Patent Publication Number: US-2013241293-A1

Title: Power control system and electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2012-057870 filed in Japan on Mar. 14, 2012. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power control system and an electronic apparatus. 
     2. Description of the Related Art 
     In electronic apparatuses such as image forming apparatuses, reduction of power consumption is an essential issue for reducing global environmental problems and running cost for users. For example, Japanese Patent Application Laid-open No. 2005-237095 discloses a technique in which, for the purpose of energy saving, a main power supply that supplies power to various parts of an equipment main unit, a photovoltaic cell, and a secondary battery are provided, and during a power saving mode, the main power supply stops operating while the various parts of the equipment main unit are supplied with power from the secondary battery. 
     However, in the technique disclosed by Japanese Patent Application Laid-open No. 2005-237095, the photovoltaic cell is built into an image forming apparatus. Therefore, there is a problem that a sufficient amount of power is not obtained when the image forming apparatus is not irradiated by a sufficient amount of sunlight. In other words, the technique disclosed by Japanese Patent Application Laid-open No. 2005-237095 cannot sufficiently reduce the power consumption. 
     Therefore, there is a need to provide a power control system and an electronic apparatus that can achieve further reduction in power consumption. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an embodiment, there is provided a power control system that includes an electronic apparatus; and a power transmission apparatus configured to send power to the electronic apparatus. The power transmission apparatus includes a power generation unit configured to generate the power; a first communication unit configured to communicate with the electronic apparatus; and a power transmission unit configured to send the power generated by the power generation unit to the electronic apparatus in response to a request from the electronic apparatus received at the first communication unit. The electronic apparatus includes a processing unit configured to perform a process; a second communication unit configured to send, to the power transmission apparatus, a request for starting or stopping supplying the power; a power receiving unit configured to receive the power from the power transmission apparatus; and a power supply control unit configured to control, in a power saving mode in which power consumption is smaller than that in a normal mode in which the power is supplied to the processing unit and the second communication unit, the power supplied to the second communication unit to a value smaller than the power supplied to the second communication unit in the normal mode. 
     According to another embodiment, there is provided an electronic apparatus that includes a processing unit configured to perform a process; a communication unit configured to send, to a power transmission apparatus configured to send generated power to an external device, a request for starting or stopping supplying the power; a power receiving unit configured to receive the power from the power transmission apparatus; and a power supply control unit configured to control, in a power saving mode in which power consumption is smaller than that in a normal mode in which the power is supplied to the processing unit and the communication unit, the power supplied to the communication unit to a value smaller than the power supplied to the communication unit in the normal mode. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a hardware configuration example of an image forming system according to a first embodiment of the present invention; 
         FIG. 2  is a flow chart illustrating an example of charge control of a power storage unit in an image forming apparatus; 
         FIGS. 3A and 3B  are flow charts illustrating an operation example of the image forming apparatus when entering a power saving mode and when returning from the power saving mode; 
         FIG. 4  is a flow chart illustrating an operation example of a power transmission apparatus; 
         FIG. 5  is a block diagram illustrating a hardware configuration example of an image forming system according to a second embodiment of the present invention; 
         FIG. 6  is a flow chart illustrating an example of charge control of a power storage unit in the power transmission apparatus; and 
         FIGS. 7A and 7B  are flow charts illustrating a modification of the operation example of the image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a power control system and an electronic apparatus according to the present invention will be described below in detail with reference to the accompanying drawings. The embodiments below will be described by giving, as the electronic apparatus, an example of an image forming apparatus that has a function of forming an image on a medium. However, not limited to this apparatus, the electronic apparatus may be of any type. For example, a digital camera or a mobile phone can be employed as the electronic apparatus. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating a hardware configuration example of an image forming system (power control system)  100  according to a first embodiment of the present invention. As illustrated in  FIG. 1 , the image forming system  100  is provided with a power transmission apparatus  10  and the image forming apparatus (MFP)  20 . 
     The power transmission apparatus  10  is an apparatus that can supply (can transmit) generated power to the image forming apparatus  20 . Specific details of the power transmission apparatus  10  will be described below. As illustrated in  FIG. 1 , the power transmission apparatus  10  is provided with a power generation unit  11 , a first communication unit  12 , an operating unit  13 , a display unit  14 , a storage unit  15 , a power transmission unit  16 , and a power transmission control unit  17 . 
     The power generation unit  11  is an apparatus that generates electric power. Although, in the present embodiment, the power generation unit  11  includes a photovoltaic cell, the power generation unit  11  is not limited to this configuration. The power generation unit  11  supplies the generated power to the power transmission control unit  17 . 
     The first communication unit  12  communicates with the image forming apparatus  20 . In the present embodiment, the first communication unit  12  receives, from the image forming apparatus  20 , information such as a power supply start request that requests a start of power supply and a power supply stop request that requests a stop of the power supply. The first communication unit  12  notifies the power transmission control unit  17  of various type of information received from the image forming apparatus  20 . Also, in response to a request received from the power transmission control unit  17 , the first communication unit  12  sends, to the image forming apparatus  20 , information such as supply availability information that indicates availability of supply of power and power supply stop information that indicates that the power supply is to be stopped. 
     For example, the first communication unit  12  can perform wireless communication with external devices such as the image forming apparatus  20  using light, electromagnetic waves, or acoustic waves. However, the communication methods are not limited to these methods. For example, a power transmission coil used for sending (transmitting) power with a magnetic field resonance technique may be used as a helical antenna to perform the communication, like a technique disclosed in Japanese Patent Application Laid-open No. 2010-141966. In other words, the power transmission unit  16  to be described later may be used to serve as the first communication unit  12 . 
     The operating unit  13  is a device for a user to perform various input operations to the power transmission apparatus  10 . The operating unit  13  may include, for example, of a keyboard, a mouse, buttons, directional keys, a jog dial, a touch panel, or a combination of these devices. However, the components are not limited to the above-listed ones. 
