Patent Publication Number: US-2013234527-A1

Title: Wireless electric power transmission device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority from the Japanese Patent Application No. 2012-49567, filed on Mar. 6, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a wireless electric power transmission device. 
     BACKGROUND 
     In recent years, a wireless electric power transmission technique for wirelessly transmitting electric power in a non-contact manner using a power transmission coil and a power receiving coil is widely employed in many apparatuses such as an IC card and a cellular phone. The power receiving apparatus is provided with a rectifying circuit for obtaining electric power for driving the power receiving apparatus from an electromagnetic wave transmitted from a power transmission apparatus, and an impedance control circuit for changing the impedance of the power receiving apparatus as seen by the power transmission apparatus. The impedance control circuit changes the impedance so that the output voltage of the rectifying circuit attains a desired value. 
     However, when the impedance of the power receiving apparatus is increased or decreased, it is not clear whether the output voltage of the rectifying circuit increases or decreases. Therefore, in the past, a step for finding change direction of the output voltage in accordance with change of the impedance (relationship between increase/decrease of the impedance and increase/decrease of the output voltage) is required before controlling the output voltage, and it is difficult to achieve high speed control of the received electric power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram illustrating a wireless electric power transmission system according to a first embodiment; 
         FIG. 2  is a figure illustrating an example of configuration of a capacitor; 
         FIG. 3  is a graph illustrating an example of relationship between received electric power of a load circuit and load resistance of a power receiving device; 
         FIG. 4  is a graph illustrating an example of relationship between received electric power of a load circuit and load resistance of a power receiving device; 
         FIG. 5  is a flowchart for explaining a method for adjusting a resonant frequency according to the first embodiment; 
         FIGS. 6A and 6B  are graphs illustrating an example of relationship between received electric power of a load circuit and load resistance of a power receiving device; 
         FIG. 7  is a schematic configuration diagram illustrating a wireless electric power transmission system according to a second embodiment; 
         FIG. 8  is a schematic configuration diagram illustrating a wireless electric power transmission system according to a modification; 
         FIGS. 9A and 9B  are graphs illustrating an example of relationship between received electric power of a load circuit and load resistance of a power receiving device; 
         FIG. 10  is a figure illustrating an example of configuration of a power transmitting resonance unit and a power receiving resonance unit; 
         FIG. 11  is a figure illustrating an example of configuration of a power transmitting resonance unit and a power receiving resonance unit; and 
         FIG. 12  is a graph illustrating an example of relationship between a load resistance value and an electric power transmission efficiency. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a wireless electric power transmission device supplies electric power, transmitted wirelessly from a first device, to a load circuit. The device includes a power receiving resonance unit, a detecting unit which detects electric power information corresponding to the electric power supplied to the load circuit, and a control unit. The control unit determines whether to adjust at least one of a resonant frequency of the power receiving resonance unit, an output frequency of an alternating current power supply of the first device, and a resonant frequency of a power transmitting resonance unit of the first device, on the basis of a relationship in terms of magnitude between first electric power information when the impedance is a first impedance and second electric power information when the impedance is a second impedance. 
     Embodiments will now be explained with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  illustrates a schematic configuration of a wireless electric power transmission system according to the first embodiment of the present invention. The wireless electric power transmission system includes a power transmission device  1  and a power receiving device  2  to which electric power is transmitted (fed) from the power transmission device  1 . 
     The power transmission device  1  includes an alternating current power supply  11  and a power transmitting resonance unit  14  having a capacitor  12  and a power transmission coil  13  connected in series. The power transmitting resonance unit  14  transmits (supplies) a signal, which is output from an alternating current power supply  11  (electric power supplied from the alternating current power supply  11 ), to the power receiving device  2  from the power transmission coil  13 . 
     The power receiving device  2  includes a power receiving resonance unit  23  having the power receiving coil  21  and the capacitor  22 , an impedance converting unit  24 , a load circuit  25 , an electric power information detecting unit  26 , and a control unit  27 . 
     In the power receiving resonance unit  23 , a resonant frequency is variable. For example, as illustrated in  FIG. 2 , the capacitor  22  includes a plurality of capacitive devices  22   a  and a plurality of switch  22   b  respectively connected to the capacitive devices  22   a,  thus capable of changing the capacity in accordance with the number of switches  22   b  turned on. By changing the capacity of the capacitor  22 , the resonant frequency of the power receiving resonance unit  23  can be changed. 
