Patent Publication Number: US-8989606-B2

Title: Image forming apparatus having preference control unit

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus provided with image forming units capable of forming an image in different colors (for example, a tandem-type color printer). 
     In a conventional image forming apparatus capable of forming an image (for example, a tandem-type color printer), there are provided image forming units in different colors such as cyan (C), magenta (M), yellow (Y) and black (K) (also referred to as an image drum formed of an integrated unit of a photosensitive member, a toner cartridge, and the like); an LED (Light Emitting Diode) head combined with each of the image forming units for exposure; a transfer portion disposed below each of the image forming units; and a fixing portion disposed a backside of the transfer portion. When the conventional image forming apparatus performs a printing operation, it is possible to perform a color printing operation at a speed the same as that in a monochrome printing operation. 
     In term of usage frequency of the conventional image forming apparatus, the printing operation tends to be performed far more often for the color printing operation than the monochrome printing operation. Accordingly, black (K) toner tends to be consumed more than other color toner. As a result, the image forming unit in black (B) is replaced more frequently for replenishing black (K) toner. 
     As one of counter measures, Patent Reference has disclosed a conventional configuration, in which image forming units in colors are converted into an image forming unit for black, and only the image forming unit for black is used in a way similar to a monochrome printer. 
     Patent Reference: Japanese Patent Publication No. 2001-130053 
     According to Patent Reference, the conventional image forming apparatus is a tandem-type printer capable of performing a color printing operation and a monochrome printing operation. The conventional image forming apparatus includes a plurality of LED heads capable of printing per line; a unit for transferring a monochrome dot data row of one line to each of the LED heads; and a print control unit for controlling each of the LED heads to print only a specific dot area in a dot area of one line according to the monochrome dot data so that the LED heads divide the monochrome dot data row of one line. It is noted that the dot data are raster data for one line to be printed on a printing medium. 
     In the conventional image forming apparatus described above, when the image forming units are changed from multiple color to monochrome, an amount of toner in the monochrome color increases, thereby prolonging life of toner. Further, when the monochrome dot data row of one line are divided and used per image forming unit, a printing area is divided and becomes smaller, thereby increasing a printing speed as opposed to a case in which one image forming unit is used for one entire line. 
     As explained above, in the conventional image forming apparatus, toner in each of the image forming units is divided and used uniformly for increasing the printing speed. Accordingly, toner in each of the image forming units is consumed uniformly. There may be a case, however, in which it is necessary to change the image forming units from monochrome to multiple color before toner in the image forming units is completely consumed. In this case, the image forming unit for multiple color is replaced with a new image forming unit. When toner still remains in the image forming unit for monochrome (K), it is possible to continue to use the image forming unit, thereby making it possible not to replace the image forming unit. 
     As explained above, when it is necessary to perform the color printing operation, it is necessary to change the image forming units from monochrome to multiple color while the image forming unit for monochrome (K) is not replaced. In this case, toner in the image forming unit for monochrome (K) is completely consumed earlier, thereby making it necessary to replace the image forming unit for monochrome (K). 
     In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     In order to attain the objects described above, according to a first aspect of the present invention, an image forming apparatus includes an image forming portion capable of mounting a plurality of first image forming units for forming images in different colors or a plurality of second image forming units for forming images in a single color. The first image forming units and the second image forming units are arranged to be exchangeable. The first image forming units retain toner in different colors, and the second image forming units retain toner in the single color. The image forming apparatus further includes a preference control unit for leaving at least one of the second image forming units unused and preferentially using rest of the second image forming units when the first image forming units are exchanged to the second image forming units in a monochrome printing operation. 
     In the first aspect of the present invention, the image forming apparatus includes the preference control unit for leaving at least the one of the second image forming units unused and preferentially using the rest of the second image forming units when the first image forming units are exchanged to the second image forming units in the monochrome printing operation. 
     In the first aspect of the present invention, when the second image forming units are switched back to the first image forming units in a color printing operation while each of the second image forming units still retains toner, the one of the second image forming units does not consume toner in the previous printing operation. Accordingly, it is possible to use the one of the second image forming units from the previous state, and to prolong life of one of the second image forming units. As a result, it is possible to reduce the number of exchanges between the first image forming units and the second image forming units. 
     According to a second aspect of the present invention, an image forming apparatus is capable of performing a limited color printing operation and a multiple color printing operation 
     In the second aspect of the present invention, the image forming apparatus includes an image forming portion for mounting a first image forming unit for forming an image in a first color. The image forming portion is further capable of mounting a second image forming unit for forming an image in the first color in the limited color printing operation. The image forming portion is further capable of mounting a third image forming unit for forming an image in a second color different from the first color in the multiple color printing operation. 
     In the second aspect of the present invention, the image forming apparatus further includes a control unit for controlling the image forming portion so that the second image forming unit is preferentially used relative to the first image forming unit in the limited color printing operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a preference control unit of an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic sectional view showing a configuration of the image forming apparatus applied to a printer according to the first embodiment of the present invention; 
         FIG. 3  is a schematic view showing an example of an operation panel of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 4  is a block diagram showing the configuration of the preference control unit of the image forming apparatus in a color printing operation thereof according to the first embodiment of the present invention; 
         FIG. 5  is a schematic sectional view showing the configuration of the image forming apparatus applied to the printer in the color printing operation thereof according to the first embodiment of the present invention; 
         FIG. 6  is a flow chart showing an operation of the image forming apparatus in a mode setting switching process thereof according to the first embodiment of the present invention; 
         FIG. 7  is a flow chart showing an operation of the image forming apparatus in a printing operation thereof according to the first embodiment of the present invention; 
         FIG. 8  is a schematic view showing an example of an operation panel of an image forming apparatus according to a second embodiment of the present invention; 
         FIG. 9  is a block diagram showing a configuration of a preference control unit of the image forming apparatus according to the second embodiment of the present invention; 
         FIG. 10  is a block diagram showing the configuration of the preference control unit of the image forming apparatus in a color printing operation thereof according to the second embodiment of the present invention; 
         FIG. 11  is a flow chart showing an operation of the image forming apparatus in a mode setting switching process thereof according to the second embodiment of the present invention; and 
         FIG. 12  is a flow chart showing an operation of the image forming apparatus in a printing operation thereof according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. 
