Patent Publication Number: US-10315525-B2

Title: Source device and method for controlling magnetic field using two source resonators in wireless power transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a Continuation based on U.S. application Ser. No. 13/592,595 filed on Aug. 23, 2012, which claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2011-0085000, filed on Aug. 25, 2011, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     1. Field 
     The following description relates to a source device and a method for controlling a shape of a magnetic field in a wireless power transmission system. 
     2. Description of Related Art 
     A wireless power refers to energy that is transferred from a wireless power transmitter to a wireless power receiver through magnetic coupling. Accordingly, a wireless power transmission system includes a source device to wirelessly transmit a power, and a target device to wirelessly receive a power. The source device may be referred to as a wireless power transmitter, and the target device may be referred to as a wireless power receiver. 
     The source device includes a source resonator, and the target device includes a target resonator. Magnetic coupling or resonance coupling may be formed between the source resonator and the target resonator. According to a characteristic of a wireless environment, the target device may be positioned around the source device. However, since the source device may not control the magnetic coupling of the source device based on a position of the target device, a transmission rate between the source resonator and the target resonator may be relatively low depending on the position of the target device. 
     SUMMARY 
     In one general aspect, there is provided a device configured to control a magnetic field, the device including resonators configured to form the magnetic field to transmit power to another device. The device further includes a magnetic field shape determining unit configured to determine a shape of the magnetic field. The device further includes a phase changing unit configured to change a phase of at least one of the resonators to form the magnetic field in the determined shape. 
     The magnetic field shape determining unit may be further configured to determine the shape of the magnetic field based on a user request. 
     The device may further include a target verification unit configured to verify a number of devices to which the power is to be transmitted, and positions of the devices. The magnetic field shape determining unit may be further configured to determine the shape of the magnetic field based on the number of the devices and the positions of the devices to optimize transmission rates between the device and the devices, respectively. 
     The phase changing unit may be further configured to set a phase difference between the resonators to zero degrees if the determined shape corresponds to a shape in which a magnitude of the magnetic field between the resonators is at a maximum. 
     The phase changing unit may be further configured to set a phase difference between the resonators to 180 degrees if the determined shape corresponds to a shape in which a magnitude of the magnetic field between the resonators is at a minimum and a magnitude of the magnetic field outside of the resonators is relatively large. 
     The phase changing unit may be further configured to set a phase difference between the resonators to be between zero degrees and 180 degrees based on the determined shape. 
     The phase changing unit may be further configured to delay transfer of a current to be input into the at least one of the resonators to change the phase. 
     In another general aspect, there is provided a device configured to control a magnetic field, the device including at least two resonators configured to form the magnetic field to transmit power to devices. The device further includes a target verification unit configured to verify a number of the devices and positions of the devices. The device further includes a magnetic field shape determining unit configured to determine a shape of the magnetic field based on the number of the devices and the positions of the devices to optimize transmission rates between the device and the devices, respectively. The device further includes a phase changing unit configured to change a phase of at least one of the at least two resonators to form the magnetic field in the determined shape. 
     In still another general aspect, there is provided a method of controlling, by a device, a magnetic field, the method including determining a shape of the magnetic field to be formed by resonators to transmit power to another device. The method further includes changing a phase of at least one of the resonators to form the magnetic field in the determined shape. 
     The determining may include determining the shape of the magnetic field based on a user request. 
     The method may further include verifying a number of devices to which the power is to be transmitted, and positions of the devices. The determining may include determining the shape of the magnetic field based on the number of the devices and the positions of the devices to optimize transmission rates between the device and the devices, respectively. 
     The changing may include setting a phase difference between the resonators to zero degrees if the determined shape corresponds to a shape in which a magnitude of the magnetic field between the resonators is at a maximum. 
     The changing may include setting a phase difference between the resonators to 180 degrees if the determined shape corresponds to a shape in which a magnitude of the magnetic field between the resonators is at a minimum and a magnitude of the magnetic field outside of the resonators is relatively large. 
     The changing may include setting a phase difference between the resonators to be between zero degrees and 180 degrees based on the determined shape. 
     The changing may include delaying transfer of a current to be input into the at least one of the resonators to change the phase. 
     A non-transitory computer-readable storage medium may store a program including instructions to cause a computer to perform the method. 
     In yet another general aspect, there is provided a method of controlling, by a device, a magnetic field, the method including verifying a number of devices to which power is to be transmitted, and positions of the devices. The method further includes determining a shape of the magnetic field to be formed by at least two resonators to transmit the power to the devices based on the number of the devices and the positions of the devices to optimize transmission rates between the device and the devices, respectively. The method further includes changing a phase of at least one of the at least two resonators to form the magnetic field in the determined shape. 
     A non-transitory computer-readable storage medium may store a program including instructions to cause a computer to perform the method. 
     In another general aspect, there is provided an electric vehicle including resonators configured to form a magnetic field to transmit power to another electric vehicle. The electric vehicle further includes a magnetic field shape determining unit configured to determine a shape of the magnetic field. The electric vehicle further includes a phase changing unit configured to change a phase of at least one of the resonators to form the magnetic field in the determined shape. 
     The electric vehicle may further include a target verification unit configured to verify a number of electric vehicles to which the power is to be transmitted, and positions of the electric vehicles. The magnetic field shape determining unit may be further configured to determine the shape of the magnetic field based on the number of the electric vehicles and the positions of the electric vehicles to optimize transmission rates between the electric vehicle and the electric vehicles, respectively. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a wireless power transmission system. 
         FIG. 2  is a diagram illustrating an example of a source device configured to control a magnetic field in a wireless power transmission system. 
         FIG. 3  is a diagram illustrating an example of a source device controlling a magnetic field. 
         FIG. 4  is a diagram illustrating another example of a source device controlling a magnetic field. 
         FIG. 5  is a diagram illustrating still another example of a source device controlling a magnetic field. 
         FIG. 6  is a diagram illustrating an example of a source device verifying transmission rates of target devices when a magnetic field is controlled by the source device. 
         FIG. 7  is a graph illustrating an example of transmission rates of target devices when a magnetic field is controlled by a source device. 
         FIG. 8  is a graph illustrating another example of transmission rates of target devices when a magnetic field is controlled by a source device. 
         FIG. 9  is a graph illustrating still another example of transmission rates of target devices when a magnetic field is controlled by a source device. 
