Patent Publication Number: US-11038373-B2

Title: Power transmission system including power transmitter apparatus, power receiver apparatus, or power transmitter and receiver apparatus easily attachable and detachable to/from transmission path

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. § 371 of International Patent Application No. PCT/JP2017/044023, filed on Dec. 7, 2017, which in turn claims the benefit of Japanese Application No. 2016-239392, filed on Dec. 9, 2016, the entire disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present disclosure relates to a power receiver apparatus which receives power via a transmission path, a power transmitter apparatus which transmits power via a transmission path, and a power transmitter and receiver apparatus which transmits and receives power via a transmission path. The present disclosure also relates to a power transmission system including at least one of the power receiver apparatus, the power transmitter apparatus, and the power transmitter and receiver apparatus. 
     BACKGROUND ART 
     In recent years, power supplies of renewable energy, typically photovoltaic power generation, wind power generation, and biofuel power generation, are increasingly used, as well as conventional power supplies provided by power companies, such as thermal power generation, hydropower generation, and nuclear power generation. In addition, apart from large-scale commercial power networks currently provided, local and small-scale power networks capable of achieving local production and local consumption of power have been being spread worldwide in order to reduce losses of long-distance power transmission. 
     In a small-scale power network, power can be supplied self-sufficiently by using a natural energy power generator, and electric load equipment capable of efficient power regeneration. This type of power network is highly promising as a power transmission system for supplying electricity to non-electrified areas, such as desert oasis and remote islands. 
     For example, each of Patent Documents 1 to 3 discloses a power transmission system which transmits power from a power supply to a load via a power line. 
     CITATION LIST 
     Patent Documents 
     PATENT DOCUMENT 1: Japanese Patent Publication No. 5612718 B 
     PATENT DOCUMENT 2: Japanese Patent Publication No. 5612920 B 
     PATENT DOCUMENT 3: Japanese Patent laid-open Publication No. 2011-091954 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     A certain type of power transmission system transmits power from a plurality of power supplies to a plurality of loads. In this case, it is required to simplify the configuration of the power transmission system using a common transmission path, rather than using individual transmission paths for respective pairs of a power supply and a load. In addition, when transmitting power from a plurality of power supplies to a plurality of loads via a common transmission path, it is required for transmission efficiency to be less likely to degrade due to multiplexed power transmission. 
     In addition, the number of power supplies and the number of loads of the power transmission system may increase or decrease in accordance with users&#39; requests, etc. In this case, it is required to be capable of easily attaching and detaching the power transmitter apparatus, the power receiver apparatus, etc., to/from the power transmission system. 
     An object of the present disclosure is to which solve the aforementioned problems, and to provide a power receiver apparatus with a simple configuration, operable at higher transmission efficiency than that of the prior art, and easily attachable and detachable to/from a power transmission system. 
     Solution to Problem 
     According an aspect of the present disclosure, a power receiver apparatus receives a code-modulated wave from a power transmitter apparatus via a transmission path, the code-modulated wave including first power modulated by code modulation using a modulation code based on a code sequence. The power receiver apparatus is provided with: a contactless connector that is coupled to the transmission path without electrical contact with the transmission path, and receives the code-modulated wave from the power transmitter apparatus via the transmission path; and a code demodulator that demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for the code modulation. 
     These generic and specific aspects may be implemented as a system, as a method, or as any combination of systems and methods. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a power receiver apparatus with a simple configuration, operable at higher transmission efficiency than that of the prior art, and easily attachable and detachable to/from a power transmission system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a power transmission system according to a first embodiment. 
         FIG. 2  is a waveform diagram showing an exemplary signal waveform of a modulated current I 2  of the power transmission system of  FIG. 1 . 
         FIG. 3  is a waveform diagram showing an exemplary signal waveform of a modulated current I 2  of a communication system according to a comparison example. 
         FIG. 4  is a waveform diagram showing exemplary signal waveforms in the power transmission system of  FIG. 1 , in which: (a) shows a signal waveform of a generated current I 1 ; (b) shows a signal waveform of a modulated current I 2 ; and (c) shows a signal waveform of a demodulated current I 3 . 
         FIG. 5  is a block diagram showing a configuration of a code modulator  2  of  FIG. 1 . 
         FIG. 6  is a block diagram showing a configuration of a code demodulator  4  of  FIG. 1 . 
         FIG. 7  is a block diagram showing configurations of a code modulation circuit  23  and a code demodulation circuit  33  of  FIG. 1 . 
         FIG. 8A  is a diagram showing an example of a modulation code of the code modulator  2  and a demodulation code of the code demodulator  4  in the power transmission system of  FIG. 1 , as a first implementation example in which direct-current power is transmitted and received. 
         FIG. 8B  is a diagram showing an example of a modulation code of the code modulator  2  and a demodulation code of the code demodulator  4  in the power transmission system of  FIG. 1 , as a second implementation example in which direct-current power is transmitted and received. 
         FIG. 9  is a waveform diagram showing exemplary signal waveforms in the power transmission system according to a second embodiment, in which: (a) shows a signal waveform of a generated current I 1 ; (b) shows a signal waveform of a modulated current I 2 ; and (c) shows a signal waveform of a demodulated current I 3 . 
         FIG. 10  is a block diagram showing a partial configuration of a code modulator  2 A of the power transmission system according to the second embodiment. 
         FIG. 11  is a block diagram showing a partial configuration of a code demodulator  4 A of the power transmission system according to the second embodiment. 
         FIG. 12A  is a diagram showing an example of a modulation code of the code modulator  2 A and a demodulation code of the code demodulator  4 A in the power transmission system according to the second embodiment, as a third implementation example in which alternating-current power is transmitted and received. 
         FIG. 12B  is a diagram showing an example of a modulation code of the code modulator  2 A and a demodulation code of the code demodulator  4 A in the power transmission system according to the second embodiment, as a fourth implementation example in which direct-current power is transmitted and received. 
         FIG. 13A  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 21 A for a code modulation circuit  23 A used in a power transmission system according to a modified embodiment of the second embodiment. 
         FIG. 13B  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 22 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 13C  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 23 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 13D  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 24 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 14A  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 31 A for a code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 14B  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 32 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 14C  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 33 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 14D  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 34 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. 
         FIG. 15  is a block diagram showing a configuration of a power transmission system according to a third embodiment. 
         FIG. 16A  is a diagram showing an example of a modulation code of a code modulator  2 A- 1  and a demodulation code of a code demodulator  4 A- 1  in the power transmission system of  FIG. 15  according to the third embodiment, in which direct-current power is transmitted and received. 
         FIG. 16B  is a diagram showing an example of a modulation code of the code modulator  2 A- 2  and a demodulation code of the code demodulator  4 A- 2  in the power transmission system of  FIG. 15  according to the third embodiment, in which direct-current power is transmitted and alternating-current power is received. 
         FIG. 17  is a waveform diagram showing exemplary signal waveforms in the power transmission system according to the third embodiment, in which: (a) shows a signal waveform of a generated current I 11 ; (b) shows a signal waveform of a generated current I 12 ; (c) shows a signal waveform of a modulated current I 2 ; (d) shows a signal waveform of a demodulated current I 31 ; and (e) shows a signal waveform of a demodulated current I 32 . 
         FIG. 18  is a block diagram showing a configuration of a power transmission system according to a fourth embodiment. 
         FIG. 19  is a diagram showing a first implementation example of a transmission path  3 A and a contactless connector  7  of  FIG. 18 . 
         FIG. 20  is a diagram showing a second implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . 
         FIG. 21  is a diagram showing a third implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . 
         FIG. 22  is a diagram showing a fourth implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . 
         FIG. 23  is a diagram showing a first modification of coils  7   d   1  and  7   d   2  of  FIG. 22 . 
         FIG. 24  is a diagram showing a second modification of the coils  7   d   1  and  7   d   2  of  FIG. 22 . 
         FIG. 25  is a diagram showing a third modification of the coils  7   d   1  and  7   d   2  of  FIG. 22 . 
         FIG. 26  is a diagram showing a fifth implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . 
         FIG. 27  is a block diagram showing a configuration of a power transmission system according to a first modification of the fourth embodiment. 
         FIG. 28  is a block diagram showing a configuration of a power transmission system according to a second modification of the fourth embodiment. 
         FIG. 29  is a block diagram showing a configuration of a power transmission system according to a third modification of the fourth embodiment. 
         FIG. 30  is a block diagram showing a configuration of a power transmission system according to a fourth modification of the fourth embodiment. 
         FIG. 31  is a block diagram showing a configuration of a power transmission system according to a fifth embodiment. 
         FIG. 32  is a block diagram showing a configuration of a code modulator  2 B of  FIG. 31 . 
         FIG. 33  is a block diagram showing a configuration of a code demodulator  4 B of  FIG. 31 . 
         FIG. 34  is a block diagram showing a configuration of a power transmission system according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Findings Underlying Present Disclosure 
     Patent Document 1 discloses an interconnection apparatus for power transmission apparatuses, the interconnection apparatus being capable of transmitting and receiving power among a plurality of power systems. 
     According to Patent Document 1, the interconnection apparatus is provided with a converter and an inverter. For power transmission, the converter converts transmitting power from alternating current to direct current, and the converted power is transmitted to the interconnection apparatus connected to a receiving power system. At the interconnection apparatus of the receiving power system, the inverter converts the power so as to have a desired frequency, thus providing power having an optimum frequency for the power system to which the interconnection apparatus is connected. Moreover, Patent Document 2 discloses a configuration further provided with a power storage apparatus, in addition to the components of Patent Document 1. 
     On the other hand, Patent Document 3 discloses a method of transmitting power from a plurality of power transmitter apparatuses to a plurality of power receiver apparatuses. According to Patent Document 3, power is transmitted from the plurality of power transmitter apparatuses to the plurality of power receiver apparatuses in a time division manner. According to Patent Document 3, control signals are wirelessly transmitted among the power transmitter apparatuses and the power receiver apparatuses in order to transmit and receive power. 
     However, according to Patent Documents 1 and 2, the interconnection apparatus is provided with the inverter and converter, and basically, individual power transmission cables are required for all combinations of the power systems transmitting and receiving power. According to Patent Documents 1 and 2, the interconnection apparatus may be configured so as to reduce the number of power transmission cables, however, in any case, a large number of power transmission cables are required. Thus, installation costs and the cables&#39; material costs increase. In addition, the interconnection apparatus should be provided with the same number of pairs of the inverter and the converter as the number of the power systems to be connected. Accordingly, the cables&#39; costs may increase, and costs may further increase due to the increased size of the interconnection apparatus. 
     In addition, according to Patent Document 3, it is possible to transmit and receive power among the plurality of power transmitter apparatuses and the plurality of power receiver apparatuses in a time division manner, and advantageously, it is possible to reduce the number of power transmission cables. However, in case of time-division power transmission, it is not possible to transmit and receive power among the plurality of power systems, simultaneously. In other words, it may not be possible to immediately handle a power demand from a load connected to a power receiver. Furthermore, when transmitting and receiving power among a large number of pairs of the power transmitter apparatuses and the power receiver apparatuses, only a short time is allocated for one pair of the power transmitter apparatus and the power receiver apparatus to transmit and receive power, and therefore, large pulse power is transmitted over the power transmission cable. Accordingly, the transmission cable with a high power durability is required, and thus, it may increase costs. In addition, since time intervals in which power can not be received occur, it may be required to provide the power receiver apparatuses with a buffer for large power. Furthermore, in order to transmit and receive power in a time division manner, time-synchronization is required among the plurality of power transmitter apparatuses and the plurality of power receiver apparatuses. In order to achieve such synchronization, very accurate controls among the apparatuses are required, and thus, it may increase the entire system costs. 
     As described above, according to both Patent Documents 1 and 2, a large number of power transmission cables are used, and therefore, it is not possible to reduce the power transmission cables by multiplexed power transmission. Further, the interconnection apparatus requires a pair of inverter and converter for each of the power transmission cables, and therefore, it is not possible to reduce the size of the interconnection apparatus. Accordingly, it is difficult to transmit and receive power among a large number of power systems. On the other hand, according to Patent Document 3, power is transmitted and received among the plurality of power transmitter apparatuses and the plurality of power receiver apparatuses over the power transmission cables in a time division manner, thus reducing the number of the power transmission cables. However, it is not possible to provide a transmission system capable of transmitting and receiving power among the plurality of power systems, simultaneously. Accordingly, there is a demand for a power transmission system with a reduced number of power transmission cables, and capable of transmitting and receiving power from a plurality of power transmitter apparatuses to a plurality of power receiver apparatuses, simultaneously, and more reliably, while reducing sizes and thicknesses of the power transmitter apparatuses and the power receiver apparatuses. 
     Further, as described above, the number of power supplies and the number of loads of the power transmission system may increase or decrease in accordance with users a users&#39; requests, etc. In this case, it is required to be capable of easily attaching and detaching the power transmitter apparatus, the power receiver apparatus, etc., to/from the power transmission system. 
     Based on the above consideration, the inventors provide the following aspects of the invention. 
     According an aspect of the present disclosure, a power receiver apparatus that receives a code-modulated wave from a power transmitter apparatus via a transmission path, the code-modulated wave including first power modulated by code modulation using a modulation code based on a code sequence. The power receiver apparatus is provided with: a contactless connector that is coupled to the transmission path without electrical contact with the transmission path, and receives the code-modulated wave from the power transmitter apparatus via the transmission path; and a code demodulator that demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for the code modulation. 
     According an aspect of the present disclosure, a power transmitter apparatus transmits power to a power receiver apparatus via a transmission path. The power transmitter apparatus is provided with: a code modulator that modulates first power to generate a code-modulated wave by code modulation using a modulation code based on a code sequence; and a contactless connector that is coupled to the transmission path without electrical contact with the transmission path, and transmits the code-modulated wave to the power receiver apparatus via the transmission path. 
     According an aspect of the present disclosure, a power transmitter and receiver apparatus transmits power to a power receiver apparatus via a transmission path and receives power from a power transmitter apparatus via the transmission path. The power transmitter and receiver apparatus is provided with: a code modulator/demodulator that modulates first power to generate a first code-modulated wave by code modulation using a first modulation code based on a first code sequence; and a contactless connector that is coupled to the transmission path without electrical contact with the transmission path, and transmits the first code-modulated wave to the power receiver apparatus via the transmission path. The contactless connector further receives a second code-modulated wave from the power transmitter apparatus via the transmission path, the second code-modulated wave including power modulated by code modulation using a second modulation code based on a second code sequence. The code modulator/demodulator further demodulates the received second code-modulated wave to generate second power by code demodulation using a demodulation code based on the second code sequence. 
     A power transmission system according to the present disclosure can actively specify combinations of a power supply and a load, and specify amounts of power to be transmitted, and then, simultaneously and independently transmit power among the combinations over one transmission path, as well as easily attach/detach and increase/decrease apparatuses involved in power transmission. 
     Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. In the following embodiments, similar constituent elements are denoted by identical reference numerals. 
     An object of the present disclosure is to provide a power receiver apparatus, a power transmitter apparatus, and a power transmitter and receiver apparatus, each having a simple configuration, operable at higher transmission efficiency than that of the prior art, and easily attachable and detachable to/from a power transmission system. In addition, an object of the present disclosure is to provide a power transmission system including at least one of the power receiver apparatus, the power transmitter apparatus, and the power transmitter and receiver apparatus. In first to third embodiments, we describe preparatory overviews of power transmission systems. Thereafter, in fourth to sixth embodiments, we describe power transmission systems which solves the problems. 
     First Embodiment 
       FIG. 1  is a block diagram showing a configuration of a power transmission system according to the first embodiment. Referring to  FIG. 1 , the power transmission system according to the first embodiment is provide with a power generator  1 , a code modulator  2 , a transmission path  3 , a code demodulator  4 , a load  5 , and a controller  10 . The transmission path  3  is, for example, a wired transmission path. 
     The controller  10  is provided with a control circuit  11  and a communication circuit  12 . The control circuit  11  communicates with the code modulator  2  and the code demodulator  4  via the communication circuit  12 , and controls operations of the code modulator  2  and the code demodulator  4 . 
     In the power transmission system of  FIG. 1 , the code modulator  2  operates as a power transmitter apparatus, and the code demodulator  4  operates as a power receiver apparatus. The code modulator  2  modulates first power to generate a code-modulated wave by code modulation using a modulation code based on a code sequence, and transmits the code-modulated wave to the code demodulator  4  via the transmission path  3 . The code demodulator  4  receives the code-modulated wave from the code modulator  2  via the transmission path  3 , and demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for the code modulation. The first power is, for example, direct-current power generated by the power generator  1 , and is shown as a generated current I 1  in  FIG. 1 . The code-modulated wave is alternating-current power modulated by code modulation, and is shown as a modulated current I 2  in  FIG. 1 . The second power is, for example, direct-current power to be supplied to the load  5 , and is shown as a demodulated current I 3  in  FIG. 1 . 
     The power transmission system of  FIG. 1  is further provided with power meters  1   m  and  5   m . The power meter  1   m  is first power measuring means which measures an amount of the first power. More specifically, the power meter  1   m  measures an amount of direct-current power generated by the power generator  1  and transmitted from the power generator  1  to the code modulator  2 . The power meter  1   m  may be provided to the power generator  1 , or disposed between the power generator  1  and the code modulator  2 . The power meter  5   m  is second power measuring means for measuring an amount of the second power. More specifically, the power meter  5   m  measures an amount of direct-current power transmitted from the code demodulator  4  to the load  5 , and used by the load  5 . The power meter  5   m  may be provided to the load  5 , or may be disposed between the code demodulator  4  and the load  5 . The amounts of powers measured by the power meters  1   m  and  5   m  are transmitted to the controller  10 . 
     The controller  10  controls operations of the code modulator  2  and the code demodulator  4  based on the amounts of powers received from the power meters  1   m  and  5   m . For example, the controller  10  transmits control signals to the code modulator  2  and the code demodulator  4 , the control signals including synchronization signals for synchronizing the code modulator  2  and the code demodulator  4  to each other, thus achieving code modulation and code demodulation of power in an accurately synchronized manner. 
     The controller  10  sets a modulation code to the code modulator  2 , and a demodulation code to the code demodulator  4 , based on one code sequence. The code sequence of the modulation code used for modulation by the code modulator  2 , and the code sequence of the demodulation code used for demodulation by the code demodulator  4  may be set in advance to the code modulator  2  and the code demodulator  4 . In addition, for example, the controller  10  may transmit, as the control signals, the code sequence of the modulation code used for modulation by the code modulator  2 , and the code sequence of the demodulation code used for demodulation by the code demodulator  4 . Further, the controller  10  may transmit, as the control signals, only information specifying the code sequences, without transmitting the code sequences themselves, so that the code modulator  2  and the code demodulator  4  to generates the code sequences, respectively. In this case, it is possible to achieve code modulation and code demodulation between the code modulator  2  and the code demodulator  4  corresponding to each other in an accurately synchronized manner. 
       FIG. 2  is a waveform diagram showing an exemplary signal waveform of the modulated current I 2  of the power transmission system of  FIG. 1 . In addition,  FIG. 3  is a waveform diagram showing an exemplary signal waveform of a modulated current I 2  of a communication system according to a comparison example. 
     The code modulator  2  of  FIG. 1  modulates a current of power, which is generated by the power generator  1 , by code modulation using a modulation code based on a code sequence. In this case, the code modulator  2  generates an alternating-current code-modulated wave made of currents flowing in directions corresponding to code values of “1” and “−1.”, respectively, as shown in  FIG. 2 . This code-modulated wave can transmit power in both periods of positive current flows, and periods of negative current flows (e.g., period T 01  of  FIG. 2 ). While the first embodiment indicates an example in which direct-current power is modulated by code modulation, alternating-current power may be modulated by code modulation as in a second embodiment described below. 
     In the data transmission system according to the comparison example, e.g., to be used for communication, code values of “1” and “0” are typically used for code modulation, as shown in  FIG. 3 . However, according to the code-modulated wave as shown in  FIG. 3 , when the code value of the modulation code is “0” (e.g., period T 02  of  FIG. 3 ), a modulated current or voltage becomes zero, that is, a period of no power transmission occurs. Such periods of no power transmission may reduce overall power transmission efficiency. More specifically, for the case of communication, since information such as data should be transmitted in an accurately synchronized manner, it is only required that the code demodulator accurately distinguish between “0” and “1”. On the other hand, for the case of power transmission, a power loss due to the period of no power transmission is not permissible from a viewpoint of efficiency in use of energy. Accordingly, by using an alternating-current code-modulated wave flowing in directions corresponding to the code values of “1” and “4”, respectively, as shown in  FIG. 2 , it is possible to transmit power with higher transmission efficiency than that of the comparison example. 
       FIG. 4  is a waveform diagram, where (a) to (c) show exemplary signal waveforms in the power transmission system of  FIG. 1 . In  FIG. 4 , (a) shows a signal waveform of the generated current I 1 , (b) shows a signal waveform of the modulated current I 2 , and (c) shows a signal waveform of the demodulated current I 3 . The power generator  1  generates the direct-current generated current I 1 . The code modulator  2  multiplies the generated current I 1  by a modulation code m 0  to generate the alternating-current modulated current I 2 . The code demodulator  4  multiplies the modulated current I 2  by a demodulation code d 0  identical to the modulation code m 0  to reproduce the direct-current power generated by the power generator  1 , and supply the reproduced direct-current power to the load  5 . 
     Referring to  FIG. 4 , T 10  indicates a period of one cycle of the modulation code m 0  and the demodulation code d 0 . The same also applies to subsequent drawings. 
     According to the exemplary signal waveform of  FIG. 4 , the direct-current generated current I 1  ( FIG. 4( a ) ) is multiplied by the modulation code m 0  having a frequency of 35 kHz, to generate the modulated current I 2  ( FIG. 4( b ) ) of the code-modulated wave. In this case, the duration of each bit of the modulation code m 0  is 1/(35 kHz)/2=14.2 microseconds. 
     Each bit of the modulation code m 0  and the demodulation code d 0  has a code value “1” or “−1”. The code value “1” of the modulation code m 0  indicates that the code modulator  2  outputs a current in the same direction as the direction of an inputted current, and the code value “−1” of the modulation code m 0  indicates that the code modulator  2  outputs a current in the direction opposite to the direction of the inputted current. Similarly, the code value “1” of the demodulation code d 0  indicates that the code demodulator  4  outputs a current in the same direction as the direction of an inputted current, and the code value “4” of the demodulation code d 0  indicates that the code demodulator  4  outputs a current in the direction opposite to the direction of the inputted current. 
     For example, the modulation code m 0  and the demodulation code d 0  are given as follows.
 
