generated in the power reception device  200  can be transmitted as tactile vibration. In addition, the notification information to the user generated on the power transmission device  100  side can also be transmitted through the wireless communication units  115  and  215  by the vibrator  217  of the power reception device  200 . 
     The human body sensor  218  includes a heart rate sensor, a blood pressure sensor, a body temperature sensor, and the like, and is worn in close contact with the user&#39;s body to detect the user&#39;s heart rate, blood pressure, and body temperature. The detected value such as the heart rate can be displayed on the display unit  213 , and can be transmitted to the power transmission device  100  through the short-range wireless communication units  215  and  115  and displayed on the display unit  113  on the power transmission device  100  side. The wearing detection unit  219  detects that the power reception device  200  is worn by the user and generates a wearing detection signal. For example, it is possible to detect whether or not the user is wearing the power reception device  200  based on the heart rate or blood pressure detected by the human body sensor  218 . 
     The position measurement unit  220  uses a GPS (Global Positioning System), which is a position information measurement system using artificial satellites, to receive radio waves from satellites and measure the current position (longitude, latitude, altitude) of the signal reception device  200 . In addition, as the position measurement unit  220 , an indoor position measurement system other than the GPS can also be used. The base station communication unit  222  is a communication interface that performs long-distance wireless communication with a mobile phone base station through the transmission and reception antenna  221 , and this enables communication of various kinds of information, voice communication, and the like. 
     The switch  230  of the power reception device  200  performs switching to the radio wave power reproduction unit  202  or the coil power reproduction unit  204  that reproduces the DC power supply power, and outputs the reproduced power to the charge processing unit  231 . The charge processing unit  231  charges the battery  232  using the input DC power supply power. In addition, regarding the switching operation of the switch  230 , power supply method information (radio wave reception method/electromagnetic induction method or magnetic resonance method) may be acquired from the power transmission device  100  side through the short-range wireless communication units  115  and  215 , and the control unit  210  may control the switch  230 . 
     With the above-described configuration and operation, when the power transmission device  100  and the power reception device  200  are in close contact with each other, the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are attracted to each other by magnetic force, so that the power transmission unit  105  (power transmission coil  103 ) of the power transmission device  100  and the power reception unit  205  (power reception coil  203 ) of the power reception device  200  are accurately aligned. In this state, the electromagnetic induction method or the magnetic resonance method can be selected to perform wireless power supply most efficiently. 
     In addition, in a state in which the power transmission device  100  and the power reception device  200  are not aligned with each other, the distance between the devices is determined by the magnetic force detection signal from the magnetic sensor  108 . When the magnetic force detection signal is equal to or higher than a predetermined level (the distance is equal to or less than a predetermined value), a radio wave reception method for wirelessly supplying power from the power transmission antenna  101  to the power reception antenna  20  is selected. In addition, when the magnetic force detection signal is less than the predetermined level (the distance is larger than the predetermined value), the wireless power supply using the radio wave reception method is stopped. 
     Thus, it is possible to select a power supply method having a highest power supply efficiency according to the alignment or distance between the devices. In addition, by stopping the power supply in a state in which the inter-device distance becomes too long and accordingly the power supply efficiency is significantly lowered, it is possible to prevent unnecessary consumption of the battery on the power transmission device side. As a result, the built-in battery is efficiently charged on the power reception device side, and many functions of the smart watch provided in the power reception device can be used for a long time. In addition, in the configuration of the present embodiment, the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are used for the alignment between the power transmission device  100  and the power reception device  200 , and the magnetic force generated by the power reception side magnets  206  and  207  is detected by the magnetic sensor  108  to determine the inter-device distance. Therefore, there is an advantage that the device configuration can be simplified. 
     Next, the operation of the power supply system of the present embodiment will be described in detail. In the following description, the power supply method based on the radio wave reception method will be referred to as an “antenna method”, and the power supply method based on the electromagnetic induction method or the magnetic resonance method will be referred to as a “coil method”. 
       FIG. 2A  is a diagram showing the relationship between the distance between a power transmission device and a power reception device and the power supply efficiency at the time of wireless power supply. The horizontal axis schematically shows an inter-device distance d, and the vertical axis schematically shows power supply efficiency p. The power supply efficiency p is the ratio of the power (received power) received by the power reception unit  205  to the power (transmitted power) transmitted from the power transmission unit  105 , and the ideal state without loss is the efficiency p 0  (=100%). A curve  301  shows the characteristics in the case of the coil method, and a curve  302  shows the characteristics in the case of the antenna method. 
