Patent Publication Number: US-2018054086-A1

Title: Method and device for adjusting position of coils in wireless power transmission system

Description:
TECHNICAL FIELD 
     The present invention relates to wireless charging, and more particularly, to a method and apparatus of adjusting a coil location for magnetic field coupling in a wireless power transmission system. 
     BACKGROUND ART 
     The wireless power transfer technology is a technology that wirelessly delivers power between a power source and an electronic device. For example, the wireless power transfer technology enables a battery of a wireless terminal to be charged by simply placing a wireless terminal such as a smart phone or a tablet on a wireless charging pad. Thus, compared to a wired charging environment using a typical wired charging connector, the mobility, convenience, and safety can be improved. In addition to wireless charging of wireless terminals, the wireless power transfer technology is attracting attention as a substitute for the existing wired power transfer environment in various fields such as electric vehicles, wearable devices such as Bluetooth earphones or 3D glasses, home appliances, furniture, underground facilities, buildings, medical devices, robots, and leisure. 
     The wireless power transfer method is also referred to as a contactless power transfer method, a no point of contact power transfer method, or a wireless charging method. The wireless power transmission system includes a wireless power transfer apparatus for supplying electric energy by a wireless power transfer method and a wireless power receiving apparatus for receiving electric energy wirelessly supplied from the wireless power transfer apparatus to charge a battery cell. 
     In a typical terminal supply method, commercial power is supplied and converted into a voltage and a current corresponding to a battery cell to supply electrical energy to the battery cell through a terminal of the corresponding battery cell. In this terminal supply method, power transfer is performed through a terminal connection between a charger and a terminal. On the other hand, in the wireless power transmission system, due to the characteristics of no point of contact charging, coupling (magnetic induction and/or magnetic resonance) between a primary coil provided in the wireless power transfer apparatus and a secondary coil provided in the wireless power receiving apparatus is required for power transmission, and the wireless power transfer apparatus transmits power to the wireless power receiving apparatus through the magnetic field coupling. Accordingly, there are inconveniences in that matching between the primary coil and the secondary coil is important for smooth wireless power transfer/reception and the wireless power receiving apparatus needs to be located at a designated place in order to maintain optimal charging efficiency. Also, in the wireless power transmission system, the wireless power receiving apparatus may be moved without being fixed. For example, the wireless power receiving apparatus may be moved by an external impact, or the wireless power receiving apparatus may be moved according to the vibration of the wireless power receiving apparatus (e.g., vibration of the cellular phone according to a call signal). Accordingly, there is a limitation in that the wireless power transfer efficiency is lowered or the wireless power transfer is interrupted. In order to overcome this limitation, there is a need for a method of adjusting the location of a coil which can optimize the wireless power transfer efficiency in the wireless power transmission system. 
     DISCLOSURE 
     Technical Problem 
     The present invention provides a method and apparatus for adjusting coil location in a wireless power transmission system. 
     The present invention also provides a wireless power transfer apparatus including a movable primary coil. 
     The present invention also provides a wireless power transmission system that supports adaptive coupling. 
     The present invention also provides a method and apparatus of detecting the location of a secondary coil. 
     The present invention also provides a method and apparatus of adjusting the location of a primary coil. 
     The present invention also provides a method and apparatus which detects an optimal location of a primary coil for wireless power transfer and moves the primary coil even when the location of the wireless power receiving apparatus is changed during the wireless power transfer. 
     Technical Solution 
     According to an aspect of the present invention, there is provided a wireless power transfer apparatus for performing adaptive coupling, the apparatus comprising: a primary coil coupled to a secondary coil of a wireless power receiving apparatus located in a charging region and transmitting wireless power; a sensing unit for sensing that the wireless power receiving apparatus is located in the charging region; a moving unit for moving the primary coil; a control unit for controlling the primary coil to sequentially emit search pings while moving; and a communication unit receiving signal strength packets corresponding to each search ping, wherein the control unit detects a target location where a degree of coupling is equal to or greater than a predetermined threshold or a relatively largest based on the signal strength packet, and controls the moving unit to move the primary coil to the target location. 