     First Embodiment 
     A first embodiment of the present invention will be explained.  FIG. 2  is a schematic sectional view showing a configuration of the image forming apparatus applied to a printer  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 2 , the printer  1  includes a housing  101  and an image forming portion  203  disposed in the housing  101 . The image forming portion  203  includes image drums  105 ,  106 ,  107  and  108  (image forming units) for different colors such as cyan (C), magenta (M), yellow (Y) and black (K); LED (Light Emitting Diode) heads  109 ,  110 ,  111  and  112  disposed above each of the image drums  105 ,  106 ,  107  and  108  for exposure; transfer portions  113 ,  114 ,  115  and  116  disposed below each of the image drums  105 ,  106 ,  107  and  108  for transferring developed images to a printing medium  103  (a sheet); and a transportation belt  104 . 
     In the embodiment, the image forming portion  203  further includes a fixing portion  117  for fixing toner transferred to the printing medium  103 ; a control circuit board  140  disposed above the image drums  105 ,  106 ,  107  and  108  and having a use image drum determination unit and a control unit integrated in an integrated circuit; an operation panel  121  disposed on an upper front surface of the housing  101  near the control circuit board  140 ; and an external interface unit  122  (an I/F) disposed on a lower backside surface of the housing  101  for externally connecting to an external device. Further, the printer  1  includes a tray  102  disposed below the image forming portion  203  for storing the printing medium  103 . 
     In the embodiment, in the image forming portion  203 , each of the image drums  105 ,  106 ,  107  and  108  includes a photosensitive member (not shown); a charging roller (not shown) disposed around an outer circumferential surface of the photosensitive member for uniformly charging the photosensitive member; a developing roller (not shown) for coating toner on a surface of the photosensitive member thus charged; the developing blade (not shown); a developing blade (not shown); a toner supply sponge roller (not shown); a cleaning blade (not shown) for removing toner on the photosensitive member after transfer; and a toner cartridge for retaining toner. The toner cartridge is configured to be detachable relative to each of the image drums  105 ,  106 ,  107  and  108 , and may be integrated with the image drums  105 ,  106 ,  107  and  108 . 
     In the embodiment, the image drums  105 ,  106 ,  107  and  108  are provided with the LED heads  109 ,  110 ,  111  and  112  for forming static latent images on charged surfaces of the photosensitive members through irradiating light with an exposure unit such as an LED; and toner cartridge memories  131 ,  132 ,  133  and  134  for storing color information and a remaining amount of toner retained in the toner cartridge. 
     As shown in  FIG. 2 , the control circuit board  140  is disposed at an upper portion or a side portion of the housing  101 . The printer  1  is connected to a host computer (a host personal computer or a host PC) through the external interface unit  122  or an LAN (Local Area Network). The control circuit board  140  includes an input output unit for transmitting and receiving print data between the printer  1  and the host PC, and for a user to set a mode and the like through a display on the operation panel  121 ; the control unit for controlling an entire portion of the printer  1  to operate each component of the printer  1  and process the print data; and a memory such as an EEPROM  141  for storing a necessary program for operating the printer  1  and the print data. The input output unit, the control unit, and the EEPROM  141  are integrated as the integrated circuit. 
     As shown in  FIG. 2 , the operation panel  121  as the input output unit is disposed on the upper front surface of the housing  101  of the printer  1 , and is connected to an operation panel control unit  201  of the control circuit board  140  (described later). The user operates a keyboard according to a screen displayed on a display portion formed of a liquid crystal display, so that the user can set the mode or make a selection. 
       FIG. 3  is a schematic view showing an example of the operation panel  121  of the image forming apparatus according to the first embodiment of the present invention. As shown in  FIG. 3 , the user can set the mode between a limited color printing mode and a multiple color printing mode. When the user selects the limited color printing mode, a valid light is turned on, thereby storing color information specifying a single color as a specified color in the EEPROM  141 . When the user selects the multiple color printing mode, an invalid light of the limited color printing mode is turned on, thereby storing the color information specifying a plurality of colors as the specified color in the EEPROM  141 . When the user specifies a certain color as the single color, the user operates the keyboard to store the certain color in the EEPROM  141 , and the certain color is displayed on the display portion. 
     A preference control unit  3  of the printer  1  will be explained next.  FIG. 1  is a block diagram showing a configuration of the preference control unit  3  of the printer  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 1 , the preference control unit  3  includes an operation panel control unit  201  for controlling the operation panel  121  as the input output unit; the EEPROM  141  for storing the specified color information according to the operation of the operation panel  121 ; a use image drum determination unit  202  for determining an image drum to be used among the image drums  105 ,  106 ,  107  and  108  according to the specified color information retrieved from the EEPROM  141 , and the color information of toner and the toner remaining amount information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 ; a toner empty state determination unit  204  for determining whether the toner cartridge retains toner or is empty; and the image forming portion  203  for receiving the print data and a control demand from the host PC  2  as the upper device, and for controlling the LED heads  109 ,  110 ,  111  and  112 , the transfer portions  113 ,  114 ,  115  and  116 , the transportation belt  104 , and the fixing portion  117 . 
     In the embodiment, the use image drum determination unit  202  is provided for determining which one of the image drums  105 ,  106 ,  107  and  108  is installed at which position, so that the use image drum determination unit  202  determines the image drum to be used according to the color information stored in the toner cartridge memories  131 ,  132 ,  133  and  134 . The use image drum determination unit  202  includes a control circuit having an information processing function such as a microprocessor and a volatile memory such as an SRAM. 