     The display unit  14  is device for displaying various types of information on the power transmission apparatus  10 . The display unit  14  displays screens such as an operation screen for making the power transmission apparatus  10  perform desired operations and a screen that indicates a status of communication with the image forming apparatus  20 , a status of power transmission, and the like. However, the screens are not limited to the above-listed ones. The display unit  14  can be composed, for example, of an LCD or an organic EL display. However, the display is not limited to the above-listed ones. 
     The storage unit  15  stores therein, for example, identification (ID) information of the image forming apparatus  20  to be supplied with power, the power supply start request, the power supply stop request, and various applications. The storage unit  15  can be composed, for example, of a magnetic recording medium such as a hard disk, or of a nonvolatile memory such as a flash memory. However, the component is not limited to the above-listed ones. 
     The power transmission unit  16  supplies (transmits) the power generated in the power generation unit  11  to the image forming apparatus  20 . Although the power can be supplied to the image forming apparatus  20  in any manner, the power transmission unit  16  of the present embodiment uses the magnetic field resonance technique under the control of the power transmission control unit  17  to send the power generated in the power generation unit  11  to the image forming apparatus  20 . Due to a principle called a magnetic field resonance phenomenon, the magnetic field resonance technique has an advantage over an electromagnetic induction technique in that the power can be transmitted to a more distant place and a drop in a transmission efficiency can be suppressed even when there is a certain degree of misalignment in position between the power transmission coil and a power receiving coil. 
     The power transmission control unit  17  controls the power transmission apparatus  10  in an integrated manner. The power transmission control unit  17  can be composed, for example, of an MPU. In the present embodiment, the power transmission control unit  17  has a maximum power point tracker (MPPT) function to track a point at which the power output from the power generation unit  11  is maximized. The power transmission control unit  17  calculates the amount of power generated in the power generation unit  11 , and, when supplying power to the image forming apparatus  20 , performs control to supply the power from the power generation unit  11  to the power transmission unit  16 , and to send the power to the image forming apparatus  20  using the magnetic field resonance technique. When stopping the power supply to the image forming apparatus  20 , the power transmission control unit  17  stops the power supply from the power generation unit  11  to the power transmission unit  16 . 
     Also, the power transmission control unit  17  makes a request for the first communication unit  12  to send the various types of information (such as the supply availability information and the power supply stop information) to the image forming apparatus  20 . Moreover, the power transmission control unit  17  performs control according to the information (such as the power supply start request and the power supply stop request) received at the first communication unit  12 . The details thereof will be described later. 
     Furthermore, the power transmission control unit  17  performs control to store, in the storage unit  15 , the information received at the first communication unit  12  and information entered from the operating unit  13 , and to display these pieces of information on the display unit  14 . 
     Next, specific details of the image forming apparatus  20  will be described. As illustrated in  FIG. 1 , the image forming apparatus  20  is provided with a communication apparatus  30 , a controller  40 , an engine  50 , and a power supply unit  60 . 
     The communication apparatus  30  is an apparatus that communicates with the power transmission apparatus  10 , and is provided separately from an external I/F that can receive image data (such as image data described in a page-description language) from a host device such as a PC. 
     The communication apparatus  30  has a second communication unit  31  that wirelessly communicates with the first communication unit  12  of the power transmission apparatus  10 . The second communication unit  31  receives the information such as the supply availability information and the power supply stop information from the power transmission apparatus  10 . The second communication unit  31  notifies a power supply control unit  65  in the power supply unit  60  of the various types of information received from the power transmission apparatus  10 . Also, in response to a request received from the power supply control unit  65 , the second communication unit  31  sends the information such as the power supply start request and the power supply stop request to the power transmission apparatus  10 . 
     For example, the second communication unit  31  can perform wireless communication with the power transmission apparatus  10  using light, electromagnetic waves, or acoustic waves. However, the communication methods are not limited to these methods. For example, the power transmission coil used for sending (transmitting) power with the magnetic field resonance technique may be used as the helical antenna to perform the communication, like the technique disclosed in Japanese Patent Application Laid-open No. 2010-141966. In other words, a power receiving unit  61  to be described later may be used to serve as the second communication unit  31 . 
     The controller  40  is an apparatus that controls the image forming apparatus  20  in an integrated manner. The controller  40  is provided with a CPU  41 , a RAM  42 , a ROM  43 , an ASIC  44 , and an external I/F  45 . The CPU  41  loads programs stored in the ROM  43  or the like into the RAM  42  and executes the loaded programs so as to control operations of the controller  40 . The ROM  43  is a nonvolatile semiconductor memory, and stores therein the control programs and various types of data. The RAM  42  is a volatile semiconductor memory that functions as work areas when the various programs stored in the ROM  43  are executed. The external I/F  45  is an interface device to which external devices such as a PC can be connected. 
     The ASIC  44  performs, for example, various types of image processing such as compression and decompression, input and output (I/O) processing, control of a power supply in the controller  40 , and various notifications (such as a power saving mode enter notification and a power saving mode return notification) to the power supply unit  60  (power supply control unit  65 ). In the present embodiment, the ASIC  44  continues to be supplied with power even in the power saving mode because of a necessity to issue the power saving mode return notification to the power supply unit  60  even after the system enters the power saving mode. In the present embodiment, a state in which all elements of the communication apparatus  30  (second communication unit  31 ), the controller  40 , and the engine  50  are supplied with power so that the image forming apparatus  20  can operate is called a “normal mode”. A state in which power is stopped being supplied to the communication apparatus  30  and the engine  50  and is supplied to only the ASIC  44  in the controller  40  is called the “power saving mode”. However, not limited to this state, the power saving mode can take any form. For example, it can be configured that the communication apparatus  30  is supplied with power even in the power saving mode, but the value of the power is set to a smaller value than the value of the power supplied to the communication apparatus  30  in the normal mode. 
     When the ASIC  44  has detected a power saving mode entering factor that represents a factor for entering the power saving mode, and when a state is established in which the controller  40  can enter the power saving mode (a state in which power can be stopped being supplied to the elements to be stopped being supplied with the power in the power saving mode), the ASIC  44  notifies the power supply unit  60  (power supply control unit  65 ) to enter the power saving mode (power saving mode enter notification). In the present embodiment, the ASIC  44  sets a power saving mode enter/return signal to be communicated to the power supply unit  60  to a high level so as to notify the power supply unit  60  to enter the power saving mode. However, the method for entering the power saving mode is not limited to this method. Any factor can be the power saving mode entering factor. For example, the factor may be an event in which an operation input of pressing a power button is given in the normal mode or an event in which a timer detects that the time to enter the power saving mode is reached. 