 m 0=[1−1 1 1 1−1 −1−1 1−1 −1−1 1 1]  (1)
 
 d 0= m 0=[1−1 1 1 1−1 −1−1 1−1 −1−1 1 1]  (2)
 
     Subsequently, the modulated current I 2  of the code-modulated wave generated by the modulation code m 0  is multiplied by the demodulation code d 0 . This multiplication is denoted as follows.
 
 m 0× d 0=[1 1 1 1 1 1 1 1 1 1 1 1 1 1]  (3)
 
     As apparent from Mathematical Expression (3), the demodulated current I 3  ( FIG. 4( c ) ) is obtained, which is direct current similarly to the original generated current I 1 . 
     As described above, it is possible to achieve direct-current power transmission in an accurately synchronized manner, without power loss, by using the code modulator  2  and the code demodulator  4  according to the present embodiment. In addition, it is possible to achieve efficient power transmission for a longer period, for example, by repeatedly using the modulation code m 0  and demodulation code d 0  as described above. 
     Further, the modulation code m 0  can be divided into its former-half code portion m 0   a , and its latter-half code portion m 0   b , as follows.
 
 m 0 a =[1 −1 1 1 1 −1 −1]  (4)
 
 m 0 b =[−1 1−1 −1−1 1 1]  (5)
 