     More specifically, the use image drum determination unit  202  is arranged to determine whether the color information is retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134  in the image drums  105 ,  106 ,  107  and  108  according to the program stored in the EEPROM  141 . When the use image drum determination unit  202  determines that the color information is not retrieved, the use image drum determination unit  202  retrieves the color information from the toner cartridge memories  131 ,  132 ,  133  and  134  through a data line  205 . It is noted that the use image drum determination unit  202  registers the color information the toner cartridge memories  131 ,  132 ,  133  and  134  based on a bar code that can be identified according to toner colors of the image drums  105 ,  106 ,  107  and  108  or the toner cartridges. 
     When the use image drum determination unit  202  determines that the color information is retrieved, the use image drum determination unit  202  is capable of determining which one of the image drums  105 ,  106 ,  107  and  108  is installed at which position. Accordingly, the use image drum determination unit  202  has a function of determining the image drum to be used among the image drums  105 ,  106 ,  107  and  108  according to the specified color information retrieved from the EEPROM  141  and the color information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 . When there is only one image drum of the specified color among the image drums  105 ,  106 ,  107  and  108 , the use image drum determination unit  202  determines the one image drum as a consumed image drum, and stores information of the one image drum among the image drums  105 ,  106 ,  107  and  108  in the SRAM in the use image drum determination unit  202 . 
     When there is a plurality of image drums of the specified color among the image drums  105 ,  106 ,  107  and  108 , the use image drum determination unit  202  determines the image drum to be used through the following process. For example, it is supposed that the specified color information is black (K) color. In this case, the use image drum determination unit  202  searches the image drums  106 ,  107  and  108  of black (K) installed at a position where the image drum of black (K) is not normally installed. It is supposed that the image drums  106 ,  107  and  108  of black (K) are installed at positions where the image drums of other colors are normally installed. Accordingly, the use image drum determination unit  202  detects the image drums  106 ,  107  and  108 . 
     In the next step, the use image drum determination unit  202  determines whether the image drums  106 ,  107  and  108  are empty according to toner empty information notified from the toner empty state determination unit  204 . When the use image drum determination unit  202  determines that the image drums  106 ,  107  and  108  are not empty, the use image drum determination unit  202  determines the image drums  106 ,  107  and  108  as a candidate for the image drum to be used. 
     In the embodiment, a remaining amount sensor is disposed on a stirring bar (not shown) arranged in each of the toner cartridges for determining whether the image drums  106 ,  107  and  108  are empty. More specifically, the remaining amount sensor detects the remaining amount of toner in each of the toner cartridges, and registers the remaining amount in the toner cartridge memories  131 ,  132 ,  133  and  134  all the time. When the image drums  106 ,  107  and  108  become the empty state, the empty state is displayed on the operation panel  121 , so that the user is alarmed. 
     In the next step, in order to determine the image drum to be used from the candidates, the use image drum determination unit  202  determines which image drum retains toner in the specified color in an order of the image drum  106  installed at a position where the image drum of yellow is normally installed, the image drum  107  installed at a position where the image drum of magenta is normally installed, and the image drum  108  installed at a position where the image drum of cyan is normally installed. 
     In the embodiment, a detection unit such as a bar code reader is provided for detecting a bar code of the image drum or the toner cartridge, so that the use image drum determination unit  202  determines which image drum retains toner in the specified color. It is supposed that the image drum  106  installed at the position where the image drum of yellow is normally installed retains toner of black (K). Accordingly, the use image drum determination unit  202  determines the image drum  106  as the consumed image drum, and registers the information in the SRAM in the use image drum determination unit  202 . As a result, the image drum  105 , i.e., the original image drum of black (K), is left unused. 
     In the embodiment, the toner empty state determination unit  204  is provided for detecting the remaining amount of toner in each of the toner cartridges with the remaining amount sensor, so that the toner empty state determination unit  204  determines whether each of the toner cartridges is empty. The toner empty state determination unit  204  includes a control circuit having an information processing function such as a microprocessor. 
     More specifically, when the printer  1  is turned on, at which the image drums  105 ,  106 ,  107  and  108  are likely replaced, or when a housing cover is opened, the toner empty state determination unit  204  is arranged to retrieve the remaining amount of toner from the toner cartridge memories  131 ,  132 ,  133  and  134  in the image drums  105 ,  106 ,  107  and  108  through a data line  206  according to the program stored in the EEPROM  141 . Accordingly, the toner empty state determination unit  204  determines whether each of the toner cartridges is empty, and transmits the toner empty information to the use image drum determination unit  202 . 
     Further, every time when the printer  1  prints one page, the image forming portion  203  transmits a consumed amount of toner to the toner empty state determination unit  204 . Accordingly, the toner empty state determination unit  204  determines whether each of the toner cartridges is empty, and transmits the toner empty information to the use image drum determination unit  202 . 
     In the embodiment, when the image forming portion  203  receives the print data from the host PC  2 , the image forming portion  203  heats the fixing portion  117 , and controls the transportation belt  104  to transport the printing medium  103 . When the printing medium  103  is transported, the image forming portion  203  uses the image drums  105 ,  106 ,  107  and  108  in the single color to form a toner layer in the single color on the photosensitive members through an electro-photography process. 
     More specifically, the image forming portion  203  retrieves the use drum information from the SRAM in the use image drum determination unit  202 . Then, the image forming portion  203  uses the specified one of the LED heads  109 ,  110 ,  111  and  112  paired with the image drums  105 ,  106 ,  107  and  108  to expose the photosensitive member, thereby forming the static latent image in black (K). Then, the image forming portion  203  uses the specified one of the image drums  105 ,  106 ,  107  and  108  to attach toner in black (K) to the surface of the photosensitive member, thereby developing the static latent image. Further, the image forming portion  203  uses the specified one of the transfer portions  113 ,  114 ,  115  and  116  to transfer the toner image formed on the photosensitive member to the printing medium  103  supplied from the tray  102 . Accordingly, the toner image in black (K) is formed on the printing medium  103 . Afterward, the fixing portion  117  heats and presses the toner image transferred to the printing medium  103 , thereby fixing the toner image to the printing medium  103 . 