     In the case of the coil method (curve  301 ), high efficiency p 1  is shown when the distance d=0, but the efficiency p drops abruptly when the power transmission device and the power reception device are apart from each other (distance d increases). On the other hand, in the case of the antenna method (curve  302 ), the efficiency p 2  at the distance d=0 is lower than the efficiency p 1  in the coil method, but the decrease in efficiency is gradual even if the distance d increases, and decreases approximately inversely proportional to the square of the distance d. Then, both the curves cross each other at a position of a distance d 1 , and the efficiency at that time is p 3 . Therefore, as a power supply method for maximizing the power supply efficiency, it is preferable to select the coil method (curve  301 ) in the range of the distance dl or less and switch to the antenna method (curve  302 ) at the distance d 1  or more. 
     In addition, a distance at which the efficiency of the antenna method decreases to p 4  with an increase in the distance d is assumed to be d 2 . This efficiency p 4  is an allowable value of the power supply efficiency. If the efficiency is less than this, the transmitted power is not effectively transmitted, and the battery power of the power transmission device  100  is wasted. Therefore, when the distance is larger than d 2 , the power supply operation is stopped as shown by the broken line  303 . The allowable value p 4  of the efficiency or the distance d 2  shown herein is determined depending on the specifications of the power transmission device  100  or the battery  131 . 
     In the following description, the state where the inter-device distance d=0 is referred to as a “close state” or “aligned state”, the range of 0&lt;d&lt;d 2  is referred to as a “short distance state”, and the range of d&gt;d 2  is referred to as a “long distance state”. In addition, the distance d 1  is referred to as an “efficiency reversal distance”, and the distance d 2  is referred to as an “efficiency limit distance”. 
       FIG. 2B  is a diagram showing the relationship between the distance between a power transmission device and a power reception device and the magnetic force detection signal of a magnetic sensor. The horizontal axis shows the inter-device distance d, and the vertical axis shows the level of a magnetic force detection signal m. The magnetic sensor  108  in the power transmission device  100  detects the magnetic force from the power reception side magnets  206  and  207  provided in the power reception device  200 , and the magnetic force detection signal m changes as shown in the curve  304 . In practice, the shape of the curve is affected by the shape of the magnet or the position of the magnetic sensor, but is shown schematically herein. 
     The magnetic force detection signal m indicates a level m 0  when the inter-device distance d=0 (close state), but decreases as the distance d increases, and decreases approximately inversely proportional to the square of the distance d. At the distance d 1  (efficiency reversal distance) described with reference to  FIG. 2A , the magnetic force detection signal m decreases to level m 1 , and at the distance d 2  (efficiency limit distance), the magnetic force detection signal m decreases to level m 2 . Therefore, the power transmission device  100  determines the inter-device distance d based on the level of the magnetic force detection signal m, and controls the operation of supplying power to the power reception device  200  in consideration of the characteristics of the power supply efficiency shown in  FIG. 2A . 
       FIG. 3  is a diagram schematically showing a state in which a power transmission device and a power reception device are closely arranged. Here, a state is shown in which the user detaches a smart watch, which is the power reception device  200 , and arranges the smart watch closely on the power transmission device  100  to supply power. The power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are attracted to each other to be positioned. The upper surface of the smart watch is the display unit  213 , and for example, the time is displayed. In this state, the electromagnetic induction method or the magnetic resonance method (coil method) having a high power supply efficiency can be selected, and the power supply operation can be performed in the vicinity of the efficiency p 1  according to the curve  301  shown in  FIG. 2A . 
       FIG. 4  is a diagram schematically showing a state in which a power transmission device and a power reception device are arranged apart from each other. Here, the power transmission device  100  is attached to the body of a user  3 . Then, the smart watch, which is the power reception device  200 , is worn on the wrist to supply power. When the user  3  moves his arm, the position of the power reception device  200  changes to  401 ,  402 , and  403 . 
     Corresponding to the inter-device distance d in  FIG. 2A , when the power reception device  200  is located at the positions  401  and  402  close to the power transmission device  100 , the inter-device distance d is the efficiency reversal distance d 1  or more and the efficiency limit distance d 2  or less. Therefore, the radio wave reception method (antenna method) is selected, and wireless power supply is performed in the range of efficiency p 3  to p 4  according to the curve  302 . When the power reception device  200  is located at the position  403  far from the power transmission device  100 , the inter-device distance d exceeds d 2  and the efficiency is less than the allowable value p 4 , so that the wireless power supply based on the radio wave reception method is stopped. In this manner, by performing control so that the power is supplied only when the inter-device distance is short, it is possible to obtain the effect of preventing the transmitted power from being wasted. 
     Next, the operation of the power supply system of the present embodiment will be described with reference to the flowcharts shown in  FIGS. 5A to 5C . Cases are divided on how to switch between the coil method and the antenna method, which are power supply methods. 
       FIG. 5A  is a flowchart when the power supply method is automatically switched so as to maximize the power supply efficiency. Here, the efficiency reversal distance d 1  (intersection of the curves  301  and  302 ) in  FIGS. 2A and 2B  is used as a threshold value for switching. The procedure will be described below along with the steps. 