     When the power receiving coil  21  electromagnetically couples with the power transmission coil  13 , the power receiving coil  21  generates induced voltage. This induced voltage is converted (increased/decreased) by the impedance converting unit  24  to be an operation voltage of the load circuit  25 . The impedance converting unit  24  may have a voltage conversion function, and, for example, a DC-DC converter can be used as the impedance converting unit  24 . The impedance converting unit  24  can change the impedance of the load circuit  25  as seen by the input side of the impedance converting unit  24 . In other words, the impedance of the power receiving device  2  as seen by the power transmission device  1  can be changed. 
     The electric power information detecting unit  26  detects information about electric power supplied to the load circuit  25  (electric power information). For example, the electric power information detecting unit  26  detects an electric power, a voltage, or a current supplied to the load circuit  25 , as the electric power information. The electric power information detecting unit  26  notifies the detected electric power information to the control unit  27 . In the explanation below, it is the electric power information detecting unit  26  that detects the electric power. 
     The control unit  27  controls the impedance converting unit  24  such that the impedance of the power receiving device  2  attains a first impedance, and the electric power information at that moment is obtained from the electric power information detecting unit  26 . The control unit  27  controls the impedance converting unit  24  such that the impedance of the power receiving device  2  attains a second impedance which is different from the first impedance, and the electric power information at that moment is obtained from the electric power information detecting unit  26 . Then, the control unit  27  uses the obtained two pieces of electric power information to determine whether to adjust the resonant frequency of the power receiving resonance unit  23 . For example, the control unit  27  can adjust the resonant frequency of the power receiving resonance unit  23  by switching the plurality of switches  22   b  as shown in  FIG. 2  to ON state and OFF state. What kind of values the first impedance and the second impedance have will be explained later. The method for determining whether to adjust the resonant frequency of the power receiving resonance unit  23  will be explained later. 
       FIG. 3  illustrates an example of relationship between the received electric power of the load circuit  25  and the load resistance (impedance) of the power receiving device  2  in a case where the frequency of the voltage supplied from the alternating current power supply  11 , the resonant frequency of the power transmitting resonance unit  14 , and the resonant frequency of the power receiving resonance unit  23  are all the same. Here, the load resistance means a load resistance value converted by the impedance converting unit  24 . 
     It is understood from  FIG. 3  that the received electric power is a protruding-type function having a peak at one point (R L ) in accordance with the load resistance value. For example, when the range of the load resistance value used in the power receiving device  2  (load control range) is smaller than the load resistance value R L  at which the received electric power is the maximum as shown in  FIG. 3 , the received electric power monotonically increases in accordance with the load resistance value in the entire load control range. Therefore, the load resistance may be increased in order to increase the received electric power, and the load resistance may be decreased in order to decrease the received electric power. The load control range is determined according to the design of the load circuit  25  and the impedance converting unit  24 . The load control range may also be determined according to, e.g., a range in which the impedance of the power receiving device  2  as seen by the power transmission device  1  may fall and a summation of the load circuit  25  of the power receiving device  2  and the resistance value of the impedance converting unit  24 . 
     When the resonant frequency of the power transmitting resonance unit  14  or the resonant frequency of the power receiving resonance unit  23  becomes different from the frequency of the voltage supplied from the alternating current power supply  11 , the load resistance value R L  at which the received electric power is the maximum is changed.  FIG. 4  illustrates an example of relationship between the received electric power of the load circuit  25  and the load resistance of the power receiving device  2  in a case where the resonant frequency of the power transmitting resonance unit  14  or the resonant frequency of the power receiving resonance unit  23  is different from the frequency of the voltage supplied from the alternating current power supply  11 . 
     In the example as shown in  FIG. 4 , when the resonant frequency of the power transmitting resonance unit  14  changes by about 5%, the load resistance value R L  at which the received electric power is the maximum is less than the maximum value of the load control range. In this case, as described above, the relationship of the monotonic increase of the received electric power according to the load resistance value in the entire load control range is not established, and it is impossible to uniquely determine whether to increase or decrease the load resistance in order to increase the received electric power. For this reason, the control cannot be performed based on the assumption that the received electric power monotonically depends on the load resistance value. 
     When the relationship of the monotonic increase of the received electric power according to the load resistance value in the load control range is not established, the control unit  27  determines that it is necessary to adjust the resonant frequency, and adjusts the resonant frequency. 
     The method for adjusting the resonant frequency will be explained with reference to the flowchart as illustrated in  FIG. 5 . When the resonant frequency is adjusted, the electric power is transmitted from the power transmission device  1  to the power receiving device  2 . 