         FIG. 10  is a flowchart illustrating an example of a method of controlling, by a source device, a magnetic field. 
         FIGS. 11A and 11B  are diagrams illustrating examples of a distribution of a magnetic field in a feeder and a resonator of a wireless power transmitter. 
         FIGS. 12A and 12B  are diagrams illustrating an example of a resonator and a feeder of a wireless power transmitter. 
         FIG. 13A  is a diagram illustrating an example of a distribution of a magnetic field in a resonator that is produced by feeding of a feeder, of a wireless power transmitter. 
         FIG. 13B  is a diagram illustrating examples of equivalent circuits of a feeder and a resonator of a wireless power transmitter. 
         FIG. 14  illustrates an example of an electric vehicle charging system. 
     
    
    
     Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience. 
     DETAILED DESCRIPTION 
     The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. Accordingly, various changes, modifications, and equivalents of the methods, apparatuses, and/or systems described herein will be suggested to those of ordinary skill in the art. The progression of processing steps and/or operations described is an example; however, the sequence of and/or operations is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps and/or operations necessarily occurring in a certain order. Also, description of well-known functions and constructions may be omitted for increased clarity and conciseness. 
       FIG. 1  is a diagram illustrating an example of a wireless power transmission system. Referring to  FIG. 1 , the wireless power transmission and charging system includes a source device  110  and a target device  120 . The source device  110  is a device supplying wireless power, and may be any of various devices that supply power, such as pads, terminals, televisions (TVs), and any other device that supplies power. The target device  120  is a device receiving wireless power, and may be any of various devices that consume power, such as terminals, TVs, vehicles, washing machines, radios, lighting systems, and any other device that consumes power. 
     The source device  110  includes an alternating current-to-direct current (AC/DC) converter  111 , a power detector  113 , a power converter  114 , a control and communication (control/communication) unit  115 , and a source resonator  116 . 
     The target device  120  includes a target resonator  121 , a rectification unit  122 , a DC-to-DC (DC/DC) converter  123 , a switch unit  124 , a device load  125 , and a control/communication unit  126 . The target device  120  may further include a communication module (not shown). In this example, the communication module may include a communication circuit, for example, a Bluetooth circuit, a wireless local area network (WLAN) circuit, and/or any other communication circuit known to one of ordinary skill in the art. 
     The AC/DC converter  111  generates a DC voltage by rectifying an AC voltage having a frequency of tens of hertz (Hz) output from a power supply  112 . The AC/DC converter  111  may output a DC voltage having a predetermined level, or may output a DC voltage having an adjustable level by the control/communication unit  115 . 
     The power detector  113  detects an output current and an output voltage of the AC/DC converter  111 , and provides, to the control/communication unit  115 , information on the detected current and the detected voltage. Additionally, the power detector  113  detects an input current and an input voltage of the power converter  114 . 
     The power converter  114  generates a power by converting the DC voltage output from the AC/DC converter  111  to an AC voltage using a switching pulse signal having a frequency of a few kilohertz (kHz) to tens of megahertz (MHz). In other words, the power converter  114  converts a DC voltage supplied to a power amplifier to an AC voltage using a reference resonance frequency F Ref , and generates a communication power to be used for communication, a charging power to be used for charging that may be used in a plurality of target devices, a tracking power to be used for tracking the resonance frequency, and/or an operation power to be used for operation of the target devices. Each of the communication power and the tracking power may be, for example, a low power of 0.1 to 1 milliwatts (mW) that may be used by a target device to perform communication, and each of the charging power and the operation power may be, for example, a high power of 1 mW to 200 Watts (W) that may be consumed by a device load of a target device. Also, the power converter  114  may generate the operation power based on a power transmission efficiency and a dissipation power of the target device  120 . 
     In this description, the term “charging” may refer to supplying power to an element or a unit that charges a battery or other rechargeable device with power. Also, the term “charging” may refer supplying power to an element or a unit that consumes power. For example, the term “charging power” may refer to power consumed by a target device while operating, or power used to charge a battery of the target device. The unit or the element may include, for example, a battery, a display device, a sound output circuit, a main processor, and various types of sensors. 
     In this description, the term “reference resonance frequency” refers to a resonance frequency that is nominally used by the source device  110 , and the term “tracking frequency” refers to a resonance frequency used by the source device  110  that has been adjusted based on a predetermined scheme. 
     The control/communication unit  115  determines the resonance frequency at which a transmission efficiency for the wireless power may be greater than or equal to a predetermined value. The control/communication unit  115  further performs power control to maintain an amount of power received by the target device  120  within a predetermined range. 
     The control/communication unit  115  may detect a reflected wave of the communication power, the charging power, the tracking power, and/or the operation power, and may detect mismatching between the target resonator  121  and the source resonator  116  based on the detected reflected wave. The control/communication unit  115  may detect the mismatching by detecting an envelope of the reflected wave, or by detecting an amount of a power of the reflected wave. 
     Also, the control/communication unit  115  may control a frequency of the switching pulse signal used by the power converter  114 . By controlling the switching pulse signal used by the power converter  114 , the control/communication unit  115  may generate a modulation signal to be transmitted to the target device  120 . In other words, the control/communication unit  115  may transmit various messages to the target device  120  via in-band communication. Additionally, the control/communication unit  115  may detect a reflected wave, and may demodulate a signal received from the target device  120  through an envelope of the reflected wave. 
     The control/communication unit  115  may generate a modulation signal for in-band communication using various schemes. To generate a modulation signal, the control/communication unit  115  may turn on or off the switching pulse signal used by the power converter  114 , or may perform delta-sigma modulation. Additionally, the control/communication unit  115  may generate a pulse-width modulation (PWM) signal having a predetermined envelope. 
     The control/communication unit  115  may perform out-of-band communication using a communication channel. The control/communication unit  115  may include a communication module, such as a ZigBee module, a Bluetooth module, or any other communication module, that the control/communication unit  115  may use to perform the out-of-band communication. The control/communication unit  115  may transmit or receive data to or from the target device  120  via the out-of-band communication. 
     The source resonator  116  transfers electromagnetic energy, such as the communication power, the charging power, the tracking power, and/or the operation power, to the target resonator  121  via a magnetic coupling with the target resonator  121 . 
     The target resonator  121  receives the electromagnetic energy, such as the communication power, the charging power, the tracking power, and/or the operation power, from the source resonator  116  via a magnetic coupling with the source resonator  116 . Additionally, the target resonator  121  receives various messages from the source device  110  via the in-band communication. 