     When wireless power supply is started by an operation input to the operation input unit  112 , the power transmission device  100  determines the arrangement state of the power transmission device  100  and the power reception device  200 . That is, it is identified and determined whether or not the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are closely arranged by the magnetic force of each other and aligned with each other (S 501 ). In this case, the alignment between the devices is determined from the detection level of the light emitted from the light emitting element  209  of the power reception device  200  received by the optical sensor  109 . 
     If the power transmission device  100  and the power reception device  200  are aligned in the determination of S 501  (S 501 , Yes), the power transmission device  100  generates alignment completion information indicating that the alignment between the power transmission device  100  and the power reception device  200  has been completed (S 502 ), and transmits the generated alignment completion information to the power reception device  200  by using the wireless communication unit  115 . Then, a power transmission operation is performed in the coil method by the power transmission coil  103 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 503 ). 
     The power reception device  200  receives and acquires the alignment completion information transmitted from the power transmission device  100  by using the wireless communication unit  215 . Then, the display unit  213  displays that the power transmission device  100  and the power reception device  200  are in the aligned state. Alternatively, the user is notified by the sound from the speaker of the sound input and output unit  216  and the tactile vibration by the vibrator  217  (S 521 ). Then, in the power reception device  200 , a power reception operation is performed by the power reception coil  203 , and the power reception operation state is displayed on the display unit  213 . Alternatively, the user is notified by the sound from the speaker of the sound input and output unit  216  and the tactile vibration by the vibrator  217  (S 522 ). 
     On the other hand, in the power transmission device  100 , if the power transmission device  100  and the power reception device  200  are not aligned in the determination of S 501  (S 501 , No), in order to determine an appropriate power supply method, the magnetic force from the power reception side magnets  206  and  207  is detected by the magnetic sensor  108 , and the magnetic force detection signal m is generated (S 504 ). Then, the generated magnetic force detection signal m is compared with the levels m 1  and m 2  in  FIG. 2B  (S 505 ). 
     If the value of the detection level m is m&gt;m 1 , the inter-device distance is d&lt;d 1  (efficiency reversal distance), so that switching to the coil method is performed to maximize the power supply efficiency (S 506 ). Then, a power transmission operation is performed by the power transmission coil  103 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 503 ). If the value of the detection level m is m 2 &lt;m&lt;m 1 , the inter-device distance is d 1 &lt;d&lt;d 2 , so that switching to the antenna method is performed to maximize the power supply efficiency (S 507 ). Then, a power transmission operation is performed from the power transmission antenna  101 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 508 ). 
     In the determination of S 505 , when the value of the detection level m is m&lt;m 2 , the inter-device distance is d&gt;d 2  (efficiency limit distance), so that the power transmission operation from the power transmission unit  105  is not performed (stopped or not started), and the display unit  113  displays that the power transmission operation is not performed to notify the user (S 540 ). In addition, in this case, the power transmission device  100  may transmit power transmission stop information indicating that power transmission is not performed to the power reception device  200 , and the power reception device  200  may display the fact on the display unit  213 . Then, when the wireless power supply process is continuously performed (S 541 , Yes), the process returns to S 501  to repeat the above process. 
     On the other hand, the power transmission device  100  transmits power supply method information selected by the determination of S 505  to the power reception device  200  by using the wireless communication unit  115 . The power reception device  200  receives and acquires the power supply method information from the power transmission device  100  by using the wireless communication unit  215  (S 523 ). The power reception device  200  identifies the acquired power supply method information (S 524 ). 
     When the power supply method is determined to be the coil method as a result of the identification in S 524 , the power reception operation is performed by the power reception coil  203  and the power reception operation state is displayed on the display unit  213  (S 522 ). Alternatively, the user is notified by the sound input and output unit  216  or the vibrator  217 . On the other hand, when the power supply method is determined to be the antenna method as a result of the identification of S 524 , the power reception operation is performed by the power reception antenna  201  and the power reception operation state is displayed on the display unit  213 . Alternatively, the user is notified by the sound input and output unit  216  or the vibrator  217  (S 525 ). 
     When the power reception device  200  receives power from the power transmission device  100 , the charge processing unit  231  charges the battery  232  with the power received by the power reception coil  203  or the power reception antenna  201 . Then, the state of charge to the battery  232  is displayed on the display unit  213 , or the user is notified of the state of charge to the battery  232  by the sound input and output unit  216  or the vibrator  217  (S 530 ). 
     When the power reception device  200  being charged receives a stop instruction during the charging operation (S 531 , Yes), the power reception device  200  stops the charging operation and notifies the user that the charging operation has been stopped through the display unit  213 , the sound input and output unit  216 , and the vibrator  217  (S 532 ). Then, charging stop information is generated and transmitted to the power transmission device  100  through the wireless communication unit  215 . 
     In the power transmission device  100 , when the charging stop information from the power reception device  200  is acquired through the wireless communication unit  215  (S 542 ), the power transmission operation from the power transmission unit  105  is stopped, and the power transmission operation stop is displayed on the display unit  113  to notify the user (S 540 ). Also in this case, when the wireless power supply process is continued (S 541 , Yes), the process returns to S 501  to repeat the process. 