     In this case, the code portion m 0   b  is generated by inverting the sign of the code value of each bit of the code portion m 0   a . More specifically, when the code value of a certain bit of the code portion m 0   a  is “1”, the code value of a corresponding bit of the code portion m 0   b  is “−1”. Similarly, when the code value of a certain bit of the code portion m 0   a  is “−1.”, the code value of a corresponding bit of the code portion m 0   b  is “1”. 
       FIG. 5  is a block diagram showing a configuration of the code modulator  2  of  FIG. 1 . Referring to  FIG. 5 , the code modulator  2  is provided with a control circuit  20 , a communication circuit  21 , a code generation circuit  22 , and a code modulation circuit  23 . The communication circuit  21  receives a synchronization signal and a control signal from the controller  10 , the control signal including a code sequence itself or information specifying the code sequence, and outputs the received signals to the control circuit  20 . In this case, the synchronization signal may be, for example, trigger signals to start and end modulation, or time information indicating a start time and an end time of modulation. Based on the control signal, the control circuit  20  controls the code generation circuit  22  so as to generate a modulation code based on a code sequence and output the modulation code to the code modulation circuit  23 , and controls start and end of operation of the code modulation circuit  23 . The code modulation circuit  23  has input terminals T 1  and T 2  connected to the power generator  1 , and output terminals T 3  and T 4  connected to the transmission path  3 . 
       FIG. 6  is a block diagram showing a configuration of the code demodulator  4  of  FIG. 1 . Referring to  FIG. 6 , The code demodulator  4  is provided with a control circuit  30 , a communication circuit  31 , a code generation circuit  32 , and a code demodulation circuit  33 . The communication circuit  31  receives a synchronization signal and a control signal from the controller  10 , the control signal including a code sequence itself or information specifying the code sequence, and outputs the received signals to the control circuit  30 . In this case, the synchronization signal may be, for example, trigger signals to start and end demodulation, or time information indicating a start time and an end time of demodulation. Based on the control signal, the control circuit  30  controls the code generation circuit  32  so as to generate a demodulation code based on a code sequence and output the demodulation code to the code demodulation circuit  33 , and controls start and end of operation of the code demodulation circuit  33 . The code demodulation circuit  33  has input terminals T 11  and T 12  connected to the transmission path  3 , and output terminals T 13  and T 14  connected to the load  5 . 
     Note that in the power transmission system of  FIG. 1 , the control signals from the controller  10  to the code modulator  2  and the code demodulator  4  may be transmitted via control signal lines different from the transmission path  3 , or may be transmitted via the transmission path  3  in a manner multiplexed with the code-modulated wave using some multiplexing scheme. In the latter case, it is possible to omit cables provided for communication from the controller  10  to the code modulator  2  and the code demodulator  4 , and reduce cost. 
       FIG. 7  is a block diagram showing configurations of the code modulation circuit  23  and the code demodulation circuit  33  of  FIG. 1 . Referring to  FIG. 7 , the code modulation circuit  23  is provided with four switch circuits SS 1  to SS 4  connected in a bridge configuration. The switch circuits SS 1  to SS 4  include unidirectional switch elements S 1  to S 4 , respectively, each made of, for example, a metal-oxide-semiconductor (MOS) transistor. In addition, the code demodulation circuit  33  is provided with four switch circuits SS 11  to SS 14  connected in a bridge configuration. The switch circuits SS 11  to SS 14  include unidirectional switch elements S 11  to S 14 , respectively, each made of, for example, an MOS transistor. 
     The code generation circuit  22  generates and outputs the modulation codes m 1  and m 2  to the code modulation circuit  23  under control of the control circuit  20 , in order to operate the code modulator  2  according to the modulation code m 0  as described above. The switch elements S 1  and S 4  of the code modulation circuit  23  are controlled according to the modulation code m 1 , and the switch elements S 2  and S 3  of the code modulation circuit  23  are controlled according to the modulation code m 2 . Each of the modulation codes m 1  and m 2  has code values “1” and “0”. For example, when a signal of the code value “1” is inputted to each of the switch elements S 1  to S 4 , each of the switch elements S 1  to S 4  is turned on. When a signal of the code value “0” is inputted to each of the switch elements S 1  to S 4 , each of the switch elements S 1  to S 4  is turned off. Note that switch elements other than the switch elements S 1  to S 4  described in the present description operate in a similar manner. In this case, the switch elements S 1  to S 4  have directionality as follows. When the switch element S 1  is turned on, the switch element S 1  outputs a generated current inputted from the terminal T 1 , to the terminal T 3 . When the switch element S 3  is turned on, the switch element S 3  outputs a generated current inputted from the terminal T 1 , to the terminal T 4 . When the switch element S 2  is turned on, the switch element S 2  outputs a modulated current inputted from the terminal T 3 , to the terminal T 2 . When the switch element S 4  is turned on, the switch element S 4  outputs a modulated current inputted from the terminal T 4 , to the terminal T 2 . 
     The code generation circuit  32  generates and outputs the demodulation codes d 1  and d 2  to the code demodulation circuit  33  under control of the control circuit  30 , in order to operate the code demodulator  4  according to the demodulation code d 0  as described above. The switch elements S 11  and S 14  of the code demodulation circuit  33  are controlled according to the demodulation code d 2 , and the switch elements S 12  and S 13  of the code demodulation circuit  33  are controlled according to the demodulation code d 1 . Each of the demodulation codes d 1  and d 2  has code values “1” and “0”. In this case, the switch elements S 11  to S 14  have directionality as described below. When the switch element S 11  is turned on, the switch element S 11  outputs a modulated current inputted from the terminal T 12 , to the terminal T 13 . When the switch element S 13  is turned on, the switch element S 13  outputs a modulated current inputted from the terminal T 11 , to the terminal T 13 . When the switch element S 12  is turned on, the switch element S 12  outputs a demodulated current inputted from the terminal T 14 , to the terminal T 12 . When the switch element S 14  is turned on, the switch element S 14  outputs a demodulated current inputted from the terminal T 14 , to the terminal T 11 . 
     In the notation of  FIG. 7 , directions of current flows in the switch elements S 11  to S 14  of the code demodulator  4  are opposite to directions of current flows in the switch elements S 1  to S 4  of the code modulator  2 . 
       FIG. 8A  is a diagram showing an example of a modulation code of the code modulator  2  and a demodulation code of the code demodulator  4  in the power transmission system of  FIG. 1 , as a first implementation example in which direct-current power is transmitted and received. More specifically,  FIG. 8A  shows an example of the modulation codes m 1  and m 2  inputted to the switch elements S 1  to S 4  of the code modulator  2 , and the demodulation codes d 1  and d 2  inputted to the switch elements S 11  to S 14  of the code demodulator  4 . 
     As shown in  FIG. 8A , the modulation code m 1  and the demodulation code d 1  are identical to each other, and each is made of a code sequence c 1   a . In addition, the modulation code m 2  and the demodulation code d 2  are identical to each other, and each is made of a code sequence c 1   b . In addition, the code sequences c 1   a  and c 1   b  are configured such that when the code value of a certain bit of the code sequence c 1   a  is “1”, the code value of a corresponding bit of the code sequence c 1   b  is “0”; and when the code value of a certain bit of the code sequence c 1   a  is “0”, the code value of a corresponding bit of the code sequence c 1   b  is “1”. 
     Accordingly, among the switch elements S 1  to S 4  and S 11  to S 14  of  FIG. 7 , when a switch element receiving the code value of a certain bit of the code sequence c 1   a  is turned on, the switch element receiving the code value of a corresponding bit of the code sequence c 1   b  is turned off. In addition, when the switch element receiving the code value of a certain bit of the code sequence c 1   a  is turned off, the switch element receiving the code value of a corresponding bit of the code sequence c 1   b  is turned on. 
     According to the code modulation circuit  23  of  FIG. 7 , when the switch elements S 1  and S 4  are turned on, the switch elements S 2  and S 3  are turned off; and when the switch elements S 1  and S 4  are turned off, the switch elements S 2  and S 3  are turned on. Thus, when the switch elements S 1  and S 4  are turned on, and the switch elements S 2  and S 3  are turned off, the modulated current I 2  flows in the transmission path  3  in a positive direction, i.e., in a direction of solid arrows. On the other hand, when the switch elements S 1  and S 4  are turned off, and the switches S 2  and S 3  are turned on, the modulated current I 2  flows in the transmission path  3  in a negative direction, i.e., in a direction of dotted arrows. Accordingly, as shown in  FIG. 4 , when the direct-current generated current I 1  is inputted to the code modulator  2 , the alternating-current modulated current I 2  can be transmitted to the transmission path  3 . 
     In the code demodulation circuit  33  of  FIG. 7 , the switch elements S 11  to S 14  are turned on or off in response to the demodulation codes d 1  and d 2  in synchronization with the code modulation circuit  23 . In this case, the switch elements S 12  and S 13  are turned on or off in accordance with the demodulation code d 1  identical to the modulation code m 1 , and the switch elements S 11  and S 14  are turned on or off in accordance with the demodulation code d 2  identical to the modulation code m 2 . Thus, when the code value of the modulation code m 1  is “1”, and the code value of the modulation code m 2  is “0”, i.e., when the modulated current I 2  flows in the transmission path  3  in the positive direction, the code value of the demodulation code d 1  is “1”, and the code value of the demodulation code d 1  is “0”. Accordingly, by turning on the switch elements S 13  and S 12  and turning off the switch elements S 11  and S 14 , the demodulated current I 3  flows at the output terminals T 13  and T 14  of the code demodulation circuit  33  in the positive direction, i.e., in the direction of the solid arrows. On the other hand, when the code value of the modulation code m 1  is “0”, and the code value of the modulation code m 2  is “1”, i.e., when the modulated current I 2  flows in the transmission path  3  in the negative direction, the code value of the demodulation code d 1  is “0”, and the code value of the demodulation code d 1  is “1”. Accordingly, by turning on the switch elements S 11  and S 14  and turning off the switch elements S 12  and S 13 , the demodulated current I 3  again flows at the output terminals T 13  and T 14  of the code demodulation circuit  33  in the positive direction, i.e., in the direction of the solid arrows. 
     As described above, when using the modulation codes m 1  and m 2  and the demodulation codes d 1  and d 2  of  FIG. 8A , equivalently, the code modulator  2  operates according to the modulation code m 0  of Mathematical Expression (1), and the code demodulator  4  operates according to the demodulation code d 0  of Mathematical Expression (2). 
     As described above, according to  FIGS. 7 and 8A , when the direct-current generated current I 1  is inputted to the code modulator  2 , it is possible to extract the demodulated current I 3  from the code demodulator  4 , the demodulated current I 3  being also a direct current similarly to the generated current I 1  inputted to the code modulator  2 . Therefore, according to the first embodiment, it is possible to modulate the direct-current generated current I 1  by code modulation into the alternating-current modulated current I 2 , and then, transmit the modulated current I 2  via the transmission path  3 , and then, demodulate the modulated current I 2  into the direct-current demodulated current I 3 . 
       FIG. 8B  is a diagram showing an example of a modulation code of the code modulator  2  and a demodulation code of the code demodulator  4  in the power transmission system of  FIG. 1 , as a second implementation example in which direct-current power is transmitted and received. When in each of the code sequences c 1   a  and c 1   b , the number of bits of the code value “1” is equal to the number of bits of the code value “0”, the modulated current I 2  being modulated by code modulation and flowing in the transmission path  3  includes, in average, no direct-current component, but includes only an alternating-current component. However, in some code sequence, the number of bits of the code value “1” is different from the number of bits of the code value “0”, and thus, a direct-current component occurs. When using such a code sequence, by concatenating the code sequence with a code sequence of bits having code values inverted from those of corresponding bits, respectively, it is possible to generate a modulation code and a demodulation code, in each of which the number of bits of the code value “1” is equal to the number of bits of the code value “0”. According to the example of  FIG. 8B , each of the modulation code m 1  and the demodulation code d 1  is a code sequence [c 1   a  c 1   b ] which is a concatenation of the code sequence c 1   a  and the code sequence c 1   b , and each of the modulation code m 2  and the demodulation code d 2  is a code sequence [c 1   b  c 1   a ] which is a concatenation of the code sequence c 1   b  and the code sequence c 1   a . As a result, the average value of the code-modulated current I 2  flowing in the transmission path  3  becomes zero, and the modulated current I 2  includes only an alternating-current component. 
     Note that the power generator  1  or the load  5  may be a power storage apparatus, such as a battery and a capacitor. When a power storage apparatus is incorporated in the power transmission system according to the present embodiment, it is possible to effectively utilize power generated during hours of low or no power consumption, and thus, improve overall power efficiency. 
     Second Embodiment 
     In the first embodiment, we have described the power transmission system which modulates and transmits a direct-current generated current by code modulation. Meanwhile, in a second embodiment, we describe a power transmission system which modulates and transmits an alternating-current generated current by code modulation. 
     The power transmission system according to the second embodiment is provided with a code modulator  2 A and a code demodulator  4 A, which will be described below with reference to  FIGS. 10 and 11 , in place of the code modulator  2  and the code demodulator  4  of  FIG. 1 . The other portions of the power transmission system according to the second embodiment are configured in a manner similar to that of the power transmission system according to the first embodiment. 
       FIG. 9  is a waveform diagram, where (a) to (c) show exemplary signal waveforms in the power transmission system according to the second embodiment. In  FIG. 9 , (a) shows a signal waveform of a generated current I 1 ; (b) shows a signal waveform of a modulated current I 2 ; and (c) shows a signal waveform of a demodulated current I 3 . More specifically,  FIG. 9  shows exemplary signal waveforms generated as follows: the code modulator  2 A modulates the alternating-current generated current I 1  by code modulation, and then, the modulated current I 2  is transmitted via a transmission path  3 , and then, the code demodulator  4 A demodulates the modulated current I 2  by code demodulation. 
     The power generator  1  generates the alternating-current generated current I 1 . For example, the alternating-current generated current I 1  has a rectangular waveform at a frequency of 5 kHz, which cyclically repeats positive and negative periods every 200 microseconds. Also in this case, the code modulator  2 A multiplies the generated current I 1  by a modulation code m 0  to generate the alternating modulated current I 2 , in a manner similar to the code modulation of the direct-current generated current I 1  as shown in  FIG. 4 . The code demodulator  4 A multiplies the modulated current I 2  by a demodulation code d 0  identical to the modulation code m 0  to reproduce the alternating-current power generated by the power generator  1 , and supply the reproduced alternating-current power to a load  5 . 
     The frequency of the modulation code m 0  and the demodulation code d 0  is set to frequencies higher than the frequency of the generated current I 1  and the frequency of the demodulated current I 3 . According to the exemplary signal waveform of  FIG. 9 , the alternating-current generated current I 1  ( FIG. 9( a ) ) is multiplied by the modulation code m 0  having a frequency of 35 kHz to generate the modulated current I 2  ( FIG. 9( b ) ) of the code-modulated wave. In this case, the duration of each bit of the modulation code m 0  is 1/(35 kHz)/2=14.2 microseconds. 
     Each bit of the modulation code m 0  and the demodulation code d 0  has a code value “1” or “−1”. In case of transmission of the alternating-current generated current I 1 , the meaning of the code value “1” or “−1” in a period when the generated current I 1  is positive (period from 0 to 100 microsecond in  FIG. 9( a ) ) is different from that of a period when the generated current I 1  is negative (period from 100 to 200 microsecond in  FIG. 9( a ) ). In the period when the generated current I 1  is positive, the code value “1” of the modulation code m 0  indicates that the code modulator  2 A outputs a current in the same direction as the direction of an inputted current, and the code value “−1” of the modulation code m 0  indicates that the code modulator  2 A outputs a current in the direction opposite to the direction of an inputted current. Similarly, in the period when the generated current I 1  is positive, the code value “1” of the demodulation code d 0  indicates that the code demodulator  4 A outputs a current in the same direction as the direction of an inputted current, and the code value “4” of the demodulation code d 0  indicates that the code demodulator  4 A outputs a current in the direction opposite to the direction of an inputted current. In the period when the generated current I 1  is negative, the code value “1” of the modulation code m 0  indicates that the code modulator  2 A outputs a current in the direction opposite to the direction of an inputted current, and the code value “−1” of the modulation code m 0  indicates that the code modulator  2 A outputs a current in the same direction as the direction of an inputted current. Similarly, in the period when the generated current I 1  is negative, the code value “1” of the demodulation code d 0  indicates that the code demodulator  4 A outputs a current in the direction opposite to the direction of an inputted current, and the code value “−1” of the demodulation code d 0  indicates that the code demodulator  4 A outputs a current in the same direction as the direction of an inputted current. 
     For example, the modulation code m 0  and the demodulation code d 0  are given as follows.
 
 m 0=[1−1 1 1 1−1 −1−1 1−1 −1−1 1 1]  (6)
 
 d 0= m 0=[1−1 1 1 1−1 −1−1 1−1 −1−1 1 1]  (7)
 
     Similarly to the code demodulation according to the first embodiment, the modulated current I 2  of the code-modulated wave generated by the modulation code m 0  is multiplied by the demodulation code d 0 . This multiplication is denoted as follows.
 
 m 0× d 0=[1 1 1 1 1 1 1 1 1 1 1 1 1 1]  (8)
 