     According to another aspect of the present invention, there is provided a wireless power transfer method for performing adaptive coupling, the method comprising: sensing a wireless power receiving apparatus; detecting a target location corresponding to a location of a secondary coil of the sensed wireless power receiving apparatus; and moving a primary coil to the detected target location, wherein the detecting of the target location comprises sequentially emitting search pings while moving the primary coil, receiving signal strength packets corresponding to each search ping, and detecting a target location where a degree of coupling of the primary coil and the secondary coil is equal to or greater than a predefined threshold value or is a largest value. 
     According to another aspect of the present invention, there is provided a wireless power transmission system for performing adaptive coupling, the system comprising: a wireless power transfer apparatus comprising a primary coil for generating a magnetic field and transmitting wireless power; and a wireless power receiving apparatus comprising a secondary coil coupled to the primary coil to receive the wireless power, wherein the wireless power transfer apparatus sequentially emits search pings while moving the primary coil, receives signal strength packets corresponding to each search ping, and moves the primary coil to a target location where a degree of coupling of the primary coil and the secondary coil is equal to or greater than a predefined threshold value or is a largest value. 
     Advantageous Effects 
     According to an embodiment of the present invention, a wireless power transfer apparatus can detect a location for optimal charging efficiency, and can improve the degree of coupling of a primary coil and a secondary coil. 
     Also, according to an embodiment of the present invention, the wireless power transfer apparatus can detect a location for optimal charging efficiency even when the wireless power receiving apparatus moves, and can move the primary coil to the detected location, thereby maintaining the wireless power transfer efficiency. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  a view illustrating components of a wireless power transmission system according to an embodiment of the present invention. 
         FIG. 2  is a view illustrating a wireless power transfer procedure according to an embodiment of the present invention. 
         FIG. 3  is a view illustrating a target location detection according to an embodiment of the present invention. 
         FIG. 4  is a view illustrating another exemplary wireless power transfer procedure according to an embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating a wireless power transfer method according to an embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating a wireless power transfer method according to another embodiment of the present invention. 
         FIG. 7  is a view illustrating a wireless power transfer apparatus and a wireless power receiving apparatus according to an embodiment of the present invention. 
     
    
    
     MODE FOR INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the accompanying drawings, it should be noted that the same components are indicated as the same reference numerals and symbols even if they are shown in different drawings. Also, detailed descriptions related to well-known functions or configurations will be ruled out in order not to unnecessarily obscure subject matters of the present invention. 
     The term ‘wireless power’ below is used to mean any type of energy associated with an electric field, a magnetic field, and an electromagnetic field transmitted from a transmitter to a receiver without the use of physical electromagnetic conductors. The wireless power may also be referred to as a power signal, and may denote an oscillating magnetic flux enclosed by a primary coil and a secondary coil. For example, power conversion in a system to wirelessly charge devices including mobile phones, cordless phones, iPods, MP 3  players, headsets and the like will be described herein. In general, the basic principles of wireless power transfer include, for example, both magnetic induction method and magnetic resonance method. 
       FIG. 1  a view illustrating components of a wireless power transmission system according to an embodiment of the present invention. 
     Referring to  FIG. 1 , a wireless power transmission system  100  may include a wireless power transfer apparatus  110  and one wireless power reception apparatus  150 - 1  or n wireless power reception apparatuses  150 - 1  to  150 - n.    
     The wireless power transfer apparatus  110  includes a primary coil  111 . The primary coil may include one or more coils. When the primary coil includes a plurality of coils, the plurality of coils may be partially overlapped. The wireless power transfer apparatus  110  may have any suitable form, but one preferred form is a flat platform with a power transfer surface. Here, each of the wireless power receiving apparatuses  150 - 1  to  150 - n  may be located on the platform or a charging area (e.g., a charging pad) nearby thereto. 
     Each of the wireless power receiving apparatuses  150 - 1  to  150 - n  is detachable from the wireless power transfer apparatus  110 , and each of the wireless power receiving apparatuses  150 - 1  to  150 - n  includes the secondary coil  151  coupled with the electromagnetic field generated by the primary coil  111  of the wireless power transfer apparatus  110  when being close to the wireless power transfer apparatus  110 . The secondary coil  151  may include one or more coils. 