     On the other hand, unless there is an instruction to stop charging from the user (S 531 , No), the power reception device  200  being charged continues the charging operation until the battery  232  is fully charged (S 533 , No). When the battery  232  is fully charged (S 533 , Yes), the charging operation is completed, and the user is notified of the completion of charging through the display unit  213 , the sound input and output unit  216 , and the vibrator  217  (S 534 ). Then, the power reception device  200  transmits charging completion information to the power transmission device  100  by using the wireless communication unit  215 . 
     When the power transmission device  100  acquires the charging completion information from the power reception device  200  through the wireless communication unit  215  (S 543 ), the power transmission device  100  ends the power transmission operation from the power transmission unit  105  and displays the end of the power transmission operation on the display unit  113  (S 544 ). Thus, the wireless power supply process ends. 
     According to the flowchart of  FIG. 5A , the power supply method can be automatically switched so that the power supply efficiency p in  FIG. 2A  is maximized. 
       FIG. 5B  is a flowchart when the switching of the power supply method in  FIG. 5A  is simplified. Here, the method is switched depending on whether or not the inter-device distance in  FIGS. 2A and 2B  is d=0 (close state). That is, the coil method of the curve  301  is selected if d=0, and the antenna method of the curve  302  is selected unless d=0. 
     When wireless power supply is started by an operation input to the operation input unit  112 , the power transmission device  100  determines the arrangement state of the power transmission device  100  and the power reception device  200 . That is, it is identified and determined whether or not the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are closely arranged by the magnetic force of each other and aligned with each other (S 501 ). The alignment between the devices is determined from the detection level of the light from the light emitting element  209  of the power reception device  200  received by the optical sensor  109 . 
     If the power transmission device  100  and the power reception device  200  are aligned in the determination of S 501  (S 501 , Yes), the power transmission device  100  generates alignment completion information indicating that the alignment between the power transmission device  100  and the power reception device  200  has been completed (S 502 ), and transmits the generated alignment completion information to the power reception device  200  by using the wireless communication unit  115 . Then, a power transmission operation is performed in the coil method by the power transmission coil  103 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 503 ). 
     The power reception device  200  receives and acquires the alignment completion information transmitted from the power transmission device  100  by using the wireless communication unit  215 . Then, the display unit  213  displays that the power transmission device  100  and the power reception device  200  are aligned, or the user is notified that the power transmission device  100  and the power reception device  200  are aligned through the sound input and output unit  216  or the vibrator  217  (S 521 ). Then, in the power reception device  200 , a power reception operation is performed by the power reception coil  203 , and the power reception operation state is displayed on the display unit  213  or the user is notified of the power reception operation state by the sound input and output unit  216  or the vibrator  217  (S 522 ). 
     On the other hand, if the power transmission device  100  and the power reception device  200  are not aligned in the determination of S 501  (S 501 , No), the power transmission device  100  determines the power supply efficiency based on the antenna method. Therefore, the magnetic sensor  108  detects the magnetic force from the power reception side magnets  206  and  207  and generates the magnetic force detection signal m (S 504 ). Then, the generated magnetic force detection signal m is compared with the level m 2  in  FIG. 2B . That is, it is determined whether or not the inter-device distance d is within the efficiency limit distance d 2  (S 510 ). 
     When the value of the detection level m is m&gt;m 2  (S 510 , Yes), the inter-device distance is d&lt;d 2  (efficiency limit distance) and the power supply efficiency is equal to or greater than the allowable value p 4 , so that the power supply method is changed to the antenna method (S 543 ). Then, a power transmission operation is performed from the power transmission antenna  101 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 508 ). 
     In addition, the power transmission device  100  transmits power supply method information, which indicates that the power supply method has been changed to the antenna method, to the power reception device  200  by using the wireless communication unit  115 . When the power reception device  200  receives the power supply method information (antenna method) transmitted from the power transmission device  100  by using the wireless communication unit  215  (S 523 ), the power reception operation is performed by the power reception antenna  201  and the power reception operation state is displayed on the display unit  213 . Alternatively, the user is notified by the sound input and output unit  216  or the vibrator  217  (S 525 ). 
     When the power reception device  200  receives power from the power transmission device  100 , the charge processing unit  231  charges the battery  232  with the power received by the power reception coil  203  or the power reception antenna  201 . Then, the state of charge to the battery  232  is displayed on the display unit  213 , or the user is notified of the state of charge to the battery  232  by the sound input and output unit  216  or the vibrator  217  (S 530 ). 