     In the embodiment, when the printer  1  is used as the multiple color (color) printer, the user selects the multiple color printing mode (invalidates the limited color printing mode) through the operation panel  121  shown in  FIG. 3 . Accordingly, the operation panel control unit  201  sets the specified color information as no specified color, and registers the specific color information in the EEPROM  141 . When the use image drum determination unit  202  receives the specified color information as no specified color, the use image drum determination unit  202  registered data with no specified color in the SRAM in the use image drum determination unit  202 . 
       FIG. 5  is a schematic sectional view showing the configuration of the image forming apparatus applied to the printer  1  in the color printing operation thereof according to the first embodiment of the present invention. As shown in  FIG. 5 , when the image forming portion  203  receives the print data from the host PC  2 , the image forming portion  203  heats the fixing portion  117 , and controls the transportation belt  104  to transport the printing medium  103 . When the printing medium  103  is transported, the image forming portion  203  uses image drums  150 ,  151 ,  152  and  153  in each color of cyan (C), magenta (M), yellow (Y), and black (K) to form the toner layer in each color on the photosensitive members through the electro-photography process. 
     More specifically, the image forming portion  203  retrieves the use drum information from the SRAM in the use image drum determination unit  202 . Since there is no specified color, the image forming portion  203  uses the LED heads  109 ,  110 ,  111  and  112  to expose the photosensitive members, thereby forming the static latent images in multiple colors according to the color information of the print data. Then, the image forming portion  203  uses the transfer portions  113 ,  114 ,  115  and  116  to transfer the toner images formed on the photosensitive members to the printing medium  103  supplied from the tray  102 . Accordingly, the toner images in multiple colors are formed on the printing medium  103 . Afterward, the fixing portion  117  heats and presses the toner images transferred to the printing medium  103 , thereby fixing the toner images to the printing medium  103 . 
     An operation of the printer  1  in an image drum selection process will be explained next with reference to flow charts.  FIG. 6  is a flow chart showing the operation of the image forming apparatus in a mode setting switching process thereof according to the first embodiment of the present invention.  FIG. 7  is a flow chart showing the operation of the image forming apparatus in a printing operation thereof after the printer  1  receives the print data from the host PC  2  according to the first embodiment of the present invention. 
     After the mode setting switching process stars, when the user switches the setting of the printing mode through the operation panel  121 , the specified color information is stored in the EEPROM  141 . Then, the use image drum determination unit  202  retrieves the specified color information from the EEPROM  141 . In step S 101 , the use image drum determination unit  202  determines whether the specified color is set from the toner cartridge memories  131 ,  132 ,  133  and  134 . In step S 110 , when the use image drum determination unit  202  determines that the specified color is not set, the use image drum determination unit  202  sets the use image drum information as that the specified color is not set, thereby completing the process. When the use image drum determination unit  202  determines that the specified color is set, the use image drum determination unit  202  proceeds the process to step S 102 . 
     In step S 102 , the use image drum determination unit  202  determines whether all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved. When the use image drum determination unit  202  determines that all of the toner cartridge memories  131 ,  132 ,  133  and  134  are not retrieved, the use image drum determination unit  202  waits until all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved. 
     In step S 103 , when all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved, the use image drum determination unit  202  determines whether a plurality of the image drums  106 ,  107  and  108  in the color same as the specified color is installed according to the color information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 . In step S 109 , when the use image drum determination unit  202  determines that only one of the image drums  106 ,  107  and  108  is installed, the use image drum determination unit  202  sets the one of the image drums  106 ,  107  and  108  as the use image drum information. 
     In step S 104 , when the use image drum determination unit  202  determines that a plurality of the image drums  106 ,  107  and  108  is installed, the use image drum determination unit  202  determines whether all of the image drums  106 ,  107  and  108  in the color same as the specified color and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state. 
     In step S 105 , when the use image drum determination unit  202  determines in step S 104  that not all of the image drums  106 ,  107  and  108  in the color same as the specified color and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state, the use image drum determination unit  202  displays the state that at least one of the image drums  106 ,  107  and  108  is in the empty state on the operation panel  121 . In step S 111 , the use image drum determination unit  202  sets the use image drum information, in which the image drum  106  not in the empty state and installed at a most upstream position among the image drums not in the empty state as the highest position in the preference order. Then, the use image drum determination unit  202  sets the use image drum information, in which the image drums  107  and  108  installed at downstream positions as the subsequent positions in the preference order. 
     In step S 106 , when the use image drum determination unit  202  determines in step S 104  that all of the image drums  106 ,  107  and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state, the use image drum determination unit  202  determines whether the image drum  105  installed at the normal position is in the empty state. 
     In step S 107 , the use image drum determination unit  202  determines in step S 106  that the image drum  105  installed at the normal position is not in the empty state, the use image drum determination unit  202  sets the image drum  105  installed at the normal position to the use image drum information, thereby completing the process. 
     In step S 108 , when the use image drum determination unit  202  determines in step S 106  that the image drum  105  installed at the normal position is in the empty state, the use image drum determination unit  202  controls the operation panel  121  through the operation panel control unit  201  to display that the image drum  105  is in the empty state. In step S 112 , the use image drum determination unit  202  stops the printer  1 , thereby completing the process. 
     The printing operation of the printer  1  will be explained next with reference to the flow chart shown in  FIG. 7 . In step S 201 , when the user operates the host PC to start the printing operation, the image forming portion  203  starts the transportation belt  104  to transport the printing medium  103  from the tray  102  according to the print data. 