     As apparent from Mathematical Expression (8), the demodulated current I 3  ( FIG. 8( c ) ) is obtained, which is an alternating current similarly to the original generated current I 1 . 
     As described above, it is possible to achieve power transmission in an accurately synchronized manner, without power loss, by using the method of code modulation and code demodulation according to the present embodiment. In addition, it is possible to achieve efficient power transmission for a longer period, for example, by repeatedly using the modulation code m 0  and demodulation code d 0  as described above. 
       FIG. 10  is a block diagram showing a partial configuration of the code modulator  2 A of the power transmission system according to the second embodiment. The code modulator  2 A of  FIG. 10  is provided with a code generation circuit  22 A and a code modulation circuit  23 A, in place of the code generation circuit  22  and the code modulation circuit  23  of  FIG. 5 . The code modulator  2 A of  FIG. 10  is further provided with a control circuit  20  and a communication circuit  21  similarly to the code modulator  2  of  FIG. 5 , which are omitted in  FIG. 10  for ease of illustration. 
     The code generation circuit  22 A and the code modulation circuit  23 A of  FIG. 10  are different from the code generation circuit  22  and the code modulation circuit  23  of  FIG. 7  in following points. 
     (1) The code generation circuit  22 A generates four modulation codes m 1  to m 4  in place of the two modulation codes m 1  and m 2 , and outputs the generated modulation codes m 1  to m 4  to the code modulation circuit  23 A. 
     (2) The code modulation circuit  23 A is provided with four bidirectional switch circuits SS 21  to SS 24  connected in a bridge configuration, in place of the unidirectional switch circuits SS 1  to SS 4 . 
     The code generation circuit  22 A generates and outputs the modulation codes m 1  to m 4  to the code modulation circuit  23 A under control of the control circuit  20 , in order to operate the code modulator  2 A according to the modulation code m 0  as described above. Each of the modulation codes m 1  to m 4  has code values “1” and “0”. 
     In the code modulation circuit  23 A, the switch circuit SS 21  is provided with the switch element S 1  of  FIG. 7  to be turned on and off in response to the modulation code m 1 , and further provided with a switch element S 21  having directionality opposite to that of the switch element S 1 , connected in parallel to the switch element S 1 , and to be turned on and off in response to the modulation code m 3 . The switch circuit SS 22  is provided with the switch element S 2  of  FIG. 7  to be turned on and off in response to the modulation code m 2 , and further provided with a switch element S 22  having directionality opposite to that of the switch element S 2 , connected in parallel to the switch element S 2 , and to be turned on and off in response to the modulation code m 4 . The switch circuit SS 23  is provided with the switch element S 3  of  FIG. 7  to be turned on and off in response to the modulation code m 2 , and further provided with a switch element S 23  having directionality opposite to that of the switch element S 3 , connected in parallel to the switch element S 3 , and to be turned on and off in response to the modulation code m 4 . The switch circuit SS 24  is provided with the switch element S 4  of  FIG. 7  to be turned on and off in response to the modulation code m 1 , and further provided with a switch element S 24  having directionality opposite to that of the switch element S 4 , connected in parallel to the switch element S 4 , and to be turned on and off in response to the modulation code m 3 . Each of the switch elements S 21  to S 24  is made of, for example, an MOS transistor. The code modulation circuit  23 A has terminals T 1  and T 2  connected to a power generator  1 , and terminals T 3  and T 4  connected to the transmission path  3 . Alternating-current power is inputted from the power generator  1  to the code modulation circuit  23 A. The code modulation circuit  23 A modulates the alternating-current power by code modulation, and then, outputs a code-modulated wave to the transmission path  3 . 
       FIG. 11  is a block diagram showing a partial configuration of the code demodulator  4 A of the power transmission system according to the second embodiment. The code demodulator  4 A of  FIG. 11  is provided with a code generation circuit  32 A and a code demodulation circuit  33 A, in place of the code generation circuit  32  and the code demodulation circuit  33  of  FIG. 6 . The code demodulator  4 A of  FIG. 11  is further provided with a control circuit  30  and a communication circuit  31  similarly to the code demodulator  4  of  FIG. 5 , which are omitted in  FIG. 11  for ease of illustration. 
     The code generation circuit  32 A and the code demodulation circuit  33 A of  FIG. 11  are different from the code generation circuit  32  and the code demodulation circuit  33  of  FIG. 7  in following points. 
     (1) The code generation circuit  32 A generates four demodulation codes d 1  to d 4  in place of the two modulation codes d 1  and d 2 , and outputs the generated demodulation codes d 1  to d 4  to the code demodulation circuit  33 A. 
     (2) The code demodulation circuit  33 A is provided with four bidirectional switch circuits SS 31  to SS 34  connected in a bridge configuration, in place of the unidirectional switch circuits SS 11  to SS 14 . 
     The code generation circuit  32 A generates and outputs the demodulation codes d 1  to d 4  to the code demodulation circuit  33 A under control of the control circuit  30 , in order to operate the code demodulator  4 A according to the demodulation code d 0  as described above. Each of the demodulation codes d 1  and d 4  has code values “1” and “0”. 
     In the code demodulation circuit  33 A, the switch circuit SS 31  is provided with the switch element S 11  of  FIG. 7  to be turned on and off in response to the demodulation code d 2 , and further provided with a switch element S 31  having directionality opposite to that of the switch element S 11 , connected in parallel to the switch element S 11 , and to be turned on and off in response to the demodulation code d 4 . The switch circuit SS 32  is provided with the switch element S 12  of  FIG. 7  to be turned on and off in response to the demodulation code d 1 , and further provided with a switch element S 32  having directionality opposite to that of the switch element S 12 , connected in parallel to the switch element S 12 , and to be turned on and off in response to the demodulation code d 3 . The switch circuit SS 33  is provided with the switch element S 13  of  FIG. 7  to be turned on and off in response to the demodulation code d 1 , and further provided with a switch element S 33  having directionality opposite to that of the switch element S 13 , connected in parallel to the switch element S 13 , and to be turned on and off in response to the demodulation code d 3 . The switch circuit SS 34  is provided with the switch element S 14  of  FIG. 7  to be turned on and off in response to the demodulation code d 2 , and further provided with a switch element S 34  having directionality opposite to that of the switch element S 14 , connected in parallel to the switch element S 14 , and to be turned on and off in response to the demodulation code d 4 . Each of the switch elements S 31  to S 34  is made of, for example, an MOS transistor. The code demodulation circuit  33 A has terminals T 11  and T 12  connected to the transmission path  3 , and terminals T 13  and T 14  connected to the load  5 . An alternating-current code-modulated wave is inputted from the transmission path  3  to the code demodulation circuit  33 A. The code demodulation circuit  33 A demodulates the code-modulated wave by code demodulation into alternating-current demodulated power, and then outputs the demodulated power to the load  5 . 
       FIG. 12A  is a diagram showing an example of a modulation code of the code modulator  2 A and a demodulation code of the code demodulator  4 A in the power transmission system according to the second embodiment, as a third implementation example in which alternating-current power is transmitted and received. More specifically,  FIG. 12A  shows an example of the modulation codes m 1  to m 4  inputted to the bidirectional switch circuits SS 21  to SS 24  of the code modulation circuit  23 A, and the demodulation codes d 1  to d 4  inputted to the bidirectional switch circuits SS 31  to SS 34  of the code demodulation circuit  33 A. 
     As shown in  FIG. 12A , the modulation code m 1  and the demodulation code d 1  are identical to each other, and the modulation code m 2  and the demodulation code d 2  are identical to each other. Similarly, the modulation code m 3  and the demodulation code d 3  are identical to each other, and the modulation code m 4  and the demodulation code d 4  are identical to each other. In addition, similarly to the case of direct-current power transmission, code sequences c 1   a  and c 1   b  are configured such that when the code value of a certain bit of the code sequence c 1   a  is “1”, the code value of a corresponding bit of the code sequence c 1   b  is “0”; and when the code value of a certain bit of the code sequence c 1   a  is “0”, the code value of a corresponding bit of the code sequence c 1   b  is “1”. 
       FIG. 12A  shows a case in which the duration of the code sequence c 1   a  and the code sequence c 1   b  is set to be equal to a half of the cycle of the alternating-current generated current I 1 . In a period when the alternating-current generated current I 1  flows in the positive direction (in example of  FIG. 12A , former half period of each cycle), the modulation codes m 1  and m 2  are the code sequences c 1   a  and c 1   b , respectively, and on the other hand, all code values of the modulation codes m 3  and m 4  are “0”. In a period when the alternating-current generated current I 1  flows in the negative direction (in example of  FIG. 12A , latter half period of each cycle), all the code values of the modulation codes m 1  and m 2  are “0”, and on the other hand, the modulation codes m 3  and m 4  are the code sequences c 1   a  and c 1   b , respectively. Each of the modulation codes m 1  to m 4  for one cycle is generated by concatenating bits for a former half of each cycle with bits for a latter half of each cycle. Accordingly, in the former half of each cycle, the switch elements S 1  to S 4  are turned on and off according to the modulation codes m 1  and m 2 , and on the other hand, the switch elements S 21  to S 24  are disconnected and no current flows. In addition, in the latter half of each cycle, the switch elements S 1  to S 4  are disconnected and no current flows, and on the other hand, the switch elements S 21  to S 24  are turned on and off according to the modulation codes m 3  and m 4 . Similarly to the modulation codes m 1  to m 4 , each of the demodulation codes d 1  to d 4  for one cycle is generated by concatenating bits for the former half of each cycle with bits for the latter half of each cycle. 
     Now, operation of the code modulation circuit  23 A is described. 
     At first, operation is described for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  in the positive direction, i.e., in a direction of solid arrows A 1 . In this case, when the switch elements S 1  and S 4  receiving the code value “1” of the modulation code m 1  are turned on, the switch elements S 2  and S 3  receiving the code value “0” of the modulation code m 2  are turned off. In addition, when the switch elements S 1  and S 4  receiving the code value “0” of the modulation code m 1  are turned off, the switch elements S 2  and S 3  receiving the code value “1” of the modulation code m 2  are turned on. Thus, when the switch elements S 1  and S 4  are turned on, and the switch elements S 2  and S 3  are turned off, the modulated current I 2  flows in the transmission path  3  in a positive direction, i.e., in a direction of the solid arrows A 1 . On the other hand, when the switch elements S 1  and S 4  are turned off, and the switch elements S 2  and S 3  are turned on, the modulated current I 2  flows in the transmission path  3  in a negative direction, i.e., in a direction of dotted arrows A 2 . Accordingly, when the current of positive period of the alternating-current generated current I 1  is inputted to the code modulation circuit  23 A, it is possible to transmit the alternating-current modulated current I 2  to the transmission path  3 , as shown in  FIG. 9( b ) . 
     Next, operation is described for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  in a negative direction, i.e., in a direction of chain arrows B 1 . In this case, when the switch elements S 21  and S 24  receiving the code value “1” of the modulation code m 3  are turned on, the switch elements S 22  and S 23  receiving the code value “0” of the modulation code m 4  are turned off. In addition, when the switch elements S 21  and S 24  receiving the code value “0” of the modulation code m 3  are turned off, the switch elements S 22  and S 23  receiving the code value “1” of the modulation code m 4  are turned on. Thus, when the switch elements S 21  and S 24  are turned on, and the switch elements S 22  and S 23  are turned off, the modulated current I 2  flows in the transmission path  3  in a negative direction, i.e., in a direction of the chain arrows B 1 . On the other hand, when the switch elements S 21  and S 24  are turned off, and the switch elements S 22  and S 23  are turned on, the modulated current I 2  flows in the transmission path  3  in a positive direction, i.e., in a direction of two-dot chain arrows B 2 . Accordingly, when the current of negative period of the alternating-current generated current I 1  is inputted to the code modulation circuit  23 A, it is possible to transmit the alternating-current modulated current I 2  to the transmission path  3 , as shown in  FIG. 9( b ) . 
     As described with reference to  FIG. 10 , the code modulation circuit  23 A can generate the alternating-current modulated current I 2 , as shown in  FIG. 9( b ) , in both the positive and negative periods of the alternating-current generated current I 1 . 
     Next, operation of the code demodulation circuit  33 A of  FIG. 11  is described. 
     At first, we consider a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the positive direction, i.e., in the direction of the solid arrows A 1 . In this case, the alternating-current modulated current I 2  flowing in the positive and negative directions is inputted to the input terminals T 11  and T 12  of the code demodulation circuit  33 A via the transmission path  3 . When the code demodulation circuit  33 A correctly performs demodulation operation, the demodulated current I 3  flows at the output terminals T 13  and T 14  of the code demodulation circuit  33 A in a positive direction, i.e., in a direction of solid arrows C 1 . These operations are described below. In this case, all code values of the demodulation code d 3  and the demodulation code d 4  are “0”, and all the switch elements S 31  to S 34  are turned off. 
     At first, operation of the code demodulation circuit  33 A is described for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the positive direction, and the modulated current I 2  flows at the input terminals T 11  and T 12  of the code demodulation circuit  33 A in the positive direction, i.e., in the direction of the solid arrows C 1 . In this case, the code value of the code sequence c 1   a  is “1”, and the code value of the code sequence c 1   b  is “0”. Accordingly, the switch elements S 12  and S 13  receiving the code value “1” of the demodulation code d 1  are turned on, and the switch elements S 11  and S 14  receiving the code value “0” of the demodulation code d 2  are turned off. Therefore, the demodulated current I 3  flows at the output terminals T 13  and T 14  in the positive direction, i.e., in the direction of the solid arrows C 1 . 
     Next, operation of the code demodulation circuit  33 A is described for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the positive direction, and the modulated current I 2  flows at the input terminals T 11  and T 12  of the code demodulation circuit  33 A in the negative direction, i.e., in the direction of dotted arrows C 2 . In this case, the code value of the code sequence c 1   a  is “0”, and the code value of the code sequence c 1   b  is “1”. Accordingly, the switch elements S 12  and S 13  receiving the code value “0” of the demodulation code d 1  are turned off, and the switch elements S 11  and S 14  receiving the code value “1” of the demodulation code d 2  are turned on. Therefore, the demodulated current I 3  flows at the output terminals T 13  and T 14  in the positive direction, i.e., in the direction of the solid arrows C 1 . Accordingly, when the current of positive period of the alternating-current generated current I 1  is inputted to the code modulation circuit  23 A, the code demodulation circuit  33 A can output the demodulated current I 3  which is correctly demodulated with positive polarity, to the load  5 , as shown in  FIG. 9( c ) . 
     Next, we consider a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the negative direction, i.e., in the direction of the chain arrows B 1 . Similarly to the above case, the alternating-current modulated current I 2  flowing in the positive and negative directions is inputted to the input terminals T 11  and T 12  of the code demodulation circuit  33 A via the transmission path  3 . When the code demodulation circuit  33 A correctly performs demodulation operation, the demodulated current I 3  flows at the output terminals T 13  and T 14  of the code demodulation circuit  33 A in the negative direction, i.e., in a direction of the dotted arrows C 2 . These operations are described below. In this case, all code values of the demodulation codes d 1  and d 2  are “0”, and all the switch elements S 11  to S 14  are turned off. 