     When the ASIC  44  has detected a returning factor that represents a factor for returning from the power saving mode to the normal mode, the ASIC  44  notifies the power supply unit  60  (power supply control unit  65 ) to return from the power saving mode (power saving mode return notification). In the present embodiment, the ASIC  44 , the ASIC  44  sets a power saving mode enter/return signal to be communicated to the power supply unit  60  to a low level so as to notify the power supply unit  60  to return from the power saving mode. However, the method for returning from the power saving mode is not limited to this method. Any factor can be the returning factor. For example, the factor may be an event in which an operation input of pressing the power button is given in the power saving mode or an event in which the timer detects that the time to return to the normal mode is reached. 
     The engine  50  is an apparatus that forms an image on a recording medium such as transfer paper. The engine  50  can also be understood as an “image forming unit”. As illustrated in  FIG. 1 , the engine  50  is provided with an image reading unit  51 , an image output unit  52 , and a data processing unit  53 . The image reading unit  51  scans a document and obtains electronic image data. The image output unit  52  prints out image data on the recording medium such as paper. The data processing unit  53  applies a plurality of processes (such as modification, correction, edit, detection, conversion) to image data. The image data entered into the data processing unit  53  is not limited to the image data obtained in the image reading unit  51 , but may be, for example, the image data (such as the image data described in the page-description language (PDL)) sent from the host device such as the PC. 
     The power supply unit  60  is an apparatus that can supply power to various units of the image forming apparatus  20 . As illustrated in  FIG. 1 , the power supply unit  60  is provided with the power receiving unit  61 , a first power storage unit  62 , a main power supply  63 , a power supply switching unit  64 , and the power supply control unit  65 . 
     The power receiving unit  61  receives the power sent from the power transmission apparatus  10 . In the present embodiment, the power receiving unit  61  uses the magnetic field resonance technique to receive the power sent from the power transmission apparatus  10 . Under the control of the power supply control unit  65 , the power receiving unit  61  can supply the power received from the power transmission apparatus  10  to the power supply control unit  65 , or can alternatively stop supplying the received power to the power supply control unit  65 . 
     Under the control of the power supply control unit  65 , the first power storage unit  62  stores (accumulates) therein the power received at the power receiving unit  61 . The first power storage unit  62  can also discharge the stored power under the control of the power supply control unit  65 . Further, the first power storage unit  62  can notify the power supply control unit  65  of an electricity storage amount of itself. The first power storage unit  62  can be composed, for example, of a secondary battery such as a lithium secondary battery or a nickel-metal hydride battery, or of a capacitor. However, the component is not limited to the above-listed ones. 
     Under the control of the power supply control unit  65 , the main power supply  63  converts power (AC power) from a commercial power source into power (DC power) usable in the communication apparatus  30 , the controller  40 , the engine  50 , and the like, and outputs the converted power (DC power) to the power supply switching unit  64 . In the present embodiment, the main power supply  63  operates according to a control signal PON_AC_N from the power supply control unit  65 . 
     The main power supply  63  is described more specifically as follows. The main power supply  63  has an input unit (not illustrated) for receiving the AC power. If the control signal PON_AC_N supplied from the power supply control unit  65  is at a low level (L), the main power supply  63  starts receiving the AC power and performs processing of converting the AC power into the DC power. Then, if the supply of the DC power to the power supply switching unit  64  is stabilized, the main power supply  63  notifies the power supply control unit  65  of a state signal POK_AC_P that is set to a low level. If, instead, the supply of the DC power to the power supply switching unit  64  is unstable, the main power supply  63  notifies the power supply control unit  65  of the state signal POK_AC_P that is set to a high level. 
     If the control signal PON_AC_N supplied from the power supply control unit  65  is at a high level (H), the main power supply  63  stops receiving the AC power and stops performing the processing of converting the AC power into the DC power. In this case, the main power supply  63  notifies the power supply control unit  65  of the state signal POK_AC_P that is set to a high level. Note that, even in this state (state where the DC power is not generated), the main power supply  63  is supplied with the power from the first power storage unit  62  in order to operate a circuit for performing the processing of converting the AC power into the DC power. 
     The main power supply  63  may be configured, for example, to have, in the input unit thereof, a relay, which in turn performs the on/off control based on the control signal PON_AC_N from the power supply control unit  65 . 
     Under the control of the power supply control unit  65 , the power supply switching unit  64  switches the source of power supplied to the units of the image forming apparatus  20  to either the power from the main power supply  63  (which can also be understood as the “power from the commercial power source”) or the power received at the power receiving unit  61  (including the power stored in the first power storage unit  62 ). If the source of supply of power is switched to the power from the main power supply  63 , the units of the image forming apparatus  20  can be supplied with the power from the main power supply  63  and cannot be supplied with the power received at the power receiving unit  61 . If, instead, the source of supply of power is switched to the power received at the power receiving unit  61 , the units of the image forming apparatus  20  can be supplied with the power received at the power receiving unit  61  and cannot be supplied with the power from the main power supply  63 . 
     Note that, not limited to this, even in the state in which the source of supply of power is switched to the power from the main power supply  63 , the power received at the power receiving unit  61 , if being received, can be supplied to the units of the image forming apparatus  20 . For example, even when the units of the image forming apparatus  20  are supplied with the power from the main power supply  63 , if the power receiving unit  61  is receiving the power from the power transmission apparatus  10 , the received power can be used to supply power to the units of the image forming apparatus  20  so as to reduce the amount of power supply from the main power supply  63  (that is, the amount of power supply from the commercial power source). In the same manner, even in the state in which the source of supply of power is switched to the power received at the power receiving unit  61 , the units of the image forming apparatus  20  can also be supplied with the power from the main power supply  63 . For example, while the units of the image forming apparatus  20  are supplied with the power received at the power receiving unit  61 , if the power supply capacity of the power transmission apparatus  10  drops, resulting in a lack of power supply, the power from the main power supply  63  can also be used to continue the power supply to the units of the image forming apparatus  20 . 