     The rectification unit  122  generates a DC voltage by rectifying an AC voltage received by the target resonator  121 . 
     The DC/DC converter  123  adjusts a level of the DC voltage output from the rectification unit  122  based on a voltage rating of the device load  125 . For example, the DC/DC converter  123  may adjust the level of the DC voltage output from the rectification unit  122  to a level in a range from 3 volts (V) to 10 V. 
     The switch unit  124  is turned on or off by the control/communication unit  126 . When the switch unit  124  is turned off, the control/communication unit  115  of the source device  110  may detect a reflected wave. In other words, when the switch unit  124  is turned off, the magnetic coupling between the source resonator  116  and the target resonator  121  is interrupted. 
     The device load  125  may include a battery. The device load  125  may charge the battery using the DC voltage output from the DC/DC converter  123 . 
     The control/communication unit  126  transmits, to the source device  110 , information about an amount of the received operation power, information about a dissipation power of the device load  125 , and information about an amount of the received tracking power. In this example, the amount of the received operation power may be maintained within the predetermined range. The control/communication unit  126  further detects information about a charging state of the battery for charging, and transmits the information about the charging state to the source device  110 . In this example, the information about the charging state may correspond to an amount of current flowing through the battery, and a voltage applied to the battery. 
     The control/communication unit  126  may perform in-band communication for transmitting or receiving data using a resonance frequency by demodulating a received signal obtained by detecting a signal between the target resonator  121  and the rectification unit  122 , or by detecting an output signal of the rectification unit  122 . In other words, the control/communication unit  126  may demodulate a message received via the in-band communication. 
     Additionally, the control/communication unit  126  may adjust an impedance of the target resonator  121  to modulate a signal to be transmitted to the source device  110 . Specifically, the control/communication unit  126  may modulate the signal to be transmitted to the source device  110  by turning the switch unit  124  on and off. For example, the control/communication unit  126  may increase the impedance of the target resonator by turning the switch unit  124  off so that a reflected wave will be detected by the control/communication unit  115  of the source device  110 . In this example, depending on whether the reflected wave is detected, the control/communication unit  115  of the source device  110  will detect a binary number “0” or “1.” 
     The control/communication unit  126  may transmit, to the source device  110 , any one or any combination of a response message including a product type of a corresponding target device, manufacturer information of the corresponding target device, a product model name of the corresponding target device, a battery type of the corresponding target device, a charging scheme of the corresponding target device, an impedance value of a load of the corresponding target device, information about a characteristic of a target resonator of the corresponding target device, information about a frequency band used the corresponding target device, an amount of power to be used by the corresponding target device, an intrinsic identifier of the corresponding target device, product version information of the corresponding target device, and standards information of the corresponding target device. 
     The control/communication unit  126  may also perform an out-of-band communication using a communication channel. The control/communication unit  126  may include a communication module, such as a ZigBee module, a Bluetooth module, or any other communication module known in the art, that the control/communication unit  126  may use to transmit or receive data to or from the source device  110  via the out-of-band communication. 
       FIG. 2  is a diagram illustrating an example of a source device  200  configured to control a magnetic field in a wireless power transmission system. Referring to  FIG. 2 , the source device  200  includes an AC/DC converter  111 , a power detector  113 , a power converter  114 , a control/communication unit  210 , a target verification unit  220 , a magnetic field shape determining unit  230 , a phase changing unit  240 , and source resonators  252  and  254 . 
     The AC/DC converter  111 , the power detector  113 , and the power converter  114  of  FIG. 2  are identical to the AC/DC converter  111 , the power detector  113 , and the power converter  114  of  FIG. 1 . Thus, detailed descriptions will be omitted for conciseness. 
     The target verification unit  220  verifies a number of target devices and positions of the target devices to which a wireless power is to be transmitted. To verify the number of the target devices and the positions of the target devices, the target verification unit  220  may perform communication with the target devices when the target devices request, from the source device  200 , the wireless power. Additionally or alternatively, to verify the number of the target devices and the positions of the target devices, the target verification unit  220  may use a sensor to sense a target device and/or a resonance characteristic used to supply the wireless power. 
     The magnetic field shape determining unit  230  determines a shape of a magnetic field formed by the source resonators  252  and  254 . The magnetic field shape determining unit  230  may determine the shape of the magnetic field based on a user request. Also, the magnetic field determining unit  230  may determine the shape of the magnetic field based on the number of the target devices and the positions of the target devices to optimize transmission rates between the source device  200  and the target devices, respectively. For example, if all or most of the target devices are positioned between the source resonators  252  and  254 , the shape of the magnetic field is determined to be a shape in which a magnitude of the magnetic field between the source resonators  252  and  254  is at a maximum. The transmission rates between the source device  200  and the target devices may refer to transmission efficiencies of power transmitted from the source device  200  to the target devices, respectively, or amounts of the power received by the target devices. 
     The phase changing unit  240  changes a phase of a current to be input into the source resonator  254  to form the magnetic field in the determined shape. In this example, the phase changing unit  240  may delay transfer of the current to be input into the source resonator  254  to change the phase of the current. Examples of a shape of a magnetic field formed by the phase changing unit  240  will be described with reference to  FIGS. 3 through 5 . 
       FIG. 3  is a diagram illustrating an example of the source device  200  controlling a magnetic field. Referring to  FIG. 3 , a shape of the magnetic field corresponds to a shape in which a magnitude of the magnetic field between the source resonators  252  and  254  is at a maximum. In this example, the phase changing unit  240  changes a phase of a current to be input into the source resonator  254  to set a phase difference between the source resonators  252  and  254  to zero degrees. 
       FIG. 4  is a diagram illustrating another example of the source device  200  controlling a magnetic field. Referring to  FIG. 4 , a shape of the magnetic field corresponds to a shape in which a magnitude of the magnetic field between the source resonators  252  and  254  is at a minimum, and a magnitude of the magnetic field outside of the source resonators  252  and  254  is relatively large. In this example, the phase changing unit  240  changes a phase of a current to be input into the source resonator  254  to set a phase difference between the source resonators  252  and  254  to 180 degrees. 