     The wireless power transfer apparatus  110  transmits power to the wireless power receiving apparatuses  150 - 1  to  150 - n  without direct electrical contact. In this case, it is said that the primary coil  111  and the secondary coil  151  are magnetically coupled (or coupled, inductively coupled, or resonance-inductively coupled) to each other. The primary coil  111  or the secondary coil  151  may have any suitable shape, but may be a copper wire wound around a formation having a high permeability, such ferrite or amorphous metal. For smooth magnetic coupling of the primary coil  111  and the secondary coil  151 , the wireless power transfer apparatus  110  may include a moving unit (not shown) for moving the primary coil  111  to a suitable position. For example, the mobile unit may be an actuator. 
     The wireless power receiving apparatuses  150 - 1  to  150 - n  are connected to an external load (not shown, here, also referred to as an actual load of the wireless power receiving apparatus), and supply power wirelessly received from the wireless power transfer apparatus  110  to the external load. For example, the wireless power receiving apparatuses  150 - 1  to  150 - n  may each carry received power to an object that consumes or stores power, such as a portable electric or electronic device, or a rechargeable battery cell or battery. 
       FIG. 2  is a view illustrating a wireless power transfer procedure according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the wireless power transfer apparatus detects that the wireless power receiving apparatus is located at a charging region in a standby mode (S 200 ). Here, there may be various methods in which the wireless power transfer apparatus senses the wireless power receiving apparatus, and the present invention is not limited to a specific method. In one example, the wireless power transfer apparatus may periodically emit a specific frequency of analog ping, and based on at least one of the detection current, the resonance shift, and the capacitance change in response to the emitted analog ping, may detect that the wireless power receiving apparatus is located at the charging region. In another example, when the wireless power transfer apparatus periodically transmits a search signal and the wireless power receiving apparatus transmits a response signal based on the search signal, based on the response signal, the wireless power transfer apparatus may detect that the wireless power receiving apparatus is located at the charging region. In another example, when the wireless power transfer apparatus periodically transmits a beacon, the wireless power receiving apparatus may transmit a searching signal or advertisement to the wireless power transfer apparatus as a response thereto. Thus, the wireless power transfer apparatus can detect the wireless power receiving apparatus. 
     When the wireless power receiving apparatus is detected, the wireless power transfer apparatus performs target location detection (S 210 ). The wireless power transfer apparatus detects the target location for the primary coil corresponding to the location of the wireless power receiving apparatus (or the secondary coil). 
     The target location detection procedure may be performed as follows. 
     The wireless power transfer apparatus controls the moving unit to move the primary coil and sequentially emits search pings through the primary coil. In this case, the wireless power transfer apparatus may control the movement path of the moving unit according to a predefined algorithm. For example, the wireless power transfer apparatus may sequentially emit the search pings while sequentially moving the entire area from any one edge of the charging area, or may sequentially emit the search pings while moving in a spiral direction from the center of the charging region. Alternatively, the wireless power transfer apparatus may sequentially emit the search pings while moving in any direction about a location where the wireless power receiving apparatus (or secondary coil) is expected to be located in accordance with any other detection unit or algorithm. Here, the search pings may be emitted at a power intensity smaller than the intensity of the power for the wireless power transfer to reduce the amount of magnetic field radiation. 
     The wireless power receiving apparatus measures the received signal strength for each of the search pings received through the secondary coil among the search pings that are sequentially transmitted. Here, the received signal strength may be information indicating the degree of coupling of the primary coil and the secondary coil. For example, the received signal strength may be a rectified voltage, an open circuit voltage, received power, and the like. The wireless power receiving apparatus transmits the respective signal strength packets indicating the degree of coupling of the primary coil and the secondary coil to the wireless power transfer apparatus based on the respective received signal strengths. The respective signal strength packets may be transmitted through the secondary coil. 
     The wireless power transfer apparatus may detect the target location based on the respective signal strength packets. For example, the wireless power transfer apparatus may detect a location where the degree of coupling (or received signal strength) is equal to or greater than a predefined threshold value or a relatively largest value based on the signal strength packets as an optimal target location. 