     Under the control of the power supply control unit  65 , the power supply switching unit  64  can also supply and stop the power individually to each of the units (such as the communication apparatus  30 , the controller  40 , and the engine  50 ) of the image forming apparatus  20 . The value of the power supplied to each of the units (such as the communication apparatus  30 , the controller  40 , and the engine  50 ) of the image forming apparatus  20  is controlled by the power supply control unit  65 . Note that, in the present embodiment, the first power storage unit  62  or the main power supply  63  supplies the power for operating the power supply switching unit  64 . 
     The power supply control unit  65  controls the power supply unit  60  in an integrated manner. The power supply control unit  65  can be composed, for example, of an MPU. In the present embodiment, in the case of entering the power saving mode, the power supply control unit  65  performs control to stop supplying power to the second communication unit  31  (communication apparatus  30 ) and to switch the source of supply of power to the power received at the power receiving unit  61 . More specifically, when having received the high-level power saving mode enter/return signal from the ASIC  44 , the power supply control unit  65  controls the power supply switching unit  64  so as to stop supplying power to the second communication unit  31 . The power supply control unit  65  also controls the power supply switching unit  64  so as to switch the source of supply of power to the power received at the power receiving unit  61 , and so as to stop supplying power to the engine  50  and to the elements other than the ASIC  44  in the controller  40 . 
     In the case of returning from the power saving mode to the normal mode, the power supply control unit  65  controls the power supply switching unit  64  so as to switch the source of supply of power to the power from the main power supply  63 , and so as to resume supplying the power to the engine  50  and to the elements other than the ASIC  44  in the controller  40 . Here, the power supply control unit  65  performs the above-described control when having received the low-level power saving mode enter/return signal from the ASIC  44 . 
     The power supply control unit  65  also performs charge/discharge control of the first power storage unit  62  based on the electricity storage amount notification from the first power storage unit  62 . The power supply control unit  65  further performs control to switch the power to be supplied to the power supply switching unit  64  to either the power supplied from the power receiving unit  61  or the power stored in the first power storage unit  62 . In addition, the power supply control unit  65  can control the power receiving unit  61  so as to supply the power received from the power transmission apparatus  10  to the power supply control unit  65 , and can control the power receiving unit  61  so as to stop supplying the received power to the power supply control unit  65 . The power supply control unit  65  also controls the generation of the DC power performed by the main power supply  63  by controlling the above-described control signal PON_AC_N. Furthermore, the power supply control unit  65  requests the second communication unit  31  to send the information such as the power supply start request and the power supply stop request to the power transmission apparatus  10 . The power supply control unit  65  also performs control according to the various types of information (such as the supply availability information and the power supply stop information) received at the second communication unit  31 . The details thereof will be described later. 
       FIG. 2  is a flow chart illustrating an example of the charge control of the first power storage unit  62 . Specific details of  FIG. 2  will be described below. As illustrated in  FIG. 2 , the power supply control unit  65  first determines whether the power received at the power receiving unit  61  is currently being charged to the first power storage unit  62  (Step S 1 ). 
     If determining that the power received at the power receiving unit  61  is currently being charged to the first power storage unit  62  (Yes at Step S 1 ), the power supply control unit  65  determines, based on the electricity storage amount notification from the first power storage unit  62 , whether the electricity storage amount of the first power storage unit  62  is not less than a first threshold (Step S 2 ). 
     If determining that the electricity storage amount of the first power storage unit  62  is not less than the first threshold (Yes at Step S 2 ), the power supply control unit  65  requests the second communication unit  31  to send the power supply stop request to the power transmission apparatus  10 . Upon receiving the request from the power supply control unit  65 , the second communication unit  31  sends the power supply stop request to the power transmission apparatus  10  (Step S 3 ). In the present embodiment, the second communication unit  31  sends, in addition to the power supply stop request, the identification (ID) information of the image forming apparatus  20  to the power transmission apparatus  10 . 
     Next, the power supply control unit  65  determines whether the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10  (Step S 4 ). In the present embodiment, the power supply control unit  65  determines whether it has been notified of the power supply stop information from the second communication unit  31  to determine whether the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10 . 
     If determining that the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10  (Yes at Step S 4 ), the power supply control unit  65  performs control to stop receiving the power from the power transmission apparatus  10  (Step S 5 ). For example, the power supply control unit  65  can also control the power receiving unit  61  so as to stop supplying the power received from the power transmission apparatus  10  to the power supply control unit  65 . If, instead, the second communication unit  31  has not received the power supply stop information from the power transmission apparatus  10  (No at Step S 4 ), and if a given period of time has passed since the power supply stop request was sent (Yes at Step S 6 ), the power supply control unit  65  also performs the control to stop receiving the power from the power transmission apparatus  10  (Step S 5 ). 
     If determining at Step S 1  described above that the power received at the power receiving unit  61  is not currently being charged to the first power storage unit  62  (No at Step S 1 ), the power supply control unit  65  determines, based on the electricity storage amount notification from the first power storage unit  62 , whether the electricity storage amount of the first power storage unit  62  is not less than a second threshold (Step S 7 ). If the electricity storage amount of the first power storage unit  62  is determined to be not less than the second threshold (Yes at Step S 7 ), the process is terminated. 
     If, instead, the electricity storage amount of the first power storage unit  62  is determined to be less than the second threshold (No at Step S 7 ), the power supply control unit  65  requests the second communication unit  31  to send the power supply start request to the power transmission apparatus  10 . Upon receiving the request from the power supply control unit  65 , the second communication unit  31  sends the power supply start request to the power transmission apparatus  10  (Step S 8 ). In the present embodiment, the second communication unit  31  sends, in addition to the power supply start request, the identification (ID) information of the image forming apparatus  20  to the power transmission apparatus  10 . 