       FIG. 5  is a diagram illustrating still another example of the source device  200  controlling a magnetic field. Referring to  FIG. 5 , a shape of the magnetic field corresponds to a shape in which the magnetic field is uniformly distributed between the source resonators  252  and  254 , and outside of the source resonators  252  and  254 . The distribution of the magnetic field may be changed based on a phase difference between the source resonators  252  and  254 . In this example, the phase changing unit  240  changes a phase of a current to be input into the source resonator  254  based on a determined shape of the magnetic field to set the phase difference between the source resonators  252  and  254  to be between zero degrees and 180 degrees. 
     Referring again to  FIG. 2 , the source resonators  252  and  254  form the magnetic field between the source resonators  252  and  254 . The source resonators  252  and  254  further transfer electromagnetic energy to a target resonator. 
     The control/communication unit  210  performs functions of the control/communication unit  115  of  FIG. 1 . Additionally, the control/communication unit  210  may perform functions of the target verification unit  220  and the magnetic field shape determining unit  230 . In  FIG. 2 , the control/communication unit  210 , the target verification unit  220 , and the magnetic field shape determining unit  230  are separately illustrated to individually describe the functions of the control/communication unit  210 , the target verification unit  220 , and the magnetic field shape determining unit  230 . Accordingly, the control/communication unit  210  may include at least one processor configured to perform all of the functions of the target verification unit  220  and the magnetic field shape determining unit  230 , or to perform only a portion of the functions of the target verification unit  220  and the magnetic field shape determining unit  230 . 
       FIG. 6  is a diagram illustrating an example of the source device  200  verifying transmission rates of target devices  610 ,  620 , and  630  when a magnetic field is controlled by the source device  200 . Referring to  FIG. 6 , a wireless power transmission system includes the source device  200  including the source resonators  252  and  254 , the target device A  610  and the target device C  630  that are positioned outside the source resonators  252  and  254 , and the target device B  620  that is positioned between the source resonators  252  and  254 . 
     Each of the target device A  610 , the target device B  620 , and the target device C  630  receive a power from the source device  200  at each of their respective positions. In this example, an amount of the power received by each of the target device A  610 , the target device B  620 , and the target device C  630  may be changed based on a transmission rate, which may be changed based on a shape of the magnetic field. 
     The source device  200  determines the shape of the magnetic field based on the positions of the target device A  610 , the target device B  620 , and the target device C  630 , and changes a phase difference between the source resonators  252  and  254  to change or form the shape of the magnetic field. For example, if the source device  200  determines the shape of the magnetic field to be the shape as shown in  FIG. 3 , the source device  200  sets the phase difference between the source resonators  252  and  254  to zero degrees. In this example, transmission rates of the target device A  610 , the target device B  620 , and the target device C  630  are shown in  FIG. 7 , respectively. 
       FIG. 7  is a graph illustrating an example of the transmission rates of the target devices  610 ,  620 , and  630  when a magnetic field is controlled by the source device  200 . The transmission rate between a source resonator and a target resonator is determined based on a power coupling rate. Accordingly, the terms “power coupling rate” and “transmission rate” may be used to denote the same meaning, and may be used interchangeably herein. Referring to  FIG. 7 , the transmission rate of the target device B  620  that is positioned between the source resonators  252  and  254  is maximized, and the transmission rates of the target device A  610  and the target device C  630  that are positioned outside the source resonators  252  and  254  are minimized. 
     Referring again to  FIG. 6 , in another example, if the source device  200  determines the shape of the magnetic field to be the shape as shown in  FIG. 4 , the source device  200  sets the phase difference between the source resonators  252  and  254  to 180 degrees. In this example, transmission rates of the target device A  610 , the target device B  620 , and the target device C  630  are shown in  FIG. 8 , respectively. 
       FIG. 8  is a graph illustrating another example of the transmission rates of the target devices  610 ,  620 , and  630  when a magnetic field is controlled by the source device  200 . Referring to  FIG. 8 , the transmission rate of the target device B  620  that is positioned between the source resonators  252  and  254  is minimized, and the transmission rates of the target device A  610  and the target device C  630  that are positioned outside the source resonators  252  and  254  are maximized. 
     Referring again to  FIG. 6 , in still another example, if the source device  200  sets the phase difference between the source resonators  252  and  254  to be between zero degrees and 180 degrees, the shape of the magnetic field may be formed as shown in  FIG. 5 , and the shape of the magnetic field may be changed based on the phase difference. In this example, transmission rates of the target device A  610 , the target device B  620 , and the target device C  630  may be changed based on the phase difference between the source resonators  252  and  254  as shown in  FIG. 9 , respectively. 
       FIG. 9  is a graph illustrating still another example of the transmission rates of the target devices  610 ,  620 , and  630  when a magnetic field is controlled by the source device  200 . Referring to  FIG. 9 , the transmission rate of the target device B  620  that is positioned between the source resonators  252  and  254  may become greater as the phase difference between the source resonators  252  and  254  becomes closer to zero degrees. The transmission rates of the target device A  610  and the target device C  630  that are positioned outside the source resonators  252  and  254  may become greater as the phase difference between the source resonators  252  and  254  becomes closer to 180 degrees. In this example, the transmission rate of the target device B  620  and the transmission rates of the target device A  610  and the target device C  630  include a trade-off relationship. 
       FIG. 10  is a flowchart illustrating an example of a method of controlling, by a source device, a magnetic field. Referring to  FIG. 10 , in operation  1010 , the source device verifies a number of target devices and positions of the target devices to which a wireless power is to be transmitted. 
     In operation  1020 , the source device determines a shape of a magnetic field formed by two source resonators of the source device based on, e.g., the number of the target devices and the positions of the target devices to optimize transmission rates between the source device and the target devices, respectively. In operation  1030 , the source device changes a phase of at least one of the two source resonators to form the magnetic field in the determined shape. 
     In the following description, the term “resonator” used in the discussion of  FIGS. 11A through 13B  refers to both a source resonator and a target resonator. 
       FIGS. 11A and 11B  are diagrams illustrating examples of a distribution of a magnetic field in a feeder and a resonator of a wireless power transmitter. When a resonator receives power supplied through a separate feeder, magnetic fields are formed in both the feeder and the resonator. 