     The emission of the search ping and the transmission of the corresponding signal strength packet may be simultaneously or alternately and repeatedly performed. For example, the wireless power transfer apparatus may emit an n-th search ping, the wireless power receiving apparatus may transmit a k-th signal strength packet regarding the n-th search ping, and then the wireless power transfer apparatus may emit a (n+1)-th search ping, the wireless power receiving apparatus may transmit a (k+1)-th signal strength packet regarding the (n+1)-th search ping. 
       FIG. 3  is a view illustrating a target location detection according to an embodiment of the present invention. In  FIG. 3 , a wireless power transfer apparatus  310  may include a moving unit  320 , and a primary coil  311  may be connected to the moving unit  320  and move together. 
     Referring to  FIG. 3 , the moving unit  320  included in the wireless power transfer apparatus  310  moves to a location A, and the primary coil  311  emits an n-th search ping at the Location A. The wireless power receiving apparatus  350  receives the n-th search ping through the secondary coil  351 , and transmits the corresponding k-th signal strength packet. The wireless power transfer apparatus  310  determines the degree of coupling at the location A based on the k-th signal strength packet. 
     The moving unit  320  included in the wireless power transfer apparatus  310  moves to a location B, and the primary coil  311  emits a (n+1)-th search ping at the location B. The wireless power receiving apparatus  350  receives the (n+1)-th search ping through the secondary coil  351 , and transmits the corresponding (k+1)-th signal strength packet. The wireless power transfer apparatus  310  determines the degree of coupling at the location B based on the k-th signal strength packet. 
     The moving unit  320  included in the wireless power transfer apparatus  310  moves to a location C, and the primary coil  311  emits a n+2th search ping at the location C. The wireless power receiving apparatus  350  receives the n+2th search ping through the secondary coil  351 , and transmits the corresponding (k+2)-th signal strength packet. The wireless power transfer apparatus  310  determines the degree of coupling at the location C based on the (k+2)-th signal strength packet. 
     The moving unit  320  included in the wireless power transfer apparatus  310  moves to a location D, and the primary coil  311  emits a (n+3)-th search ping at the location D. When the wireless power receiving apparatus  350  does not receive the n+3th search ping through the secondary coil  351  (e.g., when the primary coil  311  and the secondary coil  312  have a large difference in horizontal location), the received signal strength cannot be measured. Accordingly, a signal strength packet is not generated. That is, the wireless power transfer apparatus  310  does not receive the signal strength packet for the location D, and at the location D, it can be seen that the primary coil  311  and the secondary coil  351  are not coupled (or the degree of coupling is very low). 
     Based on the k-th signal strength packet, the (k+1)-th signal strength packet, and the (k+2)-th signal strength packet, when the degree of coupling at the location B is relatively large, the wireless power transfer apparatus  310  may detect the location B as the target location. Of course, this is an example. As an example, after detecting the target location, while the moving unit  320  is densely moved around the location B by the control of the wireless power transfer apparatus  31 , the primary coil  311  may sequentially emit search pings, and a target location having a more optimal degree of coupling may be detected. Alternatively, while the moving unit  320  is moved in an axial direction different from the existing direction or in an arbitrary direction by the control of the wireless power transfer apparatus  310 , the primary coil  311  may sequentially emit searching pings, and a target location having a more optimal degree of coupling may be detected. 
     Referring again to  FIG. 2 , the wireless power transfer apparatus moves the primary coil to the target location (S 220 ). The wireless power transfer apparatus may control the moving unit to move the primary coil to the target location. 
     The wireless power transfer apparatus transmits an information request signal to the wireless power receiving apparatus as a preparation step for wireless power transfer (S 230 ). Here, the information request signal may be a signal for requesting the required power information and the ID of the wireless power receiving apparatus. In one example, the information request signal may be transmitted in a form of data packet message. In another example, the information request signal may be transmitted in a form of digital ping according to predefined criteria between the wireless power transfer apparatus and the wireless power receiving apparatus. 