     At first, described is operation of the code demodulation circuit  33 A for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the negative direction, and the modulated current I 2  flows at the input terminals T 11  and T 12  of the code demodulation circuit  33 A in the negative direction, i.e., in the direction of dotted arrows C 2 . In this case, the code value of the code sequence c 1   a  is “1”, and the code value of the code sequence c 1   b  is “0”. Accordingly, the switch elements S 32  and S 33  receiving the code value “1” of the demodulation code d 3  are turned on, and the switch elements S 31  and S 34  receiving the code value “0” of the demodulation code d 4  are turned off. Therefore, the demodulated current I 3  flows at the output terminals T 13  and T 14  in the negative direction, i.e., in the direction of the dotted arrows C 2 . 
     Next, operation of the code demodulation circuit  33 A is described for a case in which the generated current I 1  flows at the input terminals T 1  and T 2  of the code modulation circuit  23 A in the negative direction, and the modulated current I 2  flows at the input terminals T 11  and T 12  of the code demodulation circuit  33 A in the positive direction, i.e., in the direction of the solid arrows C 1 . In this case, the code value of the code sequence c 1   a  is “0”, and the code value of the code sequence c 1   b  is “1”. Accordingly, the switch elements S 32  and S 33  receiving the code value “0” of the demodulation code d 3  are turned off, and the switch elements S 31  and S 34  receiving the code value “1” of the demodulation code d 4  are turned on. Therefore, the demodulated current I 3  flows at the output terminals T 13  and T 14  in the negative direction, i.e., in the direction of the dotted arrows C 2 . Accordingly, when the current of negative period of the alternating-current generated current I 1  is inputted to the code modulation circuit  23 A, the code demodulation circuit  33 A can output the demodulated current I 3  which is correctly demodulated with negative polarity, to the load  5 , as shown in  FIG. 9( c ) . 
     As described above, when using the modulation codes m 1  to m 4  and the demodulation codes d 1  to d 4  of  FIG. 12A , equivalently, the code modulator  2 A operates according to the modulation code m 0  of Mathematical Expression (6), and the code demodulator  4 A operates according to the demodulation code d 0  of Mathematical Expression (7). 
     As described above, according to  FIGS. 10, 11, and 12A , when the alternating-current generated current I 1  is inputted to the code modulator  2 A, it is possible to extract the demodulated current I 3  from the code demodulator  4 A, the demodulated current I 3  being also an alternating current similarly to the generated current I 1  inputted to the code modulator  2 A. Therefore, according to the second embodiment, it is possible to modulate the alternating-current generated current I 1  by code modulation into the alternating-current modulated current I 2 , and then, transmit the modulated current I 2  via the transmission path  3 , and then, demodulate the modulated current I 2  into the alternating-current demodulated current I 3 . 
       FIG. 12B  is a diagram showing an example of a modulation code of the code modulator  2 A and a demodulation code of the code demodulator  4 A in the power transmission system according to the second embodiment, as a fourth implementation example in which direct-current power is transmitted and received. In this case, in the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11 , all code values of the modulation codes m 3  and m 4  and the demodulation codes d 3  and d 4  are set to “0” as shown in  FIG. 12B , and thus, the switch elements S 21  to S 24  and S 31  to S 34  are turned off. Thus, the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11  operate as the code modulation circuit  23  and the code demodulation circuit  33  of  FIG. 7 , respectively. Accordingly, it is possible to achieve direct-current power transmission of  FIG. 4  by generating the modulation codes m 1  and m 2  and the demodulation codes d 1  and d 2  from the code sequences c 1   a  and c 1   b  as shown in  FIG. 12B . Thus, by changing the modulation codes m 1  to m 4  and the demodulation codes d 1  to d 4 , it is possible to achieve a favorable power transmission system capable of supporting both direct-current power transmission and alternating-current power transmission using the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11 . 
     The direct-current power generator  1  may be, for example, a photovoltaic power generator. The alternating-current power generator  1  may be, for example, a power generator provided with a turbine rotated by thermal power, hydraulic power, wind power, nuclear power, tidal power, or the like. 
     As described above, by using the modulation code and the demodulation code identical to each other, the power transmission system according to the second embodiment is capable of modulating and transmitting the direct-current generated current I 1  and demodulating the modulated current into the direct-current demodulated current I 3 , and is also capable of modulating and transmitting the alternating-current generated current I 1  and demodulating the modulated current into the alternating-current demodulated current I 3 . In addition, by using the demodulation code different from the modulation code, the power transmission system according to the second embodiment is capable of modulating and transmitting the direct-current generated current I 1  and demodulating the modulated current into the alternating-current demodulated current I 3 , and is also capable of modulating and transmitting the alternating-current generated current I 1  and demodulating the modulated current into the direct-current demodulated current I 3 . 
     Since the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11  are provided with the bidirectional switch circuits SS 21  to SS 24  and SS 31  to SS 34 , these circuits are reversible. More specifically, the code modulation circuit  23 A is also operable as a code demodulation circuit to demodulate a modulated current inputted from the terminals T 3  and T 4  and output the demodulated current from the terminals T 1  and T 2 . The code demodulation circuit  33 A is also operable as a code modulation circuit to modulate a generated current inputted from the terminals T 13  and T 14  and output the modulated current from the terminals T 11  and T 12 . Thus, it is possible to transmit power from the code demodulator provided with the code demodulation circuit  33 A, to the code modulator provided with the code modulation circuit  23 A. 
       FIGS. 10 to 11  show the example in which each of the bidirectional switch circuits SS 21  to SS 34  is made of a pair of switch elements connected in parallel such that currents flow in opposite directions (S 1 , S 21 ; S 2 , S 22 ; S 3 , S 23 ; S 4 , S 24 ; S 11 , S 31 ; S 12 , S 32 ; S 13 , S 33 ; S 14 , S 34 ). Alternatively, each of the bidirectional switch circuits SS 21  to SS 34  may be made of a pair of switch elements connected in series, as shown in  FIGS. 13A to 14D  (S 41 , S 51 ; S 42 , S 52 ; S 43 , S 53 ; S 44 , S 54 ). In each of  FIGS. 13A to 14D , the direction from top to bottom is referred to as a “positive direction”, and the direction from bottom to top is referred to as a “negative direction”. 
       FIG. 13A  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 21 A for a code modulation circuit  23 A used in a power transmission system according to a modified embodiment of the second embodiment. The switch circuit SS 21 A of  FIG. 13A  corresponds to the switch circuit SS 21  of  FIG. 10 , and is made of series connection of: (1) a switch element S 41  connected in parallel with a diode D 1  allowing a current to flow in the negative direction, and turned on and off in accordance with the modulation code m 1 ; and (2) a switch element S 51  connected in parallel with a diode D 11  allowing a current to flow in the positive direction, and turned on and off in accordance with the modulation code m 3 . 
       FIG. 13B  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 22 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 22 A of  FIG. 13B  corresponds to the switch circuit SS 22  of  FIG. 10 , and is made of series connection of: (1) a switch element S 42  connected in parallel with a diode D 2  allowing a current to flow in the negative direction, and turned on and off in accordance with the modulation code m 2 ; and (2) a switch element S 52  connected in parallel with a diode D 12  allowing a current to flow in the positive direction, and turned on and off in accordance with the modulation code m 4 . 
       FIG. 13C  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 23 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 23 A of  FIG. 13C  corresponds to the switch circuit SS 23  of  FIG. 10 , and is made of series connection of: (1) a switch element S 43  connected in parallel with a diode D 3  allowing a current to flow in the negative direction, and turned on and off in accordance with the modulation code m 2 ; and (2) a switch element S 53  connected in parallel with a diode D 13  allowing a current to flow in the positive direction, and turned on and off in accordance with the modulation code m 4 . 
       FIG. 13D  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 24 A for the code modulation circuit  23 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 24 A of  FIG. 13D  corresponds to the switch circuit SS 24  of  FIG. 10 , and is made of series connection of: (1) a switch element S 44  connected in parallel with a diode D 4  allowing a current to flow in the negative direction, and turned on and off in accordance with the modulation code m 1 ; and (2) a switch element S 54  connected in parallel with a diode D 14  allowing a current to flow in the positive direction, and turned on and off in accordance with the modulation code m 3 . 
       FIG. 14A  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 31 A for a code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 31 A of  FIG. 14A  corresponds to the switch circuit SS 31  of  FIG. 11 , and is made of series connection of: (1) a switch element S 61  connected in parallel with a diode D 31  allowing a current to flow in the positive direction, and turned on and off in accordance with the demodulation code d 2 ; and (2) a switch element S 71  connected in parallel with a diode D 21  allowing a current to flow in the negative direction, and turned on and off in accordance with the demodulation code d 4 . 
       FIG. 14B  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 32 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 32 A of  FIG. 14B  corresponds to the switch circuit SS 32  of  FIG. 11 , and is made of series connection of: (1) a switch element S 62  connected in parallel with a diode D 32  allowing a current to flow in the positive direction, and turned on and off in accordance with the demodulation code d 1 ; and (2) a switch element S 72  connected in parallel with a diode D 22  allowing a current to flow in the negative direction, and turned on and off in accordance with the demodulation code d 3 . 
       FIG. 14C  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 33 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 33 A of  FIG. 14C  corresponds to the switch circuit SS 33  of  FIG. 11 , and is made of series connection of: (1) a switch element S 63  connected in parallel with a diode D 33  allowing a current to flow in the positive direction, and turned on and off in accordance with the demodulation code d 1 ; and (2) a switch element S 73  connected in parallel with a diode D 23  allowing a current to flow in the negative direction, and turned on and off in accordance with the demodulation code d 3 . 
       FIG. 14D  is a circuit diagram showing a configuration of a bidirectional switch circuit SS 34 A for the code demodulation circuit  33 A used in the power transmission system according to the modified embodiment of the second embodiment. The switch circuit SS 34 A of  FIG. 14D  corresponds to the switch circuit SS 34  of  FIG. 11 , and is made of series connection of: (1) a switch element S 64  connected in parallel with a diode D 34  allowing a current to flow in the positive direction, and turned on and off in accordance with the demodulation code d 2 ; and (2) a switch element S 74  connected in parallel with a diode D 24  allowing a current to flow in the negative direction, and turned on and off in accordance with the demodulation code d 4 . 
     Referring to  FIG. 13A  to  FIG. 14D , each of the switch elements S 41  to S 74  may be made of, for example, an MOS transistor. Parallel parasitic (body) diodes D 1  to D 34  of MOS transistors may be used. For example, when each of the switch circuits SS 21 A to SS 34 A of  FIGS. 13A to 14D  is implemented by a switch element of an MOS transistor and one diode, two MOS transistors and two diodes are required for each one of the bidirectional switch circuit SS 21 A to SS 34 A. Meanwhile, packaged MOS transistors are widely available, including a built-in diode having good reverse characteristics. When using such packaged MOS transistors, each of the bidirectional switch circuits SS 21 A to SS 34 A can be made of two switch elements, and thus, size can be reduced. 
     Third Embodiment 
     In the first and second embodiments, we have described the power transmission systems which transmit power from the one power generator  1  to the one load  5 . Meanwhile, in a third embodiment, we describe a power transmission system which transmits powers from a plurality of power generators to a plurality of loads. 
       FIG. 15  is a block diagram showing a configuration of a power transmission system according to the third embodiment. Referring to  FIG. 15 , the power transmission system according to the third embodiment is provided with a plurality of power generators  1 - 1  and  1 - 2 , a plurality of code modulators  2 A- 1  and  2 A- 2 , a transmission path  3 , a plurality of code demodulators  4 A- 1  and  4 A- 2 , a plurality of loads  5 - 1  and  5 - 2 , and a controller  10 A. 
     The controller  10 A is provided with a control circuit  11  and a communication circuit  12 A. The control circuit  11  communicates with the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2  via the communication circuit  12 A, and controls operations of the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2 . 
     In the power transmission system of  FIG. 15 , each of the code modulators  2 A- 1  and  2 A- 2  operates as a power transmitter apparatus, and each of the code demodulators  4 A- 1  and  4 A- 2  operates as a power receiver apparatus. Each of a plurality of power transmitter apparatuses included in the code modulators  2 A- 1  and  2 A- 2  modulates first power to generate a code-modulated wave by code modulation using a modulation code based on a code sequence, and transmits the code-modulated wave to one of the code demodulators  4 A- 1  and  4 A- 2  via the transmission path  3 . Each one of the code demodulators  4 A- 1  and  4 A- 2  receives the code-modulated wave from one of the code modulators  2 A- 1  and  2 A- 2  via the transmission path  3 , and demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for the code modulation. The first powers are, for example, powers generated by the power generators  1 - 1  and  1 - 2 , and are shown as generated currents I 11  and I 12  in  FIG. 15 . The code-modulated wave is alternating-current power modulated by code modulation, and is shown as a modulated current I 2  in  FIG. 15 . The second power are, for example, powers to be supplied to the loads  5 - 1  and  5 - 2 , and are shown as demodulated currents I 31  and I 32  in  FIG. 1 . 
     In this case, the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2  of  FIG. 15  are configured and operated similarly to the code modulator  2 A and the code demodulator  4 A according to the second embodiment. 
     The power transmission system of  FIG. 15  is further provided with power meters  1   m - 1 ,  1   m - 2 ,  5   m - 1 , and  5   m - 2 . Each of the power meters  1   m - 1  and  1   m - 2  is first power measuring means which measures an amount of the first power. More specifically, each of the power meters  1   m - 1  and  1   m - 2  measures an amount of power generated by the power generators  1 - 1  and  1 - 2  and transmitted from the power generators  1 - 1  and  1 - 2  to the code modulators  2 A- 1  and  2 A- 2 . Each of the power meters  5   m - 1  and  5   m - 2  is second power measuring means which measures an amount of the second power. More specifically, each of the power meters  5   m - 1  and  5   m - 2  measures an amount of power transmitted from the code demodulators  4 A- 1  and  4 A- 2  to the loads  5 - 1  and  5 - 2 , and used by the loads  5 - 1  and  5 - 2 . The amounts of powers measured by the power meters  1   m - 1 ,  1   m - 2 ,  5   m - 1 , and  5   m - 2  are transmitted to the controller  10 A. 