     When the value of the detection level m is m&lt;m 2  in the determination of S 510  (S 510 , No) the inter-device distance is d&gt;d 2  (efficiency limit distance), so that the power transmission device  100  does not perform a power transmission operation from the power transmission unit  105  and displays on the display unit  113  that the power transmission operation is not performed to notify the user (S 540 ). In addition, in this case, the power transmission device  100  may transmit power transmission stop information indicating that power transmission is not performed to the power reception device  200 , and the power reception device  200  may display the fact on the display unit  213 . 
     Subsequent operations (operations after S 530  and S 540 ) are the same as the flowchart of  FIG. 5A , and accordingly the description thereof will be omitted. According to the flowchart of  FIG. 5B , it is possible to perform automatic switching to the power supply method having a high power supply efficiency p while simplifying the process of determining the inter-device distance d. 
       FIG. 5C  shows a flowchart when the user&#39;s selection is included in the switching of the power supply method in  FIG. 5B . Here, even when the inter-device distance is d=0 (close state) in  FIGS. 2A and 2B , the user selects whether to use the coil method or the antenna method. 
     When wireless power supply is started by an operation input to the operation input unit  112 , the power transmission device  100  determines the arrangement state of the power transmission device  100  and the power reception device  200 . That is, it is identified and determined whether or not the power transmission side magnets  106  and  107  and the power reception side magnets  206  and  207  are closely arranged by the magnetic force of each other and aligned with each other (S 501 ). In this case, the alignment between the devices is determined from the detection level of the light emitted from the light emitting element  209  of the power reception device  200  received by the optical sensor  109 . 
     If the power transmission device  100  and the power reception device  200  are aligned in the determination of S 501  (S 501 , Yes), the user selects a method of power supply from the power transmission device  100  to the power reception device  200 . When the user selects whether to use a coil method using a power transmission and reception coil or an antenna method using a power transmission and reception antenna by the operation input from the operation input unit  112  or the like, power supply method information indicating the selected power supply method is generated (S 511 ). 
     The power transmission device  100  identifies the generated power supply method information (S 512 ), and when the power supply method is a coil method, a power transmission operation is performed from the power transmission coil  103  and the power transmission operation state is displayed on the display unit  113  to notify the user (S 503 ). On the other hand, when the power supply method is an antenna method, a power transmission operation is performed from the power transmission antenna  101  and the power transmission operation state is displayed on the display unit  113  to notify the user (S 508 ). 
     The power transmission device  100  transmits the power supply method information generated in S 511  to the power reception device  200  by using the wireless communication unit  115 . The power reception device  200  acquires the power supply method information transmitted from the power transmission device  100  by using the wireless communication unit  215  (S 523 ), and identifies which power supply method is used (S 524 ). 
     As a result of the identification in S 524 , when the power supply method is a coil method, the power reception operation is performed by the power reception coil  203  (S 522 ). On the other hand, when the power supply method is an antenna method, the power reception operation is performed by the power reception antenna  201  (S 525 ). In both methods, the power reception operation state is displayed on the display unit  213 , or the user is notified of the power reception operation state by the sound input and output unit  216  or the vibrator  217 . 
     When the power reception device  200  receives power from the power transmission device  100 , the charge processing unit  231  charges the battery  232  with the power received by the power reception coil  203  or the power reception antenna  201 . Then, the state of charge to the battery  232  is displayed on the display unit  213 , or the user is notified of the state of charge to the battery  232  by the sound input and output unit  216  or the vibrator  217  (S 530 ). 
     On the other hand, if the power transmission device  100  and the power reception device  200  are not aligned in the determination of S 501  (S 501 , No), the power transmission device  100  determines the power supply efficiency based on the antenna method. Therefore, the magnetic sensor  108  detects the magnetic force from the power reception side magnets  206  and  207  and generates the magnetic force detection signal m (S 504 ). Then, the generated magnetic force detection signal m is compared with the level m 2  in  FIG. 2B . That is, it is determined whether or not the inter-device distance d is within the efficiency limit distance d 2  (S 510 ). 
     When the value of the detection level m is m&gt;m 2  (S 510 , Yes), the inter-device distance is d&lt;d 2  (efficiency limit distance) and the power supply efficiency is equal to or greater than the allowable value p 4 , so that the power supply method is switched to the antenna method (S 543 ). Then, a power transmission operation is performed from the power transmission antenna  101 , and the power transmission operation state is displayed on the display unit  113  to notify the user (S 508 ). 
     When the value of the detection level m is m&lt;m 2  in the determination of S 510  (S 510 , No) the inter-device distance is d&gt;d 2  (efficiency limit distance), so that the power transmission operation from the power transmission unit  105  is not performed, and the display unit  113  displays that the power transmission operation is not performed to notify the user (S 540 ). In addition, in this case, the power transmission device  100  may transmit power transmission stop information indicating that power transmission is not performed to the power reception device  200 , and the power reception device  200  may display the fact on the display unit  213 . Subsequent operations (operations after S 530  and S 540 ) are the same as the flowchart of  FIG. 5A , and accordingly the description thereof will be omitted. 