     Next, the power supply control unit  65  determines whether the second communication unit  31  has received the supply availability information from the power transmission apparatus  10  (Step S 9 ). In the present embodiment, the power supply control unit  65  determines whether it has been notified of the supply availability information from the second communication unit  31  to determine whether the second communication unit  31  has received the supply availability information from the power transmission apparatus  10 . 
     If determining that the second communication unit  31  has received the supply availability information from the power transmission apparatus  10  (Yes at Step S 9 ), the power supply control unit  65  determines whether the received supply availability information indicates that the power supply is possible (Step S 10 ). If determining that the received supply availability information indicates that the power supply is possible (Yes at Step S 10 ), the power supply control unit  65  performs control to start receiving the power from the power transmission apparatus  10  (Step S 11 ). In the present embodiment, the power supply control unit  65  controls the power receiving unit  61  so as to supply the power received from the power transmission apparatus  10  to the power supply control unit  65 . Then, the power supply control unit  65  performs control to charge the first power storage unit  62  with the power supplied from the power receiving unit  61  (Step S 12 ). The process after Step S 12  is the same as the process of Step S 2  and later described above. If, instead, it is determined that the received supply availability information indicates that power cannot be supplied (No at Step S 10 ), the process is terminated. 
     Further, if the second communication unit  31  has not received the supply availability information from the power transmission apparatus  10  (No at Step S 9 ), and if a given period of time has passed since the power supply start request was sent (Yes at Step S 13 ), the process is also terminated. 
       FIGS. 3A and 3B  are flow charts illustrating an operation example of the image forming apparatus  20  when entering the power saving mode and when returning from the power saving mode. Specific details of  FIGS. 3A and 3B  will be described below. As illustrated in  FIGS. 3A and 3B , the ASIC  44  first determines whether the power saving mode entering factor has occurred (Step S 101 ). If determining that the power saving mode entering factor has occurred (Yes at Step S 101 ), the ASIC  44  notifies the power supply control unit  65  to enter the power saving mode (power saving mode enter notification). Upon receiving the power saving mode enter notification, the power supply control unit  65  requests the second communication unit  31  to send the power supply start request to the power transmission apparatus  10 . In the present embodiment, the power supply control unit  65  requests the second communication unit  31  to send, in addition to the power supply start request, information such as the identification (ID) information of the image forming apparatus  20  and required power information that indicates an amount of power necessary for the image forming apparatus  20  in the power saving mode. Upon receiving the request from the power supply control unit  65 , the second communication unit  31  sends the power supply start request to the power transmission apparatus  10  (Step S 102 ). In the present embodiment, in the case of entering the power saving mode, the second communication unit  31  sends, to the power transmission apparatus  10 , information such as the identification (ID) information of the image forming apparatus  20  and the required power information, in addition to the power supply start request. 
     Next, the power supply control unit  65  determines whether the second communication unit  31  has received the supply availability information from the power transmission apparatus  10  (Step S 103 ). In the present embodiment, the power supply control unit  65  determines whether it has been notified of the supply availability information from the second communication unit  31  to determine whether the second communication unit  31  has received the supply availability information from the power transmission apparatus  10 . 
     If determining that the second communication unit  31  has received the supply availability information from the power transmission apparatus  10  (Yes at Step S 103 ), the power supply control unit  65  determines whether the received supply availability information indicates that the power supply is possible (Step S 104 ). If determining that the received supply availability information indicates that the power supply is possible (Yes at Step S 104 ), the power supply control unit  65  performs control to start receiving the power from the power transmission apparatus  10  (Step S 105 ). For example, the power supply control unit  65  can also control the power receiving unit  61  so as to supply the power received from the power transmission apparatus  10  to the power supply control unit  65 . Next, the power supply control unit  65  performs control to switch the power to be supplied to the power supply switching unit  64  to the power supplied from the power receiving unit  61  (currently received power) (Step S 106 ). Next, the power supply control unit  65  controls the power supply switching unit  64  so as to stop the power supply to the second communication unit  31  (Step S 107 ). Next, the power supply control unit  65  performs control to enter the power saving mode (Step S 108 ). More specifically, the power supply control unit  65  controls the power supply switching unit  64  so as to switch the source of power supplied to the units of the image forming apparatus  20  to the power received at the power receiving unit  61  (power supplied from the power supply control unit  65 ), and so as to stop supplying power to the engine  50  and to the elements other than the ASIC  44  in the controller  40 . 
     If, instead, it is determined at Step S 104  described above that the received supply availability information indicates that power cannot be supplied (No at Step S 104 ), the process moves to Step S 109 . Further, if the second communication unit  31  has not received the supply availability information from the power transmission apparatus  10  (No at Step S 103 ), and if a given period of time has passed since the power supply start request was sent (Yes at Step S 110 ), the process also moves to Step S 109 . 
     At Step S 109 , the power supply control unit  65  performs control to switch the power to be supplied to the power supply switching unit  64  to the power stored (charged) in the first power storage unit  62  (Step S 109 ). The process thereafter is the same as the process of Step S 107  and later described above. 
     If the ASIC  44  determines at Step S 101  described above that the power saving mode entering factor has not occurred (No at Step S 101 ), the process moves to Step S 111 . At Step S 111 , the ASIC  44  determines whether the returning factor has occurred. 
     If determining that the returning factor has occurred (Yes at Step S 111 ), the ASIC  44  notifies the power supply control unit  65  to return from the power saving mode (power saving mode return notification). Upon receiving the power saving mode return notification from the ASIC  44 , the power supply control unit  65  performs control to return from the power saving mode (Step S 112 ). More specifically, the power supply control unit  65  controls the power supply switching unit  64  so as to switch the source of power supplied to the units of the image forming apparatus  20  to the power from the main power supply  63  (power from the commercial power source), and so as to resume supplying the power to the engine  50  and to the elements other than the ASIC  44  in the controller  40 . Then, the power supply control unit  65  controls the power supply switching unit  64  so as to resume supplying the power to the second communication unit  31  (Step S 113 ). 