     The controller  10 A controls operations of the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2  based on the amounts of powers received from the power meters  1   m - 1 ,  1   m - 2 ,  5   m - 1 , and  5   m - 2 . For example, the controller  10 A transmits control signals to the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2 , the control signals including synchronization signals for synchronizing the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2  to each other, thus achieving code modulation and code demodulation of power in an accurately synchronized manner. 
     The controller  10 A transmits the code sequences of the modulation codes, or information specifying the code sequences, to at least one of the code modulators  2 A- 1  and  2 A- 2 , which is to transmit power, and transmits the code sequences of the demodulation codes, or information specifying the code sequences, to at least one of the code demodulators  4 A- 1  and  4 A- 2 , which is to receive power. For example, when transmitting power from the code modulator  2 A- 1  to the code demodulator  4 A- 1 , the controller  10 A sets a modulation code to the code modulator  2 A- 1 , and a demodulation code to the code demodulator  4 A- 1 , based on one code sequence. When simultaneously transmitting power from the code modulator  2 A- 2  to the code demodulator  4 A- 2 , the controller  10 A sets a modulation code to the code modulator  2 A- 2 , and a demodulation code to the code demodulator  4 A- 2 , based on another different code sequence. When simultaneously transmitting powers from the plurality of code modulators  2 A- 1  and  2 A- 2  to the plurality of code demodulators  4 A- 1  and  4 A- 2 , a plurality of low-correlated (e.g., orthogonal) code sequences may be used. 
     Thus, it is possible to transmit powers from the plurality of power generators  1 - 1  and  1 - 2  to the plurality of loads  5 - 1  and  5 - 2 . 
     Now, we describe exemplary operations of the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2  for transmitting powers generated by the power generators  1 - 1  and  1 - 2  to the loads  5 - 1  and  5 - 2 . 
     In the third embodiment, we describe a case in which the power generators  1 - 1  and  1 - 2  output direct-current powers, direct-current power is inputted to the load  5 - 1 , and alternating-current power is inputted to the load  5 - 2 . That is, when transmitting power from the power generator  1 - 2  to the load  5 - 2 , direct-current power is converted into alternating-current power. 
       FIG. 16A  is a diagram showing an example of a modulation code of the code modulator  2 A- 1  and a demodulation code of the code demodulator  4 A- 1  in the power transmission system of  FIG. 15  according to the third embodiment, in which direct-current power is transmitted and received. In addition,  FIG. 16B  is a diagram showing an example of a modulation code of the code modulator  2 A- 2  and a demodulation code of the code demodulator  4 A- 2  in the power transmission system of  FIG. 15  according to the third embodiment, in which direct-current power is transmitted and alternating-current power is received. 
       FIG. 16A  shows modulation codes and demodulation codes inputted to the switch elements S 1  to S 44  of the code modulator  2 A- 1  and the code demodulator  4 A- 1 . In this case, modulation codes m 1   a  to m 4   a  correspond to the modulation codes m 1  to m 4  of the code modulation circuit  23 A as shown in  FIG. 10 , respectively, and demodulation codes d 1   a  to d 4   a  correspond to the demodulation codes d 1  to d 4  of the code demodulation circuit  33 A as shown in  FIG. 11 , respectively. In this case, as described with reference to  FIG. 12B , by setting all the code values of the modulation codes m 3   a  and m 4   a  and the demodulation codes d 3   a  and d 4   a  to “0”, the switch elements S 21  to S 24  and S 31  to S 34  are turned off. In addition, the modulation codes m 1   a  and m 2   a  and the demodulation codes d 1   a  and d 2   a  are generated from the code sequence c 1   a  and the code sequence c 1   b , as described with reference to  FIG. 12B . 
     Further,  FIG. 16B  shows modulation codes and demodulation codes inputted to the switch elements S 1  to S 44  of the code modulator  2 A- 2  and the code demodulator  4 A- 2 . In this case, modulation codes m 1   b  to m 4   b  correspond to the modulation codes m 1  to m 4  of the code modulation circuit  23 A as shown in  FIG. 10 , respectively, and demodulation codes d 1   b  to d 4   b  correspond to the demodulation codes d 1  to d 4  of the code demodulation circuit  33 A as shown in  FIG. 11 , respectively. In this case, by setting all the code values of the modulation codes m 3   b  and m 4   b  to “0”, the switch elements S 21  to S 24  are turned off. In addition, the modulation codes m 1   b  and m 2   b  and the demodulation codes d 1   b  to d 4   b  are generated from the code sequence c 2   a  and the code sequence c 2   b . The principle of code modulation and code demodulation of currents is similar to that of the first and second embodiments, and therefore, its explanation is omitted here. 
     Now, with reference to  FIG. 17 , we describe an operation of transmitting powers from the plurality of power generators  1 - 1  and  1 - 2  to the plurality of loads  5 - 1  and  5 - 2 . 
       FIG. 17  are waveform diagrams, where (a) to (e) show exemplary signal waveforms of the power transmission system according to the third embodiment. In  FIG. 17 , (a) shows a signal waveform of a generated current I 11 , (b) shows a signal waveform of a generated current I 12 , (c) shows a signal waveform of a modulated current I 2 , (d) shows a signal waveform of a demodulated current I 31 , and (e) shows a signal waveform of a demodulated current I 32 . 
     The code modulator  2 A- 1  modulates the direct-current generated current I 11  by code modulation into an alternating-current code-modulated wave. Similarly, the code modulator  2 A- 2  modulates the direct-current generated current I 12  by code modulation into an alternating-current code-modulated wave. As shown in  FIG. 17( c ) , the code-modulated wave generated by the code modulator  2 A- 1  and the code-modulated wave generated by the code modulator  2 A- 2  are transmitted as the combined modulated current I 2  via the transmission path  3 . 
     As described above, the code modulators  2 A- 1  and  2 A- 2  have an identical configuration, and are configured in a manner similar to that of the code modulator  2 A of  FIG. 10 . In addition, the code demodulators  4 A- 1  and  4 A- 2  also have an identical configuration, and are configured in a manner similar to that of the code demodulator  4 A of  FIG. 11 . The difference between the code modulators  2 A- 1  and  2 A- 2 , and the difference between the code demodulators  4 A- 1  and  4 A- 2  reside in the use of different sets of the code sequences c 1   a  and c 1   b , and the code sequences c 2   a  and c 2   b . The code modulator  2 A- 1  and the code demodulator  4 A- 1  use the code sequences c 1   a  and c 1   b , and the code modulator  2 A- 2  and the code demodulator  4 A- 2  use the code sequences c 2   a  and c 2   b . In this case, the code sequences c 1   a  and c 2   a  are orthogonal to each other, and therefore, the code sequences c 1   b  and c 2   b  are also orthogonal to each other. In this case, Gold sequences of seven stages are adopted, and different Gold sequences are set to the code sequences c 1   a  and c 2   a.    
     The code demodulators  4 A- 1  and  4 A- 2  can demodulate the modulated current I 2  to extract powers generated by the corresponding code modulators  2 A- 1  and  2 A- 2 , respectively, by using the orthogonal code sequences c 1   a  and c 2   a . Accordingly, as shown in  FIGS. 17( d ) and ( e ) , the generated currents I 11  and  112  are inputted to the code modulators  2 A- 1  and  2 A- 2 , and then, the currents are transmitted as code-modulated waves, and then, the corresponding code demodulators  4 A- 1  and  4 A- 2  correctly demodulate and output the demodulated currents I 31  and I 32 . As a result, the demodulated currents I 31  and I 32  having desired waveforms (direct current or alternating current) and desired magnitudes are supplied to the loads  5 - 1  and  5 - 2 , respectively. 
     As described above, according to the present embodiment, it is possible to simultaneously perform two power transmissions in a multiplexed manner in the one transmission path  3 , and separate the transmitted powers from each other, by using the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2 . Accordingly, it is possible to achieve a favorable power transmission system capable of simultaneously transmitting currents of desired magnitudes from the two power generators  1 - 1  and  1 - 2  to the two loads  5 - 1  and  5 - 2 . 
     By measuring instantaneous powers at the code modulators  2 A- 1  and  2 A- 2  or the code demodulators  4 A- 1  and  4 A- 2  and comparing the instantaneous powers with the code sequences, it is possible to know which of the power generators  1 - 1  and  1 - 2  transmits power, which of the loads receives power, and what amount of power is transmitted. Accordingly, when a plurality of the different power generators  1 - 1  and  1 - 2  requiring different generation costs are connected, it is possible to conduct power business with electricity charges dependent on which of the power generators  1 - 1  and  1 - 2  transmits power. Alternatively, in case of a system having variable power transmission efficiency depending on which of the power generators  1 - 1  and  1 - 2  transmits power and which of the loads  5 - 1  and  5 - 2  receives the power, it is possible to achieve optimum power supply by managing and analyzing information on power transmission. 
     As described above, according to the present embodiment, it is possible to provide the power transmission system capable of efficiently supplying power from the one or more power generators  1 - 1  and  1 - 2  to the one or more loads  5 - 1  and  5 - 2 , by using the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2 . 
     In the above described embodiment, we indicated the example of the power transmission system including the two power generators  1 - 1  and  1 - 2  and the two loads  5 - 1  and  5 - 2 , but the present disclosure is not limited thereto. It is possible to provide power transmission systems including the one power generator  1 - 2  and the two or more loads  5 - 1  and  5 - 2 , or including two or more power generators  1 - 1  and  1 - 2  and the two or more loads  5 - 1  and  5 - 2 . In this case, it is possible to simultaneously perform a number of power transmissions using one transmission path  3 . Accordingly, it is possible to reduce costs for installation of the transmission path  3 , and reduce costs by reducing the number of transmission paths  3 , etc. 
     In the above described embodiment, we indicated the example in which each of the code modulators  2 A- 1  and  2 A- 2  of  FIG. 15  is configured as the code modulation circuit  23 A of  FIG. 10 , but the present disclosure is not limited thereto. For example, when the output powers from the power generators  1 - 1  and  1 - 2  are direct-current powers, each of the code modulators  2 A- 1  and  2 A- 2  may be configured as the code modulation circuit  23  of  FIG. 7 . In addition, when the input powers to the loads  5 - 1  and  5 - 2  are direct-current powers, each of the code demodulators  4 A- 1  and  4 A- 2  may be configured as the code demodulation circuit  33  of  FIG. 7 . In these cases, it is possible to simplify the circuit configurations of the code modulators  2 A- 1  and  2 A- 2  and the code demodulators  4 A- 1  and  4 A- 2 , and accordingly, there are advantageous effects of reducing the number of parts, reducing costs, and reducing size of the apparatuses. 
     In the third embodiment, we indicated the example of the power transmission system which transmits powers from two power generators each having direct-current output power, to one load having direct-current input power, and to one load having alternating-current input power, but the present disclosure is not limited thereto. The power transmission system may receive powers from any number of power generators each having direct-current output power, and from any number of power generators each having alternating-current output power. In addition, the power transmission system may supply powers to any number of loads each having direct-current input power, and to any number of loads each having alternating-current input power. 
     Photovoltaic power generation, which generates most of natural energy, generates direct-current power. On the other hand, wind power generation and geothermal power generation generate alternating-current power. In this case, since it is not desirable that both direct-current power supplies and alternating-current power supplies are connected to the same power network, according to conventional power transmission systems, all power generators (power supplies) and loads should be of only direct current or only alternating current. 
     On the other hand, according to the power transmission system according to the present embodiment, by using code modulation and code demodulation, it is possible simultaneously transmit powers from a direct-current power supply to a direct-current load, from a direct-current power supply to an alternating-current load, from an alternating-current power supply to a direct-current load, and from an alternating-current power supply to an alternating-current load, via one transmission path. 
     Thus, according to the first to third embodiments, it is possible to provide a favorable power transmission system capable of correctly perform code modulation and code demodulation of power, and further, capable of simultaneously performing a plurality of power transmissions in a multiplexed manner via one transmission path. 
     Fourth Embodiment 
       FIG. 18  is a block diagram showing a configuration of a power transmission system according to a fourth embodiment. Referring to  FIG. 18 , the power transmission system according to the fourth embodiment is provided with a power generator  1 , a code modulator  2 , a transmission path  3 A, a code demodulator  4 , a load  5 , and a contactless connector  7 . 
     The power generator  1 , the code modulator  2 , the code demodulator  4 , and the load  5  of  FIG. 18  are configured in a manner similar to that of the corresponding constituent elements of  FIG. 1 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 18 . 
     The contactless connector  7  is connected to the code demodulator  4 , and also is electromagnetically coupled to the transmission path  3 A without electrical contact. An electrically insulating material or a gap is provided between the transmission path  3 A and the contactless connector  7 . In addition, the transmission path  3 A and the contactless connector  7  are not mechanically joined to each other by soldering, screwing, or the like. The contactless connector  7  receives power from the transmission path  3 A via an electric field or a magnetic field between the transmission path  3 A and the contactless connector  7 , without electrical contact. 
     In the power transmission system of  FIG. 18 , the code modulator  2  operates as a power transmitter apparatus, and the contactless connector  7  and the code demodulator  4  operate as a power receiver apparatus. The code modulator  2  modulates first power to generate a code-modulated wave by code modulation using a modulation code based on a code sequence, and transmits the code-modulated wave to the transmission path  3 A. The contactless connector  7  receives the code-modulated wave from the code modulator  2  via the transmission path  3 A, and passes the code-modulated wave to the code demodulator  4 . The code demodulator  4  receives the code-modulated wave via the contactless connector  7 , and demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for code-modulation. The first power is direct-current or alternating-current power generated by the power generator  1 . The code-modulated wave is alternating current power modulated by code modulation. The second power is direct-current or alternating-current power to be supplied to the load  5 . 
       FIG. 19  is a diagram showing a first implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . The contactless connector  7  may be coupled to the transmission path  3 A via an electric field. The transmission path  3 A of  FIG. 19  is provided with power lines  3 A 1  and  3 A 2 , a conductor plate  3 Aa 1  connected to the power line  3 A 1 , and a conductor plate  3 Aa 2  connected to the power line  3 A 2 . The contactless connector  7  of  FIG. 19  is provided with conductor plates  7   a   1  and  7   a   2  opposing to the conductor plates  3 Aa 1  and  3 Aa 2 , respectively. The conductor plate  3 Aa 1  and the conductor plate  7   a   1  are capacitively coupled to each other, and the conductor plate  3 Aa 2  and the conductor plate  7   a   2  are capacitively coupled to each other. Power is transmitted via the electric field generated between the conductor plates  3 Aa 1  and  3 Aa 2  and the conductor plates  7   a   1  and  7   a   2 . 
     In order to transmit power at various positions, a plurality of pairs of the conductor plates  3 Aa 1  and  3 Aa 2  may be provided along the longitudinal direction of the transmission path  3 A, as shown in  FIG. 19 , or alternatively, lengths of the conductor plates  3 Aa 1  and  3 Aa 2  may be extended along the longitudinal direction longer than the conductor plates  7   a   1  and  7   a   2 . Thus, it is possible to uniformly transmit power at any positions of the transmission path  3 A. However, in the latter case, the longitudinal length of the conductor plates  3 Aa 1  and  3 Aa 2  is set to be shorter than a wavelength of the code-modulated wave transmitted over the transmission path  3 A, in order to prevent generation of undesirable electromagnetic radiation. 