     In step S 202 , the image forming portion  203  retrieves the use image drum information from the SRAM in the use image drum determination unit  202 , so that the image forming portion  203  determines whether the image drums  105 ,  106 ,  107  and  108  are set in the use image drum information. In step S 207 , when the image forming portion  203  determines that the image drums  105 ,  106 ,  107  and  108  are not set in the use image drum information, the image forming portion  203  uses all of the image drums  105 ,  106 ,  107  and  108  to perform the printing operation. 
     In step S 203 , when the image forming portion  203  determines that the image drums  106 ,  107  and  108  are set in the use image drum information, the image forming portion  203  uses the image drums  106 ,  107  and  108  and the LED heads  110 ,  111  and  112  paired with the image drums  106 ,  107  and  108  to perform the printing operation. 
     In step S 204 , the image forming portion  203  uses the transfer portions  114 ,  115  and  116  to transfer the toner images to the printing medium  103 . In step S 205 , the use image drum determination unit  202  determines whether the image drums  106 ,  107  and  108  thus used are in the empty state. 
     In step S 206 , when the use image drum determination unit  202  determines that the image drums  106 ,  107  and  108  thus used are in the empty state, the use image drum determination unit  202  performs the process shown in  FIG. 6  to determine the image drum to be used. When the use image drum determination unit  202  determines that the image drums  106 ,  107  and  108  thus used are not in the empty state, the use image drum determination unit  202  does not determine the image drum to be used. 
     In step S 208 , the image forming portion  203  determines whether the print jobs are completed. When the image forming portion  203  determines that the print jobs are completed, the image forming portion  203  stops the printing operation. When the image forming portion  203  determines that the print jobs are not completed, the image forming portion  203  repeats the process described above until the print jobs are completed. 
     As explained above, in the embodiment, the image drum  105  in black (K) is not used until the last step. Accordingly, toner of the image drum  105  in black (K), which is used in the monochrome printing operation, is not consumed and left unused. 
     Second Embodiment 
     A second embodiment of the present invention will be explained next. Components in the second embodiment similar to those in the first embodiment are designated with the same reference numerals, and explanations thereof are omitted. In the second embodiment, similar to the first embodiment, the image forming apparatus is applied to the printer  1 . 
     In the second embodiment, similar to the printer  1  in the first embodiment shown in  FIG. 2 , the printer  1  includes the housing  101  and an image forming portion  503  disposed in the housing  101  (refer to  FIG. 10 ). The image forming portion  503  includes the image drums  105 ,  106 ,  107  and  108  (image forming units) for different colors such as cyan (C), magenta (M), yellow (Y) and black (K); the LED (Light Emitting Diode) heads  109 ,  110 ,  111  and  112  disposed for each of the image drums  105 ,  106 ,  107  and  108  for exposure; the transfer portions  113 ,  114 ,  115  and  116  for transferring the developed images to the printing medium  103 ; and the transportation belt  104 . 
     In the embodiment, the image forming portion  503  further includes the fixing portion  117  for fixing toner transferred to the printing medium  103 ; the control circuit board  140  disposed above the image drums  105 ,  106 ,  107  and  108 ; the operation panel  121  disposed on the upper front surface of the housing  101  near the control circuit board  140 ; and the external interface unit  122  (an I/F) disposed on the lower backside surface of the housing  101  for externally connecting to an external device. Further, the printer  1  includes the tray  102  disposed below the image forming portion  503  for storing the printing medium  103 . 
     In the embodiment, each of the image drums  105 ,  106 ,  107  and  108  includes the photosensitive member (not shown); the charging roller (not shown) disposed around the outer circumferential surface of the photosensitive member for uniformly charging the photosensitive member; the developing roller (not shown) for coating toner on the surface of the photosensitive member thus charged; the developing blade (not shown); the toner supply sponge roller (not shown); the cleaning blade (not shown) for removing toner on the photosensitive member after transfer; and the toner cartridge for retaining toner. The toner cartridge is configured to be detachable relative to each of the image drums  105 ,  106 ,  107  and  108 , and may be integrated with the image drums  105 ,  106 ,  107  and  108 . 
     In the embodiment, the image drums  105 ,  106 ,  107  and  108  are provided with the LED heads  109 ,  110 ,  111  and  112  for forming the static latent images on the charged surfaces of the photosensitive members through irradiating light with the exposure unit such as the LED; and the toner cartridge memories  131 ,  132 ,  133  and  134  for storing the color information and the remaining amount of toner retained in the toner cartridge. 
     As shown in  FIG. 2 , the control circuit board  140  is disposed at the upper portion or the side portion of the housing  101 . The printer  1  is connected to the host computer (the host personal computer or the host PC) through the external interface unit  122  or the LAN (Local Area Network). The control circuit board  140  includes the input output unit for transmitting and receiving the print data between the printer  1  and the host PC, and for the user to set the mode and the like through the display on the operation panel  121 ; the control unit for controlling the entire portion of the printer  1  to operate each component of the printer  1  and process the print data; and the memory such as the EEPROM  141  for storing the necessary program for operating the printer  1  and the print data. The input output unit, the control unit, and the EEPROM  141  are integrated as the integrated circuit. 
     In the embodiment, the operation panel  121  is connected to the control unit of the control circuit board  140 . The user operates the keyboard according to the screen displayed on the display portion formed of the liquid crystal display, so that the user can set the mode or make the selection. 
       FIG. 8  is a schematic view showing an example of the operation panel  121  of the image forming apparatus according to the second embodiment of the present invention. As shown in  FIG. 8 , the user can set the mode between the limited color printing mode and the multiple color printing mode. When the user selects the limited color printing mode, a valid light is turned on, thereby storing the color information specifying the single color as the specified color in the EEPROM  141 . When the user selects the multiple color printing mode, an invalid light of the limited color printing mode is turned on, thereby storing the color information specifying a plurality of colors as the specified color in the EEPROM  141 . When the user specifies a certain color as the single color, the user operates the keyboard to store the certain color in the EEPROM  141 , and the certain color is displayed on the display portion. 