     (Step S 101 ) The control unit  27  controls the impedance converting unit  24  so that the impedance of the power receiving device  2  attains the first impedance. Here, the first impedance is the maximum value of the load control range. 
     (Step S 102 ) the electric power information detecting unit  26  detects the electric power supplied to the load circuit  25  (received electric power) as the electric power information, and notifies the control unit  27  of the electric power information. 
     (step S 103 ) the control unit  27  controls the impedance converting unit  24  so that the impedance of the power receiving device  2  attains the second impedance. The second impedance is a value slightly less than the first impedance. The second impedance may be set to any value less than the first impedance. For example, the second impedance is set so that the difference between the first impedance and the second impedance is about 10% of the load control range. 
     (Step S 104 ) the electric power information detecting unit  26  detects the electric power supplied to the load circuit  25  as the electric power information, and notifies the control unit  27  of the electric power information. 
     (step S 105 ) the control unit  27  deducts the electric power obtained in step S 104  from the electric power obtained in step S 102 , and determines whether the result of deduction is positive or negative. 
     When the result of deduction is determined to be positive, the load resistance value R L  at which the received electric power is the maximum is equal to or more than the maximum value of the load control range as illustrated in  FIG. 6A , and the received electric power monotonically increases in accordance with the load resistance value in the entire load control range. Therefore, when the result of deduction is determined to be positive, the adjustment of the resonant frequency is determined to be unnecessary, and the processing is terminated. 
     On the other hand, when the result of deduction is determined to be negative, the load resistance value R L  at which the received electric power is the maximum is less than the maximum value of the load control range as illustrated in  FIG. 6B , and the relationship of the monotonic increase of the received electric power according to the load resistance value in the entire load control range is not established. Therefore, when the result of deduction is determined to be negative, the adjustment of the resonant frequency is determined to be necessary, and step S 106  is performed. 
     (Step S 106 ) the control unit  27  adjusts the resonant frequency of the power receiving resonance unit  23 . For example, the control unit  27  adjusts the resonant frequency of the power receiving resonance unit  23  by changing the capacity of the capacitor  22 . After the resonant frequency of the power receiving resonance unit  23  is adjusted, return to step S 101 . 
     With such processing, as shown in  FIG. 6A , the relationship of the monotonic increase of the received electric power according to the load resistance value in the entire load control range can be established. After the resonant frequency has been adjusted, the power receiving device  2  causes the control unit  27  to compare desired electric power with the received electric power detected by the electric power information detecting unit  26 . When the detected received electric power is less than the desired electric power, the control unit  27  may increase the load resistance. When the detected received electric power is more than the desired electric power, the control unit  27  may decrease the load resistance. When the desired electric power is supplied to the load circuit  25 , the direction in which the load resistance is to be changed is uniquely determined, and therefore, the received electric power can be controlled at high speed. 
     Second Embodiment 
       FIG. 7  illustrates a schematic configuration of a wireless electric power transmission system according to the second embodiment of the present invention. In the above first embodiment, the resonant frequency of the power receiving resonance unit  23  is adjusted. However, in the present embodiment, further, a resonant frequency of the power transmitting resonance unit  14  and an output frequency of the alternating current power supply  11  can be adjusted. 
     As shown in  FIG. 7 , a power transmission device  1  further includes not only the power transmission device  1  according to the first embodiment as shown in  FIG. 1  but also a wireless communication unit  15  and a control unit  16 . A power receiving device  2  further includes the configuration of the power receiving device  2  according to the first embodiment as shown in  FIG. 1  but also a wireless communication unit  28 . 
     When the result of deduction is determined to be negative in step S 105  in  FIG. 5 , a control unit  27  transmits a frequency adjusting instruction to the power transmission device  1  via a wireless communication unit  28 . 
     When the control unit  16  of the power transmission device  1  receives a frequency adjusting instruction from the power receiving device  2  via the wireless communication unit  15 , the control unit  16  adjusts at least one of the resonant frequency of the power transmitting resonance unit  14  and the output frequency of the alternating current power supply  11 . For example, the control unit  16  can control the resonant frequency of the power transmitting resonance unit  14  by adjusting the capacity of the capacitor  12  having the same configuration as  FIG. 2 . 
     Like the first embodiment, the relationship of the monotonic increase of the received electric power according to the load resistance value in the entire load control range can be established by adjusting at least one of the resonant frequency of the power transmitting resonance unit  14  and the output frequency of the alternating current power supply  11 . Accordingly, when the desired electric power is supplied to the load circuit  25 , the direction in which the load resistance is to be changed is uniquely determined, and therefore, the received electric power can be controlled at high speed. 