     According to the flowchart of  FIG. 5C , since the user can select the power supply method at the time of close arrangement, it is possible to realize a convenient power supply operation according to the usage pattern of the user. For example, when the inter-device distance frequently repeats a close state and a short distance state, the user selects the antenna method so that the power supply method is not frequently switched. As a result, it is possible to perform a more stable power supply operation. 
     In any of the flowcharts of  FIGS. 5A to 5C , when the inter-device distance becomes excessive, the power transmission device does not consume unnecessary power. In addition, it is possible to notify the user and realize a convenient and comprehensively efficient power supply operation. In addition, in each flowchart, notification (display) to the user is performed each time each operation (for example, a power transmission operation, a power reception operation) starts/ends, but the notification to the user may not be provided for all and may be limited to only some operations. 
     Next, the specific configuration of a power transmission unit and a power reception unit in a power supply system will be described. 
       FIG. 6A  is a bird&#39;s-eye view showing a configuration example of a power transmission unit of a power transmission device. On the upper surface of the power transmission device  100  (a surface facing the power reception device  200 ), the power transmission antenna  101  and the power transmission coil  103  as the power transmission unit  105 , the power transmission side magnets  106  and  107 , and the magnetic sensor  108  are arranged. The power transmission antenna  101  is arranged inside the power transmission coil  103  so as to be as close to the surface as possible. A ferromagnet  601  is provided on a surface (here, a side surface or a bottom surface) other than the direction in which the magnetic sensor  108  detects the magnetic force, and the magnetic sensor  108  is magnetically shielded so that the magnetic force from the side surface or bottom surface direction does not enter. In addition, ferromagnets  602  and  603  for magnetic shielding are respectively arranged around the power transmission side magnets  106  and  107 . In addition, although the description of the configuration of the power reception unit  205  and the power reception side magnets  206  and  207  of the power reception device  200  is omitted in the diagrams, these are arranged at positions corresponding to the respective components of the power transmission device  100  in  FIG. 6A . 
       FIG. 6B  is a cross-sectional view when a power transmission device and a power reception device are closely arranged. When the power transmission device  100  and the power reception device  200  are in close contact with each other, a magnetic force  701  is concentrated between the power transmission side magnet  106  and the power reception side magnet  206  to cause a closed state and a magnetic force  702  is concentrated between the power transmission side magnet  107  and the power reception side magnet  207  to cause a closed state, so that these magnets are attracted to each other to enable accurate alignment. In addition, the alignment between the devices is determined by the light emitting element  209  and the optical sensor  109  shown in  FIGS. 1A and 1B . 
       FIG. 6C  is a cross-sectional view when a power transmission device and a power reception device are arranged apart from each other. The magnetic sensor  108  of the power transmission device  100  detects magnetic forces  606  and  607  from the power reception side magnets  206  and  207  of the facing power reception device  200 , and the magnetic force detection signal m changes with respect to the inter-device distance d as shown in the curve  304  of  FIG. 2B . At that time, since the magnetic force from a direction other than the upper surface (bottom surface or side surface direction) is shielded by the magnetic shielding action of the ferromagnet  601 , the magnetic force detection signal can be accurately detected. In addition, since the ferromagnets  602  and  603  are provided so as to surround the power transmission side magnets  106  and  107 , magnetic forces  604  and  605  from the power transmission side magnets  106  and  107  are magnetically shielded by the ferromagnets  602  and  603 . Therefore, since the magnetic sensor  108  can detect only the magnetic forces  606  and  607  from the power reception side magnets  206  and  207 , it is possible to generate an accurate magnetic force detection signal. Then, by using the curve  304  of  FIG. 2B , the inter-device distance d is calculated from the level of the magnetic force detection signal m generated by the magnetic sensor  108 . Then, by comparing the detection level m with the levels m 1  (efficiency reversal position) and m 2  (efficiency limit position) in  FIG. 2B , it is possible to accurately control the switching of the power supply method and the stop of wireless power supply. 
     In addition, in the above description, the magnetic sensor  108  is provided in the vicinity of the power transmission side magnet  107 , but a plurality of magnetic sensors may be provided. For example, as shown in  FIGS. 6B and 6C , another magnetic sensor  108 ′ (shown by a broken line) can be added in the vicinity of the power transmission side magnet  106 , so that it is possible to further improve the detection accuracy by detecting the magnetic force with a plurality of sensors. 
     The specific configuration of the power transmission unit and the power reception unit is not limited to this, and various modification examples are possible. For example, it is possible to provide only one set of a power transmission side magnet and a power reception side magnet. 
       FIG. 7A  is a bird&#39;s-eye view showing another configuration example of the power transmission unit of the power transmission device. Only one power transmission side magnet  106  is provided at the center of the upper surface, and a donut-shaped power transmission antenna  101  and a power transmission coil  103  are arranged around the power transmission side magnet  106 . The magnetic sensor  108  is arranged at a position next to the power transmission side magnet  106 . The components of the facing power reception device are also arranged at positions corresponding to the respective components of the power transmission device in  FIG. 7A . By arranging the magnetic sensor  108  near the center in this manner, it is possible to reduce the detection error of the strength of the magnetic force and the misalignment of the antenna. 