     In the embodiment, the user can selects an order of using the toner cartridges through a limited color toner consumption mode setting. When the user selects a uniform consumption, the toner cartridge remaining toner in a largest amount is used, so that all of the toner cartridges become the empty state at the same time. When the user selects a sequential consumption, the toner cartridge is completely used one by one. Each time when the toner cartridge becomes the empty state, a warning is displayed on the panel. 
     The preference control unit  3  of the printer  1  in the second embodiment will be explained next.  FIG. 9  is a block diagram showing a configuration of the preference control unit  3  of the printer  1  according to the second embodiment of the present invention. 
     As shown in  FIG. 9 , the preference control unit  3  of the printer  1  includes an operation panel control unit  501  for controlling the operation panel  121  as the input output unit; the EEPROM  141  for storing the specified color information according to the operation of the operation panel  121 ; a first use image drum determination unit  502  for determining an image drum to be used among the image drums  105 ,  106 ,  107  and  108  according to the specified color information retrieved from the EEPROM  141 , and the color information of toner and the toner remaining amount information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 ; a toner empty state determination unit  504  for determining whether the toner cartridge retains toner or is empty; and the image forming portion  503  for receiving the print data and the control demand from the host PC  2  as the upper device, and for controlling the LED heads  109 ,  110 ,  111  and  112 , the image drums  105 ,  106 ,  107  and  108 , the transfer portions  113 ,  114 ,  115  and  116 , and the fixing portion  117 , and a second use image drum determination unit  507 . 
     In the embodiment, the first use image drum determination unit  502  is provided for determining which one of the image drums  105 ,  106 ,  107  and  108  is installed at which position, so that the first use image drum determination unit  502  determines the image drum to be used according to the color information stored in the toner cartridge memories  131 ,  132 ,  133  and  134 . The use image drum determination unit  502  includes a control circuit having an information processing function such as a microprocessor and a volatile memory such as an SRAM. 
     More specifically, the first use image drum determination unit  502  is arranged to determine whether the color information is retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134  in the image drums  105 ,  106 ,  107  and  108  according to the program stored in the EEPROM  141 . When the first use image drum determination unit  502  determines that the color information is not retrieved, the first use image drum determination unit  502  retrieves the color information from the toner cartridge memories  131 ,  132 ,  133  and  134  through a data line  505 . 
     When the first use image drum determination unit  502  determines that the color information is retrieved, the use image drum determination unit  202  is capable of determining which one of the image drums  105 ,  106 ,  107  and  108  is installed at which position. Accordingly, the first use image drum determination unit  502  has a function of determining the image drum to be used among the image drums  105 ,  106 ,  107  and  108  according to the specified color information retrieved from the EEPROM  141  and the color information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 . 
     When there is only one image drum of the specified color among the image drums  105 ,  106 ,  107  and  108 , the first use image drum determination unit  502  determines the one image drum as a consumed image drum, and stores information of the one image drum among the image drums  105 ,  106 ,  107  and  108  in the SRAM in the first use image drum determination unit  502 . 
     When there is a plurality of image drums of the specified color among the image drums  105 ,  106 ,  107  and  108 , the first use image drum determination unit  502  determines the image drum to be used through the following process. For example, it is supposed that the specified color information is black (K) color. In this case, the first use image drum determination unit  502  searches the image drums  106 ,  107  and  108  of black (K) installed at a position where the image drum of black (K) is not normally installed. It is supposed that the image drums  106 ,  107  and  108  of black (K) are installed at positions where the image drums of other colors are normally installed. Accordingly, the first use image drum determination unit  502  detects the image drums  106 ,  107  and  108 . 
     In the next step, the first use image drum determination unit  502  determines whether the image drums  106 ,  107  and  108  are empty according to toner empty information notified from the toner empty state determination unit  504 . When the first use image drum determination unit  502  determines that the image drums  106 ,  107  and  108  are not empty, the first use image drum determination unit  502  determines the image drums  106 ,  107  and  108  as a candidate for the image drum to be used. 
     In the embodiment, a remaining amount sensor is disposed on a stirring bar (not shown) arranged in each of the toner cartridges for determining whether the image drums  106 ,  107  and  108  are empty. More specifically, the remaining amount sensor detects the remaining amount of toner in each of the toner cartridges, and registers the remaining amount in the toner cartridge memories  131 ,  132 ,  133  and  134  all the time. When the image drums  106 ,  107  and  108  become the empty state, the empty state is displayed on the operation panel  121 , so that the user is alarmed. 
     In the next step, in order to determine the image drum to be used from the candidates, the use image drum determination unit  202  determines which image drum retains toner in the specified color in an order of the image drum  106  installed at a position where the image drum of yellow is normally installed, the image drum  107  installed at a position where the image drum of magenta is normally installed, and the image drum  108  installed at a position where the image drum of cyan is normally installed. 
     In the embodiment, a detection unit such as a bar code reader is provided for detecting a bar code of the image drum or the toner cartridge, so that the first use image drum determination unit  502  determines which image drum retains toner in the specified color. It is supposed that the image drum  106  installed at the position where the image drum of yellow is normally installed retains toner of black (K). Accordingly, the first use image drum determination unit  502  determines the image drum  106  as the consumed image drum, and registers the information in the SRAM in the first use image drum determination unit  502 . As a result, the image drum  105 , i.e., the original image drum of black (K), is left unused. 
     In the embodiment, the second use image drum determination unit  507  includes a control circuit having an information processing function such as a microprocessor and a volatile memory such as an SRAM. The second use image drum determination unit  507  is provided for determining a method of using toner. More specifically, the second use image drum determination unit  507  is provided for selecting a sequential consumption mode or a uniform consumption mode. In the sequential consumption mode, toner in a plurality of the toner cartridges is consumed sequentially for each of the toner cartridges. In the uniform consumption mode, toner in a plurality of the toner cartridges is consumed uniformly. 