     When the result of deduction in step S 105  in  FIG. 5  is determined to be negative, at least one of the resonant frequency of the power receiving resonance unit  23 , the resonant frequency of the power transmitting resonance unit  14 , and the output frequency of the alternating current power supply  11  may be adjusted. Which parameter is to be adjusted may be determined in accordance with variation of manufacturing process of the power transmission device  1  and the power receiving device  2 . For example, when there is less variation of manufacturing process of the power transmission device  1 , and there is much variation of manufacturing process of the power receiving device  2 , the resonant frequency of the power receiving resonance unit  23  may be adjusted. 
     Such determination as to whether it is necessary to adjust the resonant frequency and adjustment of the resonant frequency may be done on every occasion before power is transmitted from the power transmission device  1 , or may be done with regular interval or with any given timing. Alternatively, as shown in  FIG. 8 , the power transmission device  1  and the power receiving device  2  may be provided with memories  17 ,  29 , and when the power transmission device  1  and the power receiving device  2  are manufactured or shipped, the determination as to whether it is necessary to adjust the resonant frequency and adjustment of the resonant frequency may be done, and the result of adjustment may be recorded to the memories  17 ,  29 . The memory  17  of the power transmission device  1  records the result of adjustment when the resonant frequency of the power transmitting resonance unit  14  and the output frequency of the alternating current power supply  11  are adjusted. The memory  29  of the power receiving device  2  records the result of adjustment when the resonant frequency of the power receiving resonance unit  23  is adjusted. 
     Third Embodiment 
     In the first, second embodiments, the relationship of the monotonic increase of the received electric power according to the load resistance value in the entire load control range is established. Alternatively, the relationship of the monotonic decrease of the received electric power according to the load resistance value in the entire load control range may be established. More specifically, the load resistance value R L  at which the received electric power is the maximum is set at a value equal to or less than the minimum value of the load control range. 
     The method for adjusting the resonant frequency according to the present embodiment is the same as the one shown in the flowchart in  FIG. 5 . However, the first impedance is the minimum value of the load control range, and the second impedance is a value slightly more than the first impedance. 
     When the result of deduction obtained by deducting the electric power when the load resistance value is the second impedance from the electric power when the load resistance value is the first impedance is positive, the load resistance value R L  at which the received electric power is the maximum is equal to or less than the minimum value of the load control range as shown in  FIG. 9A , and the received electric power monotonically decreases according to the load resistance value in the entire load control range. Therefore, when the result of deduction is determined to be positive, the adjustment of the resonant frequency is determined to be unnecessary, and the processing is terminated. 
     On the other hand, when the result of deduction is determined to be negative, the load resistance value R L  at which the received electric power is the maximum is more than the minimum value of the load control range as illustrated in  FIG. 9B , and the relationship of the monotonic decrease of the received electric power according to the load resistance value in the entire load control range is not established. Therefore, when the result of deduction is determined to be negative, the adjustment of the resonant frequency is determined to be necessary. Accordingly, at least one of the resonant frequency of the power receiving resonance unit  23 , the resonant frequency of the power transmitting resonance unit  14 , and the output frequency of the alternating current power supply  11  is adjusted. 
     With such processing, as shown in  FIG. 9A , the relationship of the monotonic decrease of the received electric power according to the load resistance value in the entire load control range can be established. After the resonant frequency has been adjusted, the power receiving device  2  causes the control unit  27  to compare desired electric power with the received electric power detected by the electric power information detecting unit  26 . When the detected received electric power is less than the desired electric power, the control unit  27  may decrease the load resistance. When the detected received electric power is more than the desired electric power, the control unit  27  may increase the load resistance. When the desired electric power is supplied to the load circuit  25 , the direction in which the load resistance is to be changed is uniquely determined, and therefore, the received electric power can be controlled at high speed. 
     As shown in  FIG. 10 , in the power transmitting resonance unit  14  of the power transmission device  1 , a capacitor  12  and a power transmission coil  13  may be connected in parallel, and in the power receiving resonance unit  23  of the power receiving device  2 , a power receiving coil  21  and a capacitor  22  may be connected in parallel. In this configuration, the load resistance value R L  at which the received electric power is the maximum is decreased. Therefore, when the configuration as shown in  FIG. 10  is employed, the relationship of the monotonic decrease of the received electric power according to the load resistance value in the entire load control range is preferably established like the above third embodiment. Alternatively, as shown in  FIG. 11 , a capacitor  12   a  connected in series to the power transmission coil  13  and a capacitor  12   b  connected in parallel with the power transmission coil  13  may be provided in the power transmitting resonance unit  14  of the power transmission device  1 . 