       FIG. 7B  is a cross-sectional view when the power transmission device and the power reception device are closely arranged in  FIG. 7A . The magnetic force  701  is concentrated between the power transmission side magnet  106  and the power reception side magnet  206  to cause a closed state, so that these magnets are attracted to each other to enable accurate alignment. In addition, the power transmission antenna  101  and the power reception antenna  201  may be donut-shaped, but a flat plate-shaped antenna may be arranged on one side of the magnets  106  and  206 . Even in this case, since the deviation in the rotational direction between the devices is regulated by the direction of the magnetic force  701  generated between the power transmission side magnet  106  and the power reception side magnet  206 , accurate alignment is also possible for the flat plate-shaped power transmission antenna  101  and power reception antenna  201 . 
     Second Embodiment 
     In a second embodiment, a pairing process between a power transmission device and a power reception device in a power supply system will be described. That is, by performing a pairing process between the power transmission device and the power reception device in advance and allowing a power supply operation only between the devices that have completed the pairing process, unnecessary power supply from the power transmission device to the power reception device that has not been paired with the power transmission device is prevented. 
       FIG. 8  is a flowchart showing a pairing process between a power transmission device and a power reception device. The power transmission device and the power reception device perform a pairing process with each other before starting the wireless power supply process. In the pairing process, each other&#39;s information is shared with each other and encryption is established using the same encryption key to share the encryption key while ensuring security. Hereinafter, the procedure will be described. 
     Upon receiving an instruction to start a pairing process from the user, first, the power transmission device  100  sets a state in which a registration request from the pairing partner (power reception device) has been received by operating the operation input unit  112  (S 800 ). On the other hand, the power reception device  200  displays a pairable partner (power transmission device) (S 801 ), and sends a request for registration to the desired pairing partner by operating the operation input unit  212 . Therefore, registration request information is generated, and the generated registration request information is transmitted to the power transmission device  100  through the wireless communication unit  215  (S 802 ). The power transmission device  100  receives and acquires the registration request information from the power reception device  200  by using the wireless communication unit  115  (S 803 ). 
     The power transmission device  100  generates a passkey, which is a randomly encrypted authentication key, according to the acquired registration request information, and transmits the generated passkey to the power reception device  200  by using the wireless communication unit  115  (S 804 ). The power reception device  200  receives and acquires the generated passkey generated by the power transmission device  100  by using the wireless communication unit  215  (S 805 ). In addition, the user may directly check and acquire the generated passkey displayed on the power transmission device  100 . The power reception device  200  receives the acquired generated passkey from the operation input unit  212 , and transmits the received passkey (input passkey) to the power transmission device  100  (S 806 ). The power transmission device  100  receives and acquires the input passkey (S 807 ). 
     Then, the power transmission device  100  compares the received input passkey with the generated passkey to determine match or mismatch (S 808 ). When the received input passkey and the generated passkey match as a result of the determination (S 808 , Yes), the establishment of pairing is displayed on the display unit  113  to provide notification, and pairing establishment information indicating that the pairing has been established is generated. Then, the generated pairing establishment information is transmitted to the power reception device  200  through the wireless communication unit  115  (S 809 ). 
     When the power reception device  200  acquires the pairing establishment information by using the wireless communication unit  215 , the establishment of pairing is displayed on the display unit  213 , or the user is notified of the establishment of pairing by the sound input and output unit  216  or the vibrator  217  (S 810 ). 
     With the establishment of pairing, both the power transmission device  100  and the power reception device  200  generate a unique link key based on the passkey and share the unique link key with each other, and store link key information (S 811 , S 812 ). Thereafter, by using the link key that is shared and stored, if the power transmission device and the power reception device are within the communicable range, it is possible to recognize each other&#39;s pairing partners. 
     On the other hand, when the received input passkey and the generated passkey do not match in the determination of S 808  (S 808 , No), the pairing fails. The power transmission device  100  displays and notifies of the pairing failure (S 813 ), generates pairing failure information and transmits the pairing failure information to the power reception device  200  (S 814 ). When the power reception device  200  receives the pairing failure information (S 815 ), the user is notified of the pairing failure by the display unit  213  or by the sound input and output unit  216  or the vibrator  217  (S 816 ). Then, the process return to S 801 . The power transmission device  100  determines an instruction as to whether or not to retry the pairing process, and returns to the start of the pairing process or ends the pairing process (S 817 ). 
     When the pairing is established between the power transmission device and the power reception device by the above pairing process and the power transmission device and the power reception device are within the communicable range, it is allowed to start the wireless power supply process described in the first embodiment. On the other hand, it is prohibited to start the wireless power supply process between the devices for which pairing has not been established. That is, the power transmission device determines whether or not the power reception device is a device, which is recognized in advance as a power supply target from the power transmission device, through the wireless communication unit, and supplies power only when the device is recognized in advance as a power reception target. 