       FIG. 11A  illustrates an example of a structure of a wireless power transmitter in which a feeder  1110  and a resonator  1120  do not have a common ground. Referring to  FIG. 11A , as an input current flows into a feeder  1110  through a terminal labeled “+” and out of the feeder  1110  through a terminal labeled “−”, a magnetic field  1130  is formed by the input current. A direction  1131  of the magnetic field  1130  inside the feeder  1110  is into the plane of  FIG. 11A , and has a phase that is opposite to a phase of a direction  1133  of the magnetic field  1130  outside the feeder  1110 . The magnetic field  1130  formed by the feeder  1110  induces a current to flow in a resonator  1120 . The direction of the induced current in the resonator  1120  is opposite to a direction of the input current in the feeder  1110  as indicated by the dashed arrows in  FIG. 11A . 
     The induced current in the resonator  1120  forms a magnetic field  1140 . Directions of the magnetic field  1140  are the same at all positions inside the resonator  1120 . Accordingly, a direction  1141  of the magnetic field  1140  formed by the resonator  1120  inside the feeder  1110  has the same phase as a direction  1143  of the magnetic field  1140  formed by the resonator  1120  outside the feeder  1110 . 
     Consequently, when the magnetic field  1130  formed by the feeder  1110  and the magnetic field  1140  formed by the resonator  1120  are combined, a strength of the total magnetic field inside the resonator  1120  decreases inside the feeder  1110  and increases outside the feeder  1110 . In an example in which power is supplied to the resonator  1120  through the feeder  1110  configured as illustrated in  FIG. 11A , the strength of the total magnetic field decreases in the center of the resonator  1120 , but increases outside the resonator  1120 . In another example in which a magnetic field is randomly distributed in the resonator  1120 , it is difficult to perform impedance matching since an input impedance will frequently vary. Additionally, when the strength of the total magnetic field increases, an efficiency of wireless power transmission increases. Conversely, when the strength of the total magnetic field is decreases, the efficiency of wireless power transmission decreases. Accordingly, the power transmission efficiency may be reduced on average. 
       FIG. 11B  illustrates an example of a structure of a wireless power transmitter in which a resonator  1150  and a feeder  1160  have a common ground. The resonator  1150  includes a capacitor  1151 . The feeder  1160  receives a radio frequency (RF) signal via a port  1161 . When the RF signal is input to the feeder  1160 , an input current is generated in the feeder  1160 . The input current flowing in the feeder  1160  forms a magnetic field, and a current is induced in the resonator  1150  by the magnetic field. Additionally, another magnetic field is formed by the induced current flowing in the resonator  1150 . In this example, a direction of the input current flowing in the feeder  1160  has a phase opposite to a phase of a direction of the induced current flowing in the resonator  1150 . Accordingly, in a region between the resonator  1150  and the feeder  1160 , a direction  1171  of the magnetic field formed by the input current has the same phase as a direction  1173  of the magnetic field formed by the induced current, and thus the strength of the total magnetic field increases in the region between the resonator  1150  and the feeder  1160 . Conversely, inside the feeder  1160 , a direction  1181  of the magnetic field formed by the input current has a phase opposite to a phase of a direction  1183  of the magnetic field formed by the induced current, and thus the strength of the total magnetic field decreases inside the feeder  1160 . Therefore, the strength of the total magnetic field decreases in the center of the resonator  1150 , but increases outside the resonator  1150 . 
     An input impedance may be adjusted by adjusting an internal area of the feeder  1160 . The input impedance refers to an impedance viewed in a direction from the feeder  1160  to the resonator  1150 . When the internal area of the feeder  1160  is increased, the input impedance is increased. Conversely, when the internal area of the feeder  1160  is decreased, the input impedance is decreased. Because the magnetic field is randomly distributed in the resonator  1150  despite a reduction in the input impedance, a value of the input impedance may vary based on a location of a target device. Accordingly, a separate matching network may be required to match the input impedance to an output impedance of a power amplifier. For example, when the input impedance is increased, a separate matching network may be used to match the increased input impedance to a relatively low output impedance of the power amplifier. 
       FIGS. 12A and 12B  are diagrams illustrating an example of a resonator and a feeder of a wireless power transmitter. Referring to  FIG. 12A , the wireless power transmitter includes a resonator  1210  and a feeder  1220 . The resonator  1210  further includes a capacitor  1211 . The feeder  1220  is electrically connected to both ends of the capacitor  1211 . 
       FIG. 12B  illustrates, in greater detail, a structure of the wireless power transmitter of  FIG. 12A . The resonator  1210  includes a first transmission line (not identified by a reference numeral in  FIG. 12B , but formed by various elements in  FIG. 12B  as discussed below), a first conductor  1241 , a second conductor  1242 , and at least one capacitor  1250 . 
     The capacitor  1250  is inserted in series between a first signal conducting portion  1231  and a second signal conducting portion  1232 , causing an electric field to be confined within the capacitor  1250 . Generally, a transmission line includes at least one conductor in an upper portion of the transmission line, and at least one conductor in a lower portion of first transmission line. A current may flow through the at least one conductor disposed in the upper portion of the first transmission line, and the at least one conductor disposed in the lower portion of the first transmission line may be electrically grounded. In this example, a conductor disposed in an upper portion of the first transmission line in  FIG. 12B  is separated into two portions that will be referred to as the first signal conducting portion  1231  and the second signal conducting portion  1232 . A conductor disposed in a lower portion of the first transmission line in  FIG. 12B  will be referred to as a first ground conducting portion  1233 . 
     As illustrated in  FIG. 12B , the resonator  1210  has a generally two-dimensional (2D) structure. The first transmission line includes the first signal conducting portion  1231  and the second signal conducting portion  1232  in the upper portion of the first transmission line, and includes the first ground conducting portion  1233  in the lower portion of the first transmission line. The first signal conducting portion  1231  and the second signal conducting portion  1232  are disposed to face the first ground conducting portion  1233 . A current flows through the first signal conducting portion  1231  and the second signal conducting portion  1232 . 
     One end of the first signal conducting portion  1231  is connected to one end of the first conductor  1241 , the other end of the first signal conducting portion  1231  is connected to the capacitor  1250 , and the other end of the first conductor  1241  is connected to one end of the first ground conducting portion  1233 . One end of the second signal conducting portion  1232  is connected to one end of the second conductor  1242 , the other end of the second signal conducting portion  1232  is connected to the other end of the capacitor  1250 , and the other end of the second conductor  1242  is connected to the other end of the ground conducting portion  1233 . Accordingly, the first signal conducting portion  1231 , the second signal conducting portion  1232 , the first ground conducting portion  1233 , the first conductor  1241 , and the second conductor  1242  are connected to each other, causing the resonator  1210  to have an electrically closed loop structure. The term “loop structure” includes a polygonal structure, a circular structure, a rectangular structure, and any other geometrical structure that is closed, i.e., that does not have any opening in its perimeter. The expression “having a loop structure” indicates a structure that is electrically closed. 