     Next, the power supply control unit  65  requests the second communication unit  31  to send the power supply stop request to the power transmission apparatus  10 . Upon receiving the request from the power supply control unit  65 , the second communication unit  31  sends the power supply stop request to the power transmission apparatus  10  (Step S 114 ). In the present embodiment, the second communication unit  31  sends, in addition to the power supply stop request, the identification (ID) information of the image forming apparatus  20  to the power transmission apparatus  10 . 
     Next, the power supply control unit  65  determines whether the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10  (Step S 115 ). In the present embodiment, the power supply control unit  65  determines whether it has been notified of the power supply stop information from the second communication unit  31  to determine whether the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10 . 
     If determining that the second communication unit  31  has received the power supply stop information from the power transmission apparatus  10  (Yes at Step S 115 ), the power supply control unit  65  performs control to stop receiving the power from the power transmission apparatus  10  (Step S 116 ). For example, the power supply control unit  65  can also control the power receiving unit  61  so as to stop supplying the power received from the power transmission apparatus  10  to the power supply control unit  65 . If, instead, the second communication unit  31  has not received the power supply stop information from the power transmission apparatus  10  (No at Step S 115 ), and if a given period of time has passed since the power supply stop request was sent (Yes at Step S 117 ), the power supply control unit  65  also performs the control to stop receiving the power from the power transmission apparatus  10  (Step S 116 ). 
       FIG. 4  is a flow chart illustrating an operation example of the power transmission apparatus  10 . Specific details of  FIG. 4  will be described below. As illustrated in  FIG. 4 , the power transmission control unit  17  first determines whether the first communication unit  12  has received the power supply start request from the image forming apparatus  20  (Step S 201 ). In the present embodiment, the power transmission control unit  17  determines whether it has been notified of the power supply start request from the first communication unit  12  to determine whether the first communication unit  12  has received the power supply start request from the image forming apparatus  20 . 
     If determining that the first communication unit  12  has received the power supply start request from the image forming apparatus  20  (Yes at Step S 201 ), the power transmission control unit  17  determines whether the power supplied from (power generated by) the power generation unit  11  is not less than a threshold (Step S 202 ). 
     If determining that the power supplied from the power generation unit  11  is not less than the threshold (Yes at Step S 202 ), the power transmission control unit  17  starts the power supply to the image forming apparatus  20  (Step S 203 ). As described above, in the present embodiment, in the case of entering the power saving mode, the second communication unit  31  sends, to the power transmission apparatus  10 , the power supply start request, the identification (ID) information of the image forming apparatus  20 , and the required power information. Therefore, at Step S 201  described above, the first communication unit  12  receives, in addition to the power supply start request, the identification (ID) information of the image forming apparatus  20  and the required power information. Then, the power transmission control unit  17  performs control to send the power generated in the power generation unit  11  to the image forming apparatus  20  so as to satisfy the amount of power indicated by the required power information received from the image forming apparatus  20 . More specifically, the power transmission control unit  17  starts to control the amount of power supplied from the power generation unit  11  to the power transmission unit  16  so as to satisfy the amount of power indicated by the required power information and to send the power supplied from the power generation unit  11  to the power transmission unit  16  to the image forming apparatus  20  using the magnetic field resonance technique. 
     Next, the power transmission control unit  17  requests the first communication unit  12  to send, to the image forming apparatus  20 , the supply availability information indicating that the power supply is possible. Upon receiving the request from the power transmission control unit  17 , the first communication unit  12  sends, to the image forming apparatus  20 , the supply availability information indicating that the power supply is possible (Step S 204 ). Next, the power transmission control unit  17  stores, in the storage unit  15 , the information on the image forming apparatus  20  which has sent the power supply start request (Step S 205 ). More specifically, the power transmission control unit  17  stores, in the storage unit  15 , the identification information of the image forming apparatus  20  which has sent the power supply start request, in addition to necessary information (such as supply status information indicating whether the power supply is currently being performed). In this case, the supply status information is set (or updated) to that indicating that the power supply is currently being performed. 
     If determining at Step S 202  described above that the power supplied from the power generation unit  11  is less than the threshold (No at Step S 202 ), the power transmission control unit  17  requests the first communication unit  12  to send, to the image forming apparatus  20 , the supply availability information indicating that the power supply is impossible. Upon receiving the request from the power transmission control unit  17 , the first communication unit  12  sends, to the image forming apparatus  20 , the supply availability information indicating that the power supply is impossible (Step S 206 ), and the process is terminated. 
     If determining at Step S 201  described above that the first communication unit  12  has not received the power supply start request from the image forming apparatus  20  (No at Step S 201 ), the power transmission control unit  17  determines whether the first communication unit  12  has received the power supply stop request (and the identification information) from the image forming apparatus  20  (Step S 207 ). In the present embodiment, the power transmission control unit  17  determines whether it has been notified of the power supply stop request from the first communication unit  12  to determine whether the first communication unit  12  has received the power supply stop request from the image forming apparatus  20 . 
     If determining that the first communication unit  12  has received the power supply stop request from the image forming apparatus  20  (Yes at Step S 207 ), the power transmission control unit  17  stops the power supply to the image forming apparatus  20  (Step S 208 ). More specifically, the power transmission control unit  17  stops supplying the power from the power generation unit  11  to the power transmission unit  16 , and stops the control of sending the power to the image forming apparatus  20  using the magnetic field resonance technique. Next, the power transmission control unit  17  requests the first communication unit  12  to send, to the image forming apparatus  20 , the power supply stop information that indicates that the power supply is to be stopped. Upon receiving the request from the power transmission control unit  17 , the first communication unit  12  sends the power supply stop information to the image forming apparatus  20  (Step S 209 ). Next, the power transmission control unit  17  stores, in the storage unit  15 , the information on the image forming apparatus  20  which has sent the power supply stop request (Step S 210 ). More specifically, the power transmission control unit  17  stores, in the storage unit  15 , the identification information of the image forming apparatus  20  which has sent the power supply stop request, in addition to the necessary information (such as the supply status information indicating whether the power supply is currently being performed). In this case, the supply status information is set (or updated) to that indicating that the power supply is currently stopped. 