     According to the second embodiment, since the power transmission device can wirelessly supply power only to the power reception device that has been paired with the power transmission device, it is possible to avoid wireless power supply to the power reception device that has not been paired with the power transmission device. As a result, not only can unnecessary power supply be prevented, but also a user-friendly power supply system can be realized by notifying the user of the success or failure of the pairing establishment. 
     In addition, although an example of the pairing process is shown in  FIG. 8 , it is needless to say that the same operation and effect can be obtained even if the pairing process is performed by another method. 
     Third Embodiment 
     In a third embodiment, the control of a power supply operation using the wearing detection unit  219  will be described. The wearing detection unit  219  of the power reception device  200  determines whether or not a smart watch, which is the power reception device  200 , is worn on the user&#39;s body (wrist) based on the detection signal of the human body sensor  218  (a heart rate sensor, a blood pressure sensor, a body temperature sensor, and the like). Then, the power reception device  200  transmits the wearing detection signal from the wearing detection unit  219  to the power transmission device  100  through the wireless communication unit  215 , and the control unit  110  of the power transmission device  100  controls the power transmission unit  105  according to the wearing detection signal received through the wireless communication unit  115 . 
     For example, the state of  FIG. 4  is a state in which the power reception device  200  (smart watch) is worn on the wrist. In this case, even if the distance between the power transmission device  100  and the power reception device  200  is a close arrangement state, power is not supplied by the coil method (electromagnetic induction method or magnetic resonance method), but switching to the antenna method (radio wave reception method) capable of supplying power in a short distance is performed. In this case, since the wrist is normally in motion, the smart watch worn on the wrist is in close contact with the power transmission device for a very short time or rarely. Therefore, even if the power supply method is switched between the coil method and the antenna method, the degree of improvement in the power supply time is extremely small. On the other hand, by using only the antenna method, frequent switching operations can be avoided and wireless power supply can be performed more stably. Undoubtedly, when the smart watch is not worn, the power supply method is switched according to the inter-device distance as described in the first embodiment. 
     According to the third embodiment, when the power reception device is worn on the user&#39;s body, wireless power supply can be performed more stably by fixing the power supply method to the antenna method. 
     In the above description of each embodiment, the case of a mobile battery as a power transmission device is taken as an example. However, it is apparent that the same operation and effect can be obtained even with a mobile information terminal such as a smartphone having a built-in battery as a power reception device. That is, when wirelessly supplying power from the built-in battery of the power transmission device to the built-in battery of the power reception device, the effect of suppressing a decrease in power supply efficiency and eliminating the waste of transmitted power can be obtained by controlling the power supply operation according to the inter-device distance. 
     In addition, the present invention is not limited to the embodiments described above, and includes various modification examples. For example, the above embodiments have been described in detail for easy understanding of the present invention, but the present invention is not necessarily limited to having all the components described above. In addition, some of the components in one embodiment can be replaced with the components in another embodiment, and the components in another embodiment can be added to the components in one embodiment. In addition, for some of the components in each embodiment, addition, removal, and replacement of other components are possible. 
     In addition, each of the above-described components, functions, processing units, processing means, and the like may be realized by hardware, for example, by designing some or all of these with an integrated circuit or the like. In addition, each of the above-described components, functions, and the like may be realized by software by the processor interpreting and executing a program for realizing each function. Information such as a program, a table, and a file for realizing each function can be placed in a memory, a recording device such as a hard disk or an SSD (Solid State Drive), or a recording medium such as an IC card, an SD card, or a DVD. 
     In addition, the control lines or the information lines are those considered to be necessary for the description, and all control lines or information lines on the product are not necessarily shown. In practice, it may be considered that almost all components are interconnected. 
     REFERENCE SIGNS LIST 
       1  Power supply system
 
 100  Power transmission device
 
 101  Power transmission antenna
 
 102  Power-to-radio wave conversion unit
 
 103  Power transmission coil
 
 104  Power-to-current conversion unit
 
 105  Power transmission unit
 
 106 ,  107  Power transmission side magnet
 
 108  Magnetic sensor
 
 109  Optical sensor
 
 110 ,  210  Control unit
 
 113 ,  213  Display unit
 
 115 ,  215  Wireless communication unit
 
       130 ,  230  Switch 
       131 ,  232  Battery 
       200  Power reception device
 
 201  Power reception antenna
 
 202  Radio wave power reproduction unit
 
 203  Power reception coil
 
 204  Coil power reproduction unit
 
 205  Power reception unit
 
 206 ,  207  Power reception side magnet
 
 209  Light emitting element
 
 216  Sound input and output unit
 
       217  Vibrator 
       218  Human body sensor
 
 219  Wearing detection unit
 
       601 ,  602 ,  603  Ferromagnet