     The capacitor  1250  is inserted into an intermediate portion of the first transmission line. In the example in  FIG. 12B , the capacitor  1250  is inserted into a space between the first signal conducting portion  1231  and the second signal conducting portion  1232 . The capacitor  1250  may be a lumped element capacitor, a distributed capacitor, or any other type of capacitor known to one of ordinary skill in the art. For example, a distributed element capacitor may include a zigzagged conductor line and a dielectric material having a relatively high permittivity disposed between parallel portions of the zigzagged conductor line. 
     The capacitor  1250  inserted into the first transmission line may cause the resonator  1210  to have a characteristic of a metamaterial. A metamaterial is a material having a predetermined electrical property that is not found in nature, and thus may have an artificially designed structure. All materials existing in nature have a magnetic permeability and permittivity. Most materials have a positive magnetic permeability and/or a positive permittivity. 
     For most materials, a right-hand rule may be applied to an electric field, a magnetic field, and a Poynting vector of the materials, so the materials may be referred to as right-handed materials (RHMs). However, a metamaterial that has a magnetic permeability and/or a permittivity that is not found in nature, and may be classified into an epsilon negative (ENG) material, a mu negative (MNG) material, a double negative (DNG) material, a negative refractive index (NRI) material, a left-handed (LH) material, and other metamaterial classifications known to one of ordinary skill in the art based on a sign of the magnetic permeability of the metamaterial and a sign of the permittivity of the metamaterial. 
     If the capacitor  1250  is a lumped element capacitor and a capacitance of the capacitor  1250  is appropriately determined, the resonator  1210  may have a characteristic of a metamaterial. If the resonator  1210  is caused to have a negative magnetic permeability by appropriately adjusting the capacitance of the capacitor  1250 , the resonator  1210  may also be referred to as an MNG resonator. Various criteria may be applied to determine the capacitance of the capacitor  1250 . For example, the various criteria may include a criterion for enabling the resonator  1210  to have the characteristic of the metamaterial, a criterion for enabling the resonator  1210  to have a negative magnetic permeability at a target frequency, a criterion for enabling the resonator  1210  to have a zeroth order resonance characteristic at the target frequency, and any other suitable criterion. Based on any one or any combination of the aforementioned criteria, the capacitance of the capacitor  1250  may be appropriately determined. 
     The resonator  1210 , hereinafter referred to as the MNG resonator  1210 , may have a zeroth order resonance characteristic of having a resonance frequency when a propagation constant is “0”. If the MNG resonator  1210  has the zeroth order resonance characteristic, the resonance frequency is independent of a physical size of the MNG resonator  1210 . By changing the capacitance of the capacitor  1250 , the resonance frequency of the MNG resonator  1210  may be changed without changing the physical size of the MNG resonator  1210 . 
     In a near field, the electric field is concentrated in the capacitor  1250  inserted into the first transmission line, causing the magnetic field to become dominant in the near field. The MNG resonator  1210  has a relatively high Q-factor when the capacitor  1250  is a lumped element, thereby increasing a power transmission efficiency. The Q-factor indicates a level of an ohmic loss or a ratio of a reactance with respect to a resistance in the wireless power transmission. As will be understood by one of ordinary skill in the art, the efficiency of the wireless power transmission will increase as the Q-factor increases. 
     Although not illustrated in  FIG. 12B , a magnetic core passing through the MNG resonator  1210  may be provided to increase a power transmission distance. 
     Referring to  FIG. 12B , the feeder  1220  includes a second transmission line (not identified by a reference numeral in  FIG. 12B , but formed by various elements in  FIG. 12B  as discussed below), a third conductor  1271 , a fourth conductor  1272 , a fifth conductor  1281 , and a sixth conductor  1282 . 
     The second transmission line includes a third signal conducting portion  1261  and a fourth signal conducting portion  1262  in an upper portion of the second transmission line, and includes a second ground conducting portion  1263  in a lower portion of the second transmission line. The third signal conducting portion  1261  and the fourth signal conducting portion  1262  are disposed to face the second ground conducting portion  1263 . A current flows through the third signal conducting portion  1261  and the fourth signal conducting portion  1262 . 
     One end of the third signal conducting portion  1261  is connected to one end of the third conductor  1271 , the other end of the third signal conducting portion  1261  is connected to one end of the fifth conductor  1281 , and the other end of the third conductor  1271  is connected to one end of the second ground conducting portion  1263 . One end of the fourth signal conducting portion  1262  is connected to one end of the fourth conductor  1272 , the other end of the fourth signal conducting portion  1262  is connected to one end the sixth conductor  1282 , and the other end of the fourth conductor  1272  is connected to the other end of the second ground conducting portion  1263 . The other end of the fifth conductor  1281  is connected to the first signal conducting portion  1231  at or near where the first signal conducting portion  1231  is connected to one end of the capacitor  1250 , and the other end of the sixth conductor  1282  is connected to the second signal conducting portion  1232  at or near where the second signal conducting portion  1232  is connected to the other end of the capacitor  1250 . Thus, the fifth conductor  1281  and the sixth conductor  1282  are connected in parallel to both ends of the capacitor  1250 . The fifth conductor  1281  and the sixth conductor  1282  are used as an input port to receive an RF signal as an input. 
     Accordingly, the third signal conducting portion  1261 , the fourth signal conducting portion  1262 , the second ground conducting portion  1263 , the third conductor  1271 , the fourth conductor  1272 , the fifth conductor  1281 , the sixth conductor  1282 , and the resonator  1210  are connected to each other, causing the resonator  1210  and the feeder  1220  to have an electrically closed loop structure. The term “loop structure” includes a polygonal structure, a circular structure, a rectangular structure, and any other geometrical structure that is closed, i.e., that does not have any opening in its perimeter. The expression “having a loop structure” indicates a structure that is electrically closed. 
     If an RF signal is input to the fifth conductor  1281  or the sixth conductor  1282 , input current flows through the feeder  1220  and the resonator  1210 , generating a magnetic field that induces a current in the resonator  1210 . A direction of the input current flowing through the feeder  1220  is identical to a direction of the induced current flowing through the resonator  1210 , thereby causing a strength of a total magnetic field to increase in the center of the resonator  1210 , and decrease near the outer periphery of the resonator  1210 . 