       FIG. 12  illustrates an example of relationship between the load resistance value and the electric power transmission efficiency. The electric power transmission efficiency is a value obtained by dividing the received electric power by the transmitted electric power. As can be understood from  FIG. 12 , the electric power transmission efficiency has a peak at one point (R X ) according to the load resistance value. The load resistance value R X  at which the electric power transmission efficiency is the maximum is configured to be included in the load control range, so that the electric power can be transmitted wirelessly with high efficiency. 
     When at least one of the resonant frequency of the power receiving resonance unit  23 , the resonant frequency of the power transmitting resonance unit  14 , and the output frequency of the alternating current power supply  11  is adjusted according to the method of the above embodiment, and as a result, the impedance of the power receiving device  2  attains a value close to an end portion of the load control range (the maximum value, the minimum value), then the voltage transmitted from the power transmission device  1  is preferably controlled, so that the impedance of the power receiving device  2  is close to the central value of the load control range. 
     For example, when the load resistance value R L  at which the received electric power is the maximum is adjusted to become larger than the maximum value of the load control range, and as a result, the impedance of the power receiving device  2  attains a value close to the maximum value of the load control range, then the control unit  27  transmits the power transmission voltage increase instruction to the power transmission device  1  via the wireless communication unit  28  (see  FIG. 7 ). When the control unit  16  of the power transmission device  1  receives a power transmission voltage increase instruction via the wireless communication unit  15 , the output voltage of the alternating current power supply  11  is raised. Accordingly, the received electric power of the power receiving device  2  increases and the impedance decreases, so that the impedance of the power receiving device  2  can be changed to a value closer to the central value of the load control range. As described above, the impedance of the power receiving device  2  can be changed to a value closer to the central value of the load control range, so that a wide control range of the impedance can be ensured when the received electric power is controlled at high speed. 
     In the above embodiment, the control unit  27  of the power receiving device  2  deducts the electric power obtained in step S 104  from the electric power obtained in step S 102  in  FIG. 5 , and determines whether to adjust at least one of the resonant frequency of the power receiving resonance unit  23 , the resonant frequency of the power transmitting resonance unit  14 , and the output frequency of the alternating current power supply  11 , on the basis of the positive/negative sign of the result of deduction. Alternatively, the control unit  16  of the power transmission device  1  (see  FIG. 7 ) may perform this determination processing. In this case, the control unit  27  of the power receiving device  2  controls the impedance of the impedance converting unit  24 , and the electric power information detected by the electric power information detecting unit  26  is transmitted to the power transmission device  1  via the wireless communication unit  28 . The control unit  16  performs the determination processing of step S 105  of  FIG. 5 . When the resonant frequency of the power receiving resonance unit  23  is determined to be adjusted, the control unit  16  gives the resonant frequency adjusting instruction to the control unit  27  of the power receiving device  2  via the wireless communication unit  28 . 
     In the flowchart as shown in  FIG. 5 , the setting value of the first impedance and the setting value of the second impedance may be set oppositely. In that case, the positive/negative sign of the result of deduction in step S 105  is treated oppositely. 
     In the above first, second embodiments, the first impedance is the maximum value of the load control range, and the second impedance is a value slightly less than the first impedance. Alternatively, the first impedance may slightly larger than the maximum value of the load control range, and the second impedance may be set the maximum value of the load control range. 
     In the above third embodiment, the first impedance is the minimum value of the load control range, and the second impedance is a value slightly larger than the first impedance. Alternatively, the first impedance may be a value slightly less than the minimum value of the load control range, and the second impedance may be the minimum value of the load control range. 
     In the above embodiment, the relationship of the monotonic increase (or monotonic decrease) of the received electric power according to the load resistance value in the entire load control range is established, and when desired electric power is supplied to the load circuit  25 , the direction in which the load resistance is changed is uniquely defined, and the received electric power is controlled at high speed. Therefore, when the change of the electric power caused by the change of the load resistance is different from the expected direction, certain malfunction and change may have occurred in the wireless electric power transmission system, and accordingly, the power transmission may be stopped. For example, the control unit  27  determines to stop the power transmission, and instructs the power transmission device  1  to stop the power transmission, or the control unit  16  determines to stop the power transmission. In this configuration, change and occurrence of abnormality of the system can be detected. In order to follow the change of the system, the processing as shown in  FIG. 5  may be performed again after the power transmission is stopped, and thereafter, the power transmission may be resumed. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.