     More specifically, in the sequential consumption mode, similar to the first embodiment, the second use image drum determination unit  507  is arranged to determine which image drum retains toner in the specified color in the order of the image drum  106  installed at the position where the image drum of yellow is normally installed, the image drum  107  installed at the position where the image drum of magenta is normally installed, and the image drum  108  installed at the position where the image drum of cyan is normally installed, so that the image drum is stored in the use drum information. When the image drums  105 ,  106 ,  107  and  108  are in the empty state, the second use image drum determination unit  507  determines that the remaining image drum of black (K) as the image drum to be used, and registers the information in the SRAM in the first use image drum determination unit  502 . 
     In the uniform consumption mode, the second use image drum determination unit  507  retrieves the use drum information from the SRAM of the first use image drum determination unit  502 . Then, the second use image drum determination unit  507  determines one of the image drums  106 ,  107  and  108  retaining toner in the largest amount, and registers the one of the image drums  106 ,  107  and  108  as the use drum information in the SRAM of the first use image drum determination unit  502 . It is supposed that the image drum  106  installed at the position where the image drum of yellow is normally installed retains toner in the largest amount, and the image drum  106  is registered in the SRAM. In other words, the image drum retaining toner in the largest amount is used first. 
     In the embodiment, the toner empty state determination unit  504  is provided for detecting the remaining amount of toner in each of the toner cartridges with the remaining amount sensor, so that the toner empty state determination unit  504  determines whether each of the toner cartridges is empty. The toner empty state determination unit  504  includes a control circuit having an information processing function such as a microprocessor. 
     More specifically, when the printer  1  is turned on, at which the image drums  105 ,  106 ,  107  and  108  are likely replaced, or when a housing cover is opened, the toner empty state determination unit  504  is arranged to retrieve the remaining amount of toner from the toner cartridge memories  131 ,  132 ,  133  and  134  in the image drums  105 ,  106 ,  107  and  108  through a data line  506  according to the program stored in the EEPROM  141 . Accordingly, the toner empty state determination unit  504  determines whether each of the toner cartridges is empty, and transmits the toner empty information to the first use image drum determination unit  502 . 
     Further, every time when the printer  1  prints one page, an image forming portion  503  transmits the consumed amount of toner to the toner empty state determination unit  504 . Accordingly, the toner empty state determination unit  504  determines whether each of the toner cartridges is empty, and transmits the toner empty information to the first use image drum determination unit  502 . 
     In the embodiment, when the printer  1  is used as the multiple color (color) printer, the user selects the multiple color printing mode (invalidates the limited color printing mode) through the operation panel  121  shown in  FIG. 8 . Accordingly, the operation panel control unit  501  sets the specified color information as no specified color, and registers the specific color information in the EEPROM  141 . When the first use image drum determination unit  502  receives the specified color information as no specified color, the first use image drum determination unit  502  registered data with no specified color in the SRAM in the first use image drum determination unit  502 . 
     In the embodiment, as shown in  FIG. 5 , when the image forming portion  503  receives the print data from the host PC  2 , the image forming portion  503  heats the fixing portion  117 , and controls the transportation belt  104  to transport the printing medium  103 . When the printing medium  103  is transported, the image forming portion  503  uses the image drums  150 ,  151 ,  152  and  153  in each color of cyan (C), magenta (M), yellow (Y), and black (K) to form the toner layer in each color on the photosensitive members through the electro-photography process. 
     More specifically, the image forming portion  503  retrieves the use drum information from the SRAM in the first use image drum determination unit  502 . Since there is no specified color, the image forming portion  503  uses the LED heads  109 ,  110 ,  111  and  112  to expose the photosensitive members, thereby forming the static latent images in multiple colors according to the color information of the print data. Then, the image forming portion  503  uses the transfer portions  113 ,  114 ,  115  and  116  to transfer the toner images formed on the photosensitive members to the printing medium  103  supplied from the tray  102 . Accordingly, the toner images in multiple colors are formed on the printing medium  103 . Afterward, the fixing portion  117  heats and presses the toner images transferred to the printing medium  103 , thereby fixing the toner images to the printing medium  103 . 
     An operation of the printer  1  in the image drum selection process will be explained next with reference to a flow chart shown in  FIG. 11 .  FIG. 11  is the flow chart showing the operation of the image forming apparatus in a mode setting switching process thereof according to the second embodiment of the present invention. It is noted that the printing operation of the printer  1  after the printer  1  receives the print data from the host PC  2  is similar to the flow chart in  FIG. 7 . 
     After the mode setting switching process stars, when the user switches the setting of the printing mode through the operation panel  121 , the specified color information is stored in the EEPROM  141 . Then, the first use image drum determination unit  502  retrieves the specified color information from the EEPROM  141 . In step S 301 , the first use image drum determination unit  502  determines whether the specified color is set from the toner cartridge memories  131 ,  132 ,  133  and  134 . In step S 311 , when the first use image drum determination unit  502  determines that the specified color is not set, the first use image drum determination unit  502  sets the use image drum information as that the specified color is not set, thereby completing the process. When the first use image drum determination unit  502  determines that the specified color is set, the first use image drum determination unit  502  proceeds the process to step S 302 . 
     In step S 302 , the first use image drum determination unit  502  determines whether all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved. When the first use image drum determination unit  502  determines that all of the toner cartridge memories  131 ,  132 ,  133  and  134  are not retrieved, the first use image drum determination unit  502  waits until all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved. 
     In step S 303 , when the first use image drum determination unit  502  determines that all of the toner cartridge memories  131 ,  132 ,  133  and  134  are retrieved in step S 302 , the first use image drum determination unit  502  determines whether a plurality of the image drums  106 ,  107  and  108  in the color same as the specified color is installed according to the color information retrieved from the toner cartridge memories  131 ,  132 ,  133  and  134 . In step S 310 , when the first use image drum determination unit  502  determines that only one of the image drums  106 ,  107  and  108  is installed, the first use image drum determination unit  502  sets the one of the image drums  106 ,  107  and  108  as the use image drum information. 