     An input impedance is determined by an area of a region between the resonator  1210  and the feeder  1220 . Accordingly, a separate matching network used to match the input impedance to an output impedance of a power amplifier may not be necessary. However, if a matching network is used, the input impedance may be adjusted by adjusting a size of the feeder  1220 , and accordingly a structure of the matching network may be simplified. The simplified structure of the matching network may reduce a matching loss of the matching network. 
     The second transmission line, the third conductor  1271 , the fourth conductor  1272 , the fifth conductor  1281 , and the sixth conductor  1282  of the feeder  1220  may have a structure identical to the structure of the resonator  1210 . For example, if the resonator  1210  has a loop structure, the feeder  1220  may also have a loop structure. As another example, if the resonator  1210  has a circular structure, the feeder  1220  may also have a circular structure. 
       FIG. 13A  is a diagram illustrating an example of a distribution of a magnetic field in a resonator that is produced by feeding of a feeder, of a wireless power transmitter.  FIG. 13A  more simply illustrates the resonator  1210  and the feeder  1220  of  FIGS. 12A and 12B , and the names of the various elements in  FIG. 12B  will be used in the following description of  FIG. 13A  without reference numerals. 
     A feeding operation may be an operation of supplying power to a source resonator in wireless power transmission, or an operation of supplying AC power to a rectification unit in wireless power transmission.  FIG. 13A  illustrates a direction of input current flowing in the feeder, and a direction of induced current flowing in the source resonator. Additionally,  FIG. 13A  illustrates a direction of a magnetic field formed by the input current of the feeder, and a direction of a magnetic field formed by the induced current of the source resonator. 
     Referring to  FIG. 13A , the fifth conductor or the sixth conductor of the feeder  1220  may be used as an input port  1310 . In  FIG. 13A , the sixth conductor of the feeder is being used as the input port  1310 . An RF signal is input to the input port  1310 . The RF signal may be output from a power amplifier. The power amplifier may increase and decrease an amplitude of the RF signal based on a power requirement of a target device. The RF signal input to the input port  1310  is represented in  FIG. 13A  as an input current flowing in the feeder. The input current flows in a clockwise direction in the feeder along the second transmission line of the feeder. The fifth conductor and the sixth conductor of the feeder are electrically connected to the resonator. More specifically, the fifth conductor of the feeder is connected to the first signal conducting portion of the resonator, and the sixth conductor of the feeder is connected to the second signal conducting portion of the resonator. Accordingly, the input current flows in both the resonator and the feeder. The input current flows in a counterclockwise direction in the resonator along the first transmission line of the resonator. The input current flowing in the resonator generates a magnetic field, and the magnetic field induces a current in the resonator due to the magnetic field. The induced current flows in a clockwise direction in the resonator along the first transmission line of the resonator. The induced current in the resonator transfers energy to the capacitor of the resonator, and also generates a magnetic field. In  FIG. 13A , the input current flowing in the feeder and the resonator is indicated by solid lines with arrowheads, and the induced current flowing in the resonator is indicated by dashed lines with arrowheads. 
     A direction of a magnetic field generated by a current is determined based on the right-hand rule. As illustrated in  FIG. 13A , within the feeder, a direction  1321  of the magnetic field generated by the input current flowing in the feeder is identical to a direction  1323  of the magnetic field generated by the induced current flowing in the resonator. Accordingly, a strength of the total magnetic field may increases inside the feeder. 
     In contrast, as illustrated in  FIG. 13A , in a region between the feeder and the resonator, a direction  1333  of the magnetic field generated by the input current flowing in the feeder is opposite to a direction  1331  of the magnetic field generated by the induced current flowing in the source resonator. Accordingly, the strength of the total magnetic field decreases in the region between the feeder and the resonator. 
     Typically, in a resonator having a loop structure, a strength of a magnetic field decreases in the center of the resonator, and increases near an outer periphery of the resonator. However, referring to  FIG. 13A , since the feeder is electrically connected to both ends of the capacitor of the resonator, the direction of the induced current in the resonator is identical to the direction of the input current in the feeder. Since the direction of the induced current in the resonator is identical to the direction of the input current in the feeder, the strength of the total magnetic field increases inside the feeder, and decreases outside the feeder. As a result, due to the feeder, the strength of the total magnetic field increases in the center of the resonator having the loop structure, and decreases near an outer periphery of the resonator, thereby compensating for the normal characteristic of the resonator having the loop structure in which the strength of the magnetic field decreases in the center of the resonator, and increases near the outer periphery of the resonator. Thus, the strength of the total magnetic field may be constant inside the resonator. 
     A power transmission efficiency for transferring wireless power from a source resonator to a target resonator is proportional to the strength of the total magnetic field generated in the source resonator. Accordingly, when the strength of the total magnetic field increases inside the source resonator, the power transmission efficiency also increases. 
       FIG. 13B  is a diagram illustrating examples of equivalent circuits of a feeder and a resonator of a wireless power transmitter. Referring to  FIG. 13B , a feeder  1340  and a resonator  1350  may be represented by the equivalent circuits in  FIG. 13B . The feeder  1340  is represented as an inductor having an inductance L f , and the resonator  1350  is represented as a series connection of an inductor having an inductance L coupled to the inductance L f  of the feeder  1340  by a mutual inductance M, a capacitor having a capacitance C, and a resistor having a resistance R. An example of an input impedance Z in  viewed in a direction from the feeder  1340  to the resonator  1350  may be expressed by the following Equation 1: 
     
       
         
           
             
               
                 
                   
                     Z 
                     in 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           M 
                         
                         ) 
                       
                       2 
                     
                     Z 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Equation 1, M denotes a mutual inductance between the feeder  1340  and the resonator  1350 , ω denotes a resonance frequency of the feeder  1340  and the resonator  1350 , and Z denotes an impedance viewed in a direction from the resonator  1350  to a target device. As can be seen from Equation 1, the input impedance Z in  is proportional to the square of the mutual inductance M. Accordingly, the input impedance Z in  may be adjusted by adjusting the mutual inductance M. The mutual inductance M depends on an area of a region between the feeder  1340  and the resonator  1350 . The area of the region between the feeder  1340  and the resonator  1350  may be adjusted by adjusting a size of the feeder  1340 , thereby adjusting the mutual inductance M and the input impedance Z in . Since the input impedance Z in  may be adjusted by adjusting the size of the feeder  1340 , it may be unnecessary to use a separate matching network to perform impedance matching with an output impedance of a power amplifier. 