     The wireless power receiving apparatus transmits the ID and configuration information to the wireless power transfer apparatus in response to the information request signal (S 240 ). Here, the configuration information may include a maximum amount of power that the wireless power receiving apparatus desires to receive. The ID and configuration information may be transmitted in a form of a data packet message. In an example, an ID packet may be generated in regard to the ID, and a configuration packet may be generated in regard to the configuration information. The ID packet and the configuration packet may be transmitted to the wireless power transfer apparatus, respectively. 
     The wireless power transfer apparatus configures parameters for power transfer based on the ID and configuration information, and performs wireless power transfer to the wireless power receiving apparatus (S 250 ). That is, the wireless power transfer apparatus creates a power transfer contract based on the ID and configuration information, and performs wireless power transfer to the wireless power receiving apparatus. The step from starting of the wireless power transfer of the wireless power transfer apparatus to the wireless power receiving apparatus to stopping of the wireless power transfer may be referred to as a (wireless) power transfer phase. 
     The wireless power receiving apparatus may supply the received wireless power to an external load such as a battery. 
     Meanwhile, the above-described target location detection operation according to an embodiment of the present invention may also be performed in the power transfer phase. This is because, in a wireless power transmission system, a wireless power receiving apparatus is not fixed and is movable. For example, the wireless power receiving apparatus can be moved by an external impact, or the wireless power receiving apparatus can be moved according to the vibration of the wireless power receiving apparatus (e.g., vibration of the cellular phone according to a call signal). Accordingly, there is a limitation in that the wireless power transfer efficiency is lowered or the wireless power transfer is interrupted, and the target location detection and the location adjustment of the primary coil need to be performed. 
       FIG. 4  is a view illustrating another exemplary wireless power transfer procedure according to an embodiment of the present invention. 
     Referring to  FIG. 4 , the wireless power transfer apparatus performs wireless power transfer to the wireless power receiving apparatus in the power transfer phase (S 450 ). 
     The wireless power transfer apparatus performs target location (re-)detection according to a defined criterion (S 460 ). For example, the wireless power transfer apparatus may perform target location (re-)detection according to a specific period. As another example, the wireless power transfer apparatus may receive a control message from the wireless power receiving apparatus, and may perform the target location (re-)detection based on the control message. Here, the control message may be a message requesting the target location adjustment or the location movement of the primary coil. For example, the control message may be a received power packet message, and a received power value included in the received power packet message may be equal to or less than a predefined threshold value. Here, the received power value may indicate an average power amount received by the wireless power receiving apparatus for a predefined period of time. 
     The wireless power transfer apparatus may perform wireless power transfer while moving the primary coil, and may perform the target location (re-)detection based on the received power packet received from the wireless power receiving apparatus. In this case, the wireless power transfer apparatus may compare the received power values included in the respective received power packets, and may (re-)detect a location where the received power value is equal to or greater than a predefined threshold value or is relatively largest as an optimal target location. 
     The wireless power transfer apparatus moves the primary coil to the target location (re-)detected in operation S 460  (S 470 ). The wireless power transfer apparatus may control the moving unit to move the primary coil to the (re-)detected target location. 
     According to an embodiment of the present invention, the wireless power transfer apparatus (re-)detects a location for optimal charging efficiency even when the wireless power receiving apparatus moves, and moves the primary coil to the (re-)detected location, thereby maintaining the wireless power transfer efficiency. 
       FIG. 5  is a flowchart illustrating a wireless power transfer method according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the wireless power transfer apparatus detects the wireless power receiving apparatus (S 500 ). A method for detecting the wireless power receiving apparatus by the wireless power transfer apparatus is as described in operation S 200 . 
     When the wireless power receiving apparatus is detected, the wireless power transfer apparatus performs target location detection (S 510 ). The wireless power transfer apparatus detects the target location for the primary coil corresponding to the location of the wireless power receiving apparatus (or the secondary coil). The target location may be a location for the primary coil in which the degree of coupling of the primary coil and the secondary coil is equal to or more than a predefined value or becomes optimal. The target location detection procedure may include operations of sequentially transmitting the search pings while changing the location of the primary coil by the wireless power transfer apparatus, receiving signal strength packets for each of the search pings from the wireless power receiving apparatus, and performing the target location detection based on the signal strength packets by the wireless power transfer apparatus. Specifically, the target location detection procedure may be performed as described in S 210  of  FIG. 2  and  FIG. 3 . 