     As has been described above, in the present embodiment, the system configuration is such that the power transmission apparatus  10  provided with the power generation unit  11  composed of the photovoltaic cell is provided separately from the image forming apparatus  20 , and the power generated in the power generation unit  11  is supplied to the image forming apparatus  20  using the magnetic field resonance technique. Accordingly, it is possible to sufficiently ensure the power generated in the power generation unit  11  by installing the power transmission apparatus  10  at a place sufficiently irradiated with sunlight. 
     Here, in the system configuration in which a power transmission apparatus having a power generation unit such as a photovoltaic cell is provided separately from an image forming apparatus, and the power generated in the power generation unit is supplied to the image forming apparatus using the magnetic field resonance technique, the image forming apparatus needs to be provided, separately from an external I/F that can receive image data or the like from a host device such as a PC, with a communication unit (corresponding to the “second communication unit  31 ” of the present embodiment) for requesting the power transmission apparatus to start or stop supplying the power. From the viewpoint of energy saving, it is desirable to suppress the power consumption of the above-mentioned communication unit as much as possible. 
     As described above, in the present embodiment, in the case of entering the power saving mode, the power supply control unit  65  performs control to stop supplying power to the second communication unit  31 . Therefore, it is possible to reduce the power consumption of the communication apparatus  30  which communicates with the power transmission apparatus  10 . Alternatively, instead of stopping supplying power to the second communication unit  31 , the power supply control unit  65  may be configured, in the case of entering the power saving mode, to control the power supplied to the second communication unit  31  to a value smaller than the power supplied to the second communication unit  31  in the normal mode. This configuration can also reduce the power consumption of the communication apparatus  30 . The essential point is that, the power supply control unit  65  only needs to control, in the case of entering the power saving mode, the power supplied to the second communication unit  31  to a value (including 0 V) smaller than the power supplied to the second communication unit  31  in the normal mode. 
     In the present embodiment, in the case of entering the power saving mode, the power supply control unit  65  also performs, in addition to the control to stop supplying power to the second communication unit  31 , the control to switch the source of power supplied to the units of the image forming apparatus  20  to the power received at the power receiving unit  61 . Therefore, it is possible to use as little power as possible from the commercial power source while reducing the power consumption of the communication apparatus  30 . According to the present embodiment described above, it is possible to provide an image forming system that can achieve further reduction in power consumption. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. The second embodiment differs from the first embodiment in that the power transmission apparatus is provided with a power storage unit that stores therein the power generated in the power generation unit. Specific description will be made below. Note that the same numerals are used for the parts in common with those in the above-described first embodiment, and description thereof will be omitted as appropriate. 
       FIG. 5  is a block diagram illustrating a hardware configuration example of an image forming system (power control system)  1000  of the second embodiment. As illustrated in  FIG. 5 , the image forming system  1000  is provided with a power transmission apparatus  1  and the image forming apparatus  20 . The power transmission apparatus  1  differs from the power transmission apparatus  10  of the first embodiment in being further provided with a second power storage unit  18 . The other configuration is the same as that of the first embodiment. 
     Under the control of the power transmission control unit  17 , the second power storage unit  18  stores (accumulates) therein the power generated in the power generation unit  11 . The second power storage unit  18  can also discharge the stored power under the control of the power transmission control unit  17 . Further, the second power storage unit  18  can notify the power supply control unit  65  of an electricity storage amount of itself. The first power storage unit  62  can be composed, for example, of a secondary battery such as a lithium secondary battery or a nickel-metal hydride battery, or of a capacitor. However, the component is not limited to the above-listed ones. 
     The power transmission control unit  17  performs charge/discharge control of the second power storage unit  18  based on the electricity storage amount notification from the second power storage unit  18 . The power transmission control unit  17  can perform control to charge the second power storage unit  18  with the power from the power generation unit  11 , or can alternatively perform control to supply (discharge) the power stored in the second power storage unit  18  to the power transmission unit  16  and to send the power to the image forming apparatus  20  using the magnetic field resonance technique. 
       FIG. 6  is a flow chart illustrating an example of the charge control of the second power storage unit  18 . As illustrated in  FIG. 6 , the power transmission control unit  17  first determines whether the power generated by the power generation unit  11  is being sent to the image forming apparatus  20  (Step S 301 ). In other words, the power transmission control unit  17  determines whether the power is currently being supplied to the image forming apparatus  20 . 
     If determining that the power is currently being supplied (Yes at Step S 301 ), the power transmission control unit  17  determines, based on the electricity storage amount notification from the second power storage unit  18 , whether the electricity storage amount of the second power storage unit  18  is not less than a threshold (Step S 302 ). 
     If determining that the electricity storage amount of the second power storage unit  18  is not less than the threshold (Yes at Step S 302 ), the power transmission control unit  17  determines whether the power generated in the power generation unit  11  is currently being charged to the second power storage unit  18  (Step S 303 ). If, instead, determining that the electricity storage amount of the second power storage unit  18  is less than the threshold (No at Step S 302 ), the power transmission control unit  17  starts performing control to charge the second power storage unit  18  with the power generated in the power generation unit  11  (Step S 305 ). 
     If determining at Step S 303  described above that the power generated in the power generation unit  11  is currently being charged to the second power storage unit  18  (Yes at Step S 303 ), the power transmission control unit  17  performs control to stop charging the second power storage unit  18  (Step S 304 ). If, instead, it is determined at Step S 303  described above that the power generated in the power generation unit  11  is not currently being charged to the second power storage unit  18  (No at Step S 303 ), the process is terminated. 
     As has been described above, in the present embodiment, the power transmission apparatus  1  is provided with the second power storage unit  18  that stores therein the power generated in the power generation unit  11 , and accordingly has an advantage that the power stored in the second power storage unit  18  can be supplied to the image forming apparatus  20  even in the case of a bad weather such as a cloudy or a rainy weather, or in the case of a nighttime when the output of the photovoltaic cell drops. 
     Modifications 
     While the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, but various modifications can be made within the scope that does not depart from the gist of the present invention. 