     In a target resonator and a feeder included in a wireless power receiver, a magnetic field may be distributed as illustrated in  FIG. 13A . For example, the target resonator may receive wireless power from a source resonator via magnetic coupling. The received wireless power induces a current in the target resonator. The induced current in the target resonator generates a magnetic field, which induces a current in the feeder. If the target resonator is connected to the feeder as illustrated in  FIG. 13A , a direction of the induced current flowing in the target resonator will be identical to a direction of the induced current flowing in the feeder. Accordingly, for the reasons discussed above in connection with  FIG. 13A , a strength of the total magnetic field will increase inside the feeder, and will decrease in a region between the feeder and the target resonator. 
       FIG. 14  illustrates an example of an electric vehicle charging system. Referring to  FIG. 14 , an electric vehicle charging system  1400  includes a source system  1410 , a source resonator  1420 , a target resonator  1430 , a target system  1440 , and an electric vehicle battery  1450 . 
     In one example, the electric vehicle charging system  1400  has a structure similar to the structure of the wireless power transmission system of  FIG. 1 . The source system  1410  and the source resonator  1420  in the electric vehicle charging system  1400  operate as a source. The target resonator  1430  and the target system  1440  in the electric vehicle charging system  1400  operate as a target. 
     In one example, the source system  1410  includes an alternating current-to-direct current (AC/DC) converter, a power detector, a power converter, a control and communication (control/communication) unit similar to those of the source device  110  of  FIG. 1 . In one example, the target system  1440  includes a rectification unit, a DC-to-DC (DC/DC) converter, a switch unit, a charging unit, and a control/communication unit similar to those of the target device  120  of  FIG. 1 . The electric vehicle battery  1450  is charged by the target system  1440 . The electric vehicle charging system  1400  may use a resonant frequency in a band of a few kHz to tens of MHz. 
     The source system  1410  generates power based on a type of the vehicle being charged, a capacity of the electric vehicle battery  1450 , and a charging state of the electric vehicle battery  1450 , and wirelessly transmits the generated power to the target system  1440  via a magnetic coupling between the source resonator  1420  and the target resonator  1430 . 
     The source system  1410  may control an alignment of the source resonator  1420  and the target resonator  1430 . For example, when the source resonator  1420  and the target resonator  1430  are not aligned, the controller of the source system  1410  may transmit a message to the target system  1440  to control the alignment of the source resonator  1420  and the target resonator  1430 . 
     For example, when the target resonator  1430  is not located in a position enabling maximum magnetic coupling, the source resonator  1420  and the target resonator  1430  are not properly aligned. When a vehicle does not stop at a proper position to accurately align the source resonator  1420  and the target resonator  1430 , the source system  1410  may instruct a position of the vehicle to be adjusted to control the source resonator  1420  and the target resonator  1430  to be aligned. However, this is just an example, and other methods of aligning the source resonator  1420  and the target resonator  1430  may be used. 
     The source system  1410  and the target system  1440  may transmit or receive an ID of a vehicle and exchange various messages by performing communication with each other. 
     The descriptions of  FIGS. 2 through 13B  are also applicable to the electric vehicle charging system  1400 . However, the electric vehicle charging system  1400  may use a resonant frequency in a band of a few kHz to tens of MHz, and may wirelessly transmit power that is equal to or higher than tens of watts to charge the electric vehicle battery  1450 . 
     According to the teachings above, there is provided a source device and a method for controlling a magnetic field using two source resonators in a wireless power transmission system. The source device determines a shape of a magnetic field formed by the two source resonators, and changes a phase of at least one of the two source resonators to form the magnetic field in the determined shape. The shape of the magnetic field is determined based on a number of the target devices and positions of the target devices to optimize transmission rates between the source device and the target devices. Also, when a device that may influence the magnetic field is around the source device, the source device forms the magnetic field to avoid the device. 
     The units described herein may be implemented using hardware components, software components, or a combination thereof. For example, the hardware components may include microphones, amplifiers, band-pass filters, audio to digital converters, and processing devices. In this example, a processing device may be implemented using one or more general-purpose or special purpose computers, such as, for example, a processor, a controller and an arithmetic logic unit, a digital signal processor, a microcomputer, a field programmable array, a programmable logic unit, a microprocessor or any other device capable of responding to and executing instructions in a defined manner. The processing device may run an operating system (OS) and one or more software applications that run on the OS. The processing device also may access, store, manipulate, process, and create data in response to execution of the software. For purpose of simplicity, the description of a processing device used is described as being singular; however, one skilled in the art will appreciated that a processing device may include multiple processing elements and multiple types of processing elements. For example, a processing device may include multiple processors or a processor and a controller. In addition, different processing configurations are possible, such as parallel processors. 
     The software may include a computer program, a piece of code, an instruction, or some combination thereof, to independently or collectively instruct or configure the processing device to operate as desired. Software and data may be embodied permanently or temporarily in any type of machine, component, physical or virtual equipment, computer storage medium or device, or in a propagated signal wave capable of providing instructions or data to or being interpreted by the processing device. The software also may be distributed over network coupled computer systems so that the software is stored and executed in a distributed fashion. The software and data may be stored by one or more computer readable recording mediums. 
     The computer readable recording medium may include any data storage device that can store data which can be thereafter read by a computer system or processing device. Examples of the non-transitory computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices. Also, functional programs, codes, and code segments of accomplishing the examples disclosed herein can be easily construed by programmers skilled in the art to which the examples pertain based on and using the flow diagrams and block diagrams of the figures and their corresponding descriptions as provided herein. 
     As a non-exhaustive illustration only, a device described herein may refer to mobile devices such as a cellular phone, a personal digital assistant (PDA), a digital camera, a portable game console, and an MP3 player, a portable/personal multimedia player (PMP), a handheld e-book, a portable laptop PC, a global positioning system (GPS) navigation, a tablet, a sensor, and devices such as a desktop PC, a high definition television (HDTV), an optical disc player, a setup box, a home appliance, and the like that are capable of wireless communication or network communication consistent with that which is disclosed herein. 
     A number of examples have been described above. Nevertheless, it should be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.