     The wireless power transfer apparatus moves the primary coil to the detected target location (S 520 ). The wireless power transfer apparatus may control the moving unit connected to the primary coil to move the primary coil to the target location. 
     The wireless power transfer apparatus creates a power transfer contract with the wireless power receiving apparatus in a state where the primary coil is moved to the target location, and perform wireless power transfer to the wireless power receiving apparatus through the primary coil (S 550 ). In this case, the wireless power transfer apparatus transmits an information request signal to the wireless power receiving apparatus as a preparation step for wireless power transfer, if necessary, and receives ID and configuration information from the wireless power receiving apparatus. Here, the configuration information may include a maximum amount of power that the wireless power receiving apparatus desires to receive. The wireless power transfer apparatus may create the power transmission contract based on the ID and the configuration information. 
       FIG. 6  is a flowchart illustrating a wireless power transfer method according to another embodiment of the present invention.  FIG. 6  shows an example in which target location detection is performed in the power transfer phase. 
     Referring to  FIG. 6 , the wireless power transfer apparatus performs wireless power transfer to the wireless power receiving apparatus in the power transfer phase (S 650 ). 
     The wireless power transfer apparatus performs target location detection/re-detection according to a defined criterion (S 660 ). The target location detection/re-detection procedure may include operations of continuously transmitting wireless power while changing the location of the primary coil by the wireless power transfer apparatus, receiving received power packets from the wireless power receiving apparatus, and performing the target location detection/re-detection based on the received power packets by the wireless power transfer apparatus. In this case, the wireless power transfer apparatus may compare the received power values included in the respective received power packets, and may detect/re-detect a location where the received power value is equal to or greater than a predefined threshold value or is relatively largest as an optimal target location. 
     For example, the wireless power transfer apparatus may perform target location detection/re-detection according to a specific period. As another example, the wireless power transfer apparatus may receive a control message from the wireless power receiving apparatus, and may perform the target location detection/re-detection based on the control message. 
     The wireless power transfer apparatus moves the primary coil to the target location detected/re-detected in operation S 660  (S 670 ). The wireless power transfer apparatus may control the moving unit to move the primary coil to the detected/re-detected target location. Thus, the wireless power transfer efficiency may be optimized even when the wireless power receiving apparatus moves. 
       FIG. 7  is a view illustrating a wireless power transfer apparatus and a wireless power receiving apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 7 , a wireless power transfer apparatus  700  includes a primary coil  705  including at least one coil, a moving unit  710  for moving the primary coil  705 , a sensing unit  715  for sensing a wireless power receiving apparatus  750 , a power conversion unit  720  connected to the primary coil  705  and applying electrical driving signals to the primary coil  705  to generate an electromagnetic field, a communication unit  730 , and a control unit  740 . 
     The wireless power transfer apparatus  700  may have any suitable form, but one preferred form is a flat platform with a power transfer surface. Here, the wireless power receiving apparatus  750  may be located on the platform or a charging area nearby thereto. 
     The moving unit  710  may move the primary coil  705  by the control of the control unit  740 . 
     The sensing unit  715  senses that the wireless power receiving apparatus  750  is located in the charging region. 
     The power conversion unit  720  may be a half-bridge inverter or a full-bridge inverter. The power conversion unit  720  may control the frequency, duty cycle, amplitude, etc. of a electric driving signal applied to the primary coil  705  through switching. 
     The communication unit  730  controls communication between the wireless power transfer apparatus  600  and the wireless power receiving apparatus  650 . In one example, the communication unit  730  may communicate with the wireless power receiving apparatus  750  through the primary coil  705 . In another example, the communication unit  730  may communicate with the wireless power receiving apparatus  750  through a separate Radio Frequency (RF) communication unit disposed in the communication unit  730  and a communication unit  780  respectively. 
     The communication unit  730  may receive a signal strength packet, an ID, configuration information, a received power packet, and the like from the wireless power receiving apparatus  750 . 