     According to the transmission path  3 A and the contactless connector  7  of  FIG. 19 , power is transmitted via the electric field. Accordingly, even when a metal is located between the transmission path  3 A and the contactless connector  7 , there is an advantageous effect of being capable of reducing heat generated from the metal as compared to that of the case that power is transmitted via a magnetic field. 
     In the case that the contactless connector  7  is coupled to the transmission path  3 A via an electric field, terminations of the transmission path  3 A may be either closed (termination resistor  6 ) or open. 
       FIG. 20  is a diagram showing a second implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . The contactless connector  7  may be magnetically coupled to the transmission path  3 A. The transmission path  3 A of  FIG. 20  is provided with the power lines  3 A 1  and  3 A 2 , and a coil  3 Ab connected across the power lines  3 A 1  and  3 A 2 . The contactless connector  7  of  FIG. 20  is provided with a coil  7   b  opposing to the coil  3 Ab. The coils  3 Ab and  7   b  include, for example, windings each circularly wound on a plane, and are disposed in parallel to each other. The coils  3 Ab and  7   b  are magnetically coupled to each other. Power is transmitted via the magnetic field between the coils  3 Ab and  7   b.    
     In order to transmit power at various positions, a plurality of the coils  3 Ab may be provided along the longitudinal direction of the transmission path  3 A, as shown in  FIG. 20 , or alternatively, the coil  3 Ab may be made larger than the coil  7   b . Thus, it is possible to uniformly transmit power at any positions of the transmission path  3 A. 
       FIG. 21  is a diagram showing a third implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . Also in the implementation example of  FIG. 21 , the contactless connector  7  is coupled to the transmission path  3 A via a magnetic field. The transmission path  3 A of  FIG. 21  is provided with the power lines  3 A 1  and  3 A 2 , and a coil  3 Ac wound solenoidally and connected across the power lines  3 A 1  and  3 A 2 . The contactless connector  7  of  FIG. 21  is provided with a coil  7   c  wound solenoidally. The coils  3 Ac and  7   c  are magnetically coupled to each other. Power is transmitted via the magnetic field between the coils  3 Ac and  7   c.    
     In order to transmit power at various positions, a plurality of the coils  3 Ac may be provided along the longitudinal direction of the transmission path  3 A, as shown in  FIG. 21 . Thus, it is possible to uniformly transmit power at any positions of the transmission path  3 A. 
       FIG. 22  is a diagram showing a fourth implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . Also in the implementation example of  FIG. 22 , the contactless connector  7  is coupled to the transmission path  3 A via a magnetic field. The transmission path  3 A of  FIG. 22  is provided with the power lines  3 A 1  and  3 A 2 . The contactless connector  7  of  FIG. 22  is provided with a coil  7   d   1  coupled with a magnetic field around the power line  3 A 1 , and a coil  7   d   2  coupled with a magnetic field around the power line  3 A 2 . The coil  7   d   1  includes, for example, a winding circularly wound on a plane including the power line  3 A 1 , and is provided at a distance from the power line  3 A 1 . The coil  7   d   2  includes, for example, a winding circularly wound on a plane including the power line  3 A 2 , and is provided at a distance from the power line  3 A 2 . As shown in  FIG. 22 , when the coils  7   d   1  and  7   d   2  are disposed in parallel to each other, the coils  7   d   1  and  7   d   2  are wound in directions opposite to each other. Thus, the power lines  3 A 1  and  3 A 2  and the coils  7   d   1  and  7   d   2  are magnetically coupled to each other. Power is transmitted via magnetic fields therebetween. According to the transmission path  3 A and the contactless connector  7  of  FIG. 22 , it is possible to uniformly transmit power at any positions of the transmission path  3 A. 
     In addition, according to the transmission path  3 A and the contactless connector  7  of  FIG. 22 , no coil is provided to the transmission path  3 A. Therefore, a foreign object between or near the coils is less likely to heat up, and accordingly, the transmission path  3 A and the contactless connector  7  of  FIG. 22  are safer than the transmission path  3 A and the contactless connector  7  of  FIGS. 20 and 21 . 
       FIG. 23  is a diagram showing a first modification of the coils  7   d   1  and  7   d   2  of  FIG. 22 .  FIG. 24  is a diagram showing a second modification of the coils  7   d   1  and  7   d   2  of  FIG. 22 .  FIG. 25  is a diagram showing a third modification of the coils  7   d   1  and  7   d   2  of  FIG. 22 . The coils of the contactless connector  7  of  FIG. 22  may include windings not limited to being wound circularly, but being wound elongately, rectangularly, elliptically, etc. By using coils shaped in such a manner, it is possible more strongly couple the contactless connector  7  and the transmission path  3 A with each other via the magnetic field as compared to the case of  FIG. 22 , thus improving efficiency of power transmission. 
       FIG. 26  is a diagram showing a fifth implementation example of the transmission path  3 A and the contactless connector  7  of  FIG. 18 . While the coils  7   d   1  and  7   d   2  of the contactless connector  7  of  FIG. 22  are wound in directions opposite to each other, the coils  7   d   1  and  7   d   2  may be wound in the same direction, as shown in  FIG. 26 . The contactless connector  7  of  FIG. 26  includes only one type of coil, and therefore, it is possible to reduce costs. 
     According to the transmission path  3 A and the contactless connector  7  of  FIGS. 19 to 26 , it is possible to transmit power using the electric field coupling or the magnetic field coupling, without electrical contact. In addition, it is possible to efficiently transmit power by electromagnetically disposing the contactless connector  7  at any position of the transmission path  3 A. 
     As described above, the power transmission system of  FIG. 18  modulates power generated by the power generator  1 , by code modulation; transmits the power modulated by code modulation, via the transmission path  3 A and the contactless connector  7 ; demodulates the transmitted power by code demodulation; and supplies the demodulated power to the load  5 , in a manner similar to that of the power transmission system of the first to third embodiments. Power can be transmitted between the transmission path  3 A and the contactless connector  7 , without electrical contact. Thus, there is an advantageous effect of being capable of easily attaching and detaching the code demodulator  4  and the load  5  to/from the transmission path  3 A, and increasing and decreasing the number of the loads  5  receiving power in the power transmission system. Furthermore, the code demodulator  4  and the load  5  can be disposed at any positions of the transmission path  3 A. Therefore, there is an advantageous effect of being capable of transmitting power even when the load  5  moves over time. 
       FIG. 27  is a block diagram showing a configuration of a power transmission system according to a first modification of the fourth embodiment. The power transmission system of  FIG. 27  is provided with the power generator  1 , the code modulator  2 , the transmission path  3 A, the code demodulator  4 , the load  5 , and the contactless connector  7 . 
     The power generator  1 , the code modulator  2 , the code demodulator  4 , and the load  5  of  FIG. 27  are configured in a manner similar to that of the corresponding constituent elements of  FIG. 1 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 27 . 
     The contactless connector  7  is connected to the code modulator  2 , and also is electromagnetically coupled to the transmission path  3 A without electrical contact. The contactless connector  7  of  FIG. 27  is configured in a manner similar to that of the contactless connector  7  of  FIG. 18  (see  FIGS. 19 to 26 ). 
     According to the power transmission system of  FIG. 27 , the code modulator  2  and the contactless connector  7  operate as a power transmitter apparatus, and the code demodulator  4  operates as a power receiver apparatus. The code modulator  2  modulates first power to generate a code-modulated wave by code modulation using a modulation code based on a code sequence, and transmits the code-modulated wave to the transmission path  3 A. The contactless connector  7  transmits the code-modulated wave to the power receiver apparatus via the transmission path  3 A. The code demodulator  4  receives the code-modulated wave from the transmission path  3 A, and demodulates the received code-modulated wave to generate second power by code demodulation using a demodulation code based on a code sequence identical to the code sequence of the modulation code used for code-modulation. The first power is direct-current or alternating-current power generated by the power generator  1 . The code-modulated wave is alternating current power modulated by code modulation. The second power is direct-current or alternating-current power to be supplied to the load  5 . 
     The power transmission system of  FIG. 27  modulates power generated by the power generator  1 , by code modulation; transmits the power modulated by code modulation, via the transmission path  3 A and the contactless connector  7 ; demodulates the transmitted power by code demodulation; and supplies the demodulated power to the load  5 , in a manner similar to that of the power transmission system of the first to third embodiments. Power can be transmitted between the contactless connector  7  and the transmission path  3 A, without electrical contact. Thus, it is possible to easily attach and detach the power generator  1  and the code modulator  2  to/from the transmission path  3 A, and increasing and decreasing the number of the power generators  1  operable as power sources in the power transmission system. Furthermore, the power generator  1  and the code modulator  2  can be disposed at any positions of the transmission path  3 A. Therefore, it is possible to transmit power even when the power generator  1  moves over time. 
       FIG. 28  is a block diagram showing a configuration of a power transmission system according to a second modification of the fourth embodiment. The power transmission system in  FIG. 28  is provided with the power generator  1 , the code modulator  2 , the transmission path  3 A, the code demodulator  4 , the load  5 , the contactless connector  7 , a code modulator/demodulator  8 , and a battery  9 . 
     The power generator  1 , the code modulator  2 , the transmission path  3 A, the code demodulator  4 , the load  5 , and the contactless connector  7  of  FIG. 28  are configured in a manner similar to that of the corresponding constituent elements of  FIG. 18 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 28 . 
     The transmission path  3 A of  FIG. 28  is connected with the battery  9  via the code modulator/demodulator  8 . As described above, the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11  are reversible. The code modulator/demodulator  8  includes circuits similar to the code modulation circuit  23 A of  FIG. 10  and the code demodulation circuit  33 A of  FIG. 11 . Accordingly, the code modulator/demodulator  8  can operate as a code modulator, and also operate as a code demodulator. 
     When power consumption of the load  5  is smaller than generated power of the power generator  1 , a modulation code and a demodulation code based on one code sequence are set to the code modulator  2  and the code demodulator  4 , respectively, such that a part of the generated power is transmitted to the load  5 , and at the same time, a modulation code and a demodulation code based on another code sequence are set to the code modulator  2  and the code modulator/demodulator  8 , respectively, such that the rest of the generated power is transmitted to the battery  9 . In this case, the code modulator  2  is provided with a plurality of code modulation circuits for simultaneously transmitting power to a plurality of destinations. In addition, in this case, the code modulator/demodulator  8  operates as a code demodulator. When simultaneously transmitting power from the code modulator  2  to the code demodulator  4  and the code modulator/demodulator  8 , two low-correlated (e.g., orthogonal) code sequences may be used. Thus, in the transmission path  3 A, it is possible to distinguish the code-modulated wave transmitted to the load  5 , and the code-modulated wave transmitted to the battery  9 , from each other, and therefore, it is possible to supply power from the power generator  1  to the load  5  and the battery  9  at a desired distribution ratio. 
     On the other hand, when power consumption of the load  5  is larger than generated power of the power generator  1 , the generated power is transmitted to the load  5 , and simultaneously, discharged power of the battery  9  is supplied to the load  5 . In this case, the code modulator/demodulator  8  operates as a code modulator. The code modulator/demodulator  8  is synchronized with the code modulator  2 , and modulates the discharged power by code modulation using the same modulation code as that used by the code modulator  2 . Thus, the code demodulator  4  can simultaneously demodulates the code-modulated wave transmitted from the code modulator  2 , and the code-modulated wave transmitted from the code modulator/demodulator  8 , by code demodulation, and transmit the demodulated power to the load  5 . 
     In addition to the advantageous effects of the power transmission system of  FIG. 18 , the power transmission system of  FIG. 28 , which is provided with the code modulator/demodulator  8  and the battery  9 , has an advantageous effect of being capable of transmitting power to the load  5  in a more flexible manner by charging and discharging the battery  9 . Thus, it is not necessary to promptly control generated power of the power generator  1  in response to variations of power consumption due to operating conditions of the load  5 . In other words, there is an advantageous effect of being capable of generating power at an efficient operation point in a more stable manner. 
       FIG. 29  is a block diagram showing a configuration of a power transmission system according to a third modification of the fourth embodiment. The power transmission system of  FIG. 29  is provided with the power generator  1 , the code modulator  2 , the transmission path  3 A, the load  5 , the contactless connector  7 , code modulator/demodulators  8 - 1  and  8 - 2 , and the battery  9 . 
     The power generator  1 , the code modulator  2 , the transmission path  3 A, the load  5 , the contactless connector  7 , and the battery  9  of  FIG. 29  are configured in a manner similar to that of the corresponding constituent elements of  FIG. 28 . The code modulator/demodulators  8 - 1  and  8 - 2  of  FIG. 29  are configured in a manner similar to that of the code modulator/demodulator  8  of  FIG. 28 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 29 . 
     In the power transmission system of  FIG. 29 , the code modulator  2  operates as a power transmitter apparatus, the code modulator/demodulator  8 - 1  operates as a power transmitter and receiver apparatus, and the contactless connector  7  and the code modulator/demodulator  8 - 2  operate as a power transmitter and receiver apparatus. The power transmission system of  FIG. 29  is provided with the code modulator/demodulator  8 - 2 , in place of the code demodulator  4  connected to the load  5  of  FIG. 28 . Here, we describe a case in which the load  5  is an electric facility which consumes and generates (regenerates) power, such as a motor, and power is transmitted via the contactless connector  7  in both directions. When the load  5  consumes power, the code modulator/demodulator  8 - 2  operates as a code demodulator. In this case, a modulation code and a demodulation code based on one code sequence are set to the code modulator  2  and the code modulator/demodulator  8 - 2 , respectively, such that a part or all of generated power of the power generator  1  is transmitted to the load  5 . On the other hand, when the load  5  generates power, the code modulator/demodulator  8 - 2  operates as a code modulator. In this case, the code modulator/demodulator  8 - 2  is synchronized with the code modulator  2 , and modulates the generated power (regenerated power) by code modulation using the same modulation code as the code used by the code modulator  2 . Thus, the code modulator/demodulator  8 - 1  can simultaneously demodulates the code-modulated wave transmitted from the code modulator  2 , and the code-modulated wave transmitted from the code modulator/demodulator  8 - 2 , by code demodulation, and transmit the demodulated power to the battery  9 . 
     In addition to the advantageous effects of the power transmission system of  FIG. 18 , the power transmission system of  FIG. 29 , which is provided with the code modulator/demodulator  8 - 2  connected to the load  5  which consumes and generates power, has an advantageous effect of achieving power transmission of the load  5  in a more flexible manner Therefore, it is also possible to charge the battery  9  with generated power of the load  5 , and thus, there is an advantageous effect of achieving more efficient power utilization. 