     (1) First Modification 
     For example, in the case of entering the power saving mode, the power supply control unit  65  can perform control to switch the source of power supplied to the units of the image forming apparatus  20  to the power stored in the first power storage unit  62  if the power stored in the first power storage unit  62  is not less than the first threshold.  FIGS. 7A and 7B  are flow charts illustrating an operation example of the image forming apparatus  20  in this case. The example of  FIGS. 7A and 7B  differs from the example of  FIGS. 3A and 3B  in that Step S 118  is newly added. Description will be made below focusing on differences from the example of  FIGS. 3A and 3B . 
     As described in  FIGS. 7A and 7B , after Step S 101 , the power supply control unit  65  determines, based on the electricity storage amount notification from the first power storage unit  62 , whether the electricity storage amount of the first power storage unit  62  is not more than the first threshold (Step S 118 ). If the electricity storage amount of the first power storage unit  62  is determined to be not more than the first threshold (Yes at Step S 118 ), the process moves to Step S 102 . Although the process of Step S 102  and later is basically the same as that of  FIG. 3A , the process differs from that of the first embodiment in that the process returns to Step S 102  if the supply availability information indicating that the power supply is impossible is received from the power transmission apparatus  10  (No at Step S 104 ), or if the given period of time has passed since the power supply start request was sent to the power transmission apparatus  10  (Yes at Step S 110 ). Specifically, in this example, in the case of entering the power saving mode, if the power transmission apparatus  10  cannot supply power and if the power stored in the first power storage unit  62  is less than the first threshold, the power supply control unit  65  performs control of waiting until the power transmission apparatus  10  becomes capable of supplying power. 
     If, instead, the electricity storage amount of the first power storage unit  62  is determined to be not less than the first threshold (No at Step S 118 ), the power supply control unit  65  performs the control to switch the power to be supplied to the power supply switching unit  64  to the power stored in the first power storage unit  62  (Step S 109 ). Next, the power supply control unit  65  controls the power supply switching unit  64  so as to stop the power supply to the second communication unit  31  (Step S 107 ). Next, the power supply control unit  65  performs the control to enter the power saving mode (Step S 108 ). More specifically, the power supply control unit  65  controls the power supply switching unit  64  so as to switch the source of power supplied to the units of the image forming apparatus  20  to the power supplied from the power supply control unit  65  (power stored in the first power storage unit  62 ), and so as to stop supplying power to the engine  50  and to the elements other than the ASIC  44  in the controller  40 . As described above, in the case of entering the power saving mode, the power supply control unit  65  performs the control to switch the source of power supplied to the units of the image forming apparatus  20  to the power stored in the first power storage unit  62  if the power stored in the first power storage unit  62  is not less than the first threshold. 
     (2) Second Modification 
     In each of the above-described embodiments, the image forming system includes only one image forming apparatus  20 . However, not limited to this configuration, the image forming system can include any number of image forming apparatuses. For example, the system configuration may be such that one power transmission apparatus supplies power to each of a plurality of image forming apparatuses. In this configuration, in the case of supplying the power to each of the image forming apparatuses, the power transmission control unit  17  can control the amount of power supplied from the power generation unit  11  to the power transmission unit  16  so as to satisfy the sum of the amounts of power each indicated by the required power information from each of the image forming apparatuses, and can perform control to send the power to the image forming apparatuses using the magnetic field resonance technique. 
     (3) Third Modification 
     For example, if the power stored in the first power storage unit  62  is less than the second threshold after the image forming apparatus  20  enters the power saving mode in the state in which the source of power supplied to the units of the image forming apparatus  20  is switched to the power stored in the first power storage unit  62 , the power supply control unit  65  can perform control to request the power transmission apparatus  10  to start supplying power. The power supply control unit  65  can, for example, request the second communication unit  31  to send the power supply start request to the power transmission apparatus  10 . With this configuration, the power supply to the elements (such as the ASIC  44 ) of the image forming apparatus  20  that are to be operated even during the power saving mode is prevented from being stopped by depletion of the power stored in the first power storage unit  62 . 
     (4) Fourth Modification 
     For example, in the power saving mode, if the power generated in the power generation unit  11  or the power stored in the second power storage unit  18  (or the sum of these powers) is reduced below a threshold, and thus, if the amount of power required by the image forming apparatus  20  (amount of power indicated by the required power information) cannot be supplied, the power supply control unit  65  can notify the image forming apparatus  20  of the impossibility of supply. If receiving this notification, the power supply control unit  65  can perform the control to switch the source of power supplied to the units of the image forming apparatus  20  to the power from the main power supply  63  (power from the commercial power source), or can alternatively perform the control to return from the power saving mode to the normal mode. 
     The embodiments and the modifications described above can be arbitrarily combined with each other. 
     The control programs executed in the image forming apparatus (power supply control unit  65 ) of each of the above-described embodiments may be configured to be provided by being recorded in a computer-readable recording medium such as a CD-ROM, a flexible disk (FD), a CD-R, or a digital versatile disc (DVD) as files in an installable or an executable format. 
     Alternatively, the control programs executed in the image forming apparatus of each of the above-described embodiments may be configured to be provided by being stored on a computer connected to a network such as the Internet and downloaded via the network. Still alternatively, the control programs executed in the image forming apparatus of each of the above-described embodiments may be configured to be provided or distributed via a network such as the Internet. Assume that an electronic apparatus (such as the image forming apparatus of each of the above-described embodiments) is provided with processing units (such as the controller  40  and the engine  50 ) that perform processing, a communication unit (such as the second communication unit  31 ) that sends, to a power transmission apparatus that can send power generated thereby to an external device, a request for starting or stopping supplying the power, and a power receiving unit that receives the power from the power transmission apparatus. Then, the programs of the present invention can also be understood as programs for making the electronic apparatus execute control steps to control, in the case of entering the power saving mode in which power consumption is smaller than that in the normal mode in which the power is supplied to each of the processing units and the communication unit, the power supplied to the communication unit to a value smaller than the power supplied to the communication unit in the normal mode. 
     According to the embodiments, it is possible to provide an advantageous effect of bringing about further reduction in power consumption. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.