     In step S 304 , when the first use image drum determination unit  502  determines that a plurality of the image drums  106 ,  107  and  108  is installed in step S 303 , the first use image drum determination unit  502  determines whether all of the image drums  106 ,  107  and  108  in the color same as the specified color and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state. 
     In step S 305 , when the first use image drum determination unit  502  determines in step S 304  that not all of the image drums  106 ,  107  and  108  in the color same as the specified color and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state, the first use image drum determination unit  502  displays the state that at least one of the image drums  106 ,  107  and  108  is in the empty state on the operation panel  121 . 
     In step S 306 , the use image drum determination unit  202  sets the image drum  106  not in the empty state and installed at a most upstream position among the image drums not in the empty state to the use image drum information. 
     In step S 307 , when the first use image drum determination unit  502  determines in step S 304  that all of the image drums  106 ,  107  and installed at positions other than the positions where the image drums  106 ,  107  and  108  are normally installed in the color printing operation are in the empty state, the first use image drum determination unit  502  determines whether the image drum  105  installed at the normal position is in the empty state. 
     In step S 308 , the first use image drum determination unit  502  determines in step S 307  that the image drum  105  installed at the normal position is not in the empty state, the first use image drum determination unit  502  sets the image drum  105  installed at the normal position to the use image drum information, thereby completing the process. 
     In step S 309 , when the first use image drum determination unit  502  determines in step S 106  that the image drum  105  installed at the normal position is in the empty state, the first use image drum determination unit  502  controls the operation panel  121  through the operation panel control unit  201  to display that the image drum  105  is in the empty state. In step S 312 , the first use image drum determination unit  502  stops the printer  1 , thereby completing the process. 
     The printing operation of the printer  1  will be explained next with reference to the flow chart shown in  FIG. 12 .  FIG. 12  is a flow chart showing the operation of the image forming apparatus in the printing operation thereof according to the second embodiment of the present invention. 
     In step S 401 , when the user operates the host PC to start the printing operation, the image forming portion  503  starts the transportation belt  104  to transport the printing medium  103  from the tray  102  according to the print data. Further, the image forming portion  503  controls the second use image drum determination unit  507  to retrieve the use image drum information. 
     In step S 402 , the second use image drum determination unit  507  retrieves the use image drum information from the SRAM in the first use image drum determination unit  502 , so that the second use image drum determination unit  507  determines whether the image drums  105 ,  106 ,  107  and  108  are set in the use image drum information. In step S 407 , when the second use image drum determination unit  507  determines that the image drums  105 ,  106 ,  107  and  108  are not set in the use image drum information, the second use image drum determination unit  507  uses all of the image drums  105 ,  106 ,  107  and  108  to perform the printing operation. 
     In step S 403 , when the second use image drum determination unit  507  determines that the image drums  106 ,  107  and  108  are set in the use image drum information, the second use image drum determination unit  507  determines the limited color toner consumption mode. When the second use image drum determination unit  507  determines that the limited color toner consumption mode is the uniform consumption mode, the second use image drum determination unit  507  determines one of the image drums  106 ,  107  and  108  retaining toner in the largest amount, and registers the one of the image drums  106 ,  107  and  108  to the use drum information. 
     In step S 409 , the image forming portion  503  uses the image drums  106 ,  107  and  108  and the LED heads  110 ,  111  and  112  paired with the image drums  106 ,  107  and  108  thus set in the second use image drum determination unit  507  to perform the printing operation. 
     In step S 410 , when the second use image drum determination unit  507  determines that the limited color toner consumption mode is the sequential consumption mode, the second use image drum determination unit  507  sets the image drum  106  installed at the most upstream position among the image drums  106 ,  107  and  108  to the use image drum information. Accordingly, the image forming portion  503  uses the image drum  106  and the LED head  110  paired with the image drum  106  thus set in the second use image drum determination unit  507  to perform the printing operation. 
     In step S 404 , the image forming portion  503  uses the transfer portions  114 ,  115  and  116  to transfer the toner images to the printing medium  103 . In step S 405 , the second use image drum determination unit  507  determines whether the image drums  106 ,  107  and  108  thus used are in the empty state. 
     In step S 406 , when the second use image drum determination unit  507  determines that the image drums  106 ,  107  and  108  thus used are in the empty state, the second use image drum determination unit  507  performs the process shown in  FIG. 11  to determine the image drum to be used. When the second use image drum determination unit  507  determines that the image drums  106 ,  107  and  108  thus used are not in the empty state, the second use image drum determination unit  507  does not change the image drum to be used. When the printer  1  performs the printing operation on the next printing medium, the second use image drum determination unit  507  determines again one of the image drums  106 ,  107  and  108  retaining toner in the largest amount, thereby performing the printing operation using the one of the image drums  106 ,  107  and  108 . 
     In step S 408 , the image forming portion  503  determines whether the print jobs are completed. When the image forming portion  503  determines that the print jobs are completed, the image forming portion  503  stops the printing operation. When the image forming portion  503  determines that the print jobs are not completed, the image forming portion  503  repeats the process described above until the print jobs are completed. 
     As explained above, in the embodiment, it is possible to select the sequential consumption mode or the uniform consumption mode. In the sequential consumption mode, each of the image drums  106 ,  107  and  108 , which are preferentially used in the monochrome printing operation, is sequentially used. In the uniform consumption mode, one of the image drums  106 ,  107  and  108  retaining toner in the largest amount is used. Accordingly, it is possible to select the case, in which the image drums  106 ,  107  and  108  to be used preferentially are replaced at the same time, and the case, in which each of the image drums  106 ,  107  and  108  is replaced when becomes the empty state. 
     The disclosure of Japanese Patent Application No. 2010-167074, filed on Jul. 26, 2010, is incorporated in the application by reference. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.