       FIG. 30  is a block diagram showing a configuration of a power transmission system according to a fourth modification of the fourth embodiment. The power transmission system of  FIG. 30  is provided with the power generator  1 , the code modulator  2 , the transmission path  3 A, the code demodulator  4 , the load  5 , termination resistors  6 - 1  and  6 - 2 , contactless connectors  7 - 1  to  7 - 3 , the code modulator/demodulator  8 , and the battery  9 . 
     The power generator  1 , the code modulator  2 , the transmission path  3 A, the code demodulator  4 , the load  5 , the code modulator/demodulator  8 , and the battery  9  of  FIG. 30  are configured in a manner similar to that of the corresponding constituent elements of  FIGS. 27 and 28 . The contactless connectors  7 - 1  to  7 - 3  of  FIG. 30  are configured in a manner similar to that of the contactless connector  7  of  FIG. 18 . The termination resistors  6 - 1  and  6 - 2  of  FIG. 30  are configured in a manner similar to that of the termination resistor  6  of  FIG. 18 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 30 . 
     As shown in  FIG. 30 , a plurality of apparatuses may be combined, the plurality of apparatuses being selected from: a power receiver apparatus similar to that of  FIG. 18  (contactless connector  7  and code demodulator  4 ), a power transmitter apparatus similar to that of  FIG. 27  (code modulator  2  and contactless connector  7 ), and a power transmitter and receiver apparatus similar to that of  FIG. 29  (contactless connector  7  and code modulator/demodulator  8 ). Thus, there is an advantageous effect of being capable of easily attaching and detaching the power generator  1 , the load  5 , and the battery  9  to/from the transmission path  3 A, and easily increasing and decreasing the numbers of the power generator  1 , the numbers of the load  5 , and the numbers of the battery  9  in the power transmission system. Furthermore, there is an advantageous effect of being capable of disposing the power generator  1 , the load  5 , and the battery  9  at any positions of the transmission path  3 A, and transmitting power even when the power generator  1 , the load  5 , and the battery  9  move over time. Thus, there is an advantageous effect of easily moving and disposing electric facilities connected via the contactless connectors, thus building a more flexible power transmission system. 
       FIG. 30  shows the example of the power transmission system including the one power generator  1 , the one load  5 , and the one battery  9 , but not limited thereto. The power transmission system may include a plurality of power generators, a plurality of loads, and a plurality of storage batteries. Thus, in addition to the aforementioned advantageous effects, there is an advantageous effect of being capable of transmitting power among a large number of pairs of power transmitter apparatus and power receiver apparatus over the one transmission path  3 A. 
       FIG. 30  shows the example in which the code demodulator  4  is connected to the load  5 , but not limited thereto. The code modulator/demodulator  8  may be provided, instead of the code demodulator  4 . Thus, in addition to the aforementioned advantageous effects, there is an advantageous effect of being capable of transmitting and receiving power to/from the load  5  in a more flexible manner, the load  5  consuming and generating power. 
     Fifth Embodiment 
       FIG. 31  is a block diagram showing a configuration of a power transmission system according to a fifth embodiment. Referring to  FIG. 31 , The power transmission system according to the fifth embodiment is provided with a power generator  1 , a code modulator  2 B, a transmission path  3 A, a code demodulator  4 B, a load  5 , and a contactless connector  7 . 
     The power generator  1 , the transmission path  3 A, the load  5 , a termination resistor  6 , and the contactless connector  7  of  FIG. 31  are configured in a manner similar to that of the corresponding constituent elements of  FIG. 18 . For ease of illustration, the power meters  1   m  and  5   m  and the controller  10  of  FIG. 1  are not shown in  FIG. 18 . 
     The power transmission system of  FIG. 31  is provided with the code modulator  2 B in place of the code modulator  2  of  FIG. 18 , and provided with the code demodulator  4 B in place of the code demodulator  4  of  FIG. 18 . The power transmission system of  FIG. 31  transmits power via the transmission path  3 A, and further transmit information signals superimposed on the power. 
       FIG. 32  is a block diagram showing a configuration of the code modulator  2 B of  FIG. 31 . The code modulator  2 B of  FIG. 32  is provided with a power line communication circuit  24 , a signal separation circuit  25 , and a coupler circuit  26 , in addition to the constituent elements of the code modulator  2  of  FIG. 5 . 
     A control circuit  20  transmits and receives information signals to/from the code demodulator  4 B, using the power line communication circuit  24 , the signal separation circuit  25 , and the coupler circuit  26 . The information signals are used for setting up power transmission. The power line communication circuit  24  is connected to the transmission path  3 A via the signal separation circuit  25  and the coupler circuit  26 . The signal separation circuit  25  is, for example, a high-frequency filter for separating the information signals from the transmitted power of power transmission. The signal separation circuit  25  is provided in order to prevent the transmitted power of power transmission from being inputted into the power line communication circuit  24 . In addition, the coupler circuit  26  is, for example, a circuit for dividing and mixing a part of the high-frequency power, such as a coupling transformer. In this case, the coupler circuit  26  is used to mix the power of the information signals and the transmitted power of power transmission. For example, when the code modulator  2 B transmits an information signal to the code demodulator  4 B, an information signal outputted from the control circuit  20  is converted into a communication signal by the power line communication circuit  24 , and the communication signal is transmitted to the transmission path  3 A via the signal separation circuit  25  and the coupler circuit  26 . On the other hand, when the code modulator  2 B receives an information signal from the code demodulator  4 B, the coupler circuit  26  and the signal separation circuit  25  separate power of a communication signal from the transmitted power of power transmission at a predetermined ratio, the power line communication circuit  24  converts the communication signal into an information signal, and passes the information signal to the control circuit  20 . The control circuit  20  performs predetermined controls and operations in accordance with this information signal. 
     Thus, according to the code modulator  2 B of  FIG. 32 , it is possible to transmit and receive the information signals over the transmission path of power transmission in a superposed manner. The transmission path can be shared between information transmission and power transmission. Therefore, there is an advantageous effect of being capable of reducing material costs of the transmission path, and reducing installation costs of the transmission path. 
       FIG. 33  is a block diagram showing a configuration of the code demodulator  4 B of  FIG. 31 . The code demodulator  4 B of  FIG. 33  is provided with a power line communication circuit  34 , a signal separation circuit  35 , and a coupler circuit  36 , in addition to the constituent elements of the code demodulator  4 B of  FIG. 6 . 
     A control circuit  30  transmits and receives information signals to/from the code modulator  2 B, using the power line communication circuit  34 , the signal separation circuit  35 , and the coupler circuit  36 . The information signals are used for setting up power transmission. The power line communication circuit  34  is connected to the transmission path  3 A via the signal separation circuit  35  and the coupler circuit  36 . The coupler circuit  36  is, for example, a circuit for dividing and mixing a part of the high-frequency power, such as a coupling transformer. In this case, the coupler circuit  36  is used to divide the power of the communication signals from the transmitted power of power transmission at a predetermined ratio. In addition, the signal separation circuit  35  is, for example, a high-frequency filter for separating the information signals from the transmitted power of power transmission. The signal separation circuit  35  is provided in order to prevent the transmitted power of power transmission from being inputted into the power line communication circuit  34 . For example, when the code demodulator  4 B receives an information signal from the code modulator  2 B, the coupler circuit  36  and the signal separation circuit  35  separate power of a communication signal from the transmitted power of power transmission at a predetermined ratio, the power line communication circuit  34  converts the communication signal into an information signal, and passes the information signal to the control circuit  30 . The control circuit  30  performs predetermined controls and operations in accordance with this information signal. On the other hand, when the code demodulator  4 B transmits an information signal to the code modulator  2 B, an information signal outputted from the control circuit  30  is converted into a communication signal by the power line communication circuit  34 , and the communication signal is transmitted to the transmission path  3 A via the signal separation circuit  35  and the coupler circuit  36 . 
     According to the code demodulator  4 B of  FIG. 33 , it is possible to transmit and receive the information signals over the transmission path of power transmission in a superposed manner. The transmission path can be shared between information transmission and power transmission. Therefore, there is an advantageous effect of being capable of reducing material costs of the transmission path, and reducing installation costs of the transmission path. 
     Here, we describe a case in which the information signals are used, for example, as control signals of the code modulator  2 B and the code demodulator  4 B. The code modulator  2 B of  FIG. 32  performs communicates required for controlling power transmission, with the controller  10 , or with another code modulator or code demodulator, using the power line communication circuit  24 . Based on control information received from the controller  10 , the control circuit  20  generates a code sequence used for code modulation, and controls the code modulation circuit  23  to modulate power by code modulation from a designated start time to a designated end time of modulation, and output the modulated power to the transmission path  3 A. The code demodulator  4 B of  FIG. 33  also performs communicates required for controlling power transmission, using the power line communication circuit  34 , in a manner similar to that of the code modulator  2 B of  FIG. 32 . Based on control information received from the controller  10 , the control circuit  30  generates a code sequence used for code demodulation, and controls the code demodulation circuit  33  to demodulate power by code demodulation from a designated start time to a designated end time od demodulation. 
     The control signals may include synchronization signals. Thus, there is an advantageous effect of being capable of establishing synchronization between the code modulator  2 B and the code demodulator  4 B, performing accurate code modulation and code demodulation of power, and achieving efficient power transmission. 
     The control signals may further include an emergency stop signal. Thus, there is an advantageous effect of being capable of immediately stop power transmission when detecting disconnection or abnormality of the transmission path, and preventing failure of the code modulator, the code demodulator, electrical facilities and equipment connected thereto, etc. 
     In addition, the code modulator  2 B may transmit and receive, via the transmission path  3 A, information signals to be transmitted and received to/from the controller  10 . Similarly, the code demodulator  4 B may also transmit and receive, via the transmission path  3 A, information signals to be transmitted and received to/from the controller  10 . 
     Thus, according to the power transmission system of the fifth embodiment, it is possible to achieve communications between the controller  10  and the control circuit  20 , between the controller  10  and the control circuit  30 , and between the control circuit  20  and the control circuit  30 , via the transmission path  3 A. Therefore, there is an advantageous effect of being capable of transmitting and receiving the information signals over the transmission path  3 A of power transmission in a superposed manner, so that the transmission path can be shared between information transmission and power transmission, and thus, reducing material costs of the transmission path, and reducing installation costs of the transmission path. Although the case of the code modulator  2 B and the code demodulator  4 B has been described, but not limited thereto, it is possible to obtain similar advantageous effects by a code modulator/demodulator provided with a similar power line communication circuit, a similar signal separation circuit, and a similar coupler circuit. 
     Sixth Embodiment 
       FIG. 34  is a block diagram showing a configuration of a power transmission system according to a sixth embodiment. The power transmission system of  FIG. 6  is provided with a power generator  101 , a transmission path  103 , and loads  105 - 1  and  105 - 2 . 
       FIG. 34  shows an exemplary system configuration of the transmission path  103  with grid arrangement. The power generator  101  is, for example, a renewable energy power generator, such as a solar cell. For example, the load  105 - 1  is a robot, and the load  105 - 2  is an automated guided vehicle (AGV). The power generator  101  and the loads  105 - 1  and  105 - 2  have contactless connectors, and transmit and receive power via the contactless connectors without electrical contact. 
     According to the power transmission system of  FIG. 34 , there is an advantageous effect of being capable of transmitting and receiving power at any position within a two-dimensional (or three-dimensional) range of the transmission path  103 , in addition to the advantageous effects described in the fourth to fifth embodiments. 
     OTHER EMBODIMENTS 
     In the second to sixth embodiments, when the power generator generates alternating-current power, the frequency of the generated power may be measured and notified to the controller. 
     In the third to sixth embodiments, a plurality of code modulators may use the same code sequence, and a plurality of code demodulators may use the same code sequence. Thus, one code modulator may transmit powers to a plurality of code demodulators, a plurality of code modulators may transmit powers to one code demodulator, and a plurality of code modulators may transmit powers to a plurality of code demodulators. 
     In the first to sixth embodiments, we have indicated the example in which power is transmitted using code modulation and code demodulation of current, but the power transmission is not limited thereto. Power may be transmitted using code modulation and code demodulation of direct-current or alternating-current voltage. In this case, similar advantageous effects can be achieved. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  1 - 1 ,  1 - 2 : POWER GENERATOR, 
               1   m ,  1   m - 1 ,  1   m - 2 : POWER METER, 
               2 ,  2 - 1 ,  2 - 2 ,  2 A,  2 A- 1 ,  2 A- 2 ,  2 B: CODE MODULATOR, 
               3 ,  3 A: TRANSMISSION PATH, 
               3 A 1 ,  3 A 2 : POWER LINE, 
               3 Aa 1 ,  3 Aa 2 : CONDUCTOR PLATE, 
               3 Ab,  3 Ac: COIL, 
               4 ,  4 - 1 ,  4 - 2 ,  4 A,  4 A- 1 ,  4 A- 2 ,  4 B: CODE DEMODULATOR, 
               5 ,  5 - 1 ,  5 - 2 : LOAD, 
               5   m ,  5   m - 1  to  5   m - 2 : POWER METER, 
               6 ,  6 - 1 ,  6 - 2 : TERMINATION RESISTOR, 
               7 ,  7 - 1  to  7 - 3 : CONTACTLESS CONNECTOR, 
               7   a   1 ,  7   a   2 : CONDUCTOR PLATE, 
               7   b ,  7   c ,  7   d   1 ,  7   d   2 : COIL, 
               8 ,  8 - 1 ,  8 - 2 : CODE MODULATOR/DEMODULATOR, 
               9 : BATTERY, 
               10 ,  10 A: CONTROLLER, 
               11 : CONTROL CIRCUIT, 
               12 ,  12 A: COMMUNICATION CIRCUIT, 
               20 : CONTROL CIRCUIT, 
               21 : COMMUNICATION CIRCUIT, 
               22 ,  22 A: CODE GENERATION CIRCUIT, 
               23 ,  23 A: CODE MODULATION CIRCUIT, 
               24 : POWER LINE COMMUNICATION CIRCUIT, 
               25 : SIGNAL SEPARATION CIRCUIT, 
               26 : COUPLER CIRCUIT, 
               30 : CONTROL CIRCUIT, 
               31 : COMMUNICATION CIRCUIT, 
               32 ,  32 A: CODE GENERATION CIRCUIT, 
               33 ,  33 A: CODE DEMODULATION CIRCUIT, 
               34 : POWER LINE COMMUNICATION CIRCUIT, 
               35 : SIGNAL SEPARATION CIRCUIT, 
               36 : COUPLER CIRCUIT, 
               101 : POWER GENERATOR, 
               103 : TRANSMISSION PATH, 
               105 - 1 ,  105 - 2 : LOAD, 
             D 1  to D 34 : DIODE, 
             S 1  to S 74 : SWITCH ELEMENT, 
             SS 1  to SS 34 , SS 21 A to SS 34 A: SWITCH CIRCUIT, and 
             T 1  to T 14 : TERMINAL.