     The control unit  740  generates a control signal for power control based on the ID and the configuration information, and transmits the control signal to the power conversion unit  720 . 
     The control unit  740  may perform the necessary control operations to implement the present invention as described above. The control unit  740  performs a control operation for target location detection. The control unit  740  may perform the control operation for the target location detection in the standby phase before the wireless power transfer and/or in the power transfer phase during the wireless power transfer. 
     By the control of the control unit  740 , the movement path of the moving unit  710  may be controlled, and the search pings may be sequentially transmitted while the location of the primary coil  705  is changing. The control unit  740  may control the movement path of the moving unit  710  according to a predefined algorithm. 
     The communication unit  730  may receive a signal strength packet for each of the search pings from the wireless power receiving apparatus  750 , and control unit  740  may detect a location where the degree of coupling (or received signal strength) is equal to or greater than a predefined threshold value or a relatively largest value based on the signal strength packets as an optimal target location. 
     Also, by the control of the control unit  740  during the wireless power transfer, the movement path of the moving unit  710  may be controlled, and the wireless power transfer may be continuously performed while changing the location of the primary coil  705 . The communication unit  730  may receive the received power packets from wireless power receiving apparatus  750 , and control unit  740  may perform the target location re-detection based on the received power packets. In this case, the control unit  740  may compare the received power values included in the respective received power packets, and may (re-)detect a location of the primary coil  730  where the received power value is equal to or greater than a predefined threshold value or is a relatively largest value as an optimal target location. 
     The control unit  740  controls the moving unit  710  such that the primary coil  705  can be moved to the detected/re-detected target location. That is, the moving unit  710  is moved to the detected/re-detected target location. 
     The wireless power receiving apparatus  750  includes a secondary coil  755  coupled to an electromagnetic field generated by the wireless power transfer apparatus  700  when the wireless power receiving apparatus  750  is in the charging region of the wireless power transfer apparatus  700 . In this manner, power may be delivered from the wireless power transfer apparatus  700  to the wireless power receiving apparatus  750  without a direct electrical contact. The wireless power receiving apparatus includes a load  770 , a power pick-up unit  760  connected to the secondary coil  755  to collect power and supplying power to the load  770 , a communication unit  780 , and a control unit  790 . 
     The secondary coil  755  includes at least one coil. The secondary coil  755  receives at least one search ping from the wireless power transfer apparatus  700 . 
     The communication unit  780  controls communication between the wireless power transfer apparatus  700  and the wireless power receiving apparatus  750 . In one example, the communication unit  780  may communicate with the wireless power transfer apparatus  700  through the secondary coil  755 . In another example, the communication unit  780  and the communication unit  730  may each have separate RF communication units, and the communication unit  780  may communicate with the wireless power transfer apparatus  700  through the RF communication unit. 
     The communication unit  780  may transmit the ID, configuration information, signal strength packet, or received power packet of the wireless power receiving apparatus  750  to the wireless power transfer apparatus  700 . 
     The control unit  790  may control the overall operations of the wireless power receiving apparatus  750 . The control unit  790  may perform calculation and processing of various kinds of information, and may control each component of the wireless power receiving apparatus  750 . 
     The control unit  790  detects the received signal strengths for each of the at least one search pings received through the secondary coil  755 . Here, each of the received signal strengths may be information indicating the degree of coupling of the primary coil and the secondary coil. For example, the received signal strength may be a rectified voltage, an open circuit voltage, received power, and the like. The control unit  790  may generate the respective signal strength packets indicating the degree of coupling of the primary coil and the secondary coil based on the respective received signal strengths. 
     The control unit  790  may receive the wireless power signal received through the secondary coil  755 , and may detect a received power value. The control unit  790  may generate a received power packet including the received power value. 
     All of the functions described above may be performed by a processor such as a microprocessor, a controller, a microcontroller, and an Application Specific Integrated Circuit (ASIC) according to software or program codes coded so as to perform the functions. The design, development and implementation of the above codes will be apparent to those skilled in the art based on the description of the present invention. 
     The invention has been described in detail with reference to exemplary embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. Therefore, the present invention covers all embodiments falling within the scope of the following claims, rather than being limited to the above-described embodiments.